Compilation of Submitted Abstracts for Magnetic Fields: CC2YSO
Abstract submissions for this conference appear below, categorized by subject matter / probable session. The names of contributors are appended with the following identifiers: an (I) if they are invited speakers, a (C) if they are being considered for a contributed talk, and a (P) if electing to contribute a poster talk.
Confirmed participants wishing to submit their abstracts, whom have not already done so,
should contact us directly at cc2yso [at] uwo [dot] ca.
Core Collapse and Inner Regions
| Daniele Galli (I) | Magnetic braking and field dissipation in the protostellar accretion phase |
| I will summarize recent theoretical work on the role of magnetic fields in the process of star formation and disk accretion. First, I will concentrate on the efficiency of magnetic braking during cloud collapse and its consequences on the formation of centrifugally supported disks around young stars. Then, I will show that the introduction of non-ideal MHD effects is a necessary step toward the development of self-consistent models for the collapse of molecular clouds and the formation and evolution of accretion disks around young stars . Finally, I will discuss the structure and evolution of magnetized accretion disks around young stars that have dragged their magnetic field in the process of gravitational collapse. | |
| Josep Miquel Girart (I) | Observations of magnetic fields in the advent of ALMA |
| Interstellar magnetic fields are known to thread the molecular clouds, but their specific role in molecular cloud evolution and in the formation of stars is a vivid matter of debate. We have observed the dust polarization at optical and submm wavelengths to study the magnetic field at different scales in molecular clouds and dense cores, from few parsecs to few hundred astronomical units (some of the results of this study are being presented in other talks and posters). In the first part of the talk, I will show the the results of a multi-wavelength study of the Pipe Nebula, a nearby massive dark cloud with a very low star formation efficiency. In the second part of the talk, I will present the results of the SMA observation toward high mass star forming dense molecular cores. | |
| Patrick Hennebelle (I) | The influence of the magnetic field on the protostellar collapse. Is there a fragmentation crisis? |
| Various works have recently demonstrated that the magnetic field is playing a crutial role during the collapse of protostellar cores. I will first show that magnetic field is largely inhibiting the disk formation but also the fragmentation in few objects, even when the magnetic intensity is sufficiently weak to allow for the formation of a disk. I will then discuss possible scenarii to account for the core fragmentation. Then, I will discuss the consequences of the magnetic field being not aligned with the rotation axis. In particular, I will show that disks can be more easily formed when the magnetic field and the rotation axis are perpendicular and I will discuss the physical reason of this. The consequences of the misalignement on the outflows will also be presented. | |
| Shu-ichiro Inutsuka (I) | Emergence of protoplanetary disks and successive formation of gaseous planet by gravitational instability |
| The early evolution of protostars are reviewed in this talk. We use resistive magnetohydrodynamical simulations with the nested grid technique to study the formation of protostars and protoplanetary disks from molecular cloud cores that provide the realistic environments for planet formation. We find that gaseous planetary-mass objects are formed much earlier than previously thought, by gravitational instability in regions that are de-coupled from the magnetic field and surrounded by the injection points of the magnetohydrodynamical outflows during the formation phase of protoplanetary disks. Magnetic de-coupling enables massive disks to form and these are subject to gravitational instability, even at ~10 AU. The frequent formation of planetary mass objects in the disk suggests the possibility of constructing a hybrid scenario of planet formation, where the rocky planets form later under the influence of the giant planets in the protoplanetary disk. | |
| Shih-Ping Lai (I) | Toroidal magnetic field revealed in the pseudodisk of NGC1333 IRAS 4A |
| We present the first map of the toroidal magnetic field structure in a pseudodisk of a star-forming core. The magnetic field in the low-mass protostellar core NGC1333 IRAS4A (hereafter IRAS4A) have an hourglass morphology in the scale of few thousands AU (Girart et al. 2006, Science). Here we further explore the magnetic field structure within the central 1000 AU region of IRAS4A with the sub-arcsecond resolution dust polarization data taken with SMA at 345 GHz. The SMA observations reveal that except for the regions perpendicular to the center of IRAS4A1, the magnetic field appears to be parallel the protostellar binary, IRAS4A1 and IRAS4A2, and perpendicular to the large scale hourglass structure. We model the observed polarization with a large-scale poloidal (hourglass-like) field and a small-scale toroidal field withing central 1000 AU, and show that the toroidal component is necessary for producing the observed field structure. This modified geometry is in agreement with the expectation of star formation theories with ideal MHD conditions and nonzero angular momentum. | |
| Zhi-Yun Li (I) | Magnetic braking and protostellar disk formation |
| I will review the work by our group on disk formation in magnetized cloud cores during the protostellar mass accretion phase. We find that even a relatively weak field can suppress the formation of a rotationally supported disk through magnetic braking in the ideal MHD limit and under the axisymmetry assumption. Ambipolar diffusion does not appear capable of weakening the braking enough to enable disk formation for realistic levels of core magnetization and cosmic ray ionization rate. We demonstrate that the formation of 100-AU scale disks can be enabled by Ohmic dissipation, although the required resistivity needs to be enhanced over the classic value. Preliminary work on the Hall effect will also be presented. | |
| Telemachos Mouschovias (I) | Magnetic fields in star formation: crucial but not omnipotent |
| From the formation of interstellar clouds (atomic and molecular) to the support of self-gravitating clouds, their fragmentation and collapse of fragments (or cores) to form stars, magnetic fields have been shown to play a central role. The theoretical and observational evidence establishing magnetic fields first as the close ally of gravity and later as its primary opponent in the star formation process is reviewed. No mechanism other than magnetic braking has been shown capable of resolving the most serious dynamical problem of star formation, namely, the angular momentum problem. And no mechanism other than ambipolar diffusion, when it reawakens at densities ~10^10 cm^-3, has been demonstrated effective enough to resolve the bulk of the magnetic flux problem, although Ohmic dissipation appears necessary at present to complete the task during the late, opaque (i.e., nonisothermal) phases of star formation. Analytical calculations and numerical simulations over the last more than thirty years have established not only the central role of magnetic fields in the star formation process but also the important role of grains in the process. Grains are not just important in the microscopic processes that establish the degree of ionization in molecular clouds and protostellar fragments and thereby affecting the dynamical processes in those objects. The microscopic processes involving grains are also affected by the macroscopic dynamics. Ignoring these nonlinear interactions leads to quantitatively and, more importantly, qualitatively incorrect conclusions. Although the two-fluid (plasma and neutral molecules) approximation has yielded (and can still yield) some important results, the complete six-fluid system (neutral molecules, atomic and molecular ions, electrons, negatively- charged, positively-charged, and neutral grains) is necessary for understanding the star formation process and for obtaining reliable, testable predictions of core collapse and the formation of young stellar objects. | |
| Ramprasad Rao (I) | An observational view of the magnetic field structure in low mass star forming regions |
| Recent measurements of the magnetic field structure in regions of low mass star formation have been made with the polarimetry system installed on the Submillimeter Array (SMA). In two of the closest and brightest systems, NGC 1333 IRAS 4A and IRAS 16293-2422, the magnetic field topology indeed resembles an hour-glass structure that is predicted from theoretical models in which the magnetic field plays a dominant role. However, there are similar regions, such as HL Tau and L1551 IRS 5 and others, where no polarization is significantly detected. We will attempt to provide an explanation for the discrepancy between the various observations. With the advent of the Atacama Large Millimeter/Submillimeter Array (ALMA) this sample of high resolution magnetic field maps can be increased. This should enable us to address, through observations, the important question of the role of magnetic fields in the process of low mass star formation. | |
| Felipe Alves (C) | Magnetic field properties of NGC 2024 FIR 5 |
| We present new SMA dust polarization data of the protostellar core NGC 2024 FIR 5. Our continuum maps show that FIR 5 is resolved into two dust condensations, FIR 5 A and B. The brightest of them, FIR 5 A, is powering an unipolar and collimated molecular outflow which may be interacting with the local medium, giving a complex CO morphology to the region. The polarization maps yield a curved magnetic filed around A. The core is in a supercritical stage, which suggests that the curved magnetic field around source FIR 5 A is delineating the expected hourglass morphology. Alternatively, we offer another possible explanation for the curved magnetic filed morphology around FIR 5: it could be tracing the compression produced by the expansion of the HII region associated with the FIR 2 massive stars. We discuss if this scenario is expected based on an energetic analysis. | |
| Benoit Commercon (C) | Protostellar collapse: magnetic and radiative feedbacks on small scales collapse and fragmentation |
| It is established that both radiative transfer and magnetic field have a strong impact on the collapse and the fragmentation of prestellar dense cores. We perform the first Radiation-Magneto-HydroDynamics (RMHD) numerical calculations at a prestellar core scale. I will first briefly discuss the RMHD solver we designed in the RAMSES code. Then I will present original AMR calculations including magnetic field (in the ideal MHD limit) and radiative transfer, within the Flux Limited Diffusion (FLD) approximation, of the collapse of a 1 solar mass dense core. We compare the results with calculations performed with a barotropic EOS. We show that radiative transfer has an important impact on the collapse and the fragmentation, through the cooling or heating of the gas, and is complementary of the magnetic field. A larger field yields a stronger magnetic braking, increasing the accretion rate and thus the effect of the radiative feedback. Even for a strongly magnetized core, where the dynamics of the collapse is dominated by the magnetic field, radiative transfer is crucial to determine the temperature and optical depth distributions, two potentially accessible observational diagnostics. A barotropic EOS cannot account for realistic fragmentation. The diffusivity of the numerical scheme, however, is found to strongly affect the output of the collapse, leading eventually to spurious fragmentation. | |
| Wolf Dapp (C) | A resolution of the magnetic braking catastrophe during the second collapse |
| We perform axisymmetric non-ideal MHD simulations of gravitational collapse of an isolated, rotating, low-mass prestellar core. Our models include magnetic braking and one or both of ambipolar diffusion and ohmic dissipation. We integrate through the 'second-collapse' phase all the way to a stellar-sized object using a barotropic equation of state. We quantify conditions under which a centrifugally-supported disk can form, and show that this happens very close to the YSO. We find that significant radial flux loss can hamper magnetic braking sufficiently to allow a disk to form. We conclude that the resolution of the 'magnetic braking catastrophe' is found at very small scales. | |
| Dennis Duffin (C) | Disks, outflows, and feedback in collapsing magnetized cores |
| The pre-stellar cores in which low mass stars form are highly magnetized. Early protostellar disks will be massive and will experience strong magnetic torques in the form of magnetic braking and protostellar outflows. Early simulations of protostellar disk formation suggest that these torques are strong enough to supress a rotationally supported structure from forming for near critical values of mass-to-flux. We demonstrate through the use of a 3D adaptive mesh refinement code -- including cooling, sink particles and magnetic fields -- that one easily produces kAU disks while simultaneously generating large outflows which leave the core region, carrying away mass and angular momentum. Early inflow/outflow rates suggest that only small fraction of the mass is lost in the initial magnetic tower/jet event. | |
| Pau Frau Méndez (C) | Fitting magnetized molecular cloud collapse models to NGC 1333 IRAS 4A |
| Magnetic fields are believed to play an important role in the star formation process. Grain alignment is one of the visible effects of the magnetic fields polarizing the dust emission perpendicularly to the field-lines. To test the influence of magnetic fields we compare high-angular resolution observations of the submillimetric polarized emission of the low-mass protostar NGC 1333 IRAS 4A with collapse models of magnetized molecular cloud cores. We assume a model density and magnetic flux function to compute the Stokes parameters and synthetic polarization maps convolved with the inteferometric response. Our synthetic maps show a magnetic field morphology in a good agreement with the data suggesting that this theoretical scenario is a plausible explanation. Consequently, instead of turbulence, well-ordered magnetic fields control the evolution of low-mass star-forming molecular cloud cores. | |
| Matthew Kunz (C) | The role of magnetic fields, dust, and radiation in star formation |
| Magnetic fields are an importunate necessity in the formulation of a theory of star formation. Not only do they play a crucial role in the fragmentation of molecular clouds and the subsequent contraction of prestellar cores, but they also significantly affect (and are affected by) the dynamics of dust grains and hence the emitted radiation. I will present results of the first radiative, 6-fluid, nonideal MHD star formation simulations to accurately track the evolution of a protostellar fragment over eleven orders of magnitude in density, from the early ambipolar-diffusion initiated fragmentation phase, through the magnetically supercritical, dynamical-contraction phase and the subsequent magnetic decoupling stage, all the way to the nonisothermal phase, including the formation and evolution of a hydrostatic core. The ability of ambipolar diffusion and/or Ohmic dissipation to resolve the magnetic flux problem of star formation will be addressed. Emphasis will be placed on detailed comparisons with both current and ongoing observations. | |
| Anaelle Maury (C) | Understanding the formation of multiple systems: a pilot subarcsecond survey of Class 0 objects and its impact on numerical simulations of protostellar collapse |
| The formation process of binary stars and multiple systems is poorly understood. To determine the typical outcome of protostellar collapse and constrain models of binary formation by core fragmentation during collapse, we carried out very high-resolution millimeter imaging of very young (Class 0) protostars. Using the most extended (A) configuration of the IRAM PdBI at 1.3 mm allowed us to probe the multiplicity of Class 0 protostars down to separations a~50 AU and circumstellar mass ratios q~0.07. I will present the results of these unprecedented observations, and show that, when combined with previous millimeter interferometric observations of Class 0 protostars, our pilot PdBI study tentatively suggests that the binary fraction in the ~ 75-1000 AU range increases from the Class 0 to the Class I stage. Finally, I will show why these recent results argue against purely hydrodynamic models of protostellar collapse, while they lend strong support to magnetized scenarios. See Maury et al. (2010) for further details. |
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| Tao-Chung Ching (P) | Magnetic fields associated with the outflows in NGC1333 IRAS4A protostellar core |
| We present the polarization map of CO J = 3-2 line in the molecular outflows launched from NGC1333 IRAS 4A protostellar core. Linearly polarized spectral line emission arises from the Goldreich-Kylafis effect, which predicts that the polarization should be either parallel or perpendicular to the magnetic field. We compare the CO J = 3-2 polarization to the dust polarization to resolve the ambiguity of magnetic field direction in Goldreich-Kylafis effect. Within IRAS 4A dust envelope, the CO J = 3-2 polarization is perpendicular to dust polarization, and therefore indicates that the outflow is parallel to magnetic field close to the origin. Away from the dust continuum, the smooth variation of CO position angles implies that the magnetic field seems to be bended over to perpendicular to the outflow. The magnetic field direction inferred from our CO J = 3-2 polarization is consistent with the field probed by CO J = 2-1 polarization over a more extended outflow region, suggesting that the deflection of the outflow may be the result of the interaction between the outflow and the magnetic field. |
YSOs and Stars
| Evelyne Alecian (I) | Magnetism and activity in Herbig Ae/Be stars |
| While it is generally well accepted that partly convective low-mass stars (M<1.5 Msun) generate their magnetic fields through convective dynamo, it is more difficult to understand the origin of the strong (~1 kG) magnetic fields in few of the primarily radiative higher mass stars (M>1.5 Msun). Our favored hypothesis is the fossil field theory implying that the magnetic fields observed among the main sequence A/B stars are relics from the magnetic fields present in the parental molecular clouds, or generated very early during the stellar formation. This theory implies also that, once the star is formed, their magnetic fields survive to the various phases of stellar evolution without regeneration. According to this theory, few of the pre-main sequence (PMS) progenitors of the A/B stars, the Herbig Ae/Be stars (HAeBe) should be magnetic. In addition, some of the HAeBe stars show indirect signs of activity that could be of magnetic origin (X-rays, rotational modulation of non-photospheric lines, emission lines of highly-ionised species). Until recently, we were strongly missing information about the magnetism in the HAeBe stars. In order to test the fossil field theory and understand the origin of the active HAeBe stars, we performed a high-resolution spectropolarimetric survey of the Herbig Ae/Be stars using the new generation instrument ESPaDOnS, installed at the Canada-France-Hawaii telescope. In this talk I will present our results and discuss them in the framework of the activity observed in some HAeBe stars. | |
| Scott Gregory (I) | The star-disk interaction as a function of spectral type |
| The large-scale magnetic topologies of classical T Tauri stars can now be probed in unprecedented detail through Zeeman-Doppler imaging. To date, magnetic maps of four accreting T Tauri stars have been published. Initial results suggest that the magnetospheres of high mass T Tauri stars (above one solar mass) are significantly more complex than those of intermediate mass stars (0.5 to 1 Msun). The intermediate mass star BP Tau, which is wholly convective, has a simple large-scale magnetic topology with a strong dipole component. In contrast, the high mass stars V2129 Oph, CR Cha and CV Cha, which have already developed radiative cores, have complex magnetic fields with many strong high order field components, and weak dipole components. More data is required to confirm if this rapid increase in field complexity, and decay in the strength of the dipole component, with increasing stellar mass, is a general result. However, it is similar to what is found in studies of the magnetic fields of low-mass main-sequence stars. Assuming that the magnetospheres of pre-main sequence stars follow similar trends, I examine the star-disk interaction across the fully convective-radiative core divide for a large sample of stars of varying spectral type (a proxy for stellar mass). I will discuss how disparate results from the molecular physics and geophysics literature can be applied to construct simple analytic models of multipolar magnetospheres. By assuming that disks are truncated where the magnetic torque from the stellar magnetosphere is comparable to the viscous torque in the disk, I find that only intermediate mass stars can truncate their disks out to the corotation radius. The disks of high mass accreting T Tauri stars are always truncated well within corotation. I conclude that the star-disk interaction is more strongly mass dependent than previously anticipated by dipolar accretion models, and the angular momentum removal mechanism that must operate in order to explain the slow rotation of accreting T Tauri stars is more efficient for earlier spectral types. | |
| Christopher Johns-Krull (I) | Magnetospheric accretion onto the Classical T Tauri Star BP Tauri |
| We report new observations of the Classical T Tauri Star (CTTS) BP Tauri made with the Zeeman Analyzer system coupled to the Robert G. Tull coude echelle spectrometer of the 2.7 m Harlan J. Smith telescope at McDonald Observatory. This CTTS was observed for 7 nights in December 2009, covering nearly an entire rotation period of this star. We combine this data with earlier time series data taken with this the same system. We analyze the photospheric absorption lines with a newly implemented least squares deconvolution analysis package. We also study the polarization behavior of the variable emission lines of this star, focusng particular attention on lines such as those of He I 587.6 nm and 667.8 nm which are believed to form primarily in the accretion shock at the stellar surface in the case of BP Tau. Recent observations have reported very different field strengths for these two lines, while models of the accretion shock on CTTSs suggest the field strengths should be nearly identical. We analyze the behavior of these two lines over the 7 nights of data recently obtained. | |
| Thierry Montmerle (I) | X-ray diagnostics of magnetospheric accretion in T Tauri stars |
| Since the discovery of soft, non-coronal X-rays from the CTTS TW Hya (Kastner et al., 2002), observations have shown that, in a relatively limited, yet important class of T Tauri stars, the X-rays are produced not by flaring activity, but by an accretion shock close to the photosphere, likely channeled by magnetic field lines connecting the star to its accretion disk. High-resolution X-ray spectra, revealing the hyperfine structure of several atomic species, give a unique access to an independent determination of the temperature and density of the infalling plasma, hence to physical conditions close the accretion shock. After a general introduction, I will present some preliminary results of an XMM ''Large Program'', done in conjunction with several ground-based observations (including ''Espadons'' at CFHT), to study the magnetospheric accretion characteristics of the solar-mass ''twin binary'' system V4046 Sgr. I will also briefly discuss the prospects for X-ray spectroscopy offered by the ''International X-ray Observatory'' (IXO), an ESA-NASA-JAXA satellite project currently under discussion. | |
| Kohji Tomisaka (I) | ''Observation'' of the first core |
| Magnetic field and rotation are two major players in the star formation process. We show the radiation magnetohydrodynamics (RMHD) simulations of cloud core collapse. In the course of star formation, a first core (the name comes from the first hydrostatic object) is formed after the core becomes optically thick against the dust thermal radiation. Fragmentation appears in the first core when it has an enough angular momentum. This explains the formation of binary stars. Since the magnetic field removes the angular momentum along the field line, it suppresses the fragmentation. We discuss the condition of fragmentation of the first core. Spherically symmetric RHD calculations gave us an estimation of the observability of the first core, which was low due to its short lifetime (~100 yr) and optically thick envelope around the first core. Observability of this object is restudied for the thermal radiation from dust grains and molecular rotation transitions. This indicates that the spherically symmetric RHD calculations underestimated the observability. | |
| Gregg Wade (I) | Magnetism in pre-main sequence and main sequence stars |
| This talk will review our general understanding of stellar magnetism at the pre-main sequence and main sequence phases, with a focus on those observations and ideas that connect to the process of star formation and the earliest stages of stellar evolution. I will concentrate in particular on what observations can and do tell us, how theory frames those observations, as well as what are the current prevailing opinions, suppositions and biases. | |
| Jonathan Braithwaite (C) | Interaction between differential rotation and magnetic fields |
| I briefly review the theory of the formation of fossil fields and their secular evolution, before presenting new results concerning the effect on fossil fields of differential rotation during the protostellar phase. It is found that the important factor is the magnetic helicity, which is conserved on the dynamical timescale and evolves only due to diffusive processes. Protostellar/accretion processes resulting in generation or destruction of helicity are therefore of crucial to the nature of the magnetic field a star possesses during the main sequence. Finally I look at a magnetic process by which differential rotation can be damped, which could potentially explain the slow rotation of stellar remnants. | |
| Jason Grunhut (C) | The Magnetism in Massive Stars Project |
| Although the existence of magnetic fields in massive stars is no longer in question, our knowledge of the basic statistical properties of massive star magnetic fields is seriously incomplete. The Magnetism in Massive Stars (MiMeS) Project is a consensus collaboration among the foremost international researchers of the physics of hot, massive stars (stars with masses greater than 8 solar masses), with the basic aim of understanding the origin, evolution and impact of magnetic fields in these objects. The cornerstone of the project is the MiMeS Large Program at the Canada-France-Hawaii Telescope, which represents a dedication of 640 hours of telescope time from 2008-2012. The MiMeS Large Program is exploiting the unique capabilities of the ESPaDOnS spectropolarimeter to obtain critical missing information about the poorly-studied magnetic properties of these important stars, to confront current models and to guide theory. This talk will present an overview of the project and discuss our current results. | |
| Gaitee Hussain (C) | Magnetic fields and binarity in the Orion Nebula Cluster |
| We present results from a recent spectroscopic study of the brightest T Tauri stars in the Orion Nebula Cluster. Our study reveals a high binary fraction, suggesting a significant fraction of close binary T Tauri systems in the cluster. We find evidence for increased flaring amongst the binary stars in our sample, which may suggest magnetospheric interaction in the coronae of their component stars. We investigate this in these and related systems. | |
| Yohko Tsuboi (C) | X-rays from massive protostars |
| We have investigated X-rays from massive protostars using Chandra and XMM-Newton satellites. In Cep A region, we detected at least three point-like sources at the hot core, each with similar X-ray properties and differing radio and submillimeter properties. The sources are HW9, HW3c, and a new source that is undetected at other wavelengths, h10. They each have inferred X-ray luminosities >= 10^31 erg s-1 with hard spectra, T >= 10^7 K, and high low-energy absorption equivalent to tens to as much as a hundred magnitudes of visual absorption. The star usually assumed to be the most massive and energetic, HW2, is not detected with an upper limit about seven times lower than the detections. As for the other massive star-forming regions, we have analized or re-analized X-ray data which are mostly archival one, and we detected 31 massive protostars within 59 samples. In our talk, we will compile the X-ray properties (time variabilities, plasma temperatures, emission measures) and discuss the origin comparing the other sources (low-mass protostars, massive or low-mass main-sequence stars, etc.). | |
| Joel Kastner (P) | V838 Mon: A violent magnetic flare from a born-again protostar |
| I summarize the results of a series of X-ray observations of the enigmatic star V838 Mon, which underwent a spectacular optical/IR outburst in 2002 January. We detected a luminous XMM source at the position of V838 Mon in 2008. If the V838 Mon eruption was the result of a stellar merger --- as the leading models for the outburst contend --- then the X-ray luminosity, temperature, and light curve of the XMM source, and the fact that it went undetected in 2003 and 2010 Chandra exposures, indicates the merger product is now undergoing violent magnetic flares akin to those observed at very young, deeply cloud-embedded protostars. Alternatively the XMM source may have arisen from interactions between ejecta from V838 Mon and its widely separated, early-type companion. We plan further X-ray and supporting ground-based monitoring observations so as to distinguish between these scenarios. | |
| David Principe (P) | X-Ray outburst in the accreting pre-main sequence star V1647 Ori: outshining its peers |
| "EXors" are a particularly interesting class of pre-main sequence stars that are known for intense optical outbursts (periods of months to years) due to sudden, large increases in the rate of mass accretion onto the star. V1647 Ori is a pre-main sequence object that has recently been monitored during a pair of EXor outbursts and was the first case in which elevated, bright X-ray emission was observed to accompany an EXor-type pre-MS accretion eruption. The enhanced X-ray emission most likely results from interactions between accretion disk and stellar magnetosphere. We will present our recent x-ray observations and analysis of V1647 Ori to highlight its unique features relative to the pre-main sequence population in its host molecular cloud (L1630). | |
| Giuseppe Sacco (P) | Modeling the X-ray emission from accretion shock on cTTSs |
| X-ray spectroscopic observations of classical T Tauri stars show the presence of a soft X-ray component produced by plasma at T=2-3 MK and n_e=10^11-10^13 cm^-3. This emission is probably due to the accretion process, but a physical model that explains all the observational results has not yet been formulated. We performed a large set of 1D HD simulations of the accretion shock on the surface of a young star and synthesized the X-ray emission from the simulation results with the aim of investigating the dependence of the X-ray emission from the shock-heated plasma on the properties of the accretion flow (velocity, density and metal abundance). We performed further 2D MHD simulations of the accretion stream impacting on the stellar surface to study the stability and the dynamics of the accretion shock for cases in which the plasma thermal pressure exceeds that of the magnetic pressure. | |
| Tomofumi Umemoto (P) | Radio flare at millimeter-wavelengths from young stellar objects |
| Recent observational researches have shown evidence of large-scale magnetic structures around young stellar objects (YSOs), by the X-ray emission, the circular and linear polarizations, and the non-thermal radio emission by VLBI observations. The non-thermal radio emission is most likely gyrosynchrotron radiation from mildly relativistic electrons in the magnetospheres. Giant radio flare from a YSO was detected just after X-ray flare. It is thought that the X-ray flare and non-thermal radio emission are caused by a large scale flare due to magnetic reconnection. That emission is highly variable, so it is difficult to know when a flare occurs. However, some YSOs show the periodic radio flares in accordance with the orbital period of the binary. V773 Tau A is the young binary system with an orbital period of 51.1033 days. This system known as the X-ray and radio luminous weak-line T Tauri star in Taurus is a highly variable radio source. We have carried out radio continuum observations at 22 GHz, 43 GHz and 86 GHz, simultaneously, using the Nobeyama 45 m telescope, then successfully detected a radio flare at all frequency bands. We found that the radio flare just occurred at the periastron passage which is expected from a binary orbital period. Our results are likely consistent with the model that the flares are caused by colliding magnetospheres in close proximity at periastron. V773 Tau A is the most promising target for detailed imaging of the magnetosphere with the next space VLBI project VSOP-2 has angular resolution of around 40 micro-arcseconds at 43 GHz corresponds to 1.2 of solar radius in the star forming region at a distance of 150 pc. Such observation will provide key information on the magnetic structure around YSOs and bring us to the new horizon of the star formation study. |
Star-Disk Interactions
| Marina Romanova (I) | Accretion onto stars with complex fields and outflows from the disk-magnetosphere boundary |
| Results of global 3D MHD simulations will be shown of disk accretion onto stars with complex magnetic fields, including examples of stars with quadrupolar and octupolar components, and example of accretion onto T Tauri stars V2129 Oph and BP Tau with a measured surface magnetic fields which were modelled with a superposition of dipole and octupole components. Simulations show that the weaker, dipole component of the field dominates at larger distances and is responsible for the disk-magnetosphere interaction and formation of funnel streams, while the octupolar component determines the shape of hot spots. The magnetic field lines, threading the disk, are twisted, inflate and form axial magnetic towers above and below the disk. Conical-type outflows from the inner disk at the disk-magnetosphere boundary were found and investigated numerically. Outflows occur when the inward motion of gas in the disk is faster than the outward diffusion of the stellar magnetic field lines and hence the magnetic flux is bunched into an X-type configuration. Enhanced outflows are expected during events of enhanced accretion rate in the disk. This work was carried out in conjunction with Min Long and Richard Lovelace. | |
| Claudio Zanni (I) | Magnetic star-disk interaction and its influence on the stellar angular momentum |
| Classical T Tauri stars are characterized by slow rotation periods, roughly corresponding to 10% of their break-up speed: a large fraction of the stellar angular momentum has been plausibly extracted during the embedded phase. Moreover, the rotation period seems to stay constant during the T Tauri phase, despite the fact that the protostar is still actively accreting and contracting: an efficient mechanism of angular momentum removal is therefore required. Making use of numerical MHD experiments of the interaction of an accretion disk with the magnetosphere of the protostar, I will review different processes that have been proposed to balance the spin-up torque associated with accretion. In particular, I will illustrate the effects of an extended star-disk magnetic connection, stellar winds and unsteady magnetospheric ejections. | |
| Caroline D'Angelo (C) | Episodic Accretion onto Young Stellar Objects |
| The T Tauri star EX Lupi is the prototype of the 'EXor' class of T Tauri stars, which show recurrent, unexplained outbursts on a timescale of several years. In this talk I will present work on a disk instability that could explain the outbursts. The instability arises when the strong magnetic field (~1 kG) of a protostar truncates the surrounding accretion disk near the co-rotation radius (where the Keplerian frequency matches the rotation rate of the star). When the disk is truncated just outside co-rotation, the interaction between the inner regions of the disk and magnetic field exerts an outward torque on the disk, allowing a reservoir of mass to build up in the disk's inner regions. The increased mass in turn increases the viscous torque in the disk, which opposes the magnetic torque and allows the inner edge of the disk to push inside the co-rotation radius, whereupon the disk can accrete freely through the magnetosphere onto the star. Once the reservoir of mass has been drained, the disk moves back outside the corotation radius, and the cycle starts again. I will discuss the mechanism and compare the predicted outburst profiles to detailed observations of EX Lupi's 2008 outburst. | |
| Ryuichi Kurosawa (C) | Radiative transfer models of hydrogen and helium line profiles based on MHD simulations of accretion and inner wind of Classical T Tauri Stars |
| We present the radiative transfer models of optical and near infrared hydrogen and helium line profiles, based on the results of 2-D and 3-D MHD simulations of the (innermost) conical wind and magnetospherical accretion of Classical T Tauri Stars (CTTSs). A direct comparison of the MHD simulations with observations is difficult, but it can be mediated by modeling emission line profiles which could constrain some basic physical parameters of the flow such as the geometry, temperature and mass accretion and outflow rates. In turn, these parameters can provide constraints on the amount of angular momentum transfer to and out of the stars. We present the radiative transfer models of rotationally induced line variability arising from complex circumstellar environment of CTTS using the results of the 3-D MHD simulations by Romanova et al., who considered accretion onto a CTTS with a misaligned dipole magnetic axis with respect to the rotational axis. We also present the results of the line profile models based on the recent 2-D MHD simulations (Romanova et al.) of the (innermost) conical winds which are launched from the disk-magnetosphere boundary of a rotating stars with dipole magnetic field. We will discuss the applicability of the MHD models to real objects based on the analysis of the line profile models. | |
| Kazuhito Motogi (C) | Observational properties of protostellar outflow within 1000 au scale |
| Protostellar outflows have important role in star formation. Besides a main process to release excess angular momentum brought in core collapse, they are important for understanding core-scale energy feedback from a protostar to its parental core. Several theoretical studies have shown that protostellar outflow can be driven by MHD process intrinsically, and this is well consistent with a case of low-mass star formation. However, observed shapes and velocities of an outflow associated with a massive young stellar object (MYSO) vary widely. Such variations seem to be caused by a contribution of effects of strong radiation and stellar wind to pure MHD process more or less. This implies a possibility that associated outflow-structure changes as a powering MYSO evolves. Several evidences supporting this possibility have been detected in our recent VLBI observations of H2O masers which are ideal tracers of three-dimensional velocity-fields in outflows. We also detected periodic blinking of the maser alignment in the early phase MYSO. Because of the large magnitude of fluctuation in the luminosity, we interpreted it as a sign of time-variation of outflow activity. This may reflect intermittent behavior of outflow-launching process. If it is the case, this time-dependency is related to au-scale MHD turbulence around MYSO. It will be a important target of VLBI-ALMA combined study. In this talk, we show several examples of outflow structure within 1000 au scale found in our VLBI observations and compare it with recent MHD simulations. |
Disks
| Eric Feigelson (I) | X-ray effects on protoplanetary disks |
| High-resolution X-ray observations of star forming regions show that solar-type magnetic reconnection flares are powerful and frequent in pre-main sequence solar-type stars. Well-defined samples in the Orion Nebula Cluster and Taurus clouds exhibit flares with peak luminosities up to 1/10 solar luminosities repeating on timescales of days. X-rays are emitted in magnetic loops extending up to many stellar radii. These X-ray emitting loops should efficiently irradiate protoplanetary disks. X-ray irradiation of disks is directly supported by fluorescent FeK 6.4 keV emission line and X-ray absorption in some young systems, and indirectly by mid-infrared [NeII] emission and molecules from disk outer layers. Hard X-rays from the most powerful flares should penetrate deeply into the disk interior where planet formation occurs. Although details depend on uncertain electron recombination rates, hard stellar flare X-rays are likely to be the dominant ionization in disk interiors, regulating the extent of MRI-induced turbulence in the outer ''active zones'' and the neutral ''dead zones''. X-ray irradiation may also be critical for the magnetic coupling of disk material to collimated outflows, and may affect disk longevity through accretion and photoevaporation rates. Although these effects are not yet convincingly established, X-rays may play a critically important role in disk physics and planet formation processes. | |
| Arieh Königl (I) | The formation and early evolution of protostellar accretion disks |
| I provide a general overview of the formation and early evolution of protostellar accretion disks that arise from the collapse of molecular cloud cores. Although the natal core could lose angular momentum during a gradual contraction phase - e.g., through the process of magnetic braking - once dynamical collapse is triggered the specific angular momentum is nearly conserved as matter falls in under self-gravity. Subsequent mass transfer from the disk that forms in this way to the nascent protostar requires the outward transport of angular momentum. The main mechanisms implicated in this process are turbulent viscosity induced by the magnetorotational instability (MRI) and gravitational torques, which lead to angular momentum transport along the plane of the disk, and torques exerted by a large-scale magnetic field that threads the disk (involving magnetic braking or a centrifugally driven wind), which lead to vertical angular momentum transport through the disk surfaces. The MRI and large-scale field mechanisms require a minimum ionization level for efficient operation, a condition that could result in the formation of ''dead zones'' within the disk if it is not fulfilled. The accretion process - particularly during early epochs - is not steady, with most of the mass evidently reaching the central protostar during intense, but relatively brief, accretion episodes. | |
| Mark Wardle (I) | Magnetic diffusion in protoplanetary disks |
| Magnetic flux dragged in during the collapse of a molecular cloud core plays a key role in the dynamics and evolution of protoplanetary disks. Magnetic diffusion is a critical factor in the ability of the field to couple effectively to shear in the disk and transport angular momentum. Magnetic diffusion is controlled by ionization and recombination processes which determine the abundances of dust grains, ions and electrons. As a result, the relative importance of hall, ambipolar, and resistive diffusion is strongly location dependent. In turn, magnetically-driven transport and mixing impacts disk chemistry, the evolution of the grain population, and planetary migration. There are considerable uncertainties in understanding and quantifying these processes. | |
| David Tilley (C) | Numerical MHD investigation of turbulence in accretion disks and the earliest stages in planet formation |
| Understanding the MHD-driven turbulence in accretion disks is a prime area of research in astrophysics. Matter that accretes onto a central star is known to form a disk as it falls towards a star. It is thought that magnetohydrodynamic stresses drive matter inwards while driving angular momentum outwards. We make a numerical investigation of this turbulence using higher order Godunov schemes. The goal of this first phase of research is to understand the interplay between the numerics and the resulting turbulence. The effective alpha is shown to decrease with increasing resolution, though the decrease is not quite as dramatic as had been found in previous studies. This decrease is related to a dependence on the numerical Reynolds number and the numerical Prandtl number. Accretion disks also play an extremely important role in planet formation. It is believed that the earliest stages of planet formation take place due to collective turbulent processes in these disks which serve to agglomerate refractory solids. Such processes are studied in detail using numerical simulations. The simulations provide evidence for a new kind of instability known as a streaming instability. This instability serves to rapidly bring together refractory grains, thus speeding up the process of planet formation to the fast rates that have been indicated by modern observations. | |
| Chad Meyer (C) | The convergence of the magnetorotational instability under increasing numerical resolution |
| The Magnetorotational Instability (MRI) is the primary mechanism by which protostellar cores can shed their angular momentum as they collapse. Most quantitative results of MRI driven turbulence come from numerical simulations. In a recent paper Fromang & Papaloizou (2007) used an isothermal version of the ZEUS code to show that the saturated results from MRI simulations depend strongly on the details of the numerical algorithm used and also on the resolution of the simulations. We have undertaken a number of simulations of the saturated MRI-driven turbulence to explore the resolution dependence using a second order Godunov algorithm. Further, we have explored the dependence on the quality of Riemann solver and reconstruction strategy. We found the higher quality Riemann solvers and reconstruction algorithms to produce markedly better results for MRI-driven turbulence. | |
| Raquel Salmeron (C) | MRI and outflows in protostellar accretion disks |
| The mechanisms responsible for transporting angular momentum in protostellar disks are not fully understood. The most promising are turbulence viscosity driven by the magnetorotational instability (MRI) and outflows accelerated centrifugally from the surfaces of the disk. Both processes are powered by the action of magnetic fields and are, in turn, likely to play key roles in the structure, dynamics and evolution of these objects. It is also expected that the relative importance of these two mechanisms change with location and with the evolutionary stage of the system. Furthermore, the weak ionization characteristic of these accreting systems may prevent the magnetic field to effectively couple to the gas and drive these processes. As a result, it is imperative that realistic models account for the disk radial and vertical structure and for the departure from ideal-MHD fluid conditions. In my talk I will examine the viability and properties of these two angular momentum transport mechanisms in stratified, partially ionized protostellar disks. I will discuss the properties of the MRI for a range of fluid conditions, paying particular attention to the effect of the magnetic diffusivity and the presence of dust grains mixed with the gas. I will also present radially-localized solutions in which the angular momentum transport is purely vertical via winds launched from the inner regions (R < 10 AU), and consider possible scenarios in which both radial and vertical transport mechanisms operate. The implications - and future applications -- of these studies are briefly discussed. | |
| George B. Trammell (C) | Magnetically-confined upper atmospheres of hot Jupiters |
| Gas giant exoplanet upper atmospheres provide a unique laboratory to study planets in an environment not found within our solar system, as they are subjected to intense heating and tidal forces from the parent star. Although previous models have assumed heated gas can escape the planet in the form of a hydrodynamic outflow, we include the effect of intrinsic planetary magnetic field on upper atmosphere structure for the first time, and discuss possible observational consequences. We find that, for magnetic field strengths comparable to Jupiter, the magnetic pressure dominates the thermal pressure in layers at which hydrogen can be significantly photoionized, leading to the formation of a ''dead-zone''. | |
| Catherine Braiding (P) | The Hall Effect in star formation |
| Magnetic fields provide pressure support against gravity and carry away angular momentum prior to and during the collapse of cloud cores into protostars. In previous models of star formation the breakdown of flux-freezing was approximated by ambipolar diffusion at low densities or resistive diffusion at high densities. The Hall effect is expected to alter the dynamics of gravitational collapse, as it has been shown to dominate the evolution of the magnetic field in molecular gas for many of the densities involved in forming stars (Wardle and Ng, 1999). We present similarity solutions to the collapse of rotating isothermal molecular cloud cores under the influence of both ambipolar and Hall diffusion. We demonstrate that the orientation of the field affects the size of the magnetically-diluted Keplerian accretion disc formed and can introduce multiple shock discontinuities into the disc region. |
Clouds and Cores
| Takahiro Kudoh (I) | Formation of collapsing cores in magnetically subcritical clouds : three-dimensional simulations |
| We employ the fully three-dimensional simulation to study the gravitationally driven fragmentation of magnetically subcritical molecular clouds, in which the initial magnetic energy is larger than the gravitational energy. The cores in an initially subcritical cloud generally develop gradually over an ambipolar diffusion time, which is theoretically estimated to be a few ×10^7 years in a typical molecular cloud. On the other hand, the formation of collapsing cores in subcritical clouds is known to be accelerated by the supersonic nonlinear flows. Here, we have demonstrated the acceleration of the core formation by three-dimensional MHD simulations. The parameter study shows that the cores form faster with increasing the initial velocity strength in the clouds. We found that the formation time is roughly proportional to the inverse of the square root of the enhanced density driven by the supersonic nonlinear flows. It means that the ambipolar diffusion time, which is estimated to be proportional to the inverse of the square root of the density, becomes shorter when the density is enhanced by the supersonic nonlinear flows. We have also demonstrated that the accelerated formation time is not strongly dependent on the initial strength of the magnetic field if the cloud is highly subcritical. In the real molecular cloud, the supersonic turbulence is observed with 3-10 times sound speed. Our simulation shows that the core formation time in magnetically subcritical clouds is shortened to be several ×10^6 years by the large scale supersonic flows (> 3 times sound speed). The core formation time of the order of 10^6 years is consistent with some observations. | |
| Hua-bai Li (I) | Anchoring magnetic fields in turbulent molecular clouds |
| Because of the sensitivities of current instruments, our knowledge of cloud magnetic fields has been mostly concentrated on cloud cores, using e.g. field morphologies, Chandrasekhar-Fermi method and Zeeman measurements. Knowing field strength from cloud cores, however, tells us nothing about the global property of a cloud because both super- and sub-Alfvenic cores can possibly develop from either super- or sub-Alfvenic clouds. Nevertheless, global field strength of a cloud are crucial initial conditions which can significantly influence the efficiency and rate of core and star formation. We compared magnetic field directions in high-density, small-scale cores (pc to sub-pc scale) with those in the low-density, large-scale inter cloud media (several hundred pc scale), and found a significant correlation. Comparing with simulations, a globally super-Alfvenic cloud does not fit into this picture. | |
| Konstantinos Tassis (I) | Deciphering B-field observations in molecular clouds |
| Different dynamical models for the formation and evolution of molecular clouds predict different orientations for the mean magnetic field with respect to the intrinsic cloud axes, as well as different cloud shapes. However, due to projection effects, the shape and magnetic field orientation cannot be determined on a cloud-to-cloud basis. I will discuss a statistical treatment based on the assumption of randomly distributed lines of sight that can be used to overcome these difficulties. This analysis finds that the most likely cloud shape is an oblate disk, with the magnetic field oriented close to the shortest cloud axis. I will also discuss reasons why complementary Zeeman observations of the magnetic field strength are at present less costraining. | |
| Robi Banerjee (C) | Formation of molecular cloud cores out of the magnetized insterstellar medium |
| In this talk, I'll present our latest results from our numerical investigations of molecular cloud formation by transonic converging flows out of the magnetized, warm neutral medium (WNM). Typically, star-forming molecular clumps and cores emerge through triggered thermal instabilities where cold clumps coexsist with the warm atomic gas. These clumps grow by outward propagation of their phase transition fronts and through accretion of the WMN. But the formation of gravitational unstable cloud cores depends strongly on the magnetization of the interstellar medium. Finite, magnetically sub-critical, gas streams of the WMN will not trigger the formation of self-gravitating clumps even if ambipolar diffusion is included in the calculations. Therefore, the magnetization of the insterstellar medium should determine the lower mass of molecular clouds. | |
| Hsin-Fang Chiang (C) | Protostellar envelopes around Class 0 YSOs: the case of L1157 |
| We present observations of the Class 0 YSO L1157-mm using the Combined Array for Research in Millimeter-wave Astronomy (CARMA). In the N2H+ line, we detect a large-scale envelope extended over a linear size of ~20,000AU flattened in the direction perpendicular to the outflow. This highly flattened N2H+ envelope shows dynamical signatures consistent with gravitational infall in the inner region, but a slow, solid-body rotation at large scales. This flattened structure is not a rotationally supported circumstellar disk; instead, it resembles a prestellar core both morphologically and kinematically, representing the early phase of a Class 0 system. Furthermore, the solid-body rotation at large scales is consistent with the model of magnetic braking. In this study, we construct a simple model to interpret the N2H+ emission and suggest a possible dynamical scenario for the overall properties of the envelope. | |
| Laura Fissel (C) | Tracing magnetic field structure in molecular clouds with BLAST-pol |
| We present a brief overview of BLAST-pol, the Balloon-borne Large Aperture Submillimeter Telescope for Polarimetry. BLAST-pol is a 1.8 meter telescope that maps linearly polarized dust emission at 250, 350 and 500 microns, with a diffraction limited beam FWHM of 30'' at 250 microns. With its unprecedented mapping speed and resolution, BLAST-pol will produce three-color polarization maps for a large number of molecular clouds. The instrument will provide a much needed bridge in spatial coverage between larger-scale, coarse resolution surveys and narrow field of view, high resolution observations of filamentary structures and dense cores within clouds. We will use the BLAST-pol maps to study magnetic field morphology across entire molecular clouds, the degree of order in the field and the relationship between field direction and the filamentary structure commonly seen within clouds. We will also compare our results with predictions from simulations of star formation. Our first science flight is scheduled for December 2010 from McMurdo Station, Antarctica. | |
| Martin Houde (C) | Characterizing the magnetized turbulent power spectrum through the dispersion of magnetic fields |
| The importance of magnetic fields in the star formation process is extremely hard to quantify due to the difficulty in making the relevant measurements. While Submillimetre polarimetry of dust emission is arguably the most important observational tool to probe magnetic fields in molecular clouds, it has mainly only been used so far to provide a measure of the geometry of the field. Accordingly, I will discuss a promising new method introduced by Houde et al. (2009) and Hildebrand et al. (2009) for characterizing magnetic fields and turbulence from such observations. More precisely, I will describe how an analysis of the difference in the orientation of pairs of polarization vectors as a function of their separation (i.e., the structure function of the polarization angle) leads to a direct determination of the magnetized turbulent power spectrum. This type of analysis thus provides a way to test potential turbulence theories. As an example, I will present an application of this technique to high spatial resolution Submillimeter Array (SMA) polarization data obtained for Orion KL, IRAS 16293 (Rao et al. 2009), and NGC 1333 IRAS 4A (Girart et al 2006). For Orion KL we determine that in the inertial range the spectrum can be approximately fitted with a power law $k^{-1.6}$ and we obtain an upper limit of 5.5 mpc for $\delta_{\mathrm{AD}}$, the high spatial frequency cutoff presumably due to turbulent ambipolar diffusion. For the same parameters we have $\sim k^{-1.5}$ and $\delta_{\mathrm{AD}}\simeq2.0$ mpc for IRAS 16293, and $\sim k^{-1.2}$ and $\delta_{\mathrm{AD}}\simeq2.2$ mpc for NGC 1333 IRAS 4A. We thus have a clear determination of the turbulent ambipolar diffusion scale directly from the magnetized turbulent power spectrum for two of our three sources. | |
| Ryo Kandori (C) | Near infrared polarimetry of nearby dense cores with SIRPOL |
| Magnetic fields are believed to play an important role in controlling the stability and contraction of molecular cloud cores. In present study, magnetic fields of 13 nearby dense cores (prestellar: 9, protostellar: 4) are mapped based on deep and wide-field near-infrared polarimetric observations of background stars with IRSF/SIRPOL. Magnetic fields are generally aligned (delta theta < 20 degree) toward all the cores. Observed magnetic fields are not straight but curved shape toward more then half of the cores, some of which show axisymmetric magnetic fields like a shape of hourglass, indicative of magnetic fields distorted by gravitationally contracting medium. These (projected) structures are confirmed to be real, because the relationships between polarization degree and stellar color (dichroic extinction) appear linear for all the cores up to Av ~ 20-30 mag. The polarized light produced by magnetically aligned dust particles deep inside the cores can be detected without any depolarization effects. Our findings on the geometry of magnetic fields as well as the polarization-extinction relationship toward nearby dense cores will be discussed in the talk. | |
| Jongsoo Kim (C) | Molecular line profiles of a core formed in a turbulent cloud |
| We performed numerical experiments designed to see core formation in a self-gravitating, magnetically supercritical, supersonically turbulent, isothermal cloud. First, we found that, due to the self-gravity, a fraction of gas whose density is larger than a maximum post-shock density value attained by isothermal shocks. It is this fraction of gas that significantly increases the core formation rate than the value expected from a lognormal density probability density function. Second, we traced out a collapsing core formed in the cloud and calculated molecular line profiles of HCO+ and C18O. Features of the profiles can be affected more significantly by coupled velocity and abundance structures in the outer region or the core than those in the inner one. During the evolution of the core, the asymmetry of line profiles could change from blue to red, and vice versa. According to our study, the observed reversed (red) asymmetry toward some starless cores could be interpreted as an outward motion in the outer region of a dense core, which is embedded in a turbulent environment and still grows in density at the center. | |
| Talayeh Hezareh (C) | CN (N=2-1) Zeeman observations in molecular clouds |
| I will talk about a new Four-Stokes-Parameter spectral line Polarimeter (FSPPol) that we designed and built for the Caltech Submillimeter Observatory (CSO) and the preliminary observations we carried out using this instrument. The simple design of the FSPPol does not include any mirrors or optical components to redirect or re-image the radiation beam and simply transmits the beam to the receiver through its retarder plates. FSPPol is currently optimized for observation in the 200-260 GHz range and measures all four Stokes parameters, I, Q, U, and V. We used FSPPol to study the Zeeman effect in the N=2-1 transition of CN in DR21(OH) for the first time. At this point we do not have a Zeeman detection, but more observations are ongoing. | |
| Barth Netterfield (C) | The mass function, lifetimes, and properties of intermediate mass cores from a 50 square degree submillimeter galactic survey in Vela using the BLAST telescope |
| We present first results from an unbiased 50 deg^2 submillimeter Galactic survey at 250, 350, and 500 micron from the 2006 flight of the Balloon-borne Large Aperture Submillimeter Telescope (BLAST). The map has resolution ranging from 36 arcsec to 60 arcsec in the three submillimeter bands spanning the thermal emission peak of cold starless cores. We determine the temperature, luminosity, and mass of more than one thousand compact sources in a range of evolutionary stages and an unbiased statistical characterization of the population. From comparison with C^(18)O data, we find the dust opacity per gas mass, kappa r = 0.16 cm^2 g^(-1) at 250 micron, for cold clumps. We find that 2% of the mass of the molecular gas over this diverse region is in cores colder than 14 K, and that the mass function for these cold cores is consistent with a power law with index alpha = -3.22 +/- 0.14 over the mass range 14 M_sun < M < 80 M_sun. Additionally, we infer a mass-dependent cold core lifetime of t_c(M) = 4E6 (M/20 M_sun)^(-0.9) years - longer than what has been found in previous surveys of either low or high mass cores, and significantly longer than free fall or likely turbulent decay times. This implies some form of non-thermal support for cold cores during this early stage of star formation. | |
| Ralph Pudritz (C) | Star formation in filamentary clouds |
| Spitzer observations increasingly show that cores and star formation are strongly associated with filamentary structure in molecular clouds. Simulations of supersonic turbulence repeatedly demonstrate that filamentary structure and the appearance of cores within them are ubiquitous. I will discuss some of the theoretical and computational aspects of filament and core formation in molecular clouds. I will then review new simulations of magnetized molecular clouds that we are doing that shed some light on how filamentary structure and collapse in turbulent clouds are linked in magnetized, turbulent clouds. | |
| Dinshaw Balsara (C) | Direct evidence for two-fluid effects in molecular clouds |
| We present simulations that examine the role of two-fluid ambipolar drift on widths of molecular lines. The dissipation provided by ion-neutral interactions can produce a significant difference between the widths of neutral molecules and the widths of ionic species, comparable in magnitude to the sound speed. We show that this effect arises when Alfven waves and certain families of magnetosonic waves become strongly damped on scales comparable to the ambipolar diffusion scale, leadinng to a reduction in the ion velocity power spectrum relative to the neutral velocity power spectrum. Following a set of motivating observations by Li & Houde (2008), we produce synthetic line width-size relations that illustrate that two-fluid effects can have an observationally detectable role in modifying the MHD turbulence in molecular clouds. | |
| Chang Won Lee (P) | Internal motions in starless dense cores |
| Internal motions in the central regions of starless dense (n_{H_2} > 10^4 cm^-2) cores are a key issue in studying initial conditions of isolated star formation because these may indicate contraction, a sign of progress toward star formation. A single pointing survey of optically thin and thick tracers such as CS 2-1 and N2H^+1-0 toward starless cores indicated a predominance of inward motions. However, mapping studies with the same tracers showed different or more complex distribution of the spectral asymmetry from what is seen toward the central regions of the cores. Our study introduces a detailed investigation of mapped molecular line profile data to know which kind of asymmetric profiles are the most typical and thus which kind of motions are the most likely. We investigated all existing data of optically thin and thick molecular lines for starless cores to assess the distribution of starless dense cores with primarily inward, outward, and mixed motions and analyzed the asymmetric patterns of these whole profiles. In total 34 starless cores are examined with the normalized velocity difference by which the optically thick spectrum is blue- or red-shifted with respect to the optically thin tracer. It was found that over 50% of the sample is dominated with blueskewed profiles, implying predominance of inward motions in starless cores, about 11% of the starless cores has the distribution of a mixture of blue and red profiles which are sometimes interpreted as oscillatory motions, and only one source is dominated with red-skewed profiles indicating outward gaseous motions, while the rest (about 35%) of the sample shows domiance of symmetric profiles. This study indicates that majority of the cores are likely to have motions which are infall-dominated. The pattern of inward motions is found to be dependent of several physical parameters. A core with higher column density tends to have better pronounced and more frequent inward motions, and a position with higher column density in a core tends to be in more chance of inward motions. Similarly, the blue profiles are generally more populated than the red profiles in the core and this tendency is stronger at smaller radius (< 0.1 pc). Whether the core is isolated or in complexes do not seem to affect on how internal motions are occurring. | |
| Katherine Lee (P) | The role of magnetic fields in star formation: a case study of a starless core in Orion |
| Do magnetic fields play an important role in star formation? Ambipolar diffusion theory tells us that magnetic fields play a dominating role in regulating star formation process, allowing fragmentations and resolving the angular momentum problem. We carried out an observation of a starless core in Orion with CARMA to study the effect of magnetic fields at early stages of star formation. The starless core shows a solid-body rotation profile, and it is near the onset of dynamical contraction. The observation and theory show remarkable consistency in the density and the angular velocity of the core, indicating that magnetic fields are playing a significant role in the star formation process. | |
| Daniel Seifried (P) | Outflows in high mass, magnetized cloud cores |
| We present collapse simulations of high mass, rotating and magnetized molecular cloud cores. The combined effect of rotation and magnetic fields leads to the occurrence of outflow phenomena. Our special interest lays in the long term evolution of large scale outflows appearing already in the early phase of stellar evolution. These outflows seem to release a significant amount of mass into the ambient medium and therefore reduce the rate of accretion onto the protostar. We are following the long term evolution of these outflows with the help of sink particles. With this technique we are able to examine whether magnetic towers and protostellar jets are persistent or just transient features. In the former case outflows should be able to open channels of low density gas through which radiation can escape. |