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,

Core Collapse and Inner Regions

YSOs and Stars

 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.