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Register a new userLow Virial Parameters in Molecular Clouds: Implications for High Mass Star Formation and Magnetic Fields
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Jens Kauffmann n Caltech
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Whether or not molecular clouds and embedded cloud fragments are stable against collapse is of utmost importance for the study of the star formation process. Only supercritical cloud fragments are able to collapse and form stars. The virial parameter, alpha=M_vir/M, which compares the virial to the actual mass, provides one way to gauge stability against collapse. Supercritical cloud fragments are characterized by alpha<2, as indicated by a comprehensive stability analysis considering perturbations in pressure and density gradients. Past research has suggested that virial parameters alpha>2 prevail in clouds. This would suggest that collapse towards star formation is a gradual and relatively slow process, and that magnetic fields are not needed to explain the observed cloud structure. Here, we review a range of very recent observational studies that derive virial parameters <<2 and compile a catalogue of 1325 virial parameter estimates. Low values of alpha are in particular observed for regions of high mass star formation (HMSF). These observations may argue for a more rapid and violent evolution during collapse. This would enable competitive accretion in HMSF, constrain some models of monolithic collapse, and might explain the absence of high--mass starless cores. Alternatively, the data could point at the presence of significant magnetic fields ~1 mG at high gas densities. We examine to what extent the derived observational properties might be biased by observational or theoretical uncertainties. For a wide range of reasonable parameters, our conclusions appear to be robust with respect to such biases.
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Ever since their discovery, Infrared dark clouds (IRDCs) are generally considered to be the sites just at the onset of high-mass (HM) star formation. In recent years, it has been realized that not all IRDCs harbour HM Young Stellar Objects (YSOs). Only those IRDCs satisfying a certain mass-size criterion, or equivalently above a certain threshold density, are found to contain HMYSOs. In all cases, IRDCs provide ideal conditions for the formation of stellar clusters. In this paper, we study the massive stellar content of IRDCs to re-address the relation between IRDCs and HM star formation. For this purpose, we have identified all IRDCs associated to a sample of 12 Galactic molecular clouds (MCs). The selected MCs have been the target of a systematic search for YSOs in an earlier study. The catalogued positions of YSOs have been used to search all YSOs embedded in each identified IRDC. In total, we have found 834 YSOs in 128 IRDCs. The sample of IRDCs have mean surface densities of 319 Mo/pc2, mean mass of 1062 Mo, and a mass function power-law slope -1.8, which are similar to the corresponding properties for the full sample of IRDCs and resulting physical properties in previous studies. We find that all those IRDCs containing at least one intermediate to high-mass young star satisfy the often-used mass-size criterion for forming HM stars. However, not all IRDCs satisfying the mass-size criterion contain HM stars. We find that the often used mass-size criterion corresponds to 35% probability of an IRDC forming a massive star. Twenty five (20%) of the IRDCs are potential sites of stellar clusters of mass more than 100 Mo.
We study the star formation (SF) law in 12 Galactic molecular clouds with ongoing high-mass star formation (HMSF) activity, as traced by the presence of a bright IRAS source and other HMSF tracers. We define the molecular cloud (MC) associated to each IRAS source using 13CO line emission, and count the young stellar objects (YSOs) within these clouds using GLIMPSE and MIPSGAL 24 micron Spitzer databases.The masses for high luminosity YSOs (Lbol>10~Lsun) are determined individually using Pre Main Sequence evolutionary tracks and the evolutionary stages of the sources, whereas a mean mass of 0.5 Msun was adopted to determine the masses in the low luminosity YSO population. The star formation rate surface density (sigsfr) corresponding to a gas surface density (siggas) in each MC is obtained by counting the number of the YSOs within successive contours of 13CO line emission. We find a break in the relation between sigsfr and siggas, with the relation being power-law (sigsfr ~ siggas^N) with the index N varying between 1.4 and 3.6 above the break. The siggas at the break is between 150-360 Msun/pc^2 for the sample clouds, which compares well with the threshold gas density found in recent studies of Galactic star-forming regions. Our clouds treated as a whole lie between the Kennicutt (1998) relation and the linear relation for Galactic and extra-galactic dense star-forming regions. We find a tendency for the high-mass YSOs to be found preferentially in dense regions at densities higher than 1200 Msun/pc^2 (~0.25 g/cm^2).
High-mass Stars are cosmic engines known to dominate the energetics in the Milky Way and other galaxies. However, their formation is still not well understood. Massive, cold, dense clouds, often appearing as Infrared Dark Clouds (IRDCs), are the nurseries of massive stars. No measurements of magnetic fields in IRDCs in a state prior to the onset of high-mass star formation (HMSF) have previously been available, and prevailing HMSF theories do not consider strong magnetic fields. Here, we report observations of magnetic fields in two of the most massive IRDCs in the Milky Way. We show that IRDCs G11.11-0.12 and G0.253+0.016 are strongly magnetized and that the strong magnetic field is as important as turbulence and gravity for HMSF. The main dense filament in G11.11-0.12 is perpendicular to the magnetic field, while the lower density filament merging onto the main filament is parallel to the magnetic field. The implied magnetic field is strong enough to suppress fragmentation sufficiently to allow HMSF. Other mechanisms reducing fragmentation, such as the entrapment of heating from young stars via high mass surface densities, are not required to facilitate HMSF.
The Magellanic Clouds provide the only laboratory to study the effect of metallicity and galaxy mass on molecular gas and star formation at high (~20 pc) resolution. We use the dust emission from HERITAGE Herschel data to map the molecular gas in the Magellanic Clouds, avoiding the known biases of CO emission as a tracer of H$_{2}$. Using our dust-based molecular gas estimates, we find molecular gas depletion times of ~0.4 Gyr in the LMC and ~0.6 SMC at 1 kpc scales. These depletion times fall within the range found for normal disk galaxies, but are shorter than the average value, which could be due to recent bursts in star formation. We find no evidence for a strong intrinsic dependence of the molecular gas depletion time on metallicity. We study the relationship between gas and star formation rate across a range in size scales from 20 pc to ~1 kpc, including how the scatter in molecular gas depletion time changes with size scale, and discuss the physical mechanisms driving the relationships. We compare the metallicity-dependent star formation models of Ostriker, McKee, and Leroy (2010) and Krumholz (2013) to our observations and find that they both predict the trend in the data, suggesting that the inclusion of a diffuse neutral medium is important at lower metallicity.
Context: The importance of magnetic fields at the onset of star formation related to the early fragmentation and collapse processes is largely unexplored today. Aims: We want to understand the magnetic field properties at the earliest evolutionary stages of high-mass star formation. Methods: The Atacama Large Millimeter Array is used at 1.3mm wavelength in full polarization mode to study the polarized emission and by that the magnetic field morphologies and strengths of the high-mass starless region IRDC18310-4. Results: The polarized emission is clearly detected in four sub-cores of the region. In general it shows a smooth distribution, also along elongated cores. Estimating the magnetic field strength via the Davis-Chandrasekhar-Fermi method and following a structure function analysis, we find comparably large magnetic field strengths between ~0.6 and 3.7mG. Comparing the data to spectral line observations, the turbulent-to-magnetic energy ratio is low, indicating that turbulence does not significantly contribute to the stability of the gas clump. A mass-to-flux ratio around the critical value 1.0 - depending on column density - indicates that the region starts to collapse which is consistent with the previous spectral line analysis of the region. Conclusions: While this high-mass region is collapsing and thus at the verge of star formation, the high magnetic field values and the smooth spatial structure indicate that the magnetic field is important for the fragmentation and collapse process. This single case study can only be the starting point for larger sample studies of magnetic fields at the onset of star formation.
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