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Register a new userInfrared Dark Clouds and high-mass star formation activity in Galactic Molecular Clouds
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R. Retes-Romero
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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.
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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 role played by gravity in the transfer of interstellar matter from molecular cloud scales to protostellar scales is still highly debated. Only detailed studies on the kinematics of large samples of star-forming clouds will settle the issue. We present new IRAM 30m observations of a sample of 27 infrared dark clouds covering a large range of sizes, masses, and aspect ratios. Preliminary results suggest that gravity is regulating the dynamical evolution of these clouds on a couple of parsec scales.
In this chapter we review the young stars and molecular clouds found at high Galactic latitudes $(|b| ge 30^circ)$. These are mostly associated with two large-scale structures on the sky, the Gould Belt and the Taurus star formation region, and a handful of molecular clouds including MBM 12 and MBM 20 which, as a population, consist of the nearest star formation sites to our Sun. There are also a few young stars that are found in apparent isolation far from any molecular cloud. The high latitude clouds are primarily translucent molecular clouds and diffuse Galactic cirrus with the majority of them seen at high latitude simply due to their proximity to the Sun. The rare exceptions are those, like the Draco and other intermediate or high velocity clouds, found significantly above or below the Galactic plane. We review the processes that result in star formation within these low density and extraplanar environments as well as the mechanisms for production of isolated T Tauri stars. We present and discuss the known high-latitude stellar nurseries and young stellar objects.
We investigate Schmidts conjecture (i.e., that the star formation rate scales in a power-law fashion with the gas density) for four well-studied local molecular clouds (GMCs). Using the Bayesian methodology we show that a local Schmidt scaling relation of the form Sigma*(A_K) = kappa x (A_K)^{beta} (protostars pc^{-2}) exists within (but not between) GMCs. Further we find that the Schmidt scaling law, by itself, does not provide an adequate description of star formation activity in GMCs. Because the total number of protostars produced by a cloud is given by the product of Sigma*(A_K) and S(> A_K), the differential surface area distribution function, integrated over the entire cloud, the clouds structure plays a fundamental role in setting the level of its star formation activity. For clouds with similar functional forms of Sigma*(A_K), observed differences in their total SFRs are primarily due to the differences in S(> A_K) between the clouds. The coupling of Sigma*(A_K) with the measured S(> A_K) in these clouds also produces a steep jump in the SFR and protostellar production above A_K ~ 0.8 magnitudes. Finally, we show that there is no global Schmidt law that relates the star formation rate and gas mass surface densities between GMCs. Consequently, the observed Kennicutt-Schmidt scaling relation for disk galaxies is likely an artifact of unresolved measurements of GMCs and not a result of any underlying physical law of star formation characterizing the molecular gas.
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