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Register a new userFire from Ice - Massive Star Birth from Infrared Dark Clouds
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Jonathan C. Tan
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I review massive star formation in our Galaxy, focusing on initial conditions in Infrared Dark Clouds (IRDCs), including the search for massive pre-stellar cores (PSCs), and modeling of later stages of massive protostars, i.e., hot molecular cores (HMCs). I highlight how developments in astrochemistry, coupled with rapidly improving theoretical/computational and observational capabilities are helping to improve our understanding of the complex process of massive star formation.
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To study the early phases of massive star formation, we present ALMA observations of SiO(5-4) emission and VLA observations of 6 cm continuum emission towards 32 Infrared Dark Cloud (IRDC) clumps, spatially resolved down to $lesssim 0.05$ pc. Out of the 32 clumps, we detect SiO emission in 20 clumps, and in 11 of them the SiO emission is relatively strong and likely tracing protostellar outflows. Some SiO outflows are collimated, while others are less ordered. For the six strongest SiO outflows, we estimate basic outflow properties. In our entire sample, where there is SiO emission, we find 1.3 mm continuum and infrared emission nearby, but not vice versa. We build the spectral energy distributions (SEDs) of cores with 1.3 mm continuum emission and fit them with radiative transfer (RT) models. The low luminosities and stellar masses returned by SED fitting suggest these are early stage protostars. We see a slight trend of increasing SiO line luminosity with bolometric luminosity, which suggests more powerful shocks in the vicinity of more massive YSOs. We do not see a clear relation between the SiO luminosity and the evolutionary stage indicated by $L/M$. We conclude that as a protostar approaches a bolometric luminosity of $sim 10^2 : L_{odot}$, the shocks in the outflow are generally strong enough to form SiO emission. The VLA 6 cm observations toward the 15 clumps with the strongest SiO emission detect emission in four clumps, which is likely shock ionized jets associated with the more massive ones of these protostellar cores.
We present high resolution (0.2, 1000 AU) 1.3 mm ALMA observations of massive infrared dark cloud clump, G028.37+00.07-C1, thought to harbor the early stages of massive star formation. Using $rm N_2D^+$(3-2) we resolve the previously identified C1-S core, separating the bulk of its emission from two nearby protostellar sources. C1-S is thus identified as a massive ($sim50:M_odot$), compact ($sim0.1:$pc diameter) starless core, e.g., with no signs of outflow activity. Being highly deuterated, this is a promising candidate for a pre-stellar core on the verge of collapse. An analysis of its dynamical state indicates a sub-virial velocity dispersion compared to a trans-Alfvenic turbulent core model. However, virial equilibrium could be achieved with sub-Alfvenic conditions involving $sim2:$mG magnetic field strengths.
Infrared dark clouds (IRDCs) are thought to be potential hosts of the elusive early phases of high-mass star formation. Here we conduct an in-depth kinematic analysis of one such IRDC, G034.43+00.24 (Cloud F), using high sensitivity and high spectral resolution IRAM-30m N$_2$H$^+$ ($1-0$) and C$^{18}$O ($1-0$) observations. To disentangle the complex velocity structure within this cloud we use Gaussian decomposition and hierarchical clustering algorithms. We find that four distinct coherent velocity components are present within Cloud F. The properties of these components are compared to those found in a similar IRDC, G035.39-00.33 (Cloud H). We find that the components in both clouds have: high densities (inferred by their identification in N$_2$H$^+$), trans-to-supersonic non-thermal velocity dispersions with Mach numbers of $sim$ $1.5-4$, a separation in velocity of $sim$3 km s$^{-1}$, and a mean red-shift of $sim$ 0.3 km s$^{-1}$ between the N$_2$H$^+$ (dense gas) and C$^{18}$O emission (envelope gas). The latter of these could suggest that these clouds share a common formation scenario. We investigate the kinematics of the larger-scale Cloud F structures, using lower-density-tracing $^{13}$CO ($1-0$) observations. A good correspondence is found between the components identified in the IRAM-30m observations and the most prominent component in the$^{13}$CO data. We find that the IRDC Cloud F is only a small part of a much larger structure, which appears to be an inter-arm filament of the Milky Way.
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.
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.
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