Protostars in Infrared Dark Clouds
Core Mass Function
We study the core mass function (CMF) within 32 dense clumps in seven infrared dark clouds (IRDCs) with the Atacama Large Millimeter/submillimeter Array via 1.3 mm continuum emission at a resolution of ~1". We have identified 107 cores with the dendrogram algorithm, with a median radius of about 0.02 pc. Their masses range from 0.261 to 178 M☉. After applying completeness corrections, we fit the combined IRDC CMF with a power law of the form dN/dlogM ∝ M^-α and derive an index of α~ 0.86 ± 0.11 for M ≥ 0.79 M☉ and α ~ 0.70 ± 0.13 for M ≥ 1.26 M☉, which is a significantly more top-heavy distribution than the Salpeter stellar initial mass function index of 1.35. We also make a direct comparison of these IRDC clump CMF results to those measured in the more evolved protocluster G286 derived with similar methods, which have α ~ 1.29 ± 0.19 and 1.08 ± 0.27 in these mass ranges, respectively. These results provide a hint that, especially for the M ≥ 1.26 M☉ range where completeness corrections are modest, the CMF in high pressure, early-stage environments of IRDC clumps may be top-heavy compared to that in the more evolved, global environment of the G286 protoclusters. However, larger samples of cores probing these different environments are needed to better establish the robustness of this potential CMF variation.
Protostellar Outflows and Radio Emission
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, which are spatially resolved down to < 0.05 pc. Out of the 32 clumps observed, we have detected SiO emission in 20 clumps, and in 11 of them it is relatively strong and likely tracing protostellar outflows. Some SiO outflows are collimated, while others are less well ordered. There is evidence for episodic ejection events, as well as multiple outflows originating from scales of 0.1 pc. For the six strongest SiO outflows, we estimate basic outflow properties and also locate their associated protostellar cores in position-velocity space by utilizing mm continuum emission and dense gas tracers DCN(3-2), DCO+(3-2) and C18O(2-1). We do not see clear dependence of the degree of collimation of the outflows on core mass, luminosity and evolutionary stage. In our entire sample, where there is SiO emission, we always find 1.3 mm continuum emission and some infrared emission nearby, but not vice versa. We build the spectral energy distributions (SEDs) of all the clumps 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 trend of increasing SiO line luminosity with bolometric luminosity, which suggests more powerful shocks in the vicinity of more massive YSOs. However, 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 ∼ 100 L⊙, 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 to be shock ionized jets associated with the more massive of these protostellar cores.
High-Mass Protostellar Population
We conduct a census of the high-mass protostellar population of the ∼ 70, 000 M⊙ Infrared Dark Cloud (IRDC) G028.37+00.07, identifying 35 sources based on their 70 μm emission, as reported in the Herschel Hi-Gal catalog of Molinari et al. (2016). We perform aperture photometry to construct spectral energy distributions (SEDs), and these SEDs are then fit with the massive protostar models of Zhang & Tan (2018). We discuss the uncertainties associated with such SED model fitting. We find that the sources span a range of isotropic luminosities from ∼ 20 to 4500 L⊙. The most luminous source, Cp23, is predicted to have a current protostellar mass of m∗ ∼ 8 M⊙ forming from a core of mass Mc ∼ 400 M⊙. The least luminous sources in our sample are predicted to be protostars with masses as low as ∼ 0.5 M⊙ forming from cores with Mc ∼ 10 M⊙, which are the minimum values explored in the protostellar model grid. The detected protostellar population has a total estimated protostellar mass of M∗ ∼ 100 M⊙. Allowing for completeness corrections, which are constrained by comparison with an ALMA study in part of the IRDC, we estimate a star formation efficiency per free-fall time of the clump of ∼ 3%. Finally, we analyze the spatial distribution of the sources, finding relatively low degrees of central concentration of the protostars. Thus, the most massive protostars do not appear to be especially centrally concentrated in the protocluster.
Related Work
The Core Mass Function in the Massive Protocluster G286.21+0.17
Publications
The Core Mass Function in the Massive Protocluster G286.21+0.17 Revealed by ALMA
The Core Mass Function across Galactic Environments. II. Infrared Dark Cloud Clumps
SiO Outflows as Tracers of Massive Star Formation in Infrared Dark Clouds
The High-mass Protostellar Population of a Massive Infrared Dark Cloud