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Register a new userStudying Infall in Infrared Dark Clouds with Multiple HCO+ Transitions
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We investigate the infall properties in a sample of 11 infrared dark clouds (IRDCs) showing blue-asymmetry signatures in HCO$^{+}$ J=1--0 line profiles. We used JCMT to conduct mapping observations in HCO$^{+}$ J=4--3 as well as single-pointing observations in HCO$^{+}$ J =3--2, towards 23 clumps in these IRDCs. We applied the HILL model to fit these observations and derived infall velocities in the range of 0.5-2.7 km s$^{-1}$, with a median value of 1.0 km s$^{-1}$, and obtained mass accretion rates of 0.5-14$times$10$^{-3}$ Msun yr$^{-1}$. These values are comparable to those found in massive star forming clumps in later evolutionary stages. These IRDC clumps are more likely to form star clusters. HCO$^{+}$ J =3--2 and HCO$^{+}$ J =1--0 were shown to trace infall signatures well in these IRDCs with comparable inferred properties. HCO$^{+}$ J=4--3, on the other hand, exhibits infall signatures only in a few very massive clumps, due to smaller opacties. No obvious correlation for these clumps was found between infall velocity and the NH3/CCS ratio.
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The study of infall motion helps us to understand the initial stages of star formation. In this paper, we use the IRAM 30-m telescope to make mapping observations of 24 infall sources confirmed in previous work. The lines we use to track gas infall motions are HCO+ (1-0) and H13CO+ (1-0). All 24 sources show HCO+ emissions, while 18 sources show H13CO+ emissions. The HCO+ integrated intensity maps of 17 sources show clear clumpy structures; for the H13CO+ line, 15 sources show clumpy structures. We estimated the column density of HCO+ and H13CO+ using the RADEX radiation transfer code, and the obtained [HCO+]/[H2] and [H13CO+]/[HCO+] of these sources are about 10^-11 ~ 10^-7 and 10^-3~1, respectively. Based on the asymmetry of the line profile of the HCO+, we distinguish these sources: 19 sources show blue asymmetric profiles, and the other sources show red profiles or symmetric peak profiles. For eight sources that have double-peaked blue line profiles and signal-to-noise ratios greater than 10, the RATRAN model is used to fit their HCO^+ (1-0) lines, and to estimate their infall parameters. The mean Vin of these sources are 0.3 ~ 1.3 km/s, and the Min are about 10^-3 ~ 10^-4 Msun/yr , which are consistent with the results of intermediate or massive star formation in previous studies. The Vin estimated from the Myers model are 0.1 ~ 1.6 km/s, and the Min are within 10^-3 ~ 10^-5 Msun/yr. In addition, some identified infall sources show other star-forming activities, such as outflows and maser emissions. Especially for those sources with a double-peaked blue asymmetric profile, most of them have both infall and outflow evidence.
Star formation in a filamentary infrared dark cloud (IRDC) is simulated over a dynamic range of 4.2 pc to 28 au for a period of $3.5times 10^5$ yr, including magnetic fields and both radiative and outflow feedback from the protostars. At the end of the simulation, the star formation efficiency is 4.3 per cent and the star formation rate per free fall time is $epsilon_{rm ff}simeq 0.04$, within the range of observed values (Krumholz et al. 2012a). The total stellar mass increases as $sim,t^2$, whereas the number of protostars increases as $sim,t^{1.5}$. We find that the density profile around most of the simulated protostars is $sim,rhopropto r^{-1.5}$, as predicted by Murray & Chang (2015). At the end of the simulation, the protostellar mass function approaches the Chabrier (2005) stellar initial mass function. We infer that the time to form a star of median mass $0.2,M_odot$ is about $1.4times 10^5$~yr from the median mass accretion rate. We find good agreement among the protostellar luminosities observed in the large sample of Dunham et al. (2013), our simulation, and a theoretical estimate, and conclude that the classical protostellar luminosity problem Kenyon et al. (1990) is resolved. The multiplicity of the stellar systems in the simulation agrees to within a factor 2 of observations of Class I young stellar objects; most of the simulated multiple systems are unbound. Bipolar protostellar outflows are launched using a sub-grid model, and extend up to 1 pc from their host star. The mass-velocity relation of the simulated outflows is consistent with both observation and theory.
Gravitational accretion accumulates the original mass, and this process is crucial for us to understand the initial phases of star formation. Using the specific infall profiles in optically thick and thin lines, we searched the clumps with infall motion from the Milky Way Imaging Scroll Painting (MWISP) CO data in previous work. In this study, we selected 133 sources of them as a sub-sample for further research and identification. The excitation temperatures of these sources are between 7.0 and 38.5 K, while the H_2 column densities are between 10^21 and 10^23 cm^-2. We have observed optically thick lines HCO+ (1-0) and HCN (1-0) using the DLH 13.7-m telescope, and found 56 sources of them with blue profile and no red profile in these two lines, which are likely to have infall motions, with the detection rate of 42%. It suggests that using CO data to restrict sample can effectively improve the infall detection rate. Among these confirmed infall sources, there are 43 associated with Class 0/I young stellar objects (YSOs), and 13 are not. These 13 sources are probably associated with the sources in earlier evolutionary stage. By comparison, the confirmed sources which are associated with Class 0/I YSOs have higher excitation temperatures and column densities, while the other sources are colder and have lower column densities. Most infall velocities of the sources we confirmed are between 10^-1 to 10^0 km s^-1, which is consistent with previous studies.
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.
Infrared Dark Clouds (IRDCs) are very dense and highly extincted regions that host the initial conditions of star and stellar cluster formation. It is crucial to study the kinematics and molecular content of IRDCs to test their formation mechanism and ultimately characterise these initial conditions. We have obtained high-sensitivity Silicon Monoxide, SiO(2-1), emission maps toward the six IRDCs, G018.82$-$00.28, G019.27+00.07, G028.53$-$00.25, G028.67+00.13, G038.95$-$00.47 and G053.11+00.05 (cloud A, B, D, E, I and J, respectively), using the 30-m antenna at the Instituto de Radioastronom{i}a Millim{e}trica (IRAM30m). We have investigated the SiO spatial distribution and kinematic structure across the six clouds to look for signatures of cloud-cloud collision events that may have formed the IRDCs and triggered star formation within them. Toward clouds A, B, D, I and J we detect spatially compact SiO emission with broad line profiles which are spatially coincident with massive cores. Toward the IRDCs A and I, we report an additional SiO component that shows narrow line profiles and that is widespread across quiescent regions. Finally, we do not detect any significant SiO emission toward cloud E. We suggest that the broad and compact SiO emission detected toward the clouds is likely associated with ongoing star formation activity within the IRDCs. However, the additional narrow and widespread SiO emission detected toward cloud A and I may have originated from the collision between the IRDCs and flows of molecular gas pushed toward the clouds by nearby HII regions.
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