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We report the identification of a sample of potential High-Mass Starless Cores (HMSCs). The cores were discovered by comparing images of the fields containing candidate High-Mass Protostellar Objects (HMPOs) at 1.2mm and mid-infrared (8.3um; MIR) wavelengths. While the HMPOs are detected at both wavelengths, several cores emitting at 1.2mm in the same fields show absorption or no emission at the MIR wavelength. We argue that the absorption is caused by cold dust. The estimated masses of a few 10^2Msun - 10^3 Msun and the lack of IR emission suggests that they may be massive cold cores in a pre-stellar phase, which could presumably form massive stars eventually. Ammonia (1,1) and (2,2) observations of the cores indicate smaller velocity dispersions and lower rotation temperatures compared to HMPOs and UCHII regions suggesting a quiescent pre-stellar stage. We propose that these newly discovered cores are good candidates for the HMSC stage in high-mass star-formation. This sample of cores will allow us to study the high-mass star and cluster formation processes at the earliest evolutionary stages.
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Young massive stars are usually found embedded in dense and massive molecular clumps and are known for being highly obscured and distant. During their formation process, deuteration is regarded as a potentially good indicator of the formation stage. Therefore, proper observations of such deuterated molecules are crucial, but still, hard to perform. In this work, we test the observability of the transition o-H$_2$D$^+(1_{10}$-$1_{11})$, using a synthetic source, to understand how the physical characteristics are reflected in observations through interferometers and single-dish telescopes. In order to perform such tests, we post-processed a magneto-hydrodynamic simulation of a collapsing magnetized core using the radiative transfer code POLARIS. Using the resulting intensity distributions as input, we performed single-dish (APEX) and interferometric (ALMA) synthetic observations at different evolutionary times, always mimicking realistic configurations. Finally, column densities were derived to compare our simulations with real observations previously performed. Our derivations for o-H$_2$D$^+$ are in agreement with values reported in the literature, in the range of 10$^{!10-11}$cm$^{!-2}$ and 10$^{!12-13}$cm$^{!-2}$ for single-dish and interferometric measurements, respectively.
Two families of models compete to explain the formation of high-mass stars. The quasi-static models predict the existence of high-mass pre-stellar cores sustained by a high degree of turbulence while competitive accretion models predict that high-mass proto-stellar cores evolve from low/intermediate mass proto-stellar cores in dynamic environments. We present ALMA (1.4 mm continuum emission and $^{12}$CO emission line) and MOPRA (HCO$^{+}$, H$^{13}$CO$^{+}$ and N$_2$H$^+$ molecular line emissions) observations of a sample of 9 starless massive dense cores (MDCs) discovered in a recent Herschel/HOBYS study that have masses and sizes ($sim$110 M$_odot$ and $rsim$0.1 pc, respectively) similar to the initial conditions used in the quasi-static models. The MOPRA molecular line features show that 3 of the starless MDCs are subvirialized with $alpha_{rm vir}sim$0.35, and 4 MDCs show sign of collapse. Our ALMA observations, on the other hand, show very little fragmentation within the MDCs whereas the observations resolve the Jeans length ($lambda_{rm Jeans}sim$0.03 pc) and are sensitive to the Jeans mass (M$_{rm Jeans}sim$0.65 M$_odot$) in the 9 starless MDCs. Only two of the starless MDCs host compact continuum sources, whose fluxes correspond to $<3$ M$_odot$ fragments. Therefore the mass reservoir of the MDCs has not yet been accreted onto compact objects, and most of the emission is filtered out by the interferometer. These observations do not support the quasi-static models for high-mass star formation since no high-mass pre-stellar core is found in NGC6334. The competitive accretion models, on the other hand, predict a level of fragmentation much higher than what we observe.
I review the basic processes that may be used to develop a chemical evolutionary sequence for low-mass starless cores. I highlight observational results from the Arizona Radio Observatory-Green Bank Survey. Observations were performed with the SMT 10-m, ARO 12-m, and GBT 100-m toward a sample of 25 nearby (D < 400 pc) low-mass starless cores which have radiative transfer models of the 850 $mu$m emission and observed SED (160 - 1300 um). The cores were observed in the lines of NH3 (1,1) and (2,2), o-NH2D 1_{11} - 1_{01}, C2S 1_2 - 2_1, C3S 4 - 3, HCN 1 - 0, HC5N 9 - 8, HC7N 21 - 20, C18O and C17O 2 - 1, and p-H2CO 1_{01} - 0_{00}.
We develop a method of analyzing radio frequency spectral line observations to derive data on the temperature, density, velocity, and molecular abundance of the emitting gas. The method incorporates a radiative transfer code with a new technique for handling overlapping hyperfine emission lines within the accelerated lambda iteration algorithm and a heuristic search algorithm based on simulated annnealing. We apply this method to new observations of N_2H^+ in three Lynds clouds thought to be starless cores in the first stages of star formation and determine their density structure. A comparison of the gas densities derived from the molecular line emission and the millimeter dust emission suggests that the required dust mass opacity is about kappa_{1.3mm}=0.04 cm^2/g, consistent with models of dust grains that have opacities enhanced by ice mantles and fluffy aggregrates.
We carry out an ALMA $rm N_2D^+$(3-2) and 1.3~mm continuum survey towards 32 high mass surface density regions in seven Infrared Dark Clouds with the aim of finding massive starless cores, which may be the initial conditions for the formation of massive stars. Cores showing strong $rm N_2D^+$(3-2) emission are expected to be highly deuterated and indicative of early, potentially pre-stellar stages of star formation. We also present maps of these regions in ancillary line tracers, including C$^{18}$O(2-1), DCN(3-2) and DCO$^+$(3-2). Over 100 $rm N_2D^+$ cores are identified with our newly developed core-finding algorithm based on connected structures in position-velocity space. The most massive core has $sim70:M_odot$ (potentially $sim170:M_odot$) and so may be representative of the initial conditions for massive star formation. The existence and dynamical properties of such cores constrain massive star formation theories. We measure the line widths and thus velocity dispersion of six of the cores with strongest $rm N_2D^+$(3-2) line emission, finding results that are generally consistent with virial equilibrium of pressure confined cores.
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