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Register a new userA Hunt for Massive Starless Cores
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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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How do stars that are more massive than the Sun form, and thus how is the stellar initial mass function (IMF) established? Such intermediate- and high-mass stars may be born from relatively massive pre-stellar gas cores, which are more massive than the thermal Jeans mass. The Turbulent Core Accretion model invokes such cores as being in approximate virial equilibrium and in approximate pressure equilibrium with their surrounding clump medium. Their internal pressure is provided by a combination of turbulence and magnetic fields. Alternatively, the Competitive Accretion model requires strongly sub-virial initial conditions that then lead to extensive fragmentation to the thermal Jeans scale, with intermediate- and high-mass stars later forming by competitive Bondi-Hoyle accretion. To test these models, we have identified four prime examples of massive (~100Msun) clumps from mid-infrared extinction mapping of infrared dark clouds (IRDCs). Fontani et al. found high deuteration fractions of N2H+ in these objects, which are consistent with them being starless. Here we present ALMA observations of these four clumps that probe the N2D+(3-2) line at 2.3 resolution. We find six N2D+ cores and determine their dynamical state. Their observed velocity dispersions and sizes are broadly consistent with the predictions of the Turbulent Core model of self-gravitating, magnetized (with Alfven Mach number m_A~1) and virialized cores that are bounded by the high pressures of their surrounding clumps. However, in the most massive cores, with masses up to ~60Msun, our results suggest that moderately enhanced magnetic fields (so that m_A~0.3) may be needed for the structures to be in virial and pressure equilibrium. Magnetically regulated core formation may thus be important in controlling the formation of massive cores, inhibiting their fragmentation, and thus helping to establish the stellar IMF.
G0.253+0.016 is a remarkable massive infrared dark cloud located within $sim$100 pc of the galactic center. With a high mass of $1.3 times 10^5 M_odot$, a compact average radius of $sim$2.8 pc and a low dust temperature of 23 K, it has been believed to be a yet starless precursor to a massive Arches-like stellar cluster. We present sensitive JVLA 1.3 and 5.6 cm radio continuum observations that reveal the presence on three compact thermal radio sources projected against this cloud. These radio sources are interpreted as HII regions powered by $sim$B0.5 ZAMS stars. We conclude that although G0.253+0.016 does not show evidence of O-type star formation, there are certainly early B-type stars embedded in it. We detect three more sources in the periphery of G0.253+0.016 with non-thermal spectral indices. We suggest that these sources may be related to the galactic center region and deserve further study.
We study deuterium fractionation in two massive starless/early-stage cores C1-N and C1-S in Infrared Dark Cloud (IRDC) G028.37+00.07, first identified by Tan et al. (2013) with ALMA. Line emission from multiple transitions of $rm N_2H^+$ and $rm N_2D^+$ were observed with the ALMA, CARMA, SMA, JCMT, NRO 45m and IRAM 30m telescopes. By simultaneously fitting the spectra, we estimate the excitation conditions and deuterium fraction, $D_{rm frac}^{rm N_2H^+} equiv [rm N_2D^+]/[N_2H^+]$, with values of $D_{rm frac}^{rm N_2H^+} simeq 0.2$--$0.7$, several orders of magnitude above the cosmic [D]/[H] ratio. Additional observations of o-H$_2$D$^+$ are also presented that help constrain the ortho-to-para ratio of $rm H_2$, which is a key quantity affecting the degree of deuteration. We then present chemodynamical modeling of the two cores, exploring especially the implications for the collapse rate relative to free-fall, $alpha_{rm ff}$. In order to reach the high level of observed deuteration of $rm N_2H^+$, we find that the most likely evolutionary history of the cores involves collapse at a relatively slow rate, $lesssim1/10$th of free-fall.
In dense starless and protostellar cores, the relative abundance of deuterated species to their non-deuterated counterparts can become orders of magnitude greater than in the local interstellar medium. This enhancement proceeds through multiple pathways in the gas phase and on dust grains, where the chemistry is strongly dependent on the physical conditions. In this Chapter, we discuss how sensitive, high resolution observations with the ngVLA of emission from deuterated molecules will trace both the dense gas structure and kinematics on the compact physical scales required to track the gravitational collapse of star-forming cores and the subsequent formation of young protostars and circumstellar accretion regions. Simultaneously, such observations will play a critical role in tracing the chemical history throughout the various phases of star and planet formation. Many low-J transitions of key deuterated species, along with their undeuterated counterparts, lie within the 60-110 GHz frequency window, the lower end of which is largely unavailable with current facilities and instrumentation. The combination of sensitivity and angular resolution provided only by the ngVLA will enable unparalleled detailed studies of the physics and chemistry of the earliest stages of star formation.
Similar to their low-mass counterparts, massive stars likely form via the collapse of pre-stellar molecular cores. Recent observations suggest that most massive cores are subvirial (i.e., not supported by turbulence) and therefore are likely unstable to gravitational collapse. Here we perform radiation hydrodynamic simulations to follow the collapse of turbulent massive pre-stellar cores with subvirial and virialized initial conditions to explore how their dynamic state affects the formation of massive stars and core fragmentation into companion stars. We find that subvirial cores undergo rapid monolithic collapse resulting in higher accretion rates at early times as compared to the collapse of virialized cores that have the same physical properties. In contrast, we find that virialized cores undergo a slower, gradual collapse and significant turbulent fragmentation at early times resulting in numerous companion stars. In the absence of strong magnetic fields and protostellar outflows we find that the faster growth rate of massive stars that are born out of subvirial cores leads to an increase in the radiative heating of the core thereby further suppressing fragmentation at early times when turbulent fragmentation occurs for virialized cores. Regardless of initial condition, we find that the massive accretion disks that form around massive stars dominant the accretion flow onto the star at late times and eventually become gravitationally unstable and fragment to form companion stars at late times.
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