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Register a new userThe ALMA Survey of 70 $mu$m dark High-mass clumps in Early Stages (ASHES). I. Pilot Survey: Clump Fragmentation
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(Abridged) ASHES has been designed to systematically characterize the earliest stages and to constrain theories of high-mass star formation. A total of 12 massive (>500 $M_{odot}$), cold (<15 K), 3.6-70 $mu$m dark prestellar clump candidates, embedded in IRDCs, were carefully selected in the pilot survey to be observed with ALMA. We mosaiced each clump (~1 arcmin^2) in dust and line emission with the 12m/7m/TP arrays at 224 GHz, resulting in ~1.2 resolution (~4800 AU). As the first paper of the series, we concentrate on the dust emission to reveal the clump fragmentation. We detect 294 cores, from which 84 (29%) are categorized as protostellar based on outflow activity or warm core line emission. The remaining 210 (71%) are considered prestellar core candidates. The number of detected cores is independent of the mass sensitivity range of the observations. On average, more massive clumps tend to form more cores. We find a large population of low-mass (<1 M) cores and no high-mass (>30 $M_{odot}$) prestellar cores. The most massive prestellar core has a mass of 11 $M_{odot}$. From the prestellar CMF, we derive a power law index of 1.17+-0.1, slightly shallower than Salpeter (1.35). We use the MST technique to characterize the separation between cores and their spatial distribution, and derive mass segregation ratios. While there is a range of core masses and separations detected in the sample, the mean separation and mass of cores are well explained by thermal fragmentation and are inconsistent with turbulent Jeans fragmentation. The core spatial distribution is well described by hierarchical subclustering rather than centrally peaked clustering. There is no conclusive evidence of mass segregation. We test several theoretical conditions, and conclude that overall, competitive accretion and global hierarchical collapse scenarios are favored over the turbulent core accretion scenario.
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With a mass of $sim$1000 $M_odot$ and a surface density of $sim$0.5 g cm$^{-2}$, G023.477+0.114 also known as IRDC 18310-4 is an infrared dark cloud (IRDC) that has the potential to form high-mass stars and has been recognized as a promising prestellar clump candidate. To characterize the early stages of high-mass star formation, we have observed G023.477+0.114 as part of the ALMA Survey of 70 $mu$m Dark High-mass Clumps in Early Stages (ASHES). We have conducted $sim$1.2 resolution observations with the Atacama Large Millimeter/submillimeter Array (ALMA) at 1.3 mm in dust continuum and molecular line emission. We identified 11 cores, whose masses range from 1.1 $M_odot$ to 19.0 $M_odot$. Ignoring magnetic fields, the virial parameters of the cores are below unity, implying that the cores are gravitationally bound. However, when magnetic fields are included, the prestellar cores are close to virial equilibrium, while the protostellar cores remain sub-virialized. Star formation activity has already started in this clump. Four collimated outflows are detected in CO and SiO. H$_2$CO and CH$_3$OH emission coincide with the high-velocity components seen in the CO and SiO emission. The outflows are randomly oriented for the natal filament and the magnetic field. The position-velocity diagrams suggest that episodic mass ejection has already begun even in this very early phase of protostellar formation. The masses of the identified cores are comparable to the expected maximum stellar mass that this IRDC could form (8-19 $M_odot$). We explore two possibilities on how IRDC G023.477+0.114 could eventually form high-mass stars in the context of theoretical scenarios.
We present a spatio-kinematical analysis of the CO~($J$=2$rightarrow$1) line emission, observed with the Atacama Large Millimter/submillimter Array (ALMA), of the outflow associated with the most massive core, ALMA1, in the 70 $mu$m dark clump G010.991$-$00.082. The position-velocity (P-V) diagram of the molecular outflow exhibits a peculiar $mathsf{S}$-shaped morphology that has not been seen in any other star forming region. We propose a spatio-kinematical model for the bipolar molecular outflow that consists of a decelerating high-velocity component surrounded by a slower component whose velocity increases with distance from the central source. The physical interpretation of the model is in terms of a jet that decelerates as it entrains material from the ambient medium, which has been predicted by calculations and numerical simulations of molecular outflows in the past. One side of the outflow is shorter and shows a stronger deceleration, suggesting that the medium through which the jet moves is significantly inhomogeneous. The age of the outflow is estimated to be $tau$$approx$1300 years, after correction for a mean inclination of the system of $approx$57$^{circ}$.
We present a study of outflows at extremely early stages of high-mass star formation obtained from the ALMA Survey of 70 $mu rm m$ dark High-mass clumps in Early Stages (ASHES). Twelve massive 3.6$-$70 $mu rm m$ dark prestellar clump candidates were observed with the Atacama Large Millimeter/submillimeter Array (ALMA) in Band 6. Forty-three outflows are identified toward 41 out of 301 dense cores using the CO and SiO emission lines, yielding a detection rate of 14%. We discover 6 episodic molecular outflows associated with low- to high-mass cores, indicating that episodic outflows (and therefore episodic accretion) begin at extremely early stages of protostellar evolution for a range of core masses. The time span between consecutive ejection events is much smaller than those found in more evolved stages, which indicates that the ejection episodicity timescale is likely not constant over time. The estimated outflow dynamical timescale appears to increase with core masses, which likely indicates that more massive cores have longer accretion timescales than less massive cores. The lower accretion rates in these 70 $mu rm m$ dark objects compared to the more evolved protostars indicate that the accretion rates increase with time. The total outflow energy rate is smaller than the turbulent energy dissipation rate, which suggests that outflow induced turbulence cannot sustain the internal clump turbulence at the current epoch. We often detect thermal SiO emission within these 70 $mu rm m$ dark clumps that is unrelated to CO outflows. This SiO emission could be produced by collisions, intersection flows, undetected protostars, or other motions.
We present observations towards a high-mass ($rm >40,M_{odot}$), low luminosity ($rm <10,L_{odot}$) $rm 70,mu$m dark molecular core G 28.34 S-A at 3.4 mm, using the IRAM 30 m telescope and the NOEMA interferometer. We report the detection of $rm SiO$ $J=rm 2rightarrow1$ line emission, which is spatially resolved in this source at a linear resolution of $sim$0.1 pc, while the 3.4 mm continuum image does not resolve any internal sub-structures. The SiO emission exhibits two W-E oriented lobes centring on the continuum peak. Corresponding to the red-shifted and blue-shifted gas with velocities up to $rm 40,km,s^{-1}$ relative to the quiescent cloud, these lobes clearly indicate the presence of a strong bipolar outflow from this $rm 70,mu$m dark core, a source previously considered as one of the best candidates of starless core. Our SiO detection is consistent with ALMA archival data of $rm SiO$ $J=rm 5rightarrow4$, whose high-velocity blue-shifted gas reveals a more compact lobe spatially closer to the dust center. This outflow indicates that the central source may be in an early evolutionary stage of forming a high-mass protostar. We also find that the low-velocity components (in the range of $rm V_{lsr}$$rm_{-5}^{+3},km,s^{-1}$) have an extended, NW-SE oriented distribution. Discussing the possible accretion scenarios of the outflow-powering young stellar object, we argue that the molecular line emission and the molecular outflows may provide a better indication of the accretion history when forming young stellar object, than that from a snapshot observations of the present bolometric luminosity. This is particularly significant for the cases of episodic accretion, which may occur during the collapse of the parent molecular core.
(Abridged) The initial physical conditions of high-mass stars and protoclusters remain poorly characterized. To this end we present the first targeted ALMA 1.3mm continuum and spectral line survey towards high-mass starless clump candidates, selecting a sample of 12 of the most massive candidates ($400-4000, M_odot$) within 5 kpc. The joint 12+7m array maps have a high spatial resolution of $sim 3000, mathrm{au}$ ($sim 0.8^{primeprime}$) and have point source mass-completeness down to $sim 0.3, M_odot$ at $6sigma$ (or $1sigma$ column density sensitivity of $1.1times10^{22}, mathrm{cm^{-2}}$). We discover previously undetected signposts of low-luminosity star formation from CO (2-1) and SiO (5-4) bipolar outflows and other signatures towards 11 out of 12 clumps, showing that current MIR/FIR Galactic Plane surveys are incomplete to low- and intermediate-mass protostars ($lesssim 50, L_odot$). We compare a subset of the observed cores with a suite of radiative transfer models of starless cores. We find a high-mass starless core candidate with a model-derived mass consistent with $29^{52}_{15}, M_odot$ when integrated over size scales of $2times10^4, mathrm{au}$. Unresolved cores are poorly fit by starless core models, supporting the interpretation that they are protostellar even without detection of outflows. Substantial fragmentation is observed towards 10 out of 12 clumps. We extract sources from the maps using a dendrogram to study the characteristic fragmentation length scale. Nearest neighbor separations when corrected for projection are consistent with being equal to the clump average thermal Jeans length. Our findings support a hierarchical fragmentation process, where the highest density regions are not strongly supported against thermal gravitational fragmentation by turbulence or magnetic fields.
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