Abstract
We present dynamical properties of 294 cores embedded in twelve IRDCs observed as part of the ASHES Survey. Protostellar cores have higher gas masses, surface densities, column densities, ...and volume densities than prestellar cores, indicating core mass growth from the prestellar to the protostellar phase. We find that ∼80% of cores with virial parameter (
α
) measurements are gravitationally bound (
α
< 2). We also find an anticorrelation between the mass and the virial parameter of cores, with massive cores having on average lower virial parameters. Protostellar cores are more gravitationally bound than prestellar cores, with an average virial parameter of 1.2 and 1.5, respectively. The observed nonthermal velocity dispersion (from N
2
D
+
or DCO
+
) is consistent with simulations in which turbulence is continuously injected, whereas the core-to-core velocity dispersion is neither in agreement with driven nor decaying turbulence simulations. We find a not significant increment in the line velocity dispersion from prestellar to protostellar cores, suggesting that the dense gas within the core traced by these deuterated molecules is not yet severely affected by turbulence injected from outflow activity at the early evolutionary stages traced in ASHES. The most massive cores are strongly self-gravitating and have greater surface density, Mach number, and velocity dispersion than cores with lower masses. Dense cores do not have significant velocity shifts relative to their low-density envelopes, suggesting that dense cores are comoving with their envelopes. We conclude that the observed core properties are more in line with the predictions of
clump-fed
scenarios rather than with those of
core-fed
scenarios.
We report observations, made with the Atacama Large Millimeter/submillimeter Array, of 3 mm dust continuum emission and molecular line emission in HCO+, CS, and N2H+, toward G305.137+0.069, a ...massive, dense, and cold clump, in order to characterize its small-scale (∼2000 au) structure. The dust continuum observations reveal the presence of twelve compact structures (cores) with masses ranging from 3.3 to 50.6 M , radii from 1800 to 5300 au, and densities from 3.1 × 106 to 3.1 × 107 cm−3. The line observations show that the molecular emission arises from a bright central region, with an angular size of ∼12″, and an extended, weaker envelope best seen in the HCO+ line. The N2H+ emission is best correlated with the continuum emission and hence with the cores. For cores with single line profiles, the line widths range from 1.9 to 3.1 km s−1 with an average value of 2.6 km s−1, indicating that they are dominated by nonthermal motions, either due to random turbulence or core-scale motions. The virial parameter of the three most massive cores are smaller than one, suggesting that they are undergoing collapse. We find that in the regime of masses probed by our observations (M > 3 M ) the shape of the core-mass function is notably different from the initial mass function, showing an overpopulation of high-mass cores. We suggest that the formation and mass distribution of the dense cores within G305.137+0.069 can be explained as the result of hierarchical or turbulent fragmentation in a gravitationally collapsing clump.
We have observed 37 Infrared Dark Clouds (IRDCs), containing a total of 159 clumps, in high-density molecular tracers at 3 mm using the 22 m ATNFMopra Telescope located in Australia. The most ...commonly detected molecular lines are Nsub 2Hsup +, HNC, HNsup 13C, HCOsup +, Hsup 13COsup +, HCN, Csub 2H, HCsub 3N, HNCO, and SiO. We investigate the behavior of the different molecular tracers and look for chemical variations as a function of an evolutionary sequence based on Spitzer IRAC and MIPS emission. We find that the molecular tracers behave differently through the evolutionary sequence and some of them can be used to yield useful relative age information. Total column densities of the different molecules, except Csub 2H, increase with the evolutionary stage of the clumps. Molecular abundances increase with the evolutionary stage for Nsub 2Hsup + and HCOsup +. The Nsub 2Hsup +/HCOsup + and Nsub 2Hsup +/HNC abundance ratios act as chemical clocks, increasing with the evolution of the clumps.
We report ALMA observations with resolution 0 5 at 3 mm of the extended Sgr B2 cloud in the Central Molecular Zone (CMZ). We detect 271 compact sources, most of which are smaller than 5000 au. By ...ruling out alternative possibilities, we conclude that these sources consist of a mix of hypercompact H ii regions and young stellar objects (YSOs). Most of the newly detected sources are YSOs with gas envelopes that, based on their luminosities, must contain objects with stellar masses M * 8 M . Their spatial distribution spread over a ∼12 × 3 pc region demonstrates that Sgr B2 is experiencing an extended star formation event, not just an isolated "starburst" within the protocluster regions. Using this new sample, we examine star formation thresholds and surface density relations in Sgr B2. While all of the YSOs reside in regions of high column density ( N ( H 2 ) 2 × 10 23 cm − 2 ), not all regions of high column density contain YSOs. The observed column density threshold for star formation is substantially higher than that in solar vicinity clouds, implying either that high-mass star formation requires a higher column density or that any star formation threshold in the CMZ must be higher than in nearby clouds. The relation between the surface density of gas and stars is incompatible with extrapolations from local clouds, and instead stellar densities in Sgr B2 follow a linear * - gas relation, shallower than that observed in local clouds. Together, these points suggest that a higher volume density threshold is required to explain star formation in CMZ clouds.
For the past few decades, there has been great interest in determining if even the most massive stars in our galaxy (namely the spectral O-type stars) are formed in a similar manner as the low- and ...intermediate-mass stars, that is, through the presence of accreting disks and powerful outflows. Here, using sensitive observations of the Atacama Large Millimeter/Submillimeter Array, we report a resolved Keplerian disk (with 15 synthesized beams across its major axis) surrounding the deeply embedded O-type protostar IRAS 16547−4247. The disk shows some asymmetries that could arise because the disk is unstable and fragmenting or because of different excitation conditions within the disk. The enclosed mass estimated from the disk Keplerian radial velocities is 25 3 M . The molecular disk is at the base of an ionized thermal radio jet and is approximately perpendicular to the jet axis orientation. We additionally find the existence of a binary system of compact dusty objects at the center of the accreting disk, which indicates the possible formation of an O-type star and a companion of lower mass. This is not surprising due to the high binary fraction reported in massive stars. Subtracting the contribution of the dusty disk plus the envelope and the companion, we estimated a mass of 20 M for the central star.
An Episodic Wide-angle Outflow in HH 46/47 Zhang, Yichen; Arce, Héctor G.; Mardones, Diego ...
The Astrophysical journal,
09/2019, Letnik:
883, Številka:
1
Journal Article
Recenzirano
Odprti dostop
During star formation, the accretion disk drives fast MHD winds, which usually contain two components, a collimated jet and a radially distributed wide-angle wind. These winds entrain the surrounding ...ambient gas producing molecular outflows. We report a recent observation of 12CO (2-1) emission of the HH 46/47 molecular outflow by the Atacama Large Millimeter/submillimeter Array, in which we identify multiple wide-angle outflowing shell structures in both the blueshifted and redshifted outflow lobes. These shells are highly coherent in position-position-velocity space, extending to 40-50 km s−1 in velocity and 104 au in space, with well-defined morphology and kinematics. We suggest these outflowing shells are the result of the entrainment of ambient gas by a series of outbursts from an intermittent wide-angle wind. Episodic outbursts in collimated jets are commonly observed, yet detection of a similar behavior in wide-angle winds has been elusive. Here we show clear evidence that the wide-angle component of the HH 46/47 protostellar outflows experiences variability similar to that seen in the collimated component.
We present molecular line observations of the high-mass molecular clump IRAS 16562−3959 taken at 3 mm using the Atacama Large Millimeter/submillimeter Array at 1 7 angular resolution (0.014 pc ...spatial resolution). This clump hosts the actively accreting high-mass young stellar object (HMYSO) G345.4938+01.4677, which is associated with a hypercompact H ii region. We identify and analyze emission lines from 22 molecular species (encompassing 34 isomers) and classify them into two groups, depending on their spatial distribution within the clump. One of these groups gathers shock tracers (e.g., SiO, SO, HNCO) and species formed in dust grains like methanol (CH3OH), ethenone or ketene (H2CCO), and acetaldehyde (CH3CHO). The second group collects species closely resembling the dust continuum emission morphology and are formed mainly in the gas phase, like hydrocarbons (CCH, c-C3H2, CH3CCH), cyanopolyynes (HC3N and HC5N), and cyanides (HCN and CH3C3N). Emission from complex organic molecules (COMs) like CH3OH, propanenitrile (CH3CH2CN), and methoxymethane (CH3OCH3) arise from gas in the vicinity of a hot molecular core (T 100 K) associated with the HMYSO. Other COMs such as propyne (CH3CCH), acrylonitrile (CH2CHCN), and acetaldehyde seem to better trace warm (T 80 K) dense gas. In addition, deuterated ammonia (NH2D) is detected mostly in the outskirts of IRAS 16562−3959 and associated with near-infrared dark globules, probably gaseous remnants of the clump's prestellar phase. The spatial distribution of molecules in IRAS 16562−3959 supports the view that in protostellar clumps, chemical tracers associated with different evolutionary stages-starless to hot cores/H ii regions-exist coevally.
Abstract
We present high spatial resolution (52 au) observations of the high-mass young stellar object (HMYSO) G345.4938+01.4677 made using the Atacama Large Millimeter/submillimeter Array. This ...O-type HMYSO is located at 2.38 kpc and is associated with a luminosity of 1.5 × 10
5
L
☉
. We detect circumstellar emission from the H38
β
hydrogen recombination line showing a compact structure rotating perpendicularly to the previously detected radio jet. We interpret this emission as tracing a photoionized accretion disk around the HMYSO. While this disk-like structure seems currently too small to sustain continued accretion, the data present direct observational evidence of how disks can effectively survive the photoionization feedback from young high-mass stars. We also report the detection of a low-mass young stellar object in the vicinity of the HMYSO and suggest that it forms a high-mass and low-mass star binary system.
Abstract
The initial conditions found in infrared dark clouds (IRDCs) provide insights on how high-mass stars and stellar clusters form. We have conducted high-angular resolution and high-sensitivity ...observations toward thirty-nine massive IRDC clumps, which have been mosaicked using the 12 and 7 m arrays from the Atacama Large Millimeter/submillimeter Array. The targets are 70
μ
m dark massive (220–4900
M
⊙
), dense (>10
4
cm
−3
), and cold (∼10–20 K) clumps located at distances between 2 and 6 kpc. We identify an unprecedented number of 839 cores, with masses between 0.05 and 81
M
⊙
using 1.3 mm dust continuum emission. About 55% of the cores are low-mass (<1
M
⊙
), whereas ≲1% (7/839) are high-mass (≳27
M
⊙
). We detect no high-mass prestellar cores. The most massive cores (MMC) identified within individual clumps lack sufficient mass to form high-mass stars without additional mass feeding. We find that the mass of the MMCs is correlated with the clump surface density, implying denser clumps produce more massive cores. There is no significant mass segregation except for a few tentative detections. In contrast, most clumps show segregation once the clump density is considered instead of mass. Although the dust continuum emission resolves clumps in a network of filaments, some of which consist of hub-filament systems, the majority of the MMCs are not found in the hubs. Our analysis shows that high-mass cores and MMCs have no preferred location with respect to low-mass cores at the earliest stages of high-mass star formation.
ABSTRACT We present Atacama Large Millimeter/sub-millimeter Array Cycle 1 observations of the HH 46/47 molecular outflow using combined 12 m array and Atacama Compact Array observations. The improved ...angular resolution and sensitivity of our multi-line maps reveal structures that help us study the entrainment process in much more detail and allow us to obtain more precise estimates of outflow properties than in previous observations. We use (1-0) and (1-0) emission to correct for the (1-0) optical depth to accurately estimate the outflow mass, momentum, and kinetic energy. This correction increases the estimates of the mass, momentum, and kinetic energy by factors of about 9, 5, and 2, respectively, with respect to estimates assuming optically thin emission. The new and data also allow us to trace denser and slower outflow material than that traced by the maps, and they reveal an outflow cavity wall at very low velocities (as low as 0.2 with respect to the core's central velocity). Adding the slower material traced only by and , there is another factor of three increase in the mass estimate and 50% increase in the momentum estimate. The estimated outflow properties indicate that the outflow is capable of dispersing the parent core within the typical lifetime of the embedded phase of a low-mass protostar and that it is responsible for a core-to-star efficiency of 1/4 to 1/3. We find that the outflow cavity wall is composed of multiple shells associated with a series of jet bow-shock events. Within about 3000 au of the protostar the and emission trace a circumstellar envelope with both rotation and infall motions, which we compare with a simple analytic model. The CS (2-1) emission reveals tentative evidence of a slowly moving rotating outflow, which we suggest is entrained not only poloidally but also toroidally by a disk wind that is launched from relatively large radii from the source.
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