Abstract Most stars form in clusters and groups rather than in isolation. We present ≲5″ angular resolution (∼2000 au, or 0.01 pc) Very Large Array NH 3 (1,1), (2,2), and (3,3) and 1.3 cm continuum ...emission observations of the dense gas within the Serpens South protocluster and extended filaments to the north and south. We identify 94 dense cores using a dendrogram analysis of the NH 3 (1,1) integrated intensity. Gas temperatures T K and nonthermal line widths σ NT both increase toward the center of the young stellar cluster, in the dense gas generally and in the cores specifically. We find that most cores (54%) are supervirial, with gravitationally bound cores located primarily in the filaments. Cores in the protocluster have higher virial parameters by a factor of ∼1.7, driven primarily by the increased core σ NT values. These cores cannot collapse to form stars unless they accrete additional mass or their core internal motions are reduced. The southern filament shows a significant velocity gradient previously interpreted as mass flow toward the cluster. We find more complex kinematics in the northern filament. We find a strong correlation between σ NT and T K , and argue that the enhanced temperatures and nonthermal motions are due to mechanical heating and interaction between the protocluster-driven outflows and the dense gas. Filament-led accretion may also contribute to the increased σ NT values. Assuming a constant fraction of core mass ends up in the young stars, future star formation in the Serpens South protocluster will shift to higher masses by a factor of ∼2.
Recent observations of global velocity gradients across and along molecular filaments have been interpreted as signs of gas accreting onto and along these filaments, potentially feeding star-forming ...cores and protoclusters. The behavior of velocity gradients in filaments, however, has not been studied in detail, particularly on small scales (<0.1 pc). In this paper, we present MUFASA, an efficient, robust, and automatic method to fit ammonia lines with multiple velocity components, generalizable to other molecular species. We also present CRISPy, a Python package to identify filament spines in 3D images (e.g., position-position-velocity cubes), along with a complementary technique to sort fitted velocity components into velocity-coherent filaments. In NGC 1333, we find a wealth of velocity gradient structures on a beam-resolved scale of ∼0.05 pc. Interestingly, these local velocity gradients are not randomly oriented with respect to filament spines and their perpendicular, i.e., radial, component decreases in magnitude toward the spine for many filaments. Together with remarkably constant velocity gradients on larger scales along many filaments, these results suggest a scenario in which gas falling onto filaments is progressively damped and redirected to flow along these filaments.
We present an overview of the first data release (DR1) and first-look science from the Green Bank Ammonia Survey (GAS). GAS is a Large Program at the Green Bank Telescope to map all Gould Belt ...star-forming regions with mag visible from the northern hemisphere in emission from NH3 and other key molecular tracers. This first release includes the data for four regions in the Gould Belt clouds: B18 in Taurus, NGC 1333 in Perseus, L1688 in Ophiuchus, and Orion A North in Orion. We compare the NH3 emission to dust continuum emission from Herschel and find that the two tracers correspond closely. We find that NH3 is present in over 60% of the lines of sight with mag in three of the four DR1 regions, in agreement with expectations from previous observations. The sole exception is B18, where NH3 is detected toward ∼40% of the lines of sight with mag. Moreover, we find that the NH3 emission is generally extended beyond the typical 0.1 pc length scales of dense cores. We produce maps of the gas kinematics, temperature, and NH3 column densities through forward modeling of the hyperfine structure of the NH3 (1, 1) and (2, 2) lines. We show that the NH3 velocity dispersion, , and gas kinetic temperature, TK, vary systematically between the regions included in this release, with an increase in both the mean value and the spread of and TK with increasing star formation activity. The data presented in this paper are publicly available (https://dataverse.harvard.edu/dataverse/GAS_DR1).
We present Atacama Large Millimeter/submillimeter Array maps of the starless molecular cloud core Ophiuchus/H-MM1 in the lines of deuterated ammonia (ortho- ), methanol ( ), and sulfur monoxide (SO). ...The dense core is seen in emission, whereas the and SO distributions form a halo surrounding the core. Because methanol is formed on grain surfaces, its emission highlights regions where desorption from grains is particularly efficient. Methanol and sulfur monoxide are most abundant in a narrow zone that follows the eastern side of the core. This side is sheltered from the stronger external radiation field coming from the west. We show that photodissociation on the illuminated side can give rise to an asymmetric methanol distribution but that the stark contrast observed in H-MM1 is hard to explain without assuming enhanced desorption on the shaded side. The region of the brightest emission has a wavy structure that rolls up at one end. This is the signature of Kelvin-Helmholtz instability occurring in sheared flows. We suggest that in this zone, methanol and sulfur are released as a result of grain-grain collisions induced by shear vorticity.
We use data on gas temperature and velocity dispersion from the Green Bank Ammonia Survey and core masses and sizes from the James Clerk Maxwell Telescope Gould Belt Survey to estimate the virial ...states of dense cores within the Orion A molecular cloud. Surprisingly, we find that almost none of the dense cores are sufficiently massive to be bound when considering only the balance between self-gravity and the thermal and non-thermal motions present in the dense gas. Including the additional pressure binding imposed by the weight of the ambient molecular cloud material and additional smaller pressure terms, however, suggests that most of the dense cores are pressure-confined.
Abstract We present new Atacama Large Millimeter/submillimeter Array (ALMA) continuum and NH 2 D, N 2 D + , and H 2 D + line emission at matched, ∼100 au resolution toward the dense star-forming ...cores SM1N and N6 within the Ophiuchus molecular cloud. We determine the density and temperature structure of SM1N based on radiative transfer modeling and simulated observations of the multiwavelength continuum emission at 0.8, 2, and 3 mm. We show that SM1N is best fit by either a broken power-law or Plummer-like density profile with high central densities ( n ∼ 10 8 cm −3 ), and an inner transition radius of only ∼80–300 au. The free-fall time of the inner region is only a few ×10 3 yr. The continuum modeling rules out the presence of an embedded first hydrostatic core (FHSC) or protostar. SM1N is therefore a dynamically unstable but still starless core. We find that NH 2 D is likely depleted at high densities within SM1N. The nonthermal velocity dispersions increase from NH 2 D to N 2 H + and H 2 D + , possibly tracing increasing (but still subsonic) infall speeds at higher densities as predicted by some models of starless core contraction. Toward N6, we confirm the previous ALMA detection of a faint, embedded point source (N6-mm) in 0.8 mm continuum emission. NH 2 D and N 2 D + avoid N6-mm within ∼100 au, while H 2 D + is not strongly detected toward N6. The distribution of these tracers is consistent with heating by a young, warm object. N6-mm thus remains one of the best candidate FHSCs detected so far, although its observed (sub)millimeter luminosity remains below predictions for FHSCs.
We develop a new "core field structure" (CFS) model to predict the magnetic field strength and magnetic field fluctuation profile of dense cores using gas kinematics. We use spatially resolved ...observations of the nonthermal velocity dispersion from the Green Bank Ammonia survey along with column density maps from SCUBA-2 to estimate the magnetic field strength across seven dense cores located in the L1688 region of Ophiuchus. The CFS model predicts the profile of the relative field fluctuation, which is related to the observable dispersion in the direction of the polarization vectors. Within the context of our model, we find that all of the cores have a transcritical mass-to-flux ratio.
The Reservoir of the Per-emb-2 Streamer Taniguchi, Kotomi; Pineda, Jaime E.; Caselli, Paola ...
The Astrophysical journal,
04/2024, Volume:
965, Issue:
2
Journal Article
Peer reviewed
Open access
Abstract Streamers bring gas from outer regions to protostellar systems and could change the chemical composition around protostars and protoplanetary disks. We have carried out mapping observations ...of carbon-chain species (HC 3 N, HC 5 N, CCH, and CCS) in the 3 mm and 7 mm bands toward the streamer flowing to the Class 0 young stellar object (YSO) Per-emb-2 with the Nobeyama 45 m radio telescope. A region with a diameter of ∼0.04 pc is located to the north at a distance of ∼20,500 au from the YSO. The streamer connects to this northern region, which is its origin. The reservoir has high density and low temperature ( n H 2 ≈ 1.9 × 10 4 cm −3 , T kin = 10 K), which are similar to those of early-stage starless cores. By comparison of the observed abundance ratios of CCS/HC 3 N to the chemical simulations, the reservoir and streamer are found to be chemically young. The total mass available for the streamer is derived to be 24–34 M ⊙ . It has been estimated that, if all of the gas in the reservoir were to accrete onto the Per-emb-2 protostellar system, the lifetime of the streamer would be (1.1–3.2) × 10 5 yr, suggesting that the mass accretion via the streamer would continue until the end of the Class I stage.
We present the observation and analysis of newly discovered coherent structures in the L1688 region of Ophiuchus and the B18 region of Taurus. Using data from the Green Bank Ammonia Survey, we ...identify regions of high density and near-constant, almost-thermal velocity dispersion. We reveal 18 coherent structures are revealed, 12 in L1688 and 6 in B18, each of which shows a sharp "transition to coherence" in velocity dispersion around its periphery. The identification of these structures provides a chance to statistically study the coherent structures in molecular clouds. The identified coherent structures have a typical radius of 0.04 pc and a typical mass of 0.4 M☉, generally smaller than previously known coherent cores identified by Goodman et al., Caselli et al., and Pineda et al. We call these structures "droplets." We find that, unlike previously known coherent cores, these structures are not virially bound by self-gravity and are instead predominantly confined by ambient pressure. The droplets have density profiles shallower than a critical Bonnor-Ebert sphere, and they have a velocity (VLSR) distribution consistent with the dense gas motions traced by NH3 emission. These results point to a potential formation mechanism through pressure compression and turbulent processes in the dense gas. We present a comparison with a magnetohydrodynamic simulation of a star-forming region, and we speculate on the relationship of droplets with larger, gravitationally bound coherent cores, as well as on the role that droplets and other coherent structures play in the star formation process.
Spectral lines of ammonia, NH3, are useful probes of the physical conditions in dense molecular cloud cores. In addition to advantages in spectroscopy, ammonia has also been suggested to be resistant ...to freezing onto grain surfaces, which should make it a superior tool for studying the interior parts of cold, dense cores. Here we present high-resolution NH3 observations with the Very Large Array and Green Bank Telescope toward a prestellar core. These observations show an outer region with a fractional NH3 abundance of X(NH3) = (1.975 ± 0.005) × 10−8 (±10% systematic), but it also reveals that, after all, the X(NH3) starts to decrease above a H2 column density of ≈2.6 × 1022 cm−2. We derive a density model for the core and find that the break point in the fractional abundance occurs at the density n(H2) ∼ 2 × 105 cm−3, and beyond this point the fractional abundance decreases with increasing density, following the power law n−1.1. This power-law behavior is well reproduced by chemical models where adsorption onto grains dominates the removal of ammonia and related species from the gas at high densities. We suggest that the break-point density changes from core to core depending on the temperature and the grain properties, but that the depletion power law is anyway likely to be close to n−1 owing to the dominance of accretion in the central parts of starless cores.
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