42 research outputs found
Turbulence and Accretion: a High-resolution Study of the B5 Filaments
High-resolution observations of the Perseus B5 "core" have previously
revealed that this subsonic region actually consists of several filaments that
are likely in the process of forming a quadruple stellar system. Since subsonic
filaments are thought to be produced at the pc sonic scale by
turbulent compression, a detailed kinematic study is crucial to test such a
scenario in the context of core and star formation. Here we present a detailed
kinematic follow-up study of the B5 filaments at a 0.009 pc resolution using
the VLA and GBT combined observations fitted with multi-component spectral
models. Using precisely identified filament spines, we find a remarkable
resemblance between the averaged width profiles of each filament and
Plummer-like functions, with filaments possessing FWHM widths of
pc. The velocity dispersion profiles of the filaments also show decreasing
trends towards the filament spines. Moreover, the velocity gradient field in B5
appears to be locally well ordered ( pc) but globally complex, with
kinematic behaviors suggestive of inhomogeneous turbulent accretion onto
filaments and longitudinal flows towards a local overdensity along one of the
filaments.Comment: 27 pages, 12 figures, published in The Astrophysical Journa
SOFIA FORCAST photometry of 12 Extended Green Objects in the Milky Way
Funding: UK STFC grant number ST/M001296/1 (CJC).Massive young stellar objects are known to undergo an evolutionary phase in which high mass accretion rates drive strong outflows. A class of objects believed to trace this phase accurately is the Galactic Legacy Infrared Midplane Survey Extraordinaire (GLIMPSE) Extended Green Object (EGO) sample, so named for the presence of extended 4.5μm emission on size scales of∼0.1pc in Spitzer images. We have been conducting a multi-wavelength examination of a sample of 12 EGOs with distances of 1-5 kpc. In this paper, we present mid-infrared images and photometry of these EGOs obtained with the Stratospheric Observatory for Infrared Astronomy and subsequently construct spectral energy distributions (SEDs) for these sources from the near-infrared to sub-millimeter regimes using additional archival data. We compare the results from graybody models and several publicly-available software packages which produce model SEDs in the context of a single massive protostar. The models yield typical R⋆ ∼10 R⊙, T⋆ ∼103-104 K, and L⋆ ∼1−40 × 103 L⊙; the median L/M for our sample is 24.7 L⊙/M⊙. Model results rarely converge for R⋆ and T⋆, but do for L⋆, which we take to be an indication of the multiplicity and inherently clustered nature of these sources even though, typically, only a single source dominates in the mid-infrared. The median L/M value for the sample suggests that these objects may be in a transitional stage between the commonly described “IR-quiet” and “IR-bright” stages of MYSO evolution. The median Tdust for the sample is less conclusive, but suggests that these objects are either in this transitional stage or occupy the cooler (and presumably younger) part of the IR-bright stage.PostprintPeer reviewe
The Green Bank Ammonia Survey (GAS): First Results of NH3 mapping the Gould Belt
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 NH and other key
molecular tracers. This first release includes the data for four regions in
Gould Belt clouds: B18 in Taurus, NGC 1333 in Perseus, L1688 in Ophiuchus, and
Orion A North in Orion. We compare the NH emission to dust continuum
emission from Herschel, and find that the two tracers correspond closely.
NH is present in over 60\% of 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 NH is detected toward ~ 40\%
of lines-of-sight with mag. Moreover, we find that the NH
emission is generally extended beyond the typical 0.1 pc length scales of dense
cores. We produce maps of the gas kinematics, temperature, and NH column
densities through forward modeling of the hyperfine structure of the NH
(1,1) and (2,2) lines. We show that the NH velocity dispersion,
, and gas kinetic temperature, , vary systematically between
the regions included in this release, with an increase in both the mean value
and spread of and with increasing star formation activity.
The data presented in this paper are publicly available.Comment: 33 pages, 27 figures, accepted to ApJS. Datasets are publicly
available: https://dataverse.harvard.edu/dataverse/GAS_DR
Droplets I: Pressure-Dominated Sub-0.1 pc Coherent Structures in L1688 and B18
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 (GAS), we identify regions of high
density and near-constant, almost-thermal, velocity dispersion. Eighteen
coherent structures are revealed, twelve in L1688 and six 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 study
the coherent structures in molecular clouds statistically. The identified
coherent structures have a typical radius of 0.04 pc and a typical mass of 0.4
Msun, generally smaller than previously known coherent cores identified by
Goodman et al. (1998), Caselli et al. (2002), and Pineda et al. (2010). 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.Comment: Accepted by ApJ in April, 201
An Interferometric View of H-MM1. I. Direct Observation of NH3 Depletion
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) x 10(-8) (+/- 10% systematic), but it also reveals that, after all, the X(NH3) starts to decrease above a H-2 column density of approximate to 2.6 x 10(22) 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(H-2) similar to 2 x 10(5) 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.Peer reviewe
Ubiquitous supersonic component in L1688 coherent cores
Context : Star formation takes place in cold dense cores in molecular clouds.
Earlier observations have found that dense cores exhibit subsonic non-thermal
velocity dispersions. In contrast, CO observations show that the ambient
large-scale cloud is warmer and has supersonic velocity dispersions. Aims : We
aim to study the ammonia () molecular line profiles with exquisite
sensitivity towards the coherent cores in L1688 in order to study their
kinematical properties in unprecedented detail. Methods : We used
(1,1) and (2,2) data from the first data release (DR1) in the Green Bank
Ammonia Survey (GAS). We first smoothed the data to a larger beam of 1' to
obtain substantially more extended maps of velocity dispersion and kinetic
temperature, compared to the DR1 maps. We then identified the coherent cores in
the cloud and analysed the averaged line profiles towards the cores. Results :
For the first time, we detected a faint (mean (1,1) peak brightness
0.25 K in ), supersonic component towards all the coherent cores in
L1688. We fitted two components, one broad and one narrow, and derived the
kinetic temperature and velocity dispersion of each component. The broad
components towards all cores have supersonic linewidths (). This component biases the estimate of the narrow dense core component's
velocity dispersion by 28% and the kinetic temperature by
10%, on average, as compared to the results from single-component
fits. Conclusions : Neglecting this ubiquitous presence of a broad component
towards all coherent cores causes the typical single-component fit to
overestimate the temperature and velocity dispersion. This affects the derived
detailed physical structure and stability of the cores estimated from observations.Comment: Accepted for publication in Astronomy & Astrophysics on 06/07/2020.
15 pages, 16 figures, 1 table. Language edits from previous versio