118 research outputs found
The NH2D/NH3 ratio toward pre-protostellar cores around the UCHII region in IRAS 20293+3952
The deuterium fractionation, Dfrac, has been proposed as an evolutionary
indicator in pre-protostellar and protostellar cores of low-mass star-forming
regions. We investigate Dfrac, with high angular resolution, in the cluster
environment surrounding the UCHII region IRAS 20293+3952. We performed high
angular resolution observations with the IRAM Plateau de Bure Interferometer
(PdBI) of the ortho-NH2D 1_{11}-1_{01} line at 85.926 GHz and compared them
with previously reported VLA NH3 data. We detected strong NH2D emission toward
the pre-protostellar cores identified in NH3 and dust emission, all located in
the vicinity of the UCHII region IRAS 20293+3952. We found high values of
Dfrac~0.1-0.8 in all the pre-protostellar cores and low values, Dfrac<0.1,
associated with young stellar objects. The high values of Dfrac in
pre-protostellar cores could be indicative of evolution, although outflow
interactions and UV radiation could also play a role.Comment: 5 pages, 3 figures. Accepted for publication in Astronomy and
Astrophysics Letter
Observing the gas temperature drop in the high-density nucleus of L 1544
Abridged: The thermal structure of a starless core is crucial for our
understanding of the physics in these objects and hence for our understanding
of star formation. Theory predicts a gas temperature drop in the inner 5000 AU
of these objects, but there has been little observational proof of this. We
performed VLA observations of the NH3 (1,1) and (2,2) transitions towards the
pre-stellar core L 1544 in order to measure the temperature gradient between
the high density core nucleus and the surrounding core envelope. Our VLA
observation for the first time provide measurements of gas temperature in a
core with a resolution smaller than 1000 AU. We have also obtained high
resolution Plateau de Bure observations of the 110 GHz 111-101 para-NH2D line
in order to further constrain the physical parameters of the high density
nucleus. We have estimated the temperature gradient using a model of the source
to fit our data in the u,v plane. We find that indeed the temperature decreases
toward the core nucleus from 12 K down to 5.5 K resulting in an increase of a
factor of 50% in the estimated density of the core from the dust continuum if
compared with the estimates done with constant temperature of 8.75 K. We also
found a remarkably high abundance of deuterated ammonia with respect to the
ammonia abundance (50%+-20%), which proves the persistence of nitrogen bearing
molecules at very high densities (2e6 cm-3) and shows that high-resolution
observations yield higher deuteration values than single-dish observations. Our
analysis of the NH3 and NH2D kinematic fields shows a decrease of specific
angular momentum from the large scales to the small scales.Comment: 12 pages, 6 figures. Accepted for publication by A&
Lack of PAH emission toward low-mass embedded young stellar objects
PAHs have been detected toward molecular clouds and some young stars with
disks, but have not yet been associated with embedded young stars. We present a
sensitive mid-IR spectroscopic survey of PAH features toward a sample of
low-mass embedded YSOs. The aim is to put constraints on the PAH abundance in
the embedded phase of star formation using radiative transfer modeling.
VLT-ISAAC L-band spectra for 39 sources and Spitzer IRS spectra for 53
sources are presented. Line intensities are compared to recent surveys of
Herbig Ae/Be and T Tauri stars. The radiative transfer codes RADMC and RADICAL
are used to model the PAH emission from embedded YSOs consisting of a PMS star
with a circumstellar disk embedded in an envelope. The dependence of the PAH
feature on PAH abundance, stellar radiation field, inclination and the
extinction by the surrounding envelope is studied.
The 3.3 micron PAH feature is undetected for the majority of the sample
(97%), with typical upper limits of 5E-16 W/m^2. Compact 11.2 micron PAH
emission is seen directly towards 1 out of the 53 Spitzer Short-High spectra,
for a source that is borderline embedded. For all 12 sources with both VLT and
Spitzer spectra, no PAH features are detected in either. In total, PAH features
are detected toward at most 1 out of 63 (candidate) embedded protostars (<~
2%), even lower than observed for class II T Tauri stars with disks (11-14%).
Assuming typical class I stellar and envelope parameters, the absence of PAHs
emission is most likely explained by the absence of emitting carriers through a
PAH abundance at least an order of magnitude lower than in molecular clouds but
similar to that found in disks. Thus, most PAHs likely enter the protoplanetary
disks frozen out in icy layers on dust grains and/or in coagulated form.Comment: 13 pages, 9 figures, accepted for publication in A&
Gas-grain models for interstellar anion chemistry
Long-chain hydrocarbon anions CnH- (n=4, 6, 8) have recently been found to be
abundant in a variety of interstellar clouds. In order to explain their large
abundances in the denser (prestellar/protostellar) environments, new chemical
models are constructed that include gas-grain interactions. Models including
accretion of gas-phase species onto dust grains and cosmic-ray-induced
desorption of atoms are able to reproduce the observed anion-to-neutral ratios,
as well as the absolute abundances of anionic and neutral carbon chains, with a
reasonable degree of accuracy. Due to their destructive effects, the depletion
of oxygen atoms onto dust results in substantially greater polyyne and anion
abundances in high-density gas (with n_{H_2} >~ 10^5 cm^{-3}). The large
abundances of carbon-chain-bearing species observed in the envelopes of
protostars such as L1527 can thus be explained without the need for warm
carbon-chain chemistry. The C6H- anion-to-neutral ratio is found to be most
sensitive to the atomic O and H abundances and the electron density. Therefore,
as a core evolves, falling atomic abundances and rising electron densities are
found to result in increasing anion-to-neutral ratios. Inclusion of cosmic-ray
desorption of atoms in high-density models delays freeze-out, which results in
a more temporally-stable anion-to-neutral ratio, in better agreement with
observations. Our models include reactions between oxygen atoms and
carbon-chain anions to produce carbon-chain-oxide species C6O, C7O, HC6O and
HC7O, the abundances of which depend on the assumed branching ratios for
associative electron detachment
H_2D^+ in the High-mass Star-forming Region Cygnus X
H_2D^+ is a primary ion that dominates the gas-phase chemistry of cold dense gas. Therefore, it is hailed as a unique tool in probing the earliest, prestellar phase of star formation. Observationally, its abundance and distribution is, however, just beginning to be understood in low-mass prestellar and cluster-forming cores. In high-mass star-forming regions, H_2D^+ has been detected only in two cores, and its spatial distribution remains unknown. Here, we present the first map of the ortho-H_2D^+J_(k^+,k^-) = 1_(1,0) → 1_(1,1) and N_2H^+ 4-3 transition in the DR21 filament of Cygnus X with the James Clerk Maxwell Telescope, and N_2D^+ 3-2 and dust continuum with the Submillimeter Array. We have discovered five very extended (≤34, 000 AU diameter) weak structures in H2D+ in the vicinity of, but distinctly offset from, embedded protostars. More surprisingly, the H_2D^+ peak is not associated with either a dust continuum or N_2D^+ peak. We have therefore uncovered extended massive cold dense gas that was undetected with previous molecular line and dust continuum surveys of the region. This work also shows that our picture of the structure of cores is too simplistic for cluster-forming cores and needs to be refined: neither dust continuum with existing capabilities nor emission in tracers like N_2D^+ can provide a complete census of the total prestellar gas in such regions. Sensitive H_2D^+ mapping of the entire DR21 filament is likely to discover more of such cold quiescent gas reservoirs in an otherwise active high-mass star-forming region
Survey of ortho-H2D+(1_{1,0}-1_{1,1}) in dense cloud cores
We present a survey of the ortho-H2D+(1_{1,0}-1_{1,1}) line toward a sample
of 10 starless cores and 6 protostellar cores, carried out at the Caltech
Submillimeter Observatory. The high diagnostic power of this line is revealed
for the study of the chemistry, and the evolutionary and dynamical status of
low-mass dense cores. The line is detected in 7 starless cores and in 4
protostellar cores. N(ortho-H2D+) ranges between 2 and 40x10^{12} cm^{-2} in
starless cores and between 2 and 9x10^{12} cm^{-2} in protostellar cores. The
brightest lines are detected toward the densest and most centrally concentrated
starless cores, where the CO depletion factor and the deuterium fractionation
are also largest. The large scatter observed in plots of N(ortho-H2D+) vs. the
observed deuterium fractionation and vs. the CO depletion factor is likely to
be due to variations in the ortho-to-para (o/p) ratio of H2D+ from >0.5 for
T_{kin} < 10 K gas in pre-stellar cores to ~0.03 (consistent with T_{kin} ~15 K
for protostellar cores). The two Ophiuchus cores in our sample also require a
relatively low o/p ratio (~0.3). Other parameters, including the cosmic-ray
ionization rate, the CO depletion factor (or, more in general, the depletion
factor of neutral species), the volume density, the fraction of dust grains and
PAHs also largely affect the ortho-H2D+ abundance. The most deuterated and
H2D+-rich objects (L429, L1544, L694-2 and L183) are reproduced by chemical
models of centrally concentrated (central densties ~10^{6} cm^{-3}) cores with
chemical ages between 10^4 and 10^6 yr. Upper limits of the para-H3O+ (1_1-
-2_1+) and para-D2H+ (1_{1,0}-1_{0,1}) lines are also given. (Abridged)Comment: 29 pages, 6 figures. To appear in A&
On the internal structure of starless cores. II. A molecular survey of L1498 and L1517B
[Abridged] We present a molecular survey of the starless cores L1498 and
L1517B. These cores have been selected for their relative isolation and
close-to-round shape, and they have been observed in a number of lines of 13
molecular species (4 already presented in the first part of this series): CO,
CS, N2H+, NH3, CH3OH, SO, C3H2, HC3N, C2S, HCN, H2CO, HCO+, and DCO+. Using a
physical model of core structure and a Monte Carlo radiative transfer code, we
determine for each core a self-consistent set abundances that fits
simultaneously the observed radial profile of integrated intensity and the
emergent spectrum towards the core center (for abundant species, optically thin
isopologues are used). From this work, we find that L1498 and L1517B have
similar abundance patterns, with most species suffering a significant drop
toward the core center. This occurs for CO, CS, CH3OH, SO, C3H2, HC3N, C2S,
HCN, H2CO, HCO+, and DCO+, which we fit with profiles having a sharp central
hole. The size of this hole varies with molecule: DCO+, HCN, and HC3N have the
smallest holes while SO, C2S and CO have the largest holes. Only N2H+ and NH3
are present in the gas phase at the core centers. From the different behavior
of molecules, we select SO, C2S, and CH3OH as the most sensitive tracers of
molecular depletion. Comparing our abundance determinations with the
predictions from current chemical models we find order of magnitude
discrepancies. Finally, we show how the ``contribution function'' can be used
to study the formation of line profiles from the different regions of a core.Comment: 22 pages, 12 figures, A&A accepte
The Initial Conditions of Clustered Star Formation III. The Deuterium Fractionation of the Ophiuchus B2 Core
We present N2D+ 3-2 (IRAM) and H2D+ 1_11 - 1_10 and N2H+ 4-3 (JCMT) maps of
the small cluster-forming Ophiuchus B2 core in the nearby Ophiuchus molecular
cloud. In conjunction with previously published N2H+ 1-0 observations, the N2D+
data reveal the deuterium fractionation in the high density gas across Oph B2.
The average deuterium fractionation R_D = N(N2D+)/N(N2H+) ~ 0.03 over Oph B2,
with several small scale R_D peaks and a maximum R_D = 0.1. The mean R_D is
consistent with previous results in isolated starless and protostellar cores.
The column density distributions of both H2D+ and N2D+ show no correlation with
total H2 column density. We find, however, an anticorrelation in deuterium
fractionation with proximity to the embedded protostars in Oph B2 to distances
>= 0.04 pc. Destruction mechanisms for deuterated molecules require gas
temperatures greater than those previously determined through NH3 observations
of Oph B2 to proceed. We present temperatures calculated for the dense core gas
through the equating of non-thermal line widths for molecules (i.e., N2D+ and
H2D+) expected to trace the same core regions, but the observed complex line
structures in B2 preclude finding a reasonable result in many locations. This
method may, however, work well in isolated cores with less complicated velocity
structures. Finally, we use R_D and the H2D+ column density across Oph B2 to
set a lower limit on the ionization fraction across the core, finding a mean
x_e, lim >= few x 10^{-8}. Our results show that care must be taken when using
deuterated species as a probe of the physical conditions of dense gas in
star-forming regions.Comment: ApJ accepte
A deeply embedded young protoplanetary disk around L1489 IRS observed by the submillimeter array
Circumstellar disks are expected to form early in the process that leads to
the formation of a young star, during the collapse of the dense molecular cloud
core. It is currently not well understood at what stage of the collapse the
disk is formed or how it subsequently evolves. We aim to identify whether an
embedded Keplerian protoplanetary disk resides in the L1489 IRS system. Given
the amount of envelope material still present, such a disk would respresent a
very young example of a protoplanetary disk. Using the Submillimeter Array
(SMA) we have observed the HCO 3--2 line with a resolution of about
1. At this resolution a protoplanetary disk with a radius of a few hundred
AUs should be detectable, if present. Radiative transfer tools are used to
model the emission from both continuum and line data. We find that these data
are consistent with theoretical models of a collapsing envelope and Keplerian
circumstellar disk. Models reproducing both the SED and the interferometric
continuum observations reveal that the disk is inclined by 40 which is
significantly different to the surrounding envelope (74). This
misalignment of the angular momentum axes may be caused by a gradient within
the angular momentum in the parental cloud or if L1489 IRS is a binary system
rather than just a single star. In the latter case, future observations looking
for variability at sub-arcsecond scales may be able to constrain these
dynamical variations directly. However, if stars form from turbulent cores, the
accreting material will not have a constant angular momentum axis (although the
average is well defined and conserved) in which case it is more likely to have
a misalignment of the angular momentum axes of the disk and the envelope.Comment: 11 pages, 13 figures, accepted by A&
Young starless cores embedded in the magnetically dominated Pipe Nebula
The Pipe Nebula is a massive, nearby dark molecular cloud with a low
star-formation efficiency which makes it a good laboratory to study the very
early stages of the star formation process. The Pipe Nebula is largely
filamentary, and appears to be threaded by a uniform magnetic field at scales
of few parsecs, perpendicular to its main axis. The field is only locally
perturbed in a few regions, such as the only active cluster forming core B59.
The aim of this study is to investigate primordial conditions in low-mass
pre-stellar cores and how they relate to the local magnetic field in the cloud.
We used the IRAM 30-m telescope to carry out a continuum and molecular survey
at 3 and 1 mm of early- and late-time molecules toward four selected starless
cores inside the Pipe Nebula. We found that the dust continuum emission maps
trace better the densest regions than previous 2MASS extinction maps, while
2MASS extinction maps trace better the diffuse gas. The properties of the cores
derived from dust emission show average radii of ~0.09 pc, densities of
~1.3x10^5 cm^-3, and core masses of ~2.5 M_sun. Our results confirm that the
Pipe Nebula starless cores studied are in a very early evolutionary stage, and
present a very young chemistry with different properties that allow us to
propose an evolutionary sequence. All of the cores present early-time molecular
emission, with CS detections toward all the sample. Two of them, Cores 40 and
109, present strong late-time molecular emission. There seems to be a
correlation between the chemical evolutionary stage of the cores and the local
magnetic properties that suggests that the evolution of the cores is ruled by a
local competition between the magnetic energy and other mechanisms, such as
turbulence.Comment: Accepted for publication in ApJ. 15 pages, 5 figures, 9 table
- …