29 research outputs found
Multi-line observations of CHOH, c-CH and HNCO towards L1544: Dissecting the core structure with chemical differentiation
Pre-stellar cores are the basic unit for the formation of stars and stellar
systems. The anatomy of the physical and chemical structures of pre-stellar
cores is critical for understanding the star formation process. L1544 is a
prototypical pre-stellar core, which shows significant chemical differentiation
surrounding the dust peak. We aim to constrain the physical conditions at the
different molecular emission peaks. This study allows us to compare the
abundance profiles predicted from chemical models together with the classical
density structure of Bonnor-Ebert (BE) sphere. We conducted multi-transition
pointed observations of CHOH, c-CH and HNCO with the IRAM 30m
telescope, towards the dust peak and the respective molecular peaks of L1544.
With non-LTE radiative transfer calculations and a 1-dimensional model, we
revisit the physical structure of L1544, and benchmark with the abundance
profiles from current chemical models. We find that the HNCO, c-CH
and CHOH lines in L1544 are tracing progressively higher density gas,
from 10 to several times 10 cm. Particularly, we find
that to produce the observed intensities and ratios of the CHOH lines, a
local gas density enhancement upon the BE sphere is required. This suggests
that the physical structure of an early-stage core may not necessarily follow a
smooth decrease of gas density profile locally, but can be intercepted by
clumpy substructures surrounding the gravitational center. Multiple transitions
of molecular lines from different molecular species can provide a tomographic
view of the density structure of pre-stellar cores. The local gas density
enhancement deviating from the BE sphere may reflect the impact of accretion
flows that appear asymmetric and are enhanced at the meeting point of
large-scale cloud structures.Comment: accepted by A&A; 22 pages, 22 figures incl. appendice
HSCO and DSCO: a multi-technique approach in the laboratory for the spectroscopy of interstellar ions
Protonated molecular species have been proven to be abundant in the
interstellar gas. This class of molecules is also pivotal for the determination
of important physical parameters for the ISM evolution (e.g. gas ionisation
fraction) or as tracers of non-polar, hence not directly observable, species.
The identification of these molecular species through radioastronomical
observations is directly linked to a precise laboratory spectral
characterisation. The goal of the present work is to extend the laboratory
measurements of the pure rotational spectrum of the ground electronic state of
protonated carbonyl sulfide (HSCO) and its deuterium substituted isotopomer
(DSCO). At the same time, we show how implementing different laboratory
techniques allows the determination of different spectroscopical properties of
asymmetric-top protonated species. Three different high-resolution experiments
were involved to detected for the first time the type rotational spectrum
of HSCO, and to extend, well into the sub-millimeter region, the type
spectrum of the same molecular species and DSCO. The electronic
ground-state of both ions have been investigated in the 273-405 GHz frequency
range, allowing the detection of 60 and 50 new rotational transitions for
HSCO and DSCO, respectively. The combination of our new measurements
with the three rotational transitions previously observed in the microwave
region permits the rest frequencies of the astronomically most relevant
transitions to be predicted to better than 100 kHz for both HSCO and
DSCO up to 500 GHz, equivalent to better than 60 m/s in terms of equivalent
radial velocity. The present work illustrates the importance of using different
laboratory techniques to spectroscopically characterise a protonated species at
high frequency, and how a similar approach can be adopted when dealing with
reactive species.Comment: 7 pages, 4 figures. Accepted for publication in Astronomy and
Astrophysic
Methanol Mapping in Cold Cores : Testing Model Predictions*
Chemical models predict that in cold cores gas-phase methanol is expected to be abundant at the outer edge of the CO depletion zone, where CO is actively adsorbed. CO adsorption correlates with volume density in cold cores, and, in nearby molecular clouds, catastrophic CO freeze-out happens at volume densities above 10(4) cm(-3). The methanol production rate is maximized there and its freeze-out rate does not overcome its production rate, while the molecules are shielded from UV destruction by gas and dust. Thus, in cold cores, methanol abundance should generally correlate with visual extinction, which depends on both volume and column density. In this work, we test the most basic model prediction that maximum methanol abundance is associated with a local A ( V ) similar to 4 mag in dense cores and constrain the model parameters with the observational data. With the IRAM 30 m antenna, we mapped the CH3OH (2-1) and (3-2) transitions toward seven dense cores in the L1495 filament in Taurus to measure the methanol abundance. We use the Herschel/SPIRE maps to estimate visual extinction, and the (CO)-O-18(2-1) maps from Tafalla & Hacar to estimate CO depletion. We explored the observed and modeled correlations between the methanol abundances, CO depletion, and visual extinction, varying the key model parameters. The modeling results show that hydrogen surface diffusion via tunneling is crucial to reproduce the observed methanol abundances, and the necessary reactive desorption efficiency matches the one deduced from laboratory experiments.Peer reviewe
Initial conditions of star formation at 2000 au: physical structure and NH depletion of three early-stage cores
Pre-stellar cores represent a critical evolutionary phase in low-mass star
formation. We aim to unveil the detailed thermal structure and density
distribution of three early-stage cores, starless core L1517B, and prestellar
core L694-2 and L429, with the high angular resolution observations of the
NH (1,1) and (2,2) inversion transitions obtained with VLA and GBT. In
addition, we explore where/if NH depletes in the central regions.
Applying the mid-infrared extinction method to the 8m
map we obtain a high angular resolution hydrogen column density map, and derive
the gas density profile to assess the variation of NH abundance as a
function of gas volume density. The measured temperature profiles of L429 and
L1517B show a minor decrease towards the core center, dropping from 9\~K
to below 8\~K, and 11 K to 10 K, while L694-2 has a rather uniform
temperature distribution around 9 K. Among the three cores, L429 has the
highest central gas density, close to sonic velocity line-width, and largest
localised velocity gradient, all indicative of an advanced evolutionary stage.
We resolve that the abundance of NH becomes two times lower in the
central region of L429, occurring around a gas density of
4.410. Compared to Ophiuchus/H-MM1 which shows an even
stronger drop of the NH abundance at 210, the
abundance variations of the three cores plus Ophiuchus/H-MM1 suggest a
progressive NH depletion with increasing central density of the core.Comment: 23 pages, 23 figures incl. appendix; A&A accepte
The complex organic molecular content in the L1517B starless core
Recent observations of the pre-stellar core L1544 and the younger starless
core L1498 have revealed that complex organic molecules (COMs) are enhanced in
the gas phase toward their outer and intermediate-density shells. Our goal is
to determine the level of chemical complexity toward the starless core L1517B,
which seems younger than L1498, and compare it with the other two previously
studied cores to see if there is a chemical evolution within the cores. We have
carried out 3 mm high-sensitivity observations toward two positions in the
L1517B starless core: the core's centre and the position where the methanol
emission peaks (at a distance of 5000 au from the core's centre). Our
observations reveal that a lower number of COMs and COM precursors are detected
in L1517B with respect to L1498 and L1544, and also show lower abundances.
Besides methanol, we only detected CHO, HCCO, CHCHO, CHCN,
CHNC, HCCCN, and HCCNC. Their measured abundances are 3 times larger
toward the methanol peak than toward the core's centre, mimicking the behaviour
found toward the more evolved cores L1544 and L1498. We propose that the
differences in the chemical complexity observed between the three studied
starless cores are a consequence of their evolution, with L1517B being the less
evolved one, followed by L1498 and L1544. Chemical complexity in these cores
seems to increase over time, with N-bearing molecules forming first and
O-bearing COMs forming at a later stage as a result of the catastrophic
depletion of CO.Comment: 18 pages, 13 figure
Laboratory and tentative interstellar detection of trans-methyl formate using the publicly available Green Bank Telescope PRIMOS survey
The rotational spectrum of the higher-energy trans conformational isomer of
methyl formate has been assigned for the first time using several pulsed-jet
Fourier transform microwave spectrometers in the 6-60 GHz frequency range. This
species has also been sought toward the Sagittarius B2(N) molecular cloud using
the publicly available PRIMOS survey from the Green Bank Telescope. We detect
seven absorption features in the survey that coincide with laboratory
transitions of trans-methyl formate, from which we derive a column density of
3.1 (+2.6, -1.2) \times 10^13 cm-2 and a rotational temperature of 7.6 \pm 1.5
K. This excitation temperature is significantly lower than that of the more
stable cis conformer in the same source but is consistent with that of other
complex molecular species recently detected in Sgr B2(N). The difference in the
rotational temperatures of the two conformers suggests that they have different
spatial distributions in this source. As the abundance of trans-methyl formate
is far higher than would be expected if the cis and trans conformers are in
thermodynamic equilibrium, processes that could preferentially form
trans-methyl formate in this region are discussed. We also discuss measurements
that could be performed to make this detection more certain. This manuscript
demonstrates how publicly available broadband radio astronomical surveys of
chemically rich molecular clouds can be used in conjunction with laboratory
rotational spectroscopy to search for new molecules in the interstellar medium.Comment: 40 pages, 7 figures, 4 tables; accepted for publication in Ap
Gas phase Elemental abundances in Molecular cloudS (GEMS). IX. Deuterated compounds of H2S in starless cores
H2S is thought to be the main sulphur reservoir in the ice, being therefore a
key molecule to understand sulphur chemistry in the star formation process and
to solve the missing sulphur problem. The H2S deuterium fraction can be used to
constrain its formation pathways. We investigate for the first time the H2S
deuteration in a large sample of starless cores (SC). We use observations of
the GEMS IRAM 30m Large Program and complementary IRAM 30m observations. We
consider a sample of 19 SC in Taurus, Perseus, and Orion, detecting HDS in 10
and D2S in five. The H2S single and double deuterium fractions are analysed
with regard to their relation with the cloud physical parameters, their
comparison with other interstellar sources, and their comparison with deuterium
fractions in early stage star-forming sources of c-C3H2, H2CS, H2O, H2CO, and
CH3OH. We obtain a range of X(HDS)/X(H2S)~0.025-0.2 and X(D2S)/X(HDS)~0.05-0.3.
H2S single deuteration shows an inverse relation with the cloud kinetic
temperature. H2S deuteration values in SC are similar to those observed in
Class 0. Comparison with other molecules in other sources reveals a general
trend of decreasing deuteration with increasing temperature. In SC and Class 0
objects H2CS and H2CO present higher deuteration fractions than c-C3H2, H2S,
H2O, and CH3OH. H2O shows single and double deuteration values one order of
magnitude lower than those of H2S and CH3OH. Differences between c-C3H2, H2CS
and H2CO deuterium fractions and those of H2S, H2O, and CH3OH are related to
deuteration processes produced in gas or solid phases, respectively. We
interpret the differences between H2S and CH3OH deuterations and that of H2O as
a consequence of differences on the formation routes in the solid phase,
particularly in terms of the different occurrence of the D-H and H-D
substitution reactions in the ice, together with the chemical desorption
processes.Comment: Accepted for publication in A&
Efficient Methanol Production on the Dark Side of a Prestellar Core
We present Atacama Large Millimeter/submillimeter Array maps of the starless molecular cloud core Ophiuchus/H-MM1 in the lines of deuterated ammonia (ortho-NH2D), methanol (CH3OH), and sulfur monoxide (SO). The dense core is seen in NH2D emission, whereas the CH3OH 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.Peer reviewe