332 research outputs found
Simultaneous Determination of the Cosmic Ray Ionization Rate and Fractional Ionization in DR21(OH)
We present a new method for the simultaneous calculation of the cosmic ray
ionization rate, zeta(H2), and the ionization fraction, chi(e), in dense
molecular clouds. A simple network of chemical reactions dominant in the
creation and destruction of HCNH+ and HCO+ is used in conjunction with observed
pairs of rotational transitions of several molecular species in order to
determine the electron abundance and the H3+ abundance. The cosmic ray
ionization rate is then calculated by taking advantage of the fact that, in
dark clouds, it governs the rate of creation of H3+. We apply this technique to
the case of the star-forming region DR21(OH), where we successfully detected
the (J=3-2) and (J=4-3) rotational transitions of HCNH+. We also determine the
C and O isotopic ratios in this source to be 12C/13C=63+-4 and 16O/18O=318+-64,
which are in good agreement with previous measurements in other clouds. The
significance of our method lies in the ability to determine N(H3+) and chi(e)
directly from observations, and estimate zeta(H2) accordingly. Our results,
zeta(H2)=3.1x10^(-18) 1/s and chi(e)=3.2x10^(-8), are consistent with recent
determinations in other objects.Comment: 22 pages, including 3 figure
Upper limit for the D2H+ ortho-to-para ratio in the prestellar core 16293E (CHESS)
The H3+ ion plays a key role in the chemistry of dense interstellar gas
clouds where stars and planets are forming. The low temperatures and high
extinctions of such clouds make direct observations of H3+ impossible, but lead
to large abundances of H2D+ and D2H+, which are very useful probes of the early
stages of star and planet formation. The ground-state rotational ortho-D2H+
111-000 transition at 1476.6 GHz in the prestellar core 16293E has been
searched for with the Herschel/HIFI instrument, within the CHESS (Chemical
HErschel Surveys of Star forming regions) Key Program. The line has not been
detected at the 21 mK km/s level (3 sigma integrated line intensity). We used
the ortho-H2D+ 110-111 transition and para-D2H+ 110-101 transition detected in
this source to determine an upper limit on the ortho-to-para D2H+ ratio as well
as the para-D2H+/ortho-H2D+ ratio from a non-LTE analysis. The comparison
between our chemical modeling and the observations suggests that the CO
depletion must be high (larger than 100), with a density between 5e5 and 1e6
cm-3. Also the upper limit on the ortho-D2H+ line is consistent with a low gas
temperature (~ 11 K) with a ortho-to-para ratio of 6 to 9, i.e. 2 to 3 times
higher than the value estimated from the chemical modeling, making it
impossible to detect this high frequency transition with the present state of
the art receivers.Comment: Accepted in A&
Depletion and low gas temperature in the L183 prestellar core: the N2H+ - N2D+ tool
Context. The study of pre-stellar cores (PSCs) suffers from a lack of undepleted species to trace the gas physical properties in their very dense inner parts. Aims. We want to carry out detailed modelling of N2H+ and N2D+ cuts across the L183 main core to evaluate the depletion of these species and their usefulness as a probe of physical conditions in PSCs. Methods. We have developed a non-LTE (NLTE) Monte-Carlo code treating the 1D radiative transfer of both N2H+ and N2D+, making use of recently published collisional coefficients with He between individual hyperfine levels. The code includes line overlap between hyperfine transitions. An extensive set of core models is calculated and compared with observations. Special attention is paid to the issue of source coupling to the antenna beam. Results. The best fitting models indicate that i) gas in the core center is very cold (7 1 K) and thermalized with dust, ii) depletion of N2H+ does occur, starting at densities 5-7E5 cm−3 and reaching a factor of 6 (+13/−3) in abundance, iii) deuterium fractionation reaches ∼70% at the core center, and iv) the density profile is proportional to r^-1 out to ∼4000 AU, and to r^−2 beyond. Conclusions. Our NLTE code could be used to (re-)interpret recent and upcoming observations of N2H+ and N2D+ in many pre-stellar cores of interest, to obtain better temperature and abundance profiles
HD line emission in Proto-Planetary Disks
%Context: {Previous studies have indicated that the 372.4 GHz ground
transition of ortho-HD might be a powerful probe of Proto-Planetary
Disks. The line could be especially suited for study of the disk mid-plane,
where the bulk of the mass resides and where planet formation takes place.}
%Aims: {Provide detailed theoretical predictions for the line intensity,
profile and maps expected for representative disk models.} %Methods: {We
determine the physical and chemical structure of the disks from the model
developed by Ceccarelli & Dominik (2005). The line emission is computed with
the new radiative transfer method developed recently by Elitzur & Asensio Ramos
(2006).} %Results: {We present intensity maps convolved with the expected ALMA
resolution, which delineate the origin of the HD 372.4 GHz line. In the
disk inner regions, the line probes the conditions in the mid-plane out to
radial distances of a few tens of AU, where Solar-like planetary systems might
form. In the disk outermost regions, the line originates from slightly above
the mid-plane. When the disk is spatially resolved, the variation of line
profile across the image provides important information about the velocity
field. Spectral profiles of the entire disk flux show a double peak shape at
most inclination angles.} %Conclusions: {Our study confirms that the 372.4 GHz
HD line provides powerful diagnostics of the mid-plane of
Proto-Planetary Disks. Current submillimeter telescopes are capable of
observing this line, though with some difficulties. The future ALMA
interferometer will have the sensitivity to observe and even spatially resolve
the HD line emission.}Comment: To appear in A&
High spectral resolution observations of HNC3 and HCCNC in the L1544 prestellar core
HCCNC and HNC3 are less commonly found isomers of cyanoacetylene, HC3N, a
molecule that is widely found in diverse astronomical sources. We want to know
if HNC3 is present in sources other than the dark cloud TMC-1 and how its
abundance is relative to that of related molecules. We used the ASAI unbiased
spectral survey at IRAM 30m towards the prototypical prestellar core L1544 to
search for HNC3 and HCCNC which are by-product of the HC3NH+ recombination,
previously detected in this source. We performed a combined analysis of
published HNC3 microwave rest frequencies with thus far unpublished millimeter
data because of issues with available rest frequency predictions. We determined
new spectroscopic parameters for HNC3, produced new predictions and detected it
towards L1544. We used a gas-grain chemical modelling to predict the abundances
of N-species and compare with the observations. The modelled abundances are
consistent with the observations, considering a late stage of the evolution of
the prestellar core. However the calculated abundance of HNC3 was found 5-10
times higher than the observed one. The HC3N, HNC3 and HCCNC versus HC3NH+
ratios are compared in the TMC-1 dark cloud and the L1544 prestellar core.Comment: Accepted in MNRAS letters. 5 pages plus 2 additional pages for the
on-line materia
Deuterium and N fractionation in NH during the formation of a Sun-like star
Although chemical models predict that the deuterium fractionation in
NH is a good evolutionary tracer in the star formation process, the
fractionation of nitrogen is still a poorly understood process. Recent models
have questioned the similar evolutionary trend expected for the two
fractionation mechanisms in NH, based on a classical scenario in which
ion-neutral reactions occurring in cold gas should have caused an enhancement
of the abundance of ND, NNH, and NNH. In the
framework of the ASAI IRAM-30m large program, we have investigated the
fractionation of deuterium and N in NH in the best known
representatives of the different evolutionary stages of the Sun-like star
formation process. The goal is to ultimately confirm (or deny) the classical
"ion-neutral reactions" scenario that predicts a similar trend for D and
N fractionation. We do not find any evolutionary trend of the
N/N ratio from both the NNH and NNH
isotopologues. Therefore, our findings confirm that, during the formation of a
Sun-like star, the core evolution is irrelevant in the fractionation of
N. The independence of the N/N ratio with time, found also
in high-mass star-forming cores, indicates that the enrichment in N
revealed in comets and protoplanetary disks is unlikely to happen at core
scales. Nevertheless, we have firmly confirmed the evolutionary trend expected
for the H/D ratio, with the NH/ND ratio decreasing before the
pre-stellar core phase, and increasing monotonically during the protostellar
phase. We have also confirmed clearly that the two fractionation mechanisms are
not related.Comment: 9 pages, 2 figures, accepted for publication in MNRA
Shedding light on the formation of the pre-biotic molecule formamide with ASAI
Formamide (NH2CHO) has been proposed as a pre-biotic precursor with a key
role in the emergence of life on Earth. While this molecule has been observed
in space, most of its detections correspond to high-mass star-forming regions.
Motivated by this lack of investigation in the low-mass regime, we searched for
formamide, as well as isocyanic acid (HNCO), in 10 low- and intermediate-mass
pre-stellar and protostellar objects. The present work is part of the IRAM
Large Programme ASAI (Astrochemical Surveys At IRAM), which makes use of
unbiased broadband spectral surveys at millimetre wavelengths. We detected HNCO
in all the sources and NH2CHO in five of them. We derived their abundances and
analysed them together with those reported in the literature for high-mass
sources. For those sources with formamide detection, we found a tight and
almost linear correlation between HNCO and NH2CHO abundances, with their ratio
being roughly constant -between 3 and 10- across 6 orders of magnitude in
luminosity. This suggests the two species are chemically related. The sources
without formamide detection, which are also the coldest and devoid of hot
corinos, fall well off the correlation, displaying a much larger amount of HNCO
relative to NH2CHO. Our results suggest that, while HNCO can be formed in the
gas phase during the cold stages of star formation, NH2CHO forms most
efficiently on the mantles of dust grains at these temperatures, where it
remains frozen until the temperature rises enough to sublimate the icy grain
mantles. We propose hydrogenation of HNCO as a likely formation route leading
to NH2CHO.Comment: 26 pages, 9 figures. Accepted by Monthly Notices of the Royal
Astronomical Societ
Detection of a dense clump in a filament interacting with W51e2
In the framework of the Herschel/PRISMAS Guaranteed Time Key Program, the
line of sight to the distant ultracompact HII region W51e2 has been observed
using several selected molecular species. Most of the detected absorption
features are not associated with the background high-mass star-forming region
and probe the diffuse matter along the line of sight. We present here the
detection of an additional narrow absorption feature at ~70 km/s in the
observed spectra of HDO, NH3 and C3. The 70 km/s feature is not uniquely
identifiable with the dynamic components (the main cloud and the large-scale
foreground filament) so-far identified toward this region. The narrow
absorption feature is similar to the one found toward low-mass protostars,
which is characteristic of the presence of a cold external envelope. The
far-infrared spectroscopic data were combined with existing ground-based
observations of 12CO, 13CO, CCH, CN, and C3H2 to characterize the 70 km/s
component. Using a non-LTE analysis of multiple transitions of NH3 and CN, we
estimated the density (n(H2) (1-5)x10^5 cm^-3) and temperature (10-30 K) for
this narrow feature. We used a gas-grain warm-up based chemical model with
physical parameters derived from the NH3 data to explain the observed
abundances of the different chemical species. We propose that the 70 km/s
narrow feature arises in a dense and cold clump that probably is undergoing
collapse to form a low-mass protostar, formed on the trailing side of the
high-velocity filament, which is thought to be interacting with the W51 main
cloud. While the fortuitous coincidence of the dense clump along the line of
sight with the continuum-bright W51e2 compact HII region has contributed to its
non-detection in the continuum images, this same attribute makes it an
appropriate source for absorption studies and in particular for ice studies of
star-forming regions.Comment: Accepted for publication in A&
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