10,405 research outputs found
The genealogical tree of ethanol: gas-phase formation of glycolaldehyde, acetic acid and formic acid
Despite the harsh conditions of the interstellar medium, chemistry thrives in
it, especially in star forming regions where several interstellar complex
organic molecules (iCOMs) have been detected. Yet, how these species are
synthesised is a mystery. The majority of current models claim that this
happens on interstellar grain surfaces. Nevertheless, evidence is mounting that
neutral gas-phase chemistry plays an important role. In this article, we
propose a new scheme for the gas-phase synthesis of glycolaldehyde, a species
with a prebiotic potential and for which no gas-phase formation route was
previously known. In the proposed scheme, the ancestor is ethanol and the
glycolaldehyde sister species are acetic acid (another iCOM with unknown
gas-phase formation routes) and formic acid. For the reactions of the new
scheme with no available data, we have performed electronic structure and
kinetics calculations deriving rate coefficients and branching ratios.
Furthermore, after a careful review of the chemistry literature, we revised the
available chemical networks, adding and correcting several reactions related to
glycolaldehyde, acetic acid and formic acid. The new chemical network has been
used in an astrochemical model to predict the abundance of glycolaldehyde,
acetic acid and formic acid. The predicted abundance of glycolaldehyde depends
on the ethanol abundance in the gas phase and is in excellent agreement with
the measured one in hot corinos and shock sites. Our new model overpredicts the
abundance of acetic acid and formic acid by about a factor of ten, which might
imply a yet incomplete reaction network
Ionization fraction and the enhanced sulfur chemistry in Barnard 1
Barnard B1b has revealed as one of the most interesting globules from the
chemical and dynamical point of view. It presents a rich molecular chemistry
characterized by large abundances of deuterated and complex molecules.
Furthermore, it hosts an extremely young Class 0 object and one candidate to
First Hydrostatic Core (FHSC). Our aim was to determine the cosmic ray
ionization rate and the depletion factors in this extremely young star forming
region. We carried out a spectral survey towards Barnard 1b as part of the IRAM
Large program ASAI using the IRAM 30-m telescope at Pico Veleta (Spain). This
provided a very complete inventory of neutral and ionic C-, N- and S- bearing
species with, up to our knowledge, the first secure detections of the
deuterated ions DCS+ and DOCO+. We used a state-of-the-art
pseudo-time-dependent gas-phase chemical model to determine the value of the
cosmic ray ionization rate and the depletion factors. The observational data
were well fitted with between 3E-17 s and 1E-16 s.
Elemental depletions were estimated to be ~10 for C and O, ~1 for N and ~25 for
S. Barnard B1b presents similar depletions of C and O than those measured in
pre-stellar cores. The depletion of sulfur is higher than that of C and O but
not as extreme as in cold cores. In fact, it is similar to the values found in
some bipolar outflows, hot cores and photon-dominated regions. Several
scenarios are discussed to account for these peculiar abundances. We propose
that it is the consequence of the initial conditions (important outflows and
enhanced UV fields in the surroundings) and a rapid collapse (~0.1 Myr) that
permits to maintain most S- and N-bearing species in gas phase to great optical
depths. The interaction of the compact outflow associated with B1b-S with the
surrounding material could enhance the abundances of S-bearing molecules, as
well.Comment: Paper accepted in Astronomy and Astrophysics; 28 pags, 21 figure
ROSAT PSPC and Hri Observations of the Composite Starburst/Seyfert 2 Galaxy NGC 1672
The nearby barred spiral galaxy NGC 1672 is thought to have a weak Seyfert
nucleus in addition to its strong starburst activity. Observations with the
PSPC and HRI instruments on board the ROSAT X-ray satellite show that three
X-ray sources with luminosities (1--2)\times 10^{40} erg/s are clearly
identified with NGC 1672. The strongest X-ray source lies at the nucleus, and
the other two lie near the ends of the prominent bar, locations that are also
bright in H-alpha and near-infrared images. The nuclear source is resolved by
the HRI on about the scale of the recently identified nuclear ring, and one of
the sources at the ends of the bar is also probably resolved. The X-ray
spectrum of the nuclear source is quite soft, having a Raymond--Smith plasma
temperature of about 0.7 keV and little evidence for intrinsic absorption. The
ROSAT band X-ray flux of the nuclear source appears to be dominated not by
X-ray binary emission but rather by diffuse gas emission. While the properties
of the nuclear source are generally supportive of a superbubble interpretation,
its large density and emission measure stretch the limits that can be
comfortably accommodated by such models. We do not detect direct emission from
the putative Seyfert nucleus, although an alternative model for the nuclear
source is thermal emission from gas that is photoionized by a hidden Seyfert
nucleus. The spectra of the other two X-ray sources are harder than that of the
nuclear source, and superbubble models for them have the same strengths and
weaknesses.Comment: 11 pages, uuencoded compressed postscript, MNRAS in pres
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