9 research outputs found
The HNC/HCN Ratio in Star-Forming Regions
HNC and HCN, typically used as dense gas tracers in molecular clouds, are a pair of isomers that have great potential as a temperature probe because of temperature dependent, isomer-specific formation and destruction pathways. Previous observations of the HNC/HCN abundance ratio show that the ratio decreases with increasing temperature, something that standard astrochemical models cannot reproduce. We have undertaken a detailed parameter study on which environmental characteristics and chemical reactions affect the HNC/HCN ratio and can thus contribute to the observed dependence. Using existing gas and gas-grain models updated with new reactions and reaction barriers, we find that in static models the H + HNC gas-phase reaction regulates the HNC/HCN ratio under all conditions, except for very early times. We quantitatively constrain the combinations of H abundance and H + HNC reaction barrier that can explain the observed HNC/HCN temperature dependence and discuss the implications in light of new quantum chemical calculations. In warm-up models, gas-grain chemistry contributes significantly to the predicted HNC/HCN ratio and understanding the dynamics of star formation is therefore key to model the HNC/HCN system.Astronom
Molecular complexity in pre-stellar cores : a 3 mm-band study of L183 and L1544
Context. Pre-stellar cores (PSCs) are units of star formation. Besides representing early stages of the dynamical evolution leading to the formation of stars and planets, PSCs also provide a substrate for incipient chemical complexity in the interstellar space. Aims. Our aim is to understand the influence of external conditions on the chemical composition of PSCs. For this purpose, we compared molecular column densities in two typical PSCs, L183 and L1544, which are embedded in different environments. Methods. A single-pointing survey of L183 at lambda = 3 mm was conducted using the IRAM 30-m single-dish antenna. This led to the detection of more than 100 emission lines from 46 molecular species. The molecular column densities and excitation temperatures derived from these lines were compared to the corresponding parameters in L1544. The data for L1544 were obtained from literature or publicly available surveys, and they were analysed using the same procedure as adopted for L183. An astrochemical model, previously developed for the interpretation of organic molecule emissions towards the methanol peak of L1544, was used to interpret the combined data. Results. Our analysis reveals clear chemical differences between the two PSCs. While L1544 is richer in carbon-bearing species, in particular carbon chains, oxygen-containing species are generally more abundant in L183. The results are well-reproduced by our chemical model. Conclusions. The observed chemical differentiation between the two PSCs is caused by the different environmental conditions: the core of L183 is deeply buried in the surrounding cloud, whereas L1544 lies close to the edge of the Taurus Molecular Cloud. The obscuration of L183 from the interstellar radiation field (ISRF) allows the carbon atoms to be locked in carbon monoxide, which ultimately leads to a large abundance of O-bearing species. In contrast, L1544, being more affected by the ISRF, can keep a fraction of carbon in atomic form, which is needed for the production of carbon chains.Peer reviewe
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
Nuclear spin ratios of deuterated ammonia in prestellar cores. LAsMA observations of H-MM1 and Oph D
We determine the ortho/para ratios of NH2D and NHD2 in two dense, starless
cores, where their formation is supposed to be dominated by gas-phase
reactions, which, in turn, is predicted to result in deviations from the
statistical spin ratios. The Large APEX sub-Millimeter Array (LAsMA) multibeam
receiver of the Atacama Pathfinder EXperiment (APEX) telescope was used to
observe the prestellar cores H-MM1 and Oph D in Ophiuchus in the ground-state
lines of ortho and para NH2D and NHD2. The fractional abundances of these
molecules were derived employing 3D radiative transfer modelling, using
different assumptions about the abundance profiles as functions of density. We
also ran gas-grain chemistry models with different scenarios concerning proton
or deuteron exchanges and chemical desorption from grains to find out if one of
these models can reproduce the observed spin ratios. The observationally
deduced ortho/para ratios of NH2D and NHD2 are in both cores within 10% of
their statistical values 3 and 2, respectively, and taking 3-sigma limits,
deviations from these of about 20% are allowed. Of the chemistry models tested
here, the model that assumes proton hop (as opposed to full scrambling) in
reactions contributing to ammonia formation, and a constant efficiency of
chemical desorption, comes nearest to the observed abundances and spin ratios.
The nuclear spin ratios derived here are in contrast with spin-state chemistry
models that assume full scrambling in proton donation and hydrogen abstraction
reactions leading to deuterated ammonia. The efficiency of chemical desorption
influences strongly the predicted abundances of NH3, NH2D, and NHD2, but has a
lesser effect on their ortho/para ratios. For these the proton exchange
scenario in the gas is decisive. We suggest that this is because of rapid
re-processing of ammonia and related cations by gas-phase ion-molecule
reactions.Comment: accepted for publication in Astronomy & Astrophysic
Complex organic molecules in protoplanetary disks
(Abridged) Protoplanetary disks are vital objects in star and planet
formation, possessing all the material which may form a planetary system
orbiting the new star. We investigate the synthesis of complex organic
molecules (COMs) in disks to constrain the achievable chemical complexity and
predict species and transitions which may be observable with ALMA. We have
coupled a 2D model of a protoplanetary disk around a T Tauri star with a
gas-grain chemical network including COMs. We compare compare synthesised line
intensities and calculated column densities with observations and determine
those COMs which may be observable in future. COMs are efficiently formed in
the disk midplane via grain-surface chemical reactions, reaching peak
grain-surface fractional abundances 1e-6 - 1e-4 that of the H nuclei number
density. COMs formed on grain surfaces are returned to the gas phase via
non-thermal desorption; however, gas-phase species reach lower fractional
abundances than their grain-surface equivalents, 1e-12 - 1e-7. Including the
irradiation of grain mantle material helps build further complexity in the ice
through the replenishment of grain-surface radicals which take part in further
grain-surface reactions. There is reasonable agreement with several line
transitions of H2CO observed towards several T Tauri star-disk systems. The
synthesised line intensities for CH3OH are consistent with upper limits
determined towards all sources. Our models suggest CH3OH should be readily
observable in nearby protoplanetary disks with ALMA; however, detection of more
complex species may prove challenging. Our grain-surface abundances are
consistent with those derived from cometary comae observations providing
additional evidence for the hypothesis that comets (and other planetesimals)
formed via the coagulation of icy grains in the Sun's natal disk.Comment: 34 pages, 12 figures, 7 table
Seeds of Life in Space (SOLIS). III. Zooming Into the Methanol Peak of the Prestellar Core L1544
International audienceToward the prestellar core L1544, the methanol (CH3OH) emission forms an asymmetric ring around the core center, where CH3OH is mostly in solid form, with a clear peak at 4000 au to the northeast of the dust continuum peak. As part of the NOEMA Large Project SOLIS (Seeds of Life in Space), the CH3OH peak has been spatially resolved to study its kinematics and physical structure and to investigate the cause behind the local enhancement. We find that methanol emission is distributed in a ridge parallel to the main axis of the dense core. The centroid velocity increases by about 0.2 km s(-1) and the velocity dispersion increases from subsonic to transonic toward the central zone of the core, where the velocity field also shows complex structure. This could be an indication of gentle accretion of material onto the core or the interaction of two filaments, producing a slow shock. We measure the rotational temperature and show that methanol is in local thermodynamic equilibrium (LTE) only close to the dust peak, where it is significantly depleted. The CH3OH column density, N-tot(CH3OH), profile has been derived with non-LTE radiative transfer modeling and compared with chemical models of a static core. The measured N (tot)(CH3OH) profile is consistent with model predictions, but the total column densities are one order of magnitude lower than those predicted by models, suggesting that the efficiency of reactive desorption or atomic hydrogen tunneling adopted in the model may be overestimated; or that an evolutionary model is needed to better reproduce methanol abundance