23 research outputs found
On Simulating the Proton-Irradiation of O and HO Ices Using Astrochemical-type Models, with Implications for Bulk Reactivity
Many astrochemical models today explicitly consider the species that comprise
the bulk of interstellar dust grain ice-mantles separately from those in the
top few monolayers. Bombardment of these ices by ionizing radiation - whether
in the form of cosmic rays, stellar winds, or radionuclide emission -
represents an astrochemically viable means of driving a rich chemistry even in
the bulk of the ice-mantle, now supported by a large body of work in laboratory
astrophysics. In this study, using an existing rate equation-based
astrochemical code modified to include a method of considering radiation
chemistry recently developed by us, we attempted to simulate two such studies
in which (a) pure O ice at 5 K and, (b) pure HO ice at 16 K and 77 K,
were bombarded by keV H ions.
Our aims are twofold: (1) to test the capability of our newly developed
method to replicate the results of ice-irradiation experiments, and (2) to
determine in such a well-constrained system how bulk chemistry is best handled
using the same gas-grain codes that are used to model the interstellar medium
(ISM). We find that our modified astrochemical model is able to reproduce both
the abundance of O in the 5 K pure O ice, as well as both the abundance
of HO in the 16 K water ice and the previously noted decrease of
hydrogen peroxide at higher temperatures. However, these results require the
assumption that radicals and other reactive species produced via radiolysis
react quickly and non-diffusively with neighbors in the ice.Comment: ApJ, accepted. 30 pages, 5 figure
Chemistry in the ISM and disks on the verge of planet formation
The general purpose of the thesis work is to improve astrochemical models in the era of ALMA. This era is characterized by the active study of protoplanetary disks and the search for extraterrestrial life. First, we study how uncertainties in the rate coefficients of chemical reactions affect the abundances and column densities of key molecules in protoplanetary disks. We isolate a group of key species which have column densities that are not very sensitive to the rate uncertainties, making them good potential tracers of physical conditions in disks. We identify about a hundred reactions with the most problematic rate coefficients, which need to be determined more accurately in order to improve the reliability of modern astrochemical models. Second, we build a realistic astrochemical model using a Monte Carlo approach to all chemical processes, which is the first time this has been done. This allows us to properly take into account the stochastic nature of grain surface chemical reactions, which are of essential importance for the formation of organic molecules -- i.e., the precursors of life as we know it. The recent modified rate approach (MRE) of Garrod et al. (2008) is shown to be the most accurate fast approach of accounting for stochastic effects in astrochemical modeling. Finally, we apply our model to the study of the chemical composition of an evolving protoplanetary disk with grain growth, in order to reveal chemical tracers of this process. For the first time, a state-of-the-art astrochemical model is coupled with a detailed model of grain growth and sedimentation
Gas mass tracers in protoplanetary disks: CO is still the best
Protoplanetary disk mass is a key parameter controlling the process of
planetary system formation. CO molecular emission is often used as a tracer of
gas mass in the disk. In this study we consider the ability of CO to trace the
gas mass over a wide range of disk structural parameters and search for
chemical species that could possibly be used as alternative mass tracers to CO.
Specifically, we apply detailed astrochemical modeling to a large set of models
of protoplanetary disks around low-mass stars, to select molecules with
abundances correlated with the disk mass and being relatively insensitive to
other disk properties. We do not consider sophisticated dust evolution models,
restricting ourselves with the standard astrochemical assumption of m
dust. We find that CO is indeed the best molecular tracer for total gas mass,
despite the fact that it is not the main carbon carrier, provided reasonable
assumptions about CO abundance in the disk are used. Typically, chemical
reprocessing lowers the abundance of CO by a factor of 3, compared to the case
of photo-dissociation and freeze-out as the only ways of CO depletion. On
average only 13% C-atoms reside in gas-phase CO, albeit with variations from 2
to 30%. CO, HO and HCO can potentially serve as alternative mass
tracers, the latter two being only applicable if disk structural parameters are
known.Comment: Accepted for publication in Ap
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
The first detections of the key prebiotic molecule PO in star-forming regions
Phosphorus is a crucial element in prebiotic chemistry, especially the PO
bond, which is key for the formation of the backbone of the deoxyribonucleic
acid. So far, PO had only been detected towards the envelope of evolved stars,
and never towards star-forming regions. We report the first detection of PO
towards two massive star-forming regions, W51 e1/e2 and W3(OH), using data from
the IRAM 30m telescope. PN has also been detected towards the two regions. The
abundance ratio PO/PN is 1.8 and 3 for W51 and W3(OH), respectively. Our
chemical model indicates that the two molecules are chemically related and are
formed via gas-phase ion-molecule and neutral-neutral reactions during the cold
collapse. The molecules freeze out onto grains at the end of the collapse and
desorb during the warm-up phase once the temperature reaches 35 K. The
observed molecular abundances of 10 are predicted by the model if a
relatively high initial abundance of phosphorus, 510, is
assumed.Comment: To appear in "Astrochemistry VII -- Through the Cosmos from Galaxies
to Planets", proceedings of the IAU Symposium No. 332, 2017, Puerto Varas,
Chile. M. Cunningham, T. Millar and Y. Aikawa, eds. (6 pages
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
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
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
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