arXiv subjects

O. Morata

Publications and source records attributed to O. Morata.

23 records · Page 2Linked to original sources

arXiv2008

Time-dependent models of dense PDRs with complex molecules

We present a study of the chemistry of a dense photon-dominated region (PDR) using a time-dependent chemical model. Our major interest is to study the spatial distribution of complex molecules such as hydrocarbons and cyanopolyynes in the cool dense material bordering regions where star formation has taken place. Our standard model uses a homogeneous cloud of density 2x10e4 cm-3 and temperature T=40 K, which is irradiated by a far-ultraviolet radiation field of intermediate intensity, given by X=100. We find that over a range of times unsaturated hydrocarbons (e.g., C2H, C4H, C3H2) have relatively high fractional abundances in the more external layers of the PDR, whereas their abundances in the innermost layers are several orders of magnitudes lower. On the other hand, molecules that are typical of late-time chemistry are usually more abundant in the inner parts of the PDR. We also present results for models with different density, temperature, intensity of the radiation field and initial fractional abundances. Our results are compared with both high- and moderate-angular resolution observations of the Horsehead nebula. Our standard model is partially successful in reproducing the observations. Additional models run with different physical parameters are able to reproduce the abundance of many of the observed molecules, but we do not find a single model that fits all the observations at the same time. We discuss the suitability of a time-dependent model of a dense PDR such as ours as an estimator of the age of a PDR, provided that enough observational data exist.

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arXiv2007

New detections of H2O masers in planetary nebulae and post-AGB stars using the Robledo-70m antenna

Aims: We investigated the possible relationship between the evolutionary stage of post-AGB stars and planetary nebulae (PNe) and the presence of water masers in their envelopes. Methods: We have used NASA's 70-m antenna in Robledo de Chavela (Spain) to search for the water maser transition at 22235.08 MHz, towards a sample of 105 sources with IRAS colour characteristic of post-AGB stars and PNe at declination >-32 deg. 83% of the sources in the sample are post-AGB stars, 15% PNe or PN candidates, while only 2% seem to be HII regions. Results: We have detected five water masers, of which four are reported for the first time: two in PNe (IRAS 17443-2949 and IRAS 18061-2505), a ``water fountain'' in a post-AGB star (IRAS 16552-3050), and one in a source previously catalogued as a PN, but whose classification is uncertain (IRAS 17580-3111). Conclusions: The unexpected detections of water masers in two objects among the small subset of PNe led us to suggest that the PNe harbouring water masers are a special type of massive, rapidly evolving PNe.

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arXiv2007

A PDR-Code Comparison Study

We present a comparison between independent computer codes, modeling the physics and chemistry of interstellar photon dominated regions (PDRs). Our goal was to understand the mutual differences in the PDR codes and their effects on the physical and chemical structure of the model clouds, and to converge the output of different codes to a common solution. A number of benchmark models have been created, covering low and high gas densities and far ultraviolet intensities. The benchmark models were computed in two ways: one set assuming constant temperatures, thus testing the consistency of the chemical network and photo-processes, and a second set determining the temperature selfconsistently. We investigated the impact of PDR geometry and agreed on the comparison of results from spherical and plane-parallel PDR models. We identified a number of key processes governing the chemical network which have been treated differently in the various codes, and defined a proper common treatment. We established a comprehensive set of reference models for ongoing and future PDR model bench-marking and were able to increase the agreement in model predictions for all benchmark models significantly.

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arXiv2006

Monte Carlo simulations of H2 formation on stochastically heated grains

Continuous-time, random-walk Monte Carlo simulations of H2 formation on grains have been performed for surfaces that are stochastically heated by photons. We have assumed diffuse cloud conditions and used a variety of grains of varying roughness and size based on olivine. The simulations were performed at different optical depths. We confirmed that small grains (r <= 0.02 micron) have low modal temperatures with strong fluctuations, which have a large effect on the efficiency of the formation of molecular hydrogen. The grain size distribution highly favours small grains and therefore H2 formation on these particles makes a large contribution to the overall formation rate for all but the roughest surfaces. We find that at A_V=0 only the roughest surfaces can produce the required amount of molecular hydrogen, but by A_V=1, smoother surfaces are possible alternatives. Use of a larger value for the evaporation energy of atomic hydrogen, but one still consistent with experiment, allows smoother surfaces to produce more H2.

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arXiv2002

Multitransitional observations of the CS core of L673

A multitransitional study with the BIMA interferometric array was carried out toward the starless core found in the L673 region, in order to study the small-size structure of the cores detected with previous single--dish observations, which provides us with a test of the predictions of the chemical model of Taylor et al. (1996; 1998). We detected emission in the CS (2-1), N2H+ (1-0), and HCO+ (1-0) lines. Several clumps of size ~0.08 pc were found for each line distributed all over the region where previous single-dish emission was found (Morata et al. 1997). Each molecular transition traces differently the clump distribution, although in some cases the detected clumps are coincident. The distribution of the N2H+ emission and the single-dish NH3 emission are coincident and compatible with an origin in the same gas. The large fraction of missing flux measured for the CS (2-1) transition can be explained if the cloud is formed by a clumpy and heterogeneous medium. Four positions were selected to derive the abundance ratios [N2H+/CS] and [HCO+/CS] from the molecular column density determinations, and to compare them with the values predicted by the chemical model. The model was able to explain the interferometric observations, and, in particular, the chemical differentiation of the detected clumps and the coincidence of the NH3 and N2H+ emissions. The lack of HCO+ towards the two selected positions that trace the more evolved clumps cannot be accounted for by the model, but it is possibly due to strong self-absorption. We propose a classification of the studied clumps according to the stage of chemical evolution indicated by the molecular abundances.

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