Translucent molecular clouds: Theory and observations

Few suitable stars behind molecular clouds have been identified. A limited survey was performed of interstellar lines toward highly reddened stars in the southern sky, using the ESO 1.4 m CAT telescope with a Reticon detector, and the Cerro Tololo 4 m telescope equipped with a GEC charge coupled device (CCD) detector. Because of the reduced extinction at longer wavelengths, molecules were searched for with transitions in the red part of the spectrum such as C2 and CN. For some lines-of-sight for which C2 was detected, the 4300 A line of CH was also observed. Absorption lines of interstellar C2 around 8750 A were detected in the spectra of about 1/4 of the 36 observed stars. The inferred C2 column densities range between 10 to the 13th power and 10 to the 14th power sq. cm., and are up to an order of magnitude larger than those found for diffuse clouds. The observed column densities of CH correlate very well with those of C2 over this range. In contrast, the measured column densities of CN vary by orders of magnitude between the various regions, and they do not correlate with those of C2 and CH. The observed rotational population distribution of C2 also provides information about the physical conditions in the clouds. Models of translucent molecular clouds have been constructed along the lines described by van Dishoeck and Black (1986) for diffuse clouds. The models compute accurately the fractions of atomic and molecular hydrogen as functions of depth into the clouds, as well as the excitation of H2 by ultraviolet pumping. They also incorporate a detailed treatment of the photodissociation processes of the molecules (cf. van Dishoeck 1986), which play an important role in the chemistry up to depths of about 3 mag

Laboratory experiments on interstellar ice analogs: The sticking and desorption of small physisorbed molecules

Molecular oxygen and nitrogen are difficult to observe since they are infrared inactive and radio quiet. The low O2 abundances found so far combined with general considerations of dense cloud conditions suggest molecular oxygen is frozen out at low temperatures (< 20 K) in the shielded inner regions of cloud cores. In solid form O2 and N2 can only be observed as adjuncts within other ice constituents, like CO. In this work we focus on fundamental properties of N2 and O2 in CO ice-gas systems, e.g. desorption characteristics and sticking probabilities at low temperatures for different ice morphologies

The translucent molecular clouds toward hd 154368 .1. Extinction, abundances, and depletions

We report the results of a comprehensive analysis of a line of sight observed using the Goddard High Resolution Spectrograph (GHRS) on the Hubble Space Telescope (HST), toward HD 154368, an 09.5 Iab star located about 800 pc away. The line of sight intersects translucent interstellar cloud material, having a color excess E(B - V) = 0.82 and known high molecular abundances from ground-based data. The HST observations consist of high-resolution spectra obtained with grating ECH-B at wavelengths longward of 1800 Angstrom, and moderate-resolution spectra from grating G160M at shorter wavelengths. We observed some 19 wavelength settings, covering the positions of numerous atomic and molecular transitions. We also incorporate IUE and ground-based data in the study, the latter coming from several observatories at visible and millimeter wavelengths. In this paper we report on the general goals and results of the study, with emphasis on the atomic abundances and depletions. We find that the column densities of most species are slightly higher, relative to the adopted total gas column density, than in some thinner clouds in which most of the absorption arises in a single dense component. Consequently the depletions of elements from the gas phase onto the dust are less than in ''typical'' diffuse clouds such as the one toward zeta Oph. Most of the gas toward HD 154368 resides in two main clouds, centered near -3.26 and -20.95 km s(-1) (heliocentric). Our profile analyses show that the depletions in these two clouds are similar. We discuss the possible relationship of this result to the ultraviolet extinction curve toward HD 154368, derived from IUE spectra

The importanc...

Aims. We study the H2O chemistry in star-forming environments under the influence of a central X-ray source and a central far ultraviolet (FUV) radiation field. The X-ray models are applied to envelopes around low-mass Class 0 and I young stellar objects (YSOs). Methods. The gas-phase water chemistry is modeled as a function of time, hydrogen density and X-ray flux. To cover a wide range of physical environments, densities between nH = 10 4 –10 9 cm −3 and temperatures between T = 10–1000 K are studied. Results. Three different regimes are found: for T &lt; 100 K, the water abundance is of order 10 −7 –10 −6 and can be somewhat enhanced or reduced due to X-rays, depending on time and density. For 100 K � T � 250 K, H2O is reduced from initial x(H2O) ≈ 10 −4 following ice evaporation to x(H2O) ≈ 10 −6 for FX � 10 −3 erg s −1 cm −2 (t = 10 4 yr) and for FX � 10 −4 erg s −1 cm −2 (t = 10 5 yr). At higher temperatures (T � 250 K) and hydrogen densities, water can persist with x(H2O) ≈ 10 −4 even for high X-ray fluxes. Water is destroyed in both Class 0 and I envelopes on relatively short timescales (t ≈ 5000 yr) for realistic X-ray fluxes, although the effect is less prominent in Class 0 envelopes due to the higher X-ray absorbing densities there. FUV photons from the central source are not effective in destroying water. Conclusions. X-rays reduce the water abundances especially in regions where the gas temperature is T � 250–300 K for fluxes FX � 10 −5 –10 −4 erg s −1 cm −2.Theaffected regions can be envelopes, disks or outflow hot spots. The average water abundance in Class I sources for LX � 10 27 erg s −1 is predicted to be x(H2O) � 10 −6. Central UV fields have a negligible influence, unless th