The Production of HI in Photodissociation Regions and A Comparison with CO(1-0) Emission

The gas at the surfaces of molecular clouds in galaxies is heated and dissociated by photons from young stars both near and far. HI resulting from the dissociation of molecular hydrogen H2 emits hyperfine line emission at 21 cm, and warmed CO emits dipole rotational lines such as the 2.6 mm line of CO(1-0). We use previously developed models for photodissociation regions (PDRs) to compute the intensities of these HI and CO(1-0) lines as a function of the total volume density n in the cloud and the far ultraviolet flux G0 incident upon it and present the results in units familiar to observers. The intensities of these two lines behave differently with changing physical conditions in the PDR, and, taken together, the two lines can provide a ground-based radio astronomy diagnostic for determining n and G0 separately in distant molecular clouds. This diagnostic is particularly useful in the range Gzero <~ 100, 10 cm^{-3} <~ n <~ 10^5 cm^{-3}, which applies to a large fraction of the volume of the interstellar medium in galaxies. If the molecular cloud is located near discrete sources of far-UV (FUV) emission, the PDR-generated HI and CO(1-0) emission on the cloud surface can be more easily identified, appearing as layered ``blankets'' or ``blisters'' on the side of the cloud nearest to the FUV source. As an illustration, we consider the Galactic object G216 -2.5, i.e. ``Maddalena's Cloud'', which has been previously identified as a large PDR in the Galaxy. We determine that this cloud has n ~ 200 cm^{-3}, G0 ~ 0.8, consistent with other data.Comment: 13 Pages, 3 Figures. Accepted for publication in the Astrophysical Journa

Formation of Narrow Dust Rings in Circumstellar Debris Disks

Narrow dust rings observed around some young stars (e.g., HR 4796A) need to be confined. We present a possible explanation for the formation and confinement of such rings in optically thin circumstellar disks, without invoking shepherding planets. If an enhancement of dust grains (e.g., due to a catastrophic collision) occurs somewhere in the disk, photoelectric emission from the grains can heat the gas to temperatures well above that of the dust. The gas orbits with super(sub)-Keplerian speeds inward (outward) of the associated pressure maximum. This tends to concentrate the grains into a narrow region. The rise in dust density leads to further heating and a stronger concentration of grains. A narrow dust ring forms as a result of this instability. We show that this mechanism not only operates around early-type stars that have high UV fluxes, but also around stars with spectral types as late as K. This implies that this process is generic and may have occurred during the lifetime of each circumstellar disk. We examine the stringent upper-limit on the H2 column density in the HR 4796A disk and find it to be compatible with the presence of a significant amount of hydrogen gas in the disk. We also compute the OI and CII infrared line fluxes expected from various debris disks and show that these will be easily detectable by the upcoming Herschel mission. Herschel will be instrumental in detecting and characterizing gas in these disks.Comment: Accepted for publication in ApJ; 14 pages, 7 figure

An Attempt to Detect the Galactic Bulge at 12 microns with IRAS

Surface brightness maps at 12 microns, derived from observations with the Infrared Astronomical Satellite (IRAS), are used to estimate the integrated flux at this wavelength from the Galactic bulge as a function of galactic latitude along the minor axis. A simple model was used to remove Galactic disk emission (e.g. unresolved stars and dust) from the IRAS measurements. The resulting estimates are compared with predictions for the 12 micron bulge surface brightness based on observations of complete samples of optically identified M giants in several minor axis bulge fields. No evidence is found for any significant component of 12m emission in the bulge other than that expected from the optically identified M star sample plus normal, lower luminosity stars. Known large amplitude variables and point sources from the IRAS catalogue contribute only a small fraction to the total 12 micron flux.Comment: Accepted for publication in ApJ; 13 pages of text including tables in MS WORD97 generated postscript; 3 figures in postscript by Sigma Plo

86 GHz SiO maser survey of late-type stars in the Inner Galaxy. IV. SiO emission and infrared data for sources in the Scutum and Sagittarius-Carina arms, 20 deg < l < 50 deg

We present an 86 GHz SiO (v = 1, J = 2 ---> 1) maser search toward late-type stars located within |b|<0.5 deg and 20 deg < l < 50 deg. This search is an extension at longer longitudes of a previously published work. We selected 135 stars from the MSX catalog using color and flux criteria and detected 92 (86 new detections). The detection rate is 68%, the same as in our previous study. The last few decades have seen the publication of several catalogs of point sources detected in infrared surveys (MSX, 2MASS, DENIS, ISOGAL, WISE, GLIMPSE, AKARI, and MIPSGAL). We searched each catalog for data on the 444 targets of our earlier survey and for the 135 in the survey reported here. We confirm that, as anticipated, most of our targets have colors typical of oxygen-rich asymptotic giant branch (AGB) stars. Only one target star may have already left the AGB. Ten stars have colors typical of carbon-rich stars, meaning a contamination of our sample with carbon stars <=1.7%.Comment: 13 pages, 6 Figures, A&A accepte

CO in OH/IR stars close to the Galactic centre

Aims: A pilot project has been carried out to measure circumstellar CO emission from three OH/IR stars close to the Galactic centre. The intention was to find out whether it would be possible to conduct a large-scale survey for mass-loss rates using, for example, the Atacama Large Millimeter Array (ALMA). Such a survey would increase our understanding of the evolution of the Galactic bulge. Methods: Two millimetre-wave instruments were used: the Nobeyama Millimeter Array at 115 GHz and the Submillimeter Array at 230 GHz. An interferometer is necessary as a `spatial filter' in this region of space because of the confusion with interstellar CO emission. Results: Towards two of the stars, CO emission was detected with positions and radial velocities coinciding within the statistical errors with the corresponding data of the associated OH sources. However, for one of the stars the line profile is not what one expects for an unresolved expanding circumstellar envelope. We believe that this CO envelope is partially resolved and that this star therefore is a foreground star not belonging to the bulge. Conclusions: The results of the observations have shown that it is possible to detect line profiles of circumstellar CO from late-type stars both within and in the direction of the Galactic bulge. ALMA will be able to detect CO emission in short integrations with sensitivity sufficient to estimate mass-loss rates from a large number of such stars.Comment: 5 pages, 3 figure

Formation of Massive Primordial Stars in a Reionized Gas

We use cosmological hydrodynamic simulations with unprecedented resolution to study the formation of primordial stars in an ionized gas at high redshifts. Our approach includes all the relevant atomic and molecular physics to follow the thermal evolution of a prestellar gas cloud to very high densities of ~10^{18} cm^{-3}. We locate a star-forming gas cloud within a reionized region in our cosmological simulation. The first run-away collapse is triggered when the gas cloud's mass is ~40 Msun. We show that the cloud core remains stable against chemo-thermal instability and also against gravitational deformation throughout its evolution. Consequently, a single proto-stellar seed is formed, which accretes the surrounding hot gas at the rate ~10^{-3} Msun/year. We carry out proto-stellar evolution calculations using the inferred accretion rate. The resulting mass of the star when it reaches the zero-age main sequence is M_ZAMS ~40 Msun. We argue that, since the obtained M_ZAMS is as large as the mass of the collapsing parent cloud, the final stellar mass should be close to this value. Such massive, rather than exceptionally massive, primordial stars are expected to cause early chemical enrichment of the Universe by exploding as black hole-forming super/hypernovae, and may also be progenitors of high redshift gamma-ray bursts. The elemental abundance patterns of recently discovered hyper metal-poor stars suggest that they might have been born from the interstellar medium that was metal-enriched by supernovae of these massive primordial stars.Comment: Revised version. To appear in ApJ

Infrared astronomical satellite (IRAS) catalogs and atlases. Volume 1: Explanatory supplement

The Infrared Astronomical Satellite (IRAS) was launched on January 26, 1983. During its 300-day mission, IRAS surveyed over 96 pct of the celestial sphere at four infrared wavelengths, centered approximately at 12, 25, 60, and 100 microns. Volume 1 describes the instrument, the mission, and data reduction

H_2 Absorption and Fluorescence for Gamma Ray Bursts in Molecular Clouds

If a gamma ray burst with strong UV emission occurs in a molecular cloud, there will be observable consequences resulting from excitation of the surrounding H2. The UV pulse from the GRB will pump H2 into vibrationally-excited levels which produce strong absorption at wavelengths < 1650 A. As a result, both the prompt flash and later afterglow will exhibit strong absorption shortward of 1650 A, with specific spectroscopic features. Such a cutoff in the emission from GRB 980329 may already have been observed by Fruchter et al.; if so, GRB 980329 was at redshift 3.0 < z < 4.4 . BVRI photometry of GRB 990510 could also be explained by H2 absorption if GRB 990510 is at redshift 1.6 < z < 2.3. The fluorescence accompanying the UV pumping of the H2 will result in UV emission from the GRB which can extend over days or months, depending on parameters of the ambient medium and beaming of the GRB flash. The 7.5-13.6 eV fluorescent luminosity is \sim 10^{41.7} erg/s for standard estimates of the parameters of the GRB and the ambient medium. Spectroscopy can distinguish this fluorescent emission from other possible sources of transient optical emission, such as a supernova.Comment: 13 pages, including 4 figures. submitted to Ap.J.(Letters

Why are massive O-rich AGB stars in our Galaxy not S-stars?

We present the main results derived from a chemical analysis carried out on a large sample of galactic O-rich AGB stars using high resolution optical spectroscopy (R~40,000-50,000) with the intention of studying their lithium abundances and/or possible s-process element enrichment. Our chemical analysis shows that some stars are lithium overabundant while others are not. The observed lithium overabundances are interpreted as a clear signature of the activation of the so-called ``Hot Bottom Burning'' (HBB) process in massive galactic O-rich AGB stars, as predicted by the models. However, these stars do not show the zirconium enhancement (taken as a representative for the s-process element enrichment) associated to the third dredge-up phase following thermal pulses. Our results suggest that the more massive O-rich AGB stars in our Galaxy behave differently from those in the Magellanic Clouds, which are both Li- and s-process-rich (S-type stars). Reasons for this unexpected result are discussed. We conclude that metallicity is probably the main responsible for the differences observed and suggest that it may play a more important role than generally assumed in the chemical evolution of AGB stars.Comment: 4 pages, 2 figures, to appear in the proceedings of the conference "Planetary Nebulae as astronomical tools" held in Gdansk, Poland, jun 28/jul 02, 200