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H I features near young star clusters in M81 are identified as the photodissociated surfaces of giant molecular clouds (GMCs) from which the young stars have recently formed. The H I column densities of these features show a weak trend, from undetectable values inside R = 3.7 kpc and increasing rapidly to values around 3 x 10(21) cm(-2) near R approximate to 7.5 kpc. This trend is similar to that of the radially averaged H I distribution in this galaxy, and implies a constant area covering factor of approximate to 0.21 for GMCs throughout M81. The incident UV fluxes G(0) of our sample of candidate PDRs decrease radially. A simple equilibrium model of the photodissociation-reformation process connects the observed values of the incident UV flux, the H I column density, and the relative dust content, permitting an independent estimate to be made of the total gas density in the GMC. Within the GMC this gas will be predominantly molecular hydrogen. Volume densities of 1

H I features near young star clusters in M81 are identified as the photodissociated surfaces of giant molecular clouds (GMCs) from which the young stars have recently formed. The H I column densities of these features show a weak trend, from undetectable values inside R = 3.7 kpc and increasing rapidly to values around 3 x 10(21) cm(-2) near R approximate to 7.5 kpc. This trend is similar to that of the radially averaged H I distribution in this galaxy, and implies a constant area covering factor of approximate to 0.21 for GMCs throughout M81. The incident UV fluxes G(0) of our sample of candidate PDRs decrease radially. A simple equilibrium model of the photodissociation-reformation process connects the observed values of the incident UV flux, the H I column density, and the relative dust content, permitting an independent estimate to be made of the total gas density in the GMC. Within the GMC this gas will be predominantly molecular hydrogen. Volume densities of 1

Aims. In order to gain new insight into the unidentified identity of the diffuse interstellar band (DIB) carriers, this paper describes research into possible links between the shape of the interstellar extinction curve ( including the 2175 angstrom bump and far-UV rise), the presence or absence of DIBs, and physical and chemical conditions of the diffuse interstellar medium ( gas and dust) in the Small Magellanic Cloud (SMC).Methods. We searched for DIB absorption features in VLT/UVES spectra of early-type stars in the SMC whose reddened lines-of-sight probe the diffuse interstellar medium of the SMC. Apparent column density profiles of interstellar atomic species (Na I, KI, Ca II and Ti II) are constructed to provide information on the distribution and conditions of the interstellar gas.Results. The characteristics of eight DIBs detected toward the SMC wing target AzV 456 are studied and upper limits are derived for the DIB equivalent widths toward the SMC stars AzV 398, AzV 214, AzV 18, AzV 65 and Sk 191. The amount of reddening is derived for these SMC sightlines, and, using R-V and the Hi column density, converted into a gas-to-dust ratio. From the atomic column density ratios we infer an indication of the strength of the interstellar radiation field, the titanium depletion level and a relative measure of turbulence/ quiescence. The presence or absence of DIBs appears to be related to the shape of the extinction curve, in particular with respect to the presence or absence of the 2175 angstrom feature. Our measurements indicate that the DIB characteristics depend on the local physical conditions and chemical composition of the interstellar medium of the SMC, which apparently determine the rate of formation (and/or) destruction of the DIB carriers. The UV radiation field ( via photoionisation and photo-destruction) and the metallicity (i.e. carbon abundance) are important factors in determining diffuse band strengths which can differ greatly both between and within galaxies.

The polytropic equation of state of an atomic hydrogen gas is examined for primordial halos with baryonic masses of M-h similar to 10(7)-10(9) M-circle dot. For roughly isothermal collapse around 10(4) K, we find that line trapping of Ly alpha (H I and He II) photons causes the polytropic exponent to stiffen to values significantly above unity. Under the assumptions of zero H-2 abundance and very modest pollution by metals ( 10(4) K, that are attained when Ly alpha photons are trapped in the dense and massive halos that we consider. An H-2-dissociating UV background is needed if positive feedback effects on H-2 formation from X-rays occur. The black hole-to-baryon mass fraction is suggestively close to what is required for these intermediate-mass black holes, of mass M-BH similar to 10(4)-10(6) M-circle dot, to act as seeds for forming the supermassive black holes of mass similar to 0.001M(spheroid) found in galaxies today.

The polytropic equation of state of an atomic hydrogen gas is examined for primordial halos with baryonic masses of M-h similar to 10(7)-10(9) M-circle dot. For roughly isothermal collapse around 10(4) K, we find that line trapping of Ly alpha (H I and He II) photons causes the polytropic exponent to stiffen to values significantly above unity. Under the assumptions of zero H-2 abundance and very modest pollution by metals ( 10(4) K, that are attained when Ly alpha photons are trapped in the dense and massive halos that we consider. An H-2-dissociating UV background is needed if positive feedback effects on H-2 formation from X-rays occur. The black hole-to-baryon mass fraction is suggestively close to what is required for these intermediate-mass black holes, of mass M-BH similar to 10(4)-10(6) M-circle dot, to act as seeds for forming the supermassive black holes of mass similar to 0.001M(spheroid) found in galaxies today.

We present observations of the infrared fine-structure lines of [Si II] (34.8 mum), [O I] (63.2 and 145.5 mum) and [C II] (157.7 mum) obtained with the ISO SWS and LWS spectrographs of nine Planetary Nebulae (PNe). These lines originate in the Photo-Dissociation Regions (PDRs) associated with the nebulae and provide useful information on the evolution and excitation conditions of the ejected material in these regions. In order to interpret the observations, the measured line intensities have been compared with those predicted by photo-dissociation models. This comparison has been done taking into account the C/O content in the nebulae. The densities derived with this comparison show a large scatter for some nebulae, probably because the density is higher than the critical density. Therefore, they are no longer sensitive to this parameter implying that transitions from other species with higher critical density should be used. The possible contribution of shocks to the observed emission characteristics of these PNe is briefly discussed and it is shown that the radiation field is the main driving force responsible for the atomic lines in the PNe that have been studied. In addition, data on the pure rotational lines of H-2 in three nebulae (NGC7027, NGC6302 and Hb 5) are also presented. Assuming local thermal equilibrium the rotational temperature and densities have been derived. We have derived the mass of atomic gas in the PDR associated with these PNe and compared those to ionic masses derived from H and molecular masses derived from low J CO observations. This comparison shows that for these nebulae, the PDR is the main reservoir of gas surrounding these objects. A comparison of the results of these evolved PNe with very young PNe from the literature suggests that as the nebula ages the relative amount of ionic gas increases at the expense of the atomic and molecular mass.

We present ISO-LWS and ground-based SEST observations which probe the Photo Dissociation Region (PDR) of W49N. The ISO-LWS observations consist of low resolution (similar to 200) grating spectra between 43 mum and 197 mum, and high resolution (similar to 10(4)) Fabry-Perot spectra of the [CII] 158 mum and [OI] 63 and 145 mum lines. The SEST observations consist of (CO)-O-16 J = 1 -->0 and (CO)-O-18 J = 2 -->1 maps of the region. Using all these observations and with the help of a detailed PDR model, we have determined the physical and chemical structure of the PDR of W49N. We find that only a tiny fraction, 2.5%, of the cloud gas is in the atomic form. This gas has an average density n=10(4) cm(-3) and a temperature T similar to 130 K. The PDR is created by a FUV field G(0) = 3x10(5) times brighter than the average FUV interstellar field, which is emitted by the stars associated with the several HII regions inside the cloud. The photoelectric heating efficiency is very low, similar to 10(4), relative to other known PDRs, probably because of the relatively high G(0)=n ratio. Overall, W49N seems to be one of the youngest giant molecular clouds actively forming stars in the Galaxy, since most of the cloud material is still molecular and dense and not yet dispersed by the several embedded newly formed massive protostars.

H I features near young star clusters in M81 are identified as the photodissociated surfaces of giant molecular clouds (GMCs) from which the young stars have recently formed. The H I column densities of these features show a weak trend, from undetectable values inside R = 3.7 kpc and increasing rapidly to values around 3 x 10(21) cm(-2) near R approximate to 7.5 kpc. This trend is similar to that of the radially averaged H I distribution in this galaxy, and implies a constant area covering factor of approximate to 0.21 for GMCs throughout M81. The incident UV fluxes G(0) of our sample of candidate PDRs decrease radially. A simple equilibrium model of the photodissociation-reformation process connects the observed values of the incident UV flux, the H I column density, and the relative dust content, permitting an independent estimate to be made of the total gas density in the GMC. Within the GMC this gas will be predominantly molecular hydrogen. Volume densities of 1

Aims.In order to gain new insight into the unidentified identity of the diffuse interstellar band (DIB) carriers, this paper describes research into possible links between the shape of the interstellar extinction curve (including the 2175 Å bump and far-UV rise), the presence or absence of DIBs, and physical and chemical conditions of the diffuse interstellar medium (gas and dust) in the Small Magellanic Cloud (SMC). Methods: We searched for DIB absorption features in VLT/UVES spectra of early-type stars in the SMC whose reddened lines-of-sight probe the diffuse interstellar medium of the SMC. Apparent column density profiles of interstellar atomic species (Na i, K i, Ca ii and Ti ii) are constructed to provide information on the distribution and conditions of the interstellar gas. Results: The characteristics of eight DIBs detected toward the SMC wing target AzV 456 are studied and upper limits are derived for the DIB equivalent widths toward the SMC stars AzV 398, AzV 214, AzV 18, AzV 65 and Sk 191. The amount of reddening is derived for these SMC sightlines, and, using RV and the H i column density, converted into a gas-to-dust ratio. From the atomic column density ratios we infer an indication of the strength of the interstellar radiation field, the titanium depletion level and a relative measure of turbulence/quiescence. The presence or absence of DIBs appears to be related to the shape of the extinction curve, in particular with respect to the presence or absence of the 2175 Å feature. Our measurements indicate that the DIB characteristics depend on the local physical conditions and chemical composition of the interstellar medium of the SMC, which apparently determine the rate of formation (and/or) destruction of the DIB carriers. The UV radiation field (via photoionisation and photo-destruction) and the metallicity (i.e. carbon abundance) are important factors in determining diffuse band strengths which can differ greatly both between and within galaxies. Based on observations collected with VLT/UVES at the European Southern Observatory, Paranal, Chile (ESO programs 67.C-0281 and 71.C-0679). Tables [see full text]-[see full text] and Fig. [see full text] are only available in electronic form at http://www.aanda.org

Far infrared fine structure line data from the ISO archive have been extracted for several hundred YSOs and their outflows, including molecular (CO) outflows, optical jets and Herbig-Haro (HH) objects. Given the importance of these lines to astrophysics, their excitation and transfer ought to be investigated in detail and, at this stage, the reliability of the diagnostic power of the fine structure transitions of O I and C II has been examined. Several issues, such as the extremely small intensity ratios of the oxygen 63 micron to 145 micron lines, are still awaiting an explanation. It is demonstrated that, in interstellar cloud conditions, the 145 micron line is prone to masing, but that this effect is likely an insufficient cause of the line ratio anomaly observed from cold dark clouds. Very optically thick emission could in principle also account for this, but would need similar, prohibitively high column densities and must therefore be abondoned as a viable explanation. One is left with [O I] 63 micron self absorption by cold and tenuous foreground gas, as has been advocated for distant luminous sources. Recent observations with the submillimeter observatory Odin support this scenario also in the case of nearby dark molecular clouds. On the basis of this large statistical material we are led to conclude that in star forming regions, the [O I] and [C II] lines generally have only limited diagnostic value.

The polytropic equation of state of an atomic hydrogen gas is examined for primordial halos with baryonic masses of M_h~10^7-10^9 Mo. For roughly isothermal collapse around 10^4 K, we find that line trapping of Lyman alpha (HI and HeII) photons causes the polytropic exponent to stiffen to values significantly above unity. Under the assumptions of zero H2 abundance and very modest pollution by metals (, Version accepted by ApJ: HeII effects now included, 8 pages