Christopher J. Skinner

METHANOL ICE IN THE PROTOSTAR GL-2136

We present ground-based spectra in the 10 and 20 mum atmospheric windows of the deeply embedded protostar GL 2136. These reveal narrow absorption features at 9.7 and 8.9 mum, which we ascribe to the CO-stretch and CH3 rock (respectively) of solid methanol in grain mantles. The peak position of the 9.7 mum band implies that methanol is an important ice mantle component (i.e., CH3OH/H2O > 0.5). However, the CH3OH/H2O abundance ratio derived from the observed column densities is only 0.1. This discrepancy suggests that the solid methanol and water ice are located in independent grain components. These independent components may reflect chemical differentiation during grain mantle formation and/or partial outgassing close to the protostar.

Discovery of an extended nebula around AFGL 2343 (HD 179821) at 10 microns

We present images at 8.5, 10.5, and 12.5 {mu}m which reveal that AFGL 2343 (HD 179821) is surrounded by a dusty nebula 4{double_prime}{minus}5{double_prime} in diameter at 10.5 and 12.5 {mu}m. This G5 Ia supergiant has long been suspected of having a detached dusty envelope because of its optical brightness and large IR excess. The nebula is ringlike and slightly elongated, with bright patches or clumps to the northeast. An 8.5 {mu}m image is dominated by the photosphere of the central star, which also appears to a lesser extent in the 10.5 and 12.5 {mu}m images. A full radiative transfer model of the nebula correctly predicts the observed inner (1{close_quote}{close_quote}.1) and outer (1{close_quote}{close_quote}.9) radii of the shell and the spectral energy distribution of the shell. With the aid of this model, we find that AFGL 2343 is almost certainly an extremely massive star at a distance of about 6 kpc, which has recently undergone an enormous mass-loss event, during which {dot {ital M}}=10{sup {minus}2} {ital M}{sub {circle_dot}} yr{sup {minus}1}, while the current mass-loss rate is less than 2{times}10{sup {minus}6} {ital M}{sub {circle_dot}} yr{sup {minus}1}. {copyright} {ital 1995 The American Astronomical Society.}

The mid-infrared radio correlation at high angular resolution : NGC 253

We present high angular resolution (1″.2), narrow-band (Δλ/λ=0.1) images of the nucleus of NGC 253 at three wavelengths in the mid-infrared (8.5, 10.0, and 12.5 μm). We find that most of the mid-IR flux in the nucleus of NGC 253 derives from a very small region ≤120 pc in diameter. Within this small region there are three spatially and spectrally distinct IR components: two bright compact sources, and a surrounding envelope of low-level, diffuse emission. The mid-IR and 6 cm radio are loosely correlated in position but not in brightness. The implication is that the mid-IR-radio correlation may begin to break down on small spatial scales relevant to individual star-forming regions and large individual sources

The nature of dust around the post-asymptotic giant branch objects HD 161796 and HD 179821

Ground-based 7.4-24-micron spectra of two post-AGB objects, HD 161796 and HD 179821, are reported, and they are compared to those of other preplanetary nebulae. HD 161796 and HD 17982 show emission features at 10-12 microns and at 10 microns, and they exhibit a very rapid increase in flux between 13 and 15.5 microns. In view of the O-rich photosphere of HD 161796 and the presence of OH maser emission around all three objects, these features are ascribed to various oxides. The observed spectral features are quite different from the canonical silicate features observed in most O-rich giants. It is argued that HD 161796 and the bipolar nebulae Roberts 22 and NGC 6302 have all undergone the third dredge-up, with most of the dredged-up carbon having been converted to nitrogen by envelope-burning. It is concluded that carbon-rich grain material, produced during the interval between the end of the third dredge-up and the moment when envelope burning finally reduced the C/O ratio below unity again, could be responsible for the UIR bands now being excited in Roberts 22 and NGC 6302. 32 refs.

ISO LWS observations of H2O from R CAS: A consistent model for its circumstellar envelope

We present an ISO LWS 43-197 µm grating spectrum of the oxygen-rich AGB star R Cas. The spectrum is rich of isolated and blended H2O lines. For their identification and in order to determine the physical parameters of the circumstellar envelope, we have constructed a model which treats radiative transfer, chemical exchange and photodissociation reactions, and various heating and cooling processes in a consistent manner. By fitting the observed line fluxes and using stellar parameters based upon the Hipparcos distance, we derived a mass-loss rate of M = 1 10-6 M⊙ yr-1 which is close to the value 6 10-7 M⊙ yr-1 previously derived for W Hya, another oxygen-rich AGB star.

Modelling of Carbon-Rich Stars with Far Infrared Flux Excess

It is now well-known that many carbon-rich stars — especially those with optically thin dust shells — show a large infrared (IR) excess at 60 and 100 μπι. It is common opinion that such a phenomenon can be explained by assuming that the star is surrounded by a cool detached dust shell, placed far away from it. However, there is no agreement in the literature about the chemical composition or typical size of such detached shells, or their distance from the star. Here we present a set of coeval broadband photometric and spectrophotometric measurements for a sample of carbon stars which show large flux excess in the far-IR. We also present the preliminary results of a spectral analysis carried out considering both oxygen-rich and carbon-rich detached shells:

ISO LWS Observations of H2O from R CAS

We present an ISO LWS 43–197 µm grating spectrum of the oxygen-rich AGB star R Cas. The spectrum is rich of isolated and blended H2O lines. For their identification and in order to determine the physical parameters of the circumstellar envelope, we have constructed a model which treats radiative transfer, chemical exchange and photodissociation reactions, and various heating and cooling processes in a consistent manner. By fitting the observed line fluxes and using stellar parameters based upon the Hipparcos distance, we derived a mass-loss rate of \(\dot M = 1\;{10^{ - 6}}{M_ \odot }y{r^{ - 1}}\) which is close to the value 6 10-7 M⊙ yr-1 previously derived for W Hya, another oxygen-rich AGB star.