arXiv subjects

F. L. Schoeier

Publications and source records attributed to F. L. Schoeier.

42 records · Page 3Linked to original sources

arXiv2002

Modelling CO emission from Mira's wind

We have modelled the circumstellar envelope of {\it o} Ceti (Mira) using new observational constraints. These are obtained from photospheric light scattered in near-IR vibrational-rotational lines of circumstellar CO molecules at 4.6 micron: absolute fluxes, the radial dependence of the scattered intensity, and two line ratios. Further observational constraints are provided by ISO observations of far-IR emission lines from highly excited rotational states of the ground vibrational state of CO, and radio observations of lines from rotational levels of low excitation of CO. A code based on the Monte-Carlo technique is used to model the circumstellar line emission. We find that it is possible to model the radio and ISO fluxes, as well as the highly asymmetric radio-line profiles, reasonably well with a spherically symmetric and smooth stellar wind model. However, it is not possible to reproduce the observed NIR line fluxes consistently with a `standard model' of the stellar wind. This is probably due to incorrectly specified conditions of the inner regions of the wind model, since the stellar flux needs to be larger than what is obtained from the standard model at the point of scattering, i.e., the intermediate regions at approximately 100-400 stellar radii (2"-7") away from the star. Thus, the optical depth in the vibrational-rotational lines from the star to the point of scattering has to be decreased. This can be accomplished in several ways. For instance, the gas close to the star (within approximately 2") could be in such a form that light is able to pass through, either due to the medium being clumpy or by the matter being in radial structures (which, further out, developes into more smooth or shell-like structures).

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arXiv2002

Does IRAS 16293-2422 have a hot core? Chemical inventory and abundance changes in its protostellar environment

A detailed radiative transfer analysis of the observed continuum and molecular line emission toward the deeply embedded young stellar object IRAS 16293-2422 is performed. The continuum modelling is used to constrain the temperature and density distributions in the envelope, enabling quantitative estimates of various molecular abundances. The molecular excitation analysis reveals that the emission from some molecular species is well reproduced assuming a constant fractional abundance throughout the envelope. The abundances and isotope ratios are generally close to typical values found in cold molecular clouds in these cases, and there is a high degree of deuterium fractionation. There are, however, a number of notable exceptions. Lines covering a wide range of excitation conditions indicate for some molecules, e.g., H2CO, CH3OH, SO, SO2 and OCS, a drastic increase in their abundances in the warm and dense inner region of the circumstellar envelope. The location at which this increase occurs is consistent with the radius at which ices are expected to thermally evaporate off the grains. In all, there is strong evidence for the presence of a `hot core' close to the protostar, whose physical properties are similar to those detected towards most high mass protostars except for a scaling factor. However, the small scale of the hot gas and the infalling nature of the envelope lead to very different chemical time scales between low mass and high mass hot cores, such that only very rapidly produced second-generation complex molecules can be formed in IRAS 16293-2422. Alternatively, the ices may be liberated due to grain-grain collisions in turbulent shear zones where the outflow interacts with the envelope.

astro-ph↗

arXiv2002

Probing the mass loss history of carbon stars using CO line and dust continuum emission

An extensive modelling of CO line emission from the circumstellar envelopes around a number of carbon stars is performed. By combining radio observations and infrared observations obtained by ISO the circumstellar envelope characteristics are probed over a large radial range. In the radiative transfer analysis the observational data are consistently reproduced assuming a spherically symmetric and smooth wind expanding at a constant velocity. The combined data set gives better determined envelope parameters, and puts constraints on the mass loss history of these carbon stars. The importance of dust in the excitation of CO is addressed using a radiative transfer analysis of the observed continuum emission, and it is found to have only minor effects on the derived line intensities. The analysis of the dust emission also puts further constraints on the mass loss rate history. The stars presented here are not likely to have experienced any drastic long-term mass loss rate modulations, at least less than a factor of about 5, over the past thousands of years. Only three, out of nine, carbon stars were observed long enough by ISO to allow a detection of CO far-infrared rotational lines.

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arXiv2001

Models of circumstellar molecular radio line emission: Mass loss rates for a sample of bright carbon stars

Using a detailed radiative transfer analysis, combined with an energy balance equation for the gas, we have performed extensive modelling of circumstellar CO radio line emission from a large sample of optically bright carbon stars. We determine some of the basic parameters that characterize circumstellar envelopes (CSEs), e.g., the stellar mass loss rate, the gas expansion velocity, and the kinetic temperature structure of the gas. The derived mass loss rates span almost four orders of magnitude, from 5E-9 up to 2E-5 solar masses per year, with the median mass loss rate being 3E-7 solar masses per year. We estimate that the estimated mass loss rates are typically accurate to 50% within the adopted circumstellar model. The physical conditions prevailing in the CSEs vary considerably over such a large range of mass loss rates. Among other things, it appears that the dust-to-gas mass ratio and/or the dust properties change with the mass loss rate. We find that the mass loss rate and the gas expansion velocity are well correlated, and that both of them clearly depend on the pulsational period and (with larger scatter) the stellar luminosity. Moreover, the mass loss rate correlates weakly with the stellar effective temperature, in the sense that the cooler stars tend to have higher mass loss rates, but there seems to be no correlation with the stellar C/O-ratio. We conclude that the mass loss rate increases with increased regular pulsation and/or luminosity, and that the expansion velocity increases as an effect of increasing mass loss rate (for low mass loss rates) and luminosity.

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arXiv2000

The 12C/13C-ratio in cool carbon stars

We present observations of circumstellar millimetre-wave 13CO line emission towards a sample of 20 cool carbon stars. Using a detailed radiative transfer model we estimate the circumstellar 12CO/13CO-ratios, which we believe accurately measure the important stellar 12C/13C-ratios. For those optically bright carbon stars where it is possible, our derived 12C/13C-ratios are compared with the photospheric results, obtained with different methods. Our estimates agree well with those of Lambert et al. (1986). It is shown that a straightforward determination of the 12CO/13CO-ratio from observed line intensity ratios is often hampered by optical depth effects, and that a detailed radiative transfer analysis is needed in order to determine reliable isotope ratios.

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arXiv1999

The carbon star IRAS 15194-5115: Circumstellar CO radio and FIR rotational lines

We have modelled radio and far-infrared rotational lines of 12^CO and 13^CO obtained from the circumstellar envelope of the infrared-bright carbon star IRAS 15194-5115. Eleven rotational lines between J=1-0 and 21-20 and nine rotational lines between J=1-0 and 22-21 in the ground vibrational states of 12^CO and 13^CO, respectively, provide the observational constraints. A model of the circumstellar envelope with a constant mass-loss rate (1 10^-5Msun/yr) and 12^CO/13^CO-ratio (5.5) is consistent with our observed FIR and radio data, suggesting that the wind characteristics have not changed significantly over the past few thousand years. Thus, IRAS 15194-5115 appears to be a highly evolved AGB-star, but the carbon star properties combined with the inferred low 12^C/13^C-ratio make the evolutionary status of this star uncertain. It may have been a J-star for which the 12^C}/13^C-ratio has remained low, or it may be a star of 5 to 8 solar masses, which has recently become a carbon star due to quenching of hot bottom burning. The dust properties or the relative amount of dust in the envelope appear to be different from those in the envelope of the well known carbon star IRC+10 216.

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