Circumstellar Shells of Late-Type Stars -- A Study at Millimeter and Infrared Wavelenghts

Citation

Sahai, Raghvendra (1985) Circumstellar Shells of Late-Type Stars -- A Study at Millimeter and Infrared Wavelenghts. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/yfv7-n741. https://resolver.caltech.edu/CaltechETD:etd-09042008-105047

Abstract

This thesis describes an investigation of the physical conditions and molecular abundances in mass-loss envelopes around late-type stars based on millimeter and near infrared spectroscopy, concentrating on the high mass loss, carbon-rich star IRC +10216.

The first part of this thesis is a multi-transition study of the rotational spectrum of SiS from the IRC +10216 envelope. The observational results are described in Chapter 2. A general numerical model to calculate the excitation and radiative transfer for rotation and vibration-rotation lines in circumstellar envelopes is developed in Chapter 3.

Applying this model to fit the observed SiS lines from IRC +10216 (Chapter 4), we find that the undissociated abundance [SiS]/[H₂] in the shielded inner regions of the circumstellar shell is ~2.4 x 10^-7, roughly a factor of 100 smaller than that predicted previously by chemical models. The SiS density falls steeply with radius, reaching one-half the undissociated abundance at r = 9 x 10¹⁶ cms. We show that a) Si, not S depletion is responsible for the low SiS abundance, and b) most of the available Si has probably been used up in forming SiC grains in the cool IRC +10216 atmosphere.

Variations in the 13 µm infrared flux over the 644 day light-cycle of IRC +10216 produce observable changes in some of the SiS lines. Sharp cusps in the J = 5-4 line wings, observed with the Onsala 20 m antenna, are explained in terms of weak maser emission at maximum light. The model predicts a different shape for this line at minimum light, in excellent agreement with a recent Onsala observation. Line coincidences, involving 13 µm transitions of HCN and C₂H₂ (not considered in the model), may be responsible for poor fits to some of the SiS rotational spectrum.

A sensitive search for SiS J = 7-6 and J = 6-5 lines in other carbon-rich, oxygen-rich, and S-type circumstellar shells was also conducted (Chapter 2). Three new SiS sources, CIT 6, CRL 2688 and IRC +20370, all of which are carbon-rich were detected, in agreement with the chemical equilibrium models predicting SiS to be significantly more abundant in a carbon-rich environment than in an oxygen-rich environment.

The second part of the thesis (Chapters 5 and 6) deals with the investigation of physical conditions in the inner (r ~ 2") envelope of IRC +10216 -- a scale size that has never been observed previously. We have devised a new observational technique, employing an annular aperture (size 2 - 3.45") to measure extended emission from resonant-scattered photons in the 4.6 µm CO vibration-rotation band from IRC +10216. In addition to the expected emission up to the expansion velocity of the envelope, both CO and ¹³CO show evidence of high velocity features in the blue wing of the line, beyond the 15 km s^-1 expansion velocity (Chapter 5).

In order to derive physical quantities from the many P and R branch lines present in the infrared spectra, we develop an analytical model to calculate the excitation of these lines due to radiative pumping by thermal emission from circumstellar dust (Chapter 6). The lines are optically thick, and trapping of line radiation has been included. The model emission intensity is found to vary in a simple manner with radius, velocity, J value of the lower state, and the kinetic temperature (which we expect characterises the populations of the V = 0 rotational levels). Our model shows the V = 1 rotational level populations to be in equilibrium with the kinetic temperature despite being excited by infrared radiation.

We derive the following results for the IRC +10216 inner envelope:

i) The kinetic temperature at radius r = 2" is ~ 244 K, roughly twice the extrapolation of the Kwan and Hill (1977) thermodynamic model. This measurement forces revision of analyses of infrared absorption lines in which the radial information was derived assuming a temperature profile (known a priori).

ii) The gas density in the inner regions of the envelope is characterised by a minimum mass loss rate of

[Equation; see abstract in scanned thesis for details].

Thesis Files