Interferometric Mapping of SIO J=2-1 V=0 Emission from the Mass-Loss Envelopes of Red Giants
Abstract
Radiation-pressure on grains is widely held responsible for accelerating the mass-outflows in red giants to their terminal velocity - however, the question of how and where circumstellar dust condenses still remains unanswered. In oxygen-rich envelopes, refractory elements like Si are the probable constituents of grains, thus observations leading to a determination of the radial abudnance of Si-bearing molecules, such as SiO, can provide a unique probe of dust-related physical processes. Though the depletion of Si can occur through the condensation of substantial amounts of solid silicates close to the star, indicated by the rpesence of a broad 9 micron spectral feature, the strong SiO masers observed in these sources require a substantial fraction of the Si coming off the star to be in SiO- an alternate hypothesis suggests that SiO is depleted by adhesion onto cold grains at large circumstellar radii (approx. 1016 cm). In order to investigate th depletion of SiO in circumstellar envelopes, we have used the Hatcreek millimeter interferometer (B-array) to map the SiO J=2-1 (v=0) 86 GHz line emission from 3 nearby red giants IK Tau, RX Boo, and Chi Cyg, with spatial resolution approx. 7". The emission is largely circulalry symmetric, and marginally resolved (Fig. 1a, 2a). The deconvolved source-sizes are roughly less than or equal to 4.5" (approx. 1016 cm). The on-source spectra show rounded profiles, indicating optically-thick emission (Figs. 1b, 2b) with line-center intensities up to approx. 20K. The small spatial extent of the emission, high brightness temperature and large optical depth, imply that [SiO]/[H2] starts decreasing significantly at radii approaching 1016 cm - favouring the depletion of SiO by adhesion onto grains at this radius. Modelling of IRAM 30m (27" HPBW) SiO J=2-1 (v=0) data for several red giants (including our objects), also shows that [SiO]/[H2] is higher in the inner regions of the envelopes. Out future A-array mapping (resolution 2") will show whether or not the SiO abundance continues to increase for r , 1016 cm. detailed modelling (with) a radiative transfer program is in progress, in order to estimate alpha - the grain sticking probability, and Tbind - the binding energy of SiO onto grains. The on-source SiO profile in IK Tau is anomalosuly shapedm with a central narrower (in velocity) compoennt on top of a broader component - the velocity width (at zero-power) of the latter is similar to taht seen in the CO emission towards this object. The high-excitation SiO J=8-7 lien sin IK Tau and RX Boo (unpublished JCMT data - Sahai and Wannier, private communication), also show the same anomalous shape. The 2-component profiles may result from the presence of large-scale inhomogeneities, a second mass-ejection, or re-accleration in the outflow due to increased opacity of accreting grains to the stellar radiation.
- Publication:
-
From Miras to Planetary Nebulae: Which Path for Stellar Evolution?
- Pub Date:
- 1990
- Bibcode:
- 1990fmpn.coll..167S
- Keywords:
-
- Mass-Loss;
- Circumstellar Matter;
- Red Giants;
- Molecular Lines;
- Interferometry;
- Dust Grains;
- Grain Depletion