CO J=3→2 Observations of IRC+10216

Abstract

The circumstellar envelope of the old star IRC+10216 has become an important testing ground of theoretical models which seek to determine the physical properties of stellar winds. It is in particular important to evaluate accurately the mass loss rate from this class of stars, as they may play an important role in the replenishment and enrichment of the interstellar medium. The mass loss rate is one of the parameters that goes into radiative transfer models which calculate the thermal structure of the gas together with molecular line excitation and emission self-consistently. The prototype of these models is that of Kwan and Linke (1982) (KL), which has been applied to the case of CO emission in its lower two rotational transitions, 3=1→30 and 3=2→1. This model can explain adequately the main features of the observed lines; however, there are some small discrepancies in the variation of the line intensities with impact parameter. The resulting best fit parameters are a compromise solution, which somewhat overestimates the intensities at the center of the source, and underestimates them in the outer envelope. KL point out that one of the possible reasons for the discrepancies is that the beam patterns of the telescopes used may have contained close- in sidelobes that picked up power from the central region of the source, even when the telescope was pointed quite far away from the central position, and thus lead to an enhanced antenna temperature when the outer regions of the source are observed. In the work of KL, the nature of these sidelobes was sufficiently uncertain that the corrections deduced were very uncertain. There is therefore an incentive to observe this source using antennas with very accurately known beam patterns; optical telescopes fulfill this requirement, since their surfaces are virtually perfect at millimeter wavelengths. Unfortunately the beam sizes obtainable at millimeter wavelengths with optical telescopes are usually too large to constitute useful probes; one therefore has to observe at the highest possible frequency to offset this problem.