3. Emission from the present mass loss gas
The central emission, present in both lines, presumably arises from the envelope formed during the present mass loss epoch. We have determined the peak positions by fitting two-dimensional Gaussians to the data in the -27.52 range. The result is (2000) = 19h40m5700, (2000) = 32o37´056 and (2000) = 19h40m5700, (2000) = 32o37´059 for the J= and J= data, respectively. These positions agree, within the uncertainty of 05, well with each other and with the Hipparcos position of TT Cyg.
The central J= and 21 brightness distributions are well fitted by circular Gaussians with deconvolved radii at half maxima of 11 (corresponding to 81015 cm at the adopted distance) and 08 (61015 cm) and peak brightnesses of 0.085 and 0.36 Jy beam-1, respectively. That is, the emission is at least partly resolved, and the smaller size in the J= line is consistent with the higher energy requirements for exciting this line. The source fluxes, i.e., the source brightnesses integrated over the source, in the -27.52 interval are 0.8 and 5.2 Jy in the J= and J= lines, respectively. This suggests, at least partly, optically thin emission, or, as expected, that the J= line emission comes from a warmer region than the J= line. The centre position spectra are shown in Fig. 3. The J= spectrum suggests partly resolved optically thin emission, but the J= spectrum indicates a higher optical depth. The systemic and gas expansion velocities are estimated to be -27.3 and 3.8, respectively. The source fluxes correspond to J= and J= line intensities of 0.03 and 0.12 K, respectively, in the IRAM 30 m telescope (in the -scale and assuming Gaussian sources with the estimated sizes). This is consistent with the observed intensities, 0.05: and 0.1 K in the J= and J= lines, respectively (Olofsson et al. 1993).
We have estimated the present mass loss of TT Cyg using these data, and a radiative transfer model that determines the excitation of the CO molecules, in a circumstellar envelope that expands with constant velocity, using the Monte Carlo method. The energy balance equation for the circumstellar gas is solved self-consistently taking into account the CO line cooling (see Schöier, PhD thesis in preparation, for details). Adopting the (uncertain) Hipparcos distance of 510 pc, a luminosity of 2300 obtained from this distance and a derived apparent bolometric magnitude, and a gas expansion velocity of 4, and assuming a stellar temperature of 2700 K, and a CO number abundance with respect to H2, , of 10-3, we estimate a mass loss loss rate of 310. The J= emission is optically thin, but the J= is, at least partly, optically thick. An outer radius of the CO envelope of 31016 cm was used in order to fit the observed radial brightness distributions. This is half of that obtained from the CO photodissociation model of Mamon et al. (1988) for the derived mass loss rate, but it must be regarded as within the uncertainties of this model when applied to low mass loss rate objects. Thus, the present mass loss of TT Cyg lies at the very low end of the mass loss rate distribution obtained by Olofsson et al. (1993) for a sample of (100) optically bright carbon stars. The same applies to the gas expansion velocity, which is only about one third of the median gas expansion velocity of this sample.
© European Southern Observatory (ESO) 2000
Online publication: December 17, 1999