1995A&A...301..439R

We recorded lunar occultation events of the two carbon stars TX Psc and Y Tau. In the case of TX Psc, seven lightcurves were recorded from different sites in the period 1992-1994, at wavelengths ranging from 0.55µm to 3.6µm. In the case of Y Tau, one occultation event was recorded independently at two sites at 2.2µm and 3.6µm. Our observations essentially confirm and refine previous photospheric angular diameter results for these two stars obtained by lunar occultations (Lasker et al. 1973; De Vegt 1974; Dunham et al. 1975 for TX Psc, and Ridgway et al. 1977; Blow 1982; Schmidtke et al. 1986 for Y Tau). The good SNR of some of the traces, and the availability of independent observations, allow us to investigate at the highest angular resolution ever achieved on these stars (≃0.0006" on average), details such as possible deviations from circular symmetry, and possible presence of circumstellar emission. The weighted average of the best fits to occultation lightcurves yields a uniform-disc diameter of 8.38±0.05 mas for TX Psc, and 8.21±0.08 for Y Tau. However, we also present the results of model-independent data analysis. At least for TX Psc these reveal substantial departure from the simple model of circular disk, which we interpret as due to the presence of warm dust immediately adjacent to the stellar photosphere, and/or of large cold spots on the photosphere itself. We suggest that a circular disc is inadequate to describe the brightness profile of TX Psc at both visual and near-infrared wavelengths. The recent determination by optical interferometry of the angular diameter of TX Psc by Quirrenbach et al. (1994), based on such an assumption, may have been biased and the effective temperature should be correspondingly revised. Our results indicate that it is in the range 3000-3150K, in good agreement with some recent theoretical estimates. In the case of Y Tau, somewhat surprisingly, no significant presence of circumstellar emission is revealed at the wavelengths of our observations (≲1% of the 3.6µ flux in the inner 0.15"). Our determination of the angular diameter is consistent with previous suggestions (Schmidtke et al. 1986) that a regular pulsation of this star's photosphere with the phase in its variability period may have been detected.