SUBMILLIARCSECOND RESOLUTION OBSERVATIONS OF 2 CARBON STARS - TX-PISCIUM AND Y-TAURI REVISITED

We recorded lunar occultation events of the two carbon stars TX Psc an d Y Tau. In the case of TX Psc, seven lightcurves were recorded from d ifferent sites in the period 1992-1994, at wavelengths ranging from 0. 55 mu m to 3.6 mu m. In the case of Y Tau, one occultation event was r ecorded independently at two sites at 2.2 mu m and 3.6 mu m. Our obser vations essentially confirm and refine previous photospheric angular d iameter results for these two stars obtained by lunar occultations (La sker et al. 1973; De Vegt 1974; Dunham et al. 1975 for TX Psc, and Rid gway et al. 1977; Blow 1982; Schmidtke et al. 1986 for Y Tau). The goo d SNR of some of the traces, and the availability of independent obser vations, allow us to investigate at the highest angular resolution eve r achieved on these stars (approximate to 0''.0006 on average), detail s such as possible deviations from circular symmetry, and possible pre sence 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 pr esent the results of model-independent data analysis. At least for TX Psc these reveal substantial departure from the simple model of circul ar disk, which we interpret as due to the presence of warm dust immedi ately adjacent to the stellar photosphere, and/or of large cold spots on the photosphere itself. We suggest that a circular disc is inadequa te to describe the brightness profile of TX Psc at both visual and nea r-infrared wavelengths. The recent determination by optical interferom etry of the angular diameter of TX Psc by Quirrenbach et al. (1994), b ased on such an assumption, may have been biased and the effective tem perature should be correspondingly revised. Our results indicate that it is in the range 3000 - 3150 K, in good agreement with some recent t heoretical estimates. In the case of Y Tau, somewhat surprisingly, no significant presence of circumstellar emission is revealed at the wave lengths of our observations (less than or similar to 1% of the 3.6 mu flux in the inner 0''.15). Our determination of the angular diameter i s consistent with previous suggestions (Schmidtke et al. 1986) that a regular pulsation of this star's photosphere with the phase in its var iability period may have been detected.