IR photometry and dust-shell models for two carbon stars

Abstract

We present JHKLM photometry of the carbon stars ST And and T Lyn acquired in 2000- 2010. Along with brightness variations due to pulsations, changes on timescales of 2000-3000 days are also observed. Our combined light curves can be satisfactorily represented with light elements derived from visual observations, but the maxima are delayed relative to the calculated times. A color-index analysis demonstrates that the dust shell of ST And is fairly weak, and is manifest only episodically, while the presence of hot dust was always detected for T Lyn. These results confirm models of spherically symmetric stellar dust shells based on mean-flux data, supplemented with observations in the intermediate IR from the IRAS and AKARI satellites. The visual optical depth of the relatively cool dust shell of ST And assuming a dust temperature at the inner edge of T1 = 510 Ki s very low:τV =0 .047. The dust shell of T Lyn is considerably hotter (T1 = 940 K), with τV =0 .95. We estimate the mass-loss rate to be 1.8 × 10 −7 M� /year for ST And and 3.7 × 10 −7 M� /year for T Lyn.