The PTI Carbon Star Angular Size Survey: Effective Temperatures, Linear Radii, and Oblateness

We report new interferometric angular diameter observations of 41 carbon stars observed with the Palomar Testbed Interferometer (PTI). Two of these stars are CH carbon stars and represent the first such measurements for this subtype. Of these, 39 have Yamashita (1972, 1975) spectral classes and are of sufficiently high quality that we may determine the dependence of effective temperature on spectral type. We find that there is a tendency for the effective temperature to increase with increasing temperature index by ~120K per step, starting at T_EF 2500K for C3,y. Overall, the median effective temperature for the carbon star sample is found to be 2800+-270K, and the median linear radius is 360+-100 R_sun. We also find agreement on average within 15K with the T_EFF determinations of Bergeat (2001, 2002a,b), and a refinement of carbon star angular size prediction based on V & K magnitudes is presented that is good to an rms of 12%. A subsample of our stars have sufficient {u,v} coverage to permit non-spherical modeling of their photospheres, and a general tendency for detection of statistically significant oblateness with increasing signal-to-noise of the interferometric data is seen. The implications of most carbon stars being spherically oblate is considered in the context of angular moment history and mass loss rates. In particular, compared to the dM/dt rates found in Claussen (1987), a possible correlation between increasing mass loss rates and increasing oblateness is observed.