The PTI Carbon Star Angular Size Survey: Effective Temperatures and Non-Sphericity

TL;DR

This study provides new interferometric measurements of 41 carbon stars, revealing their effective temperatures, radii, and non-sphericity, with implications for understanding their surface structures and evolution.

Contribution

First interferometric angular size measurements for CH carbon stars and a detailed analysis of temperature, size, and shape variations among carbon stars.

Findings

Effective temperature increases with spectral type by ~120K per step.

Median effective temperature is 2800K, median radius is 360 R_SUN.

Many carbon stars show non-spherical shapes, possibly linked to surface inhomogeneities and rotation.

Abstract

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_EFF ~= 2500K for C3,y, although there is a large amount of scatter about this relationship. 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 departures from sphericity with increasing signal-to-noise of the interferometric data is seen. The implications of most - and potentially all - carbon stars being non-spherical is considered in the context of surface inhomogeneities and a rotation-mass loss connection.