Carbon Stars in the Large Magellanic Cloud: Luminosities, Colors, and Implications for the History of Star Formation

Abstract

We present RI and JHK photometry for 888 and 204 carbon (C) stars, respectively, of the 1035 C stars found by Blanco and his collaborators in 52 fields of the Large Magellanic Cloud (LMC). The results of our analysis of the data fall into two categories: (1) Derivation of the physical properties of the stars and a comparison with models. (2) The variation in C star properties with position in the LMC and implications for the history of star formation. For the 197 stars with data in all 5 photometric bands, we derive an equation that gives m_bol_(+/-0.34 mag) from the R_0_ and I_0_ data alone. With m_bol_ for 895 LMC C stars we derive a luminosity function that is closely similar to those for previous (but an order of magnitude smaller) samples of both field and cluster LMC C stars. We find only two C stars brighter than m_bol_ = 12.5 and fewer than 10 fainter than 15.5. A comparison of our derived bolometric magnitudes and effective temperatures for the LMC C stars with the models of Lattanzio [ApJ, 311,708(1986); ApJS, 76,215 (1991)] leads us to conclude that ~1 M_sun_ is the minimum mass required to produce a Population II C star. In addition, the observed lower limit we find to the C star luminosities corresponds to the luminosity at which a 1 M_sun_ Pop II star is predicted to have its first major thermal pulse. From a comparison of field and cluster C star color-magnitude diagrams, we conclude that the range in age and metallicity of the LMC field C stars is at least as great as those from LMC clusters. The metallicity range of the field C stars, though, appears to extend to a significantly higher value based on our finding that red C stars with (J - K)_0_ > 1.9 are three to four times more common in the field sample than in cluster stars and a similar difference previously noted between field and cluster M giants [Frogel & Blanco, ApJ, 365, 168 (1990)]. For each field observed we derive a luminosity m_bol_(t) that should be related to the transition luminosity between M and C stars. We find that the m_bol_(t) values are comparable to those found by FMB for SWB type V-VI clusters and are at least a magnitude fainter than those typical of SWB II-IV clusters. Furthermore, we find that these values of m_bol_(t) get brighter with increasing distance of the field from the LMC's bar. Such a result would be expected if the upper limit to C star ages decreased as one approached the periphery of the LMC by an amount corresponding to an increase of 30% in the minimum main-sequence-turnoff mass. We do not find any other statistically significant variations with position in the properties of the C stars