HIGH-DISPERSION SPECTROSCOPY OF THE MOST LUMINOUS F-AND G- TYPE SUPERGIANTS IN THE LARGE MAGELLANIC CLOUD AND THE MILKY WAY.

Abstract

ABSTRACT High-resolution spectroscopic observations of the most luminous F- and G-type supergiant stars in the Large Magellanic Cloud and in the Galaxy are compared. High-dispersion echelle spectrograms, at dispersions of 2.5 and 5.1 A/mm, were obtained together with optical and infrared photometry for the four brightest LMC supergiants and seven Galactic supergiants of comparable spectral types and luminosities. The LMC supergiants are all observed to exhibit line doubling in the Na I D lines, while no evidence for circumstellar line cores or line doubling is seen in either Ca I 4226 A or Sr II 4077 A. The galactic yellow supergiants also show evidence for broad diffuse or doubled lines. The Ca II H and K core emission profiles along with the doubled lines indicate extensive circumstellar envelopes, and lead to estimates of mass-loss rates in excess of 0.0001 solar mass/year. It is suggested that this mass loss rate may be explained by an outer atmospheric structure consisting of an inner shell in which material circulates between an extended atmosphere and the photosphere, and an outer shell from which matter outflow takes place.