Temporal variations in UV spectra of the red giant C star, TW Hor.

Abstract

The time behavior of ultraviolet emission lines in the cool carbon star TW Hor is discussed. The emission lines were obtained over a three year period using the International Ultraviolet Explorer (IUE) spacecraft in the wavelength range 2100-3300 A. All the lines are found to vary in time (on scales of about one hour to two months) without any apparent correlation. The most important variations in line intensity were found in the Mg II U1 doublet at 2800 A, and in the Fe Vl lines around 3280 A. Some possible physical mechanisms for the observed variations are discussed, including short period acoustic waves and slow moving acoustic magnetohydrodynamic (MHD) waves. The IUE emission spectra of TW Hor are reproduced in graphic form to illustrate the time variations.