Grain Formation in the Expanding Envelopes of T Tauri Stars

Abstract

The possibility of the nucleation in the expanding envelopes of T Tauri stars is investigated by using the nucleation theory by Becker and Doring. The nucleation time scale r,. in .the gas is investigated assuming that only graphite is condensable. The expansion time scale roxo of the gas ejected from a T Tauri star is calculated assuming that the gas flow is steady and spherically symmetric. The time scale r,. is compared with roxp for Xo=l0- 2 (Xo is the concentration of carbori by mass) assuming that the gas expands adiabatically, and the mass­ loss rate needed for nucleation is found to be larger than those observed in T Tauri stars. Thi~ suggests that nucleation around T Tauri stars is rather difficult. § I, Introduction It is generally believed that grains ·are contained m interstellar and proto­ planetary gas clouds. The importance of the existence of grains has been pointed out in connection with the cooling mechanism during the contraction of the gas cloud and with the origin of protoplanets. 1> In spite of the importance, however, the formation of grains has not yet been understood well. A few authors have .Pointed out the possibility that grains are formed in atmospheres of cool stars or in expanding shells of active stars such as T Tauri stars, planetary nebulae, novae and supernovae.2>-4> This is a:lso suggested by the infrared excesses of active stars. 5>-u> Hoyle and Wickramasinghe 2>•8> have investigated the conditions for the forma­ tio'n and the growth of solid particles in the static stellar atmospheres of N-type stars. Kamijo14) and Donn et al. 15> have re-examined Hoyle and Wickramasinghe's mechanism in cool carbon stars. On the other hand, Fix16 > has studied the forma­ tion of grains in the adiabatically . pulsating atmosphere. He has shown that if the pulsation period is longer than a few months, the grains are condensed in expanding phases of pulsation. In this paper we will investigate the possibility of the formation of grains in the expanding envelopes of T Tauri stars using the nucleation theory by Becker and Doring 17> and others. 18>-20> The reasons why we take notice ofT Tauri stars are as follows: Firstly, some of them are known as infrared stars 11> so that grains may exist in the circumstellar spaces around T Tauri stars. The mass-loss rates are known from the observations together with the initial velocities of ejected matter and stellar radii. 21 > Hence we can easily compare our estimation with