Observational studies of pre-main sequence stars

Abstract

This work investigates selected young stars, paying particular attention to their photometric and polarimetric characteristics. The stars observed represent particular sub-classes of the Orion Population of young stars: stars of about one solar maSSj stars of a few solar masses; a star which is thought to be still accreting matterj and a star which lies away from a star-forming dark cloud.
Data was acquired at ultraviolet, optical and infrared wavelengths, along with optical polarimetric data. The subsequent analysis of data for the well-studied stars can be summarised as follows: the spectroscopic characteristics of the star are defined; possible mechanisms for the photometric variabi 11 ty are discussedj and given the spectral type of the star, the intrinsic flux distribution is determined and the parameters of the optical and infrared emission are thus established. The implications of any polarimetric variability found are also discussed. A possible model of polarisation is discussed and the wavelength dependence of polarisation in eleven young stars is analysed.
It is found that the circumstellar environment plays a role in many of the observed characteristics of the stars studied. Several of the stellar spectra show lines which form in a stellar envelope. Each star is found to be affected by circumstellar extinction and to exhibit infrared emission from circumstellar dust. In most cases the circumstellar dust also gives rise to the optical polarisation. The photometric andlor polarimetric variability exhibited by some of the stars is ascribable to changes in the circumstellar dust shell opacity.