Infrared Study Of Be And Herbig Ae/Be Stars

This PhD thesis mainly consists of four papers, among them, three papers deal with Be stars and one paper Herbig Ae/Be stars. A common character in these two group stars is presence of infrared excess, which is not attributed to the central star, but its circumstellar material. Thus the infrared study of these two groups has been playing the most important role in our understanding and investigating the physical nature of their circumstellar material.It is generally thought that the infrared excess of a Be star is due to the free-free and/or free-bound radiation from the circumstellar ionized gas and the infrared excess of a Herbig Ae/Be star is due to the thermal radiation from the circumstellar dust. Based on the Infrared Astronomical Satellite (IRAS), the Two Micron All Sky Survey (2MASS) and the Hipparcos Catalog and so on, we have studied almost all known Galactic Be stars. It is noted that the IRAS low-resolution spectra (IRAS LRS) are first used to study Be stars in our work. From the color-color diagrams, spectral types and spectral indices, it can be concluded that: (1) The near infrared excesses in most Be stars are due to the free-free and free-bound radiation from the circumstellar ionized gas, but the far infrared excesses in quite number of Be stars are due to the thermal radiation from the circumstellar dust or the circumstellar reflection nebulae;(2) The infrared excesses in Be stars increase with wavelengths, and there is no directly correlation between IR excess (or circumstallar material) and the basic stellar parameters (e.g. effective temperature and luminosity). (3) The mean spectral index derived for the samples is 1.50 ± 0.34 between J to K bands and 1.14 ± 0.42 between K to A bands, respectively. Based on the trigonometric parallaxes from the Hipparcos Catalog, we have derived the absolute visual magnitudes for 457 Be stars, subsequently we obtain absolute visual magnitudes calibration of Be stars for luminosity classes III, IV and V. It can be found that: (1) Be stars are generally brighter than B stars of the corresponding spectral types; (2) There is no direct correlation between absolute visual magnitudes and the stellar rotational velocity and infrared excesses. Moreover, based on IRAS and 2MASS, we have studied Herbig Ae/Be stars in infrared region. It can be found that, for few stars in far infrared region and some stars in near infrared region, infrared excesses are attributed to the free-free and/or free-bound radiation from the circumstellar ionized gas. Furthermore, the evolution scenario from Malfait et al. (1998) has been checked, our result does not support this evolution scenario.