Other
Scientific paper
Jun 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996phdt........50d&link_type=abstract
PhD thesis. Univ. Amsterdam , (1996)
Other
15
Scientific paper
The thesis consists of several studies on candidate young stars of which most material is published or in press and which can be divided into three parts roughly. Part A is about Herbig Ae/Be stars. A complete review of the observational properties of HAeBes is given in Chapter A1 together with a renewed up-to-date catalogue of HAeBes and HAeBe candidates. As an example of the selection of HAeBes from candidate stars, the observational properties of three candidates is discussed in Chapter A2. They are in particular interesting as they are relatively bright with respect to other HAeBes candidates. An advantage of bright HAeBes is that high resolution spectroscopy can be obtained. For two well know HAeBe objects with a favourable oriented disk, UX and BF Ori, a high resolution spectroscopy monitoring programme is presented in Chapters A3 and A4. First results presented indicate that the disk material of UX Ori is accreting in the form of comet-like bodies. Such pioneering results are also found for BF Ori but more details of the cometaries are given. As discussed in Chapter A1, the IR-excess is one of the fundamental discriminators for the selection of HAeBe candidates. A good understanding of the origin of the IR-excess of HAeBe candidates is necessary to study the disk material that ultimately could produce (proto-)planetary systems. Chapter A5 discusses the amount of IR-excess of HAeBe candidates and ideas about the probable origin. In Part B objects are discussed which were originally selected as HAeBe candidates, but for which a more detailed analysis of the observational characteristics show that they are probably more evolved. This group contains very interesting objects as is shown in Chapters B1, B2 and B3, in which the discovery of a new galactic Luminous Blue Variable (LBV) is reported, WRA 751. A well known B[e] star is HD 45677. The B[e]-group was collected to consist of evolved objects with masses less than those of LBVs and comparable with B[e] stars observed in the LMC. In recent publications, however, HD 45677 was described as a possible Herbig Be star. In Chapter B4 new observational evidences together with the analyses of about 100 years of known brightness measurements of this star indicate that its PMS nature must be questioned. Another object for which the PMS status is doubtful is HD 147196. A Be star located in the dark cloud region ρ Ophiuchus. In Chapter B5 we show that the emission line nature of this object is variable, which indicate the difficulties to select homogeneous samples on the bases of spectral observations. Finally in Chapter B6 we discuss the possible youth of HR 6000, an object not showing any observable peculiarities at first sight. But being the close neighbour of HR 5999, a comparable youth is likely. Indeed, a weak near-IR excess, photometric variability and being a strong X-ray source, suggest the presence of a T Tauri companion. In Parts A and B we have encountered various difficulties to make a clear and easy distinction between PMS stars and more evolved objects. In the case of young open clusters such problems are less severe. For this reason in Part C a study of the well known very young open cluster NGC 6611 is presented. The results are reported in two chapters: in Chapter C1 the stars in the cluster field are studied, from which a HRD can be constructed, giving necessary information about some cluster properties such as distance and age; we use these findings in Chapter C1 to study in detail objects which were previously recognised as PMS candidates, in order to discover true HAeBe objects. Although we find a large number of early type stars being in the PMS phase, we find only scarcely objects with clear HAeBe characteristics. It is therefore discussed that the clearing mechanism on the circumstellar material must work on a very short timescale and that not all of them go through a HAeBe-phase. This conclusion is discussed with an eye to the recent finding af EGGs in the field of this cluster.
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