Physics – Optics
Scientific paper
Mar 1998
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998phdt........36c&link_type=abstract
Ph.D. thesis, Université Grenoble I, (1998)
Physics
Optics
10
Scientific paper
Multiplicity is a major issue in stellar astrophysics. Firstly, any stellar formation theory must explain the large abundance of multiple systems among Main Sequence and young low-mass T Tauri stars. Secondly, binary studies allow the direct determination of physical parameters. In the case of Herbig Ae/Be (HAeBe) stars, the binarity status is not well known; furthermore, direct mass determination are required to test stellar evolution models for these young intermediate mass objects. The first part of the thesis presents the results of a systematic search for HAeBe binaries in both hemispheres. Two complementary techniques were used to cover a large range of orbital period P: high angular resolution imaging with Adaptive Optics (AO) (binary separation ρ between 0.12'' and few arcseconds, i.e. P ≅ many years), and high resolution visible spectroscopy to study short orbital period (P ≅few hours to few months). Among the 68 HAeBe stars observed with ADONIS--ESO and PUEO--CFH AO instruments, 30 binaries (18 discovered) have been detected. 42 HAeBe stars have been surveyed with the CES--ESO and 'ELODIE, AURéLIE--OHP spectrographs. Radial velocity variations were found in 7 targets (4 are new spectroscopic binaries, 3 d. < P < 166 d.). In addition, the 7Li 6 708 Å absorption line (absent feature in simple HAeBe stars spectra) indicates the presence of a cooler companion in 6 HAeBe spectrum binaries, 4 of which are new detections. The observed visual binary frequency for HAeBe stars is of the order of 50%. For short period spectroscopic binaries (P < 100 days), the observed frequency is about 10%. Considering observational bias effects, these estimates are regarded as lower limits for the true HAeBe binary frequency. Based on our multi-color AO images, spectral types of twenty-two visual companions have been determined. A trend is found such that companions of Ae stars are low-mass T Tauri stars (spectral type K--M), while companions of Be stars are intermediate mass stars (A--F). Companions usually have no infrared excess, nor do primaries with massive companions. Furthermore, X-ray emission in some HAeBe stars may well be explained by the presence of a T Tauri companion. However, because of bias effects, great care must be taken about these issues, and complementary observations are needed. Our observations provide clues for binary formation theories, but while fragmentation and capture via a circumstellar disk seem plausible mechanisms, disk instabilities and stellar capture scenarios cannot be ruled out. The second part of the thesis is devoted to the study of TY CrA, the unique triple spectroscopic system among Herbig Ae/Be stars. We found this previously known eclipsing binary to be also a spectroscopic binary of SB2 type (P = 2.9 days), and we obtained the first direct mass determination of an HAeBe star. The orbital motion of a third companion around the central binary has been monitored, and a complete dynamical model of the triple system has been made. Our theoretical investigations show that the stability of the hierarchical system is insured by tidal effects inside the central eclipsing binary. To explain the puzzling subsynchroneous rotation of the primary star, a peculiar orientation, in which the primary is seen pole-on and its rotational axis is perpendicular to its orbital axis, is proposed. The circumstellar environment of TY CrA has been studied. SWS, LWS--ISO data show polycyclic aromatic hydrocarbon emissions (some of them never observed from the ground in TY CrA), and O sc i 63, 146 microns and [C II] 158 micron emission lines. These features may well be explained by the presence of a compact HII region and a photodissociation region associated with TY CrA. Adaptive Optics images in the near infrared obtained with and without coronograph show that the dusty environment must be confined very close to the star (< 0.5'' = 65 AU at 130pc).
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