Other
Scientific paper
Sep 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993phdt........96f&link_type=abstract
PhD Thesis, Observatorio Astronómico de Madrid, and Universidad Autónoma de Madrid, 1993.
Other
2
Scientific paper
We have studied the optical continuum variability of 24 pre-main sequence objects and its relation to the accretion and mass loss processes. We have selected a sample of 24 low and intermediate mass pre-main sequence objects (classical T Tauri stars and Ae/Be Herbig stars), some of which show evidences of high mass loss rate. In order to study the optical continuum variability we carried out photometric observations, UBV(RI)c system, for four years (July 1988 - August 1992). We have also carried out spectroscopic observations of the Hα region (July 1989 - August 1990). The variable stars of our sample show an irregular continuum variability. This irregular behaviour is not an observational bias since the temporal interval between consecutive measurements is shorter than the time scale of the variations. We have proved that, despite this irregular nature we can get some conclusions on the origin of the variability from the comparision between the observational results and the stellar spots models or the inhomogeneous circumstellar model. From the results of these comparisions we find that for one classical T Tauri star the observational results can be reproduced by a cold spot and for another 5 by hot spots. Nevertheless, for these 5 stars the variability in the U band (3600Å) cannot be reproduced by the hot spot model, there must be another mecanism that causes the anomalous behaviour at short wavelengths. The variable Ae/Be Herbig stars do not always show a regular behavior with wavelength so we can only assign an origin for the variability of two of them. Our observational results suggest a trend: the larger the mass loss rate, the larger the amplitud of the optical continuum variations. This trend is analyzed in detail through the comparision between the amplitud of the optical continuum variations and the equivalent width of the Hα emission. The optical continuum arises from the stellar photosphere, while the Hα emission is assigned to the internal regions (close to the star) of the wind. For the Ae/Be Herbig stars there is not a clear relation between both quantities, but we have found a direct relation for the classical T Tauri stars: for those objects with Hα equivalent width lower than 30Å we found variations lower than 0.1 mag and for the others there is a direct relation between the continuum variability and the equivalent width of the Hα emission. Taking into account the correlation found by Cabrit et al. (1990) between Hα emission and the excess of infrared luminosity, that is, between mass loss and mass accretion, we conclude that there is a relation between the optical continuum variability and the accretion process: for those objects with larger accretion rate the amplitud of the variability is larger (variability due to hot spots). As has been already suggested, these hot spots could be the regions of the stellar surface where the material from the accretion disks falls.
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