Contents of: VI/111/./abstract/TPRUSTI_VARIABLE.abs

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 Understanding the circumstellar environment around young stars is of
 crucial importance for resolving the initial conditions prior to planet
 formation. There is a growing evidence of dust concentrations revolving
 around so called Algol-type young stars. These stars are somewhat more
 evolved intermediate-mass pre-main-sequence (PMS) stars thus being the
 predecessors of stars showing the Vega phenomenon. An important link
 between the PMS phase and the Vega phenomenon is the proposed cometary
 activity. Grinin et al. (1991) explained the Algol type optical
 behaviour with proto-comets while Beust et al. (1991) modelled optical
 spectral features with infalling comets. Prusti & Mitskevich (1994)
 discovered far-infrared variability in the Algol type PMS stars and
 linked it with dust arriving in the vicinity of the star along an
 elliptical orbit. Similar kind of dust replenishment mechanism may play
 a role in Vega which has too small grains, with respect to the age of
 the star, in its circumstellar environment (van der Bliek 1994).
 Intermediate mass stars clearly show activity in their circumstellar
 environment and the IRAS work by Prusti & Mitskevich (1994) has
 demonstrated that the activity is easily measurable with ISO.

 We are proposing a straightforward light curve monitoring programme for
 Algol type PMS stars and the three most prominent Vega type
 main-sequence objects. The wavelength dependence of the variation will
 give an access to the temperature of the dust causing the variation and
 the multi-wavelength amplitudes provide information about the masses
 involved. ISO is the only possibility in the near future to provide the
 crucial information about the processes in the circumstellar environment
 of intermediate mass stars: we should use this opportunity to collect
 the data and record the phenomenon.

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