Péter Ábrahám, Timea Csengeri, Attila Juhász, and Ágnes Kóspál
INFRARED VARIABILITY AS A NEW POSSIBILITY TO EXPLORE CIRCUMSTELLAR DISK STRUCTURE (Poster)

INFRARED VARIABILITY AS A NEW POSSIBILITY TO EXPLORE CIRCUMSTELLAR DISK STRUCTURE

Péter Ábrahám(1), Timea Csengeri(2), Attila Juhász(2), and Ágnes Kóspál(1)
(1) Konkoly Observatory of the Hungarian Academy of Sciences, P.O. Box 67,
H-1525 Budapest, Hungary
(2) Eötvös Loránd University, H-1518 Budapest, P.O. Box 32, Hungary

Young pre-main sequence stars typically show variability at optical wavelengths, which is mainly due to stellar spots or variable extinction along the line of sight. At longer wavelengths these effects become less important, and in the mid- and far-infrared regimes - where the bulk of emission is originated from the circumstellar matter - young stellar objects (YSOs) are usually assumed to exhibit constant brightness.

With the availability of a growing number of ground-based and space-born infrared observations, however, infrared variability among YSOs can be reconsidered. We selected several intermediate-mass Herbig Ae/Be stars as well as FU Orionis and EX Lupi type objects, and looked for temporal changes in their 1-100$\mu$m infrared spectral energy distributions (SEDs). For each objects we collected infrared photometric data from the literature (IRAS, MSX, ISO, Spitzer, as well as MNQ-band ground-based observations), and constructed SEDs for different epochs. Measurements performed by ISOPHOT, the photometer on-board the Infrared Space Observatory, were re-evaluated using methods developed in our group. Special care was taken when data from different instrument were combined (colour correction, beam differences). For several objects variability at infrared wavelengths was found in the data, and in these cases we analysed also the wavelength-dependence of the variations.

In the literature there exist models of YSOs, which assume a certain circumstellar geometry (disk, envelope) and produce an infrared SED. We show that - with some assumptions - these models can also be used to predict how the star+disk system would react, as a function of time, on variations of the emission from the central illuminating source. In those cases when the luminosity of a YSO changes with time (e.g. due to variable accretion rate) one can compare the synthetic time-dependent SEDs with multiepoch infrared observations. This comparison may place strong constrains on some basic assumptions of the model, and can serve as an efficient diagnostic tool of the circumstellar structure. In the contribution we show several examples to illustrate the application of the method.