P. Abraham 1, Ch. Leinert 1, D. Lemke 1, A. Burkert 2, & Th. Henning 2
1 Max-Planck-Institut fur Astronomie Heidelberg, Germany
2 Astrophysikalisches Institut und Universitats-Sternwarte Jena, Germany
One of the most striking discovery of IRAS was the detection of cool circumstellar matter around main-sequence stars (Vega phenomenon). The larger wavelength coverage and higher sensitivity of ISOPHOT provide a new possibility to study this phenomenon in more details.
We observed the mid- and far-infrared spectral energy distributions of 9 A-type stars from the 300 Myrs old Ursae Majoris group using ISOPHOT and the UKIRT telescope. Our goal was to determine the typical circumstellar dust parameters of intermediate mass stars at an age of 300 Myrs from the analysis of the incidence and masses of dust disks in the UMa group.
We found that only 1 out of 9 stars shows clear signature of circumstellar dust. This one case (beta UMa) was already detected by IRAS at 60 micron. Our data confirm the 60 micron excess, and provide flux densities of the circumstellar emission at 90 micron and 170 micron. The new photometric points can be well fitted by modified blackbody curves with T=50-60 K and emissivity factors of -2 to -1. The derived dust mass associated with beta UMa is estimated to be 0.2 M(moon). For the other 8 stars we derived an upper limit of 0.05 M(moon) for their circumstellar dust. This very low mass limit, as well as the low (1:8) detection rate suggest that the Vega phenomenon is the exception rather than the rule. Comparing our results with an earlier IRAS study we conclude that the relatively high incidence of Vega-like disks observed among A-type field stars in the solar neighbourhood cannot be extrapolated to other regions of the Milky Way.