The circumstellar discs around  Bootis stars

Inga Kamp  & Ernst Paunzen 

¹ Institut für Theoretische Physik und Astrophysik, Universität Kiel, D-24098 Kiel, Germany

² Institut für Astronomie der Universität Wien, Türkenschanzstr.17, A-1180 Wien, Austria

The  Bootis stars form a group of metal-weak A-type stars named after their prototype  Boo (HD125162). Their spectra reveal a wide range of metal underabundances while the lighter elements carbon, nitrogen and oxygen have approximately solar abundances. Venn & Lambert (1990, ApJ 363, 234) noted that this abundance pattern is reminiscent of the interstellar gas, in which the more refractory elements, like silicon and iron, tend to become locked up in interstellar grains, while the lighter elements carbon, nitrogen and oxygen remain in the gas phase. They proposed the ``accretion hypothesis'' according to which the metal deficiency of the  Bootis stars is due to the preferential accretion of metal-depleted gas from a circumstellar or interstellar environment.

Several facts point to an explanation of the  Bootis stars in terms of a pre-main-sequence evolutionary phase:

• the discovery of  Bootis stars in the Orion OB1 association, in young open clusters and among Ae stars (Gray & Corbally 1993, AJ 106, 632; Gray & Corbally 1997, BAAS Vol.29, No.5, 1286)
• the high incidence of circumstellar gas and the preference of rapid rotation for  Bootis stars (Holweger & Rentzsch-Holm 1995, A&A 303, 819)
• on the basis of the new Hipparcos data the  Bootis stars lie very close to the main sequence (Paunzen 1997, A&A 326, L29)

We present ISOPHOT measurements for the five  Bootis stars HD125162 ( Boo), HD142703 (HR5930), HD192640 (29Cyg), HD204041 (HR8203), and HD221756 (15And); for two of them, namely HD125162 and HD192640, we have also ISO SWS spectra.

On the basis of disc models (Rentzsch-Holm, Holweger & Bertoldi 1998, ASP Conf. Ser. 132, 275) similar to those for the more massive discs around T Tauri stars we will derive disc masses by fitting our new data and old IRAS flux measurements. Once having fixed the disc mass we obtain the chemical composition of the  Bootis circumstellar environment from a detailed chemical network implemented in the above mentioned models. This will allow us to draw further conclusions on the nature of the `` Bootis phenomenon''.