Astron. Astrophys. 328, 602-605 (1997)

2. The Ursa Major group

One of the difficulties to diagnostic a circumstellar Ca II absorption at the stellar velocity is the possible contamination by interstellar gas. In that respect, the Ursa Major Group offers a combination of favorable properties. First, the Ursa Major region is located close to the galactic pole, and very little interstellar gas is present in that direction. For example, Hobbs (1978) gave an upper limit on the interstellar Na I column density present toward UMa: cm^-2. Thus, if a gaseous absorption were detected toward one of the surveyed stars at the stellar velocity, a circumstellar origin would be very likely, although still to be demonstrated.

Moreover, as the stars belong to the same kinematic stream, they form a sample with similar properties like age, distance or metallicity. Indeed, their age is rather well constrained to be about years (Levato & Abt 1978, Eggen 1983). The Ursa Major Group is very nearby with most of the stars being within 50 parsecs. These bright nearby stars can thus be observed at high S/N ratios and high resolution in relatively short time, even with small telescopes. In addition, very little interstellar gas is expected for such short sight lines. Finally, the Ursa Major Group stars have about the same composition as the Sun ( [Fe/H] =-0.1, Soderblom & Mayor 1993).

For the present program, one of the essential properties of the Ursa Major Group is its well known age. The age of Pictoris is a matter of debate. Paresce (1991) claimed that Pictoris may be metal deficient with an age of about years. However, through observations obtained with HST-GHRS, Lanz et al. (1995) showed that Pictoris is very close to the main sequence. This result has been reinforced by the new determination of the Pictoris distance with the Hipparcos satellite (Crifo et al. 1997). But the age is only constrained to be between years and up to years. Lanz et al. prefer the youngest age because of the presence of the circumstellar disk. However, there is no evidence that this disk is "proto-planetary". Even the Pictoris disk is certainly not the residual of a proto-planetary disk but it is continuously supplied by colliding or evaporating bodies (Weissman 1984). Thus, the presence of the disk cannot allow to conclude about the age of the star. In our own Solar System, evaporating and colliding bodies were active during few years after its formation (Hartmann 1972, Soderblom et al. 1974). The age of Pictoris remains an open question.

In this context, the analysis of A-type stars in the Ursa Major Group can give new insights. The age of the group ( years) is closed to the limit of the oldest range of possible ages for Pictoris. Therefore, if Pictoris is actually in this range, one might expect that some stars in the Ursa Major Group show similar characteristics. Well before the discovery by IRAS of dust shells around a large number of main sequence stars, Witteborn et al. (1982) did a pioneering and prescient search of infrared excess expected from debris disks in the Ursa Major Group. Yuan & Backman (1993) showed that, although IRAS was unable to detect emission from individual A-Type stars in Ursa Major Group, these stars may have on average a small 12 and 25 m emission excess above their photospheric emission. Clearly evolved circumstellar disks might be present around these stars. Further comparison with Pictoris is needed to search for spectroscopic similarities.

Spectroscopy is in fact able to detect Pictoris -like activity, even where IRAS failed to detect the presence of large amount of dust. It is obviously the case of HR 10 and HR 2174 for which spectral signature are clearly detected (Lecavelier des Etangs et al. 1997b), although infrared excesses are only marginally suspected on IRAS scans (e.g. Cheng et al. 1991). The absence of correlation between the infrared excess and the presence of circumstellar gas features shows that spectroscopic survey are still justified even for targets which does not present IR excess.

© European Southern Observatory (ESO) 1997

Online publication: March 26, 1998

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