The dusty environments of main sequence stars presenting IR excess (Aumann et al. 1984) have been subject of detailed analysis (Backman & Paresce 1993). Until very recently, the Pictoris disk remained the only disk imaged around a main sequence star despite a large survey of more than hundred stars presenting an IR excess (Smith et al. 1992). Now dust has also been observed around 643, but it is a very young star and this dust may have a different nature (Kalas & Jewitt 1997). The particularity of Pictoris is not the consequence of the favorable edge-on geometry of the disk, nor of its short distance, but suggests that this star is very peculiar among the nearby ones surrounded by dust disks (Kalas & Jewitt 1996).
The gaseous counterpart of the Pictoris dust has been observed in both visible and UV lines (see review by Vidal-Madjar & Ferlet 1995). The observed spectral signatures led to the explanation that the gas content was probably produced by the evaporation of many small bodies (Vidal-Madjar et al. 1994). However, the presence of the Ca II stable component around Pictoris is still very puzzling. The radiation pressure must have expelled this gas very quickly, except if some large amount of gas is opposed to the pressure. Neither the physical process providing the stability observed over more than twelve years, nor the origin of the suspected gas were extensively explained (Lagrange 1995). Numerical modeling to solve this problem are still underway (Lagrange et al. 1997).
On the other hand, gas is routinely observed around young pre-main sequence stars. Infalling gas also has been detected in moderate resolution spectra of Herbig Ae/Be stars (Pérez et al. 1993), UX Orionis (Grinin et al. 1996), post Herbig Ae/Be stars and shell stars (Grady et al. 1996). Gas is also detected at stellar velocity around shell-stars and some Pictoris -like stars (Lagrange et al. 1990). Unfortunately, the connection with the Pictoris phenomenon is not clear and/or the stellar ages are not well constrained. In order to constrain the gas history and to have a more complete knowledge of the status of Pictoris and its gaseous disk, one should compare it with other main sequence stars of similar type and well-known age. This has already been done for the IR excess and dust thermal emission at submillimeter wavelength (Yuan & Backman 1993, Zuckerman & Becklin 1993). For spectral study of the gaseous absorption lines, a sample of stars with as large as possible is required to avoid geometrical effects which give only negative results for disks not seen edge-on.
Five major questions should be addressed concerning the status of gaseous disks.
1) What is the link between the gas disks and the dust disks? Are we observing two different phenomena, both present around Pictoris ?
2) Is there a connection between the hot gas detected through ionized gas or the corresponding redshifted absorptions, and the cold gas at much lower temperature like CO or C I (, Vidal-Madjar et al. 1994)?
3) Is Pictoris a very peculiar object with a particular configuration (for instance presence of strong secular resonances in a planetary system as suggested by Levison et al. 1994), or is it simply surrounded by a massive disk in the tail of the mass distribution of disks surrounding other similar stars?
4) What is the evolutionary status of Pictoris ? Is it a pre-main sequence star or is it older than the ZAMS stars?
5) What is the variation of the gas content as a function of time?
Here we present an optical spectroscopic survey to search for Ca II circumstellar gas at the stellar velocity in the Ursa Major Group. It will certainly not answer the above questions, but it is expected to bring some new understandings. The target properties are presented in Sect. 2; the observations and data analysis in Sect. 3; results and discussion in Sect. 4and 5respectively.
© European Southern Observatory (ESO) 1997
Online publication: March 26, 1998