Most of our targets are A-F type stars with large . It suggests that the inclination parameter is not responsible for the negative result. If we neglect that the present sample is biased toward large (almost 70% of the stars have km s-1), and if we assume that the disks have a opening angle as observed for Pictoris (Artymowicz et al. 1989), disks should have been detected around at least three stars. We can thus roughly conclude that gaseous disks with density larger than one hundredth of the Pictoris disk density are present in less than about half of the Ursa Major Stream stars.
We come back now to the five questions listed in the Introduction. Because the result is negative, we cannot of course address the two first questions about the connection between hot gas, cold gas and dust.
Concerning questions 3 and 4, some information is provided with the non-detection of circumstellar gas in any of the surveyed stars which have the same age of years. If Pictoris is older than the ZAMS stars, then it cannot be surrounded by a classic massive disk; in this respect, Pictoris is clearly a very peculiar object around which an exceptional phenomenon takes place.
However, if Pictoris is a young star ( years), the mass of its disk may merely be due to its age. Then, question 5 about the gas mass versus time can be roughly addressed. Following Zuckerman & Becklin (1993) and assuming a time variation of the form , the present results imply , which is consistent with found by Zuckerman & Becklin (1993) for the dust. This analysis ought to be extended to younger open clusters, but this may be more difficult. For example, the Pleiades cluster is about 108 years old, but Pleiades' stars are about 100 times less bright, and a large amount of absorbing material is present in this direction.
As a matter of fact, the Pictoris age is the major hypothesis to address the issues listed in the Introduction. Either its disk is characterized by an unusual large amount of gas and supplying material, or Pictoris is younger than previously thought.
© European Southern Observatory (ESO) 1997
Online publication: March 26, 1998