## 2. Estimates of the local mass density of brown dwarfs## 2.1. Calan-ESO proper motion surveyThe Calan-ESO proper motion survey has at present led to the
discovery of one brown dwarf, Kelu-1 (Ruiz et al. 1997). The
authors estimate that their effective search volume had a size of 23
pc In Eq. (1) denote the space velocity components of the brown dwarfs in the direction of the galactic center, the direction of galactic rotation and towards the galactic north pole, respectively. are the corresponding velocity dispersions. They are assumed to increase with age as like the velocity dispersions of the luminous stars (Wielen 1977).
The axial ratios of the velocity ellipsoid have been chosen according
to the most recent discussion of the kinematics of nearby stars by
Jahreiß & Wielen (1997) as . The
diffusion coefficients follow the same ratios
and have an absolute value of = 6
10 The expected number density of brown dwarfs in a cone towards the
direction of Kelu-1 ( where is the radial length of the probing
volume investigated by Ruiz et al. (1997). The observed number
density of brown dwarfs in the cone, , can be
expressed similarly to Eq. (3), if the proper motion limit of
0.25 arcsec/yr of the Calan-ESO poper motion
survey is taken into account in the integration over the velocity
components. To implement this proper motion limit one has to transform
the cartesian velocity components , and which changes to 0.28 or 0.44, if is increased or decreased by a factor of 1.6 according to an estimated uncertainty of the distance modulus of 1 mag, respectively. Up to now we have considered only bright and young brown dwarfs. Very likely there are also fainter and older brown dwarfs, which will contribute to the mass budget as well, although they have not yet been found in the surveys. Assuming again a constant star formation rate and an age of the galactic disk of 10 Gyrs we estimate from Eq. (1) Combining this with the estimate (4) we conclude that the contribution of brown dwarfs to the local mass density is 10 times the density of brown dwarfs deduced straightforward from the Calan-ESO survey. The same correction factor is given by Ruiz et al. (1997), although the authors do not explain in detail how they arrive at that estimate. ## 2.2. DENIS mini surveyThe DENIS mini survey with a field size of 230 deg ## 2.3. BRI surveyHigh resolution spectroscopy of very late type stars of the BRI
survey (Irwin et al. 1991) by Tinney (1998) has led to the
identification of the brown dwarf LP 944-20 = BRI 0337-3535. Tinney
(1996, 1998) has also measured the parallax, proper motion and radial
velocity of this brown dwarf. Its age is estimated as about 500 Myrs
(Tinney 1998). The low space velocity of 7 km/s is consistent with
such a young age (Wielen 1977). The BRI survey covers 1000
deg ## 2.4. UK Schmidt surveySpectroscopy of very red stars of the UK Schmidt survey of ESO/SERC
field 287 (Hawkins et al. 1998) has led to the identification of
three brown dwarf candidates at distances between 37 and 48 pc. The
plate limit is at 23.1 mag in the R band, and the field area is 25
deg © European Southern Observatory (ESO) 1998 Online publication: October 21, 1998 |