Received 13th March 1998
Abstract.
The spatial distribution and the population nature of subdwarf B type stars
in the Galaxy is investigated based on the kinematics of these stars.
1. The data
For 12 stars, new absolute proper motions, which were calibrated with
background galaxies, were determined. These are based on old POSS plates
and new CCD images obtained at the Observatorium
Hoher List and the Calar Alto
Observatory.
The errors of these proper motions can only be estimated, as they are based
on only one first epoch position, but they are in the order of 4 mas/yr.
With these and already published proper motions, new and available radial
velocities, distances and a galactic mass model
the orbits of 41 stars have been calculated (de Boer et al. 1997).
2. The kinematical properties
The calculated orbits are well behaved and 10 stars reach to
|z| ≥ 2 kpc.
Many orbits are very eccentric, reaching in to just 2 kpc from the
galactic centre, or veering out to beyond 20 kpc.
None of the stars can be identified uniquely as classical Population II objects.
[Click here to see Fig. 1 and 2!]
We have analysed the properties of the orbits of our sample in order to
draw conclusions about the nature and origin of the stars. These properties
are the eccentricity
the z-component of the angular momentum Iz,
the present velocity component Θ and the normalized z extent
which is a better criterium to distinguish halo from disklike orbits than
zmax alone.
The average ecc of the orbits of our sample is 0.24,
the average nze of the orbits is 0.16 while the values for the sun,
a typical representative of the thin disk, are 0.01 and 0.09 respectively.
Our sample has a velocity dispersion of
(σU=62±8 km s-1,
σV=52±7 km s-1,
σW=59±8 km s-1)
which agrees well with the values for thick disk stars
(Mihalas & Binney 1981).
The asymmetric drift of our sample is -36 km s-1 also
in line with other results for the thick disk population (Ojha et al. 1994).
This suggests that our sample of sdB stars is part of a population of
thick disk stars.
3. Estimation of the scale height
A statistical analysis of the orbits shows that the subdwarf stars
have a spatial distribution in z compatible with an exponential one
with a scale height hz ≅ 1.0 kpc.
However, since only few stars reach to large z
the spatial distribution is only well defined to z ≅ 2 kpc.
The scale height value has been tested for robustness by subdividing
the sample into parts according to criteria like l, b etc., and
taking into consideration the errors of proper motion, radial velocities
and distances.
For these subsamples scale heights are calculated and none of these
lead to drastically different values.
The distribution in z of the relative frequency N(z) for
all stars of our sample (see Fig. 2)
shows a relative minimum near z=0 pc and has maxima near
z=300 pc.
This reflects the smaller probability to find the stars in the disk
than away from the disk, as expected for any orbit reaching to larger z.
Scale height studies based on limited samples of stars in specified directions
can therefore easily be flawed when they do not reach to large enough distances
to overcome this aspect of the z-distribution.
Other studies, like Yoss et al. (1987) derive a value for the scale height of
the thick disk of ≅ 1 kpc which agrees well with our result.
4. Discussion
Both the estimated scale height and the analysis of the kinematical properties
show that the sdB stars of the sample belong to the thick disk.
Their orbits are kinematically much more heated up than those of thin disk
stars like our sun which means that the sdB stars are considerably older.
None of the stars can be regarded as an halo object. This leaves open the
question where the halo field sdB stars are found, as sdB stars are also known
to exist in halo globular clusters like M 15 (Moehler et al. 1997).
An analysis of a sample of stars fainter than previously will probably
find the halo sdB population.
References
- de Boer K.S., Aguilar-Sánchez Y., Altmann M., Geffert M.,
Odenkirchen M., Colin J., 1997, A&A 327, 577
[astro-ph/9708184]
- Mihalas D., Binney J., 1981, in 'Galactic Astronomy: Structure and
Kinematics', Freeman K., 2nd ed.
- Moehler S., Heber U., Durell P.R., 1997, A&A 317, L83
- Ojha D.K., Bienaymé O., Robin A.C., Mohan V., 1994, A&A 290, 771
- Yoss K.M., Neese C.I., Hartkopf W.I., 1987, AJ 94, 1600
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| First version: | 21st | June, | 1998
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| Last update: | 28th | September, | 1998
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Jochen M. Braun &
Tom Richtler
(E-Mail: jbraun|richtler@astro.uni-bonn.de)