*Astron. Astrophys. 327, 90-106*
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## NGP A stars - density variation, kinematics
and the total mass
density of the disk
**J. Knude**
Niels Bohr Institute for Astronomy, Geophysics and Physics,
Juliane Maries Vej 32,
DK-2100 København Ø,
Danmark
(indus@astro.ku.dk)
*Received 23 August 1996 / Accepted 26 May 1997*
**Abstract**
From an uvby photometric survey of the north galactic pole
where most A stars (A3 - A9)
above b = 70and brighter than B = 11.5 were included we report
the *observed* number density and age variation
with z(pc). Stars between 30 and 1300 pc are included.
Our sample consists of 396 sharply defined main sequence/subgiant A stars
following
the Strömgren - Crawford definition, no B, imA or F type stars are
included, approximate color limits are 0.055 - 0.220 in (b-y)_{0}.
Proper motions from the
PPM and CAMC catalogs are available for all stars implying U and V velocities.
Radial velocities for a subsample (179 stars with b 75 and
z 450 pc) are available from the literature. From a luminosity point
of view the sample is volume complete within 450 pc.
For a fit to the observed density variation the choice
of function is not obvious and it seems that these A stars may not be
fitted by a single law but requires at least two and more likely
up to four components. Assuming exponential density variation two
groups of stars seem present with scale heights around
65 and 165 pc respectively. The latter may be an underestimate due to volume
incompleteness for the coolest unevolved A stars beyond 450 pc.
The exponentials approximating the number density has a relative
normalization 1.00: 0.03 for z = 0 pc.
With these two components only the first of Perry's (1969)
h = 40 and h = 600 pc groups seem represented in our volume complete sample.
Adopting instead a gaussian variation the data may indicate three components
with scale heights 45, 75 and 155 pc and relative normalization 1.0:0.5:0.2,
respectively.
Finally a set of four sech^{2}()s with scale heights 55, 85, 165
and 420 pc and relative normalizations 1.00:0.47:0.18:0.005 for z = 0 seem to
fit the number density data well. This combination is based on
independent 75 pc bins.
A sech^{2} variation is perhaps the most obvious choice being the expected
variation for an isothermal and selfgravitating disk.
With a 100 pc running binning there are indications
that the stars older than the sample median 0.75 Gyr require a component
with a scale height 680 pc. It may also be present in the young sample,
ages less than 0.75 Gyr, but only beyond 400 pc. What perhaps is
more surprising is the formal presence
of groups of stars diplaying a density maximum at z = 200 - 250 pc, present
for both age groups, and most interestingly the angular momentum also
displays irregularities at the same z distances. The hump is not proposed
to represent
a physical entity but something like it is required for fitting the
observations better than 10. A consequence of the hump is that derivatives
of the distribution display large variations. There seems to be significant
differences between this distribution and the sum of four sech^{2}s from
the previous paragraph.
W and U dispersions seem to have a constant ratio within the z range where we
have both kinds of data. shows a homogeneous increase from
20 to
45 km s^{-1} within the completeness limit, with a 75 pc independent
binning, and to 70 km s^{-1} at 800 pc. The complete U histogram
may not be fitted with a single gaussian, three with different dispersions
and mean values provide a reasonable fit. A more detailed binning shows
/z to change in a discontinous way at
200 pc.
An application of the data might be a combination of
the density
and velocity data for a determination of the local, total disk density.
But since velocity dispersions do obey some sort
of age depence the age - z variation turns out to be of importance
and there is a very clear systematic trend of mean ages with z.
The average age increases almost linearly to 0.75 Gyr, reached
at 200 pc, and then it stays constant within the completeness limit.
On the average the younger A stars show a relative absence beyond 200 pc
implying that very different velocity dispersions might be seen on either
side of 200 pc.
The curve fitted to the density data permits a study of its derivatives.
Adopting a linear approximation to 's z variation, instead
of isothermallity, K_{z}/z's
variation within 150 pc indicates that the combined Poisson -
Jeans equation may only return estimates of the local volume mass density
that vary within a factor of two but the fit by four sech^{2}s turns out
to be equivalent with a representation by two gaussians with scale
heights 129 and 250 pc which implies that between 100 and 200 pc
= 0.120.04 pc^{-3} whereas
K_{z}/z from the same fit and a linear approximation
to the - z variation returns (z0)
0.05 pc^{-3}.
The age, density and kinematic inhomogeneities
revealed in the sample compel us to the conclusion that the A stars do
not trace the galactic potential in a simple way - probably because the
age mixing varies with distance.
**Key words: **galaxy: kinematics and dynamics - solar neighbourhood -
stellar content - structure
© European Southern Observatory (ESO) 1997
Online publication: October 1, 1997
Last change: April 8, 1998
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