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Astron. Astrophys. 327, 90-106 (1997)

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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 Kobenhavn O, Danmark (indus@astro.ku.dk)

Received 23 August 1996 / Accepted 26 May 1997

Abstract

From an uvby [FORMULA] photometric survey of the north galactic pole where most A stars (A3 - A9) above b = 70o and brighter than B = 11.5 were included we report the observed number density and age variation with z(pc). Stars between [FORMULA] 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 [FORMULA] 75 [FORMULA] and z [FORMULA] 450 pc) are available from the literature. From a luminosity point of view the sample is volume complete within [FORMULA] 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 sech2 ([FORMULA])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 sech2 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 [FORMULA] 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 [FORMULA]. 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 sech2 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. [FORMULA] shows a homogeneous increase from [FORMULA] 20 to 45 km s-1 within the completeness limit, with a 75 pc independent binning, and to [FORMULA] 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 [FORMULA] / [FORMULA] z to change in a discontinous way at [FORMULA] 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 [FORMULA] 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 [FORMULA] 's z variation, instead of isothermallity, [FORMULA] Kz / [FORMULA] z's variation within [FORMULA] 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 sech2 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 [FORMULA] = 0.12 [FORMULA] 0.04 [FORMULA] pc-3 whereas [FORMULA] Kz / [FORMULA] z from the same fit and a linear approximation to the [FORMULA] - z variation returns [FORMULA] (z [FORMULA] 0) [FORMULA] 0.05 [FORMULA] 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

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© European Southern Observatory (ESO) 1997

Online publication: April 8, 1998
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