Scale-heights of the Galactic disk subsytems with different ages and metallicities

V.Marsakov & Yu. Shevelev

1997, "in Structure and evolution of Stellar system" Int. Conf. eg. T.A.Agekian, A.A Mullari, V.V. Orlov, Petrozovodsk, 1995, p.150-154

     Usually parameters of spatial structure of the Galactic subsystems were founding either by recountig of stars at different distancies or by determination of their total number up to appropriate distance and modeling of vertical structure by exponential law subsequently. The main uncertainty of these methods is arising from the problem of determination of distance to distant objects. To exclude all possibility of error we suggest the new method using only nearest stars. It is based on the fact, that all stars cross the disk plane several times during their Galactic orbital period. So we have tryed to restore the real altitude distribution of the stars of disk subsystems on the Solar Galactocentric distance whose representatives have sufficient quantity in the Solar vicinity for obtaining statistically reliable conclusions.

     We have used our representative sample of nearby disk F2-G2 stars Marsakov & Shevelev (1995) in which fundamental stellar parameters was determined on the basis of published date on uvby-photometry, proper motion and radial velocities. Our sample of F stars is complete within 50 pc from the Sun.

     We can determine the accurate position of the Galactic plane proceeding from simple distribution on Z-coordinate of sample stars (Fig. 1а) and young metal-rich ones (Fig. 1b ), placed into vertical cylinder with foot radius 30 pc. The solid curves are approximations of gistogramms by polinome of ten oder. The calculation shows, that maximum is at Z =-9±2 pc from the Sun to the South direction.In our opinion it is just the Galactic plane position. According to Fig. 1а, the density of F stars at the Galactic plane is n_F=0.029 pc^-3, and their mass density _F=0.035M пс^-3. These values are slightly greater than founded for F stars in Kharadze et. al. (1989) (n_F=0.002 pc^-3), and in Аллен (1977) (_F=0.035 M pc^-3). We suppose that in these papers full quantity of distant F stars is underestimated.

     Represenatives of all disk subsystems are within Solar vicinity at the present moment. Stars of earch subsystem should be distributed on their orbits accidently after some time passing and should be occupied the same volume (on Z-coordinate) which all objects of given subsystem occupies at the Solar Galactocentric distance now. Thank's to the fact that meridional orbit of any disk star is fullfilled "box" consisting of large enough number of revolutions, sooner or later earch star falls into any point of perpendicular, raising to the present stellar position. We suppose that total number of stars with the same astrophysical parameters as given star (metal abundance, age, velocity etc.) equals approximately the ratio of full time of observation to time of stellar staing in the band with width as a size of volume where F stars sample is full. The restored distribution of F stars subsystem on altitude is obtained by treatment of their Z_max-distribution by calculated weights (i.j. rations), and probability density function.(Z_max is maximum remotness of the stellar orbit from the Galactic plane.) Last prosedure is needed for obtaining of accidental stellar distribution on their orbits. The stellar orbits have been calculated on the model of the Galaxy, containing bulge, disk and massive halo. (See Marsakov & Shevelev (1995b)) for restoration of Z-distribution in details).

     In Fig. 2 you can see the three stages of Z-distribution restoration for our complete sample of F stars: a - Z_max-distribution, b - waited Z_max-distribution, c - restored Z-distribution. Solid curve on Fig. 2c is approximation of gistogramm by exponentional law: n(Z)=С exp(-Z/, where is scale-height. Scale-height of disk F stars subsystem is _F2-G2=160±10 pc. This result is in good agreement with ones of Kharadze et. al. (1989), where _F0-G0=130+20 pc.

FIGURE. 1.

FIGURE. 2.

     We also have divided all sample F stars into two metallicity groups by [Fe/H]=-0.1 (the center of F stars metallicity function) and then each of them into two age subgroups by t=3 Gyr (a half the total number of our sample) and have determined scale-heights for them. It was turned out that young metal-rich stellar group has the least scale-height (=100±5 pc), but old metal-poor groop - the greatest one (=220±20 pc). Two other groups are the same intermediate size (=170±20 pc). Thus, disk subsystems compress during the time and metal-rich stars are more consentrated to the Galactic plane than metal-poor ones at any time of evolution of the Galaxy.

References

• Marsakov V.A., Shevelev Yu.G. Bull. Infform. CDS. 1995a. N 47. P. 13.//First quotation in article//Electronic version catalogue// Desription catalogue (russian)
• Kharadze E.K., Bartaya R.A., Dluzhnevskaya O.B., Piskunov A.E.,Pavlovskaya Е.Д. //Astrophys. and Sp.Sci. 1989. V. 151. P. 319.//First quotation in article
• Allen C.W.//Astrophysical quantities. Univ. of London. 1973//First quotation in article
• Marsakov V.A. & Shevelev Yu.G. //Astron.Zh. 1995b. v.72, p. 630.//First quotation in article//Full text

24.07.98