Galactic Mass Density in the Vicinity of the Sun

Abstract

It is possible to estimate the galactic mass density in the solar neighbourhood either directly by summing up the mass densities of individual subsystems of stars and interstellar matter or indirectly from dynamical considerations.Observational data on the number density of visible stars lead to mutually consistent results on the stellar component of the.,mass density. The mean of different estimates is P s t a r s = 0 « 0 5 2 -0 « 0 1 0 ^^P 0 ' • ^ adding the probable contributions of intrinsically faint_undetected objects and of interstellar matter the value p=0.09-0.02 M^pc" has been obtained for the total mass density.Dynamical estimations have given rather discrepant results _ 3 (p., =0.05*0.30 JLpc ) . The authors who obtained p^ ^0.15 M^pc" have often interpreted'their results as an indication of^Qie presence of a population of unknown objects (missing mass) in the solar vicinity.More probable explanation of the discrepancy between the direct and dynamical density determinations lies in the fact that the dynamical determinations are subject to large systematical and accidental errors. In the first approximation p, «(o^/a ) , where a* is the mean dispersion of the galactovertical velocities and a is the dispersion of the distances z from the galactic plane in a flat steady-state subsystem. The values of p -are very sensitive to any errors in the accepted values of both dispersions, a. and a . The inclusion of few halo or old disk population stars into statistical sample of young stars may significantly increase the velocity dispersion in the sample whereas the space distribution remains practically the same. Hence inhomogeneous statistical samples based on HD or MK spectral classifications lead to exaggerated values of Pg vn > Among the available determinations of p^ the lowest values are the most trustworthy. These values are in agreement with direct density estimates. Thus so far there is no real dynamical evidence for the presence of significant anount of missing mass in the solar vicinity.The full text of the contribution is published in Tartu Astron.

Galactic disc dark matter, terrestrial impact cratering and the law of large numbers

“…Analysis of the observed response of tracer stars to the Galactic z-force, using the Boltzmann and Poisson equations combined in various forms, has given widely varying results, both for the same class of stars and among different classes of stars, ever since Oort's (1932) classic study. For the older stars, the assumption that the stars are dynamically well mixed in the z-direction must be made (Oort 1932), while for the younger ones, like the O and B stars, their ages can be used explicitly (Jôeveer 1972) or else their distribution in phase space can be examined for approximate uniformity on the assumption that the gas and dust out of which they recently formed was already roughly relaxed (Stothers & Tech 1964;von Hoerner 1966;Flynn 1987).…”

The question of selective absorption of light in space viewed from the viewpoint of the dynamics of the universe

“…His results were confirmed by Kapteyn (1922), who introduced the term "dark matter" to denote the possible invisible matter in the solar neighbourhood. This problem was studied by Jeans (1922), Oort (1932) and in Tartu Observatory by Kuzmin (1952Kuzmin ( , 1955, Eelsalu (1958) and Jõeveer (1972Jõeveer ( , 1974Jõeveer ( , 1975.…”

“…This value confirmed Öpik (1915) and Kapteyn (1922) conclusion that the gravity of known stars fully explains their motion, and there is no need for local dark matter. Further determinations of C were made in Tartu by Eelsalu (1958), and Jõeveer (1972, 1974, 1975. These studies by Kuzmin, Eelsalu and Jõeveer formed their PhD theses (candidate theses according to Soviet rules).…”

Structure and Evolution of Regular Galaxies

“…In other words, there is a need for dark invisible matter. Since the matter density and possible presence of dark matter are of fundamental importance, my Tartu collaborator Jõeveer (1968Jõeveer ( , 1972 made a new analysis, using a completely different method, see Chapter 21. Ages of young stars are known, this allows to find parameters of vertical oscillations of young B stars and cepheids, which led to parameter C = 70 km/s/kpc and dynamical density ρ dyn = 0.09 M ⊙ /pc 3 .…”