scholarly journals Studies of a and F Stars in the Region of the North Galactic Pole

1977 ◽  
Vol 4 (2) ◽  
pp. 71-71
Author(s):  
R. W. Hilditch ◽  
Graham Hill
Keyword(s):  
Total Mass ◽  
Mass Density ◽  
Coma Cluster ◽  
Broad Distribution ◽  
Short Account ◽  
Component Distribution ◽  
Narrow Component ◽  
The North ◽  
F Stars ◽  
North Galactic

A short account was given of results recently published (Hill et al. 1976; Hilditch et al. 1976a, b) of a spectroscopic and photometric study of 310 A0-F8 stars within 15° of the north galactic pole. Using these data, it has been found that δmo for the programme stars is constant out to 250 pc and corresponds, via Crawford’s (1975) calibration, to the solar value of [Fe/H]. The distribution of radial velocities of A0-F8 stars is asymmetric and may be interpreted as showing a broad distribution of mean −7 kms−1 and dispersion σ ~ 11 kms−1 together with a superposed narrow distribution of mean 0 kms-1 and dispersion σ ~ 3 kms-1. The narrow component originates from a group of 37 stars which appear to be kinematically and spatially associated with the Coma cluster. Only 14 of these 37 stars are recognised members of the Coma cluster. Removal of these 37 stars from the sample results in a distribution which matches closely the W component distribution of A0-F8 stars in Gliese’s (1969) catalogue. Thus Oort’s “well-mixed” hypothesis seems to be confirmed out to 250 pc. Much additional work on the space motions is required before the hypothesis of the ‘Coma group’ can be thoroughly tested. Results of this analysis and a preliminary value for the total mass density in the solar neighbourhood is expected to be completed during the next few months.

scholarly journals Star counts, local density and KZ force

1985 ◽  
Vol 106 ◽  
pp. 153-160
Author(s):  
Bengt Strömgren
Keyword(s):  
Galactic Plane ◽  
Local Density ◽  
Mass Density ◽  
Homogeneous Groups ◽  
Local Mass ◽  
The North ◽  
Galaxy Model ◽  
F Stars ◽  
North Galactic

The approach by Bahcall and Soneira to the determination of galactic parameters through the use of star counts is referred to, and tests of the Bahcall-Soneira Galaxy model based on additional observational data are discussed.The determination of the local mass density by Hill, Hilditch and Barnes through studies of A and F stars in the region of the North Galactic Pole is briefly discussed, as is a recent investigation of the problem by Bahcall.In the determination of the galactic force Kz and the local mass density from the density distribution ν(z) and the distribution f(W) of velocities at right angles to the galactic plane for a group of tracer stars, it is important to secure homogeneity of the tracer group. This has led Hill, Hilditch and Barnes in a continuation of their investigation to use photoelectric uvby photometry to segregate homogeneous groups of F stars. A similar approach is followed by Danish astronomers, whose work is briefly described.

scholarly journals The Space Density of Faint M-Dwarfs

1974 ◽  
Vol 3 ◽  
pp. 417-418
Author(s):  
J. H. Oort
Keyword(s):  
Coming Out ◽  
Mass Density ◽  
M Dwarfs ◽  
Space Density ◽  
Gas Density ◽  
The North ◽  
Solar Motion ◽  
Objective Prism ◽  
High Space ◽  
North Galactic

Donna Weistrop, a student of Maarten Schmidt’s, has found evidence for an unexpectedly high space density of very faint M-dwarfs. This was confirmed by Murray and Sanduleak. The space density inferred from these investigations is comparable to, or possibly higher than, the local gas density. Because there is evidence that the stars concerned are young this appears to present a problem.As the investigation by Murray and Sanduleak is the simpler and more direct of the two I confine my discussion to this. The authors determined proper motions for 21 M dwarfs down to 17m found in an objective-prism survey at the Warner and Swasey Observatory in regions close to the North Galactic Pole. The distance of the stars can be inferred from the reflection of the solar motion. Assuming the ‘basic’ solar motion they found and a space density of 0.23 pc-3 for these stars, corresponding with a mass density of about 0.04 M⊚ pc-3. This is exceedingly high, in view of the fact that the average gas density near z = 0 is only 0.03 M⊚ pc-3. The problem is aggravated by the fact that the stars have low velocities, the dispersion in velocity in one coordinate coming out ± 10kms-1. This is considerably lower than the velocity dispersion of A-type stars, and shows that the stars are young, perhaps about 108 yr. How is it then that the gas has not been completely used up long ago?

scholarly journals Kinematics and properties of F stars near the North Galactic Pole - II. The isothermal disc

1988 ◽  
Vol 230 (2) ◽  
pp. 273-320 ◽  
Author(s):  
A. J. Adamson ◽  
G. Hill ◽  
W. Fisher ◽  
R. W. Hilditch ◽  
C. D. Sinclair
Keyword(s):  
The North ◽  
F Stars ◽  
North Galactic
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