A Survey of Halo RR Lyrae Stars

Citation

Saha, Abhijit (1984) A Survey of Halo RR Lyrae Stars. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/d56z-kf18. https://resolver.caltech.edu/CaltechETD:etd-09092008-080231

Abstract

A survey of RR Lyrae stars has been conducted in selected regions of the Galactic halo, using the 48 inch Schmidt telescope at Palomar. 29 RR Lyrae stars were found in three fields (each is 42 square degrees) centered at 1 = 180°, b = 24°; 1 = 180°, b = 30° and 1 = 110°, b = -30°. The faint limit for finding RR Lyraes is 19^m.5 in B, and the faintest RR Lyrae stars that were found are at Galactocentric distances of over 40 kpc. The majority of the stars are 20 to 30 kpc from the Galactic center, and typically around 18^m in B. Apart from a very few globular clusters, these stars are the most distant known probes of the outer halo.

Photoelectric photometry done with the 60 inch telescope at Palomar has been used to obtain accurate light curves and ephemerides for all the survey RR Lyraes stars. Spectrophotometry at minimum light has been used to estimate reddening and interstellar extinction. Distances to the individual RR Lyrae stars have been derived using the information thus obtained.

The space densities of RR Lyrae stars has been traced out to a Galactocentric distance R of 33 kpc. The data are combined with that obtained by others in the Galactic bulge and in the relatively nearby halo, which gives an overall picture of the run of space densities over a very wide range of R.

Spectra of many of these objects have been taken with the double spectrograph on the 200 inch telescope at Palomar. Chemical abundances and radial velocities have been derived from them. Center of mass velocities have typically been obtained to an accuracy of 35 km s^-1.

There is no discernible abundance gradient in the halo from 10 to 40 kpc, but there is a very wide range of abundances. Two RR Lyrae stars have been found which have metallicities that are unprecedentedly high ([Fe/H] ~ -0.5]) for this region of the halo. If the results are confirmed, this will imply that there is a sizeable population of moderately metal rich stars in the outer halo.

From the radial velocities, the systemic and random motions of the system of RR Lyrae stars in the distant halo have been studied. The velocity dispersion along the axis that points to the Galactic center is found to be 105 ± 17 km s^-1. The mass of the Galaxy within 25 kpc has been estimated at 3.9 (+3.4 or -2.2) x 10¹¹M_☉, assuming isotropic orbits and spherical potential at 25 kpc. This confirms that the Galactic mass continues to increase out to 25 kpc.

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