Astronomy and Astrophysics – Astronomy
Scientific paper
Jan 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995phdt........14g&link_type=abstract
Thesis (PH.D.)--YALE UNIVERSITY, 1995.Source: Dissertation Abstracts International, Volume: 56-08, Section: B, page: 4373.
Astronomy and Astrophysics
Astronomy
8
Scientific paper
A new survey of UBV photometry and absolute proper motions to B~22.5 was carried out in a small field at the South Galactic Pole near globular cluster NGC 288. The photometry was based on CTIO 4-m and 0.9-m CCD observations. Ultraviolet excesses and photometric parallaxes were obtained for several hundred stars. The proper motions were derived from the CTIO 4-m prime focus plates taken from 1981 to 1992. The optical field angle distortion of the prime focus corrector was accurately modeled relative to an astrographic plate as a flat field. The magnitude and color equations were corrected using NGC 288 members as references. The zero-point of the absolute proper motion was defined by several hundred galaxies. The stellar tangential velocities and dispersions were derived. This has enabled us to probe the stellar spatial, kinematical and chemical distributions to very high vertical distances. It was shown that the thick disk as an intermediate component is necessary and distinct from the old thin disk and spheroidal halo. It has a scale height of ~1 kpc, a local density ratio to the thin disk of ~0.03, a mean asymmetric drift velocity of 60 +/- 5 km s ^{-1}, a mean metallicity of (Fe/H) = -0.58 +/- 0.06. The thick disk to halo transition occurs at | z|~5.5 kpc. The lack of a significant rotational velocity gradient and the small or zero metallicity gradient suggest that the thick disk is the product of some heating or merger event(s) that occurred in the young Galaxy. For the halo, the local density ratio to the thin disk is 0.1% and its mean metallicity is -1.66 +/-0.09. The mean rotational velocity is V rot=32+/-7 km s ^{-1}, indicating that the halo in mean is in prograde rather than retrograde motion. No metallicity and kinematical gradients and correlation were found for the halo. The space motion of NGC 288 was determined to be (u, v, w)LSR (31 +/-15, -263+/-16, 54+/-1) km s^{-1}, indicating that it is moving on a retrograde orbit.
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