Astronomy and Astrophysics – Astronomy
Scientific paper
Oct 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996aj....112.1500t&link_type=abstract
Astronomical Journal v.112, p.1500
Astronomy and Astrophysics
Astronomy
17
Galaxy: Halo, Stars: Abundances
Scientific paper
The primary DDO metallicity indicators for stars in the [Fe/H] range of +0.3 to -1.1 have been recalibrated. It is found that the slope of the δCN metallicity transformation needs to be reduced while the zero-point is color-dependent; no color dependence is found for the slope. Over the [Fe/H] range from +0.3 to -0.5, DDO precision photometry and δCN provide [Fe/H] with an uncertainty of only ±0.05 dex. Contrary to expectation, between [Fe/H] -0.9 and -2.0, δCN exhibits some sensitivity to changes in [Fe/H], though with a much steeper slope than at solar [Fe/H]. For the critical [Fe/H] range between -0.5 and - 0.9, within the uncertainty the trend is consistent with little to no sensitivity to metallicity change. For C0(3845), the isometallicity relations in the C0(3845) - C0(4548) diagram are nonlinear at the blue end. In contrast with δCN, the metallicity sensitivity of C0(3845) is high for [Fe/H] - 0.4, but declines rapidly at higher metallicity. Using the δCN calibration, the 16 nonvariable, red giant members in 47 Tuc exhibit a range in [m/H] from +0.2 to -0.7, consistent with a large spread in CN abundance. When divided into CN-strong and CN-weak samples, the mean abundances become -0.09±0.18 (s.d.) and -0.49±0.11 (s.d.) for 6 and 10 stars, respectively. The mean abundance of the CN-weak stars is more than a factor of two higher than expected from the predicted [Fe/H]SPEC of -0.9. In the C0(3845) - C0(4548) diagram, the 47 Tue giants show no separation by CN strength, though the scatter among the bluer stars is large. Despite the scatter, [Fe/H] =-0.9 is clearly excluded by the data, while the mean of the points is best fit by the curve for [Fe/H] = -0.6±0 1. 47 Tue appears to have a DDO metallicity which is a factor of two higher than found for field stars at the same [Fe/H]SPEC; it is not representative of the field star population at the same [Fe/H]. Use of 47 Tue to calibrate field star photometric parameters remains questionable, while identification of field star counterparts to 47 Tue giants may not prove reliable. Possible explanations for the source of the discrepancy are discussed.
Anthony-Twarog Barbara J.
Twarog Bruce A.
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