DDO Paper Abstracts


Geisler D. (1986, PASP 98, 762)

Abstract

Washington photometry is presented for a number of late-type giants with Fe abundances in the range -4.5< [Fe/H]< -0.5. The new data more than double the sample of such stars available with both accurate photometry and high-resolution abundance determinations. This permits a much-improved reexamination of the sensitivity of the Washington system to Fe and CN/CH abundances. New empirical calibrations of the two abundance indices, delta(M-T1) and delta (C-M), are presented. The Fe abundance sensitivity of the delta(M-T1) index is very limited for stars more metal-poor than [Fe/H]~ -1. The delta(C-M) index, however, is very sensitive to Fe abundance throughout the entire range of known stellar abundance. Indeed, the Fe abundance sensitivity of the delta(C-M) index is comparable to or exceeds that of all other photometric or low-resolution spectroscopic abundance indices at all metallicities. In view of the very broad bands employed by the Washington system, the delta(C-M) index offers, for many purposes, the best choice for an efficient, accurate, and sensitive abundance index for normal late-type giants. The system should prove to be very useful for investigation of abundances in extragalactic system using a CCD.

The ability of the system to detect anomalous CN/CH strengths is also investigated via observations of a number of giants in the globular clusters 47 Tuc and M4 with a variety of known CN and CH strengths. The delta(C-M) index is found to be much more sensitive to Fe abundance than CN or CH strength. The system cam differentiate CN- or CH-strong giants from normal giants, but not consistently, and is most effective for giants more metal-rich than 0.1 solar.

The new calibrations yield [Fe/H] = -0.85 = 0.1 for 47 Tuc. This intermediate values is in good agreement with most recent determinations and further weakens the evidence for a significantly lower Fe abundance, since the original value of -1.25 was used by Pilachowski, Canterna, and Wallerstein to support their controversial echelle result.


Geisler D., Claria J.J., Minniti D. (1991, AJ 102, 1836)

Abstract

A revised metal abundance calibration for the Washington photometric system is presented which represents a significant improvement over previous calibrations in several respects. First, new observations of a number of field and open cluster giants allow a much more precise definition of the solar-abundance fiducial relation in the two-color diagrams from which the abundance-sensitive delta indices are derived. Second, observations of a large sample of globular cluster giants clearly demonstrate, and allow correction for, an unsuspected decrease in metallicity sensitivity for cooler giants. Third, a new abundance index, C-T1, and a new temperature index, M-T2, are introduced. The M-T2 color provides a much broader baseline than the T1-T2 color and is thus much less susceptible to photometric errors in determining abundance. Fourth, the total sample of field and cluster giants now available with Fe abundances derived from high dispersion spectroscopy is substantially larger than available previously, leading to a more extensive and accurate calibration. Various combinations of abundance and temperature indices are investigated. Metal abundance calibrations are presented for five such combinations over the range from [Fe/H] = +0.5 to -4, indicating that each is capable of deriving abundance to ~0.15 dex. The abundance indices vary by ~1 mag over this metallicity range. We confirm that the Washington system offers a unique combination of efficiency and accuracy for determining metallicity in late-type giants over the full range of stellar abundances, although the system loses sensitivity for the coolest metal-poor stars. Metallicities determined from the calibration are given for some 50 open and globular clusters with previously Washington photometry. The abundance scale established here for globular clusters is in good agreement with that of Zinn [ApJ, 293, 424 (1985)] and with that of Janes [ApJS, 39, 135 (1979)] for open clusters. Finally, comprehensive reddening and photometric error estimates are derived, as well as abundance sensitivities, and the suitability of the different indices to abundance determinations under various conditions are discussed. Under most circumstances, especially for CCD applications where substantial photometric errors may be present due to, e.g., aperture correction uncertainties, the best abundance index is the C-M color, using M-T2 as the temperature index. Thus, in many applications it is only necessary to observe in three filters, excluding T1 observations.


Paltoglu G., Bell R.A> (1994, MNRAS 268, 793)

Abstract

Synthetic colours have been calculated for the Washington photometry system from a grid of synthetic spectra. The grid of spectra covers a range in Teff, log g and abundance appropriate for Population I and II dwarfs and giants. We also examine the effects of C and N abundance variations on the colours of the giants. We compare the model colours with observations of field dwarfs and giants compiled from the literature, as well as published cluster giant branch photometry. We find that our dwarf models match the observed disc dwarf sequence very well in the (C - M) versus (T1 - T2) and (M - T1) versus (T1 - T2) colour planes, while our solar-abundance giant branch grid and isochrones are a good fit to the disc giant sequence. Our metal-poor isochrones are systematically too red when compared with globular cluster giant branches; this may be due to the parameters chosen for the computation of the isochrones. A comparison of the effects of abundance and gravity on the indices (T1 - T2) and (M - T2), used for estimating Teff, suggests that the latter should be the index of choice. Neither (C - M) nor (M - T1) is found to be particularly sensitive to variations in [C/Fe] or [N/Fe] if the abundances are anticorrelated. At solar abundance, however, (C - M), (M - T1) and (T1 - T2) are strongly affected if the abundances are not anticorrelated. We confirm empirical suggestions that (C - M) and (C - T1), are the best abundance indicators. The model colours for the gravity indicator (M - 51) match the observed sequences very well. This index is useful for estimating both gravity and abundance. It may be particularly useful for estimating the abundance of highly reddened open and globular clusters.


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Last update: 7 Novembre 1995