GMOS SV program 58

We propose to measure the equivalent width of 8600A Ca-tripled absorption in the spectra of giants fainter than the RGB-tip in the Local Group dwarf galaxy And V. And V is interesting because, at least based on the existing 4m imaging data, it falls off the luminosity - mean metallicity relation defined by the Galactic and other M31 dSphs. A mean abundance of [Fe/H] = -1.5 or so is too high by ~0.5 dex for its luminosity (see Caldwell 1999, AJ, 118, 1230 [Fig. 4]). The aim of the SV program would be to determine the mean metallicity from spectroscopic observations of member red giants at the Ca II triplet (~860nm). A measurement of the abundance dispersion is also possible but would require higher S/N spectra. The mode being tested of course is the ability to subtract off bright night sky emission lines and the ability to combine series of individual frames into useful data. The available ground-based c-m diagram is in Armandroff et al. (1998, AJ, 116, 2287). This also has a picture of And V which shows that it easily fits on the GMOS field and that there are seemingly plenty of potential OIWFS stars.

The observations needed would be V and I imaging to determine the giants to be observed spectroscopically, followed by MOS observations. Using the integration time calculator, with I=21.0 stars of type K0III-w, Av=0.5 the R831 grating with central wavelength 860nm, no filter, spatial and spectral binning of 4 pixels, 1arcsec slitlets, then 8 1800sec integrations in 50/50/80/80 conditions gives a total S/N per spectral pixel exceeding 10 in the continuum at the Ca II triplet. With a sample of at least a dozen members, this should easily provide a well determined mean metallicity. To calibrate the metallicity though requires observations of red giants in a few globular clusters, so each of these would require a preliminary I-band image and a MOS observations with the same setup. And V is well placed for observation in the fall. Calibration globular clusters could be some subset of M15, M2, NGC6981, NGC7099, NGC7492 and NGC288. Doing this calibration "right" would be a big advantage for the partner communities.