Spectroscopy of metal-rich M31 globular clusters

Abstract

Integrated CCD spectra with a wavelength resolution of ~2.1 A covering the wavelength region λλ 3850 to 4200 A have been obtained for nine metal-rich ([Fe/H >= -1) globular clusters in M31. The spectra have been analyzed using the quantitative, reddening-free spectral-classification system developed by Rose ( 1984) that compares the strengths of neighboring absorption features. The data have been combined with similar data for nine metal-rich Galactic globular clusters previously discussed by Rose and Tripicco (1986). Spectra of 72 stars with well-determined atmospheric parameters have also been obtained, with most having been observed on several nights over the different observing runs. These spectra provide fiducial sequences in the various diagnostic diagrams, monitor the stability of the instrumental system, and provide a large database of multiple observations which are used to assess the reproducibility of the spectral indices. The following results have been found: (1) The 4000 A light from the metal-rich M31 globulars is dominated by dwarfs, which typically contribute four times as much light as do giants. This has been determined from a diagram that is based on the ratio of the Sr II λ 4077 and Fe I λ 4045, λ 4063 lines and is sensitive to the integrated surface gravity of a stellar system independent of metal abundance. In contrast, the light from the mean of the metal-rich Galactic globulars is contributed equally by giants and dwarfs, in good agreement with theoretical and observed cluster luminosity function data. (2) The contribution of early-type stars to the 4000 A light from the M31 clusters is constrained to be less than 5% in most cases by an index that compares the central intensities of the Ca II H + Hɛ and Ca II K lines. The index increases sharply with spectral type in stars of types AO through mid-F and remains constant thereafter, enabling the prediction of its value in the integrated spectrum of an old metal-rich system. Even a small contribution from stars of types earlier than mid-F will cause the index to clearly deviate from the prediction. This constraint rules out the presence of hot-star populations (e.g., blue horizontal branches), which have previously been proposed to explain the anomalously strong Balmer-line absorption reported for these clusters. (3) The λ 3883 and λ 4216 CN bands are exceptionally strong in the M31 globular clusters for their integrated spectral types. The band strengths are such that no combination of CN-normal solar-metallicity stars can reproduce them, requiring giants and dwarfs having roughly order-of-magnitude CN excesses. This observation of anomalous strength in both CN bands extends the results of Burstein et al. (1984), who discovered the enhancement of the λ 4215 CN band in metal-rich M31 clusters. The most plausible and consistent explanation for the combination of dwarf domination, Balmer-line enhancement, and hot-star constraints may be that the metal-rich M31 globular clusters are substantially younger than Galactic metal-rich clusters like 47 Tuc or M71. However, this interpretation does not explain the excess CN band strengths in the integrated spectra of the M31 globulars.