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Near-IR spectroscopy of OH/IR stars in the Galactic centre
Context.Based on the expansion velocities of their circumstellar shells,Galactic centre (GC) OH/IR stars can be divided into two groups that arekinematically different and therefore believed to have evolved fromdifferent stellar populations.Aims.We studied the metallicitydistribution of the OH/IR stars population in the GC on the basis of atheoretical relation between EW(Na), EW(Ca), and EW(CO) and themetallicity.Methods.For 70 OH/IR stars in the GC, we obtainednear-IR spectra. The equivalent line-widths of Na I, Ca I,12CO(2, 0), and the curvature of the spectrum around 1.6μm due to water absorption were determined.Results.The near-IRspectrum of OH/IR stars is influenced by several physical processes.OH/IR stars are variable stars suffering high mass-loss rates. The dustthat is formed around the stars strongly influences the near-IR spectraand reduces the equivalent line-widths of Na I and Ca I. A similareffect is caused by the water content in the outer atmosphere of theOH/IR star. Because of these effects, it is not possible to determinethe metallicities of these stars with our low-resolution near-infraredspectroscopy.
| Luminosity and Temperature from Near-Infrared Spectra of Late-Type Giant Stars
We present moderate resolution (lambda / Delta lambda ~ 1380 and lambda/ Delta lambda ~ 4830) spectra of 43 K0 to M6 III stars covering 2.19 -2.34 mu m and measure equivalent widths of the strongest absorptionfeatures - Na I, Ca I, and (12) CO(2,0) - present on the spectra. Thehigh resolution Wallace & Hinkle (1996) spectral atlas shows thatour moderate resolution measurements of the atomic features havesignificant contributions from other species, such as Sc, S, Fe, Ti, Si,and V. We also find that our measured equivalent widths are affected byCN absorption present in the continuum bands. In spite of this, theequivalent widths of Na I and Ca I features at moderate resolution stillshow a strong dependence on effective temperature. The CO equivalentwidth at moderate resolution is less affected by other lines andcontinuum placement than the atomic features, because of its relativelygreater strength. We compare our data to similar data taken forlate-type dwarf stars (Ali et al. 1995) and find that a two dimensionalspectral classification can be constructed based on the near-IR spectra.The quantity log[EW(CO)/(EW(Na)+EW(Ca))] is a strong luminosityindicator independent of effective temperature, while the equivalentwidth of (12) CO(2,0) has a well-defined dependence on effectivetemperature for a given luminosity. This two dimensional spectralclassification is ideal for cool stars obscured by dust in, for example,the central part of the Galactic bulge and regions of star formation.
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