The 1.5-1.7 micrometer spectrum of cool stars: Line identifications, indices for spectral classification and the stellar content of the Seyfert galaxy NGC 1068

Astronomy and Astrophysics – Astrophysics

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Cool Stars, Infrared Spectra, Seyfert Galaxies, Star Distribution, Stellar Composition, Dwarf Stars, H Lines, Molecular Spectra, Red Shift, Supergiant Stars

Scientific paper

The first aim of this paper is to determine which lines or narrow molecular bands in the H-band spectra of cool stars could be of interest for the classification of K-M stars. For this purpose we present high quality, medium resolution (R approximates 1500) spectra of field stars (mostly K-M giants and supergiants) and compare them with detailed synthetic spectra computed on the basis of existing model atmospheres for red giants. The agreement between theoretical and observed spectra is good and virtually all the observed features can be accounted for by lines of (12)CO, (13)CO, OH, Mg, Al, Si, Ca and Fe. We analyze in detail the relative contribution of these and other species and conclude that the feature at 1.62 micrometers which is weak in early K but very strong in late M stars, is mainly due to the CO(6-3) band-head, while that at 1.59 micrometers, which is prominent in all stars later than G, is primarily attributable to silicon up to early M types, while in late M stars this feature is strongly contaminated by OH lines. We choose these two features as 'spectral classificators' and measure their equivalent widths in more than 40 G, K, M giants and supergiants. From these data it is found that CO 1.62 in giants increases rapidly and with a relatively small scatter going to later spectral types. Supergiants have deeper CO(6-3) and display a larger scatter. The (1.62)/(2.29) ratio steadily increases going to cooler stars but does not vary significantly with luminosity class. A very useful ratio is (1.62)/(1.59) which increases by a large factor from early K to late M stars and could therefore be a powerful tool to identify and estimate the average spectral type of cool stars in complex objects like active galaxy nuclei. To demonstrate such a possibility we also present long slit spectra of the Seyfert galaxy NGC 1068 where the equivalent widths of all stellar features are found to decrease in the central 4 arcsec around the nucleus but the (1.62)/(1.59) ratio, and hence the average stellar temperature, does not change significantly. The estimated average spectral type is late-K which is compatible with either an old and very metallic bulge population or a younger one associated with a recent starburst. These data also show that the non-stellar continuum accounts for approximately 30% and approximately equal to or greater than 80% of the flux at 1.62 and 2.3 micrometers respectively in the central 4.4 arcsec. The features around 1.6 micrometers are thus much less diluted than CO(2, 0) and hence offer advantages for studies of the stellar content in such objects. The non-stellar nuclear emission is very red and most probably associated with a hot (T approximately equal or greater than 800 K) dust component.

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