Diffuse interstellar band at 8620 \AA\ in RAVE: A new method for detecting the diffuse interstellar band in spectra of cool stars

摘要

Diffuse interstellar bands are usually observed in spectra of hot stars,where interstellar lines are rarely blended with stellar ones. The need for hotstars is a strong limitation in the number of sightlines we can observe and thedistribution of sightlines in the Galaxy, as hot stars are rare andconcentrated in the Galactic plane. We are introducing a new method, whereinterstellar lines can be observed in spectra of cool stars in largespectroscopic surveys. The method is completely automated and does not requireprior knowledge of the stellar parameters. If known, the stellar parametersonly reduce the computational time and are not involved in the extraction ofthe interstellar spectrum. The main step in extracting interstellar lines is aconstruction of the stellar spectrum, which is in our method done by findingother observed spectra that lack interstellar features and are otherwise verysimilar to the spectrum in question. Such spectra are then combined into asingle stellar spectrum template, which matches the stellar component in anobserved spectrum. We demonstrate the performance of this new method on asample of 482,430 spectra observed in RAVE survey. However, many spectra haveto be combined (48 on average) in order to achieve a S/N ratio high enough tomeasure the DIB's profile, hence limiting the spatial information about theISM. Only one strong interstellar line is included in the RAVE spectral range,a diffuse interstellar band at 8620 \AA. We compare its equivalent width withextinction maps and with Bayesian reddening, calculated for individual stars,and provide a linear relation between the equivalent width and reddening.Separately from the introduced method, we calculate equivalent widths of thediffuse interstellar band in spectra of hot stars with known extinction andcompare all three linear relations with each other and with relations from theliterature.