Extracting interstellar diffuse absorption bands from cool star spectra

Application to bulge clump giants in Baade’s window

¹ GEPI Observatoire de Paris, CNRS, Université Paris-Diderot, Place Jules Janssen, 92195 Meudon Cedex, France
e-mail: hui-chen.chen@obspm.fr
² Université de Nice Sophia Antipolis, CNRS, Observatoire de la Côte d’Azur, Laboratoire Cassiopée, BP 4229, 06304 Nice Cedex 4, France

Received: 19 September 2012
Accepted: 5 November 2012

Abstract

Aims. Interstellar (IS) absorption lines or diffuse bands are usually extracted from early-type star spectra because they are characterized by smooth continua. However, this introduces a strong limitation on the number of available targets, and reduces potential studies of the IS matter and the use of absorptions for cloud mapping.

Methods. We have developed a new and automated fitting method appropriate to interstellar (IS) absorptions in the spectra of cool stars that possess stellar atmospheric parameters. We applied this method to the extraction of three diffuse interstellar bands (DIBs) in high resolution VLT FLAMES/GIRAFFE spectra of red-clump stars from the bulge. By combining all stellar synthetic spectra, HITRAN-LBLRTM atmospheric transmission spectra and diffuse band empirical absorption profiles, we determined the 6196.0, 6204.5, and 6283.8 Å DIB strength toward the 219 target stars and discuss the sources of uncertainties. To test the sensitivity of the DIB extraction, we compared all three results and compared the DIB equivalent widths (EWs) with the reddening derived from an independent extinction map based on OGLE photometric data. We also derived the three independent color excess estimates based on the DIB strengths and average linear correlation coefficients previously established in the solar neighborhood and compared with the OGLE photometric results.

Results. Most stellar spectra could be reproduced well by the composite stellar, atmospheric, and IS models. Measurement uncertainties on the EWs are smaller for the broad and strong 6283.8 Å DIB, and are in the range 10−15%. Uncertainties on the two narrow and weaker DIBs are larger, as expected, and found to be highly variable from one target to the next. They strongly depend on the star’s radial velocity. DIB-DIB correlations among the three bands demonstrate that a meaningful signal is extracted. For the 6284 and 6204 Å DIBs, the star-to-star variability of the EW also reflects features of the OGLE extinction map. The three independent extinction estimates deduced from the EWs and solar neighborhood correlation coefficients agree with each other within 20%, a value which probably reflects that they belong to different families. The estimated average color excess is also compatible with the photometric determination.

Conclusions. This work demonstrates the feasibility of the method of ISM DIB extraction in cool star spectra, based on synthetic spectra. It confirms that DIB measurements and local DIB-extinction calibrations can provide rough, first-order estimates of the towards distant targets.