Effective temperatures of classical Cepheids from line-depth ratios in the H-band

TL;DR

This study expands the use of line depth ratio techniques to determine the effective temperatures of Cepheid variable stars in the near-infrared, enabling better analysis of highly reddened stars.

Contribution

The paper introduces 87 new near-infrared LDR-temperature relations for Cepheids, calibrated with simultaneous optical and near-infrared spectra, filling a gap in existing methods.

Findings

Derived 87 temperature calibrations in the 4800-6500 K range.

Achieved a typical temperature uncertainty of 60-70 K per relation.

Final combined precision within 30-50 K for temperature estimates.

Abstract

The technique of line depth ratios (LDR) is one of the methods to determine the effective temperature of a star. They are crucial in the spectroscopic studies of variable stars like Cepheids since no simultaneous photometry is usually available. A good number of LDR-temperature relations are already available in the optical domain, here we want to expand the number of relations available in the near-infrared in order to fully exploit the capabilities of current and upcoming near-infrared spectrographs. We used 115 simultaneous spectroscopic observations in the optical and the near-infrared for six Cepheids and optical line depth ratios to find new pairs of lines sensitive to temperature and to calibrate LDR-temperature relations in the near-infrared spectral range. We have derived 87 temperature calibrations valid in the [4800-6500] K range of temperatures. The typical uncertainty for a given relation is 60-70 K, and combining many of them provides a final precision within 30-50 K. We found a discrepancy between temperatures derived from optical or near-infrared LDR for pulsations phases close to phi ~ 0.0 and we discuss the possible causes for these differences. Line depth ratios in the near-infrared will allow us to spectroscopically investigate highly reddened Cepheids in the Galactic centre or in the far side of the disk.