Exploring the stellar properties of M dwarfs with high-resolution spectroscopy from the optical to the near-infrared

Abstract

Context. Being the most numerous and oldest stars in the galaxy, M dwarfs are objects of great interest for exoplanet searches. The presence of molecules in their atmosphere complicates our understanding of their atmospheric properties. But great advances have recently been made in the modeling of M dwarfs due to the revision of solar abundances. Aims. We aim to determine stellar parameters of M dwarfs using high resolution spectra (R∼90 000) simultaneously in the visible and the near-infrared. The high resolution spectra and broad wavelength coverage provide an unique opportunity to understand the onset of dust and cloud formation at cool temperatures. Furthermore, this study will help in understanding the physical processes which occur in a cool atmospheres, particularly, the redistribution of energy from the optical to the near-infrared. Methods. The stellar parameters of M dwarfs in our sample have been determined by comparing the high resolution spectra both in the optical and in the near-infrared simultaneously observed by CARMENES with the synthetic spectra obtained from the BT-Settl model atmosphere. The detailed spectral synthesis of these observed spectra both in the optical and in the near-infrared helps to understand the missing continuum opacity. Results. For the first time, we derive fundamental stellar parameters of M dwarfs using the high resolution optical and near-infrared spectra simultaneously. We determine T eff , log g and [M/H] for 292 M dwarfs of spectral type M0 to M9, where the formation of dust and clouds are important. The derived T eff for the sample ranges from 2300 to 4000 K, values of log g ranges from 4.5 ≤ log g ≤ 5.5 and the resulting metallicity ranges from -0.5 ≤ [M/H] ≤ +0.5. We have also explored the possible differences in T eff , log g and [M/H] by comparing them with other studies of the same sample of M dwarfs.

The CARMENES search for exoplanets around M dwarfs

“…3.2 are of high quality. Furthermore, both T eff agree well with the recent measurements of Mann et al (2015) and Rajpurohit et al (2018). Therefore, the reason for the mismatch to the very reliable interferometric radii must lie in the photometry.…”

“…However, the resulting photospheric parameters typically differ. Most recently, Rajpurohit et al (2018) determined effective temperatures for the CARMENES GTO targets listed in Reiners et al (2018a) using the latest BT-Settl models, which are about 100-200 K smaller than ours. Describing these differences is beyond the scope of this paper and they will be investigated in a future publication.…”

“…However, for higher T eff values, there would be a systematic offset between these derived quantities. Similarly, using the significantly and systematically differing T eff results from Rajpurohit et al (2018) would also introduce a systematic offset in radii and masses beyond our error bars.…”

“…Only the log g values derived from radii and masses published by Gaidos & Mann (2014) show a very weak correlation to ours. Since the log g values that have recently been published by Rajpurohit et al (2018) do not correlate with Pass18 or other literature values, we did not include these values in Fig. 7.…”

Effective Temperatures of Low-mass Stars from High-resolution H-band Spectroscopy

“…TRAPPIST-1 is a M8 dwarf which hosts seven Earthsized planets, three of which are in the habitable zone (Gillon et al 2017). Filippazzo et al (2015), through a precise bolometric luminosity and radius estimate from evolutionary models, derived a semi-empirical T eff for TRAPPIST-1 of 2557±64 K. With a new measurement of the trigonometric parallax of TRAPPIST-1, Van Grootel et al (2018) obtained an updated luminosity value, that they combine with revised radius estimates, to determine a T eff = 2516±41 K. The last two temperatures are in good agreement within the uncertainties, however, more recently, Rajpurohit et al (2018a) derived a cooler temperature (2400±100 K) for TRAPPIST-1.…”