---
author:
- 'V. M. Passegger$^{1}$, A. Reiners$^{1}$, S. V. Jeffers$^{1}$, S. Wende$^{1}$, P. Schöfer$^{1}$, P. J. Amado$^{2}$, J. A. Caballero$^{3}$, D. Montes$^{4}$, R. Mundt$^{5}$, I. Ribas$^{6}$, A. Quirrenbach$^{3}$, and the CARMENES Consortium'
bibliography:
- 'passegger.bib'
title: 'Spectroscopic characterisation of CARMENES target candidates from FEROS, CAFE and HRS high-resolution spectra'
---

Introduction
============

The new CARMENES instrument is mounted at the 3.5 m telescope at Calar Alto Observatory, located in the Sierra de los Filabres in southern Spain. It consists of two fibre-fed high-resolution spectrographs, operating in the visible wavelength range from 0.52 to 0.96 $\mu$m and in the near-infrared from 0.96 to 1.71 $\mu$m, having a spectral resolution of R &gt; 80,000. [@Quirrenbach2010; @Quirrenbach2012; @Quirrenbach2014] Both spectrographs will simultaneously perform high-accuracy radial-velocity measurements of about 300 M dwarfs during three years of guaranteed observing time. The aim is to detect low-mass planets within the habitable zones of these stars.\
For science preparation over 1500 high-resolution spectra have been observed with FEROS, CAFE and HRS to determine effective temperature, surface gravity and metallicity. These parameters are fundamental for characterising star-planet systems. The spectra of M dwarfs are very complex, with molecular lines forming due to the low temperatures. This makes it difficult to use a line-by-line approach and requires a full spectral synthesis, which in turn necessitates for accurate models that take into account the formation of molecules. We use the latest generation PHOENIX model grid, the PHOENIX ACES models [@Husser2013]. These models are especially designed for low temperature stellar atmospheres and use a new equation of state to accurately reproduce molecular lines.

Methods and Data
================

   Name    Resolution   Coverage \[nm\]   No. Spectra   No. Stars       Observing Period
  ------- ------------ ----------------- ------------- ----------- --------------------------
   CAFE     \~65,000        396-950           623          236      2013-01-21 to 2014-09-26
   FEROS     48,000         350-920           455          217      2012-12-31 to 2014-07-11
    HRS      60,000        420-1100           93           29       2011-09-29 to 2013-06-18

Table \[tab:obs\] summarizes the properties of the spectrographs used for observation and the data taken. Some observed spectra could not be used for analysis because of different issues, e.g. very low signal-to-noise, observation of wrong target, polluting light from close companions.

The method we use was described in detail in [@Passegger2016a]. We fit PHOENIX ACES model spectra to our observed spectra. This is done for different spectral ranges, including the $\gamma$- and $\epsilon$-TiO bands (sensitive to temperature and metallicity), the K- and Na-doublets around 768 nm and 819 nm (sensitive to surface gravity and metallicity) and two CaII-lines. Rotational velocities determined by [@Jeffers2016] are included to account for line broadening due to stellar rotation. Other than [@Passegger2016a] a downhill simplex is implemented for linear interpolation between the model grid points and a $\chi2$ -minimization determines the best fit to the data. Figure \[fig:fit\] shows an example fit to CARMENES data.

![image](fit.jpg){width="0.85\linewidth"}

Results and Discussion
======================

We obtained stellar parameters for 351 stars from 977 spectra. We find that most stars lie within 3200-3900 K, corresponding to spectral types M1V-M5V, as shown in the upper left panel of Figure \[fig:results\]. The higher the metallicity the higher the temperature for each spectral type (Figure \[fig:results\], lower left panel). This is consistent with results by [@Mann2015]. They showed that with increasing metallicity the radius increases, for fixed temperature. The spectral types have been calculated using spectral indices [@Schoefer2015]. The green squares correspond to a literature computation by [@PecautMamajek2013] for solar metallicity. A literature comparison with [@RojasAyala2012], [@GaidosMann2014] and [@Maldonado2015] shows that our values for metallicity turn out to be higher than published ones. (Figure \[fig:results\], upper right). One possible explanation for this is that PHOENIX ACES models still cannot reproduce the full depths of some lines (see Figure \[fig:fit\], 4th wavelength range), which might cause the algorithm to choose higher metallicity models to fit the lines. On the other hand it seems that the signal-to-noise ratio is also very important for parameter determination. 75 percent of the stars with \[Fe/H\] higher than 0.6 have SNRs lower than 50. We find good agreement with expected \[Fe/H\] values for SNR&gt;50 (Figure \[fig:results\], lower right). For the first four months of CARMENES data we find that the parameters show better agreement with literature, having better SNRs.

![image](results.jpg){width="0.85\linewidth"}

Acknowledgments {#acknowledgments .unnumbered}
===============

[CARMENES is an instrument for the Centro Astronómico Hispano-Alemán de Calar Alto (CAHA). CARMENES was funded by the German Max-Planck-Gesellschaft (MPG), the Spanish Consejo Superior de Investigaciones Cientícas (CSIC), the European Union through European Regional Fund (FEDER/ERF), Spanish Ministry of Economy and Competitiveness, the state of Baden-Württemberg, the German Science Foundation (DFG), the Junta de Andalucía, and by the Klaus Tschira Stiftung, with additional contributions by the members of the CARMENES Consortium (Max-Planck-Institut für Astronomie, Instituto de Astrofísica de Andalucía, Landessternwarte Königstuhl, Institut de Ciències de l’Espai, Institut für Astrophysik Göttingen, Universidad Complutense de Madrid, Thüringer Landessternwarte Tautenburg, Instituto de Astrofísica de Canarias, Hamburger Sternwarte, Centro de Astrobiología, and the Centro Astronómico Hispano-Alemán).]{}
