As mentioned in the previous sections of this chapter, some work still has to be done to completely understand inconsistencies, especially concerning treatment of different lines in synthetic model atmospheres. The short comparison in Section 5.2 already showed that there are sets of parameters for which the models can look very similar in certain lines. That degeneracy could be broken by adjusting weights, in order to get accurate parameters also for hot M dwarfs. This requires a careful analysis of the models, com-paring lines for different parameter sets and weights. A correct adjustment might also help to accurately determine loggfrom model fits instead of evolutionary models. Later
5. Discussion
on, an adaption of the models themselves could become necessary. The previous fits already showed that for some lines would need improvement, e.g. the K-band Ca triplet.
SinceCARMENESoffers the possibility to observe stars simultaneously in the VIS and NIR, parameter determination in both ranges is desirable. A comparison of both results could help to improve models and understand any unknown wavelength dependent ef-fects on stellar parameters. For this purpose the sensitivity of lines in the NIR has to be confirmed by more detailed studies. From the analysis of the VIS spectra we saw how crucial a careful and detailed study of potentially sensitive lines is. Because the CARMENESspectra have much higher resolution than spectra from TripleSpec and X-Shooter, more spectral details are revealed, which makes a careful study necessary. This can be done by comparing the change in line strength and width for models with dif-ferent parameter sets. Lines that show large changes are potentially suitable for fitting.
However, they need to be checked in stars with known parameters to see if the models recover the line shape correctly.
The Gaia mission, launched in December 2013, provides highly accurate positions of about 1 billion stars. TheCARMENESproject can also benefit from these measurements by calculating stellar masses and radii from the Gaia distances. This information is fundamental for further planet characterisation.
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6. Summary
In the past years, planet search projects focused more and more on M dwarfs. Transit searches benefit from the smaller radius ratio between star and planet and the smaller orbital periods for planets in the habitable zone. Radial-velocity searches, on the other hand, benefit from the smaller mass ratio and therefore higher radial-velocity amplitude.
The CARMENESinstrument is a next-generation instrument located at the 3.5 m tele-scope at Calar Alto Observatory in Spain. It consists of two high-resolution (R∼82 000) spectrographs operating in the visible (0.55–0.95µm) and near-infrared (0.95–1.7 µm) wavelength range with precisions of around 1 ms−1. To characterise the target sample, more than 1 700 spectra have been observed with other high-resolution spectrographs like CAFE, FEROS and HRS prior to the start of theCARMENES survey. For the de-termination of fundamental stellar parameters, i.e. effective temperature (Teff), surface gravity (logg) and metallicity [Fe/H], I developed an algorithm to fit up-to-date synthetic models to the observations. The new PHOENIX-ACES model grid especially accounts for the formation of molecules in cool stellar atmospheres. My algorithm determines the best fit using linear interpolation of the model grid, a downhill-simplex method and χ2-minimisation. With this I analyse different samples of stars, the preparation sample observed with CAFE, FEROS and HRS (1390 spectra, 484 stars), the first six month of CARMENESdata (1738 spectra, 245 stars) and near-infrared spectra taken with Triple-Spec (19 spectra/stars) and X-Shooter (13 spectra/stars). I take into accountvsinis mea-sured by Jeffers et al. (submitted) from cross-correlation. To reduce the number of free parameters in the fit and get more accurate results, loggis determined from evolutionary models by Baraffe et al. (1998) depending onTeff and [Fe/H]. I obtain good results for 323 M dwarfs.
A comparison between results of the same stars from spectra obtained by different spec-trographs mainly shows agreement. Literature values are also generally consistent with my results, however there are a few outliers. Some of these outliers can be statistically expected, also because of the different quality and SNR of the spectra. They might as well be a hint for still existing inconsistencies in synthetic models or the use of different determination methods, especially regarding metallicities. A quite promising explana-tion is supported by the finding that there are (at least) two different sets of parameters for which the PHOENIX-ACES models look very similar in some spectral regions, namely high temperature and metallicity (~3 900–4 000 K, super-solar metallicity) and low tem-perature and metallicity (~3 500–3 600 K, sub-solar metallicity). It especially concerns the K- and Na-lines. With higher weighting of these lines due to their sensitivity in metallicity, the algorithm might fall into a local minimum and prefer a cooler tempera-ture solution for hot M dwarfs (i.e. earlier than ~M1.5 V). A more detailed investigation of this behaviour is needed to find proper weights that yield precise results for all spectral types.
6. Summary
This is the first study that derives Teff, loggand [Fe/H] from fitting PHOENIX-ACES models to high-resolution optical spectra. It shows that accurate determination of stellar parameters of a large sample of M dwarfs is possible using an automated algorithm.
With the fiasco-code I obtain uncertainties of 93 K, 0.29 dex in logg and 0.25 dex in [Fe/H]. However, a lot of detailed analysis is still necessary to completely understand the behaviour of spectral lines with changing parameters, and of synthetic models in general.
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