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A photometric calibration for M dwarfs based on high-resolution infrared spectrscopy
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Observational Astronomy.
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Observational Astronomy.
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Observational Astronomy.
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Context. M dwarfs are, by number, the dominant stellar population in the local Galaxy. We have previously shown that the stellar parameters of individual M dwarfs can be determined with good accuracy using high-resolution infrared spectroscopy. Since this method is time-consuming the use of empirical calibrations based on photometry or low-resolution spectra is preferable for studies of large samples. In earlier work we showed that available photometric calibrations for M dwarfs have offsets compared to our high- resolution metallicity results.

Aims. We use a sample of 33 late K and M dwarfs with accurately determined metallicities to derive a new photometric calibration for M dwarfs. The aim is to achieve a calibration with lower uncertainty and better agreement with spectroscopic results.Methods. We explored the B, V, J, H, K, g, r, i magnitudes to find combinations that show a correlation between the metallicity and the position in a color-magnitude diagram. We used 5-fold cross-validation and the corrected Akaike information criterion to find a polynomial model that minimizes the information lost, exploring different polynomial degrees, with and without cross-terms. The coefficients were determined through a weighted least-squares solution against our calibration sample.

Results. We present a new photometric metallicity calibration for M dwarfs, where the metallicity is given by a linear relation and onecross-termoftheMKSmagnitudeandtheV−(JHK) or B−(JHK) colors. The new calibration relations have root-mean-square deviation values with respect to the calibration sample of 0.11-0.12 dex, which is about 0.1 dex lower than for previous photometric calibrations, and we also achieve a substantial improvement of the value of the adjusted squared multiple correlation coefficient. We furthermore show that our calibration compares well with two spectroscopic calibrations. Comparing synthetic calculated iso- metallicity lines with our empirical calibration, we find a good agreement of the shapes and slopes.

Conclusions. Compared to previous calibrations our new photometric calibration shows improvement in all statistical tests performed. Through our testing of several polynomials we conclude that the inclusion of a cross-term between absolute magnitude and color is important for the performance of the photometric calibration. Applying our photometric calibration to a sample of M dwarfs known to host planets points towards a possible giant planet-metallicity correlation for M dwarfs.

Keyword [en]
Stars: low-mass - Techniques: photometric - Planets and satellites: formation
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
URN: urn:nbn:se:uu:diva-332624OAI: oai:DiVA.org:uu-332624DiVA: diva2:1153594
Funder
Swedish National Space Board
Available from: 2017-10-31 Created: 2017-10-31 Last updated: 2017-11-01
In thesis
1. Metallicity determination of M dwarfs
Open this publication in new window or tab >>Metallicity determination of M dwarfs
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

M dwarfs constitute around 70% of all stars in the local Galaxy. Their multitude together with their long main-sequence lifetimes make them important for studies of global properties of the Galaxy such as the initial mass function or the structure and kinematics of stellar populations. In addition, the exoplanet community is showing an increasing interest for those small, cold stars. However, very few M dwarfs are well characterized, and in the case of exoplanetary systems the stellar parameters have a direct influence on the derived planet properties.

Stellar parameters of M dwarfs are difficult to determine because of their low surface temperatures that result in an optical spectrum dominated by molecular lines. Most previous works have therefore relied on empirical calibrations. High-resolution spectrographs operating in the infrared, a wavelength region less affected by molecular lines, have recently opened up a new window for the investigation of M dwarfs. In the two first papers of this thesis we have shown that we can determine the metallicity, and in some cases the effective temperature, using synthetic spectral fitting with improved accuracy.

This method is time consuming and therefore not practical or even feasible for studies of large samples of M dwarfs. When comparing our results from the high-resolution studies with available photometric calibrations we find systematic differences. In the third paper we therefore used our sample to determine a new photometric metallicity calibration. Compared to previous calibrations our new photometric calibration shows improved statistical characteristics, and our calibration gives similar results as spectroscopic calibrations. In a comparison with theoretical calculations we find a good agreement of the shapes and slopes of iso-metallicity lines with our empirical relation. Applying the photometric calibration to a sample of M dwarfs with confirmed exoplanets we find a possible giant planet-metallicity correlation for M dwarfs.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2017. 46 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1587
Keyword
stars: low mass - stars: abundances, fundamental parameters - technique: spectroscopic, photometric - planets and satellites: formation
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:uu:diva-332102 (URN)978-91-513-0127-3 (ISBN)
Public defence
2017-12-12, Polhemssalen, Ångströmslaboratoriet, Lägerhyddsvägen 1, Uppsala, 09:30 (English)
Opponent
Supervisors
Available from: 2017-11-20 Created: 2017-10-24 Last updated: 2017-11-20

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