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Metallicity determination of M dwarfs
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Observational Astronomy.
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. , p. 46
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1587
Keywords [en]
stars: low mass - stars: abundances, fundamental parameters - technique: spectroscopic, photometric - planets and satellites: formation
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
URN: urn:nbn:se:uu:diva-332102ISBN: 978-91-513-0127-3 (print)OAI: oai:DiVA.org:uu-332102DiVA, id: diva2:1151639
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: 2018-03-07
List of papers
1. Metallicity determination of M dwarfs: High-resolution infrared spectroscopy
Open this publication in new window or tab >>Metallicity determination of M dwarfs: High-resolution infrared spectroscopy
2016 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 586, article id A100Article in journal (Refereed) Published
Abstract [en]

Context. Several new techniques to determine the metallicity of M dwarfs with better precision have been developed over the last decades. However, most of these studies were based on empirical methods. In order to enable detailed abundance analysis, standard methods established for warmer solar-like stars, i.e. model-dependent methods using fitting of synthetic spectra, still need to be used.

Aims. In this work we continue the reliability confirmation and development of metallicity determinations of M dwarfs using high-resolution infrared spectra. The reliability was confirmed through analysis of M dwarfs in four binary systems with FGK dwarf companions and by comparison with previous optical studies of the FGK dwarfs.

Methods. The metallicity determination was based on spectra taken in the J band (1.1-1.4 mu m) with the CRIRES spectrograph. In this part of the infrared, the density of stellar molecular lines is limited, reducing the amount of blends with atomic lines enabling an accurate continuum placement. Lines of several atomic species were used to determine the stellar metallicity.

Results. All binaries show excellent agreement between the derived metallicity of the M dwarf and its binary companion. Our results are also in good agreement with values found in the literature. Furthermore, we propose an alternative way to determine the effective temperature of M dwarfs of spectral types later than M2 through synthetic spectral fitting of the FeH lines in our observed spectra.

Conclusions. We have confirmed that a reliable metallicity determination of M dwarfs can be achieved using high-resolution infrared spectroscopy. We also note that metallicites obtained with photometric metallicity calibrations available for M dwarfs only partly agree with the results we obtain from high-resolution spectroscopy.

Keywords
stars: abundances, stars: low-mass, techniques: spectroscopic
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:uu:diva-281831 (URN)10.1051/0004-6361/201526602 (DOI)000369715900111 ()
Funder
Swedish National Space Board
Available from: 2016-03-30 Created: 2016-03-30 Last updated: 2017-10-24
2. Metallicity determination of M dwarfs: Expanded parameter range in metallicity and effective temperature
Open this publication in new window or tab >>Metallicity determination of M dwarfs: Expanded parameter range in metallicity and effective temperature
2017 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 604, article id A97Article in journal (Refereed) Published
Abstract [en]

Context. Reliable metallicity values for late K and M dwarfs are important for studies of the chemical evolution of the Galaxy and advancement of planet formation theory in low-mass environments. Historically it has been challenging to determine the stellar parameters of low-mass stars because of their low surface temperature, which causes several molecules to form in the photospheric layers. In our work we use the fact that infrared high-resolution spectrographs have opened up a new window for investigating M dwarfs. This enables us to use similar methods as for warmer solar-like stars.

Aims. Metallicity determination with high-resolution spectra is more accurate than with low-resolution spectra, but it is rather time consuming. In this paper we expand our sample analyzed with this precise method both in metallicity and effective temperature to build a calibration sample for a future revised empirical calibration. Methods. Because of the relatively few molecular lines in the J band, continuum rectification is possible for high-resolution spectra, allowing the stellar parameters to be determined with greater accuracy than with optical spectra. We obtained high-resolution spectra with the CRIRES spectrograph at the Very Large Telescope (VLT). The metallicity was determined using synthetic spectral fitting of several atomic species. For M dwarfs that are cooler than 3575 K, the line strengths of FeH lines were used to determine the effective temperatures, while for warmer stars a photometric calibration was used.

Results. We analyzed 16 targets with a range of effective temperature from 3350-4550 K. The resulting metallicities lie between -0.5 < [M/H] < +0.4. A few targets have previously been analyzed using low-resolution spectra and we find a rather good agreement with our values. A comparison with available photometric calibrations shows varying agreement and the spread within all empirical calibrations is large.

Conclusions. Including the targets from our previous paper, we analyzed 28 M dwarfs with high-resolution infrared spectra. The targets spread approximately one dex in metallicity and 1400 K in effective temperature. For individual M dwarfs we achieve uncer- tainties of 0.05 dex and 100 K on average. 

Keywords
stars:low-mass–stars:abundances–techniques:spectroscopic
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy
Identifiers
urn:nbn:se:uu:diva-332015 (URN)10.1051/0004-6361/201730715 (DOI)000408480100099 ()
Funder
Swedish National Space Board
Available from: 2017-10-22 Created: 2017-10-22 Last updated: 2017-10-24Bibliographically approved
3. A photometric calibration for M dwarfs based on high-resolution infrared spectrscopy
Open this publication in new window or tab >>A photometric calibration for M dwarfs based on high-resolution infrared spectrscopy
(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.

Keywords
Stars: low-mass - Techniques: photometric - Planets and satellites: formation
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:uu:diva-332624 (URN)
Funder
Swedish National Space Board
Available from: 2017-10-31 Created: 2017-10-31 Last updated: 2017-11-01

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