3,034 research outputs found
The Effective temperature scale of M dwarfs from spectral synthesis
We present a comparison of low-resolution spectra of 60 stars covering the
whole M-dwarf sequence. Using the most recent PHOENIX BT-Settl stellar model
atmospheres (see paper by F. Allard, in this book) we do a first quantitative
compari- son to our observed spectra in the wavelength range 550-950 nm. We
perform a first confrontation between models and observations and we assign an
effective tempera- tures to the observed M-dwarfs. Teff-spectral type relations
are then compared with the published ones. This comparison also aims at
improving the models' opacities.Comment: To be published in the on-line version of the Proceedings of Cool
Stars 16 (ASP Conference Series) New version with bibliography correcte
High-contrast imaging of Sirius~A with VLT/SPHERE: Looking for giant planets down to one astronomical unit
Sirius has always attracted a lot of scientific interest, especially after
the discovery of a companion white dwarf at the end of the 19th century. Very
early on, the existence of a potential third body was put forward to explain
some of the observed properties of the system. We present new coronagraphic
observations obtained with VLT/SPHERE that explore, for the very first time,
the innermost regions of the system down to 0.2" (0.5 AU) from Sirius A. Our
observations cover the near-infrared from 0.95 to 2.3 m and they offer the
best on-sky contrast ever reached at these angular separations. After detailing
the steps of our SPHERE/IRDIFS data analysis, we present a robust method to
derive detection limits for multi-spectral data from high-contrast imagers and
spectrographs. In terms of raw performance, we report contrasts of 14.3 mag at
0.2", ~16.3 mag in the 0.4-1.0" range and down to 19 mag at 3.7". In physical
units, our observations are sensitive to giant planets down to 11 at
0.5 AU, 6-7 in the 1-2 AU range and ~4 at 10 AU. Despite
the exceptional sensitivity of our observations, we do not report the detection
of additional companions around Sirius A. Using a Monte Carlo orbital analysis,
we show that we can reject, with about 50% probability, the existence of an 8
planet orbiting at 1 AU. In addition to the results presented in the
paper, we provide our SPHERE/IFS data reduction pipeline at
http://people.lam.fr/vigan.arthur/ under the MIT license.Comment: 16 pages, 10 figures, accepted for publication in MNRA
A library of near-infrared integral field spectra of young M-L dwarfs
We present a library of near-infrared (1.1-2.45 microns) medium-resolution
(R~1500-2000) integral field spectra of 15 young M6-L0 dwarfs, composed of
companions with known ages and of isolated objects. We use it to (re)derive the
NIR spectral types, luminosities and physical parameters of the targets, and to
test (BT-SETTL, DRIFT-PHOENIX) atmospheric models. We derive infrared spectral
types L0+-1, L0+-1, M9.5+-0.5, M9.5+-0.5, M9.25+-0.25, M8+0.5-0.75, and
M8.5+-0.5 for AB Pic b, Cha J110913-773444, USco CTIO 108B, GSC 08047-00232 B,
DH Tau B, CT Cha b, and HR7329B, respectively. BT-SETTL and DRIFT-PHOENIX
models yield close Teff and log g estimates for each sources. The models seem
to evidence a 600-300+600 K drop of the effective temperature at the M-L
transition. Assuming the former temperatures are correct, we derive new mass
estimates which confirm that DH Tau B, USco CTIO 108B, AB Pic b, KPNO Tau 4,
OTS 44, and Cha1109 lay inside or at the boundary of the planetary mass range.
We combine the empirical luminosities of the M9.5-L0 sources to the Teff to
derive semi-empirical radii estimates that do not match "hot-start"
evolutionary models predictions at 1-3 Myr. We use complementary data to
demonstrate that atmospheric models are able to reproduce the combined optical
and infrared spectral energy distribution, together with the near-infrared
spectra of these sources simultaneously. But the models still fail to represent
the dominant features in the optical. This issue casts doubts on the ability of
these models to predict correct effective temperatures from near-infrared
spectra alone. We advocate the use of photometric and spectroscopic data
covering a broad range of wavelengths to study the properties of very low mass
young companions to be detected with the planet imagers (Subaru/SCExAO,
LBT/LMIRCam, Gemini/GPI, VLT/SPHERE).Comment: 27 pages, 14 tables, 19 figures, accepted for publication in
Astronomy & Astrophysic
Photospheric properties and fundamental parameters of M dwarfs
M dwarfs are an important source of information when studying and probing the
lower end of the Hertzsprung-Russell (HR) diagram, down to the hydrogen-burning
limit. Being the most numerous and oldest stars in the galaxy, they carry
fundamental information on its chemical history. The presence of molecules in
their atmospheres, along with various condensed species, complicates our
understanding of their physical properties and thus makes the determination of
their fundamental stellar parameters more challenging and difficult. The aim of
this study is to perform a detailed spectroscopic analysis of the
high-resolution H-band spectra of M dwarfs in order to determine their
fundamental stellar parameters and to validate atmospheric models. The present
study will also help us to understand various processes, including dust
formation and depletion of metals onto dust grains in M dwarf atmospheres. The
high spectral resolution also provides a unique opportunity to constrain other
chemical and physical processes that occur in a cool atmosphere The
high-resolution APOGEE spectra of M dwarfs, covering the entire H-band, provide
a unique opportunity to measure their fundamental parameters. We have performed
a detailed spectral synthesis by comparing these high-resolution H-band spectra
to that of the most recent BT-settl model and have obtained fundamental
parameters such as effective temperature, surface gravity, and metallicity
(Teff, log g and [Fe/H]) respectively.Comment: 15 pages, 10 figures, accepted for publication in A&
Metallicity of M dwarfs III. Planet-metallicity and planet-stellar mass correlations of the HARPS GTO M dwarf sample
Aims. The aim of this work is the study of the planet-metallicity and the
planet-stellar mass correlations for M dwarfs from the HARPS GTO M dwarf
subsample
Methods. We use a new method that takes advantage of the HARPS
high-resolution spectra to increase the precision of metallicity, using
previous photometric calibrations of [Fe/H] and effective temperature as
starting values.
Results. In this work we use our new calibration (rms = 0.08 dex) to study
the planet-metallicity relation of our sample. The well-known correlation for
Giant planet FGKM hosts with metallicity is present. Regarding Neptunians and
smaller hosts no correlation is found but there is a hint that an
anti-correlation with [Fe/H] may exist. We combined our sample with the
California Planet Survey late-K and M-type dwarf sample to increase our
statistics but found no new trends. We fitted a power law to the frequency
histogram of the Jovian hosts for our sample and for the combined sample, f_p =
C10^\alpha[Fe/H], using two different approaches: a direct bin fitting and a
bayesian fitting procedure. We obtained a value for C between 0.02 and 0.04 and
for \alpha between 1.26 and 2.94.
Regarding stellar mass, an hypothetical correlation with planets was
discovered, but was found to be the result of a detection bias.Comment: Accepted for publication in A&A. 18 pages, 11 Figures, 12 Table
Metallicity of M dwarfs IV. A high-precision [Fe/H] and Teff technique from high-resolution optical spectra for M dwarfs
Aims. In this work we develop a technique to obtain high precision
determinations of both metallicity and effective temperature of M dwarfs in the
optical.
Methods. A new method is presented that makes use of the information of 4104
lines in the 530-690 nm spectral region. It consists in the measurement of
pseudo equivalent widths and their correlation with established scales of
[Fe/H] and .
Results. Our technique achieves a of 0.080.01 for [Fe/H],
9113 K for , and is valid in the (-0.85, 0.26 dex), (2800, 4100
K), and (M0.0, M5.0) intervals for [Fe/H], and spectral type
respectively. We also calculated the RMSE which estimates uncertainties
of the order of 0.12 dex for the metallicity and of 293 K for the effective
temperature. The technique has an activity limit and should only be used for
stars with . Our method is available
online at \url{http://www.astro.up.pt/resources/mcal}.Comment: Accepted in Astronomy and Astrophysics. Updated one important
reference in the introduction. Some typos correcte
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