ABSTRACT
Detailed chemical composition of stars is of prime interest for a range of topics in modern stellar astrophysics, such as the chemical evolution of the Galaxy or the formation, composition, ...and structure of exoplanets. In this work, we derive the C and O abundances and update Sc, V, Mn, and Co abundances considering hyperfine structure effects and correcting for non-local thermodynamical equilibrium for a sample of 196 late-F, G-, and early-K stars with wide resolved M-dwarf companions. We accomplished this by employing the equivalent width method and high-resolution spectroscopic data. Furthermore, we investigated the distributions of X/Fe ratios and C/O as a function of metallicity (Fe/H) and kinematic population. The observed trends are consistent with previous findings reported in the literature. Additionally, we searched for confirmed exoplanets around our primary stars in the literature and found 24 exoplanets in 17 systems, while none of the M-dwarf companions in our sample presented confirmed exoplanets. In conclusion, our study provides homogeneous abundances from high-resolution spectra for a large sample of FGK primary stars, paving the way for further research on stellar abundances of the M secondaries and exoplanetary science.
Abstract
Knowledge of stellar atmospheric parameters (
T
eff
,
log
g
, Fe/H) of M dwarfs can be used to constrain both theoretical stellar models and Galactic chemical evolutionary models, and guide ...exoplanet searches, but their determination is difficult due to the complexity of the spectra of their cool atmospheres. In our ongoing effort to characterize M dwarfs, and in particular their chemical composition, we carried out multiband photometric calibrations of metallicity for early- and intermediate-type M dwarfs. The third Gaia data release provides high-precision astrometry and three-band photometry. This information, combined with the 2MASS and CatWISE2020 infrared photometric surveys and a sample of 4919 M dwarfs with metallicity values determined with high-resolution spectroscopy by
The Cannon
and APOGEE spectra, allowed us to study the effect of the metallicity in color–color and color–magnitude diagrams. We divided this sample into two subsamples: we used 1000 stars to train the calibrations with Bayesian statistics and Markov Chain Monte Carlo techniques, and the remaining 3919 stars to check the accuracy of the estimations. We derived several photometric calibrations of metallicity applicable to M dwarfs in the range of −0.45 ≤ Fe/H ≤ + 0.45 dex and spectral types down to M5.0 V that yield uncertainties down to the 0.10 dex level. Lastly, we compared our results with other photometric estimations published in the literature for an additional sample of 46 M dwarfs in wide binary systems with FGK-type primary stars and found a great predictive performance.
Context.
The CARMENES instrument, installed at the 3.5 m telescope of the Calar Alto Observatory in Almería, Spain, was conceived to deliver high-accuracy radial velocity (RV) measurements with ...long-term stability to search for temperate rocky planets around a sample of nearby cool stars. Moreover, the broad wavelength coverage was designed to provide a range of stellar activity indicators to assess the nature of potential RV signals and to provide valuable spectral information to help characterise the stellar targets.
Aims.
We describe the CARMENES guaranteed time observations (GTO), spanning from 2016 to 2020, during which 19 633 spectra for a sample of 362 targets were collected. We present the CARMENES Data Release 1 (DR1), which makes public all observations obtained during the GTO of the CARMENES survey.
Methods.
The CARMENES survey target selection was aimed at minimising biases, and about 70% of all known M dwarfs within 10 pc and accessible from Calar Alto were included. The data were pipeline-processed, and high-level data products, including 18 642 precise RVs for 345 targets, were derived. Time series data of spectroscopic activity indicators were also obtained.
Results.
We discuss the characteristics of the CARMENES data, the statistical properties of the stellar sample, and the spectroscopic measurements. We show examples of the use of CARMENES data and provide a contextual view of the exoplanet population revealed by the survey, including 33 new planets, 17 re-analysed planets, and 26 confirmed planets from transiting candidate follow-up. A subsample of 238 targets was used to derive updated planet occurrence rates, yielding an overall average of 1.44 ± 0.20 planets with 1
M
⊕
<
M
pl
sin
i <
1000
M
⊕
and 1 day <
P
orb
< 1000 days per star, and indicating that nearly every M dwarf hosts at least one planet. All the DR1 raw data, pipeline-processed data, and high-level data products are publicly available online.
Conclusions.
CARMENES data have proven very useful for identifying and measuring planetary companions. They are also suitable for a variety of additional applications, such as the determination of stellar fundamental and atmospheric properties, the characterisation of stellar activity, and the study of exoplanet atmospheres.
Deriving metallicities for solar-like stars follows well-established methods, but for cooler stars such as M dwarfs, the determination is much more complicated due to forests of molecular lines that ...are present. Several methods have been developed in recent years to determine accurate stellar parameters for these cool stars (
T
eff
≲ 4000 K). However, significant differences can be found at times when comparing metallicities for the same star derived using different methods. In this work, we determine the effective temperatures, surface gravities, and metallicities of 18 well-studied M dwarfs observed with the CARMENES high-resolution spectrograph following different approaches, including synthetic spectral fitting, analysis of pseudo-equivalent widths, and machine learning. We analyzed the discrepancies in the derived stellar parameters, including metallicity, in several analysis runs. Our goal is to minimize these discrepancies and find stellar parameters that are more consistent with the literature values. We attempted to achieve this consistency by standardizing the most commonly used components, such as wavelength ranges, synthetic model spectra, continuum normalization methods, and stellar parameters. We conclude that although such modifications work quite well for hotter main-sequence stars, they do not improve the consistency in stellar parameters for M dwarfs, leading to mean deviations of around 50–200 K in temperature and 0.1–0.3 dex in metallicity. In particular, M dwarfs are much more complex and a standardization of the aforementioned components cannot be considered as a straightforward recipe for bringing consistency to the derived parameters. Further in-depth investigations of the employed methods would be necessary in order to identify and correct for the discrepancies that remain.
Planets with orbital periods shorter than 1 day are rare and have formation histories that are not completely understood. Small ( R p < 2 R ⊕ ) ultra-short-period (USP) planets are highly irradiated, ...probably have rocky compositions with high bulk densities, and are often found in multi-planet systems. Additionally, USP planets found around small stars are excellent candidates for characterization using present-day instrumentation. Of the current full sample of approximately 5500 confirmed exoplanets, only 130 are USP planets and around 40 have mass and radius measurements. Wolf 327 (TOI-5747) is an M dwarf ( R * = 0.406 ± 0.015 R ⊙ , M * = 0.405 ± 0.019 M ⊙ , T eff = 3542 ± 70 K, and V = 13 mag) located at a distance d = 28.5 pc. NASA’s planet hunter satellite, TESS, detected transits in this star with a period of 0.573 day (13.7 h) and with a transit depth of 818 ppm. Ground-based follow-up photometry, high resolution imaging, and radial velocity (RV) measurements taken with the CARMENES spectrograph confirm the presence of this new USP planet. Wolf 327b is a super-Earth with a radius of R p = 1.24 ± 0.06 R ⊕ and a mass of M p = 2.53 ± 0.46 M ⊕ , yielding a bulk density of 7.24 ± 1.66 g cm −3 and thus suggesting a rocky composition. Owing to its close proximity to its host star ( a = 0.01 au), Wolf 327b has an equilibrium temperature of 996 ± 22 K. This planet has a mass and radius similar to K2-229b, a planet with an inferred Mercury-like internal composition. Planet interior models suggest that Wolf 327b has a large iron core, a small rocky mantle, and a negligible (if any) H/He atmosphere.
We determined effective temperatures, surface gravities, and metallicities for a sample of 343 M dwarfs observed with CARMENES, the double-channel, high-resolution spectrograph installed at the 3.5 m ...telescope at Calar Alto Observatory. We employed S
TE
P
AR
S
YN
, a Bayesian spectral synthesis implementation particularly designed to infer the stellar atmospheric parameters of late-type stars following a Markov chain Monte Carlo approach. We made use of the BT-Settl model atmospheres and the radiative transfer code
turbospectrum
to compute a grid of synthetic spectra around 75 magnetically insensitive Fe
I
and Ti
I
lines plus the TiO
γ
and
ϵ
bands. To avoid any potential degeneracy in the parameter space, we imposed Bayesian priors on
T
eff
and log
g
based on the comprehensive, multi-band photometric data available for the sample. We find that this methodology is suitable down to M7.0 V, where refractory metals such as Ti are expected to condense in the stellar photospheres. The derived
T
eff
, log
g
, and Fe/H range from 3000 to 4200 K, 4.5 to 5.3 dex, and −0.7 to 0.2 dex, respectively. Although our
T
eff
scale is in good agreement with the literature, we report large discrepancies in the Fe/H scales, which might arise from the different methodologies and sets of lines considered. However, our Fe/H is in agreement with the metallicity distribution of FGK-type stars in the solar neighbourhood and correlates well with the kinematic membership of the targets in the Galactic populations. Lastly, excellent agreement in
T
eff
is found for M dwarfs with interferometric angular diameter measurements, as well as in the Fe/H between the components in the wide physical FGK+M and M+M systems included in our sample.
We present the abundances of magnesium (Mg) and silicon (Si) for 314 dwarf stars with spectral types in the interval K7.0--M5.5 ($T_ eff $ range approx 4200--3050 K) observed with the CARMENES ...high-resolution spectrograph at the 3.5 m telescope at the Calar Alto Observatory. Our analysis employs the BT-Settl model atmospheres, the radiative transfer code Turbospectrum and a state-of-the-art selection of atomic and molecular data. These Mg and Si abundances are critical for understanding both the chemical evolution and assembly of the Milky Way and the formation and composition of rocky planets. Our chemical abundances show a line-to-line scatter at the level of 0.1\,dex for all studied spectral types. The typical error bar of our chemical abundance measurements is pm 0.11 dex (Mg) and pm 0.16 dex (Si) for all spectral types based on the comparison of the results obtained for stellar components of multiple systems. The derived abundances are consistent with the galactic evolution trends and observed chemical abundance distribution of earlier FGK-type stars in the solar neighbourhood. Besides, our analysis provides compatible abundances for stars in multiple systems. In addition, we studied the abundances of different galactic stellar populations. In this paper, we also explore the relation of the Mg and Si abundances of stars with and without known planets.
We report the discovery, mass, and radius determination of TOI-1801 b, a temperate mini-Neptune around a young M dwarf. TOI-1801 b was observed in TESS sectors 22 and 49, and the alert that this was ...a TESS planet candidate with a period of 21.3 days went out in April 2020. However, ground-based follow-up observations, including seeing-limited photometry in and outside transit together with precise radial velocity (RV) measurements with CARMENES and HIRES revealed that the true period of the planet is 10.6 days. These observations also allowed us to retrieve a mass of 5.74 ± 1.46
M
⊕
, which together with a radius of 2.08 ± 0.12
R
⊕
, means that TOI-1801 b is most probably composed of water and rock, with an upper limit of 2% by mass of H
2
in its atmosphere. The stellar rotation period of 16 days is readily detectable in our RV time series and in the ground-based photometry. We derived a likely age of 600–800 Myr for the parent star TOI-1801, which means that TOI-1801 b is the least massive young mini-Neptune with precise mass and radius determinations. Our results suggest that if TOI-1801 b had a larger atmosphere in the past, it must have been removed by some evolutionary mechanism on timescales shorter than 1 Gyr.
We report the confirmation and mass determination of a mini-Neptune transiting the M3.5 V star TOI-4438 (G 182-34) every 7.44 days. A transit signal was detected with NASA’s TESS space mission in the ...sectors 40, 52, and 53. In order to validate the planet TOI-4438 b and to determine the system properties, we combined TESS data with high-precision radial velocity measurements from the CARMENES spectrograph, spanning almost one year, and ground-based transit photometry. We found that TOI-4438 b has a radius of R b = 2.52 ± 0.13 R ⊕ (5% precision), which together with a mass of M b = 5.4 ± 1.1 M ⊕ (20% precision), results in a bulk density of ρ b = 1.85 −0.44 +0.51 g cm −3 (~28% precision), aligning the discovery with a volatile-rich planet. Our interior structure retrieval with a pure water envelope yields aminimum water mass fraction of 46% (1 σ ). TOI-4438 b is a volatile-rich mini-Neptune with likely H/He mixed with molecules, such as water, CO 2 , and CH 4 . The primary star has a J -band magnitude of 9.7, and the planet has a high transmission spectroscopy metric (TSM) of 136 ± 13. Taking into account the relatively warm equilibrium temperature of T eq = 435 ± 15 K, and the low activity level of its host star, TOI-4438 b is one of the most promising mini-Neptunes around an M dwarf for transmission spectroscopy studies.
Detailed chemical composition of stars is of prime interest for a range of
topics in modern stellar astrophysics, such as the chemical evolution of the
Galaxy or the formation, composition, and ...structure of exoplanets. In this
work, we derive the C and O abundances and update Sc, V, Mn, and Co abundances
considering hyperfine structure effects (HFS) and correcting for non-local
thermodynamical equilibrium (NLTE) for a sample of 196 late-F, G-, and early-K
stars with wide resolved M-dwarf companions. We accomplished this by employing
the equivalent width (EW) method and high-resolution spectroscopic data.
Furthermore, we investigated the distributions of X/Fe ratios and C/O as a
function of metallicity (Fe/H) and kinematic population. The observed trends
are consistent with previous findings reported in the literature. Additionally,
we searched for confirmed exoplanets around our primary stars in the literature
and found 24 exoplanets in 17 systems, while none of the M-dwarf companions in
our sample presented confirmed exoplanets. In conclusion, our study provides
homogeneous abundances from high-resolution spectra for a large sample of FGK
primary stars, paving the way for further research on stellar abundances of the
M secondaries and exoplanetary science.
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