Person: Cortés Contreras, Miriam
Universidad Complutense de Madrid
Faculty / Institute
Física de la Tierra y Astrofísica
Astronomía y Astrofísica
Now showing 1 - 10 of 58
PublicationThe CARMENES search for exoplanets around M dwarfs Mapping stellar activity indicators across the M dwarf domain(EDP Sciencies, 2021-08-04) Caballero, J. A.; Cortés Contreras, Miriam; Montes Gutiérrez, DavidContext. Stellar activity poses one of the main obstacles for the detection and characterisation of small exoplanets around cool stars, as it can induce radial velocity (RV) signals that can hide or mimic the presence of planetary companions. Several indicators of stellar activity are routinely used to identify activity-related signals in RVs, but not all indicators trace exactly the same activity effects, nor are any of them always effective in all stars. Aims. We evaluate the performance of a set of spectroscopic activity indicators for M dwarf stars with different masses and activity levels with the aim of finding a relation between the indicators and stellar properties. Methods. In a sample of 98 M dwarfs observed with CARMENES, we analyse the temporal behaviour of RVs and nine spectroscopic activity indicators: cross-correlation function (CCF) full-width-at-half-maximum (FWHM), CCF contrast, CCF bisector inverse slope (BIS), RV chromatic index (CRX), differential line width (dLW), and indices of the chromospheric lines Hα and calcium infrared triplet (IRT). Results. A total of 56 stars of the initial sample show periodic signals related to activity in at least one of these ten parameters. RV is the parameter for which most of the targets show an activity-related signal. CRX and BIS are effective activity tracers for the most active stars in the sample, especially stars with a relatively high mass, while for less active stars, chromospheric lines perform best. FWHM and dLW show a similar behaviour in all mass and activity regimes, with the highest number of activity detections in the low-mass, high-activity regime. Most of the targets for which we cannot identify any activity-related signals are stars at the low-mass end of the sample (i.e. with the latest spectral types). These low-mass stars also show the lowest RV scatter, which indicates that ultracool M dwarfs could be better candidates for planet searches than earlier types, which show larger RV jitter. Conclusions. Our results show that the spectroscopic activity indicators analysed behave differently, depending on the mass and activity level of the target star. This underlines the importance of considering different indicators of stellar activity when studying the variability of RV measurements. Therefore, when assessing the origin of an RV signal, it is critical to take into account a large set of indicators, or at least the most effective ones considering the characteristics of the star, as failing to do so may lead to false planet claims. PublicationDetection of He I λ10830 Å absorption on HD 189733 b with CARMENES high-resolution transmission spectroscopy(EDP Science, 2018-12-07) Caballero, J.A.; Sanz Forcada, J.; Cortés Contreras, Miriam; Montes Gutiérrez, DavidWe present three transit observations of HD 189733 b obtained with the high-resolution spectrograph CARMENES at Calar Alto. A strong absorption signal is detected in the near-infrared He I triplet at 10830 Å in all three transits. During mid-transit, the mean absorption level is 0.88 ± 0.04% measured in a ±10 km s^(−1) range at a net blueshift of −3.5 ± 0.4 km s s^(−1) (10829.84–10830.57 Å). The absorption signal exhibits radial velocities of +6.5 ± 3.1 km s s^(−1) and −12.6 ± 1.0 km s s^(−1) during ingress and egress, respectively; all radial velocities are measured in the planetary rest frame. We show that stellar activity related pseudo-signals interfere with the planetary atmospheric absorption signal. They could contribute as much as 80% of the observed signal and might also affect the observed radial velocity signature, but pseudo-signals are very unlikely to explain the entire signal. The observed line ratio between the two unresolved and the third line of the He I triplet is 2.8 ± 0.2, which strongly deviates from the value expected for an optically thin atmospheres. When interpreted in terms of absorption in the planetary atmosphere, this favors a compact helium atmosphere with an extent of only 0.2 planetary radii and a substantial column density on the order of 4 × 10^(12) cm s^(−2) . The observed radial velocities can be understood either in terms of atmospheric circulation with equatorial superrotation or as a sign of an asymmetric atmospheric component of evaporating material. We detect no clear signature of ongoing evaporation, like pre- or post-transit absorption, which could indicate material beyond the planetary Roche lobe, or radial velocities in excess of the escape velocity. These findings do not contradict planetary evaporation, but only show that the detected helium absorption in HD 189733 b does not trace the atmospheric layers that show pronounced escape signatures. PublicationCalibrating the metallicity of M dwarfs in wide physical binaries with F-, G-, and K-primaries -I: High-resolution spectroscopy with HERMES: stellar parameters, abundances, and kinematics(Oxford Univ Press, 2018-09) Montes Gutiérrez, David; González Peinado, R.; Tabernero, H. M.; Caballero, J. A.; Gómez Marfil, Emilio; Alonso Floriano, F. J.; Cortés Contreras, Miriam; González Hernández, J. I.; Klutsch, A.; Moreno Jodar, C.We investigated almost 500 stars distributed among 193 binary or multiple systems made of late-E, G-, or earl y-K-primaries and late-K- or M-dwarf companion candidates, For all of them, we compiled or measured coordinates, J-band magnitudes, spectral types, distances, and proper motions. With these data, we established a sample of 192 physically bound systems. In parallel, we carried out observations with HERMES/Mercator and obtained high-resolution spectra for the 192 primaries and five secondaries. We used these spectra and the automatic STEPAR code for deriving precise stellar atmospheric parameters: T-eff, log g, xi, and chemical abundances for 13 atomic species, including [Fe/H]. After computing Galactocentric space velocities for all the primary stars, we performed a kinematic analysis and classified them in different Galactic populations and stellar kinematic groups of very different ages, which match our own metallicity determinations and isochronal age estimations. In particular, we identified three systems in the halo and 33 systems in the young Local Association, Ursa Major and Castor moving groups, and IC 2391 and Hyades superclusters. We finally studied the exoplanet-metallicity relation in our 193 primaries and made a list 13 M-dwarf companions with very high metallicity that can be the targets of new dedicated exoplanet surveys. All in all, our dataset will be of great help for future works on the accurate determination of metallicity of M dwarfS. PublicationThe CARMENES search for exoplanets around M dwarfs: variability of the HeI line at 10 830 Å(EDP Sciencies, 2020-08-10) Cortés Contreras, Miriam; Montes Gutiérrez, DavidThe He I infrared (IR) triplet at 10 830 Å is known as an activity indicator in solar-type stars and has become a primary diagnostic in exoplanetary transmission spectroscopy. He I IR lines are a tracer of the stellar extreme-ultraviolet irradiation from the transition region and corona. We study the variability of the He I triplet lines in a spectral time series of 319 M dwarf stars that was obtained with the CARMENES high-resolution optical and near-infrared spectrograph at Calar Alto. We detect He I IR line variability in 18% of our sample stars, all of which show Hα in emission. Therefore, we find detectable He I variability in 78% of the sub-sample of stars with Hα emission. Detectable variability is strongly concentrated in the latest spectral sub-types, where the He I lines during quiescence are typically weak. The fraction of stars with detectable He I variation remains lower than 10% for stars earlier than M3.0 V, while it exceeds 30% for the later spectral sub-types. Flares are accompanied by particularly pronounced line variations, including strongly broadened lines with red and blue asymmetries. However, we also find evidence for enhanced He I absorption, which is potentially associated with increased high-energy irradiation levels at flare onset. Generally, He I and Hα line variations tend to be correlated, with Hα being the most sensitive indicator in terms of pseudo-equivalent width variation. This makes the He I triplet a favourable target for planetary transmission spectroscopy. PublicationReaching the boundary between stellar kinematic groups and very wide binaries III. Sixteen new stars and eight new wide systems in the β Pictoris moving group(EDP Sciencies, 2015-11) Alonso Floriano, F. J.; Caballero, J. A.; Cortés Contreras, Miriam; Solano, E.; Montes Gutiérrez, DavidAims. We look for common proper motion companions to stars of the nearby young β Pictoris moving group. Methods. First, we compiled a list of 185 β Pictoris members and candidate members from 35 representative works. Next, we used the Aladin and STILTS virtual observatory tools and the PPMXL proper motion and Washington Double Star catalogues to look for companion candidates. The resulting potential companions were subjects of a dedicated astro-photometric follow-up using public data from all-sky surveys. After discarding 67 sources by proper motion and 31 by colour-magnitude diagrams, we obtained a final list of 36 common proper motion systems. The binding energy of two of them is perhaps too small to be considered physically bound. Results. Of the 36 pairs and multiple systems, eight are new, 16 have only one stellar component previously classified as a β Pictoris member, and three have secondaries at or below the hydrogen-burning limit. Sixteen stars are reported here for the first time as moving group members. The unexpected large number of high-order multiple systems, 12 triples and two quadruples among 36 systems, may suggest a biased list of members towards close binaries or an increment of the high-order-multiple fraction for very wide systems. PublicationThe CARMENES search for exoplanets around M dwarfs Period search in H alpha, Na I D, and Ca II IRT lines(EDP Sciencies, 2019-02-27) Cortés Contreras, Miriam; Díez Alonso, Enrique; Montes Gutiérrez, DavidWe use spectra from CARMENES, the Calar Alto high-Resolution search for M dwarfs with Exo-earths with Near-infrared and optical Echelle Spectrographs, to search for periods in chromospheric indices in 16 M0–M2 dwarfs. We measure spectral indices in the Hα, the Ca II infrared triplet (IRT), and the Na I D lines to study which of these indices are best-suited to finding rotation periods in these stars. Moreover, we test a number of different period-search algorithms, namely the string length method, the phase dispersion minimisation, the generalized Lomb–Scargle periodogram, and the Gaussian process regression with quasi-periodic kernel. We find periods in four stars using Hα and in five stars using the Ca II IRT, two of which have not been found before. Our results show that both Hα and the Ca II IRT lines are well suited for period searches, with the Ca II IRT index performing slightly better than Hα. Unfortunately, the Na I D lines are strongly affected by telluric airglow, and we could not find any rotation period using this index. Further, different definitions of the line indices have no major impact on the results. Comparing the different search methods, the string length method and the phase dispersion minimisation perform worst, while Gaussian process models produce the smallest numbers of false positives and non-detections.ǀ PublicationThe CARMENES search for exoplanets around M dwarfs Photospheric parameters of target stars from high-resolution spectroscopy(EDP Sciences S A, 2018-07-03) Cortés Contreras, Miriam; Montes Gutiérrez, David; otros, ...Context. The new CARMENES instrument comprises two high-resolution and high-stability spectrographs that are used to search for habitable planets around M dwarfs in the visible and near-infrared regime via the Doppler technique. Aims. Characterising our target sample is important for constraining the physical properties of any planetary systems that are detected. The aim of this paper is to determine the fundamental stellar parameters of the CARMENES M-dwarf target sample from high-resolution spectra observed with CARMENES. We also include several M-dwarf spectra observed with other high-resolution spectrographs, that is CAFE, FEROS, and HRS, for completeness. Methods. We used a chi(2) method to derive the stellar parameters effective temperature T-eff, surface gravity log g, and metallicity [Fe/H] of the target stars by fitting the most recent version of the PHOENIX-ACES models to high-resolution spectroscopic data. These stellar atmosphere models incorporate a new equation of state to describe spectral features of low-temperature stellar atmospheres. Since Teff, log g, and [Fe/H] show degeneracies, the surface gravity is determined independently using stellar evolutionary models. Results. We derive the stellar parameters for a total of 300 stars. The fits achieve very good agreement between the PHOENIX models and observed spectra. We estimate that our method provides parameters with uncertainties of sigma(Teff) = 51 K, sigma(logg) = 0 : 07, and sigma[(Fe/H)] = 0.16, and show that atmosphere models for low-mass stars have significantly improved in the last years. Our work also provides an independent test of the new PHOENIX-ACES models, and a comparison for other methods using low-resolution spectra. In particular, our effective temperatures agree well with literature values, while metallicities determined with our method exhibit a larger spread when compared to literature results. PublicationCARMENES: data flow(SPIE-Int Soc Optical Engineering, 2016) Alonso Floriano, F.J.; Cortés Contreras, Miriam; López Santiago, Javier; Montes Gutiérrez, DavidCARMENES, the new Calar Alto spectrograph especially built for radial-velocity surveys of exoearths around M dwarfs, is a very complicated system. For reaching the goal of 1 m/s radial-velocity accuracy, it is appropriate not only to monitor stars with the best observing procedure, but to monitor also the parameters of the CARMENES subsystems and safely store all the engineer and science data. Here we describe the CARMENES data flow from the different subsystems, through the instrument control system and pipeline, to the virtual-observatory data server and astronomers. PublicationThe CARMENES search for exoplanets around M dwarfs: characterization of the nearby ultra-compact multiplanetary system YZ Ceti(EDP Sciencies, 2020-04-07) Caballero, J. A.; Cortés Contreras, Miriam; Díez Alonso, Enrique; Domínguez Fernández, A.J.; Montes Gutiérrez, DavidContext. The nearby ultra-compact multiplanetary system YZ Ceti consists of at least three planets, and a fourth tentative signal. The orbital period of each planet is the subject of discussion in the literature due to strong aliasing in the radial velocity data. The stellar activity of this M dwarf also hampers significantly the derivation of the planetary parameters. Aims. With an additional 229 radial velocity measurements obtained since the discovery publication, we reanalyze the YZ Ceti system and resolve the alias issues. Methods. We use model comparison in the framework of Bayesian statistics and periodogram simulations based on a method by Dawson and Fabrycky to resolve the aliases. We discuss additional signals in the RV data, and derive the planetary parameters by simultaneously modeling the stellar activity with a Gaussian process regression model. To constrain the planetary parameters further we apply a stability analysis on our ensemble of Keplerian fits. Results. We find no evidence for a fourth possible companion. We resolve the aliases: the three planets orbit the star with periods of 2.02 d, 3.06 d, and 4.66 d. We also investigate an effect of the stellar rotational signal on the derivation of the planetary parameters, in particular the eccentricity of the innermost planet. Using photometry we determine the stellar rotational period to be close to 68 d and we also detect this signal in the residuals of a three-planet fit to the RV data and the spectral activity indicators. From our stability analysis we derive a lower limit on the inclination of the system with the assumption of coplanar orbits which is i_(min) = 0.9 deg. From the absence of a transit event with TESS, we derive an upper limit of the inclination of i_(max) = 87.43 deg. Conclusions. YZ Ceti is a prime example of a system where strong aliasing hindered the determination of the orbital periods of exoplanets. Additionally, stellar activity influences the derivation of planetary parameters and modeling them correctly is important for the reliable estimation of the orbital parameters in this specific compact system. Stability considerations then allow additional constraints to be placed on the planetary parameters. PublicationPrecise mass and radius of a transiting super-Earth planet orbiting the M dwarf TOI-1235: a planet in the radius gap?(EDP Sciencies, 2020-07-21) Cortés Contreras, Miriam; Montes Gutiérrez, DavidWe report the confirmation of a transiting planet around the bright weakly active M0.5 V star TOI-1235 (TYC 4384–1735–1, V ≈ 11.5 mag), whose transit signal was detected in the photometric time series of sectors 14, 20, and 21 of the TESS space mission. We confirm the planetary nature of the transit signal, which has a period of 3.44 d, by using precise RV measurements with the CARMENES, HARPS-N, and iSHELL spectrographs, supplemented by high-resolution imaging and ground-based photometry. A comparison of the properties derived for TOI-1235 b with theoretical models reveals that the planet has a rocky composition, with a bulk density slightly higher than that of Earth. In particular, we measure a mass of M_(p) = 5.9 ± 0.6 M_(⊕) and a radius of Rp = 1.69 ± 0.08 R_(⊕), which together result in a density of ρ_(p) = 6.7^(+1.3)_(−1.1) g cm^(−3) . When compared with other well-characterized exoplanetary systems, the particular combination of planetary radius and mass places our discovery in the radius gap, which is a transition region between rocky planets and planets with significant atmospheric envelopes. A few examples of planets occupying the radius gap are known to date. While the exact location of the radius gap for M dwarfs is still a matter of debate, our results constrain it to be located at around 1.7 R_(⊕) or larger at the insolation levels received by TOI-1235 b (∼60 S_(⊕)). This makes it an extremely interesting object for further studies of planet formation and atmospheric evolution.