626 research outputs found

    The EBLM project. II. A very hot, low-mass M dwarf in an eccentric and long period eclipsing binary system from SuperWASP

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    In this paper, we derive the fundamental properties of 1SWASPJ011351.29+314909.7 (J0113+31), a metal-poor (-0.40 +/- 0.04 dex), eclipsing binary in an eccentric orbit (~0.3) with an orbital period of ~14.277 d. Eclipsing M dwarfs orbiting solar-type stars (EBLMs), like J0113+31, have been identified from WASP light curves and follow-up spectroscopy in the course of the transiting planet search. We present the first binary of the EBLM sample to be fully analysed, and thus, define here the methodology. The primary component with a mass of 0.945 +/- 0.045 Msun has a large radius (1.378 +/- 0.058 Rsun) indicating that the system is quite old, ~9.5 Gyr. The M-dwarf secondary mass of 0.186 +/- 0.010 Msun and radius of 0.209 +/- 0.011 Rsun are fully consistent with stellar evolutionary models. However, from the near-infrared secondary eclipse light curve, the M dwarf is found to have an effective temperature of 3922 +/- 42 K, which is ~600 K hotter than predicted by theoretical models. We discuss different scenarios to explain this temperature discrepancy. The case of J0113+31 for which we can measure mass, radius, temperature and metallicity, highlights the importance of deriving mass, radius and temperature as a function of metallicity for M dwarfs to better understand the lowest mass stars. The EBLM Project will define the relationship between mass, radius, temperature and metallicity for M dwarfs providing important empirical constraints at the bottom of the main sequence.Comment: 13 pages, 7 figures. Accepted for publication in A&

    Spin orbit alignment for KELT-7b and HAT-P-56b via Doppler tomography with TRES

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    We present Doppler tomographic analyses for the spectroscopic transits of KELT-7b and HAT-P-56b, two hot-Jupiters orbiting rapidly rotating F-dwarf host stars. These include analyses of archival TRES observations for KELT-7b, and a new TRES transit observation of HAT-P-56b. We report spin-orbit aligned geometries for KELT-7b (2.7 +/- 0.6 deg) and HAT-P-56b (8 +/- 2 deg). The host stars KELT-7 and HAT-P-56 are among some of the most rapidly rotating planet-hosting stars known. We examine the tidal re-alignment model for the evolution of the spin-orbit angle in the context of the spin rates of these stars. We find no evidence that the rotation rates of KELT-7 and HAT-P-56 have been modified by star-planet tidal interactions, suggesting that the spin-orbit angle of systems around these hot stars may represent their primordial configuration. In fact, KELT-7 and HAT-P-56 are two of three systems in super-synchronous, spin-orbit aligned states, where the rotation periods of the host stars are faster than the orbital periods of the planets.Comment: 9 pages, accepted for publication in MNRA

    Chromospheric activity among fast rotating M-dwarfs in the open cluster NGC 2516

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    We report radial velocities (RVs), projected equatorial velocities (v sin i) and CaII triplet (CaT) chromospheric activity indices for 237 late-K to mid-M candidate members of the young open cluster NGC 2516. These stars have rotation periods between 0.1 and 15 days. Intermediate resolution spectra were obtained using the Giraffe spectrograph at the Very Large Telescope. Membership was confirmed on the basis of RVs for 210 targets. We see a marked increase in the fraction of rapidly rotators as we move to cooler spectral types. About 20 per cent of M0-M1 stars have v sin i >15km/s, increasing to 90 per cent of M4 stars. Activity indices derived from the first two lines of the CaT (8498A and 8542A) show differing dependencies on rotation period and mass for stars lying above and below the fully convective boundary. Higher mass stars, of spectral type K3-M2.5, show chromospheric activity which increases with decreasing Rossby number (the ratio of period to convective turnover time), saturating for Rossby numbers <0.1. For cooler stars, which are probably fully convective and almost all of which have Rossby numbers <0.1, there is a clear decrease in chromospheric activity as (V-I)_0 increases, amounting to a fall of about a factor of 2-3 between spectral types M2.5 and M4. This decrease in activity levels at low Rossby numbers is not seen in X-ray observations of the coronae of cluster M-dwarfs or of active field M-dwarfs. There is no evidence for supersaturation of chromospheric activity for stars of any spectral type at Rossby numbers <0.01. We suggest that the fall in the limiting level of chromospheric emission beyond spectral type M3 in NGC~2516 is, like the simultaneous increase in rotation rates in field stars, associated with a change in the global magnetic topology as stars approach the fully convective boundary and not due to any decrease in dynamo-generated magnetic flux.Comment: Accepted for publication in MNRA

    Detection of Neptune-size planetary candidates with CoRoT data. Comparison with the planet occurrence rate derived from Kepler

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    [Abridged] Context. The CoRoT space mission has been searching for transiting planets since the end of December 2006. Aims. We aim to investigate the capability of CoRoT to detect small-size transiting planets in short-period orbits, and to compare the number of CoRoT planets with 2 \leq R_p \leq 4 Rearth with the occurrence rate of small-size planets provided by the distribution of Kepler planetary candidates (Howard et al. 2012). Methods. We performed a test that simulates transits of super-Earths and Neptunes in real CoRoT light curves and searches for them blindly by using the LAM transit detection pipeline. Results. The CoRoT detection rate of planets with radius between 2 and 4 Rearth and orbital period P \leq 20 days is 59% (31%) around stars brighter than r'=14.0 (15.5). By properly taking the CoRoT detection rate for Neptune-size planets and the transit probability into account, we found that according to the Kepler planet occurrence rate, CoRoT should have discovered 12 \pm 2 Neptunes orbiting G and K dwarfs with P \leq 17 days in six observational runs. This estimate must be compared with the validated Neptune CoRoT-24b and five CoRoT planetary candidates in the considered range of planetary radii. We thus found a disagreement with expectations from Kepler at 3 \sigma or 5 \sigma, assuming a blend fraction of 0% (six Neptunes) and 100% (one Neptune) for these candidates. Conclusions. This underabundance of CoRoT Neptunes with respect to Kepler may be due to several reasons. Regardless of the origin of the disagreement, which needs to be investigated in more detail, the noticeable deficiency of CoRoT Neptunes at short orbital periods seems to indirectly support the general trend found in Kepler data, i.e. that the frequency of small-size planets increases with increasing orbital periods and decreasing planet radii.Comment: 10 pages, 7 figures. Accepted for publication in A&

    CARMENES input catalogue of M dwarfs IV. New rotation periods from photometric time series

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    Aims. The main goal of this work is to measure rotation periods of the M-type dwarf stars being observed by the CARMENES exoplanet survey to help distinguish radial-velocity signals produced by magnetic activity from those produced by exoplanets. Rotation periods are also fundamental for a detailed study of the relation between activity and rotation in late-type stars. Methods. We look for significant periodic signals in 622 photometric time series of 337 bright, nearby M dwarfs obtained by long-time baseline, automated surveys (MEarth, ASAS, SuperWASP, NSVS, Catalina, ASAS-SN, K2, and HATNet) and for 20 stars which we obtained with four 0.2-0.8 m telescopes at high geographical latitudes. Results. We present 142 rotation periods (73 new) from 0.12 d to 133 d and ten long-term activity cycles (six new) from 3.0 a to 11.5 a. We compare our determinations with those in the existing literature; we investigate the distribution of P rot in the CARMENES input catalogue,the amplitude of photometric variability, and their relation to vsin i and pEW(Halfa); and we identify three very active stars with new rotation periods between 0.34 d and 23.6 d.Comment: 34 pages, 43 figures, 2 appendix table

    Discovery and characterisation of detached M-dwarf eclipsing binaries in the WFCAM Transit Survey

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    We report the discovery of 16 detached M-dwarf eclipsing binaries with J<16 mag and provide a detailed characterisation of three of them, using high-precision infrared light curves from the WFCAM Transit Survey (WTS). Such systems provide the most accurate and model-independent method for measuring the fundamental parameters of these poorly understood yet numerous stars, which currently lack sufficient observations to precisely calibrate stellar evolution models. We fully solve for the masses and radii of three of the systems, finding orbital periods in the range 1.5<P<4.9 days, with masses spanning 0.35-0.50 Msun and radii between 0.38-0.50 Rsun, with uncertainties of ~3.5-6.4% in mass and ~2.7-5.5% in radius. Close-companions in short-period binaries are expected to be tidally-locked into fast rotational velocities, resulting in high levels of magnetic activity. This is predicted to inflate their radii by inhibiting convective flow and increasing star spot coverage. The radii of the WTS systems are inflated above model predictions by ~3-12%, in agreement with the observed trend, despite an expected lower systematic contribution from star spots signals at infrared wavelengths. We searched for correlation between the orbital period and radius inflation by combining our results with all existing M-dwarf radius measurements of comparable precision, but we found no statistically significant evidence for a decrease in radius inflation for longer period, less active systems. Radius inflation continues to exists in non-synchronised systems indicating that the problem remains even for very low activity M-dwarfs. Resolving this issue is vital not only for understanding the most populous stars in the Universe, but also for characterising their planetary companions, which hold the best prospects for finding Earth-like planets in the traditional habitable zone.Comment: 30 pages, 14 figures, 16 tables, Accepted for publication in MNRA

    The mass determination challenge for exoplanetary science

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    The mass of an exoplanet is a key parameter for the characterisation of the internal structure of a planet, as well as the study of the formation and the evolution of the planet, and its atmosphere. The radial velocity technique allows for measuring the planetary mass from the radial velocity variation of its parent star. However, limitations in the property determination of exoplanets, particularly in their masses, can arise from various sources especially from astrophysical noise due to stellar variability, caused by magnetic activity, which affects the detection and characterisation of exoplanets. This PhD thesis aims to understand the impact of our knowledge of the planetary mass in the planetary atmospheric characterisation and to reduce the sources of uncertainty by a deep study of the stellar activity and by developing new techniques for stellar variability filtering. To this end, I analysed the impact of the planetary mass uncertainties of atmospheric retrievals of multiple targets from the mission reference sample of Ariel, the forthcoming ESA M4 mission aimed at studying planetary atmospheres. I simulated different spectra as observed by Ariel, assuming a primordial or secondary atmosphere of hot Jupiters, and sub-Neptunes or super-Earths, respectively, under different cloudy configurations. I estimated both the accuracy and precision necessary for each analysed target, testing also the capability of retrieval in the case of incorrect mass estimation. I verified that one of the most crucial issues is the presence of high-altitude clouds, in particular in the secondary atmosphere cases. For this reason, I tested the capability to retrieve the cloudy configuration or the presence of a secondary atmosphere during the first tier of the Ariel mission, to take an informed decision if including the planet in the Tier-2 sample. In the second part of this thesis, I described SpotCCF, a photospheric stellar model that I developed to optimise the radial velocity extraction in fast-rotating stars. This model, based on the cross-correlation function technique, takes into account the contribution of stellar activity by considering the presence of multiple spots on the stellar surface that caused deformation of the profile of the cross-correlation function. I applied this model to the HARPS-N observations of V1298 Tau, a very active K1 star, which shows strongly deformed cross-correlation function (CCF) profiles. The SpotCCF model is also able to give information about the spot configuration (latitude, longitude and area covered by the spot). In the end, I also focused my study on understanding stellar activity in M dwarfs, which is crucial for improving our understanding of the physics of stellar atmospheres and for planet search programs. Specifically, I analysed HARPS and HARPS-N observation of AD Leonis, measuring the line profiles and intensities of sensitive activity indicators, and evaluating the correlations between them. Globally, the PhD thesis highlights the importance of planetary mass characterisation and the complexity of their determination due to the effects of stellar variability. In the context of the Ariel mission, it highlights the importance of a detailed and individual analysis of each target of the mission reference sample, to be able to accurately select the Tier-2 targets and characterise their planetary atmosphere, and represents a step forward towards the preparation of the ESA M4 Ariel mission. It also shows how this work cannot be disentangled from a detailed study of the stellar variability that is crucial in the determination of the planetary mass, both in its accuracy and precision
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