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تسجيل مستخدم جديدHADES RV Programme with HARPS-N at TNG XII. The abundance signature of M dwarf stars with planets
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Most of our current knowledge on planet formation is still based on the analysis of main-sequence, solar-type stars. Conversely, detailed chemical studies of large samples of M-dwarf planet hosts are still missing. We develop for the first time a methodology to determine stellar abundances of elements others than iron for M dwarf stars from high-resolution, optical spectra. Our methodology is based on the use of principal component analysis and sparse Bayesians methods. We made use of a set of M dwarfs orbiting around an FGK primary with known abundances to train our methods. We applied our methods to derive stellar metalliticies and abundances of a large sample of M dwarfs observed within the framework of current radial velocity surveys. We then used a sample of nearby FGK stars to cross-validate our technique by comparing the derived abundance trends in the M dwarf sample with those found on the FGK stars. The metallicity distribution of the different subsamples shows that M dwarfs hosting giant planets show a planet-metallicity correlation as well as a correlation with the stellar mass. M dwarfs hosting low-mass planets do not seem to follow the planet-metallicity correlation. We also found that the frequency of low-mass planets does not depend on the mass of the stellar host. These results seem in agreement with previous works. However, we note that for giant planet hosts our metallicities predict a weaker planet metallicity correlation but a stronger mass-dependency than photometric values. We show, for the first time, that there seems to be no differences in the abundance distribution of elements different from iron between M dwarfs with and without known planets. Our data shows that low-mass stars with planets follow the same metallicity, mass, and abundance trends than their FGK counterparts.
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Context. The high number of super-Earth and Earth-like planets in the habitable zone (HZ) detected around M-dwarf stars in the last years has revealed these stellar objects to be the key for planetary radial velocity (RV) searches. Aims. Using the HA
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The distribution of exoplanets around low-mass stars is still not well understood. Such stars, however, present an excellent opportunity of reaching down to the rocky and habitable planet domains. The number of current detections used for statistical
purposes is still quite modest and different surveys, using both photometry and precise radial velocities, are searching for planets around M dwarfs. Our HARPS-N red dwarf exoplanet survey is aimed at the detection of new planets around a sample of 78 selected stars, together with the subsequent characterization of their activity properties. Here we investigate the survey performance and strategy. From 2700 observed spectra, we compare the radial velocity determinations of the HARPS-N DRS pipeline and the HARPS-TERRA code, we calculate the mean activity jitter level, we evaluate the planet detection expectations, and we address the general question of how to define the strategy of spectroscopic surveys in order to be most efficient in the detection of planets. We find that the HARPS-TERRA radial velocities show less scatter and we calculate a mean activity jitter of 2.3 m/s for our sample. For a general radial velocity survey with limited observing time, the number of observations per star is key for the detection efficiency. In the case of an early M-type target sample, we conclude that approximately 50 observations per star with exposure times of 900 s and precisions of about 1 m/s maximizes the number of planet detections.
Context. It is now well-established that small, rocky planets are common around low-mass stars. However, the detection of such planets is challenged by the short-term activity of the host stars. Aims. The HArps-N red Dwarf Exoplanet Survey (HADES) pr
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ARPS-N can provide a major contribution to the widening of the current statistics through the in-depth analysis of accurate radial velocity observations in a narrow range of spectral sub-types (79 stars, between dM0 to dM3). Spectral accuracy will enable us to reach the precision needed to detect small planets with a few earth masses. Our survey will bring a contribute to the surveys devoted to the search for planets around M-dwarfs, mainly focused on the M-dwarf population of the northern emisphere, for which we will provide an estimate of the planet occurence. Methods. We present here a long duration radial velocity monitoring of the M1 dwarf star GJ 3998 with HARPS-N to identify periodic signals in the data. Almost simultaneous photometric observations were carried out within the APACHE and EXORAP programs to characterize the stellar activity and to distinguish from the periodic signals those due to activity and to the presence of planetary companions. Results. The radial velocities have a dispersion in excess of their internal errors due to at least four superimposed signals, with periods of 30.7, 13.7, 42.5 and 2.65 days. The analysis of spectral indices based on Ca II H & K and Halpha lines demonstrates that the periods of 30.7 and 42.5 days are due to chromospheric inhomogeneities modulated by stellar rotation and differential rotation. The shorter periods of 13.74 +/- 0.02 d and 2.6498 +/- 0.0008 d are well explained with the presence of two planets, with minimum masses of 6.26 +/- 0.79 MEarth and 2.47 +/- 0.27 MEarth and distances of 0.089 AU and 0.029 AU from the host, respectively.
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ch corresponds to slow rotators. Methods. We use the luminosity coronal activity indicator (Lx) of a sample of 78 early-M dwarfs with masses in the range from 0.3 to 0.75 Msun from the HArps-N red Dwarf Exoplanet Survey (HADES) radial velocity (RV) programme collected from ROSAT and XMM-Newton. The determination of the rotation periods (P_{rot}) was done by analysing time-series of high-resolution spectroscopy of the Ca ii H & K and H{alpha} activity indicators. Our sample principally covers the slow rotation regime with rotation periods from 15 to 60 days. Results. Our work extends to the low mass regime the observed trend for more massive stars showing a continuous shift of the Lx/Lbol vs. Prot power-law towards longer rotation period values and includes the determination, in a more accurate way, of the value of the rotation period at which the saturation occurs (P_{sat}) for M dwarf stars. Conclusions. We conclude that the relations between coronal activity and stellar rotation for FGK stars also hold for early-M dwarfs in the non-saturated regime, indicating that the rotation period is sufficient to determine the ratio Lx/Lbol.
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