The HARPS search for southern extra-solar planets XIX. Characterization and dynamics of the GJ876 planetary system

Precise radial-velocity measurements for data acquired with the HARPS spectrograph infer that three planets orbit the M4 dwarf star GJ876. In particular, we confirm the existence of planet "d", which orbits every 1.93785 days. We find that its orbit may have significant eccentricity (e=0.14), and deduce a more accurate estimate of its minimum mass of 6.3 Earth masses. Dynamical modeling of the HARPS measurements combined with literature velocities from the Keck Observatory strongly constrain the orbital inclinations of the "b" and "c" planets. We find that i_b = 48.9 degrees and i_c = 48.1 degrees, which infers the true planet masses of M_b = 2.64 Jupiter masses and M_c = 0.83 Jupiter masses, respectively. Radial velocities alone, in this favorable case, can therefore fully determine the orbital architecture of a multi-planet system, without the input from astrometry or transits. The orbits of the two giant planets are nearly coplanar, and their 2:1 mean motion resonance ensures stability over at least 5 Gyr. The libration amplitude is smaller than 2 degrees, suggesting that it was damped by some dissipative process during planet formation. The system has space for a stable fourth planet in a 4:1 mean motion resonance with planet "b", with a period around 15 days. The radial velocity measurements constrain the mass of this possible additional planet to be at most that of the Earth.Comment: 10 pages, 10 figures, accepted for publication in Astronomy & Astrophysic

SOAP. A tool for the fast computation of photometry and radial velocity induced by stellar spots

We define and put at the disposal of the community SOAP, Spot Oscillation And Planet, a software tool that simulates the effect of stellar spots and plages on radial velocimetry and photometry. This paper describes the tool release and provides instructions for its use. We present detailed tests with previous computations and real data to assess the code's performance and to validate its suitability. We characterize the variations of the radial velocity, line bisector, and photometric amplitude as a function of the main variables: projected stellar rotational velocity, filling factor of the spot, resolution of the spectrograph, linear limb-darkening coefficient, latitude of the spot, and inclination of the star. Finally, we model the spot distributions on the active stars HD166435, TW Hya and HD189733 which reproduces the observations. We show that the software is remarkably fast allowing several evolutions in its capabilities that could be performed to study the next challenges in the exoplanetary field connected with the stellar variability.Comment: 9 pages, 12 figures, 4 tables, accepted for publication in A&

The very nearby M/T dwarf binary SCR 1845-6357

The recently discovered star SCR 1845-6357 is the first late M/T dwarf binary discovered. SCR 1845 is a particular object due to its tight orbit (currently around 4 AU) and its proximity to the Sun (3.85 pc). We present spatially resolved VLT/NACO images and low resolution spectra of SCR 1845 in the J, H and K near-infrared bands. Since the T dwarf companion, SCR 1845B, is so close to the primary SCR 1845A, orbital motion is evident even within a year. Following the orbital motion, the binary's mass can be measured accurately within a decade, making SCR 1845B a key T-dwarf mass-luminosity calibrator. The NIR spectra allow for accurate determination of spectral type and also for rough estimates of the object's physical parameters. The spectral type of SCR 1845B is determined by direct comparison of the flux calibrated JHK spectra with T dwarf standard template spectra and also by NIR spectral indices obtained from synthetic photometry. Constrained values for surface gravity, effective temperature and metallicity are derived by comparison with model spectra. Our data prove that SCR 1845B is a brown dwarf of spectral type T6 that is co-moving with and therefore gravitationally bound to the M8.5 primary. Fitting the NIR spectrum of SCR 1845B to model spectra yields an effective temperature of about 950K and a surface gravity log(g)=5.1 (cgs) assuming solar metallicity. Mass and age of SCR 1845B are in the range 40 to 50 Jupiter masses and 1.8 to 3.1 Gyr.Comment: 5 pages, 4 figures, accepted for publication in Astronomy & Astrophysic

M dwarf stars in the light of (future) exoplanet searches

We present a brief overview of a splinter session on M dwarf stars as planet hosts that was organized as part of the Cool Stars 17 conference. The session was devoted to reviewing our current knowledge of M dwarf stars and exoplanets in order to prepare for current and future exoplanet searches focusing in low mass stars. We review the observational and theoretical challenges to characterize M dwarf stars and the importance of accurate fundamental parameters for the proper characterization of their exoplanets and our understanding on planet formation.Comment: 4 pages, 1 figure. Summary of the splinter session "M dwarf stars in the light of (future) exoplanet searches" held at the 17th Cambridge Workshop on Cool Stars, Stellar Systems, and the Sun, June 28th 2012, Barcelona, Spain. Submitted for publication in Astronomische Nachrichten - Astronomical Notes (AN) 334, Issue 1-2, Eds Klaus Strassmeier and Mercedes L\'opez-Morale

Transit Detection in the MEarth Survey of Nearby M Dwarfs: Bridging the Clean-First, Search-Later Divide

In the effort to characterize the masses, radii, and atmospheres of potentially habitable exoplanets, there is an urgent need to find examples of such planets transiting nearby M dwarfs. The MEarth Project is an ongoing effort to do so, as a ground-based photometric survey designed to detect exoplanets as small as 2 Earth radii transiting mid-to-late M dwarfs within 33 pc of the Sun. Unfortunately, identifying transits of such planets in photometric monitoring is complicated both by the intrinsic stellar variability that is common among these stars and by the nocturnal cadence, atmospheric variations, and instrumental systematics that often plague Earth-bound observatories. Here we summarize the properties of MEarth data gathered so far, and we present a new framework to detect shallow exoplanet transits in wiggly and irregularly-spaced light curves. In contrast to previous methods that clean trends from light curves before searching for transits, this framework assesses the significance of individual transits simultaneously while modeling variability, systematics, and the photometric quality of individual nights. Our Method for Including Starspots and Systematics in the Marginalized Probability of a Lone Eclipse (MISS MarPLE) uses a computationally efficient semi-Bayesian approach to explore the vast probability space spanned by the many parameters of this model, naturally incorporating the uncertainties in these parameters into its evaluation of candidate events. We show how to combine individual transits processed by MISS MarPLE into periodic transiting planet candidates and compare our results to the popular Box-fitting Least Squares (BLS) method with simulations. By applying MISS MarPLE to observations from the MEarth Project, we demonstrate the utility of this framework for robustly assessing the false alarm probability of transit signals in real data. [slightly abridged]Comment: accepted to the Astronomical Journal, 21 pages, 12 figure

The Empirical Mass-Luminosity Relation for Low Mass Stars

This work is devoted to improving empirical mass-luminosity relations and mass-metallicity-luminosity relation for low mass stars. For these stars, observational data in the mass-luminosity plane or the mass-metallicity-luminosity space subject to non-negligible errors in all coordinates with different dimensions. Thus a reasonable weight assigning scheme is needed for obtaining more reliable results. Such a scheme is developed, with which each data point can have its own due contribution. Previous studies have shown that there exists a plateau feature in the mass-luminosity relation. Taking into account the constraints from the observational luminosity function, we find by fitting the observational data using our weight assigning scheme that the plateau spans from 0.28 to 0.50 solar mass. Three-piecewise continuous improved mass-luminosity relations in K, J, H and V bands, respectively, are obtained. The visual mass-metallicity-luminosity relation is also improved based on our K band mass-luminosity relation and the available observational metallicity data.Comment: 8 pages, 2 figures. Accepted for publication in Astrophysics & Space Scienc

Mass-luminosity relation for FGK main sequence stars: metallicity and age contributions

The stellar mass-luminosity relation (MLR) is one of the most famous empirical "laws", discovered in the beginning of the 20th century. MLR is still used to estimate stellar masses for nearby stars, particularly for those that are not binary systems, hence the mass cannot be derived directly from the observations. It's well known that the MLR has a statistical dispersion which cannot be explained exclusively due to the observational errors in luminosity (or mass). It is an intrinsic dispersion caused by the differences in age and chemical composition from star to star. In this work we discuss the impact of age and metallicity on the MLR. Using the recent data on mass, luminosity, metallicity, and age for 26 FGK stars (all members of binary systems, with observational mass-errors <= 3%), including the Sun, we derive the MLR taking into account, separately, mass-luminosity, mass-luminosity-metallicity, and mass-luminosity-metallicity-age. Our results show that the inclusion of age and metallicity in the MLR, for FGK stars, improves the individual mass estimation by 5% to 15%.Comment: 7 pages, 4 figures, 1 table, accepted in Astrophysics and Space Scienc

Stellar activity of planetary host star HD 189733

Extra-solar planet search programs require high-precision velocity measurements. They need to study how to disentangle radial-velocity variations due to Doppler motion from the noise induced by stellar activity. We monitored the active K2V star HD 189733 and its transiting planetary companion that has a 2.2-day orbital period. We used the high-resolution spectograph SOPHIE mounted on the 1.93-m telescope at the Observatoire de Haute-Provence to obtain 55 spectra of HD 189733 over nearly two months. We refined the HD 189733b orbit parameters and put limits on the eccentricity and on a long-term velocity gradient. After subtracting the orbital motion of the planet, we compared the variability of spectroscopic activity indices to the evolution of the radial-velocity residuals and the shape of spectral lines. The radial velocity, the spectral-line profile and the activity indices measured in HeI (5875.62 \AA), Halpha (6562.81 \AA) and the CaII H&K lines (3968.47 \AA and 3933.66 \AA, respectively) show a periodicity around the stellar rotation period and the correlations between them are consistent with a spotted stellar surface in rotation. We used such correlations to correct for the radial-velocity jitter due to stellar activity. This results in achieving high precision on the orbit parameters, with a semi-amplitude K = 200.56 \pm 0.88 m.s-1 and a derived planet mass of M_{P}=1.13 \pm 0.03 M$_{Jup}$.Comment: 9 pages, 2 tables, 9 figures, accepted for publication in A&A on 20/11/200

Managing physical and mental health conditions: Consumer perspectives on integrated care

Despite the growing trend of integrating primary care and mental health services, little research has documented how consumers with severe mental illnesses (SMI) manage comorbid conditions or view integrated services. We sought to better understand how consumers perceive and manage both mental and physical health conditions and their views of integrated services. We conducted semi-structured interviews with consumers receiving primary care services integrated in a community mental health setting. Consumers described a range of strategies to deal with physical health conditions and generally viewed mental and physical health conditions as impacting one another. Consumers viewed integration of primary care and mental health services favorably, specifically its convenience, friendliness, and knowledge of providers, and collaboration between providers. Although integration was viewed positively, consumers with SMI may need a myriad of strategies and supports to both initiate and sustain lifestyle changes that address common physical health problems

Planetary detection limits taking into account stellar noise. II. Effect of stellar spot groups on radial-velocities

The detection of small mass planets with the radial-velocity technique is now confronted with the interference of stellar noise. HARPS can now reach a precision below the meter-per-second, which corresponds to the amplitudes of different stellar perturbations, such as oscillation, granulation, and activity. Solar spot groups induced by activity produce a radial-velocity noise of a few meter-per-second. The aim of this paper is to simulate this activity and calculate detection limits according to different observational strategies. Based on Sun observations, we reproduce the evolution of spot groups on the surface of a rotating star. We then calculate the radial-velocity effect induced by these spot groups as a function of time. Taking into account oscillation, granulation, activity, and a HARPS instrumental error of 80 cm/s, we simulate the effect of different observational strategies in order to efficiently reduce all sources of noise. Applying three measurements per night of 10 minutes every three days, 10 nights a month seems the best tested strategy. Depending on the level of activity considered, from log(R'_HK)= -5 to -4.75, this strategy would allow us to find planets of 2.5 to 3.5 Earth masses in the habitable zone of a K1V dwarf. Using Bern's model of planetary formation, we estimate that for the same range of activity level, 15 to 35 % of the planets between 1 and 5 Earth masses and with a period between 100 and 200 days should be found with HARPS. A comparison between the performance of HARPS and ESPRESSO is also emphasized by our simulations. Using the same optimized strategy, ESPRESSO could find 1.3 Earth mass planets in the habitable zone of early-K dwarfs. In addition, 80 % of planets with mass between 1 and 5 Earth masses and with a period between 100 and 200 days could be detected.Comment: 11 pages, 11 figures, accepted for publication in A&