arXiv subjects

Christophe Lovis

Publications and source records attributed to Christophe Lovis.

At least 127 records · Page 7Linked to original sources

arXiv2015

Kuiper belt structure around nearby super-Earth host stars

We present new observations of the Kuiper belt analogues around HD 38858 and HD 20794, hosts of super-Earth mass planets within 1 au. As two of the four nearby G-type stars (with HD 69830 and 61 Vir) that form the basis of a possible correlation between low-mass planets and debris disc brightness, these systems are of particular interest. The disc around HD 38858 is well resolved with Herschel and we constrain the disc geometry and radial structure. We also present a probable JCMT sub-mm continuum detection of the disc and a CO J=2-1 upper limit. The disc around HD 20794 is much fainter and appears marginally resolved with Herschel, and is constrained to be less extended than the discs around 61 Vir and HD 38858. We also set limits on the radial location of hot dust recently detected around HD 20794 with near-IR interferometry. We present HARPS upper limits on unseen planets in these four systems, ruling out additional super-Earths within a few au, and Saturn-mass planets within 10 au. We consider the disc structure in the three systems with Kuiper belt analogues (HD 69830 has only a warm dust detection), concluding that 61 Vir and HD 38858 have greater radial disc extent than HD 20794. We speculate that the greater width is related to the greater minimum planet masses (10-20 $M_\oplus$ vs. 3-5 $M_\oplus$), arising from an eccentric planetesimal population analogous to the Solar System's scattered disc. We discuss alternative scenarios and possible means to distinguish among them.

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arXiv2015

The EChO science case

The discovery of almost 2000 exoplanets has revealed an unexpectedly diverse planet population. Observations to date have shown that our Solar System is certainly not representative of the general population of planets in our Milky Way. The key science questions that urgently need addressing are therefore: What are exoplanets made of? Why are planets as they are? What causes the exceptional diversity observed as compared to the Solar System? EChO (Exoplanet Characterisation Observatory) has been designed as a dedicated survey mission for transit and eclipse spectroscopy capable of observing a large and diverse planet sample within its four-year mission lifetime. EChO can target the atmospheres of super-Earths, Neptune-like, and Jupiter-like planets, in the very hot to temperate zones (planet temperatures of 300K-3000K) of F to M-type host stars. Over the next ten years, several new ground- and space-based transit surveys will come on-line (e.g. NGTS, CHEOPS, TESS, PLATO), which will specifically focus on finding bright, nearby systems. The current rapid rate of discovery would allow the target list to be further optimised in the years prior to EChO's launch and enable the atmospheric characterisation of hundreds of planets. Placing the satellite at L2 provides a cold and stable thermal environment, as well as a large field of regard to allow efficient time-critical observation of targets randomly distributed over the sky. A 1m class telescope is sufficiently large to achieve the necessary spectro-photometric precision. The spectral coverage (0.5-11 micron, goal 16 micron) and SNR to be achieved by EChO, thanks to its high stability and dedicated design, would enable a very accurate measurement of the atmospheric composition and structure of hundreds of exoplanets.

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arXiv2014

The Mass of Kepler-93b and The Composition of Terrestrial Planets

Kepler-93b is a 1.478 +/- 0.019 Earth radius planet with a 4.7 day period around a bright (V=10.2), astroseismically-characterized host star with a mass of 0.911+/-0.033 solar masses and a radius of 0.919+/-0.011 solar radii. Based on 86 radial velocity observations obtained with the HARPS-N spectrograph on the Telescopio Nazionale Galileo and 32 archival Keck/HIRES observations, we present a precise mass estimate of 4.02+/-0.68 Earth masses. The corresponding high density of 6.88+/-1.18 g/cc is consistent with a rocky composition of primarily iron and magnesium silicate. We compare Kepler-93b to other dense planets with well-constrained parameters and find that between 1-6 Earth masses, all dense planets including the Earth and Venus are well-described by the same fixed ratio of iron to magnesium silicate. There are as of yet no examples of such planets with masses > 6 Earth masses: All known planets in this mass regime have lower densities requiring significant fractions of volatiles or H/He gas. We also constrain the mass and period of the outer companion in the Kepler-93 system from the long-term radial velocity trend and archival adaptive optics images. As the sample of dense planets with well-constrained masses and radii continues to grow, we will be able to test whether the fixed compositional model found for the seven dense planets considered in this paper extends to the full population of 1-6 Earth mass planets.

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arXiv2014

Characterizing K2 Planet Discoveries: A super-Earth transiting the bright K-dwarf HIP 116454

We report the first planet discovery from the two-wheeled Kepler (K2) mission: HIP 116454 b. The host star HIP 116454 is a bright (V = 10.1, K = 8.0) K1-dwarf with high proper motion, and a parallax-based distance of 55.2 +/- 5.4 pc. Based on high-resolution optical spectroscopy, we find that the host star is metal-poor with [Fe/H] = -.16 +/- .18, and has a radius R = 0.716 +/- .0024 R_sun and mass M = .775 +/- .027 Msun. The star was observed by the Kepler spacecraft during its Two-Wheeled Concept Engineering Test in February 2014. During the 9 days of observations, K2 observed a single transit event. Using a new K2 photometric analysis technique we are able to correct small telescope drifts and recover the observed transit at high confidence, corresponding to a planetary radius of Rp = 2.53 +/- 0.18 Rearth. Radial velocity observations with the HARPS-N spectrograph reveal a 11.82 +/- 1.33 Mearth planet in a 9.1 day orbit, consistent with the transit depth, duration, and ephemeris. Follow-up photometric measurements from the MOST satellite confirm the transit observed in the K2 photometry and provide a refined ephemeris, making HIP 116454 b amenable for future follow-up observations of this latest addition to the growing population of transiting super-Earths around nearby, bright stars.

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arXiv2014

Near-infrared transmission spectrum of the warm-uranus GJ 3470b with the Wide Field Camera-3 on the Hubble Space Telescope

The atmospheric composition of low-mass exoplanets is the object of intense observational and theoretical investigations. GJ3470b is a warm uranus recently detected in transit across a bright late-type star. The transit of this planet has already been observed in several band passes from the ground and space, allowing observers to draw an intriguing yet incomplete transmission spectrum of the planet atmospheric limb. In particular, published data in the visible suggest the existence of a Rayleigh scattering slope, making GJ3470b a unique case among the known neptunes, while data obtained beyond 2 um are consistent with a flat infrared spectrum. The unexplored near-infrared spectral region between 1 and 2 um, is thus key to undertanding the atmospheric nature of GJ3470b. Here, we report on the first space-borne spectrum of GJ3470, obtained during one transit of the planet with WFC3 on board HST, operated in stare mode. The spectrum covers the 1.1--1.7-um region with a resolution of about 300. We retrieve the transmission spectrum of GJ3470b with a chromatic planet-to-star radius ratio precision of 0.15% (about one scale height) per 40-nm bins. At this precision, the spectrum appears featureless, in good agreement with ground-based and Spitzer infrared data at longer wavelengths, pointing to a flat transmission spectrum from 1 to 5 um. We present new simulations of transmission spectra for GJ3470b, which allow us to show that the HST/WFC3 observations rule out cloudless hydrogen-rich atmospheres (>10 sigma) as well as hydrogen-rich atmospheres with tholin haze (>5 sigma). Adding our near-infrared measurements to the full set of previously published data from 0.3 to 5 um, we find that a cloudy, hydrogen-rich atmosphere can explain the full transmission spectrum if, at the terminator, the clouds are located at low pressures (<1 mbar) or the water mixing ratio is extremely low (<1 ppm).

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arXiv2014

Rossiter-McLaughlin Observations of 55 Cnc e

We present Rossiter-McLaughlin observations of the transiting super-Earth 55 Cnc e collected during six transit events between January 2012 and November 2013 with HARPS and HARPS-N. We detect no radial-velocity signal above 35 cm/s (3-sigma) and confine the stellar v sin i to 0.2 +/- 0.5 km/s. The star appears to be a very slow rotator, producing a very low amplitude Rossiter-McLaughlin effect. Given such a low amplitude, the Rossiter-McLaughlin effect of 55 Cnc e is undetected in our data, and any spin-orbit angle of the system remains possible. We also performed Doppler tomography and reach a similar conclusion. Our results offer a glimpse of the capacity of future instrumentation to study low amplitude Rossiter-McLaughlin effects produced by super-Earths.

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arXiv2014

Telluric-line subtraction in high-accuracy velocimetry: a PCA-based approach

Optical velocimetry has led to the detection of more than 500 planets to date and there is a strong effort to push m/s velocimetry to the near-infrared to access cooler and lighter stars. The presence of numerous telluric absorption lines in the nIR brings an important challenge. As the star's barycentric velocity varies through the year, the telluric absorption lines effectively varies in velocity relative to the star's spectrum by the same amount leading to important systematic RV offsets. We present a novel principal component analysis-based approach for telluric line subtraction and demonstrated its effectiveness with archival HARPS data for GJ436 and τ Ceti, over parts of the R-band that contain strong telluric absorption lines. The main results are: 1) a better RV accuracy with excluding only a few percentage of the domain, 2) better use of the entire spectrum to measure RV and 3) a higher telescope time efficency by using A0V telluric standard from telescope archive.

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arXiv2014

Planetary companions to sdB stars

The formation of single sdB stars is an unresolved issue and the presence of close sub-stellar companions could explain the stellar envelope ejection near the tip of the RGB, that is needed to form an sdB star. In the last 6 years the first planets orbiting sdB stars were detected using different methods. In this article we discuss these recent discoveries, review the different detection methods used, and present some preliminary results obtained with Harps-N, which allows to reach an unprecedented accuracy in radial velocity measurements.

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arXiv2014

The Kepler-10 planetary system revisited by HARPS-N: A hot rocky world and a solid Neptune-mass planet

Kepler-10b was the first rocky planet detected by the Kepler satellite and con- firmed with radial velocity follow-up observations from Keck-HIRES. The mass of the planet was measured with a precision of around 30%, which was insufficient to constrain models of its internal structure and composition in detail. In addition to Kepler-10b, a second planet transiting the same star with a period of 45 days was sta- tistically validated, but the radial velocities were only good enough to set an upper limit of 20 Mearth for the mass of Kepler-10c. To improve the precision on the mass for planet b, the HARPS-N Collaboration decided to observe Kepler-10 intensively with the HARPS-N spectrograph on the Telescopio Nazionale Galileo on La Palma. In to- tal, 148 high-quality radial-velocity measurements were obtained over two observing seasons. These new data allow us to improve the precision of the mass determina- tion for Kepler-10b to 15%. With a mass of 3.33 +/- 0.49 Mearth and an updated radius of 1.47 +0.03 -0.02 Rearth, Kepler-10b has a density of 5.8 +/- 0.8 g cm-3, very close to the value -0.02 predicted by models with the same internal structure and composition as the Earth. We were also able to determine a mass for the 45-day period planet Kepler-10c, with an even better precision of 11%. With a mass of 17.2 +/- 1.9 Mearth and radius of 2.35 +0.09 -0.04 Rearth, -0.04 Kepler-10c has a density of 7.1 +/- 1.0 g cm-3. Kepler-10c appears to be the first strong evidence of a class of more massive solid planets with longer orbital periods.

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arXiv2014

The Earth as an extrasolar transiting planet - II: HARPS and UVES detection of water vapor, biogenic O$_2$, and O$_3$

The atmospheric composition of transiting exoplanets can be characterized during transit by spectroscopy. For the transit of an Earth twin, models predict that biogenic $O_2$ and $O_3$ should be detectable, as well as water vapour, a molecule linked to habitability as we know it on Earth. The aim is to measure the Earth radius versus wavelength $λ$ - or the atmosphere thickness $h(λ)$ - at the highest spectral resolution available to fully characterize the signature of Earth seen as a transiting exoplanet. We present observations of the Moon eclipse of 21-12-2010. Seen from the Moon, the Earth eclipses the Sun and opens access to the Earth atmosphere transmission spectrum. We used HARPS and UVES spectrographs to take penumbra and umbra high-resolution spectra from 3100 to 10400 Ang. A change of the quantity of water vapour above the telescope compromised the quality of the UVES data. We corrected for this effect in the data processing. We analyzed the data by 3 different methods. The 1st method is based on the analysis of pairs of penumbra spectra. The 2nd makes use of a single penumbra spectrum, and the 3rd of all penumbra and umbra spectra. Profiles $h(λ)$ are obtained with the three methods for both instruments. The 1st method gives the best result, in agreement with a model. The second method seems to be more sensitive to the Doppler shift of solar spectral lines with respect to the telluric lines. The 3rd method makes use of umbra spectra which bias the result, but it can be corrected for this a posteriori from results with the first method. The 3 methods clearly show the spectral signature of the Rayleigh scattering in the Earth atmosphere and the bands of H$_2$O, O$_2$, and O$_3$. Sodium is detected. Assuming no atmospheric perturbations, we show that the E-ELT is theoretically able to detect the $O_2$ A-band in 8~h of integration for an Earth twin at 10pc.

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arXiv2013

An Earth-sized planet with an Earth-like density

Kepler-78 (KIC 8435766) was identified by Sanchis-Ojeda et al. (2013) as harbouring a transiting planet of 1.16 times the size of the Earth and an orbital period of only 8.5 hours. While the exquisite Kepler photometry was able to determine its radius and period, the mass of the planet (and thus its mean density) remained unconstrained in the absence of precise radial-velocity measurements. Here we present an accurate mass measurement of Kepler-78b using the HARPS-N spectrograph, recently installed on the Telescopio Nazionale Galileo (INAF) at the Roque de los Muchachos Observatory, La Palma, Spain. These new data yield a mass of 1.86 Earth masses. The resulting mean density of the planet is 5.57 grams per cubic centimetre, which is similar to that of the Earth and implies a composition of iron and rock. Kepler-78b, which orbits a Sun-like star called Kepler 78 located in the Cygnus constellation at a distance of about 400 light years from us, is now the smallest exoplanet for which both the mass and radius are known accurately.

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arXiv2013

Spectroscopic direct detection of reflected light from extra-solar planets

At optical wavelengths, an exoplanet's signature is essentially reflected light from the host star - several orders of magnitude fainter. Since it is superimposed on the star spectrum its detection has been a difficult observational challenge. However, the development of a new generation of instruments like ESPRESSO and next generation telescopes like the E-ELT put us in a privileged position to detect these planets' reflected light as we will have access to extremely high signal-to-noise ratio spectra. With this work, we propose an alternative approach for the direct detection of the reflected light of an exoplanet. We simulated observations with ESPRESSO@VLT and HIRES@E-ELT of several star+planet systems, encompassing 10h of the most favourable orbital phases. To the simulated spectra we applied the Cross Correlation Function to operate in a much higher signal-to-noise ratio domain than when compared with the spectra. The use of the Cross-Correlation Function permitted us to recover the simulated the planet signals at a level above 3 σ_{noise} significance on several prototypical (e.g., Neptune type planet with a 2 days orbit with the VLT at 4.4 σ_{noise} significance) and real planetary systems (e.g., 55 Cnc e with the E-ELT at 4.9 σ_{noise} significance). Even by using a more pessimistic approach to the noise level estimation, where systematics in the spectra increase the noise 2-3 times, the detection of the reflected light from large close-orbit planets is possible. We have also shown that this kind of study is currently within reach of current instruments and telescopes (e.g., 51 Peg b with the VLT at 5.2 σ_{noise} significance), although at the limit of their capabilities.

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arXiv2013

Spitzer Observations of GJ3470b: a Very Low-density Neptune-size Planet Orbiting a Metal-rich M dwarf

We present Spitzer/IRAC 4.5-micron transit photometry of GJ3470b, a Neptune-size planet orbiting a M1.5 dwarf star with a 3.3-day period recently discovered in the course of the HARPS M-dwarf survey. We refine the stellar parameters by employing purely empirical mass-luminosity and surface brightness relations constrained by our updated value for the mean stellar density, and additional information from new near-infrared spectroscopic observations. We derive a stellar mass of M_star = 0.539+0.047-0.043 M_sun and a radius of R_star = 0.568+0.037-0.031 R_sun. We determine the host star of GJ3470b to be metal-rich, with a metallicity of [Fe/H] = +0.20 +/- 0.10 and an effective temperature of Teff = 3600 +/- 100 K. The revised stellar parameters yield a planetary radius R_pl = 4.83+0.22-0.21 R_Earth that is 13 percent larger than the value previously reported in the literature. We find a planetary mass M_pl = 13.9+1.5-1.4 M_Earth that translates to a very low planetary density, rho_pl = 0.72+0.13-0.12 gcm-3, which is 33% smaller than the original value. With a mean density half of that of GJ436b, GJ3470b is an example of a very low-density low-mass planet, similar to Kepler-11d, Kepler-11e, and Kepler-18c but orbiting a much brighter nearby star that is more conducive to follow-up studies.

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arXiv2011

Stellar Variability of the Exoplanet Hosting Star HD 63454

Of the hundreds of exoplanets discovered using the radial velocity technique, many are orbiting close to their host stars with periods less than 10 days. One of these, HD 63454, is a young active K dwarf which hosts a Jovian planet in a 2.82 day period orbit. The planet has a 14% transit probability and a predicted transit depth of 1.2%. Here we provide a re-analysis of the radial velocity data to produce an accurate transit ephemeris. We further analyse 8 nights of time series data to search for stellar activity both intrinsic to the star and induced by possible interactions of the exoplanet with the stellar magnetospheres. We establish the photometric stability of the star at the 3 millimag level despite strong Ca II emission in the spectrum. Finally, we rule out photometric signatures of both star-planet magnetosphere interactions and planetary transit signatures. From this we are able to place constraints on both the orbital and physical properties of the planet.

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arXiv2011

Spin-orbit inclinations of the exoplanetary systems HAT-P-8, HAT-P-9, HAT-P-16 and HAT-P-23

We report the measurement of the spin-orbit angle of the extra-solar planets HAT-P-8 b, HAT-P-9 b, HAT-P-16 b and HAT-P-23 b, thanks to spectroscopic observations performed at the Observatoire de Haute-Provence with the SOPHIE spectrograph on the 1.93-m telescope. Radial velocity measurements of the Rossiter-McLaughlin effect show the detection of an apparent prograde, aligned orbit for all systems. The projected spin-orbit angles are found to be lambda=-17 deg (+9.2,-11.5), -16 deg (8), -10 deg (16), +15 deg (22) for HAT-P-8, HAT-P-9, HAT-P-16 and HAT-P-23 respectively, with corresponding projected rotational velocities of 14.5 (0.8), 12.5 (1.8), 3.9 (0.8), and 7.8 (1.6) km/s. These new results increase to 37 the number of accurately measured spin-orbit angles in transiting extrasolar systems. We conclude by drawing a tentative picture of the global behaviour of orbital alignement, involving the complexity and diversity of possible mechanisms.

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arXiv2011

WASP-23b: a transiting hot Jupiter around a K dwarf and its Rossiter-McLaughlin effect

We report the discovery of a new transiting planet in the Southern Hemisphere. It has been found by the WASP-south transit survey and confirmed photometrically and spectroscopically by the 1.2m Swiss Euler telescope, LCOGT 2m Faulkes South Telescope, the 60 cm TRAPPIST telescope and the ESO 3.6m telescope. The orbital period of the planet is 2.94 days. We find it is a gas giant with a mass of 0.88 \pm 0.10 Mj and a radius estimated at 0.96 \pm 0.05 Rj . We have also obtained spectra during transit with the HARPS spectrograph and detect the Rossiter-McLaughlin effect despite its small amplitude. Because of the low signal to noise of the effect and of a small impact parameter we cannot place a constraint on the projected spin-orbit angle. We find two confiicting values for the stellar rotation. Our determination, via spectral line broadening gives v sin I = 2.2 \pm 0.3 km/s, while another method, based on the activity level using the index log R'HK, gives an equatorial rotation velocity of only v = 1.35 \pm 0.20 km/s. Using these as priors in our analysis, the planet could either be misaligned or aligned. This should send strong warnings regarding the use of such priors. There is no evidence for eccentricity nor of any radial velocity drift with time.

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arXiv2011

HD 5388 b is a 69 M_Jup companion instead of a planet

We examined six exoplanet host stars with non-standard Hipparcos astrometric solution, which may be indicative of unrecognised orbital motion. Using Hipparcos intermediate astrometric data, we detected the astrometric orbit of HD 5388 at a significance level of 99.4 % (2.7 sigma). HD 5388 is a metal-deficient star and hosts a planet candidate with a minimum mass of 1.96 M_J discovered in 2010. We determined its orbit inclination to be i = 178.3 +0.4/-0.7 deg and the corresponding mass of its companion HD 5388 b to be M_2 = 69 +/- 20 M_J. The orbit is seen almost face-on and the companion mass lies at the upper end of the brown-dwarf mass range. A mass lower than 13 M_J was excluded at the 3-sigma level. The astrometric motions of the five other stars had been investigated by other authors revealing two planetary companions, one stellar companion, and two statistically insignificant orbits. We conclude that HD 5388 b is not a planet but most likely a brown-dwarf companion. In addition, we find that the inclinations of the stellar rotation axis and the companion's orbital axis differ significantly.

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arXiv2010

The HARPS search for southern extra-solar planets: XXVIII. Two giant planets around M0 dwarfs

Fewer giants planets are found around M dwarfs than around more massive stars, and this dependence of planetary characteristics on the mass of the central star is an important observational diagnostic of planetary formation theories. In part to improve on those statistics, we are monitoring the radial velocities of nearby M dwarfs with the HARPS spectrograph on the ESO 3.6 m telescope. We present here the detection of giant planets around two nearby M0 dwarfs: planets, with minimum masses of respectively 5 Jupiter masses and 1 Saturn mass, orbit around Gl 676A and HIP 12961. The latter is, by over a factor of two, the most massive planet found by radial velocity monitoring of an M dwarf, but its being found around an early M-dwarf is in approximate line with the upper envelope of the planetary vs stellar mass diagram. HIP 12961 ([Fe/H]=-0.07) is slightly more metal-rich than the average solar neighborhood ([Fe/H]=-0.17), and Gl 676A ([Fe/H=0.18) significantly so. The two stars together therefore reinforce the growing trend for giant planets being more frequent around more metal-rich M dwarfs, and the 5~Jupiter mass Gl 676Ab being found around a metal-rich star is consistent with the expectation that the most massive planets preferentially form in disks with large condensate masses.

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