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A. Vidal-Madjar

Publications and source records attributed to A. Vidal-Madjar.

At least 37 records · Page 2Linked to original sources
arXiv2013

HST hot Jupiter Transmission Spectral Survey: A detection of Na and strong optical absorption in HAT-P-1b

We present an optical to near-infrared transmission spectrum of the hot Jupiter HAT-P-1b, based on HST observations, covering the spectral regime from 0.29 to 1.027μm with STIS, which is coupled with a recent WFC3 transit (1.087 to 1.687μm). We derive refined physical parameters of the HAT-P-1 system, including an improved orbital ephemeris. The transmission spectrum shows a strong absorption signature shortward of 0.55μm, with a strong blueward slope into the near-ultraviolet. We detect atmospheric sodium absorption at a 3.3σ significance level, but find no evidence for the potassium feature. The red data implies a marginally flat spectrum with a tentative absorption enhancement at wavelength longer than ~0.85μm. The STIS and WFC3 spectra differ significantly in absolute radius level (4.3 +/- 1.6 pressure scale heights), implying strong optical absorption in the atmosphere of HAT-P-1b. The optical to near-infrared difference cannot be explained by stellar activity, as simulta- neous stellar activity monitoring of the G0V HAT-P-1b host star and its identical companion show no significant activity that could explain the result. We compare the complete STIS and WFC3 transmission spectrum with theoretical atmospheric mod- els which include haze, sodium and an extra optical absorber. We find that both an optical absorber and a super-solar sodium to water abundance ratio might be a sce- nario explaining the HAT-P-1b observations. Our results suggest that strong optical absorbers may be a dominant atmospheric feature in some hot Jupiter exoplanets.

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arXiv2013

HST hot Jupiter transmission spectral survey: evidence for aerosols and lack of TiO in the atmosphere of WASP-12b

We present HST optical transmission spectra of the transiting hot Jupiter WASP-12b, taken with the STIS instrument. From the transmission spectra, we are able to decisively rule out prominent absorption by TiO in the exoplanet's atmosphere. Strong pressure-broadened Na and K absorption signatures are also excluded, as are significant metal-hydride features. We compare our combined broadband spectrum to a wide variety of existing aerosol-free atmospheric models, though none are satisfactory fits. However, we do find that the full transmission spectrum can be described by models which include significant opacity from aerosols: including Rayleigh scattering, Mie scattering, tholin haze, and settling dust profiles. The transmission spectrum follows an effective extinction cross section with a power-law of index alpha, with the slope of the transmission spectrum constraining the quantity alphaT = -3528+/-660 K, where T is the atmospheric temperature. Rayleigh scattering (alpha=-4) is among the best fitting models, though requires low terminator temperatures near 900 K. Sub-micron size aerosol particles can provide equally good fits to the entire transmission spectrum for a wide range of temperatures, and we explore corundum as a plausible dust aerosol. The presence of atmospheric aerosols also helps to explain the modestly bright albedo implied by Spitzer observations, as well as the near black body nature of the emission spectrum. Ti-bearing condensates on the cooler night-side is the most natural explanation for the overall lack of TiO signatures in WASP-12b, indicating the day/night cold-trap is an important effect for very hot Jupiters. These finding indicate that aerosols can play a significant atmospheric role for the entire wide range of hot-Jupiter atmospheres, potentially affecting their overall spectrum and energy balance.(abridged)

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arXiv2013

Hubble Space Telescope hot Jupiter Transmission Spectral Survey: detection of water in HAT-P-1b from Wide Field Camera 3 near-infrared spatial scan observations

We present Hubble Space Telescope near-infrared transmission spectroscopy of the transiting hot-Jupiter HAT-P-1b. We observed one transit with Wide Field Camera 3 using the G141 low-resolution grism to cover the wavelength range 1.087- 1.678 μm. These time series observations were taken with the newly available spatial scan mode that increases the duty cycle by nearly a factor of two, thus improving the resulting photometric precision of the data. We measure a planet-to-star radius ratio of Rp/R*=0.11709+/-0.00038 in the white light curve with the centre of transit occurring at 2456114.345+/-0.000133 (JD). We achieve S/N levels per exposure of 1840 (0.061%) at a resolution of {Δλ}=19.2nm (R~70) in the 1.1173 - 1.6549μm spectral region, providing the precision necessary to probe the transmission spectrum of the planet at close to the resolution limit of the instrument. We compute the transmission spectrum using both single target and differential photometry with similar results. The resultant transmission spectrum shows a significant absorption above the 5-σ level matching the 1.4μm water absorption band. In solar composition models, the water absorption is sensitive to the ~1 mbar pressure levels at the terminator. The detected absorption agrees with that predicted by an 1000 K isothermal model, as well as with that predicted by a planetary-averaged temperature model.

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arXiv2013

An HST Optical to Near-IR Transmission Spectrum of the Hot Jupiter WASP-19b: Detection of Atmospheric Water and Likely Absence of TiO

We measure the transmission spectrum of WASP-19b from 3 transits using low-resolution optical spectroscopy from the HST Space Telescope Imaging Spectrograph (STIS). The STIS spectra cover a wavelength range of 0.29-1.03 microns with resolving power R=500. The optical data are combined with archival near-IR data from the HST Wide Field Camera 3 (WFC3) G141 grism, covering the wavelength range 1.087-1.687 micron, with resolving power R=130. We obtain the transmission spectrum from 0.53-1.687 microns with S/N levels between 3000 and 11,000 in 0.1 micron bins. WASP-19 is a very active star, with optical stellar flux varying by a few per cent over time. We correct the transit light curves for the effects of stellar activity using ground-based activity monitoring with the Cerro Tololo Inter-American Observatory (CTIO). While we were not able to construct a transmission spectrum using the blue optical data due to the presence of large occulted star spots, we were able to use the spot crossings to help constrain the mean stellar spot temperature. To search for predicted features in the hot-Jupiter atmosphere, we also define spectral indices for differential radius measurements to specifically search for the presence of TiO and alkali line features. Our measurements rule out TiO features predicted for a planet of WASP-19b's equilibrium temperature (2050 K) in the transmission spectrum at the 2.7-2.9 sigma confidence level, depending on atmospheric model formalism. The WFC3 transmission spectrum shows strong absorption features due to the presence of H2O, which is detected at the 4 sigma confidence level between 1.1 and 1.4 microns. The results indicate that WASP-19b is a planet with no or low levels of TiO and without a high C/O ratio. The lack of observable TiO features are possibly due to rainout, breakdown from stellar activity or the presence of other absorbers in the optical.

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arXiv2012

GTC OSIRIS transiting exoplanet atmospheric survey: detection of sodium in XO-2b from differential long-slit spectroscopy

We present two transits of the hot-Jupiter exoplanet XO-2b using the Gran Telescopio Canarias (GTC). The time series observations were performed using long-slit spectroscopy of XO-2 and a nearby reference star with the OSIRIS instrument, enabling differential specrophotometric transit lightcurves capable of measuring the exoplanet's transmission spectrum. Two optical low-resolution grisms were used to cover the optical wavelength range from 3800 to 9300Å. We find that sub-mmag level slit losses between the target and reference star prevent full optical transmission spectra from being constructed, limiting our analysis to differential absorption depths over ~1000Å regions. Wider long slits or multi-object grism spectroscopy with wide masks will likely prove effective in minimising the observed slit-loss trends. During both transits, we detect significant absorption in the planetary atmosphere of XO-2b using a 50Å bandpass centred on the Na I doublet, with absorption depths of Delta(R_pl/R_star)^2=0.049+/-0.017 % using the R500R grism and 0.047+/-0.011 % using the R500B grism (combined 5.2-sigma significance from both transits). The sodium feature is unresolved in our low-resolution spectra, with detailed modelling also likely ruling out significant line-wing absorption over an ~800Å region surrounding the doublet. Combined with narrowband photometric measurements, XO-2b is the first hot Jupiter with evidence for both sodium and potassium present in the planet's atmosphere.

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arXiv2012

Temporal variations in the evaporating atmosphere of the exoplanet HD 189733b

Atmospheric escape has been detected from the exoplanet HD 209458b through transit observations of the hydrogen Lyman-alpha line. Here we present spectrally resolved Lyman-alpha transit observations of the exoplanet HD 189733b at two different epochs. These HST/STIS observations show for the first time, that there are significant temporal variations in the physical conditions of an evaporating planetary atmosphere. While atmospheric hydrogen is not detected in the first epoch observations, it is observed at the second epoch, producing a transit absorption depth of 14.4+/-3.6% between velocities of -230 to -140 km/s. Contrary to HD 209458b, these high velocities cannot arise from radiation pressure alone and require an additional acceleration mechanism, such as interactions with stellar wind protons. The observed absorption can be explained by an atmospheric escape rate of neutral hydrogen atoms of about 10^9 g/s, a stellar wind with a velocity of 190 km/s and a temperature of ~10^5K. An X-ray flare from the active star seen with Swift/XRT 8 hours before the second-epoch observation supports the idea that the observed changes within the upper atmosphere of the planet can be caused by variations in the stellar wind properties, or by variations in the stellar energy input to the planetary escaping gas (or a mix of the two effects). These observations provide the first indication of interaction between the exoplanet's atmosphere and stellar variations.

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arXiv2011

The SOPHIE search for northern extrasolar planets. IV. Massive companions in the planet-brown dwarf boundary

The mass domain where massive extrasolar planets and brown dwarfs lay is still poorly understood. Indeed, not even a clear dividing line between massive planets and brown dwarfs has been established yet. This is partly due to the paucity of this kind of objects orbiting close to solar-type stars, the so-called brown dwarf desert, that hinders setting up a strong observational base to compare to models and theories of formation and evolution. We search to increase the current sample of massive sub-stellar objects with precise orbital parameters, and to constrain the true mass of detected sub-stellar candidates. The initial identification of sub-stellar candidates is done using precise radial velocity measurements obtained with the SOPHIE spectrograph at the 1.93-m telescope of the Haute-Provence Observatory. Subsequent characterisation of these candidates, with the principal aim of identifying stellar companions in low-inclination orbits, is done by means of different spectroscopic diagnostics, as the measurement of the bisector velocity span and the study of the correlation mask effect. With this objective, we also employed astrometric data from the Hipparcos mission and a novel method of simulating stellar cross-correlation functions. Seven new objects with minimum masses between ~ 10 Mjup and ~90 Mjup are detected. Out of these, two are identified as low-mass stars in low-inclination orbits, and two others have masses below the theoretical deuterium-burning limit, and are therefore planetary candidates. The remaining three are brown dwarf candidates; the current upper limits for their the masses do not allow us to conclude on their nature. Additionally, we have improved on the parameters of an already-known brown dwarf (HD137510b), confirmed by astrometry.

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arXiv2011

The retrograde orbit of the HAT-P-6b exoplanet

We observed with the SOPHIE spectrograph (OHP, France) the transit of the HAT-P-6b exoplanet across its host star. The resulting stellar radial velocities display the Rossiter-McLaughlin anomaly and reveal a retrograde orbit: the planetary orbital spin and the stellar rotational spin point towards approximately opposite directions. A fit to the anomaly measures a sky-projected angle lambda = 166 +/- 10 degrees between these two spin axes. All seven known retrograde planets are hot jupiters with masses M_p < 3 M_Jup. About two thirds of the planets in this mass range however are prograde and aligned (lambda ~ 0). By contrast, most of the more massive planets (M_p > 4 M_Jup) are prograde but misaligned. Different mechanisms may therefore be responsible for planetary obliquities above and below ~3.5 M_Jup.

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arXiv2010

The upper atmosphere of the exoplanet HD209458b revealed by the sodium D lines: Temperature-pressure profile, ionization layer, and thermosphere

A complete reassessment of the HST observations of the transits of the extrasolar planet HD209458b has provided a transmission spectrum of the atmosphere over a wide range of wavelengths. Analysis of the NaI absorption line profile has already shown that the sodium abundance has to drop by at least a factor of ten above a critical altitude. Here we analyze the profile in the deep core of the NaI doublet line from HST and high-resolution ground-based spectra to further constrain the vertical structure of the HD209458b atmosphere. With a wavelength-dependent cross section that spans more than 5 orders of magnitude, we use the absorption signature of the NaI doublet as an atmospheric probe. The NaI transmission features are shown to sample the atmosphere of HD209458b over an altitude range of more than 6500km, corresponding to a pressure range of 14 scale heights spanning 1 millibar to 1e-9 bar pressures. By comparing the observations with a multi-layer model in which temperature is a free parameter at the resolution of the atmospheric scale height, we constrain the temperature vertical profile and variations in the Na abundance in the upper part of the atmosphere of HD209458b. We find a rise in temperature above the drop in sodium abundance at the 3mbar level. We also identify an isothermal atmospheric layer at 1500+/-100K spanning almost 6 scale heights in altitude, from 1e-5 to 1e-7 bar. Above this layer, the temperature rises again to 2500(+1500/-1000)K at 1e-9 bar, indicating the presence of a thermosphere. The resulting temperature-pressure (T-P) profile agrees with the Na condensation scenario at the 3 mbar level, with a possible signature of sodium ionization at higher altitudes, near the 3e-5 bar level. Our T-P profile is found to be in good agreement with the profiles obtained with aeronomical models including hydrodynamic escape.

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arXiv2010

Gran Telescopio Canarias OSIRIS Transiting Exoplanet Atmospheric Survey: Detection of potassium in XO-2b from narrowband spectrophotometry

We present Gran Telescopio Canarias (GTC) optical transit narrow-band photometry of the hot-Jupiter exoplanet XO-2b using the OSIRIS instrument. This unique instrument has the capabilities to deliver high cadence narrow-band photometric lightcurves, allowing us to probe the atmospheric composition of hot Jupiters from the ground. The observations were taken during three transit events which cover four wavelengths at spectral resolutions near 500, necessary for observing atmospheric features, and have near-photon limited sub-mmag precisions. Precision narrow-band photometry on a large aperture telescope allows for atmospheric transmission spectral features to be observed for exoplanets around much fainter stars than those of the well studied targets HD209458b and HD189733b, providing access to the majority of known transiting planets. For XO-2b, we measure planet-to-star radius contrasts of R_pl/R_star=0.10508+/-0.00052 at 6792 Ang, 0.10640+/-0.00058 at 7582 Ang, and 0.10686+/-0.00060 at 7664.9 Ang, and 0.10362+/-0.00051 at 8839 Ang. These measurements reveal significant spectral features at two wavelengths, with an absorption level of 0.067+/-0.016% at 7664.9 Ang due to atmospheric potassium in the line core (a 4.1-sigma significance level), and an absorption level of 0.058+/-0.016% at 7582 Ang, (a 3.6-sigma significance level). When comparing our measurements to hot-Jupiter atmospheric models, we find good agreement with models which are dominated in the optical by alkali metals. This is the first evidence for potassium in an extrasolar planet, an element that has long been theorized along with sodium to be a dominant source of opacity at optical wavelengths for hot Jupiters.

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arXiv2010

Transit spectrophotometry of the exoplanet HD189733b. II. New Spitzer observations at 3.6 microns

We present a new primary transit observation of the hot-jupiter HD189733b, obtained at 3.6 microns with the Infrared Array Camera (IRAC) onboard the Spitzer Space Telescope. Previous measurements at 3.6 microns suffered from strong systematics and conclusions could hardly be obtained with confidence on the water detection by comparison of the 3.6 and 5.8 microns observations. We use a high S/N Spitzer photometric transit light curve to improve the precision of the near infrared radius of the planet at 3.6 microns. The observation has been performed using high-cadence time series integrated in the subarray mode. We are able to derive accurate system parameters, including planet-to-star radius ratio, impact parameter, scale of the system, and central time of the transit from the fits of the transit light curve. We compare the results with transmission spectroscopic models and with results from previous observations at the same wavelength. We obtained the following system parameters: R_p/R_\star=0.15566+0.00011-0.00024, b=0.661+0.0053-0.0050, and a/R_\star=8.925+0.0490-0.0523 at 3.6 microns. These measurements are three times more accurate than previous studies at this wavelength because they benefit from greater observational efficiency and less statistic and systematic errors. Nonetheless, we find that the radius ratio has to be corrected for stellar activity and present a method to do so using ground-based long-duration photometric follow-up in the V-band. The resulting planet-to-star radius ratio corrected for the stellar variability is in agreement with the previous measurement obtained in the same bandpass (Desert et al. 2009). We also discuss that water vapour could not be evidenced by comparison of the planetary radius measured at 3.6 and 5.8 microns, because the radius measured at 3.6 microns is affected by absorption by other species, possibly Rayleigh scattering by haze.

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arXiv2010

The SOPHIE search for northern extrasolar planets III. A Jupiter-mass companion around HD 109246

We report the detection of a Jupiter-mass planet discovered with the SOPHIE spectrograph mounted on the 1.93-m telescope at the Haute-Provence Observatory. The new planet orbits HD109246, a G0V star slightly more metallic than the Sun. HD109246b has a minimum mass of 0.77 MJup, an orbital period of 68 days, and an eccentricity of 0.12. It is placed in a sparsely populated region of the period distribution of extrasolar planets. We also present a correction method for the so-called seeing effect that affects the SOPHIE radial velocities. We complement this discovery announcement with a description of some calibrations that are implemented in the SOPHIE automatic reduction pipeline. These calibrations allow the derivation of the photon-noise radial velocity uncertainty and some useful stellar properties (vsini, [Fe/H], logR'HK) directly from the SOPHIE data.

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arXiv2010

Observation of the full 12-hour-long transit of the exoplanet HD80606b. Warm-Spitzer photometry and SOPHIE spectroscopy

We present new observations of a transit of the 111-day-period exoplanet HD80606b. Using the Spitzer Space Telescope and its IRAC camera on the post-cryogenic mission, we performed a 19-hour-long photometric observation of HD80606 that covers the full transit of 13-14 January 2010. We complement this photometric data by new spectroscopic observations that we simultaneously performed with SOPHIE at Haute-Provence Observatory. This provides radial velocity measurements of the first half of the transit that was previously uncovered with spectroscopy. This new data set allows the parameters of this singular planetary system to be significantly refined. We obtained a planet-to-star radius ratio R_p/R_* = 0.1001 +/- 0.0006 that is slightly lower than the one measured from previous ground observations. We detected a feature in the Spitzer light curve that could be due to a stellar spot. We also found a transit timing about 20 minutes earlier than the ephemeris prediction; this could be caused by actual TTVs due to an additional body in the system or by underestimated systematic uncertainties. The sky-projected angle between the spin-axis of HD80606 and the normal to the planetary orbital plane is found to be lambda = 42 +/- 8 degrees thanks to the fit of the Rossiter-McLaughlin anomaly. This allows scenarios with aligned spin-orbit to be definitively rejected. Over the twenty planetary systems with measured spin-orbit angles, a few of them are misaligned; this is probably the signature of two different evolution scenarios for misaligned and aligned systems, depending if they experienced or not gravitational interaction with a third body. As in the case of HD80606b, most of the planetary systems including a massive planet are tilted; this could be the signature of a separate evolution scenario for massive planets in comparison with Jupiter-mass planets.

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arXiv2010

Evaporation of the planet HD189733b observed in HI Lyman-alpha

We observed three transits of the extrasolar planet HD189733b in HI Lyman-alpha and in a few other lines in the ultraviolet with HST/ACS, in the search for atmospheric signatures. We detect a transit signature in the Lyman-alpha light curve with a transit depth of 5.05 +/- 0.75 %. This depth exceeds the occultation depth produced by the planetary disk alone at the 3.5-sigma level (statistical). Other stellar emission lines are less bright, and, taken individually, they do not show the transit signature, while the whole spectra redward of the Lyman-alpha line has enough photons to show a transit signature consistent with the absorption by the planetary disk alone. The transit depth's upper limits in the emission lines are 11.1% for OI at 1305A and 5.5% for CII at 1335A. The presence of an extended exosphere of atomic hydrogen around HD189733b producing 5% absorption of the full unresolved Lyman-alpha line flux shows that the planet is losing gas. The Lyman-alpha light curve is well-fitted by a numerical simulation of escaping hydrogen in which the planetary atoms are pushed by the stellar radiation pressure. We constrain the escape rate of atomic hydrogen to be between 10^9 and 10^{11} g/s and the ionizing extreme UV flux between 2 and 40 times the solar value (1-sigma), with larger escape rates corresponding to larger EUV flux. The best fit is obtained for dM/dt=10^{10} g/s and an EUV flux F_{EUV}=20 times the solar value. HD189733b is the second extrasolar planet for which atmospheric evaporation has been detected.

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arXiv2010

The SOPHIE search for northern extrasolar planets. II. A multi-planet system around HD9446

We report the discovery of a planetary system around HD9446, performed from radial velocity measurements secured with the spectrograph SOPHIE at the 193-cm telescope of the Haute-Provence Observatory during more than two years. At least two planets orbit this G5V, active star: HD9446b has a minimum mass of 0.7 M_Jup and a slightly eccentric orbit with a period of 30 days, whereas HD9446c has a minimum mass of 1.8 M_Jup and a circular orbit with a period of 193 days. As for most of the known multi-planet systems, the HD9446-system presents a hierarchical disposition, with a massive outer planet and a lighter inner planet.

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arXiv2009

Extrasolar planets and brown dwarfs around A-F type stars - VII. Theta Cygni radial velocity variations: planets or stellar phenomenon?

(abridged) In the frame of the search for extrasolar planets and brown dwarfs around early-type main-sequence stars, we present the results obtained on the early F-type star Theta Cygni. Elodie and Sophie at OHP were used to obtain the spectra. Our dedicated radial-velocity measurement method was used to monitor the star's radial velocities over five years. We also use complementary, high angular resolution and high-contrast images taken with PUEO at CFHT. We show that Theta Cygni radial velocities are quasi-periodically variable, with a ~150-day period. These variations are not due to the ~0.35-Msun stellar companion that we detected in imaging at more than 46 AU from the star. The absence of correlation between the bisector velocity span variations and the radial velocity variations for this 7 km/s vsini star, as well as other criteria indicate that the observed radial velocity variations are not due to stellar spots. The observed amplitude of the bisector velocity span variations also seems to rule out stellar pulsations. However, we observe a peak in the bisector velocity span periodogram at the same period as the one found in the radial velocity periodogram, which indicates a probable link between these radial velocity variations and the low amplitude lineshape variations which are of stellar origin. Long-period variations are not expected from this type of star to our knowledge. If a stellar origin (hence of new type) was to be confirmed for these long-period radial velocity variations, this would have several consequences on the search for planets around main-sequence stars, both in terms of observational strategy and data analysis. An alternative explanation for these variable radial velocities is the presence of at least one planet of a few Jupiter masses orbiting at less than 1 AU. (abridged)

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arXiv2009

Transit spectrophotometry of the exoplanet HD189733b. I. Searching for water but finding haze with HST NICMOS

We present Hubble Space Telescope near-infrared transit photometry of the nearby hot-Jupiter HD189733b. The observations were taken with the NICMOS instrument during five transits, with three transits executed with a narrowband filter at 1.87 microns and two performed with a narrowband filter at 1.66 microns. Our observing strategy using narrowband filters is insensitive to the usual HST intra-orbit and orbit-to-orbit measurement of systematic errors, allowing us to accurately and robustly measure the near-IR wavelength dependance of the planetary radius. Our measurements fail to reproduce the Swain et al. absorption signature of atmospheric water below 2 microns at a 5-sigma confidence level. We measure a planet-to-star radius contrast of 0.15498+/-0.00035 at 1.66 microns and a contrast of 0.15517+/-0.00019 at 1.87 microns. Both of our near-IR planetary radii values are in excellent agreement with the levels expected from Rayleigh scattering by sub-micron haze particles, observed at optical wavelengths, indicating that upper-atmospheric haze still dominates the near-IR transmission spectra over the absorption from gaseous molecular species at least below 2 microns.

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