arXiv subjects

France Allard

Publications and source records attributed to France Allard.

At least 19 recordsLinked to original sources

arXiv2020

A medium-resolution spectrum of the exoplanet HIP 65426 b

Medium-resolution integral-field spectrographs (IFS) coupled with adaptive-optics such as Keck/OSIRIS, VLT/MUSE, or SINFONI are appearing as a new avenue for enhancing the detection and characterization capabilities of young, gas giant exoplanets at large heliocentric distances (>5 au). We analyzed K-band VLT/SINFONI medium-resolution (R_lambda~5577) observations of the young giant exoplanet HIP 65426 b. Our dedicated IFS data analysis toolkit (TExTRIS) optimized the cube building, star registration, and allowed for the extraction of the planet spectrum. A Bayesian inference with the nested sampling algorithm coupled with the self-consistent forward atmospheric models BT-SETTL15 and Exo-REM using the ForMoSA tool yields Teff=1560 +/- 100K, log(g)<4.40dex, [M/H]=0.05 +/- 0.22dex, and an upper limit on the C/O ratio (<0.55). The object is also re-detected with the so-called "molecular mapping" technique. The technique yields consistent atmospheric parameters, but the loss of the planet pseudo-continuum in the process degrades or modifies the constraints on these parameters. The solar to sub-solar C/O ratio suggests an enrichment by solids at formation if the planet was formed beyond the water snowline (>20 au) by core-accretion. However, a formation by gravitational instability can not be ruled out. The metallicity is compatible with the bulk enrichment of massive Jovian planets from the Bern planet population models. Finally, we measure a radial velocity of 26 +/- 15km/s compatible with our revised measurement on the star. This is the fourth imaged exoplanet for which a radial velocity can be evaluated, illustrating the potential of such observations for assessing the coevolution of imaged systems belonging to star forming regions, such as HIP 65426.

astro-ph.EP↗

arXiv2019

Hunting for ancient brown dwarfs: the developing field of brown dwarfs in globular clusters

The detection of brown dwarfs in globular star clusters will allow us to break the degeneracies in age, mass and composition that affect our current models, and therefore to constrain the physics of their atmospheres and interiors. Furthermore, detecting brown dwarfs will help us constrain the properties of the clusters themselves, as they carry information about the clusters' age and dynamics. Great advancements in this field are to be expected in the next ten years, thanks to the extraordinary sensitivity in the infrared of upcoming telescopes like JWST and the ELTs.

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arXiv2017

SPIRou Input Catalogue: Global properties of 440 M dwarfs observed with ESPaDOnS at CFHT

Present and future high-precision radial-velocity spectrometers dedicated to the discovery of low-mass planets orbiting low-mass dwarfs need to focus on the best selected stars to make an efficient use of telescope time. In the framework of the preparation of the SPIRou Input Catalog, the CoolSnap program aims at screening M dwarfs in the solar neighborhood against binarity, rapid rotation, activity, ... To optimize the selection, the present paper describes the methods used to compute effective temperature, metallicity, projected rotation velocity of a large sample of 440 M dwarfs observed in the visible with the high-resolution spectro-polarimeter ESPaDOnS at CFHT. It also summarizes known and newly-discovered spectroscopic binaries, and stars known to belong to visual multiple systems. A calibration of the projected rotation velocity versus measured line widths for M dwarfs observed by the ESPaDOnS spectro-polarimeter is derived, and the resulting values are compared to equatorial rotation velocities deduced from rotation periods and radii. A comparison of the derived effective temperatures and metallicities with literature values is also conducted. Finally, the radial velocity uncertainty of each star in the sample is estimated, to narrow down the selection of stars to be included into the SPIRou Input Catalogue (SPIC).

astro-ph.SR↗

arXiv2017

Simultaneous, Multi-Wavelength Variability Characterization of the Free-Floating Planetary Mass Object PSO J318.5-22

We present simultaneous HST WFC3 + Spitzer IRAC variability monitoring for the highly-variable young ($\sim$20 Myr) planetary-mass object PSO J318.5-22. Our simultaneous HST + Spitzer observations covered $\sim$2 rotation periods with Spitzer and most of a rotation period with HST. We derive a period of 8.6$\pm$0.1 hours from the Spitzer lightcurve. Combining this period with the measured $v sin i$ for this object, we find an inclination of 56.2$\pm 8.1^{\circ}$. We measure peak-to-trough variability amplitudes of 3.4$\pm$0.1$\%$ for Spitzer Channel 2 and 4.4 - 5.8$\%$ (typical 68$\%$ confidence errors of $\sim$0.3$\%$) in the near-IR bands (1.07-1.67 $μ$m) covered by the WFC3 G141 prism -- the mid-IR variability amplitude for PSO J318.5-22 one of the highest variability amplitudes measured in the mid-IR for any brown dwarf or planetary mass object. Additionally, we detect phase offsets ranging from 200--210$^{\circ}$ (typical error of $\sim$4$^{\circ}$) between synthesized near-IR lightcurves and the Spitzer mid-IR lightcurve, likely indicating depth-dependent longitudinal atmospheric structure in this atmosphere. The detection of similar variability amplitudes in wide spectral bands relative to absorption features suggests that the driver of the variability may be inhomogeneous clouds (perhaps a patchy haze layer over thick clouds), as opposed to hot spots or compositional inhomogeneities at the top-of-atmosphere level.

astro-ph.EP↗

arXiv2017

A physically motivated and empirically calibrated method to measure effective temperature, metallicity, and Ti abundance of M dwarfs

The ability to perform detailed chemical analysis of Sun-like F-, G-, and K-type stars is a powerful tool with many applications including studying the chemical evolution of the Galaxy and constraining planet formation theories. Unfortunately, complications in modeling cooler stellar atmospheres hinders similar analysis of M-dwarf stars. Empirically-calibrated methods to measure M dwarf metallicity from moderate-resolution spectra are currently limited to measuring overall metallicity and rely on astrophysical abundance correlations in stellar populations. We present a new, empirical calibration of synthetic M dwarf spectra that can be used to infer effective temperature, Fe abundance, and Ti abundance. We obtained high-resolution (R~25,000), Y-band (~1 micron) spectra of 29 M dwarfs with NIRSPEC on Keck II. Using the PHOENIX stellar atmosphere modeling code (version 15.5), we generated a grid of synthetic spectra covering a range of temperatures, metallicities, and alpha-enhancements. From our observed and synthetic spectra, we measured the equivalent widths of multiple Fe I and Ti I lines and a temperature-sensitive index based on the FeH bandhead. We used abundances measured from widely-separated solar-type companions to empirically calibrate transformations to the observed indices and equivalent widths that force agreement with the models. Our calibration achieves precisions in Teff, [Fe/H], and [Ti/Fe] of 60 K, 0.1 dex, and 0.05 dex, respectively and is calibrated for 3200 K < Teff < 4100 K, -0.7 < [Fe/H] < +0.3, and -0.05 < [Ti/Fe] < +0.3. This work is a step toward detailed chemical analysis of M dwarfs at a similar precision achieved for FGK stars.

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arXiv2017

Science cases for a visible interferometer

High spatial resolution is the key for the understanding various astrophysical phenomena. But even with the future E-ELT, single dish instruments are limited to a spatial resolution of about 4 mas in the visible. For the closest objects within our Galaxy most of the stellar photosphere remains smaller than 1 mas. With the success of long baseline interferometry these limitations were soom overcome. Today low and high resolution interferometric instruments on the VLTI and CHARA offer an immense range of astrophysical studies. Combining more telescopes and moving to visible wavelengths broadens the science cases even more. With the idea of developing strong science cases for a future visible interferometer, we organized a science group around the following topics: pre-main sequence and main sequence stars, fundamental parameters, asteroseismology and classical pulsating stars, evolved stars, massive stars, active galactic nuclei (AGNs) and imaging techniques. A meeting was organized on the 15th and 16th of January, 2015 in Nice with the support of the Action Specific in Haute Resolution Angulaire (ASHRA), the Programme National en Physique Stellaire (PNPS), the Lagrange Laboratory and the Observatoire de la Cote d'Azur, in order to present these cases and to discuss them further for future visible interferometers. This White Paper presents the outcome of the exchanges. This book is dedicated to the memory of our colleague Olivier Chesneau who passed away at the age of 41.

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arXiv2016

The physical mechanism behind M dwarf metallicity indicators and the role of C and O abundances

We present NIR synthetic spectra based on PHOENIX stellar atmosphere models of typical early and mid M dwarfs with varied C and O abundances. We apply multiple recently published methods for determining M dwarf metallicity to our models to determine the effect of C and O abundances on metallicity indicators. We find that the pseudo-continuum level is very sensitive to C/O and that all metallicity indicators show a dependence on C and O abundances, especially in lower Teff models. In some cases the inferred metallicity ranges over a full order of magnitude (>1 dex) when [C/Fe] and [O/Fe] are varied independently by +/-0.2. We also find that [(O-C)/Fe], the difference in O and C abundances, is a better tracer of the pseudo-continuum level than C/O. Models of mid-M dwarfs with [C/Fe], [O/Fe], and [M/H] that are realistic in the context of galactic chemical evolution suggest that variation in [(O-C)/Fe] is the primary physical mechanism behind the M dwarf metallicity tracers investigated here. Empirically calibrated metallicity indicators are still valid for most nearby M dwarfs due to the tight correlation between [(O-C)/Fe] and [Fe/H] evident in spectroscopic surveys of solar neighborhood FGK stars. Variations in C and O abundances also affect the spectral energy distribution of M dwarfs. Allowing [O/Fe] to be a free parameter provides better agreement between synthetic spectra and observed spectra of metal-rich M dwarfs. We suggest that flux-calibrated, low-resolution, NIR spectra can provide a path toward measuring C and O abundances in M dwarfs and breaking the degeneracy between C/O and [Fe/H] present in M dwarf metallicity indicators.

astro-ph.SR↗

arXiv2015

Atomic and Molecular Data: Division B / Commission 14

Commission 14 has been operating for almost a century. It was one of the 32 Standing IAU Commissions and established in 1919. The main purpose of Commission 14 is to foster interactions between the astronomical community and those conducting research on atoms, molecules, and solid state particles to provide data vital to reducing and analysing astronomical observations and performing theoretical investigations. Although Commission 14 terminated at the Honolulu General Assembly, the field Atomic and Molecular Data remains with the IAU. By call of the IAU, the Commission 14 OC initiated a proposal for a new Commission on Laboratory Astrophysics that was approved. Commission on Laboratory Astrophysics will be a natural evolution of Commission 14.

physics.atom-ph↗

arXiv2015

Variability in a Young, L/T Transition Planetary-Mass Object

As part of our ongoing NTT SoFI survey for variability in young free-floating planets and low mass brown dwarfs, we detect significant variability in the young, free-floating planetary mass object PSO J318.5-22, likely due to rotational modulation of inhomogeneous cloud cover. A member of the 23$\pm$3 Myr $β$ Pic moving group, PSO J318.5-22 has T$_\mathrm{eff}$ = 1160$^{+30}_{-40}$ K and a mass estimate of 8.3$\pm$0.5 M$_{Jup}$ for a 23$\pm$3 Myr age. PSO J318.5-22 is intermediate in mass between 51 Eri b and $β$ Pic b, the two known exoplanet companions in the $β$ Pic moving group. With variability amplitudes from 7-10$\%$ in J$_{S}$ at two separate epochs over 3-5 hour observations, we constrain the rotational period of this object to $>$5 hours. In K$_{S}$, we marginally detect a variability trend of up to 3$\%$ over a 3 hour observation. This is the first detection of weather on an extrasolar planetary mass object. Among L dwarfs surveyed at high-photometric precision ($<$3$\%$) this is the highest amplitude variability detection. Given the low surface gravity of this object, the high amplitude preliminarily suggests that such objects may be more variable than their high mass counterparts, although observations of a larger sample is necessary to confirm this. Measuring similar variability for directly imaged planetary companions is possible with instruments such as SPHERE and GPI and will provide important constraints on formation. Measuring variability at multiple wavelengths can help constrain cloud structure.

astro-ph.EP↗

arXiv2015

On the absolute age of the metal-rich globular M71 (NGC 6838): I. optical photometry

We investigated the absolute age of the Galactic globular cluster M71 (NGC 6838) by using optical ground-based images (u',g',r',i',z') collected with the MegaCam camera at the Canada-France-Hawaii-Telescope (CFHT). We performed a robust selection of field and cluster stars by applying a new method based on the 3D (r',u'-g',g'-r') Color-Color-Magnitude-Diagram. The comparison between the Color-Magnitude-Diagram of the candidate cluster stars and a new set of isochrones, at the locus of the Main Sequence Turn Off (MSTO), suggests an absolute age of 12+/-2 Gyr. The absolute age was also estimated using the difference in magnitude between the MSTO and the so-called main sequence knee, a well defined bending occurring in the lower main sequence. This feature was originally detected in the near-infrared (NIR) bands and explained as a consequence of an opacity mechanism (collisionally induced absorption of molecular hydrogen) in the atmosphere of cool low-mass stars (Bono et al. 2010). The same feature was also detected in the r',u'-g' and in the r',g'-r' CMD, thus supporting previous theoretical predictions by Borysow et al. (1997). The key advantage in using the Delta^(Knee)_(TO) as an age diagnostic is that it is independent of uncertainties affecting the distance, the reddening and the photometric zero-point. We found an absolute age of 12+/-1 Gyr that agrees, within the errors, with similar age estimates, but the uncertainty is on average a factor of two smaller. We also found that the Delta^(Knee)_(TO) is more sensitive to the metallicity than the MSTO, but the dependence becomes vanishing using the difference in color between the MSK and the MSTO.

astro-ph.SR↗

arXiv2014

Discovery of a wide planetary-mass companion to the young M3 star GU Psc

We present the discovery of a co-moving planetary-mass companion ~42" (~2000 AU) from a young M3 star, GU Psc, likely member of the young AB Doradus Moving Group (ABDMG). The companion was first identified via its distinctively red i - z color (> 3.5) through a survey made with Gemini-S/GMOS. Follow-up Canada-France-Hawaii Telescope/WIRCam near-infrared (NIR) imaging, Gemini-N/GNIRS NIR spectroscopy and Wide-field Infrared Survey Explorer photometry indicate a spectral type of T3.5+-1 and reveal signs of low gravity which we attribute to youth. Keck/Adaptive Optics NIR observations did not resolve the companion as a binary. A comparison with atmosphere models indicates Teff = 1000-1100 K and logg = 4.5-5.0. Based on evolution models, this temperature corresponds to a mass of 9-13 MJup for the age of ABDMG (70-130 Myr). The relatively well-constrained age of this companion and its very large angular separation to its host star will allow its thorough characterization and will make it a valuable comparison for planetary-mass companions that will be uncovered by forthcoming planet-finder instruments such as Gemini Planet Imager and SPHERE.

astro-ph.EP↗

arXiv2013

Physical parameters of young M-type stars and brown dwarfs with VOSA

Although M dwarfs are the most common stars in our stellar neighborhood they are still among the least understood. This class of objects is dominated by dramatic changes: in their interiors (fully convective, with implications in angular momentum evolution), in their atmospheres (crossing temperatures where dust settling occurs), and in their nature (including both, stellar and substellar objects). Populating efficiently our solar neighborhood, they are very well represented in the databases coming from new and more sensitive surveys that provide photometry at many wavelength ranges and cover large areas of the sky (few examples among many others are GALEX, SDSS, 2MASS, WISE and VISTA). In this context of opulence of objects and data, the Virtual Observatory comes in naturally as an excellent framework to efficiently advance in the knowledge of M-type sources. We put special emphasis in the benefits of using the new capabilities of VOSA (Virtual Observatory SED Analyzer, Bayo et al. 2008; in operation since 2008 and in constant development) to study large samples of candidate and confirmed M members of Chamaeleon I.

astro-ph.SR↗

arXiv2013

Weather on the Nearest Brown Dwarfs: Resolved Simultaneous Multi-Wavelength Variability Monitoring of WISE J104915.57-531906.1AB

We present two epochs of MPG/ESO 2.2m GROND simultaneous 6-band ($r'i'z'JHK$) photometric monitoring of the closest known L/T transition brown dwarf binary WISE J104915.57-531906.1AB. We report here the first resolved variability monitoring of both the T0.5 and L7.5 components. We obtained 4 hours of focused observations on the night of UT 2013-04-22, as well as 4 hours of defocused (unresolved) observations on the night of UT 2013-04-16. We note a number of robust trends in our light curves. The $r'$ and $i'$ light curves appear to be anticorrelated with $z'$ and $H$ for the T0.5 component and in the unresolved lightcurve. In the defocused dataset, $J$ appears correlated with $z'$ and $H$ and anticorrelated with $r'$ and $i'$, while in the focused dataset we measure no variability for $J$ at the level of our photometric precision, likely due to evolving weather phenomena. In our focused T0.5 component lightcurve, the $K$ band lightcurve displays a significant phase offset relative to both $H$ and $z'$. We argue that the measured phase offsets are correlated with atmospheric pressure probed at each band, as estimated from 1D atmospheric models. We also report low-amplitude variability in $i'$ and $z'$ intrinsic to the L7.5 component.

astro-ph.SR↗

arXiv2012

Atmospheres from very low-mass stars to extrasolar planets

Within the next few years, several instruments aiming at imaging extrasolar planets will see first light. In parallel, low mass planets are being searched around red dwarfs which offer more favorable conditions, both for radial velocity detection and transit studies, than solar-type stars. We review recent advancements in modeling the stellar to substellar transition. The revised solar oxygen abundances and cloud models allow to reproduce the photometric and spectroscopic properties of this transition to a degree never achieved before, but problems remain in the important M-L transition characteristic of the effective temperature range of characterizable exoplanets.

astro-ph.SR↗

arXiv2011

Models of Stars, Brown Dwarfs and Exoplanets

Within the next few years, GAIA and several instruments aiming at imag- ing extrasolar planets will see first light. In parallel, low mass planets are being searched around red dwarfs which offer more favourable conditions, both for radial velocity de- tection and transit studies, than solar-type stars. Authors of the model atmosphere code which has allowed the detection of water vapour in the atmosphere of Hot Jupiters re- view recent advancement in modelling the stellar to substellar transition. The revised solar oxygen abundances and cloud model allow for the first time to reproduce the pho- tometric and spectroscopic properties of this transition. Also presented are highlight results of a model atmosphere grid for stars, brown dwarfs and extrasolar planets.

astro-ph.SR↗

arXiv2010

The discovery of a very cool, very nearby brown dwarf in the Galactic plane

We report the discovery of a very cool, isolated brown dwarf, UGPS 0722-05, with the UKIDSS Galactic Plane Survey. The near-infrared spectrum displays deeper H2O and CH4 troughs than the coolest known T dwarfs and an unidentified absorption feature at 1.275 um. We provisionally classify the object as a T10 dwarf but note that it may in future come to be regarded as the first example of a new spectral type. The distance is measured by trigonometric parallax as d=4.1{-0.5}{+0.6} pc, making it the closest known isolated brown dwarf. With the aid of Spitzer/IRAC we measure H-[4.5] = 4.71. It is the coolest brown dwarf presently known -- the only known T dwarf that is redder in H-[4.5] is the peculiar T7.5 dwarf SDSS J1416+13B, which is thought to be warmer and more luminous than UGPS 0722-05. Our measurement of the luminosity, aided by Gemini/T-ReCS N band photometry, is L = 9.2 +/- 3.1x10^{-7} Lsun. Using a comparison with well studied T8.5 and T9 dwarfs we deduce Teff=520 +/- 40 K. This is supported by predictions of the Saumon & Marley models. With apparent magnitude J=16.52, UGPS 0722-05 is the brightest T dwarf discovered by UKIDSS so far. It offers opportunities for future study via high resolution near-infrared spectroscopy and spectroscopy in the thermal infrared.

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arXiv2010

Photometric characterization of exoplanets using angular and spectral differential imaging

The direct detection of exoplanets has been the subject of intensive research in the recent years. Data obtained with future high-contrast imaging instruments optimized for giant planets direct detection are strongly limited by the speckle noise. Specific observing strategies and data analysis methods, such as angular and spectral differential imaging, are required to attenuate the noise level and possibly detect the faint planet flux. Even though these methods are very efficient at suppressing the speckles, the photometry of the faint planets is dominated by the speckle residuals. The determination of the effective temperature and surface gravity of the detected planets from photometric measurements in different bands is then limited by the photometric error on the planet flux. In this work we investigate this photometric error and the consequences on the determination of the physical parameters of the detected planets. We perform detailed end-to-end simulation with the CAOS-based Software Package for SPHERE to obtain realistic data representing typical observing sequences in Y, J, H and Ks bands with a high contrast imager. The simulated data are used to measure the photometric accuracy as a function of contrast for planets detected with angular and spectral+angular differential methods. We apply this empirical accuracy to study the characterization capabilities of a high-contrast differential imager. We show that the expected photometric performances will allow the detection and characterization of exoplanets down to the Jupiter mass at angular separations of 1.0" and 0.2" respectively around high mass and low mass stars with 2 observations in different filter pairs. We also show that the determination of the planets physical parameters from photometric measurements in different filter pairs is essentialy limited by the error on the determination of the surface gravity.

astro-ph.IM↗

arXiv2010

The role of convection, overshoot, and gravity waves for the transport of dust in M dwarf and brown dwarf atmospheres

Observationally, spectra of brown dwarfs indicate the presence of dust in their atmospheres while theoretically it is not clear what prevents the dust from settling and disappearing from the regions of spectrum formation. Consequently, standard models have to rely on ad hoc assumptions about the mechanism that keeps dust grains aloft in the atmosphere. We apply hydrodynamical simulations to develop an improved physical understanding of the mixing properties of macroscopic flows in M dwarf and brown dwarf atmospheres, in particular of the influence of the underlying convection zone. We performed 2D radiation hydrodynamics simulations including a description of dust grain formation and transport with the CO5BOLD code. The simulations cover the very top of the convection zone and the photosphere including the dust layers for effective temperatures between 900K and 2800K, all with logg=5 assuming solar chemical composition. Convective overshoot occurs in the form of exponentially declining velocities with small scale heights, so that it affects only the region immediately above the almost adiabatic convective layers. From there on, mixing is provided by gravity waves that are strong enough to maintain thin dust clouds in the hotter models. With decreasing effective temperature, the amplitudes of the waves become smaller but the clouds become thicker and develop internal convective flows that are more efficient in mixing material than gravity waves. The presence of clouds leads to a highly structured appearance of the stellar surface on short temporal and small spatial scales. We identify convectively excited gravity waves as an essential mixing process in M dwarf and brown dwarf atmospheres. Under conditions of strong cloud formation, dust convection is the dominant self-sustaining mixing component.

astro-ph.SR↗