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Brice-Olivier Demory,
Guillermo Torres,
Vasco Neves,
Leslie Rogers,
Michael Gillon,
Elliott Horch,
Peter Sullivan,
Xavier Bonfils, Xavier Delfosse,
Thierry Forveille,
Christophe Lovis,
Michel Mayor,
Nuno Santos,
Sara Seager,
Barry Smalley,
Stephane Udry
[show abstract]
[hide abstract]
ABSTRACT: 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.
01/2013;
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David Ehrenreich,
Vincent Bourrier,
Xavier Bonfils,
Alain Lecavelier des Etangs,
Guillaume Hébrard,
David K. Sing,
Peter J. Wheatley,
Alfred Vidal-Madjar, Xavier Delfosse,
Stéphane Udry,
Thierry Forveille,
Claire Moutou
[show abstract]
[hide abstract]
ABSTRACT: The naked-eye star 55 Cancri hosts a planetary system with five known
planets, including a hot super-Earth (55 Cnc e) extremely close to its star and
a farther out giant planet (55 Cnc b), found in milder irradiation conditions
with respect to other known hot Jupiters. This system raises important
questions on the evolution of atmospheres for close-in exoplanets, and the
dependence with planetary mass and irradiation. These questions can be
addressed by Lyman-alpha transit observations of the extended hydrogen
planetary atmospheres, complemented by contemporaneous measurements of the
stellar X-ray flux. In fact, planet `e' has been detected in transit,
suggesting the system is seen nearly edge-on. Yet, planet `b' has not been
observed in transit so far. Here, we report on Hubble Space Telescope STIS
Lyman-alpha and Chandra ACIS-S X-ray observations of 55 Cnc. These simultaneous
observations cover two transits of 55 Cnc e and two inferior conjunctions of 55
Cnc b. They reveal the star as a bright Lyman-alpha target and a variable X-ray
source. While no significant signal is detected during the transits of 55 Cnc
e, we detect a surprising Lyman-alpha absorption of 7.5 +/- 1.8% (4.2 sigma) at
inferior conjunctions of 55 Cnc b. The absorption is only detected over the
range of Doppler velocities where the stellar radiation repels hydrogen atoms
towards the observer. We calculate a false-alarm probability of 4.4%, which
takes into account the a-priori unknown transit parameters. This result
suggests the possibility that 55 Cnc b has an extended upper H I atmosphere,
which undergoes partial transits when the planet grazes the stellar disc. If
confirmed, it would show that planets cooler than hot Jupiters can also have
extended atmospheres.
10/2012;
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[show abstract]
[hide abstract]
ABSTRACT: The close-in planet orbiting GJ 436 presents a puzzling orbital eccentricity
considering its very short orbital period. Given the age of the system, this
planet should have been tidally circularized a long time ago. Many attempts to
explain this were proposed in recent years, either involving abnormally weak
tides, or the perturbing action of a distant companion. We address here the
latter issue based on Kozai migration. We propose that GJ 436b was formerly
located further away from the star and that it underwent a migration induced by
a massive, inclined perturber via Kozai mechanism. In this context, the
perturbations by the companion trigger high amplitude variations to GJ 436b
that cause tides to act at periastron. Then the orbit tidally shrinks to reach
its present day location. We numerically integrate the 3-body system including
tides and General Relativity correction. We first show that starting from the
present-day location of GJ 436b inevitably leads to damping the Kozai
oscillations and to rapidly circularizing the planet. Conversely, starting from
5-10 times further away allows the onset of Kozai cycles. The tides act in peak
eccentricity phases and reduce the semi-major axis of the planet. The net
result is an evolution characterized by two phases: a first one with Kozai
cycles and a slowly shrinking semi-major axis, and a second one once the planet
gets out of the Kozai resonance characterized by a more rapid decrease. The
timescale of this process appears in most cases much longer than the standard
circularization time of the planet by a factor larger than 50. This model can
provide a solution to the eccentricity paradox of GJ 436b. Depending on the
various orbital configurations, it can take several Gyrs to GJ 436b to achieve
a full orbital decrease and circularization. According to this scenario, we
could be witnessing today the second phase of the scenario where the semi-major
axis is already reduced while the eccentricity is still significant. We then
explore the parameter space and derive in which conditions this model can be
realistic given the age of the system. This yields constraints on the
characteristics of the putative companion.
08/2012;
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Claire Moutou,
Rodrigo F. Diaz,
Stephane Udry,
Guillaume Hebrard,
Francois Bouchy,
Alexandre Santerne,
David Ehrenreich,
Luc Arnold,
Isabelle Boisse,
Xavier Bonfils, [......],
Patrick Martinez,
Francesco Pepe,
Christian Perrier,
Didier Queloz,
Nuno C. Santos,
Damien Segransan,
Dominique Toublanc,
Jean-Pierre Troncin,
Michael Vanhuysse,
Alfred Vidal-Madjar
[show abstract]
[hide abstract]
ABSTRACT: 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.
05/2011;
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[show abstract]
[hide abstract]
ABSTRACT: The Canada-France Brown Dwarfs Survey is an i'- and z'-band survey realized with MegaCam at the Canada-France-Hawaii Telescope that covers a surface area of 780 deg2. Image analysis is now completed while J-band follow-up campaigns are ~90% done. The survey identified about 70 T dwarf candidates, of which 43 now have near-infrared spectra obtained with NIRI and GNIRS at Gemini and ISAAC at the Very Large Telescope. Six of these were previously published and we present here the 37 new discoveries, all T dwarfs. They range from T0 to T8.5 with four being of type T7 or later. Both newly identified T8 dwarfs are possibly high log (g) massive brown dwarfs of thin disk age. One T4.5 dwarf shows signs of sub-metallicity. We present proper motions and near-infrared photometry, and discuss about the most peculiar/interesting objects in some details.
The Astronomical Journal 05/2011; 141(6):203. · 4.03 Impact Factor
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[show abstract]
[hide abstract]
ABSTRACT: Lyman-alpha (Lya) emission of neutral hydrogen (1215.67 Angstr\"om) is the
main contributor to the ultraviolet flux of low-mass stars such as M dwarfs. It
is also the main light source used in studies of the evaporating upper
atmospheres of transiting extrasolar planets with ultraviolet transmission
spectroscopy. However, there are very few observations of the Lya emissions of
quiet M dwarfs, and none exist for those hosting exoplanets. Here, we present
Lya observations of the hot-neptune host star GJ436 with the Hubble Space
Telescope Imaging Spectrograph (HST/STIS). We detect bright emission in the
first resolved and high quality spectrum of a quiet M dwarf at Lya. Using an
energy diagram for exoplanets and an N-body particle simulation, this detection
enables the possible exospheric signature of the hot neptune to be estimated as
a ~11% absorption in the Lya stellar emission, for a typical mass-loss rate of
10^10 g/s. The atmosphere of the planet GJ436b is found to be stable to
evaporation, and should be readily observable with HST. We also derive a
correlation between X-ray and Lya emissions for M dwarfs. This correlation will
be useful for predicting the evaporation signatures of planets transiting other
quiet M dwarfs.
02/2011;
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[show abstract]
[hide abstract]
ABSTRACT: We have identified CFBDS J1458+10 as a 0.11" (2.6 AU) physical binary using
Keck laser guide star adaptive optics imaging and have measured a distance of
23.1+/-2.4 pc to the system based on near-IR parallax data from CFHT. The
integrated-light near-IR spectrum indicates a spectral type of T9.5, and model
atmospheres suggest a slightly higher temperature and surface gravity than the
T10 dwarf UGPS J0722-05. Thus, CFBDS J1458+10AB is the coolest brown dwarf
binary found to date. Its secondary component has an absolute H-band magnitude
that is 1.9+/-0.3 mag fainter than UGPS J0722-05, giving an inferred spectral
type of >T10. The secondary's bolometric luminosity of ~2 x 10^{-7} L_sun makes
it the least luminous known brown dwarf by a factor of 4-5. By comparing to
evolutionary models and T9-T10 objects, we estimate a temperature of 370+/-40 K
and a mass of 6-15 Mjup for CFBDS J1458+10B. At such extremes, atmospheric
models predict the onset of novel photospheric processes, namely the appearance
of water clouds and the removal of strong alkali lines, but their impact on the
emergent spectrum is highly uncertain. Our photometry shows that strong CH4
absorption persists at H-band; the J-K color is bluer than the latest known T
dwarfs but not as blue as predicted by current models; and the J-H color
delineates a possible inflection in the blueward trend for the latest T dwarfs.
Given its low luminosity, atypical colors and cold temperature, CFBDS J1458+10B
is a promising candidate for the hypothesized Y spectral class. However,
regardless of its ultimate classification, CFBDS J1458+10AB provides a new
benchmark for measuring the properties of brown dwarfs and gas-giant planets,
testing substellar models, and constraining the low-mass limit for star
formation.
02/2011;
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Thierry Forveille,
Xavier Bonfils,
Gaspare Lo Curto, Xavier Delfosse,
Stephane Udry,
Francois Bouchy,
Christophe Lovis,
Michel Mayor,
Claire Moutou,
Dominique Naef,
Francesco Pepe,
Christian Perrier,
Didier Queloz,
Nuno Santos
[show abstract]
[hide abstract]
ABSTRACT: 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. Comment: Corrected an error in the labelling of one line in Table 2
12/2010;
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David Ehrenreich,
Anne-Marie Lagrange,
François Bouchy,
Christian Perrier,
Guillaume Hébrard,
Isabelle Boisse,
Xavier Bonfils,
Luc Arnold, Xavier Delfosse,
Morgan Desort, [......],
Thierry Forveille,
Christophe Lovis,
Claire Moutou,
Francesco Pepe,
Frédéric Pont,
Nuno C. Santos,
Alexandre Santerne,
Damien Ségransan,
Stéphane Udry,
Alfred Vidal-Madjar
[show abstract]
[hide abstract]
ABSTRACT: The Kepler mission has detected transits and occultations of a hot compact object around an early-type star, the Kepler Object of Interest KOI 74. The mass of this transiting object was photometrically assessed in a previous study using the presence of the relativistic beaming effect (so-called `Doppler boosting') in the light curve. Our aim was to provide a spectroscopic validation of this pioneering approach. We measured the radial velocity variations of the A1V star KOI 74 with the SOPHIE spectrograph at the 1.93-m telescope of the Observatoire de Haute-Provence (France). Radial velocity measurements of this star are challenging because of the high level of stellar pulsations and the few available spectral lines. Using a technique dedicated to early-type main-sequence stars, we measured radial velocity variations compatible with a companion of mass 0.252+/-0.025 Msun, in good agreement with the value derived from the Kepler light curve. This work strengthens the scenario suggesting that KOI 74 is a blue straggler orbited by a stellar core despoiled of its envelope, the low-mass white dwarf KOI 74b. Comment: 8 pages, 6 figures, 3 tables. Accepted in Astronomy and Astrophysics. Radii of KOI 74 and KOI 81 updated in Fig. 6 (thanks to J. Carter for pointing that out)
09/2010;
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Dominique Naef,
Michel Mayor,
Gaspare Lo Curto,
Francois Bouchy,
Christophe Lovis,
Claire Moutou,
Willy Benz,
Francesco Pepe,
Didier Queloz,
Nuno C. Santos,
Damien Segransan,
Stephane Udry,
Xavier Bonfils, Xavier Delfosse,
Thierry Forveille,
Guillaume Hebrard,
Christoph Mordasini,
Christian Perrier,
Isabelle Boisse,
Danuta Sosnowska
[show abstract]
[hide abstract]
ABSTRACT: In this paper, we present our HARPS radial-velocity data for eight low-activity solar-type stars belonging to the HARPS volume-limited sample: HD6718, HD8535, HD28254, HD290327, HD43197, HD44219, HD148156, and HD156411. Keplerian fits to these data reveal the presence of low-mass companions around these targets. With minimum masses ranging from 0.58 to 2.54 MJup, these companions are in the planetary mass domain. The orbital periods of these planets range from slightly less than one to almost seven years. The eight orbits presented in this paper exhibit a wide variety of eccentricities: from 0.08 to above 0.8. Comment: 8 pages, 2 figures, accepted for publication in A&A
08/2010;
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Alfred Vidal-Madjar,
Luc Arnold,
David Ehrenreich,
Roger Ferlet,
Alain Lecavelier des Etangs,
François Bouchy,
Damien Segransan,
Isabelle Boisse,
Guillaume Hébrard,
Claire Moutou, [......],
Rodrigo F. Díaz,
Anne Eggenberger,
Thierry Forveille,
Anne-Marie Lagrange,
Christophe Lovis,
Francesco Pepe,
Christian Perrier,
Frédéric Pont,
Nuno C. Santos,
Stéphane Udry
[show abstract]
[hide abstract]
ABSTRACT: An important goal within the quest for detecting an Earth-like extrasolar planet, will be to identify atmospheric gaseous bio-signatures. Observations of the light transmitted through the Earth's atmosphere, as for an extrasolar planet, will be the first step for future comparisons. We have completed observations of the Earth during a Lunar eclipse, a unique situation similar to that of a transiting planet. We aim at showing what species could be detected in its atmosphere at optical wavelengths, where a lot of photons are available in the masked stellar light. We present observations of the 2008 August 16 Moon eclipse performed with the SOPHIE spectrograph at the Observatoire de Haute-Provence. Locating the spectrograph fibers in the penumbra of the eclipse, the Moon irradiance is then a mix of direct, unabsorbed Sun light and solar light that has passed through the Earth's limb. This mixture essentially reproduces what is recorded during the transit of an extrasolar planet. We report here the clear detection of several Earth atmospheric compounds in the transmission spectra, such as ozone, molecular oxygen, and neutral sodium as well as molecular nitrogen and oxygen through the Rayleigh signature. Moreover, we present a method that allows us to derive the thickness of the atmosphere versus the wavelength for penumbra eclipse observations. We quantitatively evaluate the altitude at which the atmosphere becomes transparent for important species like molecular oxygen and ozone, two species thought to be tightly linked to the presence of life. The molecular detections presented here are an encouraging first attempt, necessary to better prepare for the future of extremely-large telescopes and transiting Earth-like planets. Instruments like SOPHIE will be mandatory when characterizing the atmospheres of transiting Earth-like planets from the ground and searching for bio-marker signatures. Comment: 15 pages, 14 figures, 2 tables. Accepted for publication in Astronomy and Astrophysics
06/2010;
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Chris J. Willott,
Philippe Delorme,
Celine Reyle,
Loic Albert,
Jacqueline Bergeron,
David Crampton, Xavier Delfosse,
Thierry Forveille,
John B. Hutchings,
Ross J. McLure,
Alain Omont,
David Schade
[show abstract]
[hide abstract]
ABSTRACT: We present discovery imaging and spectroscopy for nine new z ~ 6 quasars found in the Canada-France High-z Quasar Survey (CFHQS) bringing the total number of CFHQS quasars to 19. By combining the CFHQS with the more luminous SDSS sample we are able to derive the quasar luminosity function from a sample of 40 quasars at redshifts 5.74 < z < 6.42. Our binned luminosity function shows a slightly lower normalisation and flatter slope than found in previous work. The binned data also suggest a break in the luminosity function at M_1450 approx -25. A double power law maximum likelihood fit to the data is consistent with the binned results. The luminosity function is strongly constrained (1 sigma uncertainty < 0.1 dex) over the range -27.5 < M_1450 < -24.7. The best-fit parameters are Phi(M_1450^*) = 1.14 x 10^-8 Mpc^-3 mag^-1, break magnitude M_1450^* = -25.13 and bright end slope beta = -2.81. However the covariance between beta and M_1450^* prevents strong constraints being placed on either parameter. For a break magnitude in the range -26 < M_1450^* < -24 we find -3.8 < beta < -2.3 at 95% confidence. We calculate the z = 6 quasar intergalactic ionizing flux and show it is between 20 and 100 times lower than that necessary for reionization. Finally, we use the luminosity function to predict how many higher redshift quasars may be discovered in future near-IR imaging surveys. Comment: 15 pages, 9 figures, AJ in press
12/2009;
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David Charbonneau,
Zachory K Berta,
Jonathan Irwin,
Christopher J Burke,
Philip Nutzman,
Lars A Buchhave,
Christophe Lovis,
Xavier Bonfils,
David W Latham,
Stéphane Udry,
Ruth A Murray-Clay,
Matthew J Holman,
Emilio E Falco,
Joshua N Winn,
Didier Queloz,
Francesco Pepe,
Michel Mayor, Xavier Delfosse,
Thierry Forveille
[show abstract]
[hide abstract]
ABSTRACT: A decade ago, the detection of the first transiting extrasolar planet provided a direct constraint on its composition and opened the door to spectroscopic investigations of extrasolar planetary atmospheres. Because such characterization studies are feasible only for transiting systems that are both nearby and for which the planet-to-star radius ratio is relatively large, nearby small stars have been surveyed intensively. Doppler studies and microlensing have uncovered a population of planets with minimum masses of 1.9-10 times the Earth's mass (M[symbol:see text]), called super-Earths. The first constraint on the bulk composition of this novel class of planets was afforded by CoRoT-7b (refs 8, 9), but the distance and size of its star preclude atmospheric studies in the foreseeable future. Here we report observations of the transiting planet GJ 1214b, which has a mass of 6.55M[symbol:see text]), and a radius 2.68 times Earth's radius (R[symbol:see text]), indicating that it is intermediate in stature between Earth and the ice giants of the Solar System. We find that the planetary mass and radius are consistent with a composition of primarily water enshrouded by a hydrogen-helium envelope that is only 0.05% of the mass of the planet. The atmosphere is probably escaping hydrodynamically, indicating that it has undergone significant evolution during its history. The star is small and only 13 parsecs away, so the planetary atmosphere is amenable to study with current observatories.
Nature 12/2009; 462(7275):891-4. · 36.28 Impact Factor
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Chris J. Willott,
Philippe Delorme,
Céline Reylé,
Loic Albert,
Jacqueline Bergeron,
David Crampton, Xavier Delfosse,
Thierry Forveille,
John B. Hutchings,
Ross J. McLure,
Alain Omont,
and David Schade
[show abstract]
[hide abstract]
ABSTRACT: We present imaging and spectroscopic observations for six quasars at z ≥ 5.9 discovered by the Canada-France High-z Quasar Survey (CFHQS). The CFHQS contains subsurveys with a range of flux and area combinations to sample a wide range of quasar luminosities at z ~ 6. The new quasars have luminosities 10-75 times lower than the most luminous Sloan Digital Sky Survey quasars at this redshift. The least luminous quasar, CFHQS J0216–0455 at z = 6.01, has absolute magnitude M 1450 = –22.21, well below the likely break in the luminosity function. This quasar is not detected in a deep XMM-Newton survey showing that optical selection is still a very efficient tool for finding high-redshift quasars.
The Astronomical Journal 02/2009; 137(3):3541. · 4.03 Impact Factor
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[hide abstract]
ABSTRACT: Spectropolarimetry is the optimal tool to investigate large‐scale magnetic topologies of cool low‐mass stars. From phase resolved spectropolarimetric data sets, tomographic imaging can be used to obtain a spherical harmonics decomposition of magnetic fields at the surfaces of stars, and thus reveal, e.g., how strong and complex such fields are, to what degree they are axisymmetric and how they split between their poloidal and toroidal components.
AIP Conference Proceedings. 02/2009; 1094(1):130-139.
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[show abstract]
[hide abstract]
ABSTRACT: We present Keck HIRES spectra of six late-M dwarfs and 11 L dwarfs. Our goal is to assign effective temperatures to the objects using detailed atmospheric models and fine analysis of the alkali resonance absorption lines of Cs I and Rb I. These yield mutually consistent results (±50 K) when we use "cleared-dust" models, which account for the removal of refractory species from the molecular states but do not include dust opacities. We find a tendency for the Rb I line to imply a slightly higher temperature, which we ascribe to an incomplete treatment of the overlying molecular opacities. The final Teff we adopt are based on the Cs I fits alone, though the Rb I fits support the Cs I temperature sequence. This work, in combination with results from the infrared, hints that dust in these atmospheres has settled out of the high atmosphere but is present in the deep photosphere. We also derive radial and rotational velocities for all the objects, finding that the previously discovered trend of rapid rotation for very low mass objects is quite pervasive. To improve on our analysis, there is a clear need for better molecular line lists and a more detailed understanding of dust formation and dynamics.
The Astrophysical Journal 12/2008; 538(1):363. · 6.02 Impact Factor
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Philippe Delorme,
Chris J. Willott,
Thierry Forveille, Xavier Delfosse,
Céline Reylé,
Emmanuel Bertin,
Loic Albert,
Etienne Artigau,
Annie C. Robin,
France Allard,
Rene Doyon,
Gary J. Hill
[show abstract]
[hide abstract]
ABSTRACT: We present the first results of a wide field survey for cool brown dwarfs with the MegaCam camera on the CFHT telescope, the Canada-France Brown Dwarf Survey, hereafter CFBDS. Our objectives are to find ultracool brown dwarfs and to constrain the field-brown dwarf mass function thanks to a larger sample of L and T dwarfs. We identify candidates in CFHT/MegaCam i' and z' images using optimised psf-fitting within Source Extractor, and follow them up with pointed near-infrared imaging on several telescopes. We have so far analysed over 350 square degrees and found 770 brown dwarf candidates brighter than z'{AB}=22.5. We currently have J-band photometry for 220 of these candidates, which confirms 37% as potential L or T dwarfs. Some are among the reddest and farthest brown dwarfs currently known, including an independent identification of the recently published ULAS J003402.77-005206.7 and the discovery of a second brown dwarf later than T8, CFBDS J005910.83-011401.3. Infrared spectra of three T dwarf candidates confirm their nature, and validate the selection process. The completed survey will discover ~100 T dwarfs and ~500 L dwarfs or M dwarfs later than M8, approximately doubling the number of currently known brown dwarfs. The resulting sample will have a very well-defined selection function, and will therefore produce a very clean luminosity function. Comment: A&A in press
04/2008;
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Chris J. Willott,
Philippe Delorme,
Alain Omont,
Jacqueline Bergeron, Xavier Delfosse,
Thierry Forveille,
Loic Albert,
Céline Reylé,
Gary J. Hill,
Michael Gully-Santiago,
Phillip Vinten,
David Crampton,
John B. Hutchings,
David Schade,
Luc Simard,
Marcin Sawicki,
Alexandre Beelen,
and Pierre Cox
[show abstract]
[hide abstract]
ABSTRACT: The Canada-France High-z Quasar Survey (CFHQS) is an optical survey designed to locate quasars during the epoch of reionization. In this paper we present the discovery of the first four CFHQS quasars at redshifts greater than 6, including the most distant known quasar, CFHQS J2329-0301 at z = 6.43. We describe the observational method used to identify the quasars and present optical, infrared, and millimeter photometry and optical and near-infrared spectroscopy. We investigate the dust properties of these quasars, finding an unusual dust extinction curve for one quasar and a high far-infrared luminosity due to dust emission for another. The mean millimeter continuum flux for CFHQS quasars is substantially lower than that for SDSS quasars at the same redshift, likely due to a correlation with quasar UV luminosity. For two quasars with sufficiently high signal-to-noise ratio optical spectra, we use the spectra to investigate the ionization state of hydrogen at z > 5. For CFHQS J1509-1749 at z = 6.12 we find significant evolution (beyond a simple extrapolation of lower redshift data) in the Gunn-Peterson optical depth at z > 5.4. The line of sight to this quasar has one of the highest known optical depths at z ≈ 5.8. An analysis of the sizes of the highly ionized near-zones in the spectra of two quasars at z = 6.12 and 6.43 suggest that the intergalactic medium surrounding these quasars was substantially ionized before these quasars turned on. Together, these observations point toward an extended reionization process, but we caution that cosmic variance is still a major limitation in z > 6 quasar observations.
The Astronomical Journal 12/2007; 134(6):2435. · 4.03 Impact Factor
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[show abstract]
[hide abstract]
ABSTRACT: We present spectra for 12 new ultracool dwarfs found in the DENIS infrared survey. Seven of them have spectral types at the bottom of the M-class (M8–M9.5), and the other five belong to the cooler "L" class. We also present spectra for the two new L dwarfs found by the EROS 2 proper-motion survey. We introduce a scheme for L dwarf classification that is based on an extension to cooler spectra of a pseudocontinuum ratio previously defined for M dwarfs. For calibrating the spectral subclasses, we use a temperature scale for late-M and L dwarfs recently obtained by Basri et al. from synthetic spectrum fitting of high-resolution profiles of Cs I and Rb I resonance lines. We define that the subclass range from L0 to L6 corresponds to the temperature range from 2200 K to 1600 K. Our subclasses L0, L1, and L2 agree with recent findings by Kirkpatrick et al., but then they diverge such that our L6 is equivalent to their L8. We find that late-M and L dwarf subclasses can be assigned either in the optical with the PC3 index or in the near-infrared with the H2O H-band index. We discuss the main photospheric features present in L dwarf spectra, in particular in the region 400–650 nm, which has never been shown before. The TiO bands at 549.7, 559.7, 615.9, and 638.4 nm fade with decreasing temperature, but do not vanish until well inside the L domain (~L5). The Na I 589.0, 589.6 nm resonance doublet in our latest object (L6) becomes the broadest atomic feature ever seen in any cool dwarf. We do not detect Hα emission in our L dwarfs later than L3. We discuss the ages and masses of our objects using their temperatures and absence or presence of lithium. Finally, we compare two L1 dwarfs with different gravities (one with lithium and one without it) and discuss differences in spectral features.
The Astronomical Journal 12/2007; 118(5):2466. · 4.03 Impact Factor
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ABSTRACT: We address the issue of the dynamical evolution of the Gliese 581 planetary system. It is crucial when considering the planets' habitability because the secular evolution of the orbits may regulate their climate, even in the case where the system is stable. We have numerically integrated the planetary system over 10^8 yrs, starting from the present fitted solution. In all cases, the system appears dynamically stable, even in close to pole-on configurations. Only a limited range of inclinations can be excluded. The climate on the planets is expected to be secularly stable, thus not precluding the development of life. Gl 581 remains the best candidate for a planetary system with planets that potentially bear primitive forms of life. Comment: 7 pages. Astronomy & Astrophysics (2007) accepted
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