148 research outputs found
Dynamical evolution of the Gliese 436 planetary system - Kozai migration as a potential source for Gliese 436b's eccentricity
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.Comment: 13 pages To appear in Astronomy \& Astrophysic
Unbiased mm-wave Line Surveys of TW Hya and V4046 Sgr: The Enhanced C2H and CN Abundances of Evolved Protoplanetary Disks
We have conducted the first comprehensive mm-wave molecular emission line
surveys of the evolved circumstellar disks orbiting the nearby T Tauri stars TW
Hya and V4046 Sgr AB. Both disks are known to retain significant residual
gaseous components, despite the advanced ages of their host stars. Our unbiased
broad-band radio spectral surveys of the TW Hya and V4046 Sgr disks were
performed with the Atacama Pathfinder Experiment (APEX) 12 meter telescope and
are intended to yield a complete census of bright molecular emission lines in
the range 275-357 GHz (1.1-0.85 mm). We find that lines of 12CO, 13CO, HCN, CN,
and C2H, all of which lie in the higher-frequency range, constitute the
strongest molecular emission from both disks in the spectral region surveyed.
The molecule C2H is detected here for the first time in both disks, as is CS in
the TW Hya disk. The survey results also include the first measurements of the
full suite of hyperfine transitions of CN N=3-2 and C2H N=4-3 in both disks.
Modeling of these CN and C2H hyperfine complexes in the spectrum of TW Hya
indicates that the emission from both species is optically thick and may
originate from very cold disk regions. It furthermore appears that the
fractional abundances of CN and C2H are significantly enhanced in these evolved
protoplanetary disks relative to the fractional abundances of the same
molecules in the environments of deeply embedded protostars.Comment: 29 pages, 6 figures; to appear in Vol. 791 of The Astrophysical
Journa
An Unbiased 1.3 mm Emission Line Survey of the Protoplanetary Disk Orbiting LkCa 15
The outer (>30 AU) regions of the dusty circumstellar disk orbiting the ~2-5
Myr-old, actively accreting solar analog LkCa 15 are known to be chemically
rich, and the inner disk may host a young protoplanet within its central
cavity. To obtain a complete census of the brightest molecular line emission
emanating from the LkCa 15 disk over the 210-270 GHz (1.4 - 1.1 mm) range, we
have conducted an unbiased radio spectroscopic survey with the Institute de
Radioastronomie Millimetrique (IRAM) 30 meter telescope. The survey
demonstrates that, in this spectral region, the most readily detectable lines
are those of CO and its isotopologues 13CO and C18O, as well as HCO+, HCN, CN,
C2H, CS, and H2CO. All of these species had been previously detected in the
LkCa 15 disk; however, the present survey includes the first complete coverage
of the CN (2-1) and C2H (3-2) hyperfine complexes. Modeling of these emission
complexes indicates that the CN and C2H either reside in the coldest regions of
the disk or are subthermally excited, and that their abundances are enhanced
relative to molecular clouds and young stellar object environments. These
results highlight the value of unbiased single-dish line surveys in guiding
future high resolution interferometric imaging of disks.Comment: 35 pages, 9 figures, accepted for publication in The Astrophysical
Journa
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).Comment: Astronomy & Astrophysics, in press. 19 figures. 2 table
First results from the Canada-France High-z Quasar Survey: Constraints on the z=6 quasar luminosity function and the quasar contribution to reionization
We present preliminary results of a new quasar survey being undertaken with
multi-colour optical imaging from the Canada-France-Hawaii Telescope. The
current data consists of 3.83 sq. deg. of imaging in the i' and z' filters to a
10 sigma limit of z'<23.35. Near-infrared photometry of 24 candidate 5.7<z<6.4
quasars confirms them all to be low mass stars including two T dwarfs and four
or five L dwarfs. Photometric estimates of the spectral type of the two T
dwarfs are T3 and T6. We use the lack of high-redshift quasars in this survey
volume to constrain the z=6 quasar luminosity function. For reasonable values
of the break absolute magnitude M*_1450 and faint-end slope alpha, we determine
that the bright-end slope beta>-3.2 at 95% confidence. We find that the
comoving space-density of quasars brighter than M_1450=-23.5 declines by a
factor >25 from z=2 to z=6, mirroring the decline observed for high-luminosity
quasars. We consider the contribution of the quasar population to the ionizing
photon density at z=6 and the implications for reionization. We show that the
current constraints on the quasar population give an ionizing photon density
<<30% that of the star-forming galaxy population. We conclude that active
galactic nuclei make a negligible contribution to the reionization of hydrogen
at z~6.Comment: 9 pages, 6 figures, ApJ, in pres
Probing the impact of stellar duplicity on the frequency of giant planets: Final results of our VLT/NACO survey
If it is commonly agreed that the presence of a (moderately) close stellar companion affects the formation and the dynamical evolution of giant planets, the frequency of giant planets residing in binary systems separated by less than 100 AU is unknown. To address this issue, we have conducted with VLT/NACO a systematic adaptive optics search for moderately close stellar companions to 130 nearby solar-type stars. According to the data from Doppler surveys, half of our targets host at least one planetary companion, while the other half show no evidence for short-period giant planets. We present here the final results of our survey, which include a new series of second-epoch measurements to test for common proper motion. The new observations confirm the physical association of two companion candidates and prove the unbound status of many others. These results strengthen our former conclusion that circumstellar giant planets are slightly less frequent in binaries with mean semimajor axes between 35 and 100 AU than in wider systems or around single star
LP 714-37: A wide pair of ultracool dwarfs actually is a triple
LP 714-37 was identified by Phan-Bao et al. (2005) as one of the very few
wide pairs of very low mass (VLM) stars known to date, with a separation of 33
AU. Here we present adaptive optics imaging which resolves the secondary of the
wide pair into a tighter binary, with a projected angular separation of 0.36
arcsec, or 7 AU. The estimated spectral types of LP 714-37B and LP 714-37C are
M8.0 and M8.5. We discuss the implications of this finding for brown dwarf
formation scenarios.Comment: Accepted by ApJ Letter
VITRUV - Science Cases
VITRUV is a second generation spectro-imager for the PRIMA enabled Very Large
Telescope Interferometer. By combining simultaneously up to 8 telescopes VITRUV
makes the VLTI up to 6 times more efficient. This operational gain allows two
novel scientific methodologies: 1) massive surveys of sizes; 2) routine
interferometric imaging. The science cases presented concentrate on the
qualitatively new routine interferometric imaging methodology. The science
cases are not exhaustive but complementary to the PRIMA reference mission. The
focus is on: a) the close environment of young stars probing for the initial
conditions of planet formation and disk evolution; b) the surfaces of stars
tackling dynamos, activity, pulsation, mass-loss and evolution; c) revealing
the origin of the extraordinary morphologies of Planetary Nebulae and related
stars; d) studying the accretion-ejection structures of stellar black-holes
(microquasars) in our galaxy; e) unveiling the different interacting components
(torus, jets, BLRs) of Active Galactic Nuclei; and f) probing the environment
of nearby supermassive black-holes and relativistic effects in the Galactic
Center black-hole.Comment: 15 pages. The Power of Optical/IR Interferometry: Recent Scientific
Results and 2nd Generation VLTI Instrumentation, Allemagne (2005) in pres
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