57 research outputs found
The Implications of M Dwarf Flares on the Detection and Characterization of Exoplanets at Infrared Wavelengths
We present the results of an observational campaign which obtained high time
cadence, high precision, simultaneous optical and IR photometric observations
of three M dwarf flare stars for 47 hours. The campaign was designed to
characterize the behavior of energetic flare events, which routinely occur on M
dwarfs, at IR wavelengths to milli-magnitude precision, and quantify to what
extent such events might influence current and future efforts to detect and
characterize extrasolar planets surrounding these stars. We detected and
characterized four highly energetic optical flares having U-band total energies
of ~7.8x10^30 to ~1.3x10^32 ergs, and found no corresponding response in the J,
H, or Ks bandpasses at the precision of our data. For active dM3e stars, we
find that a ~1.3x10^32 erg U-band flare (delta Umax ~1.5 mag) will induce <8.3
(J), <8.5 (H), and <11.7 (Ks) milli-mags of a response. A flare of this energy
or greater should occur less than once per 18 hours. For active dM4.5e stars,
we find that a ~5.1x10^31 erg U-band flare (delta Umax ~1.6 mag) will induce
<7.8 (J), <8.8 (H), and <5.1 (Ks) milli-mags of a response. A flare of this
energy or greater should occur less than once per 10 hours. No evidence of
stellar variability not associated with discrete flare events was observed at
the level of ~3.9 milli-mags over 1 hour time-scales and at the level of ~5.6
milli-mags over 7.5 hour time-scales. We therefore demonstrate that most M
dwarf stellar activity and flares will not influence IR detection and
characterization studies of M dwarf exoplanets above the level of ~5-11
milli-mags, depending on the filter and spectral type. We speculate that the
most energetic megaflares on M dwarfs, which occur at rates of once per month,
are likely to be easily detected in IR observations with sensitivity of tens of
milli-mags.Comment: Accepted in Astronomical Journal, 17 pages, 6 figure
Observational evidence for a metal rich atmosphere on the super-Earth GJ1214b
We report observations of two consecutive transits of the warm super-Earth
exoplanet GJ1214b at 3.6 and 4.5 microns with the Infrared Array Camera
instrument on-board the Spitzer Space Telescope. The two transit light curves
allow for the determination of the transit parameters for this system. We find
these paremeters to be consistent with the previously determined values and no
evidence for transit timing variations. The main investigation consists of
measuring the transit depths in each bandpass to constrain the planet's
transmission spectrum. Fixing the system scale and impact parameters, we
measure R_p/R_star=0.1176 (+0.0008/-0.0009) and 0.1163 (+0.0010/-0.0008) at 3.6
and 4.5 microns, respectively. Combining these data with the previously
reported MEarth Observatory measurements in the red optical yields constraints
on the GJ1214b's transmission spectrum and allows us to rule-out a cloud-free,
solar composition (i.e., hydrogen-dominated) atmosphere at 4.5 sigma
confidence. This independently confirms a recent finding that was based on a
measurement of the planet's transmission spectrum using the VLT. The Spitzer,
MEarth, and VLT observations together yield a remarkably flat transmission
spectrum over the large wavelength domain spanned by the data. Consequently,
cloud-free atmospheric models require more than 30% metals (assumed to be in
the form of H2O by volume to be consistent with all the observations.Comment: Accepted for publication in ApJL. 13 pages, 3 figures, 1 tabl
A deep optical/near-infrared catalog of Serpens
We present a deep optical/near-infrared imaging survey of the Serpens
molecular cloud. This survey constitutes the complementary optical data to the
Spitzer "Core To Disk" (c2d) Legacy survey in this cloud. The survey was
conducted using the Wide Field Camera at the Isaac Newton Telescope. About 0.96
square degrees were imaged in the R and Z filters, covering the entire region
where most of the young stellar objects identified by the c2d survey are
located. 26524 point-like sources were detected in both R and Z bands down to
R=24.5 mag and Z=23 mag with a signal-to-noise ratio better than 3. The 95%
completeness limit of our catalog corresponds to 0.04 solar masses for members
of the Serpens star forming region (age 2 Myr and distance 260 pc) in the
absence of extinction. Adopting the typical extinction of the observed area
(Av=7 mag), we estimate a 95% completeness level down to 0.1 solar masses. The
astrometric accuracy of our catalog is 0.4 arcsec with respect to the 2MASS
catalog. Our final catalog contains J2000 celestial coordinates, magnitudes in
the R and Z bands calibrated to the SDSS photometric system and, where
possible, JHK magnitudes from 2MASS for sources in 0.96 square degrees in the
direction of Serpens. This data product has been already used within the frame
of the c2d Spitzer Legacy Project analysis in Serpens to study the star/disk
formation and evolution in this cloud; here we use it to obtain new indications
of the disk-less population in Serpens.Comment: 7 page, 5 figure
Theoretical Transit Spectra for GJ 1214b and Other "Super-Earths"
We present new calculations of transit spectra of super-Earths that allow for
atmospheres with arbitrary proportions of common molecular species and haze. We
test this method with generic spectra, reproducing the expected systematics and
absorption features, then apply it to the nearby super-Earth GJ 1214b, which
has produced conflicting observational data, leaving the questions of a
hydrogen-rich versus hydrogen-poor atmosphere and the water content of the
atmosphere ambiguous. We present representative transit spectra for a range of
classes of atmosphere models for GJ 1214b. Our analysis supports a
hydrogen-rich atmosphere with a cloud or haze layer, although a hydrogen-poor
model with less than 10% water is not ruled out. Several classes of models are
ruled out, however, including hydrogen-rich atmospheres with no haze,
hydrogen-rich atmospheres with a haze of about 0.01-micron tholin particles,
and hydrogen-poor atmospheres with major sources of absorption other than
water. We propose an observational test to distinguish hydrogen-rich from
hydrogen-poor atmospheres. Finally, we provide a library of theoretical transit
spectra for super-Earths with a broad range of parameters to facilitate future
comparison with anticipated data.Comment: 33 pages, 21 figures, 3 table
The Atmospheric Chemistry of GJ 1214b: Photochemistry and Clouds
Recent observations of the transiting super-Earth GJ 1214b reveal that its
atmosphere may be hydrogen-rich or water-rich in nature, with clouds or hazes
potentially affecting its transmission spectrum in the optical and
very-near-IR. Here we further examine the possibility that GJ 1214b does indeed
possess a hydrogen-dominated atmosphere, which is the hypothesis that is
favored by models of the bulk composition of the planet. We study the effects
of non-equilibrium chemistry (photochemistry, thermal chemistry, and mixing) on
the planet's transmission spectrum. We furthermore examine the possibility that
clouds could play a significant role in attenuating GJ 1214b's transmission
spectrum at short wavelengths. We find that non-equilibrium chemistry can have
a large effect on the overall chemical composition of GJ 1214b's atmosphere,
however these changes mostly take place above the height in the atmosphere that
is probed by transmission spectroscopy. The effects of non-equilibrium
chemistry on GJ 1214b's transmission spectrum are therefore minimal, with the
largest effects taking place if the planet's atmosphere has super-solar
metallicity and a low rate of vertical mixing. Interestingly, we find that the
best fit to the observations of GJ 1214b's atmosphere in transmission occur if
the planet's atmosphere is deficient in CH4, and possesses a cloud layer at a
pressure of ~200 mbar. This is consistent with a picture of efficient methane
photolysis, accompanied by formation of organic haze that obscures the lower
atmosphere of GJ 1214b at optical wavelengths. However, for methane to be
absent from GJ 1214b's transmission spectrum, UV photolysis of this molecule
must be efficient at pressures of greater than ~1 mbar, whereas we find that
methane only photolyzes to pressures less than 0.1 mbar, even under the most
optimistic assumptions. (Abridged)Comment: Accepted to ApJ; 32 pages, 8 figures, 1 tabl
The Hunt for Exomoons with Kepler (HEK): I. Description of a New Observational Project
Two decades ago, empirical evidence concerning the existence and frequency of
planets around stars, other than our own, was absent. Since this time, the
detection of extrasolar planets from Jupiter-sized to most recently Earth-sized
worlds has blossomed and we are finally able to shed light on the plurality of
Earth-like, habitable planets in the cosmos. Extrasolar moons may also be
frequent habitable worlds but their detection or even systematic pursuit
remains lacking in the current literature. Here, we present a description of
the first systematic search for extrasolar moons as part of a new observational
project called "The Hunt for Exomoons with Kepler" (HEK). The HEK project
distills the entire list of known transiting planet candidates found by Kepler
(2326 at the time of writing) down to the most promising candidates for hosting
a moon. Selected targets are fitted using a multimodal nested sampling
algorithm coupled with a planet-with-moon light curve modelling routine. By
comparing the Bayesian evidence of a planet-only model to that of a
planet-with-moon, the detection process is handled in a Bayesian framework. In
the case of null detections, upper limits derived from posteriors marginalised
over the entire prior volume will be provided to inform the frequency of large
moons around viable planetary hosts, eta-moon. After discussing our
methodologies for target selection, modelling, fitting and vetting, we provide
two example analyses.Comment: 21 pages, 8 figures, 4 tables, accepted in Ap
Transit Detection in the MEarth Survey of Nearby M Dwarfs: Bridging the Clean-First, Search-Later Divide
In the effort to characterize the masses, radii, and atmospheres of
potentially habitable exoplanets, there is an urgent need to find examples of
such planets transiting nearby M dwarfs. The MEarth Project is an ongoing
effort to do so, as a ground-based photometric survey designed to detect
exoplanets as small as 2 Earth radii transiting mid-to-late M dwarfs within 33
pc of the Sun. Unfortunately, identifying transits of such planets in
photometric monitoring is complicated both by the intrinsic stellar variability
that is common among these stars and by the nocturnal cadence, atmospheric
variations, and instrumental systematics that often plague Earth-bound
observatories. Here we summarize the properties of MEarth data gathered so far,
and we present a new framework to detect shallow exoplanet transits in wiggly
and irregularly-spaced light curves. In contrast to previous methods that clean
trends from light curves before searching for transits, this framework assesses
the significance of individual transits simultaneously while modeling
variability, systematics, and the photometric quality of individual nights. Our
Method for Including Starspots and Systematics in the Marginalized Probability
of a Lone Eclipse (MISS MarPLE) uses a computationally efficient semi-Bayesian
approach to explore the vast probability space spanned by the many parameters
of this model, naturally incorporating the uncertainties in these parameters
into its evaluation of candidate events. We show how to combine individual
transits processed by MISS MarPLE into periodic transiting planet candidates
and compare our results to the popular Box-fitting Least Squares (BLS) method
with simulations. By applying MISS MarPLE to observations from the MEarth
Project, we demonstrate the utility of this framework for robustly assessing
the false alarm probability of transit signals in real data. [slightly
abridged]Comment: accepted to the Astronomical Journal, 21 pages, 12 figure
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