2,460 research outputs found
The Spitzer search for the transits of HARPS low-mass planets - II. Null results for 19 planets
Short-period super-Earths and Neptunes are now known to be very frequent
around solar-type stars. Improving our understanding of these mysterious
planets requires the detection of a significant sample of objects suitable for
detailed characterization. Searching for the transits of the low-mass planets
detected by Doppler surveys is a straightforward way to achieve this goal.
Indeed, Doppler surveys target the most nearby main-sequence stars, they
regularly detect close-in low-mass planets with significant transit
probability, and their radial velocity data constrain strongly the ephemeris of
possible transits. In this context, we initiated in 2010 an ambitious Spitzer
multi-Cycle transit search project that targeted 25 low-mass planets detected
by radial velocity, focusing mainly on the shortest-period planets detected by
the HARPS spectrograph. We report here null results for 19 targets of the
project. For 16 planets out of 19, a transiting configuration is strongly
disfavored or firmly rejected by our data for most planetary compositions. We
derive a posterior probability of 83% that none of the probed 19 planets
transits (for a prior probability of 22%), which still leaves a significant
probability of 17% that at least one of them does transit. Globally, our
Spitzer project revealed or confirmed transits for three of its 25 targeted
planets, and discarded or disfavored the transiting nature of 20 of them. Our
light curves demonstrate for Warm Spitzer excellent photometric precisions: for
14 targets out of 19, we were able to reach standard deviations that were
better than 50ppm per 30 min intervals. Combined with its Earth-trailing orbit,
which makes it capable of pointing any star in the sky and to monitor it
continuously for days, this work confirms Spitzer as an optimal instrument to
detect sub-mmag-deep transits on the bright nearby stars targeted by Doppler
surveys.Comment: Accepted for publication in Astronomy and Astrophysics. 23 pages, 21
figure
Accurate Spitzer infrared radius measurement for the hot Neptune GJ 436b
We present Spitzer Space Telescope infrared photometry of a primary transit
of the hot Neptune GJ 436b. The observations were obtained using the 8 microns
band of the InfraRed Array Camera (IRAC). The high accuracy of the transit data
and the weak limb-darkening in the 8 microns IRAC band allow us to derive
(assuming M = 0.44 +- 0.04 Msun for the primary) a precise value for the
planetary radius (4.19 +0.21-0.16 Rearth), the stellar radius (0.463
+0.022-0.017 Rsun), the orbital inclination (85.90 +0.19-0.18 degrees) and
transit timing (2454280.78186 +0.00015-0.00008 HJD). Assuming current planet
models, an internal structure similar to that of Neptune with a small H/He
envelope is necessary to account for the measured radius of GJ 436b.Comment: Accepted for publication in A&A on 21/07/2007; 5 pages, 3 figure
Application of firefly luciferase assay for adenosine triphosphate (ATP) to antimicrobial drug sensitivity testing
The development of a rapid method for determining microbial susceptibilities to antibiotics using the firefly luciferase assay for adenosine triphosphate (ATP) is documented. The reduction of bacterial ATP by an antimicrobial agent was determined to be a valid measure of drug effect in most cases. The effect of 12 antibiotics on 8 different bacterial species gave a 94 percent correlation with the standard Kirby-Buer-Agar disc diffusion method. A 93 percent correlation was obtained when the ATP assay method was applied directly to 50 urine specimens from patients with urinary tract infections. Urine samples were centrifuged first to that bacterial pellets could be suspended in broth. No primary isolation or subculturing was required. Mixed cultures in which one species was predominant gave accurate results for the most abundant organism. Since the method is based on an increase in bacterial ATP with time, the presence of leukocytes did not interfere with the interpretation of results. Both the incubation procedure and the ATP assays are compatible with automation
Discovery and Characterization of Transiting SuperEarths Using an All-Sky Transit Survey and Follow-up by the James Webb Space Telescope
Doppler and transit surveys are finding extrasolar planets of ever smaller
mass and radius, and are now sampling the domain of superEarths (1-3 Earth
radii). Recent results from the Doppler surveys suggest that discovery of a
transiting superEarth in the habitable zone of a lower main sequence star may
be possible. We evaluate the prospects for an all-sky transit survey targeted
to the brightest stars, that would find the most favorable cases for
photometric and spectroscopic characterization using the James Webb Space
Telescope (JWST). We use the proposed Transiting Exoplanet Survey Satellite
(TESS) as representative of an all-sky survey. We couple the simulated TESS
yield to a sensitivity model for the MIRI and NIRSpec instruments on JWST. We
focus on the TESS planets with radii between Earth and Neptune. Our simulations
consider secondary eclipse filter photometry using JWST/MIRI, comparing the 11-
and 15-micron bands to measure CO2 absorption in superEarths, as well as
JWST/NIRSpec spectroscopy of water absorption from 1.7-3.0 microns, and CO2
absorption at 4.3-microns. We project that TESS will discover about eight
nearby habitable transiting superEarths. The principal sources of uncertainty
in the prospects for JWST characterization of habitable superEarths are
superEarth frequency and the nature of superEarth atmospheres. Based on our
estimates of these uncertainties, we project that JWST will be able to measure
the temperature, and identify molecular absorptions (water, CO2) in one to four
nearby habitable TESS superEarths.Comment: accepted for PASP; added discussion and figure for habitable planets;
abridged Abstrac
Infrared Eclipses of the Strongly Irradiated Planet WASP-33b, and Oscillations of its Host Star
We observe two secondary eclipses of the strongly irradiated transiting
planet WASP-33b in the Ks band, and one secondary eclipse each at 3.6- and 4.5
microns using Warm Spitzer. This planet orbits an A5V delta-Scuti star that is
known to exhibit low amplitude non-radial p-mode oscillations at about
0.1-percent semi-amplitude. We detect stellar oscillations in all of our
infrared eclipse data, and also in one night of observations at J-band out of
eclipse. The oscillation amplitude, in all infrared bands except Ks, is about
the same as in the optical. However, the stellar oscillations in Ks band have
about twice the amplitude as seen in the optical, possibly because the
Brackett-gamma line falls in this bandpass. We use our best-fit values for the
eclipse depth, as well as the 0.9 micron eclipse observed by Smith et al., to
explore possible states of the exoplanetary atmosphere, based on the method of
Madhusudhan and Seager. On this basis we find two possible states for the
atmospheric structure of WASP-33b. One possibility is a non-inverted
temperature structure in spite of the strong irradiance, but this model
requires an enhanced carbon abundance (C/O>1). The alternative model has solar
composition, but an inverted temperature structure. Spectroscopy of the planet
at secondary eclipse, using a spectral resolution that can resolve the water
vapor band structure, should be able to break the degeneracy between these very
different possible states of the exoplanetary atmosphere. However, both of
those model atmospheres absorb nearly all of the stellar irradiance with
minimal longitudinal re-distribution of energy, strengthening the hypothesis of
Cowan et al. that the most strongly irradiated planets circulate energy poorly.
Our measurement of the central phase of the eclipse yields e*cos(omega)=0.0003
+/-0.00013, which we regard as being consistent with a circular orbit.Comment: 23 pages, 9 figures, 3 tables, accepted for the Astrophysical Journa
Infrared Observations During the Secondary Eclipse of HD 209458b: I. 3.6-Micron Occultation Spectroscopy Using the VLT
We search for an infrared signature of the transiting extrasolar planet HD
209458b during secondary eclipse. Our method, which we call `occultation
spectroscopy,' searches for the disappearance and reappearance of weak spectral
features due to the exoplanet as it passes behind the star and later reappears.
We argue that at the longest infrared wavelengths, this technique becomes
preferable to conventional `transit spectroscopy'. We observed the system in
the wing of the strong nu-3 band of methane near 3.6 microns during two
secondary eclipses, using the VLT/ISAAC spectrometer at a spectral resolution
of 3300. Our analysis, which utilizes a model template spectrum, achieves
sufficient precision to expect detection of the spectral structure predicted by
an irradiated, low-opacity (cloudless), low-albedo, thermochemical equilibrium
model for the exoplanet atmosphere. However, our observations show no evidence
for the presence of this spectrum from the exoplanet, with the statistical
significance of the non-detection depending on the timing of the secondary
eclipse, which depends on the assumed value for the orbital eccentricity. Our
results reject certain specific models of the atmosphere of HD 209458b as
inconsistent with our observations at the 3-sigma level, given assumptions
about the stellar and planetary parameters.Comment: 26 pages, 8 figures Accepted to Astrophysical Journa
The Spitzer search for the transits of HARPS low-mass planets - I. No transit for the super-Earth HD 40307b
We have used Spitzer and its IRAC camera to search for the transit of the
super-Earth HD 40307b. The transiting nature of the planet could not be firmly
discarded from our first photometric monitoring of a transit window because of
the uncertainty coming from the modeling of the photometric baseline. To obtain
a firm result, two more transit windows were observed and a global Bayesian
analysis of the three IRAC time series and the HARPS radial velocities was
performed. Unfortunately, any transit of the planet during the observed phase
window is firmly discarded, while the probability that the planet transits but
that the eclipse was missed by our observations is nearly negligible (0.26%).Comment: Submitted to A&
Transit confirmation and improved stellar and planet parameters for the super-Earth HD 97658 b and its host star
Super-Earths transiting nearby bright stars are key objects that
simultaneously allow for accurate measurements of both their mass and radius,
providing essential constraints on their internal composition. We present here
the confirmation, based on Spitzer transit observations, that the super-Earth
HD 97658 b transits its host star. HD 97658 is a low-mass
() K1 dwarf, as determined from the Hipparcos
parallax and stellar evolution modeling. To constrain the planet parameters, we
carry out Bayesian global analyses of Keck-HIRES radial velocities, and MOST
and Spitzer photometry. HD 97658 b is a massive () and large ( at 4.5
m) super-Earth. We investigate the possible internal compositions for HD
97658 b. Our results indicate a large rocky component, by at least 60% by mass,
and very little H-He components, at most 2% by mass. We also discuss how future
asteroseismic observations can improve the knowledge of the HD 97658 system, in
particular by constraining its age. Orbiting a bright host star, HD 97658 b
will be a key target for coming space missions TESS, CHEOPS, PLATO, and also
JWST, to characterize thoroughly its structure and atmosphere.Comment: 8 figures, accepted to Ap
Toward Eclipse Mapping of Hot Jupiters
Recent Spitzer infrared measurements of hot Jupiter eclipses suggest that
eclipse mapping techniques could be used to spatially resolve the day-side
photospheric emission of these planets using partial occultations. As a first
step in this direction, we simulate ingress/egress lightcurves for the three
brightest known eclipsing hot Jupiters and evaluate the degree to which
parameterized photospheric emission models can be distinguished from each other
with repeated, noisy eclipse measurements. We find that the photometric
accuracy of Spitzer is insufficient to use this tool effectively. On the other
hand, the level of photospheric details that could be probed with a few JWST
eclipse measurements could greatly inform hot Jupiter atmospheric modeling
efforts. A JWST program focused on non-parametric eclipse map inversions for
hot Jupiters should be actively considered.Comment: 32 pages, 6 figures, 3 tables, accepted for publication in Ap
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