1,348 research outputs found
Relative photometry of HAT-P-1b occultations
We present HST STIS observations of two occultations of the transiting
exoplanet HAT-P-1b. By measuring the planet to star flux ratio near opposition,
we constrain the geometric albedo of the planet, which is strongly linked to
its atmospheric temperature gradient. An advantage of HAT-P-1 as a target is
its binary companion ADS 16402 A, which provides an excellent photometric
reference, simplifying the usual steps in removing instrumental artifacts from
HST time-series photometry. We find that without this reference star, we would
need to detrend the lightcurve with the time of the exposures as well as the
first three powers of HST orbital phase, and this would introduce a strong bias
in the results for the albedo. However, with this reference star, we only need
to detrend the data with the time of the exposures to achieve the same
per-point scatter, therefore we can avoid most of the bias associated with
detrending. Our final result is a 2 sigma upper limit of 0.64 for the geometric
albedo of HAT-P-1b between 577 and 947 nm.Comment: 8 pages, 2 figures, 3 table
Near-infrared transit photometry of the exoplanet HD 149026b
The transiting exoplanet HD 149026b is an important case for theories of
planet formation and planetary structure, because the planet's relatively small
size has been interpreted as evidence for a highly metal-enriched composition.
We present observations of 4 transits with the Near Infrared Camera and
Multi-Object Spectrometer on the Hubble Space Telescope, within a wavelength
range of 1.1--2.0 m. Analysis of the light curve gives the most precise
estimate yet of the stellar mean density, g cm. By requiring agreement between the
observed stellar properties (including ) and stellar evolutionary
models, we refine the estimate of the stellar radius: R_\sun. We also find a deeper transit than has been
measured at optical and mid-infrared wavelengths. Taken together, these
findings imply a planetary radius of , which is larger than earlier estimates. Models of the planetary interior
still require a metal-enriched composition, although the required degree of
metal enrichment is reduced. It is also possible that the deeper NICMOS transit
is caused by wavelength-dependent absorption by constituents in the planet's
atmosphere, although simple model atmospheres do not predict this effect to be
strong enough to account for the discrepancy. We use the 4 newly-measured
transit times to compute a refined transit ephemeris.Comment: 18 pages, 13 figures, accepted for publication in Ap
Hubble Space Telescope Transmission Spectroscopy of the Exoplanet HD 189733b: High-altitude atmospheric haze in the optical and near-UV with STIS
We present Hubble Space Telescope optical and near-ultraviolet transmission
spectra of the transiting hot-Jupiter HD189733b, taken with the repaired Space
Telescope Imaging Spectrograph (STIS) instrument. The resulting spectra cover
the range 2900-5700 Ang and reach per-exposure signal-to-noise levels greater
than 11,000 within a 500 Ang bandwidth. We used time series spectra obtained
during two transit events to determine the wavelength dependance of the
planetary radius and measure the exoplanet's atmospheric transmission spectrum
for the first time over this wavelength range. Our measurements, in conjunction
with existing HST spectra, now provide a broadband transmission spectrum
covering the full optical regime. The STIS data also shows unambiguous evidence
of a large occulted stellar spot during one of our transit events, which we use
to place constraints on the characteristics of the K dwarf's stellar spots,
estimating spot temperatures around Teff~4250 K. With contemporaneous
ground-based photometric monitoring of the stellar variability, we also measure
the correlation between the stellar activity level and transit-measured
planet-to-star radius contrast, which is in good agreement with predictions. We
find a planetary transmission spectrum in good agreement with that of Rayleigh
scattering from a high-altitude atmospheric haze as previously found from HST
ACS camera. The high-altitude haze is now found to cover the entire optical
regime and is well characterised by Rayleigh scattering. These findings suggest
that haze may be a globally dominant atmospheric feature of the planet which
would result in a high optical albedo at shorter optical wavelengths.Comment: 14 pages, 14 figures, 4 tables, accepted to MNRAS, revised version
has minor change
Ambient Air Pollution and Atherosclerosis in Los Angeles
Associations have been found between long-term exposure to ambient air pollution and cardiovascular morbidity and mortality. The contribution of air pollution to atherosclerosis that underlies many cardiovascular diseases has not been investigated. Animal data suggest that ambient particulate matter (PM) may contribute to atherogenesis. We used data on 798 participants from two clinical trials to investigate the association between atherosclerosis and long-term exposure to ambient PM up to 2.5 μm in aerodynamic diameter (PM(2.5)). Baseline data included assessment of the carotid intima-media thickness (CIMT), a measure of subclinical atherosclerosis. We geocoded subjects’ residential areas to assign annual mean concentrations of ambient PM(2.5). Exposure values were assigned from a PM(2.5) surface derived from a geostatistical model. Individually assigned annual mean PM(2.5) concentrations ranged from 5.2 to 26.9 μg/m3 (mean, 20.3). For a cross-sectional exposure contrast of 10 μg/m3 PM(2.5), CIMT increased by 5.9% (95% confidence interval, 1–11%). Adjustment for age reduced the coefficients, but further adjustment for covariates indicated robust estimates in the range of 3.9–4.3% (p-values, 0.05–0.1). Among older subjects (≥60 years of age), women, never smokers, and those reporting lipid-lowering treatment at baseline, the associations of PM(2.5) and CIMT were larger with the strongest associations in women ≥60 years of age (15.7%, 5.7–26.6%). These results represent the first epidemiologic evidence of an association between atherosclerosis and ambient air pollution. Given the leading role of cardiovascular disease as a cause of death and the large populations exposed to ambient PM(2.5), these findings may be important and need further confirmation
Kepler Cycle 1 Observations of Low Mass Stars: New Eclipsing Binaries, Single Star Rotation Rates, and the Nature and Frequency of Starspots
We have analyzed Kepler light curves for 849 stars with T_eff < 5200 K from
our Cycle 1 Guest Observer program. We identify six new eclipsing binaries, one
of which has an orbital period of 29.91 d, and two of which are probably W UMa
variables. In addition, we identify a candidate "warm Jupiter" exoplanet. We
further examine a subset of 670 sources for variability. Of these objects, 265
stars clearly show periodic variability that we assign to rotation of the
low-mass star. At the photometric precision level provided by Kepler, 251 of
our objects showed no evidence for variability. We were unable to determine
periods for 154 variable objects. We find that 79% of stars with T_eff < 5200 K
are variable. The rotation periods we derive for the periodic variables span
the range 0.31 < P_rot < 126.5 d. A considerable number of stars with rotation
periods similar to the solar value show activity levels that are 100 times
higher than the Sun. This is consistent with results for solar-like field
stars. As has been found in previous studies, stars with shorter rotation
periods generally exhibit larger modulations. This trend flattens beyond P_rot
= 25 d, demonstrating that even long period binaries may still have components
with high levels of activity and investigating whether the masses and radii of
the stellar components in these systems are consistent with stellar models
could remain problematic. Surprisingly, our modeling of the light curves
suggests that the active regions on these cool stars are either preferentially
located near the rotational poles, or that there are two spot groups located at
lower latitudes, but in opposing hemispheres.Comment: 48 pages, 11 figure
Asteroseismic determination of obliquities of the exoplanet systems Kepler-50 and Kepler-65
Results on the obliquity of exoplanet host stars -- the angle between the
stellar spin axis and the planetary orbital axis -- provide important
diagnostic information for theories describing planetary formation. Here we
present the first application of asteroseismology to the problem of stellar
obliquity determination in systems with transiting planets and Sun-like host
stars. We consider two systems observed by the NASA Kepler Mission which have
multiple transiting small (super-Earth sized) planets: the previously reported
Kepler-50 and a new system, Kepler-65, whose planets we validate in this paper.
Both stars show rich spectra of solar-like oscillations. From the asteroseismic
analysis we find that each host has its rotation axis nearly perpendicular to
the line of sight with the sines of the angles constrained at the 1-sigma level
to lie above 0.97 and 0.91, respectively. We use statistical arguments to show
that coplanar orbits are favoured in both systems, and that the orientations of
the planetary orbits and the stellar rotation axis are correlated.Comment: Accepted for publication in ApJ; 46 pages, 11 figure
Kepler Observations of Transiting Hot Compact Objects
Kepler photometry has revealed two unusual transiting companions orbiting an
early A-star and a late B-star. In both cases the occultation of the companion
is deeper than the transit. The occultation and transit with follow-up optical
spectroscopy reveal a 9400 K early A-star, KOI-74 (KIC 6889235), with a
companion in a 5.2 day orbit with a radius of 0.08 Rsun and a 10000 K late
B-star KOI-81 (KIC 8823868) that has a companion in a 24 day orbit with a
radius of 0.2 Rsun. We infer a temperature of 12250 K for KOI-74b and 13500 K
for KOI-81b.
We present 43 days of high duty cycle, 30 minute cadence photometry, with
models demonstrating the intriguing properties of these object, and speculate
on their nature.Comment: 12 pages, 3 figures, submitted to ApJL (updated to correct KOI74
lightcurve
Constraints on Cosmological Models from Hubble Space Telescope Observations of High-z Supernovae
We have coordinated Hubble Space Telescope photometry with ground-based
discovery for three supernovae: two SN Ia near z~0.5 (SN 1997ce, SN 1997cj) and
a third event at z=0.97 (SN 1997ck). The superb spatial resolution of HST
separates each supernova from its host galaxy and leads to good precision in
the light curves. The HST data combined with ground-based photometry provide
good temporal coverage. We use these light curves and relations between
luminosity, light curve shape, and color calibrated from low-z samples to
derive relative luminosity distances which are accurate to 10% at z~0.5 and 20%
at z=1. The redshift-distance relation is used to place constraints on the
global mean matter density, Omega_matter, and the normalized cosmological
constant, Omega_Lambda. When the HST sample is combined with the distance to SN
1995K (z=0.48), analyzed by the same precepts, it suggests that matter alone is
insufficient to produce a flat Universe. Specifically, for
Omega_matter+Omega_Lambda=1, Omega_matter is less than 1 with >95% confidence,
and our best estimate of Omega_matter is -0.1 +/- 0.5 if Omega_Lambda=0.
Although the present result is based on a very small sample whose systematics
remain to be explored, it demonstrates the power of HST measurements for high
redshift supernovae.Comment: Submitted to ApJ Letters, 3 figures, 1 plate, additional tabl
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