25,512 research outputs found
The prevalence of dust on the exoplanet HD 189733b from Hubble and Spitzer observations
The hot Jupiter HD189733b is the most extensively observed exoplanet. Its
atmosphere has been detected and characterised in transmission and eclipse
spectroscopy, and its phase curve measured at several wavelengths. This paper
brings together results of our campaign to obtain the complete transmission
spectrum of the atmosphere of this planet from UV to IR with HST, using STIS,
ACS and WFC3. We provide a new tabulation of the transmission spectrum across
the entire visible and IR range. The radius ratio in each wavelength band was
rederived to ensure a consistent treatment of the bulk transit parameters and
stellar limb-darkening. Special care was taken to correct for, and derive
realistic estimates of the uncertainties due to, both occulted and unocculted
star spots. The combined spectrum is very different from the predictions of
cloud-free models: it is dominated by Rayleigh scattering over the whole
visible and near infrared range, the only detected features being narrow Na and
K lines. We interpret this as the signature of a haze of condensate grains
extending over at least 5 scale heights. We show that a dust-dominated
atmosphere could also explain several puzzling features of the emission
spectrum and phase curves, including the large amplitude of the phase curve at
3.6um, the small hot-spot longitude shift and the hot mid-infrared emission
spectrum. We discuss possible compositions and derive some first-order
estimates for the properties of the putative condensate haze/clouds. We finish
by speculating that the dichotomy between the two observationally defined
classes of hot Jupiter atmospheres, of which HD189733b and HD209458b are the
prototypes, might not be whether they possess a temperature inversion, but
whether they are clear or dusty. We also consider the possibility of a
continuum of cloud properties between hot Jupiters, young Jupiters and L-type
brown dwarfs.Comment: Accepted for publication in MNRAS. 31 pages, 19 figures, 8 table
A Chandra Survey of the X-ray Properties of Broad Absorption Line Radio-Loud Quasars
This work presents the results of a Chandra study of 21 broad absorption line
(BAL) radio-loud quasars (RLQs). We conducted a Chandra snapshot survey of 12
bright BAL RLQs selected from SDSS/FIRST data and possessing a wide range of
radio and CIV absorption properties. Optical spectra were obtained nearly
contemporaneously with the Hobby-Eberly Telescope; no strong flux or BAL
variability was seen between epochs. We also include in our sample 9 additional
BAL RLQs possessing archival Chandra coverage. We compare the properties of
(predominantly high-ionization) BAL RLQs to those of non-BAL RLQs as well as to
BAL radio-quiet quasars (RQQs) and non-BAL RQQs for context.
All 12 snapshot and 8/9 archival BAL RLQs are detected, with observed X-ray
luminosities less than those of non-BAL RLQs having comparable optical/UV
luminosities by typical factors of 4.1-8.5. (BAL RLQs are also X-ray weak by
typical factors of 2.0-4.5 relative to non-BAL RLQs having both comparable
optical/UV and radio luminosities.) However, BAL RLQs are not as X-ray weak
relative to non-BAL RLQs as are BAL RQQs relative to non-BAL RQQs. While some
BAL RLQs have harder X-ray spectra than typical non-BAL RLQs, some have
hardness ratios consistent with those of non-BAL RLQs, and there does not
appear to be a correlation between X-ray weakness and spectral hardness, in
contrast to the situation for BAL RQQs. RLQs are expected to have X-ray
continuum contributions from both disk-corona and small-scale jet emission.
While the entire X-ray continuum in BAL RLQs cannot be obscured to the same
degree as in BAL RQQs, we calculate that the jet is likely partially covered in
many BAL RLQs. We comment briefly on implications for geometries and source
ages in BAL RLQs.Comment: 48 pages, 5 tables, 14 figures, accepted by Ap
The optical transmission spectrum of the hot Jupiter HAT-P-32b: clouds explain the absence of broad spectral features?
We report Gemini-North GMOS observations of the inflated hot Jupiter
HAT-P-32b during two primary transits. We simultaneously observed two
comparison stars and used differential spectro-photometry to produce
multi-wavelength light curves. 'White' light curves and 29 'spectral' light
curves were extracted for each transit and analysed to refine the system
parameters and produce transmission spectra from 520-930nm in ~14nm bins. The
light curves contain time-varying white noise as well as time-correlated noise,
and we used a Gaussian process model to fit this complex noise model. Common
mode corrections derived from the white light curve fits were applied to the
spectral light curves which significantly improved our precision, reaching
typical uncertainties in the transit depth of ~2x10^-4, corresponding to about
half a pressure scale height. The low resolution transmission spectra are
consistent with a featureless model, and we can confidently rule out broad
features larger than about one scale height. The absence of Na/K wings or
prominent TiO/VO features is most easily explained by grey absorption from
clouds in the upper atmosphere, masking the spectral features. However, we
cannot confidently rule out clear atmosphere models with low abundances (~10^-3
solar) of TiO, VO or even metal hydrides masking the Na and K wings. A smaller
scale height or ionisation could also contribute to muted spectral features,
but alone are unable to to account for the absence of features reported here.Comment: 17 pages, 11 figures, 2 tables, accepted for publication in MNRA
A Gemini ground-based transmission spectrum of WASP-29b: a featureless spectrum from 515 to 720nm
We report Gemini-South GMOS observations of the exoplanet system WASP-29
during primary transit as a test case for differential spectrophotometry. We
use the multi-object spectrograph to observe the target star and a comparison
star simultaneously to produce multiple light curves at varying wavelengths.
The 'white' light curve and fifteen 'spectral' light curves are analysed to
refine the system parameters and produce a transmission spectrum from 515 to
720nm. All light curves exhibit time-correlated noise, which we model using a
variety of techniques. These include a simple noise rescaling, a Gaussian
process model, and a wavelet based method. These methods all produce consistent
results, although with different uncertainties. The precision of the
transmission spectrum is improved by subtracting a common signal from all the
spectral light curves, reaching a typical precision of ~1x10^-4 in transit
depth. The transmission spectrum is free of spectral features, and given the
non-detection of a pressure broadened Na feature, we can rule out the presence
of a Na rich atmosphere free of clouds or hazes, although we cannot rule out a
narrow Na core. This indicates that Na is not present in the atmosphere, and/or
that clouds/hazes play a significant role in the atmosphere and mask the broad
wings of the Na feature, although the former is a more likely explanation given
WASP-29b's equilibrium temperature of ~970 K, at which Na can form various
compounds. We also briefly discuss the use of Gaussian process and wavelet
methods to account for time correlated noise in transit light curves.Comment: 15 pages, 9 figures, 3 tables. Published in MNRAS. Figure 2 corrected
in version
Diagnostics of Coronal Magnetic Fields Through the Hanle Effect in UV and IR Lines
The plasma thermodynamics in the solar upper atmosphere, particularly in the
corona, are dominated by the magnetic field, which controls the flow and
dissipation of energy. The relative lack of knowledge of the coronal vector
magnetic field is a major handicap for progress in coronal physics. This makes
the development of measurement methods of coronal magnetic fields a high
priority in solar physics. The Hanle effect in the UV and IR spectral lines is
a largely unexplored diagnostic. We use magnetohydrodynamic (MHD) simulations
to study the magnitude of the signal to be expected for typical coronal
magnetic fields for selected spectral lines in the UV and IR wavelength ranges,
namely the H I Ly- and the He I 10830 {\AA} lines. We show that the
selected lines are useful for reliable diagnosis of coronal magnetic fields.
The results show that the combination of polarization measurements of spectral
lines with different sensitivities to the Hanle effect may be most appropriate
for deducing coronal magnetic properties from future observations.Comment: 15 pages, 5 figures, Frontiers in Astronomy and Space Sciences, 201
A new look at NICMOS transmission spectroscopy of HD189733, GJ-436 and XO-1: no conclusive evidence for molecular features
We present a re-analysis of archival HST/NICMOS transmission spectroscopy of
three exoplanet systems; HD 189733, GJ-436 and XO-1. Detections of several
molecules, including H20, CH4 and CO2, have been claimed for HD 189733 and
XO-1, but similarly sized features are attributed to systematic noise for
GJ-436. The data consist of time-series grism spectra covering a planetary
transit. After extracting light curves in independent wavelength channels, we
use a linear decorrelation technique account for instrumental systematics
(which is becoming standard in the field), and measure the planet-to-star
radius ratio as a function of wavelength. For HD 189733, the uncertainties in
the transmission spectrum are significantly larger than those previously
reported. We also find the transmission spectrum is considerably altered when
using different out-of-transit orbits to remove the systematics, when some
parameters are left out of the decorrelation procedure, or when we perform the
decorrelation with quadratic functions rather than linear functions. Given that
there is no physical reason to believe the baseline flux should be modelled as
a linear function of any particular set of parameters, we interpret this as
evidence that the linear decorrelation technique is not a robust method to
remove systematic effects from the light curves for each wavelength channel.
For XO-1, the parameters measured to decorrelate the light curves would require
extrapolation to the in-transit orbit to remove the systematics, and we cannot
reproduce the previously reported results. We conclude that the resulting
NICMOS transmission spectra are too dependent on the method used to remove
systematics to be considered robust detections of molecular species in
planetary atmospheres, although the presence of these molecules is not ruled
out.Comment: 17 pages, 28 figures, accepted in MNRA
X-raying the Winds of Luminous Active Galaxies
We briefly describe some recent observational results, mainly at X-ray
wavelengths, on the winds of luminous active galactic nuclei (AGNs). These
winds likely play a significant role in galaxy feedback. Topics covered include
(1) Relations between X-ray and UV absorption in Broad Absorption Line (BAL)
and mini-BAL quasars; (2) X-ray absorption in radio-loud BAL quasars; and (3)
Evidence for relativistic iron K BALs in the X-ray spectra of a few bright
quasars. We also mention some key outstanding problems and prospects for future
advances; e.g., with the International X-ray Observatory (IXO).Comment: 7 pages, 3 figures, to appear in proceedings of the conference "The
Monster's Fiery Breath: Feedback in Galaxies, Groups, and Clusters", June
2009, Madison, Wisconsi
Evidence for massive bulk Dirac Fermions in PbSnSe from Nernst and thermopower experiments
The lead chalcogenides (Pb,Sn)Te and (Pb,Sn)Se are the first examples of
topological crystalline insulators (TCI) predicted \cite{Fu,Hsieh} (and
confirmed \cite{Hasan,Story,Takahashi}) to display topological surface Dirac
states (SDS) that are protected by mirror symmetry. A starting premise
\cite{Hsieh} is that the SDS arise from bulk states describable as massive
Dirac states \cite{Wallis,Svane}, but this assumption is untested. Here we show
that the thermoelectric response of the bulk states display features specific
to the Dirac spectrum. We show that, in the quantum limit, the lowest Landau
Level (LL) is singly spin-degenerate, whereas higher levels are doubly
degenerate. The abrupt change in spin degeneracy leads to a large step-decrease
in the thermopower . In the lowest LL, displays a striking
linear increase vs. magnetic field. In addition, the Nernst signal undergoes an
anomalous sign change when the bulk gap inverts at 180 K.Comment: 16 pages, 8 figure
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