528 research outputs found
Stellar Activity and Coronal Heating: an overview of recent results
Observations of the coronae of the Sun and of solar-like stars provide
complementary information to advance our understanding of stellar magnetic
activity, and of the processes leading to the heating of their outer
atmospheres. While solar observations allow us to study the corona at high
spatial and temporal resolution, the study of stellar coronae allows us to
probe stellar activity over a wide range of ages and stellar parameters.
Stellar studies therefore provide us with additional tools for understanding
coronal heating processes, as well as the long-term evolution of solar X-ray
activity. We discuss how recent studies of stellar magnetic fields and coronae
contribute to our understanding of the phenomenon of activity and coronal
heating in late-type stars.Comment: Accepted for publication on Philosophical Transactions A. 29 pages, 5
figure
3.8-Micron Photometry During the Secondary Eclipse of the Extrasolar Planet HD 209458b
We report infrared photometry of the extrasolar planet HD 209458b during the
time of secondary eclipse (planet passing behind the star). Observations were
acquired during two secondary eclipses at the NASA Infrared Telescope Facility
(IRTF) in September 2003. We used a circular variable filter (1.5-percent
bandpass) centered at 3.8 microns to isolate the predicted flux peak of the
planet at this wavelength. Residual telluric absorption and instrument
variations were removed by offsetting the telescope to nearby bright comparison
stars at a high temporal cadence. Our results give a secondary eclipse depth of
0.0013 +/- 0.0011, not yet sufficient precision to detect the eclipse, whose
expected depth is approximately 0.002 - 0.003. We here elucidate the current
observational limitations to this technique, and discuss the approach needed to
achieve detections of hot Jupiter secondary eclipses at 3.8 microns from the
ground.Comment: 5 pages, 5 figures, in press for MNRA
On the Detectability of Oxygen X-ray Fluorescence and its Use as a Solar Photospheric Abundance Diagnostic
Monte Carlo calculations of the O Kalpha line fluoresced by coronal X-rays
and emitted just above the temperature minimum region of the solar atmosphere
have been employed to investigate the use of this feature as an abundance
diagnostic. While quite weak, we estimate line equivalent widths in the range
0.02-0.2 AA, depending on the X-ray plasma temperature. The line remains
essentially uncontaminated by blends for coronal temperatures T =< 3e6 K and
should be quite observable, with a flux >~ 2 ph/s/arcmin^2. Model calculations
for solar chemical mixtures with an O abundance adjusted up and down by a
factor of 2 indicate 35-60% changes in O Kalpha line equivalent width,
providing a potentially useful O abundance diagnostic. Sensitivity of
equivalent width to differences between recently recommended chemical
compositions with ``high'' and ``low'' complements of the CNO trio important
for interpreting helioseismological observations is less accute, amounting to
20-26% at coronal temperatures T ~< 2e6 K. While still feasible for
discriminating between these two mixtures, uncertainties in measured line
equivalent widths and in the models used for interpretation would need to be
significantly less than 20%. Provided a sensitive X-ray spectrometer with
resolving power >= 1000 and suitably well-behaved instrumental profile can be
built, X-ray fluorescence presents a viable means for resolving the solar
``oxygen crisis''.Comment: To appear in the Astrophysical Journa
A Ground-Based Search for Thermal Emission from the Exoplanet TrES-1
Eclipsing planetary systems give us an important window on extrasolar planet
atmospheres. By measuring the depth of the secondary eclipse, when the planet
moves behind the star, we can estimate the strength of the thermal emission
from the day side of the planet. Attaining a ground-based detection of one of
these eclipses has proven to be a significant challenge, as time-dependent
variations in instrument throughput and atmospheric seeing and absorption
overwhelm the small signal of the eclipse at infrared wavelengths. We gathered
a series of simultaneous L grism spectra of the transiting planet system TrES-1
and a nearby comparison star of comparable brightness, allowing us to correct
for these effects in principle. Combining the data from two eclipses, we
demonstrate a detection sensitivity of 0.15% in the eclipse depth relative to
the stellar flux. This approaches the sensitivity required to detect the
planetary emission, which theoretical models predict should lie between
0.05-0.1% of the stellar flux in our 2.9-4.3 micron bandpass. We explore the
factors that ultimately limit the precision of this technique, and discuss
potential avenues for future improvements.Comment: 10 pages, 1 table, four figures, accepted for publication in PAS
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