2,723 research outputs found
Re-visit of HST FUV observations of hot-Jupiter system HD 209458: No Si III detection and the need for COS transit observations
The discovery of OI atoms and CII ions in the upper atmosphere of HD 209458b,
made with the Hubble Space Telescope Imaging Spectrograph (STIS) using the
G140L grating, showed that these heavy species fill an area comparable to the
planet's Roche lobe. The derived ~10% transit absorption depths require
super-thermal processes and/or supersolar abundances. From subsequent Cosmic
Origins Spectrograph (COS) observations, CII absorption was reported with
tentative velocity signatures, and absorption by SiIII ions was also claimed in
disagreement with a negative STIS G140L detection. Here, we revisit the COS
dataset showing a severe limitation in the published results from having
contrasted the in-transit spectrum against a stellar spectrum averaged from
separate observations, at planetary phases 0.27, 0.72, and 0.49. We find
variable stellar SiIII and CII emissions that were significantly depressed not
only during transit but also at phase 0.27 compared to phases 0.72 and 0.49.
Their respective off-transit 7.5 and 3.1% flux variations are large compared to
their reported 8.2+/-1.4% and 7.8+/-1.3% transit absorptions. Significant
variations also appear in the stellar line shapes, questioning reported
velocity signatures. We furthermore present archive STIS G140M transit data
consistent with no SiIII absorption, with a negative result of 1.7+/-18.7
including ~15% variability. Silicon may still be present at lower ionization
states, in parallel with the recent detection of extended magnesium, as MgI
atoms. In this frame, the firm detection of OI and CII implying solar or
supersolar abundances contradicts the recent inference of potential x20-125
subsolar metallicity for HD 209458b.Comment: Accepted for publication in Ap
Prominence seismology using the period ratio of transverse thread oscillations
The ratio of the period of the fundamental mode to that of the first overtone
of kink oscillations, from here on the "period ratio", is a seismology tool
that can be used to infer information about the spatial variation of density
along solar magnetic flux tubes. The period ratio is 2 in longitudinally
homogeneous thin tubes, but it differs from 2 due to longitudinal
inhomogeneity. In this paper we investigate the period ratio in longitudinally
inhomogeneous prominence threads and explore its implications for prominence
seismology. We numerically solve the two-dimensional eigenvalue problem of kink
oscillations in a model of a prominence thread. We take into account three
nonuniform density profiles along the thread. In agreement with previous works
that used simple piecewise constant density profiles, we find that the period
ratio is larger than 2 in prominence threads. When the ratio of the central
density to that at the footpoints is fixed, the period ratio depends strongly
on the form of the density profile along the thread. The more concentrated the
dense prominence plasma near the center of the tube, the larger the period
ratio. However, the period ratio is found to be independent of the specific
density profile when the spatially averaged density in the thread is the same
for all the profiles. An empirical fit of the dependence of the period ratio on
the average density is given and its use for prominence seismology is
discussed.Comment: Accepted for publication in A&
Time damping of non-adiabatic magnetohydrodynamic waves in a partially ionized prominence plasma: Effect of helium
Prominences are partially ionized, magnetized plasmas embedded in the solar
corona. Damped oscillations and propagating waves are commonly observed. These
oscillations have been interpreted in terms of magnetohydrodynamic (MHD) waves.
Ion-neutral collisions and non-adiabatic effects (radiation losses and thermal
conduction) have been proposed as damping mechanisms. We study the effect of
the presence of helium on the time damping of non-adiabatic MHD waves in a
plasma composed by electrons, protons, neutral hydrogen, neutral helium (He I),
and singly ionized helium (He II) in the single-fluid approximation. The
dispersion relation of linear non-adiabatic MHD waves in a homogeneous,
unbounded, and partially ionized prominence medium is derived. The period and
the damping time of Alfven, slow, fast, and thermal waves are computed. A
parametric study of the ratio of the damping time to the period with respect to
the helium abundance is performed. The efficiency of ion-neutral collisions as
well as thermal conduction is increased by the presence of helium. However, if
realistic abundances of helium in prominences (~10%) are considered, this
effect has a minor influence on the wave damping. The presence of helium can be
safely neglected in studies of MHD waves in partially ionized prominence
plasmas.Comment: Research note submitted in A&
The spatial damping of magnetohydrodynamic waves in a flowing partially ionised prominence plasma
Solar prominences are partially ionised plasmas displaying flows and
oscillations. These oscillations show time and spatial damping and, commonly,
have been explained in terms of magnetohydrodynamic (MHD) waves. We study the
spatial damping of linear non-adiabatic MHD waves in a flowing partially
ionised plasma, having prominence-like physical properties. We consider single
fluid equations for a partially ionised hydrogen plasma including in the energy
equation optically thin radiation, thermal conduction by electrons and
neutrals, and heating. Keeping the frequency real and fixed, we have solved the
obtained dispersion relations for the complex wavenumber, k, and have analysed
the behaviour of the damping length, wavelength and the ratio of the damping
length to the wavelength, versus period, for Alfven, fast, slow and thermal
waves.Comment: 28 pages, 9 figure
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