2,502 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
Exoplanet HD 209458b : Evaporation strengthened
Following re-analysis of Hubble Space Telescope observations of primary
transits of the extrasolar planet HD209458b at Lyman-alpha, Ben-Jaffel (2007,
BJ007) claims that no sign of evaporation is observed. Here we show that, in
fact, this new analysis is consistent with the one of Vidal-Madjar et al.
(2003, VM003) and supports the detection of evaporation. The apparent
disagreement is mainly due to the disparate wavelength ranges that are used to
derive the transit absorption depth. VM003 derives a (15+/-4)% absorption depth
during transit over the core of the stellar Lyman-alpha line (from -130 km/s to
+100 km/s), and this result agrees with the (8.9+/-2.1)% absorption depth
reported by BJ007 from a slightly expanded dataset but over a larger wavelength
range (+/-200 km/s). These measurements agree also with the (5+/-2)% absorption
reported by Vidal-Madjar et al. (2004) over the whole Lyman-alpha line from
independent, lower-resolution data. We show that stellar Lyman-alpha
variability is unlikely to significantly affect those detections. The HI atoms
must necessarily have velocities above the escape velocities and/or be outside
the Roche lobe, given the lobe shape and orientation. Absorption by HI in
HD209458b's atmosphere has thus been detected with different datasets, and now
with independent analyses. All these results strengthen the concept of
evaporating hot-Jupiters, as well as the modelization of this phenomenon.Comment: To be published in ApJ
Jovian equatorial H2 emission from 1979-1987
Ninety two IUE observations of the Jovian equatorial region taken between 2 Dec. 1978 and 1 Feb. 1988 were averaged together by date of observation, resulting in 22 averaged spectra which were fit with a model to determine the amount of H2 Lyman band emission in the region 1552 to 1624A. The data suggest that the H2 emission may vary with time. Especially suggestive is the marked downward trend of the emission between 1983 and 1987, during which time the strength of the emission in the 1552 to 1624A region decreases by a factor of 10. Uncertainty in the existing data and a gap in the data in 1980 and 1981 preclude a positive identification of a correlation between the brightness of the H2 emission and the major solar cycle
Analyse chimique continue de la mer: II. Mission internationale <i>Mechelen</i> 1968 en Méditerranée
Results of automatic and continuous analysis of both physical and chemical parameters of sea water between Marbella (Malaga) and Brest (Mechelen cruise, Mediterranean 1968, supported by the « Institut royal des Sciences naturelles de Belgique ») are studied in this paper.Two conclusions are reached: 1° The distribution of salinity, temperature and nitrate, sometimes anomalous, appears to follow some rythm or law. 2° There appears a close spacial relationship too between these anomalies and fishery. Then, it is possible to use the continuous automatic method for the exploratory work on fishery technology
Real-time depth sectioning: Isolating the effect of stress on structure development in pressure-driven flow
Transient structure development at a specific distance from the channel wall in a pressure-driven flow is obtained from a set of real-time measurements that integrate contributions throughout the thickness of a rectangular channel. This “depth sectioning method” retains the advantages of pressure-driven flow while revealing flow-induced structures as a function of stress. The method is illustrated by applying it to isothermal shear-induced crystallization of an isotactic polypropylene using both synchrotron x-ray scattering and optical retardance. Real-time, depth-resolved information about the development of oriented precursors reveals features that cannot be extracted from ex-situ observation of the final morphology and that are obscured in the depth-averaged in-situ measurements. For example, at 137 °C and at the highest shear stress examined (65 kPa), oriented thread-like nuclei formed rapidly, saturated within the first 7 s of flow, developed significant crystalline overgrowth during flow and did not relax after cessation of shear. At lower stresses, threads formed later and increased at a slower rate. The depth sectioning method can be applied to the flow-induced structure development in diverse complex fluids, including block copolymers, colloidal systems, and liquid-crystalline polymers
Long-term variation in the Sun's activity caused by magnetic Rossby waves in the tachocline
Long-term records of sunspot number and concentrations of cosmogenic
radionuclides (10Be and 14C) on the Earth reveal the variation of the Sun's
magnetic activity over hundreds and thousands of years. We identify several
clear periods in sunspot, 10Be, and 14C data as 1000, 500, 350, 200 and 100
years. We found that the periods of the first five spherical harmonics of the
slow magnetic Rossby mode in the presence of a steady toroidal magnetic field
of 1200-1300 G in the lower tachocline are in perfect agreement with the time
scales of observed variations. The steady toroidal magnetic field can be
generated in the lower tachocline either due to the steady dynamo magnetic
field for low magnetic diffusivity or due to the action of the latitudinal
differential rotation on the weak poloidal primordial magnetic field, which
penetrates from the radiative interior. The slow magnetic Rossby waves lead to
variations of the steady toroidal magnetic field in the lower tachocline, which
modulate the dynamo magnetic field and consequently the solar cycle strength.
This result constitutes a key point for long-term prediction of the cycle
strength. According to our model, the next deep minimum in solar activity is
expected during the first half of this century.Comment: 4 pages, 4 figures, accepted in ApJ
Who Contributes to the Knowledge Sharing Economy?
Information sharing dynamics of social networks rely on a small set of
influencers to effectively reach a large audience. Our recent results and
observations demonstrate that the shape and identity of this elite, especially
those contributing \emph{original} content, is difficult to predict.
Information acquisition is often cited as an example of a public good. However,
this emerging and powerful theory has yet to provably offer qualitative
insights on how specialization of users into active and passive participants
occurs.
This paper bridges, for the first time, the theory of public goods and the
analysis of diffusion in social media. We introduce a non-linear model of
\emph{perishable} public goods, leveraging new observations about sharing of
media sources. The primary contribution of this work is to show that
\emph{shelf time}, which characterizes the rate at which content get renewed,
is a critical factor in audience participation. Our model proves a fundamental
\emph{dichotomy} in information diffusion: While short-lived content has simple
and predictable diffusion, long-lived content has complex specialization. This
occurs even when all information seekers are \emph{ex ante} identical and could
be a contributing factor to the difficulty of predicting social network
participation and evolution.Comment: 15 pages in ACM Conference on Online Social Networks 201
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