3,446 research outputs found
A bimodal correlation between host star chromospheric emission and the surface gravity of hot Jupiters
The chromospheric activity index logR'HK of stars hosting transiting hot
Jupiters appears to be correlated with the planets' surface gravity. One of the
possible explanations is based on the presence of condensations of planetary
evaporated material located in a circumstellar cloud that absorbs the CaII H&K
and MgII h&k resonance line emission flux, used to measure chromospheric
activity. A larger column density in the condensations, or equivalently a
stronger absorption in the chromospheric lines, is obtained when the
evaporation rate of the planet is larger, which occurs for a lower gravity of
the planet. We analyze here a sample of stars hosting transiting hot Jupiters
tuned in order to minimize systematic effects (e.g., interstellar medium
absorption). Using a mixture model, we find that the data are best fit by a
two-linear-regression model. We interpret this result in terms of the
Vaughan-Preston gap. We use a Monte Carlo approach to best take into account
the uncertainties, finding that the two intercepts fit the observed peaks of
the distribution of logR'HK for main-sequence solar-like stars. We also find
that the intercepts are correlated with the slopes, as predicted by the model
based on the condensations of planetary evaporated material. Our findings bring
further support to this model, although we cannot firmly exclude different
explanations. A precise determination of the slopes of the two linear
components would allow one to estimate the average effective stellar flux
powering planetary evaporation, which can then be used for theoretical
population and evolution studies of close-in planets.Comment: 23 pages, 4 figures, 1 table, accepted for publication in ApJ
A theoretical unifying scheme for gamma-ray bright blazars
The phenomenology of gamma-ray bright blazars can be accounted for by a
sequence in the source power and intensity of the diffuse radiation field
surrounding the relativistic jet. Correspondingly, the equilibrium particle
distribution peaks at different energies. This leads to a trend in the observed
properties: an increase of the observed power corresponds to: 1) a decrease in
the frequencies of the synchrotron and inverse Compton peaks; 2) an increase in
the ratio of the powers of the high and low energy spectral components. Objects
along this sequence would be observationally classified respectively as high
frequency BL Lac objects, low frequency BL Lac objects, highly polarized
quasars and lowly polarized quasars. The proposed scheme is based on the
correlations among the physical parameters derived in the present paper by
applying to 51 gamma ray loud blazars two of the most accepted scenarios for
the broad band emission of blazars, namely the synchrotron self--Compton and
external Compton models, and explains the observational trends presented by
Fossati et al. (1998) in a companion paper, dealing with the spectral energy
distributions of all blazars. This gives us confidence that our scheme applies
to all blazars as a class.Comment: 25 pages, 11 figures, uses mn.sty and psfig.tex. Accepted for
publication in MNRA
BeppoSAX Observations of Mkn 421: clues on the particle acceleration ?
Mkn 421 was repeatedly observed with BeppoSAX in 1997-1998. We present
highlights of the results of the thorough temporal and spectral analysis
discussed by Fossati et al. (1999) and Maraschi et al. (1999), focusing on the
flare of April 1998, which was simultaneously observed also at TeV energies.
The detailed study of the flare in different energy bands reveals a few very
important new results: (a) hard photons lag the soft ones by 2-3 ks *a behavior
opposite to what is normally found in High energy peak BL Lacs X-ray spectra*;
(b) the flux decay of the flare can be intrinsically achromatic if a stationary
underlying emission component is present. Moreover the spectral evolution
during the flare has been followed by extracting X-ray spectra on few ks
intervals, allowing to detect for the first time the peak of the synchrotron
component shifting to higher energies during the rising phase, and then
receding. The spectral analysis confirms the delay in the flare at the higher
energies, as above a few keV the spectrum changes only after the peak of the
outburst has occurred. The spectral and temporal information obtained challenge
the simplest models currently adopted for the (synchrotron) emission and most
importantly provide clues on the particle acceleration process. A theoretical
picture accounting for all the observational constraints is discussed, where
electrons are injected at low energies and then progressively accelerated
during the development of the flare.Comment: 5 pages, LaTeX, uses aipproc.sty, 3 PostScript figures, to appear in
the Proceedings of the "Fifth Compton Symposium", held in Portsmouth (NH),
1999 Septembe
Signs of strong Na and K absorption in the transmission spectrum of WASP-103b
Context: Transmission spectroscopy has become a prominent tool for
characterizing the atmospheric properties on close-in transiting planets.
Recent observations have revealed a remarkable diversity in exoplanet spectra,
which show absorption signatures of Na, K and , in some cases
partially or fully attenuated by atmospheric aerosols. Aerosols (clouds and
hazes) themselves have been detected in the transmission spectra of several
planets thanks to wavelength-dependent slopes caused by the particles'
scattering properties. Aims: We present an optical 550 - 960 nm transmission
spectrum of the extremely irradiated hot Jupiter WASP-103b, one of the hottest
(2500 K) and most massive (1.5 ) planets yet to be studied with this
technique. WASP-103b orbits its star at a separation of less than 1.2 times the
Roche limit and is predicted to be strongly tidally distorted. Methods: We have
used Gemini/GMOS to obtain multi-object spectroscopy hroughout three transits
of WASP-103b. We used relative spectrophotometry and bin sizes between 20 and 2
nm to infer the planet's transmission spectrum. Results: We find that WASP-103b
shows increased absorption in the cores of the alkali (Na, K) line features. We
do not confirm the presence of any strong scattering slope as previously
suggested, pointing towards a clear atmosphere for the highly irradiated,
massive exoplanet WASP-103b. We constrain the upper boundary of any potential
cloud deck to reside at pressure levels above 0.01 bar. This finding is in line
with previous studies on cloud occurrence on exoplanets which find that clouds
dominate the transmission spectra of cool, low surface gravity planets while
hot, high surface gravity planets are either cloud-free, or possess clouds
located below the altitudes probed by transmission spectra.Comment: Accepted for publication in A&
Prospects for Detection of Exoplanet Magnetic Fields Through Bow-Shock Observations During Transits
An asymmetry between the ingress and egress times was observed in the near-UV
light curve of the transit planet WASP-12b. Such asymmetry led us to suggest
that the early ingress in the UV light curve of WASP-12b, compared to the
optical observations, is caused by a shock around the planet, and that shocks
should be a common feature in transiting systems. Here, we classify all the
transiting systems known to date according to their potential for producing
shocks that could cause observable light curve asymmetries. We found that 36/92
of known transiting systems would lie above a reasonable detection threshold
and that the most promising candidates to present shocks are: WASP-19b,
WASP-4b, WASP-18b, CoRoT-7b, HAT-P-7b, CoRoT-1b, TrES-3, and WASP-5b. For
prograde planets orbiting outside the co-rotation radius of fast rotating
stars, the shock position, instead of being ahead of the planetary motion as in
WASP-12b, trails the planet. In this case, we predict that the light curve of
the planet should present a late-egress asymmetry. We show that CoRoT-11b is a
potential candidate to host such a behind shock and show a late egress. If
observed, these asymmetries can provide constraints on planetary magnetic
fields. For instance, for a planet that has a magnetic field intensity similar
to Jupiter's field (~ 14 G) orbiting a star whose magnetic field is between 1
and 100G, the stand-off distance between the shock and the planet, which we
take to be the size of the planet's magnetosphere, ranges from 1 to 40
planetary radii.Comment: 7 pages (including the complete version of Table 1), 2 Tables, 3
Figures. Accepted by MNRAS Letter
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