2,185 research outputs found
Transit observations of the Hot Jupiter HD 189733b at X-ray wavelengths
We present new X-ray observations obtained with Chandra ACIS-S of the HD
189733 system, consisting of a K-type star orbited by a transiting Hot Jupiter
and an M-type stellar companion. We report a detection of the planetary transit
in soft X-rays with a significantly larger transit depth than observed in the
optical. The X-ray data favor a transit depth of 6-8%, versus a broadband
optical transit depth of 2.41%. While we are able to exclude several possible
stellar origins for this deep transit, additional observations will be
necessary to fully exclude the possibility that coronal inhomogeneities
influence the result. From the available data, we interpret the deep X-ray
transit to be caused by a thin outer planetary atmosphere which is transparent
at optical wavelengths, but dense enough to be opaque to X-rays. The X-ray
radius appears to be larger than the radius observed at far-UV wavelengths,
most likely due to high temperatures in the outer atmosphere at which hydrogen
is mostly ionized. We furthermore detect the stellar companion HD 189733B in
X-rays for the first time with an X-ray luminosity of log LX = 26.67 erg/s. We
show that the magnetic activity level of the companion is at odds with the
activity level observed for the planet-hosting primary. The discrepancy may be
caused by tidal interaction between the Hot Jupiter and its host star.Comment: 15 pages, accepted for publication in The Astrophysical Journa
Commission des Communautes Europeennes: Groupe du Porte-Parole. Rendez-vous de midi et Reunion de la Commission - 7.12.1988 (C.D. Ehlermann) = Commission of European Communities: Spokesman Group. Appointment at noon and Meeting of the Commission - 7.12.1988 (C.D. Ehlermann). Press Spokesman Service Note to National Offices Bio No. (88) 390, 7 December 1988
Fomalhaut b is a directly imaged object in the debris disk of the star
Fomalhaut. It has been hypothesized to be a planet, however there are issues
with the observed colours of the object that do not fit planetary models. An
alternative hypothesis is that the object is a neutron star in the near fore-
or background of Fomalhaut's disk. We test if Fomalhaut b could be a neutron
star using X-ray observations with Chandra's HRC-I instrument in the energy
range of 0.08-10 keV. We do not detect X-ray emission from either Fomalhaut b
or the star Fomalhaut itself. Our nondetection corresponds to an upper limit on
the X-ray flux of Fomalhaut b of F_X < 1.3e-14 erg/cm/s^2 in the energy range
0.08-10 keV. For the A-type central star Fomalhaut, we derive an X-ray upper
limit of L_X < 2e25 erg/s in the energy range 0.08-10 keV. Fomalhaut b's X-ray
non-detection constrains the parameter space for a possible neutron star
significantly, implying surface temperatures lower than 91000 K and distances
closer than 13.3 pc to the solar system. In addition we find that reflected
starlight from the central star fits the available optical detections of
Fomalhaut b; a smaller planet with a large ring system might explain such a
scenario.Comment: 7 page
Aetiology and pathogenesis of hidradenitis suppurativa
Hidradenitis suppurativa (HS) is a chronic inflammatory disorder. Patients develop inflamed nodules and abscesses and, at later stages of disease, epithelialized tunnels and scars in skinfolds of axillary, inguinal, gluteal and perianal areas. Quality of life is affected due to severe pain, purulent secretion, restricted mobility and systemic involvement. Genetics and lifestyle factors including smoking and obesity contribute to the development of HS. These factors lead to microbiome alteration, subclinical inflammation around the terminal hair follicles, and infundibular hyperkeratosis, resulting in plugging and rupture of the follicles. Cell-damage-associated molecules and propagating bacteria trigger inflammation and lead to massive immune cell infiltration that clinically manifests as inflamed nodules and abscesses. The immune system plays a key role also in the progression and chronification of skin alterations. Innate proinflammatory cytokines (e.g. interleukin-1 beta and tumour necrosis factor-alpha), mediators of activated T helper (Th)1 and Th17 cells (e.g. interleukin-17 and interferon-gamma), and effector mechanisms of neutrophilic granulocytes, macrophages and plasma cells are involved. Simultaneously, skin lesions contain anti-inflammatory mediators (e.g. interleukin-10) and show limited activity of Th22 and regulatory T cells. The inflammatory vicious circle finally results in pain, purulence, tissue destruction and scarring. Chronic inflammation in patients with HS is also frequently detected in organs other than the skin, as indicated by their comorbidities. All these aspects represent a challenge for the development of therapeutic approaches, which are urgently needed for this debilitating disease. This scholarly review focuses on the causes and pathogenetic mechanisms of HS and the potential therapeutic value of this knowledge
X-Ray Determination of the Variable Rate of Mass Accretion onto TW Hydrae
Diagnostics of electron temperature (T_e), electron density (n_e), and
hydrogen column density (N_H) from the Chandra High Energy Transmission Grating
spectrum of He-like Ne IX in TW Hydrae (TW Hya), in conjunction with a
classical accretion model, allow us to infer the accretion rate onto the star
directly from measurements of the accreting material. The new method introduces
the use of the absorption of Ne IX lines as a measure of the column density of
the intervening, accreting material. On average, the derived mass accretion
rate for TW Hya is 1.5 x 10^{-9} M_{\odot} yr^{-1}, for a stellar magnetic
field strength of 600 Gauss and a filling factor of 3.5%. Three individual
Chandra exposures show statistically significant differences in the Ne IX line
ratios, indicating changes in N_H, T_e, and n_e by factors of 0.28, 1.6, and
1.3, respectively. In exposures separated by 2.7 days, the observations
reported here suggest a five-fold reduction in the accretion rate. This
powerful new technique promises to substantially improve our understanding of
the accretion process in young stars
Magnetic cycles in a dynamo simulation of the fully convective M-star Proxima Centauri
The recent discovery of an Earth-like exoplanet around Proxima Centauri has
shined a spot light on slowly rotating fully convective M-stars. When such
stars rotate rapidly (period days), they are known to generate
very high levels of activity that is powered by a magnetic field much stronger
than the solar magnetic field. Recent theoretical efforts are beginning to
understand the dynamo process that generates such strong magnetic fields.
However, the observational and theoretical landscape remains relatively
uncharted for fully convective M-stars that rotate slowly. Here we present an
anelastic dynamo simulation designed to mimic some of the physical
characteristics of Proxima Centauri, a representative case for slowly rotating
fully convective M-stars. The rotating convection spontaneously generates
differential rotation in the convection zone which drives coherent magnetic
cycles where the axisymmetric magnetic field repeatedly changes polarity at all
latitudes as time progress. The typical length of the `activity' cycle in the
simulation is about nine years, in good agreement with the recently proposed
activity cycle length of about seven years for Proxima Centauri. Comparing our
results with earlier work, we hypothesis that the dynamo mechanism undergoes a
fundamental change in nature as fully convective stars spin down with age.Comment: 8 pages, 4 figures, double column; Accepted in ApJ Letter
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