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Hamburg, Germany

Ruszkowski M.,University of Michigan | Bruggen M.,Hamburger Sternwarte | Lee D.,University of Chicago | Shin M.-S.,University of Oxford
Astrophysical Journal | Year: 2014

Ram pressure stripping can remove significant amounts of gas from galaxies in clusters and massive groups and thus has a large impact on the evolution of cluster galaxies. Recent observations have shown that key properties of ram-pressure-stripped tails of galaxies, such as their width and structure, are in conflict with predictions by simulations. To increase the realism of existing simulations, we simulated for the first time a disk galaxy exposed to a uniformly magnetized wind including radiative cooling and self-gravity of the gas. We find that magnetic fields have a strong effect on the morphology of the gas in the tail of the galaxy. While in the purely hydrodynamical case the tail is very clumpy, the magnetohydrodynamical case shows very filamentary structures in the tail. The filaments can be strongly supported by magnetic pressure and, wherever this is the case, the magnetic fields vectors tend to be aligned with the filaments. The ram pressure stripping process may lead to the formation of magnetized density tails that appear as bifurcated in the plane of the sky and resemble the double tails observed in ESO 137-001 and ESO 137-002. Such tails can be formed under a variety of situations, both for the disks oriented face-on with respect to the intracluster medium (ICM) wind and for the tilted ones. While this bifurcation is the consequence of the generic tendency for the magnetic fields to produce very filamentary tail morphology, the tail properties are further shaped by the combination of the magnetic field orientation and the sliding of the field past the disk surface exposed to the wind. Despite the fact that the effect of the magnetic field on the morphology of the tail is strong, magnetic draping does not strongly change the rate of gas stripping. For a face-on galaxy, the field tends to reduce the amount of gas stripping compared to the pure hydrodynamical case, and is associated with the formation of a stable magnetic draping layer on the side of the galaxy exposed to the incoming ICM wind. For significantly tilted disks, the situation may be reversed and the stripping rate may be enhanced by the "scraping" of the disk surface by the magnetic fields sliding past the ISM/ICM interface. Instabilities, such as gravitational instabilities, undo the protective effect of this layer and allow the gas to leak out of the galaxy. © 2014. The American Astronomical Society. All rights reserved.. Source


Poppenhaeger K.,Harvard - Smithsonian Center for Astrophysics | Schmitt J.H.M.M.,Hamburger Sternwarte | Wolk S.J.,Harvard - Smithsonian Center for Astrophysics
Astrophysical Journal | Year: 2013

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 deeper 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-1. 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. © 2013. The American Astronomical Society. All rights reserved. Source


Claret A.,Institute Astrofisica Of Andalucia | Hauschildt P.H.,Hamburger Sternwarte | Witte S.,Hamburger Sternwarte
Astronomy and Astrophysics | Year: 2012

Aims. The knowledge of how the specific intensity is distributed over the stellar disk is crucial for interpreting the light curves of extrasolar transiting planets, double-lined eclipsing binaries, and other astrophysical phenomena. To provide theoretical inputs for light curve modelling codes, we present new calculations of limb-darkening coefficients for the spherically symmetric phoenix models. Methods. The limb-darkening coefficients were computed by covering the transmission curves of Kepler, CoRoT, and Spitzer space missions, as well as the passbands of the Strömgren, Johnson-Cousins, Sloan, and 2MASS. These computations adopted the least-square method. In addition, we also calculated the linear and bi-parametric approximations by adopting the flux conservation method as an additional tool for estimating the theoretical error bars in the limb-darkening coefficients. Results. Six laws were used to describe the specific intensity distribution: linear, quadratic, square root, logarithmic, exponential, and a more general one with 4 terms. The computations are presented for the solar chemical composition, with log g varying between 2.5 and 5.5 and effective temperatures between 1500-4800 K. The adopted microturbulent velocity and the mixing-length parameters are 2.0 kmâ ‰ s -1 and 2.0, respectively. ©2012 ESO. Source


Claret A.,Institute Astrofisica Of Andalucia | Hauschildt P.H.,Hamburger Sternwarte | Witte S.,Hamburger Sternwarte
Astronomy and Astrophysics | Year: 2013

Aims. We present an extension of our investigations on limb-darkening coefficients computed with spherical symmetrical Phoenix models. The models investigated in this paper cover the range 5000 K ≤ Teff ≤ 10 000 K and complete our previous studies of low effective temperatures computed with the same code. Methods. The limb-darkening coefficients are computed for the transmission curves of the Kepler, CoRoT, and Spitzer space missions and the Strömgren, Johnson-Cousins, Sloan, and 2MASS passbands. These computations were performed by adopting the least-squares method. Results. We have used six laws to describe the specific intensity distribution: linear, quadratic, square root, logarithmic, exponential, and a general law with four terms. The computations are presented for the solar chemical composition and cover the range 3.0 ≤ logg ≤ 5.5. The adopted microturbulent velocity and the mixing-length parameter are 2.0 kms-1 and 2.0. © 2013 ESO. Source


Vazza F.,Hamburger Sternwarte | Vazza F.,National institute for astrophysics | Bruggen M.,Hamburger Sternwarte
Monthly Notices of the Royal Astronomical Society | Year: 2014

Radio relics in galaxy clusters are thought to be associated with powerful shock waves that accelerate particles via diffusive shock acceleration (DSA). Among the particles accelerated by DSA, relativistic protons should outnumber electrons by a large factor.While the relativistic electrons emit synchrotron emission detectable in the radio band, the protons interact with the thermal gas to produce gamma-rays in hadronic interactions. Using simple models for the propagation of shock waves through clusters, the distribution of thermal gas and the efficiency of DSA, we find that the resulting hadronic gamma-ray emission lies very close or above the upper limits from the Fermi data on nearby clusters. This suggests that the relative acceleration efficiency of electrons and protons is at odds with predictions from DSA. The inclusion of re-accelerated 'fossil' particles does not seem to solve the problem. Our study highlights a possible tension of the commonly assumed scenario for the formation of radio relics and we discuss possible solutions to the problem. © 2013 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society. Source

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