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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..

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.

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.

Engels D.,Hamburger Sternwarte | Bunzel F.,Hamburger Sternwarte
Astronomy and Astrophysics | Year: 2015

We present a new database of circumstellar OH masers at 1612, 1665, and 1667 MHz in the Milky Way galaxy. The database (version 2.4) contains 13 655 observations and 2341 different stars detected in at least one transition. Detections at 1612 MHz are considered to be complete until the end of 2014 as long as they were published in refereed papers. Detections of the main lines (1665 and 1667 MHz) and non-detections in all transitions are included only if published after 1983. The database contains flux densities and velocities of the two strongest maser peaks, the expansion velocity of the shell, and the radial velocity of the star. Links are provided for about 100 stars (<5% of all stars with OH masers) to interferometric observations and monitoring programs of the maser emission published since their beginnings in the 1970s. Access to the database is possible over the Web (, allowing cone searches for individual sources and lists of sources. A general search is possible in selected regions of the sky and by defining ranges of flux densities and/or velocities. Alternative ways to access the data are via the German Virtual Observatory and the CDS. © ESO, 2015.

Poppenhaeger K.,Hamburger Sternwarte | Schmitt J.H.M.M.,Hamburger Sternwarte
Astrophysical Journal | Year: 2011

The activity levels of stars are influenced by several stellar properties, such as stellar rotation, spectral type, and the presence of stellar companions. Analogous to binaries, planetary companions are also thought to be able to cause higher activity levels in their host stars, although at lower levels. Especially in X-rays, such influences are hard to detect because coronae of cool stars exhibit a considerable amount of intrinsic variability. Recently, a correlation between the mass of close-in exoplanets and their host star's X-ray luminosity has been detected, based on archival X-ray data from the ROSAT All-Sky Survey. This finding has been interpreted as evidence for star-planet interactions. We show in our analysis that this correlation is caused by selection effects due to the flux limit of the X-ray data used and due to the intrinsic planet detectability of the radial velocity method, and thus does not trace possible planet-induced effects. We also show that the correlation is not present in a corresponding complete sample derived from combined XMM-Newton and ROSAT data. © 2011. The American Astronomical Society. All rights reserved..

Kohl S.,Hamburger Sternwarte
Contributions of the Astronomical Observatory Skalnate Pleso | Year: 2014

The performance of a near infrared camera for photometry is investigated by observations of multiple samples of spatially close non-variable stars. Objects with a J-band magnitude less than 9.2 have been observed by the application of differential photometry. For the faint objects a statistical scatter of 7.9 mmag has been determined. Possible future applications include the examination of planets orbiting late type stars by an eclipse or a timing analysis.

Ian Short C.,Saint Mary's University, Halifax | Hauschildt P.H.,Hamburger Sternwarte
Astrophysical Journal | Year: 2010

We present a grid of LTE atmospheric models and synthetic spectra that covers the spectral class range from mid-G to mid-K, and luminosity classes from V to III, that is dense in Teff sampling (ΔTeff = 62.5 K), for stars of solar metallicity and moderately metal-poor scaled solar abundance ([ A/H ] = 0.0 and -0.5). All models have been computed with two choices of atomic line list: (1) the "big" line lists of Kurucz that best reproduce the broadband solar blue and near-UV fλ level, and (2) the "small" lists of Kurucz & Peytremann that provide the best fit to the high-resolution solar blue and near-UV spectrum. We compare our model spectral energy distributions to a sample of stars carefully selected from the large catalog of uniformly re-calibrated spectrophotometry of Burnashev with the goal of determining how the quality of fit varies with stellar parameters, especially in the historically troublesome blue and near-UV bands. We confirm that our models computed with the "big" line list recover the derived Teff values of the PHOENIX NextGen grid, but find that the models computed with the "small" line list provide greater internal self-consistency among different spectral bands, and closer agreement with the empirical Teff scale of Ramirez & Melendez, but not to the interferometrically derived Teff values of Baines et al. We find no evidence that the near-UV band discrepancy between models and observations for Arcturus (α Boo) reported in two works by Short & Hauschildt is pervasive, and that Arcturus may be peculiar in this regard. © 2010. The American Astronomical Society. All rights reserved.

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.

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.

Ogrean G.A.,Hamburger Sternwarte | Bruggen M.,Hamburger Sternwarte
Monthly Notices of the Royal Astronomical Society | Year: 2013

The Coma cluster is one of the nearest galaxy clusters, and the first one in which a peripheral radio relic was discovered. However, X-ray observations of the plasma near the relic have been scarce. Here, we present results from a re-analysis of a 22 ks archival XMM-Newton observation. Across the relic, we detect a temperature discontinuity indicative of a shock of Mach number ~2. This challenges the previously suggested hypothesis that the relic was formed by turbulence. Furthermore, multiwavelength observations and numerical models do not support the scenario in which the shock is an outgoing cluster-merger shock. Instead, our results lend support to the idea that the relic coincides with an infall shock front formed just as a 'wall' of galaxies, possibly associated with NGC 4839, falls on to the cluster along a cosmic filament. © 2013.

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