Stellar activity and planetary atmosphere evolution in tight binary star systems

Context. In tight binary star systems, tidal interactions can significantly influence the rotational and orbital evolution of both stars, and therefore their activity evolution. This can have strong effects on the atmospheric evolution of planets that are orbiting the two stars. Aims. In this paper, we aim to study the evolution of stellar rotation and of X-ray and ultraviolet (XUV) radiation in tight binary systems consisting of two solar mass stars and use our results to study planetary atmosphere evolution in the habitable zones of these systems. Methods. We have applied a rotation model developed for single stars to binary systems, taking into account the effects of tidal interactions on the rotational and orbital evolution of both stars. We used empirical rotation-activity relations to predict XUV evolution tracks for the stars, which we used to model hydrodynamic escape of hydrogen dominated atmospheres. Results. When significant, tidal interactions increase the total amount of XUV energy emitted, and in the most extreme cases by up to factor of $\sim$50. We find that in the systems that we study, habitable zone planets with masses of 1~M$_\oplus$ can lose huge hydrogen atmospheres due to the extended high levels of XUV emission, and the time that is needed to lose these atmospheres depends on the binary orbital separation.For some orbital separations, and when the stars are born as rapid rotators, it is also possible for tidal interactions to protect atmospheres from erosion by quickly spinning down the stars. For very small orbital separations, the loss of orbital angular momentum by stellar winds causes the two stars to merge. We suggest that the merging of the two stars could cause previously frozen planets to become habitable due to the habitable zone boundaries moving outwards.Comment: Accepted for publication by A&

Colliding Winds in Low-Mass Binary Star Systems: wind interactions and implications for habitable planets

Context. In binary star systems, the winds from the two components impact each other, leading to strong shocks and regions of enhanced density and temperature. Potentially habitable circumbinary planets must continually be exposed to these interactions regions. Aims. We study, for the first time, the interactions between winds from low-mass stars in a binary system, to show the wind conditions seen by potentially habitable circumbinary planets. Methods. We use the advanced 3D numerical hydrodynamic code Nurgush to model the wind interactions of two identical winds from two solar mass stars with circular orbits and a binary separation of 0.5 AU. As input into this model, we use a 1D hydrodynamic simulation of the solar wind, run using the Versatile Advection Code. We derive the locations of stable and habitable orbits in this system to explore what wind conditions potentially habitable planets will be exposed to during their orbits. Results. Our wind interaction simulations result in the formation of two strong shock waves separated by a region of enhanced density and temperature. The wind-wind interaction region has a spiral shape due to Coriolis forces generated by the orbital motions of the two stars. The stable and habitable zone in this system extends from approximately 1.4 AU to 2.4 AU. (TRUNCATED)Comment: 15 pages, 11 figures, to be published in A&

Antigen Presentation and Autophagy in Teleost Adaptive Immunity

Infectious diseases are a burden for aquaculture. Antigen processing and presentation (APP) to the immune effector cells that fight pathogens is key in the adaptive immune response. At the core of the adaptive immunity that appeared in lower vertebrates during evolution are the variable genes encoding the major histocompatibility complex (MHC). MHC class I molecules mainly present peptides processed in the cytosol by the proteasome and transported to the cell surface of all cells through secretory compartments. Professional antigen-presenting cells (pAPC) also express MHC class II molecules, which normally present peptides processed from exogenous antigens through lysosomal pathways. Autophagy is an intracellular self-degradation process that is conserved in all eukaryotes and is induced by starvation to contribute to cellular homeostasis. Self-digestion during autophagy mainly occurs by the fusion of autophagosomes, which engulf portions of cytosol and fuse with lysosomes (macroautophagy) or assisted by chaperones (chaperone-mediated autophagy, CMA) that deliver proteins to lysosomes. Thus, during self-degradation, antigens can be processed to be presented by the MHC to immune effector cells, thus, linking autophagy to APP. This review is focused on the essential components of the APP that are conserved in teleost fish and the increasing evidence related to the modulation of APP and autophagy during pathogen infection.Versión del editor2,46

ASCA and ROSAT observations of nearby cluster cooling flows

We present a detailed analysis of the X-ray properties of the cooling flows in a sample of nearby, X-ray bright clusters of galaxies using high-quality ASCA spectra and ROSAT X-ray images. We demonstrate the need for multiphase models to consistently explain the spectral and imaging X-ray data for the clusters. The mass deposition rates of the cooling flows, independently determined from the ASCA spectra and ROSAT images, exhibit reasonable agreement. We confirm the presence of intrinsic X-ray absorption in the clusters using a variety of spectral models. We also report detections of extended $100\mu$m infrared emission, spatially coincident with the cooling flows, in several of the systems studied. The observed infrared fluxes and flux limits are in good agreement with the predicted values due to reprocessed X-ray emission from the cooling flows. We present precise measurements of the abundances of iron, magnesium, silicon and sulphur in the central regions of the Virgo and Centaurus clusters. Our results firmly favour models in which a high mass fraction (70-80 per cent) of the iron in the X-ray gas in these regions is due to Type Ia supernovae. Finally, we present a series of methods which may be used to measure the ages of cooling flows from the X-ray data. The results for the present sample of clusters indicate ages of between 2.5 and 7 Gyr. If the ages of cooling flows are primarily set by subcluster merger events, then our results suggest that in the largest clusters, mergers with subclusters with masses of approximately 30 per cent of the final cluster mass are likely to disrupt cooling flows.Comment: Final version. MNRAS, in press. 36 pages, 9 figs, 14 tables in MNRAS LaTex styl

Young planets under extreme UV irradiation. I. Upper atmosphere modelling of the young exoplanet K2-33b

The K2-33 planetary system hosts one transiting ~5 R_E planet orbiting the young M-type host star. The planet's mass is still unknown, with an estimated upper limit of 5.4 M_J. The extreme youth of the system (<20 Myr) gives the unprecedented opportunity to study the earliest phases of planetary evolution, at a stage when the planet is exposed to an extremely high level of high-energy radiation emitted by the host star. We perform a series of 1D hydrodynamic simulations of the planet's upper atmosphere considering a range of possible planetary masses, from 2 to 40 M_E, and equilibrium temperatures, from 850 to 1300 K, to account for internal heating as a result of contraction. We obtain temperature profiles mostly controlled by the planet's mass, while the equilibrium temperature has a secondary effect. For planetary masses below 7-10 M_E, the atmosphere is subject to extremely high escape rates, driven by the planet's weak gravity and high thermal energy, which increase with decreasing mass and/or increasing temperature. For higher masses, the escape is instead driven by the absorption of the high-energy stellar radiation. A rough comparison of the timescales for complete atmospheric escape and age of the system indicates that the planet is more massive than 10 M_E.Comment: 11 pages, 7 figure

A Deep Look at the Emission-Line Nebula in Abell 2597

The close correlation between cooling flows and emission-line nebulae in clusters of galaxies has been recognized for over a decade and a half, but the physical reason for this connection remains unclear. Here we present deep optical spectra of the nebula in Abell 2597, one of the nearest strong cooling-flow clusters. These spectra reveal the density, temperature, and metal abundances of the line-emitting gas. The abundances are roughly half-solar, and dust produces an extinction of at least a magnitude in V. The absence of [O III] 4363 emission rules out shocks as a major ionizing mechanism, and the weakness of He II 4686 rules out a hard ionizing source, such as an active galactic nucleus or cooling intracluster gas. Hot stars are therefore the best candidate for producing the ionization. However, even the hottest O stars cannot power a nebula as hot as the one we see. Some other nonionizing source of heat appears to contribute a comparable amount of power. We show that the energy flux from a confining medium can become important when the ionization level of a nebula drops to the low levels seen in cooling-flow nebulae. We suggest that this kind of phenomenon, in which energy fluxes from the surrounding medium augment photoelectric heating, might be the common feature underlying the diverse group of objects classified as LINERS.Comment: 33 Latex pages, including 16 Postscript figures, to appear in 1997 September 1 Astrophysical Journa

Hydrogen Two-Photon Continuum Emission from the Horseshoe Filament in NGC 1275

Far ultraviolet emission has been detected from a knot of Halpha emission in the Horseshoe filament, far out in the NGC 1275 nebula. The flux detected relative to the brightness of the Halpha line in the same spatial region is very close to that expected from Hydrogen two-photon continuum emission in the particle heating model of Ferland et al. (2009) if reddening internal to the filaments is taken into account. We find no need to invoke other sources of far ultraviolet emission such as hot stars or emission lines from CIV in intermediate temperature gas to explain these data.Comment: 9 pages, 8 figures. Accepted for publication in MNRA

Current Star Formation in the Ophiuchus and Perseus Molecular Clouds: Constraints and Comparisons from Unbiased Submillimeter and Mid-Infrared Surveys. II

We present a census of the population of deeply embedded young stellar objects (YSOs) in the Ophiuchus molecular cloud complex based on a combination of Spitzer Space Telescope mid-infrared data from the "Cores to Disks" (c2d) legacy team and JCMT/SCUBA submillimeter maps from the COMPLETE team. We have applied a method developed for identifying embedded protostars in Perseus to these datasets and in this way construct a relatively unbiased sample of 27 candidate embedded protostars with envelopes more massive than our sensitivity limit (about 0.1 M_sun). Embedded YSOs are found in 35% of the SCUBA cores - less than in Perseus (58%). On the other hand the mid-infrared sources in Ophiuchus have less red mid-infrared colors, possibly indicating that they are less embedded. We apply a nearest neighbor surface density algorithm to define the substructure in each of the clouds and calculate characteristic numbers for each subregion - including masses, star formation efficiencies, fraction of embedded sources etc. Generally the main clusters in Ophiuchus and Perseus (L1688, NGC1333 and IC348) are found to have higher star formation efficiencies than small groups such as B1, L1455 and L1448, which on the other hand are completely dominated by deeply embedded protostars. We discuss possible explanations for the differences between the regions in Perseus and Ophiuchus, such as different evolutionary timescales for the YSOs or differences, e.g., in the accretion in the two clouds.Comment: Accepted for publication in ApJ (56 pages, 13 figures; abstract abridged). Version with full-resolution figures available at http://www.astro.uni-bonn.de/~jes/paper120.pd