M. Cuntz

On Stellar Activity Enhancement Due to Interactions with Extrasolar Giant Planets.

We present a first attempt to identify and quantify possible interactions between recently discovered extrasolar giant planets (and brown dwarfs) and their host stars, resulting in activity enhancement in the stellar outer atmospheres. Many extrasolar planets have masses comparable to or larger than Jupiter and are within a distance of 0.5 AU, suggesting the possibility of their significant influence on stellar winds, coronae, and even chromospheres. Beyond the well-known rotational synchronization, the interactions include tidal effects (in which enhanced flows and turbulence in the tidal bulge lead to increased magnetoacoustic heating and dynamo action) and direct magnetic interaction between the stellar and planetary magnetic fields. We discuss relevant parameters for selected systems and give preliminary estimates of the relative interaction strengths.

The habitability of super-Earths in Gliese 581

Aims. The planetary system around the M star Gliese 581 consists of a hot Neptune (Gl 581b) and two super-Earths (Gl 581c and Gl 581d). The habitability of this system with respect to the super-Earths is investigated following a concept that studies the long-term possibility of photosynthetic biomass production on a dynamically active planet. Methods. A thermal evolution model for a super-Earth is used to calculate the sources and sinks of atmospheric carbon dioxide. The habitable zone is determined by the limits of photosynthetic life on the planetary surface. Models with different ratios of land / ocean coverage are investigated. Results. The super-Earth Gl 581c is clearly outside the habitable zone, since it is too close to the star. In contrast, Gl 581d is a tidally locked habitable super-Earth near the outer edge of the habitable zone. Despite the adverse conditions on this planet, at least some primitive forms of life may be able to exist on its surface. Therefore, Gl 581d is an interesting target for the planned TPF/Darwin missions to search for biomarkers in planetary atmospheres.

Further Results of TiO-Band Observations of Starspots

We present measurements of starspot parameters (temperature and filling factor) on five highly active stars, using absorption bands of TiO, from observations made between 1998 March and 2001 December. We determined starspot parameters by fitting TiO bands using spectra of inactive G and K stars as proxies for the unspotted photospheres of the active stars and spectra of M stars as proxies for the spots. For three evolved RS CVn systems, we find spot filling factors between 0.28 and 0.42 for DM UMa, 0.22 and 0.40 for IN Vir, and 0.31 and 0.35 for XX Tri; these values are similar to those found by other investigators using photometry and Doppler imaging. Among active dwarfs, we measured a lower spot temperature (3350 K) for EQ Vir than found in a previous study of TiO bands, and for EK Dra a lower spot temperature (~3800 K) than found through photometry. For all active stars but XX Tri, we achieved good phase coverage through a stellar rotational period. We also present our final, extensive grid of spot and nonspot proxy stars.

Two-Component Theoretical Chromosphere Models for K Dwarfs of Different Magnetic Activity: Exploring the Ca II Emission-Stellar Rotation Relationship

We compute two-component theoretical chromosphere models for K2 V stars with diUerent levels of magnetic activity. The two components are a nonmagnetic component heated by acoustic waves and a magnetic component heated by longitudinal tube waves. The —lling factor for the magnetic component is determined from an observational relationship between the measured magnetic area coverage and the stellar rotation period. We consider stellar rotation periods between 10 and 40 days. We investigate two diUerent geometrical distributions of magnetic —ux tubes: uniformly distributed tubes, and tubes arranged as a chromospheric network embedded in the nonmagnetic region. The chromosphere models are constructed by performing state-of-the-art calculations for the generation of acoustic and magnetic energy in stellar convection zones, the propagation and dissipation of this energy at the diUerent atmo- spheric heights, and the formation of speci—c chromospheric emission lines that are then compared to the observational data. In all these steps, the two-component structure of stellar photospheres and chromospheres is fully taken into account. We —nd that heating and chromospheric emission is signi—- cantly increased in the magnetic component and is strongest in —ux tubes that spread the least with height, expected to occur on rapidly rotating stars with high magnetic —lling factors. For stars with very slow rotation, we are able to reproduce the basal —ux limit of chromospheric emission previously identi- —ed with nonmagnetic regions. Most importantly, however, we —nd that the relationship between the Ca II H)K emission and the stellar rotation rate deduced from our models is consistent with the relationship given by observations. Subject headings: line: formationMHDstars: activitystars: chromospheres ¨ stars: late-typestars: rotation

Orbital Stability of Terrestrial Planets inside the Habitable Zones of Extrasolar Planetary Systems

We investigate orbital stability of terrestrial planets inside the habitable zones of three stellar systems, i.e., 51 Peg, 47 UMa, and HD 210277, with recently discovered giant planets. These systems have similar habitable zones; however, their giant planets have different masses and significantly different orbital parameters. It is shown that stable orbits of terrestrial planets exist in the entire habitable zone of 51 Peg as well as in the inner part of the habitable zone of 47 UMa, but no stable orbits are found in the habitable zone of HD 210277. The obtained results allow us to draw general conclusions on the existence of stable orbits in the habitable zones of newly found extra-solar planetary systems.

A critical test of empirical mass loss formulas applied to individual giants and supergiants

To test our new, improved Reimers-type mass-loss relation, given by Schroder & Cuntz in 2005 (ApJ, 630, L73), we take a look at the best studied galactic giants and supergiants – particularly those with spatially resolved circumstellar shells and winds, obtained directly or by means of a companion acting as a probing light source. Together with well-known physical parameters, the selected stars provide the most powerful and critical observational venues for assessing the validity of parameterized mass-loss relations for cool winds not driven by molecules or dust. In this study, star by star, we compare our previously published relation with the original Reimers relation (1975, Mem. Roy. Soc. Liege 6. Ser. 8, 369), the Lamers relation (1981, ApJ, 245, 593), and the two relations by de Jager and his group (1988, AA 1990, A&A, 231, 134). The input data, especially the stellar masses, have been constrained using detailed stellar evolution models. We find that only the relationship by Schroder & Cuntz agrees, within the error bars, with the observed mass-loss rates for all giants and supergiants.

Stability of planetary orbits in binary systems

Stability of S-type and P-type planetary orbits in binary systems of different mass and separation ratios is inves- tigated. Criteria for stable, marginally stable and unstable planetary orbits are specified. These criteria are used to determine regions of stability of planetary orbits in different binary systems with Jupiter-type planets. The obtained results show that the regions of stability for S-type orbits depend on the distance ratio between the star and planet, and the stellar companions, in the range of 0.22 and 0.46, depending on the mass ratio. For P-type orbits, the regions of stability also depend on that distance ratio, in the range of 1.75 and 2.45, again depending on the the mass ratio. Applications of these results to three observed binary systems with giant planets, namely, τ Boo, HD 195019 and GJ 86, show that the orbits of the giant planets in those systems can be classified as stable, as expected.

S-TYPE AND P-TYPE HABITABILITY IN STELLAR BINARY SYSTEMS: A COMPREHENSIVE APPROACH. I. METHOD AND APPLICATIONS

A comprehensive approach is provided for the study of both S-type and P-type habitability in stellar binary systems, which in principle can also be expanded to systems of higher order. P-type orbits occur when the planet orbits both binary components, whereas in the case of S-type orbits, the planet orbits only one of the binary components with the second component considered a perturbator. The selected approach encapsulates a variety of different aspects, which include: (1) the consideration of a joint constraint, including orbital stability and a habitable region for a putative system planet through the stellar radiative energy fluxes (radiative habitable zone; RHZ), needs to be met; (2) the treatment of conservative, general, and extended zones of habitability for the various systems as defined for the solar system and beyond; (3) the provision of a combined formalism for the assessment of both S-type and P-type habitability; in particular, mathematical criteria are presented for the kind of system in which S-type and P-type habitability is realized; (4) applications of the attained theoretical approach to standard (theoretical) main-sequence stars. In principle, five different cases of habitability are identified, which are S-type and P-type habitability provided by the full extent of the RHZs; habitability, where the RHZs are truncated by the additional constraint of planetary orbital stability (referred to as ST- and PT-type, respectively); and cases of no habitability at all. Regarding the treatment of planetary orbital stability, we utilize the formulae of Holman & Wiegert as also used in previous studies. In this work, we focus on binary systems in circular orbits. Future applications will also consider binary systems in elliptical orbits and provide thorough comparisons to other methods and results given in the literature.

HABITABILITY OF EARTH-MASS PLANETS AND MOONS IN THE KEPLER-16 SYSTEM

We demonstrate that habitable Earth-mass planets and moons can exist in the Kepler-16 system, known to host a Saturn-mass planet around a stellar binary, by investigating their orbital stability in the standard and extended habitable zone (HZ). We find that Earth-mass planets in satellite-like (S-type) orbits are possible within the standard HZ in direct vicinity of Kepler-16b, thus constituting habitable exomoons. However, Earth-mass planets cannot exist in planetary-like (P-type) orbits around the two stellar components within the standard HZ. Yet, P-type Earth-mass planets can exist superior to the Saturnian planet in the extended HZ pertaining to considerably enhanced back-warming in the planetary atmosphere if facilitated. We briefly discuss the potential detectability of such habitable Earth-mass moons and planets positioned in satellite and planetary orbits, respectively. The range of inferior and superior P-type orbits in the HZ is between 0.657-0.71 AU and 0.95-1.02 AU, respectively.

On the possibility of Earth-type habitable planets around 47 UMa

Abstract We investigate whether Earth-type habitable planets can in principle exist in the planetary system of 47 UMa. The system of 47 UMa consists of two Jupiter-size planets beyond the outer edge of the stellar habitable zone, and thus resembles our own Solar System most closely compared to all exosolar planetary systems discovered so far. Our study of habitability deliberately follows an Earth-based view according to the concept of Franck and colleagues, which assumes the long-term possibility of photosynthetic biomass production under geodynamic conditions. Consequently, a broad variety of climatological, biogeochemical, and geodynamical processes involved in the generation of photosynthesis-driven life conditions is taken into account. The stellar luminosity and the age of the star/planet system are of fundamental importance for planetary habitability. Our study considers different types of planetary continental growth models and takes into account a careful assessment of the stellar parameters. In the event of successful formation and orbital stability, two subjects of intense research, we find that Earth-type habitable planets around 47 UMa are in principle possible! The likelihood of those planets is increased if assumed that 47 UMa is relatively young (≲6 Gyr) and has a relatively small stellar luminosity as permitted by the observational range of those parameters.