Steven H. Saar

ACTIVITY-RELATED RADIAL VELOCITY VARIATION IN COOL STARS

Planets have been detected orbiting several solar-type stars with the use of high-precision radial velocity (vr) measurements. While changes in vr can be measured with an accuracy of a few meters per second, there has been relatively little study of how other astrophysical processes, such as magnetic activity, may affect the observed velocities. In this paper, we use published data and simple models to explore the contributions to vr from two activity-related sources, starspots and convective inhomogeneities, as these features rotate across the disk and evolve in time. Radial velocity perturbations due to both of these sources increase with rotation and the level of surface activity. Our models indicate that for solar-age G stars, the amplitude of perturbations due to spots is AS 5 m s-1, increasing to AS ~ 30-50 m s-1 for Hyades-age G stars. If fS is the starspot area coverage, we find that A -->Sf -->0.9S v sin i. The effects of convective inhomogeneities (as observed in line bisector variations) appear to depend on both rotation and spectral type. Young (active) F and G dwarfs can have convective vr perturbations with amplitudes AC 50 m s-1, while vr amplitudes are reduced for stars with lower v sin i and cooler Teff. We show that vr data from the literature display similar trends with v sin i and Teff. AS and AC will be strongest at or near timescales related to magnetic activity variations: rotation, active region growth and decay, and activity cycles. Thus, knowledge of these timescales and typical AS and AC values are important in searching for extrasolar planets, especially those around younger, more active stars or those with small vr reflex amplitudes (i.e., 20 m s-1). We discuss implications of our results for current planet detections and planet search strategies.

Time Evolution of the Magnetic Activity Cycle Period. II. Results for an Expanded Stellar Sample

We further explore nondimensional relationships between the magnetic dynamo cycle period Pcyc, the rotational period Prot, the activity level (as observed in Ca II HK), and other stellar properties by expanding the stellar sample studied in the first paper in this series. We do this by adding photometric and other cycles seen in active stars and the secondaries of CV systems and by selectively adding less certain cycles from the Mount Wilson HK survey; evolved stars, long-term HK trends and secondary Pcyc are also considered. We confirm that most stars with age t 0.1 Gyr occupy two roughly parallel branches, separated by a factor of ~6 in Pcyc, with the ratio of cycle and rotational frequencies ωcyc/Ω Ro-0.5, where Ro is the Rossby number. Using the model of the first paper in this series, this result implies that the α effect increases with mean magnetic field (contrary to the traditional α-quenching concept) and that α and ωcyc decrease with t. Stars are not strictly segregated onto one or the other branch by activity level, though the high-ωcyc/Ω branch is primarily composed of inactive stars. The expanded data set suggests that for t 1 Gyr, stars can have cycles on one or both branches, though among older stars, those with higher (lower) mass tend to have their primary Pcyc on the lower (upper) ωcyc/Ω branch. The Suns ~80 yr Gleissberg cycle agrees with this scenario, suggesting that long-term activity trends in many stars may be segments of long (Pcyc ~ 50-100 yr) cycles not yet resolved by the data. Most very active stars (Prot < 3 days) appear to occupy a new, third branch with ωcyc/Ω Ro0.4. Many RS CVn variables lie in a transition region between the two most active branches. We compare our results with various models, discuss their implications for dynamo theory and evolution, and use them to predict Pcyc for three groups: stars with long-term HK trends, stars in young open clusters, and stars that may be in Maunder-like magnetic minima.

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.

TESTING A PREDICTIVE THEORETICAL MODEL FOR THE MASS LOSS RATES OF COOL STARS

The basic mechanisms responsible for producing winds from cool, late-type stars are still largely unknown. We take inspiration from recent progress in understanding solar wind acceleration to develop a physically motivated model of the time-steady mass loss rates of cool main-sequence stars and evolved giants. This model follows the energy flux of magnetohydrodynamic turbulence from a subsurface convection zone to its eventual dissipation and escape through open magnetic flux tubes. We show how Alfven waves and turbulence can produce winds in either a hot corona or a cool extended chromosphere, and we specify the conditions that determine whether or not coronal heating occurs. These models do not utilize arbitrary normalization factors, but instead predict the mass loss rate directly from a stars fundamental properties. We take account of stellar magnetic activity by extending standard age-activity-rotation indicators to include the evolution of the filling factor of strong photospheric magnetic fields. We compared the predicted mass loss rates with observed values for 47 stars and found significantly better agreement than was obtained from the popular scaling laws of Reimers, Schroder, and Cuntz. The algorithm used to compute cool-star mass loss rates is provided as a self-contained and efficient computer code. We anticipate that the results from this kind of model can be incorporated straightforwardly into stellar evolution calculations and population synthesis techniques.

Extrasolar Giant Planets and X-Ray Activity

We have carried out a survey of X-ray emission from stars with giant planets, combining both archival and targeted surveys. Over 230 stars have been currently identified as possessing planets, and roughly one-third of these have been detected in X-rays. We carry out detailed statistical analysis on a volume-limited sample of main-sequence star systems with detected planets, comparing subsamples of stars that have close-in planets with stars that have more distant planets. This analysis reveals strong evidence that stars with close-in giant planets are on average more X-ray active by a factor of 4 than those with planets that are more distant. This result persists for various sample selections. We find that even after accounting for observational sample bias, a significant residual difference still remains. This observational result is consistent with the hypothesis that giant planets in close proximity to the primary stars influence the stellar magnetic activity.

Time Evolution of the Magnetic Activity Cycle Period

We propose a new interpretation of the relationships between the dynamo cycle period (Pcyc) as observed in Ca II H and K, the rotational period (Prot), the activity level, and other stellar properties. Viewed within this framework, the data suggest that the dynamo α-parameter increases with magnetic field strength, contrary to the conventional idea of α-quenching. The data also suggest a possibly discontinuous dependence of the ratio of cycle to rotation frequency, ωcyc/Ω, as a function of Rossby number, Ro (or equivalently, activity or age). Stars evolve with ωcyc/Ω ∝ t−0.35 (or Ro-0.7), until age t ≈ 2-3 Gyr (roughly at the Vaughan-Preston gap), where a sharp transition occurs, in which ωcyc/Ω increases by a factor of ≈ 6. Thereafter, evolution with ωcyc/Ω ∝ t−0.35 continues. The age at which transition occurs may be mass dependent, with K stars making the transition first.

Confirmation of the Planet Hypothesis for the Long-period Radial Velocity Variations of Beta Geminorum

Aims. Our aim is to confirm the nature of the long period radial velocity measurements for β Gem first found by Hatzes & Cochran (1993). Methods. We present precise stellar radial velocity measurements for the K giant star β Gem spanning over 25 years. An examination of the Ca II K emission, spectral line shapes from high resolution data (

The low-level radial velocity variability in Barnard's star (= GJ 699) - Secular acceleration, indications for convective redshift, and planet mass limits

R = 210\,000

Improved methods for the measurement and analysis of stellar magnetic fields

), and Hipparcos photometry was also made to discern the true nature of the long period radial velocity variations. Results. The radial velocity data show that the long period, low amplitude radial velocity variations found by Hatzes & Cochran (1993) are long-lived and coherent. Furthermore, the Ca II K emission, spectral line bisectors, and Hipparcos photometry show no significant variations of these quantities with the radial velocity period. An orbital solution assuming a stellar mass of 1.7

Measurements of Starspot Parameters on Active Stars using Molecular Bands in Echelle Spectra

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