E. F. Guinan

Atmospheric Loss of Exoplanets Resulting from Stellar X-Ray and Extreme-Ultraviolet Heating

Past studies addressing the thermal atmospheric escape of hydrogen from hot Jupiters have been based on the planets effective temperature, which, as we show here, is not physically relevant for loss processes. In consequence, these studies led to significant underestimations of the atmospheric escape rate (≤103 g s-1) and to the conclusion of long-term atmospheric stability. From more realistic exospheric temperatures, determined from X-ray and extreme-ultraviolet (XUV) irradiation and thermal conduction in the thermosphere, we find that energy-limited escape and atmospheric expansion arise, leading to much higher estimations for the loss rates (≈1012 g s-1). These fluxes are in good agreement with recent determinations for HD 209458b based on observations of its extended exosphere. We also show that for young solar-type stars, which emit stronger XUV fluxes, the inferred loss rates are significantly higher. Thus, hydrogen-rich giant exoplanets under such strong XUV irradiances may evaporate down to their core sizes or shrink to levels at which heavier atmospheric constituents may prevent hydrodynamic escape. These results could explain the apparent paucity of exoplanets so far detected at orbital distances less than 0.04 AU.

The effect of tidal locking on the magnetospheric and atmospheric evolution of "Hot Jupiters"

We study the interaction between the planetary magnetosphere and atmosphere of the close-in extrasolar planets HD 209458b and OGLE-TR-56b with the stellar wind during the evolution of their host stars. Recent astrophysical observations of solar-like stars indicate that the radiation and particle environments of young stars are orders of magnitudes larger than for stars with ages comparable to the sun (∼4.6 Gyr). We model the interaction for the present and for early evolutionary stages, showing that it is possible that Hot Jupiters have an ionosphere-stellar wind interaction like Venus, Our study suggests that the internal magnetic field of exoplanets orbiting close to their host stars may be very weak due to tidal locking. The magnetic moments can be less than one tenth of the value presently observed for the rapidly rotating planet Jupiter. We find that the stronger stellar wind of younger solar-type stars compresses the magnetosphere to a standoff distance at which the ionized part of the upper atmosphere, hydrodynamically expanded by the XUV-flux, builds an obstacle. Because of a much larger stellar wind particle flux during the first ∼0.5 Gyr after the host stars arrived on the Zero-Age-Main-Sequence, Hot Jupiters may have not been protected by their intrinsic magnetic fields, even if one neglects the effect of tidal locking. In such a case, the unshielded upper atmosphere will be affected by different ionization and non-thermal ion loss processes. This contributes to the estimated neutral hydrogen loss rates of about ≥10 10 g/s of the observed expanded exosphere of HD 209458b (Vidal-Madjar et al. 2003) and will be an ionized part of the estimated upper energy-limited neutral hydrogen loss rates of about 10 12 g/s (Lammer et al. 2003a).

YSOVAR: THE FIRST SENSITIVE, WIDE-AREA, MID-INFRARED PHOTOMETRIC MONITORING OF THE ORION NEBULA CLUSTER

We present initial results from time-series imaging at infrared wavelengths of 0.9 deg^2 in the Orion Nebula Cluster (ONC). During Fall 2009 we obtained 81 epochs of Spitzer 3.6 and 4.5 μm data over 40 consecutive days. We extracted light curves with ~3% photometric accuracy for ~2000 ONC members ranging from several solar masses down to well below the hydrogen-burning mass limit. For many of the stars, we also have time-series photometry obtained at optical (I_c) and/or near-infrared (JK_s ) wavelengths. Our data set can be mined to determine stellar rotation periods, identify new pre-main-sequence eclipsing binaries, search for new substellar Orion members, and help better determine the frequency of circumstellar disks as a function of stellar mass in the ONC. Our primary focus is the unique ability of 3.6 and 4.5 μm variability information to improve our understanding of inner disk processes and structure in the Class I and II young stellar objects (YSOs). In this paper, we provide a brief overview of the YSOVAR Orion data obtained in Fall 2009 and highlight our light curves for AA-Tau analogs—YSOs with narrow dips in flux, most probably due to disk density structures passing through our line of sight. Detailed follow-up observations are needed in order to better quantify the nature of the obscuring bodies and what this implies for the structure of the inner disks of YSOs.

RACE-OC project: Rotation and variability of young stellar associations within 100 pc

Context. Examining the angular momentum of stars and its interplay with their magnetic fields represent a promising way to probe the stellar internal structure and evolution of low-mass stars. Aims. We attempt to determine the rotational and magnetic-related activity properties of stars at different stages of evolution.We focused our attention primarily on members of clusters and young stellar associations of known ages. In this study, our targets are 6 young loose stellar associations within 100 pc and with ages in the range 8-70 Myr: TW Hydrae (~8 Myr), β Pictoris (~10 Myr), Tucana/Horologium, Columba, Carina (~30 Myr), and AB Doradus (~70 Myr). Additional rotational data for α Persei and the Pleiades from the literature are also considered. Methods. Rotational periods of stars exhibiting rotational modulation due to photospheric magnetic activity (i.e., starspots) were determined by applying the Lomb-Scargle periodogram technique to photometric time-series data obtained by the All Sky Automated Survey (ASAS). The magnetic activity level was derived from the amplitude of the V lightcurves. The statistical significance of the rotational evolution at different ages was inferred by applying a two-sided Kolmogorov-Smirnov test to subsequent age-bins. Results. We detected the rotational modulation and measured the rotation periods of 93 stars for the first time, and confirmed the periods of 41 stars already known from the literature. For an additional 10 stars, we revised the period determinations by other authors. The sample was augmented with periods of 21 additional stars retrieved from the literature. In this way, for the first time we were able to determine the largest set of rotation periods at ages of ~8, ~10 and ~30 Myr, as well as increase by 150% the number of known periodic members of AB Dor. Conclusions. The analysis of the rotation periods in young stellar associations, supplemented by Orion Nebula Cluster (ONC) and NGC 2264 data from the literature, has allowed us to find that in the 0.6-1.2 M ⊙ range the most significant variations in the rotation period distributipn are the spin-up between 9 and 30 Myr and the spin-down between 70 and 110 Myr. Variations of between 30 and 70 Myr are rather doubtful, despite the median period indicating a significant spin-up. The photospheric activity level is found to be correlated with rotation at ages greater than ~70 Myr and to show some additional age dependence besides that related to rotation and mass.

Fundamental Properties and Distances of the Large Magellanic Cloud from Eclipsing Binaries. II. HV 982

We have determined the distance to a second eclipsing binary (EB) system in the Large Magellanic Cloud, HV 982 (DB1 IVV ) DB1 IVV). The measurement of the distanceamong other properties of the systemis based on optical photometry and spectroscopy and space-based UV/optical spectropho- tometry. The analysis combines the ii classical ˇˇ EB study of light and radial velocity curves, which yields the stellar masses and radii, with a new analysis of the observed energy distribution, which yields the eUective temperature, metallicity, and reddening of the system plus the distance ii attenuation factor,ˇˇ essentially (radius/distance)2. Combining the results gives the distance to HV 982, which is 50.2 ^ 1.2 kpc. This distance determination consists of a detailed study of well-understood objects (B stars) in a well-understood evolutionary phase (core H burning). The results are entirely consistent withbut do not depend onstellar evolution calculations. There are no ii zero-point ˇˇ uncertainties as, for example, with the use of Cepheid variables. Neither is the result subject to sampling biases, as may aUect tech- niques that utilize whole stellar populations, such as red giant branch stars. Moreover, the analysis is insensitive to stellar metallicity (although the metallicity of the stars is explicitly determined), and the eUects of interstellar extinction are determined for each object studied. After correcting for the location of HV 982, we —nd an implied distance to the optical center of the LMCs bar of kpc. d LMC \ 50.7 ^ 1.2 This result diUers by nearly 5 kpc from our earlier result for the EB HV 2274, which implies a bar distance of 45.9 kpc. These results may either re—ect marginally compatible measures of a unique LMC distance or, alternatively, suggest a signi—cant depth to the stellar distribution in the LMC. Some evi- dence for this latter hypothesis is discussed. Subject headings: binaries: eclipsingdistance scaleMagellanic Cloudsstars: distances ¨ stars: fundamental parametersstars: individual (HV 982)

Intrinsic Properties of the Young Stellar Object SU Aurigae

Intensive long-term photometric Q1 observations of the archetypical young G2 IIIe stellar object SU Aurigae are analyzed to determine many of its photometric and physical properties. Combining nearly 2000 Stromgren uvby measures obtained using the 0.8 m Four College Automatic Photoelectric Telescope, previously published photometry, and recently obtained high-resolution echelle spectra has led to the deter- mination of its effective temperature, surface gravity, luminosity, mass, age, rotation period, and absolute radius. Since the disk of SU Aur is seen at a high inclination angle (i.e., nearly edge-on), this complicates the observations with significant (DVDyd0:80 mag), and apparently random, drops in observed mean light. These are possibly due to the transits of protoplanetary bodies, protocomets, or associated accretion halos. In this study, an effort has been made to separate the contributions of the circumstellar disk from the intrinsic properties of the stellar core itself. Furthermore, photometry has been simultaneously obtained for the nearby young A0 Vpe stellar companion AB Aurigae. Analysis has shown that SU Aur and AB Aur are a coeval proper-motion pair. Also included are certain photometric and physical properties of AB Aur. Q2

Dependence of coronal X-ray emission on spot-induced brightness variations in cool main sequence stars

The maximum amplitude ( A max ) of spot-induced brightness variations from long-term V -band photometry and the ratio L X / L bol between X-ray and bolometric luminosities are suitable indicators of the level of magnetic activity in the photosphere and in the corona of late-type stars, respectively. By using these activity indicators we investigate the dependence of coronal X-ray emission on the level of photospheric starspot activity in a homogeneous sample of low mass main sequence field and cluster stars of different ages (IC 2602, IC 4665, IC 2391, α Persei, Pleiades and Hyades). First, the activity-rotation connection at the photospheric level is re-analysed, as well as its dependence on spectral type and age. The upper envelope of

Artificial Intelligence Approach to the Determination of Physical Properties of Eclipsing Binaries. I. The EBAI Project

A_{\rm max}

RACE-OC project: rotation and variability in the ϵ Chamaeleontis, Octans, and Argus stellar associations

increases monotonically with decreasing rotational period ( P ) and Rossby number ( R 0 ) showing a break around 1.1 d that separates two rotation regimes where the starspot activity shows different behaviours. The

THE PERIOD CHANGE OF THE CEPHEID POLARIS SUGGESTS ENHANCED MASS LOSS

A_{\rm max}