We report on the results of the Sun in Time multi-wavelength program (X-rays to the UV) of solar analogs with ages covering ∼0.1-7 Gyr. The chief science goals are to study the solar magnetic dynamo and to determine the radiative and magnetic properties of the Sun during its evolution across the main sequence. The present paper focuses on the latter goal, which has the ultimate purpose of providing the spectral irradiance evolution of solar-type stars to be used in the study and modeling of planetary atmospheres. The results from the Sun in Time program suggest that the coronal X-ray-EUV emissions of the young main-sequence Sun were ∼100-1000 times stronger than those of the present Sun. Similarly, the transition region and chromospheric FUV-UV emissions of the young Sun are expected to be 20-60 and 10-20 times stronger, respectively, than at present. When considering the integrated high-energy emission from 1 to 1200 A the resulting relationship indicates that the solar high-energy flux was about 2.5 times the present value 2.5 Gyr ago and about 6 times the present value about 3.5 Gyr ago (when life supposedly arose on Earth). The strong radiation emissions inferred should have had major influences on the thermal structure, photochemistry, and photoionization of planetary atmospheres and also played an important role in the development of primitive life in the Solar System. Some examples of the application of the Sun in Time results on exoplanets and on early Solar System planets are discussed.

No abstract

We have used the ASCA and ROSAT X-ray satellites to probe the coronae of a sample of nine solarlike G stars. These stars are all ostensibly single with ages ranging from 70 Myr to 9 Gyr and have X-ray luminosities ranging from 1 to 500 times that of the quiet Sun. SpeciÐcally, we investigate the dependence of the coronal temperature and emission measure structure of these stars on age and rotation period.In the younger stars, a considerable portion of the volume emission measure resides at very high temperatures, reaching up to D20È30 MK in EK Dra. Such temperatures are comparable to temperatures that are achieved on the Sun during short Ñaring episodes. In two-temperature Ðts to ROSAT data, the higher temperature decays rapidly within the Ðrst few 100 Myr ; the decay may be described by an inverse power law,We also Ðnd a power-law dependence between the total X-ray lumi-T hot P age~0.3. nosity and the higher temperatureWe interpret this as evidence of a decrease in the efficiency L X P T hot 4 . of high-temperature coronal heating as a solar-like star ages and its rotation slows down. A reconstruction of the coronal di †erential emission measure (DEM) distribution in three of the stars using ASCA data indicates a bimodal distribution in temperature, with the hotter plasma at 12È30 MK and the cooler plasma below 10 MK. We infer, for the Ðrst time, a consistent evolution of the DEM structure in a solar-type star. The emission measure of the hotter component rapidly decreases with age and becomes unimportant at ages beyond D500 Myr. The emitted X-ray emission of the young Sun thus rapidly softened, which had important implications for the young planetary atmospheres. We suggest that the high-temperature component is the result of superimposed but temporally unresolved Ñaring events and support this picture by reconstructing the time-integrated (average) emission measure distribution of a typical solar X-ray Ñare. Radio observations of active stars Ðt well into this picture and suggest that the presence of nonthermal electrons in coronae is linked to the presence of hot ([10 MK) plasma, very much the same situation as in solar Ñares. We Ðnd, however, that radio emission saturates, if at all, at smaller rotation periods than does X-ray emission.

OO Dra is a short-period Algol system with a δ Sct-like pulsator. We obtained time-series spectra between 2016 February and May to derive the fundamental parameters of the binary star and to study its evolutionary scenario. The radial velocity (RV) curves for both components were presented, and the effective temperature of the hotter and more massive primary was determined to be T eff,1 = 8260 ± 210 K by comparing the disentangling spectrum and the Kurucz models. Our RV measurements were solved with the BV light curves of Zhang et al. (2014) using the Wilson-Devinney binary code. The absolute dimensions of each component are determined as follows: M 1 = 2.03 ± 0.06 M ⊙ , M 2 = 0.19 ± 0.01 M ⊙ , R 1 = 2.08 ± 0.03 R ⊙ , R 2 = 1.20 ± 0.02 R ⊙ , L 1 = 18 ± 2 L ⊙ , and L 2 = 2.0 ± 0.2 L ⊙. Comparison with stellar evolution models indicated that the primary star resides inside the δ Sct instability strip on the main sequence, while the cool secondary component is noticeably overluminous and oversized. We demonstrated that OO Dra is an oscillating post-mass transfer R CMa-type binary; the originally more massive star became the low-mass secondary component through mass loss caused by stellar wind and mass transfer, and the gainer became the pulsating primary as the result of mass accretion. The R CMa stars, such as OO Dra, are thought to have formed by non-conservative binary evolution and ultimately to evolve into EL CVn stars.

Radial velocity monitoring has found the signature of a M sin i = 1.3 M ⊕ planet located within the Habitable Zone (HZ) of Proxima Centauri (Anglada-Escudé et al. 2016). Despite a hotter past and an active host star the planet Proxima b could have retained enough volatiles to sustain surface habitability (Ribas et al. 2016). Here we use a 3D Global Climate Model (GCM) to simulate Proxima b's atmosphere and water cycle for its two likely rotation modes (1:1 and 3:2 spin-orbit resonances) while varying the unconstrained surface water inventory and atmospheric greenhouse effect. Any low-obliquity low-eccentricity planet within the HZ of its star should be in one of the climate regimes discussed here. We find that a broad range of atmospheric compositions allow surface liquid water. On a tidally-locked planet with sufficient surface water inventory, liquid water is always present, at least in the substellar region. With a non-synchronous rotation, this requires a minimum greenhouse warming (∼10 mbar of CO 2 and 1 bar of N 2). If the planet is dryer, ∼0.5 bar/1.5 bars of CO 2 (respectively for asynchronous/synchronous rotation) suffice to prevent the trapping of any arbitrary small water inventory into polar/nightside ice caps. We produce reflection/emission spectra and phase curves for the simulated climates. We find that atmospheric characterization will be possible by direct imaging with forthcoming large telescopes. The angular separation of 7λ/D at 1 µm (with the E-ELT) and a contrast of ∼10 −7 will enable high-resolution spectroscopy and the search for molecular signatures, including H 2 O, O 2 , and CO 2. The observation of thermal phase curves can be attempted with JWST, thanks to a contrast of 2 × 10 −5 at 10 µm. Proxima b will also be an exceptional target for future IR interferometers. Within a decade it will be possible to image Proxima b and possibly determine whether this exoplanet's surface is habitable.

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.

We investigate the EUV and X-ray flare rate distribution in radiated energy of the late-type active star AD Leo. Occurrence rates of solar flares have previously been found to be distributed in energy according to a power law, dN/dE ∝ E −α , with a power-law index α in the range 1.5−2.6. If α ≥ 2, then an extrapolation of the flare distribution to low flare energies may be sufficient to heat the complete observable X-ray/EUV corona.We have obtained long observations of AD Leo with the EUVE and Bep-poSAX satellites. Numerous flares have been detected, ranging over almost two orders of magnitude in their radiated energy. We compare the observed light curves with light curves synthesized from model flares that are distributed in energy according to a power law with selectable index α. Two methods are applied, the first comparing flux distributions of the binned data, and the second using the distributions of photon arrival time differences in the unbinned data (for EUVE). Subsets of the light curves are tested individually, and the quiescent flux has optionally been treated as a superposition of flares from the same flare distribution. We find acceptable α values between 2.0−2.5 for the EUVE DS and the BeppoSAX LECS data. Some variation is found depending on whether or not a strong and long-lasting flare occurring in the EUVE data is included. The BeppoSAX MECS data indicate a somewhat shallower energy distribution (smaller α) than the simultaneously observed LECS data, which is attributed to the harder range of sensitivity of the MECS detector and the increasing peak temperatures of flares with increasing total (radiative) energy. The results suggest that flares can play an important role in the energy release of this active corona. We discuss caveats related to time variability, total energy, and multiple power-law distributions. Studying the limiting case of a corona that is entirely heated by a population of flares, we derive an expression for the time-averaged coronal differential emission measure distribution (DEM) that can be used as a diagnostic for the flare energy distribution. The shape of the analytical DEM agrees with previously published DEMs from observations of active stars.

Context. Angular momentum and its interplay with magnetic fields represent a promising tool to probe the stellar internal structure and evolution of low-mass stars Aims. Our goal is to determine the rotational and magnetic-related activity properties of stars at different stages of evolution. For this reason, we have focussed 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 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 data on α Persei and the Pleiades from the literature is also considered. Methods. Rotational periods of stars showing rotational modulation due to photospheric magnetic activity (i.e. starspots) have been determined applying the Lomb-Scargle periodogram technique to photometric time-series obtained by the All Sky Automated Survey (ASAS). The magnetic activity level has been derived from the amplitude of the V lightcurves. The statistical significance of the rotational evolution at different ages has been inferred applying a twosided 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 further 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 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 of the rotation period distribution are the spin-up between 9 and 30 Myr and the spin-down between 70 and 110 Myr. Variations between 30 and 70 Myr are rather doubtful, despite the median period indicates a significant spin-up. The photospheric activity level is found to be correlated to rotation at ages greater than ∼70 Myr and to show some additional age dependence beside that related to rotation and mass.