Effects of XUV radiation on circumbinary planets

Several circumbinary planets have recently been discovered. The orbit of a planet around a binary stellar system poses several dynamic constraints. The effects that radiation from the host stars may have on the planet atmospheres must be considered. Because of the configuration of a close binary system, these stars have a high rotation rate, which causes a permanent state of high stellar activity and copious XUV radiation. The accumulated effects are stronger than for exoplanets around single stars, and cause a faster evaporation of their atmospheres. We evaluate the effects that stellar radiation has on the evaporation of exoplanets around binary systems and on the survival of these planets. We considered the XUV spectral range to account for the photons that are easily absorbed by a planet atmosphere that is mainly composed of hydrogen. A more complex atmospheric composition is expected to absorb this radiation more efficiently. We used direct X-ray observations to evaluate the energy in the X-rays range and coronal models to calculate the (nondetectable) EUV part of the spectrum. The simulations show that exoplanets in a close orbit will suffer strong photoevaporation that may cause a total loss of atmosphere in a short time. A binary system of two solar-like stars will be highly efficient in evaporating the atmosphere of the planet. These systems will be difficult to find, even if they are dynamically stable. Still, planets may orbit around binary systems of low mass stars for wider orbits. Currently known circumbinary planets are not substantially affected by thermal photoevaporation processes, unless Kepler-47 b has an inflated atmosphere. The distribution of the orbital periods of circumbinary planets is shifted to much longer periods than the average of Kepler planets, which supports a scenario of strong photoevaporation in close-in circumbinary planets.Comment: Accepted by A&A. 8 pages, 5 figure

The EUVE point of view of AD Leo

All the Extreme Ultraviolet Explorer (EUVE) observations of AD Leo, totalling 1.1 Ms of exposure time, have been employed to analyze the corona of this single M dwarf. The light curves show a well defined quiescent stage, and a distribution of amplitude of variability following a power law with a ~-2.4 index. The flaring behavior exhibits much similarity with other M active stars like FK Aqr or YY Gem, and flares behave differently from late type active giants and subgiants. The Emission Measure Distribution (EMD) of the summed spectrum, as well as that of quiescent and flaring stages, were obtained using a line-based method. The average EMD is dominated by material at log T(K)~6.9, with a second peak around log T(K)~6.3, and a large increase in the amount of material with log T(K)>~7.1 during flares, material almost absent during quiescence. The results are interpreted as the combination of three families of loops with maximum temperatures at log T(K)~6.3, ~6.9 and somewhere beyond log T(K)>~7.1. A value of the abundance of [Ne/Fe]=1.05+-0.08 was measured at log T(K)~5.9. No significative increment of Neon abundance was detected between quiescence and flaring states.Comment: Full PS version can be found also at http://www.astropa.unipa.it/~jsanz/papers0002.htm

Eclipsed X-ray flares in binary stars: geometrical constraints on the flare's location and size

The observation of eclipses during X-rays flares taking place in active cool stars binaries allows us to calculate the position and size of the flares. This information cannot be derived by analyzing the decay of the flares, a frequently used approach in the literature that requires the assumption of a physical model. We make use of the eclipsing light curve to constrain the set of possible solutions, from the geometrical point of view, in two flares of Algol, and one flare in VW Cep. We make use of a technique developed with the system SV Cam (i~90 deg) and generalize it to binary systems with arbitrary inclination. The method simulates all possible geometrical situations that can produce the times of the four contacts of the eclipse. As an approximation we assume that the emitting region has a spherical shape that remains unchanged during the eclipse. We however show that this is a good approximation for the problem. The solutions observed indicate that in two of the three cases the flare cannot be polar (lat<55 deg) and in a third one the flare can be placed either near the pole or at other latitudes. The emitting regions must have a small size (0.002-0.5 R_*), but if interpreted as the apex of coronal loops, their length could actually be up to 3.1 R_* for one of the Algol flares. These measurements imply a lower limit to the electron density in the emitting region between log n_e(cm^-3) 10.4 and 14.0, and a magnetic field between 70 and 3500 G. Similar results are found if the emitting region is assumed to be loop-shaped.Comment: 8 pages (17 in online version), 9 figures (18 in online version). Figure 12 is a set of animated GIF, available also at http://laeff.esa.es/users/jsanz/Papers/Eclipses/Videosweb . Accepted for publication in A&

Estimation of the XUV radiation onto close planets and their evaporation

Context: The current distribution of planet mass vs. incident stellar X-ray flux supports the idea that photoevaporation of the atmosphere may take place in close-in planets. Integrated effects have to be accounted for. A proper calculation of the mass loss rate due to photoevaporation requires to estimate the total irradiation from the whole XUV range. Aims: The purpose of this paper is to extend the analysis of the photoevaporation in planetary atmospheres from the accessible X-rays to the mostly unobserved EUV range by using the coronal models of stars to calculate the EUV contribution to the stellar spectra. The mass evolution of planets can be traced assuming that thermal losses dominate the mass loss of their atmospheres. Methods: We determine coronal models for 82 stars with exoplanets that have X-ray observations available. Then a synthetic spectrum is produced for the whole XUV range (~1-912 {\AA}). The determination of the EUV stellar flux, calibrated with real EUV data, allows us to calculate the accumulated effects of the XUV irradiation on the planet atmosphere with time, as well as the mass evolution for planets with known density. Results: We calibrate for the first time a relation of the EUV luminosity with stellar age valid for late-type stars. In a sample of 109 exoplanets, few planets with masses larger than ~1.5 Mj receive high XUV flux, suggesting that intense photoevaporation takes place in a short period of time, as previously found in X-rays. The scenario is also consistent with the observed distribution of planet masses with density. The accumulated effects of photoevaporation over time indicate that HD 209458b may have lost 0.2 Mj since an age of 20 Myr. Conclusions: Coronal radiation produces rapid photoevaporation of the atmospheres of planets close to young late-type stars. More complex models are needed to explain fully the observations.Comment: Accepted by A&A. 10 pages, 8 figures, 7 Tables (2 online). Additional online material includes 7 pages, 6 figures and 6 tables, all include

Coronal versus photospheric abundances of stars with different activity levels

We report a detailed analysis of the coronal abundance of 4 stars with varying levels of activity and with accurately known photospheric abundances. The coronal abundance is determined here using a line flux analysis and a full determination of the differential emission measure. Photospheric abundance values are taken from literature works. Previous coronal abundance determinations have generally been compared to solar photospheric abundances; from this a number of general properties have been inferred, such as the presence of a coronal metal depletion with an inverse First Ionization Potential correlated with activity level. Here we show that, when coronal abundances are compared with real photospheric values for the individual stars, the resulting pattern can be very different. Also, we present evidence that, in some cases, the coronal metal abundance may not be uniform in the corona; in particular it can vary with the temperature of the emitting plasma.Comment: 10 pages, Online material: 8 pages (online material is included in the file). Accepted by A&

The flaring and quiescent components of the solar corona

The solar corona is a template to understand stellar activity. The Sun is a moderately active star, and its corona differs from active stars: active stellar coronae have a double-peaked EM(T) with the hot peak at 8-20 MK, while the non flaring solar corona has one peak at 1-2 MK. We study the average contribution of flares to the solar EM(T) to investigate indirectly the hypothesis that the hot peak of the EM(T) of active stellar coronae is due to a large number of unresolved solar-like flares, and to infer properties on the flare distribution from nano- to macro-flares. We measure the disk-integrated time-averaged emission measure, EM_F(T), of an unbiased sample of solar flares analyzing uninterrupted GOES/XRS light curves over time intervals of one month. We obtain the EM_Q(T) of quiescent corona for the same time intervals from the Yohkoh/SXT data. To investigate how EM_F(T) and EM_Q(T) vary with the solar cycle, we evaluate them at different phases of the cycle (from Dec. 1991 to Apr. 1998). Irrespective of the solar cycle phase, EM_F(T) appears like a peak of the distribution significantly larger than the values of EM_Q(T) for T~5-10 MK. As a result the time-averaged EM(T) of the whole solar corona is double-peaked, with the hot peak, due to time-averaged flares, located at temperature similar of that of active stars, but less enhanced. The EM_F(T) shape supports the hypothesis that the hot EM(T) peak of active coronae is due to unresolved solar-like flares. If this is the case, quiescent and flare components should follow different scaling laws for increasing stellar activity. In the assumption that the heating of the corona is entirely due to flares, from nano- to macro-flares, then either the flare distribution or the confined plasma response to flares, or both, are bimodal.Comment: 8 pages, 7 postscript figures, accepted for publication in Astronomy and Astrophysic

A scenario of planet erosion by coronal radiation

Context: According to theory, high-energy emission from the coronae of cool stars can severely erode the atmospheres of orbiting planets. No observational tests of the long term effects of erosion have yet been made. Aims: To analyze the current distribution of planetary mass with X-ray irradiation of the atmospheres in order to make an observational assessment of the effects of erosion by coronal radiation. Methods: We study a large sample of planet-hosting stars with XMM-Newton, Chandra and ROSAT; make a careful identification of X-ray counterparts; and fit their spectra to make accurately measurements of the stellar X-ray flux. Results: The distribution of the planetary masses with X-ray flux suggests that erosion has taken place: most surviving massive planets, (M_p sin i >1.5 M_J), have been exposed to lower accumulated irradiation. Heavy erosion during the initial stages of stellar evolution is followed by a phase of much weaker erosion. A line dividing these two phases could be present, showing a strong dependence on planet mass. Although a larger sample will be required to establish a well-defined erosion line, the distribution found is very suggestive. Conclusions: The distribution of planetary mass with X-ray flux is consistent with a scenario in which planet atmospheres have suffered the effects of erosion by coronal X-ray and EUV emission. The erosion line is an observational constraint to models of atmospheric erosion.Comment: A&A 511, L8 (2010). 4 pages, 3 figures, 1 online table (included). Language edited; corrected a wrong unit conversion (g/s -> M_J/Gyr); corrected values in column 12 of Table 1 (slightly underestimated in first version), and Figure 2 updated accordingl

Three years in the coronal life of AB Dor. I. Plasma emission measure distributions and abundances at different activity levels

The young active star AB Dor (K1 IV-V) has been observed 16 times in the last three years with the XMM-Newton and Chandra observatories, totalling 650 ks of high-resolution X-ray spectra. The XMM/RGS observations with the highest and lowest average emission levels have been selected to study the coronal properties of AB Dor in two different activity levels. We compare the results based on the XMM data with those obtained from a higher resolution Chandra/HETG spectrum, using the same line-based analysis technique. We have reconstructed the plasma Emission Measure Distribution vs. temperature (EMD) in the range log T(K) ~ 6.1-7.6, and we have determined the coronal abundances of AB Dor, obtaining consistent results between the two instruments.The overall shape of the EMD is also consistent with the one previously inferred from EUVE data. The EMD shows a steep increase up to the peak at log T (K) ~ 6.9 and a substantial amount of plasma in the range log T (K) ~ 6.9-7.3. The coronal abundances show a clear trend of increasing depletion with respect to solar photospheric values, for elements with increasing First Ionization Potential (FIP), down to the Fe value ([Fe/H]=-0.57), followed by a more gradual recovery of the photospheric values for elements with higher FIP. He-like triplets and Fe XXI and Fe XXII lines ratios indicate electron densities log ne~10.8 cm^-3 at log T(K) ~ 6.3 and log ne ~ 12.5 at log T(K) ~ 7, implying plasma pressures steeply increasing with temperature. These results are interpreted in the framework of a corona composed of different families of magnetic loop structures, shorter than the stellar radius and in isobaric conditions, having pressures increasing with the maximum plasma temperature, and which occupy a small fraction (f ~ 10^-4 - 10^-6) of the stellar surface.Comment: Accepted by A&

Biocompatible and thermo-responsive nanocapsules through vesicle templating

Thermo-responsive and biocompatible cross-linked nanocapsules were synthesized through dimethyldioctadecylammonium bromide (DODAB) vesicle templating. For this, firstly two random copolymers, N-vinylcaprolactam (VCL) and acrylic acid (AA), with different chain lengths but using the same monomer ratio, were synthesized by reversible addition–fragmentation chain transfer (RAFT) polymerization. These anionic random copolymers were adsorbed onto cationic DODAB vesicles. Then, biocompatible and thermo-responsive nanocapsules were obtained by semicontinuous emulsion polymerization under monomer-starved conditions for both the main monomer (VCL) and the cross-linker. Although in all the cases the typical thermal behavior of PVCL-based nanocapsules was observed, hysteresis between cooling and heating cycles was observed at low temperature in the case of non-cross-linked nanocapsules. This behavior was reduced using different types and amounts of cross-linkers. In addition, transmission electron microscopy (TEM) characterization demonstrated the successful formation of nanocapsules either with short or long random copolymers. The formation of stable nanocapsules was confirmed below and above the volume phase transition temperature (VPTT) by surfactant lysis experiments through optical density and DLS measurements in all the nanocapsules synthesized. These biocompatible and thermo-responsive nanocapsules could be suitable and potentially useful as nanocarriers for drug delivery