G2 Star Research Articles

Feature-rich transmission spectrum for WASP-127b

Context. WASP-127b is a planet with one of the lowest densities discovered to date. With a sub-Saturn mass (Mp = 0.18 ± 0.02 MJ) and super-Jupiter radius (Rp = 1.37 ± 0.04 RJ), it orbits a bright G5 star that is about to leave the main-sequence.

THE FORMATION OF SECONDARY STELLAR GENERATIONS IN MASSIVE YOUNG STAR CLUSTERS FROM RAPIDLY COOLING SHOCKED STELLAR WINDS

Abstract We study a model of rapidly cooling shocked stellar winds in young massive clusters and estimate the circumstances under which secondary star formation, out of the reinserted winds from a first stellar generation (1G), is possible. We have used two implementations of the model: a highly idealized, computationally inexpensive, spherically symmetric semi-analytic model, and a complex, three-dimensional radiation-hydrodynamic, simulation; they are in a good mutual agreement. The results confirm our previous findings that, in a cluster with 1G mass 107 M ⊙ and half-mass–radius 2.38 pc, the shocked stellar winds become thermally unstable, collapse into dense gaseous structures that partially accumulate inside the cluster, self-shield against ionizing stellar radiation, and form the second generation (2G) of stars. We have used the semi-analytic model to explore a subset of the parameter space covering a wide range of the observationally poorly constrained parameters: the heating efficiency, η he, and the mass loading, η ml. The results show that the fraction of the 1G stellar winds accumulating inside the cluster can be larger than 50% if η he ≲ 10%, which is suggested by the observations. Furthermore, for low η he, the model provides a self-consistent mechanism predicting 2G stars forming only in the central zones of the cluster. Finally, we have calculated the accumulated warm gas emission in the H30α recombination line, analyzed its velocity profile, and estimated its intensity for super star clusters in interacting galaxies NGC4038/9 (Antennae) showing that the warm gas should be detectable with ALMA.

TheHubble Space TelescopeUV Legacy Survey of Galactic globular clusters – IX. The Atlas of multiple stellar populations

We use high-precision photometry of red-giant-branch (RGB) stars in 57 Galactic globular clusters (GCs), mostly from the `Hubble Space Telescope (HST) UV Legacy Survey of Galactic globular clusters', to identify and characterize their multiple stellar populations. For each cluster the pseudo two-color diagram (or `chromosome map') is presented, built with a suitable combination of stellar magnitudes in the F275W, F336W, F438W and F814W filters that maximizes the separation between multiple populations. In the chromosome map of most GCs (Type I clusters), stars separate in two distinct groups that we identify with the first (1G) and the second generation (2G). This identification is further supported by noticing that 1G stars have primordial (oxygen-rich, sodium-poor) chemical composition, whereas 2G stars are enhanced in sodium and depleted in oxygen. This 1G-2G separation is not possible for a few GCs where the two sequences have apparently merged into an extended, continuous sequence. In some GCs (Type II clusters) the 1G and/or the 2G sequences appear to be split, hence displaying more complex chromosome maps. These clusters exhibit multiple SGBs also in purely optical color-magnitude diagrams, with the fainter SGB joining into a red RGB which is populated by stars with enhanced heavy-element abundance. We measure the RGB width by using appropriate colors and pseudo-colors. When the metallicity dependence is removed, the RGB width correlates with the cluster mass. The fraction of 1G stars ranges from ~8% to ~67% and anticorrelates with the cluster mass, indicating that incidence and complexity of the multiple population phenomenon both increase with cluster mass.

WASP-120 b, WASP-122 b, and WASP-123 b: Three Newly Discovered Planets from the WASP-South Survey

We present the discovery by the WASP-South survey of three planets transiting moderately bright stars (V ≈ 11). WASP-120 b is a massive (4.85 MJup) planet in a 3.6-day orbit that we find likely to be eccentric () around an F5 star. WASP-122 b is a hot Jupiter (1.28 MJup, 1.74 RJup) in a 1.7-day orbit about a G4 star. Our predicted transit depth variation caused by the atmosphere of WASP-122 b suggests it is well suited to characterization. WASP-123 b is a hot Jupiter (0.90 MJup, 1.32 RJup) in a 3.0-day orbit around an old (∼7 Gyr) G5 star.

KELT-14b AND KELT-15b: AN INDEPENDENT DISCOVERY OF WASP-122b AND A NEW HOT JUPITER

ABSTRACT We report the discovery of KELT-14b and KELT-15b, two hot Jupiters from the KELT-South survey. KELT-14b, an independent discovery of the recently announced WASP-122b, is an inflated Jupiter mass planet that orbits a ∼ 5.0 − 0.7 + 0.3 Gyr, V = 11.0, G2 star that is near the main sequence turnoff. The host star, KELT-14 (TYC 7638-981-1), has an inferred mass M * = 1.18 − 0.07 + 0.05 M ⊙ and radius R * = 1.37 ± − 0.08 R ⊙, and has T eff = 5802 − 92 + 95 K, log g * = 4.23 − 0.04 + 0.05 and [ Fe / H ] = 0.33 ± −0.09. The planet orbits with a period of 1.7100588 ± 0.0000025 days (T 0 = 2457091.02863 ± 0.00047) and has a radius R p = 1.52 − 0.11 + 0.12 R J and mass M p = 1.196 ± 0.072 M J, and the eccentricity is consistent with zero. KELT-15b is another inflated Jupiter mass planet that orbits a ∼ 4.6 − 0.4 + 0.5 Gyr, V = 11.2, G0 star (TYC 8146-86-1) that is near the “blue hook” stage of evolution prior to the Hertzsprung gap, and has an inferred mass M * = 1.181 − 0.050 + 0.051 M ⊙ and radius R * = 1.48 − 0.04 + 0.09 R ⊙, and T eff = 6003 − 52 + 56 K, log g * = 4.17 − 0.04 + 0.02 and [ Fe / H ] = 0.05 ± 0.03. The planet orbits on a period of 3.329441 ± 0.000016 days (T 0 = 2457029.1663 ± 0.0073) and has a radius R p = 1.443 − 0.057 + 0.11 R J and mass M p = 0.91 − 0.22 + 0.21 M J and an eccentricity consistent with zero. KELT-14b has the second largest expected emission signal in the K-band for known transiting planets brighter than K < 10.5. Both KELT-14b and KELT-15b are predicted to have large enough emission signals that their secondary eclipses should be detectable using ground-based observatories.

FIRST IMAGES OF DEBRIS DISKS AROUND TWA 7, TWA 25, HD 35650, AND HD 377

ABSTRACT We present the first images of four debris disks observed in scattered light around the young (4–250 Myr old) M dwarfs TWA 7 and TWA 25, the K6 star HD 35650, and the G2 star HD 377. We obtained these images by reprocessing archival Hubble Space Telescope NICMOS coronagraph data with modern post-processing techniques as part of the Archival Legacy Investigation of Circumstellar Environments program. All four disks appear faint and compact compared with other debris disks resolved in scattered light. The disks around TWA 25, HD 35650, and HD 377 appear very inclined, while TWA 7's disk is viewed nearly face-on. The surface brightness of HD 35650's disk is strongly asymmetric. These new detections raise the number of disks resolved in scattered light around M and late-K stars from one (the AU Mic system) to four. This new sample of resolved disks enables comparative studies of heretofore scarce debris disks around low-mass stars relative to solar-type stars.

The Protective and Curative Role of Star Anise Extract against Margarine-Induced Atherosclerosis in Male Rats

The Protective and Curative Role of Star Anise Extract against Margarine-Induced Atherosclerosis in Male Rats Introduction: The diet and lifestyle of the Mediterranean populations have led to decreased rates of diabetes, cancer, and heart disease. Margarine is important constituents of our diet that contain trans fatty acids as major sources of dietary free radicals. Objective: The present study investigated the protective and curative effect of star anise extract against margarine induced hyperlipidemia in rats. Method: A total of 40 albino male rats were divided into 4 groups (10 rats for each group) as G1 (Control), G2 (Star anise), G3 (Experimental margarine group), G4 (Post treated group (Margarine + Star anise extract)). Results and Discussion: A significant increase in levels of total cholesterol, triglycerides, LDL cholesterol, Creatine kinase, Lactate dehydrogenase and Acid phosphatase, total protein and malondialdehyde in margarine group when compared with control, while HDL cholesterol and catalase data showed a significant decrease in margarine group when compared with control. Treatment with star anise extract helps in improving the adverse effect of margarine. Aorta sections of margarine group revealed abnormal straight tunica media with small subendothelial deposits of lipid enter the artery wall. There is also smooth muscle proliferation and migration from tunica media to intimae. Our histological and biochemical study confirms that; treatment with star anise extract helps in improving the adverse effect of margarine. So the present study recommends all people to keep star anise extract in all diets to be protected against atherosclerosis.

THREE WASP-SOUTH TRANSITING EXOPLANETS: WASP-74b, WASP-83b, AND WASP-89b

We report the discovery of three new transiting hot Jupiters by WASP-South together with the TRAPPIST photometer and the Euler/CORALIE spectrograph. WASP-74b orbits a star of V = 9.7, making it one of the brighter systems accessible to Southern telescopes. It is a 0.95 MJup planet with a moderately bloated radius of 1.5 RJup in a 2-d orbit around a slightly evolved F9 star. WASP-83b is a Saturn-mass planet at 0.3 MJup with a radius of 1.0 RJup. It is in a 5-d orbit around a fainter (V = 12.9) G8 star. WASP-89b is a 6 MJup planet in a 3-d orbit with an eccentricity of e = 0.2. It is thus similar to massive, eccentric planets such as XO-3b and HAT-P-2b, except that those planets orbit F stars whereas WASP-89 is a K star. The V = 13.1 host star is magnetically active, showing a rotation period of 20.2 d, while star spots are visible in the transits. There are indications that the planet’s orbit is aligned with the stellar spin. WASP-89 is a good target for an extensive study of transits of star spots. Subject headings: planetary systems — stars: individual (WASP-74, WASP-83, WASP-89)

GW Orionis: Inner disk readjustments in a triple system

(abridged) We study the young stellar system GW Ori, concentrating on its accretion/wind activity by using our high-resolution optical spectra and $U$-band photometry. We also characterize the disk properties of GW Ori by modeling its spectral energy distribution (SED). By comparing our data to the synthetical spectra, we classify GW Ori as a G8 star. Based on the RVs derived from the spectra, we confirm the previous result as a close companion in GW Ori with a period of ~242 days and an orbital semi-major axis of ~1 AU. The RV residuals after the subtraction of the orbital solution with the equivalent widths of accretion-related emission lines vary with periods of 5-6.7 days during short time intervals, which are caused by the rotational modulation. The H$\alpha$ and H$\beta$ line profiles of GW Ori can be decomposed in two central-peaked emission components and one blue-shifted absorption component. The absorption components are due to a disk wind modulated by the orbital motion of the close companion. Therefore, the systems like GW Ori can be used to study the extent of disk winds. We find that the accretion rates of GW Ori are rather constant but can occasionally be enhanced by a factor of 2-3. We reproduce the SED of GW Ori by using disk models with gaps ~25-55 AU in size. A small population of tiny dust particles within the gap produces the excess emission at near-infrared bands and the strong and sharp silicate feature at 10 $\mu$m. The SED of GW Ori exhibits dramatic changes on timescales of ~20 yr in the near-infrared bands, which can be explained as the change in the amount and distribution of small dust grains in the gap. We collect a sample of binary/multiple systems with disks in the literature and find a strong positive correlation between their gap sizes and separations from the primaries to companions, which is generally consistent with the prediction from the theory.

Transiting exoplanets from the CoRoT space mission

We report the discovery of a massive and dense transiting planet CoRoT-27b on a 3.58 day orbit around a 4.2 Gyr-old G2 star. The planet candidate was identified from the CoRoT photometry, and was confirmed as a planet with ground-based spectroscopy. The confirmation of the planet candidate is based on radial velocity observations combined with imaging to rule out blends. The characterisation of the planet and its host star is carried out using a Bayesian approach where all the data (CoRoT photometry, radial velocities, and spectroscopic characterisation of the star) are used jointly. The Bayesian analysis includes a study whether the assumption of white normally distributed noise holds for the CoRoT photometry, and whether the use of a non-normal noise distribution offers advantages in parameter estimation and model selection. CoRoT-27b has a mass of $10.39 \pm 0.55$ $\mathrm{M}_{\rm Jup}$, a radius of $1.01 \pm 0.04$ $\mathrm{R}_{\rm Jup}$, a mean density of $12.6_{-1.67}^{+1.92}$ $\mathrm{g\,cm^{-3}}$, and an effective temperature of $1500 \pm 130$ K. The planet orbits around its host star, a 4.2 Gyr-old G2-star with a mass $M_{\star}=1.06$ $M_{\odot}$, and a radius $R_{\star}=1.05$ $R_{\odot}$, on a $0.048 \pm 0.007$ AU orbit every 3.58 days. The radial velocity observations allow us to exclude highly eccentric orbits, namely, $e<0.065$ with a 99% confidence. Given its high mass and density, theoretical modelling of CoRoT-27b is demanding. We identify two solutions with heavy element mass fractions of $0.11\pm0.08$ $\mathrm{M_{\oplus}}$ and $0.07\pm0.06$ $\mathrm{M_{\oplus}}$, but even solutions void of heavy elements cannot be excluded. We carry out a secondary eclipse search from the CoRoT photometry using a method based on Bayesian model selection, but conclude that the noise level is too high to detect eclipses shallower than 9% of the transit depth.

New companions in the stellar systems of DI Cha, Sz 22, CHXR 32, and Cha Hα5 in the Chamaeleon I star-forming region

The star-forming regions in Chamaeleon (Cha) are among the nearest (distance ~165 pc) and youngest (age ~2 Myrs) conglomerates of recently formed stars and among the ideal targets for studies of star formation. We search for new, hitherto unknown binary or multiple-star components and investigate their membership in Cha and their gravitationally bound nature. We used the NACO instrument at the VLT UT 4/YEPUN of the Paranal Observatory, at 2 or 3 different epochs, in order to obtain relative and absolute astrometric measurements, as well as differential photometry in the J, H, and Ks band. On the basis of known proper motions and these observations, we analysed the astrometric results in proper motion diagrams to eliminate possible (non-moving) background stars and establish co-moving binaries and multiples. DI Cha turns out to be a quadruple system with a hierachical structure, consisting of two binaries: a G2/M6 pair and a co-moving pair of two M5.5 dwarfs. For both pairs we detected orbital motion (P~130 and ~65 years), although in opposite directions. Sz 22 is a binary whose main component is embedded in a circumstellar disc or reflection nebula, accompanied by a co-moving M4.5 dwarf. CHXR 32 is a triple system, consisting of a single G5 star, weakened by an edge-on disc and a co-moving pair of M1/M3.5 dwarfs whose components show significant variations in their angular separation. Finally, Cha Halpha 5 is a binary consisting of two unresolved M6.5 dwarfs whose strong variations in position angle at its projected separation of only 8 AU imply an orbital period of ~46 years. DI Cha D and Cha Halpha 5 A&B are right at the stellar mass limit and could possibly be brown dwarfs. In spite of various previously published studies of the star-forming regions in Cha we found four hitherto unknown components in young low-mass binaries and multiple systems. (abridged)

Transiting exoplanets from the CoRoT space mission

We report the discovery of two transiting exoplanets, CoRoT-25b and CoRoT-26b, both of low density, one of which is in the Saturn mass-regime. For each star, ground-based complementary observations through optical photometry and radial velocity measurements secured the planetary nature of the transiting body and allowed us to fully characterize them. For CoRoT-25b we found a planetary mass of 0.27 ± 0.04 M Jup , a radius of 1.08-0.10 +0.3 R Jup and hence a mean density of 0.15-0.06 +0.15 g cm-3 . The planet orbits an F9 main-sequence star in a 4.86-day period, that has a V magnitude of 15.0, solar metallicity, and an age of 4.5-2.0 +1.8 -Gyr. CoRoT-26b orbits a slightly evolved G5 star of 9.06 ± 1.5-Gyr age in a 4.20-day period that hassolar metallicity and a V magnitude of 15.8. With a mass of 0.52 ± 0.05 M Jup , a radius of 1.26-0.07 +0.13 R Jup , and a mean density of 0.28-0.07 +0.09 g cm-3 , it belongs to the low-mass hot-Jupiter population. Planetary evolution models allowed us to estimate a core mass of a few tens of Earth mass for the two planets with heavy-element mass fractions of 0.52-0.15 +0.08 and 0.26-0.08 +0.05 , respectively, assuming that a small fraction of the incoming flux is dissipated at the center of the planet. In addition, these models indicate that CoRoT-26b is anomalously large compared with what standard models could account for, indicating that dissipation from stellar heating could cause this size.

Superbubble dynamics in globular cluster infancy

The picture of the early evolution of globular clusters has been significantly revised in recent years. Current scenarios require at least two generations of stars of which the first generation (1G), and therefore also the protocluster cloud, has been much more massive than the currently predominating second generation (2G). Fast gas expulsion is thought to unbind the majority of the 1G stars. Gas expulsion is also mandatory to remove metal-enriched supernova ejecta, which are not found in the 2G stars. It has long been thought that the supernovae themselves are the agent of the gas expulsion, based on crude energetics arguments. Here, we assume that gas expulsion happens via the formation of a superbubble, and describe the kinematics by a thin-shell model. We find that supernova- driven shells are destroyed by the Rayleigh-Taylor instability before they reach escape speed for all but perhaps the least massive and most extended clusters. More power is required to expel the gas, which might plausibly be provided by a coherent onset of accretion onto the stellar remnants. The resulting kpc-sized bubbles might be observable in Faraday rotation maps with the planned Square Kilometre Array radio telescope against polarised background radio lobes if a globular cluster would happen to form in front of such a radio lobe.

The Hyper-MUCHFUSS project: probing the Galactic halo with sdB stars

High-velocity stars in the Galactic halo, e.g. the so-called hyper-velocity stars (HVS), are important tracers of the properties of the dark matter halo, in particular its mass. Based on the SDSS DR6 spectral database a search for the fastest stars among hot subdwarfs (sdB) in the halo is carried out to identify HVS, unbound to the Galaxy, and bound population II stars in order to derive a lower limit to the halo mass. The radial velocity measurements were verified at several telescopes to exclude radial velocity variable stars. Out of 88 stars observed in the follow-up campaign 39 stars were found to have constant radial velocities. For twelve of them we measured a proper motion significantly different from zero and obtained spectroscopic distances from quantitative spectral analysis to construct the full 6D phase space information for a kinematical study. The programme sdBs can be distinguished into two kinematical groups, one (G1) with low Galactic rotation typical of halo stars and a second one (G2) with rapid retrograde motion. The G1 objects crossed the Galactic plane in the central bulge, whereas the G2 stars did in the outer Galactic disc. J1211+1437 (G2) is a HVS candidate, as it is unbound to the Galaxy if the standard Galactic potential is adopted. We conclude that in the ejection scenario G1 stars might have been formed via the slingshot mechanism that invokes acceleration by tidal interaction of a binary with the central supermassive black hole. The G2 stars, however, would originate in the outskirts of the Galactic disc and not in the central bulge. J1211+1437 is the first unbound subdwarf B star, for which we can rule out the slingshot mechanism. Alternatively, we may assume that the stars are old population II stars and therefore have to be bound. Then the kinematics of J1211+1437 set a lower limit of 2 x 10^12 Msun to the mass of the Galactic dark matter halo.

FIVE LONG-PERIOD EXTRASOLAR PLANETS IN ECCENTRIC ORBITS FROM THE MAGELLAN PLANET SEARCH PROGRAM

Five new planets orbiting G and K dwarfs have emerged from the Magellan velocity survey. These companions are jovian-mass planets in eccentric (e \geq 0.24) intermediate and long-period orbits. HD 86226b orbits a solar metallicity G2 dwarf. The MP sin i mass of the planet is 1.5 MJUP, the semi-major axis is 2.6 AU, and the eccentricity 0.73. HD 129445b orbits a metal rich G6 dwarf. The minimum mass of the planet is MP sin i =1.6 MJUP, the semi-major axis is 2.9 AU, and the eccentricity 0.70. HD 164604b orbits a K2 dwarf. The MP sin i mass is 2.7 MJUP, semi-major axis is 1.3 AU, and the eccentricity is 0.24. HD 175167b orbits a metal rich G5 star. The MP sin i mass is 7.8 MJUP, the semi-major axis is 2.4 AU, and the eccentricity 0.54. HD 152079b orbits a G6 dwarf. The MP sin i mass of the planet is 3 MJUP, the semi-major axis is 3.2 AU, and the eccentricity is 0.60.

Photometric and polarimetric clues to the circumstellar environment of RY Lupi

<i>Aims. <i/>We investigate the stellar and circumstellar properties of the bright southern T Tauri star RY Lup, a G-type star showing type III variability.<i>Methods. <i/>We report simultaneous <i>BV<i/> polarimetric and <i>UBV<i/> photometric observations obtained during 12 consecutive nights on the 1.0 m and 50 cm telescopes of the European Southern Observatory at La Silla. We compare these data to models.<i>Results. <i/>The polarization is high (<i>≈<i/>3.0%) when the star is faint and red (, ), and it is low (<i>≈<i/>0.5%) when it is bright and bluer (, ). The photometric and polarimetric variations share a common period of 3.75 d. Irregular light variations, larger at shorter wavelengths, are also superposed on the cyclic variations and may be due to processes different than the one producing the periodic variations. The linear polarization is produced by dust scattering in an asymmetric (flat) circumstellar envelope. The photometric and polarimetric variations can be explained with an almost edge-on circumstellar disk that is warped close to the star, where it interacts with the star's magnetosphere. The inhomogeneous disk matter contained in the warp corotates with the star and partially occults it during part of the rotation period, which explains the dips in luminosity and the accompanying increase in polarization. All the information available on RY Lup is consistent with a system comprising a G8 star surrounded by an edge-on disk, and we find that the mass of RY Lup is , while its age is .

Detection of a Third Planet in the HD 74156 System Using the Hobby‐Eberly Telescope

We report the discovery of a third planetary mass companion to the G0 star HD 74156. High precision radial velocity measurements made with the Hobby-Eberly Telescope aided the detection of this object. The best fit triple Keplerian model to all the available velocity data yields an orbital period of 347 days and minimum mass of 0.4 M_Jup for the new planet. We determine revised orbital periods of 51.7 and 2477 days, and minimum masses of 1.9 and 8.0 M_Jup respectively for the previously known planets. Preliminary calculations indicate that the derived orbits are stable, although all three planets have significant orbital eccentricities (e = 0.64, 0.43, and 0.25). With our detection, HD 74156 becomes the eighth normal star known to host three or more planets. Further study of this system's dynamical characteristics will likely give important insight to planet formation and evolutionary processes.

The Transit Light Curve Project. VII. The Not-So-Bloated Exoplanet HAT-P-1b

We present photometry of the G0 star HAT-P-1 during six transits of its close-in giant planet, and we refine the estimates of the system parameters. Relative to Jupiter's properties, HAT-P-1b is 1.20 ± 0.05 times larger, and its surface gravity is 2.7 ± 0.2 times weaker. Although it remains the case that HAT-P-1b is among the least dense of the known sample of transiting exoplanets, its properties are in accord with previously published models of strongly irradiated, coreless, solar-composition giant planets. The times of the transits have a typical accuracy of 1 minute and do not depart significantly from a constant period.

Formation and Evolution of Planetary Systems: Upper Limits to the Gas Mass in HD 105

We report infrared spectroscopic observations of HD 105, a nearby (~40 pc) and relatively young (~30 Myr) G0 star with excess infrared continuum emission, which has been modeled as arising from an optically thin circumstellar dust disk with an inner hole of size 13 AU. We have used the high spectral resolution mode of the Infrared Spectrometer (IRS) on the Spitzer Space Telescope to search for gas emission lines from the disk. The observations reported here provide upper limits to the fluxes of H2 S(0) 28 μm, H2 S(1) 17 μm, H2 S(2) 12 μm, [Fe II] 26 μm, [Si II] 35 μm, and [S I] 25 μm infrared emission lines. The H2 line upper limits place direct constraints on the mass of warm molecular gas in the disk: M(H2) < 4.6, 3.8 × 10-2, and 3.0 × 10-3 MJ at T = 50, 100, and 200 K, respectively. We also compare the line flux upper limits to predictions from detailed thermal/chemical models of various gas distributions in the disk. These comparisons indicate that if the gas distribution has an inner hole with radius ri,gas, the surface density at that inner radius is limited to values ranging from 3 g cm-2 at ri,gas = 0.5 AU to 0.1 g cm-2 at ri,gas = 5-20 AU. These values are considerably below the value for a minimum mass solar nebula, and suggest that less than 1 Jupiter mass (MJ) of gas (at any temperature) exists in the 1-40 AU planet-forming region. Therefore, it is unlikely that there is sufficient gas for gas giant planet formation to occur in HD 105 at this time.

X-rays from α Centauri – The darkening of the solar twin

We present first results from five XMM-Newton observations of the binary system α Centauri, which has been observed in snapshot like exposures of roughly two hours each during the last two years. In all our observations the X-ray emission of the system is dominated by α Cen B, a K1 star. The derived light curves of the individual components reveal variability on short timescales and a flare was discovered on α Cen B during one observation. A PSF fitting algorithm is applied to the event distribution to determine the brightness of each component during the observations. We perform a spectral analysis with multi-temperature models to calculate the X-ray luminosities. We investigate long term variability and possible activity cycles of both stars and find the optically brighter component α Cen A, a G2 star very similar to our Sun, to have fainted in X-rays by at least an order of magnitude during the observation program, a behaviour never observed before on α Cen A, but rather similar to the X-ray behaviour observed with XMM-Newton on HD 81809. We also compare our data with earlier spatially resolved observations performed over the last 25 years.