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The physical properties of transiting exoplanets are connected with the physical properties of their host stars. We present a homogeneous spectroscopic analysis based on the spectra of FGK-type stars observed with the Hydra spectrograph on the WIYN telescope. We derived the effective temperatures, surface gravities, and metallicities, for 81 stars observed by K2 and 33 by Kepler 1. We constructed an Fe i and ii line list that is adequate for the analysis of R ∼ 18,000 spectra covering 6050–6350 Å and adopted the spectroscopic technique based on equivalent-width measurements. The calculations were done in LTE using Kurucz model atmospheres and the qoyllur-quipu (q 2) package. We validated our methodology via an analysis of a benchmark solar twin and solar proxies, which are used as a solar reference. We estimated the effects that including Zeeman-sensitive Fe i lines have on the derived stellar parameters for young and possibly active stars in our sample and found them not to be significant. Stellar masses and radii were derived by combining the stellar parameters with Gaia EDR3 and V magnitudes and isochrones. The measured stellar radii have a 4.2% median internal precision, leading to a median internal uncertainty of 4.4% in the derived planetary radii. With our sample of 83 confirmed planets orbiting K2 host stars, the radius gap near R planet ∼ 1.9 R ⊕ is detected, in agreement with previous findings. Relations between the planetary radius, orbital period, and metallicity are explored and these also confirm previous findings for Kepler 1 systems. [ABSTRACT FROM AUTHOR]

The physical properties of transiting exoplanets are connected with the physical properties of their host stars. We present a homogeneous spectroscopic analysis based on the spectra of FGK-type stars observed with the Hydra spectrograph on the WIYN telescope. We derived the effective temperatures, surface gravities, and metallicities, for 81 stars observed by K2 and 33 by Kepler 1. We constructed an Fe i and ii line list that is adequate for the analysis of R ∼ 18,000 spectra covering 6050–6350 Å and adopted the spectroscopic technique based on equivalent-width measurements. The calculations were done in LTE using Kurucz model atmospheres and the qoyllur-quipu ( q ^2 ) package. We validated our methodology via an analysis of a benchmark solar twin and solar proxies, which are used as a solar reference. We estimated the effects that including Zeeman-sensitive Fe i lines have on the derived stellar parameters for young and possibly active stars in our sample and found them not to be significant. Stellar masses and radii were derived by combining the stellar parameters with Gaia EDR3 and V magnitudes and isochrones. The measured stellar radii have a 4.2% median internal precision, leading to a median internal uncertainty of 4.4% in the derived planetary radii. With our sample of 83 confirmed planets orbiting K2 host stars, the radius gap near R _planet ∼ 1.9 R _⊕ is detected, in agreement with previous findings. Relations between the planetary radius, orbital period, and metallicity are explored and these also confirm previous findings for Kepler 1 systems.

We present high cadence detections of two superflares from a bright G8 star (V = 11.56) with the Next Generation Transit Survey (NGTS).We improve upon previous super are detections by resolving the flare rise and peak, allowing us to fit a solar flare inspired model without the need for arbitrary break points between rise and decay. Our data also enables us to identify substructure in the flares. From changing star-spot modulation in the NGTS data, we detect a stellar rotation period of 59 h, along with evidence for differential rotation. We combine this rotation period with the observed ROSATX-ray flux to determine that the star's X-ray activity is saturated. We calculate the flare bolometric energies as 5.4+0.8-0.7 x 1034 and 2.6+0.4-0.3 x 1034 erg and compare our detections with G star superflares detected in the Kepler survey. We find our main flare to be one of the largest amplitude superflares detected from a bright G star. With energies more than 100 times greater than the Carrington event, our flare detections demonstrate the role that ground-based instruments such as NGTS can have in assessing the habitability of Earth-like exoplanets, particularly in the era of PLATO. [ABSTRACT FROM AUTHOR]

It is known that the strength of neutral oxygen triplet lines at 7771-5 Å shows a luminosity effect in evolved A through G stars. However, its general behavior across the HR diagram is not yet well understood, since the applicability limit of the relations proposed by various previous work (tending to be biased toward supergiants) still remains unclear. Besides, our understanding on the nature of atmospheric micro-scale turbulence, which is considered to play a significant role (along with the non-LTE line intensification) for the cause of this effect, is still insufficient. Towards clarifying these problems, we carried out an extensive non-LTE spectrum-fitting analysis of O I 7771-5 lines for unbiased sample of 75 evolved A-, F,- and G-type stars over wide luminosity classes (from subgiants through supergiants) including rapid rotators, from which the total equivalent width (W77) was derived and the microturbulence (ξ) was determined by two different (profileand abundance-based) methods for each star. While we confirmed that W77 tends to increase in the global sense as a star's absolutemagnitude (MV) becomesmore luminous, distinctly different trends were found between lower-gravity (log g ≤ 2.5) and highergravity (log g ≥ 2.5) stars, in the sense that the MV vs. W77 formulas proposed by past studies are applicable only to the former supergiant group. In case of using W77 for empirical MV evaluation by such simple formulas, it is recommended to confine only to supergiants of -5 ≥ MV ≥ -10. Regarding the microturbulence significantly controlling W77, it roughly shows an increasing tendencywith a decrease in surface gravity. However, the trend is not monotonic but rather intricate (e.g., hump, stagnation, or discontinuously large increase) depending on the stellar type and evolutionary stage. [ABSTRACT FROM AUTHOR]

We present the results of our observations of the maser radio emission source G188.946+0.886 in hydroxyl (OH) molecular lines with the radio telescope of the Nançay Observatory (France) and in the HO line at λ = 1.35 cm with the RT-22 radio telescope at the Pushchino Observatory (Russia). An emission feature in the 1720-MHz satellite line of the OH ground state has been detected for the first time. The radial velocity of the feature, V = 3.6 km s, has a 'blue' shift relative to the range of emission velocities in the main 1665- and 1667-MHz OH lines, which is 8-11 km s. This suggests a probable connection of the observed feature in the 1720-MHz line with the 'blue' wing of the bipolar outflow observed in this region in the CO line. We have estimated the magnetic field strength for three features (0.90 and 0.8 mG for 1665 MHz and 0.25 mG for 1720 MHz) from the Zeeman splitting in the 1665- and 1720-MHz lines. No emission and (or) absorption has been detected in the other 1612-MHz satellite OH line. Three cycles of HO maser activity have been revealed. The variability is quasi-periodic in pattern. There is a general tendency for the maser activity to decrease. Some clusters of H2O maser spots can form organized structures, for example, chains and other forms. [ABSTRACT FROM AUTHOR]

Aims. We investigate the magnetic field at the surface of 48 red giants selected as promising for detection of Stokes V Zeeman signatures in their spectral lines. In our sample, 24 stars are identified from the literature as presenting moderate to strong signs of magnetic activity. An additional 7 stars are identified as those in which thermohaline mixing appears not to have occured, which could be due to hosting a strong magnetic field. Finally, we observed 17 additional very bright stars which enable a sensitive search to be performed with the spectropolarimetric technique. Methods. We use the spectropolarimeters Narval and ESPaDOnS to detect circular polarization within the photospheric absorption lines of our targets. We treat the spectropolarimetric data using the least-squares deconvolution method to create high signal-to-noise ratio mean Stokes V profiles. We also measure the classical S-index activity indicator for the Ca Π H&K lines, and the stellar radial velocity. To infer the evolutionary status of our giants and to interpret our results, we use state-of-the-art stellar evolutionary models with predictions of convective turnover times. Results. We unambiguously detect magnetic fields via Zeeman signatures in 29 of the 48 red giants in our sample. Zeeman signatures are found in all but one of the 24 red giants exhibiting signs of activity, as well as 6 out of 17 bright giant stars. However no detections were obtained in the 7 thermohaline deviant giants. The majority of the magnetically detected giants are either in the first dredge up phase or at the beginning of core He burning, i.e. phases when the convective turnover time is at a maximum: this corresponds to a "magnetic strip" for red giants in the Hertzsprung-Russell diagram. A close study of the 16 giants with known rotational periods shows that the measured magnetic field strength is tightly correlated with the rotational properties, namely to the rotational period and to the Rossby number Ro. Our results show that the magnetic fields of these giants are produced by a dynamo, possibly of α-ω origin since Ro is in general smaller than unity. Four stars for which the magnetic field is measured to be outstandingly strong with respect to that expected from the rotational period/magnetic field relation or their evolutionary status are interpreted as being probable descendants of magnetic Ap stars. In addition to the weak-field giant Pollux, 4 bright giants (Aldebaran, Alphard, Arcturus, η Psc) are detected with magnetic field strength at the sub-Gauss level. Besides Arcturus, these stars were not considered to be active giants before this study and are very similar in other respects to ordinary giants, with S-index indicating consistency with basal chromospheric flux. [ABSTRACT FROM AUTHOR]

Aims. We assess the importance of tidal evolution and its interplay with magnetic braking in the population of hot-Jupiter planetary systems. Methods. By minimizing the total mechanical energy of a given system under the constraint of stellar angular momentum loss, we rigorously find the conditions for the existence of dynamical equilibrium states. We estimate their duration, in particular when the wind torque spinning down the star is almost compensated for by the tidal torque spinning it up. We introduce dimensionless variables to characterize the tidal evolution of observed hot Jupiter systems and discuss their spin and orbital states using generalized Darwin diagrams based on our new approach Results. We show that their orbital properties are related to the effective temperature of their host stars. The long-term evolution of planets orbiting F- and G-type stars is significantly different owing to the combined effect of magnetic braking and tidal dissipation. The existence of a quasi-stationary state, in the case of short-period planets, can significantly delay their tidal evolution that would otherwise bring the planet to fall into its host star. Most of the planets known to orbit F-type stars are presently found to be near this stationary state, probably in a configuration not too far from what they had when their host star settled on the zero-age main sequence. Considering the importance of angular momentum loss in the early stages of stellar evolution, our results indicate that it has to be considered to properly test the migration scenarios of planetary system formation. [ABSTRACT FROM AUTHOR]

Modern observations of magnetic fields confirm basic ideas obtained earlier by A.B. Severny about field changes that occur over the course of a flare. For interpretation of new data of the magnetic field vector, the methods of the non-linear force-free field (NLFFF) extrapolation were developed. As a test of these methods, we carried out the NLFFF extrapolation for the active region AR 11158 (February 2011) using the algorithm by Rudenko and Myshyakov (2009). This NLFFF approximation gives information on the magnetic field structure in the corona of active regions (AR). When considering the only total energy of non-stationary processes, it is possible to shift from the equations describing the structural features to considering the magnetic virial theorem. We obtain new analytic expression for the free energy of the solar corona as a whole and discuss circumstances, which prevent using this analytic expression as a measure of the free energy of a size-limited AR, when magnetic fields of the photosphere of AR are not force-free. Observations and NLFFF extrapolation give the upper estimate of the energy of flares, which are able to occur in a given large active region. For the Sun, this value does not exceed 3 × 10 32 ergs. Because the average longitudinal magnetic fields of G stars (at an age of 1–2 Gyr) are ten-times stronger than the maximum magnetic field of the Sun as a star, the energy of solar-type stellar flares can only slightly exceed 10 34 ergs. These flares are associated with the energy deposit above the chromosphere and its subsequent release. Phenomena with the total energy up to 10 36 ergs which were registered also on the younger G stars have the nature which differs substantially from those non-stationary processes occurring on the Sun. [ABSTRACT FROM AUTHOR]

We discuss basic properties of the solar activity in the modern epoch and the further development of ideas about estimate the age of stars by the level of activity (gyrochronology). From comparison of properties of solar activity and processes on other G stars we have determined activity levels of the chromosphere and corona of the young Sun in its age of 1-2 Gyr. So, activity of the Sun in such an age is closest to the processes in the G stars, HD 152391 (V2292 Oph) and HD 1835 (BE Cet) with the rotational periods of 11 and 8 days correspondingly. The total spot area is of 2-3 orders of magnitude higher than the current value at the maximum of the solar cycle, the activity levels of the chromosphere and the corona are quite high and similar to the Hyades, but it did not reach the saturation level. We have analyzed new measurements of longitudinal magnetic fields and distribution of the toroidal and poloidal fields on the G main sequence stars. The mean value of the longitudinal fields for active G stars is of one order of magnitudes stronger than the mean daily magnetic field of the Sun as a star, for instance those during the 21th cycle maximum. Besides, the detection of large-scale toroidal magnetic field in such stars is important. Taking into account the activity levels and magnetic fields, we analyzed data on superflares on G stars registered with the Kepler mission. We have defined the upper limit of energy of flares that can occur in the magnetic fields of active G stars. We evaluated the frequency of flares with the total energy 10 erg on active G stars: one flare every 5 years and one flare in 500 years for the stars with periods of about 3 days and about 12-15 days, respectively. This determines the range of the frequency of occurrence of such events for the young Sun. If flares of this energy could still be the result of evolution of the magnetic fields, the nature of the more powerful events should differ significantly from flares that occur in the present era. We evaluate the mass loss of the young Sun as much as 10 M/year, and the contribution of CME in this value was significantly higher than it is now. [ABSTRACT FROM AUTHOR]

The entire evolution of the Milky Way, including its mass-assembly and star-formation history, is imprinted onto the chemo-dynamical distribution function of its member stars, f(x, v, [X/H]), in the multi-dimensional phase space spanned by position, velocity, and elemental abundance ratios. In particular, the chemo-dynamical distribution functions for low-mass stars (e.g., G- or K-type dwarfs) are precious tracers of the earliest stages of the Milky Way's formation, since their main-sequence lifetimes approach or exceed the age of the universe. A basic tenet of essentially all previous analyses is that the stellar metallicity, usually parameterized as [Fe/H], is conserved over time for main-sequence stars (at least those that have not been polluted due to mass transfer from binary companions). If this holds true, any correlations between metallicity and kinematics for long-lived main-sequence stars of different masses, effective temperatures, or spectral types must strictly be the same, since they reflect the same mass-assembly and star-formation histories. By analyzing a sample of nearby metal-poor halo and thick-disk stars on the main sequence, taken from Data Release 8 of the Sloan Digital Sky Survey, we find that the median metallicity of G-type dwarfs is systematically higher (by about 0.2 dex) than that of K-type dwarfs having the same median rotational velocity about the Galactic center. If it can be confirmed, this finding may invalidate the long-accepted assumption that the atmospheric metallicities of long-lived stars are conserved over time. [ABSTRACT FROM AUTHOR]

Extensive X-ray and EUV photometric observations of the eclipsing RS CVn system AR Lac were obtained over the years 1997-2013 with the Chandra X-Ray ObservatoryExtreme-Ultraviolet Explorer (EUVE). During primary eclipse, High Resolution Camera count rates decrease by ∼40%. A similar minimum is seen during one primary eclipse observed by EUVE but not in others owing to intrinsic source variability. Little evidence for secondary eclipses is present in either the X-ray or EUV data, reminiscent of earlier X-ray and EUV observations. Primary eclipses allow us to estimate the extent of a spherically symmetric corona on the primary G star of about 1.3 R☼, or 0.86 R⋆, and indicate that the G star is likely brighter than the K component by a factor of 2-5. Brightness changes not attributable to eclipses appear to be dominated by stochastic variability and are generally non-repeating. X-ray and EUV light curves cannot therefore be reliably used to reconstruct the spatial distribution of emission assuming that only eclipses and rotational modulation are at work. Moderate flaring is observed, where count rates increase by up to a factor of three above quiescence. Combined with older ASCA, Einstein, EXOSAT, ROSAT, and BeppoSAX observations, the data show that the level of quiescent coronal emission at X-ray wavelengths has remained remarkably constant over 33 yr, with no sign of variation due to magnetic cycles. Variations in base level X-ray emission seen by Chandra over 13 yr are only ∼10%, while variations back to pioneering Einstein observations in 1980 amount to a maximum of 45% and more typically about 15%. [ABSTRACT FROM AUTHOR]

The origin of solar-type activity for low-mass stars of late spectral types is considered. Spectroscopic data were used to study the dependence of the activity level log R′ HK on the lithium abundance log A(Li) and axial rotation rate. A close correlation between log A(Li) and log R′ HK is found for two groups of G stars, hotter and cooler than the Sun. This relation is most clearly expressed in the case of high activity, and is somewhat more strongly expressed for G6-K3 dwarfs, which includes many BY Dra variables, than for F8-G5 stars. It is confirmed that, for stars with high activity, both the lithium abundance and the activity level are determined by the rotation rate, which depends on the age. The lithium abundance exhibits different dependences on the chromospheric activity, depending on the level of this activity. Cooler stars, with detectable lithium and solar-like chromospheres, possess much stronger coronas. This change in the relationship between the relative luminosities of the chromosphere and corona can be reliably traced using larger datasets. The different ratios between the activity of the choromosphere and corona for cooler and hotter G stars may reflect the fact that their convective zones become deeper or shallower than some critical value. This is consistent with observations of parameters describing rotational modulation and the correlation and anti-correlation of chromospheric and photospheric activity indices for stars hotter and cooler than the Sun. Physically, this means that the character of the activity could be related to a changing contribution of the large-scale and local magnetic fields to the generation of the activity. The results of this study confirm the earlier idea that there may be different evolution paths associated with solar-type activity. The results can be used to refine methods for estimating ages of stars from their activity levels (gyrochronology). [ABSTRACT FROM AUTHOR]

We determined the chemical composition of a large sample of weak G band stars-a rare class of G and K giants of intermediate mass with unusual abundances of C, N, and Li.We have observed 24 weak G band stars with the 2.7 m Harlan J. Smith Telescope at theMcDonald Observatory and derived spectroscopic abundances for C, N, O, and Li, as well as for selected elements from Na-Eu. The results show that the atmospheres of weak G band stars are highly contaminated with CN-cycle products. The C underabundance is about a factor of 20 larger than for normal giants and the 12C/13C ratio approaches the CN-cycle equilibrium value. In addition to the striking CN-cycle signature the strong N overabundance may indicate the presence of partially ON-cycled material in the atmospheres of the weak G band stars. The exact mechanism responsible for the transport of the elements to the surface has yet to be identified but could be induced by rapid rotation of the main sequence progenitors of the stars. The unusually high Li abundances in some of the stars are an indicator for Li production by the Cameron-Fowler mechanism. A quantitative prediction of a weak G band star's Li abundance is complicated by the strong temperature sensitivity of the mechanism and its participants. In addition to the unusual abundances of CN-cycle elements and Li, we find an overabundance of Na that is in accordance with the NeNa chain running in parallel with the CN cycle. Apart from these peculiarities, the element abundances in a weak G band star's atmosphere are consistent with those of normal giants. [ABSTRACT FROM AUTHOR]

Context. Surveys of binary populations in the solar neighbourhood have discovered that the periods of G- and M-type stars are log-normally distributed in the range 0.1-1011 days. However, observations of young binary populations in various star-forming regions have instead inferred a log-uniform distribution. Some process(es) must clearly be responsible for this change in the period distribution over time. Most stars form in star clusters, so it is here that the(se) process(es) take place. Aims. In dense young clusters, two important dynamical processes occur: i) the gas-induced orbital decay of embedded binary systems and ii) the destruction of soft binaries in three-body interactions. The emphasis in this work is on orbital decay as its influence on the binary distribution in clustered environments has largely been neglected so far. Methods. We performed Monte Carlo simulations of binary populations to model the process of orbital decay due to friction between the gas and binary stars. In addition, the destruction of soft binaries in young dense star clusters was simulated using N-body modelling of binary populations. Results. It is known that the cluster dynamics destroy the number of wide binaries, but leave short-period binaries basically undisturbed. Here we demonstrate that this result is also valid for an initially log-uniform period binary distribution. In contrast, the process of orbital decay significantly reduces the number and changes the properties of short-period binaries, leaving wide binaries largely uneffected. Until now, it has been unclear whether the short period distribution of the field has remained unaltered since its formation. We show here, that if any alteration took place, then orbital decay is a prime candidate for this task. In combination, the dynamics of these two processes, convert even an initial log-uniform distribution into a log-normal period distribution. The probability is 94% that the evolved period distribution and the observed period distribution have been sampled from the same parent distribution. Conclusions. Our results provide a new picture for the development of the field binary population: binaries can be formed as a result of the star-formation process in star clusters with periods that are sampled from the log-uniform distribution. As the cluster evolves, short-period binaries merge to form single stars by means of gas-induced orbital decay, while the dynamical evolution in the cluster destroys wide binaries. The combination of these two equally important processes reshapes an initial log-uniform period distribution to the log-normal period distribution that is observed in the field. [ABSTRACT FROM AUTHOR]

Context. While we have ample evidence of stellar analogues to the solar activity cycle for chromospheric activity, very little is known about stellar coronal cycles and their possible similarities to the solar behavior. Aims. An ongoing X-ray monitoring program of solar-like stars with XMM-Newton is performed to investigate coronal activity cycles. Methods. We used X-ray observations of the nearby binaries 61 Cyg A/B (K5V and K7V) and αCen A/B (G2V and K1V) to study the long-term evolution of magnetic activity in weakly to moderately active G + K dwarfs over nearly a decade. Specifically we searched for X-ray activity cycles and related coronal changes and compared them to the solar behavior. Results. For 61Cyg A we find a regular coronal activity cycle analog to its 7.3 yr chromospheric cycle. The X-ray brightness variations are with a factor of three significantly lower than on the Sun, yet the changes of coronal properties resemble the solar behavior, with stronger variations occurring in the respective hotter plasma components. 61Cyg B does not show a clear cyclic coronal trend so far, but the X-ray data match the more irregular chromospheric cycle. The two αCen stars exhibit significant long-term X-ray variability. αCen A shows indications for cyclic variability of an order of magnitude with a period of about 12-15 years; the αCen B data suggest an X-ray cycle with an amplitude of about six to eight and a period of 8-9 years. The sample stars exhibit X-ray luminosities ranging between LX ≤ 1 × 1026 - 3 × 1027 erg s-1 in the 0.2-2.0 keV band and have coronae dominated by cool plasma with variable average temperatures of around 1.0 - 2.5 MK. Conclusions. Coronal activity cycles are apparently a common phenomenon in older, slowly rotating G and K stars. The spectral changes of the coronal X-ray emission over the cycles are solar-like in all studied targets. [ABSTRACT FROM AUTHOR]

ABSTRACT We have discovered a dwarf nova (DN) of type SU UMa in Kepler data which is 7.0 arcsec from the G-type exoplanet survey target KIC 4378554. The DN appears as a background source in the pixel aperture of the foreground G star. We extracted only the pixels where the DN is present and observed the source to undergo five outbursts - one a superoutburst - over a time span of 22 months. The superoutburst was triggered by a normal outburst, a feature that has been seen in all DNe superoutbursts observed by Kepler. Superhumps during the superoutburst had a period of 1.842 ± 0.004 h, and we see a transition from disc-dominated superhump signal to a mix of disc and accretion stream impact. Predictions of the number of DNe present in Kepler data based on previously published space densities vary from 0.3 to 258. An investigation of the background pixel targets would lead to firmer constraints on the space density of DNe. [ABSTRACT FROM AUTHOR]

We present the photometry and spectropolarimetry of the pre-main-sequence star HD 106506. A photometric rotational period of 1.416 0.133 d has been derived using observations at Mount Kent Observatory (MKO). Spectropolarimetric data obtained with the 3.9-m Anglo-Australian Telescope (AAT) were used to derive spot occupancy and magnetic maps of the star through the technique of Zeeman Doppler imaging (ZDI). The resulting brightness maps indicate that HD 106506 displays photospheric spots at all latitudes including a predominant polar spot. Azimuthal and radial magnetic images of this star have been derived, and a significant azimuthal magnetic field is indicated, in line with other active young stars. A solar-like differential rotation law was incorporated into the imaging process. Using Stokes I information the equatorial rotation rate, , was found to be 4.54 0.01 , with a photospheric shear of . This equates to an equatorial rotation period of 1.39 0.01 d, with the equatorial region lapping the poles every d. Using the magnetic features, the equatorial rotation rate, , was found to be 4.51 0.01 , with a photospheric shear of 0.24 0.03 . This differential rotation is approximately four times that observed on the Sun. [ABSTRACT FROM AUTHOR]

Spectropolarimetric observations of the pre-main sequence early-G star HD 141943 were made at three observing epochs (2007, 2009 and 2010). The observations were made using the 3.9-m Anglo-Australian Telescope with the UCLES echelle spectrograph and the SEMPOL spectropolarimeter visitor instrument. The brightness and surface magnetic field topologies (given in Paper I) were used to determine the star's surface differential rotation and reconstruct the coronal magnetic field of the star. The coronal magnetic field at the three epochs shows on the largest scales that the field structure is dominated by the dipole component with possible evidence for the tilt of the dipole axis shifting between observations. We find very high levels of differential rotation on HD 141943 (8 times the solar value for the magnetic features and 5 times solar for the brightness features), similar to that evidenced by another young early-G star, HD 171488. These results indicate that a significant increase in the level of differential rotation occurs for young stars around a spectral type of early-G. We also find for the 2010 observations that there is a large difference in the differential rotation measured from the brightness and magnetic features, similar to that seen on early-K stars, but with the difference being much larger. We find only tentative evidence for temporal evolution in the differential rotation of HD 141943. [ABSTRACT FROM AUTHOR]

Spectroscopic and spectropolarimetric observations of the pre-main sequence early-G star HD 141943 were obtained at four observing epochs (in 2006, 2007, 2009 and 2010). The observations were undertaken at the 3.9-m Anglo-Australian Telescope using the UCLES echelle spectrograph and the SEMPOL spectropolarimeter visitor instrument. Brightness and surface magnetic field topologies were reconstructed for the star using the technique of least-squares deconvolution to increase the signal-to-noise ratio of the data. The reconstructed brightness maps show that HD 141943 had a weak polar spot and a significant amount of low-latitude features, with little change in the latitude distribution of the spots over the 4 yr of observations. The surface magnetic field was reconstructed at three of the epochs from a high-order () spherical harmonic expansion of the spectropolarimetric observations. The reconstructed magnetic topologies show that in 2007 and 2010 the surface magnetic field was reasonably balanced between poloidal and toroidal components. However, we find tentative evidence of a change in the poloidal/toroidal ratio in 2009 with the poloidal component becoming more dominant. At all epochs the radial magnetic field is predominantly non-axisymmetric while the azimuthal field is predominantly axisymmetric with a ring of positive azimuthal field around the pole similar to that seen on other active stars. [ABSTRACT FROM AUTHOR]

Abstract: We estimated the galactic model parameters for a set of 20 intermediate-latitude fields with galactic longitudes and , included in the currently available Data Release 6 (DR6) of the Sloan Digital Sky Survey (SDSS), to explore their possible variation with galactic longitude. The local space densities of the thick disc and halo are almost the same for all fields, and , respectively, a result different than the one cited for high latitude fields. The thin disc’s scaleheight is 325pc in the galactic centre and it changes to 369pc in the third quadrant, which confirms the existence of disc flare, whereas the thick disc scaleheight is as large as 952pc at galactic longitude and 10% lower at , which confirms the existence of the disc long bar in the direction . Finally, the variation of the axis ratio of the halo with galactic longitude is almost flat, , except a slight minimum and a small maximum in the second and third quadrants, respectively, indicating an effect of the long bar which seems plausible for a shallow halo. We estimated the metallicities of unevolved G-type stars and discussed the metallicity gradient for different vertical distances. The metallicity gradient is for short distances, confirming the formation of this region of the Galaxy by dissipational collapse. However, its change is steeper in the transition regions of different galactic components. The metallicity gradient is almost zero for inner halo ( kpc), indicating a formation of merger or accretion of numerous fragments such as dwarf galaxies. [Copyright &y& Elsevier]

Using photometric data from the Two-Micron All-Sky Survey (2MASS) and GLIMPSE catalogues, I investigate the incidence of mid-infrared (mid-IR) excesses in G and K stars of luminosity class III. In order to obtain a large sample size, stars are selected using a near-IR colour–magnitude diagram. Sources which are candidates for showing mid-IR excess are carefully examined and modelled to determined whether they are likely to be G/K giants. It is found that mid-IR excesses are present at a level of . While the origin of these excesses remains uncertain, it is plausible that they arise from debris discs around these stars. I note that the measured incidence is consistent with a scenario in which dust lifetimes in debris discs are determined by Poynting–Robertson drag rather than by collisions. [ABSTRACT FROM AUTHOR]

In this note we consider a variable G cosmology which is consistent with observation and which had successfully predicted an ever expanding accelerating universe with a small cosmological constant amongst other things. Three further tests are proposed in this note: First, the inexplicable anomalous accelerations of the Pioneer spacecrafts can be explained. It is then shown that the observed shortening of the orbital periods of binary pulsars is in good agreement with this model. Finally, more general changes in orbital parameters are deduced, which may be observed in the future. [ABSTRACT FROM AUTHOR]

In order to trace the origin and evolution of carbon in the Galactic disc, we have determined carbon abundances in 51 nearby F and G dwarf stars. The sample is divided into two kinematically distinct subsamples with 35 and 16 stars that are representative of the Galactic thin and thick discs, respectively. The analysis is based on spectral synthesis of the forbidden [C i] line at 872.7 nm using spectra of very high resolution and high signal-to-noise ratio that were obtained with the Coudé Echelle Spectrograph (CES) spectrograph by the European Southern Observatory (ESO) 3.6-m telescope at La Silla in Chile. We find that [C/Fe] versus [Fe/H] trends for the thin and thick discs are totally merged and flat for subsolar metallicities. The thin disc that extends to higher metallicities than the thick disc shows a shallow decline in [C/Fe] from and up to . The [C/O] versus [O/H] trends are well separated between the two discs (due to differences in the oxygen abundances) and bear a great resemblance to the [Fe/O] versus [O/H] trends. Our interpretation of our abundance trends is that the sources that are responsible for the carbon enrichment in the Galactic thin and thick discs have operated on a time-scale very similar to those that are responsible for the Fe and Y enrichment [i.e. SN Ia and asymptotic giant branch (AGB) stars, respectively]. We further note that there exist other observational data in the literature that favour massive stars as the main sources for carbon. In order to match our carbon trends, we believe that the carbon yields from massive stars then must be very dependent on metallicity for the C, Fe and Y trends to be so finely tuned in the two disc populations. Such metallicity-dependent yields are no longer supported by the new stellar models in the recent literature. For the Galaxy, we hence conclude that the carbon enrichment at metallicities typical of the disc is mainly due to low- and intermediate-mass stars, while massive stars are still the main carbon contributor at low metallicities (halo and metal-poor thick disc). [ABSTRACT FROM AUTHOR]

We discuss whether the hypothesis that “all (or most) subdwarfs are in close binaries” is supported by the frequently reported observations of photometrically or spectroscopically composite character of many hot subdwarf stars. By way of a possible counter-argument, we focus on resolved companions (optical pairs) of hot subdwarf stars. On a statistical basis, many of these are physically associated with the hot subdwarfs, i.e. they are common proper motion pairs. These resolved pairs make a several percent contribution to the catalog of hot subdwarf stars per decade in projected separation. If they are just the relatively wide members of a binary population similar to the local G-dwarf binary population (A&A, 248, 485), which has a very wide distribution of orbital separations, then many of the unresolved but composite hot subdwarf binaries may not be “close” in the astrophysical sense. In that case, binary channels for hot subdwarf formation may be less important than expected, or must involve companions (white dwarfs) that do not result in a composite spectrum system. [ABSTRACT FROM AUTHOR]

We present the results of ultraviolet echelle spectroscopy of a sample of 59 F, G, K and M stars from the Anglo-Australian Planet Search target list. Ca II activity indices, which are essential in the interpretation of planet detection claims, have been determined for these stars and placed on the Mount Wilson R′[sub HK] system. [ABSTRACT FROM AUTHOR]

The space velocities and Galactic orbital elements of stars calculated from the currently available high-accuracy observations in our compiled catalogue of spectroscopic magnesium abundances in dwarfs and subgiants in the solar neighbourhood are used to identify thick-disc objects. We analyse the relations between chemical, spatial and kinematic parameters of F—G stars in the identified subsystem. [ABSTRACT FROM AUTHOR]

We present a multiplicity census for a volume-complete all-sky survey of 422 stars with distances less than 25 pc and primary main-sequence effective temperatures Teff ≥ 5300 K. Very similar to previous results that have been presented for various subsets of this survey, we confirm the positive correlation of the stellar multiplicities with primary mass. We find for the F- and G-type Population I stars that 58% are non-single and 21% are in triple or higher level systems. For the old intermediate-disk and Population II stars—virtually all of G type and less massive—even two out of three sources prove to be non-single. These numbers being lower limits because of the continuous flow of new discoveries, the unbiased survey clearly demonstrates that the standard case for solar-type field stars is a hydrogen-burning source with at least one ordinary or degenerate stellar companion, and a surprisingly large number of stars are organized in multiple systems. A principal consequence is that orbital evolution, including the formation of blue straggler stars, is a potentially important issue on all spatial scales and timescales for a significant percentage of the stellar systems, in particular among Population II stars. We discuss a number of recent observations of known or suspected companions in the local survey, including a new detection of a double-lined Ba-Bb subsystem to the visual binary HR 8635. [ABSTRACT FROM AUTHOR]

We use a 440.5 ks Chandra observation of the ≈500 Myr old open cluster M37 to derive the X-ray luminosity functions of its ≤1.2 stars. Combining detections of 162 M37 members with upper limits for 160 non-detections, we find that its G, K, and M stars have a similar median (0.5–7 keV) X-ray luminosity , whereas the -to-bolometric-luminosity ratio () indicates that M stars are more active than G and K stars by order of magnitude at 500 Myr. To characterize the evolution of magnetic activity in low-mass stars over their first , we consolidate X-ray and optical data from the literature for stars in six other open clusters: from youngest to oldest they are, the Orion Nebula Cluster (ONC), NGC 2547, NGC 2516, the Pleiades, NGC 6475, and the Hyades. For these, we homogenize the conversion of instrumental count rates to by applying the same one-temperature emission model as for M37, and obtain masses using the same empirical mass-absolute magnitude relation (except for the ONC). We find that for G and K stars X-ray activity decreases orders of magnitude over their first 600 Myr, and for M stars, ≈1.5. The decay rate of the median follows the relation , where for G stars, −0.82 ± 0.16 for K stars, and −0.40 ± 0.17 for M stars. In space, the slopes are −0.68 ± 0.12, −0.81 ± 0.19, and −0.61 ± 0.12, respectively. These results suggest that for low-mass stars the age-activity relation steepens after , consistent with the faster decay in activity observed in solar analogs at . [ABSTRACT FROM AUTHOR]

Keck/HIRES precision radial velocities of HD 207832 indicate the presence of two Jovian-type planetary companions in Keplerian orbits around this G star. The planets have minimum masses of Msin i = 0.56 MJup and 0.73 MJup, with orbital periods of ∼162 and ∼1156 days, and eccentricities of 0.13 and 0.27, respectively. Strömgren b and y photometry reveals a clear stellar rotation signature of the host star with a period of 17.8 days, well separated from the period of the radial velocity variations, reinforcing their Keplerian origin. The values of the semimajor axes of the planets suggest that these objects have migrated from the region of giant planet formation to closer orbits. In order to examine the possibility of the existence of additional (small) planets in the system, we studied the orbital stability of hypothetical terrestrial-sized objects in the region between the two planets and interior to the orbit of the inner body. Results indicated that stable orbits exist only in a small region interior to planet b. However, the current observational data offer no evidence for the existence of additional objects in this system. [ABSTRACT FROM AUTHOR]

We investigate the stability of super-Earth atmospheres around M stars using a seven-parameter, analytical framework. We construct stability diagrams in the parameter space of exoplanetary radius versus semimajor axis and elucidate the regions in which the atmospheres are stable against the condensation of their major constituents, out of the gas phase, on their permanent nightside hemispheres. We find that super-Earth atmospheres that are nitrogen-dominated (Earth-like) occupy a smaller region of allowed parameter space, compared to hydrogen-dominated atmospheres, because of the dual effects of diminished advection and enhanced radiative cooling. Furthermore, some super-Earths which reside within the habitable zones of M stars may not possess stable atmospheres, depending on the mean molecular weight and infrared photospheric pressure of their atmospheres. We apply our stability diagrams to GJ 436b and GJ 1214b, and demonstrate that atmospheric compositions with high mean molecular weights are disfavored if these exoplanets possess solid surfaces and shallow atmospheres. Finally, we construct stability diagrams tailored to the Kepler data set, for G and K stars, and predict that about half of the exoplanet candidates are expected to harbor stable atmospheres if Earth-like conditions are assumed. We include 55 Cancri e and CoRoT-7b in our stability diagram for G stars. [ABSTRACT FROM AUTHOR]

The article focuses on the "Encyclopedia of Astrobiology, Astronomy and Spaceflight," from astronomer by David Darling of Brainerd, Minnesota. The average lifetime of a G type star like sun is 10 billion years, whereas a torrid giant blue star like Alnitak will perish after a mere 10 million years. Pages offer brief biographies of luminaries in astronomy, rocketry and related fields, such as the eccentric Swiss-American Fritz Zwicky, who inferred the presence of dark matter.