תוכן
תמונות
הוסף תמונה שלך
DSS Images Other Images
מאמרים קשורים
Astrophysics in 2005
We bring you, as usual, the Sun and Moon and stars, plus some galaxiesand a new section on astrobiology. Some highlights are short (the newlyidentified class of gamma-ray bursts, and the Deep Impact on Comet9P/Tempel 1), some long (the age of the universe, which will be found tohave the Earth at its center), and a few metonymic, for instance theterm ``down-sizing'' to describe the evolution of star formation rateswith redshift.
| Roche tomography of cataclysmic variables - III. Star-spots on AE Aqr
We present a Roche tomography reconstruction of the secondary star inthe cataclysmic variable AE Aqr. The tomogram reveals several surfaceinhomogeneities that are due to the presence of large, cool star-spots.In addition to a number of lower latitude spots, the maps also show thepresence of a large, high-latitude spot similar to that seen in Dopplerimages of rapidly rotating isolated stars, and a relative paucity ofspots at a latitude of 40°. In total, we estimate that some 18 percent of the Northern hemisphere of AE Aqr is spotted.We have also applied the entropy landscape technique to determineaccurate parameters for the binary system. We obtain optimal massesM1 = 0.74Msolar, M2 =0.50Msolar, a systemic velocity γ = -63kms-1and an orbital inclination i = 66°.Given that this is the first study to successfully image star-spots onthe secondary star in a cataclysmic variable, we discuss the role thatfurther studies of this kind may play in our understanding of thesebinaries.
| Simulated X-ray cycles in rapidly rotating solar-like stars
It is generally accepted that the presence of a hot magnetic coronaprovides the source of X-ray emission in cool stars. With thisconnection one could expect to see the variation of magnetic flux in theactivity cycle of a star mirrored by a similar variation in the starsX-ray emission. Using magnetic maps produced from flux emergence andtransport simulations and assuming a potential field for the corona, wecan extrapolate the coronal magnetic field and hence calculate thevariation of the X-ray emission. We consider three types of activitycycle that successfully reproduce the pattern of intermingled magneticflux at high latitudes, a feature observed with Zeeman-Doppler imaging.The three different cycles take the form of (1) an enhanced butterflypattern where flux emergence is extended to a latitude of 70°, (2)an extended emergence profile as before but with an overlap of 4 yr inthe butterfly diagram and (3) where no butterfly diagram is used. Thecyclic variation in the X-ray emission is around two orders of magnitudefor cases (1) and (3), but less than one order of magnitude for case(2). For all three cases, the rotational modulation of the X-rayemission is greatest at cycle minimum, but the emission measure weighteddensity varies little over the cycle. For cases (1) and (2) the fractionof the total flux that is open (along which a wind can escape) varieslittle over the cycle, but for case (3) this is three times larger atcycle minimum than at maximum. Our results clearly show that althoughmagnetic cycles may exist for stars they are not necessarily observablein the X-ray emission.
| Spectral synthesis analysis and radial velocity study of the northern F-, G- and K-type flare stars
In this paper, we present a study of the general physical and chemicalproperties and radial velocity monitoring of young active stars. Wederive temperatures, logg, [Fe/H], v sini and Rspec valuesfor eight stars. The detailed analysis reveals that the stars are nothomogeneous in their principal physical parameters or in the agedistribution. In 4/5, we found a periodic radial velocity signal whichoriginates in surface features; the fifth is surprisingly inactive andshows little variation.
| A Comparative Study of Flaring Loops in Active Stars
Dynamo activity in stars of different types is expected to generatemagnetic fields with different characteristics. As a result, adifferential study of the characteristics of magnetic loops in a broadsample of stars may yield information about dynamo systematics. In theabsence of direct imaging, certain physical parameters of a stellarmagnetic loop can be extracted if a flare occurs in that loop. In thispaper we employ a simple nonhydrodynamic approach introduced by Haisch,to analyze a homogeneous sample of all of the flares we could identifyin the EUVE DS database: a total of 134 flares that occurred on 44 starsranging in spectral type from F to M and in luminosity class from V toIII. All of the flare light curves that have been used in the presentstudy were obtained by a single instrument (EUVE DS). For each flare, wehave applied Haisch's simplified approach (HSA) in order to determineloop length, temperature, electron density, and magnetic field. For eachof our target stars, a literature survey has been performed to determinequantitatively the extent to which our results are consistent withindependent studies. The results obtained by HSA are found to be wellsupported by results obtained by other methods. Our survey suggeststhat, on the main sequence, short loops (with lengths<=0.5R*) may be found in stars of all classes, while thelargest loops (with lengths up to 2R*) appear to be confinedto M dwarfs. Based on EUVE data, the transition from small to largeloops on the main sequence appears to occur between spectral types K2and M0. We discuss the implications of this result for dynamo theories.
| The Nearest Young Moving Groups
The latest results in the research of forming planetary systems have ledseveral authors to compile a sample of candidates for searching forplanets in the vicinity of the Sun. Young stellar associations areindeed excellent laboratories for this study, but some of them are notclose enough to allow the detection of planets through adaptive opticstechniques. However, the existence of very close young moving groups cansolve this problem. Here we have compiled the members of the nearestyoung moving groups, as well as a list of new candidates from ourcatalog of late-type stars that are possible members of young stellarkinematic groups, studying their membership through spectroscopic andphotometric criteria.
| Starspot activity in late stars: Methods and results
Three types of methods for studying the surface inhomogeneities of coolstars and the results of their use on type BY Dra, RS CVn, FK Com, and TTau variables are discussed. The current relevance of traditionalphotometric methods and the advantages of the zonal spottedness modelare pointed out. Dependences of the maximum total areas, averagelatitudes, and temperatures of spots on the global parameters of thestars are given. Analogs of the solar cycle in the variations of theareas and latitudes of starspots are examined, as well as the effects ofdifferential rotation and active longitudes.
| The RS CVn binary HK Lacertae: long-term photometry from Sonneberg sky-patrol plates
Long-term photographic photometry of the active long-period RS CVnbinary HK Lac (HD 209813) was obtained from more than 2000 SonnebergSky-Patrol plates taken between 1956 and 1996. We achieve an internalaccuracy of 0.07 m. The correspondence with contemporaneoushigh-precision photoelectric photometry from automatic telescopes isstriking and successfully demonstrates the feasibility of our approach.Based on a Bayesian time series analysis, we improve the previouslypublished cycle period to 13.37± 0.08 years, and present evidenceof an additional period of 9.48± 0.13 years. This establishes themulti-periodicity of dynamo action in these overactive stars as comparedto the Sun. The already known 6.7-years cycle turns out to be anovertone of the dominating 13.4-years cycle. Our long-term photographicphotometry even allowed the detection of the star's mean rotationalperiod of 24.35 days.
| X-ray and optical bursts and flares in YSOs: results from a 5-day XMM-Newton monitoring campaign of L1551
We present the results of a five-day monitoring campaign with XMM-Newtonof six X-ray bright young stellar objects (YSOs) in the star-formingcomplex L1551 in Taurus. All stars present significant variability onthe five-day time scale. Modulation of the light curve on time scalescomparable with the star's rotational period appeared to be present inthe case of one weak-lined T Tauri star. Significant spectral variationsbetween the 2000 and the 2004 observations were detected in the(unresolved) classical T Tauri binary system XZ Tau: a hot plasmacomponent which was present in the X-ray spectrum in 2000 hadsignificantly weakened in 2004. As XZ Tau N was undergoing a strongoptical outburst in 2000, which had terminated since then, we speculateon the possible relationship between episodic, burst accretion, andX-ray heating. The transition object HL Tau underwent a strong flarewith a complex temperature evolution, which is indicative of an eventconfined within a very large magnetic structure (few stellar radii),similar to the ones found in YSOs in the Orion Nebula Cluster.
| Differential Photometry of EK Dra and 29 Dra
We report on BVR precise photometry of active late-type stars EK Dra and29 Dra in the spring of 2004. The analysis suggests a reasonable fitwith single spot model for both cases in the season as far as theV-curve is concerned. The spots are generally cooler than surroundingphotosphere and are at high latitudes. The maximal brightness changesfor both stars implying a non-rotating component of activity signatures,the spots. We expect that the spot configuration is stable over theperiod of approximately three months.
| High-Resolution Spectroscopy of some Active Southern Stars
High-resolution échelle spectra of 42 nearby southern solar-typestars have been obtained, in a search for young, single, active, andrapidly rotating sun-like stars suitable for Doppler imaging and ZeemanDoppler imaging studies. As a result of this survey, 13 stars weredetermined to be youthful with ages less than 600Myr (Hyades age) andeight of these were found to have projected rotational velocities inexcess of 15kms-1. In addition, five spectroscopic binarysystems were identified. Of those stars observed for this survey, HD106506 is the most outstanding target for mapping active regions. It isan apparently young and single star with rapid rotation (v sin i~80kms-1), strong Hα chromospheric activity (logR'Hα~-4.2), and deformation of the spectral lineprofiles indicating the presence of large starspots.
| The dependence of the Rossby number and XUV-Lyα emission flux with age for solar-like G-type stars
Stellar parameters of 11 G-type stars with ages ranging from 0.1 to 8.5Gyr, from the Sun in Time programme, were used to compute the Rossbynumber, ℜ, for each star. The Rossby number for each star wascalculated from the rotation period and the convective overturn timederived from spectral type (B-V). It was found to vary essentially ast0.5, where t is the stellar age in Gyr. The Rossby number isused as an index of X-ray-ultraviolet (XUV) (1-1200 Å) andLyα activity, defined as the ratio of the total emission flux inthese spectral regions to the total bolometric emission. Expressions forthe ratio of the stellar surface XUV and Lyα emission fluxrelative to present mean solar surface flux values are given in terms ofℜ. It is shown that the observed activity in these stars varies asℜ-β, where β takes values of 2.5 and 1.5 forXUV and Lyα, respectively. Expressions for deriving the Rossbynumber from B-V and age are also given. Thus, one can use the stellarB-V and effective temperature variation with age to calculate the XUVand Lyα emission flux relative to present solar values. As anexample, the evolution of the solar XUV and Lyα with age from 0.1to 8.5 Gyr is given. The variation of the stellar ultraviolet flux withage can be used in photochemical models to study the evolution ofplanetary atmospheres orbiting such stars.
| Rotational Modulation of X-Ray Emission in Orion Nebula Young Stars
We investigate the spatial distribution of X-ray-emitting plasma in asample of young Orion Nebula Cluster stars by modulation of their X-raylight curves due to stellar rotation. The study, part of the ChandraOrion Ultradeep Project (COUP), is made possible by the exceptionallength of the observation: 10 days of ACIS integration during a timespan of 13 days, yielding a total of 1616 detected sources in the17'×17' field of view. We here focus on asubsample of 233 X-ray-bright stars with known rotational periods. Wesearch for X-ray modulation using the Lomb Normalized Periodogrammethod. X-ray modulation related to the rotation period is detected inat least 23 stars with periods between 2 and 12 days and relativeamplitudes ranging from 20% to 70%. In 16 cases, the X-ray modulationperiod is similar to the stellar rotation period, while in 7 cases it isabout half that value, possibly due to the presence of X-ray-emittingstructures at opposite stellar longitudes. These results constitute thelargest sample of low-mass stars in which X-ray rotational modulationhas been observed. The detection of rotational modulation indicates thatthe X-ray-emitting regions are distributed inhomogeneneously inlongitude and do not extend to distances significantly larger than thestellar radius. Modulation is observed in stars with saturated activitylevels (LX/Lbol~10-3) showing thatsaturation is not due to the filling of the stellar surface withX-ray-emitting regions.Smithsonian Astrophysical Observatory, 60 Garden Street, Cambridge, MA02138.Universe Division, Science Mission Directorate, NASA Headquarters.
| Spectroscopic Properties of Cool Stars (SPOCS). I. 1040 F, G, and K Dwarfs from Keck, Lick, and AAT Planet Search Programs
We present a uniform catalog of stellar properties for 1040 nearby F, G,and K stars that have been observed by the Keck, Lick, and AAT planetsearch programs. Fitting observed echelle spectra with synthetic spectrayielded effective temperature, surface gravity, metallicity, projectedrotational velocity, and abundances of the elements Na, Si, Ti, Fe, andNi, for every star in the catalog. Combining V-band photometry andHipparcos parallaxes with a bolometric correction based on thespectroscopic results yielded stellar luminosity, radius, and mass.Interpolating Yonsei-Yale isochrones to the luminosity, effectivetemperature, metallicity, and α-element enhancement of each staryielded a theoretical mass, radius, gravity, and age range for moststars in the catalog. Automated tools provide uniform results and makeanalysis of such a large sample practical. Our analysis method differsfrom traditional abundance analyses in that we fit the observed spectrumdirectly, rather than trying to match equivalent widths, and wedetermine effective temperature and surface gravity from the spectrumitself, rather than adopting values based on measured photometry orparallax. As part of our analysis, we determined a new relationshipbetween macroturbulence and effective temperature on the main sequence.Detailed error analysis revealed small systematic offsets with respectto the Sun and spurious abundance trends as a function of effectivetemperature that would be inobvious in smaller samples. We attempted toremove these errors by applying empirical corrections, achieving aprecision per spectrum of 44 K in effective temperature, 0.03 dex inmetallicity, 0.06 dex in the logarithm of gravity, and 0.5 kms-1 in projected rotational velocity. Comparisons withprevious studies show only small discrepancies. Our spectroscopicallydetermined masses have a median fractional precision of 15%, but theyare systematically 10% higher than masses obtained by interpolatingisochrones. Our spectroscopic radii have a median fractional precisionof 3%. Our ages from isochrones have a precision that variesdramatically with location in the Hertzsprung-Russell diagram. We planto extend the catalog by applying our automated analysis technique toother large stellar samples.
| Stellar Coronal Abundances at Intermediate-Activity Levels: ξ UMa
We present an analysis of Chandra High Energy Transmission Grating(HETG) spectra of the corona of the critical intermediate-activity,quadruple stellar system ξ UMa. Using the maximum A/B subsystemseparation in 2001, we attempted to resolve for the first time in X-raysthe two components using the HETG Medium Energy Grating (MEG). We foundthe Aa component of the system to be at least 2 orders of magnitudefainter than the Bb component. We used the Markov Chain Monte Carlomethod to reconstruct differential emission measures (DEMs) andabundances and discuss first ionization potential (FIP) issues. Thereconstructed DEMs showed two pronounced peaks at logT~6.5 and logT~7.0K, respectively, similar to brighter solar active regions. A plot ofcoronal abundances, with respect to stellar photospheric, versus FIPreveals that the FIP behavior of ξ UMa is intermediate between lessand more active stars, commensurate with its X-ray luminosity. Theabundances of Mg, Fe, and Si are just subsolar, with Fe having thelowest abundance of these. The low-FIP elements Na and Al have similarabundances, and they are both higher than the Mg, Fe, and Si group. Theabundances show a minimum at S, rising gradually to Ne, which is higherthan photospheric, as has been seen by other authors on similar stars.Some of the FIP behavior seen here is predicted by the new ponderomotiveforce model of Laming.
| Evolution of the Solar Activity over Time and Effects on Planetary Atmospheres. I. High-Energy Irradiances (1-1700 Å)
We report on the results of the Sun in Time multiwavelength program(X-rays to UV) of solar analogs with ages covering ~0.1-7 Gyr. The chiefscience goals are to study the solar magnetic dynamo and to determinethe radiative and magnetic properties of the Sun during its evolutionacross the main sequence. The present paper focuses on the latter goal,which has the ultimate purpose of providing the spectral irradianceevolution of solar-type stars to be used in the study and modeling ofplanetary atmospheres. The results from the Sun in Time program suggestthat the coronal X-ray-EUV emissions of the young main-sequence Sun were~100-1000 times stronger than those of the present Sun. Similarly, thetransition region and chromospheric FUV-UV emissions of the young Sunare expected to be 20-60 and 10-20 times stronger, respectively, than atpresent. When we consider the integrated high-energy emission from 1 to1200 Å, the resulting relationship indicates that about 2.5 Gyrago the solar high-energy flux was about 2.5 times the present value andabout 3.5 Gyr ago was about 6 times the present value (when lifesupposedly arose on Earth). The strong radiation emissions inferredshould have had major influences on the thermal structure,photochemistry, and photoionization of planetary atmospheres and haveplayed an important role in the development of primitive life in thesolar system. Some examples of the application of the Sun in Timeresults on exoplanets and on early solar system planets are discussed.
| Coronal Evolution of the Sun in Time: High-Resolution X-Ray Spectroscopy of Solar Analogs with Different Ages
We investigate the long-term evolution of X-ray coronae of solar analogsbased on high-resolution X-ray spectroscopy and photometry withXMM-Newton. Six nearby main-sequence G stars with ages between ~0.1 and~1.6 Gyr and rotation periods between ~1 and 12.4 days have beenobserved. We use the X-ray spectra to derive coronal element abundancesof C, N, O, Ne, Mg, Si, S, and Fe and the coronal emission measuredistribution (EMD). We find that the abundances change from an inversefirst ionization potential (FIP) distribution in stars with ages around0.1 Gyr to a solar-type FIP distribution in stars at ages of 0.3 Gyr andbeyond. This transformation is coincident with a steep decline ofnonthermal radio emission. The results are in qualitative agreement witha simple model in which the stream of electrons in magnetic fieldssuppresses diffusion of low-FIP ions from the chromosphere into thecorona. The coronal emission measure distributions show shapescharacterized by power laws on each side of the EMD peak. The lattershifts from temperatures of about 10 MK in the most rapidly rotating,young stars to temperatures around 4 MK in the oldest target consideredhere. The power-law index on the cooler side of the EMD exceeds expectedslopes for static loops, with typical values being 1.5-3. We interpretthis slope with a model in which the coronal emission is due to asuperposition of stochastically occurring flares, with an occurrencerate that is distributed in radiated energy E as a power law,dN/dE~E-α, as previously found for solar and stellarflares. We obtain the relevant power-law index α from the slope ofthe high-temperature tail of the EMD. Our EMDs indicate α~2.2-2.8,in excellent agreement with values previously derived from light curvesof magnetically active stars. Modulation with timescales reminiscent offlares is found in the light curves of all our targets. Several strongflares are also observed. We use our α-values to simulate lightcurves and compare them with the observed light curves. We thus derivethe range of flare energies required to explain the light-curvemodulation. More active stars require a larger range of flare energiesthan less active stars within the framework of this simplistic model. Inan overall scenario, we propose that flaring activity plays a largerrole in more active stars. In this model, the higher flare rate isresponsible both for the higher average coronal temperature and the highcoronal X-ray luminosity, two parameters that are indeed found to becorrelated.
| Spots, activity cycles, and differential rotation on cool stars
The first results are reported from a search for activity cycles instars similar to the sun based on modelling their spotting with analgorithm developed at the Crimean Astrophysical Observatory. Of themore than thirty program stars, 10 manifested a cyclical variation intheir central latitudes and total starspot area. The observed cycleshave durations of 4-15 years, i.e., analogous to the 11 year Schwabesunspot cycle. Most of the stars have a rough analog of the solarbutterfly pattern, with a reduction in the average latitude of the spotsas their area increases. A flip-flop effect during the epoch of themaximum average latitude is noted in a number of these objects (e.g.,the analog LQ Hya of the young sun or the RS CVn-type variable V711Tau), as well as a reduction in the photometric rotation period of astar as the spots drift toward the equator, an analog of thedifferential rotation effect in the sun. Unlike in the sun, the observedspot formation cycles do not correlate uniquely with other indicators ofactivity chromospheric emission in the CaII HK lines (Be Cet, EKDra, Dx Leo), H line emission (LQ Hya, VY Ari, EV Lac), or cyclicalflare activity (EV Lac). In V833 Tau, BY Dra, EK Dra, and VY Ari shortSchwabe cycles coexist with long cycles that are analogous to theGleissberg solar cycle, in which the spotted area can approach half theentire area of the star.
| Spot activity cycles and flip-flops on young solar analogs
We discuss activity phenomena observed in the spot distribution on youngsingle solar-type stars LQ Hya, AB Dor and EK Dra. These include cyclicvariations of the mean activity level, differential rotation, persistentactive longitudes separated by 180 ° and flip-flop cycles. Acomparison with the sunspot activity reveals many similarities betweenthe Sun and the young solar analogs. Our results confirm the presence ofcyclic activity in very young dwarfs and allow for studying evolution ofthe stellar magnetic activity during the main-sequence stage.
| Flip-flop phenomenon: observations and theory
In many active stars the spots concentrate on two permanent activelongitudes which are 180 ° apart. In some of these stars thedominant part of the spot activity changes the longitude every fewyears. This so-called flip-flop phenomenon has up to now been reportedin 11 stars, both single and binary alike, and including also the Sun.To explain this phenomenon, a non-axisymmetric dynamo mode, giving riseto two permanent active longitudes at opposite stellar hemispheres, isneeded together with an oscillating axisymmetric magnetic field. Here wediscuss the observed characteristics of the flip-flop phenomenon andpresent a dynamo solution to explain them.
| Statistical Constraints for Astrometric Binaries with Nonlinear Motion
Useful constraints on the orbits and mass ratios of astrometric binariesin the Hipparcos catalog are derived from the measured proper motiondifferences of Hipparcos and Tycho-2 (Δμ), accelerations ofproper motions (μ˙), and second derivatives of proper motions(μ̈). It is shown how, in some cases, statistical bounds can beestimated for the masses of the secondary components. Two catalogs ofastrometric binaries are generated, one of binaries with significantproper motion differences and the other of binaries with significantaccelerations of their proper motions. Mathematical relations betweenthe astrometric observables Δμ, μ˙, and μ̈ andthe orbital elements are derived in the appendices. We find a remarkabledifference between the distribution of spectral types of stars withlarge accelerations but small proper motion differences and that ofstars with large proper motion differences but insignificantaccelerations. The spectral type distribution for the former sample ofbinaries is the same as the general distribution of all stars in theHipparcos catalog, whereas the latter sample is clearly dominated bysolar-type stars, with an obvious dearth of blue stars. We point outthat the latter set includes mostly binaries with long periods (longerthan about 6 yr).
| Evolution of Cold Circumstellar Dust around Solar-type Stars
We present submillimeter (Caltech Submillimeter Observatory 350 μm)and millimeter (Swedish-ESO Submillimetre Telescope [SEST] 1.2 mm, OwensValley Radio Observatory [OVRO] 3 mm) photometry for 127 solar-typestars from the Formation and Evolution of Planetary Systems SpitzerLegacy program that have masses between ~0.5 and 2.0 Msolarand ages from ~3 Myr to 3 Gyr. Continuum emission was detected towardfour stars with a signal-to-noise ratio>=3: the classical T Tauristars RX J1842.9-3532, RX J1852.3-3700, and PDS 66 with SEST, and thedebris-disk system HD 107146 with OVRO. RX J1842.9-3532 and RXJ1852.3-3700 are located in projection near the R CrA molecular cloud,with estimated ages of ~10 Myr (Neuhäuser et al.), whereas PDS 66is a probable member of the ~20 Myr old Lower Centaurus-Crux subgroup ofthe Scorpius-Centaurus OB association (Mamajek et al.). The continuumemission toward these three sources is unresolved at the 24" SESTresolution and likely originates from circumstellar accretion disks,each with estimated dust masses of ~5×10-5Msolar. Analysis of the visibility data toward HD 107146(age~80-200 Myr) indicates that the 3 mm continuum emission is centeredon the star within the astrometric uncertainties and resolved with aGaussian-fit FWHM size of (6.5"+/-1.4")×(4.2"+/-1.3"), or185AU×120 AU. The results from our continuum survey are combinedwith published observations to quantify the evolution of dust mass withtime by comparing the mass distributions for samples with differentstellar ages. The frequency distribution of circumstellar dust massesaround solar-type stars in the Taurus molecular cloud (age~2 Myr) isdistinguished from that around 3-10 Myr and 10-30 Myr old stars at asignificance level of ~1.5 and ~3 σ, respectively. These resultssuggest a decrease in the mass of dust contained in small dust grainsand/or changes in the grain properties by stellar ages of 10-30 Myr,consistent with previous conclusions. Further observations are needed todetermine if the evolution in the amount of cold dust occurs on evenshorter timescales.
| Spots on EK Draconis. Active longitudes and cycles from long-term photometry
We analyse photometric observations of the young active star EK Dra,altogether about 21 years. Similar to the ZAMS stars LQ Hya and AB Dor,EK Dra shows long-lived, non-axisymmetric spot distribution with activelongitudes on opposite hemispheres. At least two activity cycle can befound from the data. The first cycle originates from repeated switchesof the activity between two active longitudes in about (2-2.25)-yearintervals, resulting in a cycle of about (4-4.5) years. The second cycleis of the order of 10.5 years and comes from migration of the activelongitudes. Our data cover two consecutive cycles. The periodicity isalso present in the maximum, the mean and the minimum stellarbrightness. Additionally, there is a long-term trend. If we combine ourdata with the Sonneberg plate measurements, we can conclude that theoverall brightness of EK Dra has been continuously decreasing at leastfor the last 45 years. A comparison with current sunspot activityreveals many similarities between the Sun and EK Dra.
| The influence of stellar wind conditions on the detectability of planetary radio emissions
Magnetized giant exoplanets in close orbits around their host star areexpected to be strong nonthermal radio emitters. The anticipated radioflux is strong enough to allow its detection on Earth using the nextgeneration of instruments. However, the measured quantity will not bethe planetary radio flux but the sum of planetary and stellar emission.We compare the expected stellar and planetary radio signal for stellarsystems of different ages. Solar-like stellar wind parameters as well asconditions corresponding to the young solar system (i.e. with increasedstellar wind density and velocity) are considered. For young stellarsystems, conditions appear to be more favorable than for older stellarsystems. It is shown that configurations exist where the separation ofthe planetary signal from the stellar emission seems feasible.
| The young active binary star EK Draconis
EK Dra (HD 129333) is a young,active, nearby star that is orbited by a low mass companion. Bycombining new speckle observations with old and new radial velocitymeasurements we find that the orbit is highly eccentric withe=0.82±0.03, and we derive the true masses of both components.The masses are 0.9±0.1 Mȯ and 0.5±0.1Mȯ, for the primary and secondary, respectively. Fromhigh resolution spectra we derive a new Teff of5700±70 K and a log g of 4.37±0.10, which is different toprevious estimates. However, the new spectroscopic distance differs byonly 5.8% to the distance derived by parallax measurement by theHipparcos satellite and thus the stellar parameters are presumably morerealistic than older determinations. We derive a somewhat higher valuefor the metallicity of [Fe/H]=-0.16±0.07. EKDra turns out to be one of the few nearby young stars thatwill evolve similarly to the Sun. The precise radial velocitymeasurements taken in the course of this program also allow us to shedmore light on the activity of this star. In 2001 and 2002 we find radialvelocity variation with a period of 2.767±0.005 days which weinterpret as the rotation period. This signal vanishes in 2003. Howeverthe signal can be recovered if only the spectra in which thephotospheric lines are asymmetric are used. On the other hand, we do notfind a close correlation between the asymmetry of photospheric lines andthe radial velocity.
| Coronal properties of G-type stars in different evolutionary phases
We report on the analysis of XMM-Newton observations of three G-typestars in very different evolutionary phases: the weak-lined T Tauri starHD 283572, the Zero Age Main Sequence starEK Dra and the Hertzsprung-gap giant star 31Com. They all have high X-ray luminosity (~1031 ergs-1 for HD 283572 and 31 Com and ~1030 ergs-1 for EK Dra). We compare the Emission MeasureDistributions (EMDs) of these active coronal sources, derived fromhigh-resolution XMM-Newton grating spectra, as well as the pattern ofelemental abundances vs. First Ionzation Potential (FIP). We alsoperform time-resolved spectroscopy of a flare detected by XMM from EKDra. We interpret the observed EMDs as the result of the emission ofensembles of magnetically confined loop-like structures with differentapex temperatures. Our analysis indicates that the coronae of HD 283572and 31 Com are very similar in terms of dominant coronal magneticstructures, in spite of differences in the evolutionary phase, surfacegravity and metallicity. In the case of EK Dra the distribution appearsto be slightly flatter than in the previous two cases, although the peaktemperature is similar.
| Mg II chromospheric radiative loss rates in cool active and quiet stars
The Mg II k emission line is a good indicator of the level ofchromospheric activity in late-type stars. We investigate the dependenceof this activity indicator on fundamental stellar parameters. To thispurpose we use IUE observations of the Mg II k line in 225 late-typestars of luminosity classes I-V, with different levels of chromosphericactivity. We first re-analyse the relation between Mg II k lineluminosity and stellar absolute magnitude, performing linear fits to thepoints. The ratio of Mg II surface flux to total surface flux is foundto be independent of stellar luminosity for evolved stars and toincrease with decreasing luminosity for dwarfs. We also analyse the MgII k line surface flux-metallicity connection. The Mg II k emissionlevel turns out to be not dependent on metallicity. Finally, the Mg II kline surface flux-temperature relation is investigated by treatingseparately, for the first time, a large sample of very active and normalstars. The stellar surface fluxes in the k line of normal stars arefound to be strongly dependent on the temperature and slightly dependenton the gravity, thus confirming the validity of recently proposedmodels. In contrast, data relative to RS CVn binaries and BY Dra stars,which show very strong chromospheric activity, are not justified in theframework of a description based only on acoustic waves and uniformlydistributed magnetic flux tubes so that they require more detailedmodels.
| Differential Rotation of the Active G5 V Star κ1 Ceti: Photometry from the MOST Satellite
About 30.5 days of nearly uninterrupted broadband photometry of thesolar-type star κ1 Ceti, obtained with the MOST(Microvariability and Oscillations of Stars) satellite, shows evidencefor two large starspots with different rotation periods of 8.9 and ~9.3days (ΔΩ/Ω~=4%). Ground-based measurements of Ca II Hand K emission in 2002 and 2003 reveal variations in chromosphericactivity with a period of about 9.3 days. The data were obtained duringthe MOST commissioning phase. When the data are combined with historicalobservations, they indicate that the 9.3 day spot has been stable in itsperiod for over 30 yr. The photometry, with a sampling rate ofapproximately once per minute, was also used to search for acoustic(p-mode) oscillations in the star. We detect no clear evidence forp-modes in the κ1 Ceti photometry, with a noise levelaround 7-9 μmag at frequencies in the range of 0.5-4 mHz (3 σdetection limit of 21-27 μmag). There were no flares or planetarytransits during 30.5 days of MOST monitoring with light amplitudesgreater than 2 mmag (durations >200 minutes) and 3 mmag (2-200 minutedurations). While this rules out any close-in planets with Jupiterdiameters >=0.5 and orbital inclinations close to 90°, thescatter in differential radial velocities permit a close giant planet ina more highly inclined orbit.Based on data obtained with the MOST satellite, a Canadian Space Agencymission jointly operated by Dynacon, Inc., the University of TorontoInstitute of Aerospace Studies, and the University of British Columbia.
| High-resolution Doppler images of the spotted contact binary AE Phe
We present Doppler images of the short period (P= 0.362 d) W UMa binaryAE Phe. In order to obtain the necessary S/N ratio and time resolutionrequired to see individual star-spot features in highly rotationallybroadened profiles, we use least-squares deconvolution, which makes useof the information content of the several thousand lines in a typicalechelle spectrum. This yields a single rotation profile (free ofsidelobes due to blending) per spectrum with a typical S/N ratio ofseveral thousand.We use radial velocity curves, generated from standard profile fittingtechniques, to measure velocity amplitudes and the mass ratio. Failureto model star-spots with this method leads to a biased set of values,and we show that an imaging code is essential if accurate systemparameters are to be derived.Images are reconstructed from four nights of data which revealstar-spots at most latitudes on both components of the common envelopesystem. Our model requires that the primary component be several hundredK cooler than the secondary in order to reproduce the profile depthchanges with phase. In a two-temperature imaging model, we interpretthis as being due to 27 per cent greater - but unresolved - spot fillingon the primary relative to the secondary component. The images revealthat dark spots are present on both stars at various latitudes andlongitudes. Star-spots are also found in the neck region of bothcomponents, which appear to be darker on the side of each star leadingin rotation phase - particularly on the secondary component. Weinvestigate the reproducibility of the images from night to night andconclude that the star-spots evolve significantly on very shorttime-scales, of the order of 1 d. This is significantly faster than theweek time-scales found on active single stars and the Sun.
| Chromospheric Ca II Emission in Nearby F, G, K, and M Stars
We present chromospheric Ca II H and K activity measurements, rotationperiods, and ages for ~1200 F, G, K, and M type main-sequence stars from~18,000 archival spectra taken at Keck and Lick Observatories as a partof the California and Carnegie Planet Search Project. We have calibratedour chromospheric S-values against the Mount Wilson chromosphericactivity data. From these measurements we have calculated medianactivity levels and derived R'HK, stellar ages,and rotation periods from general parameterizations for 1228 stars,~1000 of which have no previously published S-values. We also presentprecise time series of activity measurements for these stars.Based on observations obtained at Lick Observatory, which is operated bythe University of California, and on observations obtained at the W. M.Keck Observatory, which is operated jointly by the University ofCalifornia and the California Institute of Technology. The KeckObservatory was made possible by the generous financial support of theW. M. Keck Foundation.
|
הכנס מאמר חדש
לינקים קשורים
הכנס לינק חדש
משמש של הקבוצה הבאה
|
תצפית ומידע אסטרומטרי
| קבוצת-כוכבים: | דרקון |
| התרוממות ימנית: | 14h39m00.22s |
| סירוב: | +64°17'29.8" |
| גודל גלוי: | 7.613 |
| מרחק: | 33.944 פארסק |
| תנועה נכונה: | -135.9 |
| תנועה נכונה: | -25.3 |
| B-T magnitude: | 8.385 |
| V-T magnitude: | 7.677 |
קטלוגים וכינוים:
|
