Ýçindekiler
Görüntüler
Resim Yükleyin
DSS Images Other Images
Ýlgili Makaleler
Are the W Ursae Majoris-type systems EK Comae Berenices and UX Eridani surrounded by circumstellar matter?
The variations of the orbital periods of two nearly neglected W UMa-typeeclipsing binaries, EK Comae Berenices and UX Eridani, are presentedthrough a detailed analysis of the O C diagrams. It is found that theorbital period of EK Com is decreasing and the period of UX Eridani isincreasing, and several sudden jumps have occurred in the orbitalperiods of both binaries. We analyze the mechanism(s), which mightunderlie the changes of the orbital periods of both systems, and obtainsome new results. The long-term decrease of the orbital period of EKComae Berenices might be caused by the decrease of the orbital angularmomentum due to a magnetic stellar wind (MSW) or by mass transfer fromthe more massive to the less massive component. The secular increase inthe orbital period of UX Eridani might be caused by mass transfer fromthe less massive to the more massive star. The possible mechanisms,which underlie the sudden changes in the orbital periods of the closebinary systems are as the followings: (1) the variations of thestructure due to the variation of the magnetic field; (2) the rapid massexchange between the close binaries and their circumstellar matter.Finally, the evolutionary status of the systems EK Comae Berenices andUX Eridani is discussed.
| Observations of variables.
Not Available
| 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.
| On the Temperature-Emission Measure Distribution in Stellar Coronae
Strong peaks in the emission measure-temperature (EM-T ) distributionsin the coronae of some binary stars are associated with the presence ofhot (107 K), dense (up to 1013 cm -3)plasma. These peaks are very reminiscent of those predicted to arise inan impulsively heated solar corona. A coronal model comprised of manyimpulsively heated strands is adapted to stellar parameters. It is shownthat the properties of the EM-T distribution can be accounted for ingeneral terms provided the emission comes from many very small loops(length under 103 km) with intense magnetic fields (1 kG)distributed across part of the surface of the star. The heating requiresevents that generally dissipate between 1026 and 1028 ergs, which is in the range of solar microflares. This impliesthat such stars must be capable of generating regions of localizedintense magnetic fields.
| Contact Binaries with Additional Components. II. A Spectroscopic Search for Faint Tertiaries
It is unclear how very close binary stars form, given that during thepre-main-sequence phase the component stars would have been inside eachother. One hypothesis is that they formed farther apart but were broughtin closer after formation by gravitational interaction with a thirdmember of the system. If so, all close binaries should be members oftriple (or higher order) systems. As a test of this prediction, wepresent a search for the signature of third components in archivalspectra of close binaries. In our sample of 75 objects, 23 show evidencefor the presence of a third component, down to a detection limit oftertiary flux contributions of about 0.8% at 5200 Å (consideringonly contact and semidetached binaries, we find 20 out of 66). In ahomogeneous subset of 59 contact binaries, we are fairly confident thatthe 15 tertiaries we have detected are all tertiaries present with massratios 0.28<~M3/M12<~0.75 and implied outerperiods P<~106 days. We find that if the frequency oftertiaries were the same as that of binary companions to solar-typestars, one would expect to detect about 12 tertiaries. In contrast, ifall contact binaries were in triple systems, one would expect about 20.Thus, our results are not conclusive but are sufficiently suggestive towarrant further studies.
| Dwarfs in the Local Region
We present lithium, carbon, and oxygen abundance data for a sample ofnearby dwarfs-a total of 216 stars-including samples within 15 pc of theSun, as well as a sample of local close giant planet (CGP) hosts (55stars) and comparison stars. The spectroscopic data for this work have aresolution of R~60,000, a signal-to-noise ratio >150, and spectralcoverage from 475 to 685 nm. We have redetermined parameters and derivedadditional abundances (Z>10) for the CGP host and comparison samples.From our abundances for elements with Z>6 we determine the meanabundance of all elements in the CGP hosts to range from 0.1 to 0.2 dexhigher than nonhosts. However, when relative abundances ([x/Fe]) areconsidered we detect no differences in the samples. We find nodifference in the lithium contents of the hosts versus the nonhosts. Theplanet hosts appear to be the metal-rich extension of local regionabundances, and overall trends in the abundances are dominated byGalactic chemical evolution. A consideration of the kinematics of thesample shows that the planet hosts are spread through velocity space;they are not exclusively stars of the thin disk.
| Contact Binaries with Additional Components. I. The Extant Data
We have attempted to establish observational evidence for the presenceof distant companions that may have acquired and/or absorbed angularmomentum during the evolution of multiple systems, thus facilitating orenabling the formation of contact binaries. In this preliminaryinvestigation we use several techniques (some of themdistance-independent) and mostly disregard the detection biases ofindividual techniques in an attempt to establish a lower limit to thefrequency of triple systems. While the whole sample of 151 contactbinary stars brighter than Vmax=10 mag gives a firm lowerlimit of 42%+/-5%, the corresponding number for the much better observednorthern-sky subsample is 59%+/-8%. These estimates indicate that mostcontact binary stars exist in multiple systems.
| Lithium Abundances of F-, G-, and K-Type Stars: Profile-Fitting Analysis of the Li I 6708 Doublet
An extensive profile-fitting analysis was performed for the Li(+Fe)6707-6708Å feature of nearby 160 F-K dwarfs/subgiants (including27 planet-host stars) in the Galactic disk ( 7000 K ≳Teff ≳ 5000 K, -1 ≲ [Fe/H] ≲ +0.4), in orderto establish the photospheric lithium abundances of these stars. Thenon-LTE effect (though quantitatively insignificant) was taken intoaccount based on our statistical equilibrium calculations, which werecarried out on an adequate grid of models. Our results confirmed most ofthe interesting observational characteristics revealed by recentlypublished studies, such as the bimodal distribution of the Li abundancesfor stars at Teff ≳ 6000 K, the satisfactory agreementof the upper envelope of the A(Li) vs. [Fe/H] distribution with thetheoretical models, the existence of a positive correlation betweenA(Li) and the stellar mass, and the tendency of lower lithium abundancesof planet-host stars (as compared to stars without planets) at thenarrow ``transition'' region of 5900 K ≳ Teff ≳5800 K. The solar Li abundance derived from this analysis is 0.92 (H =12.00), which is by 0.24dex lower than the widely referenced standardvalue of 1.16.
| Spectroscopic Study on the Atmospheric Parameters of Nearby F--K Dwarfs and Subgiants
Based on a collection of high-dispersion spectra obtained at OkayamaAstrophysical Observatory, the atmospheric parameters (Teff,log g, vt, and [Fe/H]) of 160 mid-F through early-K starswere extensively determined by the spectroscopic method using theequivalent widths of Fe I and Fe II lines along with the numericaltechnique of Takeda et al. (2002, PASJ, 54, 451). The results arecomprehensively discussed and compared with the parameter values derivedby different approaches (e.g., photometric colors, theoreticalevolutionary tracks, Hipparcos parallaxes, etc.) as well as with thepublished values found in various literature. It has been confirmed thatour purely spectroscopic approach yields fairly reliable and consistentresults.
| The `solar model problem' solved by the abundance of neon in nearby stars
The interior structure of the Sun can be studied with great accuracyusing observations of its oscillations, similar to seismology of theEarth. Precise agreement between helioseismological measurements andpredictions of theoretical solar models has been a triumph of modernastrophysics. A recent downward revision by 25-35 per cent of the solarabundances of light elements such as C, N, O and Ne (ref. 2) has,however, broken this accordance: models adopting the new abundancesincorrectly predict the depth of the convection zone, the depth profilesof sound speed and density, and the helium abundance. The discrepanciesare far beyond the uncertainties in either the data or the modelpredictions. Here we report neon-to-oxygen ratios measured in a sampleof nearby solar-like stars, using their X-ray spectra. The abundanceratios are all very similar and substantially larger than the recentlyrevised solar value. The neon abundance in the Sun is quite poorlydetermined. If the Ne/O abundance in these stars is adopted for the Sun,the models are brought back into agreement with helioseismologymeasurements.
| Kinematics of W Ursae Majoris type binaries and evidence of the two types of formation
We study the kinematics of 129 W UMa binaries and we discuss itsimplications on the contact binary evolution. The sample is found to beheterogeneous in the velocity space. That is, kinematically younger andolder contact binaries exist in the sample. A kinematically young (0.5Gyr) subsample (moving group) is formed by selecting the systems thatsatisfy the kinematical criteria of moving groups. After removing thepossible moving group members and the systems that are known to bemembers of open clusters, the rest of the sample is called the fieldcontact binary (FCB) group. The FCB group is further divided into fourgroups according to the orbital period ranges. Then, a correlation isfound in the sense that shorter-period less-massive systems have largervelocity dispersions than the longer-period more-massive systems.Dispersions in the velocity space indicate a 5.47-Gyr kinematical agefor the FCB group. Compared with the field chromospherically activebinaries (CABs), presumably detached binary progenitors of the contactsystems, the FCB group appears to be 1.61 Gyr older. Assuming anequilibrium in the formation and destruction of CAB and W UMa systems inthe Galaxy, this age difference is treated as an empirically deducedlifetime of the contact stage. Because the kinematical ages (3.21, 3.51,7.14 and 8.89 Gyr) of the four subgroups of the FCB group are muchlonger than the 1.61-Gyr lifetime of the contact stage, the pre-contactstages of the FCB group must dominantly be producing the largedispersions. The kinematically young (0.5 Gyr) moving group covers thesame total mass, period and spectral ranges as the FCB group. However,the very young age of this group does not leave enough room forpre-contact stages, and thus it is most likely that these systems wereformed in the beginning of the main sequence or during thepre-main-sequence contraction phase, either by a fission process or mostprobably by fast spiralling in of two components in a common envelope.
| New Minima of Selected Eclipsing Close Binaries
We present 180 CCD and photoelectric times of minima of selected closeeclipsing binaries.
| Inferring Coronal Structure from X-Ray Light Curves and Doppler Shifts: A Chandra Study of AB Doradus
The Chandra X-Ray Observatory continuously monitored the single coolstar AB Dor for a period lasting 88 ks (1.98Prot) in 2002December with the Low-Energy Transmission Grating HRC-S. The X-ray lightcurve shows rotational modulation with three peaks that repeat in twoconsecutive rotation cycles. These peaks may indicate the presence ofcompact emitting regions in the quiescent corona. Centroid shifts as afunction of phase in the strongest line profile, O VIII λ18.97,indicate Doppler rotational velocities with a semiamplitude of 30+/-10km s-1. By taking these diagnostics into account along withconstraints on the rotational broadening of line profiles (provided byarchival Chandra High-Energy Transmission Grating Fe XVII and FarUltraviolet Spectroscopic Explorer Fe XVIII profiles), we can constructa simple model of the X-ray corona that requires two components. One ofthese components is responsible for 80% of the X-ray emission and arisesfrom the pole and/or a homogeneously distributed corona. The secondcomponent consists of two or three compact active regions that causemodulation in the light curve and contribute to the O VIII centroidshifts. These compact regions account for 16% of the emission and arelocated near the stellar surface with heights of less than0.3R*. At least one of the compact active regions is locatedin the partially obscured hemisphere of the inclined star, while anotherof the active regions may be located at 40°. High-quality X-ray datasuch as these can test the models of the coronal magnetic fieldconfiguration as inferred from magnetic Zeeman Doppler imaging.
| Stars within 15 Parsecs: Abundances for a Northern Sample
We present an abundance analysis for stars within 15 pc of the Sunlocated north of -30° declination. We have limited our abundancesample to absolute magnitudes brighter than +7.5 and have eliminatedseveral A stars in the local vicinity. Our final analysis list numbers114 stars. Unlike Allende Prieto et al. in their consideration of a verysimilar sample, we have enforced strict spectroscopic criteria in thedetermination of atmospheric parameters. Nevertheless, our results arevery similar to theirs. We determine the mean metallicity of the localregion to be <[Fe/H]>=-0.07 using all stars and -0.04 when interlopersfrom the thick disk are eliminated.
| X-ray observations of the old open stellar cluster NGC 188
I present the analysis results from XMM-Newton observations of the oldopen stellar cluster NGC 188, which has an age of about 7 Gyr and a nearsolar metallicity. 58 X-ray sources were detected in the field of viewof the EPIC MOS and pn cameras, and 46 sources are new X-ray detections.Visible counterparts were found for 20 sources including the variablestar WV 28, the W UMa-type binaries V371 Cep and V372 Cep, and the redgiant V11. 9 X-ray sources are identified with probable clusternon-members, while 43 X-ray sources are of unknown membership. X-rayemission was detected from 6 stars with high membership probabilityabove a luminosity threshold of 1030 erg s-1. Thisindicates the presence of very active late-type stars in NGC 188 inspite of its old age. The HR diagram positions of two of these starsjust above the main sequence are reminiscent of those for W UrsaeMajoris-type contact binaries. Two other sources could be either membersof close binary systems or the product of the coalescence of W UMa typebinaries into single stars. One X-ray source in NGC 188 is located atthe bottom of the red giant branch in an evolutionary status similar tothat of an FK Comae-type star. Another X-ray source detected in NGC 188has the HR diagram position of an M type star. Its X-ray to bolometricluminosity ratio, greater than the canonical 10-3 saturationlevel, suggests that the star was flaring during XMM-Newtonobservations. M stars are most likely the most numerous X-ray sources inNGC 188 at lower X-ray luminosity thresholds.
| New V Light Curve and Ephemeris of the Binary System 44i Bootis
This paper presents the results of the photometric observations of the WUMa-type eclipsing binary 44i Bootis in V band, carried out in 1992. Acomplete light curve together with times of minima was obtained. Lightand period variations of the system are also discussed.
| Period and light variations for the cool, overcontact binary BX Pegasi
New charge-coupled device photometric observations of the W UMa-typebinary BX Pegasi (BX Peg) were collected on four nights from 1999October to 2000 September. The light curve was covered completely ineach season. Seven new times of minimum light were determined. It wasfound that the orbital period of the system has varied recently in asinusoidal way, superimposed on a downward parabolic variation. Thelong-term period decrease rate is deduced as dP/dt=-8.62 or 9.59 ×10-8 d yr-1, which can be interpreted as eithermass transfer from the more massive cool star to the less massive hotcomponent, or as the combination of mass transfer and angular momentumloss due to a magnetic stellar wind. The period and amplitude of thesinusoidal period variation were calculated to be about 35.3 yr and0.015 d, respectively. The light curves of BX Peg are asymmetric andshow year-to-year light variability. A spot model has been applied toanalyse these light curves. After using the light curves of 1999 asreference ones, we solve those of 2000 by adjusting only the spotparameters. One cool-spot model on the cool secondary satisfies theobserved light curves of both 1999 and 2000 quite well and shows a goodrepresentation of the BX Peg system for both the photospheric and spotdescriptions. The brightness variations of BX Peg are not coincidentwith the period variations and so do not conform to a prediction of theApplegate mechanism. We think the most likely cause of the cyclicalvariation is the light-time effect due to a third body, although nothird light was detected in the light-curve analysis. If it exists, thehypothetical object could be a very red main-sequence star or a whitedwarf. We have solved anew the historical published light curve for onlythe spot parameters and these closely resemble our spot parameters. Wespeculate that this result is associated with the small coronalsaturation of the cool star of the system.
| CCD Times of Minima of Selected Eclipsing Binaries
682 CCD minima observations of 259 eclipsing binaries made mainly byauthor are presented. The observed stars were chosen mainly fromcatalogue BRKA of observing programme of BRNO-Variable Star Section ofCAS.
| Eclipsing Binaries in the Blue Envelope of the Period-Color Diagram
Several interesting close eclipsing binaries in the short-period blueenvelope of the period-color diagram are investigated. Their l O--Cdiagrams are discussed and in several cases new solutions are given.
| The Density of Coronal Plasma in Active Stellar Coronae
We have analyzed high-resolution X-ray spectra of a sample of 22 activestars observed with the High Energy Transmission Grating Spectrometer onChandra in order to investigate their coronal plasma density. Densitieswere investigated using the lines of the He-like ions O VII, Mg XI, andSi XIII. Si XIII lines in all stars of the sample are compatible withthe low-density limit (i.e., ne<~1013cm-3), casting some doubt on results based on lowerresolution Extreme Ultraviolet Explorer (EUVE) spectra finding densitiesne>1013 cm-3. Mg XI lines betray thepresence of high plasma densities up to a few times 1012cm-3 for most of the sources with higher X-ray luminosity(>~1030 ergs s-1) stars with higherLX and LX/Lbol tend to have higherdensities at high temperatures. Ratios of O VII lines yield much lowerdensities of a few times 1010 cm-3, indicatingthat the ``hot'' and ``cool'' plasma resides in physically differentstructures. In the cases of EV Lac, HD 223460, Canopus, μ Vel, TYPyx, and IM Peg, our results represent the first spectroscopic estimatesof coronal density. No trends in density-sensitive line ratios withstellar parameters effective temperature and surface gravity were found,indicating that plasma densities are remarkably similar for stars withpressure scale heights differing by up to 3 orders of magnitude. Ourfindings imply remarkably compact coronal structures, especially for thehotter (~7 MK) plasma emitting the Mg XI lines characterized by thecoronal surface filling factor, fMgXI, ranging from10-4 to 10-1, while we find fOVIIvalues from a few times 10-3 up to ~1 for the cooler (~2 MK)plasma emitting the O VII lines. We find that fOVIIapproaches unity at the same stellar surface X-ray flux level ascharacterizes solar active regions, suggesting that these stars becomecompletely covered by active regions. At the same surface flux level,fMgXI is seen to increase more sharply with increasingsurface flux. These results appear to support earlier suggestions thathot 107 K plasma in active coronae arises from flaringactivity and that this flaring activity increases markedly once thestellar surface becomes covered with active regions. Comparison of ourmeasured line fluxes with theoretical models suggests that significantresidual model inaccuracies might be present and, in particular, thatcascade contributions to forbidden and intercombination lines resultingfrom dielectronic recombination might be to blame.
| Nearby stars of the Galactic disk and halo. III.
High-resolution spectroscopic observations of about 150 nearby stars orstar systems are presented and discussed. The study of these and another100 objects of the previous papers of this series implies that theGalaxy became reality 13 or 14 Gyr ago with the implementation of amassive, rotationally-supported population of thick-disk stars. The veryhigh star formation rate in that phase gave rise to a rapid metalenrichment and an expulsion of gas in supernovae-driven Galactic winds,but was followed by a star formation gap for no less than three billionyears at the Sun's galactocentric distance. In a second phase, then, thethin disk - our ``familiar Milky Way'' - came on stage. Nowadays ittraces the bright side of the Galaxy, but it is also embedded in a hugecoffin of dead thick-disk stars that account for a large amount ofbaryonic dark matter. As opposed to this, cold-dark-matter-dominatedcosmologies that suggest a more gradual hierarchical buildup throughmergers of minor structures, though popular, are a poor description forthe Milky Way Galaxy - and by inference many other spirals as well - if,as the sample implies, the fossil records of its long-lived stars do notstick to this paradigm. Apart from this general picture that emergeswith reference to the entire sample stars, a good deal of the presentwork is however also concerned with detailed discussions of manyindividual objects. Among the most interesting we mention the bluestraggler or merger candidates HD 165401 and HD 137763/HD 137778, thelikely accretion of a giant planet or brown dwarf on 59 Vir in itsrecent history, and HD 63433 that proves to be a young solar analog at\tau200 Myr. Likewise, the secondary to HR 4867, formerly suspectednon-single from the Hipparcos astrometry, is directly detectable in thehigh-resolution spectroscopic tracings, whereas the visual binary \chiCet is instead at least triple, and presumably even quadruple. Withrespect to the nearby young stars a complete account of the Ursa MajorAssociation is presented, and we provide as well plain evidence foranother, the ``Hercules-Lyra Association'', the likely existence ofwhich was only realized in recent years. On account of its rotation,chemistry, and age we do confirm that the Sun is very typical among itsG-type neighbors; as to its kinematics, it appears however not unlikelythat the Sun's known low peculiar space velocity could indeed be thecause for the weak paleontological record of mass extinctions and majorimpact events on our parent planet during the most recent Galactic planepassage of the solar system. Although the significance of thiscorrelation certainly remains a matter of debate for years to come, wepoint in this context to the principal importance of the thick disk fora complete census with respect to the local surface and volumedensities. Other important effects that can be ascribed to this darkstellar population comprise (i) the observed plateau in the shape of theluminosity function of the local FGK stars, (ii) a small thoughsystematic effect on the basic solar motion, (iii) a reassessment of theterm ``asymmetrical drift velocity'' for the remainder (i.e. the thindisk) of the stellar objects, (iv) its ability to account for the bulkof the recently discovered high-velocity blue white dwarfs, (v) itsmajor contribution to the Sun's 220 km s-1 rotationalvelocity around the Galactic center, and (vi) the significant flatteningthat it imposes on the Milky Way's rotation curve. Finally we note ahigh multiplicity fraction in the small but volume-complete local sampleof stars of this ancient population. This in turn is highly suggestivefor a star formation scenario wherein the few existing single stellarobjects might only arise from either late mergers or the dynamicalejection of former triple or higher level star systems.
| On the sizes of stellar X-ray coronae
Spatial information from stellar X-ray coronae cannot be assesseddirectly, but scaling laws from the solar corona make it possible toestimate sizes of stellar coronae from the physical parameterstemperature and density. While coronal plasma temperatures have longbeen available, we concentrate on the newly available densitymeasurements from line fluxes of X-ray lines measured for a large sampleof stellar coronae with the Chandra and XMM-Newton gratings. We compileda set of 64 grating spectra of 42 stellar coronae. Line counts of strongH-like and He-like ions and Fe XXI lines were measured with the CORAsingle-purpose line fitting tool by \cite{newi02}. Densities areestimated from He-like f/i flux ratios of O VII and Ne IX representingthe cooler (1-6 MK) plasma components. The densities scatter between logne ≈ 9.5-11 from the O VII triplet and between logne ≈ 10.5-12 from the Ne IX triplet, but we caution thatthe latter triplet may be biased by contamination from Fe XIX and Fe XXIlines. We find that low-activity stars (as parameterized by thecharacteristic temperature derived from H- and He-like line flux ratios)tend to show densities derived from O VII of no more than a few times1010 cm-3, whereas no definitive trend is foundfor the more active stars. Investigating the densities of the hotterplasma with various Fe XXI line ratios, we found that none of thespectra consistently indicates the presence of very high densities. Weargue that our measurements are compatible with the low-density limitfor the respective ratios (≈ 5× 1012cm-3). These upper limits are in line with constant pressurein the emitting active regions. We focus on the commonly used \cite{rtv}scaling law to derive loop lengths from temperatures and densitiesassuming loop-like structures as identical building blocks. We derivethe emitting volumes from direct measurements of ion-specific emissionmeasures and densities. Available volumes are calculated from theloop-lengths and stellar radii, and are compared with the emittingvolumes to infer filling factors. For all stages of activity we findsimilar filling factors up to 0.1.Appendix A is only available in electronic form athttp://www.edpsciences.org
| X-ray spectroscopy of the W UMa-type binary 44 Bootis
44 Boo B, a W UMa-type binary system, was observed in June 2001 duringone entire revolution period with the XMM-Newton observatory. The countrate in the 0.3 to 2 keV band is constant in average with 5 to 20% countrate increases reminiscent of flares. Spectral fitting of the EPICspectra indicates a corona configuration with little contribution fromquiet regions, similar to the Sun. On the contrary, the (2-9) ×106 K temperature range of the ``cool'' plasma suggests thatthe active corona around the two companions is densely filled withlow-lying loops similar to those found in solar-type active regions. The44 Boo O VII He-like triplet constrains the electron density to an upperlimit ne < 8.6 × 1010 cm-3. Weargue that this low-lying loop system may be overlaid by larger loops.Magnetic reconnection phenomena in this large loops system may explainthe characteristic flare decay time in the light curve that implies looplengths of about 16 × 109 cm. An extended corona around44 Boo would explain the absence of eclipses in its X-ray light curve.The average element abundance in 44 Boo corona is found to be lower thanthe solar photospheric value. The spectral analysis indicates enhancedabundances of oxygen and neon relative to iron which suggest an inverseFIP effect. Compared with other active binary systems such as RSCVn orBY Dra, 44 Boo has relatively less material at temperatures higher than107 K and the temperature of its hottest plasma componentappears to be lower.
| On the properties of contact binary stars
We have compiled a catalogue of light curve solutions of contact binarystars. It contains the results of 159 light curve solutions. Theproperties of contact binary stars were studied using the cataloguedata. As is well known since Lucy's (\cite{Lucy68a},b) and Mochnacki's(\cite{Mochnacki81}) studies, primary components transfer their ownenergy to the secondary star via the common envelope around the twostars. This transfer was parameterized by a transfer parameter (ratio ofthe observed and intrinsic luminosities of the primary star). We provethat this transfer parameter is a simple function of the mass andluminosity ratios. We introduced a new type of contact binary stars: Hsubtype systems which have a large mass ratio (q>0.72). These systemsshow behaviour in the luminosity ratio- transfer parameter diagram thatis very different from that of other systems and according to ourresults the energy transfer rate is less efficient in them than in othertypes of contact binary stars. We also show that different types ofcontact binaries have well defined locations on the mass ratio -luminosity ratio diagram. Several contact binary systems do not followLucy's relation (L2/L1 =(M2/M1)0.92). No strict mass ratio -luminosity ratio relation of contact binary stars exists.Tables 2 and 3 are available in electronic form athttp://www.edpsciences.org
| S4N: A spectroscopic survey of stars in the solar neighborhood. The Nearest 15 pc
We report the results of a high-resolution spectroscopic survey of allthe stars more luminous than M_V = 6.5 mag within 14.5 pc from the Sun.The Hipparcos catalog's completeness limits guarantee that our survey iscomprehensive and free from some of the selection effects in othersamples of nearby stars. The resulting spectroscopic database, which wehave made publicly available, includes spectra for 118 stars obtainedwith a resolving power of R ≃ 50 000, continuous spectral coveragebetween 362-921 nm, and typical signal-to-noise ratios in therange 150-600. We derive stellar parameters and perform a preliminaryabundance and kinematic analysis of the F-G-K stars in the sample. Theinferred metallicity ([Fe/H]) distribution is centered at about -0.1dex, and shows a standard deviation of 0.2 dex. A comparison with largersamples of Hipparcos stars, some of which have been part of previousabundance studies, suggests that our limited sample is representative ofa larger volume of the local thin disk. We identify a number ofmetal-rich K-type stars which appear to be very old, confirming theclaims for the existence of such stars in the solar neighborhood. Withatmospheric effective temperatures and gravities derived independentlyof the spectra, we find that our classical LTE model-atmosphere analysisof metal-rich (and mainly K-type) stars provides discrepant abundancesfrom neutral and ionized lines of several metals. This ionizationimbalance could be a sign of departures from LTE or inhomogeneousstructure, which are ignored in the interpretation of the spectra.Alternatively, but seemingly unlikely, the mismatch could be explainedby systematic errors in the scale of effective temperatures. Based ontransitions of majority species, we discuss abundances of 16 chemicalelements. In agreement with earlier studies we find that the abundanceratios to iron of Si, Sc, Ti, Co, and Zn become smaller as the ironabundance increases until approaching the solar values, but the trendsreverse for higher iron abundances. At any given metallicity, stars witha low galactic rotational velocity tend to have high abundances of Mg,Si, Ca, Sc, Ti, Co, Zn, and Eu, but low abundances of Ba, Ce, and Nd.The Sun appears deficient by roughly 0.1 dex in O, Si, Ca, Sc, Ti, Y,Ce, Nd, and Eu, compared to its immediate neighbors with similar ironabundances.Based on observations made with the 2.7 m telescope at the McDonaldObservatory of the University of Texas at Austin (Texas), and the 1.52 mtelescope at the European Southern Observatory (La Silla, Chile) underthe agreement with the CNPq/Observatorio Nacional (Brazil).Tables 3-5 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/420/183
| The Geneva-Copenhagen survey of the Solar neighbourhood. Ages, metallicities, and kinematic properties of 14 000 F and G dwarfs
We present and discuss new determinations of metallicity, rotation, age,kinematics, and Galactic orbits for a complete, magnitude-limited, andkinematically unbiased sample of 16 682 nearby F and G dwarf stars. Our63 000 new, accurate radial-velocity observations for nearly 13 500stars allow identification of most of the binary stars in the sampleand, together with published uvbyβ photometry, Hipparcosparallaxes, Tycho-2 proper motions, and a few earlier radial velocities,complete the kinematic information for 14 139 stars. These high-qualityvelocity data are supplemented by effective temperatures andmetallicities newly derived from recent and/or revised calibrations. Theremaining stars either lack Hipparcos data or have fast rotation. Amajor effort has been devoted to the determination of new isochrone agesfor all stars for which this is possible. Particular attention has beengiven to a realistic treatment of statistical biases and errorestimates, as standard techniques tend to underestimate these effectsand introduce spurious features in the age distributions. Our ages agreewell with those by Edvardsson et al. (\cite{edv93}), despite severalastrophysical and computational improvements since then. We demonstrate,however, how strong observational and theoretical biases cause thedistribution of the observed ages to be very different from that of thetrue age distribution of the sample. Among the many basic relations ofthe Galactic disk that can be reinvestigated from the data presentedhere, we revisit the metallicity distribution of the G dwarfs and theage-metallicity, age-velocity, and metallicity-velocity relations of theSolar neighbourhood. Our first results confirm the lack of metal-poor Gdwarfs relative to closed-box model predictions (the ``G dwarfproblem''), the existence of radial metallicity gradients in the disk,the small change in mean metallicity of the thin disk since itsformation and the substantial scatter in metallicity at all ages, andthe continuing kinematic heating of the thin disk with an efficiencyconsistent with that expected for a combination of spiral arms and giantmolecular clouds. Distinct features in the distribution of the Vcomponent of the space motion are extended in age and metallicity,corresponding to the effects of stochastic spiral waves rather thanclassical moving groups, and may complicate the identification ofthick-disk stars from kinematic criteria. More advanced analyses of thisrich material will require careful simulations of the selection criteriafor the sample and the distribution of observational errors.Based on observations made with the Danish 1.5-m telescope at ESO, LaSilla, Chile, and with the Swiss 1-m telescope at Observatoire deHaute-Provence, France.Complete Tables 1 and 2 are only available in electronic form at the CDSvia anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/418/989
| Stellar Coronal Astronomy
Coronal astronomy is by now a fairly mature discipline, with a quartercentury having gone by since the detection of the first stellar X-raycoronal source (Capella), and having benefitted from a series of majororbiting observing facilities. Serveral observational characteristics ofcoronal X-ray and EUV emission have been solidly established throughextensive observations, and are by now common, almost text-book,knowledge. At the same time the implications of coronal astronomy forbroader astrophysical questions (e.g.Galactic structure, stellarformation, stellar structure, etc.) have become appreciated. Theinterpretation of stellar coronal properties is however still often opento debate, and will need qualitatively new observational data to bookfurther progress. In the present review we try to recapitulate our viewon the status of the field at the beginning of a new era, in which thehigh sensitivity and the high spectral resolution provided by Chandraand SMM-Newton will address new questions which were not accessiblebefore.
| Period Changes of Two W UMa-Type Contact Binaries: RW Comae Berenices and CC Comae Berenices
From the present times of minimum light and those collected from theliterature, changes in the orbital period of the two W UMa-type contactbinaries RW Com and CC Com are analyzed. The results reveal that theperiod changes of these two systems show the same natures, with ashort-term oscillation superposed on the secular decrease. For RW Com,its period shows a secular decrease at a rate ofdP/dt=0.43×10-7 days yr-1. An oscillationwith a periodicity of 13.7 yr and an amplitude ofΔP=5.4×10-7 days is superposed on the seculardecrease. For CC Com, its period shows a secular decrease at a rate ofdP/dt=0.40×10-7 days yr-1. An oscillationwith a periodicity of 16.1 yr and an amplitude ofΔP=2.8×10-7 days is superposed on the seculardecrease. The period secular decreases of the two systems may beexplained by a mass-transfer rate of dm/dt=0.29×10-7Msolar yr-1 for RW Com anddm/dt=0.52×10-7 Msolar yr-1 forCC Com. The period short-term oscillations of the two systems may beexplained by the magnetic activity cycle model given by Applegate, andthe parameters for the magnetic activity cycle model are presented.
| Some anomalies in the occurrence of debris discs around main-sequence A and G stars
Debris discs consist of large dust grains that are generated bycollisions of comets or asteroids around main-sequence stars, and thequantity and distribution of debris may be used to detect the presenceof perturbing planets akin to Neptune. We use stellar and disc surveysto compare the material seen around A- and G-type main-sequence stars.Debris is detected much more commonly towards A stars, even when acomparison is made only with G stars of comparable age. Detection ratesare consistent with disc durations of ~0.5 Gyr, which may occur at anytime during the main sequence. The higher detection rate for A stars canresult from this duration being a larger fraction of the main-sequencelifetime, possibly boosted by a globally slightly larger disc mass thanfor the G-type counterparts. The disc mass range at any given age is afactor of at least ~100 and any systematic decline with time is slow,with a power law estimated to not be steeper than t-1/2.Comparison with models shows that dust can be expected as late as a fewGyr when perturbing planetesimals form slowly at large orbital radii.Currently, the Solar system has little dust because the radius of theKuiper Belt is small and hence the time-scale to produce planetesimalswas less than 1 Gyr. However, the apparently constant duration of ~0.5Gyr when dust is visible is not predicted by the models.
| Minimum Times of Several Eclipsing Binaries
We present 26 minima times of 11 eclipsing binaries, observed between1996 and 1999.
|
Yeni bir Makale Öner
Ýlgili Baðlantýlar
Yeni Bir Baðlantý Öner
sonraki gruplarýn üyesi:
|
Gözlemler ve gökölçümü verileri
Kataloglar ve belirtme:
|
