תוכן
תמונות
הוסף תמונה שלך
DSS Images Other Images
מאמרים קשורים
From the Solar Corona to Clusters of Galaxies: The Radio Astronomy of Bruce Slee
Owen Bruce Slee is one of the pioneers of Australian radio astronomy.During World War II he independently discovered solar radio emission,and, after joining the CSIRO Division of Radiophysics, used a successionof increasingly more sophisticated radio telescopes to examine anamazing variety of celestial objects and phenomena. These ranged fromthe solar corona and other targets in our solar system, to differenttypes of stars and the ISM in our Galaxy, and beyond to distant galaxiesand clusters of galaxies. Although long retired, Slee continues to carryout research, with emphasis on active stars and clusters of galaxies. Aquiet and unassuming man, Slee has spent more than half a century makingan important, wide-ranging contribution to astronomy, and his workdeserves to be more widely known.
| The photospheric abundances of active binaries. II. Atmospheric parameters and abundance patterns for 6 single-lined RS CVn systems
Photospheric parameters and abundances are presented for a sample ofsingle-lined chromospherically active binaries from a differential LTEanalysis of high-resolution spectra. Abundances have been derived for 13chemical species, including several key elements such as Li, Mg, and Ca.Two methods have been used. The effective temperatures, surfacegravities and microturbulent velocities were first derived from a fullyself-consistent analysis of the spectra, whereby the temperature isdetermined from the excitation equilibrium of the Fe I lines. The secondapproach relies on temperatures derived from the (B-V) colour index.These two methods give broadly consistent results for the stars in oursample, suggesting that the neutral iron lines are formed underconditions close to LTE. We discuss the reliability in the context ofchromospherically active stars of various colour indices used astemperature indicators, and conclude that the (V-R) and (V-I) coloursare likely to be significantly affected by activity processes.Irrespective of the method used, our results indicate that the X-rayactive binaries studied are not as metal poor as previously claimed, butare at most mildly iron-depleted relative to the Sun (-0.41protect <~[Fe/H]protect la +0.11). A significant overabundance of several chemicalspecies is observed (e.g., the alpha -synthezised elements). Theseabundance patterns are discussed in relation to stellar activity.Based on observations collected at ESO (La Silla, Chile).Table A.1 is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/412/495
| ICCD Speckle Observations of Binary Stars. XXIII. Measurements during 1982-1997 from Six Telescopes, with 14 New Orbits
We present 2017 observations of 1286 binary stars, observed by means ofspeckle interferometry using six telescopes over a 15 year period from1982 April to 1997 June. These measurements constitute the 23dinstallment in CHARA's speckle program at 2 to 4 m class telescopes andinclude the second major collection of measurements from the MountWilson 100 inch (2.5 m) Hooker Telescope. Orbital elements are alsopresented for 14 systems, seven of which have had no previouslypublished orbital analyses.
| Ca II activity and rotation in F-K evolved stars
Ca II H and K high resolution observations for 60 evolved stars in thefield and in 5 open clusters are presented. From these spectrachromospheric fluxes are derived, and a homogeneous sample of more than100 giants is built adding data from the literature. In addition, formost stars, rotational velocities were derived from CORAVELobservations. By comparing chromospheric emission in the cluster starswe confirm the results of Pasquini & Brocato (1992): chromosphericactivity depends on the stellar effective temperature, and mass, whenintermediate mass stars (M ~ 4 Msun) are considered. TheHyades and the Praesepe clump giants show the same level of activity, asexpected from stars with similar masses and effective temperatures. Adifference of up to 0.4 dex in the chromospheric fluxes among the Hyadesgiants is recorded and this sets a clear limit to the intrinsic spreadof stellar activity in evolved giants. These differences in otherwisevery similar stars are likely due to stellar cycles and/or differencesin the stellar initial angular momentum. Among the field stars none ofthe giants with (V-R)o < 0.4 and Ia supergiants observedshows a signature of Ca II activity; this can be due either to the realabsence of a chromosphere, but also to other causes which preclude theappearance of Ca II reversal. By analyzing the whole sample we find thatchromospheric activity scales linearly with stellar rotational velocityand a high power of stellar effective temperature: F'k ~Teff7.7 (Vsini)0.9. This result can beinterpreted as the effect of two chromospheric components of differentnature: one mechanical and one magnetic. Alternatively, by using theHipparcos parallaxes and evolutionary tracks, we divide the sampleaccording to the stellar masses, and we follow the objects along anevolutionary track. For each range of masses activity can simply beexpressed as a function of only one parameter: either theTeff or the angular rotation Omega , with laws F'k~ Omega alpha , because angular velocity decreases witheffective temperature along an evolutionary track. By using theevolutionary tracks and the observed Vsini we investigate the evolutionof the angular momentum for evolved stars in the range 1-5Msun. For the 1.6-3 solar mass stars the data are consistentwith the IOmega =const law while lower and higher masses follow a lawsimilar to IOmega 2=const, where I is the computed stellarmomentum of inertia. We find it intriguing that Vsini remains almostconstant for 1Msun stars along their evolution; if a similarbehavior is shared by Pop II stars, this could explain the relativelyhigh degree of activity observed in Pop II giants. Finally, through theuse of models, we have verified the consistency of the F'k ~Omega alpha and the IOmega beta = Const lawsderived, finding an excellent agreement. This representation, albeitcrude (the models do not consider, for instance, mass losses) representsthe evolution of Ca II activity and of the angular momentum in asatisfactory way in most of the portion of HR diagram analyzed.Different predictions could be tested with observations in selectedclusters. Based on observations collected at ESO, La Silla. Tables 1-3are only available in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| Coordinated multiwavelength observations of the RS CVn system CF TUC
The relatively bright eclipsing RS CVn binary CF Tuc was observed inboth optical (photometry and spectroscopy) and microwave ranges in a`multisite, multiwavelength' campaign in 1996. The microwave data,gathered using the Australia Telescope Compact Array, covered slightlymore than one complete orbital cycle at 4.8 and 8.64 GHz in onecontinuous run. There was also limited coverage at 1.38 and 2.38 GHz.High-dispersion spectroscopy was obtained using the McLellan 1-mtelescope and echelle spectrograph at Mt John University Observatory(New Zealand). Supporting photometry came from smaller scale facilitiesassociated with the Royal Astronomical Society of New Zealand (RASNZ).The data show the correlation between microwave signal enhancement andphotometric flux diminution (maculation effect) noted previously insimilar studies. The spectroscopy also points to the phenomena beingrelated to a very enhanced active region on the secondary star locatedclose to the maculation region. Cross-correlation of the microwaveemission data between the 4.8- and 8.64-GHz ranges shows a strongpositive correlation, with a significant lag of the 4.8-GHz behind the8.64-GHz variations. This indicates energized waves propagating throughthe corona of the K4 subgiant at the electron sound speed. The emittingregion may have been eclipsed by the primary star, but such eclipseeffects are of marginal significance for the present analysis. Anattempt is made to put all the evidence into one coherent picture of theactive region.
| A catalog of rotational and radial velocities for evolved stars
Rotational and radial velocities have been measured for about 2000evolved stars of luminosity classes IV, III, II and Ib covering thespectral region F, G and K. The survey was carried out with the CORAVELspectrometer. The precision for the radial velocities is better than0.30 km s-1, whereas for the rotational velocity measurementsthe uncertainties are typically 1.0 km s-1 for subgiants andgiants and 2.0 km s-1 for class II giants and Ib supergiants.These data will add constraints to studies of the rotational behaviourof evolved stars as well as solid informations concerning the presenceof external rotational brakes, tidal interactions in evolved binarysystems and on the link between rotation, chemical abundance and stellaractivity. In this paper we present the rotational velocity v sin i andthe mean radial velocity for the stars of luminosity classes IV, III andII. Based on observations collected at the Haute--Provence Observatory,Saint--Michel, France and at the European Southern Observatory, LaSilla, Chile. Table \ref{tab5} also available in electronic form at CDSvia anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| Ultraviolet and Optical Studies of Binaries with Luminous Cool Primaries and Hot Companions. V. The Entire IUE Sample
We have obtained or retrieved IUE spectra for over 100 middle- andlate-type giant and supergiant stars whose spectra indicate the presenceof a hot component earlier than type F2. The hot companions areclassified accurately by temperature class from their far-UV spectra.The interstellar extinction of each system and the relative luminositiesof the components are derived from analysis of the UV and opticalfluxes, using a grid of UV intrinsic colors for hot dwarfs. We find thatthere is fair agreement in general between current UV spectralclassification and ground-based hot component types, in spite of thedifficulties of assigning the latter. There are a few cases in which thecool component optical classifications disagree considerably with thetemperature classes inferred from our analysis of UV and opticalphotometry. The extinction parameter agrees moderately well with otherdeterminations of B-V color excess. Many systems are worthy of furtherstudy especially to establish their spectroscopic orbits. Further workis planned to estimate luminosities of the cool components from the dataherein; in many cases, these luminosities' accuracies should becomparable to or exceed those of the Hipparcos parallaxes.
| EUV Emission from RS Canum Venaticorum binaries.
We performed a study of 104 RS CVn systems in the extreme ultraviolet(EUV) using the all-sky survey data obtained by the Extreme UltravioletExplorer (EUVE). The present sample includes several new RS CVndetections; 11 more than in the published EUVE catalogs, and 8 more thanin the ROSAT Wide Field Camera catalog. The ratio of detections tonon-detections remained constant throughout the sky, implying that ourdetections are not limited by the exposure time but are most likelylimited by absorption from the interstellar medium. A general trend ofincreasing Lex/B (50-180Å) flux with decreasing rotational periodis clear. The dwarf systems exhibit a leveling-off for the fasterrotators. In contrast, the evolved systems exhibit no such effect. Forthe RS CVn systems the losses in the EUV represent a smaller fraction ofthe coronal radiative losses, as compared to active late-type dwarfs.
| A photometric and orbital analysis of GT MUSCAE
GT Mus is a quadruple system comprising a long-period RS CVn-type binary(HD 101379) and a pair of eclipsing A dwarfs (HD 101380). Six and a halfyears of UBV (RI)_C photometry obtained at the Mt John UniversityObservatory has enabled identification of four distinct types ofphotometric variability in this system. These are (1) a slowly changingmean magnitude, which probably arises from an activity-cycle-like effectin the active component of HD 101379, (2) a periodic variation(P_rot~64d), which is attributed to rotational modulation due to spotson the active star, (3) a periodic variation (P_eclipse=2.7546d) due tothe eclipses of HD 101380, and (4) an excess in the I band, which occurson a short time-scale (<1d) and is probably associated with HD 101379activity. The evolution of the light curve of HD 101379 is fast withrespect to the rotational period, suggesting rapid spot evolution forwhich we anticipate a possible model. The colours of HD 101379, even atmaximum brightness, are excessively red for its spectral type, unlessthere is significant reddening by dust. Radial velocity measurements ofHD 101379 are also presented, along with an improved determination ofthe orbit of this somewhat long-period (P_orb=61.448d) system.
| The radio corona of HD 101379
A programme to investigate the coronae of late-type stars usingfour-frequency radio continuum spectra from the Australia TelescopeCompact Array has yielded several spectra of HD 101379, including twoflare spectra in which fluxes in excess of 30 mJy were observed. In oneof the flares the degree of circular polarization was less than 1 percent, but in the other ~=11 per cent circular polarization was observedat 8.64 GHz. The spectra are interpreted in terms of the gyrosynchrotrontheory. Our modelling cannot reproduce the polarization and spectralproperties of these fluxes with a simple magnetic field configurationsuch as a dipole or single loop. Instead, we are forced to assume thatthe radio flux comes from a large number of relatively thin magneticloops, whose footprints cover about 2 per cent of the stellar surface.The loops probably extend out to 4 stellar radii from the star's centre.Also, we must assume mixed polarity loops to explain the low degree ofpolarization observed.
| A Photometric Analysis of the Active Chromosphere Star GT MUSCAE
Not Available
| Vitesses radiales. Catalogue WEB: Wilson Evans Batten. Subtittle: Radial velocities: The Wilson-Evans-Batten catalogue.
We give a common version of the two catalogues of Mean Radial Velocitiesby Wilson (1963) and Evans (1978) to which we have added the catalogueof spectroscopic binary systems (Batten et al. 1989). For each star,when possible, we give: 1) an acronym to enter SIMBAD (Set ofIdentifications Measurements and Bibliography for Astronomical Data) ofthe CDS (Centre de Donnees Astronomiques de Strasbourg). 2) the numberHIC of the HIPPARCOS catalogue (Turon 1992). 3) the CCDM number(Catalogue des Composantes des etoiles Doubles et Multiples) byDommanget & Nys (1994). For the cluster stars, a precise study hasbeen done, on the identificator numbers. Numerous remarks point out theproblems we have had to deal with.
| The active dynamo stars: RS CVn, BY Dra, FK Com, Algol, W UMa, and T Tau
Not Available
| Do we have a radio powerful star without a chromosphere?
Two radio stars are selected, both equally powerful in microwave radioemission, but one of which, HD 101379, with a superpower chromosphere,and the other one, HD 132742, without a noticable chromosphere. OnInternational Ultraviolet Explorer (IUE) recordings of HD 132742, abinary system like HD 101319, the doublet 2800 Mg II appears inabsorption. However, in the last case the faint emission traces of k andh Mg II have been discovered in the lowest parts of absorption profiles.The reconstructed profiles of these emission details seem to be shiftedto the relation of absorption profiles, which may be interpreted as aconfirmation of the non-chromospheric nature of these details. In thecase of radio star HD 101379, also a binary system, on the contrary, thedoublet 2800 Mg II appears as a strong emission, without shift. Theselimiting cases may be combined assuming that the sources of both typesof emission, radio and magnesium doublet, are located in the spacebetween the components of binary systems. The parameters of thesesources are obtained.
| 2800 MG2 emission in the most active radio stars
Spectral recordings in the region near 2800 A are examined from theInternational Ultraviolet Explorer (IUE) archives for the twenty mostactive radio stars, which are also close binary systems. In all of thesespectra, the doublet k and h Mg 11 is seen in strong emission. We findthat the observed fluxes of the magnesium doublet emission are directlyproportional to the mean fluxes of the observed radio emission at thefrequency of 8.4 GHz. The same correlation is found between the absoluteluminosities of the radio emission and magnesium doublet emissions. Itis argued that the source of both emissions, radio and magnesiumdoublet, is related to a high temperature stratified cloud locatedbetween the components of the binary system. The fluxes of the magnesiumdoublet emission of these sources are much larger by one or two ordersof magnitude, compared with the usual chromospheric magnesium emissionseen in single stars.
| Are the Rotational Axes Perpendicular to the Orbital Planes in the Binary Systems. I. Asynchronous Long-Period RS CVn Stars
We show that only 12 out of 27 analyzed binaries have data good enoughfor reliable determination of iorb and irot. In 4systems (out of these 12) assumption about coplanarity of rotationalequatorial plane and orbital plane is justified. In the remaining casesrotational axes are not perpendicular to the orbital planes(non-coplanar case). The commonly accepted assumption about thecoplanarity of rotational and orbital planes should be revised.
| Lithium in Rs-Canum Binaries and Related Chromospherically Active Stars - Part Three - Northern Rs-Canum Systems
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1994A&A...283..893R&db_key=AST
| The ROSAT All-Sky Survey of active binary coronae. I - Quiescent fluxes for the RS Canum Venaticorum systems
One hundred and thirty-six RS CV(n) active binary systems were observedwith the ROSAT Position Sensitive Proportional Counter (PSPC) during theAll-Sky Survey component of the mission. The entire sky was surveyed,which represents the largest sample of RS CV(n) systems observed to dateat any wavelength, including X-rays. X-ray surface fluxes for the RSCV(n) systems are found to lie in the range 10 exp 4 to 10 exp 8 ergs/sqcm seconds. Surface flux as a function of (B - V) color is reported. Adecrease in surface flux with increasing rotation period for the entiresample is observed. The rotation period provides the best stellar ororbital parameter to predict the X-ray surface flux level. The absenceof correlation of F(x) or L(x) with Gamma is noted due to the fact thatthe coronal heating mechanism for these active stars must be magnetic incharacter, and the magnetic field depends on the interaction betweenconvection and differential rotation inside the star. X-ray propertiesof the RS CV(n) systems with 6 cm radio and C IV UV emission systems iscompared.
| The ROSAT All-Sky Survey of active binary coronae. II - Coronal temperatures of the RS Canum Venaticorum systems
We present the results from an analysis of X-ray spectra of 44 RS CVnsystems obtained during the ROSAT All-Sky Survey with the PositionSensitive Proportional Counter (PSPC). Thermal plasma models with twotemperature components are found to reproduce the observations betterthan single or continuous temperature models. We typically find that abimodal distribution of temperatures centered near 2 x 10 exp 6 and 1.6x 10 exp 7 K fit the data best. We show that the PSPC temperatures agreewell with those from similar low-resolution measurements, althoughdifferences exist, primarily due to differing detector bandpasses. Aftercomparing coronal (either temperature or emission measure)characteristics with stellar parameters including rotation period anddynamo number, we find no compelling relationship. The height-integratedemission measures of the components in the two-temperature models,including a gravity term, are found to be well correlated withtemperature.
| ICCD speckle observations of binary stars. VIII - Measurements during 1989-1991 from the Cerro Tololo 4 M telescope
One-thousand eighty-eight observations of 947 binary star systems,observed by means of speckle interferometry with the 4 m telescope onCerro Tololo, are presented. These measurements, made during the period1989-1991, comprise the second installment of results stemming from theexpansion of our speckle program to the southern hemisphere.
| Lithium in Rs-Canum Binaries and Related Chromospherically Active Stars - Part Two - Spectrum Synthesis Analysis
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1993A&A...273..194R&db_key=AST
| Statistical Analysis of a Sample of Spectroscopic Binaries Containing Late Type Giants
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1993A&A...271..125B&db_key=AST
| Lithium in RS CVn binaries and related chromospherically active stars. I - Observational results
The present survey of the Li I 6708 A line in a sample of spectral typeG and K stars with luminosity classes III, IV, and V shows that manyK-type stars in the sample, including a large number of RS CVn binaries,show an anomalously high Li abundance relative to typical inactive starsof the same spectral type. Only a few stars in the sample are likely tobe premain sequence objects of stars which have recently arrived on themain sequence. Mechanisms that could lead to the enhanced Li absorptionin chromospherically active stars are discussed; these encompass largecool spots on the stellar surface, the production of Li in stellarflares by spallation reactions, and the evolution from main-sequenceprogenitors without, or with very shallow, outer convective zones.
| The non-synchronous systems among RS CVn stars
An RS CVn star is considered as nonsynchronous if its orbital perioddiffers by more than 4 percent from its photometric period. Of 88systems examined according to this criterion, 67 are synchronous, 21 arenonsynchronous. For the 21 nonsynchronous systems, it is found thattheir spectral types are most around KO-2 III and that their orbitaleccentricities are larger and their chromospheric activity lower thanthe average level.
| The interpretation of the Wilson-Bappu effect
The present study investigates the physical interpretation of theWilson-Bappu effect. A postulate is advanced according to which thiseffect expresses the dependence of the complete kinetic energy ofturbulent motions in the medium-generating magnesium emission in thedoublet 2800 Mg II on the velocity of turbulent motion. The experimentis carried out in relation to ten radio stars which are close binarysystems of the RS CVn-type. The measured magnitudes of the turbulentvelocity and the log-turbulent velocity for these objects are presented.The numerical magnitudes of absolute stellar luminosities for thesestars are obtained and the results as well as the absolute magnitudesfor the same stars are given. It is concluded that the magnitudesobtained have no relation to the chromosphere of the radio stars andthat the observed magnesium emission in these objects are generated outof the limits of their chromospheres.
| Close binaries observed polarimetrically
Not Available
| An astrometric catalogue of radio stars
The first part is presented of a radio star catalog encompassing 186objects whose selection was guided by the priority criteria of theHipparchos Input Catalogue Consortium. Since these criteria are wellsuited to the need for linkage of ground-based optical systems to radioreference frames, this first selection is also considered a suitablebase for the catalog. Seventeen categories of stellar parameters arefurnished for each of the stars, including optical and radio positions,optical and radio parallax, radial velocity, type of variability, andoptical structure.
| Stellar radio luminosity and stellar rotation
Two measurements of the microwave surface brightness of 63 activechromospheric stars, one related to surface flux and one related to fluxratio, are correlated with rotational parameters. Although both methodsare found to provide similar results, the surface flux method accountsfor more of the variance in the regression. The consistency of theobserved surface brightnesses of the sun and three BY-Dra-type flarestars with the present rotational relationships is considered.
|
הכנס מאמר חדש
לינקים קשורים
הכנס לינק חדש
משמש של הקבוצה הבאה
|
תצפית ומידע אסטרומטרי
| קבוצת-כוכבים: | זבוב |
| התרוממות ימנית: | 11h39m29.40s |
| סירוב: | -65°23'52.0" |
| גודל גלוי: | 5.17 |
| מרחק: | 172.117 פארסק |
| תנועה נכונה: | -30 |
| תנועה נכונה: | -5 |
| B-T magnitude: | 6.046 |
| V-T magnitude: | 5.181 |
קטלוגים וכינוים:
|
