Contenidos
Imágenes
Subir su imagen
DSS Images Other Images
Artículos relacionados
Bisectors of the cross-correlation function applied to stellar spectra. Discriminating stellar activity, oscillations and planets
Context: .Bisectors of strong, single spectral lines, usually the Fe I6252 line, have traditionally been used to examine the velocity fieldsin stellar atmospheres. This requires high S/N often achieved by summingmany individual spectra. Aims: .We investigate whether bisectorsderived from cross-correlation functions (CCF) of single-exposurespectra can be used to provide information on stellar atmospheres, andwhether they can be used to discriminate between radial velocity changescaused by planets, magnetic activity and oscillations. Methods:.Using a sample of bright stars observed with the HARPS spectrograph, weexamine the shapes of the bisectors of individual strong spectral linesin summed spectra, comparing with similar studies in the literature.Moreover, we examine four different quantitative CCF bisector measuresfor correlations with radial velocity and stellar parameters.Results: .We show that CCF bisector measures can be used forquantitative analysis, employing both the absolute values and thevariations. From absolute values, log g and absolute magnitude can beapproximated, and from the correlations with radial velocity one candistinguish between magnetic activity, oscillations and orbiting planetsas the probable cause of radial velocity variations. We confirm thatdifferent isolated spectral lines show different bisector shapes, evenbetween lines of the same element, calling for caution in trying toderive global stellar properties from the bisector of a CCF. For theactive star HR 1362 we suggest from the bisector shape an extraphotospheric heating caused by the chromosphere of several hundreddegrees. We confirm the fill-in of spectral lines of the Sun taken onthe daylight sky caused by Rayleigh-Brillouin and aerosol scattering,and we show for the first time that the fill-in has an asymmetriccomponent.
| Binarity, activity and metallicity among late-type stars. I. Methodology and application to HD 27536 and HD 216803
We present the first in a series of papers that attempt to investigatethe relation between binarity, magnetic activity, and chemical surfaceabundances of cool stars. In the current paper, we lay out and test twoabundance analysis methods and apply them to two well-known, active,single stars, HD 27536 (G8IV-III) and HD216803 (K5V), presenting photospheric fundamental parametersand abundances of Li, Al, Ca, Si, Sc, Ti, V, Cr, Fe, Co and Ni. Theabundances from the two methods agree within the errors for all elementsexcept calcium in HD 216803, which means that either method yields thesame fundamental model parameters and the same abundances. Activity isdescribed by the radiative loss in the Ca ii H and K lines with respectto the bolometric luminosity, through the activity index R_HK. Binarityis established by very precise radial velocity (RV) measurements usingHARPS spectra. The spectral line bisectors are examined for correlationsbetween RV and bisector shape to distinguish between the effects ofstellar activity and unseen companions. We show that HD 27536 exhibit RVvariations mimicking the effect of a low-mass (m 4 M_J) companionin a relatively close (a 1 AU) orbit. The variation is stronglycorrelated with the activity, and consistent with the known photometricperiod P = 306.9 d, demonstrating a remarkable coherence between R_HKand the bisector shape, i.e. between the photosphere and thechromosphere. We discuss the complications involved in distinguishingbetween companion and activity induced RV variations.
| New periodic variables from the Hipparcos epoch photometry
Two selection statistics are used to extract new candidate periodicvariables from the epoch photometry of the Hipparcos catalogue. Theprimary selection criterion is a signal-to-noise ratio. The dependenceof this statistic on the number of observations is calibrated usingabout 30000 randomly permuted Hipparcos data sets. A significance levelof 0.1 per cent is used to extract a first batch of candidate variables.The second criterion requires that the optimal frequency be unaffectedif the data are de-trended by low-order polynomials. We find 2675 newcandidate periodic variables, of which the majority (2082) are from theHipparcos`unsolved' variables. Potential problems with theinterpretation of the data (e.g. aliasing) are discussed.
| Measuring starspots on magnetically active stars with the VLTI
We present feasibility studies to directly image stellar surfacefeatures, which are caused by magnetic activity, with the Very LargeTelescope Interferometer (VLTI). We concentrate on late typemagnetically active stars, for which the distribution of starspots onthe surface has been inferred from photometric and spectroscopic imaginganalysis. The study of the surface spot evolution during consecutiverotation cycles will allow first direct measurements (apart from theSun) of differential rotation which is the central ingredient ofmagnetic dynamo processes. The VLTI will provide baselines of up to 200m, and two scientific instruments for interferometric studies at near-and mid-infrared wavelengths. Imaging capabilities will be made possibleby closure-phase techniques. We conclude that a realistically modeledcool surface spot can be detected on stars with angular diametersexceeding ~ 2 mas using the VLTI with the first generation instrumentAMBER. The spot parameters can then be derived with reasonable accuracy.We discuss that the lack of knowledge of magnetically active stars ofthe required angular size, especially in the southern hemisphere, is acurrent limitation for VLTI observations of these surface features.
| The Wolfgang and Amadeus Automatic Photoelectric Telescopes. A ``Kleine-Nacht-Musik'' during the first five years of routine operation
We present a summary of five years of continuous operation of theUniversity of Vienna twin Automatic Photoelectric Telescopes (APTs) --Wolfgang and Amadeus. These two telescopes are part of the FairbornObservatory facility located in the Sonoran desert close to WashingtonCamp in southern Arizona. The detection and distinction procedurebetween weather-induced data-quality loss and systematic data-qualityloss turned out to be a crucial task. Therefore, special emphasis islaid on the data quality monitoring tools developed throughout theyears. Furthermore, we summarize the scientific highlights from thefirst five years of operation
| On the Variability of G0-G9 Stars
We investigate the Hipparcos Satellite photometry of G0-G9 stars. Mostare not particularly variable over the 3 year observing period, but maybe over a longer time. Stars for which further study is desirable areidentified.
| The ROSAT Bright Survey: II. Catalogue of all high-galactic latitude RASS sources with PSPC countrate CR > 0.2 s-1
We present a summary of an identification program of the more than 2000X-ray sources detected during the ROSAT All-Sky Survey (Voges et al.1999) at high galactic latitude, |b| > 30degr , with countrate above0.2 s-1. This program, termed the ROSAT Bright Survey RBS, isto more than 99.5% complete. A sub-sample of 931 sources with countrateabove 0.2 s-1 in the hard spectral band between 0.5 and 2.0keV is to 100% identified. The total survey area comprises 20391deg2 at a flux limit of 2.4 x 10-12 ergcm-2 s-1 in the 0.5 - 2.0 keV band. About 1500sources of the complete sample could be identified by correlating theRBS with SIMBAD and the NED. The remaining ~ 500 sources were identifiedby low-resolution optical spectroscopy and CCD imaging utilizingtelescopes at La Silla, Calar Alto, Zelenchukskaya and Mauna Kea. Apartfrom completely untouched sources, catalogued clusters and galaxieswithout published redshift as well as catalogued galaxies with unusualhigh X-ray luminosity were included in the spectroscopic identificationprogram. Details of the observations with an on-line presentation of thefinding charts and the optical spectra will be published separately.Here we summarize our identifications in a table which contains opticaland X-ray information for each source. As a result we present the mostmassive complete sample of X-ray selected AGNs with a total of 669members and a well populated X-ray selected sample of 302 clusters ofgalaxies with redshifts up to 0.70. Three fields studied by us remainwithout optical counterpart (RBS0378, RBS1223, RBS1556). While the firstis a possible X-ray transient, the two latter are isolated neutron starcandidates (Motch et al. 1999, Schwope et al. 1999).
| Lithium and rotation on the subgiant branch. II. Theoretical analysis of observations
Lithium abundances and rotation, determined for 120 subgiant stars inLèbre et al. (1999) are analyzed. To this purpose, theevolutionary status of the sample as well as the individual masses havebeen determined using the HIPPARCOS trigonometric parallax measurementsto locate very precisely our sample stars in the HR diagram. We look atthe distributions of A_Li and Vsini with mass when stars evolve from themain sequence to the subgiant branch. For most of the stars in oursample we find good agreement with the dilution predictions. However,the more massive cool stars with upper limits of Li abundances show asignificant discrepancy with the theoretical predictions, even if theNon-LTE effects are taken into account. For the rotation behaviour, ouranalysis confirms that low mass stars leave the main sequence with a lowrotational rate, while more massive stars are slowed down only whenreaching the subgiant branch. We also checked the connection between theobserved rotation behaviour and the magnetic braking due to thedeepening of the convective envelope. Our results shed new light on thelithium and rotation discontinuities in the evolved phase.
| The accretion of brown dwarfs and planets by giant stars - II. Solar-mass stars on the red giant branch
This paper extends our previous study of planet/brown dwarf accretion bygiant stars to solar-mass stars located on the red giant branch. Themodel assumes that the planet is dissipated at the bottom of theconvective envelope of the giant star. The evolution of the giant isthen followed in detail. We analyse the effects of different accretionrates and different initial conditions. The computations indicate thatthe accretion process is accompanied by a substantial expansion of thestar, and, in the case of high accretion rates, hot bottom burning canbe activated. The possible observational signatures that accompany theengulfing of a planet are also extensively investigated. They includethe ejection of a shell and a subsequent phase of IR emission, anincrease in the ^7Li surface abundance and a potential stellarmetallicity enrichment, spin-up of the star because of the deposition oforbital angular momentum, the possible generation of magnetic fields andthe related X-ray activity caused by the development of shear at thebase of the convective envelope, and the effects on the morphology ofthe horizontal branch in globular clusters. We propose that the IRexcess and high Li abundance observed in 4-8per cent of the G and Kgiants originate from the accretion of a giant planet, a brown dwarf ora very low-mass star.
| Starspot photometry with robotic telescopes. UBV(RI)_C and by light curves of 47 active stars in 1996/97
We present continuous multicolor photometry for 47 stars from October1996 through June 1997. Altogether, 7073 V(RI)_c, UBV, and by datapoints, each the average of three individual readings, were acquiredwith three automatic photoelectric telescopes (APTs) at FairbornObservatory in southern Arizona. Most of our targets arechromospherically active single and binary stars of spectral type G to Kbut there are also four pre-main-sequence objects and three pulsatingstars in our sample. The light variability is generally due torotational modulation of an asymmetrically spotted stellar surface andtherefore precise rotational periods and their seasonal variations aredetermined from Fourier analysis. We also report on photometricvariations of gamma CrB (A0V) with a period of 0.44534 days. All dataare available in numerical form. All data are available from CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| 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
| Lithium and rotation on the subgiant branch. I. Observations and spectral analysis
We have obtained new high resolution spectroscopic observations of thelithium line at 6707.81 Angstroms and derived lithium abundances (A_Li )by spectral synthesis for a sample of about 120 F-, G- and K-typePopulation I subgiant stars. For each of these stars, high precisionrotational velocity obtained with the CORAVEL spectrometer is available.We present the behavior of the lithium abundance as a function ofeffective temperature, which shows a sort of discontinuity around 5600K, somewhat later than the well known rotational discontinuity. based onobservations collected at the Observatoire de Haute--Provence (France)and at the European Southern Observatory, La Silla (Chile).
| Evolved, single, slowly rotating ... but magnetically active. The G8-giant HR 1362 = EK Eridani revisited
We rediscuss the unusual case of the slowly rotating late-type giant HR1362, which exhibits a very unusual high level of magnetic activity. NewBVRI photometry from two robotic telescopes from 1991 through 1998together with previously published photometric data gives a very precisephotometric period of 306.9+/-0.4 days. With the aid of high-resolution(R=120,000) optical spectra and the Hipparcos parallax we redeterminethe absolute parameters of HR 1362 and find it to be a single G8IV-IIIstar of 14 L_sun and a mass of 1.85 M_sun with T_eff=5125 K, log g=3.25,and solar abundances. Lithium is not significantly different from solarand we obtain log n(Li)=1.14 from a detailed spectrum synthesisincluding both isotopes at 6708 Angstroms. Vsin i and macroturbulenceare determined from fits of disk-integrated models to the observed lineprofiles as well as their Fourier transforms and are 1.5+/-0.5 km s(-1)\ and ~5 km s(-1) , respectively. The minimum radius from vsin i andP_rot is only then in agreement with the spectral classification and thebolometric luminosity from the Hipparcos parallax if the inclination ofthe stellar rotation axis is nearly 90°. We concur with thearguments of Stȩpień (\cite{step}) that HR 1362 is anevolved Ap star, and therefore the magnetic field is possibly ofgalactic origin rather than dynamo generated.
| Physical properties of active stars and stellar systems
We study spectra of 18 active stars and stellar systems, drawn fromlists of known active binaries or newly discovered extreme-ultravioletand soft X-ray sources. Using spectral synthesis and template modelling,we measure (often for the first time) a variety of physical propertiesof the stars, including spectral types, luminosities and vsini values.We report direct observation of previously undetected secondarycomponents in three systems, and refine the orbital elements for two ofthese (HR 7275 and HD 217344).
| The ROSAT all-sky survey catalogue of optically bright main-sequence stars and subgiant stars
We present X-ray data for all main-sequence and subgiant stars ofspectral types A, F, G, and K and luminosity classes IV and V listed inthe Bright Star Catalogue that have been detected as X-ray sources inthe ROSAT all-sky survey; several stars without luminosity class arealso included. The catalogue contains 980 entries yielding an averagedetection rate of 32 percent. In addition to count rates, sourcedetection parameters, hardness ratios, and X-ray fluxes we also listX-ray luminosities derived from Hipparcos parallaxes. The catalogue isalso available in electronic form via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| Doppler imaging of stellar surface structure. VIII. The effectively single and rapidly-rotating G8-giant HD 51066 = CM Camelopardalis
We present first Doppler images of HD 51066 from observations in 1994,1995, 1996 and 1997, and find evidence for a vanishing polar spot inaccordance with the system's long-term brightness increase. Severalsmall spots with Delta T~500 K appear also at low latitudes. Ourcross-correlation maps indicate a latitude-dependent phase-shift patternbetween annual maps. New and continuous BVRI photometry from 1996 to1998 is presented and suggest a photometric period of 16.053+/-0.004days, that we interpret to be the stellar rotation period. Additionaloptical spectroscopy and the Hipparcos data are used to obtain absolutestellar parameters for HD 51066. A comparison with evolutionary tracksand the assumption of angular-momentum and magnetic-flux conservationsuggest that the main-sequence progenitor was a very rapidly-rotating Bpstar with a several kiloGauß\ magnetic field. We also examine theHα line profiles in this star and find some evidence that itsequivalent width is modulated with the stellar rotation period in phasewith the photospheric starspots. Our radial velocities indicate that HD51066 is likely a long-period (~10 yrs) spectroscopic binary and apreliminary orbit is presented. We emphasize that HD 51066 is aninteresting target for studies of evolutionary angular-momentum lossbecause it is effectively single, significantly evolved but stillrapidly rotating.
| An All-Sky Catalog of Faint Extreme Ultraviolet Sources
We present a list of 534 objects detected jointly in the ExtremeUltraviolet Explorer (EUVE) 100 Angstroms all-sky survey and in theROSAT X-Ray Telescope 0.25 keV band. The joint selection criterionpermits use of a low count rate threshold in each survey. This lowthreshold is roughly 60% of the threshold used in the previous EUVEall-sky surveys, and 166 of the objects listed here are new EUV sources,appearing in neither the Second EUVE Source Catalog nor the ROSAT WideField Camera Second Catalog. The spatial distribution of this all-skycatalog shows three features: an enhanced concentration of objects inUrsa Major, where the Galactic integrated H I column reaches its globalminimum; an enhanced concentration in the third quadrant of the Galaxy(lII from 180 deg to 270 deg) including the Canis Major tunnel, whereparticularly low H I columns are found to distances beyond 200 pc; and aparticularly low number of faint objects in the direction of the fourthquadrant of the Galaxy, where nearby intervening H I columns areappreciable. Of particular interest is the composition of the 166detections not previously reported in any EUV catalog. We offerpreliminary identifications for 105 of these sources. By far the mostnumerous (81) of the identifications are late-type stars (F, G, K, M),while 18 are other stellar types, only five are white dwarfs (WDs), andnone are extragalactic. The paucity of WDs and extragalactic objects maybe explained by a strong horizon effect wherein interstellar absorptionstrongly limits the effective new-source search volume and, thereby,selectively favors low-luminosity nearby sources over more luminous butdistant objects.
| Accurate Positions for Radio Stars as Determined from CCD Observations in the Extragalactic Reference Frame.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1997AJ....114.1679S&db_key=AST
| On the link between rotation and lithium depletion in subgiant stars.
We present projected rotational velocity Vsini for 65 F, G and Ksubgiant stars. In this work we study the link between rotation andLithium abundance in such class of luminosity. In particular, we lookfor possible effects of rotational braking on the Lithium depletion. Weshow that a sharp decrease in Lithium abundances parallels therotational discontinuity, located at F8IV, in all its aspects. We havefound some dependence of Lithium abundance upon rotation for singlesubgiants, but there is no evident correlation between Lithium abundanceand rotational velocity for binary subgiant stars.
| A Search for Optical Flares in Chromospherically Active Stars
We have surveyed four years of archival UBV photometry of 69chromospherically active stars obtained with the Phoenix-10 automaticphotoelectric telescope in search of optical flares on these stars. Wedemonstrate taht, while flares can be detected in these data, the numberof events observed on evolved stars remains very small. Flares werefound only on UX Arietis, II Pegasi, and AR Piscium. (SECTION: Stars)
| Ca II H and K Filter Photometry on the UVBY System. II. The Catalog of Observations
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1995AJ....109.2828T&db_key=AST
| Chromospherically Active Stars. XIV. A Rediscussion of the Properties and Evolutionary State of HD 181943
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1995AJ....109.2821F&db_key=AST
| Spectrophotometry of blue straggler candidates in NGC 6397
An attempt to determine spectroscopically the mass/luminosity ratios,and hence masses of candidate blue straggler (BS) stars in the globularcluster NGC 6397 is described. Measures of the radial velocities ofstars selected from the photometry of Alcaino et al. (1987) are combinedwith abundances derived from the Ca II K line to identify 4 probable and3 possible cluster members from a list of 19 photometric candidates. Themass of these stars is found to range from 0.62 to 1.15 solar mass. Themasses are generally higher than the turnoff mass of the cluster,supporting binary hypotheses of the origin of BS stars, and areconsistent with the relative central concentration of BS stars inglobular clusters.
| A ROSAT XUV pointed phase source catalogue.
We present a catalogue of XUV sources from observations by the WideField Camera (WFC) on ROSAT during the pointed phase. The ROSAT WFC is atelescope sensitive in the extreme UV range (17-210eV) which observes inparallel with the ROSAT X-Ray Telescope (XRT). The 5916 pointedobservations processed are from the calibration and verification phasein June 1990 and from the period 9 Feb. 1991 to 15 July 1994. Thecatalogue contains 1022 independent source detections which correspondto 328 individual sources, many of which have been observed repeatedly.Each observation was done with one of four filters S_1_, S_2_, P_1_ andP_2_. Of the 328 sources 113 are new sources (they are not listed in the"2RE" catalogue) and 274 have been identified with optical counterparts.The catalogue contains coordinates, observed count rates, normalizedsource count rates and the proposed optical counterpart with itsspectral class. For observations with filters S_1_ and S_2_ an in-flightcalibration has been applied using data of White Dwarfs which wereassumed to have a constant flux and which were observed repeatedly (inpointed observations and/or during the All Sky Survey). In this way, itwas possible to correct for the time dependant degradation of thedetector efficiency and to normalize the count rates to those valid atthe beginning of the mission.
| Radio continuum emission from stars: a catalogue update.
An updated version of my catalogue of radio stars is presented. Somestatistics and availability are discussed.
| HR 1362: The Evolved 53 Camelopardalis
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1993ApJ...416..368S&db_key=AST
| Lithium and rapid rotation in chromospherically active single giants
The rotational velocities presently obtained via spectroscopicobservations of a group of moderately rapidly rotating, chomosphericallyactive single giants indicate that Gray's (1989) rotostat hypothesisrequires modification. Their rapid rotation appears to be due to high Liabundances, and results in increased chromospheric activity. A scenariois projected in which the surface convection zone reaches the rapidlyrotating core just as a star begins its first ascent of the giantbranch, and dredges both high angular momentum material and freshlysynthesized Li to the surface.
| 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
| MG II absolute line profiles for late-type stars and for spatially-resolved solar regions
We present the results of a comparison of Mg II h and k line profilesand fluxes for late-type stars observed with IUE and for spatiallyresolved solar regions observed at comparable resolution by the NRLSpectrograph on Skylab. We show that the spatially resolved solarobservations are in good agreement with the disk integrated stellarobservations, thus suggesting that different Mg II emission levelsobserved in stars of similar spectral type are due to differentfractions of their surface covered by magnetic regions. We confirm thatthe Mg II stellar fluxes are well correlated with other chromosphericand transition-region fluxes, and we show that the spatially resolvedsolar data obey the same flux-flux relationships as the stellar datawith virtually the same slope. We compare some important line diagnosticparameters in disk integrated stellar data and in spatially resolvedsolar regions, finding no significant discrepancies between the twocases. We discuss the possibility and limitations of using thecomparison of solar and stellar Mg II fluxes to infer the fraction of astar's surface covered by magnetic regions.
| 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.
|
Enviar un nuevo artículo
Enlaces relacionados
- - No se han encontrado enlaces -
En viar un nuevo enlace
Miembro de los siguientes grupos:
|
Datos observacionales y astrométricos
| Constelación: | Erídano |
| Ascensión Recta: | 04h20m38.70s |
| Declinación: | -06°14'44.0" |
| Magnitud Aparente: | 6.27 |
| Distancia: | 63.171 parsecs |
| Movimiento Propio en Ascensión Recta: | 92.4 |
| Movimiento Propio en Declinación: | -55.1 |
| B-T magnitude: | 7.257 |
| V-T magnitude: | 6.252 |
Catálogos y designaciones:
|
