Contenuti
Immagini
Carica la tua immagine
DSS Images Other Images
Articoli relazionati
On the Use of Line Depth Ratios to Measure Starspot Properties on Magnetically Active Stars
Photometric and spectroscopic techniques have proven to be effectiveways to measure the properties of dark, cool starspots on magneticallyactive stars. Recently, a technique was introduced using atomic linedepth ratios (LDRs) to measure starspot properties. Carefullyreproducing this technique using a new set of spectroscopic observationsof active stars, we find that the LDR technique encounters difficulties,specifically by overestimating spot temperatures (because the atomiclines blend with titanium oxide absorption in cooler spots) and by nottightly constraining the filling factor of spots. While the use of LDRsfor active star studies has great promise, we believe that theseconcerns need to be addressed before the technique is more widelyapplied.This paper includes data taken at McDonald Observatory of the Universityof Texas at Austin.
| Newly discovered active binaries in the RasTyc sample of stellar X-ray sources. I. Orbital and physical parameters of six new binaries
We present the first results from follow-up optical observations, bothphotometric and spectroscopic, of stellar X-ray sources, selected fromthe RasTyc sample, resulting from the cross-correlation of ROSAT All-SkySurvey (RASS) and TYCHO catalogues. In particular, we report on thediscovery of six late-type binaries, for which we obtained good radialvelocity curves and solved their orbits. We performed an automaticspectral classification of both single-lined and double-lined binarieswith codes developed by us and found two binaries composed of twomain-sequence stars and four binaries with an evolved (giant orsubgiant) component. Filled-in or pure emission Hα profilesindicative of a moderate or high level of chromospheric activity wereobserved. In nearly all the systems, we also detected a photometricmodulation ascribable to surface inhomogeneities that is correlated withthe orbital period, suggesting a synchronization between rotational andorbital periods. The position on the HR diagram of the components of thefive sources with a known parallax indicates three binaries containingonly main-sequence stars and two single-lined systems with a giantcomponent. The kinematical properties of two, or possibly four, of theobserved systems are consistent with a young disk population.
| Photospheric and chromospheric active regions on three single-lined RS CVn binaries
A monitoring of three active RS CVn binaries has been performed withmedium resolution spectroscopy with the aim of investigating thebehavior of chromospheric and photospheric inhomogeneities. Surfacetemperature, as recovered from line-depth ratios (LDRs), allowed us tomap the photospheric spots, while the Hα emission has been used asan indicator of chromospheric inhomogeneities. We have found that therotational modulation of the Hα emission is always in anti-phasewith the temperature wave, i.e. at the time of our observations activeregions at chromospheric and photospheric levels are closely spatiallyassociated in these active stars. The residual Hα profiles,obtained as the difference between the observed spectra and non-activetemplates, are well reproduced by a two Gaussian fitting. The broademission component, responsible for the wide emission wings in near allthe spectra, is often blue-shifted with respect to the center of thestellar disk. The narrow Hα emission displays a phase-dependentvariation in all stars and is anti-correlated with the photosphericdiagnostics, while the broad one displays no or little rotationalmodulation. We suggest that the broad emission component is mainlyrelated to physical phenomena, like micro-flaring or strongchromospheric velocity fields, occurring all over the star disk, whilethe central narrow emission is more affected by chromospheric plages. Wehave also detected a modulation of the intensity of the He I D3 linewith the star rotation, suggesting surface features also in the upperchromosphere of these stars.
| Astrometric orbits of SB^9 stars
Hipparcos Intermediate Astrometric Data (IAD) have been used to deriveastrometric orbital elements for spectroscopic binaries from the newlyreleased Ninth Catalogue of Spectroscopic Binary Orbits(SB^9). This endeavour is justified by the fact that (i) theastrometric orbital motion is often difficult to detect without theprior knowledge of the spectroscopic orbital elements, and (ii) suchknowledge was not available at the time of the construction of theHipparcos Catalogue for the spectroscopic binaries which were recentlyadded to the SB^9 catalogue. Among the 1374 binaries fromSB^9 which have an HIP entry (excluding binaries with visualcompanions, or DMSA/C in the Double and Multiple Stars Annex), 282 havedetectable orbital astrometric motion (at the 5% significance level).Among those, only 70 have astrometric orbital elements that are reliablydetermined (according to specific statistical tests), and for the firsttime for 20 systems. This represents a 8.5% increase of the number ofastrometric systems with known orbital elements (The Double and MultipleSystems Annex contains 235 of those DMSA/O systems). The detection ofthe astrometric orbital motion when the Hipparcos IAD are supplementedby the spectroscopic orbital elements is close to 100% for binaries withonly one visible component, provided that the period is in the 50-1000 drange and the parallax is >5 mas. This result is an interestingtestbed to guide the choice of algorithms and statistical tests to beused in the search for astrometric binaries during the forthcoming ESAGaia mission. Finally, orbital inclinations provided by the presentanalysis have been used to derive several astrophysical quantities. Forinstance, 29 among the 70 systems with reliable astrometric orbitalelements involve main sequence stars for which the companion mass couldbe derived. Some interesting conclusions may be drawn from this new setof stellar masses, like the enigmatic nature of the companion to theHyades F dwarf HIP 20935. This system has a mass ratio of 0.98 but thecompanion remains elusive.
| Local kinematics of K and M giants from CORAVEL/Hipparcos/Tycho-2 data. Revisiting the concept of superclusters
The availability of the Hipparcos Catalogue has triggered many kinematicand dynamical studies of the solar neighbourhood. Nevertheless, thosestudies generally lacked the third component of the space velocities,i.e., the radial velocities. This work presents the kinematic analysisof 5952 K and 739 M giants in the solar neighbourhood which includes forthe first time radial velocity data from a large survey performed withthe CORAVEL spectrovelocimeter. It also uses proper motions from theTycho-2 catalogue, which are expected to be more accurate than theHipparcos ones. An important by-product of this study is the observedfraction of only 5.7% of spectroscopic binaries among M giants ascompared to 13.7% for K giants. After excluding the binaries for whichno center-of-mass velocity could be estimated, 5311 K and 719 M giantsremain in the final sample. The UV-plane constructed from these datafor the stars with precise parallaxes (σπ/π≤20%) reveals a rich small-scale structure, with several clumpscorresponding to the Hercules stream, the Sirius moving group, and theHyades and Pleiades superclusters. A maximum-likelihood method, based ona Bayesian approach, has been applied to the data, in order to make fulluse of all the available stars (not only those with precise parallaxes)and to derive the kinematic properties of these subgroups. Isochrones inthe Hertzsprung-Russell diagram reveal a very wide range of ages forstars belonging to these groups. These groups are most probably relatedto the dynamical perturbation by transient spiral waves (as recentlymodelled by De Simone et al. \cite{Simone2004}) rather than to clusterremnants. A possible explanation for the presence of younggroup/clusters in the same area of the UV-plane is that they have beenput there by the spiral wave associated with their formation, while thekinematics of the older stars of our sample has also been disturbed bythe same wave. The emerging picture is thus one of dynamical streamspervading the solar neighbourhood and travelling in the Galaxy withsimilar space velocities. The term dynamical stream is more appropriatethan the traditional term supercluster since it involves stars ofdifferent ages, not born at the same place nor at the same time. Theposition of those streams in the UV-plane is responsible for the vertexdeviation of 16.2o ± 5.6o for the wholesample. Our study suggests that the vertex deviation for youngerpopulations could have the same dynamical origin. The underlyingvelocity ellipsoid, extracted by the maximum-likelihood method afterremoval of the streams, is not centered on the value commonly acceptedfor the radial antisolar motion: it is centered on < U > =-2.78±1.07 km s-1. However, the full data set(including the various streams) does yield the usual value for theradial solar motion, when properly accounting for the biases inherent tothis kind of analysis (namely, < U > = -10.25±0.15 kms-1). This discrepancy clearly raises the essential questionof how to derive the solar motion in the presence of dynamicalperturbations altering the kinematics of the solar neighbourhood: doesthere exist in the solar neighbourhood a subset of stars having no netradial motion which can be used as a reference against which to measurethe solar motion?Based on observations performed at the Swiss 1m-telescope at OHP,France, and on data from the ESA Hipparcos astrometry satellite.Full Table \ref{taba1} is 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/430/165}
| Kinematics of chromospherically active binaries and evidence of an orbital period decrease in binary evolution
The kinematics of 237 chromospherically active binaries (CABs) werestudied. The sample is heterogeneous with different orbits andphysically different components from F to M spectral-type main-sequencestars to G and K giants and supergiants. The computed U, V, W spacevelocities indicate that the sample is also heterogeneous in velocityspace. That is, both kinematically younger and older systems exist amongthe non-evolved main sequence and the evolved binaries containing giantsand subgiants. The kinematically young (0.95 Gyr) subsample (N= 95),which is formed according to the kinematical criteria of moving groups,was compared with the rest (N= 142) of the sample (3.86 Gyr) toinvestigate any observational clues of binary evolution. Comparing theorbital period histograms between the younger and older subsamples,evidence was found supporting the finding of Demircan that the CABs losemass (and angular momentum) and evolve towards shorter orbital periods.The evidence of mass loss is noticeable on the histograms of the totalmass (Mh+Mc), which is compared between theyounger (only N= 53 systems available) and older subsamples (only N= 66systems available). The orbital period decrease during binary evolutionis found to be clearly indicated by the kinematical ages of 6.69, 5.19and 3.02 Gyr which were found in the subsamples according to the periodranges of logP<= 0.8, 0.8 < logP<= 1.7 and 1.7 < logP<=3, respectively, among the binaries in the older subsample.
| Spot sizes on Sun-like stars
The total area coverage by starspots is of interest for a variety ofreasons, but direct techniques only provide estimates of this importantquantity. Sunspot areas exhibit a lognormal size distributionirrespective of the phase of the activity cycle, implying that mostsunspots are small. Here we explore the consequences if starspot areaswere similarly distributed. The solar data allow for an increase in thefraction of larger sunspots with increasing activity. Taking thisdifference between the size distribution at sunspot maximum and minimum,we extrapolate to higher activity levels, assuming different dependencesof the parameters of the lognormal distribution on total spot coverage.We find that, even for very heavily spotted (hypothetical) stars, alarge fraction of the spots are smaller than the current resolutionlimit of Doppler images and hence might be missed on traditional Dopplermaps.
| Further Results of TiO-Band Observations of Starspots
We present measurements of starspot parameters (temperature and fillingfactor) on five highly active stars, using absorption bands of TiO, fromobservations made between 1998 March and 2001 December. We determinedstarspot parameters by fitting TiO bands using spectra of inactive G andK stars as proxies for the unspotted photospheres of the active starsand spectra of M stars as proxies for the spots. For three evolved RSCVn systems, we find spot filling factors between 0.28 and 0.42 for DMUMa, 0.22 and 0.40 for IN Vir, and 0.31 and 0.35 for XX Tri; thesevalues are similar to those found by other investigators usingphotometry and Doppler imaging. Among active dwarfs, we measured a lowerspot temperature (3350 K) for EQ Vir than found in a previous study ofTiO bands, and for EK Dra a lower spot temperature (~3800 K) than foundthrough photometry. For all active stars but XX Tri, we achieved goodphase coverage through a stellar rotational period. We also present ourfinal, extensive grid of spot and nonspot proxy stars.This paper includes data taken at McDonald Observatory of the Universityof Texas at Austin.
| Doppler imaging of stellar surface structure. XX. The rapidly-rotating single K2-giant HD 31993 = V1192 Orionis
We present two Doppler images from two consecutive stellar rotations ofthe single K2-giant HD 31993. Each Doppler image is reconstructed fromspectra obtained within a single stellar rotation. With its 25-dayrotational period and a radius of ~18 solar radii, HD 31993 isconsidered a very rapidly rotating star and thus allows the applicationof the Doppler-imaging technique, despite the unusually long period. Allmaps reveal 7 isolated, predominantly low-latitude spots with atemperature difference, photosphere minus spot, of just ~200 K. No polarspot or high-latitude activity above, say, +60° is seen. A largewarm feature is detected at high latitude and is believed to be real.These spots act as tracers for a cross correlation analysis and yield aclear signature of anti-solar differential surface rotation, i.e. thepolar regions rotating faster than the equator, with alpha =0.125+/-0.05 corresponding to a lap time of ~200 days. A detailed parameterstudy is carried out to verify the reality of the HD 31993 maps.
| The Hamburg/RASS Catalogue of optical identifications. Northern high-galactic latitude ROSAT Bright Source Catalogue X-ray sources
We present the Hamburg/RASS Catalogue (HRC) of optical identificationsof X-ray sources at high-galactic latitude. The HRC includes all X-raysources from the ROSAT Bright Source Catalogue (RASS-BSC) with galacticlatitude |b| >=30degr and declination delta >=0degr . In thispart of the sky covering ~ 10 000 deg2 the RASS-BSC contains5341 X-ray sources. For the optical identification we used blue Schmidtprism and direct plates taken for the northern hemisphere Hamburg QuasarSurvey (HQS) which are now available in digitized form. The limitingmagnitudes are 18.5 and 20, respectively. For 82% of the selectedRASS-BSC an identification could be given. For the rest either nocounterpart was visible in the error circle or a plausibleidentification was not possible. With ~ 42% AGN represent the largestgroup of X-ray emitters, ~ 31% have a stellar counterpart, whereasgalaxies and cluster of galaxies comprise only ~ 4% and ~ 5%,respectively. In ~ 3% of the RASS-BSC sources no object was visible onour blue direct plates within 40\arcsec around the X-ray sourceposition. The catalogue is used as a source for the selection of(nearly) complete samples of the various classes of X-ray emitters.
| VLA multifrequency observations of RS CVn binaries
We present multiepoch Very Large Array (VLA) observations at 1.4 GHz,4.9 GHz, 8.5 GHz and 14.9 GHz for a sample of eight RS CVn binarysystems. Circular polarization measurements of these systems are alsoreported. Most of the fluxes observed are consistent with incoherentemission from mildly relativistic electrons. Several systems show anincrease of the degree of circular polarization with increasingfrequency in the optically thin regime, in conflict with predictions bygyrosynchrotron models. We observed a reversal in the sense of circularpolarization with increasing frequency in three non-eclipsing systems:EI Eri, DM UMa and HD8358. We find clear evidence for coherent plasma emission at1.4 GHz in the quiescent spectrum of HD 8358 duringthe helicity reversal. The degrees of polarization of the other twosystems could also be accounted for by a coherent emission process. Theobservations of ER Vul revealed two U-shaped fluxspectra at the highest frequencies. The U-shape of the spectra may beaccounted for by an optically thin gyrosynchrotron source for the lowfrequency part whereas the high frequency part is dominated by a thermalemission component.
| A systematic study of X-ray variability in the ROSAT all-sky survey
We present a systematic search for variability among the ROSAT All-SkySurvey (RASS) X-ray sources. We generated lightcurves for about 30 000X-ray point sources detected sufficiently high above background. For ourvariability study different search algorithms were developed in order torecognize flares, periods and trends, respectively. The variable X-raysources were optically identified with counterparts in the SIMBAD, theUSNO-A2.0 and NED data bases, but a significant part of the X-raysources remains without cataloged optical counterparts. Out of the 1207sources classified as variable 767 (63.5%) were identified with stars,118 (9.8%) are of extragalactic origin, 10 (0.8%) are identified withother sources and 312 (25.8%) could not uniquely be identified withentries in optical catalogs. We give a statistical analysis of thevariable X-ray population and present some outstanding examples of X-rayvariability detected in the ROSAT all-sky survey. Most prominent amongthese sources are white dwarfs, apparently single, yet neverthelessshowing periodic variability. Many flares from hitherto unrecognisedflare stars have been detected as well as long term variability in theBL Lac 1E1757.7+7034.The complete version of Table 7 is only available in electronic form atthe CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/403/247
| The HRX-BL Lac sample - Evolution of BL Lac objects
The unification of X-ray and radio selected BL Lacs has been anoutstanding problem in the blazar research in the past years. Recentinvestigations have shown that the gap between the two classes can befilled with intermediate objects and that apparently all differences canbe explained by mutual shifts of the peak frequencies of the synchrotronand inverse Compton component of the emission. We study the consequencesof this scheme using a new sample of X-ray selected BL Lac objectscomprising 104 objects with z<0.9 and a mean redshift bar {z} = 0.34.77 BL Lacs, of which the redshift could be determined for 64 (83%)objects, form a complete sample. The new data could not confirm ourearlier result, drawn from a subsample, that the negative evolutionvanishes below a synchrotron peak frequency log nupeak =16.5. The complete sample shows negative evolution at the 2sigma level(< Ve/Va > = 0.42 +/- 0.04). We concludethat the observed properties of the HRX BL Lac sample show typicalbehaviour for X-ray selected BL Lacs. They support an evolutionarymodel, in which flat-spectrum radio quasars (FSRQ) with high energeticjets evolve towards low frequency peaked (mostly radio-selected) BL Lacobjects and later on to high frequency peaked (mostly X-ray selected) BLLacs.Appendix (Tables 8 and 9, Fig. 8) is only available in electronic format http://www.edpsciences.org
| Simultaneous optical and X-ray observations of flares and rotational modulation on the RS CVn binary HR 1099 (V711 Tau) from the MUSICOS 1998 campaign
We present simultaneous and continuous observations of the Hα,Hβ, He I D3, Na I D1, D2 doubletand the Ca II H & K lines for the RS CVn system HR 1099. Thespectroscopic observations were obtained during the MUSICOS 1998campaign involving several observatories and instruments, both echelleand long-slit spectrographs. During this campaign, HR 1099 was observedalmost continuously for more than 8 orbits of 2.8 deg. Two large opticalflares were observed, both showing an increase in the emission ofHα, Ca II H & K, Hβ and He I D3 and a strongfilling-in of the Na I D1, D2 doublet.Contemporary photometric observations were carried out with the robotictelescopes APT-80 of Catania and Phoenix-25 of Fairborn Observatories.Maps of the distribution of the spotted regions on the photosphere ofthe binary components were derived using the Maximum Entropy andTikhonov photometric regularization criteria. Rotational modulation wasobserved in Hα and He I D3 in anti-correlation with thephotometric light curves. Both flares occurred at the same binary phase(0.85), suggesting that these events took place in the same activeregion. Simultaneous X-ray observations, performed by ASM on board RXTE,show several flare-like events, some of which correlate well with theobserved optical flares. Rotational modulation in the X-ray light curvehas been detected with minimum flux when the less active G5 V star wasin front. A possible periodicity in the X-ray flare-like events was alsofound.Based on observations obtained during the MUSICOS 98 MUlti-SIteCOntinuous Spectroscopic campaign from Observatoire de Haute-Provence,France, Kitt Peak National Observatory, USA, ESO La Silla, Chile, Mt.Stromlo Observatory, Australia, Xinglong National Observatory, China,Isaac Newton Telescope, Spain, Laboratório Nacional deAstrofísica, Brazil, and South African Astronomical Observatory,South Africa. Contemporaneous observations from Catania, Italy andFairborn Observatories, USA, and on data obtained with the Rossi X-rayTiming Explorer.
| Doppler images of starspots
I present a literature survey of the currently available Doppler imagesof cool stars. The 65 individual stars with Doppler images consist of 29single stars and 36 components in close binaries. Out of the total, 31were observed only once but 12 stars are (or were) being monitored foryears. Each image for each star is identified with the time when it wasobserved, whether photometry was used in the imaging, the inclination ofthe stellar rotation axis, the vsin i, the stellar rotation period, andwhether a polar spot and/or a high-latitude or low-latitude spot wasseen. The type of variable star and its M-K spectral classification isalso listed to identify the evolutionary status. The sample consists of3 classical T Tauri stars, 8 weak-lined T Tauri's, 27 main-sequencestars, 9 subgiants, and 18 giants. The total number of Doppler images is245 as of June 2002.
| 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.
| Multiwavelength optical observations of chromospherically active binary systems. IV. The X-ray/EUV selected binary BK Psc (2RE J0039+103)
We present high resolution echelle spectra taken during four observingruns from 1999 to 2001 of the recently X-ray/EUV selectedchromospherically active binary BK Psc (2RE J0039+103). Our observationsconfirm the single-lined spectroscopic binary (SB1) nature of thissystem and allow us to obtain, for the first time, the orbital solutionof the system as in the case of a SB2 system. We have determined preciseradial velocities of both components: for the primary by using the crosscorrelation technique, and for the secondary by using its chromosphericemission lines. We have obtained a circular orbit with an orbital periodof 2.1663 days, very close to its photometric period of 2.24 days(indicating synchronous rotation). The spectral type (K5V) we determinedfor our spectra and the mass ratio (1.8) and minimum masses (Msin3i) resulting from the orbital solution are compatible withthe observed K5V primary and an unseen M3V secondary. Using thisspectral classification, the projected rotational velocity (vsin i, of17.1 km s-1) obtained from the width of the cross-correlationfunction and the data provided by HIPPARCOS, we have derived otherfundamental stellar parameters. The kinematics and the non-detection ofthe Li I line indicate that it is an old star. The analysis of theoptical chromospheric activity indicators from the Ca II H & K to CaII IRT lines, by using the spectral subtraction technique, indicatesthat both components of the binary system show high levels ofchromospheric activity. Hα emission above the continuum from bothcomponents is a persistent feature of this system during the period 1999to 2001 of our observations as well as in previous observations. TheHα and Hβ emission seems to arise from prominence-likematerial, and the Ca II IRT emission from plage-like regions. Based onobservations made with the 2.2 m telescope of the German-SpanishAstronomical Centre, Calar Alto (Almería, Spain), operated by theMax-Planck-Institute for Astronomy, Heidelberg, jointly with the SpanishNational Commission for Astronomy, with the Nordic Optical Telescope(NOT), operated on the island of La Palma jointly by Denmark, Finland,Iceland, Norway and Sweden, in the Spanish Observatorio del Roque de LosMuchachos of the Instituto de Astrofísica de Canarias, and withthe Isaac Newton Telescope (INT) operated on the island of La Palma bythe Isaac Newton Group in the Spanish Observatorio del Roque de LosMuchachos of the Instituto de Astrofísica de Canarias.
| Doppler imaging of stellar surface structure. XVIII. The very active RS CVn binary UZ Librae revisited
We present eight Doppler images of UZ Librae from the years 1994, 1996,1997, 1998 and 2000. These maps indicate a preferred temperaturedistribution from one year to the other and suggest preferred spotlocations for at least the seven years of our observations. Twoequatorial active regions appear facing towards and opposite the(unseen) companion star. A large and cool polar spot, with two or maybethree appendages extending down to a latitude of 40 degr -50 degr , ispresent and causes the long-term light variability of UZ Lib. Theappendages appear at longitudes very similar to the two equatorialactive regions and sometimes are possibly even attached to them. Nocoherent sign of differential rotation is found for the equatorialregions up to a latitude of approximately +50 degr , but the polarappendages may indicate a general acceleration of the polar regions. Ifinterpreted as being due to differential rotation, if real at all, thenUZ Lib clearly exhibits a non-solar flow pattern.
| Chromospheric activity on the RS CVn-type binary UX Arietis
High resolution spectroscopic observations of UX Ari made during threeobserving seasons in 2000 have been analyzed by means of the spectralsubtraction technique. The information about chromospheric activity ofUX Ari has been obtained from several optical chromospheric activityindicators such as the HeI D3, NaI D1,D2, Halpha, and CaII IRT lines. The resultssuggest the chromospheric activity of the system is associated with thecooler secondary. An orbital phase modulation of chromospheric emissionsis found, and consequently the activity longitude area of the system isobtained, that corresponds to the phase interval near the secondquadrature of the binary system. A flare-like event is detected duringour new observations, characterized by the dramatic increase in theH_alpha and CaII IRT emission lines, that is confirmed by the emissionof the HeI D_3 line and the filled-in cores of the NaI D_1, D_2 lines.It is found that the values of the EW8542/EW8498ratio decrease when the activity of the system increases, especiallywhen the flare occurs. The small values of theEW8542/EW8498 ratio indicate that the CaII IRTemissions come from plage-like regions. The high activity level of UXAri around the quadrature may originate with the coupling between thechromospheric activity of the secondary and the mass transfer activityof the two components.
| A complete sample of Seyfert galaxies selected at 0.25 keV
We have used the ROSAT Bright Source Catalogue to extract a completesample of sources selected in the band from 0.1-0.4keV. This0.25keV-selected sample is composed of 54 Seyfert galaxies, 25 BLLacertae objects, 4 clusters and 27 Galactic stars or binaries.Seyfert-type galaxies with `ultrasoft' X-ray spectra can very often beclassed optically as narrow-line Seyfert 1s (NLS1s). Such objects arereadily detected in 0.25keV surveys; the sample reported here contains20 NLS1s, corresponding to a 40 per cent fraction of the Seyferts.Optical spectra of the Seyfert galaxies were gathered for correlativeanalysis, which confirmed the well-known relations between X-ray slopeand optical spectral properties (e.g. [Oiii]/Hβ ratio; Feiistrength, Hβ width). The various intercorrelations are most likelydriven, fundamentally, by the shape of the photoionizing continuum inSeyfert nuclei. We argue that a steep X-ray spectrum is a betterindicator of an `extreme' set of physical properties in Seyfert galaxiesthan is the narrowness of the optical Hβ line. The correlationstudies were also used to isolate a number of Seyfert galaxies withapparently `anomalous' properties. Of particular interest are the sixobjects with relatively weak permitted line emission (Hβ and Feii)and weak optical continua. Such objects are rare in most surveys, buttwo of these (IC 3599 and NGC 5905) are known to be transient activegalactic nuclei in which the X-ray flux has faded by factors ~100. Ifthe other four objects also turn out to be transient, this woulddemonstrate that 0.25keV surveys provide an efficient way of finding aninteresting class of object. Finally, the luminosity function of the0.25keV-selected Seyfert galaxies was determined and broken down intosubsamples to investigate the relative space densities of Seyferts whenseparated on the basis of either X-ray slope or Hβ linewidth.
| The Catania Automatic Photoelectric Telescope on Mt. Etna: a systematic study of magnetically active stars
A photometric monitoring of about 50 magnetically active stars, that arespread almost all over the H-R diagram, was initiated at the mountainstation of Catania Observatory on Mt. Etna (1750-m a.s.l.) in 1992 withan 80-cm robotic telescope (APT-80) built by AutoScope Co. (USA). Thissystematic survey is now approaching its 10th year anniversary. For mostof the stars, quite well defined solar-like spot maps have been derivedfrom UBV data obtained in different epochs. These data have allowed usto investigate some relevant characteristics of spot activity andvariability on stars, and to obtain clear evidence of long-term activitycycles, in the range from a few to about 10 years, on some of theobserved targets. Starspot maps are constructed by using advanced tools,such as massive parallel computing and are based on Maximum Entropy andTikhonov regularization criteria. Selected results are here presented.Our systematic observation program is still underway and a secondAPT80/2, equipped with a CCD camera, will pair the APT80/1 on the samesite. Its operation is foreseen for mid 2002.
| The T Tauri star RX J1608.6-3922 - not an eclipsing binary but a spotted single star
High-resolution spectroscopy and photometric monitoring of the pre-mainsequence star RX J1608.6-3922 shows that it is not aneclipsing binary, as previously claimed. Radial velocity measurementscovering suitable time spans in order to detect a spectroscopic binarywith the claimed period of about 7 days have been performed. The scatterof the radial velocity does not exceed 2.4 km s-1, definingan upper mass limit of 24 M_Jupiter} for any eclipsing companionorbiting this star with the claimed period. Photometric observations ofRX J1608.6-3922 in 7 consecutive nights (i.e. as long as the claimedorbital period) reveal brightness variations of the order of 0.2 magwith a period of 3.6 days. The shape of the detected light curve differsfrom a light curve of the star recorded in 1996. The small variations ofthe radial velocity, the variable shape of the light curve, as well as(B-V) color variations suggest that the flux of RX J1608.6-3922 ismodulated by spots on the stellar surface with a rotational period of3.6 days. The stellar activity of this star seems to be highly variable,taking into account the variable shape of the light curve, with anamplitude varying from 0.5 to 0.2 mag in a few years, as well as hintsfor a variable Halpha equivalent width. Based onobservations obtained at the European Southern Observatory at La Silla,Chile in program 62.I-0418, 63.I-0096, 64.I-0294, 65.I-0275 and onobservations obtained at the MSSSO Observatory at Siding Springs,Australia.
| 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).
| The BeppoSAX 2-10 keV survey
We present the results of a 2-10 keV BeppoSAX survey based on 140 highgalactic latitude MECS fields, 12 of which are deep exposures of``blank'' parts of the sky. The limiting sensitivity is5*E-14 erg cm -2s-1 where about 25% ofthe Cosmic X-ray Background (CXB) is resolved into discrete sources. ThelogN-logS function, built with a statistically complete sample of 177sources, is steep and in good agreement with the counts derived fromASCA surveys. A CXB fluctuation analysis allowed us to probe thelogN-logS down to about 1.5*E-14 erg cm-2s-1 where the contribution of discrete sourcesto the CXB grows to ~ 40-50%. A hardness ratio analysis reveals thepresence of a wide range of spectral shapes and that a fairly largefraction of sources appear to be heavily absorbed, some of which showingsoft components. A comparison of the flux distribution of differentsubsamples confirms the existence of a spectral hardening withdecreasing flux. This effect is probably due to an increasing percentageof absorbed sources at faint fluxes, rather than to a gradual flatteningof the spectral slope. Nearly all the sources for which adequate ROSATexposures exist, have been detected in the soft X-rays. This confirmsthat soft spectral components are present even in strongly absorbedobjects, and that a large population of sources undetectable below a fewkeV does not exist. A Ve/Va test provides evidencefor the presence of cosmological evolution of a magnitude similar tothat found in soft X-ray extragalactic sources. Evolution is presentboth in normal and absorbed sources, with the latter population possiblyevolving faster, although this effect could also be the result ofcomplex selection effects. Table 1 is only available in electronic format the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) orvia http://cdsweb.u-strasbg.fr/Abstract.html
| A multi-wavelength study of pre-main sequence stars in the Taurus-Auriga star-forming region
Although many lowmass pre-main sequence stars are strong X-ray sources,the origin of the X-ray emission is not well known. Since these objectsare variable at all frequencies, simultaneous observations in X-rays andin other wavelengths are able to constrain the properties of the X-rayemitting regions. In this paper, we report quasi-simultaneousobservations in X-rays, the optical, and the radio regime for classicaland weak-line T Tauri stars from the Taurus-Auriga star-forming region.We find that all detected T Tauri stars show significant night-to-nightvariations of the X-ray emission. For three of the stars, FMTau and CW Tau, both classical T Tauristars, and V773 Tau, a weak-line T Tauri star, thevariations are especially large. From observations taken simultaneously,we also find that there is some correspondence between the strength ofHα and the X-ray brightness in V773 Tau. Thelack of a strong correlation leads us to conclude that the X-rayemission of V773 Tau is not a superposition offlares. However, we suggest that a weak correlation occurs becausechromospherically active regions and regions of strong X-ray emissionare generally related. V773 Tau was detected at 8.46GHz as a weakly circularly polarised but highly variable source. We alsofind that the X-ray emission and the equivalent width of Hαremained unchanged, while large variations of the flux density in theradio regime were observed. This clearly indicates that the emittingregions are different. Using optical spectroscopy we detected a flare inHα and event which showed a flare-like light-curve of thecontinuum brightness in FM Tau. However, ROSAT didnot observe the field at the times of these flares. Nevertheless, aninteresting X-ray event was observed in V773 Tau,during which the flux increased for about 8 hours and then decreasedback to the same level in 5 hours. We interpret this as a long-durationevent similar to those seen on the sun and other active stars. In thecourse of the observations, we discovered a new weak-line T Tauri star,GSC-1839-5674. Results are also presented for several other stars in theROSAT field.
| On X-Ray Variability in Active Binary Stars
We have compared the X-ray emissions of active binary stars observed atvarious epochs by the Einstein and ROSAT satellites in order toinvestigate the nature of their X-ray variability. The primary aim ofthis work is to determine whether or not active binaries exhibitlong-term variations in X-ray emission, perhaps analogous to theobserved cyclic behavior of solar magnetic activity. We find that, whilethe mean level of emission of the sample remains steady, comparison ofdifferent ROSAT observations of the same stars shows significantvariation on timescales <~2 yr, with an ``effective variability''ΔI/I=0.32+/-0.04, where I and ΔI represent the mean emissionand variation from the mean emission, respectively. A comparison of theROSAT All-Sky Survey and later pointed observations with earlierobservations of the same stars carried out with Einstein yields onlymarginal evidence for a larger variation (ΔI/I=0.38+/-0.04 forEinstein vs. ROSAT All-Sky Survey and 0.46+/-0.05 for Einstein vs. ROSATpointed) at these longer timescales (~10 yr), thus indicating thepossible presence of a long-term component to the variability. Whetheror not this long-term component is due to the presence of cyclicvariability cannot be decided on the basis of existing data. However,assuming that this component is analogous to the observed cyclicvariability of the Sun, we find that the relative magnitude of thecyclic component in the ROSAT passband can, at most, be a factor of 4,i.e., I_cyc/I_min<4. This is to be compared with the correspondingbut significantly higher solar value of ~10-10^2 derived from GOES,Yohkoh, and Solrad data. These results are consistent with thesuggestions of earlier studies that a turbulent or distributive dynamomight be responsible for the observed magnetic activity on the mostactive, rapidly rotating stars.
| Observations of short-duration X-ray transients by WATCH on GRANAT
During 1990-92, the WATCH all-sky X-ray monitor on Granat discovered sixshort-duration X-ray transients. In this paper we discuss their possiblerelationship to peculiar stars. Only one of the fast (few hours) X-raytransients (GRS 1100-771) might be tentatively ascribed to a superflarearising from a young stellar object in the Chamaeleon I star-formingcloud. At the distance of ~ 150 pc, Lx = 1.35 x 10(34) ergs(-1) (8-15 keV), or 2.6 x 10(34) erg s(-1) (0.1-2.4 keV) assuming athermal spectrum with kT ~ 10 keV, a temperature higher than thosepreviously seen in T Tauri stars (Tsuboi et al. 1998). The peak X-rayluminosity is at least 2 times higher than that derived for theprotostar IRS 43 (Grosso et al. 1997) which would make -to ourknowledge- the strongest flare ever seen in a YSO. However, thepossibility of GRS 1100-771 being an isolated neutron star unrelated tothe cloud cannot be excluded, given the relatively large error boxprovided by WATCH. Regarding the longer duration ( ~ 1 day) X-raytransients, none of them seem to be related to known objects. We suggestthat the latter are likely to have originated from compact objects inlow-mass or high-mass X-ray binaries, similarly to XTE J0421+560.
| X-ray coronal activities of active binaries.
Not Available
| On the rotation-activity correlation for active binary stars
We present an investigation of rotation-activity correlations usingInternational Ultraviolet Explorer (IUE) SWP measurements of the Civemission line at 1550Angstroms for 72 active binary systems. We use astandard stellar evolution code to derive non-empirical Rossby numbers,R_0, for each star in our sample and compare the resulting Civrotation-activity correlation to that found for empirically derivedvalues of the Rossby number and that based on rotation alone. For dwarfstars our values of R_0 do not differ greatly from empirical ones and wefind a corresponding lack of improvement in correlation. Only a marginalimprovement in correlation is found for evolved components in oursample. We discuss possible additional factors, other than rotation orconvection, that may influence the activity levels in active binaries.Our observational data imply, in contrast to the theoretical predictionsof convective motions, that activity is only weakly related to mass inevolved stars. We conclude that current dynamo theory is limited in itsapplication to the study of active stars because of the uncertainty inthe angular velocity-depth profile in stellar interiors and the unknowneffects of binarity and surface gravity.
| Measurements of Starspot Parameters on Active Stars using Molecular Bands in Echelle Spectra
We present results from a study of starspot areas (f_S) and temperatures(T_S), primarily on active, single-lined spectroscopic binaries,determined using molecular absorption bands. Expanding upon our previousstudies, we have analyzed multiorder echelle spectra of eight systems tosimultaneously measure several different molecular bands andchromospheric emission lines. We determined starspot parameters byfitting the molecular bands of interest, using spectra of inactive G andK stars as proxies for the nonspotted photosphere of the active stars,and using spectra of M stars as proxies for the spots. At least twobands with different T_eff sensitivities are required. We found thatfitting bands other than the TiO 7055 and 8860 Å features does notgreatly extend the temperature range or sensitivity of our technique.The 8860 Å band is particularly important because of its sharplydifferent temperature sensitivity. We did not find any substantialdepartures from f_S or T_S that we have measured previously based onsingle-order spectra. We refined our derived spot parameters usingcontemporaneous photometry where available. We found that using M giantsas spot proxies for subgiant active stars often underestimates f_Sneeded to fit the photometry; this is presumably due to the increase instrength of the TiO bands with decreasing gravity. We also investigatedcorrelations between f_S and chromospheric emission, and we developed asimple method to measure nonspot temperature (T_Q) solely from ourechelle spectra.
|
Sottometti un nuovo articolo
Link relazionati
Sottometti un nuovo link
Membro dei seguenti gruppi:
|
Osservazione e dati astrometrici
| Costellazione: | Orsa Maggiore |
| Ascensione retta: | 10h55m43.54s |
| Declinazione: | +60°28'09.7" |
| Magnitudine apparente: | 9.299 |
| Moto proprio RA: | -37.7 |
| Moto proprio Dec: | -7 |
| B-T magnitude: | 10.581 |
| V-T magnitude: | 9.405 |
Cataloghi e designazioni:
|
