Contenuti
Immagini
Carica la tua immagine
DSS Images Other Images
Articoli relazionati
Spitzer IRS Spectroscopy of IRAS-discovered Debris Disks
We have obtained Spitzer Space Telescope Infrared Spectrograph (IRS)5.5-35 μm spectra of 59 main-sequence stars that possess IRAS 60μm excess. The spectra of five objects possess spectral features thatare well-modeled using micron-sized grains and silicates withcrystalline mass fractions 0%-80%, consistent with T Tauri and HerbigAeBe stars. With the exception of η Crv, these objects are youngwith ages <=50 Myr. Our fits require the presence of a cool blackbodycontinuum, Tgr=80-200 K, in addition to hot, amorphous, andcrystalline silicates, Tgr=290-600 K, suggesting thatmultiple parent body belts are present in some debris disks, analogousto the asteroid and Kuiper belts in our solar system. The spectra forthe majority of objects are featureless, suggesting that the emittinggrains probably have radii a>10 μm. We have modeled the excesscontinua using a continuous disk with a uniform surface densitydistribution, expected if Poynting-Robertson and stellar wind drag arethe dominant grain removal processes, and using a single-temperatureblackbody, expected if the dust is located in a narrow ring around thestar. The IRS spectra of many objects are better modeled with asingle-temperature blackbody, suggesting that the disks possess innerholes. The distribution of grain temperatures, based on our blackbodyfits, peaks at Tgr=110-120 K. Since the timescale for icesublimation of micron-sized grains with Tgr>110 K is afraction of a Myr, the lack of warmer material may be explained if thegrains are icy. If planets dynamically clear the central portions ofdebris disks, then the frequency of planets around other stars isprobably high. We estimate that the majority of debris disk systemspossess parent body masses, MPB<1 M⊕. Thelow inferred parent body masses suggest that planet formation is anefficient process.Based on observations with the NASA Spitzer Space Telescope, which isoperated by the California Institute of Technology for NASA.
| Investigating Disk Evolution: A High Spatial Resolution Mid-Infrared Survey of T Tauri Stars
We present a high spatial resolution, 10-20 μm survey of 65 T Tauribinary stars in Taurus, Ophiuchus, and Corona Australis using the Keck10 m telescopes. Designed to probe the inner ~1 AU region of thecircumstellar disks around the individual stellar components in thesebinary systems, this study increases the number of binaries withspatially resolved measurements at 10 μm by a factor of ~5. Combinedwith resolved near-infrared photometry and spectroscopic accretiondiagnostics, we find that ~10% of stars with a mid-infrared excess donot appear to be accreting. In contrast to an actively accreting disksystem, these passive disks have significantly lower near-infraredcolors that are, in most cases, consistent with photospheric emission,suggesting the presence of an inner disk hole. In addition, thereappears to be a spectral type/mass dependence associated with thepresence of a passive disk, with all passive disks occurring aroundM-type stars. The presence of a passive disk does not appear to berelated to the fact that these objects are in visual binary systems; thepassive disk systems span the entire range of binary separations presentin the sample, and a similar fraction of passive disks is observed in asample of single stars. The possibility that the passive disks arecaused by the presence of an as yet undetected companion at a smallseparation (0.3-3 AU) is possible for any individual system; however, itcannot account for the spectral type dependence of the passive disksample as a whole. We propose that these passive disks represent asubset of T Tauri stars that are undergoing significant disk evolution.The fraction of observed passive disks and the observed spectral typedependence can both be explained by models of disk evolution thatinclude disk photoevaporation from the central star.
| Measurements of Binary Stars, Including Two New Discoveries, with the Lick Observatory Adaptive Optics System
We present astronomical results from observations for a number ofmultiple star systems observed with the Lick Observatory natural guidestar adaptive optics system. We have discovered and classified a fifthcomponent in the ι Cas system and a third component for the widebinary WDS 00310+2839. Using two different data reduction techniques, wedemonstrate relative astrometric precision to 2-3 mas and photometricprecision to within 0.05 mag. The binary stars enable anisoplanatism tobe measured, from which a mean turbulence height over Lick Observatoryof 1.5-3 km is determined.
| Wavefront outer scale deduced from interferometric dispersed fringes
In addition to site characterization, measurements of criticalatmospheric parameters are required to design and to optimize futureadaptive optic systems and long-baseline interferometers. It is possibleto estimate seeing conditions by processing data obtained with existingHigh Angular Resolution instruments. We report the results of jointobservations with the GI2T interferometer and the GSM site-testingmonitor performed over a period of several nights.We compared estimates of the wavefront outer scale done at variousbaselines as well as the seeing (Fried's parameter). We processedinterferometric data by calculating power spectra of dispersed fringeimages. Deduced measurements of the optical path difference lead to theestimates of the outer scale. We found that the outer scale valuesobtained from the GI2T data are mostly in the 5-30m range, in goodagreement with GSM measurements.
| The epoch of the constellations on the Farnese Atlas and their origin in Hipparchus's lost catalogue
Not Available
| A Mid-Infrared Imaging Survey of Embedded Young Stellar Objects in the ρ Ophiuchi Cloud Core
Results of a comprehensive, new, ground-based mid-infrared imagingsurvey of the young stellar population of the ρ Ophiuchi cloud arepresented. Data were acquired at the Palomar 5 m and at the Keck 10 mtelescopes with the MIRLIN and LWS instruments, at 0.5" and 0.25"resolutions, respectively. Of 172 survey objects, 85 were detected.Among the 22 multiple systems observed, 15 were resolved and theirindividual component fluxes determined. A plot of the frequencydistribution of the detected objects with SED spectral slope shows thatYSOs spend ~4×105 yr in the flat-spectrum phase,clearing out their remnant infall envelopes. Mid-infrared variability isfound among a significant fraction of the surveyed objects and is foundto occur for all SED classes with optically thick disks. Large-amplitudenear-infrared variability, also found for all SED classes with opticallythick disks, seems to occur with somewhat higher frequency at theearlier evolutionary stages. Although a general trend of mid-infraredexcess and near-infrared veiling exists progressing through SED classes,with Class I objects generally exhibiting rK>=1,flat-spectrum objects with rK>=0.58, and Class III objectswith rK=0, Class II objects exhibit the widest range ofrK values, ranging from 0<=rK<=4.5. However,the highly variable value of veiling that a single source can exhibit inany of the SED classes in which active disk accretion can take place isstriking and is direct observational evidence for highly time-variableaccretion activity in disks. Finally, by comparing mid-infrared versusnear-infrared excesses in a subsample with well-determined effectivetemperatures and extinction values, disk-clearing mechanisms areexplored. The results are consistent with disk clearing proceeding fromthe inside out.
| Observed Orbital Eccentricities
For 391 spectroscopic and visual binaries with known orbital elementsand having B0-F0 IV or V primaries, we collected the derivedeccentricities. As has been found by others, those binaries with periodsof a few days have been circularized. However, those with periods up toabout 1000 or more days show reduced eccentricities that asymptoticallyapproach a mean value of 0.5 for the longest periods. For those binarieswith periods greater than 1000 days their distribution of eccentricitiesis flat from 0 to nearly 1, indicating that in the formation of binariesthere is no preferential eccentricity. The binaries with intermediateperiods (10-100 days) lack highly eccentric orbits.
| Decay of Planetary Debris Disks
We report new Spitzer 24 μm photometry of 76 main-sequence A-typestars. We combine these results with previously reported Spitzer 24μm data and 24 and 25 μm photometry from the Infrared SpaceObservatory and the Infrared Astronomy Satellite. The result is a sampleof 266 stars with mass close to 2.5 Msolar, all detected toat least the ~7 σ level relative to their photospheric emission.We culled ages for the entire sample from the literature and/orestimated them using the H-R diagram and isochrones; they range from 5to 850 Myr. We identified excess thermal emission using an internallyderived K-24 (or 25) μm photospheric color and then compared allstars in the sample to that color. Because we have excluded stars withstrong emission lines or extended emission (associated with nearbyinterstellar gas), these excesses are likely to be generated by debrisdisks. Younger stars in the sample exhibit excess thermal emission morefrequently and with higher fractional excess than do the older stars.However, as many as 50% of the younger stars do not show excessemission. The decline in the magnitude of excess emission, for thosestars that show it, has a roughly t0/time dependence, witht0~150 Myr. If anything, stars in binary systems (includingAlgol-type stars) and λ Boo stars show less excess emission thanthe other members of the sample. Our results indicate that (1) there issubstantial variety among debris disks, including that a significantnumber of stars emerge from the protoplanetary stage of evolution withlittle remaining disk in the 10-60 AU region and (2) in addition, it islikely that much of the dust we detect is generated episodically bycollisions of large planetesimals during the planet accretion end game,and that individual events often dominate the radiometric properties ofa debris system. This latter behavior agrees generally with what we knowabout the evolution of the solar system, and also with theoreticalmodels of planetary system formation.
| A new look at the position of the 1604 Supernova (V843 Ophiuchi)
The position of the supernova of 1604 (V843 Oph) is re-assessed, withrelevant discussion pertaining to the present-day remnant, 3C 358.
| 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.
| Detection of X-ray emission from β Pictoris with XMM-Newton: a cool corona, a boundary layer or what?
β Pictoris (HR 2020) is the most prominentprototype of stars with circumstellar disks and has generated particularinterest in the framework of young planetary systems. Given its spectraltype A5, stellar activity is not expected. Nevertheless, resonance linesof C iii and O vi typical for a chromosphere and transition region havebeen unambiguously detected with FUSE. We present results from anXMM-Newton observation of β Pic and find evidence for X-rayemission. In particular, we detected an emission of O vii at 21.6Å with the MOS detectors. These findings present a challenge forthe development of both stellar activity and disk models. We discuss andinvestigate various models to explain the observed emission includingthe presence of a cool corona and a boundary layer.
| A near-infrared stellar spectral library: I. H-band spectra.
This paper presents the H band near-infrared (NIR) spectral library of135 solar type stars covering spectral types O5-M3 and luminosityclasses I-V as per MK classification. The observations were carried outwith 1.2 meter Gurushikhar Infrared Telescope (GIRT), at Mt. Abu, Indiausing a NICMOS3 HgCdTe 256 x 256 NIR array based spectrometer. Thespectra have a moderate resolution of 1000 (about 16 A) at the H bandand have been continuum shape corrected to their respective effectivetemperatures. This library and the remaining ones in J and K bands oncereleased will serve as an important database for stellar populationsynthesis and other applications in conjunction with the newly formedlarge optical coude feed stellar spectral library of Valdes et al.(2004). The complete H-Band library is available online at: http://vo.iucaa.ernet.in/~voi/NIR_Header.html
| Classification of Spectra from the Infrared Space Observatory PHT-S Database
We have classified over 1500 infrared spectra obtained with the PHT-Sspectrometer aboard the Infrared Space Observatory according to thesystem developed for the Short Wavelength Spectrometer (SWS) spectra byKraemer et al. The majority of these spectra contribute to subclassesthat are either underrepresented in the SWS spectral database or containsources that are too faint, such as M dwarfs, to have been observed byeither the SWS or the Infrared Astronomical Satellite Low ResolutionSpectrometer. There is strong overall agreement about the chemistry ofobjects observed with both instruments. Discrepancies can usually betraced to the different wavelength ranges and sensitivities of theinstruments. Finally, a large subset of the observations (~=250 spectra)exhibit a featureless, red continuum that is consistent with emissionfrom zodiacal dust and suggest directions for further analysis of thisserendipitous measurement of the zodiacal background.Based on observations with the Infrared Space Observatory (ISO), aEuropean Space Agency (ESA) project with instruments funded by ESAMember States (especially the Principle Investigator countries: France,Germany, Netherlands, and United Kingdom) and with the participation ofthe Institute of Space and Astronautical Science (ISAS) and the NationalAeronautics and Space Administration (NASA).
| Tidal Effects in Binaries of Various Periods
We found in the published literature the rotational velocities for 162B0-B9.5, 152 A0-A5, and 86 A6-F0 stars, all of luminosity classes V orIV, that are in spectroscopic or visual binaries with known orbitalelements. The data show that stars in binaries with periods of less thanabout 4 days have synchronized rotational and orbital motions. Stars inbinaries with periods of more than about 500 days have the samerotational velocities as single stars. However, the primaries inbinaries with periods of between 4 and 500 days have substantiallysmaller rotational velocities than single stars, implying that they havelost one-third to two-thirds of their angular momentum, presumablybecause of tidal interactions. The angular momentum losses increase withdecreasing binary separations or periods and increase with increasingage or decreasing mass.
| Detection of Cool Dust around the G2 V Star HD 107146
We report the detection of dust emission at submillimeter wavelengthsfrom HD 107146, a G2 V star with an age estimated to lie between 80 and200 Myr. The emission is resolved at 450 μm with a size300AU×210AU. A fit to the spectral energy distribution gives adust temperature of 51 K and a dust mass of 0.10M⊕. Noexcess emission above the photosphere was detected at 18 μm, showingthat there is very little warm dust and implying the presence of a largeinner hole, at least 31 AU (~1") in radius, around the star. Theproperties of this star-disk system are compared with similarobservations of other systems. We also discuss prospects for futureobservations that may be able to determine whether the inner hole ismaintained by the dynamical effect of an unseen orbiting companion.
| Nearby stars of the Galactic disk and halo. III.
High-resolution spectroscopic observations of about 150 nearby stars orstar systems are presented and discussed. The study of these and another100 objects of the previous papers of this series implies that theGalaxy became reality 13 or 14 Gyr ago with the implementation of amassive, rotationally-supported population of thick-disk stars. The veryhigh star formation rate in that phase gave rise to a rapid metalenrichment and an expulsion of gas in supernovae-driven Galactic winds,but was followed by a star formation gap for no less than three billionyears at the Sun's galactocentric distance. In a second phase, then, thethin disk - our ``familiar Milky Way'' - came on stage. Nowadays ittraces the bright side of the Galaxy, but it is also embedded in a hugecoffin of dead thick-disk stars that account for a large amount ofbaryonic dark matter. As opposed to this, cold-dark-matter-dominatedcosmologies that suggest a more gradual hierarchical buildup throughmergers of minor structures, though popular, are a poor description forthe Milky Way Galaxy - and by inference many other spirals as well - if,as the sample implies, the fossil records of its long-lived stars do notstick to this paradigm. Apart from this general picture that emergeswith reference to the entire sample stars, a good deal of the presentwork is however also concerned with detailed discussions of manyindividual objects. Among the most interesting we mention the bluestraggler or merger candidates HD 165401 and HD 137763/HD 137778, thelikely accretion of a giant planet or brown dwarf on 59 Vir in itsrecent history, and HD 63433 that proves to be a young solar analog at\tau200 Myr. Likewise, the secondary to HR 4867, formerly suspectednon-single from the Hipparcos astrometry, is directly detectable in thehigh-resolution spectroscopic tracings, whereas the visual binary \chiCet is instead at least triple, and presumably even quadruple. Withrespect to the nearby young stars a complete account of the Ursa MajorAssociation is presented, and we provide as well plain evidence foranother, the ``Hercules-Lyra Association'', the likely existence ofwhich was only realized in recent years. On account of its rotation,chemistry, and age we do confirm that the Sun is very typical among itsG-type neighbors; as to its kinematics, it appears however not unlikelythat the Sun's known low peculiar space velocity could indeed be thecause for the weak paleontological record of mass extinctions and majorimpact events on our parent planet during the most recent Galactic planepassage of the solar system. Although the significance of thiscorrelation certainly remains a matter of debate for years to come, wepoint in this context to the principal importance of the thick disk fora complete census with respect to the local surface and volumedensities. Other important effects that can be ascribed to this darkstellar population comprise (i) the observed plateau in the shape of theluminosity function of the local FGK stars, (ii) a small thoughsystematic effect on the basic solar motion, (iii) a reassessment of theterm ``asymmetrical drift velocity'' for the remainder (i.e. the thindisk) of the stellar objects, (iv) its ability to account for the bulkof the recently discovered high-velocity blue white dwarfs, (v) itsmajor contribution to the Sun's 220 km s-1 rotationalvelocity around the Galactic center, and (vi) the significant flatteningthat it imposes on the Milky Way's rotation curve. Finally we note ahigh multiplicity fraction in the small but volume-complete local sampleof stars of this ancient population. This in turn is highly suggestivefor a star formation scenario wherein the few existing single stellarobjects might only arise from either late mergers or the dynamicalejection of former triple or higher level star systems.
| X-ray astronomy of stellar coronae
X-ray emission from stars in the cool half of the Hertzsprung-Russelldiagram is generally attributed to the presence of a magnetic coronathat contains plasma at temperatures exceeding 1 million K. Coronae areubiquitous among these stars, yet many fundamental mechanisms operatingin their magnetic fields still elude an interpretation through adetailed physical description. Stellar X-ray astronomy is thereforecontributing toward a deeper understanding of the generation of magneticfields in magnetohydrodynamic dynamos, the release of energy in tenuousastrophysical plasmas through various plasma-physical processes, and theinteractions of high-energy radiation with the stellar environment.Stellar X-ray emission also provides important diagnostics to study thestructure and evolution of stellar magnetic fields from the first daysof a protostellar life to the latest stages of stellar evolution amonggiants and supergiants. The discipline of stellar coronal X-rayastronomy has now reached a level of sophistication that makes tests ofadvanced theories in stellar physics possible. This development is basedon the rapidly advancing instrumental possibilities that today allow usto obtain images with sub-arcsecond resolution and spectra withresolving powers exceeding 1000. High-resolution X-ray spectroscopy has,in fact, opened new windows into astrophysical sources, and has played afundamental role in coronal research.
| Stellar Coronal Astronomy
Coronal astronomy is by now a fairly mature discipline, with a quartercentury having gone by since the detection of the first stellar X-raycoronal source (Capella), and having benefitted from a series of majororbiting observing facilities. Serveral observational characteristics ofcoronal X-ray and EUV emission have been solidly established throughextensive observations, and are by now common, almost text-book,knowledge. At the same time the implications of coronal astronomy forbroader astrophysical questions (e.g.Galactic structure, stellarformation, stellar structure, etc.) have become appreciated. Theinterpretation of stellar coronal properties is however still often opento debate, and will need qualitatively new observational data to bookfurther progress. In the present review we try to recapitulate our viewon the status of the field at the beginning of a new era, in which thehigh sensitivity and the high spectral resolution provided by Chandraand SMM-Newton will address new questions which were not accessiblebefore.
| Some anomalies in the occurrence of debris discs around main-sequence A and G stars
Debris discs consist of large dust grains that are generated bycollisions of comets or asteroids around main-sequence stars, and thequantity and distribution of debris may be used to detect the presenceof perturbing planets akin to Neptune. We use stellar and disc surveysto compare the material seen around A- and G-type main-sequence stars.Debris is detected much more commonly towards A stars, even when acomparison is made only with G stars of comparable age. Detection ratesare consistent with disc durations of ~0.5 Gyr, which may occur at anytime during the main sequence. The higher detection rate for A stars canresult from this duration being a larger fraction of the main-sequencelifetime, possibly boosted by a globally slightly larger disc mass thanfor the G-type counterparts. The disc mass range at any given age is afactor of at least ~100 and any systematic decline with time is slow,with a power law estimated to not be steeper than t-1/2.Comparison with models shows that dust can be expected as late as a fewGyr when perturbing planetesimals form slowly at large orbital radii.Currently, the Solar system has little dust because the radius of theKuiper Belt is small and hence the time-scale to produce planetesimalswas less than 1 Gyr. However, the apparently constant duration of ~0.5Gyr when dust is visible is not predicted by the models.
| Mid-Infrared Imaging of the First-Ascent Giant HD 233517
We present 10.8 and 18.2 μm images of HD 233517 (K2 III) made withthe Observatory Spectrometer Camera for the Infrared on Keck II.Previous observations with a 3.8 m telescope indicated that a highlyinclined disk with FWHM of 1.5" surrounds this star, but we find that HD233517 is unresolved in the mid-IR at the highest resolution currentlypossible at these wavelengths. We measure the source to have FWHM lessthan 0.31" at 10.8 μm and less than 0.41" at 18.2 μm. Combinedwith more recent analyses and conclusions by others, HD 233517 appearsto be a first-ascent giant star with spatially unresolved mid-IR dustemission.
| Apsidal Motion in Binaries: Rotation of the Components
A sample of 51 separated binary systems with measured apsidal periodsand rotational velocities of the components is examined. The ranges ofthe angles of inclination of the equatorial planes of the components tothe orbital plane are estimated for these systems. The observed apsidalvelocities can be explained by assuming that the axes of rotation of thestars are nonorthogonal to the orbital plane in roughly 47% of thesystems (24 of the 51) and the rotation of the components is notsynchronized with the orbital motion in roughly 59% of the systems (30of 51). Nonorthogonality and nonsynchrony are defined as deviations from90° and a synchronized angular velocity, respectively, at levels of1 or more.
| Contributions to the Nearby Stars (NStars) Project: Spectroscopy of Stars Earlier than M0 within 40 Parsecs: The Northern Sample. I.
We have embarked on a project, under the aegis of the Nearby Stars(NStars)/Space Interferometry Mission Preparatory Science Program, toobtain spectra, spectral types, and, where feasible, basic physicalparameters for the 3600 dwarf and giant stars earlier than M0 within 40pc of the Sun. In this paper, we report on the results of this projectfor the first 664 stars in the northern hemisphere. These resultsinclude precise, homogeneous spectral types, basic physical parameters(including the effective temperature, surface gravity, and overallmetallicity [M/H]), and measures of the chromospheric activity of ourprogram stars. Observed and derived data presented in this paper arealso available on the project's Web site.
| Stellar Kinematic Groups. II. A Reexamination of the Membership, Activity, and Age of the Ursa Major Group
Utilizing Hipparcos parallaxes, original radial velocities and recentliterature values, new Ca II H and K emission measurements,literature-based abundance estimates, and updated photometry (includingrecent resolved measurements of close doubles), we revisit the UrsaMajor moving group membership status of some 220 stars to produce afinal clean list of nearly 60 assured members, based on kinematic andphotometric criteria. Scatter in the velocity dispersions and H-Rdiagram is correlated with trial activity-based membership assignments,indicating the usefulness of criteria based on photometric andchromospheric emission to examine membership. Closer inspection,however, shows that activity is considerably more robust at excludingmembership, failing to do so only for <=15% of objects, perhapsconsiderably less. Our UMa members demonstrate nonzero vertex deviationin the Bottlinger diagram, behavior seen in older and recent studies ofnearby young disk stars and perhaps related to Galactic spiralstructure. Comparison of isochrones and our final UMa group membersindicates an age of 500+/-100 Myr, some 200 Myr older than thecanonically quoted UMa age. Our UMa kinematic/photometric members' meanchromospheric emission levels, rotational velocities, and scattertherein are indistinguishable from values in the Hyades and smaller thanthose evinced by members of the younger Pleiades and M34 clusters,suggesting these characteristics decline rapidly with age over 200-500Myr. None of our UMa members demonstrate inordinately low absolutevalues of chromospheric emission, but several may show residual fluxes afactor of >=2 below a Hyades-defined lower envelope. If one defines aMaunder-like minimum in a relative sense, then the UMa results maysuggest that solar-type stars spend 10% of their entire main-sequencelives in periods of precipitously low activity, which is consistent withestimates from older field stars. As related asides, we note six evolvedstars (among our UMa nonmembers) with distinctive kinematics that liealong a 2 Gyr isochrone and appear to be late-type counterparts to diskF stars defining intermediate-age star streams in previous studies,identify a small number of potentially very young but isolated fieldstars, note that active stars (whether UMa members or not) in our samplelie very close to the solar composition zero-age main sequence, unlikeHipparcos-based positions in the H-R diagram of Pleiades dwarfs, andargue that some extant transformations of activity indices are notadequate for cool dwarfs, for which Ca II infrared triplet emissionseems to be a better proxy than Hα-based values for Ca II H and Kindices.
| A spatially resolved limb flare on Algol B observed with XMM-Newton
We report XMM-Newton observations of the eclipsing binary Algol A (B8V)and B (K2III). The XMM-Newton data cover the phase interval 0.35-0.58,i.e., specifically the time of optical secondary minimum, when the X-raydark B-type star occults a major fraction of the X-ray bright K-typestar. During the eclipse a flare was observed with complete light curvecoverage. The decay part of the flare can be well described with anexponential decay law allowing a rectification of the light curve and areconstruction of the flaring plasma region. The flare occurred near thelimb of Algol B at a height of about 0.1 R* with plasmadensities of a few times 1011 cm-3 consistent withspectroscopic density estimates. No eclipse of the quiescent X-rayemission is observed leading us to the conclusion that the overallcoronal filling factor of Algol B is small.
| Tomography of a stellar X-ray corona: alpha Coronae Borealis
We interpret the X-ray light curve obtained by XMM-Newton during a totalX-ray eclipse in the nearby binary star alpha CoronaeBorealis. This system consists of a G5 V main-sequence starorbiting an X-ray dark A0 star. The secondary G star is a young, activesolar analog with an age of a few 100 Myr. As the primary A stareclipses active regions on the X-ray bright companion, the light curvedrops in consecutive steps to zero; as individual active regionsreappear during egress, the flux rises in similar steps. The ingress andegress light curves are combined to reconstruct the 2D distribution ofX-ray brightness on and around the G star. Three different methods areapplied, and variations due to statistical noise and uncertainties inthe binary system parameters are discussed. Although the solutions arenon-unique, all reconstructions reveal a similar distribution of X-raybright regions and large areas with little flux. We present plausibleestimates of (lower limits to) the electron densities in the brightregions, obtaining characteristic values between 109-3x1010 cm-3.Based on observations obtained with XMM-Newton, an ESA science missionwith instruments and contributions directly funded by ESA Member Statesand the USA (NASA).
| The radii and spectra of the nearest stars
We discuss direct measurements of the radii of 36 stars located closerthan 25 parsecs to the Sun. We present the data on 307 radii and 326spectral types and luminosity classes for the nearest stars locatedinside the sphere with a radius of 10 parsecs.
| The Velocity Distribution of the Nearest Interstellar Gas
The bulk flow velocity for the cluster of interstellar cloudlets within~30 pc of the Sun is determined from optical and ultraviolet absorptionline data, after omitting from the sample stars with circumstellar disksor variable emission lines and the active variable HR 1099. A total of96 velocity components toward the remaining 60 stars yield a streamingvelocity through the local standard of rest of -17.0+/-4.6 kms-1, with an upstream direction of l=2.3d, b=-5.2d (usingHipparcos values for the solar apex motion). The velocity dispersion ofthe interstellar matter (ISM) within 30 pc is consistent with that ofnearby diffuse clouds, but present statistics are inadequate todistinguish between a Gaussian or exponential distribution about thebulk flow velocity. The upstream direction of the bulk flow vectorsuggests an origin associated with the Loop I supernova remnant.Groupings of component velocities by region are seen, indicatingregional departures from the bulk flow velocity or possibly separateclouds. The absorption components from the cloudlet feeding ISM into thesolar system form one of the regional features. The nominal gradientbetween the velocities of upstream and downstream gas may be an artifactof the Sun's location near the edge of the local cloud complex. The Sunmay emerge from the surrounding gas patch within several thousand years.
| Apsidal Motion in Detached Binary Stars: Comparison of Theory and Observations
A list of 62 detached binaries having reliable data on the rotation ofthe line of apsides is considered. Theoretical estimates of the rate ofapsidal motion are obtained. These estimates are compared withobservational data. It is shown that cases in which the theoreticalestimate exceeds the observed value are several times more frequent thancases in which the theoretical value is lower than the observed one.This discrepancy increases when systems with more reliable observationaldata are considered.
| 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.
|
Sottometti un nuovo articolo
Link relazionati
Sottometti un nuovo link
Membro dei seguenti gruppi:
|
Osservazione e dati astrometrici
Cataloghi e designazioni:
|
