Contents
Images
Upload your image
DSS Images Other Images
Related articles
Mapping the Shores of the Brown Dwarf Desert. III. Young Moving Groups
We present the results of an aperture-masking interferometry survey forsubstellar companions around 67 members of the young (~8-200 Myr) nearby(~5-86 pc) AB Doradus, ? Pictoris, Hercules-Lyra, TW Hya, andTucana-Horologium stellar associations. Observations were made atnear-infrared wavelengths between 1.2 and 3.8 ?m using the adaptiveoptics facilities of the Keck II, Very Large Telescope UT4, and PalomarHale Telescopes. Typical contrast ratios of ~100-200 were achieved atangular separations between ~40 and 320 mas, with our survey being 100%complete for companions with masses below ~0.25 M &sun;across this range. We report the discovery of a 0.52 ± 0.09 M&sun; companion to HIP 14807, as well as the detections andorbits of previously known stellar companions to HD 16760, HD 113449,and HD 160934. We show that the companion to HD 16760 is in a face-onorbit, resulting in an upward revision of its mass from M2sin i ~ 14 M J to M 2 = 0.28 ±0.04 M &sun;. No substellar companions were detected aroundany of our sample members, despite our ability to detect companions withmasses below 80 M J for 50 of our targets: of these, oursensitivity extended down to 40 M J around 30 targets, with asubset of 22 subject to the still more stringent limit of 20 MJ. A statistical analysis of our non-detection of substellarcompanions allows us to place constraints on their frequency around~0.2-1.5 M &sun; stars. In particular, consideringcompanion mass distributions that have been proposed in the literature,we obtain an upper limit estimate of ~9%-11% for the frequency of 20-80M J companions between 3 and 30 AU at 95% confidence,assuming that their semimajor axes are distributed according to d {N}/da\propto a^{-1} in this range.
| Potential Members of Stellar Kinematic Groups within 30 pc of the Sun
We analyze the kinematic histories of stars within 30 pc of the Sun, forwhich three-dimensional spatial coordinates and three-dimensionalvelocity vectors are available. From this sample, we extract members ofstellar kinematic groups (SKGs) in the following manner. First, weconsider in the three-dimensional velocity space centered on the localstandard of rest, a sphere with a radius of 8 km s-1centered on the mean velocity vector of a particular SKG. Around eachSKG velocity center, we have found a significant excess of starscompared to background field stars. For each candidate, in thethree-dimensional spatial coordinate space, its trajectory is tracedback in time by the age of the relevant SKG to obtain the estimateddistance from the SKG center at the time of the SKG's birth by theepicyclic approximation and harmonic vertical motion. It often happensthat a star is a candidate member of multiple SKGs. Then we rank thecandidacy to multiple SKGs based on the smallness of distanceseparations. In this manner, we have kinematically selected 238candidates. We further impose at least one of the following qualitativecriteria for being a member: spectral type A or B, variability, or EUVand X-ray emission. We have finally selected 137 candidate members ofSKGs out of a sample of 966 stars.
| The Search for Super-saturation in Chromospheric Emission
We investigate if the super-saturation phenomenon observed at X-raywavelengths for the corona exists in the chromosphere for rapidlyrotating late-type stars. Moderate resolution optical spectra offast-rotating EUV- and X-ray-selected late-type stars were obtained.Stars in ? Per were observed in the northern hemisphere with theIsaac Newton 2.5 m telescope and Intermediate Dispersion Spectrograph.Selected objects from IC 2391 and IC 2602 were observed in the southernhemisphere with the Blanco 4 m telescope and R-C spectrograph at CTIO.Ca II H and K fluxes were measured for all stars in our sample. We findthe saturation level for Ca II K at log (L CaK/Lbol) = -4.08. The Ca II K flux does not show a decrease as afunction of increased rotational velocity or smaller Rossby number asobserved in the X-ray. This lack of "super-saturation" supports the ideaof coronal stripping as the cause of saturation and super-saturation instellar chromospheres and coronae, but the detailed underlying mechanismis still under investigation.
| Starspots on the fastest rotators in the ? Pictoris moving group
Aims: We carried out high-resolution spectroscopy andBV(I)C photometric monitoring of the two fastest late-typerotators in the nearby ? Pictoris moving group, HD 199143 (F7V) andCD-64°1208 (K7V). The motivation for this work is to investigate therotation periods and photospheric spot patterns of these very youngstars, with a longer term view to probing the evolution of rotation andmagnetic activity during the early phases of main-sequence evolution. Wealso aim to derive information on key physical parameters, such asrotational velocity and rotation period. Methods: We appliedmaximum entropy (ME) and Tikhonov regularizing (TR) criteria to derivethe surface spot map distributions of the optical modulation observed inHD 199143 (F7 V) and CD-64°1208 (K7 V). We also usedcross-correlation techniques to determine stellar parameters such asradial velocities and rotational velocities. Lomb-Scargle periodogramswere used to obtain the rotational periods from differential magnitudetime series. Results: We find periods and inclinations of 0.356days and 21.5 deg for HD 199143, and 0.355 days and 50.1 deg forCD-64°1208. The spot maps of HD 199143 obtained from the ME and TRmethods are very similar, although the latter gives a smootherdistribution of the filling factor. Maps obtained at two differentepochs three weeks apart show a remarkable increase in spot coverageamounting to ~7% of the surface of the photosphere over a time period ofonly ~20 days. The spot maps of CD-64°1208 from the two methods showgood longitudinal agreement, whereas the latitude range of the spots isextended to cover the whole visible hemisphere in the TR map. Thedistributions obtained from the first light curve of HD 199143 show thepresence of an extended and asymmetric active longitude with the maximumfilling factor at longitude ~325°. A secondary active longitude ispresent at ~100°. The spotted area distributions on CD-64°1208show two active longitudes separated by about 180°, which is notunusual on such very active stars.Figures 7-10 are available in electronic form at http://www.aanda.org
| RACE-OC project: rotation and variability in the ? Chamaeleontis, Octans, and Argus stellar associations
Context. Rotational properties of late-type low-mass members ofassociations of known age provide a fundamental source of information onstellar internal structure and its evolution. Aims: We aim atdetermining the rotational and magnetic-related activity properties ofstars at different stages of evolution. We focus our attention primarilyon members of young stellar associations of known ages. Specifically, weextend our previous analysis in Paper I (Messina et al. 2010, A&A520, A15) to 3 additional young stellar associations beyond 100 pc andwith ages in the range 6-40 Myr: ? Chamaeleontis (~6 Myr),Octans (~20 Myr), and Argus (~40 Myr). Additional rotational data of? Chamaeleontis and IC 2391 clusters are also considered. Methods: Rotational periods were determined by applying the Lomb-Scargleperiodogram technique to photometric time-series data obtained by theAll Sky Automated Survey (ASAS) and the Wide Angle Search for Planets(SuperWASP) archives. The magnetic activity level was derived from theamplitude of the V light curves. Results: We detected therotational modulation and measured the rotation periods of 56 stars forthe first time, confirmed 11 and revised 3 rotation periods alreadyknown from the literature. Adding the periods of 10 additional starsretrieved from the literature we determined a sample of 80 periodicstars at ages of ~6, ~20, and ~40 Myr. Using the SuperWASP data we alsorevisited some of the targets studied in Paper I. Conclusions:With the present study we have completed the analysis of the rotationalproperties of the late-type members of all known young looseassociations in the solar neighbourhood. Considering also the results ofPaper I, we have derived the rotation periods of 241 targets: 171confirmed, 44 likely, 26 uncertain. The period of the remaining 50 starsknown to be part of loose associations still remains unknown. Therotation period distributions we provided in the 0.8-1.2M&sun; mass range span nine different ages from 1 to ~100Myr. This rotation period catalogue, and specifically the newinformation presented in this paper at ~6, 20, and 40 Myr, contributessignificantly to a better observational description of the angularmomentum evolution of young stars. The results of the angular momentumevolution model based on this period database will be presented inforthcoming papers.Based on the All Sky Automated Survey (ASAS) and Wide Angle Search forPlanets (SuperWASP) photometric data.Appendices A and B are available inelectronic form at http://www.aanda.org
| Effect of magnetic activity saturation in chromospheric flux-flux relationships
We present a homogeneous study of chromospheric and coronal flux-fluxrelationships using a sample of 298 late-type dwarf active stars withspectral types F to M. The chromospheric lines were observedsimultaneously in each star to avoid spread as a result of long-termvariability. Unlike other works, we subtract the basal chromosphericcontribution in all the spectral lines studied. For the first time, wequantify the departure of dMe stars from the general relations. We showthat dK and dKe stars also deviate from the general trend. Studying theflux-colour diagrams, we demonstrate that the stars deviating from thegeneral relations are those with saturated X-ray emission and we showthat these stars also present saturation in the H? line. Usingseveral age spectral indicators, we show that these are younger starsthan those following the general relationships. The non-universality offlux-flux relationships found in this work should be taken into accountwhen converting between fluxes in different chromospheric activityindicators.
| The Tucana/Horologium, Columba, AB Doradus, and Argus Associations: New Members and Dusty Debris Disks
We propose 35 star systems within ~70 pc of Earth as newly identifiedmembers of nearby young stellar kinematic groups; these identificationsinclude the first A- and late-B-type members of the AB Doradus movinggroup and field Argus Association. All but one of the 35 systems containa bright solar- or earlier-type star that should make an excellenttarget for the next generation of adaptive optics (AO) imaging systemson large telescopes. AO imaging has revealed four massive planets inorbit around the ? Boo star HR 8799. Initially, the planets wereof uncertain mass due in large part to the uncertain age of the star. Wefind that HR 8799 is a likely member of the ~30 Myr old ColumbaAssociation, implying planet masses ~6 times that of Jupiter. Weconsider Spitzer Space Telescope MIPS photometry of stars in the ~30 Myrold Tucana/Horologium and Columba Associations, the ~40 Myr old fieldArgus Association, and the ~70 Myr old AB Doradus moving group. Thepercentage of stars in these young stellar groups that display excessemission above the stellar photosphere at 24 and 70 ?mwavelengths—indicative of the presence of a dusty debrisdisk—is compared with corresponding percentages for members of 11open clusters and stellar associations with ages between 8 and 750 Myr,thus elucidating the decay of debris disks with time.
| Long-term photometry of the ultra-fast rotator LO Peg
LO Peg is a young main-sequence star of spectral type K3. With itsequatorial rotation velocity of 65 km s-1 it is amongst theultra-fast rotators. Its high equatorial rotation velocity and rapidlychanging surface activity features make it an important object in termsof both stellar activity and the evolution of stellar rotation andangular momentum. Since its discovery as a variable star, it has mostlybeen subject to spectral surface mapping studies such as DopplerImaging, while there have been very few photometric studies on it. Thispaper aims to present the first long-term photometric observations andits results covering the years between 2003 and 2009. The U BV R Johnsonwide band photometric data showed that the surface activity structuresof LO Peg vary in timescales changing between days and months, andparallel to this, the mean, maximum and minimum brightness andamplitudes change dramatically between years and sometimes even withinthe same observation season. Long-term changes in system brightness andcolours, both characteristic features of active stars, were also seen inthis ultra-fast young star. The active longitudes, which has a life timeof ˜1.3 years and an activity cycle period of ˜4.8 years forLO Peg were estimated.
| RACE-OC project: Rotation and variability of young stellar associations within 100 pc
Context. Examining the angular momentum of stars and its interplay withtheir magnetic fields represent a promising way to probe the stellarinternal structure and evolution of low-mass stars. Aims: Weattempt to determine the rotational and magnetic-related activityproperties of stars at different stages of evolution.We focused ourattention primarily on members of clusters and young stellarassociations of known ages. In this study, our targets are 6 young loosestellar associations within 100 pc and with ages in the range 8-70 Myr:TW Hydrae (~8 Myr), ? Pictoris (~10 Myr), Tucana/Horologium,Columba, Carina (~30 Myr), and AB Doradus (~70 Myr). Additionalrotational data for ? Persei and the Pleiades from the literatureare also considered. Methods: Rotational periods of starsexhibiting rotational modulation due to photospheric magnetic activity(i.e., starspots) were determined by applying the Lomb-Scargleperiodogram technique to photometric time-series data obtained by theAll Sky Automated Survey (ASAS). The magnetic activity level was derivedfrom the amplitude of the V lightcurves. The statistical significance ofthe rotational evolution at different ages was inferred by applying atwo-sided Kolmogorov-Smirnov test to subsequent age-bins. Results: We detected the rotational modulation and measured the rotationperiods of 93 stars for the first time, and confirmed the periods of 41stars already known from the literature. For an additional 10 stars, werevised the period determinations by other authors. The sample wasaugmented with periods of 21 additional stars retrieved from theliterature. In this way, for the first time we were able to determinethe largest set of rotation periods at ages of ~8, ~10 and ~30 Myr, aswell as increase by 150% the number of known periodic members of AB Dor. Conclusions: The analysis of the rotation periods in youngstellar associations, supplemented by Orion Nebula Cluster (ONC) and NGC2264 data from the literature, has allowed us to find that in the0.6-1.2 M? range the most significant variations in therotation period distribution are the spin-up between 9 and 30 Myr andthe spin-down between 70 and 110 Myr. Variations of between 30 and 70Myr are rather doubtful, despite the median period indicating asignificant spin-up. The photospheric activity level is found to becorrelated with rotation at ages greater than ~70 Myr and to show someadditional age dependence besides that related to rotation and mass.Tables 1.1-1.7 and Figs. 1.1-1.22 are only available in electronic format http://www.aanda.orgBased on theAll Sky Automated Survey photometric data.
| The Effect of Magnetic Activity on Low-Mass Stars in Eclipsing Binaries
In recent years, analyses of eclipsing binary systems have unveileddifferences between the observed fundamental properties of low-massstars and those predicted by stellar structure models. Particularly,radius and effective temperatures computed from models are ~5%-10% lowerand ~3%-5% higher than observed, respectively. These discrepancies havebeen attributed to different factors, notably the high levels ofmagnetic activity present on these stars. In this paper, we test theeffect of magnetic activity both on models and on the observationalanalysis of eclipsing binaries using a sample of such systems withaccurate fundamental properties. Regarding stellar models, we have foundthat unrealistically high spot coverages need to be assumed to reproducethe observations. Tests considering metallicity effects and missingopacities on models indicate that these are not able to explain theradius discrepancies observed. With respect to the observations, we havetested the effect of several spot distributions on the light curveanalysis. Our results show that spots cause systematic deviations on thestellar radii derived from light curve analysis when mainly distributedover the stellar poles. Assuming the existence of polar spots, overallagreement between models and observations is reached when ~35% spotcoverage is considered on stellar models. Such spot coverage induces asystematic deviation in the radius determination from the light curveanalysis of ~3% and is also compatible with the modulations observed onthe light curves of these systems. Finally, we have found that theeffect of activity or rotation on convective transport in partiallyradiative stars may also contribute to the explanation of thedifferences seen in some of the systems with shorter orbital periods.
| A Uniform Analysis of 118 Stars with High-contrast Imaging: Long-period Extrasolar Giant Planets are Rare Around Sun-like Stars
We expand on the results of Nielsen et al., using the null result forgiant extrasolar planets around the 118 target stars from the Very LargeTelescope (VLT) NACO H- and Ks-band planet search (conducted byMasciadri and collaborators in 2003 and 2004), the VLT and MMTSimultaneous Differential Imager survey, and the Gemini Deep PlanetSurvey to set constraints on the population of giant extrasolar planets.Our analysis is extended to include the planet luminosity models ofFortney et al., as well as the correlation between stellar mass andfrequency of giant planets found by Johnson et al. Doubling the samplesize of FGKM stars strengthens our conclusions: a model for extrasolargiant planets with power laws for mass and semimajor axis as given byCumming et al. cannot, with 95% confidence, have planets beyond 65 AU,compared to the value of 94 AU reported by Nielsen et al., using themodels of Baraffe et al. When the Johnson et al. correction for stellarmass (which gives fewer Jupiter-mass companions to M stars with respectto solar-type stars) is applied, however, this limit moves out to 82 AU.For the relatively new Fortney et al. models, which predict fainterplanets across most of parameter space, these upper limits, with andwithout a correction for stellar mass, are 182 and 234 AU, respectively.
| A high-resolution spectroscopic survey of late-type stars: chromospheric activity, rotation, kinematics, and age
Aims: We present a compilation of spectroscopic data from asurvey of 144 chromospherically active young stars in the solarneighborhood, which may be used to investigate different aspects of itsformation and evolution in terms of kinematics and stellar formationhistory. The data have already been used by us in several studies. Withthis paper, we make all these data accessible to the scientificcommunity for future studies on different topics. Methods: Weperformed spectroscopic observations with echelle spectrographs to coverthe entirety of the optical spectral range simultaneously. Standard datareduction was performed with the IRAF echelle package. We applied thespectral subtraction technique to reveal chromospheric emission in thestars of the sample. The equivalent width of chromospheric emissionlines was measured in the subtracted spectra and then converted tofluxes using equivalent width-flux relationships. Radial and rotationalvelocities were determined by the cross-correlation technique.Kinematics, equivalent widths of the lithium line ?6707.8 Åand spectral types were also determined. Results: A catalog ofspectroscopic data is compiled: radial and rotational velocities, spacemotion, equivalent widths of optical chromospheric activity indicatorsfrom Ca II H & K to the calcium infrared triplet and the lithiumline in ?6708 Å. Fluxes in the chromospheric emission linesand R'_HK are also determined for each observation of a star in thesample. We used these data to investigate the emission levels of ourstars. The study of the H? emission line revealed two differentpopulations of chromospheric emitters in the sample, clearly separatedin the logFH?/Fbol - (V-J) diagram. Thedichotomy may be associated with the age of the stars.Based on observations made with the 2.2 m telescope of theGerman-Spanish Astronomical Centre, Calar Alto (Almería, Spain),operated jointly by the Max-Planck-Institute for Astronomy, Heidelberg,and the Spanish National Commission for Astronomy; the Nordic OpticalTelescope (NOT), operated on the island of La Palma jointly by Denmark,Finland, Iceland, Norway and Sweden, in the Spanish Observatorio delRoque de Los Muchachos of the Instituto de Astrofísica deCanarias; the Isaac Newton Telescope (INT) operated on the island of LaPalma by the Isaac Newton Group in the Spanish Observatorio del Roque deLos Muchachos of the Instituto de Astrofísica de Canarias; withthe Italian Telescopio Nazionale Galileo (TNG) operated on the island ofLa Palma by the Centro Galileo Galilei of the INAF (Istituto Nazionaledi Astrofisica) at the Spanish Observatorio del Roque de Los Muchachosof the Instituto de Astrofísica de Canarias; and with theHobby-Eberly Telescope (HET) operated by McDonald Observatory on behalfof The University of Texas at Austin, the Pennsylvania State University,Stanford University, Ludwig-Maximilians-Universität München,and Georg-August-Universität Göttingen. This research has madeuse of the SIMBAD database and VizieR catalog access tool, operated atCDS, Strasbourg, France.Tables A.1-A.4 and reduced spectra are alsoavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/514/A97
| Evidence of a massive planet candidate orbiting the young active K5V star BD+20 1790
Context. BD+20 1790 is a young active, metal-rich, late-type K5Ve star.We have undertaken a study of stellar activity and kinematics for thisstar over the past few years. Previous results show a high level ofstellar activity, with the presence of prominence-like structures, spotson the surface, and strong flare events, despite the moderate rotationalvelocity of the star. In addition, radial velocity variations with asemi-amplitude of up to 1 km s-1 were detected. Aims:We investigate the nature of these radial velocity variations, in orderto determine whether they are due to stellar activity or the reflexmotion of the star induced by a companion. Methods: We haveanalysed high-resolution echelle spectra by measuring stellar activityindicators and computing radial velocity (RV) and bisector velocityspans. Two-band photometry was also obtained to produce the light curveand determine the photometric period. Results: Based upon theanalysis of the bisector velocity span, as well as spectroscopic indicesof chromospheric indicators, Ca ii H & K, H?, and taking thephotometric analysis into account, we report that the best explanationfor the RV variation is the presence of a substellar companion. TheKeplerian fit of the RV data yields a solution for a close-in massiveplanet with an orbital period of 7.78 days. The presence of the close-inmassive planet could also be an interpretation for the high level ofstellar activity detected. Since the RV data are not part of a planetsearch programme, we can consider our results as a serendipitousevidence of a planetary companion. To date, this is the youngest mainsequence star for which a planetary candidate has been reported.Based on observations collected at the German-Spanish AstronomicalCenter, Calar Alto, jointly operated by the Max-Planck-Institut fürAstronomie Heidelberg and the Instituto de Astrofísica deAndalucía (CSIC). Based on observations made with the ItalianTelescopio Nazionale Galileo (TNG) operated on the island of La Palma bythe Fundación Galileo Galilei of the INAF (Istituto Nazionale diAstrofisica) at the Spanish Observatorio del Roque de los Muchachos ofthe Instituto de Astrofísica de Canarias. Based on observationsmade with the Liverpool Telescope operated on the island of La Palma byLiverpool John Moores University in the Spanish Observatorio del Roquede los Muchachos of the Instituto de Astrofísica de Canarias withfinancial support from the UK Science and Technology FacilitiesCouncil.Our radial velocity data from Table 3 are available inelectronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/512/A45
| Li I and K I Scatter in Cool Pleiades Dwarfs
We utilize high-resolution (R ~ 60,000), high signal-to-noise ratio(~100) spectroscopy of 17 cool Pleiades dwarfs to examine theconfounding star-to-star scatter in the ?6707 Li I line strengthsin this young cluster. Our Pleiades, selected for their small projectedrotational velocity and modest chromospheric emission, evincesubstantial scatter in the line strengths of ?6707 Li I featurethat is absent in the ?7699 K I resonance line. The Li I scatteris not correlated with that in the high-excitation ?7774 O Ifeature, and the magnitude of the former is greater than the latterdespite the larger temperature sensitivity of the O I feature. Theseresults suggest that systematic errors in line strength measurements dueto blending, color (or color-based T eff) errors, or lineformation effects related to an overlying chromosphere are not theprincipal source of Li I scatter in our stars. There do exist analyticspot models that can produce, via line formation effects, the observedLi scatter without introducing scatter in the K I line strengths or thecolor-magnitude diagram. However, these models predict factor of >=3differences in abundances derived from the subordinate ?6104 andresonance ?6707 Li I features; we find no difference in theabundances determined from these two features. These analytic spotmodels also predict CN line strengths significantly larger than weobserve in our spectra. The simplest explanation of the Li, K, CN, andphotometric data is that there must be a real abundance component to thePleiades Li dispersion. We suggest that this real abundance component isthe manifestation of relic differences in erstwhile pre-main-sequence Liburning caused by effects of surface activity on stellar structure. Wediscuss observational predictions of these effects, which may be relatedto other anomalous stellar phenomena.Based on observations obtained with the High Resolution Spectrograph onthe Hobby-Eberly Telescope, which is operated by McDonald Observatory onbehalf of the University of Texas at Austin, Pennsylvania StateUniversity, Stanford University, the Ludwig-Maximillians-Universitaet,Munich, and the George-August-Universitaet, Goettingen. Public Accesstime was available on the Hobby-Eberly Telescope through an agreementwith the National Science Foundation.
| XID II: Statistical Cross-Association of ROSAT Bright Source Catalog X-ray Sources with 2MASS Point Source Catalog Near-Infrared Sources
The 18,806 ROSAT All Sky Survey Bright Source Catalog (RASS/BSC) X-raysources are quantitatively cross-associated with near-infrared (NIR)sources from the Two Micron All Sky Survey Point Source Catalog(2MASS/PSC). An association catalog is presented, listing the mostlikely counterpart for each RASS/BSC source, the probability Pid that the NIR source and X-ray source are uniquelyassociated, and the probability P no-id that none of the2MASS/PSC sources are associated with the X-ray source. The catalogincludes 3853 high quality (P id>0.98) X-ray-NIR matches,2280 medium quality (0.98 >= P id>0.9) matches, and4153 low quality (0.9 >= P id>0.5) matches. Of the highquality matches, 1418 are associations that are not listed in the SIMBADdatabase, and for which no high quality match with a USNO-A2 opticalsource was presented for the RASS/BSC source in previous work. Thepresent work offers a significant number of new associations withRASS/BSC objects that will require optical/NIR spectroscopy forclassification. For example, of the 6133 P id>0.92MASS/PSC counterparts presented in the association catalog, 2411 haveno classification listed in the SIMBAD database. These 2MASS/PSC sourceswill likely include scientifically useful examples of known sourceclasses of X-ray emitters (white dwarfs, coronally active stars, activegalactic nuclei), but may also contain previously unknown sourceclasses. It is determined that all coronally active stars in theRASS/BSC should have a counterpart in the 2MASS/PSC, and that the uniqueassociation of these RASS/BSC sources with their NIR counterparts thusis confusion limited.
| Starspots
Starspots are created by local magnetic fields on the surfaces of stars,just as sunspots. Their fields are strong enough to suppress theoverturning convective motion and thus block or redirect the flow ofenergy from the stellar interior outwards to the surface andconsequently appear as locally cool and therefore dark regions againstan otherwise bright photosphere (Biermann in Astronomische Nachrichten264:361, 1938; Z Astrophysik 25:135, 1948). As such, starspots areobservable tracers of the yet unknown internal dynamo activity and allowa glimpse into the complex internal stellar magnetic field structure.Starspots also enable the precise measurement of stellar rotation whichis among the key ingredients for the expected internal magnetictopology. But whether starspots are just blown-up sunspot analogs, we donot know yet. This article is an attempt to review our current knowledgeof starspots. A comparison of a white-light image of the Sun (G2V, 5Gyr) with a Doppler image of a young solar-like star (EK Draconis;G1.5V, age 100 Myr, rotation 10 × ? Sun) and witha mean-field dynamo simulation suggests that starspots can be ofsignificantly different appearance and cannot be explained with ascaling of the solar model, even for a star of same mass and effectivetemperature. Starspots, their surface location and migration pattern,and their link with the stellar dynamo and its internal energytransport, may have far reaching impact also for our understanding oflow-mass stellar evolution and formation. Emphasis is given in thisreview to their importance as activity tracers in particular in thelight of more and more precise exoplanet detections around solar-like,and therefore likely spotted, host stars.
| Photospheric and chromospheric activity in V405 Andromedae. An M dwarf binary with components on the two sides of the full convection limit
We investigate the fast-rotating (P_orb=P_rot=0.465d), active dwarfbinary V405 And (M0V+M5V) using photometric BV(RI)C andoptical spectroscopic data. The light variation is caused by thecombined effect of spottedness and binarity with a small eclipse. Weestimate the system parameters from the available light and radialvelocity curves. Three flare events occurred during the observations:two were found in the spectroscopic data and one was observedphotometrically in BV(RI)C colours. An interesting eruptivephenomenon emerged from the photometric measurements that can beinterpreted as a series of post-flare eruptions lasting for at least 3orbits (rotations) of the system, originating from trans-equatorialmagnetic loops, which connect the active regions in the two hemispheres.The two components of V405 And have masses well over and below thetheoretical limit of full convection. This rare property makes thebinary an ideal target for observing and testing models for stellardynamo action.
| A spectroscopic survey of the youngest field stars in the solar neighbourhood. I. The optically bright sample
Aims: We present the first results of an ambitious ground-basedobservation programme conducted on 1-4 m class telescopes. Our sampleconsists of 1097 active and presumably young stars, all of them beingoptical (Tycho Catalogue) counterparts of ROSAT All-Sky Survey X-raysources in the northern hemisphere. In this paper, we concentrate on theoptically brightest (V_T?slant 9.5 m) candidates (704 objects). Weacquired high-resolution optical spectroscopy in the H? and/orlithium spectral regions for 426 of such stars without relevant data inthe literature. We describe the star sample and the observations and westart to discuss the physical properties of the investigated stars. Methods: We used a cross-correlation technique and other toolsdeveloped by us to derive accurate radial and rotational velocities andto perform an automatic spectral classification for both single starsand double-lined systems. The spectral subtraction technique was used toderive chromospheric activity levels and lithium abundances. Weestimated the fraction of young single stars and multiple systems instellar soft X-ray surveys and the contamination by more evolvedsystems, like RS CVn binaries. We classified stars on the basis of theirlithium abundance and give a glimpse of their sky distribution. Results: The sample appears to be a mixture of quite youngPleiades-like and Hyades-like stars plus an older lithium-poorpopulation probably born within the last 1-2 Gyr. Seven stars with alithium abundance compatible with the age of IC 2602 (about 30 Myr) oreven younger were detected as well, although two appear to belithium-rich giants.The discovery of a large number of highly ormoderately lithium-rich giants is another outcome of the present survey. Conclusions: The contamination of soft X-ray surveys by oldsystems in which the activity level is enhanced by tidal synchronisationis not negligible, especially for K-type stars. Five stars with lithiumcontent close to the primordial abundance are probably associated withalready known moving groups in the solar neighbourhood. Some of them aregood post-T Tauri candidates according to their positions in the HRdiagram.Based on observations collected at the Observatoire de Haute Provence(France) and the Catania Astrophysical Observatory (Italy). Data for the426 stars is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/504/829
| LO Pegasi: an investigation of multiband optical polarization
We present BVR polarimetric study of the cool active star LO Pegasi (LOPeg) for the first time. LO Peg was found to be highly polarized amongthe cool active stars. Our observations yield average values ofpolarization in LO Peg: PB = 0.387 +/- 0.004 per cent,θB = 88° +/- 1° PV = 0.351 +/-0.004 per cent, θV = 91° +/- 1° andPR = 0.335 +/- 0.003 per cent, θR = 91°+/- 1°. Both the degree of polarization and the position angle arefound to be variable. The semi-amplitude of the polarization variabilityin B, V and R bands is found to be 0.18 +/- 0.02, 0.13 +/- 0.01 and 0.10+/- 0.02 per cent, respectively. We suggest that the levels ofpolarization observed in LO Peg could be the result of scattering of ananisotropic stellar radiation field by an optically thin circumstellarenvelope or scattering of the stellar radiation by prominence-likestructures.
| New Debris Disks Around Young, Low-Mass Stars Discovered with the Spitzer Space Telescope
We present 24 μm and 70 μm Multiband Imaging Photometer forSpitzer (MIPS) observations of 70 A through M-type dwarfs with estimatedages from 8 Myr to 1.1 Gyr, as part of a Spitzer guaranteed timeprogram, including a re-analysis of some previously published sourcephotometry. Our sample is selected from stars with common youthindicators such as lithium abundance, X-ray activity, chromosphericactivity, and rapid rotation. We compare our MIPS observations toempirically derived Ks -[24] colors as a function of thestellar effective temperature to identify 24 μm and 70 μmexcesses. We place constraints or upper limits on dust temperatures andfractional infrared luminosities with a simple blackbody dust model. Weconfirm the previously published 70 μm excesses for HD 92945, HD112429, and AU Mic, and provide updated flux density measurements forthese sources. We present the discovery of 70 μm excesses for fivestars: HD 7590, HD 10008, HD 59967, HD 73350, and HD 135599. HD 135599is also a known Spitzer IRS (InfraRed Spectrograph) excess source, andwe confirm the excess at 24 μm. We also present the detection of 24μm excesses for 10 stars: HD 10008, GJ 3400A, HD 73350, HD 112429, HD123998, HD 175742, AT Mic, BO Mic, HD 358623 and Gl 907.1. We find thatlarge 70 μm excesses are less common around stars with effectivetemperatures of less than 5000 K (3.7+7.6–1.1%) than around stars with effective temperaturesbetween 5000 K and 6000 K (21.4+9.5 –5.7%),despite the cooler stars having a younger median age in our sample (12Myr vs. 340 Myr). We find that the previously reported excess for TWA13A at 70 μm is due to a nearby background galaxy, and the previouslyreported excess for HD 177724 is due to saturation of the near-infraredphotometry used to predict the mid-infrared stellar flux contribution.In the Appendix, we present an updated analysis of dust grain removaltimescales due to grain-grain collisions and radiation pressure,Poynting-Robertson (P-R) drag, stellar wind drag, and planet-dustdynamical interaction. We find that drag forces can be important fordisk dynamics relative to grain-grain collisions for L IR/L* < 10–4, and that stellar wind drag ismore important than P-R drag for K and M dwarfs, and possibly for young(<1 Gyr) G dwarfs as well.
| High levels of surface differential rotation on the young G0 dwarf HD171488
We present high-resolution images of the young, rapidly rotating G0dwarf HD171488, using both Stokes I and Stokes V data. The observationswere secured with the MuSiCoS spectropolarimeter at Telescope BernardLyot from 2005 May 31 to June 10. The photospheric surface brightnessdistributions show a strong and slightly decentred polar cap thatdominates over weak high- and low-latitude spot features. Thelarge-scale magnetic field topology shows a strong ring of anticlockwiseazimuthal field with a latitudinal dependence on polarity and largeregions of radial field with negative polarity at all latitudes. Usingthe good phase coverage of our data, we measure the differentialrotation on HD171488. The results indicate that the equator laps thepole every 12 days for brightness data and 13 days for magnetic data,which is the highest measurement of differential rotation obtained usingZeeman-Doppler imaging techniques.Spectropolarimetric observations were obtained, from 2005 May 31 to June10, with the MuSiCoS echelle spectropolarimeter at the Telescope BernardLyot (Observatoire du Pic du Midi, France).E-mail: s.v.jeffers@uu.nl
| Doppler imaging of the young late-type star LO Pegasi (BD+22°4409) in 2003 September
A Doppler image of the zero-age main-sequence (ZAMS) late-type rapidlyrotating star LO Pegasi, based on spectra acquired between 2003September 12 and 15 is presented. The least-squares deconvolutiontechnique is applied to enhance the signal-to-noise ratio of the meanrotational broadened line profiles extracted from the observed spectra.In the present application, an unbroadened spectrum is used as areference, instead of a simple line list, to improve the deconvolutiontechnique applied to extract the mean profiles. The reconstructed imageis similar to those previously obtained from observations taken in 1993and 1998, and shows that LO Peg photospheric activity is dominated byhigh-latitude spots with a non-uniform polar cap. The latter seems to bea persistent feature as it has been observed since 1993 with littlemodifications. Small spots, observed between ~10° and ~60° oflatitude, appears to be different with respect to those present in the1993 and 1998 maps.Based on observations made with the Italian Telescopio Nazionale Galileooperated on the island of La Palma by the Centro Galileo Galilei of INAF(Istituto Nazionale di Astrofisica) at the Spanish Observatorio delRoque del los Muchachos of the Instituto de Astrofísica deCanarias.E-mail: nicolo.piluso@oact.inaf.it (NP); nuccio.lanza@oact.inaf.it(AFL); isabella.pagano@oact.inaf.it (IP);alessandro.lanzafame@oact.inaf.it (ACL); donati@ast.obs-mip.fr (J-FD)
| Doppler images and chromospheric variability of TWA 6
We present Doppler imaging and Balmer line analysis of the weak-line TTauri star TWA 6. Using these data we have made one of the firstattempts to measure differential rotation in a T Tauri star, and thefirst detection of a slingshot prominence in such a star. We also showthe most direct evidence to date of the existence of solar-type plagesin a star other than the Sun.Observations were made over six nights: 2006 February 11-13 and 2006February 18-20, when spectra were taken with the UCL EchelleSpectrograph on the 3.9-m Anglo-Australian Telescope. Usingleast-squares deconvolution to improve the effective signal-to-noiseratio we produced two Doppler maps. These show similar features to mapsof other rapidly rotating T Tauri stars, i.e. a polar spot with morespots extending out of it down to equator. Comparison of the two mapswas carried out to measure the differential rotation. Cross-correlationand parameter fitting indicates that TWA 6 does not have detectabledifferential rotation.The Balmer emission of the star was studied. The mean Hα profilehas a narrow component consistent with rotational broadening and a broadcomponent extending out to +/-250kms-1. The variability inHα suggests that the chromosphere has active regions that arecospatial with the spots in the photosphere, similar to the `plages'observed on the Sun. In addition the star has at least one slingshotprominence 3R* above the surface - the first such detectionin a T Tauri star.
| On the kinematic evolution of young local associations and the Scorpius-Centaurus complex
Context: Over the last decade, several groups of young (mainly low-mass)stars have been discovered in the solar neighbourhood (closer than ~100pc), thanks to cross-correlation between X-ray, optical spectroscopy andkinematic data. These young local associations - including an importantfraction whose members are Hipparcos stars - offer insights into thestar formation process in low-density environments, shed light on thesubstellar domain, and could have played an important role in the recenthistory of the local interstellar medium. Aims: To study the kinematicevolution of young local associations and their relation to other youngstellar groups and structures in the local interstellar medium, thuscasting new light on recent star formation processes in the solarneighbourhood. Methods: We compiled the data published in theliterature for young local associations. Using a realistic Galacticpotential we integrated the orbits for these associations and theSco-Cen complex back in time. Results: Combining these data with thespatial structure of the Local Bubble and the spiral structure of theGalaxy, we propose a recent history of star formation in the solarneighbourhood. We suggest that both the Sco-Cen complex and young localassociations originated as a result of the impact of the inner spiralarm shock wave against a giant molecular cloud. The core of the giantmolecular cloud formed the Sco-Cen complex, and some small cloudlets ina halo around the giant molecular cloud formed young local associationsseveral million years later. We also propose a supernova in young localassociations a few million years ago as the most likely candidate tohave reheated the Local Bubble to its present temperature.
| The Gemini Deep Planet Survey
We present the results of the Gemini Deep Planet Survey, a near-infraredadaptive optics search for giant planets and brown dwarfs around 85nearby young stars. The observations were obtained with the Altairadaptive optics system at the Gemini North telescope, and angulardifferential imaging was used to suppress the speckle noise of thecentral star. Typically, the observations are sensitive to angularseparations beyond 0.5" with 5 σ contrast sensitivities inmagnitude difference at 1.6 μm of 9.5 at 0.5", 12.9 at 1", 15.0 at2", and 16.5 at 5". These sensitivities are sufficient to detect planetsmore massive than 2 MJ with a projected separation in therange 40-200 AU around a typical target. Second-epoch observations of 48stars with candidates (out of 54) have confirmed that all candidates areunrelated background stars. A detailed statistical analysis of thesurvey results is presented. Assuming a planet mass distributiondn/dm~m-1.2 and a semimajor-axis distributiondn/da~a-1, the 95% credible upper limits on the fraction ofstars with at least one planet of mass 0.5-13 MJ are 0.28 forthe range 10-25 AU, 0.13 for 25-50 AU, and 0.093 for 50-250 AU; thisresult is weakly dependent on the semimajor-axis distribution power-lawindex. The 95% credible interval for the fraction of stars with at leastone brown dwarf companion having a semimajor axis in the range 25-250 AUis 0.019+0.083-0.015, irrespective of anyassumption on the mass and semimajor-axis distributions. Theobservations made as part of this survey have resolved the stars HD14802, HD 166181, and HD 213845 into binaries for the first time.Based on observations obtained at the Gemini Observatory, which isoperated by the Association of Universities for Research in Astronomy,Inc., under a cooperative agreement with the NSF on behalf of the Geminipartnership: the National Science Foundation (United States), theParticle Physics and Astronomy Research Council (United Kingdom), theNational Research Council (Canada), CONICYT (Chile), the AustralianResearch Council (Australia), CNPq (Brazil), and CONICET (Argentina).
| The coronal structure of Speedy Mic - II. Prominence masses and off-disc emission
Observations of stellar prominences on young rapidly rotating starsprovide unique probes of their magnetic fields out to many stellarradii. We compare two independently obtained data sets of the K3 dwarfSpeedy Mic (BO Mic, HD 197890) using the Anglo-Australian Telescope(AAT) and the European Southern Observatory (ESO) Very Large Telescope(VLT). Taken more than a fortnight apart, they provide the first insightinto the evolution of the prominence system on such a young rapidlyrotating star. The largest prominences observed transiting the stellardisc are found at very similar rotational phases between the epochs.This suggests that the magnetic structures supporting the prominencesretain their identity on a two to three week time-scale. By takingadvantage of the high signal-to-noise ratio and large wavelength rangeof the VLT observations, we identify prominences as transient absorptionfeatures in all lines of the hydrogen Balmer series down toH10. We use the ratios of the prominence equivalent widths(EWs) in these lines to determine their column densities in the firstexcited state of hydrogen. We determine the optical depths, findingprominences to be rather optically thick (τ ~ 20) in the Hαline. The total hydrogen column density and thus the prominence massesare determined via observations of the CaII H&K lines. We findtypical masses for four of the largest prominences to be in the range0.5-2.3 × 1014 kg, slightly larger than giant solarprominence masses. Rotationally modulated emission is seen outside ofthe Hα line. These loops of emission are shown to be caused byprominences seen off the stellar disc. We find that all of the largeemission loops can be associated with prominences we see transiting thestellar disc. This, combined with the fact that many prominences appearto eclipse the off-disc emission of others, strongly suggests that theprominence system is highly flattened and likely confined to low stellarlatitudes.Based on observations made with the ESO telescopes obtained from the ESOScience Archive Facility.E-mail: njd2@st-andrews.ac.uk
| Pulkovo compilation of radial velocities for 35495 stars in a common system.
Not Available
| The Nearest Young Moving Groups
The latest results in the research of forming planetary systems have ledseveral authors to compile a sample of candidates for searching forplanets in the vicinity of the Sun. Young stellar associations areindeed excellent laboratories for this study, but some of them are notclose enough to allow the detection of planets through adaptive opticstechniques. However, the existence of very close young moving groups cansolve this problem. Here we have compiled the members of the nearestyoung moving groups, as well as a list of new candidates from ourcatalog of late-type stars that are possible members of young stellarkinematic groups, studying their membership through spectroscopic andphotometric criteria.
| Roche tomography of cataclysmic variables - III. Star-spots on AE Aqr
We present a Roche tomography reconstruction of the secondary star inthe cataclysmic variable AE Aqr. The tomogram reveals several surfaceinhomogeneities that are due to the presence of large, cool star-spots.In addition to a number of lower latitude spots, the maps also show thepresence of a large, high-latitude spot similar to that seen in Dopplerimages of rapidly rotating isolated stars, and a relative paucity ofspots at a latitude of 40°. In total, we estimate that some 18 percent of the Northern hemisphere of AE Aqr is spotted.We have also applied the entropy landscape technique to determineaccurate parameters for the binary system. We obtain optimal massesM1 = 0.74Msolar, M2 =0.50Msolar, a systemic velocity γ = -63kms-1and an orbital inclination i = 66°.Given that this is the first study to successfully image star-spots onthe secondary star in a cataclysmic variable, we discuss the role thatfurther studies of this kind may play in our understanding of thesebinaries.
| Aryabhatta Research Institute of Observational Sciences: reincarnation of a 50 year old State Observatory of Nainital
The fifty year old State Observatory, well known as U.P. StateObservatory till the formation of Uttaranchal in November 2000, was reincarnated on March 22, 2004 as Aryabhatta Research Institute ofObservational Sciences with acronym ARIES, an autonomous institute,under the Department of Science & Technology, Government of India.The growth of academic and technical activities and new mandate of theInstitute are briefly described. In early 60's, the Institute was one ofthe 12 centres established by the Smithsonian AstrophysicalObservatory, USA, all over the globe but the only centre in India forimaging artificial earth satellites. Commensurating with its observingcapabilities, the Institute started a number of front-line researchprogrammes during the last decade, e.g., optical follow up observationsof GRB afterglows, radio and space borne astronomical resources,intra-night optical variability in active galactic nuclei as well asgravitational microlensing and milli-magnitude variations in the rapidlyoscillating peculiar A type stars. As a part of atmospheric studies,characterisation of aerosol at an altitude of about 2 km is going onsince 2002. ARIES has plans for establishing modern observing facilities equipped with latest backend instruments in the area of bothastrophysics and atmospheric science. Formation of ARIES, thereforeaugurs well for the overall development of astrophysics and atmosphericscience in India.
|
Submit a new article
Related links
Submit a new link
Member of following groups:
|
Observation and Astrometry data
| Constellation: | Πήγασος |
| Right ascension: | 21h31m01.71s |
| Declination: | +23°20'07.4" |
| Apparent magnitude: | 9.251 |
| Distance: | 25.056 parsecs |
| Proper motion RA: | 135 |
| Proper motion Dec: | -144.1 |
| B-T magnitude: | 10.503 |
| V-T magnitude: | 9.355 |
Catalogs and designations:
|
