תוכן
תמונות
הוסף תמונה שלך
DSS Images Other Images
מאמרים קשורים
High-resolution Spectroscopy During Eclipse of the Young Substellar Eclipsing Binary 2MASS 0535-0546. I. Primary Spectrum: Cool Spots Versus Opacity Uncertainties
We present high-resolution Keck optical spectra of the very youngsubstellar eclipsing binary 2MASS J05352184-0546085, obtainedduring eclipse of the lower-mass (secondary) brown dwarf. Theobservations yield the spectrum of the higher-mass (primary) brown dwarfalone, with negligible (~1.6%) contamination by the secondary. Weperform a simultaneous fine analysis of the TiO-epsilon band and the redlobe of the K I doublet, using state-of-the-art PHOENIX DUSTY and CONDsynthetic spectra. Comparing the effective temperature and surfacegravity derived from these fits to the empirically determined surfacegravity of the primary (log g = 3.5) then allows us to test the modelspectra as well as probe the prevailing photospheric conditions. We findthat: (1) fits to TiO-epsilon alone imply T eff = 2500± 50 K; (2) at this T eff, fits to K I imply log g =3.0, 0.5 dex lower than the true value; and (3) at the true log g, K Ifits yield T eff = 2650 ± 50 K, ~150 K higher thanfrom TiO-epsilon alone. On the one hand, these are the trends expectedin the presence of cool spots covering a large fraction of the primary'ssurface (as theorized previously to explain the observed Teff reversal between the primary and secondary).Specifically, our results can be reproduced by an unspotted stellarphotosphere with T eff = 2700 K and (empirical) log g = 3.5,coupled with axisymmetric cool spots that are 15% cooler (2300 K), havean effective log g = 3.0 (0.5 dex lower than photospheric), and cover70% of the surface. On the other hand, the trends in our analysis canalso be reproduced by model opacity errors: there are lacks in thesynthetic TiO-epsilon opacities, at least for higher-gravity fielddwarfs. Stringently discriminating between the two possibilitiesrequires combining the present results with an equivalent analysis ofthe secondary (predicted to be relatively unspotted compared to theprimary).
| Simulations of Winds of Weak-lined T Tauri Stars. II. The Effects of a Tilted Magnetosphere and Planetary Interactions
Based on our previous work, we investigate here the effects on the windand magnetospheric structures of weak-lined T Tauri stars due to amisalignment between the axis of rotation of the star and its magneticdipole moment vector. In such a configuration, the system loses theaxisymmetry presented in the aligned case, requiring a fullythree-dimensional (3D) approach. We perform 3D numericalmagnetohydrodynamic simulations of stellar winds and study the effectscaused by different model parameters, namely the misalignment angle? t , the stellar period of rotation, theplasma-?, and the heating index ?. Our simulations take intoaccount the interplay between the wind and the stellar magnetic fieldduring the time evolution. The system reaches a periodic behavior withthe same rotational period of the star. We show that the magnetic fieldlines present an oscillatory pattern. Furthermore, we obtain that byincreasing ? t , the wind velocity increases,especially in the case of strong magnetic field and relatively rapidstellar rotation. Our 3D, time-dependent wind models allow us to studythe interaction of a magnetized wind with a magnetized extrasolarplanet. Such interaction gives rise to reconnection, generatingelectrons that propagate along the planet's magnetic field lines andproduce electron cyclotron radiation at radio wavelengths. The powerreleased in the interaction depends on the planet's magnetic fieldintensity, its orbital radius, and on the stellar wind localcharacteristics. We find that a close-in Jupiter-like planet orbiting at0.05 AU presents a radio power that is ~5 orders of magnitude largerthan the one observed in Jupiter, which suggests that the stellar windfrom a young star has the potential to generate strong planetary radioemission that could be detected in the near future with LOFAR. Thisradio power varies according to the phase of rotation of the star. Forthree selected simulations, we find a variation of the radio power of afactor 1.3-3.7, depending on ? t . Moreover, we extendthe investigation done in Vidotto et al. and analyze whether winds frommisaligned stellar magnetospheres could cause a significant effect onplanetary migration. Compared to the aligned case, we show that thetimescale ? w for an appreciable radial motion of theplanet is shorter for larger misalignment angles. While for the alignedcase ? w ~= 100 Myr, for a stellar magnetosphere tiltedby ? t = 30°, ? w ranges from ~40to 70 Myr for a planet located at a radius of 0.05 AU. Further reductionon ? w might occur for even larger misalignment anglesand/or different wind parameters.
| Age Estimation And Mass Functions of TTauri Stars in The Taurus Auriga Molecular Cloud
In this work the present day mass functions (PDMF) of T-Tauri Stars(TTS) which are in the pre-main sequence (PMS) evolutionary phase oftheir evolution in the Taurus-Auriga Molecular Cloud Complex have beencalculated. For this purpose, by applying our modified stellarevolutionary code, stellar models in the mass range 0.1 - 2.5 M_?are used to determine the mass and age mass of TTS. The obtained massfunction is compared with the mass function of Miller & Scalo(1979). The age found for TTS is around 1-3×10^6 yr and the massfunction is about 0.644 ? 0.348. From these results, we havecalculated the stellar birthrate as about 1.3×10^{-7} M_?yr^{-1} in this region.
| Dynamo processes in the T Tauri star V410 Tau
We present new brightness and magnetic images of the weak-line T Tauristar V410 Tau, made by using data from the Narval spectropolarimeter atTélescope Bernard Lyot (TBL). The brightness image shows a largepolar spot and significant spot coverage at lower latitudes. Themagnetic maps show a field that is predominantly dipolar andnon-axisymmetric with a strong azimuthal component. The field is 50 percent poloidal and 50 per cent toroidal, and very little differentialrotation is apparent from the magnetic images.A photometric monitoring campaign on this star has previously revealedV-band variability of up to 0.6 mag, but in 2009 the light curve is muchflatter. The Doppler image presented here is consistent with this lowvariability. Calculating the flux predicted by the mapped spotdistribution gives a peak-to-peak variability of 0.04 mag. The reductionin the amplitude of the light curve, compared with previousobservations, appears to be related to a change in the distribution ofthe spots rather than the number or area.This paper is the first from a Zeeman-Doppler imaging campaign beingcarried out on V410 Tau between 2009 and 2012 at the TBL. During thistime, it is expected that the light curve will return to ahigh-amplitude state, allowing us to ascertain whether the photometricchanges are accompanied by a change in the magnetic field topology.Based on observations obtained at the Télescope Bernard Lyot(TBL), operated by Centre national de la recherche scientifique/Institutnational des sciences de l'Univers (CNRS/INSU)E-mail: mskelly@ast.obs-mip.fr
| A Detailed Study of the Rise Phase of a Long Duration X-Ray Flare in the Young Star TWA 11B
We analyzed a long duration flare observed in a serendipitous XMM-Newtondetection of the M star CD-39 7717B (TWA 11B), member of the youngstellar association TW Hya (~8 Myr). Only the rise phase (with aduration of ~35 ks) and possibly the flare peak were observed. We tookadvantage of the high count rate of the X-ray source to carry out adetailed analysis of its spectrum during the whole exposure. After acareful analysis, we interpreted the rise phase as resulting from theignition of a first group of loops (event A) which triggered asubsequent two-ribbon flare (event B). Event A was analyzed using asingle-loop model, while a two-ribbon model was applied for event B.Loop semi-lengths of ~4 R * were obtained. Such largestructures had been previously observed in very young stellar objects(~1-4 Myr). This is the first time that they have been inferred in aslightly more evolved star. The fluorescent iron emission line at 6.4keV was detected during event B. Since TWA 11B seems to have no disk,the most plausible explanation found for its presence in the X-rayspectrum of this star is collisional- or photo-ionization. As far as weare concerned, this is only the third clear detection of Fe photosphericfluorescence in stars other than the Sun.
| The Taurus Spitzer Survey: New Candidate Taurus Members Selected Using Sensitive Mid-Infrared Photometry
We report on the properties of pre-main-sequence objects in the Taurusmolecular clouds as observed in seven mid- and far-infrared bands withthe Spitzer Space Telescope. There are 215 previously identified membersof the Taurus star-forming region in our ~44 deg2 map thesemembers exhibit a range of Spitzer colors that we take to define youngstars still surrounded by circumstellar dust (noting that ~20% of thebona fide Taurus members exhibit no detectable dust excesses). We lookedfor new objects in the survey field with similar Spitzer properties,aided by extensive optical, X-ray, and ultraviolet imaging, and found148 new candidate members of Taurus. We have obtained follow-upspectroscopy for about half the candidate sample, thus far confirming 34new members, three probable new members, and 10 possible new members, anincrease of 15%-20% in Taurus members. Of the objects for which we havespectroscopy, seven are now confirmed extragalactic objects, and one isa background Be star. The remaining 93 candidate objects awaitadditional analysis and/or data to be confirmed or rejected as Taurusmembers. Most of the new members are Class II M stars and are locatedalong the same cloud filaments as the previously identified Taurusmembers. Among non-members with Spitzer colors similar to young, dustystars are evolved Be stars, planetary nebulae, carbon stars, galaxies,and active galactic nuclei.
| The Disk Population of the Taurus Star-Forming Region
We have analyzed nearly all images of the Taurus star-forming region at3.6, 4.5, 5.8, 8.0, and 24 ?m that were obtained during the cryogenicmission of the Spitzer Space Telescope (46 deg2) and havemeasured photometry for all known members of the region that are withinthese data, corresponding to 348 sources, or 99% of the known stellarpopulation. By combining these measurements with previous observationswith the Spitzer Infrared Spectrograph and other facilities, we haveclassified the members of Taurus according to whether they show evidenceof circumstellar disks and envelopes (classes I, II, and III). Throughthese classifications, we find that the disk fraction in Taurus,N(II)/N(II+III), is ~75% for solar-mass stars and declines to ~45% forlow-mass stars and brown dwarfs (0.01-0.3 M sun). Thisdependence on stellar mass is similar to that measured for Chamaeleon I,although the disk fraction in Taurus is slightly higher overall,probably because of its younger age (1 Myr versus 2-3 Myr). Incomparison, the disk fraction for solar-mass stars is much lower (~20%)in IC 348 and ? Ori, which are denser than Taurus and Chamaeleon Iand are roughly coeval with the latter. These data indicate that disklifetimes for solar-mass stars are longer in star-forming regions thathave lower stellar densities. Through an analysis of multiple epochs ofSpitzer photometry that are available for ~200 Taurus members, we findthat stars with disks exhibit significantly greater mid-infrared(mid-IR) variability than diskless stars, which agrees with the resultsof similar variability measurements for a smaller sample of stars inChamaeleon I. The variability fraction for stars with disks is higher inTaurus than in Chamaeleon I, indicating that the IR variability of disksdecreases with age. Finally, we have used our data in Taurus to refinethe observational criteria for primordial, evolved, and transitionaldisks. The ratio of the number of evolved and transitional disks to thenumber of primordial disks in Taurus is 15/98 for spectral types ofK5-M5, indicating a timescale of 0.15 × ?primordial~ 0.45 Myr for the clearing of the inner regions of optically thickdisks. After applying the same criteria to older clusters andassociations (2-10 Myr) that have been observed with Spitzer, we findthat the proportions of evolved and transitional disks in thosepopulations are consistent with the measurements in Taurus when theirstar formation histories are properly taken into account.Based on observations performed with the Spitzer Space Telescope.
| HST/STIS observations of the RW Aurigae bipolar jet: mapping the physical parameters close to the source
Context: We present the results of new spectral diagnosticinvestigations applied to high-resolution long-slit spectra obtainedwith the Hubble Space Telescope Imaging Spectrograph (HST/STIS) of thejet from the T Tauri star RW Aur. Aims: Our primary goal is todetermine basic physical parameters (electron density ne andelectron temperature Te, hydrogen ionisation fractionxe, total hydrogen density nH, radial velocityvr and the mass outflow rate dot Mj) along boththe red- and blueshifted lobes of the RW Aur jet. Methods: Theinput dataset consists of seven long-slit spectra, of 0.1 arcsec spatialresolution, taken with the STIS slit parallel to the jet, and steppedacross it. We use the Bacciotti & Eislöffel (1999, A&A,342, 717) method to analyse the forbidden doublets [OI]??6300,6363, [S II]?? 6716,6731, and [NII]?? 6548,6583 Å to extract n_e, T_e, x_e, and n_H.Results: We were able to extract the parameters as far as 3.9arcsec in the red- and 2.1 arcsec in the blueshifted beam. The electrondensity at the base of both lobes is close to the critical density for[S II] emission but then it decreases gradually with distance from thesource. The range of electron temperatures derived for this jet(Te = 10^4-2×104 K) is similar to thosegenerally found in other outflows from young stars. The ionisationfraction xe varies between 0.04 and 0.4, increasing withinthe first few arcseconds and then decreasing in both lobes. The totalhydrogen density, derived as nH = ne / x_e, is onaverage 3.2×104 cm-3 and shows a gradualdecrease along the beam. Variations of the above quantities along thejet lobes appear to be correlated with the position of knots. Combiningthe derived parameters with vr measured from the HST spectraand other characteristics available for this jet, we estimate dotMj following two different procedures. The mass-outflow ratedot Mj is moderate and similar in the two lobes, despite thefact that the well-known asymmetry in the radial velocity persists closeto the source. Using the results of the BE diagnostics we find averagesalong the first 2.1 arcsec of both flows (a region presumably not yetaffected by interaction with the jet environment) of2.6×10-9 M_? yr-1 for the red lobe and2.0×10-9 M_? yr-1 for the blueshiftedflow, with an uncertainty of ± log M_? = 1.6. Conclusions: The fact that the derived mass outflow rate is similar inthe two lobes appears to indicate that the central engine is constrainedon the two sides of the system and that the observed asymmetries are dueto environmental conditions. Possible suggestions for the origin of thedifferences are discussed. The RW Aur jet appears to be the seconddensest outflow from a T Tauri star studied so far, but its otherproperties are quite similar to those found in other jets from youngstars, suggesting that a common acceleration mechanism operates in thesesources.Based on observations made with the NASA/ESA Hubble Space Telescope,obtained at the Space Telescope Science Institute, which is operated bythe Association of Universities for Research in Astronomy, Inc., underNASA contract NAS5-26555.
| 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.
| No Transition Disk? Infrared Excess, PAH, H2, and X-Rays from the Weak-Lined T Tauri Star DoAr 21
As part of a program to understand disk dispersal and the interplaybetween circumstellar disks and X-ray emission, we present newhigh-resolution mid-infrared (IR) imaging, high-resolution opticalspectroscopy, and Chandra grating X-ray spectroscopy of the weak-lined TTauri star DoAr 21. DoAr 21 (age <106 yr and mass ~2.2 Msun based on evolutionary tracks) is a strong X-ray emitter,with conflicting evidence in the literature about its disk properties.It shows weak but broad Hα emission (reported here for the firsttime since the 1950s); polarimetric variability; polycyclic aromatichydrocarbon (PAH) and H2 emission; and a strong, spatiallyresolved 24 μm excess in archival Spitzer photometry. Geminisub-arcsecond-resolution 9-18 μm images show that there is little orno excess mid-IR emission within 100 AU of the star; the excess emissionis extended over several arcseconds and is quite asymmetric. Theextended emission is bright in the ultraviolet (UV)-excited λ =11.3 μm PAH emission feature. A new high-resolution X-ray gratingspectrum from Chandra shows that the stellar X-ray emission is very hardand dominated by continuum emission; it is well fit by amulti-temperature thermal model, typical of hard coronal sources, andshows no evidence of unusually high densities. A flare during the X-rayobservation shows a temperature approaching 108 K. We arguethat the far-UV emission from the transition region is sufficient toexcite the observed extended PAH and continuum emission, and that theH2 emission may be similarly extended and excited. While thisextended emission may be a disk in the final stages of clearing, it alsocould be more akin to a small-scale photodissociation region than aprotoplanetary disk, highlighting both the very young ages(<106 yr) at which some stars are found without disks andthe extreme radiation environment around even late-typepre-main-sequence stars.
| The Distance to NGC 2264
We determine the distance to the open cluster NGC 2264 using astatistical analysis of cluster member inclinations. We derivedistance-dependent values of sin i (where i is the inclination angle)for 97 stars in NGC 2264 from the rotation periods, luminosities,effective temperatures, and projected equatorial rotation velocities,vsin i, measured for these stars. We have measured 96 of the vsin ivalues in our sample by analyzing high-resolution spectra with across-correlation technique. We model the observed distribution of sin ifor the cluster by assuming that member stars have random axialorientations and by adopting prescriptions for the measurement errors inour sample. By adjusting the distance assumed in the observed sin idistribution until it matches the modeled distribution, we obtain abest-fit distance for the cluster. We find the data to be consistentwith a distance to NGC 2264 of 913 pc. Quantitative tests of ouranalysis reveal uncertainties of 40 and 110 pc due to sampling andsystematic effects, respectively. This distance estimate suggests arevised age for the cluster of ~1.5 Myr, although more detailedinvestigations of the full cluster membership are required to drawstrong conclusions.
| X-ray spectroscopy of stars
Non-degenerate stars of essentially all spectral classes are soft X-raysources. Their X-ray spectra have been important in constrainingphysical processes that heat plasma in stellar environments totemperatures exceeding one million degrees. Low-mass stars on the coolerpart of the main sequence and their pre-main sequence predecessorsdefine the dominant stellar population in the galaxy by number. TheirX-ray spectra are reminiscent, in the broadest sense, of X-ray spectrafrom the solar corona. The Sun itself as a typical example of amain-sequence cool star has been a pivotal testbed for physical modelsto be applied to cool stars. X-ray emission from cool stars is indeedascribed to magnetically trapped hot gas analogous to the solar coronalplasma, although plasma parameters such as temperature, density, andelement abundances vary widely. Coronal structure, its thermalstratification and geometric extent can also be interpreted based onvarious spectral diagnostics. New features have been identified inpre-main sequence stars; some of these may be related to accretionshocks on the stellar surface, fluorescence on circumstellar disks dueto X-ray irradiation, or shock heating in stellar outflows. Massive, hotstars clearly dominate the interaction with the galactic interstellarmedium: they are the main sources of ionizing radiation, mechanicalenergy and chemical enrichment in galaxies. High-energy emission permitsto probe some of the most important processes at work in these stars,and put constraints on their most peculiar feature: the stellar wind.Medium and high- resolution spectroscopy have shed new light on theseobjects as well. Here, we review recent advances in our understanding ofcool and hot stars through the study of X-ray spectra, in particularhigh-resolution spectra now available from XMM -Newton and Chandra. Weaddress issues related to coronal structure, flares, the composition ofcoronal plasma, X-ray production in accretion streams and outflows,X-rays from single OB-type stars, massive binaries, magnetic hot objectsand evolved WR stars.
| 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.
| Variability of the transitional T Tauri star T Chamaeleontis
Context: It is known that for solar-mass stars planet formation beginsin a circumstellar disc. The study of transitional objects exhibitingclear signs of evolution in their discs, such as the growth of dustparticles and beginning of disc dispersal, is fundamental tounderstanding the processes governing dust-grain coagulation and theonset of planet formation. Aims: We attempt to characterise thephysical properties of T Chamaeleontis, a transitional T Tauri starshowing UX Ori-type variability, and of its associated disc, and probethe possible effects of disc-clearing processes. Methods:Different spectral diagnostics were examined, based on a rich collectionof optical high- and low-resolution spectra. The cross-correlationtechnique was used to determine radial and projected rotationalvelocities, shape changes of photospheric lines were analysed viabisector-method applied to the cross-correlation profile, and theequivalent widths of both the Li i ?6708 Å photosphericabsorption and the most prominent emission lines (e.g., H?,H? and [Oi] 6300 Å) were measured. Variability in the mainemission features was inspected by means of line-profile correlationmatrices. Available optical and near-infrared photometry combined withinfrared data from public catalogues was used to construct the spectralenergy distribution (SED) and infer basic stellar and disc properties.Results: Remarkable variability on timescale of days in the mainemission lines, H? changing from pure emission to nearlyphotospheric absorption, is correlated with variations in visualextinction of over three magnitudes, while the photospheric absorptionspectrum shows no major changes. The strength of emission in H?and H? is highly variable and well correlated with that of the [Oi]lines. The structure of the H? line-profile also varies on a dailytime-span, while the absence of continuum veiling suggests very low orno mass accretion. Variations of up to nearly 10 km s-1 inthe radial velocity of the star are measured on analogous timescales,but with no apparent periodicity. SED modelling confirms the existenceof a gap in the disc. Conclusions: Variable circumstellarextinction is inferred to be responsible for the conspicuous variationsobserved in the stellar continuum flux and for concomitant changes inthe emission features by contrast effect. Clumpy structures,incorporating large dust grains and orbiting the star within a fewtenths of AU, obscure episodically the star and, eventually, part of theinner circumstellar zone, while the bulk of the hydrogen-lineemitting-zone and outer low-density wind region traced by the [Oi]remain unaffected. In agreement with this scenario, the detected radialvelocity changes are also explainable in terms of clumpy materialtransiting and partially obscuring the star.Based on observations obtained at the European Southern Observatoryat La Silla, Chile in program 63.I-0045(A); 65.I-0089(A); 66.C-0616(A);67.C-0155(A); 67.C-0155(B); 68.C-0292(A); 69.C-0207(A); 70.C-0163(A);073.C-0355(A); 074.A-9018(A); 075.C-0399(A-F).Tables 1, 2 and 4, and Figs. 21-32 are only available in electronic formvia http://www.aanda.org
| Multiple and changing cycles of active stars. II. Results
Aims. We study the time variations in the cycles of 20 active starsbased on decade-long photometric or spectroscopic observations. Methods:A method of time-frequency analysis, as discussed in a companion paper,is applied to the data. Results: Fifteen stars definitely show multiplecycles, but the records of the rest are too short to verify a timescalefor a second cycle. The cycles typically show systematic changes. Forthree stars, we found two cycles in each of them that are not harmonicsand vary in parallel, indicating a common physical mechanism arisingfrom a dynamo construct. The positive relation between the rotationaland cycle periods is confirmed for the inhomogeneous set of activestars. Conclusions: Stellar activity cycles are generally multiple andvariable.
| Dust Processing and Grain Growth in Protoplanetary Disks in the Taurus-Auriga Star-Forming Region
Mid-infrared spectra of 65 T Tauri stars (TTS) taken with the InfraredSpectrograph (IRS) on board the Spitzer Space Telescope are modeledusing populations of optically thin warm and cool grains to probe theradial variation in dust composition in the uppermost layers ofprotoplanetary disks. Most spectra with narrow emission featuresassociated with crystalline silicates require Mg-rich minerals andsilica, while a few spectra with these features suggest the presence ofother components. IRS spectra indicating the presence of large amountsof warm enstatite of ~400-500 K require crystalline silicates (enstatiteor forsterite) at temperatures lower than the median temperature of thecool dust in the models, ~127 K; spectra showing a high abundance ofother crystalline silicates (forsterite or silica) typically do not. Afew spectra show 10 ?m complexes of very small equivalent width. Theyare fit well using abundant crystalline silicates but very few largegrains, inconsistent with the expectation that a low peak-to-continuumratio of the 10 ?m complex always indicates grain growth. Most of thespectra in our sample are fit well without using the opacities of largecrystalline silicate grains. If large grains grow by agglomeration ofsubmicron grains of all dust types, the amorphous silicate components ofthese aggregates must typically be more abundant than the crystallinesilicate components. We also find that the more there is of onecrystalline dust species, the more there is of the others. This couldsuggest that crystalline silicates are processed directly from amorphoussilicates, whether through evaporation of the amorphous grains andcondensation in chemical equilibrium or by annealing of the amorphousprecursors. Alternatively, if one kind of crystalline silicatetransforms into another kind, it suggests that the intermediate speciestransforms into the end-product species at a slower rate than theprecursor transforms into the intermediate species; otherwise, therewould be a negligible abundance of intermediate species. It is alsofound that the crystalline silicate abundance is correlated tightly withdisk geometry, in the sense of higher crystallinity accompanyingmore-settled disks, which are commonly associated with growth andsettling of grains. The abundance of large grains is also correlatedwith disks that are more highly settled, but with a wide range of largegrain abundance for a given degree of settling. We interpret this rangeas that the settling of large grains is sensitive to individual diskproperties. We also find that lower-mass stars have higher abundances oflarge grains in their inner regions.
| Stellar Rotation in M35: Mass-Period Relations, Spin-Down Rates, and Gyrochronology
We present the results of a five month photometric time-series surveyfor stellar rotation over a 40' × 40' field centered on the 150Myr open cluster M35. We report rotation periods for 441 stars withinthis field and determine their cluster membership and binarity based ona decade-long radial velocity survey, proper-motion measurements, andmultiband photometric observations. We find that 310 of the stars withmeasured rotation periods are late-type members of M35. The distributionof rotation periods for cluster members span more than 2 orders ofmagnitude from ~0.1 to 15 days, not constrained by the samplingfrequency and the timespan of the survey. With an age between thezero-age main sequence and the Hyades, and with ~6 times more rotationperiods than measured in the Pleiades, M35 permit detailed studies ofearly rotational evolution of late-type stars. Nearly 80% of the 310rotators lie on two distinct sequences in the color-period plane, anddefine clear relations between stellar rotation period and color (mass).The M35 color-period diagram enables us to determine timescales for thetransition between the two rotational states, of ~60 Myr and ~140 Myrfor G and K dwarfs, respectively. These timescales are inversely relatedto the mass of the convective envelope, and offer constraints on therates of internal and external angular momentum transport and of theevolution of stellar dynamos. A comparison to the Hyades, confirm theSkumanich spin-down dependence for G dwarfs on one rotational state, butsuggest that K dwarfs spin down more slowly. The locations of therotational sequences in the M35 color-period diagram support the use ofrotational isochrones to determine ages for coeval stellar populations.We use such gyrochronology to determine "gyro-ages" for M35 from 134 Myrto 161 Myr. We use the M35 data to evaluate new color dependences forthe rotational isochrones.WIYN Open Cluster Study. XXXV.
| A catalogue of stars suspected of bright active regions
We present a catalogue of field stars across the HR diagram suspectedof bright active regions in their atmospheres. We aim at developing thefirst version of a database of active stars with bright regions (brightspots). Using a variety of databases and the internet we found andgathered all relevant archival data starting about 1973 and beingimportant for developing such a catalogue. We found that the phenomenonstarspot is now common to a variety of spectral type and luminosityclasses. Our primary goal was to identify active solar and late typestars suspicious of bright active regions but the search offers expandedresults including young T Tauri stars, eclipsing binaries with equal ormixed spectral types components (Algols, W UMa stars) and in some casesother types of objects. Moreover, the light curves analyses foreclipsing binaries offer reliable estimates for spot properties and itwas found that 20% of binaries in the catalogue had a spot located nearthe L point (neck zone). At present, the catalogue consists of 134 starsand overall characteristics for them are organised in several files inASCII format. The catalogue is electronically available.
| Dynamo model with a small number of modes and magnetic activity of T Tauri stars
We suggest a model based on the representation of the stellar magneticfield as a superposition of a finite number of poloidal and toroidalfree decay modes to describe the dynamo action in fully convectivestars. For the adopted law of stellar differential rotation, wedetermined the dynamo number in exceeding which the generation of acyclically varying magnetic field is possible in stars without aradiative core and derived an expression for the period of the cycle.The dynamo cycles in fully convective stars and in stars with thinconvective envelopes are shown to differ qualitatively: first, thedistributions of spots in latitude during the cycle are different forthese two types of stars and, second, the model predicts a greatweakening of the spot formation in fully convective stars at certainphases of the cycle. To compare the theory with observations, we haveanalyzed the historical light curve for the weak-line T Tauri star V410Tau and found that its long-term activity is not a well-defined cyclewith a definite period—its activity is more likely quasi-cyclicwith a characteristic time of ˜4 yr and with a chaotic componentsuperimposed. we have also concluded that a redistribution of spots inlongitude is responsible for the secular brightness variations in thestar. This does not allow the results of photometric observations to bedirectly compared with predictions of ourmodel, in which, forsimplicity, we assumed a symmetry in longitude and investigated thetemporal evolution of the spot distribution in latitude. Therefore, wediscuss the questions of what and how observations can be compared withpredictions of the dynamo theory.
| Spectrophotometric analysis of the T Tauri star GQ Lupi A
Context: GQ Lup A is a classical T Tauri star that shows clear signs ofaccretion through the presence of inverse P Cygni profiles in its mainemission lines. Recently, Neuhäuser et al. (2005, A&A, 435,L13) found a co-moving sub-stellar companion of GQ Lup A, raising theimportance of determining precise stellar parameters, system age, anddistance to GQ Lup. Aims: Our main objective is to usespectrophotometric time series to determine GQ Lup A stellar parametersand predict its photospheric spectral distribution. The excess spectralluminosity can then be measured and employed to test predictions ofmagnetospheric accretion models of classical T Tauri stars. Methods: We present the analysis of 18 spectrophotometric observationsof the T Tauri star GQ Lup A (K7) obtained with the Boller & Chivensspectrograph at the 1.52 m ESO telescope in La Silla. We also revisitedarchival photometric data of this star, comparing previous light curvevariability with our more recent data. Results: We determined thephotospheric flux of GQ Lup A on each observing night and obtained thestellar radius (Rstar = 1.8 ± 0.3 Rȯ),adopting a mean distance of 150 ± 20 pc to the Lupus 1 cloud.Assuming a K7 V temperature of 4060 K, the luminosity of GQ Lup A isLstar = 0.8 ± 0.3 Lȯ. Standardevolutionary models indicate a stellar mass of Mstar = 0.8± 0.2 Mȯ and an age of 3 ± 2 Myr. GQ Lup Aspectral lines are consistent with a projected rotational velocity of vsin i = 6.5 ± 2.0 km s-1. We measured the excessemission - veiling - and used the resulting photospheric spectraldistribution to calculate a stellar extinction (A_V) of 0.5 ±0.1. The veiling was found to be variable and periodic at 10.7 ±1.6 days, which is consistent with the period of GQ Lup A obtained fromarchival B band photometric data (10.43 ± 0.12 days). The starexhibits strong emission lines with substantial variability in flux. Theemission line fluxes are strongly correlated with one another but notwith veiling.
| Comparative statistics and origin of triple and quadruple stars
The statistics of catalogued quadruple stars consisting of two binaries(hierarchy 2 + 2), is studied in comparison with triple stars, withrespective sample sizes of 81 and 724. Seven representative quadruplesystems are discussed in greater detail. The main conclusions are asfollows. (i) Quadruple systems of ? Lyr type with similar massesand inner periods are common, in 42 per cent of the sample the outermass ratio is above 0.5 and the inner periods differ by less than 10times. (ii) The distributions of the inner periods in triple andquadruple stars are similar and bimodal. The inner mass ratios do notcorrelate with the inner periods. (iii) The statistics of outer periodsand mass ratios in triples and quadruples are different. The medianouter mass ratio in triples is 0.39 independently of the outer period,which has a smooth distribution. In contrast, the outer periods of 25per cent quadruples concentrate in the narrow range from 10 to 100yr,the outer mass ratios of these tight quadruples are above 0.6 and theirtwo inner periods are similar to each other. (iv) The outer and innermass ratios in triple and quadruple stars are not mutually correlated.In 13 per cent of quadruples both inner mass ratios are above 0.85(double twins). (v) The inner and outer orbital angular momenta andperiods in triple and quadruple systems with inner periods above 30dshow some correlation, the ratio of outer-to-inner periods is mostlycomprised between 5 and 104. In the systems with small periodratios the directions of the orbital spins are correlated, while in thesystems with large ratios they are not. The properties of multiple starsdo not correspond to the products of dynamical decay of small clusters,hence the N-body dynamics is not the dominant process of theirformation. On the other hand, rotationally driven (cascade)fragmentation possibly followed by migration of inner and/or outerorbits to shorter periods is a promising scenario to explain the originof triple and quadruple stars.
| An assessment of Li abundances in weak-lined and classical T Tauri stars of the Taurus-Auriga association
Context: Accurate measurements of lithium abundances in young low-massstars provide an independent and reliable age diagnostics. Previousstudies of nearby star forming regions have identified significantnumbers of Li-depleted stars, often at levels inconsistent with the agesindicated by their luminosity. Aims: We aim at a new and accurateanalysis of Li abundances in a sample of ~100 pre-main sequence stars inTaurus-Auriga using a homogeneous and updated set of stellar parametersand model atmospheres appropriate for the spectral types of the samplestars. Methods: We compute Li abundances using published values of theequivalent widths of the Li λ6708 Å doublet obtained frommedium/high resolution spectra. Results: We find that the number ofsignificantly Li-depleted stars in Taurus-Auriga is greatly reduced withrespect to earlier results. Only 13 stars have abundances lower than theinterstellar value by a factor of 5 or greater. All of them areweak-lined T Tauri stars drawn from X-ray surveys; with the exception offour stars located near the L1551 and L1489 dark clouds, all theLi-depleted stars belong to the class of dispersed low-mass stars,distributed around the main sites of current star formation. If locatedat the distance of Taurus-Auriga, the stellar ages implied by thederived Li abundances are in the range 3-30 Myr, greater than the bulkof the Li-rich population with implication on the star formation historyof the region. Conclusions: In order to derive firm conclusions aboutthe fraction of Li-depleted stars of Taurus-Auriga, Li measurements ofthe remaining members of the association should be obtained, inparticular of the group of stars that fall in the Li-burning region ofthe HR diagram.Table [see full text] is only available in electronic form at http://www.aanda.org
| Multiwavelength studies of the gas and dust disc of IRAS 04158+2805
We present a study of the circumstellar environment of IRAS04158+2805 based on multi-wavelength observations and models.Images in the optical and near-infrared, a polarisation map in theoptical, and mid-infrared spectra were obtained with VLT-FORS1, CFHT-IR,and Spitzer-IRS. Additionally we used an X-ray spectrum observed withChandra. We interpret the observations in terms of a central starsurrounded by an axisymmetric circumstellar disc, but without anenvelope, to test the validity of this simple geometry. We estimate thestructural properties of the disc and its gas and dust content. Wemodelled the dust disc with a 3D continuum radiative transfer code,MCFOST, based on a Monte-Carlo method that provides synthetic scatteredlight images and polarisation maps, as well as spectral energydistributions. We find that the disc images and spectral energydistribution narrowly constrain many of the disc model parameters, suchas a total dust mass of 1.0-1.75×10-4 M_? and aninclination of 62°-63°. The maximum grain size required to fitall available data is of the order of 1.6-2.8 ?m although the upperend of this range is loosely constrained. The observed opticalpolarisation map is reproduced well by the same disc model, suggestingthat the geometry we find is adequate and the optical properties arerepresentative of the visible dust content. We compare the inferred dustcolumn density to the gas column density derived from the X-ray spectrumand find a gas-to-dust ratio along the line of sight that is consistentwith the ISM value. To our knowledge, this measurement is the first todirectly compare dust and gas column densities in a protoplanetary disc.Based on observations obtained at the Canada-France-Hawaii Telescope(CFHT) which is operated by the National Research Council of Canada, theInstitut National des Sciences de l'Univers of the Centre National de laRecherche Scientifique of France, and the University of Hawaii. Basedalso on data collected at ESO/VLT during observation program 68-C.0171.
| Starspot-Induced Radial Velocity Variability in LkCa 19
We describe a new radial velocity survey of T Tauri stars and presentthe first results. Our search is motivated by an interest in detectingmassive young planets, as well as investigating the origin of the browndwarf desert. As part of this survey, we discovered large-amplitude,periodic, radial velocity variations in the spectrum of the weak-line TTauri star LkCa 19. Using line bisector analysis and a new simulation ofthe effect of starspots on the photometric and radial velocityvariability of T Tauri stars, we show that our measured radialvelocities for LkCa 19 are fully consistent with variations caused bythe presence of large starspots on this rapidly rotating young star.These results illustrate the level of activity-induced radial velocitynoise associated with at least some very young stars. Thisactivity-induced noise will set lower limits on the mass of a companiondetectable around LkCa 19 and similarly active young stars.
| 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.
| The SCUBA Legacy Catalogues: Submillimeter-Continuum Objects Detected by SCUBA
We present the SCUBA Legacy Catalogues, two comprehensive sets ofcontinuum maps (and catalogs) using data at 850 and 450 μm of thevarious astronomical objects obtained with the Submillimetre Common UserBolometer Array (SCUBA). The Fundamental Map Data Set contains data onlywhere superior atmospheric opacity calibration data were available. TheExtended Map Data Set contains data regardless of the quality of theopacity calibration. Each data set contains1.2deg×1.2deg maps at locations where dataexisted in the JCMT archive, imaged using the matrix inversion method.The Fundamental Data Set is composed of 1423 maps at 850 μm and 1357maps at 450 μm. The Extended Data Set is composed of 1547 maps at 850μm. Neither data set includes high sensitivity, single-chop SCUBAmaps of ``cosmological fields'' nor solar system objects. Each data setwas used to determine a respective object catalogue, consisting ofobjects identified within the respective 850 μm maps using anautomated identification algorithm. The Fundamental and Extended MapObject Catalogues contain 5061 and 6118 objects, respectively. Objectsare named based on their respective J2000.0 position of peak 850 μmintensity. The catalogues provide for each object the respective maximum850 μm intensity, estimates of total 850 μm flux and size, andtentative identifications from the SIMBAD Database. Where possible, thecatalogues also provide for each object its maximum 450 μm intensityand total 450 μm flux and flux ratios.
| Results of the ROTOR-program. II. The long-term photometric variability of weak-line T Tauri stars
Context: T Tauri stars exhibit variability on all timescales, whoseorigin is still debated. On WTTS the variability is fairly simple andattributed to long-lived, ubiquitous cool spots. Aims: We investigatethe long term variability of WTTS, extending up to 20 years in somecases, characterize it statistically and discuss its implications forour understanding of these stars. Methods: We have obtained a unique,homogeneous database of photometric measurements for WTTS extending upto 20 years. It contains more than 9000 UBVR observations of 48 WTTS.All the data were collected at Mount Maidanak Observatory (Uzbekistan)and they constitute the longest homogeneous record of accurate WTTSphotometry ever assembled. Results: Definitive rotation periods for 35of the 48 stars are obtained. Phased light curves over 5 to 20 seasonsare now available for analysis. Light curve shapes, amplitudes andcolour variations are obtained for this sample and various behaviorsexhibited, discussed and interpreted. Conclusions: Our main conclusionis that most WTTS have very stable long term variability with relativelysmall changes of amplitude or mean light level. The long termvariability seen reflects modulation in the cold spot distributions.Photometric periods are stable over many years, and the phase of minimumlight can be stable as well for several years. On the long term, spotproperties do change in subtle ways, leading to secular variations inthe shape and amplitudes of the light curves.UBVR photometric data described in Table [see full textsee full textseefull textsee full textsee full textsee full textsee full text] are onlyavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/479/827 Tables [see fulltextsee full textsee full textsee full textsee full textsee full textseefull text]-[see full textsee full textsee full textsee full textsee fulltextsee full textsee full text] are only available in electronic form athttp://www.aanda.org
| New Variable Star in the Field of the Seyfert Galaxy Mrk 290
CCD BVRI phase light curves of the new variable are almost sinusoidaland have a period of 1.51785 days. This period and the relationshipbetween V and color indices indicate that this star may belong to theclass of spotted variable stars.
| X-rays from young stars: A summary of highlights from the XMM-Newton Extended Survey of the Taurus Molecular Cloud (XEST)
The XMM-Newton Extended Survey of the Taurus Molecular Cloud (XEST)is a survey of the nearest large star-forming region, the TaurusMolecular Cloud (TMC), making use of all instruments on board theXMM-Newton X-ray observatory. The survey, presently still growing, hasprovided unprecedented spectroscopic results from nearly every observedT Tauri star, and from ?50% of the studied brown dwarfs andprotostars. The survey includes the first coherent statistical sample ofhigh-resolution spectra of T Tauri stars, and is accompanied by anU-band/ultraviolet imaging photometric survey of the TMC. XEST led tothe discovery of new, systematic X-ray features not possible before withsmaller samples, in particular the X-ray soft excess in classical TTauri stars and the Two-Absorber X-ray (TAX) spectra of jet-driving TTauri stars. This paper summarizes highlights from XEST and reviews thekey role of this large project.
| Applicability of colour index calibrations to T Tauri stars
We examine the applicability of effective temperature scales of several broad band colours to T Tauri stars (TTS). We take into account different colour systems as well as stellar parameters like metallicity and surface gravity which influence the conversion from colour indices or spectral type to effective temperature. For a large sample of TTS, we derive temperatures from broad band colour indices and check if they are consistent in a statistical sense with temperatures inferred from spectral types. There are some scales (for V-H, V-K, I-J, J-H, and J-K) which indeed predict the same temperatures as the spectral types and therefore can be at least used to confirm effective temperatures. Furthermore, we examine whether TTS with dynamically derived masses can be used for a test of evolutionary models and effective temperature calibrations. We compare the observed parameters of the eclipsing T Tauri binary V1642 Ori A to the predictions of evolutionary models in both the H-R and the Kiel diagram using temperatures derived with several colour index scales. We check whether the evolutionary models and the colour index scales are consistent with coevality and the dynamical masses of the binary components. It turns out that the Kiel diagram offers a stricter test than the H-R diagram. Only the evolutionary models of \cite {BCAH98} with mixing length parameter ?=1.9 and of \cite{DM94,DM97} show consistent results in the Kiel diagram in combination with some conversion scales of \cite{HBS00} and of \cite{KH95}.
|
הכנס מאמר חדש
לינקים קשורים
הכנס לינק חדש
משמש של הקבוצה הבאה
|
תצפית ומידע אסטרומטרי
| קבוצת-כוכבים: | שור |
| התרוממות ימנית: | 04h18m31.10s |
| סירוב: | +28°27'16.1" |
| גודל גלוי: | 10.759 |
| תנועה נכונה: | 4.5 |
| תנועה נכונה: | -31.2 |
| B-T magnitude: | 12.553 |
| V-T magnitude: | 10.908 |
קטלוגים וכינוים:
|
