Садржај
Слике
Уплоадјуј своје слике
DSS Images Other Images
Везани чланци
Recurring millimeter flares as evidence for star-star magnetic reconnection events in the DQ Tauri PMS binary system
Observations of the T Tauri spectroscopic binary DQ Tau in April 2008captured an unusual flare at 3 mm, which peaked at an observed maximumflux of 0.5 Jy (about 27 times the quiescent value). Here we presentfollow-up millimeter observations that demonstrate a periodicity to thephenomenon. While monitoring 3 new periastron encounters, we havedetected flares within 17.5 h (or 4.6%) of the orbital phase of thefirst reported flare and constrained the main emitting region to astellar height of 3.7-6.8 R_star. The recorded activity is consistentwith the proposed picture for synchrotron emission initiated by amagnetic reconnection event when the two stellar magnetospheres of thehighly eccentric (e = 0.556) binary are believed to collide nearperiastron as the stars approach a minimum separation of 8 R_star ( 13R_?). The similar light curve decay profiles allow us to estimatean average flare duration of 30 h. Assuming one millimeter flare perorbit, DQ Tau could spend approximately 8% of its 15.8-day orbitalperiod in an elevated flux state. These findings continue to serve as asmall caution for millimeter flux points in spectral energydistributions that could contain unrecognized flare contributions. Ouranalysis of the millimeter emission provides an upper limit of 5% on thelinear polarization. We discuss the extent to which a severely entangledmagnetic field structure and Faraday rotation effects are likely toreduce the observed polarization fraction. We also predict that, for thecurrent picture, the stellar magnetospheres must be misaligned at asignificant angle or, alternatively, that the topologies of the outermagnetospheres are poorly described by a well-ordered dipole inside aradius of 7 R_star. Finally, to investigate whether reorganization ofthe magnetic field during the interaction affects mass accretion, wealso present simultaneous optical (VRI) monitoring of the binary, as anestablished tracer of accretion activity in this system. We find that anaccretion event can occur coincident in both time and duration with thesynchrotron fallout of a magnetic reconnection event. While the pulsedaccretion mechanism has been attributed previously to the dynamicalmotions of the stars alone, the similarities between the millimeter andoptical light curves evoke the possibility of a causal or co-dependentrelationship between the magnetospheric and dynamical processes.Appendices are only available in electronic form at http://www.aanda.org
| Physical Processes in Magnetically Driven Flares on the Sun, Stars, and Young Stellar Objects
The first flare on the Sun was observed exactly 150 years ago. Duringmost of the long history, only secondary effects have been noticed, soflares remained a riddle. Now the primary flare products, high-energyelectrons and ions, can be spatially resolved in hard X-rays (HXRs) andgamma rays on the Sun. Soft X-rays (SXRs) are observed from most stars,including young stellar objects. Structure and bulk motions of thecorona are imaged on the Sun in high temperature lines and are inferredfrom line shifts in stellar coronae. Magnetic reconnection is thetrigger for reorganization of the magnetic field into a lower energyconfiguration. A large fraction of the energy is converted intononthermal particles that transport the energy to higher density gas,heating it to SXR-emitting temperatures. Flares on young stars areseveral orders of magnitude more luminous and more frequent; theysignificantly ionize protoplanetary disks and planetary ionospheres.
| AKARI's infrared view on nearby stars. Using AKARI infrared camera all-sky survey, 2MASS, and Hipparcos catalogs
Context. The AKARI, a Japanese infrared space mission, has performed anAll-Sky Survey in six infrared-bands from 9 to 180 ?m with higherspatial resolutions and better sensitivities than IRAS. Aims: Weinvestigate the mid-infrared (9 and 18 ?m) point source catalog (PSC)obtained with the infrared camera (IRC) onboard AKARI, in order tounderstand the infrared nature of the known objects and to identifypreviously unknown objects. Methods: Color-color diagramsand a color-magnitude diagram were plotted with the AKARI-IRC PSCand other available all-sky survey catalogs. We combined the Hipparcosastrometric catalog and the 2MASS all-sky survey catalog with theAKARI-IRC PSC. We furthermore searched literature and SIMBADastronomical database for object types, spectral types, and luminosityclasses. We identified the locations of representative stars and objectson the color-magnitude and color-color diagram schemes. Theproperties of unclassified sources can be inferred from their locationson these diagrams. Results: We found that the (B-V) vs.(V-S9W) color-color diagram is useful for identifying thestars with infrared excess emerged from circumstellar envelopes ordisks. Be stars with infrared excess are separated well from other typesof stars in this diagram. Whereas (J-L18W) vs. (S9W-L18W)diagram is a powerful tool for classifying several object types.Carbon-rich asymptotic giant branch (AGB) stars and OH/IR stars formdistinct sequences in this color-color diagram. Young stellarobjects (YSOs), pre-main sequence (PMS) stars, post-AGB stars, andplanetary nebulae (PNe) have the largest mid-infrared color excess andcan be identified in the infrared catalog. Finally, we plot the L18W vs.(S9W-L18W) color-magnitude diagram, using the AKARI data togetherwith Hipparcos parallaxes. This diagram can be used to identify low-massYSOs and AGB stars. We found that this diagram is comparable to the [24]vs. ([8.0]-[24]) diagram of Large Magellanic Cloud sources usingthe Spitzer Space Telescope data. Our understanding of Galactic objectswill be used to interpret color-magnitude diagram of stellar populationsin the nearby galaxies that Spitzer Space Telescope observed. Conclusions: Our study of the AKARI color-color andcolor-magnitude diagrams will be used to explore properties ofunknown objects in the future. In addition, our analysis highlights afuture key project to understand stellar evolution with a circumstellarenvelope, once the forthcoming astronometrical data with GAIA areavailable.Catalog (full Tables 3 and 4) are only available in electronic form atthe CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/514/A2
| 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.
| 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.
| Dust amorphization in protoplanetary disks
Aims. High-energy irradiation of circumstellar material might impact thestructure and the composition of a protoplanetary disk and hence theprocess of planet formation. In this paper, we present a study of thepossible influence of stellar irradiation, indicated by X-ray emission,on the crystalline structure of circumstellar dust. Methods: Thedust crystallinity is measured for 42 class II T Tauri stars in theTaurus star-forming region using a decomposition fit of the 10 ?msilicate feature, measured with the spitzer IRS instrument. Since thesample includes objects with disks of various evolutionary stages, wefurther confine the target selection, using the age of the objects as aselection parameter. Results: We correlate the X-ray luminosityand the X-ray hardness of the central object with the crystalline massfraction of the circumstellar dust and find a significantanti-correlation for 20 objects within an age range of approx. 1 to 4.5Myr. We postulate that X-rays represent the stellar activity andconsequently the energetic ions of the stellar winds which interact withthe circumstellar disk. We show that the fluxes around 1 AU and ionenergies of the present solar wind are sufficient to amorphize the upperlayer of dust grains very efficiently, leading to an observablereduction of the crystalline mass fraction of the circumstellar,sub-micron sized dust. This effect could also erase other relationsbetween crystallinity and disk/star parameters such as age or spectraltype.Appendices are only available in electronic form at http://www.aanda.org
| Interferometric science results on young stellar objects
Long-baseline interferometry at infrared wavelengths allows theinnermost regions around young stars to be observed. These observationsdirectly probe the location of the dust and gas in the disks. Thecharacteristic sizes of these regions found are larger than previouslythought. These results have motivated in part a new class of models ofthe inner disk structure, but the precise understanding of the origin ofthese low visibilities is still in debate. Mid-infrared observationsprobe disk emission over a larger range of scales revealing mineralogygradients in the disk. Recent spectrally resolved observations allow thedust and gas to be studied separately showing that the Brackett gammaemission can find its origin either in a wind or in a magnetosphere andthat there is probably no correlation between the location of theBrackett gamma emission and accretion. In a certain number of cases, thevery high spatial resolution reveals very close companions and candetermine their masses. Overall, these results provide essentialinformation on the structure and the physical properties of closeregions surrounding young stars especially where planet formation issuspected to occur.
| The Coevality of Young Binary Systems
Multiple star systems are commonly assumed to form coevally; they thusprovide the anchor for most calibrations of stellar evolutionary models.In this paper, we study the binary population of the Taurus-Aurigaassociation, using the component positions in an HR diagram in order toquantify the frequency and degree of coevality in young binary systems.After identifying and rejecting the systems that are known to beaffected by systematic errors (due to further multiplicity orobscuration by circumstellar material), we find that the relative binaryages, |Δlog τ|, have an overall dispersionσ|Δlog τ|~ 0.40 dex. Random pairs of Taurusmembers are coeval only to within σ|Δlog τ|~0.58 dex, indicating that Taurus binaries are indeed more coeval thanthe association as a whole. However, the distribution of |Δlogτ| suggests two populations, with ~2/3 of the sample appearingcoeval to within the errors (σ|Δlog τ|~ 0.16dex) and the other ~1/3 distributed in an extended tail reaching|Δlog τ|~ 0.4-0.9 dex. To explain the finding of a multipeakeddistribution, we suggest that the tail of the differential agedistribution includes unrecognized hierarchical multiples, stars seen inscattered light, or stars with disk contamination; additional follow-upis required to rule out or correct for these explanations. The relativecoevality of binary systems does not depend significantly on the systemmass, mass ratio, or separation. Indeed, any pair of Taurus memberswider than ~10' (~0.7 pc) shows the full age spread of the association.
| Disk Evolution in the Three Nearby Star-forming Regions of Taurus, Chamaeleon, and Ophiuchus
We analyze samples of Spitzer Infrared Spectrograph spectra of T Tauristars in the Ophiuchus, Taurus, and Chamaeleon I star-forming regions,whose median ages lie in the <1-2 Myr range. The median mid-infraredspectra of objects in these three regions are similar in shape,suggesting, on average, similar disk structures. When normalized to thesame stellar luminosity, the medians follow each other closely, implyingcomparable mid-infrared excess emission from the circumstellar disks. Weuse the spectral index between 13 and 31 μm and the equivalent widthof the 10 μm silicate emission feature to identify objects whose diskconfiguration departs from that of a continuous, optically thickaccretion disk. Transitional disks, whose steep 13-31 μm spectralslope and near-IR flux deficit reveal inner disk clearing, occur withabout the same frequency of a few percent in all three regions. Objectswith unusually large 10 μm equivalent widths are more common(20%-30%); they could reveal the presence of disk gaps filled withoptically thin dust. Based on their medians and fraction of evolveddisks, T Tauri stars in Taurus and Chamaeleon I are very alike. Diskevolution sets in early, since already the youngest region, theOphiuchus core (L1688), has more settled disks with larger grains. Ourresults indicate that protoplanetary disks show clear signs of dustevolution at an age of a few Myr, even as early as ~1 Myr, but age isnot the only factor determining the degree of evolution during the firstfew million years of a disk's lifetime.
| Simulations of Winds of Weak-Lined T Tauri Stars: The Magnetic Field Geometry and the Influence of the Wind on Giant Planet Migration
By means of numerical simulations, we investigate magnetized stellarwinds of pre-main-sequence stars. In particular, we analyze under whichcircumstances these stars will present elongated magnetic features(e.g., helmet streamers, slingshot prominences, etc). We focus onweak-lined T Tauri stars, as the presence of the tenuous accretion diskis not expected to have strong influence on the structure of the stellarwind. We show that the plasma-? parameter (the ratio of thermal tomagnetic energy densities) is a decisive factor in defining the magneticconfiguration of the stellar wind. Using initial parameters within theobserved range for these stars, we show that the coronal magnetic fieldconfiguration can vary between a dipole-like configuration and aconfiguration with strong collimated polar lines and closed streamers atthe equator (multicomponent configuration for the magnetic field). Weshow that elongated magnetic features will only be present if theplasma-? parameter at the coronal base is ?0 Lt 1.Using our self-consistent three-dimensional magnetohydrodynamics model,we estimate for these stellar winds the timescale of planet migrationdue to drag forces exerted by the stellar wind on a hot-Jupiter. Incontrast to the findings of Lovelace et al., who estimated suchtimescales using the Weber and Davis model, our model suggests that thestellar wind of these multicomponent coronae are not expected to havesignificant influence on hot-Jupiters migration. Further simulations arenecessary to investigate this result under more intense surface magneticfield strengths (~2-3 kG) and higher coronal base densities, as well asin a tilted stellar magnetosphere.
| Unusually Wide Binaries: Are They Wide or Unusual?
We describe an astrometric and spectroscopic campaign to confirm theyouth and association of a complete sample of candidate wide companionsin Taurus and Upper Sco. Our survey found 15 new binary systems (threein Taurus and 12 in Upper Sco) with separations of 3''-30'' (500-5000AU) among all of the known members with masses of 2.5-0.012 Msun. The total sample of 49 wide systems in these two regionsconforms to only some expectations from field multiplicity surveys.Higher mass stars have a higher frequency of wide binary companions, andthere is a marked paucity of wide binary systems near the substellarregime. However, the separation distribution appears to be log-flat,rather than declining as in the field, and the mass ratio distributionis more biased toward similar-mass companions than the initial massfunction or the field G-dwarf distribution. The maximum separation alsoshows no evidence of a limit at lsim5000 AU until the abrupt cessationof any wide binary formation at system masses of ~0.3 M sun.We attribute this result to the post-natal dynamical sculpting thatoccurs for most field systems; our binary systems will escape to thefield intact, but most field stars are formed in denser clusters andundergo significant dynamical evolution. In summary, only wide binarysystems with total masses lsim0.3 M sun appear to be"unusually wide."
| 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.
| VLBA Determination of the Distance to Nearby Star-Forming Regions. III. HP TAU/G2 and the Three-Dimensional Structure of Taurus
Using multiepoch Very Long Baseline Array (VLBA) observations, we havemeasured the trigonometric parallax of the weak-line T Tauri star HPTau/G2 in Taurus. The best fit yields a distance of 161.2 ± 0.9pc, suggesting that the eastern portion of Taurus (where HP Tau/G2 islocated) corresponds to the far side of the complex. Previous VLBAobservations have shown that T Tau, to the south of the complex, is atan intermediate distance of about 147 pc, whereas the region aroundL1495 corresponds to the near side at roughly 130 pc. Our observationsof only four sources are still too coarse to enable a reliabledetermination of the three-dimensional structure of the entire Taurusstar-forming complex. They do demonstrate, however, that VLBAobservations of multiple sources in a given star-forming region have thepotential not only to provide a very accurate estimate of its meandistance, but also to reveal its internal structure. The proper motionmeasurements obtained simultaneously with the parallax allowed us tostudy the kinematics of the young stars in Taurus. Combining the fourobservations available so far, we estimate the peculiar velocity ofTaurus to be about 10.6 km s-1 almost completely in adirection parallel to the Galactic plane. Using our improved distancemeasurement, we have refined the determination of the position on theH-R diagram of HP Tau/G2, and of two other members of the HP Tau group(HP Tau itself and HP Tau/G3). Most pre-main-sequence evolutionarymodels predict significantly discrepant ages (by 5 Myr) for those threestars—expected to be coeval. Only in the models of Palla &Stahler do they fall on a single isochrone (at 3 Myr).
| Interferometric Evidence for Resolved Warm Dust in the DQ Tau System
We report on near-infrared (IR) interferometric observations of thedouble-lined pre-main sequence binary system DQ Tau. We model these datawith a visual orbit for DQ Tau supported by the spectroscopic orbit andanalysis of Mathieu et al. Further, DQ Tau exhibits significant near-IRexcess; modeling our data requires inclusion of near-IR light from an"excess" source. Remarkably, the excess source is resolved in our data,similar in scale to the binary itself (~0.2 AU at apastron), rather thanthe larger circumbinary disk (~0.4 AU radius). Our observations supportthe Mathieu et al. and Carr et al. inference of significant warmmaterial near the DQ Tau binary.
| Primordial Circumstellar Disks in Binary Systems: Evidence for Reduced Lifetimes
We combine the results from several multiplicity surveys ofpre-main-sequence stars located in four nearby star-forming regions withSpitzer data from three different Legacy Projects. This allows us toconstruct a sample of 349 targets, including 125 binaries, which we useto to investigate the effect of companions on the evolution ofcircumstellar disks. We find that the distribution of projectedseparations of systems with Spitzer excesses is significantly different(P~ 2.4e-5, according to the K-S test for binaries with separations lessthan 400 AU) from that of systems lacking evidence for a disk. Asexpected, systems with projected separations less than 40 AU are half aslikely to retain at least one disk than are systems with projectedseparations in the 40-400 AU range. These results represent the firststatistically significant evidence for a correlation between binaryseparation and the presence of an inner disk (r~ 1 AU). Several factors(e.g., the incompleteness of the census of close binaries, the use ofunresolved disk indicators, and projection effects) have previouslymasked this correlation in smaller samples. We discuss the implicationsof our findings for circumstellar disk lifetimes and the formation ofplanets in multiple systems.
| Updated pre-main sequence tracks at low metallicities for 0.1 ? M/M _? ? 1.5
Context: Young populations at Z < Z? are beingexamined to understand the role of metallicity in the first phases ofstellar evolution. For the analysis it is necessary to assign mass andage to Pre-Main Sequence (PMS) stars. While it is well known that themass and age determination of PMS stars is strongly affected by theconvection treatment, extending any calibration to metallicitiesdifferent from solar is very artificial, in the absence of anycalibrators for the convective parameters. For solar abundance, MixingLength Theory models have been calibrated by using the results of 2Dradiative-hydrodynamical models (MLT-?^2D), that are very similarto those computed with non-grey ATLAS9 atmosphere boundary conditionsand a full spectrum of turbulence (FST) convection model both in theatmosphere and in the interior (NEMO-FST models). Aims: WhileMLT-?^2D models are not available for lower metallicities, weextend to lower Z the NEMO-FST models, under the hypothesis that in sucha way we are simulating the results of MLT-?^2D models also atsmaller Z. Methods: We use standard stellar computationtechniques in which the atmospheric boundary conditions are derivedmaking use of model atmosphere grids. This allows us to take intoaccount the non greyness of the atmosphere, but adds a new parameter tothe stellar structure uncertainty, namely the efficiency of convectionin the atmospheric structure, if convection is computed in theatmospheric grid by a model different to the model adopted for theinterior integration. Results: We present PMS models for low massstars from 0.1 to 1.5 M? for metallicities [Fe/H] =-0.5, -1.0 and -2.0. The calculations include the most recent interiorphysics and the latest generation of non-grey atmosphere models. Atfixed luminosity more metal poor isochrones are hotter than solar onesby ? log T_eff/? log Z ~ 0.03-0.05 in the range in Z from0.02 to 0.0002 and for ages from 105 to 107 yr.
| Large grains in discs around young stars: ATCA observations of WW Chamaeleontis, RU Lupi, and CS Chamaeleontis
Context: Grains in discs around young stars grow from interstellarsubmicron sizes to planetesimals, up to thousands of kilometres in size,over the course of several Myr. Thermal emission of large grains orpebbles can be best observed at centimetre wavelengths. However, otheremission mechanisms can contribute, most notably free-free emission fromstellar winds and chromospheric activity. Aims: We aim todetermine the mechanisms of centimetre emission for three T Tauri stars.WW Cha and RU Lup have recentlybeen found to have grain growth at least up to millimetre sizes in theircircumstellar discs, based on millimetre data up to 3.3 mm. CSCha has similar indications of grain growth in itscircumbinary disc. Methods: The T Tauri stars WWCha and RU Lup were monitored over thecourse of several years at millimetre and centimetre wavelengths, usingthe Australia Telescope Compact Array (ATCA). The new ATCA 7 mm systemwas also used to observe CS Cha at 7 mm. Results: WW Cha was detected on several occasions at 7 and 16 mm. Weobtained one detection of WW Cha at 3.5 cm and upperlimits only for 6.3 cm. The emission at 16 mm was stable over periods ofdays, months, and years, whereas the emission at 3.5 cm is found to bevariable. A second young stellar object, Ced 112 IRS4, was found in the field of WW Cha at 16mm. RU Lup was detected at 7 mm. It was observed at16 mm three times and at 3 and 6 cm four times and found to be variablein all three wavebands. CS Cha was detected at 7 mm,but the signal-to-noise ratio was not high enough to resolve the gap inthe circumbinary disc. The typical resolution of the 7 and 16 mmobservations was 5-10 arcsec with rms ~0.2 mJy. Conclusions: Theemission at 3, 7, and 16 mm for WW Cha is explainedwell by thermal emission from millimetre and centimetre-sized“pebbles”. The cm spectral index between 3.5 and 6.3 cm isconsistent with the emission from an optically-thick ionised wind,although the high variability of the cm emission points to a non-thermalcontribution. The spectral energy distributions of both RULup and CS Cha from 1 to 7 mm areconsistent with thermal emission from mm-sized grains. The variabilityof the longer-wavelength emission for RU Lup and thenegative spectral index suggests non-thermal emission, arising from anoptically-thin plasma.
| High-Resolution Spectroscopy of T Tauri Stars
Not Available
| Multiwavelength Signatures of Magnetic Activity from Young Stellar Objects in the LkH? 101 Cluster
We describe the results of our multiwavelength observing campaign on theyoung stellar objects in the LkH? 101 cluster. Our simultaneousX-ray and multifrequency radio observations are unique in providingsimultaneous constraints on short-timescale variability at bothwavelengths as well as constraints on the thermal or nonthermal natureof radio emission from young stars. Focussing on radio-emitting objectsand the multiwavelength data obtained for them, we find thatmultifrequency radio data indicate nonthermal emission even in objectswith infrared evidence for disks. We find radio variability ontimescales of decades, days, and hours. About half of the objects withX-ray and radio detections were variable at X-ray wavelengths, despitelacking large-scale flares or large variations. Variability appears tobe a bigger factor affecting radio emission than X-ray emission. A starwith infrared evidence for a disk, [BW88] 3, was observed in the decayphase of radio flare. In this object and another ([BW88] 1), we find aninverse correlation between radio flux and spectral index, whichcontrasts with behavior seen in the Sun and active stars. We interpretthis behavior as the repopulation of the hardest energy electrons due toparticle acceleration. A radio and X-ray source lacking an infraredcounterpart, [BW88] 1, may be near the substellar limit; its radioproperties are similar to other cluster members, but its much higherradio to X-ray luminosity ratio is reminiscent of behavior in nearbyvery low-mass stars/brown dwarfs. We find no correspondence betweensignatures of particle acceleration and those of plasma heating, bothtime-averaged and time-variable. The lack of correlated temporalvariability in multiwavelength behavior, the breakdown ofmultiwavelength correlations of time-averaged luminosities, and theoptical thickness of X-ray emitting material at radio wavelengthssupport the idea that radio and X-ray emission on young stars arephysically and/or energetically distinct.
| Crystalline Silicates and Dust Processing in the Protoplanetary Disks of the Taurus Young Cluster
We characterize the crystalline-silicate content and spatialdistribution of small dust grains in a large sample of protoplanetarydisks in the Taurus-Auriga young cluster, using the Spitzer SpaceTelescope mid-IR spectra. In turn we use the results to analyze theevolution of structure and composition of these 1-2 Myr old disks aroundSolar- and later-type young stars, and test the standard models of dustprocessing which result in the conversion of originally amorphous dustinto minerals. We find strong evidence of evolution of thedust-crystalline mass fraction in parallel with that of the structure ofthe disks, in the sense that increasing crystalline mass fraction isstrongly linked to dust settling to the disk midplane. We also confirmthat the crystalline silicates are confined to small radii, r lsim 10AU. However, we see no significant correlation of crystalline massfraction with stellar mass or luminosity, stellar-accretion rate, diskmass, or disk/star mass ratio, as would be expected in the standardmodels of dust processing based upon photoevaporation and condensationclose to the central star, accretion-heating-driven annealing at r lsim1 AU, or spiral-shock heating at r lsim 10 AU, with or without effectivelarge-scale radial mixing mechanisms. Either another grain-crystallizingmechanism dominates over these, or another process must be at workwithin the disks to erase the correlations they produce. We propose oneof each sort that seems to be worth further investigation, namely X-rayheating and annealing of dust grains, and modulation of disk structureby giant-planetary formation and migration.
| The mass of young stellar objects.
Not Available
| Milli-arcsecond Binaries
As part of an astrometric program, we have used the Very Long BaselineArray to measure with great accuracy the trigonometric parallax ofseveral young stars in the Taurus and Ophiuchus star-forming regions.Additionally, we have obtained an unprecedented sample ofhigh-resolution (~1 mas) images of several young stellar systems. Theseimages revealed that about 70% of the stars in our sample are very tightbinary stars (with separations of a few mas). Since it is highlyunlikely that 70% of all stars are such tight binaries, we argue thatselection effects are at work.
| Very Long Baseline Array astrometry of low-mass young stellar objects
Multi-epoch radio-interferometric observations of young stellar objectscan be used to measure their displacement over the celestial sphere witha level of precision that currently cannot be attained at any otherwavelength. In particular, the accuracy achieved using carefullycalibrated, phase-referenced observations with the Very Long BaselineArray is better than 50 micro-arcseconds. This is sufficient to measurethe trigonometric parallax and the proper motion of any radio-emittingyoung star within several hundred parsecs of the Sun with an accuracybetter than a few percent. Taking advantage of this situation, we haveinitiated a large project aimed mainly at measuring the distance to thenearest regions of star-formation (Taurus, Ophiuchus, Perseus, etc.).Here, we will present the results for several stars in Taurus andOphiuchus, and show that the accuracy obtained is already more than oneorder of magnitude better than that of previous estimates. The propermotion obtained from the data can also provide important information,particularly in multiple stellar systems. To illustrate this point, wewill present the case of the famous system T Tauri, where the VLBA dataprovide crucial information for the characterization of the orbitalpath.
| Captured at millimeter wavelengths: a flare from the classical T Tauri star DQ Tauri
For several hours on 2008 April 19, the T Tauri spectroscopic binaryDQ Tau was observed to brighten, reaching a maximumdetected flux of 468 mJy and likely making it (briefly) the brightestobject at 3 mm in the Taurus star-forming region. We present the lightcurve of a rarely before observed millimeter flare originating in theregion around a pre-main-sequence star, and the first from a classical TTauri star. We discuss the properties and nature of the flaring behaviorin the context of pulsed accretion flows (the current picture basedlargely on studies of this object's optically variable spectrum), aswell as magnetospheric re-connection models (a separate theory thatpredicts millimeter flares for close binaries of high orbitaleccentricity). We believe that the flare mechanism is linked to thebinary orbit, and therefore periodic. DQ Tau makes a strong case formulti-wavelength follow-up studies, performed in parallel, of futureflares to help determine whether magnetospheric and dynamicalinteractions in a proto-binary system are independent.
| X-Ray Flares in Orion Young Stars. II. Flares, Magnetospheres, and Protoplanetary Disks
We study the properties of powerful X-ray flares from 161pre-main-sequence (PMS) stars observed with the Chandra X-rayObservatory in the Orion Nebula region. Relationships between flareproperties, protoplanetary disks, and accretion are examined in detailto test models of star-disk interactions at the inner edge of theaccretion disks. Previous studies found no differences in flaringbetween disk-free and accreting systems other than a small overalldiminution of X-ray luminosity in accreting systems. The most importantfinding is that X-ray coronal extents in fast-rotating disk-free starscan significantly exceed the Keplerian corotation radius, whereas X-rayloop sizes in disky and accreting systems do not exceed the corotationradius. This is consistent with models of star-disk magnetic interactionin which the inner disk truncates and confines the PMS stellarmagnetosphere. We also find two differences between flares in accretingand disk-free PMS stars. First, a subclass of superhot flares with peakplasma temperatures exceeding 100 MK are preferentially present inaccreting systems. Second, we tentatively find that accreting starsproduce flares with shorter durations. Both results may be consequencesof the distortion and destabilization of the stellar magnetosphere bythe interacting disk. Finally, we find no evidence that any flare types,even slow-rise top-flat flares, are produced in star-disk magneticloops. All are consistent with enhanced solar long-duration events withboth footpoints anchored in the stellar surface.
| X-Ray Flares in Orion Young Stars. I. Flare Characteristics
Pre-main-sequence (PMS) stars are known to produce powerful X-rayflares, which resemble magnetic reconnection solar flares scaled byfactors up to 104. However, there are numerous puzzles,including the structure of X-ray-emitting coronae and magnetospheres,the effects of protoplanetary disks, and the effects of stellarrotation. To investigate these issues in detail, we examine 216 of thebrightest flares from 161 PMS stars observed in the Chandra OrionUltradeep Project (COUP). These constitute the largest homogeneous dataset of PMS, or indeed stellar flares at any stellar age, ever acquired.Our effort is based on a new flare spectral analysis technique thatavoids nonlinear parametric modeling. We provide a catalog of derivedproperties and an electronic atlas for this unique collection of flares.The current study (Paper I) examines the flare morphologies, andprovides a general comparison of COUP flare characteristics with thoseof other active X-ray stars and the Sun. Paper II will concentrate onrelationships between flare behavior, protoplanetary disks, and otherstellar properties. Several results are obtained. First, COUP flares areamong the most powerful, longest, and hottest, corresponding to thelargest known coronal structures. Second, no significant statisticaldifferences in peak flare luminosity or temperature distributions arefound among different morphological flare classes, suggesting a commonunderlying mechanism for all flares. Third, comparison with the generalsolar scaling laws indicates that COUP flares may not adequately fitproposed power-temperature and duration-temperature solar-stellar fits.Fourth, COUP superhot flares are found to be brighter but shorter thancooler COUP flares. Finally, the majority of bright COUP flares can beviewed as enhanced analogs of the rare solar ``long-duration events.''
| 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
| IPHAS discoveries of young stars towards Cyg OB2 and its southern periphery
We report on the discovery of over 50 strong H? emitting objectstowards the large OB association Cyg OB2 and the HII region DR 15 on itssouthern periphery. This was achieved using the INT Photometric H?Survey of the Northern Galactic Plane (IPHAS), combined with follow-upspectroscopy using the MMT multi-object spectrometer HectoSpec. Wepresent optical spectra, supplemented with optical r',i' and H?photometry from IPHAS, and near-infrared J,H and K photometry from TwoMicron All Sky Survey. The position of the objects in the (J - H) versus(H - K) diagram strongly suggests most of them are young. Many show CaIIinfrared triplet emission indicating that they are in apre-main-sequence phase of evolution of T Tauri and Herbig Ae nature.Among these, we have uncovered pronounced clustering of T Tauri starsroughly a degree south of the centre of Cyg OB2, in an arc close to theHII region DR 15, and the radio ring nebula G79.29+0.46, for which wediscuss its candidacy as a luminous blue variable. The emission-lineobjects towards Cyg OB2 itself could be the brightest most prominentcomponent of a population of lower mass pre-main-sequence stars that hasyet to be uncovered. Finally, we discuss the nature of the ongoing starformation in Cyg OB2 and the possibility that the central OB stars havetriggered star formation in the periphery.
|
Додај нови чланак
Линкови у сродству са темом
Додај нови линк
Чланови следећих група \:
|
Посматрања и Астрометриски подаци
| Сазвежђа: | Бик |
| Ректацензија: | 04h14m12.92s |
| Deклинација: | +28°12'12.3" |
| Apparent магнитуда: | 10.735 |
| Proper motion RA: | 7.7 |
| Proper motion Dec: | -23.7 |
| B-T magnitude: | 12.339 |
| V-T magnitude: | 10.868 |
Каталог и designations:
|
