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Is the silicate emission feature only influenced by grain size?
The flattening of the 10 μm silicate emission feature observed in thespectra of T Tauri and Herbig Ae/Be stars is usually interpreted as anindicator of grain growth. We show in this paper that a similarbehaviour of the feature shape occurs when the porosity of compositegrains varies. We modelled the fluffy aggregates with inclusions ofdifferent sizes with multi-layered spheres consisting of amorphouscarbon, amorphous silicate, and vacuum. We also found that the inclusionof crystalline silicates in composite porous particles can lead to ashift of the known resonances and production of new ones.Table 1 is only available in electronic form at http://www.aanda.org
| On the formation and migration of giant planets in circumbinary discs
Aims. We present the results of hydrodynamic simulations of theformation and subsequent orbital evolution of giant planets embedded ina circumbinary disc. The aim is to examine whether or not giant planetscan be found to orbit stably in close binary systems. Methods: Weperformed numerical simulations using a grid-based hydrodynamics code.We assume that a 20 M_⊕ core has migrated to the edge of the innercavity formed by the binary where it remains trapped by corotationtorques. This core is then allowed to accrete gas from the disc, and westudy its orbital evolution as it grows in mass. For each of the twoaccretion time scales we considered, we performed three simulations. Intwo of the three simulations, we stopped the accretion onto the planetonce its mass became characteristic of that of Saturn or Jupiter. In theremaining case, the planet accreted disc material freely in such a waythat its mass became higher than Jupiter's. Results: The simulationsshow different outcomes depending on the final mass mp of thegiant. For m_p=1~MS (where MS is Saturn's mass),we find that the planet migrates inward through its interaction with thedisc until its eccentricity becomes high enough to induce a torquereversal. The planet then migrates outward, and the system remainsstable on long time scales. For mp ≥ 1~MJ(where MJ is Jupiter's mass) we observed two differentoutcomes. In each case the planet enters the 4:1 resonance with thebinary, and resonant interaction drives up the eccentricity of theplanet until it undergoes a close encounter with the secondary star,leading to scattering. The result can either be ejection from the systemor scattering out into the disc followed by a prolonged period ofoutward migration. These results suggest that circumbinary planets aremore likely to be quite common in the Saturn-mass range. Jupiter-masscircumbinary planets are likely to be less common because of their lessstable evolution, but if present are likely to orbit at large distancesfrom the central binary.
| The X-Ray Origin of Herbig AeBe Systems: New Insights
We present a statistical study of the X-ray emission toward 22 HerbigAeBe stars (HAEBE) using the Chandra archive. We probe the origin of theX-rays toward Herbig stars: are they intrinsic?This question isaddressed by correlations between the physical stellar properties andthe X-ray emission. There is a weak correlation between the continuumradio emission at λ = 3.6 cm and L X, which suggeststhat the X-ray emission depends upon the source. On the other hand, nocorrelation was found with the stellar rotational period, but that onlyexcludes solar-like magnetic activity as the origin of the X-rays. Mostimportantly, the X-ray luminosity of HAEBE has a different distributionfrom that of T Tauri stars, suggesting that X-ray emission from anunseen late-type star companion can be ruled out with an 80% confidencelevel. This implies that the HAEBE must have magnetic activity. Inaddition, we report the observation of five sources for the first time,in three detections.
| On the evolution of multiple low mass planets embedded in a circumbinary disc
Context: Previous work has shown that the tidal interaction between abinary system and a circumbinary disc leads to the formation of a largeinner cavity in the disc. Subsequent formation and inward migration of alow mass planet causes it to become trapped at the cavity edge, where itorbits until further mass growth or disc dispersal. The question of howsystems of multiple planets in circumbinary discs evolve has not yetbeen addressed. Aims: We present the results of hydrodynamicsimulations of multiple low mass planets embedded in a circumbinarydisc. The aim is to examine their long term evolution as they approachand become trapped at the edge of the tidally truncated inner cavity. Methods: A grid-based hydrodynamics code was used to compute simulationsof 2D circumbinary disc models with embedded planets. The 3D evolutionof the planet orbits was computed, and inclination damping due to thedisc was calculated using prescribed forces. We present a suite ofsimulations which study the evolution of pairs of planets migrating inthe disc. We also present the results of hydrodynamic simulations offive-planet systems, and study their long term evolution after discdispersal using a N-body code. Results: For the two-planet simulationswe assume that the innermost planet has migrated to the edge of theinner cavity and remains trapped there, and study the subsequentevolution of the system as the outermost planet migrates inward. We findthat the outcomes largely depend on the mass ratio q=m_i/m_o, wheremi (m_o) is the mass of the innermost (outermost) planet. Forq < 1, planets usually undergo dynamical scattering or orbitalexchange. For values of q > 1 the systems reach equilibriumconfigurations in which the planets are locked into mean motionresonances, and remain trapped at the edge of the inner cavity withoutfurther migration. Most simulations of five-planet systems we performedresulted in collisions and scattering events, such that only a singleplanet remained in orbit about the binary. In one case however, amultiplanet resonant system was found to be dynamically stable over longtime scales, suggesting that such systems may be observed in planetsearches focussed on close binary systems.
| The Rise and Fall of Debris Disks: MIPS Observations of h and χ Persei and the Evolution of Mid-IR Emission from Planet Formation
We describe Spitzer MIPS observations of the double cluster, h and χPersei, covering a ~0.6 deg2 area surrounding the cores ofboth clusters. The data are combined with IRAC and 2MASS data toinvestigate ~616 sources from 1.25-24 μm. We use the long-baselineKs-[24] color to identify two populations with IR excessindicative of circumstellar material: Be stars with 24 μm excess fromoptically thin free-free emission, and 17 fainter sources (J~14-15) with[24] excess consistent with a circumstellar disk. The frequency of IRexcess for the fainter sources increases from 4.5 to 24 μm. The IRexcess is likely due to debris from the planet formation process. Thewavelength-dependent behavior is consistent with an inside-out clearingof circumstellar disks. A comparison of the 24 μm excess populationin h and χ Per sources with results for other clusters shows that 24μm emission from debris disks ``rises'' from 5 to 10 Myr, peaks at~10-15 Myr, and then ``falls'' from ~15-20 Myr to 1 Gyr.
| Discovery of Gas Accretion onto Stars in 13 Myr old h and χ Persei
We report the discovery of accretion disks associated with ~13 Myr oldintermediate/low-mass stars in h and χ Persei. Optical spectroscopyof ~5000 stars in these clusters and a surrounding halo populationreveal 32 A-K stars with Hα emission. Matching these stars with2MASS and optical photometry yields 25 stars with the highestprobability of cluster membership and EW(Hα) >= 5 Å.Sixteen of these sources have EW(Hα) >= 10 Å. Thepopulation of accreting sources is strongly spectral type dependent:Hα emission characteristic of accretion, especially strongaccretion [EW(Hα) >= 10 Å], is much more prevalent aroundstars later than G0. Strong Hα emission from accretion istypically associated with redder Ks-[8] colors. The existenceof accreting pre-main-sequence stars in h and χ Persei implies thatcircumstellar gas in some systems, especially those with primaries laterthan G5 spectral type, can last longer than 10-15 Myr.
| Relative velocities among accreting planetesimals in binary systems: the circumbinary case
We numerically investigate the possibility of planetesimal accretion incircumbinary discs, under the coupled influence of both stars' secularperturbations and friction due to the gaseous component of theprotoplanetary disc. We focus on one crucial parameter: the distributionof encounter velocities between planetesimals in the 0.5-100km sizerange. An extended range of binary systems with differing orbitalparameters is explored. The resulting encounter velocities are comparedto the threshold velocities below which the net outcome of a collisionis accumulation into a larger body instead of mass erosion. For eachbinary configuration, we derive the critical radial distance from thebinary barycentre beyond which planetesimal accretion is possible. Thiscritical radial distance is smallest for equal-mass binaries on almostcircular orbits. It shifts to larger values for increasingeccentricities and decreasing mass ratio. The importance of theplanetesimals' orbital alignments of planetesimals due to gas drageffects is discussed.
| On the migration of protoplanets embedded in circumbinary disks
Aims.We present the results of hydrodynamical simulations of low massprotoplanets embedded in circumbinary accretion disks. The aim is toexamine the migration and long term orbital evolution of theprotoplanets, in order to establish the stability properties of planetsthat form in circumbinary disks. Methods: Simulations were performedusing a grid-based hydrodynamics code. First we present a set ofcalculations that study how a binary interacts with a circumbinary disk.We evolve the system for ~ 105 binary orbits, which is thetime needed for the system to reach a quasi-equilibrium state. From thistime onward the apsidal lines of the disk and the binary are aligned,and the binary eccentricity remains essentially unchanged with a valueof eb ~ 0.08. Once this stationary state is obtained, weembed a low mass protoplanet in the disk and let it evolve under theaction of the binary and disk forces. We consider protoplanets withmasses of mp = 5, 10 and 20 M_⊕. Results: In eachcase, we find that inward migration of the protoplanet is stopped at theedge of the tidally truncated cavity formed by the binary. This effectis due to positive corotation torques, which can counterbalance the netnegative Lindblad torques in disk regions where the surface densityprofile has a sufficiently large positive gradient. Halting of migrationoccurs in a region of long-term stability, suggesting that low masscircumbinary planets may be common, and that gas giant circumbinaryplanets should be able to form in circumbinary disks.
| Periodic Accretion from a Circumbinary Disk in the Young Binary UZ Tau E
Close pre-main-sequence binary stars are expected to clear central holesin their protoplanetary disks, but the extent to which material can flowfrom the circumbinary disk across the gap onto the individualcircumstellar disks has been unclear. In binaries with eccentric orbits,periodic perturbation of the outer disk is predicted to induce mass flowacross the gap, resulting in accretion that varies with the binaryperiod. This accretion may manifest itself observationally as periodicchanges in luminosity. Here we present a search for such periodicaccretion in the pre-main-sequence spectroscopic binary UZ Tau E. Wepresent BVRI photometry spanning 3 years; we find that the brightness ofUZ Tau E is clearly periodic, with a best-fit period of 19.16+/-0.04days. This is consistent with the spectroscopic binary period of 19.13days, refined here from analysis of new and existing radial velocitydata. The brightness of UZ Tau E shows significant random variability,but the overall periodic pattern is a broad peak in enhanced brightness,spanning more than half the binary orbital period. The variability ofthe Hα line is not as clearly periodic, but given the sparsenessof the data, some periodic component is not ruled out. The photometricvariations are in good agreement with predictions from simulations ofbinaries with orbital parameters similar to those of UZ Tau E,suggesting that periodic accretion does occur from circumbinary disks,replenishing the inner circumstellar disks and possibly extending thetimescale over which they might form planets.
| Binary Black Hole Accretion Flows in Merged Galactic Nuclei
We consider accretion flows from circumbinary disks onto supermassivebinary black holes on a subparsec scale of the galactic center based ona smoothed particles hydrodynamics (SPH) code. Simulation models arepresented for four cases; that is, a circular binary and an eccentricone, each with equal and unequal masses. We find that thecircumblack-hole disks are formed around each black hole regardless ofthe simulation parameters. There are two-step mechanisms that causeaccretion flow. First, tidally induced elongation of the circumbinarydisk triggers mass inflow toward two closest points on the circumbinarydisk. Then, the gas is increasingly accumulated on these two pointsowing to the gravitational attraction of black holes. Second, when thegas can pass across the maximum loci of the effective binary potential,it starts to overflow via their two points, and freely infalls to eachblack hole. In circular binaries, the gas continues to be supplied fromthe circumbinary disk, (i.e., the gap between the circumbinary disk andthe binary black hole is always closed). In eccentric cases, the masssupply undergoes periodic on/off transitions during one orbital periodbecause of the variation of the periodic potential. The gap starts toclose after the apoastron, and to open again after the next periastronpassage. Due to the gap closing/opening cycles, the mass-capture ratesare eventually strongly phase dependent. This could provide observablediagnosis for the presence of supermassive binary black holes in mergedgalactic nuclei.
| The Frequency of Mid-Infrared Excess Sources in Galactic Surveys
We have identified 230 Tycho-2 Spectral Catalog stars that exhibit 8μm mid-IR extraphotospheric excesses in the MSX and Spitzer GLIMPSEsurveys. Of these, 183 are either OB stars earlier than B8 in which theexcess plausibly arises from a thermal bremsstrahlung component orevolved stars in which the excess may be explained by an atmosphericdust component. The remaining 47 stars have spectral classifications B8or later and appear to be main-sequence or late pre-main-sequenceobjects harboring circumstellar disks. Six of the 47 stars exhibitmultiple signatures characteristic of pre-main-sequence circumstellardisks, including emission lines, near-IR K-band excesses, and X-rayemission. Approximately one-third of the remaining 41 sources haveemission lines suggesting relative youth. We modeled the excesses in 26stars having two or more measurements in excess of the expectedphotospheres as single-component blackbodies. We determine probable disktemperatures and fractional IR luminosities in the range 191K
| Spitzer 24 μm Observations of Open Cluster IC 2391 and Debris Disk Evolution of FGK Stars
We present 24 μm Spitzer MIPS photometric observations of the ~50 Myropen cluster IC 2391. Thirty-four cluster members ranging in spectraltype from B3 to M5 were observed in the central square degree of thecluster. Excesses indicative of debris disks were discovered around oneA star, six FGK stars, and possibly one M dwarf. For the cluster membersobserved to their photospheric limit, we find a debris disk frequency of10+17-3% for B-A stars and31+13-9% for FGK stars using a 15% relative excessthreshold. Relative to a model of decaying excess frequency, thefrequency of debris disks around A-type stars appears marginally low forthe cluster's age while that of FGK stars appears consistent. Scenariosthat may qualitatively explain this result are examined. We concludethat planetesimal activity in the terrestrial region of FGK stars iscommon in the first ~50 Myr and decays on timescales of ~100 Myr.Despite luminosity differences, debris disk evolution does not appear todepend strongly on stellar mass.
| On the origin of the X-ray emission from Herbig Ae/Be stars
Context: .Herbig Ae/Be stars are fully radiative and not expected tosupport dynamo action analogous to their convective lower-masscounterparts, the T Tauri stars. Alternative X-ray productionmechanisms, related to stellar winds or star-disk magnetospheres havebeen proposed, but their X-ray emission has remained a mystery. Aims: .A study of Herbig Ae/Be stars' global X-ray properties (such asdetection rate, luminosity, temperature, variability), helps toconstrain the emission mechanism by comparison to other types of stars,e.g. similar-age but lower-mass T Tauri stars, similar-mass but moreevolved main-sequence A- and B-type stars, and with respect to modelpredictions. Methods: .We performed a systematic search forChandra archival observations of Herbig Ae/Be stars. The superiorspatial resolution of this satellite with respect to previous X-rayinstrumentation has allowed us to also examine the possible role oflate-type companions in generating the observed X-rays. Results:.In the total sample of 17 Herbig Ae/Be stars, 8 are resolved from X-rayemitting faint companions or other unrelated X-ray bright objects within10''. The detection fraction of Herbig Ae/Be stars is 76%, but decreasesto 35% if all emission is attributed to further known and unresolvedcompanions. The spectral analysis confirms the high X-ray temperatures( 20 MK) and large range of fractional X-ray luminosities(log{L_x/L_*}) of this class derived from earlier studies of individualobjects. Conclusions: .Radiative winds are ruled out as anemission mechanism on the basis of the high temperatures. The X-rayproperties of Herbig Ae/Be stars are not vastly different from those oftheir late-type companion stars (if such are known), nor from otheryoung late-type stars used for comparison. Therefore, either a similarkind of process takes place in both classes of objects, or there must beas yet undiscovered companion stars.
| A survey for nanodiamond features in the 3 micron spectra of Herbig Ae/Be stars
Aims.We have carried out a survey of 60 Herbig Ae/Be stars in the 3micron wavelength region in search for the rare spectral features at3.43 and 3.53 micron. These features have been attributed to thepresence of large, hot, hydrogen-terminated nanodiamonds. Only twoHerbig Ae/Be stars, HD 97048 and Elias3-1 are known to display both these features. Methods:.We have obtained medium-resolution spectra (R 2500) with the ESOnear-IR instrument ISAAC in the 3.15-3.65 micron range. Results:.In our sample, no new examples of sources with prominent nanodiamondfeatures in their 3 micron spectra were discovered. Less than 4% of theHerbig targets show the prominent emission features at 3.43 and/or 3.53μm. Both features are detected in our spectrum of HD 97048. Weconfirm the detection of the 3.53 μm feature and the non-detection ofthe 3.43 μm feature in MWC 297. Furthermore, we report tentative 3.53μm detections in V921 Sco, HD 163296 and T CrA. The sources whichdisplay the nanodiamond features are not exceptional in the group ofHerbig stars with respect to disk properties, stellar characteristics,or disk and stellar activity. Moreover, the nanodiamond sources are verydifferent from each other in terms of these parameters. We do not findevidence for a recent supernova in the vicinity of any of thenanodiamond sources. We have analyzed the PAH 3.3 μm feature and thePfund δ hydrogen emission line, two other spectral features whichoccur in the 3 micron wavelength range. We reinforce the conclusion ofprevious authors that flared-disk systems display significantly more PAHemission than self-shadowed-disk sources. The Pf δ line detectionrate is higher in self-shadowed-disk sources than in the flared-disksystems. Conclusions: . We discuss the possible origin and paucityof the (nano)diamond features in Herbig stars. Different creationmechanisms have been proposed in the literature, amongst others in-situand supernova-induced formation. Our data set is inconclusive in provingor disproving either formation mechanism.
| Modeling of PMS Ae/Fe stars using UV spectra
Context: .Spectral classification of AeFe stars, based on visualobservations, may lead to ambiguous conclusions. Aims: . We aimto reduce these ambiguities by using UV spectra for the classificationof these stars, because the rise of the continuum in the UV is highlysensitive to the stellar spectral type of A/F-type stars. Methods: . We analyse the low-resolution UV spectra in terms of a3-component model, that consists of spectra of a central star, of anoptically-thick accretion disc, and of a boundary-layer between the discand star. The disc-component was calculated as a juxtaposition of Planckspectra, while the 2 other components were simulated by thelow-resolution UV spectra of well-classified standard stars (taken fromthe IUE spectral atlases). The hot boundary-layer shows strongsimilarities to the spectra of late-B type supergiants (see Appendix A). Results: . We modeled the low-resolution UV spectra of 37 AeFestars. Each spectral match provides 8 model parameters: spectral typeand luminosity-class of photosphere and boundary-layer, temperature andwidth of the boundary-layer, disc-inclination and circumstellarextinction. From the results of these analyses, combined with availabletheoretical PMS evolutionary tracks, we could estimate their masses andages and derive their mass-accretion rates. For a number of analysed PMSstars we calculated the corresponding SEDs and compared these with theobserved SEDs. Conclusions: . All stars (except βPic) showindications of accretion, that affect the resulting spectral type of thestellar photosphere. Formerly this led to ambiguities in classificatonof PMS stars as the boundary-layer was not taken into consideration. Wegive evidence for an increase of the mass-accretion rate with stellarmass and for a decreases of this rate with stellar age.
| Analysis of the dust evolution in the circumstellar disks of T Tauri stars
Aims.We present a compositional analysis of 8{-}13 μ m spectra of 32young stellar objects (YSOs). Our sample consists of 5 intermediate-massstars and 27 low-mass stars. Although some previous studies give reasonsfor the similarity between the dust in circumstellar disks of T Tauristars and Herbig Ae/Be stars, a quantitative comparison has been lackingso far. Therefore, we include a discussion of the results of the 10μm spectroscopic survey of van Boekel et al. (2005, A&A, 437,189), who focus on Herbig Ae/Be stars, the higher mass counterparts of TTauri stars, and draw comparisons to this and other studies. Methods: .While the spectra of our 32 objects and first scientificresults have already been published elsewhere we perform a more detailedanalysis of the 10 μ m silicate feature. In our analysis weassume that this emission feature can be represented by a linearsuperposition of the wavelength-dependent opacity κ_abs(λ)describing the optical properties of silicate grains with differentchemical composition, structure, and grain size. Determining an adequatefitting equation is another goal of this study. Using a restrictednumber of fitting parameters, we investigate which silicate species arenecessary for the compositional fitting. Particles, with radii of 0.1μm- and 1.5 μm and consisting of amorphous olivine and pyroxene,forsterite, enstatite, and quartz are considered. Only compact,homogeneous dust grains are used in the presented fitting procedures. Inthis context we show that acceptable fitting results can also beachieved if emission properties of porous silicate grains are consideredinstead. Results: .Our analysis shows - in terms of the propertiesof the circumstellar dust-like crystallinity - T Tauri systems are acontinuation of HAeBe systems at their lower mass end. However, a weakcorrelation between grain growth and stellar luminosity could be found,in contrast to HAeBe systems.
| Photometric monitoring of the pre-main sequence binary AK Sco
Not Available
| c2d Spitzer IRS Spectra of Disks around T Tauri Stars. I. Silicate Emission and Grain Growth
Infrared ~5-35 μm spectra for 40 solar mass T Tauri stars and 7intermediate-mass Herbig Ae stars with circumstellar disks were obtainedusing the Spitzer Space Telescope as part of the c2d IRS survey. Thiswork complements prior spectroscopic studies of silicate infraredemission from disks, which were focused on intermediate-mass stars, withobservations of solar mass stars limited primarily to the 10 μmregion. The observed 10 and 20 μm silicate feature strengths/shapesare consistent with source-to-source variations in grain size. A largefraction of the features are weak and flat, consistent with micron-sizedgrains indicating fast grain growth (from 0.1 to 1.0 μm in radius).In addition, approximately half of the T Tauri star spectra showcrystalline silicate features near 28 and 33 μm, indicatingsignificant processing when compared to interstellar grains. A fewsources show large 10-to-20 μm ratios and require even larger grainsemitting at 20 μm than at 10 μm. This size difference may arisefrom the difference in the depth into the disk probed by the twosilicate emission bands in disks where dust settling has occurred. The10 μm feature strength versus shape trend is not correlated with ageor Hα equivalent width, suggesting that some amount of turbulentmixing and regeneration of small grains is occurring. The strengthversus shape trend is related to spectral type, however, with M starsshowing significantly flatter 10 μm features (larger grain sizes)than A/B stars. The connection between spectral type and grain size isinterpreted in terms of the variation in the silicate emission radius asa function of stellar luminosity, but could also be indicative of otherspectral-type-dependent factors (e.g., X-rays, UV radiation, andstellar/disk winds).
| Investigating Disk Evolution: A High Spatial Resolution Mid-Infrared Survey of T Tauri Stars
We present a high spatial resolution, 10-20 μm survey of 65 T Tauribinary stars in Taurus, Ophiuchus, and Corona Australis using the Keck10 m telescopes. Designed to probe the inner ~1 AU region of thecircumstellar disks around the individual stellar components in thesebinary systems, this study increases the number of binaries withspatially resolved measurements at 10 μm by a factor of ~5. Combinedwith resolved near-infrared photometry and spectroscopic accretiondiagnostics, we find that ~10% of stars with a mid-infrared excess donot appear to be accreting. In contrast to an actively accreting disksystem, these passive disks have significantly lower near-infraredcolors that are, in most cases, consistent with photospheric emission,suggesting the presence of an inner disk hole. In addition, thereappears to be a spectral type/mass dependence associated with thepresence of a passive disk, with all passive disks occurring aroundM-type stars. The presence of a passive disk does not appear to berelated to the fact that these objects are in visual binary systems; thepassive disk systems span the entire range of binary separations presentin the sample, and a similar fraction of passive disks is observed in asample of single stars. The possibility that the passive disks arecaused by the presence of an as yet undetected companion at a smallseparation (0.3-3 AU) is possible for any individual system; however, itcannot account for the spectral type dependence of the passive disksample as a whole. We propose that these passive disks represent asubset of T Tauri stars that are undergoing significant disk evolution.The fraction of observed passive disks and the observed spectral typedependence can both be explained by models of disk evolution thatinclude disk photoevaporation from the central star.
| Pulkovo compilation of radial velocities for 35495 stars in a common system.
Not Available
| Pre-main sequence star Proper Motion Catalogue
We measured the proper motions of 1250 pre-main sequence (PMS) stars andof 104 PMS candidates spread over all-sky major star-forming regions.This work is the continuation of a previous effort where we obtainedproper motions for 213 PMS stars located in the major southernstar-forming regions. These stars are now included in this present workwith refined astrometry. The major upgrade presented here is theextension of proper motion measurements to other northern and southernstar-forming regions including the well-studied Orion and Taurus-Aurigaregions for objects as faint as V≤16.5. We improve the precision ofthe proper motions which benefited from the inclusion of newobservational material. In the PMS proper motion catalogue presentedhere, we provide for each star the mean position and proper motion aswell as important photometric information when available. We providealso the most common identifier. The rms of proper motions vary from 2to 5 mas/yr depending on the available sources of ancient positions anddepending also on the embedding and binarity of the source. With thiswork, we present the first all-sky catalogue of proper motions of PMSstars.
| [O I] 6300 Å emission in Herbig Ae/Be systems: Signature of Keplerian rotation
We present high spectral-resolution optical spectra of 49 Herbig Ae/Bestars in a search for the [O i] 6300 Å line. The vast majority ofthe stars in our sample show narrow ({FWHM} < 100 km s-1)emission lines, centered on the stellar radial velocity. In only threesources is the feature much broader ( 400 km s-1), andstrongly blueshifted (-200 km s-1) compared to the stellarradial velocity. Some stars in our sample show double-peaked lineprofiles, with peak-to-peak separations of 10 km s-1. Thepresence and strength of the [O i] line emission appears to becorrelated with the far-infrared energy distribution of each source:stars with a strong excess at 60 μm have in general stronger [O i]emission than stars with weaker 60 μm excesses. We interpret thesenarrow [O i] 6300 Å line profiles as arising in the surface layersof the protoplanetary disks surrounding Herbig Ae/Be stars. A simplemodel for [O i] 6300 Å line emission due to the photodissociationof OH molecules shows that our results are in quantitative agreementwith that expected from the emission of a flared disk if the fractionalOH abundance is 5 × 10-7.
| Evolution of a Protobinary: Accretion Rates of the Primary and Secondary
We reexamine accretion onto a protobinary based on two-dimensionalnumerical simulations with high spatial resolution. We focus ourattention on the ratio of the primary and secondary accretion rates.Fifty-eight models are made for studying the dependence of the accretionrates on the specific angular momentum of infalling gas jinf,the mass ratio of the binary q, and the sound speed cs. Whenjinf is small, the binary accretes the gas mainly through twochannels (type I): one through the Lagrange point L2 and the otherthrough L3. When jinf is large, the binary accretes the gasonly through the L2 point (type II). The primary accretes more than thesecondary in both the cases, although the L2 point is closer to thesecondary. After flowing through the L2 point, the gas flows halfwayaround the secondary and through the L1 point to the primary. Only asmall amount of gas flows back to the secondary, and the rest forms acircumstellar ring around the primary. The boundary between types I andII depends on q. When jinf is very large, the accretionbegins after several rotations (type III). The beginning of theaccretion is later when jinf is larger and cs issmaller. Our result that the primary accretion rate is higher for alarge jinf is qualitatively different from results of earliersimulations. The difference is mainly due to limited spatial resolutionand large numerical viscosity in the numerical simulations thus far.
| A Spitzer Study of Dusty Disks in the Scorpius-Centaurus OB Association
We have obtained Spitzer Space Telescope MIPS observations of 40 F- andG-type common proper motion members of the Scorpius-Centaurus OBassociation with ages between 5 and 20 Myr at 24 and 70 μm. We reportthe detection of 14 objects that possess 24 μm fluxes >=30% largerthan their predicted photospheres, tentatively corresponding to a diskfraction of >=35%, including seven objects that also possess 70 μmexcesses >=100 times larger than their predicted photospheres. The 24μm plus 70 μm excess sources possess high fractional infraredluminosities, LIR/L*=7×10-4 to3×10-3 either they possess optically thin, dusty βPictoris-like disks or compact, opaque HD 98800-like disks.
| Polarimetric Variations of Binary Stars. VI. Orbit-Induced Variations in the Pre-Main-Sequence Binary AK Scorpii
We present simultaneous UBV polarimetric and photometric observations ofthe pre-main-sequence binary AK Sco, obtained over 12 nights, slightlyless than the orbital period of 13.6 days. The polarization is a sum ofinterstellar and intrinsic polarization, with a significant intrinsicpolarization of 1% at 5250 Å, indicating the presence ofcircumstellar matter distributed in an asymmetric geometry. Thepolarization and its position angle are clearly variable on timescalesof hours and nights in all three wavelengths, with a behavior related tothe orbital motion. The variations have the highest amplitudes seen sofar for pre-main-sequence binaries (~1% and ~30°) and are sinusoidalwith periods similar to the orbital period and half of it. Thepolarization variations are generally correlated with the photometricones: when the star gets fainter, it also gets redder, and itspolarization increases. The (B-V, V) color-magnitude diagram exhibits aratio of total to selective absorption R=4.3, higher than in normalinterstellar clouds (R=3.1). The interpretation of the simultaneousphotometric and polarimetric observations is that a cloud ofcircumstellar matter passes in front of the star, decreasing the amountof direct, unpolarized light and hence increasing the contribution ofscattered (blue) light. We show that the large amplitude of thepolarization variations cannot be reproduced with a single-scatteringmodel and axially symmetric circumbinary or circumstellar disks.Based on observations made with the ESO telescopes at the La SillaObservatory.
| The pre-main sequence spectroscopic binary UZ Tau East: Improved orbital parameters and accretion phase dependence
We present radial-velocity measurements obtained using high- andintermediate-resolution spectroscopic observations of the classical TTauri star UZ Tau East from 1994 to 1996. We also provide measurementsof Hα equivalent widths and optical veiling. Combining ourradial-velocity data with those recently reported by Prato et al.(2002), we improve the orbital elements for this spectroscopic binary.The orbital period is 18.979±0.007 days and the eccentricity ise=0.14. We find variability in the Hα emission andveiling, signposts of accretion, but at periastron passage the accretionis not as clearly enhanced as in the case of the binary DQ Tau. Thedifference in the behaviour of these two binaries is consistent with thehydrodynamical models of accretion from circumbinary disks because UZTau East has lower eccentricity than DQ Tau. It seems that enhancedperiastron accretion may occur only in systems with very higheccentricity (e>0.5).Based on observations made with the Isaac Newton Telescope, operated onthe island of La Palma by the Isaac Newton Group in the SpanishObservatorio del Roque de los Muchachos of the Instituto deAstrofísica de Canarias, the ESO 3.6-m telescope at La SillaObservatory in Chile, and the Shane 3-m telescope at Lick observatory inCalifornia.
| New Perspectives on the X-Ray Emission of HD 104237 and Other Nearby Herbig Ae/Be Stars from XMM-Newton and Chandra
The origin of the X-ray emission from Herbig Ae/Be stars is not yetknown. These intermediate-mass pre-main-sequence stars lie on radiativetracks and are not expected to emit X-rays via solar-like magneticprocesses, nor are their winds powerful enough to produce X-rays byradiative wind shocks as in more massive O-type stars. The emissioncould originate in unseen low-mass companions, or it may be intrinsic tothe Herbig stars themselves if they still have primordial magneticfields or can sustain magnetic activity via a nonsolar dynamo. Wepresent new X-ray observations of the nearby Herbig Ae star HD 104237(=DX Cha) with XMM-Newton, whose objective is to clarify the origin ofthe emission. Several X-ray emission lines are clearly visible in theCCD spectra, including the high-temperature Fe Kα complex. Theemission can be accurately modeled as a multitemperature thermal plasmawith cool (kT<1 keV) and hot (kT>~3 keV) components. The presenceof a hot component is compelling evidence that the X-rays originate inmagnetically confined plasma, either in the Herbig star itself or in thecorona of an as yet unseen late-type companion. The X-ray temperaturesand luminosity (logLX=30.5 ergs s-1) are withinthe range expected for a T Tauri companion, but high-resolution Chandraand Hubble Space Telescope images constrain the separation of a putativecompanion to less than 1". We place these new results into broadercontext by comparing the X-ray and bolometric luminosities of a sampleof nearby Herbig stars with those of T Tauri stars and classicalmain-sequence Be stars. We also test the predictions of a model thatattributes the X-ray emission of Herbig stars to magnetic activity thatis sustained by a shear-powered dynamo.
| ISO spectroscopy of disks around Herbig Ae/Be stars
We have investigated the infrared spectra of all 46 Herbig Ae/Be starsfor which spectroscopic data are available in the ISO data archive. Ourquantitative analysis of these spectra focuses on the emission bands at3.3, 6.2, ``7.7'', 8.6 and 11.2 micron, linked to polycyclic aromatichydrocarbons (PAHs), the nanodiamond-related features at 3.4 and 3.5micron, the amorphous 10 micron silicate band and the crystallinesilicate band at 11.3 micron. We have detected PAH emission in 57% ofthe Herbig stars in our sample. Although for most of these sources thePAH spectra are similar, there are clear examples of differences in thePAH spectra within our sample which can be explained by differences inPAH size, chemistry and/or ionization. Amorphous silicate emission wasdetected in the spectra of 52% of the sample stars, amorphous silicateabsorption in 13%. We have detected crystalline silicate emission in 11stars (24% of our sample), of which four (9%) also display strong PAHemission. We have classified the sample sources according to thestrength of their mid-IR energy distribution. The systems with strongermid-infared (20-100 μm) excesses relative to their near-infrared (1-5μm) excess display significantly more PAH emission than those withweaker mid-infrared excesses. There are no pronounced differences in thebehaviour of the silicate feature between the two groups. This providesstrong observational support for the disk models by \citet{dullemond01},in which systems with a flaring disk geometry display a strongmid-infrared excess, whereas those with disks that are strongly shadowedby the puffed-up inner rim of the disk only display modest amounts ofmid-infrared emission. Since the silicates are expected to be producedmainly in the warm inner disk regions, no large differences in silicatebehaviour are expected between the two groups. In contrast to this, thePAH emission is expected to be produced mainly in the part of the diskatmosphere that is directly exposed to radiation from the central star.In this model, self-shadowed disks should display weaker PAH emissionthan flared disks, consistent with our observations.Based on observations with ISO, an ESA project with instruments fundedby ESA Member States (especially the PI countries: France, Germany, TheNetherlands and the United Kingdom) and with the participation of ISASand NASA.Tables 1, 3-6 and Appendix are only available in electronic form athttp://www.edpsciences.org
| Evolution of irradiated circumbinary disks
We study the evolution and emission of circumbinary disks around closeclassical T Tauri binary systems. High resolution numericalhydrodynamical simulations are employed to model a system consisting ofa central eccentric binary star within an irradiated accretion disk. Adetailed energy balance including viscous heating, radiative cooling andirradiation from the central star is applied to calculate accurately theemitted spectral energy distribution.Numerical simulations using two different methods, the previouslydeveloped Dual-Grid technique with a finite difference discretization,and the Smoothed Particle Hydrodynamics method are employed to comparethe hydrodynamical features and strengthen our conclusions.Physical parameters of the setup are chosen to model the close systemsof DQ Tau and AK Sco. Using theself-consistent models, we are able to fit the observed spectral energydistributions by constraining parameters such as disk mass, densityprofile and radial extension for those systems. We find that theincorporation of irradiation effects is necessary to obtain correct disktemperatures.
| Correlation between grain growth and disk geometry in Herbig Ae/Be systems
We have calculated the (sub-)mm spectral indices of 26 Herbig Ae/Bestars, for which we can determine the infrared spectral energydistribution (SED). We find a clear correlation between the strength ofthe ratio of the near- to mid-infrared excess of these sources, and theslope of the (sub-)mm energy distribution. Based on earliermulti-dimensional modeling of disks around Herbig Ae stars, we interpretthis as a correlation between the geometry of the disk (flared orself-shadowed) and the size of the grains: self-shadowed disks have, onaverage, larger grains than their flared counterparts. These datasuggest that the geometry of a young stellar disk evolves from flared toself-shadowed.
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Observation and Astrometry data
| Constellation: | Scorpius |
| Right ascension: | 16h54m44.85s |
| Declination: | -36°53'18.6" |
| Apparent magnitude: | 9.166 |
| Proper motion RA: | -8.4 |
| Proper motion Dec: | -29.5 |
| B-T magnitude: | 10.047 |
| V-T magnitude: | 9.239 |
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INHABITED SKY
