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Unraveling the Origins of Nearby Young Stars
A systematic search for close conjunctions and clusterings in the pastof nearby stars younger than the Pleiades is undertaken, which mayreveal the time, location, and mechanism of formation of these oftenisolated, disconnected from clusters and star-forming regions, objects.The sample under investigation includes 101 T Tauri, post-TT, andmain-sequence stars and stellar systems with signs of youth, culled fromthe literature. Their Galactic orbits are traced back in time and nearapproaches are evaluated in time, distance, and relative velocity.Numerous clustering events are detected, providing clues to the originof very young, isolated stars. Each star's orbit is also matched withthose of nearby young open clusters, OB and TT associations andstar-forming molecular clouds, including the Ophiuchus, Lupus, CoronaAustralis, and Chamaeleon regions. Ejection of young stars from openclusters is ruled out for nearly all investigated objects, but thenearest OB associations in Scorpius-Centaurus, and especially, the denseclouds in Ophiuchus and Corona Australis have likely played a major rolein the generation of the local streams (TWA, Beta Pic, andTucana-Horologium) that happen to be close to the Sun today. The core ofthe Tucana-Horologium association probably originated from the vicinityof the Upper Scorpius association 28 Myr ago. A few proposed members ofthe AB Dor moving group were in conjunction with the coeval Cepheus OB6association 38 Myr ago.
| Transience of Hot Dust around Sun-like Stars
In this paper a simple model for the steady state evolution of debrisdisks due to collisions is developed and confronted with the propertiesof the emerging population of seven Sun-like stars that have hot dust at<10 AU. The model shows that there is a maximum possible disk mass ata given age, since more massive primordial disks process their massfaster. The corresponding maximum dust luminosity isfmax=0.16×10-3r7/3t-1age,where r is disk radius in AU and tage is system age in Myr.The majority (4/7) of the hot disks exceed this limit by >>1000and so cannot be the products of massive asteroid belts; rather, thefollowing systems must be undergoing transient events characterized byan unusually high dust content near the star: η Corvi, HD 69830, HD72905, and BD +20 307. It is also shown that the hot dust cannotoriginate in a recent collision in an asteroid belt, since there is alsoa maximum rate at which collisions of sufficient magnitude to reproducea given dust luminosity can occur. The planetesimal belt feeding thedust in these systems must be located farther from the star than thedust, typically at >>2 AU. Other notable properties of the fourhot dust systems are as follows: two also have a planetesimal belt at>10 AU (η Corvi and HD 72905); one has three Neptune mass planetsat <1 AU (HD 69830); all exhibit strong mid-IR silicate features. Weconsider the most likely origin for this transient dust to be adynamical instability that scattered planetesimals inward from a moredistant planetesimal belt in an event akin to the late heavy bombardmentin our own system, the dust being released from such planetesimals incollisions and sublimation.
| Deficit of Wide Binaries in the η Chamaeleontis Young Cluster
We have carried out a sensitive high-resolution imaging survey of starsin the young (6-8 Myr), nearby (97 pc) compact cluster around ηChamaeleontis to search for stellar and substellar companions. Our datawere obtained using the NACO adaptive optics system on the ESO VeryLarge Telescope (VLT). Given its youth and proximity, any substellarcompanions are expected to be luminous, especially in the near-infrared,and thus easier to detect next to their parent stars. Here, we presentVLT NACO adaptive optics imaging with companion detection limits for 17η Cha cluster members, and follow-up VLT ISAAC near-infraredspectroscopy for companion candidates. The widest binary detected is~0.2", corresponding to the projected separation 20 AU, despite oursurvey being sensitive down to substellar companions outside 0.3", andplanetary-mass objects outside 0.5". This implies that the stellarcompanion probability outside 0.3" and the brown dwarf companionprobability outside 0.5" are less than 0.16 with 95% confidence. Wecompare the wide binary frequency of η Cha to that of the similarlyaged TW Hydrae association and estimate the statistical likelihood thatthe wide binary probability is equal in both groups to be less than2×10-4. Even though the η Cha cluster is relativelydense, stellar encounters in its present configuration cannot accountfor the relative deficit of wide binaries. We thus conclude that thedifference in wide binary probability in these two groups providesstrong evidence for multiplicity properties being dependent onenvironment. In two appendices we derive the projected separationprobability distribution for binaries, used to constrain physicalseparations from observed projected separations, and summarizestatistical tools useful for multiplicity studies.
| On the age of the TW Hydrae association and 2M1207334-393254
Aims.We have estimated the age of the young moving group TW HydraeAssociation, a cohort of a few dozen stars and brown dwarfs located nearthe Sun which share the same kinematic properties and, presumably, thesame origin and age. Methods: .The chronology has been determinedby analyzing different properties (magnitudes, colors, activity,lithium) of its members and comparing them with several well-known starforming regions and open clusters, as well as theoretical models. Inaddition, by using medium-resolution optical spectra of two M8 membersof the association (2M1139 and 2M1207 - an accreting brown dwarf with aplanetary mass companion), we have derived spectral types and measuredHα and lithium equivalent widths. We have also estimated theireffective temperature and gravity, which were used to produce anindependent age estimation for these two brown dwarfs. We have alsocollected spectra of 2M1315, a candidate member with a L5 spectral typeand measured its Hα equivalent width. Results: .Our ageestimate for the association, 10+10-7 Myr, agreeswith previous values cited in the literature. In the case of the twobrown dwarfs, we have derived an age of 15+15-10Myr, which also agree with our estimate for the whole group.Conclusions: .We compared our results with recent articles published onthe same subject using other techniques, and discuss the limits of theage-dating techniques.
| Optical polarimetry of infrared excess stars
We present UBRVI polarimetry measurements for a group of 38 IRASinfrared excess stars and complement these observations with V-band datataken from the literature for 87 additional objects. After correctingthe observed values by the interstellar contribution, we find that 48%of the analyzed sample has polarization excess. In addition, thepolarization of these stars may correlate with infrared color excesses,particularly at 60 and 100 μm. We caution, however, that poor IRASdata quality at longer wavelengths affects this correlation. We analyzethe wavelength dependence of the linear polarization of 15 polarizedobjects in relation to Serkowski's empirical interstellar law. We findthat for 6 to 7 objects (depending on the interstellar model) themeasured polarization differs significantly from the empiricalinterstellar law, suggesting an intrinsic origin. We analyze thepolarimetry distribution of IRAS infrared excess objects in relation tothe Exoplanet host stars (i.e., stars associated with at least onelikely planetary mass object). The corresponding polarimetrydistributions are different within a high confidence level. Finally, wecompare the metallicity distributions of F and G IRAS infrared excess,Exoplanet host and field main sequence stars, and find that F-G IRASinfrared excess objects have metallicities quite similar (although notidentical) to field main sequence stars and significantly different fromthe Exoplanet host group.
| Multi-aperture photometry of extended IR sources with ISOPHOT. I. The nature of extended IR emission of planetary Nebulae
Context: .ISOPHOT multi-aperture photometry is an efficient method toresolve compact sources or to detect extended emission down torelatively faint levels with single detectors in the wavelength range 3to 100 μm. Aims: .Using ISOPHOT multi-aperture photometry andcomplementary ISO spectra and IR spectral energy distributions wediscuss the nature of the extended IR emission of the two PNe NGC 6543and NGC 7008. Methods: .In the on-line appendix we describe thedata reduction, calibration and interpretation methods based on asimultaneous determination of the IR source and background contributionsfrom the on-source multi-aperture sequences. Normalized profiles enabledirect comparison with point source and flat-sky references. Modellingthe intensity distribution offers a quantitative method to assess sourceextent and angular scales of the main structures and is helpful inreconstructing the total source flux, if the source extends beyond aradius of 1 arcmin. The photometric calibration is described and typicalaccuracies are derived. General uncertainty, quality and reliabilityissues are addressed, too. Transient fitting to non-stabilised signaltime series, by means of combinations of exponential functions withdifferent time constants, improves the actual average signals andreduces their uncertainty. Results: .The emission of NGC 6543 inthe 3.6 μm band coincides with the core region of the optical nebulaand is homogeneously distributed. It is comprised of 65% continuum and35% atomic hydrogen line emission. In the 12 μm band a resolved butcompact double source is surrounded by a fainter ring structure with allemission confined to the optical core region. Strong line emission of[ArIII] at 8.99 μm and in particular [SIV] at 10.51 μm shapes thisspatial profile. The unresolved 60 μm emission originates from dust.It is described by a modified (emissivity index β = 1.5) blackbodywith a temperature of 85 K, suggesting that warm dust with a mass of 6.4× 10-4 Mȯ is mixed with the ionisedgas. The gas-to-dust mass ratio is about 220. The 25 μm emission ofNGC 7008 is characterised by a FWHM of about 50´´ with anadditional spot-like or ring-like enhancement at the bright rim of theoptical nebula. The 60 μm emission exhibits a similar shape, but isabout twice as extended. Analysis of the spectral energy distributionsuggests that the 25 μm emission is associated with 120 K warm dust,while the 60 μm emission is dominated by a second dust component with55 K. The dust mass associated with this latter component amounts to 1.2× 10-3 Mȯ, significantly higher thanpreviously derived. The gas-to-dust mass ratio is 59 which, compared tothe average value of 160 for the Milky Way, hints at dust enrichment bythis object.
| Dynamical Evolution of the TW Hydrae Association
Using Galactic dynamics we have determined the age of the low-masspost-T Tauri stars in the TW Hya Association (TWA). To do so we appliedthe method of Ortega and coworkers to five stars of the association withHipparcos-measured distances (TWA 1, TWA 4, TWA 9, TWA 11, and TWA 19).The method is based on the calculation of the past three-dimensionalorbits of the stars. Of these stars, only TWA 9 presents a quitedifferent orbit so that it does not appear to be a dynamical member ofthe TWA. The four remaining stars have their first maximum orbitalconfinement at the age of -8.3+/-0.8 Myr, which is considered thedynamical age of the TWA. This confinement fixes the probablethree-dimensional forming region of the TWA within a mean radius of 14.5pc. This region is related to the older subgroups of the Sco-Cen OBassociation, Lower Centaurus Crux and Upper Centaurus Lupus, both with amean age of about 18 Myr. This dynamical age of the TWA and that of theβ Pic Moving Group, 11 Myr, also discussed here, introduce a moreprecise temporal scale for studies of disk evolution and planetaryformation around some stars of these associations. Using the retracedorbit of the runaway star HIP 82868 we examine the possibility that theformation of the TWA was triggered by a supernova explosion. It is shownthat for the four considered TWA stars, the expansion in volume is afactor of 5 from their origin to the present state. This is mainly dueto the currently more distant star TWA 19.
| Spitzer Observations of Two TW Hydrae Association Brown Dwarfs
We present Spitzer Space Telescope observations of two TW Hydraeassociation brown dwarfs, 2MASSW J1207334-393254 and 2MASSWJ1139511-315921, in the IRAC and MIPS 24 μm bands. On the basis oftheir IRAC colors, we have classified them as classical and weak-line TTauri stars, respectively. For 2MASSW J1207334-393254, we have foundthat a flat-disk model fits the data very well. This brown dwarf showsthe presence of warm (T>~100 K) circumstellar dust close (R<~0.2AU) to it and does not display any signs of cleansing of dust withinseveral AU of the star. In comparison with other TWA members that showexcess in IR, we suggest that there exists a different diskevolution/dust processing mechanism for stellar and substellar objects.The star 2MASSW J1139511-315921 does not show any significant excess inany of the IRAC bands but a small one at 24 μm, which is notsignificant enough to suggest the presence of warm dust around thisstar. It shows signs of dust cleansing in the inner several AU, similarto most of the other TWA members.
| Near-infrared imaging polarimetry of dusty young stars
We have carried out JHK polarimetric observations of 11 dusty youngstars, by using the polarimeter module IRPOL2 with the near-infraredcamera UIST on the 3.8-m United Kingdom Infrared Telescope (UKIRT). Oursample targeted systems for which UKIRT-resolvable discs had beenpredicted by model fits to their spectral energy distributions. Ourobservations have confirmed the presence of extended polarized emissionaround TW Hya and around HD 169142. HD 150193 and HD 142666 show thelargest polarization values among our sample, but no extended structurewas resolved. By combining our observations with Hubble Space Telescope(HST) coronographic data from the literature, we derive the J- andH-band intrinsic polarization radial dependences of the disc of TW Hya.We find the polarizing efficiency of the disc is higher at H than at J,and we confirm that the J- and H-band percentage polarizations arereasonably constant with radius in the region between 0.9 and 1.3arcsecfrom the star. We find that the objects for which we have detectedextended polarizations are those for which previous modelling hassuggested the presence of flared discs, which are predicted to bebrighter than flat discs and thus would be easier to detectpolarimetrically.
| The Formation and Evolution of Planetary Systems (FEPS): Discovery of an Unusual Debris System Associated with HD 12039
We report the discovery of a debris system associated with the ~30 Myrold G3/5V star HD 12039 using Spitzer Space Telescope observations from3.6-160 μm. An observed infrared excess(LIR/L*=1×10-4) above theexpected photosphere for λ>~14 μm is fit by thermallyemitting material with a color temperature of T~110 K, warmer than themajority of debris disks identified to date around Sun-like stars. Theobject is not detected at 70 μm with a 3 σ upper limit 6 timesthe expected photospheric flux. The spectrum of the infrared excess canbe explained by warm, optically thin material comprised ofblackbody-like grains of size >~7 μm that reside in a beltorbiting the star at 4-6 AU. An alternate model dominated by smallergrains, near the blowout size a~0.5 μm, located at 30-40 AU is alsopossible but requires the dust to have been produced recently, sincesuch small grains will be expelled from the system by radiation pressurein approximately a few times 102 yr.
| Dynamical Masses for Low-Mass Pre-Main-Sequence Stars: A Preliminary Physical Orbit for HD 98800 B
We report on Keck Interferometer observations of the double-lined binary(B) component of the quadruple pre-main-sequence (PMS) system HD 98800.With these interferometric observations, combined with astrometricmeasurements made by the Hubble Space Telescope (HST) Fine GuidanceSensors (FGS) and published radial velocity observations, we haveestimated preliminary visual and physical orbits of the HD 98800 Bsubsystem. Our orbit model calls for an inclination of 66.8d+/-3.2d andallows us to infer the masses and luminosities of the individualcomponents. In particular we find component masses of 0.699+/-0.064 and0.582+/-0.051 Msolar for the Ba (primary) and Bb (secondary)components, respectively. Spectral energy distribution (SED) modeling ofthe B subsystem suggests that the B circumstellar material is a sourceof extinction along the line of sight to the B components. This seems tocorroborate a conjecture by Tokovinin that the B subsystem is viewedthrough circumbinary material, but it raises important questions aboutthe morphology of that circumbinary material. Our modeling of thesubsystem component SEDs finds temperatures and luminosities inagreement with previous studies, and coupled with the component massestimates allows for comparison with PMS models in the low-mass regimewith few empirical constraints. Solar abundance models seem tounderpredict the inferred component temperatures and luminosities, whileassuming slightly subsolar abundances brings the models and observationsinto better agreement. The current preliminary orbit does not yet placesignificant constraints on existing PMS stellar models, but prospectsfor additional observations improving the orbit model and componentparameters are very good.
| A Moving Cluster Distance to the Exoplanet 2M1207b in the TW Hydrae Association
A candidate extrasolar planet companion to the young brown dwarf 2MASSWJ1207334-393254 (hereafter 2M1207) was recently discovered by Chauvin etal. They find that the temperature and luminosity of 2M1207b areconsistent with a young, ~5MJ planet. The 2M1207 system ispurported to be a member of the TW Hya association (TWA) and situated~70 pc away. Using a revised space motion vector for TWA and improvedproper motion for 2M1207, I use the moving cluster method to estimatethe distance to the 2M1207 system and other TWA members. The deriveddistance for 2M1207 (53+/-6 pc) forces the brown dwarf and planet to behalf as luminous as previously thought. The inferred masses for 2M1207Aand 2M1207b decrease to ~21 and ~3-4MJ, respectively, withthe mass of 2M1207b well below the observed tip of the planetary massfunction and the theoretical deuterium-burning limit. After removingprobable Lower Centaurus Crux (LCC) members from the TWA sample, as wellas the probable nonmember TWA 22, the remaining TWA membership is foundto have distances of 49+/-3 (s.e.m.)+/-12 (1 σ) pc and an internalone-dimensional velocity dispersion of 0.8+0.3-0.2km s-1. There is weak evidence that the TWA is expanding, andthe data are consistent with a lower limit on the expansion age of 10Myr (95% confidence).
| Exploring Terrestrial Planet Formation in the TW Hydrae Association
Spitzer Space Telescope infrared measurements are presented for 24members of the TW Hya association (TWA). High signal-to-noise ratio 24μm photometry is presented for all these stars, including 20 starsthat were not detected by IRAS. Among these 20 stars, only a singleobject, TWA 7, shows excess emission at 24 μm at the level of only40% above the star's photosphere. TWA 7 also exhibits a strong 70 μmexcess that is a factor of 40 brighter than the stellar photosphere atthis wavelength. At 70 μm, an excess of similar magnitude is detectedfor TWA 13, although no 24 μm excess was detected for this binary.For the 18 stars that failed to show measurable IR excesses, thesensitivity of the current 70 μm observations does not rule outsubstantial cool excesses at levels 10-40 times above their stellarcontinua. Measurements of two T Tauri stars, TW Hya and Hen 6-300,confirm that their spectacular IR spectral energy distributions (SEDs)do not turn over even by 160 μm, consistent with the expectation fortheir active accretion disks. In contrast, the Spitzer data for theluminous planetary debris systems in the TWA, HD 98800B and HR 4796A,are consistent with single-temperature blackbody SEDs and agree withprevious IR, submillimeter, and millimeter measurements. The major newresult of this study is the dramatic bimodal distribution found for theassociation in the form of excess emission at a wavelength of 24 μm,indicating negligible amounts of warm (>~100 K) dust and debrisaround 20 of 24 stars in this group of very young stars. This bimodaldistribution is especially striking given that the four stars in theassociation with strong IR excesses are >~100 times brighter at 24μm than their photospheres. Clearly, two terrestrial planetarysystems, HD 98800B and HR 4796A, exist in some form. In addition, thereare at least two active accreting objects, TW Hya and Hen 6-300, thatmay still be forming planetesimals. The remaining stars may possesssignificant amounts of cold dust, as in TWA 7 and 13, that have yet tobe found.
| An Infrared Coronagraphic Survey for Substellar Companions
We have used the F160W filter (1.4-1.8 μm) and the coronagraph on theNear-Infrared Camera and Multi-Object Spectrometer (NICMOS) on theHubble Space Telescope to survey 45 single stars with a median age of0.15 Gyr, an average distance of 30 pc, and an average H magnitude of 7mag. For the median age we were capable of detecting a 30MJcompanion at separations between 15 and 200 AU. A 5MJ objectcould have been detected at 30 AU around 36% of our primaries. Forseveral of our targets that were less than 30 Myr old, the lower masslimit was as low as 1MJ, well into the high mass planetregion. Results of the entire survey include the proper-motionverification of five low-mass stellar companions, two brown dwarfs(HR7329B and TWA5B), and one possible brown dwarf binary (Gl 577B/C).
| Expansion of the TW Hydrae association and the encounter with Vega
We investigate the paths of several probable members of the youngassociation around the star TW Hydrae (TWA) with accurate distances,proper motions and radial velocities. We find that three of thepreviously identified members, TWA 1, TWA 4 and TWA 11, together withtwo other young nearby stars, HD 139084 and HD 220476, form a rapidlyexpanding association with an expansion age of 4.7 +/- 0.6 Myr. Initialvelocities of member stars with respect to the common centre of massrange from 4 to 10km s-1. A characteristic size of theassociation in the initial configuration is 21 pc, which may be somewhatbiased upwards due to the uncertainties in the observational data. TheLower Centaurus Crux (LCC) OB association passed near TWA, at a distanceof 36 +/- 6 pc, 11 Myr ago. A plausible scenario, which accounts for thedifference between the isochrone age (~=10 Myr) and expansion age (5Myr), is that star formation was stimulated in the TWA progenitor cloudby the near passage of the LCC, but that the newly formed stars were notreleased from the cloud until a subsequent collision with one of theother molecular clouds in the North Ophiuchus region. Vega was insidethe TWA association, and close to its centre of gravity, at the time ofmaximum compression 4.7 Myr ago. If this alignment is a chanceencounter, the powerful particular disc around Vega could have beenenhanced by the passage through the TWA progenitor cloud at 8 kms-1.
| XMM-Newton spectroscopy of the metal depleted T Tauri star TWA 5
We present results of X-ray spectroscopy for TWA 5, amember of the young TW Hydrae association, observed with XMM-Newton.TWA 5 is a multiple system which shows Hαemission, a signature typical of classical T Tauri stars, but noinfrared excess. From this analysis of the RGS and EPIC spectra, we havederived the emission measure distribution vs. temperature of the X-rayemitting plasma, its abundances, and the electron density. Thecharacteristic temperature and density of the plasma suggest a coronasimilar to that of weak-line T Tauri stars and active late-type mainsequence stars. TWA 5 also shows low iron abundance(~0.1 times the solar photospheric one) and a pattern of increasingabundances for elements with increasing first ionization potentialreminiscent of the inverse FIP effect observed in highly active stars.The especially high ratio Ne/Fe10 is similar to that of theclassical T Tauri star TW Hya, where the accretingmaterial has been held responsible for the X-ray emission. We discussthe possible role of an accretion process in this scenario. Since all TTauri stars in the TW Hydrae association studied so far have very highNe/Fe ratios, we also propose that environmental conditions may causethis effect.
| Extreme collisions between planetesimals as the origin of warm dust around a Sun-like star
The slow but persistent collisions between asteroids in our Solar Systemgenerate a tenuous cloud of dust known as the zodiacal light (because ofthe light the dust reflects). In the young Solar System, such collisionswere more common and the dust production rate should have been manytimes larger. Yet copious dust in the zodiacal region around stars muchyounger than the Sun has rarely been found. Dust is known to orbitaround several hundred main-sequence stars, but this dust is cold andcomes from a Kuiper-belt analogous region out beyond the orbit ofNeptune. Despite many searches, only a few main-sequence stars revealwarm (> 120K) dust analogous to zodiacal dust near the Earth. Signsof planet formation (in the form of collisions between bodies) in theregions of stars corresponding to the orbits of the terrestrial planetsin our Solar System have therefore been elusive. Here we report anexceptionally large amount of warm, small, silicate dust particlesaround the solar-type star BD+20 307 (HIP8920, SAO75016). Thecomposition and quantity of dust could be explained by recent frequentor huge collisions between asteroids or other `planetesimals' whoseorbits are being perturbed by a nearby planet.
| X-Ray Diagnostics of Grain Depletion in Matter Accreting onto T Tauri Stars
Recent analysis of high-resolution Chandra X-ray spectra has shown thatthe Ne/O abundance ratio is remarkably constant in stellar coronae.Based on this result, we point out the utility of the Ne/O ratio as adiscriminant for accretion-related X-rays from T Tauri stars and forprobing the measure of grain depletion of the accreting material in theinner disk. We apply the Ne/O diagnostic to the classical T Tauri starsBP Tau and TW Hya-the two stars found to date whose X-ray emissionappears to originate, at least in part, from accretion activity. We showthat TW Hya appears to be accreting material that is significantlydepleted in O relative to Ne. In contrast, BP Tau has an Ne/O abundanceratio consistent with that observed for post-T Tauri stars. We interpretthis result in terms of the different ages and evolutionary states ofthe circumstellar disks of these stars. In the young BP Tau disk (age~0.6 Myr), dust is still present near the disk corotation radius and canbe ionized and accreted, rereleasing elements depleted onto grains. Inthe more evolved TW Hya disk (age ~10 Myr), evidence points to ongoingcoagulation of grains into much larger bodies, and possibly planets,that can resist the drag of inward-migrating gas, and the accreting gasis consequently depleted of grain-forming elements.
| CO emission from discs around isolated HAeBe and Vega-excess stars
We describe results from a survey for J = 3-2 12CO emissionfrom visible stars classified as having an infrared excess. The line isclearly detected in 21 objects, and significant molecular gas(>=10-3 Jupiter masses) is found to be common in targetswith infrared excesses >=0.01 (>=56 per cent of objects), but rarefor those with smaller excesses (~10 per cent of objects).A simple geometrical argument based on the infrared excess implies thatdisc opening angles are typically >=12° for objects with detectedCO; within this angle, the disc is optically thick to stellar radiationand shields the CO from photodissociation. Two or three CO discs have anunusually low infrared excess (<=0.01), implying the shielding discis physically very thin (<=1°).Around 50 per cent of the detected line profiles are double-peaked,while many of the rest have significantly broadened lines, attributed todiscs in Keplerian rotation. Simple model fits to the line profilesindicate outer radii in the range 30-300 au, larger than found throughfitting continuum SEDs, but similar to the sizes of debris discs aroundmain-sequence stars. As many as five have outer radii smaller than theSolar System (50 au), with a further four showing evidence of gas in thedisc at radii smaller than 20 au. The outer disc radius is independentof the stellar spectral type (from K through to B9), but there isevidence of a correlation between radius and total dust mass. Also themean disc size appears to decrease with time: discs around stars of age3-7 Myr have a mean radius ~210 au, whereas discs of age 7-20 Myr are afactor of three smaller. This shows that a significant mass of gas (atleast 2 M⊕) exists beyond the region of planetformation for up to ~7 Myr, and may remain for a further ~10Myr withinthis region.The only bona fide debris disc with detected CO is HD9672; this shows adouble-peaked CO profile and is the most compact gas disc observed, witha modelled outer radius of 17 au. In the case of HD141569, detailedmodelling of the line profile indicates gas may lie in two rings, withradii of 90 and 250 au, similar to the dust structure seen in scatteredlight and the mid-infrared. In both AB Aur and HD163296 we also findthat the sizes of the molecular disc and the dust scattering disc aresimilar; this suggests that the molecular gas and small dust grains areclosely co-located.
| HD 34700 is a T Tauri multiple system
We have imaged the young double-lined spectroscopic binary HD 34700 inthe near-infrared and find evidence for two faint, visual, stellarcomponents at 5.2 Arcsec and 9.2 arcsec distance. High-resolutionechelle spectroscopy of both stars shows strong Li I 6708 Åabsorption and Hα emission. The spectral types of the companionsare estimated from the spectra and photometry as M1-M2 and M2-M3. Theirradial velocities are similar to the center-of-mass velocity of thecentral SB2; hence all four stars are most probably physically bound andconstitute a young quadruple stellar system with an inner short-periodbinary. We provide a list of pre-main sequence spectroscopic binarieswith additional components of which HD 34700 is yet another example. Theavailable statistics strengthen the suspicion that dynamical effects inmultiple systems play a key role in the formation of very close binariesearly in their evolution.
| 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.
| Warm gas in the cold diffuse interstellar medium: Spectral signatures in the H2 pure rotational lines
We present ISO-SWS observations of five pure rotational lines ofH2 along a line of sight through the Galaxy which avoidsregions of massive star formation. It samples 30 mag of gas, half of it(i.e. 15 mag) being diffuse gas running from the solar neighbourhood tothe molecular ring, up to the far side of the Galaxy. The intensities ofthe S(1) and S(2) lines are too large relative to S(0) to be produced byUV excitation in the known radiation field of the Galaxy. The excitationof these transitions has to tap a more powerful source of energy. Weinvestigate the possibility that it takes place in a large number ofmagneto-hydrodynamic (MHD) shocks or coherent small-scale vortices, twoprocesses responsible for the intermittent dissipation of MHDturbulence. These dissipation bursts locally and temporarily heat thediffuse gas to temperatures (Tk 103 K) wellabove that of the ambient diffuse gas. We compute the spectroscopicsignatures of these processes in the H2 lines. Not only arethe computed relative line intensities in good agreement with theobservations, but the few percent of warm gas involved is consistentwith other independent determinations. We find that the fraction of warmH2 in the diffuse gas (i.e. H2 molecules inJu ≥ 3 levels) on that line of sight,N(H2*)/Av ≈ 4 ×1017 cm-2 mag-1, is the same as thatfound from far UV spectroscopy in the direction of nearby stars. It isalso the same as that estimated in the solar neighbourhood to reproducethe large observed abundances of molecules like CH+. Theseresults suggest that the existence, within the cold neutral medium(CNM), of a few percent of warm gas, for which UV photons cannot be thesole heating source, is ubiquitous and presumably traces theintermittent dissipation of MHD turbulence in the cold diffuse gas.
| Rotation periods for stars of the TW Hydrae association: the evidence for two spatially and rotationally distinct pre-main-sequence populations
We have conducted a photometric study of late-type members of the TWHydrae association (TWA) and measured the rotation periods for 16 starsin 12 systems. For TWA stars listed by Webb et al. and Sterzik et al.(TWA 1-13 led by TW Hya = TWA 1) we find a median period of 4.7 d.However, for stars that we measured in the TWA 14-19 group identified byZuckerman et al., we find a median period of only 0.7 d. The perioddistributions of the two groups cannot be reconciled at the 3σsignificance level. Using photometric arguments supported by theHipparcos distance to HD 102458 (= TWA 19A), we find that TWA 14-19reside at an average distance of d~ 90 pc, spatially at the nearboundary of the Lower Centaurus-Crux (LCC) subgroup of theOphiuchus-Scorpius-Centaurus OB-star association. Proper motions for HD102458, TWA 14, 18 and 19B link these stars to the LCC subgroup. FromHertzsprung-Russell diagram placement, we derive an age of ~17 Myr forthe HD 102458 system that may be the representative age for the TWA14-19 group. Merging various lines of evidence, we conclude that thesestars form a spatially and rotationally distinct population of olderpre-main-sequence (PMS) stars, rather than being an extension of the TWAbeyond those stars associated with TW Hya that have an age of ~10 Myrand reside at d~ 55 pc. Instead, TWA 14-19 likely represent thepopulation of low-mass stars still physically associated with the LCCsubgroup.
| Mid-IR observations of circumstellar disks. I. Pre-main sequence objects
We present new N-band photometry and spectroscopy for a sample of eightpre-main sequence stars including T Tauri, Herbig Ae/Be stars and FU Oriobjects using the ESO TIMMI2 camera at the La Silla observatory (Chile).For some objects this is their first N-band spectroscopic observationever. The FU Ori stars V 346 Nor, V 883 Ori and Z CMa show a broadabsorption band which we attribute to silicates, while for BBW 76 wefind silicate emission. A comparison with ISO-SWS spectra of V 346 Norand Z CMa taken in 1996/1997 reveals no differences in spectral shape.All T Tauri and Herbig Ae/Be stars possess N-band emission features. Wemodel the emission spectra with a mixture of silicates consisting ofdifferent grain sizes and composition. The Herbig Ae star HD 34282 showsstrong features of PAHs but none of silicate, while the emissionspectrum of the Herbig Ae star HD 72106 resembles those of solar-systemcomets and known Herbig sources of evolved dust. We demonstrate that HD72106 is host to highly processed silicates and find evidence forenstatite, which is not common in young objects. Evolved dust is alsoseen in the T Tauri stars HD 98800 and MP Mus. We further detected MPMus at 1200 μm with the bolometer array SIMBA at the SEST in LaSilla. The findings of our analysis are given in the context of previousdust studies of young stellar objects.Based on observations collected at the European Southern Observatory, LaSilla, Chile (69.C-0073, 70.C-0468, 71.C-0001, 73.C-0372).
| Radial Distribution of Dust Grains around HR 4796A
We present high dynamic range images of circumstellar dust around HR4796A that were obtained with MIRLIN at the Keck II telescope atλ=7.9, 10.3, 12.5, and 24.5 μm. We also present a newcontinuum measurement at 350 μm obtained at the Caltech SubmillimeterObservatory. Emission is resolved in Keck images at 12.5 and 24.5 μmwith point-spread function FWHM values of 0.37" and 0.55", respectively,and confirms the presence of an outer ring centered at 70 AU. Unresolvedexcess infrared emission is also detected at the stellar position andmust originate well within 13 AU of the star. A model of dust emissionfitted to flux densities at 12.5, 20.8, and 24.5 μm indicates thatdust grains are located 4+3-2 AU from the starwith effective size 28+/-6 μm and an associated temperature of260+/-40 K. We simulate all extant data with a simple model ofexozodiacal dust and an outer exo-Kuiper ring. A two-component outerring is necessary to fit both Keck thermal infrared and Hubble SpaceTelescope scattered-light images. Bayesian parameter estimates yield atotal cross-sectional area of 0.055 AU2 for grains roughly 4AU from the star, and an outer-dust disk composed of a narrowlarge-grain ring embedded within a wider ring of smaller grains. Thenarrow ring is 14+/-1 AU wide with inner radius 66+/-1 AU and totalcross-sectional area 245 AU2. The outer ring is 80+/-15 AUwide with inner radius 45+/-5 AU and total cross-sectional area 90AU2. Dust grains in the narrow ring are about 10 times largerand have lower albedos than those in the wider ring. These propertiesare consistent with a picture in which radiation pressure dominates thedispersal of an exo-Kuiper belt.
| SSSPM J1102-3431: A probable new young brown dwarf member of the TW Hydrae Association
We have used archival survey data, most importantly from the SuperCOSMOSSky Surveys, to derive accurate proper motions and thus confirm themembership of two previously announced isolated brown dwarfs in thenearby, ˜10 Myr old TW Hydrae Association (TWA), 2MASSWJ1207334-393254 and 2MASSW 1139511-315921 (Gizis \cite{gizis02}), and todiscover a new substellar candidate. This new object, SSSPM J1102-3431,has extremely red optical, optical-to-infrared, and near-infraredcolours (R-I=3.2, I-J=3.4, J-Ks=1.15), while low-resolutionnot only classifies it as a late-type (M8.5) object, but also showsspectral signatures of low gravity as expected for a young brown dwarf.If it is a true TWA member, brown dwarf cooling models predict its massto be ˜ 20 MJup. SSSPM J1102-3431 lies only ˜12arcmin away from the T Tauri star TW Hya and their proper motions agreeto within the errors, and thus it is possible that the two objects forman extremely wide binary system, with a separation of ˜40 000 AUat d=56 pc. We have also obtained improved proper motions for SSS120727.4-324700 and SSS 101726.7-535428, recent additions to the list ofTWA members (Song et al. \cite{song03}). The relatively large propermotion of the latter object is consistent with the suggested proximity(d˜22 pc) of this lowest-mass stellar representative of the TWA.Based on archival data from the SuperCOSMOS Sky Surveys, 2MASS, andDENIS, and on spectroscopic observations with the ESO 3.6-m telescope(ESO 072.C-0630).
| High-resolution x-ray spectroscopy of the accreting weak-line T Tauri star DoAr 21
High-resolution grating spectroscopy of young stars can reveal physicalconditions associated with these stars' x-ray emission, providinginformation about stellar activity, dynamo mechanisms, and the effectsof x-rays on disks during the planet formation epoch. Unfortunately,most very young stars are too faint in x-rays to yield high-qualitygrating spectra. We present a high-resolution Chandra grating spectrumof the low-mass pre-main-sequence star DoAr 21, one of the brightestx-ray sources in the ρ Oph region and, at an age of 1--3 Myr, one ofthe youngest stars for which a high-resolution x-ray spectrum has beenobtained.The x-ray spectrum of DoAr 21 shows substantial flaring and is veryhard, with strong continuum and relatively weak lines. The entireobservation is well-fit with a two-temperature thermal model withcomponents at 1.5 keV (T ≈ 17 MK) and 5.5 keV (T ≈ 64 MK). Duringa large flare, the hot component increased in temperature from 2.8 keVto 5.8 keV in less than two hours.The strongest line in the spectrum (Si XIV 6.18 Å) is marginallyresolved, with a FWHM of 450 ± 120 km/s. The other strong linesin the spectrum are consistent with having similarly broad widths, butgiven the noise level, zero broadening cannot be ruled out.New high-resolution optical spectra of DoAr 21 show filled-in Hαwith broad emission wings, and a transient, weak Hα emissioncomponent with a FWHM of 300 km/s, indicating continued low-levelaccretion onto DoAr 21. Combined with its weak mid-infrared excess, thissuggests that the star is in the process of losing its circumstellardisk. Interestingly, the density-sensitive forbidden-to-intercombinationline ratio in the x-ray spectrum shows a value intermediate between thehigh-density limit seen in the classical T Tauri star TW Hya and thelow-density limit seen in the diskless T Tauri star HD 98800 A.
| Young Stars Near the Sun
Until the late 1990s the rich Hyades and the sparse UMa clusters werethe only coeval, comoving concentrations of stars known within 60 pc ofEarth. Both are hundreds of millions of years old. Then beginning in thelate 1990s the TW Hydrae Association, the Tucana/Horologium Association,the Pictoris Moving Group, and the AB Doradus Moving Group wereidentified within 60 pc of Earth, and the Chamaeleontis cluster wasfound at 97 pc. These young groups (ages 8 50 Myr), along with othernearby, young stars, will enable imaging and spectroscopic studies ofthe origin and early evolution of planetary systems.
| The State of Protoplanetary Material 10 Million years after Stellar Formation: Circumstellar Disks in the TW Hydrae Association
We have used the Spitzer Space Telescope Infrared Spectrograph toobserve seven members of the TW Hya association, the nearest stellarassociation whose age (~10 Myr) is similar to the timescales thought toapply to planet formation and disk dissipation. Only two of the seventargets display infrared excess emission, indicating that substantialamounts of dust still exist closer to the stars than is characteristicof debris disks; however, in both objects we confirm an abruptshort-wavelength edge to the excess, as is seen in disks withcleared-out central regions. The mid-infrared excesses in the spectra ofHen 3-600 and TW Hya include crystalline silicate emission features,indicating that the grains have undergone significant thermalprocessing. We offer a detailed comparison between the spectra of TW Hyaand Hen 3-600, and a model that corroborates the spectral shape and ourprevious understanding of the radial structure of these protoplanetarydisks.
| Resolving X-Ray Sources from B Stars Spectroscopically: The Example of μ Leporis
We present high-resolution X-ray observations of the chemically peculiarlate-type B star μ Lep. However, we find spectroscopic andastrometric evidence, which shows that the X-rays are not traced back tothe B star itself but rather to a previously unresolved companion, μLep-B, whose X-ray spectrum resembles that of a coronally active source.We discuss the possibility that μ Lep-B is a pre-main-sequencecompanion, most likely of the nonaccreting magnetically active type.
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