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C2D Spitzer-IRS spectra of disks around T Tauri stars. IV. Crystalline silicates
Aims: Dust grains in the planet-forming regions around young stars areexpected to be heavily processed due to coagulation, fragmentation, andcrystallization. This paper focuses on the crystalline silicate dustgrains in protoplanetary disks for a statistically significant number ofTTauri stars (96). Methods: As part of the cores to disks (c2d)legacy program, we obtained more than a hundred Spitzer/IRS spectra ofTTauri stars, over a spectral range of 5-35 μm where many silicateamorphous and crystalline solid-state features are present. At thesewavelengths, observations probe the upper layers of accretion disks upto distances of a dozen AU from the central object. Results: Morethan 3/4 of our objects show at least one crystalline silicate emissionfeature that can be essentially attributed to Mg-rich silicates. TheFe-rich crystalline silicates are largely absent in the c2d IRS spectra.The strength and detection frequency of the crystalline features seen atλ > 20 μm correlate with each other, while they are largelyuncorrelated with the observational properties of the amorphous silicate10 μm feature. This supports the idea that the IRS spectraessentially probe two independent disk regions: a warm zone (≤1 AU)emitting at λ ~ 10 μm and a much colder region emitting atλ > 20 μm (≤10 AU). We identify a crystallinity paradox,as the long-wavelength (λ > 20 μm) crystalline silicatefeatures are detected 3.5 times more frequently (~55% vs. ~15%) than thecrystalline features arising from much warmer disk regions (λ ~10μm). This suggests that the disk has an inhomogeneous dustcomposition within ~10 AU. The analysis of the shape and strength ofboth the amorphous 10 μm feature and the crystalline feature around23 μm provides evidence for the prevalence of μm-sized (amorphousand crystalline) grains in upper layers of disks. Conclusions:The abundant crystalline silicates found far from their presumedformation regions suggest efficient outward radial transport mechanismsin the disks around TTauri stars. The presence of μm-sized grains indisk atmospheres, despite the short timescales for settling to themidplane, suggests efficient (turbulent) vertical diffusion, probablyaccompanied by grain-grain fragmentation to balance the expectedefficient growth. In this scenario, the depletion of submicron-sizedgrains in the upper layers of the disks points toward removal mechanismssuch as stellar winds or radiation pressure.Tables [see full textsee full textsee full textsee full text], [see fulltextsee full textsee full textsee full text] and Figs. [see full textseefull textsee full textsee full text]-[see full textsee full textsee fulltextsee full text] are only available in electronic form athttp://www.aanda.org
| Stringent Limits on the Polarized Submillimeter Emission from Protoplanetary Disks
We present arcsecond-resolution Submillimeter Array (SMA) polarimetricobservations of the 880 μm continuum emission from the protoplanetarydisks around two nearby stars, HD 163296 and TW Hydrae. Althoughprevious observations and theoretical work have suggested that a 2%-3%polarization fraction should be common for the millimeter continuumemission from such disks, we detect no polarized continuum emissionabove a 3σ upper limit of 7 mJy in each arcsecond-scale beam, or<1% in integrated continuum emission. We compare the SMA upper limitswith the predictions from the exploratory Cho & Lazarian model ofpolarized emission from T Tauri disks threaded by toroidal magneticfields, and rule out their fiducial model at the ~10σ level. Weexplore some potential causes for this discrepancy, focusing on modelparameters that describe the shape, magnetic field alignment, and sizedistribution of grains in the disk. We also investigate related effectslike the magnetic field strength and geometry, scattering off of largegrains, and the efficiency of grain alignment, including recent advancesin grain alignment theory, which are not considered in the fiducialmodel. We discuss the impact each parameter would have on the data anddetermine that the suppression of polarized emission plausibly arisesfrom rounding of large grains, reduced efficiency of grain alignmentwith the magnetic field, and/or some degree of magnetic field tangling(perhaps due to turbulence). A poloidal magnetic field geometry couldalso reduce the polarization signal, particularly for a face-on viewinggeometry like the TW Hya disk. The data provided here offer the moststringent limits to-date on the polarized millimeter-wavelength emissionfrom disks around young stars.
| Tracing the Inner Edge of the Disk Around HD 100546 with Rovibrational CO Emission Lines
In this paper we present high-resolution 4.7 μm spectra of theisolated Herbig Be star HD 100546. HD 100546 has been the subject ofintense scrutiny because it is a young nearby star with a transitionaldisk. We observe the Δv = 1 rovibrational CO transitions in orderto clarify the distribution of warm gas in the inner disk. Modeling ofthe CO spectrum indicates that the gas is vibrationally excited bycollisions and UV fluorescence. The observed emission extends from 13 to100 AU. The inner edge of the molecular gas emission is consistent withthe inner edge of the optically thick dust disk indicating that theinner hole is not simply a hole in the dust opacity but is likelycleared of gas as well. The rotational temperature of the CO is ~1000K—much hotter than the ~200 K CO in the otherwise similartransitional disk surrounding HD 141569. The origin of this discrepancyis likely linked to the brighter polycyclic aromatic hydrocarbonemission observed toward HD 100546. We use the excitation of the CO toconstrain the geometry of the inner disk and comment on the evolutionarystate of the system.
| Abundant Circumstellar Silica Dust and SiO Gas Created by a Giant Hypervelocity Collision in the ~12 Myr HD172555 System
The fine dust detected by infrared (IR) emission around the nearbyβ Pic analog star HD172555 is very peculiar. The dust mineralogy iscomposed primarily of highly refractory, nonequilibrium materials, withapproximately three quarters of the Si atoms in silica (SiO2)species. Tektite and obsidian lab thermal emission spectra(nonequilibrium glassy silicas found in impact and magmatic systems) arerequired to fit the data. The best-fit model size distribution for theobserved fine dust is dn/da = a -3.95±0.10.While IR photometry of the system has stayed stable since the 1983 IRASmission, this steep a size distribution, with abundant micron-sizedparticles, argues for a fresh source of material within the last0.1 Myr. The location of the dust with respect to the star is at5.8 ± 0.6 AU (equivalent to 1.9 ± 0.2 AU from the Sun),within the terrestrial planet formation region but at the outer edge ofany possible terrestrial habitability zone. The mass of fine dust is 4× 1019-2 × 1020 kg, equivalent toa 150-200 km radius asteroid. Significant emission featurescentered at 4 and 8 μm due to fluorescing SiO gas are alsofound. Roughly 1022 kg of SiO gas, formed by vaporizingsilicate rock, is also present in the system, and a separate populationof very large, cool grains, massing 1021-1022 kgand equivalent to the largest sized asteroid currently found in thesolar system's main asteroid belt, dominates the solid circumstellarmaterial by mass. The makeup of the observed dust and gas, and the notedlack of a dense circumstellar gas disk, strong stellar X-ray activity,and an extended disk of β meteoroids argues that the source of theobserved circumstellar materials is a giant hypervelocity(>10 km s-1) impact between large rockyplanetesimals, similar to the ones which formed the Moon and whichstripped the surface crustal material off of Mercury's surface.
| Structure and Evolution of Pre-main-sequence Circumstellar Disks
We present new subarcsecond (~0farcs7) Combined Array for Research inMillimeter-wave Astronomy (CARMA) observations of the 1.3 mm continuumemission from circumstellar disks around 11 low- and intermediate-masspre-main-sequence stars. High-resolution observations for threeadditional sources were obtained from the literature. In all cases thedisk emission is spatially resolved. We adopt a self-consistentaccretion disk model based on the similarity solution for the disksurface density and constrain the dust radial density distribution onspatial scales of about 40 AU. Disk surface densities appear to becorrelated with the stellar ages where the characteristic disk radiusincreases from ~20 AU to ~100 AU over about 5 Myr. This disk expansionis accompanied by a decrease in the mass accretion rate, suggesting thatour sample disks form an evolutionary sequence. Interpreting our resultsin terms of the temporal evolution of a viscous α-disk, weestimate (1) that at the beginning of the disk evolution about 60% ofthe circumstellar material was located inside radii of 25-40 AU, (2)that disks formed with masses from 0.05 to 0.4 M sun, and (3)that the viscous timescale at the disk initial radius is about 0.1-0.3Myr. Viscous disk models tightly link the surface density Σ(R)with the radial profile of the disk viscosity ν(R) vprop Rγ. We find values of γ ranging from -0.8 to0.8, suggesting that the viscosity dependence on the orbital radius canbe very different in the observed disks. Adopting the αparameterization for the viscosity, we argue that α must decreasewith the orbital radius and that it may vary between 0.5 and10-4. From the inferred disk initial radii we derivespecific angular momenta, j, for parent cores of (0.8 - 4) ×10-4 km s-1 pc. Comparison with thevalues of j in dense cores suggests that about 10% of core angularmomentum and 30% of the core mass are conserved in the formation of thestar/disk system. We demonstrate that the similarity solution for thesurface density for γ < 0 can explain the properties of some"transitional disks" without requiring discontinuities in the disksurface density. In the case of LkCa 15, a smooth distribution ofmaterial from few stellar radii to about 240 AU can produce both theobserved spectral energy distribution and the spatially resolvedcontinuum emission at millimeter wavelengths. Finally we show that amongthe observed sample, TW Hya is the only object that has a disk radiuscomparable with the early solar nebula.
| Mid-Infrared Size Survey of Young Stellar Objects: Description of Keck Segment-Tilting Experiment and Basic Results
The mid-infrared properties of pre-planetary disks are sensitive to thetemperature and flaring profiles of disks for the regions where planetformation is expected to occur. In order to constrain theories of planetformation, we have carried out a mid-infrared (λ = 10.7 μm)size survey of young stellar objects using the segmented Keck telescopein a novel configuration. We introduced a customized pattern of tilts toindividual mirror segments to allow efficient sparse-apertureinterferometry, allowing full aperture synthesis imaging with highercalibration precision than traditional imaging. In contrast to previoussurveys on smaller telescopes and with poorer calibration precision, wefind that most objects in our sample are partially resolved. Here, wepresent the main observational results of our survey of five embeddedmassive protostars, 25 Herbig Ae/Be stars, 3 T Tauri stars, 1 FU Orisystem, and five emission-line objects of uncertain classification. Theobserved mid-infrared sizes do not obey the size-luminosity relationfound at near-infrared wavelengths and a companion paper will providefurther modeling analysis of this sample. In addition, we report imagingresults for a few of the most resolved objects, including complexemission around embedded massive protostars, the photoevaporatingcircumbinary disk around MWC 361A, and the subarcsecond binaries T Tau,FU Ori, and MWC 1080.
| Revealing the Structure of a Pre-Transitional Disk: The Case of the Herbig F Star SAO 206462 (HD 135344B)
SAO 206462 (HD 135344B) has previously been identified as a Herbig Fstar with a circumstellar disk with a dip in its infrared excess near 10μm. In combination with a low accretion rate estimated from Brγ, it may represent a gapped, but otherwise primordial or"pre-transitional" disk. We test this hypothesis with Hubble SpaceTelescope coronagraphic imagery, FUV spectroscopy and imagery andarchival X-ray data, and spectral energy distribution (SED) modelingconstrained by the observed system inclination, disk outer radius, andouter disk radial surface brightness (SB) profile using the WhitneyMonte Carlo Radiative Transfer Code. The essentially face-on (i lsim20°) disk is detected in scattered light from 0farcs4 to 1farcs15(56-160 AU), with a steep (r -9.6) radial SB profilefrom 0farcs6 to 0farcs93. Fitting the SB data requires a concave upwardor anti-flared outer disk, indicating substantial dust grain growth andsettling by 8 ± 4 Myr. The warm dust component is significantlyvariable in near to mid-IR excess and in temperature. At its warmest, itappears confined to a narrow belt from 0.08 to 0.2 AU. The steep SED forthis dust component is consistent with grains with a<= 2.5 μm. Forcosmic carbon to silicate dust composition, conspicuous 10 μmsilicate emission would be expected and is not observed. This mayindicate an elevated carbon to silicate ratio for the warm dust, whichis not required to fit the outer disk. At its coolest, the warm dust canbe fit with a disk from 0.14 to 0.31 AU, but with a higher inclinationthan either the outer disk or the gaseous disk, providing confirmationof the high inclination inferred from mid-IR interferometry. In tandem,the compositional and inclination difference between the warm dust andthe outer dust disk suggests that the warm dust may be ofsecond-generation origin, rather than a remnant of a primordial diskcomponent. With its near face-on inclination, SAO 206462's disk is aprime location for planet searches.Based in part on data collected at Subaru Telescope, which is operatedby the National Astronomical Observatory of Japan.
| Searching for a link between the magnetic nature and other observed properties of Herbig Ae/Be stars and stars with debris disks
Context: Recently, evidence for the presence of weak magnetic fields inHerbig Ae/Be stars has been found in several studies. Aims: Weseek to expand the sample of intermediate-mass pre-main sequence starswith circular polarization data to measure their magnetic fields, and todetermine whether magnetic field properties in these stars arecorrelated with mass-accretion rate, disk inclination, companions,silicates, PAHs, or show a correlation with age and X-ray emission asexpected for the decay of a remnant dynamo. Methods:Spectropolarimetric observations of 21 Herbig Ae/Be stars and six debrisdisk stars have been obtained at the European Southern Observatory withFORS 1 mounted on the 8 m Kueyen telescope of the VLT. With the GRISM600B in the wavelength range 3250-6215 Å we were able to cover allhydrogen Balmer lines from Hβ to the Balmer jump. In allobservations a slit width of 0.4 arcsec was used to obtain a spectralresolving power of R ≈ 2000. Results: Among the 21 HerbigAe/Be stars studied, new detections of a magnetic field were achieved insix stars. For three Herbig Ae/Be stars, we confirm previous magneticfield detections. The largest longitudinal magnetic field, =-454±42 G, was detected in the Herbig Ae/Be star HD 101412 usinghydrogen lines. No field detection at a significance level of 3σwas achieved in stars with debris disks. Our study does not indicate anycorrelation of the strength of the longitudinal magnetic field with diskorientation, disk geometry, or the presence of a companion. We also donot see any simple dependence on the mass-accretion rate. However, it islikely that the range of observed field values qualitatively supportsthe expectations from magnetospheric accretion models giving support fordipole-like field geometries. Both the magnetic field strength and theX-ray emission show hints of a decline with age in the range of ~2-14Myr probed by our sample, supporting a dynamo mechanism that decays withage. However, our study of rotation does not show any obvious trend ofthe strength of the longitudinal magnetic field with rotation period.Furthermore, the stars seem to obey the universal power-law relationbetween magnetic flux and X-ray luminosity established for the Sun andmain-sequence active dwarf stars.Based on observations obtained at the European Southern Observatory,Paranal, Chile (ESO programmes 077.C-0521(A) and 081.C-0410(A)).
| A break in the gas and dust surface density of the disc around the T Tauri star IM Lupi
Aims: We study the distribution and physical properties of molecular gasin the disc around the T Tauri star IM Lup on scales close to 200 AU. Weinvestigate how well the gas and dust distributions compare and worktowards a unified disc model that can explain both gas and dustemission. Methods: 12CO, 13CO, andC18O J=2-1 line emission, as well as the dust continuum at1.3 mm, is observed at 1.8 arcsec resolution towards IM Lup using theSubmillimeter Array. A detailed disc model based on the dust emission istested against these observations with the aid of a molecular excitationand radiative transfer code. Apparent discrepancies between the gas anddust distribution are investigated by adopting simple modifications tothe existing model. Results: The disc is seen at an inclinationof 54° ± 3° and is in Keplerian rotation around a 0.8-1.6M_ȯ star. The outer disc radius traced by molecular gas emission is900 AU, while the dust continuum emission and scattered light imageslimit the amount of dust present beyond 400 AU and are consistent withthe existing model that assumes a 400 AU radius. Our observationsrequire a drastic density decrease close to 400 AU with the vertical gascolumn density at 900 AU in the range of 5×1020-1022 cm-2. We derive a gas-to-dustmass ratio of 100 or higher in disc regions beyond 400 AU. Within 400 AUfrom the star our observations are consistent with a gas-to-dust ratioof 100 but other values are not ruled out.
| The Alignment of the Polarization of Herbig Ae/Be Stars with the Interstellar Magnetic Field
We present a study of the correlation between the direction of thesymmetry axis of the circumstellar material around intermediate massyoung stellar objects and that of the interstellar magnetic field. Weuse CCD polarimetric data on 100 Herbig Ae/Be stars. A large numberof them show intrinsic polarization, which indicates that theircircumstellar envelopes are not spherical. The interstellar magneticfield direction is estimated from the polarization of field stars. Thereis an alignment between the position angle of the Herbig Ae/Be starpolarization and that of the field stars for the most polarized objects.This may be an evidence that the ambient interstellar magnetic fieldplays a role in shaping the circumstellar material around young stars ofintermediate mass and/or in defining their angular momentum axis.Based on observations made at the Observatório do Pico dos Dias,Brazil, operated by the Laboratório Nacional deAstrofísica.
| HD 100453: A Link Between Gas-Rich Protoplanetary Disks and Gas-Poor Debris Disks
HD 100453 has an IR spectral energy distribution (SED) which can be fitwith a power law plus a blackbody. Previous analysis of the SED suggeststhat the system is a young Herbig Ae star with a gas-rich, flared disk.We reexamine the evolutionary state of the HD 100453 system by refiningits age (based on a candidate low-mass companion) and by examininglimits on the disk extent, mass accretion rate, and gas content of thedisk environment. We confirm that HD 100453B is a common proper motioncompanion to HD 100453A, with a spectral type of M4.0V-M4.5V, and derivean age of 10 ± 2 Myr. We find no evidence of mass accretion ontothe star. Chandra ACIS-S imagery shows that the Herbig Ae star has Lx/L bol and an X-ray spectrum similar tononaccreting β Pic Moving Group early F stars. Moreover, the disklacks the conspicuous Fe II emission and excess FUV continuum seen inspectra of actively accreting Herbig Ae stars, and from the FUVcontinuum, we find the accretion rate is < 1.4 ×10–9 M sun yr–1. Asensitive upper limit to the CO J = 3–2 intensity indicates thatthe gas in the outer disk is likely optically thin. Assuming a[CO]/[H2] abundance of 1 × 10–4 and adepletion factor of 103, we find that the mass of coldmolecular gas is less than ~0.33 M J and that the gas-to-dustratio is no more than ~4:1 in the outer disk. The combination of a highfractional IR excess luminosity, a relatively old age, an absence ofaccretion signatures, and an absence of detectable circumstellarmolecular gas suggests that the HD 100453 system is in an unusual stateof evolution between a gas-rich protoplanetary disk and a gas-poordebris disk.
| Circumstellar disks around Herbig Be stars
Aims. Our goal is to investigate the properties of the circumstellardisks around intermediate mass stars to determine their occurrence,lifetime and evolution. Methods: We completed a search forcircumstellar disks around Herbig Be stars using the NRAO Very LargeArray (VLA) and the IRAM Plateau de Bure (PdB) interferometers. Thusfar, we have observed 6 objects with 4 successful detections. Theresults towards 3 of these stars (R Mon, MWC 1080, MWC 137) werepresented elsewhere. We present our new VLA and PdBI data for the threeobjects MWC 297, Z CMa, and LKHα 215. We constructed the SED fromnear-IR to centimeter wavelengths by adding our millimeter andcentimeter data to the available data at other wavelengths, mainlySpitzer images. The entire SED was fitted using a disk+envelope model.In addition, we compiled all the disk millimeter observations in theliterature and completed a statistical analysis of all the data. Results: We show that the disk mass is usually only a small percentage(less than 10%) of the mass of the entire envelope in HBe stars. For thedisks, there are large source-to-source variations. Two disks in oursample, R Mon and Z CMa, have similar sizes and masses to those found inT Tauri and Herbig Ae stars. The disks around MWC 1080 and MWC 297 are,however, smaller (r_out < 100 AU). We did not detect the diskstowards MWC 137 and LkHα 215 at millimeter wavelengths, whichlimits the mass and the size of the possible circumstellar disks. Conclusions: A comparison between our data and previous results for TTauri and Herbig Ae stars indicates that although massive disks (~0.1M_ȯ) are found in young objects (~104 yr), the masses ofthe disks around Herbig Be stars are usually 5-10 times lower than thosearound lower mass stars. We propose that disk photoevaporation isresponsible for this behavior. In Herbig Be stars, the UV radiationdisperses the gas in the outer disk on a timescale of a few105 yr. Once the outer part of the disk has vanished, theentire gaseous disk is photoevaporated on a very short timescale(~105 yr) and only a small, dusty disk consisting of largegrains remains.
| Spatially Resolved 3 μm Spectroscopy of Elias 1: Origin of Diamonds in Protoplanetary Disks
We present spatially resolved 3 μm spectra of Elias 1 obtained withan adaptive optics system. The central part of the disk is almost devoidof polycyclic aromatic hydrocarbon (PAH) emission at 3.3 μm it showsup only at 30 AU and beyond. The PAH emission extends up to 100 AU, atleast to the outer boundary of our observation. The diamond emission, incontrast, is more centrally concentrated, with the column density peakedaround 30 AU from the star. There are only three Herbig Ae/Be starsknown to date that show diamond emission at 3.53 μm. Two of them havelow-mass companions likely responsible for the large X-ray flaresobserved toward the Herbig Ae/Be stars. We speculate on the origin ofdiamonds in circumstellar disks in terms of the graphitic material beingtransformed into diamond under the irradiation of highly energeticparticles.Based on data collected at Subaru Telescope, which is operated by theNational Astronomical Observatory of Japan.
| Parameters of Herbig Ae/Be and Vega-type stars
Context: This work presents the characterization of 27 young early-typestars, most of them in the age range 1-10 Myr, and three suspected hotcompanions of post-T Tauri stars belonging to the Lindroos binarysample. Most of these objects show IR excesses in their spectral energydistributions, which are indicative of the presence of disks. The workis relevant in the fields of stellar physics, physics of disks andformation of planetary systems. Aims: The aim of the work is thedetermination of the effective temperature, gravity, metallicity, mass,luminosity and age of these stars. An accurate modelling of their disksrequires the knowledge of most of these parameters, since they willdetermine the energy input received by the disk and hence, its geometryand global properties. Methods: Spectral energy distributions andmid-resolution spectra were used to estimate T_eff, the effectivetemperature. The comparison of the profiles of the Balmer lines withsynthetic profiles provides the value of the stellar gravity, g_*.High-resolution optical observations and synthetic spectra are used toestimate the metallicity, [M/H]. Once Teff, g* and[M/H] are known for each star, evolutionary tracks and isochronesprovide estimations of the mass, luminosity, age and distances (or upperlimits in some cases). The method is original in the sense that it isdistance-independent, i.e. the estimation of the stellar parameters doesnot require the knowledge of the distance to the object. Results:Stellar parameters (effective temperature, gravity, metallicity, mass,luminosity, age and distances - or upper limits) are obtained for thesample of stars mentioned above. A detailed discussion of someindividual objects, in particular VV Ser, RR Tau, 49 Cet and the threesuspected hot companions of post-T Tauris, is presented. Conclusions:These results, apart from their intrinsic interest, would be extremelyvaluable to model the disks surrounding the stars. The paper also showsthe difficulty posed by the morphology and behaviour of the systemstar+disk in the computation of the stellar parameters.Based on observations collected at the German-Spanish AstronomicalCentre, Calar Alto, jointly operated by the Max-Planck Institut fürAstronomie and the Instituto de Astrofísica de Andalucía(CSIC), and on observations made with the WHT telescope operated on theisland of La Palma by the Isaac Newton Group in the Spanish Observatoriodel Roque de los Muchachos of the Instituto de Astrofísica deCanarias.
| Dynamical Mass of the Substellar Benchmark Binary HD 130948BC
We present Keck adaptive optics imaging of the L4+L4 binary HD 130948BCalong with archival Hubble Space Telescope and Gemini Northobservations, which together span ≈ 70% of the binary's orbitalperiod. From the relative orbit, we determine a total dynamical mass of0.109 ± 0.003 M sun (114 ± 3 M Jup).The flux ratio of HD 130948BC is near unity, so both components areunambiguously substellar for any plausible mass ratio. An independentconstraint on the age of the system is available from the primary HD130948A (G2V, [M/H] = 0.0). The ensemble of available indicatorssuggests an age comparable to Hyades, with the most precise age being0.79+0.22 -0.15 Gyr based on gyrochronology.Therefore, HD 130948BC is now a unique benchmark among field L and Tdwarfs, with a well-determined mass, luminosity, and age. We find thatsubstellar theoretical models disagree with our observations. (1) Bothcomponents of HD 130948BC appear to be overluminous by a factor of ≈2-3 times compared to evolutionary models. The age of the system wouldhave to be notably younger than the gyro age to ameliorate theluminosity disagreement. (2) Effective temperatures derived fromevolutionary models for HD 130948B and C are inconsistent withtemperatures determined from spectral synthesis for objects of similarspectral type. Overall, regardless of the adopted age, evolutionary andatmospheric models give inconsistent results, which indicate systematicerrors in at least one class of models, possibly both. The masses of HD130948BC happen to be very near the theoretical mass limit for lithiumburning, and thus measuring the differential lithium depletion between Band C will provide a uniquely discriminating test of theoretical models.The potential underestimate of luminosities by evolutionary models wouldhave wide-ranging implications; therefore, a more refined estimate agefor HD 130948A is critically needed.Based on observations obtained at the Gemini Observatory (program IDsGN-2001A-DD-2, GN-2001A-C-24, GN-2001B-DD-1, and GN-2002A- DD-1), whichis operated by the Association of Universities for Research inAstronomy, Inc., under a cooperative agreement with the NSF on behalf ofthe Gemini partnership: the National Science Foundation (United States),the Science and Technology Facilities Council (United Kingdom), theNational Research Council (Canada), CONICYT (Chile), the AustralianResearch Council (Australia), Ministério da Ciência eTecnologia (Brazil) and SECYT (Argentina).
| Spatially Resolved Spectroscopy of Sub-AU-Sized Regions of T Tauri and Herbig Ae/Be Disks
We present spatially resolved near-IR spectroscopic observations of 15young stars. Using a grism spectrometer behind the Keck interferometer,we obtained an angular resolution of a few milliarcseconds and aspectral resolution of 230, enabling probes of both gas and dust in theinner disks surrounding the target stars. We find that the angular sizeof the near-IR emission typically increases with wavelength, indicatinghot, presumably gaseous material within the dust sublimation radius. Ourdata also clearly indicate Brγ emission arising from hot hydrogengas, and suggest the presence of water vapor and carbon monoxide gas inthe inner disks of several objects. This gaseous emission is morecompact than the dust continuum emission in all cases. We constructsimple physical models of the inner disk and fit them to our data toconstrain the spatial distribution and temperature of dust and gasemission components.
| The enigmatic X-rays from the Herbig star HD 163296: Jet, accretion, or corona?
Context: Herbig Ae/Be stars (HAeBe) are pre-main sequence objects in themass range 2 Mȯ < M* < 8Mȯ. Their X-ray properties are uncertain and, as yet,unexplained. Aims: We want to elucidate the X-ray generatingmechanism in HAeBes. Methods: We present an XMM-Newtonobservation of the HAeBe HD 163296. We analyse the light curve, thebroad band and the grating spectra, fit emission measures and abundancesand apply models for accretion and wind shocks. Results: We findthree temperature components ranging from 0.2 keV to 2.7 keV. The O VIIHe-like triplet indicates an X-ray formation region in a low densityenvironment with a weak UV photon field, i.e. above the stellar surface.This makes an origin in an accretion shock unlikely; instead we suggesta shock at the base of the jet for the soft component and a coronalorigin for the hot component. A mass outflow of dot M_shock ≈10-10 Mȯ yr-1 is sufficient topower the soft X-rays. Conclusions: HD 163296 is thought to besingle, so this data represent genuine HAeBe X-ray emission. HD 163296might be prototypical for its class.Based on observations obtained with XMM-Newton, an ESA science missionwith instruments and contributions directly funded by ESA Member Statesand NASA.
| Spectropolarimetric Observations of Herbig Ae/Be Stars. II. Comparison of Spectropolarimetric Surveys: Haebe, Be and Other Emission-Line Stars
The polarization of light across individual spectral lines containsinformation about the circumstellar environment on very small spatialscales. We have obtained a large number of high-precision,high-resolution spectropolarimetric observations of Herbig Ae/Be,Classical Be and other emission-line stars collected on 117 nights ofobservations with the Hi-Resolution Visible spectropolarimeter at aresolution of R = 13, 000 on the 3.67 m Advanced Electro-Optical Systemtelescope. We also have many observations from the ESPaDOnSspectropolarimeter at a resolution of R = 68, 000 on the 3.6 mCanada-France-Hawaii Telescope. In roughly ~2/3 of the so-called "windy"or "disky" Herbig Ae/Be stars, the detected Hα linearpolarization varies from our typical detection threshold near 0.1% toover 2%. In all but one HAe/Be star, the detected polarization effect isnot coincident with the Hα emission peak but isdetected in and around the obvious absorptive part of the line profile.The qu-loops are dominated by the polarization in this absorptiveregion. In several stars, the polarization varies in time mostly in theabsorptive component and is not necessarily tied to correspondingvariations in intensity. This is a new result not seen at lowerresolution. In the Be and emission-line stars, ten out of a sample of 30show a typical broad depolarization effect but four of these ten showweaker effects only visible at high resolution. Another five of 30 showsmaller amplitude, more complex signatures. Six stars of alternateclassification showed large amplitude (1%-3%) absorptive polarizationeffects. These detections are largely inconsistent with the traditionaldisk-scattering and depolarization models.
| New X-ray detections of Herbig stars
Context: The interpretation of X-ray detections from Herbig Ae/Be starsis disputed as it is not clear whether these intermediate-mass pre-mainsequence stars are able to drive a dynamo and ensuing phenomena ofmagnetic activity. Alternative X-ray production mechanisms, related tostellar winds, star-disk magnetospheres, or unresolved late-type T Tauristar companions have been proposed. Aims: The companionhypothesis can be tested by resolving Herbig stars in X-rays from theirknown visual secondaries. Furthermore, their global X-ray properties(such as detection rate, luminosity, temperature, variability) may giveclues to 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: In a series of papers we have beeninvestigating high-resolution X-ray Chandra images of Herbig Ae/Be andmain-sequence B-type stars where known close visual companions arespatially separated from the primaries. Results: Here we reporton six as yet unpublished Chandra exposures from our X-ray survey ofHerbig stars. The target list comprises six Herbig stars with known coolcompanions, and three other A/B-type stars that are serendipitously inthe Chandra field-of-view. In this sample we record a detection rate of100%; i.e. all A/B-type stars display X-ray emission at levels oflog(L_x/L_bol) ~ -5...-7. The analysis of hardness ratios confirms thatHAeBes have hotter and/or more absorbed X-ray emitting plasma than moreevolved B-type stars. Conclusions: Radiative winds are ruled outas an exclusive emission mechanism on the basis of the high X-raytemperatures. Confirming earlier results, the X-ray properties of HerbigAe/Be stars are not vastly different from those of their late-typecompanion stars (if such are known). The diagnostics provided by thepresently available data leave it open whether the hard X-ray emissionof Herbig stars is due to young age or indicative of further coronallyactive low-mass companion stars. In the latter case, our detectionstatistics imply a high fraction of higher order multiple systems amongHerbig stars.
| A Tale of Two Herbig Ae Stars, MWC 275 and AB Aurigae: Comprehensive Models for Spectral Energy Distribution and Interferometry
We present comprehensive models for the Herbig Ae stars MWC 275 and ABAur that aim to explain their spectral energy distribution (from UV tomillimeter) and long-baseline interferometry (from near-infrared tomillimeter) simultaneously. Data from the literature, combined with newmid-infrared (MIR) interferometry from the Keck Segment TiltingExperiment, are modeled using an axisymmetric Monte Carlo radiativetransfer code. Models in which most of the near-infrared (NIR) emissionarises from a dust rim fail to fit the NIR spectral energy distribution(SED) and sub-milliarcsecond NIR CHARA interferometry. Following recentwork, we include an additional gas emission component with similar sizescale to the dust rim, inside the sublimation radius, to fit the NIR SEDand long-baseline NIR interferometry on MWC 275 and AB Aur. In theabsence of shielding of starlight by gas, we show that the gas-dusttransition region in these YSOs will have to contain highly refractorydust, sublimating at ~1850 K. Despite having nearly identical structurein the thermal NIR, the outer disks of MWC 275 and AB Aur differsubstantially. In contrast to the AB Aur disk, MWC 275 lacks smallgrains in the disk atmosphere capable of producing significant 10-20μm emission beyond ~7 AU, forcing the outer regions into the``shadow'' of the inner disk.
| The TEXES Survey for H2 Emission from Protoplanetary Disks
We report the results of a search for pure rotational molecular hydrogenemission from the circumstellar environments of young stellar objectswith disks using the Texas Echelon Cross Echelle Spectrograph (TEXES) onthe NASA Infrared Telescope Facility and the Gemini North Observatory.We searched for mid-infrared H2 emission in the S(1), S(2),and S(4) transitions. Keck/NIRSPEC observations of the H2S(9) transition were included for some sources as an additionalconstraint on the gas temperature. We detected H2 emissionfrom 6 of 29 sources observed: AB Aur, DoAr 21, Elias 29, GSS 30 IRS 1,GV Tau N, and HL Tau. Four of the six targets with detected emission areclass I sources that show evidence for surrounding material in anenvelope in addition to a circumstellar disk. In these cases, we showthat accretion shock heating is a plausible excitation mechanism. Thedetected emission lines are narrow (~10 km s-1), centered atthe stellar velocity, and spatially unresolved at scales of 0.4", whichis consistent with origin from a disk at radii 10-50 AU from the star.In cases where we detect multiple emission lines, we derive temperatures>~500 K from ~1 M⊕ of gas. Our upper limits for thenondetections place upper limits on the amount of H2 gas withT>500 K of less than a few Earth masses. Such warm gas temperaturesare significantly higher than the equilibrium dust temperatures at theseradii, suggesting that the gas is decoupled from the dust in the regionsthat we are studying and that processes such as UV, X-ray, and accretionheating may be important.
| A parameter study of self-consistent disk models around Herbig AeBe stars
We present a parameter study of self-consistent models of protoplanetarydisks around Herbig AeBe stars. We use the code developed by Dullemondand Dominik, which solves the 2D radiative transfer problem including aniteration for the vertical hydrostatic structure of the disk. This gridof models will be used for several studies on disk emission andmineralogy in follow-up papers. In this paper we take a first look onthe new models, compare them with previous modeling attempts and focuson the effects of various parameters on the overall structure of the SEDthat leads to the classification of Herbig AeBe stars into two groups,with a flaring (group I) or self-shadowed (group II) SED. We find thatthe parameter of overriding importance to the SED is the total mass ingrains smaller than 25 μm, confirming the earlier results byDullemond and Dominik. All other parameters studied have only minorinfluences, and will alter the SED type only in borderline cases. Wefind that there is no natural dichotomy between group I and II. From amodeling point of view, the transition is a continuous function of thesmall dust mass. We also show that moderate grain growth producesspectra with weak or no 10 μm feature, both for flaring (Group I) andnon-flaring (Group II) sources. The fact that sources with weak featureshave been found mostly in Group I sources is therefore surprising andmust be due to observational biases or evolutionary effects.
| X-Ray Emission from Young Stellar Objects in the ɛ Chamaeleontis Group: The Herbig Ae Star HD 104237 and Associated Low-Mass Stars
We present Chandra HETGS observations of the Herbig Ae star HD 104237and the associated young stars comprising lower mass stars, in the0.15-1.75 Msolar mass range, in their pre-main-sequencephase. The brightest X-ray source in the association is the centralsystem harboring the Herbig Ae primary and a K3 companion. Its X-rayvariability indicates modulation possibly on timescales of the rotationperiod of the Herbig Ae star, and this would imply that the primarysignificantly contributes to the overall emission. The spectrum of theHerbig Ae+K3 system shows a soft component significantly more pronouncedthan in other K-type young stars. This soft emission is reminiscent ofthe unusually soft spectra observed for the single Herbig Ae stars HD163296 and AB Aur, and therefore we tentatively attribute it to theHerbig Ae of the binary system. The HETGS spectrum shows strong emissionlines corresponding to a wide range of plasma temperatures. The He-liketriplet of Mg XI and Ne IX suggests the presence of plasma at densitiesof about 1012 cm-3, possibly indicating anaccretion-related X-ray production mechanism. The analysis of thezeroth-order spectra of the other sources indicates X-ray emissioncharacteristics typical of pre-main-sequence stars of similar spectraltype, with the exception of the T Tauri HD 104237-D, whose extremelysoft emission is very similar to the emission of the classical T Tauristar TW Hya and suggests X-ray production by shocked accreting plasma.
| Gas and dust mass in the disc around the Herbig Ae star HD 169142
Context: Spatially resolved observations of circumstellar discs atmillimetre wavelengths allow detailed comparisons with theoreticalmodels for the radial and vertical distribution of the material. Aims: We investigate the physical structure of the gas component ofthe disc around the pre-main-sequence star HD 169142 and test the discmodel derived from the spectral energy distribution. Methods: The13CO and C18O J = 2-1 line emission was observedfrom the disc with 1.4 arcsec resolution using the Submillimeter Array.We adopted the disc physical structure derived from a model that fitsthe spectral energy distribution of HD 169142. We obtained the fullthree-dimensional information on the CO emission with the aid of amolecular excitation and radiative transfer code. This information wasused for the analysis of our observations and previous 12CO J= 2-1 and 1.3 mm continuum data. Results: The spatially resolved13CO and C18O emission shows a Keplerian velocitypattern. The disc is seen at an inclination close to 13° fromface-on. We conclude that the regions traced by different COisotopologues are distinct in terms of their vertical location withinthe disc, their temperature, and their column densities. With the givendisc structure, we find that freeze-out is not efficient enough toremove a significant amount of CO from the gas phase. Both observedlines match the model prediction both in flux and in the spatialstructure of the emission. Therefore we use our data to derive the13CO and C18O mass and consequently the12CO mass with standard isotopic ratios. We constrain thetotal disc gas mass to (0.6-3.0) × 10-2Mȯ. Adopting a maximum dust opacity of 2 cm2g-1_dust we derive a minimum dust mass of 2.16 ×10-4 Mȯ from the fit to the 1.3 mm data.Comparison of the derived gas and dust mass shows that the gas-to-dustmass ratio of 100 is only possible under the assumption of a dustopacity of 2 cm2 g-1 and 12CO abundanceof 10-4 with respect to H2. However, our data are alsocompatible with a gas-to-dust ratio of 25, with a dust opacity of 1cm2 g-1 and 12CO abundance of 2 ×10-4.
| A GLIMPSE into the Nature of Galactic Mid-IR Excesses
We investigate the nature of the mid-IR excess for 31 intermediate-massstars that exhibit an 8 μm excess in either the Galactic LegacyInfrared Mid-Plane Survey Extraordinaire or the Mid-Course SpaceExperiment using high-resolution optical spectra to identify starssurrounded by warm circumstellar dust. From these data we determineprojected stellar rotational velocities and estimate stellar effectivetemperatures for the sample. We estimate stellar ages from thesetemperatures, parallactic distances, and evolutionary models. Using MIPS[24] measurements and stellar parameters we determine the nature of theinfrared excess for 19 GLIMPSE stars. We find that 15 stars exhibitHα emission and four exhibit Hα absorption. Assuming thatthe mid-IR excesses arise in circumstellar disks, we use the Hαfluxes to model and estimate the relative contributions of dust andfree-free emission. Six stars exhibit Hα fluxes that implyfree-free emission can plausibly explain the infrared excess at [24].These stars are candidate classical Be stars. Nine stars exhibitHα emission, but their Hα fluxes are insufficient to explainthe infrared excesses at [24], suggesting the presence of acircumstellar dust component. After the removal of the free-freecomponent in these sources, we determine probable disk dust temperaturesof Tdisk~=300-800 K and fractional infrared luminosities ofLIR/L*~=10-3. These nine stars may bepre-main-sequence stars with transitional disks undergoing diskclearing. Three of the four sources showing Hα absorption exhibitcircumstellar disk temperatures ~=300-400 K,LIR/L*~=10-3, IR colors K-[24]<3.3,and are warm debris disk candidates. One of the four Hα absorptionsources has K-[24]>3.3 implying an optically thick outer disk and isa transition disk candidate.
| The origin of hydrogen line emission for five Herbig Ae/Be stars spatially resolved by VLTI/AMBER spectro-interferometry
Context: Accretion and outflow processes are of fundamental importancefor our understanding of the formation of stars and planetary systems.To trace these processes, diagnostic spectral lines such as theBrγ 2.166 μm line are widely used, although due to a lack ofspatial resolution, the origin of the line emission is still unclear.Aims: Employing the AU-scale spatial resolution which can beachieved with infrared long-baseline interferometry, we aim todistinguish between theoretical models which associate the Brγline emission with mass infall (magnetospheric accretion, gaseous innerdisks) or mass outflow processes (stellar winds, X-winds, or diskwinds). Methods: Using the VLTI/AMBER instrument, we spatiallyand spectrally (λ/Δλ = 1500) resolved the inner(⪉5 AU) environment of five Herbig Ae/Be stars (HD 163296, HD104237, HD 98922, MWC 297, V921 Sco) in the Brγ emission line aswell as in the adjacent continuum. From the measuredwavelength-dependent visibilities, we derive the characteristic size ofthe continuum and Brγ line-emitting region. Additional informationis provided by the closure phase, which we could measure both in thecontinuum wavelength regime (for four objects) as well as in thespectrally resolved Brγ emission line (for one object). Thespectro-interferometric data is supplemented by archival and newVLT/ISAAC spectroscopy. Results: For all objects (except MWC297), we measure an increase of visibility within the Brγ emissionline, indicating that the Brγ-emitting region in these objects ismore compact than the dust sublimation radius. For HD 98922, ourquantitative analysis reveals that the line-emitting region is compactenough to be consistent with the magnetospheric accretion scenario. ForHD 163296, HD 104237, MWC 297, and V921 Sco we identify an extendedstellar wind or a disk wind as the most likely line-emitting mechanism.Since the stars in our sample cover a wide range of stellar parameters,we also search for general trends and find that the size of theBrγ-emitting region does not seem to depend on the basic stellarparameters (such as the stellar luminosity), but correlates withspectroscopic properties, in particular with the Hα line profileshape. Conclusions: By performing the first high-resolutionspectro-interferometric survey on Herbig Ae/Be stars, we find evidencefor at least two distinct Brγ line-formation mechanisms. Mostsignificant, stars with a P-Cygni Hα line profile and a highmass-accretion rate seem to show particularly compact Brγ-emittingregions (RBrγ/Rcont < 0.2), while starswith a double-peaked or single-peaked Hα-line profile show asignificantly more extended Brγ-emitting region (0.6 ⪉RBrγ/Rcont ⪉ 1.4), possibly tracing astellar wind or a disk wind.Based on observationsmade with ESO telescopes at the La Silla Paranal Observatory under opentime programme IDs 077.C-0694, 078.C-0360, and 078.C-0680.
| Disks around CQ Tauri and MWC 758: dense PDR or gas dispersal?
Context: The overall properties of disks surrounding intermediate PMSstars (HAe) are not yet well constrained by current observations. Thedisk inclination, which significantly affects spectral energydistribution modeling, is often unknown. Aims: We attempted toresolve the disks around CQ Tau and MWC 758 to provide accurateconstraints on the disk parameters, in particular the temperature andsurface density distribution. Methods: We report arcsecondresolution observations of dust and CO line emissions with the IRAMarray. We also searched for the HCO+ J = 1 → 0transition. The disk properties are derived using a standard disk model.We use the Meudon PDR code to study the chemistry. Results: Thetwo disks share some common properties. The mean CO abundance is lowdespite disk temperatures above the CO condensation temperature.Furthermore, the CO surface density and dust opacity have differentradial dependence. The CQ Tau disk appears warmer and perhaps less densethan that of MWC 758. Modeling the chemistry, we find thatphotodissociation of CO is a viable mechanism to explain its lowabundance. The photospheric flux is not sufficient for this: a strong UVexcess is required. In CQ Tau, the high temperature is consistent withthe expectation for a PDR. The PDR model has difficulty explaining themild temperatures obtained in MWC 758, for which a low gas-to-dust ratiois preferred. A yet, unexplored alternative could be that, despitecurrently high gas temperatures CO remains trapped in grains, as themodels suggest that large grains can be cold enough to prevent thermaldesorption of CO. The low inclination of the CQ Tau disk, ~30°,challenges previous interpretations given for UX Ori - like luminosityvariations of this star. Conclusions: We conclude that CO cannotbe used as a simple tracer of gas-to-dust ratio, the CO abundance beingaffected by photodissociation and grain growth.Based on observations carried out with the IRAM Plateau de BureInterferometer. IRAM is supported by INSU/CNRS (France), MPG (Germany)and IGN (Spain).Appendices A-C are only available in electronic form athttp://www.aanda.org
| The HD 163296 Circumstellar Disk in Scattered Light: Evidence of Time-Variable Self-Shadowing
We present the first multicolor view of the scattered light disk of theHerbig Ae star HD 163296, based on coronagraphic observations from theHubble Space Telescope Advanced Camera for Surveys (HST ACS). Radialprofile fits of the surface brightness along the disk's semimajor axisindicate that the disk is not continuously flared, and extends to ~540AU. The disk's color (V-I)=1.1 at a radial distance of 3.5" is redderthan the observed stellar color (V-I)=0.15. This red disk color might beindicative of either an evolution in the grain size distribution (i.e.,grain growth) and/or composition, both of which would be consistent withthe observed nonflared geometry of the outer disk. We also identify asingle ansa morphological structure in our F435W ACS data, which isabsent from earlier epoch F606W and F814W ACS data, but corresponds toone of the two ansae observed in archival HST Space Telescope ImagingSpectrograph (STIS) coronagraphic data. Following transformation tosimilar bandpasses, we find that the scattered light disk of HD 163296is 1 mag arcsec-2 fainter at 3.5" in the STIS data than inthe ACS data. Moreover, variations are seen in (1) the visibility of theansa(e) structures, (2) the relative surface brightness of the ansa(e)structures, and (3) the (known) intrinsic polarization of the system.These results indicate that the scattered light from the HD 163296 diskis variable. We speculate that the inner disk wall, which Sitko et al.suggests has a variable scale height as diagnosed by near-IR SEDvariability, induces variable self-shadowing of the outer disk. Wefurther speculate that the observed surface brightness variability ofthe ansa(e) structures may indicate that the inner disk wall isazimuthally asymmetric.
| Characterizing the nature of embedded young stellar objects through silicate, ice and millimeter observations
Context: Determining the evolutionary stage of a Young Stellar Object(YSO) is of fundamental importance to test star formation theories.Classification schemes for YSOs are based on evaluating the degree ofdissipation of the surrounding envelope, whose main effects are theextinction of the optical radiation from the central YSO and re-emissionin the far-infrared to millimeter part of the electromagnetic spectrum.Since extinction is a property of column density along the line ofsight, the presence of a protoplanetary disk may lead to amisclassification of pre-main sequence stars with disks when viewededge-on. Aims: We performed radiative transfer calculations toshow the effects of different geometries on the main indicators of YSOevolutionary stage. In particular we tested not only the effects on theinfrared colors, like the slope α of the flux between 2.2 and 24μm, but also on other popular indicators of YSO evolutionary stage,such as the bolometric temperature and the optical depth of silicatesand ices. Methods: We used the axisymmetric 3D radiative transfercodes RADMC and RADICAL to calculate the spectral energy distributionincluding silicates and ice features in a grid of models covering therange of physical properties typical of embedded and pre-main sequencesources. Results: Our set of models compares well with existingobservations, supporting the assumed density parametrization and theadopted dust opacities. We show that for systems viewed at intermediateangles (25°-70°) the “classical” indicators ofevolution are able to classify the degree of evolution of young stellarobjects since they accurately trace the envelope column density, andthey all agree with each other. On the other hand, edge-on system aremisclassified for inclinations larger than 65° ± 5°,where the spread is mostly due to the range of mass and the flaringdegree of the disk. In particular, silicate emission, typical ofpre-main sequence stars with disks, turns into silicate absorption whenthe disk column density along the line of sight reaches 1 ×1022 cm-2, corresponding e.g. to a 5 ×10-3 M_ȯ flaring disk viewed at 64°. A similareffect is noticed in all the other classification indicators studied:α, T_bol, and the H{2}O and CO{2} ice absorption strengths. Thismisclassification has a large impact on the nature of the flat-spectrumsources (α ≃ 0), whose number can be explained by simplegeometrical arguments without invoking evolution. A reliableclassification scheme using a minimal number of observations isconstituted by observations of the millimeter flux with both a singledish and an interferometer.Table 2 is only available in electronic form at http://www.aanda.org
| Molecules in the circumstellar disk orbiting BP Piscium
Context: BP Psc is a puzzling late-type, emission-line field star withlarge infrared excess. The star is encircled and enshrouded by a nearlyedge-on, dusty circumstellar disk, and displays an extensive jet systemsimilar to those associated with pre-main sequence (pre-MS) stars.However, the photospheric absorption features of the star itself appearmore consistent with post-main sequence status. Aims: We seek tocharacterize the molecular gas component of the BP Psc disk, so as tocompare the properties of its molecular disk with those of well-studiedpre-main sequence stars. Methods: We conducted a mm-wave molecular linesurvey of BP Psc with the 30 m telescope of the Institut de RadioAstronomie Millimetrique (IRAM). We use these data to investigate thekinematics, gas mass, and chemical constituents of the BP Psc disk. Results: We detected lines of 12CO and 13CO and,possibly, very weak emission from HCO+ and CN; HCN, H2CO, andSiO are not detected. The CO line profiles of BP Psc are well fit by amodel invoking a disk in Keplerian rotation. The mimumum disk gas mass,inferred from the 12CO line intensity and13CO/12CO line ratio, is ~0.1 Jupiter masses. Conclusions: The weakness of HCO+ and CN (relative to13CO) stands in sharp contrast to the strong HCO+and CN emission that characterizes most low-mass, pre-main sequencestars that have been the subjects of molecular emission-line surveys,and is suggestive of a very low level of X-ray-induced molecularionization within the BP Psc disk. These results lend some support tothe notion that BP Psc is an evolved star whose circumstellar disk hasits origins in a catastrophic interaction with a close companion.
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