Ýçindekiler
Görüntüler
Resim Yükleyin
DSS Images Other Images
Ýlgili Makaleler
CO and H+3 Toward MWC 1080, MWC 349, and LkH? 101
We present high-resolution, near-infrared NIRSPEC observations of thefundamental rovibrational CO and H+ 3 R(1,0),R(1,1) u , and Q(1,0) transitions toward three early-typeyoung stars: MWC 1080, MWC 349, and LkH? 101. These observationswere performed for the purpose of constraining the physicalcharacteristics of the interstellar material along each line of sight.Toward MWC 1080, we detected strong CO absorption and determined acolumn density upper limit of 1.4 × 1014cm-2 for H+ 3. We infer thatthere is very little diffuse material along the line of sight toward MWC1080 and that the CO absorption is consistent with an origin in thedispersing natal cloud. We detected both cold CO and H+3 toward MWC 349, consistent with a diffuse cloud origin.Similarly, both CO and H+ 3 were detected towardLkH? 101. Using a recently revised value for the cosmic rayionization rate, we conclude that the CO absorption is consistent with adense cloud origin while the H+ 3 could originatein either the dense or diffuse interstellar medium. We also find noevidence for CO fractionation toward LkH? 101 as reported by Gotoet al.
| The mass ratio and formation mechanisms of Herbig Ae/Be star binary systems
We present B- and R-band spectroastrometry of a sample of 45 HerbigAe/Be (HAe/Be) stars in order to study their binary properties. All butone of the targets known to be binary systems with a separation of~0.1-2.0 arcsec are detected by a distinctive spectroastrometricsignature. Some objects in the sample exhibit spectroastrometricfeatures that do not appear attributable to a binary system. We findthat these may be due to light reflected from dusty haloes or materialentrained in winds. We present eight new binary detections and fourdetections of an unknown component in previously discovered binarysystems. The data confirm previous reports that HAe/Be stars have a highbinary fraction, 74 +/- 6 per cent in the sample presented here. We usea spectroastrometric deconvolution technique to separate the spatiallyunresolved binary spectra into the individual constituent spectra. Theseparated spectra allow us to ascertain the spectral type of theindividual binary components, which in turn allows the mass ratio ofthese systems to be determined. In addition, we appraise the method usedand the effects of contaminant sources of flux. We find that thedistribution of system mass ratios is inconsistent with random pairingfrom the initial mass function, and that this appears robust despite adetection bias. Instead, the mass ratio distribution is broadlyconsistent with the scenario of binary formation via disc fragmentation.Based on observations made with the William Herschel Telescope (WHT) andthe Isaac Newton Telescope (INT) operated on the island of La Palma bythe Isaac Newton Group in the Spanish Observatorio del Roque de losMuchachos of the Instituto de Astrofísica de Canarias.E-mail: pyhew@leeds.ac.uk
| 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.
| Evidence for an Edge-on Disk Around the Young Star MWC 778 from Infrared Imaging and Polarimetry
MWC 778 is an unusual and little-studied young stellar object located inthe IC 2144 nebula. Recent spectroscopy by Herbig & Vacca suggestedthe presence of an edge-on circumstellar disk around it. We presentnear-infrared adaptive optics imaging polarimetry and mid-infraredimaging which directly confirm the suspected nearly edge-on disk aroundMWC 778 (i ~ 70°-80°) plus reveal a more extensive envelopepierced by bipolar outflow cavities. In addition, our mid-infraredimages and near-infrared polarization maps detect a spiral-shapedstructure surrounding MWC 778, with arms that extend beyond 6'' oneither side of the star. Although MWC 778 has previously been classifiedas a Herbig Ae/Be star, the properties of its central source (includingits spectral type) remain fairly uncertain. Herbig & Vacca suggestedan F or G spectral type based on the presence of metallic absorptionlines in the optical spectrum, which implies that MWC 778 may belong tothe fairly rare class of intermediate-mass T Tauri Stars (IMTTSs) whichare the evolutionary precursors to Herbig Ae/Be objects. Yet itsintegrated bolometric luminosity, gsim750 L sun (for anassumed distance of 1 kpc) is surprisingly high for an F or G spectraltype, even for an IMTTS. We speculate on several possible explanationsfor this discrepancy, including its true distance being much closer than1 kpc, the presence of a binary companion, and/or a nonstellar originfor the observed absorption lines.
| 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 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.
| Maximum stellar mass versus cluster membership number revisited
We have made a new compilation of observations of maximum stellar massversus cluster membership number from the literature, which we analysefor consistency with the predictions of a simple random drawinghypothesis for stellar mass selection in clusters. Previously, Weidnerand Kroupa have suggested that the maximum stellar mass is lower, inlow-mass clusters, than would be expected on the basis of randomdrawing, and have pointed out that this could have importantimplications for steepening the integrated galactic initial massfunction (IGIMF) at high masses. Our compilation demonstrates how theobserved distribution in the plane of maximum stellar mass versusmembership number is affected by the method of target selection; inparticular, rather low n clusters with large maximum stellar masses areabundant in observational data sets that specifically seek clusters inthe environs of high-mass stars. Although we do not consider ourcompilation to be either complete or unbiased, we discuss the method bywhich such data should be statistically analysed. Our very provisionalconclusion is that the data are not indicating any striking deviationfrom the expectations of random drawing.
| 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.
| Dense Molecular Gas in a Young Cluster around MWC 1080: Rule of the Massive Star
We present CS J=2-->1, 13CO J=1-->0, andC18O J=1-->0, observations with the 10 element BerkeleyIllinois Maryland Association (BIMA) array toward the young clusteraround the Be star MWC 1080. These observations reveal a biconicaloutflow cavity with size ~0.3 and 0.05 pc for the semimajor andsemiminor axis and ~45° position angle. These transitions trace thedense gas, which is likely the swept-up gas of the outflow cavity,rather than the remaining natal gas or the outflow gas. The gas isclumpy; 32 clumps are identified. The identified clumps areapproximately gravitationally bound and consistent with a standardisothermal sphere density, which suggests that they are likelycollapsing protostellar cores. The gas kinematics suggests that velocitygradients exist that imply effects from the inclination of the cavityand MWC 1080. The kinematics of dense gas has also been affected byeither outflows or stellar winds from MWC 1080, and lower mass clumpsare possibly under stronger effects from MWC 1080 than higher massclumps. In addition, low-mass cluster members tend to be formed in thedenser and more turbulent cores, compared to isolated low-massstar-forming cores. This results from the contributions of nearbyforming massive stars, such as outflows or stellar winds. Therefore, weconclude that in clusters like the MWC 1080 system, effects from massivestars dominate the star-forming environment in both the kinematics anddynamics of the natal cloud and the formation of low-mass clustermembers. This study provides insights into the effects of MWC 1080 onits natal cloud, and suggests a different low-mass star-formingenvironment in clusters compared to isolated star formation.
| Extended shells around B[e] stars. Implications for B[e] star evolution
Aims.The position of B[e] stars in the upper left part of theHertzsprung-Russell diagram creates a quandary. Are these stars youngstars evolving onto the main sequence or old stars that are evolving offof it? Spectral characteristics suggest that B[e] stars can be placedinto five subclasses and are not a homogeneous set. Suchsub-classification is believed to coincide with varying origins anddifferent evolutions. However, the evolutionary connection of B[e] stars- and notably sgB[e] - to other stars is unclear, particularly toevolved massive stars. We attempt to provide insight into theevolutionary past of B[e] stars. Methods: We performed an Hαnarrow-band CCD imaging survey of B[e] stars, in the northernhemisphere. Prior to the current work, no emission-line survey of B[e]stars had yet been made, while only two B[e] stars appeared to have ashell nebula as seen in the Digital Sky Survey. Of nebulae around B[e]stars, only the ring nebula around MWC 137 has been previously observedextensively. Results: In this presentation we report the findings fromour narrow-band optical imaging survey of the environments of 25 B[e]stars. Of the objects surveyed, 7 show bipolar or uni- polar structuresup to 15 arcmin across; 5 show faint, large, or filamentary shells; and2 are compact planetary nebula-type systems. The most spectacular systemobserved is a large bipolar structure associated with MWC 314. Conclusions: The possible links between B[e] stars and other evolvedstars, implied by our observations, are investigated.
| Massive stars, globular clusters and elliptical galaxies
Globular clusters as omega Cen and NGC 2808 appear to have a populationof very He-rich stars. From a theoretical point of view, one expects thepresence of He-rich stars in all globular clusters showing anoxygen-sodium (O-Na) anticorrelation. In this paper, we briefly recallhow fast rotating massive stars could be the main source of the materialfrom which He-rich stars have formed. We speculate that the UV-upturnphenomenon observed in all elliptical galaxies might be due to He-richstars. If this hypothesis is correct then fast rotating massive starsmay have played in the early evolutionary phases of these systems asimilar role as the one they played in the nascent globular clusters.
| Water Maser and Ammonia Survey toward IRAS Sources in the Galaxy I. H2O Maser Data
We present H2O maser data from a survey toward IRAS sourcesin the Galaxy with the Nobeyama 45m telescope. This survey had a1σ noise level as small as 0.24Jy, resulting in one of the mostsensitive water-maser surveys. The maximum distance of the masers to bedetected by our survey is estimated to be 3kpc for sources withFν,1kpc < 10Jy and 10kpc for those with 10Jy ≤Fν,1kpc < 100Jy, where Fν,1kpc is themaser flux density converted at a distance of 1kpc. For strong maserswith Fν,1kpc ≥ 100Jy, our survey could detect allsources in the Galaxy. We carried out a total of 2229 observationstoward 1563 sources and detected water-maser emission toward 222sources. Our survey newly found masers from 75 of the 222 sources. Themaser spectra of the new sources are shown in addition to the lineparameters of all the detected sources. Furthermore, we discovered anextremely high-velocity component with VLSR = ‑146 kms‑1 toward a well-known source, NGC 7538 IRS 11. Forthe three sources of NGC 1333 IRAS 4A/B, IRAS 05329‑0512, and06053‑0622, we succeeded to spatially separate multiple-velocitycomponents.
| Discovery of a New Dusty B[e] Star in the Small Magellanic Cloud
We present new optical spectroscopic and archival Spitzer IRACphotometric observations of a B-type star in the SMC cluster NGC 346,NGC 346:KWBBe 200. We detect numerous Fe II, [O I], and [Fe II] lines,as well as strong P Cygni profile H I emission lines in its opticalspectrum. The star's near-IR color and optical to IR spectral energydistribution clearly indicate the presence of an infrared excess,consistent with the presence of gas and warm, T~800 K, circumstellardust. Based on a crude estimate of the star's luminosity and theobserved spectroscopic line profile morphologies, we find that the staris likely to be a B-type supergiant. We suggest that NGC 346:KWBBe 200is a newly discovered B[e] supergiant star, and represents the fifthsuch object to be identified in the SMC.
| An Evolved Disk Surrounding the Massive Main-Sequence Star MWC 297?
We present the results of the interferometric observations of thecircumstellar disk surrounding MWC 297 in the continuum at 230 GHz (1.3mm) and in the (J=2-1) rotational transitions of12CO,13CO, and C18O using theSubmillimeter Array. At a distance of 250 pc, MWC 297 is one of theclosest, young massive stars (M*~10 Msolar) to us.Compact continuum emission is detected toward MWC 297 from which weestimate a disk mass (gas+dust) of 0.07 Msolar and a diskradius of <=80 AU. Our result demonstrates that circumstellar diskscan survive around massive stars well into their main-sequence phaseeven after they have become optically visible. Complementing ourobservations with the data compiled from the literature, we find thesubmillimeter dust opacity index ? to be between 0.1 and 0.3. Ifthe emission is optically thin, the low value of ? indicates thepresence of relatively large grains in the disk, possibly because ofgrain growth. We do not detect any CO emission associated with thecontinuum source. We argue that the 13CO emission from thedisk is likely optically thin, in which case we derive an upper limit tothe gas mass that implies significant depletion of molecular gas in thedisk. The mass of this disk and the evolutionary trends observed aresimilar to those found for intermediate-mass Herbig Ae stars andlow-mass T Tauri stars.
| Optical and infrared properties of V1647 Orionis during the 2003-2006 outburst. II. Temporal evolution of the eruptive source
Aims.The occurrence of new FU Orionis-like objects is fundamental tounderstand the outburst mechanism in young stars and their role in starformation and disk evolution. Our work is aimed at investigating theproperties of the recent outburst of V1647 Ori. Methods: Using opticaland mid infrared long slit spectroscopy we monitored V1647Orionis in outburst between February 2004 and January 2006. Results: The optical spectrum is characterized by Hα and Hβin P-Cygni profile and by many weak Fe i and Fe ii emission lines. Shorttimescale variability was measured in the continuum and line emission.In January 2006 we detected for the first time forbidden emission lines([O i], [S ii] and [Fe ii]). These lines are likely produced by aHerbig-Haro object driven by V1647 Ori. The mid infrared spectrum ofV1647 Ori is flat and featureless at all epochs. The SED changeddrastically: the source was much redder in the early outburst than inthe final phase. The magnitude rise and the SED of V1647 Ori resemblesthat of a FUor while the duration and recurrence of the outburstresemble that of a EXor. The optical spectrum is clearly distinct fromeither the absorption line spectrum of a FUor or the T Tauri-likespectrum of an EXor. Conclusions: Our data are consistent with a diskinstability event which led to an increase of the mass accretion rate.The data also suggest the presence of a circumstellar envelope aroundthe star+disk system. The peculiar N band spectrum might be explained bydust sublimation in the outer layers of the disk. The presence of theenvelope and the outburst statistics suggest that these instabilityevents occur only in a specific stage of a Class I source (e.g. in thetransition phase to an optically visible star surrounded by aprotoplanetary disk). We discuss the outburst mechanisms in term of thethermal instability model.
| On the difference between Herbig Ae and Herbig Be stars
We present linear spectropolarimetric data for eight Herbig Be and fourHerbig Ae stars at Hα, Hβ and Hγ. Changes in the linearpolarization are detected across all Balmer lines for a large fractionof the observed objects, confirming that the small-scale regionssurrounding these objects are flattened (i.e. disc-like). Furthermore,all objects with detections show similar characteristics at the threespectral lines, despite differences in transition probability andoptical depth going from Hα to Hγ. A large fraction of earlyHerbig Be stars (B0-B3) observed show line-depolarization effects.However, the early Herbig Ae stars (A0-A2), observed for comparison,show intrinsic line-polarization signatures. Our data suggest that thepopular magnetic accretion scenario for T Tauri objects may be extendedto Herbig Ae stars, but that it may not be extended to early Herbig Bestars, for which the available data are consistent with disc accretion.
| A survey of OH masers towards high mass protostellar objects
Context: Masers are important tracers of the early evolution of younghigh mass stars, but the relationship between different types of maserand the evolutionary state of the exciting source remains unclear. Aims: To determine whether OH masers are common towards candidate highmass protostellar objects. Methods: We present a survey of OH maseremission towards a sample of high mass protostellar objects made usingthe Nançay and GBT telescopes. Results: OH maser emission wasdetected towards 63 objects with 36 new detections. There are 56star-forming regions and 7 OH/IR candidates. Nearly half of the detectedsources have OH flux densities ⪉1 Jy. There is no evidence thatsources with OH masers have a different range of luminosities from thenon-maser sources. The results of this survey are compared with previousH2O and class II CH3OH maser observations of the same objects. Some ofthe detected sources are only associated with OH masers and some sourcesare only associated with the 1720 MHz OH maser line. The velocity rangeof the maser emission suggests that the water maser sources may bedivided into two groups. The detection rates and velocity range of theOH and Class II CH3OH masers support the idea that there is a spatialassociation of the OH and Class II CH3OH masers. The sources span a widerange in R, the ratio of the methanol maser peak flux to OH 1665 MHzmaser peak flux, however there are only a few sources with intermediatevalues of R, 8
| Relation between the Luminosity of Young Stellar Objects and Their Circumstellar Environment
We present a new model-independent method of comparison of NIRvisibility data of YSOs. The method is based on scaling the measuredbaseline with the YSO's distance and luminosity, which removes thedependence of visibility on these two variables. We use this method tocompare all available NIR visibility data and demonstrate that itdistinguishes YSOs of luminosity L*<~103Lsolar (low L) from YSOs of L*>~103Lsolar (high L). This confirms earlier suggestions, based onfits of image models to the visibility data, for the difference betweenthe NIR sizes of these two luminosity groups. When plotted against the``scaled'' baseline, the visibility creates the following data clusters:low-L Herbig Ae/Be stars, T Tauri stars, and high-L Herbig Be stars. Wemodel the shape and size of clusters with different image models andfind that low-L Herbig stars are best explained by the uniformbrightness ring and the halo model, T Tauri stars with the halo model,and high-L Herbig stars with the accretion disk model. However, theplausibility of each model is not well established. Therefore, we try tobuild a descriptive model of the circumstellar environment consistentwith various observed properties of YSOs. We argue that low-L YSOs haveoptically thick disks with the optically thin inner dust sublimationcavity and an optically thin dusty outflow above the inner disk regions.High-L YSOs have optically thick accretion disks with high accretionrates enabling gas to dominate the NIR emission over dust. Althoughobservations would favor such a description of YSOs, the required dustdistribution is not supported by our current understanding of dustdynamics.
| Spectrally Dispersed K-Band Interferometric Observations of Herbig Ae/Be Sources: Inner Disk Temperature Profiles
We use spectrally dispersed near-IR interferometry data to constrain thetemperature profiles of sub-AU-sized regions of 11 Herbig Ae/Be sources.We find that a single-temperature ring does not reproduce the data well.Rather, models incorporating radial temperature gradients are preferred.These gradients may arise in a dusty disk, or may reflect separate gasand dust components with different temperatures and spatialdistributions. Comparison of our models with broadband spectral energydistributions suggests the latter explanation. The data support the viewthat the near-IR emission of Herbig Ae/Be sources arises from hotcircumstellar dust and gas in sub-AU-sized disk regions. Intriguingly,our derived temperature gradients appear systematically steeper fordisks around higher mass stars. It is not clear, however, whether thisreflects trends in relative dust/gas contributions or gradients withinindividual components.
| Optical spectroscopy of close companions to nearby Herbig Ae/Be and T Tauri stars
We present VLT-FORS2 optical (5700-9400 Å) spectroscopy of close(r < 1.5´´) companions to three nearby (d < 200 pc)Herbig Ae/Be stars (HD 144432, HD 150193, KK Oph) and one T Tauri star(S CrA). We report the detection of Li I (6707 Å) in absorptionand emission lines (Hα, Ca II triplet) in the spectra of thecompanions. Our observations strongly suggest that the companions arephysically associated pre-main-sequence stars. The spectral type derivedfor the companions is K5Ve for HD 144432 B, F9Ve for HD 150193 B, andG6Ve for KK Oph B. S CrA A and B were observed simultaneously. Thespatially resolved spectra indicate that S CrA A (primary, north) is a Gstar and that S CrA B (secondary, south) is a K star. Using photometryfrom the literature and estimations of the R and I magnitude derivedfrom the spectra, we localized primaries and companions in the HRdiagram, derived their masses and assuming coevality constrained the ageof the systems. KK Oph B (7 Myr) and S CrA B (2 Myr) are activelyaccreting T Tauri stars and are very likely surrounded by disks. HD150193 B (10 Myr) and HD 144432 B (8 Myr) are weak-line T Tauri stars.Three of the four systems studied (HD 144432, HD 150193, KK Oph) haveages >7 Myr. These systems retained their disks for a longer timethan typical of a young star. Our results suggest that binarity may be akey issue in understanding the lifetime of disks.Based on observations collected at the European Southern Observatory,Chile (program ID 075.C-0395(A)). Figure 3 is only available inelectronic form at http://www.aanda.org
| Evolution of Emission-Line Activity in Intermediate-Mass Young Stars
We present optical spectra of 45 intermediate-mass Herbig Ae/Be stars.Together with the multiepoch spectroscopic and photometric data compiledfor a large sample of these stars and ages estimated for individualstars by using pre-main-sequence evolutionary tracks, we have studiedthe evolution of emission-line activity in them. We find that, onaverage, the Hα emission line strength decreases with increasingstellar age in Herbig Ae/Be stars, indicating that the accretionactivity gradually declines during the pre-main-sequence phase. Thiswould hint at a relatively long-lived (a few Myr) process beingresponsible for the cessation of accretion in Herbig Ae/Be stars. Wealso find that the accretion activity in these stars drops substantiallyby ~3 Myr. This is comparable to the timescale in which mostintermediate-mass stars are thought to lose their inner disks,suggesting that inner disks in intermediate-mass stars are dissipatedrapidly after the accretion activity has fallen below a certain level.We further find a relatively tight correlation between strength of theemission line and near-infrared excess due to inner disks in HerbigAe/Be stars, indicating that the disks around Herbig Ae/Be stars cannotbe entirely passive. We suggest that this correlation can be understoodwithin the framework of the puffed-up inner rim disk models if theradiation from the accretion shock is also responsible for the diskheating.
| A Keplerian Gaseous Disk around the B0 Star R Monocerotis
We present high angular resolution observations of the circumstellardisk around the massive Herbig Be star R Mon (M*~8Msolar) in the continuum at 2.7 and 1.3 mm and the12CO 1-->0 and 2-->1 rotational lines. On the basis ofthe new 1.3 mm continuum image, we estimate a disk mass (gas+dust) of0.007 Msolar and an outer radius of <150 AU. Our CO imagesare consistent with the existence of a Keplerian rotating gaseous diskaround this star. Up to our knowledge, this is the most clear evidencefor the existence of Keplerian disks around massive stars reported thusfar. The mass and physical characteristics of this disk are similar tothose of the more evolved T Tauri stars and indicate a shorter timescalefor the evolution and dispersal of circumstellar disks around massivestars which lose most of their mass before the star becomes visible.
| Few Skewed Disks Found in First Closure-Phase Survey of Herbig Ae/Be Stars
Using the three-telescope IOTA interferometer on Mount Hopkins, wereport results from the first near-infrared (λ=1.65 μm)closure-phase survey of young stellar objects (YSOs). These closurephases allow us to unambiguously detect departures from centrosymmetry(i.e., skew) in the emission pattern from YSO disks on the scale of ~4mas, expected from generic ``flared disk'' models. Six of 14 targetsshowed small, yet statistically significant nonzero closure phases, withlargest values from the young binary system MWC 361-A and the(pre-main-sequence?) Be star HD 45677. Our observations are quitesensitive to the vertical structure of the inner disk, and we confrontthe predictions of the ``puffed-up inner wall'' models of Dullemond,Dominik, & Natta (DDN). Our data support disk models with curvedinner rims because the expected emission appears symmetricallydistributed around the star over a wide range of inclination angles. Incontrast, our results are incompatible with the models possessingvertical inner walls because they predict extreme skewness (i.e., largeclosure phases) from the near-IR disk emission that is not seen in ourdata. In addition, we also present the discovery of mysterious H-band``halos'' (~5%-10% of light on scales 0.01"-0.50") around a few objects,a preliminary ``parametric imaging'' study for HD 45677, and the firstastrometric orbit for the young binary MWC 361-A.
| VLT/NACO adaptive optics imaging of the Herbig Ae star HD 100453
We report the detection of a companion candidate to the Herbig Ae starHD 100453, using the NACO instrument at the VLT UT4. The separationbetween the companion candidate and HD 100453 is 1.6 arcsec, and theirbrightness contrast is 5.1 mag at Ks and 4.2 mag atBrγ. We discuss the possible nature of the newly detected starbased on near-infrared photometry. The photometric data are consistentwith a 0.3 M_ȯ companion to HD 100453, corresponding to a spectraltype of M3-M5. This detection could have important implications for thestructure of the disk around HD 100453.
| A search for shock-excited optical emission from the outflows of massive young stellar objects
We have searched for optical shock-excited emission lines in the outerparts of the bipolar outflows from massive young stellar objects wherethe flow terminates and the extinction is expected to be low. The TaurusTunable Filter (TTF) at the Anglo-Australian Telescope (AAT) was used toobtain narrow-band (12-15 Å) images of the Hα, [Nii] 6583,and [Sii] 6717/6731 lines around Mon R2 IRS3,S255 IRS1/3, GL 961 andGL 989. No clear examples of shocked emission werefound. A bow shock feature in the GL 989 region mayberelated with the molecular outflow NGC 2264 D. A weak optical bow-likefeature is seen in GL 961, but which of the stars inthe cluster is the driving source cannot be uniquely identified. Otheremission line features were found in Mon R2 and S255 that are consistentwith an Hii region origin. The observed fluxes and the upper limits werecompared with predictions from radiative shock models available in theliterature. Any head-on collisions of a jet with speeds of the order of500 km s-1 should have been detected if the extinctionAv < 10 mag. Estimates of the extinction in the outerparts of the molecular clouds are somewhat lower than this, although itis possible that it could be higher local to any interaction regions dueto swept up material. If the extinction is low, then the shocks musteither be slower (<150 km s-1) or very oblique(>75°), which would argue against the presence of highlycollimated jets from high mass young stellar objects.
| Pre-main sequence star Proper Motion Catalogue
We measured the proper motions of 1250 pre-main sequence (PMS) stars andof 104 PMS candidates spread over all-sky major star-forming regions.This work is the continuation of a previous effort where we obtainedproper motions for 213 PMS stars located in the major southernstar-forming regions. These stars are now included in this present workwith refined astrometry. The major upgrade presented here is theextension of proper motion measurements to other northern and southernstar-forming regions including the well-studied Orion and Taurus-Aurigaregions for objects as faint as V≤16.5. We improve the precision ofthe proper motions which benefited from the inclusion of newobservational material. In the PMS proper motion catalogue presentedhere, we provide for each star the mean position and proper motion aswell as important photometric information when available. We providealso the most common identifier. The rms of proper motions vary from 2to 5 mas/yr depending on the available sources of ancient positions anddepending also on the embedding and binarity of the source. With thiswork, we present the first all-sky catalogue of proper motions of PMSstars.
| An ISO-LWS two-colour diagram of Herbig Ae/Be stars
In this paper, we present and discuss an infrared two-colour diagrambuilt with the 60, 100, and 170 μm photometry of the whole sample ofHerbig Ae/Be stars observed by the spectrometers on board of ISOsatellite. An overview of this diagram reveals a certain degree ofhomogeneity in the behaviour of these stars and their IR-emittingenvironments, with some exceptions. In particular, we account for theobjects located to the left of the blackbody line. In addition, theinfrared colours obtained with ISO gave us the opportunity to comparewith the IRAS measurements, which generally appear in good agreement.Finally, a simple spherically symmetric model of pre-ZAMS circumstellarenvironment is used to obtain the two-colour diagram, as a diagnostictool complementary to the best-fit of the spectra in investigating thedistribution of matter around these stars.
| [O I] 6300 Å emission in Herbig Ae/Be systems: Signature of Keplerian rotation
We present high spectral-resolution optical spectra of 49 Herbig Ae/Bestars in a search for the [O i] 6300 Å line. The vast majority ofthe stars in our sample show narrow ({FWHM} < 100 km s-1)emission lines, centered on the stellar radial velocity. In only threesources is the feature much broader ( 400 km s-1), andstrongly blueshifted (-200 km s-1) compared to the stellarradial velocity. Some stars in our sample show double-peaked lineprofiles, with peak-to-peak separations of 10 km s-1. Thepresence and strength of the [O i] line emission appears to becorrelated with the far-infrared energy distribution of each source:stars with a strong excess at 60 μm have in general stronger [O i]emission than stars with weaker 60 μm excesses. We interpret thesenarrow [O i] 6300 Å line profiles as arising in the surface layersof the protoplanetary disks surrounding Herbig Ae/Be stars. A simplemodel for [O i] 6300 Å line emission due to the photodissociationof OH molecules shows that our results are in quantitative agreementwith that expected from the emission of a flared disk if the fractionalOH abundance is 5 × 10-7.
| The Near-Infrared Size-Luminosity Relations for Herbig Ae/Be Disks
We report the results of a sensitive K-band survey of Herbig Ae/Be disksizes using the 85 m baseline Keck Interferometer. Targets were chosento span the maximum range of stellar properties to probe the disk sizedependence on luminosity and effective temperature. For most targets,the measured near-infrared sizes (ranging from 0.2 to 4 AU) support asimple disk model possessing a central optically thin (dust-free)cavity, ringed by hot dust emitting at the expected sublimationtemperatures (Ts~1000-1500 K). Furthermore, we find a tightcorrelation of disk size with source luminosity R~L1/2 for Aeand late Be systems (valid over more than two decades in luminosity),confirming earlier suggestions based on lower quality data.Interestingly, the inferred dust-free inner cavities of the highestluminosity sources (Herbig B0-B3 stars) are undersized compared topredictions of the ``optically thin cavity'' model, likely because ofoptically thick gas within the inner AU.
| On the nature of pre-main sequence candidate stars in the Large Magellanic Cloud
We investigate a sample of 18 Large Magellanic CloudHerbig Ae/Be candidate stars looking at their (1) spectral types (2)brightness variability mechanism and (3) near infra-red JHK emission. Wefind that the majority of the target stars have Hα emission, areof spectral type early- to mid-B and lack strong JHK excess emission.Their Balmer decrements are found to be similar to that of Galactic Bestars in general. Their erratic brightness variability is evaluated byusing the observed optical color excess and the color gradient from thelight curves and is subsequently interpreted as being due to variabledust obscuration or variable bf-ff emission from circumstellar ionizedgas. For approximately half of the target stars in our sample the typeof variability seems to be dissimilar to the mechanism involvingbound-free and free-free emission, but could be interpreted as caused byvariable dust obscuration, as we have proposed in earlier studies. It istherefore suggested that they are pre-main sequence objects, despite thefact that they nearly all lack thermal dust emission in the nearinfra-red; mid/far infra-red observations for these objects arewarranted. One star is observed to have JHK excess emission and aninspection of its 7.5 year MACHO light curve confirms its erraticphotometric behavior. The object displays deep photometric minima with aquasi-period of 191.3 days, as generally seen in the Galactic pre-mainsequence subgroup of the UX Orionis stars.Based on observations collected at ESO, La Silla.
|
Yeni bir Makale Öner
Ýlgili Baðlantýlar
Yeni Bir Baðlantý Öner
sonraki gruplarýn üyesi:
|
Gözlemler ve gökölçümü verileri
Kataloglar ve belirtme:
|
INHABITED SKY
