目录
图像
上传图像
DSS Images Other Images
相关文章
A multi-wavelength study of the young star V1118 Orionis in outburst
Context. The accretion history of low-mass young stars is not smooth butshows spikes of accretion that can last from months and years to decadesand centuries. Aims: Observations of young stars in outbursts canhelp us understand the temporal evolution of accreting stars and theinterplay between the accretion disk and the stellar magnetosphere. Methods: The young late-type star V1118 Orionis was in outburst from2005 to 2006. We followed the outburst with optical and near-infraredphotometry. The X-ray emission was further probed with observationstaken with XMM-Newton and Chandra during and after the outburst. Inaddition, we obtained mid-infrared photometry and spectroscopy withSpitzer at the peak of the outburst and in the post-outburst phase. Results: The spectral energy distribution of V1118 Ori variedsignificantly over the course of the outburst. The optical flux showedthe largest variations, most likely caused by enhanced emission by a hotspot. The hot spot dominated the optical and near-infrared emission atthe peak of the outburst, while the disk emission dominated in themid-infrared. The emission silicate feature in V1118 Ori is flat anddoes not vary in shape, but was slightly brighter at the peak of theoutburst compared to the post-outburst spectrum. The X-ray fluxcorrelated with the optical and infrared fluxes, indicating thataccretion affected the magnetically active corona and the stellarmagnetosphere. The thermal structure of the corona was variable withsome indication of a cooling of the coronal temperature in the earlyphase of the outburst with a gradual return to normal values.Color-color diagrams in the optical and infrared showed variationsduring the outburst, with no obvious signature of reddening caused bycircumstellar matter. Using Monte-Carlo realizations ofstar+disk+hotspot models to fit the spectral energy distributions in“quiescence” and at the peak of the outburst, we determinedthat the mass accretion rate varied from about 2.5 ×10-7 M_? yr-1 to 1.0 × 10-6M_? yr-1; in addition, the fractional area of thehotspot increased significantly. Conclusions: Themulti-wavelength study of the V1118 Ori outburst helped us to understandthe variations in spectral energy distributions and demonstrated theinterplay between the disk and the stellar magnetosphere in a young,strongly accreting star.Appendices are only available in electronic form at http://www.aanda.orgTwo tables ofphotometry are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/511/A63
| The nature of the recent extreme outburst of the Herbig Be/FU Orionis binary Z Canis Majoris
Context. Z CMa is a binary system which consists of two young stars: aHerbig AeBe component Z CMa NW embedded in a dust cocoon and a lessmassive component Z CMa SE, which is classified as a FU Orionis typestar. Associated to the binary system is a giant parsec-size jet. Pastspectropolarimetric observations showed that the position angle of thelinear optical polarization is perpendicular to the jet axis, indicatingthat the visual light escapes the cocoon via cavities aligned with thejet axis and is then scattered back into the line of sight of theobserver. Recently the system showed the largest outburst reportedduring the almost 90 years of available observations. Aims: Wepresent new spectrophotometric and spectropolarimetric data obtained in2008 during the recent outburst phase. Methods: The data obtainedin the visual spectral range at medium spectral resolution were used tostudy the geometry of the system from the linear polarization spectra aswell as its magnetic field from the circular polarization spectra. Results: During the recent outburst we detected that the Z CMa systemis polarized by 2.6% in the continuum and emission line spectrum, with aposition angle still perpendicular to the jet. From the high level ofpolarization we concluded that the outburst is associated with thedust-embedded Herbig AeBe NW component. The deep absorption componentsof the Balmer lines in the velocity frame which extend from zerovelocity and reach a wind velocity of 700 km s-1, togetherwith the absence of a red-shifted broad emission at similar velocities,indicate a bi-polar wind. We did not detect a significant meanlongitudinal magnetic field during the outburst, but in the dataobtained in 2004 we detected the possible presence of a rather strongmagnetic field of the order of 1 kG. However, we critically review theapplied method of magnetic field measurements in the presence of astrong stellar wind. The main result of our studies is that thebolometric luminosity of Z CMa remained surprisingly constant during therecent outburst. We conclude that either the geometry of the cavitythrough which the light escapes from the cocoon has opened a new path orthat the screen of dust, which reflects the light toward the observerbecame more efficient, causing the observed increase of the visualbrightness by about 2fm5.Based on observations made with ESO Telescopes at the ParanalObservatories under programme IDs: 074.C-0442, 081.C-0410, and282.C-5041.Appendix A, Tables 1 and 2, and Fig. 6 are only available athttp://www.aanda.org
| A systematic study of variability among OB-stars based on HIPPARCOS photometry
Context: Variability is a key factor for understanding the nature of themost massive stars, the OB stars. Such stars lie closest to the unstableupper limit of star formation. Aims: In terms of statistics, thedata from the HIPPARCOS satellite are unique because of time coverageand uniformity. They are ideal to study variability in this large,uniform sample of OB stars. Methods: We used statisticaltechniques to determine an independant threshold of variabilitycorresponding to our sample of OB stars, and then applied an automaticalgorithm to search for periods in the data of stars that are locatedabove this threshold. We separated the sample stars into 4 maincategories of variability: 3 intrinsic and 1 extrinsic. The intrinsiccategories are: OB main sequence stars (~2/3 of the sample), OBe stars(~10%) and OB Supergiant stars (~1/4).The extrinsic category refers toeclipsing binaries. Results: We classified about 30% of the wholesample as variable, although the fraction depends on magnitude level dueto instrumental limitations. OBe stars tend to be much more variable(≈80%) than the average sample star, while OBMS stars are belowaverage and OBSG stars are average. Types of variables include αCyg, β Cep, slowly pulsating stars and other types from the generalcatalog of variable stars. As for eclipsing binaries, there arerelatively more contact than detached systems among the OBMS and OBestars, and about equal numbers among OBSG stars.
| Mid-IR observations of circumstellar disks. Part III. A mixed sample of PMS stars and Vega-type objects
We present new mid-infrared spectra of 15 targets (1 FU Orionis object,4 Herbig Ae stars, 5 T Tauri stars, and 5 Vega-type stars), obtainedwith the TIMMI2 camera at La Silla Observatory (ESO). Three targets aremembers of the ? Pic moving group (HD 155 555,HD 181 296, and HD 319 139). PAHbands are observed towards the T Tauri star HD 34 700and the Herbig Ae star PDS 144 N. For HD 34700, the band profiles indicate processed PAHs. The spectrumof the Vega-type object ? Corvi (HD 109 085), forwhich a resolved disk at sub-mm wavelengths is known, appears stellarbetween 8-13 ?m, but a small excess emission was reported by Spitzerobservations. Similarly, no indication of circumstellar matter atmid-infrared wavelengths is found towards the Vega-like starsHD 3003, HD 80 951, HD181 296, and, surprisingly, the T Tauri system HD 155555. The silicate emission features of the remaining eightsources are modelled with a mixture of silicates of different grainsizes and composition. Unprocessed dust dominates FUOri, HD 143 006, and CD-43344. Large amorphous grains are the main dust component aroundHD 190 073, HD 319 139,KK Oph, and PDS 144 S. Both smallgrains and crystalline dust is found for the Vega-type HD 123356, with a dominance of small amorphous grains. We show thatthe infrared emission of the binary HD 123 356 isdominated by its late-type secondary, but optical spectroscopy is stillrequired to confirm the age of the system and the spectral class of thecompanion. For most targets, this is their first mid-infraredspectroscopic observation. We investigate trends between stellar, disk,and silicate properties and confirm correlations identified in previousstudies. Several objects present an exciting potential for follow-uphigh-resolution disk studies.Based on observations made at the European Southern Observatory,obtained under program IDs 076.C-0634(A) and 077.C-0054(A),during TIMMI2 technical time as well as collected from the ESO/ST-ECFScience Archive Facility.
| Star formation history of Canis Major R1. I. Wide-Field X-ray study of the young stellar population
Aims: The CMa R1 star-forming region contains several compact clustersas well as many young early-B stars. It is associated with a well-knownbright rimmed nebula, the nature of which is unclear (fossil HII regionor supernova remnant). To help elucidate the nature of the nebula, ourgoal was to reconstruct the star-formation history of the CMa R1 region,including the previously unknown older, fainter low-mass stellarpopulation, using X-rays. Methods: We analyzed images obtainedwith the ROSAT satellite, covering 5 sq. deg. Complementary VRIphotometry was performed with the Gemini South telescope.Colour-magnitude and colour-colour diagrams were used in conjunctionwith pre-main sequence evolutionary tracks to derive the masses and agesof the X-ray sources. Results: The ROSAT images show two distinctclusters. One is associated with the known optical clusters near Z CMa,to which 40 members are added. The other, which we name the “GUCMa” cluster, is new, and contains 60 members. The ROSAT sourcesare young stars with masses down to M_star 0.5 M_?, and ages upto 10 Myr. The mass functions of the two clusters are similar, but theGU CMa cluster is older than the cluster around Z CMa by at least a fewMyr. Also, the GU CMa cluster is away from any molecular cloud, implyingthat star formation must have ceased; on the contrary (as alreadyknown), star formation is very active in the Z CMa region.Based in part on observations obtained at the Gemini Observatory, 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 Ministerio de Ciencia, Tecnología eInnovación Productiva (Argentina).
| New Extreme Outburst of Z CMa
Not Available
| Spatially resolved mid-infrared observations of the triple system T Tauri
Aims: The aim of this study is to enhance our knowledge of thecharacteristics and distribution of the circumstellar dust associatedwith the individual components of the young hierarchical triple system TTau. Methods: To reach this goal, observations in the N-band(8-13 ?m) with the two-telescope interferometric instrument MIDI atthe VLTI were performed. For the northern component of the T Tau system,projected baseline lengths of 43 m, 62 m, and 85 m were used. For thesouthern binary projected baseline lengths of equivalent resolutioncould be utilised. Our study is based on both the interferometric andthe spectrophotometric measurements and is supplemented by new visualand infrared photometry. Also, the phases were investigated to determinethe dominating mid-infrared source in the close southern binary. Thedata were fit with the help of a sophisticated physical disc model. Thismodel utilises the radiative transfer code MC3D that is based on theMonte-Carlo method. Results: Extended mid-infrared emission isfound around all three components of the system. Simultaneous fits tothe photometric and interferometric data confirm the picture of analmost face-on circumstellar disc around T Tau N. Towards this star, thesilicate band is seen in emission. This emission feature is used tomodel the dust content of the circumstellar disc. Clear signs of dustprocessing are found. Towards T Tau S, the silicate band is seen inabsorption. This absorption is strongly pronounced towards the infraredcompanion T Tau Sa as can be seen from the first individual N-bandspectra for the two southern components. Our fits support the previoussuggestion that an almost edge-on disc is present around T Tau Sa. Thisdisc is thus misaligned with respect to the circumstellar disc around TTau N. The interferometric data indicate that the disc around T Tau Sais oriented in the north-south direction, which favours this source aslaunching site for the east-western jet. We further determine from theinterferometric data the relative positions of the components of thesouthern binary in the N-band. We find good agreement with recentposition measurements in the near-infrared.Based on observations with the Very Large Telescope Interferometer(VLTI, proposal 074. C-0209(A) and 077.C-0176(C)).
| 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.
| The RMS survey. 6 cm continuum VLA observations towards candidate massive YSOs in the northern hemisphere
Context: The Red MSX Source (RMS) survey is an ongoing multi-wavelengthobservational programme designed to return a large, well-selected sampleof massive young stellar objects (MYSOs). We have identified ~2000 MYSOcandidates located throughout the Galaxy by comparing the colours of MSXand 2MASS point sources to those of known MYSOs. The aim of thesefollow-up observations is to identify other objects with similar colourssuch as ultra compact (UC) HII regions, evolved stars and planetarynebulae (PNe) and distinguish between genuine MYSOs and nearby low-massYSOs. Aims: To identify the populations of UCHII regions and PNewithin the sample and examine their Galactic distribution. Methods: We have conducted high resolution radio continuumobservations at 6 cm towards 659 MYSO candidates in the northernhemisphere (10° < l < 250°) using the Very Large Array(VLA). These observations have a spatial resolution of~1-2´´ and typical image rms noise values of ~0.22 mJy -sensitive enough to detect a HII region powered by B0.5 star at the farside of the Galaxy. In addition to these targeted observations wepresent archival data towards a further 315 RMS sources extracted from aprevious VLA survey of the inner Galaxy. Results: We present theresults of radio continuum observations made towards 974 MYSOcandidates, 272 (~27% of the observed sample) of which are found to beassociated with radio emission above a 4? detection limit (~1mJy). Using results from other parts of our multi-wavelength survey weseparate these RMS-radio associations into two distinct types ofobjects, classifying 51 as PNe and a further 208 as either compact or UCHII regions. Including all HII regions and PNe identified either fromthe literature or from the multi-wavelength data these numbers increaseto 391 and 79, respectively. Using this well selected sample of HIIregions we estimate their Galactic scale height to be 0.6°. Inaddition to the RMS-radio associations we are able to set upper limitson the radio emission of ?1 mJy for the 702 non-detections, which isbelow the level expected if they had already begun to ionise theirsurroundings. Conclusions: Using radio continuum and archivaldata we have identified 79 PNe and 391 HII regions within the northernRMS catalogue. We estimate the total fraction of contamination by PNe inthe RMS sample is of order 10%. The sample of HII regions is probablythe best representation to date of the Galactic population of HIIregions as a whole.Full Tables 3-5 and Full Figs. 2, 4, and 7 are only available inelectronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/501/539
| 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.
| The X-ray emission from Z Canis Majoris during an FUor-like outburst and the detection of its X-ray jet
Accretion shocks have been recognized as an important X-ray emissionmechanism for pre-main sequence stars, and yet the X-ray properties ofFUor outbursts, events that are caused by violent accretion, have beengiven little attention. We observed the FUor object Z CMa during opticaloutburst and quiescence with Chandra. No significant changes in X-raybrightness and spectral shape were found, suggesting that the X-rayemission is coronal. The binary nature of Z CMa makes the origin of theX-ray source ambiguous. However, the moderate hydrogen column densityderived from our data makes it unlikely that the embedded primary staris the X-ray source. The secondary star, which is the FUor object, isthus responsible for both the X-ray emission and the ongoing accretionoutburst, which seem, however, to be unrelated phenomena. The secondaryis also known to drive a large outflow and jet, which we detect here forthe first time in X-rays. The distance of the X-ray emitting outflowsource to the central star is greater than in jets of low-mass stars.
| A new spectroscopic and interferometric study of the young stellar object V645 Cygni
Aims: We present the results of high-resolution optical spectroscopy,low-resolution near-IR spectroscopy and near-infrared speckleinterferometry of the massive young stellar object candidate V645 Cyg,acquired to refine its fundamental parameters and the properties of itscircumstellar envelope. Methods: Speckle interferometry in the H-and K-bands and an optical spectrum in the range 5200-6680 Å witha spectral resolving power of R = 60 000 were obtained at the 6 mtelescope of the Russian Academy of Sciences. Another optical spectrumin the range 4300-10 500 Å with R = 79 000 was obtained at the 3.6m CFHT. Low-resolution spectra in the ranges 0.46-1.4 ?m and 1.4-2.5?m with R ~ 800 and ~700, respectively, were obtained at the 3 mShane telescope of the Lick Observatory. Results: Using a novelkinematical method based on the non-linear modeling of the neutralhydrogen density profile in the direction toward the object, we proposea distance of D = 4.2 ± 0.2 kpc. We also suggest a revisedestimate of the star's effective temperature, T_eff ~ 25 000 K. Weresolved the object in both H- and K-bands. Using a two-component ringfit, we derived a compact component size of 14 mas and 12 mas in the H-and K-band, respectively, which correspond to 29 and 26 AU at therevised distance. Analysis of our own and previously published dataindicates a ~2 mag decrease in the near-infrared brightness of V645 Cygat the beginning of the 1980's. At the same time, the cometary nebularcondensation N1 appears to fade in this wavelength range with respect tothe N0 object, representing the star with a nearly pole-onoptically-thick disk and an optically-thin envelope. Conclusions:We conclude that V645 Cyg is a young, massive, main-sequence star, whichrecently emerged from its cocoon and has already experienced itsprotostellar accretion stage. The presence of accretion is not necessaryto account for the high observed luminosity of (2-6) ×104 M? yr-1. The receding part ofa strong, mostly uniform outflow with a terminal velocity of ~800 kms-1 is only blocked from view far from the star, whereforbidden lines form. The near-infrared size of the source is consistentwith the dust sublimation distance close to this hot and luminous starand is the largest among all young stellar objects observedinterferometrically to-date.Partially based on data obtained at the 6-m BTA telescope of the RussianAcademy of Sciences, 3.6-m Canada-France-Hawaii telescope, and 3-m Shanetelescope of the Lick Observatory.
| 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.
| 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.
| Encounter-driven accretion in young stellar clusters - A connection to FUors?
Context: The brightness of FUors increases by several magnitudes withinone to several years. The currently favoured explanation for thisbrightness boost is that of dramatically rising accretion from the discmaterial around a young star. The mechanism leading to this accretionincrease is a point of debate. Aims: Choosing the Orion nebulacluster as representative, we simulate accretion bursts driven byencounters in dense stellar environments. We investigate whetherproperties like rise and decay times, event frequency, etc., speak forencounters as a possible cause for FUor phenomena. Methods: Wecombine cluster simulations performed with the Nbody6++ code withparticle simulations that describe the effect of a fly-by on the discaround a young star to determine the induced mass accretion. Results: The induced accretion rates, the overall temporal accretionprofile, the decay time, and possibly the binarity rate we obtain forencounter-induced accretion agree very well with observations of FUors.However, the rise time of one year observed in some FUors is difficultto achieve in our simulations unless the matter is stored somewhereclose to the star and then released after a certain mass limit istransgressed. The severest argument against the FUors phenomenon beingcaused by encounters is that most FUors are found in environments of lowstellar density. We extend the discussion to eccentric binaries andgravitationally unstable discs and find that both models have similarproblems in achieving the necessary rise times. Conclusions: Wefind no conclusive answer as to whether the observed FUors are triggeredby encounters. However, it seems an intense accretion burst phase shouldexist - possibly an FU phase - early on in the development of denseclusters. We predict that in dense young clusters these outbursts shouldhappen predominantly close to the cluster centre and with high massratios between the involved stars.
| The Local Environment of the FU-Orionis-Like Objects AR 6A and 6B
We present new 12CO J = 3-2 and HCN J = 3-2 molecular linemaps of the region surrounding the young star AR 6 using the 15 m JamesClerk Maxwell Telescope (JCMT). AR 6 was previously found to be a doublesource with both components exhibiting several characteristics of FUOrionis (FUor) eruptive variable stars. The aims of this investigationwere to determine if the AR 6 sources are associated with molecularoutflows and if a significant reservoir of natal molecular gas and dustexists around the stars. These observations form a part of a large-scalestudy of the outflow and circumstellar environment characteristics ofFUors and FUor-like objects to place constrains on the age andevolutionary state of sources exhibiting FUor-like tendencies. Our dataindicate that AR 6, like FUor itself, does not possess a CO outflow andlikewise, does not show evidence for large amounts of molecular gas inits circumstellar environment. In fact, AR 6 seems to lie in a localminimum of HCN emission. This is also found in 850 ?m dust emissionseen in JCMT archival data. We conclude that from the near-infrared tothe sub-millimeter, AR 6 is similar to FUor in several respects. Weinterpret the lack of significant dust and molecular gas in thecircumstellar environment of AR 6, together with the large near-infrared(NIR) thermal excess, as evidence that the sources have exhausted theirnatal envelopes, that they have at least small hot circumstellar disks,and that they are more evolved than Class I protostars. This, in itself,suggests that, since FUor eruptions have also been observed in starswith large dust mass envelopes (e.g., V346 Nor) and with CO outflows(e.g., L1551 IRS5), FUor events probably occur at many different stagesin the early, formative phase of a star's life, and lends support to theidea that FUor outbursts are repetitive like their shorter-livedrelatives occurring in EXor eruptive variables. Finally, we study thestellar environment around AR 6 using Two Micron All Sky Survey, SpitzerInfrared Array Camera, and Chandra ACIS images and show that, being partof the "Spokes" young stellar cluster, AR 6 is unlike many FUors whichare typically located in more sparsely populated regions.
| MWC 297: a young high-mass star rotating at critical velocity
Context: MWC 297 is a nearby young massive B[e] star.The central star is attenuated by 8 mag in the optical and has a highprojected rotational velocity of 350 km s-1. Despite thewealth of published observations, the nature of this object and itscircumstellar environment is not understood very well. Aims: Withthe present paper, we intend to shed light on the geometrical structureof the circumstellar matter that is responsible for the near- tomid-infrared flux excess. Methods: The H-band (1.6-2.0 ?m),K-band (2.0-2.5 ?m), and N-band (8-13 ?m) brightness distributionof MWC 297 was probed with the ESO interferometric spectrographs AMBERand MIDI, mounted on the VLTI in Paranal, Chile. We obtained visibilitymeasurements on 3 AMBER and 12 MIDI baselines, covering a wide range ofspatial frequencies. Different models (parametrized circumstellar disks,a dusty halo) were invoked to fit the data, all of which fail to do soin a satisfying way. We approximated the brightness distribution in H,K, and N with a geometric model consisting of three Gaussian disks withdifferent extents and brightness temperatures. This model can accountfor the entire near- to mid-IR emission of MWC 297. Results: Thecircumstellar matter around MWC 297 is resolved on all baselines. Thenear- and mid-IR emission, including the silicate emission at 10 micron,emanates from a very compact region (FWHM < 1.5 AU) around thecentral star. Conclusions: We argue that the extinction towardsthe MWC 297 star+disk system is interstellar and most likely due toremnants of the natal cloud from which MWC 297 was formed. Furthermore,we argue that the circumstellar matter in the MWC 297 system isorganized in a circumstellar disk, seen under moderate (i < 40°)inclination. The disk displays no inner emission-free gap at theresolution of our interferometric observations. The low inclination ofthe disk implies that the already high projected rotational velocity ofthe star corresponds to an actual rotational velocity that exceeds thecritical velocity of the star. This result shows that stars can obtainsuch high rotation rates at birth. We discuss the impact of this resultin terms of the formation of high-mass stars and the main-sequenceevolution of classical Be stars.Based on observations made with ESO telescopes at the La Silla ParanalObservatory underprogram IDs 077.D-0071(B-C), 077.D-0095(C-F), 079.C-0012(A-H)and 079.C-0207(A).
| The X-Ray Origin of Herbig AeBe Systems: New Insights
We present a statistical study of the X-ray emission toward 22 HerbigAeBe stars (HAEBE) using the Chandra archive. We probe the origin of theX-rays toward Herbig stars: are they intrinsic?This question isaddressed by correlations between the physical stellar properties andthe X-ray emission. There is a weak correlation between the continuumradio emission at λ = 3.6 cm and L X, which suggeststhat the X-ray emission depends upon the source. On the other hand, nocorrelation was found with the stellar rotational period, but that onlyexcludes solar-like magnetic activity as the origin of the X-rays. Mostimportantly, the X-ray luminosity of HAEBE has a different distributionfrom that of T Tauri stars, suggesting that X-ray emission from anunseen late-type star companion can be ruled out with an 80% confidencelevel. This implies that the HAEBE must have magnetic activity. Inaddition, we report the observation of five sources for the first time,in three detections.
| De-biasing interferometric visibilities in VLTI-AMBER data of low SNR observations
Aims: We have found that the interferometric visibilities of VLTI-AMBERobservations, extracted via the standard reduction package, aresignificantly biased when faint targets are concerned. The visibilitybiases derive from a time variable fringing effect (correlated noise)appearing on the detector. Methods: We have developed a methodto correct this bias that consists in a subtraction of the extra powercaused by such correlated noise, so that the real power spectrum at thespatial frequencies of the fringing artifact can be restored. Results: This pre-processing procedure is implemented in a softwareprogram called AMDC, available to the community, which is to be runbefore the standard reduction package. Results obtained on simulated andreal observations are presented and discussed.Based on VLTI-AMBER observations.
| Interacting jets from binary protostars
Aims: We investigate potential models that could explain why multipleproto-stellar systems predominantly show single jets. During theirformation, stars most frequently produce energetic outflows and jets.However, binary jets have only been observed in a very small number ofsystems. Methods: We model numerically 3D binary jets for variousoutflow parameters. We also model the propagation of jets from aspecific source, namely L1551 IRS 5, known to have two jets, usingrecent observations as constraints for simulations with a new MHD code.We examine their morphology and dynamics, and produce synthetic emissionmaps. Results: We find that the two jets interfere up to thestage where one of them is almost destroyed or engulfed into the secondone. We are able to reproduce some of the observational features ofL1551 such as the bending of the secondary jet. Conclusions:While the effects of orbital motion are negligible over the jetsdynamical timeline, their interaction has significant impact on theirmorphology. If the jets are not strictly parallel, as in most observedcases, we show that the magnetic field can help the collimation andrefocusing of both of the two jets.
| V1647 Orionis: One Year Into Quiescence
We present new optical, near-IR, and mid-IR observations of the youngeruptive variable star V1647 Orionis that went into outburst in late2004 for approximately two years. Our observations, taken one year afterthe star had faded to its pre-outburst optical brightness, show thatV1647 Ori is still actively accreting circumstellar material. We compareand contrast these data with existing observations of the source fromboth pre-outburst and outburst phases. From near-IR spectroscopy weidentify photospheric absorption features for the first time that allowus to constrain the classification of the young star itself. Ourbest-fit spectral type is M0 ± 2 sub-classes with a visualextinction of 19 ± 2 magnitudes and a K-band veiling ofrK ~ 1.5 ± 0.2. We estimate that V1647 Ori has aquiescent bolometric luminosity of ~ 9.5 Lsun and a massaccretion rate of ~ 1 × 10-6 Msunyr-1. Our derived mass and age, from comparison withevolutionary models, are 0.8 ± 0.2 Msun and lsim0.5Myr, respectively. The presence toward the star of shock-excited optical[S II] and [Fe II] emission as well as near-IR H2 and [Fe II]emission perhaps suggests that a new Herbig-Haro flow is becomingvisible close to the star.
| 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.
| Evolution of Dust and Ice Features around FU Orionis Objects
We present spectra for a sample of 14 FUors and two T Tauri starsobserved with Spitzer or ISO. Based on the appearance of the 10 μmsilicate feature, we define two categories of FUors. Objects showing thesilicate feature in absorption (Category 1) are still embedded in acircumstellar envelope. The shape of the silicate bands is in goodagreement with typical dust compositions of the ISM. Only one objectappears too rich in amorphous pyroxene dust, but a superposed emissionfeature can explain the observed shape. We derive optical depths andextinction values from the silicate and additional ice bands.Particularly, the analysis of the CO2 ice band at 15.2 μmallows us to search for evidence for ice processing and to constrainwhether the absorbing material is physically linked to the centralobject. For objects showing the silicate band in emission (Category 2),we argue that the feature comes from the surface layer of accretiondisks. We find evidence that grain growth has already taken place withinthe disks, but no clear indications for crystallization are present. Wediscuss how these observations fit into the picture of a young andactive accretion disk. Finally, a framework is proposed how the twocategories of FUors can be understood in a general paradigm of theevolution of young, low-mass stars. As one object (Parsamian 21) showsPAH emission features typical for evolved stars, we question its statusas a FUor. Additionally, two spectra (RNO 1B and L1551 IRS 5) show [FeII] emission lines that are attributed to hot, dense, or shockedmaterial related to outflows.Based on observations with ISO, an ESA project with instruments fundedby ESA Member States (especially the PI countries: France, Germany, theNetherlands, and the United Kingdom) and with the participation of ISASand NASA. This work is based in part on observations made with theSpitzer Space Telescope, which is operated by the Jet PropulsionLaboratory, California Institute of Technology under a contract withNASA.
| 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.
| A search for strong, ordered magnetic fields in Herbig Ae/Be stars
The origin of magnetic fields in intermediate- and high-mass stars isfundamentally a mystery. Clues towards solving this basic astrophysicalproblem can likely be found at the pre-main-sequence (PMS) evolutionarystage. With this work, we perform the largest and most sensitive searchfor magnetic fields in PMS Herbig Ae/Be (HAeBe) stars. We seek todetermine whether strong, ordered magnetic fields, similar to those ofmain-sequence Ap/Bp stars, can be detected in these objects, and if so,to determine the intensities, geometrical characteristics, andstatistical incidence of such fields. 68 observations of 50 HAeBe starshave been obtained in circularly polarized light using the FORS1spectropolarimeter at the ESO VLT. An analysis of both Balmer andmetallic lines reveals the possible presence of weak longitudinalmagnetic fields in photospheric lines of two HAeBe stars, HD 101412 andBF Ori. Results for two additional stars, CPD-53 295 and HD 36112, aresuggestive of the presence of magnetic fields, but no firm conclusionscan be drawn based on the available data. The intensity of thelongitudinal fields detected in HD 101412 and BF Ori suggest that theycorrespond to globally ordered magnetic fields with surface intensitiesof order 1 kG. On the other hand, no magnetic field is detected in 4other HAeBe stars in our sample in which magnetic fields had previouslybeen confirmed. Monte Carlo simulations of the longitudinal fieldmeasurements of the undetected stars allow us to place an upper limit ofabout 300 G on the general presence of aligned magnetic dipole magneticfields, and of about 500 G on perpendicular dipole fields. Taking intoaccount the results of our survey and other published results, we findthat the observed bulk incidence of magnetic HAeBe stars in our sampleis 8-12 per cent, in good agreement with that of magnetic main-sequencestars of similar masses. We also find that the rms longitudinal fieldintensity of magnetically detected HAeBe stars is about 200 G, similarto that of Ap stars and consistent with magnetic flux conservationduring stellar evolution. These results are all in agreement with thehypothesis that the magnetic fields of main-sequence Ap/Bp stars arefossils, which already exist within the stars at the PMS stage. Finally,we explore the ability of our new magnetic data to constrainmagnetospheric accretion in Herbig Ae/Be stars, showing that ourmagnetic data are not consistent with the general occurrence in HAeBestars of magnetospheric accretion as described by the theories ofKönigl and Shu et al..Based on observations from the ESO telescopes at the La Silla ParanalObservatory under programme ID 072.C-0447, DDT-272.C-5063, 074.C-0442.E-mail: wade-g@rmc.ca
| 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.
| V733 Cep (Persson's Star): A New FU Orionis Object in Cepheus
Persson recently found that a faint star had appeared in a cloud inCepheus. A CCD image shows a R~17.3 nebulous star, now known as V733Cep, located in the L1216 = Cep F cloud at the apex of a cavity in thecloud. Infrared photometry indicates a modest infrared excess. Opticalspectroscopy shows a well-defined Li I λ6707 line, andblueshifted absorption troughs at the Hα and Na I D linesextending to at least 200 km s-1, indicative of a massivefast wind. An infrared 1-4 μm spectrum of V733 Cep shows the presenceof strong water vapor features, and is almost identical to a similar IRspectrum of FU Ori that is reddened by AV=8 mag. Assuming anintrinsic energy distribution similar to that of FU Ori, V733 Cep has aluminosity of about 135 Lsolar at the assumed distance of 800pc. The star was detected by the Midcourse Space Experiment satellite at8.3 μm, but not by IRAS. Nor is it detected at 850 μm, indicatingthat while the star possesses circumstellar material it is notsurrounded by a significant cool envelope. A 12CO(3-2) mapshows what appears to be a small molecular outflow along the same axisas the cavity seen in optical images. There is evidence for a limitedamount of other low- and medium-mass star formation in the Cep F cloud.The totality of the evidence strongly indicates that V733 Cep is a newFU Ori-type object that must have erupted sometime between 1953 and1984.
| Disk and wind interaction in the young stellar object MWC 297 spatially resolved with AMBER/VLTI
The young stellar object MWC 297 is an embeddedB1.5Ve star exhibiting strong hydrogen emission lines and a strongnear-infrared continuum excess. This object has been observed with theVLT interferometer equipped with the AMBER instrument during its firstcommissioning run. AMBER/VLTI is currently the only near infraredinterferometer that can observe spectrally dispersed visibilities.MWC 297 has been spatially resolved in the continuumwith a visibility of 0.50+0.08-0.10 as well as inthe Br? emission line where the visibility decreases to0.33±0.06. This change in the visibility with wavelength can beinterpreted by the presence of an optically thick disk responsible forthe visibility in the continuum and of a stellar wind traced by theBr? emission line and whose apparent size is 40% larger. Wevalidate this interpretation by building a model of the stellarenvironment that combines a geometrically thin, optically thickaccretion disk model consisting of gas and dust, and alatitude-dependent stellar wind outflowing above the disk surface. Thecontinuum emission and visibilities obtained from this model are fullyconsistent with the interferometric AMBER data. They agree also withexisting optical, near-infrared spectra and other broad-bandnear-infrared interferometric visibilities. We also reproduce the shapeof the visibilities in the Br? line as well as the profile of thisline obtained at an higher spectral resolution with the VLT/ISAACspectrograph, and those of the H? and H? lines. The disk andwind models yield a consistent inclination of the system ofapproximately 20°. A picture emerges in which MWC297 is surrounded by an equatorial flat disk that is possiblystill accreting and an outflowing wind that has a much higher velocityin the polar region than at the equator. The AMBER/VLTI uniquecapability of measuring spectral visibilities therefore allows us forthe first time to compare the apparent geometry of a wind with the diskstructure in a young stellar system.Based on observations collected at the European Southern Observatory,Paranal, Chile, within the commissioning programme 60.A-9054(A).
| 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.
|
提交文章
相关链接
提交链接
下列团体成员
|
观测天体数据
目录:
|
INHABITED SKY
