Tartalom
Képek
Kép feltöltése
DSS Images Other Images
Kapcsolódó cikkek
Mid-infrared Spectral Variability Atlas of Young Stellar Objects
Optical and near-infrared variability is a well-known property of youngstellar objects. However, a growing number of recent studies claim thata considerable fraction of them also exhibit mid-infrared flux changes.With the aim of studying and interpreting variability on a decadaltimescale, here we present a mid-infrared spectral atlas containingobservations of 68 low- and intermediate-mass young stellar objects. Theatlas consists of 2.5-11.6 ?m low-resolution spectra obtained withthe ISOPHOT-S instrument on board the Infrared Space Observatory (ISO)between 1996 and 1998, as well as 5.2-14.5 ?m low-resolution spectraobtained with the Infrared Spectrograph instrument on board the SpitzerSpace Telescope between 2004 and 2007. The observations were retrievedfrom the ISO and Spitzer archives and were post-processed interactivelyby our own routines. For those 47 objects where multi-epoch spectra wereavailable, we analyze mid-infrared spectral variability on annual and/ordecadal timescales. We identify 37 variable candidate sources. Manystars show wavelength-independent flux changes, possibly due to variableaccretion rates. In several systems, all exhibiting 10 ?m silicateemission, the variability of the 6-8 ?m continuum, and the silicatefeature exhibit different amplitudes. A possible explanation is variableshadowing of the silicate-emitting region by an inner disk structure ofchanging height or extra silicate emission from dust clouds in the diskatmosphere. Our results suggest that mid-infrared variability, inparticular, the wavelength-dependent changes, is more ubiquitous thanwas known before. Interpreting this variability is a new possibility forexploring the structure of the disk and its dynamical processes.This work is based on observations made with the Infrared SpaceObservatory (ISO) and with the Spitzer Space Telescope. ISO is an ESAproject with instruments funded by ESA Member States (especially the PIcountries: France, Germany, the Netherlands, and UK) and with theparticipation of ISAS and NASA. Spitzer is operated by the JetPropulsion Laboratory, California Institute of Technology under acontract with NASA.
| GREAT [C ii] and CO observations of the BD+40°4124 region
The BD+40°4124 region was observed with high angular and spectralresolution with the German heterodyne instrument GREAT in CO J = 13? 12 and [C ii] on SOFIA. These observations show that the [C ii]emission is very strong in the reflection nebula surrounding the youngHerbig Ae/Be star BD+40°4124. A strip map over the nebula shows thatthe [C ii] emission approximately coincides with the optical nebulosity.The strongest [C ii] emission is centered on the B2 star and a deepspectrum shows that it has faint wings, which suggests that the ionizedgas is expanding. We also see faint CO J = 13 ? 12 at the positionof BD+40°4124, which suggests that the star may still be surroundedby an accretion disk. We also detected [C ii] emission and strong CO J =13 ? 12 toward V 1318 Cyg. Here the [C i] emission is fainter thanin BD+40°4124 and appears to come from the outflow, since it showsred and blue wings with very little emission at the systemic velocity,where the CO emission is quite strong. It therefore appears that in thebroad ISO beam the [C ii] emission was dominated by the reflectionnebula surrounding BD+40°4124, while the high J CO lines originatedfrom the adjacent younger and more deeply embedded binary system V 1318Cyg.
| Near-infrared interferometric observation of the Herbig Ae star HD 144432 with VLTI/AMBER
Aims: We study the sub-AU-scale circumstellar environment of theHerbig Ae star HD 144432 with near-infrared VLTI/AMBER observations toinvestigate the structure of its inner dust disk. Methods: Theinterferometric observations were carried out with the AMBER instrumentin the H and K band. We interpret the measured H- and K-bandvisibilities, the near- and mid-infrared visibilities from theliterature, and the spectral energy distribution (SED) of HD 144432 byusing geometric ring models and ring-shaped temperature-gradient diskmodels with power-law temperature distributions. Results: Wederive a K-band ring-fit radius of 0.17 ± 0.01 AU and an H-bandradius of 0.18 ± 0.01 AU (for a distance of 145 pc). Thismeasured K-band radius of ~0.17 AU lies in the range between the dustsublimation radius of ~0.13 AU (predicted for a dust sublimationtemperature of 1500 K and gray dust) and the prediction of modelsincluding backwarming (~0.27 AU). We find that an additional extendedhalo component is required in both the geometric andtemperature-gradient modeling. In the best-fit temperature-gradientmodel, the disk consists of two components. The inner part of the diskis a thin ring with an inner radius of ~0.21 AU, a temperature of ~1600K, and a ring thickness ~0.02 AU. The outer part extends from ~1 AU to~10 AU with an inner temperature of ~400 K. We find that the disk isnearly face-on with an inclination angle of <28°. Conclusions: Our temperature-gradient modeling suggests that thenear-infrared excess is dominated by emission from a narrow, bright rimlocated at the dust sublimation radius, while an extended halo componentcontributes ~6% to the total flux at 2 ?m. The mid-infrared modelemission has a two-component structure with ~20% of the flux originatingfrom the inner ring and the rest from the outer parts. Thistwo-component structure is indicative of a disk gap, which is possiblycaused by the shadow of a puffed-up inner rim.Based on observations made with ESO telescopes at Paranal Observatoryunder program ID 083.D-0224(C) and 085.C-0126(A).
| Science with the Vo: Spectroscopic Studies of Herbig Ae/Be Stars
The Virtual Observatory (VO) is opening up new ways of exploiting thehuge amount of data provided by the ever growing number of ground-basedand space facilities. Using VOSpec, a multi-wavelength spectral analysistool developed by the ESA-VO Team at ESAC, and new developments onscripting with VOSpec (VOScript), we have started to undertake acomprehensive study of spectroscopic and photometric data in the VO onHerbig Ae/Be stars. By studying line strengths, variabilities andspectral energy distributions, from the X-ray to sub-millimeter ranges,we aim to gain insights into processes and disk properties of a largesample of these objects. This paper presents initial findings of thespectroscopic analysis and initial spectral energy distributionclassifications.
| Astronomical Optical Interferometry. II. Astrophysical Results
Optical interferometry is entering a new age with several ground-basedlong-baseline observatories now making observations of unprecedentedspatial resolution. Based on a great leap forward in the quality andquantity of interferometric data, the astrophysical applications are notlimited anymore to classical subjects, such as determination offundamental properties of stars; namely, their effective temperatures,radii, luminosities and masses, but the present rapid development inthis field allowed to move to a situation where optical interferometryis a general tool in studies of many astrophysical phenomena.Particularly, the advent of long-baseline interferometers making use ofvery large pupils has opened the way to faint objects science and firstresults on extragalactic objects have made it a reality. The firstdecade of XXI century is also remarkable for aperture synthesis in thevisual and near-infrared wavelength regimes, which provided imagereconstructions from stellar surfaces to Active Galactic Nuclei. Here Ireview the numerous astrophysical results obtained up to date, exceptfor binary and multiple stars milli-arcsecond astrometry, which shouldbe a subject of an independent detailed review, taking into account itsimportance and expected results at micro-arcsecond precision level. Tothe results obtained with currently available interferometers, Iassociate the adopted instrumental settings in order to provide a guidefor potential users concerning the appropriate instruments which can beused to obtain the desired astrophysical information.
| A Multi-epoch, Simultaneous Water and Methanol Maser Survey toward Intermediate-mass Young Stellar Objects
We report a multi-epoch, simultaneous 22 GHz H2O and 44 GHzClass I CH3OH maser line survey toward 180 intermediate-massyoung stellar objects, including 14 Class 0 and 19 Class I objects, and147 Herbig Ae/Be stars. We detected H2O and CH3OHmaser emission toward 16 (9%) and 10 (6%) sources with one newH2O and six new CH3OH maser sources. The detectionrates of both masers rapidly decrease as the central (proto)starsevolve, which is contrary to the trends in high-mass star-formingregions. This suggests that the excitations of the two masers areclosely related to the evolutionary stage of the central (proto)starsand the circumstellar environments. H2O maser velocitiesdeviate on average 9 km s-1 from the ambient gasvelocities whereas CH3OH maser velocities match quite wellwith the ambient gas velocities. For both maser emissions, largevelocity differences (| v_{H_{2O}} - v_{sys} | > 10 {\,km\,s^{-1}}and | v_{CH_{3OH}} - v_{sys} | > 1 {\,km\,s^{-1}}) are mostlyconfined to Class 0 objects. The formation and disappearance ofH2O masers is frequent and their integrated intensitieschange by up to two orders of magnitude. In contrast, CH3OHmaser lines usually show no significant change in intensity, shape, orvelocity. This is consistent with the previous suggestion thatH2O maser emission originates from the base of an outflowwhile 44 GHz Class I CH3OH maser emission arises from theinteraction region of the outflow with the ambient gas. The isotropicmaser luminosities are well correlated with the bolometric luminositiesof the central objects. The fitted relations are L_{H_{2O}} = 1.71\times 10^{-9} (L_{bol})^{0.97} and L_{CH_{3OH}} = 1.71 \times 10^{-10}(L_{bol})^{1.22}.
| Mid-infrared interferometry of the massive young stellar object NGC 2264 IRS 1
Context. The optically invisible infrared-source NGC 2264 IRS 1 lyingnorth of the Cone Nebula is thought to be a massive young stellar object(~10 M&sun;). Although strong infrared excess clearly showsthat the central object is surrounded by large amounts of circumstellarmaterial, no information about the spatial distribution of thiscircumstellar material has been available until now. Aims: Weused the ESO Very Large Telescope Interferometer to performlong-baseline interferometric observations of NGC 2264 IRS 1 in themid-infrared regime. Our observations resolve the circumstellar materialaround NGC 2264 IRS 1, provide the first direct measurement of theangular size of the mid-infrared emission, and yield direct constraintson the spatial distribution of the dust. Methods: We analyze thespectrally dispersed interferometric data taken with MIDI at twodifferent position angles and baseline lengths. We use differentapproaches (a geometrical model, a temperature-gradient model, andradiative transfer models) to jointly model the observed interferometricvisibilities and the spectral energy distribution. Results: Thederived visibility values between ~0.02 and ~0.3 show that themid-infrared emission is clearly resolved. The characteristic size ofthe MIR-emission region is ~30-60 AU; this value is typical for otherYSOs with similar or somewhat lower luminosities. A comparison of thesizes for the two position angles shows a significant elongation of thedust distribution. Simple spherical envelope models are thereforeinconsistent with the data. The radiative transfer modeling of our datasuggests that we observe a geometrically thin and optically thickcircumstellar disk with a mass of about 0.1 M&sun;. Conclusions: Our modeling indicates that NGC 2264 IRS 1 is surrounded bya flat circumstellar disk that has properties similar to disks typicallyfound around lower-mass young stellar objects. This result supports theassumption that massive young stellar objects form via accretion fromcircumstellar disks.Based on observations collected at the European Organisation forAstronomical Research in the Southern Hemisphere, Chile, observingprograms 076.C-0725(B) and 082.C-899(A).
| On the alignment between the circumstellar disks and orbital planes of Herbig Ae/Be binary systems
Context. The majority of the intermediate mass, pre-main-sequence HerbigAe/Be stars reside in binary systems. As these systems are young, theirproperties may contain an imprint of the star formation process atintermediate masses (2-15 M&sun;). However, these systems aregenerally spatially unresolved, making it difficult to probe theircircumstellar environment to search for manifestations of theirformation process, such as accretion disks. Aims: Here weinvestigate the formation mechanism of Herbig Ae/Be (HAe/Be) binarysystems by studying the relative orientation of their binary orbits andcircumstellar disks. Methods: We present linearspectropolarimetric observations of HAe/Be stars over the H? line,which are used to determine the orientation of their circumstellardisks. In conjunction with data from the literature, we obtain a sampleof 20 binaries with known disk position angles (PAs). We subsequentlycompare our disk PA data to a model to investigate whether HAe/Be binarysystems and their disks are co-planar. Moreover, in the light of arelatively recent suggestion that some HAe/Be star spectropolarimetricsignatures may not necessarily be related to circumstellar disks, were-assess the relationship between spectropolarimetric signatures anddisk PAs. We do this by comparing spectropolarimetric and high spatialresolution observations of young stellar objects (both HAe/Be and TTauri stars). Results: We find that spectropolarimetricobservations of pre-main-sequence stars do indeed trace circumstellardisks. This finding is significant above the 3? level. Inaddition, our data are entirely consistent with the situation in whichHAe/Be binary systems and circumstellar disks are co-planar, whilerandom orientations can be rejected at the 2.2? level. Conclusions: The conclusive alignment (at more than 3?) betweenthe disk PAs derived from linear spectropolarimetry and high spatialresolution observations indicates that linear spectropolarimetry tracesdisks. This in turn allows us to conclude that the orbital planes ofHAe/Be binary systems and the disks around the primaries are likely tobe co-planar, which is consistent with the notion that these systemsform via monolithic collapse and subsequent disk fragmentation.
| Evolution of the Dust/Gas Environment around Herbig Ae/Be Stars
Using the KOSMA 3 m telescope, 54 Herbig Ae/Be (HAe/Be) stars weresurveyed in CO and 13CO emission lines. The properties of thestars and their circumstellar environments were studied by fittingspectral energy distributions (SEDs). The mean line width of13CO (2-1) lines of this sample is 1.87 km s-1.The average column density of H2 is found to be 4.9 ×1021 cm-2 for stars younger than 106yr, while this drops to 2.5 × 1021 cm-2 forthose older than 106 yr. No significant difference is foundamong the SEDs of HAe and HBe stars of the same age. Infrared excessdecreases with age, envelope masses and envelope accretion rates deceasewith age after 105 yr, the average disk mass of the sample is3.3 × 10-2 M sun, the disk accretion ratedecreases more slowly than the envelope accretion rate, and a strongcorrelation between the CO line intensity and the envelope mass isfound.
| New H? stars. NGC 6910 region. II.
A search for H? objects in four fields with an overall area ofabout 0.14 square degrees in the region of NGC 6910 (the core of theknown Cyg OB9 association) has led to the discovery of 64 emissionstars, of which 49 are newly detected. The high surface density isindicative of a very rich T association or, possibly, of the projectionof several T associations onto one another in this direction. The latterpossibility is also indicated by the range of the R photometric stellarmagnitudes of the emission stars, 9m.68-19m.42. Acomparison with earlier observations shows that for many of the stars,the emission line intensity is variable. A P Cyg H? line profileis recorded for the star V 1515 Cyg with an emission componentequivalent width that greatly exceeds the equivalent absorption width.
| Bayesian inference of stellar parameters and interstellar extinction using parallaxes and multiband photometry
Astrometric surveys provide the opportunity to measure the absolutemagnitudes of large numbers of stars, but only if the individualline-of-sight extinctions are known. Unfortunately, extinction is highlydegenerate with stellar effective temperature when estimated frombroad-band optical/infrared photometry. To address this problem, Iintroduce a Bayesian method for estimating the intrinsic parameters of astar and its line-of-sight extinction. It uses both photometry andparallaxes in a self-consistent manner in order to provide anon-parametric posterior probability distribution over the parameters.The method makes explicit use of domain knowledge by employing theHertzsprung-Russell Diagram (HRD) to constrain solutions and to ensurethat they respect stellar physics. I first demonstrate this method byusing it to estimate effective temperature and extinction from BVJHKdata for a set of artificially reddened Hipparcos stars, for whichaccurate effective temperatures have been estimated from high-resolutionspectroscopy. Using just the four colours, we see the expected strongdegeneracy (positive correlation) between the temperature andextinction. Introducing the parallax, apparent magnitude and the HRDreduces this degeneracy and improves both the precision (reduces theerror bars) and the accuracy of the parameter estimates, the latter byabout 35 per cent. The resulting accuracy is about 200 K in temperatureand 0.2 mag in extinction. I then apply the method to estimate theseparameters and absolute magnitudes for some 47 000 F, G, K Hipparcosstars which have been cross-matched with Two-Micron All-Sky Survey(2MASS). The method can easily be extended to incorporate the estimationof other parameters, in particular metallicity and surface gravity,making it particularly suitable for the analysis of the 109stars from Gaia.
| A catalogue of young runaway Hipparcos stars within 3 kpc from the Sun
Traditionally, runaway stars are O- and B-type stars with large peculiarvelocities. We would like to extend this definition to young stars (upto ?50 Myr) of any spectral type and to identify those present in theHipparcos catalogue by applying different selection criteria, such aspeculiar space velocities or peculiar one-dimensional velocities.Runaway stars are important for studying the evolution of multiple starsystems or star clusters, as well as for identifying the origins ofneutron stars. We compile the distances, proper motions, spectral types,luminosity classes, V magnitudes and B-V colours, and we utilizeevolutionary models from different authors to obtain star ages. We studya sample of 7663 young Hipparcos stars within 3 kpc from the Sun. Theradial velocities are obtained from the literature. We investigate thedistributions of the peculiar spatial velocity and the peculiar radialvelocity as well as the peculiar tangential velocity and itsone-dimensional components and we obtain runaway star probabilities foreach star in the sample. In addition, we look for stars that aresituated outside any OB association or OB cluster and the Galactic planeas well as stars for which the velocity vector points away from themedian velocity vector of neighbouring stars or the surrounding local OBassociation/cluster (although the absolute velocity might be small). Wefind a total of 2547 runaway star candidates (with a contamination ofnormal Population I stars of 20 per cent at most). Thus, aftersubtracting these 20 per cent, the runaway frequency among young starsis about 27 per cent. We compile a catalogue of runaway stars, which isavailable via VizieR.
| Search for and study of hot circumstellar dust envelopes
Long-term (1984-2008) JHKLM photometry for 254 objects is presented. Theobservations were carried out in the standard JHKLM photometric systemusing an original method and a modern IR photometer designed and builtat the Sternberg Astronomical Institute. Our program of studies includedsearches for and studies of relatively hot circumstellar dust envelopes.The most important results obtained using these observations include thefollowing. We have detected relatively hot dust envelopes in a number ofobjects for the first time, including the RCB star UV Cas, RX Cas,several classical symbiotic stars, etc. A model has been calculated forthe dust envelope of FG Sge, which formed around the star as a result ofseveral successive cycles of dust condensation beginning in Autumn 1992.Several dust-condensation episodes have been traced in the envelopes ofsymbiotic systems (CH Cyg, V1016 Cyg, HM Sge, etc.), as well as the roleof the hot component in the formation of the dust envelopes. We haveestablished from variations of the IR emission that the cool componentsin the symbiotic novae V1016 Cyg and HM Sge, and possibly CH Cyg, areMiras. The binarity of V1016 Cyg and HM Sge has also been firmlyestablished. The variability of a whole series of object has beenstudied, including the stellar components of close binary systems andseveral dozen Mira and semi-regular variables. The ellipsoidality of thecomponents in the RX Cas system (a prototype W Ser star) and the coolcomponent in the symbiotic systems CI Cyg and BF Cyg has been firmlyestablished. We have obtained the first IR light curve for the eclipsingsystem V444 Cyg (WN5+O6), and determined the wavelength dependence ofthe obtained parameters of the WN5 star. Analysis of the IR light curvesof several novae indicate the condensation of dust envelopes in thetransition periods of Cygnus 1992, Aquila 1993, and Aquila 1995. The IRlight curve of R CrB has been obtained over a long period and analyzed.IR observations of the nova-like variable V4334 Sgr have been carriedout over four years, over which the star passed through four stagesduring its motion along its post-AGB evolutionary track; the star'sbolometric flux and optical depth of its dust envelope have beenestimated, and the structure and mass of the dust layer determined. Wehave analyzed the IR variability of the symbiotic star V407 Cyg over 14years, and found its cool component to be a Mira with a period of 745days. The observed pulsations and trend are associated with theluminosity and temperature variations of the Mira, as well as theoptical depth of the dust envelope. The size of the dust grains andmass-loss rate of the Mira have been determined. We have obtained JHKLlight curves for the Seyfert galaxy NGC 4151 over 23 years. The IRbrightness of the galaxy grew from 1985 through 1996 (by ˜0.9m at 1.25 µm, ˜1 m at 1.65 µm,˜1.1 m at 2.2 µm, and ˜1.3 m at 3.5 µm), while the galaxy simultaneously reddened. The "cool"variable source in NGC 4151 was still in the active state in 1998,although its luminosity had decreased by approximately 15%-20%. If the"cool" component of the variable source in this galaxy is a dustenvelope heated by the central "hot" source, it should be optically thinto the radiation of this source: its mean optical depth is in the range0.05-0.15. Emission from dust particles heated to temperatures of600-800 K was observed in the near IR at a distance of several parsecsfrom the nucleus during the period of activity in 1995-1998; theinferred mass of emitting dust was 5-20 M ?. In1994-2003, we observed a tendency for NGC 4151 to become bluer at1.25-1.65 µm while simultaneously reddening at 2.2-3.5 µm.Beginning in Autumn 2000, the galaxy began to emerge from a minimum,which lasted from March 2000 through April 2001 in the IR; a flare ofthe galactic nucleus was observed and followed in detail in the IR inthis same period. We confirm the IR variability of the nucleus of theSeyfert galaxy NGC 1068, which can be located in various stages ofactivity. The variability of NGC 1068 is associated with a complexsource. A periodic component has been detected in the J brightnessvariations of the oxygen Mira V2108 Oph; we have shown that this star isimmersed in a fairly dense dust envelope, and have calculated amodel forthis envelope. We have calculated an (axially symmetrical) dust-envelopemodel for the carbon semi-regular variable RWLMi with a densitydistribution characteristic for the "superwind" stage. This envelopemodel is able to reproduce the observed fluxes over the entireobservable spectrum, and displays a good agreement with theobservational data. The three hot supergiants V482 Cas, QZ Sge, and HD338926 may be variable in the IR. Long-term photometry of eightplanetary nebulae in the near IR (1.25-5 µm) has enabled us tofirmly establish the IR brightness and color variability of thesenebulae on time scales from several tens of days to six-to-eight years.We have analyzed long-term JK photometry of the X-ray binary Cyg X-1(V1357 Cyg). Periodicity with a characteristic time scale of ˜11.5years is visible in the JK brightness variations of Cyg X-1 in1984-2007, possibly due to periodic variations of the temperature,radius and luminosity of the optical component of the binary with P˜11.5 yr. Fourteen-year IR light curves of five RV Tau stars (RSge, RV Tau, AC Her, V Vul, and R Sct) and the yellow supergiant andprotoplanetary-nebula candidate V1027 Cyg have been analyzed. Aspherically symmetrical dust-envelope model has been calculated forV1027 Cyg.
| A Submillimeter Mapping Survey of Herbig AeBe Stars
We have acquired submillimeter observations of 33 fields containing 37Herbig Ae/Be (HAEBE) stars or potential HAEBE stars, including SCUBAmaps of all but two of these stars. Nine target stars show extended dustemission. The other 18 are unresolved, suggesting that the dustenvelopes or disks around these stars are less than a few arcseconds inangular size. In several cases, we find that the strongest submillimeteremission originates from younger, heavily embedded sources rather thanfrom the HAEBE star, which means that previous models must be viewedwith caution. These new data, in combination with far-infrared fluxmeasurements available in the literature, yield spectral energydistributions (SEDs) from far-infrared to millimeter wavelengths for allthe observed objects. Isothermal fits to these SEDs demonstrateexcellent fits, in most cases, to the flux densities longward of 100?m. We find that a smaller proportion of B-type stars than A- andF-type stars are surrounded by circumstellar disks, suggesting thatdisks around B stars dissipate on shorter timescales than those aroundlater spectral types. Our models also reveal that the mass of thecircumstellar material and the value of ? are correlated, with lowmasses corresponding to low values of ?. Since low values of ?imply large grain sizes, our results suggest that a large fraction ofthe mass in low-? sources is locked up in very large grains.Several of the isolated HAEBE stars have disks with very flatsubmillimeter SEDs. These disks may be on the verge of forming planetarysystems.
| A Mapping Survey of Dense Clumps Associated with Embedded Clusters. II. Can Clump-Clump Collisions Induce Stellar Clusters?
We report the H13CO+ (J = 1-0) survey observationstoward embedded clusters obtained using the Nobeyama 45 m telescope,which were performed to follow up our previous study in theC18O survey with a dense gas tracer. Our aim is to addressthe evolution of cluster-forming clumps. We observed the same 14clusters in C18O, which are located at distances from 0.3 to2.1 kpc with a 27'' resolution (corresponding to the Jeans length formost of our targets) in H13CO+. We detected the 13clumps in H13CO+ line emission and obtained thephysical parameters of the clumps with radii of 0.24-0.75 pc, masses of100-1400 M sun, and velocity widths in FWHM of 1.5-4.0 kms-1. The mean density is ~3.9 × 104cm-3 and the equivalent Jeans length is ~0.13 pc at 20 K. Weclassified the H13CO+ clumps into three types,type A, type B, and type C according to the relative locations of theH13CO+ clumps and the clusters (see our previousstudy). Our classification represents an evolutionary trend ofcluster-forming clumps because dense clumps are expected to be convertedinto stellar constituents, or dispersed by stellar activities. We founda similar, but clearer trend than our previous results, for derived starformation efficiencies to increase from type A to C in theH13CO+ data, and for the dense gas regions withinthe clumps traced by H13CO+ to be sensitive to thephysical evolution of the clump-cluster systems. In addition, we foundthat 4 out of 13 H13CO+ clumps, which we named"Distinct Velocity Structure Objects" (DVSOs), have distinct velocitygradients at their central parts, i.e., at the location of the embeddedclusters. Assuming that the velocity gradients represent the rigid-likerotation of the clumps, we calculated the virial parameter of theH13CO+ clumps by taking into account thecontribution of the rotation and found that the DVSOs tend to begravitationally unbound. In order to explain the above physicalproperties for DVSOs in a consistent way, we propose a clump-clumpcollision model as a possible mechanism for triggering the formation ofclusters.
| Self-Correlation Analysis of the Photometric Variability of T Tauri Stars. II. A Survey
We have used archival CCD data, Fourier analysis, and self-correlationanalysis to study the photometric time variability of 162 T Tauri starsand related objects, including Herbig Ae/Be stars. Many show periodicvariability, presumably due to rotation of a spotted star in most cases.For the nonperiodic stars, we have estimated an upper limit to theperiodic variability. We have also analyzed 26 stars suspected to haveperiods longer than about 10 days. Of these 26 stars, at least 14 haveperiods significantly longer than 10 days; their variability may be dueto processes in the disk, or the effect of a companion, since most (butnot all) T Tauri stars have rotational periods less than this value. Fora few of the rotational variables, namely AA Tau, DK Tau, DL Tau, DNTau, GK Tau, GM Aur, Rox 29, V1121 Oph, V410 Tau, and V649 Ori, we havefound long-term variability of the amplitude, presumably due to activitycycles; the time scales are 1500-4000 days.
| Masses and luminosities of O- and B-type stars and red supergiants
Massive stars are of interest as progenitors of supernovae, i.e.neutron stars and black holes, which can be sources of gravitationalwaves. Recent population synthesis models can predict neutron star andgravitational wave observations but deal with a fixed supernova rate oran assumed initial mass function for the population of massive stars. Here we investigate those massive stars, which are supernovaprogenitors, i.e. with O- and early B-type stars, and also allsupergiants within 3 kpc. We restrict our sample to those massive starsdetected both in 2MASS and observed by Hipparcos, i.e. only those starswith parallax and precise photometry. To determine the luminositieswe calculated the extinctions from published multi-colour photometry,spectral types, luminosity class, all corrected for multiplicity andrecently revised Hipparcos distances. We use luminosities andtemperatures to estimate the masses and ages of these stars usingdifferent models from different authors. Having estimated theluminosities of all our stars within 3 kpc, in particular for all O- andearly B-type stars, we have determined the median and mean luminositiesfor all spectral types for luminosity classes I, III, and V. Ourluminosity values for supergiants deviate from earlier results: Previouswork generally overestimates distances and luminosities compared to ourdata, this is likely due to Hipparcos parallaxes (generally moreaccurate and larger than previous ground-based data) and the fact thatmany massive stars have recently been resolved into multiples of lowermasses and luminosities. From luminosities and effective temperatureswe derived masses and ages using mass tracks and isochrones fromdifferent authors. From masses and ages we estimated lifetimes andderived a lower limit for the supernova rate of ?20 events/Myraveraged over the next 10 Myr within 600 pc from the sun. These data arethen used to search for areas in the sky with higher likelihood for asupernova or gravitational wave event (like OB associations).
| 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
| The relation between the most-massive star and its parental star cluster mass
We present a thorough literature study of the most-massive star,mmax, in several young star clusters in order to assesswhether or not star clusters are populated from the stellar initial massfunction (IMF) by random sampling over the mass range 0.01 <= m <=150Msolar without being constrained by the cluster mass,Mecl. The data reveal a partition of the sample into lowestmass objects (Mecl <= 102Msolar),moderate mass clusters (102Msolar
| A Spitzer Survey of Young Stellar Clusters Within One Kiloparsec of the Sun: Cluster Core Extraction and Basic Structural Analysis
We present a uniform mid-infrared imaging and photometric survey of 36young, nearby, star-forming clusters and groups using Spitzer IRAC andMIPS. We have confidently identified and classified 2548 young stellarobjects (YSOs) using recently established mid-infrared color-basedmethods. We have devised and applied a new algorithm for the isolationof local surface density enhancements from point source distributions,enabling us to extract the overdense cores of the observed star-formingregions for further analysis. We have compiled several basic structuralmeasurements of these cluster cores from the data, such as mean surfacedensities of sources, cluster core radii, and aspect ratios, in order tocharacterize the ranges for these quantities. We find that a typicalcluster core is 0.39 pc in radius, has 26 members with infrared excessin a ratio of Class II to Class I sources of 3.7, is embedded in aAK = 0.8 mag cloud clump, and has a surface density of 60pc-2. We examine the nearest neighbor distances among theYSOs in several ways, demonstrating similarity in the spacings betweenClass II and Class I sources but large member clusters appear more densethan smaller clusters. We demonstrate that near-uniform source spacingsin cluster cores are common, suggesting that simple Jeans fragmentationof parsec-scale cloud clumps may be the dominant process governing starformation in nearby clusters and groups. Finally, we compare our resultsto other similar surveys in the literature and discuss potential biasesin the data to guide further interpretation.
| 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.
| A spatial study of the mid-IR emission features in four Herbig Ae/Be stars
Context: Infrared (IR) spectroscopy and imaging provide a prime tool tostudy the characteristics of polycyclic aromatic hydrocarbon (PAH)molecules and the mineralogy in regions of star formation. Herbig Ae/Bestars are known to have varying amounts of natal cloud material presentin their vicinity. Aims: Our aim is to study the characteristicsof the mid-IR emission originating in Herbig Ae/Be stars, especially theextent of the emission and how this relates to the (proto-)stellarcharacteristics. Methods: Today's powerful ground- andspace-based telescopes provide images and spectra at unprecedentedspectral and spatial resolution. We analyse the images and spectra fromfour Herbig Ae/Be stars (IRAS 06084-0611,CD-42 11721, TY CrA, andHD 176386), as obtained with TIMMI2 on the ESO 3.6 mtelescope and VISIR on the VLT. These observations are supplemented withdata from ISO-SWS and Spitzer-IRAC. Results: We find evidence forlarge-scale structure and extended emission in all four sources, exceptfor HD 176386, which only shows silicate emission, all sources show PAHemission in their spectra. In addition, a spatially resolved silicateand PAH spectrum could be extracted for TY CrA. Conclusions: Thevariety in emission scales distinguishes two classes. In the first, themorphology and spectral characteristics resemble those of reflectionnebulae. In the second, the characteristics are in-line with Herbig Astars. This separation simply reflects a difference in stellarcharacteristics (e.g. luminosity). In Herbig B stars, dust emission fromthe surroundings dominates, where for Herbig A stars, the disk dominatesthe emission. In this scheme, IRAS 06084-0611 and CD-42 11721 resemblereflection nebulae and HD 176386 a more typical Herbig Ae/Be star. TYCrA shows characteristics common to both genuine reflection nebulae andHerbig B stars. We propose a geometry for TY CrA, with most notably, a~70 AU inner gap in the 340 AU circumtertiary disk cleared by a fourthstellar companion.
| 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.
| What can we learn about protoplanetary disks from analysis of mid-infrared carbonaceous dust emission?
Context: The disks of gas and dust that form around young stars and canlead to planet formation contain polycyclic aromatic hydrocarbons (PAHs)and very small grains (VSGs). Aims: In this paper we analyze themid-infrared (mid-IR) emission of these very small dust particles in asample of 12 protoplanetary disks. Our goal is twofold: first we want tocharacterize the properties of these particles in disks and see how theyare connected to interstellar matter, and second we investigate thepossibility that their emission can be used as a probe of the physicalconditions and evolution of the disk. Methods: We define a basis madeof three mid-IR template spectra: PAH^0, PAH^+, and VSGs that werederived from the analysis of reflection nebulae, and an additionalPAHx spectrum that was introduced recently for analysis ofthe spectra of planetary nebulae. Results: From the optimization of thefit of 12 star+disk spectra, using a linear combination of the 4template spectra, we found that an additional small grain component witha broad feature at 8.3 μm is needed. We find that the fraction of VSGemission in disks decreases with increasing stellar temperature. VSGsappear to be destroyed by UV photons at the surface of disks, thusreleasing free PAH molecules, which are eventually ionized as observedin photodissociation regions. In contrast, we observe that the fractionof PAHx increases with increasing star temperature except inthe case of B stars where they are absent. We argue that this iscompatible with the identification of PAHx as large ionizedPAHs, most likely emitting in regions of the disk that are close to thestar. Finally, we provide a UV-dependent scheme to explain the evolutionof PAHs and VSGs in protoplanetary disks. These results allow us to putnew constraints on the properties of two sources: IRS 48 and“Gomez's Hamburger” which are poorly characterized. Conclusions: Very small dust particles incorporated into protoplanetarydisks are processed while exposed to the intense radiation field of thecentral star. The resulting shape of the mid-IR spectrum can reveal thisprocessing and be used as an efficient probe of the radiation field i.e.luminosity of central star.This work is based on observations made with the Spitzer SpaceTelescope, which is operated by the Jet Propulsion Laboratory,California Institute of Technology under a contract with NASA. Based onobservations with ISO, an ESA project with instruments funded by ESAMember States (especially the PI countries: France, Germany, theNetherlands and the UK) and with the participation of ISAS and NASA.
| 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.
| IPHAS and the symbiotic stars. I. Selection method and first discoveries
Context: The study of symbiotic stars is essential to understandimportant aspects of stellar evolution in interacting binaries. Theirobserved population in the Galaxy is however poorly known, and is one tothree orders of magnitudes smaller than the predicted population size.Aims: IPHAS, the INT Photometric Hα survey of the NorthernGalactic plane, gives us the opportunity to make a systematic, completesearch for symbiotic stars in a magnitude-limited volume, and discover asignificant number of new systems. Methods: A method of selectingcandidate symbiotic stars by combining IPHAS and near-IR (2MASS) coloursis presented. It allows us to distinguish symbiotic binaries from normalstars and most of the other types of Hα emission line stars in theGalaxy. The only exception are T Tauri stars, which can however berecognized because of their concentration in star forming regions. Results: Using these selection criteria, we discuss the classificationof a list of 4338 IPHAS stars with Hα in emission. 1500 to 2000 ofthem are likely to be Be stars. Among the remaining objects, 1183fulfill our photometric constraints to be considered candidate symbioticstars. The spectroscopic confirmation of three of these objects, whichare the first new symbiotic stars discovered by IPHAS, proves thepotential of the survey and selection method.Based on observations obtained at the 2.5 m INT telescope of the IsaacNewton Group of Telescopes in the Spanish Observatorio del Roque de LosMuchachos of the Instituto de Astrofísica de Canarias. Thispublication makes use of data products from the Two Micron All SkySurvey, which is a joint project of the University of Massachusetts andthe Infrared Processing and Analysis Center/California Institute ofTechnology, funded by the National Aeronautics and Space Administrationand the National Science Foundation. This research has also made use ofthe SIMBAD database, operated at CDS, Strasbourg, France. Table 1 isonly available in electronic form at http://www.aanda.org
| The Molecular Gas Environment around Two Herbig Ae/Be Stars: Resolving the Outflows of LkH? 198 and LkH? 225S
Observations of outflows associated with pre-main-sequence stars revealdetails about morphology, binarity, and evolutionary states of youngstellar objects. We present molecular line data from theBerkeley-Illinois-Maryland Association array and Five Colleges RadioAstronomical Observatory toward the regions containing the Herbig Ae/Bestars LkH? 198 and LkH? 225S. Single-dish observations of12CO J=1-0, 13CO J=1-0, N2H+J=1-0, and CS J=2-1 were made over a field of4.3'×4.3' for each species. 12COJ=1-0 data from FCRAO were combined with high-resolution BIMA array datato achieve a naturally weighted synthesized beam of6.75''×5.5'' toward LkH? 198 and5.7''×3.95'' toward LkH? 225S,representing resolution improvements of factors of approximately 10 and5 over existing data. By using uniform weighting, we achieved anotherfactor of 2 improvement. The outflow around LkH? 198 resolves intoat least four outflows, none of which are centered on LkH? 198-IR,but even at our resolution we cannot exclude the possibility of anoutflow associated with this source. In the LkH? 225S region wefind evidence for two outflows associated with LkH? 225S itself,and a third outflow is likely driven by this source. Identification ofthe driving sources is still resolution limited and is also complicatedby the presence of three clouds along the line of sight toward theCygnus molecular cloud. 13CO J=1-0 is present in theenvironments of both stars along with cold, dense gas as traced by CSJ=2-1 and (in LkH? 225S) N2H+ J=1-0. No 2.6mm continuum is detected in either region in relatively shallow mapscompared to existing continuum observations.
| 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.
| Young Stellar Groups around Herbig Ae/Be Stars: A Low-Mass YSO Census
We present near-IR and mid-IR observations of eight embedded youngstellar groups around Herbig Ae/Be stars (HAEBEs) using archived SpitzerIRAC data and 2MASS data. These young stellar groups are nearby (<=1kpc) and still embedded within their molecular clouds. In order toidentify the young stellar objects in our sample, we use the color-colordiagram of J-[3.6] versus Ks-[4.5]. The Spitzer images of oursample show that the groups around HAEBEs, spectral types earlier thanB8, are usually associated with bright infrared nebulosity. Within this,there are normally 10-50 young stars distributed close to the HAEBEs(<1 pc). Not only are there young stars around the HAEBEs, there arealso young stellar populations throughout the whole cloud, some of whichare distributed and some of which are clumped. The groups around theHAEBEs are substructures of the large young population within themolecular cloud. The sizes of groups are also comparable with thosesubstructures seen in massive clusters. Young stars in groups aroundHAEBEs have generally larger SED slopes compared to those outside, whichsuggests that the young stars in groups are probably younger than thedistributed systems. This might imply that there is usually a higher andmore continuous star-forming rate in groups, that the formation ofgroups initiates later, or that low-mass stars in groups form slowerthan those outside. Finally, there is no obvious trend between the SEDslopes and the distance to the HAEBEs for those young stars within thegroups. This suggests that the clustering of young stars dominates overthe effect of massive stars on the low-mass young stars at the scale ofour study.
|
Új cikk hozzáadása
Kapcsolódó hivatkozások
- - (nincs kapcsolódó hivatkozás) -
Új link hozzáadása
Besorolás csoportokba:
|
Pozíciós és asztrometriai adatok
| Csillagkép: | Hattyú |
| Rektaszcenzió: | 20h20m28.24s |
| Deklináció: | +41°21'51.5" |
| Vizuális fényesség: | 10.889 |
| RA sajátmozgás: | -1.8 |
| Dec sajátmozgás: | -7.7 |
| B-T magnitude: | 11.609 |
| V-T magnitude: | 10.949 |
Katalógusok és elnevezések:
|
