תוכן
תמונות
הוסף תמונה שלך
DSS Images Other Images
מאמרים קשורים
Spectral atlas of massive stars around He I 10 830 Å
We present a digital atlas of peculiar, high-luminosity massive stars inthe near-infrared region (10 470-11 000 Å) at medium resolution(R≃7000). The spectra are centered around He I 10 830 Å,which is formed in the wind of those stars, and is a crucial line toobtain their physical parameters. The instrumental configuration alsosampled a rich variety of emission lines of Fe II, Mg II, C I, N I, andPa γ. Secure identifications for most spectral lines are given,based on synthetic atmosphere models calculated by our group. We alsopropose that two unidentified absorption features have interstellarand/or circumstellar origin. For the strongest one (10 780 Å) anempirical calibration between E(B-V) and equivalent width is provided.The atlas displays the spectra of massive stars organized in fourcategories, namely Be stars, OBA Iape (or luminous blue variables, LBVcandidates and ex/dormant LBVs), OB supergiants and Wolf-Rayet stars.For comparison, the photospheric spectra of non emission-line stars arepresented. Selected LBVs were observed in different epochs from 2001 to2004, and their spectral variability reveals that some stars, such asη Car, AG Car and HR Car, suffered dramatic spectroscopic changesduring this time interval.Based on observations made at Observatório do Pico dos Dias/LNA(Brazil). Figures 5 to 18 are only available in electronic form athttp://www.aanda.org Electronic version of the spectra (fichiers FITS)is only available in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/465/993
| The Structure of the Homunculus. II. Modeling the Physical Conditions in η Carinae's Molecular Shell
We present models that reproduce the observed double-shell structure ofthe Homunculus Nebula around η Carinae, including the stratificationof infrared H2 and [Fe II] emission seen in data obtainedwith the Phoenix spectrograph on Gemini South, as well as thecorresponding stratified grain temperature seen in thermal-infrareddata. Tuning the model to match the observed shell thickness allows usto determine the threshold density that permits survival ofH2. An average density ofnH~=(0.5-1)×107 cm-3 in the outerzone is required to allow H2 to exist at all latitudes in thenebula, and for Fe+ to recombine to Fe0. Thisgives independent confirmation of the very large mass of the Homunculus,indicating a total of roughly 15-35 Msolar (although we notereasons why the lower end of this range is favored). At the interfacebetween the atomic and molecular zones, we predict a sharp drop in thedust temperature, in agreement with the bimodal dust color temperaturesobserved in the two zones. In the outer molecular shell, the dusttemperature drops to nearly the blackbody temperature, and becomesindependent of grain size because of self-shielding at shorter UVwavelengths and increased heating at longer wavelengths. This relaxesconstraints on large grain sizes suggested by near-blackbody colortemperatures. Finally, from the strength of infrared [Fe II] emission inthe inner shell we find that the gas-phase Fe abundance is roughlysolar. This is astonishing in such a dusty object, where one normallyexpects gaseous iron to be depleted by 2 orders of magnitude.Based in part on observations obtained at the Gemini Observatory, whichis operated by AURA, under a cooperative agreement with the NSF onbehalf of the Gemini partnership: the National Science Foundation (US),the Particle Physics and Astronomy Research Council (UK), the NationalResearch Council (Canada), CONICYT (Chile), the Australian ResearchCouncil (Australia), CNPq (Brazil), and CONICET (Argentina).
| Submillimetre emission from η Carinae
We present critical, long-wavelength observations of η Carinae inthe submillimetre using Submillimetre Common-User Bolometer Array on theJames Clerk Maxwell Telescope at 850 and 450 μm to confirm thepresence of a large mass of warm dust around the central star. We fit atwo-component blackbody to the infrared-submm spectral energydistribution and estimate between 0.3 and 0.7 Msolar of dustexists in the nebula depending on the dust absorption properties and theextent of contamination from free-free emission at the SCUBAwavelengths. These results provide further evidence that η Car'scircumstellar nebula contains >10Msolar of gas, althoughthis may have been ejected on a longer time-scale than previouslythought.
| The present status of four luminous variables in M 33
Context: . Understanding the origin of the instabilities of LBVs isimportant for shedding light on the late evolutionary stages of massivestars and on the chemical evolution of galaxies. Aims: . Toinvestigate the physical nature of variable stars in the upper H-Rdiagram, we performed a spectrophotometric study of the Romano's star GR290 and the Hubble-Sandage variables A, B, and C in the close galaxy M33. Methods: . New spectroscopic and photometric data wereemployed in conjunction with already published data of these stars inorder to derive spectral types, energy distribution and bolometricluminosities. Results: . The yellow hypergiant Var A is still atminimum, with a ~G-type spectrum and strong Hα emission (W_eq≃ -35 Å). Var B is in a low luminosity hot state (V=17.5,B-V = -0.15) with very strong Hα emission (W_eq=-310 Å). Itsabsolute bolometric luminosity is 0.6× 106Lȯ. Var C, at V=16.4, is fainter than in the mid 1980s,but its spectrum shows the typical features of LBVs at maximum, aspectrum that is very rich in Fe ii emission lines. Its L_bol is about0.7× 106 Lȯ. The Romano's star GR 290has a rich hot emission-line spectrum and is very bright withL_bol=3× 106 Lȯ. During 2004 the starbrightened by ~half magnitude in each of the BVR filters. Conclusions: . Our observations confirm that Var A probably is anintermediate type hypergiant star surrounded by an expanding envelopewith a collisionally excited hydrogen emission, largely obscured bydusty disk and nebula. In recent years, Var B has undergone a bluewardtransition in the H-R diagram, probably at constant bolometricluminosity, while Var C is in a post-maximum phase with an ηCar-type spectrum. GR 290 is notable for its spectrum and luminosity,and it is likely to develop ample spectral variations in the nearfuture, similar to those observed in AG Car.
| On the Role of Continuum-driven Eruptions in the Evolution of Very Massive Stars and Population III Stars
We suggest that the mass lost during the evolution of very massive starsmay be dominated by optically thick, continuum-driven outbursts orexplosions, instead of by steady line-driven winds. In order for amassive star to become a Wolf-Rayet star, it must shed its hydrogenenvelope, but new estimates of the effects of clumping in winds fromO-type stars indicate that line driving is vastly insufficient. Wediscuss massive stars above roughly 40-50 Msolar, which donot become red supergiants and for which the best alternative is massloss during brief eruptions of luminous blue variables (LBVs). Ourclearest example of this phenomenon is the 19th century outburst ofη Carinae, when the star shed 12-20 Msolar or more inless than a decade. Other examples are circumstellar nebulae of LBVs andLBV candidates, extragalactic η Car analogs (the so-called supernovaimpostors), and massive shells around supernovae and gamma-ray bursters.We do not yet fully understand what triggers LBV outbursts or whatsupplies their energy, but they occur nonetheless, and they present afundamental mystery in stellar astrophysics. Since line opacity frommetals becomes too saturated, the extreme mass loss probably arises froma continuum-driven wind or a hydrodynamic explosion, both of which areinsensitive to metallicity. As such, eruptive mass loss could haveplayed a pivotal role in the evolution and ultimate fate of massivemetal-poor stars in the early universe. If they occur in thesePopulation III stars, such eruptions would also profoundly affect thechemical yield and types of remnants from early supernovae andhypernovae thought to be the origin of long gamma-ray bursts.
| Cleaning Up η Carinae: Detection of Ammonia in the Homunculus Nebula
We report the first detection of ammonia in the Homunculus Nebula aroundη Carinae, which is also the first detection of emission from apolyatomic molecule in this or any other luminous blue variable (LBV)nebula. Observations of the NH3 (J,K)=(3,3) inversiontransition made with the Australia Telescope Compact Array revealemission at locations where infrared H2 emission had beendetected previously, near the strongest dust emission in the core of theHomunculus. We also detect ammonia emission from the so-called strontiumfilament in the equatorial disk. The presence of NH3 aroundη Car hints that molecular shells around some Wolf-Rayet stars couldhave originated in prior LBV eruptions, rather than in cool redsupergiant winds or the ambient interstellar medium. Combined with thelack of any CO detection, NH3 seems to suggest that theHomunculus is nitrogen-rich like the ionized ejecta around η Car. Italso indicates that the Homunculus is a unique laboratory in which tostudy unusual molecule and dust chemistry, as well as their rapidformation in a nitrogen-rich environment around a hot star. We encouragefuture observations of other transitions like NH 3 (1, 1) and(2, 2), related molecules like N2H+, and renewedattempts to detect CO.
| The Asymmetrical Wind of the Candidate Luminous Blue Variable MWC 314
We present the results of long-term spectropolarimetric andspectroscopic monitoring of MWC 314, a candidate luminous blue variablestar. We detect the first evidence of Hα variability in MWC 314and find no apparent periodicity in this emission. The total R-bandpolarization is observed to vary between 2.21% and 3.00% at a positionangle consistently around ~0°, indicating the presence of atime-variable intrinsic polarization component, and hence anasymmetrical circumstellar envelope. We find suggestive evidence thatMWC 314's intrinsic polarization exhibits a wavelength-independentmagnitude varying between 0.09% and 0.58% at a wavelength-independentposition angle covering all four quadrants of the Stokes Q-U plane.Electron scattering off of density clumps in MWC 314's wind isconsidered as the probable mechanism responsible for these variations.
| The UV Scattering Halo of the Central Source Associated with η Carinae
We have made an extensive study of the UV spectrum of η Carinae andfind that we do not directly observe the star and its wind in the UV.Because of dust along our line of sight, the UV light that we observearises from bound-bound scattering at large impact parameters. We obtaina reasonable fit to the UV spectrum by using only the flux thatoriginates outside 0.033". This explains why we can still observe theprimary star in the UV despite the large optical extinction: it is dueto the presence of an intrinsic coronagraph in the η Car system andto the extension of the UV-emitting region. It is not due to peculiardust properties alone. We have computed the spectrum of the purportedcompanion star and show that it could only be directly detected in theUV spectrum, preferentially in the FUSE spectral region (912-1175Å). However, we find no direct evidence for a companion star, withthe properties indicated by X-ray studies and studies of the Weigeltblobs, in UV spectra. This might be due to reprocessing of thecompanion's light by the dense stellar wind of the primary. Broad Fe IIand [Fe II] emission lines, which form in the stellar wind, are detectedin spectra taken in the southeastern lobe, 0.2" from the central star.The wind spectrum shows some similarities to the spectra of the B and DWeigelt blobs but also shows some marked differences in that linespumped by Lyα are not seen. The detection of the broad lines lendssupport to our interpretation of the UV spectrum and to our model forη Car.Based on observations with the NASA/ESA Hubble Space Telescope, obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS5-2655.
| Early-type stars observed in the ESO UVES Paranal Observatory Project - I. Interstellar NaI UV, TiII and CaII K observations*
We present an analysis of interstellar NaI (λair=3302.37 and 3302.98 Å), TiII(λair= 3383.76Å) and CaII K (λair= 3933.66 Å) absorptionfeatures for 74 sightlines towards O- and B-type stars in the Galacticdisc. The data were obtained from the Ultraviolet and Visual EchelleSpectrograph Paranal Observatory Project, at a spectral resolution of3.75 km s-1 and with mean signal-to-noise ratios per pixel of260, 300 and 430 for the NaI, TiII and CaII observations, respectively.Interstellar features were detected in all but one of the TiIIsightlines and all of the CaII sightlines. The dependence of the columndensity of these three species with distance, height relative to theGalactic plane, HI column density, reddening and depletion relative tothe solar abundance has been investigated. We also examine the accuracyof using the NaI column density as an indicator of that for HI. Ingeneral, we find similar strong correlations for both Ti and Ca, andweaker correlations for Na. Our results confirm the general belief thatTi and Ca occur in the same regions of the interstellar medium (ISM) andalso that the TiII/CaII ratio is constant over all parameters. We henceconclude that the absorption properties of Ti and Ca are essentiallyconstant under the general ISM conditions of the Galactic disc.
| GCIRS34W: an irregular variable in the Galactic Centre
We report the results of time-resolved photometric and spectroscopicnear-infrared observations of the Ofpe/WN9 star and LBV candidateGCIRS34W in the Galactic Centre star cluster. Diffraction limitedresolution photometric observations obtained in H- and K-bands show astrong, non-periodic variability on time scales from months to years inboth bands, accompanied by variations of the stellar colour. ThreeK-band spectra obtained in 1996, 2003, and 2004 with integral fieldspectrometers are identical within their accuracies and excludesignificant spectroscopic variability. The most probable explanation ofthe stellar photometric variability is obscuration by circumstellarmaterial ejected by the star. The approximated position of GCIRS34W inan HR diagram is located between O supergiants and LBVs, suggesting thatthis star is a transitional object between these two phases of stellarevolution.
| The Discovery of a P Cygni Analog in M31
We present spectroscopy and discuss the photometric history of apreviously obscure star in M31. The spectrum of the star is an extremelyclose match to that of P Cygni, one of the archetypes of luminous bluevariables (LBVs). The star has not shown much variability over the past40 years (<0.2 mag), although small-scale (0.05 mag) variations overa year appear to be real. Nevertheless, the presence of a subarcsecondextension around the star is indicative of a past outburst, and from thenebula's size (0.5 pc diameter) we estimate that the outburst took placeroughly 2000 years ago. P Cygni itself exhibits a similar photometricbehavior and has a similar nebula (0.2 pc diameter). We argue that thismay be more typical behavior for LBVs than commonly assumed. The star'slocation in the H-R diagram offers substantial support for stellarevolutionary models that include the effects of rotation, as the star isjust at a juncture in the evolutionary track of a 85 Msolarstar. The star is likely in a transition from an O star to a late-typeWN Wolf-Rayet star.Based in part on observations made with the NASA/ESA Hubble SpaceTelescope (HST), obtained from the Data Archive at the Space TelescopeScience Institute, which is operated by the Association for Universitiesfor Research in Astronomy (AURA), Inc., under NASA contract NAS 5-26555.
| AG Carinae: A Luminous Blue Variable with a High Rotational Velocity
We report the detection of broad absorptions due to Si IVλλ4088-4116 in the luminous blue variable (LBV) AG Carinaeduring its last hot phase (2001-2003). Our non-LTE spectral analysis,with the radiative transfer code CMFGEN, revealed the photosphericnature of these lines, predicting, however, much narrower and deeperabsorption profiles than observed. Using a recently developed code tocompute synthetic spectra in two-dimensional geometry allowing for theeffects of rotation, we were able to match the broad absorptions with ahigh projected rotational velocity of 190+/-30 km s-1 in 2001April. Analysis of spectra obtained in 2002 March and 2003 January, whenthe star was cooling, yielded projected rotational velocities of110+/-10 and 85+/-10 km s-1, respectively. The derivedrotational velocities are proportional to R-1*, asexpected from angular momentum conservation. We discuss the effects ofsuch high rotation on the spectral analysis of AG Car and on the windterminal velocity. Our results show direct spectroscopic evidence, forthe first time, that an LBV may rotate at a significant fraction of itsbreakup velocity. Thus, AG Car (and possibly other LBVs) is indeed closeto the ΓΩ limit, as predicted by theoretical studies ofLBVs.Based on observations made with the 1.52 m ESO telescope at La Silla,Chile, and the 8 m ESO Very Large Telescope (VLT) at Paranal, Chile.
| Infrared [Fe II] Emission from P Cygni's Nebula: Atomic Data, Mass, Kinematics, and the 1600 AD Outburst
We present moderate- and high-dispersion 1-2.5 μm spectra of the ~10"radius nebula around P Cygni, dominated by bright emission lines of [FeII]. Observed [Fe II] line ratios disagree with theoretical transitionrates in the literature, so we use the spectrum of P Cyg's nebula toconstrain the atomic data for low-lying levels of [Fe II]. Of particularinterest is the ratio [Fe II] λ12567/λ16435, often used asa reddening indicator, for which we empirically derive an intrinsicvalue of 1.49, which is 10%-40% higher than previous estimates.High-dispersion spectra of [Fe II] λ16435 constrain the geometry,detailed structure, and kinematics of P Cyg's nebula, which is the majorproduct of P Cyg's outburst in 1600 AD. We use the [N II]/[N I] lineratio to conclude that the nebula is mostly ionized, with a total massof ~0.1 Msolar, more than the mass lost by the stellar windsince the eruption. For this mass, we would expect a larger infraredexcess than observed. We propose that the dust that obscured the starafter the outburst has since been largely destroyed, releasing Fe intothe gas phase to produce the bright [Fe II] emission. The kinetic energyof this shell is ~1046.3 ergs, far less than the kineticenergy released during the giant eruption of η Car in the 1840s, butclose to the value for η Car's smaller 1890 outburst. In thisrespect, it is interesting that the infrared spectrum of P Cyg's nebularesembles that of the ``Little Homunculus'' around η Car, ejected inthat star's 1890 eruption. The mass and kinetic energy in the nebulae ofη Car and P Cyg give insight into the range of parameters expectedfor extragalactic η Car-like eruptions.
| The Detection of Crystalline Silicates in Ultraluminous Infrared Galaxies
Silicates are an important component of interstellar dust, and thestructure of these grains (amorphous or crystalline) is sensitive to thelocal physical conditions. We have studied the infrared spectra of asample of ultraluminous infrared galaxies (ULIRGs). Here we report thediscovery of weak, narrow absorption features at 11, 16, 19, 23, and 28μm, characteristic of crystalline silicates, superimposed on thebroad absorption bands at 10 and 18 μm due to amorphous silicates ina subset of this sample. These features betray the presence offorsterite (Mg2SiO4), the magnesium-rich endmember of the olivines. Previously, crystalline silicates have only beenobserved in circumstellar environments. The derived fraction offorsterite to amorphous silicates is typically 0.1 in these ULIRGs. Thisis much larger than the upper limit for this ratio in the interstellarmedium of the Milky Way, 0.01. These results suggest that the timescalefor injection of crystalline silicates into the ISM is short in amerger-driven starburst environment (e.g., as compared to the total timeto dissipate the gas), pointing toward massive stars as a prominentsource of crystalline silicates. Furthermore, amorphization due tocosmic rays, which is thought to be of prime importance for the localISM, lags in vigorous starburst environments.
| MWC 930 - a new luminous blue variable candidate
We present the results of optical high-resolution and near-infraredlow-resolution spectroscopy and multicolour optical and near-infraredphotometry of the emission-line star MWC 930. The spectrum is rich withFeII emissions, most of which have P Cyg-type profiles. The emissionlines are strong and narrow, indicating a powerful stellar wind with alow terminal velocity (v&infy;~ 140kms-1). Thephotospheric absorption lines are broad and show splitting, which mightbe due to the object's binarity. MWC 930 is most probably located in theNorma spiral arm at a distance of D= 3-4kpc. This strong and slow windas well as the star's luminosity (logL/Lsolar~ 5.5) and theinfrared excess shape suggest that MWC 930 is an unusual B-typesupergiant, most likely undergoing the luminous blue variableevolutionary phase.
| Stellar Winds and Embedded Star Formation in the Galactic Center Quintuplet and Arches Clusters: Multifrequency Radio Observations
A multifrequency, multiconfiguration study has been made of the compactradio sources in the Galactic center Quintuplet and Arches stellarclusters using the Very Large Array. Ten radio sources have beendetected in the Quintuplet cluster. The majority of these radio sourceshave rising spectral indices and are positionally coincident with youngmassive stars that are known to have powerful stellar winds. We concludethat the three most compact of these sources are produced by stellarwind emission; thus, mass-loss rates can be derived and have an averagevalue of 3×10-5 Msolar yr-1. Theremainder of the sources are likely to be a combination of stellar windemission and free-free emission from surrounding ionized gas. In threecases, the radio sources have no stellar counterpart, and the radioemission is thought to arise from compact or ultracompact H II regions.If so, these sources would be the first detections of embedded massivestars to be discovered in the Galactic center clusters. The radio nebulaassociated with the Pistol star resembles the nebula surrounding theluminous blue variable star η Car and may be related to the stellarwind of the Pistol star. Ten compact radio sources are also detected inthe Arches cluster and are interpreted to be stellar wind sources,consistent with previous findings. Several of the sources show moderatevariability (10%-30%) in their flux density, possibly related to anonthermal component in the wind emission. A number of radio sources inboth clusters have X-ray counterparts, which have been interpreted to bethe shocked, colliding winds of massive binary systems.
| Asphericity and clumpiness in the winds of Luminous Blue Variables
We present the first systematic spectropolarimetric study of LuminousBlue Variables (LBVs) in the Galaxy and the Magellanic Clouds, in orderto investigate the geometries of their winds. We find that at least halfof our sample show changes in polarization across the strong Hαemission line, indicating that the light from the stars is intrinsicallypolarized and therefore that asphericity already exists at the base ofthe wind. Multi-epoch spectropolarimetry on four targets revealsvariability in their intrinsic polarization. Three of these, AG Car, HRCar and P Cyg, show a position angle (PA) of polarization which appearsrandom with time. Such behaviour can be explained by the presence ofstrong wind-inhomogeneities, or “clumps” within the wind.Only one star, R 127, shows variability at a constant PA, and henceevidence for axi-symmetry as well as clumpiness. However, if viewed atlow inclination, and at limited temporal sampling, such a wind wouldproduce a seemingly random polarization of the type observed in theother three stars. Time-resolved spectropolarimetric monitoring of LBVsis therefore required to determine if LBV winds are axi-symmetric ingeneral. The high fraction of LBVs (>50%) showing intrinsicpolarization is to be compared with the lower ~20-25% for similarstudies of their evolutionary neighbours, O supergiants and Wolf-Rayetstars. We anticipate that this higher incidence is due to the lowereffective gravities of the LBVs, coupled with their variabletemperatures within the bi-stability jump regime. This is alsoconsistent with the higher incidence of wind asphericity that we find inLBVs with strong Hα emission and recent (last ~10 years) strongvariability.
| Detection of a Massive Dust Shell around the Type II Supernova SN 2002hh
Dust emission from the Type II supernova SN 2002hh in NGC 6946 has beendetected at mid-infrared wavelengths by the Spitzer Space Telescope from590 to 758 days after outburst and confirmed by higher angularresolution Gemini North mid-IR observations. The day 600 5.8-24 μmemission can be fit by a 290 K blackbody having a luminosity of1.6×107 Lsolar. The minimum emitting radiusof 1.1×1017 cm is too large for the emitting dust tohave been formed in the supernova ejecta. Using radiative transfermodels and realistic dust grain parameters, fits to the observed fluxdistribution could be obtained with an optically thick dust shell havinga mass of 0.10-0.15 Msolar, corresponding to a total dust+gasmass in excess of 10 Msolar, suggesting a massive Msupergiant or luminous blue variable precursor to this self-obscuredobject.
| Crowded field 3D spectroscopy of LBV candidates in M 33
We present integral field spectroscopy of the LBV candidate stars B 416and v 532 in the local group galaxy M 33. B 416 is surrounded by anelongated ring-like nebula, which has a projected radius of 20 ×30 pc. From the datacube we create ionization and radial velocity mapsof the nebula. The excitation of the gas decreases towards the outerpart of the ring, while the inner part of the nebula is filled with amore excited gas. In the EW direction the ring is seen to expand with amaximum projected velocity amplitude of about 40 km s-1. Theeastern part approaches the observer. We estimate the nebula dynamicallifetime ∼ 8 × 105 years. It could be a residual MSbubble, which indicates a main-sequence or pre-LBV status of the star.We classify B 416 as an "iron star" or B[e]-supergiant. In v 532 anelongated nebula has been marginally detected. The total projected sizeof the nebula along the main axis is 30 pc, and the total radialvelocity gradient is 44 ± 11 km s-1. v 532 exhibitsboth strong photometric and spectral variability. At the time of ourobservations it was in an intermediate brightness state with a richnitrogen spectrum. We classify v 532 as an LBV, showing LBV≤ftrightarrow Ofpe/WN transitions. We stress the importance ofintegral field spectroscopy as the optimal technique for studyingnebulae and the evolution of LBV-like stars in nearby galaxies.
| A Model of the Circumstellar Envelope of Luminous Blue Variables
The continuum energy distributions of the luminous blue variables R127and R110 in the outburst phase are fitted with a circumstellar envelopemodel. Both stars show two peaks in their continuum, one near1250Å and the other in the optical band. We suggest that their UVand optical fluxes may have different origins: the UV flux comes fromthe central star while the optical flux comes from an expandingcircumstellar envelope. We construct a model for LBVs consisting of twoLTE atmosphere models with different temperatures, and find it to be inagreement with the observed spectral energy distributions of R127 andR110. According to our numerical experiments, R127's continuum iscomposed of fluxes from a circumstellar envelope of Teff =8000 K, R = 485Rodot, and log g = 1, and from a central starof Teff = 17 000 K, R = 135Rodot, and log g = 2.5with a permeating factor f = 0.5; while R110's continuum can be fittedby a circumstellar envelope of Teff = 7000 K, R =350Rodot, and log g = 0.5, and a central star ofTeff = 25 000 K, R = 27Rodot, and log g = 3.0 witha permeating factor f = 0.65. Both models show that the non-sphericallysymmetric, optically thick regions are formed surrounding the centralstar in the outburst phase. The light of the central star is shielded bythe circumstellar envelope so that the visual brightness increases withthe decrease/increase of the temperature/radius of the optically thickregions.
| On the population of galactic Luminous Blue Variables
We report the first results of a long term infrared monitoring campaignof known and candidate galactic Luminous Blue Variables (LBVs). Inparticular, we are able to confirm the LBV nature ofG24.73+0.69, a luminous mid-B supergiant associatedwith a dusty ejection nebula. We find that prior to 2003 SeptemberG24.73+0.69 exhibited low amplitude (Δ JHK 0.4 mag) variability, but in the ~200 day period between 2003September-2004 April it abruptly brightened by ~0.7 mag in the broadbandJ filter. Subsequently, a further ~0.4 mag increase was observed between2004 April-October, resulting in an overall difference of ~1.1 magbetween (current) photometric mimimum and maximum; similar variabilityalso being observed in the H and K bands. In light of the numerousrecent IR studies of the galactic hot star population we also compile anupdated census of confirmed and candidate galactic LBVs, reporting 12and 23 members respectively for each class. Finally, we utilise this newcensus to construct an H-R diagram for the galactic LBV population,resulting in a striking confirmation of the LBV-minimum light strip.
| Numerical Modeling of η Carinae Bipolar Outflows
In this paper, we present two-dimensional gasdynamic simulations of theformation and evolution of the η Car bipolar outflows. Adopting theinteracting nonspherical winds model, we have carried outhigh-resolution numerical simulations, which include explicitly computedtime-dependent radiative cooling, for different possible scenarios ofthe colliding winds. In our simulations, we consider different degreesof nonspherical symmetry for the preoutburst wind and the great eruptionof the 1840s produced by the η Car wind. Different models showimportant differences in the shape and kinematical properties of theHomunculus structure. In particular, we search for the appropriatecombination of wind parameters (which control the degree of nonsphericalsymmetry) to obtain the numerical results that best match both theobserved morphology and the expansion velocity of the η Car bipolarshell. In addition, our numerical simulations show the formation of abipolar nebula embedded within the Homunculus (the little Homunculus)that developed from a secondary eruptive event suffered by the star inthe 1890s, and also the development of tenuous, high-velocity ejectionsin the equatorial region that resulted from the impact of the eruptivewind of the 1840s with the preoutburst wind; these ejections couldexplain some of the high-speed features observed in the equatorialejecta. The models were, however, unable to produce the equatorialejections associated with the second eruptive event.
| Kinematical Structure of Wolf-Rayet Winds. II. Internal Velocity Scatter in WN Stars
The shortward edge of the absorption core velocities - v_black asdetermined from low resolution archived IUE spectra from the INESdatabase are presented for three P Cyg profiles of NV 1240, HeII 1640and NIV 1720 for 51 Galactic and 64 LMC Wolf-Rayet stars of the WNsubtype. These data, together with v_black of CIV 1550 line presented inNiedzielski and Skorzynski (2002) are discussed. Evidences are presentedthat v_black of CIV 1550 rarely displays the largest wind velocity amongthe four lines studied in detail and therefore its application as anestimator of the terminal wind velocity in WN stars is questioned. Anaverage v_black of several lines is suggested instead but it is pointedout that v_black of HeII 1640 usually reveals the highest observablewind velocity in Galactic and LMC WN stars. It is shown that thestratification strength decreases from WNL to WNE stars and that for WNLstars there exists a positive relation between v_black and theIonization Potential. The velocity scatter between v_black obtained fromdifferent UV lines is found to correlate well with the X-ray luminosityof single WN stars (correlation coefficient R=0.82 for the data obtainedfrom the high resolution IUE spectra) and therefore two clumpy windmodels of single WN stars are presented that allow the velocity scatterto persist up to very large distances from the stellar surface (r approx500-1000 R_*). These models are used to explain the specific features ofsingle WN stars like broad absorption troughs of strong lines havingdifferent v_black, X-ray fluxes, IR/radio continua and stratificationrelations.
| Infrared Observations of the Candidate LBV 1806-20 and Nearby Cluster Stars1,
We report near-infrared photometry, spectroscopy, and speckle imaging ofthe hot, luminous star we identify as candidate LBV 1806-20. We alsopresent photometry and spectroscopy of three nearby stars, which aremembers of the same star cluster containing LBV 1806-20 and SGR 1806-20.The spectroscopy and photometry show that LBV 1806-20 is similar in manyrespects to the luminous ``Pistol star,'' albeit with some importantdifferences. They also provide estimates of the effective temperatureand reddening of LBV 1806-20 and confirm distance estimates, leading toa best estimate for the luminosity of this star of greater than5×106Lsolar. The nearby cluster stars havespectral types and inferred absolute magnitudes that confirm thedistance (and thus luminosity) estimate for LBV 1806-20. If we dropkinematic measurements of the distance(15.1+1.8-1.3 kpc), we have a lower limit on thedistance of greater than 9.5 kpc and on the luminosity of greater than2×106Lsolar, based on the cluster stars. Ifwe drop both the kinematic and cluster star indicators for distance, anammonia absorption feature sets yet another lower limit to the distanceof greater than 5.7 kpc, with a corresponding luminosity estimate ofgreater than 7×105 Lsolar for the candidateLBV 1806-20. Furthermore, on the absis of very high angular resolutionspeckle images, we determine that LBV 1806-20 is not a cluster of starsbut is rather a single star or binary system. Simple arguments based onthe Eddington luminosity lead to an estimate of the total mass of LBV1806-20 (single or binary) exceeding 190Msolar. We discussthe possible uncertainties in these results and their implications forthe star formation history of this cluster.Based on data obtained at the Palomar Observatory 200 inch telescope,which is operated by the California Institute of Technology, the JetPropulsion Laboratory, and Cornell University.This publication 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.
| The Missing Luminous Blue Variables and the Bistability Jump
We discuss an interesting feature of the distribution of luminous bluevariables (LBVs) on the H-R diagram, and we propose a connection withthe bistability jump seen in the winds of early-type supergiants. Thereappears to be a deficiency of quiescent LBVs on the S Doradusinstability strip at luminosities between log(L/Lsolar)~=5.6and 5.8. The upper boundary, interestingly, is also where thetemperature-dependent S Doradus instability strip intersects thebistability jump at about Teff~=21,000 K. Because ofincreased opacity, winds of early-type supergiants are slower and denseron the cool side of the bistability jump, and we postulate that this maytrigger optically thick winds that inhibit quiescent LBVs from residingthere. We conduct numerical simulations of radiation-driven winds for arange of temperatures, masses, and velocity laws atlog(L/Lsolar)=5.7 to see what effect the bistability jumpshould have. We find that for relatively low stellar masses, theorder-of-magnitude increase in the wind density at the bistability jumpleads to the formation of a modest to strong pseudophotosphere thatmight alter a star's apparent position on the H-R diagram. The effect isstrongest for LBVs approaching 10 Msolar, where thepseudophotospheres are sufficiently extended to make an early B-typestar appear as a yellow hypergiant. Thus, the proposed mechanism will bemost relevant for LBVs that are post-red supergiants [curiously, theupper boundary at log(L/Lsolar)~=5.8 coincides with the upperluminosity limit for red supergiants]. Further work is obviously needed,especially with regard to a possible evolutionary connection between the``missing'' LBVs and the most luminous red supergiants and yellowhypergiants. Specifically, yellow hypergiants such as IRC +10420 andρ Cas occupy the same luminosity range as the missing LBVs and showapparent temperature variations at constant luminosity. If these yellowhypergiants do eventually become Wolf-Rayet stars, we speculate thatthey may skip the normal LBV phase, at least as far as their apparentpositions on the H-R diagram are concerned.
| On the Nature of the LBV/WR Eclipsing Binary System HD 5980
HD 5980 is a multiple star system in the Small Magellanic Cloud thatunderwent a luminous blue variable-like eruption in 1993-1994. Theerupting star (star A) forms part of a close eclipsing binary whosecompanion's (star B) Wolf-Rayet nature has recently been questioned. Thedata analyzed in this paper cover the period 1979-2002, and support theWNE classification of star B. The orbital phase-dependent variations ofultraviolet Pabsorption lines imply that the wind speed of star A is1700-2200s-1 and that of star B is 2600-3100s-1. These allow an estimateof the wind-wind collision (wwc) shock cone geometry and line profilesfrom the wwc region. Given the assumed mass-loss rates, only a smallfraction of the observed emission line profiles may originate in the wwcregion. The star A+star B binary pair is likely the result of evolutionwith mass transfer and/or evolution with rapid rotation, and provides amagnificent opportunity for studying the processes that occur in massivemultiple systems at the stages of stellar evolution just prior to thesupernova event.
| Planetary nebula distances re-examined: an improved statistical scale
The distances of planetary nebulae (PNe) are still quite uncertain.Although observational estimates are available for a small proportion ofPNe, based on statistical parallax and the like, such distances are verypoorly determined for the majority of galactic PNe. In particular,estimates of so-called `statistical' distance appear to differ byfactors of ~2.7.We point out that there is a well-defined correlation between the 5-GHzluminosity of the sources, L5, and their brightnesstemperatures, TB. This represents a different trend to thoseinvestigated in previous statistical analyses, and permits us todetermine independent distances to a further 449 outflows. Thesedistances are shown to be closely comparable to those determined using aTB-R correlation, providing that the latter trend is taken tobe non-linear.This non-linearity in the TB-R plane has not been noted inprevious analyses, and is likely responsible for the broad (andconflicting) ranges of distance that have previously been published.Finally, we point out that there is a close accord between observedtrends within the L5-TB and TB-Rplanes, and the variation predicted through nebular evolutionarymodelling. This is used to suggest that observational biases areprobably modest, and that our revised distance scale is reasonablytrustworthy.
| CO-to-H2 Abundance Ratio of the Foreground Gas of the Carina Nebula
We analyze CO and H2 absorption lines of the foregroundmolecular cloud in the Carina nebula. We use HST-STIS(Hubble SpaceTelescope - Space Telescope Imaging Spectrograph) & IUE(International Ultraviolet Explorer) INES data to analyze the A-X(v=0→2) absorption band of CO for several hot stars toward theCarina nebula, while 9 stars of them have FUSE (Far UltravioletSpectroscopic Explorer) spectra to analyze the (v=0→4) vibrationalband in the Lyman series of H2. The column densities of COand H2 varies in the vicinity of N(CO) 1013cm-2 and N(H2) 1019cm-2, respectively. The resultant CO-to-H2abundance ratio is about 10-6. We investigate the variationof the abundance ratio according to the relative position of the targetstars to morphology the molecular cloud in the Carina nebula.
| The Nature of SN 1961V
The nature of SN 1961V has been uncertain. Its peculiar optical lightcurve and slow expansion velocity are similar to those of superoutburstsof luminous blue variables (LBVs), but its nonthermal radio spectralindex and declining radio luminosity are consistent with decades-oldsupernovae (SNe). We have obtained Hubble Space Telescope STIS imagesand spectra of the stars in the vicinity of SN 1961V and find object 7identified by Filippenko and coworkers to be closest to the optical andradio positions of SN 1961V. Object 7 is the only point source detectedin our STIS spectra, and only its Hα emission is detected; itcannot be the SN or its remnant because of the absence of forbiddenlines. While the Hα line profile of object 7 is remarkably similarto that of η Carinae, the blue color (similar to an A2 Ibsupergiant) and lack of appreciable variability are unlike knownpostoutburst LBVs. We have also obtained Very Long Baseline Arrayobservations of SN 1961V at 18 cm. The nondetection of SN 1961V places alower limit on the size of the radio-emitting region, 7.6 mas or 0.34pc, which implies an average expansion velocity in excess of 4400 kms-1, much higher than the optical expansion velocity measuredin 1961. We conclude the following: (1) An SN occurred in the vicinityof SN 1961V a few decades ago. (2) If the SN 1961V light maximumoriginates from a giant eruption of a massive star, object 7 is the mostprobable candidate for the survivor, but its blue color and lack ofsignificant variability are different from a postoutburst η Car. (3)The radio SN and object 7 could be physically associated with each otherthrough a binary system. (4) Object 7 needs to be monitored to determineits nature and relationship to SN 1961V.
| Discovery and Evolution of an Unusual Luminous Variable Star in NGC 3432 (Supernova 2000ch)
We present photometric and spectroscopic observations of SN 2000ch, anunusual and extremely luminous variable star located in the galaxy NGC3432. The object was discovered on 2000 May 3.2 during the course of theLick Observatory Supernova Search, at an unfiltered magnitude of about17.4. Prediscovery images obtained in 1997, 1998, and 2000 April showthe object with R=19.2-19.5 mag. Optical spectra obtained beginning on2000 May 6 show a smooth, flat continuum and strong, broad hydrogenBalmer emission lines at wavelengths consistent with the catalogedredshift of NGC 3432, strengthening the association of the variable withthe galaxy. Photometric monitoring reveals a complex and erratic lightcurve over a time span of ~10 days. Subsequent optical spectra over thenext ~3 months continued to show strong Balmer emission lines with amean full width at half-maximum intensity ~1550 km s-1 and adistinct red asymmetry. A spectrum obtained 9 months after the outburstis similar to the previous spectra, but the integrated flux in Hαis nearly half that observed during the outburst. The object'sphotometric behavior, spectrum, and luminosity suggest that it is a verymassive and luminous variable star and might be related to some luminousblue variable stars such as η Carinae and SN 1997bs in NGC 3627. Thebrightest apparent magnitude implies an absolute magnitude ofMV~-12.7 at the distance of NGC 3432, a value that iscomparable to η Car during its outburst in the mid-19th century.
|
הכנס מאמר חדש
לינקים קשורים
הכנס לינק חדש
משמש של הקבוצה הבאה
|
תצפית ומידע אסטרומטרי
| קבוצת-כוכבים: | שדרית |
| התרוממות ימנית: | 10h56m11.58s |
| סירוב: | -60°27'12.8" |
| גודל גלוי: | 7.226 |
| מרחק: | 10000000 פארסק |
| תנועה נכונה: | -3.8 |
| תנועה נכונה: | 1.4 |
| B-T magnitude: | 7.78 |
| V-T magnitude: | 7.272 |
קטלוגים וכינוים:
|
INHABITED SKY
