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A Symmetric Bipolar Nebula Around MWC 922
We report regular and symmetric structure around dust-enshrouded Be starMWC 922 with infrared imaging. Biconical lobes that appear nearly squarein aspect, forming this “Red Square” nebula, are crossed bya series of rungs that terminate in bright knots or“vortices,” while an equatorial dark band crossing the coredelimits twin hyperbolic arcs. The intricate yet cleanly constructedforms that comprise the skeleton of the object argue for minimalperturbation from global turbulent or chaotic effects. We also reportthe presence of a linear comb structure, which may arise from opticallyprojected shadows of a periodic feature in the inner regions, such ascorrugations in the rim of a circumstellar disk. The sequence of nestedpolar rings draws comparison with the triple-ring system seen around theonly naked-eye supernova in recent history: SN1987A.
| 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
| Discovery of a Nearby Twin of SN 1987A's Nebula around the Luminous Blue Variable HD 168625: Was Sk -69 202 an LBV?
Spitzer Space Telescope images of the luminous blue variable (LBV)candidate HD 168625 reveal the existence of a bipolar nebula severaltimes larger than its previously known equatorial dust torus. The outernebula of HD 168625 has a full extent of ~80" or 0.85 pc, and one of thelobes has a well-defined polar ring. The nebula is a near twin of thetriple-ring system around SN 1987A. Because of these polar rings, andaccounting for stellar/progenitor luminosity, HD 168625 is an evencloser twin of SN 1987A than the B supergiant Sher 25 in NGC 3603. HD168625's nebula was probably ejected during a giant LBV eruption and notduring a red supergiant phase, so its similarity to the nebula around SN1987A may open new possibilities for the creation of SN 1987A's rings.Namely, the hypothesis that Sk -69 202 suffered an LBV-like eruptionwould avert the complete surrender of single-star models for its bipolarnebula by offering an alternative to an unlikely binary merger scenario.It also hints that LBVs are the likely progenitors of some Type IIsupernovae, and that HD 168625's nebula is a good example of apre-explosion environment.Based on observations made with the Spitzer Space Telescope, which isoperated by the Jet Propulsion Laboratory, California Institute ofTechnology, under NASA contract 1407.
| 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.
| Radio observations of the planetary nebula around the OH/IR star OH354.88-0.54 (V1018 Sco)
We present radio observations of the unique, recently formed, planetarynebula (PN) associated with a very long-period OH/IR variable star V1018Sco that is unequivocally still in its asymptotic giant branch phase.Two regions within the optical nebula are clearly detected innon-thermal radio continuum emission, with radio spectral indicescomparable to those seen in colliding-wind Wolf-Rayet binaries. Wesuggest that these represent shocked interactions between the hot, faststellar wind and the cold nebular shell that represents the PN's slowwind moving away from the central star. This same interface producesboth synchrotron radio continuum and the optical PN emission. The fastwind is neither spherical in geometry nor aligned with any obviousoptical or radio axis. We also report the detection of transientH2O maser emission in this nebula.
| The infrared void in the Lupus dark clouds revisited: a polarimetric approach
The results of B-band CCD imaging linear polarimetry obtained for starsfrom the Hipparcos catalogue are used to re-examine the distribution ofthe local interstellar medium towards the IRAS 100-μm emission voidin the Lupus dark clouds. The analysis of the obtainedparallax-polarization diagram assigns to the dark cloud Lupus1 adistance between 130 and 150pc and assures the existence of a low columndensity region coincident with the observed infrared void. Moreover,there are clear indications of the existence of absorbing material atdistances closer than 60-100pc, which may be associated with theinterface boundary between the Local Bubble and its neighbourhood LoopIsuperbubble.
| 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.
| 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.
| Polarization Measurements of Post-Asymptotic Giant Branch Candidates and Related Stars
We have obtained UBVRI polarization measurements of 26 post-asymptoticgiant branch (post-AGB) candidates and related stars. The extremelymetal-poor post-AGB star HR 4049 has been observed several times. Inmost cases we find the objects to be intrinsically polarized. Thepolarization measurements presented in this paper indicate asymmetriccircumstellar dust shells and disks around these stars. For some objectsthe steep percent polarization λ-dependence and large degree ofpolarization suggest that scattering by circumstellar dust grains may beresponsible for the observed polarizations in the blue.
| 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.
| The Optical Spectrum of an LBV Candidate in the Cyg OB2 Association
We have obtained the first high-spectral-resolution (R=15000 and 60000)optical spectra for the extremely luminous star No. 12, identified withthe IR source IRAS 20308+4104 in the Cyg OB2 association. We haveidentified about 200 spectral features at 4552 7939 Å, includingthe interstellar NaI, KI lines and numerous DIBs, which are thestrongest absorption lines in the spectrum, along with the HeI, CII, andSiII lines. A two-dimensional spectral classification indicates that thestar's spectral type is B5±0.5 Ia+. Our analysis of theradial-velocity pattern shows the presence of a radial-velocity gradientin the stellar atmosphere, due to the infall of matter onto the star.The strong Hα emission line displays broad Thompson wings andtime-variable core absorption, providing evidence that the stellar windis inhomogeneous, and a slightly blue-shifted P Cygni absorptionprofile. We conclude that the wind is time-variable.
| 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.
| Spatial Closeness of the White Hypergiants HD 168607 and HD 168625
Our spectroscopic monitoring of the hypergiants HD 168607 (B9.5 Ia-0)and HD 168625 (B5.5 Ia-0) with resolutions from 15000 to 70000 confirmsthat both stars belong to the Ser OB1 association, proves their spatialcloseness, and increases the probability that they constitute a physicalpair.
| HD 183143: A Hypergiant
We present spectroscopic evidence that the luminosity of HD 183143 ishigher by one magnitude than thought previously. The star is yet anotherB6-8 Ia-0 white hypergiant of the Galaxy. Its absolute visual magnitudeis close to -8 mag, and its distance is close to 2 kpc. We describespectroscopic manifestations of the nonstationary behavior of itsatmosphere and wind.
| The Dust Ring of Luminous Blue Variable Candidate HD 168625: Infrared Observations and Model Calculations
We present a 2.218 μm image from the Hubble Space Telescope/NearInfrared Camera and Multi-Object Spectrometer (NICMOS) and a 55 μmimage from ISOPHOT of the dust ring surrounding the luminous bluevariable (LBV) candidate HD 168625, together with new temperature andoptical depth maps derived from mid-IR images. The shell is detachedfrom the star in the near-IR, and substructure in the overall toroidalshell is visible. The far-IR image constrains the extent of the dustshell to ~25" in diameter, providing an upper radius limit for modeling.The temperature maps and the NICMOS image show evidence for very smalltransiently heated dust grains in the shell. The opacity maps showhigher optical depth in the limbs, consistent with interpretation of thedust shell as an equatorially enhanced torus inclined ~60° withrespect to the observer. An overall trend in the dust emission locationwith wavelength is observed and interpreted as a variation with respectto location in the nebula of either the dust grain size distribution orgas-to-dust mass ratio. Radiative transfer calculations using 2-DUSTindicate that a mass-loss event occurred ~5700 yr ago with a rate of(1.9+/-0.1)×10-4Msolaryr-1,creating a dust torus that currently has a τV~0.22 in theequatorial plane and a dust mass of(2.5+/-0.1)×10-3Msolar. Using publishedvalues for the gas mass, we find a gas-to-dust mass ratio of 840, whichis ~4 times higher than current estimates for the interstellar medium.In addition to a high equator-to-pole density ratio (~31) torus, anelliptical midshell is needed to reproduce the appearance and spectralenergy distribution of the dust. Therefore, HD 168625 is an excellentexample of proposed models of LBV nebulae in which a stellar windinteracts with a preexisting density contrast and creates a blowout inthe polar direction perpendicular to the equatorial ring. Thecircumstellar shell is much lower in mass than that of LBV ηCarinae, suggesting that HD 168625 had a lower mass progenitor.Based in part on observations made with the NASA/ESA Hubble SpaceTelescope, obtained at the Space Telescope Science Institute, which isoperated by the Association of Universities for Research in Astronomy,Inc., under NASA contract NAS5-26555 these observations are associatedwith proposal 7898. Based also on observations with ISO, an ESA projectwith instruments funded by ESA member states (especially the PIcountries: France, Germany, the Netherlands, and the United Kingdom) andwith the participation of ISAS and NASA.
| Dusty ring nebulae around new candidate Luminous Blue Variables
We report on the discovery of a further two ring nebulae in theMidcourse Space Experiment (MSX) Galactic Plane Survey;G24.73+0.69 and G26.47+0.02.Morphologically, both appear similar to the nebulae found around theLuminous Blue Variable (LBV) candidates G79.29+0.46and Wra 17-96. A central, unresolved point source wasidentified in both cases - positional coincidence with the starStRS 237 was found forG26.47+0.02, while no optical counterpart could beidentified for G24.73+0.69. However, subsequent nearIR broadband imaging of the G24.73+0.69 fieldidentified a very red - (J-K) ~ 2 mag - stellar counterpart to thecentral object. Near-IR spectroscopy of both objects reveal richemission line spectra dominated by H I, He I and low excitation metals,suggesting classification as luminous B supergiants and revealing astriking superficial similarity to the other MSX ring sources and knownLBVs. We utilised a NLTE model atomsphere code to model the K bandspectra and near-IR spectral energy distributions of the central starsin order to determine their physical parameters. Adopting a distance, d=5.2 kpc to G24.73+0.69 yields a temperature, T= 12kK, luminosity, log (L/Lsun) = 5.6 and mass loss rate, dot{M}=1x 10-5 Msun yr-1.G26.47+0.02 appears to be a more extreme object;adopting d= 6.5 kpc results in T= 17 kK, log (L/Lsun) = 6.0and dot {M}=9x 10-5 Msun yr-1, placingit at the Humphreys-Davidson limit for massive stellar objects. Analysisof the spatially resolved mid-IR fluxes of both objects reveal extendedperiods of enhanced mass loss, resulting in comparatively low massnebulae, with chemistries dominated by O-rich dust (with a population ofsmall Fe grains existing co-spatially with the silicate dust).Comparison to the other MSX ring nebulae sources reveals a homogeneousgroup of objects, with both stellar and nebular properties consistentwith known LBVs. With both spectroscopic and/or photometric variabilityobserved for those sources with multiepoch observations, we propose aclose affinity between both classes of object and suggest that long termmonitoring of the MSX sources will reveal them to be bona fide LBVs.
| On the structure and kinematics of nebulae around LBVs and LBV candidates in the LMC
We present a detailed analysis of the morphology and kinematics ofnebulae around LBVs and LBV candidates in the Large Magellanic Cloud.HST images and high-resolution Echelle Spectra were used to determinethe size, shape, brightness, and expansion velocities of the LBV nebulaearound R 127, R 143, and S 61. For S Dor, R 71, R 99, and R 84 wediscuss the possible presence of nebular emission, and derive upperlimits for the size and lower limits on the expansion velocities ofpossible nebulae. Including earlier results for the LBV candidates S 119and Sk-69o279 we find that in general the nebulae around LBVsin the LMC are comparable in size to those found in the Milky Way. Theexpansion velocities of the LMC nebulae, however, are significantlylower - by about a factor of 3 to 4 - than those of galactic nebulae ofcomparable size. Galactic and LMC nebulae show about the same diversityof morphologies, but only in the LMC do we find nebulae with outflow.Bipolarity - at least to some degree - is found in nebulae in the LMC aswell as in the Milky Way, and manifests a much more general featureamong LBV nebulae than previously known.
| An atlas of spectra of B6-A2 hypergiants and supergiants from 4800 to 6700 Å
We present an atlas of spectra of 5 emission-line stars: thelow-luminosity luminous blue variables (LBVs) HD 168625 and HD 160529,the white hypergiants (and LBV candidates) HD 168607 and AS 314, and thesupergiant HD 183143. The spectra were obtained with 2 echellespectrometers at the 6-m telescope of the Russian Academy of Sciences inthe spectral range 4800 to 6700 Å, with a resolution of 0.4Å. We have identified 380 spectral lines and diffuse interstellarbands within the spectra. Specific spectral features of the objects aredescribed.The complete atlas and Table 2 are only available in electronic form atthe CDS via anonymous ftp to\ cdsarc.u-strasbg.fr (130.79.128.5) or via\http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/397/1035
| Luminous Blue Variables, cool hypergiants and some impostors in the H-R diagram
Current observations of the S Dor/LBVs and candidates and theimplications for their important role in massive star evolution arereviewed. Recent observations of the cool hypergiants are altering ourideas about their evolutionary state, their atmospheres and winds, andthe possible mechanisms for their asymmetric high mass loss episodeswhich may involve surface activity and magnetic fields. Recent resultsfor IRC+10 420, ρ Cas and VY CMa are highlighted. S Dor/LBVs ineruption, and the cool hypergiants in their high mass loss phases withtheir optically thick winds are not what their apparent spectra andtemperatures imply; they are then `impostors' on the H-R diagram. Theimportance of the very most massive stars, like η Carinae and the`supernovae impostors' are also discussed.
| The total-to-selective extinction ratio determined from near IR photometry of OB stars
The paper presents an extensive list of the total to selectiveextinction ratios R calculated from the infrared magnitudes of 597 O andB stars using the extrapolation method. The IR magnitudes of these starswere taken from the literature. The IR colour excesses are determinedwith the aid of "artificial standards" - Wegner (1994). The individualand mean values of total to selective extinction ratios R differ in mostcases from the average value R=3.10 +/-0.05 - Wegner (1993) in differentOB associations. The relation between total to selective extinctionratios R determined in this paper and those calculated using the "methodof variable extinction" and the Cardelli et al. (1989) formulae isdiscussed. The R values presented in this paper can be used to determineindividual absolute magnitudes of reddened OB stars with knowntrigonometric parallaxes.
| The Dust Ring of LBV Candidate HD 168625
We present infrared observations and model calculations of the dustshell surrounding HD 168625 (IRAS 18184-1623). This object is acandidate Luminous Blue Variable due to its photometric variations andthe presence of a detached dust shell indicating a major mass-lossepisode several hundred years ago. We have conducted a multi-wavelengthinfrared study of HD 168625, and present here high-resolutionnear-infrared imaging from NICMOS on HST, which shows substructure in ashell that is clearly detached from the star. We also presenttemperature and optical depth maps derived from mid-infrared images,which illustrate the toroidal geometry of the dust shell. In additionthe these observations, we have modelled the dust shell of HD 168625using the radiative transfer code 2-Dust, using observationallydetermined stellar parameters and geometric dust shell parameters as astarting point. We present here a comparison of the model spectralenergy distribution to infrared photometry data, and compare model anddata images of the dust shell. The shell can be modelled by a dustdistribution that is largely disk-like (equator-to-pole density ratio ~30) but with an elliptical midshell, as would be formed from theinteraction of a fast wind with a pre-existing density constrast.
| Infrared [FeII] emission in the circumstellar nebulae of luminous blue variables
After a serendipitous discovery of bright [FeII]λ16435 emissionin nebulae around η Carinae and P Cygni, infrared spectra of otherluminous blue variables (LBV) and LBV candidates were obtained. Brightinfrared [FeII] emission appears to be a common property among LBVs withprominent nebulae; this is an interesting discovery because strong[FeII]λ16435 is typically seen in shock-excited objects likesupernova remnants and outflows from newly formed massive stars, as wellas in active galactic nuclei (AGN), where the excitation mechanism isuncertain. This paper presents spectra in the H-band (1.5 to 1.75 μm)for the central stars and nebulae of η Car, AG Car, P Cyg, Wra 751,HR Car, HD 168625, HD 160529, R 127 and S Doradus. Seven of nine targetsshow bright [FeII]λ16435 in their nebulae, while it is absent inall central stars except the LBV candidate Wra 751. The two objects (SDor and HD 160529) without prominent [FeII]λ16435 are not yetknown to have nebulae detected in optical images, and both lack brightthermal infrared emission from dust. The possible excitation mechanismsfor this line and the implications of its discovery in LBV nebulae arediscussed; there are good reasons to expect shock excitation in someobjects, but other mechanisms cannot be ruled out.
| Multiwavelength Study of the Nebula Associated with the Galactic LBV Candidate HD 168625
We present high-resolution HST imaging of the nebula associated with theGalactic luminous blue variable candidate HD 168625, together with ISOimaging and Anglo-Australian Telescope echelle spectroscopy. The overallnebular morphology is elliptical, with the major axis at P.A.~=120°.The dimensions of the nebula are 12"×16.7" at Hα and15.5"×23.5" at 4 μm. In the HST Hα image the nebula isresolved into a complex structure of filaments and arcs of differentbrightness. The asymmetry is lost in the HST continuum image, where thenebula appears more diffuse and richer in filaments and clumps with theshape of cometary tails. At 11.3 μm the nebular emission peaks in twodiametrically opposite lobes located on the nebula boundaries and alongits major axis. A very faint loop is also visible at optical wavelengthsnorth and south of the shell. We suggest that the nebula is an ellipsoidwith projected sizes of 14" and 9" (0.19 pc×0.12 pc) along theright ascension and declination directions, respectively. This ellipsoidis expanding at 19 km s-1 and is dynamically as old as ~=4800yr; it probably interacts with the stellar wind and the loop so thatpolycyclic aromatic hydrocarbon emission is detected from its caps,i.e., the lobes seen in the ISO images. The chemistry of the loopsuggests that it is composed of unprocessed material, probably from thelocal interstellar medium swept by the stellar wind. Based in part onobservations with the NASA/ESA Hubble Space Telescope, obtained at theSpace Telescope Science Institute (STScI), which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS 5-26555. Also based on observations with the Infrared SpaceObservatory, an ESA project with instruments funded by ESA member states(especially the PI countries: France, Germany, Netherlands, and the UK),and with the participation of ISAS and NASA. Also based on observationsobtained at the Anglo-Australian Observatory, Siding Spring.
| An Infrared Ring Nebula around MSX5C G358.5391+00.1305: The True Nature of Suspected Planetary Nebula Wray 17-96 Determined via Direct Imaging and Spectroscopy
The Midcourse Space Experiment (MSX) Galactic plane survey discovered anearly perfectly circular ring nebula around the suspected planetarynebula Wray 17-96. Using near-IR spectral typing and modeling of themid-IR nebula, we find that Wray 17-96 is more likely a candidate to bea luminous blue variable (LBV) surrounded by a large spherical ejectashell. It is very similar to the G79.29+0.46 LBV candidate in Cygnus andthe Pistol Star. The K-band spectrum and the mid-IR data indicate astellar temperature of 13,000 K. The most likely distance to the sourceis 4.5 kpc, leading to a luminosity of 1.8×106Lsolar. We suggest that the nebula consists of multipleshells and that an evolution from oxygen-rich to carbon-rich chemistrymay be indicated.
| Radio images of four luminous blue variable stars
We have used the Australia Telescope Compact Array to image foursouthern luminous blue variable stars: AG Car, He3-519, HR Car and WRA751, at wavelengths of 3 and 6cm, and resolutions of 1 and 2arcsecrespectively. With the partial exception of HR Car, all radio imagesshow an unresolved stellar core surrounded by a large ionized gaseousnebula, and agree well with published Hα and [Nii] optical images.The image of WRA 751 shows a stellar torus or disc. HR Car's radio imageis unusual, and seems best explained by the presence of a hot binarycompanion.
| Discovery of an Extended Dust Emission around IRAS 18576+0341 (AFGL 2298) At 10.3 and 18.0 Microns: A New Luminous Blue Variable Candidate?
We report the detection of extended mid-infrared emission from IRAS18576+0341 (AFGL 2298). The object shows a dusty circumstellar shellthat has diameter of >~7" at 10.3 and 18.0 μm. The dust nebulashows two emission peaks concentrically elongated and symmetricallyoriented on the opposite sides of the third, central peak, which appearsto be the central star of the system. The observed mid-infraredmorphology indicates that the circumstellar dust shell has anequatorially enhanced material distribution, which is a common signatureof stellar objects that have experienced mass loss. Radiative transfermodel calculations suggest that the central star is an extremely bright(L*=106.4 Lsolar) star at a distance ofabout 10 kpc: this object is best described as a new luminous bluevariable candidate. The circumstellar dust shell seems to have beengenerated by an equatorially-enhanced mass loss process withM>=6.8×10-6 Msolar yr-1 andMpole/Meq~0.5.
| S Doradus variables in the Galaxy and the Magellanic Clouds
The goal in writing this paper is five fold: (1) to summarize thescientific achievements in the 20th century on S Dor variables (orLBVs); (2) to present an inventory of these variables in the Galaxy andthe Magellanic Clouds with a description of their physical state andinstability properties; (3) to emphasize the photometric achievements ofthe various types of instabilities. Generally this seems to be aneglected item resulting in a number of misunderstandings continuouslywandering through literature; (4) to investigate the structure of the SDor-area on the HR-diagram; (5) to estimate the total numbers of S Dorvariables in the three stellar systems. The position of the strongactive S Dor variables in minimum brightness obey the following linearrelation on the HR-diagram:log L/Lsun = 1.37 log T_eff -0.03. The relatively small dispersion of less active and supposed ex-and dormant S Dor variables with respect to this relation is twice aslarge at the blue side than at the red side. This might be caused byevolution to the WR stage and/or to high rotation. S Dor variables canbe subject to five types of instabilities: the very rare genuineeruptive episodes (the ``SD-eruptions''), two different brighteningphases caused by slow pulsations (the ``SD-phases''): one on a timescale of years, the other on a time scale of decades at a more or lessconstant luminosity and two types of microvariations: one on a timescale of weeks, the other on a time scale of about 100 d. So far, noperiodicities of light curve characteristics of any of theseinstabilities have ever been found. The durations of active andnon-active stages are estimated for about half of the sample based onscattered magnitude estimations such as from historical records, and onmodern monitoring campaigns. It would be a misunderstanding to believethat all S Dor variables should be always spectacular. It is estimatedthat most of them will not be spectacular at all for at least 70% oftheir lifetime as an S Dor variable. Tables 1 to 6 and 8 to 17 are onlyavailable in electronic form at http://www.edpsciences.org, Table 7 isonly available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/366/508. Figures 2--10,12, 14, 15, 17--19 are only available in electronic form athttp://www.edpsciences.org, see Note added in proof
| Spectroscopy of post-AGB candidates
We have carried out an analysis of the high resolution optical spectraof post-AGB candidates SAO 85766 (IRAS 18062+2410), SAO 34504 (IRAS22272+5435), HD 187885 (IRAS 19500-1709), HD 331319 (IRAS 19475+3119),HD 168625 (IRAS 18184-1623) and IRAS 10215-5916. All these objects havefar-infrared colors similar to planetary nebulae and proto- planetarynebulae. Some of these are high galactic latitude stars. The spectrum ofSAO 85766 shows several permitted and forbidden emission lines and lowexcitation nebular emission lines. We find the chemical composition ofSAO 85766 to be similar to that of high galatcic latitude hot post-AGBstars with under abundance of carbon. The variations observed in thespectrum of SAO 85766 indicates that it is rapidly evolving into aplanetary nebula ( similar to that SAO 244567 (Hen 1357)). SAO 34504 andHD 187885 are post-AGB stars with 21 micron emission feature. We havedetermined the atmospheric parameters and chemical composition of thesestars and confirm the over abundance of carbon and s-process elements.These two stars appear to have evovled from the carbon star phase on theAGB and are now carbon-rich post-AGB stars. The evolutionary status ofHD 331319, HD 168625 and IRAS 10215-5916 is not clear. They may be lowmass stars in post-AGB phase of evolution, or, they may be massive popIsupergiants. The chemical composition analysis of all the abovementioned stars is presented.
| The Radio Nebula around HR Carinae
We present a sensitive radio image of the nebula associated with theluminous blue variable star HR Carinae. This nebula is small anddifficult to observe optically because of the presence of the brightstar. The radio image shows the filaments in the outer regions of thenebula as seen in optical coronagraphic images. The core of the nebulais elongated north-south on the sky. A compact source associated with HRCar is clearly detected at the western edge of the nebula, but thenebula is very asymmetric with respect to the star, lying almostentirely to the east. The inner nebula shows no evidence for the bipolarstructure inferred to exist from observations of the outer nebula: thesymmetry axes in the inner nebula are 45° away from the bipolaraxes. If the compact radio emission at the location of the star is aclassical stellar wind source, we estimate a mass-loss rate for ionizedgas of 1.8×10-5 Msolar yr-1. Themass in the central core of the nebula is about 0.3 Msolar,while the outer nebula may contain as much as 0.5 Msolar. Webelieve that a colliding winds explanation of the nebula is unlikely; asymbiotic-like explanation in which ionization of neutral ejecta isprovided by a hot companion star may be possible if the companion staris on the far side of the nebula and heavily extincted, but it is notclear whether the properties required of this star are compatible withthe fact that it is not detected in a 10 μm image. A B0 V companionwithout much dust may be consistent with the data.
| AS 314: A dusty A-type hypergiant
We present the results of our observations of the poorly-studiedemission-line star AS 314 which include high-resolution spectroscopy,obtained at the 6 -meter telescope of the Russian Academy of Sciences,multicolor optical and near-infrared photometry. The strong Hαline, Hβ and Hγ of moderate strength and a number of weak Feii lines were detected in emission. The Balmer lines and most of the Feii lines show narrow P Cyg-type profiles which implies a very lowterminal velocity of the stellar wind. Very weak signs of emission arefound in Hdelta . Photospheric lines detected for the first time allowedus to determine the object's spectral type, A0. The luminosity, M_bol ~-8.0 mag was estimated using several methods and implies that AS 314 isa hypergiant, which is located at about 10 kpc from the Sun and has aninitial mass of ~ 20; Msun . Modeling of the Balmer lineprofiles resulted in the following parameters of the stellar wind: dot{M} = 2\ 10-5;Msun yr-1, v_infty =75;km s-1. The star is located within the LBV strip in theHRD. Its noticeable far-IR excess is due to the circumstellar dustemission and is likely evidence of an LBV-type outburst in the past.Table 5 is only available in electronic form in the online edition ofthe present issue.
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Osservazione e dati astrometrici
| Costellazione: | Sagittario |
| Ascensione retta: | 18h21m19.55s |
| Declinazione: | -16°22'26.1" |
| Magnitudine apparente: | 8.486 |
| Distanza: | 411.523 parsec |
| Moto proprio RA: | 0.7 |
| Moto proprio Dec: | -1.5 |
| B-T magnitude: | 10.128 |
| V-T magnitude: | 8.622 |
Cataloghi e designazioni:
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