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|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.
|Variable Unidentified Emission near 6307 Å in η Carinae|
We have discovered a conspicuous unidentified variable feature near 6307Å in the spectrum of η Carinae that is spatially unresolvedfrom the central star and its wind (r<~200-300 AU). It is significantfor two reasons: first, such prominent unidentified lines are now rarein this object, and second, this feature varies strongly andsystematically. It exhibits a combination of characteristics that, sofar as we know, are unique in η Carinae's spectrum. It may provideinsights into the recurrent spectroscopic events and the star'slong-term brightening.This research is part of the Hubble Space Telescope Treasury Project forη Carinae, supported by grants GO-9420 and GO-9973 from the SpaceTelescope Science Institute (STScI), which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS 5-26555.Partially based on observations obtained with UVES at the ESO Very LargeTelescope, Paranal, Chile (proposals 70.D-0607[A], 71.D-0168[A], and72.D-0524[A]).
|New and Confirmed Variables in the Field of Eta Carinae. Discovery of New Massive O-type Eclipsing Binaries|
A search for massive eclipsing binaries in the eta Carinae area resultedin the discovery or confirmation of eighteen variable stars. Six of themare in fact eclipsing binaries and three are O-type systems.
|Outflows from evolved stars with OASIS, NAOMI and GLAS|
I discuss the potential of integral-field spectroscopy (IFS) withadaptive optics in the study of the outflows from evolved stars ofdifferent masses. With IFS, detailed 3-D spatio-kinematical models ofthe outflows can be built, providing excellent observational datasets tobe confronted with the existing dynamical theories. In addition, ifmulti-epoch observations are able to resolve the apparent expansion ofthe nebulae in the plane of the sky, then their dynamics can be furtherconstrained, and other basic quantities like the distance via theexpansion parallax, can be determined. The kind of results that can beobtained are illustrated by recent HST and VLT observations of the ringnebula around the symbiotic nova He 2-147. Given the presentcapabilities of the OASIS integral-field spectrograph of the IsaacNewton Group of Telescopes (ING), classical novae ejecta are the mostappealing targets for such kind of studies, provided that its AO systemNAOMI is complemented with the forthcoming laser guide star system GLAS.IFS+AO is also a powerful technique to detect faint ionized nebulaearound bright stars, like for instance the outflows from luminous bluevariables.
|Bubbles in planetary nebulae and clusters of galaxies: jet bending|
We study the bending of jets in binary stellar systems. A compactcompanion accretes mass from the slow wind of the mass-losing primarystar, forms an accretion disc and blows two opposite jets. These fastjets are bent by the slow wind. Disregarding the orbital motion, we findthe dependence of the bending angle on the properties of the slow windand the jets. Bending of jets is observed in planetary nebulae which arethought to be the descendants of interacting binary stars. For example,in some of these planetary nebulae, the two bubbles (lobes) which areinflated by the two opposite jets are displaced to the same side of thesymmetry axis of the nebula. Similar displacements are observed inbubble pairs in the centre of some clusters and groups of galaxies. Wecompare the bending of jets in binary stellar systems with that inclusters of galaxies.
|Faint supernovae and supernova impostors: case studies of SN 2002kg/NGC 2403-V37 and SN 2003gm|
Photometric and spectroscopic observations of the faint Supernovae (SNe)2002kg and 2003gm, and their precursors, in NGC 2403 and NGC 5334,respectively, are presented. The properties of these SNe are discussedin the context of previously proposed scenarios for faint SNe: low-massprogenitors producing underenergetic SNe; SNe with ejecta constrained bya circumstellar medium; and outbursts of massive Luminous Blue Variables(LBVs). The last scenario has been referred to as `Type V SNe', `SNimpostors' or `fake SNe'.The faint SN 2002kg reached a maximum brightness of MV =-9.6, much fainter than normal Type II SNe. The precursor of SN 2002kgis confirmed to be, as shown in previous work, the LBV NGC 2403-V37.Late-time photometry of SN 2002kg shows it to be only 0.6 mag fainter at500 d than at the epoch of discovery. Two spectra of SN 2002kg, with anapproximately 1-yr interval between observations, show only minordifferences. Strong FeII lines are observed in the spectra of SN 2002kg,similar to both the LBV NGC 2363-V1 and the Type IIn SN 1995G. Thespectrum of SN 2002kg does show strong resolved [NII] atλλ6549,6583 Å. The identified progenitor of SN2003gm is a bright yellow star, consistent with a F5-G2 supergiant,similar to the identified progenitor of SN 2004et. SN 2003gm, at theepoch of discovery, was of similar brightness to the possible fake SN1997bs and the Type IIP SNe 1999br and 2005cs. Photometrically SN 2003gmshows the same decrease in brightness, over the same time period as SN1997bs. The light curve and the spectral properties of SN 2003gm arealso consistent with some intrinsically faint and low-velocity Type IISNe. The early-time spectra of SN 2003gm are dominated by Balmeremission lines, which at the observed resolution, appear similar to SN2000ch. On the basis of the post-discovery photometric and spectroscopicobservations presented here, we suggest that SN 2003gm is a similarevent to SN 1997bs, although the SN/LBV nature of both of these objectsis debated. At 226 d post-discovery the spectrum of SN 2003gm isstrongle contaminated by HII region emission lines, and it cannot beconfirmed that the precursor star has disappeared. The presence ofstrong [NII] lines, near Hα, is suggested as a possible means ofidentifying objects such as SN 2002kg/NGC 2403-V37 as being LBVs -although not as a general classification criterion of all LBVsmasquerading as SNe.
|Optical spectroscopy of X-Mega targets in the Carina nebula - VI. FO15: a new O-type double-lined eclipsing binary|
We report the discovery of a new O-type double-lined spectroscopicbinary with a short orbital period of 1.4 d. We find the primarycomponent of this binary, FO15, to have an approximate spectral typeO5.5Vz, i.e. a zero-age main-sequence (ZAMS) star. The secondary appearsto be of spectral type O9.5V. We have performed a numerical model fit tothe public All Sky Automated Survey photometry, which shows that FO15 isalso an eclipsing binary. We find an orbital inclination of ~80°.From a simultaneous light curve and radial velocity solution, we findthe masses and radii of the two components to be 30 +/- 1 and 16 +/-1Msolar and 7.5 +/- 0.5 and 5.3 +/- 0.5Rsolar.These radii, and hence also the luminosities, are smaller than those ofnormal O-type stars, but similar to recently born ZAMS O-type stars. Theabsolute magnitudes derived from our analysis locate FO15 at the samedistance as η Carinæ. From Chandra and XMM X-ray images, wealso find that there are two close X-ray sources: one coincident withFO15 and the other one without optical counterpart. The latter seems tobe a highly variable source, presumably due to a pre-main-sequencestellar neighbour of FO15.
|A census of the Carina Nebula - I. Cumulative energy input from massive stars|
The Carina Nebula (NGC 3372) is our richest nearby laboratory in whichto study feedback through ultraviolet radiation and stellar winds fromvery massive stars during the formation of an OB association, at anearly phase in the evolution of the surrounding proto-superbubble beforesupernova explosions have influenced the environment. This feedback istriggering successive generations of new star formation around theperiphery of the nebula, while simultaneously evaporating the gas anddust reservoirs out of which young stars are trying to accrete material.This paper takes inventory of the combined effect from all the knownmassive stars that power the Carina Nebula through their total ionizingflux and integrated mechanical energy from their stellar winds. Carinais close enough and accessible enough that spectral types for individualstars are available, and many close binary and multiple systems haverecently been spatially resolved, so that one can simply add them.Adopting values from the literature for corresponding spectral types,the present-day total ionizing photon luminosity produced by the 65 Ostars and three WNL stars in Carina is QH~=1051s-1, the total bolometric luminosity of allstars earlier than B2 is 2.5 × 107Lsolar,and the total mechanical luminosity of stellar winds is LSW~=105Lsolar. The total QH was about 25per cent higher when η Carinae was on the main sequence, before itand its companion were surrounded by its obscuring dust shell; for thefirst 3Myr, the net ionizing flux of the 70 O stars in Carina was about150 times greater than in the Orion Nebula. About400-500Msolar has been contributed to the HII region bystellar wind mass-loss during the past 3Myr. Values for QHand LSW are also given for the individual clusters Tr14, 15and 16, and Bo10 and 11, which are more relevant on smaller spatialscales than the total values for the whole nebula.
|Anisotropic mass ejection in binary mergers|
We investigate the mass loss from a rotationally distorted envelopefollowing the early, rapid in-spiral of a companion star inside a commonenvelope. For initially wide, massive binaries(M1+M2= 20Msolar, P~10yr), the primaryhas a convective envelope at the onset of mass transfer and is able tostore much of the available orbital angular momentum in its expandedenvelope. Three-dimensional smoothed particle hydrodynamics calculationsshow that mass loss is enhanced at mid-latitudes due to shock reflectionfrom a torus-shaped outer envelope. Mass ejection in the equatorialplane is completely suppressed if the shock wave is too weak topenetrate the outer envelope in the equatorial direction (typically whenthe energy deposited in the star is less than about one-third of thebinding energy of the envelope). We present a parameter study to showhow the geometry of the ejecta depends on the angular momentum and theenergy deposited in the envelope during a merging event. Applications tothe nearly axisymmetric, but very non-spherical nebulae around SN1987Aand Sheridan 25 are discussed, as well as possible links to RY Scuti andthe Small Magellanic Cloud object R4.
|Is the universe teeming with super civilizations ?|
|Eta Carinae across the 2003.5 Minimum: The Character and Variability of the Ejecta Absorption in the Near-Ultraviolet|
We present Hubble Space Telescope (HST) Space Telescope ImagingSpectrograph (STIS) high-resolution near-ultraviolet (NUV) spectra ofη Car's central source recorded before, during and after the 2003.5spectroscopic minimum. Our focus is on the narrow absorption linesformed in multiple circumstellar shells, superimposed on the broad PCygni stellar wind features, and how they respond to the flux variationsof η Car across the minimum. Over 30 separate narrow-line velocitycomponents are identified: three around -146 km s-1, manybetween -323 and -587 km s-1, and a few exceeding -1000 kms-1. In general, excitation decreases with increasingvelocity indicating that the primary excitation is by UV stellar photonsand that the slower shells are located closer to the central source. Twowell-isolated velocity systems at -146 and -513 km s-1display very different spectral characteristics regardingionization/excitation and respond differently to η Car'sspectroscopic minimum. The -146 km s-1 shell, associated withthe (internal) Little Homunculus, is ionized across the broadspectroscopic maximum but relaxes during the few months long minimum.The -146 km s-1 component is joined by adjacent velocitycomponents in lines of singly ionized iron-group elements. Ti II and VII absorptions appear during the minimum, most likely caused by adecrease in Lyman-ionizing flux. The -513 km s-1 component,part of the (outer) Homunculus, does not show significant changes inatomic absorptions, but intermediate-velocity components between -369and -503 km s-1 vary during the minimum. We present the NUVspectrum and describe its general characteristics, but we focus onidentifications of the nebular absorption lines and their variationsacross the 2003.5 minimum. The complete spectrum between 2380 and 3160Å with line identifications is available in the electronic editionof this paper.
|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.
|Discovery of Luminous Pulsed Hard X-Ray Emission from Anomalous X-Ray Pulsars 1RXS J1708-4009, 4U 0142+61, and 1E 2259+586 by INTEGRAL and RXTE|
We investigated the time-averaged high-energy spectral characteristicsof the persistent anomalous X-ray pulsars (AXPs) 1RXS J1708-4009, 4U0142+61, 1E 2259+586, and 1E 1048.1-5937, using RXTE PCA (2-60 keV),RXTE HEXTE (15-250 keV), and INTEGRAL IBIS ISGRI (20-300 keV) data. Wediscovered hard spectral tails for energies above 10 keV in the totaland pulsed spectra of AXPs 1RXS J1708-4009, 4U 0142+4009, and 1E2259+586, but 1E 1048.1-5937 appeared to be too weak to be detected.Improved hard X-ray spectral information for AXP 1E 1841-045 is alsopresented. The pulsed and total spectra above 10 keV have power-lawshapes, and there is so far no significant evidence for spectral breaksor bends up to ~150 keV. The pulsed spectra above 10 keV areexceptionally hard with indices for four AXPs in the range -1.0 to 1.0.Below 10 keV these indices were in the range 2.0-4.3, indicating thevery drastic spectral changes in a narrow energy interval around 10 keV.The best-fit power-law models to the total spectra between ~10 and 150keV are significantly softer, with indices measured for 1E 1841-045,1RXS J1708-4009, and 4U 0142+61 in the range 1.0-1.4. For the latterAXPs the pulsed fractions are consistent with 100% around 100 keV butare different at 10 keV: ~10% for 4U 0142+61, ~25% for 1E 1841-045, andconsistent with 100% for 1RXS J1708-4009. The luminosities of thesetotal and pulsed spectral tails (10-150 keV) largely exceed the totalavailable spin-down powers by factors ranging from ~100 to ~600. We alsoreanalyzed archival CGRO COMPTEL (0.75-30 MeV) data: no AXP detectionscan be claimed, and the obtained upper limits indicate for 1RXSJ1708-4009, 4U 0142+61, and 1E 1841-045 that strong breaks or bends mustoccur somewhere between ~150 and 750 keV. We discuss predictions fromfirst attempts to model our hard X-ray and soft gamma-ray spectra in thecontext of the magnetar model.
|The Structure of the Homunculus. I. Shape and Latitude Dependence from H2 and [Fe II] Velocity Maps of η Carinae|
High-resolution long-slit spectra obtained with the Phoenix spectrographon Gemini South provide our most accurate probe of the three-dimensionalstructure of the Homunculus Nebula around η Carinae. The newnear-infrared spectra dramatically confirm the double-shell structureinferred previously from thermal dust emission, resolving the nebulainto a very thin outer shell seen in H2 and a warmer, thickerinner layer seen in [Fe II]. The remarkably thin and uniform H2 skin hints that the most important mass loss during the 19thcentury eruption had a very short duration of <~5 yr. H2emission traces the majority of the more than 10 Msolar ofmaterial in the nebula and has an average density of ordernH>~106.5 cm-3. This emission, inturn, yields our first definitive picture of the exact shape of thenebula, plus a distance of 2350+/-50 pc and an inclination angle of~41°. The distribution of the H2 emission provides thefirst measure of the latitude dependence of the speed, mass loss, andkinetic energy associated with η Car's 19th century explosion.Almost 75% of the total mass and more than 90% of the kinetic energy inthe ejecta were released at high latitudes between 45° and the polaraxis. This rules out a model for the bipolar shape in which an otherwisespherical explosion was pinched at the waist by a circumstellar torus.Instead, most of the mass appears to have been directed poleward by theexplosion itself. H2 emission also provides our firstreliable picture of the critical innermost waist of the Homunculus,yielding clues to the observed morphology of the core and the moreextended equatorial debris.Based on observations obtained at the Gemini Observatory, which isoperated by AURA, Inc., under a cooperative agreement with the NSF onbehalf of the Gemini partnership: the National Science Foundation (USA),the Particle Physics and Astronomy Research Council (UK), the NationalResearch Council (Canada), CONICYT (Chile), the Australian ResearchCouncil (Australia), CNPq (Brazil), and CONICET (Argentina).
|Lens-Aided Multi-Angle Spectroscopy (LAMAS) Reveals Small-Scale Outflow Structure in Quasars|
Spectral differences between lensed quasar image components are common.Since lensing is intrinsically achromatic, these differences aretypically explained as the effect of either microlensing, or as lightpath time delays sampling intrinsic quasar spectral variability. Here weadvance a novel third hypothesis: some spectral differences are due tosmall line-of-sight differences through quasar disk wind outflows. Inparticular, we propose that variable spectral differences seen only incomponent A of the widest separation lens SDSS J1004+4112 are due todifferential absorption along the sight lines. The absorber propertiesrequired by this hypothesis are akin to known broad absorption line(BAL) outflows but must have a broader, smoother velocity profile. Weinterpret the observed C IV emission-line variability as furtherevidence for spatial fine structure transverse to the line of sight.Since outflows are likely to be rotating, such absorber fine structurecan consistently explain some of the UV and X-ray variability seen inAGNs. The implications are many: (1) Spectroscopic differences in otherlensed objects may be due to this ``lens-aided multi-anglespectroscopy'' (LAMAS). (2) Outflows have fine structure on size scalesof arcseconds, as seen from the nucleus. (3) Assuming either broadabsorption line region sizes proposed in recent wind models, ortypically assumed continuum emission region sizes, LAMAS and/orvariability provide broadly consistent absorber size scale estimates of~1015 cm. (4) Very broad smooth absorption may be ubiquitousin quasar spectra, even when no obvious troughs are seen.
|Accretion onto the Companion of η Carinae during the Spectroscopic Event. II. X-Ray Emission Cycle|
We calculate the X-ray luminosity and light curve for the stellar binarysystem η Car for the entire orbital period of 5.54 yr. By using anew approach we find, as suggested in previous works, that the collisionof the winds blown by the two stars can explain the X-ray emission andtemporal behavior. Most X-ray emission in the 2-10 keV band results fromthe shocked secondary stellar wind. The observed rise in X-rayluminosity just before minimum is due to the increase in density andsubsequent decrease in radiative cooling time of the shocked fastsecondary wind. Absorption, particularly of the soft X-rays from theprimary wind, increases as the system approaches periastron and theshocks are produced deep inside the primary wind. However, absorptioncannot account for the drastic X-ray minimum. The 70 day minimum isassumed to result from the collapse of the collision region of the twowinds onto the secondary star. This process is assumed to shut down thesecondary wind, and hence the main X-ray source. We show that thisassumption provides a phenomenological description of the X-ray behavioraround the minimum.
|Hubble Space Telescope Observations of Oxygen-rich Supernova Remnants in the Magellanic Clouds. III. WFPC2 Imaging of the Young, Crab-like Supernova Remnant SNR 0540-69.3|
HST images with WFPC2 of the young, oxygen-rich, Crab-like supernovaremnant SNR 0540-69.3 in the Large Magellanic Cloud (LMC) reveal detailsof the emission distribution and the relationship between the expandingejecta and synchrotron nebula. The emission distributions appear verysimilar to those seen in the Crab Nebula, with the ejecta located in athin envelope surrounding the synchrotron nebula. The [O III] emissionis more extended than other tracers, forming a faint ``skin'' around thedenser filaments and synchrotron nebula, as also observed in the Crab.The [O III] exhibits somewhat different kinematic structure in long-slitspectra, including a more extended high-velocity emission halo not seenin images. Yet even the fastest expansion speeds in SNR 0540-69.3's haloare slow when compared to most other young supernova remnants, althoughthe Crab Nebula has similar slow expansion speeds. We show a strikingcorrespondence between the morphology of the synchrotron nebula observedin an optical continuum filter with that recently resolved in ChandraX-ray images. We argue that the multicomponent kinematics andfilamentary morphology of the optical emission-line features likelyresult from magnetic Rayleigh-Taylor instabilities that form as thesynchrotron nebula expands and sweeps up ejecta, as seen in the CrabNebula. Our images and spectra help to refine our understanding of SNR0540-69.3 in several more detailed respects: they confirm theidentification of Hα+[N II] in the red spectrum, show that thesystemic velocity of SNR 0540-69.3 is not significantly different fromthat of the LMC, and hint at a lower Ne abundance than the Crab(potentially indicating a more massive progenitor star).Based on observations with the NASA/ESA Hubble Space Telescope obtainedat the Space Telescope Science Institute, which is operated by AURA,Inc., for NASA under contract NAS5-26555.
|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.
|Proper-Motion Measurements of the Cygnus Egg Nebula|
We present the results of proper-motion measurements of the dust shellstructure in the Egg Nebula (AFGL 2688, CRL 2688, V1610 Cyg), based onthe archived two-epoch data at 2 μm taken with the Hubble SpaceTelescope. We measured the amount of motion of local structures in thenebula by determining their relative shifts over an interval of 5.5 yr.The dynamical age of the nebula is found to be roughly 350 yr based onthe overall Hubble-law-esque motion of the nebula. By adopting thedeprojected velocity of 45 km s-1 at the tips of the bipolarlobes, our proper-motion measurements indicate that the distance to theEgg Nebula is about 420 pc and that the lobes are inclined at 7.7d withrespect to the plane of the sky. The refined distance estimate yields aluminosity of the central star of 3.3×103Lsolar, a total shell mass of 1.2 Msolar, and amass-loss rate (the upper limit) of 3.6×10-3Msolar yr-1. Assuming a 0.6 Msolarcentral post-AGB stellar mass, the initial mass of the Egg is 1.8Msolar. Upon analysis, we also discovered that (1) thecentral star of the Egg Nebula has a proper motion of its own at a rateof 17 mas yr-1, (2) the tips of the lobes increased theirvelocity due to shock acceleration, and (3) the apparent bipolar lobesconsist of multiple outflows at distinct inclination angles projectedonto each other.
|Optical Structure and Proper-Motion Age of the Oxygen-rich Supernova Remnant 1E 0102-7219 in the Small Magellanic Cloud|
We present new optical emission-line images of the young SNR 1E0102-7219 in the SMC obtained with the ACS on HST. This object is amember of the oxygen-rich class of SNRs showing strong oxygen, neon, andother metal-line emissions in its optical and X-ray spectra, and anabsence of hydrogen and helium. The progenitor of 1E 0102-7219 may havebeen a Wolf-Rayet star that underwent considerable mass loss prior toexploding as a Type Ib/c or IIL/b supernova. The ejecta in this SNR aregenerally fast-moving (V>1000 km s-1) and emit as they arecompressed and heated in the reverse shock. In 2003 we obtained optical[O III], Hα, and continuum images with the ACS Wide Field Camera.The [O III] image through the F475W filter captures the full velocityrange of the ejecta and shows considerable high-velocity emissionprojected in the middle of the SNR that was Doppler-shifted out of thenarrow F502N bandpass of a previous WFPC2 image from 1995. Using thesetwo epochs separated by ~8.5 yr, we measure the transverse expansion ofthe ejecta around the outer rim in this SNR for the first time atvisible wavelengths. From proper-motion measurements of 12 ejectafilaments, we estimate a mean expansion velocity for the bright ejectaof ~2000 km s-1 and an inferred kinematic age for the SNR of~2050+/-600 yr. The age we derive from HST data is about twice thatinferred by Hughes et al. from X-ray data, although our 1 σ errorbars overlap. Our proper-motion age is consistent with an independentoptical kinematic age derived by Eriksen et al. in 2003 using spatiallyresolved [O III] radial-velocity data. We derive an expansion centerthat lies very close to conspicuous X-ray and radio hot spots, whichcould indicate the presence of a compact remnant (neutron star or blackhole).Based on observations with the NASA/ESA Hubble Space Telescope (HST),obtained at the Space Telescope Science Institute (STScI), which isoperated by the Association of Universities for Research in Astronomy(AURA), Inc., under NASA contract NAS5-26555.
|The Ecology of Star Clusters and Intermediate-Mass Black Holes in the Galactic Bulge|
We simulate the inner 100 pc of the Milky Way to study the formation andevolution of the population of star clusters and intermediate-mass blackholes (IMBHs). For this study we perform extensive direct N-bodysimulations of the star clusters that reside in the bulge, and of theinner few tenth of parsecs of the supermassive black hole in theGalactic center. In our N-body simulations the dynamical friction of thestar cluster in the tidal field of the bulge are taken into account viasemianalytic solutions. The N-body calculations are used to calibrate asemianalytic model of the formation and evolution of the bulge. We findthat ~10% of the clusters born within ~100 pc of the Galactic centerundergo core collapse during their inward migration and form IMBHs viarunaway stellar merging. After the clusters dissolve, these IMBHscontinue their inward drift, carrying a few of the most massive starswith them. We predict that a region within ~10 pc of the supermassiveblack hole (SMBH) is populated by ~50 IMBHs of ~1000 Msolar.Several of these are still expected to be accompanied by some of themost massive stars from the star cluster. We also find that within a fewmilliparsecs of the SMBH there is a steady population of several IMBHs.This population drives the merger rate between IMBHs and the SMBH at arate of about one per 10 Myr, sufficient to build the accumulatedmajority of mass of the SMBH. Mergers of IMBHs with SMBHs throughout theuniverse are detectable by LISA at a rate of about two per week.
|On the He II Emission in η Carinae and the Origin of Its Spectroscopic Events|
We describe and analyze Hubble Space Telescope (HST) observations oftransient emission near 4680 Å in η Car, reported earlier bySteiner & Damineli. If, as seems probable, this is He IIλ4687, then it is a unique clue to η Car's 5.5 yr cycle.According to our analysis, several aspects of this feature support amass-ejection model of the observed spectroscopic events, and not aneclipse model. The He II emission appeared in early 2003, grew to abrief maximum during the 2003.5 spectroscopic event, and then abruptlydisappeared. It did not appear in any other HST spectra before or afterthe event. The peak brightness was larger than previously reported andis difficult to explain even if one allows for an uncertainty factor oforder 3. The stellar wind must provide a temporary larger-than-normalenergy supply, and we describe a special form of radiative amplificationthat may also be needed. These characteristics are consistent with aclass of mass-ejection or wind-disturbance scenarios, which haveimplications for the physical structure and stability of η Car.This research is part of the Hubble Space Telescope Treasury Project forη Car, supported by grants GO-9420 and GO-9973 from the SpaceTelescope Science Institute (STScI), which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS 5-26555.
|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.
|Isolated, Massive Supergiants near the Galactic Center|
We have carried out a pilot project to assess the feasibility of usingradio, infrared, and X-ray emission to identify young, massive starslocated between 1 and 25 pc from the Galactic center. We first comparedcatalogs compiled from the VLA, Chandra, and 2MASS. We identified twomassive, young stars: the previously identified star that is associatedwith the radio H II region H2 and a newly identified star that we referto as CXOGC J174516.1-290315. The infrared spectra of both stars exhibitvery strong Brγ and He I lines and resemble those of massivesupergiants that have evolved off of the main sequence but not yetreached the Wolf-Rayet phase. We estimate that each star has abolometric luminosity >~106 Lsolar. These twostars are also associated with bright mid-infrared sources from the MSXsurvey, although the origin of this emission is uncertain. Likewise, thedetection of these two sources in X-rays is surprising because stars atsimilar evolutionary states are not uniformly bright X-ray sources.Therefore, we suggest that both stars are in binary systems that containeither OB stars whose winds collide with those of the luminoussupergiants or compact objects that are accreting from the winds of thesupergiants. We also identify X-ray emission from a nitrogen-typeWolf-Rayet star and place upper limits on the X-ray luminosities ofthree more evolved, massive stars that previously have been identifiedbetween 1 and 25 pc from Sgr A*. Finally, we briefly discuss theimplications that future searches for young stars will have for ourunderstanding of the recent history of star formation near the Galacticcenter.
|Oscillator Strengths of Near-Infrared Lines of Fe II|
We present extensive configuration interaction calculations ofoscillator strengths and transition rates of certain near-infraredemission lines of Fe II observed in the BD Weigelt blobs of ηCarinae. Comparison is made between our work and results available inexisting databases. We offer arguments in favor of our results being themost accurate, where differences occur.
|M33's Variable A: A Hypergiant Star More Than 35 YEARS in Eruption|
Variable A in M33 is a member of a rare class of highly luminous,evolved stars near the upper luminosity boundary that show sudden anddramatic shifts in apparent temperature due to the formation ofoptically thick winds in high mass loss episodes. Recent optical andinfrared spectroscopy and imaging reveal that its ``eruption,'' begun in~1950, has ended, having lasted ~45 yr. Our current observations showmajor changes in its wind from a cool, dense envelope to a much warmerstate surrounded by low-density gas with rare emission lines of Ca II,[Ca II], and K I. Its spectral energy distribution has unexpectedlychanged, especially at the long wavelengths, with a significant decreasein its apparent flux, while the star remains optically obscured. Weconclude that much of its radiation is now escaping out of our line ofsight. We attribute this to the changing structure and distribution ofits circumstellar ejecta, corresponding to the altered state of its windas the star recovers from a high mass loss event.
|The polar wind of the fast rotating Be star Achernar. VINCI/VLTI interferometric observations of an elongated polar envelope|
Context: .Be stars show evidence of mass loss and circumstellarenvelopes (CSE) from UV resonance lines, near-IR excesses, and thepresence of episodic hydrogen emission lines. The geometry of theseenvelopes is still uncertain, although it is often assumed that they areformed by a disk around the stellar equator and a hot polar wind.Aims: .We probe the close environment of the fast rotating Be starAchernar at angular scales of a few milliarcseconds (mas) in theinfrared, in order to constrain the geometry of a possible polar CSE. Methods: .We obtained long-baseline interferometric observations ofAchernar with the VINCI/VLTI beam combiner in the H and K bands, usingvarious telescope configurations and baseline lengths with a wideazimuthal coverage. Results: .The observed visibility measurementsalong the polar direction are significantly lower than the visibilityfunction of the photosphere of the star alone, in particular at lowspatial frequencies. This points to the presence of an asymmetricdiffuse CSE elongated along the polar direction of the star. To ourdata, we fit a simple model consisting of two components: a 2Delliptical Gaussian superimposed on a uniform ellipse representing thedistorted photosphere of the fast rotating star. Conclusions: .Weclearly detected a CSE elongated along the polar axis of the star, aswell as rotational flattening of the stellar photosphere. For theuniform-ellipse photosphere we derive a major axis of θ_eq = 2.13± 0.05 mas and a minor axis of θ_pol = 1.51 ± 0.02mas. The relative near-IR flux measured for the CSE compared to thestellar photosphere is f = 4.7 ± 0.3%. Its angular dimensions areloosely constrained by the available data at ρ_eq = 2.7 ± 1.3mas and ρ_pol = 17.6 ± 4.9 mas. This CSE could be linked tofree-free emission from the radiative pressure driven wind originatingfrom the hot polar caps of the star.
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