Contenuti
Immagini
Carica la tua immagine
DSS Images Other Images
Articoli relazionati
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.
| NAOMI/OASIS on-sky performance
NAOMI is the AO system of the 4.2-m William Herschel Telescope on LaPalma. It delivers an AO-corrected image to a lenslet array at the focalplane of the optical integral-field spectrograph OASIS. The resulting1100 spectra are imaged onto a high-QE, low-fringing MITLL3 CCD. A rangeof spectroscopic and spatial (0.09 0.42 arcsec/lenslet) configurationsis available. At wavelength 0.7 μm, the NAOMI-corrected FWHM istypically half that of the natural seeing. Scheduled OASIS observingbegan in semester 2004B, with 9 programmes awarded a total of 26 nightsduring the first year of operation. A Rayleigh laser guide star is underdevelopment, with first light expected summer 2006. In conjunction withNAOMI/OASIS, this will provide a unique facility: AO-corrected opticalintegral-field spectroscopy anywhere on the northern sky.
| 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.
| 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.
| 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.
| 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).
| 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.
| 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.
| 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.
| A Survey of Local Group Galaxies Currently Forming Stars. I. UBVRI Photometry of Stars in M31 and M33
We present UBVRI photometry obtained from Mosaic images of M31 and M33using the Kitt Peak National Observatory 4 m telescope. We describe ourdata reduction and automated photometry techniques in some detail, as wewill shortly perform a similar analysis of other Local Group galaxies.The present study covered 2.2 deg2 along the major axis ofM31 and 0.8 deg2 on M33, chosen so as to include all of theregions currently active in forming massive stars. We calibrated ourdata using photometry from the Lowell 1.1 m telescope, and this externalmethod resulted in millimagnitude differences in the photometry ofoverlapping fields, providing some assurance that our photometry isreliable. The final catalog contains 371,781 and 146,622 stars in M31and M33, respectively, where every star has a counterpart in (at least)the B, V, and R passbands. Our survey goes deep enough to achieve 1%-2%photometry at 21 mag (corresponding to stars more massive than 20Msolar) and achieves <10% errors at U~B~V~R~I~23 mag.Although our typical seeing was only modest (0.8"-1.4", with median1.0") by some standards, we find excellent correspondence between ourcatalog sources and those we see in our Hubble Space Telescope ACS datafor OB48, a crowded region in M31. We compare our final photometry withthat of others and find good agreement with the CCD catalog of M31 starsby Magnier et al., although our study covers twice the area and goesabout 2 mag deeper. There is also excellent agreement with the CCD``DIRECT'' surveys of M31 and M33. The photographic studies of othersfare less well, particularly at the faint end in V, where accuratebackground subtraction is needed for good photometry. We providecross-references to the stars confirmed as members by spectroscopy andcompare the locations of these to the complete set in color-magnitudediagrams. While follow-up spectroscopy is needed for many projects, wedemonstrate the success of our photometry in being able to distinguishM31/M33 members from foreground Galactic stars. Finally, we present theresults of a single night of spectroscopy on the WIYN 3.5 m telescope,examining the brightest likely members of M31. The spectra identify 34newly confirmed members, including B-A supergiants, the earliest O starknown in M31, and two new luminous blue variable candidates whosespectra are similar to that of P Cygni.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(AURA), Inc., under NASA contract NAS5-26555. These observations areassociated with program GO-9794.
| A deep mosaic of [O III]5007 Å CCD images of the environment of the LBV star P Cygni
A mosaic of six, deep, CCD images in the light of the [O iii]5007Å nebular emission line has been obtained with the 1.3-m Skinakas(Crete) telescope of the filamentary nebulosity surrounding P Cygni. The[O iii]5007 Å line discriminates against confusing galactic Hiiregions along the same sight-lines and the new mosaic did not includethe 4.8 mag. central star; a source of artifacts in the previous lowerangular resolution observations. New giant “lobes” and“shells” are found to be clustered around P Cygni which mustbe the relics of historic eruptions between 2400 and up to≈105 yr ago.
| The interior of the SNR RX J0852.0-4622 (Vela Jr) at radio wavelengths
Aims.We observed the center of the supernova remnant Vela Jr in radiocontinuum in order to search for a counterpart to the compact centralX-ray source CXOU J085201.4-461753, possibly a neutron star candidatewhich could be the remnant of the supernovaexplosion.Methods.Observations were made with the Australia TelescopeCompact Array at 13 and 20 cm. Spectral indices were obtained using fluxdensity correlations of the data which were spatially filtered to havethe same u-v coverage. A multiwavelength search for counterparts to thecompact central X-ray source was made.Results.We compiled a newcatalogue of 31 small diameter radio sources, including the previouslyknown source PMN J0853-4620, listing the integrated flux densities at 20cm and, for half of the sources, the flux densities at 13 cm with thecorresponding spectral indices. All sources are unresolved at thepresent angular resolution except for Source 18, which is clearlyelongated and lies strikingly close to CXOU J085201.4-461753. Ourobservations show no evidence for the existence of a pulsar wind drivennebula associated with the point X-ray source. Furthermore, Source 18has a thermal spectrum with index α = +0.8 ± 0.4 (S ∝να), and appears to be the counterpart of theoptical source Wray 16-30. In spite of the absence of [O iii] emissionlines as reported in the literature, we find that this object could beexplained as a low emission planetary nebula belonging to the"butterfly" morphological class.Conclusions.We conclude that if theradio source 18 is actually a planetary nebula, then CXOUJ085201.4-461753 is more likely to be related to it rather than to VelaJr.
| 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.
| Physical parameters and wind properties of galactic early B supergiants
We present optical studies of the physical and wind properties, plus CNOchemical abundances, of 25 O9.5-B3 Galactic supergiants. We employnon-LTE, line blanketed, extended model atmospheres, which provide amodest downward revision in the effective temperature scale of early Bsupergiants of up to 1-2 kK relative to previous non-blanketed results.The so-called "bistability jump" at B1 (Teff 21 kK)from Lamers et al. is rather a more gradual trend (with large scatter)from v&infy;/vesc3.4 for B0-0.5 supergiantsabove 24 kK to v&infy;/vesc 2.5 for B0.7-1supergiants with 20 kK ≤ Teff ≤ 24 kK, andv&infy;/vesc 1.9 for B1.5-3 supergiants below20 kK. This, in part, explains the break in observed UV spectralcharacteristics between B0.5 and B0.7 subtypes as discussed by Walbornet al. We compare derived (homogeneous) wind densities with recentresults for Magellanic Cloud B supergiants and generally confirmtheoretical expectations for stronger winds amongst Galacticsupergiants. However, winds are substantially weaker than predictionsfrom current radiatively driven wind theory, especially at mid-Bsubtypes, a problem which is exacerbated if winds are already clumped inthe Hα line forming region. In general, CNO elemental abundancesreveal strongly processed material at the surface of Galactic Bsupergiants, with mean N/C and N/O abundances 10 and 5 times higher thanthe Solar value, respectively, with HD 2905 (BC0.7 Ia) indicating thelowest degree of processing in our sample, and HD 152236 (B1.5Ia+) the highest.
| Determination of the Mass Loss Rate and the Terminal Velocity of Stellar Winds. I. Genetic Algorithm for Automatic Line Profile Fitting
A new method for automatic fitting of Pline profiles in UV spectra ofstellar winds is presented. The line source function is calculated usingSobolev's approximation and the emergent flux is obtained by exactintegration of the equation of the radiation transport (similar to theSEI method described by Lamers et al. (1987)). The quality of the fit isevaluated using the likelihood estimator. The maximization of thelikelihood is done by a genetic algorithm. The advantages of our methodwith respect to other similar approaches are its robustness and itsinsensibility to the initial guess. In addition, the algorithmguarantees the localization of the global maximum of the likelihoodhypersurface, which is not the case for classical minimizationalgorithms. Here we present an implementation of the genetic algorithmfor line profile fitting, its tests on both synthetic and real data andan estimation of the confidence limits of the results.
| Supernova 1954J (Variable 12) in NGC 2403 Unmasked
We have confirmed that the precursor star of the unusual supernova 1954J(also known as Variable 12) in NGC 2403 survived what appears to havebeen a superoutburst, similar to the 1843 Great Eruption of ηCarinae in the Galaxy. The apparent survivor has changed little inbrightness and color over the last 8 years, and a Keck spectrum revealscharacteristics broadly similar to those of η Car. This is furthersuggested by our identification of the actual outburst-surviving star inhigh-resolution images obtained with the Advanced Camera for Surveys onthe Hubble Space Telescope. We reveal this ``supernova impostor'' as ahighly luminous (M0V~-8.0 mag), very massive(Minitial>~25 Msolar) eruptive star, nowsurrounded by a dusty (AV~4 mag) nebula, similar to ηCar's famous Homunculus.Based in part on observations with the NASA/ESA Hubble Space Telescope,obtained at the Space Telescope Science Institute (STScI), which isoperated by AURA, Inc., under NASA contract NAS5-26555.
| 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.
| Evolution of X-ray emission from young massive star clusters
The evolution of X-ray emission from young massive star clusters ismodelled, taking into account the emission from the stars as well asfrom the cluster wind. It is shown that the level and character of thesoft (0.2-10 keV) X-ray emission change drastically with cluster age andare tightly linked with stellar evolution. Using the modern X-rayobservations of massive stars, we show that the correlation betweenbolometric and X-ray luminosity known for single O stars also holds forO+O and (Wolf-Rayet) WR+O binaries. The diffuse emission originates fromthe cluster wind heated by the kinetic energy of stellar winds andsupernova explosions. To model the evolution of the cluster wind, themass and energy yields from a population synthesis are used as input toa hydrodynamic model. It is shown that in a very young cluster theemission from the cluster wind is low. When the cluster evolves, WRstars are formed. Their strong stellar winds power an increasing X-rayemission of the cluster wind. Subsequent supernova explosions pump thelevel of diffuse emission even higher. Clusters at this evolutionarystage may have no X-ray-bright stellar point sources, but a relativelyhigh level of diffuse emission. A supernova remnant may become adominant X-ray source, but only for a short time interval of a fewthousand years. We retrieve and analyse Chandra and XMM-Newtonobservations of six massive star clusters located in the LargeMagellanic Cloud (LMC). Our model reproduces the observed diffuse andpoint-source emission from these LMC clusters, as well as from theGalactic clusters Arches, Quintuplet and NGC 3603.
| Correlation patterns between 11 diffuse interstellar bands and ultraviolet extinction
We relate the equivalent widths of 11 diffuse interstellar bands,measured in the spectra of 49 stars, to different colour excesses in theultraviolet. We find that most of the observed bands correlatepositively with the extinction in the neighbourhood of the2175-Åbump. Correlation with colour excesses in other parts of theextinction curve is more variable from one diffuse interstellar band toanother; we find that some diffuse bands (5797, 5850 and 6376 Å)correlate positively with the overall slope of the extinction curve,while others (5780 and 6284 Å) exhibit negative correlation. Wediscuss the implications of these results on the links between thediffuse interstellar band carriers and the properties of theinterstellar grains.
| Polarization Effects in the Radiation of Magnetized Envelopes and Extended Accretion Structures
We have calculated the degree and position angle of the polarization ofradiation scattered in a magnetized, optically thin or optically thickenvelope around a central source, taking into account Faraday rotationof the plane of polarization during the propagation of the scatteredradiation and the finite size of the radiation source. The wavelengthdependence of the degree of polarization can be used to estimate themagnetic field of the source (a star, the region around a neutron star,or a black hole), and we have used our calculations to estimate themagnetic fields in a number of individual objects: several hot O andWolf-Rayet stars, compact objects in X-ray close binaries with blackholes (SS 433, Cyg X-1), and supernovae. The spectrum of the linearpolarization can be used to determine the magnetic field in the vicinityof a central supermassive black hole, where the polarized opticalradiation is generated. In a real physical model, this value can beextrapolated to the region of the last stable orbit. In the future, theproposed technique will make it possible to directly estimate themagnetic field in the region of the last stable orbit of a supermassiveblack hole using X-ray polarimetry.
| The Ultraviolet Spectrum of η Carinae: Investigation of the Ejecta Absorption
We have investigated the far- through mid-UV (1150-2360 Å)spectrum of η Carinae during the late stages of its broad maximumusing the Hubble Space Telescope (HST) Space Telescope ImagingSpectrograph (STIS) moderate dispersion echelle modes. The η Carspectrum is a mixture of absorption and emission lines from thesurrounding nebula superimposed on broad stellar wind features. Thispaper provides a description of the observed spectrum including the windfeatures, the interstellar absorption, and the emission spectrum fromthe surrounding nebula, but with the emphasis on the absorption spectrumformed in the foreground ejecta. The ejecta absorption spectrum has acomplicated velocity structure in which two velocity structures, at -146and -513 km s-1, are easily distinguished. These two velocitycomponents, formed in different regions of the η Car nebula, have inan earlier analysis been identified and demonstrated to have verydifferent spectral characteristics. The slower velocity component istime variable over the spectroscopic period and is characterized byspectral lines from mainly singly ionized iron-peak elements, while thefaster one shows transitions from neutral and singly ionized elements inaddition to molecular lines from the hydrogen Lyman bands. Thehigh-velocity H2 lines dominate great parts of the spectrumwith over 800 identified transitions from energy levels up to 30,000cm-1. The STIS MAMA data provide the tool for spatialinvestigations of the central parts of η Car. H I Lyα pumpedand semiforbidden emission lines are observed to be formed east of thecentral source toward Weigelt blobs B and C, located up to 0.2" from thecentral source. The complete spectrum, with nebular and interstellarline identifications, is available in the electronic edition of thepaper.Laboratory for Astronomy and Solar Physics, Code 681, Goddard SpaceFlight Center, Greenbelt, MD 20771.
| On the detection of chemically peculiar stars using Δa photometry
We have summarized all Δ a measurements for galactic field stars(1474 objects) from the literature published over more than two decades.These measurements were, for the first time, compiled and homogeneouslyanalyzed. The Δ a intermediate band photometric system samples thedepth of the 5200 Å flux depression by comparing the flux at thecenter with the adjacent regions with bandwidths of 110 Å to 230Å. Because it was slightly modified over the last three decades,we checked for systematic trends for the different measurements butfound no correlations whatsoever. The Δ a photometric system ismost suitable to detecting magnetic chemically peculiar (CP) stars withhigh efficiency, but is also capable of detecting a small percentage ofnon-magnetic CP objects. Furthermore, the groups of (metal-weak)λ Bootis, as well as classical Be/shell stars, can besuccessfully investigated. In addition, we also analyzed the behaviourof supergiants (luminosity class I and II). On the basis of apparentnormal type objects, the correlation of the 3σ significance limitand the percentage of positive detection for all groups was derived. Wecompared the capability of the Δ a photometric system with theΔ (V1 - G) and Z indices of the Geneva 7-color system to detectpeculiar objects. Both photometric systems show the same efficiency forthe detection of CP and λ Bootis stars, while the indices in theGeneva system are even more efficient at detecting Be/shell objects. Onthe basis of this statistical analysis it is possible to derive theincidence of CP stars in galactic open cluster and extragalactic systemsincluding the former unknown bias of undetected objects. This isespecially important in order to make a sound statistical analysis ofthe correlation between the occurrence of these objects andastrophysical parameters such as the age, metallicity, and strength ofglobal, as well as local, magnetic fields.
| Interstellar 12C/13C ratios through CH^+λλ 3957,4232 absorption in local clouds: incomplete mixing in the ISM
The 12C/13C isotope ratio is a tracer of stellaryields and the efficiency of mixing in the ISM.12CH+/13CH+ is not affectedby interstellar chemistry, and is the most secure way of measuring12C/13C in the diffuse ISM.R=12C/13C is 90 in the solar system. Previousmeasurements of 12CH+λλ3957.7,4232.3and 13CH+λλ3958.2,4232.0 absorptiontoward nearby stars indicate some variations in12C/13C, with values ranging from 40 to 90suggesting inefficient mixing. Except for the cloud toward ζOph,these R values are strongly affected by noise. With UVES on the VLT wehave improved on the previous interstellar 12C/13Cmeasurements. The weighted 12C/13C ratio in thelocal ISM is 78.27 ± 1.83, while the weighted dispersion of ourmeasurements is 12.7, giving a 6.9σ scatter. Thus we report on a6.9σ detection of 16.2% root-mean-square variations in the carbonisotopic ratio on scales of ~100 pc: R= 74.7 ± 2.3 in theζOph cloud, while R = 88.6 ± 3.0 toward HD 152235 in theLupus clouds, R = 62.2 ± 5.3 towards HD 110432 in the Coalsack,and R = 98.9 ± 10.1 toward HD 170740. The observed variations in13C/12C are the first significant detection ofchemical heterogeneity in the local ISM.
| 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.
| Current day mass loss rate for Luminous Blue Variable IRAS 18576+0341
In this letter we report on the first set of multi-frequency and highangular resolution radio observations of IRAS 18576+0341. The radioobservations have revealed an extended, asymmetric and quite structuredradio nebula and allowed us to locate the central core of this highlyobscured new galactic Luminous Blue Variable. From the analysis of radioproperties of IRAS 18576+0341 estimates of important physical parametersof the central star have been determined. In particular, an effectivetemperature of Teff (2.6 ± 0.2) ×104 K, corresponding to a B0-B0.5 supergiant, has beenderived. The most notable result is the determination of the current daymass-loss (dot {M}= 3.7 × 10-5 Mȯyr-1), which is of particular importance since mass-loss ratedetermination from radio observations appears to be a more reliablemethod compared to the others based on different diagnostics.
| 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.
| CHARM2: An updated Catalog of High Angular Resolution Measurements
We present an update of the Catalog of High Angular ResolutionMeasurements (CHARM, Richichi & Percheron \cite{CHARM}, A&A,386, 492), which includes results available until July 2004. CHARM2 is acompilation of direct measurements by high angular resolution methods,as well as indirect estimates of stellar diameters. Its main goal is toprovide a reference list of sources which can be used for calibrationand verification observations with long-baseline optical and near-IRinterferometers. Single and binary stars are included, as are complexobjects from circumstellar shells to extragalactic sources. The presentupdate provides an increase of almost a factor of two over the previousedition. Additionally, it includes several corrections and improvements,as well as a cross-check with the valuable public release observationsof the ESO Very Large Telescope Interferometer (VLTI). A total of 8231entries for 3238 unique sources are now present in CHARM2. Thisrepresents an increase of a factor of 3.4 and 2.0, respectively, overthe contents of the previous version of CHARM.The catalog is only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/431/773
| SN 2002kg - the brightening of LBV V37 in NGC 2403
SN 2002kg is a type IIn supernova, detected in October 2002 in thenearby spiral galaxy NGC 2403. We show that the position of SN 2002kgagrees within the errors with the position of the LBV V37. Ground basedand HST ACS images however show that V37 is still present after the SN2002kg event. We compiled a lightcurve of V37 which underlines thevariablity of the object, and shows that SN 2002kg was the brighteningof V37 and not a supernova. The recent brightening is not a gianteruption, but more likely part of an S Dor phase. V37 shows strongHα +[N II] emission in recent images and in the SN2002kg spectrum, which we interprete as the signature of the presence ofan LBV nebula. A historic spectrum lacks emission, which may hint thatwe are witnessing the formation of an LBV nebula.
| Very Large Array Observations of Winds from Massive Stars
The classical model for free-free emission from ionized stellar winds isbased on the assumption of a stationary, isotropic and homogeneous wind.However, since there exist objects whose wind behavior deviates from thestandard model, these assumptions have been questioned in the lastdecade. In this work, we present results for 3 bright sources: P Cyg(B1Ia, LBV), Cyg OB2 No.12 (B5Ie, LBV?) and WR 147 (WN8h+B0.5V). Thesehot massive objects have been reported to possess winds that deviatefrom the basic assumptions. We have obtained radio flux densities,sizes, spectral indices and mass loss rates for each of the targets.These parameters allow us to analyze possible asymmetries,inhomogeneities and time variations in the flux densities. Thesefeatures confirm the nonclassical behaviour of the winds of these hotmassive stars.
|
Sottometti un nuovo articolo
Link relazionati
Sottometti un nuovo link
Membro dei seguenti gruppi:
|
Osservazione e dati astrometrici
| Costellazione: | Cigno |
| Ascensione retta: | 20h17m47.20s |
| Declinazione: | +38°01'59.0" |
| Magnitudine apparente: | 4.81 |
| Distanza: | 1923.077 parsec |
| Moto proprio RA: | -4.4 |
| Moto proprio Dec: | -6.6 |
| B-T magnitude: | 5.221 |
| V-T magnitude: | 4.837 |
Cataloghi e designazioni:
|
