Садржај
Слике
Уплоадјуј своје слике
DSS Images Other Images
Везани чланци
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 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.
| 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.
| Grey extinction in the solar neighbourhood?
Some of the close O and B dwarfs appear to be fainter than indicated bytheir Hipparcos distances, intrinsic absolute magnitudes attributed totheir spectral types, and estimated selective interstellar extinction.This discrepancy is explained in the paper by the grey (neutral)interstellar extinction in the visual range of spectrum. The measure ofsuch an effect is related to discrete features of the interstellarmatter.
| 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.
| 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.
| Outflow-induced Dynamical and Radiative Instability in Stellar Envelopes with an Application to Luminous Blue Variables and Wolf-Rayet Stars
Theoretical models of the remnants of massive stars in a very hot,post-red-supergiant phase display no obvious instability if standardassumptions are made. However, the brightest observed classical luminousblue variables (LBVs) may well belong to such a phase. A simpletime-dependent theory of moving stellar envelopes is developed in orderto treat deep hydrodynamical disturbances caused by surface mass lossand to test the moving envelopes for dynamical instability. In the caseof steady state outflow, the theory reduces to the equivalent of theCastor, Abbott, & Klein formulation for optically thick winds atdistances well above the sonic point. The time-dependent versionindicates that the brightest and hottest LBVs are both dynamically andradiatively unstable, as a result of the substantial lowering of thegeneralized Eddington luminosity limit by the mass-loss acceleration. Itis suggested that dynamical instability, by triggering secular cycles ofmass loss, is primarily what differentiates LBVs from the purelyradiatively unstable Wolf-Rayet stars. Furthermore, when accuratemain-sequence mass-loss rates are used to calculate the evolutionarytracks, the predicted surface hydrogen and nitrogen abundances of theblue remnants agree much better with observations of the brightest LBVsthan before.
| Observations of H3 in the Diffuse Interstellar Medium
Surprisingly large column densities of H+3 havebeen detected using infrared absorption spectroscopy in seven diffusecloud sight lines (Cygnus OB2 12, Cygnus OB2 5, HD 183143, HD 20041, WR104, WR 118, and WR 121), demonstrating that H+3is ubiquitous in the diffuse interstellar medium. Using the standardmodel of diffuse cloud chemistry, our H+3 columndensities imply unreasonably long path lengths (~1 kpc) and lowdensities (~3 cm-3). Complimentary millimeter-wave, infrared,and visible observations of related species suggest that the chemicalmodel is incorrect and that the number density ofH+3 must be increased by 1-2 orders of magnitude.Possible solutions include a reduced electron fraction, an enhanced rateof H2 ionization, and/or a smaller value of theH+3 dissociative recombination rate constant thanimplied by laboratory experiments.
| 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
| On the identification of the C60+ interstellar features
The identity of the carriers of the diffuse interstellar bands (DIBs) isone of the most fascinating puzzles of modern spectroscopy. Over thelast few years the number of known DIBs has grown substantially. In thispaper we discuss the two recently discovered near-infrared weakinterstellar features which have already been proposed as fingerprintsof the buckminsterfullerene [formmu2] We present and discussmeasurements of the two related DIBs within a larger sample of reddenedtargets, observed with different spectrometers, telescopes and siteconditions. We provide additional arguments in favour of theinterstellar origin of the two bands. We find evidence around the9577-Å DIB of far-wing structures, which may affect broad-bandmeasurements. We estimate corrections and errors for telluric andstellar blends, and show that the cores of the two DIBs are wellcorrelated with a ratio near unity within 20per cent. Finally, wediscuss their relation to the laboratory spectra of [formmu3] and thesearch for two expected weaker [formmu4] transitions.
| The Milton Bureau Revisited
Under the direction of Cecilia Payne-Gaposchkin and Sergei Gaposchkin, aprogram was subsidized by the Milton Fund of Harvard Observatory in 1937for the study of all variable stars then known to be brighter than tenthphotographic magnitude at maximum. This included some 1512 stars forwhich a grand total of 1,263,562 estimates of magnitude were made,ranging from a low of 16 (except for a few novae) to 4084 observationsper star. The sky had been divided into 54 fields, and the results ofthe measurements presented field by field in two volumes of the Annalsof Harvard Observatory. Then, in another volume, the results werediscussed in four sections, each dealing with a particular class ofvariable: 1, those of RV Tauri type; 2, the eclipsing variables; 3,Cepheids and RR Lyrae variables, and 4, the red variables, especiallyMira-type and semiregular variables.For the present paper, many of these results have been compared withmodern determinations in the 1985-87 version of the "General Catalogueof Variable Stars (GCVS)". In particular, there are numerous instancesof disagreement as to whether a star should be classified RV or SR.Although there are many instances where the Milton Bureau determinationsof types of variability differ from the types given in moderncatalogues, the reasons for the differences are generallyunderstandable.For 17 RV Tauri type stars in this survey multiple periods have now beendetermined. Many of these still deserve continued observations in orderto ascertain the constance of the periods and improve the accuracy oftheir longest reported periods.
| 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.
| Narrow Diffuse Interstellar Bands: A Survey with Precise Wavelengths
We present an atlas of 271 diffuse interstellar bands (DIBs) between4460 and 8800 Å based on echelle spectra (R=45,000), of which morethan 100 are new DIBs discovered in this survey. The atlas is restrictedmostly to narrow features, and we describe the tests for an interstellarorigin. The rest wavelength of each DIB was determined in a high-qualitycomposite spectrum of the star HD 23180 using the interstellar Na I (D1and D2) lines to establish the radial velocity of the single interveningcloud. DIB wavelengths are quoted to 0.01 Å and are probablyaccurate to ~0.1 Å. Other, weak DIBs found in the spectra ofheavily reddened stars are included with a lower wavelength precision.
| Gas-Phase Electronic Spectra of Carbon-Chain Radicals Compared with Diffuse Interstellar Band Observations
This paper compares laboratory gas-phase spectra of neutral and cationiclinear carbon-chain radicals with astronomical diffuse interstellar band(DIB) spectra. The origin bands of the strong electronic transitions ofthe studied species, C2nH (n=3-6),HC2nH+ (n=2-4), NC2n-2N+(n=3, 4), do not coincide with detectable DIBs, except forHC2n-1N+ (n=3), which possibly matches a weakfeature. It is concluded that the column densities of these speciesthrough the observed diffuse clouds are below 2x1012cm-2. The profile change and the shift of the maxima of theabsorption bands with temperature are illustrated. The relation toradio-astronomical detection of carbon chains in diffuse media is alsodiscussed.
| The OB Zoo: A Digital Atlas of Peculiar Spectra
A digital atlas of 20 high-luminosity, peculiar OB spectra in the3800-4900 Å range is presented. The atlas is organized anddiscussed in terms of the following four categories: WN-A or WNL stars,OB Iape or very late WN (WNVL) stars, iron stars, and B-supergiantluminous blue variables (LBVs). Several objects in the earliercategories are also active or quiescent LBVs. Some (but not all) ofthese objects have been well studied, and extensive references areprovided, as are comprehensive spectral-line identifications. Severalnew morphological relationships among the objects have been recognizedthrough this presentation. In particular, attention is drawn to theoccurrence of spatial pairing between nearly identical, unusual spectra,which may have implications for a particular mode of massive-starformation. This small sample includes one or both members of at leastfive such pairs. Physical explanations of these peculiar, likelytransitional spectra and the relationships among them are essential fora complete understanding of massive stellar evolution.
| Checking the yellow evolutionary void. Three evolutionary critical Hypergiants: HD 33579, HR 8752 & IRC +10420
We have checked the reality of the yellow evolutionary void (which is anarea in the Hertzsprung-Russell diagram where atmospheres of bluewardevolving super- and hypergiants are moderately unstable), by comparingone star inside the void: HD 33579 (= R76), and two at thelow-temperature border of it: HR 8752 (= HD 217476, V509 Cas) andIRC+10420. We found that the first star has a large mass and a fairlystable behaviour over time. These aspects suggest, together withabundance determinations by others, that it is a fairly young, stillredward-evolving supergiant. For such a star the void is not forbidden.The two other stars, HR 8752 resp. IRC+10420, have low masses whichplaces them in the post-red blueward loop. They show indications of theexpected bouncing effect for blueward returning red supergiants: whenapproaching the void they eject mass, resulting in a sudden reduction ofT_eff and a decrease of the atmospheric acceleration to g_eff =~ zero.Thereafter T_eff and g_eff increase again. For HR 8752 two such recent`bounces' have been identified. The photometric variations of HR 8752and of HD 33579 are due to high-l gravity-wave pulsations. Based onobservations at the La Palma Observatory and the ESO Observatory inChili.
| Interstellar C_2 absorption lines towards CH+ forming regions
Interstellar C_2 absorption line observations towards the southern OBassociations NGC 2439, Vela OB1, and Cen OB1 are used to infergaskinetic temperatures Tkin and densities nctowards lines of sight with previously determined large CH andCH+ column densities. Towards NGC 2439, the material ischaracterised by temperatures of Tkin = 75-85 K and densitiesexceeding nc > 1000 C_2m, and a fractional C_2 abundanceof about x(C_2) = 1.5 10-8. Temperatures and densitiesinferred towards two stars in Vela OB1 are Tkin = 65-85 K andnc >= 600 C_2m. C_2 fractional abundances in the cloudcomplex obscuring Vela OB1 are about half the value found in NGC 2439.For the lines of sight towards Cen OB1, values of Tkin =70-95 K and fractional abundances of x(C_2) ~ 5 10-9 arefound, and towards HD 114213, Tkin = 25 K and nc =400+/-100 C_2m. The observations demonstrate that C_2 resides in coolgas at temperature of Tkin < 100 K. The C_2 columndensities are correlated with those of CH, which is expected fromtheoretical models. This suggests that CH is efficiently formed in thecool material as well, together with C_2. This finding does not supportexpectations from recent MHD and vortex models of CH+formation, which stipulate that neutrals such as CH are abundantlyproduced in hot regions of many 100 K temperature.
| Light variations of the blue hypergiants HD 168607 and HD 168625 (1973-1999)
Strömgren differential photometry of HD 168607 and HD 168625collected during the previous 27 years is analysed. We bring to an endour quarter of a century monitoring of these objects and conclude thatHD 168607 shows typical variability of an uneventful LBV, whereas HD168625 remains to be classified as a variable hypergiant, or amarginally-dormant LBV. The long-term light- and colour behaviour of HD168607 indicates the possible presence of a very slowly-developing S Dorphase - that is, a bright phase that coincides with redder colours.Besides the alpha Cygni-type microvariability, no other signs ofultrashort-periodic variability has been observed. Based on observationsobtained at the European Southern Observatory at La Silla, Chile(applications ESO 56D-0249, 58D-0118, 60D-0148, 61D-0128 and 62H-0110)
| Five-colour photometry of OB-stars in the Southern Hemisphere
Observations of OB-stars, made in 1959 and 1960 at the Leiden SouthernStation near Hartebeespoortdam, South Africa, with the VBLUW photometerattached to the 90 cm light-collector, are given in this paper. They arecompared with photometry obtained by \cite[Graham (1968),]{gra68}\cite[Walraven & Walraven (1977),]{wal77} \cite[Lub & Pel(1977)]{lub77} and \cite[Van Genderen et al. (1984).]{gen84} Formulaefor the transformation of the present observations to those of\cite[Walraven & Walraven (1977)]{wal77} and \cite[Lub & Pel(1977)]{lub77} are given. Table 4 is only available in electronic format the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) orvia http://cdsweb.u-strasbg.fr/Abstract.html
| The orientation within the Galaxy and the Large Magellanic Cloud of nebulae ejected by massive stars
The orientation of nebulae ejected by massive stars (Luminous BlueVariables, WR stars, SN1987A) is investigated with respect to thestructure of the galaxy to which they belong. In the Galaxy, we findthat the projected long axes of the nebulae most often align with thegalactic plane, and then also with the galactic magnetic field. Thisalignment is statistically significant. In addition, a few nebulae areapparently oriented perpendicular to the galactic plane. In the LargeMagellanic Cloud, the nebular axes are found to closely follow thespiral magnetic field. With different inclinations, the Galaxy and theLarge Magellanic Cloud probably offer complementary views of the samephenomenon. Although the sample studied thus far is small and thestatistics limited, these results suggest that the orientation ofmassive star ejecta depends on galactic magnetic fields. Since thenebular axes are apparently correlated to the symmetry axes of the starsthemselves, and since, in the early evolutionary stages, alignments ofaccretion disk axes with the interstellar magnetic field have beenreported, it is argued that the observed alignment effect results fromthe star formation process. Based in part on observations collected atthe European Southern Observatory (ESO, La Silla)
| Warm Dust around Blue Hypergiants: Mid-Infrared Imaging of the Luminous Blue Variable HD 168625
We present the first high spatial resolution images of thermal emissionfrom the nebula surrounding the Galactic luminous blue variable (LBV) HD168625. Four images, centered at 4.7, 10.1, 11.6, and 19.9 mu m, havebeen obtained at UKIRT with MAX, the new mid-IR camera of theMax-Planck-Institut fur Astronomie. A Br gamma image taken at UKIRT withIRCAM3 is also presented. On the basis of the availablespectrophotometric data and of our photometry of the central star, werevise the distance currently assumed for this source from 2.2 to 1.2kpc. The dust emission, which is optically thin and therefore sensitiveto the mass distribution, clearly indicates the bipolar structure of thenebula. The majority of the infrared (IR) flux comes from ageometrically thin layer of dust tracing the outer edges of the nebularlobes. An inner shell or ring along the equatorial plane is alsodetected. From the mid-IR spectral energy distribution we constrain thecomposition and temperature of the grains. Through a simple geometricalmodel we estimate the dust density and total mass in the shell.Considering the structure of the nebula from an evolutionary point ofview, we investigate the formation mechanism (novalike ejection orinteracting stellar winds) and the recent past of the LBV precursor. Wefind that the interaction of the present wind with a slower dusty wind(M dot ~=2x10^{-5} Mȯ yr-1, v ~= 10 km s-1) ejected by the stellarprecursor in a previous phase is by far the most viable formationmechanism for the nebula around HD 168625. Finally, the generaloccurrence of a redward evolution for the LBV precursors is discussed.
| UBV beta Database for Case-Hamburg Northern and Southern Luminous Stars
A database of photoelectric UBV beta photometry for stars listed in theCase-Hamburg northern and southern Milky Way luminous stars surveys hasbeen compiled from the original research literature. Consisting of over16,000 observations of some 7300 stars from over 500 sources, thisdatabase constitutes the most complete compilation of such photometryavailable for intrinsically luminous stars around the Galactic plane.Over 5000 stars listed in the Case-Hamburg surveys still lackfundamental photometric data.
| HIPPARCOS photometry of 24 variable massive stars (alpha Cygni variables)
The photometric variability of 24 alpha Cyg variables, i.e. variablesuper-and hypergiants, observed by the Hipparcos satellite is described.Three of the selected stars are situated in the SMC, 12 in the LMC andthe remaining 9 in the Galactic plane. Four of them are hot S Dor-typevariables, or LBVs (luminous blue variables) and two are possiblemembers of this class. Light curves are presented for all stars. Forfive stars, among which one LBV, the variability was discovered from theHipparcos photometric data. Results of period searches are presentedand, when relevant, folded light curves are shown. The linear ephemerisof two variables was revised. For most of the program stars theHipparcos magnitude scale (Hp) differs from the V of the UBV system byla 0fm1 . For all variables temperatures and M_bol are given. Galacticforeground reddening for the objects in the Magellanic Clouds are givenbased on IRAS maps.
| The yellow hypergiants
We list the main stellar data of known hypergiants and similarlyluminous stars, and then concentrate on a review of the yellowhypergiants. These stars are post-red supergiants evolving alongblueward loops in the Hertzsprung-Russell diagram. Their properties,their location in the Hertzsprung-Russell diagram and their occasionalmass ejections are related to a region of atmospheric instability in theH-R diagram, the Yellow Void. The `bouncing against the border of theVoid' of three objects: ρ Cas, HR8752 and IRC+10420, is described.The apparent atmospheric instability of yellow hypergiants is related tothe atmospheric pulsations. There are indications that the approach tothe Void is associated with an increased amplitude of the pulsation andwith enhanced mass loss. The observed small-scale motion field is onlyapparently strongly supersonic; the observed large stochastic velocitiesare the quasi-stochastically varying thermal motions in the many hotsheets that occur in the wakes of many small shocks, while the realhydrodynamic velocity component is small and subsonic. This shock-wavefield is also responsible for the observed rate of mass loss and foremission in the wings of Hα . Most yellow hypergiants haveenvelopes containing gas and dust, but a thick extended envelope,presumably dissipating and showing bipolar outflow, is only known aroundIRC+10420. At the interface of the bipolar wind and the interstellarmedium one or more stationary shocks may develop as is observed in thecase of IRC+10420 and suspected with ρ Cas.
| A Radial Velocity Database for Stephenson-Sanduleak Southern Luminous Stars
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1997AJ....113..823R&db_key=AST
| H-beta Photometry of Southern Luminous Stars
H-beta photometry for 116 stars listed in Stephenson and Sanduleak's(1971) catalog of _Luminous Stars in the Southern Milky Way_ isreported. The vast majority of these objects are O and B-type stars; afew are A, F, or Wolf-Rayet stars. A systematic difference of on theorder of 30 mmag between the present results and those of otherinvestigators is noted. (SECTION: Stars)
|
Додај нови чланак
Линкови у сродству са темом
Додај нови линк
Чланови следећих група \:
|
Посматрања и Астрометриски подаци
| Сазвежђа: | Стрелац |
| Ректацензија: | 18h21m14.89s |
| Deклинација: | -16°22'31.7" |
| Apparent магнитуда: | 8.258 |
| Даљина: | 495.05 parsecs |
| Proper motion RA: | 2.4 |
| Proper motion Dec: | 0.3 |
| B-T magnitude: | 9.962 |
| V-T magnitude: | 8.399 |
Каталог и designations:
|
