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מאמרים קשורים
The Circumstellar Environments of NML Cygni and the Cool Hypergiants
We present high-resolution HST WFPC2 images of compact nebulositysurrounding the cool M-type hypergiants NML Cyg, VX Sgr, and S Per. Thepowerful OH/IR source NML Cyg exhibits a peculiar bean-shaped asymmetricnebula that is coincident with the distribution of its H2Ovapor masers. We show that NML Cyg's circumstellar envelope is likelyshaped by photodissociation from the powerful, nearby association CygOB2 inside the Cygnus X superbubble. The OH/IR sources VX Sgr and S Perhave marginally resolved envelopes. S Per's circumstellar nebula appearselongated in a northeast/southwest orientation similar to that for itsOH and H2O masers, while VX Sgr is embedded in a spheroidalenvelope. We find no evidence for circumstellar nebulosity around theintermediate-type hypergiants ρ Cas, HR 8752, and HR 5171a, noraround the normal M-type supergiant μ Cep. We conclude that there isno evidence for high mass loss events prior to 500-1000 years ago forthese four stars.
| Light variations of massive stars (α Cyg variables). XIX. The late-type supergiants R 59, HDE 268822, HDE 269355, HDE 269612 and HDE 270025 in the LMC
We present and discuss VBLUW photometry (Walraven system) of fivesupergiants in the LMC. For one well-known variable, the hypergiant R 59= HDE 268757 (G7 Ia+) also Hipparcos photometry and numerousvisual observations are available. The second variable is HDE 269612 (F0Ia), and a third one is HDE 268822 (F6 Ia). Two F6 Ia supergiants turnedout to be constant: HDE 269355 and HDE 270025.Partly based on observations obtained at the European SouthernObservatory at La Silla, Chile.
| A gallery of cool hypergiants --- imaging their circumstellar environments
High resolution imaging with HST-WFPC2 of seven of the most luminousevolved cool stars shows that the three OH/IR stars NML Cyg, VX Sgr andS Per have circumstellar nebulae while no ejecta was detected for theM-type supergiant μ Cep and the yellow hypergiants ρ Cas, HR 8752and HR 5171a.
| New periodic variables from the Hipparcos epoch photometry
Two selection statistics are used to extract new candidate periodicvariables from the epoch photometry of the Hipparcos catalogue. Theprimary selection criterion is a signal-to-noise ratio. The dependenceof this statistic on the number of observations is calibrated usingabout 30000 randomly permuted Hipparcos data sets. A significance levelof 0.1 per cent is used to extract a first batch of candidate variables.The second criterion requires that the optimal frequency be unaffectedif the data are de-trended by low-order polynomials. We find 2675 newcandidate periodic variables, of which the majority (2082) are from theHipparcos`unsolved' variables. Potential problems with theinterpretation of the data (e.g. aliasing) are discussed.
| Yellow Hypergiants as Dynamically Unstable Post-Red Supergiant Stars
According to recent theoretical studies, the majority of single starsmore massive than 30 Msolar successfully evolve into redsupergiants but then lose most of their hydrogen envelopes andmetamorphose into hot blue remnants. While they are cool, they becomedynamically unstable as a result of high radiation pressure and partialionization of the gases in their outer layers. It is shown here thatthese unstable red supergiant models repeatedly shrink and reexpand on athermal timescale when perturbed by heavy bursts of mass loss.Consequently, they fill up the domain of yellow hypergiants on theHertzsprung-Russell diagram and display very fast rates of evolutionthere, as observed.
| 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.
| Observational investigation of mass loss of M supergiants
We present the analysis of infrared photometry and millimeterspectroscopy of a sample of 74 late-type supergiants. These observationsare particularly suitable to study the mass loss and the circumstellarenvelopes of evolved massive stars. In particular, we quantify thecircumstellar infrared excess, the relation of mass loss with stellarproperties, using the K-[12] colour index as mass-loss indicator. We donot find any clear correlation between mass loss rate and luminosity. Wealso show that the K-band magnitude is a simple luminosity indicator,because of the relative constancy of the K-band bolometric correction.Based on observations collected at the European Southern Observatory, LaSilla, Chile within program ESO 54.E-0914, and on observations collectedwith the IRAM 30m telescope. Tables A1 to A3 are only available inelectronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or via http://cdsweb.u-strasbg.fr/Abstract.html
| G309.2-00.6 and jets in supernova remnants
We present Australia Telescope Compact Array observations of thesupernova remnant (SNR) G309.2-00.6. In a 1.3-GHz continuum image theremnant appears as a near-circular shell, but with two brightened anddistorted arcs of emission on opposite sides. Hi absorption against theSNR yields a distance in the range 5.4 to 14.1 kpc, corresponding to anage (1-20)x10^3 yr. On the basis of the morphology of the SNR we arguethat it is a younger analogue of the W 50/SS 433 system, and that itsunusual appearance is a result of opposed jets or outflows from acentral source. A jet-like feature and breaks in the shell can both beseen along the axis of proposed outflow, providing further support forthis interpretation; the central source itself is not detected. The SNRmay be interacting with the adjacent Hii region RCW 80 through anextension of the proposed outflow beyond its shell. This would put theSNR at the lower limit of its distance range and would imply an age<~4000 yr. We consider other SNRs similar to G309.2-00.6, and proposeremnants whose shells are affected by jets as one of several classes ofSNR from which the presence of a central source can be inferred.
| 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.
| Classification and Identification of IRAS Sources with Low-Resolution Spectra
IRAS low-resolution spectra were extracted for 11,224 IRAS sources.These spectra were classified into astrophysical classes, based on thepresence of emission and absorption features and on the shape of thecontinuum. Counterparts of these IRAS sources in existing optical andinfrared catalogs are identified, and their optical spectral types arelisted if they are known. The correlations between thephotospheric/optical and circumstellar/infrared classification arediscussed.
| A catalogue of [Fe/H] determinations: 1996 edition
A fifth Edition of the Catalogue of [Fe/H] determinations is presentedherewith. It contains 5946 determinations for 3247 stars, including 751stars in 84 associations, clusters or galaxies. The literature iscomplete up to December 1995. The 700 bibliographical referencescorrespond to [Fe/H] determinations obtained from high resolutionspectroscopic observations and detailed analyses, most of them carriedout with the help of model-atmospheres. The Catalogue is made up ofthree formatted files: File 1: field stars, File 2: stars in galacticassociations and clusters, and stars in SMC, LMC, M33, File 3: numberedlist of bibliographical references The three files are only available inelectronic form at the Centre de Donnees Stellaires in Strasbourg, viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5), or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| Colour excesses of F-G supergiants and Cepheids from Geneva photometry.
A reddening scale for F-G supergiants and Cepheids is presented.Supergiants with low reddenings or in clusters form the basis of thecalibration. In this sense, it is entirely empirical. The data have beenobtained in the Geneva photometric system. Comparisons with otherreddening scales show no disagreement. The only problem is with Fernie'sscale for Cepheids (1990), where a systematic trend exists. Its originis not clear. It is suggested to extend the number of supergiants withindependently obtained colour excesses in order to test the existence ofa possible luminosity dependence of the calibration. A period-colourrelation for Cepheids is deduced, on the basis of the present reddeningcorrections. It gives strong support for V473 Lyr being a secondovertone pulsator.
| The shock strength in super- and hypergiant atmospheres derived from microturbulence data
This study is based on evidence that small-scale motions, observed asmicroturbulent line broadening, in the atmospheres of super- andhypergiants can possibly be interpreted as fields of shock waves movingoutwards through the atmosphere. We derive values for the average shockstrength M_1_ for a number of well-observed stars. We found that thevalue of M_1_ increases strongly with stellar luminosity, suggestingthat shock wave pressure is relevant for the stability limit of mostevolved stars (Humphreys-Davidson Limit) and for stellar evolutioncalculations. Our results however apply only to those stars in whichphotospheric shock wave patterns exist. For some stars we studied thedependence of M_1'_ on the Rosseland optical depths τ_R_ and wefound that M_1'_(τ_R_) does not change very much. We compared theenergy fluxes associated with the shocks with acoustic energy fluxesgiven by Bohn and with radiative energy losses from the chromospheres(only possible for cool giants and supergiants). We found that thesefluxes are compatible for cool stars. The energy fluxes of Bohn cannotbe used for super- and hypergiants with T_eff_ > 6500 K as all thesestars are lacking convection zones.
| Massive stars: setting the stage
Not Available
| The 1.5-1.7 micrometer spectrum of cool stars: Line identifications, indices for spectral classification and the stellar content of the Seyfert galaxy NGC 1068
The first aim of this paper is to determine which lines or narrowmolecular bands in the H-band spectra of cool stars could be of interestfor the classification of K-M stars. For this purpose we present highquality, medium resolution (R approximates 1500) spectra of field stars(mostly K-M giants and supergiants) and compare them with detailedsynthetic spectra computed on the basis of existing model atmospheresfor red giants. The agreement between theoretical and observed spectrais good and virtually all the observed features can be accounted for bylines of (12)CO, (13)CO, OH, Mg, Al, Si, Ca and Fe. We analyze in detailthe relative contribution of these and other species and conclude thatthe feature at 1.62 micrometers which is weak in early K but very strongin late M stars, is mainly due to the CO(6-3) band-head, while that at1.59 micrometers, which is prominent in all stars later than G, isprimarily attributable to silicon up to early M types, while in late Mstars this feature is strongly contaminated by OH lines. We choose thesetwo features as 'spectral classificators' and measure their equivalentwidths in more than 40 G, K, M giants and supergiants. From these datait is found that CO 1.62 in giants increases rapidly and with arelatively small scatter going to later spectral types. Supergiants havedeeper CO(6-3) and display a larger scatter. The (1.62)/(2.29) ratiosteadily increases going to cooler stars but does not vary significantlywith luminosity class. A very useful ratio is (1.62)/(1.59) whichincreases by a large factor from early K to late M stars and couldtherefore be a powerful tool to identify and estimate the averagespectral type of cool stars in complex objects like active galaxynuclei. To demonstrate such a possibility we also present long slitspectra of the Seyfert galaxy NGC 1068 where the equivalent widths ofall stellar features are found to decrease in the central 4 arcsecaround the nucleus but the (1.62)/(1.59) ratio, and hence the averagestellar temperature, does not change significantly. The estimatedaverage spectral type is late-K which is compatible with either an oldand very metallic bulge population or a younger one associated with arecent starburst. These data also show that the non-stellar continuumaccounts for approximately 30% and approximately equal to or greaterthan 80% of the flux at 1.62 and 2.3 micrometers respectively in thecentral 4.4 arcsec. The features around 1.6 micrometers are thus muchless diluted than CO(2, 0) and hence offer advantages for studies of thestellar content in such objects. The non-stellar nuclear emission isvery red and most probably associated with a hot (T approximately equalor greater than 800 K) dust component.
| New features of IRSPEC.
Not Available
| Recent Variability in the S Doradus - Type Star AG Carinae [errata: 1993MNSSA..52....2C]
Not Available
| A catalogue of Fe/H determinations - 1991 edition
A revised version of the catalog of Fe/H determinations published by G.Cayrel et al. (1985) is presented. The catalog contains 3252 Fe/Hdeterminations for 1676 stars. The literature is complete up to December1990. The catalog includes only Fe/H determinations obtained from highresolution spectroscopic observations based on detailed spectroscopicanalyses, most of them carried out with model atmospheres. The catalogcontains a good number of Fe/H determinations for stars from open andglobular clusters and for some supergiants in the Magellanic Clouds.
| Light variations of massive stars (Alpha Cygni variables). XII - The photometric history of the G8Ia(+) hypergiant V766 CEN (= HR 5171A) during the years 1953-1991 and its interpretation
VBLUW photometry of the G8 Ia(+) hypergiant V 766 Cen (= HR 5171A) madein the interval 1980-1991 is presented and discussed, together with UBVphotometry taken from the literature, which goes back in time until1953. The star shows a very strong reddening of about 0.65 m between1953 and 1981, a brightness oscillation with a quasi-period of 494 d, inthe interval 1973-1991, and a drop in brightness between 1973 and 1977.Whether the strong reddening is caused by an increasing circumstellardust density or by evolution to the red in the HR diagram is discussed.The radial velocity curve (1979-1981) appears to be in phase with thelight curve. This peculiar behavior is shared with the variable F-typesupergiant BL Tel(F). The radius variation amounts to about 8 percent.
| 1612 MHz OH survey of IRAS point sources. I - Observations made at Dwingeloo, Effelsberg and Parkes
The data from a large sky survey are presented including a northernpilot study and a detailed southern study in which detections are biasedtoward the most evolved sources and distant sources. Both areinvestigated at the 1612-MHz transition of OH to take advantage of thestrongest line for AGB stars with optically thick dust shells. The IRASsources are chosen by considering their IR colors related to fluxes at12, 25, and 60 microns. Observations are reported for 2703 IRAS pointsources at the 1612-MHz transition, and 738 OH/IR stars are detected.The survey identifies 597 of the sources as previously unidentified, and95 percent of the OH profiles observed have twin-peak masercharacteristics which are related to emission from expandingcircumstellar shells. The other 5 percent of the sources are concludedto be transition objects between OH/IR stars and planetary nebulae.
| The atmospheric motion field in super- and hypergiants
The paper presents an investigation of the motion fields in the outeratmospheres of a few well-studied supergiants and hypergiants for whichgood observational information is available from spectral studies. Thefollowing topics are examined: the existence of pressure and gravitywaves; the dissipation of gravity-wave energy by radiative damping; thereduction of the radiative damping of gravity waves by layer curvature;the large-scale motion of two moderate supergiants (Alpha Per and 41Cyg); pressure waves and their transformation into shock waves; weakshock dissipation of wave energy; the spectrum of shock-wave energy; andthe gravity wave and shock wave components of motion in supergiants andhypergiants.
| A survey of Balmer-line profiles and IRAS fluxes in forty yellow supergiants
Results of a survey of yellow supergiants (predominantly F and G) thatwas undertaken to search for distorted Balmer lines are presented. Acomparison between the photometrically variable and nonvariablesupergiants is made with respect to the distorted and undistortedBalmer-line profiles. To complement the analysis, IRAS data are used todetect excess infrared emission, to infer circumstellar dust, and tosuggest a possible prior M supergiant evolutionary phase. Plans forfuture work are briefly discussed.
| Walraven photometry of nearby southern OB associations
Homogeneous Walraven (VBLUW) photometry is presented for 5260 stars inthe regions of five nearby southern OB associations: Scorpio Centaurus(Sco OB2), Orion OB1, Canis Major OB1, Monoceros OB1, and Scutum OB2.Derived V and (B - V) in the Johnson system are included.
| Colour excesses and absolute magnitudes for non-Cepheid F-G supergiants from uvbybeta photometry
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1990A&A...239..205A&db_key=AST
| The circumstellar silicate dust as seen by IRAS. I - Opacity and temperature
The study deals with objects included in the IRAS low-resolution spectra(LRS) catalog and exhibiting the 10-micron feature. A radiative transfermodel is outlined, and the parameters involved are defined along withthe initial choice of the proposed investigating procedure. Thenumerical models computed with the final choice of the best values ofopacities and grain condensation radii are presented. A principle of thetailoring procedure leading to the final choice is covered with emphasisplaced on the 10-micron feature intensity as a function of the (25/12)color and red supergiants. It is noted that the models agree withtwo-color diagrams and with the observed spectral shapes, and that thedust properties are uniform except for the sparsely populated LRS class6n.
| SiO and CO emission from carbon stars with silicate features and southern IRAS sources
Forty-one southern IRAS sources have been observed in the CO J = 1-0 andSiO J = 2-1, v = 1 lines with the 15-m Swedish European SubmillimeterTelescope, and CO or SiO emission has been detected in 20 sources. Among41 sources, southern carbon stars with silicate features were observedbut no CO or SiO emission was detected except in IRAS 13442-6109 forwhich the identification with the carbon star C2123 is questionable.Nondetection of SiO masers in carbon stars with silicate features favorsthe transition-phase hypothesis for the origin of these stars. Some SiOemission has been detected in stars with IRAS LRS class 4n, confirmingthat silicate absorption can mimic SiC emission in the coldest 4nsources. The detection rate and chemistry of circumstellar envelopes arediscussed.
| Water maser emission from southern IRAS sources
A search for water maser emission in 112 bright, southern IRAS sourceswas conducted. Water maser emission was detected in 27 cases. Thesesources include M-type stars and previously unidentified IRAS objects.The detection rate is discussed with reference to IRAS low-resolutionspectral (LRS) class and the (60-25) micron color index. Water maseremission was detected in three sources which have LRS class 4n,indicating the presence of 11-micron SiC emission. The O/C-richness ofthe circumstellar envelopes of these sources is discussed, and it isconcluded that these sources are probably M-type stars whose LRS classis misassigned.
| A list of MK standard stars
Not Available
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הכנס מאמר חדש
לינקים קשורים
הכנס לינק חדש
משמש של הקבוצה הבאה
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תצפית ומידע אסטרומטרי
| קבוצת-כוכבים: | קנטאור |
| התרוממות ימנית: | 13h47m10.70s |
| סירוב: | -62°35'24.0" |
| גודל גלוי: | 6.51 |
| מרחק: | 3571.429 פארסק |
| תנועה נכונה: | 4.3 |
| תנועה נכונה: | -14.4 |
| B-T magnitude: | 10.265 |
| V-T magnitude: | 7.187 |
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INHABITED SKY
