תוכן
תמונות
הוסף תמונה שלך
DSS Images Other Images
מאמרים קשורים
M33's Variable A: A Hypergiant Star More Than 35 YEARS in Eruption
Variable A in M33 is a member of a rare class of highly luminous,evolved stars near the upper luminosity boundary that show sudden anddramatic shifts in apparent temperature due to the formation ofoptically thick winds in high mass loss episodes. Recent optical andinfrared spectroscopy and imaging reveal that its ``eruption,'' begun in~1950, has ended, having lasted ~45 yr. Our current observations showmajor changes in its wind from a cool, dense envelope to a much warmerstate surrounded by low-density gas with rare emission lines of Ca II,[Ca II], and K I. Its spectral energy distribution has unexpectedlychanged, especially at the long wavelengths, with a significant decreasein its apparent flux, while the star remains optically obscured. Weconclude that much of its radiation is now escaping out of our line ofsight. We attribute this to the changing structure and distribution ofits circumstellar ejecta, corresponding to the altered state of its windas the star recovers from a high mass loss event.
| The 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.
| 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.
| A Revised Calibration of the MV-W(O I 7774) Relationship using Hipparcos Data: Its Application to Cepheids and Evolved Stars
A new calibration of the MV-W(O I 7774) relationship hasbeen calculated using better reddening and distance estimates for asample of 27 calibrator stars of spectral types A to G, based onaccurate parallaxes and proper motions from the Hipparcos and Tychocatalogues. The present calibration predicts absolute magnitude withaccuracies of +/-0.38mag for a sample covering a large range ofMV, from -9.5 to +0.35 mag. The color term included in aprevious paper has been dropped since its inclusion does not lead to anysignificant improvement in the calibration. The variation of the O I7774 feature in the classical cepheid SS Sct has been studied. Wecalculated a phase-dependent correction to random phase OI featurestrengths in Cepheids, such that it predicts mean absolute magnitudesusing the above calibration. After applying such a correction, we couldincrease the list of calibrators to 58 by adding MV and O Itriplet strength data for 31 classical Cepheids. The standard error ofthe calibration using the composite sample was comparable to thatobtained from the primary 27 calibrators, showing that it is possible tocalculate mean Cepheid luminosities from random phase observations ofthe O I 7774 feature. We use our derived calibrations to estimateMV for a set of evolved objects to be able to locate theirpositions in the HR diagram.
| Empirical calibration of the near-infrared CaII triplet - IV. The stellar population synthesis models
We present a new evolutionary stellar population synthesis model, whichpredicts spectral energy distributions for single-age single-metallicitystellar populations (SSPs) at resolution 1.5 Å (FWHM) in thespectral region of the near-infrared CaII triplet feature. The mainingredient of the model is a new extensive empirical stellar spectrallibrary that has been recently presented by Cenarro et al., which iscomposed of more than 600 stars with an unprecedented coverage of thestellar atmospheric parameters.Two main products of interest for stellar population analysis arepresented. The first is a spectral library for SSPs with metallicities-1.7 < [Fe/H] < +0.2, a large range of ages (0.1-18 Gyr) andinitial mass function (IMF) types. They are well suited to modellinggalaxy data, since the SSP spectra, with flux-calibrated responsecurves, can be smoothed to the resolution of the observational data,taking into account the internal velocity dispersion of the galaxy,allowing the user to analyse the observed spectrum in its own system. Wealso produce integrated absorption-line indices (namely CaT*, CaT andPaT) for the same SSPs in the form of equivalent widths.We find the following behaviour for the CaII triplet feature in old-agedSSPs: (i) the strength of the CaT* index does not change much with timefor all metallicities for ages larger than ~3 Gyr; (ii) this index showsa strong dependence on metallicity for values below [M/H]~-0.5 and (iii)for larger metallicities this feature does not show a significantdependence either on age or on the metallicity, being more sensitive tochanges in the slope of power-like IMF shapes.The SSP spectra have been calibrated with measurements for globularclusters by Armandroff & Zinn, which are well reproduced, probingthe validity of using the integrated CaII triplet feature fordetermining the metallicities of these systems. Fitting the models totwo early-type galaxies of different luminosities (NGC 4478 and 4365),we find that the CaII triplet measurements cannot be fitted unless avery dwarf-dominated IMF is imposed, or if the Ca abundance is evenlower than the Fe abundance. More details can be found in work byCenarro et al.
| 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.
| 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.
| High-Resolution Spectroscopy of the Yellow Hypergiant ρ Cassiopeiae from 1993 through the Outburst of 2000-2001
We present an overview of the spectral variability of the peculiarF-type hypergiant ρ Cas, obtained from our long-term monitoringcampaigns over the past 8.5 yr with four spectrographs in the northernhemisphere. Between 2000 June and September an exceptional variabilityphase occurred when the V brightness dimmed by about a full magnitude.The star recovered from this deep minimum by 2001 April. It is the thirdoutburst of ρ Cas on record in the last century. We observe TiOabsorption bands in high-resolution near-IR spectra obtained with theUtrecht Echelle Spectrograph during the summer of 2000. TiO formation inthe outer atmosphere occurred before the deep brightness minimum.Atmospheric models reveal that the effective temperature decreases by atleast 3000 K, and the TiO shell is driven supersonically withM~=5.4×10-2 Msolar yr-1. Strongepisodic mass loss and TiO have also been observed during the outburstsof 1945-1947 and 1985-1986. A detailed analysis of the exceptionaloutburst spectra is provided, by comparing with high-resolution opticalspectra of the early M-type supergiants μ Cep (Ia) and Betelgeuse(Iab). During the outburst, central emission appears above the localcontinuum level in the split Na D lines. A prominent optical emissionline spectrum appears in variability phases of fast wind expansion. Theradial velocity curves of Hα and of photospheric metal absorptionlines signal a very extended and velocity-stratified dynamic atmosphere.The outburst spectra indicate the formation of a low-temperature,optically thick circumstellar gas shell of 3×10-2Msolar during 200 days, caused by dynamic instability of theupper atmosphere of this pulsating massive supergiant near the Eddingtonluminosity limit. We observe that the mass-loss rate during the outburstis of the same order of magnitude as has been proposed for the outburstsof η Carinae. We present calculations that correctly predict theoutburst timescale, whereby the shell ejection is driven by the releaseof hydrogen ionization recombination energy.
| The Evolutionary State of Stars in the NGC 1333S Star Formation Region
We present 2 μm near-IR spectroscopic observations of a sample of 33objects in the NGC 1333S active star-forming cluster centered on thepre-main-sequence star SSV 13. We have previously studied this regionphotometrically in the optical and near-IR, and with the addition ofthese near-IR spectra, we further probe the pre-main-sequence clustermembership and evolutionary state. From the atomic and molecularabsorption features observed, together with the earlier photometry, wederive spectral types, effective temperatures, masses, and ages of thestars and conclude that almost all (90%) the stars observed in thissample are pre-main-sequence objects. This result significantly refinesthe evolutionary information obtained from photometric evidence alone.Comparison with theoretical evolutionary tracks and isochrones suggeststhat our survey has sampled sources with masses in the range 0.2-2Msolar and stellar ages between 7×104 and1×108 yr with a preponderance of sources around3×106 yr. This implies the presence of low- tointermediate-mass T Tauri stars of evolutionary designation Class I toClass III. We conclude that star formation seems to have occurred inlikely several bursts rather than occurring coevally. Star formation insuch a region as NGC 1333S is likely significantly affected by the largenumber of active molecular outflows in the region, which could provide amechanism for cloud turbulence and the onset of subsequent starformation.
| The association of IRAS sources and 12CO emission in the outer Galaxy
We have revisited the question of the association of CO emission withIRAS sources in the outer Galaxy using data from the FCRAO Outer GalaxySurvey (OGS). The availability of a large-scale high-resolution COsurvey allows us to approach the question of IRAS-CO associations from anew direction - namely we examined all of the IRAS sources within theOGS region for associated molecular material. By investigating theassociation of molecular material with random lines of sight in the OGSregion we were able to construct a quantitative means to judge thelikelihood that any given IRAS-CO association is valid and todisentangle multiple emission components along the line of sight. Thepaper presents a list of all of the IRAS-CO associations in the OGSregion. We show that, within the OGS region, there is a significantincrease ( ~ 22%) in the number of probable star forming regions overprevious targeted CO surveys towards IRAS sources. As a demonstration ofthe utility of the IRAS-CO association table we present the results ofthree brief studies on candidate zone-of-avoidance galaxies with IRAScounterparts, far outer Galaxy CO clouds, and very bright CO clouds withno associated IRAS sources. We find that ~ 25% of such candidate ZOAGsare Galactic objects. We have discovered two new far outer Galaxystar-forming regions, and have discovered six bright molecular cloudsthat we believe are ideal targets for the investigation of the earlieststages of sequential star formation around HII regions. Finally, thispaper provides readers with the necessary data to compare othercatalogued data sets with the OGS data.Tables 1, 2 and A1 are 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/399/1083
| Empirical calibration of the near-infrared Ca II triplet - III. Fitting functions
Using a near-infrared stellar library of 706 stars with a wide coverageof atmospheric parameters, we study the behaviour of the CaII tripletstrength in terms of effective temperature, surface gravity andmetallicity. Empirical fitting functions for recently definedline-strength indices, namely CaT*, CaT and PaT, are provided. Thesefunctions can be easily implemented into stellar population models toprovide accurate predictions for integrated CaII strengths. We alsopresent a thorough study of the various error sources and their relationto the residuals of the derived fitting functions. Finally, the derivedfunctional forms and the behaviour of the predicted CaII are comparedwith those of previous works in the field.
| Crossing the Yellow Void: Spatially Resolved Spectroscopy of the Post-Red Supergiant IRC +10420 and Its Circumstellar Ejecta
IRC +10420 is one of the extreme hypergiant stars that define theempirical upper luminosity boundary in the H-R diagram. During theirpost-red supergiant evolution, these massive stars enter a temperaturerange (6000-9000 K) of increased dynamical instability, high mass loss,and increasing opacity, a semiforbidden region that de Jager and hiscollaborators have called the ``yellow void.'' We report HST/STISspatially resolved spectroscopy of IRC +10420 and its reflection nebulawith some surprising results. Long-slit spectroscopy of the reflectedspectrum allows us to effectively view the star from differentdirections. Measurements of the double-peaked Hα emission profileshow a uniform outflow of gas in a nearly spherical distribution,contrary to previous models with an equatorial disk or bipolar outflow.Based on the temperature and mass-loss rate estimates that are usuallyquoted for this object, the wind is optically thick to the continuum atsome and possibly all wavelengths. Consequently, the observed variationsin apparent spectral type and inferred temperature are changes in thewind and do not necessarily mean that the underlying stellar radius andinterior structure are evolving on such a short timescale. To explainthe evidence for simultaneous outflow and infall of material near thestar, we propose a ``rain'' model, in which blobs of gas condense inregions of lowered opacity outside the dense wind. With the apparentwarming of its wind, the recent appearance of strong emission, and adecline in the mass-loss rate, IRC +10420 may be about to shed itsopaque wind, cross the yellow void, and emerge as a hotter star. Basedon observations with the NASA/ESA Hubble Space Telescope, obtained atthe Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS 5-26555.
| P Cygni 2000: 400 Years of Progress
Conference was held in Armagh, Northern Ireland, 2000 August 20-23. TheProceedings will be edited by Mart de Groot and Christiaan Sterken andpublished in the ASP Conference Series.
| Instability regions in the upper HR diagram
The following instability regions for blueward evolving-supergiants areoutlined and compared. (1) Areas in the Hertzsprung-Russell (HR) diagramwhere stars are dynamically unstable. (2) Areas where the effectiveacceleration in the upper part of the photospheres is negative, hencedirected outward. (3) Areas where the sonic points of the stellar winds(where vwind=vsound) are situated inside thephotospheres, at a level deeper than τRoss=0.01. Wecompare the results with the positions of actual stars in the HR diagramand we find evidence that the recent strong contraction of the yellowhypergiant HR 8752 was initiated in a period during which<0, whereupon the star became dynamicallyunstable. The instability and extreme shells around IRC+10420 aresuggested to be related to three factors: <0the sonic point is situated inside the photosphere; and the star isdynamically unstable.
| Near-infrared spectroscopy of the circumnuclear star formation regions in M100: evidence for sequential triggering
We present low-resolution (R~450) K-band spectroscopy for 16 of the 43circumnuclear star-forming knots in M100 identified by Ryder &Knapen. We compare our measurements of equivalent widths for theBrγ emission line and CO 2.29-μm absorption band in each knotwith the predictions of starburst models from the literature, and deriveages and burst parameters for the knots. The majority of these knots arebest explained by the result of short, localized bursts of starformation between 8 and 10Myr ago. By examining both radial andazimuthal trends in the age distribution, we present a case forsequential triggering of star formation, most likely resulting from theaction of a large-scale shock. In an appendix, we draw attention to thefact that the growth in the CO spectroscopic index with decreasingtemperature in supergiant stars is not as regular as is commonlyassumed.
| 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.
| On the Dynamic Stability of Cool Supergiant Atmospheres
We have developed a new formalism to compute the thermodynamiccoefficient Γ1 in the theory of stellar and atmosphericstability. We generalize the classical derivation of the first adiabaticindex, which is based on the assumption of thermal ionization andequilibrium between gas and radiation temperature, toward an expressionthat incorporates photoionization due to radiation with a temperatureTrad different from the local kinetic gas temperature. Ourformalism considers the important non-LTE conditions in the extendedatmospheres of supergiant stars. An application to the Kurucz grid ofcool supergiant atmospheres demonstrates that models withTrad~=Teff between 6500 and 7500 K become mostunstable against dynamic perturbations, according to Ledoux' stabilityintegral <Γ1>. This results fromΓ1 and <Γ1> acquiring very lowvalues, below 4/3, throughout the entire stellar atmosphere, whichcauses very high gas compression ratios around these effectivetemperatures. Based on detailed non-LTE calculations, we discussatmospheric instability of pulsating massive yellow supergiants, such asthe hypergiant ρ Cas (Ia+), which exist in the extensionof the Cepheid instability strip, near the Eddington luminosity limit.
| The comparative accuracy of photographic observations of radio stars observed at the Engelhardt Astronomical Observatory
At the Engelhardt Astronomical Observatory (EAO), we observedphotographic positions of 113 Galactic Radio Sources (GRS) in the systemPPM catalogue (Rizvanov & Dautov 1998). Analysis of their accuracyis made by comparison with the Hipparcos catalogue (Perryman et al.1997) and astrometric catalogue of radio stars in the radio window fromthe article of Walter et al. (1991). Table 2 is only available inelectronic form 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/375/670
| Catalogue of Apparent Diameters and Absolute Radii of Stars (CADARS) - Third edition - Comments and statistics
The Catalogue, available at the Centre de Données Stellaires deStrasbourg, consists of 13 573 records concerning the results obtainedfrom different methods for 7778 stars, reported in the literature. Thefollowing data are listed for each star: identifications, apparentmagnitude, spectral type, apparent diameter in arcsec, absolute radiusin solar units, method of determination, reference, remarks. Commentsand statistics obtained from CADARS are given. The Catalogue isavailable 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/qcar?J/A+A/367/521
| 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
| Sixth Catalogue of Fundamental Stars (FK6). Part III. Additional fundamental stars with direct solutions
The FK6 is a suitable combination of the results of the HIPPARCOSastrometry satellite with ground-based data, measured over a longinterval of time and summarized mainly in the FK5. Part III of the FK6(abbreviated FK6(III)) contains additional fundamental stars with directsolutions. Such direct solutions are appropriate for single stars or forobjects which can be treated like single stars. Part III of the FK6contains in total 3272 stars. Their ground-based data stem from thebright extension of the FK5 (735 stars), from the catalogue of remainingSup stars (RSup, 732 stars), and from the faint extension of the FK5(1805 stars). From the 3272 stars in Part III, we have selected 1928objects as "astrometrically excellent stars", since their instantaneousproper motions and their mean (time-averaged) ones do not differsignificantly. Hence most of the astrometrically excellent stars arewell-behaving "single-star candidates" with good astrometric data. Thesestars are most suited for high-precision astrometry. On the other hand,354 of the stars in Part III are Δμ binaries in the sense ofWielen et al. (1999). Many of them are newly discovered probablebinaries with no other hitherto known indication of binarity. The FK6gives, besides the classical "single-star mode" solutions (SI mode),other solutions which take into account the fact that hidden astrometricbinaries among "apparently single-stars" introduce sizable "cosmicerrors" into the quasi-instantaneously measured HIPPARCOS proper motionsand positions. The FK6 gives, in addition to the SI mode, the "long-termprediction (LTP) mode" and the "short-term prediction (STP) mode". TheseLTP and STP modes are on average the most precise solutions forapparently single stars, depending on the epoch difference with respectto the HIPPARCOS epoch of about 1991. The typical mean error of anFK6(III) proper motion in the single-star mode is 0.59 mas/year. This isa factor of 1.34 better than the typical HIPPARCOS errors for thesestars of 0.79 mas/year. In the long-term prediction mode, in whichcosmic errors are taken into account, the FK6(III) proper motions have atypical mean error of 0.93 mas/year, which is by a factor of about 2better than the corresponding error for the HIPPARCOS values of 1.83mas/year (cosmic errors included).
| Near-Infrared Classification Spectroscopy: J-Band Spectra of Fundamental MK Standards
We present a catalog of J-band spectra for 88 fundamental MK standardstars observed at a resolving power of R~3000. This contribution servesas a companion atlas to the K-band spectra recently published by Wallace& Hinkle and the H-band atlas by Meyer and coworkers. We report datafrom 7400 to 9550 cm-1 (1.05-1.34 μm) for stars ofspectral types O7-M6 and luminosity classes I-V as defined in the MKsystem. In reducing these data, special care has been taken to removetime-variable telluric features of water vapor. We identify atomic andmolecular indexes that are both temperature and luminosity sensitivethat aid in the classification of stellar spectra in the J band. Inaddition to being useful in the classification of late-type stars, the Jband contains several features of interest in the study of early-typestellar photospheres. These data are available electronically foranonymous FTP in addition to being served on the World Wide Web.
| The Star Clusters in the Starburst Irregular Galaxy NGC 1569
We examine star clusters in the irregular starburst galaxy NGC 1569 fromHubble Space Telescope images taken with filters F336W, F555W, andF814W. In addition to the two super-star clusters that are well known,we identify 45 other clusters that are compact but resolved. IntegratedUVI colors of the clusters span a large range, and comparison withcoeval evolutionary models suggests that the ages range from 2-3 Myr to1 Gyr. Most of the clusters have colors consistent with ages of <=30Myr, placing them at the end of the recent burst of star formation. Weexamine the radial surface brightness profiles of four of the clustersand fit King models to three of them. The colors of the clusters areapproximately constant with radius. The four clusters have half-lightradii and core radii that are in the range observed in present-dayglobular clusters in our Galaxy. However, they are somewhat lessconcentrated than the average globular cluster. The two well-knownsuper-star clusters have luminosities (and one has a known mass) thatare comparable to those of typical globular clusters. The other twoclusters and likely numerous others in the sample are similar to a smallglobular cluster and to R136 in the LMC. The conditions that producedthe recent starburst, therefore, have also been those necessary forproducing compact bright star clusters. We examine resolved stars in theouter parts of the super-star clusters. We find that cluster A containsmany bright blue stars. Some of the blue stars are bright enough to beevolved massive stars. There is also a small population of redsupergiants. Components A1 and A2 within cluster A have similar colors,and a two-dimensional color map does not offer evidence that onecomponent is dominated by red supergiants and the other is not. Theapparent contradiction of the presence of red supergiants withWolf-Rayet stars is allowed by the evolution of massive stars or mayinstead be due to an age spread within cluster A. The stars that weresolve around cluster B, on the other hand, contain a small populationof more normal blue massive stars and a large population of redsupergiants. The presence of the red supergiants is consistent with theview that cluster B is in its red supergiant phase. The presence of thered supergiant stars in clusters A and B is also verified innear-infrared spectra, where we find strong stellar CO absorptionfeatures. The various age indicators are consistent with a picture inwhich cluster B is of order 10-20 Myr old, the older stars in cluster Aare >=7 Myr old. The timescale to form the holes seen in Hα andH I is comparable to the age of cluster B. Based on observations withthe NASA/ESA Hubble Space Telescope, obtained at the Space TelescopeScience Institute, which is operated by the Association of Universitiesfor Research in Astronomy, Inc., under NASA contract NAS 5-26555.
| 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.
| The Yellow Hypergiants HR 8752 and rho Cassiopeiae near the Evolutionary Border of Instability
High-resolution near-ultraviolet spectra of the yellow hypergiants HR8752 and rho Cassiopeiae indicate high effective temperatures, placingboth stars near the low-T_eff border of the ``yellow evolutionaryvoid.'' At present, the temperature of HR 8752 is higher than ever. Forthis star we found T_eff=7900+/-200 K, whereas rho Cassiopeiae hasT_eff=7300+/-200 K. Both HR 8752 and rho Cassiopeiae have developedstrong stellar winds with V_&infy;~=120 and V_&infy;~=100 km s^-1,respectively. For HR 8752 we estimate an upper limit for the sphericallysymmetric mass loss of 6.7x10^-6 M_solar yr^-1. Over the past decadestwo yellow hypergiants appear to have approached an evolutionary phase,which has never been observed before. We present the first spectroscopicevidence of the blueward motion of a cool super/hypergiant on the H-Rdiagram.
| A Second Catalog of Orbiting Astronomical Observatory 2 Filter Photometry: Ultraviolet Photometry of 614 Stars
Ultraviolet photometry from the Wisconsin Experiment Package on theOrbiting Astronomical Observatory 2 (OAO 2) is presented for 614 stars.Previously unpublished magnitudes from 12 filter bandpasses withwavelengths ranging from 1330 to 4250 Å have been placed on thewhite dwarf model atmosphere absolute flux scale. The fluxes wereconverted to magnitudes using V=0 for F(V)=3.46x10^-9 ergs cm^-2 s^-1Å^-1, or m_lambda=-2.5logF_lambda-21.15. This second catalogeffectively doubles the amount of OAO 2 photometry available in theliterature and includes many objects too bright to be observed withmodern space observatories.
| Stars with ISM Polarization Observed with HPOL
Polarization data are given for stars whose polarizations are mostlyinterstellar which were observed for various programs with theUniversity of Wisconsin spectropolarimeter (HPOL) during 1989-1994.
| Empirical calibration of the lambda 4000 Å break
Empirical fitting functions, describing the behaviour of the lambda 4000Ä break, D4000, in terms of effective temperature,metallicity and surface gravity, are presented. For this purpose, thebreak has been measured in 392 stars from the Lick/IDS Library. We havefollowed a very detailed error treatment in the reduction and fittingprocedures, allowing for a reliable estimation of the breakuncertainties. This calibration can be easily incorporated into stellarpopulation models to provide accurate predictions of the break amplitudefor, relatively old, composite systems. Table 1 is 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
| Absolute declinations with the photoelectric astrolabe at Calern Observatory (OCA)
A regular observational programme with a photoelectric astrolabe havebeen performed at ``Observatoire du Calern" (Observatoire de laCôte d'Azur, OCA, phi = +43() o44′55.011″; lambda =-0() h27() m42.44() s, Calern, Caussols, France) for the last twentyyears. It has been almost fully automatized between 1984 and 1987. Since1988 the photoelectric astrolabe was used without any modification. Inaddition to determining the daily orientation of the local vertical, theyearly analysis of the residuals permits to derive corrections to theused star catalogue \cite[(Vigouroux et al. 1992)]{vig92}. A globalreduction method was applied for the ASPHO observations. The new form ofthe equations \cite[(Martin & Leister 1997)]{mar97} give us thepossibility of using the entire set of the observing program using datataken at two zenith distances (30() o and 45() o). The program containsabout 41648 stars' transits of 269 different stars taken at``Observatoire du Calern" (OCA). The reduction was based on theHIPPARCOS system. We discuss the possibility of computing absolutedeclinations through stars belonging simultaneously to the 30() o and45() o zenith distances programmes. The absolute declination correctionswere determined for 185 stars with precision of 0.027arcsec and thevalue of the determined equator correction is -0.018arcsec +/-0.005arcsec . The instrumental effects were also determined. The meanepoch is 1995.29. Catalogue only available at CDS in electronic from viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| Catalogue of H-alpha emission stars in the Northern Milky Way
The ``Catalogue of Stars in the Northern Milky Way Having H-alpha inEmission" appears in Abhandlungen aus der Hamburger Sternwarte, Band XIin the year 1997. It contains 4174 stars, range {32degr <= l() II< 214degr , -10degr < b() II < +10degr } having the Hαline in emission. HBH stars and stars of further 99 lists taken from theliterature till the end of 1994 were included in the catalogue. We givethe cross-identification of stars from all lists used. The catalogue isalso available in the Centre de Données, Strasbourg ftp130.79.128.5 or http://cdsweb.u-strasbg.fr and at the HamburgObservatory via internet.
|
הכנס מאמר חדש
לינקים קשורים
הכנס לינק חדש
משמש של הקבוצה הבאה
|
תצפית ומידע אסטרומטרי
| קבוצת-כוכבים: | קאסיופיאה |
| התרוממות ימנית: | 23h00m05.10s |
| סירוב: | +56°56'43.0" |
| גודל גלוי: | 5 |
| מרחק: | 10000000 פארסק |
| תנועה נכונה: | -1.7 |
| תנועה נכונה: | -2.7 |
| B-T magnitude: | 6.372 |
| V-T magnitude: | 5.21 |
קטלוגים וכינוים:
|
INHABITED SKY
