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The Reddening of Red Supergiants: When Smoke Gets in Your Eyes
Deriving the physical properties of red supergiants (RSGs) depends onaccurate corrections for reddening by dust. We use our recent modelingof the optical spectra of RSGs to address this topic. First, we findthat previous broadband studies have underestimated the correction forextinction in the visible, and hence the luminosities (if derived fromV); the shift in the effective wavelengths of the standard B and Vbandpasses necessitates using an effective value of the ratioR'V=4.2 to correct broadband photometry of RSGs ifRV=3.1 for early-type stars viewed through the same dust,where we have assumed the standard reddening law of Cardelli andcoauthors. Use of the Fitzpatrick reddening law would lead toR'V=3.8, as well as slightly lower values ofextinction derived from spectrophotometry, but results in slightlypoorer fits. Second, we find that a significant fraction of RSGs inGalactic OB associations and clusters show up to several magnitudes ofexcess visual extinction compared to OB stars in the same regions; weargue that this is likely due to circumstellar dust around the RSGs. Wealso show that the RSG dust production rate (as indicated by the 12μm excess) is well correlated with bolometric luminosity, contrary towhat has been found by earlier studies. The stars with the highestamount of extra visual extinction also show significant near-UV (NUV)excesses compared to the stellar models reddened by the standardreddening law. This NUV excess is likely due to scattering of the star'slight by the dust and/or a larger average grain size than that typicalof grains found in the diffuse interstellar medium. Similar excesseshave been attributed to circumstellar dust around R Coronae Borealisstars. Finally, we estimate that the RSGs contribute dust grains at therate of 3×10-8Msolar yr-1kpc-2 in the solar neighborhood, comparable to what weestimate for late-type WCs, 1×10-7Msolaryr-1 kpc-2. In the solar neighborhood thisrepresents only a few percent of the dust production (which is dominatedby low-mass AGBs), but we note that in low-metallicity starbursts, dustproduction by RSGs would likely dominate over other sources.
| The Effective Temperature Scale of Galactic Red Supergiants: Cool, but Not As Cool As We Thought
We use moderate-resolution optical spectrophotometry and the new MARCSstellar atmosphere models to determine the effective temperatures of 74Galactic red supergiants (RSGs). The stars are mostly members of OBassociations or clusters with known distances, allowing a criticalcomparison with modern stellar evolutionary tracks. We find we canachieve excellent matches between the observations and the reddenedmodel fluxes and molecular transitions, although the atomic lines Ca Iλ4226 and Ca II H and K are found to be unrealistically strong inthe models. Our new effective temperature scale is significantly warmerthan those in the literature, with the differences amounting to 400 Kfor the latest type M supergiants (i.e., M5 I). We show that the newlyderived temperatures and bolometric corrections give much betteragreement with stellar evolutionary tracks. This agreement provides acompletely independent verification of our new temperature scale. Thecombination of effective temperature and bolometric luminosities allowsus to calculate stellar radii; the coolest and most luminous stars (KWSgr, Case 75, KY Cyg, HD 206936=μ Cep) have radii of roughly 1500Rsolar (7 AU), in excellent accordance with the largeststellar radii predicted from current evolutionary theory, althoughsmaller than that found by others for the binary VV Cep and for thepeculiar star VY CMa. We find that similar results are obtained for theeffective temperatures and bolometric luminosities using only thedereddened V-K colors, providing a powerful demonstration of theself-consistency of the MARCS models.
| The Evolution of Massive Stars. I. Red Supergiants in the Magellanic Clouds
We investigate the red supergiant (RSG) content of the SMC and LMC usingmultiobject spectroscopy on a sample of red stars previously identifiedby BVR CCD photometry. We obtained high-accuracy (<1 kms-1) radial velocities for 118 red stars seen toward the SMCand 167 red stars seen toward the LMC, confirming most of these (89% and95%, respectively) as red supergiants. Spectral types were alsodetermined for most of these RSGs. We find that the distribution ofspectral types is skewed toward earlier type at lower metallicities: theaverage (median) spectral type is K5-K7 I in the SMC, M1 I in the LMC,and M2 I in the Milky Way. Our examination of the Kurucz ATLAS9 modelatmospheres suggests that the effect that metallicity has on theappearance on the TiO lines is probably sufficient to account for thiseffect, and we argue that RSGs in the Magellanic Clouds are 100 K (LMC)and 300 K (SMC) cooler than Galactic stars of the same spectral types.The colors of the Kurucz models are not consistent with thisinterpretation for the SMC, although other models (e.g., Bessell et al.)show good agreement. A finer grid of higher resolution synthetic spectraappropriate to cool supergiants is needed to better determine theeffective temperature scale. We compare the distribution of RSGs in theH-R diagram to that of various stellar evolutionary models; we find thatnone of the models produce RSGs as cool and luminous as what is actuallyobserved. This result is much larger than any uncertainty in theeffective temperature scale. We note that, were we to simply adopt theuncorrected Galactic effective scale for RSGs and apply this to oursample, then the SMC's RSGs would be underluminous compared with theLMC's, contrary to what we expect from stellar evolution considerations.In all of our H-R diagrams, however, there is an elegant sequence ofdecreasing effective temperatures with increasing luminosities;explaining this will be an important test of future stellar evolutionarymodels. Finally, we compute the blue-to-red supergiant ratio in the SMCand LMC, finding that the values are indistinguishable (~15) for the twoClouds. We emphasize that ``observed'' B/R values must be carefullydetermined if a comparison with that predicted by stellar models is tobe meaningful. The nonrotation Geneva models overestimate the number ofblue to red supergiants for the SMC, but underestimate it for the LMC;however, given the inability to produce high-luminosity RSGs in themodels that match what is observed in the H-R diagram, such adisagreement is not surprising.
| 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.
| Dynamical masses of young star clusters in NGC 4038/4039
In order to estimate the masses of the compact, young star clusters inthe merging galaxy pair, NGC 4038/4039 (``the Antennae''), we haveobtained medium and high resolution spectroscopy using ISAAC on VLT-UT1and UVES on VLT-UT2 of five such clusters. The velocity dispersions wereestimated using the stellar absorption features of CO at 2.29 mu m andmetal absorption lines at around 8500 Å, including lines of theCalcium Triplet. The size scales and light profiles were measured fromHST images. From these data and assuming Virial equilibrium, weestimated the masses of five clusters. The resulting masses range from6.5 x 105 to 4.7 x 106 Msun. Thesemasses are large, a factor of a few to more than 10 larger than thetypical mass of a globular cluster in the Milky Way. The mass-to-lightratios for these clusters in the V- and K-bands in comparison withstellar synthesis models suggest that to first order the IMF slopes areapproximately consistent with Salpeter for a mass range of 0.1 to 100Msun. However, the clusters show a significant range ofpossible IMF slopes or lower mass cut-offs and that these variations maycorrelate with the interstellar environment of the cluster. Comparisonwith the results of Fokker-Planck simulations of compact clusters withproperties similar to the clusters studied here suggest that they arelikely to be long-lived and may lose a substantial fraction of theirtotal mass. This mass loss would make the star clusters obtain masseswhich are comparable to the typical mass of a globular cluster. Based onobservations collected at the European Southern Observatory, Chile.
| Polarimetry of 167 Cool Variable Stars: Data
Multicolor photoelectric polarimetry is presented for 167 stars, most ofwhich are variable stars. The observations constitute a data set thatfor some stars covers a time span of 35 yr. Complex variations are foundover time and wavelength and in both the amount of polarization and itsposition angle, providing constraints for understanding the polarizingenvironments in and around these cool stars.
| Studies of Yellow Semiregular(SRd) Variables
We have used the Hipparcos database of epoch photometry to study thevariability of several yellow semiregular(SRd) variables. For some ofthe stars (V847 Cas, RW Cep, BM Sco, CE Vir), the results wereinconclusive. For SX Lac and TY Vir, the periods found were consistentwith the General Catalogue of Variable Stars (190 and 50 daysrespectively). For UU Her, the known periods of about 45 and 72 dayswere recovered. For WY And, RU Cep, and SX Her, reliable periods werefound; we used archival data to construct (O-C) diagrams to study boththe random and systematic period changes in these three stars.
| On the Variability of Early K Stars
We investigate the Hipparcos Satellite photometry of K0-K4 stars tostudy their pattern of activity. Most are not particularly variable. Afew stars for which further study is desirable are identified.
| Absolute proper motions of open clusters. I. Observational data
Mean proper motions and parallaxes of 205 open clusters were determinedfrom their member stars found in the Hipparcos Catalogue. 360 clusterswere searched for possible members, excluding nearby clusters withdistances D < 200 pc. Members were selected using ground basedinformation (photometry, radial velocity, proper motion, distance fromthe cluster centre) and information provided by Hipparcos (propermotion, parallax). Altogether 630 certain and 100 possible members werefound. A comparison of the Hipparcos parallaxes with photometricdistances of open clusters shows good agreement. The Hipparcos dataconfirm or reject the membership of several Cepheids in the studiedclusters. Tables 1 and 2 are only available in electronic form at theCDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| Magnitudes absolues des étoiles standards MK des types G à M à partir des parallaxes Hipparcos. The absolute magnitudes of the G to M type MK standards from the Hipparcos parallaxes
We analyse a sample of about 500 MK standards of cool spectral types (Gto M) for to compare the visual absolute magnitudes obtained from bothHipparcos data and Schmidt-Kaler calibrations. Our purpose is tovalidate our spectroscopic work \cite[(Ginestet et al. 1997, 1999)]{G97}on stars with composite spectra with the help of Hipparcos data.Contrary to what is claimed in other papers, the absolute magnitudedomain devoted to the giant stars does not overlap the domain of dwarfs.We find that the discrepancies between absolute magnitudes fromHipparcos data and absolute magnitudes deduced from Schmidt-Kalercalibrations increase with the relative error sigma (pi )/pi on theparallaxes. So, for sigma (pi )/pi <= 0.05 only 3% of the starspresent a discrepancy of one luminosity class, while this percentagereaches 54% for 0.25 < sigma (pi )/pi <= 0.50. Curiously, theluminosity of the giants seems to increase with the distance of thestars, whereas the supergiants of the sample appear underluminous atleast for d < 600 pc! We point out a list of 14 MK standards whoseluminosity classes may be erroneous and need a new spectralclassification, in the near infrared. The case of composite-spectrumbinaries is also discussed. Most of these are too distant for accurateparallaxes even with Hipparcos: only sixteen stars have sigma (pi )/pi<= 0.10; for these, we give new spectral classifications in agreementwith both our classifications in the near infrared of the coolcomponents and Hipparcos data. Finally, for stars having high-precisionparallaxes (sigma (pi )/pi <= 5%) there is no serious problem forSchmidt-Kaler calibrations whith respect to Hipparcos data. The datacorresponding to parallaxes of lower precisions should be used withcaution and only for statistical analyses.
| 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.
| Mass-losing M supergiants in the solar neighborhood
A list of the 21 mass-losing red supergiants (20 M type, one G type; Lgreater than 100,000 solar luminosities) within 2.5 kpc of the sun iscompiled. These supergiants are highly evolved descendants ofmain-sequence stars with initial masses larger than 20 solar masses. Thesurface density is between about 1 and 2/sq kpc. As found previously,these stars are much less concentrated toward the Galactic center thanW-R stars, which are also highly evolved massive stars. Although withconsiderable uncertainty, it is estimated that the mass return by the Msupergiants is somewhere between 0.00001 and 0.00003 solar mass/sq kpcyr. In the hemisphere facing the Galactic center there is much less massloss from M supergiants than from W-R stars, but, in the anticenterdirection, the M supergiants return more mass than do the W-R stars. Theduration of the M supergiant phase appears to be between 200,000 and400,000 yr. During this phase, a star of initially at least 20 solarmasses returns perhaps 3-10 solar masses into the interstellar medium.
| Mass loss from stars : the universal formula for mass loss rate
Not Available
| A list of MK standard stars
Not Available
| The Perkins catalog of revised MK types for the cooler stars
A catalog is presented listing the spectral types of the G, K, M, and Sstars that have been classified at the Perkins Observatory in therevised MK system. Extensive comparisons have been made to ensureconsistency between the MK spectral types of stars in the Northern andSouthern Hemispheres. Different classification spectrograms have beengradually improved in spite of some inherent limitations. In thecatalog, the full subclasses used are the following: G0, G5, G8, K0, K1,K2, K3, K4, K5, M0, M1, M2, M3, M4, M5, M6, M7, and M8. Theirregularities are the price paid for keeping the general scheme of theoriginal Henry Draper classification.
| New LRS spectra for 356 bright IRAS sources
The low-resolution spectra of all IRAS point sources with F(nu) (12microns) greater than 40 Jy that were not included in the Atlas ofLow-Resolution Spectra are presented. These have been classified intoeight groups based upon the spectral morphology. Silicate emissionspectra and red-continuum spectra associated with H II region sourcesform about 60 percent of this sample. All types of spectra in the LRSAtlas are represented in the sample except for emission-line sources.The sample is used to test a recent classification scheme for IRASsources based on broadband colors. The spectra is used to test a recentclassification scheme for IRAS sources based on broadband colors. Thespectra are consistent with the classifications from the colors in mostcases.
| Infrared circumstellar shells - Origins, and clues to the evolution of massive stars
The infrared fluxes, spatial and spectral characteristics for a sampleof 111 supergiant stars of spectral types F0 through M5 are tabulated,and correlations examined with respect to the nature of theircircumstellar envelopes. One-fourth of these objects were spatialyresolved by IRAS at 60 microns and possess extended circumstellar shellmaterial, with implied expansion ages of about 10 to the 5th yr.Inferences about the production of dust, mass loss, and the relation ofthese characteristics of the evolution of massive stars, are discussed.
| 1988 Revised MK Spectral Standards for Stars GO and Later
Not Available
| A unified formula for mass-loss rate of O to M stars
A formula for stellar mass-loss rate has been derived using conservationequations of mass and momentum for coronal and continuumradiation-driven wind. The derived mass-loss rate formula has been foundto be consistent with the observed mass-loss rates for stars from O to Mspectral type. Two constant parameters appearing in the mass-loss rateformula have been found to have values for special group of stars suchas Be-stars, which are different from the majority of stars.
| Far-infrared circumstellar 'debris' shell of red supergiant stars
An examination of IRAS data of red supergiant stars in the field and ingalactic OB star associations indicates the presence of substantialamounts of 60-micron emitting material extending several arcminutesaround many such sources. The characteristics of these large shells arediscussed in terms of remnants of ongoing mass loss, and a simple modeldeveloped for the case of Alpha Orionis, in particular.
| Mass-losing red giants in open clusters
Mass-losing stars in open clusters with main-sequence turn-offs atintermediate mass have been searched for by using the IRAS data base.The absence of many strong 60 micron sources in open clusters impliesthat intermediate-mass stars lose much of their mass during an intensewind phase of rather short duration. For stars of about seven solarmasses, this phase, if it exists at all, lasts for not much more than100,000 yr. For stars of about four solar masses, the intense wind phaseappears to last considerably less than 10 million yr; it may well lastfor less than a million yr.
| Valinhos 2.2 micron survey of the southern galactic plane. II - Near-IR photometry, IRAS identifications and nature of the sources
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1987A&AS...71...39E&db_key=AST
| Ultraviolet and infrared observations of stars with 'quenched' chromospheres and the nature of mass loss
Previous observational evidence implies that the presence of Ca IIemission, a chromospheric indicator, is correlated with the gas/dustratio in the envelopes of red giant and supergiant stars. An attempt ismade to determine whether this correlation can be generalized to allchromospheric activity indicators and the gas/dust ratio. Newultraviolet observations address the strength of UV emission featuresand the fraction of the total chromospheric flux emitted in variouslines. Evidence is found that chromospheres are not completely quenchedin the presence of dust, but that significant alteration of relativeradiative loss patterns may occur. These observations are interpreted interms of an instability that converts warm, chromospheric gas intonear-surface dust grains and cool gas capable of supporting molecularmasing. This supports the dust-driven mass loss scenario for red giantwinds.
| Spectral types and luminosities of supergiants in open clusters
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1985PASP...97..297K&db_key=AST
| 1985 revised MK spectral standards : stars GO and later
Not Available
| The radial velocities of 116 southern red stars
Using a photoelectric speedometer based on the radial velocityspectrometer of Griffin (1967), radial velocities of 116 southern redstars, many of them semi-regular variables, were measured. Theinstrument was placed at the Newtonian focus of the 130-inch (4.2-m)camera of the Coude spectrograph of the Mount Stromlo 74-inch (1.88-m)reflector. The stellar spectrum was focused on a mask 50 mm in length,designed to match the spectrum of the M-giant Beta Pegasi. The inversedispersion was 2.5 angstroms per millimeter in the second order. Thewavelength range used (5338-5449 angstroms) was chosen because it isbetween two TiO bands and so suffers less blanketing in M stars. Resultsare tabulated and compared with standard values.
| Revised MK Spectral Standard Stars Later than G0
Not Available
| The circumstellar envelopes and chromospheres of cool giants and supergiants
A survey of the circumstellar (CS) envelopes of cool giants andsupergiants shows no correlation between the quantities of CS gas anddust, indicating that radiation pressure on dust grains is not theprincipal mechanism of mass loss. The observed quantity of dust is notsufficient to drive the observed mass loss. Stars with high dust-to-gasratios were also seen to show a high proportion of neutral to ionizedgas and a lack of Ca II H and K emission, and in many cases, Balmeremission, and/or molecular masing. It is argued that shock waves may belinked to dust formation. Mass loss rates were derived for the programstars and found to correlate with K4 absorption width, indicating thatthe mass loss mechanism may be coupled to turbulence.
| Erratum - Some Errata in the Fifth General Catalogue of MK Spectral Classifications
Not Available
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Pozíciós és asztrometriai adatok
| Csillagkép: | Skorpió |
| Rektaszcenzió: | 17h40m58.55s |
| Deklináció: | -32°12'52.1" |
| Vizuális fényesség: | 6.041 |
| Távolság: | 469.484 parszek |
| RA sajátmozgás: | -1.4 |
| Dec sajátmozgás: | -6.1 |
| B-T magnitude: | 8.273 |
| V-T magnitude: | 6.226 |
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