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1985Apjs...57...91E the Astrophysical Journal Supplement Series, 57:91-131, 1985 January © 1985. the American Astronomical Soci The Astrophysical Journal Supplement Series, 57:91-131, 1985 January © 1985. The American Astronomical Society. All rights reserved. Printed in U.S.A. M SUPERGIANTS IN THE MILKY WAY AND THE MAGELLANIC CLOUDS: 1985ApJS...57...91E COLORS, SPECTRAL TYPES, AND LUMINOSITIES J. H. Elias Cerro Tololo Inter-American Observatory, National Optical Astronomy Observatories,1 and California Institute of Technology Jay A. Frogel2 Cerro Tololo Inter-American Observatory, National Optical Astronomy Observatories1 AND Roberta M. Humphreys3 University of Minnesota Received 1984 January 16; accepted 1984 June 1 ABSTRACT New BVRI photometry for 116 red supergiant candidates in the Small Magellanic Cloud (SMC) and 11 luminous red supergiants in the Large Magellanic Cloud (LMC) is presented. New infrared photometry for 65 SMC and 96 LMC red supergiant candidates is also presented. Spectroscopic confirmation for 65 of the SMC candidates is given. These data are combined with existing photometric and spectroscopic MC data obtained by Humphreys. Available data for Milky Way supergiants are reexamined, and all data are put on uniform photometric and spectroscopic systems to enable comparison with the MC data. The main results of this study of red supergiant colors and spectral types are as follows: 1. The median MK spectral type in the SMC is earlier than MO. It is Ml in the LMC and M2-3 in the Milky Way. The width of the spectral type distribution increases along this sequence of three galaxies. The shifts in the median values are attributed to a combination of the well-known shift in the Hayashi track as a function of metallicity and to probable small differences in TiO band strengths at a given effective temperature for the different metallicities in the three galaxies. 2. Measurements at 3.5 and 10 /am are used to investigate mass-loss rates. It is found that the typical 10 j^m excess is highest among Milky Way red supergiants and lowest in the SMC; LMC excesses are intermediate. These excesses appear proportional to the mean metallicities of the three systems, suggesting that mass-loss rates for luminous M supergiants are similar in all three galaxies. 3. A new reddening law for galactic M supergiants is derived. It differs from that of Lee because of the heterogeneous nature of the spectral types he used and because of biases introduced by Lee’s analysis procedure. Our data base is still inadequate to determine whether or not the red supergiant reddening law in the MCs differs from that in the Milky Way. 4. With a new reddening law and a homogeneous set of spectral types it is possible to derive a new set of intrinsic colors for the red supergiants in the three galaxies. a) For the Milky Way significant differences from previous studies are found both in the color-spectral type relation and in the dependence of color on luminosity. These differences are attributed to the new reddening law and the revised set of spectral types. b) The intrinsic colors of the red supergiants vary among the three galaxies in a complicated and not fully understood manner. Most of the differences can be qualitatively understood, though, as arising from luminosity and abundance differences between the three galaxies. Differences in specific results between the present paper and that of McGregor and Hyland are due to our much larger body of data and to the fact that McGregor and Hyland use J - K color as a pure temperature indicator, whereas we show that there are luminosity and abundance effects for this color. 5. Extinction values and bolometric magnitudes are derived for individual supergiants in the MCs. Median values for ^ are 0.5 and 0.7 mag for the SMC and LMC, respectively. Many stars in the LMC have Av 'm excess of 1.0. These underscore the effect that reddening can have on studies of the young population component of the MCs. As previously shown by Elias et al, the brightest LMC supergiants are 0.3-0.4 mag brighter in Mbol than those in the SMC. 1 Operated by the Association of Universities for Research in Astronomy, Inc., under contract with the National Science Foundation. 2 Guest Investigator, Mount Wilson and Las Campanas Observatories, which are operated by the Carnegie Institution of Washington. 3 Guest Investigator and Visiting Resident Scientist, Cerro Tololo Inter-American Observatory. 91 © American Astronomical Society • Provided by the NASA Astrophysics Data System 92 ELIAS, FROGEL, AND HUMPHREYS 6. Multiple IR and visual observations of many of the stars in the MCs permit examination of their variability and of the way in which star color changes as they vary. On the average, the supergiants in the LMC are more variable than those in the SMC. Stars generally get redder, with stronger H20 indices, as they become fainter. A notable exception to this rule is that the LMC supergiants develop bluer B — V colors as they become fainter. 7. The SMC sample of supergiants has a small subset of large ( > 0.5 mag at K) amplitude variables with large 1985ApJS...57...91E ( > 0.4 mag) H20 indices. There are no counterparts to these stars in the LMC sample. They are most likely asymptotic giant branch stars of low mass rather than true supergiants. 8. In three appendices details are given on the color transformations employed, the least-squares technique used, and how the sample of Milky Way M supergiants was produced. Subject headings: galaxies: Magellanic Clouds — galaxies: stellar content — infrared: general — stars: late-type I. INTRODUCTION the red, did intermediate resolution spectroscopy in the blue, Late-type supergiants are among the most luminous stars and compared these results and the infrared photometry with visible in nearby galaxies. The Large and Small Magellanic calculated model atmospheres. In the absence of spectral types Clouds (the LMC and SMC) are the two nearest galaxies in for the Magellanic Cloud stars, McGregor and Hyland used a which there is a large population of K and M supergiants. dereddened J — K color as a temperature indicator. They Both Clouds are less massive than the Milky Way and have obtained four main results relevant to the present investiga- lower metallicity. They are thus ideal for studying the effects tion. of differing metallicity and galactic environment on the ob- 1. They find that the H magnitudes are depressed relative served properties of M supergiants. M supergiants have been to the J and K magnitudes at increased luminosity, and that proposed as standard candles for extragalactic distance de- the depression is greater in the Milky Way stars than in the terminations (see Elias etal 1981; Sandage and Tammann Magellanic Cloud stars. The depression is attributed to the 1982; Humphreys 1983), and it is therefore useful to study effects of CN absorption, and the differences to differences in their properties in different galaxies in order to test their CN abundance. suitability for this use. 2. At a given (/- 7f)0, both CO and TiO band strengths In this paper we report the final results of a coordinated are weaker in the Magellanic Clouds than in the Milky Way; program of spectral classification and infrared and visual the SMC stars show weaker CO than do the LMC stars. photometry of the late-type supergiants in the SMC and 3. The (V— K)0 colors are bluer for a given (J- K)0 in LMC. Most of the visible-wavelength LMC data used in the the Magellanic Clouds than they are in the Milky Way. analyses have been previously published (Humphreys 1979a), 4. The differences seen between the Milky Way and the but the infrared photometry and nearly all of the SMC data Magellanic Clouds can be accounted for by lower metal are new. Existing data for galactic M supergiants have been abundances in the Magellanic Clouds; from analysis of the reanalyzed; new values for intrinsic colors and the galactic supergiant data, McGregor (1981) obtains an estimate of a reddening law have been derived which supersede those ob- factor of 2 to 5 deficiency in the LMC and a factor of 3 to 10 tained by Lee (1970). The improvements are due to elimina- deficiency in the SMC, with respect to the Milky Way. tion of a number of biases present in the original analysis and Differences between the results of this paper and of to an increase in the amount of available data. McGregor and Hyland stem primarily from the use of The data presented here include a number of 10 /im ob- (J—K)0 as a temperature indicator. Improved dereddening servations. These show that the 10 /im excesses are larger in (§ IV) of both galactic and Magellanic Cloud stars shows that the LMC than in the SMC, and that the Milky Way stars the / - color has a definite luminosity dependence and a show still larger excesses. This can be explained as being due probable metal abundance dependence (§ VI), so it is not a to lower dust abundances (due to lower metal abundances) in completely rehable temperature indicator for M supergiants. the Magellanic Clouds (§ V). Since the effective temperatures inferred from MK spectral The MK spectral types allow accurate determinations of the types are also affected by metal abundance, they require extinctions toward the individual stars; these are shown to be corrections when comparing stars in the three galaxies with typically almost 1 mag at V (§ VII). one another (§ III). Assuming these corrections to be roughly Also, the amplitude of variability of the stars and the way right, the weakening of both CO and TiO absorption (at a in which the colors of M supergiants vary can be studied given effective temperature) in the Magellanic Clouds is con- (§ VIII).
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