32J .7 The Astrophysical Journal, 309:732-736,1986 October 15 kf 1986. The American Astronomical Society. All rights reserved. Printed in U.S.A. .309. 6ApJ. 198 RV TAURI STARS AS POST-ASYMPTOTIG GIANT BRANCH OBJECTS M.Jura Department of Astronomy, University of California, Los Angeles Received 1986 February 5 ; accepted 1986 April 8 ABSTRACT RV Tau stars are rare, luminous pulsators of spectral types F, G, and K. Analysis of the IRAS data shows that the mass-loss rates from RV Tau stars have apparently significantly decreased during the past ~500 yr -5 -1 from about 10 M0 yr , depending upon the metallicity of the stars and the grain emissivity at 60 /mi. It seems likely that these stars have just evolved from the phase of rapid mass loss, characteristic of the last stages of the asymptotic giant branch (AGB). The birthrate of RV Tau stars in the solar neighborhood is very roughly 6 x 10"4 kpc-3 yr-1, about a tenth of the birthrate of all planetary nebulae, and this is consistent with the view that we are witnessing the subset of stars undergoing post-AGB evolution that are low mass and at least in some cases of low metallicity. Most RV Tau stars will probably become planetary nebulae; others, however, may evolve sufficiently slowly that their envelopes will dissipate before being photoionized. Subject headings: infrared: sources — stars: evolution — stars: mass loss — stars: RV Tauri — stars : stellar statistics I. INTRODUCTION and a small bolometric correction, then, from equation (1), 3 The evolution of stars from the asymptotic giant branch again, L ~ 10 L0. Finally, from the correlation between (AGB) to the white dwarf phase is still not well understood (see, molecular outflow velocity and luminosity for OH/IR stars for example, Iben and Renzini 1983). One diagnostic of AGB (Jones, Hyland, and Gatley 1983; Jura 1984a), the outflow stars is that in their very late stages, they undergo extensive velocity inferred from observations of OH—of about 10 km s~1 for RV Tau (Fix and Claussen 1984)—is consistent with a mass loss which results in substantial circumstellar envelopes 3 that are most profitably studied in the radio and infrared (see, luminosity of ~ 10 L0. These stars therefore seem to have the for example, Olofsson 1985). With the IRAS survey, it is pos- effective temperatures and luminosities of the less massive sible to perform systematic studies of circumstellar dust and post-AGB stars (Gingold 1974; Iben and Renzini 1983; Schon- therefore identify both candidate AGB stars and stars that berner 1983). have just evolved out of the AGB phase and still retain some Gehrz (1972) and Lloyd Evans (1985) found that RV Tau circumstellar material. It has long been thought that RV Tau stars quite often have circumstellar dust because they display stars are highly evolved objects, and here we use the IRAS significant amounts of infrared radiation. Here, we suggest that fluxes to constrain their evolutionary histories. most of the dust is not being currently ejected. Some stars, such as the Egg nebula (CRL 2688) or CRL 618 The basic argument of this paper comes from using the well- have previously been identified as post-AGB preplanetary known properties of circumstellar dust shells (see, for example, nebula stars (Zuckerman et al 1976), and models have been Sopka et al 1985). In the IRAS bandpasses, cool dust mainly proposed to describe their evolution (see Kwok 1982). Very contributes to the 100 gm band while warmer dust contributes young planetary nebulae with some residual molecular to the emission at 12 jam. By comparing the relative amounts of material have also been identified such as NGC 7027 (see Jura long and short wavelength emission, it is possible to estimate 19846). Here, we aim to increase our understanding of post- the density of material that is nearby to the star compared to AGB objects by studying the properties of RV Tau stars. that which is more distant, and thus determine whether the The structure of this paper is as follows. In § II, we argue that mass-loss rate is constant or changing with time. the RV Tau stars are probably post-AGB objects, as have Because Gehrz (1972) could only perform observations from Gingold and Eggen (1986) on independent grounds. In § III, the ground out to 20 jam, he had insufficient information to we present detailed models for the circumstellar envelopes to study the time history of the mass loss—he could only establish develop some insight into the stars’ histories. In § IV we the existence of circumstellar material. discuss the evolutionary status of these stars and in § V, we From the comprehensive variable star catalog of Kukarkin present our conclusions. et al (1969), we have identified 17 RV Tau stars which were detected by IRAS at 60 jam, the wavelength that is probably II. RV TAURI STARS AS POST-AGB OBJECTS most suitable for estimating the dust loss rate (see Jura 1986). The detected stars and the IRAS fluxes are listed in Table 1 ; Preston et al (1963) have described the basic properties of this list includes about 20% of all known RV Tau stars. We fit RV Tau stars; they are luminous pulsators of spectral class F, a power law to the observed fluxes such that F varies as vq, G, and K. From their optical classification, they estimate that v 3 and we list the derived values of q in Table 1. For stars which Mv ^ — 3 mag which indicates a luminosity of ~ 10 L©. Con- are detected in all four IRAS bands, we list q for the spectral sistent with this result, Barnes and Dupuy (1975) have pro- region between 12 jim and 100 /xm; otherwise we list q for the posed that M == -5.3 + 0.021P , (1) range between 12 jam and 60 jim. v Jura (1986) found that for mass-losing carbon stars, the where P is the period in days. With a typical period of 75 days average value of q is 1.54, while the power-law fits to oxygen- 732 © American Astronomical Society • Provided by the NASA Astrophysics Data System 32J .7 No. 2, 1986 RV TAURI STARS 733 .309. TABLE 1 . Av = 0 and Ay — 1 mag in order to bracket the proposed RV Tauri Stars Detected at 60 /¿m by IRÁS values for this quantity (see Baird and Cardelli 1985 and below). 6ApJ. Star 12 fim 25 60 /¿m 100 /mi Following the method described by Sopka et al (1985), we 198 TW Cam . 8.3 5.6 1.8 <1.7 0.95 can compute the temperature of carbon grains at, say, 1" from UY CMa. 3.5 2.5 0.57 <1.0 1.13 the star to give GK Car .. 2.9 2.5 0.78 <12 0.82 IW Car ... 101 96 34 13 0.97 T(l") = 123 K , Av = 0 (3a) RU Cen .. 5.4 11 5.6 2.0 0.47 SX Cen ... 6.0 3.6 1.1 <1.5 1.05 T(l") = 144 K , Av=l . (3b) SU Gem . 7.9 5.7 2.2 <12 0.79 AC Her .. 41 65 21 7.8 0.78 If we assume a constant mass-loss rate from this star, we U Mon ... 124 88 26 9.2 1.23 would derive from equation (3) of Jura (1986) and the observed CT Ori ... 6.2 5.6 1.2 <1.5 1.02 IRAS flux at 60 fim DYOri .. 12 15 4.1 <11 0.67 9 AR Pup .. 131 94 26 12 1.13 M = 5.9 x lO^v.oD^lSO/xeo) g s“1 , A = 0 (4a) R Sge 10.6 7.6 2.1 <1.7 1.01 dust v AI Seo ... 18 11 2.9 <46 1.13 M = 4.0 x 1017z; D (150/ o) g s“1 , ^1 = 1 . (4b) R Set 21 9.3 8.1 <139 0.59 dust 1() kpc Z6 F RV Tau .. 22.5 18 6.4 2.5 1.04 1 In equation (4), v10 is the outflow velocity in units of 10 km s ~ 12 5.7 1.3 <7.0 1.38 s 2 -1 V Vul .... and Xóo i the opacity of the dust (cm g ) at 60 ¿un. We Note.—We exclude BI Cep even though it was detected by IRAS at assume that i^o = 1 as seems appropriate for mass-losing 60 /im because its period is longer than 200 days and Kukarkin et al. carbon-rich red giants (Knapp and Morris 1985; Zuckerman (1969) list it as an M star as well as an RV Tau variable. We assume it and Dyck 1986; Zuckerman, Dyck, and Claussen 1986), and is an unusual Mira. for RV Tau where OH has been detected (Fix and Claussen 1984). rich stars are, on the average, even steeper (Hacking et al Because of the relatively low flux at 12 ¿mi, the simple model 1985). In contrast, for the RV Tau stars shown in Table 1, the of a constant mass-loss rate with time is not correct. Assume, maximum value of q is only 1.41 while the average value is 0.98. for simplicity, a model with a constant mass-loss rate with Therefore, the RV Tau stars have significantly flatter infrared time, until, the star abruptly evolves off the AGB and the mass spectra than do most mass-losing red giants. loss stops. If we further assume a simple free expansion of the The flux distribution in the infrared depends upon the material, then density distribution of dust grains and the evmissivity of the p = 0 r <r (5a) grains as a function of wavelength.