Pub. Astron. Soc. Pacific, Volume 85, October 1973 VISUAL AND

Pub. Astron. Soc. Pacific, Volume 85, October 1973

VISUAL AND INFRARED PHOTOMETRY OF RY SAGITTARII NEAR THE PHASE OF DEEP MINIMUM

THOMAS A. LEE Lunar and Planetary Laboratory, The University of Arizona Received 18 June 1973

Photometric measures near the deep minimum phase are presented for the R CrB-type star, RY Sgr. While the visual brightness was some 4 magnitudes fainter than observed in 1969, the infrared flux levels (λ 3.4 μ) remain essentially unchanged. Key words: photometry — infrared — R CrB stars

RY Sagittarii is a member of the R Coronae The spectral energy distribution of RY Sgr, Borealis class of variables — stars which charac- obtained from the observed magnitudes in Table teristically show an overabundance of carbon I and the absolute calibration of Johnson (1966), and large, irregular variations (up to 6 or 7 mag- is shown in Figure 1. The figure shows the nitudes) in the visual region. Sizable infrared marked change in the overall energy spectrum excesses have been reported for both RY Sgr of RY Sgr between the two epochs of the obser- (Lee and Feast 1969) and R CrB (Stein et al. vations, 1969 and 1972. While the visual flux 1969), and subsequently for eleven other R CrB- decreased by a factor of ~35, the radiation at type stars (Feast and Glass 1973). The exhaustive the longest wavelengths appears to have remain- study of RY Sgr by Alexander et al. (1972) shows ed unchanged. The argument for constant flux that the photometric and spectroscopic behavior at λ = 3.4 μ is not the strongest, yet such con- of the star is very complicated, with erratic stancy is surely suggested by the 3.4 μ and 10 μ changes in color and spectral features superim- measurements made in 1969 and 1972. The dis- posed upon a well-defined pulsation whose cordance in the 1μ to 3.4 μ measures of Feast period and visual amplitude are ~39 days and and Glass (1973) and the author for 1972 are -^0^5, respectively. The irregularly occurring larger than expected from the usual observational phases of deep minimum that distinguish the errors; the dates of the observations are not iden- R CrB-type variables are thought to be due to tical and, therefore, the differences at these the ejection (or condensation) of clouds of car- wavelengths could be real. For simplicity, how- bon particles by the star —in agreement with ever, one curve has been drawn through these the so-called Loreta — O'Keefe hypothesis (Lor- points to represent the mean energy distribution eta 1934; O'Keefe 1939; Feast and Glass 1973). at the near deep minimum phase. Photometric observations of RY Sgr near the The spectral distribution in 1969, with its phase of deep minimum were obtained during double maxima, is easily resolvable into two the summer of 1972 with the NASA-University components; one that is stellar and radiates in of Arizona 1.5-meter metal mirror telescope atop the visual region and another which is extra- or Mount Lemmon (elevation 2500 meters) near circumstellar and produces the infrared flux. Tucson, Arizona. The measurements, which Lee and Feast (1969) found a good fit to the extend from 0.36 μ to 3.4μ, were made on the stellar component with a normally reddened UBVRI-JHKL system of Johnson et al. (1966) GO lb star (EB _ ν = 0^45), as shown in Figure 1. and are summarized in Table I. Also included The excess radiation at longer wavelengths is in the table are some observations made the thought to be due to an absorbing and reradiating previous year, 1971, as well as the published data circumstellar shell whose temperature is -700° Κ of Lee and Feast (1969) and Feast and Glass and whose composition is (perhaps) largely (1973). All of the new observations were made graphite particles. However, the flux gradients at an air mass of ~ 2.4. at wavelengths longward and shortward of 5 μ 637

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RY Sgr Δ FEAST and GLASS (1975)- ίχ(νν cm"2 μ"') O AUTHOR - 1972 • LEE «and FEAST (1969)

Fig. 1—The solid and dashed lines show the spectral energy distribution for RY Sgr observed in 1969 and 1972. The dotted curve (and crosses) represent the reddened GO lb star used by Lee and Feast (1969) to fit the visual observations in 1969. The solid squares represent the excess radiation observed (over the GO lb star) for the 1969 epoch. The error bars on the far infrared points represent the mean error (stand- ard deviation).

are not consistent with a single temperature, but discordance can be due entirely to the develop- imply instead a range of temperatures for the ment of strong emission lines (relative to the shell. One can easily see, as mentioned above, continuum) and/or an abnormal stellar continuum that only the stellar component has faded (from at certain phases, or whether large variations in 1969 to 1972); the infrared (circumstellar) com- the extinction law are also taking place. Con- ponent is essentially the same. current spectroscopy and photometry are required Large discrepancies appear when one tries to to setde this question; yet, one can visualize that account for the change in the visual brightness large changes in the opacity of the shell would ( Δ V) seen in Figure 1 by simply adding attenua- be accompanied by varying physical conditions tion whose wavelength dependence is that found therein which could alter the size distribution by Alexander et al. (1972), with Ay = AV. This of the grains and, therefore, the observed ex- extinction law {R = 4.3; EU_BIEB_V = 1.3) was tinction law. Nonuniform coverage of the stellar derived from some 200 UBV observations, but surface by a small, but completely opaque, cloud applies only to those epochs of normal spectro- could produce the same result. scopic appearance — which occur primarily when Finally, the data plotted in Figure 1 are very V á ICW. While this law is likely to be a reliable significant regarding the models proposed for mean relation, the observations in Table I for the R CrB-type variables. For example, Lee and 1969 and 1972 give AV = 3^9 and A(ß-V) = Feast (1969) reported changes in brightness in 0^14. Such a large departure from the above the ν(0.55μ) and L{3Ap) bands that were extinction law prompts one to ask whether the anticorrelated, and from these data concluded

© Astronomical Society of the Pacific · Provided by the NASA Astrophysics Data System 640 THOMAS A. LEE that the total flux of RY Sgr tended to be con- troscopic constraints found by Alexander et al. served. These variations, however, were likely (1972), as well as the infrared photometry. If a part of the 39-day pulsation, and not associated a uniform spherical shell, whose thickness varies with the drop to deep minimum. In the case with time, is at work in RY Sgr, then at maximum of R CrB, Gillett and Stein (1971) and more re- visual brightness (V = 6^5) the 3.4 μ radiation cently Forrest, Gillett, and Stein (1972) reported should be down by a factor of three. Thus, de- totally uncorrelated variations in the visual and tailed monitoring of RY Sgr in the far infrared, infrared flux and, thus, no conservation of lum- with emphasis on the periods of visual maxima, inosity. These authors concluded that a "patchy is of primary importance for the resolution of or discrete cloud model" was most applicable to the nature of this fascinating star. R CrB —their finding of no anticorrelated changes in the visual and infrared radiation rul- The author extends his thanks to the National ing out any possibility of a uniform symmetric Aeronautics and Space Administration for sup- shell. However, the patchy cloud model is unable porting this work, and to M. W. Feast for several to adequately account for the asymmetry seen helpful comments. in the visual light curve near deep minimum (Feast and Glass 1973). In the case of RY Sgr, Figure 1 shows that the energy distribution changes dramatically when the star fades in the REFERENCES visual region as it nears the phase of deep min- Alexander, J. B., Andrews, P. J., Catchpole, R. M., Feast, imum. Yet, the change in the integrated flux M. W., Lloyd Evans, T., Menzies, J. W., Wisse, P. N. J, and Wisse, M. 1972, M.N.fí.A.S. 158, 305. (from λ = 0.36 μ to 3.4 μ) between the 1969 Feast, M. W., and Glass, 1. S. 1973, M.N.R.A.S. 161, and 1972 epochs in Figure 1 is only 10%. The 293. far infrared (λ > 3.4 μ) monitoring of RY Sgr Forrest, W. J., Gillett, F. C., and Stein, W. A. 1972, done to date is insufficient to accurately docu- Ap. J. (Letters) 178, L129. ment changes in brightness at these wavelengths. Gillett, F. C., and Stein, W. A. 1971, Ap. J. 164, 77. For example, only a small percentage increase Johnson, H. L. 1966, Annual Rev. of Astr. and Astro- physics 4, 193. in flux at λ > 3.4μ would be required to off- Johnson, H. L., Mitchell, R. I., Iriarte, B., and Wis- set the large drop seen in the visual region near niewski, W. Z. 1966, Comm. Lunar and Planetary deep minimum. Thus, at this point, we cannot Lab. 4, 99. reach a definite conclusion regarding the con- Lee, Τ. Α., and Feast, M. W. 1969, Ap. J, (Letters) 157, servation of the total luminosity of RY Sgr, and L173. Loreta, E. 1934, A.N. 254, 151. similarly the possibility of a uniform circum- O'Keefe, J. A. 1939, Ap./. 90, 294. stellar shell for this star. The ultimate model for Stein, W. Α., Gaustad, J. E., Gillett, F. C., and Knacke, RY Sgr must, of course, satisfy the many spec- R. F. 1969, Ap. J. (Letters) 155, L3.