RESULTS FROM OPTICAL INTERFEROMETRY Jacques M. Beckers Advanced Development Program National Optical Astronomy Observatory * Tucson, Arizona 85726 ABSTRACT. The techniques of optical speckle and Michelson interferometry have both contributed significant information on the diameter, the atmospheres, and the envelopes of red supergiant stars. In this paper, I summarize the observations obtained to date and preview future plans. 1. INTRODUCTION Next to the sun, the largest angular diameter stars are red supergiant and Mira variable stars. Their diameters are as much as 50 milliseconds of arc which is well below the angular resolution of even the space telescope but which is within the resolution capability of interferometric techniques using large telescopes or multielement interferometers. The techniques of speckle and Michelson interferometry have been described elsewhere (e.g. Labeyrie 1970, 1978). In this summary, I will report on the results of observations using these techniques. Observations are generally limited to the larger and brighter objects (a ORI, a SCO, a HER, a TAU, a BOO, 0 CET, etc.) and mostly refer to either diameter observations of the star as a function of wavelength and time or attempts at observations of the 2D image structure. Since the techniques are very data intensive and complex the number of research groups involved is small and the results are still very limited. 2. STELLAR DIAMETERS Stellar diameters are derived from the variation of fringe visibility of the object as a function of angular frequency using either two element Michelson interferometers or speckle images. Instrumental and atmospheric fringe visibility effects are removed by calibration *Operated by the Association of Universities for Research in Astronomy, Inc., under contract with the National Science Foundation. M. Morris and B. Zuckerman (eds.), Mass Loss from Red Giants, 57-61. 1985 by D. Reidel Publishing Company. 57
58 J. M. BECKERS against unresolved stars. The fringe visibility spectrum is interpreted in terms of the object diameter and its center to limb variation in intensity. Both measurement and interpretation include uncertainties. These are probably responsible for much of the a ORI diameter scatter shown in figure 1 taken from a recent paper by Balega et al. (1982). Diameters range from 30 to 77 millisecond of arc. Some of these variations are related to spectral features. All observers agree on an increase of diameter of 10-20% in the TiD molecular bands and perhaps 1.5 Figure 1: 5000 6000 7000 o )"IA) Uniform disc angular diameter of Betelgeuse normalized with respect to the diameter at A 7520 A (0 7520 = 50 msa) versus wavelength. Symbols refer to: _ this paper, 0 Lynds et al. (1976) and Welter and Worden (1980), + Roddier et al. (1981), and x other measurements reported by White (1980). The straight lines are linear regressions: a / 7520 = -10-~ (±3 10-6) A (A) + 1.49 (±0.02) with a correlation coefficient r = - 0.48. Excluding data from Lynds et al. and from Welter and Worden would lead to: b 0/07520 = 5 x 10~ (±6 10-6) A (A) + 0.65 (±0.03) with r=o.23 (from Balega et al. 1982). more in spectral lines like Ha. These increases are believed to be due to the extended atmosphere of this type of star and will in the future lead to improved stellar atmospheric models of these stars. It has been suggested that some of the other variations in the diameter of a ORI are caused by temporal changes in the diameter of the star. 3. STELLAR IMAGES A number of techniques have been tried with limited success to give images of supergiant stars and their envelopes. The well known
RESULTS FROM OPTICAL INTERFEROMETRY 59 image of a ORI by Lynds et al. (1976) was derived by means of the socalled shift-and-add algorithm of speckle images. It uses data from the 3.8 meter telescope at Kitt Peak and is therefore limited,in resolution to the radius of its Airy disk (- 33 millisecond of arc at 600 nm) which means less than 1..5 resolution elements per stellar diameter. To really adequately resolve the stellar disks of stars like a ORI, a SCO and a HER, one needs a substantially larger telescope. The phased MMT with its 686 cm baseline has a resolution of 18 msa at 600 nm which is substantially better and an improvement in this type of imaging may therefore be expected in the future. The University of Arizona and Nice groups have been observing twodimensional fringe visibilities of a ORI. Figure 2 from Roddier et~ (1983) is a two-dimensional map of the fringe visibility of a ORI. This type of observational data cannot distinguish between stellar N s baselin~ Clrcsec.\ o o 10 Figure 2: Two-dimensional map of the visibility of the fringes produced by Betelgeuse at A = 5348 A (~A = 90 A) on 1980 November 30 (from Roddier and Roddier 1983). image structure 180 0 in position angle apart. The data in Figure 2, therefore, demonstrates anisotropic image structure with an extension to either the SW or NE direction, but it cannot distinguish between structure in these two directions. Computational techniques have been proposed which derive image information from two dimensional visibility functions as shown in figure 2. They depend on the fact that the scene to be imaged is positive everywhere. So far, those techniques have been used on stellar data only with limited success.
J.M.BECKERS Figure 3: rmage of a ORr obtained from interferograms in the pupil plane (courtesy C. and F. Roddier). The Roddiers are presently experimenting with pupil interferometry observations of a ORr and other stars. They use a pupil rotating shearing interferometer (Roddier et al. 1978) to obtain an array of both fringe visibility and relative phase at different 2D Fourier frequencies. Figure 3 is an image of a ORI obtained from pupil interferograms. It, for the first time, shows a structure in the continuum outside the stellar disk at 2.5 ~ with an intensity of about 10% of the stellar disk. The Roddiers interpret this as evidence for the existence of a highly anisotropic dust layer around the star with a temperature of about 1750 K. We (Beckers et al. 1982, 1983) using the technique of Differential Speckle Interferometry obtained images of the Ha envelope around the star extending at the 1% level to about 2 ~ and at the 0.1% level to about 3~. We find, however, that the results are very sensitive to proper detector calibration. New observations using a phased MMT and good calibrations are now being analysed. Because the MMT has an altazimuth mount, the image rotates across the field of view of the speckle camera so that it will be possible to separate instrumentally caused anisotropies from object anisotropies. 4. ANTICIPATED FUTURE DEVELOPEMENTS It appears that the field of red supergiant imaging in optical wavelengths is ready for major advances. Pupil interferometry will result in unambiguous images of supergiants in continuum wavelengths. Differential speckle interferometry is developing rapidly and will result in images in lines. Images in opposite wings of emission lines will result in Doppler shift observations. With the phased MMT those observations can be done at resolutions of 15-20 msa. With the NNTT in
RESULTS FROM OPTICAL INTERFEROMETRY 61 the future, 5 msa resolution observations are a possibility. We also anticipate that observations of the supergiants a SCO and a HER will compete and maybe overtake the a ORr observations. These two stars are accompanied by bluer, small angular size, relatively bright companions. The angular distance of these companions from the red supergiant (2.9 and 4.9 arc sec respectively), is within the so-called isoplanatic patch so that the bluer companion can be used to determine by deconvolution or crosscorrelation the supergiant image. REFERENCES Balega, Y., Blazit, A., Bonneau, D., Koechlin, L., Foy, R., and Labeyrie, A. 1982, A8t~. and A8t~ophy8. 115, 253. Beckers, J. M., Hege, E. K., Murphy. H.P., and Burnette, F. 1982, Butt. Am. A8t~on. Soo. 14, 918. Beckers, J. M., and Hege, E. K. 1983, SprE Proceedings, 445, "Instrumentation in Astronomy V," p. 462. Labeyrie, A., 1970 A8t~on. and A8t~ophy8. 6, 85. Labeyrie, A., 1978 Ann. Rev. Astpon. and Astpophys., 16, 77. Lynds, C. R., Worden, S. P., and Harvey, J. W. 1976, A8t~phys. J. 207, 174. Roddier, F., Roddier, C., and Demarcq, J. 1978, J. Optias (Paris) 9, 145 Roddier, C., Roddier, F, and Vernin, J. 1981, Proc. ESO Conference on "Scientific Importance of High Angular Resolution at Infrared and Optical Wavelengths," p. 165. Roddier, C., and Roddier, F. 1983, A8t~ophy8. J. 270, L 23. Welter, G. L. and Worden, S. P. 1980, Astpophys. J. 242, 673. White, N.M. 1980, A8t~ophy8. J. 242. 646. Comment added by the Author: This paper was not intended to be a full review of the application of interferometric techniques to the imaging of red giants and supergiants. I therefore have not discussed all published observations. My intent was to summarize my knowledge of the sizing and imaging of these objects.