COMMISSION 27 OF THE I. A. U. INFORMATION BULLETIN ON VARIABLE STARS Number 1326 Konkoly Observatory Budapest 1977 August 15 THE LINEAR POLARIZATION OF u HERCULIS The eclipsing variable, u Her (68 Her, HR 6431, HD 1566383, BD +33d2864), was observed with the polarimeter described by Koch and Pfeiffer (1976) mounted on the Pennsylvania 72-cm reflector. The instrumental polarization, which was small for all bandpasses, was evaluated from the observations of 16 zero-polarization standard stars. The notation of Koch and Pfeiffer is used in the present paper. Table I lists the instrumental responses and the integration and housekeeping intervals. It may be noted that T = 2800 sec. corresponds to 0.016 P. The journal of the observations appears in Table II for which phases have been computed from the ephemeris: Pr. Min. (hel.) = 2405830.033 + 2.051027 E. The first purpose for obtaining the present observations was to compare and contrast the present blue polarization state of u Her against the similar data of Rudy and Kemp (1977), which are much more numerous and of greater observational weight and were obtained in the previous season. The following remarks may be made. (1) For both series most of the signal is contained in the Q-parameter. (2) The phases and magnitudes of minimum polarization are essentially the same for the two seasons. The minimum polarization of about 0.02% is consistent with the interstellar component (Mathewson and Ford 1970) to be expected at a distance of about 200 pc for the galactic coordinates of u Her. (3) For both seasons one polarization maximum of 0.06% occurs near phase 0.75 P. (4) In both sets of data a second polarization maximum occurs at phase 0.3 P. The peak-to-peak scatter of both the Oregon and Pennsylvania observations is about 0.04%. The mean of the Pennsylvania measures from 0.24 P to 0.41 P is greater than the Oregon mean by 0.02% +- 0.006%. The scatter in both data sets and the imprecision of the small systematic difference permit only a weak conclusion: the systematic difference, if Table I. Instrumental responses and observing intervals for u Her Filter lambda eff FWHM t (sec.) T (sec.) (Angstrom) (Angstrom) Narrow red 7450 200 1920 2800 Red 6520 850 320 840 Green 5350 800 320 840 Blue 4300 760 320 840 Ultraviolet 3710 310 1920 2800 Table II. Polarization observations of u Her Filter J.D.(hel.)- Phase Q(%) U(%) P(%) 2443200 Blue 49.822 0.414 -.02 (.02) -.06 (.02) 0.06 (.02) Green 49.840 .423 -.08 (.02) .00 (.02) .08 (.02) Blue 51.778 .368 -.04 (.01) -.02 (.01) .05 (.01) Red 51.799 .378 +.01 (.01) -.04 (.01) .04 (.01) Red 51.814 .385 -.03 (.02) -.07 (.02) .07 (.02) Blue 51.833 .394 -.05 (.02) -.05 (.02) .07 (.02) Blue 64.736 .686 -.06 (.01) +.04 (.01) .08 (.01) Ultraviolet 64.780 .707 +.01 (.03) -.02 (.03) .02 (.03) Ultraviolet 64.820 .727 -.06 (.03) -.06 (.03) .09 (.03) Blue 64.858 .745 -.056 (.009) -.001 (.009) .056 (.009) Blue 75.733 .047 -.01 (.01) +.01 (.01) .02 (.01) Blue 75.827 .093 -.03 (.01) +.03 (.01) .04 (.01) Blue 78.637 .463 +.02 (.01) -.02 (.01) .02 (.01) Narrow red 78.690 .489 +.04 (.04) -.05 (.04) .06 (.04) Green 78.739 .513 -.06 (.01) +.02 (.01) .06 (.01) Green 78.758 .523 -.02 (.01) .00 (.01) .02 (.01) Blue 78.780 .533 -.01 (.02) .00 (.02) .01 (.02) Blue 79.635 .950 -.05 (.01) .00 (.01) .05 (.01) Blue 85.625 .871 -.05 (.02) .00 (.02) .05 (.02) Table II. (cont.) Filter J.D.(hel.)- Phase Q(%) U(%) P(%) 2443200 Red 85.649 .882 -.04 (.02) -.01 (.02) .04 (.02) Red 85.665 .890 -.01 (.03) -.01 (.03) .02 (.03) Blue 85.684 .899 -.06 (.02) -.02 (.02) .06 (.02) Ultraviolet 85.736 .925 +.01 (.02) -.01 (.02) .02 (.02) Blue 85.789 .951 +.04 (.01) -.04 (.01) .06 (.01) Blue 90.622 .307 -.10 (.01) -.01 (.01) .10 (.01) Green 90.642 .317 -.09 (.01) +.01 (.01) .09 (.01) Green 90.664 .327 -.02 (.02) .00 (.02) .02 (.02) Blue 90.682 .336 -.08 (.02) -.04 (.02) .09 (.02) Narrow red 90.735 .362 +.04 (.06) -.04 (.06) .06 (.06) Blue 90.780 .384 -.08 (.02) +.02 (.02) .08 (.02) Blue 91.619 .795 -.05 (.02) +.02 (.02) .05 (.02) Ultraviolet 91.731 .850 -.08 (.03) +.03 (.03) .08 (.03) Blue 91.768 .868 -.04 (.02) +.01 (.02) .04 (.02) Red 91.792 .879 +.02 (.01) +.04 (.01) .04 (.01) Red 91.816 .891 -.04 (.02) +.01 (.02) .04 (.02) Blue 91.837 .902 -.04 (.01) .00 (.01) .04 (.01) Blue 98.687 .240 -.08 (.01) .00 (.01) .08 (.01) Blue 98.807 .299 -.05 (.03) -.04 (.03) .07 (.03) Green 98.835 0.312 -.11 (.02) -.03 (.02) 0.12 (.02) real, cannot be due to the familiar close binary reflection effect. (5) It is similarly true that the small systematic difference between the Pennsylvania polarizations at the two maxima cannot be due to the reflection effect. (6) Over the phase interval, 0.89 P to 0.95 P, the Pennsylvania results are 0.02% +- 0.006% greater than the Oregon mean. (7) The limiting measure by Hall and Mikesell (1950) sheds no light on possible intrinsic variability. The second reason for obtaining the new observations was to determine a polarization spectrum. This accounts for the sequence of the data of Table II which typically permitted interpolating the blue measures to the times of those for other filters. Two procedures were used to calculate the spectrum: (1) normalize by division a non-blue measure by the interpolated blue value, and (2) establish a fictitiously constant blue polarization, correct all real blue measures to that constant value, and apply the same additive correction to the non-blue values. The two procedures yielded the identical result that, within a precision of 0.02%, the polarization spectrum of u Her is flat from lambda3700 to lambda7450. The wavelength dependence of the interstellar component cannot be recognized for a value as small as 0.02%. Additionally, there is no reason to suppose that a thick shell is associated with the u Her system so hydrogen self absorption should be absent. The best interpretation appears to be that electron scattering is truly the cause of the systemic polarization as Rudy and Kemp have postulated. These authors also note that the polarization should decrease with increasing wavelength if the scattering mechanism is the reflection effect seated in the cool (B5) binary component. From lambda3700 to lambda7450, a 35% polarization decrease is suggested by the models given in Gingerich (1969) if the reflection effect is really the cause of the scattering. The present ultraviolet and narrow red observations have not been made at the same phases. When they are corrected for the mean phase dependence of Rudy and Kemp's blue data, no polarization gradient emerges. A realistic test would have to distinguish between, say, 0.05% at lambda3700 and 0.03% at lambda7450. This is impossible for the present data since the error of the mean at lambda7450 is +- 0.03%. The possible intrinsic polarization variability of u Her would be qualitatively consistent with the evidence of streaming gas developed by Kovachev and Reinhardt (1975). Since the binary is no longer a ZAMS one, variability is also compatible with the statistical summary by Pfeiffer and Koch (1977). There remains only the question of what fraction of the variability is due to scattering in the circumstellar volume, and it is reasonable to suppose that a lower limit to this value is displayed by the intrinsic seasonal variability which remains poorly known at present. We are indebted to J. C. Kemp and R. J. Rudy for communicating their results in advance of publication. NSF Grant MPS 74-01656 A01 supported this work. ROBERT H. KOCH RAYMOND J. PFEIFFER Department of Astronomy Department of Physics University of Pennsylvania Trenton State College Philadelphia, PA 19104, U.S.A. Trenton, NJ 08560, U.S.A. References: Gingerich, O. (1969). Theory and Observation of Normal Stellar Atmospheres: Proceedings of the Third Harvard-Smithsonian Conference on Stellar Atmospheres (M.I.T. Press, Cambridge, Mass.). [BIBCODE 1969tons.conf.....G ] Hall, J. S. and Mikesell, A. (1950). Publ. U. S. N. O. 17, 1. [BIBCODE 1950PUSNO..17....1H ] Koch, R. H. and Pfeiffer, R. J. (1976). Astrophys. J. 204, L47. [BIBCODE 1976ApJ...204L..47K ] Kovachev, B. J. and Reinhardt, M. (1975). Acta Astron. 25, 133. [BIBCODE 1975AcA....25..133K ] Mathewson, D. S. and Ford, V. L. (1970). Mem. Roy. Astr. Soc. 74 (5), 139. [BIBCODE 1970MmRAS..74..139M ] Pfeiffer, R. J. and Koch, R. H. (1977). Publ. Astron. Soc. Pac. 89, 147. [BIBCODE 1977PASP...89..147P ] Rudy, R. J. and Kemp, J. C. (1977). Astrophys. J. (in press).