COMMISSION 27 OF THE I.A.U. INFORMATION BULLETIN ON VARIABLE STARS Number 3226 Konkoly Observatory Budapest 8 August 1988 HU ISSN 0374 - 0676 LIGHT VARIABILITY OF THE HELIUM-STRONG STAR HD 96446 HD 96446 [V = 6.68, B-V = -0.16, U-B = -0.87, sp. B1IVp - B2Vp (Hoffleit and Jaschek 1982)] was first recognized as a helium-strong star by Jaschek and Jaschek (1959). Its spectrum indicates that helium is roughly equal in number abundance to hydrogen and oxygen is deficient by a factor of ~13, while other metal abundances appear normal (Wolf 1973). We had included HD 96446 in a list of 7 candidates to search for Beta Cephei variables among the He-strong stars (see Matthews and Bohlender 1988) during an observing run at the Las Campanas Observatory in March 1987. Because much of the observing time was instead devoted to studying the recently discovered SN 1987, and additional time was lost to nonphotometric weather, only HD 96446 was observed often enough to produce meaningful results. The presence of variations at Beta Cephei timescales cannot be established from our data alone. The observations do, however, clearly demonstrate that HD 96446 is a broadband light variable. We have used these data to address the question of this star's rotation period. Measurements were obtained by J.M.M. during UT 19 - 22 March 1987 with the 0.6-m telescope of the University of Toronto located on Las Campanas, Chile, using a photometer equipped with an S25 phototube and UBV filter set. In addition to the programme star HD 96446, a comparison star [HR 4342: V= 6.87, B-V = -0.09, U-B = -0.44, sp. B7III] and a check star [HR 4361: V= 5.73, B-V = -0.12, U-B = -0.71, sp. B3] (Hoffleit and Jaschek 1982) were also monitored. The standard observing routine was: HD 96446, sky, HR 4342, sky, HD 96446, sky, HR 4361, sky, HD 96446, sky, (repeat). For each star, 30- second integrations through U, B, and V filters were recorded in sequence. TABLE I. Differential photometry of HD 96446 HJD (2446800+) V B U HD 96446 - HR 4342 73.78011 -0.310 -- -0.784 73.78761 -0.319 -0.361 -0.801 73.79483 -0.319 -- -- 73.50808 -0.312 -0.360 -- 73.81650 -0.313 -0.365 -0.785 73.82267 -0.311 -0.347 -0.784 73.83525 -0.309 -0.350 -0.777 73.84744 -0.311 - -0.772 73.85389 -0.314 -0.347 -0.775 73.85994 -0.310 -0.354 -0.779 73.86575 -0.313 -0.352 -0.779 73.87628 -0.311 -- -0.775 73.88273 -0.310 -0.354 -0.784 73.88833 -0.318 -0.343 -0.775 74.67382 -0.320 -0.364 -- 74.68134 -0.319 -0.363 -- 74.68764 -0.321 -0.367 -- 74.70911 -0.320 -0.364 -0.793 74.71515 -0.315 -0.365 -0.799 74.72122 -0.313 -0.365 -0.797 74.75543 -0.316 -0.359 -0.791 74.76110 -0.320 -0.367 -0.804 74.76690 -0.315 -0.356 -0.798 74.78460 -0.321 -0.367 -0.801 74.79134 -0.316 -0.362 -0.796 74.79684 -0.315 -0.359 -0.797 74.80199 -0.314 -0.355 -0.790 74.80801 -0.317 -0.353 -0.789 75.77596 -0.307 -0.350 -0.792 75.77753 -0.307 -0.351 -0.790 75.78607 -0.305 -0.351 -0.790 75.79278 -0.309 -0.348 -0.783 75.80291 -0.309 -0.359 -0.791 75.80923 -0.305 -0.352 -0.786 75.81551 -0.306 -0.360 -0.791 75.82175 -0.299 -0.356 -0.786 75.83845 -0.298 -0.344 -0.772 75.84406 -0.296 -0.342 -0.778 75.85232 -0.301 -0.342 -0.778 75.85850 -0.303 -0.344 -0.776 75.87151 -0.295 -0.341 -0.772 75.87781 -0.298 -0.343 -0.785 76.80495 -0.297 -0.344 -0.780 76.81127 -0.293 -0.345 -0.773 76.81731 -0.292 -0.337 -0.776 76.82756 -0.296 -0.344 -0.783 76.84217 -0.293 -0.343 -0.760 76.84773 -0.290 -0.331 -0.762 76.85364 -0.296 -0.331 -0.767 76.85967 -0.295 -0.338 -0.775 76.86813 -0.290 -0.338 -- HR 4342 - HR 4361 73.78398 1.188 1.161 -- 73.81253 1.183 1.158 1.428 73.83818 1.189 1.165 1.421 73.85676 1.188 1.167 1.430 73.87946 1.192 1.165 1.430 74.67794 1.188 1.159 1.434 74.71204 1.186 1.166 1.424 74.75816 1.186 1.163 1.421 74.78831 1.185 1.163 1.424 74.80447 1.185 1.162 1.419 75.78131 1.188 1.166 1.428 75.80636 1.190 1.171 1.435 75.81875 1.182 1.169 1.425 75.83563 -- 1.164 1.426 75.84918 1.184 1.163 -- 75.87449 1.185 1.171 1.427 76.80851 1.186 1.171 1.429 76.82487 1.188 1.165 1.434 76.83862 1.186 1.163 1.426 76.85052 1.188 1.165 1.425 76.86431 1.185 1.165 1.431 [FIGURE 1] [FIGURE 2] [FIGURE 3] The raw counts were corrected for coincidence-counting ("dead time") effects and sky background, and converted into instrumental magnitudes. The comparison and check star values were plotted against air mass to derive local extinction coefficients, which were then used to correct the magnitudes for mean sir mass extinction. Finally, interpolated values of the comparison and check star magnitudes were determined to produce the differences (HD 96446 - HR 4342) and (HR 4342 - HR 4361) in the three bandpasses. These data are listed in Table I. There is no indication of variability in HR 4342 or HR 4361; the standard deviations of the magnitude differences are sigma_V = 0.0025m, sigma_B = 0.0032m and sigma_U = 0.0044m. The larger scatter in U is expected, since sky stability usually worsens at shorter wavelengths. To search for periodicities in the data, the measurements of HD 96446 were analysed using a Fourier periodogram routine for unequally spaced time series (Matthews and Wehlau 1985). A Fourier amplitude spectrum of the differential V data, spanning the frequency range 0-50 d^-1, is shown in Figure 1a; the range 0-10 d^-1 is reproduced at a larger scale in Figure 1b. The largest peak is centred at a frequency of nu_1 = 1.178 +/- 0.001 d^-1, corresponding to a period of P_1 = 0.8490 +/- 0.0007 d. This peak is accompanied by an extended comb of 1 d^-1 aliases, including alias peaks "reflected" about zero frequency. The UBV measurements are displayed in Figure 2, folded at the 0.8490 d period. Both the programme-comparison and comparison-check differences are plotted. The variability of HD 96446 is obvious from these phase diagrams. When a sinusoid of period 0.8490 d and amplitude 0.011m is removed from the V data of Figure 2a, the periodogram of the residuals (Figure 1c) shows no signs of additional variations with amplitudes much greater than the observational scatter. There are indications of power - suggested by 1 d^-1 aliasing patterns just barely rising above the noise - at frequencies of 8.047 and 39.219 d^-1. We will return to these shortly. The +/-1 d^-1 aliases of the 0.8490-d periods (at periods of 5.6225 and 0.4592 d) are also possible solutions to the data, although the residual Fourier spectra and fits to the phase diagrams are noticeably worse. Even though the 0.8490-d period accounts well for the V variations, it is apparent from Figures 2b and c that the scatter of the B and particularly the U measurements about such a sinusoid exceeds that indicated by the (comparison - check) star data. When a sinusoid of period 0.8490 d and amplitude 0m.012 is subtracted from the U data, frequency analysis of the residuals shows evidence for periodicity with a timescale between roughly 0.25-0.35 d and a peak-to-peak amplitude in U of 0.02m. A phase diagram of the best fit to the residuals, at a period of P_2 = 0.2570 d is shown in Figure 3. (We note that when a sinusoid of this period and an amplitude of only 0.0025m is filtered from the V residuals, the two possible aliasing patterns - centred near P_2/2 and P_2/10 - are no longer obvious in the periodogram (Figure 1d).) We believe this may indicate variability in HD 96446 comparable to that seen in the Beta Cephei stars. Unfortunately, the strongest indicator of such short-term variations in our data is the U residual curve. Although the (comparison - check) U values are well-behaved, the U transparency of the atmosphere is notoriously unreliable even at good sites. Therefore, we must await more extensive photometric (and hopefully corroborating radial velocity measurements) to confirm if these variations are indeed real. Many of the He-strong stars exhibit photometric variations which are believed to correspond to their rotation periods, in the context of the oblique rotator model. Well-determined periods are available for at least 9 He-strong stars (Hunger 1986; Bohlender et al. 1987), ranging from 0.90 to 9.53 d. The value of 0.849 d we propose for HD 96446 falls at the low end of this range; the alias period of 5.623 d is also consistent with this range. If this star is an oblique rotator, its spectroscopic variations (if any) should have the same period as the light variability. Pedersen and Thomsen (1977) searched for periodic variability in the He I lambda4026 line strength among several He-strong stars, including HD 96446. They obtained 21 observations of this star from which they derived a period of 23 +/- 6 d. Their data show no correlation with either the 0.849107-d or the 5.623-d period, nor do any of the aliases of their period correspond to these values. It is possible that He line strength variations in this star are not linked to rotation as simply as in some other He-strong stars (Landstreet, private communication). Even so, we have some concerns about the reliability of their R index data for the purpose of period determination in the case of HD 96446. The changes they detected in this star were the smallest of any in their sample for which they claimed variability, and the amplitude was roughly the same as the standard error in each measurement. If HD 96446 is rotating with a period of 0.849 d, we can use Wolf's (1973) estimate of the radius of the star (R = 3.6 +/- 0.8 R_sun) to derive an equatorial rotation velocity v_eq = 200 +/- 30 km s-1. This is consistent with the range of rotation rates determined for early BIV-V stars by Wolff et al. (1982). However, so too is the value v_eq = 33 km s-1 which we find by using the 5.623-d period, if the star belongs to the subset of early B stars with slow rotation inferred by Wolff et al. On the other hand, Pedersen and Thomsen's period of 23 +/- 6 d yields v_eq = 9 +/- 4 km s-1, which is exceptionally low for such a star. Walborn (1983) has set an upper limit on the projected rotation velocity of HD 96446 at v sini < 30 km s-1. Of the 9 He-strong stars with accurately- determined periods, six have periods less than 2 d; of these, the values of v sini range from 170 to <30 km s-1. The three remaining stars all have v sini < 30 km s-1. Therefore, the existing sample is compatible with periods of 0.85 or 5.6 d and a v sini of < 30 km s-1. This work was financed by grants from the Natural Sciences and Engineering Research Council of Canada to Drs. W.H. Wehlau, J.D. Landstreet, and R. Mitalas. JAYMIE M. MATTHEWS Dept. of Geophysics & Astronomy University of British Columbia Vancouver, B.C., V6T 1W5 Canada DAVID A. BOHLENDER Dept. of Astronomy University of Western Ontario London, Ontario, N6A 3K7 Canada References: Bohlender, D.A., Brown, D.N., Landstreet, J.D. and Thompson, I.B.: 1987, Ap. J. 323, 325. 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