COMMISSION 27 OF THE I.A.U. INFORMATION BULLETIN ON VARIABLE STARS Number 3227 Konkoly Observatory Budapest 11 August 1988 HU ISSN 0374 - 0676 PHOTOMETRY OF ALPHA Ori (BETELGEUSE), GAMMA Ori, AND PHI-2 Ori We present photometry of Betelgeuse with respect to two comparison stars, Gamma Ori (V=1.64, B-V=-0.22) and Phi-2 Ori (V=4.09, B-V=0.95, Johnson et al. 1966, Table 9). Previous data has been published by Krisciunas (1986, and references therein). For V-band data reduction we adopted B-V=1.84 for Betelgeuse, as given by Johnson et al. Data by Fisher, given in Table I, was obtained at Berwick, Ontario, Canada, using a 20 cm Celestron reflector and an Optec SSp-3 solid state photometer. Transformation to the UBV system (determining epsilon_V) was accomplished by differential measurements of 28 and 27 LMi (for which DeltaV=0.378, Delta(B-V)=-1.03, Hall 1983) Differential magnitudes with respect to Phi-2 Ori were then added to the known V magnitude of Phi-2 Ori to obtain the V magnitude of Betelgeuse. Fisher's epsilon_v was typically -0.01. A value of V-band extinction of 0.30 mag/air mass was used in the data reduction. Data by Krisciunas, given in Tables II and III, was obtained at the 2800 m level of Mauna Kea on the island of Hawaii. Krisciunas used a 15 cm f/5.82 Newtonian reflector, an uncooled RCA 931A photomultiplier tube, operated at -1050 V, and UBV filters by Estafilter. His transformation coefficients epsilon_V and mu were obtained from all - sky measurements of UBV standards, typically 9 observations on a given night, but on one occasion 26 observations of 15 stars. Typically epsilon_V ranged from -0.04 to -0.06 but on JD 7205 and 7222 epsilon_v was -0.092 and -0.081, respectively. The average value of mu was 0.940. On JD 7242 Krisciunas found epsilon_v=-0.060 +/-0.004 from observations of 8 standards. That was followed by 10 differential measurements of 27 vs. 28 LMi, for which epsilon_v=-0.046 +/-0.006. From 6 differential measurements on the same pair on JD 7262 he found epsilon_v=-0.050 +/-0.007. On nights when epsilon_v and mu were not determined, recently determined values were adopted. Krisciunas also measured the extinction and reddening as often as possible. From observations on 20 nights from September 1985 to April 1988 the median V-band extinction at the 2800-m level of Mauna Kea on nights without cirrus is 0.173 mag/air mass. It was assumed that k"(v) was 0.00. Table I Photometry of Alpha Ori by Fisher (comp star phi-2 Ori) Date Julian Date V 5/6 Jan 1986 2446936.54 0.666 13/19 Jan 1986 6444.69 0.610 12/13 Feb 1986 6474.54 0.442 29/30 Nov 1986 6764.62 0.542 30 Nov/1 Dec 1986 6765.65 0.539 6/7 Dec 1986 6771.64 0.555 16/17 Jan 1987 6812.60 0.536 25/26 Jan 1987 6821.55 0.531 4/5 Feb 1987 6831.63 0.491 9/10 Feb 1987 6836.61 0.446 14/15 Feb 1987 6841.54 0.465 24/25 Feb 1987 6851.55 0.466 Table II Photometry of alpha Ori by Krisciunas (comp star gamma Ori) Date Julian Date V B-V n_v n_bv 11/12 Oct 1986 1158 2496716.00 0.435 +-0.020 3 29/30 Nov 1986 1097 6764.95 0.544 0.020 3 21/22 Dec 1986 0839 6786.86 0.531 0.014 4 27/28 Dec 1986 0830 6792.87 0.543 0.007 4 21/22 Jan 1987 0932 6817.90 0.562 0.007 1.797 +-0.004 3 3 8/9 Feb 1987 0716 6835.80 0.523 0.014 1.795 0.017 3 3 29/30 Mar 1987 0707 6884.80 0.383 0.024 1.958 0.033 3 3 5/6 Apr 1987 0598 6891.74 0.428 0.011 5 16/17 Apr 1987 0619 6902.76 0.451 0.010 2 24/25 Oct 1987 1041 7093.95 0.555 0.005 1.923 3 1 12/13 Nov 1987 0929 7112.90 0.538 0.009 1.857 3 1 27/28 Dec 1987 0829 7157.85 0.610 0.007 1.857 3 1 23/24 Jan 1988 0708 7184.80 0.602 0.005 1.819 0.010 4 2 13/14 Feb 1988 0839 7205.86 0.478 0.021 1.809 0.025 3 3 1/2 Mar 1988 0710 7222.80 0.511 0.008 1.751 3 1 21/22 Mar 1988 0651 7242.79 0.402 0.010 1.869 3 1 10/11 Apr 1988 0614 7262.76 0.396 0.007 3 16/17 Apr 1988 0625 7268.77 0.385 0.023 3 Table III Photometry of Alpha Ori by Krisciunas (comp star phi-2 Ori) Date Julian Date V B-V n_v n_bv 21/22 Dec 1986 0839 2446786.86 0.508 +-0.008 4 27/28 Dec 1986 0830 6792.87 0.487 0.007 4 21/22 Jan 1987 0932 6817.90 0.535 0.017 1.858 +-0.015 3 3 8/9 Feb 1987 0716 6835.80 0.443 0.005 1.871 0.009 3 3 29/30 Mar 1987 0707 6884.80 0.337 0.005 1.925 0.022 3 3 24/25 Oct 1987 1041 7093.95 0.505 1.989 1 1 12/13 Nov 1987 0929 7112.90 0.482 1.958 1 1 27/28 Dec 1987 0829 7157.85 0.609 1.853 1 1 23/24 Jan 1988 0708 7184.80 0.569 0.004 1.805 0.066 3 2 13/14 Feb 1988 0839 7205.86 0.488 0.009 1.856 3 1 1/2 Mar 1988 0710 7222.80 0.434 0.014 1.790 3 1 21/22 Mac 1988 0651 7242.79 0.408 0.013 1.822 2 1 10/11 Apr 1988 0619 7262.76 0.357 0.011 4 16/17 Apr 1988 0625 7268.77 0.363 0.011 4 Table IV Differential photometry of Alpha Ori vs. phi-2 Ori Date Julian Date DeltaV n_v n_bv 29/30 Nov 1986 0252 2446764.62 -2.519 30 Nov/1 Dec 86 0336 6765.65 -2.475 6/7 Dec 1986 0321 6771.64 -2.510 Krisciunas data: 21/22 Dec 1986 0841 6786.86 -2.486 +-0.008 4 27/28 Dec 1986 0832 6792.86 -2.513 0.011 4 21/22 Jan 1987 0927 6817.89 -2.464 0.013 -1.111+-0.018 3 3 8/9 Feb 1987 0721 6835.81 -2.519 0.003 -1.097 0.010 3 3 29/30 Mar 1987 0712 6884.80 -2.516 0.016 -1.223 0.024 3 3 24/25 Oct 1987 1053 7093.95 -2.500 -1.109 1 1 12/13 Nov 1987 0943 7112.90 -2.520 -1.065 1 1 27/28 Dec 1987 0848 7157.87 -2.446 -1.173 1 1 23/24 Jan 1988 0723 7184.81 -2.470 0.011 -1.184 0.076 4 2 13/14 Feb 1988 0844 7205.86 -2.444 0.028 -1.092 3 1 1/2 Mar 1988 0710 7222.80 -2.505 0.023 -1.122 3 1 21/22 Mar 1988 0658 7242.79 -2.436 0.012 -1.239 2 1 10/11 Apr 1988 0612 7262.76 -2.484 0.009 3 16/17 Apr 1988 0623 7268.77 -2.463 0.015 3 Table V Mean star color differences Pair n_bv Johnson et al. value Alpha Ori - Gamma Ori 2.061 +-0.016 19 2.06 Alpha Ori - Phi-2 Ori 0.923 0.015 17 0.89 Gamma Ori - Phi-2 Ori -1.139 0.015 17 -1.17 [FIGURE 1] Figure 1. V magnitude of Alpha Ori vs. Julian Date for 1986-1987 observing season. Circles: Krisciunas data using Gamma Ori as comparison star. X's: Krisciunas data using Phi-2 Ori as comparison star. +'s: Fisher data using Phi-2 Ori as comparison star. For the data presented here we used a value of k(v) appropriate for the night. For all the B-V color determinations we used the mean coefficients of out-of-atmosphere reddening parameters of: k'(bv) = 0.10; k"(bv) =-0.05. In Table II we give data on Betelgeuse by Krisciunas, reduced using Gamma Ori as comparison star. Given are the local date/UT date, the mean Universal Time, the geocentric Julian Date, the mean V magnitude, B-V color, and numbers of differential v and b-v observations made. Table III gives [FIGURE 2] Figure 2. V magnitude of Alpha Ori vs. Julian Date for 1987-1988 observing season. Symbols same as in Figure 1. [FIGURE 3] Figure 3. Differential V magnitude for Gamma Ori minus Phi-2 Ori, November 1986 to April 1988. +'s: Fisher data. Circles: Krisciunas data. The dotted line corresponds to data given by Johnson et al. (1966). Betelgeuse data by Krisciunas, reduced using Phi-2 Ori as comparison star. Table IV gives data by both authors on Gamma Ori minus Phi-2 Ori. In Tables II to V the errors quoted are the mean error of the mean, without the uncertainties of gain values, transformation coefficients, or atmospheric parameters folded (i.e., they are internal random errors). Figure 1 is the V-band light curve of Betelgeuse in 1986-1987. Figure 2 is for Betelgeuse during 1987-1988. Figure 3 shows Gamma Ori minus Phi-2 Ori from November 1986 to April 1988. Data obtained by Fisher in early 1986 for Betelgeuse is 0.153 magnitudes fainter than values interpolated from data previously published by Krisciunas. Fisher used Phi-2 Ori as the comp star, while Krisciunas used Gamma Ori. The most likely possible sources of error are: 1/ Saturation of Fisher's detector (he used the larger telescope). 2/ Systematic errors in the adopted values of epsilon_v. 3/ Wrong catalog value(s) for comparison star(s). 4/ Variability of comparison star(s). 5/ Systematic errors in adopted gain values for amplifier, given that observations by Krisciunas of Betelgeuse and comparison stars were made on different gains. Krisciunas recalibrated his amplifier gain steps in the lab and double checked the values using observations of standard stars. If these same gain values were correct for the 1985-1986 observing season, that would make Krisciunas's previously published values from 1985-1986 fainter by 0.057 magnitudes. We note that photometry of Alpha Ori vs. Phi-2 Ori by Fisher in January and February of 1987 is in excellent agreement with the two points of Krisciunas obtained in those months, using the same comparison star. Consider the following facts: 1/ Alpha Ori vs. Phi-2 Ori in 1987-1988 shows a relatively smooth light curve. 2/ Alpha Ori vs. Gamma Ori in 1987-1988 shows a relatively ragged light curve. 3/ Gamma Ori vs. Phi-2 Ori from 1986-1988 is ragged. 4/ Other photometry by Stebbins (see Goldberg 1984) indicates that Alpha Ori usually varies smoothly. This would seem to indicate that Gamma Ori is a "standard" star that is variable. Further, Krisciunas finds that all-sky photometry of Phi-2 Ori gives values not statistically significantly different than V=5.09, while Gamma Ori has shown standardized values up to 0.07 brighter than its catalog value. (However, Gamma Ori is usually the bluest star observed, and it is assumed that the V-band transformation is linear) Given our list of 5 sources of error above, we feel that numbers 1, 2, and 5 are under control. Gamma Ori, of spectral type B3 III, could be a Beta Cephei star, and seems variable. Phi-2 Ori, of spectral type K0 III, is not suspected to be variable. Finally, a word about B-V colors. Table V gives mean color differences between pairs of stars, obtained from observations when the stars were high in the sky, so as to minimize the effect of the k" (bv) term. There is considerable scatter of the nightly means given in Tables II, III, and IV. We do not believe these variations are due to actual color variations versus time. The scatter comes from star centering, filter positioning, possible systematic errors in the adopted value of mu, and possible variations of k'(bv) and k"(bv). KEVIN KRISCIUNAS DAVID FISHER Joint Astronomy Centre Geophysics Institute 665 Komohana Street Haraldsgade 6 Hilo, Hawaii 96720 USA 2200 Copenhagen N., Denmark References: Goldberg, L., 1984, Publ. Astr. Soc. Pacific 96, 366. [BIBCODE 1984PASP...96..366G ] Hall, D.S., 1983, I.A.P.P. Communic.No. 11, 3. [BIBCODE 1983IAPPP..11....3H ] Johnson, H.L. et al., 1966, Communic. Lunar and Planetary Lab. 4, 99. [BIBCODE 1966CoLPL...4...99J ] Krisciunas, K.L., 1986, J.A.A.V.S.O. 15, 15. [BIBCODE 1986JAVSO..15...15K ]