COMMISSION 27 OF THE I. A. U. INFORMATION BULLETIN ON VARIABLE STARS Number 3477 Konkoly Observatory Budapest 12 June 1990 HU ISSN 0374 - 0676 PHOTOMETRY OF ALPHA Ori (SEP 1988 TO APR 1990) AND A REASSESSMENT OF SOME PREVIOUSLY PUBLISHED DATA We present photometry of alpha Ori (Betelgeuse), carried out differentially with respect to phi-2 Ori (V = 4.09, B-V = 0.95). The data were obtained using a 15-cm f/5.82 reflector and photometer employing an RCA 931A photomultiplier tube, operated at -1050 volts, and standard UBV filters. Other recent data are published by Krisciunas (1986) and Krisciunas and Fisher (1988), which we shall refer to as Paper I and Paper II. In Table I we give the local date at sunset/UT date, the mean Universal Time of the observations, the geocentric Julian Date, the mean observed V magnitude and B-V color, and the numbers of differential v and b-v observations made. Differential extinction corrections were calculated with measured values of extinction when the stars were at high air mass and either measured or mean extinction values when the stars were high in the sky. On two of the nights (JD's 2447879 and 1880) the data were obtained at a site at elevation 75-m, 25 km south-southeast of Hilo. All other data were obtained at the 2800-m level of Mauna Kea. The mean V-band extinction at the 2800-m level is kv = 0.18 mag/air mass. The mean reddening coefficients are k'bv = 0.11, k"bv = -0.07. The coefficients for transformation to the UBV system were obtained from all-sky observations of UBV standards, and also from differential photometry of 27 and 28 LMi, for which Delta V = 0.378, Delta (B-V)= -1.03. Observations of 27 and 28 LMi from March 1986 through March 1990 give a mean V- band transformation coefficient of epsilon v= -0.054 +/- 0.003. All-sky results give epsilon v = -0.062 +/- 0.005. The mean B-V transformation coefficient is mu = 0.94. In Fig. 1 we present the data of Table I. The check star was gamma Ori. From JD 2447415 to 7574 for gamma Ori we find < V > = 1.614, and from JD 2447599 to 7978 we find < V > = 1.647. The variations of about 0.08 mag in V for gamma Ori (see Paper II) are not confirmed. In Paper II we mentioned that data of Fisher was 0.153 mag fainter Table I Photometry of alpha Ori (comp star phi-2 Ori) Date Julian Date V B-V nv nbv 9/10 Sep 1988 1344 2447415.07 0.739+-0.010 1.862 4 1 12/13 Nov 1988 0940 7478.90 0.757 0.009 1.893 3 1 1/2 Dec 1988 0851 7497.87 0.810 0.012 1.908 3 1 10/11 Dec 1988 0831 7506.85 0.838 0.007 1.865 3 1 16/17 Feb 1989 0647 7574.78 1.003 0.008 3 13/14 Mar 1989 0806 7599.84 0.931 0.007 3 25/26 Mar 1989 0717 7611.80 0.846 0.005 2 3/4 Sep 1989 1430 7774.10 0.702 0.027 11/12 Sep 1989 1996 7782.12 0.732 0.008 1.852 3 1 20/21 Sep 1989 1353 7791.08 0.723 0.015 3 8/9 Nov 1989 1012 7839.93 0.831 0.010 1.832 3 1 4/5 Dec 1989 0824 7865.85 0.750 0.008 3 18/19 Dec 1989 0726 7879.81 0.689 0.013 3 19/20 Dec 1989 0831 7880.85 0.630 0.037 2 11/12 Feb 1990 0729 7934.81 0.574 0.008 2 20/21 Mar 1990 0654 7971.79 0.550 0.013 3 27/28 Mar 1990 0746 7978.82 0.590 0.015 4 17/18 Apr 1990 0620 7999.76 0.606 0.006 3 than data presented in Paper I which was obtained during the same months by Krisciunas. This was due in part to a systematic error in Krisciunas' gain table of 0.057 mag (Paper II). I believe that the rest of the discrepancy is due to a systematic error in the adopted value of epsilon v used to reduce the data from JD's 2446379 to 6533. Epsilon v = -0.094 had been used, a value obtained from all-sky measurements. However, epsilon v = -0.052 was also obtained on 8/9 March 1986 from differential measurements of 27 and 28 LMi, which is close to the long-term average of -0.054 found from that pair. The data from Paper I from JD's 2446379 to 6533 may then be systematically in error by (-0.094) - (-0.054) = 0.040 times Delta (B-V) = 2.061 (the color difference of alpha Ori minus gamma Ori from Paper II), or an additional 0.082 mag. We give in Fig. 2 the data from September 1982 to April 1986, reduced differentially with respect to gamma Ori, using epsilon v = - -0.054 for all nights, and using our updated gain table. Fisher's data from Paper II are also shown in Fig. 2. The agreement is now very good. [FIGURE 1] Figure 1. Photometry of alpha Ori (open circles) and gamma Ori (large dots). Comparison star was phi-2 Ori. [FIGURE 2] Figure 2. Dots: photometry of alpha Ori reduced with respect to gamma Ori. Plusses: data of Fisher from Paper II (reduced with respect to phi-2 Ori). In Paper I and previously published papers we reduced the alpha Ori data using gamma Ori as the comparison star. The great difference in color of these two stars leaves open the possibility of large systematic errors in the results owing to any error in the transformation coefficient. Using phi-2 Ori as the comparison star, as was done in Paper II and here, cuts any such systematic error in half. KEVIN KRISCIUNAS Joint Astronomy Centre 665 Komohana Street Hilo, Hawaii 96720 USA References: Krisciunas, K.: 1986, J.A.A.V.S.O., 15, 15. [BIBCODE 1986JAVSO..15...15K ] Krisciunas, K. and Fisher, D.: 1988, I.B.V.S., No. 3227.