COMMISSION 27 OF THE I. A. U. INFORMATION BULLETIN ON VARIABLE STARS Number 3009 Konkoly Observatory Budapest 13 April 1987 HU ISSN 0374-0676 On the Constancy of Some Stars in NGC 2287 = Messier 41 The Pleiades-age open cluster NGC 2287 contains several luminous red bright-giants and spectroscopically interesting stars. Three of these stars have previously been suspected of low-level variability. In order to confirm this and ascertain the nature of the variations, differential photometry was obtained over two observing seasons. The stars were observed with the Lowell Observatory 53cm photometric telescope. Two groups of stars were arranged in trios observed in the pattern: star 2-1-3-2..., switching filters at the completion of each cycle. A single nightly observation consisted of one cycle of measurement in each color, the measurements being the mean of six ten-second integrations on 'star' compensated by the mean of two integrations on "sky." Diaphragms of 29 or 49 arcsec diameter were used depending on seeing. The data were reduced to instrumental magnitudes, accounting for differential extinction by the use of mean monthly extinction coefficients determined with this specific telescope/filters/detector system in an ongoing program initiated in 1972 (Lockwood and Thompson 1986). The first group observed in the cluster consisted of three K-type bright-giants, identified by designations assigned by Cox (1954). Pertinent information about the stars is given in Table I. Table I Membership Star HD V B-V MK probability Remarks Group 1 1=Cox 21 49091 6.908 1.503 K3 IIb 0.81 =ADS 5437 2=Cox 97 49105 7.803 1.147 K0 IIab-IIb 0.89 3=Cox 75 49068 7.436 1.254 K1.5 II-III 0.90 MK standard Group 2 1=Cox 102=`a' 49126 7.275 0.592 F8 IV-V + B9.5 V 0.87 2=Cox 21 49091 6.908 1.503 K3 IIb 0.81 =ADS 5437 3=Cox 107=`f' 49212 7.781 1.136 K0 IIab 0.90 BV photometry and cluster proper-motion membership probabilities are drawn from Ianna et al (1987); the MK classifications are by Levato and Malaroda (1979), except Cox 75, which is an MK standard in the 1985 list of Keenan and Yorka (1985). Of this group, Schmidt (1984) suspected variability in Cox 75 and 97 based on DDO photometry on two nights. The group was observed in the present program using Stromgren y and b filters on thirteen nights from 1985 March 31 to November 7 UT. The second group included two bright-giants (Cox 21 carried over as a tie-in to group 1) and Cox 102, which has a composite spectrum of hotter stars. In the course of their photometric study of the cluster, Feinstein et al (1978) found Cox 102 to be variable with a V range of 0.09 mag from ten observations over a four-month interval. This trio was measured on twelve nights between 1985 November 20 and 1986 March 1 UT using Stromgren y, b, and v filters. Table II Pair Delta y Delta b Delta v sigma sigma sigma Group 1 -0.909 -0.672 - 1-2 6 6 1-3 -0.539 -0.362 - 5 6 2-3 0.370 0.310 - 7 4 Group 2 1-2 0.394 -0.094 -1.340 4 5 9 1-3 -0.495 -0.745 -1.445 4 4 6 2-3 -0.889 -0.651 -0.105 6 6 11 Table II summarizes the photometry, showing pairwise differential magnitudes and the standard deviation of these means on the second line of each entry. Because Messier 41 culminates at 1.8 airmasses from Flagstaff, the precision of the data (sigma ~ 5 millimagnitudes in y and b) is roughly half what we expect for constant stars of similar brightness located north of the celestial Equator (cf. Skiff and Lockwood 1986). Nevertheless the consistency of sigma for each filter suggests it is reasonable to conclude that all five stars were constant to better than 1% over the interval covered by the observations. Indeed, for group 1, whose measures were split between two observing seasons, the interseasonal means differ on average by only 2.5 millimagnitudes. Sowell (1986) also reports differential UBV photometry of Cox 21, 97, and 107. A dozen measures were obtained over a four-day baseline in November 1985. These indicate short-term (less than several hours) constancy to within a few millimagnitudes. These results exclude variations with time scales of hours to months, but longer-term changes are not ruled out. Brian A. Skiff Lowell Observatory Mars Hill Road, 1400 West Flagstaff AZ USA 86001 References: Cox, A.N. 1954, Ap.J., 119, 118. [BIBCODE 1954ApJ...119..188C ] Feinstein, A., Cabrera, A.L., and Claria, J.J. 1978, Astr. & Ap. Suppl., 34, 241. [BIBCODE 1978A&AS...34..241F ] Ianna, P.A., Adler, D.A., and Faudree, E.F. 1987, A.J., 93, 347. [BIBCODE 1987AJ.....93..347I ] Keenan, P.C., and Yorka, S.B. 1985, CDS Bull. Inf., 29, 25. [BIBCODE 1985BICDS..29...25K ] Levato, H., and Malaroda, S.1979, Pubs.A.S.P., 91, 636. [BIBCODE 1979PASP...91..636L ] Lockwood, G.W., and Thompson, D.T. 1986, A.J., 92, 976. [BIBCODE 1986AJ.....92..976L ] Schmidt, E.G. 1984, Ap.J. Suppl., 55, 455. [BIBCODE 1984ApJS...55..455S ] Skiff, B.A., and Lockwood, G.W. 1986, Pubs.A.S.P., 98, 338. [BIBCODE 1986PASP...98..338S ] Sowell, J.R. 1986, Ph.D. dissertation, Yellow Supergiants in Open Clusters, University of Michigan. [BIBCODE 1986PhDT.........4S ]