COMMISSION 27 OF THE I. A. U. INFORMATION BULLETIN ON VARIABLE STARS NUMBER 294 Konkoly Observatory Budapest 1968 August 21 OBSERVATIONS OF THE FLARE STARS G3-33 AND G24-16 The stars G3-33 and G24-16 from the catalog of Giclas, Burnham, and Thomas (1963) were monitored with the sixty-inch reflector of the Cerro Tololo Inter-American Observatory in continuous U-band photoelectric photometry, with the following results. G3-33 This star, of spectral type dM5e (Eggen and Greenstein, 1965) was monitored on the night of 26 July, 1968 from 08^h29.^m4 to 09^h26.^m3 U.T. Against a detection threshold of U_lim=16.6 (defined as a magnitude equal to three times the standard deviation of the noise during the quiescent state recorded with a one second time constant) an event was recorded at 08^h49.^m5 U.T. that reached a peak light U_Peak=14.66 (Delta U=1.06). The logarithms of the decay times measured at 1, 2, and 3 magnitudes below peak light are logt_1=-0.11+-.10, logt_2=-0.81+-.10, and logt_3=-0.91+-.20. ^x ---------------------------------------------------------------------------- x The measurement of a slope 3 magnitudes below peak light, i.e., at U=17.66, is not inconsistent with a detection threshold U_lim=16.6. Measurement of flare decay rates are curve fitting problems. In this instance a straight line was fit to a four minute portion of the record, involving 240 independent sample points. The detection threshold is a statistic governing a single sample point. ---------------------------------------------------------------------------- The decay parameters most nearly resemble those of the flare star AD Leo (Kunkel 1967), a star whose absolute magnitude is 11.07, or 2.83 magnitudes brighter than G3-33 (Lippincott 1967). If the relation between flare decay times and stellar luminosity proposed by Kunkel (1968) applies, then the event observed on G3-33 was one of unusually long duration for this star. [FIGURE 1] G24-16 During the short interval in which this star was monitored, from 05^h45.^m0 to 06^h53.^m6 U.T. on 27 July, 1968, four flares were recorded against a detection threshold U_lim=16.7. The flare data are summarized in table 1 below. TABLE I. Flares of G24-16 Event U.T. U_peak delta U logt_1 logt_2 logt_3 6^h36.^m72 15.94 0.8 +0.64 +0.50 - 6^h37.^m11 15.60 1.0 +0.60 +0.43: -0.54: 6^h48.^m19 16.41 0.6 +0.71 - - 6^h48.^m86 15.11 1.2 +0.41 +0.06 -0.28 The probable error in logt is 0.1 except where a colon is used to indicate 0.2. The decay characteristics are those of a low luminosity star. They may be compared with those of W Ceti, for which logt_1=+0.55, logt_2= +0.13, and logt_3=-0.28. The empirical relation between the flare decay parameter logt_3 and the bolometric magnitude, together with Johnson's (1965) bolometric correction lead to an absolute magnitude of 14.24 for this star, and hence a parallax of 0".17 +-.04. The example offered by the flare of G3-33 should serve as a warning that since only two flares yield the average of logt_3, such a parallax should be considered ad highly tentative. Finding Charts and Comparison Stars The finding charts were redrawn from Giclas et al (1963) to assist the identification of comparison stars. The photometry for all stars is based on a single observation, so that probable errors in V are +-.03, in B-V they are +-.02, and in U-B they are +-.04. TABLE 2. Photoelectric Measures Star V B-V U-B G3-33 12.19 1.76 1.23 A 11.71 .53 +.03 B 13.88 .71 .24 C 13.25 1.15 .48 G24-16 13.07 1.70 1.25 A 11.60 .36 +.18 B 12.37 .34 +.30 C 12.43 1.50 2.07: The U-B measure for G24-16/C is uncertain by 0.15 magnitudes. References: Eggen, O.J., and Greenstein, J.L. 1965, Ap.J., 142, 915. Giclas, H.L., Burnham, R., and Thomas, N.G., 1963, Lowell Observatory Bulletin No. 120. Johnson. H.L. 1965, Ap.J., 141, 170. Kunkel, W.E. 1967, Unpublished Ph.D. dissertation, University of Texas. ____,1968, Proceedings of the Colloquium on Low Luminosity Stars, March 28 - 30, 1968, Charlottesville. Lippincott, S.L. 1967. A.J., 72, 840. August 3, 1968 WILLIAM E. KUNKEL Cerro Tololo Inter-American Observatory