COMMISSION 27 OF THE I. A. U. INFORMATION BULLETIN ON VARIABLE STARS NUMBER 462 Konkoly Observatory Budapest 1970 September 1 FLARE ACTIVITY OF PROXIMA CENTAURI, 1969 Flare activity of Proxima Centauri was discovered by Shapley (1949, 1951, 1954) from a search of Harvard plate material. The activity reported is considerable, with delta m_pg greater than 0. 5 magnitudes about six percent of the time. Photoelectric monitoring of Proxima was initiated at Cerro Tololo in 1969 to determine the present level of activity, and further, to expand the meager data on flare activity of the least luminous flare stars. A listing of events recorded in 23.6 hours of U-band photometry during the 1969 observing season appears in table 2, following observational procedures described elsewhere (Kunkel 1968). A 90-cm reflector wast used on the first two nights, and one of 40-cm aperture on the remaining nights. Comparison stars listed in table 1 and identified in figure 1 were used on all nights. Probable errors in V and (B-V) are 0.01, Table 1. Comparison Stars for Proxima Star V B- V U-B A 11.77 0.85 0.54 B 11.75 0.20 -0.33 C 11.33 0.75 0.26 and in (U-B) 0.015 magnitudes. Given for each night in table 2 are the event U. T., the corresponding airmass, and the U-magnitudes at peak light, columns (1) through (3). Columns (4) and (5) list the durations at 0.5 and 0.1 peak light, respectively. R.m. s. uncertainties greater than ten percent are indicated by a colon. Decay rates at 1, 2, and 3 magnitudes below peak light appear in columns (6) through (8). They are expressed as the common logarithm of the decay in magnitudes per minute. A colon is used to indicate r.m.s. uncertainties greater than 0.1. A letter "c" indicates a complex time-history, such as events with multiple peaks or rapid changes in slope, denoting situations in which a meaningful measurement was difficult to make. Completeness of the sample is bounded by two lines: (1) For each night sampling is at least 90 percent complete for events with u = 14.4 or brighter. (2) Descriptive parameters of flare light are not significantly biased by the recorder response (with a one second time constant) for events with T_0,5 greater than 0.08 minutes. Since no flares of so short a duration were recorded, the influence of instrumental response on estimates of peak light is assumed to be negligible. A flare incidence relation giving the event rate for flares brighter than magnitude u, taking the form R(u) = exp [a(u - u_o)] events hr -1 represents the observations well (see figure 2) above a 90 percent completeness level of u_lim = 14.4. The constants take the values a = 0.78 _+ 0.12 and u_o - 14.69 _+ 0.15. Table 2. Flare Abstract, Proxima Event Airmass U_peak T_0.5 T_0.1 tau_1 tau_2 t_3 Notes U.T. 7 Feb. 1969 7h01m0 - 9h00m3 3 events T=1h988 8h22m60 1.24 13.58 0.15 0.68 +0.99 +0.8: 8 23.40 1.24 13.91 .52 2.6: c 8 43.53 1.22 14.46 .37 - 8 Feb. 1969 7h03m1 - 9h00m5 1 event T=1h957 8h48m16 1.21 15.05 0.13 - 14 April 1969 1h52.6 - 9h41m4 11 events T=7h813 2h10m87 1.47 14.29 1.5 -0.27 2 29.32 1.40 12.10 .13 1.52 +0.96 -0.08 -0.46 note 1 3 02.98 1.33 11.87 - 1.9: - - -0.09 note 2 3 54.51 1.25 13.18 .31 1.35 +0.49 +0.45 .0: 4 57.85 1.18 13.75 .40 +0.12 5 29.0 1.18 14.17 13.2 32: -1.05 5 31.13 1.18 12.89 .39 2.0 +0.36 0.0 -0.21 5 45.33 1.18 14.20 .18 +0.09 5 46.96 1.18 13.03 .45 1.45 +0.5 c +0.23 -0.21: 7 50.69 1.28 15.26 2.2 - 9 14.3 1.46 15.58 2.2 - 15 April 1969 0h25m4 - 3h52m4 3 events T=3h450 1h02m 1.70 14.34 4. - 1 22.5 1.60 12.72c 7.2 15.8 -0.68 -0.71 -0.82 note 3 3 01 1.32 15.02 11.5 - 17 April 1969 0h57m6 - 9h18m5 10 events T=8h348 1h09.65 1.62 12.77 0.14 3.7 +0.86 -0.41 -0.41 1 32.78 1.54 14.64 .18 - 1 50.35 1.48 14.54 .5: - 1 54.98 1.46 14.37c .6,c - note 4 2 55.83 1.32 12.73 .4 4.4 +0.26 +0.07 -0.55 4 08.20 1.22 14.05 .58 +0.18 5 14.29 1.18 14.93 .3 - 6 43.61 1.18 14.31 .09 - 6 53.86 1.22 14.77 .40 - 7 05.68 1.23 13.27 .10 1.04,c +1.36 +0.57 Note 1 tau_4 = -0.99: 2 tau_4 = -0.67:"fast event", peak lost while seeking proper gain setting. 3 Three peaks 4 Double peak separated by 2.5 minutes in time, [FIGURE 1] [FIGURE 2] A direct comparison of the activity of Proxima Centauri with that of other flare stars of low luminosity is facilitated by a transformation of the flare activity parameter u_o to a scale of absolute magnitudes, M_u,o. If we assume that the constant "a" takes of a unique value, for all stars (data presently available are not sufficient to reject this hypothesis), the value of M_u,o serves as a comparative measure of flare activity: lower values denoting greater activity. Three stars of low luminosity for which measures of M_u,o are available are Wolf 424 A, B, UV Ceti, and Wolf 359, with M_u,o = 17.8, 17.2 and 18.15 respectively. Activity of the binaries is assumed to be divided equally between components since the components are of practically equal luminosity. The value of M_u,o for Proxima is 19.1, indicating a level of flare activity that is only moderate. It appears quite possible that flare activity was more pronounced at the time of Shapley' s observations, since the upper limit in flare activity proposed by Kunkel (1970) for stars of the luminosity of Proxima is about M_u,o = 17. Cerro Tololo Inter-American Observatory + 1970 August 21 WILLIAM E. KUNKEL + Operated by the Association of Universities for Research in Astronomy, Inc. , under contract with the National Science Foundation. References Kunkel, W.E. 1968, I.B.V.S. No. 315. Kunkel, W.E. 1970, in press. Shapley, H. 1949, Harvard Announcement Card No. 1010. Shapley, H. 1951, Proc. National Academy of Sciences 37, 15. [BIBCODE 1951PNAS...37...15S ] Shapley, H. 1954, A.J. 59, 118. [BIBCODE 1954AJ.....59..118S ]