COMMISSION 27 OF THE I. A. U. INFORMATION BULLETIN ON VARIABLE STARS Number 3631 Konkoly Observatory Budapest 8 July 1991 HU ISSN 0374-0676 U and I band observations of stellar flares Introduction: Most photometric observations of stellar flares were obtained with a time resolution of about 10 s, allowing for successive measurements in different spectral bands (e.g. UBV). In the case of the observations reported here, we preferentially observed only in one given band in order to obtain well defined light curves and study the flare fine structure. This allowed us to discover two new types of stellar flares of great interest that will be investigated elsewhere (see also Houdebine, 1990). Observations: The observations were acquired in March 1990 with the 1m telescope at the European Southern Observatory equipped with its standard photometer. 4 and 2 nights were dedicated to observations respectively in the U and I Johnson's bands. Weather conditions were good to excellent. We give a compendium of the flare observations in Table 1. The second column gives the total duration of the observations for a given star. The sky count level is given in the last column. The flare magnitudes were computed with respect to the stellar quiescent level prior to a given flare. Only one flare was observed in the I band. In the U band the time exposure was 3s and the effective time resolution lies between 4 and 5 seconds due to the data printing time delay. In the I-band, the exposure time was 1s which was almost equal to the effective time resolution. Other stars were observed on which no flare were detected: CC Eri, Gl182, Gl494, Gl516 AB, Ross 845 and V371 Ori were observed in the U-band respectively for 0.77h (on the 06/03-1991), 0.78h (07/03), 0.7h (04/03) and 1.0h (06/03), 1.25h (05/03) and 1.15h (07/03), 1.5h (06/03), 1.4h (06/03). Similarly, Ross 614, AD Leo, Gl 494, V371 Ori, Wolf 424 AB, YZ CMi were observed in the I-band for respectively 2.3h (08/03), 0.9h (08/03) and 2.9h (09/03), 1.45h (09/03), 1.9h (08/03), 0.9h (08/03) and 2.1h (09/03). The light curves of the main flares are shown in Figure 1. Note that the ordinate axis only gives the photon count rate. We refer to Table I for the sky count level. During the night of the 6 March 1990, Ross 614 exhibited an almost permanent flaring state. The largest flare (in magnitude) was observed on YZ CMi with a 2.87 mag increase. The maximum brightness is reached only 28 seconds after the flare onset. Before this major event, two flare precursors were detected at 09:04:17 and 09:07:36 Sidereal Time. Wolf 424 AB and Gl 644 AB large flares exhibit secular light curves. The former displays evidence for periodic changes in brightness, and the latter seems a combination of homologous and sympathetic flares. These flares will be discussed in forthcoming studies (see also Houdebine, 1990). [FIGURE 1] fig. 1: Flare light curves. [FIGURE 2] fig. 1 continued: Table 1: Magnitude, observed duration, and decay time of the flares. star Obs. Date t_1/e Duration mag. UT Band Sky name (h) UT (min) (min) h :min:s AD Leo 3.45 04/03 - 2.3 0.04 23:37:53 U 103 dM4.5e 05/03 2.376 15 1.18 00:36.0* U " 06/03 - - - - U 268 07/03 0.95 2.2 0.21 04:07:31 U 252 " - 0.9 0.07 04:21:21 U " " - 1 0.07 04:29:29 U " " 0.552 7 0.66 05:02:06 U " - 0.6 0.18 05:21:03 U " GL 644AB 2.5 07/03 0.787 60 1.20 07:27:45 U 45 dM3.5e 07/03 0.428 60 1.67 07:39:33 U " Prox Cen 1.85 05/03 - - - - U 130 dM5e 0.8 06/03 0.9 4 0.59 09:04:40 U 122 2.1 08/03 - 9 0.10 08:07:00 I 340 Ross 614 0.5 05/03 - - - - U 260 dM4.5e 1.15 06/03 - 2.5 0.27 02:25:59 U 387 06/03 - 2.2 0.35 02:34:34 U " 06/03 - 2.1 0.32 02:49:55 U " 06/03 - 9.7 0.60 02:52:40 U " 06/03 - 3.7 0.52 03:07:11 U " Wolf 424 1.9 04/03 1.43 60 2.50 01:27:47 U 113 dM5.5e 04/03 - - 1.16 01:49:30 U " 04/03 - 1.12 02:00:31 U " 04/03 - - 1.03 02:09:05 U " 04/03 - - 1.42 02:24:42 U " 04/03 - - 0.59 02:56:48 U " YZ CMi 1.3 04/03 0.94 21.7 2.87 09:08:22** U 791 dM4.5e 04/03 - - 0.36 09:04:08** U " 0.9 07/03 2.17 - 0.64 02:00:32 U 1030 * at maximum **Sidereal Time E.R. HOUDEBINE DUNSINK OBSERVATORY DUBLIN INSTITUTE FOR ADVANCED STUDIES CASTLEKNOCK, DUBLIN 15, IRELAND. Reference : HOUDEBINE, E.R.: 1990, PhD. Thesis of the Universite d'Orsay - Paris XI, France.