COMMISSION 27 OF THE I. A. U. INFORMATION BULLETIN ON VARIABLE STARS Number 839 Konkoly Observatory Budapest 1973 October 25 SEVENTEEN NEW FLARE STARS IN THE PRAESEPE-REGION We have continued the survey for flare stars in the region of the Praesepe cluster initiated by G. Haro and E. Chavira (1, 2) and L. Rosino (3). The observations centred at RA = 8h37m05s, D = 19d51'(1950) have been carried out with the Schmidt telescopes of the Byurakan Observatory (40" and 21" telescopes) and the Konkoly Observatory (24"). 17 new flare stars have been discovered during 247,8 hours of effective coverage. One (No. 4 Haro) of the already known stars showed three more flares. The intention of this communication is to report briefly about the observations. Table 1 gives some data concerning the used telescopes and the plate-filter combinations. Table 1 Tel. Scale Field Plate-filter Average square deg. combinations limiting magnitudes u pg Kodak 103aO Kodak 103aO 40" 97"/mm 16.3 Orwo ZU-2 Orwo ZU-2 17.8 18.0 UG-2 2mm 24" 112"/mm 17.2 Kodak 103aO Kodak 103aO 17.2 17.3 UG-1 1mm 21" 114"/mm 22.6 - Kodak 103aO - 16.7 Orwo ZU-2 The method of observations was the common one of multiple and equal exposures of 4,5 or 10 minutes in pg and 10 minutes in U. Table 2 shows the distribution of effective observational times, the number of the exposures, the number of the plates as well as the number of the discovered flares for each telescope. 40" telescope 21" telescope 24" telescope Synchr.with 21" Synchr. with 40" pg u u Sp pg pg pg u Total Obs. 23h15m 44h05m 15h51m 1h40m 109h25m 17h31m 19h52m 33h50m 265h20m time Numb. 279 255 95 10 1144 105 293 203 2384 of exp. Numb. of 52 54 17 5 156 19 38 26 367 plates Numb. of 2+1* 4 1** - 7 1** 2 3+1* 20 flares Remark: * The same flare star. ** The same flare star. Table 3 gives some data of the observed flares as follow: Column 1. The serial number of the flare stars discovered at the Byurakan (By) and Konkoly (K) Observatories. Column 2 and 3. Coordinates for 1950. Column 4. The approximate photographic magnitudes at minimum. Column 5. The observed amplitude of the flare in pg or in U light. Column 6 and 7. The date of the flare-up and the telescope use. Table 3 Designation RA D mpg Delta_mpg Date of Tele- 1950.0 flare-up scope By 1 8h33m9 19d29'4 17.9 3.3 22.03.1971 21" By 2 40.5 19 22.9 19.1 2.6 22.04.1971 40" By 4 39.2 20 7.9 17.5 1.4 10.92.1972 21" By 5 39.7 18 44.6 17.4 1.2 10.02.1972 21" By 6 29.7 19 34.7 16.1 0.9 13.02.1972 21" By 7 44.5 19 37.6 18.0 1.9 13.02.1972 21" By 8 27.7 19 12.3 15.6 0.7 09.03.1972 21" K 2 38.3 17 59.7 18.5 1.0 14.03.1972 24" By 9 35.7 21 56.5 16.6 3.4u 17.03.1972 40",21" By10 38.3 18 56.0 18.3 0.8 19.03.1972 40" By11 40.0 18 24.9 18.6 2.5 06.04.1972 21" By12 41.2 21 14.7 18.2 2.0u 05.01.1973 40" By13 44.9 18 32.4 18.3 3.9u 07.01.1973 40"' K 3 35.7 19 26.1 18.9 4.8u 29.01.1973 24" By14 38.6 18 24.8 17.3 2.5u 27.02.1973 40" By15=K4 38.7 18 55.5 18.4 1.9 05.03.1973 40",24" K 5 39.7 20 52.5 15.3 1.6u 24.03.1973 24" Table 4 shows the repeated flares of the star No. 4 of Haro's list (2). Table 4 Designation RA D mpg Delta_mpg Date of Tele- 1950.0 flare-up scope K 1 8h37m9 18d35'2 18.9 1.6 21.01.1972 24" 4.3u 29.11.1972 24" By3. 4.7u 10.02.1973 40" =No.4 Haro Our observations augment the number of the known flare stars in the Praesepe region to thirty. One of them (No. 4 of Haro's list is probably not a cluster member as the difference between its positions on the Palomar Sky Survey and on our plates reveals a rather large proper motion for this star. At present the total effective coverage for the Praesepe cluster is about 390h (1,2,3 and this paper). Three or more flare-ups of the same star have not been observed (exception No. 4 Haro). A short calculation according to the formula given in (5) suggests that the number of flare stars in the Praesepe cluster must be close to N = summa for k (n_k) = 146 (k = 0,1,2, ...) Coming out from the earlier experience (4, 5, 6, 7) it is probable that the number of flare stars in Praesepe calculated in this way is close to the real one. Already these preliminary results show a lower flare activity in the Praesepe cluster than in the Pleiades. Details of our observations, identification charts light curves of the flares and discussions will be published in forthcoming papers. I. JANKOVICS Byurakan and Konkoly Observatories References: 1. G. Haro, R. Chavira, Vistas in Astronomy Vol. 8, 89. [BIBCODE 1966VA......8...89H ] 2. G. Haro, Flare Stars, Stars and Stellar Systems, Vol. VII. p. 141. 3. L. Rosino, Asiago Contr. 189, 1966. 4. V. Ambartsumian, L. Mirzoyan, etc. Astrofizika 6, 7, 1970. [BIBCODE 1970Afz.....6....7A ] 5. V. Ambartsumian, L. Mirzoyan, etc. Astrofizika 7, 319, 1971. [BIBCODE 1971Afz.....7..319A ] 6. V. Ambartsumian, L. Mirzoyan, etc. Astrofizika 8, 485, 1972. [BIBCODE 1972Afz.....8..485A ] 7. V. Ambartsumian, L. Mirzoyan, etc. Preprint of Byurakan Observatory, 6. 1973. 8. G. Haro, E. Chavira, Ton. Bull., Vol. 5, 31, 1969. [BIBCODE 1969BOTT....5...23H ]