Long-Term Monitoring of Active Stars

ASTRONOMY & ASTROPHYSICS JANUARY II 1998, PAGE 207 SUPPLEMENT SERIES Astron. Astrophys. Suppl. Ser. 127, 207-215 (1998) Long-term monitoring of active stars ?,?? VII. UBV(RI)c photometry collected in March 1991 G. Cutispoto Osservatorio Astrofisico di Catania, V.le A.Doria 6, I-95125 Catania, Italy e-mail: [email protected] Received April 21; accepted May 5, 1997 Abstract. In the framework of an extensive program luminosity, thus supporting the cool starspot hypothe- focusing on the global properties and evolution of sis. However, anticorrelation of the U − B and B − V active stars, high-precision UBV(RI)c photometry of color indices with respect to the V-band light modu- 19 selected stars, collected at the European Southern lation has been observed for some stars such as V711 Observatory over the 15-31 March 1991 interval is pre- Tau, TW Lep and UX Ari (Cutispoto 1992; Rodonò& sented. Significant evolution of the light curves, period Cutispoto 1992; this paper). The orbital/photometric pe- variations and evidence for long-term variability of the riods of active stars span from less than one day to sev- global degree of spottedness are found. Most of the spec- eral weeks and the photometric waves can undergo notice- tral classifications are discussed. A flare event was de- able changes over time scales as short as few stellar rota- tected for the star HD 127535 = V 841 Cen. These obser- tions (cf. Figs. 12, 18 and 23 in Cutispoto 1995). Hence, vations contribute to the establishment of a time-extended in order to investigate the physical characteristics and photometric database which can give important clues on evolution of spotted areas and the time scale of activ- topics such as the stability of spotted areas, differential ity cycles, active stars must be observed systematically. rotation, solar-like cycles and the correlation between in- This continuous monitoring program, already started at homogeneities at different atmospheric levels1. Catania Astrophysical Observatory in the early Sixties, is being carried out by using the 0.5 and 1.0 m telescopes Key words: stars: activity — stars: flare — stars: of the European Southern Observatory (ESO, La Silla, late-type — stars: variables — techniques: Chile), the 0.25 m Automatic Photoelectric Telescope of photometric — stars: HD 127535 Franklin & Marshall College at Washington Camp. (AZ, U.S.A.) and the 0.8 m Automatic Photoelectric Telescope of Catania Astrophysical Observatory on Mt. Etna (Italy). It is aimed at establishing the time-extended database that is essential to investigate fundamental topics such 1. Introduction as the evolution of spotted areas and spot lifetimes, the presence of photospheric solar-like activity cycles One of the most typical signatures of stellar activity is and differential rotation (see, among others, Cutispoto & the presence of photospheric inhomogeneities, whose vis- Rodonò 1992; Budding & Zeilik 1995; Rodonò 1992a,b; ibility, modulated by the stellar rotation, produces peri- Lanza et al. 1997), the temporal and/or spatial corre- odic or quasi-periodic light variation typically of the or- lation between inhomogeneities at different atmospheric der of 0.1 − 0.2 magnitudes in the V-band (cf. Rodonò levels (Pagano et al. 1992, 1993; Pallavicini et al. 1993; 1992a,b and references therein). Multicolor photometry Kürster 1996; Kürster et al. 1994, 1997; Catalano et al. shows, in most cases, a reddening of the star at minimum 1996). This paper reports on data obtained by using the Send offprint requests to:G.Cutispoto 0.5 m ESO telescope and is organized as follows: the de- ? based on data collected at the European Southern tails on the equipment, observations and reduction proce- Observatory, La Silla, Chile. dures are given in Sect. 2, the results and the discussion ?? Tables and the complete data set are also available in on individual stars are presented in Sect. 3. electronic form at the CDS via anonymous ftp to cdsarc.u- strasbg.fr or via http://cdsweb.u-strasbg.fr/Abstract.html 1 Figure 1 to 24 are only available on the on-line version of A&A at http://www.ed-phys.fr 208 G. Cutispoto: Long-term monitoring of active stars. VII. Table 1. Comparison (c)andcheck(ck) stars for each program star (v) and standard deviations (σ) for the v − c and ck − c V-band differential magnitudes for each series of N nights Program star (c) (ck) σ σ N (v) (v − c) (ck − c) HD 32918 = YY Men HD 33763 HD 33747 54 4 13 HD 36705 = AB Dor HD 35230 HD 36316 17 6 11 HD 37847 = TW Lep HD 37653 SAO 170680 41 4 16 HD 39917 = SZ Pic HD 39901 HD 39962 38 5 13 HD 61245 = V344 Pup HD 61390 HD 61333 22 5 12 HD 81410 = IL Hya HD 81904 HD 80991 46 4 14 HD 82558 = LQ Hya HD 82508 HD 82447 25 4 12 HD 86005 HD 86034 HD 85849 4 2 10 HD 98712 = SZ Crt HD 97977 HD 98251 20 4 11 HD 101309 = V829 Cen HD 101679 HD 101215 50 4 13 HD 102077 = V838 Cen HD 102076 HD 102202 16 4 13 HD 106225 = HU Vir HD 106270 HD 105796 72 4 16 HD 119285 = V851 Cen HD 119164 HD 119076 14 2 13 HD 127535 = V841 Cen HD 128227 HD 128618 33 4 17 HD 136905 = GX Lib HD 136480 HD 137241 49 4 14 HD 139084 = V343 Nor HD 139070 HD 138363 60 4 13 HD 155555 = V824 Ara HD 157427 HD 154775 25 7 16 HD 174429 = PZ Tel HD 173560 HD 176557 21 5 17 HD 197481 = AU Mic HD 197237 HD 197339 15 8 9 ported in Table 2. These coefficients were obtained few Table 2. Mean atmospheric extinction coefficients for La Silla months before the June 1991 eruption of Mt. Pinatubo. site, obtained over the 15-31 Mar. 1991 period Transformation coefficients were inferred by observing E- U B V R I region standard stars (Menzies et al. 1989). The typical Mag/airmass .520 .274 .157 .107 .053 error of the differential photometry is of the order of 0.005 magnitudes, with somewhat larger values (up to 0.01 magnitudes) in the U-band due to the low photon count- 2. The observations ing level. The standard deviations (σ)forthev−cand ck − c mean differential V-band magnitudes obtained over The present observations were carried out at the European N nights are reported in Table 1. The V magnitudes and Southern Observatory (La Silla, Chile) over the 15-31 colors of the comparison and check stars were obtained March 1991 period, by using the 0.5 m ESO telescope via standard stars (Menzies & Laing 1988; Menzies et al. equipped with a single-channel photon-counting photome- 1989; Menzies et al. 1991) and are given in Table 3. For ter, a thermoelectrically cooled Hamamatzu R-943/02 each program star the brighest V magnitude and corre- photomultiplier and standard ESO filters matching the sponding colors are listed in Table 4. Taking into account UBV(RI)c system. In order to obtain accurate differen- the accuracy of the standard stars data and the extinction tial photometry, for each program star (v) a comparison and transformation errors, the typical accuracy of the ab- (c) and a check (ck) star were also observed (see Table 1). solute photometry in Tables 3 and 4 is of the order of The measurement of a star consisted in the average of 0.01 magnitudes, with somewhat larger values (up to 0.02 10 − 15 1-s integrations in each filter, according to the magnitudes) for U − B colors. U − B − V − Rc − Ic color sequence. A complete obser- vation consisted in sequential c − v − v − v − v − ck − c measurements. From these data, after accurate sky sub- 3. Results traction, four v − c and one ck − c differential magnitudes were computed; the four v −c values were finally averaged The present multicolor photometry has been used to in- to obtain one data point. The observations were corrected vestigate the light curve evolutions, to search for the pres- for atmospheric extinction and transformed into the stan- ence of photospheric solar-like activity cycles and, using dard UBV(RI)c system. The nightly atmospheric extinc- the method described by Cutispoto et al. (1996), to in- tion coefficients were obtained by observing two standards fer or further constrain the spectral type and luminosity of very different spectral types in the 1 − 2.5airmass class of program stars (see Table 4). Color indices of active range. Their mean values over the whole period are re- stars have to be taken prudently when used for spectral G. Cutispoto: Long-term monitoring of active stars. VII. 209 and references therein). It has shown two of the most Table 3. V magnitude and colors for the c and ck stars derived intense and longest duration radio flares (Slee et al. from standard stars. Errors are of the order of 0.01 magnitudes. The symbol “:” denotes errors of the order of 0.02 magnitudes 1987a; Bunton et al. 1989) and probably the longest op- tical flare ever recorded for any class of active stars (Cutispoto et al. 1992). The data obtained over the 15-29 c or ck V U − B B − V V − Rc V − I c Mar. interval are shown in Fig. 1, where phases are reck- HD 33747 8.76 0.72 0.98 0.53 1.03 oned from the 9.5476-day photometric period computed HD 33763 8.37 0.50 0.89 0.49 0.95 by Collier (1982a). The asymmetric V-band light curve is HD 35230 7.58 0.47 0.88 0.48 0.93 HD 36316 7.95 1.72 1.46 0.79 1.50 single-peaked and its maximum results about 0.045 mag- HD 37653 8.24 0.50 0.89 0.48 0.93 nitudes fainter than the brighest values observed to date SAO 170680 8.97 0.91 1.04 0.54 1.01 (cf.

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