2. The observations

The present observations were carried out at the European Southern Observatory (La Silla, Chile) over the 15-31 March 1991 period, by using the 0.5 m ESO telescope equipped with a single-channel photon-counting photometer, a thermoelectrically cooled Hamamatzu R-943/02 photomultiplier and standard ESO filters matching the system. In order to obtain accurate differential photometry, for each program star (v) a comparison (c) and a check (ck) star were also observed (see Table 1 (click here)). The measurement of a star consisted in the average of 10-15 1-s integrations in each filter, according to the color sequence. A complete observation consisted in sequential c-v-v-v-v-ck-c measurements. From these data, after accurate sky subtraction, four v-c and one ck-c differential magnitudes were computed; the four v-c values were finally averaged to obtain one data point. The observations were corrected for atmospheric extinction and transformed into the standard system. The nightly atmospheric extinction coefficients were obtained by observing two standards of very different spectral types in the 1-2.5 air mass range. Their mean values over the whole period are reported in Table 2 (click here). These coefficients were obtained few months before the June 1991 eruption of Mt. Pinatubo. Transformation coefficients were inferred by observing E-region standard stars (Menzies et al. 1989). The typical 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 counting level. The standard deviations () for the v-c and ck-c mean differential V-band magnitudes obtained over N nights are reported in Table 1 (click here). The V magnitudes and colors of the comparison and check stars were obtained via standard stars (Menzies & Laing 1988; Menzies et al. 1989; Menzies et al. 1991) and are given in Table 3 (click here). For each program star the brighest V magnitude and corresponding colors are listed in Table 4 (click here). Taking into account the accuracy of the standard stars data and the extinction and transformation errors, the typical accuracy of the absolute photometry in Tables 3 (click here) and 4 (click here) is of the order of 0.01 magnitudes, with somewhat larger values (up to 0.02 magnitudes) for U-B colors.

 

Program star (c) (ck) 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

Table 1: Comparison (c) and check (ck) stars for each program star (v) and standard deviations () for the v-c and ck-c V-band differential magnitudes for each series of N nights

 

 

U B V R I

Mag/airmass .520 .274 .157 .107 .053

Table 2: Mean atmospheric extinction coefficients for La Silla site, obtained over the 15-31 Mar. 1991 period

 

 

c or ck V U-B B-V

HD 33747 8.76 0.72 0.98 0.53 1.03

HD 33763 8.37 0.50 0.89 0.49 0.95

HD 35230 7.58 0.47 0.88 0.48 0.93

HD 36316 7.95 1.72 1.46 0.79 1.50

HD 37653 8.24 0.50 0.89 0.48 0.93

SAO 170680 8.97 0.91 1.04 0.54 1.01

HD 39901 6.54 1.63 1.37 0.71 1.32

HD 39962 7.97 -.06: 0.40 0.26 0.50

HD 61333 7.18 -.64: -.11 -.05 -.09

HD 61390 7.92 0.70 1.01 0.52 1.03

HD 81904 8.02 0.72: 0.98 0.50 0.98

HD 80991 8.51 0.83: 1.04 0.52 1.02

HD 82477 6.11 1.18 1.18 0.61 1.17

HD 82508 7.58 0.35 0.71 0.40 0.80

HD 85849 8.52 1.07 1.14 0.58 1.12

HD 86034 7.89 1.32 1.26 0.63 1.20

HD 97977 8.85 1.66: 1.42 0.75 1.42

HD 98251 9.21 1.47: 1.31 0.69 1.28

HD 101215 7.79 0.07 0.15 0.10 0.20

HD 101679 8.12 0.79 1.09 0.59 1.15

HD 102076 7.10 0.75 1.00 0.51 0.98

HD 102202 8.86 0.19 0.67 0.38 0.73

HD 105796 8.06 0.93 1.06 0.55 1.04

HD 106270 7.58 0.29 0.72 0.40 0.77

HD 119076 6.87 0.88 1.15 0.60 1.15

HD 119164 7.20 1.14 1.29 0.64 1.22

HD 128227 8.33 0.82 1.07 0.57 1.10

HD 128618 8.03 1.63: 1.46 0.77 1.47

HD 136480 7.35 1.12: 1.16 0.60 1.15

HD 137241 7.36 1.10: 1.14 0.59 1.11

HD 138363 7.12 0.95 1.14 0.61 1.15

HD 139070 8.71 0.80 1.12 0.60 1.16

HD 154775 7.60 1.97 1.59 0.87 1.75

HD 157427 7.41 1.69 1.50 0.81 1.55

HD 173560 8.72 0.11: 0.64 0.36 0.70

HD 176557 7.19 1.76: 1.48 0.78 1.47

HD 197237 9.02 0.83: 0.98 0.51 0.96

HD 197339 7.39 1.44: 1.29: 0.66 1.23

Table 3: V magnitude and colors for the c and ck stars derived from standard stars. Errors are of the order of 0.01 magnitudes. The symbol ":" denotes errors of the order of 0.02 magnitudes

 

 

Program Star U-B B-V Spectral Type

HD 32918 = YY Men 7.98 0.67 1.04 0.59 1.14 K1 III

HD 36705 = AB Dor 6.89 0.37 0.83 0.49 0.97 K0 IV/V (PMS)

HD 37847 = TW Lep 7.35 0.76 1.09 0.60 1.16 K2/3 III + F6 IV

HD 39917 = SZ Pic 7.85 0.31 0.81 0.47 0.91 K0/1 IV + G5 IV

HD 61245 = V344 Pup 6.85 0.80 1.04 0.55 1.05 K1 III

HD 81410 = IL Hya 7.33 0.74 1.03 0.55 1.09 K2 IV

HD 82558 = LQ Hya 7.81 0.56 0.91 0.52 1.02 K2 V

HD 86005 7.18 1.01 1.31 0.69 1.34 K4 III + K1/2 III

HD 98712 AB = SZ Crt 8.60 1.21 1.33 0.86 1.65 K7 V + M3 V

HD 98712 A 8.74 1.30 0.83 1.56 K7 V

HD 98712 B 11.25 1.6 1.1 2.4 M3 V

HD 101309 = V829 Cen 7.82 0.49 0.93 0.53 1.04 K1/2 IV + G5 V

HD 102077 = V838 Cen 8.92 0.53 0.91 0.54 1.08 K1/2 V + K2 V

HD 106225 = HU Vir 8.59 0.58 0.98 0.57 1.13 K1 IV/III

HD 119285 = V851 Cen 7.69 0.81 1.09 0.61 1.20 K2/3 IV

HD 127535 = V841 Cen 8.55 0.82 1.08 0.61 1.18 K2/3 IV

HD 136905 = GX Lib 7.30 0.73 1.03 0.57 1.10 K1 III + G5 IV

HD 139084 = V343 Nor 8.06 0.37 0.82 0.48 0.93 K0 IV/V (PMS)

HD 155555 = V825 Ara 6.87 0.38 0.82 0.49 0.93 G5 IV + K0 IV (PMS)

HD 174429 = PZ Tel 8.44 0.31 0.78 0.45 0.87 G9 IV/V (PMS)

HD 197481 = AU Mic 8.73 1.12 1.45 0.94 2.06 M1 V

Table 4: Maximum luminosity () and corresponding colors for the program stars and inferred spectral classification (Spectral Type)