SUPPLEMENT SERIES Astron. Astrophys. Suppl. Ser. 124, 353-357 (1997) UBV absolute CCD photometry and differential astrometry of close visual double stars, with G-type primaries?,?? T. Nakos1,2, D. Sinachopoulos1, and E. van Dessel1 1 Royal Belgian Observatory, Ring Laan 3, B-1180 Ukkel, Brussels, Belgium 2 University of Thessaloniki, Department of Physics, Section of Astrophysics Astronomy and Mechanics, GR-54006 Thessaloniki, Greece Received May 29; accepted November 27, 1996 Abstract. We present V magnitudes, (B − V )and 2. Observations (U−B) colours of close visual double star components, and their differences as well as separations and position The observations were carried out using the 90 cm Dutch angles of 40 visual double stars. The common properties telescope and its CCD camera at ESO La Silla, Chile, of the members of this sample are the small angular sep- between the 25th of April and 1st of May, 1994. The ESO aration ρ, which is less than 700, and the common spectral No. 29 CCD chip was installed in the camera. It is a chip type of the primaries, which are of G-type. The observa- of the type TEK 512CB, thinned and AR coated for im- tions made with the Bessel U, B and V filters, while the proved quantum efficiency. Its dimensions are 512 × 512 astrometry was performed in the V filter only. For the ob- pixels of 27 × 27 microns each. Its linearity goes up to servations, the CCD camera attached to the Cassegrain 265000 e−/pixel remaining always better than 0.5%, which focus of the 90 cm Dutch telescope, at La Silla, Chile, had makes it an excellent tool for observing relatively bright been used. From the analysis of the data we concluded targets, as the binaries of our sample. that from the sample of the fourty double stars observed We have selected a fixed window 200 × 200 pixels wide seven proved to be physical pairs, while the rest must have on the CCD chip and we centered all double stars observed common origin components. and the photometric standard stars used in this window before starting the exposures. We could save a lot of time Key words: binaries: visual — catalog reading out only this part of the CCD after each exposure. This improved significantly the efficiency of our observa- tions. The window remained the same all nights. On the other hand, we used the whole surface of the CCD chip for the exposures aimed at its astrometric calibration. 1. Introduction According to the schedule followed, all visual binaries Until recently and due to technical difficulties (Rakos presented here were observed in only one filter during each 1982) only few photometric measurements of close vi- night. Observing a target in different filters in the same sual binaries have been published. For these reasons, our night with CCDs is a time consuming method: as the focal knowledge on this stellar system is still insufficient. length of the telescope depends on the filter properties, it Under the visual binaries those systems with solar type is obvious that the instrument must be refocused when- components are especially interesting for understanding ever a different filter is used. On the contrary, using the the evolution of the Sun. The present work is the first same filter during the whole night increases significantly attempt to study them systematically: this is the first pa- the efficiency of the observations. per in which we present absolute photometry of close vi- Eight exposures per double star and the extinction star sual double stars with G - type primaries. The sample has were taken in V in the night of April 26 and four in the been chosen in such a way that the secondaries are usually night of April 30. The duration of the exposures was as G - type stars as well. long as possible, at least two seconds and typically around Send offprint requests to:D.Sinachopoulos five seconds. Eight exposures per double star and the ex- ? Based on observations made at ESO La Silla, Chile. tinction star were taken in B in the night of April 27 and ?? Tables 1 and 2 are only available in electronic form at the four in the night of May 1. The duration of the exposures CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) was at least three seconds and typically around six sec- or via http://cdsweb.u-strasbg.fr/Abstract.html onds. Finally, only three exposures per double star and the extinction star were taken in U in the night of April In order to estimate the scale of the CCD we used sev- 28, since the duration of the exposures was between fifty eral stars on images of M 16 taken in the same night of and ninety seconds. The lower efficiency of the observa- April 26 through the V filter as well. A detailed astromet- tions during the last three nights is due to the installation ric work on this cluster has been published in the paper of and tests of the new data acquisition system of the CCD Tucholke et al. (1986). According to it the typical internal camera, which is based on a new camera controller and a errors of α and δ positions are both 000. 03. The minimum DEC Alpha workstation computer. We should note that accuracy (for δ = 0) in the position of one star, is at the 29th of April a lot of time was lost due to major 2 2 2 2 problems of this newly installed data aquisition system of σpos = (σα cos δ) +(σδ) ' (σα) +(σδ) the telescope. In addition, the night was not photometric. √ = pσδ 2. p We preferred to use some of the stars in the central re- Since the error in the angular separation of two stars of gion of M 16 open cluster for the estimation of the scale of the M 16 cluster depends on the accuracy in the position the CCD. For the calculation of the instrumental position of each one of them, the standard deviation will be angle of the CCD camera we used traces of stars in M 16 √ as well, taken on separate CCD frames. 2 2 σdd = (σpos1) +(σpos2) = 2 σpos1 =2σδ During the first night (of April 25 to 26) 18 photomet- q00 ric standard stars from the E regions were observed, in =0.06. all filters, for the calculation of the transformation coef- According to the same paper, the mean error in the proper ficients into the UBV system. Five exposures per filter motion for the stars observed is 100. 4/1000 yrs in both com- were taken. ponents. Since the difference of time between the epoch of The atmospheric conditions under which the observa- the observations and the epoch to which the paper refers tions were carried out were very good. During all nights is ∆t = 1994.4 − 1950 = 44.4 years, the error in both seeing at the 90 cm Dutch telescope was varying from 1.2 components is to 1.5 arcseconds but at the NTT (new technology tele- scope at ESO La Silla) was usually much better, while 00 σµ = σµ =0.0616 the temperature was fluctuating from 12 ◦C − 14 ◦C. δ α cos δ Furthermore, the data reduction showed that all nights and as we are interested in the angular separation of two used for photometric observations were indeed photomet- stars, the error due to the proper motion uncertainty will ric. be By the end of the observations more than 1500 CCD 00 frames were taken. The data reduction was performed by σp.m. =0.1232. using the MIDAS image processing software, while in some cases, we also used DAOPHOT. It is evident that a bias Moreover the error in the difference in the position of two offset has been subtracted and a flat-field correction has stars does not depend on the error in the proper motion, also been performed. the standard deviation in the angular separation will be σ = (σ )2 +(σ )2 =000.137. 3. Astrometric data reduction ρ dd p.m. q The night of April 26 was chosen for the astrometric part We used several star pairs in M16 having angular sepa- of this work, due to the good seeing value at Cerro La ration around 600 pixels on the CCD chip, in order to Silla of far below one arcsecond. Good seeing produces determine the telescope scale with high accuracy. The er- stable star traces of high quality, which enable an accurate ror we derived for the scale factor in that way is determination of the right ascension direction. As it has σρ been already mentioned, the angular separation (ρ) as well σ = ' 0.00022 (00/pixel). scale 600 as the position angle (θ) for every binary was calculated in the V filter only on frames taken in this night. By comparing the positions of the stars detected by In order to define the right ascension direction on the DAOPHOT with the values presented in the paper men- CCD chip, several trails of stars in M 16 open cluster were tioned above we calculated the scale factor of our op- taken during this night using the V filter. Obviously, all tical system, which was s =0.46249 ± 0.0002200/pixel the CCD surface was used for taking these traces. In an (27 µ/pixel). This agrees with the scale calculated for this earlier work, Sinachopoulos (1988) presented the tech- telescope by van Dessel & Sinachopoulos (1993). nique of star traces for CCD cameras. Using several star For the error of the angular separation ρ due to the trails per frame on six different frames taken one after the atmospheric refraction we took σr(ρ) = 0.