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CCD Standards for U and I in the Open Cluster NGC 7790?,??,???

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CCD Standards for U and I in the Open Cluster NGC 7790?,??,??? A&A 376, 745–750 (2001) Astronomy DOI: 10.1051/0004-6361:20011001 & c ESO 2001 Astrophysics CCD standards for U and I in the open cluster NGC 7790?,??,??? G. Petrov1, W. Seggewiss2,A.Dieball2,andB.Kovachev1 1 Institute of Astronomy, Bulgarian Academy of Sciences, Sofia, Bulgaria e-mail: [email protected]; [email protected] 2 Universit¨atssternwarte Bonn, Auf dem H¨ugel 71, 53121 Bonn, Germany e-mail: [email protected]; [email protected] Received 17 November 2000 / Accepted 12 June 2001 Abstract. Photometric U and I standard sequences in the field of the open cluster NGC 7790 are presented. The intention is to achieve wide ranges in magnitude and colour, making these sequences suitable for calibrating deep CCD photometry. The 84 standard stars extend the BVR sequences of Odewahn et al. (1992) to the near UV and IR, respectively. Key words. techniques: photometric – astronomical data bases: miscellaneous – open clusters: individual: NGC 7790 1. Introduction summarized under the names Johnson-Kron-Cousins (for its intricate history, see Landolt 1983). 1.1. Photometric standard sequences for imaging In 1985, Christian and co-workers published six photo- detectors electric BVRI standard sequences suitable for video cam- Photometric calibration of the two-dimensional CCD de- era and CCD calibration. They had selected between 6 and tectors has to be based on standard sequences which 12 stars in or near 6 clusters (M 92, NGC 2264, NGC 2419, should fulfil a number of fundamental requirements: the NGC 4147, NGC 7006, and NGC 7790). Seven years later, standard stars should cover as wide as possible a range in Odewahn et al. (1992, OBH) extended three of the pre- colour and should reach to faint magnitudes. The field of vious sequences (NGC 4147, NGC 7006, and NGC 7790) view should have the typical dimensions of a CCD field, to fainter limits and to wider ranges in colour by means approximately 50 × 50, and the crowding of stellar images of CCD observations. However, they restricted the pho- should be a minimum. Of course, the internal and exter- tometric bands to B, V ,andR. A suitable U standard nal errors of magnitudes and colours should approach the sequence as well as a wide standard range in I were, so limits of feasibility. far, still missing. Practically all modern calibrations refer ultimately to 1.2. Standard stars in the open cluster NGC 7790 the homogeneous photoelectrically observed set of stan- dard stars by A. U. Landolt (1983). The underlying pho- In 1995, the project “Structure of the Galaxy: evolu- tometric system is often called UBVRI system for sim- tion and kinematics of open clusters in the anticentre plicity. However, it relies on a combination of systems region” was started as a common investigation of the from the northern and southern hemispheres that can be Universit¨atssternwarte Bonn, Germany, and the Institute of Astronomy of the Bulgarian Academy of Sciences, Sofia. Send offprint requests to: W. Seggewiss, (Details and results of the project will be given later.) For e-mail: [email protected] calibration purposes we used several well-known standard ? Based on observations collected at the National sequences in star clusters, e.g. in M 67 (Montgomery et al. Astronomical Observatory Rozhen, Bulgaria. 1993), and in M 92 (Majewski et al. 1994), but most of ?? This work is part of a common project between the our photometry was calibrated with standard stars in the Universit¨atssternwarte Bonn, Germany, and the Institute of open cluster NGC 7790. Astronomy of the Bulgarian Academy of Sciences to study bi- The coordinates of NGC 7790 are RA = 23h58m. 4and nary open star clusters and clusters in the direction of the ◦ 0 Galactic anticentre. Dec = +61 13 (2000). Therefore, most of the year it can ??? Full Table 4 is only available in electronic form at the CDS be observed from northern sky observatories. Being an via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) intermediate-age open cluster (approx. 120 Myr, Gupta or via et al. 2000), it is suitable for calibration of different types http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/376/745 of astronomical objects, like clusters and distant galaxies. Article published by EDP Sciences and available at http://www.aanda.org or http://dx.doi.org/10.1051/0004-6361:20011001 746 G. Petrov et al.: U and I standard sequences in NGC 7790 Table 1. Observational data for NGC 7790 from the 2 m RC 16 stars from Sandage’s list with reference to the Landolt telescope. standards; – Pedreros et al. (1984, PMF): photographic observations date filter No. of frames scale seeing calibrated by Sandage’s U sequence which they had cor- per filter [00/px] [00] rected by 0.075 mag due to an apparent offset in the U 1998-02-28 U, B, V , R, I 20.62≤2 scale (Sandage’s observations are too blue). 1998-08-23 U, B, V , R, I 4 0.62 1.5 ... 2 The stars of these lists are spread over an area of about 100 × 100 around the centre of the cluster. The dynamical 1998-08-23 U, B, V , R, I 2 0.31 1.5 ... 2 interval of these data – in magnitudes and colours – is not 1998-09-06 U, B, V , R, I 10 0.62 2.5 ... 3 large enough for CCD receivers and improved techniques of data reduction. From these lists we find 4 AAF stars and 9 PMF stars The basic sample of our U and I calibration is a list (from the corrected Sandage sequence) which coincide of 13 stars (“primary standards”) in NGC 7790 with ob- with BVR standard stars from OBH. We chose these served magnitudes in the passband U which refer to the 13 stars as primary standards in the passband U.They fundamental standards of Landolt (1983); see Sect. 3. This are listed in Table 2 with their OBH numbers; the first list is enlarged to a total of 84 stars which are in com- 4 stars are those from AAF, the following 9 from PMF. mon with the improved BVR standards from OBH (1992); Note that we did not use star no. OBH-31 (= AAF-36), see Sect. 5. The ranges in magnitudes and colours are, because this star has an elliptical shape and is always re- e.g., 13.15 <V <18.52, 0.39 <B− V<1.71, and jected automatically in the reduction process. 0.25 <V− R<1.28 (always given in mag). The stars We used the same 13 stars to construct our primary are spread over a field of view of 50 × 3.05 to the south-east standard sequence in the passband I. The first 4 stars (see of the cluster’s centre. Table 2) have I magnitudes from the CCD work of Romeo We note for the sake of completeness only that et al. (1989). For the remaining stars we can refer to the Schmidt (1981) published Str¨omgren photometry of stars basic sequence of Christian et al. (1985). in NGC 7790. Recently Stetson (2000) published Landolt calibrated BVRI data for about 240 stars in a field of 60×60 centred on NGC 7790. 4. Methods of calibration For the final calibration we used a three-step iteration 2. Observations and data reduction method for the following reason: (1) The UBVRI data of the 13 primary standards are taken from different sources The basic observational data for NGC 7790 are presented with different reliability. The aim of the process is to ho- in Table 1. These frames have been taken with the mogenize the mixed sample and to minimize the influ- “Photometrics” CCD camera at the 2 m RC telescope ence of bright primary stars determined with lower ac- of Rozhen observatory. The detector SITE SI003AB has curacy. (2) The original sequence covers only a narrow 2 1024×1024 px, with a pixel size of 24×24 µ . Two regimes interval of colours and the distribution in magnitude is of observations are at disposal to the observer: (1) Gain = rather nonuniform. The iteration allows us to enlarge the 4.93, RON = 1.05 ADUs and (2) Gain = 1.21, RON = interval of magnitudes and colours of the standards. 00 2.73 ADUs. The scale is 0. 31/px without binning and Photometric transformation coefficients were deter- 000. 62/px with binning. At the RC focus of the 2 m tele- 0 0 mined in three steps using the following transformation scope the field of view is 5 × 5 . The filter system is close relations: to Johnson’s UBV (including a read-leak suppression fil- ter), and Kron/Cousins’ RI: Ust = a0U + a1U · Uin + a2U · (Uin − Bin)(1) U: 2 mm UG1 + 1 mm BG39; Ist = a0I + a1I · Iin + a2I · (Rin − Iin)(2) B: 1 mm BG14 + 1 mm GG1 + 1 mm BG23; V : 2 mm GG495 + 1 mm GG11; where Mst are the photometrically calibrated magnitudes R: 1 mm OG570 + 1 mm KG3; and Min are the instrumental ones. As the standards and I: 3 mm RG9. After standard image reduction with the programme stars are located in the same field, no ex- MIDAS, profile fitting photometry was carried out with tinction correction is needed (see e.g. Hardie 1962; Strayzis DAOPHOT II (Stetson 1991) running under MIDAS. 1977). • Step (1): Determine the transformation coefficients using the above 13 stars discussed and compute the First Step 3. Constructing the U and I standard sequences Standard Magnitudes (FSSM) for all the objects in the Unfortunately, there are no suitable standard stars for U field of interest.
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