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2 0 0 3MNRAS.345. .2 6 9H Mon. Not. R. Astron. Soc. 345 9H 6 .2 Mon. Not. R. Astron. Soc. 345, 269-291 (2003) Distances to Cepheid open clusters via optical and A^-band imaging 3MNRAS.345. 0 0 2 Fiona Hoyle,1* Tom Shanks1 and N. R. Tanvir2 1 Department of Physics, Science Laboratories, South Road, Durham DH1 3LE 2 Department of Astrophysical Sciences, University of Hertfordshire, College Lane, Hatfield ALIO 9AB Accepted 2003 June 20. Received 2003 June 20; in original form 1999 December 29 ABSTRACT We investigate the reddening and main-sequence-fitted distances to 11 young, Galactic open clusters that contain Cepheids. Each cluster contains or is associated with at least one Cepheid variable star. Reddening to the clusters is estimated using \htU — B\B — V colours of the OB stars and the distance modulus to the cluster is estimated via B — V:V and V — K:V colour- magnitude diagrams. Our main-sequence fitting assumes that the solar-metallicity zero-age main sequence of Allen applies universally to all the open clusters, although this point is controversial at present. In this way we proceed to calibrate the Cepheid period-luminosity (PL) relation and find Mv = —2.87 x log P — 1.243 zb 0.09, MK = —3.44 x log P — 2.21 ± 0.10 and absolute distance moduli to the Large Magellanic Cloud (LMC) of 18.54 zb 0.10 from the L-band and 18.48 ± 0.10 from the /Gband giving an overall distance modulus to the LMC of /jLq = 18.51 zb 0.10. This is in good agreement with the previous Cepheid ŸL-K result of Laney & Stobie at /xq = 18.51 zb 0.09 and with the Hipparcos parallax-calibrated Cepheid PL estimate of Feast & Catchpole at /¿o = 18.66 ± 0.10 when no account is taken of the LMC metallicity. We also find that the two-colour U — B:B — V diagrams of two important clusters are not well fitted by the standard main-sequence line. In one case, NGC 7790, we find that the F stars show a UV excess and in the second case, NGC 6664, they are too red in U — B. Previous spectroscopic estimates of the metallicity of the Cepheids in these clusters appear to suggest that the effects are not due to metallicity variations. Other possible explanations for these anomalies are positional variations in the dust reddening law and contamination by foreground or background stars. Key words: Cepheids - open clusters and associations: general - Magellanic Clouds - distance scale. recently challenged in a paper by Feast & Catchpole ( 1997) who de- 1 INTRODUCTION termined the distance modulus to the LMC as 18.70 ± 0.1. This small Determining the value of Hubble’s constant, H0, has been a chal- difference in the distance modulus causes a 10 per cent decrease in lenge to astronomers since the discovery of the Universal expansion estimates of Hubble’s constant. This discrepancy has further moti- in 1927. It is sometimes argued that we are now at the fine-tuning vated us to check the Galactic zero-point of the PL relation. We do stage and many measurements give values for H0 which lie be- this by checking the values of the distance modulus and reddening of tween the hotly argued values of 50 km s_1 Mpc-1 (Sandage) and a sample of 11 Galactic clusters that contain Cepheids via zero-age 100 km s-1 Mpc-1 (de Vaucouleurs), e.g. Tanvir, Ferguson & Shanks main-sequence (ZAMS) fitting. Unfortunately, the clusters contain- -1 -1 (1999) calculated H0 = 67 ± 7 km s Mpc . However, many ing the Cepheids CS Vel and S Nor are not included due to their not of these measurements are based on secondary indicator methods being available for observation during our allocated telescope time. which in turn are dependent on the accuracy of primary indicators Previous work on measuring the reddening and distance to young of distance, such as the Cepheid period-luminosity (PL) relation. open clusters which contain Cepheids via ZAMS fitting has been The well-studied PL relation in the Large Magellanic Cloud (LMC) done using photoelectric and photographic data in optical wave- is usually calibrated via the distance modulus to the LMC and the bands. It is time-consuming to observe a large number of stars previously accepted value was around 18.50. However, this has been using photoelectric observations as each star has to be observed individually, and photographic data can give relatively inaccurate magnitudes and colours. However, charge-coupled devices (CCDs) * E-mail : hoy le @ venus .physics. drexel.edu now make it possible to observe a large number of stars in many © 2003 RAS © Royal Astronomical Society • Provided by the NASA Astrophysics Data System 9H 6 .2 270 F. Hoyle, T. Shanks and N. R. Tanvir different wavebands quickly and accurately. Although CCDs have Table 1. Approximate pointings for the clusters in the study in all already been used for open-cluster studies e.g. Walker (1985a,b), wavebands. Romeo et al. (1989), these have mainly been carried out in BVRL Cluster Pointing 3MNRAS.345. Recently CCDs with improved f/-band sensitivity and more impor- 0 0 tantly, with smaller and better-behaved colour terms with respect to NGC 6649 Star 19 (Madore & Van den Bergh 1975) 2 Johnson photoelectric photometry, have become available, allow- M25 Star 95 (Sandage 1960) ing CCD t/-band photometry to be included in this study. Infrared NGC 6664 Star 5 (Arp 1958) imaging detectors are also now available and although some of the WZ Sgr WZ Sgr detectors used here do not cover as wide an area as optical CCDs, ob- Lynga 6 TW Nor serving the full extent of an open cluster with a mosaic of pointings NGC 6067 Star 136 (Thackeray et al. 1962) is a practical proposition. vdBergh 1 CV Mon TR35 5 arcsec south of TR35 Until fairly recently, good-quality infrared measurements of the NGC 6823 Star j (Guetter 1992) Cepheids themselves were not available and the Cepheid PL relation NGC 129 Star 113 (Arp et al. 1959) has been primarily calibrated in the V-band. Laney & Stobie (1993, NGC 7790 Star E (Romeo et al. 1989) 1994) present infrared data along with V-band magnitudes for a large number of southern hemisphere Galactic Cepheids. Using data in the literature to obtain values for the distance modulus and reddening to the clusters, they calibrated the Cepheid PL relation in the V- and 6 x 300 s in the U-band. Due to the southerly declination of some of /Ubands. However, any errors in the determination of the distance the objects, they had to be observed at high airmass. However, these modulus and the reddening in the previous work would cause an observations were normally used to obtain relative photometry, and error in the PL relation as determined by Laney and Stobie. Hence, calibration frames were observed at as low an airmass as possible in this work, we check the previous measurements of the reddening or during a later observing run at CTIO. The pointings are given and distance modulus obtained from the open clusters. in Table 1. For calibration purposes, standard stars from Landolt The layout of this paper is as follows. In Section 2 we present (1992) were observed. On the one fully photometric night (1997 the observational data and we test the accuracy of the photometry September 21) six Landolt fields were observed at regular intervals and calibration of the data. In Section 3.1 we describe how the throughout the night, with most of these fields containing several reddenings and distances to the open clusters are obtained and in standard stars. Section 3.2 we discuss each cluster individually. In Section 4 we use these values with the magnitudes of the Cepheids to calibrate the 2.1.2 CTIO Cepheid PL relation. In Section 5 we discuss the implications of the results, particularly for the clusters whose U — B:B — V diagrams The observations were made using the CTIO 0.9 m during an ob- do not appear to follow the canonical locus. In Section 6 we draw serving run during 1998 September 24-29. These observations were conclusions. carried out using the 2048 x 2048 Tek#3 CCD with pixel scale 0.384 arcsec pixel-1. The Tek#3 CCD has low readout noise (four electrons) and good quantum efficiency in the t/-band. A CuS04 2 DATA blocked U filter and Harris B, V filters were used. The resulting U passband, corrected for atmospheric extinction, peaks at ^3750 A, 2.1 Observations similar to photoelectric U but with a narrower FWHM of ^350 Â The observations of the Galactic open clusters were taken dur- as opposed to 570 A. The average seeing during the observations ing five observing runs on the Jacobus Kapetyn Telescope (JKT), was around 1.4 arcsec. Approximate pointings are again given in United Kingdom Infrared Telescope (UKIRT), at Cerro Tololo Inter- Table 1. Standard E-region fields from Graham (1982) and standard American Observatory (CTIO), at Calar Alto and on the William stars from Landolt (1992) were observed throughout the night. The Herschel Telescope (WHT) over a 2-yr period. The spread in dec- only photometric night was 1998 September 28 and on this night lination of the clusters and the multiwavelength nature of the study six E-region standard fields and one Landolt standard field con- meant that many different telescopes were required. taining four standard stars were observed. Three new clusters were observed at CTIO and further observations of clusters observed at the JKT were made in cases where the clusters had been observed 2.1.1 JKT in non-photometric conditions only.
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