2. Observations and data reduction
2.1. Instrumentation and observations
The CCD frames discussed here were acquired at the Vatican Advanced Technology Telescope (VATT) on Mt. Graham, in Arizona, using the Columbia SpectraSource CCD camera, during the nights of December 2 and 3, 1996. The conditions were photometric during both nights. A short summary of the optical characteristics of the telescope is given in Table 1. The CCD was a 2048 pixel square Tektronic chip. At the plate scale of the VATT, each pixel spans about 0.33 arcsec in the sky, yielding a square field of view of about 11 arcmin on a side. The filters used closely reproduce the standard passbands of the Johnson-Cousin B, V, R and I filters.
Table 1. Optical characteristics of the VATT telescope.
The square field shown in Fig. 1 was surveyed with a mosaic made with four different and partially overlapping pointings. Each of the four fields has been observed with the four filters and with four different exposure times, ranging from 1 to 200 seconds, in order to enhance the dynamic range of the measurements preventing the saturation of the CCD. A total of 64 CCD frames were thus obtained, in addition to the standard calibration frames and to observations of 19 standard stars from Landolt (1992). A list of the CCD frames analyzed here is shown in Table 2.
Table 2. Summary of observed fields
2.2. Data reduction and photometry
The reduction of the data was performed using the IRAF software and in particular the tasks provided in the CCDRED package. At the time of the observations discussed here the VATT was not yet fully operational, and in particular the collimation of the optics was still being worked on. As a result, the optical quality of the individual CCD frames is not ideal, and several different approaches were explored to flat-field the data and to determine the shutter travel time. To optimally reduce the data, use was made of the program frames in addition to the calibration frames. The data reduction process is discussed in detail in Flaccomio (1996).
The `daofind' source detection algorithm was used to detect individual stars in each frame, as well as to determine their position. An analysis of the magnitude distribution of the detected sources (Fig. 2a) shows that the detected sample is complete down to magnitude , with completeness falling rapidly at the fainter magnitudes. We have also established that this completeness limit is quite uniform over our field of view by variously dividing the whole area in several sections and repeating the same kind of semi-quantitative analysis.
The magnitude of each star in the field of view was determined through synthetic aperture photometry, using the IRAF 'daophot' package, using a circular aperture with a radius of 5 arcsec, and a background determined from a 3.3 arcsec wide concentric annulus whose inner radius was 8.3 arcsec. Each program star was then checked for contamination from nearby visible and resolved stars. The instrumental magnitudes thus obtained were converted to the Johnson-Cousin system through the use of the Landolt (1992) photometric standards.
The residuals of the transformations indicate, for stars brighter than , a photometric precision of better than 0.05 mags for a single measurement in each of the bands. In the vast majority of cases, we averaged the results obtained for the same star from all the available frames, leading to an estimated final uncertainty on the resulting magnitudes of 0.05 mags at .
We compared our magnitudes with those of Sung et al. (1997) for the 70 stars in common between the two studies. The median differences (this work vs. Sung at al. 1997) in the B, V and I magnitudes are , and respectively.
The results of the photometry are summarized in Table 3 where the columns represent, in order: a running identification number, the sky coordinates of the object, and the derived magnitudes. A colon beside the identification number indicates those stars whose photometry is uncertain, in most cases because of contamination from nearby stars.
We then proceeded to cross-match our star list with the lists of probable members of NGC 2264 from various works in the literature. This yielded a list of 110 stars in common between our list and those of the examined papers: 52 of these stars are suspected of belonging to the SFR on the basis of their line emission and appear in the lists of either Herbig (1954), Marcy (1980), Ogura (1984) or Sung et al. (1997); another 39 stars have, according to the proper motion study by Vasilevskis et al. (1965), membership probabilities greater than 50%. Finally, 61 of our stars were found to be strong () X-ray emitters by Flaccomio et al. (in prep.). Distributions of V magnitude for these three samples are given in Fig. 2b,c,d.
© European Southern Observatory (ESO) 1999
Online publication: April 19, 1999