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Astron. Astrophys. 364, 712-722 (2000)

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2. Observations

Our observations were taken during the nights of June 29 and June 30, 1993 at the Palomar Observatory, using the 1.5-meter telescope and Palomar 6 Tektronix CCD camera, using a 1024x1024 array with 24 micron pixels and low readout noise (8.2 e-). The telescope and CCD provide a field of view of 6.3 arcminutes, with a plate scale of 15.48 arcsecs/mm. The limited field of view of the CCD allowed us to select fields which were contained exclusively within regions of the cloud which were bright in IRAS 100µm emission, as well as within regions of detectable emission in 12CO. Our 12CO maps were derived from the Bell Labs 10-meter telescope, and were kindly provided in advance of publication by Dr. Marc Pound. A view of the selected areas for our survey may be seen in Fig. 1, which presents an IRAS 100µm image from the ISSA survey (Wheelock et al. 1994), and the 20 selected areas indicated on the figure. The survey region concentrates on the Southern edge of the cloud, which is brightest and presumedly the densest portion of the cloud. Fig. 2 presents a more detailed view of the region studied, with the entire field shown from a mosaic of Digital Sky Survey plates, and the selected areas indicated. The coordinates and identifications for the selected area are presented below in Sect. 3.

[FIGURE] Fig. 1. IRAS 100µ image of the Draco Cloud, showing the wispy "cometary" nature of the cloud, and the selected areas for this work.

[FIGURE] Fig. 2. Digital Sky Survey of the central Draco Cloud Region, showing in more detail the locations of our CCD fields observed with the Palomar Observatory 1.5-meter telescope

CCD frames were taken for all of the 20 shown fields in the V and B filters during the first night, and a subset of these fields was observed during the second night in the U, V, B, and H[FORMULA] filters. The filters used were multilayer custom U, B, and V filters designed by Dr. Jim Schombert of Caltech. The H[FORMULA] filter was a 10 nm wide narrow-band filter centered at the rest wavelength of 6562 Å.

Each field was observed for at least 10 minutes in each of the filters, and duplicate frames were combined to improve the detectability of fainter stars. A summary of the observations is presented in Table 1. The fields were flat-fielded using dome flats and comic-ray and bias offsets were removed from the images. Instrumental magnitudes were obtained using the DAOFIND and DAOPHOT routines in IRAF, after carefully adjusting aperture sizes and testing background subtraction algorithms. Table 2 summarizes the fields studied. The 20 selected areas were chosen for their proximity to the dense cores in the Draco cloud, and were arranged in a grid of pointings spaced by the CCD field of view, and centered on the position at [FORMULA] where radio maps indicate the peak of CO emission from the Draco Cloud. The arrangements of the CCD fields is shown in Fig. 2, which includes a digitized POSS image of the region near the Draco Cloud.


Table 1. Summary of observations.
1) These 362 stars with good B and V photometry form the basis of the catalog in Appendix A. The 465 stars with V magnitudes were used for star-count analysis.
2) These 75 stars with detectable U, H[FORMULA], B and V flux form the basis of Appendix B.


Table 2. Coordinates of selected area CCD fields

During the first night observations CCD photometric standard stars from Landolt (1992) were observed frequently at various airmasses to allow for removal of extinction and to derive transformation coefficients for converting the instrumental magnitudes into the standard Johnson B and V colors. Extinction corrections were performed using the standard star PG1525+071 observed at three different airmasses, and a linear least squares fit to the instrumental magnitudes indicated that the extinction correction for the range of airmass in our observations is [FORMULA]V [FORMULA] 0.1 magnitudes.

A total of 47 separate standard star observations during the first night were used to perform a linear least squares fit between instrumental and Johnson V and B magnitudes. The transformation coefficients are defined by:



where small letters indicate instrumental magnitudes, and the transformation coefficients [FORMULA],µ,[FORMULA], and [FORMULA] are presented in Table 3. The transformation to the standard system appears accurate for our standard stars to within 0.1 magnitudes over the range of values of -0.25 [FORMULA] [FORMULA] [FORMULA] 2.5.


Table 3. Transformation coefficients for BV photometry.
a) Uncertainties indicated in parentheses

A total of 465 stars were measured in V for the first night, and these stars were used for the star-count analysis of Sect. 3. A slightly smaller sample of 362 stars had good photometry in the B band, and V and [FORMULA] magnitudes are presented for this sample in Appendix A. A histogram of V magnitudes for the 362 stars of the catalog (solid line), and the 465 stars with V magnitudes (dashed line) is presented in Fig. 3a, while a similar histogram of [FORMULA] magnitudes of the 362 stars in our catalog is presented in Fig. 3b.

[FIGURE] Fig. 3. (above) Histogram of the V magnitudes for the 362 stars of the catalog in Appendix A (solid line), and the 465 stars with V magnitudes (dashed line). (below) Histogram of the [FORMULA] magnitudes for the catalog.

During the second night, we observed a subset of stars in all four passbands U, B, V, and H[FORMULA]. The quantum efficiency for the CCD was too low to allow for useful determination of [FORMULA] for all of the stars, so we extracted magnitudes in U, V, B, and [FORMULA] for the 75 stars in 7 fields which had detectable U magnitudes. The results for these stars are presented in Appendix 2, and 35 of these stars were included in the larger catalog of Appendix 1.

Fig. 4 shows a comparison between V magnitudes between the two nights, using the transformation coefficients in Table 3. The two nights agree to well within the uncertainty of 0.1 magnitudes, although there appears to be a systematic offset between the two nights of 0.04 magnitudes in V. We believe that the systematic error arises from the night 2 photometry, which had fewer standard star observations than the night 1 photometry.

[FIGURE] Fig. 4. Comparison between V values derived from the two nights of photometry, using the sample of stars common to both nights with detectable U fluxes. With the exception of a systematic offset of 0.04 magnitudes, the two nights agree to well within the stated photometric uncertainty of 0.1 magnitudes.

Fig. 5 shows a comparison between [FORMULA] colors between the two nights, using the transformation coefficients in Table 3. The two nights again agree to within the uncertainty of 0.1 magnitudes, with a similar small systematic offset between the two nights of 0.04 magnitudes.

[FIGURE] Fig. 5. Comparison of [FORMULA] colors derived from the two nights of photometry, using the stars common to both nights with detectable U fluxes. Again, a systematic offset of 0.04 magnitudes is seen, but the photometry is consistent within the estimated uncertainty of 0.1 magnitudes.

In addition to the standard V and [FORMULA] magnitudes, we have derived an index [FORMULA] which combines the observations at H[FORMULA] with the V magnitude. The H[FORMULA] counts for each star were converted to instrumental magnitudes, and for simplicity we used the same transformation constant CV for getting the [FORMULA] magnitude. The [FORMULA] index was observed as a possible basis for photometric classification, and can be used to detect chromospherically active stars (Grebel et al. 1992). We also discuss the effects of reddening on the [FORMULA] index and its possible use for photometric spectral classification in Sect. 3.3.

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Online publication: January 29, 2001