2 The data and data reduction

Figure 1: Digitized sky survey image of NGC6231 (20 $\hbox {$^\prime $ }$$\times$20 $\hbox {$^\prime $ }$, N is up, E is to the left). The three superimposed boxes show the fields covered by our observations. The smallest field was observed with the Dutch 0.9-m telescope (ESO), the medium-size field was observed with the Danish 1.54-m telescope (ESO) in 1998 June and the large field was observed in 2000 June and July, also with the Danish 1.54-m. See Table 1 for further details.

Data were obtained at ESO during 6 individual observing runs in 1998-2000, as outlined in Table 1. The observations in 1998-1999 were optimised for the fainter stars (13$^{\rm m}$-14$^{\rm m}$) while the observations in 2000 were optimised for the several magnitudes brighter $\beta$ Cephei stars. Two different ESO telescopes were used, the Dutch 0.9-m (D90) and the Danish 1.54-m (D154). Three overlapping fields of different sizes were covered by the time-series observations, as shown in Fig. 1.

The data obtained at the D90 were bias-corrected and flat fielded using (standard) techniques described in e.g. Arentoft & Sterken (2000). Observations were made in the V passband and covered about 2 hours of time series during 13 individual nights.

Two output amplifiers were used for the CCD readout at the D154 in 1998 June and also in this case only the V-filter was used. The time was shared with another programme and only during one night was a time string longer than 6 hours collected. Significant pointing offsets from night to night imply that some stars are only present on some of the frames. As the severe overexposure of the bright stars caused charge bleeding on the CCD, a correction for cross-talk between the two amplifiers had to be applied. Because the number of useful V sky-flats from the individual nights was limited, we used one combined (median) master flat for the whole run. Science and flatfield images were corrected for the amplifier cross-talk following Freyhammer et al. (2002). Then bias subtraction was made by subtracting constants measured in the overscan regions and finally the science images were corrected for gain variations using the combined flatfield.

The observations in 2000 June and July with the D154 were done in uvby. Also in this case, significant pointing offsets occurred from night to night. Owing to bad weather during parts of the two runs, relatively few flatfields in each filter were collected. We therefore combined the y flatfields from one good night of both runs and created master flats for June and July, respectively. For uv and b, we created one master flat in each filter and applied it to both the June and July data. The images did not contain an overscan strip so the zero-point levels were estimated from evening, night and morning BIAS frames. A linear change in the zero-point level during the night was found and corrected for.

The photometric reductions were carried out in all cases using MOMF (Kjeldsen & Frandsen 1992). This very robust, semi-automatic program combines PSF and aperture photometry, uses a local sky-background determination and calculates differential magnitudes relative to a weighted mean level of all detected stars.

From the data obtained with the D90 we extracted light curves of 210 stars; from the 1998 June data 928 stars were processed. The data obtained in 2000 produced light curves of 376 stars. As the observed fields (Fig. 1) overlap, we can search for suspected variability of some stars in independent data sets.

 

 

Table 2: New (except for SBL0455) faint variable stars in NGC6231 based on the deep V-data. See the text for discussion of the individual stars. The frequency solutions were obtained to fit our data as well as possible, but are only tentative. "Segg.'' refers to the numbering system of Seggewiss (1968). f₁-f₅ are detected frequencies (in cd^-1), S/N is the signal-to-noise ratio of the frequency having the highest (semi-) amplitude (a_V, in mmag). The frequency analysis is typically based on 700 to 800 datapoints per star.

SBL	BVF	Segg.	BL95	V	B-V	f1	f2	f3	f4	f5	S/N	aV	Notes
0306		247	14	13.987	0.668	15.6	9.8	23.8			5.9	6.5	$\delta$ Sct star
0332				15.713	1.217	1.9	1.1	5.3	4.4		69.8	57.2
0417	279			14.618	1.154	1.1					10.4	18.7	PMS (SBL)
0432	287	84		14.692	0.696	30.8	31.0	38.4	4.5	0.3	7.1	2.6	$\delta$ Sct star
0455	178	237	41	13.455	0.611	23.8	21.8	18.9			15.1	10.7	$\delta$ Sct star (BL95)
0508	237	233	50	14.120	1.082	0.3	3.1	4.7			30.8	12.3
0595		100	1	14.334	0.585	41.4	36.2	41.1			5.5	1.6	$\delta$ Sct star
0598		101	2	13.701	0.448	3.6	1.05				7.6	2.7
0628		98		15.033	1.258	1.2	3.4				9.3	36.1	$\gamma$ Dor star?
0636	324			15.118	0.886	1.9	4.5	8.9			4.7	1.6
0646	372			15.671	0.828	2.1	3.7	4.8			14.0	4.8	$\gamma$ Dor star?
1949	949			18.28	1.55	0.67	1.01				23.6	111.0
ASK0171*				16.2		1.2	0.96	1.5			9.4	48.8	$\gamma$ Dor star?

* Not in SBL; $\alpha_{2000}=3.174$, $\delta_{2000}=-5.585$ (coordinates in arcminutes relative to $\alpha_{2000}=16^{\rm h}$54$^{\rm m}$12$^{\rm s}$, $\delta_{2000}=-41^{\circ}$50 $\hbox {$^\prime $ }$30 ${\hbox{$^{\prime\prime}$ }}$, as in SBL).