2 Photometry

Observations of NGC 6604 have been carried out with the CCD camera mounted on the 1.0 m telescope of the South Africa Astronomical Observatory (SAAO) at Sutherland on July 2, 1992. The journal of observations is given in Table 1 and a finding chart is presented in Fig. 1. The surveyed area corresponds to $2.1\times 3.3$ arcmin centered on the Of star HD 167971. The reader is referred to Munari & Carraro (1995) for the observing technique employed. As usual, we observed several E-regions standard stars to determine the extinction coefficients ( $K_{B}=0.77\pm0.04$, $K_{V}=0.52\pm0.01$ and $K_{I}=0.42\pm0.02$) and the color equations:

B - b = -0.002(B-V) - 4.204 (1)

V - v = -0.025(B-V) - 4.387 (2)

I - i = -0.055(V-I) - 4.954 (3)

B - V = +1.024(b-v) + 0.187 (4)

V - I = +1.019(v-i) + 0.584. (5)

The resulting $BVI\rm _C$ magnitudes are listed in Table 2 together with their internal errors. These ones average at $\sigma \sim$ 0.0015 mag and increase only for stars fainter than V = 14 mag for which photon statistics becomes the dominant source of noise.

Comparison between our CCD profile photometry (DAOPHOT) and the photoelectric observations of MV yields for the seven stars in common:

$$V - V_{{\rm MV}} = 0.034 (\sigma=0.055)$$ (6)

$$(B-V) - (B-V)_{{\rm MV}} = 0.033 (\sigma=0.020).$$ (7)

   

Table 1: Journal of observations. The last column is the FWHM of stellar images as measured on the CCD frames

Date	Filter	Exp. time	Seeing
		(sec)	( $\prime\prime$)
July 9, 1992	V	30	2.1
	V	180	2.2
	B	300	2.4
	B	60	2.3
	I	3	2.1
	I	30	1.9

Figure 1: Identification map for the stars with $BVI_{\rm C}$ photometry in Table 2. The imaged field covers $2.1\times 3.3$ arcmin. North is up, East to the left

The comparison with the photoelectric observations of FD gives for the seven stars in common:

$$V - V_{{\rm FD}} = 0.039 (\sigma=0.047)$$ (8)

$$(B-V) - (B-V)_{{\rm FD}} = 0.092 (\sigma=0.028).$$ (9)

Our CCD profile photometry appears slightly fainter and redder than older aperture photometry. It is our opinion that fainter and therefore generally redder stars (quite abundant in a crowded field like that of NGC 6604) have entered and contaminated the older aperture photometry, performed with fixed diaphragms. The different standard stars used (Landolt' equatorial stars for older photometric works, Cousin's E-regions for us) may add a little to the differences in the photometry. The excellent agreement in the interstellar reddening found below via three independent methods gives some support to such arguments.

Figure 2: Identification map for low resolution slitless spectroscopy. North is at the top, East to the left. The identification map for the $BVI_{\rm C}$ photometry in Fig. 1 covers the central part of this collage of Vband frames obtained in parallel with the slitless spectroscopic observations

   

Table 2: $BVI_{\rm C}$ photometry of NGC 6604. Columns gives our identification number and those used by MV and FD, the X and Y positions on the chip, the corresponding J2000.0 equatorial coordinates (linked to the GSC I system), and the BVI magnitudes and colours with associated internal errors as provided by the DAOPHOT package

ID	MV	FD	X	Y	$\alpha_{{\rm J}2000}$	$\delta_{{\rm J}2000}$	V	$\sigma_V$	B-V	$\sigma_{B-V}$	V-I	$\sigma_{V-I}$
1		D	276.51	63.89	18 18 11.26	-12 13 54.71	12.101	0.001	0.763	0.001	1.025	0.002
2		M	128.33	144.03	18 18 09.12	-12 14 52.47	12.201	0.001	0.800	0.001	1.065	0.002
3	11		142.21	293.03	18 18 05.15	-12 14 46.96	12.993	0.001	0.840	0.002	1.271	0.013
4	10		171.07	310.88	18 18 04.68	-12 14 35.69	12.996	0.002	0.807	0.003	1.108	0.004
5		51	69.04	321.60	18 18 04.39	-12 15 15.48	14.683	0.005	0.943	0.009	1.175	0.004
6			40.66	331.84	18 18 04.12	-12 15 26.55	12.873	0.001	0.839	0.002	1.138	0.003
7		J	21.64	373.07	18 18 03.02	-12 15 33.94	12.227	0.001	0.807	0.001	1.094	0.002
8			154.74	378.57	18 18 02.88	-12 14 42.02	13.025	0.002	0.767	0.003	1.051	0.004
9			160.40	388.27	18 18 02.62	-12 14 39.80	12.411	0.001	0.751	0.001	1.052	0.003
10		2	283.59	417.69	18 18 01.84	-12 13 51.72	15.894	0.013	1.020	0.026	2.586	0.025
11		72	94.91	453.76	18 18 00.87	-12 15 05.31	15.987	0.013	1.026	0.026	1.394	0.035
12	9	Z	254.86	469.18	18 18 00.47	-12 14 02.90	11.658	0.001	0.772	0.001	1.057	0.001
13		52	5.97	148.61	18 18 08.99	-12 15 40.20	15.187	0.008	0.943	0.014	1.236	0.026
14		49	121.83	185.94	18 18 08.00	-12 14 54.98	14.971	0.006	1.176	0.013	1.505	0.014
15		31	308.73	294.42	18 18 05.12	-12 13 42.00	14.478	0.004	0.989	0.007	1.185	0.011
16		71	135.02	424.02	18 18 01.67	-12 14 49.68	15.282	0.007	0.909	0.013	1.200	0.021
17		76	50.75	484.57	18 18 00.05	-12 15 22.52	14.259	0.003	0.872	0.005	1.239	0.009
18			46.77	322.57	18 18 04.36	-12 15 24.17	14.083	0.003	0.869	0.006	1.185	0.009
19			143.02	271.89	18 18 05.72	-12 14 46.66	14.265	0.006	0.892	0.008	1.271	0.013
20		50	89.39	319.79	18 18 04.44	-12 15 07.55	14.790	0.009	0.963	0.013	1.240	0.016
21	2	H	253.65	244.38	18 18 06.45	-12 14 03.52	10.127	0.001	0.707	0.001	0.921	0.001
22			278.91	250.70	18 18 06.29	-12 13 53.66	15.454	0.011	1.037	0.019	1.238	0.027
23	3		175.74	369.74	18 18 03.11	-12 14 33.83	9.681	0.001	0.729	0.001	0.972	0.001
24	4	Y	228.36	444.68	18 18 01.12	-12 14 13.25	10.426	0.001	0.735	0.001	0.984	0.001
25			208.44	445.42	18 18 01.10	-12 14 21.02	15.112	0.010	0.853	0.016	1.233	0.019
26	1	X	179.19	265.92	18 18 05.88	-12 14 32.55	7.524	0.001	0.812	0.001	1.116	0.001
27			195.26	90.86	18 18 10.54	-12 14 26.39	13.716	0.017			2.890	0.017