2. Observations and results
2.1. Observations and data reduction
The spectroscopic observations were carried out between June 7 and June 11 in 1994 with the 2.16 m telescope at Xinglong station of Beijing Astronomical Observatory, equipped with a thin Tektronix 512512 CCD and a UNIVASAL spectrograph. A grating with a dispersion of 195Å/mm was used. The spectral resolution is 5.26Å/pixel, and the wavelength coverage is from 4205 to 6898Å.
The stars were selected from Sanders' list of NGC 6913. All the stars in the sample have a membership probability greater than 50%. A membership probability of 50% guarantees that most of the non-cluster members have been removed. We will discuss the problem of membership in Sect. 3 of this paper. A cross identification between Sanders' finding chart and ST Atlas was made so as to get the accurate coordinates of the stars.
The spectroscopic data were reduced on a Sun sparc-2 workstation with the ESO Midas package. Every spectrum was corrected from the CCD flat and dark by using the usual methods. The contamination of the nearby sky was subtracted.
Fig. 1 shows the reduced spectra that we plot in order of spectral type. Three of them were found to be emission spectra, i.e. #33, 125 and 151. One emission line star (#159) in Mermilliod's catalogue (Mermilliod 1986) does not show emission lines in its spectrum. Fig. 1 is available as supplementary data in electronic form.
Table 1 presents the results of our spectral classification as well as other observational data for NGC 6913. The method we used to make the spectral classification has been described by Shi & Hu (1999). They discussed the MK standards and an extension of the MK classification to the yellow-red region based on CCD spectra, obtained with the 2.16 m telescope of BAO, of 125 MK standards covering the spectral types from O to M and luminosity classes from V to I. They listed the main features for spectral classification in the yellow-red region. We also refer to the Library of stellar spectra observed by Jacoby et al. (1984) to classify the 100 cluster stars. Besides, we observed some standard MK stars to compare accuracy with our program stars. We find that the accuracy of classification is better than one spectral subclass. The values of V, (B-V), (U-B) and Ep(B-V) are taken from Joshi's photoelectric UBV photometry. The values of (B-V)0, (U-B)0, E(B-V) and E(U-B) in this table are calculated from the spectral type.
Table 1. Observational data for the young open cluster NGC 6913 (M29).
Table 1. (continued)
The distribution of spectral type of the stars we observed is shown in Fig. 2. From Figs. 1 and 2, it can be seen that there is a broad spectral type distribution in our sample, which covers spectral type O7 to K3. There are only 2 O-type stars in the OB association. Most of the stars belong to B or A type, namely 40 and 33 percent of the total number of stars respectively. And the F, G, K stars make up 15, 6, 4 percent respectively.
© European Southern Observatory (ESO) 2000
Online publication: March 28, 2000