ASTRONOMY & ASTROPHYSICS APRIL II 1999, PAGE 313 SUPPLEMENT SERIES Astron. Astrophys. Suppl. Ser. 136, 313–331 (1999) Spectroscopic observations of young open clusters: IC 1805, NGC 654 and NGC 6823 H.M. Shi1,2 and J.Y. Hu2,3 1 Shanghai Astronomical observatory, Chinese Academy of Sciences, Shanghai 200030, China 2 Joint Laboratories for Optical Astronomy, Chinese Academy of Sciences, China 3 Beijing Astronomical observatory, Chinese Academy of Sciences, Beijing, 100080, China Received September 30, 1998; accepted January 11, 1999 Abstract. Spectroscopic observations were made for a Table 1. Spectrographs and gratings sample of 263 stars from three young open clusters spectrograph grating dispersion central wavelenth IC 1805, NGC 654 and NGC 6823 with CCD detectors in name lines/mm A/mm˚ A/pixel˚ A˚ the blue and yellow-red spectral regions. MKK types were UNIVERSAL 300 195 4.7 6000 properly determined for these stars by carefully compar- OMR 300 200 4.8 6000 600 100 2.4 5000 ing spectra of program stars with those of a complete grid 1200 50 1.2 4375 of MKK standards, which were observed with the same in- struments as the program stars. For most of the observed stars precise MKK types were assigned for the first time. A few of early-type emission-line stars were discovered or status, to just cite a few. The study of clusters and as- confirmed. The presence of Be stars in the studied clusters sociations deeply relies upon the accumulation of various is discussed. Spectra of the observed MKK standards also kinds of precise data, be it astrometric, photometric or reveal lots of features showing temperature or luminosity spectroscopic. While large efforts have been spent on ob- effect in the yellow-red region, which helped in our stellar taining magnitudes and colours in various systems such as spectral classification. UBV and uvby for stars in clusters, the situation about spectroscopic data is still far from satisfactory. Reliable Key words: stars: fundamental parameters — stars: MKK spectral types, for example, are often available in emission-line, Be — open clusters and associations: published literature only to few brightest stars of a clus- IC 1805, NGC 654 and NGC 6823 ter (Mermilliod 1986). Accurate two-dimensional spectral classification (tem- perature type and luminosity class) of stars is essential to the study of young open clusters. Differential redden- 1. Introduction ings often affect member stars in young clusters, caus- ing scatters in color-magnitude diagrams. Spectroscopy Open clusters are natural laboratories for stellar evolution, provides the means of precisely detecting any differential because stars in clusters share a common origin from the reddening. By combination with photometric data (mag- same progenitor molecular cloud. Young open clusters are nitudes and colors), spectral classification may be used to particularly suited to study the processes of star forma- construct individually-dereddened high-quality H-R dia- tion and early evolution of stars over a wide mass spec- grams, from which the distances and evolutionary ages of trum. Many questions related to young clusters and/or clusters can be reliably determined and even the effects associations remain to be answered, e.g., the constancy of of binarity (usually indistinguishable from small diffential IMF (at higher masses), the coevality of star formation, reddenings) may be examined. Spectral classification can the evolution of massive stars and its influence on the also be used to investigate the nature of reddening and ex- environment, and the presence of various peculiar stars tinction, determining if any abnormality exists and if the (emission-line stars, for example) and their evolutionary differential reddening is caused by interstellar, intraclus- ter or circumstellar extinction material (Sagar 1987; van Send offprint requests to: H.M. Shi den Ancker et al. 1997). Peculiar stars (e.g., emission-line e-mail: [email protected] stars) may also be discovered from spectral classification. 314 H.M. Shi and J.Y. Hu: Spectroscopic observations of young open clusters Traditionally spectral classification is made photo- A total of 263 program stars, 140 from IC 1805, 46 graphically in the blue spectral region of 3900 A˚ through from NGC 654 and 77 from NGC 6823, were observed. 4900 A.˚ Reliable two-dimensional spectral types can be They are mainly proper motion cluster members (See Sect. gained only for brighter stars due to the low sensitivity of 3.2). The magnitude limit is around V =14m. Spectra photographic plates. With the use of CCDs spectroscopy were taken with the 300 lines/mm grating for nearly all can be done on much fainter objects and on a broader program stars. For the brighter stars spectra were also spectral coverage. The peak of the quantum efficiency of taken with the 600 lines/mm grating, and for the few CCDs is usually in the yellow-red spectral region, which brightest stars additional spectra were obtained with the is advantageous to the observation of late type stars or 1200 lines/mm grating. The exposure times ranged from highly-reddened early type stars such as those in young a few minutes to about an hour, depending on the magni- open clusters. With the spectral coverage extended from tude of the object, the seeing, and the grating used. The the blue into the yellow-red and even the near infrared, ad- resulting signal-to-noise (S/N) ratios usually exceeded ditional spectral features suitable for stellar classification 100. The observed cluster stars with their identifying num- also appear in these regions of longer wavelength (Jacoby bers are listed in Tables 2, 3, and 4 for IC 1805, NGC 654 et al. 1984; Turnshek et al. 1985; Torres-Dodgen & Weaver and NGC 6823 respectively. 1993; Danks & Dennefeld 1994; Shi & Hu 1999). We also observed a complete grid of MKK standards We have begun a project of systematically investigat- (up to 200) with the same instruments (telescope and ing the properties and stellar contents of young open clus- spectrograph with same gratings) and under nearly iden- ters and/or associations, especially those with severe dif- tical observing conditions as our program stars. The S/N ferential reddening. The study is to be based upon more was kept comparable to that of the program stars. These complete and homogeneous sets of combined data. As a standard stars are mainly from Keenan & McNeil (1976); part of this program and the first step, we engage our- Morgan et al. (1978) and Morgan & Keenan (1973). selves in making accurate two-dimensional spectral classi- Reobserving MKK standards here is necessary for the fication for large samples of stars in young clusters. The spectral comparison and classification. first three clusters selected by us are IC 1805 (C0228+612, All the spectra were reduced using MIDAS. The raw h m ◦ 0 α1950 =02 28. 9, δ1950 =61 54 ), NGC 654 (C0140+616, CCD spectral images were corrected using average Bias h m ◦ 0 α1950 =0140. 6, δ1950 =6138 ) and NGC 6823 and high S/N dome-flat frames. The dark current of the h m ◦ 0 (C1941+231, α1950 =1941. 0, δ1950 =2311 ). They used CCDs was negligible. Sky subtraction was achieved are of very young ages and all showed differential red- by subtracting a third degree polynomial fitted to the denings according to earlier investigators (Moffat 1972; spectrum in the spatial direction. This kind of sky sub- Turner 1979; Stone 1980; Sagar & Joshi 1981; Joshi & traction proved to be effective in eliminating the emission Sagar 1983a,b). Most of the cluster stars lack MKK types. from surrounding nebulosity. The sky-subtracted spec- In this work a total of 263 stars from these three clusters tral images were then wavelength-calibrated using the were spectroscopically observed by slit spectrograh and comparison spectra (Fe-Ar or He-Ne-Ar lamps) and one- CCD detectors in the blue and yellow-red spectral regions. dimensional spectra were extracted. For the purpose of Results of the MKK classification are given in this paper. spectral classification all one-dimensional spectra were The presence of emission-line stars is discussed. Further continuum-normalized instead of flux-calibrated. Data re- studies upon these clusters are underway. duction for program stars and MKK standard stars were the same. Representative spectra of MKK standards are illustrated in Fig. ?? and Fig. ??. Spectra of all the pro- 2. Observation and data reduction gram stars from the three clusters are shown in Fig. ?? through Fig. ??. Spectroscopic data were obtained during two observing runs, in 1995 and in 1997, with the 2.16 m telescope at Beijing Astronomical Observatory. In 1995 the telescope 3. Spectral classification was equipped with a Tektronix 1024 CCD mounted at an UNIVASAL spectrograph, whereas in 1997 with an- 3.1. Methods other Tektronix 1024 CCD at an OMR (Optomechanics Research Inc.) spectrograph. The pixel size of the two To properly classify stellar spectra in the MKK system, Tektronix CCDs were both 24 µm. For the UNIVASAL the spectra of program stars have to be carefully com- spectrograph a grating of 300 lines/mm were used, and pared with those of MKK standards. MKK standards for the OMR spectrograph three gratings of 300 lines/mm, should be observed with the same instrument and under 600 lines/mm and 1200 lines/mm were used. The corre- the same conditions as the program stars. The reobser- sponding dispersion and central wavelength are given in vation of MKK standards is necessary to link the local Table 1. A slit with a width of about 200 was used during instrumental system to the MKK system which is defined all the observations. by its standards (Morgan & Keenan 1973; Morgan 1984). H.M. Shi and J.Y. Hu: Spectroscopic observations of young open clusters 315 Table 2.