On the nature of early-type emission line objects in NGC6611
Christophe Martayan, Michele Floquet, Anne-Marie Hubert, Juan Fabregat, Yves Fremat, Dietrich Baade, Coralie Neiner
aa r X i v : . [ a s t r o - ph ] S e p SF2A 2007
J. Bouvier, A. Chalabaev, C. Charbonnel (eds)
ON THE NATURE OF EARLY-TYPE EMISSION LINE OBJECTS IN NGC6611
C. Martayan , , M. Floquet , A.-M. Hubert , J. Fabregat , Y. Fr´emat , D. Baade and C. Neiner Abstract.
The number and the nature of emission line stars in the young open cluster NGC6611 is stillthe object of debates. Due to the presence of a strong and variable nebulosity in the cluster, the numberof emission line stars is highly depending on the technique and the resolution used for the observations.Thanks to observations with the ESO-WFI, in slitless spectroscopic mode, and with the VLT-GIRAFFE wehave been able to disentangle the circumstellar and nebular emissions. We confirm the small number of trueemission line objects and we precise their nature: mainly Herbig Be stars.
NGC6611 is a young open cluster with log(age) = 6.2 or 6.8, depending on the authors. According to Hillenbrandet al. (1993) and de Winter et al. (1997) it contains a great number of emission line stars (ELS), whereasHerbig & Dahm (2001) only found a small number. It is however worth noticing that the two first studies werecarried out using slit spectrographs, while Herbig & Dahm (2001) used a slitless instrument not sensitive to thesurrounding emission originating from the Eagle nebula.In order to further investigate the occurence of emission line stars in NGC6611 and to determine the natureof objects, we used the Wide Field Imager (WFI) at ESO in slitless spectroscopic mode associated to a 200 nmpassband H α filter. We further also made use of the multi-object spectrograph GIRAFFE at the VLT inMEDUSA mode. The spectra obtained in this way allowed us: 1) to determine the stellar parameters, 2) todisentangle the circumstellar and nebular emissions, and 3) to determine the true nature of the targets. With WFI, we obtained ∼ The 100 objects finally observed with GIRAFFE are shown in Fig. 1.Among them, only 9 were identified as “true” CS ELS. The main part of the previously known ELS hadtheir spectra contaminated by nebular emission, as shown in Figure 2 where the H α regions obtained with WFIand GIRAFFE are displayed for two cases: W483, a true ELS, and W371, misidentified as ELS in previous slitspectroscopy. Royal Observatory of Belgium, 3 avenue circulaire, 1180 Brussels, Belgium GEPI, Observatoire de Paris, CNRS, Universit´e Paris Diderot; 5 place Jules Janssen, 92195 Meudon Cedex, France Observatorio Astron´omico de Valencia, edifici Instituts d’investigaci´o, Poligon la Coma, 46980 Paterna Valencia, Spain European Organisation for Astronomical Research in the Southern Hemisphere, Karl-Schwarzschild-Str. 2, D-85748 Garchingb. Muenchen, Germany c (cid:13)
Soci´et´e Francaise d’Astronomie et d’Astrophysique (SF2A) 2007
SF2A 2007
Fig. 1.
Location of the stars observed with the VLT-GIRAFFE in the field of NGC6611. The image is a RGB mosaicwith 3 channels of SPITZER (B: 3.6 µ m, G: 4.5 µ m, and R: 8 µ m). The small white circles are for the non-ELS stars, thegreen boxes are for the ELS stars. The large white circle shows the field of GIRAFFE and the large white box showsthe field of the WFI-spectro. WFI017 stands for WFI[N6611]017. To investigate the nature of the stars observed with GIRAFFE, we determined their fundamental parametersand studied their spectral energy distributions (SED).
We determined the fundamental parameters (T eff , log g and V sin i ) by fitting the GIRAFFE data with syntheticspectra following a procedure described in Fr´emat et al. (2006). Other parameters (mass, radius, age, lumi-nosity) were then estimated by interpolation in theoretical evolutionary tracks computed for a solar metallicity(Schaller et al. 1992). A part of stars in the sample (mainly the massive stars) are young and lie close to theZAMS. However, the analysis of our results demonstrates that a group of intermediate mass stars are too oldn the nature of early-type emission line objects in NGC6611 3
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Fig. 2.