The Be Star Newsletter, Volume 37 - May 2003

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A spectroscopic survey of NGC 663 I. Negueruela1, C. Motch2, O. Herent2, & A. Marco1 1 Departamento de Física, Ingeniería de Sistemas y Teoría de la Señal Universidad de Alicante, Apdo. 99, E03080 Alicante, Spain 2 Observatoire de Strasbourg 11 rue de l'Université, F67000 Strasbourg, France email:  ignacio@dfists.ua.es Received: May 6, 2003

ABSTRACT As part of our ongoing programme to search for Be+WD binaries in young open clusters and understanding the role of binarity in the Be phenomenon, we have conducted a spectroscopic survey of the "Be-rich" open cluster NGC 663. We have obtained intermediate resolution spectra of 140 likely members, down to B15.5. Unfortunately, we find that the total distance modulus to NGC 663 is rather larger than previously estimated, meaning that the sample is not complete down to the latest B spectral types. We find 5 previously undetected Be stars and thus conclude that photometric searches are only moderately successful at detecting Be stars with weak emission.

Observations were obtained during runs at the 2.5-m Isaac Newton Telescope at the Observatorio del Roque de los Muchachos (Spain) and the 1.93-m telescope at the Observatoire de Haute Provence (France), between October 25th and November 7th, 2002. In total, 149 stars (of which 140 turn out to be likely members) were observed at 1.4  Å/pixel (INT) or 1.8  Å/pixel (OHP) over the 3600—6900 Å range. In addition, 32 of the brightest stars were observed at 0.4 Å/pixel in the blue. Obviously such a large dataset provides an enormous deal of information, especially when combined with other observations. Here we just give some preliminary results concerning the Be content of the cluster. In the following, we will use the numbering system adopted in the Webda database (Mermilliod 1995), with star numbers preceded by an "S". 1. Cluster parameters We have derived spectral types for all the objects in our sample. The earliest spectral type found on the main sequence (MS) is B2V, in good agreement with photometric determinations indicating ages around 20 Myr (e.g., Pigulski, Kopacki, & Kolaczkowski 2001). The vast majority of cluster members form a consistent evolutionary sequence, with an MS extending up to B2V, subgiants in the B2-2.5IV range, giants in the B2-3III range (with a single B3II star, S221) and a supergiant sequence starting at B4. The most massive stars, however, are too young in comparison with this evolutionary sequence (the most extreme case being S162, O9.5V), a situation also found in h Per (e.g., Marco & Bernabeu 2001). Phelps & Janes (1994) have argued for an extended epoch of star formation, but the fact that one of the blue stragglers is S194 (LS I +61° 235), the optical counterpart to the Be/X-ray system RX J0146.9+6121 (Motch et al. 1991), suggests that these systems may be the result of massive binary evolution. By combining the spectral types derived with the CCD BV photometry of Pigulski et al. (2001) and appropriate calibrations, we have calculated spectroscopic parallaxes for objects without emission lines. The average distance modulus for 53 MS stars in the B2-B5 range is (m-M)0=12.4±0.5 (1- standard deviation). However, the distribution in (m-M)0 appears bimodal, with ~30% of the objects giving (m-M)011.7. Though the possibility of two different populations being superposed in the same area needs to be studied, we will preliminarily assume that the brightest objects represent the binary population in the cluster. The rest of the population gives then (m-M)0=12.5±0.2. Though this value is larger than most recent determinations, we caution that its derivation explicitly assumes standard interstellar absorption (R=3.1), while several authors have claimed anomalous extinction in this area. In any case, the total (reddened) distance modulus for most stars is (m-MV)15.0, rather larger than previously estimated, meaning that most members of spectral types B8 and later are too faint to have been included in our magnitude-limited sample. 2. Known Be stars Sanduleak (1979) gives a list of Be stars in NGC 663. We have observed all the objects in that list, except for S297=San 27, which is very far away from the cluster centre. None of the objects removed by Sanduleak (1990) from the list of Be stars, namely, S100=San 15=BD +60°344, S30=San 18, S67=San 19, and S147=San 24=BD +60°347 shows any trace of emission. In particular, S147 is an A0 supergiant and cannot be, by definition, a Be star. In addition, S107=San 11=BD +60°340, considered by Sanduleak a doubtful Be star, does not show any emission either. We detect all other 21 confirmed Be stars in the list of Sanduleak (1990), with the following caveats: The brighter component of BD +60°332, S92=BD +60°332A, is a Be star, while S91=BD +60°332B is a normal star. Sanduleak (1979) identified BD +60°332A=San 6 and BD +60°332B=San 7, but later removed San 7 from the list of Be stars. In the Webda, S91 is given as the brighter component, but from our spectra, it is the star identified in both Simbad and the Webda as S92, which is brighter and a Be star, in agreement with Sanduleak. In both Simbad and the Webda, San 13=BD +60°341 is identified with S144. This is not a Be star. The correct identification is the rather brighter S141. S7191 does not correspond to any star. BD +60°333 has only two components. BD +60°333A=S54 is a supergiant, while BD +60°333B=S53=San 8 is a Be star. S40 does not correspond to any star. It is an old (offset) position for BD +60°343. The faintest component, BD +60°343B=S8072=San 16, is a Be star, while the brightest component, BD +60°343A=S830, is a supergiant. S39 is another misidentification for the same double or one of its components. Sanduleak (1990) states that no emission has been visible since the 50s, but S8072 is a rather strong Be star in our spectra. In the Webda, S120 is given as the identification of VES 624=San 21, but it is a foreground late-type star. The correct identification is its neighbour S121. Pigulski et al. (2001), using H filters, identify four new likely Be stars. We confirm that S61, S128, and S151 are emission line B stars. S240, on the other hand, is a foreground G-type star. Independently Capilla, Fabregat, & Baines (1999), using H and H filters, identify S61, S102, and S175 as candidate Be stars. However, we do not find any sign of emission in either S102 or S175, which were not detected by Pigulski et al. (2001) either. 3. Be star content We find 3 new objects showing clear, if weak, emission in H, namely S101, S132, and S181. S131 shows considerable emission infilling, not reaching the continuum level, while S97 shows asymmetric H absorption, likely to be filled in by emission, though the case of a double-lined spectroscopic binary cannot be ruled out at this resolution. Therefore we have to conclude that photometric methods are only moderately effective at detecting stars with weak emission, as Pigulski et al. (2001) failed to detect at least four Be stars and mistook one foreground object for a Be star. This low efficiency has an immediate bearing on recent claims of a clear decrease in the Be fraction at later spectral types, as late-type Be stars tend to have much weaker emission than early B objects (e.g., Steele, Negueruela, & Clark 1999). In our sample, we find 3 Be stars among 30 stars later than B4. Such counts hint at the existence of a decrease, but, as the sample is very far from complete, the statistical significance of this result has yet to be established. ACKNOWLEDGMENTS IN is a researcher of the programme Ramón y Cajal, funded by the Spanish Ministerio de Ciencia y Tecnología and the University of Alicante. This research is partially supported by the Spanish Ministerio de Ciencia y Tecnología under grant AYA2002-00814. We acknowledge the help of Dr. J. C. Mermilliod, who has updated the WEBDA database. REFERENCES Eyer, L. & Genton, M.G. 1999, A&AS, 136, 421 Capilla, G., Fabregat, J., & Baines, D. 1999, in: Smith M., Henrichs H.F., & Fabregat J. (eds.) IAU Colloq. 175, The Be Phenomenon in Early-Type Stars. ASP, San Francisco, p. 63 Marco, A. & Bernabeu, G. 2001, A&A 372, 477 Mermilliod, J.C. 1995, in: Egret D. & Albrecht, M.A. (eds.) Information and On-line Data in Astronomy. Kluwer Academic Press, Dordrecht, p. 127 Motch, C. et al. 1991 A&A 246, L24 Phelps, R.L. & Janes, K.A. 1994, ApJS 90, 31 Pigulski, A., Kopacki, G., & Kolaczkowski, Z. 2001, A&A 376, 144 Sanduleak, N. 1979, AJ 84, 1319 Sanduleak, N. 1990, AJ 100, 1239 Steele, I.A., Negueruela, I., & Clark, J.S. 1999, A&AS 137, 147