A spectroscopic investigation of early-type stars in the young open cluster Westerlund 2

binaries: eclipsing; stars: early-type; open clusters and associations: individual: Westerlund 2; Sun: fundamental parameters; stars: individual: WR 20b; stars: individual: Westerlund 2 MSP 18

Abstract :

[en] Context. The distance of the very young open cluster <ASTROBJ>Westerlund 2</ASTROBJ>, which contains the very massive binary system <ASTROBJ>WR 20a</ASTROBJ> and is likely associated with a TeV source, has been the subject of much debate. <BR /> Aims: We attempt a joint analysis of spectroscopic and photometric data of eclipsing binaries in the cluster to constrain its distance. <BR /> Methods: A sample of 15 stars, including three eclipsing binaries (<ASTROBJ>MSP 44</ASTROBJ>, <ASTROBJ>MSP 96</ASTROBJ>, and <ASTROBJ>MSP 223</ASTROBJ>) was monitored with the FLAMES multi-object spectrograph. The spectroscopic data are analysed together with existing B V photometry. <BR /> Results: The analysis of the three eclipsing binaries clearly supports the larger values of the distance, around 8 kpc, and rules out values of about 2.4 - 2.8 kpc that have been suggested in the literature. Furthermore, our spectroscopic monitoring reveals no clear signature of binarity with periods shorter than 50 days in either the WN6ha star <ASTROBJ>WR 20b</ASTROBJ>, the early O-type stars <ASTROBJ>MSP 18</ASTROBJ>, <ASTROBJ>MSP 171</ASTROBJ>, <ASTROBJ>MSP 182</ASTROBJ>, <ASTROBJ>MSP 183</ASTROBJ>, <ASTROBJ>MSP 199</ASTROBJ>, and <ASTROBJ>MSP 203</ASTROBJ>, or three previously unknown mid O-type stars. The only newly identified candidate binary system is <ASTROBJ>MSP 167</ASTROBJ>. The absence of a binary signature is especially surprising for WR 20b and MSP 18, which were previously found to be bright X-ray sources. <BR /> Conclusions: The distance of Westerlund 2 is confirmed to be around 8 kpc as previously suggested based on the spectrophotometry of its population of O-type stars and the analysis of the light curve of WR 20a. Our results suggest that short-period binary systems are not likely to be common, at least not among the population of O-type stars in the cluster. Based on observations collected at the European Southern Observatory (Cerro Paranal, Chile).Appendix A is available in electronic form at <A href="http://www.aanda.org">http://www.aanda.org</A>

Disciplines :

Space science, astronomy & astrophysics

Author, co-author :

Rauw, Grégor ; Université de Liège - ULiège > Département d'astrophys., géophysique et océanographie (AGO) > Sciences spatiales

Sana, H.; Sterrenkundig Instituut "Anton Pannekoek", Universiteit van Amsterdam, Science Park 904, 1098 XH, Amsterdam, The Netherlands

Nazé, Yaël ; Université de Liège - ULiège > Département d'astrophys., géophysique et océanographie (AGO) > Astrophysique observationnelle (sol et espace)

Language :

English

Title :

A spectroscopic investigation of early-type stars in the young open cluster Westerlund 2

Publication date :

01 November 2011

Journal title :

Astronomy and Astrophysics

ISSN :

0004-6361

eISSN :

1432-0746

Publisher :

EDP Sciences, Les Ulis, France

Volume :

535

Pages :

A40

Peer reviewed :

Peer Reviewed verified by ORBi

Additional URL :

http://adsabs.harvard.edu/abs/2011A%26A...535A..40R

Commentary :

http://de.arxiv.org/abs/1109.1086

Available on ORBi :

since 12 December 2011

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