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Register a new userDetection of additional Wolf-Rayet stars in the starburst cluster Westerlund 1 with SOAR
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J. H. Groh
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We report the detection of 3 additional Wolf-Rayet stars in the young cluster Westerlund 1. They were selected as emission-line star candidates based on 1 micron narrow-band imaging of the cluster carried out at OPD/LNA (Brazil), and then confirmed as Wolf-Rayet stars by K-band spectroscopy performed at the 4.1 m SOAR telescope (Chile). Together with previous works, this increases the population of Wolf-Rayet stars detected in the cluster to 22 members. Moreover, it is presented for the first time a K-band spectrum of the luminous blue variable W243, which apparently implies in a higher temperature than that derived from optical spectra taken in 2003. The WC9 star WR-F was also observed, showing clear evidence of dust emission in the K-band.
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We stress the importance of Wolf-Rayet stars for the understanding of the AGN phenomenon in galaxies. WR stars provide an unique opportunity to explore from the ground whether non-thermal nuclear activity and circumnuclear starbursts are connected. We review the known reported WR signatures observed so far in AGNs and point out some intrincacies related to the analysis of the spectra, linked to reddening correction, the origin of the Hbeta line, etc. Finally, we advocate that integral field spectroscopy is a very promising tool to study this problem and present preliminary results of a long-term project that have been obtained at the CFHT in 1998.
Meynet and Arnould (1993) have suggested that Wolf-Rayet (WR) stars could significantly contaminate the Galaxy with 19F. In their scenario, 19F is synthesized at the beginning of the He-burning phase from the 14N left over by the previous CNO-burning core, and is ejected in the interstellar medium when the star enters its WC phase. Recourse to CNO seeds makes the 19F yields metallicity-dependent. These yields are calculated on grounds of detailed stellar evolutionary sequences for an extended range of initial masses (from 25 to 120 Msol) and metallicities (Z = 0.008, 0.02 and 0.04). The adopted mass loss rate prescription enables to account for the observed variations of WR populations in different environments. The 19F abundance in the WR winds of 60 Msol model stars is found to be about 10 to 70 times higher than its initial value, depending on the metallicity. This prediction is used in conjunction with a very simple model for the chemical evolution of the Galaxy to predict that WR stars could be significant (dominant?) contributors to the solar system fluorine content. We also briefly discuss the implications of our model on the possible detection of fluorine at high redshift.
The Wolf-Rayet (WR) phenomenon is widespread in astronomy. It involves classical WRs, very massive stars (VMS), WR central stars of planetary nebula CSPN [WRs], and supernovae (SNe). But what is the root cause for a certain type of object to turn into an emission-line star? In this contribution, I discuss the basic aspects of radiation-driven winds that might reveal the ultimate difference between WR stars and canonical O-type stars. I discuss the aspects of (i) self-enrichment via CNO elements, (ii) high effective temperatures Teff, (iii) an increase in the helium abundance Y, and finally (iv) the Eddington factor Gamma. Over the last couple of years, we have made a breakthrough in our understanding of Gamma-dependent mass loss, which will have far-reaching consequences for the evolution and fate of the most massive stars in the Universe. Finally, I discuss the prospects for studies of the WR phenomenon in the highest redshift Ly-alpha and He II emitting galaxies.
Westerlund 1 is the most important starburst cluster in the Galaxy due to its massive star content. We have performed BVIc and JKs photometry to investigate the initial mass function (IMF). By comparing the observed color with the spectral type - intrinsic color relation, we obtain the mean interstellar reddening of <E(B-V)>=4.19+/-0.23 and <E(J-Ks)>=1.70+/-0.21. Due to the heavy extinction toward the cluster, the zero-age main sequence fitting method based on optical photometry proved to be inappropriate for the distance determination, while the near-infrared photometry gave a reliable distance to the cluster -- 3.8 kpc from the empirical relation. Using the recent theoretical stellar evolution models with rotation, the age of the cluster is estimated to be 5.0+/-1.0 Myr. We derived the IMF in the massive part and obtained a fairly shallow slope of {Gamma} = -0.8 +/- 0.1. The integration of the IMF gave a total mass for the cluster in excess of 5.0 x 10^4 solar mass. The IMF shows a clear radial variation indicating the presence of mass segregation. We also discuss the possible star formation history of Westerlund 1 from the presence of red supergiants and relatively low-luminosity yellow hypergiants.
Radioisotopes are natural clocks which can be used to estimate the age of the solar system. They also influence the shape of supernova light curves. In addition, the diffuse emission at 1.8 MeV from the decay of 26Al may provide a measure of the present day nucleosynthetic activity in the Galaxy. Therefore, even if radionuclides represent only a tiny fraction of the cosmic matter, they carry a unique piece of information. A large number of radioisotopes are produced by massive stars at the time of their supernova explosion. A more or less substantial fraction of them are also synthesized during the previous hydrostatic burning phases. These nuclides are then ejected either at the time of the supernova event, or through stellar winds during their hydrostatic burning phases. This paper focusses of the non explosive ejection of radionuclides by non-rotating or rotating Wolf-Rayet stars.
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