4 Discussion

We argue that the initial mass of the progenitor of 4U1700-37 was $\geq$ 30⁺³⁰_-10 $M_{\odot}$. This is relevant for the discussion which stars leave black holes and which stars end up as neutron stars. It is commonly believed that the most massive stars form black holes, while massive stars with a mass below a certain limit ( $M_{\rm BH}$) form neutron stars. This mass limit is under strong debate (e.g. Ergma & Van den Heuvel 1998). Maeder (1992) suggested that the observed helium and overall metal abundance is best reproduced if $M_{\rm BH} \simeq 20$ $M_{\odot}$, while Timmes et al. (1996) set this limit at $\sim$30 $M_{\odot}$. Whether the mass limit for black-hole formation in single stars can be compared to that in massive binaries is not clear (Brown et al. 1996). Kaper et al. (1995) set the lower limit for black-hole formation in a massive binary at $\sim$50 $M_{\odot}$ based on observations of Wray 977 and X-ray pulsar companion GX301-2. But Wellstein & Langer (1999) propose that the initial mass of the neutron star in this system was much less, about 26 $M_{\odot}$. The same authors derive for single stars that $M_{\rm BH} \le 25$ $M_{\odot}$.

However, for 4U1700-37 we now have an independent estimate of its progenitor mass, based on the age of its parent OB association. The only drawback is that it is not clear whether 4U1700-37 is a neutron star or a black hole. Up to now, X-ray pulsations, which would immediately identify the compact star as a neutron star, have not been detected. The presence of a cyclotron feature in the X-ray spectrum would also classify the X-ray source as a neutron star. Reynolds et al. (1999) modeled the X-ray spectrum of 4U1700-37, obtained with BeppoSAX, and report the presence of a possible cyclotron feature at an energy of $\sim$37 keV. If real, this observation yields a magnetic field strength of about 5  10¹¹ G, so that 4U1700-37 must be a neutron star. Without confirmation, the alternative that 4U1700-37 is a low-mass black hole cannot be excluded. If 4U1700-37 is a neutron star, a lower limit for black-hole formation in a massive binary derived from this system would be $M_{\rm BH} = 30^{+30}_{-10}$ $M_{\odot}$.

Acknowledgements

We thank the referee Dany Vanbeveren for carefully reading the manuscript. LK is supported by a fellowship of the Royal Netherlands Academy of Arts and Sciences. JD acknowledges NWO Spinoza grant 08-0 to E. P. J. Van den Heuvel. Ed Van den Heuvel and Gijs Nelemans are thanked for stimulating discussions.