4U1700-37

6 Summary

We have performed a sophisticated NLTE analysis on the O6.5 Iaf+ star HD 153919, the primary of the HMXB 4U1700-37 and used the results to constrain the masses of both components of the system via a Monte Carlo simulation. Our NLTE model atmosphere analysis leads to parameters for HD 153919 of log( $L_{\ast}/L_{\odot})=5.82 \pm 0.07$, $T_{\rm eff}=35~000 \pm 1000$ K, $R_{\ast} =21.9^{+1.3}_{-0.5}$ $R_{\odot }$, $\dot{M}=9.5\times 10^{-6}~M_{\odot}$ yr^-1, and an overabundance of nitrogen and possibly carbon over solar metallicities. Combined with the short orbital period of the system this implies a common envelope phase of pre-SN evolution - despite the mass of the SN progenitor apparently precluding a RSG phase - leading to the formation of a close WC+O star binary, with the carbon enrichment of HD 153919 a result of the impact of the stellar wind of the WC star on the surface of the O star.

The Monte Carlo simulations result in masses for the O star and compact object of $M_{\ast} = 58 \pm 11~M_{\odot}$ and $M_x =2.44\pm 0.27~M_{\odot}$, with none of the 10⁶ trials resulting in $M_x \leq1.65~M_{\odot}$, while only 3.5 per cent of the trials result in $M_x \leq2~M_{\odot}$. Given that no significant mass transfer via RLOF has occured this implies that the initial mass of the SN precursor must have been $\ga$60 $M_{\odot }$. Thus even very massive stars can effectively "melt down'' to leave rather low mass post-SN remnants. Equally, the masses of both components imply that it is impossible for stars of $\sim$60 $M_{\odot }$ to leave 5-10 $M_{\odot }$ remnants via Case A or B evolution, suggesting that most high mass black holes are instead formed via Case C mass transfer.

The mass of the compact object is found to lie in between the range of masses observed for neutron stars and black holes. Given that $M_{\ast }$and M_x are strongly correlated, forcing consistency between M_xand either type of object results in significant discrepancies between $M_{\ast }$ and evolutionary predictions for the mass of HD 153919.

In order to produce consistency with the range of masses observed for neutron stars the O star has to be significantly undermassive for its spectral type and luminosity class. While theoretical evolutionary tracks for massive ($\ga$60 $M_{\odot }$) stars suggest that after a redwards excursion the star will evolve bluewards again with a substantially reduced mass, such a star would show significant chemical enrichment (and H depletion) which is not observed. Equally, the surface gravity determined from modeling excludes such an anomolously low mass. Forcing consistency between M_x and the masses of known black hole candidates ( $M_{x} \ga 4.4$ $M_{\odot }$) results in $M_{\ast} \ga$100 $M_{\odot }$. Stars of such extreme masses are not expected to go through an O supergiant phase, rather evolving into H depleted WR stars via a H-rich pseudo WR phase, where their high mass loss rate simulates the spectrum of a WR star. Once again, the constraint implied by the surface gravity also appears to exclude this possibility.

We are therefore left with the conclusion that no solution is fully consistent with present expectations for stellar evolution and the chemical abundances and surface gravity of HD 153919. If the compact object has a mass consistent with the observed range of neutron star masses, the O star is significantly undermassive, while if it consistent with the lower limit to black hole masses the O star is overmassive by a similar (or larger) factor. Finally if - as the Monte Carlo analysis implies - the probable mass of the O star is consistent with evolutionary predictions and the measured surface gravity, the mass of the compact object lies in between the two alternatives.

While our results do not allow us to distinguish between a massive neutron star or a low mass black hole, the existence of a neutron star of mass in the mass range $2.44\pm 0.27~M_{\odot}$ would significantly constrain the high density nuclear equation of state and provide details about the QCD phase transition complementary to information about the temperature induced transition which will be obtained at RHIC and LHC. Phenomena which might occur deep in the star such as the appearance of hyperons or a quark matter core would be strongly constrained as the existence of these phases might result in an equation of state too soft to support such a high mass star.

Acknowledgements

This paper is partially based on observations collected at the European Southern Observatory, Chile. The United Kingdom Infrared Telescope is operated by the Joint Astronomy Centre on behalf of the U.K. Particle Physics and Astronomy Research Council. JSC, SPG & CB gratefully acknowledge PPARC funding. LK is supported by a fellowship of the Royal Accademy of Arts and Sciences in The Netherlands. MF is supported by the FNRS and was helped by conversations with Nicolas Borghini; we also thank John Porter, Marten van Kerkwijk and Phil Charles for their helpful comments.

4U1700-37