New Astronomy Reviews 54 (2010) 181–182
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Compact binaries, hypernovae, and GRBs Melvyn B. Davies a,⇑, Andrew Levan b a b
Lund Observatory, Box 43, SE-221 00 Lund, Sweden Centre for Astrophysics Research, University of Hertfordshire, College Lane, Hatﬁeld AL10 9AB, UK
a r t i c l e
i n f o
Article history: Available online 1 October 2010
a b s t r a c t The collapse of a massive stellar core may lead to the production of a black hole surrounded by a torus of material. Such a system is a potential source for the so-called long gamma-ray bursts (GRBs). A torus will form around the black hole if the infalling material contains sufﬁcient angular momentum. This however requires that the core of the massive star rotates extremely rapidly prior to collapse. Here we explore whether tidal locking within binaries can spin stars up sufﬁciently. We show that the binaries are required to have separations 63–4 R , hence the massive star would have lost its outer envelope (for example in a common envelope phase). In addition, the companions to the massive stars must themselves be compact. Comparison with observed tight binaries, which contain either two neutron stars or a neutron star and a white dwarf, shows that angular momentum is likely to have played an important role during the core collapse of the secondary in about half the systems, including the recently-discovered neutron star binary J0737-3039. Even if these systems failed to produce a GRB, as they do not contain a black hole, they are relevant to the problem of GRB production as a very similar evolutionary pathway (but with a slightly more massive helium star core) may well produce a GRB. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction Hypernovae may be the source of the so-called long gamma-ray bursts (GRBs), see for example Woosley (1993). Hypernovae are a subset of core-collapse supernovae. The picture being that the core of a massive star collapses forming a central black hole. If the core contained sufﬁcient angular momentum, then further infall will be arrested and the infalling material will form a torus around the black hole, as is shown schematically in Fig. 1. Energy is extracted from the material in the torus, this energy being ejected from the star along two jets perpendicular to the torus plane. For the case of a single, massive, star, the spin of the core may be insufﬁcient to form a torus, see for example Petrovic et al. (2005). However it has been recently suggested that rapidly-rotating metal-poor stars may retain enough angular momentum (Yoon and Langer, 2005). Here we explore the idea that the core of a massive star is spunup by tidal locking within a tight binary. The massive star in such a binary would earlier have lost its hydrogen envelope (probably through a common envelope phase), leaving its helium core.
object when core collapse occurs. Following Podsiadlowski et al. (2004) and Izzard et al. (2004), we will assume that material within a core undergoing free-fall collapse will carry its angular momentum with it. In order to form a torus around the central black hole, this material must have sufﬁcient angular momentum to place it on a circular orbit with a distance greater than 6Rbh. Here we will consider the case where the helium star core has a mass of 5 M , and where a torus forms after the central collapsed core has accumulated 2 M of material. A torus will form providing the material has sufﬁcient angular momentum. Assuming the helium star to have been tidally locked, this criteria sets a maximum separation for the binary which will be a function of secondary mass. In Fig. 2, we plot the maximum separation of the binary allowed, if the collapse is to lead to the formation of a torus. It is clear from this ﬁgure that only extremely tight binaries can spin up the helium star sufﬁciently. The Roche lobe of the secondary is about 1–2 R thus we conclude that the companion must be a compact object already when the helium star collapses and explodes as a supernova. 3. Comparison with observed systems
2. The required binary properties In this section we consider the properties of the binary required if the core of a massive star is to form a torus around the central ⇑ Corresponding author. E-mail address: [email protected]
(M.B. Davies). 1387-6473/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.newar.2010.09.004
Do compact binaries exist which satisfy the criteria illustrated in Fig. 2? Currently there are no known compact binaries which contain a black hole. However, we can consider systems containing either two neutron stars or a neutron star and white dwarf. The current eccentricities and semi-major axes of the known systems are shown in Fig. 3. The error bars on this ﬁgure show the allowed
M.B. Davies, A. Levan / New Astronomy Reviews 54 (2010) 181–182
Fig. 1. A cartoon showing schematically the collapse of a rotating core. If the infalling material carries with it its original angular momentum, the infall of some material may be halted forming a torus around the central object providing the core has sufﬁcient initial spin.
Fig. 3. The eccentricity and semi-major axis of observed systems containing either two neutron stars, or a neutron star and white dwarf. The error bars give an indication of the allowed range of initial separations between the two stars when the second one exploded as a supernova. Angular momentum contained within the helium star core will be signiﬁcant in systems located to the left of the line.
Fig. 2. The maximum separation allowed for binaries containing a helium core of a massive star as a function of secondary mass, such that core collapse of the helium star leads to the formation of a torus of material around the central core, as shown schematically in Fig. 1, assuming that the stars are tidally locked.
We have shown that tidal locking in tight binaries could spin up the helium core of a massive star sufﬁciently to produce a torus of material around a central object when core collapse occurs. Such binaries are sufﬁciently tight that the companion in the binary would have to be a compact object (i.e. a white dwarf, neutron star or black hole). Comparison with observed tight binaries, which contain either two neutron stars or a neutron star and white dwarf, suggest that angular momentum would have played an important role in the core collapse of the secondary in about half the systems. Thus the production of hypernovae and long gamma-ray bursts may be related to the production of compact binaries. Those systems tight enough to spin-up the helium star signiﬁcantly, are likely to produce systems which will merge via the effects of gravitational radiation within a few hundred Myr, see for example Davies et al. (2002). Thus progenitors of long gamma-ray bursts, may later evolve to produce short gamma-ray bursts when a neutron star and black hole merge. Acknowledgments
range of separations between the two stars when the second one exploded as a supernova. Systems to the left of the line are likely to have satisﬁed the criteria shown in Fig. 2. In other words, angular momentum within the cores of the helium stars is likely to have been important during the second supernova in about half of the observed systems (including for example, the recently-discovered system, J0737-3039). If we assume that GRBs are only produced in supernovae that produce black holes, then none of these systems would have produced GRBs. Even so, systems containing slightly more-massive helium stars which would produce black holes would have followed a similar evolutionary path, hence comparison with those observed systems is relevant.
MBD is a Royal Swedish Academy Research Fellow supported by a grant from the Knut and Alice Wallenberg Foundation. AL is supported by PPARC via a postdoctoral fellowship. References Davies, M.B., Ritter, H., King, A., 2002. MNRAS 335, 369. Izzard, R.G., Ramirez-Ruiz, E., Tout, C.A., 2004. MNRAS 348, 1215. Petrovic, J., Langer, N., Yoon, S.-C., Heger, A., 2005. A&A 435, 247. Podsiadlowski, P., Mazzali, P.A., Nomoto, K., Lazzati, D., Cappellaro, E., 2004. ApJ 607, L17. Woosley, S.E., 1993. ApJ 405, 273. Yoon, S.-C., Langer, N., 2005. A&A 443, 643.