Emilios T. Harlaftis,5,6
Jonay I. Gonza´lez Herna´ndez,1,2,3 Rafael Rebolo,1,4 Garik Israelian,1 Alexei V. Filippenko,7 and Ryan Chornock7
Received 2006 April 3; accepted 2006 April 26; published 2006 May 25
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4. Discussion and Conclusions If we include the metallicity distribution of halo and thick-disk stars (Allende Prieto et al. 2006) and thin-disk stars (Allende Prieto et al. 2004) in the equations based on kinematics for establishing the relative likehoods of belonging to the halo, thick disk, or thin disk (Bensby et al. 2003), the probability that a star with the Galactic space velocity components of this system (Mirabel et al. 2001; U = −105 ± 16 km s−1 , V = −98 ± 16 km s−1 , W = −21 ± 10 km s−1 ) and metallicity [Fe/H] = 0.18 belongs to the Galactic halo is less than 0.1%. Moreover, the kinematics alone suggest thick-disk rather than halo membership, although the high metallicity of the secondary star favors thin-disk membership. If, however, the progenitor was a massive star in the halo or thick disk, the high metallicity of the secondary rules out the hypothesis that the black hole was formed by direct collapse of the massive star; instead, a supernova explosion origin is strongly suggested. Given its present orbital distance from the black hole (ac ≈ 3 R⊙, Wagner et al. 2001), it is plausible that the secondary star captured a significant fraction of the matter ejected in a supernova explosion. The chemical composition of the secondary may provide crucial information on nucleosynthesis in the progenitor and the formation mechanism of the black hole. We consider two possible scenarios for the origin of the compact object: either it formed in the Galactic halo or thick disk as a result of the explosion of a metal-poor massive progenitor which enriched the secondary star from the typical abundances of halo/thick-disk stars up to the observed supersolar values, or alternatively, it formed in the Galactic thin disk with a natal kick imparted during the supernova explosion which propelled the binary into its current orbit. In the first case, the similarity with the kinematics of halo and thick-disk stars makes unnecessary a significant kick during the black hole formation process. However, since the typical metallicities of halo stars and thick-disk stars are significantly lower than solar, it is required that the secondary captured enough matter from the ejecta to reach the current abundances. A metal-poor ∼1 M⊙ secondary star, initially placed at an orbital distance of ∼6 R⊙ (after tidal circularization of the orbit), would need to capture roughly 5–10% of the matter ejected in a spherically symmetric corecollapse supernova explosion of a 16 M⊙ helium core to achieve the observed iron abundance. We have considered supernova and hypernova models of metal-poor progenitors with different masses, metallicities, mass cuts (i.e., the mass above which the matter is expelled at the time of the supernova explosion), fallback (i.e., amount of mass which is eventually accreted by the compact core), and mixing (Umeda & Nomoto, 2002, 2005; Tominaga, Umeda & Nomoto 2006, in preparation), and assumed that all the fallback material is well mixed with the ejecta. We find that a sufficiently large Fe enrichment is possible for mass cuts in the range 2–4 M⊙ [and explosion energies (1 − 30) × 1051 ergs]. The abundance ratios of the other elements are only marginally reproduced by these models. An origin of XTE J1118+480 in the Galactic halo or in the Galactic thick disk cannot be discarded by the present observations, nor can it be con- firmed. Further higher-quality observations and extensive model calculations are required to fully explore this possibility. Further details of analysis will put together in Gon´alez Hern´andez et al. (2006, in preparation).
In the second scenario, the system had to acquire a peculiar space velocity of ∼ 180 km s−1 , to change from a Galactic thin disk orbit to the currently observed orbit (Gualandris et al. 2005), requiring an asymmetric kick. Such kicks imparted during the birth of nascent neutron stars, due to asymmetric mass ejection and/or an asymmetry in the neutrino emission (Lai et al. 2001), have been proposed to explain the large transverse motions of neutron stars in the plane of the sky (Lyne & Lorimer 1994). The black hole could have formed in a two-stage process where the initial collapse led to the formation of a neutron star accompanied by a substantial kick and the final mass of the compact remnant was achieved by matter that fell back after the initial collapse as proposed to explain the origin of the black hole in Nova Sco 1994 (Podsiadloski et al. 2002; Brandt et al. 1995). Spherically symmetric explosion models are able to explain the observed metal enrichment. A modest amount of ejecta (∼ 0.02) M⊙ captured by a secondary with initial solar abundance is suf- ficient if vigorous mixing (Kifonidis et al. 2000) between the fallback matter (which is necessarily large given the mass of the black hole) and the ejecta took place. In this scenario the black hole would be formed in a mild explosion with fallback as in collapsar models (MacFadyen et al. 2001) associated with rapidly rotating massive stars. However, in the pure spherical supernova explosion scenario for XTE J1118+480, the maximum allowed ejected mass to keep the secondary star gravitationally bound is ∆M ≈ 8 M⊙. This leads to a system velocity of ∼60 km s−1 (there could be extreme cases with system velocities as high as ∼ 100 km s−1 requiring very special model parameters such as higher secondary and black hole masses just after the explosion; see further details in Sec. 3.2 of Gualandris et al. 2005), much lower than the observed value, and a neutrino-induced kick is required if we are to explain the kinematics of the system. The chemical composition of the ejecta in a non-spherically symmetric supernova explosion is strongly dependent on direction. In particular, if we assume that the jet is collimated perpendicular to the orbital plane of the binary, where the secondary star is located, elements such as Ti, Ni, and Fe are mainly ejected in the jet direction, while Al, O, Si, S, and Mg are preferentially ejected near the equatorial plane of the helium star (Maeda et al. 2002). Using predictions for an aspherical explosion model of a 16 M⊙ He core metal-rich progenitor, we can explain the observed abundances in the secondary. Complete lateral mixing (Podsiadlowski et al. 2002) is required to account for the similar enhancement of Mg and Ni and the observed abundances can be reproduced for all mass cuts in the range 2–8 M⊙. It is therefore plausible that the black hole in XTE J1118+480 formed in the Galactic thin disk from a massive metal-rich progenitor and was launched into its current orbit either by mass ejection in an asymmetric supernova/hypernova explosion or by a neutrino-induced kick. In addition, ultraviolet observations of the accretion disk in this system suggest that the material accreted onto the compact object is substantially CNO processed (Haswell et al. 2002), indicating that the zero-age mass of the secondary star could have been ∼ 1.5 M⊙. By means of binary evolution calculations this may constrain the age of the system in the range 2–5.5 Gyr (Gualandris et al. 2005). Future chemical studies of the secondary star during periods of quiescence may provide accurate abundances of elements (e.g., C, Ti, Si) whose lines either are not available in the observed spectrum or are present in spectral regions where the signalto-noise ratio is too low for an accurate chemical analysis. These may reveal further details of the formation mechanism of the black hole in this system.
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