A 3D numerical model for Kepler's supernova remnant

MNRAS 442, 229–238 (2014) doi:10.1093/mnras/stu880 A 3D numerical model for Kepler’s supernova remnant J. C. Toledo-Roy,1‹ A. Esquivel,1‹ P. F. Velázquez1‹ and E. M. Reynoso2,3‹ 1 Instituto de Ciencias Nucleares, Universidad Nacional Autónoma de México, Ap.P. 70–543, 04510 D.F., México de Astronomı́a y Fı́sica del Espacio (IAFE), C.C. 67, Suc. 28, 1428 Buenos Aires, Argentina 3 Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Argentina 2 Instituto Accepted 2014 May 1. Received 2014 April 28; in original form 2014 January 6 We present new 3D numerical simulations for Kepler’s supernova remnant. In this work we revisit the possibility that the asymmetric shape of the remnant in X-rays is the product of a Type Ia supernova explosion which occurs inside the wind bubble previously created by an AGB companion star. Due to the large peculiar velocity of the system, the interaction of the strong AGB wind with the interstellar medium results in a bow shock structure. In this new model we propose that the AGB wind is anisotropic, with properties such as mass-loss rate and density having a latitude dependence, and that the orientation of the polar axis of the AGB star is not aligned with the direction of motion. The ejecta from the Type Ia supernova explosion is modelled using a power-law density profile, and we let the remnant evolve for 400 yr. We computed synthetic X-ray maps from the numerical results. We find that the estimated size and peculiar X-ray morphology of Kepler’s supernova remnant are well reproduced by considering an AGB mass-loss rate of 10−5 M yr−1 , a wind terminal velocity of 10 km s−1 , an ambient medium density of 10−3 cm−3 and an explosion energy of 7 × 1050 erg. The obtained total X-ray luminosity of the remnant in this model reaches 6 × 1050 erg, which is within a factor of 2 of the observed value, and the time evolution of the luminosity shows a rate of decrease in recent decades of ∼2.4 per cent yr−1 that is consistent with the observations. Key words: hydrodynamics – radiation mechanisms: thermal – methods: numerical – ISM: supernova remnants – X-rays: ISM. Kepler’s supernova remnant (SNR; SN 1604, G004.5+06.8, V 843 Ophiuchi) is one of the few supernova events in our Galaxy that have been observed in historical times. Just above 400 years old, this young remnant has been observed by a multitude of scientific instruments in different spectral bands over the past decades: several VLA radio observations (e.g. Reynoso & Goss 1999; DeLaney et al. 2002), a submillimetric dust study by Morgan et al. (2003), optical imaging and spectroscopy using both ground-based telescopes and the Hubble Space Telescope (e.g. Blair, Long & Vancura 1991; Green et al. 1997; Sollerman et al. 2003; Sankrit et al. 2008), Spitzer (Blair et al. 2007) and near-infrared (Gerardy & Fesen 2001) imaging and spectroscopy, as well as multiple X-ray studies using the space telescopes Einstein (White & Long 1983), ROSAT (Predehl & Schmitt 1995), ASCA (Kinugasa & Tsunemi 1999), XMM–Newton (Cassam-Chenaı̈ et al. 2004) and Chandra (e.g. Bamba et al. 2005; Reynolds et al. 2007). The observations across multiple wavelengths reveal a complex morphology which is prominently asymmetrical. In Fig. 1 we present a radio image at 1.42 GHz (Fig. 1a, left panel) reprocessed from archival VLA1 observations, and an X-ray image from the Chandra catalogue (Fig. 1b, right panel) in the band 2–10 keV. In both bands, the remnant appears as an incomplete shell which is considerably brighter to the North-west and displays two protuberances (‘ears’) that are roughly aligned with the Southeast–North-west axis (Dickel et al. 1998). Another remarkable feature is a curved bar-like structure that spans across the remnant roughly from its South to North limbs. The diameter of the shell is approximately 3.3 arcmin (Blair 2005). In the optical (e.g. Sankrit et al. 2008), the emission comes from dense, knotty structures (radiative shocks) with an excess of nitrogen, and from thin Hα filaments (non-radiative shocks). Overall, the optical emission roughly corresponds to the bright NW regions in radio and X-rays. The nature of Kepler’s supernova explosion has been debated a lot. The relatively high Galactic latitude of the remnant (l = 6.◦ 8)  E-mail: (JCTR); esquivel@nucleares. unam.mx (AE); (PFV); (EMR) 1 The Very Large Array (VLA) is operated by the National radio Astronomy Observatory, which is a facility of the National Science Foundation, operated under cooperative agreement by Associated Universities, Inc. 1 I N T RO D U C T I O N  C 2014 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society ABSTRACT 230 J. C. Toledo-Roy et al. and the reconstructed light curve based on historical observations (Baade 1943) initially prompted a Type Ia identification. However, its high spatial motion away from the Galactic plane and the presence of nitrogen in optical observations (which is usually associated with stellar winds) challenged this view, suggesting a core-collapse scenario instead (e.g. Borkowski, Sarazin & Blondin 1992). Nevertheless, X-ray studies have generally provided evidence in favour of the initially suggested Type Ia scenario. In particular, the large iron abundance (Kinugasa & Tsunemi 1999; Cassam-Chenaı̈ et al. 2004), which is consistent with Type Ia supernovae, as well as the absence of a neutron star (Reynolds et al. 2007), which would be an additional indication of a core-collapse event, has led us (Reynolds et al. 2007) to conclude that Kepler’s SNR is the result of a Type Ia explosion. Furthermore, the hydrodynamical models of Borkowski et al. (1992) and Velázquez et al. (2006) found that a Type Ia origin produces numerical results that are a closer match to the observations. The most likely explanation for the asymmetric brightness of the northwestern region is that the medium in which the ejected material expands is inhomogeneous. Since strong density gradients are not expected at such high altitude above the Galactic plane, this suggests that the progenitor system had an important role in shaping the circumstellar medium (CSM). Van den Bergh & Kamper (1977) analysed the proper motions of optical knots within the nebula and determined that the progenitor must have had a high peculiar velocity relative to the ambient medium. Using more recent data, Bandiera & van den Bergh (1991) determined this velocity to be around 280 km s−1 . This prompted the idea that the northwestern region is the product of the interaction between the supernova shock wave and the bow shock structure produced by the stellar wind of an evolved star as it travels through the ambient medium. Bandiera (1987) and Borkowski et al. (1992) investigated this ‘runaway progenitor’ hypothesis in depth and found that it can explain not only the brightness of the northwestern region, but also the absence of a sharp limb on the southern edge. This idea can (...truncated)