No surviving non-compact stellar companion to Cassiopeia A

Abstract

Massive stars in binaries can give rise to extreme phenomena such as X-ray binaries and gravitational wave sources after one or both stars end their lives as core-collapse supernovae. Stars in close orbit around a stellar or compact companion are expected to explode as "stripped-envelope supernovae", showing no (Type Ib/c) or little (Type IIb) signs of hydrogen in the spectra, because hydrogen-rich progenitors are too large to fit. The physical processes responsible for the stripping process and the fate of the companion are still very poorly understood. Aiming to find new clues, we investigate Cas A, which is a very young (∼340 yr) and near (∼3.4 kpc) remnant of a core-collapse supernova. Cas A has been subject to several searches for possible companions, all unsuccessfully. We present new measurements of the proper motions and photometry of stars in the vicinity based on deep HST ACS/WFC and WFC3-IR data. We identify stellar sources that are close enough in projection but using their proper motions we show that none are compatible with being at the location of center at the time of explosion, in agreement with earlier findings. Our photometric measurements allow us to place much deeper (order-of-magnitude) upper limits on the brightness of possible undetected companions. We systematically compare them with model predictions for a wide variety of scenarios. We can confidently rule out the presence of any stellar companion of any reasonable mass and age (main sequence, pre main sequence or stripped) ruling out what many considered to be likely evolutionary scenarios for Type IIb supernova (SN IIb). More exotic scenarios that predict the presence of a compact companion (white dwarf, neutron star or black hole) are still possible as well as scenarios where the progenitor of Cas A was single at the moment of explosion (either because it was truly single, or resulted from a binary that was disrupted, or from a binary merger). The presence of a compact companion would imply that Cas A is of interest to study exotic outcomes of binary evolution. The single-at-death solution would still require fine-tuning of the process that removed most of the envelope through a mass-loss mechanism yet to be identified. We discuss how future constraints from Gaia and even deeper photometric studies may help to place further constraints.

Massive runaway and walkaway stars

“…This strategy has been successfully applied to search for runaways by van den Bergh (1980), Guseinov et al (2005), Tetzlaff et al (2013Tetzlaff et al ( , 2014, Dinçel et al (2015), Boubert et al (2017b). Kerzendorf et al (2019) also used the predicted kinematics of disrupted binaries to search for companions of the star that exploded producing the supernova remnant Cas A. Tetzlaff et al (2011) also suggested that, with precise astrometry, it might be possible to relate the ejected companion to the remnant of the companion up to ∼5 Myr after the SN explosion, provided that the compact remnant of the former companion is visible (e.g., as a pulsar).…”

The forward and reverse shock dynamics of Cassiopeia A

“…In particular the cause of its large mass loss rate is often attributed to a closely interacting binary system. But there is no evidence for a surviving companion star (Kerzendorf et al 2019). Nevertheless, the shock dynamics reported here provide important hints on the late mass-loss history of the progenitor, be it in the form of a partial, asymmetric shell from episodic mass loss, an aspherical cavity created by a brief WR phase wind, or perhaps even a combination of both.…”

Formation pathway for lonely stripped-envelope supernova progenitors: implications for Cassiopeia A

“…Recent observations have attempted to find this companion that should not be too far from the centre of expansion. However, they instead place extremely strong upper limits on any remaining companion and conclude that there was no stellar companion to the progenitor upon explosion (Kochanek 2018;Kerzendorf et al 2019). Most binary evolution scenarios have been ruled out because of this, and the only remaining channels are binary mergers (Nomoto et al 1995;Lohev et al 2019;Soker 2019), binary disruption after mass transfer from the secondary (Zapartas et al 2017), or the companion is a compact object.…”

“…The inferred ejecta mass is ∼ 2-4 M (Willingale et al 2003;Hwang & Laming 2012), which is typical of a type IIb SN and is consistent with binary progenitor models. However, the upper limits placed on any optical counterpart are so strong that no stellar companion of reasonable mass and age is allowed other than a white dwarf, neutron star or black hole (Kochanek 2018;Kerzendorf et al 2019). Another case is SNR RX J1713.7-3946 (G347.3-0.5), which is inferred to be a type Ib/c SNR (Katsuda et al 2015).…”