Freely Expanding Knots of X-ray Emitting Ejecta in Kepler's Supernova Remnant

TL;DR

This study measures the velocities, composition, and expansion of X-ray knots in Kepler's supernova remnant, revealing high-speed ejecta consistent with Type Ia supernova models and constraining the explosion site and progenitor scenarios.

Contribution

It provides detailed kinematic and compositional analysis of ejecta knots in Kepler's SNR, offering new insights into explosion dynamics and progenitor constraints.

Findings

High-velocity knots with speeds up to ~10,000 km/s.

Ejecta knots originate from the partial Si-burning zone.

Accurate explosion site reduces search area for surviving donor stars.

Abstract

We report measurements of proper motion, radial velocity, and elemental composition for 14 X-ray knots in Kepler's supernova remnant (SNR) using Chandra data. The highest speed knots show both large proper motions (0.11-0.14 "/yr) and high radial velocities (v ~ 8,700--10,020 km/s) with estimated space velocities comparable to the typical Si velocity (~10,000 km/s) seen in SN Ia near maximum light. High speed ejecta knots appear only in specific locations and are morphologically and kinematically distinct from the rest of the ejecta. The proper motions of five knots extrapolate back over the age of Kepler's SNR to a consistent central position that agrees well with previous determinations, but is less subject to systematic errors. These five knots are expanding at close to the free expansion rate (expansion indices of 0.75 <~ m <~ 1.0), which we argue indicates either that they were formed in the explosion with a high density contrast (more than 100 times the ambient density) or that they have propagated through relatively low density (n_H < 0.1 cm^-3) regions in the ambient medium. X-ray spectral analysis shows that the undecelerated knots have high Si and S abundances, a lower Fe abundance and very low O abundance, pointing to an origin in the partial Si-burning zone, which occurs in the outer layer of the exploding white dwarf for SN Ia models. Other knots show slower speeds and expansion indices consistent with decelerated ejecta knots or features in the ambient medium overrun by the forward shock. Our new accurate location for the explosion site has well-defined positional uncertainties allowing for a great reduction in the area to be searched for faint surviving donor stars under non-traditional single-degenerate SN Ia scenarios; because of the lack of bright stars in the search area the traditional scenario remains ruled out.