Mixing of hot shocked plasma with cold gas in Nova YZ Ret 2020
The Astrophysical Journal(2024)
摘要
The origin of bright X-ray emission lines that appear late in a nova eruption
remains largely a puzzle. We present two high-resolution X-ray grating spectra
of the classical nova YZ Ret, observed 77 and 115 days post-eruption, using
XMM-Newton and Chandra , respectively. Both spectra feature resolved emission
lines blueshifted by v = -1500 km s^-1 and broadened by σ_v=500 km
s^-1. The two spectra are well described by a collisionally ionized plasma
of kT∼ 70 eV that dimmed by a factor of ∼40 between the two
exposures. The spectra also show narrow radiative recombination continua (RRCs)
of C^+4, C^+5, and N^+5, indicating the interaction of the hot
ionized plasma with cold electrons of kT∼ 2 eV. The high-n Rydberg
series of C^+4 is anomalously bright, allowing us to measure the electron
density through continuum lowering, which is in agreement with the He-like
N^+5 density diagnostic of n_e=(1.7±0.4)×10^11 cm^-3. The
high population of these high-n levels constitutes the best evidence to date
of charge exchange (CX) with neutral H in an astrophysical ionized plasma. The
remarkable fact that the velocity and plasma temperature are the same after 38
days, despite the high density and decreasing flux is evidence for ongoing
heating. We suggest the heating is due to a reverse shock in the nova ejecta,
which forms a thin X-ray shell. The narrow RRCs and CX are attributed to direct
mixing with cold gas, which overtakes the hot plasma either from the shock
front, or through the contact discontinuity.
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关键词
Novae,Spectroscopy,Atomic physics,Shocks,Charge exchange recombination,Radiative recombination
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