Astronomy & Astrophysics (A&A) is an international journal which publishes papers on all aspects of astronomy and astrophysics

The recurrent nova (RN) V3890 Sgr was observed during the seventh day after the onset of its most recent outburst, with the Chandra ACIS-S camera and High Energy Transmission Gratings. A rich emission line spectrum was detected, due to transitions of Fe-L and K-shell ions ranging from neon to iron. The measured absorbed flux is ≈10⁻¹⁰ erg cm⁻² s⁻¹ in the 1.4–15 Å range (0.77–8.86 keV). The line profiles are asymmetric, blueshifted, and skewed toward the blue side, as if the ejecta moving toward us are less absorbed than the receding ejecta. The full width at half-maximum of most emission lines is 1000–1200 km s⁻¹, with some extended blue wings. The spectrum is thermal and consistent with a plasma in collisional ionization equilibrium with column density 1.3 × 10²² cm⁻² and at least two components at temperatures of about 1 and 4 keV, possibly a forward and a reverse shock, or regions with differently mixed ejecta and a red giant wind. The spectrum is remarkably similar to the symbiotic RNe V745 Sco and RS Oph, but we cannot distinguish whether the shocks occurred at a distance of a few au from the red giant, or near the giant’s photosphere, in a high-density medium containing only a low mass. The ratios of the flux in lines of aluminum, magnesium, and neon relative to the flux in lines of silicon and iron probably indicate a carbon–oxygen white dwarf.

Nova Her 2021 (V1674 Her), which erupted on 2021 June 12, reached naked-eye brightness and has been detected from radio to γ-rays. An extremely fast optical decline of 2 magnitudes in 1.2 days and strong Ne lines imply a high-mass white dwarf. The optical pre-outburst detection of a 501.42 s oscillation suggests a magnetic white dwarf. This is the first time that an oscillation of this magnitude has been detected in a classical nova prior to outburst. We report X-ray outburst observations from Swift and Chandra that uniquely show (1) a very strong modulation of supersoft X-rays at a different period from reported optical periods, (2) strong pulse profile variations and the possible presence of period variations of the order of 0.1–0.3 s, and (3) rich grating spectra that vary with modulation phase and show P Cygni–type emission lines with two dominant blueshifted absorption components at ∼3000 and 9000 km s⁻¹ indicating expansion velocities up to 11,000 km s⁻¹. X-ray oscillations most likely arise from inhomogeneous photospheric emission related to the magnetic field. Period differences between reported pre- and post-outburst optical observations, if not due to other period drift mechanisms, suggest a large ejected mass for such a fast nova, in the range 2 × 10⁻⁵–2 × 10⁻⁴ M _⊙. A difference between the period found in the Chandra data and a reported contemporaneous post-outburst optical period, as well as the presence of period drifts, could be due to weakly nonrigid photospheric rotation.

Estimates of the accretion rate in symbiotic recurrent novae (RNe) often fall short of theoretical expectations by orders of magnitude. This apparent discrepancy can be resolved if the accumulation of mass by the white dwarf (WD) is highly sporadic, and most observations are performed during low states. Here we use a re-analysis of archival data from the Digital Access to a Sky Century @Harvard survey to argue that the most recent nova eruption in symbiotic RN T CrB, in 1946, occurred during—and was therefore triggered by—a transient accretion high state. Based on similarities in the optical light curve around 1946 and the time of the prior eruption, in 1866, we suggest that the WD in T CrB accumulates most of the fuel needed to ignite the thermonuclear runaways (TNRs) during accretion high states. A natural origin for such states is dwarf-nova like accretion-disk instabilities, which are expected in the presumably large disks in symbiotic binaries. The timing of the TNRs in symbiotic RNe could thus be set by the stability properties of their accretion disks. T CrB is in the midst of an accretion high state like the ones we posit led to the past two nova eruptions. Combined with the approach of the time at which a TNR would be expected based on the 80 yr interval between the prior two novae (2026 ± 3), the current accretion high state increases the likelihood of a TNR occurring in T CrB in the next few years.

We use a new interdisciplinary approach to study the UV surface habitability of Proxima b under quiescent and flaring stellar conditions. We assumed planetary atmospheric compositions based on CO₂ and N₂ and surface pressures from 100 to 5000 mbar. Our results show that the combination of these atmospheric compositions and pressures provide enough shielding from the most damaging UV wavelengths, expanding the ”UV-protective” planetary atmospheric compositions beyond ozone. Additionally, we show that the UV radiation reaching the surface of Proxima b during quiescent conditions would be negligible from the biological point of view, even without an atmosphere. Given that high UV fluxes could challenge the existence of life, then, we experimentally tested the effect that flares would have on microorganisms in a ”worst case scenario” (no UV-shielding). Our results show the impact that a typical flare and a superflare would have on life: when microorganisms receive very high fluences of UVC, such as those expected to reach the surface of Proxima b after a typical flare or a superflare, a fraction of the population is able to survive. Our study suggests that life could cope with highly UV irradiated environments in exoplanets under conditions that cannot be found on Earth.