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    Understanding the radii of massive stars throughout their evolution is important to answering numerous questions about stellar physics, from binary interactions on the main sequence to the pre-supernova radii. One important factor determining a star’s radius is the fraction of its mass in elements heavier than Helium (metallicity, Z). However, the metallicity enters stellar evolution through several distinct microphysical processes, and which dominates can change throughout stellar evolution and with the overall magnitude of Z. We perform a series of numerical experiments with 15 $\, \mathrm{M}_{\odot }$mesa models computed doubling separately the metallicity entering the radiative opacity, the equation of state, and the nuclear reaction network to isolate the impact of each on stellar radii. We explore separately models centred around two metallicity values: one near solar Z = 0.02 and another sub-solar Z ∼ 10−3, and consider several key epochs from the end of the main sequence to core carbon depletion. We find that the metallicity entering the opacity dominates at most epochs for the solar metallicity models, contributing to on average ∼60–90 per cent of the total change in stellar radius. Nuclear reactions have a larger impact (∼50–70 per cent) during most epochs in the subsolar Z models. The methodology introduced heremore »can be employed more generally to propagate known microphysics errors into uncertainties on macrophysical observables including stellar radii.

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  2. ABSTRACT The Legacy Survey of Space and Time (LSST) by the Vera C. Rubin Observatory is expected to discover tens of millions of quasars. A significant fraction of these could be powered by coalescing massive black hole (MBH) binaries, since many quasars are believed to be triggered by mergers. We show that under plausible assumptions about the luminosity functions, lifetimes, and binary fractions of quasars, we expect the full LSST quasar catalogue to contain between 20 and 100 million compact MBH binaries with masses M = 105–9M⊙, redshifts z = 0–6, and orbital periods P = 1–70 d. Their light-curves are expected to be distinctly periodic, which can be confidently distinguished from stochastic red-noise variability, because LSST will cover dozens, or even hundreds of cycles. A very small subset of 10–150 ultracompact (P ≲ 1 d) binary quasars among these will, over ∼5–15 yr, evolve into the mHz gravitational-wave frequency band and can be detected by LISA. They can therefore be regarded as ‘LISA verification binaries’, analogous to short-period Galactic compact-object binaries. The practical question is how to find these handful of ‘needles in the haystack’ among the large number of quasars: this will likely require a tailored co-adding analysis optimized for thismore »purpose.« less
  3. ABSTRACT The variability of quasars across multiple wavelengths is a useful probe of physical conditions in active galactic nuclei. In particular, variable accretion rates, instabilities, and reverberation effects in the accretion disc of a supermassive black hole are expected to produce correlated flux variations in ultraviolet (UV) and optical bands. Recent work has further argued that binary quasars should exhibit strongly correlated UV and optical periodicities. Strong UV–optical correlations have indeed been established in small samples of (N ≲ 30) quasars with well-sampled light curves, and have extended the ‘bluer-when-brighter’ trend previously found within the optical bands. Here, we further test the nature of quasar variability by examining the observed-frame UV–optical correlations among bright quasars extracted from the Half Million Quasars (HMQ) catalogue. We identified a large sample of 1315 quasars in HMQ with overlapping UV and optical light curves from the Galaxy Evolution Explorer and the Catalina Real-time Transient Survey, respectively. We find that strong correlations exist in this much larger sample, but we rule out, at ∼95 per cent confidence, the simple hypothesis that the intrinsic UV and optical variations of all quasars are fully correlated. Our results therefore imply the existence of physical mechanism(s) that can generate uncorrelated opticalmore »and UV flux variations.« less
  4. ABSTRACT The bright quasar PG1302-102 has been identified as a candidate supermassive black hole binary from its near-sinusoidal optical variability. While the significance of its optical periodicity has been debated due to the stochastic variability of quasars, its multiwavelength variability in the ultraviolet (UV) and optical bands is consistent with relativistic Doppler boost caused by the orbital motion in a binary. However, this conclusion was based previously on sparse UV data that were not taken simultaneously with the optical data. Here, we report simultaneous follow-up observations of PG1302-102 with the Ultraviolet Optical Telescope on the Neil Gehrels Swift Observatory in six optical + UV bands. The additional nine Swift observations produce light curves roughly consistent with the trend under the Doppler boost hypothesis, which predicts that UV variability should track the optical, but with a ∼2.2 times higher amplitude. We perform a statistical analysis to quantitatively test this hypothesis. We find that the data are consistent with the Doppler boost hypothesis when we compare the the amplitudes in optical B-band and UV light curves. However, the ratio of UV to V-band variability is larger than expected and is consistent with the Doppler model, only if either the UV/optical spectral slopes vary,more »the stochastic variability makes a large contribution in the UV, or the sparse new optical data underestimate the true optical variability. We have evidence for the latter from comparison with the optical light curve from All-Sky Automated Survey for Supernovae. Additionally, the simultaneous analysis of all four bands strongly disfavours the Doppler boost model whenever Swift V band is involved. Additional, simultaneous optical + UV observations tracing out another cycle of the 5.2-yr proposed periodicity should lead to a definitive conclusion.« less