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<jats:title>Abstract</jats:title> <jats:p>Models of solar-like oscillators yield acoustic modes at different frequencies than would be seen in actual stars possessing identical interior structure, due to modeling error near the surface. This asteroseismic “surface term” must be corrected when mode frequencies are used to infer stellar structure. Subgiants exhibit oscillations of mixed acoustic (<jats:italic>p</jats:italic>-mode) and gravity (<jats:italic>g</jats:italic>-mode) character, which defy description by the traditional <jats:italic>p</jats:italic>-mode asymptotic relation. Since nonparametric diagnostics of the surface term rely on this description, they cannot be applied to subgiants directly. In Paper I, we generalized such nonparametric methods to mixed modes, and showed that traditional surface-term corrections only account for mixed-mode coupling to, at best, first order in a perturbative expansion. Here, we apply those results, modeling subgiants using asteroseismic data. We demonstrate that, for grid-based inference of subgiant properties using individual mode frequencies, neglecting higher-order effects of mode coupling in the surface term results in significant systematic differences in the inferred stellar masses, and measurable systematics in other fundamental properties. While these systematics are smaller than those resulting from other choices of model construction, they persist for both parametric and nonparametric formulations of the surface term. This suggests that mode coupling should be fully accounted for when correcting for the surface term in seismic modeling with mixed modes, irrespective of the choice of correction used. The inferred properties of subgiants, in particular masses and ages, also depend on the choice of surface-term correction, in a different manner from those of both main-sequence and red giant stars.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We explore a possible time variation of the fine structure constant (<jats:italic>α</jats:italic> ≡ <jats:italic>e</jats:italic> <jats:sup>2</jats:sup>/<jats:italic>ℏ</jats:italic> <jats:italic>c</jats:italic>) using the Sunyaev–Zel’dovich effect measurements of galaxy clusters along with their X-ray observations. Specifically, the ratio of the integrated Comptonization parameter <jats:inline-formula> <jats:tex-math> <?CDATA ${Y}_{\mathrm{SZ}}{D}_{A}^{2}$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>Y</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>SZ</mml:mi> </mml:mrow> </mml:msub> <mml:msubsup> <mml:mrow> <mml:mi>D</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>A</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjac2150ieqn1.gif" xlink:type="simple" /> </jats:inline-formula> and its X-ray counterpart <jats:italic>Y</jats:italic> <jats:sub> <jats:italic>X</jats:italic> </jats:sub> is used as an observable to constrain the bounds on the variation of <jats:italic>α</jats:italic>. Considering the violation of the cosmic distance duality relation, this ratio depends on the fine structure constant of ∼ <jats:italic>α</jats:italic> <jats:sup>3</jats:sup>. We use the quintessence model to provide the origin of <jats:italic>α</jats:italic> time variation. In order to give a robust test on <jats:italic>α</jats:italic> variation, two galaxy cluster samples, the 61 clusters provided by the Planck collaboration and the 58 clusters detected by the South Pole Telescope (SPT), are collected for analysis. Their X-ray observations are given by the XMM-Newton survey. Our results give <jats:inline-formula> <jats:tex-math> <?CDATA $\zeta =-{0.203}_{-0.099}^{+0.101}$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>ζ</mml:mi> <mml:mo>=</mml:mo> <mml:mo>−</mml:mo> <mml:msubsup> <mml:mrow> <mml:mn>0.203</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.099</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.101</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjac2150ieqn2.gif" xlink:type="simple" /> </jats:inline-formula> for the Planck sample and <jats:inline-formula> <jats:tex-math> <?CDATA $\zeta =-{0.043}_{-0.148}^{+0.165}$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>ζ</mml:mi> <mml:mo>=</mml:mo> <mml:mo>−</mml:mo> <mml:msubsup> <mml:mrow> <mml:mn>0.043</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.148</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.165</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjac2150ieqn3.gif" xlink:type="simple" /> </jats:inline-formula> for the SPT sample, indicating that <jats:italic>α</jats:italic> is constant with redshift within 3<jats:italic>σ</jats:italic> and 1<jats:italic>σ</jats:italic> for the two samples, respectively.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Messier 15 (NGC 7078) is an old and metal-poor post core-collapse globular cluster that hosts a rich population of variable stars. We report new optical (<jats:italic>gi</jats:italic>) and near-infrared (NIR, <jats:italic>JK</jats:italic> <jats:sub> <jats:italic>s</jats:italic> </jats:sub>) multi-epoch observations for 129 RR Lyrae, 4 Population II Cepheids (3 BL Herculis, 1 W Virginis), and 1 anomalous Cepheid variable candidate in M15 obtained using the MegaCam and the WIRCam instruments on the 3.6 m Canada–France–Hawaii Telescope. Multi-band data are used to improve the periods and classification of variable stars, and determine accurate mean magnitudes and pulsational amplitudes from the light curves fitted with optical and NIR templates. We derive optical and NIR period–luminosity relations for RR Lyrae stars which are best constrained in the <jats:italic>K</jats:italic> <jats:sub> <jats:italic>s </jats:italic> </jats:sub>band, <jats:inline-formula> <jats:tex-math> <?CDATA ${m}_{{K}_{s}}=-2.333\,(0.054)\mathrm{log}P+13.948\,(0.015)$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>m</mml:mi> </mml:mrow> <mml:mrow> <mml:msub> <mml:mrow> <mml:mi>K</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>s</mml:mi> </mml:mrow> </mml:msub> </mml:mrow> </mml:msub> <mml:mo>=</mml:mo> <mml:mo>−</mml:mo> <mml:mn>2.333</mml:mn> <mml:mspace width="0.25em" /> <mml:mo stretchy="false">(</mml:mo> <mml:mn>0.054</mml:mn> <mml:mo stretchy="false">)</mml:mo> <mml:mi>log</mml:mi> <mml:mi>P</mml:mi> <mml:mo>+</mml:mo> <mml:mn>13.948</mml:mn> <mml:mspace width="0.50em" /> <mml:mo stretchy="false">(</mml:mo> <mml:mn>0.015</mml:mn> <mml:mo stretchy="false">)</mml:mo> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjac214dieqn1.gif" xlink:type="simple" /> </jats:inline-formula> with a scatter of only 0.037 mag. Theoretical and empirical calibrations of RR Lyrae period–luminosity–metallicity relations are used to derive a true distance modulus to M15: 15.196 ± 0.026 (statistical) ± 0.039 (systematic) mag. Our precise distance moduli based on RR Lyrae stars and Population II Cepheid variables are mutually consistent and agree with recent distance measurements in the literature based on Gaia parallaxes and other independent methods.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We present deep Chandra X-ray observations of two nearby Type Ia supernovae, SN 2017cbv and SN 2020nlb, which reveal no X-ray emission down to a luminosity <jats:italic>L</jats:italic> <jats:sub>X</jats:sub> ≲ 5.3 × 10<jats:sup>37</jats:sup> and ≲ 5.4 × 10<jats:sup>37</jats:sup> erg s<jats:sup>−1</jats:sup> (0.3–10 keV), respectively, at ∼16–18 days after the explosion. With these limits, we constrain the pre-explosion mass-loss rate of the progenitor system to be <jats:inline-formula> <jats:tex-math> <?CDATA $\dot{M}$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mover accent="true"> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>̇</mml:mo> </mml:mrow> </mml:mover> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjac20daieqn1.gif" xlink:type="simple" /> </jats:inline-formula> &lt; 7.2 × 10<jats:sup>−9</jats:sup> and &lt; 9.7 × 10<jats:sup>−9</jats:sup> <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub> yr<jats:sup>−1</jats:sup> for each (at a wind velocity <jats:italic>v</jats:italic> <jats:sub> <jats:italic>w</jats:italic> </jats:sub> = 100 km s<jats:sup>−1</jats:sup> and a radius of <jats:italic>R</jats:italic> ≈ 10<jats:sup>16</jats:sup> cm), assuming any X-ray emission would originate from inverse Compton emission from optical photons upscattered by the supernova shock. If the supernova environment was a constant-density medium, we would find a number density limit of <jats:italic>n</jats:italic> <jats:sub>CSM</jats:sub> &lt; 36 and &lt; 65 cm<jats:sup>−3</jats:sup>, respectively. These X-ray limits rule out all plausible symbiotic progenitor systems, as well as large swathes of parameter space associated with the single degenerate scenario, such as mass loss at the outer Lagrange point and accretion winds. We also present late-time optical spectroscopy of SN 2020nlb, and set strong limits on any swept up hydrogen (<jats:italic>L</jats:italic> <jats:sub>H<jats:italic>α</jats:italic> </jats:sub> &lt; 2.7 × 10<jats:sup>37</jats:sup> erg s<jats:sup>−1</jats:sup>) and helium (<jats:italic>L</jats:italic> <jats:sub>He,<jats:italic>λ</jats:italic>6678</jats:sub> &lt; 2.7 × 10<jats:sup>37</jats:sup> erg s<jats:sup>−1</jats:sup>) from a nondegenerate companion, corresponding to <jats:italic>M</jats:italic> <jats:sub> <jats:italic>H</jats:italic> </jats:sub> ≲ 0.7–2 × 10<jats:sup>−3</jats:sup> <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub> and <jats:italic>M</jats:italic> <jats:sub>He</jats:sub> ≲ 4 × 10<jats:sup>−3</jats:sup> <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>. Radio observations of SN 2020nlb at 14.6 days after explosion also yield a non-detection, ruling out most plausible symbiotic progenitor systems. While we have doubled the sample of normal Type Ia supernovae with deep X-ray limits, more observations are needed to sample the full range of luminosities and subtypes of these explosions, and set statistical constraints on their circumbinary environments.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>A number of gamma-ray bursts (GRBs) exhibit the simultaneous bumps in their optical and X-ray afterglows around the jet break. These bumps are similar to the afterglows of GRB 170817A, except preceded by a long shallow decay. Its origin is unclear. We suggest that these late simultaneous bumps may sound a transition of circumburst environment from a free-wind medium to a constant density medium, e.g., the shocked-wind medium. In this paper, we study the emission of an external-forward shock propagating in a free-to-shocked wind environment at different viewing angles. The late simultaneous bumps/plateaux followed by a steep decay are found in the optical and X-ray afterglows for high-viewing-angle observers. In addition, these theoretical bumps are preceded by a long plateau or shallow decay, which is formed during the external-forward shock propagating in the free-wind environment. For low-viewing-angle observers, the above bumps also appear but only in the situation where the structured jet has a low characteristic angle and the deceleration radius of the in-core jet flow is at around or beyond the free-wind boundary. We search GRBs for afterglows with the late simultaneous optical and X-ray bumps followed by a steep decay. GRBs 120326A, 100901A, 100814A, and 120404A are obtained. We find that an off-core (in-core) observed external-forward shock in a free-to-shocked wind environment can well explain the optical and X-ray afterglows in GRBs 120326A, 100901A, and 100814A (GRB 120404A).</jats:p>