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<jats:title>Abstract</jats:title> <jats:p>We study the evolution and gravitational wave emission of white dwarf–black hole accreting binaries with a semianalytical model. These systems will evolve across the mHz gravitational wave frequency band and potentially be detected by the Laser Interferometer Space Antenna (LISA). We identify new universal relations for this class of binaries, which relate the component masses to the gravitational wave frequency and its first derivative. Combined with the high precision measurements possible with LISA, these relations could allow us to infer the component masses and the luminosity distance of the source. LISA has therefore the potential to detect and characterize a virtually unexplored binary population.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The thermal structure of protoplanetary disks is a fundamental characteristic of the system that has wide-reaching effects on disk evolution and planet formation. In this study, we constrain the 2D thermal structure of the protoplanetary disk TW Hya structure utilizing images of seven CO lines. This includes new ALMA observations of <jats:sup>12</jats:sup>CO <jats:italic>J</jats:italic> = 2–1 and C<jats:sup>18</jats:sup>O <jats:italic>J</jats:italic> = 2–1 as well as archival ALMA observations of <jats:sup>12</jats:sup>CO <jats:italic>J</jats:italic> = 3–2, <jats:sup>13</jats:sup>CO <jats:italic>J</jats:italic> = 3–2 and 6–5, and C<jats:sup>18</jats:sup>O <jats:italic>J</jats:italic> = 3–2 and 6–5. Additionally, we reproduce a Herschel observation of the HD <jats:italic>J</jats:italic> = 1–0 line flux and the spectral energy distribution and utilize a recent quantification of CO radial depletion in TW Hya. These observations were modeled using the thermochemical code RAC2D, and our best-fit model reproduces all spatially resolved CO surface brightness profiles. The resulting thermal profile finds a disk mass of 0.025 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub> and a thin upper layer of gas depleted of small dust with a thickness of ∼1.2% of the corresponding radius. Using our final thermal structure, we find that CO alone is not a viable mass tracer, as its abundance is degenerate with the total H<jats:sub>2</jats:sub> surface density. Different mass models can readily match the spatially resolved CO line profiles with disparate abundance assumptions. Mass determination requires additional knowledge, and, in this work, HD provides the additional constraint to derive the gas mass and support the inference of CO depletion in the TW Hya disk. Our final thermal structure confirms the use of HD as a powerful probe of protoplanetary disk mass. Additionally, the method laid out in this paper is an employable strategy for extraction of disk temperatures and masses in the future.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>As a gamma-ray burst (GRB) jet drills its way through the collapsing star, it traps a baryonic “cork” ahead of it. Here we explore a prompt emission model for GRBs in which the jet does not cross the cork, but rather photons that are emitted deep in the flow largely by pair annihilation are scattered inside the expanding cork and escape largely from the back end of it as they push it from behind. Due to the relativistic motion of the cork, these photons are easily seen by an observer close to the jet axis peaking at <jats:italic>ε</jats:italic> <jats:sub>peak</jats:sub> ∼ few ×100 keV. We show that this model naturally explains several key observational features: (1) a high-energy power-law index <jats:italic>β</jats:italic> <jats:sub>1</jats:sub> − 2 to − 5 with an intermediate thermal spectral region; (2) decay of the prompt emission light curve as ∼ <jats:italic>t</jats:italic> <jats:sup>−2</jats:sup>; (3) delay of soft photons; (4) a peak energy–isotropic energy (the so-called “Amati”) correlation, <jats:inline-formula> <jats:tex-math> <?CDATA ${\varepsilon }_{\mathrm{peak}}\sim {\varepsilon }_{\mathrm{iso}}^{m}$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>ε</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>peak</mml:mi> </mml:mrow> </mml:msub> <mml:mo>∼</mml:mo> <mml:msubsup> <mml:mrow> <mml:mi>ε</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>iso</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>m</mml:mi> </mml:mrow> </mml:msubsup> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabd242ieqn1.gif" xlink:type="simple" /> </jats:inline-formula>, with <jats:italic>m</jats:italic> ∼ 0.45, resulting from different viewing angles (at low luminosities, our model predicts an observable turnoff in the Amati relation); (5) an anticorrelation between the spectral FWHM and time as <jats:italic>t</jats:italic> <jats:sup>−1</jats:sup>; (6) temporal evolution <jats:italic>ε</jats:italic> <jats:sub>peak</jats:sub> ∼ <jats:italic>t</jats:italic> <jats:sup>−1</jats:sup>; and (7) distribution of peak energies <jats:italic>ε</jats:italic> <jats:sub>peak</jats:sub> in the observed GRB population. The model is applicable for single-pulse GRB light curves and their respective spectra. We discuss the consequences of our model in view of current and future prompt emission observations.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We conducted submillimeter observations with the Atacama Large Millimeter/submillimeter Array (ALMA) of star-forming galaxies at <jats:italic>z</jats:italic> ∼ 3.3, whose gas-phase metallicities have been measured previously. We investigated the dust and gas contents of the galaxies at <jats:italic>z</jats:italic> ∼ 3.3 and studied the interaction of galaxies with their circumgalactic or intergalactic medium at this epoch by probing their gas mass fractions and gas-phase metallicities. Single-band dust continuum emission tracing dust mass and the relation between the gas-phase metallicity and gas-to-dust mass ratio were used to estimate the gas masses. The estimated gas mass fractions and depletion timescales are <jats:italic>f</jats:italic> <jats:sub>gas</jats:sub>= 0.20–0.75 and <jats:italic>t</jats:italic> <jats:sub>dep</jats:sub>= 0.09–1.55 Gyr. Although the galaxies appear to be tightly distributed around the star-forming main sequence at <jats:italic>z</jats:italic> ∼ 3.3, both quantities show a wider spread at a fixed stellar mass than expected from the scaling relation, suggesting a large diversity of fundamental gas properties in star-forming galaxies that apparently lie on the main sequence. When we compared gas mass fraction and gas-phase metallicity in star-forming galaxies at <jats:italic>z</jats:italic> ∼ 3.3 and at lower redshifts, star-forming galaxies at <jats:italic>z</jats:italic> ∼ 3.3 appear to be more metal poor than local galaxies with similar gas mass fractions. Using the gas regulator model to interpret this offset, we find that this can be explained by a higher mass-loading factor, suggesting that the mass-loading factor in outflows increases at earlier cosmic times.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We present deep Hubble Space Telescope (HST) photometry of the ultra-faint dwarf galaxy Eridanus II (Eri II). Eri II, which has an absolute magnitude of <jats:italic>M</jats:italic> <jats:sub> <jats:italic>V</jats:italic> </jats:sub> = −7.1, is located at a distance of 339 kpc, just beyond the virial radius of the Milky Way. We determine the star formation history of Eri II and measure the structure of the galaxy and its star cluster. We find that a star formation history consisting of two bursts, constrained to match the spectroscopic metallicity distribution of the galaxy, accurately describes the Eri II stellar population. The best-fit model implies a rapid truncation of star formation at early times, with &gt;80% of the stellar mass in place before <jats:italic>z</jats:italic> ∼ 6. A small fraction of the stars could be as young as 8 Gyr, but this population is not statistically significant; Monte Carlo simulations recover a component younger than 9 Gyr only 15% of the time, where they represent an average of 7 ± 4% of the population. These results are consistent with theoretical expectations for quenching by reionization. The HST depth and angular resolution enable us to show that Eri II’s cluster is offset from the center of the galaxy by a projected distance of 23 ± 3 pc. This offset could be an indication of a small (∼50–75 pc) dark matter core in Eri II. Moreover, we demonstrate that the cluster has a high ellipticity of <jats:inline-formula> <jats:tex-math> <?CDATA ${0.31}_{-0.06}^{+0.05}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabd31bieqn1.gif" xlink:type="simple" /> </jats:inline-formula> and is aligned with the orientation of Eri II within 3° ± 6°, likely due to tides. The stellar population of the cluster is indistinguishable from that of Eri II itself.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We performed the largest and most homogeneous spectroscopic survey of field RR Lyraes (RRLs). We secured ≈6300 high-resolution (HR, <jats:italic>R</jats:italic> ∼ 35,000) spectra for 143 RRLs (111 fundamental, RRab; 32 first-overtone, RRc). The atmospheric parameters were estimated by using the traditional approach and the iron abundances were measured by using an LTE line analysis. The resulting iron distribution shows a well-defined metal-rich tail approaching solar iron abundance. This suggests that field RRLs experienced a complex chemical enrichment in the early halo formation. We used these data to develop a new calibration of the Δ<jats:italic>S</jats:italic> method. This diagnostic, based on the equivalent widths of Ca <jats:sc>ii</jats:sc> K and three Balmer (H<jats:sub> <jats:italic>δ</jats:italic>,<jats:italic>γ</jats:italic>,<jats:italic>β</jats:italic> </jats:sub>) lines, traces the metallicity of RRLs. For the first time, the new empirical calibration: (i) includes spectra collected over the entire pulsation cycle; (ii) includes RRc variables; (iii) relies on spectroscopic calibrators covering more than three dex in iron abundance; and (iv) provides independent calibrations based on one/two/three Balmer lines. The new calibrations were applied to a data set of both SEGUE-SDSS and degraded HR spectra totalling 6451 low-resolution (<jats:italic>R</jats:italic> ∼ 2000) spectra for 5001 RRLs (3439 RRab, 1562 RRc). This resulted in an iron distribution with a median <jats:italic>η</jats:italic> = −1.55 ± 0.01 and <jats:italic>σ</jats:italic> = 0.51 dex, in good agreement with literature values. We also found that RRc are 0.10 dex more metal-poor than RRab variables, and have a distribution with a smoother metal-poor tail. This finding supports theoretical prescriptions suggesting a steady decrease in the RRc number when moving from metal-poor to metal-rich stellar environments.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>A likely detection of <jats:italic>γ</jats:italic>-ray emission from the region of Kepler’s Supernova Remnant (SNR) is reported by analyzing ∼12 yr of Pass 8 data of the Fermi Large Area Telescope. Its photon flux is (4.85 ± 0.60) × 10<jats:sup>−10</jats:sup> ph cm<jats:sup>−2</jats:sup> s<jats:sup>−1</jats:sup> with ∼4<jats:italic>σ</jats:italic> significance in 0.2−500 GeV. Moreover, our results show that there is no significant variability in the light curve of ∼12 yr, and its position can well overlap with the observation result of Chandra in hard X-ray band with a good spatial resolution of 0.″5, so the source is likely to be the GeV <jats:italic>γ</jats:italic>-ray counterpart of Kepler’s SNR. The spectral energy distribution of <jats:italic>γ</jats:italic>-rays from Kepler’s SNR favors a hadronic origin in GeV band. Through analyzing multi-band data from radio to <jats:italic>γ</jats:italic>-ray and surveying the distribution from the surrounding CO molecules cloud, we found that if this <jats:italic>γ</jats:italic>-ray emission is from Kepler’s SNR, then it may originate from interactions between the relativistic protons escaping from the shock of Kepler’s SNR and surrounding CO gas molecules. However, more observation data are necessary to firmly confirm the association between the <jats:italic>γ</jats:italic>-ray source and Kepler’s SNR in the future.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We address the shocks from acoustic pulses and wave trains in general one-dimensional flows, with an emphasis on the application to super-Eddington outbursts in massive stars. Using approximate adiabatic invariants, we generalize the classical equal-area technique in its integral and differential forms. We predict shock evolution for the case of an initially sinusoidal but finite wave train, with separate solutions for internal shocks and head or tail shocks, and demonstrate detailed agreement with numerical simulations. Our internal shock solution motivates improved expressions for the shock-heating rate. Our solution for head and tail shocks demonstrates that these preserve dramatically more wave energy to large radii and have a greater potential for the direct ejection of matter. This difference highlights the importance of the waveform for shock dynamics. Our weak-shock analysis predicts when shocks will become strong and provides a basis from which this transition can be addressed. We use it to estimate the mass ejected by sudden sound pulses and weak central explosions.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We present an in-depth study of surface brightness fluctuations (SBFs) in low-luminosity stellar systems. Using the MIST models, we compute theoretical predictions for absolute SBF magnitudes in the LSST, HST ACS/WFC, and proposed Roman Space Telescope filter systems. We compare our calculations to observed SBF–color relations of systems that span a wide range of age and metallicity. Consistent with previous studies, we find that single-age population models show excellent agreement with observations of low-mass galaxies with 0.5 ≲ <jats:italic>g</jats:italic> − <jats:italic>i</jats:italic> ≲ 0.9. For bluer galaxies, the observed relation is better fit by models with composite stellar populations. To study SBF recovery from low-luminosity systems, we perform detailed image simulations in which we inject fully populated model galaxies into deep ground-based images from real observations. Our simulations show that LSST will provide data of sufficient quality and depth to measure SBF magnitudes with precisions of ∼0.2–0.5 mag in ultra-faint <jats:inline-formula> <jats:tex-math> <?CDATA $\left({10}^{4}\leqslant {M}_{\star }/{M}_{\odot }\leqslant {10}^{5}\right)$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabd030ieqn1.gif" xlink:type="simple" /> </jats:inline-formula> and low-mass classical (<jats:italic>M</jats:italic> <jats:sub>⋆</jats:sub> ≤ 10<jats:sup>7</jats:sup> <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>) dwarf galaxies out to ∼4 Mpc and  ∼25 Mpc, respectively, within the first few years of its deep-wide-fast survey. Many significant practical challenges and systematic uncertainties remain, including an irreducible “sampling scatter” in the SBFs of ultra-faint dwarfs due to their undersampled stellar mass functions. We nonetheless conclude that SBFs in the new generation of wide-field imaging surveys have the potential to play a critical role in the efficient confirmation and characterization of dwarf galaxies in the nearby universe.</jats:p>