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<jats:title>Abstract</jats:title> <jats:p>We investigate the orbital motions of dwarf spheroidal galaxies (dSphs) in the halo of the Milky Way (MW) to understand their possible effects on the diversity of the star formation histories seen in these MW satellites. In this work, we explicitly consider a time-varying gravitational potential due to the growth of the MW’s dark halo mass to calculate the long-term orbital evolutions of the dSphs, guided with Gaia DR2 proper motions, over the past 13.5 billion years. We find that the infall time of a satellite, defined as when the galaxy first crosses within the growing virial radius of the MW’s halo, coincides well with the time when the star formation rate (SFR) is peaked for the sample of classical dSphs. On the other hand, ultra-faint dSphs already finished their SF activity prior to their infall times as already suggested in previous works, but there is a signature that their earlier SF histories are affected by interaction with the growing MW’s halo to some extent. We also find, for classical dSphs, that the relative fraction of stars formed after the peak of the SFR to the current stellar mass is smaller for the smaller pericentric radius of the galaxy at its first infall. These results suggest that the infalling properties of the dSphs into the MW and the resultant environmental effects, such as ram pressure stripping and/or tidal disturbance in the MW’s dark halo containing hot gas, play important roles in their star formation histories.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We analyzed the long-cadence (LC) light curves of 5435 stars with effective temperatures of 7000–10,500 K in Data Release 25 of Kepler to extend the samples of A-type flare stars and study their flare properties. A total of 103 flare stars are detected, of which 72 have the Large Sky Area Multi-Object fiber Spectroscopic Telescope spectral types, resulting in 51 A-type stars, including 17 new and 34 known. A total of 352 flares are detected from the 51 stars. The flare durations, amplitudes, and energies in the Kepler band are calculated, with medians of 2.5 hr, 2.1 mmag, and 10<jats:sup>35.8</jats:sup> erg, respectively. We give the flare frequency distribution (FFD) of the A-type stars. Within the same energy range, the flare rates of our FFD are lower than those in previous research, which was also based on Kepler LC data, and the FFD slope is consistent with that in the research. The activity–rotation relation of the A-type flare stars is shown for the first time, in which the flare activity decreases as the rotation period increases. The relation is similar to that of the late-type stars in previous studies. For the two relatively bright A-type flare stars, KIC 5360548 and KIC 9468475, three high-resolution spectra are observed on different nights for each of them, and radial velocities are calculated and compared with that of the previous research. The comparisons suggest that there is no evidence of the KIC 5360548 companion, and KIC 9468475 could have a low-mass companion.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Faint <jats:italic>z</jats:italic> ∼ 5 quasars with <jats:italic>M</jats:italic> <jats:sub>1450</jats:sub> ∼ −23 mag are known to be potentially important contributors to the ultraviolet ionizing background in the postreionization era. However, their number density has not been well determined, making it difficult to assess their role in the early ionization of the intergalactic medium (IGM). In this work, we present the updated results of our <jats:italic>z</jats:italic> ∼ 5 quasar survey using the Infrared Medium-deep Survey (IMS), a near-infrared imaging survey covering an area of 85 deg<jats:sup>2</jats:sup>. From our spectroscopic observations with the Gemini Multi-Object Spectrograph on the Gemini-South 8 m telescope, we discovered eight new quasars at <jats:italic>z</jats:italic> ∼ 5 with  −26.1 ≤ <jats:italic>M</jats:italic> <jats:sub>1450</jats:sub> ≤ −23.3. Combining our IMS faint quasars (<jats:italic>M</jats:italic> <jats:sub>1450</jats:sub> &gt; −27 mag) with the brighter Sloan Digital Sky Survey quasars (<jats:italic>M</jats:italic> <jats:sub>1450</jats:sub> &lt; −27 mag), we derive the <jats:italic>z</jats:italic> ∼ 5 quasar luminosity function (QLF) without any fixed parameters down to the magnitude limit of <jats:italic>M</jats:italic> <jats:sub>1450</jats:sub> = −23 mag. We find that the faint-end slope of the QLF is very flat (<jats:inline-formula> <jats:tex-math> <?CDATA $\alpha =-{1.2}_{-0.6}^{+1.4}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabc0eaieqn1.gif" xlink:type="simple" /> </jats:inline-formula>), with a characteristic luminosity of <jats:inline-formula> <jats:tex-math> <?CDATA ${M}_{1450}^{* }=-{25.8}_{-1.1}^{+1.4}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabc0eaieqn2.gif" xlink:type="simple" /> </jats:inline-formula> mag. The number density of <jats:italic>z</jats:italic> ∼ 5 quasars from the QLF gives an ionizing emissivity at 912 Å of <jats:italic>ϵ</jats:italic> <jats:sub>912</jats:sub> = (3.7–7.1) × 10<jats:sup>23</jats:sup> erg s<jats:sup>−1</jats:sup> Hz<jats:sup>−1</jats:sup> Mpc<jats:sup>−3</jats:sup> and an ionizing photon density of <jats:inline-formula> <jats:tex-math> <?CDATA ${\dot{n}}_{\mathrm{ion}}=(3.0\mbox{--}5.7)\,\times {10}^{49}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabc0eaieqn3.gif" xlink:type="simple" /> </jats:inline-formula> Mpc<jats:sup>−3</jats:sup> s<jats:sup>−1</jats:sup>. These results imply that quasars are responsible for only 10%–20% (up to 50% even in the extreme case) of the photons required to completely ionize the IGM at <jats:italic>z</jats:italic> ∼ 5, disfavoring the idea that quasars alone could have ionized the IGM at <jats:italic>z</jats:italic> ∼ 5.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Stars often reside in binary configurations. The nuclear star cluster surrounding the supermassive black hole (SMBH) in the Galactic Center (GC) is expected to include a binary population. In this dense environment, a binary frequently encounters and interacts with neighboring stars. These interactions vary from small perturbations to violent collisions. In the former case, weak gravitational interactions unbind a soft binary over the evaporation timescale, which depends on the binary properties as well as the density of surrounding objects and velocity dispersion. Similarly, collisions can also unbind a binary, and the collision rate depends on the density. Thus, the detection of a binary with known properties can constrain the density profile in the GC with implications for the number of compact objects, which are otherwise challenging to detect. We estimate the density necessary to unbind a binary within its lifetime for an orbit of arbitrary eccentricity about the SMBH. We find that the eccentricity has a minimal impact on the density constraint. In this proof of concept, we demonstrate that this procedure can probe the density in the GC using hypothetical young and old binaries as examples. Similarly, a known density profile provides constraints on the binary orbital separation. Our results highlight the need to consider multiple dynamical processes in tandem. In certain cases, often closer to the SMBH, the collision timescale rather than the evaporation timescale gives the more stringent density constraint, while other binaries farther from the SMBH provide unreliable density constraints because they migrate inward due to mass segregation.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>There are only a handful of known short-period pre-main-sequence spectroscopic binaries with significant accretion rates (Class II sources). AK Sco stands out in this list because the system is composed of two equal mass F5 stars in a highly eccentric orbit thus both stars get as close as 11 stellar radii at periastron passage. This configuration is optimal for accretion studies because enhanced accretion events can be precisely timed at periastron passage. In this work, we present the results from the monitoring of the AK Sco system with Hubble during three consecutive periastron passages. These data provide a unique data set to spectroscopically characterize accretion and evaluate the intercycle variability of the system. Clear evidence of accretion rate enhancement was observed in cycles 1 and 3: the blueing of the near-UV continuum, the sudden flux increase of important accretion tracers, such as the N <jats:sc>V</jats:sc>, Si <jats:sc>IV,</jats:sc> and C <jats:sc>IV</jats:sc> lines, and also of neutral/singly ionized species such as O <jats:sc>I</jats:sc> and C <jats:sc>II</jats:sc>. Also, variations in the Si <jats:sc>III</jats:sc>]/C <jats:sc>III</jats:sc>] ratio reveal an enhancement of the electron density by an order of magnitude during the periastron passage. Moreover, in cycle 3, the spectral resolution of the observations obtained with the Cosmic Origins Spectrograph enabled us to discern that the flow was channeled preferentially into one of the two components. The most remarkable feature in the cycle-to-cycle variations was the detection of a notable increase of the UV flux from cycle 1 to cycle 2 that was not accompanied by enhanced accretion signatures.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Millimeter observations of disks around young stars reveal substructures indicative of gas pressure traps that may aid grain growth and planet formation. We present Submillimeter Array observations of HD 34700: two Herbig Ae stars in a close binary system (Aa/Ab, ∼0.25 au), surrounded by a disk presenting a large cavity and spiral arms seen in scattered light, and two distant, lower-mass companions. These observations include 1.3 mm continuum emission and the <jats:sup>12</jats:sup>CO 2–1 line at ∼05 (178 au) resolution. They resolve a prominent azimuthal asymmetry in the continuum and Keplerian rotation of a circumbinary disk in the <jats:sup>12</jats:sup>CO line. The asymmetry is located at a radius of 155<jats:sup>+11</jats:sup> <jats:sub>−7</jats:sub> au, consistent with the edge of the scattered-light cavity, being resolved in both radius (72 <jats:sup>+14</jats:sup> <jats:sub>−15</jats:sub> au) and azimuth (FWHM = 64<jats:sup>°+8</jats:sup> <jats:sub>−7</jats:sub>). The strong asymmetry in millimeter continuum emission could be evidence for a dust trap, together with the more symmetric morphology of <jats:sup>12</jats:sup>CO emission and small grains. We hypothesize an unseen circumbinary companion responsible for the cavity in scattered light and creating a vortex at the cavity edge that manifests in dust trapping. The disk mass has limitations imposed by the detection of <jats:sup>12</jats:sup>CO and nondetection of <jats:sup>13</jats:sup>CO. We discuss its consequences for the potential past gravitational instability of this system, likely accounting for the rapid formation of a circumbinary companion. We also report the discovery of resolved continuum emission associated with HD 34700B (projected separation ∼1850 au), which we explain through a circumstellar disk.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Gravitational microlensing is a powerful tool to search for a population of invisible black holes (BHs) in the Milky Way (MW), including isolated BHs and binary BHs at wide orbits that are complementary to gravitational wave observations. By monitoring highly populated regions of source stars like the MW bulge region, one can pursue microlensing events due to these BHs. We find that if BHs have a Salpeter-like mass function extended beyond 30<jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub> and a similar velocity and spatial structure to stars in the Galactic bulge and disk regions, the BH population is a dominant source of microlensing events at long timescales of the microlensing light curve  ≳100 days. This is due to a boosted sensitivity of the microlensing event rate to lens mass, given as <jats:italic>M</jats:italic> <jats:sup>2</jats:sup>, for such long-timescale events. A monitoring observation of 2 × 10<jats:sup>10</jats:sup> stars in the bulge region over 10 yr with the Rubin Observatory Legacy Survey of Space and Time (LSST) would enable one to find about 6 × 10<jats:sup>5</jats:sup> BH microlensing events. We evaluate the efficiency of potential LSST cadences for characterizing the light curves of BH microlensing and find that nearly all events of long timescales can be detected.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Characterization of the gas component in debris disks is of fundamental importance for understanding their origin. To address this goal, we have conducted non-LTE (local thermodynamic equilibrium) analyses of the rotational spectral lines of CO including those of rare isotopologues (<jats:sup>13</jats:sup>CO and C<jats:sup>18</jats:sup>O) that have been observed toward the gaseous debris disks of 49 Ceti and HD 21997 with the Atacama Large Millimeter/submillimeter Array (ALMA) and the Atacama Compact Array (ACA). The analyses have been carried out for a wide range of H<jats:sub>2</jats:sub> density, and the observed line intensities are found to be reproduced as long as the H<jats:sub>2</jats:sub> density is higher than 10<jats:sup>3</jats:sup> cm<jats:sup>−3</jats:sup>. The CO column density and the gas temperature are evaluated to be (1.8–5.9) × 10<jats:sup>17</jats:sup> cm<jats:sup>−2</jats:sup> and 8–11 K for 49 Ceti and (2.6–15) × 10<jats:sup>17</jats:sup> cm<jats:sup>−2</jats:sup> and 8–12 K for HD 21997, respectively, where the H<jats:sub>2</jats:sub> collision is assumed for the rotational excitation of CO. The results do not change significantly even if electron collision is considered. Thus, CO molecules can be excited under environments containing no H<jats:sub>2</jats:sub> or a small number of H<jats:sub>2</jats:sub> molecules, even where collisions with CO, C, O, and C<jats:sup>+</jats:sup> would make an important contribution to the CO excitation in addition to H<jats:sub>2</jats:sub>. Meanwhile, our result does not rule out the case of abundant H<jats:sub>2</jats:sub> molecules. The low gas temperature observed in the debris disks is discussed in terms of inefficient heating by interstellar and stellar UV radiation.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We report on the investigation of a very high energy (VHE), Galactic <jats:italic>γ</jats:italic>-ray source recently discovered at &gt;50 GeV using the Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope (Fermi). This object, 2FHL J1703.4–4145, displays a very hard &gt;50 GeV spectrum with a photon index Γ<jats:sub> <jats:italic>γ</jats:italic> </jats:sub> ∼ 1.2 in the 2FHL catalog and, as such, is one of the most extreme sources in the 2FHL subsample of Galactic objects. A detailed analysis of the available multiwavelength data shows that this source is located on the western edge of the supernova remnant (SNR) G344.7–0.1, along with extended TeV source, HESS J1702–420. The observations and the spectral energy distribution modeling support a scenario where this <jats:italic>γ</jats:italic>-ray source is the byproduct of the interaction between the SNR shock and the dense surrounding medium, with escaping cosmic rays (CRs) diffusing into the dense environment and interacting with a large local cloud, generating the observed TeV emission. If confirmed, an interaction between the SNR CRs and a nearby cloud would make 2FHL J1703.4–4145 another promising candidate for efficient particle acceleration of the 2FHL Galactic sample, following the first candidate from our previous investigation of a likely shock–cloud interaction occurring on the western edge of the Vela SNR.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Asymmetrical features in disks provide indirect evidence of embedded objects, such as planets. Observed with the Atacama Large Millimeter/submillimeter Array (ALMA), the circumstellar disk in MWC 758 traced with thermal dust continuum emission at wavelengths of 0.9 mm with an angular resolution up to 01 (15 au) exhibits an asymmetrical dust ring with additional features. In order to analyze the structures azimuthally and radially, we split the dust ring into small segments in azimuth. For each segment, we fit two-Gaussian functions to the radial intensity profile. The obtained best-fit parameters as a function of azimuth are analyzed. Three spiral-like arm structures are identified. When fitting the 0.9 mm features with the spiral density wave theory using the WKB approximation, two sets of disk aspect ratios are found: one solution gives relatively low values (∼0.03) while the other solution is at the upper bound of the free parameter (∼0.2). The planet locations suggested by the upper-bound result are similar to the ones determined by Benisty et al. for the NIR polarized intensity image. Comparing the reported spiral-like structures with the higher angular resolution (004) ALMA image in Dong et al., we identify different structures in the west of the disk due to differences in the adopted analysis methods and the respective resolutions of the images.</jats:p>