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<jats:title>Abstract</jats:title> <jats:p>The X-ray source 3XMM J150052.0+015452 was discovered as a spectacular tidal disruption event candidate during a prolonged (&gt;11 yr) outburst. It exhibited unique quasi-soft X-ray spectra of characteristic temperature <jats:italic>kT</jats:italic> ∼ 0.3 keV for several years at the peak, but in a recent Chandra observation (10 yr into the outburst) a super-soft X-ray spectrum of <jats:italic>kT</jats:italic> ∼ 0.15 keV was detected. Such dramatic spectral softening could signal the transition from the super-Eddington to thermal state or the temporary presence of a warm absorber. Here we report on our study of four new XMM-Newton follow-up observations of the source. We found that they all showed super-soft spectra, suggesting that the source had remained super-soft for &gt;5 yr. Then its spectral change is best explained as due to the super-Eddington to thermal spectral state transition. The fits to the thermal-state spectra suggested a smaller absorption toward the source than that obtained in Lin et al. This led us to update the modeling of the event as due to the disruption of a 0.75 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub> star by a massive black hole of <jats:italic>a few</jats:italic> × 10<jats:sup>5</jats:sup> <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>. We also obtained two HST images in the F606W and F814W filters and found that the dwarf star-forming host galaxy can be resolved into a dominant disk and a smaller bulge. No central point source was clearly seen in either filter, ruling out strong optical emission associated with the X-ray activity.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Current cosmological observations point to a serious discrepancy between the observed Hubble parameter obtained using direct versus cosmic microwave background radiation measurements. Besides this so-called Hubble–Lemaître tension, we also find considerable evidence in diverse cosmological observables that indicate violation of the cosmological principle. In this paper, we suggest that both these discrepancies are related and can be explained by invoking superhorizon perturbations in the universe. We implement this by considering a single superhorizon mode and showing that it leads to both a dipole in large-scale structures and a shift in the Hubble–Lemaître parameter. Furthermore, the shift is found to be independent of redshift up to a certain distance. This is nicely consistent with the data.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>A highly misaligned gas disk around one component of a binary star system can undergo global Kozai–Lidov (KL) oscillations for which the disk inclination and eccentricity are exchanged. With hydrodynamical simulations of a gas and dust disk we explore the effects of these oscillations on the dust density distribution. For dust that is marginally coupled to the gas (St ≈ 1), we find that the dust undergoes dynamical behavior similar to that of the gas disk but the radial distribution of dust may be very different from that of the gas. The inward radial drift of the dust is faster in an eccentric disk leading to a smaller outer dust disk radius. The dust breaks into multiple narrow eccentric rings during the highly eccentric disk phase. Eccentric dust-ring formation may have significant implications for the formation of planets in misaligned disks. We suggest that multiple dust rings may generally occur within gas disks that have sufficiently strong eccentricity peaks at intermediate radii.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Following the discovery of the first exoplanet candidate transiting a white dwarf (WD), a “white dwarf opportunity” for characterizing the atmospheres of terrestrial exoplanets around WDs is emerging. Large planet-to-star size ratios and hence large transit depths make transiting WD exoplanets favorable targets for transmission spectroscopy; conclusive detection of spectral features on an Earth-like planet transiting a close-by WD can be achieved within a medium James Webb Space Telescope program. Despite the apparently promising opportunity, however, the post-main sequence evolutionary history of a first-generation WD exoplanet has never been incorporated in atmospheric modeling. Furthermore, second-generation planets formed in WD debris disks have never been studied from a photochemical perspective. We demonstrate that transmission spectroscopy can identify a second-generation rocky WD exoplanet with a thick (∼1 bar) H<jats:sub>2</jats:sub>-dominated atmosphere. In addition, we can infer outgassing activities of a WD exoplanet based on its transmission spectra and test photochemical runaway by studying CH<jats:sub>4</jats:sub> buildup.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>V1298 Tau is a young (20–30 Myr) solar-mass K star hosting four transiting exoplanets with sizes between 0.5 and 0.9 <jats:italic>R</jats:italic> <jats:sub>J</jats:sub>. Given the system’s youth, it provides a unique opportunity to understand the evolution of planetary radii at different separations in the same stellar environment. V1298 Tau was originally observed 6 yr ago during K2 Campaign 4. Now, V1298 Tau has been re-observed during the extended mission of NASA’s Transiting Exoplanet Survey Satellite (TESS). Here, we present new photometric observations of V1298 Tau from TESS. We use the TESS data to update the ephemerides for V1298 Tau bcde as well as compare newly observed radii to those measured from the K2 light curve, finding shallower transits for V1298 Tau bcd in the redder TESS bandpass at the 1–2<jats:italic>σ</jats:italic> level. We suspect the difference in radii is due to starspot crossing events or contamination from nearby faint stars on the same pixels as V1298 Tau. Additionally, we catch a second transit of V1298 Tau e and present a new method for deriving the marginalized posterior probability of a planet’s period from two transits observed years apart. We find the highest probability period for V1298 Tau e to be in a near 2:1 mean motion resonance with V1298 Tau b which, if confirmed, could make V1298 Tau bcde a four-planet resonant chain. V1298 Tau is the target of several ongoing and future observations. These updated ephemerides will be crucial for scheduling future transit observations and interpreting future Doppler tomographic or transmission spectroscopy signals.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Know thy star, know thy planetary atmosphere. Every exoplanet with atmospheric measurements orbits around a star, and the stellar environment directly affects the planetary atmosphere. Here we present the emission spectrum of ultra-hot Jupiter KELT-20b which provides an observational link between host-star properties and planet atmospheric thermal structure. It is currently the only planet with thermal emission measurements in the <jats:italic>T</jats:italic> <jats:sub>eq</jats:sub> ∼ 2200 K range that orbits around an early A-type star. By comparing it with other similar ultra-hot Jupiters around FGK stars, we can better understand how different host-star types influence planetary atmospheres. The emission spectrum covers 0.6–4.5 <jats:italic>μ</jats:italic>m with data from TESS, HST WFC3/G141, and Spitzer 4.5 <jats:italic>μ</jats:italic>m channel. KELT-20b has a 1.4 <jats:italic>μ</jats:italic>m water feature strength metric of <jats:inline-formula> <jats:tex-math> <?CDATA ${{\rm{S}}}_{{{\rm{H}}}_{2}{\rm{O}}}$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi mathvariant="normal">S</mml:mi> </mml:mrow> <mml:mrow> <mml:msub> <mml:mrow> <mml:mi mathvariant="normal">H</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> </mml:msub> <mml:mi mathvariant="normal">O</mml:mi> </mml:mrow> </mml:msub> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjlac4968ieqn1.gif" xlink:type="simple" /> </jats:inline-formula> = −0.097 ± 0.02 and a blackbody brightness temperature difference of 528 K between WFC3/G141 (T<jats:sub> <jats:italic>b</jats:italic> </jats:sub> = 2402 ± 14 K) and Spitzer 4.5 <jats:italic>μ</jats:italic>m channel (T<jats:sub> <jats:italic>b</jats:italic> </jats:sub> = 2930 ± 59 K). These very large H<jats:sub>2</jats:sub>O and CO emission features combined with the A-type host star make KELT-20b a unique planet among other similar hot Jupiters. The abundant FUV, NUV, and optical radiation from its host star (T<jats:sub>eff</jats:sub> = 8720 ± 250 K) is expected to be the key that drives its strong thermal inversion and prominent emission features based on previous PHOENIX model calculations.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Born-again stars allow probing stellar evolution in human timescales and provide the most promising path for the formation of hydrogen-deficient post-asymptotic giant branch objects, but their cold and molecular components remain poorly explored. Here we present ALMA observations of V 605 Aql that unveil for the first time the spatio-kinematic distribution of the molecular material associated with a born-again star. Both the continuum and molecular line emission exhibit a clumpy ring-like structure with a total extent of ≈1″ in diameter. The bulk of the molecular emission is interpreted as being produced in a radially expanding disk-like structure with an expansion velocity <jats:inline-formula> <jats:tex-math> <?CDATA ${v}_{\exp }$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>v</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>exp</mml:mi> </mml:mrow> </mml:msub> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjlac4a5bieqn1.gif" xlink:type="simple" /> </jats:inline-formula> ∼ 90 km s<jats:sup>−1</jats:sup> and an inclination <jats:italic>i</jats:italic> ≈ 60° with respect to the line of sight. The observations also reveal a compact high-velocity component, <jats:inline-formula> <jats:tex-math> <?CDATA ${v}_{\exp }$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>v</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>exp</mml:mi> </mml:mrow> </mml:msub> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjlac4a5bieqn2.gif" xlink:type="simple" /> </jats:inline-formula> ∼ 280 km s<jats:sup>−1</jats:sup>, that is aligned perpendicularly to the expanding disk. This component is interpreted as a bipolar outflow with a kinematical age <jats:italic>τ</jats:italic> ≲ 20 yr, which could either be material that is currently being ejected from V 605 Aql, or is being dragged from the inner parts of the disk by a stellar wind. The dust mass of the disk is in the range <jats:italic>M</jats:italic> <jats:sub>dust</jats:sub> ∼ 0.2–8 × 10<jats:sup>−3</jats:sup> <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>, depending on the dust absorption coefficient. The mass of the CO is <jats:italic>M</jats:italic> <jats:sub>CO</jats:sub> ≈ 1.1 × 10<jats:sup>−5</jats:sup> <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>, which is more than three orders of magnitude larger than the mass of the other detected molecules. We estimate a <jats:sup>12</jats:sup>C/<jats:sup>13</jats:sup>C ratio of 5.6 ± 0.6, which is consistent with the single stellar evolution scenario in which the star experienced a very late thermal pulse instead of a nova-like event as previously suggested.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The equator of star K2-290A was recently found to be inclined by 124° ± 6° relative to the orbits of both its known transiting planets. The presence of a companion star B at ∼100 au suggested that the birth protoplanetary disk could have tilted, thus providing an explanation for the peculiar retrograde state of this multi-planet system. In this work, we show that a primordial misalignment is not required and that the observed retrograde state is a natural consequence of the chaotic stellar obliquity evolution driven by a wider-orbit companion C at ≳2000 au long after the disk disperses. The star C drives eccentricity and/or inclination oscillations on the inner binary orbit, leading to widespread chaos from the periodic resonance passages between the stellar spin and planetary secular modes. Based on a population synthesis study, we find that the observed stellar obliquity is reached in ∼40%–70% of the systems, making this mechanism a robust outcome of the secular dynamics, regardless of the spin-down history of the central star. This work highlights the unusual role that very distant companions can have on the orbits of close-in planets and the host star’s spin evolution, connecting four orders of magnitude in distance scale over billions of orbits. We finally comment on the application to other exoplanet systems, including multi-planet systems in wide binaries.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>For nearly 40 yr, studies of exosphere formation on airless bodies have been hindered by uncertainties in our understanding of the underlying ion collisional sputtering by the solar wind (SW). These ion impacts on airless bodies play an important role in altering their surface properties and surrounding environment. Much of the collisional sputtering data needed for exosphere studies come from binary collision approximation (BCA) sputtering models. These depend on the surface binding energy (SBE) for the atoms sputtered from the impacted material. However, the SBE is not reliably known for many materials important for planetary science, such as plagioclase feldspars and sodium pyroxenes. BCA models typically approximate the SBE using the cohesive energy for a monoelemental solid. We use molecular dynamics (MD) to provide the first accurate SBE data we are aware of for Na sputtered from the above silicate minerals, which are expected to be important for exospheric formation at Mercury and the Moon. The MD SBE values are ∼8 times larger than the Na monoelemental cohesive energy. This has a significant effect on the predicted SW ion sputtering yield and energy distribution of Na and the formation of the corresponding Na exosphere. We also find that the SBE is correlated with the coordination number of the Na atoms within the substrate and with the cohesive energy of the Na-bearing silicate. Our MD SBE results will enable more accurate BCA predictions for the SW ion sputtering contribution to the Na exosphere of Mercury and the Moon.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The Sun often produces coronal mass ejections with similar structure repeatedly from the same source region, and how these homologous eruptions are initiated remains an open question. Here, by using a new magnetohydrodynamic simulation, we show that homologous solar eruptions can be efficiently produced by recurring formation and disruption of a coronal current sheet as driven by the continuous shearing of the same polarity inversion line within a single bipolar configuration. These eruptions are initiated by the same mechanism, in which an internal current sheet forms slowly in a gradually sheared bipolar field and reconnection of the current sheet triggers and drives the eruption. Each of the eruptions does not release all of the free energy, leaving a large amount in the post-flare arcade below the erupting flux rope. Thus, a new current sheet can be more easily formed by further shearing of the post-flare arcade than by shearing a potential field arcade, and this is favorable for producing the next eruption. Furthermore, it is found that the new eruption is stronger since the newly formed current sheet has a larger current density and a lower height. In addition, our results also indicate the existence of a magnetic energy threshold for a given flux distribution, and eruption occurs once this threshold is approached.</jats:p>