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<jats:title>Abstract</jats:title> <jats:p>The Cygnus Superbubble (CSB) is a region of soft X-ray emission approximately 13 degrees wide in the direction of the local spiral arm. Such a large region might be the result of strong stellar winds and supernovae from nearby stellar nurseries, or it could be the result of a single event—a hypernova. HaloSat observed four nonoverlapping 10 degree diameter fields in the CSB region over the 0.4-7 keV band. The CSB absorption and temperature was found to be consistent over all four fields, with a weighted average of 6.1 × 10<jats:sup>21</jats:sup> cm<jats:sup>−2</jats:sup> and 0.190 keV, respectively. These observations suggest that the CSB is a cohesive object with a singular origin. The total thermal energy for the CSB is estimated at 4 × 10<jats:sup>52</jats:sup> erg, based upon a shell-like physical model of the CSB. Absorption and distance estimates to Cyg OB associations are examined. The CSB absorption is found to be most consistent with the absorption seen in Cyg OB1, implying that the CSB lies at a similar distance of 1.1–1.4 kpc.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>In this paper, we analyze the high-resolution UV spectra for a C1.7 solar flare (SOL2017-09-09T06:51) observed by the Interface Region Imaging Spectrograph (IRIS). We focus on the spectroscopic observations at the locations where the cool lines of Si <jats:sc>iv</jats:sc> 1402.8 Å (∼10<jats:sup>4.8</jats:sup> K) and C <jats:sc>ii</jats:sc> 1334.5/1335.7 Å (∼10<jats:sup>4.4</jats:sup> K) reveal significant redshifts with Doppler velocities up to ∼150 km s<jats:sup>−1</jats:sup>. These redshifts appear in the rise phase of the flare, then increase rapidly, reach the maximum in a few minutes, and proceed into the decay phase. Combining the images from IRIS and Atmospheric Imaging Assembly on board the Solar Dynamics Observatory, we propose that the redshifts in the cool lines are caused by the downflows in the transition region and upper chromospheric layers, which likely result from a magnetic reconnection leading to the flare. In addition, the cool Si <jats:sc>iv</jats:sc> and C <jats:sc>ii</jats:sc> lines show gentle redshifts (a few tens of km s<jats:sup>−1</jats:sup>) at some other locations, which manifest some distinct features from the above locations. This is supposed to originate from a different physical process.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>A large fraction of gamma-ray burst (GRB) lightcurves (LCs) show X-ray plateaus. We analyze all GRBs with known redshifts presenting plateaus observed by The Neil Gehrels Swift Observatory from its launch until 2019 August. The fundamental plane relation between the rest-frame time and X-ray luminosity at the end of the plateau emission and the peak prompt luminosity holds for all the GRB classes when selection biases and cosmological evolutions are applied. We have discovered two important findings: (1) a new class of long GRBs with good data coverage: the platinum sample; and (2) the platinum, the SNe-LGRB, and the KN-SGRB samples, yield the smallest intrinsic scatter with <jats:italic>σ</jats:italic> <jats:sub>platinum</jats:sub>,<jats:sub>GRB−SNe</jats:sub> = 0.22 ± 0.10 and <jats:italic>σ</jats:italic> <jats:sub>KN−SGRB </jats:sub>= 0.24 ± 0.12. The SNe-LGRBs are composed of GRBs associated spectroscopically with the SNe Ib,c, the KN-SGRBs are composed by eight GRBs associated with kilonovae or where there could have been such an association. The highest correlation coefficients are yielded for the SN-LGRB-ABC sample, which includes GRBs spectroscopically associated with SNe Ib/c or with a clear optical bump in the LC resembling the SNe Ib/c, (<jats:inline-formula> <jats:tex-math> <?CDATA ${R}_{\mathrm{SN} \mbox{-} \mathrm{LGRB} \mbox{-} \mathrm{ABC}}^{2}=0.95$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabbe8aieqn1.gif" xlink:type="simple" /> </jats:inline-formula>), the SN-LGRBs (<jats:inline-formula> <jats:tex-math> <?CDATA ${R}_{\mathrm{SN} \mbox{-} \mathrm{LGRB}}^{2}=0.91$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabbe8aieqn2.gif" xlink:type="simple" /> </jats:inline-formula>), and the KN-SGRBs (<jats:inline-formula> <jats:tex-math> <?CDATA ${R}_{\mathrm{KN} \mbox{-} \mathrm{SGRB}}^{2}=0.90$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabbe8aieqn3.gif" xlink:type="simple" /> </jats:inline-formula>) when the redshift evolution is considered. These category planes are reliable candidates to use as cosmological tools. Furthermore, the distance from the gold fundamental plane is a crucial discriminant among classes. In fact, we find that the distributions of the distances of the SNe-LGRB, SNe-LGRB-ABC, KN-SGRB, and SGRB samples from the gold fundamental plane are statistically different from the distribution of the gold GRBs’ distances from the gold fundamental plane with and without considering evolution cases.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Narrow emission-line galaxies can be distinguished in the well-known BPT diagrams through narrow emission-line properties. However, there are no boundaries visible to the naked eye between type-2 active galactic nuclei (AGN) and H <jats:sc>ii</jats:sc> galaxies in BPT diagrams, besides the extreme dividing lines expected by theoretical photoionization models. Here, based on the powerful t-SNE technique applied to the local narrow emission-line galaxies in the Sloan Digital Sky Survey Data Release 15, type-2 AGN and H <jats:sc>ii</jats:sc> galaxies can be clearly separated in the t-SNE determined two-dimensional projected map, and then the dividing lines can be mathematically determined in BPT diagrams, leading to charming harmonization of the theoretical expectations and the actual results from real observed properties. The results not only provide an interesting and robust method to determine the dividing lines in BPT diagrams through the powerful t-SNE technique, but also lead to further confirmation on previously defined composite galaxies more efficiently classified in the BPT diagram of [O <jats:sc>iii</jats:sc>]/H<jats:italic>β</jats:italic> versus [N <jats:sc>ii</jats:sc>]/H<jats:italic>α</jats:italic>.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We present ZTF20aajnksq (AT 2020blt), a fast-fading (Δ<jats:italic>r</jats:italic> = 2.3 mag in Δ<jats:italic>t</jats:italic> = 1.3 days) red (<jats:italic>g</jats:italic> − <jats:italic>r</jats:italic> ≈ 0.6 mag) and luminous (<jats:italic>M</jats:italic> <jats:sub>1626 Å</jats:sub> = −25.9 mag) optical transient at <jats:italic>z</jats:italic> = 2.9 discovered by the Zwicky Transient Facility (ZTF). AT 2020blt shares several features in common with afterglows to long-duration gamma-ray bursts (GRBs): (1) an optical light curve well-described by a broken power law with a break at <jats:italic>t</jats:italic> <jats:sub>j</jats:sub> = 1 d (observer frame); (2) a luminous (<jats:italic>L</jats:italic> <jats:sub>0.3–10 KeV</jats:sub> = 10<jats:sup>46</jats:sup> erg s<jats:sup>−1</jats:sup>) X-ray counterpart; and (3) luminous (<jats:italic>L</jats:italic> <jats:sub>10 GHz</jats:sub> = 4 × 10<jats:sup>31</jats:sup> erg s<jats:sup>−1</jats:sup> Hz<jats:sup>−1</jats:sup>) radio emission. However, no GRB was detected in the 0.74 days between the last ZTF nondetection (<jats:italic>r</jats:italic> &gt; 21.36 mag) and the first ZTF detection (<jats:italic>r</jats:italic> = 19.60 mag), with an upper limit on the isotropic-equivalent gamma-ray energy release of <jats:italic>E</jats:italic> <jats:sub> <jats:italic>γ</jats:italic>,iso</jats:sub> &lt; 7 × 10<jats:sup>52</jats:sup> erg. AT 2020blt is thus the third afterglow-like transient discovered without a detected GRB counterpart (after PTF11agg and ZTF19abvizsw) and the second (after ZTF19abvizsw) with a redshift measurement. We conclude that the properties of AT 2020blt are consistent with a classical (initial Lorentz factor Γ<jats:sub>0</jats:sub> ≳ 100) on-axis GRB that was missed by high-energy satellites. Furthermore, by estimating the rate of transients with light curves similar to that of AT 2020blt in ZTF high-cadence data, we agree with previous results that there is no evidence for an afterglow-like phenomenon that is significantly more common than classical GRBs, such as dirty fireballs. We conclude by discussing the status and future of fast-transient searches in wide-field high-cadence optical surveys.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We present the first predictions for the <jats:italic>L</jats:italic> <jats:sub>[C <jats:sc>ii</jats:sc>]</jats:sub>–SFR relation and [C<jats:sc>ii</jats:sc>] luminosity function (LF) in the epoch of reionization (EOR) based on cosmological hydrodynamics simulations using the <jats:sc>simba</jats:sc> suite plus radiative transfer calculations via <jats:sc>sígame</jats:sc>. The sample consists of 11,137 galaxies covering halo mass log <jats:italic>M</jats:italic> <jats:sub>halo</jats:sub> ∈ [9, 12.4] <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>, star formation rate SFR ∈ [0.01, 330] <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub> yr<jats:sup>−1</jats:sup>, and metallicity 〈<jats:italic>Z</jats:italic> <jats:sub>gas</jats:sub>〉<jats:sub>SFR</jats:sub> ∈ [0.1, 1.9] <jats:italic>Z</jats:italic> <jats:sub>⊙</jats:sub>. The simulated <jats:italic>L</jats:italic> <jats:sub>[C <jats:sc>ii</jats:sc>]</jats:sub>–SFR relation is consistent with the range observed, but with a spread of ≃0.3 dex at the high end of SFR (&gt;100 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub> yr<jats:sup>−1</jats:sup>) and ≃0.6 dex at the lower end, and there is tension between our predictions and the values of <jats:italic>L</jats:italic> <jats:sub>[C <jats:sc>ii</jats:sc>]</jats:sub> above 10<jats:sup>8.5</jats:sup> <jats:italic>L</jats:italic> <jats:sub>⊙</jats:sub> observed in some galaxies reported in the literature. The scatter in the <jats:italic>L</jats:italic> <jats:sub>[C <jats:sc>ii</jats:sc>]</jats:sub>–SFR relation is mostly driven by galaxy properties, such that at a given SFR galaxies with higher molecular mass and metallicity have higher <jats:italic>L</jats:italic> <jats:sub>[C <jats:sc>ii</jats:sc>]</jats:sub>. The [C<jats:sc>ii</jats:sc>] LF predicted by <jats:sc>simba</jats:sc> is consistent with the upper limits placed by the only existing untargeted flux-limited [C<jats:sc>ii</jats:sc>] survey at the EOR and those predicted by semianalytic models. We compare our results with existing models and discuss the differences responsible for the discrepant slopes in the <jats:italic>L</jats:italic> <jats:sub>[C <jats:sc>ii</jats:sc>]</jats:sub>–SFR relation.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Hydrodynamical simulations show that circumbinary disks become eccentric, even when the binary is circular. Here we demonstrate that, in steady state, the disk’s eccentricity behaves as a long-lived free mode trapped by turning points that naturally arise from a continuously truncated density profile. Consequently, both the disk’s precession rate and eccentricity profile may be calculated via the simple linear theory for perturbed pressure-supported disks. By formulating and solving the linear theory, we find that (i) surprisingly, the precession rate is roughly determined by the binary’s quadrupole, even when the quadrupole is very weak relative to pressure; (ii) the eccentricity profile is largest near the inner edge of the disk and falls exponentially outward; and (iii) the results from linear theory indeed agree with what is found in simulations. Understanding the development of eccentric modes in circumbinary disks is a crucial first step for understanding the long-term (secular) exchange of eccentricity, angular momentum, and mass between the binary and the gas. Potential applications include the search for a characteristic kinematic signature in disks around candidate binaries and precession-induced modulation of accretion over long timescales.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Stellar associations should be co-eval, but some associations exhibit apparently discrepant ages: M stars <jats:italic>appear</jats:italic> younger than FG stars. Among M stars in the Upper Sco association (age ≈ 11 Myr), magnetic effects have been shown to slow down evolution of the lowest mass stars, thereby alleviating the age discrepancy. A recent study of pre-main-sequence (PMS) stars in the much younger Taurus star-forming region (SFR) suggest that an age discrepancy may also exist there. Our goal is to compare two approaches to magnetic/nonmagnetic modeling of the Taurus stars. In one approach, Simon et al. concluded that an age discrepancy exists, but it can be resolved by magnetic models if one imposes an “equipartition condition” on the photospheric field of each star. In this paper, we examine an independent sample of seven Taurus PMS stars reported by Rizzuto et al. Our approach to modeling magneto-convection imposes no condition on the photospheric field strength. We find that nonmagnetic isochrones can successfully fit the stars in the Rizzuto et al. sample with ages 1.6–2.4 Myr, without the need to invoke magnetic effects. An upper limit on the vertical field strength on the surfaces of the Rizzuto stars is found to be ∼300 G for the low-mass secondaries of FF Tau and HP Tau/G2 and ∼100 G for the other, more massive stars. The Taurus SFR is so large (extending over many square degrees in the sky) that magnetic conditions may plausibly differ from star to star.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Future exoplanet direct imaging missions, such as HabEx and LUVOIR, will select target stars to maximize the number of Earth-like exoplanets that can have their atmospheric compositions characterized. Because one of these missions’ aims is to detect biosignatures, they should also consider the expected biosignature yield of planets around these stars. In this work, we develop a method of computing relative biosignature yields among potential target stars, given a model of habitability and biosignature genesis, and using a star’s habitability history. As an illustration and first application of this method, we use MESA stellar models to calculate the time evolution of the habitable zone and examine three simple models for biosignature genesis to calculate the relative biosignature yield for different stars. We find that the relative merits of K stars versus F stars depend sensitively on model choice. In particular, use of the present-day habitable zone as a proxy for biosignature detectability favors young, luminous stars lacking the potential for long-term habitability. Biosignature yields are also sensitive to whether life can arise on Cold Start exoplanets that enter the habitable zone after formation, an open question deserving of more attention. Using the case study of biosignature yields calculated for <jats:italic>θ</jats:italic> Cygni and 55 Cancri, we find that robust mission design and target selection for HabEx and LUVOIR depend on choosing a specific model of biosignature appearance with time, the terrestrial planet occurrence rate as a function of orbital separation, precise knowledge of stellar properties, and accurate stellar evolutionary histories.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Several studies have documented periodic and quasi-periodic signals from the time series of dMe flare stars and other stellar sources. Such periodic signals, observed within quiescent phases (i.e., devoid of larger-scale microflare or flare activity), range in a period from 1 to 1000 s and hence have been tentatively linked to ubiquitous <jats:italic>p</jats:italic>-mode oscillations generated in the convective layers of the star. As such, most interpretations for the observed periodicities have been framed in terms of magnetohydrodynamic wave behavior. However, we propose that a series of continuous nanoflares, based upon a power-law distribution, can provide a similar periodic signal in the associated time series. Adapting previous statistical analyses of solar nanoflare signals, we find the first statistical evidence for stellar nanoflare signals embedded within the noise envelope of M-type stellar lightcurves. Employing data collected by the Next Generation Transit Survey (NGTS), we find evidence for stellar nanoflare activity demonstrating a flaring power-law index of 3.25 ± 0.20, alongside a decay timescale of 200 ± 100 s. We also find that synthetic time series, consistent with the observations of dMe flare star lightcurves, are capable of producing quasi-periodic signals in the same frequency range as <jats:italic>p</jats:italic>-mode signals, despite being purely composed of impulsive signatures. Phenomena traditionally considered a consequence of wave behavior may be described by a number of high-frequency but discrete nanoflare energy events. This new physical interpretation presents a novel diagnostic capability, by linking observed periodic signals to given nanoflare model conditions.</jats:p>