Table of contents

We present ALMA 1.3 mm continuum observations at $0\buildrel{\prime\prime}\over{.} 2$ (25 au) resolution of Elias 2–24, one of the largest and brightest protoplanetary disks in the Ophiuchus Molecular Cloud, and we report the presence of three partially resolved concentric gaps located at ∼20, 52, and 87 au from the star. We perform radiative transfer modeling of the disk to constrain its surface density and temperature radial profile and place the disk structure in the context of mechanisms capable of forming narrow gaps such as condensation fronts and dynamical clearing by actively forming planets. In particular, we estimate the disk temperature at the locations of the gaps to be 23, 15, and 12 K (at 20, 52, and 87 au, respectively), very close to the expected snowlines of CO (23–28 K) and N₂ (12–15 K). Similarly, by assuming that the widths of the gaps correspond to 4–8× the Hill radii of forming planets (as suggested by numerical simulations), we estimate planet masses in the range of $0.2\mbox{--}1.5\,{M}_{\mathrm{Jup}}$, $1.0\mbox{--}8.0\,{M}_{\mathrm{Jup}}$, and $0.02\mbox{--}0.15\,{M}_{\mathrm{Jup}}$ for the inner, middle, and outer gap, respectively. Given the surface density profile of the disk, the amount of "missing mass" at the location of each one of these gaps (between 4 and 20 ${M}_{\mathrm{Jup}}$) is more than sufficient to account for the formation of such planets.

We present our study on the spatially resolved Hα and M_* relation for 536 star-forming and 424 quiescent galaxies taken from the MaNGA survey. We show that the star formation rate surface density (${{\rm{\Sigma }}}_{\mathrm{SFR}}$), derived based on the Hα emissions, is strongly correlated with the M_* surface density (${{\rm{\Sigma }}}_{* }$) on kiloparsec scales for star-forming galaxies and can be directly connected to the global star-forming sequence. This suggests that the global main sequence may be a consequence of a more fundamental relation on small scales. On the other hand, our result suggests that ∼20% of quiescent galaxies in our sample still have star formation activities in the outer region with lower specific star formation rate (SSFR) than typical star-forming galaxies. Meanwhile, we also find a tight correlation between ${{\rm{\Sigma }}}_{{\rm{H}}\alpha }$ and ${{\rm{\Sigma }}}_{* }$ for LI(N)ER regions, named the resolved "LI(N)ER" sequence, in quiescent galaxies, which is consistent with the scenario that LI(N)ER emissions are primarily powered by the hot, evolved stars as suggested in the literature.

In LIGO's O1 and O2 observational runs, the detectors were sensitive to stellar-mass binary black hole (BBH) coalescences with component masses up to $100\,{M}_{\odot }$, with binaries with primary masses above $40\,{M}_{\odot }$ representing ≳90% of the total accessible sensitive volume. Nonetheless, of the 5.9 detections (GW150914, LVT151012, GW151226, GW170104, GW170608, and GW170814) reported by LIGO-Virgo, the most massive binary detected was GW150914 with a primary component mass of $\sim 36\,{M}_{\odot }$, far below the detection mass limit. Furthermore, there are theoretical arguments in favor of an upper mass gap, predicting an absence of black holes in the mass range $50\lesssim M\lesssim 135\,{M}_{\odot }$. We argue that the absence of detected binary systems with component masses heavier than $\sim 40\,{M}_{\odot }$ may be preliminary evidence for this upper mass gap. By allowing for the presence of a mass gap, we find weaker constraints on the shape of the underlying mass distribution of BBHs. We fit a power-law distribution with an upper mass cutoff to real and simulated BBH mass measurements, finding that the first 3.9 BBHs favor shallow power-law slopes $\alpha \lesssim 3$ and an upper mass cutoff ${M}_{\max }\sim 40\,{M}_{\odot }$. This inferred distribution is entirely consistent with the two recently reported detections, GW170608 and GW170814. We show that with ∼10 additional LIGO-Virgo BBH detections, fitting the BH mass distribution will provide strong evidence for an upper mass gap if one exists.

Multi-band phase variations, in principle, allow us to infer the longitudinal temperature distributions of planets as a function of height in their atmospheres. For example, 3.6 μm emission originates from deeper layers of the atmosphere than 4.5 μm due to greater water vapor absorption at the longer wavelength. Because heat transport efficiency increases with pressure, we expect thermal phase curves at 3.6 μm to exhibit smaller amplitudes and greater phase offsets than at 4.5 μm—yet this trend is not observed. Of the seven hot Jupiters with full-orbit phase curves at 3.6 and 4.5 μm, all of them have greater phase amplitude at 3.6 μm than at 4.5 μm, while four of the seven exhibit a greater phase offset at 3.6 μm. We use a 3D radiative-hydrodynamic model to calculate theoretical phase curves of HD 189733b, assuming thermo-chemical equilibrium. The model exhibits temperature, pressure, and wavelength-dependent opacity, primarily driven by carbon chemistry: CO is energetically favored on the dayside, while CH₄ is favored on the cooler nightside. Infrared opacity, therefore, changes by orders of magnitude between day and night, producing dramatic vertical shifts in the wavelength-specific photospheres, which would complicate eclipse or phase mapping with spectral data. The model predicts greater relative phase amplitude and greater phase offset at 3.6 μm than at 4.5 μm, in agreement with the data. Our model qualitatively explains the observed phase curves, but it is in tension with current thermo-chemical kinetics models that predict zonally uniform atmospheric composition due to the transport of CO from the hot regions of the atmosphere.

We performed a search for eclipsing and dipping sources in the archive of the EXTraS project—a systematic characterization of the temporal behavior of XMM-Newton point sources. We discovered dips in the X-ray light curve of 3XMM J004232.1+411314, which has been recently associated with the hard X-ray source dominating the emission of M31. A systematic analysis of XMM-Newton observations revealed 13 dips in 40 observations (total exposure time of ∼0.8 Ms). Among them, four observations show two dips, separated by ∼4.01 hr. Dip depths and durations are variable. The dips occur only during low-luminosity states (${L}_{0.2\mbox{--}12}\lt 1\times {10}^{38}$ erg s⁻¹), while the source reaches ${L}_{0.2\mbox{--}12}\sim 2.8\times {10}^{38}$ erg s⁻¹. We propose that this system is a new dipping low-mass X-ray binary in M31 seen at high inclination (60°–80°); the observed dipping periodicity is the orbital period of the system. A blue HST source within the Chandra error circle is the most likely optical counterpart of the accretion disk. The high luminosity of the system makes it the most luminous (not ULX) dipper known to date.

The transient neutron star (NS) low-mass X-ray binary MAXI J0556−332 provides a rare opportunity to study NS crust heating and subsequent cooling for multiple outbursts of the same source. We examine MAXI, Swift, Chandra, and XMM-Newton data of MAXI J0556−332 obtained during and after three accretion outbursts of different durations and brightnesses. We report on new data obtained after outburst III. The source has been tracked up to ∼1800 days after the end of outburst I. Outburst I heated the crust strongly, but no significant reheating was observed during outburst II. Cooling from ∼333 eV to ∼146 eV was observed during the first ∼1200 days. Outburst III reheated the crust up to ∼167 eV, after which the crust cooled again to ∼131 eV in ∼350 days. We model the thermal evolution of the crust and find that this source required a different strength and depth of shallow heating during each of the three outbursts. The shallow heating released during outburst I was ∼17 MeV nucleon⁻¹ and outburst III required ∼0.3 MeV nucleon⁻¹. These cooling observations could not be explained without shallow heating. The shallow heating for outburst II was not well constrained and could vary from ∼0 to 2.2 MeV nucleon⁻¹, i.e., this outburst could in principle be explained without invoking shallow heating. We discuss the nature of the shallow heating and why it may occur at different strengths and depths during different outbursts.

In this work, we report the discovery and characterization of PSR J1411+2551, a new binary pulsar discovered in the Arecibo 327 MHz Drift Pulsar Survey. Our timing observations of the radio pulsar in the system span a period of about 2.5 years. This timing campaign allowed a precise measurement of its spin period (62.4 ms) and its derivative (9.6 ± 0.7) × 10⁻²⁰ s s⁻¹; from these, we derive a characteristic age of >9.1 Gyr and a surface magnetic field strength of <2.6 × 10⁹ G. These numbers indicate that this pulsar was mildly recycled by accretion of matter from the progenitor of the companion star. The system has an eccentric (e = 0.17) 2.61 day orbit. This eccentricity allows a highly significant measurement of the rate of advance of periastron, $\dot{\omega }\,=0.07686\pm 0.00046^\circ \,{\mathrm{yr}}^{-1}$. Assuming general relativity accurately describes the orbital motion, this implies a total system mass M = 2.538 ± 0.022 M_⊙. The minimum companion mass is 0.92 M_⊙ and the maximum pulsar mass is 1.62 M_⊙. The large companion mass and the orbital eccentricity suggest that PSR J1411+2551 is a double neutron star system; the lightest known to date including the DNS merger GW170817. Furthermore, the relatively low orbital eccentricity and small proper motion limits suggest that the second supernova had a relatively small associated kick; this and the low system mass suggest that it was an ultra-stripped supernova.

We present a series of near-infrared spectra of Nova Ophiuchus 2017 in the K band that record the evolution of the first overtone CO emission in unprecedented detail. Starting from 11.7 days after maximum, when CO is first detected at great strength, the spectra track the CO emission to +25.6 days by which time it is found to have rapidly declined in strength by almost a factor of ∼35. The cause for the rapid destruction of CO is examined in the framework of different mechanisms for CO destruction, namely, an increase in photoionizating flux, chemical pathways of destruction, or destruction by energetic nonthermal particles created in shocks. From LTE modeling of the CO emission, the ¹²C/¹³C ratio is determined to be 1.6 ± 0.3. This is consistent with the expected value of this parameter from nucleosynthesis theory for a nova eruption occuring on a low mass ($\sim 0.6\,{M}_{\odot }$) carbon–oxygen core white dwarf. The present ¹²C/¹³C estimate constitutes one of the most secure estimates of this ratio in a classical nova.

We observed the newly discovered hyperbolic minor planet 1I/'Oumuamua (2017 U1) on 2017 October 30 with Lowell Observatory's 4.3 m Discovery Channel Telescope. From these observations, we derived a partial lightcurve with a peak-to-trough amplitude of at least 1.2 mag. This lightcurve segment rules out rotation periods less than 3 hr and suggests that the period is at least 5 hr. On the assumption that the variability is due to a changing cross-section, the axial ratio is at least 3:1. We saw no evidence for a coma or tail in either individual images or in a stacked image having an equivalent exposure time of 9000 s.

It has been suggested that short gamma-ray bursts (GRBs) have shorter or undetectable spectral lags than longer GRBs because for the former, the observer's line of sight makes a larger angle with the GRB jet axis than it does for the latter. It is proposed that simultaneous gravitational-wave–short-GRB events could provide a simple test of this hypothesis. Multimessenger astronomy eventually may test whether event horizons are a necessary ingredient for GRBs.

The "kinematic" morphology–density relation for early-type galaxies posits that those galaxies with low angular momentum are preferentially found in the highest-density regions of the universe. We use a large sample of galaxy groups with halo masses ${10}^{12.5}\lt {M}_{\mathrm{halo}}\lt {10}^{14.5}\,{h}^{-1}\,{M}_{\odot }$ observed with the Mapping Nearby Galaxies at APO (MaNGA) survey to examine whether there is a correlation between local environment and rotational support that is independent of stellar mass. We find no compelling evidence for a relationship between the angular momentum content of early-type galaxies and either local overdensity or radial position within the group at fixed stellar mass.

Accreting neutron stars can power a wide range of astrophysical phenomena including short- and long-duration gamma-ray bursts, ultra-luminous X-ray sources, and X-ray binaries. Numerical simulations are a valuable tool for studying the accretion-disk–magnetosphere interaction that is central to these problems, most clearly for the recently discovered transitional millisecond pulsars. However, magnetohydrodynamic (MHD) methods, widely used for simulating accretion, have difficulty in highly magnetized stellar magnetospheres, while force-free methods, suitable for such regions, cannot include the accreting gas. We present an MHD method that can stably evolve essentially force-free, highly magnetized regions, and describe the first time-dependent relativistic simulations of magnetized accretion onto millisecond pulsars. Our axisymmetric general-relativistic MHD simulations for the first time demonstrate how the interaction of a turbulent accretion flow with a pulsar's electromagnetic wind can lead to the transition of an isolated pulsar to the accreting state. This transition naturally leads to the formation of relativistic jets, whose power can greatly exceed the power of the isolated pulsar's wind. If the accretion rate is below a critical value, the pulsar instead expels the accretion stream. More generally, our simulations produce for the first time the four possible accretion regimes, in order of decreasing mass accretion rate: (a) crushed magnetosphere and direct accretion; (b) magnetically channeled accretion onto the stellar poles; (c) the propeller state, where material enters through the light cylinder but is prevented from accreting by the centrifugal barrier; (d) almost perfect exclusion of the accretion flow from the light cylinder by the pulsar wind.

On 2017 June 8 at 02:01:16.49 UTC, a gravitational-wave (GW) signal from the merger of two stellar-mass black holes was observed by the two Advanced Laser Interferometer Gravitational-Wave Observatory detectors with a network signal-to-noise ratio of 13. This system is the lightest black hole binary so far observed, with component masses of ${12}_{-2}^{+7}\,{M}_{\odot }$ and ${7}_{-2}^{+2}\,{M}_{\odot }$ (90% credible intervals). These lie in the range of measured black hole masses in low-mass X-ray binaries, thus allowing us to compare black holes detected through GWs with electromagnetic observations. The source's luminosity distance is ${340}_{-140}^{+140}\,\mathrm{Mpc}$, corresponding to redshift ${0.07}_{-0.03}^{+0.03}$. We verify that the signal waveform is consistent with the predictions of general relativity.

The luminosity distance measurement of GW170817 derived from gravitational-wave analysis in Abbott et al. (2017a, hereafter A17:H0) is highly correlated with the measured inclination of the NS–NS system. To improve the precision of the distance measurement, we attempt to constrain the inclination by modeling the broadband X-ray-to-radio emission from GW170817, which is dominated by the interaction of the jet with the environment. We update our previous analysis and we consider the radio and X-ray data obtained at t < 40 days since merger. We find that the afterglow emission from GW170817 is consistent with an off-axis relativistic jet with energy E_k ∼ 10⁴⁸ −3 × 10⁵⁰ erg propagating into an environment with density n ∼ 10⁻²–10⁻⁴ cm⁻³, with preference for wider jets (opening angle θ_j = 15°). For these jets, our modeling indicates an off-axis angle θ_obs ∼ 25°–50°. We combine our constraints on θ_obs with the joint distance–inclination constraint from LIGO. Using the same ∼170 km s⁻¹ peculiar velocity uncertainty assumed in A17:H0 but with an inclination constraint from the afterglow data, we get a value of ${H}_{0}=74.0\pm \tfrac{11.5}{7.5}$ km s⁻¹ Mpc⁻¹, which is higher than the value of ${H}_{0}=70.0\pm \tfrac{12.0}{8.0}$ km s⁻¹ Mpc⁻¹ found in A17:H0. Further, using a more realistic peculiar velocity uncertainty of 250 km s⁻¹ derived from previous work, we find ${H}_{0}=75.5\pm \tfrac{11.6}{9.6}$ km s⁻¹ Mpc⁻¹ for H₀ from this system. This is in modestly better agreement with the local distance ladder than the Planck cosmic microwave background, though such a significant discrimination will require ∼50 such events. Measurements at t > 100 days of the X-ray and radio emission will lead to tighter constraints.

Over the course of the last decade, the Nice model has dramatically changed our view of the solar system's formation and early evolution. Within the context of this model, a transient period of planet–planet scattering is triggered by gravitational interactions between the giant planets and a massive primordial planetesimal disk, leading to a successful reproduction of the solar system's present-day architecture. In typical realizations of the Nice model, self-gravity of the planetesimal disk is routinely neglected, as it poses a computational bottleneck to the calculations. Recent analyses have shown, however, that a self-gravitating disk can exhibit behavior that is dynamically distinct, and this disparity may have significant implications for the solar system's evolutionary path. In this work, we explore this discrepancy utilizing a large suite of Nice model simulations with and without a self-gravitating planetesimal disk, taking advantage of the inherently parallel nature of graphic processing units. Our simulations demonstrate that self-consistent modeling of particle interactions does not lead to significantly different final planetary orbits from those obtained within conventional simulations. Moreover, self-gravitating calculations show similar planetesimal evolution to non-self-gravitating numerical experiments after dynamical instability is triggered, suggesting that the orbital clustering observed in the distant Kuiper Belt is unlikely to have a self-gravitational origin.

The recent discovery by Pan-STARRS1 of 1I/2017 U1 ('Oumuamua), on an unbound and hyperbolic orbit, offers a rare opportunity to explore the planetary formation processes of other stars and the effect of the interstellar environment on a planetesimal surface. 1I/'Oumuamua's close encounter with the inner solar system in 2017 October was a unique chance to make observations matching those used to characterize the small-body populations of our own solar system. We present near-simultaneous g', r', and J photometry and colors of 1I/'Oumuamua from the 8.1 m Frederick C. Gillett Gemini-North Telescope and gri photometry from the 4.2 m William Herschel Telescope. Our g'r'J observations are directly comparable to those from the high-precision Colours of the Outer Solar System Origins Survey (Col-OSSOS), which offer unique diagnostic information for distinguishing between outer solar system surfaces. The J-band data also provide the highest signal-to-noise measurements made of 1I/'Oumuamua in the near-infrared. Substantial, correlated near-infrared and optical variability is present, with the same trend in both near-infrared and optical. Our observations are consistent with 1I/'Oumuamua rotating with a double-peaked period of 8.10 ± 0.42 hr and being a highly elongated body with an axial ratio of at least 5.3:1, implying that it has significant internal cohesion. The color of the first interstellar planetesimal is at the neutral end of the range of solar system g − r and r − J solar-reflectance colors: it is like that of some dynamically excited objects in the Kuiper Belt and the less-red Jupiter Trojans.

The recent discovery of a roughly simultaneous periodic variability in the light curves of the BL Lac object PG 1553+113 at several electromagnetic bands represents the first case of such odd behavior reported in the literature. Motivated by this, we analyzed 15 GHz interferometric maps of the parsec-scale radio jet of PG 1553+113 to verify the presence of a possible counterpart of this periodic variability. We used the Cross-entropy statistical technique to obtain the structural parameters of the Gaussian components present in the radio maps of this source. We kinematically identified seven jet components formed coincidentally with flare-like features seen in the γ-ray light curve. From the derived jet component positions in the sky plane and their kinematics (ejection epochs, proper motions, and sky position angles), we modeled their temporal changes in terms of a relativistic jet that is steadily precessing in time. Our results indicate a precession period in the observer's reference frame of 2.24 ± 0.03 years, compatible with the periodicity detected in the light curves of PG 1553+113. However, the maxima of the jet Doppler boosting factor are systematically delayed relative to the peaks of the main γ-ray flares. We propose two scenarios that could explain this delay, both based on the existence of a supermassive black hole binary system in PG 1553+113. We estimated the characteristics of this putative binary system that also would be responsible for driving the inferred jet precession.

We present high spatial resolution (FWHM ∼ 014) observations of the CO(8–7) line in GDS-14876, a compact star-forming galaxy at z = 2.3 with a total stellar mass of log(M_⋆/M_⊙) = 10.9. The spatially resolved velocity map of the inner r ≲ 1 kpc reveals a continuous velocity gradient consistent with the kinematics of a rotating disk with v_rot(r = 1 kpc) = 163 ± 5 km s⁻¹ and v_rot/σ ∼ 2.5. The gas-to-stellar ratios estimated from CO(8–7) and the dust continuum emission span a broad range, ${f}_{\mathrm{gas}}^{\mathrm{CO}}={M}_{\mathrm{gas}}/{M}_{\star }=13 \% \mbox{--}45 \%$ and ${f}_{\mathrm{gas}}^{\mathrm{cont}}=50 \% \mbox{--}67 \%$, but are nonetheless consistent given the uncertainties in the conversion factors. The dynamical modeling yields a dynamical mass of $\mathrm{log}({M}_{\mathrm{dyn}}/{M}_{\odot })={10.58}_{-0.2}^{+0.5}$, which is lower, but still consistent with the baryonic mass, $\mathrm{log}({M}_{\mathrm{bar}}={M}_{\star }+{M}_{\mathrm{gas}}^{\mathrm{CO}}/{M}_{\odot })=11.0$, if the smallest CO-based gas fraction is assumed. Despite a low, overall gas fraction, the small physical extent of the dense, star-forming gas probed by CO(8–7), ∼3× smaller than the stellar size, implies a strong relative concentration that increases the gas fraction up to ${f}_{\mathrm{gas}}^{\mathrm{CO},1\,\mathrm{kpc}}\sim 85 \%$ in the central 1 kpc. Such a gas-rich center, coupled with a high star formation rate (SFR) ∼ 500 M_⊙ yr⁻¹, suggests that GDS-14876 is quickly assembling a dense stellar component (bulge) in a strong nuclear starburst. Assuming its gas reservoir is depleted without replenishment, GDS-14876 will quickly (t_depl ∼ 27 Myr) become a compact quiescent galaxy that could retain some fraction of the observed rotational support.

We report on white light observations of high latitude tethered prominences acquired during the total solar eclipses of 2012 November 13 and 2013 November 3, at solar maximum, with a field of view spanning several solar radii. Distinguished by their pinkish hue, characteristic of emission from neutral hydrogen and helium, the four tethered prominences were akin to twisted flux ropes, stretching out to the limit of the field of view, while remaining anchored at the Sun. Cotemporal observations in the extreme ultraviolet from the Solar Dynamics Observatory (SDO/AIA) clearly showed that the pinkish emission from the cool ($\approx {10}^{4}-{10}^{5}$ K) filamentary prominences was cospatial with the 30.4 nm He II emission, and was directly linked to filamentary structures emitting at coronal temperatures $\geqslant {10}^{6}$ K in 17.1 and 19.3 nm. The tethered prominences evolved from typical tornado types. Each one formed the core of different types of coronal mass ejections (CMEs), as inferred from coordinated LASCO C2, C3, and STEREO A and B coronagraph observations. Two of them evolved into a series of faint, unstructured puffs. One was a normal CME. The most striking one was a "light-bulb" type CME, whose three-dimensional structure was confirmed from all four coronagraphs. These first uninterrupted detections of prominence-CME systems anchored at the Sun, and stretching out to at least the edge of the field of view of LASCO C3, provide the first observational confirmation for the source of counter-streaming electron fluxes measured in interplanetary CMEs, or ICMEs.

Following our previous study of Time History of Events and Macroscale Interactions during Substorms (THEMIS) data, we consider intermittent turbulence in the magnetosheath depending on various conditions of the magnetized plasma behind the Earth's bow shock and now also near the magnetopause. Namely, we look at the fluctuations of the components of the Elsässer variables in the plane perpendicular to the scale-dependent background magnetic fields and along the local average ambient magnetic fields. We have shown that Alfvén fluctuations often exhibit strong anisotropic non-gyrotropic turbulent intermittent behavior resulting in substantial deviations of the probability density functions from a normal Gaussian distribution with a large kurtosis. In particular, for very high Alfvénic Mach numbers and high plasma beta, we have clear anisotropy with non-Gaussian statistics in the transverse directions. However, along the magnetic field, the kurtosis is small and the plasma is close to equilibrium. On the other hand, intermittency becomes weaker for moderate Alfvén Mach numbers and lower values of the plasma parameter beta. It also seems that the degree of intermittency of turbulence for the outgoing fluctuations propagating relative to the ambient magnetic field is usually similar as for the ingoing fluctuations, which is in agreement with approximate equipartition of energy between these oppositely propagating Alfvén waves. We believe that the different characteristics of this intermittent anisotropic turbulent behavior in various regions of space and astrophysical plasmas can help identify nonlinear structures responsible for deviations of the plasma from equilibrium.

Nearby type Ia supernovae (SNe Ia), such as SN 2017cbv, are useful events to address the question of what the elusive progenitor systems of the explosions are. Hosseinzadeh et al. suggested that the early blue excess of the light curve of SN 2017cbv could be due to the supernova ejecta interacting with a non-degenerate companion star. Some SN Ia progenitor models suggest the existence of circumstellar (CS) environments in which strong outflows create low-density cavities of different radii. Matter deposited at the edges of the cavities should be at distances at which photoionization due to early ultraviolet (UV) radiation of SNe Ia causes detectable changes to the observable Na i D and Ca ii H&K absorption lines. To study possible narrow absorption lines from such material, we obtained a time series of high-resolution spectra of SN 2017cbv at phases between −14.8 and +83 days with respect to B-band maximum, covering the time at which photoionization is predicted to occur. Both narrow Na i D and Ca ii H&K are detected in all spectra, with no measurable changes between the epochs. We use photoionization models to rule out the presence of Na i and Ca ii gas clouds along the line of sight of SN 2017cbv between ∼8 × 10¹⁶–2 × 10¹⁹ cm and ∼10¹⁵–10¹⁷ cm, respectively. Assuming typical abundances, the mass of a homogeneous spherical CS gas shell with radius R must be limited to ${M}_{{\rm{H}}\,{\rm{I}}}^{\mathrm{CSM}}\lt 3\times {10}^{-4}\times {(R/{10}^{17}[\mathrm{cm}])}^{2}$${M}_{\odot }$. The bounds point to progenitor models that deposit little gas in their CS environment.

Galaxies with stellar masses $\lt {10}^{7.4}\ {M}_{\odot }$ and specific star formation rates $\mathrm{sSFR}\gt {10}^{-7.4}$ yr⁻¹ were examined on images of the Hubble Space Telescope Frontier Field Parallels for Abell 2744 and MACS J0416.1-02403. They appear as unresolved "Little Blue Dots" (LBDs). They are less massive and have higher specific star formation rates (sSFRs) than "blueberries" studied by Yang et al. and higher sSFRs than "Blue Nuggets" studied by Tacchella et al. We divided the LBDs into three redshift bins and, for each, stacked the B435, V606, and I814 images convolved to the same stellar point-spread function (PSF). Their radii were determined from PSF deconvolution to be ∼80 to ∼180 pc. The high sSFRs suggest that their entire stellar mass has formed in only 1% of the local age of the universe. The sSFRs at similar epochs in local dwarf galaxies are lower by a factor of ∼100. Assuming that the star formation rate is ${\epsilon }_{\mathrm{ff}}{M}_{\mathrm{gas}}/{t}_{\mathrm{ff}}$ for efficiency ${\epsilon }_{\mathrm{ff}}$, gas mass M_gas, and free-fall time, t_ff, the gas mass and gas-to-star mass ratio are determined. This ratio exceeds 1 for reasonable efficiencies, and is likely to be ∼5 even with a high ${\epsilon }_{\mathrm{ff}}$ of 0.1. We consider whether these regions are forming today's globular clusters. With their observed stellar masses, the maximum likely cluster mass is $\sim {10}^{5}\ {M}_{\odot }$, but if star formation continues at the current rate for $\sim 10{t}_{\mathrm{ff}}\sim 50\,\mathrm{Myr}$ before feedback and gas exhaustion stop it, then the maximum cluster mass could become $\sim {10}^{6}\ {M}_{\odot }$.

Recently, the LIGO–Virgo Collaborations reported their first detection of gravitational-wave (GW) signals from the low-mass compact binary merger GW170817, which is most likely due to a double neutron star (NS) merger. With the GW signals only, the chirp mass of the binary is precisely constrained to ${1.188}_{-0.002}^{+0.004}\,{M}_{\odot }$, but the mass ratio is loosely constrained in the range 0.4–1, so that a very rough estimation of the individual NS masses (1.36 M_⊙ < M₁ < 2.26 M_⊙ and 0.86 M_⊙ < M₂ < 1.36 M_⊙) was obtained. Here, we propose that if one can constrain the dynamical ejecta mass through performing kilonova modeling of the optical/IR data, by utilizing an empirical relation between the dynamical ejecta mass and the mass ratio of NS binaries, one may place a more stringent constraint on the mass ratio of the system. For instance, considering that the red "kilonova" component is powered by the dynamical ejecta, we reach a tight constraint on the mass ratio in the range of 0.46–0.59. Alternatively, if the blue "kilonova" component is powered by the dynamical ejecta, the mass ratio would be constrained in the range of 0.53–0.67. Overall, such a multi-messenger approach could narrow down the mass ratio of GW170817 system to the range of 0.46–0.67, which gives a more precise estimation of the individual NS mass than pure GW signal analysis, i.e., 1.61 M_⊙ < M₁ < 2.11 M_⊙ and 0.90 M_⊙ < M₂ < 1.16 M_⊙.

We report the detection of interstellar methoxymethanol (CH₃OCH₂OH) in Atacama Large Millimeter/submillimeter Array (ALMA) Bands 6 and 7 toward the MM1 core in the high-mass star-forming region NGC 6334I at ∼01–1'' spatial resolution. A column density of 4(2) × 10¹⁸ cm⁻² at T_ex = 200 K is derived toward MM1, ∼34 times less abundant than methanol (CH₃OH), and significantly higher than predicted by astrochemical models. Probable formation and destruction pathways are discussed, primarily through the reaction of the CH₃OH photodissociation products, the methoxy (CH₃O) and hydroxymethyl (CH₂OH) radicals. Finally, we comment on the implications of these mechanisms on gas-phase versus grain-surface routes operative in the region, and the possibility of electron-induced dissociation of CH₃OH rather than photodissociation.

We present late-time observations by Swift and XMM-Newton of the tidal disruption event (TDE) ASASSN-15oi that reveal that the source brightened in the X-rays by a factor of ∼10 one year after its discovery, while it faded in the UV/optical by a factor of ∼100. The XMM-Newton observations measure a soft X-ray blackbody component with ${{kT}}_{\mathrm{bb}}\sim 45\,\mathrm{eV}$, corresponding to radiation from several gravitational radii of a central $\sim {10}^{6}\,{M}_{\odot }$ black hole. The last Swift epoch taken almost 600 days after discovery shows that the X-ray source has faded back to its levels during the UV/optical peak. The timescale of the X-ray brightening suggests that the X-ray emission could be coming from delayed accretion through a newly forming debris disk and that the prompt UV/optical emission is from the prior circularization of the disk through stream–stream collisions. The lack of spectral evolution during the X-ray brightening disfavors ionization breakout of a TDE "veiled" by obscuring material. This is the first time a TDE has been shown to have a delayed peak in soft X-rays relative to the UV/optical peak, which may be the first clear signature of the real-time assembly of a nascent accretion disk, and provides strong evidence for the origin of the UV/optical emission from circularization, as opposed to reprocessed emission of accretion radiation.

Binary neutron star mergers are important in understanding stellar evolution, the chemical enrichment of the universe via the r-process, the physics of short gamma-ray bursts, gravitational waves, and pulsars. The rates at which these coalescences happen is uncertain, but it can be constrained in different ways. One of those is to search for the optical transients produced at the moment of the merging, called a kilonova, in ongoing supernova (SN) searches. However, until now, only theoretical models for a kilonova light curve were available to estimate their rates. The recent kilonova discovery of AT 2017gfo/DLT17ck gives us the opportunity to constrain the rate of kilonovae using the light curve of a real event. We constrain the rate of binary neutron star mergers using the DLT40 Supernova search and the native AT 2017gfo/DLT17ck light curve obtained with the same telescope and software system. Excluding AT 2017gfo/DLT17ck due to visibility issues, which was only discovered thanks to the aLIGO/aVirgo trigger, no other similar transients were detected during the 13 months of daily cadence observations of ∼2200 nearby (<40 Mpc) galaxies. We find that the rate of BNS mergers is lower than 0.47–0.55 kilonovae per 100 years per 10¹⁰${L}_{{B}_{\odot }}$ (depending on the adopted extinction distribution). In volume, this translates to $\lt 0.99\times {10}^{-4}{}_{-0.15}^{+0.19},\,{\mathrm{Mpc}}^{-3}\,{\mathrm{yr}}^{-1}$ (SNe Ia–like extinction distribution), consistent with previous BNS coalescence rates. Based on our rate limit, and the sensitivity of aLIGO/aVirgo during O2, it is very unlikely that kilonova events are lurking in old pointed galaxy SN search data sets.

We present the first detection of gas-phase S₂H in the Horsehead, a moderately UV-irradiated nebula. This confirms the presence of doubly sulfuretted species in the interstellar medium and opens a new challenge for sulfur chemistry. The observed S₂H abundance is ∼5 × 10⁻¹¹, only a factor of 4–6 lower than that of the widespread H₂S molecule. H₂S and S₂H are efficiently formed on the UV-irradiated icy grain mantles. We performed ice irradiation experiments to determine the H₂S and S₂H photodesorption yields. The obtained values are ∼1.2 × 10⁻³ and <1 × 10⁻⁵ molecules per incident photon for H₂S and S₂H, respectively. Our upper limit to the S₂H photodesorption yield suggests that photodesorption is not a competitive mechanism to release the S₂H molecules to the gas phase. Other desorption mechanisms such as chemical desorption, cosmic-ray desorption, and grain shattering can increase the gaseous S₂H abundance to some extent. Alternatively, S₂H can be formed via gas-phase reactions involving gaseous H₂S and the abundant ions S⁺ and SH⁺. The detection of S₂H in this nebula therefore could be the result of the coexistence of an active grain-surface chemistry and gaseous photochemistry.

Type Ia supernovae (SNe Ia) exhibit a wide diversity of peak luminosities and light curve shapes: the faintest SNe Ia are 10 times less luminous and evolve more rapidly than the brightest SNe Ia. Their differing characteristics also extend to their stellar age distributions, with fainter SNe Ia preferentially occurring in old stellar populations and vice versa. In this Letter, we quantify this SN Ia luminosity–stellar age connection using data from the Lick Observatory Supernova Search (LOSS). Our binary population synthesis calculations agree qualitatively with the observed trend in the $\gt 1\,\mathrm{Gyr}$ old populations probed by LOSS if the majority of SNe Ia arise from prompt detonations of sub-Chandrasekhar-mass white dwarfs (WDs) in double WD systems. Under appropriate assumptions, we show that double WD systems with less massive primaries, which yield fainter SNe Ia, interact and explode at older ages than those with more massive primaries. We find that prompt detonations in double WD systems are capable of reproducing the observed evolution of the SN Ia luminosity function, a constraint that any SN Ia progenitor scenario must confront.