Table of contents

Despite numerous observational and theoretical attempts, the heating problem of the solar chromosphere still remains unsolved. We develop a novel 3D two-fluid model that accounts for dynamics of charged species and neutrals, and use it to perform the numerical simulations of granulation driven jets and associated waves in a quiet region of the solar chromosphere. The energy carried by the waves is dissipated through ion–neutral collisions, which are sufficient to balance radiative energy losses and to sustain the quasi-stationary atmosphere whose ion and neutral number densities, ionization fraction, and temperature profiles are relatively close to the observationally based semi-empirical model. Additional verification of our results is provided by a good fit of the numerically predicted waveperiod variations with height to the recent observational data. These observational validations of the numerical results demonstrate that the wave heating problem of a quiet region of the chromosphere may be solved.

The discovery rate of fast radio bursts (FRBs) is increasing dramatically thanks to new radio facilities. Meanwhile, wide-field instruments such as the 47 deg² Zwicky Transient Facility (ZTF) survey the optical sky to study transient and variable sources. We present serendipitous ZTF observations of the Canadian Hydrogen Intensity Mapping Experiment (CHIME) repeating source FRB 180916.J0158+65 that was localized to a spiral galaxy 149 Mpc away and is the first FRB suggesting periodic modulation in its activity. While 147 ZTF exposures corresponded to expected high-activity periods of this FRB, no single ZTF exposure was at the same time as a CHIME detection. No >3σ optical source was found at the FRB location in 683 ZTF exposures, totaling 5.69 hr of integration time. We combined ZTF upper limits and expected repetitions from FRB 180916.J0158+65 in a statistical framework using a Weibull distribution, agnostic of periodic modulation priors. The analysis yielded a constraint on the ratio between the optical and radio fluences of η ≲ 200, corresponding to an optical energy E_opt ≲ 3 × 10⁴⁶ erg for a fiducial 10 Jy ms FRB (90% confidence). A deeper (but less statistically robust) constraint of η ≲ 3 can be placed assuming a rate of $r(\gt 5\,\mathrm{Jy}\,\mathrm{ms})=1\,{\mathrm{hr}}^{-1}$ and 1.2 ± 1.1 FRB occurring during exposures taken in high-activity windows. The constraint can be improved with shorter per-image exposures and longer integration time, or observing FRBs at higher Galactic latitudes. This work demonstrated how current surveys can statistically constrain multiwavelength counterparts to FRBs even without deliberately scheduled simultaneous radio observation.

After almost 20 years of hunting, only about a dozen hot corinos, hot regions enriched in interstellar complex organic molecules (iCOMs), are known. Of them, many are binary systems with the two components showing drastically different molecular spectra. Two obvious questions arise. Why are hot corinos so difficult to find and why do their binary components seem chemically different? The answer to both questions could be a high dust opacity that would hide the molecular lines. To test this hypothesis, we observed methanol lines at centimeter wavelengths, where dust opacity is negligible, using the Very Large Array interferometer. We targeted the NGC 1333 IRAS 4A binary system, for which one of the two components, 4A1, has a spectrum deprived of iCOMs lines when observed at millimeter wavelengths, while the other component, 4A2, is very rich in iCOMs. We found that centimeter methanol lines are similarly bright toward 4A1 and 4A2. Their non-LTE analysis indicates gas density and temperature ($\geqslant 2\times {10}^{6}$ cm⁻³ and 100–190 K), methanol column density (∼10¹⁹ cm⁻²), and extent (∼35 au in radius) similar in 4A1 and 4A2, proving that both are hot corinos. Furthermore, the comparison with previous methanol line millimeter observations allows us to estimate the optical depth of the dust in front of 4A1 and 4A2, respectively. The obtained values explain the absence of iCOMs line emission toward 4A1 at millimeter wavelengths and indicate that the abundances toward 4A2 are underestimated by ∼30%. Therefore, centimeter observations are crucial for the correct study of hot corinos, their census, and their molecular abundances.

Type Ia supernovae (SNe Ia) are powerful standardizable candles for constraining cosmological models and provided the first evidence of the accelerated expansion of the universe. Their precision derives from empirical correlations, now measured from >1000 SNe Ia, between their luminosities, light-curve shapes, colors, and most recently with the stellar mass of their host galaxy. As mass correlates with other galaxy properties, alternative parameters have been investigated to improve SN Ia standardization though none have been shown to significantly alter the determination of cosmological parameters. We re-examine a recent claim, based on 34 SN Ia in nearby passive host galaxies, of a 0.05 mag Gyr⁻¹ dependence of standardized SN Ia luminosity on host age, which, if extrapolated to higher redshifts, would be a bias up to 0.25 mag, challenging the inference of dark energy. We reanalyze this sample of hosts using both the original method and a Bayesian hierarchical model and find after a fuller accounting of the uncertainties the significance of a dependence on age to be ≤2σ and ∼1σ after the removal of a single poorly sampled SN Ia. To test the claim that a trend seen in old stellar populations can be applied to younger ages, we extend our analysis to a larger sample that includes young hosts. We find the residual dependence of host age (after all standardization typically employed for cosmological measurements) to be consistent with zero for 254 SNe Ia from the Pantheon sample, ruling out the large but low significance trend seen in passive hosts.

The energy spectrum of solar wind strahl, halo, and superhalo electrons likely carries crucial information on their possible origin and acceleration at the Sun. Here we statistically investigate the energy spectrum of solar wind strahl/halo electrons at ∼0.1–1.5 keV and superhalo electrons at ∼20–200 keV measured by Wind/3D Plasma and Energetic Particle during quiet times from 1998 to 2014, according to the types of their Potential Field Source Surface–mapped coronal source regions (CSRs). We adopt the classification scheme developed by Zhao et al. to categorize the CSRs into four types: active region (AR), quiet Sun (QS), coronal hole (CH), and helmet-streamer associated region (HS). We find that for the quiet-time strahl, the AR and HS (QS and CH) correspond to a smaller (larger) kappa index κ_strahl with the most frequent value of 7–8.5 (8.5–10) and a larger (smaller) n_strahl with the most frequent value of 0.013–0.026 cm⁻³ (0.006–0.0013 cm⁻³). For the quiet-time halo, κ_halo behaves similarly to κ_strahl, but n_halo appears similar among the four CSR types. For the superhalo, the AR (QS) corresponds to a larger (smaller) power-law index β with the most frequent value of 2.2–2.4 (1.8–2.0), while the HS and CH have a β not different from either the AR or QS; n_sup appears similar, with the most frequent value of 3 × 10⁻⁸–3 × 10⁻⁷ cm⁻³, among the four CSR types. These results suggest that the strahl (superhalo) from the hotter CSRs tends to be more (less) efficiently accelerated.

We present 3D general relativistic magnetohydrodynamic (GRMHD) simulations of the accretion flow surrounding Sagittarius A* that are initialized using larger-scale MHD simulations of the ∼30 Wolf–Rayet (WR) stellar winds in the Galactic center. The properties of the resulting accretion flow on horizon scales are set not by ad hoc initial conditions but by the observationally constrained properties of the WR winds with limited free parameters. For this initial study we assume a non-spinning black hole. Our simulations naturally produce a ∼10⁻⁸M_⊙ yr⁻¹ accretion rate, consistent with previous phenomenological estimates. We find that a magnetically arrested flow is formed by the continuous accretion of coherent magnetic field being fed from large radii. Near the event horizon, the magnetic field is so strong that it tilts the gas with respect to the initial angular momentum and concentrates the originally quasi-spherical flow to a narrow disk-like structure. We also present 230 GHz images calculated from our simulations where the inclination angle and physical accretion rate are not free parameters but are determined by the properties of the WR stellar winds. The image morphology is highly time variable. Linear polarization on horizon scales is coherent with weak internal Faraday rotation.

We report on new X-ray and optical observations of PSR J2030+4415, a gamma-ray pulsar with an Hα bow shock. These data reveal the velocity structure of the bow shock apex and resolve unusual X-ray structure in its interior. In addition the system displays a very long, thin filament, extending at least 5' at ∼130° to the pulsar motion vector. Careful astrometry, compared with a short archival exposure, detects the pulsar proper motion at 85 mas yr⁻¹. With the Hα velocity structure this allows us to estimate the distance as 0.75 kpc.

'Oumuamua (I1 2017) was the first macroscopic (l ∼ 100 m) body observed to traverse the inner solar system on an unbound hyperbolic orbit. Its light curve displayed strong periodic variation, and it showed no hint of a coma or emission from molecular outgassing. Astrometric measurements indicate that 'Oumuamua experienced nongravitational acceleration on its outbound trajectory, but energy balance arguments indicate this acceleration is inconsistent with a water ice sublimation-driven jet of the type exhibited by solar system comets. We show that all of 'Oumaumua's observed properties can be explained if it contained a significant fraction of molecular hydrogen (H₂) ice. H₂ sublimation at a rate proportional to the incident solar flux generates a surface-covering jet that reproduces the observed acceleration. Mass wasting from sublimation leads to monotonic increase in the body axis ratio, explaining 'Oumuamua's shape. Back-tracing 'Oumuamua's trajectory through the solar system permits calculation of its mass and aspect ratio prior to encountering the Sun. We show that H₂-rich bodies plausibly form in the coldest dense cores of giant molecular clouds, where number densities are of order n ∼ 10⁵, and temperatures approach the T = 3 K background. Post-formation exposure to galactic cosmic rays implies a τ ∼ 100 Myr age, explaining the kinematics of 'Oumuamua's inbound trajectory.

Magnetic flux generated and intensified by the solar dynamo emerges into the solar atmosphere, forming active regions (ARs) including sunspots. Existing theories of flux emergence suggest that the magnetic flux can rise buoyantly through the convection zone but is trapped at the photosphere, while its further rising into the atmosphere resorts to the Parker buoyancy instability. To trigger such an instability, the Lorentz force in the photosphere needs to be as large as the gas pressure gradient to hold up an extra amount of mass against gravity. This naturally results in a strongly non-force-free photosphere, which is indeed shown in typical idealized numerical simulations of flux tube buoyancy from below the photosphere into the corona. Here we conduct a statistical study of the extents of normalized Lorentz forces and torques in the emerging photospheric magnetic field with a substantially large sample of Solar Dynamics Observatory/Helioseismic and Magnetic Imager vector magnetograms. We found that the photospheric field has a rather small Lorentz force and torque on average, and thus is very close to a force-free state, which is not consistent with theories as well as idealized simulations of flux emergence. Furthermore, the small extents of forces and torques seem not to be influenced by the emerging AR's size, the emergence rate, or the nonpotentiality of the field. This result puts an important constraint on future development of theories and simulations of flux emergence.

We explore the possibility that GW190412, a binary black hole merger with a non-equal-mass ratio and significantly spinning primary, was formed through repeated black hole mergers in a dense super star cluster. Using a combination of semianalytic prescriptions for the remnant spin and recoil kick of black hole mergers, we show that the mass ratio and spin of GW190412 are consistent with a binary black hole whose primary component has undergone two successive mergers from a population of $\sim 10{M}_{\odot }$ black holes in a high-metallicity environment. We then explore the production of GW190412-like analogs in the CMC Cluster Catalog, a grid of 148 N-body star cluster models, as well as a new model, behemoth, with nearly 10⁷ particles and initial conditions taken from a cosmological MHD simulation of galaxy formation. We show that, if the spins of black holes born from stars are small, the production of binaries with GW190412-like masses and spins is dominated by massive super star clusters with high metallicities and large central escape speeds. While many are observed in the local universe, our results suggest that a careful treatment of these massive clusters, many of which may have been disrupted before the present day, is necessary to characterize the production of unique gravitational-wave events produced through dynamics.

Fast radio bursts (FRBs) are bright radio transients with millisecond duration at cosmological distances. Since compact dark matter/objects (COs) could act as lenses and cause splitting of these kinds of very short duration signals, Muñoz et al. have proposed a novel method to probe COs with lensing of FRBs. In this Letter, we for the first time apply this method to real data and give constraints of the nature of COs with currently available FRB observations. We emphasize that the information from dynamic spectra of FRBs is quite necessary for identifying any lensed signals and find no echoes in the existing data. The null search gives a constraint comparable to that from galactic wide binaries, though the methods of redshift inference from the dispersion measure would impact a little. Furthermore, we make an improved forecast based on the distributions of real data for the ongoing and upcoming telescopes. Finally, we discuss the situation where one or more lensed signals will be detected. In such a case, the parameter space of COs can be pinned down very well since the lens mass can be directly determined through the observed flux ratio and time delay between split images.

A new generation mechanism of the magnetic field in an inhomogeneous collisionless plasma with a beam component is proposed. We show that even though the current and charge neutralities are initially satisfied, the current neutrality is eventually violated if there is an inhomogeneity, so that the magnetic field is generated. By conducting ab initio two-dimensional particle-in-cell simulations, we demonstrate that the magnetic field is generated as expected. The new generation mechanism of the magnetic field can play an important role in the current universe because cosmic rays can be regarded as the beam component in the astrophysical plasma. We propose that the first cosmic rays generate the magnetic field with a large scale at the redshift of z ≈ 20.

While many studies have shown a correlation between properties of the light curves of SNe Ia and properties of their host galaxies, it remains unclear what is driving these correlations. We introduce a new direct method to study these correlations by analyzing "parent" galaxies that host multiple SNe Ia "siblings." Here, we search the Dark Energy Survey SN sample, one of the largest samples of discovered SNe, and find eight galaxies that hosted two likely SNe Ia. Comparing the light-curve properties of these SNe and recovered distances from the light curves, we find no better agreement between properties of SNe in the same galaxy as any random pair of galaxies, with the exception of the SN light-curve stretch. We show at 2.8σ significance that at least one-half of the intrinsic scatter of SNe Ia distance modulus residuals is not from common host properties. We also discuss the robustness with which we could make this evaluation with LSST, which will find 100× more pairs of galaxies, and pave a new line of study on the consistency of SNe Ia in the same parent galaxies. Finally, we argue that it is unlikely that some of these SNe are actually single, lensed SN with multiple images.

The large-scale magnetic field of the Milky Way is thought to be created by an αΩ dynamo, which implies that it should have opposite handedness north and south of the Galactic midplane. Here we attempt to detect a variation in handedness using polarization data from the Wilkinson Microwave Anisotropy Probe. Previous analyzes of the parity-even and parity-odd parts of linear polarization of the global dust and synchrotron emission have focused on quadratic correlations in spectral space of, and between, these two components. Here, by contrast, we analyze the parity-odd polarization itself and show that it has, on average, opposite signs in northern and southern Galactic hemispheres. Comparison with a Galactic mean-field dynamo model shows broad qualitative agreement and reveals that the sign of the observed hemispheric dependence of the azimuthally averaged parity-odd polarization is not determined by the sign of α, but by the sense of differential rotation.

We collected over 6000 high-resolution spectra of four dozen field RR Lyrae (RRL) variables pulsating either in the fundamental (39 RRab) or in the first overtone (9 RRc) mode. We measured radial velocities (RVs) of four strong metallic and four Balmer lines along the entire pulsational cycle and derived RV amplitudes with accuracies better than 1–2 km s⁻¹. The new amplitudes were combined with literature data for 23 RRab and 3 RRc stars (total sample of 74 RRLs), which allowed us to investigate the variation of the Bailey diagram (photometric amplitude versus period) when moving from optical to mid-infrared bands and to recast the Bailey diagram in terms of RV amplitudes. We found that RV amplitudes for RRab are minimally affected by nonlinear phenomena (shocks) and multiperiodicity (Blazhko effect). The RV slope (logP–A(V_r)) when compared with the visual slope (logP–A(V)) is shallower, and the dispersion, at fixed period, decreases by a factor of two. We constructed homogeneous sets of horizontal branch evolutionary models and nonlinear, convective pulsation models of RRLs to constrain the impact of evolutionary effects on their pulsation properties. Evolution causes, on the Bailey diagram based on RV amplitudes, a modest variation in pulsation period and a large dispersion in amplitude. The broad dispersion in period of the Bailey diagram is mainly caused by variation in RRL intrinsic parameters (stellar mass, chemical composition). Empirical evidence indicates that RV amplitudes are an optimal diagnostic for tracing the mean effective temperature across the RRab instability strip.

The potential habitability of tidally locked planets orbiting M-dwarf stars has been widely investigated in recent work, typically with a nondynamic ocean and without continents. On Earth, ocean dynamics are a primary means of heat and nutrient distribution. Continents are a critical source of nutrients, strongly influence ocean dynamics, and participate in climate regulation. In this work, we investigate how the size of a substellar land mass affects the oceans ability to transport heat and upwell nutrients on the tidally locked planet Proxima Centauri b using the ROCKE-3D coupled ocean-atmosphere general circulation model (GCM). We find that dayside ice-free ocean and nutrient delivery to the mixed layer via upwelling are maintained across all continent sizes. We also find that Proxima Centauri b's climate is more sensitive to differences among atmospheric GCMs than to the inclusion of ocean dynamics in ROCKE-3D. Finally, we find that Proxima Centauri b transitions from a "lobster" state where ocean heat transport distributes heat away from the substellar point to an "eyeball" state where heat transport is restricted and surface temperature decreases symmetrically from the substellar point when the continent size exceeds ∼20% of the surface area. Our work suggests that both a dynamic ocean and continents are unlikely to decrease the habitability prospects of nearby tidally locked targets like Proxima Centauri b that could be investigated with future observations by the James Webb Space Telescope.

The position of galaxies on the stellar mass, star formation rate (SFR) plane with respect to the star-forming main sequence at each redshift is a convenient way to infer where the galaxy is in its evolution compared to the rest of the population. We use Hubble Space Telescope high-resolution images in the GOODS-S field from the the Cosmic Assembly Near-IR Deep Extragalactic Legacy Survey (CANDELS) and fit multiwavelength lights in resolution elements of galaxies with stellar population synthesis models. We then construct resolved kpc-scale stellar mass, SFR surface density curves for galaxies at z ∼ 1. Fitting these resolved main sequence curves with Schechter functions, we parameterize and explain the multiwavelength structure of galaxies with three variables: ϕ*, α, and M*. For quenched galaxies below the main sequence, we find an average high-mass slope (α) of the resolved main sequence curves to be ∼−0.4. The scatter of this slope is higher among the lower mass star-forming galaxies and those above the main sequence compared to quenched galaxies, due to lack of an evolved bulge. Our findings agree well with an inside-out quenching of star formation. We find that the knee of the Schechter fits (M*) for galaxies below the main sequence occurs at lower stellar mass surface densities compared to star-forming galaxies, which hints at how far quenching has proceeded outward.