adsabs/SciX_UAT_keywords huggingface dataset:
ASASSN-16oh: A Nova Outburst with No Mass Ejection—A New Type of Supersoft X-Ray Source in Old Populations

ASASSN-16oh is a peculiar transient supersoft X-ray source without a mass-ejection signature in the field of the Small Magellanic Cloud. Maccarone et al. concluded that ASASSN-16oh is the first dwarf nova with supersoft X-ray that originated from an equatorial accretion belt on a white dwarf (WD). Hillman et al. proposed a thermonuclear runaway model that both the X-rays and V/I photons are emitted from the hot WD. We propose a nova model induced by a high rate of mass accretion during a dwarf nova outburst, I.e., the X-rays originate from the surface of the hydrogen-burning WD whereas the V/I photons are from the irradiated disk. Our model explains the main observational properties of ASASSN-16oh. We also obtained thermonuclear runaway models with no mass ejection for a wide range of parameters of the WD mass and mass-accretion rates including both natural and forced novae in low-metal environments of Z = 0.001 and Z = 0.0001. They are a new type of periodic supersoft X-ray sources with no mass ejection and also a bright transient in V/I bands if they have a large disk. We suggest that such objects are candidates for Type Ia supernova progenitors because its mass is increasing at a very high efficiency (∼100%).

Keywords: novae; population ii stars; dwarf novae
Density and Velocity Fluctuations of Alpha Particles in Magnetic Switchbacks

Magnetic switchbacks, or sudden reversals in the magnetic field's radial direction, are one of the more striking observations of the Parker Solar Probe (PSP) in its mission thus far. While their precise production mechanisms are still unknown, the two main theories are via interchange reconnection events and in situ generation. In this work, density and abundance variations of alpha particles are studied inside and outside individual switchbacks. We find no consistent compositional differences in the alpha particle abundance ratio, n α p , inside versus outside switchbacks, nor do we observe any signature when separating the switchbacks according to V α p /V pw , the ratio of the alpha-proton differential speed to the wave phase speed (the speed at which the switchback is traveling). We argue that these measurements cannot be used to rule in favor of one production mechanism over the other, due to the distance between PSP and the postulated interchange reconnection events. In addition, we examine the 3D velocity fluctuations of protons and alpha particles within individual switchbacks. While switchbacks are always associated with increases in proton velocity, alpha velocities may be enhanced, unchanged, or decrease. This is due to the interplay between V pw and V α p , with the Alfvénic motion of the alpha particles vanishing as the difference ∣V pw - V α p ∣ decreases. We show how the Alfvénic motion of both the alphas and the protons through switchbacks can be understood as an approximately rigid arm rotation about the location of the wave frame, and illustrate that the wave frame can therefore be estimated using particle measurements alone, via sphere fitting.

Keywords: heliosphere; solar wind; space plasmas
SOFIA Observations of Variability in Jupiter's Para-H2 Distribution and Subsurface Emission Characteristics of the Galilean Satellites

We observed Jupiter's thermal emission with SOFIA/FORCAST in 2018 August and 2019 July. Both broad-band images (8-37 μm) and spectra (17-37 μm) were obtained. We used the shape of the Jovian spectra to determine the latitudinal distribution of temperatures and para-H2 in the upper troposphere, and compared this to similar data obtained in Fletcher et al. (2017). The two data sets were taken approximately half a Jovian year apart, the first (2014) during northern summer (Ls = 158°) and the second (2019) during southern summer (Ls = 304°). During both epochs the high northern latitudes are cooler than the south. Para-H2 is observed in sub-equilibrium at the equator and in super-equilibrium near the poles during all epochs. The largest difference between the two epochs is the detection of high-para-H2 content at high southern latitudes in 2019, in contrast to the earlier (2014) observations. This implies that the high-latitude para-H2 appears to vary over multi-year timescales. In addition to aiding in the calibration of and providing context to the Jovian spectra, the images were used to determine spectra of the four Galilean satellites. Over the 8-35 μm wavelength range the brightness temperatures of all four satellites drop with increasing wavelength. Although this is expected as longer wavelengths probe the satellites' deeper, cooler layers, our data quantify the brightness temperature gradient with wavelength.

Keywords: jupiter
Magnetic Field Structure and Faraday Rotation of the Plerionic Supernova Remnant G21.5-0.9

We present a polarimetric study of the pulsar wind nebula (PWN) in supernova remnant G21.5-0.9, using archival Very Large Array data taken at 5 and 7.4 GHz. The rotation measure (RM) map of the PWN shows a symmetric pattern that aligns with the presumed pulsar spin axis direction, implying a significant contribution to the RM from the nebula. We suggest that the spatial variation of the internal RM is mostly caused by the nonuniform distribution of electrons originating from the supernova ejecta. Our high-resolution radio polarization map reveals a global radial B-field. We show that a simple model with overall radial field and turbulence on a small scale can reproduce many observed features of the PWN, including the polarization pattern and polarized fraction. The modeling results also reject a strong large-scale toroidal B-field, suggesting that the toroidal field observed in the inner PWN cannot propagate to the entire nebula. Lastly, our model predicts that the internal Faraday rotation would break the linear relation between the polarization angle and the square of the wavelength, and cause severe depolarization at low frequencies.

Keywords: supernova remnants; radio interferometry; polarimetry; magnetic fields; pulsars
Three-dimensional Climate Simulations for the Detectability of Proxima Centauri b

The discovery of a planet orbiting around Proxima Centauri, the closest star to the Sun, opens new avenues for the remote observations of the atmosphere and surface of an exoplanet, Proxima b. To date, three-dimensional (3D) general circulation models (GCMs) are the best available tools to investigate the properties of the exo-atmospheres, waiting for the next generation of space- and ground-based telescopes. In this work, we use the Planet Simulator (PlaSim), an intermediate-complexity, flexible and fast 3D GCM, suited to handle all the orbital and physical parameters of a planet and to study the dynamics of its atmosphere. Assuming an Earth-like atmosphere and a 1:1 spin/orbit configuration (tidal locking), our simulations of Proxima b are consistent with a dayside open ocean planet with a superrotating atmosphere. Moreover, because of the limited representation of the radiative transfer in PlaSim, we compute the spectrum of the exoplanet with an offline radiative transfer code with a spectral resolution of 1 nm. This spectrum is used to derive the thermal phase curves for different orbital inclination angles. In combination with instrumental detection sensitivities, the different thermal phase curves are used to evaluate observation conditions at ground level (e.g., ELT) or in space (e.g., James Webb Space Telescope (JWST)). We estimated the exposure time to detect the Proxima b (assuming an Earth-like atmosphere) thermal phase curve in the far-IR with JWST with signal-to-noise ratio ≃1. Under the hypothesis of total noise dominated by shot noise, neglecting other possible extra contribution producing a noise floor, the exposure time is equal to 5 hr for each orbital epoch.

Keywords: exoplanets; exoplanet atmospheres; planetary climates; ocean planets; astronomical simulations; exoplanet detection methods; infrared photometry; infrared telescopes; light curves; two-color diagrams
The Effect of Spatially Varying Collision Frequency on the Development of the Rayleigh–Taylor Instability

The Rayleigh–Taylor (RT) instability is ubiquitously observed, yet has traditionally been studied using ideal fluid models. Collisionality can vary strongly across the fluid interface, and previous work demonstrates the necessity of kinetic models to completely capture dynamics in certain collisional regimes. Where previous kinetic simulations used spatially and temporally constant collision frequency, this work presents five-dimensional (two spatial, three velocity dimensions) continuum-kinetic simulations of the RT instability using a more realistic spatially varying collision frequency. Three cases of collisional variation are explored for two Atwood numbers: low to intermediate, intermediate to high, and low to high. The low-to-intermediate case exhibits no RT instability growth, while the intermediate-to-high case is similar to a fluid-limit kinetic case with interface widening biased toward the lower-collisionality region. A novel contribution of this work is the low-to-high collisionality case that shows significantly altered instability growth through an upward movement of the interface and damped spike growth due to increased free-streaming particle diffusion in the lower region. Contributions to the energy flux from the non-Maxwellian portions of the distribution function are not accessible to fluid models and are greatest in magnitude in the spike and regions of low collisionality. Increasing the Atwood number results in greater RT instability growth and reduced upward interface movement. Deviation of the distribution function from Maxwellian is inversely proportional to collision frequency and concentrated around the fluid interface. The linear phase of RT instability growth is well described by theoretical linear growth rates accounting for viscosity and diffusion.

Keywords: plasma physics; astrophysical fluid dynamics; hydrodynamics
Tidal Wave Breaking in the Eccentric Lead-in to Mass Transfer and Common Envelope Phases

The evolution of many close binary and multiple star systems is defined by phases of mass exchange and interaction. As these systems evolve into contact, tidal dissipation is not always sufficient to bring them into circular, synchronous orbits. In these cases, encounters of increasing strength occur while the orbit remains eccentric. This paper focuses on the outcomes of close tidal passages in eccentric orbits. Close eccentric passages excite dynamical oscillations about the stars' equilibrium configurations. These tidal oscillations arise from the transfer of orbital energy into oscillation mode energy. When these oscillations reach sufficient amplitude, they break near the stellar surface. The surface wave-breaking layer forms a shock-heated atmosphere that surrounds the object. The continuing oscillations in the star's interior launch shocks that dissipate into the atmosphere, damping the tidal oscillations. We show that the rapid, nonlinear dissipation associated with the wave breaking of fundamental oscillation modes therefore comes with coupled mass loss to the wave-breaking atmosphere. The mass ratio is an important characteristic that defines the relative importance of mass loss and energy dissipation and therefore determines the fate of systems evolving under the influence of nonlinear dissipation. The outcome can be rapid tidal circularization (q ≪ 1) or runaway mass exchange (q ≫ 1).

Keywords: hydrodynamical simulations; tidal interaction; close binary stars; common envelope evolution
Fast Magnetic Reconnection with Turbulence in High Lundquist Number Limit

We use extensive 3D resistive MHD simulations to study how large-scale current sheets will undergo fast reconnection in the high Lundquist number S limit (above ∼104), when the system is subject to different externally driven turbulence levels and the self-generated turbulence produced by 3D reconnection dynamics. We find that the normalized global reconnection rate ∼0.01-0.13 is weakly dependent on S. Global reconnection with the classic inflow/outflow configurations is observed, and 3D flux ropes are hierarchically formed and ejected from reconnection regions. A statistical separation of the reconnected magnetic field lines follows a superdiffusive behavior, from which the rate is measured to be very similar to that obtained from the mixing of tracer populations. We find that the reconnection rate scales roughly linearly with the turbulence level during the peak of reconnection. This scaling is consistent with the turbulence properties produced by both the externally driven and self-generation processes. These results imply that large-scale thin current sheets tend to undergo rigorous reconnection.

Keywords: solar magnetic reconnection; interplanetary turbulence; computational methods; solar corona
Ultra-low-frequency Gravitational Waves from Massive Clusters at z 1

Recent progress with pulsar timing array experiments, especially from the NANOGrav collaboration, indicates that we are on the cusp of detecting significant signals from the inspiral of supermassive black hole (BH) binaries. While recent analysis has focused on nearby galaxies as possible sources of the loudest signals, we show that mergers in identified clusters at z ~ 1 can have larger strain amplitudes. We make an estimate comparing the nearby Two Micron All Sky Survey redshift survey (2MASS) galaxy sample with the more distant Massive and Distant Clusters of WISE Survey (MaDCoWs) cluster sample, showing that the latter might be expected to contribute more, and louder, gravitational-wave events. Thus, the first individual source detections may well be from ultramassive BHs in clusters at z ~ 1, rather than nearby galaxies.

Keywords: gravitational wave sources; high-redshift galaxy clusters
X-Ray Scaling Relations for a Representative Sample of Planck-selected Clusters Observed with XMM-Newton

We report the scaling relations derived by fitting the X-ray parameters determined from analyzing the XMM-Newton observations of 120 galaxy clusters in the Planck Early Sunyaev-Zel'dovich (SZ) sample spanning the redshift range of 0.059 < z < 0.546. We find that the slopes of all the investigated scaling relations significantly deviate from the self-similar predictions, if self-similar redshift evolution is assumed. When the redshift evolution is left free to vary, the derived slopes are more in agreement with the self-similar predictions. Relaxed clusters have on average ∼30% higher X-ray luminosity than disturbed clusters at a given mass, a difference that, depending on the relative fraction of relaxed and disturbed clusters in the samples (e.g., SZ vs. X-ray selected), has a strong impact on the normalization obtained in different studies. Using the core-excised cluster luminosities reduces the scatter and brings into better agreement the L-Mtot and L-T relations determined for different samples. Mtot-T, Mtot-YX, and Mtot-Mgas relations show little dependence on the dynamical state of the clusters, but the normalizations of these relations may depend on the mass range investigated. Although most of the clusters investigated in this work reside at relatively low redshift, the fits prefer values of γ, the parameter accounting for the redshift evolution, different from the self-similar predictions. This suggests an evolution (<2σ level, with the exception of the Mtot-T relation) of the scaling relations. For the first time, we find significant evolution (>3σ) of the Mtot-T relation, pointing to an increase of the kinetic-to-thermal energy ratio with redshift. This is consistent with a scenario in which higher-redshift clusters are on average more disturbed than their lower-redshift counterparts.

Keywords: galaxy clusters; intracluster medium; observational cosmology; x-ray astronomy
The Imprint of Convection on Type I X-Ray Bursts: Pauses in Photospheric Radius Expansion Lightcurves

Motivated by the recent observation by NICER of a type I X-ray burst from SAX J1808.4-3658 with a distinct "pause" feature during its rise, we show that bursts which ignite in a helium layer underneath a hydrogen-rich shell naturally give rise to such pauses, as long as enough energy is produced to eject the outer layers of the envelope by super-Eddington winds. The length of the pause is determined by the extent of the convection generated after ignition, while the rate of change of luminosity following the pause is set by the hydrogen gradient left behind by convection. Using the MESA stellar evolution code, we simulate the accumulation, nuclear burning, and convective mixing prior to and throughout the ignition of the burst, followed by the hydrodynamic wind. We show that the results are sensitive to the treatment of convection adopted within the code. In particular, the efficiency of mixing at the H/He interface plays a key role in determining the shape of the lightcurve. The data from SAX J1808.4-3658 favor strong mixing scenarios. Multidimensional simulations will be needed to properly model the interaction between convection and nuclear burning during these bursts, which will then enable a new way to use X-ray burst lightcurves to study neutron star surfaces.

Keywords: neutron stars
Ubiquitous Molecular Outflows in z > 4 Massive, Dusty Galaxies. II. Momentum-driven Winds Powered by Star Formation in the Early Universe

Galactic outflows of molecular gas are a common occurrence in galaxies and may represent a mechanism by which galaxies self-regulate their growth, redistributing gas that could otherwise have formed stars. We previously presented the first survey of molecular outflows at z > 4 toward a sample of massive, dusty galaxies. Here we characterize the physical properties of the molecular outflows discovered in our survey. Using low-redshift outflows as a training set, we find agreement at the factor of 2 level between several outflow rate estimates. We find molecular outflow rates of 150-800 ${M}_{\odot }$ yr-1 and infer mass loading factors just below unity. Among the high-redshift sources, the molecular mass loading factor shows no strong correlations with any other measured quantity. The outflow energetics are consistent with expectations for momentum-driven winds with star formation as the driving source, with no need for energy-conserving phases. There is no evidence for active galactic nucleus activity in our sample, and while we cannot rule out deeply buried active galactic nuclei, their presence is not required to explain the outflow energetics, in contrast to nearby obscured galaxies with fast outflows. The fraction of the outflowing gas that will escape into the circumgalactic medium (CGM), though highly uncertain, may be as high as 50%. This nevertheless constitutes only a small fraction of the total cool CGM mass based on a comparison to z ∼ 2-3 quasar absorption line studies, but could represent ≳10% of the CGM metal mass. Our survey offers the first statistical characterization of molecular outflow properties in the very early universe.

Keywords: high-redshift galaxies; galactic winds; gravitational lensing; galaxy evolution
Newly Improved Ionization Corrections for the Neutral Interstellar Medium: Enabling Accurate Abundance Determinations in Star-forming Galaxies throughout the Universe

Studies measuring the chemical abundances of the neutral gas in star-forming galaxies (SFGs) require ionization correction factors (ICFs) to accurately measure their metal contents. In the work presented here, we calculate newly improved ICFs for a sample of SFGs. These new corrections include both the contaminating ionized gas along the line of sight (ICFionized) and unaccounted for higher ionization stages in the neutral gas (ICFneutral). We make use of recently acquired spectroscopic observations taken with the Cosmic Origins Spectrograph on board Hubble to measure column densities for Fe II and Fe III. Using the Fe III/Fe II ratios as well as other physical properties (I.e., log[LUV], N(H I), T, and Z), we generate ad hoc photoionization models with CLOUDY to quantify the corrections required for each of the targets. We identify a luminosity threshold of log[LUV] ∼ 40.75 erg s-1 above which the ICFneutral values for nitrogen are relatively higher (ICFneutral=0.05-0.7) than those for the rest of the elements (ICFneutral ∼ 0.01). This behavior indicates that, for the high UV luminosity objects, N II is found in non-negligible quantities in the neutral gas, making these ICFneutral corrections critical for determining the true abundances in the interstellar medium. In addition, we calculate ICFs from a uniform grid of models covering a wide range of physical properties typically observed in studies of SFGs and extragalactic H II regions. We provide the community with tabulated ICF values for the neutral gas abundances measured from a variety of environments and applicable to chemical studies of the high-redshift universe.

Keywords: interstellar medium; chemical abundances; starburst galaxies
Multiheight Observations of Atmospheric Gravity Waves at Solar Disk Center

Atmospheric gravity waves (AGWs) are low-frequency, buoyancy-driven waves that are generated by turbulent convection and propagate obliquely throughout the solar atmosphere. Their proposed energy contribution to the lower solar atmosphere and sensitivity to atmospheric parameters (e.g., magnetic fields and radiative damping) highlight their diagnostic potential. We investigate AGWs near a quiet-Sun disk center region using multiwavelength data from the Interferometric Bidimensional Spectrometer and the Solar Dynamics Observatory. These observations showcase the complex wave behavior present in the entire acoustic-gravity wave spectrum. Using Fourier spectral analysis and local helioseismology techniques on simultaneously observed line core Doppler velocity and intensity fluctuations, we study both the vertical and horizontal properties of AGWs. Propagating AGWs with perpendicular group and phase velocities are detected at the expected temporal and spatial scales throughout the lower solar atmosphere. We also find previously unobserved, varied phase difference distributions among our velocity and intensity diagnostic combinations. Time-distance analysis indicates that AGWs travel with an average group speed of 4.5 km s-1, which is only partially described by a simple simulation, suggesting that high-frequency AGWs dominate the signal. Analysis of the median magnetic field (4.2 G) suggests that propagating AGWs are not significantly affected by quiet-Sun photospheric magnetic fields. Our results illustrate the importance of multiheight observations and the necessity of future work to properly characterize this observed behavior.

Keywords: quiet sun; solar photosphere; solar atmosphere; solar oscillations; solar physics
TESS as a Low-surface-brightness Observatory: Cutouts from Wide-area Coadded Images

We present a mosaic of those co-added Full Frame Images acquired by the TESS satellite that had been released in 2020 April. The mosaic shows substantial stray light over the sky. Yet over spatial scales of a few degrees, the background appears uniform. This result indicates that TESS has considerable potential as a Low Surface Brightness Observatory. The co-added images are freely available as a High Level Science Product (HLSP) at MAST and accessible through a Jupyter Notebook.

Keywords: computational methods; diffuse radiation; astronomy software; low surface brightness galaxies; computational astronomy; cloud computing