adsabs/SciX_UAT_keywords huggingface dataset:
Anisotropy of Magnetic Field and Velocity Fluctuations in the Solar Wind

We present a large statistical study of the fluctuation anisotropy in minimum variance (MV) frames of the magnetic field and solar wind velocity. We use 2, 10, 20, and 40 minute intervals of simultaneous magnetic field (the Wind spacecraft) and velocity (the Spektr-R spacecraft) observations. Our study confirms that magnetic turbulence is a composite of fluctuations varying along the mean magnetic field and those changing in the direction perpendicular to the mean field. Regardless of the length scale within the studied range of spacecraft-frame frequencies, 90 of the observed magnetic field fluctuations exhibit an MV direction aligned with the mean magnetic field, 10 of events have the MV direction perpendicular to the background field, and a negligible portion of fluctuations has no preferential direction. On the other hand, the MV direction of velocity fluctuations tends to be distributed more uniformly. An analysis of magnetic compressibility and density fluctuations suggests that the fluctuations resemble properties of Alfvnic fluctuations if the MV direction is aligned with background magnetic field whereas slow-mode-like fluctuations have the MV direction perpendicular to the background field. The proportion between Alfvnic and slow-mode-like fluctuations depends on plasma and length scale: the dependence on the solar wind speed is weak. We present 3D numerical MHD simulations and show that the numerical results are compatible with our experimental results.

Keywords: solar wind; fast solar wind; slow solar wind; heliosphere; interplanetary turbulence; alfven waves
The North/South Asymmetry of the Galaxy: Possible Connection to the Vertical Phase-space Snail

The galaxy is found to be in disequilibrium based on recent findings of the north/south (N/S) asymmetry and the phase mixing signatures, such as a phase spiral (snail) structure in the vertical phase space (z - V z ). We show that the N/S asymmetry in a tracer population of dwarfs may be quantitatively modeled with a simple phase snail model superimposed on a smooth equilibrium background. As the phase snail intersects with the z-axis, the number density is enhanced, and the velocity dispersion (σ z ) is decreased relative to the other side of the Galactic plane. Fitting only to the observed asymmetric N/S σ z profiles, we obtain reasonable parameters for the phase-space snail and the potential utilized in modeling the background, despite the complex dependence of the model on the potential parameters and the significant selection effects of the data. Both the snail shape and the N/S number density difference given by our best-fit model are consistent with previous observations. The equilibrium background implies a local dark matter density of ${0.0151}_{-0.0051}^{+0.0050}$ M ⊙ pc-3. The vertical bulk motion of our model is similar to the observation, but with a ~1.2 km s-1 shift. Our work demonstrates the strong correlation between the phase-space snail and the N/S asymmetry. Future observational constraints will facilitate more comprehensive snail models to unravel the Milky Way potential and the perturbation history encoded in the snail feature.

Keywords: galaxy structure; milky way dynamics; milky way disk; stellar kinematics; stellar dynamics
The Zwicky Transient Facility Bright Transient Survey. I. Spectroscopic Classification and the Redshift Completeness of Local Galaxy Catalogs

The Zwicky Transient Facility (ZTF) is performing a three-day cadence survey of the visible northern sky (∼3π) with newly found transient candidates announced via public alerts. The ZTF Bright Transient Survey (BTS) is a large spectroscopic campaign to complement the photometric survey. BTS endeavors to spectroscopically classify all extragalactic transients with mpeak ≤ 18.5 mag in either the gZTF or rZTF filters, and publicly announce said classifications. BTS discoveries are predominantly supernovae (SNe), making this the largest flux-limited SN survey to date. Here we present a catalog of 761 SNe, classified during the first nine months of ZTF (2018 April 1-2018 December 31). We report BTS SN redshifts from SN template matching and spectroscopic host-galaxy redshifts when available. We analyze the redshift completeness of local galaxy catalogs, the redshift completeness fraction (RCF; the ratio of SN host galaxies with known spectroscopic redshift prior to SN discovery to the total number of SN hosts). Of the 512 host galaxies with SNe Ia, 227 had previously known spectroscopic redshifts, yielding an RCF estimate of 44% ± 4%. The RCF decreases with increasing distance and decreasing galaxy luminosity (for z < 0.05, or ∼200 Mpc, RCF ≈ 0.6). Prospects for dramatically increasing the RCF are limited to new multifiber spectroscopic instruments or wide-field narrowband surveys. Existing galaxy redshift catalogs are only ∼50% complete at r ≈ 16.9 mag. Pushing this limit several magnitudes deeper will pay huge dividends when searching for electromagnetic counterparts to gravitational wave events or sources of ultra-high-energy cosmic rays or neutrinos.

Keywords: supernovae; galaxies; redshift surveys; surveys
Spectroscopic Studies of 30 Short-period Cataclysmic Variable Stars and Remarks on the Evolution and Population of Similar Objects

We present spectroscopy and orbital periods Porb for 30 apparently nonmagnetic cataclysmic binaries with periods below ∼3 hr, nearly all of which are dwarf novae, mostly of the SU Ursae Majoris subclass. We then turn to the evidence supporting the prediction that short-period dwarf novae evolve toward longer periods after passing through a minimum period—the "period bounce" phenomenon. Plotting data from the literature reveals that for superhump period excess ɛ = (Psh - Porb)/Porb below ∼0.015, the period appears to increase with decreasing ɛ, agreeing at least qualitatively with the predicted behavior. Next, motivated by the long (decadal) outburst intervals of the WZ Sagittae subclass of short-period dwarf novae, we ask whether there could be a sizable population of "lurkers"—systems that resemble dwarf novae at minimum light, but which do not outburst over accessible timescales (or at all) and therefore do not draw attention to themselves. By examining the outburst history of the Sloan Digital Sky Survey sample of CVs, which were selected by color and not by outburst, we find that a large majority of the color-selected dwarf-nova-like objects have been observed to outburst, and we conclude that "lurkers," if they exist, are a relatively minor part of the CV population.

Keywords: semi-detached binary stars; su ursae majoris stars; wz sagittae stars; cataclysmic variable stars; dwarf novae; close binary stars
Reenergization of Radio Halo Electrons in the Merging Galaxy Cluster A2163

Radio halos in galaxy clusters are among the largest known radio-emitting objects in the universe. Their faint and amorphous structure and lack of emission elsewhere in the electromagnetic spectrum have yielded few clues to their origin. The strong association of halos with X-ray-luminous merging clusters has sustained the belief that mergers kindle halos. We report the detection of a sharply defined, central Ridge of halo emission in the galaxy cluster A2163, which is undergoing a merger. This "shock front"-like Ridge lies in between and is oriented perpendicular to the two merging subclusters, and is not associated with any active radio galaxy. This Ridge has the flattest spectrum and the brightest emission in the entire halo, suggesting that it is the principal site of energization, and appears to be a direct link between the merger geometry and halo energization. We also discuss the possibility that the Ridge is a relic seen in projection against the center of tthe halo. However, the configuration of the Ridge relative to the merging subclusters suggests that it is at the center of the halo. Based on our observations we suggest that a halo may simply be the merger-induced collective reincarnation of a large number of dead radio lobes.

Keywords: intracluster medium; galaxy clusters; non-thermal radiation sources; radio continuum emission; extragalactic radio sources; abell clusters
Radio and γ-Ray Variability in Blazar S5 0716+714: A Year-like Quasi-periodic Oscillation in the Radio Light Curve

The nearly 33 yr long-term radio light curve obtained with the Metsähovi Radio Observatory 14 m telescope at 37 GHz and the recent 12.7 yr γ-ray light curve of the blazar S5 0716+714 at 0.1-300 GeV from the Fermi Large Area Telescope (Fermi-LAT) were analyzed by using the Lomb-Scargle periodogram and the weighted wavelet Z-transform techniques. In the radio light curve, we discovered a possible quasi-periodic oscillation (QPO) signal of about 352 ± 23 days at a confidence level of ~3σ. We recalculated the periodicity and its significance in a chosen time range that has higher variability and denser sampling, and then found that the significance had increased to a confidence level of 99.996% (~4.1σ). This QPO component was further confirmed by fitting a linear autoregressive integrated moving average model to the selected radio light curve. A possible QPO of 960 ± 80 days at a 99.35% level (~2.7σ) was found in the γ-ray light curve, which generally agrees with the earlier QPO claims of S5 0716+714. This paper discusses possible mechanisms for this potential year-like QPO. One possibility is a pure geometrical scenario with blobs moving helically inside the jet. Another is a supermassive binary black hole involving a gravitational wave-driven regime. In the latter scenario, we derived a milliparsec separation in the binary system that undergoes coalescence within a century due to the emission of low-frequency gravitational waves.

Keywords: active galactic nuclei; blazars; time series analysis; relativistic jets
Spitzer Infrared Observations of the Galactic Classical Nova V2615 Ophiuchus

Classical novae contribute to the cycle of chemical enrichment of the interstellar medium through through explosive nucleosynthesis and the violent ejection of material dredged from the white dwarf progenitor and mixed with the accreted surface layers. Analysis of archival NASA Spitzer spectroscopic observations of V2615 Ophiuchus shows that [O IV] 25.89 μm is a strong coolant and that no evidence for deceleration of the ejecta was present ~1000 days after outburst, based on observed Spitzer InfraRed Spectrograph emission line average FWHM velocities of ≃600 km s-1.

Keywords: classical novae; spectroscopy
The Wind beneath My Wings. I. Spectral Types and Multiplicity of the Central Stars Supporting Stellar Bow Shock Nebulae

Arcuate mid-infrared nebulae known as stellar bow shock nebulae (SBNe) have been previously hypothesized to be supported by the strong stellar winds and/or luminosity of massive early-type stars. We present an optical spectroscopic survey of 84 stars identified from mid-infrared images as candidate SBN-supporting stars. Eighty-one of 84 sources, 96%, are O or early-B spectral types. K-band luminosities for a larger sample of 289 stars at the centers of bow shock nebulae are overwhelmingly consistent with OB stars. This affirms both that SBNe are supported by massive stars and that arcuate mid-infrared nebulae are reliable indicators of the presence of a massive star. The radial velocity dispersions of these systems and detections of double-lined systems indicate that at least 27 of the 74 systems with multiple observations (>36%) are candidate multiple-star systems. This rate is consistent with observed multiplicity rates of field OB stars detected with similar radial velocity surveys and lower than, but not inconsistent with, the multiplicity rates of OB stars in clusters and associations.

Keywords: massive stars; o stars; b stars; interstellar medium; stellar-interstellar interactions; stellar wind bubbles; dust nebulae; runaway stars; stellar bow shocks
Comparative Analysis of the 2020 November 29 Solar Energetic Particle Event Observed by Parker Solar Probe

We analyze two specific features of the intense solar energetic particle (SEP) event observed by Parker Solar Probe (PSP) between 2020 November 29 and 2020 December 2. The interplanetary counterpart of the coronal mass ejection (CME) on 2020 November 29 that generated the SEP event (hereafter ICME-2) arrived at PSP (located at 0.8 au from the Sun) on 2020 December 1. ICME-2 was preceded by the passage of an interplanetary shock at 18:35 UT on 2020 November 30 (hereafter S2), that in turn was preceded by another ICME (i.e., ICME-1) observed in situ on 2020 November 30. The two interesting features of this SEP event at PSP are the following: First, the presence of the intervening ICME-1 affected the evolution of the ≲8 MeV proton intensity-time profiles resulting in the observation of inverted energy spectra throughout the passage of ICME-1. Second, the sheath region preceding ICME-2 was characterized by weak magnetic fields compared to those measured immediately after the passage of the shock S2 and during the passage of ICME-2. Comparison with prior SEP events measured at 1 au but with similar characteristics indicates that (1) low-energy particles accelerated by S2 were excluded from propagating throughout ICME-1, and (2) the low magnetic fields measured in the sheath of ICME-2 resulted from the properties of the upstream solar wind encountered by ICME-2 that was propagated into the sheath, whereas the energy density of the high-energy particles in the sheath did not play a dominant role in the formation of these low magnetic fields.

Keywords: solar energetic particles; interplanetary shocks; solar coronal mass ejections; interplanetary magnetic fields
Cosmic Sands: The Origin of Dusty, Star-forming Galaxies in the Epoch of Reionization

We present the Cosmic Sands suite of cosmological zoom-in simulations based on the SIMBA galaxy formation model in order to study the buildup of the first massive and dusty galaxies in the early universe. Residing in the most massive halos, we find that the compact proto-massive galaxies undergo nearly continuous mergers with smaller subhalos, boosting star formation rates (SFRs) and the buildup of stellar mass. The galaxies are already appreciably chemically evolved by z = 7, with modeled dust masses comparable to those inferred from observations in the same epoch, except for the most extreme systems. We track gas accretion onto the galaxies to understand how extreme SFRs can be sustained by these early systems. We find that smooth gas accretion can maintain SFRs above 250 M ⊙ yr-1, but to achieve SFRs that boost galaxies well above the main sequence, a larger perturbation like a gas-rich major merger is necessary to trigger a starburst episode. Post-processing the Cosmic Sands simulations with dust RT, we find that, while the infrared luminosities of the most-dust-rich galaxies are comparable to local ULIRGs, they are substantially dimmer than classical z = 2 submillimeter galaxies. We end with a discussion on the possible reasons for this discrepancy at the highest masses and the future work we intend to carry out to study the chemical enrichment of the earliest dusty galaxies.

Keywords: galaxies; hydrodynamical simulations; ultraluminous infrared galaxies; starburst galaxies
Energetic Neutral Atom Flux from the Inner Heliosheath and Its Connection to Termination Shock Properties

We present statistical comparisons between energetic neutral atom (ENA) fluxes obtained using a global simulation of the heliosphere and data collected by the Interstellar Boundary Explorer (IBEX) spacecraft. The simulation of the inner heliosheath (IHS) ENA flux is based on a 3D steady-state heliosphere, while the data are from the IBEX-Hi instrument over the time period 2009-2015. The statistical comparison is performed by calculating the chi-square value between the simulated ENA fluxes and the data for each line of sight in the sky. A comparison with exposure-averaged data for solar minimum and solar maximum conditions is also performed to see the effect of solar wind (SW) properties on the IHS ENA fluxes. The model matches well with the data in the flanks and parts of the nose of the heliosphere, whereas the match is poor in the downwind tail, ribbon, and polar regions. We interpret these results to mean that (I) heliosheath plasma in the polar region consists of advected fast (or slow) SW during the solar minimum (or maximum) condition, and (II) heliospheric termination shock parameters are likely different over the poles. A poor match at around 30° north and south of the downwind direction is likely due to the existence of a mixture of plasma that comes from fast and slow SW. While our results are consistent with a single heliotail, the shape of the heliosphere continues to be an area of active research, and more data and further modeling are needed to determine its true structure.

Keywords: termination shock; heliosphere; pickup ions; heliosheath
Constraining Global Solar Models through Helioseismic Analysis

Global hydrodynamic simulations of internal solar dynamics have focused on replicating the conditions for solar-like (equator rotating faster than the poles) differential rotation and meridional circulation using the results of helioseismic inversions as a constraint. Inferences of meridional circulation, however, have provided controversial results showing the possibility of one, two, or multiple cells along the radius. To help address this controversy and develop a more robust understanding of global flow regimes in the solar interior, we apply a "forward-modeling" approach to the analysis of helioseismic signatures of meridional circulation profiles obtained from numerical simulations. We employ the global acoustic modeling code GALE to simulate the propagation of acoustic waves through regimes of mean mass-flows generated by global hydrodynamic and magnetohydrodynamic models: EULAG, the Pencil code, and the Rayleigh code. These models are used to create synthetic Dopplergram data products, used as inputs for local time-distance helioseismology techniques. Helioseismic travel-time signals from solutions obtained through global numerical simulations are compared directly with inferences from solar observations, in order to set additional constraints on global model parameters in a direct way. We show that even though these models are able to replicate solar-like differential rotation, the resulting rotationally constrained convection develops a multicell global meridional circulation profile that is measurably inconsistent with local time-distance inferences of solar observations. However, we find that the development of rotationally unconstrained convection close to the model surface is able to maintain solar-like differential rotation, while having a significant impact on the helioseismic travel-time signal, replicating solar observations within one standard deviation of the error due to noise.

Keywords: helioseismology; solar convective zone; solar meridional circulation; hydrodynamical simulations
Gas-phase Formation of Cationic Fullerene/Amino Acid Clusters: Evidence for the "Magic Number" Chemical Reactivity of Fullerene Cations

An experimental and theoretical investigation on the chemical reactivity of fullerene cations (Cn+, n = [36, 60]) with amino acid molecules (e.g., isoleucine, C6H13NO2) is performed. The results show that, in the gas phase, fullerene cations can react with amino acid molecules to form fullerene/amino acid cluster cations with high efficiency. The formation rate constants for the ion-molecule collision reactions between fullerene cations and isoleucine are estimated under the pseudo-first-order reaction condition. We find the formation rate constants increase gradually with a declining C-atom number of fullerene cations (Cn+, n = [46, 60]), and depict a plateau in the reactivity for smaller fullerene cations (Cn+, n = [36, 44]). More importantly, by comparing to its neighbor fullerenes, the magic number of C-atom counts (54, 58) that process with enhanced chemical reactivity are determined. We also obtained the molecular structures and binding energies for the fullerene-monoisoleucine adducts by quantum chemical calculations, which give a good explanation for the magic number chemical reactivity and the enhanced chemical reactivity of smaller fullerene cations. We infer that our results demonstrate the importance of ion-molecule reactions to the formation of large and complex fullerene-amino acid derivatives in the ISM. The high reactivity of fullerene species may indicate that amino acid molecules or other related prebiotic compounds can accrete on small interstellar carbon dust grains.

Keywords: interstellar medium; molecular physics; interstellar dust processes; polycyclic aromatic hydrocarbons; astrochemistry
Long-term Near-infrared Brightening of Nonvariable OH/IR Stars

Nonvariable OH/IR stars are thought to have just left the asymptotic giant branch (AGB) phase. In this conventional picture, they must still show strong circumstellar extinction caused by the dust ejected during the AGB phase, and the extinction is expected to decrease over time because of the dispersal of the circumstellar dust after the cessation of stellar mass loss. The reduction of extinction makes the stars become apparently brighter and bluer with time, especially in the near-infrared (NIR) range. We look for such long-term brightening of nonvariable OH/IR stars by using 2MASS, UKIDSS, and OAOWFC survey data. As such, we obtain multiepoch NIR data taken over a 20 yr period (1997-2017) for 6 of 16 nonvariable OH/IR stars, and all 6 objects are found to be brightening. The K-band brightening rate of five objects ranges from 0.010 to 0.130 mag yr-1, which is reasonably explained with the conventional picture. However, one OH/IR star, OH 31.0-0.2, shows a rapid brightening, which cannot be explained only by the dispersal of the dust shell. Multicolor (J-, H-, and K-band) data are obtained for three objects, OH 25.1-0.3, OH 53.6-0.2, and OH 77.9+0.2. Surprisingly, none of them appears to have become bluer, and OH 53.6-0.2 is found to have reddened at a rate of 0.013 mag yr-1 in (J - K). Our findings suggest other mechanisms such as rapid changes in stellar properties (temperature or luminosity) or a generation of a new batch of dust grains.

Keywords: late-type stars; post-asymptotic giant branch stars; circumstellar matter; asymptotic giant branch stars; stellar mass loss; stellar evolution
VERITAS Discovery of VHE Emission from the Radio Galaxy 3C 264: A Multiwavelength Study

The radio source 3C 264, hosted by the giant elliptical galaxy NGC 3862, was observed with the Very Energetic Radiation Imaging Telescope Array System (VERITAS) between 2017 February and 2019 May. These deep observations resulted in the discovery of very high energy (VHE; E > 100 GeV) γ-ray emission from this active galaxy. An analysis of ∼57 hr of quality-selected live time yields a detection at the position of the source, corresponding to a statistical significance of 7.8 standard deviations above background. The observed VHE flux is variable on monthly timescales, with an elevated flux seen in 2018 observations. The VHE emission during this elevated state is well characterized by a power-law spectrum with a photon index Γ = 2.20 ± 0.27 and flux F(>315 GeV) = ( $7.6\pm {1.2}_{\mathrm{stat}}\pm {2.3}_{\mathrm{syst}})\times {10}^{-13}$ cm-2 s-1, or approximately 0.7% of the Crab Nebula flux above the same threshold. 3C 264 (z = 0.0217) is the most distant radio galaxy detected at VHE, and the elevated state is thought to be similar to that of the famously outbursting jet in M87. Consequently, extensive contemporaneous multiwavelength data were acquired in 2018 at the time of the VHE high state. An analysis of these data, including Very Long Baseline Array, Very Large Array, Hubble Space Telescope, Chandra, and Swift observations in addition to the VERITAS data, is presented, along with a discussion of the resulting spectral energy distribution.

Keywords: relativistic jets; high energy astrophysics; active galactic nuclei