adsabs/SciX_UAT_keywords huggingface dataset:
Accretion and Intercycle Variations in the PMS Interacting Binary AK Sco

There are only a handful of known short-period pre-main-sequence spectroscopic binaries with significant accretion rates (Class II sources). AK Sco stands out in this list because the system is composed of two equal mass F5 stars in a highly eccentric orbit thus both stars get as close as 11 stellar radii at periastron passage. This configuration is optimal for accretion studies because enhanced accretion events can be precisely timed at periastron passage. In this work, we present the results from the monitoring of the AK Sco system with Hubble during three consecutive periastron passages. These data provide a unique data set to spectroscopically characterize accretion and evaluate the intercycle variability of the system. Clear evidence of accretion rate enhancement was observed in cycles 1 and 3: the blueing of the near-UV continuum, the sudden flux increase of important accretion tracers, such as the N V, Si IV, and C IV lines, and also of neutral/singly ionized species such as O I and C II. Also, variations in the Si III]/C III] ratio reveal an enhancement of the electron density by an order of magnitude during the periastron passage. Moreover, in cycle 3, the spectral resolution of the observations obtained with the Cosmic Origins Spectrograph enabled us to discern that the flow was channeled preferentially into one of the two components. The most remarkable feature in the cycle-to-cycle variations was the detection of a notable increase of the UV flux from cycle 1 to cycle 2 that was not accompanied by enhanced accretion signatures.

Keywords: interacting binary stars; f stars; pre-main sequence stars; stellar accretion disks; stellar jets; circumstellar disks
Searching for the Transit Signal of the Exoplanet Gl 514 b in the TESS Photometry

We report an analysis of the TESS light curve of the nearby M dwarf Gl 514 that was observed during Sector 50. We search for a transit of the likely super-Earth Gl 514 b that was detected in 2022 in radial velocity data. The planet moves on an eccentric orbit that partly resides within the habitable zone of its host. According to the predictions, a transit may occur with high probability within the observing window covered by Sector 50, and being easily detected, therefore offering a great opportunity for an independent confirmation of the existence of Gl 514 b, and for a better characterization of the planet.

Keywords: exoplanets
Particle Dynamics in 3D Self-gravitating Disks. I. Spirals

Spiral arms are distinctive features of many circumstellar disks, observed in scattered light, which traces the disk surface; millimeter dust emission, which probes the disk midplane; as well as molecular emission. The two leading explanations for spirals are wakes generated by a massive planet and the density waves excited by disk self-gravity. We use stratified 3D hydrodynamic shearing-box simulations including dust particles and disk self-gravity to investigate how gas and dust spirals in a self-gravitating disk depend on the simulation size, the cooling efficiency, and the aerodynamic properties of particles. We find that the opening angles of spirals are universal (∼10°) and not significantly affected by the size of the computational domain, the cooling time, or the particle size. In simulations with the biggest domain, the spirals in the gaseous disk become slightly more open with a higher cooling efficiency. Small dust follows the gaseous spirals very well, while intermediate-sized dust with dimensionless stopping time (St) close to 1 is more concentrated in the spirals and shows stronger spirals. However, large dust with St > 1 also shows spirals, which is different from some previous simulations. We identify that this is due to the gravity from the gas to the dust component. We show that when St ≳ Q, the gravitational force from the gaseous spirals on the dust particles becomes stronger than the particles' aerodynamic drag force, so that the gas significantly affects these large particles through gravitational interaction. This has important implications for both spiral observations and planetesimal formation/dynamics.

Keywords: protoplanetary disks; hydrodynamics; circumstellar dust; gravitation
Magnetospheric Multiscale Observations of the Off-equatorial Dipolarization Front Dynamics in the Terrestrial Magnetotail

We report the Magnetospheric Multiscale observations of dynamics at an off-equatorial dipolarization front (DF) in the Terrestrial Magnetotail. Three different plasma waves, namely electromagnetic ion cyclotron (EMIC) waves, lower hybrid drift waves, and electrostatic solitary waves (ESWs), associated with different electron pitch angle distributions were detected at different subregions of a single DF. It is interesting to note that the EMIC wave was linearly polarized, associating with a parallel current as a result of the antiparallel drift of electrons in the energy range of about 0.3-2 keV. These suggest that the wave was most likely to be locally generated. This generation could be explained by the current-driven kink-like instability due to the electron drift. The current-driven instabilities may dissipate the energy of the field-aligned current at the DF and thus play important roles in the magnetosphere-ionosphere coupling. On the other hand, the detected ESWs are interpreted as multidimensional electromagnetic electron holes (EHs) which are manifestations of several distinguishing features in electric and magnetic field perturbation. The EHs with a strong positive central potential suggested the likelihood of nonlinear behavior.

Keywords: space plasmas; plasma astrophysics
Continuum-fitting the X-Ray Spectra of Tidal Disruption Events

We develop a new model for X-ray emission from tidal disruption events (TDEs), applying stationary general relativistic "slim disk" accretion solutions to supermassive black holes (SMBHs) and then ray-tracing the photon trajectories from the image plane to the disk surface, including gravitational redshift, Doppler, and lensing effects self-consistently. We simultaneously and successfully fit the multi-epoch XMM-Newton X-ray spectra for two TDEs: ASASSN-14li and ASASSN-15oi. We test explanations for the observed, unexpectedly slow X-ray brightening of ASASSN-15oi, including delayed disk formation and variable obscuration by a reprocessing layer. We propose a new mechanism that better fits the data: a "slimming disk" scenario in which accretion onto an edge-on disk slows, reducing the disk height and exposing more X-rays from the inner disk to the sightline over time. For ASASSN-15oi, we constrain the SMBH mass to ${4.0}_{-3.1}^{+2.5}\times {10}^{6}{M}_{\odot }$ . For ASASSN-14li, the SMBH mass is ${10}_{-7}^{+1}\times {10}^{6}{M}_{\odot }$ , and the spin is >0.3. For both TDEs, our fitted masses are consistent with independent estimates; for ASASSN-14li, application of the external mass constraint narrows our spin constraint to >0.85. The mass accretion rate of ASASSN-14li decays slowly, as ∝t-1.1, perhaps due to inefficient debris circularization. Over ≈1100 days, its SMBH has accreted ΔM ≈ 0.17M⊙, implying a progenitor star mass of >0.34M⊙, I.e., no "missing energy problem." For both TDEs, the hydrogen column density declines to the host galaxy plus Milky Way value after a few hundred days, suggesting a characteristic timescale for the depletion or removal of obscuring gas.

Keywords: supermassive black holes; black hole physics; accretion; x-ray transient sources; tidal disruption
Nanoflare Theory and Stochastic Reconnection

Local magnetic reversals are an inseparable part of magnetohydrodynamic turbulence whose collective outcome may lead to a global reconnection with a rate independent of the small scale physics—stochastic reconnection. We show that this picture is related to the nanoflare theory, which is one of the most plausible models to solve the coronal heating problem. The magnetic field follows the turbulent flow in a statistical sense by means of stochastic flux freezing. Hence the turbulence, which bends and stretches the initially smooth field, will tend to increase the field's spatial complexity. Strong magnetic shears associated with such a highly tangled field can trigger local reversals and field annihilations on a wide range of inertial scales, which convert magnetic energy into kinetic and thermal energy respectively. The former enhances the turbulence while the latter enhances heat generation on any inertial scale. These theoretical predictions are supported by scaling laws and simulations.

Keywords: solar corona; solar coronal heating; solar magnetic reconnection
Confirming the Explosive Outflow in G5.89 with ALMA

The explosive molecular outflow detected decades ago in the Orion BN/KL region of massive star formation was considered to be a bizarre event. This belief was strengthened by the nondetection of similar cases over the years with the only exception of the marginal case of DR21. Here, we confirm a similar explosive outflow associated with the UCHII region G5.89-0.39 that indicates that this phenomenon is not unique to Orion or DR21. Sensitive and high angular resolution (∼0"1) Atacama Large Millimeter/submillimeter Array (ALMA) CO(2-1) and SiO(5-4) observations show that the molecular outflow in the massive star-forming region G5.89-0.39 is indeed an explosive outflow with an age of about 1000 yr and a liberated kinetic energy of 1046-49 erg. Our new CO(2-1) ALMA observations revealed over 30 molecular filaments, with Hubble-like expansion motions, pointing to the center of UCHII region. In addition, the SiO(5-4) observations reveal warmer and strong shocks very close to the origin of the explosion, confirming the true nature of the flow. A simple estimation for the occurrence of these explosive events during the formation of the massive stars indicates an event rate of once every ∼100 yr, which is close to the supernovae rate.

Keywords: star formation; aperture synthesis; massive stars; stellar jets; stellar-interstellar interactions
A Recipe for Unbiased Background Modeling in Deep Wide-field Astronomical Images

Unbiased sky background modeling is crucial for the analysis of deep wide-field images, but it remains a major challenge in low surface brightness astronomy. Traditional image processing algorithms are often designed to produce artificially flat backgrounds, erasing astrophysically meaningful structures. In this paper, we present three ideas that can be combined to produce wide-field astronomical data that preserve accurate representations of the background sky: (1) Use of all-sky infrared/submillimeter data to remove the large-scale time-varying components while leaving the scattered light from Galactic cirrus intact, with the assumptions of (a) the underlying background has little power on small scales, and (b) the Galactic cirrus in the field is optically thin on large scales; (2) Censoring of frames contaminated by anomalously prominent wings in the wide-angle point-spread function; and (3) Incorporation of spatial covariance in image stacking that controls the local background consistency. We demonstrate these methods using example data sets obtained with the Dragonfly Telephoto Array, but these general techniques are prospective to be applied to improve sky models in data obtained from other wide-field imaging surveys, including those from the upcoming Vera Rubin Telescope.

Keywords: low surface brightness galaxies; astronomy data reduction; interstellar dust; astronomy image processing; astronomical techniques; sky surveys; diffuse nebulae
Young Faithful: The Eruptions of EC 53 as It Cycles through Filling and Draining the Inner Disk

While young stellar objects sometimes undergo bursts of accretion, these bursts usually occur sporadically, making them challenging to study observationally and to explain theoretically. We build a schematic description of cyclical bursts of the young stellar object EC 53 (also known as V371 Ser) using near-IR and submillimeter monitoring obtained over six cycles, each lasting ≈530 days. EC 53 brightens over 0.12 yr by 0.3 mag at 850 μm, by 2 mag at 3.35 μm, and by 1.5 mag at near-IR wavelengths, to a maximum luminosity consistent with an accretion rate of ∼8 × 10-6 M⊙ yr-1. The emission then decays with an e-folding timescale of $\approx 0.74\,\mathrm{yr}$ until the accretion rate is ∼1 × 10-6 M⊙ yr-1. The next eruption then occurs, likely triggered by the buildup of a ∼5 × 10-6 M⊙ mass in the inner disk, enough for it to become unstable and drain onto the star. Just before outburst, when the disk is almost replenished, the near-IR colors become redder, indicating an increase in the geometrical height of the disk by this mass buildup. The reddening disappears soon after the initial burst, as much of the mass is drained from the disk. We quantify physical parameters related to the accretion process in EC 53 by assuming an α-disk formulation, constrained by the observed disk properties and accretion rate. While we can only speculate about the possible trigger for these faithful eruptions, we hope that our quantified schematic will motivate theorists to test the hypothesized mechanisms that could cause the cyclical buildup and draining of mass in the inner disk.

Keywords: star formation; protoplanetary disks; circumstellar disks; protostars; variable stars; stellar accretion; periodic variable stars; young stellar objects
Analysis of Spectral Lines in Large Databases of Synthetic Spectra for Massive Stars

In this paper, we describe a program that identifies in the optical spectrum the main parameters of a spectral line, namely the initial and final wavelengths, and the line depth. Moreover, using numerical calculations, it identifies and removes adjacent lines. Next, the program calculates the equivalent width and the FWHM. The software was tested in a sample of 300 lines in two databases of synthetic spectra generated by the CMFGEN and PoWR codes, and 300 lines in observed spectra from the IACOB database, showing a Gaussian distribution of relative errors, from which it is inferred that 80% of the measured lines have errors less than 17% and only 5% of the lines have errors greater than 26%. The program was also run on the entire database of 45,000 CMFGEN and 202 POWR synthetic spectra, generating a library of H I, He I, and He II lines necessary to feed the FITspec code for the derivation of stellar parameters: effective temperature, surface gravity, and luminosity.

Keywords: stellar atmospheres
Left Ringing: Betelgeuse Illuminates the Connection between Convective Outbursts, Mode Switching, and Mass Ejection in Red Supergiants

Betelgeuse, the nearest red supergiant, dimmed to an unprecedented level in early 2020. The star emerged from this "Great Dimming" episode with its typical, roughly 400 days pulsation cycle halved, and a new dominant period of around 200 days. The dimming event has been attributed to a surface mass ejection, in which rising material drove shocks through the stellar atmosphere and expelled some material, partially obscuring the star as it formed molecules and dust. In this paper, we use hydrodynamic simulations to reveal the connections between Betelgeuse's vigorously convective envelope, the surface mass ejection, and the pulsation mode switching that ensued. An anomalously hot convective plume, generated rarely but naturally in the star's turbulent envelope, can rise and break free from the surface, powering an upwelling that becomes the surface mass ejection. The rising plume also breaks the phase coherence of the star's pulsation, causing the surface to keep expanding even as the deeper layers contract. This drives a switch from the 400 days fundamental mode of pulsation, in which the whole star expands and contracts synchronously, to the 200 days first overtone, where a radial node separates the interior and exterior of the envelope moving in opposite phase. We predict that the star's convective motions will damp the overtone oscillation and Betelgeuse will return to its previous, 400 days fundamental-mode pulsation in the next 5-10 yr. With its resolved surface and unprecedentedly detailed characterization, Betelgeuse opens a window to episodic surface mass ejection in the late-stage evolution of massive stars.

Keywords: hydrodynamics; pulsating variable stars; red supergiant stars; stellar convection envelopes
A Comparative Analysis of the Solar Ultraviolet Spectral Irradiance Measured from Earth and Mars: Toward a General Empirical Model for the Study of Planetary Aeronomy

Accurate estimation of the solar vacuum ultraviolet irradiance between 0.1 and 200 nm is critical for the study of planetary aeronomy. Previous empirical models have relied on a limited number of reference spectra, or on multiple data sets with various degrees of uncertainty, and on an empirical selection of solar proxies. Here we propose a novel method for the development of empirical models based on Fourier transform and least-squares fitting of the long-term measurements from the Solar EUV Experiment on the Thermosphere Ionosphere Mesosphere Energetics and Dynamics mission. A Fourier transform analysis is performed to examine a large number of solar proxies, which reveals that the solar radio flux at 10.7 cm and the solar Lyα flux at 121.6 nm are better proxies for solar irradiance below and above ∼120 nm, respectively. Using these two proxies, a nonlinear empirical model is developed through Fourier transform and least-squares fitting of solar irradiance measurements, which can reproduce the solar irradiance with uncertainties of only ∼1%–2% above ∼120 nm, ∼2%–4% within ∼45–120 nm, and ∼4%–8% below ∼45 nm. Comparison with measurements from the Extreme Ultraviolet Monitor on the Mars Atmosphere and Volatile Evolution mission indicates that the solar irradiance at Mars can be predicted with uncertainties of less than ∼8% by geometric extrapolation of the solar irradiance measured from Earth, provided that the measurements from Earth can be calibrated accurately. Our study provides a general method for the development of empirical models using long-term observations in planetary aeronomy.

Keywords: solar spectral irradiance; solar ultraviolet emission; mars; aeronomy
FRAMEx. III. Radio Emission in the Immediate Vicinity of Radio-quiet AGNs

We present follow-up results from the first Fundamental Reference AGN Monitoring Experiment (FRAMEx) X-ray/radio snapshot program of a volume-complete sample of local hard X-ray-selected active galactic nuclei (AGNs). Here, we added nine new sources to our previous volume-complete snapshot campaign, two of which are detected in 6 cm Very Long Baseline Array (VLBA) observations. We also obtained deeper VLBA observations for a sample of nine AGNs not detected by our previous snapshot campaign. We recovered three sources with approximately twice the observing sensitivity. In contrast with lower-angular-resolution Very Large Array (VLA) studies, the majority of our sources continue to be undetected with the VLBA. The subparsec radio (6 cm) and X-ray (2-10 keV) emission shows no significant correlation, with L R/L X ranging from 10-8 to 10-4, and the majority of our sample lies well below the fiducial 10-5 relationship for coronal synchrotron emission. Additionally, our sources are not aligned with any of the proposed "fundamental" planes of black hole activity, which purport to unify black hole accretion in the M BH-L X-L R parameter space. The new detections in our deeper observations suggest that the radio emission may be produced by the synchrotron radiation of particles accelerated in low-luminosity outflows. Nondetections may be a result of synchrotron self-absorption at 6 cm in the radio core, similar to what has been observed in X-ray binaries transitioning from the radiatively inefficient state to a radiatively efficient state.

Keywords: radio astrometry; active galaxies; radio active galactic nuclei; x-ray active galactic nuclei
Herschel/PACS OH Spectroscopy of Seyfert, LINER, and Starburst Galaxies

We investigated the 65, 71, 79, 84, 119, and 163 μm OH doublets of 178 local (0 < z < 0.35) galaxies. They were observed using the Herschel/Photoconductor Array Camera and Spectrometer, including Seyfert galaxies, low-ionization nuclear emission-line regions, and star-forming galaxies. We observe these doublets exclusively in absorption (OH71), primarily in absorption (OH65, OH84), mostly in emission (OH79), only in emission (OH163), and an approximately even mix of the both (OH119). In 19 galaxies we find P Cygni or reverse P Cygni line profiles in the OH doublets. We use several galaxy observables to probe spectral classification, brightness of a central active galactic nucleus (AGN)/starburst component, and radiation field strength. We find that OH79, OH119, and OH163 are more likely to display strong emission for bright, unobscured AGNs. For less luminous, obscured AGNs and nonactive galaxies, we find populations of strong absorption (OH119), weaker emission (OH163), and a mix of weak emission and weak absorption (OH79). For OH65, OH71, and OH84, we do not find significant correlations with the observables listed above. For OH79 and OH119, we find relationships with both the 9.7 μm silicate feature and Balmer decrement dust extinction tracers in which more dust leads to weaker emission/stronger absorption. The origin of emission for the observed OH doublets, whether from collisional excitation or from radiative pumping by infrared photons, is discussed. * Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.

Keywords: active galaxies; seyfert galaxies; galaxy counts; infrared galaxies
Identification of Active Magnetic Reconnection Using Magnetic Flux Transport in Plasma Turbulence

Magnetic reconnection has been suggested to play an important role in the dynamics and energetics of plasma turbulence by spacecraft observations, simulations, and theory over the past two decades, and recently, by magnetosheath observations of MMS. A new method based on magnetic flux transport (MFT) has been developed to identify reconnection activity in turbulent plasmas. This method is applied to a gyrokinetic simulation of two-dimensional (2D) plasma turbulence. Results on the identification of three active reconnection X-points are reported. The first two X-points have developed bidirectional electron outflow jets. Beyond the category of electron-only reconnection, the third X-point does not have bidirectional electron outflow jets because the flow is modified by turbulence. In all cases, this method successfully identifies active reconnection through clear inward and outward flux transport around the X-points. This transport pattern defines reconnection and produces a new quadrupolar structure in the divergence of MFT. This method is expected to be applicable to spacecraft missions such as MMS, Parker Solar Probe, and Solar Orbiter.

Keywords: solar magnetic reconnection; space plasmas; interplanetary turbulence
A Magnetized, Moon-forming Giant Impact

The Moon is believed to have formed in the aftermath of a giant impact between a planetary-mass body and the proto-Earth. In a typical giant impact scenario, a disk of vapor, liquid, and solid debris forms around the proto-Earth and—after possibly decades of evolution—condenses to form the Moon. Using state-of-the-art numerical simulations, we investigate the dynamical effects of magnetic fields on the Moon-forming giant impact. We show that turbulence generated by the collision itself, shear in the boundary layer between the post-impact debris field and the proto-Earth, and turbulence in the vapor component of the disk amplify the field to dynamically significant strengths. Magnetically driven turbulence promotes angular momentum transport in the protolunar disk. Debris material is accreted onto the proto-Earth, making Moon formation less efficient, while the disk is forced to spread to larger radii, cooling at its outer edge. Magnetic fields speed the evolution of the vapor component of the protolunar disk and hasten the formation of the Moon.

Keywords: magnetohydrodynamical simulations; natural satellite formation; magnetic fields