adsabs/SciX_UAT_keywords huggingface dataset:
Surviving in Ocean Worlds: Experimental Characterization of Fiber Optic Tethers across Europa-like Ice Faults and Unraveling the Sliding Behavior of Ice

As an initial step toward in situ exploration of the interiors of Ocean Worlds to search for life using cryobot architectures, we test how various communication tethers behave under potential Europa-like stress conditions. By freezing two types of pretensioned insulated fiber optic cables inside ice blocks, we simulate tethers being refrozen in a probe's wake as it traverses through an Ocean World's ice shell. Using a cryogenic biaxial apparatus, we simulate shear motion on preexisting faults at various velocities and temperatures. These shear tests are used to evaluate the mechanical behavior of ice, characterize the behavior of communication tethers, and explore their limitations for deployment by a melt probe. We determine (a) the maximum shear stress tethers can sustain from an ice fault, prior to failure (viable/unviable regimes for deployment), and (b) optical tether performance for communications. We find that these tethers are fairly robust across a range of temperature and velocity conditions expected on Europa (T = 95-260 K, velocity = 5 × 10-7 m s-1 to 3 × 10-4 m s-1). However, damage to the outer jackets of the tethers and stretching of inner fibers at the coldest temperatures tested both indicate a need for further tether prototype development. Overall, these studies constrain the behavior of optical tethers for use at Ocean Worlds, improve the ability to probe thermomechanical properties of dynamic ice shells likely to be encountered by landed missions, and guide future technology development for accessing the interiors of (potentially habitable ± inhabited) Ocean Worlds.

Keywords: europa; galilean satellites; tidal friction; planetary interior; planetary probes; space probes
Constraining Galactic Structure with the LISA White Dwarf Foreground

White dwarfs (WDs) comprise 95% of all stellar remnants, and are thus an excellent tracer of old stellar populations in the Milky Way. Current and planned telescopes are not able to directly probe the WD population in its entirety due to its inherently low luminosity. However, the Galactic population of double white dwarf binaries (DWDs) gives rise to a millihertz gravitational-wave foreground detectable by the Laser Interferometer Space Antenna (LISA). Here we show how characterizing the angular power of the WD foreground will enable probes of the Galactic structure in a novel way to determine whether the Galactic WD population traces the spatial distribution of young, bright stars, or traces a vertically heated spatial distribution associated with the Galaxy's oldest stellar populations. We do this using a binary population synthesis study that incorporates different Galactic spatial distributions for the DWD population. We find that the level of anisotropy in the WD foreground's angular power spectrum is dependent on the vertical scale height of the population, but show that multipole coefficients from the spherical harmonic decomposition must be considered individually because of LISA's angular resolution. Finally, we show that LISA can probe the vertical scale height of the Galactic WD population with an accuracy of 300 pc, using the hexadecapole moment of the WD foreground.

Keywords: galaxy disks; galaxy dynamics; gravitational wave astronomy; white dwarf stars; stellar populations
The Proper Motion of Sagittarius A*. III. The Case for a Supermassive Black Hole

We report measurements with the Very Long Baseline Array of the proper motion of Sgr A* relative to two extragalactic radio sources spanning 18 yr. The apparent motion of Sgr A* is -6.411 ± 0.008 mas yr-1 along the Galactic plane and -0.219 ± 0.007 mas yr-1 toward the North Galactic Pole. This apparent motion can almost entirely be attributed to the effects of the Sun's orbit about the Galactic center. Removing these effects yields residuals of -0.58 ± 2.23 km s-1 in the direction of Galactic rotation and -0.85 ± 0.75 km s-1 toward the North Galactic Pole. A maximum-likelihood analysis of the motion, both in the Galactic plane and perpendicular to it, expected for a massive object within the Galactic center stellar cluster indicates that the radiative source, Sgr A*, contains more than about 25% of the gravitational mass of 4 × 106 M⊙ deduced from stellar orbits. The intrinsic size of Sgr A* is comparable to its Schwarzschild radius, and the implied mass density of ≳ 4× 1023 M⊙ pc-3 is very close to that expected for a black hole, providing overwhelming evidence that it is indeed a supermassive black hole. Finally, "intermediate mass" black holes more massive than ≈3 × 104 M⊙ between approximately 0.003 and 0.1 pc from Sgr A* are excluded.

Keywords: astrometry; galactic center; milky way galaxy physics; milky way dynamics; supermassive black holes
A Spatially Resolved Study of Hard X-Ray Emission in Kepler's Supernova Remnant: Indications of Different Regimes of Particle Acceleration

Synchrotron X-ray emission in young supernova remnants (SNRs) is a powerful diagnostic tool to study the population of high-energy electrons accelerated at the shock front and the acceleration process. We performed a spatially resolved spectral analysis of NuSTAR and XMM-Newton observations of the young Kepler's SNR, aiming to study in detail its nonthermal emission in hard X-rays. We selected a set of regions all around the rim of the shell and extracted the corresponding spectra. The spectra were analyzed by adopting a model of synchrotron radiation in the loss-limited regime, to constrain the dependence of the cutoff energy of the synchrotron radiation on the shock velocity. We identify two different regimes of particle acceleration, characterized by different Bohm factors. In the north, where the shock interacts with a dense circumstellar medium (CSM), we found a more efficient acceleration than in the south, where the shock velocity is higher and there are no signs of shock interaction with the dense CSM. Our results suggest an enhanced efficiency of the acceleration process in regions where the shock-CSM interaction generates an amplified and turbulent magnetic field. By combining hard X-ray spectra with radio and γ-ray observations of Kepler's SNR, we modeled the spectral energy distribution. In the light of our results we propose that the observed γ-ray emission is mainly hadronic and originates in the northern part of the shell.

Keywords: cosmic ray sources; supernova remnants
Constraints on Magnetic Braking from the G8 Dwarf Stars 61 UMa and τ Cet

During the first half of their main-sequence lifetimes, stars rapidly lose angular momentum to their magnetized winds, a process known as magnetic braking. Recent observations suggest a substantial decrease in the magnetic braking efficiency when stars reach a critical value of the Rossby number, the stellar rotation period normalized by the convective overturn timescale. Cooler stars have deeper convection zones with longer overturn times, reaching this critical Rossby number at slower rotation rates. The nature and timing of the transition to weakened magnetic braking have previously been constrained by several solar analogs and two slightly hotter stars. In this Letter, we derive the first direct constraints from stars cooler than the Sun. We present new spectropolarimetry of the old G8 dwarf τ Cet from the Large Binocular Telescope, and we reanalyze a published Zeeman Doppler image of the younger G8 star 61 UMa, yielding the large-scale magnetic field strengths and morphologies. We estimate mass-loss rates using archival X-ray observations and inferences from Lyα measurements, and we adopt other stellar properties from asteroseismology and spectral energy distribution fitting. The resulting calculations of the wind braking torque demonstrate that the rate of angular momentum loss drops by a factor of 300 between the ages of these two stars (1.4-9 Gyr), well above theoretical expectations. We summarize the available data to help constrain the value of the critical Rossby number, and we identify a new signature of the long-period detection edge in recent measurements from the Kepler mission.

Keywords: spectropolarimetry; stellar evolution; stellar magnetic fields; stellar rotation; stellar winds
Stability of Parker's Steady Solar Wind Solution in the Subcritical Region

Parker's steady solar wind solution (PSSWS) is a physically acceptable solution describing a smooth acceleration of the solar wind to supersonic speeds. Parker proposed that PSSWS possesses an intrinsic stability, like a "stable attractor" of this dynamical system. With a view to give a systematic analytical development, we restrict ourselves to the subcritical region inside the Parker critical point (PCP) where the solar wind goes through sonic flow conditions. This enables one to avoid the singularity at PCP plaguing the linear stability problem. Following Parker, we approximate the corona in the subcritical region by a static atmosphere and amend it to include an azimuthal flow and a weak radial flow. These physical simplifications enable us to pose a Sturm-Liouville problem for linearized perturbations about PSSWS. PSSWS is shown to have an intrinsic stability in the subcritical region, while leaving the solar coronal base in a state of (1) rest, (2) corotation with the Sun, and (3) slow radial motion. This result is also shown to hold when a diabatic flow in near-isothermal conditions is included in Parker's model to explicitly account for the extended coronal heating. The isothermal gas assumption in Parker's model is then relaxed, and a more realistic barotropic fluid representing variable extended active coronal heating conditions is considered for the gas flow. The stability of PSSWS, as the solar wind flow emerges from a state of rest at the solar surface, is shown to continue to hold.

Keywords: solar wind
Delayed X-Ray Brightening Accompanied by Variable Ionized Absorption Following a Tidal Disruption Event

Supermassive black holes can experience super-Eddington peak mass fallback rates following the tidal disruption of a star. The theoretical expectation is that part of the infalling material is expelled by means of an accretion disk wind, whose observational signature includes blueshifted absorption lines of highly ionized species in X-ray spectra. To date, however, only one such ultrafast outflow (UFO) has been reported in the tidal disruption event (TDE) ASASSN–14li. Here we report on the discovery of a transient absorption-like signature in X-ray spectra of the TDE AT2020ksf/Gaia20cjk (at a redshift of z = 0.092), following an X-ray brightening ∼230 days after UV/optical peak. We find that while no statistically significant absorption features are present initially, they appear on a timescale of several days and remain detected up to 770 days after peak. Simple thermal continuum models, combined with a power-law or neutral absorber, do not describe these features well. Adding a partial-covering, low-velocity ionized absorber improves the fit at early times but fails at late times. A high-velocity (v w ∼ 42,000 km s‑1), ionized absorber (UFO) provides a good fit to all data. The few-day timescale of variability is consistent with expectations for a clumpy wind. We discuss several scenarios that could explain the X-ray delay, as well as the potential for larger-scale wind feedback. The serendipitous nature of the discovery could suggest a high incidence of UFOs in TDEs, alleviating some of the tension with theoretical expectations.

Keywords: x-ray transient sources; ultraviolet transient sources; tidal disruption
ALMA Observations of Multiple CO and C Lines toward the Active Galactic Nucleus of NGC 7469: An X-Ray-dominated Region Caught in the Act

We used the Atacama Large Millimeter/submillimeter Array to map 12CO(J = 1-0), 12CO(J = 2-1), 12CO(J = 3-2), 13CO(J = 2-1), and [C I](3P1-3P0) emission lines around the type 1 active galactic nucleus (AGN) of NGC 7469 (z = 0.0164) at ∼100 pc resolutions. The CO lines are bright in both the circumnuclear disk (central ∼300 pc) and the surrounding starburst (SB) ring (∼1 kpc diameter), with two bright peaks on either side of the AGN. By contrast, the [C I](3P1-3P0) line is strongly peaked on the AGN. Consequently, the brightness temperature ratio of [C I](3P1-3P0) to 13CO(2-1) is ∼20 at the AGN, as compared to ∼2 in the SB ring. Our local thermodynamic equilibrium (LTE) and non-LTE models indicate that the enhanced line ratios (or C I enhancement) are due to an elevated C0/CO abundance ratio (∼3-10) and temperature (∼100-500 K) around the AGN as compared to the SB ring (abundance ratio ∼1, temperature ≲100 K), which accords with the picture of the X-ray-dominated region. Based on dynamical modelings, we also provide CO(1-0)-to- and [C I](3P1-3P0)-to-molecular mass conversion factors at the central ∼100 pc of this AGN as αCO = 4.1 and αC I = 4.4 M⊙ (K km s-1 pc2)-1, respectively. Our results suggest that the C I enhancement is potentially a good marker of AGNs that could be used in a new submillimeter diagnostic method toward dusty environments.

Keywords: agn host galaxies; seyfert galaxies; astrochemistry; interstellar medium; starburst galaxies; active galactic nuclei
Infrared Emission from Cold Gas Dusty Disks in Massive Ellipticals

What is the expected infrared output of elliptical galaxies? We report the latest findings obtained in this high time resolution (∼10 yr) and high spatial resolution (2.5 pc at center) study. We add a set of grain physics to the Massive active galactic nucleus (AGN) Controlled Ellipticals Resolved code, including (a) dust grains made in passive stellar evolution; (b) dust grain growth due to collision and sticking; (c) grain destruction due to thermal sputtering; (d) dust cooling of hot gas via inelastic collisions; and (e) radiation pressure on dust grains. The code improvements enable us to analyze metal depletion and AGN obscuration due to dust, and to assess its infrared output. We simulate a representative massive elliptical galaxy of a central stellar velocity dispersion ∼260 km s-1 and modest rotation. We find that: (1) the circumnuclear disk (∼1 kpc in diameter) is dusty in its outer region where most of the metals are contained in dust grains, while in the inner disk, dust grains are mostly destroyed by the AGN irradiation; (2) the dusty disk is optically thick to both the starlight within the disk and the radiation from the central AGN; thus the AGN is obscured behind the disk, and the covering factor is ∼0.2; and (3) the duty cycles of the AGN activities, star formation, and the dust infrared luminosity roughly match observations; e.g., in most of its lifetime, the simulated galaxy is a stereotypical "quiescent" elliptical galaxy with ${L}_{\mathrm{IR}}\sim {10}^{11}{L}_{\odot }$ , but it can reach ≳1046 erg s-1 during outbursts with a star formation rate $\gtrsim 250\,{M}_{\odot }\,{\mathrm{yr}}^{-1}$ .

Keywords: black hole physics; elliptical galaxies; e+a galaxies; galaxy evolution; interstellar medium; interstellar dust; metallicity; dust continuum emission
Characterizing Protoclusters and Protogroups at z 2.5 Using Lyα Tomography

Lyα tomography surveys have begun to produce 3D maps of the intergalactic medium opacity at z ~ 2.5 with megaparsec resolution. These surveys provide an exciting new way to discover and characterize high-redshift overdensities, including the progenitors of today's massive groups and clusters of galaxies, known as protogroups and protoclusters. We use the IllustrisTNG-300 hydrodynamical simulation to build mock maps that realistically mimic those observed in the Lyα Tomographic IMACS Survey. We introduce a novel method for delineating the boundaries of structures detected in 3D Lyα flux maps by applying the watershed algorithm. We provide estimators for the dark matter masses of these structures (at z ~ 2.5), their descendant halo masses at z = 0, and the corresponding uncertainties. We also investigate the completeness of this method for the detection of protogroups and protoclusters. Compared to earlier work, we apply and characterize our method over a wider mass range that extends to massive protogroups. We also assess the widely used fluctuating Gunn-Peterson approximation applied to dark-matter-only simulations; we conclude that while it is adequate for estimating the Lyα absorption signal from moderate-to-massive protoclusters (≳1014.2 h -1 M ⊙), it artificially merges a minority of lower-mass structures with more massive neighbors. Our methods will be applied to current and future Lyα tomography surveys to create catalogs of overdensities and study environment-dependent galactic evolution in the Cosmic Noon era.

Keywords: dark matter distribution; large-scale structure of the universe; galaxy environments; protoclusters; lyman alpha forest; intergalactic medium; dark matter density; magnetohydrodynamical simulations
Variability of Late-time Radio Emission in the Superluminous Supernova PTF10hgi

We report the time variability of the late-time radio emission in a Type I superluminous supernova (SLSN), PTF10hgi, at z = 0.0987. The Karl G. Jansky Very Large Array 3 GHz observations at 8.6 and 10 yr after the explosion both detected radio emission with a ∼40% decrease in flux density in the second epoch. This is the first report of a significant variability of the late-time radio light curve in an SLSN. Through combination with previous measurements in two other epochs, we constrained both the rise and decay phases of the radio light curve over three years, peaking at approximately 8-9 yr after the explosion with a peak luminosity of L3 GHz = 2 × 1021 W Hz-1. Possible scenarios for the origin of the variability are an active galactic nucleus (AGN) in the host galaxy, an afterglow caused by the interaction between an off-axis jet and circumstellar medium, and a wind nebula powered by a newly born magnetar. Comparisons with models show that the radio light curve can be reproduced by both the afterglow model and magnetar wind nebula model. Considering the flat radio spectrum at 1-15 GHz and an upper limit at 0.6 GHz obtained in previous studies, plausible scenarios are a low-luminosity flat-spectrum AGN or a magnetar wind nebula with a shallow injection spectral index.

Keywords: radio continuum emission; extragalactic radio sources; radio transient sources; supernovae; very large array; radio astronomy
Probing the Stellar Wind of the Wolf-Rayet Star in IC 10 X-1

IC 10 X-1 is an eclipsing high-mass X-ray binary containing a stellar-mass black hole (BH) and a Wolf-Rayet (WR) donor star with an orbital period of P = 34.9 hr. This binary belongs to a group of systems that can be the progenitors of gravitational-wave sources; hence understanding the dynamics of systems such as IC 10 X-1 is of paramount importance. The prominent He II 4686 emission line (previously used in mass estimates of the BH) is out of phase with the X-ray eclipse, suggesting that this line originates somewhere in the ionized wind of the WR star or in the accretion disk. We obtained 52 spectra from the GEMINI/GMOS archive, observed between 2001 and 2019. We analyzed the spectra both individually, and after binning them by orbital phase to improve the signal-to-noise ratio. The radial-velocity curve from the stacked data is similar to historical results, indicating the overall parameters of the binary have remained constant. However, the He II line profile shows a correlation with the X-ray hardness-ratio values; also, we report a pronounced skewness of the line profile, and the skewness varies with orbital phase. These results support a paradigm wherein the He II line tracks structures in the stellar wind that are produced by interactions with the BH's ionizing radiation and the accretion flow. We compare the observable signatures of two alternative hypotheses proposed in the literature: wind irradiation plus shadowing, and accretion disk hotspot; and we explore how the line-profile variations fit into each of these models.

Keywords: black hole physics; eclipsing binary stars; spectroscopic binary stars; stellar spectral lines; stellar winds; wolf-rayet stars
Constraints on the Abundance of 0.01 c Stellar Engines in the Milky Way

Stellar engines are hypothesized megastructures that extract energy from the host star, typically with the purpose of generating thrust and accelerating the stellar system. We explore the maximum potential speeds that could be realizable by stellar engines, and determine that speeds up to ∼0.1 c might perhaps be attainable under optimal conditions. In contrast, natural astrophysical phenomena in the Milky Way are very unlikely to produce such speeds. Hence, astrometric surveys of hypervelocity stars may be utilized to conduct commensal searches for high-speed stellar engines in the Milky Way. It may be possible to derive bounds on their abundance, but this requires certain assumptions regarding the spatiotemporal distribution of such engines, which are not guaranteed to be valid.

Keywords: astrobiology; search for extraterrestrial intelligence; technosignatures; astrometry; surveys
WIYN Open Cluster Study 89. M48 (NGC 2548) 2: Lithium Abundances in the 420 Myr Open Cluster M48 from Giants through K Dwarfs

We consider WIYN/Hydra spectra of 329 photometric candidate members of the 420 Myr old open cluster M48 and report lithium detections or upper limits for 234 members and likely members. The 171 single members define a number of notable Li-mass trends, some delineated even more clearly than in Hyades/Praesepe: the giants are consistent with subgiant Li dilution and prior MS Li depletion due to rotational mixing. A dwarfs (8600-7700 K) have upper limits higher than the presumed initial cluster Li abundance. Two of five late A dwarfs (7700-7200 K) are Li-rich, possibly due to diffusion, planetesimal accretion, and/or engulfment of hydrogen-poor planets. Early F dwarfs already show evidence of Li depletion seen in older clusters. The Li-T eff trends of the Li Dip (6675-6200 K), Li Plateau (6200-6000 K), and G and K dwarfs (6000-4000 K) are very clearly delineated and are intermediate to those of the 120 Myr old Pleiades and 650 Myr old Hyades/Praesepe, which suggests a sequence of Li depletion with age. The cool side of the Li Dip is especially well defined with little scatter. The Li-T eff trend is very tight in the Li Plateau and early G dwarfs, but scatter increases gradually for cooler dwarfs. These patterns support and constrain models of the universally dominant Li depletion mechanism for FGK dwarfs, namely rotational mixing due to angular momentum loss; we discuss how diffusion and gravity-wave-driven mixing may also play roles. For late G/K dwarfs, faster rotators show higher Li than slower rotators, and we discuss possible connections between angular momentum loss and Li depletion.

The WIYN Observatory is a joint facility of the University of Wisconsin-Madison, Indiana University, the National Optical Astronomy Observatory, and the University of Missouri.

Keywords: stellar spectral lines; stellar spectral types; stellar abundances; stellar evolution; open star clusters; stellar interiors; stellar rotation