adsabs/SciX_UAT_keywords huggingface dataset:
The Detection of Higher-order Millimeter Hydrogen Recombination Lines in the Large Magellanic Cloud

We report the first extragalactic detection of the higher-order millimeter hydrogen recombination lines (Δn > 2). The γ-, ϵ-, and η-transitions have been detected toward the millimeter continuum source N 105-1 A in the star-forming region N 105 in the Large Magellanic Cloud with the Atacama Large Millimeter/submillimeter Array. We use the H40α line, the brightest of the detected recombination lines (H40α, H36β, H50β, H41γ, H57γ, H49ϵ, H53η, and H54η), to determine the electron temperature and study ionized gas kinematics in the region, and the 3 mm free-free continuum emission to determine the physical parameters: the size, emission measure, and electron density. We compare the physical properties of N 105-1 A to a large sample of Galactic compact and ultracompact (UC) H II regions and conclude that N 105-1 A is similar to the most luminous (L > 105 L ⊙) UC H II regions in the Galaxy. N 105-1 A is ionized by an O5.5 V star; it is deeply embedded in its natal molecular clump, and likely associated with a (proto)cluster. We incorporate high-resolution molecular line data including CS, SO, SO2, and CH3OH (~0.12 pc), and HCO+ and CO (~0.087 pc) to explore the molecular environment of N 105-1 A. Based on the CO data, we find evidence for a cloud-cloud collision that likely triggered star formation in the region. We find no clear outflow signatures, but the presence of filaments and streamers indicates ongoing accretion onto the clump hosting the UC H II region. Sulfur chemistry in N 105-1 A is consistent with the accretion shock model predictions.

Keywords: star formation; magellanic clouds; star forming regions; astrochemistry; molecular clouds; h ii regions
A Short Intense Dynamo at the Onset of Crystallization in White Dwarfs

The origin of large magnetic fields (≳106 G) in isolated white dwarfs is not clear. One possible explanation is that crystallization of the star's core drives compositional convection, which when combined with the star's rotation, can drive a dynamo. However, whether convection is efficient enough to explain the large intensity of the observed magnetic fields is still under debate. Recent work has shown that convection in cooling white dwarfs spans two regimes: efficient convection at the onset of crystallization, and thermohaline convection during most of the star's cooling history. Here, we calculate the properties of crystallization-driven convection for cooling models of several white dwarfs of different masses. We combine mixing-length theory with scalings from magnetorotational convection to estimate the typical magnitude of the convective velocity and induced magnetic field for both scenarios. In the thermohaline regime, we find velocities ∼10‑6–10‑5 cm s‑1, with fields restricted to ≲ 100 G. However, when convection is efficient, the flow velocity can reach magnitudes of ∼102–103 cm s‑1, with fields of ∼106–108 G, independent of the star's rotation rate. Thus, dynamos driven at the onset of crystallization could explain the large intensity magnetic fields measured for single white dwarfs.

Keywords: stellar convective zones; white dwarf stars; stellar interiors; stellar magnetic fields
Cluster Cepheids with High Precision Gaia Parallaxes, Low Zero-point Uncertainties, and Hubble Space Telescope Photometry

We present Hubble Space Telescope (HST) photometry of 17 Cepheids in open clusters and their cluster mean parallaxes from Gaia EDR3. These parallaxes are more precise than those from individual Cepheids (G < 8 mag) previously used to measure the Hubble constant because they are derived from an average of >300 stars per cluster. Cluster parallaxes also have smaller systematic uncertainty because their stars lie in the range (G > 13 mag) where the Gaia parallax calibration is the most comprehensive. Cepheid photometry employed in the period-luminosity relation was measured using the same HST instrument (WFC3) and filters (F555W, F814W, F160W) as extragalactic Cepheids in Type Ia supernova hosts. We find no evidence of residual parallax offset in this magnitude range, zp = -3 ± 4 μas, consistent with the results from Lindegren et al. and most studies. The Cepheid luminosity (at P = 10 d and solar metallicity) in the HST near-infrared, Wesenheit magnitude system derived from the cluster sample is ${M}_{H,1}^{W}=-5.902\pm 0.025$ mag and -5.890 ± 0.018 mag with or without simultaneous determination of a parallax offset, respectively. These results are similar to measurements from field Cepheids, confirming the accuracy of the Gaia parallaxes over a broad range of magnitudes. The SH0ES distance ladder calibrated only from this sample gives H 0 = 72.9 ± 1.3 and H 0 = 73.3 ± 1.1 km s-1 Mpc-1 with or without offset marginalization; combined with all other anchors we find H 0 = 73.01 ± 0.99 and 73.15 ± 0.97 km s-1 Mpc-1, respectively, a 5% or 7% reduction in the uncertainty in H 0 and a ~5.3σ Hubble tension relative to Planck+ΛCDM. It appears increasingly difficult to reconcile two of the best-measured cosmic scales, parallaxes from Gaia and the angular size of the acoustic scale of the cosmic microwave background, using the simplest form of ΛCDM to connect the two.

Keywords: hubble constant; cepheid distance
Photoevaporation and Close Encounters: How the Environment around Cygnus OB2 Affects the Evolution of Protoplanetary Disks

In our Galaxy, star formation occurs in a variety of environments, with a large fraction of stars formed in clusters hosting massive stars. OB stars have an important feedback on the evolution of protoplanetary disks orbiting around nearby young stars and likely on the process of planet formation occurring in them. The nearby massive association Cyg OB2 is an outstanding laboratory to study this feedback. It is the closest massive association to our Sun and hosts hundreds of massive stars and thousands of low-mass members, both with and without disks. In this paper, we analyze the spatial variation of the disk fraction (i.e., the fraction of cluster members bearing a disk) in Cyg OB2 and study its correlation with the local values of far-ultraviolet (FUV) and extreme-ultraviolet (EUV) radiation fields and the local stellar surface density. We present definitive evidence that disks are more rapidly dissipated in the regions of the association characterized by intense local UV fields and large stellar density. In particular, the FUV radiation dominates disk dissipation timescales in the proximity (i.e., within 0.5 pc) of the O stars. In the rest of the association, EUV photons potentially induce a significant mass loss from the irradiated disks across the entire association, but the efficiency of this process is reduced at increasing distances from the massive stars owing to absorption by the intervening intracluster material. We find that disk dissipation due to close stellar encounters is negligible in Cyg OB2 and likely to have affected 1% or fewer of the stellar population. Disk dissipation is instead dominated by photoevaporation. We also compare our results to what has been found in other young clusters with different massive populations, concluding that massive associations like Cyg OB2 are potentially hostile to protoplanetary disks but that the environments where disks can safely evolve in planetary systems are likely quite common in our Galaxy.

Keywords: star formation; pre-main sequence stars
An Atlas of Dynamical Evolution Models of 361 Fanaroff-Riley Type II Radio Sources

Dynamical evolution models of 361 extragalactic Fanaroff-Riley type II radio sources selected from the Cambridge 3CRR, 6CE, 5C6, and 5C7 Sky Surveys, as well as the Bologna B2, Green Bank GB, and GB2 Surveys, are presented. Their spectra, compiled mostly from the recent catalogs of radio sources and the available NASA/IPAC and Astrophysical Catalogs Support System databases, along with morphological characteristics of the sources determined from their radio maps, have been modeled using the DYNAGE algorithm and/or its extension (KDA EXT) for the hypothetical case of further evolution after the jet's termination. The best-fit models provide estimates of a number of important physical parameters of the sources, as (i) the jet power, (ii) the density distribution of the external gaseous medium surrounding the radio core and the jet propagating through it, (iii) the initial energy distribution of the relativistic particles accelerated at the shock fronts, and (iv) the age of the observed radio structure. Additionally, estimates of some derivative parameters are provided, e.g., the radio lobes' pressure, their longitudinal expansion velocity, the magnetic field strength, and the total energy deposited in the lobes. The observed spectra and their best-fit models are included. Finally, one of the useful applications of the above models is presented, namely a strong correlation between the ambient medium density and the rest-frame two-point spectral index available directly from the observed spectra.

Keywords: active galaxies; dynamical evolution; radio galaxies
Introducing MEGASIM: Multitudinous Earth Greek/Trojan Asteroid SIMulation

We announce the completion of two new Earth Trojan Asteroid (ETA) simulations. ETAs co-orbit the Sun with Earth and remain loosely bound to the fourth or fifth Lagrange points. To date, two ETAs have been detected (2010 TK7 and 2020 XL5); however, based on their orbits, these are likely temporary captures rather than members of a primordial population. The motivation for our simulations is multi-faceted: (1) the lack of a detected population despite literature claims of stability over the age of the solar system, (2) ETAs are important for both solar system science and planetary defense, and (3) the proximity of ETAs makes them prime candidates for space exploration missions. These simulations are of extremely high fidelity, large in number and long in simulation time. We will be releasing a series of papers based upon these simulations. *There is no indication that L4 Earth co-orbitals are referred to as a "Greek Camp" as in the Jupiter Trojan population. We merely preferred MEGASIM to METASIM.

Keywords: trojan asteroids; earth trojans; n-body simulations; n-body problem; lagrangian points; astronomical simulations; asteroid dynamics
Solar Eruptions in Nested Magnetic Flux Systems

The magnetic topology of erupting regions on the Sun is a key factor in the energy buildup and release, and the subsequent evolution of flares and coronal mass ejections (CMEs). The presence/absence of null points and separatrices dictates whether and where current sheets form and magnetic reconnection occurs. Numerical simulations show that energy buildup and release via reconnection in the simplest configuration with a null, the embedded bipole, is a universal mechanism for solar eruptions. Here we demonstrate that a magnetic topology with nested bipoles and two nulls can account for more complex dynamics, such as failed eruptions and CME–jet interactions. We investigate the stalled eruption of a nested configuration on 2013 July 13 in NOAA Active Region 11791, in which a small bipole is embedded within a large transequatorial pseudo-streamer containing a null. In the studied event, the inner active region erupted, ejecting a small flux rope behind a shock accompanied by a flare; the flux rope then reconnected with pseudo-streamer flux and, rather than escaping intact, mainly distorted the pseudo-streamer null into a current sheet. EUV and coronagraph images revealed a weak shock and a faint collimated outflow from the pseudo-streamer. We analyzed Solar Dynamics Observatory and Solar TErrestrial RElations Observatory observations and compared the inferred magnetic evolution and dynamics with three-dimensional magnetohydrodynamics simulations of a simplified representation of this nested fan-spine system. The results suggest that the difference between breakout reconnection at the inner null and at the outer null naturally accounts for the observed weak jet and stalled ejection. We discuss the general implications of our results for failed eruptions.

Keywords: solar coronal mass ejections; solar activity; solar magnetic reconnection; solar magnetic fields
AGILE Observations of Two Repeating Fast Radio Bursts with Low Intrinsic Dispersion Measures

We focus on two repeating fast radio bursts (FRBs) recently detected by the CHIME/FRB experiment in 2018-2019 (Source 1: 180916.J0158+65, and Source 2: 181030.J1054+73). These sources have low excess dispersion measures ( $\lt 100\,\mathrm{pc}\,{\mathrm{cm}}^{-3}$ and $\lt 20\,\mathrm{pc}\,{\mathrm{cm}}^{-3}$ , respectively), implying relatively small maximal distances. They were repeatedly observed by AGILE in the MeV-GeV energy range. We do not detect prompt emission simultaneously with these repeating events. This search is particularly significant for the submillisecond and millisecond integrations obtainable by AGILE. The sources are constrained to emit a MeV-fluence in the millisecond range below $F{{\prime} }_{\mathrm{MeV}}={10}^{-8}\,\mathrm{erg}\,{\mathrm{cm}}^{-2}$ corresponding to an isotropic energy near ${E}_{\mathrm{MeV},\mathrm{UL}}\simeq 2\times {10}^{46}$ erg for a distance of 150 Mpc (applicable to Source 1). We also searched for γ-ray emission for time intervals up to 100 days, obtaining 3σ upper limits (ULs) for the average isotropic luminosity above 50 MeV, ${L}_{\gamma ,\mathrm{UL}}\,\simeq $ (5-10) $\,\times \,{10}^{43}\,\mathrm{erg}\,{{\rm{s}}}^{-1}$ . For a source distance near 100 kpc (possibly applicable to Source 2), our ULs imply ${E}_{\mathrm{MeV},\mathrm{UL}}\simeq {10}^{40}\,\mathrm{erg}$ , and ${L}_{\gamma ,\mathrm{UL}}\,\simeq $ 2 $\times {10}^{37}\,\mathrm{erg}\,{{\rm{s}}}^{-1}$ . Our results are significant in constraining the high-energy emission of underlying sources such as magnetars, or other phenomena related to extragalactic compact objects, and show the prompt emission to be lower than the peak of the 2004 magnetar outburst of SGR 1806-20 for source distances less than about 100 Mpc.

Keywords: gamma-ray transient sources; radio transient sources; x-ray transient sources; magnetars
A New Estimate for the Orientation of Underlying Heliospheric Magnetic Field Associated with Alfvénic Fluctuations

In the solar wind Alfvénic fluctuations are coupled fluctuations in the transverse components of plasma velocity, ${\boldsymbol{V}}$, and magnetic field, ${\boldsymbol{B}}$, in which $\left|{\boldsymbol{B}}\right|$ remains nearly constant. Consequently, the fluctuations in the component of ${\boldsymbol{V}}$ and ${\boldsymbol{B}}$ along the direction of the underlying magnetic field, ${{\boldsymbol{B}}}_{0}$, are always one-sided relative to base values rather than mean values. This paper proposes a new method for estimating ${{\boldsymbol{B}}}_{0}$ and verifies it using simulation data. The new approach is to find a consistent minimum variance direction of magnetic field for two different data subsets divided by a threshold angle from the original data set. Simulation results show that (1) the direction of ${{\boldsymbol{B}}}_{0}$ predicted by the new method is generally more accurate than the average magnetic field, ${{\boldsymbol{b}}}_{\mathrm{av}}$, and that (2) ${{\boldsymbol{b}}}_{\mathrm{av}}$ does not correspond to ${{\boldsymbol{B}}}_{0}$ when the averages of the transverse field fluctuations are nonzero. Two events of one-sided Alfvénic fluctuations in the solar wind observed at 0.36 and 1 au, where a small but significant mean appears in the transverse field fluctuation, are demonstrated for the new method. Results show that the new estimate is more radial than ${{\boldsymbol{b}}}_{\mathrm{av}}$ and also different from that predicted by Parker's model of the heliospheric magnetic field (HMF). This new technique will be useful for comparing solar wind observations with the global HMF model and for determining the fluctuation k-vector, which is presumably directed along ${{\boldsymbol{B}}}_{0}$ .

Keywords: interplanetary magnetic fields; solar wind; alfven waves
The Deeper Studies of IC 2488 and IC 2714

In this paper, we use two methods, an algorithm of the density-based spatial clustering of applications with noise combined with the Gaussian mixture model, and the maximum likelihood method, to study two open clusters: IC 2488 and IC 2714 with Gaia Data Release 3 data. We compare the two methods in terms of cluster star number, isochrone fitting, cluster mass, and radius. The comparison shows that the selections of cluster stars by the two methods are consistent. The parameters obtained by the two methods are very close. But there are slight differences between the two methods for faint stars in IC 2714. The maximum likelihood method selects more faint stars outside of the main-sequence stripe. We conclude that the maximum likelihood method is more likely affected by field stars.

Keywords: open star clusters; clustering
On the Detection of Habitable Trojan Planets in the Kepler Circumbinary Systems

We present the results of a study of the prospect of detecting habitable Trojan planets in the Kepler Habitable Zone circumbinary planetary systems (Kepler-16, -47, -453, -1647, and -1661). We integrated the orbits of 10,000 separate N-body systems (N = 4, 6), each with a one Earth-mass body in a randomly selected orbit near the L4 and L5 Lagrangian points of the host HZ circumbinary planet. We find that stable Trojan planets are restricted to a narrow range of semimajor axes in all five systems and limited to small eccentricities in Kepler-16, -47, and -1661. To assess the prospect of the detection of these habitable Trojan planets, we calculated the amplitudes of the variations they cause in the transit timing of their host bodies. Results show that the mean amplitudes of the transit timing variations (TTVs) correlate with the mass of the transiting planet and range from 70 minutes for Kepler-16b to 390 minutes for Kepler-47c. Our analysis indicates that the TTVs of the circumbinary planets caused by these Trojan bodies fall within the detectable range of timing precision obtained from the Kepler telescope's long-cadence data. The latter points to Kepler data as a viable source to search for habitable Trojan planets.

Keywords: n-body simulations; transit timing variation method; trojan planets
Earth as a Proxy Exoplanet: Deconstructing and Reconstructing Spectrophotometric Light Curves

Point-source spectrophotometric (single-point) light curves of Earth-like planets contain a surprising amount of information about the spatial features of those worlds. Spatially resolving these light curves is important for assessing time-varying surface features and the existence of an atmosphere, which in turn is critical to life on Earth and significant for determining habitability on exoplanets. Given that Earth is the only celestial body confirmed to harbor life, treating it as a proxy exoplanet by analyzing time-resolved spectral images provides a benchmark in the search for habitable exoplanets. The Earth Polychromatic Imaging Camera (EPIC) on the Deep Space Climate Observatory (DSCOVR) provides such an opportunity, with observations of ∼5000 full-disk sunlit Earth images each year at 10 wavelengths with high temporal frequency. We disk-integrate these spectral images to create single-point light curves and decompose them into principal components (PCs). Using machine-learning techniques to relate the PCs to six preselected spatial features, we find that the first and fourth PCs of the single-point light curves, contributing ∼83.23% of the light-curve variability, contain information about low and high clouds, respectively. Surface information relevant to the contrast between land and ocean reflectance is contained in the second PC, while individual land subtypes are not easily distinguishable (<0.1% total light-curve variation). We build an Earth model by systematically altering the spatial features to derive causal relationships to the PCs. This model can serve as a baseline for analyzing Earth-like exoplanets and guide wavelength selection and sampling strategies for future observations.

Keywords: exoplanet astronomy; exoplanet atmospheres; exoplanet surfaces; exoplanets; exoplanet structure; exoplanet surface variability; extrasolar rocky planets; habitable planets; exoplanet surface characteristics; exoplanet atmospheric composition; exoplanet atmospheric variability; exoplanet surface composition
Powerful Yet Lonely: Is 3C 297 a High-redshift Fossil Group?

The environment of the high-redshift (z = 1.408), powerful radio-loud galaxy 3C 297 has several distinctive features of a galaxy cluster. Among them, a characteristic halo of hot gas revealed by Chandra X-ray observations. In addition, a radio map obtained with the Very Large Array shows a bright hotspot in the northwestern direction, created by the interaction of the active galactic nucleus (AGN) jet arising from 3C 297 with its environment. In the X-ray images, emission cospatial with the northwestern radio lobe is detected, and peaks at the position of the radio hotspot. The extended, complex X-ray emission observed with our new Chandra data is largely unrelated to its radio structure. Despite having attributes of a galaxy cluster, no companion galaxies have been identified from 39 new spectra of neighboring targets of 3C 297 obtained with the Gemini Multi-Object Spectrograph. None of the 19 galaxies for which a redshift was determined lies at the same distance as 3C 297. The optical spectral analysis of the new Gemini spectrum of 3C 297 reveals an isolated Type II radio-loud AGN. We also detected line broadening in [O II] λ3728 with a FWHM about 1700 km s-1 and possible line shifts of up to 500-600 km s-1. We postulate that the host galaxy of 3C 297 is a fossil group, in which most of the stellar mass has merged into a single object, leaving behind an X-ray halo.

Keywords: active galactic nuclei; brightest cluster galaxies; galaxy clusters; intracluster medium
Plasma Composition Measurements in an Active Region from Solar Orbiter/SPICE and Hinode/EIS

A key goal of the Solar Orbiter mission is to connect elemental abundance measurements of the solar wind enveloping the spacecraft with extreme-UV (EUV) spectroscopic observations of their solar sources, but this is not an easy exercise. Observations from previous missions have revealed a highly complex picture of spatial and temporal variations of elemental abundances in the solar corona. We have used coordinated observations from Hinode and Solar Orbiter to attempt new abundance measurements with the Spectral Imaging of the Coronal Environment (SPICE) instrument, and benchmark them against standard analyses from the EUV Imaging Spectrometer (EIS). We use observations of several solar features in active region (AR) 12781 taken from an Earth-facing view by EIS on 2020 November 10, and SPICE data obtained one week later on 2020 November 17, when the AR had rotated into the Solar Orbiter field of view. We identify a range of spectral lines that are useful for determining the transition region and low-coronal-temperature structure with SPICE, and demonstrate that SPICE measurements are able to differentiate between photospheric and coronal magnesium/neon abundances. The combination of SPICE and EIS is able to establish the atmospheric composition structure of a fan loop/outflow area at the AR edge. We also discuss the problem of resolving the degree of elemental fractionation with SPICE, which is more challenging without further constraints on the temperature structure, and comment on what that can tell us about the sources of the solar wind and solar energetic particles.

Keywords: solar physics; active sun; bessel photometry; atomic spectroscopy