adsabs/SciX_UAT_keywords huggingface dataset:
Distance and Tangential Velocity of the Main Ionizing Star in the North America/Pelican Nebulae with Gaia EDR3

The Bajamar Star is an early O star that ionizes the North America/Pelican Nebulae. In projection, it is near the geometric center of the H II region, but appears to lie outside any of the main stellar subgroups. Furthermore, in Gaia DR2, there were slight discrepancies between this star and the rest of the system in parallax (2σ larger) and relative tangential velocity (∼6 km s-1). Using Gaia EDR3, we find that the parallax discrepancy has disappeared, but the velocity difference remains. These results are consistent with the star having escaped from a subgroup.

Keywords: o stars; stellar associations; astrometry; stellar kinematics
How to Train Your Flare Prediction Model: Revisiting Robust Sampling of Rare Events

We present a case study of solar flare forecasting by means of metadata feature time series, by treating it as a prominent class-imbalance and temporally coherent problem. Taking full advantage of pre-flare time series in solar active regions is made possible via the Space Weather Analytics for Solar Flares (SWAN-SF) benchmark data set, a partitioned collection of multivariate time series of active region properties comprising 4075 regions and spanning over 9 yr of the Solar Dynamics Observatory period of operations. We showcase the general concept of temporal coherence triggered by the demand of continuity in time series forecasting and show that lack of proper understanding of this effect may spuriously enhance models' performance. We further address another well-known challenge in rare-event prediction, namely, the class-imbalance issue. The SWAN-SF is an appropriate data set for this, with a 60:1 imbalance ratio for GOES M- and X-class flares and an 800:1 imbalance ratio for X-class flares against flare-quiet instances. We revisit the main remedies for these challenges and present several experiments to illustrate the exact impact that each of these remedies may have on performance. Moreover, we acknowledge that some basic data manipulation tasks such as data normalization and cross validation may also impact the performance; we discuss these problems as well. In this framework we also review the primary advantages and disadvantages of using true skill statistic and Heidke skill score, two widely used performance verification metrics for the flare-forecasting task. In conclusion, we show and advocate for the benefits of time series versus point-in-time forecasting, provided that the above challenges are measurably and quantitatively addressed.

Keywords: solar flares; solar physics; classification
NuSTAR Observations of G11.2-0.3

We present in this paper the hard X-ray view of the pulsar wind nebula in G11.2-0.3 and its central pulsar powered pulsar J1811-1925 as seen by NuSTAR. We complement the data with Chandra for a more complete picture and confirm the existence of a hard, power-law component in the shell with photon index ${\rm{\Gamma }}=2.1\pm 0.1$ , which we attribute to synchrotron emission. Our imaging observations of the shell show a slightly smaller radius at higher energies, consistent with Chandra results, and we find shrinkage as a function of increased energy along the jet direction, indicating that the electron outflow in the PWN may be simpler than that seen in other young PWNe. Combining NuSTAR with INTEGRAL, we find that the pulsar spectrum can be fit by a power law with ${\rm{\Gamma }}=1.32\pm 0.07$ up to 300 keV without evidence of curvature.

Keywords: rotation powered pulsars; supernova remnants; single x-ray stars; young disk cepheid variable stars
Probing Disk Ice Content and Polycyclic Aromatic Hydrocarbon Emission through Multiband MagAO+Clio Images of HD 141569

We present resolved images of the inner disk component around HD 141569A using the Magellan adaptive optics system with the Clio2 1-5 μm camera, offering a glimpse of a complex system thought to be in a short evolutionary phase between protoplanetary and debris disk stages. We use a reference star along with the Karhunen-Loéve image projection (KLIP) algorithm for point-spread function subtraction to detect the disk inward to about 0.″24 (~25 au assuming a distance of 111 pc) at high signal-to-noise ratios at $L^{\prime} $ (3.8 μm), Ls (3.3 μm), and narrowband Ice (3.1 μm). We identify an arc or spiral arm structure at the southeast extremity, consistent with previous studies. We implement forward modeling with a simple disk model within the framework of a Markov Chain Monte Carlo sampler to better constrain the geometrical attributes and photometry using our KLIP-reduced disk images. We then leverage these modeling results to facilitate a comparison of the measured brightness in each passband to find a reduction in scattered light from the disk in the Ice filter, implying significant absorption due to water ice in the dust. Additionally, our best-fit disk models exhibit peak brightness in the southwestern, back-scattering region of the disk, which we suggest to be possible evidence of 3.3 μm polycyclic aromatic hydrocarbon emission. However, we point out the need for additional observations with bluer filters and more complex modeling to confirm these hypotheses.

Keywords: high contrast techniques; direct imaging; protoplanetary disks; adaptive optics
Gaia-based Isochronal, Kinematic, and Spatial Analysis of the ɛ Cha Association

The precise parallax, proper motion, and photometric measurements contained in Gaia Data Release 2 (DR2) offer the opportunity to reexamine the membership and ages of nearby young moving groups (NYMGs), i.e., loose groups of stars of age ≲100 Myr in the solar vicinity. Here, we analyze the available DR2 data for members and candidate members of the ɛ Cha Association (ɛCA) which, at an estimated age of ∼3-5 Myr, has previously been identified as among the youngest NYMGs. The several dozen confirmed members of ɛCA include MP Mus and T Cha, two of the nearest stars of roughly solar mass that are known to host primordial protoplanetary disks, and the Herbig Ae/Be star HD 104237A. We have used Gaia DR2 data to ascertain the Galactic positions and kinematics and color-magnitude diagram positions of ɛCA members and candidates so as to reassess their membership status and thereby refine estimates of the distance, age, multiplicity, and disk fraction of the group. Our analysis yields 36 bona fide ɛCA members, as well as 20 provisional members, including 3 new members identified here as comoving companions to previously known ɛCA stars. We determine a mean distance to ɛCA of 101.0 ± 4.6 pc and confirm that, at an age of ${5}_{-2}^{+3}$ Myr, ɛCA represents the youngest stellar group within ∼100 pc of Earth. We identify several new photometric binary candidates, bringing the overall multiplicity fraction (MF) of ɛCA to 40%, intermediate between the MFs of young T associations and the field.

Keywords: stellar associations; pre-main sequence stars
Starspot Modeling and Flare Analysis on Selected Main-sequence M-type Stars

We studied the light curves of GJ 1243, YZ CMi, and V374 Peg, as observed by TESS, for the presence of stellar spots and stellar flares. One of the main goals was to model the light curves of the spotted stars to estimate the number of spots, along with their parameters, using our original BASSMAN software. The modeled light curves were subtracted from the observations to increase the efficiency of the flare detection. The flares were detected automatically with our new dedicated software, WARPFINDER. We estimated the presence of two spots on GJ 1243, with a mean temperature of about 2800 K and a spottedness varying between 3% and 4% of the stellar surface, and two spots on V374 Peg, with a mean temperature of about 3000 K and a spottedness of about 6% of the stellar surface. On YZ CMi, we found two different models for two light curves separated in time by 1.5 yr. One of them was a three-spot model, with a mean temperature of about 3000 K and a spottedness of about 9% of the stellar surface. The second was a four-spot model, with a mean temperature of about 2800 K and a spottedness of about 7% of the stellar surface. We tested whether the flares were distributed homogeneously in phase and where there was any correlation between the presence of spots and the distribution of the flares. For YZ CMi, one spot was in anticorrelation with the distribution of the flares, while GJ 1243 shows the nonhomogeneous distribution of flares.

Keywords: starspots; stellar flares; low mass stars; stellar activity
An Improved Hertzsprung-Russell Diagram for the Orion Trapezium Cluster

We present a study of the Trapezium cluster in Orion. We analyze flux-calibrated Very Large Telescope/Multi-Unit Spectroscopic Explorer spectra of 361 stars to simultaneously measure the spectral types, reddening, and the optical veiling due to accretion. We find that the extinction law from Cardelli et al. with a total-to-selective extinction value of RV = 5.5 is more suitable for this cluster. For 68% of the sample the new spectral types are consistent with literature spectral types within two subclasses but, as expected, we derive systematically later types than the literature by one to two subclasses for the sources with significant accretion levels. Here we present an improved Hertzsprung-Russell (H-R) diagram of the Trapezium cluster, in which the contamination by optical veiling on spectral types and stellar luminosities has been properly removed. A comparison of the locations of the stars in the H-R diagram with the non-magnetic and magnetic pre-main-sequence evolutionary tracks indicates an age of 1-2 Myr. The magnetic pre-main-sequence evolutionary tracks can better explain the luminosities of the low-mass stars. In the H-R diagram, the cluster exhibits a large luminosity spread (σ(Log L⋆/L⊙) ∼ 0.3). By collecting a sample of 14 clusters/groups with different ages, we find that the luminosity spread tends to be constant (σ(Log L⋆/L⊙) ∼ 0.2-0.25) after 2 Myr, which suggests that age spread is not the main cause of the luminosity spread. There are ∼0.1 dex larger luminosity spreads for the younger clusters, e.g., the Trapezium cluster, than the older clusters, which can be explained by the starspots, accretion history, and circumstellar disk orientations.

Keywords: pre-main sequence stars; stellar accretion disks; protoplanetary disks; hertzsprung russell diagram; stellar classification
Synchrotron Polarization of Gamma-Ray Burst Afterglow Shocks with Hydrodynamic-scale Turbulent Magnetic Field

Afterglows of gamma-ray bursts (GRBs) are emitted from expanding forward shocks, which are expected to have magnetic fields much stronger than the interstellar field, although the origin of the field is a long-standing problem. Two field amplification mechanisms, plasma kinetic instabilities and magnetohydrodynamic instabilities, have been discussed so far. The coherence-length scales of the fields amplified by these two processes are different by 7-10 orders of magnitude, and the polarimetric observations may distinguish them. We construct a semi-analytic model of the forward-shock afterglow polarization under the assumption of hydrodynamic-scale turbulent magnetic field. We perform numerical calculations of synchrotron polarization for the isotropic turbulence and the zero viewing angle. We find that the polarization degrees are ~1% when the field coherence-length scale in the fluid co-moving frame is of the order of the thickness of the shocked regions. This range of polarization degree is comparable to that of the observed late-phase optical afterglows. Our model also shows that the radio polarization degrees are comparable to the optical ones on average but can be higher than the optical ones at some time intervals. The polarization angles are shown to vary randomly and continuously. These polarimetric properties are clearly different from the case of plasma kinetic instability. Simultaneous polarimetric observations of GRB afterglows at the radio and optical bands have recently started, which will help us constrain the magnetic field amplification mechanism.

Keywords: gamma-ray bursts; magnetic fields; non-thermal radiation sources; jets
The Atacama Cosmology Telescope: Summary of DR4 and DR5 Data Products and Data Access

Two recent large data releases for the Atacama Cosmology Telescope (ACT), called DR4 and DR5, are available for public access. These data include temperature and polarization maps that cover nearly half the sky at arcminute resolution in three frequency bands; lensing maps and component-separated maps covering ~2100 deg2 of sky; derived power spectra and cosmological likelihoods; a catalog of over 4000 galaxy clusters; and supporting ancillary products including beam functions and masks. The data and products are described in a suite of ACT papers; here we provide a summary. In order to facilitate ease of access to these data, we present a set of Jupyter IPython notebooks developed to introduce users to DR4, DR5, and the tools needed to analyze these data. The data products (excluding simulations) and the set of notebooks are publicly available on the NASA Legacy Archive for Microwave Background Data Analysis; simulation products are available on the National Energy Research Scientific Computing Center.

Keywords: cosmology; cosmic microwave background radiation
The Behavior of the Bp Star 36 Lynxis in the Far-UV

The analysis of twenty-four high resolution SWP spectra of the magnetic Bp star HD 79158 (36 Lynxis) recorded by the International Ultraviolet Explorer (IUE) reveals that the far-ultraviolet flux of this star mostly remained constant over the various observations with IUE. Only a few spectra show less flux shortwards of 1600 Å and enhanced line absorption compared to the mean spectrum, which could be interpreted as non uniform distribution of certain chemical elements over the surface of 36 Lynxis. The TESS lightcurve of HD 79158 reveals a complex structure with an estimated rotational period of about 3.832 ± 0.019 days, close to previous determinations.

Keywords: chemically peculiar stars
Radial Spectral Evolution of the Elsässer Variable z ‑ in the Slow Solar Wind

The nature of the Elsässer variable z ‑ plays a significant role in the understanding of solar wind turbulence. The radial spectral evolution of z ‑ observed by the Helios spacecraft suggests that z ‑ has a persistent background spectrum in the low-frequency range from 0.3 to 1 au. However, the radial evolution in the near-Sun solar wind remains unknown. Here we use Parker Solar Probe measurements to analyze the averaged spectra of z ‑ in the slow solar wind from 0.1 to 0.5 au and compare the results with the WIND observations at 1 au. We find that the averaged spectra of z ‑ do not show the low-frequency break at the low-frequency breakpoint on the power spectra of z +, magnetic field, and velocity. The energy in the frequency range of 10‑4–10‑2 Hz demonstrates a radial decay from 0.1 to 0.5 au and keeps nearly constant to 1 au. The spectral index of z ‑ gradually decreases from ‑1.27 at 0.1–0.2 au to ‑1.59 at 0.4–0.5 au and then slowly declines to ‑1.70 at 1 au. The radial behaviors of the ∥, ⊥1, and ⊥2 components of z ‑ in the field-aligned coordinates are similar to that of the trace of z ‑. Our findings suggest a faster radial spectral evolution of z ‑ in the near-Sun solar wind and provide strong observational constraints for the solar wind turbulence model.

Keywords: interplanetary turbulence; slow solar wind; magnetohydrodynamics; heliosphere
Energetic Constraints on Ocean Circulations of Icy Ocean Worlds

Globally ice-covered oceans have been found on multiple moons in the solar system and may also have been a feature of Earth's past. However, relatively little is understood about the dynamics of these ice-covered oceans, which affect not only the physical environment but also any potential life and its detectability. A number of studies have simulated the circulation of icy-world oceans, but have come to seemingly widely different conclusions. To better understand and narrow down these diverging results, we discuss the energetic constraints for the circulation on ice-covered oceans, focusing in particular on Snowball Earth, Europa, and Enceladus. The energy input that can drive ocean circulation on ice-covered bodies can be associated with heat and salt fluxes at the boundaries as well as ocean tides and librations. We show that heating from the solid core balanced by heat loss through the ice sheet can drive an ocean circulation, but the resulting flows would be relatively weak and strongly affected by rotation. Salt fluxes associated with freezing and melting at the ice sheet boundary are unlikely to energetically drive a circulation, although they can shape the large-scale circulation when combined with turbulent mixing. Ocean tides and librations may provide an energy source for such turbulence, but the magnitude of this energy source remains highly uncertain for the icy moons, which poses a major obstacle to predicting the ocean dynamics of icy worlds and remains an important topic for future research.

Keywords: ocean planets; saturnian satellites; europa; hydrosphere; planetary science
Computational Insight into Diatomic Molecules as Probes to Measure the Variation of the Proton-to-electron Mass Ratio

Astrophysical molecular spectroscopy is an important means of searching for new physics through probing the variation of the proton-to-electron mass ratio, μ. New molecular probes could provide tighter constraints on the variation of μ and better direction for theories of new physics. Here we summarize our previous paper for astronomers, highlighting the importance of accurate estimates of peak molecular abundance and temperature as well as spectral resolution and sensitivity of telescopes in different regions of the electromagnetic spectrum. While none of the 11 astrophysical diatomic molecules we investigated showed enhanced sensitive rovibronic transitions at observable intensities for astrophysical environments, we have gained a better understanding of the factors that contribute to high sensitivities. From our results, CN, CP, SiN and SiC have shown the most promise of all astrophysical diatomic molecules for further investigation, with further work currently being done on CN.

Keywords: molecular spectroscopy; spectroscopy; high resolution spectroscopy; galactic and extragalactic astronomy
Did a Kilonova Set Off in Our Galactic Backyard 3.5 Myr ago?

The recent detection of the live isotopes 60Fe and 244Pu in deep ocean sediments dating back to the past 3–4 Myr poses a serious challenge to the identification of their production site(s). While 60Fe is usually attributed to standard core-collapse supernovae, actinides are r-process nucleosynthesis yields, which are believed to be synthesized in rare events, such as special classes of supernovae or binary mergers involving at least one neutron star. Previous works concluded that a single binary neutron star merger cannot explain the observed isotopic ratio. In this work, we consider a set of numerical simulations of binary neutron star mergers producing long-lived massive remnants expelling both dynamical and spiral-wave wind ejecta. The latter, due to a stronger neutrino irradiation, also produce iron-group elements. Assuming that large-scale mixing is inefficient before the fading of the kilonova remnant and that the spiral-wave wind is sustained over a 100–200 ms timescale, the ejecta emitted at mid-high latitudes provide a 244Pu over 60Fe ratio compatible with observations. The merger could have happened 80–150 pc away from the Earth and between 3.5 and 4.5 Myr ago. We also compute expected isotopic ratios for eight other live radioactive nuclides showing that the proposed binary neutron star merger scenario is distinguishable from other scenarios proposed in the literature.

Keywords: neutron stars; compact binary stars; nucleosynthesis; r-process; solar system evolution
GS 2000+25: The Least Luminous Black Hole X-Ray Binary

Little is known about the properties of the accretion flows and jets of the lowest-luminosity quiescent black holes. We report new, strictly simultaneous radio and X-ray observations of the nearby stellar-mass black hole X-ray binary GS 2000+25 in its quiescent state. In deep Chandra observations we detect the system at a faint X-ray luminosity of ${L}_{{\rm{X}}}={1.1}_{-0.7}^{+1.0}\times {10}^{30}{(d/2\mathrm{kpc})}^{2}$ erg s-1 (1-10 keV). This is the lowest X-ray luminosity yet observed for a quiescent black hole X-ray binary, corresponding to an Eddington ratio LX/LEdd ∼ 10-9. In 15 hours of observations with the Karl G. Jansky Very Large Array, no radio continuum emission is detected to a 3σ limit of <2.8 μJy at 6 GHz. Including GS 2000+25, four quiescent stellar-mass black holes with LX < 1032 erg s-1 have deep simultaneous radio and X-ray observations and known distances. These sources all have radio to X-ray luminosity ratios generally consistent with, but slightly lower than, the low-state radio/X-ray correlation for stellar-mass black holes with LX > 1032 erg s-1. Observations of these sources tax the limits of our current X-ray and radio facilities, and new routes to black hole discovery are needed to study the lowest-luminosity black holes.

Keywords: astrophysical black holes; low-mass x-ray binary stars
Three-dimensional Reconstruction of Coronal Mass Ejections by the Correlation-aided Reconstruction Technique through Different Stereoscopic Angles of the Solar Terrestrial Relations Observatory Twin Spacecraft

Recently, we developed the correlation-aided reconstruction (CORAR) method to reconstruct solar wind inhomogeneous structures, or transients, using dual-view white-light images. This method is proved to be useful for studying the morphological and dynamical properties of transients like blobs and coronal mass ejection (CME), but the accuracy of the reconstruction may be affected by the separation angle between the two spacecrafts. Based on the dual-view CME events from the Heliospheric Imager CME Join Catalogue in the Heliospheric Cataloguing, Analysis and Techniques Service (HELCATS) project, we study the quality of the reconstruction of CME using the CORAR method under different STEREO stereoscopic angles. We find that when the separation angle of the spacecraft is around 150°, most CME events can be well reconstructed. If the collinear effect is considered, the optimal separation angle should locate between 120° and 150°. Compared with the direction of the CME given in the Heliospheric Imager Geometrical Catalogue from HELCATS, the parameters of the CME obtained by the CORAR method are reasonable. However, the CORAR-obtained directions deviate toward the meridian plane in longitude, and toward the equatorial plane in latitude. An empirical formula is proposed to correct these deviations. This study provides the basis for the configuration of the spacecraft of our recently proposed Solar Ring mission concept.

Keywords: solar coronal mass ejections; solar wind; heliosphere; optical observation