adsabs/SciX_UAT_keywords huggingface dataset:
Unveiling the Hierarchical Structure of Open Star Clusters: The Perseus Double Cluster

We introduce a new kinematic method to investigate the structure of open star clusters. We adopt a hierarchical clustering algorithm that uses the celestial coordinates and the proper motions of the stars in the field of view of the cluster to estimate a proxy of the pairwise binding energy of the stars and arrange them in a binary tree. The cluster substructures and their members are identified by trimming the tree at two thresholds, according to the σ-plateau method. Testing the algorithm on 100 mock catalogs shows that, on average, the membership of the identified clusters is (91.5 ± 3.5)% complete and the fraction of unrelated stars is (10.4 ± 2.0)%. We apply the algorithm to the stars in the field of view of the Perseus double cluster from the Data Release 2 of Gaia. This approach identifies a single structure, Sub1, that separates into two substructures, Sub1-1 and Sub1-2. These substructures coincide with h Per and χ Per: the distributions of the proper motions and the color-magnitude diagrams of the members of Sub1-1 and Sub1-2 are fully consistent with those of h Per and χ Per reported in the literature. These results suggest that our hierarchical clustering algorithm can be a powerful tool to unveil the complex kinematic information of star clusters.

Keywords: stellar kinematics; single-linkage hierarchical clustering; open star clusters; astronomy data analysis; astronomy data visualization
Faint but Not Forgotten. I. First Results from a Search for Astrospheres around AGB Stars in the Far-ultraviolet

Using the GALEX archive, we have discovered extended structures around ten asymptotic giant branch (AGB) stars (out of a total 92 searched) emitting in the far-ultraviolet (FUV) band. In all but one, we find the typical morphology expected for a spherical wind moving relative to, and interacting with, the interstellar medium (ISM) to produce an astrosphere. The exception is V Hya whose mass ejection is known to be highly aspherical, where we find evidence of its large parabolic outflows interacting with the ISM, and its collimated, extreme velocity outflows interacting with the circumstellar medium. For eight objects with relatively large proper motions, we find (as expected) that the termination-shock region lies in a hemisphere that contains the proper motion vector. Radial intensity cuts for each source have been used to locate the termination shock and the astropause's outer edge. In a few objects, the cuts also reveal faint emission just outside the astropause that likely arises in shocked ISM material. We have used these data, together with published mass-loss rates and wind expansion velocities, to determine the total mass lost and duration for each source-we find that the duration of and total mass in the shocked wind are significantly larger than their corresponding values for the unshocked wind. The combination of FUV and far-IR data on AGB astrospheres provides a unique database for theoretical studies (numerical simulations) of wind-ISM interactions. We show that a Cyclical Spatial Heterodyne Spectrometer on a small space-based telescope can provide high-resolution spectra of astrospheres to confirm the emission mechanism.

Keywords: stellar mass loss; asymptotic giant branch stars; ultraviolet astronomy; astrospheres; stellar bow shocks; circumstellar envelopes; stellar winds
A New Martian Crater Chronology: Implications for Jezero Crater

Crater chronologies are a fundamental tool to assess the relative and absolute ages of planetary surfaces when direct radiometric dating is not available. Martian crater chronologies are derived from lunar crater spatial densities on terrains with known radiometric ages, and thus they critically depend on the Moon-to-Mars extrapolation. This extrapolation requires knowledge of the time evolution of the impact flux, including contributions from various impactor populations, factors that are not trivially connected to the dynamical evolution of the early Solar System. In this paper, we will present a new Martian crater chronology based on current dynamical models, and consider the main sources of uncertainties (e.g., impactor size-frequency distribution; dynamical models with late and early instabilities, etc.). The resulting "envelope" of Martian crater chronologies significantly differs from previous chronologies. The new Martian crater chronology is discussed using two interesting applications: Jezero crater's dark terrain (relevant to the NASA Mars 2020 mission) and the southern heavily cratered highlands. Our results indicate that Jezero's dark terrain may have formed ∼3.1 Ga, i.e., up to 0.5 Gyr older than previously thought. In addition, available crater chronologies (including our own) overestimate the number of craters larger than 150 km on the southern highlands, suggesting either that large craters have been efficiently erased over Martian history or that dynamical models need further refinement. Further, our chronology constrains the age of Isidis basin to be 4.05-4.2 Ga and that of the Borealis basin to be 4.35-4.40 Ga; these are predictions that can be tested with future sample and return missions.

Keywords: mars; solar system terrestrial planets
XMM-Newton and NuSTAR Observations of the Compact Millisecond Pulsar Binary PSR J1653-0158

We have presented the first joint XMM-Newton and NuSTAR analysis of the millisecond pulsar (MSP) binary PSR J1653-0158. The 75 minute orbital period inferred from optical and gamma-ray observations together with the 1.97 ms pulsation in the gamma-rays indicate that this system is the most compact Black Widow MSP system known to date. The orbital period was not detected in the XMM-Newton and NuSTAR data, probably due to insufficient photon counts obtained in the observations. Fitting the joint X-ray spectrum of PSR J1653-0158 with a power law gives a photon index Γ = 1.71 ± 0.09. The X-ray luminosity of the source in the (0.2-40) keV band is deduced to be 1.18 × 1031 erg s-1, for an adopted distance of 0.84 kpc. We have shown that the broadband X-ray spectrum can be explained by synchrotron radiation from electrons accelerated in the intrabinary shock, and the gamma-rays detected in the Fermi data are curvature radiations from electrons and positrons in the pulsar magnetosphere. Our kinematic analysis of the Tidarren systems PSR J1653-0158 and PSR J1311-3430 indicates that the two Tidarren systems are likely to have originated in the Galactic disk.

Keywords: millisecond pulsars; x-ray binary stars; gamma-ray sources
Spatial and Temporal Analysis of 3 minute Oscillations in the Chromosphere Associated with the X2.2 Solar Flare on 2011 February 15

Three minute oscillations in the chromosphere are attributed to both slow magnetoacoustic waves propagating from the photosphere and to oscillations generated within the chromosphere itself at its natural frequency as a response to a disturbance. Here we present an investigation of the spatial and temporal behavior of the chromospheric 3 minute oscillations before, during, and after the SOL2011-02-15T01:56 X2.2 flare. Observations in ultraviolet emission centered on 1600 and 1700 Å obtained at 24 s cadence from the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory are used to create power maps as functions of both space and time. We observe higher 3 minute power during the flare, spatially concentrated in small areas ∼10 pixels ( $\sim 4^{\prime\prime} $ ) across. This implies that the chromospheric plasma is not oscillating globally as a single body. The locations of increased 3 minute power are consistent with observations of HXR flare emission from previous studies, suggesting that these small areas are manifestations of the chromosphere responding to injection of energy by nonthermal particles. This supports the theory that the chromosphere oscillates at the acoustic cutoff frequency in response to a disturbance.

Keywords: solar chromosphere; solar oscillations
Evolution of Subsonic Shock Waves Associated with Reconnection Jets in Earth's Magnetotail

Motivated by the signatures of nonlinear electrostatic waves observed by the Magnetospheric Multiscale spacecraft mission in reconnection jet regions of Earth's magnetotail, we have explored the dynamical features of ion-acoustic shock waves in the magnetotail. In this investigation, we have examined the dynamics and characteristics of ion-acoustic subsonic shock waves in non-Maxwellian space plasma comprising of two counterstreaming ion beams with suprathermal electrons, assumed to follow a kappa (κ) distribution. A reductive perturbation technique has been adopted to establish an evolution equation for small amplitude electrostatic shock structures. Importantly, subsonic waves only exist when the beam velocity exceeds a certain threshold, beyond which supersonic and subsonic waves may coexist. The combined effects of the beam velocity and the non-Maxwellian electron statistics have been analyzed to examine the characteristics of subsonic shock waves. Both symmetric and asymmetric (in relative beam density) models have been considered, leading to distinct possibilities in the evolution of subsonic shock waves. The findings of the investigation will help unfold the relatively unexplored dynamical characteristics of subsonic shock waves that may form and propagate in the magnetosphere.

Keywords: plasma physics; space plasmas
A Systematic Observational Study on Galactic Interstellar Ratio 18O/17O. I. C18O and C17O J = 1-0 Data Analysis

The interstellar oxygen isotopic ratio of 18O/17O can reflect the relative amount of the secular enrichment by ejecta from high-mass versus intermediate-mass stars. Previous observations found a Galactic gradient of 18O/17O, i.e., low ratios in the Galactic center and large values in the Galactic disk, which supports the inside-out formation scenario of our Galaxy. However, there are not many observed objects and, in particular, there are not many at large galactocentric distances. For this reason, we started a systematic study on Galactic interstellar 18O/17O, through observations of C18O and C17O multi-transition lines toward a large sample of 286 sources (at least one order of magnitude larger than previous ones), from the Galactic center region to the far outer Galaxy (∼22 kpc). In this article, we present our observations of J = 1-0 lines of C18O and C17O, with the 12 m antenna of the Arizona Radio Observatory (ARO 12 m) and the Institut de Radio Astronomie Millimétrique (IRAM) 30 m telescopes. Among our IRAM 30 m sample of 50 targets, we detected successfully both C18O and C17O 1-0 lines for 34 sources. Similarly, our sample of 260 targets for ARO 12 m observations resulted in the detection of both lines for 166 sources. The C18O optical depth effect on our ratio results, evaluated by fitting results of C17O spectra with hyperfine components (assuming ${\tau }_{C18O}=4{\tau }_{C17O}$ ) and our radiative transfer and excitation model nonlocal thermodynamic equilibrium (non-LTE) model calculation for the strongest source, was found to be insignificant. Beam dilution does not seem to be a problem either, which was supported by the fact that there is no systematic variation between the isotopic ratio and the heliocentric distance, and ratios are consistently measured from two telescopes for most of those detected sources. With this study we obtained 18O/17O isotopic ratios for a large sample of molecular clouds with different galactocentric distances. Our results, though there are still very few detections made for sources in the outer Galaxy, confirm the apparent 18O/17O gradient of 18O/17O=(0.10 ± 0.03)RGC+(2.95 ± 0.30), with a Pearson's rank correlation coefficient of R = 0.69. This is supported by the newest Galactic chemical evolution model including the impact of massive stellar rotators and novae. Our future J = 2-1 and J = 3-2 observations of C18O and C17O toward the same sample would be important to determine their physical parameters (opacities, abundances, etc.) and further accurately determine the Galactic radial gradient of the isotopic ratio 18O/17O.

Keywords: interstellar molecules; isotopic abundances; galaxy chemical evolution; milky way evolution
Evolution of the Magnetic Field in High- and Low-β Disks with Initially Toroidal Fields

We present the results from a pair of high-resolution, long-timescale (~105 GM/c 3), global, three-dimensional magnetohydrodynamical accretion disk simulations with differing initial magnetic plasma β in order to study the effects of the initial toroidal field strength on the production of a large-scale poloidal field. We initialize our disks in approximate equilibrium with purely toroidal magnetic fields of strength β 0 = 5 and 200. We also perform a limited resolution study. We find that simulations of differing field strengths diverge early in their evolution and remain distinct over the time studied, indicating that the initial magnetic conditions leave a persistent imprint in our simulations. Neither simulation enters the magnetically arrested disk regime. Both simulations are able to produce poloidal fields from initially toroidal fields, with the β 0 = 5 simulation evolving clear signs of a large-scale poloidal field. We make a cautionary note that computational artifacts in the form of large-scale vortices may be introduced in the combination of initially weak field and disk-internal mesh refinement boundaries, as evidenced by the production of an m = 1 mode overdensity in the weak field simulation. Our results demonstrate that the initial toroidal field strength plays a vital role in the simulated disk evolution for the models studied.

Keywords: accretion; black hole physics; magnetic fields
Limits on X-Ray Polarization at the Core of Centaurus A as Observed with the Imaging X-Ray Polarimetry Explorer

We present measurements of the polarization of X-rays in the 2-8 keV band from the nucleus of the radio galaxy Centaurus A (Cen A), using a 100 ks observation from the Imaging X-ray Polarimetry Explorer (IXPE). Nearly simultaneous observations of Cen A were also taken with the Swift, NuSTAR, and INTEGRAL observatories. No statistically significant degree of polarization is detected with IXPE. These observations have a minimum detectable polarization at 99% confidence (MDP99) of 6.5% using a weighted, spectral model-independent calculation in the 2-8 keV band. The polarization angle ψ is consequently unconstrained. Spectral fitting across three orders of magnitude in X-ray energy (0.3-400 keV) demonstrates that the SED of Cen A is well described by a simple power law with moderate intrinsic absorption (N H ~ 1023 cm-2) and a Fe Kα emission line, although a second unabsorbed power law is required to account for the observed spectrum at energies below 2 keV. This spectrum suggests that the reprocessing material responsible for this emission line is optically thin and distant from the central black hole. Our upper limits on the X-ray polarization are consistent with the predictions of Compton scattering, although the specific seed photon population responsible for the production of the X-rays cannot be identified. The low polarization degree, variability in the core emission, and the relative lack of variability in the Fe Kα emission line support a picture where electrons are accelerated in a region of highly disordered magnetic fields surrounding the innermost jet.

Keywords: polarimetry; x-ray quasars; radio galaxies
A Model for Nonthermal Particle Acceleration in Relativistic Magnetic Reconnection

The past decade has seen an outstanding development of nonthermal particle acceleration in magnetic reconnection in magnetically dominated systems, with clear signatures of power-law energy distributions as a common outcome of first-principles kinetic simulations. Here we propose a semianalytical model for systematically investigating nonthermal particle acceleration in reconnection. We show particle energy distributions are well determined by particle injection, acceleration, and escape processes. Using a series of kinetic simulations, we accurately evaluate the energy- and time-dependent model coefficients. The resulting spectral characteristics, including the spectral index and lower and upper bounds of the power-law distribution, agree well with the simulation results. Finally, we apply the model to predict the power-law indices and break energies in astrophysical reconnection systems.

Keywords: high energy astrophysics; plasma astrophysics
Correlation between Supergranular Lane Widths and Sunspot Number; A Simple Way to Predict the Amplitude of Sunspot Cycle

The network structure seen in the solar images is the outline of supergranulation, which is the large-scale convection in the Sun with a size of about 30 Mm and a lifetime of 24 hr. We have obtained the supergranulation lane widths from the autocorrelation function of image windows from the Ca II K spectroheliograms. The images are obtained from the 100 yr Kodaikanal data, which contains information on more than nine solar cycles. The lane widths are known to show a positive correlation with the sunspot number. It is now found that the lane widths, obtained near the mid-latitudes during the sunspot cycle minima, are strongly correlated to the following sunspot number maxima. A straight-line fit adequately describes the variation. It is also found that the correlation is weak or insignificant at other times. The strong correlation of the two parameters thus provides a simple way to predict the maximum sunspot number about 4-5 yr in advance. The results are important in space weather predictions and solar irradiance variations.

Keywords: solar cycle; solar atmosphere
Probing Jet Launching in Neutron Star X-Ray Binaries: The Variable and Polarized Jet of SAX J1808.4-3658

We report on an optical photometric and polarimetric campaign on the accreting millisecond X-ray pulsar (AMXP) SAX J1808.4-3658 during its 2019 outburst. The emergence of a low-frequency excess in the spectral energy distribution in the form of a red excess above the disk spectrum (seen most prominently in the z, i, and R bands) is observed as the outburst evolves. This is indicative of optically thin synchrotron emission due to a jet, as seen previously in this source and in other AMXPs during outburst. At the end of the outburst decay, the source enters a reflaring state. The low-frequency excess is still observed during the reflares. Our optical (BVRI) polarimetric campaign shows variable linear polarization (LP) throughout the outburst. We show that this is intrinsic to the source, with low-level but significant detections (0.2%-2%) in all bands. The LP spectrum is red during both the main outburst and the reflaring state, favoring a jet origin for this variable polarization over other interpretations, such as Thomson scattering with free electrons from the disk or the propelled matter. During the reflaring state, a few episodes with stronger LP levels (1%-2%) are observed. The low-level, variable LP is suggestive of strongly tangled magnetic fields near the base of the jet. These results clearly demonstrate how polarimetry is a powerful tool for probing the magnetic field structure in X-ray binary jets, as for active galactic nuclei jets.

Keywords: jets; low-mass x-ray binary stars; neutron stars; compact objects; stellar accretion disks
Nuclear Activity in the Low-metallicity Dwarf Galaxy SDSS J0944-0038 : A Glimpse into the Primordial Universe

Local low-metallicity dwarf galaxies are relics of the early universe and are thought to hold clues into the origins of supermassive black holes. While recent studies are uncovering a growing population of active galactic nuclei (AGNs) in dwarf galaxies, the vast majority reside in galaxies with solar or supersolar metallicities and stellar masses comparable to that of the LMC. Using Multi-Unit Spectroscopic Explorer (MUSE) and Very Large Telescope observations, we report the detection of [Fe X] λ6374 coronal line emission and a broad Hα line in the nucleus of SDSS J094401.87-003832.1, a nearby (z = 0.0049) metal-poor dwarf galaxy almost 500 times less massive than the LMC. Unlike the emission from the lower-ionization nebular lines, the [Fe X] λ6374 emission is compact and centered on the brightest nuclear source, with a spatial extent of ≈100 pc, similar to that seen in well-known AGNs. The [Fe X] luminosity is ≈1037 erg s-1, within the range seen in previously identified AGNs in the dwarf-galaxy population. The [Fe X] emission has persisted over the roughly 19 yr time period between the SDSS and MUSE observations, ruling out supernovae as the origin for the emission. The FWHM of the broad component of the Hα line is 446 ± 17 km s-1 and its luminosity is ≈1.5 × 1038 erg s-1, corresponding to a black hole mass of ≈ 3150 M ⊙, in line with its stellar mass if virial mass relations and black hole-galaxy scaling relations apply in this mass regime. These observations, together with previously reported multiwavelength observations, can most plausibly be explained by the presence of an accreting intermediate-mass black hole in a primordial galaxy analog.

Keywords: dwarf irregular galaxies; supermassive black holes; agn host galaxies
Microinstabilities in the Transition Region of Weak Quasi-perpendicular Intracluster Shocks

Microinstabilities play important roles in both entropy generation and particle acceleration in collisionless shocks. Recent studies have suggested that in the transition region of quasi-perpendicular (Q⊥) shocks in the high-beta (β = Pgas/PB) intracluster medium (ICM), the ion temperature anisotropy due to the reflected-gyrating ions could trigger the Alfvén ion cyclotron (AIC) instability and the ion-mirror instability, while the electron temperature anisotropy induced by magnetic field compression could excite the whistler instability and the electron-mirror instability. Adopting the numerical estimates for ion and electron temperature anisotropies found in the particle-in-cell (PIC) simulations of Q⊥ shocks with sonic Mach numbers, Ms = 2-3, we carry out a linear stability analysis for these microinstabilities. The kinetic properties of the microinstabilities and the ensuing plasma waves on both ion and electron scales are described for wide ranges of parameters, including β and the ion-to-electron mass ratio. In addition, the nonlinear evolution of the induced plasma waves are examined by performing 2D PIC simulations with periodic boundary conditions. We find that for β ≈ 20-100, the AIC instability could induce ion-scale waves and generate shock surface ripples in supercritical shocks above the AIC critical Mach number, ${M}_{\mathrm{AIC}}^{* }\approx 2.3$ . Also, electron-scale waves are generated primarily by the whistler instability in these high-β shocks. The resulting multiscale waves from electron to ion scales are thought to be essential in the electron injection to diffusive shock acceleration in Q⊥ shocks in the ICM.

Keywords: shocks; plasma astrophysics