adsabs/SciX_UAT_keywords huggingface dataset:
Intermediate-mass Black Hole Progenitors from Stellar Collisions in Dense Star Clusters

Very massive stars (VMSs) formed via a sequence of stellar collisions in dense star clusters have been proposed as the progenitors of massive black hole seeds. VMSs could indeed collapse to form intermediate-mass black holes, which would then grow by accretion to become the supermassive black holes observed at the centers of galaxies and powering high-redshift quasars. Previous studies have investigated how different cluster initial conditions affect the formation of a VMS, including mass segregation, stellar collisions, and binaries, among others. In this study, we investigate the growth of VMSs with a new grid of Cluster Monte Carlo star cluster simulations—the most expansive to date. The simulations span a wide range of initial conditions, varying the number of stars, cluster density, stellar initial mass function (IMF), and primordial binary fraction. We find a gradual shift in the mass of the most massive collision product across the parameter space; in particular, denser clusters born with top-heavy IMFs provide strong collisional regimes that form VMSs with masses easily exceeding 1000 M ⊙. Our results are used to derive a fitting formula that can predict the typical mass of a VMS formed as a function of the star cluster properties. Additionally, we study the stochasticity of this process and derive a statistical distribution for the mass of the VMS formed in one of our models, recomputing the model 50 times with different initial random seeds.

Keywords: star clusters; massive stars; intermediate-mass black holes; young star clusters; black holes; stellar mergers; n-body simulations; initial mass function; binary stars
Detecting Reconnection Events in Kinetic Vlasov Hybrid Simulations Using Clustering Techniques

Kinetic turbulence in magnetized space plasmas has been extensively studied via in situ observations, numerical simulations, and theoretical models. In this context, a key point concerns the formation of coherent current structures and their disruption through magnetic reconnection. We present automatic techniques aimed at detecting reconnection events in a large data set of numerical simulations. We make use of clustering techniques known as K-means and DBscan (usually referred to in literature as unsupervised machine-learning approaches), and other methods based on thresholds of standard reconnection proxies. All our techniques also use a threshold on the aspect ratio of the regions selected. We test the performance of our algorithms. We propose an optimal aspect ratio to be used in the automated machine-learning algorithm: AR = 18. The performance of the unsupervised approach results in it being strongly competitive with respect to those of other methods based on thresholds of standard reconnection proxies.

Keywords: solar magnetic reconnection; astronomy data analysis; astronomy data modeling; computational methods; clustering
Effects of Pseudostreamer Boundary Dynamics on Heliospheric Field and Wind

Interchange reconnection has been proposed as a mechanism for the generation of the slow solar wind, and a key contributor to determining its characteristic qualities. In this paper we study the implications of interchange reconnection for the structure of the plasma and field in the heliosphere. We use the Adaptively Refined Magnetohydrodynamic Solver to simulate the coronal magnetic evolution in a coronal topology containing both a pseudostreamer and helmet streamer. We begin with a geometry containing a low-latitude coronal hole that is separated from the main polar coronal hole by a pseudostreamer. We drive the system by imposing rotating flows at the solar surface within and around the low-latitude coronal hole, which leads to a corrugation (at low altitudes) of the separatrix surfaces that separate open from closed magnetic flux. Interchange reconnection is induced both at the null points and separators of the pseudostreamer, and at the global helmet streamer. We demonstrate that a preferential occurrence of interchange reconnection in the "lanes" between our driving cells leads to a filamentary pattern of newly opened flux in the heliosphere. These flux bundles connect to but extend far from the separatrix-web (S-Web) arcs at the source surface. We propose that the pattern of granular and supergranular flows on the photosphere should leave an observable imprint in the heliosphere.

Keywords: slow solar wind; solar magnetic reconnection; magnetohydrodynamics; solar corona
Multiwavelength Observations of SDSS J105621.45+313822.1, a Broad-line, Low-metallicity AGN

In contrast to massive galaxies with solar or super-solar gas phase metallicities, very few active galactic nuclei (AGNs) are found in low-metallicity dwarf galaxies. Such a population could provide insight into the origins of supermassive black holes. Here we report near-IR spectroscopic and X-ray observations of SDSS J105621.45+313822.1, a low-mass, low-metallicity galaxy with optical narrow line ratios consistent with star-forming galaxies but a broad Hα line and mid-infrared colors consistent with an AGN. We detect the [Si VI] 1.96 μm coronal line and a broad Paα line with an FWHM of 850 ± 25 km s-1. Together with the optical broad lines and coronal lines seen in the Sloan Digital Sky Survey (SDSS) spectrum, we confirm the presence of a highly accreting black hole with mass (2.2 ± 1.3) × 106 M⊙, with a bolometric luminosity of ≈1044 erg s-1 based on the coronal line luminosity, implying a highly accreting AGN. Chandra observations reveal a weak nuclear point source with ${L}_{{\rm{X}},2\mbox{--}10\mathrm{keV}}=(2.3\pm 1.2)\times {10}^{41}$ erg s-1, ∼2 orders of magnitude lower than that predicted by the mid-infrared luminosity, suggesting that the AGN is highly obscured despite showing broad lines in the optical spectrum. The low X-ray luminosity and optical narrow line ratios of J1056+3138 highlight the limitations of commonly employed diagnostics in the hunt for AGNs in the low-metallicity, low-mass regime.

Keywords: supermassive black holes; agn host galaxies
Deriving X-Ray Line Profiles for Massive-star Winds from Momentum-conserving Dynamical Working Surface Solutions

We present a general procedure for deriving a line-profile model for massive-star X-ray spectra that captures the dynamics of the wind more directly. The basis of the model is the analytic solution to the problem of variable jets in Herbig–Haro objects given by Cantó et al. In deriving our model, we generalize this jet solution to include flows with a prescribed nonzero acceleration for the context of radiatively driven winds. We provide example line profiles generated from our model for the case of sinusoidal velocity and mass-ejection variations. The example profiles show the expected shape of massive-star X-ray emission lines, as well as interesting but complicated trends with the model parameters. This establishes the possibility that observed X-rays could be a result of temporal variations seeded at the wind base, rather than purely generated intrinsically within the wind volume, and can be described via a quantitative language that connects with the physical attributes of those variations, consistently with the downstream momentum-conserving nature of radiatively cooled shocked radial flows.

Keywords: massive stars; stellar winds; x-ray astronomy; analytical mathematics
On the Impact of 22Ne on the Pulsation Periods of Carbon-Oxygen White Dwarfs with Helium-dominated Atmospheres

We explore changes in the adiabatic low-order g-mode pulsation periods of 0.526, 0.560, and 0.729 M⊙ carbon-oxygen white dwarf models with helium-dominated envelopes due to the presence, absence, and enhancement of 22Ne in the interior. The observed g-mode pulsation periods of such white dwarfs are typically given to 6-7 significant figures of precision. Usually white dwarf models without 22Ne are fit to the observed periods and other properties. The rms residuals to the ≃150-400 s low-order g-mode periods are typically in the range of σrms ≲ 0.3 s, for a fit precision of σrms/P ≲ 0.3%. We find average relative period shifts of ΔP/P ≃ ±0.5% for the low-order dipole and quadrupole g-mode pulsations within the observed effective temperature window, with the range of ΔP/P depending on the specific g-mode, abundance of 22Ne, effective temperature, and the mass of the white dwarf model. This finding suggests a systematic offset may be present in the fitting process of specific white dwarfs when 22Ne is absent. As part of the fitting processes involves adjusting the composition profiles of a white dwarf model, our study on the impact of 22Ne can provide new inferences on the derived interior mass fraction profiles. We encourage routinely including 22Ne mass fraction profiles, informed by stellar evolution models, to future generations of white dwarf model-fitting processes.

Keywords: white dwarf stars; stellar pulsations; stellar evolution; non-radial pulsations; stellar physics
Constraining the Neutron Star Mass-Radius Relation and Dense Matter Equation of State with NICER. III. Model Description and Verification of Parameter Estimation Codes

We describe the X-ray pulse profile models we use and how we use them to analyze Neutron Star Interior Composition Explorer (NICER) observations of rotation-powered millisecond pulsars to obtain information about the mass-radius relation of neutron stars and the equation of state of the dense matter in their cores. Here we detail our modeling of the observed profile of PSR J0030+0451 that we analyzed in Miller et al. and Riley et al. and describe a cross-verification of computations of the pulse profiles of a star with R/M 3, in case stars this compact need to be considered in future analyses. We also present our early cross-verification efforts of the parameter estimation procedures used by Miller et al. and Riley et al. by analyzing two distinct synthetic data sets. Both codes yielded credible regions in the mass-radius plane that are statistically consistent with one another, and both gave posterior distributions for model parameter values consistent with the values that were used to generate the data. We also summarize the additional tests of the parameter estimation procedure of Miller et al. that used synthetic pulse profiles and the NICER pulse profile of PSR J0030+0451. We then illustrate how the precision of mass and radius estimates depends on the pulsar's spin rate and the size of its hot spot by analyzing four different synthetic pulse profiles. Finally, we assess possible sources of systematic error in the estimates made using this technique, some of which may warrant further investigation.

Keywords: neutron stars; compact objects; nuclear astrophysics; pulsars; neutron star cores; x-ray astronomy; x-ray sources; diffuse x-ray background; stellar atmospheres
A Fast, Two-dimensional Gaussian Process Method Based on Celerite: Applications to Transiting Exoplanet Discovery and Characterization

Gaussian processes (GPs) are commonly used as a model of stochastic variability in astrophysical time series. In particular, GPs are frequently employed to account for correlated stellar variability in planetary transit light curves. The efficient application of GPs to light curves containing thousands to tens of thousands of data points has been made possible by recent advances in GP methods, including the celerite method. Here we present an extension of the celerite method to two input dimensions where, typically, the second dimension is small. This method scales linearly with the total number of data points when the noise in each large dimension is proportional to the same celerite kernel and only the amplitude of the correlated noise varies in the second dimension. We demonstrate the application of this method to the problem of measuring precise transit parameters from multiwavelength light curves and show that it has the potential to improve transit parameters measurements by orders of magnitude. Applications of this method include transit spectroscopy and exomoon detection, as well a broader set of astronomical problems.

Keywords: exoplanets; natural satellites (extrasolar); transmission spectroscopy; transits; gaussian processes regression; astrostatistics; bayesian statistics
Search for Neutrinos from the Tidal Disruption Events AT2019dsg and AT2019fdr with the ANTARES Telescope

On 2019 October 1, the IceCube Collaboration detected a muon track neutrino with a high probability of being of astrophysical origin, IC191001A. After a few hours, the tidal disruption event (TDE) AT2019dsg, observed by the Zwicky Transient Facility (ZTF), was indicated as the most likely counterpart of the IceCube track. More recently, the follow-up campaign of the IceCube alerts by ZTF suggested a second TDE, AT2019fdr, as a promising counterpart of another IceCube muon track candidate, IC200530A, detected on 2020 May 30. Here, these intriguing associations are followed-up by searching for neutrinos in the ANTARES detector from the directions of AT2019dsg and AT2019fdr using a time-integrated approach. As no significant evidence for space clustering is found in the ANTARES data, upper limits on the one-flavor neutrino flux and fluence are set.

Keywords: neutrino astronomy
Long-duration Gamma-Ray Burst Progenitors and Magnetar Formation

Millisecond magnetars produced in the center of dying massive stars are one prominent model to power gamma-ray bursts (GRBs). However, their detailed nature remains a mystery. To explore the effects of the initial mass, rotation rate, wind mass loss, and metallicity on the GRB progenitors and the newborn magnetar properties, we evolve 227 of 10-30 M ⊙ single star models from the pre-main sequence to core collapse by using the stellar evolution code MESA. The presupernova properties, the compactness parameter, and the magnetar characteristics of models with different initial parameters are presented. The compactness parameter remains a nonmonotonic function of the initial mass and initial rotation rate when the effects of varying metallicity and the "Dutch" wind scale factor are taken into account. We find that the initial rotation rate and mass play the dominant roles in whether a star can evolve into a GRB progenitor. The minimum rotation rate necessary to generate a magnetar gradually reduces as the initial mass increases. The greater the initial metallicity and "Dutch" wind scale factor, the larger the minimum rotation rate required to produce a magnetar. In other words, massive stars with low metallicity are more likely to harbor magnetars. Furthermore, we present the estimated period, magnetic field strength, and masses of magnetars in all cases. The typical rotational energy of these millisecond magnetars is sufficient to power long-duration GRBs.

Keywords: massive stars; stellar evolutionary models; gamma-ray bursts; magnetars
Dependence of Ion Temperatures on Alpha-Proton Differential Flow Vector and Heating Mechanisms in the Solar Wind

According to Wind observations between 2004 June and 2019 May, this Letter investigates the proton and alpha particle temperatures in the space of (θd, Vd/VA) for the first time, where θd and Vd are the radial angle and magnitude of alpha-proton differential flow vector ${{\boldsymbol{V}}}_{d}$ , respectively, VA is the local Alfvén speed. Results show that the temperatures significantly depend on θd as well as Vd/VA. In case of low proton parallel beta (βp∥ < 1), it is found that the proton perpendicular temperature is clearly enhanced when θd is small (≲45°) and Vd/VA ≳ 0.5. On the contrary, the perpendicular temperature of alpha particles is considerably enhanced when θd is large (≳90°) or Vd/VA is sufficiently small. The maximum of proton parallel temperature takes place at θd ∼ 90° accompanied by higher βp∥ and by larger turbulence amplitude of magnetic fluctuations in inertial range. This study should present strong evidence for cyclotron resonance heating of protons and alpha particles in the solar wind. Other mechanisms including Landau resonance and stochastic heating are also proposed, which tend to have different (θd, Vd/VA) spaces than cyclotron resonance heating.

Keywords: solar coronal heating; alfven waves; interplanetary turbulence; solar wind; space plasmas; plasma physics
The CaFe Project: Optical Fe II and Near-infrared Ca II Triplet Emission in Active Galaxies. II. The Driver(s) of the Ca II and Fe II and Its Potential Use as a Chemical Clock

In this second paper in the series, we carefully analyze the observational properties of the optical Fe II and near-IR Ca II triplet in active galactic nuclei (AGNs), as well as the luminosity, black hole mass, and Eddington ratio in order to define the driving mechanism behind the properties of our sample. The Ca II shows an inverse Baldwin effect, bringing out the particular behavior of this ion with respect to the other low-ionization lines such as Hβ. We performed a principal component analysis, where 81.2% of the variance can be explained by the first three principal components drawn from the FWHMs, luminosity, and equivalent widths. The first principal component (PC1) is primarily driven by the combination of black hole mass and luminosity with a significance over 99.9%, which in turn is reflected in the strong correlation of the PC1 with the Eddington ratio. The observational correlations are better represented by the Eddington ratio; thus, it could be the primary mechanism behind the strong correlations observed in the Ca II-Fe II sample. Since calcium belongs to the α-elements, the Fe II/Ca II flux ratio can be used as a chemical clock for determining the metal content in AGNs and trace the evolution of the host galaxies. We confirm the de-enhancement of the ratio Fe II/Ca II by the Eddington ratio, suggesting a metal enrichment of the BLR in intermediate-z with respect to low-z objects. A larger sample, particularly at z > 2, is needed to confirm the present results.

Keywords: quasars; active galactic nuclei; spectroscopy; supermassive black holes
The Spherical Fast Multipole Method (sFMM) for Gravitational Lensing Simulation

In this paper, we present a spherical Fast Multipole Method (sFMM) for ray-tracing simulation of gravitational lensing on a curved sky. The sFMM is a nontrivial extension of the Fast Multiple Method to sphere ${{\mathbb{S}}}^{2}$ , and it can accurately solve the Poisson equation with time complexity of $O(N)\mathrm{log}(N)$ , where N is the number of particles. It is found that the time complexity of the sFMM is near O(N) and the computational accuracy can reach 10-10 in our test. In addition, compared with the Fast Spherical Harmonic Transform, the sFMM is not only faster but also more accurate, as it has the ability to reserve high-frequency components of the density field. These merits make the sFMM an optimum method to simulate the gravitational lensing on a curved sky, which is the case for upcoming large-area sky surveys, such as the Vera Rubin Observatory and the China Space Station Telescope.

Keywords: gravitational lensing; algorithms; n-body simulations
A Radiatively Quiet Glitch and Anti-glitch in the Magnetar 1E 2259+586

We report on the timing and spectral properties of the soft X-ray emission from the magnetar 1E 2259+586 from 2013 January, ∼8 months after the detection of an anti-glitch, until 2019 September, using the Neil Gehrels Swift and Neutron star Interior Composition ExploreR (NICER) observatories. During this time span, we detect two timing discontinuities. The first, occurring around 5 yr after the 2012 April anti-glitch, is a relatively large spin-up glitch with a fractional amplitude ${\rm{\Delta }}\nu /\nu =1.24(2)\times {10}^{-6}$ . We find no evidence for flux enhancement or change in the spectral or pulse-profile shape around the time of this glitch. This is consistent with the picture that a significant number of magnetar spin-up glitches are radiatively quiet. Approximately 1.5 yr later in 2019 April, 1E 2259+586 exhibited an anti-glitch with spin-down of a fractional amplitude ${\rm{\Delta }}\nu /\nu =-5.8(1)\times {10}^{-7}$ , similar to the fractional change detected in 2012. We do not, however, detect any change to the pulse-profile shape or increase in the rms pulsed flux of the source, nor do we see any possible bursts from its direction around the time of the anti-glitch, all of which occurred during the 2012 event. Hence, similar to spin-up glitches, anti-glitches can occur silently. This may suggest that these phenomena originate in the neutron star interior, and that their locale and triggering mechanism do not necessarily have to be connected to the magnetosphere. Last, our observations suggest that the occurrence rate of spin-up and spin-down glitches is about the same in 1E 2259+586, with the former having a larger net fractional change.

Keywords: magnetars; neutron stars; compact objects
A Rotating Accretion Disk around MWC 297, a Young B1.5Ve Star

High-resolution spectra with iSHELL on IRTF in the K and M bands of the young, heavily accreting B1.5Ve star MWC 297 show numerous double-peaked CO lines. These CO lines originate in an inclined gaseous disk in Keplerian rotation. MWC 297 is the only early B star known to show a Keplerian disk in CO. Analysis of the spectra shows that 12CO 1-0 is optically thick for the low excitation lines. Even the 13CO 1-0 and 12CO 2-1 have somewhat optically thick lines at low J levels. We find that the CO emission in the disk can be fitted with CO being in a narrow ring at a radius of 12 au, with a temperature of 1500 K and a CO column density of 1.6 × 1018 cm-2. This model underestimates the line strength of high-J lines, indicating that they are excited by fluorescence. The CO overtone lines have a similar temperature. The 13CO lines are much brighter than expected from interstellar isotope ratios. The 13CO lines are wider than the 12CO ones, suggesting different excitation conditions. The same is true for 12CO 2-1. We see strong absorption in 12CO and 13CO 1-0 at low J levels, which is due to two cold foreground clouds. These clouds, one with a temperature of 8.3 K and a column density of 6.7 1017 cm-2 and the other one colder and with lower column density, can fully account for the observed extinction toward MWC 297.

Keywords: star formation; star forming regions; massive stars; circumstellar disks; far infrared astronomy
Mapping Gaia Parallax Systematic Errors over the Sky with Faint Milky Way Stars

Parallaxes measured by the Gaia mission have huge significance for astronomy, but parallaxes in Gaia DR2 are known to have systematic errors that depend on the source position and other quantities. We use the abundant information in faint Milky Way stars, along with the Gaia Object Generator simulation of the Gaia catalog, to probe the spatial dependence of Gaia DR2 parallax systematic errors in an empirical way. The parallax signal, concentrated in thick disk turnoff stars with magnitude G ∼ 17, is sufficient to construct maps of the parallax systematic error over the majority of the sky. These maps show a locally regular "waffle pattern" on ∼1° scales following Gaia scan directions, stronger linear "scar" features, and coherent variations on larger scales. The parallax bias maps also retain traces of astrophysical effects such as dust clouds. The waffle pattern, known from earlier maps of the Magellanic Clouds, extends over the entire sky; its local rms amplitude averages 15 μas and varies by about a factor of two. The strength of this pattern increases by a factor ∼6 from magnitude G = 13 to G = 20. Correlations with parallaxes of quasars and of stars with independent distance estimates support our bias estimates. Using similar methods, we map systematic errors in the proper motion and examine the relationship with the parallax systematics. We provide a code package to access and query our bias maps. Similar tests on the general stellar population should be useful in quantifying systematic errors in future Gaia releases.

Keywords: space astrometry; parallax; space observatories; astronomy databases; milky way galaxy