adsabs/SciX_UAT_keywords huggingface dataset:
Optical Linear Polarization Study toward Czernik 3 Open Cluster at Different Spatial Scales

We present the optical linear polarization observation of stars toward the core of the Czernik 3 cluster in the Sloan i band. The data were obtained using the EMPOL instrument on the 1.2 m telescope at Mount Abu Observatory. We study the dust distribution toward this cluster by combining the results from our polarization observations with the data from Gaia EDR3, WISE, and the H I, 12CO surveys. In addition, we use the polarimetric data of previously studied clusters within 15° of Czernik 3 to understand the large-scale dust distribution. The observational results of Czernik 3 show a large range in the degree of polarization, indicating that the dust is not uniformly distributed over the plane of the sky, even on a small scale. The distance to Czernik 3 is constrained to 3.6 ± 0.8 kpc using the member stars in the core region identified from Gaia EDR3 astrometry. This makes it one of the most distant clusters observed for optical polarization so far. The variation of observed degree of polarization and extinction toward this cluster direction suggests the presence of at least two dust layers along this line of sight at distances of ~1 and ~3.4 kpc. There is an indication of the presence of dust in the center of the cluster as seen from an increase in the degree of polarization and WISE W4 flux. The large-scale distribution of dust reveals the presence of a region of low dust content between the Local Arm and the Perseus arm.

Keywords: interstellar dust extinction; interstellar medium; starlight polarization; milky way disk; open star clusters
Evolution of Flare Activity in GKM Stars Younger Than 300 Myr over Five Years of TESS Observations

Stellar flares are short-duration (< hours) bursts of radiation associated with surface magnetic reconnection events. Stellar magnetic activity generally decreases as a function of both the age and Rossby number, R 0, a measure of the relative importance of the convective and rotational dynamos. Young stars (<300 Myr) have typically been overlooked in population-level flare studies due to challenges with flare-detection methods. Here, we select a sample of stars that are members of 26 nearby moving groups, clusters, or associations with ages <300 Myr that have been observed by the Transiting Exoplanet Survey Satellite at 2 minute cadence. We identified 26,355 flares originating from 3160 stars and robustly measured the rotation periods of 1847 stars. We measure and find the flare frequency distribution slope, α, saturates for all spectral types at α ∼ ‑0.5 and is constant over 300 Myr. Additionally, we find that flare rates for stars t age = 50–250 Myr are saturated below R 0 < 0.14, which is consistent with other indicators of magnetic activity. We find evidence of annual flare rate variability in eleven stars, potentially correlated with long-term stellar activity cycles. Additionally, we crossmatch our entire sample with the Galaxy Evolution Explorer and find no correlation between flare rate and far- and near-ultraviolet flux. Finally, we find the flare rates of planet-hosting stars are relatively lower than comparable, larger samples of stars, which may have ramifications for the atmospheric evolution of short-period exoplanets.

Keywords: pre-main sequence stars; optical flares; stellar rotation; stellar activity; time series analysis
First Systematic Study Reporting the Changes in Eclipse Cutoff Frequency for Pulsar J1544+4937

We present results from long-term monitoring of frequency-dependent eclipses of the radio emission from PSR J1544+4937, which is a black widow spider millisecond pulsar (MSP) in a compact binary system. The majority of such systems often exhibit relatively long-duration radio eclipses caused by ablated material from their companion stars. With the wide spectral bandwidth of the upgraded Giant Metrewave Radio Telescope, we present the first systematic study of temporal variation of eclipse cutoff frequency. With decade-long monitoring of 39 eclipses for PSR J1544+4937, we notice significant changes in the observed cutoff frequency ranging from 343 ± 7 to ≥740 MHz. We also monitored changes in eclipse cutoff frequency on timescales of tens of days and observed a maximum change of ≥315 MHz between observations that were separated by 22 days. In addition, we observed a change of ~47 MHz in eclipse cutoff frequency between adjacent orbits, i.e., on timescales of ~2.9 hr. We infer that such changes in the eclipse cutoff frequency depict an eclipse environment for the PSR J1544+4937 system that is dynamically evolving, where, along with the change in electron density, the magnetic field could also be varying. We also report a significant correlation between the eclipse cutoff frequency and the mass-loss rate of the companion. This study provides the first direct evidence of mass-loss rate affecting the frequency-dependent eclipsing in a spider MSP.

Keywords: binary pulsars; radio pulsars; millisecond pulsars; pulsars
EMRI + TDE = QPE: Periodic X-Ray Flares from Star-Disk Collisions in Galactic Nuclei

Roughly half of the quasiperiodic eruption (QPE) sources in galactic nuclei exhibit a remarkably regular alternating "long-short" pattern of recurrence times between consecutive flares. We show that a main-sequence star (brought into the nucleus as an extreme mass-ratio inspiral; EMRI) that passes twice per orbit through the accretion disk of the supermassive black hole (SMBH) on a mildly eccentric inclined orbit, each time shocking and ejecting optically thick gas clouds above and below the midplane, naturally reproduces observed properties of QPE flares. Inefficient photon production in the ejecta renders the QPE emission much harder than the blackbody temperature, enabling the flares to stick out from the softer quiescent disk spectrum. Destruction of the star via mass ablation limits the QPE lifetime to decades, precluding a long-lived AGN as the gaseous disk. By contrast, a tidal disruption event (TDE) naturally provides a transient gaseous disk on the requisite radial scale, with a rate exceeding the EMRI inward migration rate, suggesting that many TDEs should host a QPE. This picture is consistent with the X-ray TDE observed several years prior to the QPE appearance from GSN 069. Remarkably, a second TDE-like flare was observed from this event, starting immediately after detectable QPE activity ceased; this event could plausibly result from the (partial or complete) destruction of the QPE-generating star triggered by runaway mass loss, though other explanations cannot be excluded. Our model can also be applied to black hole-disk collisions, such as those invoked in the context of the candidate SMBH binary OJ 287.

Keywords: tidal disruption; x-ray transient sources; supermassive black holes; gravitational waves; stellar dynamics
Precessing Flaring Magnetar as a Source of Repeating FRB 180916.J0158+65

Recently, the Canadian Hydrogen Intensity Mapping Experiment detected periodicity in the bursting rate of the repeating FRB 180916.J0158+65. In a popular class of models, the fast radio bursts (FRBs) are created by magnetic flares of a hyperactive magnetar driven by fast ambipolar diffusion in the core. We point out that in this scenario the magnetar is expected to precess freely with a period of weeks to months. The internal magnetic field B ∼ 1016 G deforms the star, and magnetic flares induce sudden changes in magnetic stresses. The resulting torques and displacements of the principal axes of inertia are capable of pumping a significant amplitude of precession. The anisotropy of the flaring FRB activity, combined with precession, implies a strong periodic modulation of the visible bursting rate. The ultrastrong field invoked in the magnetar model provides: (1) energy for the frequent giant flares, (2) the high rate of ambipolar diffusion, releasing the magnetic energy on the timescale ∼109 s, (3) the core temperature T ≈ 109 K, likely above the critical temperature for neutron superfluidity, (4) strong magnetospheric torques, which efficiently spin down the star, and (5) deformation with ellipticity ɛ ≳ 10-6, much greater than the rotational deformation. These conditions result in a precession with negligible viscous damping, and can explain the observed 16 day period in FRB 180916.J0158+65. The increase of precession period due to the magnetar spindown should become measurable in the near future.

Keywords: magnetars; radio transient sources; neutron stars
A Radial Decrease in Kinetic Temperature Measured with H2CO in 30 Doradus

Feedback from star formation is a critical component of the evolution of galaxies and their interstellar medium. At parsec scales internal to molecular clouds, however, the observed signatures of that feedback on the physical properties of CO-emitting gas have often been weak or inconclusive. We present subparsec observations of H2CO in the 30 Doradus region, which contains the massive star cluster R136 that is clearly exerting feedback on its neighboring gas. H2CO provides a direct measure of gas kinetic temperature, and we find a trend of decreasing temperature with projected distance from R136 that may be indicative of gas heating by the stars. While it has been suggested that mechanical heating affects H2CO-measured temperature, we do not observe any correlation between T K and line width. The lack of an enhancement in mechanical feedback close to R136 is consistent with the absence of a radial trend in gravitational boundedness seen the Atacama Large Millimeter/submillimeter Array CO observations. Estimates of cosmic-ray flux in the region are quite uncertain, but can plausibly explain the observed temperatures if R136 itself is the dominant local source of energetic protons. The observations presented here are also consistent with the H2CO-emitting gas near R136 being dominated by direct radiation from R136 and photoelectric heating in the photodissociation regions.

Keywords: star formation; molecular clouds; stellar feedback; interstellar molecules; h ii regions
Discovery of GeV Gamma-Ray Emission from Pulsar Wind Nebula Kes 75 and PSR J1846-0258

We report the detection of gamma-ray emission from pulsar wind nebula (PWN) Kes 75 and PSR J1846-0258. Through modeling the spectral energy distribution incorporating the new Fermi-LAT data, we find that the observed gamma-ray emission is likely a combination of both the PWN and pulsar magnetosphere. The spectral shape of this magnetospheric emission is similar to the γ-ray spectrum of rotation-powered pulsars detected by Fermi-LAT, and the results from our best-fit model suggest that the pulsar's magnetospheric emission accounts for 1% of the current spin-down luminosity. Prior works attempted to characterize the properties of this system and found a low supernova (SN) explosion energy and low SN ejecta mass. We reanalyze the broadband emission incorporating the new Fermi emission and compare the implications of our results to prior reports. The best-fit gamma-ray emission model suggests a second very hot photon field possibly generated by the stellar wind of a Wolf-Rayet star embedded within the nebula, which supports the low ejecta mass found for the progenitor in prior reports and here in the scenario of binary mass transfer.

Keywords: supernova remnants; pulsar wind nebulae; neutron stars; rotation powered pulsars; magnetars; gamma-ray sources; gamma-ray astronomy
Constraining the γ-Ray Emission Region for Fermi-detected FSRQs by the Seed Photon Approach

The location of γ-ray-emitting region in blazars has been an open issue for several decades and is still being debated. We use the Paliya et al. sample of 619 γ-ray-loud flat-spectrum radio quasars with available spectral energy distributions and employ a seed photon factor approach to locate the γ-ray production region. This method efficiently sets up a relation between the peak frequencies and luminosities for the synchrotron emission and inverse Compton scattering, together with a combination of the energy density and characteristic energy for the external seed photon field, namely, $\sqrt{{U}_{0}}/{\epsilon }_{0}$ , an indicative factor of seed photons in units of Gauss. By comparing it with canonical values of the broad-line region (BLR) and molecular dusty torus (DT), we principally ascertain that the GeV emission originated far beyond the BLR and close to the DT-farther out at parsec scales from the central black hole, which supports a far-site scenario for γ-ray blazars. We probe the idea that inverse Compton scattering of infrared seed photons is happening in the Thomson regime. This approach and our findings are based on the validity of the external Compton model, which is applicable in understanding the GeV emission mechanism in FSRQs. However, the completeness of this framework has been challenged by reports of neutrino emission from blazars. Thus, we also shed new light on the neutrino production region by using our derived results because blazars are promising neutrino emitters.

Keywords: active galaxies; flat-spectrum radio quasars; gamma-ray sources
Deriving Median Disk Lifetimes from Disk Lifetime Distributions

Observations show that individual protoplanetary disk lifetimes vary from <1 Myr to ≫20 Myr. The disk lifetime distribution is currently unknown. For the example of a Gaussian distribution of the disk lifetime, I suggest a simple method for deducing such a disk lifetimes distribution. The median disk lifetimes inferred with this method is also shown.

Keywords: planet formation
Physical Origin of GeV Emission in the Early Phase of GRB 170405A: Clues from Emission Onsets with Multiwavelength Observations

The origin of GeV emission from the early epoch of gamma-ray bursts (GRBs) is a widely discussed issue. The long gamma-ray burst GRB 170405A, observed by the Fermi Gamma-ray Space Telescope, showed high-energy emission delayed by ∼20 s with respect to the X-ray emission, followed by temporally fading gamma-ray emission lasting for ∼1000 s, as commonly observed in high-energy GRBs. In addition, a high-energy spectral cutoff at ∼50 MeV was detected in the prompt-emission phase. If this feature is caused by pair-production opacity, the bulk Lorentz factor of the GRB ejecta can be estimated to be Γbulk = 170-420. Simultaneously with Fermi, GRB 170405A was observed by the Swift/Burst Alert Telescope and X-ray telescope, and a clear optical onset was detected ∼200 s after the burst by the Swift/Ultraviolet Optical Telescope. By coupling the deceleration time to the derived bulk Lorentz factor, the deceleration time was found to correspond to the delayed onset in the optical band. While the delayed onset in the optical band is evidence that this emission had an external shock origin, the temporally extended emission in the GeV band before the optical onset is hard to reconcile with the standard synchrotron emission from the same external shock. This may imply that the common feature of GeV emission with a power-law decay does not necessarily have the same origin as the optical afterglow in all GRBs detected by the Fermi/Large Area Telescope, particularly in their early epoch. Another mechanism to explain the GeV emission in GRB 170405A may be required, such as an internal shock or inverse Compton emission.

Keywords: gamma-ray bursts; astronomy data analysis
Generation of Subion Scale Magnetic Holes from Electron Shear Flow Instabilities in Plasma Turbulence

Magnetic holes (MHs) are coherent structures associated with strong magnetic field depressions in magnetized plasmas. They are observed in many astrophysical environments at a wide range of scales, but their origin is still under debate. In this work, we investigate the formation of subion scale MHs using a fully kinetic 2D simulation of plasma turbulence initialized with parameters typical of the Earth's magnetosheath. Our analysis shows that the turbulence is capable of generating subion scale MHs from large scale fluctuations via the following mechanism: first, the nonlinear large scale dynamics spontaneously leads to the development of thin and elongated electron velocity shears; these structures then become unstable to the electron Kelvin-Helmholtz instability and break up into small scale electron vortices; the electric current carried by these vortices locally reduces the magnetic field, inducing the formation of subion scale MHs. The MHs thus produced exhibit features consistent with satellite observations and with previous numerical studies. We finally discuss the kinetic properties of the observed subion scale MHs, showing that they are characterized by complex non-Maxwellian electron velocity distributions exhibiting anisotropic and agyrotropic features.

Keywords: plasma physics; plasma astrophysics; space plasmas
The Once-canceled Habitable-zone Super-Earth Gliese 581d Might Indeed Exist!

Recent studies indicate that Gliese 581d, a proposed habitable zone (HZ) super-Earth planet, does not exist, as the respective data denote that the planet is an artifact of stellar activity. Here we report evidence to the contrary considering that those studies were based on inaccurate spectroscopic measurements of the stellar rotation period regarding the planet's inactive host star (dM3). Gliese 581d, if real, is of particular interest as it constitutes the first planet identified to be in a stellar HZ outside of the solar system based on studies in 2007. If confirmed as a true planet, at 20.5 lt-yr, it would also be one of the nearest potentially habitable super-Earths.

Keywords: exoplanet astronomy; m dwarf stars; exoplanet detection methods; stellar rotation; astronomy data reduction; habitable zone; exoplanets; stellar activity; super earths
Tracing Lyα and LyC Escape in Galaxies with Mg II Emission

Star-forming galaxies are considered the likeliest source of the H I ionizing Lyman continuum (LyC) photons that reionized the intergalactic medium at high redshifts. However, above z ≳ 6, the neutral intergalactic medium prevents direct observations of LyC. Therefore, recent years have seen the development of indirect indicators for LyC that can be calibrated at lower redshifts and applied in the epoch of reionization. Emission from the Mg II λλ2796, 2803 doublet has been proposed as a promising LyC proxy. In this paper, we present new Hubble Space Telescope/Cosmic Origins Spectrograph observations for eight LyC emitter candidates, selected to have strong Mg II emission lines. We securely detect LyC emission in 50% (4/8) of the galaxies with 2σ significance. This high detection rate suggests that strong Mg II emitters might be more likely to leak LyC than similar galaxies without strong Mg II. Using photoionization models, we constrain the escape fraction of Mg II as ~15%-60%. We confirm that the escape fraction of Mg II correlates tightly with that of Lyα, which we interpret as an indication that the escape fraction of both species is controlled by resonant scattering in the same low column density gas. Furthermore, we show that the combination of the Mg II emission and dust attenuation can be used to estimate the escape fraction of LyC statistically. These findings confirm that Mg II emission can be adopted to estimate the escape fraction of Lyα and LyC in local star-forming galaxies and may serve as a useful indirect indicator at the epoch of reionization.

Keywords: ultraviolet astronomy; starburst galaxies; interstellar dust; reionization; emission line galaxies
Anisotropic Electron Heating in Turbulence-driven Magnetic Reconnection in the Near-Sun Solar Wind

We perform a high-resolution, 2D, fully kinetic numerical simulation of a turbulent plasma system with observation-driven conditions, in order to investigate the interplay between turbulence, magnetic reconnection, and particle heating from ion to subelectron scales in the near-Sun solar wind. We find that the power spectra of the turbulent plasma and electromagnetic fluctuations show multiple power-law intervals down to scales smaller than the electron gyroradius. Magnetic reconnection is observed to occur in correspondence of current sheets with a thickness of the order of the electron inertial length, which form and shrink owing to interacting ion-scale vortices. In some cases, both ion and electron outflows are observed (the classic reconnection scenario), while in others-typically for the shortest current sheets-only electron jets are present ("electron-only reconnection"). At the onset of reconnection, the electron temperature starts to increase and a strong parallel temperature anisotropy develops. This suggests that in strong turbulence electron-scale coherent structures may play a significant role for electron heating, as impulsive and localized phenomena such as magnetic reconnection can efficiently transfer energy from the electromagnetic fields to particles.

Keywords: space plasmas; plasma astrophysics; solar wind; interplanetary turbulence
29P/Schwassmann-Wachmann 1: A Rosetta Stone for Amorphous Water Ice and CO ↔ CO2 Conversion in Centaurs and Comets?

Centaur 29P/Schwassmann-Wachmann 1 (SW1) is a highly active object orbiting in the transitional "Gateway" region between the Centaur and Jupiter-family comet (JFC) regions. SW1 is unique among the Centaurs in that it experiences quasi-regular major outbursts and produces CO emission continuously; however, the source of the CO is unclear. We argue that, due to its very large size (~32 km radius), SW1 is likely still responding, via amorphous water ice (AWI) conversion to crystalline water ice (CWI), to the "sudden" change in its external thermal environment produced by its Myrs-long dynamical migration from the Kuiper Belt to its current location at the inner edge of the Centaur region. It is this conversion process that is the source of the abundant CO and dust released from the object during its quiescent and outburst phases. If correct, these arguments have a number of important predictions testable via remote sensing and in situ spacecraft characterization, including the quick release on Myr timescales of CO from AWI conversion for any few kilometer-scale scattered disk Kuiper Belt Objects transiting into the inner system; that to date SW1 has only converted between 50% and 65% of its nuclear AWI to CWI; that volume changes on AWI conversion could have caused subsidence and cave-ins, but not significant mass wasting or crater loss; that SW1's coma should contain abundant amounts of CWI+CO2 "dust" particles; and that when SW1 transits into the inner system within the next 10,000 yr, it will be a very different kind of JFC.

Keywords: centaur group; comet surfaces; comet volatiles; comet interiors; surface ices; comet nuclei; ice formation