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Published By American Astronomical Society

1538-4357, 0004-637x
Updated Wednesday, 19 January 2022

2022 ◽  
Vol 924 (1) ◽  
pp. 3
Author(s):  
Ziyan Xu ◽  
Xue-Ning Bai

Abstract Planetesimal formation is a crucial yet poorly understood process in planet formation. It is widely believed that planetesimal formation is the outcome of dust clumping by the streaming instability (SI). However, recent analytical and numerical studies have shown that the SI can be damped or suppressed by external turbulence, and at least the outer regions of protoplanetary disks are likely weakly turbulent due to magneto-rotational instability (MRI). We conduct high-resolution local shearing-box simulations of hybrid particle-gas magnetohydrodynamics (MHD), incorporating ambipolar diffusion as the dominant nonideal MHD effect, applicable to outer disk regions. We first show that dust backreaction enhances dust settling toward the midplane by reducing turbulence correlation time. Under modest level of MRI turbulence, we find that dust clumping is in fact easier than the conventional SI case, in the sense that the threshold of solid abundance for clumping is lower. The key to dust clumping includes dust backreaction and the presence of local pressure maxima, which in our work is formed by the MRI zonal flows overcoming background pressure gradient. Overall, our results support planetesimal formation in the MRI-turbulent outer protoplanetary disks, especially in ring-like substructures.


2022 ◽  
Vol 924 (1) ◽  
pp. 6
Author(s):  
Julien Salmon ◽  
Robin M. Canup

Abstract We investigate aspects of the co-accretion + giant impact scenario proposed by Morbidelli et al. (2012) for the origin of the Uranian satellites. In this model, a regular satellite system formed during gas accretion is impulsively destabilized by a Uranus-tipping impact, producing debris that ultimately re-orients to the planet’s new equatorial plane and re-accumulates into Uranus’ current large moons. We first investigate the nodal randomization of a disk of debris resulting from disruptive collisions between the hypothesized prior satellites. Consistent with Morbidelli et al., we find that an impact-generated interior c-disk with mass ≥10−2 Uranus masses is needed to cause sufficient nodal randomization to appropriately realign the outer debris disk. We then simulate the reaccumulation of the outer debris disk into satellites and find that disks with larger initial radii are needed to produce an outer debris disk that extends to Oberon’s distance, and that Uranus’ obliquity prior to the giant impact must have been substantial, ≥40°, if its original co-accreted satellite system was broadly similar in radial scale to those at Jupiter and Saturn today. Finally, we explore the subsequent evolution of a massive, water-dominated inner c-disk as it condenses, collisionally spreads, and spawns new moons beyond the Roche limit. We find that intense tidal dissipation in Uranus (i.e., ( Q / k 2 ) U ≤ 10 2 ) is needed to prevent large icy moons spawned from the inner disk from expanding beyond the synchronous orbit, where they would be long lived and inconsistent with the lack of massive inner moons at Uranus today. We conclude that while a co-accretion + giant impact is viable it requires rather specific conditions.


2022 ◽  
Vol 924 (2) ◽  
pp. 64
Author(s):  
Alessandro Ignesti ◽  
Benedetta Vulcani ◽  
Bianca M. Poggianti ◽  
Rosita Paladino ◽  
Timothy Shimwell ◽  
...  

Abstract Ram pressure stripping is a crucial evolutionary driver for cluster galaxies. It is thought to be able to accelerate the evolution of their star formation, trigger the activity of their central active galactic nucleus (AGN) and the interplay between galactic and environmental gas, and eventually dissipate their gas reservoirs. We explored the outcomes of ram pressure stripping by studying the nonthermal radio emission of the jellyfish galaxy JW100 in the cluster A2626 (z = 0.055), by combining LOw Frequency Array, MeerKAT, and Very Large Array observations from 0.144 to 5.5 GHz. We studied the integrated spectra of the stellar disk, the stripped tail, and the AGN; mapped the spectral index over the galaxy; and constrained the magnetic field intensity to between 11 and 18 μG in the disk and <10 μG in the tail. The stellar disk radio emission is dominated by a radiatively old plasma, likely related to an older phase of a high star formation rate. This suggests that the star formation was quickly quenched by a factor of 4 in a few 107 yr. The radio emission in the tail is consistent with the stripping scenario, where the radio plasma that originally accelerated in the disk is subsequently displaced in the tail. The morphology of the radio and X-ray emissions supports the scenario of the accretion of magnetized environmental plasma onto the galaxy. The AGN nonthermal spectrum indicates that relativistic electron acceleration may have occurred simultaneously with a central ionized gas outflow, thus suggesting a physical connection between the two processes.


2022 ◽  
Vol 924 (2) ◽  
pp. 41
Author(s):  
Bernard J. Vasquez ◽  
Sergei A. Markovskii ◽  
Charles W. Smith

Abstract Three-dimensional hybrid kinetic simulations are conducted with particle protons and warm fluid electrons. Alfvénic fluctuations initialized at large scales and with wavevectors that are highly oblique with respect to the background magnetic field evolve into a turbulent energy cascade that dissipates at proton kinetic scales. Accompanying the proton scales is a spectral magnetic helicity signature with a peak in magnitude. A series of simulation runs are made with different large-scale cross helicity and different initial fluctuation phases and wavevector configurations. From the simulations a so-called total magnetic helicity peak is evaluated by summing contributions at a wavenumber perpendicular to the background magnetic field. The total is then compared with the reduced magnetic helicity calculated along spacecraft-like trajectories through the simulation box. The reduced combines the helicity from different perpendicular wavenumbers and depends on the sampling direction. The total is then the better physical quantity to characterize the turbulence. On average the ratio of reduced to total is 0.45. The total magnetic helicity and the reduced magnetic helicity show intrinsic variability based on initial fluctuation conditions. This variability can contribute to the scatter found in the observed distribution of solar wind reduced magnetic helicity as a function of cross helicity.


2022 ◽  
Vol 924 (1) ◽  
pp. 27
Author(s):  
Joseph Patterson ◽  
Jonathan Kemp ◽  
Berto Monard ◽  
Gordon Myers ◽  
Enrique de Miguel ◽  
...  

Abstract We present a study of the orbital light curves of the recurrent nova IM Normae since its 2002 outburst. The broad “eclipses” recur with a 2.46 hr period, which increases on a timescale of 1.28(16) × 106 yr. Under the assumption of conservative mass transfer, this suggests a rate near 10−7 M ⊙ yr−1, and this agrees with the estimated accretion rate of the postnova, based on our estimate of luminosity. IM Nor appears to be a close match to the famous recurrent nova T Pyxidis. Both stars appear to have very high accretion rates, sufficient to drive the recurrent-nova events. Both have quiescent light curves, which suggest strong heating of the low-mass secondary, and very wide orbital minima, which suggest obscuration of a large “corona” around the primary. And both have very rapid orbital period increases, as expected from a short-period binary with high mass transfer from the low-mass component. These two stars may represent a final stage of nova—and cataclysmic variable—evolution, in which irradiation-driven winds drive a high rate of mass transfer, thereby evaporating the donor star in a paroxysm of nova outbursts.


2022 ◽  
Vol 924 (2) ◽  
pp. 47
Author(s):  
Abhishek Paswan ◽  
Kanak Saha ◽  
Claus Leitherer ◽  
Daniel Schaerer

Abstract Using integral field unit spectroscopy, we present here the spatially resolved morphologies of [S ii]λ6717,6731/Hα and [S ii]λ6717,6731/[O iii]λ5007 emission line ratios for the first time in a blueberry Lyα emitter (BBLAE) at z ∼ 0.047. Our derived morphologies show that the extreme starburst region of the BBLAE, populated by young (≤10 Myr), massive Wolf–Rayet stars, is [S ii] deficient, while the rest of the galaxy is [S ii] enhanced. We infer that the extreme starburst region is density-bounded (i.e., optically thin to ionizing photons), and the rest of the galaxy is ionization-bounded, indicating a Blister-type morphology. We find that the previously reported small escape fraction (10%) of Lyα photons is from our identified density-bounded H ii region of the BBLAE. This escape fraction is likely constrained by a porous dust distribution. We further report a moderate correlation between [S ii] deficiency and inferred Lyman continuum (LyC) escape fraction using a sample of confirmed LyC leakers studied in the literature, including the BBLAE studied here. The observed correlation also reveals its dependency on the stellar mass and gas-phase metallicity of the leaky galaxies. Finally, the future scope and implications of our work are discussed in detail.


2022 ◽  
Vol 924 (2) ◽  
pp. 52
Author(s):  
Mario Riquelme ◽  
Alvaro Osorio ◽  
Daniel Verscharen ◽  
Lorenzo Sironi

Abstract Using 2D particle-in-cell plasma simulations, we study electron acceleration by temperature anisotropy instabilities, assuming conditions typical of above-the-loop-top sources in solar flares. We focus on the long-term effect of T e,⊥ > T e,∥ instabilities by driving the anisotropy growth during the entire simulation time through imposing a shearing or a compressing plasma velocity (T e,⊥ and T e,∥ are the temperatures perpendicular and parallel to the magnetic field). This magnetic growth makes T e,⊥/T e,∥ grow due to electron magnetic moment conservation, and amplifies the ratio ω ce/ω pe from ∼0.53 to ∼2 (ω ce and ω pe are the electron cyclotron and plasma frequencies, respectively). In the regime ω ce/ω pe ≲ 1.2–1.7, the instability is dominated by oblique, quasi-electrostatic modes, and the acceleration is inefficient. When ω ce/ω pe has grown to ω ce/ω pe ≳ 1.2–1.7, electrons are efficiently accelerated by the inelastic scattering provided by unstable parallel, electromagnetic z modes. After ω ce/ω pe reaches ∼2, the electron energy spectra show nonthermal tails that differ between the shearing and compressing cases. In the shearing case, the tail resembles a power law of index α s ∼ 2.9 plus a high-energy bump reaching ∼300 keV. In the compressing runs, α s ∼ 3.7 with a spectral break above ∼500 keV. This difference can be explained by the different temperature evolutions in these two types of simulations, suggesting that a critical role is played by the type of anisotropy driving, ω ce/ω pe, and the electron temperature in the efficiency of the acceleration.


2022 ◽  
Vol 924 (2) ◽  
pp. 46
Author(s):  
Lia Medeiros ◽  
Chi-Kwan Chan ◽  
Ramesh Narayan ◽  
Feryal Özel ◽  
Dimitrios Psaltis

Abstract The Event Horizon Telescope recently captured images of the supermassive black hole in the center of the M87 galaxy, which shows a ring-like emission structure with the south side only slightly brighter than the north side. This relatively weak asymmetry in the brightness profile along the ring has been interpreted as a consequence of the low inclination of the observer (around 17° for M87), which suppresses the Doppler beaming and boosting effects that might otherwise be expected due to the nearly relativistic velocities of the orbiting plasma. In this work, we use a large suite of general relativistic magnetohydrodynamic simulations to reassess the validity of this argument. By constructing explicit counterexamples, we show that low inclination is a sufficient but not necessary condition for images to have low brightness asymmetry. Accretion flow models with high accumulated magnetic flux close to the black hole horizon (the so-called magnetically arrested disks) and low black hole spins have angular velocities that are substantially smaller than the orbital velocities of test particles at the same location. As a result, such models can produce images with low brightness asymmetry even when viewed edge on.


2022 ◽  
Vol 924 (2) ◽  
pp. 87
Author(s):  
J. Christopher Mihos ◽  
Patrick R. Durrell ◽  
Elisa Toloba ◽  
Patrick Côté ◽  
Laura Ferrarese ◽  
...  

Abstract We use deep Hubble Space Telescope imaging to derive a distance to the Virgo Cluster ultradiffuse galaxy (UDG) VCC 615 using the tip of the red giant branch (TRGB) distance estimator. We detect 5023 stars within the galaxy, down to a 50% completeness limit of F814W ≈ 28.0, using counts in the surrounding field to correct for contamination due to background sources and Virgo intracluster stars. We derive an extinction-corrected F814W tip magnitude of m tip , 0 = 27.19 − 0.05 + 0.07 , yielding a distance of d = 17.7 − 0.4 + 0.6 Mpc. This places VCC 615 on the far side of the Virgo Cluster (d Virgo = 16.5 Mpc), at a Virgocentric distance of 1.3 Mpc and near the virial radius of the main body of Virgo. Coupling this distance with the galaxy’s observed radial velocity, we find that VCC 615 is on an outbound trajectory, having survived a recent passage through the inner parts of the cluster. Indeed, our orbit modeling gives a 50% chance the galaxy passed inside the Virgo core (r < 620 kpc) within the past gigayear, although very close passages directly through the cluster center (r < 200 kpc) are unlikely. Given VCC 615's undisturbed morphology, we argue that the galaxy has experienced no recent and sudden transformation into a UDG due to the cluster potential, but rather is a long-lived UDG whose relatively wide orbit and large dynamical mass protect it from stripping and destruction by the Virgo cluster tides. Finally, we also describe the serendipitous discovery of a nearby Virgo dwarf galaxy projected 90″ (7.2 kpc) away from VCC 615.


2022 ◽  
Vol 924 (2) ◽  
pp. 93
Author(s):  
J. Andrew Casey-Clyde ◽  
Chiara M. F. Mingarelli ◽  
Jenny E. Greene ◽  
Kris Pardo ◽  
Morgan Nañez ◽  
...  

Abstract The nanohertz gravitational wave background (GWB) is believed to be dominated by GW emission from supermassive black hole binaries (SMBHBs). Observations of several dual-active galactic nuclei (AGN) strongly suggest a link between AGN and SMBHBs, given that these dual-AGN systems will eventually form bound binary pairs. Here we develop an exploratory SMBHB population model based on empirically constrained quasar populations, allowing us to decompose the GWB amplitude into an underlying distribution of SMBH masses, SMBHB number density, and volume enclosing the GWB. Our approach also allows us to self-consistently predict the number of local SMBHB systems from the GWB amplitude. Interestingly, we find the local number density of SMBHBs implied by the common-process signal in the NANOGrav 12.5-yr data set to be roughly five times larger than previously predicted by other models. We also find that at most ∼25% of SMBHBs can be associated with quasars. Furthermore, our quasar-based approach predicts ≳95% of the GWB signal comes from z ≲ 2.5, and that SMBHBs contributing to the GWB have masses ≳108 M ⊙. We also explore how different empirical galaxy–black hole scaling relations affect the local number density of GW sources, and find that relations predicting more massive black holes decrease the local number density of SMBHBs. Overall, our results point to the important role that a measurement of the GWB will play in directly constraining the cosmic population of SMBHBs, as well as their connections to quasars and galaxy mergers.


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