733 results match your criteria Astrophysical Journal[Journal]

On the Dynamics of Overshooting Convection in Spherical Shells: Effect of Density Stratification and Rotation.

Astrophys J 2021 Dec;923(1)

Southwest Research Institute, Department of the Space Studies, Boulder, CO 80302, USA.

Overshooting of turbulent motions from convective regions into adjacent stably stratified zones plays a significant role in stellar interior dynamics, as this process may lead to mixing of chemical species and contribute to the transport of angular momentum and magnetic fields. We present a series of fully nonlinear, three-dimensional (3D) anelastic simulations of overshooting convection in a spherical shell that are focused on the dependence of the overshooting dynamics on the density stratification and the rotation, both key ingredients in stars that however have not been studied systematically together via global simulations. We demonstrate that the overshoot lengthscale is not simply a monotonic function of the density stratification in the convective region, but instead it depends on the ratio of the density stratifications in the two zones. Read More

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December 2021

FOXSI-2 Solar Microflares. II. Hard X-ray Imaging Spectroscopy and Flare Energetics.

Astrophys J 2021 May 19;913(1). Epub 2021 May 19.

Space Sciences Laboratory, University of California at Berkeley, Berkeley, CA, USA.

We study the nature of energy release and transfer for two sub-A class solar microflares observed during the second Focusing Optics X-ray Solar Imager (FOXSI-2) sounding rocket flight on 2014 December 11. FOXSI is the first solar-dedicated instrument to utilize focusing optics to image the Sun in the hard X-ray (HXR) regime, sensitive to energies of 4-20 keV. Through spectral analysis of the microflares using an optically thin isothermal plasma model, we find evidence for plasma heated to ~10 MK and emission measures down to ~10 cm. Read More

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NuSTAR Observation of Energy Release in 11 Solar Microflares.

Astrophys J 2021 Feb 9;908(1). Epub 2021 Feb 9.

University of Glasgow, Glasgow, UK.

Solar flares are explosive releases of magnetic energy. Hard X-ray (HXR) flare emission originates from both hot (millions of Kelvin) plasma and nonthermal accelerated particles, giving insight into flare energy release. The Nuclear Spectroscopic Telescope ARray (NuSTAR) utilizes direct-focusing optics to attain much higher sensitivity in the HXR range than that of previous indirect imagers. Read More

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February 2021

Signatures of Recent Cosmic-Ray Acceleration in the High-latitude Gamma-Ray Sky.

Astrophys J 2021 Aug 11;917(1). Epub 2021 Aug 11.

W.W. Hansen Experimental Physics Laboratory and Kavli Institute for Particle Astrophysics and Cosmology, Stanford University, Stanford, CA 94305, USA.

Cosmic-ray (CR) sources temporarily enhance the relativistic particle density in their vicinity over the background distribution accumulated from the Galaxy-wide past injection activity and propagation. If individual sources are close enough to the solar system, their localized enhancements may present as features in the measured spectra of the CRs and in the associated secondary electromagnetic emissions. Large-scale loop-like structures visible in the radio sky are possible signatures of such nearby CR sources. Read More

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Laboratory Detection of Cyanoacetic Acid: A Jet-Cooled Rotational Study.

Astrophys J 2021 Jul 8;915(2). Epub 2021 Jul 8.

Grupo de Espectroscopía Molecular (GEM), Edificio Quifima, Área de Química-Física, Laboratorios de Espectroscopía y Bioespectroscopía, Parque Científico UVa, Unidad Asociada CSIC, E-47011 Valladolid, Spain.

Herein we present a laboratory rotational study of cyanoacetic acid (CH(CN)C(O)OH), an organic acid as well as a -CN bearing molecule, that is a candidate molecular system to be detected in the interstellar medium (ISM). Our investigation aims to provide direct experimental frequencies of cyanoacetic acid to guide its eventual astronomical search in low-frequency surveys. Using different jet-cooled rotational spectroscopic techniques in the time domain, we have determined a precise set of the relevant rotational spectroscopic constants, including the N nuclear quadrupole coupling constants for the two distinct structures, - and - cyanoacetic acid. Read More

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On a Possible Solution to the Tidal Realignment Problem for Hot Jupiters.

Astrophys J 2021 Jun 15;914(1). Epub 2021 Jun 15.

Department of Physics, The University of Texas at Dallas, 800 West Campbell Road, Richardson, TX 75080-3021, USA.

Hot stars with hot Jupiters have a wide range of obliquities, while cool stars with hot Jupiters tend to have low obliquities. An enticing explanation for this pattern is tidal realignment of the cool host stars, although this explanation assumes that obliquity damping occurs faster than orbital decay, an assumption that needs further exploration. Here we revisit this tidal realignment problem, building on previous work identifying a low-frequency component of the time-variable tidal potential that affects the obliquity but not the orbital separation. Read More

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The Discovery of a Low-energy Excess in Cosmic-Ray Iron: Evidence of the Past Supernova Activity in the Local Bubble.

Astrophys J 2021 May 18;913(1). Epub 2021 May 18.

INFN, Milano-Bicocca, Milano, Italy.

Since its launch, the Alpha Magnetic Spectrometer-02 (AMS-02) has delivered outstanding quality measurements of the spectra of cosmic-ray (CR) species ( , , and nuclei, H-O, Ne, Mg, Si) which resulted in a number of breakthroughs. One of the latest long-awaited surprises is the spectrum of Fe just published by AMS-02. Because of the large fragmentation cross section and large ionization energy losses, most of CR iron at low energies is local and may harbor some features associated with relatively recent supernova (SN) activity in the solar neighborhood. Read More

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The TeV Cosmic-Ray Bump: A Message from the Epsilon Indi or Epsilon Eridani Star?

Astrophys J 2021 Apr 28;911(2). Epub 2021 Apr 28.

Hansen Experimental Physics Laboratory and Kavli Institute for Particle Astrophysics and Cosmology, Stanford University, Stanford, CA 94305, USA.

A recently observed bump in the cosmic-ray (CR) spectrum from 0.3 to 30 TV is likely caused by a stellar bow shock that reaccelerates preexisting CRs, which further propagate to the Sun along the magnetic field lines. Along their way, these particles generate an Iroshnikov-Kraichnan (I-K) turbulence that controls their propagation and sustains the bump. Read More

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Elemental abundances of major elements in the solar wind as measured in Genesis targets and implications on solar wind fractionation.

Astrophys J 2021 Jan;907(1)

California Institute of Technology, Division of Geological and Planetary Sciences, Pasadena CA 91125, USA.

We present elemental abundance data of C, N, O, Na, Mg, Al, Ca, and Cr in Genesis silicon targets. For Na, Mg, Al, and Ca, data from three different SW regimes are also presented. Data were obtained by backside depth profiling using Secondary Ion Mass Spectrometry. Read More

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January 2021

Probing the Solar Meridional Circulation Using Fourier Legendre Decomposition.

Astrophys J 2021 Apr 15;911(1). Epub 2021 Apr 15.

National Center for Atmospheric Research, HAO Division, 3080 Center Green Drive, Boulder, CO 80301, USA.

We apply the helioseismic methodology of Fourier Legendre decomposition to 88 months of Dopplergrams obtained by the Helioseismic and Magnetic Imager (HMI) as the basis of inferring the depth variation of the mean meridional flow, as averaged between 20° and 60° latitude and in time, in both the northern and southern hemispheres. We develop and apply control procedures designed to assess and remove center-to-limb artifacts using measurements obtained by performing the analysis with respect to artificial poles at the east and west limbs. Forward modeling is carried out using sensitivity functions proportional to the mode kinetic energy density to evaluate the consistency of the corrected frequency shifts with models of the depth variation of the meridional circulation in the top half of the convection zone. Read More

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Scaling Laws for Dynamic Solar Loops.

Astrophys J 2020 Dec 27;904(2). Epub 2020 Nov 27.

Department of Physics & Astronomy, Western Kentucky University, Bowling Green, KY 42101, USA.

The scaling laws which relate the peak temperature and volumetric heating rate to the pressure and length for static coronal loops were established over 40 years ago; they have proved to be of immense value in a wide range of studies. Here we extend these scaling laws to loops, where enthalpy flux becomes important to the energy balance, and study impulsive heating/filling characterized by upward enthalpy flows. We show that for collision-dominated thermal conduction, the functional dependencies of the scaling laws are the same as for the static case, when the radiative losses scale as , but with a different constant of proportionality that depends on the Mach number of the flow. Read More

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December 2020

Silicon and Hydrogen Chemistry under Laboratory Conditions Mimicking the Atmosphere of Evolved Stars.

Astrophys J 2021 Jan 5;906(1). Epub 2021 Jan 5.

Instituto de Ciencia de Materiales de Madrid (ICMM. CSIC). Materials Science Factory. Structure of Nanoscopic Systems Group. c/Sor Juana Inés de la Cruz 3, 28049 Cantoblanco, Madrid, Spain.

Silicon is present in interstellar dust grains, meteorites and asteroids, and to date thirteen silicon-bearing molecules have been detected in the gas-phase towards late-type stars or molecular clouds, including silane and silane derivatives. In this work, we have experimentally studied the interaction between atomic silicon and hydrogen under physical conditions mimicking those at the atmosphere of evolved stars. We have found that the chemistry of Si, H and H efficiently produces silane (SiH), disilane (SiH) and amorphous hydrogenated silicon (a-Si:H) grains. Read More

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January 2021

From Supernova to Remnant: Tracking the Evolution of the Oldest Known X-Ray Supernovae.

Astrophys J 2020 Oct 29;901(2). Epub 2020 Sep 29.

Department of Astronomy and Astrophysics, University of Chicago 5640 S Ellis Avenue, Chicago, IL 60637, USA.

Core-collapse supernovae (SNe) expand into a medium created by winds from the pre-SN progenitor. The SN explosion and resulting shock wave(s) heat up the surrounding plasma, giving rise to thermal X-ray emission, which depends on the density of the emitting material. Tracking the variation of the X-ray luminosity over long periods of time thus allows for investigation of the kinematics of the SN shock waves, the structure of the surrounding medium, and the nature of the progenitor star. Read More

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October 2020

The Chemistry of Cosmic Dust Analogues from C, C, and CH in C-Rich Circumstellar Envelopes.

Astrophys J 2020 Jun 2;895(2). Epub 2020 Jun 2.

Instituto de Ciencia de Materiales de Madrid (ICMM. CSIC). Materials Science Factory. Structure of Nanoscopic Systems Group. c/ Sor Juana Inés de la Cruz 3, 28049 Cantoblanco, Madrid, Spain.

Interstellar carbonaceous dust is mainly formed in the innermost regions of circumstellar envelopes around carbon-rich asymptotic giant branch (AGB) stars. In these highly chemically stratified regions, atomic and diatomic carbon, along with acetylene are the most abundant species after H and CO. In a previous study, we addressed the chemistry of carbon (C and C) with H showing that acetylene and aliphatic species form efficiently in the dust formation region of carbon-rich AGBs whereas aromatics do not. Read More

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Subsecond Spikes in Fermi GBM X-Ray Flux as a Probe for Solar Flare Particle Acceleration.

Astrophys J 2020 Nov 2;903(1). Epub 2020 Nov 2.

The School of Physics and Astronomy, University of Minnesota, USA.

Solar flares are known to release a large amount of energy into accelerating electrons. Studying small timescale (⩽2s) fluctuations in nonthermal X-ray flux offers the opportunity to probe the nature of those acceleration mechanisms. By comparing the durations, differences in timing between energy bands, and the periodicity of these spikes against the relevant timescales called for by various acceleration mechanisms, a test for each mechanism's validity can be made. Read More

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November 2020

Albedos, Equilibrium Temperatures, and Surface Temperatures of Habitable Planets.

Astrophys J 2019 Oct 14;884(1). Epub 2019 Oct 14.

NASA Goddard Institute for Space Studies, 2880 Broadway, New York, NY 10025, USA.

The potential habitability of known exoplanets is often categorized by a nominal equilibrium temperature assuming a Bond albedo of either ∼0.3, similar to Earth, or 0. As an indicator of habitability, this leaves much to be desired, because albedos of other planets can be very different, and because surface temperature exceeds equilibrium temperature due to the atmospheric greenhouse effect. Read More

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October 2019

Non-Maxwellian velocity distribution functions for Coulombic systems out of equilibrium.

Brent M Randol

Astrophys J 2019 Oct 4;883(2). Epub 2019 Oct 4.

NASA's Goddard Space Flight Center Heliophysics Science Division, 8800 Greenbelt Rd., Greenbelt, MD, 20771, USA.

The velocity distribution function (VDF) of ions in the solar wind, as observed by spacecraft at 1 AU and elsewhere in the heliosphere, exhibits a consistent trend: at low energies in the solar wind frame, the distribution is largely Maxwellian-the core; at higher but still modest energies in the solar wind frame, the distribution follows a power law ( ∝ , where is the VDF, is the speed in the solar wind frame, and is an arbitrary spectral index parameter)-the tail-with a spectral index of ≈ 5 being extremely common. Several theories have been proposed to explain this common index. Among these theories is that the tail is a natural consequence of an ensemble of particles obeying Coulomb's law (Randol & Christian 2014, 2016). Read More

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October 2019

Revisiting The Spectral and Timing Properties of NGC 4151.

Astrophys J 2019 Oct 8;884(1). Epub 2019 Oct 8.

Department of Physics, Wayne State University, Detroit, MI 48201, USA.

NGC 4151 is the brightest Seyfert 1 nucleus in X-rays. It was the first object to show short time delays in the Fe K band, which were attributed to relativistic reverberation, providing a new tool for probing regions at the black hole scale. Here, we report the results of a large XMM-Newton campaign in 2015 to study these short delays further. Read More

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October 2019

A Census of Star Formation in the Outer Galaxy: The SMOG Field.

Astrophys J 2019 Jul 18;880(1). Epub 2019 Jul 18.

Center for Astrophysics Harvard & Smithsonian, 60 Garden St., Cambridge MA 02138, USA.

In this paper we undertake a study of the 21 deg SMOG field, a cryogenic mission Legacy program to map a region of the outer Milky Way toward the Perseus and outer spiral arms with the IRAC and MIPS instruments. We identify 4648 YSOs across the field. Using the DBSCAN method, we identify 68 clusters or aggregations of YSOs in the region, having eight or more members. Read More

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Testing The Lamp-Post and Wind Reverberation Models with XMM-Newton Observations of NGC 5506.

Astrophys J 2020 Apr;893(2)

Centre for Extragalactic Astronomy, Department of Physics, University of Durham, South Road, Durham, DH1 3LE, UK.

The lamp-post geometry is often used to model X-ray data of accreting black holes. Despite its simple assumptions, it has proven to be powerful in inferring fundamental black hole properties such as the spin. Early results of X-ray reverberations showed support for such a simple picture, though wind-reverberation models have also been shown to explain the observed delays. Read More

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Multiwavelength Light Curves of Two Remarkable Sagittarius A* Flares.

Astrophys J 2019 Sep 30;864(1). Epub 2019 Aug 30.

Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, MS-65, Cambridge, MA 02138, USA.

Sgr A*, the supermassive black hole (SMBH) at the center of our Milky Way Galaxy, is known to be a variable source of X-ray, near-infrared (NIR), and submillimeter radiation and therefore a prime candidate to study the electromagnetic radiation generated by mass accretion flow onto a black hole and/or a related jet. Disentangling the power source and emission mechanisms of this variability is a central challenge to our understanding of accretion flows around SMBHs. Simultaneous multiwavelength observations of the flux variations and their time correlations can play an important role in obtaining a better understanding of possible emission mechanisms and their origin. Read More

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September 2019

Magnetic Field Geometry and Composition Variation in Slow Solar Winds: The Case of Sulfur.

Astrophys J 2020 May 21;895(1). Epub 2020 May 21.

Space Science Division, Code 7684, Naval Research Laboratory, Washington DC 20375, USA.

We present an examination of the first ionization potential (FIP) fractionation scenario, invoking the ponderomotive force in the chromosphere and its implications for the source(s) of slow-speed solar winds by using observations from The Advanced Composition Explorer (ACE). Following a recent conjecture that the abundance enhancements of intermediate FIP elements, S, P, and C, in slow solar winds can be explained by the release of plasma fractionated on open fields, though from regions of stronger magnetic field than usually associated with fast solar wind source regions, we identify a period in 2008 containing four solar rotation cycles that show repeated pattern of sulfur abundance enhancement corresponding to a decrease in solar wind speed. We identify the source regions of these slow winds in global magnetic field models, and find that they lie at the boundaries between a coronal hole and its adjacent active region, with origins in both closed and open initial field configurations. Read More

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Element Abundances: A New Diagnostic for the Solar Wind.

Astrophys J 2019 Jul 15;879(2):124. Epub 2019 Jul 15.

Space Science Division, Code 7685, Naval Research Laboratory, Washington, DC 20375, USA.

We examine the different element abundances exhibited by the closed loop solar corona and the slow speed solar wind. Both are subject to the first ionization potential (FIP) effect, the enhancement in coronal abundance of elements with FIP below 10 eV (e.g. Read More

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Beyond Optical Depth: Future Determination of Ionization History from the Cosmic Microwave Background.

Astrophys J 2020 Feb 30;889(2). Epub 2020 Jan 30.

JHU Department of Physics and Astronomy, 3701 San Martin Drive, Baltimore MD, 21218, USA.

We explore the fundamental limits to which reionization histories can be constrained using only large-scale cosmic microwave background (CMB) anisotropy measurements. The redshift distribution of the fractional ionization () affects the angular distribution of CMB polarization. We project constraints on the reionization history of the universe using low-noise full-sky temperature and E-mode measurements of the CMB. Read More

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February 2020

Analytic Calculation of Covariance between Cosmological Parameters from Correlated Data Sets, with an Application to SPTpol.

Astrophys J 2020 Jan 31;888(1). Epub 2019 Dec 31.

Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218, USA.

Consistency checks of cosmological data sets are an important tool because they may suggest systematic errors or the type of modifications to ΛCDM necessary to resolve current tensions. In this work, we derive an analytic method for calculating the level of correlations between model parameters from two correlated cosmological data sets, which complements more computationally expensive simulations. This method is an extension of the Fisher analysis that assumes a Gaussian likelihood and a known data covariance matrix. Read More

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January 2020

Modeling a Carrington-scale Stellar Superflare and Coronal Mass Ejection from .

Astrophys J 2019 Aug 30;880(2). Epub 2019 Jul 30.

Space Sciences Laboratory, University of California-Berkeley, Berkeley, CA 94720, USA.

Observations from the mission have revealed frequent superflares on young and active solar-like stars. Superflares result from the large-scale restructuring of stellar magnetic fields, and are associated with the eruption of coronal material (a coronal mass ejection, or CME) and energy release that can be orders of magnitude greater than those observed in the largest solar flares. These catastrophic events, if frequent, can significantly impact the potential habitability of terrestrial exoplanets through atmospheric erosion or intense radiation exposure at the surface. Read More

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Gravity-dominated Collisions: A Model for the Largest Remnant Masses with Treatment for "Hit and Run" and Density Stratification.

Astrophys J 2020 Mar 24;892(1):40. Epub 2020 Mar 24.

Physikalisches Institut, Universität Bern, Sidlerstrasse 5, CH-3012, Bern, Switzerland.

We develop empirical relationships for the accretion and erosion of colliding gravity-dominated bodies of various compositions under conditions expected in late-stage solar system formation. These are fast, easily coded relationships based on a large database of smoothed particle hydrodynamics (SPH) simulations of collisions between bodies of different compositions, including those that are water rich. The accuracy of these relations is also comparable to the deviations of results between different SPH codes and initial thermal/rotational conditions. Read More

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Testing Densities and Refractive Indices of Extraterrestrial Ice Components Using Molecular Structures - Organic Compounds and Molar Refractions.

Astrophys J 2020 Mar 28;891(1). Epub 2020 Feb 28.

Astrochemistry Laboratory, NASA Goddard Space Flight Center, Greenbelt, MD, 20771 USA.

The use of infrared spectra to determine molecular abundances of icy astronomical objects and to study their chemistry requires laboratory measurements of reference spectra and related quantities, such as the index of refraction () and density () of candidate ices. Here we present new and measurements on ices involving C-, H-, and O-containing compounds, both acyclic and cyclic, representing seven chemical families. We examine the results in a way that is rare in the astrochemical literature, namely one in which data from an ice formed from molecules of a particular chemical family are compared to measurements on another member of the same family, such as of a homologous series or a pair of isomers. Read More

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Analysis of Small-scale Magnetic Flux Ropes Covering the Whole Mission.

Astrophys J 2019 13;881(1). Epub 2019 Aug 13.

Department of Space Science, The University of Alabama in Huntsville, Huntsville, AL 35805, USA.

Small-scale magnetic flux ropes in the solar wind have been studied for decades via both simulation and observation. Statistical analysis utilizing various in situ spacecraft measurements is the main observational approach, which helps investigate the generation and evolution of these small-scale structures. In this study, we extend the automated detection of small-scale flux ropes based on the Grad-Shafranov reconstruction to the complete data set of in situ measurements of the spacecraft. Read More

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Turbulence in the Local Interstellar Medium and the Ribbon.

Astrophys J 2020 Jan;888(1)

Department of Mathematics and Statistics, University of Waikato, Hamilton, New Zealand.

The effects of turbulence in the very local interstellar medium (VLISM) have been proposed by Giacalone & Jokipii (2015) to be important in determining the structure of the () ribbon via particle trapping by magnetic mirroring. We further explore this effect by simulating the motion of charged particles in a turbulent magnetic field superposed on a large-scale mean field, which we consider to be either spatially-uniform or a draped field derived from a 3D MHD simulation. We find that the ribbon is not double-peaked, in contrast to Giacalone & Jokipii (2015). Read More

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January 2020