Hasil untuk "Astrophysics"

Michael Perryman

These lecture notes are intended for an advanced astrophysics course on Stellar Structure and Evolution given at Utrecht University (NS-AP434M). Their goal is to provide an overview of the physics of stellar interiors and its application to the theory of stellar structure and evolution, at a level appropriate for a third-year Bachelor student or beginning Master student in astronomy. To a large extent these notes draw on the classical textbook by Kippenhahn & Weigert (1990; see below), but leaving out unnecessary detail while incorporating recent astrophysical insights and up-to-date results. At the same time I have aimed to concentrate on physical insight rather than rigorous derivations, and to present the material in a logical order, following in part the very lucid but somewhat more basic textbook by Prialnik (2000). Finally, I have borrowed some ideas from the textbooks by Hansen, Kawaler & Trimble (2004), Salaris & Cassissi (2005) and the recent book by Maeder (2009). These lecture notes are evolving and I try to keep them up to date. If you find any errors or inconsistencies, I would be grateful if you could notify me by email ( O.R.Pols@uu.nl ).

C. Bailer-Jones, J. Rybizki, M. Fouesneau et al.

Stellar distances constitute a foundational pillar of astrophysics. The publication of 1.47 billion stellar parallaxes from Gaia is a major contribution to this. Yet despite Gaia's precision, the majority of these stars are so distant or faint that their fractional parallax uncertainties are large, thereby precluding a simple inversion of parallax to provide a distance. Here we take a probabilistic approach to estimating stellar distances that uses a prior constructed from a three-dimensional model of our Galaxy. This model includes interstellar extinction and Gaia's variable magnitude limit. We infer two types of distance. The first, geometric, uses the parallax together with a direction-dependent prior on distance. The second, photogeometric, additionally uses the colour and apparent magnitude of a star, by exploiting the fact that stars of a given colour have a restricted range of probable absolute magnitudes (plus extinction). Tests on simulated data and external validations show that the photogeometric estimates generally have higher accuracy and precision for stars with poor parallaxes. We provide a catalogue of 1.47 billion geometric and 1.35 billion photogeometric distances together with asymmetric uncertainty measures. Our estimates are quantiles of a posterior probability distribution, so they transform invariably and can therefore also be used directly in the distance modulus (5log10(r)-5). The catalogue may be downloaded or queried using ADQL at various sites (see this http URL) where it can also be cross-matched with the Gaia catalogue.

L. F. Barrett

J. Allison, K. Amako, J. Apostolakis et al.

Geant4 is a software toolkit for the simulation of the passage of particles through matter. It is used by a large number of experiments and projects in a variety of application domains, including high energy physics, astrophysics and space science, medical physics and radiation protection. Over the past several years, major changes have been made to the toolkit in order to accommodate the needs of these user communities, and to efficiently exploit the growth of computing power made available by advances in technology. The adaptation of Geant4 to multithreading, advances in physics, detector modeling and visualization, extensions to the toolkit, including biasing and reverse Monte Carlo, and tools for physics and release validation are discussed here.

J. Bland-Hawthorn, O. Gerhard

Our Galaxy, the Milky Way, is a benchmark for understanding disk galaxies. It is the only galaxy whose formation history can be studied using the full distribution of stars from faint dwarfs to supergiants. The oldest components provide us with unique insight into how galaxies form and evolve over billions of years. The Galaxy is a luminous (L⋆) barred spiral with a central box/peanut bulge, a dominant disk, and a diffuse stellar halo. Based on global properties, it falls in the sparsely populated “green valley” region of the galaxy color-magnitude diagram. Here we review the key integrated, structural and kinematic parameters of the Galaxy, and point to uncertainties as well as directions for future progress. Galactic studies will continue to play a fundamental role far into the future because there are measurements that can only be made in the near field and much of contemporary astrophysics depends on such observations.

Jieun Choi, A. Dotter, C. Conroy et al.

This is the first of a series of papers presenting the Modules for Experiments in Stellar Astrophysics (MESA) Isochrones and Stellar Tracks (MIST) project, a new comprehensive set of stellar evolutionary tracks and isochrones computed using MESA, a state-of-the-art open-source 1D stellar evolution package. In this work, we present models with solar-scaled abundance ratios covering a wide range of ages ( ), masses ( ), and metallicities ( ). The models are self-consistently and continuously evolved from the pre-main sequence (PMS) to the end of hydrogen burning, the white dwarf cooling sequence, or the end of carbon burning, depending on the initial mass. We also provide a grid of models evolved from the PMS to the end of core helium burning for . We showcase extensive comparisons with observational constraints as well as with some of the most widely used existing models in the literature. The evolutionary tracks and isochrones can be downloaded from the project website at http://waps.cfa.harvard.edu/MIST/.

B. Draine

L. Rothman, I. Gordon, R. Barber et al.

S. Hawking, W. Israel

T. Sotiriou, V. Faraoni

Modified gravity theories have received increased attention lately due to combined motivation coming from high-energy physics, cosmology, and astrophysics. Among numerous alternatives to Einstein's theory of gravity, theories that include higher-order curvature invariants, and specifically the particular class of $f(R)$ theories, have a long history. In the last five years there has been a new stimulus for their study, leading to a number of interesting results. Here $f(R)$ theories of gravity are reviewed in an attempt to comprehensively present their most important aspects and cover the largest possible portion of the relevant literature. All known formalisms are presented\char22{}metric, Palatini, and metric affine\char22{}and the following topics are discussed: motivation; actions, field equations, and theoretical aspects; equivalence with other theories; cosmological aspects and constraints; viability criteria; and astrophysical applications.

K. Novoselov, A. Geim, S. Morozov et al.

Quantum electrodynamics (resulting from the merger of quantum mechanics and relativity theory) has provided a clear understanding of phenomena ranging from particle physics to cosmology and from astrophysics to quantum chemistry. The ideas underlying quantum electrodynamics also influence the theory of condensed matter, but quantum relativistic effects are usually minute in the known experimental systems that can be described accurately by the non-relativistic Schrödinger equation. Here we report an experimental study of a condensed-matter system (graphene, a single atomic layer of carbon) in which electron transport is essentially governed by Dirac's (relativistic) equation. The charge carriers in graphene mimic relativistic particles with zero rest mass and have an effective ‘speed of light’ c* ≈ 106 m s-1. Our study reveals a variety of unusual phenomena that are characteristic of two-dimensional Dirac fermions. In particular we have observed the following: first, graphene's conductivity never falls below a minimum value corresponding to the quantum unit of conductance, even when concentrations of charge carriers tend to zero; second, the integer quantum Hall effect in graphene is anomalous in that it occurs at half-integer filling factors; and third, the cyclotron mass mc of massless carriers in graphene is described by E = mcc*2. This two-dimensional system is not only interesting in itself but also allows access to the subtle and rich physics of quantum electrodynamics in a bench-top experiment.

James Binney, Scott Tremaine, D. Lynden-Bell

Lee Smolin

We describe the basic assumptions and key results of loop quantum gravity, which is a background independent approach to quantum gravity. The emphasis is on the basic physical principles and how one deduces predictions from them, at a level suitable for physicsts in other areas such as string theory, cosmology, particle physics, astrophysics and condensed matter physics. No details are given, but references are provided to guide the interested reader to the literature. The present state of knowledge is summarized in a list of 35 key results on topics including the hamiltonian and path integral quantizations, coupling to matter, extensions to supergravity and higher dimensional theories, as well as applications to black holes, cosmology and Plank scale phenomenology. We describe the near term prospects for observational tests of quantum theories of gravity and the expectations that loop quantum gravity may provide predictions for their outcomes. Finally, we provide answers to frequently asked questions and a list of key open problems.

Yongyun Chen, Qiusheng Gu, Junhui Fan et al.

Accretion supermassive black holes in the center of active galaxies usually produce “jet”-collimated bipolar outflows of relativistic particles. Magnetic fields near the black hole event horizon may play a crucial role in the formation of jets/outflows. Both theory and observation indicate that jets/outflows driven by centrally active supermassive black holes have a feedback effect on the overall properties of the host galaxies. Therefore, the magnetic field is a key ingredient for the formation and evolution of galaxies. Here, we report a clear correlation between the magnetic field of jets and star formation rate for a large sample of 96 galaxies hosting supermassive black holes, which suggests that the star formation of active galactic nuclei host galaxies may be powered by the jets.

Douglas Grion Filho, Puragra Guhathakurta, Stanley M. Rinehart V et al.

The recent discovery that red supergiants in M31 and M33 contain a weak CN spectral absorption feature at ​​​​​​∼8000 Å prompted us to analyze a large CTIO 4 m Hydra spectral database of stars in the Large Magellanic Cloud (LMC). We find 751 confirmed weak CN stars in the LMC, present an overview of their spectral and photometric properties, and compare them with the weak CN stars in M31 and M33. We use a method for automatically identifying weak CN stars based on a spectral template and find that weak CN stars are almost exclusively short-lived massive stars in the core He-burning evolutionary phase. We derive a mean main-sequence age of 40 Myr for these stars through a Bayesian approach to isochrone fitting that is consistent across all three galaxies. We then examine possible evolutionary processes and atmospheric conditions that could lead to weak CN stars using a suite of low-resolution model spectra. We find a preferred range of values for T _eff and log g in O-rich stars that would result in the CN absorption and also speculate that this feature could arise due to the N enhancement expected in fast-rotating stars, though more targeted modeling is required.

Lei Wang, Wenjun Song, Jiali Di et al.

With the increasing importance of securing images during network transmission, this paper introduces a novel image encryption algorithm that integrates a 3D chaotic system with V-shaped scrambling techniques. The proposed method begins by constructing a unique 3D chaotic system to generate chaotic sequences for encryption. These sequences determine a random starting point for V-shaped scrambling, which facilitates the transformation of image pixels into quaternary numbers. Subsequently, four innovative bit-level scrambling strategies are employed to enhance encryption strength. To further improve randomness, DNA encoding is applied to both the image and chaotic sequences, with chaotic sequences directing crossover and DNA operations. Ciphertext feedback is then utilized to propagate changes across the image, ensuring increased complexity and security. Extensive simulation experiments validate the algorithm’s robust encryption performance for grayscale images, yielding uniformly distributed histograms, near-zero correlation values, and an information entropy value of 7.9975, approaching the ideal threshold. The algorithm also features a large key space, providing robust protection against brute force attacks while effectively resisting statistical, differential, noise, and cropping attacks. These results affirm the algorithm’s reliability and security for image communication and transmission.

M. A. Cafolla, S. C. Chapman, N. W. Watkins et al.

Abstract Total Electron Content (TEC) is central to characterizing ionospheric response to solar and geomagnetic activity. Variations in TEC structures over time provide insight into underlying physical processes and inform monitoring of space weather events, which pose a risk to navigation and communication systems. JPL processed GNSS observations over 20 years provide a series of 15‐min Global Ionospheric Maps (GIMs) of spatial resolution 1°×1° longitude/latitude. We translate these into geomagnetic coordinates centered about the sub‐solar point and we isolate the top 1% of TEC values in each map to define High Density Regions (HDRs) of TEC. Image processing tools are used to develop an algorithm that detects and tracks these to compile a set of contiguous, uniquely labeled space‐time TEC HDRs. We find that HDRs naturally divide into two populations by peak area, separated by a size of 8.0×106km2, which is around the continental scale. These populations are studied for different storm conditions—quiet (Kp <4), moderate (4≤ Kp <7) and extreme (Kp ≥7): small HDRs form primarily around four magnetic latitude bands and move roughly parallel to lines of constant magnetic latitude toward later MLT. Large HDRs form around the same latitude bands but follow more complex paths. The statistical nature of these results could be used in predictive ionospheric models and identify reproducible trends on these spatial/temporal scales.

J. J. Curto, A. Segarra, J. M. Torta et al.

Abstract Geomagnetic storms produce global variations in the geomagnetic field that are measured at magnetic observatories. Roughly one half of magnetic storms are preceded by sudden increases in the horizontal component of the magnetic field world‐wide. These increases, called storm sudden commencements (SSC), produce geomagnetically induced currents and cause other space weather disturbances whose study is paramount due to the technological dependence of our society. SSC event lists date back to 1868 and provide invaluable information about interplanetary conditions over centennial time scales. Since 1975, the Service of Rapid Magnetic Variations has been responsible for the maintenance and consistency of the SSC list. Here we will review the significant changes in the definition and methods of SSC detection that have been introduced over time and will analyze and discuss whether those changes have affected the homogeneity of the SSC series. Alerted by the greatly reduced number of SSCs in solar cycle 24, we have reanalyzed SSC occurrence in the period 2006–2017. As a result, we found a 26% increase in the number of SSCs, which motivates a change in the adopted SSC definition but leaves the SSC level exceptionally low during this period. We completed the study by examining the relation and dependency of SSCs with solar sunspot numbers and the temporal variation of the horizontal magnetic field.

R. Abbasi, M. Ackermann, J. Adams et al.

Hao Li, Tanausú del Pino Alemán, Javier Trujillo Bueno et al.

We study the circular polarization of the magnetic-field-induced transition (MIT) between the 3 d ^5 ( ^6 S )4 d ^7 D _2 and 3 d ^5 ( ^6 S )4 p ^7 P _4 ° states of Cr i at 533.03 nm (wavelength in air). The fractional circular polarization V / I of this spectral line resulting from the solution of the radiation transfer problem in a sunspot model permeated by a homogeneous magnetic field of 3 kG shows amplitudes of about 2%. Spectropolarimetric observations of two sunspots were obtained with the Zurich Imaging Polarimeter-3 at the Istituto ricerche solari Aldo e Cele Daccò observatory in Locarno, Switzerland. The observed V / I profiles show approximately antisymmetrical shapes with an amplitude of around 0.1% and 0.2% for the two sunspots. The center of this profile coincides with the wavelengths predicted for the abovementioned MIT. We apply an inversion code to the spectropolarimetric data of the Cr i -permitted lines at 532.91 and 532.98 nm, as well as to the MIT line at 533.03 nm, to infer a stratification of the emitting atmosphere. We compare the V / I profiles synthesized in the inferred atmosphere models with the observations, showing that the observed signal likely corresponds to the MIT line.