Skip to main content Accessibility help

We use cookies to distinguish you from other users and to provide you with a better experience on our websites. Close this message to accept cookies or find out how to manage your cookie settings.

Close cookie message
×
  1. Home
  2. Browse subjects
  3. Physics and Astronomy
  4. Astrophysics
  5. Content Listing

Refine search

Refine search

Access:
Content type:
Publication date:
Apply   From and To filters
Subject: Show more
Journals Show more
Publishers: Show more
Societies Show more
Series: Show more
Collections: Show more

Actions for selected content:

Select all | Deselect all
  • View selected items
  • Export citations
  • Download PDF (zip)
  • Save to Kindle
  • Save to Dropbox
  • Save to Google Drive

Save Search

You can save your searches here and later view and run them again in "My saved searches".

Please provide a title, maximum of 40 characters.
Cancel
×

27023 results in Astrophysics

Page 182 of 1352

Bayesian Astrophysics
  • Edited by Andrés Asensio Ramos, Íñigo Arregui
  • Published online:
    28 April 2018
    Print publication:
    26 April 2018
  • Bayesian methods are being increasingly employed in many different areas of research in the physical sciences. In astrophysics, models are used to make predictions to be compared to observations. These observations offer information that is incomplete and uncertain, so the comparison has to be pursued by following a probabilistic approach. With contributions from leading experts, this volume covers the foundations of Bayesian inference, a description of computational methods, and recent results from their application to areas such as exoplanet detection and characterisation, image reconstruction, and cosmology. It appeals to both young researchers seeking to learn about Bayesian methods as well as to astronomers wishing to incorporate these approaches in their research areas. It provides the next generation of researchers with the tools of modern data analysis that are already becoming standard in current astrophysical research.

Cosmic Magnetic Fields
  • Edited by Jorge Sánchez Almeida, María Jesús Martínez González
  • Published online:
    21 April 2018
    Print publication:
    12 April 2018
  • Magnetic fields pervade the universe and play an important role in many astrophysical processes. However, they require specialised observational tools, and are challenging to model and understand. This volume provides a unified view of magnetic fields across astrophysical and cosmological contexts, drawing together disparate topics that are rarely covered together. Written by the lecturers of the XXV Canary Islands Winter School, it offers a self-contained introduction to cosmic magnetic fields on a range of scales. The connections between the behaviours of magnetic fields in these varying contexts are particularly emphasised, from the relatively small and close ranges of the Sun, planets and stars, to galaxies and clusters of galaxies, as well as on cosmological scales. Aimed at young researchers and graduate students, this up-to-date review uniquely brings together a subject often tackled by disconnected communities, conveying the latest advances as well as highlighting the limits of our current understanding.

  • 3 - Stellar Magnetic Fields

  • Edited by Jorge Sánchez Almeida, Instituto de Astrofísica de Canarias, Tenerife, María Jesús Martínez González, Instituto de Astrofísica de Canarias, Tenerife
  • Book:
    Cosmic Magnetic Fields
    Published online:
    21 April 2018
    Print publication:
    12 April 2018, pp 47-86

  • Summary

    Abstract

    Recent developments in astrophysical spectropolarimetry have significantly expanded our understanding of stellar magnetism. Using modern instrumentation and data analysis methods it is becoming possible to detect and characterise surface magnetic fields not just for a few peculiar objects but for essentially all types of stars, ranging from massive OB stars to brown dwarfs. The intention of this chapter is to give a general overview of the stellar magnetic field diagnostic methods and to present results of the application of these methods to different classes of stars. First, the chapter discusses the physical processes that lead to appearance of magnetic signatures in stellar spectra. Based on this discussion, the most commonly used magnetic observables are introduced for the intensity and polarisation spectra. The methods of interpretation of magnetic measurements, starting with robust field detection techniques and leading to reconstruction of the physically realistic vector maps of surface magnetic fields, are discussed. Finally, the current knowledge about the incidence, geometries and the origin of magnetic fields in stars of different masses and ages is reviewed and the astrophysical significance of magnetism in the context of stellar structure and evolution is discussed.

    Introduction

    This chapter presents an introduction to the studies of stellar magnetic fields. We explore the questions of how magnetic fields can be detected on the stellar surfaces, what information about magnetic field topologies can be extracted from different observations, and what are the observed properties of magnetic fields in stars of different masses and ages. The origin of magnetism in different types of stars and its relation to other physical processes operating in stellar interiors and atmospheres are briefly discussed.

    The following sections start by presenting the essential theoretical foundations of the stellar magnetic field diagnostic and modelling. We consider the Zeeman effect in spectral lines and information contained in the polarisation characteristics of stellar radiation. A connection between the local magnetic field parameters and morphology of the stellar polarisation profiles is established using both detailed numerical calculations and simplified analytical considerations. Based on this discussion, we introduce different methodologies suitable for the detection and characterisation of stellar magnetic fields.

  • Preface

  • Edited by Jorge Sánchez Almeida, Instituto de Astrofísica de Canarias, Tenerife, María Jesús Martínez González, Instituto de Astrofísica de Canarias, Tenerife
  • Book:
    Cosmic Magnetic Fields
    Published online:
    21 April 2018
    Print publication:
    12 April 2018, pp xiii-xiii

  • Summary

    Magnetic fields play an important role in many astrophysical processes. They are difficult to detect and characterize because often their properties have to be inferred through interpreting the polarization of the light. Magnetic fields are also challenging to model and understand. Magnetized plasmas behave following highly non-linear differential equations having no general solution, so that every astrophysical problem represents a special case to be studied independently. Hence, magnetic fields are often an inconvenient subject that is overlooked or simply neglected (the elephant in the room, as they are dubbed on posters in the XXV Canary Islands Winter School of Astrophysics). Such a difficulty burdens the research on magnetic fields, which has evolved to become a very technical subject, with many small disconnected communities studying specific aspects and details. The school tried to amend the situation by providing a unifying view of the subject. The students had a chance to understand the behavior of magnetic fields in all astrophysical contexts, from cosmology to the Sun, and from starbursts to AGNs. The school was planned to present a balanced yet complete review of our knowledge, with excursions into the unknown to point out present and future lines of research.

    The subject of Cosmic Magnetic Fields was split into seven different topics: cosmic magnetic field essentials, solar magnetic fields, stellar magnetic fields, the role of magnetic fields on AGN feedback, magnetic fields in galaxies, magnetic fields in galaxy clusters and at larger scales, and primordial magnetic fields and magnetic fields in the early Universe. The corresponding lectures were delivered by seven well known and experienced scientists that have played key roles in the major advances of the field during the last years: F. Cattaneo, P. Judge, O. Kochukhov, R. Keppens, R. Beck, K. Dolag, and F. Finelli. Their lectures were recorded and are freely available at the IAC website: http://iactalks.iac.es/talks/serie/19. Together with the reviews included in the present volume, they form a unique resource for both students and professional researchers. They provide a global view of this very compartmentalized, yet fundamental, field of research.

  • 5 - Magnetic Fields in Galaxies

    • By Rainer Beck, Max Planck Institut für Radioastronomie, Germany
  • Edited by Jorge Sánchez Almeida, Instituto de Astrofísica de Canarias, Tenerife, María Jesús Martínez González, Instituto de Astrofísica de Canarias, Tenerife
  • Book:
    Cosmic Magnetic Fields
    Published online:
    21 April 2018
    Print publication:
    12 April 2018, pp 123-160

  • Summary

    Introduction

    Magnetism is a fundamental force with special properties.

  • • Most baryonic matter is ionized. Magnetic fields are easy to generate.

  • • Magnetic monopoles do not exist or are extremely rare. Magnetic fields are hard to destroy.

  • • Magnetic fields need illumination. Magnetic fields are difficult to observe.

    • The scarceness of data leaves many questions on cosmic magnetic fields.

  • • When and how were the first fields generated?

  • • Did significant fields exist before galaxies formed?

  • • How and how fast were the fields amplified?

  • • How did fields affect the evolution of stars, planets, galaxies and galaxy clusters?

  • • How strongly is intergalactic space magnetized?

    • Magnetic fields are often ignored in astrophysics, in particular in models of galaxies and the interstellar medium (ISM), although they are a major agent in the ISM and in galaxy halos, and important for the structure and evolution of galaxies.

  • • Magnetic fields contribute significantly to the total pressure which balances the gas disk of galaxies against gravitation (Fletcher & Shukurov, 2001).

  • • Magnetic turbulence distributes energy from supernova explosions within the ISM (Subramanian, 1998).

  • • Magnetic reconnection is a possible heating source for the ISM and halo gas (Birk et al., 1998).

  • • Magnetic fields increase angular momentum transport and hence the gas inflow rate in barred galaxies (Beck et al., 2005; Kim & Stone, 2012).

  • • Magnetic fields affect the dynamics of the turbulent ISM (de Avillez & Breitschwerdt, 2005) and the gas flows in spiral arms (Gómez & Cox, 2002).

  • • Magnetic fields make the gaseous spiral arms more patchy and drive gas outflows into the halo (Pakmor & Springel, 2013).

  • • The shock strength in spiral density waves is decreased and structure formation is reduced in the presence of strong fields (Dobbs & Price, 2008; Fletcher et al., 2011).

  • • Magnetic fields stabilize gas clouds and reduce the star-formation efficiency to the observed low values (Vázquez-Semadeni et al., 2005; Price & Bate, 2008).

  • • Magnetic fields are essential for the onset of star formation as they enable the removal of angular momentum from the protostellar cloud via ambipolar diffusion (Heitsch et al., 2004).

  • 6 - Primordial Magnetic Fields in the Early Universe and Cosmic Microwave Background Anisotropies

  • Edited by Jorge Sánchez Almeida, Instituto de Astrofísica de Canarias, Tenerife, María Jesús Martínez González, Instituto de Astrofísica de Canarias, Tenerife
  • Book:
    Cosmic Magnetic Fields
    Published online:
    21 April 2018
    Print publication:
    12 April 2018, pp 161-190

  • Summary

    Introductory Remarks

    Our aim in the talks given at the XXV Canary Islands Winter School of Astrophysics in 2013 was to present models for the generation of primordial magnetic fields in the early Universe and the imprints that these leave in cosmological observables, with an emphasis on Cosmic Microwave Background (CMB) anisotropies.

    Introduction

    The origin of the large-scale magnetic fields (LSMF) observed in galaxies, clusters of galaxies, with also hints of their presence in cosmic voids and on even larger scales in filaments, is an open issue of great importance in modern astrophysics (seeWidrow, 2002, for reviews). Both large-scale and stochastic components are present in magnetic fields observed in galaxies (usually dependent on the morphology of the host) with magnitudes from a few to several microGauss (μG). In clusters of galaxies, stochastic magnetic fields from a few to several μG strength are present with a correlation scale of the order of magnitude of ten kiloparsec (Clarke et al., 2001; Enslin & Vogt, 2006). More recently, the presence of intergalactic magnetic fields even in cosmic voids of the large-scale structure was proposed as a possible explanation for the gamma-ray observations of a couple of blazars (Neronov & Vovk, 2010; Taylor et al., 2011; Vovk et al., 2012; Tavecchio et al., 2010, 2011; Dolag et al., 2011). As a possible explanation for the lack of TeV photons observed in the high-energy spectrum from these blazars a lower bound of 10−18−10−15 G was derived for such pervasive intergalactic magnetic fields.

    In light of this multitude of observations of LSMF of different magnitudes and different coherence lengths, a primordial hypothesis for generating the seed magnetic fields, which are amplified afterwards by adiabatic compression and dynamo during structure formation, is a viable possibility (Widrow, 2002), also taking in consideration the recent observations of strong magnetic fields in galaxies at high redshift (Bernet et al., 2008; Wolfe et al., 2008). Recently, initial seeds motivated as primordial magnetic fields (PMF) have been used in N-body simulations to reproduce magnetic fields in clusters of galaxies (Govoni et al., 2013; Xu et al., 2012).

  • 4 - The Role of Magnetic Fields in AGN Activity and Feedback

  • Edited by Jorge Sánchez Almeida, Instituto de Astrofísica de Canarias, Tenerife, María Jesús Martínez González, Instituto de Astrofísica de Canarias, Tenerife
  • Book:
    Cosmic Magnetic Fields
    Published online:
    21 April 2018
    Print publication:
    12 April 2018, pp 87-122

  • Summary

    Abstract

    Active galactic nuclei (AGNs), the luminous, compact core regions of galaxies where accretion occurs onto supermassive black holes, can dramatically influence their entire host galaxy evolution by a process referred to as AGN feedback. Energy feedback to the galaxy is the result of combined radiation fields and directed outflows, and especially radio-loud active galaxies show pronounced jets and lobes. Their synchrotron radio emission indicates that dynamically important magnetic fields are at play in AGN jet collimation, stability, energy transfer to the intergalactic medium and their overall morphological appearance. Current knowledge on the launching mechanisms for such highly energetic relativistic jets, as well as the near black-hole accretion processes themselves, all invoke magnetic fields as active agents in angular momentum, mass and energy redistributions. In this review, we cover aspects of AGN feedback and the role played by magnetic fields, almost necessarily studied at vastly different length and timescales. We emphasize how typical large-scale galaxy interaction studies rely on parametric prescriptions for feedback, while detailed dedicated studies for near black-hole dynamics and relativistic jet propagation exist which take full account of magnetic field influences. We discuss representative hydro to magnetohydrodynamic (MHD) numerical simulations that exploit analogies with less energetic X-ray binary sources or even protostellar accretion-ejection systems, emphasize relativistic MHD descriptions, and point out that magnetic fields in accretion disks yield many linear instability routes to turbulence that have scarcely been recognized in the astrophysical community. In combination, they serve to show that magnetic field influences in AGN accretion, jet launch, energy feedback, and overall evolution are still far from completely understood, although many aspects have been disclosed by advanced analytical and numerical relativistic MHD studies.

    Motivation: Astrophysical Jets

    Radio galaxies confront us with dramatic views on energy redistributions at all scales, as mediated by central massive black holes lurking in their nucleus. A clear example is provided by the elliptical galaxy NGC5532, a nearby (red shift z = 0.0237, type S0) galaxy where the stellar distribution is in sharp contrast with its double-jetted appearance in radio images.

  • Where is Population II?

  • J. Mould, F. Bianchini, Duncan A. Forbes, C. L. Reichardt
  • Journal:
    Publications of the Astronomical Society of Australia / Volume 35 / 2018
    Published online by Cambridge University Press:
    28 March 2018, e016
    • Article
      • You have access
    • PDF
    • HTML
    • Export citation

  • The use of roman numerals for stellar populations represents a classification approach to galaxy formation which is now well behind us. Nevertheless, the concept of a pristine generation of stars, followed by a protogalactic era, and finally the mainstream stellar population is a plausible starting point for testing our physical understanding of early star formation. This will be observationally driven as never before in the coming decade. In this paper, we search out observational tests of an idealised coeval and homogeneous distribution of population II stars. We examine the spatial distribution of quasars, globular clusters, and the integrated free electron density of the intergalactic medium, in order to test the assumption of homogeneity. Any real inhomogeneity implies a population II that is not coeval.

  • A Multi-Frequency Study of the Milky Way-Like Spiral Galaxy NGC 6744

  • Miranda Yew, Miroslav D. Filipović, Quentin Roper, Jordan D. Collier, Evan J. Crawford, Thomas H. Jarrett, Nicholas F. H. Tothill, Andrew N. O’Brien, Marko Z. Pavlović, Thomas G. Pannuti, Timothy J. Galvin, Anna D. Kapińska, Michelle E. Cluver, Julie K. Banfield, Eric M. Schlegel, Nigel Maxted, Kevin R. Grieve
  • Journal:
    Publications of the Astronomical Society of Australia / Volume 35 / 2018
    Published online by Cambridge University Press:
    28 March 2018, e015
    • Article
      • You have access
    • PDF
    • HTML
    • Export citation

  • We present a multi-frequency study of the intermediate spiral SAB(r)bc type galaxy NGC 6744, using available data from the Chandra X-Ray telescope, radio continuum data from the Australia Telescope Compact Array and Murchison Widefield Array, and Wide-field Infrared Survey Explorer infrared observations. We identify 117 X-ray sources and 280 radio sources. Of these, we find nine sources in common between the X-ray and radio catalogues, one of which is a faint central black hole with a bolometric radio luminosity similar to the Milky Way’s central black hole. We classify 5 objects as supernova remnant (SNR) candidates, 2 objects as likely SNRs, 17 as H ii regions, 1 source as an AGN; the remaining 255 radio sources are categorised as background objects and one X-ray source is classified as a foreground star. We find the star-formation rate (SFR) of NGC 6744 to be in the range 2.8–4.7 M⊙~yr − 1 signifying the galaxy is still actively forming stars. The specific SFR of NGC 6744 is greater than that of late-type spirals such as the Milky Way, but considerably less that that of a typical starburst galaxy.

  • Accuracy of Shack–Hartmann wavefront sensor using a coherent wound fibre image bundle

  • Jessica R. Zheng, Michael Goodwin, Jon Lawrence
  • Journal:
    Publications of the Astronomical Society of Australia / Volume 35 / 2018
    Published online by Cambridge University Press:
    26 March 2018, e014
    • Article
      • You have access
    • PDF
    • HTML
    • Export citation

  • Shack–Hartmannwavefront sensors using wound fibre image bundles are desired for multi-object adaptive optical systems to provide large multiplex positioned by Starbugs. The use of a large-sized wound fibre image bundle provides the flexibility to use more sub-apertures wavefront sensor for ELTs. These compact wavefront sensors take advantage of large focal surfaces such as the Giant Magellan Telescope. The focus of this paper is to study the wound fibre image bundle structure defects effect on the centroid measurement accuracy of a Shack–Hartmann wavefront sensor. We use the first moment centroid method to estimate the centroid of a focused Gaussian beam sampled by a simulated bundle. Spot estimation accuracy with wound fibre image bundle and its structure impact on wavefront measurement accuracy statistics are addressed. Our results show that when the measurement signal-to-noise ratio is high, the centroid measurement accuracy is dominated by the wound fibre image bundle structure, e.g. tile angle and gap spacing. For the measurement with low signal-to-noise ratio, its accuracy is influenced by the read noise of the detector instead of the wound fibre image bundle structure defects. We demonstrate this both with simulation and experimentally. We provide a statistical model of the centroid and wavefront error of a wound fibre image bundle found through experiment.

Page 182 of 1352