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The Planck satellite (Planck 2015 Results I) has mapped the polarized microwave sky (from 30 GHz to 353 GHz) with unprecedented sensitivity and angular resolution. This wealth of data yields the first complete map of polarized thermal emission from dust in our own Galaxy (Planck Intermediate Results XIX),, shedding new light on the formation of dense cold structures within which new stars and planetary systems are born, under the combined effects of gravity, turbulence and magnetic fields. We present a statistical analysis of this polarized emission from nearby molecular clouds, with an emphasis on the evolution of the maximum polarization fraction observed as a function of column density, and on the anti-correlation between the polarization fraction and the local dispersion of polarization angles. To interpret this data, numerical simulations of anisotropic MHD turbulence (Fromang, Hennebelle, & Teyssier 2006, Hennebelle et al. 2008) underline the essential role played by the topology of the interstellar magnetic field, in particular its large-scale component (Planck Intermediate Results XX). Indeed, the polarization of dust thermal emission at the scales observed by Planck is essentially related to the geometry of the magnetic field. Polarization fractions anti-correlate with column densities, which may be due to a succession of variously polarized structures on the line of sight. They also anti-correlate with the local dispersion of polarization angles. These features are well reproduced by MHD simulations of the diffuse ISM, with comparable correlation coefficients. As an extension to this work published in Planck Intermediate Results XX, the statistical properties of the random component of the interstellar magnetic field are explored using a toy model of the turbulent magnetized ISM based on fractional Brownian motion (fBm) fields. A least-squares analysis to retrieve the statistical properties of the interstellar magnetic field from Planck observations is pursued. Application of this method on the toy model shows good promise, and we are currently working towards its application on Planck data.
Super Asymptotic Giant Branch (Super-AGB) stars reside in the mass range ≈ 6.5-10 M⊙ and bridge the divide between low/intermediate-mass and massive stars. They are characterised by off-centre carbon ignition prior to a thermally pulsing phase which can consist of many tens to even thousands of thermal pulses. With their high luminosities and very large, cool, red stellar envelopes, these stars appear seemingly identical to their slightly more massive red supergiant counterparts. Due to their similarities, super-AGB stars may therefore act as stellar imposters and contaminate red supergiant surveys. The final fate of super-AGB stars is also quite uncertain and depends primarily on the competition between the core growth and mass-loss rates. If the stellar envelope is removed prior to the core reaching ≈ 1.375 M⊙, an O-Ne white dwarf will remain, otherwise the star will undergo an electron-capture supernova (EC-SN) leaving behind a neutron star. We determine the relative fraction of super-AGB stars that end life as either an O-Ne white dwarf or as a neutron star, and provide a mass limit for the lowest mass supernova over a broad range of metallicities from the Z=0.02 to 0.0001.
Direct ultraviolet imaging and spectroscopy of Alpha Orionis (Betelgeuse) reveals variable chromospheric structures and mass motions. Spectroscopy also demonstrates the changes of wind opacity, speeds, and mass loss in luminous stars. Cool stars have complex chromospheres that need to be considered in construction of stellar atmospheric models and subsequent spectral analyses.
We review the most standard impact monitoring techniques. Linear methods are the fastest approach but their applicability regime is limited because of the chaotic dynamics of near-Earth asteroids. Among nonlinear methods, Monte Carlo algorithms are the most reliable ones but also most computationally intensive and so unpractical for routine impact monitoring. In the last 15 years, the Line of Variations method has been the most successful technique thanks to its computational efficiency and capability of detecting low probability events deep in the nonlinear regime. We also present some more recent techniques developed to deal with the new challenges arising in the impact hazard assessment problem. In particular, we describe keyhole maps as a tool to go beyond strongly scattering encounters and how to account for nongravitational perturbations, especially the Yarkovsky effect, when their contribution is the main source of prediction uncertainty. Finally, we discuss systematic ranging to deal with the short-term hazard assessment problem for newly discovered asteroids, when only a short observed arc is available thus leading to severe degeneracies in the orbit estimation process.
This paper is an introduction to Focus Meeting 5 held at the IAU's General Assembly held in Honolulu in August 2015. It describes the rationale for the meeting, and summarizes the introductory talk to FM5, which contained a description of the Planck data products released by ESA and the Planck Collaboration in the first half of 2015.
In the Astronomy in Focus series, volume XXIXA, the posters from Focus Meeting 12 should have been published online but were omitted. We apologise for this error. The missing posters are now published online.
We present results of Herschel PACS imaging spectroscopy data toward ten massive young stellar objects taken as part of the WISH project. Our sample consists of four high mass protostellar objects (HMPOs), two hot molecular cores (HMCs), and four ultracompact HII regions (UCHIIs), and the spectra cover a broad range of wavelengths (55 to 210 μm) imaged over an ~50” field with 5×5 spaxels. By fitting the continua utilizing a modified black-body formula we estimate mass-weighted dust temperature and column density distributions of warm dust and find that UCHII regions are hottest and HMCs are most deeply embedded. We also estimate rotational temperature and column density distributions of warm CO gas using the rotational diagram analysis, which are comparable over targets in contrast to continuum results. By comparing high J CO line fluxes to the RATRAN estimates of centrally heated envelope models, we find that majority of warm CO originates from bipolar outflow shocks.
We study the mass–metallicity relation and fundamental relation (FMR) for infrared bright galaxies (IR galaxies) at z ~ 0.9 discovered by AKARI NEP-Deep survey. The main result of this work is that metallicity of IR galaxies surprisingly match optical selected galaxies at a given mass even their star formation rates are different, which may imply that optical and IR selected galaxies follow similar star formation histories, and the starbursts in the IR galaxies do not give a strong impact in changing metallicity because of the short duration time.
A small flare ribbon above a sunspot umbra in active region 12205 was observed on November 7, 2014, at 12:00 UT in the blue imaging channel of the 1.5-m GREGOR telescope, using a 0.1 nm Ca II H interference filter. Context observations from SDO/AIA, Hinode/SOT, and IRIS show that the ribbon is a part of a larger one that extends through the neighboring positive polarities and also participates in several other flares within the active region. A 140 second long time series of Ca II H images was reconstructed by means of the Multi-Frame Blind Deconvolution method, giving the respective spatial and temporal resolutions of 0”.1 and 1 s. Light curves and horizontal velocities of small-scale bright knots in the observed flare ribbon were measured. Some knots are stationary but three move along the ribbon with speeds of 7–11 km s−1. Two of them move in the opposite direction and exhibit highly correlated intensity changes, providing evidence for the presence of slipping reconnection at small spatial scales.
M105 in the Leo I Group is a textbook example of a standard elliptical galaxy. It is only one of the few elliptical galaxies for which we can study their stellar halos using the resolved stars. It is an ideal target to study the structure and composition of stellar halos in elliptical galaxies. We present photometry and metallicity of the resolved stars in the inner and outer regions of M105. These provide strong evidence that there are two distinct stellar halos in this galaxy, a metal-poor (blue) halo and a metal-rich (red) halo. Then we compare them with those in other early-type galaxies and use the dual halo mode formation scenario to describe how massive galaxies formed.
The Astronomy For Development Focus Meeting 20 at the IAU General Assembly encompassed an ‘Unconference’ session as part of the proceedings. Unstructured conferences, with their potential to unleash innovative ideas, are gaining traction in various conferences and symposia. Astronomy For Development is a field that is applicable to the entire Astronomy community (and even beyond) and hence an unconference inviting ideas and fostering frank dialogue is very pertinent.
Officially one of the final sessions of the the 2015 General Assembly, the unconference session was intended to provide a balanced platform for a diverse set of participants and act as an informal setting to promote open discussion on topics of relevance to Astronomy for Development.
Asteroid disk-integrated sparse-in-time photometry can be used for determination of shapes and spin states of asteroids by the lightcurve inversion method. To clearly distinguish the correct solution of the rotation period from other minima in the parameter space, data with good photometric accuracy are needed. We show that if the low-quality sparse photometry obtained from ground-based astrometric surveys is combined with data from the Wide-field Infrared Survey Explorer (WISE) satellite, the correct rotation period can be successfully derived. Although WISE observed in mid-IR wavelengths, we show that for the period and spin determination, these data can be modelled as reflected light. The absolute fluxes are not required since only relative variation of the flux over the rotation is sufficient to determine the period. We also discuss the potential of combining all WISE data with the Lowell photometric database to create physical models of thousands of asteroids.
The evidence from the Earth's bow shock and other solar system shocks is that collisionless shocks, at least for the observed parameter range, are not characterized by strong electron heating. Similarly, they are generally not responsible for strong electron acceleration, either in terms of fluxes or maximum energy. On the other hand, there is clear observational evidence for some shock acceleration of electrons to moderate energies at the bow shock and at interplanetary shocks. In both cases there is an electron foreshock populated by superthermal, energized electrons. The average thermal speed of the solar wind electron distribution is approximately 2000 km s–1 that the speed of electrons accelerated to only a few thermal energiesis much greater than the solar wind convection speed. Consequently the upstream edge of the electron foreshock can be taken as the tangent surface of magnetic field lines touching the shock surface. Accelerated ions, with their relatively slower speeds, are more affected by the solar wind convection and swept downstream. Thus, there is a region, downstream of the magnetic field tangent line surface and upstream of the ion foreshock, which can only be accessed by shock-accelerated electrons. This part of the electron foreshock, magnetically connected to the quasi-perpendicular shock, has the advantage, in terms of analysis, of not being disturbed by waves driven by accelerated ions. Direct particle observations at the electron foreshock at the Earth's bow shock gives evidence of shock acceleration from suprathermal energies to tens of keV. A review of electron foreshock observations and models is given by Fitzenreiter (1995).
Apart from direct measurements, the presence of energetic electrons can be inferred from the observations of Langmuir turbulence and other plasma waves driven via electron beams instabilities. At the Earth the electron foreshock is also a radio source at first and second multiples of the local plasma frequency, so-called fp and 2fp emission. Here the generation mechanism depends on electron beams which drive electrostatic Langmuir waves with frequency close to fpe via a beam instability. Some of the Langmuir wave energy is converted into fp and 2fp radiation by various linear or nonlinear wave–wave processes (Melrose, 1986). The fact that foreshock radio emission can be related to shock acceleration of electron beams gives the opportunity for remote sensing or characterizing shocks on larger scales than observable by a single spacecraft.
The study of shock waves in collisionless plasmas has a long history of over 50 years. That of shock waves in gas dynamics has roots which go back to the foundations of applied mathematics in the twentieth century, the nature of hyperbolic systems of equations and the physics of blast waves. Much of this early work was associated with military research, and some of the earliest work on shock waves in plasmas started from a similar background. However, in the early 1960s it was in space that truly collisionless shocks were first observed. With the advent of high-resolution space observations a fundamental challenge came into being: how can nonlinear collective processes replace the action of particle collisions and lead to thin shock waves in a collisionless plasma? In other words: how do collisionless shocks work?
With the growing exploration of space and better understanding of the plasma physics of the heliosphere, the importance of shocks has become evident. Shocks are formed around the planets in the supersonic flow of the solar wind; they are formed ahead of the impulsive flows of coronal mass ejections, and at the steady interaction regions between solar wind with different speeds; the entire region of the solar wind is defined by an outer boundary, the solar wind termination shock, where the flow transitions to subsonic as it comes into balance with the interstellar medium. In parallel to our increasing understanding of solar system shocks, it has become obvious that shock waves will arise in many other astrophysical systems, and that often the physics will be dominated by collisionless processes. A widely cited example is the shock wave driven by a supernova remnant; such shocks are understood to be vital for explaining the majority of cosmic ray acceleration.
In writing this book we had three aims in sight. We felt there was a need for a graduate level textbook that brought together the physics of collisionless shocks as found in the heliosphere, with an emphasis on the theoretical underlying physics of shock waves in plasmas.
Observations of diffuse ions at heliospheric shocks
Virtually all shocks observed either in situ or indirectly in the solar system are accompanied by energetic particles, i.e., protons, heavy ions and electrons, with energies up to 1000MeV and higher. The most intense solar energetic particle (SEP) events are produced by acceleration at interplanetary shock waves driven by coronal mass ejections (CMEs). The intensity–time profiles of electrons and protons in CME as sociated particle events have usually a fast rise and a decay phase extending over several days. The shock passage at 1 au occurs early in the decay phase and is often accompanied by a peak in the lower-energy ions (˜1MeV). Since these events are long-lasting (several days) they have been termed gradual events, as opposed to impulsive events of a duration of approximately a few hours. The latter are associated with impulsive X-ray flares and type III radio bursts. X-ray emission from flares indicates plasma heating to temperatures of order ten million kelvin, and solar type III radio bursts imply the impulsive production of electrons travelling at 0.1–0.25 times the speed of light. Acceleration at the CME driven shocks populate magnetic field lines over a broad range of longitudes, while the impulsive event sare generally detected when the observer is magnetically connected to the flare site. Figure 6.1a shows the intensity–time profiles of particle fluxes (one electron and three proton energy channels) in a ‘pure’ gradual event accompanied by a CME and Fig. 6.1b shows an impulsive event due to a series of flares (Reames, 1999).
There is clear evidence that energetic ions are accelerated at interplanetary shocks out of the solar wind thermal population. Gosling et al. (1981) have transformed the ion distribution function measured over a wide energy range (10 eV to 1.6MeV) behind an interplanetary shock into the solar wind frame. Figure 6.2 shows a cut of the distribution function along the Sun–Earth line which demonstrates that the suprathermal distribution emerges smoothly out of the solar wind thermal distribution, which can be taken as evidence that the solar wind ions are accelerated to high energies.