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  • 1
    Publication Date: 2017-09-14
    Description: This study examines the cause of the spread of extratropical circulation responses to the inclusion of atmospheric cloud radiative effects (ACRE) across atmospheric general circulation models. The ensemble of Clouds On-Off Klimate Intercomparison Experiment aquaplanet simulations shows that these responses include both equatorward and poleward shifts of the eddy-driven jet of varying magnitudes. These disparate extratropical responses occur despite the relatively consistent response in the tropics: a heating in the upper troposphere, which leads to a strengthening of the Hadley cell. It is argued that the eddy-driven jet response is a competition between two effects: the local influence of clouds driving shifts of the jet through meridional gradients in ACRE and the remote impact of a strengthened Hadley cell causing an equatorward shift of the eddy-driven jet. Simulations in which cloud radiative effects are separately turned on in the tropics and extratropics demonstrate this explicitly.
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 2
    Publication Date: 2016-10-17
    Description: Accretion-driven luminosity outbursts are a vivid manifestation of variable mass accretion on to protostars. They are known as the so-called FU Orionis phenomenon in the context of low-mass protostars. More recently, this process has been found in models of primordial star formation. Using numerical radiation hydrodynamics simulations, we stress that present-day forming massive stars also experience variable accretion and show that this process is accompanied by luminous outbursts induced by the episodic accretion of gaseous clumps falling from the circumstellar disc on to the protostar. Consequently, the process of accretion-induced luminous flares is also conceivable in the high-mass regime of star formation and we propose to regard this phenomenon as a general mechanism that can affect protostars regardless of their mass and/or the chemical properties of the parent environment in which they form. In addition to the commonness of accretion-driven outbursts in the star formation machinery, we conjecture that luminous flares from regions hosting forming high-mass stars may be an observational implication of the fragmentation of their accretion discs.
    Print ISSN: 1745-3925
    Electronic ISSN: 1745-3933
    Topics: Physics
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  • 3
    Publication Date: 2016-11-13
    Description: A significant fraction of OB-type, main-sequence massive stars are classified as runaway and move supersonically through the interstellar medium (ISM). Their strong stellar winds interact with their surroundings, where the typical strength of the local ISM magnetic field is about 3.5–7 μG, which can result in the formation of bow shock nebulae. We investigate the effects of such magnetic fields, aligned with the motion of the flow, on the formation and emission properties of these circumstellar structures. Our axisymmetric, magneto-hydrodynamical simulations with optically thin radiative cooling, heating and anisotropic thermal conduction show that the presence of the background ISM magnetic field affects the projected optical emission of our bow shocks at Hα and [O iii ] 5007 which become fainter by about 1–2 orders of magnitude, respectively. Radiative transfer calculations against dust opacity indicate that the magnetic field slightly diminishes their projected infrared emission and that our bow shocks emit brightly at 60 μm. This may explain why the bow shocks generated by ionizing runaway massive stars are often difficult to identify. Finally, we discuss our results in the context of the bow shock of Ophiuchi and we support the interpretation of its imperfect morphology as an evidence of the presence of an ISM magnetic field not aligned with the motion of its driving star.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
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  • 4
    Publication Date: 2015-05-13
    Description: After the death of a runaway massive star, its supernova shock wave interacts with the bow shocks produced by its defunct progenitor, and may lose energy, momentum and its spherical symmetry before expanding into the local interstellar medium (ISM). We investigate whether the initial mass and space velocity of these progenitors can be associated with asymmetric supernova remnants. We run hydrodynamical models of supernovae exploding in the pre-shaped medium of moving Galactic core-collapse progenitors. We find that bow shocks that accumulate more than about 1.5 M generate asymmetric remnants. The shock wave first collides with these bow shocks 160-750 yr after the supernova, and the collision lasts until 830-4900 yr. The shock wave is then located 1.35-5 pc from the centre of the explosion, and it expands freely into the ISM, whereas in the opposite direction it is channelled into the region of undisturbed wind material. This applies to an initially 20 M progenitor moving with velocity 20 km s -1 and to our initially 40 M progenitor. These remnants generate mixing of ISM gas, stellar wind and supernova ejecta that is particularly important upstream from the centre of the explosion. Their light curves are dominated by emission from optically thin cooling and by X-ray emission of the shocked ISM gas. We find that these remnants are likely to be observed in the [O iii ] 5007 spectral line emission or in the soft energy-band of X-rays. Finally, we discuss our results in the context of observed Galactic supernova remnants such as 3C 391 and the Cygnus Loop.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
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  • 5
    Publication Date: 2014-09-17
    Description: At least 5 per cent of the massive stars are moving supersonically through the interstellar medium (ISM) and are expected to produce a stellar wind bow shock. We explore how the mass-loss and space velocity of massive runaway stars affect the morphology of their bow shocks. We run two-dimensional axisymmetric hydrodynamical simulations following the evolution of the circumstellar medium of these stars in the Galactic plane from the main sequence to the red supergiant phase. We find that thermal conduction is an important process governing the shape, size and structure of the bow shocks around hot stars, and that they have an optical luminosity mainly produced by forbidden lines, e.g. [O iii ]. The Hα emission of the bow shocks around hot stars originates from near their contact discontinuity. The Hα emission of bow shocks around cool stars originates from their forward shock, and is too faint to be observed for the bow shocks that we simulate. The emission of optically thin radiation mainly comes from the shocked ISM material. All bow shock models are brighter in the infrared, i.e. the infrared is the most appropriate waveband to search for bow shocks. Our study suggests that the infrared emission comes from near the contact discontinuity for bow shocks of hot stars and from the inner region of shocked wind for bow shocks around cool stars. We predict that, in the Galactic plane, the brightest, i.e. the most easily detectable bow shocks are produced by high-mass stars moving with small space velocities.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
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  • 6
    Publication Date: 2016-04-23
    Description: Massive stars that have been ejected from their parent cluster and supersonically sailing away through the interstellar medium (ISM) are classified as exiled. They generate circumstellar bow-shock nebulae that can be observed. We present two-dimensional, axisymmetric hydrodynamical simulations of a representative sample of stellar wind bow shocks from Galactic OB stars in an ambient medium of densities ranging from n ISM  = 0.01 up to 10.0 cm – 3 . Independently of their location in the Galaxy, we confirm that the infrared is the most appropriated waveband to search for bow shocks from massive stars. Their spectral energy distribution is the convenient tool to analyse them since their emission does not depend on the temporary effects which could affect unstable, thin-shelled bow shocks. Our numerical models of Galactic bow shocks generated by high-mass ( 40 M ) runaway stars yield H α fluxes which could be observed by facilities such as the SuperCOSMOS H-Alpha Survey. The brightest bow-shock nebulae are produced in the denser regions of the ISM. We predict that bow shocks in the field observed at H α by means of Rayleigh-sensitive facilities are formed around stars of initial mass larger than about 20 M . Our models of bow shocks from OB stars have the emission maximum in the wavelength range 3 ≤ ≤ 50 μm which can be up to several orders of magnitude brighter than the runaway stars themselves, particularly for stars of initial mass larger than 20 M .
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
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  • 7
    Publication Date: 2014-03-02
    Description: In this Letter, we explore the hypothesis that the smooth appearance of bow shocks around some red supergiants (RSGs) might be caused by the ionization of their winds by external sources of radiation. Our numerical simulations of the bow shock generated by IRC –10414 (the first-ever RSG with an optically detected bow shock) show that the ionization of the wind results in its acceleration by a factor of 2, which reduces the difference between the wind and space velocities of the star and makes the contact discontinuity of the bow shock stable for a range of stellar space velocities and mass-loss rates. Our best-fitting model reproduces the overall shape and surface brightness of the observed bow shock and suggests that the space velocity and mass-loss rate of IRC –10414 are 50 km s –1 and 10 –6 M yr –1 , respectively, and that the number density of the local interstellar medium is 3 cm –3 . It also shows that the bow shock emission comes mainly from the shocked stellar wind. This naturally explains the enhanced nitrogen abundance in the line-emitting material, derived from the spectroscopy of the bow shock. We found that photoionized bow shocks are 15–50 times brighter in optical line emission than their neutral counterparts, from which we conclude that the bow shock of IRC –10414 must be photoionized.
    Print ISSN: 1745-3925
    Electronic ISSN: 1745-3933
    Topics: Physics
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  • 8
    Publication Date: 2013-12-06
    Description: Most runaway OB stars, like the majority of massive stars residing in their parent clusters, go through the red supergiant (RSG) phase during their lifetimes. Nonetheless, although many dozens of massive runaways were found to be associated with bow shocks, only two RSG bow-shock-producing stars, Betelgeuse and μ Cep, are known to date. In this paper, we report the discovery of an arc-like nebula around the late M-type star IRC –10414 using the SuperCOSMOS H-alpha Survey. Our spectroscopic follow-up of IRC –10414 with the Southern African Large Telescope (SALT) showed that it is a M7 supergiant, which supports previous claims on the RSG nature of this star based on observations of its maser emission. This was reinforced by our new radio- and (sub)millimetre-wavelength molecular line observations made with the Atacama Pathfinder Experiment 12-m telescope and the Effelsberg 100-m radio telescope, respectively. The SALT spectrum of the nebula indicates that its emission is the result of shock excitation. This finding along with the arc-like shape of the nebula and an estimate of the space velocity of IRC –10414 (70 ± 20 km s –1 ) imply the bow shock interpretation for the nebula. Thus, IRC –10414 represents the third case of a bow-shock-producing RSG and the first one with a bow shock visible at optical wavelengths. We discuss the smooth appearance of the bow shocks around IRC –10414 and Betelgeuse and propose that one of the necessary conditions for stability of bow shocks generated by RSGs is the ionization of the stellar wind. Possible ionization sources of the wind of IRC –10414 are proposed and discussed.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
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  • 9
    Publication Date: 2011-08-19
    Description: Very precise observations (with S/N greater than 2000) of the 3874-angstrom band of interstellar CN toward zeta Per and omicron Per are presented. In the zeta Oph, zeta Per, and omicron Per lines of sight, the saturation-corrected CN line strengths yield respective excitation temperatures of 2.72 plus or minus 0.05 K, 2.76 plus or minus 0.05 K, and 2.78 plus or minus 0.07 K for the J = 0-1 rotational transition at 2.64 mm. By confirming the blackbody character of the cosmic microwave background spectrum at wavelengths near the peak of its flux, the simplest explanation of the background as primeval fireball radiation from a hot bang is reinforced.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 297; 119-132
    Format: text
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  • 10
    Publication Date: 2011-08-19
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 294; 238-241
    Format: text
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