| 1. |
- Brandenburg, Axel, et al.
(författare)
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Hemispheric Handedness in the Galactic Synchrotron Polarization Foreground
- 2020
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Ingår i: Astrophysical Journal Letters. - : American Astronomical Society. - 2041-8205 .- 2041-8213. ; 896:1
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Tidskriftsartikel (refereegranskat)abstract
- The large-scale magnetic field of the Milky Way is thought to be created by an alpha omega dynamo, which implies that it should have opposite handedness north and south of the Galactic midplane. Here we attempt to detect a variation in handedness using polarization data from the Wilkinson Microwave Anisotropy Probe. Previous analyzes of the parity-even and parity-odd parts of linear polarization of the global dust and synchrotron emission have focused on quadratic correlations in spectral space of, and between, these two components. Here, by contrast, we analyze the parity-odd polarization itself and show that it has, on average, opposite signs in northern and southern Galactic hemispheres. Comparison with a Galactic mean-field dynamo model shows broad qualitative agreement and reveals that the sign of the observed hemispheric dependence of the azimuthally averaged parity-odd polarization is not determined by the sign of alpha, but by the sense of differential rotation.
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| 2. |
- Dan, Marius, et al.
(författare)
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The structure and fate of white dwarf merger remnants
- 2014
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 438:1, s. 14-34
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Tidskriftsartikel (refereegranskat)abstract
- We present a large parameter study where we investigate the structure of white dwarf (WD) merger remnants after the dynamical phase. A wide range of WD masses and compositions are explored, and we also probe the effect of different initial conditions. We investigated the degree of mixing between the WDs, the conditions for detonations as well as the amount of gas ejected. We find that systems with lower mass ratios have more total angular momentum and as a result more mass is flung out in a tidal tail. Nuclear burning can affect the amount of mass ejected. Many WD binaries that contain a helium-rich WD achieve the conditions to trigger a detonation. In contrast, for carbon-oxygen-transferring systems, only the most massive mergers with a total mass M greater than or similar to 2.1 M-circle dot detonate. Even systems with lower mass may detonate long after the merger if the remnant remains above the Chandrasekhar mass and carbon is ignited at the centre. Finally, our findings are discussed in the context of several possible observed astrophysical events and stellar systems, such as hot subdwarfs, R Coronae Borealis stars, single massive WDs, supernovae of Type Ia and other transient events. A large data base containing 225 WD merger remnants is made available via a dedicated web page.
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| 3. |
- Morsony, Brian J., et al.
(författare)
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Simulations of bent-double radio sources in galaxy groups
- 2013
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 431:1, s. 781-792
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Tidskriftsartikel (refereegranskat)abstract
- Bent-double radio sources have been used as a probe to measure the density of intergalactic gas in galaxy groups. We carry out a series of high-resolution, three-dimensional simulations of active galactic nucleus (AGN) jets moving through an external medium with a constant density in order to develop a general formula for the radius of curvature of the jets, and to determine how accurately the density of the intra-group medium (IGM) can be measured. Our simulations produce curved jets ending in bright radio lobes with an extended trail of low surface brightness radio emission. The radius of curvature of the jets varies with time by only about 25 per cent. The radio trail seen in our simulations is typically not detected in known sources, but may be detectable in lower resolution radio observations. The length of this tail can be used to determine the age of the AGN. We also use our simulation data to derive a formula for the kinetic luminosity of observed jets in terms of the radius of curvature and jet pressure. In characterizing how well observations can measure the IGM density, we find that the limited resolution of typical radio observations leads to a systematic underestimate of the IGM density of about 50 per cent. The unknown angles between the observer and the direction of jet propagation and direction of AGN motion through the IGM lead to an uncertainty of about +/- 50 per cent in estimates of the IGM density. Previous conclusions drawn using these sources, indicating that galaxy groups contain significant reservoirs of baryons in their IGM, are still valid when considering this level of uncertainty. In addition, we model the X-ray emission expected from bent-double radio sources. We find that known sources in reasonably dense environments should be detectable in similar to 100 ks Chandra observations. X-ray observations of these sources would place constraints on the IGM density and AGN velocity that are complementary to radio observations.
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