Monthly Notices of the Royal Astronomical Society Letters

Bucciantini, N.; Quataert, E.; Arons, J.; Metzger, B. D.; Thompson, T. A.

doi: 10.1111/j.1745-3933.2007.00403.xpmid: N/A

We present time-dependent axisymmetric magnetohydrodynamic simulations of the interaction of a relativistic magnetized wind produced by a proto-magnetar with a surrounding stellar envelope, in the first ∼10 s after core collapse. We inject a super-magnetosonic wind with into a cavity created by an outgoing supernova shock. A strong toroidal magnetic field builds up in the bubble of plasma and magnetic field that is at first inertially confined by the progenitor star. This drives a jet out along the polar axis of the star, even though the star and the magnetar wind are each spherically symmetric. The jet has the properties needed to produce a long-duration gamma-ray burst (GRB). At ∼5 s after core bounce, the jet has escaped the host star and the Lorentz factor of the material in the jet at large radii ∼1011 cm is similar to that in the magnetar wind near the source. Most of the spindown power of the central magnetar escapes via the relativistic jet. There are fluctuations in the Lorentz factor and energy flux in the jet on a ∼ 0.01–0.1 s time-scale. These may contribute to variability in GRB emission (e.g. via internal shocks).

Angus, G. W.; Famaey, B.; Tiret, O.; Combes, F.; Zhao, H. S.

doi: 10.1111/j.1745-3933.2007.00393.xpmid: N/A

The recent Sloan Digital Sky Survey measured velocity distribution of satellite galaxies has been modelled in the context of Modified Newtonian Dynamics (MOND). We show that even when the extra constraint of adhering to the projected satellite number density profile is added, the two line-of-sight (los) velocity dispersion profiles presented in Klypin & Prada can be matched simply with a radially varying anisotropy. Interestingly, the anisotropies required to fit the los velocity dispersions are remarkably similar to the anisotropies generated by dissipationless collapse simulations in MOND. The mass-to-light ratios of the two host galaxies used are sensible, and positivity of the distribution function is satisfied.

Mysen, E.

doi: 10.1111/j.1745-3933.2007.00405.xpmid: N/A

An analytical model for the diurnal YORP and Yarkovsky effects is described, which reproduces the overall results from previous numerical studies. Important shape parameters are identified and general forms of the differential equations are presented. The force cosines for comet orbit solutions are modified in order to include both thermal lag and geometrical effects.

Froebrich, D.; Meusinger, H.; Davis, C. J.

doi: 10.1111/j.1745-3933.2007.00409.xpmid: N/A

We are conducting a large programme to classify newly discovered Milky Way star cluster candidates from Froebrich et al. Here we present near-infrared follow-up observations of FSR 0190 (, J2000). The cluster is situated close to the Galactic plane . It shows a circular shape, and a relatively large number of core helium burning stars – which clearly distinguishes the cluster from the rich field – but no centrally condensed star density profile. We derive an age of more than 7 Gyr, a Galactocentric distance of 10.5 kpc, a distance of 10 kpc from the Sun, and an extinction of AK= 0.8 mag. The estimated mass is at least of the order of 105 M⊙, and the absolute brightness is MV≤−4.7 mag; both are rather typical properties for Palomar-type globular clusters.

Svensson, Karl M.; Church, Ross P.; Davies, Melvyn B.

doi: 10.1111/j.1745-3933.2007.00400.xpmid: N/A

We have computed the Galactic trajectories of 12 hypervelocity stars (HVSs) under the assumption that they originated in the Galactic Centre. We show that eight of these 12 stars are bound to the Galaxy. We consider the subsequent trajectories of the bound stars to compute their characteristic orbital period, which is 2 Gyr. All eight bound stars are moving away from the centre of the Galaxy, which implies that the stars' lifetimes are less than 2 Gyr. We thus infer that the observed HVSs are massive main-sequence stars, rather than blue horizontal branch stars. The observations suggest that blue HVSs are ejected from the Galactic Centre roughly every 15 Myr. This is consistent with the observed population of blue stars in extremely tight orbits round the central supermassive black hole (SMBH), the so-called S-stars, if we assume that the HVSs are produced by the breakup of binaries. One of the stars in such a binary is ejected at high velocities to form a HVS; the other remains bound to the SMBH as an S-star.We further show that the one high-velocity system observed to be moving towards the Galactic Centre, SDSS J172226.55+594155.9, could not have originated in the Galactic Centre; rather, we identify it as a halo object.

Gaburov, E.; Lombardi, J. C.; Portegies Zwart, S.

doi: 10.1111/j.1745-3933.2007.00399.xpmid: N/A

The early evolution of dense star clusters is possibly dominated by close interactions between stars, and physical collisions between stars may occur quite frequently. Simulating a stellar collision event can be an intensive numerical task, as detailed calculations of this process require hydrodynamic simulations in three dimensions. We present a computationally inexpensive method in which we approximate the merger process, including shock heating, hydrodynamic mixing and mass loss, with a simple algorithm based on conservation laws and a basic qualitative understanding of the hydrodynamics of stellar mergers. The algorithm relies on Archimedes' principle to dictate the distribution of the fluid in the stable equilibrium situation. We calibrate and apply the method to mergers of massive stars, as these are expected to occur in young and dense star clusters. We find that without the effects of microscopic mixing, the temperature and chemical composition profiles in a collision product can become double-valued functions of enclosed mass. Such an unphysical situation is mended by simulating microscopic mixing as a post-collision effect. In this way we find that head-on collisions between stars of the same spectral type result in substantial mixing, while mergers between stars of different spectral type, such as type B and O stars (∼10 and ∼40 M⊙ respectively), are subject to relatively little hydrodynamic mixing. Our algorithm has been implemented in an easy-to-use software package, which we have made publicly available for download.1

Davis, D. S.; Richer, H. B.; King, I. R.; Anderson, J.; Coffey, J.; Fahlman, G. G.; Hurley, J.; Kalirai, J. S.

doi: 10.1111/j.1745-3933.2007.00402.xpmid: N/A

We have examined the radial distribution of white dwarfs over a single Hubble Space Telescope/Advanced Camera for Surveys field in the nearby globular cluster NGC 6397. In relaxed populations, such as in a globular cluster, stellar velocity dispersion, and hence radial distribution, is directly dependent on stellar masses. The progenitors of very young cluster white dwarfs had a mass of ∼0.8 M⊙, while the white dwarfs themselves have a mass of ∼0.5 M⊙. We thus expect young white dwarfs to have a concentrated radial distribution (like that of their progenitors) that becomes more extended over several relaxation times to mimic that of ∼0.5-M⊙ main-sequence stars. However, we observe young white dwarfs to have a significantly extended radial distribution compared both with the most massive main-sequence stars in the cluster and with old white dwarfs.

Auger, M. W.

doi: 10.1111/j.1745-3933.2007.00408.xpmid: N/A

We report on an investigation of the environments of the Sloan Lens ACS Survey (SLACS) sample of gravitational lenses. The local and global environments of the lenses are characterized using Sloan Digital Sky Survey (SDSS) photometry and, when available, spectroscopy. We find that the lens systems that are best modelled with steeper than isothermal density profiles are more likely to have close companions than lenses with shallower than isothermal profiles. This suggests that the profile steepening may be caused by interactions with a companion galaxy as indicated by N-body simulations of group galaxies. The global environments of the SLACS lenses are typical of non-lensing SDSS galaxies with comparable properties to the lenses, and the richnesses of the lens groups are not as strongly correlated with the lens density profiles as are the local environments. Furthermore, we investigate the possibility of line-of-sight contamination affecting the lens models, but do not find a significant over-density of sources compared to lines of sight without lenses.

Teodoro, Luís F. A.; Diver, Declan A.; Hendry, Martin A.

doi: 10.1111/j.1745-3933.2007.00406.xpmid: N/A

This note is concerned with potentially misleading concepts in the treatment of cosmological magnetic fields by magnetohydrodynamical (MHD) modelling. It is not a criticism of MHD itself but rather a cautionary comment on the validity of its use in cosmology. Now that the relevant cosmological data are greatly improved compared with a few decades ago, and even better data are imminent, it makes sense to revisit original modelling assumptions and examine critically their shortcomings in respect of modern science. Specifically, this Letter argues that ideal MHD is a poor approximation around recombination, since it inherently restricts evolutionary time-scales, and is often misapplied in the existing literature.

Nord, B.; Stanek, R.; Rasia, E.; Evrard, A. E.

doi: 10.1111/j.1745-3933.2007.00407.xpmid: N/A

We explore how the behaviour of galaxy cluster scaling relations are affected by flux-limited selection biases and intrinsic covariance among observable properties. Our models presume log-normal covariance between luminosity (L) and temperature (T) at fixed mass (M), centred on evolving, power-law mean relations as a function of host halo mass. Selection can mimic evolution; the L—M and L—T relations from shallow X-ray flux-limited samples will deviate from mass-limited expectations at nearly all scales while the relations from deep surveys (10−14 erg s−1 cm−2) become complete, and therefore unbiased, at masses above ∼2 × 1014h−1 M⊙. We derive expressions for low-order moments of the luminosity distribution at fixed temperature, and show that the slope and scatter of the L—T relation observed in flux-limited samples is sensitive to the assumed L—T correlation coefficient. In addition, L—T covariance affects the redshift behaviour of halo counts and mean luminosity in a manner that is nearly degenerate with intrinsic population evolution.