Monthly Notices of the Royal Astronomical Society Letters

Catala, C.; Donati, J.-F.; Shkolnik, E.; Bohlender, D.; Alecian, E.

doi: 10.1111/j.1745-3933.2006.00261.xpmid: N/A

We have obtained high-resolution spectropolarimetric data for the planet-hosting star τ Bootis, using the ESPaDOnS spectropolarimeter at the Canada–France–Hawaii Telescope (CFHT). A weak but clear Stokes V signature is detected on three of the four nights of 2006 June during which we have recorded data. This polarimetric signature indicates with no ambiguity the presence of a magnetic field at the star's surface, with intensity of just a few gauss.The analysis of the photospheric lines of τ Boo at ultra-high signal-to-noise ratio reveals the presence of an 18 per cent relative differential rotation. Tentative Zeeman–Doppler imaging, using our spectropolarimetric observations covering only a fraction of the star's rotational phase, indicates a magnetic field with a dominant potential field component. The data are best fitted when a 3.1-d period of modulation and an intermediate inclination are assumed. Considering the level of differential rotation of τ Boo, this implies a rotation period of 3.0 d at the equator and of 3.7 d at the pole, and a topology of the magnetic field where its main non-axisymmetric part is located at low latitudes.The planet is probably synchronized with the star's rotation at intermediate latitudes, while the non-axisymmetric part of the magnetic field seems located at lower latitudes. Our limited data do not provide sufficient constraints on the magnetic field to study a possible interaction of the planet with the star's magnetosphere. Investigating this issue will require data with much better phase coverage. Similar studies should also be performed for other stars hosting close-in giant planets.

Done, Chris; Sobolewska, Małgorzata A.; Gierliński, Marek; Schurch, Nicholas J.

doi: 10.1111/j.1745-3933.2006.00255.xpmid: N/A

Narrow line Seyfert 1 galaxies are generally accreting at high fractions of the Eddington limit. They can show complex X-ray spectra, with a strong ‘soft excess’ below 2 keV and a sharp drop at ∼7 keV. There is strong evidence linking the soft excess to either reflection or absorption from relativistic, partially ionized material close to the black hole. The reflection models can also simultaneously produce the 7-keV feature from fluorescent iron Kα line emission from the disc. Here we show that absorption can also produce a sharp feature at 7 keV from the P Cygni profile which results from absorption/scattering/emission of He- and H-like iron Kα resonance lines in the wind. We demonstrate this explicitly by fitting the iron feature seen in XMM–Newton data from 1H 0707-495 to a P Cygni profile. The resulting column and ionization required to produce this feature are probably larger than those needed to produce the soft excess. Nonetheless, the absorbing material could still be a single structure with stratified ionization such as that produced by the ionization instability.

Evans, A.; Kerr, T.; Yang, Bin; Matsuoka, Y.; Tsuzuki, Y.; Bode, M. F.; Eyres, S. P. S.; Geballe, T. R.; Woodward, C. E.; Gehrz, R. D.; Lynch, D. K.; Rudy, R. J.; Russell, R. W.; O'Brien, T. J.; Starrfield, S. G.; Davis, R. J.; Ness, J.-U.; Drake, J.; Osborne, J. P.; Page, K. L.; Adamson, A.;

Babich, Daniel; Loeb, Abraham

doi: 10.1111/j.1745-3933.2006.00257.xpmid: N/A

Thomson scattering of the cosmic microwave background (CMB) on moving electrons in the outflows of Lyman break galaxies (LBGs) at redshifts 2–8 contributes to the small-scale CMB anisotropies. The net effect produced by each outflow depends on its level of deviation from spherical symmetry, caused either by an anisotropic energy injection from the nuclear starburst or quasar activity, or by an inhomogeneous intergalactic environment. We find that for plausible outflow parameters consistent with spectroscopic observations of LBGs, the induced CMB anisotropies on arcminute scales reach up to ∼1 μK, comparable to the level produced during the epoch of reionization.

Pan, Jun

doi: 10.1111/j.1745-3933.2006.00253.xpmid: N/A

Fractional Brownian motion with index α is introduced to construct the fractional excursion set model. A new mass function with single parameter α is derived within the formalism, of which the Press–Schechter (PS) mass function is a special case when α= 1/2. Although the new mass function is computed assuming spherical collapse, comparison with the Sheth–Tormen (ST) fitting function shows that the new mass function of α≈ 0.435 agrees with the ST function remarkably well in the high-mass regime, while it predicts more low-mass haloes than the ST function but less than the PS function. The index α is the Hurst exponent, the exact value of which in the context of structure formation is modulated by properties of the smoothing window function and the shape of the power spectrum. It is conjectured that the halo merging rate and merging history in the fractional set theory might be imprinted with the interplay between haloes at small scales and their large-scale environment. Also, the mass function in the high-mass regime can be a good tool to detect the non-Gaussianity of the initial density fluctuation.

Klessen, Ralf S.; Spaans, Marco; Jappsen, Anne-Katharina

doi: 10.1111/j.1745-3933.2006.00258.xpmid: N/A

Understanding the origin of stellar masses is a key problem in astrophysics. In the solar neighbourhood, the mass distribution of stars follows a seemingly universal pattern. In the centre of the Milky Way, however, there are indications for strong deviations and the same may be true for the nuclei of distant starburst galaxies. Here we present the first numerical hydrodynamical calculations of stars formed in a molecular region with chemical and thermodynamic properties similar to those of warm and dusty circumnuclear starburst regions. The resulting initial mass function is top-heavy with a peak at ∼15 M⊙, a sharp turn-down below ∼7 M⊙ and a power-law decline at high masses. We find a natural explanation for our results in terms of the temperature dependence of the Jeans mass, with collapse occurring at a temperature of ∼100 K and an H2 density of a few ×105 cm−3, and discuss possible implications for galaxy formation and evolution.

King, Andrew; Olsson, Emma; Davies, Melvyn B.

doi: 10.1111/j.1745-3933.2006.00259.xpmid: N/A

We consider gamma-ray bursts (GRBs) produced by the merger of a massive white dwarf with a neutron star. We show that these are likely to produce long-duration GRBs, in some cases definitely without an accompanying supernova, as observed recently. This class of burst would have a strong correlation with star formation, and occur close to the host galaxy. However, rare members of the class need not be near star-forming regions, and could have any type of host galaxy. Thus a long-duration burst far from any star-forming region would also be a signature of this class. Estimates based on the existence of a known progenitor suggest that our proposed class may be an important contributor to the observed GRB rate.

King, Lindsay; Corless, Virginia

doi: 10.1111/j.1745-3933.2006.00260.xpmid: N/A

The distribution of mass on galaxy cluster scales is an important test of structure formation scenarios, providing constraints on the nature of dark matter itself. Several techniques have been used to probe the mass distributions of clusters, sometimes yielding results which are discrepant, or at odds with clusters formed in simulations – for example giving Navarro–Frenk–White (NFW) concentration parameters much higher than expected in the standard cold dark matter (CDM) model. In addition, the velocity fields of some well-studied galaxy clusters reveal the presence of several structures close to the line of sight, often not dynamically bound to the cluster itself. We investigate what impact such neighbouring but unbound massive structures would have on the determination of cluster profiles using weak gravitational lensing. Depending on its concentration and mass ratio to the primary halo, one secondary halo close to the line-of-sight can cause the estimated NFW concentration parameter to be significantly higher than that of the primary halo, and also cause the estimated mass to be biased high. Although it is difficult to envisage how this mechanism alone could yield concentrations as high as reported for some clusters, multiple haloes close to the line-of-sight, such as in the case of Abell 1689, can substantially increase the concentration parameter estimate. Together with the fact that clusters are triaxial, and that including baryonic physics also leads to an increase in the concentration of a dark matter halo, the tension between observations and the standard CDM model is eased. Additionally, we note that if the alignment with the secondary structure is imprecise, then the estimated concentration parameter can also be even lower than that of the primary halo, reinforcing the importance of identifying structures in cluster fields.

Rushton, A.; Spencer, R. E.; Strong, M.; Campbell, R. M.; Casey, S.; Fender, R. P.; Garrett, M. A.; Miller-Jones, J. C. A.; Pooley, G. G.; Reynolds, C.; Szomoru, A.; Tudose, V.; Paragi, Z.

doi: 10.1111/j.1745-3933.2006.00262.xpmid: N/A

We present results from the first successful open call electronic very-long-baseline interferometry (e-VLBI) science run, observing the X-ray binary GRS 1915+105. e-VLBI science allows the rapid production of VLBI radio maps, within hours of an observation rather than weeks, facilitating a decision for follow-up observations. A total of six telescopes observing at 5 GHz across the European VLBI Network (EVN) were correlated in real time at the Joint Institute for VLBI in Europe (JIVE). Constant data rates of 128 Mbps were transferred from each telescope, giving 4 TB of raw sampled data over the 12 hours of the whole experiment. Throughout this, GRS 1915+105 was observed for a total of 5.5 h, producing 2.8 GB of visibilities of correlated data. A weak flare occurred during our observations, and we detected a slightly resolved component of 2.7 × 1.2 ms with a position angle of 140°± 2°. The peak brightness was 10.2 mJy per beam, with a total integrated radio flux of 11.1 mJy.

Li, Li-Xin

doi: 10.1111/j.1745-3933.2006.00256.xpmid: N/A

In this Letter we show that there is a redshift degeneracy in the Eiso–Epeak relation of gamma-ray bursts (GRBs). If a GRB has a redshift solved from the Eiso–Epeak relation that lies in the range of 0.9 < z < 20, a GRB that has the same observed fluence and peak spectral energy but is at a different redshift in that range also satisfies the Eiso–Epeak relation within a 1σ error, implying an extremely large error in the calculated redshift. Even if the data scatter in the Eiso–Epeak relation is reduced by a factor of 2, the error in the predicted redshift is still large enough to prevent one from constraining the redshift meaningfully. Hence, the Eiso–Epeak relation is not useful for determining the GRB distance.