Galactic Escape Speeds in Mirror and Cold Dark Matter Models

The mirror dark matter (MDM) model of Berezhiani et al. has been shown to reproduce observed galactic rotational curves for a variety of spiral galaxies, and has been presented as an alternative to cold dark matter (CDM) models. We investigate possible additional tests involving the properties of stellar orbits, which may be used to discriminate between the two models. We demonstrate that in MDM and CDM models fitted equally well to a galactic rotational curve, one generally expects predictable differences in escape speeds from the disc. The recent radial velocity (RAVE) survey of the Milky Way has pinned down the escape speed from the solar neighbourhood to $v_{esc}=544^{+64}_{-46}$ km s$^{-1}$, placing an additional constraint on dark matter models. We have constructed an MDM model for the Milky Way based on its rotational curve, and find an escape speed that is just consistent with the observed value given the current errors, which lends credence to the viability of the MDM model. The Gaia-ESO spectroscopic survey is expected to lead to an even more precise estimate of the escape speed that will further constrain dark matter models. However, the largest differences in stellar escape speeds between both models are predicted for dark matter dominated dwarf galaxies such as DDO 154, and kinematical studies of such galaxies could prove key in establishing, or abolishing, the validity of the MDM model.Comment: Accepted for publication in the European Physical Journal

The effects of tidally induced disc structure on white dwarf accretion in intermediate polars

We investigate the effects of tidally induced asymmetric disc structure on accretion onto the white dwarf in intermediate polars. Using numerical simulation, we show that it is possible for tidally induced spiral waves to propagate sufficiently far into the disc of an intermediate polar that accretion onto the central white dwarf could be modulated as a result. We suggest that accretion from the resulting asymmetric inner disc may contribute to the observed X-ray and optical periodicities in the light curves of these systems. In contrast to the stream-fed accretion model for these periodicities, the tidal picture predicts that modulation can exist even for systems with weaker magnetic fields where the magnetospheric radius is smaller than the radius of periastron of the mass transfer stream. We also predict that additional periodic components should exist in the emission from low mass ratio intermediate polars displaying superhumps.Comment: 9 pages, 5 figures, accepted for publication in MNRA

Superconducting Superstructure for the TESLA Collider

We discuss the new layout of a cavity chain (superstructure) allowing, we hope, significant cost reduction of the RF system of both linacs of the TESLA linear collider. The proposed scheme increases the fill factor and thus makes an effective gradient of an accelerator higher. We present mainly computations we have performed up to now and which encouraged us to order the copper model of the scheme, still keeping in mind that experiments with a beam will be necessary to prove if the proposed solution can be used for the acceleration.Comment: 11 page

The open cluster initial-final mass relationship and the high-mass tail of the white dwarf distribution

Recent studies of white dwarfs in open clusters have provided new constraints on the initial - final mass relationship (IFMR) for main sequence stars with masses in the range 2.5 - 6.5 Mo. We re-evaluate the ensemble of data that determines the IFMR and argue that the IFMR can be characterised by a mean initial-final mass relationship about which there is an intrinsic scatter. We investigate the consequences of the IFMR for the observed mass distribution of field white dwarfs using population synthesis calculations. We show that while a linear IFMR predicts a mass distribution that is in reasonable agreement with the recent results from the PG survey, the data are better fitted by an IFMR with some curvature. Our calculations indicate that a significant (~28%) percentage of white dwarfs originating from single star evolution have masses in excess of ~0.8 Mo, obviating the necessity for postulating the existence of a dominant population of high-mass white dwarfs that arise from binary star mergers.Comment: 5 pages, 2 color Postscript figures. Accepted for publication in MNRA

WD1953-011 - a magnetic white dwarf with peculiar field structure

We present H-alpha spectra of the magnetic white dwarf star WD1953-011 which confirm the presence of the broad Zeeman components corresponding to a field strength of about 500kG found by Maxted & Marsh (1999). We also find that the line profile is variable over a timescale of a day or less. The core of the H-alpha line also shows a narrow Zeeman triplet corresponding to a field strength of of about 100kG which appears to be almost constant in shape. These observations suggest that the magnetic field on WD1953-011 has a complex structure and that the star has a rotational period of hours or days which causes the observed variability of the spectra. We argue that neither an offset dipole model nor a double-dipole model are sufficient to explain our observations. Instead, we propose a two component model consisting of a high field region of magnetic field strength of about 500kG covering about 10% of the surface area of the star superimposed on an underlying dipolar field of mean field strength of about 70kG. Radial velocity measurements of the narrow Zeeman triplet show that the radial velocity is constant to within a few km/s so this star is unlikely to be a close binary.Comment: Accpeted for publication in MNRAS. 4 pages, 2 figure

V405 Aurigae: A High Magnetic Field Intermediate Polar

Our simultaneous multicolor (UBVRI) circular polarimetry has revealed nearly sinusoidal variation over the WD spin cycle, and almost symmetric positive and negative polarization excursions. Maximum amplitudes are observed in the B and V bands (+-3 %). This is the first time that polarization peaking in the blue has been discovered in an IP, and suggests that V405 Aur is the highest magnetic field IP found so far. The polarized flux spectrum is similar to those found in polars with magnetic fields in the range B ~ 25-50 MG. Our low resolution circular spectropolarimetry has given evidence of transient features which can be fitted by cyclotron harmonics n = 6, 7, and 8, at a field of B = 31.5 +- 0.8 MG, consistent with the broad-band polarized flux spectrum. Timings of the circular polarization zero crossovers put strict upper limits on WD spin period changes and indicate that the WD in V405 Aur is currently accreting closely at the spin equilibrium rate, with very long synchronization timescales, T_s > 10^9 yr. For the observed spin to orbital period ratio, P_{spin}/P_{orb} = 0.0365, and P_{orb} ~ 4.15 hr, existing numerical accretion models predict spin equilibrium condition with B ~ 30 MG if the mass ratio of the binary components is q_1 ~ 0.4. The high magnetic field makes V405 Aur a likely candidate as a progenitor of a polar.Comment: To appear in The Astrophysical Journal, September 1 Issue (2008), 9 pages, 10 figure

Observations of radio pulses from CU Virginis

The magnetic chemically peculiar star CU Virginis is a unique astrophysical laboratory for stellar magnetospheres and coherent emission processes. It is the only known main sequence star to emit a radio pulse every rotation period. Here we report on new observations of the CU Virginis pulse profile in the 13 and 20\,cm radio bands. The profile is known to be characterised by two peaks of 100$\%$ circularly polarised emission that are thought to arise in an electron-cyclotron maser mechanism. We find that the trailing peak is stable at both 13 and 20\,cm, whereas the leading peak is intermittent at 13\,cm. Our measured pulse arrival times confirm the discrepancy previously reported between the putative stellar rotation rates measured with optical data and with radio observations. We suggest that this period discrepancy might be caused by an unknown companion or by instabilities in the emission region. Regular long-term pulse timing and simultaneous multi-wavelength observations are essential to clarify the behaviour of this emerging class of transient radio source.Comment: Accepted by MNRAS Letters; 5 pages, 2 figures, 3 table

Towards a Conceptualization of Sociomaterial Entanglement

In knowledge representation, socio-technical systems can be modeled as multiagent systems in which the local knowledge of each individual agent can be seen as a context. In this paper we propose formal ontologies as a means to describe the assumptions driving the construction of contexts as local theories and to enable interoperability among them. In particular, we present two alternative conceptualizations of the notion of sociomateriality (and entanglement), which is central in the recent debates on socio-technical systems in the social sciences, namely critical and agential realism. We thus start by providing a model of entanglement according to the critical realist view, representing it as a property of objects that are essentially dependent on different modules of an already given ontology. We refine then our treatment by proposing a taxonomy of sociomaterial entanglements that distinguishes between ontological and epistemological entanglement. In the final section, we discuss the second perspective, which is more challenging form the point of view of knowledge representation, and we show that the very distinction of information into modules can be at least in principle built out of the assumption of an entangled reality