3,441 research outputs found
Massive stars in the hinterland of the young cluster, Westerlund 2
Accepted for publication in MNRAS, 13 July 2018. 16 pages, plus one-page table in an appendix.An unsettled question concerning the formation and distribution of massive stars is whether they must be born in massive clusters and, if found in less dense environments, whether they must have migrated there. With the advent of wide-area digital photometric surveys, it is now possible to identify massive stars away from prominent Galactic clusters without bias. In this study we consider 40 candidate OB stars found in the field around the young massive cluster, Westerlund 2, by Mohr-Smith et al.: these are located inside a box of 1.5 × 1.5 deg 2 and are selected on the basis of their extinctions and K magnitudes.We present VLT/X-shooter spectra of two of the hottest O stars, respectively 11 and 22 arcmin from the centre of Westerlund 2. They are confirmed as O4V stars, with stellar masses likely to be in excess of 40 M ·. Their radial velocities relative to the non-binary reference object, MSP 182, in Westerlund 2 are -29.4 ± 1.7 and -14.4 ± 2.2 km s -1, respectively. Using Gaia DR2 proper motions we find that between 8 and 11 early O/WR stars in the studied region (including the two VLT targets, plus WR 20c and WR 20aa) could have been ejected fromWesterlund 2 in the last one million years. This represents an efficiency of massive-star ejection of up to ~ 25 per cent. On sky, the positions of these stars and their proper motions show a near N-S alignment. We discuss the possibility that these results are a consequence of prior sub-cluster merging combining with dynamical ejection.Peer reviewe
The physical properties of AM CVn stars: new insights from Gaia DR2
AM CVn binaries are hydrogen deficient compact binaries with an orbital
period in the 5-65 min range and are predicted to be strong sources of
persistent gravitational wave radiation. Using Gaia Data Release 2, we present
the parallaxes and proper motions of 41 out of the 56 known systems. Compared
to the parallax determined using the HST Fine Guidance Sensor we find that the
archetype star, AM CVn, is significantly closer than previously thought. This
resolves the high luminosity and mass accretion rate which models had
difficulty in explaining. Using Pan-STARRS1 data we determine the absolute
magnitude of the AM CVn stars. There is some evidence that donor stars have a
higher mass and radius than expected for white dwarfs or that the donors are
not white dwarfs. Using the distances to the known AM CVn stars we find strong
evidence that a large population of AM CVn stars have still to be discovered.
As this value sets the background to the gravitational wave signal of LISA,
this is of wide interest. We determine the mass transfer rate for 15 AM CVn
stars and find that the majority have a rate significantly greater than
expected from standard models. This is further evidence that the donor star has
a greater size than expected.Comment: Accepted by A&A in main journa
Electro-Mechanical Fredericks Effects in Nematic Gels
The solid nematic equivalent of the Fredericks transition is found to depend
on a critical field rather than a critical voltage as in the classical case.
This arises because director anchoring is principally to the solid rubbery
matrix of the nematic gel rather than to the sample surfaces. Moreover, above
the threshold field, we find a competition between quartic (soft) and
conventional harmonic elasticity which dictates the director response. By
including a small degree of initial director misorientation, the calculated
field variation of optical anisotropy agrees well with the conoscopy
measurements of Chang et al (Phys.Rev.E56, 595, 1997) of the electro-optical
response of nematic gels.Comment: Latex (revtex style), 5 EPS figures, submitted to PRE, corrections to
discussion of fig.3, cosmetic change
Aging and memory phenomena in magnetic and transport properties of vortex matter: a brief review
There is mounting experimental evidence that strong off-equilibrium
phenomena, such as ``memory'' or ``aging'' effects, play a crucial role in the
physics of vortices in type II superconductors. We give a short review, based
on a recently introduced schematic vortex model, of current progresses in
understanding out of equilibrium vortex behaviours. We develop a unified
description of ``memory'' phenomena in magnetic and transport properties, such
as magnetisation loops and their ``anomalous'' 2nd peak, logarithmic creep,
``anomalous'' finite creep rate in the limit of vanishing temperature,
``memory'' and ``irreversibility'' in I-V characteristics, time dependent
critical currents, ``rejuvenation'' and ``aging'' of the system response.Comment: updated versio
Temperature and magnetic-field dependence of the conductivity of YBaCuO films in the vicinity of superconducting transition: Effect of Tc-inhomogeneity
Temperature and magnetic field dependences of the conductivity of YBaCuO
films in the transition region are analyzed taking into account spatial
inhomogeneity in transition temperature, Tc.
(i) An expression for the superconducting contribution to conductivity,
\sigma_s(T,H,Tc), of a homogeneous superconductor for H<<Hc2(T=0) is obtained
using the solution of the Ginzburg-Landau equation in form of perturbation
expansions [S.Ullah, A.T.Dorsey, PRB 44, 262 (1991)].
(ii) The error in \sigma_s(T,H,Tc) occurring due to the presence of
Tc-inhomogeneity is calculated and plotted on an H-T plane diagram. These
calculations use an effective medium approximation and a Gaussian distribution
of Tc.
(iii) Measuring the temperature dependences of a voltage, induced by a
focused electron beam, we determine spatial distributions of the critical
temperature for YBaCuO microbridges with a 2 micron resolution. A typical
Tc-distribution dispersion is found to be approximately 1K. For such
dispersion, error in \sigma_s(T,H,Tc) due to Tc-inhomogeneity exceeds 30% for
magnetic fields H < 1 T and temperatures |T-Tc| < 0.5 K.
(iv) Experimental R(T,H) dependences of resistance are well described by a
numerical solution of a set of Kirchoff equations for the resistor network
based on the measured spatial distributions of Tc and the expression for
\sigma_s(T,H,Tc).Comment: REVTeX, 12 pages including 7 figures, resubmitted to Phys. Rev.
iPTF16geu: A multiply imaged, gravitationally lensed type Ia supernova
We report the discovery of a multiply-imaged gravitationally lensed Type Ia
supernova, iPTF16geu (SN 2016geu), at redshift . This phenomenon could
be identified because the light from the stellar explosion was magnified more
than fifty times by the curvature of space around matter in an intervening
galaxy. We used high spatial resolution observations to resolve four images of
the lensed supernova, approximately 0.3" from the center of the foreground
galaxy. The observations probe a physical scale of 1 kiloparsec, smaller
than what is typical in other studies of extragalactic gravitational lensing.
The large magnification and symmetric image configuration implies close
alignment between the line-of-sight to the supernova and the lens. The relative
magnifications of the four images provide evidence for sub-structures in the
lensing galaxy.Comment: Matches published versio
Phase-Resolved Spectroscopy of Gaia14aae: Line Emission From Near the White Dwarf Surface
AM CVn binaries are a class of ultracompact, hydrogen-deficient binaries,
each consisting of a white dwarf accreting helium-dominated material from a
degenerate or semi-degenerate donor star. Of the 56 known systems, only
Gaia14aae undergoes complete eclipses of its central white dwarf, allowing the
parameters of its stellar components to be tightly constrained. Here, we
present phase-resolved optical spectroscopy of Gaia14aae. We use the spectra to
test the assumption that the narrow emission feature known as the `central
spike' traces the motion of the central white dwarf. We measure a central spike
velocity amplitude of km/s, which agrees at the 1 level
with the predicted value of km/s based on eclipse-derived system
parameters. The orbital phase offset of the central spike from its expected
position is , consistent with 0 . Doppler maps of
the He I lines in Gaia14aae show two accretion disc bright spots, as seen in
many AM CVn systems. The formation mechanism for the second spot remains
unclear. We detect no hydrogen in the system, but we estimate a 3
limit on H emission with an equivalent width of -1.14 \AA. Our
detection of nitrogen and oxygen with no corresponding detection of carbon, in
conjunction with evidence from recent studies, mildly favours a formation
channel in which Gaia14aae is descended from a cataclysmic variable with a
significantly evolved donor.Comment: 16 pages, accepted by MNRA
Unified order-disorder vortex phase transition in high-Tc superconductors
The diversity of vortex melting and solid-solid transition lines measured in
different high-T superconductors is explained, postulating a unified
order-disorder phase transition driven by both thermally- and disorder-induced
fluctuations. The temperature dependence of the transition line and the nature
of the disordered phase (solid, liquid, or pinned liquid) are determined by the
relative contributions of these fluctuations and by the pinning mechanism. By
varying the pinning mechanism and the pinning strength one obtains a spectrum
of monotonic and non-monotonic transition lines similar to those measured in
BiSrCaCuO, YBaCuO,
NdCeCuO,
BiPbSrCaCuO and (LaSr)CuOComment: To be published in Phys. Rev. B Rapid Com
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