445 research outputs found
Photometry of two DQ white dwarfs - search for spots
Aims. The intensity profiles of the C_2 Swan bands in cool DQ white dwarfs
cannot be adequately fitted with models that otherwise succesfully reproduce
spectral features of the molecule CH in these stars. Initial modelling showed
that a two-component atmosphere in the style of a spot might be able to solve
the problem.
Methods. We photometrically observed the two cool DQ white dwarfs GJ1117 and
EGGR78 to search for variability caused by stellar spots.
Results. We have not found any such variability, but we estimate the effects
of hypothetical spots on lightcurves. We also estimate detection probabilities
for spots in different configurations. Alternative explanations of the problem
are needed and briefly discussed.Comment: 5 pages, 6 figures, published in A&
Fluidity Onset in Graphene
Viscous electron fluids have emerged recently as a new paradigm of
strongly-correlated electron transport in solids. Here we report on a direct
observation of the transition to this long-sought-for state of matter in a
high-mobility electron system in graphene. Unexpectedly, the electron flow is
found to be interaction-dominated but non-hydrodynamic (quasiballistic) in a
wide temperature range, showing signatures of viscous flows only at relatively
high temperatures. The transition between the two regimes is characterized by a
sharp maximum of negative resistance, probed in proximity to the current
injector. The resistance decreases as the system goes deeper into the
hydrodynamic regime. In a perfect darkness-before-daybreak manner, the
interaction-dominated negative response is strongest at the transition to the
quasiballistic regime. Our work provides the first demonstration of how the
viscous fluid behavior emerges in an interacting electron system.Comment: 8pgs, 4fg
Measuring Hall Viscosity of Graphene's Electron Fluid
Materials subjected to a magnetic field exhibit the Hall effect, a phenomenon
studied and understood in fine detail. Here we report a qualitative breach of
this classical behavior in electron systems with high viscosity. The viscous
fluid in graphene is found to respond to non-quantizing magnetic fields by
producing an electric field opposite to that generated by the classical Hall
effect. The viscous contribution is large and identified by studying local
voltages that arise in the vicinity of current-injecting contacts. We analyze
the anomaly over a wide range of temperatures and carrier densities and extract
the Hall viscosity, a dissipationless transport coefficient that was long
identified theoretically but remained elusive in experiment. Good agreement
with theory suggests further opportunities for studying electron
magnetohydrodynamics.Comment: 18 pages, 9 figure
Evidence for Extremely High Dust Polarization Efficiency in NGC 3184
Recent studies have found the Type II-plateau supernova (SN) 1999gi to be
highly polarized (p_max = 5.8%, where p_max is the highest degree of
polarization measured in the optical bandpass; Leonard & Filippenko 2001) and
minimally reddened (E[B-V] = 0.21 +/- 0.09 mag; Leonard et al. 2002). From
multiple lines of evidence, including the convincing fit of a ``Serkowski''
interstellar polarization (ISP) curve to the continuum polarization shape, we
conclude that the bulk of the observed polarization is likely due to dust along
the line of sight (l-o-s), and is not intrinsic to SN 1999gi. We present new
spectropolarimetric observations of four distant Galactic stars close to the
l-o-s to SN 1999gi (two are within 0.02 degrees), and find that all are null to
within 0.2%, effectively eliminating Galactic dust as the cause of the high
polarization. The high ISP coupled with the low reddening implies an
extraordinarily high polarization efficiency for the dust along this l-o-s in
NGC 3184: ISP / E(B-V) = 31^{+22}_{-9} % mag^{-1}. This is inconsistent with
the empirical Galactic limit (ISP / E[B-V] < 9% mag^{-1}), and represents the
highest polarization efficiency yet confirmed for a single sight line in either
the Milky Way or an external galaxy.Comment: 27 pages, accepted for publication by the Astronomical Journa
Statistical Properties of Galactic Starlight Polarization
We present a statistical analysis of Galactic interstellar polarization from
the largest compilation available of starlight data. The data comprises ~ 9300
stars of which we have selected ~ 5500 for our analysis. We find a nearly
linear growth of mean polarization degree with extinction. The amplitude of
this correlation shows that interstellar grains are not fully aligned with the
Galactic magnetic field, which can be interpreted as the effect of a large
random component of the field. In agreement with earlier studies of more
limited scope, we estimate the ratio of the uniform to the random
plane-of-the-sky components of the magnetic field to be B_u/B_r = 0.8.
Moreover, a clear correlation exists between polarization degree and
polarization angle what provides evidence that the magnetic field geometry
follows Galactic structures on large-scales. The angular power spectrum C_l of
the starlight polarization degree for Galactic plane data (|b| < 10 deg) is
consistent with a power-law, C_l ~ l^{-1.5} (where l ~ 180 deg/\theta is the
multipole order), for all angular scales \theta > 10 arcmin. An investigation
of sparse and inhomogeneous sampling of the data shows that the starlight data
analyzed traces an underlying polarized continuum that has the same power
spectrum slope, C_l ~ l^{-1.5}. Our findings suggest that starlight data can be
safely used for the modeling of Galactic polarized continuum emission at other
wavelengths.Comment: 31 pages, 11 figures. Minor corrections and some clarifications
included. Matches version accepted for publication by the Astrophysical
Journa
Spectropolarimetric observations of cool DQ white dwarfs
Following our recent discovery of a new magnetic DQ white dwarf (WD) with CH molecular features, we report the results for the rest of the DQ WDs from our survey. We use high signal-to-noise spectropolarimetric data to search for magnetic fields in a sample of 11 objects. One object in our sample, WD1235+422, shows the signs of continuum circular polarization that is similar to some peculiar DQs with unidentified molecular absorption bands, but the low S/N and spectral resolution of these data make more observations necessary to reveal the true nature of this object
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