4,647 research outputs found
Mass loss rates from mid-IR excesses in LMC and SMC O stars
We use a combination of BVJHK and Spitzer [3.6], [5.8] and [8.0] photometry
to determine IR excesses for a sample of 58 LMC and 46 SMC O stars. This sample
is ideal for determining IR excesses because the very small line of sight
reddening minimizes uncertainties due to extinction corrections. We use the
core-halo model developed by Lamers & Waters (1984a) to translate the excesses
into mass loss rates and demonstrate that the results of this simple model
agree with the more sophisticated CMFGEN models to within a factor of 2. Taken
at face value, the derived mass loss rates are larger than those predicted by
Vink et al. (2001), and the magnitude of the disagreement increases with
decreasing luminosity. However, the IR excesses need not imply large mass loss
rates. Instead, we argue that they probably indicate that the outer atmospheres
of O stars contain complex structures and that their winds are launched with
much smaller velocity gradients than normally assumed. If this is the case, it
could affect the theoretical and observational interpretations of the "weak
wind" problem, where classical mass loss indicators suggest that the mass loss
rates of lower luminosity O stars are far less than expected.Comment: 15 pages, 10 figures. Accepted for publication in MNRA
The effects of clumping on wind line variability
We review the effects of clumping on the profiles of resonance doublets. By
allowing the ratio of the doublet oscillator strenghts to be a free parameter,
we demonstrate that doublet profiles contain more information than is normally
utilized. In clumped (or porous) winds, this ratio can lies between unity and
the ratio of the f-values, and can change as a function of velocity and time,
depending on the fraction of the stellar disk that is covered by material
moving at a particular velocity at a given moment. Using these insights, we
present the results of SEI modeling of a sample of B supergiants, zeta Pup and
a time series for a star whose terminal velocity is low enough to make the
components of its Si IV 1400 doublet independent. These results are interpreted
within the framework of the Oskinova et al. (2007) model, and demonstrate how
the doublet profiles can be used to extract infromation about wind structure.Comment: 3 pages, to appear in Clumping in Hot Star Winds, W.-R. Hamann, A.
Feldmeier & L. Oskinova, eds., Potsdam: Univ.-Verl., 2007, URN:
http://nbn-resolving.de/urn:nbn:de:kobv:517-opus-1398
Social Collaborative Retrieval
Socially-based recommendation systems have recently attracted significant
interest, and a number of studies have shown that social information can
dramatically improve a system's predictions of user interests. Meanwhile, there
are now many potential applications that involve aspects of both recommendation
and information retrieval, and the task of collaborative retrieval---a
combination of these two traditional problems---has recently been introduced.
Successful collaborative retrieval requires overcoming severe data sparsity,
making additional sources of information, such as social graphs, particularly
valuable. In this paper we propose a new model for collaborative retrieval, and
show that our algorithm outperforms current state-of-the-art approaches by
incorporating information from social networks. We also provide empirical
analyses of the ways in which cultural interests propagate along a social graph
using a real-world music dataset.Comment: 10 page
Structure and clumping in the fast wind of NGC6543
Far-UV spectroscopy from the FUSE satellite is analysed to uniquely probe
spatial structure and clumping in the fast wind of the central star of the
H-rich planetary nebula NGC6543 (HD164963). Time-series data of the unsaturated
PV 1118, 1128 resonance line P Cygni profiles provide a very sensitive
diagnostic of variable wind conditions in the outflow. We report on the
discovery of episodic and recurrent optical depth enhancements in the PV
absorption troughs, with some evidence for a 0.17-day modulation time-scale.
SEI line-synthesis modelling is used to derive physical properties, including
the optical depth evolution of individual `events'. The characteristics of
these features are essentially identical to the `discrete absorption
components' (DACs) commonly seen in the UV lines of massive OB stars. We have
also employed the unified model atmosphere code CMFGEN to explore spectroscopic
signatures of clumping, and report in particular on the clear sensitivity of
the PV lines to the clump volume filling factor. The results presented here
have implications for the downward revision of mass-loss rates in PN central
stars. We conclude that the temporal structures seen in the PV lines of NGC6543
likely have a physical origin that is similar to that operating in massive,
luminous stars, and may be related to near-surface perturbations caused by
stellar pulsation and/or magnetic fields.Comment: 11 pages, 11 figures. Accepted for publication in MNRA
Semi-Teleparallel Theories of Gravitation
A class of theories of gravitation that naturally incorporates preferred
frames of reference is presented. The underlying space-time geometry consists
of a partial parallelization of space-time and has properties of Riemann-Cartan
as well as teleparallel geometry. Within this geometry, the kinematic
quantities of preferred frames are associated with torsion fields. Using a
variational method, it is shown in which way action functionals for this
geometry can be constructed. For a special action the field equations are
derived and the coupling to spinor fields is discussed.Comment: 14 pages, LaTe
CIR Modulation of the X-ray Flux from the O7.5 III(n)((f)) Star xi Persei?
We analyze a 162 ks HETG Chandra observation of the O7.5 III(n)((f)) star xi
Per, together with contemporaneous H alpha observations. The X-ray spectrum of
this star is similar to other single O stars, and not pathological in any way.
Its UV wind lines are known to display cyclical time variability, with a period
of 2.086 days, which is thought to be associated with co-rotating interaction
regions (CIRs). We examine the Chandra and H alpha data for variability on this
time scale. We find that the X-rays vary by about 15% over the course of the
observations and that this variability is out of phase with variable absorption
on the blue wing of the H alpha profiles (assumed to be a surrogate for the UV
absorption associated with CIRs). While not conclusive, both sets of data are
consistent with models where the CIRs are either a source of X-rays or modulate
them.Comment: Accepted by MNRAS. 9 pages, 9 figure
Comparison of the INRIM and PTB lattice-spacing standards
To base the kilogram definition on the atomic mass of the silicon 28 atom,
the present relative uncertainty of the silicon 28 lattice parameter must
lowered to 3E-9. To achieve this goal, a new experimental apparatus capable of
a centimetre measurement-baseline has been made at the INRIM. The comparison
between the determinations of the lattice parameter of crystals MO*4 of INRIM
and WASO4.2a of PTB is intended to verify the measurement capabilities and to
assess the limits of this experiment.Comment: 10 pages, 8 figures, submitted to Metrologi
The Influence of Stellar Wind Variability on Measurements of Interstellar O VI Along Sightlines to Early-Type Stars
A primary goal of the FUSE mission is to understand the origin of the O VI
ion in the interstellar medium of the Galaxy and the Magellanic Clouds. Along
sightlines to OB-type stars, these interstellar components are usually blended
with O VI stellar wind profiles, which frequently vary in shape. In order to
assess the effects of this time-dependent blending on measurements of the
interstellar O VI lines, we have undertaken a mini-survey of repeated
observations toward OB-type stars in the Galaxy and the Large Magellanic Cloud.
These sparse time series, which consist of 2-3 observations separated by
intervals ranging from a few days to several months, show that wind variability
occurs commonly in O VI (about 60% of a sample of 50 stars), as indeed it does
in other resonance lines. However, in the interstellar O VI 1032
region, the O VI 1038 wind varies only in 30% of the cases. By
examining cases exhibiting large amplitude variations, we conclude that
stellar-wind variability {\em generally} introduces negligible uncertainty for
single interstellar O VI components along Galactic lines of sight, but can
result in substantial errors in measurements of broader components or blends of
components like those typically observed toward stars in the Large Magellanic
Cloud. Due to possible contamination by discrete absorption components in the
stellar O VI line, stars with terminal velocities greater than or equal to the
doublet separation (1654 km/s) should be treated with care.Comment: Accepted for publication in the Astrophysical Journal Lette
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