2,696 research outputs found
The VLT-FLAMES Survey of Massive Stars: Observations centered on the Magellanic Cloud clusters NGC 330, NGC 346, NGC 2004, and the N11 region
We present new observations of 470 stars using the Fibre Large Array
Multi-Element Spectrograph (FLAMES) instrument in fields centered on the
clusters NGC 330 and NGC 346 in the Small Magellanic Cloud (SMC), and NGC 2004
and the N11 region in the Large Magellanic Cloud (LMC). A further 14 stars were
observed in the N11 and NGC 330 fields using the Ultraviolet and Visual Echelle
Spectrograph (UVES) for a separate programme. Spectral classifications and
stellar radial velocities are given for each target, with careful attention to
checks for binarity. In particular we have investigated previously unexplored
regions around the central LH9/LH10 complex of N11, finding ~25 new O-type
stars from our spectroscopy. We have observed a relatively large number of
Be-type stars that display permitted Fe II emission lines. These are primarily
not in the cluster cores and appear to be associated with classical Be-type
stars, rather than pre main-sequence objects. The presence of the Fe II
emission, as compared to the equivalent width of H, is not obviously
dependent on metallicity. We have also explored the relative fraction of Be- to
normal B-type stars in the field-regions near to NGC 330 and NGC 2004, finding
no strong evidence of a trend with metallicity when compared to Galactic
results. A consequence of service observations is that we have reasonable
time-sampling in three of our FLAMES fields. We find lower limits to the binary
fraction of O- and early B-type stars of 23 to 36%. One of our targets
(NGC346-013) is especially interesting with a massive, apparently hotter, less
luminous secondary component.Comment: 35 pages, 17 figures (some reduced in size). Replacement copy,
includes an erratum on the final page. A copy with full res. & embedded
figures is at http://www.roe.ac.uk/~cje/flamesMC.ps.g
Absolute linear instability in laminar and turbulent gas/liquid two-layer channel flow
We study two-phase stratified flow where the bottom layer is a thin laminar
liquid and the upper layer is a fully-developed gas flow. The gas flow can be
laminar or turbulent. To determine the boundary between convective and absolute
instability, we use Orr--Sommerfeld stability theory, and a combination of
linear modal analysis and ray analysis. For turbulent gas flow, and for the
density ratio r=1000, we find large regions of parameter space that produce
absolute instability. These parameter regimes involve viscosity ratios of
direct relevance to oil/gas flows. If, instead, the gas layer is laminar,
absolute instability persists for the density ratio r=1000, although the
convective/absolute stability boundary occurs at a viscosity ratio that is an
order of magnitude smaller than in the turbulent case. Two further unstable
temporal modes exist in both the laminar and the turbulent cases, one of which
can exclude absolute instability. We compare our results with an
experimentally-determined flow-regime map, and discuss the potential
application of the present method to non-linear analyses.Comment: 33 pages, 20 figure
Disentangling the spatial substructure of Cygnus OB2 from Gaia DR2
© 2019 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical SocietyFor the first time, we have explored the spatial substructure of the Cygnus OB2 association using parallaxes from the recent second Gaia data release. We find significant line-of-sight substructure within the association, which we quantify using a parametrized model that reproduces the observed parallax distribution. This inference approach is necessary due to the non-linearity of the parallax distance transformation and the asymmetry of the resulting probability distribution. Using a Markov Chain Monte Carlo ensemble sampler and an unbinned maximum likelihood test, we identify two different stellar groups superposed on the association. We find the main Cygnus OB2 group at ∼1760 pc, further away than recent estimates have envisaged, and a foreground group at ∼1350 pc. We also calculate individual membership probabilities and identify outliers as possible non-members of the association.Peer reviewe
Terminal velocities of luminous, early-type SMC stars
Ultraviolet spectra from the Space Telescope Imaging Spectrograph (STIS) are
used to determine terminal velocities for 11 O and B-type giants and
supergiants in the Small Magellanic Cloud (SMC) from the Si IV and C IV
resonance lines. Using archival data from observations with the Goddard
High-Resolution Spectrograph and the International Ultraviolet Explorer
telescope, terminal velocities are obtained for a further five B-type
supergiants. We discuss the metallicity dependence of stellar terminal
velocities, finding no evidence for a significant scaling between Galactic and
SMC metallicities for Teff < 30,000 K, consistent with the predictions of
radiation driven wind theory for supergiant stars. A comparison of the
ratio between the SMC and Galactic samples, while
consistent with the above statement, emphasizes that the uncertainties in the
distances to galactic O-stars are a serious obstacle to a detailed comparison
with theory. For the SMC sample there is considerable scatter in this ratio at
a given effective temperature, perhaps indicative of uncertainties in stellar
masses.Comment: 28 pages, 8 figures, accepted by ApJ; minor revisions prior to
acceptanc
Singular solutions of a modified two-component Camassa-Holm equation
The Camassa-Holm equation (CH) is a well known integrable equation describing
the velocity dynamics of shallow water waves. This equation exhibits
spontaneous emergence of singular solutions (peakons) from smooth initial
conditions. The CH equation has been recently extended to a two-component
integrable system (CH2), which includes both velocity and density variables in
the dynamics. Although possessing peakon solutions in the velocity, the CH2
equation does not admit singular solutions in the density profile. We modify
the CH2 system to allow dependence on average density as well as pointwise
density. The modified CH2 system (MCH2) does admit peakon solutions in velocity
and average density. We analytically identify the steepening mechanism that
allows the singular solutions to emerge from smooth spatially-confined initial
data. Numerical results for MCH2 are given and compared with the pure CH2 case.
These numerics show that the modification in MCH2 to introduce average density
has little short-time effect on the emergent dynamical properties. However, an
analytical and numerical study of pairwise peakon interactions for MCH2 shows a
new asymptotic feature. Namely, besides the expected soliton scattering
behavior seen in overtaking and head-on peakon collisions, MCH2 also allows the
phase shift of the peakon collision to diverge in certain parameter regimes.Comment: 25 pages, 11 figure
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