51,259 research outputs found
Radial Velocity Curves of Ellipsoidal Red Giant Binaries in the Large Magellanic Cloud
Ellipsoidal red giant binaries are close binary systems where an unseen,
relatively close companion distorts the red giant, leading to light variations
as the red giant moves around its orbit. These binaries are likely to be the
immediate evolutionary precursors of close binary planetary nebula and
post-asymptotic giant branch and post-red giant branch stars. Due to the MACHO
and OGLE photometric monitoring projects, the light variability nature of these
ellipsoidal variables has been well studied. However, due to the lack of radial
velocity curves, the nature of their masses, separations, and other orbital
details has so far remained largely unknown. In order to improve this
situation, we have carried out spectral monitoring observations of a large
sample of 80 ellipsoidal variables in the Large Magellanic Cloud and we have
derived radial velocity curves. At least 12 radial velocity points with good
quality were obtained for most of the ellipsoidal variables. The radial
velocity data are provided with this paper. Combining the photometric and
radial velocity data, we present some statistical results related to the binary
properties of these ellipsoidal variables.Comment: 10 pages, 8 figures, 3 table
Experimental investigation of leading-edge thrust at supersonic speeds
Wings, designed for leading edge thrust at supersonic speeds, were investigated in the Unitary Plan Wind Tunnel at Mach numbers of 1.60, 1.80, 2.00, 2.16, and 2.36. Experimental data were obtained on a uncambered wing which had three interchangeable leading edges that varied from sharp to blunt. The leading edge thrust concept was evaluated. Results from the investigation showed that leading edge flow separation characteristics of all wings tested agree well with theoretical predictions. The experimental data showed that significant changes in wing leading edge bluntness did not affect the zero lift drag of the uncambered wings
Optically visible post-AGB/RGB stars and young stellar objects in the Small Magellanic Cloud: candidate selection, spectral energy distributions and spectroscopic examination
We have carried out a search for optically visible post-AGB candidates in the
Small Magellanic Cloud (SMC). We used mid-IR observations from the Spitzer
Space Telescope to select optically visible candidates with a mid-IR excess. We
obtained low-resolution optical spectra for 801 candidates. After removing
contaminants and poor quality spectra, the final sample comprised of 63
post-AGB/RGB candidates of A - F spectral type. Using the spectra, we estimated
the stellar parameters: effective temperature, surface gravity and [Fe/H]. We
also estimated the reddening and deduced the luminosity using the stellar
parameters combined with photometry. Based on a luminosity criterion, 42 of
these 63 sources were classified as post-RGB candidates and the remaining as
post-AGB candidates. From the spectral energy distributions we found that 6 of
the 63 post-AGB/RGB candidates have a circumstellar shell suggesting that they
are single stars, while 27 of them have a surrounding disc, suggesting that
they are binaries. For the remaining candidates the nature of the circumstellar
environment was unclear. Variability is displayed by 38 post-AGB/RGB candidates
with common variability types being the Population II Cepheids (including
RV-Tauri stars) and semi-regular variables. This study has also revealed a new
s-process enriched RV Tauri star (J005107.19-734133.3). From the numbers of
post-AGB/RGB stars in the SMC, we were able to estimate evolutionary rates that
are in good agreement with the stellar evolution models with mass loss in the
post-AGB phase and re-accretion in the post-RGB phase. This study also resulted
in a new sample of 40 luminous young stellar objects (YSOs) of A - F spectral
type. Additionally, we also identified a group of 63 objects whose spectra are
dominated by emission lines and in some cases, a UV continuum. These objects
are likely to be either hot post-AGB/RGBs or luminous YSOs.Comment: 67 pages, 26 figures, 20 tables, 3 appendices + online supporting
information on CD
Magellanic Cloud stars with TiO bands in emission: binary post-RGB/AGB stars or young stellar objects?
Fourteen stars from a sample of Magellanic Cloud objects selected to have a
mid-infrared flux excess have been found to also show TiO bands in emission.
The mid-infrared dust emission and the TiO band emission indicate that these
stars have large amounts of hot circumstellar dust and gas in close proximity
to the central star. The luminosities of the sources are typically several
thousand L_sun while the effective temperatures are 4000-8000 K. Such stars
could be post-AGB stars of mass 0.4-0.8 M_sun or pre-main-sequence stars (young
stellar objects) with masses of 7-19 M_sun. If the stars are pre-main-sequence
stars, they are substantially cooler and younger than stars at the birth line
where Galactic protostars are first supposed to become optically visible out of
their molecular clouds. They should therefore be hidden in their present
evolutionary state. The second explanation for these stars is that they are
post-AGB or post-RGB stars that have recently undergone a binary interaction
when the red giant of the binary system filled its Roche lobe. Being
oxygen-rich, they have gone through this process before becoming carbon stars.
Most of the stars vary slowly on timescales of 1000 days or more suggesting a
changing circumstellar environment. Apart from the slow variations, most stars
also show variability with periods of tens to hundreds of days. One star shows
a period that is rapidly decreasing and we speculate that this star may have
accreted a large blob of gas and dust onto a disk whose orbital radius is
shrinking rapidly. Another star has Cepheid-like pulsations of rapidly
increasing amplitude suggesting a rapid rate of evolution. Seven stars show
quasi-periodic variability and one star has a light curve similar to that of an
eclipsing binary.Comment: 15 pages, 2 tables, 8 figures, MNRAS, in pres
General Position Subsets and Independent Hyperplanes in d-Space
Erd\H{o}s asked what is the maximum number such that every set of
points in the plane with no four on a line contains points in
general position. We consider variants of this question for -dimensional
point sets and generalize previously known bounds. In particular, we prove the
following two results for fixed :
- Every set of hyperplanes in contains a subset
of size at least , for some
constant , such that no cell of the arrangement of is bounded by
hyperplanes of only.
- Every set of points in , for some constant
, contains a subset of cohyperplanar points or points in
general position.
Two-dimensional versions of the above results were respectively proved by
Ackerman et al. [Electronic J. Combinatorics, 2014] and by Payne and Wood [SIAM
J. Discrete Math., 2013].Comment: 8 page
Elastic stress concentration at radial crossholes in pressurised thick cylinders
Results of a parametric finite element analysis investigation of stress concentration at radial crossholes in pressurized cylinders are presented in numerical and graphical form. The analysis shows that the location of maximum stress does not generally occur at the junction between the bores, as is commonly supposed, but at some small distance up the crosshole from the junction. Maximum stress concentration factors (SCFs) are defined on the basis of the maximum principal stress, von Mises equivalent stress, and stress intensity. Three-dimensional plots of the SCF against the cylinder radius ratio b/a and the crosshole-to-main-bore-radius ratio c/a are presented. The SCFs were found to vary across the range of geometries considered with local minima identified within the parameter range in most cases. The results therefore allow designers to select optimum b/a and c/a ratios to minimize stress concentration in real problems
The platinum nuclei: concealed configuration mixing and shape coexistence
The role of configuration mixing in the Pt region is investigated. For this
chain of isotopes, the nature of the ground state changes smoothly, being
spherical around mass and and deformed around the
mid-shell N=104 region. This has a dramatic effect on the systematics of the
energy spectra as compared to the systematics in the Pb and Hg nuclei.
Interacting Boson Model with configuration mixing calculations are presented
for gyromagnetic factors, -decay hindrance factors, and isotope shifts.
The necessity of incorporating intruder configurations to obtain an accurate
description of the latter properties becomes evident.Comment: Accepted in Physical Review
A newly discovered stellar type: dusty post-red giant branch stars in the Magellanic Clouds
Context: We present a newly discovered class of low-luminosity, dusty,
evolved objects in the Magellanic Clouds. These objects have dust excesses,
stellar parameters, and spectral energy distributions similar to those of dusty
post-asymptotic giant branch (post-AGB) stars. However, they have lower
luminosities and hence lower masses. We suggest that they have evolved off the
red giant branch (RGB) instead of the AGB as a result of binary interaction.
Aims: In this study we aim to place these objects in an evolutionary context
and establish an evolutionary connection between RGB binaries (such as the
sequence E variables) and our new sample of objects. Methods: We compared the
theoretically predicted birthrates of the progeny of RGB binaries to the
observational birthrates of the new sample of objects. Results: We find that
there is order-of-magnitude agreement between the observed and predicted
birthrates of post-RGB stars. The sources of uncertainty in the birthrates are
discussed; the most important sources are probably the observational
incompleteness factor and the post-RGB evolution rates. We also note that
mergers are relatively common low on the RGB and that stars low on the RGB with
mid-IR excesses may recently have undergone a merger. Conclusions: Our sample
of dusty post-RGB stars most likely provides the first observational evidence
for a newly discovered phase in binary evolution: post-RGB binaries with
circumstellar dust.Comment: Accepted for publication in Astronomy and Astrophysics Letter
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