3,341 research outputs found
Differential Proper-Motion Study of the Circumstellar Dust Shell of the Enigmatic Object, HD 179821
HD179821 is an enigmatic evolved star that possesses characteristics of both
a post-asymptotic giant branch star and a yellow hyper-giant, and there has
been no evidence that unambiguously defines its nature. These two hypotheses
are products of an indeterminate distance, presumed to be 1 kpc or 6 kpc. We
have obtained the two-epoch Hubble Space Telescope WFPC2 data of its
circumstellar shell, which shows multiple concentric arcs extending out to
about 8 arcsec. We have performed differential proper-motion measurements on
distinct structures within the circumstellar shell of this mysterious star in
hopes of determining the distance to the object, and thereby distinguishing the
nature of this enigmatic stellar source. Upon investigation, rather than
azimuthal radially symmetric expansion, we discovered a bulk motion of the
circumstellar shell of (2.41+-0.43, 2.97+-0.32) mas/yr. This corresponded to a
translational ISM flow of (1.28+-0.95, 7.27+-0.75) mas/yr local to the star.
This finding implies that the distance to HD 179821 should be rather small in
order for its circumstellar shell to preserve its highly intact spherical
structure in the presence of the distorting ISM flow, therefore favoring the
proposition that HD 179821 is a post-AGB object.Comment: Accepted for publication in ApJ
Cometary Astropause of Mira Revealed in the Far-Infrared
Evolved mass-losing stars such as Mira enrich the interstellar medium (ISM)
significantly by their dust-rich molecular wind. When these stars move fast
enough relative to the ISM, the interaction between the wind and ISM generates
the structure known as the astropause (a stellar analog of the heliopause),
which is a cometary stellar wind cavity bounded by the contact discontinuity
surface between the wind and ISM. Far-infrared observations of Mira spatially
resolve the structure of its astropause for the first time, distinguishing the
contact surface between Mira's wind and the ISM and the termination shock due
to Mira's wind colliding with the ISM. The physical size of the astropause and
the estimated speed of the termination shock suggest the age of the astropause
to be about 40,000 yr, confirming a theoretical prediction of the shock
re-establishment time after Mira has entered the Local Bubble.Comment: 9 pages including 2 figures. To appear in the Astrophysical Journal
Letter
Capital depreciation and waste accumulation in capital-resource economies
In this note, we investigate how the depreciation of a manufactured stock and the related accumulation of a waste stock can alter the optimal path of the capital-resource economy. It is shown that the optimal consumption path is aected by how the disuse pattern of the manufactured stock in question is distributed, specifically with forward-looking terms.capital depreciation; waste accumulation; capital-resource model; optimal control
Entangled-photon generation in nano-to-bulk crossover regime
We have theoretically investigated a generation of entangled photons from
biexcitons in a semiconductor film with thickness in nano-to-bulk crossover
regime. In contrast to the cases of quantum dots and bulk materials, we can
highly control the generated state of entangled photons by the design of
peculiar energy structure of exciton-photon coupled modes in the thickness
range between nanometers and micrometers. Owing to the enhancement of radiative
decay rate of excitons at this thickness range, the statistical accuracy of
generated photon pairs can be increased beyond the trade-off problem with the
signal intensity. Implementing an optical cavity structure, the generation
efficiency can be enhanced with keeping the high statistical accuracy.Comment: 9 pages, 3 figure
Evidence of grain growth in the disk of the bipolar proto-planetary nebula M 1--92
We investigate the dust size and dust shell structure of the bipolar
proto-planetary nebula M 1--92 by means of radiative transfer modeling. Our
models consists of a disk and bipolar lobes that are surrounded by an AGB
shell, each component having different dust characteristics. The upper limit of
the grain size in the lobes is estimated to be m from
the polarization value in the bipolar lobe. The value of the
disk is constrained with the disk mass (0.2 M_{\sun}), which was estimated
from a previous CO emission line observation. We find a good model with
m, which provides an approximated disk mass of 0.15
M_{\sun}. Even taking into account uncertainties such as the gas-to-dust mass
ratio, a significantly larger dust of m, comparing to
the dust in the lobe, is expected.
We also estimated the disk inner radius, the disk outer radius, and the
envelope mass to be 30 (=9 AU), 4500 AU, and 4 M_{\sun},
respectively, where is the expansion velocity. If the dust
existing in the lobes in large separations from the central star undergoes
little dust processing, the dust sizes preserves the ones in the dust
formation. Submicron-sized grains are found in many objects besides M 1--92,
suggesting that the size does not depend much on the object properties, such as
initial mass of the central star and chemical composition of the stellar
system. On the other hand, the grain sizes in the disk do. Evidence of large
grains has been reported in many bipolar PPNs, including M 1--92. This result
suggests that disks play an important role in grain growth.Comment: 8 pages with 3 figures. Accepted for publication in A&
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