5,228 research outputs found
How Well Do We Know the Beta-Decay of 16N and Oxygen Formation in Helium Burning
We review the status of the 12C(a,g)16O reaction rate, of importance for
stellar processes in a progenitor star prior to a super-nova collapse. Several
attempts to constrain the p-wave S-factor of the 12C(a,g)16O reaction at Helium
burning temperatures (200 MK) using the beta-delayed alpha-particle emission of
16N have been made, and it is claimed that this S-factor is known, as quoted by
the TRIUMF collaboration. In contrast reanalyses (by G.M. hale) of all thus far
available data (including the 16N data) does not rule out a small S-factor
solution. Furthermore, we improved our previous Yale-UConn study of the beta-
delayed alpha-particle emission of \n16 by improving our statistical sample (by
more than a factor of 5), improving the energy resolution of the experiment (by
20%), and in understanding our line shape, deduced from measured quantities.
Our newly measured spectrum of the beta-delayed alpha-particle emission of 16N
is not consistent with the TRIUMF('94) data, but is consistent with the
Seattle('95) data, as well as the earlier (unaltered !) data of Mainz('71). The
implication of this discrepancies for the extracted astrophysical p-wave
s-factor is briefly discussed.Comment: 6 pages, 4 figures, Invited Talk, Physics With Radioactive Beams,
Puri, India, Jan. 12-17, 1998, Work Supported by USDOE Grant No.
DE-FG02-94ER4087
Taking historical chemistry to the bench: A new perspective for modern chemists through the re-creation and analysis of 19th-century Scottish Turkey red dyed textiles
UV spectroscopy of the blue supergiant SBW1: the remarkably weak wind of a SN 1987A analog
The Galactic blue supergiant SBW1 with its circumstellar ring nebula
represents the best known analog of the progenitor of SN 1987A. High-resolution
imaging has shown H-alpha and IR structures arising in an ionized flow that
partly fills the ring's interior. To constrain the influence of the stellar
wind on this structure, we obtained an ultraviolet (UV) spectrum of the central
star of SBW1 with the HST Cosmic Origins Spectrograph (COS). The UV spectrum
shows none of the typical wind signatures, indicating a very low mass-loss
rate. Radiative transfer models suggest an extremely low rate below 10
Msun/yr, although we find that cooling timescales probably become comparable to
or longer than the flow time below 10 Msun/yr. We therefore adopt this
latter value as a conservative upper limit. For the central star, the model
yields =21,0001000 K, 510 ,
and roughly Solar composition except for enhanced N abundance. SBW1's very low
mass-loss rate may hinder the wind's ability to shape the surrounding nebula.
The very low mass-loss rate also impairs the wind's ability to shed angular
momentum; the spin-down timescale for magnetic breaking is more than 500 times
longer than the age of the ring. This, combined with the star's slow rotation
rate, constrain merger scenarios to form ring nebulae. The mass-loss rate is at
least 10 times lower than expected from mass-loss recipes, without any account
of clumping. The physical explanation for why SBW1's wind is so weak presents
an interesting mystery.Comment: 12 pages, 6 figs. submitted to MNRAS. comments welom
Authenticating Turkey Red Textiles through Material Investigations by FTIR and UHPLC
Nineteenth-century Turkey red, a cotton textile
dyed by a peculiar and unique process, is found
in many collections around the world. It was
known for its bright colour and remarkable fastness
to light exposure and washing. Light fading
is a significant concern in the display of historical
textiles, and understanding more about the
properties of these objects may increase the
accessibility of collections. This research explored
the identification of historical Turkey red
through non-invasive Fourier transform infrared
(FTIR) spectroscopy to detect the presence of
oil – a necessary step in the process – on the
fibres. Around 1869, Turkey red dyers began to
transition from using madder and garancine
to synthetic alizarin, which was investigated
through ultra-high-performance liquid chromatography
(UHPLC). The chemical profiles of
19th-century samples and references of known
dye source were used to predict whether Turkey
red of unknown date was dyed with natural or
synthetic dye
Parallel Implementation of the PHOENIX Generalized Stellar Atmosphere Program
We describe the parallel implementation of our generalized stellar atmosphere
and NLTE radiative transfer computer program PHOENIX. We discuss the parallel
algorithms we have developed for radiative transfer, spectral line opacity, and
NLTE opacity and rate calculations. Our implementation uses a MIMD design based
on a relatively small number of MPI library calls. We report the results of
test calculations on a number of different parallel computers and discuss the
results of scalability tests.Comment: To appear in ApJ, 1997, vol 483. LaTeX, 34 pages, 3 Figures, uses
AASTeX macros and styles natbib.sty, and psfig.st
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