7,805 research outputs found
Structural and mechanical effects of interstitial sinks Interim technical report, 8 Mar. 1968 - 8 Mar. 1969
Structural and mechanical effects of interstitial sinks in niobium and alloys of niobium and tantalu
Structural and mechanical effects of interstitial sinks
Structural changes in niobium base alloys induced by exposure to titanium interstitial sink at elevated temperatur
The asteroseismological potential of the pulsating DB white dwarf stars CBS 114 and PG 1456+103
We have acquired 65 h of single-site time-resolved CCD photometry of the
pulsating DB white dwarf star CBS 114 and 62 h of two-site high-speed CCD
photometry of another DBV, PG 1456+103. The pulsation spectrum of PG 1456+103
is complicated and variable on time scales of about one week and could only
partly be deciphered with our measurements. The modes of CBS 114 are more
stable in time and we were able to arrive at a frequency solution somewhat
affected by aliasing, but still satisfactory, involving seven independent modes
and two combination frequencies. These frequencies also explain the discovery
data of the star, taken 13 years earlier. We find a mean period spacing of 37.1
+/- 0.7 s significant at the 98% level between the independent modes of CBS 114
and argue that they are due to nonradial g-mode pulsations of spherical degree
l=1. We performed a global search for asteroseismological models of CBS 114
using a genetic algorithm, and we examined the susceptibility of the results to
the uncertainties of the observational frequency determinations and mode
identifications (we could not provide m values). The families of possible
solutions are identified correctly even without knowledge of m. Our optimal
model suggests Teff = 21,000 K and M_* = 0.730 M_sun as well as log(M_He/M_*) =
-6.66, X_O = 0.61. This measurement of the central oxygen mass fraction implies
a rate for the ^12C(alpha,gamma)^16O nuclear reaction near S_300=180 keV b,
consistent with laboratory measurements.Comment: 10 pages, 10 embedded figures, 3 embedded tables. Accepted for
publication in MNRA
Structural and mechanical effects of interstitial sinks
Changes in structure and mechanical properties due to loss of interstitials to reactive metal coatings studied in dispersion strengthened niobium alloy
Asteroseismic Signatures of Stellar Magnetic Activity Cycles
Observations of stellar activity cycles provide an opportunity to study
magnetic dynamos under many different physical conditions. Space-based
asteroseismology missions will soon yield useful constraints on the interior
conditions that nurture such magnetic cycles, and will be sensitive enough to
detect shifts in the oscillation frequencies due to the magnetic variations. We
derive a method for predicting these shifts from changes in the Mg II activity
index by scaling from solar data. We demonstrate this technique on the
solar-type subgiant beta Hyi, using archival International Ultraviolet Explorer
spectra and two epochs of ground-based asteroseismic observations. We find
qualitative evidence of the expected frequency shifts and predict the optimal
timing for future asteroseismic observations of this star.Comment: 5 pages including 3 figures and 1 table, MNRAS Letters accepte
Measuring ^{12}C(&alpha,&gamma)^{16}O from White Dwarf Asteroseismology
During helium burning in the core of a red giant, the relative rates of the
3&alpha and ^{12}C(&alpha,&gamma)^{16}O reactions largely determine the final
ratio of carbon to oxygen in the resulting white dwarf star. The uncertainty in
the 3&alpha reaction at stellar energies due to the extrapolation from
high-energy laboratory measurements is relatively small, but this is not the
case for the ^{12}C(&alpha,&gamma)^{16}O reaction. Recent advances in the
analysis of asteroseismological data on pulsating white dwarf stars now make it
possible to obtain precise measurements of the central ratio of carbon to
oxygen, providing a more direct way to measure the ^{12}C(&alpha,&gamma)^{16}O
reaction rate at stellar energies. We assess the systematic uncertainties of
this approach and quantify small shifts in the measured central oxygen
abundance originating from the observations and from model settings that are
kept fixed during the optimization. Using new calculations of white dwarf
internal chemical profiles, we find a rate for the ^{12}C(&alpha,&gamma)^{16}O
reaction that is significantly higher than most published values. The accuracy
of this method may improve as we modify some of the details of our description
of white dwarf interiors that were not accessible through previous
model-fitting methods.Comment: 8 pages, 4 figures, 3 tables, uses emulateapj5.sty, Accepted for
publication in the Astrophysical Journa
Galaxy number counts- IV. surveying the Herschel deep field in the near-infrared
(abridged) We present results from two new near-infrared imaging surveys. One
covers 47.2 arcmin^2 to K(3\sigma)<20 whilst a second, deeper survey covers a
sub-area of 1.8 arcmin^2 to K(3\sigma)<22.75. Over the entire area we have
extremely deep UBRI photometry. Our K- counts are consistent with the
predictions of non-evolving models with 0 < q0 <0.5. The K-selected (B-K)
galaxy colour distributions move sharply bluewards fainter than K~20 and at at
brighter magnitudes (K<20) our observed colour distributions indicate a
deficiency of red, early-type galaxies at z~1 in comparison with passively
evolving models. This implies either a pure luminosity evolution (PLE) model
with a low level of continuing star-formation following an an initial burst, or
dynamical merging. At fainter magnitudes, the continuing bluewards trend
observed in (B-K) can be explained purely in terms of passively evolving PLE
models. Our observed numbers of (I-K)>4 galaxies at K<20 exhibit the same
deficiency, suggesting that at least part of the larger deficit observed in
(B-K) at K<20 may be due to star-formation rather than dynamical merging.
Finally, as we and others have noted, the number-redshift distribution at
18<K<19 of recent, deep K- selected redshift surveys is well fitted by
non-evolving models; passively evolving models with a Salpeter or Scalo initial
mass functions overpredict the numbers of galaxies with z>1. Dynamical merging
is one possible solution to reduce the numbers of these galaxies but a
dwarf-dominated IMF for early-type galaxies could offer an alternative
explanation; we show that this model reproduces both the optical-infrared
colour distributions and the K- band galaxy counts.Comment: 15 pages, 9 figures, revised version, requires astrobib.sty,
mn-abs.sty, submitted to MNRA
Proof Theory and Ordered Groups
Ordering theorems, characterizing when partial orders of a group extend to
total orders, are used to generate hypersequent calculi for varieties of
lattice-ordered groups (l-groups). These calculi are then used to provide new
proofs of theorems arising in the theory of ordered groups. More precisely: an
analytic calculus for abelian l-groups is generated using an ordering theorem
for abelian groups; a calculus is generated for l-groups and new decidability
proofs are obtained for the equational theory of this variety and extending
finite subsets of free groups to right orders; and a calculus for representable
l-groups is generated and a new proof is obtained that free groups are
orderable
A maximum-likelihood method for improving faint source flux and color estimates
Flux estimates for faint sources or transients are systematically biased high
because there are far more truly faint sources than bright. Corrections which
account for this effect are presented as a function of signal-to-noise ratio
and the (true) slope of the faint-source number-flux relation. The corrections
depend on the source being originally identified in the image in which it is
being photometered. If a source has been identified in other data, the
corrections are different; a prescription for calculating the corrections is
presented. Implications of these corrections for analyses of surveys are
discussed; the most important is that sources identified at signal-to-noise
ratios of four or less are practically useless.Comment: 9 pp., accepted for publication in PAS
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