19,145 research outputs found
Surface-effect corrections for oscillation frequencies of evolved stars
Accurate modelling of solar-like oscillators requires that modelled mode
frequencies are corrected for the systematic shift caused by improper modelling
of the near-surface layers, known as the surface effect. ... We investigate how
much additional uncertainty is introduced to stellar model parameters by our
uncertainty about the functional form of the surface effect. At the same time,
we test whether any of the parametrizations is significantly better or worse at
modelling observed subgiants and low-luminosity red giants. We model six stars
observed by Kepler that show clear mixed modes. We fix the input physics of the
stellar models and vary the choice of surface correction ... Models using a
solar-calibrated power law correction consistently fit the observations more
poorly than the other four corrections. Models with the remaining four
corrections generally fit ... about equally well, with the combined surface
correction by Ball & Gizon perhaps being marginally superior. The fits broadly
agree on the model parameters within about the uncertainties, with
discrepancies between the modified Lorentzian and free power law corrections
occasionally exceeding the level. Relative to the best-fitting
values, the total uncertainties on the masses, radii and ages of the stars are
all less than 2, 1 and 6 per cent, respectively. A solar-calibrated power law
... appears unsuitable for use with more evolved solar-like oscillators. Among
the remaining surface corrections, the uncertainty in the model parameters
introduced by the surface effects is about twice as large as the uncertainty in
the individual fits for these six stars. Though the fits are thus somewhat less
certain because of our uncertainty of how to manage the surface effect, these
results also demonstrate that it is feasible to model the individual mode
frequencies of subgiants and low-luminosity red giants. ...Comment: Accepted for publication in Astronomy & Astrophysics. 13 pages, 6
figures, 5 tables. Abstract slightly abridged to meet arXiv's 1920 character
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A simple solution of sound transmission through an elastic wall to a rectangular enclosure, including wall damping and air viscosity effects
A simple solution to the problem of the acoustical coupling between a rectangular structure, its air content, and an external noise source is presented. This solution is a mathematical expression for the normalized acoustic pressure inside the structure. Numerical results for the sound-pressure response for a specified set of parameters are also presented
Constraining differential rotation of Sun-like stars from asteroseismic and starspot rotation periods
In previous work we identified six Sun-like stars observed by Kepler with
exceptionally clear asteroseismic signatures of rotation. Here, we show that
five of these stars exhibit surface variability suitable for measuring
rotation. In order to further constrain differential rotation, we compare the
rotation periods obtained from light-curve variability with those from
asteroseismology. The two rotation measurement methods are found to agree
within uncertainties, suggesting that radial differential rotation is weak, as
is the case for the Sun. Furthermore, we find significant discrepancies between
ages from asteroseismology and from three different gyrochronology relations,
implying that stellar age estimation is problematic even for Sun-like stars.Comment: Accepted for publication in A&A. 5 pages, 4 figure
Flow field prediction and analysis study for project RAM B3 Final report
Flow field properties in shock layer surrounding Ram B3 vehicl
Parametrizing the time-variation of the "surface term" of stellar p-mode frequencies: application to helioseismic data
The solar-cyle variation of acoustic mode frequencies has a frequency
dependence related to the inverse mode inertia. The discrepancy between model
predictions and measured oscillation frequencies for solar and solar-type
stellar acoustic modes includes a significant frequency-dependent term known as
the surface term that is also related to the inverse mode inertia. We
parametrize both the surface term and the frequency variations for low-degree
solar data from Birmingham Solar-Oscillations Network (BiSON) and medium-degree
data from the Global Oscillations Network Group (GONG) using the mode inertia
together with cubic and inverse frequency terms. We find that for the central
frequency of rotationally split multiplets the cubic term dominates both the
average surface term and the temporal variation, but for the medium-degree case
the inverse term improves the fit to the temporal variation. We also examine
the variation of the even-order splitting coefficients for the medium-degree
data and find that, as for the central frequency, the latitude-dependent
frequency variation, which reflects the changing latitudinal distribution of
magnetic activity over the solar cycle, can be described by the combination of
a cubic and an inverse function of frequency scaled by inverse mode inertia.
The results suggest that this simple parametrization could be used to assess
the activity-related frequency variation in solar-like asteroseismic targets.Comment: 13 pages, 11 figures. Accepted by MNRAS 13 October 201
The Sun in transition? Persistence of near-surface structural changes through Cycle 24
We examine the frequency shifts in low-degree helioseismic modes from the
Birmingham Solar-Oscillations Network (BiSON) covering the period from 1985 -
2016, and compare them with a number of global activity proxies well as a
latitudinally-resolved magnetic index. As well as looking at frequency shifts
in different frequency bands, we look at a parametrization of the shift as a
cubic function of frequency. While the shifts in the medium- and highfrequency
bands are very well correlated with all of the activity indices (with the best
correlation being with the 10.7 cm radio flux), we confirm earlier findings
that there appears to have been a change in the frequency response to activity
during solar cycle 23, and the low frequency shifts are less correlated with
activity in the last two cycles than they were in Cycle 22. At the same time,
the more recent cycles show a slight increase in their sensitivity to activity
levels at medium and higher frequencies, perhaps because a greater proportion
of activity is composed of weaker or more ephemeral regions. This lends weight
to the speculation that a fundamental change in the nature of the solar dynamo
may be in progress.Comment: 9 pages, 6 figures. Accepted by MNRAS 24 May 201
Matrix isolation as a tool for studying interstellar chemical reactions
Since the identification of the OH radical as an interstellar species, over 50 molecular species were identified as interstellar denizens. While identification of new species appears straightforward, an explanation for their mechanisms of formation is not. Most astronomers concede that large bodies like interstellar dust grains are necessary for adsorption of molecules and their energies of reactions, but many of the mechanistic steps are unknown and speculative. It is proposed that data from matrix isolation experiments involving the reactions of refractory materials (especially C, Si, and Fe atoms and clusters) with small molecules (mainly H2, H2O, CO, CO2) are particularly applicable to explaining mechanistic details of likely interstellar chemical reactions. In many cases, matrix isolation techniques are the sole method of studying such reactions; also in many cases, complexations and bond rearrangements yield molecules never before observed. The study of these reactions thus provides a logical basis for the mechanisms of interstellar reactions. A list of reactions is presented that would simulate interstellar chemical reactions. These reactions were studied using FTIR-matrix isolation techniques
Upper wing surface boundary layer measurements and static aerodynamic data obtained on a 0.015-scale model (42-0) or the SSV orbiter configuration 140A/B in the LTV HSWT at a Mach number of 4.6 (LA58)
The purpose of the test was to investigate the nature of the Orbiter boundary layer characteristics at angles of attack from -4 to 32 degrees at a Mach number of 4.6. The effect of large grit, employed as transition strips, on both the nature of the boundary layer and the force and moment characteristics were investigated along with the effects of large negative elevon deflection on lee side separation. In addition, laminar and turbulent boundary layer separation phenomena which could cause asymmetric flow separation were investigated
Non-existence of Skyrmion-Skyrmion and Skyrmion-anti-Skyrmion static equilibria
We consider classical static Skyrmion-anti-Skyrmion and Skyrmion-Skyrmion
configurations, symmetric with respect to a reflection plane, or symmetric up
to a -parity transformation respectively. We show that the stress tensor
component completely normal to the reflection plane, and hence its integral
over the plane, is negative definite or positive definite respectively.
Classical Skyrmions always repel classical Skyrmions and classical Skyrmions
always attract classical anti-Skyrmions and thus no static equilibrium, whether
stable or unstable, is possible in either case. No other symmetry assumption is
made and so our results also apply to multi-Skyrmion configurations. Our
results are consistent with existing analyses of Skyrmion forces at large
separation, and with numerical results on Skymion-anti-Skyrmion configurations
in the literature which admit a different reflection symmetry. They also hold
for the massive Skyrme model. We also point out that reflection symmetric
self-gravitating Skyrmions or black holes with Skyrmion hair cannot rest in
symmetric equilibrium with self-gravitating anti-Skyrmions.Comment: v2 Typos corrected, refs added. v3 Journal versio
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