9,055 research outputs found
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
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
The core/envelope symmetry in pulsating stars
We demonstrate that there is an inherent symmetry in the way high-overtone
stellar pulsations sample the core and the envelope, which can potentially lead
to an ambiguity in the asteroseismologically derived locations of internal
structures. We provide an intuitive example of the source of this symmetry by
analogy with a vibrating string. For the stellar case, we focus on the white
dwarf stars, establishing the practical consequences for high-order white dwarf
pulsations both analytically and numerically. In addition, we verify the effect
empirically by cross-fitting two different structural models, and we discuss
the consequences that this approximate symmetry may have for past and present
asteroseismological fits of the pulsating DBV, GD 358. Finally, we show how the
signatures of composition transition zones that are brought about by physically
distinct processes may be used to help alleviate this potential ambiguity in
our asteroseismological interpretation of the pulsation frequencies observed in
white dwarf stars.Comment: Accepted for publication in MNRAS; 8 pages, 8 figure
The CANADA-FRANCE REDSHIFT SURVEY XIII: The luminosity density and star-formation history of the Universe to z ~ 1
The comoving luminosity density of the Universe is estimated from the CFRS
faint galaxy sample in three wavebands (2800A, 4400A and 1 micron) over the
redshift range 0 < z < 1. In all three wavebands, the comoving luminosity
density increases markedly with redshift. For a (q_0 = 0.5, Omega = 1.0)
cosmological model, the comoving luminosity density increases as at 1 micron, as at 4400A and as at 2800A, these exponents being reduced by 0.43 and 1.12 for (0.05,0.1)
and (-0.85,0.1) cosmological models respectively. The variation of the
luminosity density with epoch can be reasonably well modelled by an actively
evolving stellar population with a Salpeter initial mass function (IMF)
extending to 125 M_sun, a star-formation rate declining with a power 2.5, and a
turn-on of star-formation at early epochs. A Scalo (1986) IMF extending to the
same mass limit produces too many long-lived low mass stars. This rapid
evolution of the star-formation rate and comoving luminosity density of the
Universe is in good agreement with the conclusions of Pei and Fall (1995) from
their analysis of the evolving metallicity of the Universe. One consequence of
this evolution is that the physical luminosity density at short wavelengths has
probably declined by two orders of magnitude since z ~ 1.Comment: uuencoded compressed tar file containing 8 page Tex file, 2
postscript figures and 2 tables. Ap J Letters, in press. Also available at
http://www.astro.utoronto.ca/~lilly/CFRS/papers.htm
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