294 research outputs found
Meet My Mentor : A Collection of Personal Reminiscences
Contributors describe the mentoring they received as law librarians. Individually the pieces offer fascinating glimpses of individuals and relationships. Collectively, they demonstrate how important - and how varied - the process of mentoring has been and continues to be for the growth and evolution of the profession
The effect of different opacity data and chemical element mixture on the Petersen diagram
The Petersen diagram is a frequently used tool to constrain model parameters
such as metallicity of radial double-mode pulsators. In this diagram the period
ratio of the radial first overtone to the fundamental mode, P_1/P_0, is plotted
against the period of the fundamental mode. The period ratio is sensitive to
the chemical composition as well as to the rotational velocity of a star. In
the present study we compute stellar pulsation models to demonstrate the
sensitivity of the radial period ratio to the opacity data (OPAL and OP tables)
and we also examine the effect of different relative abundances of heavy
elements. We conclude that the comparison with observed period ratios could be
used successfully to test the opacity data.Comment: 5 pages, 5 figures, 1 table; to be published in the Proceedings of
the Conference 'Unsolved Problems in Stellar Physics', Cambridge, 2-6 July
200
NEAR-SURFACE EFFECTS IN MODELLING OSCILLATIONS OF ETA BOO
Following the report of solar-like oscillations in the G0 V star eta Boo
(Kjeldsen et al. 1995, AJ 109, 1313), a first attempt to model the observed
frequencies was made by Christensen-Dalsgaard et al. (1995, ApJ Letters, in
press). This attempt succeeded in reproducing the observed frequency
separations, although there remained a difference of about 10 microHz between
observed and computed frequencies. In those models, the near-surface region of
the star was treated rather crudely. Here we consider more sophisticated models
that include non-local mixing-length theory, turbulent pressure and
nonadiabatic oscillations.Comment: uuencoded and compressed Postscript (2 pages, including figure); To
appear in Proceedings of IAU Colloquium 155, "Astrophysical Applications of
Stellar Pulsation", Cape Town, South Afric
The acoustic cut-off frequency of the Sun and the solar magnetic activity cycle
The acoustic cut-off frequency -the highest frequency for acoustic solar
eigenmodes- is an important parameter of the solar atmosphere as it determines
the upper boundary of the p-mode resonant cavities. At frequencies beyond this
value, acoustic disturbances are no longer trapped but traveling waves.
Interference amongst them give rise to higher-frequency peaks -the pseudomodes-
in the solar acoustic spectrum. The pseudomodes are shifted slightly in
frequency with respect to p modes making possible the use of pseudomodes to
determine the acoustic cut-off frequency. Using data from GOLF and VIRGO
instruments on board the SOHO spacecraft, we calculate the acoustic cut-off
frequency using the coherence function between both the velocity and intensity
sets of data. By using data gathered by these instruments during the entire
lifetime of the mission (1996 till the present), a variation in the acoustic
cut-off frequency with the solar magnetic activity cycle is found.Comment: Paper accepted in ApJ. 26 Pages, 9 figure
Stellar turbulence and mode physics
An overview of selected topical problems on modelling oscillation properties
in solar-like stars is presented. High-quality oscillation data from both
space-borne intensity observations and ground-based spectroscopic measurements
provide first tests of the still-ill-understood, superficial layers in distant
stars. Emphasis will be given to modelling the pulsation dynamics of the
stellar surface layers, the stochastic excitation processes and the associated
dynamics of the turbulent fluxes of heat and momentum.Comment: Proc. HELAS Workshop on 'Synergies between solar and stellar
modelling', eds M. Marconi, D. Cardini, M. P. Di Mauro, Astrophys. Space
Sci., in the pres
Stellar Model Analysis of the Oscillation Spectrum of eta Bootis Obtained from MOST
Eight consecutive low-frequency radial p-modes are identified in the G0 IV
star eta Bootis based on 27 days of ultraprecise rapid photometry obtained by
the MOST (Microvariability & Oscillations of Stars) satellite. The MOST data
extend smoothly to lower overtones the sequence of radial p-modes reported in
earlier groundbased spectroscopy by other groups. The lower-overtone modes from
the MOST data constrain the interior structure of the model of eta Boo. With
the interior fit anchored by the lower-overtone modes seen by MOST, standard
models are not able to fit the higher-overtone modes with the same level of
accuracy. The discrepancy is similar to the discrepancy that exists between the
Sun's observed p-mode frequencies and the p-mode frequencies of the standard
solar model. This discrepancy promises to be a powerful constraint on models of
3D convection.Comment: 30 pages with 14 figures. Accepted for publication in Ap
Evidence for solar-like oscillations in beta Hydri
We have made a clear detection of excess power, providing strong evidence for
solar-like oscillations in the G2 subgiant beta Hyi. We observed this star over
five nights with the UCLES echelle spectrograph on the 3.9-m Anglo-Australian
Telescope, using an iodine absorption cell as a velocity reference. The time
series of 1196 velocity measurements shows an rms scatter of 3.30 m/s, and the
mean noise level in the amplitude spectrum at frequencies above 0.5 mHz is 0.11
m/s. We see a clear excess of power centred at 1.0 mHz, with peak amplitudes of
about 0.5 m/s, in agreement with expectations for this star. Fitting the
asymptotic relation to the power spectrum indicates the most likely value for
the large separation is 56.2 microHz, also in good agreement with the known
properties of beta Hyi.Comment: Accepted by ApJ Letter
Interaction Between Convection and Pulsation
This article reviews our current understanding of modelling convection
dynamics in stars. Several semi-analytical time-dependent convection models
have been proposed for pulsating one-dimensional stellar structures with
different formulations for how the convective turbulent velocity field couples
with the global stellar oscillations. In this review we put emphasis on two,
widely used, time-dependent convection formulations for estimating pulsation
properties in one-dimensional stellar models. Applications to pulsating stars
are presented with results for oscillation properties, such as the effects of
convection dynamics on the oscillation frequencies, or the stability of
pulsation modes, in classical pulsators and in stars supporting solar-type
oscillations.Comment: Invited review article for Living Reviews in Solar Physics. 88 pages,
14 figure
Amplitudes and lifetimes of solar-like oscillations observed by CoRoT* Red-giant versus main-sequence stars
Context. The advent of space-borne missions such as CoRoT or Kepler providing
photometric data has brought new possibilities for asteroseismology across the
H-R diagram. Solar-like oscillations are now observed in many stars, including
red giants and main- sequence stars. Aims. Based on several hundred identified
pulsating red giants, we aim to characterize their oscillation amplitudes and
widths. These observables are compared with those of main-sequence stars in
order to test trends and scaling laws for these parameters for both
main-sequence stars and red giants. Methods. An automated fitting procedure is
used to analyze several hundred Fourier spectra. For each star, a modeled
spectrum is fitted to the observed oscillation spectrum, and mode parameters
are derived. Results. Amplitudes and widths of red-giant solar-like
oscillations are estimated for several hundred modes of oscillation. Amplitudes
are relatively high (several hundred ppm) and widths relatively small (very few
tenths of a {\mu}Hz). Conclusions. Widths measured in main-sequence stars show
a different variation with the effective temperature than red giants. A single
scaling law is derived for mode amplitudes of both red giants and main-sequence
stars versus their luminosity to mass ratio. However, our results suggest that
two regimes may also be compatible with the observations.Comment: Accepted in A&A on 2011 February 8th, now includes corrections
(results now more precise on \Gamma and A_max in Section 4.3 and 4.4, fig. 7
corrected consequently
Prospects for asteroseismology
The observational basis for asteroseismology is being dramatically
strengthened, through more than two years of data from the CoRoT satellite, the
flood of data coming from the Kepler mission and, in the slightly longer term,
from dedicated ground-based facilities. Our ability to utilize these data
depends on further development of techniques for basic data analysis, as well
as on an improved understanding of the relation between the observed
frequencies and the underlying properties of the stars. Also, stellar modelling
must be further developed, to match the increasing diagnostic potential of the
data. Here we discuss some aspects of data interpretation and modelling,
focussing on the important case of stars with solar-like oscillations.Comment: Proc. HELAS Workshop on 'Synergies between solar and stellar
modelling', eds M. Marconi, D. Cardini & M. P. Di Mauro, Astrophys. Space
Sci., in the press Revision: correcting abscissa labels on Figs 1 and
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