TY - JOUR
T1 - Frequency dependence of the large frequency separation of solar-like oscillators: influence of the helium second-ionization zone
AU - Hekker, S
AU - Basu, S
AU - Elsworth, Yvonne
AU - Chaplin, William
PY - 2011/11/21
Y1 - 2011/11/21
N2 - The large frequency separation (Δν) between modes of the same
degree and consecutive orders in a star is approximately proportional to
the square root of its mean density. To determine Δν as
accurately as possible, a mean large frequency separation
() computed over several orders is often used. It is,
however, known that Δν varies with frequency in a second-order
effect. From observations, it has been shown that this frequency
dependence is more important for main-sequence stars than it is for red
giant stars. Here we use YREC models to verify and explain this
observational result. We find that for stars with R ≳ 8
R&sun;, the effect of the helium second-ionization zone (He
II zone) is relatively small. For these stars, the deep location of the
He II zone induces a frequency modulation covering only a few
Δν, while the amplitude of the modulation is low due to the
relatively weak and extended He II layer, causing a shallow wide
depression in the first adiabatic exponent (Γ1). For
less evolved stars, the He II zone is located closer to the surface, and
it is more confined, i.e. a deep narrow depression in
Γ1. This causes frequency modulations with relatively
high amplitudes covering up to about 20Δν, inducing a
relatively large frequency modulation. Additionally, we find that for
less evolved stars, the He II zone is stronger and more localized for
more massive stars and for stars with low metallicities further
increasing the amplitude of the frequency modulation.
AB - The large frequency separation (Δν) between modes of the same
degree and consecutive orders in a star is approximately proportional to
the square root of its mean density. To determine Δν as
accurately as possible, a mean large frequency separation
() computed over several orders is often used. It is,
however, known that Δν varies with frequency in a second-order
effect. From observations, it has been shown that this frequency
dependence is more important for main-sequence stars than it is for red
giant stars. Here we use YREC models to verify and explain this
observational result. We find that for stars with R ≳ 8
R&sun;, the effect of the helium second-ionization zone (He
II zone) is relatively small. For these stars, the deep location of the
He II zone induces a frequency modulation covering only a few
Δν, while the amplitude of the modulation is low due to the
relatively weak and extended He II layer, causing a shallow wide
depression in the first adiabatic exponent (Γ1). For
less evolved stars, the He II zone is located closer to the surface, and
it is more confined, i.e. a deep narrow depression in
Γ1. This causes frequency modulations with relatively
high amplitudes covering up to about 20Δν, inducing a
relatively large frequency modulation. Additionally, we find that for
less evolved stars, the He II zone is stronger and more localized for
more massive stars and for stars with low metallicities further
increasing the amplitude of the frequency modulation.
KW - stars: oscillations
KW - asteroseismology
KW - stars: interiors
KW - stars: late type
U2 - 10.1111/j.1745-3933.2011.01156.x
DO - 10.1111/j.1745-3933.2011.01156.x
M3 - Article
VL - 418
SP - L119-L123
JO - Monthly Notices of the Royal Astronomical Society: Letters
JF - Monthly Notices of the Royal Astronomical Society: Letters
IS - 1
ER -