Pulsating Am stars
• Introduction to Am Stars
• Am stars are chemically peculiar A-type stars.
• The "Am" stands for metallic-lined stars.
• Display peculiar abundance of metals like zinc, strontium,
and barium.
• Slower rotation compared to normal A-type stars, leading
to unusual chemical distribution.
What Are
Pulsating Stars?
Stars that undergo periodic expansion and
contraction due to internal instability.
Pulsations lead to observable brightness variations.
Common pulsating variables include Cepheids, and
RR Lyrae, RV Tauri, Long-Period Variables (LPVs)
stars.
The
Instability
Strip and its
important
Pulsating Stars in the Instability Strip
• Stars in this region are subject
to pulsations caused by the partial
ionization of helium in their outer layers.
This ionization creates pressure that
causes the star to expand and contract
in cycles, leading to brightness
variations.
• Delta Scuti variables are one of
the main types of stars found in this
part of the instability strip. These are Aand
F-type stars that are slightly
evolved or near the end of their mainsequence
life. Their internal structure
makes them unstable to pulsations,
causing them to vary in brightness on
short timescales (hours to minutes).
Am Stars and Their Slow Rotation
• Am stars rotate slower than typical A-type stars.
• Slow rotation allows for atomic diffusion, leading to chemical
peculiarities.
• Tidal forces in close binary systems can cause this slow rotation,
influencing pulsation behavior.
Pulsating Am
Stars
• Pulsating Am stars challenge the traditional view that
Am stars don't pulsate due to slow rotation.
• Not all Am stars pulsate, but some are found to
pulsate, often exhibiting Delta Scuti-like behavior.
Delta Scuti Stars
and Am Stars
• Delta Scuti stars are pulsating variables found near
the instability strip.
• Some Am stars show similar pulsations, behaving
like Delta Scuti variables
• Pulsations typically have periods of a few hours and
small brightness variations.
Am stars in Kepler
field
• SuperWASP survey overlaps
with a large fraction of the
Kepler field.
• amplitudes >∼1 mmag
• the amplitudes found using SuperWASP lightcurves
are slightly higher than those from Kepler
pulsation modes of Am stars are
low radial order
pulsation amplitude increases with
height and decreases with depth.
In Am stars, the microturbulence
velocity is also peculiar
The photometric
amplitudes found in Am
stars are consistent with
atmospheric pulsation
radial velocity amplitudes
of a few km s−1.
The loss of helium by
gravitational settling from
the He ii ionisation zone
reduces driving, but does
not suppress it entirely.
• high-frequency (>∼20 d−1)
pulsations
Am/Fm star categories
• – γ Doradus pulsators
• – δ Scuti pulsators
• – Hybrid pulsators
• – Eclipsing binaries
• – Constant
• – Other variability
The typical uncertainties for Teff and luminosity are
shown in the lower part of the plot
LAMOST sample of Am/Fm stars
Discussion