- 4151 - MEASURE
GALAXY DISTANCES ?
Astronomers propose novel method of measuring galaxy distances. The universe consists of hundreds of billions
of galaxies, and for nearby galaxies, distances are mainly measured by
classical Cepheids and RR Lyrae (RR Lyr) stars.
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-------------- 4151 - MEASURE GALAXY DISTANCES ?
- RR Lyr stars are pulsating variable stars
that are 100 times brighter than our sun and
more than twice as old. They can be used as a standard candle for
measuring the distances of galaxies, due to the tight relationship between
their pulsation period and luminosity at the same elemental abundance.
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- Based on this knowledge, astronomers are
using double-period RR Lyr stars to measure the distances of galaxies, with the
distance error of galaxies being optimized to 1–2%.
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- About 5% of RR Lyr stars pulsate with more
than one period. Double-period RR Lyr stars are unique because the two periods
are associated with stellar properties such as mass and elemental abundance.
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- Compared to measurements of elemental
abundance, measurements of period are both easy and accurate. It provides a method by which distance
measurements of nearby galaxies can be obtained from photometry alone, without
relying on spectroscopic observations.
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- Double-period RR Lyr stars are valuable
probes that provide not only reliable distances but also information about
elemental abundances. Using a distance
measurement method based on double-period RR Lyr stars, astronomers expect to
see a 3D intuitive map of the Local Group and obtain a high-precision Hubble
constant.
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- RR Lyrae stars are old, pulsating stars that
burn helium in their cores for fuel. Their progenitors were similar to our sun,
but they are now much larger and brighter than our sun. We know of hundreds of
thousands of RR Lyrae stars within the Milky Way, and we can use them to study
how our galaxy has evolved since its formation.
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- RR Lyrae stars are also pulsating stars,
with pulsations driven by a "stellar engine" known as the “kappa
mechanism” which is a process by which a
sub-surface layer inside the star containing partially ionized helium modulates
the amount of energy that can leave the stellar core.
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- This layer acts as a heat engine, so the
envelopes of pulsating stars expand and contract periodically, as waves
propagate through their interiors. This phenomenon is observed by astronomers
as periodic changes in brightness.
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- The kinds of waves that propagate through a
particular star depend on the physical conditions inside the star. As a result,
we can study a star's internal properties based on its pulsations, much like
studying the inside of the Earth through seismology. In the case of stars, this
field is called “asteroseismology”.
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- For many years, RR Lyrae stars were thought
to be simple radial pulsators: stars whose spherical symmetry is preserved
during pulsation. With only one two pulsation modes present, it was not
possible to study them by asteroseismology, which requires the identification
of multiple modes of pulsation.
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- In the last decade excellent ground and
space-based observations revolutionized our view on RR Lyrae stars. Such modern
observations have revealed that RR Lyrae stars can have additional, non-radial
periodicities, just with very small amplitudes.
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- Radial pulsation drives the characteristic
brightness variations of RR Lyrae stars. High-precision photometry can reveal
the signs of small-amplitude non-radial pulsations hidden beneath that large
signal.
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- The origin of these modes was unknown for a
long time, but the growing number of stars showing these periodicities
facilitated the development of a hypothesis that the observed signals were due
to non-radial modes of pulsations of degrees 8 or 9. This means that 8 or 9
nodal lines divide the surface into separate zones. Modes at such degrees were
previously thought to be unobservable for distant stars.
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- These newly identified pulsation modes
enabled the application of asteroseismic methods to study RR Lyrae stars, and
in particular, to address the long-standing problem of mass measurements.
Fortunately, with these additional non-radial modes, if the theory correct, the
masses of RR Lyrae stars can be estimated using asteroseismology.
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- Using independent observations, astronomers
determined constraints on physical parameters such as the observed brightness
or metallicity of these stars. They then computed a grid of theoretical models
of RR Lyrae stars along various parameters (such as luminosity, metallicity,
temperature, and, most importantly, mass) with the MESA-RSP stellar pulsation
code and tested whether such models would show pulsations and, if so, what the
pulsation periods of the different modes would be.
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- The results show that periodicities in RR
Lyrae stars can be successfully reproduced using the proposed identification of
8–9 degree non-radial modes. They
successfully derived asteroseismic masses for the stars studied, ranging from
0.5 to 0.85 solar masses, in good agreement with the predictions of stellar
evolution models and other indirect means of estimating mass.
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September 12,
2023 MEASURE GALAXY
DISTANCES ? 4151
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