Review of “Miras”
by L. A. Willson and M. Marengo
JAAVSO Volume 40, 2012
http://www.aavso.org/sites/default/files/jaavso/v40n1/516.pdf
Motivation
I am new to astronomy and new to AAVSO. I have not yet made any AAVSO observations. The
AAVSO Website contains good information on how to get started observing variable stars. Long
Period Variables (e.g. Miras) would seem to be a good group to get started with because of
their availability and high visual amplitudes allowing observations to be made with minimal aid.
The AAVSO LPV Legacy Program contains many Mira variables. I have decided that my initial
observations will be chosen from those variables found on the LPV Legacy Program list. I
choose this article to gain a better understanding of Miras.
Overview
This article presents the fundamentals of Mira variables, modeling of Miras, methods of
observing Miras (e.g. interferometry), light curves, relationships to other variable classes, and a
discussion of surveys involving Miras. The story of Miras is presented in a historical way with
reference to many publications. Many of these references appear to be classic papers on Miras
dating back as early as 1931. This approach allows for the discussion of earlier conclusions to
be revised as later discoveries were made.
The article appears to be written for amateur variable star observers as it is not as technically
challenging as might be expected in other professional journals. This level of writing seems to
be appropriate for an AAVSO publication and is similar to many other papers that I have read in
AAVSO journals. For me, the overview of Mira variables presented in this paper is at a level
that is appropriate and allows for some comprehension of the fundamentals of Mira variables
as well as gain an interesting historical perspective.
Observations
The following are some of the fundamentals that I learned while reading through this paper:
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The high visual amplitudes over the cycle are directly related to the opacity of the
atmosphere.
The pulsation is driven by changes in the opacity of the atmosphere and the
dependence of pressure on temperature and density when hydrogen and helium are
ionizing or recombining.
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There has been some debate as to whether Miras have both a fundamental and a first
overtone mode with the first overtone mode having a shorter effective wavelength.
The mass is less than or equal to the mass of the star when it was on the main
sequence.
Miras are assigned in to an appropriate population (old, young) based on their galactic
orbits and on their metal content.
Mira progenitor stars with periods of 200 to 250 days were main sequence stars with
masses near 1 solar mass while those with 400 to 600 day periods come from stars with
progenitor masses greater than 2 solar masses.
There are three categories of models in the study of Miras—the evolutionary model, the
pulsation model, and the modeling of atmosphere and wind.
Interferometry is a technique for seeing detail not possible with a telescope. Use of this
technique has helped in resolving the changes in the diameter of Miras.
Long term changes to the amplitudes and periods of some Miras is not well understood.
A study (Templeton and others, 2005) using AAVSO data showed that about 10% of
Miras contain significant long term variability of their periods.
RV Tauri stars are thought to be post-Mira stars and possibly in pre-planetary-Nebula
phase of evolution.
The fact that most planetary nebulae are not spherical hints that a single Mira star was
probably not the progenitor. It is now believed that some form of a binary system may
be involved in forming planetary nebulae.
Conclusions
This journal paper reviews the fundamentals of Mira variables with much of the article done in
a historical way with reference to many other papers. The article presents a reader like me
with an understanding of Mira basics along with providing a roadmap (i.e. the references) for
further study. The author believes that the use of evolutionary modeling, infrared
observations, and statistical analysis of large population of stars will advance the understanding
of stellar evolution in general and as well as mass loss associated with Miras. The author also
concludes that the visual observations performed by organizations like AAVSO “continue to play
a central role in the study of Miras.” That type of acknowledgement from a professional
astronomer is why I have recently decided to get involved with observing variable stars. I have
concluded from this paper that much is known about Miras but much is still to be learned and I
hope that I can help contribute to this learning.