Date: October 7, 2009

Title: Mira and the Pulsating Red Giants

Podcaster: Mike Simonsen with Virginia Renehan and Rebecca Turner

Organization: American Association of Variable Star Observers – http://www.aavso.org

Description: Today we’re going to introduce you to a group of stars known as long period variables, or LPVs for short. The most prominent of the long-period variables are the Mira variables, which pulsate with periods between 80 and 1000 days, and have amplitudes larger than 2.5 magnitudes. Mira was the first known periodic variable star. Dutch clergyman and amateur astronomer David Fabricius is credited with the discovery of this famous variable in 1596.

Bio: The AAVSO is an international non-profit organization whose mission is: to observe and analyze variable stars; to collect and archive observations for worldwide access; to forge strong collaborations between amateur and professional astronomers; and to promote scientific research and education using variable star data.

Today’s sponsor: This episode of “365 Days of Astronomy” is sponsored by Steve Nerlich for Joanne, Baz and Lilly – oh, and Cheap Astronomy (www.cheapastro.com).

Transcript:

Restless Universe October 7, 2009

Long Period Variables – Mira

Mike: Hi again, and welcome to the Restless Universe, the podcast of the American Association of Variable Star Observers. You can find us on the web at www.aavso.org.

I’m Mike Simonsen, and with me today are Virginia Renehan and Rebecca Turner. We’re going to introduce you to a group of stars known as long period variables, or LPVs for short. And to do that, we are going to tell you all about one of the most famous of these stars, Mira.

Rebecca: An LPV, or Long-Period Variable, is a type of variable star that varies in brightness with periods longer than approximately 100 days. This definition is very loose, but covers most of what we consider the long period stars. The cause of the variation in brightness in these stars is primarily pulsation, where the surface of the star expands and contracts over time, and the effective temperature changes along with the star’s size.

The most prominent of the long-period variables are the Mira variables, which pulsate with periods between 80 and 1000 days, and have amplitudes larger than 2.5 magnitudes in the visual band. The Mira variables, named for the prototype, Mira (omicron Ceti), are the most regular of the LPVs, although their light curves still vary from cycle-to-cycle.

Virginia: Mira holds the high honor of being the first known periodic variable star. Dutch clergyman and amateur astronomer David Fabricius is credited with the discovery of this famous variable in 1596. Fabricius had used the star as a comparison to determine the position of Mercury. Noting a change in brightness, he assumed the star to be a nova. Named omicron Ceti by Johann Bayer in 1603, the variable was virtually forgotten until 1638 when Johann Fokkens Holwarda determined its period of 11 months. In 1642, omi Cet received its more common name when Johannes Hevelius called the star Mira, The Wonderful.

As the first to be discovered, Mira serves as the prototype, lending its name to an entire group of variable stars called the Mira-type variables. Such stars were once much like our own Sun, but have since evolved toward the end of their stellar lives. As cool red giant stars, they are found in the high luminosity portion of the Asymptotic Giant Branch in the H-R diagram. These stars generally have larger radii, higher luminosities, lower temperatures, and lower surface gravities than our Sun. As a result of the low surface gravity, the outer atmosphere is tenuous and loosely bound and forms an envelope around the star. Although it is not fully understood, pulsations of this cool, weak outer atmosphere seem to give rise to the brightness variations seen in Mira stars.

Rebecca: In the case of Mira, it reaches magnitude 3.5 at maximum, on average. Maxima can go as high as magnitude 2.0 and as low as 4.9. Minima range much less, and have historically been between 8.6 and 10.1. Interestingly, since Mira emits the vast majority of its radiation in the infrared, its variability in that band is only about two magnitudes. Like many stars in this class, the shape of its light curve shows a relatively fast rise to maximum and a much longer fade to minimum.

Mike: Miras have rapid mass loss, giving them a major role in galactic evolution through enrichment of the interstellar medium in heavy elements. Through mass loss these massive stars avoid exploding as supernovae. Some Miras are progenitors of planetary nebula, while others evolve directly to the white dwarf stage. Mira variables have extended atmospheres and a large number of them have circumstellar dust shells. They are close relatives of semiregular variables, and are sources of both hydroxyl (OH) and infrared (IR) emissions.

The period, the time interval between two consecutive maxima in days, is a very important parameter in Mira variables, in that the period gives an indication of their size and luminosity, as well as their age, initial metallicity, mode of pulsation, and evolution.

Virginia: In a considerable number of Mira variables, the visual light curves exhibit complex variations of both maxima and minima from cycle to cycle and also on a long-time scale. This multi-periodic modulation has been attributed to circumstellar dust shells.

There appears to be a correlation between the period and the shape of a Mira light curve. Miras with periods less than 200 days have symmetrical light curves and small amplitudes. Mira variables with periods longer than 200 days have larger amplitudes and steeper rising branches of the light curve, as in Mira itself, with a period of 333 days. Mira variables that have periods longer than 300 days have large amplitudes, and many show standstills or “bumps,” particularly in the rising branch of the light curve.

Mike: Because Mira variables are intrinsically bright, their amplitudes are large and their periods are long, they are particularly well suited for visual observing. In fact, the light variations of Miras are almost always determined from visual observations compiled by the AAVSO or other variable star observer groups.

Rebecca: At the beginning of the 20th century, variable stars in general were a mystery. Many had been discovered, but the mechanism of their variability and their precise nature was unknown.

In order to obtain regular observations of these stars over months and years, Harvard College Observatory Director, Edward Pickering, enlisted amateur astronomers in the early 1900’s. His original list of 372 LPVs and the names of some of these observers were published as the Harvard College Observatory Circular.

Many of that original group of observers went on to become the nucleus of the AAVSO. That group of 372 variable stars went on too, forming the bulk of the AAVSO’s LPV observing program for nearly a century. These LPVs have periods in some cases approaching two years. Many patient years of gathering data are necessary to properly describe the behavior of these stars.

Mike: Mira is one of the few LPVs associated with a close companion star. The companion star, a variable star in its own right, was originally thought to be a white dwarf, surrounded by an accretion disk of matter drawn from the pulsating giant. This star is designated VZ Ceti. According to the General Catalogue of Variable Stars, VZ Cet varies with a range of 9.5-12 and has a possible periodicity of about 13 years. The companion’s orbital period around Mira is approximately 400 years.

As a pair, the system is referred to as Mira AB, with Mira itself as Mira A and VZ Cet as Mira B. In 1995 the Hubble Space Telescope Faint Object Camera was used to resolve Mira AB both spatially and spectrally at UV and optical wavelengths. This was the first time the interacting components could be studied individually.

Rebecca: You might think that since we have been studying these stars for so long, and they pulsate slowly and fairly regularly that we know just about all there is to know about these stars. But Mira herself has revealed new things in just the last few years we didn’t know before.

Infrared studies by Karovska et al. (2002) indicate strong deviations in the symmetry of Mira’s shell in the direction of the companion, implying that Mira B will most likely effect the shape of the loose, outer shell of Mira A as it evolves toward the Planetary Nebula stage of stellar life. In the same paper it was noted that the UV spectrum of Mira B has changed in recent years. While the cause for the change is not fully understood, it may be the result of a possible disruption of the accretions disk, a possibility that is supported by HST observations of a decreased accretion rate onto Mira B.

Virginia: One of the most exciting recent findings is a 2005 report on the discovery of X-ray emissions from Mira. What makes this news so fascinating is that while X-rays are expected to be produced by a white dwarf’s hot accretion disk, astronomers found a flare of X-ray emission separate from the accretion disk that was coming from Mira A itself, an event that has never been seen in any Asymptotic Giant Branch star, let alone Mira!

More recently, ultra-violet studies of Mira by NASA’s Galaxy Evolution Explorer space telescope (GALEX) have revealed that it is shedding a trail of material from its outer envelope, creating a tail 13 light-years long, formed over tens of thousands of years. Pictures of Mira and her tail look like a comet.

Rebecca: In 2007, observations showed a protoplanetary disc around the companion, Mira B. This disc is being accreted from material in the solar wind from Mira and could eventually form new planets. These observations also revealed that the companion is most likely a main sequence star of around 0.7 solar masses and spectral type K, instead of a white dwarf as previously believed.

If you’d like to observe Mira yourself, the next maximum occurs around Nov 21-30, 2009, October 21-31 in 2010, right around Halloween, and again in September in 2011.

Mike: There are dozens of bright, well-studied, fascinating Mira variables, like chi Cygni, the occasional extra star in the body of Cygnus, the swan; and R Leonis, tucked in next to a triangle of stars due west of Regulus. And then there are the cousins of Mira stars, the semi-regular variables, whose members include some incredible, famous, gigantic stars like Betelgeuse and Herschel’s Garnet Star, mu Cephei.

We’ve just begun to scratch the surface of the personalities and wonders of these majestic, slowly pulsating giant stars sprinkled throughout our Restless Universe, but that’s all we have time for today. If you’d like to learn more about variable stars and the role that amateurs play, contributing to science by observing, recording and archiving their observations, visit the AAVSO on the web at www.aavso.org.

From all of us to all of you, thank you and good-bye.

End of podcast:

365 Days of Astronomy
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