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Mira - the wonderful star: Eyes on the Sky


Video aimed at northern hemisphere observers about the variable star Mira in the constellation Cetus, that has recently peaked in brightness.
Mira is at it's brightest this peak, at around magnitude 2.1 in October of 2011. It will decrease by about one magnitude per month, likely diminishing to it's typical low magnitude of around 9.



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Title: The evolutionary state of Miras with changing pulsation periods
Authors: Stefan Uttenthaler (1,2), Koen Van Stiphout (1,3), Kevin Voet (1), Hans Van Winckel (1), Sophie Van Eck (4), Alain Jorissen (4), Franz Kerschbaum (2), Gert Raskin (1), Saskia Prins (1), Wim Pessemier (1), Christoffel Waelkens (1), Yves Frémat (5), Herman Hensberge (5), Louis Dumortier (5), Holger Lehmann (6) ((1) Instituut voor Sterrenkunde, University of Leuven, Belgium, (2) University of Vienna, Department of Astronomy, Austria, (3) Instituut voor Kern- en Stralingsfysica, University of Leuven, Belgium, (4) Institut d'Astronomie et d'Astrophysique, Université Libre de Bruxelles, Belgium, (5) Royal Observatory of Belgium, Brussels, Belgium, (6) Thüringer Landessternwarte Tautenburg, Tautenburg, Germany)

Context: Miras are long-period variables thought to be in the asymptotic giant branch (AGB) phase of evolution. In about one percent of known Miras, the pulsation period is changing. It has been speculated that this changing period is the consequence of a recent thermal pulse in these stars.
Aims: We aim to clarify the evolutionary state of these stars, and to determine in particular whether or not they are in the thermally-pulsing (TP-)AGB phase.
Methods: One important piece of information that has been neglected so far when determining the evolutionary state is the presence of the radio-active s-process element technetium (Tc). We obtained high-resolution, high signal-to-noise-ratio optical spectra of a dozen prominent Mira variables with changing pulsation period to search for this indicator of TPs and dredge-up. We also use the spectra to measure lithium (Li) abundances. Furthermore, we establish the evolutionary states of our sample stars by means of their present-day periods and luminosities.
Results: Among the twelve sample stars observed in this programme, five were found to show absorption lines of Tc. BH Cru is found to be a carbon-star, its period increase in the past decades possibly having stopped by now. We report a possible switch in the pulsation mode of T UMi from Mira-like to semi-regular variability in the past two years. R Nor, on the other hand, is probably a fairly massive AGB star, which could be true for all meandering Miras. Finally, we assign RU Vul to the metal-poor thick disk with properties very similar to the short-period, metal-poor Miras.
Conclusions: We conclude that there is no clear correlation between period change class and Tc presence. The stars that are most likely to have experienced a recent TP are BH Cru and R Hya, although their rates of period change are quite different.

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R Hydrae
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University of Denver astronomers shed new light on dying star

NASA’s Spitzer Space Telescope reveals R Hydrae’s bow shock wave
When University of Denver astronomers Toshiya Ueta and Robert Stencel pointed NASA’s Spitzer Space Telescope at a dying star named R Hydrae (R Hya) in the constellation Hydra, they expected to see a spherical shell of low-temperature gas and dust ejected from the star by its stellar wind. Instead, they found a curved shell — called a bow shock wave — in front of the moving star, like the foamy breakers churned up in front of a ship under sail.

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R Hya _medium
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Credit NASA

Bow shocks are formed where the stellar wind from a star are pushed into a bow shape (illustration, right panel) as the star plunges through the gas and dust between stars. Our own Sun has a bow shock, but prior to this image one had never been observed around this particular class of red giant star.
R Hya moves through space at approximately 50 kilometres per second. As it does so, it discharges dust and gas into space. Because the star is relatively cool, that ejecta quickly assumes a solid state and collides with the interstellar medium. The resulting dusty nebula is invisible to the naked eye but can be detected using an infrared telescope.
This bow shock is 16,295 AU from the star to the apex and 6,188 AU thick. 1 AU is the distance between the Sun and the Earth. The mass of the bow shock is about 400 times the mass of the Earth.
The false-colour Spitzer image shows infrared emissions at 70 microns. Brighter colours represent greater intensities of infrared light at that wavelength. The location of the star itself is drawn onto the picture in the black "unobserved" region in the centre.

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R Virginis
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Title: Stellar and Molecular Radii of a Mira Star: First Observations with the Keck Interferometer Grism
Authors: J.A. Eisner, J.R. Graham, R.L. Akeson, E.R. Ligon, M.M. Colavita, G. Basri, K. Summers, S. Ragland, A. Booth

Using a new grism at the Keck Interferometer, we obtained spectrally dispersed (R ~ 230) interferometric measurements of the Mira star R Vir. These data show that the measured radius of the emission varies substantially from 2.0-2.4 microns. Simple models can reproduce these wavelength-dependent variations using extended molecular layers, which absorb stellar radiation and re-emit it at longer wavelengths. Because we observe spectral regions with and without substantial molecular opacity, we determine the stellar photospheric radius, uncontaminated by molecular emission. We infer that most of the molecular opacity arises at approximately twice the radius of the stellar photosphere.

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Title: Detection of a Far-Infrared Bow-Shock Nebula Around R Hydrae: the First MIRIAD Results
Authors: T. Ueta (NASA Ames/SOFIA), A. K. Speck (U. of Missouri), R. E. Stencel (U. of Denver), F. Herwig (LANL), R. D. Gehrz (U. of Minnesota), R. Szczerba (CAMK, Poland), H. Izumiura (OAO/NAO, Japan), A. A. Zijlstra (U. of Manchester, UK), W. B. Latter (NHSC/Caltech), M. Matsuura (NAO, Japan), M. Meixner (STScI), M. Steffen (AIP, Germany), M. Elitzur (U. of Kentucky)

Researchers present the first results of the MIRIAD (MIPS (Multiband Imaging Photometer for Spitzer) Infra-Red Imaging of AGB (asymptotic giant branch)Dustshells) project using the Spitzer Space Telescope. The primary aim of the project is to probe the material distribution in the extended circumstellar envelopes (CSE) of evolved stars and recover the fossil record of their mass loss history. Hence, they must map the whole of the CSEs plus the surrounding sky for background subtraction, while avoiding the central star that is brighter than the detector saturation limit. With their unique mapping strategy, the researchers have achieved better than one MJy/sr sensitivity in three hours of integration and successfully detected a faint (< 5 MJy/sr), extended (~400 arcsec) far-infrared nebula around the AGB star R Hydrae.
Based on the parabolic structure of the nebula, the direction of the space motion of the star with respect to the nebula shape, and the presence of extended H alpha emission co-spatial to the nebula, they suggest that the detected far-IR nebula is due to a bow shock at the interface of the interstellar medium and the AGB wind of this moving star. This is the first detection of the stellar-wind bow-shock interaction for an AGB star and exemplifies the potential of Spitzer as a tool to examine the detailed structure of extended far-IR nebulae around bright central sources.

R Hydraecurve

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An arc-shaped nebula near the star R Hydrae is the first bow shock ever seen around a pulsating red giant, say astronomers using the Spitzer Space Telescope. The bow shock arises as material streaming off the star slams into the interstellar medium.
R Hydrae is near the end of its life and is one of the brightest Mira-type variable stars in the sky. As the star expands and contracts, it brightens and fades. When brightest, R Hydrae shines at fifth magnitude, making it visible to the naked eye; but when dimmest, the star can only be seen by using optical aid.
On February 26, 2006, Toshiya Ueta of NASA's Ames Research Centre near San Francisco, Angela Speck of the University of Missouri at Columbia, and their colleagues in the United States, Europe, and Japan took aim at R Hydrae with the Spitzer Space Telescope. They discovered far-infrared radiation from a faint nebula located just a quarter of a light-year from the star.

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Mira variables pulsate with a period between 80 and 1000 days.
In visual light, the amplitude of the light change is in general between 2.5 and 10 magnitudes.
In infrared the amplitude is significantly smaller. Miras are of spectral type M, S or C, dependent on the ratio between the amount of carbon and oxygen in their photosphere.
M-type Mira stars have less carbon than oxygen; S-types contain roughly an equal amount of both elements, while C-types are over abundant in carbon. M, S and C type stars have a noticeable orange colour, especially around maximum. This colour points to a relatively low surface temperature of 3000K. Special care has to be taken while estimating Miras near maximum.
The orange colour makes it difficult to make a reliable estimate of its brightness. This explains the large scatter in brightness estimates of Miras near maximum. some observers use the "quick glance method", while others use the "out-of-focus" method.
It is probably best to observe maxima of Miras with an as small as possible instrument, or to use a diaphragm to reduce the aperture of the telescope. The mass of Mira variables is comparable to the Sun, but with a diameter 200 to 300 times as big. This large diameter causes the big luminosity.
Miras radiate 3000 to 4000 times as much light as the Sun.

Mass loss

As a Mira star expands, the diameter increases greatly. Therefore, the escape velocity in the outer parts of the photosphere will be very small. A part of the hot plasma (ionized gas) at the edge of the star will move fast enough (10 to 20 kilometres per second) to escape to interstellar space.
This outflow of plasma is connected to shockwaves moving through the star and convection in the outer layers of the star. The out flowing gasses will condense at a distance of 2 to 6 AU from the star. Here they react with the dust present. Infrared satellites have proven the existence of this dust.
The dust particles measure about one micron and are mainly composed of silicon dioxide. This makes them grains of sand, "polluted" with iron, magnesium and aluminium compounds. Radio telescopes have shown the presence of hydroxyl (OH) around Miras.
This way Miras loose about one millionth of a solar mass a year. When you realize that Miras have a mass of about one solar mass, then it becomes obvious that the Mira stage is only a short period in the evolution of these stars.
Mira variables play an important role enriching interstellar space with heavy elements. Because of their continuous mass loss the are predecessors of planetary nebula and as such also of white dwarf stars.

Evolution of Miras

Stars spend most of their time on the main branch of the HR diagram. In this phase they convert hydrogen to helium. When the nuclear fuel runs out, the star expands and gets brighter. The star changes into a giant.
At a certain stage the radiation pressure of the star cannot withstand gravity anymore. The star compresses and the brightness decreases. The density and the temperature in the nucleus increase. At this higher temperature helium can be converted into carbon.
The star reaches the horizontal branch above the main branch of the HR diagram. When the helium in the nucleus is depleted, the star turns into a giant again. The nucleus contains carbon and
hydrogen, surrounded by two nuclear fusion zones, one in which helium is converted to carbon, surrounded by a layer where hydrogen is fused to helium. The star reaches a second horizontal branch in the HR diagram.
This Asymptotic Giant Branch (AGB) lies above and parallel to the first giant branch. The star becomes bigger and more luminous than the first time. In the AGB you can find the Mira stars and the SRa and SRb stars as well.

Thermal Flash

Some Miras do not have a constant period. Some show an increasing period and others a decreasing period, sometimes after a prolonged time with a constant period. Astronomers suspect that the change in period is connected to a thermal pulse. During a thermal pulse there is a short period with an enhanced fusion in the helium layer.
This releases extra energy.
When the star processes this extra energy, the carbon/oxygen nucleus and the helium and the hydrogen layers mix a little bit, enriching the convection layer of the star with carbon and oxygen. This change in interior structure influences the pulsation period of the star. It could even be that a Mira star stops its pulsations, to become a Mira variable again later on.

Of the about 6000 Mira variables in the General Catalogue of Variable Stars (GCVS), less than 1000 are regularly observed. Of these, only a few handfuls show a clear period change.

The most illustrative examples are discussed below.

T Ursae Minoris

Mrs. L. Ceraski discovered T UMi on February 13, 1902. Until 1968 the period stayed almost constant at about 315 days. Starting in 1968, the period of this star started to decrease.
Nowadays the period is roughly 240 days and the decrease hasn't stopped yet. T UMi can reach magnitude + 7.8, and can get as faint as +15.2. The average magnitude range lies between +9.2 and +14.0.
The maxima are easy to observe in a small scope, but for a faint minimum you need at least a 30 cm (12") telescope. For a big part of the northern hemisphere this object is circumpolar. T UMi is easy to find starting from beta UMi.

R Hydrae

Maraldi discovered the variability of R Hya in 1704. Until 1770, the period was nearly constant at 495 days. From 1770 to 1950 the period decreased to 395 days and has remained constant since that date. R Hya can be as bright as + 3.7 and as faint as + 10.3. On average the brightness lies between +4.5 and +9.5.
Although this star has a southern declination, it is also very easy to observe from a big part of the northern hemisphere. At maximum it is a naked eye or a binocular object, at minimum only a small telescope is needed R Hya is very easy to find starting from gamma Hya.

R Aquilae

This star near delta Aql was discovered by astronomers in Bonn, Germany in 1856. Since its discovery the period decreased to about 270 days nowadays.
The star varies between magnitude +6.1 and +11.5.
Although this star has a northern declination, it can be observed from all major landmasses from the southern hemisphere. At maximum binoculars are sufficient, but at minimum at least a 11 cm (4.5") telescope is needed.

Z Tauri

This star can reach a maximum of +9.0, but the minimum could be as low as +14.8. On average the brightness lies between +9.8 and +13.9.
Around 1925 the period was about 500 days, but it has decreased to 450 days at the moment. Z Tau can be observed from both hemispheres, as it lies near the border between Taurus and Orion.

R Centauri

The period of R Cen remained virtually constant at around 550 days until 1950. Nowadays it has decreased to 510 days. R Cen is a so-called "double-peaked" Mira, whose light curve shows two maxima per cycle. Besides the period, also the amplitude of this star has decreased.
The amplitude is nowadays one third of what it was in the beginning of the twentieth century.
R Cen can only be observed from the equatorial region and the southern
hemisphere. It is located near beta Cen. This star seems an easy target, but there are many faint stars out there to confuse you. Take care that you identify R Cen properly. This star is observable in small telescopes.

LX Cygni

There are also Mira's that increase their period. In 1968 the period of LX Cyg was around 480 days, but it has increased to 580 days since then. LX Cyg is only visible from the northern hemisphere, and is located near the open cluster NGC7209.
The star is located near the Cygnus / Lacerta border, in a region of the sky devoid of bright stars. This makes LX Cyg a difficult target for beginners. At maximum a small telescope is sufficient, but to follow this star as it goes to its minimum you need a 25 cm (10") telescope.

BH Crucis

Also BH Cru is a variable that has increased in period. In 1979 it had a period of 490 days; nowadays the period has increased to 530 days. BH Cru can only be observed from equatorial regions and the southern hemisphere.
The star is easy to locate near gamma and delta Cru. At maximum a binocular is sufficient, around minimum a small telescope will do. There is a good chart available from Sebastián Otero's website: http://ar.geocities.com/varsao/Carta_BH_Cru.htm

W Draco

The last star in this discussion is W Dra. This star also increased in period, from 255 days in the beginning of the twentieth century to 280 days now. W Dra has a declination of +66 degrees, and is therefore only observable form the northern hemisphere. Unfortunately, this star lies in a region with no bright stars and is for beginners difficult to locate.
At maximum a small scope will do, but at minimum you will need a 25 cm (10") telescope at least.


Extract from “E Y E P I E C E V I E W S “ June, 2005
(electronic magazine)






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