M Mira stars

- These are long period variable giants with characteristic late-type emission spectra (Me, Ce, Se) and light amplitudes from 2".'5 to 11"' in V. Their periodicity is well pronounced, and the periods lie in the range 80''-1000d. Infrared amplitudes are usually less pronounced than in the visible and may be < 2".'5. If the amplitudes are > lm-l™5, but it is not certain that the true light amplitude is > 2"!5, then the symbol "M" is followed by a colon, or the star is attributed to the semiregular class with a colon following the symbol for that type (SR). GCVS

Observation Key

Bright stars [yrj large amplitudes

Long periods <3> Visual

figure 4.7. light curve 0flhe Mirotype variable star R Leo. Julia" da,es ore indicated along the horizontal oxis. Daio provided by VSNET. Used with permission

40

50

60

K k.

L \ f

70

\ \ i \

80

A \ I \

\ \ I

90

? > * V

V V ^

10

Tt ▼ v

110

5000050200 50400 50600 50800 5100051200 5140051600

Mira-type" variable stars, also known as long period variable (LPV) stars, are all similar (astronomers prefer to say "homogeneous") and probably the best studied of the pulsating red variables. The GCVS suggests three defining characteristics of the Mira variables: the spectral type is M[e], S[e] or C[e], the visual or photographic light amplitude must exceed 2™5, and the period should be in the range 80-1000 days (Figure 4.7).

The spectral type indicates that Mira atmospheres contain strong molecular absorption features and are therefore cool because only within cool stars will molecules form. The outer atmospheres of these stars have temperatures below about 3800 K.9 The atmospheres of these cool stars may be oxygen-rich (M[e)), carbon-rich (C[e)) or intermediate (S[e]). The emission lines, whose presence is signified by the "[e]," are an important characteristic of this type of variability as they are the signature of shock waves associated with pulsation. The amplitude boundary is essentially arbitrary and as a result a few stars that are physically similar to the Mira variables are classified as semi-regular (SR) type variables because their amplitudes fall short of 2"?5. The infrared light curves of the Mira variables, where most of the energy is emitted in these stars, and the total integrated luminosity have smaller amplitudes than those of visible light, although they are mostly over 0'!'5. The large visual amplitudes arise from the combination that we are observing temperature variations from the blue side of the star's energy-distribution peak (the steepest portion of the visible energy window of the Planck curve at this temperature) and from changes in molecular-band strengths asso-

8Mira, the "wonderful," named by the Lutheran pastor and amateur astronomer named David Fabricius in 1595.

'When measured from absolute zero, temperature in degrees is properly called kelvins, not degrees Kelvin or Kelvin degrees.

dated with these temperature changes. The very long periods tell us that Miras have very large radii. The upper limit to the period boundary is probably irrelevant. There are certainly stars with periods in the 1000-2000 day range that can be considered Mira variables.

The Mira variable stars are of great interest to astronomers, primarily because they represent a very short-lived phase in stellar evolution and in the HR diagram they are found at the very tip of the asymptotic giant branch (AGB), so their next evolutionary step is expected to be a rapid move across the diagram to become planetary nebulae. There are various studies that suggest the period of a Mira variable is a good indicator of the stellar population to which it belongs. Long-period variables with periods of around 200 days belong to the same, old population as do the metal-rich globular clusters. Longer-period Mira variables are more massive and/or more metal rich. Consistent with this picture, but contrary to popular belief, there is no evidence that Miras systematically evolve to longer periods as they age. Miras are also useful as distance indicators as they obey a period-luminosity relation that can be expressed either in terms of the total (bolometric) luminosity or in terms of the near-infrared magnitudes.

It remains uncertain whether Miras pulsate in the fundamental or first overtone modes. While there are theoretical reasons for favoring the fundamental mode, the observational evidence favors the overtone. Miras are rapidly losing mass, perhaps as much as 108-104 M0 yr\ although the mechanism for this is not well understood. The mass-loss rates are statistically correlated with the pulsation period, the bolometric light amplitude and the shape of the light curve. The most highly evolved Miras are surrounded by the material they have ejected, rendering them optically faint but strong infrared sources. The very long period Miras that have evolved from the most massive progenitors and have the most mass to lose, have particularly thick shells. Some of these circumstellar shells also produce SiO, H20, and/or OH maser emissions that are detectable at radio frequencies.

With careful examination, you will discover that the light curves for Mira variables are not identical from cycle to cycle and the brightening at maximum often varies by a magnitude or more from one cycle to the next. Period changes are also observed in some of these stars. Period change can be seen particularly clearly in R Aql and R Hya, because they may be undergoing helium shell flashes.

Within the GCVS, there are over 5,200 known Mira variables, with another 940 suspected. Clearly, there is a sufficient number of long period variables to study for many years to come.

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