Secular Evolution in Mira Variable Pulsations

Abstract

Stellar evolution theory predicts that asymptotic giant branch (AGB) stars undergo a series of short thermal pulses that significantly change their luminosity and mass on timescales of hundreds to thousands of years. These pulses are confirmed observationally by the existence of the short-lived radioisotope technetium in the spectra of some of these stars, but other observational consequences of thermal pulses are subtle and may only be detected over many years of observations. Secular changes in these stars resulting from thermal pulses can be detected as measurable changes in period if the star is undergoing Mira pulsations. It is known that a small fraction of Mira variables exhibit large secular period changes, and the detection of these changes among a larger sample of stars could therefore be useful in evolutionary studies of these stars. The American Association of Variable Star Observers (AAVSO) International Database currently contains visual data for over 1500 Mira variables. Light curves for these stars span nearly a century in some cases, making it possible to study the secular evolution of the pulsation behavior on these timescales. In this paper we present the results of our study of period change in 547 Mira variables using data from the AAVSO. We use wavelet analysis to measure the period changes in individual Mira stars over the span of available data. By making linear fits to the period versus time measurements, we determine the average rates of period change, d ln P/dt, for each of these stars. We find nonzero d ln P/dt at the 2 significance level in 57 of the 547 stars, at the 3 level in 21 stars, and at the level of 6 or greater in eight stars. The latter eight stars have been previously noted in the literature, and our derived rates of period change largely agree with published values. The largest and most statistically significant d ln P/dt are consistent with the rates of period change expected during thermal pulses on the AGB. A number of other stars exhibit nonmonotonic period change on decades-long timescales, the cause of which is not yet known. In the majority of stars, the period variations are smaller than our detection threshold, meaning the available data are not sufficient to unambiguously measure slow evolutionary changes in the pulsation period. It is unlikely that more stars with large period changes will be found among heretofore well-observed Mira stars in the short term, but continued monitoring of these and other Mira stars may reveal new and serendipitous candidates in the future.

The evolutionary state of Miras with changing pulsation periods

“…Among the SCh change type stars, BH Cru seems to have stopped or maybe even reversed its period increase. A deceleration of the period increase was proposed by Templeton et al (2005). RU Vul has continued its period decrease (also found by Templeton et al 2008), and the pulsation is very weak now.…”

“…Figure 1 shows the Coralie spectrum of BH Cru, smoothed to the resolution of classification spectra to make the Notes. The meaning of the columns is as follows: (1) name listed in Simbad; (2) period change type; (3) range of spectral type; (4) heliocentric radial velocity; (5) presence of Tc, this paper; (6) presence of Tc, previous studies; (7) effective temperature; (8) abundance of Li; (9) rate of period change; (10) period as given in Templeton et al (2005); (11) period found in this study; (12) mean J − K colour; (13) absolute K magnitude; (14) bolometric magnitude. References of previous studies on Tc content: (1) Little-Marenin & Little (1979); (2) Little et al (1987); (3) Vanture et al (1991); (4) Merrill (1952); (5) Uttenthaler & Lebzelter (2010); (6) Lebzelter & Hron (1999).…”

“…They are prominent representatives of the period change classes introduced by Zijlstra & Bedding (2002), and most are also discussed in detail in Templeton et al (2005). In total, twelve stars were selected for the observations.…”

Finding the most variable stars in the Orion Belt with the All Sky Automated Survey

“…It is an M8-9-type Mira Ceti variable found by Wolf (1904) with P = 424.75±1.77 d (Templeton et al 2005) and silicate dust emission (Sloan & Price 1998;Speck et al 2000). In particular, the 9.7 and 18 µm silicate dust feature originates from the circumstellar envelopes of oxygenrich asymptotic giant branch stars (Kwok et al 1997).…”

LONG-PERIOD VARIABILITY INoCETI

“…Large period changes, though rare, have been well documented in a few stars since the earliest work of Sterne & Campbell (1937), and centuries-long historical light curves for a few Miras show significant changes over time (Sterken et al 1999;Percy & Au 1999;, even if periods are relatively stable at present. Templeton et al (2005) used the American Association of Variable Star Observers (AAVSO) data archives to search for large, longterm period changes in 547 Mira variables, including o Ceti itself. Although o Ceti was not flagged as having a large period change, the significance criteria used in that paper were very high, and did not rule out period changes at the level of a few percent.…”