Semi-regular red giants as distance indicators

Abstract

Context. Semi-regular variables (SRVs) are similar to Miras in brightness, and they also follow one or more period–luminosity relations (PLRs), though not necessarily the same one as Miras. As potential standard candles they are more challenging than Miras because of their smaller variability amplitudes and less regular light curves, but they are substantially more numerous and especially promising for probing old stellar populations. Aims. We aim to characterise the variability of SRVs, specifically focusing on their connection with Miras, in order to prepare the ground for investigating their potential as distance indicators. Methods. We examine SRVs and Miras in the Magellanic Clouds from OGLE-III observations, with data from Gaia and 2MASS. After cleaning the sample of variability periods unrelated to pulsation, we classify each source by chemical type and combination of pulsation modes. We examine the results in terms of global photometric and pulsation properties. Results. We identify four SRV groups that fit the general evolutionary scenario predicted by theory. SRVs dominated by fundamental-mode pulsation are very similar to Miras, especially if mono-periodic. They further split into two subgroups, one of which follows the same sequence as Miras in the period–luminosity and period–amplitude diagrams, without discontinuity. Conclusions. The similarities between Miras and SRVs suggest that the latter can be adopted as distance indicators in a way that is complementary to the use of the former, thereby at least doubling the available number of long-period variables (LPVs) suitable for use as distance indicators. The traditional amplitude-based separation between Miras and SRVs is not necessarily appropriate, and a more physically sound criterion should also involve pulsation periods. While this would require comparatively longer time-series, they are expected to become accessible in the coming years even for weak sources thanks to current and future large-scale surveys. The table of reclassified LPVs is made public.

A Fresh Look at AGB Stars in Galactic Open Clusters with Gaia: Impact on Stellar Models and the Initial–Final Mass Relation

“…(2017, and references therein; see also Trabucchi et al 2021a), we are able to identify the pulsation mode responsible for each period. We identify nine sources with a period due to the fundamental mode, eight sources pulsating predominantly in the first-overtone mode, and one star (S1 * 338) whose period is most likely due to pulsation in the second-overtone mode.…”

“…However, it should be noted that the traditional distinction between Miras and semiregular variables, according to which the former have visual amplitude ΔV > 2.5 mag, has been criticized by a number of studies (e.g., Kerschbaum & Hron 1992;Kiss et al 2000;Lebzelter & Hinkle 2002). Recently, Trabucchi et al (2021a) have shown that semiregular variables in the LMC that pulsate only in the fundamental mode follow the same sequence as Miras both in the PLD and in the period-amplitude diagram, suggesting that it would be incorrect to assign them different variability types only because their amplitude is smaller than an arbitrary, although reasonable, threshold.…”

Distance estimates for AGB stars from parallax measurements

“…Therefore, the reliability of the distances derived with this method can be arguable. More recently, Trabucchi et al (2021) investigated the suitability of SRs as distance indicators and found that a subgroup of SRs follows the same sequence as Mira variables in the PL diagram. However, they concluded that long-time series are necessary to properly classify SRs according to their pulsation periods.…”

Mira variables in the Milky Way’s nuclear stellar disc: discovery and classification

“…Physically they are often linked with high-amplitude fundamental mode pulsation and hence membership of a particular period-luminosity sequence. From an observational perspective, this definition is often approximated as a pure amplitude cut (e.g Soszyński et al 2013) although Trabucchi et al (2021) acknowledge that this simple consideration removes lower amplitude stars on the same fundamental period-luminosity relation as the higher amplitude systems. In the M09 analysis, stars are considered as non-Mira variables if the amplitude in any of J, H or K s is less than 0.4 although this removes only ∼ 10 per cent of the stars in their sample.…”