Period-luminosity relations of pulsating M giants in the solar neighbourhood and the Magellanic Clouds

Abstract

We analyse the results of a 5.5‐yr photometric campaign that monitored 247 southern, semiregular variables with relatively precise Hipparcos parallaxes to demonstrate an unambiguous detection of red giant branch (RGB) pulsations in the solar neighbourhood. We show that Sequence A′ contains a mixture of asymptotic giant branch and RGB stars, as indicated by a temperature‐related shift at the tip of the red giant branch. Large Magellanic Cloud (LMC) and Galactic sequences are compared in several ways to show that the period–luminosity sequence zero‐points have a negligible metallicity dependence. We describe a new method to determine absolute magnitudes from pulsation periods and calibrate the LMC distance modulus using Hipparcos parallaxes to find μLMC= 18.54 ± 0.03 mag. Several sources of systematic error are discussed to explain discrepancies between the Massive Compact Halo Object and Optical Gravitational Lensing Experiment sequences in the LMC. We derive a relative distance modulus of the Small Magellanic Cloud relative to the LMC of Δμ= 0.41 ± 0.02 mag. A comparison of other pulsation properties, including period–amplitude and luminosity–amplitude relations, confirms that RGB pulsation properties are consistent and universal, indicating that the RGB sequences are suitable as high‐precision distance indicators. The M giants with the shortest periods bridge the gap between G and K giant solar‐like oscillations and M‐giant pulsation, revealing a smooth continuity as we ascend the giant branch.

The variability behaviour of CoRoT M-giant stars

“…3 (upper panels) for the stronger pulsation (open circles) follows the trend observed by Tabur et al (2010) closely (red line in the diagram), which corresponds to stars pulsating in overtones. The other modes of oscillation agree well with the amplitude-ν max relation of Mosser et al (2013) for ν < 1 μHz, corresponding to log P > 1.06 days.…”

“…The red line between the two red crosses roughly depict the trend of the overtone (AB) pulsators taken from Fig. 15 of Tabur et al (2010). The blue line between the two blue crosses represents the area defined by Bányai et al (2013), selected from the top panel of Fig.…”

“…In this context, Tabur et al (2010) show that, as we advance through the RGB, the observed low amplitude of stochastically excited solar-type oscillations, which is typical of the GK-type giants, progress to a mixture of Mira-like and solar-like variability, as found in semi-regular (SR) variables, and ending with stable, mono-periodic, Mira-like pulsations. Afterwards, Mosser et al (2013) studied global oscillation parameters of Kepler red giants, concluding that the main excitation mechanism in M giant SR variables are solar-like oscillations, thus confirming the findings of Dziembowski & Soszyński (2010), but they were unable to disentangle RGB from AGB stars.…”

“…For each LC, we selected all the independent periods that exhibited a significance level greater than 99%, i.e., a false alarm probability (FAP) less than 0.01. Therefore, a collective set of independent periods for each target was retained to perform an analysis that was similar to that of Tabur et al (2010) and Bányai et al (2013), where we retained the first three periods above the FAP, as shown in Table 2. The FAP was computed based on the Eaton et al (1995) method, which consists of randomising the temporal bins of the original LC and computing the resulting power spectra (see also Affer et al 2012).…”

“…Our recent understanding of how M giants oscillate was obtained from ground-based surveys such as MACHO, OGLE, and EROS (e.g., Wood & Sebo 1996;Alard et al 2001;Lebzelter et al 2002;Kiss & Bedding 2003;Soszynski et al 2007;Riebel et al 2010;Wiśniewski et al 2011;Soszyński et al 2013), as well as solar neighbourhood observations (Tabur et al 2010).…”