WP2: Pulsation

Main contributors:
Michele Trabucchi, Joesfina Montalbán

New pulsation models from WP2 can be found here.

The main goal of WP2 is to produce an extended grid of pulsation models covering the range of stellar parameters necessary to study and interpret the observed pulsation properties of TP-AGB stars in old globular clusters and in the Magellanic Clouds (MCs). In particular, the different sequences identified in the Period-Luminosity (PL) diagram for Long Period Variables (LPVs) in the LMC OGLE-III Catalogue of Variable Stars (Soszyńsky et al., 2009).

The properties (mode amplitude, complexity of the oscillation spectra, period domain, etc.) of oscillation spectra depend on global parameters of the stars, such as chemical composition, effective temperature and luminosity (or radius), and also on details of the internal distribution of matter resulting from current and previous physical processes inside the star. Therefore, the study of pulsation and the comparison with theoretical modelling predictions allows us to constrain the properties of those physical processes. On the other hand, the dependence of oscillation periods on stellar global parameters convert pulsastions into useful tools to estimate stellar mass and radius.

To link the oscillation features with stellar properties, an identification of the observed oscillation modes and of the driving mechanism at their origin is highly recommended. There is a long standing controversy concerning the latter: stochastic excitation (also called solar-like oscillations; e.g. Christensen-Dalsgaard et al., 2001) self-excited pulsations, or driving from different effects of convection, as suggested by Xiong & Deng, 2007, with the main driving agent changing with stellar luminosity.

Recent identification of substructure in some of the PL sequences (e.g. Soszyński et al., 2004) together with CoRoT and Kepler observations of Galactic red giants, strongly suggest that, at least low-luminosity, low-period pulsations show indeed stochastic excitation properties (i.e. Mosser et al., 2013; Stello et al., 2014).

Concerning mode identification, the main PL sequences (from A to C, following the nomenclature used in Wood, 2015) are traditionally interpreted as due to radial pulsation modes with a different radial order for each sequence. There is however no consensus about the assignment of the radial order to PL sequences (see Wood, 2015 for a summary). Traditionally, the oscillations of very evolved stars, such as AGB ones, have been limited to radial oscillations, that are expected to have larger amplitudes than non-radial ones. However, the improvement of instrumentation has allowed the detection of tiny luminosity variations in evolved stars that could result from the presence of non-radial oscillations rather than radial ones. Unfortunately, different attempts to the interpretation of these structures as non-radial modes have not been completely successful so far (i.e., Dziembowski & Soszyńsky, 2010)

The primary golas of WP2 are:

  • To compute an extended grid of stellar envelope models widely covering the parameters space to describe AGB evolution, and for which a study of pulsastion properties will be performed. For each grid point the theoretical modelling will involve the computation of radial, non-adiabatic pulsation properties as well as non-radial, adiabatic oscillation periods.
  • To define. with the help of such a set, scaling relations (or fitting relations) linking the stellar parameters to pulsation properties in order to introduce efficiently this information in the parameteric estimates of mass loss, and in the stellar population synthesis code TRILEGAL-
  • To develop a kit-tool allowing versatile computation of pulsation models that includes the updated of the physical inputs with large impact for this models, such as radiative opacity tables (from the AESOPUS code) for the specific chemical composition of study cases.
  • To analyse the aforementioned models and tools to determine the dependencies of different pulsation parameters, or combinations of them, on the global stellar parameters (mass, radius, age, evolutionary state, chemical composition) and on the properties of the stellar structure (core mass, density contrast between centre and the envelope, etc.)
  • To combine non-adiabatic information on radial modes with adiabatic radial and non-radial periods to study and fit the period-luminosity sequences of long-period variables resulting from variability surveys sich as OGLE and Gaia.