Self-Excited Pulsations and the Instability Strip of Long-Period Variables: the Transition from Small-Amplitude Red Giants to Semi-Regular Variables

Abstract

We use one-dimensional hydrodynamic calculations combined with synthetic stellar population models of the Magellanic Clouds to study the onset of self-excited pulsation in luminous red giants. By comparing the results with OGLE observations in the period-luminosity diagram we are able to link the transition from small-amplitude red giants to semi-regular variables with a shift from stochastic driving to self-excited pulsations. This is consistent with previous studies relating this transition with an increase in mass-loss rate, dust formation, and the appearance of long secondary periods. The luminosity and effective temperature at the onset of pulsation are found to depend on metallicity, hydrogen content, and the adopted mixing length parameter. This confirms the role of partial hydrogen ionization in driving the pulsation, supporting the idea of a heat mechanism similar to that of classical pulsators. We examine the impact of turbulent viscosity, and find clear evidence that it must be adjusted according to the stellar chemical and physical parameters to fully match observations. In order to improve the predictive power of pulsation models, the turbulent viscosity and the temperature scale of pulsating red giants must be jointly calibrated. This is critical for model-based studies of the period-luminosity relations of evolved stars and to exploit their potential as distance and age indicators, in particular given the sensitivity of the onset of pulsation to the envelope composition. The grid of models is made publicly available with a companion interpolation routine.

Revised with publishing corrections. 14 pages, 6 figures, accepted for publication in the Astrophysical Journal; additional material available at https://zenodo.org/records/14002294

References in corpus (27)

• Array Programming with NumPy
• The Astropy Project: Sustaining and Growing a Community-oriented Open-source Project and the Latest Major Release (v5.0) of the Core Package
• Constraining the thermally-pulsing asymptotic giant branch phase with resolved stellar populations in the Small Magellanic Cloud
• Variability in red supergiant stars: pulsations, long secondary periods and convection noise
• Constraining the thermally pulsing asymptotic giant branch phase with resolved stellar populations in the Large Magellanic Cloud
• The pulsation modes, masses and evolution of luminous red giants
• Non-radial oscillations in M-giant semi-regular variables: Stellar models and Kepler observations
• Gaia Data Release 3: The second Gaia catalogue of Long-Period Variable candidates
• A new interpretation of the period-luminosity sequences of long-period variables
• Binarity as the Origin of Long Secondary Periods in Red Giant Stars
• Asteroseismology of luminous red giants with Kepler I: Long Period Variables with radial and non-radial modes
• An updated metal-dependent theoretical scenario for Classical Cepheids
• The onset of the AGB wind tied to a transition between sequences in the period-luminosity diagram
• Modelling Long-Period Variables -- II. Fundamental mode pulsation in the nonlinear regime
• Toward an internally consistent astronomical distance scale
• Low-Temperature Gas Opacities with AESOPUS 2.0
• Mira variables in the Milky Way's nuclear stellar disc: discovery and classification
• Properties of self-excited pulsations in 3D simulations of AGB stars and red supergiants
• Simulating the Legacy Survey of Space and Time stellar content with TRILEGAL
• The Optical Gravitational Lensing Experiment. Small Amplitude Variable Red Giants in the Magellanic Clouds
• The period-age relation of long-period variables
• The far side of the Galactic bar/bulge revealed through semi-regular variables
• Semi-regular red giants as distance indicators I. The period-luminosity relations of semi-regular variables revisited
• From Solar-like to Mira stars: a unifying description of stellar pulsators in the presence of stochastic noise
• Kepler and the Long Period Variables
• The first comprehensive Milky Way stellar mock catalogue for the Chinese Space Station Telescope Survey Camera
• A new dimension in the variability of AGB stars: convection patterns size changes with pulsation

Cited by in corpus (2)

• Evidence for the Keplerian orbit of a close companion around a giant star
• Interferometric view into RT Pav's long secondary period. binary vs oscillatory convective modes