Theoretical amplitudes and lifetimes of non-radial solar-like oscillations in red giants

Abstract

Context. Solar-like oscillations have been observed in numerous red giants from ground and from space. An important question arises: could we expect to detect non-radial modes probing the internal structure of these stars? Aims. We investigate under what physical circumstances non-radial modes could be observable in red giants; what would be their amplitudes, lifetimes and heights in the power spectrum (PS)? Methods. Using a non-radial non-adiabatic pulsation code including a non-local time-dependent treatment of convection, we compute the theoretical lifetimes of radial and non-radial modes in several red giant models. Next, using a stochastic excitation model, we compute the amplitudes of these modes and their heights in the PS. Results. Distinct cases appear. Case A corresponds to subgiants and stars at the bottom of the ascending giant branch. Our results show that the lifetimes of the modes are mainly proportional to the inertia I, which is modulated by the mode trapping. The predicted amplitudes are lower for non-radial modes. But the height of the peaks in the PS are of the same order for radial and non-radial modes as long as they can be resolved. The resulting frequency spectrum is complex. Case B corresponds to intermediate models in the red giant branch. In these models, the radiative damping becomes high enough to destroy the non-radial modes trapped in the core. Hence, only modes trapped in the envelope have significant heights in the PS and could be observed. The resulting frequency spectrum of detectable modes is regular for = 0 and 2, but a little more complex for = 1 modes because of less efficient trapping. Case C corresponds to models of even higher luminosity. In these models the radiative damping of non-radial modes is even larger than in the previous case and only radial and non-radial modes completely trapped in the envelope could be observed. The frequency pattern is very regular for these stars. The comparison between the predictions for radial and non-radial modes is very different if we consider the heights in the PS instead of the amplitudes. This is important as the heights (not the amplitudes) are used as detection criterion.

Pulsating red giant stars in eccentric binary systems discovered fromKeplerspace-based photometry

“…At some stages of stellar evolution, p modes and g modes can couple and become a mixed mode. The firm identification of such mixed modes in Kepler and CoRoT data by Beck et al (2011), Bedding et al (2011), and Mosser et al (2011a) extended the sensitivity of the seismic analysis towards the core regions of red giant stars (Dupret et al 2009, and references therein). The analysis of mixed modes allows us to determine the evolutionary state Mosser et al 2011a) and constrain the core rotation of red giants (Beck et al 2012;Deheuvels et al 2012).…”

Asteroseismic Signatures of Evolving Internal Stellar Magnetic Fields

“…In red giants, mixed modes are generally observable when the evanescent region separating the core and envelope is quite narrow, which occurs for RGB stars below the luminosity bump (Dupret et al 2009;Grosjean et al 2014). As stars evolve further up the RGB and expand, the evanescent region thickens, and mixed modes become undetectable.…”

“…This technique utilizes observations of mixed modes, oscillations that behave as pressure waves (p-modes) near the stellar surface and gravity waves (g-modes) in the stellar core (Scuflaire 1974;Osaki 1975;Aizenman et al 1977;Dziembowski et al 2001;Christensen-Dalsgaard 2004;Dupret et al 2009). Mixed modes have been used successfully to determine the evolutionary status of red giant stars Stello et al 2013;Mosser et al 2014), and to measure their internal rotation rate (Beck et al 2012;Mosser et al 2012;Deheuvels et al 2014Deheuvels et al , 2015.…”

Fast core rotation in red-giant stars as revealed by gravity-dominated mixed modes

“…Mixed modes in the wings of the dipole forest are predicted to have large amplitudes in the central regions of the star and, therefore, larger values of inertia and lifetime. These modes have narrower mode profiles in the frequency spectrum than have the centre modes that are predominantly trapped in the outer cavity 11 . This behaviour of the mode profiles ( Supplementary Fig.…”