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What element drives pulsations on T Tauri stars? I understand that T Tauri stars are variable and pulsate, and I also know that other stars pulsate as well. My thinking is that a T Tauri star would pulsate due to doubly ionized helium, like Cepheids, but I'm not entirely sure. How do T Tauri stars pulsate?

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    $\begingroup$ Which pulsations are you referring to? $\endgroup$
    – ProfRob
    Apr 3 at 7:02
  • $\begingroup$ The pulsations that cause these stars to become variable stars. $\endgroup$
    – Astrovis
    Apr 4 at 4:51

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T-Tauri stars are not pulsational variables. Their variability, on timescales of minutes to decades, is associated with: spots and magnetically active regions on their surfaces; with variability of the accretion flow that is impacting their surfaces; and with variations in extinction caused by changes in the orientation of their discs or accretion flows (e.g., Siwak et al. 2018; Robinson et al. 2021.)

T-Tauri stars are very young, low mass objects (usually less than $2M\odot$ protostars with cool spectral types) that are contracting towards the main sequence and are often, though not always, actively accreting from a surrounding disc of gas and dust. If we were to admit higher mass young, accreting objects - often referred to as Herbig Ae/Be stars - into the definition, then some of these do occupy the instability strip in the Hertzsprung-Russell diagram and show pulsational variability that is similar to delta Scuti variables or in a few cases the gamma Doradus variables.

The pulsation mechanism in delta Scuti variables is known as the kappa mechanism and is associated with the periodic ionisation and recombination of helium in a doubly ionised zone beneath the photosphere, in common with the mechanism causing the variability in Cepheids (see for example Handler 2021). The gamma Doradus star pulsations are driven by an instability in transferring heat across the boundary between the radiative interior and the thin convective outer layer, rather than changes in opacity due to the ionisation of particular elements. Of order 50 pulsating Herbig Ae/Be pre main sequence stars have been identified, the vast majority are of the delta Scuti type with a few gamma Doradus types (Kurtz and Muller 2001; Fumel & Bohm 2012; Zwintz 2019).

There is however an interesting theoretical prediction that there could be a pulsational variability associated with the deuterium burning phase in very low-mass T-Tauri stars (or brown dwarfs) - see Palla & Baraffe (2005). This is caused by the high temperature sensitivity of the D-burning nuclear reaction that takes place when the contracting cores of fully convective pre main sequence objects reach about $10^6$ K. Despite quite sensitive searches (e.g., Cody & Hillenbrand 2014) no evidence for these pulsations has been detected.

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