Pulsational instability of pre-main-sequence models from accreting protostars

Abstract

Context. The picture of pre-main-sequence evolution is often simplified by the application of classical initial models. Such models have large initial radii and sufficient uniform contraction to make them fully convective, however, real stars are born as small protostellar seeds in collapsing molecular clouds and obtain their final mass by means of accretion. Aims. We aim to constrain the input physics of accretion on protostellar seeds with the observed spectroscopic parameters and stellar pulsations of young stellar objects and pre-main-sequence stars. Methods. We conducted a literature search for spectroscopic samples of young stellar objects and pre-main-sequence stars, including all previously known pulsators. The sample size of pulsating pre-main-sequence stars was increased by analysing TESS observations and presenting additional discoveries in the CoRoT data. We employed Modules for Experiments in Stellar Astrophysics and GYRE to calculate evolutionary tracks of accreting protostellar seeds in a constant accretion scenario, the subsequent pre-main-sequence evolution, and their pulsation properties. We then compared the results with the observations to constrain the input physics. Results. We discuss 16 formerly unknown pulsating pre-main-sequence stars and candidates that are of any of the following type: slowly pulsating B-stars, δ Scuti, γ Doradus, or a δ Scuti - γ Doradus hybrid type. We find that evolutionary tracks with a mass accretion rate of 5 × 10−6 M⊙ yr−1 and a fraction of injected accretion energy of β = 0.1 provide the best results for enveloping the spectroscopic parameters of pre-main-sequence stars in the constant accretion scenario. The calculated instability regions constrain the atmospheric boundary conditions to Eddington Gray atmospheres. Here, we discuss the future potential for additional constraints by instability regions that are dependent on radial order. Finally, we present a possible candidate star for pulsations in M-type young stellar objects. Conclusions. We show that evolutionary calculations of accreting protostellar seeds match the observed spectroscopic parameters of pre-main-sequence stars. Future observations that will allow for the identification of radial orders in particular will present opportunities for additional constraints.

The Pre-main Sequence: Challenges and Prospects for Asteroseismology

“…As a consequence, SPB pulsators before the onset of hydrogen core burning are observationally harder to find. The expected temperature range for pre-main sequence SPB stars is 11,100-18,700 K (Steindl et al, 2021a).…”

“…δ Scuti type. The pulsation periods of these intermediate-mass pre-main sequence stars with effective temperatures from 6,300 to 10,300 K (Steindl et al, 2021a) lie between ~18 min and 7 h (Zwintz, 2019). Pre-main sequence δ Scuti stars show p-modes driven by the heat-engine (κ) mechanism in the ionisation zones of hydrogen and helium (Aerts et al, 2010).…”

“…Pre-main sequence γ Doradus stars have early F spectral types. Their expected range in effective temperature lies between 5,200 and 7,650 K (Steindl et al, 2021a). First theoretical predictions for this type of pulsations in pre-main sequence stars have been conducted by Bouabid et al (2011) without observational evidence.…”

“…The concept of such a birthline is hence outdated, with state-of-the-art models of the pre-mainsequence providing a very different picture. The latter has been manifested by many authors (e.g., Hartmann and Kenyon, 1996;Hartmann et al, 1997;Wuchterl, 2001;Baraffe et al, 2009;Baraffe and Chabrier, 2010;Hosokawa et al, 2011;Baraffe et al, 2012;Kunitomo et al, 2017;Jensen and Haugbølle, 2018;Elbakyan et al, 2019), but only recently arrived in the field of asteroseismology (Steindl et al, 2021a.…”

“…Most notably is the existence of a temperature inversion towards the centre of the star (see e.g. Figure 8 of Steindl et al, 2021a). An imprint of star formation on the internal structure remains throughout the pre-main sequence phase and at least until the ZAMS; this should provide the opportunity to probe such diskmediated evolution models with asteroseismology .…”