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

I. Constraining the input physics for accretion with spectroscopic parameters and stellar pulsations

¹ Institut für Astro- und Teilchenphysik, Universität Innsbruck, Technikerstraße 25, 6020 Innsbruck, Austria
e-mail: thomas.steindl@uibk.ac.at
² The University of Texas at Austin, McDonald Observatory, 2515 Speedway, Stop C1402, Austin, TX 78712-1206, USA
³ Department of Astrophysics, University of Vienna, Vienna 1180, Austria
⁴ Research Institute of Physics, Southern Federal University, Rostov-on-Don 344090, Russia

Received: 16 March 2021
Accepted: 14 July 2021

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.