Effect of accretion on the pre-main-sequence evolution of low-mass stars and brown dwarfs

Abstract

Aims. The pre-main-sequence evolution of low-mass stars and brown dwarfs is studied numerically starting from the formation of a protostellar/proto-brown dwarf seed and taking into account the mass accretion onto the central object during the initial several Myr of evolution. Methods. The stellar evolution was computed using the STELLAR evolution code developed by Yorke & Bodenheimer with recent modifications by Hosokawa et al. The mass accretion rates were taken from numerical hydrodynamics models of Vorobyov & Basu computing the circumstellar disk evolution starting from the gravitational collapse of pre-stellar cloud cores of various mass and angular momentum. The resulting stellar evolution tracks were compared with the isochrones and isomasses calculated using non-accreting models.Results. We find that mass accretion in the initial several Myr of protostellar evolution can have a strong effect on the subsequent evolution of young stars and brown dwarfs. The disagreement between accreting and non-accreting models in terms of the total stellar luminosity L * , stellar radius R * and effective temperature T eff depends on the thermal efficiency of accretion, i.e., on the fraction of accretion energy absorbed by the central object. The largest mismatch is found for the cold accretion case, in which essentially all accretion energy is radiated away. The relative deviations in L * and R * in this case can reach 50% for 1.0-Myr-old objects and remain notable even for 10-Myr-old objects. In the hot and hybrid accretion cases, in which a constant fraction of accretion energy is absorbed, the disagreement between accreting and non-accreting models becomes less pronounced, but still remains notable for 1.0-Myr-old objects. These disagreements may lead to the wrong age estimate for objects of (sub-)solar mass when using the isochrones based on non-accreting models, as was also previously noted by Baraffe et al. and Hosokawa et al. We find that objects with strong luminosity bursts exhibit notable excursions in the L * -T eff diagram, but the character of these excursions is distinct for hybrid/hot and cold accretion scenarios. In particular, the cold accretion scenario predicts peak luminosities that are greater than those of known FU-Orionis-type outbursts, which implies that cold accretion is physically less realistic. Conclusions. Mass accretion during the early stages of star and brown dwarf evolution is an important factor, but its effect depends on the details of how accretion energy is distributed within the star.

Gravitoviscous protoplanetary disks with a dust component

“…The mass accretion rates could be estimated from the accretion luminosity and the stellar mass and radius (see Eq. (8) in Vorobyov et al 2017). The accretion rates are found to correlate with the stellar mass (Calvet et al 2004;Herczeg & Hillenbrand 2008;Manara et al 2015) and stellar age (Hartmann et al 1998;Manara et al 2012;Venuti et al 2014).…”

“…where Σ d,sm and Σ d,gr are the surface densities of small and grown dust; u p describes the planar components of the grown dust velocity; S (a r ) is the rate of dust growth per unit surface area, the expression for which can be found in ; and a r is the maximum radius of grown dust. The properties of the central protostar (e.g., radius and photospheric luminosity) are calculated using the STELLAR evolution code (Yorke & Bodenheimer 2008;Hosokawa et al 2013;Vorobyov et al 2017;Elbakyan et al 2019). The evolution of the central protostar and the circumstellar disk are connected self-consistently.…”

The chemistry of episodic accretion in embedded objects

“…Finally, variable accretion with episodic bursts can help to resolve the "luminosity problem" of embedded protostars (Dunham & Vorobyov 2012), explain the existence of the very low luminosity objects (VELLOs) in the protostellar phase (Vorobyov et al 2017a), and affect the positions of A&A 604, A15 (2017) pre-main-sequence stars on the HR diagram (Baraffe et al 2012;Hosokawa et al 2011;Vorobyov et al 2017b). Until recently, episodic bursts were a feature exclusively attributed to lowmass star formation, but recent numerical models and observations demonstrated that massive stars can also have accretion bursts (Burns et al 2016;Caratti o Garatti et al 2016;Meyer et al 2017).…”

An analysis of four stellar rings

“…However, we are not able to, a priori, rule out a premain sequence nature of this object. According to the absolute magnitude and thus the luminosity, it has to be in the very early stages of its evolution (Vorobyov et al ). This stage is characterized by a significant IR‐excess (Meyer et al ).…”