Parameter study for the burst mode of accretion in massive star formation

Abstract

It is now a widely held view that, in their formation and early evolution, stars build up mass in bursts. The burst mode of star formation scenario proposes that the stars grow in mass via episodic accretion of fragments migrating from their gravitationally-unstable circumstellar discs and it naturally explains the existence of observed pre-main-sequence bursts from high-mass protostars. We present a parameter study of hydrodynamical models of massive young stellar objects (MYSOs) that explores the initial masses of the collapsing clouds (Mc = 60–$200\, \rm M_{\odot }$) and ratio of rotational-to-gravitational energies (β = 0.005 − −0.33). An increase in Mc and/or β produces protostellar accretion discs that are more prone to develop gravitational instability and to experience bursts. We find that all MYSOs have bursts even if their pre-stellar core is such that β ≤ 0.01. Within our assumptions, the lack of stable discs is therefore a major difference between low- and high-mass star formation mechanisms. All our disc masses and disk-to-star mass ratios Md/M⋆ > 1 scale as a power-law with the stellar mass. Our results confirm that massive protostars accrete about 40-60% of their mass in the burst mode. The distribution of time periods between two consecutive bursts is bimodal: there is a short duration ($\sim 1-10~\rm yr$) peak corresponding to the short, faintest bursts and a long-duration peak (at $\sim 10^{3}-10^{4} \rm yr$) corresponding to the long, FU-Orionis-type bursts appearing in later disc evolution, i.e., around $30\, \rm kyr$ after disc formation. We discuss this bimodality in the context of the structure of massive protostellar jets as potential signatures of accretion burst history.

The Extraordinary Outburst in the Massive Protostellar System NGC6334I-MM1: Strong Increase in Mid-Infrared Continuum Emission

“…Since the brightness of MM2 has remained unchanged (Hunter et al 2017), we compute MM1's pre-outburst luminosity as L MM1preburst =2900±600 L and its outburst luminosity as L MM1burst = 47600 ± 7800 L (assuming 20% uncertainty on r preburst ). Thus, the outburst ratio is 16.3±4.4, consistent with a magnitude 3 outburst (Meyer et al 2021).…”

“…with the accretion rate ( Ṁacc ) increasing during outbursts while having a persistent "background" value in between outbursts (Meyer et al 2021). If we assume that half of L MM1preburst arises from a ZAMS photosphere with solar metallicity, then the progenitor's properties are: mass M * =6.7 M , radius R * =2.6 R , and effective temperature T eff =22000 K (Tout et al 1996), which lies between spectral type B1.5V and B2V (Pecaut & Mamajek 2013).…”

Accretion bursts in high-mass protostars: A new test bed for models of episodic accretion

“…Simulations (e.g. Meyer et al 2021) predict that brown dwarfs and very young low-mass protostars could also be driven rapidly into the inner disc. While the mass budget of bursts observed from HMYSOs so far is in the planetary mass regime, it may be possible for a more massive object to lose only a part of its envelope if its mass-radius relation, r(M), is different from the simple power law we used here (Eq.…”

“…This example calculation illustrates that the planet migration may indeed be sufficiently short to transport planet-mass objects into the very inner disc during a typical HMYSO evolutionary timescale, expected to be ∼5×10 4 yr (Meyer et al 2021). Furthermore, for simplicity we assumed here that the protostar mass is M * = 15 M at the time of disc fragmentation.…”

“…Most recently, Meyer et al (2021) performed a large parameter study of the model and showed that mass accreted per burst varies from ∼10 M J to ∼1 M . In Sect.…”