IN SITU FORMATION OF SgrA* STARS VIA DISK FRAGMENTATION: PARENT CLOUD PROPERTIES AND THERMODYNAMICS

Abstract

The formation of the massive young stars surrounding SgrA * is still an open question. In this paper, we simulate the infall of a turbulent molecular cloud towards the Galactic Center (GC). We adopt two different cloud masses (4.3 × 10 4 M ⊙ and 1.3 × 10 5 M ⊙ ). We run five simulations: the gas is assumed to be isothermal in four runs, whereas radiative cooling is included in the fifth run. In all the simulations, the molecular cloud is tidally disrupted, spirals towards the GC, and forms a small, dense and eccentric disk around SgrA * . With high resolution simulations, we follow the fragmentation of the gaseous disk. Star candidates form in a ring at ∼ 0.1 − 0.4 pc from the super-massive black hole (SMBH) and have moderately eccentric orbits (e ∼ 0.2−0.4), in good agreement with the observations. The mass function of star candidates is top-heavy only if the local gas temperature is high ( > ∼ 100 K) during the star formation and if the parent cloud is sufficiently massive ( > ∼ 10 5 M ⊙ ). Thus, this study indicates that the infall of a massive molecular cloud is a viable scenario for the formation of massive stars around SgrA * , provided that the gas temperature is kept sufficiently high ( > ∼ 100 K).

Modelling the formation of the circumnuclear ring in the Galactic centre

“…On the other hand, we can speculate that for a smaller impact parameter of the cloud (e.g. the one adopted in Mapelli et al 2012) and/or for a lower initial orbital velocity of the cloud, the mass of the inner ring might be consistent with the expectations for the formation of the CW disc.…”

“…We simulate the infall of a molecular cloud towards the SMBH, adopting the same technique as discussed in Mapelli et al (2012Mapelli et al ( , 2013. The molecular cloud model is a spherical cloud with homogeneous density and a radius of 15 pc.…”

“…mass function and age) as the members of the CW disc but do not belong to any discs. Recent work suggested that the formation of the young stars in the innermost parsec is connected with the disruption of a molecular cloud by the SMBH (Bonnell & Rice 2008;Mapelli et al 2008;Alig et al 2011;Mapelli et al 2012;Lucas et al 2013;Alig et al 2013; see Mapelli & Gualandris 2015 for a review). Furthermore, several studies highlight that the CNR might have played an important role in the dynamical evolution of the CW disc (e.g.…”

Young stars in the Galactic center

“…The observed eccentricity distribution peaks at e=0.3 Yelda et al 2013). Scenarios invoking more rapid gas inflow, such as the infall of a single molecular cloud or the collision of two molecular clouds, could produce disk stars with large initial eccentricities and have the added bonus of producing a large population of young stars off the disk (Bonnell & Rice 2008;Wardle & Yusef-Zadeh 2008;Hobbs & Nayakshin 2009;Mapelli et al 2012), similar to what has been observed (Paumard et al 2006;Bartko et al 2009;Lu et al 2009;Yelda et al 2013). However, the evolution of an initially circular disk over 4-6 Myr depends on the initial mass function (IMF) of the stars and it is possible to evolve to today's observed distribution for moderately top-heavy IMFs .…”

“…These models predict extreme gas conditions compared to the disk of the Milky Way with higher temperatures, pressures, densities, and ambient radiation fields (Nayakshin 2006;Alexander et al 2007Alexander et al , 2008Cuadra et al 2008;Mapelli et al 2012). Analysis of the observed luminosity function of the spectroscopically identified young stars and careful correction for incompleteness has resulted in two different estimates for the IMF.…”