The violent life of the Galactic Centre

The Galactic Centre (GC) is an exciting laboratory for astrophysics, as it hosts the closest supermassive black hole (SMBH), and recent multi-wavelength observations provide a huge amount of information about it. Despite the strong tidal field exerted by the SMBH, more than a hundred young massive stars are observed in the central parsec of the Galaxy. The majority are O-type and Wolf–Rayet (WR) stars of which about two-thirds show clockwise motion when projected on the plane of the sky while the remainder show counterclockwise motion (Bartko et al. 2009). The WR/O stars in the larger group orbit the SMBH in a thin disc, usually referred to as the clockwise (CW) disc, with an estimated age of ~6 Myr and an average eccentricity of ~0.4. The smaller group of stars on counter-rotating orbits may indicate the presence of an additional dissolving disc. The CW disc shows a sharp inner truncation at about 1 arcsec (0.04 pc).

The S-cluster (Gillessen et al. 2009) located in the innermost arcsecond is a group of about 20 B-type stars moving on eccentric and randomly oriented orbits, with estimated ages of 20-100 Myr. The origin of the S-cluster is largely controversial, with main models including the inspiral of a young star cluster, and the tidal disruption of binaries on low angular momentum orbits.

The formation of the CW disc is also largely debated. Recent studies attribute it to in situ star formation from an infalling gas cloud (Morris 1993; Nayakshin & Cuadra 2005; Bonnell & Rice 2008; Mapelli et al. 2008, 2012; Wardle & Yusef-Zadeh 2008; Hobbs & Nayakshin 2009; Alig et al. 2011). At OAPd, we recently performed  N-body/smooth particle hydrodynamics (SPH) simulations of the infall of a turbulent molecular cloud towards the GC and studied the formation of stars in the region (Mapelli et al. 2012). We find that the tidal disruption of the cloud results in the formation of a small (radius <0.5 pc), dense (>10^8 atoms cm^-3) and eccentric (e ~ 0-0.6) gas disc around the SMBH (see the Figure). The simulations by Mapelli et al. 2012 are the first following the fragmentation of the gas disc into self-bound clumps. Stars originate from such clumps in a ring at a distance of 0.1-0.4 pc with moderately eccentric orbits (e ∼ 0.2-0.4). The mass function of the stars is top-heavy if the local background temperature is sufficiently high (>100 K) and if the parent cloud is sufficiently massive (>10^5 Msun). These properties are in good agreement with observations of the CW disc (Paumard et al. 2006; Bartko et al. 2009), and lend further support to the in situ formation model for the disc.

Many questions remain open, such as the mass distribution of the stars, their orbital properties, the formation of the S-stars (the ~20 stars closest to the Galactic centre), the properties of the progenitor molecular cloud, etc. A Master/PhD thesis is available, to understand the physics of the stars surrounding a SMBH, by means of N-body/SPH and N-body/AMR simulations. This project will be done in collaboration with  Dr. Alessia Gualandris at the University of Leicester, and with Dr. Tristen Hayfield at the Max-Planck-Institut for Astronomy (Heidelberg).


People: Michela Mapelli (http://web.pd.astro.it/mapelli/)

Collaborations:  Alessia Gualandris (University of Surrey, UK); Tristen Hayfield (Max-Planck-Institut for Astronomy, Heidelberg); Lucio Mayer (University of Zurich); Hagai Perets (Technion - Israel Institute of Science).

Recent publications: Mapelli et al. (2012, ApJ, 749, 168); Gualandris, Mapelli & Perets (2012, MNRAS, 427, 1793)

                                                                                                            
cloudx20pcbis

Figure:  Density map of the gas in run E of Mapelli et al. (2012) at t = 4.8 x 10^5  yr. The simulation was projected in the plane where the gaseous disc (at the centre) is seen face-on. The box measures 20 pc per edge.



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