Clump formation through colliding stellar winds in the Galactic Centre

et al. 2019

ApJL

The central super-massive black hole of the Milky Way, Sgr A*, accretes at a very low rate making it a very underluminous galactic nucleus. Despite the tens of Wolf-Rayet stars present within the inner parsec supplying ∼10 −3 M yr −1 in stellar winds, only a negligible fraction of this material (< 10 −4 ) ends up being accreted onto Sgr A*. The recent discovery of cold gas (∼10 4 K) in its vicinity raised questions about how such material could settle in the hostile (∼10 7 K) environment near Sgr A*. In this work we show that the system of mass-losing stars blowing winds can naturally account for both the hot, inefficient accretion flow, as well as the formation of a cold disk-like structure. We run hydrodynamical simulations using the grid-based code Ramses starting as early in the past as possible to observe the state of the system at the present time. Our results show that the system reaches a quasisteady state in about ∼500 yr with material being captured at a rate of ∼10 −6 M yr −1 at scales of ∼10 −4 pc, consistent with the observations and previous models. However, on longer timescales ( 3000 yr) the material accumulates close to the black hole in the form of a disk. Considering the duration of the Wolf-Rayet phase (∼10 5 yr), we conclude that this scenario likely has already happened, and could be responsible for the more active past of Sgr A*, and/or its current outflow. We argue that the hypothesis of the mass-losing stars being the main regulator of the activity of the black hole deserves further consideration.

Section: Discussionmentioning

confidence: 99%

Stellar Winds Pump the Heart of the Milky Way

Calderón

Cuadra

et al. 2019

ApJL

“…A similar stream of clumps might, in principle, also be produced by the collision of winds from binary systems like IRS 16SW (Calderón et al 2016).…”

Section: Introductionmentioning

confidence: 99%

THE G2+G2t COMPLEX AS A FAST AND MASSIVE OUTFLOW?

Ballone

Burkert

et al. 2016

ApJL

Self Cite

Observations of the gas component of the cloud G2 in the Galactic Center have revealed its connection to a tail (G2t) lying on the same orbit. More recent studies indicate a connection between G2 and G1, another cloud detected on the blueshifted side of G2ʼs orbit, suggesting a scenario in which G2 is a denser clump in a stream of gas. In this Letter, we show that a simulation of an outflow by a central source (possibly a T Tauri star) moving on G2ʼs orbit and interacting with a hot atmosphere surrounding SgrA * can have G2 and G2t as a byproduct. G2 would be the bow shock formed in the head of the source, while G2t might be the result of the stripping of the rest of the shocked material by the ram pressure of the surrounding hot gas and of its successive accumulation in the trailing region. Mock position-velocity (PV) diagrams for the Brγ emission for this simulation can indeed reproduce the correct position and velocity of G2t, as well as the more tenuous material in between. Though some tension between the observations and the simulated model remains, we argue that this might be due to issues in the construction of observed PV diagrams and/or to a poor treatment of some physical processes-like hydrodynamic mixing-in our simulation.

“…In this section we proceed to compare the results of this work with previous analytical estimates mainly from Calderón et al (2016). Furthermore, we discuss the choice of the resolution employed in our simulations, and study the convergence of the results.…”

Section: Discussionmentioning

confidence: 94%

Three-dimensional simulations of clump formation in stellar wind collisions

Calderón

Cuadra

Monthly Notices of the Royal Astronomical Society

et al. 2020

The inner parsec of our Galaxy contains tens of Wolf-Rayet stars whose powerful outflows are constantly interacting while filling the region with hot, diffuse plasma. Theoretical models have shown that, in some cases, the collision of stellar winds can generate cold, dense material in the form of clumps. However, their formation process and properties are not well-understood yet. In this work we present, for the first time, a statistical study of the clump formation process in unstable wind collisions. We study systems with dense outflows (Ṁ ∼ 10 −5 M yr −1 ), wind speeds of ∼ 500-1500 km s −1 , and stellar separations of ∼ 20-200 au. We develop 3D high resolution hydrodynamical simulations of stellar wind collisions, making use of the adaptive-mesh refinement grid-based code Ramses. We aim to characterise the initial properties of clumps that form through hydrodynamic instabilities, mostly via the non-linear thin shell instability. Our results confirm that more massive clumps are formed in systems whose winds are close to the transition between the radiative and adiabatic regimes, as long as such collisions are capable of creating cold, thin shells. Also, we find that increasing either the wind speed or the degree of asymmetry in the wind interaction increases the dispersion of the clump masses and ejection speed distribution. Nevertheless, our findings show that the most massive clumps are very light (∼ 10 −3 -10 −2 M ⊕ ), approximately three orders of magnitude less massive than theoretical upper limits. We apply our results to the central parsec of our Galaxy finding that clumps formed are not heavy enough neither to affect the thermodynamic state of the region nor to survive for long enough in order to fall onto the central super-massive black hole before being destroyed.