Conditions for the Thermal Instability in the Galactic Centre Mini-spiral region

TL;DR

This study investigates the conditions under which thermal instability can cause clump formation in the mini-spiral region near the Galactic centre, considering past activity levels of Sgr A* and the effects of dust and plasma states.

Contribution

It identifies the parameter ranges for thermal instability in the Galactic centre's mini-spiral, highlighting the influence of past luminosity, dust destruction, and plasma conditions on clump formation.

Findings

Thermal instability does not operate at current low activity levels of Sgr A*.

High past luminosity could produce a two-phase medium up to 1.4 pc from the centre.

Clump masses are likely in the range of 1 to 100 Earth masses.

Abstract

We explore the conditions for the thermal instability to operate in the mini-spiral region of the Galactic centre (Sgr A*), where both the hot and cold media are known to coexist. The photoionisation Cloudy calculations are performed for different physical states of plasma. We neglect the dynamics of the material and concentrate on the study of the parameter ranges where the thermal instability may operate, taking into account the past history of Sgr A* bolometric luminosity. We show that the thermal instability does not operate at the present very low level of the Sgr A* activity. However, Sgr A* was much more luminous in the past. For the highest luminosity states the two-phase medium can be created up to 1.4 pc from the centre. The presence of dust grains tends to suppress the instability, but the dust is destroyed in the presence of strong radiation field and hot plasma. The clumpiness is thus induced in the high activity period, and the cooling/heating timescales are long enough to preserve later the past multi-phase structure. The instability enhances the clumpiness of the mini-spiral medium and creates a possibility of episodes of enhanced accretion of cold clumps towards Sgr A*. The mechanism determines the range of masses and sizes of clouds; under the conditions of Sgr A*, the likely values come out $1$ - $10^2M_{\oplus}$ for the cloud typical mass.