Galactic center G objects as dust-enshrouded stars near the supermassive black hole

TL;DR

This paper revisits models of dust-enshrouded stars near the supermassive black hole at the Galactic center, providing observational evidence and discussing their stability, spectral features, and possible origins.

Contribution

It offers a revised model explaining the nature and stability of G objects as dust-enshrouded stars close to the SMBH, supported by observational data.

Findings

G objects can approach the SMBH within a few thousand gravitational radii.

Infrared spectral energy distributions are consistent with stable gaseous-dusty envelopes.

Broad emission lines originate from circumstellar accretion streams or outflows.

Abstract

In this contribution, we revisit the model of a dust-enshrouded star orbiting a low-luminosity galactic nucleus (Zaja\v{c}ek et al. 2014, 2016, 2017). Although it is quite challenging for dust to survive in hot X-ray-emitting plasma surrounding supermassive black holes (SMBHs), we now have an observational evidence that compact dusty objects or ``G'' objects can approach the SMBH in the Galactic center (Sgr A*) on the scale of a few 1000 gravitational radii. Since there are about ten G objects in the Galactic center, it is more likely that they are dust-enshrouded stars whose gaseous-dusty envelopes are stable within the corresponding tidal (Hill) radii of the order of a few astronomical units. Such a length-scale is consistent with their infrared broad-band spectral energy distributions. Broad emission lines, in particular Br$\gamma$ recombination line, can be interpreted to arise within the accretion stream from the circumstellar envelopes forming a compact disc that is truncated by the stellar magnetic field. Alternatively, they could also be associated with circumstellar accretion-disc outflows as well as the material within a denser bow shock ahead of the star. In comparison with the line origin in the photoionized envelopes that can generally be tidally stretched, the scenario involving the circumstellar accretion-disc inflow or outflow can ensure that the line luminosity is rather stable, except for the viewing-angle effects. We speculate about the origin of dust-enshrouded stars that could be young stellar objects or binary-merger products.