Nature of the Galactic centre NIR-excess sources. I. What can we learn from the continuum observations of the DSO/G2 source?

Abstract

The Dusty S-cluster Object (DSO/G2) orbiting the supermassive black hole (Sgr A*) in the Galactic centre has been monitored in both near-infrared continuum and line emission. There has been a dispute about the character and the compactness of the object: interpreting it as either a gas cloud or a dust-enshrouded star. A recent analysis of polarimetry data in $K_{\mathrm{s}}$-band ($2.2\mu \mathrm{m}$) allows us to put further constraints on the geometry of the DSO. The purpose of this paper is to constrain the nature and the geometry of the DSO. We compare 3D radiative transfer models of the DSO with the NIR continuum data including polarimetry. In the analysis, we use basic dust continuum radiative transfer theory implemented in the 3D Monte Carlo code Hyperion. Moreover, we implement analytical results of the two-body problem mechanics and the theory of non-thermal processes. We present a composite model of the DSO -- a dust-enshrouded star that consists of a stellar source, dusty, optically thick envelope, bipolar cavities, and a bow shock. This scheme can match the NIR total as well as polarized properties of the observed spectral energy distribution (SED). The SED may be also explained in theory by a young pulsar wind nebula that typically exhibits a large linear polarization degree due to magnetospheric synchrotron emission. The analysis of NIR polarimetry data combined with the radiative transfer modelling shows that the DSO is a peculiar source of compact nature in the S cluster $(r\lesssim 0.04\mathrm{pc})$. It is most probably a young stellar object embedded in a non-spherical dusty envelope, whose components include optically thick dusty envelope, bipolar cavities, and a bow shock. Alternatively, the continuum emission could be of a non-thermal origin due to the presence of a young neutron star and its wind nebula.

21 pages, 18 figures, accepted for publication in Astronomy & Astrophysics

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• Monitoring dusty sources in the vicinity of Sgr A*
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• A binary system in the S cluster close to the supermassive black hole Sagittarius A*
• A new bow-shock source with bipolar morphology in the vicinity of Sgr A*
• The apparent tail of the Galactic center object G2/DSO
• First observed interaction of the circumstellar envelope of an S-star with the environment of Sgr A*
• The evolution of circumstellar discs in the Galactic Centre: an application to the G-clouds
• Dusty plasma in active galactic nuclei
• Polarization simulations of stellar wind bow shock nebulae. I. The case of electron scattering
• X3: a high-mass Young Stellar Object close to the supermassive black hole Sgr~A*
• Polarization simulations of stellar wind bow shock nebulae. II. The case of dust scattering
• The Evaporating Massive Embedded Stellar Cluster IRS 13 Close to Sgr A*. II. Kinematic structure
• Candidate Young Stellar Objects in the S-cluster: The Kinematic Analysis of a Sub-population of the Low-mass G-objects close to Sgr A*
• 3D AMR hydrosimulations of a compact source scenario for the Galactic Centre cloud G2
• Closing the gap: Follow-up observations of peculiar dusty objects close to Sgr A* using ERIS
• Enhanced Doppler Beaming for Dust-Enshrouded Objects and Pulsars in the Galactic Center
• Non-thermal emission from fall-back clouds in the Broad-Line Region of Active Galactic Nuclei