The Sgr A* Stellar Cluster: New NIR Imaging and Spectroscopy

TL;DR

This paper presents new near-infrared imaging and spectroscopy of the Sgr A* stellar cluster, revealing stellar types, emission features, and proper motions, enhancing understanding of the cluster's composition and dynamics.

Contribution

It provides the first detailed spectral analysis of the central cluster stars and reports proper motion measurements confirming stellar dynamics near Sgr A*.

Findings

Stars in the northern cluster are likely O9 - B0.5 type.

No strong CO absorption in the northern cluster region.

Detection of Br gamma emission possibly linked to the mini-spiral.

Abstract

We report preliminary results based on new near-infrared observations of the central stellar cluster of our Galaxy conducted with the infrared spectrometer ISAAC at the ESO VLT UT1 and the MPE speckle camera SHARP at the ESO NTT. We obtained lambda / dlambda >= 3000 K-band spectroscopy of the 2.058 um He I, 2.165 um Br gamma emission lines, and 2.29 um CO bandhead absorption line. These data demonstrate clearly that there is no strong CO bandhead absorption originating in the northern part (S1/S2 area) of the central stellar cluster at the position of Sgr A*. This makes it likely that these K~14.5 stars are O9 - B0.5 stars with masses of 15 to 20 M(sun). Weaker CO bandhead absorption in the southern part of the cluster (S10/S11 area) could be due to contributions from neighbouring stars. We also report the detection of Br gamma line emission at the position of the central stellar cluster which could be associated with the 'mini-spiral' rather than with the Sgr A* cluster itself. At the NTT we obtained another epoch epoch of proper motion measurments. The changes of source positions are consistent with the proper motion velocities derived from earlier epochs.