[Paper Review] The Sgr A* Stellar Cluster: New NIR Imaging and Spectroscopy

A. Eckart, Thomas Ott+1 more|arXiv (Cornell University)|Nov 2, 1999

Stellar, planetary, and galactic studies3 citations

TL;DR

This study presents new near-infrared (NIR) imaging and spectroscopy of the Sgr A* stellar cluster using the VLT/ISAAC and NTT/SHARP instruments, revealing no strong CO bandhead absorption in the northern cluster region (S1/S2), indicating these K~14.5 stars are likely O9-B0.5 spectral type with masses of 15–20 M☉. Brγ emission at the cluster center may originate from the 'mini-spiral' rather than the cluster itself, and proper motion measurements confirm consistent kinematics across epochs.

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.

Motivation & Objective

• To determine the stellar spectral types and masses of the K~14.5 stars in the Sgr A* stellar cluster using high-resolution NIR spectroscopy.
• To investigate the presence and origin of CO bandhead absorption in the central cluster region, particularly in the S1/S2 and S10/S11 areas.
• To assess whether the Brγ emission line detected at the cluster center originates from the stellar population or from surrounding structures like the 'mini-spiral'.
• To verify the consistency of proper motion measurements across multiple epochs using high-precision NIR imaging.
• To clarify the nature of the central stellar cluster by distinguishing contributions from individual stars versus extended emission features.

Proposed method

• Acquired high-resolution K-band spectroscopy (λ/Δλ ≥ 3000) using the VLT/ISAAC instrument to analyze the 2.058 µm He I, 2.165 µm Brγ, and 2.29 µm CO bandhead lines.
• Performed near-infrared imaging with the MPE speckle camera SHARP at the ESO NTT to measure precise source positions and proper motions.
• Compared observed CO bandhead absorption in the S1/S2 and S10/S11 regions to infer the presence of cooler, lower-mass stars or contributions from neighboring sources.
• Analyzed the spatial distribution and kinematics of the Brγ emission line to distinguish between emission from the stellar cluster and the surrounding mini-spiral structure.
• Conducted multi-epoch observations to track changes in source positions and validate proper motion velocities derived from earlier data.
• Used spectral line profiles and depth to infer stellar atmospheric parameters and constrain the mass and spectral type of the central stars.

Experimental results

Research questions

• RQ1 What is the spectral type and mass of the K~14.5 stars in the Sgr A* stellar cluster, based on their NIR spectroscopic features?
• RQ2 Is there significant CO bandhead absorption in the northern (S1/S2) and southern (S10/S11) regions of the cluster, and what does its absence or presence imply about the stellar population?
• RQ3 Does the observed Brγ emission at the cluster center originate from the stellar population or from the surrounding 'mini-spiral' structure?
• RQ4 Are the proper motion measurements of the cluster stars consistent across multiple observational epochs?
• RQ5 To what extent do neighboring stars contribute to the observed CO bandhead absorption in the southern cluster region?

Key findings

• No strong CO bandhead absorption was detected in the northern part of the cluster (S1/S2 area), indicating the K~14.5 stars are not cool giants or dwarfs but likely early B-type stars.
• The absence of strong CO absorption implies the stars in the S1/S2 region are O9–B0.5 spectral type with masses of 15 to 20 M☉.
• Weaker CO bandhead absorption in the southern region (S10/S11) may result from contributions by nearby stars rather than the cluster's central stars.
• Brγ line emission was detected at the cluster center, but its origin is likely associated with the 'mini-spiral' rather than the stellar cluster itself.
• Proper motion measurements from the second epoch are consistent with velocities derived from earlier observations, confirming kinematic stability.
• The high spectral resolution (λ/Δλ ≥ 3000) enabled precise identification of emission and absorption features, supporting robust stellar classification.