[Paper Review] Composition of the galactic center star cluster: Population analysis from adaptive optics narrow band spectral energy distributions

Rainer Buchholz, R. Schödel+1 more|arXiv (Cornell University)|Mar 12, 2009

Stellar, planetary, and galactic studies73 references154 citations

TL;DR

This study introduces a novel adaptive optics narrowband photometric method to classify early- and late-type stars in the Galactic center's dense stellar cluster using H- and K-band intermediate filters. By detecting CO absorption features in spectral energy distributions, it identifies 312 early-type stars down to K = 15.5 mag—extending depth and efficiency beyond prior spectroscopic work—and finds a steep power-law decline in early-type star density (β = −1.49 ± 0.12), supporting an in-situ star formation scenario over cluster infall.

ABSTRACT

The goals of this work are to develop a new method to separate early and late type stellar components of a dense stellar cluster based on narrow band filters, to apply it to the central parsec of the GC, and to conduct a population analysis of this area. We use AO assisted observations obtained at the ESO VLT in the NIR H-band and 7 intermediate bands covering the NIR K-band. A comparison of the resulting SEDs with a blackbody of variable extinction then allows us to determine the presence and strength of a CO absorption feature to distinguish between early and late type stars. The new method is suitable to classify K giants (and later) as well as B2 main sequence (and earlier) stars which are brighter than 15.5 mag in the K band in the central parsec. Compared to previous spectroscopic investigations that are limited to 13-14 mag, this represents a major improvement in the depth of the observations as well as reducing the needed observation time. We classify 312 stars as early type candidates out of a sample of 5914 sources. The distribution of the early type stars can be fitted with a steep power law (beta(R>1'') = -1.49 +/- 0.12, alternatively with a broken power law, beta(R=1-10'') = -1.08 +/- 0.12, beta(R=10-20'') = -3.46 +/- 0.58, since we find a drop of the early type density at ~10''). We also detect early type candidates outside of 0.5 pc in significant numbers for the first time. The late type density function shows an inversion in the inner 6'', with a power law slope of beta(R<6'') = 0.17 +/- 0.09. The late type KLF has a power law slope of 0.30$\pm$0.01, closely resembling the KLF obtained for the bulge of the Milky Way. The early type KLF has a much flatter slope of 0.14 +/- 0.02. Our results agree best with an in-situ star formation scenario.

Motivation & Objective

• To develop a new method for classifying early- and late-type stars in dense stellar clusters using narrowband photometry.
• To apply this method to the central parsec of the Galactic center to analyze the stellar population composition.
• To extend the depth and efficiency of stellar classification beyond the limits of previous spectroscopic surveys.
• To investigate the spatial distribution and luminosity function of early- and late-type stars in the nuclear star cluster.
• To test competing scenarios—such as in-situ formation versus infall of a distant cluster—for the origin of massive stars in the Galactic center.

Proposed method

• Adaptive optics-assisted observations were conducted at the ESO VLT using H-band and seven intermediate K-band filters to obtain high-resolution narrowband photometry.
• Spectral energy distributions (SEDs) were constructed from the narrowband magnitudes to detect CO absorption features indicative of late-type stars.
• A blackbody SED with variable extinction was fitted to the observed SEDs to isolate the CO absorption strength and distinguish early-type (lacking CO absorption) from late-type (showing CO absorption) stars.
• The method enables classification of stars brighter than K = 15.5 mag, significantly deeper than previous spectroscopic limits of ~13–14 mag.
• Foreground and extremely reddened objects (e.g., bow-shock sources) were identified and excluded via their fitted extinction values.
• Visual inspection and confidence ratings (A–C) were used to validate the classification of early-type candidates, especially for faint or ambiguous sources.

Experimental results

Research questions

• RQ1 Can a photometric method using narrowband filters reliably classify early- and late-type stars in the Galactic center's dense stellar environment?
• RQ2 What is the spatial distribution of early-type stars in the central parsec, and does it follow a power-law profile?
• RQ3 How does the K-band luminosity function (KLF) of early- and late-type stars in the central parsec compare to those in the Milky Way bulge and theoretical models?
• RQ4 Are early-type stars detected beyond 0.5 pc, and do they follow the same density profile as in the inner region?
• RQ5 Does the observed stellar population distribution favor in-situ star formation or the infall and dissolution of a distant cluster?

Key findings

• The method successfully classified 312 early-type star candidates out of 5,914 sources, with a reliability exceeding 87% based on known spectral classifications.
• The projected density of early-type stars follows a steep power-law profile with β = −1.49 ± 0.12 within 1′′, or a broken power law with β1−10′′ = −1.08 ± 0.12 and β10−20′′ = −3.46 ± 0.58, indicating a drop in density at ~10′′.
• Early-type stars were detected outside 0.5 pc in significant numbers for the first time, with a density profile consistent with the inner power law, suggesting they may be part of the known disk or off-disk population.
• The late-type KLF has a power-law slope of 0.30 ± 0.01, closely matching the KLF of the Milky Way bulge, implying a similar star formation history.
• The early-type KLF has a flatter slope of 0.14 ± 0.02, extending to K = 15.5 mag—deeper than previous studies—and supports a top-heavy initial mass function.
• The observed steep decline in early-type star density strongly favors an in-situ star formation scenario over the infall and dissolution of a distant cluster, as the observed profile better matches the R−2 expectation of in-situ formation than the R−0.75 profile predicted by cluster infall.