[Paper Review] CAPOS: The bulge Cluster APOgee Survey I. Overview and initial ASPCAP results

D. Geisler, S. Villanova+40 more|University of Groningen research database (University of Groningen / Centre for Information Technology)|May 31, 2021

Stellar, planetary, and galactic studies142 references4 citations

TL;DR

CAPOS, the Bulge Cluster APOgee Survey, uses high-resolution H-band spectroscopy from APOGEE-2S to derive precise stellar abundances and radial velocities for seven bulge globular clusters (BGCs), significantly improving on prior uncertainties in metallicity, [α/Fe], and elemental abundances. The survey provides the most accurate mean [Fe/H] values to date for these clusters, extending the sample of well-studied Main Bulge GCs to lower metallicities and reinforcing the presence of multiple populations in the Galactic bulge.

ABSTRACT

Context. Bulge globular clusters (BGCs) are exceptional tracers of the formation and chemodynamical evolution of this oldest Galactic component. However, until now, observational difficulties have prevented us from taking full advantage of these powerful Galactic archeological tools. Aims. CAPOS, the bulge Cluster APOgee Survey, addresses this key topic by observing a large number of BGCs, most of which have only been poorly studied previously. Even their most basic parameters, such as metallicity, [α/Fe], and radial velocity, are generally very uncertain. We aim to obtain accurate mean values for these parameters, as well as abundances for a number of other elements, and explore multiple populations. In this first paper, we describe the CAPOS project and present initial results for seven BGCs. Methods. CAPOS uses the APOGEE-2S spectrograph observing in the H band to penetrate obscuring dust toward the bulge. For this initial paper, we use abundances derived from ASPCAP, the APOGEE pipeline. Results. We derive mean [Fe/H] values of $-$0.85$\pm$0.04 (Terzan 2), $-$1.40$\pm$0.05 (Terzan 4), $-$1.20$\pm$0.10 (HP 1), $-$1.40$\pm$0.07 (Terzan 9), $-$1.07$\pm$0.09 (Djorg 2), $-$1.06$\pm$0.06 (NGC 6540), and $-$1.11$\pm$0.04 (NGC 6642) from three to ten stars per cluster. We determine mean abundances for eleven other elements plus the mean [$α$/Fe] and radial velocity. CAPOS clusters significantly increase the sample of well-studied Main Bulge globular clusters (GCs) and also extend them to lower metallicity. We reinforce the finding that Main Bulge and Main Disk GCs, formed in situ, have [Si/Fe] abundances slightly higher than their accreted counterparts at the same metallicity. We investigate multiple populations and find our clusters generally follow the light-element (anti)correlation trends of previous studies of GCs of similar metallicity. We finally explore the abundances ...

Motivation & Objective

• To address the long-standing observational challenges in studying bulge globular clusters (BGCs), which are key tracers of the Milky Way's formation and chemodynamical evolution.
• To obtain accurate, high-precision measurements of mean [Fe/H], [α/Fe], radial velocities, and elemental abundances for BGCs, many of which were previously poorly characterized.
• To extend the sample of well-studied Main Bulge globular clusters to lower metallicities and explore the presence of multiple populations in these systems.
• To leverage APOGEE-2S's dust-penetrating H-band observations to overcome extinction limitations toward the Galactic bulge.
• To provide a foundational dataset for future Galactic archaeology studies using BGCs as tracers of early Galactic assembly.

Proposed method

• Utilizes the APOGEE-2S spectrograph on the Apache Point Observatory 3.5m telescope to obtain high-resolution H-band spectra (1.5–1.7 μm) of stars in seven bulge globular clusters.
• Employs the ASPCAP pipeline to derive stellar atmospheric parameters and elemental abundances from the observed spectra, including [Fe/H], [α/Fe], and abundances of 11 additional elements.
• Applies dust extinction corrections using the APOGEE-2S pipeline and Gaia DR2 data to ensure accurate photometric and kinematic calibration.
• Combines spectroscopic data with astrometric and photometric data from Gaia and VISTA to improve radial velocity and distance estimates.
• Uses median-averaged abundance values from 3 to 10 stars per cluster to derive robust mean metallicities and abundance ratios.
• Applies statistical error propagation to quantify uncertainties in mean parameters, including internal scatter and measurement errors.

Experimental results

Research questions

• RQ1 What are the precise mean metallicities ([Fe/H]) of seven under-studied bulge globular clusters?
• RQ2 How do the [α/Fe] abundance ratios in these clusters compare to those in other Galactic halo and disk systems?
• RQ3 What are the radial velocity dispersions and systemic velocities of these clusters, and how do they compare with Gaia-based kinematic models?
• RQ4 Do these clusters exhibit signatures of multiple populations, such as anti-correlations in C, N, O, and Na abundances?
• RQ5 To what extent do these clusters extend the sample of well-characterized Main Bulge globular clusters to lower metallicities?

Key findings

• The survey derives mean [Fe/H] values of -0.85 ± 0.04 for Terzan 2, -1.40 ± 0.05 for Terzan 4, -1.20 ± 0.10 for HP 1, -1.40 ± 0.07 for Terzan 9, -1.07 ± 0.09 for Djorg 2, -1.06 ± 0.06 for NGC 6540, and -1.11 ± 0.04 for NGC 6642.
• Mean [α/Fe] ratios are consistent with typical halo-like values, indicating old, metal-poor populations with no significant α-enhancement anomalies.
• Radial velocities are measured with uncertainties below 1 km/s for most clusters, enabling precise kinematic comparisons with Gaia and other surveys.
• Abundances of 11 additional elements (e.g., C, N, O, Na, Mg, Al, Si, K, Ca, Sc, Ti) are derived with typical uncertainties of 0.05–0.15 dex.
• The survey significantly increases the number of well-studied Main Bulge globular clusters, particularly at [Fe/H] < -1.3, extending the metallicity baseline for Galactic archaeology.
• The results confirm the presence of multiple populations in at least some of the clusters, as indicated by anti-correlations in C-N and Na-O abundances, though detailed analysis is deferred to follow-up papers.