Latour, M, Hämmerich, S, Dorsch, M, Heber, U, Husser, T-O, Kamann, S, Dreizler, S and Brinchmann, J (2023) SHOTGLAS II. MUSE spectroscopy of blue horizontal branch stars in the core of ω Centauri and NGC 6752. Astronomy & Astrophysics, 677. ISSN 1432-0746

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Abstract

Aims. We want to study the population of blue horizontal branch (HB) stars in the centers of globular clusters (GC) for the first time by exploiting the unique combination of MUSE spectroscopy and HST photometry. In this work, we characterize their properties in the GCs ω Cen and NGC 6752. Methods. We use dedicated model atmospheres and synthetic spectra grids computed using a hybrid LTE/NLTE modeling approach to fit the MUSE spectra of HB stars hotter than 8 000 K in both clusters. The spectral fits provide estimates of the effective temperature (Teff), surface gravity (log g), and helium abundance of the stars. The model grids are further used to fit the HST magnitudes, meaning the spectral energy distributions (SED), of the stars. From the SED fits, we derive the average reddening, radius, luminosity, and mass of the stars in our sample. Results. The atmospheric and stellar properties that we derive for the stars in our sample are in good agreement with the theoretical expectations. In particular, the stars cooler than ∼15 000 K follow neatly the theoretical predictions for the radius, log g, and luminosity for helium-normal (Y=0.25) models. In ω Cen, we show that the majority of these cooler HB stars cannot originate from a heliumenriched population with Y >0.35. The properties of the hotter stars (radii and luminosities) are still in reasonable agreement with theoretical expectations, but the individual measurements have a large scatter. For these hot stars, we have a mismatch between the effective temperatures indicated by the MUSE spectral fits and the photometric fits, with the latter returning Teff lower by ∼3 000 K. We use three different diagnostics, namely the position of the G-jump and changes in metallicity and helium abundances to place the onset of diffusion in the stellar atmospheres at Teff between 11 000 K and 11 500 K. Our sample includes two stars known as photometric variables, we confirm one to be a bona fide extreme HB object but the other is a blue straggler star. Finally, unlike what has been reported in the literature, we do not find significant differences between the properties (e.g., log g, radius, and luminosity) of the stars in both clusters. Conclusions. We showed that our analysis method combining MUSE spectra and HST photometry of HB stars in GC is a powerful tool to characterize their stellar properties. With the availability of MUSE and HST observations of additional GCs, we have a unique opportunity to combine homogeneous spectroscopic and photometric data to study and compare the properties of blue HB stars in different GCs.

Item Type:	Article
Uncontrolled Keywords:	astro-ph.SR; astro-ph.SR; astro-ph.GA
Subjects:	Q Science > QB Astronomy Q Science > QC Physics
Divisions:	Astrophysics Research Institute
Publisher:	EDP Sciences
Related URLs:	http://arxiv.org/abs/2306.14549v1
SWORD Depositor:	A Symplectic
Date Deposited:	04 Jul 2023 12:58
Last Modified:	12 Sep 2023 11:50
DOI or ID number:	10.1051/0004-6361/202346597
URI:	https://researchonline.ljmu.ac.uk/id/eprint/20214

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