Observations of the Hot Horizontal-Branch Stars in the Metal-Rich Bulge Globular Cluster NGC 6388 - Indications of Helium Enrichment and a Lesson in Crowded Field Spectroscopy

TL;DR

This study investigates hot horizontal-branch stars in the metal-rich globular cluster NGC 6388, providing insights into helium enrichment and highlighting challenges in crowded field spectroscopy.

Contribution

The paper presents medium resolution spectroscopic analysis of blue HB stars in NGC 6388, revealing evidence for helium enrichment and discussing data reduction challenges in crowded fields.

Findings

Support for helium pollution scenario in 10,000 K stars

Low gravities and masses likely due to background subtraction issues

Illustrates difficulties in spectroscopic analysis of hot HB stars in crowded fields

Abstract

(abridged) The metal-rich bulge globular cluster NGC6388 shows a distinct blue horizontal-branch tail with a strong upward tilt in its colour-magnitude diagram. Several noncanonical scenarios have been proposed to explain these puzzling observations. In order to test the predictions of these scenarios, we have obtained medium resolution spectra to determine the atmospheric parameters of a sample of the blue HB stars in NGC 6388. Using the medium resolution spectra, we determine effective temperatures, surface gravities and helium abundances by fitting the observed Balmer and helium lines with appropriate theoretical stellar spectra. As we know the distance to the cluster, we can verify our results by determining masses for the stars. During the data reduction we took special care in subtracting the background, which is dominated by the overlapping spectra of cool stars. The physical parameters of the blue HB stars near 10,000 K support the helium pollution scenario. The low gravities and masses found for the hot blue tail stars, however, are probably caused by problems with the data reduction, most likely due to remaining background light in the spectra, which would affect the fainter hot blue tail stars much more strongly than the brighter cool blue tail stars. Our study of the hot blue tail stars in NGC 6388 illustrates the obstacles which are encountered when attempting to determine the atmospheric parameters of hot HB stars in very crowded fields using ground-based observations. We discuss these obstacles and offer possible solutions for future projects.