A Universal Transition in Atmospheric Diffusion for Hot Subdwarfs Near 18,000 K

TL;DR

This study investigates the atmospheric changes in hot subdwarfs near 18,000 K, revealing a universal transition characterized by a decrease in atmospheric iron that explains the Momany jump in globular clusters.

Contribution

It identifies the physical cause of the Momany jump as a decrease in atmospheric iron in hot subdwarfs, providing a new understanding of stellar atmospheric phenomena near 18,000 K.

Findings

The Momany jump is primarily due to a decrease in atmospheric Fe.

Spectroscopic data confirms the Fe decrease in stars hotter than 18,000 K.

The transition is a universal feature in hot subdwarfs near 18,000 K.

Abstract

In the color-magnitude diagrams (CMDs) of globular clusters, when the locus of stars on the horizontal branch (HB) extends to hot temperatures, discontinuities are observed at colors corresponding to ~12,000 K and ~18,000 K. The former is the "Grundahl jump" that is associated with the onset of radiative levitation in the atmospheres of hot subdwarfs. The latter is the "Momany jump" that has remained unexplained. Using the Space Telescope Imaging Spectrograph on the Hubble Space Telescope, we have obtained ultraviolet and blue spectroscopy of six hot subdwarfs straddling the Momany jump in the massive globular cluster omega Cen. By comparison to model atmospheres and synthetic spectra, we find that the feature is due primarily to a decrease in atmospheric Fe for stars hotter than the feature, amplified by the temperature dependence of the Fe absorption at these effective temperatures.