On the impossible NGC 4372 V1 and V2: an extended AGB to the [Fe/H] = -2.2 cluster

TL;DR

This paper investigates the extended asymptotic giant branch (AGB) in the metal-poor globular cluster NGC 4372, providing new spectra of luminous AGB stars and proposing a delay in mass-loss processes due to inhibited atmospheric pulsation.

Contribution

It presents the first spectra of very cool, luminous AGB stars in NGC 4372 and hypothesizes a mechanism delaying mass loss, explaining the high white dwarf mass without stellar mergers.

Findings

AGB stars V1 and V2 are likely cluster members.

V1 and V2 are undergoing the superwind phase.

Delayed mass loss due to inhibited pulsation explains high white dwarf mass.

Abstract

The asymptotic giant branch (AGB) of the globular cluster NGC 4372 appears to extend to unexpectedly high luminosities. We show, on the basis of proper motions and spatial distribution, that the extended AGB is indeed a likely part of the cluster. We also present the first spectra of the very cool (2600 K), very luminous (8000 Lsun), very dusty, oxygen-rich, purported long-period variable stars V1 and V2 that define the AGB tip. In particular, on the basis of their radial velocities, we conclude that V1 and V2 are probably members. We find that V1 and V2 are likely undergoing the superwind phase that terminates their nuclear-burning evolution. We hypothesise that the mass-loss processes that terminate the AGB are inhibited in NGC 4372 due to a lack of atmospheric pulsation and the high gas-to-dust ratio in the ejecta, leading to a delay in the associated enhanced mass loss and dust production. Previously predicted, but never observed, this explains the high mass of the white dwarf in Pease 1 in M15 without the need to invoke a stellar merger. If commonplace, this phenomenon has implications for the mass return from stars, the production of carbon stars and supernovae through the Universe's history, and the AGB contribution to light from unresolved metal-poor populations.