The Spectral Types of White Dwarfs in Messier 4

TL;DR

This study spectroscopically analyzed 24 white dwarfs in Messier 4, finding all to be hydrogen-dominated DA types, suggesting that the cluster environment suppresses the formation of helium-dominated DB white dwarfs.

Contribution

First spectroscopic survey of white dwarf spectral types in Messier 4, revealing a lack of DB types and suggesting environmental suppression of DB formation in clusters.

Findings

All 24 white dwarfs are DA type.

The probability of no DBs occurring by chance is very low.

Cluster environments may suppress DB white dwarf formation.

Abstract

We present the spectra of 24 white dwarfs in the direction of the globular cluster Messier 4 obtained with the Keck/LRIS and Gemini/GMOS spectrographs. Determining the spectral types of the stars in this sample, we find 24 type DA and 0 type DB (i.e., atmospheres dominated by hydrogen and helium respectively). Assuming the ratio of DA/DB observed in the field with effective temperature between 15,000 - 25,000 K, i.e., 4.2:1, holds for the cluster environment, the chance of finding no DBs in our sample due simply to statistical fluctuations is only 6 X 10^(-3). The spectral types of the ~100 white dwarfs previously identified in open clusters indicate that DB formation is strongly suppressed in that environment. Furthermore, all the ~10 white dwarfs previously identified in other globular clusters are exclusively type DA. In the context of these two facts, this finding suggests that DB formation is suppressed in the cluster environment in general. Though no satisfactory explanation for this phenomenon exists, we discuss several possibilities.