Exploring Ancient Stars: What White Dwarfs Tell Us About the Universe

This study, led by Luigi R. Bedin and colleagues, delves into the properties of white dwarfs in the globular cluster M 4 using cutting-edge observations from the James Webb Space Telescope (JWST). The researchers compare these new findings with earlier observations from the Hubble Space Telescope (HST), offering fresh insights into the cooling patterns of white dwarfs, cluster ages, and potential mysteries like infrared excess light from these ancient stars.

Introduction

Globular clusters, like M 4, are some of the oldest objects in the universe, housing stars with nearly identical ages and chemical compositions. These clusters are excellent laboratories for studying how stars evolve. White dwarfs, the final remnants of stars like our Sun, gradually cool and fade over billions of years. Their brightness and temperature can help scientists determine the age of the cluster they belong to.

This paper builds on earlier research and investigates M 4's white dwarfs using JWST’s infrared capabilities, which are more effective at minimizing the effects of dust and foreground stars compared to optical data.

Observations and Data Collection

The team combined infrared data from JWST, collected in 2023, with optical images taken by HST roughly two decades earlier. By comparing the positions of stars in these datasets, they identified which stars belonged to M 4. JWST’s Near Infrared Camera (NIRCam) provided clearer images of the faintest white dwarfs, especially in crowded regions near the cluster's core.

However, challenges arose. JWST’s diffraction spikes (artifacts from bright stars) and the crowded nature of M 4’s core made it difficult to measure the faintest stars. Despite this, the researchers gathered enough data to extend previous studies of the cluster’s white dwarf cooling sequence into the infrared.

White Dwarfs and Their Cooling Sequence

The cooling sequence of white dwarfs provides a snapshot of their temperatures and brightnesses, which reveal the cluster’s age. The study confirmed that the faintest white dwarfs in M 4 align with theoretical models, showing a clear "turn" in the cooling sequence due to effects in their atmospheres, such as collision-induced absorption of hydrogen molecules. This feature is more pronounced in infrared light, offering an unambiguous test of these models.

By comparing M 4 to another globular cluster, NGC 6397, the researchers found that M 4 might be slightly younger by 0.8 billion years, placing its age at around 12.2 billion years.

A Mysterious Infrared Excess

One intriguing discovery is the broader-than-expected color distribution of faint white dwarfs in M 4. Some of these stars appeared to emit more infrared light than predicted. A similar finding was noted in NGC 6397, where infrared excess might suggest the presence of debris disks or faint companions like brown dwarfs. In M 4, the infrared excess was less pronounced and limited to specific filters, leaving the cause unclear.

Conclusions and Future Work

This study highlights JWST’s potential to study faint stellar populations and improve our understanding of star clusters. The findings not only confirm previous white dwarf models but also raise questions about unexplained features, such as the infrared excess. Future observations with additional filters and instruments, like JWST’s MIRI, could reveal whether these excesses are due to companion stars, dust, or another unknown phenomenon.

Source: Bedin