Astronomers Discover Hot Post-Asymptotic Giant Branch Star in Globular Cluster ESO 37-1

A new post-asymptotic giant branch (PAGB) star has been discovered in the Galactic globular cluster known as ESO 37-1, or E3. This discovery, detailed in a research paper published on March 19, has shed light on the fundamental parameters of this rare star.

UVIT F169M filter image of E3. Credit: Kumar et al, 2024

PAGB stars are luminous supergiant stars of intermediate mass that represent a very late phase of stellar evolution. Despite their rarity in globular clusters (GCs) due to their short lifetime of less than 500,000 years, they stand out as the most luminous objects in these clusters.

The discovery was made by a team of astronomers led by Ranjan Kumar of the Physical Research Laboratory (PRL) in Ahmedabad, India. The team used the AstroSat and Gaia satellites to investigate E3, one of the faintest and least massive GCs in the Milky Way galaxy, located approximately 32,600 light years away. The star was identified by analyzing data from AstroSat's Ultraviolet Imaging Telescope (UVIT) and from Gaia's Early Data Release 3 (EDR3).

The newly discovered PAGB star in E3 is about 4.61 times larger than the sun, has a mass of about 0.51–0.55 solar masses, and its effective temperature is 17,500 K. The star has a metallicity of approximately -0.7 dex, which aligns with the cluster's metallicity, and is enriched with carbon and oxygen.

The radial velocities of the newly detected star show a variation of about 6 km/s between the two epochs of observation, indicating a binary nature of the star. Simulations suggest a binary period of 39.12 or 17.83 days, with the companion object likely being an evolved post-main sequence star with an estimated mass of 0.5–0.8 solar masses.

The astronomers noted that the new PAGB star has carbon and oxygen abundances generally similar to those PAGB stars of other GCs that have undergone the so-called third dredge-up (3DU). This suggests that the PAGB star in E3 also underwent the 3DU during its asymptotic giant branch evolution, enriching its carbon and oxygen abundances.

The authors of the paper emphasized the need for future multi-epoch spectroscopic observations to better understand the binary nature of the newly detected PAGB star in the E3 cluster and to constrain its period.