Nineteen new Wolf-Rayet stars discovered in the Andromeda galaxy

Using the 4.3-m Lowell Discovery Telescope (LDT), astronomers Kathryn F. Neugent and Philip Massey, have observed the nearby Andromeda galaxy. As a result, they discovered 19 new Wolf-Rayet stars in the galaxy. The findings were reported in a paper published on June 22 on the print server arXiv.

Wolf-Rayet (WR) stars are extremely hot and luminous with strong, broad helium lines. Observations show that WRs are massive stars at high rates of stellar evolution and mass loss at very high rates. To date, only a few hundred such stars have been found, mostly in the Milky Way.

Now, Kathryn F. Neugent from the Harvard-Smithsonian Center for Astrophysics (CfA) and Philip Massey from the Lowell Observatory, report the discovery of another group of WR stars. They discovered 19 new objects of this type in the nearby Andromeda galaxy (also known as Messier 31, or M31) using the LDST’s Large Monolithic Imager (LMI).

“Our high-resolution imaging and optical alignments have confirmed 19 new WRs in three subregions of M31,” the researchers wrote in the paper.

The scientists searched for 30 WRs with LMI at the end of 2021 and confirmed 19 of them using the 6.5-m MMT telescope in the second half of 2022, Most of them were found to be of the WN type (rich in nitrogen). The newly discovered WRs are usually much smaller than the previously known stars in the Andromeda galaxy, due to the slight increase in redshift.

It was noted that many of the newly discovered WR stars were not detected in previous galaxy-wide surveys due to a lack of observational sensitivity. The astronomers added that the average magnitude (in the WN filter of the LMI) is 21.0, compared to the 150 WN magnitude previously known, which is 19.2.

The authors of the paper estimate that 60 more Wolf-Rayet stars are left to be discovered in the Andromeda galaxy. Since WRs are not evenly distributed in the galaxy, but are usually located in their birthplaces, researchers plan to focus on these regions to identify more stars of this type.

Summarizing the results, the astronomers said that the total ratio of WN-type to WC-type (carbon-rich) WRs for Andromeda did not change with the new findings. They plan to continue the current research, with the goal of completing the census of WR stars in the galaxy.

“We hope to continue this research in the coming seasons…. Our updated models will disrupt the RSG/WR [red supergiant/Wolf-Rayet] the ratio discussed in Massey et al. (2021a), allow accurate comparisons with evolutionary models, and ultimately help us understand the evolution and fate of massive stars,” the researchers concluded.