Newly found carbon-enhanced metal-poor stars reveal early stellar populations near the Milky Way

Using the Baryons Oscillation Spectroscopic Survey (BOSS) spectrograph, astronomers have discovered five new carbon-enhanced metal-poor stars in the Large Magellanic Cloud (LMC). This is the first time such stars have been identified in this galaxy. The discovery was reported in a paper published January 15 on the arXiv pre-print server.

Metal-poor but enhanced with carbon

Metal-poor stars are rare objects, as only a few thousand stars with iron abundances [Fe/H] below -2.0 have been discovered to date. Expanding the still-short list of metal-poor stars is of high importance for astronomers, as such objects have the potential to improve our knowledge of the chemical evolution of the universe.

Observations show that a significant fraction of these stars exhibit a large overabundance of carbon; therefore, they are known as carbon-enhanced metal-poor (CEMP) stars.

However, although many CEMP stars have been identified in our galaxy, there is an observational deficit of these objects in dwarf spheroidal galaxies. For instance, so far no such star has been detected in the Large Magellanic Cloud (LMC)—a dwarf satellite of the Milky Way.

Is there a CEMP star in the LMC?

In order to change this, a team of astronomers led by Madeline Lucey of the University of Pennsylvania, conducted a search for CEMP stars by analyzing the data from the Sloan Digital Sky Survey (SDSS).

«In this work, we use data from the SDSS-V Magellanic Genesis program to search for CEMP stars in the LMC. (…) We present the discovery of the first five CEMP stars in the Milky Way’s largest dwarf companion, the LMC, using SDSS-V spectra from the BOSS instrument,» the researchers write.

Five promising candidates

Lucey’s team analyzed 381 stars in the SDSS-V DR20 Magellanic Genesis catalog, which are classified as CEMP candidates based on their spectra. Out of this sample, they chose five stars with the lowest metallicities and the highest carbon to iron [C/Fe] abundance rate for further investigation.

The selected stars are designated SDSS 92278782, 96041179, 98320880, 98332219 and 98357416. The stars have metallicities between -3.21 and -2.09, while their [C/Fe] abundance ratios range from 1.23 to 2.41. The effective temperatures of these five stars were found to be within the range of 4,600–4,850 K.

The authors of the paper underline that the five stars reported in the study are most likely CEMP stars. However, it is unclear whether the CEMP classification would apply beyond the Milky Way, taking into account the differences in chemical evolution between galaxies of various masses. Therefore, higher-resolution spectra to confirm their neutron-capture element abundances are required to draw final conclusions regarding the status of these stars.

Future research

Summing up the results, the researchers underscore the importance of their finding for future studies.

«The discovery of these five stars represents a crucial step towards understanding the chemistry of the most metal-poor stars in the LMC. In future work, we plan to homogeneously analyze all of the BOSS spectra of the LMC and SMC [Small Magellanic Cloud] from SDSS-V to robustly measure the occurrence rate of CEMP in these galaxies,» the scientists write.

More information: arXiv