A New Class of Wolf-Rayet Stars: WN3/O3s

TL;DR

This paper reports the discovery of a new class of Wolf-Rayet stars, WN3/O3, in the Magellanic Clouds, which challenges existing understanding of stellar evolution due to their unique properties and potential implications for WR population estimates.

Contribution

The identification and preliminary analysis of a novel WN3/O3 class of Wolf-Rayet stars, expanding the known diversity and challenging current evolutionary models.

Findings

Discovered 9 new Wolf-Rayet stars in the LMC, suggesting underestimation of the total WR population.

Found that WN3/O3 stars have normal bolometric luminosities but unusually low mass-loss rates.

Revealed that the observed WC to WN ratio is biased and that these stars are likely evolved with enriched N and He.

Abstract

Our new survey for Wolf-Rayet stars in the Magellanic Clouds is only 15% complete but has already found 9 new WRs in the LMC. This suggests that the total WR population in the LMC may be underestimated by 10-40%. Eight of the nine are WNs, demonstrating that the "observed" WC to WN ratio is too large, and is biased towards WC stars. The ninth is another rare WO star, the second we have found in the LMC in the past two years. Five (and possibly six) of the 8 WNs are of a new class of WRs, which pose a significant challenge to our understanding. Naively we would classify these stars as "WN3+O3V," but there are several reasons why such a pairing is unlikely, not the least of which is that the absolute visual magnitudes of these stars are faint, with M_V~ -2.3 to -3.1. We have performed a preliminary analysis with CMFGEN, and we find that (despite the faint visual magnitudes) the bolometric luminosities of these stars are normal for early-type WNs. Our fitting suggests that these stars are evolved, with significantly enriched N and He. Their effective temperatures are also normal for early-type WNs. What is unusual about these stars is that they have a surprisingly small mass-loss rate compared to other early-type WNs. How these stars got to be the way they are (single star evolution? binary evolution?) remains an open question. For now, we are designating this class as WN3/O3, in analogy to the late-type WN "slash" stars.