Year: 2021
Type: article
Abstract: ABSTRACT An analysis of the Large Magellanic Cloud (LMC) WC4 star BAT99-9 (HD 32125, FD 4, Brey 7, WS 3) shows that the star still contains photospheric nitrogen. Three N emission features (N v λλ1238, 1242, N iv λ1719, and N iv λλ3479–3485) are unambiguously identified in the spectrum. cmfgen models of the star yield an N/C ratio of 0.004 ± 0.002 (by number) and a C/He ratio of $0.15_{-0.05}^{+0.10}$. Due to the similarity of BAT99-9 to other WC4 stars, and the good fit achieved by cmfgen to both the classic WC4 spectrum and the N lines, we argue that the N lines are intrinsic to BAT99-9. An examination of a limited set of rotating models for single-star evolution at LMC and Galactic metallicities shows that a model with a Galactic metallicity (z = 0.014) and a progenitor mass of around 50 M⊙ can have an N/C ratio similar to, or larger than, what we observe for a significant fraction of its lifetime. However, the LMC models (z = 0.006) are inconsistent with the observations. Both the single and binary BPASS models predict that many WC stars can have an N/C ratio similar to, or larger than, what we observe for a significant fraction of their lifetime. While the binary models cover a wider range of luminosities and provide a somewhat better match to BAT99-9, it is not currently possible to rule out BAT99-9 being formed through single-star evolution, given the uncertainties in mass-loss rates, and the treatment of convection and mixing processes.
Language: English
Subfield: Astronomy and Astrophysics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: green
Awards: G1038393546, G1262850075, Massive Star Birth, Life and Death: Closing the Loop in the Local Group of Galaxies, G2044427836, THE MULTI-MISSION ARCHIVE AT SPACE SCIENCE INSTITUTE:, G58316358, MASSIVE STARS ARE THE COSMIC ENGINES THAT POWER THE FAR-INFRARED LUMINOSITIES OF DISTANT GALAXIES AND DOMINATE THE IONIZATION OF NEARBY HII REGIONS. THEY ARE THE PRIMARY SOURCE OF CARBON AND OXYGEN IN THE UNIVERSE AND THEIR CORE COLLAPSES MANUFACTURE ALL OF THE ELEMENTS HEAVIER THAN FE. THE RE-IONIZATION OF THE EARLY UNIVERSE WAS THANKS TO POPULATION III MASSIVE STARS AND THE SUPER-MASSIVE BLACK HOLES WE FIND IN THE CORES OF GALAXIES TODAY WERE SEEDED AS A RESULT OF THE BLACK HOLES THAT FORMED FROM THE FIRST GENERATIONS OF MASSIVE STARS. UNDERSTANDING MASSIVE STAR EVOLUTION IS THE KEY TO UNLOCKING MANY ASTROPHYSICAL PROBLEMS. THE LARGEST UNCERTAINTY IN MASSIVE STAR EVOLUTION IS THE QUESTION OF HOW WOLF-RAYET (WR) STARS FORM. OUR PROPOSAL WILL DETERMINE THE FUNDAMENTAL PHYSICAL PROPERTIES OF WRS USING FOUR ARCHIVAL NASA DATA SETS FOR A CRITICAL COMPARISON WITH PRESENT DAY EVOLUTION MODELS. IT IS GENERALLY ASSUMED THAT MASSIVE STARS SPEND MOST OF THEIR POST-MAIN-SEQUENCE LIVES WRS. FOR DECADES WE HAVE BELIEVED THAT WRS FORM AS A RESULT OF STELLAR WINDS STRIPPING OFF THE H-RICH OUTER LAYERS OF A STAR LEAVING BEHIND A BARE STELLAR CORE. IN THIS PICTURE WRS ARE A NORMAL STAGE IN THE EVOLUTION OF THE MOST MASSIVE STARS. RECENTLY THIS SCENARIO HAS BEEN CALLED INTO QUESTION. STELLAR WIND MASSLOSS RATES ARE NOW KNOWN TO BE SIGNIFICANTLY LOWER THAN PREVIOUSLY THOUGHT ALTHOUGH WHETHER THIS IS A FACTOR OF 3 OR 10 REMAINS UNCLEAR. IF THE LATTER IS CORRECT THEN THIS POSES A SERIOUS PROBLEM FOR THE FORMATION OF WRS. THIS HAS CREATED A PARADIGM SHIFT WITH INCREASED IMPORTANCE ATTACHED TO THE ROLE OF BINARY EVOLUTION WITH ROCHE-LOBE OVERFLOW PERFORMING THE STRIPPING. ATTEMPTS TO DISTINGUISH WHICH SCENARIO IS MORE PREVALENT IS COMPLICATED BY THE POSSIBILITY OF PAST MERGERS; I.E. JUST BECAUSE A WR IS NOT A BINARY TODAY DOES NOT PROVE IT WAS NOT ONE IN THE PAST. WE WILL TACKLE THIS QUESTION FROM A FRESH PERSPECTIVE DETERMINING RELIABLE FUNDAMENTAL PHYSICAL PROPERTIES OF WRS AND SEEING WHETHER THEY BETTER MATCH THE SINGLE OR BINARY STAR EVOLUTIONARY MODELS. IF THEY AGREE WITH THE SINGLE-STAR MODELS THAT IS COMPELLING EVIDENCE THAT WRS ARE A NORMAL PART OF THE EVOLUTION OF MASSIVE STARS. IF THEY DISAGREE PERHAPS EITHER BINARY EVOLUTION PLAYS AN IMPORTANT ROLE IN THE FORMATION OF WRS OR THE SINGLE STAR MODELS COULD BE IMPROVED. FOR INSTANCE WE KNOW THAT THE MASS-LOSS RATES DURING THE LUMINOUS BLUE VARIABLE AND RED SUPERGIANT PHASES ARE POORLY CONSTRAINED BY OBSERVATIONS. IF HIGHER MASS LOSS RATES DURING THESE PHASES WERE INCLUDED COULD WE ACCOUNT FOR ALL OF THE WR PHYSICAL PROPERTIES (INCLUDING CHEMICAL ABUNDANCES) THAT WE FIND? EITHER RESULT WILL HELP US LEARN MORE ABOUT THE ORIGIN OF WRS WHILE ALSO TESTING AND HELPING IMPROVE THE EVOLUTIONARY MODELS. FOR THIS TEST TO BE MEANINGFUL WE MUST HAVE ACCURATE MEASUREMENTS OF THE FUNDAMENTAL PHYSICAL PROPERTIES OF WRS (SUCH AS EFFECTIVE TEMPERATURES BOLOMETRIC LUMINOSITIES AND CHEMICAL ABUNDANCES) AS WELL AS HAVING A GOOD UNDERSTANDING OF THE UNCERTAINTIES ON THESE QUANTITIES. TO ACHIEVE THIS WE HAVE A SELECTED A STATISTICALLY LARGE SAMPLE OF 27 WRS IN THE SMALL AND LARGE MAGELLANIC CLOUDS WHICH POSSESS EXCELLENT UV SPECTRA IN THE MAST IUE ARCHIVE. THIS WAVELENGTH IS CRUCIAL AS IT CONTAINS KEY DIAGNOSTIC RESONANCE LINES SUCH AS CIV 1550. TO THESE WE ADD OUR OWN HIGH QUALITY MAGELLAN OPTICAL (AND WHEN NEEDED NEAR-IR) SPECTROPHOTOMETRY. FORTY PERCENT OF OUR SAMPLE HAS ALSO BEEN OBSERVED IN THE FAR-UV WITH FUSE PROVIDING ADDITIONAL DIAGNOSTICS. FINALLY WE WILL INCORPORATE NASA 2MASS AND SPITZER IPAC PHOTOMETRY WHICH EXTEND THE SPECTRAL ENERGY DISTRIBUTION INTO THE IR. WE WILL MODEL EACH OF THESE COMBINED DATA SETS USING CMFGEN A STELLAR ATMOSPHERE CODE THAT INCLUDES THE MANY COMPLICATIONS NEEDED TO MODEL THE SPECTRA OF THESE STARS. THE USE OF THIS COMBINED DATA SET ACHIEVES WHAT ONE COULD NOT HOPE TO DO FROM ANY ONE OF THEM CONSISTENT WITH THE AIMS OF THE ADAP.
OpenAlex ID: W3134534202