In 1982, aspectrum was obtained of one of theH II regions within IC 1613 named S3, as a comparison object for observing asupernova remnant within the same galaxy. The spectrum displayed broademission lines atwavelengths of 4650 and 5810Å, indicative of the presence of either a singular WC star with peculiar emission features or a WC+WN binary.[7] Also in 1982, Davidson & Kinman suggested that it was rather an WO star due to the presence of OVI (O5+) lines.[8] A 1991 paper gave it a tentative WO4spectral type,[9] which was revised to WO3 in 1993.[10] In 1985, it was listed as a WR candidate alongside seven other objects in IC 1613,[11] but all but DR1 turned out not to be WR stars.[12] As of 2019, it remains the only confirmed WR star in the galaxy,[13] though in 2021, a candidatedust-producing WC star designated SPIRITS14bqe was found, which may resemble the WC5+O9binary systemWR 19.[14]
DR1, ofstellar classification WO3, is one of thehottest stars discovered with aneffective temperature of 150,000 K. As a consequence, it is very luminous, radiating 479,000 times theluminosity of the Sun. It is a very dense star, weighing 20M☉ despite only being 6% larger than theSun in radius. The star has the lowestmetallicity known for a WO star at just 15% the solar value. Intensestellar winds, with aterminal velocity of 2,750 km/s, are causing DR1 to lose mass at a rate of1.8×10−5M☉/yr.[5] This is several hundred million times faster than the Sun's, which is at (2–3)×10−14M☉ per year.[15]
The brightest HII region in IC 1613, S3 was first catalogued byAllan Sandage in 1971.[16][17] It is approximately108 pc by33 pc and is thought to be ionised solely by DR1.[7] With anelectron temperature of 17,100 K, it is among the hottest HII regions in theLocal Group.[2] Unusually for HII regions, it displays nebular HeII (He+) lines at 4686 Å.[3] It has abipolar structure, similar in appearance to theplanetary nebulaNGC 2346, with two lobes that stretch northwest and southeast, the northwest one being larger. Twosuperbubbles, named R16 and R17, have been detected in the vicinity of the northwestern lobe, which could be ionised by DR1. The WO star is nestled in its bright central region, thus it is probable that the nebula was formed by DR1 while it was still amain-sequence star.[18] A shell of neutral gas has been discovered surrounding S3, which is also likely to have been ejected from DR1 during its main-sequence phase.[19]
WO stars are the lastevolutionary stage of the most massive stars.[20] From its position in theHertzsprung–Russell diagram, DR1 is inferred to have started its life as a 120M☉ star,[2] which has now lost most of its mass and is at its last stages ofstellar nucleosynthesis, near the end ofhelium burning.[5] Stars with a final mass above 10M☉, such as DR1, are expected to formblack holes, producing a faintsupernova or no supernova at all. However, if the star is rapidlyrotating like the galactic WO starsWR 102 andWR 142 analyzed by Sander et al. (2012),[21] it may instead produce a bright type Ib/c supernova perhaps accompanied by agamma-ray burst. The latter scenario is more likely at lower metallicities as seen in DR1.[2] The supernova is estimated to occur within 17,000 years.[5]
^abKingsburgh, Robin L.; Barlow, M. J. (March 1995). "DR 1: a WO3 star in IC 1613 and its surrounding nebula, S3".Astronomy & Astrophysics.295:171–182.Bibcode:1995A&A...295..171K.
^abD'Odorico, S.; Rosa, M. (January 1982). "Wolf-Rayet stars in extragalactic HII regions: discovery of a peculiar WR in IC 1613/#3".Astronomy & Astrophysics.105:410–412.Bibcode:1982A&A...105..410D.
^Kingsburgh, Robin L.; Barlow, M. J.; Storey, P. J. (1994). "Oxygen and carbon abundances for the WO stars".Space Science Reviews.66 (1–4):277–280.doi:10.1007/BF00771077.ISSN0038-6308.
^Azzopardi, M.; Lequeux, J.; Maeder, A. (January 1988). "The number of Wolf-Rayet stars in Local Group galaxies".Astronomy & Astrophysics.189:34–38.Bibcode:1988A&A...189...34A.
^Carroll, Bradley W.; Ostlie, Dale A. (1995).An Introduction to Modern Astrophysics (revised 2nd ed.). Benjamin Cummings. p. 409.ISBN978-0-201-54730-6.
^Tautvaišienė, Gražina; Geisler, Doug; Wallerstein, George; Borissova, Jura; Bizyaev, Dmitry; Pagel, Bernard E. J.; Charbonnel, Corinne; Smith, Verne (2007). "First Stellar Abundances in the Dwarf Irregular Galaxy IC 1613".The Astronomical Journal.134 (6): 2318.Bibcode:2007AJ....134.2318T.doi:10.1086/523630.
