W40 IRS 1A South

W40 IRS 1A South is a young O-type star in the H II region, Westerhout 40, one of the closest O type stars to the Sun at just over 1,400 light years away. It is the only known O-type star in W40 and is responsible for most of W40's ionisation, and its stellar wind has carved out a 4 pc wide bubble around it, which can be seen in wide field mid-IR images.[2] W40 IRS 1A South is separated from its northern counterpart, W40 IRS 1A North, which is a heavily reddened Herbig Be star, by approximately 2".

W40 IRS 1A South
Observation data
Epoch J2000      Equinox J2000
Constellation Serpens
Right ascension 18h 31m 27.827s[1]
Declination −2° 5′ 22.86″[1]
Apparent magnitude (V) 15
Characteristics
Evolutionary stage O-type main-sequence star
Spectral type O9.5[2]
Astrometry
Distance436[3] pc
Absolute magnitude (MV)-4.25
Details
Radius7.71[2] R☉
Luminosity52,500 L☉
Temperature31,500[4] K
Other designations
(W40 IRS 1A:) CXOW40 J183127.84-020523.5, 2MASS J18312782-0205228
Database references
SIMBAD(IRS 1a) data

Properties

Not much is known about IRS 1A South, given how reddened and extinguished it is (by 10.6 magnitudes, which is why it is so dim despite being such a massive star so close to us, i.e. only 0.0058% of its light reaches us).[2] So far attempts to derive its parameters have used averages from its spectral type of O9.5 (V), and different models have given different parameters. The most recent temperature value for IRS 1A South is 31,500 Kelvin,[4] and the most recent radius value is 7.71 solar radii.[2] Considering these parameters and the Stefan–Boltzmann law derives a luminosity of about 52,500 solar luminosities. Assuming the distance of 436 parsecs,[3] the apparent magnitude of about 15 and the extinction of 10.6 magnitudes,[2] IRS 1A South has an absolute magnitude of -4.25, i.e. it's about 4,300 times brighter than the Sun in the visible wavelength.

References
  1. Cutri, R. M.; Skrutskie, M. F.; van Dyk, S.; Beichman, C. A.; Carpenter, J. M.; Chester, T.; Cambresy, L.; Evans, T.; Fowler, J.; Gizis, J.; Howard, E. (2003-06-01). "VizieR Online Data Catalog: 2MASS All-Sky Catalog of Point Sources (Cutri+ 2003)". VizieR Online Data Catalog: II/246. Bibcode:2003yCat.2246....0C.
  2. Shuping, R. Y.; Vacca, William D.; Kassis, Marc; Yu, Ka Chun (2012-10-01). "Spectral Classification of the Brightest Objects in the Galactic Star-forming Region W40". The Astronomical Journal. 144 (4): 116. arXiv:1208.4648. Bibcode:2012AJ....144..116S. doi:10.1088/0004-6256/144/4/116. ISSN 0004-6256. S2CID 119227485.
  3. Ortiz-León, G. N.; et al. (2016). "The Gould's Belt Distances Survey (GOBELINS) III. The distance to the Serpens/Aquila Molecular Complex". Astrophysical Journal. 834 (2): 143. arXiv:1610.03128. Bibcode:2017ApJ...834..143O. doi:10.3847/1538-4357/834/2/143. S2CID 10802135.
  4. Schneider, N.; Simon, R.; Guevara, C.; Buchbender, C.; Higgins, R. D.; Okada, Y.; Stutzki, J.; Güsten, R.; Anderson, L. D.; Bally, J.; Beuther, H. (2020-10-01). "FEEDBACK: a SOFIA Legacy Program to Study Stellar Feedback in Regions of Massive Star Formation". Publications of the Astronomical Society of the Pacific. 132 (1016): 104301. arXiv:2009.08730. Bibcode:2020PASP..132j4301S. doi:10.1088/1538-3873/aba840. ISSN 0004-6280. S2CID 221802315.
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