9 Sagittarii

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9 Sagittarii
Location of 9 Sagittarius (circled)
Observation data
Epoch J2000      Equinox J2000
Constellation Sagittarius
Right ascension 18h 03m 52.44501s[1]
Declination −24° 21′ 38.6323″[1]
Apparent magnitude (V) 5.97[2]
Characteristics
Spectral type O4V((f))z[3] (O3.5V((f+)) + O5–5.5V((f))[4])
U−B color index −0.89[2]
B−V color index 0.00[2]
Distance
4,100 ± 400 ly
(1,200 ± 100 pc)
Absolute magnitude (MV)−6.20[4]
Semi-amplitude
(K1)
(primary)		36+4
−1 km/s
Semi-amplitude (K2)
(secondary)		49±3 km/s
Details
Myr
CD−24°13814, 2MASS J18035245-2421386, WDS
 J18039-2422
Database references
SIMBADdata

9 Sagittarii (9 Sgr) is a massive binary star in the constellation Sagittarius. It has an apparent magnitude of 5.97. Both components are highly luminous O-type main-sequence stars.

Surroundings

9 Sgr is the brightest star in the image, just left of the intense Hourglass Nebula core of the Lagoon Nebula. The scattering of stars on the left is NGC 6530.

9 Sgr is a naked eye star lying in the direction of the Lagoon Nebula (Messier 8), very close to the central condensation of the Hourglass Nebula around Herschel 36. It was not catalogued by Bayer with a Greek letter, but was listed by Flamsteed as number 9 in the constellation Sagittarius. It lies close to the open cluster NGC 6530. 9 Sgr is the main source of ionisation for much of the visible nebulosity in the region, although the young O star Herschel 36 ionises the dense Hourglass Nebula region.[8]

9 Sgr itself is surrounded by an ionised

NGC 6523 and NGC 6533. This ionised region lies in front of a denser molecular cloud.[8]

The distances to 9 Sgr, M8, and NGC 6530 are uncertain, but generally estimated to be between 1,200 and 1,800 parsecs. Recent studies derive a distance around 1,250 pc for the M8 region.[8] Erosion of the front of the molecular cloud apparently caused by 9 Sgr suggests that it lies in front of the cloud, but studies of 9 Sgr as a binary star give a distance of 1,790 pc.[4][9] A 2021 study of the binary system derives a distance of 1,310±60 pc. On this basis, it is thought to be a member of the NGC 6530 open cluster.[6]

System

9 Sagittarii is a binary system with the longest known period for a pair of class O stars at 9.1 years. The orbit is eccentric and the separation between the stars varies from 11 AU to 27 AU. The large separation means that the stellar winds of the two stars do not impact strongly and so the pair are not a strong source of x-rays. Lack of x-rays, low orbital velocities, and similar spectral types merging to a combined spectrum of O4V, mean that 9 Sgr was only confirmed to be a binary in 2012.[4] Earlier clues such as nonthermal radio emission and periodic spectral line profile variations had prompted the detail search for a companion.[10][11]

The two stars have been resolved using the

PIONIER instrument.[12][13]

Properties

Both components of 9 Sgr are massive

luminosity of the Sun. At an age of about one million years, both are still main sequence stars fusing hydrogen in their cores, although strong convection have enriched the surface with nitrogen and helium which are visible in the spectrum. The powerful stellar winds produce emission lines in their spectra.[4]

References

  1. ^
    S2CID 244398875
    . Gaia DR3 record for this source at VizieR.
  2. ^
    Bibcode:2002yCat.2237....0D
    .
  3. S2CID 118847528
    .
  4. ^
    S2CID 19967446
    .
  5. S2CID 119231169
    .
  6. ^
    S2CID 235125785
    .
  7. S2CID 119120927
    .
  8. ^
    Bibcode:2008hsf2.book..533T
    .
  9. S2CID 55429395
    .
  10. doi:10.1086/162040
    .
  11. doi:10.1051/0004-6361:20020926
    .
  12. S2CID 58911264
    .
  13. S2CID 53500788
    .
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