S Monocerotis

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S Monocerotis
Monoceros constellation map.svg
S Monocerotis lies in NGC 2264 at the very northern edge of Monoceros.
Observation data
Epoch J2000       Equinox J2000
Constellation Monoceros
A
Right ascension 06h 40m 58.656s [1]
Declination +09° 53 44.71 [1]
Apparent magnitude  (V)4.66 [2] (4.62 - 4.68 [3] ) + 5.90 [2]
B
Right ascension 06h 40m 58.566s [1]
Declination +09° 53 42.20 [1]
Apparent magnitude  (V)7.830 [1]
Characteristics
Spectral type O7V((f))zvar [4] + O9.5Vn [5] + B2V [6]
U−B color index −1.034 [7]
B−V color index −0.261 [7]
Variable type Ia [3]
Astrometry
A
Radial velocity (Rv)22.00 [8]  km/s
Proper motion (μ)RA: −2.61 [9]   mas/yr
Dec.: −1.61 [9]   mas/yr
Parallax (π)3.55 ± 0.50  mas [9]
Distance 720 [5]   pc
Absolute magnitude  (MV)−5.21 [10]
B
Proper motion (μ)RA: −1.971 [11]   mas/yr
Dec.: −4.225 [11]   mas/yr
Parallax (π)1.4019 ± 0.0984  mas [11]
Distance 2,300 ± 200  ly
(710 ± 50  pc)
C
Proper motion (μ)RA: −1.464 [12]   mas/yr
Dec.: −2.746 [12]   mas/yr
Parallax (π)1.5438 ± 0.0394  mas [12]
Distance 2,110 ± 50  ly
(650 ± 20  pc)
Orbit [13]
PrimaryAa
CompanionAb
Period (P)108±12 yr
Semi-major axis (a)112.5±6  mas
Eccentricity (e)0.770+0.023
−0.030
Inclination (i)47±2°
Longitude of the node (Ω)60±3°
Periastron epoch (T)B 1996.05+0.15
−0.10
Argument of periastron (ω)
(secondary)
63±4°
Details
Aa
Mass 29.1 [2]   M
Radius 9.9 [14]   R
Luminosity 214,000 [14]   L
Surface gravity (log g)4.5 [15]   cgs
Temperature 38,500 [14]   K
Rotational velocity (v sin i)120 [16]  km/s
Ab
Mass 21.3 [2]   M
Age 3.1 [17]   Myr
Other designations
15 Monocerotis, HD 47839, HIP 31978, HR 2456, SAO 114258, BD+10°1220
Database references
SIMBAD data
ARICNS data
NGC 2264 including the Cone Nebula and the Christmas Tree Cluster (upside down in this image) with S Monocerotis at the very top of the image (and the base of the Christmas tree) NGC 2264 cluster.jpg
NGC 2264 including the Cone Nebula and the Christmas Tree Cluster (upside down in this image) with S Monocerotis at the very top of the image (and the base of the Christmas tree)

S Monocerotis, also known as 15 Monocerotis, is a massive multiple and variable star system located in the constellation Monoceros. It is the brightest star in the Christmas Tree open cluster in the area catalogued as NGC 2264.

S Monocerotis is found within an open cluster and the Washington Double Star Catalog lists many companion stars. [18] The closest and brightest is S Mon B, magnitude 7.8 and 3 arcseconds away. It is classified as B2 main sequence star with a mass of 7.31  M. Designated component C is an 11th-magnitude B8V star. [6] The cluster contains another dozen or so 9th and 10th magnitude stars and many fainter stars.

S Monocerotis A is a spectroscopic binary system with an eccentric orbit of about 112 years. [13] Since 1943, the spectrum of this star has served as the MK standard for O7 by which other stars are classified. [19] It is also an irregular variable star with a range of less than a tenth of a magnitude. The orbital parameters can be used to derive the masses of the two stars, giving 31 M and 11 M. [6]

The distance to S Monocerotis and NGC 2264 has been derived in various ways, including dynamical parallax and isochrone fitting. These consistently give estimates of 700 - 900 parsecs, although this is double the likely distance derived from the Hipparcos parallax measurements. [9] Gaia Early Data Release 3 contains parallaxes for the companions components B and C of 1.4  mas and 1.5 mas respectively, consistent with the expected distance to the cluster.

Related Research Articles

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<span class="mw-page-title-main">R Monocerotis</span> Variable star in the constellation Monoceros

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<span class="mw-page-title-main">18 Monocerotis</span> Star in the constellation Monoceros

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<span class="mw-page-title-main">CD Crucis</span> Eclipsing binary star system in the constellation Crux

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<span class="mw-page-title-main">HD 151932</span> Star in the constellation of Scorpius

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<span class="mw-page-title-main">HD 193322</span> Star group in the constellation Cygnus

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<span class="mw-page-title-main">WR 120</span> Binary star system in the constellation Scutum

WR 120 is a binary containing two Wolf-Rayet stars in the constellation of Scutum, around 10,000 light years away. The primary is a hydrogen-free weak-lined WN7 star, the secondary is a hydrogen-free WN3 or 4 star, and the system is a possible member of the cluster Dolidze 33. From our point of view, WR 120 is reddened by 4.82 magnitudes, and it has the variable designation of V462 Scuti.

References

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