WR 124

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WR 124
M1-67 & WR124.png
Hubble Space Telescope image of nebula M1-67 around Wolf–Rayet star WR 124.
Observation data
Epoch J2000      Equinox J2000
Constellation Sagitta
Right ascension 19h 11m 30.876s[1]
Declination +16° 51′ 38.168″[1]
Apparent magnitude (V) 11.50 ± 0.11[2]
Characteristics
Spectral type WN8h[3]
B−V color index 0.69[2]
Variable type Eruptive (WR)[4]
Astrometry
Radial velocity (Rv) 190 ± 7.4[5] km/s
Proper motion (μ) RA: -1.81 ± 2.48[1] mas/yr
Dec.: −9.29 ± 2.82[1] mas/yr
Parallax (π) 0.21 ± 2.59[1] mas
Distance 10,900 ly
(3,350[6] pc)
Absolute magnitude (MV) -5.3[6]
Details
Mass 9[6] M
Radius 10.1[6] R
Luminosity 150,000[6] L
Temperature 35,900[6] K
Age 8.6M[6] years
Other designations
Merrill's Star, QR Sge, GSC 01586-00411, Sh 2-80, Hen 2-427
Database references
SIMBAD data

WR 124 is a Wolf–Rayet star in the constellation of Sagitta surrounded by a ring nebula of expelled material known as M1-67.[7] It is one of the fastest runaway stars in the galaxy with a radial velocity around 200 km/s. It was discovered by Paul W. Merrill in 1938, identified as a high velocity Wolf Rayet star.[8] It is listed in the General Catalogue of Variable Stars as variable with a range of 0.08 magnitudes.[4]

Distance[edit]

WR 124 has a Hipparcos parallax, but the quoted margin of error is much larger than the parallax itself, so it isn't really a useful measurement.

A recent study of WR 124 directly measured the expansion rate of the M1-67 nebula expelled from the star using WFPC2 camera images taken 11 years apart, and compared that rate to the expansion velocity measured by the Doppler shift of the nebular emission lines.[6] This yields a direct geometric measurement of the distance to WR 124, something that has only been done for one other WR star (Gamma Velorum), and which should be less subject to error than other methods of distance measurement.

The distance calculated from the nebular expansion rate is 3.35kpc, which is less than previous studies, and the resulting luminosity of 150,000 times the sun (L) is much lower than previously calculated. The luminosity is also lower than predicted by models for a star of this spectral class. Previous studies found distances of 5kpc[7] to 8.4kpc,[3] with corresponding luminosities of 338,000-1,000,000 L, as expected for a typical WN8h which is a very young star just moving away from the main sequence. This may be an uncommon post-red-supergiant WN star with some hydrogen still remaining. In this case WR 124 would be much older, around 8.6 million years, but the luminosity is still lower than predicted by theory.[6] WR stars of lower metallicity can form from lower mass progenitors and have lower luminosity, but this would be unusual for a Population I star within the Milky Way.

Physical Characteristics[edit]

Assuming a distance of 3.35kpc, WR 124 is estimated to have a current mass of 9 M, with a probable initial mass around 25 M. It has blown away a large portion of its mass through the intense stellar winds of the Wolf-Rayet phase, and possibly also earlier hypergiant and luminous blue variable phases. The temperature of around 36,000K means that most of its energy is emitted at ultraviolet wavelengths, the visual absolute magnitude is -5.3 (or up to -7.2 if it is further away), and WR 124 is ten times the radius of the sun.

WR 124 is measured to still be about 15% hydrogen with most of the remaining mass being helium. A young highly massive and luminous WN8h star would still be burning hydrogen in its core, but a less luminous and older star would be burning helium in its core.[9] The result of modelling the star purely from its observed characteristics is a luminosity of 1,000,000 L and a mass of 33 M, corresponding to a relatively young hydrogen-burning star at around 8 kpc[3] In either case, it has only a few hundred thousand years before it explodes as a type Ib or Ic supernova.

The mass loss rate is 10-5 M - 10-4 M per year, depending on the distance and properties determined for the star.[7]

Nebula[edit]

WR 124 is surrounded by an intensely hot nebula formed from the star's extreme stellar wind. The nebula M1-67 is expanding at a rate of over 150,000 km/h (100,000 mph) and is nearly 6 light-years across, leading to the dynamical age of 20,000 years. M1-67 has little internal structure, though large clumps of material have been detected, some of which have 30 times the mass of Earth and stretch out up to 150 billion km (90 billion miles). If placed in the Solar System, one of these clumps would span the distance from the Sun to Saturn. WR 124 can be seen as a glowing body in the center of a gigantic fireball.[7]

External links[edit]

References[edit]

  1. ^ a b c d e Van Leeuwen, F. (2007). "Validation of the new Hipparcos reduction". Astronomy and Astrophysics 474 (2): 653. Bibcode:2007A&A...474..653V. doi:10.1051/0004-6361:20078357. 
  2. ^ a b Høg, E.; Fabricius, C.; Makarov, V. V.; Urban, S.; Corbin, T.; Wycoff, G.; Bastian, U.; Schwekendiek, P.; Wicenec, A. (2000). "The Tycho-2 catalogue of the 2.5 million brightest stars". Astronomy and Astrophysics 355: L27. Bibcode:2000A&A...355L..27H. 
  3. ^ a b c Hamann, W.-R.; Gräfener, G.; Liermann, A. (2006). "The Galactic WN stars". Astronomy and Astrophysics 457 (3): 1015. Bibcode:2006A&A...457.1015H. doi:10.1051/0004-6361:20065052. 
  4. ^ a b Kukarkin, B. V.; Kholopov, P. N.; Pskovsky, Y. P.; Efremov, Y. N.; Kukarkina, N. P.; Kurochkin, N. E.; Medvedeva, G. I. (1971). "The third edition containing information on 20437 variable stars discovered and designated till 1968". General Catalogue of Variable Stars: 0. Bibcode:1971GCVS3.C......0K. 
  5. ^ Kharchenko, N. V.; Scholz, R.-D.; Piskunov, A. E.; Röser, S.; Schilbach, E. (2007). "Astrophysical supplements to the ASCC-2.5: Ia. Radial velocities of ˜55000 stars and mean radial velocities of 516 Galactic open clusters and associations". Astronomische Nachrichten 328 (9): 889. Bibcode:2007AN....328..889K. doi:10.1002/asna.200710776. 
  6. ^ a b c d e f g h i Marchenko, S. V.; Moffat, A. F. J.; Crowther, P. A. (2010). "Population I Wolf-Rayet Runaway Stars: The Case of Wr124 and Its Expanding Nebula M1-67". The Astrophysical Journal 724: L90. Bibcode:2010ApJ...724L..90M. doi:10.1088/2041-8205/724/1/L90. 
  7. ^ a b c d Crowther, Paul A.; Pasquali, A.; De Marco, Orsola; Schmutz, W.; Hillier, D. J.; De Koter, A. (1999). "Wolf-Rayet nebulae as tracers of stellar ionizing fluxes. I. M1-67". Astronomy and Astrophysics 350: 1007. Bibcode:1999A&A...350.1007C. 
  8. ^ Merrill, P. W. (1938). "A Wolf-Rayet Star with High Velocity". Publications of the Astronomical Society of the Pacific 50: 350. Bibcode:1938PASP...50..350M. doi:10.1086/124982. 
  9. ^ Meynet, G.; Maeder, A. (2003). "Stellar evolution with rotation". Astronomy and Astrophysics 404 (3): 975. Bibcode:2003A&A...404..975M. doi:10.1051/0004-6361:20030512.