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MWC 656

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MWC 656
Observation data
Epoch J2000.0      Equinox J2000.0
Constellation Lacerta
Right ascension 22h 42m 57.303s[1]
Declination +44° 43′ 18.26″[1]
Apparent magnitude (V) 8.75[2]
Characteristics
Spectral type B1.5-2III[3] or B5:ne[4]
U−B color index −0.77[2]
B−V color index 0.090±0.015[2]
Astrometry
Proper motion (μ) RA: −3.478 mas/yr[1]
Dec.: −3.159 mas/yr[1]
Parallax (π)0.4860 ± 0.0185 mas[1]
Distance6,700 ± 300 ly
(2,060 ± 80 pc)
Orbit[5]
Period (P)59.028±0.011 d
Eccentricity (e)0 (fixed)
Periastron epoch (T)2,457,509.20±0.31 BJD
Argument of periastron (ω)
(secondary)		90 (fixed)°
Semi-amplitude (K1)
(primary)		11.6±3.7 km/s
Semi-amplitude (K2)
(secondary)		91.2±2.9 km/s
Details
Primary
Mass7.8±2.0[5] M☉
Radius8.8±0.5[6] R☉
Surface gravity (log g)2.941[7] cgs
Temperature5,782[7] K
Rotation1.12±0.03 d[6]
Rotational velocity (v sin i)305[8] km/s
Secondary
Mass0.94±0.34[5] M☉
Other designations
AGL J2241+4454, BD+43° 4279, HD 215227, HIP 112148, SAO 52294, PPM 63303, WDS J22430+4443A[9]
Database references
SIMBADdata

MWC 656 is an X-ray binary star system in the northern constellation of Lacerta. It has the identifier HD 215227 from the Henry Draper Catalogue.[9] With an apparent visual magnitude of 8.75,[2] it is too faint to be viewed with the naked eye. Based on parallax measurements, it is located at a distance of approximately 6,700 light years from the Sun.[1] At one time it was considered a member of the Lacerta OB1 association of co-moving stars,[4] but the distance estimate places it well past that group.[10]

Observations

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On August 11, 1935, R. F. Sanford found a weak emission line of Hydrogen–β in the spectrum of this star, and it was included in the 1943 supplement to the Mount Wilson catalogue of similar stars with the identifier MWC 656.[11] In 1964, it was assigned a stellar classification of B5:ne, where 'B5' indicates this is a B-type star, 'n' means it displays 'nebular' lines due to rapid rotation, and 'e' shows it has emission lines. The ':' suffix indicates some uncertainty about the classification.[12] It was included in a catalogue of Be stars in 1982.[13] In 2005 it was found to have high projected rotational velocity of 305 km/s.[8]

A light curve for MWC 656. The blue points show the Hipparcos data.[14] The red curve shows a sine function fit to that data, and the green points show the averages of the data within 10 bins across each period.

In 2009, the AGILE satellite discovered a nearby source of gamma-ray emission above 100 MeV. This source was given the identifier AGL J2241+4454. HD 215227 is the only suitable optical counterpart to lay within the 0.6° error circle. Spectra from the star showed evidence of emission from a circumstellar disk, as well as absorption from a shell feature. Rapid changes in emission line variability suggest an orbiting companion that is tidally interacting with the disk. Hipparcos light curve data indicated an orbital period of 60.37±0.04 d.[10] This was confirmed in 2012 via radial velocity measurements of helium lines in photosphere of the Be star.[15]

Refined radial velocity measurements in 2014 indicated a massive companion in the range of 3.8–6.9 M☉, assuming the Be star has a mass of 10–16 M☉. A main sequence companion with a mass this high should be readily visible in the optical band. Likewise, a subdwarf or a stripped helium core from a massive progenitor star don't fit the observations. The mass is too high for a white dwarf or a neutron star, leaving a stellar mass black hole as the only viable candidate.[16][17] A faint X-ray emission was detected later the same year with a total luminosity of (3.7±1.7)×1031 erg·s−1, making this a high mass X-ray binary system. This luminosity is consistent with a stellar black hole in quiescence – meaning very little material is being fed into the black hole from the primary star.[18]

This was the first reported binary system combining a black hole with a Be star.[6] However, many Be stars are now found to have subdwarf OC companions, and the properties of these appear similar to MWC 656. The 2022 discovery of tidal distortion of the disk orbiting the Be star invalidated the original radial velocity amplitude, which called into question the 2014 mass estimates. The correction for this probably rules out a black hole companion. Emission from ionized helium near the companion appears double-peaked, indicating there is an orbiting accretion disk being fed from the disk orbiting the Be star.[19] Revised measurements reported in 2023 found a mass range of 0.94 to 2.4 M☉ for the companion, which means this is instead a neutron star, a white dwarf, or a hot helium star.[5]

The position of the star at a distance of 2,300 light-years (0.71 kpc) below the galactic plane suggests this is a runaway star system, since it is a young star not located near any star forming region.[10] This scenario favors the neutron star companion.[5]

References

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  1. ^ a b c d e Vallenari, A.; et al. (Gaia collaboration) (2023). "Gaia Data Release 3. Summary of the content and survey properties". Astronomy and Astrophysics. 674: A1. arXiv:2208.00211. Bibcode:2023A&A...674A...1G. doi:10.1051/0004-6361/202243940. S2CID 244398875. Gaia DR3 record for this source at VizieR.
  2. ^ a b c d Anderson, E.; Francis, Ch. (2012), "XHIP: An extended hipparcos compilation", Astronomy Letters, 38 (5): 331, arXiv:1108.4971, Bibcode:2012AstL...38..331A, doi:10.1134/S1063773712050015, S2CID 119257644.
  3. ^ Casares, J.; et al. (January 2014), "A Be-type star with a black-hole companion", Nature, 505 (7483): 378–381, arXiv:1401.3711, Bibcode:2014Natur.505..378C, doi:10.1038/nature12916, PMID 24429635.
  4. ^ a b Chen, W. P.; Lee, H. T. (December 2008), Reipurth, Bo (ed.), "The Lacerta OB1 Association", Handbook of Star Forming Regions, Volume I: The Northern Sky, ASP Monograph Publications, vol. 4, p. 124, arXiv:0811.0443, Bibcode:2008hsf1.book..124C.
  5. ^ a b c d e Janssens, S.; et al. (September 2023), "MWC 656 is unlikely to contain a black hole", Astronomy & Astrophysics, 677, id. L9, arXiv:2308.08642, Bibcode:2023A&A...677L...9J, doi:10.1051/0004-6361/202347318.
  6. ^ a b c Zamanov, Radoslav K.; et al. (March 2021), "Radius, rotational period, and inclination of the Be stars in the Be/gamma ray binaries MWC 148 and MWC 656", Astronomische Nachrichten, 342 (3): 531–537, arXiv:2102.01971, Bibcode:2021AN....342..531Z, doi:10.1002/asna.202123856.
  7. ^ a b Ting, Yuan-Sen; et al. (July 8, 2019), "The Payne : Self-consistent ab initio Fitting of Stellar Spectra", The Astrophysical Journal, 879 (2): 69, arXiv:1804.01530, Bibcode:2019ApJ...879...69T, doi:10.3847/1538-4357/ab2331, ISSN 1538-4357, S2CID 54588218.
  8. ^ a b Glebocki, R.; Gnacinski, P. (2005), "Catalog of Stellar Rotational Velocities", VizieR On-line Data Catalog: III/244. Originally Published in: 2005csss...13..571G; 2005yCat.3244....0G, 3244, Bibcode:2005yCat.3244....0G.
  9. ^ a b "MWC 656", SIMBAD, Centre de données astronomiques de Strasbourg, retrieved 2024-12-15.
  10. ^ a b c Williams, S. J.; et al. (November 2010), "The Be Star HD 215227: A Candidate Gamma-ray Binary", The Astrophysical Journal Letters, 723 (1): L93–L97, arXiv:1009.4947, Bibcode:2010ApJ...723L..93W, doi:10.1088/2041-8205/723/1/L93.
  11. ^ Merrill, Paul W.; Burwell, Cora G. (1943), "No. 682. Supplement to the Mount Wilson catalogue and bibliography of stars of classes B and A whose spectra have bright hydrogen lines", Contributions from the Mount Wilson Observatory, 682, Carnegie Institution of Washington: 1–32, Bibcode:1943CMWCI.682....1M.
  12. ^ Jaschek, Carlos; et al. (1964), "Catalogue of stellar spectra classified in the Morgan-Keenan system", Observatorio Astronomico de la Universidad de la Plata, Serie Astronomica, La Plata, vol. 28, p. 1, Bibcode:1964PLPla..28....1J.
  13. ^ Jaschek, M.; Egret, D. (April 1982), "Catalog of Be stars", IAU Symposium, vol. 98, p. 261, Bibcode:1982IAUS...98..261J.
  14. ^ EAS (1997). "The HIPPARCOS and TYCHO catalogues". Astrometric and Photometric Star Catalogues Derived from the ESA Hipparcos Space Astrometry Mission. ESA SP Series. 1200. Noordwijk, Netherlands: ESA Publications Division. Bibcode:1997ESASP1200.....E. ISBN 9290923997. Retrieved 17 December 2024.
  15. ^ Casares, J.; et al. (April 2012), "On the binary nature of the γ-ray sources AGL J2241+4454 (= MWC 656) and HESS J0632+057 (= MWC 148)", Monthly Notices of the Royal Astronomical Society, 421 (2): 1103–1112, arXiv:1201.1726, Bibcode:2012MNRAS.421.1103C, doi:10.1111/j.1365-2966.2011.20368.x.
  16. ^ Casares, J.; et al. (January 2014), "A Be-type star with a black-hole companion", Nature, 505 (7483): 378–381, arXiv:1401.3711, Bibcode:2014Natur.505..378C, doi:10.1038/nature12916, PMID 24429635.
  17. ^ Wall, Mike (January 15, 2014), "Oddly Quiet Black Hole Spotted Around Fast-Spinning Star", space.com, retrieved 2024-12-15.
  18. ^ Munar-Adrover, P.; et al. (May 2014), "Discovery of X-Ray Emission from the First Be/Black Hole System", The Astrophysical Journal Letters, 786 (2), id. L11, arXiv:1404.0901, Bibcode:2014ApJ...786L..11M, doi:10.1088/2041-8205/786/2/L11.
  19. ^ Rivinius, Th.; et al. (August 2022), MWC656: A Be+BH or a Be+sdO?, arXiv:2208.12315, Bibcode:2022arXiv220812315R.

Further reading

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