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Westbrook Nebula
Reflection nebula
Protoplanetary nebula
	Hubble image of the Westbrook Nebula
Observation data: J2000 epoch
Right ascension	04h 42m 53.64s
Declination	+36° 06′ 53.4″
Distance	3600 ± 700 ly (1100 ± 200 pc)
Constellation	Auriga
Designations	CRL 618, IRAS 04395+3601, 2MASS J04425364+3606534, RAFGL 618
	See also: Lists of nebulae

Westbrook Nebula (CRL 618) is a bipolar protoplanetary nebula which is located in the constellation Auriga. It is being formed by a star that has passed through the red giant phase and has ceased nuclear fusion at its core. This star is concealed at the center of the nebula, and is ejecting gas and dust at velocities of up to 200 km/s.^[3] The nebula is named after William E. Westbrook, who died in 1975.^[4]

Two spectra of carbon monoxide in the Westbrook Nebula are shown. The component shown in yellow arises from the slow wind from the star's AGB phase. The component shown in red is from the faster wind expelled after the star left the AGB. Adapted from Gammie *et al.*.^[5]

This nebula began to form about 200 years ago, and primarily consists of molecular gas. The outer part of the nebula is the result of interaction between rapid bipolar outflow and the gas that was ejected when the star was passing through its asymptotic giant branch phase. The lobes are inclined about 24° to the line of sight. The energy being radiated from the nebula consists of scattered light from the star at the core, light being emitted from a compact HII region surrounding the star, and energy from the shock-excited gas in the lobes.^[6]

The dynamics of the molecular gas envelope can be studied by examining microwave emission-line spectra from carbon monoxide rotational transitions. These spectra show two distinct velocity components. The narrow cores of the spectral lines show the familiar parabolic profile of a slow (20 km/sec), high optical depth stellar wind from an Asymptotic Giant Branch (AGB) star. This material was expelled before the object became a protoplanetary nebula and constitutes the bulk of the nebula's mass. A second component arising from the far faster post-AGB wind is also visible. The fast (>190 km/sec) wind component becomes more prominent in higher frequency, higher energy spectral lines, because the fast wind has a higher temperature than the slow AGB wind.^[5]

The central star is believed to be of spectral class B0 and has 12,200 times the solar luminosity.^[6] The photosphere of this star is now hot enough to have begun ionizing the nebula, and the ionization region is expanding rapidly. The size and rate of growth indicates that ionization began around the year 1971.^[7] Once a sizeable portion of the nebula has been ionized, it will have become a planetary nebula. This means the Westbrook Nebula is at a somewhat more advanced evolutionary stage than the Egg Nebula, whose spectral class F5 star has not yet begun to ionize the nebula material.

References

^ ^a ^b ^c "Westbrook Nebula -- Post-AGB Star". SIMBAD. Centre de Données astronomiques de Strasbourg. Retrieved 2009-04-16.
^ Cerrigone, Luciano; Umana, Grazia; Trigilio, Corrado; Menten, Karl M.; Bordiu, Cristobal; Ingallinera, Adriano; Leto, Paolo; Buemi, Carla S.; Bufano, Filomena; Cavallaro, Francesco; Loru, Sara; Riggi, Simone (2024). "The distance to CRL 618 through its radio expansion parallax". Monthly Notices of the Royal Astronomical Society. 529 (2): 1579–1584. arXiv:2403.04647. Bibcode:2024MNRAS.529.1579C. doi:10.1093/mnras/stae689.
^ Christensen, Lars Lindberg; Tielens, A.G.G.M. (2000-08-31). "A stellar cocoon soon to hatch to a butterfly". European Space Agency. Retrieved 2013-06-12.
^ Westbrook, W. E.; Gezari, D. Y.; Hauser, M. G.; Werner, M. W.; Elias, J. H.; Neugebauer, G.; et al. (1976). "One-millimeter continuum emission studies of four molecular clouds". Astrophysical Journal. 209: 94–101. Bibcode:1976ApJ...209...94W. doi:10.1086/154695.
^ ^a ^b Gammie, C.F.; Knapp, G.R.; Young, K.; Phillips, T.G.; Falgarone, E. (October 1989). "A Very Fast Molecular Outflow from the Proto-Planetary Nebula CRL 618". The Astrophysical Journal. 345: L87–L89. Bibcode:1989ApJ...345L..87G. doi:10.1086/185559. Retrieved 15 November 2020.
^ ^a ^b Contreras, C. S.; Sahai, R.; Gil de Paz, A. (October 2002). "Physical structure of the protoplanetary nebula CRL618. I. Optical long-slit spectroscopy and imaging". Astrophysical Journal. 578 (1): 269–289. arXiv:astro-ph/0206200. Bibcode:2002ApJ...578..269S. doi:10.1086/342316. S2CID 6798962.
^ Tafoya, D.; Loinard, L.; Vlemmings, W.H.T.; Marti-Vidal, I.; Pech, G. (22 July 2013). "Rapid angular expansion of the ionized core of CRL 618" (PDF). Astronomy and Astrophysics. 556: A35. arXiv:1306.2630. Bibcode:2013A&A...556A..35T. doi:10.1051/0004-6361/201321704. Retrieved 15 November 2020.