RCW 36

RCW 36

Young stars in RCW 36 are revealed in the X-ray (blue), while infrared images (red and green) show both stars and gas.
Object type	H II region
Other designations	RCW 36, Gum 20, BBW 217[1][2]
Observation data (Epoch J2000)
Constellation	Vela
Right ascension	08h 59m 00.9s
Declination	−43° 44′ 10″
Distance	2300 ly[3] / 700 pc
In visual light (V)
Apparent magnitude	15.2
Size	5 arcmin
Estimated age	1.1±0.6 Myr[4]
Related media on Wikimedia Commons
[edit on Wikidata]

RCW 36 (also designated Gum 20)^[5] is an emission nebula containing an open cluster in the constellation Vela. This H II region is part of a larger-scale star-forming complex known as the Vela Molecular Ridge (VMR), a collection of molecular clouds in the Milky Way that contain multiple sites of ongoing star-formation activity.^[1] The VMR is made up of several distinct clouds, and RCW 36 is embedded in the VMR Cloud C.

RCW 36 is one of the sites of massive-star formation closest to the Solar System,^[6] whose distance of approximately 700 parsecs (2300 light-years). The most massive stars in the star cluster are two stars with late-O or early-B spectral types, but the cluster also contains hundreds of lower-mass stars.^[4] This region is also home to objects with Herbig–Haro jets, HH 1042 and HH 1043.^[7]

Star formation in RCW 36

Like most star-forming regions, the interstellar medium around RCW 36 contains both the gas from which stars form and some newly formed young stars.^[1] Here, young stellar clusters form in giant molecular clouds.^[8] Molecular clouds are the coldest, densest form of interstellar gas and are composed mostly of molecular hydrogen (H₂), but also include more complex molecules, cosmic dust, and atomic helium. Stars form when the mass gas in part of a cloud becomes too great, causing it to collapse due to the Jeans instability.^[9] Most stars do not form alone, but in groups containing hundreds or thousands of other stars.^[10] RCW 36 is an example of this type of "clustered" star formation.^[3]

Molecular cloud and H II region

The Vela Molecular Ridge can be subdivided into several smaller clouds, each of which in turn can be subdivided into cloud "clumps". The molecular cloud clump from which the RCW 36 stars are forming is Clump 6 in the VMR C cloud.^[11]

Early maps of the region were produced by radio telescopes that traced emission from several types of molecules found in the clouds, including CO, OH, and H₂CO.^[12][13] More detailed CO maps were produced in the 1990s by a team of Japanese astronomers using the NANTEN millimeter-wavelength telescope. Using emission from C¹⁸O, they estimated the total mass of Cloud C to be 44,000 M_☉.^[11] The cloud maps suggest that Cloud C is the youngest component of the VMR because of an ultra-compact H II region associated with RCW 36 and several sites of embedded protostars, while H II regions in other VMR clouds are more evolved.^[1] Observations from the Herschel Space Telescope show that the material within the cloud is organized into filaments and RCW 36 sits near the south end of a 10-parsec long filament.^{[14][15][16][17]}

Star formation in RCW 36 is currently ongoing. In the dense gas at the western edge of RCW 36, where the far-infrared emission is greatest, are found protostellar cores, the Herbig Haro objects, and an ultra-compact H II region. However, more deeply embedded star-formation is obscured by dust, so radiation can only escape from the cloud surface and not from the embedded objects themselves.^[4]

The H II region is an area around the cluster in which hydrogen atoms in the interstellar medium have been ionized by ultraviolet light from O- and B-type stars. The H II region in RCW 36 has an hourglass morphology,^[14] similar to the shape of H II regions around other young stellar clusters like W40 or Sh2-106. In addition, an ultra-compact H II region surrounds IRAS source 08576−4333.^[18]

Star cluster

Due to the youth of RCW 36, most of the stars in the cluster are at an early stage of stellar evolution where they are known as young stellar objects or pre-main-sequence stars. These stars are still in the process of contraction before they reach the main sequence, and they may still have gas accreting onto them from either a circumstellar disk or envelope.

Cluster members in RCW 36 have been identified through both infrared and X-ray observations. Bright infrared sources, attributed to massive stars, were first identified by the TIFR 100-cm balloon-born telescope from the National Balloon Facility in Hyderabad, India.^[19] In the early 2000s, infrared images in the J, H, and K_s bands have suggested at least 350 cluster members.^[3] Observations by NASA's Spitzer Space Telescope and Chandra X-ray Observatory were used to identify cluster members as part of the MYStIX survey of nearby star-forming regions.^[6] In the MYStIX catalog of 384 probable young stellar members of RCW 36, more than 300 of the stars are detected by X-ray sources.^[20] Modeling of the brightnesses of stars at various infrared wavelengths has shown 132 young stellar objects to have infrared excess consistent with circumstellar disks or envelopes.^[21]

The cluster has been noted by Baba et al. for having a high density of stars, with star counts (the number of stars within an angular area of the sky) exceeding 3000 stars per square parsec at the center of the cluster.^[3] A measurement of central area density using the MYStIX catalog suggested approximately 10,000 stars per square parsec at the cluster center, but this study also suggested that such densities are not unusual for massive star-forming regions.^[22] The spatial distribution of stars has been described as a King profile^[3] or alternatively as a "core-halo" structure.^[23]

Stellar density near the center of RCW 36 has been estimated to be approximately 300,000 stars per cubic parsec (or 10,000 stars per cubic light year).^[24] In contrast, the density of stars in the Solar neighborhood is only 0.14 star per cubic parsec,^[25][26] so the density of stars at the center of RCW 36 is about 2 million times greater. It has been calculated that for young stellar clusters with more than 10⁴ stars pc.⁻³ close encounters between stars can lead to interactions between protoplanetary disks that affect developing planetary systems.^[27]

Young stellar objects

Several special types of young stellar object have been identified in RCW 36, and are described in more detail below. The properties of these stars are related to their extreme youth.

Two stars in RCW 36 have Herbig-Haro jets (HH 1042 and HH 1043).^[28] Jets of gas flowing out from young stars can be produced by accretion onto a star.^[29] In RCW 36 these jets were seen in a number of spectral lines, including lines from hydrogen, helium, oxygen, nitrogen, sulfur, nickel, calcium, and iron. Mass loss rates from the jets have been estimated to be on the order of 10⁻⁷ M_☉ solar masses per year. Inhomogeneities in the jets have been attributed to variable accretion rate on timescales of approximately 100 years.^[28]

The young star 2MASS J08592851-4346029 has been classified as a Herbig Ae star. Stars in this class are pre-main-sequence, intermediate-mass stars (spectral type A) with emission lines in their spectra from hydrogen. Observations indicate that 2MASS J08592851-4346029 has a bloated radius as would be expected for a young star that is still contracting. Some of the lines in its spectrum have a P-Cygni Profile indicating the presence of a stellar wind.^[4]

The young star CXOANC J085932.2−434602 was observed by the Chandra X-ray Observatory to have produced a large flare with a peak temperature greater than 100 million kelvins.^[30] Such "super hot" flares from young stars have been seen in other star-forming regions like the Orion Nebula.^[31]

See also

Wikimedia Commons has media related to RCW 36.

1. ^ ^{a b c d} Pettersson, Bertil (2008). "Young Stars and Dust Clouds in Puppis and Vela". In Reipurth, B. (ed.). Handbook of Star Forming Regions, Volume II: The Southern Sky ASP Monograph Publications. Vol. 5. p. 43. Bibcode:2008hsf2.book..683R. ISBN 978-1-58381-670-7.
2. ^ "RCW 36". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved February 19, 2017.
3. ^ ^{a b c d e} Baba; et al. (2004). "Deep Near-Infrared Imaging toward the Vela Molecular Ridge C. I. A Remarkable Embedded Cluster in RCW 36". The Astrophysical Journal. 614 (2): 818–826. arXiv:astro-ph/0406645. Bibcode:2004ApJ...614..818B. doi:10.1086/423705. S2CID 8037661.
4. ^ ^{a b c d} Ellerbroek; et al. (2013). "RCW36: characterizing the outcome of massive star formation". Astronomy and Astrophysics. 558: A102. arXiv:1308.3238. Bibcode:2013A&A...558A.102E. doi:10.1051/0004-6361/201321752. S2CID 55117416.
5. ^ Lang, Kenneth R. (2012-12-06). Astrophysical Data: Planets and Stars. Springer Science & Business Media. ISBN 978-1-4684-0640-5.
6. ^ ^{a b} Feigelson; et al. (2013). "Overview of the Massive Young Star-Forming Complex Study in Infrared and X-Ray (MYStIX) Project". The Astrophysical Journal Supplement Series. 209 (2): 26. arXiv:1309.4483. Bibcode:2013ApJS..209...26F. doi:10.1088/0067-0049/209/2/26. S2CID 56189137.
7. ^ Ellerbroek, L. E.; et al. (2012). "The Star Formation History of RCW 36". ASP Conference Proceedings. 464: 351. arXiv:1205.1513. Bibcode:2012ASPC..464..351E.
8. ^ Carpenter (2004). "Embedded Clusters in Giant Molecular Clouds". The Formation and Evolution of Massive Young Star Clusters. 322: 319. Bibcode:2004ASPC..322..319C.
9. ^ Stahler, Steven W.; Palla, Francesco (2008). The Formation of Stars. Wiley-VCH. ISBN 978-3-527-61868-2.
10. ^ Lada; et al. (2003). "Embedded Clusters in Molecular Clouds". Annual Review of Astronomy and Astrophysics. 41: 57–115. arXiv:astro-ph/0301540. Bibcode:2003ARA&A..41...57L. doi:10.1146/annurev.astro.41.011802.094844. S2CID 16752089.
11. ^ ^{a b} Yamaguchi; et al. (1999). "A Study of Dense Molecular Gas and Star Formation toward the Vela Molecular Ridge with NANTEN". Publications of the Astronomical Society of Japan. 51 (6): 775–790. Bibcode:1999PASJ...51..775Y. doi:10.1093/pasj/51.6.775.
12. ^ Brand; et al. (1984). "CO (J = 2-1) observations of molecular clouds associated with H II regions from the southern hemisphere". Astronomy and Astrophysics. 139: 181. Bibcode:1984A&A...139..181B.
13. ^ Whiteoak; et al. (1977). "H2CO and OH observations of a molecular cloud near RCW 36". Proceedings of the Astronomical Society of Australia. 3 (2): 147–150. Bibcode:1977PASA....3..147W. doi:10.1017/S1323358000015162. S2CID 118151316.
14. ^ ^{a b} Tremblin; et al. (2014). "Ionization compression impact on dense gas distribution and star formation. Probability density functions around H II regions as seen by Herschel" (PDF). Astronomy and Astrophysics. 564: A106. arXiv:1401.7333. Bibcode:2014A&A...564A.106T. doi:10.1051/0004-6361/201322700. S2CID 316729.
15. ^ Hill; et al. (2011). "Filaments and ridges in Vela C revealed by Herschel: from low-mass to high-mass star-forming sites". Astronomy and Astrophysics. 533: A94. arXiv:1108.0941. Bibcode:2011A&A...533A..94H. doi:10.1051/0004-6361/201117315. S2CID 17662468.
16. ^ Hill; et al. (2012). "Resolving the Vela C ridge with P-ArTeMiS and Herschel". Astronomy and Astrophysics. 548: L6. arXiv:1211.0275. Bibcode:2012A&A...548L...6H. doi:10.1051/0004-6361/201220504. S2CID 118376263.
17. ^ Minier; et al. (2013). "Ionisation impact of high-mass stars on interstellar filaments. A Herschel study of the RCW 36 bipolar nebula in Vela C". Astronomy and Astrophysics. 550: A50. Bibcode:2013A&A...550A..50M. doi:10.1051/0004-6361/201219423.
18. ^ Walsh; et al. (1998). "Studies of ultracompact HII regions – II. High-resolution radio continuum and methanol maser survey". Monthly Notices of the Royal Astronomical Society. 301 (3): 640–698. Bibcode:1998MNRAS.301..640W. doi:10.1046/j.1365-8711.1998.02014.x.
19. ^ Verma; et al. (1994). "Far-infrared observations of three galactic star-forming regions: RCW 36, IRAS 10361-5830 and IRAS 10365-5803". Astronomy and Astrophysics. 284: 936. Bibcode:1994A&A...284..936V.
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24. ^ Kuhn; et al. (2015). "The Spatial Structure of Young Stellar Clusters. III. Physical Properties and Evolutionary States". The Astrophysical Journal. 812 (2): 131. arXiv:1507.05653. Bibcode:2015ApJ...812..131K. doi:10.1088/0004-637X/812/2/131. S2CID 54687747.
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26. ^ Max-Planck-Institut für Astronomie (2002) [October 9–13, 2000]. Eva K. Grebel; Wolfgang Brandner (eds.). Modes of star formation and the origin of field populations: proceedings of a workshop. Astronomical Society of the Pacific conference series. Vol. 285. Max-Planck Institute of Astronomy, Heidelberg, Germany: Astronomical Society of the Pacific. p. 165. ISBN 1-58381-128-1.
27. ^ Gutermuth; et al. (2005). "The Initial Configuration of Young Stellar Clusters: A K-Band Number Counts Analysis of the Surface Density of Stars". The Astrophysical Journal. 632 (1): 397–420. arXiv:astro-ph/0410750. Bibcode:2005ApJ...632..397G. doi:10.1086/432460. S2CID 421304.
28. ^ ^{a b} Ellerbroek; et al. (2013). "The outflow history of two Herbig-Haro jets in RCW 36: HH 1042 and HH 1043". Astronomy and Astrophysics. 551: A5. arXiv:1212.4144. Bibcode:2013A&A...551A...5E. doi:10.1051/0004-6361/201220635. S2CID 216080264.
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