Westerhout 40


Westerhout 40 or W40 (also designated Sharpless 64, Sh2-64, or RCW 174) is a star-forming region in the Milky Way located in the constellation Serpens. In this region, interstellar gas forming a diffuse nebula surrounds a cluster of several hundred new-born stars.[2][4][5] The distance to W40 is 436 ± 9 pc (1420 ± 30 light years),[1] making it one of the closest sites of formation of high-mass O-type and B-type stars.[6] The ionizing radiation from the massive OB stars has created an H II region,[7] which has an hour-glass morphology.[4]

W 40
The Spitzer Space Telescope's view of W40.jpg
W40 as seen in the infrared by the Spitzer Space Telescope.
Object typeH ii region, star cluster edit this on wikidata
Other designationsW40, Sh2-64, RCW 174, LBN 90
Observation data
(Epoch J2000)
ConstellationSerpens Cauda
18 31 29
Declination-02 05.4
Distance1420±30 ly[1] / 436±9 pc

In visual light (V)
8 arcminutes[2]

Estimated age0.8–1.5 Myr[3]
Related media on Wikimedia Commons

Dust from the molecular cloud in which W40 formed obscures the nebula, rendering W40 difficult to observe at visible wavelengths of light.[2][8] Thus, X-ray, infrared, and radio observations have been used to see through the molecular cloud to study the star-formation processes going on within.[2][9][10]

W40 appears near to several other star-forming regions in the sky, including an infrared dark cloud designated Serpens South[11] and a young stellar cluster designated the Serpens Main Cluster.[12] Similar distances measured for these three star-forming regions suggests that they are near to each other and part of the same larger-scale collection of clouds known as the Serpens Molecular Cloud.[1]

On the SkyEdit

The W40 star-forming region is projected on the sky in the direction of the Serpens-Aquila Rift, a mass of dark clouds above the Galactic plane in the constellations Aquila, Serpens, and eastern Ophiuchus.[13] The high extinction from interstellar clouds means that the nebula looks unimpressive in visible light, despite being one of the nearest sites of massive star formation.

Location of W 40 on the sky

Star Formation in W40Edit

Like all star-forming regions, W40 is made up of several components: the cluster of young stars and the gaseous material from which these stars form (the interstellar medium). Most of the gas in W40 is in the form of molecular clouds, the coldest, densest phase of the interstellar medium, which is made up of mostly molecular hydrogen (H2).[14] Stars form in molecular clouds when the gas mass in part of a cloud becomes too great, causing it to collapse due to the Jeans instability.[15] Stars usually do not form in isolation, but rather in groups containing hundreds or thousands of other stars,[16] as is the case of W40.

In W40, feedback from the star cluster has ionized some of the gas and blown a bipolar bubble in the cloud around the cluster.[4] Such feedback effects may trigger further star-formation but can also lead to the eventual destruction of the molecular cloud and an end of star-formation activity.[17]

Star clusterEdit

A cluster of young stars lies at the center of the W40 HII region containing approximately 520 stars[2][18] down to 0.1 solar masses (M). Age estimates for the stars indicate that the stars in the center of the cluster are approximately 0.8 million years old, while the stars on the outside are slightly older at 1.5 million years.[3] The cluster is roughly spherically symmetric and is mass segregated, with the more massive stars relatively more likely to be found near the center of the cluster.[2] The cause of mass segregation in very young star clusters, like W40, is an open theoretical question in star-formation theory because timescales for mass segregation through two-body interactions between stars are typically too long.[19][20]

The cloud is ionized by several O and B-type stars.[21] Near-infrared spectroscopy has identified one late-O type star named IRS 1A South, and 3 early B-type stars, IRS 2B, IRS 3A, and IRS 5. In addition, IRS 1A North and IRS 2A are Herbig Ae/Be stars.[6] Radio emission from several of these stars is observed with the Very Large Array, and may be evidence for ultra-compact H II regions.[22]

Excess light in the infrared indicates that a number of stars in the cluster have circumstellar disks, which may be in the process of forming planets.[2] Millimeter observations from the IRAM 30m telescope show 9 Class-0 protostars in the Serpens South region and 3 Class-0 protostars in W40,[23] supporting the view that the region is very young and actively forming stars.

Interstellar mediumEdit

W40 lies in a molecular cloud with an estimated mass of 104 M.[4] The core of the molecular cloud has a shape like a shepherd's crook and is currently producing new stars.[23][24] The cluster of OB and pre–main-sequence (PMS) stars lies just eastward of the bend in this filament. The cloud core was also observed in radio light produced by CO, which allows the mass of the core to be estimated at 200–300 M. A weak, bipolar outflow of gas flows out of the core, likely driven by a young stellar object, with two lobes differing in velocity by 0.5 km/s.[25]

View of the core of the molecular cloud as seen by Herschel/SPIRE at 500 µm. Overlaid (white circles) are young stars detected by the Chandra X-ray Observatory.[26][27]

It was in this region where the striking prevalence of filamentary cloud structures seen by ESA's Herschel Space Observatory was first noted.[28] These filaments of cloud have dense "cores" of gas embedded within them—many of which are likely to gravitationally collapse and form stars. The Herschel results for this region, and subsequently reported results for other star-forming regions, imply that fragmentation of molecular-cloud filaments are fundamental to the star-formation process. The Herschel results for W40 and the Aquila Rift, compared to those for molecular clouds in the Polaris region, suggest that star-formation occurs when the linear density (mass per unit length) exceeds a threshold making them susceptible to gravitational instability. This accounts for the high star-formation rate in W40 and the Aquila Rift, in contrast to the low star-formation rate in the Polaris clouds. These observational results complement computer simulations of star-formation, which also emphasize the role that molecular-cloud filaments play in the birth of stars.[29]

Observations by the space-based Chandra X-ray Observatory have shown a diffuse X-ray glow from the H II region, which is likely due to the presence of a multi-million Kelvin plasma.[2][30] Such hot plasmas can be produced by winds from massive stars, which become shock heated.


See alsoEdit


  1. ^ a b c Ortiz-León, G. N.; et al. (2016). "The Gould's Belt Distances Survey (GOBELINS) III. The distance to the Serpens/Aquila Molecular Complex". Astrophysical Journal. 834 (2): 143. arXiv:1610.03128. Bibcode:2017ApJ...834..143O. doi:10.3847/1538-4357/834/2/143. S2CID 10802135.
  2. ^ a b c d e f g h i Kuhn, M. A.; et al. (2010). "A Chandra Observation of the Obscured Star-forming Complex W40". Astrophysical Journal. 725 (2): 2485–2506. arXiv:1010.5434. Bibcode:2010ApJ...725.2485K. doi:10.1088/0004-637X/725/2/2485. S2CID 119192761.
  3. ^ a b Getman, K. V.; et al. (2014). "Age Gradients in the Stellar Populations of Massive Star Forming Regions Based on a New Stellar Chronometer". Astrophysical Journal. 787 (2): 108. arXiv:1403.2741. Bibcode:2014ApJ...787..108G. doi:10.1088/0004-637X/787/2/108. S2CID 118626928.
  4. ^ a b c d Rodney, S. A.; Reipurth, B. (2008). "The W40 Cloud Complex". 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.
  5. ^ Mallick, K. K.; et al. (2013). "The W40 Region in the Gould Belt: An Embedded Cluster and H II Region at the Junction of Filaments". Astrophysical Journal. 779 (2): 113. arXiv:1309.7127. Bibcode:2013ApJ...779..113M. doi:10.1088/0004-637X/779/2/113. S2CID 118353815.
  6. ^ a b Shuping, R. Y.; et al. (2012). "Spectral Classification of the Brightest Objects in the Galactic Star-forming Region W40". Astronomical Journal. 144 (4): 116. arXiv:1208.4648. Bibcode:2012AJ....144..116S. doi:10.1088/0004-6256/144/4/116. S2CID 119227485.
  7. ^ Vallee, J. P. (1987). "The warm C II region between the hot ionized region S 64 = W 40 and the cold molecular cloud G 28.74 + 3.52". Astronomy & Astrophysics. 178: 237. Bibcode:1987A&A...178..237V.
  8. ^ a b Hagenauer, Beth; Veronico, Nicholas (Nov 21, 2011). "NASA'S SOFIA Airborne Observatory Views Star Forming Region W40" (Press release). Moffett Field, CA. NASA. Archived from the original on March 21, 2016. Retrieved Mar 8, 2015.
  9. ^ Rumble, D.; et al. (2016). "The JCMT Gould Belt Survey: evidence for radiative heating and contamination in the W40 complex". Monthly Notices of the Royal Astronomical Society. 460 (4): 4150–4175. arXiv:1605.04842. Bibcode:2016MNRAS.460.4150R. doi:10.1093/mnras/stw1100.
  10. ^ Shimoikura, T.; et al. (2015). "Dense Clumps and Candidates for Molecular Outflows in W40". The Astrophysical Journal. 806 (2): 201. arXiv:1505.02486. Bibcode:2015ApJ...806..201S. doi:10.1088/0004-637X/806/2/201. S2CID 118440764.
  11. ^ Gutermuth, R. A.; et al. (2008). "The Spitzer Gould Belt Survey of Large Nearby Interstellar Clouds: Discovery of a Dense Embedded Cluster in the Serpens-Aquila Rift". Astrophysical Journal. 673 (2): L151–L154. arXiv:0712.3303. Bibcode:2008ApJ...673L.151G. doi:10.1086/528710. S2CID 339753.
  12. ^ "NAME Serpens Cluster". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 24 February 2017.
  13. ^ Straižys, V.; et al. (1996). "Interstellar extinction in the area of the Serpens Cauda molecular cloud". Baltic Astronomy. 5 (1): 125–147. Bibcode:1996BaltA...5..125S. doi:10.1515/astro-1996-0106.
  14. ^ Zeilik, M. II; Lada, C. J. (1978). "Near-infrared and CO observations of W40 and W48". The Astrophysical Journal. 222: 896. Bibcode:1978ApJ...222..896Z. doi:10.1086/156207.
  15. ^ Stahler, Steven W.; Palla, Francesco (2008). The Formation of Stars. Wiley-VCH. ISBN 978-3-527-61868-2. Archived from the original on 2017-01-26. Retrieved 2017-02-24.
  16. ^ 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.
  17. ^ Pirogov; et al. (2015). "The region of triggered star formation W40: Observations and model". Astronomy Reports. 59 (5): 360–365. arXiv:1503.08010. Bibcode:2015ARep...59..360P. doi:10.1134/S1063772915050078. S2CID 118412224.
  18. ^ Kuhn, M. A.; Getman, K. V.; Feigelson, E. D. (2015). "The Spatial Structure of Young Stellar Clusters. II. Total Young Stellar Populations". Astrophysical Journal. 802 (1): 60. arXiv:1501.05300. Bibcode:2015ApJ...802...60K. doi:10.1088/0004-637X/802/1/60. S2CID 119309858.
  19. ^ Küpper, A. H. W.; et al. (2011). "Mass segregation and fractal substructure in young massive clusters - I. The McLuster code and method calibration". Monthly Notices of the Royal Astronomical Society. 417 (3): 2300–2317. arXiv:1107.2395. Bibcode:2011MNRAS.417.2300K. doi:10.1111/j.1365-2966.2011.19412.x. S2CID 119259635.
  20. ^ Krumholz, M. R. (2014). "The Big Problems in Star Formation: the Star Formation Rate, Stellar Clustering, and the Initial Mass Function". Physics Reports. 539 (2): 49–134. arXiv:1402.0867. Bibcode:2014PhR...539...49K. doi:10.1016/j.physrep.2014.02.001. S2CID 119230647.
  21. ^ Smith, J.; et al. (1985). "Infrared sources and excitation of the W40 complex". Astrophysical Journal. 291: 571–580. Bibcode:1985ApJ...291..571S. doi:10.1086/163097.
  22. ^ Rodríguez, L. F.; et al. (2011). "A Cluster of Compact Radio Sources in W40". Astronomical Journal. 140 (4): 968–972. arXiv:1007.4974. Bibcode:2010AJ....140..968R. doi:10.1088/0004-6256/140/4/968. S2CID 14827799.
  23. ^ a b Maury, A. J.; et al. (2011). "The formation of active protoclusters in the Aquila rift: a millimeter continuum view". Astronomy & Astrophysics. 535: 77. arXiv:1108.0668. Bibcode:2011A&A...535A..77M. doi:10.1051/0004-6361/201117132. S2CID 119285813.
  24. ^ Pirogov, L. (2013). "Molecular line and continuum study of the W40 cloud". Monthly Notices of the Royal Astronomical Society. 436 (4): 3186–3199. arXiv:1309.6188. Bibcode:2013MNRAS.436.3186P. doi:10.1093/mnras/stt1802.
  25. ^ Zhu, L.; et al. (2006). "A Study of the Molecular Cloud S64 with Multiple Lines of CO Isotopes". Chinese Journal of Astronomy and Astrophysics. 6 (1): 61–68. Bibcode:2006ChJAA...6...61Z. doi:10.1088/1009-9271/6/1/007.
  26. ^ Feigelson, E. D.; et al. (2013). "Overview of the Massive Young Star-Forming Complex Study in Infrared and X-Ray (MYStIX) Project". Astrophysical Journal Supplement. 209 (2): 26. arXiv:1309.4483. Bibcode:2013ApJS..209...26F. doi:10.1088/0067-0049/209/2/26. S2CID 56189137.
  27. ^ Broos, P. S.; et al. (2013). "Identifying Young Stars in Massive Star-forming Regions for the MYStIX Project". Astrophysical Journal Supplement. 209 (2): 32. arXiv:1309.4500. Bibcode:2013ApJS..209...32B. doi:10.1088/0067-0049/209/2/32. S2CID 67827240.
  28. ^ a b André, Ph.; et al. (2010). "From filamentary clouds to prestellar cores to the stellar IMF: Initial highlights from the Herschel Gould Belt Survey". Astronomy & Astrophysics. 518: 102. arXiv:1005.2618. Bibcode:2010A&A...518L.102A. doi:10.1051/0004-6361/201014666. S2CID 248768.
  29. ^ Bate, M. R.; et al. (2003). "The formation of a star cluster: predicting the properties of stars and brown dwarfs". Monthly Notices of the Royal Astronomical Society. 339 (3): 577–599. arXiv:astro-ph/0212380. Bibcode:2003MNRAS.339..577B. doi:10.1046/j.1365-8711.2003.06210.x. S2CID 15346562.
  30. ^ Townsley, L. K.; et al. (2014). "The Massive Star-Forming Regions Omnibus X-Ray Catalog". Astrophysical Journal Supplement. 213 (1): 1. arXiv:1403.2576. Bibcode:2014ApJS..213....1T. doi:10.1088/0067-0049/213/1/1. S2CID 76658453.
  31. ^ Povich, M. S.; et al. (2013). "The MYStIX Infrared-Excess Source Catalog". Astrophysical Journal Supplement. 209 (2): 31. arXiv:1309.4497. Bibcode:2013ApJS..209...31P. doi:10.1088/0067-0049/209/2/31. S2CID 62807763.
  32. ^ Block, A. (2013). "SH2-64". www.caelumobservatory.com. Archived from the original on November 30, 2019. Retrieved September 26, 2015.