The Sagittarius OB7 region comprises several extended ionized gas nebulae of various sizes linked to a small association of young and hot stars, the OB association Sagittarius OB7, also known as Sagittarius R1; it is named after the constellation in which it is located, Sagittarius. The association lies in the Sagittarius Arm at a distance of about 1,700 parsecs (5,540 light-years) from the Solar System and appears associated with the nebulae Sh2-35 and Sh2-37; the large number of infrared sources and molecular jets embedded in the gases of these clouds indicate that star formation phenomena are still active in the region.
Characteristics and structure The Sagittarius OB7 region is dominated by the presence of a vast complex of giant molecular clouds, clearly visible thanks to the background star field, which is obscured by them; this complex has three entries in the LDN catalogue, which thus treats them as separate clouds: LDN 291, LDN 314 and LDN 315. However, identifying these three clouds as distinct objects is rather difficult; therefore they are often referred to by the single designation LDN 291, which gives its name to the entire complex. The stellar components of the association were identified in 1978. The brightest is HD 167264, also known as 15 Sagittarii; it is a blue supergiant of spectral class B0Ia and apparent magnitude 5.38. It is followed by the blue giant HD 167263 (16 Sagittarii), of class O9.5II-III and magnitude 5.98, BD-20°5053, of class O6 and magnitude 9.52, and BD-20°5060, of class B0IV and magnitude 8.84. Among the bright nebulae linked to the association, IC 1284, also known as Sh2-37, stands out; its light filters through a kind of window that opens in the dark nebula complex, making it appear distinctly separated from the surrounding star fields. Sh2-35, on the other hand, appears as a long and faint diffuse nebulosity whose northeastern edge is well traced by the overlapping dark clouds. Also linked to the brightest stars are several reflection nebulae, cataloged as vdB 118 and vdB 119; their presence is the reason for the nomenclature, sometimes used for the association, of Sagittarius R1. It is believed that the sources of ionization for the gases of the emission nebulae include not only the two blue giants but also some sources deeply embedded within the gas and obscured. Although there are some uncertainties regarding the distance to the complex, the generally accepted value is approximately 1700 parsecs (5540 light-years); this estimate appears to be a compromise between the value calculated by Humphreys for the Sagittarius OB7 association (1750 parsecs) and the value derived from the study of the reflection nebulae (1600 parsecs). The complex measures approximately 80x20 parsecs, and its mass would exceed one million solar masses. The largest ionized cloud (Sh2-35) appears ionized by about twenty young, hot stars and coincides with a hollow structure 110 parsecs in diameter of, which is thought to have been created by the combined action of the stellar wind of the ionizing sources and the explosion of several supernovae that occurred about 3 million years ago.
Star formation phenomena Star formation phenomena within the Sagittarius OB7 region are still active, as evidenced by the presence of several infrared radiation sources, often associated with HH objects; among these, the IRAS 18162−2048 source stands out, to which the two objects HH 80/81 are associated, considered the intrinsically brightest HH objects known, and the HH 80N counterpart, detected in the radio continuum and deeply embedded in the gases. HH 80 has a very luminous core and is associated with a group of denser knots, while HH 81 appears to be in a more isolated position. The jet of the two objects is well-collimated both from the source and outside the objects themselves and extends within a network of faint currents excited by the expansion shock wave, which terminate in a large bow shock; it is believed that such structures form when material ejected by the jets expands violently into the less dense surrounding medium. A compact H II region and maser with OH and H2O emissions are also associated with the infrared source. The other eight IRAS sources identified in the region exhibit typical characteristics of Class I young stellar objects, i.e. very young protostars; these are very luminous sources that confirm the presence of massive-mass star formation processes. In addition to these sources, 22 stars with strong emissions in the Hα band are known, among which ESO Hα 285 and ESO Hα 295 stand out, probably two T Tauri stars, demonstrating that low-mass star formation phenomena is also active in the region. Others among these are likely Herbig Ae/Be stars, such as ESO Hα 292.
See also OB association Sagittarius (constellation) H II region
References
Bibliography
General texts Robert Burnham Jr. (1978). Burnham's Celestial Handbook: Volume Two [Burnham's Celestial Handbook: Volume Two]. New York: Dover Publications, Inc. Thomas T. Arny (2007). Explorations: An Introduction to Astronomy [Explorations: An Introduction to Astronomy] (3 updated ed.). Boston: McGraw-Hill. ISBN 978-0-07-321369-9. AA.VV (2002). L'Universo - Grande enciclopedia dell'astronomia [The Universe - Great encyclopedia of astronomy]. Novara: De Agostini. Gribbin, J. (2005). Enciclopedia di astronomia e cosmologia [Encyclopedia of astronomy and cosmology]. Milano: Garzanti. ISBN 88-11-50517-8. Owen, W.; et al. (2006). Atlante illustrato dell'Universo [Illustrated atlas of the Universe]. Milano: Il Viaggiatore. ISBN 88-365-3679-4.
Specific texts
On stellar evolution Lada, C. J.; N. D. Kylafits (1999). The Origin of Stars and Planetary Systems [The Origin of Stars and Planetary Systems]. Kluwer Academic Publishers. ISBN 0-7923-5909-7. De Blasi, A. (2002). Le stelle: nascita, evoluzione e morte [Stars: birth, evolution and death]. Bologna: CLUEB. ISBN 88-491-1832-5. Abbondi, C. (2007). Universo in evoluzione dalla nascita alla morte delle stelle [Evolutionary universe from the birth to the death of stars]. Sandit. ISBN 978-88-89150-32-0.
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