Cepheus OB3 Region

The region of the sky where the Cepheus OB3 association is observed lies in the southeastern part of the constellation Cepheus, along the northern Milky Way, near the border with Cassiopeia; its blue stars easily merge with the rich background star fields visible in this direction. The brightest components are of visual magnitude 7 and 8 and cannot be seen with the naked eye, but a pair of binoculars is sufficient to observe them, provided they can be distinguished among the numerous stars of similar brightness visible in this direction. The brightest one, HD 217312, is of magnitude 7.42.^[3]

The declination of the association is strongly northern, favouring its observation from the Northern Hemisphere, where it is circumpolar down to low latitudes; from the Southern Hemisphere, the observation period is therefore very limited, and even the areas where it is visible are restricted to the tropical belt. The suitable months for observing it in the evening sky are between July and December.^[a][3]

Cepheus OB3 contains about forty young, bright stars,^[4] among which three blue main-sequence stars, two blue main-sequence stars, and one blue giant of class B0III stand out. Among these, the brightest in terms of absolute magnitude are HD 217086, of class O7V and magnitude −4.9, and the aforementioned giant, of magnitude −4.8.^[5] Photometric studies conducted in the 1990s refined this list of components, extending it to fainter stars.^[3] It should be noted, however, that a 2005 study indicates that two of the O-class components listed as belonging to the association might be part of a more distant open cluster, designated ASCC 1125, with an age of 10 million years and located 1,500 parsecs from the Sun.^[6]

In this association there is evidence of a division of its member stars into two subgroups, catalogued as Cepheus OB3a and Cepheus OB3b, with age serving as the distinguishing characteristic: Cepheus OB3a would in fact be the younger subgroup, with an age initially estimated at about 4 million years, while the second would have about 8 million years;^[7] with the refinement of detection techniques, an age of 5.5 million years for the former and 7.5 million years for the latter was subsequently indicated.^[3] The brightest stars of the Cepheus OB3b subgroup excite the gases of a nearby nebula, the ionized hydrogen region Sh2-155 (the famous Cave Nebula).^[7] According to a study conducted using data from the Hipparcos satellite, it has been suggested that the runaway star Lambda Cephei may originate from this association, rather than from the adjacent Cepheus OB2.^[8]

In the Cepheus OB3 association, over fifty X-ray sources are known, many of which were detected by ROSAT; it is suspected that most of them originate from T Tauri stars located in the regions surrounding the molecular cloud (but not inside it).^[9] Also in X-rays, using the Chandra X-ray Observatory, two very rich open clusters were discovered in the 2000s, composed of 321 pre-main-sequence stars; one of these extends outside the molecular cloud and is part of the Cepheus OB3b association. These observations suggest that the X-ray luminosity function of this association differs from that obtained for other similar clusters, such as that in the Orion Nebula: indeed, in Cepheus OB3b there are more stars with masses below 0.3 M_☉.^[10]

Through mapping in ¹²CO of the association's area, a molecular nebular complex extending 20×60 parsecs has been identified since the 1970s, within which several denser nebular areas can be distinguished, catalogued with progressive letters from Cepheus A to Cepheus F; in some of these, especially the first one, star formation would be very active, caused by the interaction of the cloud itself with the expanding H II region Sh2-155.^[11] Cepheus OB3 can therefore be considered an example of sequential star formation.^[12]

Cepheus A is one of the most studied nebular regions in the sky: it is an extremely active high-mass star-forming region, within which several thermal and non-thermal sources are known, some small H II regions, and strong infrared radiation emissions, all in an area of sky smaller than an arcminute.

The cloud Cepheus B is instead located in front of the nebula Sh2-155, whose ionization is due to the brightest components of the Cepheus OB3b subgroup; this nebula is the most active in the region and it is believed that the formation of the third generation of stars within Cepheus OB3 is taking place here. Studies in the radio continuum have shown that the compact ionized region located between Sh2-155 and the Cepheus B cloud is divided into four distinct regions; the most extensive of these, called the Ridge, is excited by the ionization front coming from very luminous young stellar objects. The other three parts, on the other hand, hide protostars still well wrapped in the cocoons in which they are forming.^[13]

Associated with Sh2-155 might also be the dark nebula LDN 1211, which contains within it a bipolar jet structure originating from an infrared source and a small compact Herbig–Haro object, HH 363, plus three other infrared sources.^[14] In total, around fifty infrared sources catalogued by IRAS^[15] and 24 masers are known in the region around Sh2-155, of which six with OH emissions, fourteen with H₂O emissions, and four with CH₃OH emissions.^[16]