Draft:Cepheus OB4 Region

The Cepheus OB4 association is located in the eastern part of the constellation Cepheus, along the northern Milky Way, near the border with Cassiopeia. Its brightest components are of visual magnitude 9 and are easily confused with those of the background star fields, nor can they be seen with the naked eye; however, a pair of binoculars is sufficient to notice them, in the region around the large nebula Ced 214. The brightest of them, BD+66 1675, has a magnitude of 9.05. The association is centered around the open cluster Berkeley 59, which consists of about twenty stars of magnitude between 9 and 13.^[2]

The association's declination is strongly northern, favoring 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 its observation in the evening sky are between July and December.^[a][2]

The Cepheus OB4 association was discovered in 1959, with the identification of 16 young and hot stars within a small region of sky at galactic coordinates l=118°, b=+4°, all centered around the open cluster Berkeley 59; it appears connected to a very dense and irregular dark nebula containing some emission regions, among which stands out the large H II region Ced 214 (W1), whose northernmost extremity bears the designation NGC 7822.^[5] The association would be composed of 42 blue giants, located at a distance of 840 parsecs (2740 light-years) from the Sun; there is also a correlation between the luminosity and the reddening of these components: in fact, the O and B spectral type stars of Cepheus OB4 seem to be located only in the inner regions of the cloud, while the more whitish B-type components appear to be outside, although this may be mainly due to the incompleteness of surveys conducted in this region.^[6] The age of the association's stars is estimated to be between 0.6 and 6 million years.^[6]

The open cluster Berkeley 59 constitutes the most massive stellar concentration of the association and contains nine blue stars with spectral classes ranging from O7 to B3, among which stands out BD+66 1673, one of the hottest stars located within a radius of 1000 parsecs from the Sun; it is an eclipsing binary (with designation V747 Cephei^[7]) in which the primary star, of class O5V, has a surface temperature of 45000 K and a luminosity of 100,000 L_⊙.^[4]

Cepheus OB4, along with the nearby Cepheus OB2 and Cepheus OB3 regions, consists of two areas with different dynamic and evolutionary properties: the older and more dispersed of these extends over an area of 15' (about 4 parsecs) in diameter and is located on the southern edge of a circular nebular structure called the Cepheus Loop; through the proper motion of its components, its possible formation point has been identified, located near the center of what is now the Cepheus Loop. The Cepheus Loop would thus have been originated by the stellar wind of the stars in the Cepheus OB2 association and by the explosion of one or more supernovae contained within it, subsequently expanding until it encountered other molecular clouds; from the outcome of these collisions, the open cluster Berkeley 59 would then have formed.^[8]

By studying the morphology and dynamics of the H II regions associated with Cepheus OB4 at various emission lines, the existence of two expanding shell structures was discovered: one of these, with a radius of 0.7°, contains the nebulae Ced 214 and NGC 7822, plus most of the association's stars, whose stellar wind promotes its expansion; the other structure, with a radius of 1.5°, is centered on the second nebula and could be the result of a supernova explosion or the action of the stellar wind from the most massive stars.^[5] By investigating instead the spatial distribution and dynamic properties of the interstellar medium in the area, a further bubble structure was discovered, whose center falls at galactic coordinates l=122°, b=+10°; given its distance of 800 parsecs, it was calculated that the bubble's radius should be about 100 parsecs, with an expansion velocity of 0.4 km/s and a neutral hydrogen mass of 99,000 M_☉.^[9]

Within the dark clouds present in this region, several sources with Hα emission lines have been identified, some of which would be young T Tauri stars. Almost all of the star formation phenomena in this region are concentrated around the Ced 214 nebula.^[6]

The intense ultraviolet radiation from the stars of Cepheus OB4 is responsible for the ionization of the gas throughout the nebula and produces an ionization front that extends over the surface of two dense molecular clouds, identified through their CO emissions;^[10] the compression resulting from the shock wave of the front destabilizes the internal equilibrium of the clouds, causing them to collapse at multiple points. For this reason, the Ced 214 complex is a site of great importance for studying the processes of low-mass star formation stimulated by the action of nearby massive stars.^[11]

In the region of the Berkeley 59 cluster, 48 stars with Hα emissions have been identified, as well as four young stellar objects that exhibit emissions in the near infrared, a sign that they are still deeply embedded in the cloud from which they are forming; while the age of the cluster's massive stars is on average between 1 and 2 million years, that of the young stellar objects is less than 1 million years, with the exception of a single star. Also, over 30% of the Hα stars exhibit an excess of infrared emissions, indicating that their inner circumstellar disc has not yet dissipated.^[11]

Also in the vicinity of the cluster, two concentrations, designated C1 and C2 (from the English clump), have been identified. The first contains a dense bright-rimmed cloud (BRC) in which star formation processes are caused by collapse due to the shock wave of the ionization front.^[12] In both concentrations, infrared emissions are present, also cataloged by IRAS.^[11][13] The mass of the young forming stars is generally between 0.8 and 3 M_⊙, although the components located in the central region of Berkeley 59 tend to be on average the most massive, and six of these have a mass greater than 2 M_⊙; by contrast, in the westernmost part of the complex there is only one star with a mass greater than 2 M_⊙.^[11]

In the northern part of the nebular complex, jagged structures called elephant trunks have been identified, that is, globules of matter being eroded by the strong stellar wind coming from the massive stars of Berkeley 59.^[14]

• OB association
• Cepheus (constellation)
• H II region