Scorpius–Centaurus association
The Scorpius–Centaurus association is the closest OB association to the Solar System, composed of three subgroup located at a distance of 420 light-years from the Sun. Analysis using improved Hipparcos data has brought the number of known members to 436. The cluster shows a continuous spread of stars with no apparent need for subclassification.
The Sco–Cen subgroups range in age from 11 million years to roughly 15 million years. Many of the bright stars in the constellations of Scorpius, Lupus, Centaurus, and Crux are in fact the brightest members of the Sco–Cen association, including the red supergiant star Antares, and most of the stars in the Southern Cross. Hundreds of stars have been identified as members of Sco-Cen, with masses ranging from roughly 15 solar masses down to below the hydrogen-burning limit, and the total stellar population in each of the three subgroups is probably of the order 1000–2000and the total number of stars in the association exceeds 10,000.
The stellar members of the Sco–Cen association have convergent proper motions of approximately 0.02–0.04 arcseconds per year, indicative that the stars have nearly parallel velocity vectors, moving at about 20 km/s with respect to the Sun. The dispersion of the velocities within the subgroups are only of order 1–2 km/s, and the group is most likely gravitationally unbound. Several supernovae have exploded in Sco–Cen over the past 15 million years, leaving a network of expanding gas superbubbles around the group, including the Loop I Bubble.
To explain the presence of radioactive 60Fe in deep ocean ferromanganese crusts and in biogenic magnetite crystals within Pacific Ocean sediments it has been hypothesized that a nearby supernova, possibly a member of Sco–Cen, exploded in the Sun's vicinity roughly 3 million years ago, causing the Pliocene–Pleistocene boundary marine extinction. However, other findings cite the distance at which this supernova occurred at more than 100 parsec, maintaining that it is not likely not to have contributed to this extinction through the mechanism of what is known as the ultra-violet B catastrophe.
In 2019, researchers found interstellar iron in Antarctica which they relate to the Local Interstellar Cloud, which might have been formed near the Sco-Cen association.
Observation
The Scorpius-Centaurus Association is one of the most conspicuous and easily recognizable galactic structures in the entire celestial vault, as well as, in terms of apparent size, the largest OB association visible from Earth: it extends for over fifty degrees and includes a large number of very bright blue stars, whose apparent magnitudes can reach first magnitude, making them visible even from the centers of large cities. Generally speaking, it can be said that almost all the bright stars that make up the constellations of Lupus, the northern part of Scorpius, Centaurus and the Southern Cross belong to this association, only a few stars are exceptions, such as Alpha Centauri, Gacrux, Theta and Iota Centauri, which appear in this region of the sky only due to perspective effects, as they are located in the foreground with respect to the association. The association appears tangent to the luminous trail of the Milky Way, inclined with respect to it by more than 20°.The Scorpius-Centaurus Association lies entirely in the Southern Celestial Hemisphere. The northernmost part, coinciding with the Antares region, lies at an average declination of −23° and is visible without excessive difficulty even from much of the Northern Hemisphere, the central segments, coinciding with the constellations of Lupus and Northern Centaurus, lie on average at 40°S and can only be observed from lower temperate, Mediterranean, and subtropical latitudes. The southern section, on the other hand, extends into the southernmost part of Centaurus and the Southern Cross, until it touches Musca and ends in Carina, with the bright open cluster of the IC 2602, this section is easily observable only from tropical regions, and it is no coincidence that from the Southern Hemisphere it appears circumpolar throughout the extratropical belt.
Due to its enormous extension across the starry sky, a complete and clear view of the association can only be obtained from regions in the Southern Hemisphere, where the portion of the Milky Way affected by its presence, which coincides with the southernmost part, appears high on the horizon. However, global views are also possible north of the equator, in the lower tropical zone, provided the southern horizon is completely clear of obstacles. The ideal time for its observation in the evening hours coincides with the months between March and June, its presence high in the sky during southern nights indicates the advance of the autumn season, while in the Northern Hemisphere, the northern parts of Centaurus and Scorpius appearing in the southeast indicate the imminent arrival of the summer season.
Due to the precession of the equinoxes, the south celestial pole is slowly moving towards the southwestern part of the association, between the Southern Cross and Carina, within a few thousand years, when the south celestial pole will point towards the Milky Way and the False Cross asterism, the stars of the Scorpius-Centaurus association will have reached their southernmost point. As the Earth's axis moves away from that region of the sky, the constellations of Scorpius and Centaurus will assume increasingly northerly declinations, until they even move partly north of the celestial equator.
Characterstics
The Scorpius–Centaurus association region exemplifies a medium-scale star formation process, wherein a giant molecular cloud produces stars across a broad mass spectrum—from the most massive, clustered in an OB association, to the least massive—before disintegrating under the influence of stellar winds and supernova explosions from its progeny, which accumulate, compact, and erode residual gas and dust to trigger additional, often marginal, star formation episodes. As the most prominent component of a vast complex of recent and ongoing star formation, the Sco–Cen OB association encompasses numerous nearby molecular clouds at distances of approximately 120–200 parsecs, including the Rho Ophiuchi, Pipe Nebula, Barnard 68, Chamaeleon, Lupus, Corona Australis, and Coalsack complexes, these peripheral clouds, aligned along the association's inclination relative to the Milky Way, host relatively subdued low-mass star formation and form the edges of the broader Scorpius–Centaurus complex, featuring western structures like the Chamaeleon and Coalsack Nebula, and eastern ones such as the Lupus, Corona Australis, Rho Ophiuchi, and Pipe nebula, oriented toward the galactic bulge. Surrounding Sco–Cen are several less dense young stellar groups, including the ~3–5 million-year-old Epsilon Chamaeleontis group, ~7 million-year-old Eta Chamaeleontis moving group, ~8 million-year-old TW Hydrae association, ~12 million-year-old Beta Pictoris moving group, and possibly the ~30–50 million-year-old IC 2602 open cluster.The three large subregions into which the association appears to be divided are almost completely devoid of interstellar gas, which has been completely swept away, the stars present here also show a very low extinction rate, an indication of the low obscuration due to dust. A complex ring-like structure has thus formed around the association, a low-density bubble whose edges are composed of molecular hydrogen, whose mass is around 300,000 and could in fact coincide with what remains of the large molecular cloud from which the association itself originated.
Compared to other regions of the Galaxy of more or less contemporary origin, the Scorpius-Centaurus association hosts a higher percentage of double or multiple systems, up to a rate 1.16 times higher than the average, this percentage increases significantly if low-mass pre-main sequence stars, such as red dwarfs, are considered. A study conducted on almost 200 members of the first spectral classes located mainly in the northernmost part of the association, showed the presence of 176 companion stars, of which at least eighty are physically linked to the major stars around which they have been observed. On average, the masses of these minor stellar and substellar components identified vary from 0.03 to 1.2.
Evolution
The dynamics that led to the formation of the various subgroups of the association are very complex and partly still unknown, in particular, the processes that took place in the northern part of the association, visible in the direction of Scorpius, are known, while those that generated the two southernmost subgroups remain almost completely unknown. It is known that the age of the stars in the association increases as one proceeds in a westerly direction, a sign that the oldest star formation phenomena took place mainly in the region occupied by the constellation of Centaurus, it is also known that the stars located north of the galactic equator have a younger age than those located to the south. Furthermore, the southern part appears to be at a distance of 109 parsecs, slightly closer than the northern part, located at 123 parsecs.According to some very simplified models, star formation would have initially taken place in the northern part of the upper Centaurus-Lupus group, about 17 million years ago, and would have then extended southwards, to the lower Centaurus-Crux group, reaching its peak about 12 million years ago; the new stars would have been initially concentrated in small clusters and filaments surrounded by gas, containing tens or hundreds of stars. The residual gas of the progenitor molecular cloud would have subsequently been swept away by the combined action of the stellar wind and the supernova explosion of the most massive components, which completed their life cycle very quickly. About 6 million years ago these generative processes extended to the clouds located south of the galactic equator, in particular in the region of the Chameleon Cloud and of the stars Epsilon and Eta Chamaeleontis.
Starting from 12 million years ago, the bubble originated by the wind emitted by the young stars of the upper Centaurus-Lupus group began its expansion, perhaps further accelerated by the explosion of some supernova at a later time. About 5 million years ago the great pressure generated by the expansion front of the bubble compressed the molecular cloud located in correspondence with the current group of stars that form the head of Scorpius, generating the youngest part of the association, the upper Scorpius group, also catalogued as Scorpius OB2. Through the knowledge of the physical distance between the two groups of the association, equal to about 60 parsecs, it has been calculated that the expansion velocity of this bubble was about 25 km/s.
The burst of star formation in the Northern Scorpius Cloud generated a total of approximately 2,500 stars, including some particularly massive ones with masses exceeding 10. These stars rapidly evolved and subsequently exploded as supernovae, generating, among others, the pulsar PSR J1932+1059. The powerful shock wave generated by these explosions almost completely swept away the residual gas of the ancient molecular cloud, whose remnants are visible in the delicate filaments known as Sh 2-1 and Sh 2-7. The shock wave also impacted the adjacent Rho Ophiuchi Cloud over the last million years, fueling the intense star formation activity in this region that can still be observed today. The same shock wave may also be responsible for the initiation of stellar genesis phenomena in the Lupus Molecular Cloud, where the oldest stellar components show an age less than 1 million years.
Structure
The Scorpius-Centaurus association is traditionally divided into three groups, which show slightly different ages and characteristics. The northern section is called Upper Scorpius and includes all the blue stars in the northwestern part of Scorpius, including Antares, the central section, the largest, is called Upper Centaurus-Lupus and includes almost all the stars of Lupus and most of the northern and central stars of Centaurus. The southernmost part of the association is called Lower Centaurus-Crux, this section lies in the wake of the Milky Way and includes the southern part of Centaurus with the exception of Alpha Centauri, and the Southern Cross, excluding Gacrux. The southwestern edge of the association coincides with the bright cluster of the Southern Pleiades.Upper Scorpius association
The Upper Scorpius Association is the youngest part of the Scorpius-Centaurus Association, it formed about 5 million years ago and contains about 120 massive stellar components dispersed over a 35 parsec region that is about 145 parsecs from the Sun. The spectral class of these components varies between B0.5 and G5 and most of them lie on the main sequence, although there are some stars already in an advanced evolutionary phase, among which the red supergiant Antares stands out. Despite the young age of the group, no star formation is currently taking place in the region, which is instead active in the adjacent Rho Ophiuchi cloud. The original most massive components have already completed their life cycle and have already exploded as supernovae, Among these, the most massive was probably the progenitor of the pulsar PSR J1932+1059, which exploded about 1.5 million years ago, and whose original mass was probably around 50. Although it was speculated that this star was the larger companion of the runaway star Zeta Ophiuchi, subsequent measurements suggest that they are two stars separated since their origin.In addition to the high-mass stars, several hundred low and medium mass stars, as well as some T Tauri stars, have been discovered, however, some of these stars are not physically related to the association, but are part of the Rho Ophiuchi star-forming region. A portion of the low-mass objects may fall into the category of brown dwarfs: their masses are in fact between 0.3 and 0.007 M⊙, and a dozen of these have a mass equal to or less than 15 Jupiter masses.
In December 2021, around 70 new rogue planets were discovered in the Upper Scorpius association.
Upper Centaurus-Lupus association
The Upper Centaurus-Lupus association constitutes the central body of the association, it includes hundreds of large and medium mass stellar components, many of which are clearly visible even to the naked eye, which make up almost the entire constellation of Lupus and part of Scorpius and Centaurus. Unlike the Upper Scorpius association, the study of this large group of stars is made more difficult by several factors: first, its position much closer to the galactic plane, which makes it more difficult to correctly determine the group's true physical components with respect to the background stars, added to this is the greater dispersion of its stellar components and the almost total absence of pre-main sequence stars shrouded in nebulosity, due to the group's greater age. Finally, its declination means that the part of the sky in which the group lies is clearly visible and studyable only by observers located at southern or, at least, tropical latitudes. The parallax measurement of the UCL components indicates a distance between approximately 90 and 200 parsecs, this large range is due to the great depth extension of the group itself, therefore some components appear to be closer and others further away.The most massive components of the group belong to the spectral class B and have an apparent magnitude between 2.0 and 3.5 on average. The easternmost components are found in the central part of Scorpius, in particular the famous optical pair formed by the stars Mu¹ and Mu² Scorpii, among the most massive components of the group, Alpha and Beta Lupi stand out. These are mostly blue giants and blue subgiants, while, unlike the upper Scorpius group, there are no components of the lower spectral classes. This absence can be explained by the older age of the upper Centaurus-Lupus group, estimated at around 17 million years: the most massive components originally part of the group have in fact already exploded as supernovae in the past, The supernova remnant associated with these explosions coincides with a large expanding superbubble with a diameter of 200 parsecs, the edges of which are clearly visible in the neutral hydrogen band. The powerful shock wave generated by the explosions, combined with the action of the stellar wind of the hottest stars in the group, has completely dissolved the original molecular cloud that generated them. According to some studies, the supernovae that exploded in this region and provided the energy for the expansion of the superbubble visible in the HI band could be at least seven.
The population of low-mass stars was detected especially in the late 1990s, when the launch of the ROSAT satellite allowed the study of the sky in X-rays, attention was concentrated on the Lupus Cloud, adjacent to the UCL group. In this region, 136 stars with characteristics similar to T Tauri stars were identified in 1997. The stars identified in this region show different ages depending on their position: the stars scattered outside the cloud are less young than those known inside it, this difference in age, respectively 5-27 million years and 1 million years, testifies that these belong to two distinct stellar populations, thus originating from different molecular clouds.
The low and medium mass stellar components of class G and K, mostly in a pre-main sequence phase, show a large amount of lithium. Based on the study of more than fifty of these components, an average age of between 15 and 22 million years has been determined.
Lower Centaurus-Crux association
The Lower Centaurus-Crux association is the least known and studied part of the Scorpius-Centaurus Association, although it is also the closest: its distance is estimated to be around 118 parsecs. It extends in the southwestern part of Centaurus and occupies the area of sky visible in the direction of the Southern Cross and the Fly, until it reaches the northeastern edge of Carina. The age of the stars in the group varies according to their position, the components in the northeastern part, the closest to the UCL group, have an age of about 17 million years, very similar to that of the previous group. The more southern components instead have a younger age, equal to about 12 million years; this reveals that the star formation phenomena have extended from north to south.The northern part has a higher density than the southern part, which instead appears more dispersed, among the higher-mass components, several are clearly visible even to the naked eye, and indeed appear as very bright stars: among these are six very massive stars, referred to as the "super Cen-Cru six", namely Hadar, one of the brightest stars in the sky, halfway between this and the previous group, Acrux, Mimosa, Delta, Epsilon and Rho Centauri, to these are added other easily observed stars, such as Gamma Centauri, Alpha and Beta Muscae. However, some of these stars show anomalies in their proper motion and their exact membership in the association is still a matter of study. The southwestern edge is represented by the open cluster IC 2602. Among the massive components there are also two Herbig Ae/Be stars, catalogued with the acronyms HD 100546 and HD 100453, of sixth and seventh magnitude respectively, their age would be around 10 million years and one of the two, HD 100453, seems to have a red pre-main sequence companion, although its presence has not been confirmed spectroscopically.
The group contains about 120 low-mass stellar components, whose magnitudes are mainly between the eighth and eleventh magnitude, these are mainly main sequence, pre-main sequence and T Tauri stars belonging to the G and K classes, identified through various studies conducted since the nineties through observations made using the ROSAT satellite.
Members stars and Exoplanets
Almost all bright stars in Constellation of Scorpius, Centaurus, Lupus and Crux are members of Scorpius-Centaurus association.The subgroups of the Scorpius–Centaurus association contains the youngest transiting exoplanets: K2-33 b, TOI-1227 b and HIP 67522 b. It also contains directly imaged exoplanets such as UScoCTIO 108 b and the PDS 70 system.
Galactic environment
The Scorpius-Centaurus association is located on the inner edge of the Orion Arm and is part of a large ring of young stars called Gould's Belt, which groups together several OB associations, among which Vela OB2, Orion OB1, Perseus OB2 and Lacerta OB1 stand out. With respect to the Sun, the Sco-Cen association coincides with the closest part of this ring.Other small star groups that show a similar age to each other are linked to the association; among these, the TW Hydrae association, the Beta Pictoris association and the Eta Chamaeleontis association stand out. All these groups, although they are found in the proximity of the association, are not in physical contact with it, and indeed their proper motion indicates that they are moving away from it, this therefore means that in relatively recent times, from an astronomical point of view, they were found in the same region. One theory suggests that these groups, now isolated and dispersed, were formed together with the association, but in peripheral regions of the large molecular cloud from which the stars of the UCL group were also born.
Among the star groups closest to the solar system is that of TW Hydrae, located at a distance of 50 parsecs, it is a group of young stars, approximately 16 million years old, which, thanks to its proximity, is of great importance in the study of the accretion phenomena that accompany the formation of stars and planets: many of its stars, in fact, are surrounded by protoplanetary disks and their current evolutionary phase is considered crucial for the subsequent formation of planets. Among its components is the young brown dwarf 2M1207, famous for being the first star around which an extrasolar planet was observed directly from Earth through the 2MASS mapping.
In the line of sight of the Scorpius-Centaurus association lie several bright stars and objects completely unrelated to it, among the former, Alpha Centauri stands out, whose great brightness is due exclusively to the fact that it is the closest star to the Sun, located at only 4 light-years. At about 88 light-years away lies Gacrux, a red giant that overlaps with the LCC group; both stars exhibit a large proper motion oriented in the direction opposite to that of the association: Alpha Centauri will appear almost superimposed on Hadar within 4000 years, while in a few tens of thousands of years it will find itself outside the boundaries of Centaurus, Gacrux will follow the same fate. In addition to these stars, there are also two nebulae that do not belong to the association, although they are perspectively aligned with it: The most notable is IC 2944, visible southwest of the Southern Cross, in the direction of the LCC group: it is a large H II region located at about 2000 parsecs in the Sagittarius Arm. The second is the Circinus Cloud, a dense, unilluminated clump of gas in which vigorous star formation is active, its distance is about 700 parsecs and it is visible just southeast of Alpha Centauri.