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Handbook of Star Forming Regions Vol. I
Astronomical Society of the Pacific, c(cid:13) 2008
Bo Reipurth, ed.

Overview of the Orion Complex

John Bally

Center for Astrophysics and Space Astronomy, University of Colorado
Boulder, CO 80389, USA

The Orion star formation complex is the nearest region of on-going star
Abstract.
formation that continues to produce both low and high mass stars. Orion is discussed in
the larger context of star formation in the Solar vicinity over the last 100 Myr. The Orion
complex is located on the far side of the Gould’s Belt system of clouds and young stars
through which our Solar system is drifting. A review is given of the overall structure and
properties of the Orion star forming complex, the best studied OB association. Over the
last 12 Myr, Orion has given birth to at least ten thousand stars contained in a half dozen
sub-groups and short-lived clusters. The Orion OB association has been the source of
several massive, high-velocity run-away stars, including µ Columbae and AE Aurigae.
Some of Orion’s most massive members died in supernova explosions that created the
300 pc diameter Orion / Eridanus super-bubble whose near wall may be as close as
180 pc. The combined effects of UV radiation, stellar winds, and supernovae have
impacted surviving molecular clouds in Orion. The large Orion A, IC 2118 molecular
clouds and dozens of smaller clouds strewn throughout the interior of the superbubble
have cometary shapes pointing back towards the center of the Orion OB association.
Most are forming stars in the compressed layers facing the bubble interior.

1.

Introduction

Orion is the best studied region of star formation in the sky. Its young stars and gas
provide important clues about the physics of star formation, the formation, evolution,
and destruction of star forming clouds, the dynamics and energetics of the interstellar
medium, and the role that OB associations and high mass stars play in the cycling of
gas between the various phases of the ISM.

In this chapter, we start with an overview of the Solar vicinity that establishes the
context in which nearby star forming regions must be understood. Then we discuss
the Orion complex of star forming regions as an example of star formation in an OB
association.

2. Spiral Arms, Superbubbles, GMCs, and Star Formation in the Solar Vicinity

The Solar vicinity (within about 0.5 to 1 kpc of the Sun) is the only place where it is
possible to investigate the distributions and motions of young stars and associated gas
in 3 dimensions and to probe the history of star formation and the interstellar medium
(ISM). While HII regions and low mass T-Tauri stars trace the youngest (< 3 to 5 Myr old) sites of on-going or recent star formation, OB associations can trace star formation history back nearly 100 Myr. The least massive stars that end their lives in supernova explosions have masses of about 8 M⊙, main-sequence lifetimes of about 40 Myr, and 459 460 Bally spectral type B3 which can excite small HII regions. Thus, groups containing B3 and earlier type stars trace sites of star birth younger than 40 Myr. OB associations whose most massive members have later spectral types can identify locales where stars formed more than 40 Myr ago. The mass spectra, locations, velocities, and ages of young stars and the properties of gas in nearby associations provide clues about the history of star formation and the origin, evolution, and destruction of molecular clouds over the last 100 Myr. Such studies enable us to decode the recent history of the interstellar medium and associated star birth in our portion of the Galaxy. − 15 to − 35 and Currently, the Solar vicinity appears to be located in an inter-arm spur of gas and dust between two major spiral arms of the Galaxy. Looking towards the Galactic anti- center, the Perseus Arm lies about 2 kpc beyond the Solar circle. Many well-known star forming complexes are embedded in this arm. They include giant cloud complexes near l = 111◦ that contain NGC 7538 and the Cas A supernova remnant, the W3/W4/W5 complexes near l = 134◦, and the Auriga and Gem OB1 clouds in the anti-center direc- tion (see chapters by Kun et al., Megeath et al., and Reipurth & Yan). Looking towards the inner Galaxy, the Sagittarius Arm is located about 2 kpc inside the Solar circle and contains the M8, M16, and M17 star forming complexes, see chapters by Tothill et al., Oliveira, and Chini & Hoffmeister. Millimeter-wavelength molecular absorption against the Galactic center clouds shows that the Sagittarius arm has a blueshifted radial 20 km s−1. The more distant Scutum and 3 kpc Arms velocity of VLSR = − 60 km s−1. appear at VLSR = − Superimposed on Galactic differential rotation, most of the ISM in the Solar vicin- ity (GMCs and HI) is expanding with a mean velocity of 2 to 5 km s−1 from a point located near l = 150o, b = 0o, d = 200 pc, the approximate centroid of the 50 Myr old Cas–Tau group, a “fossil” OB association (Blaauw 1991). This systematic expansion of the local gas was first identified by Lindblad (1967, 1973) and is sometimes called ‘Lindblad’s ring’ of HI, but it is even more apparent in the kinematics of nearby molec- ular gas (Dame et al. 1987, 2001; Taylor, Dickman, & Scoville 1987; Poppel et al. 1994). The Sun appears to be well inside this expanding ring. The nearest OB associ- ations, such as Sco-Cen (d 400), ≈ 500 pc), and the B and A stars that trace the so-called ‘Gould’s and Lac OB1b (d Belt’ of nearby young and intermediate age stars are all associated with Lindblad’s ring (Lesh 1968; De Zeeuw et al. 1999). Lindblad’s ring appears to be a 30 to 60 Myr old fossil supershell driven into the local ISM by the Cas–Tau group and the associated α Persei cluster (Blaauw 1991). The Gould’s Belt of stars, nearby OB associations, and star forming dark clouds may thus represent secondary star formation in clouds that condensed from the ancient Lindblad ring supershell. Figure 1 shows a schematic face- on view of the Solar vicinity. Support for this view comes from the agreement between the observed radial velocity fields of the local HI emission and nearby CO clouds (Fig- ure 2) with models of an expanding and tidally sheared 30 to 60 Myr old superbubble powered by the Cas-Tau group (Poppel et al. 1994). 300 pc), Orion OB1 (d 150 pc), Per OB2 (d ≈ ≈ ≈ The distribution of dust in the COBE and IRAS data and the kinematics of high- latitude HI provide additional evidence for an ancient super-shell created by a super- bubble centered on the Cas–Tau group that swept-up the local ISM, and blew out of the Galaxy orthogonal to the Galactic plane. The lines of sight with the lowest column densities of HI (the Lockman Hole; Figure 12 in Stark et al. 1992) and dust (Baade’s 35o, providing Hole) lie above and below the Cas–Tau group near l = 150o and b ± OrionOverview 461 A Sun-centric schematic face-on view of the Solar vicinity showing the Figure 1. older OB associations from de Zeeuw et al. (1999; solid grey disks), the approxi- mate location of the Lindblad Ring, (blue oval) and the three major OB associations younger than about 20 Myr; Sco-Cen, Per OB2, and Orion OB1 (solid blue disks) . The approximate outer boundaries of the supershells powered by each young asso- ciation are shown as red ovals. The Cas-Tau fossil OB association whose center is marked by the α-Persei cluster, is marked by the grey oval inside the Gould’s Belt / Lindblad Ring. evidence that an ancient bubble burst out of the Galactic plane at about one dust scale- height above and below the Cas–Tau “fossil” OB association. Gas expelled from the Galactic disk 20 to 50 Myr ago is now expected to be falling back. Indeed, the 21 cm line profiles formed by averaging all HI emission produced in both the northern and southern Galactic hemispheres show excess emission at low negative velocities in the range vlsr = –10 to –40 km s−1 (Stark et al. 1992) indicating an excess of infalling material at low velocities. Since the ambient pressure of the ISM orthogonal to the Galactic plane declines as an exponential, an expanding pressure driven supershell will accelerate once its radius becomes larger than the gas layer scale- height. As a supershell bursts out of the Galactic plane, it is subject to Rayleigh-Taylor 462 Bally | b The location of CO clouds in the l–v plane with brightness temperature Figure 2. shown in grey-scale (the CO data are from Dame et al. 1987). To reduce confusion < 2.0o is not shown. The location from distant gas in the disk, emission with | of the nearby OB associations are shown by asterisks (*). Association velocities are derived from their longitudes, distances, and the Galactic rotation curve. Included are Orion at l = 210o, Sco-Cen at l = 300o to 360o, Per OB2 at l = 165o, Lac OB1 at l = 100o, the α Persei cluster at l = 150o, and the expected location of the asso- ciation that created GSH238-00+9 (Heiles 1998). Models of the supershell powered by the “fossil” Cas-Tau group (centered near the α Persei cluster) are shown by two curves extending horizontally across the figure. Models of the younger supershells are shown by the closed loops. Dashed lines show the expected velocity fields of stationary supershells assuming that their velocities reflect only the motion produced by Galactic differential rotation (e.g. the shells are not expanding). The solid lines show the velocity fields produced by both Galactic differential rotation and expan- sion powered by the energy released by OB stars. For the Gould’s Belt/Lindblad ring, an expansion velocity of 4.5 km s−1 is assumed from a point located about 170 pc from the Sun towards l = 131o. While this is 20 degrees from the estimated center of the Cas–Tau fossil association, the original position of Cas–Tau is uncer- tain due to its 200 pc extent along the Galactic plane at a distance of 160 pc and 4 km s−1) peculiar velocity could have moved it this angular because a small ( distance during the past 15 Myr. The expansion velocities used for the other bubbles are 5.0 km s−1 for the “NSB” (GSH238+00+09) identified by Heiles at l = 240◦, 5.0 km s−1 for Sco-Cen, 5.0 km s−1 for Orion, 10.0 km s−1 for Perseus, and 4.0 km s−1 for Lac OB1. ≈ OrionOverview 463 fragmentation instabilities (MacLow & McCray 1987). After blow–out, the resulting dense clumps will move ballistically in the gravitational potential of the Galactic disk. Up to about 500 pc above the plane, the gravitational field of the disk is reasonably well represented by a harmonic oscillator potential with a z–oscillation time of about 80 to 100 Myr (Spitzer 1978). Clumps ejected from near the Galactic plane at less than 40 km s−1 will stop at heights of less than 500 pc within 20 – 25 Myr of their formation. During the next 20 – 25 Myr, they fall back towards the plane. Thus, the dynamical age of the low velocity infalling HI is comparable to that of the Cas–Tau group. Figures 3 and 4 show a schematic view of super-bubble, super-shell, and super-ring evolution. In summary, the Sun appears to be in the interior of an ancient supershell that may have produced the parent clouds from which Orion and most of the other nearby star forming regions originated. There are several lines of evidence to support this view: The expanding network of HI clouds (the Lindblad Ring) and associated molec- The Gould’s Belt of young stars that appears to be associated with the Lindblad The locations of the lowest column densities of dust and HI are situated above and below the centers of the ring. The excess of infalling, low negative velocity HI towards the North and South • ular clouds. • Ring. • • polar caps of the Galaxy. • α-Per group. The presence of the Cas-Tau fossil OB association and its central cluster, the The age of the Cas-Tau group, estimated to be between 40 to 90 Myr, may indicate that it formed when the Solar vicinity made its last passage through a major spiral arm of the Galaxy. Since the density of gas is higher in an arm, it is likely that so too was the star formation rate. Thus, the Cas-Tau group may have produced more stars and a larger super-bubble than the second generation clouds and OB associations such as the Sco-Cen and Orion OB associations, that were spawned from its super-ring. The stellar content of Cas-Tau remain poorly determined. Future parallax, proper motion, radial velocity, and spectroscopic surveys are needed to establish membership. 3. Stars in the Orion Complex The Orion OB association consists of a sequence of stellar groups of different ages that are partially superimposed along our line-of-sight (see chapter by Briceno and Figures 5 and 6). Traditionally, OB sub-group boundaries have been drawn to segregate each into a well defined and contiguous regions on the plane of the sky (see chapter by Brice˜no). However, when various stellar aggregates are superimposed on the plane of the sky, it make sense to also incorporate ages into the sub-group classification. Although there are differences in the estimated ages of the various groups, most workers agree that the oldest group, Orion OB1a, is located northwest of Orion’s Belt, and has an age of about 8 to 12 Myr (Blaauw 1991; Brown et al. 1994). The OB1b subgroup is centered on the Belt and has been estimated to have an age ranging from 1.7 to 8 Myr. However, the younger age is inconsistent with the presence of the three supergiants that form the naked-eye Belt stars which must be at least 5 Myrs old given their masses. A recently discovered cluster of roughly 7 to 10 Myr old stars is centered around 25 Ori at the northwestern end of Orion’s Belt (Brice˜no et al. 2007). Although formally a part of the Orion OB1a sub-group, the 25 Ori cluster has a distinct radial velocity, being about 10 464 Bally Figure 3. A schematic cartoon showing a super-bubble blowing out of the Galaxy during its early evolution (top). UV radiation, stellar winds, and multiple supernova explosions in the parent OB association inject energy at a roughly steady rate for about 40 Myr as stellar mortality depopulates the upper-end of the mass function down to a mass of 8 M⊙, the least massive star that can explode. The hot expanding bubble displaces ambient gas and sweeps it into a dense shell. As the swept-up shell reaches a radius larger than the scale-height of the gas layer, it blows out of the Galactic plane (bottom). The densest part of the shell forms a ring in the plane of the gas layer. As the shell expands into the exponentially declining density gradient above the plane, Rayleigh-Taylor instabilities can cause the shell to fragment into dense clumps at high latitudes. OrionOverview 465 Figure 4. A schematic cartoon showing the late phases in the evolution of a super- bubble-driven super-shell and ring. Dense clumps formed at high latitudes (top) by instabilities in the expanding shell start to fall back towards the Galactic plane after 20 to 25 Myr due to the gravitational potential of the Galactic disk. To first order, the gravitational field of the disk in the Solar vicinity is well represented by a harmonic oscillator potential in the vertical direction that gives rise of oscillations about the mid-plane with a period of about 80 to 100 Myr. Thus, clumps created by fragmentation of the shell will tend to fall-back into the mid-plane in about 40 to 50 Myr. As the shell sweeps-up the ISM in the Galactic mid-plane and decelerates, the expanding ring of gas can become unstable to gravitational collapse (middle). This occurs when the local spreading velocity caused by shell expansion drops below the gravitational escape speed from that portion of the ring. For typical OB association and super-bubble parameters in the Solar vicinity, instabilities set in at a ring-age of about 30 to 50 Myr after formation. The first self-gravitating objects tend to have masses of order 105 M⊙ (McCray & Kafatos 1987). Note the similarity of time- scales on which high latitude debris returns to the plane and the onset of gravitational instabilities. These clumps may soon evolve into GMCs that form their own OB associations and new super-bubbles (bottom). 466 Bally km s−1 lower than the traditional members of the 1a subgroup. It has been proposed that the 25 Ori group was formed as the HII region created by the 1a subgroup expanded into surrounding gas and triggered a burst of star formation. The 25 Ori group may thus represent a group that formed between Ori 1a and 1b. The 2 to 6 Myr old OB1c subgroup consists of stars located in Orion’s Sword about 4o below the Belt and directly in front of the Orion Nebula. This subgroups contains two loose clusters, NGC 1980 at the southern end of the Sword, and NGC 1981 at the northern end (Figure 6). The older stars in the Sword are superimposed on the much younger stellar populations associated with the Orion Nebula, M43, NGC 1977, and the OMC1, 2, and 3 regions in the Integral Shaped Filament at the northern end of the Orion A molecular cloud (see Figure 2 in the chapter by O’Dell et al.). Thus, it is hard to separate these two stellar populations and it is unclear weather the 1c and 1d sub- groups represent different populations, or merely older and younger stellar groups that formed from the Orion A cloud at different times. See the chapter by Muench et al. for further discussion. The λ Ori group (see chapter by Mathieu) may also have been triggered by the expansion of the bubble created by Orion OB1a. This group has an age similar to the Orion OB1b or the older stars in OB1c and, as illustrated in Figure 5, the λ Ori group is located at approximately the same distance from the center of Ori OB1a as Ori OB1c Thus, the λ Ori group could be considered to be a disjoint portion of OB1c. The sub-cluster of stars centered on σ Ori, located just below the eastern end of Orion’s Belt (Figures 5 & 6), has been assigned to OB 1b based on its spatial proximity. However, the ages of the σ Ori stars indicate that it is considerably younger than most of the stars in the Belt region (Walter et al. 1997). Because of its similar age, I prefer to assign the loose cluster of 2 to 4 Myr old stars centered on σ Orionis about 1o south of ζ Ori at the east end of the Belt (see the chapter by Walter et al. ) to the OB 1c sub-group. The Orion Nebula Cluster (ONC) in the Orion A molecular cloud and NGC 2024 in the Orion B molecular cloud are the two largest clusters in the youngest subgroup (known as OB1d) with ages less than 2 Myr (see chapters by Muench et al. and O’Dell et al. for the ONC and Meyer et al. for NGC 2024). In addition to these large clus- ters, the 1d subgroup contains a dozen smaller clusters and a background distribution of relatively isolated stars forming in cores throughout the Orion molecular clouds (e.g. NGC 2068 and 2071 in Orion B, see chapter by Gibb, and L1641, see chapter by Allen & Davis). Several thousand (mostly low mass) members of the OB1d subgroup formed from the “integral shaped filament” (Bally et al. 1987; Johnstone & Bally 1999) in the northern part of the Orion A molecular cloud that contains the Orion Nebula. About 2,000 of these stars with ages < 106 years are concentrated around the massive Trapez- ium stars in the Orion Nebula itself (Hillenbrand 1997). Hundreds more are forming in the dense OMC2 and 3 cores north of the Orion Nebula (see chapter by Peterson & Megeath). Finally, there are dozens of small, mostly cometary clouds spread around the Orion region that form small aggregates (see chapter by Alcala et al.). Though the full membership of the OB association has not been established, between 5000 and 20,000 stars are likely to have formed in the Orion region within the last 15 Myr. The ages and locations of Orion’s various subgroups indicate that star formation has propagated through the proto-Orion cloud in a sequential manner. The older subgroups in Orion lie closer to us than the younger subgroups. While the distances to the brighter members of the older 1a and 1b subgroups range from 320