The Formation of Massive StarsHenrik BeutherMax-Planck-Institute for Astronomy, HeidelbergEdward B. ChurchwellUniversity of Wisconsin, MadisonChristopher F. McKeeUniversity of California, BerkeleyJonathan C. TanUniversity of Florida, GainesvilleMassive stars have a profound influence on the Universe, but their formation remains poorlyunderstood. We review the current status of observational and theoretical research in thisfield, describing the various stages of an evolutionary sequence that begins with cold, massivegas cores and ends with the dispersal and ionization of gas by the newly-formed star. Thephysical processes in massive star formation are described and related to their observationalmanifestations. Feedback processes and the relation of massive stars to star cluster formationare also discussed. We identify key observational and theoretical questions that future studiesshould address.1. INTRODUCTIONMassive star formation has drawn considerable interestfor several decades, but the last 10 years have witnesseda strong acceleration of theoretical and observational re-search in this field. One of the major conceptual problemsin massive star formation arises from the radiation pres-sure massive stars exert on the surrounding dust and gascore (e.g., Kahn, 1974; Wolfire and Cassinelli, 1987; Jijinaand Adams, 1996; Yorke and Sonnhalter, 2002; Krumholzet al., 2005b). In principle, this radiation pressure couldbe strong enough to stop further accretion, which wouldimply that the standard theory of low-mass star formationhad to be adapted to account for the formation of mas-sive stars. Two primary approaches have been followedto overcome these problems: the first and more straight-forward approach is to modify the standard theory quanti-tatively rather than qualitatively. Theories have been pro-posed that invoke varying dust properties (e.g., Wolfire andCassinelli, 1987), increasing accretion rates in turbulentcloud cores of the order 10−4− 10−3Myr−1comparedto ∼ 10−6Myr−1for low-mass star formation (e.g., Nor-berg and Maeder, 2000; McKee and Tan, 2003), accretionvia disks (e.g., Jijina and Adams, 1996; Yorke and Sonnhal-ter, 2002), accretion through the evolving hypercompactHII region (Keto, 2003; Keto and Wood, 2006), the es-cape of radiation through wind-blown cavities (Krumholzet al., 2005a) or radiatively driven Rayleigh-Taylor insta-bilities (Krumholz et al., 2005b). These variations to thestandard picture of low-mass star formation suggest thatmassive stars can form within an accretion-based pictureof star formation. Contrary to this, a paradigm change forthe formation of massive stars has been proposed based onthe observational fact that massive stars always form at thedense centers of stellar clusters: the coalescence scenario.In this scenario, the protostellar and stellar densities of aforming massive cluster are high enough (∼ 108pc−3) thatprotostars undergo physical collisions and merge, therebyavoiding the effects of radiation pressure (Bonnell et al.,1998; Bally and Zinnecker, 2005). Variants of the coales-cence model that operate at lower stellar densities have beenproposed by Stahler at al. (2000) and by Bonnell and Bate(2005). A less dramatic approach suggests that the bulkof the stellar mass is accreted via competitive accretion ina clustered environment (Bonnell et al., 2004). This doesnot necessarily require the coalescence of protostars, but themass accretion rates of the massive cluster members wouldbe directly linked to the number of their stellar companions,implying a causal relationship between the cluster forma-tion process and the formation of higher-mass stars therein.We propose an evolutionary scenario for massive starformation, and then discuss the various stages in more de-tail. Following Williams et al. (2000), we use the termclumps for condensations associated with cluster formation,and the term cores for molecular condensations that formsingle or gravitationally bound multiple massive protostars.The evolutionary sequence we propose for high-mass star-forming cores is:1High-Mass Starless Cores (HMSCs)→ High-Mass Cores harboring accreting Low/Intermediate-Mass Protostar(s) destined to become a high-mass star(s)→ High-Mass Protostellar Objects (HMPOs)→ Final Stars.The term HMPO is used here in a literal sense, i.e., accret-ing high-mass protostars. Hence, the HMPO group consistsof protostars >8 M, which early on have not necessar-ily formed a detectable Hot Molecular Core (HMC) and/orHypercompact HII region (HCHIIs, size < 0.01 pc). HMCsand HCHIIs might coexist simultaneously. UltracompactHII regions (UCHIIs, size < 0.1 pc) are a transition group:some of them may still harbor accreting protostars (henceare at the end of the HMPO stage), but many have likelyalready ceased accretion (hence are part of the Final-Starclass). High-mass stars can be on the main sequence whilethey are deeply embedded and actively accreting as well asafter they cease accreting and become Final Stars. The classof High-Mass Cores harboring accreting Low/Intermediate-Mass Protostars has not been well studied yet, but there hasto be a stage between the HMSCs and the HMPOs, consist-ing of high-mass cores with embedded low/intermediate-mass objects. On the cluster/clump scale the proposed evo-lutionary sequence is:Massive Starless Clumps→ Protoclusters→ Stellar Clusters.By definition, Massive Starless Clumps can harbor onlyHMSCs (and low-mass starless cores), whereas Protoclus-ters in principle can harbor all sorts of smaller-scale entities(low- and intermediate-mass protostars, HMPOs, HMCs,HCHIIs, UCHIIs and even HMSCs).This review discusses the evolutionary stages and theirassociated physical processes (§2, 3, 4, 6), feedback pro-cesses (§4), and cluster formation (§5), always from an ob-servational and theoretical perspective. We restrict our-selves to present day massive star formation in a typicalGalactic environment. Primordial star formation, lowermetallicities or different dust properties may change thispicture (e.g., Bromm and Loeb, 2004; Draine, 2003). Thedirect comparison of the theoretical predictions with the ob-servational evidences and indications shows the potentialsand limitations of our current understanding of high-massstar formation. We also refer to the IAU227 Proceedingsdedicated to Massive Star Birth (Cesaroni et al., 2005b).2. INITIAL CONDITIONS2.1. Observational resultsThe largest structures within our Galaxy are
View Full Document
Unlocking...