5 Aug 2004 22:8 AR AR222-AA42-03.tex AR222-AA42-03.sgm LaTeX2e(2002/01/18) P1: IKH10.1146/annurev.astro.42.053102.134034Annu. Rev. Astron. Astrophys. 2004. 42:79–118doi: 10.1146/annurev.astro.42.053102.134034Copyrightc 2004 by Annual Reviews. All rights reservedFirst published online as a Review in Advance on May 19, 2004THE FIRST STARSVolker BrommDepartment of Astronomy, Harvard University, Cambridge, Massachusetts 02138;email: [email protected] B. LarsonDepartment of Astronomy, Yale University, New Haven, Connecticut 06520-8101;email: [email protected], intergalactic medium, initial mass function,star formation, Population III■ Abstract We review recent theoretical results on the formation of the first starsin the universe, and emphasize related open questions. In particular, we discuss theinitial conditions for Population III star formation, as given by variants of the colddark matter cosmology. Numerical simulations have investigated the collapse and thefragmentation of metal-free gas, showing that the first stars were predominantly verymassive. The exact determination of the stellar masses, and the precise form of theprimordial initial mass function, is still hampered by our limited understanding of theaccretion physics and the protostellar feedback effects. We address the importance ofheavy elements in bringing about the transition from an early star formation modedominated by massive stars to the familiar mode dominated by low-mass stars at latertimes. We show how complementary observations, both at high redshifts and in ourlocal cosmic neighborhood, can be utilized to probe the first epoch of star formation.1. INTRODUCTIONThe first stars to form out of unenriched H/He gas marked the crucial transitionfrom a homogeneous, simple universe to a highly structured, complex one atthe end of the cosmic dark ages. Extending the familiar scheme of classifyingstellar populations in the local universe (Baade 1944) to the extreme case of zerometallicity, the first stars constitute the so-called Population III (e.g., Bond 1981;Cayrel 1986; Carr 1987, 1994). The quest for Population III stars has fascinatedastronomers for many decades, back to the first tentative ideas of Schwarzschild& Spitzer (1953). Recently, the subject has attracted increased interest, both froma theoretical and an observational perspective. New empirical probes of the high-redshift universe have become available, and our ability to carry out sophisticatednumerical simulations has improved dramatically. In this review, we summarizethe current state of this rapidly evolving field.0066-4146/04/0922-0079$14.00 795 Aug 2004 22:8 AR AR222-AA42-03.tex AR222-AA42-03.sgm LaTeX2e(2002/01/18) P1: IKH80 BROMMLARSONThe first generation of stars had important effects on subsequent galaxy for-mation (see Carr et al. 1984 for an early overview). On the one hand, PopulationIII stars produced copious amounts of UV photons to reionize the universe (e.g.,Tumlinson & Shull 2000, Bromm et al. 2001b; Schaerer 2002, 2003; Tumlinsonet al. 2003; Venkatesan & Truran 2003; Venkatesan et al. 2003). Recently, theWilkinson Microwave Anisotropy Probe (WMAP) observed the large-angle polar-ization anisotropy of the cosmic microwave background (CMB), thus constrainingthe total ionizing photon production from the first stars (e.g., Cen 2003a,b; Ciardiet al. 2003; Haiman & Holder 2003; Holder et al. 2003; Kaplinghat et al. 2003;Kogut et al. 2003; Sokasian et al. 2003a,b; Wyithe & Loeb 2003a,b). The largeoptical depth to Thomson scattering measured by WMAP has been interpreted asa signature of a substantial early activity of massive star formation at redshiftsz 15 (see Section 5.1). On the other hand, the supernova (SN) explosions thatended the lives of the first stars were responsible for the initial enrichment of theintergalactic medium (IGM) with heavy elements (e.g., Ostriker & Gnedin 1996,Gnedin & Ostriker 1997, Ferrara et al. 2000, Madau et al. 2001, Mori et al. 2002,Thacker et al. 2002, Bromm et al. 2003, Furlanetto & Loeb 2003, Mackey et al.2003, Scannapieco et al. 2003, Wada & Venkatesan 2003, Yoshida et al. 2004). Anintriguing possibility unique to zero-metallicity massive stars is the complete dis-ruption of the progenitor in a pair-instability supernova (PISN), which is predictedto leave no remnant behind (e.g., Barkat et al. 1967, Ober et al. 1983, Bond et al.1984, Fryer et al. 2001, Heger & Woosley 2002, Heger et al. 2003). This peculiarexplosion mode could have played an important role in quickly seeding the IGMwith the first metals (see Section 4.2).To place the study of the first stars into the appropriate cosmological context,one has to ask (see Section 2), When and how did the cosmic dark ages end? Duringthis time between the emission of the CMB half a million years after the big bang,when the CMB photons shifted into the infrared, and the formation of the firstsources of light, the universe would have appeared completely dark to a humanobserver (see Barkana & Loeb 2001, Loeb & Barkana 2001, Miralda-Escud´e 2003for comprehensive reviews of this cosmic epoch). Historically, the evocative—andnow widely used—dark age analogy was first introduced into the literature brieflyafter the ascent of nonbaryonic, cold dark matter (CDM) scenarios (e.g., Sargent1986, Rees 1993). In the context of popular CDM models of hierarchical structureformation, the first stars are predicted to have formed in dark matter (DM) halosof mass ∼106Mthat collapsed at redshifts z 20–30 (e.g., Tegmark et al. 1997,Yoshida et al. 2003a). The first quasars, however (e.g., Umemura et al. 1993, Loeb& Rasio 1994, Eisenstein & Loeb 1995), are likely to have formed in more massivehost systems, at redshifts z 10 (Haiman & Loeb 2001, Bromm & Loeb 2003a),and certainly before z ∼ 6.4, which is the redshift of the most distant quasar known(Fan et al. 2003). Here, we do not discuss quasar formation in depth, and we referthe reader to the references above.The most fundamental question about the first stars is how massive they typicallywere (see Section 3). Results from recent numerical simulations of the collapse5 Aug 2004 22:8 AR AR222-AA42-03.tex AR222-AA42-03.sgm LaTeX2e(2002/01/18) P1: IKHFIRST STARS 81and fragmentation of primordial gas clouds suggest that the first stars were pre-dominantly very massive, with typical masses M∗ 100 M(Bromm et al. 1999,2002; Nakamura & Umemura 2001; Abel et al. 2000,