Multiple Main Sequence of Globular Clusters as a Result of Inhomogeneous Big Bang Nucleosynthesis Takashi Moriya ∗ Department of Astronomy, Graduate School of Science, University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan. and Institute for the Physics and Mathematics of the Universe, University of Tokyo, Kashiwanoha 5-1-5, Kashiwa, Chiba 277-8568, Japan. Toshikazu Shigeyama Research Center for the Early Universe, Graduate School of Science, University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan. A new mechanism for enhancing the helium abundance in the blue main sequence stars of ω Centauri and NGC 2808 is investigated. We suggest that helium enhancement was caused by the inhomogeneous big bang nucleosynthesis. Regions with extremely high baryon-to-photon ratios are assumed to be caused by the baryogenesis. Its mass scale is also assumed to be 10 $^{6}$M ⊙ . An example of the mechanisms to realize these two things was already proposed as the Affleck-Dine baryogenesis. As the baryon-to-photon ratio becomes larger, the primordial helium abundance is enhanced. We calculated the big bang nucleosynthesis and found that there exists a parameter region yielding enough helium to account for the split of the main sequence in the aforementioned globular clusters while keeping the abundances of other elements compatible with observations. Our mechanism predicts that heavy elements with the mass number of around 100 is enhanced in the blue main sequence stars. We estimate the timescales of diffusion of the enhanced helium and mass accretion in several stages after the nucleosynthesis to investigate whether these processes diminish the enhancement of helium. We found that the diffusion does not influence the helium content. A cloud with a sufficiently large baryon-to-photon ratio to account for the multiple main sequence collapsed immediately after the recombination. Subsequently, the cloud accreted the ambient matter with the normal helium content. If the star formation occurred both in the collapsed core and the accreted envelope, then the resultant star cluster has a double main sequence. PACS numbers: 98.20.Gm, 26.35.+c, 98.80.Ft, 13.60.Rj A globular cluster (GC) is a gravitationally bound sys- tem of stars with a spherical shape. The typical radius is of the order of 10 pc and its mass is about 10 $^{4}$−$^{6}$M ⊙ . Stars in a GC are known to be very old and their chemi- cal compositions roughly represent that of the primordial universe. Among known ∼ 150 GCs, ω Centauri is notorious for its unusual properties. Its mass is about 3 × 10 $^{6}$M ⊙ [1] and it is the most massive GC in the Milky Way galaxy. One of the most astonishing facts about ω Centauri is that its dwarf stars split into two sequences [2–4], the blue main sequence (bMS) and the red main sequence (rMS). 25%-35% of the dwarf stars are populated by the bMS [3]. Although enhanced metallicities might split a main sequence by lowering the surface temperature of metal-rich stars, it is reported that stars in the bMS are more metal-rich than those in the rMS in ω Centauri [5]. Recently, it is reported that there also exists the third redder MS [6]. This MS is suggested to be populated by super metal-rich stars [7] and seems to have nothing to The origin of the enhanced helium abundances in the bMS stars has been a mystery. It was suggested, for example, that the ejecta descended from massive asymp- totic giant branch (AGB) stars would enhance the he- lium abundances of the main sequence stars and split the main sequence [11]. However, it is difficult for massive AGB stars to produce as much helium as Y ∼ 0 . 35 [12]. What is more, the amount of helium supplied from AGB stars is not enough to increase the helium content of all stars in the bMS up to ∼ 0 . 35 [12]. Helium enhance- ment by massive stars is also suggested to explain the MS splitting [13]. Rotating low-metal massive stars can enhance the helium abundance in the stellar winds but such stars should accelerate the helium-rich matter by the subsequent supernova explosions. The helium rich ejecta might be too energetic to be trapped in a GC and do with their helium contents. Furthermore, observations by the Hubble Space Telescope discovered the triple main sequence in another GC, NGC 2808 [8]. NGC 2808 is also massive, with its mass above 10 $^{6}$M ⊙ [9]. It was found that, in either GCs, the isochrone that matches the properties of the bMS can be produced by assuming very high helium abundances ( Y ), 0 . 35 < Y < 0 . 45 [5, 10]. As other parameters do not have much effect on colors, it is very likely that the helium enrichment is the cause of the multiple main sequence. I. INTRODUCTION arXiv:1001.1790v1 [astro-ph.CO] 12 Jan 2010 $^{∗}$takashi.moriya@ipmu.jp