Three modes of metal-enriched star formation in the early universe

Britton D. Smith, Matthew J. Turk, Steinn Sigurdsson, Brian W. O'Shea, Michael L. Norman

Research output: Contribution to journalArticle

113 Citations (Scopus)

Abstract

Simulations of the formation of Population III (Pop III) stars suggest that they were much more massive than the Pop II and Pop I stars observed today. This is due to the collapse dynamics of metal-free gas, which is regulated by the radiative cooling of molecular hydrogen. We study how the collapse of gas clouds is altered by the addition of metals to the star-forming environment by performing a series of simulations of pre-enriched star formation at various metallicities. To make a clean comparison with metal-free star formation, we use initial conditions identical to a Pop III star formation simulation, with low ionization and no external radiation other than the cosmic microwave background (CMB). For metallicities below the critical metallicity, Z®cr, collapse proceeds similar to the metal-free case, and only massive objects form. For metallicities well above Z®cr, efficient cooling rapidly lowers the gas temperature to the temperature of the CMB. The gas is unable to radiatively cool below the CMB temperature, and becomes thermally stable. For high metallicities, Z ≳ 10-2.5 Z®, this occurs early in the evolution of the gas cloud, when the density is still relatively low. The resulting cloud cores show little or no fragmentation, and would most likely form massive stars. If the metallicity is not vastly above Z ®cr, the cloud cools efficiently but does not reach the CMB temperature, and fragmentation into multiple objects occurs. We conclude that there were three distinct modes of star formation at high redshift (z ≳ 4): a "primordial" mode, producing massive stars (10s to 100s of M ®) at very low metallicities (Z ≲ 10-3.75 Z ®); a CMB-regulated mode, producing moderate mass (10s of M ®) stars at high metallicities (Z ≳ 10-2.5 Z ® at redshift z∼ 15-20); and a low-mass (a few M ®) mode existing between these two metallicities. As the universe ages and the CMB temperature decreases, the range of the low-mass mode extends to higher metallicities, eventually becoming the only mode of star formation.

Original languageEnglish (US)
Pages (from-to)441-451
Number of pages11
JournalAstrophysical Journal
Volume691
Issue number1
DOIs
StatePublished - Jan 20 2009

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metallicity
star formation
universe
metal
metals
gas
microwaves
temperature
fragmentation
cooling
simulation
massive stars
gases
stars
microwave
ionization
M stars
hydrogen
gas temperature
radiation

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science

Cite this

Smith, Britton D. ; Turk, Matthew J. ; Sigurdsson, Steinn ; O'Shea, Brian W. ; Norman, Michael L. / Three modes of metal-enriched star formation in the early universe. In: Astrophysical Journal. 2009 ; Vol. 691, No. 1. pp. 441-451.
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title = "Three modes of metal-enriched star formation in the early universe",
abstract = "Simulations of the formation of Population III (Pop III) stars suggest that they were much more massive than the Pop II and Pop I stars observed today. This is due to the collapse dynamics of metal-free gas, which is regulated by the radiative cooling of molecular hydrogen. We study how the collapse of gas clouds is altered by the addition of metals to the star-forming environment by performing a series of simulations of pre-enriched star formation at various metallicities. To make a clean comparison with metal-free star formation, we use initial conditions identical to a Pop III star formation simulation, with low ionization and no external radiation other than the cosmic microwave background (CMB). For metallicities below the critical metallicity, Z{\circledR}cr, collapse proceeds similar to the metal-free case, and only massive objects form. For metallicities well above Z{\circledR}cr, efficient cooling rapidly lowers the gas temperature to the temperature of the CMB. The gas is unable to radiatively cool below the CMB temperature, and becomes thermally stable. For high metallicities, Z ≳ 10-2.5 Z{\circledR}, this occurs early in the evolution of the gas cloud, when the density is still relatively low. The resulting cloud cores show little or no fragmentation, and would most likely form massive stars. If the metallicity is not vastly above Z {\circledR}cr, the cloud cools efficiently but does not reach the CMB temperature, and fragmentation into multiple objects occurs. We conclude that there were three distinct modes of star formation at high redshift (z ≳ 4): a {"}primordial{"} mode, producing massive stars (10s to 100s of M {\circledR}) at very low metallicities (Z ≲ 10-3.75 Z {\circledR}); a CMB-regulated mode, producing moderate mass (10s of M {\circledR}) stars at high metallicities (Z ≳ 10-2.5 Z {\circledR} at redshift z∼ 15-20); and a low-mass (a few M {\circledR}) mode existing between these two metallicities. As the universe ages and the CMB temperature decreases, the range of the low-mass mode extends to higher metallicities, eventually becoming the only mode of star formation.",
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Three modes of metal-enriched star formation in the early universe. / Smith, Britton D.; Turk, Matthew J.; Sigurdsson, Steinn; O'Shea, Brian W.; Norman, Michael L.

In: Astrophysical Journal, Vol. 691, No. 1, 20.01.2009, p. 441-451.

Research output: Contribution to journalArticle

TY - JOUR

T1 - Three modes of metal-enriched star formation in the early universe

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AU - Turk, Matthew J.

AU - Sigurdsson, Steinn

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N2 - Simulations of the formation of Population III (Pop III) stars suggest that they were much more massive than the Pop II and Pop I stars observed today. This is due to the collapse dynamics of metal-free gas, which is regulated by the radiative cooling of molecular hydrogen. We study how the collapse of gas clouds is altered by the addition of metals to the star-forming environment by performing a series of simulations of pre-enriched star formation at various metallicities. To make a clean comparison with metal-free star formation, we use initial conditions identical to a Pop III star formation simulation, with low ionization and no external radiation other than the cosmic microwave background (CMB). For metallicities below the critical metallicity, Z®cr, collapse proceeds similar to the metal-free case, and only massive objects form. For metallicities well above Z®cr, efficient cooling rapidly lowers the gas temperature to the temperature of the CMB. The gas is unable to radiatively cool below the CMB temperature, and becomes thermally stable. For high metallicities, Z ≳ 10-2.5 Z®, this occurs early in the evolution of the gas cloud, when the density is still relatively low. The resulting cloud cores show little or no fragmentation, and would most likely form massive stars. If the metallicity is not vastly above Z ®cr, the cloud cools efficiently but does not reach the CMB temperature, and fragmentation into multiple objects occurs. We conclude that there were three distinct modes of star formation at high redshift (z ≳ 4): a "primordial" mode, producing massive stars (10s to 100s of M ®) at very low metallicities (Z ≲ 10-3.75 Z ®); a CMB-regulated mode, producing moderate mass (10s of M ®) stars at high metallicities (Z ≳ 10-2.5 Z ® at redshift z∼ 15-20); and a low-mass (a few M ®) mode existing between these two metallicities. As the universe ages and the CMB temperature decreases, the range of the low-mass mode extends to higher metallicities, eventually becoming the only mode of star formation.

AB - Simulations of the formation of Population III (Pop III) stars suggest that they were much more massive than the Pop II and Pop I stars observed today. This is due to the collapse dynamics of metal-free gas, which is regulated by the radiative cooling of molecular hydrogen. We study how the collapse of gas clouds is altered by the addition of metals to the star-forming environment by performing a series of simulations of pre-enriched star formation at various metallicities. To make a clean comparison with metal-free star formation, we use initial conditions identical to a Pop III star formation simulation, with low ionization and no external radiation other than the cosmic microwave background (CMB). For metallicities below the critical metallicity, Z®cr, collapse proceeds similar to the metal-free case, and only massive objects form. For metallicities well above Z®cr, efficient cooling rapidly lowers the gas temperature to the temperature of the CMB. The gas is unable to radiatively cool below the CMB temperature, and becomes thermally stable. For high metallicities, Z ≳ 10-2.5 Z®, this occurs early in the evolution of the gas cloud, when the density is still relatively low. The resulting cloud cores show little or no fragmentation, and would most likely form massive stars. If the metallicity is not vastly above Z ®cr, the cloud cools efficiently but does not reach the CMB temperature, and fragmentation into multiple objects occurs. We conclude that there were three distinct modes of star formation at high redshift (z ≳ 4): a "primordial" mode, producing massive stars (10s to 100s of M ®) at very low metallicities (Z ≲ 10-3.75 Z ®); a CMB-regulated mode, producing moderate mass (10s of M ®) stars at high metallicities (Z ≳ 10-2.5 Z ® at redshift z∼ 15-20); and a low-mass (a few M ®) mode existing between these two metallicities. As the universe ages and the CMB temperature decreases, the range of the low-mass mode extends to higher metallicities, eventually becoming the only mode of star formation.

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