The spitzer c2d legacy results

Star-formation rates and efficiencies; evolution and lifetimes

Neal J. Evans, Michael M. Dunham, Jes K. Jørgensen, Melissa L. Enoch, Bruno Merín, Ewine F. Van Dishoeck, Juan M. Alcalaá, Philip C. Myers, Karl R. Stapelfeldt, Tracy L. Huard, Lori E. Allen, Paul M. Harvey, T. I M Van Kempen, Geoffrey A. Blake, David W Koerner, Lee G. Mundy, Deborah L. Padgett, Anneila I. Sargent

Research output: Contribution to journalArticle

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Abstract

The c2d Spitzer Legacy project obtained images and photometry with both IRAC and MIPS instruments for five large, nearby molecular clouds. Three of the clouds were also mapped in dust continuum emission at 1.1 mm, and optical spectroscopy has been obtained for some clouds. This paper combines information drawn from studies of individual clouds into a combined and updated statistical analysis of star-formation rates and efficiencies, numbers and lifetimes for spectral energy distribution (SED) classes, and clustering properties. Current star-formation efficiencies range from 3% to 6%; if star formation continues at current rates for 10 Myr, efficiencies could reach 15-30%. Star-formation rates and rates per unit area vary from cloud to cloud; taken together, the five clouds are producing about 260 M0 of stars per Myr. The star-formation surface density is more than an order of magnitude larger than would be predicted from the Kennicutt relation used in extragalactic studies, reflecting the fact that those relations apply to larger scales, where more diffuse matter is included in the gas surface density. Measured against the dense gas probed by the maps of dust continuum emission, the efficiencies are much higher, with stellar masses similar to masses of dense gas, and the current stock of dense cores would be exhausted in 1.8 Myr on average. Nonetheless, star formation is still slow compared to that expected in a free-fall time, even in the dense cores. The derived lifetime for the Class I phase is 0.54 Myr, considerably longer than some estimates. Similarly, the lifetime for the Class 0 SED class, 0.16 Myr, with the notable exception of the Ophiuchus cloud, is longer than early estimates. If photometry is corrected for estimated extinction before calculating class indicators, the lifetimes drop to 0.44 Myr for Class I and to 0.10 for Class 0. These lifetimes assume a continuous flow through the Class II phase and should be considered median lifetimes or half-lives. Star formation is highly concentrated to regions of high extinction, and the youngest objects are very strongly associated with dense cores. The great majority (90%) of young stars lie within loose clusters with at least 35 members and a stellar density of 1 M⊙ pc-3. Accretion at the sound speed from an isothermal sphere over the lifetime derived for the Class I phase could build a star of about 0.25 M0, given an efficiency of 0.3. Building larger mass stars by using higher mass accretion rates could be problematic, as our data confirm and aggravate the "luminosity problem" for protostars. At a given T b01, the values for Lboi are mostly less than predicted by standard infall models and scatter over several orders of magnitude. These results strongly suggest that accretion is time variable, with prolonged periods of very low accretion. Based on a very simple model and this sample of sources, half the mass of a star would be accreted during only 7% of the Class I lifetime, as represented by the eight most luminous objects.

Original languageEnglish (US)
Pages (from-to)321-350
Number of pages30
JournalAstrophysical Journal, Supplement Series
Volume181
Issue number2
DOIs
StatePublished - 2009

Fingerprint

star formation rate
life (durability)
star formation
accretion
stars
extinction
gas
spectral energy distribution
dust
photometry
gases
rate
continuums
half life
energy
free fall
statistical analysis
protostars
spectroscopy
estimates

Keywords

  • Dust, extinction
  • Infrared: Stars
  • Ism: Clouds
  • Stars: Formation

ASJC Scopus subject areas

  • Space and Planetary Science
  • Astronomy and Astrophysics

Cite this

Evans, N. J., Dunham, M. M., Jørgensen, J. K., Enoch, M. L., Merín, B., Van Dishoeck, E. F., ... Sargent, A. I. (2009). The spitzer c2d legacy results: Star-formation rates and efficiencies; evolution and lifetimes. Astrophysical Journal, Supplement Series, 181(2), 321-350. https://doi.org/10.1088/0067-0049/181/2/321

The spitzer c2d legacy results : Star-formation rates and efficiencies; evolution and lifetimes. / Evans, Neal J.; Dunham, Michael M.; Jørgensen, Jes K.; Enoch, Melissa L.; Merín, Bruno; Van Dishoeck, Ewine F.; Alcalaá, Juan M.; Myers, Philip C.; Stapelfeldt, Karl R.; Huard, Tracy L.; Allen, Lori E.; Harvey, Paul M.; Van Kempen, T. I M; Blake, Geoffrey A.; Koerner, David W; Mundy, Lee G.; Padgett, Deborah L.; Sargent, Anneila I.

In: Astrophysical Journal, Supplement Series, Vol. 181, No. 2, 2009, p. 321-350.

Research output: Contribution to journalArticle

Evans, NJ, Dunham, MM, Jørgensen, JK, Enoch, ML, Merín, B, Van Dishoeck, EF, Alcalaá, JM, Myers, PC, Stapelfeldt, KR, Huard, TL, Allen, LE, Harvey, PM, Van Kempen, TIM, Blake, GA, Koerner, DW, Mundy, LG, Padgett, DL & Sargent, AI 2009, 'The spitzer c2d legacy results: Star-formation rates and efficiencies; evolution and lifetimes', Astrophysical Journal, Supplement Series, vol. 181, no. 2, pp. 321-350. https://doi.org/10.1088/0067-0049/181/2/321
Evans, Neal J. ; Dunham, Michael M. ; Jørgensen, Jes K. ; Enoch, Melissa L. ; Merín, Bruno ; Van Dishoeck, Ewine F. ; Alcalaá, Juan M. ; Myers, Philip C. ; Stapelfeldt, Karl R. ; Huard, Tracy L. ; Allen, Lori E. ; Harvey, Paul M. ; Van Kempen, T. I M ; Blake, Geoffrey A. ; Koerner, David W ; Mundy, Lee G. ; Padgett, Deborah L. ; Sargent, Anneila I. / The spitzer c2d legacy results : Star-formation rates and efficiencies; evolution and lifetimes. In: Astrophysical Journal, Supplement Series. 2009 ; Vol. 181, No. 2. pp. 321-350.
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T2 - Star-formation rates and efficiencies; evolution and lifetimes

AU - Evans, Neal J.

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AU - Jørgensen, Jes K.

AU - Enoch, Melissa L.

AU - Merín, Bruno

AU - Van Dishoeck, Ewine F.

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AU - Myers, Philip C.

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N2 - The c2d Spitzer Legacy project obtained images and photometry with both IRAC and MIPS instruments for five large, nearby molecular clouds. Three of the clouds were also mapped in dust continuum emission at 1.1 mm, and optical spectroscopy has been obtained for some clouds. This paper combines information drawn from studies of individual clouds into a combined and updated statistical analysis of star-formation rates and efficiencies, numbers and lifetimes for spectral energy distribution (SED) classes, and clustering properties. Current star-formation efficiencies range from 3% to 6%; if star formation continues at current rates for 10 Myr, efficiencies could reach 15-30%. Star-formation rates and rates per unit area vary from cloud to cloud; taken together, the five clouds are producing about 260 M0 of stars per Myr. The star-formation surface density is more than an order of magnitude larger than would be predicted from the Kennicutt relation used in extragalactic studies, reflecting the fact that those relations apply to larger scales, where more diffuse matter is included in the gas surface density. Measured against the dense gas probed by the maps of dust continuum emission, the efficiencies are much higher, with stellar masses similar to masses of dense gas, and the current stock of dense cores would be exhausted in 1.8 Myr on average. Nonetheless, star formation is still slow compared to that expected in a free-fall time, even in the dense cores. The derived lifetime for the Class I phase is 0.54 Myr, considerably longer than some estimates. Similarly, the lifetime for the Class 0 SED class, 0.16 Myr, with the notable exception of the Ophiuchus cloud, is longer than early estimates. If photometry is corrected for estimated extinction before calculating class indicators, the lifetimes drop to 0.44 Myr for Class I and to 0.10 for Class 0. These lifetimes assume a continuous flow through the Class II phase and should be considered median lifetimes or half-lives. Star formation is highly concentrated to regions of high extinction, and the youngest objects are very strongly associated with dense cores. The great majority (90%) of young stars lie within loose clusters with at least 35 members and a stellar density of 1 M⊙ pc-3. Accretion at the sound speed from an isothermal sphere over the lifetime derived for the Class I phase could build a star of about 0.25 M0, given an efficiency of 0.3. Building larger mass stars by using higher mass accretion rates could be problematic, as our data confirm and aggravate the "luminosity problem" for protostars. At a given T b01, the values for Lboi are mostly less than predicted by standard infall models and scatter over several orders of magnitude. These results strongly suggest that accretion is time variable, with prolonged periods of very low accretion. Based on a very simple model and this sample of sources, half the mass of a star would be accreted during only 7% of the Class I lifetime, as represented by the eight most luminous objects.

AB - The c2d Spitzer Legacy project obtained images and photometry with both IRAC and MIPS instruments for five large, nearby molecular clouds. Three of the clouds were also mapped in dust continuum emission at 1.1 mm, and optical spectroscopy has been obtained for some clouds. This paper combines information drawn from studies of individual clouds into a combined and updated statistical analysis of star-formation rates and efficiencies, numbers and lifetimes for spectral energy distribution (SED) classes, and clustering properties. Current star-formation efficiencies range from 3% to 6%; if star formation continues at current rates for 10 Myr, efficiencies could reach 15-30%. Star-formation rates and rates per unit area vary from cloud to cloud; taken together, the five clouds are producing about 260 M0 of stars per Myr. The star-formation surface density is more than an order of magnitude larger than would be predicted from the Kennicutt relation used in extragalactic studies, reflecting the fact that those relations apply to larger scales, where more diffuse matter is included in the gas surface density. Measured against the dense gas probed by the maps of dust continuum emission, the efficiencies are much higher, with stellar masses similar to masses of dense gas, and the current stock of dense cores would be exhausted in 1.8 Myr on average. Nonetheless, star formation is still slow compared to that expected in a free-fall time, even in the dense cores. The derived lifetime for the Class I phase is 0.54 Myr, considerably longer than some estimates. Similarly, the lifetime for the Class 0 SED class, 0.16 Myr, with the notable exception of the Ophiuchus cloud, is longer than early estimates. If photometry is corrected for estimated extinction before calculating class indicators, the lifetimes drop to 0.44 Myr for Class I and to 0.10 for Class 0. These lifetimes assume a continuous flow through the Class II phase and should be considered median lifetimes or half-lives. Star formation is highly concentrated to regions of high extinction, and the youngest objects are very strongly associated with dense cores. The great majority (90%) of young stars lie within loose clusters with at least 35 members and a stellar density of 1 M⊙ pc-3. Accretion at the sound speed from an isothermal sphere over the lifetime derived for the Class I phase could build a star of about 0.25 M0, given an efficiency of 0.3. Building larger mass stars by using higher mass accretion rates could be problematic, as our data confirm and aggravate the "luminosity problem" for protostars. At a given T b01, the values for Lboi are mostly less than predicted by standard infall models and scatter over several orders of magnitude. These results strongly suggest that accretion is time variable, with prolonged periods of very low accretion. Based on a very simple model and this sample of sources, half the mass of a star would be accreted during only 7% of the Class I lifetime, as represented by the eight most luminous objects.

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