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H 2 Formation in the Perseus Molecular Cloud: Observations Meet Theory

H 2 Formation in the Perseus Molecular Cloud: Observations Meet Theory. Min-Young Lee University of Wisconsin-Madison, USA Collaborators S. Stanimirović 1 , K. Douglas 2 , L. Knee 3,4 , J. Di Francesco 4 , S. Gibson 5 , A. Begum 1 ,

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H 2 Formation in the Perseus Molecular Cloud: Observations Meet Theory

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  1. H2 Formation in the Perseus Molecular Cloud:Observations Meet Theory Min-Young Lee University of Wisconsin-Madison, USA Collaborators S. Stanimirović1, K. Douglas2, L. Knee3,4, J. Di Francesco4, S. Gibson5, A. Begum1, J. Grcevich6, C. Heiles7, E. Korpela7, A. Leroy8, J. Peek6, N. Pingel1, M. Putman6, D. Saul6 (1) UW-Madison (2) Arecibo Observatory (3) ALMA (4) Herzberg Institute of Astrophysics (5) Western Kenturky U (6) Columbia U (7) UC-Berkeley (8) NRAO

  2. Motivation (1) Observations (2) Theory • Strong correlation between star formation rate and H2 surface density • Constant SF efficiency in molecular clouds • Ability to form H2 controls the evolution of individual galaxies! • Krumholz et al. (2009) • Analytic solution for H2 content in an atomic-molecular complex • No direct comparison to individual molecular clouds in the MW! A high resolution study of the HI–H2 transition across a molecular cloud Perseus molecular cloud D ~ 300 pc and solar Z Low mass (~104 M) with intermediate SF log ΣSFR (M yr-1 kpc-2) • Estimate RH2 = ΣH2 / ΣHI • Investigate how RH2 spatially changes 30nearby spiral galaxies Bigiel et al. (2011) log ΣH2 (M pc-2)

  3. Background: Analytic Modeling of H2 Formation in a PDR • Krumholz et al. (2009; KMT) model CNM Pressure equilibrium with WNM H2 Uniform isotropic ISRF Sharp HI-H2 transition Equilibrium H2 formation: Formation on dust grains = Photodissociation by LW photons

  4. Background: Analytic Modeling of H2 Formation in a PDR • KMT's predictions: (1) Minimum ΣHI to shield H2 against ISRF 10 M pc-2 ΣHI~ 10 Mpc-2 for solar Z log ΣHI (M pc-2) (2) H2-to-HI ratio (RH2) MH2 / M log ΣHI + ΣH2 (M pc-2) RH2 is determined by CNM property, metallicity, gas surface density, and is independent of ISRF.

  5. RH2 = ΣH2 / ΣHI for Perseus • ΣHI : GALFA-HI DR1 data • ΣH2 : IRAS 60, 100 μm, Schelegel et al. Tdust, 2MASS AV images IRAS 100 μm image (~4.3': ~0.4 pc at D = 300 pc) GALFA-HI N(HI) image (~4')

  6. RH2 image 12CO contours Dark regions Star-forming regions B5 B1 NGC1333 IC348 B1E Lee et al. (2011, submitted)

  7. ΣHI vs ΣHI + H2 IC348 (Star-forming region) General results HI-dominated H2-dominated 1) Uniform ΣHI ~ 6–8 M pc-2 Consistent with KMT's prediction of ΣHI~ 10 Mpc-2 for solarZ! ΣHI (M pc-2) B1E (Dark region) 3σ H2-dominated HI-dominated 3σ ΣHI + ΣH2 (M pc-2) ΣHI (M pc-2) 2) No detection of turnover HI envelopes are highly extended (> 30 pc)! 3σ 3σ ΣHI + ΣH2 (M pc-2)

  8. RH2 vs ΣHI + H2 IC348 (Star-forming region) General results 3) Agreement with KMT on sub-pc scales 3σ 4) Best-fit parameter ΦCNM = 6–10 TCNM ~ 70 K , consistent with observed CNM properties (Heiles & Troland 2003)! RH2 = ΣH2 / ΣHI B1E (Dark region) 3σ 3σ ΣHI + ΣH2 (M pc-2) RH2 = ΣH2 / ΣHI 5) HI–H2 transition (RH2 ~ 0.25) at N(HI + H2) = (8–10) × 1020 cm-2 3σ Consistent with previous estimates in the Galaxy (e.g., Savage et al. 1977)! ΣHI + ΣH2 (M pc-2)

  9. Discussion: Equilibrium vs Non-equilibrium H2 Formation • Equilibrium H2 formation • τH2 = 10–30 Myr (e.g., Goldsmith et al. 2007) ≥ Lifetime of GMCs • Role of turbulence: non-equilibrium H2 formation? Equilibrium: RH2 ~ constant Non-equilibrium: RH2 keeps increasing Turbulence may play a secondary role! RH2 = ΣH2 / ΣHI Mac Low & Glover (2011) Time (Myr)

  10. Discussion: Importance of WNM / Internal Radiation Field • Importance of WNM for shielding H2 •  Importance of internal RF KMT: all CNM Perseus: WNM about 50% Perseus – Uniform external RF, negligible internal RF Tdust ~ 17 K Tdust image Lee et al. (2011, submitted)

  11. Summary The dark and star-forming regions have uniform ΣHI ~ 6–8 M pc-2. The purely HI envelopes are highly extended (> 30 pc). HI–H2 transition occurs at N(HI) + 2N(H2) = (8–10) × 1020 cm-2. KMT's equilibrium model captures the fundamental principles of H2 formation on sub-pc scales! The importance of WNM for H2 shielding, internal RF, and the timescale for H2 formation still remain as open questions.

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