HH22 Formation in the Perseus Molecular Formation in the Perseus Molecular Cloud:Cloud:

Observations Meet Theory Observations Meet Theory

MotivationMotivation(2) Theory

• Krumholz et al. (2009)• Analytic solution for H2 content in

an atomic-molecular complex• No direct comparison to individual

molecular clouds in the MW!

(1) Observations • 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!

log

ΣS

FR (

M y

r-1 k

pc-2

)

log ΣH2 (M pc-2)

30 nearby spiral galaxies Bigiel et al. (2011)

A high resolution study of the HI–H2 transition across a molecular cloud

• Estimate RH2 = ΣH2 / ΣHI

• Investigate how RH2 spatially changes

Perseus molecular cloudD ~ 300 pc and solar Z Low mass (~104 M) with

intermediate SF

Background:Background:Analytic Modeling of HAnalytic Modeling of H22 Formation in Formation in

a PDR a PDR • Krumholz et al. (2009; KMT) model

H2

CNM Pressure equilibrium with WNM

Sharp HI-H2 transition

Uniform isotropic ISRF

Equilibrium H2 formation: Formation on dust grains = Photodissociation by LW photons

Background:Background:Analytic Modeling of HAnalytic Modeling of H22 Formation in Formation in

a PDR a PDR • KMT's predictions:

RH2 = fH2 / fHI

= 1+s

11⎛⎝⎜

⎞⎠⎟

3

+125 + s

96 + s⎛⎝⎜

⎞⎠⎟

3⎡

⎣⎢⎢

⎤

⎦⎥⎥

1/3

−1 where s ~Σ total Z

χ

= f (nCNM , Z, Σ total )

RH2 is determined by CNM property, metallicity, gas surface density, and is independent of ISRF.

log ΣHI + ΣH2 (M pc-2)

log

ΣH

I (M

pc-2

) M

H2 /

M

(1) Minimum ΣHI to shield H2 against ISRF ΣHI ~ 10 M pc-2 for solar Z

(2) H2-to-HI ratio (RH2)

10 M pc-2

IRAS 100 μm image (~4.3': ~0.4 pc at D = 300 pc)

GALFA-HI N(HI) image (~4')

RRH2H2 = = ΣΣH2H2 / / ΣΣHI HI for Perseus for Perseus

• ΣHI : GALFA-HI DR1 data

• ΣH2 : IRAS 60, 100 μm, Schelegel et al. Tdust, 2MASS AV images

RRH2H2 image image

12CO contoursDark regions

Star-forming regions

B5

B1E

B1

IC348 NGC1333

Lee et al. (2011, submitted)

ΣΣHIHI vs vs ΣΣHI + H2 HI + H2

1) Uniform ΣHI ~ 6–8 M pc-2

General results

Consistent with KMT's prediction of ΣHI ~ 10 M pc-2 for solar Z!

2) No detection of turnover HI envelopes are highly extended (> 30 pc)!

ΣH

I (M

pc-2

)

ΣHI + ΣH2 (M pc-2)

3σ

3σ

IC348(Star-forming region)

HI-dominated H2-dominated

Σ

HI (M

pc-2

)

ΣHI + ΣH2 (M pc-2)

3σ3σ

B1E(Dark region)

HI-dominated H2-dominated

RRH2H2 vs vs ΣΣHI + H2 HI + H2

5) HI–H2 transition (RH2 ~ 0.25) at N(HI + H2) = (8–10) × 1020

cm-2 Consistent with previous estimates in the Galaxy (e.g., Savage et al. 1977)!

B1E(Dark region)

RH

2 = Σ

H2 /

ΣH

I

ΣHI + ΣH2 (M pc-2)

IC348(Star-forming region)

RH

2 = Σ

H2 /

ΣH

I

ΣHI + ΣH2 (M pc-2)

3σ

3σ

3σ

3σ

General results

4) Best-fit parameter ΦCNM = 6–10 TCNM ~ 70 K , consistent with observed

CNM properties (Heiles & Troland 2003)!

3) Agreement with KMT on sub-pc scales

Discussion:Discussion:Equilibrium vs Non-equilibrium HEquilibrium vs Non-equilibrium H22

FormationFormation• Equilibrium H2 formation

τH2 = 10–30 Myr (e.g., Goldsmith et al. 2007) ≥ Lifetime of GMCs

• Role of turbulence: non-equilibrium H2 formation?

Time (Myr)

RH

2 = Σ

H2 /

ΣH

I

Mac Low & Glover (2011)

Equilibrium: RH2 ~ constant Non-equilibrium: RH2 keeps increasing Turbulence may play a secondary role!

Discussion:Discussion:Importance of WNM / Internal Radiation Importance of WNM / Internal Radiation

FieldField• Importance of WNM for shielding H2

Importance of internal RF

Tdust image Lee et al. (2011, submitted)

Tdust ~ 17 K

KMT: all CNM Perseus: WNM about

50%

Perseus – Uniform external RF, negligible internal RF

SummarySummary

1) The dark and star-forming regions have uniform ΣHI ~ 6–8 M pc-2.

2) The purely HI envelopes are highly extended (> 30 pc).

3) HI–H2 transition occurs at N(HI) + 2N(H2) = (8–10) × 1020 cm-2.

4) KMT's equilibrium model captures the fundamental principles of H2 formation on sub-pc scales!

5) The importance of WNM for H2 shielding, internal RF, and the timescale for H2 formation still remain as open questions.