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.