Äîêóìåíò âçÿò èç êýøà ïîèñêîâîé ìàøèíû. Àäðåñ îðèãèíàëüíîãî äîêóìåíòà : http://www.sao.ru/hq/katya/InterCloud/wiebe.pdf Äàòà èçìåíåíèÿ: Wed Feb 24 16:36:00 2010 Äàòà èíäåêñèðîâàíèÿ: Tue Oct 2 02:45:45 2012 Êîäèðîâêà: Ïîèñêîâûå ñëîâà: m 106

Formation of molecules in interstellar clouds
D. Wiebe (INASAN, Moscow, Russia)


In collaboration with...
B.M. Shustov, V.I. Shematovich, Ya.N. Pavlyuchenkov, A.I. Vasyunin, D.A. Semenov, Z.-Y. Li, Th. Henning, R. Launhardt

M.S. Kirsanova, A.M. Sobolev, W.D. Watson, R.M. Crutcher


"Stars are among the most fundamental building blocks of the universe, and yet the processes by which they are formed are not understood."
Derek Ward-Thompson Science, January 4, 2002


Star formation flow chart

Initial conditions for star formation?


Molecular clouds
· · · · Masses -- up to 6 · 106 M Sizes -- tens of pc Temperature -- 10­50 Density -- > 200 cm­3

Total mass -- some 109 M Stark et al. (2006)

51


Palla & Stahler (2002)


Stars are forming too slow

· ·

Molecular clouds are gravitationally (Jeans) unstable SFR in the Galaxy is 3 orders of magnitude lower than we would expect.

"Standard" model of star formation · · · Molecular clouds are long-living entities They are supported by the magnetic field (turbulence dissipates too fast) Magnetic support is gradually lost due to ambipolar diffusion (Mestel & Spitzer 1956)


Stars are forming too fast
· · We do not know molecular clouds without star formation (except for, may be, one) In star forming regions with molecular gas typical ages of young stars do not exceed 3 Myr

Gravoturbulent model of star formation · · · Molecular clouds are transient entities Magnetic field either is not important or, at least, is not a major factor (Crutcher, Hakobian, & Troland 2008) Prestellar cores (and, may be, molecular clouds themselves) form due to convergence of turbulent flows


Prestellar cores

Density profile Dust/gas temperature Magnetic field Velocity field Chemical composition (chemical clock)


Density distribution

Alves et al. (2001) Kirk et al. (2005)

Bonnor-Ebert profile


Dynamic cores in hydrostatic disguise

Of all artificial cores, about 65% have apparent BE profiles. Nearly half of these cores would be classified as gravitationally stable, even though in reality they are neither stable nor static.

Ballesteros-Paredes et al. (2003)


Goodman et al. (1992)


Magnetic field morphology

Myers & Goodman (1991)


Magnetic field strength
· Zeeman effect · Chandrasekhar-Fermi relation

1E-007

1E-008 X(OH) 1E-009

Crutcher & Troland (2000)
103
2 3 4 5 6 78

104

2

3

r, a.e.


Ward-Thompson et al. (2000)

L1544 L1498 L1517B
Kirk et al. (2006)


Velocity field

B68: Lada et al. (2003), Maret et al. (2007)

Tafalla et al. (2004)


Molecular composition

Tafalla et al. (2002)


Molecular lines as a discriminator for star formation models
Kinematics (quiescent vs transsonic) Magnetic field (OH vs CN) Age (short timescale vs long timescale) Chemistry is important!

It is complicated, it has thousands of free parameters, but it cannot be avoided...


Equations of chemical kinetics
dg g g ni (r , t ) = K lj nlg n g - nig K ij n g - k acc nig + kides nid j j j jl j dt dd d d ni (r , t ) = K lj nld n d - nid K ij n d + k acc nig - kides nid j j j jl j dt

· Gas-Phase Chemistry · Gas-Dust Interaction · Surface Chemistry


Initial Conditions for Star Formation Atomic gas Diffuse clouds Molecular clouds Prestellar cores Protostellar objects Young stars Initial conditions are especially important for chemistry


What is a diffuse cloud?

H

+

H
Diffuse atomic

H

2

C
Translucent

+

C

CO

Diffuse molecular

Dense molecular

Property A
v

f(H2) < 0.1 0 10­100 30­100 UV, visible, 21 cm

f(H2) > 0.1 f(C+) > 0.5 0.2 100­500 30­100 UV, visible, IR, mm (abs)

f(C+) < 0.5 f(CO) < 0.9 1­2 500­5000 15­50 UV, visible, IR, mm (abs) & mm (emis)

f(CO) > 0.9 5­10 > 5000 10­50 IR (abs), mm (emis)

nH (cm­3) T (K) Observational techniques

Snow & McCall (2006)

AV = 0m ­1

m

AV = 1m ­5

m


Molecules in diffuse clouds
Weight 2 3 3 13 13 14 15 17 24 25 26 27 27 28 28 29 29 29 29 30 30 36 36 38 44 64 Species H2 HD H3+ CH CH+ 13CH+ NH OH C2 C2H CN HCN HNC N2 CO HCO+ HOC+ 13CO C17O C18O H2CO C3 HCl C3H2 CS SO2 Method UV UV IR Optical Optical Optical Optical UV Optical mm abs. Optical mm abs. mm abs. UV UV mm abs. mm abs. UV UV UV mm abs. Optical UV mm abs. mm abs. mm abs. Target Oph Oph Per Oph Oph Oph Oph Oph Oph BL Lac Oph BL Lac BL Lac HD 124314 X Per BL Lac BL Lac X Per X Per X Per BL Lac Oph Oph BL Lac BL Lac BL Lac N(X)/NH 0.56 4.5 (-7) 5.1 (-8) 1.5 (-9) 2.4 (-8) 3.5 (-10) 6.2 (-10) 3.3 (-8) 1.3 (-8) 1.8 (-8) 1.9 (-9) 2.6 (-9) 4.4 (-10) 3.1 (-8) 6.4 (-6) 1.5 (-9) 2.2 (-11) 8.9 (-8) 7.4 (-10): 2.1 (-9): 3.7 (-9) 1.1 (-9) 1.9 (-10) 6.4 (-10) 1.6 (-9) 8.2 (-10)

Snow & McCall (2006)

+ H2S, HCS+ (Lucas & Liszt 2002), NH3 (Liszt et al. 2006), HOC+ (Liszt et al. 2004)


Molecular composition of diffuse clouds
1. Physical conditions in diffuse clouds 2. Dynamical evolution of molecular clouds 3. Initial conditions for chemical evolution in prestellar cores

Hydrogen is not entirely molecular

Other molecules are abundant


What is used?
Study Initial density 3 · 103 3 · 104 103 Initial abundances

Bergin & Langer (1997) Aikawa et al. (2001) Shematovich et al. (2003) Lee at al. (2004) Pavlyuchenkov et al. (2006) Tsamis et al. (2008)

103 5 · 103 103

Hydrogen is totally molecular; other elements are atoms or ions Hydrogen is totally molecular; other elements are atoms or ions Hydrogen is totally molecular; other elements are atoms or ions; some C and O are converted to CO Hydrogen is totally molecular; other elements are atoms or ions Hydrogen is totally molecular; other elements are atoms Chemical equilibrium


Role of molecular hydrogen
C + + H 2 4500 K CH + + H CH + + e - C + H CH + + H C + + H CH + + H 2 CH
+ 2 2

+H

The oldest unsolved astrophysical problem (Liszt 2006): C-shocks? Diffusion? Turbulent dissipation?


Dark cloud chemistry in initially H-rich regions
Rawlings et al. (2002) XC -- H molecular XD -- H atomic


Model H-atomic

0.3 0.2 0.1 0.0 103 10-4 10-5 10-6 10-7 10-8 10-9 10-1
0

TMC-1 H atomic H molec ular

104

n, cm

105 ­3

106

CO
10-6 10-7 103 10-7 10-8 10-9

Initial abundances Hydrogen is totally atomic; other elements are atoms H-molecular Hydrogen is totally molecular; other elements are atoms TMC-1 Complex molecular composition, but hydrogen is still totally molecular

0.6 0.5 0.4

10-4

10-5

H2

104

n, cm

105 ­3

106

CS
3 4 5 6

10-1 10-1 10-1 10-1

C

0

1

2

3

10

10

n, cm

10 ­3

10

103

104

n, cm

105 ­3

106


1 = m

-0 -200
2

104

-400 -800

m u = - 2 - 2 m b 1 .4 = m ff u=

c

nH, cm­3

b2 + 2 c 2 b 2 2 b m

v, cm/s

-600

-1000 -1200 -1400 -1600 10
17

103
f(H)=0.1 f(H)=0.2 f(H)=1.0

r, cm

10

18

102

1017

r, cm

1018

= =

B t
ff ni

8 c a

c

2

ff



=

i 4 G
x(C+)

10-5 10-6 10-6
e

Shematovich et al. (2003)

10-7

x
1017 1018

10-7 10-8 1017 1018

r, cm

r, cm


Why do we take wrong initial conditions?


Gas-phase 2 formation
Three-body reaction: H + H + H H2 + H Very slow reaction: H+ + H H2+ + h H2 + + H H2 + H+ Very slow reaction: H + e­ H­ + h H+ + H­ H2 H2 + + H­ H2 + H


Dust surface H2 formation

1.

H atoms stick to a grain.

2.

They migrate over its surface, collide, and form H2 molecule.

3.

Newly formed H2 desorbs into the gas-phase.

It is necessary to include surface chemistry in the model


Dust surface H2 formation
Even on surfaces it is a very slow process Effective only at dust temperatures between 10 and 20 Newly formed H2 is dissociated by UV photons


Dissociation

2

Hosokawa & Inutsuka (2006)

Dixon et al. (1998) Jenkins et al. (2000) Self-shielding


H2 accumulation in quiescent molecular clouds
Glover & McLow (2007a)


H2 accumulation in turbulent molecular clouds
Glover & McLow (2007b)


Dobbs & Bonnell (2007)

Other molecules (in particular, CO)
·Tracers ·Heating & cooling ·Fractional ionization


Other molecules: role of surface chemistry

E

D

E

b

Hasegawa, Herbst, Leung (1992) Rate equation approach to surface chemistry


Unified Monte Carlo treatment of gasgrain chemistry
1. The Chemical Master Equation, universal and very slow

2. Rate Equations, very fast, but not universal

Unified treatment of gas-phase and surface chemistry in large chemical networks -- Vasyunin et al. (submitted)


50K

Model T, 104 years
0. 9

50K 30K

Model T, 105 years
0.8 5

50K 30K 25K 20K

Model T, 106 years

Temperature

30K 25K 20K
0.85 0.9 0.9 5 0.97

0.9

0 .9 0.85 0.8 0.75

25K 20K

0.8 0.85

0.8
0.85

0.9

15K
10K

RE
2 103

15K
10K

15K

RE
2 103 2 10

0.85

10K
4

0.9

RE
2 10
3

2 102

2 104 2 102

2 102

2 104

Density, cm­3
10-9 10-1
0

H2O
Abundance
10-1
0

10-11

NH

3

Abundance
MC RE

10-1

2

10-13 10-1 10-1
4

10-11

5

10-12 102

103

104

105

106

10-16 102

103

104

105

106

Time, yrs


Model T-RE, 10(6) yrs 50
Temperature, K

30 25 20 15 10 2(2)
10-4

-0 .3

0.3 0

1

0

2(3) Gas density, cm

2(4)
-3

Abundance

10-5

10-6

MC RE

10-7 10

2

10

3

10

4

10

5

10

6

Time, yrs


10

-6 -7 -8

Abundance

10 10 10

gH2O

10
2

-5

10 10

-6

O2

MC RE

-7

-9 0

10-1 10

10

-8

-11 2

10-1

Model T-RE 4 ­ n = 2 · 10 cm T = 10 K
10
3

3

10 10 105 10
6

-9

-10

Model T-RE 4 ­ n = 2 · 10 cm T = 30 K
10
3

3

10-13 102

10

4

10-11 102

10

4

105

10

6

Time, yrs
Incorrect treatment of surface reaction leads to significant errors in computed abundances of important molecules at densities and temperatures typical of translucent clouds. Grain size, composition, structure...


Evolutionary status of diffuse clouds
Warm medium Cold (diffuse) medium

Cold (diffuse) medium

Molecular cloud


Collapse vs expansion

Price et al. (2003)


Conclusions
1. Diffuse and translucent clouds represent truly initial conditions for prestellar cores. 2. Abundances in prestellar cores (and conclusions which are based on these abundances) sensitively depend on the initial fraction of molecular hydrogen. 3. Incorrect treatment of surface reaction leads to significant errors in computed abundances of important molecules at densities and temperatures typical of translucent clouds.