Nitrogen chemistry in the inner regions of
protoplanetary disks
Catherine Walsh (Leiden Observatory)
Hideko Nomura (Kyoto University)
Ewine van Dishoeck (Leiden Observatory)
Wednesday, 25 September 13
1
Outline
✴ Introduction
✴ Protoplanetary disk model
✴ Results
✴ Conclusions and further work
Wednesday, 25 September 13
2
Introduction
Wednesday, 25 September 13
3
Low-mass star
formation
Schematic view of low-mass star formation and planetary system formation
Outflow
X 1000
Wednesday, 25 September 13
Infall
Cloud Collapse
Protoplanetary Disk
Planet Formation
Planetary System
4
Protoplanetary
disks
Protoplanetary disks are physically and chemically complex environments
Figure adapted from Mumma & Charnley 2011
Ion-molecule
chemistry
Molecular
layer
Photochemistry
Surface
chemistry
Migration
Mixing
CH+,
H2O, OH,
CO, HCN,
HNC, SO, H2CO, HC3N, cC3H2, CN, CS, C2H, HCO+, N2H
+, isotopologues
100 AU
Far-IR and (sub)mm
Wednesday, 25 September 13
Surface
Midplane
Cosmic
rays
Thermochemistry
CO, HCN, H2O,
OH, C2H2
1 AU
Near-IR
Hot and diffuse photo-dominated
surface abundant in ions and
atoms
10 AU
Mid-IR
Warm molecule-rich region
CO, HCN, H2O,
OH, C2H2
CH4, CO2
Dense and frozen midplane where young
planets likely begin to form
5
Motivation
Zooming into the planet forming region (< 10 AU) ...
Median of continuum subtracted normalized spectra
Spitzer IRS spectra
Sun-like stars
show
C2H2 emission at 13.7 μm
AND
HCN emission at 14.0 μm
HCN
Sun-like star (scaled)
3
Cool Star
2
Cool stars
show
C2H2 emission at 13.7 um
AND NO
HCN emission at 14.0 um
C2H2
1
The question is why?
0
13.4
13.6
13.8
14.0
Wavelength [micron]
14.2
14.4
Figure adapted from Pasucci et al. 2009
Wednesday, 25 September 13
6
Nitrogen
chemistry
In the warm molecular regions
of protoplanetary disks,
nitrogen is mainly in the form
of N2
N2 is not observable in cold/
warm gas so need to use
proxies such as N2H+ and CN/
HCN
Many complex
organic molecules are
N-containing species,
e.g., the simplest
amino acid, glycine,
NH2CH2COOH
Photodissociation of
N2 releases N for
incorporation into
HCN
Complex Organic
Molecules
Figure adapted from HilyBlant et al. 2013
Wednesday, 25 September 13
7
Questions
Does the radiation field present in the disk influence the nitrogen
chemistry through the photodissociation of N2?
If so, can this explain the trend seen in the strength of HCN line
emission at 14 μm?
M
Cool
3000 K
Weak FUV
Wednesday, 25 September 13
K
G
F
A
Hot
10,000 K
Strong FUV
8
Protoplanetary disk
model
Wednesday, 25 September 13
9
Model: Physics
Spectral type
M Dwarf
T Tauri
Herbig Ae
Effective
temperature
3000 K
4000 K
10,000 K
Stellar mass
0.1 Mo
0.5 Mo
2.0 Mo
Stellar radius
0.7 Ro
2.0 Ro
2.0 Ro
UV excess1
No
Yes
No
X-ray model2
TW Hya
TW Hya
Lx ~ 1029 erg s-1
1T
Tauri stars have a strong UV excess thought to be triggered by accretion
2 Low-mass stars are more X-ray bright than higher-mass stars (TW Hya, L ~ 1030 erg s-1)
x
Wednesday, 25 September 13
10
Model: Chemistry
✴ Gas-phase chemistry
‣ UMIST Database for Astrochemistry, ‘RATE12’ (McElroy et al. 2013)
‣ Photochemistry (http://home.strw.leidenuniv.nl/~ewine/photo/)
‣ Hot H2 reactions (Bruderer et al. 2013)
‣ H2, CO, and N2 self- and mutual shielding (Visser et al. 2011; Li et al. 2013)
✴ Gas-grain interactions (Walsh et al. 2010,2012,2013a)
‣ Freeze out
‣ Thermal desorption
‣ Non-thermal desorption (cosmic rays, UV photons, chemical/reactive)
✴ Grain-surface chemistry (Walsh et al. 2013b)
‣ Two-body association reactions (Garrod et al. 2006, 2008)
Wednesday, 25 September 13
11
Model results
Wednesday, 25 September 13
12
Results: disk
structure
M Dwarf
T Tauri
M Dwarf Disk
Gas Temperature (K)
7
6
10
5
4
3
10
3
2
2
6
10
5
4
3
10
2
1
0
106
105
104
103
102
101
100
10-1
10-2
10-3
10-4
10-5
10-6
9
10
0
1
2
3
9
4
5
6
Radius (AU)
7
8
9
10
M Dwarf Disk
8
-2 -1
FUV Flux (erg cm s )
7
Height (AU)
6
5
4
3
2
1
0
0
1
2
3
4
5
6
Radius (AU)
7
8
9
10
103
6
5
4
102
3
2
1
=1
0
Height (AU)
7
3
2
1
Herbig Ae Disk
Gas Temperature (K)
8
1
=1
10
0
1
2
3
4
5
6
Radius (AU)
7
8
9
10
T Tauri Disk
8
-2 -1
FUV Flux (erg cm s )
7
6
1
106
105
104
103
102
101
100
10-1
10-2
10-3
10-4
10-5
10-6
Height (AU)
Height (AU)
7
104
9
T Tauri Disk
Gas Temperature (K)
8
Height (AU)
8
104
9
Height (AU)
104
9
Herbig Ae
5
4
3
2
1
0
0
1
2
3
4
5
6
Radius (AU)
7
8
9
10
=1
101
0
0
1
2
3
9
4
5
6
Radius (AU)
8
Herbig Ae Disk
7
FUV Flux (erg cm-2 s-1)
7
8
9
10
7
8
9
10
6
5
4
3
2
1
0
0
1
2
3
4
5
6
Radius (AU)
106
105
104
103
102
101
100
10-1
10-2
10-3
10-4
10-5
10-6
Increasing gas temperature
Increasing FUV flux
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13
Results: molecular
structure
T Tauri
10
M Dwarf Disk
x(HCN)
8
6
10-7
5
10-8
4
1
T Tauri Disk
x(HCN)
6
5
4
0
1
2
3
9
M Dwarf Disk
x(C2H2)
8
7
4
5
6
Radius (AU)
7
8
9
10
10-5
8
10-6
7
Height (AU)
6
9
10-7
5
10-8
4
10-9
3
=1
1
1
2
3
T Tauri Disk
x(C2H2)
4
5
6
Radius (AU)
7
8
=1
9
10
6
5
4
0
1
2
3
4
5
6
Radius (AU)
7
8
9
10
Molecular layer closest to disk
“surface”
10-7
6
0
1
2
3
4
5
6
Radius (AU)
7
8
9
10
Herbig Ae Disk
x(HCN)
10-5
10-6
10-7
10-8
4
10-9
3
10-10
2
1
10-12
0
10-4
9
10-5
8
10-6
7
10-7
6
10-11
=1
10-12
0
10-10
=1
10-4
5
10-11
10-9
3
10-12 0
0
7
10-8
10-10 2
10-11 1
2
10-6
10-10
0
10-4
8
10-9
10-12 0
0
10-5
10-8
10-11 1
=1
9
10
10-9 3
10-10 2
3
2
Height (AU)
9
10-5 8
10-6 7
Height (AU)
Height (AU)
7
Herbig Ae
-4
Height (AU)
9
-4
Height (AU)
M Dwarf
1
2
3
Herbig Ae Disk
x(C2H2)
4
5
6
Radius (AU)
7
8
9
10
10-5
10-6
10-7
5
10-8
4
10-9
3
10-10
2
10-11
1
10-12
0
10-4
10-11
=1
10-12
0
1
2
3
4
5
6
Radius (AU)
7
8
9
10
Deeper, much narrower molecular layer
HCN and C2H2 are both abundant in the disk above the τ = 1 surface (N(H2) ~ 5 x 1021 cm-2)
How does this affect the column densities?
Wednesday, 25 September 13
14
Results: molecular
structure
10
20
10
19
1021
M Dwarf
T Tauri
Herbig Ae
N(HCN)
Column density (cm-2)
Column density (cm-2)
1021
1018
1017
1016
1015
1014
1013
1012
10
20
10
19
1018
1017
1016
1015
1014
1013
1012
N( = 1)
1011
M Dwarf
T Tauri
Herbig Ae
N(C2H2)
N( = 1)
1011
0.1
1
Radius (AU)
10
0.1
1
Radius (AU)
10
Largest column density of both HCN and C2H2 in “observable” layer in M Dwarf model
Lower FUV flux
Decreased photodissociation of N2 AND HCN
Photodissociation is not crucial for the conversion from N2 to HCN
Wednesday, 25 September 13
15
Results: molecular
structure
1021
1019
10
Column density (cm-2)
Column density (cm-2)
10
M Dwarf
T Tauri
Herbig Ae
N(HCN)
20
1021
1018
1017
1016
1015
1014
1013
1012
1019
1018
1017
1016
1015
1014
1013
1012
N( = 1)
1011
M Dwarf
T Tauri
Herbig Ae
N(C2H2)
20
N( = 1)
1011
0.1
1
Radius (AU)
10
0.1
1
Radius (AU)
10
Migration of HCN ‘snowline’ outwards with increasing spectral type
Wednesday, 25 September 13
16
Results: N2 shielding
Does N2 shielding play a role in the nitrogen chemistry?
9
10
T Tauri Disk
N2
Height (AU)
6
5
T Tauri Disk
HCN
8
No shielding
------------------With shielding
7
101
9
10
4
10
0
10
-1
3
2
7
1
1
Height (AU)
8
2
6
5
No shielding
------------------With shielding
100
4
3
2
1
10-2
0
0
1
2
3
4
5
6
Radius (AU)
7
8
9
10
10-1
0
0
1
2
3
4
5
6
Radius (AU)
7
8
9
10
Neglect of N2 shielding
Decrease in N2 fractional abundance (~ 100) and small increase in HCN abundance (~ a few)
N2 shielding hinders the production of HCN over a very narrow region of this disk
Wednesday, 25 September 13
17
Summary
Wednesday, 25 September 13
18
Summary
✴ Stellar spectral type influences the molecular composition of the disk
✴ Disks around cooler stars have a greater fractional abundance of molecules in the
upper disk atmosphere
✴ HCN and C2H2 are most abundant in the disk around the M Dwarf star in contradiction
with mid-IR observations
✴ Decreased FUV flux leads to decreased photodissociation of both N2 and HCN
✴ Photodissociation of N2 (releasing N) is not crucial for the formation of HCN
✴ N2 shielding hinders the formation of N-bearing species in a narrow region of the disk
molecular layer only
Wednesday, 25 September 13
19
Further
questions
Why the apparent disagreement between models and observations?
Median of continuum subtracted normalized spectra
HCN
Sun-like star (scaled)
3
Could HCN excitation
conditions be responsible for
the lack of HCN emission
from disks around cool stars?
Cool Star
2
Cool stars are also bright in X-rays
C2H2
Are X-rays helping release N from
N2 to form other N-containing
species such as HCN?
1
0
13.4
13.6
13.8
14.0
Wavelength [micron]
14.2
14.4
Figure adapted from Pasucci et al. 2009
Wednesday, 25 September 13
20