Photodissociation of N2 and its impact on nitrogen
chemistry in protoplanetary disks
Catherine Walsh
NWO Veni Fellow
Leiden Observatory
Hideko Nomura (Tokyo Institute of Technology)
Ewine van Dishoeck (Leiden Observatory/MPE)
Protoplanetary Disks
Protoplanetary disks are chemically layered
Surface
Ion-molecule
chemistry
Molecular
layer
Photochemistry
Surface
chemistry
Migration
Mixing
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
100 AU
Far-IR and (sub)mm
Mid-IR
Warm molecule-rich region
CO, HCN, H2O,
OH, C2H2
CH4, CO2
Dense and frozen midplane where young
planets likely begin to form
Adapted from Mumma & Charnley, 2011, ARA&A, 49, 471
Protoplanetary Disks
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
0
13.4
13.6
13.8
14.0
Wavelength [micron]
14.2
14.4
Adapted from Pasucci et al. 2009, ApJ, 696, 143
Protoplanetary Disks
Zooming into the planet forming region (< 10 AU) ...
Higher spectral resolution Spitzer IRS spectra
Brown dwarf disks
T Tauri disks
Models
X-ray
luminosity
C/O
Adapted from Pascucci et al. 2013, ApJ, 779, 178
Nitrogen Chemistry
In the warm molecular regions
of protoplanetary disks,
nitrogen is mainly in the form
of N2 gas
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
Adapted from Hily-Blant et al. 2013, Icarus, 223, 585
Protoplanetary Disks
Does the radiation field present in the disk influence the nitrogen
chemistry through the photodissociation and shielding 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
K
G
F
A
Hot
10,000 K
Strong FUV
Protoplanetary Disks
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
Yes
Yes
No
X-ray model2
TW Hya
TW Hya
Lx ~ 1029 erg s-1
1 Young
low-mass stars have a strong UV excess thought to be triggered by accretion
stars are more X-ray bright than higher-mass stars (TW Hya, Lx ~ 1030 erg s-1)
2 Low-mass
Radiation Fields
Radiation fields at R = 1 AU
UV Flux Density (photons cm−2 s−1 Å−1)
1015
10
Ly−
Herbig Ae disk
14
1013
T Tauri disk
1012
1011
M Dwarf disk
1010
109
1000
Adapted from Walsh et al. 2015, in prep
1200
1400
1600
Wavelength (Å)
1800
2000 from Li et
Shielding
functions
al. 2013, A&A, 555, A14
Disk Physical
Structure
Adapted from Walsh et al. 2015, in prep
M Dwarf
T Tauri
10
103
4
3
2
10
(14 um) = 1
2
103
3
10
(14 um) = 1
2
1
101
0
1
2
3
4
5
6
Radius (AU)
7
8
9
10
M Dwarf Disk
5
FUV Flux (erg cm-2 s-1)
4
3
2
1
0
0
1
2
3
4
5
6
Radius (AU)
7
8
9
10
101
0
106
5
10
104
3
10
102
101
100
10-1
10-2
-3
10
10-4
10-5
10-6
6
103
4
3
2
102
(14 um) = 1
1
0
1
2
3
4
5
6
Radius (AU)
7
8
9
5
FUV Flux (erg cm-2 s-1)
4
3
2
1
0
0
1
2
3
4
5
6
Radius (AU)
0
106
5
10
104
3
10
102
101
100
10-1
10-2
-3
10
10-4
10-5
10-6
T Tauri Disk
7
8
9
10
Increasing gas temperature
Increasing FUV flux
101
0
10
6
Height (AU)
0
Herbig Ae Disk
Gas Temperature (K)
5
4
2
104
6
1
2
3
4
5
6
Radius (AU)
7
8
9
10
106
5
10
104
3
10
102
101
100
10-1
10-2
-3
10
10-4
10-5
10-6
6
Herbig Ae Disk
5
Height (AU)
1
Height (AU)
10
T Tauri Disk
Gas Temperature (K)
5
Height (AU)
Height (AU)
6
M Dwarf Disk
Gas Temperature (K)
5
Herbig Ae
4
Height (AU)
6
4
FUV Flux (erg cm-2 s-1)
4
3
2
1
0
0
1
2
3
4
5
6
Radius (AU)
7
8
9
10
Disk Molecular
Structure
Figures from Walsh et al. 2015, in prep
Dwarf Disk
MM Dwarf
Disk
TT Tauri
Tauri
Herbig Ae Disk
Herbig
Ae
0.0
10-4
10-5
-6
10
10-7
10-8
10-9
10-10
-11
10
-12
10
0.4
10
0.4
C 2H 2
C 2H 2
C 2H 2
Z/R
0.3
0.2
0.1
HCN
HCN
HCN
10-5
10-6
0.3
10
Z/R
-4
-7
10-8
0.2
10-9
0.1
0.0
0.1
1
R (AU)
10 0.1
1
R (AU)
10 0.1
1
R (AU)
Decrease in molecules in disk atmosphere with increasing spectral type
C2H2 and HCN relatively abundant in atmosphere of M Dwarf disk
10
10
-10
10
-11
10
-12
Disk Molecular
Structure
Figures from Walsh et al. 2015, in prep
Column Density (cm-2)
C 2H 2
10
20
10
19
1018
M Dwarf
Dwarf Disk
Disk
M
T Tauri
Tauri Disk
Disk
T
Herbig Ae
Ae Disk
Disk
Herbig
T Tauri C2H2/HCNC2Hratio
agrees
2
(14 µm) = 1
well with observations
M Dwarf C2H2/HCN ratio too low
by one order of magnitude
M Dwarf Disk
T Tauri Disk
Herbig Ae Disk
C2H
H
C
2 22
(14 µm) = 1
1017
10
16
10
15
10
14
Are X-rays or shielding playing a
role?
1013
10
12
10
11
1010
10
10
21
1020
10
19
10
18
10
17
10
16
HCN
HCN
(14 µm) = 1
1015
1014
1013
1
(AU)
M Dwarf Disk
T Tauri Disk
Herbig Ae Disk
N(C2H2)/N(HCN)
Column Density (cm-2)
HCN
22
1012
10 0.1
0.1
10
2
10
1
M Dwarf Disk
T Tauri Disk
M Dwarf Disk
T Tauri Disk
Herbig Ae Disk
HCN
(14 µm) = 1
M Dwarf observed
100
T Tauri observed
10-1
10-2
M Dwarf Disk
T Tauri Disk
Herbig Ae Disk
10-3
11
R
(AU)
R (AU)
10
10
0.1
0.1
1
R (AU)
1
R (AU)
10
10
n(X)/n(H2)
M Dwarf Disk
10
-3
10
-4
Fiducial
No shielding
No X-rays
N
N
Shielding and
X-rays
10-5
10
-6
10
-7
Figures from Walsh et al. 2015, in prep
M Dwarf
10-9
10-3
10
n(X)/n(H2)
Herbig Ae Disk
N
10-8
T Tauri
N2
Fiducial
No shielding
No X-rays
Fiducial
No shielding
No X-rays
Herbig Ae
N2
N2
-4
10-5
10-6
10-7
10-8
Fiducial
No shielding
No X-rays
10-9
n(X)/n(H2)
T Tauri Disk
10
-3
10
-4
10
-5
Fiducial
No shielding
No X-rays
Fiducial
No shielding
No X-rays
Fiducial
No shielding
No X-rays
Fiducial
No shielding
No X-rays
HCN
HCN
HCN
10-6
10-7
10-8
10
Fiducial
No shielding
No X-rays
-9
0.0
0.1
0.2
Z/R
0.3
0.4 0.0
0.1
0.2
Z/R
0.3
0.4 0.0
0.1
0.2
0.3
0.4
Z/R
ShieldingNdoes
increase
becomes
in importance
increasingly
in outer
more
disk
important
where gradients in
2 shielding
X-ray
density
chemistry
and UV becomes
are shallower
increasingly
(see e.g.,
less
Li et
important
al. 2013)
R = 1AU
M Dwarf Disk
10-3
T Tauri Disk
N2
Herbig Ae Disk
N2
N2
n(X)/n(H2)
10-4
Nitrogen
Reservoirs
10-5
10-6
10-7
Fiducial
N
N2
N2 ice
NH3 ice
10-8
M Dwarf
n(X)/n(H2)
10-9
10
-3
10
-4
Figures from Walsh et al.
Fiducial
N
N2
N2 ice
NH3 ice
10-5
Fiducial
N
2015,
N2 in prep
N2 ice
NH3 ice
T Tauri
NH3
NH3
NH3
Fiducial
N
N2
N2 ice
NH3 ice
Fiducial
N
N2
N2 ice
NH3 ice
HCN
HCN
Fiducial
N
N2
N2 ice
NH3 ice
Fiducial
N
N2
N2 ice
NH3 ice
10-6
10-7
10-8
10-9
10-3
10
n(X)/n(H2)
Herbig Ae
Fiducial
N
N2
N2 ice
NH3 ice
Fiducial
N
N2
N2 ice
NH3 ice
-4
10-5
HCN
10-6
10-7
10-8
10-9
0.0
0.1
R = 1AU
0.2
Z/R
0.3
0.4 0.0
0.1
0.2
Z/R
0.3
0.4 0.0
0.1
0.2
Z/R
Initial nitrogen reservoir not important in disk atmosphere
Is important in the disk midplane: building blocks of planets
0.3
0.4
Summary
✴ Stellar spectral type influences the molecular composition of the disk
✴ Disks around cooler stars have a higher fractional abundance of molecules in
the upper disk atmosphere: weak FUV is amenable to molecule synthesis
✴ C2H2/HCN trend reproduced for T Tauri stars: ratio for M Dwarfs low compared
with observations
✴ N2 shielding hinders the formation of N-bearing species in a narrow region of
the disk molecular layer only: effect is enhanced in disks around hotter stars
✴ Exclusion of X-rays helps to build more complexity in disk atmosphere around
cooler stars
✴ Initial nitrogen reservoir important in disk midplane only: building blocks for
planets
✴ Observations suggest higher C/O ratio in disks around cooler stars: indicative of
mixing in atmosphere?