Nitrogen isotopic ratio in the ISM and star
forming region
M. Gerin, E. Roueff, N. Biver, N. Marcelino, M. Elkeurti ,
L. Coudert, D. Bockelee-Morvan, J. Cernicharo, D. Lis,
What is the primordial 14N/15N ratio in the solar system?
Are there evidences for N fractionation ?
Are there evidences for Galactic gradient of 14N/15N ?
N/15N used to determine the Nitrogen sources of
solar system bodies : N2 vs NH3 and the evolution of the
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atmospheres
Observations of N/ N ?
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N+ 15N+
N2 -> N2H+ N2D+
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NNH+ , N15NH+ 15NND+ , N15ND+
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NO 15NO
NH3, NH2D,ND2H,ND3 15NH3, 15NH2D , 15ND2H
HCN, DCN HC15N DC15N
HNC, DNC H15NC
CN C15N
HC3N, HC315N, DC315N
CH3CN CH3C15N
CH3CH2CN CH3CH2C15N
Many pairs with 14N/15N; but very few with 2 isotopic
substitutions. Restricted source sample for most species
Previous surveys of 14N/15N : N2H+
Linke et al (1983) 1st detection of 15NNH+ and N15NH+
4 upper limits
DR21, massive star forming region. Analysis to infer
opacity of N2H+, not the 14N/15N ratio, with asumption
14N/15N ~ 270
Womack et al (1992) survey of N2H+, 15NNH+ & N15NH+
towards massive star forming regions.
weak 15NNH+ and N15NH+ lines, limited S/N ratio
Turner (2001): tentative detections in TMC-1 and L183
(20mK), N2H+/15NNH+ ~ N2H+/N1NH+ ~ 500 with large
uncertainty depending on the source density structure
Previous surveys of 14N/15N : HCN
expected H13CN/HC15N ~ 4.5 if
12C/13C = 60 & 14N/15N = 270
Ikeda et al : 3 low mass cores.
H13CN/HC15N ~ 3, 5 & 10
=> fractionation ?
Pillai et al (2007) : 12 cold,
massive cores (progenitors of
massive stars ?)
H13CN/HC15N = 17 +/- 7
14N/15N ~ 700 ?
Previous surveys of 14N/15N : HCN
Diffuse interstellar clouds
(Lucas & Liszt 1998)
1 source, 2 velocity
components
14N/15N = 235+/- 27
close to Earth value ?
Previous surveys : NH3
Güsten and Ungerechts (85)
DR21 & S140 14N/15N ~ 450 ;
W3(OH) 14N/15N ~ 300 - 360
Galactic Center 14N/15N ~ 1000
Wilson et al (00)
Orion hot core. Analysis consistent
with 14N/15N = 270
Evidence for Galactic Gradients
(HCN)
Dahmen et al ('95)
Wilson & Rood ('94)
Wielen & Wilson ('97)
Small variation with Radius
but large dispersion of data
points : S/N ratio + intrinsic
variation ?
New measurements of N/ N in cold
molecules : N2H+ & NH2D
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NNH+ and N15NH+ in CDMS : Hyperfine splitting.
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NH2D and 15NH2D Spectroscopy by Elkeurti, Coudert et al.
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No hfs for 15NH2D when compared with NH2D (same as for
NH3 or HCN)
sources : pre-stellar (Barnard 1b, L183, L1689N), low mass
(NGC1333-IRAS4A, LDN483), high mass (W33A)
Selected for high deuteration (ND2H, ND3) except
W33A (massive core, ~20K low deuteration)
W33A
N2H+
Strong N2H+
weak N2D+
LTE analysis :
N2H+/15NNH+ ~ 300
N2H+/N15NH+ ~ 300
N2H+/N2D+ > 150
N15NH+ &
HC3N v7
H13CN/HC15N ~ 7
NNH+
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Barnard 1:
N2H+
N2H+
Strong N2H+
strong N2D+
N2H+/15NNH+ ~> 500
N2H+/N15NH+ > 500
N2H+/N2D+ ~ 5
NNH+
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N15NH+
(if similar excitation
for all isotopologues)
Radiative transfer with hfs (Daniel &
Cernicharo 08)
Red : without overlap,
Black with overlap
The difference
increases with
abundance => effect
more important for
main isotopologue
1 models for W33A and B1 (F. Daniel & J.
Cernicharo)
st
Depends on source model
(density, T, v structure + local abundances)
1 Detection
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of NH2D
st
NH2D
Barnard 1:
Strong NH2D
ND3/NH3 ~ 0.1 %
N2HD/15NH2D ~ 430
NH2D
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NH2D/NH2D ~ 2.34(-3)
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(same as Jupiter)
15NNH+
Survey of N molecules for combined
analysis of N and D chemistry
more 15NH2D detections !
NH2D/15NH2D = 370
(NGC1333) ; ~ 600 (L1689N)
NGC1333
upper limits NGC1333-IRAS4A,
LDN1544
H13CN/HC15N = 5 to 8
depending on the source
! very preliminary !
L1689N
Mechanisms of Nitrogen Fractionation (see
talk by15 E.14Roueff)
How to derive the true N/ N ratio ?
Can we understand the observed 15N/14N in various
molecules and the origin of Nitrogen ?
Gas phase fractionation (Terzevia & Herbst '00 +
Charnley & Rodgers '02 , Rodgers & Charnley '08) :
N+ + N2 -> 15N14N + 14N+
15 +
N + NO -> 15NO + 14N+
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N + N2H+ -> 15NNH+ + 14N
-> N15NH+ + 14N
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N + CNC+ -> 14N + C15N1C+
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N + HCNH+ -> 14N + HC15NH+
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(ΔE = 28.3 K)
(ΔE = 24.3 K)
(ΔE = 27.7 K)
(ΔE = 36.1 K)
(ΔE = 36 K)
(ΔE = 39 K) ...
Compared to D fractionation : lower ΔE, smaller
fractionation for 15N.
Model predictions (Charnley & Rodgers '08)
N2=100%
N2=63%
N2=10%
T= 10 K(full)
7K(dashed
)
N2H+/15NNH+ ~N2H+/N15NH+ >> 2 x 14N/15N
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NH3/15NH3 starts ~ 14N/15N then increases
with time until freeze-out
Sensitivity to T , depletions & initial N/N2
Model predictions (E. Roueff)
T = 10K
N/15N = 400
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C, O
depletions
increases
with n
Coupled 15N and D chemistry : N fractionation can operate
simultaneously with D fractionation.
●
●
Strongest 15N fractionation in N2 and N2H+. 15N depletion in N,
CN, HCN & HNC
Conclusion - 1
1. Detection of multiply substituted molecules
(15NH2D, D13CN, DN13C ...)
--> new (weak) lines in cold & dense cores
--> very sensitive probes of chemistry
but
--> Improve the spectroscopic data bases (D15NC etc. )
2. Upcoming more sensitive instruments
IRAM : better receivers next year, ALMA
Herschel or SOFIA : access to 14N+ and 15N+ ? (fine structure
lines at 205 and 122 µm)
Conclusion - 2
2. Evidences for 15N fractionation: where is the
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N reservoir ? Do we understand N chemistry ?
NNH+ & N15NH+ : similar abundances but lower than
predicted ?
Variation of H13CN/HC15N between sources at same Galactic
Distance ?
What are the best probes of the true 14N/15N ?
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=> talk by E Roueff tomorrow
Solar system
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N/ N
N/14N = 2.35e(-3)
derived from Jupiter data.
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In meteorites 15N/14N not
uniform : hot spots 15N + hot
spots D/H
N Fractionation in comets.
Hypothesis : 15NH3 rich solid
material + C2H2 produces
HCN/HNC polymers, which
get degraded in HCN, HNC &
CN under solar radiation.
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Meibom et al. (2007)
Observations of N/ N ?
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Main nitrogen bearing molecules in CDMS and JPL
N2 -> N2H+ N2D+ 15NNH+ , N15NH+
NO
NH3, NH2D,ND2H,ND3 15NH3, 15NH2D , (15ND2H)
HCN, DCN HC15N (DC15N)
HNC, DNC H15NC
CN C15N
HC3N, HC315N, (DC315N)
CH3CN CH3C15N
Many pairs with 14N/15N; but very few with 2 isotopic
substitutions.
N/ N in dense cores ?
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Surveys of HC15N (Dahmen '95, Ikeda '02, Pillai'07).
different ratios for H13CN/HC15N .
But HCN affected by depletion + 13C; 15N fractionation ?
N/15N in non depleted molecules ?
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Strong D fractionation of NH3 & N2H+ in dense cores : NH3
and N2H+ are at most moderately affected by depletions
--> Good targets for 14N/15N.
1st detection 15NNH+ & N15NH+ in DR21(OH) (Linke et al 83)
Other detections in the N2H+ survey by Womack et al. '92.
N2H+/15NNH+ ~ N2H+/N15NH+ ~ 300 for massive cores