Isotopologues ; probes of ISM environments and chemical processes. data needs for ALMA

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1 Isotopologues ; probes of ISM environments and chemical processes 1

2 Isotopologues? Main elements involved in interstellar & circumstellar molecules : H,D 12C 13C 14N 15N 16O 17O 18O 24Mg, 25Mg, 26Mg 28Si 29Si 30Si 32S 33S 34S 36S 35Cl 37Cl Many molecules with different isotopes of the same element : isotopologues

3 Finding and identifying Isotopologues? Deuterated molecules H3+ : H2D+, D2H+, N2H+,N2D+ H2O : HDO & D2O, H2CO : HDCO, D2CO NH3 : NH2D, ND2H, ND3 HCO+ : DCO+, HCN : DCN HNC : DNC CCH : CCD c-c3h2 : c-c3hd H2S : HDS, D3S C4H : C4D CH3OH : CH2DOH, CHD2OH, CD3OH, CH3OD HCOOH, HCOOD, DCOOH H2CS, HDCS, D2CS CH3CCH : CH2DCCH, CH3CCD CH3CN, CH2DCN, HC3N : DC3N HC5N : DC5N

4 Deuterated molecules Specific environments : cold cores (eg L1544) T ~ 10K T,, n > 104 cm-3, quiet, D/H ~10% hot corinos near protostellar sources (eg NGC1333 IRAS4A, IRAS ) D/H > 10% in organics hot cores (eg Orion-IRc2), D/H ~ 0.1 to 1%) Pety et al 07, Goicoechea et al 09

5 Deuterium fractionation reactions Rule : abundant neutral (reservoir), ion ~abundant (not reacting with H2) + rapid reaction in the forward direction (see eg Roberts, Herbst et al 2004, Flower et al 2004, Roueff et al 2005, 2006, etc.). H3+ + HD H2D+ + H K HD2+ + HD D2H+ + H K D2H+ HD D3+ + H K CH3+ + HD CH2D+ + H K CH2D+ + HD CHD2+ + H K CHD3+ + HD CD3+ + H K C2H2+ + HD C2HD3+ + H K

6 Gas phase deuterium chemistry H3+ deuteration dominates at low T, eg in pre-stellar cores But DCN and other molecules are detected in warm gas + ( ~50K) deuteration by CH2D (Roueff et al. 2006) Different patterns of D/H ratios depending on the temperature. DCO+, N2H+,NH2D trace H2D+ DCN, HDCO, CCD are related to CH2D+ (C2HD+) and H2D+ Multiply deuterated neutral species : branching ratio in dissociative recombination Need for more information on spectroscopy + chemical reaction rates & branching ratios data as needs a function of T for ALMA

7 Solid phase deuterium chemistry Origin of deuteration in star forming regions? Process in several steps depletions in the pre-stellar phase with freeze-out (this enhance D/H in the gas phase for H3+) Competition of deuteration and hydrogenation in the mantles because D/H is large (> 1%) in the gas Desorption of highly deuterated species whith the activity of the protostar. But : no solid HDO or CD OH detected so far (massive stars) 3 (Dartois et al 2003, Parise et al 2003) (D/H < %) Need for more experiments (fractionation, thermal & non-thermal desorption thermal, + spectroscopy of D in organics)

8 Spatial distribution & radiative transfer example with N2H+ and N2D+ (Daniel et al, Pagani et al) + N2H is well correlated with submm dust continuum : trace cold dense cores. Molecules with hyperfine structure : anomalous intensity ratio of components => treat line overlap in radiative transfer. 2 Need for collisional excitation rates including hyperfine components

9 Radiative transfer with hfs (Daniel & Cernicharo 08) Red : without overlap, Black with overlap The difference increases with abundance => effect more important for main isotopologue

10 Disks + DCO & DCN in TW Hya (Qi et al 2008) Radial variations of D/H for DCO+ Same fractionation processes as in dense cores? possibility to characterize the mid plane if the chemistry & excitation are well understood

11 Finding and identifying Isotopologues? Deuterated molecules Detections of molecules with ~ small number of atoms spectroscopic data in data bases OK for detected molecules ==>Les good situation for most of interstellar molecules, even molecules in hot corinos and dense cores : HNCO? HCOOCH3? Can these lines contribute to the line forest? a priori yes if D/H as high as for CH3OH (> 10% in hot corinos, 0.1 to 1% in hot cores). study of D fractionation of IS matter and connection with enrichment of organic phase of solid material needs knowledge of D/H in more complex molecules.(ideally all detected IS molecules)

12 Nitrogen isotopologues 14N/15N used to probe the Nitrogen sources of solar system bodies : N2 vs NH3? What is the primordial 14N/15N ratio? What is the evolution of the atmosphere? What are the escape mechanism? 14N/15N also interesting for probing nucleosynthesis processes, together with 16O/17O, 16O/18O, 12C/13C, etc. Few measurements of 14N/15N so far in HCN, CN, NH3. limited S/N. Tentative evidence for some variations between sources and molecules

13 Solar system 15N/14N Solar system 15N/14N ~ 2.3e(-3) from Jupiter, Solar wind & CAI data. Earth : 15N/14N = 3.67e(-3) high 15N/14N and D/H in comets. Hypothesis 15NH3 rich solid material Meibom et al. (2007), Marty et al 2009 In meteorites 15N/14N not homogeneous : 15N hot spots + D hot spots not coincident spatially

14 The mechanisms of Nitrogen Fractionation Gas phase fractionation (Terzevia & Herbst '00 + Charnley & Rodgers '02, Rodgers & Charnley '08) : + 15N + N2 15N + N2H+ + -> 15N14N + 14N (ΔE = 28.3 K) -> 15NNH+ + 14N (ΔE = 27.7 K) -> N15NH+ + 14N (ΔE = 36.1 K)... Compared to D f : lower ΔE, smaller fractionation for 15N. Still many unknown in the reaction rates and branching ratios

15 Model predictions from E. Roueff) (see also Charnley & Rodgers) Coupled 15N and D chemistry : N fractionation can operate simultaneously with D fractionation Sensitivity to T and depletions + Strongest predicted fractionation in N and N H.

16 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 CH3CH2CN CH3CH2C15N HNCO CH2NH, etc. Many pairs with 14N/15N; but very few with 2 isotopic substitutions.

17 Barnard 1b NH2D Measuring 14N/15N in cold molecules : NH3 & NH2D 14NH2D and 15NH2D Spectroscopy by El Keurti et al. (2008) No or weak hfs for 15NH2D when compared with NH2D 15NH2D (same as for NH3 or HCN) NH2D/15NH2D ~ 470 (consistent with Jupiter) (Gerin et al 2009) 15NNH+ NH3/15NH3 ~ 360 (GBT) (Lis et al in prep)

18 Oxygen isotopologues Selective photodissociation of CO => anomalous isotopic ratio at low Av with CO/C18O > 16O/18O well characterized in the local ISM. Measurements of 12C18O and 13C18O Could be at work in protoplanetary disks (Lyons et al 09) CO enriched in 16O -> O enriched in 18O and 17O -> heavy H2O -> heavy silicates Very few information on other molecules related to CO or H2O (eg H2C18O, CH318OH, else?) Need for accurate photodissociation data (cross section as a function of FUV wavelengths, rates, etc.)

19 C isotopologues 13C fractionation at low Av due to reaction with 13C+ 13C+ + 12CO -> 13CO + 12C+ + ΔE (35 K) Different abundances of molecules with more than 2 carbon atoms (eg 13CCH and C13CH) => contraints on the formation process ()

20 Isotopologues in hot cores Very rich spectra : isotopologues contribute to the line forest eg HCOOCH3 and CH3CH2CN with 13C or 15N contribute to more than 1500 lines in the Orion Irc2 line survey ( GHz) (Carjaval et al,, Margulès et al, Demyk et al ) Many lines : good measurement of the molecule excitation Isotopic ratio and/or opacity of the main isotopologue

21 Isotopologues in hot cores Carjaval et al, '09 (HCOO13CH3, H13COOCH3, Margules et al '09 (CH3CH2C15N)

22 Isotopologues in hot cores Jacq et al, 1990, H218O and HDO Variation of hot core spectra : evolution? environment? + small scale structure

23 Isotopic ratio for stellar nucleosythesis Milam et al,09 Rich line spectra in circumstellar shells : probes of past events of metal production. Sensitive constraints on nucleosynthesis in late stages of evolution C, O, N, Si, S, Mg, Cl.

24 Isotopologues at high redshift (Müller et al 06, 09) Absorption spectroscopy => Probe chemical evolution through O isotopic ratios ( C, N, S within reach )

25 Conclusion Isotopologue detection is limited by sensitivity and line confusion With Alma : detection of multiply substituted molecules + possible (CD3OH, 15NH2D, D13CO, DN13C...) --> new (weak) lines in cold & dense cores, hot cores, hot corinos, AGBs & PPNs,, up to high z galaxies --> very sensitive probes of chemistry and physical processes --> can be used to understand the history but --> Improve the spectroscopic data bases with more isotopologues (D, 13C, 15N, 18O, 17O, etc.) --> Improve the collision cross section with hyperfine levels --> Improve the chemical network (rates, branching ratios) --> Improve Photodissociation rates for isotopologues