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Collisions Moléculaires Collisions moléculaires: théorie, expérience et observation Alexandre Faure Institut de Planétologie et d’Astrophysique de Grenoble CNRS / Université de Grenoble Alpes (France) Journées de la SF2A 2019–Nice, 14-17 mai 2019 Outline Introduction I. Collisional excitation: theory vs experiment + II. Molecular diagnostics: CN, CH , CH2NH Conclusion Chemistry starts at z ≃1000 CHEMISTRY + + (HeH , H2 , H2, etc.) (April 2019) LETTER https://doi.org/10.1038/s41586-019-1090-x Astrophysical detection of the helium hydride ion HeH+ Rolf Güsten1*, Helmut Wiesemeyer1, David Neufeld2, Karl M. Menten1, Urs U. Graf3, Karl Jacobs3, Bernd Klein1,4, Oliver Ricken1, Christophe Risacher1,5 & Jürgen Stutzki3 During the dawn of chemistry1RE,2, whenSEAR CHthe temperatureLETTER of the young reaction networks19,20 in local plasmas, and might ultimately invalidate Universe had fallen below some 4,000HeH kelvin,+ @149.1 the ions of the ! mlight towards present modelsNGC of the7027 early Universe. elements produced in Big Bang nucleosynthesis recombined in reverse The deployment of the German Receiver for Astronomy at Terahertz 9 106 order of their ionization potential. WithNGC their 7027 higher ionization Frequencies+ (GREAT) heterodyne spectrometer on board the 2+0.10 + HeH J = 1–0 10 potentials, the helium ions He and He were the first to combine StratosphericCO J = Observatory 11–10 (×0.04) for Infrared Astronomy (SOFIA) has now with free electrons, forming the first neutral atoms; the recombination opened up new opportunities. Although 10the5 HeH+ J = 1–0 transition at of hydrogen followed. In this metal-free and low-density environment, 149.137 µm (2010.183873 GHz; ref. 21) cannot be observed from ground- neutral helium atoms formed the Universe’s first molecular bond in based observatories, skies become transparent104 during high-altitude 0.05 ne + (K) the helium hydride ion HeH through radiative association with flights with SOFIA. The latest advances in terahertzTemperature technologies have mb T + + 22 protons. As recombination progressed, the destruction of HeH enabled the operation of the high-resolution103 spectrometern(He ) upGREAT at + created a path to the formation of molecular hydrogen. Despite its frequencies above 2 THz, allowing the) or temperature (K) HeH J = 1–0n(H) line to be targeted. –3 0 λ10∆6 ×λ n≈(HeH+7) unquestioned importance in the evolution of the early Universe, the The resolving power of this heterodyne instrument,102 / 10 , permits HeH+ ion has so far eluded unequivocal detection in interstellar space. the HeH+ J = 1–0 line to be distinguished unambiguously from other, In the laboratory the ion was discovered3 as long ago as 1925, but only nearby spectral features such as the CH Λ-doublet mentioned previously. + 101 in the late 1970s was the possibility that HeH–50 might exist in0 local During50 three flights100 in May 2016, theDensity (cm telescope was pointed towards 4–7 astrophysical plasmas discussed . In particular, the conditionsVelocity in (km NGCs–1) 7027 (the total on-target integration time was 71 min). Weak planetary nebulae were shown to be suitable for producing potentially emission in the HeH+ J = 1–0 line was clearly100 detected (Fig. 1), as was Fig. 1 | Spectrum of the HeH+ J = 1−0 ground-state rotational 0.014 0.016 0.018 0.0200.022 detectable column densities of HeH+. Here we report observations, emission from the nearby CH doublet. Notably, the lines are well sepa- transition, observed with upGREAT onboard SOFIA pointed towards Distance (pc) based on advances in terahertz spectroscopy8,9 and a high-altitude rated in frequency (Extended Data Fig. 1). The velocity profile of the 10 NGC 7027. ‘Contaminating’ emission+ from the nearby+ but well separated observatory , of the rotational ground-stateΛ transition of HeH at a HeH line matches nicely that of the excited CO J = 11–10 transition, CH -doublet has been removed from the data (see Methods for details 106 wavelength of 149.1 micrometresof data in theprocessing). planetary The nebula spectrum NGC has 7027. been rebinnedwhich was to a observed resolution in parallel. The velocity-integrated line brightness −+1 −1 This confirmation of the existenceof 3.6 of km HeH s (24in nearby MHz). interstellarFor comparison, space the COtemperature, J = 11–10 ∫ lineTmb isd v = 3.6 ± 0.7 K km s , corresponds to a line flux of 5 constrains our understandingsuperimposed of the chemical (at anetworks spectral resolution that control of 0.58 1.63 km s×− 110); −this13 erg transition s−1 cm− was2. Because the 14.310″ half-power beam response of the formation of this molecularobserved ion, in particularin parallel andthe ratesprobes of the radiative dense inner upGREAT edge of the includes molecular most of the NGC 7027 ionized gas sphere, this result + association and dissociative recombination.envelope near the ionization front from whichwill the be HeH close emission to the total is HeH+ flux emitted10 in4 the J = 1–0 line. The flux is − − − The planetary nebula NGCexpected 7027 seems to originate. a natural Tmb candidate, main-beam for brightness a somewhat temperature. higher thanThe greythe upper limit (1.26 × 10 13 erg s 1 cm 2) assigned + + 16 search for HeH : the nebula shadingis very youngshows the(with area a abovekinematic and below age of the zeroto any line residual for each HeH spectral contribution in the ISO103 observations, in the attempt 11 channel. ) or temperature (K) only 600 years) , and its shell of released stellar material is still rather to separate the line from its blend with –3 the CH doublet (see Extended Data n compact and dense. The central star is one of the hottest known (with Table 1 for the fluxes observed with upGREAT102 during this experiment). e 13 + Temperature an effective temperature, Teff, of some 190,000 K) and is very luminous We have modelled the HeH abundance across NGC 7027. We approx- We confirm the conclusion reached previously that, in the plane- n(He+) (with a luminosity of 1.0 × 104 L , where L is the luminosity of the imated+ the nebula+ as a constant-pressure, spherically symmetric shell, tary⊙ nebula environment,⊙ the reaction He + H → HeH + hν—which 101 n(H) Sun)12. Under these conditions, the Strömgren +spheres that are created and adjusted the pressure to obtain a StrömgrenDensity (cm sphere of angular radius dominates HeH formation in the early Universe—can be neglected, 106 n(HeH+) + + ″ × by the hard intense radiationas field can ofthe the reaction nebula’s H2 central + He hot→ HeH white + H4.6 (h—theν is the geometric photon energy). mean deduced from the0 1.4 GHz radio continuum 23 + 10 4 dwarf are not yet fully developed,Moreover, and the we radiation confirm field that drives the photodissociation ioniza- image of. WeHeH adopted is slow a stellar com- luminosity of 1.0 ×0.0210 10 L⊙, a stellar 0.0215effective 0.0220 0.0225 + + 5 12 11 tion fronts into the molecular envelope. The He Strömgren sphere will temperature of 1.9 × 10 K (ref. ), a source distance of 980 parsecsDistance, (pc) +pared with reactions (2) and (3) in the region in which the HeH emis- extend slightly beyond the H sion zone, arises, and itand is incan this therefore thin overlap also be layer neglected. and Fora He the abundance reactions of (1), 0.12 (2) relative to H. Using the CLOUDY photoion- that HeH+ will be produced. Detailed calculations13 led to predictions ization code24, we calculated profilesFig. 2 of | Temperaturetemperature andand densitydensity (for profiles H, for NGC 7027, as and (3), we critically reviewed the most recent available estimates for the predicted by our astrochemical model for this source. The bottom panel for the intensities of the v = 1–0 R(0) and P(2) rotational–vibrational He+ and electrons) as a function of position across the shell (Fig. 2). The rate coefficients, as detailed in Extended Data Table 2. Published values presents an expanded view of the top panel. The model transitions in the near-infrared; however, these predictions have not mean electron density within the ionized shell is 4.9 × 104 cm−3, and that for the rate14 coefficient,15 k1 for the16 radiative association reaction+ (reaction assumes a dissociative4 −3 recombination rate (reaction (2)) of been confirmed, despite deep searches . Observations with the in the H/He−16 3overlap−1 layer is roughly= 2 ×× 10 − cm10 . 4 −0.47 3 −1 (1)) vary widely; here we adopt a value of 1.4 × 10 cm s , based on k2 3.0 10 (T/10 K) + cm s (see Extended Data Table 2), and 25 26,27 −16 3 −1 Infrared Space Observatory (ISO)’sthe most Long recently Wavelength published Spectrometer cross-section . WeExperimental then computed studies the equilibrium a radiative abundance association of HeHrate (reaction, including (1)) of k1 = 6.0 × 10 cm s , of the pure rotational J = 1–0 ground-state transition at 149.137 µm the three reactions identified asadjusted being important to fit the observedin the layers J = in1–0 which line intensity. T, temperature; n, of the dissociative recombination reaction (reaction+ (2))—involving7,13 (where J is total angular momentum)measurements were impaired of the cross-section by the limited at energies HeH up is tomost 40 abundantelectronvolts: density. resolving power of the spectrometer (∆λ = 0.6 µm), which did not + (eV)—derive values for the thermal rate coefficient (k2) that are plainly Λ ++ −15 −1 −2 allow the HeH transition to beinconsistent separated from with thethe nearby cross-sections -doublet presented in that same study.He + AHH →predictedeH + inh νa 0.87 × 10.3″ slit is 5.9(1 ×) 10 erg s cm , comparable of the methylidyne radical (CH) at 149.0926 µm and 149.39 µm. −10 3 −1 15 −15 −1 −2 reanalysis of those+ measurements yields k2 = 3.0 × 10 cm s (at to the reported upper limit of 5 × 10 erg s cm .
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