Free Access
Volume 567, July 2014
Article Number A130
Number of page(s) 13
Section Interstellar and circumstellar matter
Published online 29 July 2014

Online material

Appendix A: Modelling the CH2NH absorption lines

CH2NH has two transitions with similar line strengths close to NH+ at 1013 GHz: 33,1−22,0 (1012.531 GHz) with El = 41 K only +8.5 MHz (2.5 km s-1) from NH+, and 33,0−22,1 (1012.661 GHz) seen as a narrow absorption at 21 km s-1. It should be noted that this line blends with the SO2415,37−404,36 (1012.673 GHz) emission. We model the SO2 emission, both CH2NH absorptions, and the o-NH242,2−41,3 emission line wing (seen at ~70 km s-1) in order to subtract these lines in the search for any remaining weak NH+ absorption. All modelled lines are shown in Fig. A.2 together with the original data in black and the resulting spectra after subtraction of the above described lines is shown in red.

thumbnail Fig. A.1

Sgr B2 (M). Gaussian fits of NH+ 1013 and 1019 GHz line profiles, including all hfs components, to the observations. The arrows mark the velocity range in which the fit was made.

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thumbnail Fig. A.2

Sgr B2 (M). Observed NH+ 1 013 GHz search spectrum (black) along with models of the two absorbing CH2NH transitions (green and cyan), the expected SO2 (blue) and NH2 (pink) emissions, and the remaining spectrum after removal of their expected contributions (red; more details in Sect. 3).

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The numerous SO2 lines observed in our band (e.g. 395,35−384,34 and 435,39−424,38), as well as in the HEXOS spectral line survey, are used to reconstruct the SO2 1012.673 GHz line. The true CH2NH absorption is then found by comparing the reconstructed SO2 emission with the observed line profile. Finally, the modelled CH2NH absorption is used as a template for the CH2NH absorption at 1012.531 GHz since their line strengths are similar. We use RADEX to check our modelled CH2NH lines together with the 225 GHz (11,1−00,0) line observed in absorption by Nummelin et al. (2000). The integrated opacities of these three lines are matched using a density of n(H2) ~ 105 cm-3, a kinetic temperature of ~100 K, a column density N(CH2NH) ~ 1 × 1015 cm-2, and a line width of ~15 km s-1 which supports the above modelling and results. After subtracting all modelled lines, we find a weak remaining absorption feature at VLSR = 69 km s-1 with an integrated opacity of 0.08 km s-1, which is used as an upper limit to NH+.

thumbnail Fig. A.3

Absorption feature found at a velocity of +18.5 km s-1 is tentatively identified as p-NH from the source molecular cloud. We use the resulting integrated opacity as an upper limit to N(NH). The emission line at 110 km s-1 is methanol 92,7–81,8 (rest frequency 933.693 GHz).

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Appendix B: NH chemistry

The NH anion can form via the dissociative attachment process through electron-impact on NH3(B.1)where e represents an energetic electron. The energetic threshold for this process is ε = 3.857 eV, potentially leading to an unusually high production rate. At kinetic temperatures of the order of 100 K or less, the thermal electrons have a characteristic energy less than 9 meV. Therefore, the hot electrons required to form the anion are extremely superthermal. A self-consistent treatment of the electron speed distribution in the weakly ionised interstellar medium is currently being investigated (Black, in prep.). The crucial energy range for reaction (B.3) is 3.8 to 8.5 eV. In photon-dominated regions, including diffuse molecular clouds, such electrons are produced mainly by the same photoelectric effect involving dust and large molecules that dominates the heating of the gas. Energetic electrons are thermalised primarily by collisions with neutrals (H and H2), rather than by elastic collisions with thermal electrons, as long as the fractional ionisation is less than 10-3. The cross-section, σDA, for the dissociative attachment process has a peak value of 1.6 Mb near ε = 5.8 eV, with vanishing values at ε< 4.2 and at ε> 8.5 eV (Sharp & Dowell 1969; Rawat et al. 2008). The number density of electrons integrated over the interval 3.857 to 8.5 eV is 9.9 × 10-8 cm-3. We find a production rate for NH by reaction (B.3) of (B.2)The possible destruction processes of NH include photo-detachment, reactions with H2, and mutual neutralisation in reactions with the most abundant positive ions. The reaction with H2, (B.3)is known to be rapid at low temperatures around 20 K but to decrease with increasing temperature (Otto et al. 2008). If the reaction with H2 is the dominant loss process, then the density of NH at T = 50−100 K will be of the order of (B.4)which immediately suggests a very low NH abundance.

A second formation route of NH is via slow radiative attachment of electrons to NH2(B.5)Radiative attachment via emission from excited vibrational states has been calculated to be an efficient process to produce negative molecular ions with large electron affinities (34 eV) and at least four atoms (Herbst & Osamura 2008). The NH anion is smaller than this limit and has only a moderate electron affinity of 0.771 eV (Wickham-Jones et al. 1989). Using Eq. (11)

in Herbst & Osamura (2008), we estimate the rate coefficient for radiative attachment via the vibrational mechanism to be only 1 × 10-17(T/ 300 K)−1 / 2 cm3 s-1. The process is at most competitive with dissociative attachment of NH3 via non-thermal electrons, and does not change the conclusion that the NH anion has a low abundance. Finally, if the anion is formed in a local region rich in atomic rather than molecular hydrogen, it can be destroyed by associative attachment with atomic hydrogen, (B.6)or competitively via photo-detachment.

Appendix C: Chemical models

thumbnail Fig. C.1

Temporal evolution of the NH+ abundance for all four models (Table 3) where each model is plotted with a high metal abundance (dashed lines) and with a low metal abundance (solid lines). The translucent and dense models are plotted for TK = 30 K alone. The observed upper limits are indicated with dashed horizontal lines following the respective model colour code.

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thumbnail Fig. C.2

Temporal evolution of the nitrogen hydride abundances. Upper left: dense gas; upper right: translucent gas; lower left: diffuse gas; lower right: Sgr B2 (M) envelope model (see Table 3). The dot-dashed and dotted lines for NH represent the estimated X(NH for the TK = 30 K and 50 K (translucent) and TK = 30 K and 10 K (dense) models. The observed abundances and upper limits corresponding to the different models are indicated with solid and dashed horizontal lines, respectively, following the respective species colour code.

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thumbnail Fig. C.3

Translucent gas conditions in all models (see Table 3). Upper left: pure gas phase chemistry. Upper right: gas and surface chemistry and inactive non-thermal desorption efficiency aRRK = 0. Lower left: gas and surface chemistry and active non-thermal desorption with the typical efficiency aRRK = 0.01. Lower right: gas and surface chemistry and high active non-thermal desorption (aRRK = 0.1). The observed abundances and upper limits are indicated with solid and dashed horizontal lines, respectively, following the respective species colour code. The dot-dashed and dotted lines for NH represent the estimated X(NH for the TK = 30 K and 50 K models. If the reactive desorption mechanism is active with the typical aRRK = 0.01, each NH, NH2 and NH3 species that is formed on the grain through a hydrogenation reaction has a probability of 9.3 × 10-3, 7.6 × 10-3, and 5.2 × 10-3, respectively, to desorb into the gas phase. There it will become available for detection and for follow-up reactions. Experiments by Dulieu et al. (2013) indicate that this type of non-thermal desorption could be much more efficient on bare grains than aRRK = 0.01. As shown, our models are not very sensitive to the exact value of the desorption probability, since the model with aRRK = 0.1 gives very similar results to the model using aRRK = 0.01.

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thumbnail Fig. C.4

Temporal evolution of the NH+ abundance for different cosmic ray ionisation rates for diffuse gas conditions (Table 3). The dashed black line represents the observed upper NH+ limit in the diffuse gas.

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Appendix D: Tables

Table D.1

Hyperfine structure components of NH+N = 1−1,J = 1.5−0.5+.

Table D.2

Hyperfine structure components of NH+N = 1−1,J = 1.5+−0.5.

Table D.3

Para-NH, JKa,Kc = 11,1−00,0. Spectroscopic data from Tack et al. (1986). See Sect. 2 for more details.

Table D.4

Herschel OBSID’s of the observed transitions analysed in this paper.

Table D.5

Initial elemental abundances.

© ESO, 2014

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