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Appendix A: CASSIS LTE modeling

CASSIS provides a Jython script which minimizes the χ² to find the best parameters to adjust the observations. With a spectrum i of N points, the χ² is derived as where I_obs and I_model are respectively the observed and the modeled intensities, I_cont is the continuum intensity, cal_i the calibration uncertainty for the spectrum i and N is the total number of points of the spectrum. The reduced χ² is the calculated as with N_spectra the number of spectra, N_ind the number of independent points, and dof the degree of freedom.

The adjustable parameters are the column density, temperature, FWHM, size of the source, and v_LSR. In the case of CF⁺, the column density varies between 10¹⁰ and 10¹⁴ cm^-2 with 500 steps; the other parameters were fixed. The temperature was set to 10 K, as in Neufeld et al. (2006) and Guzmán et al. (2012). The FWHM was measured on the spectrum as 1.6 km s^-1. No source size was assumed, so we kept the 30 m beam size of 25′′. The velocity of the source is around − 4 km s^-1. CASSIS also takes into account the beam size variation as a function of the frequency. The obtained is 3.22 and the column density is (4 ± 1) × 10¹¹ cm^-2.

The V_LSR and FWHM derived for the J = 1−0 line are then used to predict the intensity of the J = 2−1 line. We detected 202 CH₃CHO lines in the whole coverage and used a population diagram to derive a mean excitation temperature of 40 K and a column density of 3 × 10¹³ cm^-2. These values were used to model the CH₃CHO lines as shown in the bottom panel of Fig. 1.

Appendix B: VLA observations

CygX-N63 was observed on 27 April 2003 with the 27 antennas of the VLA in D configuration at 8.4 GHz (band X) (project AB1073). The image shown in Fig. B.1 corresponds to a total integration time of 25 mins spread over 4.5 h (track sharing technique to improve UV coverage). It has been cleaned with the clean algorithm of AIPS. The resulting rms is equal to 60.2 μJy/beam.

In the direction of CygX-N63, a weak peak of 0.14mJy, i.e. at a level of 2.3σ, is found. Below 3σ it cannot be considered as a possible detection, especially because the side lobes (large stripes in the image of Fig. B.1) are roughly of the same intensity.

Fig. B.1 VLA image at 8.4 GHz of the same region than in Fig. 2 with color scale from − 0.1 to 0.7 mJy/beam. The dashed ellipse indicates the VLA primary beam and the green circle shows the IRAM 30 m beam at 102 GHz. The rms in this image stays large, of the order of 0.06 mJy/beam, due to strong sidelobes originating from to DR22, a strong radio source (a compact HII region) outside the imaged field (~5 arcmin south).

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Appendix C: Carbon ionization equilibrium

At equilibrium in spherical geometry from an initial radius r_i to the radius of the CII region (where mostly all carbons are ionized) r_CII the equilibrium of carbon ionization can be expressed as where Q_CII is the total rate of ionizing photons (in s^-1), k_rec is the carbon recombination rate that we take as equal to 2.36 × 10^-12(T/ 300)^-0.29 exp(− 17.7 /T) cm³s^-1 (UMIST 2012 database McElroy et al. 2013), and n_{c +} and n_{e −} are the densities of C⁺ and of electrons. For a large range of gas temperature (10 to 4000 K), k_rec stays within a factor of less than 2 around the adopted value of 1.74 × 10^-12s^-1 (ranging from 1.08 to 2.80 × 10^-12s^-1 with the maximum obtained at ~60 K).

If carbon ionization dominates and assuming C⁺/H equal to the standard C/H ratio of 1.6 × 10^-4 (if most of the carbon is in atomic form), we have n_{e −} = n_{C +} = 1.6 × 10^-4 × n_H. Assuming a r^-2 density law and having 44.3 M_⊙ inside a FWHM of 2500 AU (Duarte-Cabral et al. 2013), we here get an H₂ density of 1.95 × 10¹⁰ × (r/ 100 AU)^-2cm^-3. For this simple model, we get the following expression of the extent of the ionized region in the inner envelope, and the radii expressed in AU. For the case of Sect. 4.3 (Q_CII = 2.63 × 10⁴⁵s^-1), one gets a very small value of 9.4 × 10^-6 for η. For r_i = 100 AU, r_CII is then equal to 100.1 AU. Only the very first layers are ionized for values of Q_CII significantly lower than ~10⁵⁰s^-1.

The ionization equilibrium can also be expressed as with N_{c +} being the number of ionized carbons. It shows that k_recn_{e −} is the fraction of ionized carbons that recombines per second. For n_{e −} = 1.6 × 10^-4 × n_H, we can then derive the typical density (assuming constant density) required to keep an amount of ionized carbon N_{c +} ionized under an ionizing rate Q_CII. Here is the expression of this density n_H^CII for our adopted numerical values: For the case of Sect. 4.3 with Q_CII = 2.63 × 10⁴⁵s^-1 and N_{c +} = 1.2 × 10⁵³, one gets n_H^CII = 7.9 × 10⁷cm^-3.

Appendix D: Centimetric free-free emission

As discussed in Sect. 4.6, any ionized gas is expected to emit free-free emission.

The centimetric flux of free-free emission in the optically thin case can be expressed as a function of EM_V, the emission measure (see André 1987; Curiel et al. 1987), with The optical depth is expressed as a function of the linear emission measure EM_L (),

which is equal to ~8.8 × 10⁶ for n_{e −} = n_H = 2.2 × 10⁹cm^-3, l = 100 AU (see Sect. 4.4), 10⁴K (the opacity is even larger for lower temperatures) and at 8.4 GHz (frequency of the VLA observation from Appendix B).

In the optically thick case the emission depends on the area of emission expressed in physical surface A_em if distance scaling is included, which is equal to 0.3 mJy for A_em = πR_em² with R_em = 19.2 AU (Sect. 4.4).

Curiel et al. (1987, 1989) also expressed the expected flux and optical depth in the case of a dissociative shock as a function of pre-shock density n₀ and shock velocity V_s (for V_s ≳ 60 km s^-1), which is found equal to 5 × 10^-4 for n₀ = 3 × 10^-3cm^-3, V_s = 200 km s^-1 (see Sect. 4.5), 10⁴K and at 8.4 GHz.

© ESO, 2015