Erratum

Erratum to: A&A 591, A52 (2016), DOI: 10.1051/0004-6361/201528001

1. Introduction

In Table A.5 (page 17) of our original publication (Esplugues et al. 2016), the rate coefficient considered for the CO ice photodesorption was 2.2 × 10^-15 s^-1, however we should have considered a coefficient of 3.67 × 10^-10 s^-1 according to recent results (Fayolle et al. 2011; Muñoz-Caro et al. 2016). We also update here the values for the solid species H₂O and H₂CO considering a coefficient of 3.67 × 10^-11 s^-1 for both of them (see Table 1) instead of 2.16 × 10^-11 s^-1. In particular, the photo-process reaction rate, R_photo (cm^-3 s^-1), is calculated for these cases as (1)where n_i is the number density of the photodissociated species, f_ss is the self-shielding factor, and k_photo (s^-1) is the photo-process rate coefficient as follows: (2)following Chaparro-Molano & Kamp (2012). In expression (2), χ is the UV field strength (Draine 1978)¹, and the photon flux produced by this field per unit area is F_Draine = 1.921× 10⁸ cm^-2 s^-1 (Woitke et al. 2009). Furthermore, n_surf = 1.11× 10¹⁵ cm^-2 is the surface density of available absorption sites per unit grain area assuming 3 Å separation between sites, N_lay = 2 is the assumed number of ice layers that photons can penetrate for photodesorption (Andersson et al. 2006; Arasa et al. 2010; Muñoz-Caro et al. 2016), and Y_i is the photodesorption yield per photon (see Table 2).

These corrections lead to variations in some of the results included in the original publication. The variations are mainly produced at visual extinctions A_V ≳ 4 mag. In particular, significant differences are found for Model 1 (n_H = 10⁴ cm^-3, G₀ = 10⁴), while results for Model 2 (n_H = 10⁶ cm^-3, G₀ = 10⁴) and Model 3 (n_H = 10⁶ cm^-3, G₀ = 10²) are barely affected. We show the new figures for those cases affected by the new rate coefficients below. We also compare these new figures with those from the original paper. In spite of these variations, all the conclusions obtained in Esplugues et al. (2016) remain the same.

2. Cooling

The corrected version of the paper, Fig. 1 (left), shows that cooling by CO becomes dominant at 3 <A_V ≲ 5 mag. In the original paper (right), the cooling is dominated by CO and [OI] 63 μm at 3 <A_V ≲ 4 mag and by gas-grain collisions at 4 <A_V ≲ 5 mag.

3. Chemical structure

3.1. Gas-phase species

The corrected version of the paper, Fig. 2 (top), shows high CO gas-phase abundances in comparison with those obtained in the original paper (bottom) at 4 ≲ A_V ≲ 5 mag. For this visual extinction range, the gas temperature is also lower in the corrected version due to the higher CO contribution to the cooling of the region. The abundances of the other species showed in Fig. 2 present insignificant differences (less than one order of magnitude) between both plots.

3.2. Dust-phase species

Figure 3 (left) shows that, taking the updated rate coefficients into account, one full monolayer of CO₂ and H₂O ice at A_V ≲ 5 mag does not form for Model 1, unlike the original (right) paper (see also Fig. 4 with the exact number of monolayers formed in each case). We also find a significant difference in the abundances of solid H₂O₂. In particular, with the new photodesorption rate coefficients (left), the H₂O₂ abundances remain high at 4 <A_V ≲ 5 mag in comparison with the original publication (right).

3.3. Ice species formation rates

For chemical reactions forming water ice (Fig. 5), the main difference between the original paper (right) and the corrected version (left) is found in the rates of the reaction between solid H and solid H₂O₂ for Model 1 at large A_V. In spite of this difference, however, we still obtain that the main chemical reaction forming water ice at A_V> 4 mag for Model 1 is the reaction between solid H and solid OH.

Figure 6 shows the rates of the reactions forming CO₂ ice in the original paper (right) and in the corrected version (left). The main differences between both plots are found once the maximum number of CO₂ monolayers is reached (at ~3.5 mag and ~1.5 mag for Models 2 and 3, respectively). For these cases, the CO₂ ice formation is dominated by the reaction of solid CO with solid O and solid OH. For Model 1, in the corrected version, we obtain that CO₂ is mainly formed only through the reaction between solid CO and solid O at A_V ≲ 5 mag. In the original version of the paper, however, we obtain that CO₂ ice is formed through solid OH and solid CO at 4.5 ≲ A_V ≲ 5 mag.

4. Desorption probabilities

Figure 7 shows abundances for several gas-phase species considering two distinct desorption probabilities (δ_ice). The main differences between both plots are found for CO at large visual extinctions (A_V> 4 mag). In particular, while the CO abundances drop at A_V ~ 4 mag in the original paper (right), this drop occurs deeper in the cloud (A_V ~ 6 mag) in the corrected version (left). It leads to a difference of ~3 orders of magnitude in the CO abundances between both δ_ice in the corrected version of the paper. For H₂CO and CH₃OH, we still obtain differences of up to two and three orders of magnitude, respectively, between both δ_ice in the corrected paper as in the original publication.

5. Effect of dust in the chemical composition of PDRs

Figures 8 and 9 show a comparison of gas-phase abundances for several species with and without dust chemistry between the original (right) and the corrected paper (left). The main differences are found for CO, CO₂, HCO⁺, and CH₃OH. In the corrected version of the paper, the abundances of these species are higher with dust chemistry than without dust chemistry at 4 <A_V< 6 mag, unlike the original paper.

6. Comparison with the original Meijerink PDR code

Figures 10 and 11 show gas-phase abundances for several species obtained with this PDR code and with the version from Meijerink & Spaans (2005) for the original paper (right) and the corrected paper (left). In the corrected version, the abundances of HCO⁺, H₂CO, and CH₃OH (Fig. 10) are up to two orders of magnitude larger than in the original publication at 4 ≲ A_V ≲ 5 mag. In Fig. 11, we find the main difference in the CO abundances between the original paper (right) and the corrected version (left) at 4 ≲ A_V ≲ 5 mag. Other species shown in Fig. 11 present differences that are lower than one order of magnitude at 4 ≲ A_V ≲ 5 mag. For lower visual extinctions, all the abundances remain unchanged.

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Acknowledgments

The authors thank Evelyne Roueff for pointing out the error of the rate coefficient for the CO photodesorption in our original publication.

References

© ESO, 2017

All Tables

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