The Sensitivity of Gas‐Phase Chemical Models of Interstellar Clouds to C and O Elemental Abundances and to a New Formation Mechanism for Ammonia

Abstract

In the paper "The Sensitivity of Gas-Phase Chemical Models of Interstellar Clouds to C and O Elemental Abundances and to a New Formation Mechanism for Ammonia" by R. Terzieva and Eric Herbst (ApJ, 501, 207 [1998]), an omission in Table 2 needs to be corrected and the criterion for agreement between models and observations should be modified. Table 2 is missing inequality signs before the upper limits for molecules searched for but not detected in the sources TMC-1 and, especially, L134N. The correct inequalities are shown in the paper by M. Ohishi, W. M. Irvine, and N. Kaifu (in Astrochemistry of Cosmic Phenomena, ed. P. D. Singh [Dordrecht: Kluwer], 171 [1992]). Furthermore, in the determination of the accuracy of various chemical models, the criterion for agreement between model and observation used by Terzieva & Herbst contains the assumption that the upper limits are equivalent to determined abundances. Since this criterion is probably overly stringent, the authors have amended it so that the model is in agreement with an observed upper limit if the calculated abundance is less than or equal to 10 times the upper limit. With this amended criterion, an assortment of models, especially those containing a number of neutral-neutral reactions (M4M and NNNM), can now fit the data in L134N much better than with their previous criterion. The improvement in agreement is shown in Figure 1 (below). All of the model networks are in excellent agreement with observation, especially if C < O and equal depletions of a factor of 5 for C and O are used. To distinguish among the models in L134N will require detection of the molecules for which only upper limits are currently available or further diminution in these upper limits. The level of agreement between model and observation for TMC-1, on the other hand, is virtually unchanged from that discussed in the original paper.