A Chemical Survey of the Dark Clouds TMC-1 and L134N

P. Pratap, J.E. Dickens, R.L. Snell, M.P. Miralles W.M. Irvine, F.P. Schloerb, & E.A. Bergin

Five College Radio Astronomy Observatory
Instituto Nacional de Astrofisica, Optica y Electronica
Harvard-Smithsonian} Center for Astrophysics

Chemical abundances in molecular clouds have often been derived from observations of single molecular transitions, assuming a Boltzmann population distribution at a fixed rotational temperature, which itself was estimated from independent determinations of kinetic temperature and molecular hydrogen density. In order to test chemical models, it is important to determine simultaneously the physical parameters which control excitation and a wide range of chemical abundances from a homogeneous data set. Such measurements over an extended portion of a molecular cloud allow the separation and intercomparison of gradients in physical properties and chemical abundances. For the massive star-formation regions in Orion A, M17, and Cepheus A just such an approach has been implemented at the Five College Radio Astronomy Observatory 14m telescope (see FCRAO Newsletter, Volume 3). A complementary study of more quiescent dark clouds is currently underway.

Extended regions of the dark clouds TMC-1 and L134N have been observed in 30 different molecular transitions of some 20 different molecular species and isotopomers, supplemented by some lower frequency data taken at the Haystack Observatory. The spectral resolution (0.05 km s) is sufficient to resolve possible different velocity components along the line of sight. The species have been chosen to give physical conditions such as density (from HCN and CS) and temperature (CHCCH, CO and NH), as well as chemical information. The derived density and temperature are used to obtain abundances using a full statistical equilibrium, LVG analysis. Thus far, the dataset is complete for TMC-1 and nearly complete for L134N.

Figure 1: The log of the H density for the TMC-1 ridge points, found from statistical equilibrium modelling of three transitions of HCN, is plotted versus distance from the (0,0) position [RA 43830; DEC 25° 38" 16""]. The filled square is the cyano-polyyne peak (CP), the filled triangle is the ammonia peak (NH), the filled circle is the SO peak, and the star is the source IRAS 04381+2540.

In Figure 1, we show the density structure along the TMC-1 ridge shown on the cover figure, obtained from multi-transition modelling of the dense gas tracer, HCN. Between the "cyanopolyyne" (CP) position on the southeastern end of the ridge and the "ammonia" (NH) position on the northwest end, there is little variation in the density; however, there is a a slight decrease in density to the southeast of the CP peak. Despite the relatively constant density along the ridge, the emission from several molecules varies as a function of position. We attribute these variations in emission to local fluctuations in abundance, and plot the relative abundances of several molecules as a function of position in Figure 2. SO exhibits an increase in relative abundance by a factor of three along the ridge from the southeast to the northwest, while the CS abundance decreases by a nearly similar factor. HCN and CHCCH (not shown) have a trend similar to that of CS, with a drop in their relative abundances by a factor of approximately three between the CP and NH positions. The remaining molecules (CO, CHOH, NH$^{+}$, CH, HCN, and CN) show little abundance variation along the ridge, and are similar to that of HNC shown in Figure 2.

The relative abundances along the TMC-1 ridge show the well-known anti-correlation between carbon-containing species such as CS, HCN, and CHCCH and oxygen-containing molecules such as SO. Calculations from a time-dependent model by Bergin show CS is preferentially produced early in a cloud's life, while SO is mainly produced at later times. Therefore, the CS/SO ratio can be a sensitive function of time. A comparison of this ratio along the ridge with calculations from the models show that a rather small gradient in the evolutionary state along the ridge can match the observations reasonably well. This is consistent with the hypothesis that the TMC-1 ridge is composed of clumps at different ages, with the clump toward the CP peak younger than that toward the NH peak.

Figure 2 : The relative abundance of several molecules is plotted versus distance along the TMC-1 ridge from southeast (left) to northwest (right). The plotted symbols are the same as in Fig. 1. The relative abundances are normalized to the weighted mean of the ridge points. The molecule is indicated in the lower left-hand corner of the panel. Note the strong anti-correlation between CS and SO abundances.