E. C. Sutton

Molecular line survey of Orion A from 215 to 247 GHz

Molecular line emission from the core of the Orion molecular cloud has been surveyed from 215 to 247 GHz to an average sensitivity of about 0.2 K. A total of 544 resolvable lines were detected, of which 517 are identified and attributed to 25 distinct chemical species. A large fraction of the lines are partially blended with other identified transitions. Because of the large line width in the Orion core, the spectrum is near the confusion limit for the weakest lines identified (≈ 0.2 K). The most abundant complex molecules present are HCOOCH_3, CH_30CH_3, and C_2H_5CN, with beam-averaged column densities of about 3X10^(15) cm^(-2). Together with the simpler species S0_2, CH_30H, and CH_3CN, they account for approximately 70% of the lines in the spectrum. Relatively few unidentified lines are present. There are 27 lines clearly present in the spectrum which are currently unidentified. However, many of these are thought to be high-lying transitions of complex asymmetric rotors such as CH_30H. Present spectroscopic data are inadequate to predict the frequencies of such transitions with sufficient accuracy.

THE ROTATIONAL EMISSION-LINE SPECTRUM OF ORION A BETWEEN 247 AND 263 GHZ

Results are presented from a molecular line survey of the core of the Orion molecular cloud between 247 and 263 GHz. The spectrum contains a total of 243 resolvable lines from 23 different chemical species. When combined with the earlier survey of Orion from 215 to 247 GHz by Sutton et al. (1985), the complete data set includes over 780 emission features from 29 distinct molecules. Of the 23 molecules detected in this survey, only NO, CCH, and HCO^+ were not identified in the lower frequency data. As a result of the supporting laboratory spectroscopy performed to supplement existing millimeter-wave spectral line catalogs, only 33 of the more than 780 lines remain unidentified, of which 16 occur in the upper frequency band. A significant chance remains that a number of these unidentified lines are due to transitions between states of either isotopically substituted or highly excited abundant and complex molecules such as CH_3OH, CH_3OCH_3, and HCOOCH_3, whose rotational spectra are poorly known at present. The very small percentage and weak strength of the unidentified lines implies that the dominant chemical constituents visible at millimeter wavelengths have been identified in the Orion molecular cloud.

Molecular line survey of Sagittarius B2(M) from 330 to 355 GHz and comparison with Sagittarius B2(N)

We have surveyed molecular line emission from Sgr B2 over the range from 330 to 355 GHz at the position designated Sgr B2(M). This position is prominent in millimeter continuum maps of the region and is associated with a compact H II region, a hot NH_3 core, and sources of H_2O and OH maser emission. We have also obtained observations contrasting the submillimeter molecular emission from Sgr B2(M) and Sgr B2(N), an additional center of activity thought to be a dense protostellar core. The picture of the interstellar chemistry of these regions which we derive is substantially different from that determined from previous observations at lower frequencies and with lower spatial resolution. In particular, molecules such as SO_2 and CH_3OH dominate the submillimeter spectrum to a much greater extent than they do the low-frequency observations. Much of this difference is due to the higher spatial resolution of the submillimeter observations, which makes them much more sensitive to emission from compact, dense cores. The millimeter data were most effective at sampling material in the surrounding lower density regions. The chemistry of the core sources in Sgr B2 appears similar to that of other dense cores, such as the core of the Orion molecular cloud. The spectral differences between Sgr B2(M) and Sgr B2(N) primarily relate to differences in excitation and column density. For most molecular species the northern source (N) has a column density significantly higher than that found in the middle source (M), often by a factor of about 5. The principal exceptions are the species SO and SO_2 which seem to be substantially more abundant in the middle source. Generally excitation seems to be higher in the northern source, suggesting a somewhat higher density core, although there are some departures indicating that the excitation situation is more complicated. High optical depths in many of the submillimeter transitions systematically bias the interpretation of both column densities and excitation. Many of the millimeter lines may also have high optical depths, particularly those lines arising from the compact core sources.

Excitation of methyl cyanide in the hot core of the Orion

The excitation of CH_3CN in the hot core of Orion is examined using high-sensitivity observational data at 1.3 mm. Observed line fluxes are analyzed by means of multilevel statistical equilibrium (SE) calculations which incorporate current theoretical values of the collisional excitation rates. The analysis is applied to both optically thin models of the hot core region and models with significant optical depths. Trapping is found to play a critical role in the excitation of CH_3CN. An optically thin analysis yields a kinetic temperature of 275 K and a cloud density of 2 x 10^6 cm^(-3). Unequal column densities are deduced in this case for the two symmetry species: N_A = 1.4 x 10^(14) cm^(-2) and N_E = 2.0 x 10^(14) cm^(-2). The deduced cloud density and temperature are lowered to 1.5 x 10^6 cm^(-3) and 240 K. The model with trapping is favored because of the agreement with measured sizes of the hot core source and the more plausible N_A/N_E ratio. Analysis of radiative excitation in the hot core indicates it is unlikely to significantly affect the ground vibrational state populations of CH_3CN. It most likely is significant for excitation of the V_8 band.

On the Interpretation of the broad-band millimeter-wave flux from Orion

Spectral observations of the core of Orion A at wavelengths around 1.3 mm show a high density of strong, broad emission lines. The combined flux in lines with peak antenna temperatures stronger than 0.2 K accounts for approximately 40 percent of the broad-band millimeter-wave flux from the region. Thus the broad-band flux from Orion A is in large part due to sources other than dust emission.

The distribution of the CO J=2-1 emission from M82

The CO J = 2-1 emission has been mapped for the first time at high spatial resolution in the central region of M82. The region of strong CO emission has an extent of 750 x 550 pc and has a radial velocity distribution which is centered about a mean systemic velocity of v/sub LSR/ = 220 km s/sup -1/ and which is in agreement with the (Ne II) rotation curve. anomalous emission is found at v/sub LSR/ = 170 km s/sup -1/ which is displaced by 250 pc (0.25) to the SW of the center of the CO emission and has a significant minor axis extension. This emission may be related to the compact radio source 41.9+58, the optical filaments, or possibly to the intergalactic gas cloud with which M82 is thought to be colliding.

CO (2−1) Observations of Maffei 2

Maffei 2 is a highly obscured galaxy, probably of type Sbc, at a distance of 5 Mpc (Allen and Raimond 1972; Spinrad et al. 1973). Since it lies close to the Galactic plane, there is considerable confusion in infrared and 21-cm HI observations due to Galactic emission, but investigations of its structure can be carried out at millimeter wavelengths where the Galaxy contribution is confined to a limited velocity range. The high resolution (30″) of our CO J=2−l observations permits both a detailed examination of Maffei 2 and a study of the nature of the gas in its nucleus, through comparison with the CO J=l−0 observations.