David M. Mehringer

Long-term Evolution of the Outgassing of Comet Hale-Bopp From Radio Observations

C/1995 O1 (Hale-Bopp) has been observed on a regular basis since August 1995 at millimetre and submillimetre wavelengths using IRAM, JCMT, CSO and SEST radio telescopes. The production rates of eight molecular species (CO, HCN, CH3OH, H2CO,H2S, CS, CH3CN,HNC) have been monitored as a function of heliocentric distance(rh from 7 AU pre-perihelion to 4 AU post-perihelion. As comet Hale-Bopp approached and receded from the Sun, these species displayed different behaviours. Far from the Sun, the most volatile species were found in general relatively more abundant in the coma. In comparison to other species, HNC, H2CO and CS showed a much steeper increase of the production rate with decreasing rh. Less than 1.5 AU from the Sun, the relative abundances were fairly stable and approached those found in other comets near 1 AU.The kinetic temperature of the coma, estimated from the relative intensities of the CH3OH and CO lines, increased with decreasing rh, from about10 K at 7 AU to 110 K around perihelion. The expansion velocity of the gaseous species, derived from the line shapes, also increased with a law close torh3.

Detection and Confirmation of Interstellar Acetic Acid

We have detected acetic acid (CH3COOH) in the Sgr B2 Large Molecule Heimat source using the Berkeley-Illinois-Maryland Association (BIMA) Array and the Caltech Owens Valley Radio Observatory (OVRO) Millimeter Array. With the BIMA array, we initially detected the 8*,8-7*,7 A blend near 90.2 GHz. The corresponding line from the E symmetry species was sought but may be blended with a line from another species. Interstellar CH3COOH was confirmed using the OVRO array, with which we detected the 9*,9-8*,8 E blend near 100.9 GHz. The corresponding line from the A symmetry species was sought but was found to be blended with the 71-70 E line of CH3SH. Our CH3COOH observations represent the first detection and confirmation of an interstellar molecule using interferometric arrays; all past detections and confirmations of new molecules have been made on the basis of single-element telescope observations.

A Molecular Line Survey of Orion KL in the 350 Micron Band

With the Caltech Submillimeter Observatory, we have carried out an unbiased spectral line survey of Orion KL throughout the 350 μm band (from 795 to 903 GHz). This is the first systematic study of molecular radiation in this frequency range. A total of 541 features, resulting from 929 transitions from a total of 26 species, have been detected. High-excitation transitions from CH3OH, CH3CN, H2CO, HNCO, and C2H5CN indicate the presence of a very hot (~250 K) component at the systemic velocity characteristic of the Hot Core. Physical parameters (column density and rotational temperature) relative to a number of species have been estimated by fitting, in the LTE approximation, the whole 100 GHz spectrum at once, thus taking line blending and optical depth effects properly into account. We also report the tentative detection, for the first time outside the Galactic center region, of the radical NH2, one of the building blocks of the chemistry of ammonia.

Observations of Formic Acid in Hot Molecular Cores

We present BIMA Array observations of formic acid (HCOOH) in Galactic hot molecular cores. It has been found that among nearly 120 interstellar and circumstellar molecular species identified to date, the more complex and saturated organic species are usually observed in hot molecular cores—dense and warm molecular condensations associated with active star formation regions inside molecular clouds. Formic acid, one of the molecules in this category, shares common structural elements with both methyl formate (HCOOCH3) and acetic acid (CH3COOH). In this study, we successfully mapped HCOOH emission in three regions: Orion KL, Sgr B2, and W51. Column densities of HCOOH are above 1015 cm-2 in these sources. The derived HCOOH column density in Sgr B2(N-LMH) is comparable to the CH3COOH column density found by Mehringer et al. in 1997. Ethyl cyanide (C2H5CN) and HCOOCH3 emission spectra were also detected in several sources. The distribution of HCOOH emission is consistent with a surface chemistry origin for the species. The abundance ratios of HCOOH to C2H5CN and to HCOOCH3 vary by nearly 2 orders of magnitude from source to source.

Complex molecules in Sagittarius B2(N): The importance of grain chemistry

The complex molecules vinyl cyanide (CH2CHCN), methyl formate (HCOOCH3), and ethyl cyanide (CH3CH2CN) were observed in the Sgr B2 star-forming region with the BIMA millimeter wavelength array. A region with diameter less than 0.1 pc toward the Sgr B2(N) molecular core is found to be the major source of these molecules. Also, this source is coincident with continuum emission from dust and a center of H2O maser activity. Ultracompact (UC) H 11 regions are located within 0.1 pc. Strikingly, none of these molecules is detected toward Sgr B2(M), a core located 1 minute south of Sgr B2(N). The existence of complex molecules, a large mass of dust, high-velocity H2O masers, and UC H 11 regions strongly suggests that the Sgr B2(N) region has just begun to form stars, while the absence of strong dust emission and large molecules suggests Sgr B2(M) is more evolved. The detection of large molecules coincident with continuum emission from dust supports the idea found in current chemical models that grain chemistry is of crucial importance for the formation of these molecules.

Confirmation of Interstellar Acetone

We present new observations of interstellar acetone [(CH3)2CO] from both the NRAO 12 m and the BIMA array. We report NRAO 12 m detections of 13 new acetone emission features that can be assigned to 20 acetone transitions. These assignments are based on the measured and calculated frequencies in 2002 of Groner and coworkers, and they confirm the interstellar acetone identification in 1987 by Combes and coworkers. In addition, our BIMA array observations show that acetone emission is concentrated in the vicinity of the hot molecular core Sgr B2 (N-LMH). The beam-averaged column density for acetone is NT = 2.9(3) × 1016 cm-2. This value is consistent with the 1990 conclusions of Herbst, Giles, & Smith that the observed acetone abundance is too high to be explained by gas-phase synthesis reactions.

Acetic Acid in the Hot Cores of Sagitarrius B2(N) and W51

We have detected interstellar acetic acid (CH3COOH) toward the hot core source W51e2. This is the first new source of interstellar CH3COOH since its discovery by Mehringer et al. toward the hot core source Sgr B2(N-LMH). In this paper, we report CH3COOH observations at two new frequencies toward Sgr B2(N-LMH) with the OVRO array and at 10 frequencies toward W51e2 with the Berkeley-Illinois-Maryland Association array. Toward Sgr B2(N-LMH) the agreement in positions, intensities, and velocities between the two lines from the previous study and the two new lines strongly indicates that all four CH3COOH lines are coming from a common source. Using all four detected transitions, we find an average column density of 6.1(6) × 1015 cm-2, a fractional abundance of (0.8-6) × 10-10 relative to H2 and (3-6) × 10-2 relative to its isomer methyl formate (HCOOCH3). Toward W51e2, we find a CH3COOH column density of 1.7(5) × 1016 cm-2 with a fractional abundance of 1.7 × 10-9 relative to H2 and (1-6) × 10-2 relative to HCOOCH3. Furthermore, we find the distribution of CH3COOH toward W51e2 is coincident with HCOOCH3, thus suggesting a similar formation mechanism.

The 1995-2002 Long-Term Monitoring of Comet C/1995 O1 (HALE-BOPP) at Radio Wavelength

The bright comet Hale-Bopp provided the first opportunity to follow the outgassing rates of a number of molecular species over a large range of heliocentric distances. We present the results of our observing campaign at radio wavelengths which began in August 1995 and ended in January 2002. The observations were carried out with the telescopes of Nancay, IRAM, JCMT, CSO and, since September 1997, SEST. The lines of nine molecules (OH, CO, HCN, CH3OH, H2CO, H2S, CS, CH3CN and HNC) were monitored. CS, H2S, H2CO, CH3CN were detected up to r h = 3–4 AU from the Sun, while HCN and CH3OH were detected up to 6 AU. CO, which is the main driver of cometary activity at heliocentric distances larger than 3–4 AU, was last detected in August 2001, at r h = 14 AU.

NEW CLASS II METHANOL MASERS IN W3(OH)

We report interferometric observations of nine class II methanol maser candidate lines toward W3(OH). Narrow maser emission spikes at vLSR = -43.1 km s-1 are present in three of the lines: 31-40 A+, 72-63 A+, and 72-63 A-. For all three lines the maser position is near the northern edge of the W3(OH) ultracompact H II region (maser emission is also seen near the southern edge in the 31-40 A+ line). For the remaining six lines there is no obvious counterpart to the narrow maser spike at -43.1 km s-1. Additional spatially extended emission is present in all nine lines over the range from -41 to -48 km s-1. By comparing our observed flux densities with an extensive set of model calculations, we infer physical characteristics of the maser region. In these calculations the methanol is excited by infrared radiation from warm dust, and this excited gas amplifies the free-free background emission from the ultracompact H II region. The gas forming the narrow maser spikes appears to have both high kinetic temperature, Tkin ≥ 110 K, and high density, n ≈ 107 cm-3. Low-temperature solutions are ruled out by the observed line ratios and low-density solutions by the unphysically large path length that would be required. The gas is rich in methanol (2NM = NA + NE 10-6N), and the methanol column density in the tangential direction for each symmetry species (divided by line width) is NM/ΔV ≈ 1012 cm-3 s. Somewhat lower values of n and NM/ΔV are also acceptable. The size of the region emitting the maser spike is of order 100 × 1000 AU. In most of the lines the broad emission from -41 to -48 km s-1 can also be attributed to weak maser action, produced in gas with similar physical conditions (high density and temperature). It differs from the narrow spike emission mainly through a beaming factor that can be interpreted as an elongation factor for clumps of maser gas. The combination of narrow and broad emission can arise naturally from an ensemble of clumps of different elongations and orientations. In this unified picture the best fit to the data is provided by n ≈ 2 × 106 cm-3 and NM/ΔV ≈ 4 × 1011 cm-3 s, somewhat lower than the values obtained for just the spike component. The methanol maser clumps may be present in an expanding shell surrounding the H II region, similar to the material producing OH maser emission in this source.

New Molecular Species in Comet C/1995 O1 (Hale-Bopp) Observed with the Caltech Ssubmillimeter Observatory

We present millimeter-wave observations of HNCO, HC3N, SO, NH2CHO, H13CN, and H3O+ in comet C/1995 O1 (Hale-Bopp)obtained in February–April, 1997 with the Caltech Submillimeter Observatory (CSO). HNCO, first detected at the CSO in comet C/1996B2 (Hyakutake), is securely confirmed in comet Hale-Bopp via observations of three rotational transitions. The derived abundance with respect to H2O is (4-13) × 10-4. HC3N, SO, and NH2CHO are detected for the first time in a comet. The fractional abundance of HC3N based on observations of three rotational lines is (1.9 ± 0.2) × 10-4. Four transitions of SO are detected and the derived fractional abundance, (2-8) ×10-3, is higher than the upper limits derived from UV observations of previous comets. Observations of NH2CHO imply a fractional abundance of (1-8) × 10-4. H3O is detected for the first time from the ground. The H13CN (3-2)transition is also detected and the derived HCN/H13CN abundance ratio is 90 ± 15, consistent with the terrestrial12C/13C ratio. In addition, a number of other molecular species are detected, including HNC, OCS, HCO+, CO+, and CN(the last two are first detections in a comet at radio wavelengths).