top partition function should be multiplied by a factor of two. Rotational constants and vibrational energies have been determined for the nine vibrationally excited states of methyl glycolate reported in reference [51], and so these states can be included in the partition function analysis. For dimethyl carbonate, however, only a ground state partition function can be calculated at this time. This should be a good approximation for the total molecular partition function since no excited state lines were observed in the laboratory spectra. The partition functions for these two molecules have been calculated at a range of temperatures and are presented in Table 5.4.
Table 5.4: Dimethyl carbonate and methyl glycolate molecular partition function values at various temperatures.
Temperature (K) Dimethyl Carbonate Qgs Methyl Glycolate Qtotal
300 4017203 1265051
200 2186688 688606
150 1420296 447263
50 273336 86076
10 24448 7699
as the detection limits are inversely proportional to the square of this quantity. The detection limits would therefore be a factor of 400 higher for dimethyl carbonate than dihydroxyacetone from the dipole moment ratios alone. Likewise, the dimethyl carbonate partition function is seven times that of dihydroxyacetone at 200 K. The line strengths are also quite weak for this molecule in comparison to dihydroxyacetone. Dimethyl carbonate would have to be at least five orders of magnitude more abundant than dihydroxyacetone before its emission would be detectable. This would require its abundance to be higher than nearly all organic species in the Sgr B2(N-LMH) source, which is an unlikely scenario given its level of molecular complexity.
Methyl glycolate, on the other hand, has a larger dipole moment than dihydroxyacetone (µa=2.68 D, µb=1.02 D; [51]), and the line strengths are much stronger than those for dihydroxyacetone. The partition function is three times that of dihydroxyacetone at 200 K.
Detection of methyl glycolate should therefore be quite straightforward in the Sgr B2(N- LMH) source if it is present at a column density on the order of or higher than that found for dihydroxyacetone.
The first step in the search for any species in this source is to compare the spectral information to the unidentified lines in existing line surveys. A single-dish survey in the 1 mm region [24] and a combined single-dish and interferometric survey in the 3 mm region [4]
are available. Unfortunately the RMS level reached in the 1 mm survey is above the level required for complex molecule identification. The 3 mm survey, however, is near the level required for such detections, and∼55% of the lines observed in this survey remain unidentified [4]. No dimethyl carbonate lines match any of the unidentified lines in this survey, but several methyl glycolate line positions are covered in this survey, and emission features are indeed seen at these frequencies. The (x,y) data from this survey as well as maps
of each spectral line have been obtained (D. N. Freidel 2004, private communication). A spectral window containing many possible methyl glycolate lines is shown in Figure 5.5 along with aTrot=200 K simulated spectrum. The relative intensities, line center frequencies, and linewidths of the methyl glycolate lines were fixed and the intensities scaled to best match the observed spectrum.
Figure 5.5: The simulated spectrum of methyl glycolate at 200 K (red) compared to a Sgr B2(N-LMH) 3 mm survey spectrum. The vertical lines correspond to line center positions, with red representing methyl glycolate lines and blue representing formic acid lines. Formic acid is the only identified species with lines in this window; all other emission features are unidentified.
The higher frequency component of the emission feature centered at 89814.7 MHz is clearly affected by the other line in the spectrum. Each of the other possible methyl glycolate lines
are in excellent agreement with the observed spectrum. Another unidentified line observed at 109960 MHz, which is not shown in Figure 5.5, also closely matches the methyl glycolate prediction.
The column density for methyl glycolate in this source can be calculated from the emission features at 89815.3, 89993.6, 90075.0, and 109960.7 MHz, and the results of this analysis are presented in Table 5.5. The parameters for these lines, specifically the transition quantum numbers, rest frequencies, Einstein A-coefficients times the upper state degeneracy, and upper state energies, are listed. All observed lines are transitions within the ground vibrational state. A rotation diagram approach cannot be used in this case because the upper state energies of these transitions are very similar, and so a rotational temperature of 200 K, the most commonly quoted rotational temperature for this source [28], was used for these calculations. These lines are asymmetry doublets and also contain bothA and E state components. Only one frequency has been listed for asymmetry doublets occurring at the same frequency; for all others, the frequency of each individual component has been listed. The line strengths were calculated for the blended A/E states in the same manner as is described in Section 3.3 for asymmetry doublets. The vibrational state contributions to the partition function were included in this analysis. An average column density of 1.92(85)×1016 cm−2 is determined from these data.
Each of these emission features has also been mapped, and these results further strengthen the case for a methyl glycolate detection, as all of these emission features have the same spatial distribution. The map for the emission feature at 89815 MHz is shown in Figure 5.6. The emission is on the same order as the beam size, and so beam dilution corrections are not necessary for these derived column densities.
These results indicate that methyl glycolate could indeed be present in the Sgr B2(N-
Table 5.5: Methyl glycolate column density in Sgr B2(N-LMH) determined from lines observed in the 3 mm line survey [4].
J0K0
a,Kc0−J00K00
a,Kc00 ν0a Agu×102 Eu TMBb NT upper limitb (MHz) (s−1) (K) (K) (×10−16 cm−2) A state
226,16→216,15 89815.3393 1.5203 63.17 0.76(19) 1.62(43)
226,17→216,16 63.17
E state
226,17→216,16 89815.3409 63.17
226,16→216,15 63.17
A state
224,19→214,18 89993.5702 1.1979 55.64 0.76(19) 1.25(35) E state
224,18→214,17 89993.5702 55.64
224,19→214,18 55.64
A state
232,22→222,21 90075.9787 1.2839 54.20 0.76(19) 2.88(76) E state
232,22→222,21 90075.0186 54.20
232,21→222,20 54.20
A state
279,19→269,18 109960.6897 4.9341 104.6 2.98(35) 0.21(07)
279,18→269,17 104.6
E state
279,18→269,17 109960.6897 104.6
279,19→269,18 104.6
a Oneσ uncertainties are 0.1 MHz.
b Uncertainties are listed in parentheses in units of last significant figure.
LMH) hot core at a column density higher than any other complex organic yet detected.
The spatial distribution is also indicative of a hot core molecule. Further observational investigation of methyl glycolate is clearly warranted before a definitive detection can be claimed, and searches with the CSO and GBT are planned. If these results are substantiated, however, the relative abundances of glyceraldehyde, dihydroxyacetone, and methyl glycolate follow similar trends to the 2C structural isomers. Their formation is governed by non-
Figure 5.6: A map of the possible methyl glycolate emission feature at 89815 MHz in Sgr B2(N-LMH).
kinetic processes that are similar to the formation routes for the 2C compounds, which are thought to form on grain surfaces. The implications of these results for interstellar grain surface chemistry are discussed in Chapter 8.
Chapter 6
Glycolaldehyde
6.1 Introduction
The results of the dihydroxyacetone study have made it clear that a detailed determination of the vibrational contribution to the molecular partition function for sugar- related species is required for accurate column density determinations (see Chapter 4 and reference [55]). While the excited vibrational state partition function contribution is often negligible for small organics, this is not the case for more complex species with low energy torsional modes. Vibrational state excitation can be strongly influenced by the far-infrared radiation field in and near hot cores, and as a result low-lying states can be significantly populated, often by large factors over purely collisional excitation processes (see [56, 57]). The molecular partition function used to determine the column density of dihydroxyacetone included terms from several vibrationally excited states in addition to the ground vibrational state [55], while the glycolaldehyde column density reported by Hollis et al. was calculated using only the ground vibrational state molecular partition function [15]. A direct comparison of these two molecules therefore requires that vibrational state contributions to the glycolaldehyde molecular partition function also be determined.
The original interstellar detection of glycolaldehyde was based on an extrapolation of
earlier microwave rotational studies [58, 59], with subsequent millimeter and submillimeter measurements extending the laboratory database over the 128 - 354 GHz interval [60]. Three excited vibrational states were identified in the original microwave work, and some spectral assignments were included [58]. Gas phase infrared studies of this molecule have only been completed above 500 cm−1 [61], and the original microwave work included energies for only two of the three vibrational states [59]. While a rough estimate of the partition function can be made from ab initio predictions of vibrational state energies, the torsional energies determined in such studies often have uncertainties of>20%.
A much more precise determination of these vibrational state energies can be made from comparison of the relative intensities of the excited vibrational state lines to ground vibrational state lines. The rotational and centrifugal distortion constants derived from higher frequency spectral assignments can also be used to determine a more complete partition function. In addition, spectral lines from other low-lying torsional states not identified in the original microwave work (i.e., combination and/or overtone bands) could be present in millimeter and submillimeter spectra. If populated, these states could contribute significantly to the molecular partition function in a hot core environment. To test these possibilities, the millimeter spectrum of glycolaldehyde from 72–122.5 GHz has been obtained. The combined millimeter and submillimeter pure rotational analysis of the vibrationally excited states of glycolaldehyde has also been completed and this information used to determine a more complete molecular partition function.