The Astrophysical Journal, 743:60 (12pp), 2011 December 10
C 2011.
doi:10.1088/0004-637X/743/1/60
The American Astronomical Society. All rights reserved. Printed in the U.S.A.
FORMATION OF PEPTIDE BONDS IN SPACE: A COMPREHENSIVE STUDY
OF FORMAMIDE AND ACETAMIDE IN Sgr B2(N)
1
D. T. Halfen1,2,3 , V. Ilyushin4 , and L. M. Ziurys1,2,3
Departments of Chemistry and Astronomy, University of Arizona, Tucson, AZ 85721, USA; [email protected], [email protected]
2 Arizona Radio Observatory, University of Arizona, Tucson, AZ 85721, USA
3 Steward Observatory, University of Arizona, Tucson, AZ 85721, USA
4 Institute of Radio Astronomy of the National Academy of Sciences Ukraine, Chervonopraporna 4, 61002 Kharkov, Ukraine
Received 2011 June 29; accepted 2011 August 25; published 2011 November 22
ABSTRACT
Extensive observations of acetamide (CH3 CONH2 ) and formamide (NH2 CHO) have been conducted toward Sgr
B2(N) at 1, 2, and 3 mm using the Submillimeter Telescope (SMT) and the 12 m antenna of the Arizona Radio
Observatory. Over the frequency range 65–280 GHz, 132 transitions of acetamide have been observed as individual,
distinguishable features, although in some cases they are partially blended. The unblended transitions in acetamide
indicate VLSR = 63.2 ± 2.8 km s−1 and ΔV1/2 = 12.5 ± 2.9 km s−1 , line parameters that are very similar to that
of formamide (NH2 CHO) and other organic species in Sgr B2(N). For formamide, 79 individual transitions were
identified over the same frequency region. Rotational diagram analyses indicate the presence of two components
for both species in Sgr B2(N). For acetamide, the colder component (Eu < 40 K) exhibits a rotational temperature
of Trot = 17 ± 4 K and a column density of Ntot = 5.2 ± 3.5 × 1013 cm−2 ; the higher energy component has Trot =
171 ± 4 K and Ntot = 6.4 ± 4.7 × 1014 cm−2 . In the case of formamide, Trot = 26 ± 4 K and Ntot = 1.6 ± 0.7 ×
1014 cm−2 for the colder component with Trot = 134 ± 17 K and Ntot = 4.0 ± 1.2 × 1014 cm−2 for the warmer
region. The fractional abundances of acetamide are f (H2 ) = 1.7 × 10−11 and 2.1 × 10−10 for the cold and warm
components, and in formamide, f (H2 ) = 5.3 × 10−11 and 1.3 × 10−10 . The similarity between the abundances
and distributions of CH3 CONH2 and NH2 CHO suggests a synthetic connection. The abundance of acetamide,
moreover, is only a factor of three lower than that of formaldehyde, and very similar to acetaldehyde and ketene.
CH3 CONH2 is therefore one of the most abundant complex organic species in Sgr B2(N), and could be a possible
source of larger peptide molecules, as opposed to amino acids.
Key words: astrobiology – astrochemistry – ISM: molecules – line: identification – methods: laboratory –
molecular data
the cold (∼8 K) halo gas surrounding the Sgr B2(N) hot core,
while formamide, a related species, was present in both core
and halo regions.
Because of the importance of acetamide and formamide
in prebiotic chemistry, we have conducted a comprehensive
observational study of these species toward Sgr B2(N) at 1,
2, and 3 mm. Several hundred favorable transitions of both
molecules occur in the frequency range observed, and we
have analyzed the complete data set. Here we present our
observations, analysis of the spectra, and discuss implications
for interstellar chemistry.
1. INTRODUCTION
Proteins are an essential component of all living systems.
These compounds form the majority of the structural components of living cells, and regulate most of the chemical processes
(Morrison & Boyd 1992). Proteins are polymers of amino acids
(NH2 CH(R)COOH, R = H, CH3 , etc.) joined together by the
peptide bond, –NHCO–. Thus far, amino acids have not been
conclusively identified in the interstellar medium (ISM; e.g.,
Snyder et al. 2005). However, a few species with the peptide
moiety have been detected. One is HNCO itself, which has
been found in many regions in our Galaxy (e.g., Bisschop et al.
2007), as well as external galaxies (e.g., Martı́n et al. 2009).
The second molecule, perhaps more important for proteins, is
formamide, NH2 CHO, which has previously been found in two
giant molecular clouds, Orion-KL and Sgr B2 (e.g., Turner 1989;
Nummelin et al. 1998). Recently, this molecule has been shown
to be present in over a dozen molecular clouds throughout our
Galaxy (G. Adande et al. 2011, in preparation). Hence, species
with peptide bonds exist throughout the ISM.
Another simple species with a peptide bond is acetamide,
CH3 CONH2 . In 2006, Hollis et al. reported the identification
of this molecule toward Sgr B2(N). These authors searched for
eight transitions of this species using the NRAO 100 m GBT
in the range 9–47 GHz. They detected weak absorption lines
at most of the frequencies, with the lowest energy features observed in emission. Hollis et al. (2006) suggested that acetamide
existed at three LSR velocities of 64, 73, and 82 km s−1 , respectively. They also concluded that this species was only found in
2. SPECTROSCOPY OF ACETAMIDE AND FORMAMIDE
Acetamide (CH3 CONH2 ) is an asymmetric top species,
containing a methyl group (CH3 ) internal rotor with a low
(∼25 cm−1 or ∼36 K) three-fold barrier, i.e., a potential energy
surface with three minima and maxima. As a consequence,
the spectrum of this molecule is quite complicated. Multiple
laboratory studies of acetamide have been conducted in the past,
in part to unravel its complex torsion–rotation spectrum caused
by the methyl rotor. Kojima et al. (1987), for example, used Stark
modulation techniques to measure the rotational transitions of
acetamide from 12 to 40 GHz, and to determine dipole moments
of the molecule, found to be μa = 1.79 D and μb = 3.22 D.
More recently, the Fourier transform microwave spectrum of
this species was measured in the range 8–26 GHz by Suenram
et al. (2001), who also recorded the millimeter spectrum from
82 to 118 GHz, with transitions as high as J = 9 and Ka = 6.
Yamaguchi et al. (2002) extended these measurements up to
1
The Astrophysical Journal, 743:60 (12pp), 2011 December 10
Halfen, Ilyushin, & Ziurys
200 GHz, and for energy levels up to J = 15 and Ka = 7. These
authors employed the internal axis method to fit the A and E
species lines, but with high residuals. Moreover, the E state lines
above Ka = 3 could not be assigned. (Note that A and E labels
correspond to singly and doubly degenerate internal rotation
states.) Measurements for this species were then conducted by
Ilyushin et al. (2004), who studied spectra arising from the first
two excited torsional levels vt = 1 and 2, as well as vt = 0, in
the range 49–149 GHz, using direct absorption methods. These
authors used the rho axis method (RAM) to analyze all available
acetamide data, including lines with J 20 and Ka 11 for
both A and E species. The major difficulty in this study was
assigning quantum numbers to the E species levels, as level
crossings resulted in mixing of states (Ilyushin et al. 2004).
However, using the RAM analysis, the data were fit to within
experimental accuracy with apparently good predictive ability.
Transition frequencies above 200 GHz could be estimated with
errors of less than 100 kHz (Ilyushin et al. 2004).
Formamide (NH2 CHO) is a planar asymmetric top species
with an energy level structure that is much simpler than
acetamide because it lacks a methyl rotor. The spectrum of this
molecule was initially measured in 1957 by Kurland & Wilson,
and numerous studies have been performed since then. A review
of these works is given in Kryvda et al. (2009). The rotational
transitions of this species from 16 GHz up to 500 GHz are
therefore well characterized. The dipole moments of formamide
are μa = 3.62 D and μb = 0.85 D (Kurland & Wilson 1957).
backend employed was a 2048 channel filter bank operating in
parallel mode (2 × 1024) with 1 MHz resolution.
The beam size varied from 91 to 37 at the 12 m and
30 to 23 at the SMT. All observations were conducted in
position-switching mode toward Sgr B2(N) (α = 17h 44m 09.s 5;
δ = −28◦ 21 20 ; B1950) with an OFF position 30 west in
azimuth. Two methods were used to ensure identification of
any image contamination: (1) a 10–20 MHz local oscillator
shift and (2) direct observation of the image sideband. The
pointing accuracy is estimated to be ±5 –10 at the 12 m and
±1 –2 at the SMT. Pointing of the telescopes was checked
on planets and continuum sources, such as 1921–293. Rest
frequencies and telescope parameters are given in Table 1.
4. RESULTS
Spectral lines arising from acetamide measured in this study
are listed in Table 1. Also given are the rest frequencies, their
quantum numbers, upper state energy (Eu ), and the product
of the square of the dipole moment μ with the line strength
(μ2 S). Out of 454 possible transitions in the observed frequency
range with ΔJ = 0 and + 1 and μ2 S 10 D2 , 132 were
clearly identified in this data set, including both a- and btype transitions. These features were checked for contamination
by other molecules from known spectral-line databases (Lovas
2004; Pickett et al. 1998; Müller et al. 2005). The energy range
covered in the identified lines is Eu = 9–173 K (6–120 cm−1 ),
with J = 4–25 and Ka = 0–5 for A state lines and |Ka | =
0–5 for E state data. Both a- and b-type dipole transitions
were observed, but b-type were predominant. The remaining
312 acetamide transitions are either completely blended with
other spectral features, or totally contaminated by emission from
more abundant molecules. There are no “missing” transitions or
internal inconsistencies among this data set.
For formamide, 121 transitions occur in the observed frequency range. In this case, only 79 of these lines had individual,
resolved features, which are listed in Table 2. The other transitions were completely blended with other stronger emission
peaks, such that an individual identification could not be reliably made. All of the identified lines are R-branch a-type dipole
transitions (μa = 3.62 D), with J = 4–13 and Ka = 0–6, and
μ2 S 10 D2 . Forty-five of these transitions had been previously
reported by Nummelin et al. (1998) and Friedel et al. (2004).
Representative spectra of acetamide are presented in Figure 1;
resolution is 1 MHz. The acetamide spectra are ordered in
increasing frequency (and energy) starting with the top panel
of Figure 1(a), where spectra of the JKa,Kc = 70,7 → 61,6 A
and 71,7 → 60,6 A and the JKa,Kc = 73,5 → 62,5 E and 72,6 →
63,4 E transitions are displayed. Figure 1(a) shows low energy
transitions from Eu = 11–48 K, while Figure 1(b) displays
higher energy lines with Eu = 63–126 K. The positions of
the respective features are indicated underneath the spectra
assuming the LSR velocities in Table 1. It is clear in these data
that acetamide appears to exhibit only one velocity component,
not three as suggested by Hollis et al. (2006), as there is no
obvious structure in these lines. The data set here, however,
selects transitions with Eu 9 K, while Hollis et al. were
observing lower energy lines. Additional spectra of acetamide
will be published elsewhere with the entire survey of Sgr B2(N)
(D. T. Halfen et al. 2011, in preparation).
The line parameters (intensity in TR∗ or TA∗ , VLSR , and the
linewidth ΔV1/2 ) of the acetamide features were determined
by fitting the lines to Gaussian profiles. As the table shows, the
LSR velocities for the acetamide features vary from VLSR = 58.0
3. OBSERVATIONS
Measurements of acetamide and formamide were conducted
as part of an extensive survey of the 1, 2, and 3 mm spectrum of Sgr B2(N). The data were taken during the period 2002
September to 2010 December using the Arizona Radio Observatory 12 m telescope on Kitt Peak and the Submillimeter Telescope (SMT) on Mount Graham.4 At the 12 m, the receivers used
were dual-channel, cooled SIS mixers covering the 2 and 3 mm
bands (65–180 GHz). The mixers were tuned single-sideband
with image rejections typically 18 dB. Data were also taken
using a new dual-polarization receiver, utilizing ALMA Band 3
(83–116 GHz) sideband-separating (SBS) mixers. Here the typical image rejection was usually 16 dB, achieved within the
mixer assembly. The temperature scale was determined by the
chopper wheel method, corrected for forward spillover losses,
and is given as TR∗ . The radiation temperature, TR , assuming the
source fills only the main beam, is then TR = TR∗ /ηc , where ηc is
the main beam efficiency corrected for forward spillover losses.
The backends used for the observations were two 256 channel
filter banks of 500 kHz and 1 MHz resolution, respectively, each
operated in parallel mode (2 × 128). An autocorrelator (MAC)
was also employed with either 390 kHz or 781 kHz resolution,
and a bandwidth of 600 MHz per receiver channel. The MAC
data were smoothed to a resolution of 1 MHz using a standard
cubic spline routine.
At the SMT, 1 mm observations (210–280 GHz) were
conducted with a dual-polarization receiver, which employs
ALMA Band 6 SBS mixers with image rejection of at least
16 dB. The temperature scale was determined by the chopper
wheel method and is given as TA∗ . The radiation temperature TR
is then TR = TA∗ /ηb , where ηb is the main beam efficiency. The
4
The 12 m telescope and the SMT are operated by the Arizona Radio
Observatory (ARO), Steward Observatory, University of Arizona.
2
The Astrophysical Journal, 743:60 (12pp), 2011 December 10
Halfen, Ilyushin, & Ziurys
Table 1
Observed Rotational Transitions of Acetamide in Sgr B2(N)
J
Ka
Kc
→
J Ka
Kc
A/E
Frequency
(MHz)
θ b ( )
ηc /ηb
4
6
7
7
7
7
6
13
12
6
10
10
7
8
8
7
6
6
13
13
7
8
9
9
8
9
9
4
5
13
13
7
14
14
13
13
9
9
8
10
10
9
9
10
10
8
5
5
12
12
11
11
12
12
16
16
13
13
8
10
12
12
14
14
3
1
0
1
3
2
2
5
5
2
2
3
2
0
1
1
3
5
4
5
2
1
3
2
1
0
1
−4
4
−1
6
5
4
5
3
4
4
1
2
3
2
1
2
0
1
5
4
5
4
1
2
3
1
2
0
5
1
0
5
4
2
3
2
2
2
5
7
7
5
6
5
8
8
4
8
8
6
8
8
7
4
2
9
9
5
8
7
8
7
9
9
0
2
12
8
3
10
10
10
10
6
9
6
8
9
8
8
10
10
4
1
1
9
12
9
9
11
11
16
12
13
13
4
6
10
10
13
13
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
3
5
6
6
6
6
5
13
12
5
10
10
6
7
7
6
5
5
13
13
6
7
8
8
7
8
8
3
4
13
13
6
14
14
13
13
8
8
7
9
9
8
8
9
9
7
4
4
11
11
10
10
11
11
16
16
12
12
7
9
11
11
13
13
2
2
1
0
2
3
1
4
4
3
1
2
1
1
0
4
2
0
3
4
3
4
2
3
2
1
0
−3
3
0
5
0
3
4
2
3
1
4
3
2
3
2
1
1
0
0
3
4
1
4
3
2
2
1
4
1
0
1
4
5
3
2
2
3
1
4
6
6
5
4
4
9
9
3
9
9
5
7
7
3
3
5
10
10
4
4
7
6
6
8
8
0
1
13
9
6
11
11
11
11
8
5
5
8
7
7
7
9
9
7
2
0
11
8
8
8
10
10
13
16
12
12
3
5
9
9
12
11
A
A
A
A
E
E
A
A
A
A
A
A
A
A
A
E
A
E
A
A
A
E
E
E
A
A
A
E
A
E
A
E
A
A
A
A
E
E
A
E
E
A
A
A
A
E
A
A
E
E
A
A
A
A
E
E
A
A
A
A
A
A
A
A
74172.528b
77199.071b
77320.854b
77321.414b
77329.950b
77331.272
77435.420b
84089.205
85159.932
85746.007b
86483.134
86484.383
87629.758b
87632.435
87632.511
87792.211
89872.354b
93680.897
95522.169
95529.037
97469.793b
97825.586
97893.417b
97893.437b
97905.699b
97943.874b
97943.884b
98113.339
99085.822b
99377.866
99378.082
100178.170
105863.585
105864.862
106514.523
106514.749
107988.282
107990.325
108136.866b
108190.195b
108190.198b
108214.106b
108215.231b
108255.234b
108255.236b
108606.160
109194.575b
110952.603
138706.227
138706.232
139110.526b
139112.000b
139138.723b
139138.726b
139167.290
139167.290
139188.423b
139188.423b
139573.536
148358.642
149410.310b
149410.545b
149411.185
149411.185
85
81
81
81
81
81
81
75
74
73
73
73
72
72
72
72
70
67
66
66
64
64
64
64
64
64
64
64
63
63
63
63
59
59
59
59
58
58
58
58
58
58
58
58
58
58
58
57
45
45
45
45
45
45
45
45
45
45
45
42
42
42
42
42
0.95
0.94
0.94
0.94
0.94
0.94
0.94
0.93
0.93
0.93
0.93
0.93
0.93
0.93
0.93
0.93
0.92
0.92
0.91
0.91
0.91
0.91
0.91
0.91
0.91
0.91
0.91
0.91
0.91
0.91
0.91
0.91
0.90
0.90
0.89
0.89
0.89
0.89
0.89
0.89
0.89
0.89
0.89
0.89
0.89
0.89
0.89
0.89
0.83
0.83
0.83
0.83
0.83
0.83
0.83
0.83
0.83
0.83
0.82
0.80
0.80
0.80
0.80
0.80
TR∗ /TA∗
(K)
a
0.018 ± 0.003
0.020 ± 0.006
0.044 ± 0.006
0.044 ± 0.006
0.036 ± 0.006
0.036 ± 0.006
0.020 ± 0.006
0.013 ± 0.004
0.067 ± 0.003
0.040 ± 0.005
0.028 ± 0.004
0.028 ± 0.004
0.032 ± 0.004
0.044 ± 0.004
0.044 ± 0.004
0.059 ± 0.004
0.018 ± 0.004
0.015 ± 0.004
0.023 ± 0.004
0.021 ± 0.004
0.051 ± 0.003
0.037 ± 0.004
0.051 ± 0.003
0.051 ± 0.003
0.017 ± 0.003
0.066 ± 0.003
0.066 ± 0.003
0.034 ± 0.004
0.042 ± 0.004
0.010 ± 0.003
0.010 ± 0.003
0.019 ± 0.003
0.042 ± 0.004
0.042 ± 0.004
0.041 ± 0.003
0.041 ± 0.003
0.026 ± 0.004
0.026 ± 0.004
0.025 ± 0.004
0.024 ± 0.004
0.024 ± 0.004
0.047 ± 0.004
0.047 ± 0.004
0.028 ± 0.004
0.028 ± 0.004
0.024 ± 0.005
0.037 ± 0.005
0.031 ± 0.004
0.087 ± 0.006
0.087 ± 0.006
0.024 ± 0.005
0.024 ± 0.005
0.045 ± 0.005
0.045 ± 0.005
0.062 ± 0.005
0.062 ± 0.005
0.053 ± 0.004
0.053 ± 0.004
0.034 ± 0.004
0.065 ± 0.008
0.080 ± 0.005
0.080 ± 0.005
0.080 ± 0.005
0.080 ± 0.005
3
ΔV1/2
(km s−1 )
VLSR
(km s−1 )
Eu
(K)
μ2 S (D2 )
16.2 ± 4.0
14.2 ± 3.9
15.5 ± 3.9
15.5 ± 3.9
19.4 ± 3.9
19.4 ± 3.9
19.2 ± 3.9
14.3 ± 3.6
10.6 ± 3.5
14.0 ± 3.5
10.4 ± 3.5
10.4 ± 3.5
12.2 ± 3.4
12.2 ± 3.4
12.2 ± 3.4
13.7 ± 3.4
20.0 ± 3.3
19.2 ± 3.1
12.0 ± 3.1
9.4 ± 3.1
15.4 ± 3.1
15.3 ± 3.1
18.4 ± 3.1
18.4 ± 3.1
15.3 ± 3.1
19.8 ± 3.1
19.8 ± 3.1
18.3 ± 3.1
18.2 ± 3.0
12.1 ± 3.0
12.1 ± 3.0
12.0 ± 3.0
12.2 ± 2.8
12.2 ± 2.8
11.3 ± 2.8
11.3 ± 2.8
11.1 ± 2.8
11.1 ± 2.8
13.9 ± 2.8
16.6 ± 2.8
16.6 ± 2.8
11.1 ± 2.8
11.1 ± 2.8
16.6 ± 2.8
16.6 ± 2.8
11.0 ± 2.8
16.0 ± 2.7
20.0 ± 2.7
15.7 ± 2.2
15.7 ± 2.2
13.0 ± 2.2
13.0 ± 2.2
12.9 ± 2.2
12.9 ± 2.2
10.8 ± 2.2
10.8 ± 2.2
12.7 ± 2.2
12.7 ± 2.2
10.7 ± 2.1
12.1 ± 2.0
16.0 ± 2.0
16.0 ± 2.0
16.0 ± 2.0
16.0 ± 2.0
66.3 ± 4.0
58.0 ± 3.9
64.0 ± 3.9
64.0 ± 3.9
67.9 ± 3.9
67.9 ± 3.9
64.7 ± 3.9
65.5 ± 3.6
65.1 ± 3.5
69.0 ± 3.5
61.0 ± 3.5
61.0 ± 3.5
64.6 ± 3.4
67.4 ± 3.4
67.4 ± 3.4
62.7 ± 3.4
63.2 ± 3.3
61.6 ± 3.1
62.5 ± 3.1
62.1 ± 3.1
68.1 ± 3.1
63.7 ± 3.1
61.9 ± 3.1
61.9 ± 3.1
61.1 ± 3.1
66.3 ± 3.1
66.3 ± 3.1
60.4 ± 3.1
70.1 ± 3.0
64.6 ± 3.0
64.6 ± 3.0
61.7 ± 3.0
62.6 ± 2.8
62.6 ± 2.8
63.8 ± 2.8
63.8 ± 2.8
66.2 ± 2.8
66.2 ± 2.8
67.6 ± 2.8
65.4 ± 2.8
65.4 ± 2.8
65.0 ± 2.8
65.0 ± 2.8
65.0 ± 2.8
65.0 ± 2.8
60.6 ± 2.8
62.6 ± 2.7
62.5 ± 2.7
60.8 ± 2.2
60.8 ± 2.2
65.8 ± 2.2
65.8 ± 2.2
65.0 ± 2.2
65.0 ± 2.2
59.9 ± 2.2
59.9 ± 2.2
63.7 ± 2.2
63.7 ± 2.2
61.2 ± 2.1
64.4 ± 2.0
61.3 ± 2.0
61.3 ± 2.0
61.3 ± 2.0
61.3 ± 2.0
9.01
14.57
15.59
15.59
21.65
21.65
14.57
73.79
61.08
16.71
38.52
38.52
18.78
19.80
19.80
24.64
16.76
22.38
69.75
69.75
21.44
29.34
30.56
30.56
23.48
24.50
24.50
19.62
14.08
70.28
70.28
27.09
78.90
78.90
65.16
65.16
34.53
34.53
26.64
35.76
35.76
28.67
28.67
29.70
29.70
32.29
14.26
15.70
53.03
53.03
45.20
45.20
47.23
47.23
91.29
91.29
48.27
48.27
31.48
45.21
52.37
52.37
61.49
61.49
26.15
54.17
83.05
83.05
84.33
84.33
54.26
70.59
57.31
36.37
30.02
30.02
67.33
96.11
96.11
67.88
38.85
32.80
57.73
57.72
51.09
81.59
110.76
110.76
80.37
109.17
109.17
41.24
33.46
42.30
42.30
48.65
58.10
58.10
44.44
44.44
95.01
95.01
64.62
123.90
123.90
93.42
93.42
122.24
122.24
64.73
25.92
53.32
134.66
134.66
103.90
103.90
132.53
132.53
29.50
29.50
163.49
163.49
38.01
57.06
116.93
116.93
13.00
168.16
Comment
Partially blended
Partially blended
Partially blended
Partially blended
Partially blended
Partially blended
Partially blended
Partially blended
Partially blended
Partially blended
Partially blended
Partially blended
Partially blended
Partially blended
Partially blended
Partially blended
Partially blended
Partially blended
Partially blended
Partially blended
Partially blended
Partially blended
Partially blended
Partially blended
Partially blended
Partially blended
Partially blended
Partially blended
Partially blended
Partially blended
Partially blended
Partially blended
Partially blended
Partially blended
Partially blended
Partially blended
The Astrophysical Journal, 743:60 (12pp), 2011 December 10
Halfen, Ilyushin, & Ziurys
Table 1
(Continued)
J
Ka
Kc
→
J Ka
Kc
A/E
Frequency
(MHz)
θ b ( )
ηc /ηb
14
14
13
13
20
20
18
18
17
17
12
12
14
14
15
15
11
14
14
16
16
16
16
19
19
19
19
19
19
21
21
21
21
20
20
20
20
19
19
22
22
22
22
22
22
22
22
21
21
21
21
20
20
23
23
23
23
21
21
25
25
24
24
24
24
3
3
1
2
5
6
3
4
3
2
3
4
2
1
1
0
5
2
3
1
0
0
1
4
1
1
4
5
0
3
2
2
3
2
1
1
2
4
3
3
2
2
3
1
4
4
1
3
2
2
3
5
4
1
2
2
1
5
4
2
3
1
2
2
1
12
12
12
12
15
15
15
15
15
15
9
9
13
13
15
15
7
12
12
16
16
16
16
16
19
19
16
15
19
19
20
20
19
19
19
19
19
16
16
20
21
21
20
22
19
19
22
19
19
19
19
16
16
22
22
22
22
17
17
24
23
23
23
23
23
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
13
13
12
12
20
20
18
18
17
17
11
11
13
13
14
14
10
13
13
15
15
15
15
18
18
18
18
18
18
20
20
20
20
19
19
19
19
18
18
21
21
21
21
21
21
21
21
20
20
20
20
19
19
22
22
22
22
20
20
24
24
23
23
23
23
2
3
2
1
4
5
2
3
1
2
4
3
1
2
0
1
4
3
2
0
1
0
1
1
4
1
4
0
5
2
3
2
3
1
2
1
2
3
4
2
3
2
3
1
1
4
4
2
3
2
3
4
5
1
2
1
2
4
5
2
3
1
2
1
2
12
11
11
11
16
16
16
16
16
16
8
8
12
12
14
14
6
11
11
15
15
15
15
18
15
18
15
18
14
19
18
19
18
18
18
18
18
15
15
20
19
20
19
21
21
18
18
18
18
18
18
15
15
21
21
21
21
16
16
23
22
22
22
22
22
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
E
E
E
E
E
E
E
E
E
E
A
A
A
A
A
A
E
E
E
E
E
E
E
E
A
A
A
A
A
A
A
A
A
A
A
A
E
E
A
A
A
A
149411.185
149411.185
149447.505b
149447.506b
156505.353
156505.355
157801.954
157801.954
158308.483
158308.483
159722.677b
159731.714b
159756.291b
159756.291b
159809.477b
159809.477b
160479.347
170015.013b
170015.018b
170119.607
170119.607
170119.607
170119.607
210635.104
210635.104
210635.104
210635.104
219601.559
219601.559
221577.829
221577.829
221577.829
221577.829
221604.719
221604.719
221604.719
221604.719
231767.233
231767.233
231887.141
231887.141
231887.141
231887.141
241463.202
241463.202
241463.202
241463.202
242142.596
242142.596
242142.596
242142.596
252286.207
252286.207
252524.307
252524.307
252524.307
252524.307
262570.237
262570.237
262813.176
262813.176
262829.961
262829.961
262829.961
262829.961
42
42
42
42
40
40
40
40
40
40
39
39
39
39
39
39
39
37
37
37
37
37
37
30
30
30
30
29
29
28
28
28
28
28
28
28
28
27
27
27
27
27
27
26
26
26
26
26
26
26
26
25
25
25
25
25
25
24
24
24
24
24
24
24
24
0.80
0.80
0.80
0.80
0.78
0.78
0.78
0.78
0.78
0.78
0.78
0.78
0.78
0.78
0.78
0.78
0.78
0.75
0.75
0.75
0.75
0.75
0.75
0.76
0.76
0.76
0.76
0.76
0.76
0.76
0.76
0.76
0.76
0.76
0.76
0.76
0.76
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
TR∗ /TA∗
(K)
a
0.080 ± 0.005
0.080 ± 0.005
0.028 ± 0.005
0.028 ± 0.005
0.027 ± 0.006
0.027 ± 0.006
0.032 ± 0.005
0.032 ± 0.005
0.029 ± 0.006
0.029 ± 0.006
0.060 ± 0.007
0.081 ± 0.007
0.063 ± 0.007
0.063 ± 0.007
0.141 ± 0.007
0.141 ± 0.007
0.080 ± 0.009
0.249 ± 0.010
0.249 ± 0.010
0.085 ± 0.010
0.085 ± 0.010
0.085 ± 0.010
0.085 ± 0.010
0.185 ± 0.011
0.185 ± 0.011
0.185 ± 0.011
0.185 ± 0.011
0.145 ± 0.008
0.145 ± 0.008
0.068 ± 0.008
0.068 ± 0.008
0.068 ± 0.008
0.068 ± 0.008
0.091 ± 0.008
0.091 ± 0.008
0.091 ± 0.008
0.091 ± 0.008
0.330 ± 0.007
0.330 ± 0.007
0.289 ± 0.007
0.289 ± 0.007
0.289 ± 0.007
0.289 ± 0.007
0.110 ± 0.007
0.110 ± 0.007
0.110 ± 0.007
0.110 ± 0.007
0.101 ± 0.006
0.101 ± 0.006
0.101 ± 0.006
0.101 ± 0.006
0.099 ± 0.007
0.099 ± 0.007
0.189 ± 0.008
0.189 ± 0.008
0.189 ± 0.008
0.189 ± 0.008
0.091 ± 0.007
0.091 ± 0.007
0.169 ± 0.007
0.169 ± 0.007
0.186 ± 0.007
0.186 ± 0.007
0.186 ± 0.007
0.168 ± 0.007
4
ΔV1/2
(km s−1 )
VLSR
(km s−1 )
Eu
(K)
μ2 S (D2 )
16.0 ± 2.0
16.0 ± 2.0
12.0 ± 2.0
12.0 ± 2.0
11.4 ± 1.9
11.4 ± 1.9
11.4 ± 1.9
11.4 ± 1.9
11.4 ± 1.9
11.4 ± 1.9
11.3 ± 1.9
9.4 ± 1.9
11.3 ± 1.9
11.3 ± 1.9
15.0 ± 1.9
15.0 ± 1.9
13.1 ± 1.9
10.6 ± 1.8
10.6 ± 1.8
14.1 ± 1.8
14.1 ± 1.8
14.1 ± 1.8
14.1 ± 1.8
11.4 ± 1.4
11.4 ± 1.4
11.4 ± 1.4
11.4 ± 1.4
9.6 ± 1.4
9.6 ± 1.4
10.8 ± 1.4
10.8 ± 1.4
10.8 ± 1.4
10.8 ± 1.4
10.8 ± 1.4
10.8 ± 1.4
10.8 ± 1.4
10.8 ± 1.4
9.1 ± 1.3
9.1 ± 1.3
10.3 ± 1.3
10.3 ± 1.3
10.3 ± 1.3
10.3 ± 1.3
10.0 ± 1.2
10.0 ± 1.2
10.0 ± 1.2
10.0 ± 1.2
9.9 ± 1.2
9.9 ± 1.2
9.9 ± 1.2
9.9 ± 1.2
9.5 ± 1.2
9.5 ± 1.2
9.5 ± 1.2
9.5 ± 1.2
9.5 ± 1.2
9.5 ± 1.2
9.1 ± 1.1
9.1 ± 1.1
11.4 ± 1.1
11.4 ± 1.1
10.3 ± 1.1
10.3 ± 1.1
10.3 ± 1.1
10.3 ± 1.1
61.3 ± 2.0
61.3 ± 2.0
62.9 ± 2.0
62.9 ± 2.0
63.6 ± 1.9
63.6 ± 1.9
61.3 ± 1.9
61.3 ± 1.9
60.7 ± 1.9
60.7 ± 1.9
63.8 ± 1.9
59.6 ± 1.9
67.0 ± 1.9
67.0 ± 1.9
60.2 ± 1.9
60.2 ± 1.9
64.5 ± 1.9
62.0 ± 1.8
62.0 ± 1.8
66.4 ± 1.8
66.4 ± 1.8
66.4 ± 1.8
66.4 ± 1.8
61.4 ± 1.4
61.4 ± 1.4
61.4 ± 1.4
61.4 ± 1.4
62.8 ± 1.4
62.8 ± 1.4
63.2 ± 1.4
63.2 ± 1.4
63.2 ± 1.4
63.2 ± 1.4
61.0 ± 1.4
61.0 ± 1.4
61.0 ± 1.4
61.0 ± 1.4
64.8 ± 1.3
64.8 ± 1.3
63.7 ± 1.3
63.7 ± 1.3
63.7 ± 1.3
63.7 ± 1.3
61.0 ± 1.2
61.0 ± 1.2
61.0 ± 1.2
61.0 ± 1.2
59.2 ± 1.2
59.2 ± 1.2
59.2 ± 1.2
59.2 ± 1.2
61.7 ± 1.2
61.7 ± 1.2
66.1 ± 1.2
66.1 ± 1.2
66.1 ± 1.2
66.1 ± 1.2
64.3 ± 1.1
64.3 ± 1.1
62.7 ± 1.1
62.7 ± 1.1
60.3 ± 1.1
60.3 ± 1.1
60.3 ± 1.1
60.3 ± 1.1
61.49
61.49
54.41
54.41
151.70
151.70
113.39
113.39
95.67
95.67
57.00
57.00
62.08
62.08
63.12
63.12
52.92
68.21
68.21
71.29
71.29
71.29
71.29
113.47
113.47
113.47
113.47
121.47
121.47
125.58
125.58
125.58
125.58
118.51
118.51
118.51
118.51
124.52
124.52
136.71
136.71
136.71
136.71
146.78
146.78
146.78
146.78
139.21
139.21
139.21
139.21
144.19
144.19
153.40
153.40
153.40
153.40
156.80
156.80
173.09
173.09
166.03
166.03
166.03
166.03
168.16
13.00
145.58
145.58
73.28
73.28
45.70
45.70
31.23
31.23
101.35
101.35
158.64
158.64
192.45
192.45
71.89
142.99
142.99
168.35
168.35
40.09
40.09
36.33
36.33
197.16
197.16
215.75
215.75
249.00
249.00
27.14
27.14
119.97
119.97
126.36
126.36
192.72
192.72
254.12
254.12
35.58
35.58
242.54
31.74
242.54
31.74
177.99
177.99
65.77
65.77
190.25
190.25
195.74
195.74
91.22
91.22
203.40
203.40
300.73
300.73
38.71
38.71
261.79
261.79
Comment
Partially blended
Partially blended
Partially blended
Partially blended
Partially blended
Partially blended
Partially blended
Partially blended
Partially blended
Partially blended
Partially blended
Partially blended
Partially blended
Partially blended
Partially blended
Partially blended
Partially blended
Partially blended
Partially blended
Partially blended
Partially blended
Partially blended
Partially blended
Partially blended
Partially blended
Partially blended
Partially blended
Partially blended
Partially blended
Partially blended
Partially blended
Partially blended
Partially blended
Partially blended
Partially blended
Partially blended
Partially blended
Partially blended
Partially blended
Partially blended
Partially blended
Partially blended
Partially blended
Partially blended
Partially blended
Partially blended
Partially blended
The Astrophysical Journal, 743:60 (12pp), 2011 December 10
Halfen, Ilyushin, & Ziurys
Table 1
(Continued)
J
Ka
Kc
→
J Ka
Kc
A/E
Frequency
(MHz)
θ b ( )
ηc /ηb
25
25
25
25
1
0
0
1
25
25
25
25
→
→
→
→
24
24
24
24
0
1
0
1
24
24
24
24
A
A
A
A
262895.259
262895.259
262895.259
262895.259
24
24
24
24
0.75
0.75
0.75
0.75
TR∗ /TA∗
(K)
ΔV1/2
(km s−1 )
VLSR
(km s−1 )
Eu
(K)
μ2 S (D2 )
Comment
10.2 ± 1.1
10.2 ± 1.1
10.2 ± 1.1
10.2 ± 1.1
63.5 ± 1.1
63.5 ± 1.1
63.5 ± 1.1
63.5 ± 1.1
167.09
167.09
167.09
167.09
57.98
57.98
272.43
272.43
Partially blended
Partially blended
Partially blended
Partially blended
a
0.368 ± 0.007
0.368 ± 0.007
0.368 ± 0.007
0.368 ± 0.007
Notes. a TR∗ pertains to frequencies less than 180 GHz (12 m data), and TA∗ applies at higher frequencies (SMT observations).
b Transition frequency measured in laboratory (e.g. Ilyushin et al. 2004).
70,7
71,7
61,6 A
60,6 A
0.21
0.14
U
U
13
CH2CH CN
HCOOCH3
13
CH2 CHCN
0.07
0.02
U
62,5
0.18
51,4 A
0.04
0.12
U
0.06
0.00
139188.4 MHz
131,13
130,13
U
HCOOCH3
0.16
13
0.08
160,16
120,12 A 16
5,12
121,12 A
164,13 E
161,13 E 121,11
122,11
CH3CH2CN
252524.3 MHz
U
U
0.03
191,18 A
192,18 A
191,18 A
192,18 A
U
CH3NH2
U
U
U
0.00
0.30
U
U
U
0.45
112,10 A
111,10 A
HCCCHO
U
CH3CN v8=1
HNCO
U 20
1,19
202,19
202,19
201,19
0.15
231,22
232,22
232,22
231,22
SO2
221,21 A
222,21 A
221,21 A
222,21 A
CH3OH
U
CH3NH2
U
0.00
U
U
U
CH3SH
0.06
61,5 A
U
CH2CHCN
0.09
72,6
HCOOCH3 U
CH3NH2
0.02
0.00
0.12
CH3OH
U
221604.7 MHz
13
U
0.24
CH2 CHCN
0.04
71,7 A
70,7 A
HCOOCH3
80,8
81,8
CH3CH2CN
CH3CH2CN
CH3OCH3
CH3OCH3
CH3OCH3
CH3CH2CN
0.06
U U
150,15 A
151,15 A
150,15 A U
151,15 A
0.00
13
87632.5 MHz
TA* (K)
TR* (K)
0.02
170119.6 MHz
13
U
161,16
160,16
160,16
161,16
CH2CHCN
U
CH3COCH3
77435.4 MHz
U
0.00
CH3COCH3
0.06
CH2 CHCN
TR* (K)
0.00
HCOOCH3
NH2CN
62,5 E
63,4 E
NH2CN
73,5
72,6
140,14 A
141,14 A
HCOOCH3
0.04
77321.2 MHz
Sgr B2(N)
159809.5 MHz 151,15
150,15
CH3CH2OH
HCOOCH3
0.06
CH3CONH2
Sgr B2(N)
CH3CH2CN
CH3CONH2
SO2
U
0.00
-58
-18
22
62
102
142
-58
182
-18
22
62
102
142
182
VLSR (km/s)
VLSR (km/s)
(a)
(b)
Figure 1. Representative spectra for acetamide obtained toward Sgr B2(N) with the ARO 12 m telescope at 2 and 3 mm and the ARO SMT at 1 mm. Frequencies are
indicated underneath the spectra, assuming the LSR velocities in Table 1. The spectral resolution is 1 MHz or 3.9–1.2 km s−1 over the range 77–252 GHz. (a) Lower
energy lines of acetamide with Eu ∼ 11–48 K. (b) Higher energy transitions with Eu ∼ 63–126 K.
to 70.1 km s−1 , while the line widths fall in the range ΔV1/2 =
9.1–20.0 km s−1 , with averages of VLSR = 63.2 ± 2.8 km s−1 and
ΔV1/2 = 12.5 ± 2.9 km s−1 . There is a definite trend to narrower
linewidths as the energy above ground state increases, as seen
in Figure 2. Here the line widths of acetamide from Table 1 are
plotted versus their upper state energy. With Eu < 40 K, ΔV1/2 =
15.2 ± 3.1 km s−1 , although there is considerable scatter in
these data, while above 40 K the linewidths narrow to ΔV1/2 =
11.3 ± 1.7 km s−1 . This trend is suggestive of a denser, hotter
core surrounded by cooler, more clumpy material. However,
even the lower energy data have much narrower linewidths that
were observed by Hollis et al. (2006), who found ΔV1/2 ∼
30–40 km s−1 for acetamide. The LSR velocities, in contrast,
do not vary significantly with energy, with average values of
VLSR = 64.2 ± 2.8 km s−1 (Eu < 40 K) and VLSR = 62.7 ±
2.1 km s−1 (Eu > 40 K).
The variation in linewidths is much more pronounced for
formamide, most likely because the data have higher signalto-noise ratios. As shown in Figure 3, there is a distinct
discontinuity near 50 K at which the broader width of ΔV1/2 ∼
20–25 km s−1 decreases to ΔV1/2 ∼ 14.3 ± 1.1 km s−1 . The
LSR velocities of formamide, on the other hand, show no such
5
6
Ka
Kc
→
J Ka
Kc
Frequency
(MHz)
θ b ( )
ηc /ηb
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
6
7
7
7
7
7
7
7
7
7
7
7
7
7
8
8
8
8
8
8
8
8
8
8
8
8
1
0
2
3
3
2
1
1
0
2
4
4
3
3
2
1
1
1
0
2
6
6
5
5
4
4
3
3
2
1
1
0
2
7
7
6
6
5
5
4
4
3
4
4
3
2
1
2
3
5
5
4
2
1
3
2
3
4
5
7
7
6
1
2
3
2
4
3
5
4
5
6
8
8
7
1
2
2
3
4
3
5
4
6
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
5
6
6
6
6
6
6
6
6
6
6
6
6
6
7
7
7
7
7
7
7
7
7
7
7
7
1
0
2
3
3
2
1
1
0
2
4
4
3
3
2
1
1
1
0
2
6
6
5
5
4
4
3
3
2
1
1
0
2
7
7
6
6
5
5
4
4
3
3
3
2
1
0
1
2
4
4
3
1
0
2
1
2
3
4
6
6
5
0
1
2
1
3
2
4
3
4
5
7
7
6
0
1
1
2
3
2
4
3
5
81693.45b
84542.33b
84807.79b
84888.99b
84890.99b
85093.27b
87848.87b
102064.27b
105464.22b
105972.60b
106107.87b
106107.89b
106134.43b
106141.40b
106541.68b
109753.50b
131617.90b
142701.32b
146871.47b
148223.14b
148555.85b
148555.85b
148566.82b
148566.82b
148598.97
148599.35
148667.30b
148709.02b
149792.57b
153432.17
162958.66
167320.70
169299.15
169785.87
169785.87
169790.68
169790.68
169810.71
169810.71
169861.47
169862.52
169955.83
77
74
74
74
74
74
72
62
60
59
59
59
59
59
59
57
48
44
43
42
42
42
42
42
42
42
42
42
42
41
39
38
37
37
37
37
37
37
37
37
37
37
0.94
0.93
0.93
0.93
0.93
0.93
0.93
0.90
0.90
0.89
0.89
0.89
0.89
0.89
0.89
0.89
0.84
0.82
0.81
0.80
0.80
0.80
0.80
0.80
0.80
0.80
0.80
0.80
0.80
0.79
0.77
0.76
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
TR∗ /TA∗
(K)
a
0.420 ± 0.003
0.390 ± 0.004
0.371 ± 0.004
0.282 ± 0.004
0.282 ± 0.004
0.357 ± 0.004
0.501 ± 0.003
0.551 ± 0.004
0.563 ± 0.005
0.514 ± 0.004
0.288 ± 0.003
0.288 ± 0.003
0.341 ± 0.003
0.357 ± 0.003
0.472 ± 0.003
0.611 ± 0.005
0.500 ± 0.014
0.681 ± 0.010
0.476 ± 0.006
0.450 ± 0.005
0.211 ± 0.008
0.211 ± 0.008
0.275 ± 0.008
0.275 ± 0.008
0.379 ± 0.008
0.379 ± 0.008
0.472 ± 0.003
0.431 ± 0.003
0.586 ± 0.003
0.484 ± 0.007
0.473 ± 0.012
0.643 ± 0.007
0.320 ± 0.013
0.331 ± 0.009
0.331 ± 0.009
0.341 ± 0.009
0.341 ± 0.009
0.396 ± 0.009
0.396 ± 0.009
0.528 ± 0.010
0.528 ± 0.010
0.462 ± 0.010
ΔV1/2
(km s−1 )
VLSR
(km s−1 )
Eu
(K)
μ2 S (D2 )
22.0 ± 3.7
24.8 ± 3.5
17.7 ± 3.5
17.7 ± 3.5
17.7 ± 3.5
17.6 ± 3.5
20.5 ± 3.4
20.6 ± 2.9
22.8 ± 2.8
17.0 ± 2.8
14.1 ± 2.8
14.1 ± 2.8
16.9 ± 2.8
14.1 ± 2.8
16.9 ± 2.8
19.1 ± 2.7
20.5 ± 2.3
14.7 ± 2.1
14.3 ± 2.0
18.2 ± 2.0
12.1 ± 2.0
12.1 ± 2.0
12.1 ± 2.0
12.1 ± 2.0
14.1 ± 2.0
14.1 ± 2.0
16.1 ± 2.0
16.1 ± 2.0
14.0 ± 2.0
15.6 ± 2.0
14.7 ± 1.8
14.3 ± 1.8
12.4 ± 1.8
14.0 ± 1.8
14.0 ± 1.8
14.1 ± 1.8
14.1 ± 1.8
12.4 ± 1.8
12.4 ± 1.8
14.1 ± 1.8
14.1 ± 1.8
14.1 ± 1.8
61.9 ± 3.7
67.1 ± 3.5
65.2 ± 3.5
65.0 ± 3.5
65.0 ± 3.5
64.9 ± 3.5
65.5 ± 3.4
64.6 ± 2.9
61.6 ± 2.8
63.9 ± 2.8
64.3 ± 2.8
64.3 ± 2.8
64.7 ± 2.8
65.0 ± 2.8
64.7 ± 2.8
63.7 ± 2.7
66.4 ± 2.3
64.5 ± 2.1
66.0 ± 2.0
66.0 ± 2.0
63.3 ± 2.0
63.3 ± 2.0
64.3 ± 2.0
63.3 ± 2.0
64.8 ± 2.0
64.8 ± 2.0
65.5 ± 2.0
64.5 ± 2.0
61.0 ± 2.0
64.9 ± 2.0
64.3 ± 1.8
64.2 ± 1.8
64.1 ± 1.8
64.7 ± 1.8
64.7 ± 1.8
65.1 ± 1.8
65.1 ± 1.8
63.4 ± 1.8
63.4 ± 1.8
64.0 ± 1.8
64.0 ± 1.8
64.0 ± 1.8
12.79
10.16
22.11
37.03
37.03
22.13
13.53
17.70
15.23
27.20
62.99
62.99
42.12
42.13
27.25
18.80
25.12
30.43
28.34
40.42
135.82
135.82
103.06
103.06
76.24
76.24
55.38
55.39
40.60
32.49
38.25
36.38
48.55
182.67
182.67
143.97
143.97
111.21
111.21
84.40
84.40
63.54
49.00
52.25
39.20
22.87
22.87
39.20
49.00
62.71
65.28
54.88
23.52
23.52
41.82
41.82
54.88
62.72
76.20
89.59
91.32
83.99
24.28
24.28
44.81
44.81
61.62
61.61
74.68
74.69
84.00
89.57
102.87
104.32
98.00
24.51
24.51
45.75
45.75
63.72
63.72
78.41
78.41
89.85
Comment
Shoulder on formamide line
Partially blended with other Ka = 3
Partially blended with other Ka = 3
Partially blended
Partially blended
Partially blended
Blended with Ka = 6
Blended with Ka = 6
Partially blended
Partially blended
Partially blended with two other features
Partially blended with two other features
Halfen, Ilyushin, & Ziurys
J
The Astrophysical Journal, 743:60 (12pp), 2011 December 10
Table 2
Observed Rotational Transitions of Formamide in Sgr B2(N)
7
Ka
Kc
→
J Ka
Kc
Frequency
(MHz)
θ b ( )
ηc /ηb
8
8
10
10
10
10
10
10
10
10
10
10
11
11
11
11
11
11
11
11
11
12
12
12
12
12
12
12
12
12
12
13
13
13
13
13
13
13
13
3
2
2
6
6
5
5
4
4
3
3
2
1
0
2
4
4
3
3
2
1
1
0
2
6
6
5
5
3
2
1
1
0
2
6
6
5
5
3
5
6
9
5
4
6
5
7
6
8
7
8
11
11
10
8
7
9
8
9
10
12
12
11
7
6
8
7
9
10
11
13
13
12
8
7
9
8
11
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
7
7
9
9
9
9
9
9
9
9
9
9
10
10
10
10
10
10
10
10
10
11
11
11
11
11
11
11
11
11
11
12
12
12
12
12
12
12
12
3
2
2
6
6
5
5
4
4
3
3
2
1
0
2
4
4
3
3
2
1
1
0
2
6
6
5
5
3
2
1
1
0
2
6
6
5
5
3
4
5
8
4
3
5
4
6
5
7
6
7
10
10
9
7
6
8
7
8
9
11
11
10
6
5
7
6
8
9
10
12
12
11
7
6
8
7
10
170039.07
171620.76
211328.96b
212276.04b
212276.04b
212323.56b
212323.56b
212428.02b
212433.45b
212572.84b
212832.31b
215687.69b
223452.51b
227605.66b
232273.65b
233734.72b
233745.61b
233896.58b
234315.50b
237896.68
239951.80b
243521.04b
247390.72b
253165.79b
254786.44
254786.45
254876.33
254876.65
255871.83
260189.09
261327.45
263542.24
267062.61
274001.45b
276052.64b
276052.64b
276170.23b
276170.84b
276555.33b
37
37
30
30
30
30
30
30
30
30
30
29
28
28
27
27
27
27
27
26
26
26
25
25
25
25
25
25
25
24
24
24
24
23
23
23
23
23
23
0.75
0.75
0.76
0.76
0.76
0.76
0.76
0.76
0.76
0.76
0.76
0.76
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
TR∗ /TA∗
(K)
a
0.545 ± 0.010
0.612 ± 0.006
0.532 ± 0.011
0.545 ± 0.009
0.545 ± 0.009
0.586 ± 0.009
0.586 ± 0.009
0.641 ± 0.009
0.592 ± 0.009
0.730 ± 0.007
0.696 ± 0.007
0.601 ± 0.007
0.693 ± 0.007
0.662 ± 0.005
0.792 ± 0.007
0.504 ± 0.013
0.624 ± 0.013
0.557 ± 0.013
0.638 ± 0.013
0.752 ± 0.006
0.782 ± 0.006
0.661 ± 0.008
0.854 ± 0.006
0.565 ± 0.008
0.596 ± 0.009
0.596 ± 0.009
0.704 ± 0.009
0.704 ± 0.009
0.659 ± 0.007
1.097 ± 0.006
0.901 ± 0.007
0.955 ± 0.008
0.624 ± 0.006
0.763 ± 0.007
0.749 ± 0.007
0.749 ± 0.007
0.849 ± 0.007
0.849 ± 0.007
0.733 ± 0.008
ΔV1/2
(km s−1 )
VLSR
(km s−1 )
Eu
(K)
μ2 S (D2 )
Comment
15.9 ± 1.8
14.0 ± 1.7
14.2 ± 1.4
14.1 ± 1.4
14.1 ± 1.4
15.5 ± 1.4
15.5 ± 1.4
14.1 ± 1.4
14.1 ± 1.4
15.5 ± 1.4
14.1 ± 1.4
13.8 ± 1.4
13.4 ± 1.3
13.2 ± 1.3
15.4 ± 1.3
15.4 ± 1.3
15.4 ± 1.3
12.8 ± 1.3
14.1 ± 1.3
15.1 ± 1.3
13.7 ± 1.2
14.8 ± 1.2
13.3 ± 1.2
14.2 ± 1.2
14.1 ± 1.2
14.1 ± 1.2
14.1 ± 1.2
14.1 ± 1.2
12.9 ± 1.2
16.1 ± 1.2
13.8 ± 1.1
14.8 ± 1.1
14.6 ± 1.1
15.3 ± 1.1
14.1 ± 1.1
14.1 ± 1.1
16.3 ± 1.1
16.3 ± 1.1
15.2 ± 1.1
63.3 ± 1.8
64.1 ± 1.7
66.2 ± 1.4
64.3 ± 1.4
64.3 ± 1.4
64.8 ± 1.4
64.8 ± 1.4
63.3 ± 1.4
66.1 ± 1.4
63.9 ± 1.4
64.3 ± 1.4
63.7 ± 1.4
65.1 ± 1.3
64.2 ± 1.3
65.2 ± 1.3
63.1 ± 1.3
63.1 ± 1.3
64.3 ± 1.3
63.6 ± 1.3
65.7 ± 1.3
64.7 ± 1.2
65.1 ± 1.2
64.0 ± 1.2
64.0 ± 1.2
64.3 ± 1.2
64.3 ± 1.2
64.3 ± 1.2
64.3 ± 1.2
63.9 ± 1.2
62.5 ± 1.2
63.9 ± 1.1
63.7 ± 1.1
63.4 ± 1.1
64.5 ± 1.1
63.3 ± 1.1
63.3 ± 1.1
64.5 ± 1.1
64.5 ± 1.1
63.8 ± 1.1
63.55
48.84
67.84
163.34
163.34
130.58
130.58
103.78
103.78
82.94
82.96
68.49
67.54
66.29
79.00
115.00
115.00
94.17
94.22
79.92
72.37
79.24
78.17
91.16
186.79
186.79
154.04
154.04
106.50
92.41
84.92
91.89
91.00
104.31
200.05
200.05
167.30
167.30
119.71
89.85
98.00
125.42
83.65
83.65
98.03
98.03
109.78
109.80
118.92
118.94
125.46
142.48
143.28
138.96
124.77
124.76
133.06
133.06
139.01
142.42
155.62
156.24
152.39
117.65
117.65
129.62
129.62
147.04
152.49
155.55
168.76
169.24
165.79
133.76
133.76
144.79
144.79
160.84
Partially blended
Notes. a TR∗ pertains to frequencies less than 180 GHz (12 m data), and TA∗ applies at higher frequencies (SMT observations).
b Transition frequency measured in laboratory by Kryvda et al. (2009).
Partially blended with two other features
Partially blended, other features present
Partially blended, other features present
Blended with next formamide line
Blended with previous formamide line
Partially blended
Partially blended
Partially blended
Partially blended
Partially blended
Partially blended
Partially blended
Partially blended
Partially blended
Partially blended
Partially blended
Partially blended
Halfen, Ilyushin, & Ziurys
J
The Astrophysical Journal, 743:60 (12pp), 2011 December 10
Table 2
(Continued)
The Astrophysical Journal, 743:60 (12pp), 2011 December 10
25
Halfen, Ilyushin, & Ziurys
Linewidths for CH3CONH2
→
20
V1/2
15
→
→
10
5
0
0
20
60
40
80
100
120
140
160
180
Eu (K)
Figure 2. Graph of the half-power linewidth ΔV1/2 of the uncontaminated
transitions of acetamide in Sgr B2(N) vs. energy of the upper state. The linewidth
appears to decrease from ∼20 km s−1 to ∼12 km s−1 as the energy increases
from 10 to 40 K.
30
Figure 4. Representative spectra of formamide (top panel: JKa,Kc = 51,5 →
41,4 ) and acetamide (bottom panel: JKa,Kc = 93,7 → 82,7 E and 92,8 → 83,6 E),
illustrating the similarity of line profiles for both species. The transitions for
both molecules are at comparable energies of ∼18 K for formamide and ∼30 K
for acetamide.
Linewidths for NH2CHO
25
1997; Nummelin et al. 2000), as well. In addition, all of these
species exhibit spectra that appear to have single as opposed to
multiple velocity components.
V1/2
20
15
5. DISCUSSION
10
5.1. Column Densities for Acetamide and Formamide
Column densities for both NH2 CHO and CH3 CONH2 were
determined using the standard rotational temperature diagram
method; see Figures 5 and 6, respectively. It was assumed for
the analysis that the sources of acetamide and formamide fill the
respective beams of the telescope, i.e., θ s 91 . Acetamide has
never been mapped in the Sgr B2 region, but formamide has been
found to be extended over at least a 120 area toward Sgr B2(N)
(Nummelin et al. 1998; G. Adande et al. 2011, in preparation).
All emission was assumed to be optically thin, as well, which
is very likely considering the number of transitions exhibited
by both molecules, and their respective partition functions. As
shown in Figures 5 and 6, the diagram for acetamide has greater
point-to-point variation, most likely due to the lower signalto-noise ratios of the spectra, as mentioned. The formamide
diagram appears to consist of two distinct components: one
dominated by energies approximately less than 50 K and the
other with Eu > 50 K. The data for NH2 CHO were fit with a twocomponent model, resulting in a rotational temperature of Trot =
26 ± 4 K and a column density of Ntot = 1.6 ± 0.7 × 1014 cm−2
for the lower energy component; for the higher energy lines,
the analysis yielded Trot = 134 ± 17 K and Ntot = 4.0 ± 1.2 ×
1014 cm−2 (see Figure 5). The higher energy transitions are likely
probing deeper into the core of Sgr B2(N), hence, the narrower
linewidths. For acetamide, the lower energy component is less
obvious in the diagram (see Figure 6), but a fit to the data
below 40 K results in Trot = 17 ± 4 K and Ntot = 5.2 ± 3.5 ×
1013 cm−2 . (Note that the uncertainties were determined from a
linear regression fitting routine and are 1σ errors.) A rotational
temperature of Trot = 171 ± 47 K and a column density of Ntot =
6.4 ± 4.7 × 1014 cm−2 were determined for the higher energy
data. Therefore, there is relatively good agreement between the
5
0
0
20
40
60
80
100
120
140
160
180
200
Eu (K)
Figure 3. Graph of the linewidth ΔV1/2 of the uncontaminated transitions
of formamide vs. energy of the upper state. The line width decreases from
∼25 km s−1 to an average value of ∼14 km s−1 , as the energy increases from
10 to 50 K.
trend, and fall in the general range VLSR = 62.5–67.1 km s−1
with an average value of VLSR = 64.3 km s−1 , very similar to
that of acetamide.
The similarity in line profiles between formamide and acetamide is illustrated in Figure 4. Here the JKa,Kc = 51,5 →
41,4 transition of formamide with Eu ∼ 18 K (top panel) and the
JKa,Kc = 93,7 → 82,7 E and 92,8 → 83,6 E lines of acetamide with
Eu ∼ 30 K (lower panel) are displayed. The linewidths of both
features are equal within the measurement errors at 20.6 ± 2.9
and 18.4 ± 3.1 km s−1 , respectively, as are the LSR velocities
(see Tables 1 and 2).
The linewidths and LSR velocities of formamide and acetamide are characteristic of organic species in Sgr B2(N).
Acetaldehyde (CH3 CHO), formic acid (HCOOH), and ketene
(H2 CCO), for example, have typical line widths of ΔV1/2 = 13,
12, and 11 km s−1 , as well as average source velocities of VLSR =
64, 63, and 63 km s−1 (Nummelin et al. 1998; D. T. Halfen
et al. 2011, in preparation). Ethylene oxide (c-CH2 OCH2 ) and
ethanol (C2 H5 OH) also have similar velocities of VLSR = 64
and 62 km s−1 and ΔV1/2 = 16 and 13 km s−1 (Dickens et al.
8
The Astrophysical Journal, 743:60 (12pp), 2011 December 10
13
Halfen, Ilyushin, & Ziurys
Rotational Temperature Diagram for Formamide
12
S 2)
Trot = 26 ± 4 K
Ntot = 1.6 ± 0.7 x 1014 cm-2
log (3kW/8
3
Trot = 134 ± 17 K
Ntot = 4.0 ± 1.2 x 1014 cm-2
11
10
9
0
20
40
60
80
100
120
140
160
180
200
Eu (K)
Figure 5. Rotational temperature diagram of formamide in Sgr B2(N) constructed from the uncontaminated transitions, assuming the source fills the telescope beam.
The analysis indicates Ntot = 1.6 ± 0.7 × 1014 cm−2 and 26 ± 4 K below ∼50 K in energy and Ntot = 4.0 ± 1.2 × 1014 cm−2 and Trot = 134 ± 17 K above 50 K.
12
Rotational Temperature Diagram for Acetamide
Trot = 17 ± 4 K
Ntot = 5.2 ± 3.5 x 1013 cm-2
S 2)
11
log (3kW/8
3
Trot = 171 ± 47 K
Ntot = 6.4 ± 4.7 x 1014 cm-2
10
9
8
0
20
40
60
80
100
120
140
160
180
Eu (K)
Figure 6. Rotational temperature diagram of acetamide in Sgr B2(N) constructed from the uncontaminated transitions, assuming the source fills the telescope beam.
The data suggest Ntot = 5.2 ± 3.5 × 1013 cm−2 and 17 ± 4 K for Eu < 40 K and Ntot = 6.4 ± 4.7 × 1014 cm−2 and Trot = 171 ± 47 K when Eu > 40 K.
gas temperatures characterizing acetamide and formamide in
this simplistic two-component model.
The previously reported column density for formamide is
Ntot = 5.9(1) × 1014 cm−2 , while for acetamide, the average
value is Ntot = 1.8(9) × 1014 cm−2 , assuming a rotational
temperature of 8 K for both molecules (Hollis et al. 2006).
(Column densities for the individual three velocity components
were not separately determined in the analysis of Hollis et al.)
Nummelin et al. (2000) reported a similar value for formamide
of Ntot = 5.6(1.3) × 1014 cm−2 . These values are in reasonable
agreement with those determined here for the warmer component of NH2 CHO and the colder one for acetamide. It should be
noted that Hollis et al. (2006) only observed lines with energies
less than 9 K.
5.2. Acetamide: A Remarkably Abundant Organic Molecule
Given a molecular hydrogen column density of Ntot ∼ 3 ×
1024 cm−2 for Sgr B2(N) (Nummelin et al. 2000), the fractional
abundances of acetamide are f ∼ 1.7 × 10−11 and 2.1 ×
10−10 , for the lower and higher energy regions, respectively. The
amounts of formamide have a similar range, with f ∼ 5.2 × 10−11
and 1.3 × 10−10 in the two-component model. The values are
summarized in Table 3. Therefore, despite being a more complex
molecule, CH3 CONH2 has an abundance virtually equivalent to
that of NH2 CHO in the warmer gas of Sgr B2(N). Furthermore,
its abundance is very similar to that of ketene (H2 CCO) and
acetaldehyde, CH3 CHO, whose amounts are estimated to be
f ∼ 3.7 × 10−11 and 1.1 × 10−10 , based on this survey data
9
The Astrophysical Journal, 743:60 (12pp), 2011 December 10
Halfen, Ilyushin, & Ziurys
Table 3
Abundances of Organic Species in Sgr B2(N)a
Name
Formaldehyde
Ketene
Acetaldehyde
Formamide
Acetamide
N-methyl formamide
Formula
H2 CO
H2 CCO
CH3 CHO
NH2 CHO
CH3 CONH2
CH3 NHCHO
This Work
10−10 b
5.3 ×
3.7 × 10−10
1.7 × 10−10
0.53–1.3 × 10−10
0.17–2.1 × 10−10
< 3.1–9.5 × 10−12 c
Nummelin et al.
10−10
5×
2 × 10−10
2 × 10−10
2 × 10−10
...
...
Hollis et al.
...
...
...
2 × 10−10
6 × 10−11
...
Notes.
a Assuming an H column density of 3 × 1024 cm−2 (Nummelin et al. 2000).
2
b From Halfen et al. (2006).
c Range assumes T
rot = 25 and 100 K.
(D. T. Halfen et al. 2011, in preparation). Nummelin et al. (2000)
calculate f ∼ 2 × 10−10 and 2 × 10−10 for these two species as
well, in relative agreement, see Table 3. In contrast, the estimated
abundance of H2 CO in Sgr B2(N) is f ∼ 5 × 10−10 , based on
the 18 O isotopologue (Halfen et al. 2006). Using formaldehyde
as a benchmark species, CH3 CONH2 is a remarkably prevalent
molecule.
The spectrum of acetamide is also very rich across the 1,
2, and 3 mm bands. The number of identified transitions in
the 1 mm window alone is 54, not counting those lines that
are completely blended or contaminated. Based on the derived
rotational temperature, it is expected that the spectrum of
acetamide will contribute significant emission in the 0.8 mm
window, as well. A cursory search through the data of Nummelin
et al. (1998) reveals at least 34 transitions of acetamide.
An isomer of acetamide, N-methyl formamide
(CH3 NHCHO), was also searched for in our survey (D. T. Halfen
et al. 2011, in preparation). From the current data, an upper limit
to the abundance was found to be 3.1 × 10−12 or 9.5 × 10−12 ,
assuming a rotational temperature of 25 K or 100 K, respectively. Therefore, this species is at least a factor of 5–22 lower
in abundance than acetamide.
CH3 CHONH+2 + e− → CH3 CONH2 + H.
A similar scheme was proposed for acetamide involving acetaldehyde as a precursor, as well. The rates of these reactions
have not been studied, but were assumed by Quan & Herbst
(2007) to be 10−9 cm3 s−1 at 10 K for the radiative association
reactions and 10−7 cm3 s−1 for the recombination processes.
With these mechanisms considered, however, the model of Quan
& Herbst could only produce an abundance of acetamide of
∼10−15 , significantly lower than the value determined in this
work or by Hollis et al. (2006).
Instead of radiative association reactions forming formamide
and acetamide, ion–molecule processes might lead to both
species. Formamide could be produced from the reaction of
formaldehyde and protonated ammonia, creating protonated
formamide, followed by dissociative electron recombination:
The similarity between acetamide and formamide in terms
of line parameters and general abundance suggest that the
two species may be connected synthetically. Hollis et al.
(2006) proposed, in fact, such a related formation scheme,
creating acetamide from the addition of the methylene radical
to formamide:
While this reaction is overall exothermic, it involves a spin
flip on the CH2 radical; consequently, it likely has an energy
barrier > 1000 K, and thus would not typically proceed in gas
that is ∼130–170 K (Quan & Herbst 2007).
Quan & Herbst (2007) propose that both NH2 CHO and
CH3 CONH2 are created from radiative association reactions.
The formation of formamide first occurs via
(2)
NH4 CH2 O+ + e− → NH2 CHO + H + H2 .
(3)
Formamide then leads to acetamide:
NH2 CHO + CH+3 → CH3 CHONH+2 + hv
(6)
NH3 CHO+ + e− → NH2 CHO + H.
(7)
NH2 CHO + CH+5 → CH3 CONH+3 + H2
(8)
CH3 CONH+3 + e− → CH3 CONH2 + H.
(9)
Both initial ion–molecule reactions have never been studied
experimentally, but could proceed near the Langevin rate of
∼10−9 cm3 s−1 (Landau & Lifshitz 1965). The typical dissociative electron recombination rate is ∼10−7 cm3 s−1 (Bardsley
& Bondi 1970). Reaction (9) could also lead to an isomer of
acetamide, N-methyl formamide (CH3 NHCHO), depending on
the branching ratio.
It should be noted that the electron recombination reactions
given in the equations above are quite likely to produce a
variety of products. Recombination experiments often lead
predominantly to fragmented species (Geppert & Larsson 2008),
where the bonds between the heavy atoms are broken. However,
a small fraction of these reactions (∼3%–5%) will result in
the larger product. The proposed reactions have not yet been
measured, so it is difficult to ascertain the percent yield of
possible products.
Many models neglect radical–neutral reactions. Garrod et al.
(2008), however, propose that H2 CO + NH2 leads to NH2 CHO
in the gas phase. It is also possible that NH2 CHO and CH3
(1)
H2 CO + NH+4 → NH4 CH2 O+ + hv
H2 CO + NH+4 → NH3 CHO+ + H2
A reasonable formation mechanism for acetamide begins with
the reaction of formamide and protonated methane (CH5 + ), with
electron recombination as the next step:
5.3. Possible Formation Mechanisms for
Acetamide in Sgr B2(N)
NH2 CHO + CH2 → CH3 CONH2 + hv.
(5)
(4)
10
The Astrophysical Journal, 743:60 (12pp), 2011 December 10
Halfen, Ilyushin, & Ziurys
react to form CH3 CONH2 . (NH2 and CH3 are both radical
species.) These types of reactions could have rates as high as of
∼10−11 cm3 s−1 (Woodall et al. 2007).
Acetamide and formamide could also be produced on ice
mantles of interstellar grains. Quan & Herbst (2007) conclude
that most organic species in Sgr B2(N) are formed on grain
surfaces, since the abundances of several complex molecules
could not reproduced by their gas-phase models. Garrod et al.
(2008) generate formamide in ices by hydrogenation (addition
of H atoms) starting with OCN, and also by the reaction
of HCO and NH2 . Acetamide is formed in the ices by the
addition of CH3 to HNCO with subsequent hydrogenation.
These models reproduce the abundance of acetamide well, with
f = 0.4–2.7 × 10−10 , but formamide is predicted to be quite
overabundant at f = 1.3–2.4 × 10−6 (Garrod et al. 2008). Jones
et al. (2011) experimentally produced formamide from CO and
NH3 in simulated interstellar ices using high-energy electrons.
In addition, Berger (1961) showed that acetamide could be
synthesized in irradiated ices containing a mixture of CO, NH3 ,
and CH4 at 77 K. Until more gas-phase and surface reaction rates
involving these species are measured, however, it is difficult to
establish the exact synthesis routes.
(Shimoyawa & Ogasawara 2002). Consequently, amino acids
may not be strictly needed to form peptide bonds on early Earth,
if they can be generated from acetamide and other species.
The next largest compound in the formamide/acetamide
series is propionamide, CH3 CH2 CONH2 . Identification of this
species in interstellar gas will be challenging, especially given
the high density of rotational levels. However, the b-type
dipole moment of propionamide is substantial: μb = 3.50 D
(Marstokk et al. 1996). Therefore, its rotational spectrum might
be detectable, despite its chemical complexity. To date, only
the microwave spectrum of this molecule has been measured
and in a limited range of 21.4–39.0 GHz (Marstokk et al. 1996).
Thus, higher frequency measurements are clearly needed before
a viable interstellar search for this more complex alkyl amide
can be conducted.
6. CONCLUSION
Observations across a broad frequency range toward Sgr
B2(N) have resulted in the identification of 133 new transitions
of acetamide (CH3 CONH2 ) in this object at the 30–200 mK
level. In addition, 79 transitions of formamide (NH2 CHO) were
observed at a sensitivity of 0.2–1.1 K. Both molecules, therefore,
are prevalent in the spectrum of Sgr B2(N) at λ 1 mm,
and may contribute to emission at even shorter wavelengths.
In contrast, the higher-lying isomer of acetamide, N-methyl
formamide (CH3 NHCHO) was not detected in this source,
despite the presence of numerous favorable transitions in the
observed frequency range. These data are further evidence that
interstellar chemistry is not combinatorial. The high abundances
of acetamide and formamide in Sgr B2(N) additionally suggest
that there might be other plausible synthetic routes to simple
peptide polymers that do not involve amino acids.
5.4. Implications for Interstellar and Prebiotic Chemistry
The high abundance of acetamide in Sgr B2(N) suggests
species with peptide bonds are prevalent in dense clouds. In
the absence of amino acids, acetamide could pose an alternative
route to small peptides compounds. Amino acids form peptide
bonds, i.e. –NHCO–, through bimolecular reactions, such as the
following with glycine (Morrison & Boyd 1992):
NH2 CH2 COOH + NH2 CH2 COOH
→ NH2 CH2 CONHCH2 COOH + H2 O.
(10)
We thank the staff of ARO for making these observations
possible. We also thank F. J. Lovas for helpful comments on
the manuscript. This work was supported by NASA Exobiology
Grant NNX10AR83G.
This same di-peptide species could alternatively be created from
acetamide:
CH3 CONH2 + CH3 → CH3 CONHCH3 + H
(11)
REFERENCES
CH3 CONHCH3 + NH2 → NH2 CH2 CONHCH3 + H
(12)
Bardsley, J. M., & Bondi, M. A. 1970, Adv. Atom Mol. Phys., 6, 1
Berger, R. 1961, Proc. Natl Acad. Sci., 47, 1434
Bisschop, S. E., Jørgensen, J. K., van Dishoeck, E. F., & de Wachter, E. B. M.
2007, A&A, 465, 913
Cooper, G. W., & Cronin, J. R. 1995, Geochim. Cosmochim. Acta, 59, 1003
Dickens, J. E., Irvine, W. M., Ohishi, M., et al. 1997, ApJ, 489, 753
Friedel, D. N., Snyder, L. E., Turner, B. E., & Remijan, A. 2004, ApJ, 600, 234
Garrod, R. T., Widicus Weaver, S. L., & Herbst, E. 2008, ApJ, 682, 283
Geppert, W. D., & Larsson, M. 2008, Mol. Phys., 106, 2199
Halfen, D. T., Apponi, A. J., Woolf, N. J., Polt, R., & Ziurys, L. M. 2006, ApJ,
639, 237
Halfen, D. T., Apponi, A. J., & Ziurys, L. M. 2001, ApJ, 561, 244
Hollis, J. M., Lovas, F. J., Remijan, A. J., et al. 2006, ApJ, 643, L25
Ilyushin, V. V., Alekseev, E. A., Dyubko, S. F., Kleiner, I., & Hougen, J. T.
2004, J. Mol. Spectrosc., 227, 115
Jones, B. M., Bennett, C. J., & Kaiser, R. I. 2011, ApJ, 734, 78
Kojima, T., Yano, E., Nakagawa, K., & Tsunekawa, S. 1987, J. Mol. Spectrosc.,
122, 408
Kryvda, A. V., Gerasimov, V. G., Dyubko, S. F., Alekseev, E. A., & Motiyenko,
R. A. 2009, J. Mol. Spectrosc., 254, 28
Kurland, R. J., & Wilson, E. B, Jr. 1957, J. Chem. Phys., 27, 585
Landau, L. D., & Lifshitz, E. M. 1965, Quantum Mechanics (Oxford: Pergamon)
Lovas, F. J. 2004, J. Phys. Chem. Ref. Data, 33, 117
Marstokk, K.-M., Møllendal, H., & Samdal, S. 1996, J. Mol. Struct., 376,
11
Martı́n, S., Martı́n-Pintado, J., & Mauersberger, R. 2009, ApJ, 694, 610
Morrison, R. T., & Boyd, R. N. 1992, Organic Chemistry (6th ed.; Edgewood
Cliffs, NJ: Prentice Hall)
NH2 CH2 CONHCH3 + CO2 → NH2 CH2 CONHCH2 COOH.
(13)
The first two reactions are neutral–neutral processes involving a
radical precursor and could have rates as high as 10−11 cm3 s−1
(e.g., Woodall et al. 2007); the third reaction would be substantially slower. The first two processes are possible in the gas
phase in interstellar clouds, while the third is more plausible on
the surfaces of meteorites in the liquid phase.
Alkyl amides, such as acetamide and propionamide, have
been identified in the carbonaceous chondrites (Cooper &
Cronin 1995), but quantitative abundances have not yet been determined. Larger amides with up to 12 carbon atoms are known
to be present in the Murchison meteorite, as well. Unfortunately,
smaller amides, such as formamide, are easily lost in the preparation process and are difficult to detect in meteoritic material
(S. Pizzarello 2011, private communication). Therefore,
comparing the relative amounts of formamide and acetamide in carbonaceous chondrites has not been done.
It should additionally be noted that the di-glycine peptide (NH2 CH2 CONHCH2 COOH) has been detected in the
Murchison and Yamato-791198 meteorites in low quantities
11
The Astrophysical Journal, 743:60 (12pp), 2011 December 10
Halfen, Ilyushin, & Ziurys
Snyder, L. E., Lovas, F. J., Hollis, J. M., et al. 2005, ApJ, 619, 914
Suenram, R. D., Golubiatnikov, G. Yu., Leonov, I. I., et al. 2001, J. Mol.
Spectrosc., 208, 188
Turner, B. E. 1989, ApJS, 70, 539
Turner, B. E. 1991, ApJS, 76, 617
Woodall, J., Agundez, M., Markwick-Kemper, A. J., & Millar, T. J. 2007, A&A,
466, 1197
Yamaguchi, A., Hagiwara, S., Odashima, H., Takagi, K., & Tsunekawa, S.
2002, J. Mol. Spectrosc., 215, 144
Müller, H. S. P., Schlöder, S. F., Stutzki, J., & Winnewisser, G. 2005, J. Mol.
Struct., 742, 215
Nummelin, A., Bergman, P., Hjalmarson, A., et al. 1998, ApJS, 117, 427
Nummelin, A., Bergman, P., Hjalmarson, Å., et al. 2000, ApJS, 128, 213
Pickett, H. M., Poynter, R. L., Cohen, E. A., et al. 1998, J. Quantum Spectrosc.
Radiat. Transfer, 60, 883
Quan, D., & Herbst, E. 2007, A&A, 474, 521
Shimoyawa, A., & Ogasawara, R. 2002, Orig. Life Evol. Biosph., 32,
165
12