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The Astrophysical Journal, 652:1787Y1795, 2006 December 1
# 2006. The American Astronomical Society. All rights reserved. Printed in U.S.A.
HYDROXYACETONE (CH3COCH2OH): A COMBINED MICROWAVE AND MILLIMETER-WAVE
LABORATORY STUDY AND ASSOCIATED ASTRONOMICAL SEARCH
A. J. Apponi, J. J. Hoy, D. T. Halfen,1 and L. M. Ziurys
Departments of Chemistry and Astronomy and Arizona Radio Observatory, Steward Observatory,
University of Arizona, 933 North Cherry Avenue, Tucson, AZ, 85721
and
M. A. Brewster
Sfyri, Inc., P.O. Box 19742, Seattle, WA 98109
Received 2006 June 29; accepted 2006 July 27
ABSTRACT
A combined laboratory and astronomical investigation has been conducted on the methyl sugar hydroxyacetone
(CH3COCH2OH ). Rotational transitions of this species in the ground torsional state (vt = 0) were recorded using both
millimeter-wave direct absorption techniques and Fourier transform microwave spectroscopy. A total of 1145 lines of
CH3COCH2OH were analyzed in the frequency range 4 to 180 GHz, including transitions arising from both A- and Esymmetry species. A modified rho-axis method Hamiltonian was needed for the analysis because of the presence of
perturbations resulting from the torsional motion of the methyl group in this molecule. Assignment of the E-species was
particularly problematic as a consequence of significant mixing between the ground and torsionally excited levels. The
complete data set was fitted using 21 spectroscopic parameters and had a global rms of 90 kHz; the barrier to internal
rotation was established to be 65.3560(22) cm1. An astronomical search was subsequently conducted for hydroxyacetone at 2 and 3 mm using the 12 m telescope of the Arizona Radio Observatory. Twenty-eight favorable transitions
arising from both A- and E-species, each consisting of collapsed quartets, were searched for toward Sgr B2(N). Although credible features were detected at several frequencies of hydroxyacetone, there were a sufficient number of missing lines to rule out an interstellar detection. An upper limit to the column density of Ntot < 5 ; 1012 cm2 was derived
for CH3COCH2OH in Sgr B2(N), indicating that this species is an order of magnitude less abundant than glycolaldehyde
(CH2OHCHO).
Subject headingg
s: astrobiology — astrochemistry — ISM: abundances — ISM: individual (Sagittarius B2) —
ISM: molecules — line: identification — methods: laboratory
Online material: machine-readable table
1. INTRODUCTION
sured a total of 51 lines both in the A- and E-species. More recently,
Braakman et al. (2005) reported measurements of 578 A-state lines
in the 85 to 115 and 220 to 376 GHz ranges; these authors also
assigned 288 E-state lines, but they could not adequately fit rotational constants to these data. This study remains unpublished.
Without characterization of the E-species, astrophysical identification of hydroxyacetone is subject to large uncertainties.
In order to carry out a viable astronomical search for hydroxyacetone, we have measured rotational spectra of this molecule
in the range of 4 to 20 GHz and 65 to 180 GHz, using a combination of direct absorption millimeter-wave spectroscopy and
Fourier transform microwave ( FTM ) spectroscopy. Our work
was conducted independently of the Braakman et al. (2005) study.
We have been able to characterize the E-species of hydroxyacetone
to high precision using a modified rho-axis method Hamiltonian.
This method has also been successfully applied to a number of
other methyl rotor problems: acetamide, acetic acid, acetaldehyde,
methanol, and ethyl acetamidoacetate (Ilyushin et al. 2004). Here
we present our spectroscopic measurements, as well as a combined
A- and E-species analysis, which required the use of highly mixed
eigenfunctions. A detailed search for this species in the dense molecular cloud Sgr B2(N) was subsequently conducted. Results of
the astronomical observations are also presented.
Hydroxyacetone, CH3COCH2OH, is an interesting molecule
from an astrophysical perspective. This compound exhibits the
next level of molecular complexity in species that have carbon
backbones with adjacent oxygen atoms, namely, glycolaldehyde,
CH2OHCHO (Hollis et al. 2000; Halfen et al. 2006), and ethylene
glycol, CH2OHCH2OH (Hollis et al. 2002; Crovisier et al. 2004).
In the laboratory, hydroxyacetone is an important starting material
in the organic synthesis of gem-diols, acetals, and ketals (see, e.g.,
Solomons 1984). If such molecules are synthesized in the interstellar medium, they could lead to the production of complex prebiotic species. For example, the reaction of hydroxyacetone with
HCN could produce a -hydroxy cyanohydrin. This process would
result in an additional carbon atom and the creation of an active site
for the synthesis of hydroxy amines, hydroxy aldehydes (sugars),
and hydroxy acids, all of which possess chiral centers.
A decisive search for hydroxyacetone in interstellar space has
yet to be conducted. Unlike the other stable molecules with the
chemical formula C3H6O2, hydroxyacetone still requires further
spectroscopic investigation. This molecule has proved very difficult to characterize owing to the presence of an internal methyl
group rotor, which has a very low barrier to internal rotation of
about 65 cm1. Although the rotational spectrum of hydroxyacetone was first recorded by Kattija-Ari & Harmony (1980) in
the microwave region (26 to 43 GHz), these authors only mea1
2. EXPERIMENTAL MEASUREMENTS
The microwave measurements of hydroxyacetone were recorded
in the frequency range of 4 to 20 GHz with a Balle-FlygareYtype
NSF Astronomy and Astrophysics Postdoctoral Fellow.
1787
1788
APPONI ET AL.
Fourier transform microwave spectrometer (Balle & Flygare 1981)
recently built in the Ziurys lab, which uses a version of the popular
FTMW++ software package 2 that is compatible with our hardware.
A gas sample of hydroxyacetone was prepared by placing 5 ml of
the commercially available liquid into an evacuated 30 liter gas
cylinder and pressurizing the container to 150 psi (absolute) with
argon. The partial pressure of hydroxyacetone was high enough in
the mixture to produce strong signals without further processing
and was also sufficiently stable to allow for many subsequent dilutions without affecting the signal strength. The gas was introduced into the Fabry-Pérot cavity at a 40 angle relative to the
cavity axis using a pulsed solenoid valve. This angle was chosen
as a trade-off between the advantages of a coaxial molecular beam
and the mechanical difficulties of introducing the gas through the
mirror. This arrangement improved the signal strength by a factor
of 2 to 5 versus a perpendicular expansion. Optimal signals were
achieved in an expansion using a 40 psi absolute stagnation
pressure behind a 0.8 mm nozzle orifice and a 10 Hz repetition
rate with a total mass flow of about 50 standard cubic centimeters
per minute. Time-domain spectra were recorded using a digital
oscilloscope at 0.5 s intervals and averaged until a sufficient
signal-to-noise ratio was achieved, typically 100 to 1000 shots.
Fourier transforms of the averaged signals resulted in spectra with
2 kHz resolution. Transitions of hydroxyacetone were observed
as Doppler doublets with a full width at half-maximum (FWHM)
of 5 kHz per feature. The resultant Doppler components are similar to those expected for a coaxial expansion owing to a highly divergent molecular beam and a relatively small interaction region.
The rest frequencies are simply taken as the average of the two
Doppler peaks; this method reproduces the rest frequencies of
OCS and its isotopic species to 0.2 kHz or less.
The millimeter-wave measurements of hydroxyacetone were
conducted in one of the Ziurys group direct absorption spectrometers ( Ziurys et al. 1994). The instrument consists of Gunn
oscillatorYSchottky diode multiplier radiation sources, an absorption cell, and a liquid-heliumYcooled InSb detector. Phasesensitive detection was employed, by frequency modulation of
the radiation source and demodulation with a lock-in amplifier.
A partial pressure of 2 to 10 millitorr of hydroxyacetone was maintained in the gas cell by pumping on the room-temperature liquid
and throttling the flow with a valve. A higher signal-to-noise ratio
could be achieved by heating the liquid hydroxyacetone to about
40 C. A total of 115 GHz was contiguously recorded from 65 to
180 GHz at step intervals of 90 kHz; typical FWHM line widths
were 400 to 500 kHz.
3. LABORATORY RESULTS AND ANALYSIS
The room-temperature rotational spectrum of hydroxyacetone
is complicated because of its near-prolate asymmetric structure,
a low-barrier internal methyl rotor, and nearly equal dipole moments along the a- and b-axes (a = 2.22 D and b = 2.17 D;
Kattija-Ari & Harmony 1980; see Fig. 1). These characteristics
generate a confused spectrum consisting of many highly mixed,
low-lying torsional states in both A- and E-symmetry species,
which cannot be adequately analyzed using the usual asymmetric
reduction of the Watson Hamilitonian. As shown in Figure 2, the
millimeter-wave spectrum exhibits a line density of about 50 lines
per 100 MHz coverage—of which less than 20% in the end could
be unambiguously assigned to ground-state transitions.
The millimeter-wave spectrum of hydroxyacetone fortunately
possesses a series of accidental quartets comprising two a-type
2
See http://www.pci.uni-hannover.de/~lgpca/spectroscopy/ftmw.
Fig. 1.—Minimum-energy structure for hydroxyacetone, showing the internal hydrogen bond (dashed cylinder), which gives rise to the low-barrier methyl
rotor by removing electron density from the carbonyl oxygen (black, carbon;
dark gray, oxygen; light gray, hydrogen).
and two b-type transitions of adjacent Ka levels. This pattern was
easily identifiable in the early stages of the project. The quartets
also slowly collapse with increasing J in both the A- and E-species
and thus provided a means to visually connect K-stacks over several J-values; they also gave insight into the value of K in the quartet series (the lowest K-values have the smallest splittings for a
given J ). Figure 2 illustrates this progression in the observed quartet patterns of the vt = 0 state; it also shows one additional quartet
exhibiting this same behavior, which probably belongs to a lowlying torsional state.
It was relatively easy to predict the frequencies of the A-species
starting with the constants provided by Kattija-Ari & Harmony
(1980) using a conventional reduced Watson Hamilitonian across
the entire 4 to 180 GHz region studied. Hence, assignment of the
A-species lines was relatively straightforward. Constants derived
from their E-species data, however, were unable to predict the frequencies of other E-species lines either to lower or to higher frequency. Calculations were attempted with an internal-axis method
(Hartwig & Dreizler 1996), but this technique still failed to reproduce the frequencies at low J. In fact, an unambiguous assignment
was impossible at this stage. Although several series of quartets
had been identified above 100 GHz, none of them could be used to
successfully predict the low-frequency lines measured by KattijaAri & Harmony (1980); Braakman et al. (2005) reported similar
difficulties in fitting the E-species. Since the perturbations of the
torsional motion of the methyl group primarily affect low-J transitions, these lines also hold the most information in fitting a set of
meaningful torsion-rotation parameters to the transition frequencies. An extensive search for those transitions of the E-species was
subsequently conducted using the FTM spectrometer and led to
the assignment of the strongest E-species lines by intensity comparison (see Fig. 3). In some cases, the transitions were found more
than a gigahertz away from their predicted frequencies.
A modified rho-axis method (RAM ) Hamiltonian was employed in the final analysis. This Hamiltonian reproduced the data
within the measurement uncertainty (2 to 100 kHz; see Table 1) in
a global fit of both the A- and E-species. This method was originally based on the work of Kirtman (1962) and later refined in its
application toward solving the torsional-rotational spectrum of
methanol ( Lees & Baker 1968; Herbst et al. 1984). The details of
Fig. 2.—Loomis-Woods plot of the Jupper = 16 Y20 transitions of the millimeter-wave spectrum of hydroxyacetone measured here, illustrating how the asymmetry
quartets collapse with increasing J. Three progressions are indicated with dashed lines. The progressions on the left and in the middle are the E- and A-species in the
ground torsional state, and the one on the right is possibly the E-species of the first exited state of the methyl torsion. Each spectrum shows 150 MHz of raw data
centered on the A-species lines. Also noteworthy is the very high line density in the hydroxyacetone spectrum, upward of 50 lines per 100 MHz.
1790
APPONI ET AL.
Vol. 652
Fig. 3.—Spectra of the JKa ; Kc = 30,3Y20,2 transition in both the A- and E-species of hydroxyacetone, measured here with the FTM system. The top panels are the free
induction decay of the recorded signal, and the bottom panels are the fast Fourier transform of the time-domain signal. The double-peaked spectra result from the
interaction of the molecular beam with the standing wave of the cavity, producing two Doppler components. The intensity of the E-species transition is the same as that
for the A-species, as expected.
this method have been described in the literature (e.g., Herbst et al.
1984; Hougen et al. 1994). To summarize, the technique separates
the Hamiltonian into two parts, a rotational and a torsional contribution: Ĥ ¼ Ĥrot þ Ĥtors . The RAM Hamiltonian defines
the molecular coordinates so as to eliminate the (2FP b Pb þ
2FP c Pc )-terms in the torsional kinetic energy operator, which
simplifies to
T̂tors ¼ F(P̂ þ ˆ P̂)2 :
ð1Þ
This change in the coordinate system creates off-diagonal elements in the rotational Hamiltonian; equation (2) shows the
form of the rotational Hamiltonian used, including the three offdiagonal constants, Dab , Dac , and Dbc :
Ĥrot ¼ 12 (B þ C)(P̂b2 þ P̂c2 ) þ AP̂a2
where F is the rotational constant for the methyl rotor, V3 is the
barrier energy of the threefold axis, is the phase angle of the
threefold axis, P̂ is the angular momentum operator of the internal rotor, and is the coupling constant or degree of coupling
between the a inertial axis and the internal methyl rotor.
Labeling of quantum numbers in the E-species of molecules
possessing low-barrier internal methyl rotors is problematic,
especially at high J- and Ka -values. Extensive mixing of the freerotor states of the methyl group and the symmetric-top wave
functions occurs. In this regime, Ka loses its usual meaning because no dominant basis in a given Ka quantum number exists in
the eigenvector. Hydroxyacetone falls into this category, requiring special treatment to assign the appropriate quantum
numbers to the eigenvectors. The method used involves two
diagonalization steps. The first is performed in the free-rotor
basis
þ 12 (B C)(P̂b2 P̂c2 ) þ Dab (P̂a P̂b þ P̂b P̂a )
þ Dac (P̂a P̂c þ P̂c P̂a ) þ Dbc (P̂b P̂c þ P̂c P̂b );
ð2Þ
jK; vt ; i ¼
10
X
(
)
i 3kþ t
AK;v
;
3kþ e
ð4Þ
k ¼10
where A, B, and C are the normal asymmetric-top rotational constants and P̂n is the angular momentum operator for the principal
axes, with n 2 {a, b, c}. The resultant torsional Hamiltonian describing the threefold potential of the methyl rotor then simplifies
to
Ĥtors ¼ F(P̂ þ P̂a )2 þ 12 V3 (1 cos );
ð3Þ
a ;vt
represent the eigenfunctions for a
where the coefficients AK3kþ
given K-value summed over a 21-dimensional plane wave basis
and is a symmetry parameter that has the values 0 (A symmetry) and 1 (E symmetry). This treatment results in a signed
K quantum number that is retained here for consistency. The
torsional eigenfunctions from the first step are then multiplied
No. 2, 2006
STUDY OF AND SEARCH FOR HYDROXYACETONE
TABLE 1
Assigned Rotational Transitions of Hydroxyacetone in Its Ground State
J
0
Ka0
Kc0
p
0
J
00
Ka00
Kc00
p
a
( MHz)
00
obs calc
A-State Transitions (rms = 0.081 MHz)
1
1
3
3
4
6
7
7
7
7
7
7
7
8
1
1
2
1
1
2
5
6
2
2
3
2
4
5
1
0
1
2
3
4
3
2
5
5
4
6
3
4
þ
þ
þ
þ
þ
þ
þ
þ
0
1
3
3
4
5
6
6
6
7
7
7
6
7
0
0
1
1
0
3
4
5
3
1
2
1
3
4
0
1
2
3
4
3
2
1
4
6
5
7
4
3
þ
þ
þ
þ
þ
þ
þ
þ
þ
þ
þ
13207.762
7477.569
18680.536
5717.580
12887.568
9616.541
109762.263
123698.800
18837.559
18945.264
29634.660
35045.810
96006.451
116411.581
(2)
(2)
(2)
(2)
(2)
(2)
(50)
(100)
(2)
(2)
(100)
(100)
(50)
(50)
0.001
0.001
0.006
0.003
0.003
0.004
0.040
0.042
0.004
0.002
0.050
0.019
0.068
0.015
6473.660 (2)
8715.410 (2)
15322.721 (2)
15511.217 (2)
13297.979 (2)
16372.299 (2)
16905.649 (2)
19353.220 (2)
29519.840 (100)
36790.110 (100)
37294.980 (100)
38267.560 (100)
152526.127 (50)
158822.450 (50)
65827.400 (50)
67804.333 (50)
67455.972 (50)
0.002
0.001
0.003
0.001
0.004
0.001
0.002
0.001
0.008
0.020
0.048
0.054
0.205
0.041
0.063
0.016
0.155
E-State Transitions (rms = 0.105 MHz)
1
1
2
2
3
3
3
3
5
6
6
6
8
9
10
10
10
0
þ1
þ2
þ1
1
þ2
0
þ1
0
þ1
0
þ1
5
5
1
þ5
5
1
1
1
2
2
2
3
3
5
6
6
6
3
4
9
6
5
...
...
...
...
...
...
...
...
...
...
...
...
...
...
...
...
...
0
0
2
1
3
3
2
2
4
5
5
5
7
8
9
9
9
0
0
þ1
0
0
þ1
þ1
þ1
þ1
þ1
0
0
þ4
þ4
1
þ5
5
0
0
2
1
3
3
2
2
4
5
5
5
4
5
8
5
4
...
...
...
...
...
...
...
...
...
...
...
...
...
...
...
...
...
Notes.—Table 1 is published in its entirety in the electronic edition of the
Astrophysical Journal. A portion is shown here for guidance regarding its form
and content.
a
Measurement uncertainties are included in parentheses in kilohertz. Uncertainties listed as 2 kHz were measured with the FTM spectrometer, those
listed as 50 kHz were measured with the millimeter-wave spectrometer, and
those listed as 100 kHz were taken from Kattija-Ari & Harmony (1980). Other
values (in the full table) are estimates of the uncertainties for unresolved blended
lines. The combined rms of the A- and E-species transitions is 0.090 MHz.
by those of a normal symmetric top and diagonalized in the
second step:
jJ ; Ka ; Kc ; vt ; i ¼
X
J ;Ka ;Kc vt ;
CK;v
jK; vt0 ; ijJ ; Ki:
0
K¼J ; J
vt0 ¼ 0;Nt 1
t
J; K ;v ;
ð5Þ
The resulting coefficients CK;vt0 a t are the eigenvectors of the
torsion-rotation states, where Nt is the number of torsional states
retained in the solution. The result is an Nt (2J þ 1) by Nt (2J þ 1)
matrix for each J- combination with Nt (2J þ 1) eigenvalues that
no longer have easily assignable Ka and vt quanta, owing to the
mixing of the pure states. However, it is desirable to comprehensively label the energy levels in this basis while maintaining physically meaningful connections within Ka -ladders.
1791
TABLE 2
Torsion-Rotation Parameters for the Global Fit
of the A- and E-States of Hydroxyacetone
Operator
Parameter
Valuea
1
2(1 cos 3).................................................
P2 ..................................................................
P Pa ..............................................................
Pa2 ..................................................................
Pb2 ..................................................................
Pc2 ..................................................................
Pa Pb þ Pb Pa .................................................
(1 cos 3)P2 ..............................................
(1 cos 3)Pa2 .............................................
(1 cos 3)(Pb2 Pa2 ) .................................
(1 cos 3)(Pa Pb þ Pb Pa ) ..........................
P2 Pa2 .............................................................
2P2 (Pb2 Pa2 ) ...............................................
P2 (Pa Pb þ Pb Pa ) ..........................................
P (Pa2 Pb þ Pb Pa2 ) .........................................
P 4 ...............................................................
P 2 Pb2 ..........................................................
Pa4 ...............................................................
2P2(Pb2 Pa2 ).............................................
{Pa2 , Pb2 Pa2 } ..........................................
Pa3 Pb þ Pb Pa3 .................................................
V3
F
Ab
Bb
Cb
Dab
FV
k5
c2
dab
k2
c1
ab
ab
DJ
DJK
DK
J
K
DabK
65.3560(22)
159118.2(4.0)
0.0587793(26)
9879.136(50)
4012.9939(95)
2866.6157(100)
1089.287(44)
1.772(12)
26.585(85)
1.746(14)
12.696(70)
0.981(20)
0.03230(71)
0.4750(85)
0.08446(93)
0.0008399(17)
0.008984(95)
0.02958(19)
0.00024274(85)
0.002599(36)
0.01841(15)
a
All values are in megahertz except for , which is unitless, and V3 , which
is in wavenumbers.
b
Values obtained before diagonalization of the rotational tensor. The
diagonalized values are A = 10074.875(51), B = 3817.2550(90), and C =
2866.6157(100).
Ordering the eigenvalues for hydroxyacetone was accomplished by closely following the methods outlined by Mekhtiev &
Hougen (1998) and Ilyushin (2004). After the first diagonalization
step, eigenvector composition was used to rearrange the columns
and corresponding rows of the diagonalized torsional matrix to
satisfy one of the three overlap integrals in equations (6)Y(8) (i.e.,
the eigenvectors with the largest overlap for adjacent values of K
were assigned to the same torsional state):
hvt0 ; K 1jvT ; Ki 0:7;
hvt0 ; K 2jvT ; Ki 0:7;
ð6Þ
ð7Þ
hvt0 ; K 1jvT ; K þ 1i 0:7:
ð8Þ
The term vT represents the torsional levels of the diagonalized
matrix, which, through this method, were transferred from the
original basis, vt (the prime symbol indicates that all combinations of levels were considered). Some additional optimization
was necessary to obtain the best results, similar to the methods
discussed by Mekhtiev & Hougen (1998).
The mixing for low-barrier internal rotors after the second
diagonalization step is too severe to label the rotational-torsional
eigenvalues solely by an analogous method. To analyze the rotational spectrum of acetamide, Ilyushin (2004) developed a general method that incorporates a ‘‘similarity factor’’ to physically
connect eigenvectors of adjacent J-values when assignment by
dominant eigenvector composition fails. Ilyushin noted that dominant composition was sufficient to uniquely label Ka at low J (see
Fig. 2 of Ilyushin 2004), and that eigenvector composition varied
slowly with increasing J. Therefore, even in cases where no individual K-coefficient accounts for more than 10% of the total magnitude, the truncated overlap integral (the similarity factor) between
the corresponding Ka level of adjacent J remained large, often
1792
APPONI ET AL.
Vol. 652
TABLE 3
Upper Limits for Hydroxyacetone A- and E-Transitions toward Sagittarius B2(N)
log I a
Frequency
( MHz)
(1)
220 K
(2)
50 K
(3)
Elower
(cm1)
(4)
100017d .....................
100081d .....................
105746d .....................
105808d .....................
111475d .....................
111536d .....................
134389.404................
134447.705................
140117.448................
140175.150................
145845.227................
145902.416................
151572.703................
151629.491................
157299.838................
157356.364................
163026.592................
163083.022................
133856.278................
134080.462................
145304.654................
145529.265................
151029.030................
151253.953................
156753.425................
156978.716................
162477.775................
162703.491................
15.03
15.03
15.00
15.00
14.96
14.96
14.83
14.83
14.82
14.82
14.80
14.80
14.77
14.77
14.76
14.76
14.74
14.74
14.89
14.89
14.85
14.85
14.82
14.82
14.80
14.80
14.78
14.79
14.07
14.07
14.06
14.06
14.06
14.06
14.10
14.10
14.12
14.12
14.15
14.15
14.17
14.17
14.21
14.21
14.25
14.25
14.15
14.15
14.19
14.19
14.21
14.21
14.24
14.24
14.27
14.28
27.3
27.3
30.7
30.7
34.3
34.3
50.2
50.3
54.7
54.8
59.4
59.4
64.3
64.3
69.3
69.4
74.6
74.6
49.4
49.5
58.5
58.7
63.4
63.5
68.4
68.6
73.6
73.8
b,c
Transition
(JK00a ; Kc ! JK0 a ; Kc )
(5)
17,17 ! 16,16
17,17 ! 16,16
18,18 ! 17,17
18,18 ! 17,17
19,19 ! 18,18
19,19 ! 18,18
23,23 ! 22,22
23,23 ! 22,22
24,24 ! 23,23
24,24 ! 23,23
25,25 ! 24,24
25,25 ! 24,24
26,26 ! 25,25
26,26 ! 25,25
27,27 ! 26,26
27,27 ! 26,26
28,28 ! 27,27
28,28 ! 27,27
22,21 ! 21,20
22,21 ! 21,20
24,23 ! 23,22
24,23 ! 23,22
25,24 ! 24,23
25,24 ! 24,23
26,25 ! 25,24
26,25 ! 25,24
27,26 ! 26,25
27,26 ! 26,25
E
A
E
A
E
A
E
A
E
A
E
A
E
A
E
A
E
A
E
A
E
A
E
A
E
A
E
A
TR or 3 Upper Limit
(mK)
(6)
Comment
(7)
40
4000
40
20
<12
40
190
150
90
70
<15
30
110
65
30
40
<30
60
260
<24
220
<36
<12
60
<27
80
150
75
Shoulder of an unidentified line: possible match
Contaminated with HCCCN
Within 2 MHz of 40 mK blended feature
Near strong CH2NH, Trms = 6 mK
Trms = 4 mK
Broad line, possibly CH3CCCN
Contaminated with EtCN
Contaminated with EtCN
Shoulder of CH2CO line, coincident with CH3OCHO
Shoulder of 80 mK CCS/CH3OD lines
Trms = 5 mK
Shoulder of 300 mK HC3N
Contaminated by VyCN
Good match
Near absorption feature, possibly CH3OH
Shoulder of c-C2H4O
Trms = 10 mK
Shoulder of HCC13CN line: good match
Contaminated with CH3CHO
Trms = 8 mK
Contaminated with VyCN
Trms = 12 mK
Trms = 4 mK
Very close to C2H5OH and unidentified lines
Trms = 9 mK
Shoulder of 130 mK CCS line
Shoulder of 580 mK EtCN line
Good match
a
I is the normalized radiation temperature per unit column density and has units of K cm2 (see eq. [9]). To compute the total column density from a given
transition, divide the observed line temperature or upper limit by I(T ) for that transition.
b
Normal unsigned JKa ;Kc labels are used for E-species in this table for simplicity.
c
Asterisks signify a collapsed quartet of two b-type and two a-type transitions where Ka = Kc J and Kc J þ 1. The value of log I represents the sum of the
four coincident lines.
d
Average frequency of quartet resolved in laboratory spectrum, which would not be split in the Sgr B2( N) spectrum.
greater than 0.9. Labeling of the eigenvectors is accomplished by
first ordering the lowest one or two Ka levels in energy, then by
eigenvalue composition until the overlap integrals fall below 0.7
for a given K, and finally by ‘‘similarity factor’’ until all J-values
are exhausted.
Table 1 lists the lines assigned to the ground torsional state of hydroxyacetone; 740 A-species lines and 405 E-species lines are included. The microwave E-species lines were particularly difficult to
locate owing to large perturbations of the low-J lines by the internal
methyl rotor as described; therefore, fewer lines of this species
could be assigned. Quantum number labels consistent with the
RAM Hamiltonian were adopted (see Ilyushin et al. 2004), which
uses parity for the A-species and a signed K for the E-species.
Twenty-one parameters were required in order to accurately
reproduce the transition frequencies to within the measurement
uncertainty. Those constants and their mathematical dependence
are listed in Table 2 along with their 1 uncertainties. The global
rms was 90 kHz; individual rms values for the A- and E-species
were 81 and 105 kHz, respectively. The largest residuals were
found on lines that were only partially resolved or completely
blended with other nearby transitions, such as those exhibited in
Figure 2. Uncertainties on these lines were increased to as high
as 200 kHz. Four leading-order rotational and three leading-order
torsional constants were used in the fit. Only 14 fourth-order dis-
tortion constants were needed to reproduce the data even though
transitions as high as J = 30 and Ka = 14 were included in the
analysis. Nonetheless, the fit will likely improve once we fully
assign the vt > 1 states, as was shown for acetamide. However, a
sufficient number of lines have been unambiguously assigned in
the laboratory spectrum to facilitate an astronomical search. The
spectral analysis of the excited states of hydroxyacetone will be
presented in a forthcoming paper.
Our diagonalized A, B, and C rotational constants, as well as
the barrier to internal rotation, are in good agreement with those
determined by Kattija-Ari & Harmony (1980). These authors measured 41 lines in the A-species and 10 transitions in the E-species.
The rms of their analysis was 20 kHz and 9 MHz for the A- and
E-species, respectively.
4. ASTRONOMICAL OBSERVATIONS
Sgr B2(N ) is a dense core that exhibits chemical complexity
found in no other molecular cloud. For example, species such as
glycolaldehyde, ethanol, and ethylene glycol are found only in Sgr
B2(N). Since these compounds closely resemble hydroxyacetone,
it is the most likely source to search for this and other complex
organic molecules.
As part of an ongoing effort to survey the millimeter-wave spectrum of Sgr B2(N) down to the confusion limit, a search for the
No. 2, 2006
STUDY OF AND SEARCH FOR HYDROXYACETONE
1793
Fig. 4.—Spectra measured at the ARO 12 m telescope at 2 and 3 mm toward Sgr B2(N). Spectra are labeled (a) through (n) in order of increasing frequency; an
LSR velocity of VLSR = 62 km s1 was assumed. The temperature scale is in terms of TR (K), and the theoretical rms noise is indicated for each spectrum. Transitions
of hydroxyacetone searched for in this work are indicated in each panel. Other molecules are identified in the spectra, and obvious unidentified features are labeled
‘‘U.’’
lowest-lying rotational transitions of hydroxyacetone was conducted in this source, using our newly measured frequencies.
The observations were conducted at the Arizona Radio Observatory (ARO) 12 m telescope3 on Kitt Peak during the period
2002 October to 2006 April. The receivers used were dual-channel,
cooled SIS mixers, operated in single-sideband mode with at least
16 dB image rejection. The back ends used were (1) 256-channel
filter banks with 500 kHz and 1 MHz resolution and (2) a millimeter
autocorrelator in 781 kHz resolution mode. All spectrometers were
operated in parallel mode to accommodate both receiver channels.
The temperature scale, TR , was determined by the chopper-wheel
method and corrected for forward spillover losses. Conversion to
radiation temperature TR is then TR = TR /c , where c is the corrected beam efficiency. Over the frequency range of 100 to
162 GHz, the beam size of the ARO 12 m ranged from 6300 to
3900 and the beam efficiency from 0.9 to 0.7. All observations were
conducted in position-switching mode toward Sgr B2(N) ( =
17h44m10.s1, = 28 2101700 ; B1950) with the OFF position 300
west in azimuth. A 10 MHz local oscillator shift was performed
for all measurements to monitor any image contamination.
3
The 12 m telescope is operated by the Arizona Radio Observatory, Steward
Observatory, University of Arizona, with partial support from the Research
Corporation.
As shown in Table 3, measurements were conducted for 28
collapsed, or nearly collapsed, spectral quartets of hydroxyacetone, consisting of two a-type and two b-type transitions (Ka = 0
and 1 for one set, and Ka = 1 and 2 for the other). These transitions were selected because the added intensities of the collapsed
quartet make them far stronger than other favorable individual
lines. Hence, this set of transitions enables the greatest possibility of an interstellar detection. The logarithm of the theoretical
radiation temperature per unit column density for the observed
transitions is listed in Table 3 for Trot = 220 K (col. [2]) and
Trot = 50 K (col. [3]). The normalized intensity per unit column
density as a function of temperature was computed by rearrangement of the usual column density expression assuming h TkT
and Tbg T Tex :
00
TR
8
3 Sx2 eEg =kT
I(T ) ¼
¼
:
Ntot
3kV1=2 Qrot
ð9Þ
Here TR is the radiation temperature of the transition, V1/2 is
the line width in units of velocity, is the transition frequency, S
is the line strength, 2x is the dipole moment along the transition
axis, Eg00 is the energy above ground for the lower state, Ntot is
the total column density, and Qrot is the rotational partition function at temperature T. The partition function Qrot equals 21,129
1794
APPONI ET AL.
Vol. 652
Fig. 4.—Continued
and 5282, respectively, at 220 and 50 K. This temperature range
is characteristic of this source, as indicated by other studies (e.g.,
Nummelin et al. 1998, 2000). Noise levels (3 ) of less than 20 mK
were achieved for most of the frequencies covered, although they
range between 8 and 36 mK. Each of the listed values represents
the total intensity of the four collapsed transitions. None of the observed quartets would be significantly resolved in Sgr B2( N ),
which is assumed to have an intrinsic line width of 8 km s1
(see Table 1 for the exact transition frequencies). Also listed in
the Table 3 are the observed antenna temperatures (col. [6]) or
their equivalent 3 noise levels.
5. ASTRONOMICAL RESULTS AND DISCUSSION
Spectra from the hydroxyacetone search are presented in Figure 4. Of the 28 transitions studied, 19 are coincident (or nearly
so) with observed spectral features. Of these features, 15 could
be identified as transitions of known interstellar molecules. This
result is completely expected because of the high spectral density of Sgr B2( N ). In fact, the spectrum is so crowded that there
are few clean regions of only baseline. Consider the spectrum in
Figure 4j, which covers 151520 to 151680 MHz in frequency.
There are 16 distinct peaks and shoulders in this spectrum with
only 10 regions of apparent baseline. Moreover, only about five
of these baseline sections are wide enough to contain a spectral
feature. Hence, there is about a 3-to-1 chance that any random frequency will be nearly coincident with an emission feature. Based
on these statistics, about 18 of the 28 hydroxyacetone transitions should have frequencies that accidentally coincide with
observed features, or as shoulders on such lines, even if this molecule has no significant abundance.
Spectral lines are observed at four transitions of hydroxyacetone
that appear to be uncontaminated, namely, those at 100017,
151629, 162703, and 163083 MHz. This result might suggest
that hydroxyacetone is present in Sgr B2( N ). The more pertinent issue, however, is the number of missing transitions. One
example is the J = 22Y21 quartet near 134 GHz (Fig. 4d ), which
lies 49 cm1 above ground state. Given the physical conditions
in Sgr B2( N ), both A- and E-transitions should be present with
intensities that are virtually equivalent. While there is a possible
feature for the E-species, lying on the shoulder of a CH3CHO
line, the A-species is clearly missing. A similar situation occurs
for the J = 28Y27 transition ( Fig. 4n). Here the A-species line is
coincident with a weaker, shoulder-like feature; no equivalent
line is present for the E-species. The remaining transitions were
simply not observed at the 3 noise level. Nine transitions are
missing in total. At 50 K, the intensities of all 28 hydroxyacetone
transitions are within 50% of each other; at 225 K, they all lie
within a factor of 2 of one another. Because the energy manifold
of hydroxyacetone is very dense (levels separated by only 3 to 5 K
over the region searched), non-LTE conditions that would selectively target the four ‘‘clean’’ transitions observed are difficult to
create. It is, therefore, extremely doubtful that these observations
No. 2, 2006
STUDY OF AND SEARCH FOR HYDROXYACETONE
1795
Fig. 4.— Continued
indicate the presence of hydroxyacetone in Sgr B2(N). The upper
limit to the column density of hydroxyacetone based on the missing features is 5 ; 1012 cm2, assuming local thermodynamic
equilibrium. The abundances roughly follow a monotonic decrease
from formaldehyde (Ntot = 1.6 ; 1015 cm2), to glycolaldehyde
(Ntot = 6 ; 1013 cm2) to hydroxyacetone (5 ; 1012 cm2)
(Apponi et al. 2006; Halfen et al. 2006). The limit on hydroxyacetone is about the same as that for dihydroxyacetone.
The conclusions drawn from this study directly contrast those
of Fuchs et al. (2005) in their investigation of ethyl methyl ether
( EME) in W51 e2. These authors based their identification of
EME on seven clean transitions. Even though these authors remark that ‘‘it is not astonishing that 77% of the EME lines in the
observed bands are blended by strong lines of other known molecules,’’ they still disregard the seven favorable transitions of
EME where no significant emission was detected. For these seven
transitions, the authors were ‘‘expecting slightly higher intensi-
ties,’’ and gave very large uncertainties to the brightness temperatures associated with what was termed ‘‘noisy-like’’ data (presumably these were upper limits). With a 77% chance of a false match
owing to contamination, more weight should have been placed on
the seven missing transitions. Based on the criteria of Fuchs et al.
(2005), we could claim a detection of hydroxyacetone. Such an
identification, however, would require a biased selection of only
part of the complete data set.
This material is based on work supported by the National Aeronautics and Space Administration through the NASA Astrobiology
Institute under cooperative agreement CAN-02-OSS-02 issued
through the Office of Space Science. D. T. H. is supported by an
NSF Astronomy and Astrophysics Postdoctoral Fellowship under
award AST 06-02282.
REFERENCES
Apponi, A. J., Halfen, D. T., Ziurys, L. M., Hollis, J. M., Remijan, A. J., &
Hougen, J. T., Kleiner, I., & Godefroid, M. 1994, J. Mol. Spectrosc., 163, 559
Lovas, F. J. 2006, ApJ, 643, L29
Ilyushin, V. 2004, J. Mol. Spectrosc., 227, 140
Balle, T. J., & Flygare, W. 1981, Rev. Sci. Instrum., 52, 33
Ilyushin, V., Alekseev, E. A., Dyubko, S. F., Kleiner, I., & Hougen, J. T. 2004,
Braakman, R., Drouin, B. J., Widicus, S. L., & Blake, G. A. 2005, in 60th Ohio
J. Mol. Spectrosc., 227, 115
State Univ. Int. Symp. on Molecular Spectroscopy (Abstract Book) (Columbus:
Kattija-Ari, M., & Harmony, M. D. 1980, Int. J. Quantum Chem. Symp., No.
Ohio State Univ.), No. RA07
14, 443
Crovisier, J., Bockelée-Morvan, D., Biver, N., Colom, P., Despois, D., & Lis,
Kirtman, B. 1962, J. Chem. Phys., 37, 2516
D. C. 2004, A&A, 418, L35
Lees, R. M., & Baker, J. G. 1968, J. Chem. Phys., 48, 5299
Fuchs, G. W., Fuchs, U., Giesen, T. F., & Wyrowski, F. 2005, A&A, 444, 521
Mekhtiev, M. A., & Hougen, J. T. 1998, J. Mol. Spectrosc., 187, 49
Halfen, D. T., Apponi, A. J., Woolf, N., Polt, R., & Ziurys, L. M. 2006, ApJ,
Nummelin, A., Bergman, P., Hjalmarson, 8., Friberg, P., Irvine, W. M., Millar,
639, 237
T. J., Ohishi, M., & Saito, S. 1998, ApJS, 117, 427
Hartwig, H., & Dreizler, H. 1996, Z. Naturforsch. A, 51, 923
———. 2000, ApJS, 128, 213 (erratum 132, 127 [2001])
Herbst, E., Messer, J. K., De Lucia, F. C., & Helminger, P. 1984, J. Mol.
Solomons, T. W. G. 1984, Organic Chemistry (3rd ed.; New York: Wiley)
Spectrosc., 108, 42
Ziurys, L. M., Barclay, W. L., Jr., Anderson, M. A., Fletcher, D. A., & Lamb, J. W.
Hollis, J. M., Lovas, F. J., & Jewell, P. R. 2000, ApJ, 540, L107
1994, Rev. Sci. Instrum., 65, 1517
Hollis, J. M., Lovas, F. J., Jewell, P. R., & Coudert, L. H. 2002, ApJ, 571, L59