The Astrophysical Journal, 643:L37–L40, 2006 May 20
䉷 2006. The American Astronomical Society. All rights reserved. Printed in U.S.A.
METHYLTRIACETYLENE (CH3C6H) TOWARD TMC-1: THE LARGEST DETECTED SYMMETRIC TOP
Anthony J. Remijan,1 J. M. Hollis,2 L. E. Snyder,3 P. R. Jewell,1 and F. J. Lovas4
Received 2006 March 17; accepted 2006 April 6; published 2006 April 28
ABSTRACT
We report the detection of a new interstellar methylpolyyne, CH3C6H (methyltriacetylene), with the 100 m
Green Bank Telescope. Ten spectral lines of this species were detected toward the Taurus molecular cloud
(TMC-1): the K p 0 and K p 1 components of the 12,K–11,K, 13,K–12,K, 14,K–13,K, 15,K–14,K, and 16,K–
15,K transitions. Also observed were the K p 0 and K p 1 components of the 6,K–5,K transition of CH3C4H
(methyldiacetylene). For both methylpolyynes, no higher energy K-components were detected, which is consistent
with the 10 K kinetic temperature of the TMC-1 dark cloud. Moreover, radio spectral line data of the cyanopolyyne,
methylcyanopolyyne, and methylpolyyne carbon-chain sequences were studied, and strong correlations are found
among the values of the three different carbon-chain slopes when total column densities of sequence members
are plotted against the number of carbon atoms in the carbon chain. This result suggests that the formation
chemistry for all these carbon-chain sequences is common, and the total column density of the next larger,
undetected species in each of the three carbon chain sequences is predicted.
Subject headings: ISM: abundances — ISM: clouds — ISM: individual (TMC-1) — ISM: molecules —
radio lines: ISM
amine the slopes of the total column densities plotted against
the number of atoms in the carbon chain among the methylpolyyne, cyanopolyyne, and methylcyanopolyyne sequences,
and make specific predictions on detecting larger carbon chain
molecules toward TMC-1.
1. INTRODUCTION
The Taurus molecular cloud (TMC-1) is a prototypical dark
cloud that has been a primary target of several molecular line
surveys (e.g., Kaifu et al. 2004; Kalenskii et al. 2004; references
therein). Such surveys document that TMC-1 contains a number of different carbon-chain sequences, including the cyanopolyynes (HC2n⫺1N, n p 1, 2, 3, 4, 5, 6), which have been
extensively investigated by Bell et al. (1997); the methylcyanopolyynes (CH3C2n⫺1N, n p 1, 2, 3); and the methylpolyynes
(CH3C2nH, n p 1, 2). In the methylcyanopolyyne sequence,
CH3CN, CH3C3N, and CH3C5N were detected by Matthews &
Sears (1983), Broten et al. (1984), and Snyder et al. (2006),
respectively. In the methylpolyyne sequence, CH3C2H and
CH3C4H were detected by Irvine et al. (1981) and Walmsley
et al. (1984), MacLeod et al. (1984), and Loren et al. (1984),
respectively. Since 1984, no larger methylpolyynes have been
detected.
Snyder et al. (1984) reported the first attempt to detect interstellar CH3C6H toward TMC-1 by means of observations of
the K p 0 component of the 12,K–11,K and 16,K–15,K transitions. These observations were conducted with the Effelsberg
100 m telescope of the Max-Planck-Institut für Radioastronomie. No spectral features were detected, and a total column
density upper limit of 8.8 #1012 cm⫺2 for an excitation temperature of 10 K was reported (Snyder et al. 1984). Until the
present work, no additional observations have been reported
regarding attempts to detect interstellar CH3C6H.
The identification of interstellar CH3C6H provides essential
data regarding the formation chemistry for the methylpolyyne
sequence in cold, dark clouds, including the possibility that
simple carbon-addition reactions are taking place on the surfaces of dust grains (Snyder et al. 2006). In this Letter, we
report the detection of interstellar CH3C6H toward TMC-1, ex-
2. OBSERVATIONS AND RESULTS
Observations of CH3C4H and CH3C6H were made between
2004 March 15 and April 5, 2005 November 10 and 12, and
2006 March 1 and 5 with the NRAO5 100 m Robert C. Byrd
Green Bank Telescope (GBT). Table 1 lists the rotational transitions sought. The transition quantum numbers, calculated rest
frequencies, half-power beamwidths (vB), beam efficiencies
(hB), transition line strengths S(J, K), and lower level energies
(El) are listed in the first six columns. The K-band receiver is
divided into two frequency ranges with separate feed and amplifier sets covering 18–22.5 and 22–26.5 GHz. The GBT spectrometer was configured in its eight intermediate-frequency
(IF), 50 MHz, nine-level mode, which provides observations
of four 50 MHz frequency bands at a time in two polarizations
through the use of offset oscillators in the IF. This mode
affords 6.1 kHz channel separation. Antenna temperatures are
on the TA∗ scale (Ulich & Haas 1976) with estimated 20%
uncertainties. The TMC-1 J2000 pointing position employed
s
was a p 04h41m 42.89,
d p ⫹25⬚41⬘27⬙, and an LSR source
velocity of ⫹5.8 km s⫺1 was assumed. Data were taken in the
OFF-ON position-switching mode, with the OFF position 60⬘
east in azimuth with respect to the ON-source position. A single
scan consisted of 2 minutes in the OFF-source position followed by 2 minutes in the ON-source position. Pointing and
focusing corrections were made utilizing the calibrator
J0431⫹206. The two polarization outputs from the spectrometer were averaged to improve the signal-to-noise ratio (S/N).
Table 1 also summarizes the CH3C4H and CH3C6H detection
results toward TMC-1 in terms of Gaussian fitting intensities
and widths, which appear in columns (7) and (8), respectively.
1
National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903-2475.
2
Computational and Information Sciences and Technology Office, Code
606, NASA Goddard Space Flight Center, Greenbelt, MD 20771.
3
Department of Astronomy, University of Illinois, 1002 West Green Street,
Urbana, IL 61801.
4
Optical Technology Division, National Institute of Standards and Technology, 100 Bureau Drive, Gaithersburg, MD 20899-8440.
5
The National Radio Astronomy Observatory is a facility of the National
Science Foundation, operated under cooperative agreement by Associated Universities, Inc.
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REMIJAN ET AL.
Vol. 643
TABLE 1
Summary of Observations toward TMC-1
Frequencya
(MHz)
(2)
Transition
(J ,K–J ,K)
(1)
vB
(arcsec)
(3)
hB
(4)
S(J, K)
(5)
El
(cm⫺1)
(6)
DTA∗ b
(mK)
(7)
DV b
(km s⫺1)
(8)
7.28
2.04
81.0(27)
87.5(27)
0.47(1)
0.48(1)
8.67
3.43
9.30
4.05
9.97
4.73
10.70
5.45
11.48
6.23
8.0(16)
8.2(12)
8.0(11)
12.2(13)
6.3(16)e
6.4(17)e
8.0(12)
11.7(16)
10.8(15)
11.6(16)
0.37(5)
0.61(6)
0.58(5)
0.43(3)
0.49(8)
0.41(8)
0.58(6)
0.37(3)
0.56(5)
0.47(5)
Methyldiacetylenec (CH3C4H)
a
6,1–5,1 . . . . . . . . .
6,0–5,0 . . . . . . . . .
24428.652(20)
24428.886(20)
30
30
0.73
0.73
5.83
6.00
Methyltriacetylened (CH3C6H)
12,1–11,1
12,0–11,0
13,1–12,1
13,0–12,0
14,1–13,1
14,0–13,0
15,1–14,1
15,0–14,0
16,1–15,1
16,0–15,0
......
......
......
......
......
......
......
......
......
......
18677.699(13)
18677.805(13)
20234.161(13)
20234.277(13)
21790.621(13)
21790.746(13)
23347.078(13)
23347.212(13)
24903.532(13)
24903.674(13)
40
40
37
37
34
34
32
32
30
30
0.80
0.80
0.78
0.78
0.77
0.77
0.75
0.75
0.73
0.73
11.92
12.00
12.92
13.00
13.93
14.00
14.93
15.00
15.94
16.00
a
Uncertainties in parentheses refer to the least significant digit and are 2 j values (Taylor & Kuyatt 1994).
Gaussian fit values with 1 j uncertainties.
c
Rest frequencies from MacLeod et al. (1984); ma p 1.2071(10) D (Bester et al. 1984).
d
Rest frequencies from Alexander et al. (1978); ma ∼ 1.5(1) D (Bester et al. 1984).
e
Intensity and line widths measured from Hanning-smoothed data over three spectral channels.
b
The spectrum for the CH3C4H transitions in Table 1 is shown
in Figure 1, and similarly, the spectra for CH3C6H transitions
are shown in Figure 2. For all transitions observed, only K p
0 and K p 1 components were detected, since the K ≥ 2 components require ≥30 K of excitation energy and the kinetic
temperature of TMC-1 is ≤10 K.
In order to estimate the total column density of the symmetric
top CH3C6H at low excitation temperatures, we used the formalism outlined in Snyder et al. (2006). In that work, the
column density in a given K-ladder is given by
NK ≈
(Nu )JK (kTex /hB)e hB[J(J⫹1)⫺K(K⫹1)]/kTex
,
2J ⫹ 1
(1)
where
(Nu )JK p gu
冑p/(4 ln 2)DTA∗DV/hB
e h(n)JK /kTex ⫺ 1
1
⫺
(8p 3/3k)(n)JK Sm2
e h(n)JK /kTbg ⫺ 1
[
⫺1
]
(2)
and where cgs units are employed throughout (cf. Table 1 parameters); Tbg p 2.7 K, and all parameters can be obtained directly
or derived from the parameters in Table 1 [e.g., DTA∗, DV, vB ,
hB , El , n, S(J, K)]. The rotational constant B is 2035.747 MHz
for CH3C4H and 778.25 MHz for CH3C6H, and the electric dipole moment (ma) of CH3C4H was measured as 1.2071(10) D,
while for CH3C6H ma is estimated to be 1.5 D (Bester et al.
1984).
For each observed methylpolyyne transition, the population
column densities in the upper energy levels (Nu)JK are obtained
from equation (2) and NK ladder column densities from equation (1). Since DK ( 0 transitions are radiatively forbidden,
each K-ladder is treated as a separate linear molecule; thus, for
each transition, the sum of NK column densities from the two
observed K-components represent the total column density (NT)
of the molecule, since no higher K-components are observed.
The energy levels of the CH3C6H transitions in each K-ladder
are not well spaced (see Table 1, col. [6]), and therefore, the
rotational diagram method was not used to obtain an excitation
temperature. Since the observed K-components are weak for
the five transitions of CH3C6H, observed integrated line intensities for a K-ladder were formed from Table 1 columns (7)
and (8) and compared with the integrated line intensities calculated for the optically thin case with the assumption that the
line width is the same for all K-components. This method shows
that an excitation temperature of ∼6 K minimizes the rms difference between the observed and calculated integrated intensities. A similar 6 K result was obtained for the methylpolyyne
CH3C2H from the rotational diagram method (Askne et al.
1984). Thus, Tex p 6 K is employed herein to determine column densities for methylpolyynes. Table 2 contains the results
for each transition of CH3C4H and CH3C6H observed (col. [1]);
the K p 1 and K p 0 ladder column densities appear in columns (2) and (3), respectively, and column (4) contains the
total column density.
3. DISCUSSION
Fig. 1.—Methyldiacetylene (CH3C4H) spectra toward TMC-1 at 6.1 kHz
channel spacing. The rest-frequency axis reflects an assumed source velocity
of ⫹5.8 km s⫺1. Transition quantum numbers are labeled.
TMC-1 is the prototypical source for investigating the formation of carbon-chain species because of its rich inventory
of cyanopolyynes, methylpolyynes, and methylcyanaopolyynes. Furthermore, there appears to be a general decrease in
No. 1, 2006
METHYLTRIACETYLENE TOWARD TMC-1
L39
TABLE 2
K -Ladder and Total Column Densities
Transition
(J ,K–J ,K)
(1)
N1
(1012 cm⫺2)
(2)
N0
(1012 cm⫺2)
(3)
NT
(1012 cm⫺2)
(4)
CH3C4H (Tex p 6 K)
6,K–5,K . . . . . . . . .
8.72(48)
9.68(48)
18.40(68)
CH3C6H (Tex p 6 K)
12,K–11,K
13,K–12,K
14,K–13,K
15,K–14,K
16,K–15,K
......
......
......
......
......
0.93(34)
1.49(52)
1.02(45)
1.65(44)
2.36(56)
1.58(41)
1.70(31)
0.88(45)
1.55(36)
2.14(56)
2.52(53)
3.18(60)
1.90(64)
3.20(57)
4.50(79)
core (Bell et al. 1998), thereby filling the telescope beams
employed.
In order to predict the total column densities (NT) of larger,
undetected cyanopolyynes toward TMC-1, we used the extensive data set collected with the Nobeyama 45 m radio telescope
by Kaifu et al. (2004), who observed the known cyanopolyynes
HC2n⫺1N for n p 2, 3, 4, and 5. We calculated NT for each
observed cyanopolyyne and used a excitation temperature of
8.5 K, which provided the best agreement between observed
and expected intensities for the optically thin case for all observed cyanopolyynes. Figure 3 shows the logarithm (base 10)
of each observed cyanopolyyne NT as a function of the number
of carbon-chain atoms in the cyanopolyyne sequence. The cyanopolyyne sequence shown in Figure 3 is highly linear, suggesting that extrapolation to larger, undetected species is justified. Based on the linearity of the Figure 3 cyanopolyyne data,
we predicted the NT for the known interstellar cyanopolyyne
HC11N, obtaining ∼8.5 #1011 cm⫺2, which agrees to within a
factor of 3 with the NT determination of Bell et al. (1997), who
originally discovered this interstellar species. Proceeding
further, we predict that HC13N, which has not yet been detected
Fig. 2.—Methyltriacetylene (CH3C6H) spectra toward TMC-1 at 6.1 kHz
channel spacing. Because of the absorption and noise resulting from the
22 GHz water line in Earth’s atmosphere, the S/N of the 21.8 GHz data was
lower than the other transitions and was thus Hanning-smoothed to an effective
resolution of 12.2 kHz. Transition quantum numbers and the rest frequency
of the K p 0 component are shown in each panel. The abscissa is the radial
velocity with respect to the LSR calculated for the K p 0 rest frequency at
an assumed source velocity of ⫹5.8 km s⫺1 (dashed line).
the column densities of successively larger members in a given
carbon-chain sequence in TMC-1. The basic premise of this
work is that the relative abundance ratios among molecular
members of a carbon-chain sequence can be determined from
corresponding beam-averaged column density ratios under the
assumption that all molecules in the sequence are cospatial (i.e.,
occupy the same volume) and the TMC-1 molecular emission
is uniformly distributed in a spatially extended, high-density
Fig. 3.—Plot of the logarithm of the total column densities of the cyanopolyynes, methylpolyynes, and methylcyanopolyynes toward TMC-1. The ordinate is the logarithm (base 10) of the total column density (NT), and the
abscissa is p, the number of carbon atoms in the carbon chain. The column
densities of the cyanopolyynes are calculated at an excitation temperature of
8.5 K, the column densities of the methylpolyynes are calculated at an excitation temperature of 6 K, and the total column densities of the methylcyanopolyynes are calculated at an excitation temperature of 4 K. The filled symbols
are the values derived from observations, whereas the empty symbols are
extrapolations. Errors on predicted column densities were estimated under the
assumption that all three polyyne sequences have the same slope of ⫺0.26(3),
which results from a fit to the cyanopolyyne data.
L40
REMIJAN ET AL.
as an interstellar species, should have NT ∼ 2 #1011 cm⫺2. It is
illustrative to compute the time required for detection with the
GBT. Assuming a TMC-1 line width of ∼0.5 km s⫺1, the predicted line intensity (DTA∗ ) at the GBT would be ∼3.2 mK for
the J p 40–39 transition at 8.6 GHz. This transition should be
detectable by the GBT at S/N 1 3 in ∼50 hr of observing time.
For predicting total column densities of larger, undetected
methylcyanopolyynes toward TMC-1, we use the already published data of the resolved hyperfine spectra of CH3C3N from
Lovas et al. (2006) and the spectra of CH3C5N from Snyder
et al. (2006). The excitation temperature for methylcyanopolyynes in TMC-1 has been determined rather consistently to
be 4 K (Broten et al. 1984; Snyder et al. 2006). As a consequence, the NT for CH3C3N is 1.8(2) #1012 cm⫺2 (Lovas et al.
[2006] data reanalyzed in Snyder et al. [2006]) and NT for
CH3C5N is 8.4(28) #1011 cm⫺2 (Snyder et al. 2006). Figure 3
shows log NT for each of the two observed methylcyanopolyynes as a function of the number of carbon atoms in the methylcyanopolyyne carbon chain. Note that the slope of these two
methylcyanopolyyne points in Figure 3 correlates well with the
better determined slope of the cyanopolyyne sequence, suggesting that extrapolation to larger, undetected species in the
methylcyanopolyyne sequence is reasonable. As a consequence, we predict that NT for CH3C7N, which has not yet been
detected as an interstellar species, is ∼3.9 #1011 cm⫺2. This
molecule would require more than 100 hr of observing time
to detect with the GBT at S/N 1 3, assuming the J p 28–27,
K p 0 transition at 20.9 GHz were sought.
For predicting total column densities of larger, undetected
methylpolyynes toward TMC-1, we use the data in Table 2 of
this work. The Table 2 column densities for CH3C4H and
CH3C6H were computed for an excitation temperature of 6 K
(see § 2). We average the NT’s for CH3C6H to obtain an ANT S
of 3.1(10) #1012 cm⫺2. Figure 3 also shows log NT for each of
the two observed methylpolyynes as a function of the number
of carbon atoms in the methylpolyyne carbon chain. Note that
in Figure 3 the somewhat steeper slope determined by these
two methylpolyyne points strongly correlates with the slopes
of the cyanopolyyne sequence and the two methylcyanopolyyne
points. Using the slope thus derived, we predict that the NT for
CH3C8H, which has not yet been detected as an interstellar
species, is ∼5.2 #1011 cm⫺2. This molecule would require more
Vol. 643
than 1000 hr of observing time to detect with the GBT at
S/N 1 3, assuming the J p 28–27, K p 0 transition at 21.1
GHz were sought. If this prediction is valid, then, clearly, this
molecule must wait for the next generation of technology.
The only hope of detecting either CH3C7N or CH3C8H is
that the prediction curves in Figure 3 eventually turn upward,
and McCarthy et al. (1998) have shown in laboratory experiments that longer carbon chain molecules are more abundant
than extrapolation from the abundances of shorter carbon chain
molecules suggest. The important result emanating from Figure 3 is that the slopes of the three carbon-chain sequences are
highly correlated, suggesting that the mechanism of forming
larger species in a sequence from smaller ones is probably the
same. In any case, these results provide severe constraints on
theoretical chemistry that purports to explain these three carbon-chain sequences, and since carbon-chain molecules are
candidate carriers of diffuse interstellar bands, continued observations are important.
In summary, we have identified a new interstellar methylpolyyne, CH3C6H, which is presently the largest symmetrictop molecule detected in space. We have detected 10 spectral
lines of this species toward TMC-1: K p 0 and K p 1 components of the 12,K–11,K, 13,K–12,K, 14,K–13,K, 15,K–14,K,
and 16,K–15,K transitions. No higher energy K-components
were detected, which is consistent with the 10 K kinetic temperature of the TMC-1 dark cloud. We also observed the K
p 0 and K p 1 components of the 6,K–5,K transition of
CH3C4H. We studied radio spectral line data of the cyanopolyyne, methylcyanopolyyne, and methylpolyyne carbon-chain sequences and found strong correlations among the values of the
three different carbon-chain slopes when total column densities
of sequence members were plotted against the number of carbon atoms in the carbon chain. We suggest that this has important implications for formation chemistry that may be common to all three carbon-chain sequences. The total column
density for the next larger, but as of yet undetected, species in
each of the three carbon chain sequences is predicted.
We thank an anonymous referee for useful comments and a
very favorable review of this work. L. E. S. acknowledges
support from the Laboratory of Astronomical Imaging at the
University of Illinois and NSF grant AST 02-28953.
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