The Astrophysical Journal Letters, 770:L5 (6pp), 2013 June 10
C 2013.
doi:10.1088/2041-8205/770/1/L5
The American Astronomical Society. All rights reserved. Printed in the U.S.A.
THE REMARKABLE MOLECULAR CONTENT OF THE RED SPIDER NEBULA (NGC 6537)
1
J. L. Edwards1 and L. M. Ziurys1,2,3
Department of Chemistry, The University of Arizona, P.O. Box 210041, Tucson, AZ 85721, USA; [email protected]
2 Department of Astronomy and Steward Observatory, Arizona Radio Observatory,
The University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721, USA
Received 2013 March 10; accepted 2013 May 2; published 2013 May 22
ABSTRACT
Millimeter and sub-millimeter molecular-line observations of planetary nebula (PN) NGC 6537 (Red Spider) have
been carried out using the Sub-Millimeter Telescope and the 12 m antenna of the Arizona Radio Observatory in the
frequency range 86–692 GHz. CN, HCN, HNC, CCH, CS, SO, H2 CO, HCO+ and N2 H+ , along with the J = 3 →
2 and 6 → 5 lines of CO and those of several isotopologues, were detected toward the Red Spider, estimated to be
∼1600 yr old. This extremely high excitation PN evidently fosters a rich molecular environment. The presence of
CS and SO suggest that sulfur may be sequestered in molecular form in such nebulae. A radiative transfer analysis
of the CO and CS spectra indicate a kinetic temperature of TK ∼ 60–80 K and gas densities of n(H2 ) ∼ 1–8 ×
105 cm−3 in NGC 6537. Column densities of the molecules in the nebula and their fractional abundances relative to
H2 ranged from Ntot ∼ 1016 cm−2 and f ∼ 10−4 for CO, to ∼7 × 1011 cm−2 and f ∼ 8 × 10−9 for the least abundant
species, N2 H+ . For SO and CS, Ntot ∼ 2 × 1012 cm−2 and 1013 cm−2 , respectively, with f ∼ 10−7 and 2 × 10−8 . It
was also found that HCN/HNC ≈ 2. A low 12 C/13 C ratio of ∼4 was measured, indicative of hot-bottom burning.
These results, coupled with past observations, suggest that molecular abundances in PNe are governed principally
by the physical and chemical properties of the individual object and its progenitor star, rather than nebular age.
Key words: astrochemistry – ISM: molecules – nuclear reactions, nucleosynthesis, abundances – planetary
nebulae: individual (NGC 6537) – radio lines: ISM
studies of this source in the optical and UV regimes (e.g.,
Rowlands et al. 1994; Cuesta et al. 1995), including observations
of He ii, O iii, N v, [K iv], and [Ar iv] (Aller et al. 1999; Pottasch
et al. 2000). Molecular hydrogen and CO have been observed
as well in this object (Kastner et al. 1996; Davis et al. 2003;
Huggins et al. 2005). The Red Spider is estimated to be ∼1600 yr
old (Matsuura et al. 2005), with a progenitor star near 6 M , and
a C/O ratio of 0.95, making NGC 6537 slightly oxygen-rich
(Pottasch et al. 2000).
In order to further understand the chemical content of PNe,
we have conducted new molecular-line observations of the Red
Spider Nebula. Surprisingly, this very hot object (T∗ ∼ 1.5–2.5 ×
105 K; Matsuura et al. 2005) contains an array of unexpected
chemical species, including H2 CO, CS, and SO. In this Letter we
present our results, their analysis, and discuss the implications
for the chemistry in PNe.
1. INTRODUCTION
Intermediate and low mass stars evolve off the asymptotic
giant branch (AGB), becoming proto-planetary and then planetary nebulae (PNe; e.g., Kwok 2000). During the AGB stage,
significant mass loss occurs, creating circumstellar envelopes
that foster a rich, gas-phase chemistry. Observations of AGB
envelopes have resulted in the detection of over 70 different
chemical species (e.g., Cernicharo et al. 2000; Ziurys et al. 2007;
He et al. 2008). A number of these molecules are also present in
the proto-PN stage (e.g., Pardo et al. 2007; Park et al. 2008), and
some in the PN phase, as well (e.g., Bachiller et al. 1997; Zhang
et al. 2008; Tenenbaum et al. 2009). Although the global chemical content of PNe has not been extensively studied, there have
been surveys of CO in these objects (e.g., Huggins et al. 1996,
2005). HCN, HNC, CN, and HCO+ have been identified in a
number of these sources, as well (Bachiller et al. 1997; Josselin
& Bachiller 2003). The most detailed molecular study of a PN
to date is that of Zhang et al. (2008), who conducted a sensitive
(1σ rms < 8 mK) spectral line survey of the very young, carbonrich source NGC 7027, a nebula roughly 700 yr old (Masson
1989; Zijlstra et al. 2008). This survey showed the presence
of CO, CN, CCH, HCN, HCO+ , HC3 N, N2 H+ , and possibly
C3 H2 , but a lack of CS and HNC. OH, CH, CH+ , and CO+ have
also been identified in this object (e.g., Liu et al. 1996). In the
Helix Nebula, the oldest PN known with an age of ∼12,000 yr
(Meaburn et al. 2008), CO, HCN, CN, and HCO+ , H2 CO, C3 H2 ,
and CCH have been detected (Bachiller et al. 1997; Tenenbaum
et al. 2009). Recent observations have additionally shown that
H2 CO and HCO+ are widespread in this object (Zack & Ziurys
2013).
NGC 6537, known as the “Red Spider” Nebula, is one of the
highest excitation PNe known, as demonstrated by the presence
of [Si vi] emission (Ashley & Hyland 1988). There are numerous
3
2. OBSERVATIONS
The measurements were conducted between 2010 January
and 2012 May using the telescopes of the Arizona Radio
Observatory (ARO). The 2 and 3 mm observations were made
using the ARO 12 m at Kitt Peak, AZ. The receiver at 2 mm
consisted of dual polarization, single-sideband SIS mixers with
image rejection of >16 dB. Measurements at 3 mm were made
using a receiver equipped with dual polarization ALMA-type
Band 3 sideband-separating (SBS) mixers with image rejection
18 dB, intrinsic in the mixer architecture. The backends used
were two sets of 512 channel filter banks with 1 MHz and
2 MHz resolutions, respectively, configured in parallel mode
(2 × 256 channels) to accommodate both receiver polarizations.
The temperature scale TR∗ was determined by the chopperwheel method, corrected for forward spillover losses, where
TR = TR∗ /ηc and ηc is the corrected beam efficiency.
Measurements at 1 mm, 0.8 mm, and 0.4 mm were made
using the ARO 10 m Sub-Millimeter Telescope (SMT) on
Author to whom any correspondence should be addressed.
1
The Astrophysical Journal Letters, 770:L5 (6pp), 2013 June 10
Edwards & Ziurys
Table 1
Line Parameters for Molecules Observed in NGC 6537a
Molecule
CO
13 CO
C18 O
CN
13 CN
HCN
H13 CN
HNC
HN13 C
CCH
CS
SO
H2 CO
HCO+
N2 H+
Transition
J=6→5
J=3→2
J=2→1
J=2→1
J=2→1
N=2→1
J = 3/2 → 3/2
F = 5/2 → 5/2
J = 3/2 → 1/2
F = 3/2 → 3/2
F = 5/2 → 3/2
1/2 → 1/2
F = 3/2 → 1/2
J = 5/2 → 3/2
F = 5/2 → 3/2
7/2 → 5/2
3/2 → 1/2
F = 3/2 → 3/2
5/2 → 5/2
N=2→1
F1 = 0 → 0
F2 = 2 → 1
F1 = 1 → 1
F2 = 2 → 1
F2 = 3 → 2
J=3→2
J=1→0
J=3→2
J=1→0
J=3→2
J=1→0
J=3→2
J=1→0
N=3→2
J = 7/2 → 5/2c
J = 5/2 → 3/2c
J=5→4
J=3→2
J=2→1
N J = 6 7 → 56
N J = 5 6 → 45
JKa,Kc = 31,2 → 21,1
JKa,Kc = 30,3 → 20,2
J=3→2
J=1→0
J=3→2
J=1→0
ηb or ηc
VLSR
(km s−1 )
TA∗ or TR∗
(K)
ΔV1/2
(km s−1 )
691473.1
345796.0
230538.0
220398.7
219560.4
0.60
0.66
0.76
0.76
0.76
10.1 ± 0.4
9.7 ± 0.8
8.4 ± 2.0
9.3 ± 1.2
9.3 ± 3.6
0.75 ± 0.10
0.96 ± 0.03
0.69 ± 0.01
0.452 ± 0.006
0.012 ± 0.006
12.9 ± 0.8
13.5 ± 2.2
13.4 ± 2.4
13.6 ± 2.4
8.6 ± 5.2
∼226300b
226359.8
0.76
0.76
∼10
10.1 ± 0.8
0.010 ± 0.006
0.021 ± 0.006
∼60b
12.1 ± 2.0
226632.2
226661.8c
226661.8c
226679.4
0.76
0.76
9.1 ± 1.4
10.8 ± 0.4
0.022 ± 0.010
0.069 ± 0.010
14.1 ± 3.0
15.0 ± 3.0
0.76
9.5 ± 0.8
0.033 ± 0.010
11.7 ± 2.2
226875.0c
226875.0c
226875.0c
226889.7c
226889.7c
0.76
10.0 ± 0.4
0.16 ± 0.01
12.5 ± 0.8
0.76
9.0 ± 0.8
0.045 ± 0.010
17.2 ± 5.8
217300.3d
0.76
∼10
0.018 ± 0.004
∼20d
217436.2d
217467.9e
265886.4
88631.6
259011.8
86339.9
271981.1
90663.6
261263.3
87090.9
0.76
0.76
0.76
0.89
0.76
0.91
0.76
0.89
0.76
0.91
∼11
11.1 ± 0.6
10.0 ± 0.4
9.8 ± 0.8
9.9 ± 0.6
10.1 ± 1.2
10.1 ± 0.4
9.1 ± 1.2
9.3 ± 0.6
10.5 ± 2.6
0.011 ± 0.004
0.022 ± 0.004
0.21 ± 0.02
0.046 ± 0.006
0.060 ± 0.008
0.022 ± 0.006
0.090 ± 0.008
0.027 ± 0.006
0.020 ± 0.006
0.008 ± 0.004
∼40d
14.1 ± 2.6
11.3 ± 1.2
15.4 ± 3.0
9.0 ± 0.8
12.6 ± 2.8
10.5 ± 0.4
10.7 ± 2.4
9.5 ± 1.4
13.3 ± 5.1
262005.4
262066.3
244935.6
146969.0
97981.0
261843.7
219949.4
225697.8
218222.2
267557.6
89188.5
279511.7
93173.4
0.76
0.76
0.76
0.75
0.80
0.76
0.76
0.76
0.76
0.76
0.89
0.76
0.88
9.5 ± 1.2
10.1 ± 1.0
10.1 ± 0.4
9.3 ± 0.6
9.9 ± 2.0
10.0 ± 1.2
10.8 ± 0.8
10.4 ± 0.8
10.0 ± 0.8
9.3 ± 0.8
8.8 ± 1.2
9.2 ± 1.6
8.4 ± 2.8
0.011 ± 0.004
0.008 ± 0.004
0.035 ± 0.004
0.044 ± 0.004
0.034 ± 0.010
0.009 ± 0.004
0.014 ± 0.006
0.017 ± 0.006
0.012 ± 0.006
0.128 ± 0.006
0.034 ± 0.010
0.022 ± 0.014
0.008 ± 0.004
11.0 ± 2.2
10.0 ± 3.4
10.1 ± 0.6
12.1 ± 1.8
12.3 ± 3.4
10.1 ± 2.4
7.0 ± 3.0
10.1 ± 1.6
9.6 ± 2.0
11.6 ± 0.4
10.7 ± 2.0
11.1 ± 3.2
17.1 ± 5.6
ν rest
(MHz)
Notes.
a Measured with 1 MHz resolution; uncertainties are ±2σ .
b Blend of F = 1/2 → 1/2, 1/2 → 3/2, 3/2 → 1/2, 3/2 → 3/2, 3/2 → 5/2, 5/2 → 3/2 hyperfine components.
c Blended line; average frequency of hyperfine components.
d Blend of F = 3 → 2, 2 → 1, 1 → 0 hyperfine components; average frequency.
e Blend of F = 4 → 3, 3 → 2, 2 → 1 hyperfine components; average frequency
Mount Graham, AZ. The dual polarization 1 mm receiver
employs ALMA-type Band 6 SBS SIS mixers. Image rejection was typically >18 dB. The 0.8 mm receiver consists
of dual polarization, double-sideband (DSB) SIS mixers. The
dual polarization 0.4 mm receiver consists of ALMA-type
Band 9 DSB mixers. The temperature scale at the SMT is
TA∗ , determined by the chopper wheel method, where TA =
TA∗ /ηb , and ηb is the beam efficiency. The backend used was a
2048 channel 1 MHz resolution filter bank configured in parallel
mode (2 × 1024 channels).
Observations were conducted toward α = 18h 05m 13.s 1, δ =
−19◦ 50 34. 9 (J2000.0) in position-switching mode with an
azimuth offset of 5 . Local oscillator shifts were done to test
for image contamination. Pointing and focus were determined
on planets or strong continuum sources. Observing frequencies
and beam efficiencies are listed in Table 1.
2
The Astrophysical Journal Letters, 770:L5 (6pp), 2013 June 10
Edwards & Ziurys
3. RESULTS AND ANALYSIS
0.18
A summary of the observations toward the Red Spider is given
in Table 1. As the table shows, CN, HCN, HNC, CCH, CS, SO,
H2 CO, HCO+ and N2 H+ were detected toward NGC 6537, as
well as 13 CO, C18 O, 13 CN, H13 CN and HN13 C. At least two
transitions per molecule were measured, typically the 1 → 0
and the 3 → 2 lines; for CCH and CN, the observed fine and/or
hyperfine structure confirmed the identification. The J = 3 →
2 and J = 6 → 5 lines of CO at 345 and 690 GHz were also
measured. All detected features appear near an LSR velocity of
∼10 km s−1 , as expected (Huggins et al. 2005), with ΔV1/2 ∼
10–14 km s−1 , neglecting hyperfine structure. Searches were
also conducted for 1 mm transitions of SiO, SiS, SiN, SiC2 ,
C3 H, C3 N, HC3 N, and c-C3 H2 , with upper (3σ ) limits of TA∗ 6 mK (TA∗ 12 mK for HC3 N).
The J = 3 → 2 and J = 1 → 0 transitions of HCN, HNC,
and their 13 C isotopologues, observed toward the Red Spider
Nebula, are presented in Figure 1. The J = 1 → 0 lines of HCN
and HNC show evidence of Galactic contamination, labeled by
“G,” and are slightly broader due to nitrogen hyperfine structure.
A blueshifted “shoulder” may be present in these line profiles,
as well, in particular HN13 C.
In Figure 2, spectra of CS and SO are shown. The J = 2 → 1
line of CS was also detected in this source (see Table 1). Again,
galactic contamination is present in the CS, J = 3 → 2 spectrum.
The J = 5 → 4 line of CS also appears to exhibit a blueshifted
velocity component, similar to HN13 C. The SO line profiles are
somewhat narrower than those of CS, but this difference can be
attributed to a higher noise level.
Figure 3 presents observations of N2 H+ , H2 CO, CCH, and CN
in the Red Spider. The J = 3 → 2 and J = 1 → 0 lines of N2 H+
are displayed, as well as two asymmetry components of the
J = 3 → 2 transition of formaldehyde. In the CCH spectrum,
the spin-rotation doublets of the N = 3 → 2 transitions are
visible, indicated by quantum number J; the CN data consists of
multiple, blended hyperfine components of the N = 2 → 1, J =
5/2 → 3/2 spin component. Additional hyperfine lines were
detected for CN and several for 13 CN (see Table 1).
The data were analyzed using the non-local thermal equilibrium radiative transfer program RADEX (van der Tak et al.
2007). In this code, gas kinetic temperatures TK , densities n(H2 ),
and molecular column densities Ntot are obtained by matching the model predictions to the observed spectral intensities.
The assumption of uniform temperature and density is justified,
based on analyses of large scale maps of the Helix (Zack &
Ziurys 2013), the Dumbbell (J. L. Edwards et al., in preparation), and the Ring Nebulae (Bachiller et al. 1989). For both
CO and CS, all three parameters could be independently determined from RADEX because three rotational transitions were
measured. The modeling was conducted over ranges of 5–260 K
and 104 –107 cm−3 . It was assumed that the source size was 30 ,
based on the optical image of the nebula (Cuesta et al. 1995).
Studies of the Helix have shown that molecular gas coincides
with the lower excitation optical lines (Zack & Ziurys 2013), so
this assumption is likely valid. Preliminary maps of the J = 3 →
2 transition of CO in NGC 6537 show that the molecular gas
appears to trace the bipolar morphology observed in the optical
image (J. L. Edwards & L. M. Ziurys, in preparation).
The results of the RADEX analysis are listed in Table 2. From
the CO data, modeling resulted in TK ≈ 62 K, n(H2 ) ≈ 2 ×
105 cm−3 , and Ntot ≈ 1.0 × 1016 cm−2 ; the CS analysis yielded
TK ≈ 76 K, n(H2 ) ≈ 105 cm−3 , and Ntot ≈ 9.9 × 1012 cm−2 .
HCN
J=3
2
J=1
0
J=3
2
J=1
0
J=3
2
J=1
0
J=3
2
J=1
0
0.00
G
0.05
0.00
0.06
H13 CN
T A* (K) or T R * (K)
0.00
0.02
0.00
0.08
HNC
0.00
G
0.024
0.000
0.02
HN13 C
0.00
0.01
0.00
-90
-40
10
60
110
V LSR (km/s)
Figure 1. Spectra of HCN, HNC, and their 13 C-isotopolgues observed toward
NGC 6537. The J = 1 → 0 and J = 3 → 2 transitions were measured with the
ARO 12m (spectral resolution 500 kHz or 1.7 km s−1 ; temperature scale TR∗ )
and the SMT (spectral resolution 1 MHz or 1.1 km s−1 ; temperature scale TA∗ ),
respectively. Galactic contamination is indicated by “G.”
For verification, a rotational diagram analysis was also carried
out for these two molecules. The diagrams yielded Ntot = 9.4 ×
1015 cm−2 and Trot = 50 K for CO and Ntot = 1.0 × 1013 cm−2
and Trot = 8 K for CS, in excellent agreement, noting that Trot <
TK for higher dipole moment molecules such as CS.
For the other molecules studied, only two transitions were
observed. Therefore, the kinetic temperature was held fixed at
TK = 62 K, as determined by CO, and the H2 density and
column density were varied. For H2 CO, an ortho:para ratio of 3
was assumed. The resulting densities fell in the range n(H2 ) ≈
1.7–7.7 × 105 cm−3 , in good agreement with those established
from CO (see Table 2). In cases where only one rotational
transition was observed (CCH and 13 CN), the column density
3
The Astrophysical Journal Letters, 770:L5 (6pp), 2013 June 10
Edwards & Ziurys
Table 2
Comparison of Molecular Abundances in Planetary Nebulae
Molecule
TK
(K)
n(H2 )
(cm−3 )
Ntot a
(cm−2 )
f(X)a,b
f(X) NGC 7027c
CO
62
62e
62e
62e
2.0 × 105
2.0 × 105e
2.0 × 105e
6.5 × 105
62e
62e
62e
62e
7.7 × 105
5.1 × 105
6.1 × 105
4.4 × 105
76
62e
62e
62e
62e
62e
62e
1.0 × 105
6.5 × 105
4.0 × 105
1.7 × 105
2.2 × 105
2.0 × 105e
2.0 × 105e
1.0 × 1016
7.4 × 1015
1.3 × 1014
1.6 × 1013
8.3 × 1012g
5.5 × 1012
2.3 × 1012
2.8 × 1012
8.0 × 1011
6.1 × 1012g
9.9 × 1012
1.9 × 1012
2.0 × 1012
2.3 × 1012
6.9 × 1011
<8 × 1011
<1 × 1013
1.1 × 10−4
8.3 × 10−5
1.5 × 10−6
1.8 × 10−7
9.3 × 10−8
6.2 × 10−8
2.6 × 10−8
3.1 × 10−8
9.0 × 10−9
6.9 × 10−8
1.1 × 10−7
2.1 × 10−8
2.2 × 10−8
2.6 × 10−8
7.8 × 10−9
<9.0 × 10−9
<1.1 × 10−7
1.1 × 10−4
7.4 × 10−6
3.8 × 10−7
7.0 × 10−8
...
4.5 × 10−8
3.9 × 10−9
<4.1 × 10−11
...
5.4 × 10−8
<5.5 × 10−11
...
<1.3 × 10−10
4.8 × 10−8
3.8 × 10−9
8.5 × 10−9
8.3 × 10−9
13 CO
C18 O
CN
13 CN
HCN
H13 CN
HNC
HN13 C
CCH
CS
SO
H2 CO
HCO+
N2 H+
HC3 N
c-C3 H2
f(X) NGC 7293
2.5 × 10−4d
2.7 × 10−5f
...
8.7 × 10−7f
...
1.3 × 10−7f
...
8.0 × 10−8f
...
1.9 × 10−6h
<8.0 × 10−9f
...
2.5 × 10−7d
5.3 × 10−8d
...
<1.9 × 10−8f
4 × 10−8h
Notes.
a Assuming a source size of ∼30 ; see the text.
b Assumes H column density of 8.9 × 1019 cm−2 ; see the text.
2
c From Zhang et al. (2008).
d From Zack & Ziurys (2013), average over the nebula.
e Fixed to value from CO fit.
f From Bachiller et al. (1997); scaled from HCO+ abundance of Zack & Ziurys (2013).
g Calculated assuming rotational temperature of 10 K.
h From Tenenbaum et al. (2009); one position in Helix.
was calculated analytically assuming a rotational temperature
of 10 K. The derived values are given in Table 2.
In order to estimate fractional abundances, an H2 column density of Ntot ≈ 8.9 × 1019 cm−3 was assumed. This value was calculated from the observed H2 surface brightness in the Red Spider (Davis et al. 2003), and employing the brightness–column
density relationship of O’Dell et al. (2005). The fractional abundances thus derived, relative to H2 , are also listed in Table 2.
5. DISCUSSION
5.1. Physical Conditions and Chemical Content
of the Red Spider Nebula
The molecular material in the Red Spider appears to be
present in warm, dense gas, with TK ∼ 60–75 K and n(H2 ) ∼
1–8 × 105 cm−3 . These physical conditions are comparable
to what has been observed in other PNe of comparable age.
Analysis of HIFI data for NGC 7027 suggests TK ∼ 25–100 K,
with densities of n(H2 ) ∼ 0.3–4 × 104 cm−3 for the middle and outer shell regions (Santander-Garcia et al. 2012).
Millimeter-wave molecular observations of NGC 7027 indicate n(H2 ) ∼ 1–5 × 105 cm−3 , with Tex ∼ 35–40 K (Zhang
et al. 2008; Hasegawa & Kwok 2001). In NCG 6302 (estimated
age ∼1900 yr; Meaburn et al. 2005), TK ∼ 40 K and n(H2 ) ∼
2 × 104 cm−3 , modeled for the “fast outflow” (Bujarrabal et al.
2012). Note that in the highly evolved Helix Nebula, TK ∼
16–40 K and n(H2 ) ∼ 105 cm−3 , based on studies of CO, HCO+ ,
and H2 CO (Zack & Ziurys 2013).
The molecular content in the Red Spider is surprisingly rich.
Apart from CO, HCN, CN, HNC, and HCO+ , which have been
observed in other PNe, including the Helix (e.g., Bachiller et al.
1997; Zack & Ziurys 2013), SO, N2 H+ , CCH, H2 CO, and CS
have been detected in this source. As shown in Table 2, the most
Figure 2. Spectra of the J = 5 → 4 and J = 3 → 2 transitions of CS (top two
panels) and the NJ = 67 → 56 and NJ = 56 → 45 transitions of SO (lower
two panels), observed toward NGC 6537. The J = 3 → 2 transition of CS was
measured with the ARO 12 m (temperature scale TR∗ ), and the other lines with
the ARO SMT (temperature scale TA∗ ). The spectral resolution is 1 MHz or 1.1–
1.4 km s−1 , except for the J = 3 → 2 transition of CS (resolution: 2.0 km s−1 ).
Galactic contamination is indicated by “G.”
4
The Astrophysical Journal Letters, 770:L5 (6pp), 2013 June 10
0.03
+
N2 H
Edwards & Ziurys
J=3
2
J=1
0
are NGC 7027 and the Helix, age ∼700 and ∼12,000 yr,
respectively. For comparison, molecular abundances determined
for these two objects are given also in Table 2. As the table
shows, the more common species CO, HCN, and HCO+ have
similar abundances to within a factor of two or three among
all three sources. CN and CCH are present in the 3 nebulae, as
well, but abundances vary by at least a factor of 10. These two
species are most prevalent in the Helix, and have comparable
abundances in the Red Spider and NGC 7027. N2 H+ has not yet
been searched for in the Helix, but has a similarly low abundance
(f ∼ 4–8 × 10−9 ) in the other two nebulae.
More striking differences occur for the remaining species.
In the Helix, H2 CO has an abundance of f ∼ 2.5 × 10−7 ,
but is an order of magnitude less prevalent in the Red Spider.
Formaldehyde has not been observed in NGC 7027 to limits
of <10−10 . A similar trend is found in HNC, which is most
prominent in the Helix, a factor of two less abundant in the
Spider, but notably absent from NGC 7027. The formaldehyde
result may arise because the Red Spider and the Helix are
slightly oxygen-rich. NGC 7027, on the other hand, is clearly
carbon-rich (Keyes et al. 1990; Bernard-Salas et al. 2001),
although it is surprisingly deficient in HNC. Finally, CS remains
undetected in both NGC 7027 and the Helix, with abundance
upper limits of 10–1000 less than that observed in the Red
Spider. SO is not expected to be present in C-rich NGC 7027,
and it has never been searched for in the Helix, so a meaningful
comparison cannot be made. The identification of SO in the Red
Spider is further evidence for an O-rich environment.
Both CS and HNC have substantial abundances in C-rich
circumstellar shells (Olofsson 2005). Bachiller et al. (1997)
propose that CS is destroyed by photo-dissociation in the
transition from proto-PN to PN, because the molecule is
apparently absent in NGC 7027, the Ring, NGC 6781, and the
Helix Nebula. They additionally suggest that the abundance
of HNC, relative to HCN, increases with nebular age due
to enhanced photo-ionization. Chemical modeling by Redman
et al. (2003) also indicates a decline in the HCN/HNC ratio with
age, except the abundances of both molecules are predicted
to drop to 10−12 at 10,500 yr. Ali et al. (2001) predict an
HCN/HNC ratio near 2 for evolved PNe.
The Red Spider observations, coupled with those of the Helix
and other older nebulae, suggest that the abundance of CS, as
well as the value of the HCN/HNC ratio, is not dependent on the
evolutionary stage of a given object. CS is reasonably prevalent
in the Red Spider and other older PNe (M2–48, Dumbbell,
Ring; ages 3000–10,000 yr; J. L. Edwards & L. M. Ziurys, in
preparation), but is absent in NGC 7027 and the Helix. The
HCN/HNC ratio is >1000 in NGC 7027 (Zhang et al. 2008),
but is ∼2 in both the Red Spider and the Helix (Bachiller et al.
1997). These results suggest that molecular abundances in PNe
principally reflect the physical and chemical properties of the
PN and its progenitor star.
0.00
0.01
0.00
T A* (K) or T R * (K)
0.02
H2 CO
JKa,Kc =31,2
21,1
JKa,Kc =30,3
20,2
0.00
0.012
0.000
0.01
CCH
J=5/2
3/2
N=3 2
J=7/2 5/2
0.00
0.14
N=2 1
J=5/2 3/2
CN
F=3/2
F=7/2
3/2
5/2
0.00
-90
-40
10
60
110
V LSR (km/s)
Figure 3. Spectra of N2 H+ , H2 CO, CCH, and CN observed toward the planetary
nebula NGC 6537, measured with the ARO SMT (temperature scale TA∗ ), or
the ARO 12 m (J = 1 → 0 transition of N2 H+ only: temperature scale TR∗ ).
Spectral resolution for the SMT data is 1 MHz (1.1–1.4 km s−1 ); resolution of
the 12 m spectrum is 500 kHz (1.6 km s−1 ). The CCH spectrum consists of
two spin-rotation doublets, labeled by quantum number J. In the CN spectrum,
only two hyperfine components are labeled by quantum number F; it is actually
composed of five hyperfine transitions, indicated by lines underneath the data
(see Table 1).
abundant molecules after CO are the diatomic species CN and
CS, with Ntot ∼ 1013 cm−2 and f ∼ 10−7 . The triatomic molecules
CCH and HCN are about a factor of five less abundant than CS
and CN, with Ntot ∼ 6 × 1012 cm−2 and f ∼ 6–7 × 10−8 .
The abundances of SO, HNC, H2 CO and HCO+ are comparable
(Ntot ∼ 2–3 × 1012 cm−2 and f ∼ 2–3 × 10−8 ). With Ntot ∼
7 × 1011 cm−2 and f ∼ 8 × 10−9 , the molecular ion N2 H+ is
the least abundant species observed, while the column density
upper limits for c-C3 H2 and HC3 N are Ntot < 1012 –1013 cm−2 .
CS and SO are the first definitive detections of sulfur containing
molecules in a PN. Zhang et al. (2008) reported a tentative
detection of HCS+ in NGC 7027, based on a single weak
transition.
5.3. The 12 C/13 C Ratio in NGC 6537:
Evidence for Hot-bottom Burning
Comparison of the column densities for HNC and HCN and
their 13 C isotopolgues yields 12 C/13 C ratios of 3.5 and 2.4,
respectively. Both molecules exhibit optically thin emission
such that line opacities cannot be influencing this ratio. This
value is significantly lower than those determined for other PNe
from CO observations. For example, Bachiller et al. (1997)
established ratios of 12 C/13 C∼9–23 for four evolved PNe and a
5.2. Molecular Abundances and Evolutionary State
While there are numerous studies of proto-PNe, few PNe
have been investigated in any chemical detail. The exceptions
5
The Astrophysical Journal Letters, 770:L5 (6pp), 2013 June 10
Edwards & Ziurys
with support through the NSF University Radio Observatories
program (URO): AST-1140030.
lower limit of 25 for NGC 7027. Palla et al. (2000) found ratios
14–23 for four other nebulae.
The low ratio in NGC 6537 can be understood in terms
of nucleosynthesis. The main sequence mass estimate for the
progenitor star of the Red Spider Nebula is >4 M (Pottasch
et al. 2000; Matsuura et al. 2005). Indeed, the extremely high
energy photons required to produce [Ne v] and [Si vi] indicate
that NGC 6537 formed from a star that marginally avoided the
supernova stage (Davis et al. 2003). More massive AGB stars
that evolve to PNe (>4 M ) can undergo “hot-bottom burning”
or HBB (e.g., Herwig 2005). HBB occurs when the bottom of
the convective envelope reaches into the active H-burning shell
during the interpulse phase, mixing the by-products of the CNO
cycle into the envelope and eventually to the stellar surface.
Models show that the convective turnover timescale is rapid
such that the entire convective envelope passes through the
“hot-bottom” multiple times (Lattanzio et al. 1997). Thus, in
HBB, equilibrium CNO abundances can be generated in the
convective envelope and the excess 12 C needed to create a
carbon-rich star is destroyed (e.g., Frost et al. 1998).
Observations of atomic lines toward NGC 6537 indicate high
He and N abundances, and a C/O ratio near 0.95 (Pottasch
et al. 2000)—all evidence for HBB in the progenitor star.
The molecular data also support HBB. The observed 12 C/13 C
ratio established here is near the CNO equilibrium value of
3.4, and the presence of SO and H2 CO indicates an O-rich
environment.
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5.4. Presence of SO and CS: Sulfur in Planetary Nebulae
Systematically low sulfur abundances are often found in
the ionized gas in PNe, sometimes attributed to elemental
sequestering in dust and/or molecules (Henry et al. 2012). These
observations show that sulfur can be contained in molecular
form in PNe. However, the Red Spider is not sulfur-deficient.
Searching for species such as CS or SO in sulfur-deficient PNe
would be of interest.
There appear to be significant discrepancies between the total
mass of a PN and that of the main-sequence progenitor, based
on ionized material (e.g., Kwok 1994). Consideration of CO
does appear to account for some of the missing material (e.g.,
Bernard-Salas & Tielens 2005). Kimura et al. (2012) suggest
that a key issue in this regard is the evaluation of the H2 /CO
ratio and the chemistry that produces it, both which can be
significantly different from that in molecular clouds. These
authors have therefore constructed a chemical model involving
CO, HCO+ , CN, HCN and HNC, in which molecular synthesis
is primarily controlled by X-rays. Their predictions of H2 /CO
ratios substantially deviate from past estimates. It is now
becoming evident, however, that heavier molecules, such as
CS, SO, and even SO2 (J. L. Edwards & L. M. Ziurys, in
preparation) exist in PNe. Modeling and neutral mass estimates
for PNe need to consider a more diverse chemical environment
than previously supposed.
This research was supported by NSF grants AST-1140030 and
AST-1211502. The SMT and Kitt Peak 12 m are operated by
the Arizona Radio Observatory (ARO), University of Arizona,
6