The Astrophysical Journal, 765:112 (14pp), 2013 March 10
C 2013.
doi:10.1088/0004-637X/765/2/112
The American Astronomical Society. All rights reserved. Printed in the U.S.A.
CHEMICAL COMPLEXITY IN THE HELIX NEBULA: MULTI-LINE
OBSERVATIONS OF H2 CO, HCO + , AND CO
L. N. Zack1,2,3 and L. M. Ziurys1,2
1
Department of Chemistry, University of Arizona, P.O. Box 210041, Tucson, AZ 85721, USA; [email protected]
2 Department of Astronomy and Steward Observatory, Arizona Radio Observatory, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721, USA
Received 2012 July 17; accepted 2013 January 28; published 2013 February 25
ABSTRACT
+
Observations of CO, HCO , and H2 CO have been carried out at nine positions across the Helix Nebula (NGC 7293)
using the Submillimeter Telescope and the 12 m antenna of the Arizona Radio Observatory. Measurements of the
J = 1 → 0, 2 → 1, and 3 →2 transitions of CO, two transitions of HCO + (J = 1 → 0 and 3 →2), and
five lines of H2 CO (JKa, Kc = 10, 1 → 00, 0 , 21, 2 → 11, 1 , 20, 2 → 10, 1 , 21, 1 → 11, 0 , and 30, 3 →20, 2 ) were
conducted in the 0.8, 1, 2, and 3 mm bands toward this highly evolved planetary nebula. HCO + and H2 CO
were detected at all positions, along with three transitions of CO. From a radiative transfer analysis, the kinetic
temperature was found to be TK ∼ 15–40 K across the Helix with a gas density of n(H2 ) ∼ 0.1–5 × 105 cm−3 .
The warmer gas appears to be closer to the central star, but high density material is distributed throughout the
nebula. For CO, the column density was found to be Ntot ∼ 0.25–4.5 × 1015 cm−2 , with a fractional abundance of
f (CO/H2 ) ∼ 0.3–6 × 10−4 . Column densities for HCO + and H2 CO were determined to be Ntot ∼ 0.2–5.5 ×
1011 cm−2 and 0.2–1.6 × 1012 cm−2 , respectively, with fractional abundances of f (HCO + /H2 ) ∼ 0.3–7.3 × 10−8
and f (H2 CO/H2 ) ∼ 0.3–2.1 × 10−7 —several orders of magnitude higher than predicted by chemical models.
Polyatomic molecules in the Helix appear to be well-protected from photodissociation and may actually seed the
diffuse interstellar medium.
Key words: astrochemistry – ISM: molecules – planetary nebulae: individual (NGC 7293) – radio lines: ISM
Much of the previous work on the Helix Nebula has focused
on its structure and composition, as deduced from optical atomic
and infrared H2 transitions (e.g., see O’Dell et al. 2005, 2007).
These observations have demonstrated that the inner region
of the nebula (r 125 ), which appears as a hole on the
classic optical image, is characterized by emission lines from
highly ionized gas, such as from Hα, He ii, [Ne iii], [O iv], and
[O iii] (Cox et al. 1998; Speck et al. 2002; O’Dell et al. 2004).
The classic optical “ring” region is rich in neutral and ionized
atomic material, as traced by H i, [S ii], [N ii], C i, and [O i]
(e.g., Rodrı́guez et al. 2002; O’Dell et al. 2004, 2005; Young
et al. 1997), and molecular gas in the form of H2 and CO (e.g.,
Meixner et al. 2005; Hora et al. 2006; Young et al. 1999; Healy
& Huggins 1990). Large-scale maps of the Helix show that
both the J = 2 → 1 transition of CO and the 2.12 μm line of
vibrationally excited H2 extend out to a radius r ∼ 500 from the
central star (Young et al. 1999; Speck et al. 2002). As gleaned
from IR molecular hydrogen observations, dense globules are
present throughout the ring regions, often appearing as striking
“cometary knots” (e.g., O’Dell et al. 2007; Hora et al. 2006;
Matsuura et al. 2009). It is estimated that over 20,000 such
globules exist in the Helix nebula, with sizes on the order
of ∼0. 1–1 (O’Dell et al. 2007). Densities in the clumps are
estimated to be in the range 104 –106 cm−3 (Matsuura et al.
2007). Modeling of molecular hydrogen emission indicates gas
temperatures between 900 K and 1800 K, suggesting that the
globules are quite warm (O’Dell et al. 2007; Matsuura et al.
2007).
The Helix Nebula is thus characterized by thousands of very
dense, possibly self-shielding clumps of gas where molecular
material is present, albeit chiefly diatomic species. O’Dell et al.
(2007) postulate that CO is simply an abundant tracer of more
complex molecules, which have yet to be found. This idea
has been supported by the detection of polyatomic species
1. INTRODUCTION
The final phase of stellar evolution for most intermediatemass stars (0.5–8 M ) is the planetary nebula (PN) stage
(e.g., Kwok 2000). PNe are formed after the star leaves the
main sequence and passes through the red giant (RG) and
asymptotic giant branch (AGB) phases. During the RG, and
particularly on the AGB, significant mass loss occurs, creating
a large, dusty envelope around the central star (e.g., Huggins
et al. 2005). By the PN stage, the star has shed most of its
original mass, becoming a white dwarf, an intense emitter of
ultraviolet radiation (Kwok 2008). Consequently, the material
of the remnant AGB shell becomes highly ionized. PNe are thus
usually characterized by bright atomic emission lines, arising
from [O iii], [O ii], C ii, Ne ii, He ii, and [N ii] (e.g., Kwok 2000;
Meaburn et al. 2008). More recently, it has become clear that
there are significant quantities of neutral gas, as traced by the
2.12 μm lines of vibrationally excited H2 (Speck et al. 2002;
Kwok 2008). This neutral gas appears to exist in a clumpy
medium, often exhibiting cometary-like structures (Gonçalves
et al. 2001; Meixner et al. 2005; O’Dell et al. 2007).
One of the best-studied PNe is NGC 7293, the Helix. It is
thought to be the oldest known PNe, with an estimated age
of ∼11,000–12,000 years (Meaburn et al. 2008). At a distance
of ∼213 pc (Harris et al. 2007), it has an angular extent of
over 1000 in optical emission lines, H2 , and CO (Meaburn
et al. 2008; Matsuura et al. 2009; Young et al. 1999). From
the numerous studies at optical and infrared wavelengths (see
Matsuura et al. 2009), the geometry of the Helix is thought to
consist of two rings, one tilting about 23◦ with respect to the
plane of the sky, and a second oriented about 90◦ with respect
to the first (O’Dell et al. 2004).
3 Current address: Department of Chemistry, University of Basel,
Klingelbergstrasse 80, CH-4056 Basel, Switzerland.
1
The Astrophysical Journal, 765:112 (14pp), 2013 March 10
Zack & Ziurys
Table 2
Frequencies and Telescope Parameters of Observed Transitions
Table 1
Positions Observed in the Helix Nebula
Offset from Central Stara
(R.A. , decl. )
α(J2000.0)
(h m s )
δ(J2000.0)
(◦ )
Molecule
(130, −180)
(390, −30)
(−15, 270)
(−372, 0)b
(125, 185)
(−120, 240)
(−435, 75)
(−300, −200)
(−240, −100)
22 29 47.9
22 30 06.3
22 29 37.5
22 29 12.1
22 29 47.4
22 29 29.9
22 29 07.6
22 29 17.2
22 29 21.5
−20 53 18
−20 50 47
−20 45 49
−20 50 20
−20 47 13
−20 46 18
−20 49 04
−20 53 38
−20 51 58
CO
HCO +
H2 CO
Notes.
a Central position (J2000.0): α = 22h 29m 38s.6, δ = −20◦ 5018 .
b Position previously studied (Bachiller et al. 1997; Tenenbaum et al. 2009).
Transition
ν
(MHz)
Telescope
θb
( )
ηc or ηb
J=1→0
J=2→1
J=3→2
J=1→0
J=3→2
JKa, Kc = 10, 1 → 00, 0
JKa, Kc = 21, 2 → 11, 1
JKa, Kc = 20, 2 → 10, 1
JKa, Kc = 21, 1 → 11, 0
JKa, Kc = 30, 3 → 20, 2
115271.2
230538.0
345796.0
89188.5
267557.6
72837.9
140839.5
145602.9
150498.3
218222.2
ARO 12 m
ARO SMT
ARO SMT
ARO 12 m
ARO SMT
ARO 12 m
ARO 12 m
ARO 12 m
ARO 12 m
ARO SMT
55
33
22
70
28
86
45
43
42
35
0.84
0.77
0.70
0.89
0.76
0.94
0.76
0.75
0.74
0.78
separating (SBS) mixers. Image rejection was typically 15 dB
for all of the receivers. The backends used were 256 channel filter banks operated in the parallel mode (2 × 128 channels) with
250, 500, or 1000 kHz resolution, depending on the frequency.
The temperature scale was determined by the chopper-wheel
method, correcting for forward spillover losses, and is given as
TR ∗ , which is defined as TR = TR ∗ /ηc , where ηc is the corrected
beam efficiency.
The ARO 10 m Submillimeter Telescope (SMT) on Mt.
Graham, AZ was used for the 1 and 0.8 mm observations. The
1 mm receiver at the SMT employs dual-polarization ALMAtype Band 6 SBS SIS mixers, with image rejection typically
>15 dB. The dual-polarization 0.8 mm receiver is doublesideband (DSB), with SIS mixers based on an SMA design. The
backends used were 250 and 1000 kHz resolution filter banks
configured in parallel mode (2 × 256 channels and 2 × 1024
channels, respectively). The temperature is given as TA ∗ , where
TR = TA ∗ /ηb , as determined by the chopper-wheel method, and
ηb is the beam efficiency. It was assumed that the DSB receiver
has equal gains in the signal and image sidebands. Estimated
calibration uncertainty at both telescopes is 15%.
The observations were conducted at nine separate positions
in the Helix Nebula, offset from that of the central star at α =
s
22h 29m 38.6,
δ = −20◦ 5018. 0 (J2000); coordinates are listed
in Table 1. The observations were done in position-switching
mode with a reference offset of 30 west in azimuth for 1, 2, and
3 mm measurements and an offset of 15 in elevation at 0.8 mm.
For the HCO + and H2 CO data, local oscillator shifts were
done to test for image contamination. Pointing and focusing
were checked every ∼1.5–2 hr on planets or strong continuum
sources. Telescope parameters are given in Table 2, along with
rest frequencies of observed lines.
in the Helix at one position near the western part of the
rings. Here, Bachiller et al. (1997) observed CN, HCN, HNC,
and HCO + . More recently, Tenenbaum et al. (2009) detected
multiple transitions of H2 CO, C2 H, and c-C3 H2 toward this
position. It could be argued, however, that this particular region
is unique, and not representative of the nebula as a whole. It
would be enlightening to investigate the presence of polyatomic
molecules in other regions of the Helix Nebula.
Results from chemical models of the Helix, conducted in a
clumpy medium (Redman et al. 2003; Ali et al. 2001), suggest
that molecular content in this object and other PNe should be
severely limited due to the destructive effect of the central star’s
radiation field. For example, Redman et al. (2003) predict at
least a 104 decrease in abundance for common triatomic species
such as HCN and CCH as the nebular age increases from 2000
to 10,500 years. It has certainly been a common perception that
PNe are shells of only photoionized gas (Kwok 2010), especially
for highly evolved objects like the Helix.
In order to further elucidate its chemical content, we have
conducted observations of HCO + and H2 CO toward eight new
positions across the Helix nebula via multiple transitions at
1, 2, and 3 mm. Measurements of the J = 1 → 0, 2 → 1,
and 3 → 2 lines of CO were made at these positions, as well.
Additional data were also obtained at the location studied by
Bachiller et al. (1997) and Tenenbaum et al. (2009). HCO +
and H2 CO have been detected at all eight positions, as well
as the three lines of CO. With these new data, the gas kinetic
temperature and density were established across the nebula, as
well as molecular abundances. In this paper, we present our
observations and data analysis, and discuss their implications
for the chemistry in evolved PNe. We also speculate on the
relationship of this molecular material in the overall life cycle
of interstellar matter.
3. RESULTS
CO and HCO + have simple rotational manifolds, but that of
H2 CO is more complicated, as it is an asymmetric top. In this
case, each energy level splits into 2J + 1 components, labeled
by the quantum numbers J, Ka , and Kc . The presence of two
identical protons in H2 CO also gives rise to ortho (Ka = 1, 3, 5,
. . .) and para (Ka = 0, 2, 4, . . .) spin states. Note that the highest
energy levels probed in this work are for CO (E(J = 3) ∼ 33 K).
For HCO + and H2 CO, the highest levels probed lie 25 K.
Three transitions of CO (J = 1 → 0, 2 → 1, and 3 →
2) and one of HCO + (J = 1 → 0) were detected at all nine
positions. The J = 3 →2 line of HCO + was detected at only
five out of nine positions—perhaps not unexpected, given the
high dipole moment (μ = 3.9 D) for this molecular ion. For
2. OBSERVATIONS
The measurements were carried out between 2008 April and
2011 December using the telescopes of the Arizona Radio Observatory (ARO). Observations at 2 and 3 mm were taken with
the ARO 12 m telescope on Kitt Peak, AZ, which utilized three
different receiver systems over the course of the observations.
Receivers with dual-polarization, single-sideband SIS mixers
were used for the majority of the observations, one at 2 mm
(130–175 GHz) and another at 3 mm (65–115 GHz). Measurements made after 2008 May at 89 GHz (HCO + , J = 1→0)
employed a new receiver with ALMA-type Band 3 sideband2
The Astrophysical Journal, 765:112 (14pp), 2013 March 10
Zack & Ziurys
Figure 1. Spectra of the J = 1 → 0 transitions of HCO + (top panel) at 89 GHz and the JKa,Kc = 21,2 →11,1 line of H2 CO (lower panel) near 140 GHz measured with
the ARO 12 m telescope at eight positions in the Helix. The positions are indicated on the upper panel by the offset in R.A. and decl. (in arcseconds) from that of the
central star (J2000.0: α = 22h 29m 38s.6, δ = −20◦ 5018 ), and are marked on an R-band optical image of the Helix (Young et al. 1999). An asterisk marks the position
(−372, 0), previously studied by Bachiller et al. (1997) and Tenenbaum et al. (2009). Overlaid on the optical image is the lowest contour of the peak brightness
temperature of the J = 1 → 0 transition of HCO + (N. Zeigler et al. 2013, in preparation). The beam size at 89 GHz is shown below the image. Spectral resolution
is 250 kHz (0.84 km s−1 ) for the HCO + data, except at position (−120, 240), where it is 500 kHz (1.7 km s−1 ). For H2 CO, the resolution is 500 kHz (1.1 km s−1 ),
except at (−300, −200), where a 1 MHz (2.2 km s−1 ) resolution spectrum is shown. Both H2 CO and HCO + are clearly present at all eight positions.
H2 CO, the five transitions of interest (JKa,Kc = 10,1 → 00,0 ,
21,2 → 11,1 , 20,2 → 10,1 , 21,1 → 11,0 , and 30,3 →20,2 ) were
initially searched for and detected at four positions. As it became
apparent that this molecule was present across the nebula, only
one transition (JKa,Kc = 21,2 → 11,1 ) was studied at the remaining
five positions, where it was clearly visible.
Figure 1 shows spectra of the J = 1 → 0 transition of
HCO + (upper) and the JKa,Kc = 21,2 → 11,1 line of H2 CO
(lower) measured at the eight new positions across the Helix
Nebula. The velocity resolutions are typically 0.84 km s−1 and
1.1 km s−1 for HCO + and H2 CO, respectively. The location of
the eight positions within the nebula is indicated on an optical
R-band image taken from Young et al. (1999). Overlying the
image is the lowest contour of HCO + emission in the Helix,
extracted from a fully sampled map made across the entire
nebula (N. Zeigler et al. 2013, in preparation). The typical
beam size used for the measurements is shown on the lower
left-hand corner of the image. HCO + and H2 CO are clearly
present at each position, and the spectra of the two molecules
closely resemble each other. Multiple velocity components are
also visible in the data at several of the positions. More detailed
spectra at four of these positions are shown in Figures 2–5,
and the CO data at the remaining positions are presented
in Figure 6.
A summary of the observations is given in Table 3. Here, line
intensities (in uncorrected antenna temperatures TA ∗ or TR ∗ , as
well as TR ), linewidths at full width at half-maximum (ΔV1/2 ),
and LSR velocities are given for each feature observed. Note that
multiple components typically exist at almost every position,
as also observed by Young et al. (1999). Line parameters are
listed individually for each resolved component. The CO spectra
(J = 2→1) measured here and those of Young et al. (1999) are
in very good agreement.
Figures 2–5 consist of multiple panels. The left panels display
the three observed transitions of CO and the two of HCO + ; the
five H2 CO transitions are shown in the right panels. The spectra
are centered at the LSR velocity of −24 km s−1 (velocity of the
central star), and the temperature scale is in mK, TR ∗ for the
12 m data or TA ∗ for the SMT data. Integration times for each
spectra range from 2 to 50 hr.
The spectra in Figure 2, observed toward offset (130,
−180), clearly display two distinct velocity components near
−10 km s−1 and −47 km s−1 , as also found in the CO J =
2 → 1 line by Young et al. (1999). In the three CO transitions,
the velocity component near −47 km s−1 is typically twice the
intensity of that at −10 km s−1 . In contrast, the intensities of
the two components are roughly equivalent in the HCO + and
H2 CO data.
Figure 3 shows spectra taken toward offset (390, −30). At
this position, there is only one apparent velocity component,
centered at VLSR = −33 km s−1 , but it typically has a broader
linewidth than those at other positions (6–8 km s−1 versus
3–4 km s−1 : see Table 3). Young et al. (1999) observed a second
narrow feature at −17 km s−1 in CO near this position; in our
data, it appears as a “shoulder” on the CO spectra, contributing
to the overall linewidth. Note that the J = 3 → 2 line of HCO +
is quite narrow relative to the other features and may trace only
one velocity component.
3
The Astrophysical Journal, 765:112 (14pp), 2013 March 10
Zack & Ziurys
Figure 2. Spectra of the J = 1 → 0, 2 → 1, and 3 →2 transitions of CO, the J = 1 → 0 and 3 →2 lines of HCO + (left panels), and the JKa,Kc = 10,1 → 00,0 , 21,2 →
11,1 , 20,2 → 10,1 , 21,1 → 11,0 , and 30,3 →20,2 transitions of H2 CO (right panels), measured toward the Helix Nebula at offset position (130, −180). The data at 1 and
0.8 mm were measured with the ARO SMT (spectral resolution 1 MHz, or 0.9–1.4 km s−1 ) and the other spectra with the ARO 12 m telescope (resolution 500 kHz
or 1.0–2.1 km s−1 ). Two velocity components, located at VLSR = −10 and −47 km s−1 , are clearly present in every transition for all three molecules.
Two closely spaced velocity components at −27 and
−35 km s−1 are apparent toward the offset position (−15, 270),
as shown in Figure 4. Both components have similarly narrow
linewidths near 3–4 km s−1 (see Table 3). However, the relative
intensities of the two features in the three CO transitions vary,
with the −35 km s−1 component steadily decreasing in antenna
temperature with respect to that at −27 km s−1 . In comparison,
the two features in HCO + and H2 CO generally have similar
intensities.
The data for offset (−372, 0) are displayed in Figure 5. This
position has been the focus of previous Helix observations at
millimeter wavelengths (Bachiller et al. 1997; Tenenbaum et al.
2009). The p-H2 CO spectra shown are taken from Tenenbaum
et al. (2009). At this position, there appears to be a single
prominent velocity component at VLSR ≈ −15 km s−1 , with
a linewidth of 3.3 km s−1 . This feature may consist of multiple
blended velocity components.
The CO spectra (J = 1 → 0, 2 → 1, and 3 → 2) for the other
five positions are displayed in Figure 6. The data for the (125,
185), (−120, 240), and (−435, 75) positions show multiple
velocity components in all three transitions (also see Table 3).
At (−300, −200), a second component is visible in the J =
3 → 2 spectrum near −27 km s−1 , which on closer inspection,
is present in the other two transitions but at a lower signal level,
likely a result of beam dilution. At the (−240, −100) position,
a second component is apparent in the J = 1 → 0 spectrum at
−17 km s−1 , which is very weak in the other two transitions. It
likely traces lower densities where the other two lines are not
readily excited.
4. ANALYSIS
The data were analyzed by two methods. First, rotational
diagrams were constructed for each molecule for the positions
where at least two transitions were detected. This criterion was
met by CO at all of the positions and about half of the positions
for HCO + and H2 CO. A separate diagram was constructed for
each velocity component, when possible. The ortho and para
data were combined for the analysis, adopting an ortho:para ratio
of three. A uniform filling factor was assumed. This assumption
is justified based on a complete map of the Helix in HCO + ,
which showed extended molecular emission across the entire
optical image of the nebula (see Figure 1 and N. Zeigler et al.
2013, in preparation). The rotational partition function used for
the asymmetric top H2 CO was Qrot ∼ 1/2[π Trot 3 /ABC]1/2 ,
where A, B, and C are the rotational constants, in K (see Turner
1991; Gordy & Cook 1984). The rotational temperatures Trot and
column densities Ntot derived from this analysis are presented in
Table 4. The rotational diagrams indicate optically thin emission
in all three species.
The measured line intensities of the three molecules at a
given position were also modeled with the non-LTE radiative
transfer program RADEX (van der Tak et al. 2007). In this code,
collisional excitation is assumed and balanced with radiative
4
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Zack & Ziurys
Figure 3. Same as Figure 2, but measured toward the offset position (390, −30). Only one velocity component is evident in these data, located at VLSR ∼ −33 km s−1 .
This component is slightly broader than the features at other positions, with an average ΔV1/2 ∼ 7 km s−1 (see Table 3), likely indicating blended velocity features.
from HCO + were consistent with those derived from CO and
H2 CO, within the uncertainties.
The RADEX modeling results were found to be consistent
with the rotational diagram analysis (see Tables 4 and 5).
The column densities agree to within the uncertainties for
all three species. Furthermore, the rotational temperatures Trot
derived from the diagrams are typically less than the gas kinetic
temperatures (Trot TK ), as expected for optically thin emission.
At the positions where only one transition each of HCO +
and H2 CO were measured, column densities were also derived
using RADEX, but fixing the kinetic temperature and hydrogen
density to that determined from the CO analysis. For HCO + ,
values were derived on the basis of the J = 1 → 0 transition; the
JKa,Kc = 21,2 → 11,1 ortho line was used for formaldehyde, and
then scaled by an ortho:para ratio of three. The resulting total
column densities are also listed in Table 5.
decay to solve for statistical equilibrium, with an escape velocity
formalism that assumes an isothermal homogenous medium.
Gas kinetic temperatures TK , densities n(H2 ), and molecular
column densities Ntot are derived by matching the model
predictions to the observed spectral intensities. Each velocity
component specified in Table 3 was modeled separately. A
uniform filling factor was assumed in all of the cases, and the
only collision partner considered was para-H2 . Modeling results
for CO, H2 CO, and HCO + are summarized in Table 5.
CO was analyzed first, as it was observed in three transitions
at all nine positions. The modeling was conducted over temperature and density ranges of 10–70 K and 103 –107 cm−3 . Using
values of these parameters outside these ranges resulted in line
intensities vastly different from those observed.
For H2 CO, ortho and para lines were modeled separately. The
temperature, gas density, and column density were determined
independently from the modeling for p-H2 CO, where three transitions were observed. In general, the kinetic temperatures and
gas densities determined from H2 CO were in excellent agreement with those from the CO analysis, within the uncertainties.
For the o-H2 CO species, TK and n(H2 ) were held fixed to the
values of the para species, and Ntot was modeled. The ortho:para
ratios for H2 CO that resulted from the analysis were found in
the range 2.0 ± 0.7 to 3.4 ± 1.1, for an average value of 2.9 ±
0.7—close to the equilibrium value of 3 (see Table 6).
RADEX was also used to determine the column density for
HCO + . Because only two transitions were measured, the H2
density was modeled but the kinetic temperature was held fixed
to values established for CO. Again, gas densities determined
5. DISCUSSION
5.1. Physical Conditions in the Molecular Gas
Previous estimates of the physical properties of the molecular
gas in the Helix have been obtained primarily from modeling
observed transitions of H2 in the infrared (e.g., O’Dell et al.
2007). Based on such studies, the gas densities in this nebula
were estimated to be n(H2 ) ∼ 104 –105 cm−3 (e.g., Meixner
et al. 2005; Matsuura et al. 2007). However, densities up to
106 cm−3 have been derived for the dense cores of the cometary
globules (O’Dell et al. 2005). Based on CO, CN, HCO + , and
HCN observations, Bachiller et al. (1997) estimated densities in
5
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Zack & Ziurys
Figure 4. Same as Figure 2, but measured toward offset (−15, 270). The JKa,Kc = 30,3 → 20,2 spectra of H2 CO has been smoothed to a resolution of 500 kHz from
250 kHz. Two closely spaced velocity components at VLSR = −27 and −35 km s−1 are present in the spectra, but exhibit different relative intensities depending on
the molecule.
several PNe as high as 1–4 × 105 cm−3 , including one position
in the Helix. Huggins et al. (2002) conducted interferometer
observations of CO in one globule in the Helix and found
n(H2 ) 2 × 104 cm−3 . There have been fewer measurements
of the gas kinetic temperature. Modeling of the vibrationally
excited lines of molecular hydrogen indicates an excitation
temperature between 900 and 1800 K (O’Dell et al. 2007;
Matsuura et al. 2007). Bachiller et al. (1997) suggest a general
kinetic temperature of 25–60 K for evolved PNe, including the
Helix, based on rotational lines of CO, HCN, HCO + , and CN,
while Huggins et al. (2002) estimated Tex ∼ 18–40 K for CO in
the one Helix globule.
A cursory look at the temperatures within the Helix can be
obtained from the rotational diagram analysis, summarized in
Table 4. For CO, the range of rotational temperatures was Trot ∼
10–31 K across all of the positions. For HCO + , the analysis
yielded Trot ∼ 5–6 K, while in the case of H2 CO, Trot ∼
8–11 K—consistent with the higher dipole moments of both
molecules (μ = 3.9 D and 2.33 D, respectively). From the
radiative transfer modeling of CO and H2 CO, the gas kinetic
temperature was found to be TK ≈ 15–40 K, varying as a
function of position. The molecular gas is thus warm, but
not nearly as hot as that traced by vibrationally excited H2
(T ∼ 900–1800 K; Matsuura et al. 2007). The variation of
TK for different velocity components at a given position can
be accounted for by projection effects. There may be a radial
dependence of the temperature as well, with positions closer to
the star having a somewhat higher TK than those further away.
For example, at position (125, 185), the velocity component
near 1 km s−1 is on the inner ring (see Young et al. 1999), and
has TK ∼ 40 K. At (−300, −200), the 18 km s−1 component
appears to lie on the outer ring, and has TK ∼ 20 K. A more
detailed study is required to quantify the gradient, however.
The gas densities n(H2 ) could only be established from the
radiative transfer analysis. The values derived from CO fall in
the range 0.1–5 × 105 cm−3 , and those obtained from HCO + and
H2 CO at a given position are similar, with n(H2 ) ∼ 0.7–1.6 ×
105 cm−3 and 0.9–2.0 × 105 cm−3 , respectively. There is no
apparent radial gradient for the gas density. Certain positions in
the outer nebula have substantial densities (n(H2 ) ∼ 105 cm−3 ),
comparable to inner ones. For example, one can compare the
−29 km s−1 component at (−435, 75) or the −33 km s−1
feature at (390, −30) to the 1 km s−1 feature at (125, 185).
This clumpiness may reflect the structure of the remnant AGB
shell. These results indicate that the polyatomic molecules trace
dense molecular gas in both the inner and outer regions of the
nebula.
The velocity components found in this study lie in the range
VLSR = + 4 km s−1 to −47 km s−1 , centered on the systemic
velocity of the nebula near −23 km s−1 (Huggins et al. 2002).
This velocity spread and its variation as a function of position
closely matches that found in the J = 2 → 1 line of CO by Young
et al. (1999). At most positions, multiple velocity components
were observed in all three molecules, H2 CO, HCO + , and CO,
indicating that the denser gas is well-mixed. These data and the
wide extent of the molecular emission suggest that that HCO +
6
The Astrophysical Journal, 765:112 (14pp), 2013 March 10
Zack & Ziurys
Figure 5. Same as Figure 2, but measured toward the offset position (−372, 0). One velocity component, located at VLSR = −15 km s−1 , is present in all spectra. At
this position, previous studies by Bachiller et al. (1997) and Tenenbaum et al. (2009) showed the presence of HCN, HNC, HCO + , CN, CCH, C3 H2 , and H2 CO. The
p-H2 CO spectra were taken from Tenenbaum et al. (2009); see Table 3.
Figure 6. Spectra of the J = 1 → 0, 2 → 1, and 3 →2 transitions of CO, observed toward the other five positions. The J = 2 → 1 and 3 →2 data were measured
with the ARO SMT (spectral resolution 1 MHz or 1.3 km s−1 at 230 GHz and 0.9 km s−1 at 345 GHz). The J = 1 → 0 data were obtained with the ARO 12 m dish
(resolution 500 kHz or 1.3 km s−1 ). All of the positions show evidence of multiple velocity components.
a molecular hydrogen column density of 7.5 × 1018 cm−2 was
assumed, based on infrared observations of vibrationally excited
H2. These data are typically modeled to estimate a total column
density for molecular hydrogen, averaged over several globules
(e.g., O’Dell et al. 2007). Estimates of Ntot (H2 ) in the Helix
from infrared measurements vary from ∼1 × 1017 cm−2 to
6 × 1019 cm−2 (Matsuura et al. 2007; O’Dell et al. 2005,
2007; Cox et al. 1998). One of the most recent estimates is
and H2 CO may be located in the cometary knots that contain H2
(Speck et al. 2002; Matsuura et al. 2007, 2009; Meixner et al.
2005; O’Dell et al. 2007).
5.2. Column Densities and Molecular Abundances
Total column densities Ntot for the three molecules were derived from a radiative transfer analysis, as described above (see
Section 4 and Table 5). To determine the fractional abundances,
7
The Astrophysical Journal, 765:112 (14pp), 2013 March 10
Zack & Ziurys
Table 3
Observed Line Parameters for CO, HCO + , and H2 CO in the Helixa
Offset
(Δα, Δδ)
(130, −180)
Molecule
Transition
TR ∗ or TA ∗
(mK)
TR
(mK)
ΔV1/2
(km s−1 )
VLSR
(km s−1 )
CO
J=1→0
250 ± 39
560 ± 25
540 ± 13
900 ± 13
660 ± 88
1480 ± 88
60 ± 16
70 ± 16
15 ± 4
9±4
10 ± 2
10 ± 2
21 ± 5
26 ± 5
13 ± 4
14 ± 4
14 ± 3
13 ± 3
4±2
4±2
450 ± 29
830 ± 31
620 ± 86
75 ± 10
12 ± 5
12 ± 4
10 ± 3
6±3
6±3
5±2
320 ± 30
160 ± 25
800 ± 59
320 ± 20
1130 ± 74
280 ± 74
55 ± 10
40 ± 10
13 ± 5
10 ± 5
5±2
5±3
14 ± 5
8±5
8±4
6±4
10 ± 3
8±3
3±2
5±2
850 ± 50
1500 ± 50
1460 ± 80
110 ± 20
25 ± 5
25 ± 6
60 ± 5
35 ± 7
33 ± 10
10 ± 5
85 ± 23
180 ± 23
110 ± 23
75 ± 23
298 ± 45
667 ± 30
702 ± 17
1169 ± 17
943 ± 126
2114 ±126
67 ± 18
79 ± 18
20 ± 5
12 ± 5
11 ± 2
11 ± 2
28 ± 7
34 ± 7
17 ± 5
19 ± 5
19 ± 4
18 ± 4
5±3
5±3
536 ± 35
1078 ± 40
886 ± 123
84 ± 11
16 ± 7
13 ± 5
13 ± 4
8±4
8±4
6±3
381 ± 36
190 ± 30
1039 ± 76
416 ± 26
1614 ± 106
400 ± 106
62 ± 11
45 ± 11
17 ± 6
13 ± 6
5±2
5±3
18 ± 6
11 ± 6
11 ± 5
8±5
14 ± 4
11 ± 4
4±3
6±3
1011 ± 60
1948 ± 65
2086 ± 114
124 ± 22
33 ± 7
27 ± 6
79 ± 6
47 ± 9
45 ± 14
13 ± 6
101 ± 27
214 ± 27
131 ± 27
89 ± 27
3.3 ± 1.3
3.3 ± 1.3
2.6 ± 1.3
3.3 ± 1.3
2.7 ± 0.9
2.3 ± 0.9
3.4 ± 1.7
5.0 ± 1.7
3.3 ± 1.1
2.8 ± 1.1
4.1 ± 2.1
6.2 ± 2.1
4.2 ± 1.1
3.2 ± 1.1
2.1 ± 1.0
4.1 ± 1.0
3.0 ± 1.0
4.5 ± 1.0
4.2 ± 1.4
2.8 ± 1.4
6.5 ± 1.3
6.5 ± 1.3
7.2 ± 0.9
8.4 ± 1.7
3.3 ± 2.2
8.2 ± 2.1
6.4 ± 1.1
7.2 ± 2.1
8.0 ± 2.0
8.4 ± 1.4
3.9 ± 1.3
3.9 ± 1.3
2.6 ± 1.3
3.9 ± 1.3
2.7 ± 0.9
3.6 ± 0.9
5.0 ± 1.7
5.0 ± 1.7
3.3 ± 1.1
3.3 ± 1.1
6.2 ± 2.1
4.1 ± 2.1
4.2 ± 1.1
3.2 ± 1.1
4.2 ± 1.0
4.2 ± 1.0
3.5 ± 1.0
4.0 ± 1.0
2.0 ± 0.6
2.0 ± 0.6
3.3 ± 1.3
3.3 ± 1.3
4.5 ± 0.9
5.0 ± 1.7
3.3 ± 1.1
4.1 ± 2.1
3.2 ± 1.1
3.1 ± 1.0
3.0 ± 1.0
4.1 ± 1.4
3.9 ± 1.3
2.6 ± 1.3
3.9 ± 1.3
5.2 ± 1.3
−10.1 ± 1.3
−47.0 ± 1.3
−9.6 ± 1.3
−47.6 ± 1.3
−9.5 ± 0.9
−47.0 ± 0.9
−9.1 ± 1.7
−46.6 ± 1.7
−9.8 ± 1.1
−47.6 ± 1.1
−9.8 ± 2.1
−47.8 ± 2.1
−9.4 ± 1.1
−47.2 ± 1.1
−9.7 ± 1.0
−46.7 ± 1.0
−10.2 ± 1.0
−47.9 ± 1.0
−7.9 ± 1.4
−47.4 ± 1.4
−33.7 ± 1.3
−33.9 ± 1.3
−34.2 ± 0.9
−32.8 ± 1.7
−33.7 ± 1.1
−33.8 ± 2.1
−33.4 ± 1.1
−32.6 ± 2.1
−34.7 ± 2.0
−33.6 ± 1.4
−28.2 ± 1.3
−35.1 ± 1.3
−27.9 ± 1.3
−35.3 ± 1.3
−27.9 ± 0.9
−35.3 ± 0.9
−28.0 ± 1.7
−34.9 ± 1.7
−27.9 ± 1.1
−35.1 ± 1.1
−27.3 ± 2.1
−35.7 ± 2.1
−27.5 ± 1.1
−35.4 ± 1.1
−28.0 ± 1.0
−35.8 ± 1.0
−28.8 ± 1.0
−35.4 ± 1.0
−27.2± 0.6
−36.0 ± 0.6
−15.0 ± 1.3
−15.1 ± 1.3
−15.9 ± 0.9
−14.4 ± 1.7
−14.8 ± 1.1
−14.1 ± 2.1
−15.2 ± 1.1
−15.0 ± 1.0
−14.4 ± 1.0
−14.3 ± 1.4
1.0 ± 1.3
−12.4 ± 1.3
−20.9 ± 1.3
−25.6 ± 1.3
J=2→1
J=3→2
HCO +
J=1→0
J=3→2
H2 CO
JKa,Kc = 10,1 → 00,0
JKa,Kc = 21,2 → 11,1
JKa,Kc = 20,2 → 10,1
JKa,Kc = 21,1 → 11,0
JKa,Kc = 30,3 → 20,2
(390, −30)
CO
HCO +
H2 CO
(−15, 270)
CO
J=1→0
J=2→1
J=3→2
J=1→0
J=3→2
JKa,Kc = 10,1 → 00,0
JKa,Kc = 21,2 → 11,1
JKa,Kc = 20,2 → 10,1
JKa,Kc = 21,1 → 11,0
JKa,Kc = 30,3 → 20,2
J=1→0
J=2→1
J=3→2
HCO +
J=1→0
J = 3 → 2b
H2 CO
JKa,Kc = 10,1 → 00,0
JKa,Kc = 21,2 → 11,1
JKa,Kc = 20,2 → 10,1
JKa,Kc = 21,1 → 11,0
JKa,Kc = 30,3 → 20,2 b
(−372, 0)
CO
HCO +
H2 CO
(125, 185)
CO
J=1→0
J=2→1
J=3→2
J=1→0
J=3→2
JKa,Kc = 10,1 → 00,0 c
JKa,Kc = 21,2 → 11,1
JKa,Kc = 20,2 → 10,1 c
JKa,Kc = 21,1 → 11,0
JKa,Kc = 30,3 → 20,2 c
J=1→0
8
The Astrophysical Journal, 765:112 (14pp), 2013 March 10
Zack & Ziurys
Table 3
(Continued)
Offset
(Δα, Δδ)
Molecule
Transition
TR ∗ or TA ∗
(mK)
TR
(mK)
ΔV1/2
(km s−1 )
VLSR
(km s−1 )
J=2→1
200 ± 23
400 ± 40
95 ± 15
55 ± 15
300 ± 34
240 ± 34
70 ± 34
70 ± 34
∼ 10
46 ± 7
∼ 10
∼ 10
6±4
16 ± 4
250 ± 30
160 ± 21
460 ± 15
340 ± 15
530 ± 52
460 ± 70
24 ± 5
26 ± 5
<5
8±2
170 ± 30
260 ± 30
140 ± 30
400 ± 50
600 ± 15
360 ± 15
100 ± 70
440 ± 51
280 ± 51
40 ± 8
60 ± 8
21 ± 8
<8
10 ± 3
90 ± 20
60 ± 25
80 ± 30
80 ± 30
24 ± 5
<8
5±2
60 ± 25
120 ± 25
380 ± 10
430 ± 36
24 ± 7
<8
8± 4
260 ± 30
519 ± 52
110 ± 19
71 ± 19
429 ± 49
343 ± 49
100 ± 49
100 ± 49
∼11
52 ± 8
∼11
∼11
8±5
21 ± 5
298 ±36
190 ± 25
597 ± 19
442 ± 19
757 ± 74
657 ± 100
27 ± 6
29 ± 6
<7
11 ± 3
202 ± 36
310 ± 36
167 ± 36
519 ± 65
779 ± 19
467 ± 19
143 ± 100
629 ± 73
400 ± 73
45 ± 9
67 ± 9
24 ± 9
<11
13 ± 4
107 ± 24
78 ± 32
114 ± 43
114 ± 43
27 ± 6
<10
6±2
71 ± 30
143 ± 30
493 ± 13
614 ± 51
27 ± 8
<10
11 ± 5
2.6 ± 1.3
2.6 ± 1.3
3.9 ± 1.3
2.6 ± 1.3
3.2 ± 0.9
2.7 ± 0.9
∼3
∼3
∼2
3.0 ± 1.0
∼2
3.4 ± 1.0
4.4 ± 1.1
2.1 ± 1.1
3.3 ± 1.3
6.5 ± 1.3
3.3 ± 1.3
5.2 ± 1.3
2.7 ± 0.9
4.1 ± 0.9
6.7 ± 1.7
8.4 ± 1.7
...
6.4 ± 1.1
∼4
∼4
∼4
2.6 ± 1.3
5.2 ± 1.3
3.9 ± 1.3
∼4
∼3
∼4
∼5
∼7
∼7
...
6.4 ± 1.1
5.2 ± 1.3
9.1 ± 2.6
6.8 ± 0.9
3.2 ± 0.9
8.4 ± 1.7
...
∼5
6.5 ± 1.3
7.8 ± 2.6
3.9 ± 1.3
3.6 ± 0.9
8.4 ± 1.7
...
3.2 ± 1.1
1.1 ± 1.3
−12.7 ± 1.3
−20.7 ± 1.3
−24.5 ± 1.3
0.9 ± 0.9
−12.3 ± 0.9
−20.6 ± 0.9
−24.6 ± 0.9
∼1
−11.4 ± 1.0
∼ −20
−25.9 ± 1.0
−12.9 ± 1.1
−12.0 ± 1.1
4.0 ± 1.3
−33.9 ± 1.3
3.5 ± 1.3
−33.9 ± 1.3
3.6 ± 0.9
−34.4 ± 0.9
6.1 ± 1.7
−32.2 ± 1.7
...
−33.5 ± 1.1
−11.5 ± 1.3
−21.5 ± 1.3
−29.0 ± 1.3
−10.8 ± 1.3
−20.2 ± 1.3
−29.9 ± 1.3
−10.7 ± 0.9
−21.8 ± 0.9
−30.4 ± 0.9
∼ −11
∼ −20
∼ −30
...
−19.4 ± 1.1
−17.5 ± 1.3
−18.7 ± 1.3
−17.1 ± 0.9
−26.2 ± 0.9
−16.8 ± 1.7
...
∼ −17
−16.8 ± 1.3
−36.9 ± 1.3
−38.8 ± 1.3
−39.1 ± 0.9
−38.3 ± 1.7
...
−38.1 ± 1.1
J=3→2
(−120, 240)
HCO +
J = 1 → 0b
H2 CO
CO
J=3→2
JKa,Kc = 21,2 → 11,1
J=1→0
J=2→1
J=3→2
(−435, 75)
HCO +
J=1→0
H2 CO
CO
J=3→2
JKa,Kc = 21,2 → 11,1 d
J = 1 → 0e
J=2→1
J=3→2
HCO +
(−300, −200)
H2 CO
CO
HCO +
(−240, −100)
H2 CO
CO
HCO +
H2 CO
J = 1 → 0e
J=3→2
JKa,Kc = 21,2 → 11,1
J = 1 → 0d
J=2→1
J=3→2
J=1→0
J=3→2
JKa,Kc = 21,2 → 11,1 d
J=1→0
J=2→1
J=3→2
J=1→0
J=3→2
JKa,Kc = 21,2 → 11,1
Notes.
a Unless otherwise noted, 2 mm and 3 mm lines measured with 500 kHz resolution, with temperature scale T ∗ ; 1 mm and 0.8 mm lines measured with 1 MHz
R
resolution, with temperature scale TA ∗ .
b Measured with 250 kHz resolution.
c From Tenenbaum et al. (2009).
d Measured with 1 MHz resolution.
e Velocity components blended.
9
The Astrophysical Journal, 765:112 (14pp), 2013 March 10
Zack & Ziurys
Table 4
Column Densities and Rotational Temperatures from Rotational Diagram Analysis
Position
VLSR
(km
(130, −180)
(390, −30)
(−15, 270)
(−372, 0)
(125, 185)
(−120, 240)
(−435, 75)
(−300, −200)
(−240, −100)
−10
−47
−33
−27
−35
−15
1
−13
−20
−25
4
−33
−11
−21
−29
−18
−38
HCO +
CO
s−1 )
Trot (K)
23 ± 10
19 ± 7
18 ± 1
26 ± 13
18 ± 1
24 ± 9
31 ± 12
16 ± 2
10 ± 3
11 ± 7
19 ± 2
20 ± 4
11 ± 1
15 ± 7
21 ± 3
15 ± 5
19 ± 4
Ntot
(1015
cm−2 )
Trot (K)
1.1 ± 0.3
2.3 ± 0.7
3.6 ± 0.3
1.8 ± 0.6
0.79 ± 0.01
4.1 ± 1.2
0.53 ± 0.27
0.6 ± 0.1
0.3 ± 0.2
0.22 ± 0.17
1.0 ± 0.1
1.3 ± 0.2
0.7 ± 0.1
1.4 ± 0.8
0.84 ± 0.12
0.44 ± 0.19
1.1 ± 0.3
6±3
5±2
5±2
6±2
6±3
5±1
...
6±3
...
...
...
...
...
...
...
...
...
Ntot (H2 ) ≈ 1 × 1019 cm−2 , calculated by O’Dell et al. (2007).
Modeling of the H2 emission suggest that this column density
exists in gas with temperatures near 40 K, not just in the 900 K
material where vibrationally excited H2 emission arises (O’Dell
et al. 2005). Because the millimeter beam sizes are generally
larger than those used for IR observations, the H2 column density
was corrected by a 75% clump filling factor, relative to the millimeter beams, based on observations by Meixner et al. (2005)
for positions similar to those investigated in this work. With
Ntot (H2 ) = 7.5 × 1018 cm−2 , the average fractional abundance
of CO in the Helix was calculated to be f (CO/H2 ) ∼ 2 × 10−4 ,
in excellent agreement with previous estimates of f(CO/H2 ) ∼
2–3 × 10−4 (Healy & Huggins 1990; Huggins et al.
2002; Bachiller et al. 1997). Fractional abundances for all
three molecules relative to H2 , as well as HCO + /CO and
H2 CO/CO ratios are reported in Table 6.
Ntot
H2 CO
(1011
cm−2 )
1.6 ± 0.9
2.6 ± 1.1
4.8 ± 2.1
2.1 ± 0.8
1.6 ± 0.7
4.2 ± 0.9
...
1.1 ± 0.7
...
...
...
...
...
...
...
...
...
Trot (K)
Ntot (1011 cm−2 )
11 ± 4
8±4
8±2
9±3
10 ± 5
10 ± 4
...
...
...
...
...
...
...
...
...
...
...
3.5 ± 2.0
4.4 ± 3.6
5.7 ± 4.1
2.7 ± 1.9
1.9 ± 1.1
10 ± 5
...
...
...
...
...
...
...
...
...
...
...
5.2.2. HCO +
The column density of HCO + at the nine positions in the
Helix falls in the range Ntot ∼ 0.2–5.5 × 1011 cm−2 . The peak
value was found at the (−300, −200) position. Bachiller et al.
(1997) also observed the J = 1 → 0 transition of HCO +
at (−372, 0). They estimated a column density of 1.9 ×
1012 cm−2 , based on the one transition and assuming Tex >
25 K. Our detection of the J = 3 → 2 transition at this position
indicates a much lower excitation temperature, Tex ∼ Trot ∼ 5 K
(see Table 4), hence resulting in a smaller column density of
4.1 × 1011 cm−2 . The position with the lowest column density
for HCO + is at the offset (125,185), in the 1 km s−1 component.
Younger PNe have values of Ntot (HCO + ) ∼ 1.7 × 1011 to 1.7 ×
1012 cm−2 , while that of the young PN NGC 7027 has
Ntot = 4.3 × 1013 cm−2 (e.g., Josselin & Bachiller 2003).
The fractional abundances for HCO + in the Helix are
f(HCO + /H2 ) ∼ 0.27–7.3 × 10−8 . The larger values for HCO +
in the Helix are very similar to what has been observed in
the young PN NGC 7027 (4.8 × 10−8 ; Zhang et al. 2008)
and the PPN OH 231.8 + 2.4 (8.4 × 10−8 ; Morris et al. 1987).
The amount of HCO + does not appear to significantly decrease
with nebular age, even over a period of over 10,000 years. The
HCO + /CO ratio varies by a factor of 20 across the Helix, with
an average value of ∼ 2 × 10−4 (see Table 6).
5.2.1. CO
CO column densities were found to be in the range Ntot ∼
0.25–4.5 × 1015 cm−2 from the radiative transfer calculations,
in good agreement with those obtained independently from the
rotational diagrams (see Tables 4 and 5). The position with the
lowest column density was (125, 185), located toward the northeast part of the nebula, where Ntot ∼ 0.25 ×1015 cm−2 for the
−25 km s−1 component. At (390, −30), also toward the east,
the column depth was highest with Ntot ∼ 4.5 × 1015 cm−2 . The
average value in the Helix is Ntot ∼ 1–2 × 1015 cm−2 . Young
et al. (1997) observed position (−435, 75) in 13 CO, 12 CO, and
C i, resolving several distinct velocity components; their estimate of the CO column densities for the components at −21
and −11 km s−1 are 2.6 × 1015 cm−2 and 0.87 × 1015 cm−2 ,
respectively, in good agreement with the values found in this
work of 1.7 × 1015 cm−2 and 0.8 × 1015 cm−2 (see Table 5).
The fractional abundance of CO in the Helix ranged from
f(CO/H2 ) = 3.3 × 10−5 to 6.0 × 10−4 , in good agreement with
other estimates, as mentioned (e.g., Healy & Huggins 1990;
Bachiller et al. 1997). This value is also similar to what has
been found in the young PN NGC 7027, where f(CO/H2 ) ∼
1.1 × 10−4 (Zhang et al. 2008).
5.2.3. H2 CO
There have been few studies of H2 CO in PNe, with the exception of Tenenbaum et al. (2009), who detected the molecule
at the (−372, 0) position in the Helix. The column densities of
this molecule in the Helix are in the range Ntot = 0.19–1.6 ×
1012 cm−2 . The (−372, 0) and (−435, 75) positions have the
largest values and (−240, −100) has the smallest. At (−372, 0),
H2 CO is a factor of four more abundant than HCO + . In contrast,
H2 CO was not detected in NGC 7027, where an upper limit of
Ntot < 4.5 × 1012 cm−2 was established, based on the JKa,Kc =
31,2 → 21,1 transition (Zhang et al. 2008).
Fractional abundances for H2 CO span the range
f(H2 CO/H2 ) ≈ 0.25 – 2.1 × 10−7 . The average H2 CO/CO
ratio is 4.3 × 10−4 (Table 6). Formaldehyde is thus typically
a factor of two more abundant than HCO + in the Helix. The
10
Position
VLSR
(km
(130, −180)
(390, −30)
(−15, 270)
(−372, 0)
(125, 185)
11
(−120, 240)
(−435, 75)
(−300, −200)
(−240, −100)
s−1 )
−10
−47
−33
−27
−35
−15
1
−13
−20
−25
4
−33
−11
−21
−29
−18
−38
TK
(K)
30 ± 3
30 ± 4
20 ± 3
40 ± 2
25 ± 7
40 ± 10
40 ± 5
20 ± 2
15 ± 2
15 ± 2
30 ± 2
30 ± 1
15 ± 3
20 ± 2
20 ± 3
20 ± 3
30 ± 2
HCO +
CO
n(H2 )
(105
cm−3 )
2.0 ± 0.5
2.0 ± 1.1
1.0 ± 0.3
3.0 ± 2.0
1.0 ± 0.3
1.5 ± 0.5
5.0 ± 3.0
0.3 ± 0.1
0.1 ± 0.01
0.1 ± 0.02
0.8 ± 0.1
0.9 ± 0.1
0.3 ± 0.1
0.5 ± 0.1
1.4 ± 0.6
0.15 ± 0.06
2.2 ± 0.8
Ntot
(1015
cm−2 )
1.4 ± 0.1
3.3 ± 1.0
4.5 ± 0.5
2.3 ± 0.1
0.9 ± 0.1
4.3 ± 1.0
0.6 ± 0.1
0.7 ± 0.1
0.4 ± 0.1
0.25 ± 0.1
1.2 ± 0.1
1.4 ± 0.1
0.8 ± 0.2
1.65 ± 0.05
1.0 ± 0.5
0.8 ± 0.2
1.5 ± 0.1
n(H2 )
(105
cm−3 )
1.4 ± 0.4
0.7 ± 0.2
1.3 ± 0.5
0.9 ± 0.3
1.6 ± 0.9
0.9 ± 0.2
...
1.0 ± 0.4
...
...
...
...
...
...
...
...
...
p-H2 CO
Ntot
(1011
cm−2 )
1.8 ± 0.3
2.8 ± 0.6
4.0 ± 0.5
2.1 ± 0.3
1.7 ± 0.5
4.1 ± 0.7
∼ 0.2b
1.6 ± 0.2
∼1.3b
∼1.5b
1.5± 0.3b
2.0 ± 0.4b
3.8 ± 0.8b
5.3 ± 0.7b
1.4 ± 0.5b
5.5 ± 1.1b
1.8 ± 0.5b
TK (K)
30 ± 7
28 ± 4
18 ± 3
40± 10
25a
30 ± 2
n(H2
)a
(105
cm−3 )
1.3 ± 0.3
1.4 ± 0.1
0.9 ± 0.2
1.6 ± 0.4
2.0 ± 1.5
1.26 ± 0.02
...
...
...
...
...
...
...
...
...
...
...
o-H2 CO
Ntot(p)
(1011
cm−2 )
1.3 ± 0.2
1.5 ± 0.1
3.0 ± 0.5
0.85 ± 0.15
0.75 ± 0.25
3.7 ± 0.1
...
...
...
...
...
...
...
...
...
...
...
Ntot(o)
(1011
cm−2 )
4.1 ± 0.8
5.0 ± 0.4
5.9 ± 1.8
2.9 ± 0.8
1.9 ± 0.5
12 ± 1
...
5.4 ± 1.2b
...
...
...
3.5 ± 1.0b
...
7.0 ± 2.0b
...
∼6.5b
1.4 ± 0.6b
H2 CO
Ntot (1011 cm−2 )
5.4 ± 0.8
6.5 ± 0.4
8.9 ± 1.9
3.8 ± 0.8
2.7 ± 0.6
16 ± 1
...
7.2 ± 1.2c
...
...
...
4.7 ± 1.0c
...
9.3 ± 2.0c
...
∼8.7c
1.9 ± 0.6c
The Astrophysical Journal, 765:112 (14pp), 2013 March 10
Table 5
Physical Parameters and Column Densities from Radiative Transfer Analysis
Notes.
a Held fixed.
b Based on one measured transition, with T and n(H ) from CO (see text and Table 3).
K
2
c Assuming ortho/para ratio of three.
Zack & Ziurys
The Astrophysical Journal, 765:112 (14pp), 2013 March 10
Zack & Ziurys
Table 6
Molecular Abundances in the Helix Nebula
Position
(130, −180)
(390, −30)
(−15, 270)
(−372, 0)
(125, 185)
(−120, 240)
(−435, 75)
(−300, −200)
(−240, −100)
VLSR (km s−1 )
f(CO/H2 )a
f(HCO + /H2 )a
f(H2 CO/H2 )a
HCO + /CO (10−4 )
H2 CO/CO (10−4 )
H2 CO o/p Ratio
−10
−47
−33
−27
−35
−15
1
−13
−20
−25
4
−33
−11
−21
−29
−18
−38
1.9 × 10−4
4.4 × 10−4
6.0 × 10−4
3.1 × 10−4
1.2 × 10−4
5.7 × 10−4
8.0 × 10−5
9.3 × 10−5
5.3 × 10−5
3.3 × 10−5
1.6 × 10−4
1.9 × 10−4
1.1 × 10−4
2.2 × 10−4
1.3 × 10−4
1.1 × 10−4
2.0 × 10−4
2.4 × 10−8
3.7 × 10−8
5.3 × 10−8
2.8 × 10−8
2.3 × 10−8
5.5 × 10−8
2.7 × 10−9
2.1 × 10−8
1.7 × 10−8
2.0 × 10−8
2.0 × 10−8
2.7 × 10−8
5.1 × 10−8
7.1 × 10−8
1.9 × 10−8
7.3 × 10−8
2.4 × 10−8
7.2 × 10−8
8.7 × 10−8
1.2 × 10−7
5.1 × 10−8
3.6 × 10−8
2.1 × 10−7
...
9.6 × 10−8
...
...
...
6.3 × 10−8
...
1.2 × 10−7
...
1.2 × 10−7
2.5 × 10−8
1.3
0.8
0.9
0.9
1.9
1.0
0.3
2.3
3.2
6.1
1.3
1.4
4.6
3.2
1.5
6.7
1.2
3.8
2.0
2.0
1.6
3.0
3.7
...
10
...
...
...
3.3
...
5.5
...
11
1.3
3.2 ± 0.9
3.3 ± 0.3
2.0 ± 0.7
3.4 ± 1.1
2.5 ± 1.1
3.2 ± 0.3
Note. a Assuming Ntot (H2 ) = 7.5 × 1018 cm−2 (see text).
presence of H2 CO throughout the Helix Nebula in detectable
quantities has not been predicted by any chemical model. The
PDR-like environment of this source would seem to favor ionic
species such as HCO + , not more complex polyatomic molecules
like H2 CO. Formaldehyde has been detected in PPNe, including CRL618 (Pardo et al. 2007). For example, an abundance of
f(H2 CO/H2 ) = 4 × 10−8 was measured in OH 231.8 + 4.2, an
O-rich PPN (Lindqvist et al. 1992).
et al. 2012). Thus, the models would seem to suggest that the
chemistries of CO, HCO + , and H2 CO are linked, so it is perhaps
not surprising to find all three species in the Helix in the same
gas. However, the apparent robustness of H2 CO has yet to be
explained by any model.
5.4. The Link between AGB Envelopes and Diffuse Clouds?
There has been some debate as to the origin of the molecular
hydrogen in the Helix Nebula. As discussed by Matsuura et al.
(2009), H2 could have formed in situ in the nebula itself,
well after the ionization began. Alternatively, the H2 could
be remnant material of the previous AGB phase that survived
the transition to the late PN stage. Calculations by Aleman &
Gruenwald (2004) suggest that the latter is the case, because
efficient production of H2 cannot occur within the lifetime of
the Helix, given the densities present.
If the H2 is remnant material, other molecules may also have
a significant primordial AGB component. Their survival can
occur because they are protected in the high density clumps.
A comparison with AGB and young PNe abundances should
reveal whether this hypothesis is feasible. The CO abundance in
the Helix, f ∼ 2 × 10−4 , is certainly very similar to what is found
in circumstellar shells. In the envelopes of O-rich AGB stars,
f(CO/H2 ) is typically 4 × 10−4 (Ziurys et al. 2009), while in
carbon-rich stars, f(CO/H2 ) ∼7 × 10−4 (Ramstedt et al. 2008).
CO may not, however, be the most sensitive tracer of chemical
processes.
HCO + has been detected in both carbon- and oxygen-rich
circumstellar envelopes and protoplanetary/young PNe. In the
envelopes of O-rich AGB stars, f(HCO + /H2 ) falls in the range
2.4 × 10−8 to 1.3 × 10−7 (Pulliam et al. 2011); in the O-rich
PPN OH231.8, it is 8.4 × 10−8 (Morris et al. 1987). HCO + has
only been identified in the shell of one C-rich star, IRC + 10216,
where f ∼ 4.1 × 10−9 (Pulliam et al. 2011). The abundance range
in the Helix (f ∼ 0.27–7.3 × 10−8 ) is comparable to that in O-rich
AGB envelopes and PPNe. It is thus possible that HCO + in the
Helix represents remnant material from the AGB phase, albeit an
oxygen-rich environment. In the carbon-rich PPN CRL618, the
HCO + abundance is >7 × 10−8 (Sánchez-Contreras & Sahai
2004), and in the PN NGC 7027, f(HCO + /H2 ) ∼ 4.8 × 10−8
(Zhang et al. 2008). The values are consistent with the Helix
5.3. Comparison with Chemical Models
There are only a few models that predict abundances of
polyatomic species in PNe. Both Ali et al. (2001) and Redman
et al. (2003) have carried out calculations for HCO + , using timedependent chemistry in a dense (n ∼104 –105 cm−3 ), clumpy
medium. Ali et al. (2001) suggest f(HCO + /CO) ∼ 5 × 10−5
at an age of 12,000 years, applicable to the Helix—about a
factor of four less than the average ratio observed here. Redman
et al. (2003) predict f(HCO + /H2 ) ∼ 6 × 10−12 at 10,500 years,
roughly three orders of magnitude less than the observed
value, although the model suggests f(CO) ∼ 1.8 × 10−4 at the
same epoch, in good agreement with these observations (see
Table 6). A very recent model by Kimura et al. (2012) calculates
f(HCO + /H2 ) ∼ 10−10 for Ntot (CO) ∼ 1015 cm−2 , also much
lower than the observed value. Surprisingly, the predicted
abundance for the young PN NGC 7027 is f(HCO + /H2 ) ∼
3 × 10−9 (Hasegawa et al. 2000), in closest agreement with the
Helix observations.
Only one model considering H2 CO abundances in PNe
appears to exist in the literature—that of Hasegawa et al.
(2000). Using a steady-state code, these authors predict an
H2 CO column density of ∼ 2 × 109 cm−2 and an abundance of
f ∼ 5 × 10−13 . The observed values of 0.19–1.6 × 1012 cm−2
and f ∼ 0.25–2.1 × 10−7 are several orders of magnitude higher
than these estimates.
Hasegawa et al. (2000) suggest that the low abundance of
H2 CO arises from its fast destruction in hot, PDR-like gas via
reactions with H and C + , a pathway that would contribute to
the formation of HCO + . Ali et al. (2001), in contrast, consider
the disappearance of H2 CO as arising from photodissociation,
which produces CO. The predicted source of HCO + is thought
to be CO + , which is generated from CO (Ali et al. 2001; Kimura
12
The Astrophysical Journal, 765:112 (14pp), 2013 March 10
Zack & Ziurys
+
abundances; they also are at least a factor of ten larger than that
in the C-rich AGB envelope of IRC + 10216. The increase in the
HCO + abundance into the young PN stage relative to the AGB
in the C-rich case could result from chemistry induced by the
enhanced ionization by the central star. The observed abundance
of HCO + in the Helix may alternatively be a product of PDRtype chemistry generated in the PPN and early PN phases.
The origin of H2 CO is still unclear. It has only been detected
in one AGB envelope, that of the C-rich star IRC + 10216,
with an abundance of 1.3 × 10−8 (Ford et al. 2004). There
are no current identifications of formaldehyde in O-rich AGB
shells. The molecule has also been observed in a few PPNe,
as mentioned. Surprisingly, H2 CO has not yet been identified
in the young PNe NGC 7027 down to a significant lower limit
(Zhang et al. 2008). The H2 CO abundance of 0.3–2.1 × 10−7
in the Helix is comparable to what is found in PPN OH231.8
(f(H2 CO/H2 ) = 4 × 10−8 ), suggesting a possible connection to
remnant AGB material of an O-rich progenitor. There is some
thought that HCO + , and perhaps H2 CO, are created by shockinduced chemistry in OH231.8. H2 CO in the Helix may be a
product of shock chemistry, as well.
The composition of the progenitor star in the Helix is
itself a subject of debate. Measurements by Henry et al.
(1999) of atomic emission lines in the nebula suggest that star
went through three dredge-up phases, including the carbonenriching third dredge-up. The atomic abundances indicate that
C/O ∼ 0.9, suggesting that hot-bottom burning on the late AGB
converted much of the 12 C into 14 N. However, the presence of
CN, HCN, HNC, c-C3 H2 , and C2 H in the Helix indicates a
C-rich environment, at least toward one position (e.g.,
Tenenbaum et al. 2009). It is not clear that these molecules
are present throughout the nebula, but could indicate a C-rich
clump generated at the beginning of the TP-AGB.
The presence of significant amounts of polyatomic molecules
in a highly evolved PNe has implications for the overall evolution of the molecular interstellar medium (ISM).
As noted by O’Dell et al. (2007), the clumps containing H2 , HCO + , and H2 CO must seed the diffuse ISM,
influencing its general properties. If the globules are rich
in their molecular content, then they could have an important influence on diffuse cloud chemistry. Recent observations against strong background sources by Liszt &
co-workers (2006, 2008) show the indisputable presence of
polyatomic molecules in diffuse clouds. The molecules are
detected in absorption, because emission lines cannot be excited given the low densities (n(H2 ) 100 cm−3 ). No quiescent
ion–molecule chemistry can reproduce the observed abundances
under these conditions (Liszt et al. 2008).
HCO + and H2 CO are two of the prominent species found by
Liszt and co-workers in diffuse gas, with average abundances of
2–3 × 10−9 and 4 × 10−9 , respectively. If the globules from the
Helix slowly dispersed into the ISM, then a drop in molecular
abundances would occur. In the Helix, these two molecules
are typically at least a factor of ten more abundant than in
diffuse clouds, consistent with this scenario. Furthermore, the
H2 CO/HCO + ratio toward representative lines of sight in
diffuse gas is ∼1–5 (Liszt et al. 2008), as is observed in the
Helix.
The molecular connection is not just confined to H2 CO and
HCO + . With the recent detections of CS, SiO, and SO (Edwards
& Ziurys 2013), almost every molecule observed by Liszt and
co-workers at millimeter wavelengths in diffuse cloud has now
been identified in evolved PNe (see also Tenenbaum et al. 2009).
The two current exceptions are HCS and H2 S (Lucas & Liszt
2002). Searches are in progress for these two species in PNe.
6. CONCLUSION
The polyatomic molecules HCO + and H2 CO have been
found to have a widespread distribution in the Helix Nebula.
The gas in which they are present is dense and warm, with
n(H2 ) ∼ 104 –105 cm−3 and TK ∼ 15–40 K. The complex spectra
exhibited by these species, with multiple and physically distinct
velocity components, suggest that the polyatomic molecules
exist across the intricate ring structures observed in CO and
H2 . Abundances for both HCO + and H2 CO are surprisingly
high—many orders of magnitude greater than models predict
at this stage of PN evolution. The abundance of HCO + in
the Helix may have been enhanced by photochemistry during
the PPNe and early PNe phases. The presence and abundance
of formaldehyde is more difficult to explain, and could be
representative of remnant material from the AGB phase, as
is postulated for H2 . The molecular content in the Helix
suggests that evolved PNe may be seeding the diffuse ISM with
polyatomic species that have been observed toward many lines
of sight. Additional studies of the chemical content of evolved
PNe are necessary to establish the actual connection between
their molecular ejecta and diffuse interstellar gas.
This research was supported by NSF grants AST-0906534,
AST-1140030, and AST-1211502. The SMT and Kitt Peak 12 m
are operated by the Arizona Radio Observatory (ARO), Steward Observatory, University of Arizona, with support through
the NSF University Radio Observatories program (URO:
AST-1140030). We thank the ARO telescope operators, engineers, and staff for their assistance with the observations and N.
Zeigler and T. Folkers for assistance with modeling the data.
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