The Astrophysical Journal, 597:1065–1069, 2003 November 10
# 2003. The American Astronomical Society. All rights reserved. Printed in U.S.A.
DETECTION OF MgNC IN CRL 618: TRACING METAL CHEMISTRY WITH ASYMPTOTIC
GIANT BRANCH EVOLUTION
J. L. Highberger and L. M. Ziurys
Department of Astronomy, Department of Chemistry, Steward Observatory, and Arizona Radio Observatory,
University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721
Received 2003 June 5; accepted 2003 July 15
ABSTRACT
The MgNC radical has been detected toward the circumstellar shell of CRL 618, the first metal-bearing
molecule (in the chemist’s sense) observed in this proto–planetary nebula. Five rotational transitions of this
2 species were measured using the Arizona Radio Observatory (ARO) 12 m and IRAM 30 m telescopes,
several which clearly display the characteristic spin-rotation structure. Searches for NaCN, NaCl, AlF, and
AlCl in CRL 618, in contrast, proved negative. The line profiles of MgNC in this source, measured at the
30 m, were somewhat U-shaped and showed no evidence of high-velocity wings, indicating that this radical
arises from the remnant asymptotic giant branch (AGB) wind. The column density of MgNC in CRL 618 is
Ntot ¼ 2:4 1012 cm2 , corresponding to a fractional abundance, relative to H2 , of f 5:3 109 . The
upper limits for the sodium and aluminum species are typically f < 109 to 108. The MgNC abundance in
CRL 618 is comparable to those measured in CRL 2688 and IRC +10216; the upper limits for the sodium
and aluminum compounds are at least a factor of 10 lower in CRL 618 relative to abundances in IRC
+10216, but similar to those in CRL 2688. These data suggest that metal-bearing molecules produced by
LTE chemistry (NaCl, AlCl, AlF) are destroyed by the events associated with the second stage of AGB mass
loss, while radicals such as MgNC survive in the outer envelope for a significant time in the proto–planetary
nebula phase.
Subject headings: astrochemistry — circumstellar matter — stars: AGB and post-AGB —
stars: individual (CRL 618)
tral star (Sánchez Contreras, Sahai, & Gil de Paz 2002).
Recent Hubble Space Telescope (HST) images have
revealed the presence of multiple outflows in these bipolar
lobes, as traced by H and atomic forbidden lines
(Trammell & Goodrich 2002). Such outflows are also
observed in high-velocity molecular gas, which exhibit line
wings extending out to 200 km s1 (Neri et al. 1992). Radio
continuum measurements indicate the presence of a small
(0>4 0>1) H ii region near the central star (Kwok &
Bignell 1984), as well. Surrounding the energetic outflows,
which extend over 700 from the central star, is the remnant
AGB envelope, estimated to be as large as 9000 6000
(Meixner et al. 1998). Atomic forbidden lines and H2 S(1)
emission, which trace shocked material, are thought to arise
from the edges of the outflows, where they have collided
with the remnant shell (Cox et al. 2003; Sánchez Contreras
et al. 2002).
In the millimeter region, a variety of molecules have been
observed toward CRL 618, contributing to its complex
morphology. While CO and CS trace the remnant shell
(Meixner et al. 1998; Yamamura et al. 1994; Hajian,
Phillips, & Terzian 1995), species such as HCO+, OH, and
HNC arise close to the H ii region and are thought to be
products of photodissociation region (PDR) chemistry
(Herpin & Cernicharo 2000). Several molecules also exhibit
P Cygni–type profiles, in particular lines of vibrationally
excited HC3N (Wyrowski et al. 2003).
As part of an ongoing program, we have been
investigating the chemistry of metal-bearing species in circumstellar envelopes. Recently, we detected NaCl, NaCN,
AlF, and MgNC toward the proto–planetary nebula CRL
2688 (Highberger et al. 2001, 2003), the first time that
1. INTRODUCTION
The circumstellar envelopes of asymptotic giant branch
(AGB) stars are known to exhibit a rich chemistry, as
demonstrated by observations of IRC +10216. The circumstellar shell of this carbon-rich object has been found to
contain a wide variety of carbon chains, exotic silicon
species, and molecules bearing the metallic elements such as
magnesium and sodium (e.g., Cernicharo, Gúelin, &
Kahane 2000). In fact, until recently, it has been the only
source where such metal-bearing species have been
observed. AGB stars eventually evolve into planetary
nebulae (PNs), in the process undergoing additional, more
energetic mass loss that exposes the central star (Kwok
1993). Large amounts of UV radiation from the star subsequently penetrates the surrounding material, transforming
the molecular material into a heavily ionized medium.
The chemical evolution of circumstellar envelopes from
the AGB phase into the PN stage is naturally of interest.
Various observational studies have been conducted to
examine the survival of molecules during this transition
(e.g., Bachiller et al. 1997; Herpin et al. 2002). Thus far,
these investigations have shown that as a star leaves the
AGB, the higher temperatures and larger flux of UV photons favor the production of ions such as HCOþ , radicals
such as CN, and metastable isomers as indicated by HNC
(Bachiller et al. 1997). In contrast, the abundances of
silicon-bearing molecules such as SiO and SiC2 decrease.
One source of particular interest for the study of AGB/
PN chemistry is the proto–planetary nebula (PPN) CRL
618. The dominant characteristic of this object is the
presence of a bipolar reflection nebula surrounding the cen1065
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HIGHBERGER & ZIURYS
metal-containing molecules had been observed in a source
other than IRC +10216. Here we present the results of
searches for these species toward CRL 618.
2. OBSERVATIONS
Measurements of MgNC were conducted between 1999
December and 2003 April using the Arizona Radio
Observatory’s1 12 m telescope located at Kitt Peak,
Arizona. The N ¼ 8 ! 7, 11 ! 10, 12 ! 11, 13 ! 12, and
14 ! 13 rotational transitions of this molecule at 2 and 3
mm were observed toward CRL 618 ( = 4h39m34 90,
= 36 010 1600 [B1950.0]). Transition frequencies, beam
sizes, and main beam efficiencies ðc Þ are listed in Table 1.
The receivers used were dual-channel SIS mixers operated
in single-sideband mode with at least 20 dB rejection of the
image sideband. The back ends employed were sets of 256
channel filter banks with 1 and 2 MHz resolutions configured in parallel mode (2 128 channels). The temperature
scale at the 12 m is given as TR ; the radiation temperature
TR is then TR ¼ TR =c . All data were taken in beamswitching mode with a subreflector throw of 20 . Searches
were also conducted for the J ¼ 5 ! 4 line of AlF at
164,868 MHz, and the JKa ;Kc ¼ 90;9 ! 80;8 and 100;10 ! 90;9
transitions of NaCN at 138,652 and 153,558 MHz during
this time interval with an identical setup.
Additional observations of MgNC, AlCl, and NaCl were
carried out in 2000 October and 2001 September using the
IRAM 30 m telescope near Pico Veleta, Spain. The
N ¼ 12 ! 11 and 13 ! 12 transitions of MgNC at 2 mm
1 Arizona Radio Observatory (ARO) is operated by Steward
Observatory, University of Arizona, with partial funding from the National
Science Foundation and the Research Corporation.
Vol. 597
were measured, and searches were conducted for the
J ¼ 7 ! 6 and J ¼ 10 ! 9 transitions of NaCl at 91,170
and 130,224 MHz, as well as the AlCl J ¼ 11 ! 10 line at
160,312 MHz. Dual polarization, single-sideband SIS
mixers were used, with the image rejection 11–25 dB. The
back end used was a 1024 channel filter bank with 1 MHz
resolution, typically split into 4 256 channels for 256
MHz coverage per mixer channel. An autocorrelator with
1.25 MHz resolution was also employed. The temperature
scale at the 30 m is given as TA , and TR ¼ TA =b . All data
were obtained by wobble switching with a 1<5 throw.
3. RESULTS
The MgNC observations are summarized in Table 1. As
shown, five transitions of this molecule were detected
toward CRL 618 using the 12 m, of which two were also
confirmed at the 30 m. Because MgNC has a 2 ground
state, each transition consists of two spin-rotation components separated by 15.2 MHz, which are labeled by quantum number J. These doublets were resolved at several of
the observing frequencies, making the identification of
MgNC unambiguous. (Several transitions were contaminated by other features.) Unblended lines typically exhibit
LSR velocities in the range VLSR ¼ 22:0 to 23.5 km s1
and line widths of DV1=2 ¼ 25:1 31:4 km s1. These line
parameters are typical for the remnant wind of CRL 618
(Bujarrabal et al. 1988; Bachiller et al. 1997).
Figure 1 displays the five rotational transitions of MgNC
observed with the 12 m telescope. The centroid of each spinrotation doublet corresponds to VLSR ¼ 22:0 km s1. The
two spin-rotation components, indicated by arrows underneath the spectra, are clearly resolved in the N ¼ 8 ! 7 and
N ¼ 12 ! 11 transitions. The N ¼ 11 ! 10 and
TABLE 1
Observations of MgNC toward CRL 618
Transition
N = 8!7:
J = 7.5!6.5 .................
J = 8.5!7.5 .................
N = 11!10:
J = 10.5!9.5................
J = 11.5!10.5..............
N = 12!11:
J = 11.5!10.5..............
J = 12.5!11.5..............
N = 13!12:
J = 12.5!11.5..............
J = 13.5!12.5..............
N = 14!13:
J = 13.5!12.5..............
J = 14.5!13.5..............
TR or TA
(mK)
VLSR
(km s1)
DV1/2
(km s1)
0.89
21
21
23.4 6.3
23.3 6.3
25.1 6.3
31.4 6.3
48
0.80
31
3b
22
22
25
25
44
0.77
17c
0.69c
51
41
12 2c
13 2c
22.0 4.2
23.5 4.2
22.6 2.1c
22.5 2.1c
25.1 4.2
25.1 4.2
25.1 2.1c
27.2 2.1c
40
0.73
16c
0.67c
2d
2d
20c,d
15c,d
22
22
22c
22c
25
25
25c
25c
38
0.70
2d
2
22
22
25
25
Frequency
(MHz)
hb
(arcsec)
95,454.1
95,469.3
66
131,241.6
131,256.8
143,168.7
143,183.9
155,094.6
155,109.8
167,019.1
167,034.4
a
Note.—Data taken with the ARO 12 m telescope with 2 MHz resolution, unless noted. Temperature scale for the
12 m in TR . All errors are 3 .
a Main-beam efficiency value; for the ARO 12 m, ¼ , and for the IRAM 30 m, ¼ (see text).
c
b
b Blended with the J ¼ 27:5 ! 26:5 transition of C H and an unidentified line.
5
c IRAM 30 m data with 1 MHz resolution; temperature scale in T .
A
d Blended with an unidentified feature.
No. 2, 2003
DETECTION OF MgNC IN CRL 618
Fig. 1.—Spectra of the N ¼ 8 ! 7, N ¼ 11 ! 10, N ¼ 12 ! 11,
N ¼ 13 ! 12, and N ¼ 14 ! 13 rotational transitions of MgNC detected
toward CRL 618 using the ARO 12 m telescope at 3 and 2 mm. Each transition is composed of two spin-rotation components, whose frequencies are
indicated by arrows underneath the spectrum. These doublets are clearly
visible in the N ¼ 8 ! 7 and N ¼ 12 ! 11 transitions, which secures the
identification of this molecule in CRL 618. The rest of the transitions are
blended with other features. The spectrum of N ¼ 13 ! 12 line contains
various P Cygni profiles, some arising from vibrationally excited HC3N, as
indicated by asterisks. Filter resolution is 2 MHz.
N ¼ 14 ! 13 transitions are blended with other spectral
features (unidentified lines, labeled ‘‘ U,’’ and C5 H), which
mask the MgNC doublets. The N ¼ 13 ! 12 line exhibits
an unusual profile, which at first glance appears to be caused
by a U line located between the doublets. There are also several P Cygni lines apparent in this spectrum, mostly due to
HC3N, as indicated by asterisks. Two MgNC transitions
were also observed at the IRAM 30 m; the results are displayed in Figure 2. The spin-rotation doublets of the
N ¼ 12 ! 11 transition (top panel), as indicated by the two
arrows, appear somewhat U-shaped. These U-shaped line
profiles also appear in the autocorrelator, as shown in the
insert. The N ¼ 13 ! 12 lines (Fig. 2, lower panel),
observed here with 1 MHz resolution, exhibit peculiar
absorption dips. Because many P Cygni features are present
in these data (see insert where the full spectrum is shown),
this line shape may result from P Cygni line superposed on
the MgNC doublets.
1067
Fig. 2.—N ¼ 12 ! 11 and N ¼ 13 ! 12 transitions of MgNC in CRL
618 measured with the IRAM 30 m telescope at 2 mm. Resolution is
1 MHz. The N ¼ 12 ! 11 doublets (top panel) appear to be U-shaped, suggestive of resolved emission originating in a shell. The insert shows the same
spectrum observed with the autocorrelator, where the U-shapes are even
more prominent. The N ¼ 13 ! 12 transition (lower panel ) displays the
same unusual profile as seen with the 12 m telescope (see Fig. 1), which
likely results from a blend with one or more P Cygni lines. The insert here
shows the full spectrum, which contains many vibrationally excited HC3N
transitions, indicated by asterisks, as well as some unidentified lines.
Searches for other metal-bearing species in CRL 618
proved negative. Using the 12 m, the TR limit (3 ) obtained
for AlF ðJ ¼ 5 ! 4Þ was 5 mK, and 3 mK for NaCN
(J ¼ 9 ! 8 and J ¼ 10 ! 9, Ka ¼ 0). At IRAM, AlCl
ðJ ¼ 11 ! 10Þ was not observed to a noise level of 8 mK
ðTA Þ, and a limit of 6 mK for NaCl (J ¼ 7 ! 6 and
J ¼ 10 ! 9) was achieved.
4. DISCUSSION
4.1. MgNC in CRL 618: An AGB Remnant Molecule
The unblended line profiles of MgNC in CRL 618 have
LSR velocities and line widths typical of the remnant AGB
wind. They do not exhibit broad line wings (>100 km s1)
indicative of the high-velocity flow; nor do they show sharp
absorption dips near 27.2 or 40 km s1, as seen in other
molecules (e.g., Neri et al. 1992; Wyrowski et al. 2003).
Moreover, the uncontaminated N ¼ 12 ! 11 transition
observed at the 30 m appears to exhibit a U-shaped profile,
clearly visible in the lower frequency ðJ ¼ 11:5 ! 10:5Þ
spin-rotation component, and the autocorrelator spectrum
as well. These data suggest that MgNC emission is spatially
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HIGHBERGER & ZIURYS
resolved in the 1700 beam of the 30 m and has a shell-like
structure. In contrast, the MgNC profiles, measured with
the 12 m, are roughly flat-topped, indicating an unresolved
source. Comparison of the antenna temperatures at the two
telescopes suggests a distribution roughly 2000 –2500 in extent.
This size is consistent with the remnant AGB wind that
borders the outflows (e.g., Sánchez Contreras et al. 2002).
In fact, the N ¼ 12 ! 11 line profile from the 30 m can be
reproduced with a shell source with rmax 1000 and a width
of 500 , using the model of Bieging & Tafalla (1993), which
assumes an r2 density gradient. This model predicts
Tk 50 K and nðH2 Þ 105 cm3 at r 500 , which is consistent with the physical conditions predicted by Herpin &
Cernicharo (2000) and Herpin et al. (2002) at this radius.
The unusual profile of the N ¼ 13 ! 12 line of MgNC,
observed at IRAM, can be reproduced by the superposition of a single P Cygni profile on top of the two
MgNC doublets, centered at 155,108 MHz ðVLSR ¼
22 km s1 Þ, arising from a U-line. (There are other Ulines in this spectrum that have P Cygni shapes.) The line
profile could also be modeled with two identical P Cygni
profiles superimposed over the emission features, separated by the approximate MgNC spin-rotation splitting.
This latter modeling suggests that MgNC has a component close to the star near the PDR region, analogous to
HC3N (Wyrowski et al. 2003). However, this scenario is
unlikely because such features were not observed in the
other transitions.
4.2. Where Are the Sodium- and Aluminum-bearing
Molecules in CRL 618?
The column density of MgNC in CRL 618 was established from a rotational diagram analysis using both IRAM
and 12 m data. For s ¼ 2000 , this analysis yields
Vol. 597
Ntot ¼ 2:4 1012 cm2 and Trot ¼ 21 K. This rotational
temperature is consistent with the kinetic temperature (50
K) for this region (Herpin et al. 2002), and the dipole
moment of MgNC (5.3 D). From this column density, a
fractional abundance, relative to H2 , of f ðMgNCÞ 5:3 109 was calculated, assuming a shell with an outer
radius of 1000 , a width of 500 , mass-loss rate of 1 104 M
yr1 , and a distance of 1.7 kpc (Meixner et al. 1998). These
values are listed in Table 2, along with abundances of
MgNC in CRL 2688 ð4:1 109 Þ and IRC +10216
(8:9 109 ). (The MgNC value for IRC +10216 is an
updated number obtained using the data of Guélin et al.
[1995] in a rotational diagram analysis.) As the table illustrates, the abundance of MgNC is comparable in all three
sources.
Also listed in Table 2 are the upper limits to the column
densities and fractional abundances for NaCl, NaCN, AlF,
and AlCl in CRL 618. Two possible geometries were
chosen: a 2000 shell source analogous to MgNC with an
assumed rotational temperature of 20 K, and a 1000 spherical
source with Trot ¼ 50 K, similar to what has been found for
these molecules in IRC +10216 (Guélin, Lucas, & Neri
1997). A range of values is therefore given for the upper
limits, which are comparable to the abundances found in
CRL 2688.
The differences between these two PPNs and the AGB
star IRC +10216 are more dramatic. In IRC +10216, both
AlCl and AlF have abundances near 107 relative to H2 ,
1–2 orders of magnitude higher than the upper limits found
in the PPNs (or in the case of AlF in CRL 2688, the
observed abundance). NaCl and NaCN are at least a factor
of 10 more prevalent in IRC +10216 as well. Clearly, the
closed-shell halide molecules are produced (or are
preserved) more efficiently in the AGB envelope.
TABLE 2
Abundances of Metal-containing Molecules in CRL 618, CRL 2688,
and IRC +10216
Source
Molecule
Source Size
(arcsec)
CRL 618 ....................
MgNC
AlF
AlCl
NaCl
NaCN
MgNC
AlF
AlCl
NaCl
NaCN
MgNC
AlF
AlCl
NaCl
NaCN
20b
10, 20b,c
10, 20b,c
10, 20b,c
10, 20b,c
30d
10d
25e
25e
25e
40f
5d
5e
5e
5e
CRL 2688 ..................
IRC + 10216 ..............
Ntot
(cm2)
2.4 1012
<4–20 1012
<5–13 1012
<1–4 1011
<1–5 1012
3.7 1012
8.8 1012
<1.9 1012
1.9 1011
6.0 1012
9.3 1012
1.1 1015
6.7 1014
3.0 1013
1.6 1014
Fractional
Abundancea
5.3 109
<1–3 108
<1–2 108
<2–5 1010
<3–7 109
4.1 109
3.5 109
<1.7 109
1.6 1010
5.2 109
8.9 109
1.5 107
9.4 108
4.3 109
2.3 108
a Relative to H . Assumed mass-loss rates are 3 105 , 1:7 104 , and 1:0 104 M
2
yr1 for IRC +10216, CRL 2688, and CRL 618, respectively (Guélin et al. 1997; TruongBach et al. 1990; Meixner et al. 1998; Yamamura et al. 1994).
b Using a shell of radius 1000 and width of 500 .
c Spherical source of radius 500 .
d Highberger et al. 2001.
e Highberger et al. 2003.
f From a rotational analysis of data in Guélin et al. 1995; shell of 1000 width assumed.
No. 2, 2003
DETECTION OF MgNC IN CRL 618
4.3. The Evolving Chemistry of Metal-containing Molecules
in AGB/PPN Shells
Chemical modeling of the inner envelopes of AGB stars
suggests that molecular abundances are governed by LTE
(Tsuji 1973). Closed-shell metal halide species such as AlCl
and AlF are therefore predicted to be abundant in these
regions. Such chemical rationale applies to NaCl and NaCN
as well. It is thus not surprising that these molecules have
been observed in the inner shell of IRC +10216 (e.g.,
Guélin et al. 1997). MgNC, on the other hand, is a radical,
and therefore is found in the outer envelope in IRC +10216,
where photochemical and ion-molecule reactions create
open-shell species (e.g., Glassgold 1996).
The inner regions of CRL 618 and CRL 2688 have both
been impacted by the second phase of mass loss. Large
amounts of material have been swept up in high-velocity
flows from the central stars (e.g., Trammel & Goodrich
2002; Sahai et al. 1998), creating shock waves in the molecular gas (Cox et al. 1997, 2003). CRL 618 is sufficiently
evolved to have developed a small H ii region as well, with
an associated PDR. These phenomena must significantly
alter the chemistry in the inner envelopes of these objects.
1069
The LTE abundances of metal-bearing species also must
be affected by the shocks, outflows, and enhanced UV radiation. Sodium and aluminum-bearing compounds in CRL
618 are likely destroyed by such events. NaCl, NaCN, and
AlF are still present in CRL 2688, but this object is not as
evolved as CRL 618. Moreover, recent studies by
Highberger et al. (2003) have shown that the two sodium
compounds are actually present in a shell-like source at a
considerable distance from the star ðr 1000 1200 Þ, tracing
the edges of the outflows. These molecules may have been
survivors of previous LTE chemistry or they may have been
recreated in the shocks. Independent of their origin, these
species are no longer confined to the inner shell.
MgNC is thought to be exclusively created in the outer
envelope from radiative association reactions of Mgþ (Petrie
1996), although the exact mechanism is uncertain. The abundance of this molecule in CRL 618 and CRL 2688 is not
affected by the shocks and outflows. which have yet to impact
the outer shell. Unlike other metal-bearing species, MgNC
survives in the remnant AGB wind. Hence, this radical still
exists in a highly evolved source such as CRL 618.
This research is supported by NSF grant AST 02-04913.
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