---
title: 'CRL 618: Carbon-Rich Pre-Planetary Nebula'
url: https://www.emergentmind.com/topics/crl-618
type: topic
---

# CRL 618: Carbon-Rich Pre-Planetary Nebula

CRL 618 is a carbon-rich pre-planetary nebula, and also an object described as a post-AGB star that has started to ionize its ejecta, placing it in the brief transition between the asymptotic giant branch and the planetary-nebula phase. Its observational phenomenology combines a compact and rapidly expanding H II region, multiple highly collimated optical and molecular lobes, a dense equatorial core or torus embedded in a more extended AGB halo, rich carbon-chain and other molecular chemistry, and ordered magnetic fields traced by dust polarization [1305.1731] [1306.2630] [1409.2531].

## 1. Evolutionary status, distance, and characteristic timescales

CRL 618 is consistently identified as a very young object in the post-AGB to planetary-nebula transition. The ionization of the circumstellar envelope is inferred to have begun around \(1971\pm2\) yr, a date that has been taken as the start of the planetary-nebula phase on the basis of the secular rise of the radio continuum flux and the angular size of the compact H II region [1306.2630]. Independent radio expansion-parallax work at 43 GHz yields an angular expansion rate of \(4.0\pm0.4\) mas yr\(^{-1}\) along the major axis and a geometric distance of \(1.1\pm0.2\) kpc; within errors, this agrees with the value of \(\sim 900\) pc derived from the expansion of the optical lobes [2403.04647].

Several ages coexist because they refer to different physical components. The dense central core mapped at 350 GHz has a dynamical age of \(\sim 400\) yr and is interpreted as a recent enhanced heavy mass-loss episode that ends the AGB phase [1305.1731]. The optical lobes and fast molecular outflows at their tips have expansion ages of order \(100\) yr, while additional compact outflows near the source have ages of \(\sim 45\) yr, implying at least two recent dynamical episodes [1603.00199]. High-\(J\) CO spectroscopy further indicates that the fast bipolar outflow has kinematic ages of \(\sim 30\)–\(70\) yr, consistent with very recent acceleration by shocks [1309.6206]. Mid-infrared molecular spectroscopy characterizes CRL 618 as a young \((<200\ {\rm yr})\) PPN moving rapidly from the AGB into its ionized phase [1012.0010].

A frequent misconception is that these age estimates are mutually inconsistent. They are instead attached to distinct layers and tracers: the dense core records the terminal AGB mass-loss history, the bipolar lobes and bullets trace post-AGB shaping events, and the ionized core traces the onset of the planetary-nebula phase [1305.1731] [1306.2630] [1603.00199].

## 2. Nebular morphology and kinematics

High-resolution optical, infrared, and submillimeter observations reveal a strongly structured nebula. HST imaging shows five eastern lobes, E1–E5, with projected lengths from \(\simeq 2.3''\) for the smaller lobes up to \(\simeq 6.8''\) for the two major lobes E1 and E2; proper-motion measurements give transverse speeds \(\sim 150\)–\(300\) km s\(^{-1}\) [1603.00199]. CO \(J=3\)–2 and HCN \(J=4\)–3 imaging at up to \(\sim 0.3''\) resolution spatially resolves the fast molecular-outflow region into multiple outflows oriented along different optical lobes, and also detects a pair of equatorial outflows inside the dense torus [1308.6332].

Herschel/HIFI-based modeling separates the molecular envelope into four components: a diffuse spherical halo, a pair of double empty shells (“ellipses”), a dense central core, and a fast bipolar outflow [1309.6206]. Their kinematics span a wide range. The dense central core expands slowly, with \(V_{\rm exp}(r)=7(r/R_{\rm out})\) km s\(^{-1}\), giving \(<7\) km s\(^{-1}\) near the center and a strong velocity gradient [1309.6206]. The fast bipolar outflow extends to \(h_{\max}=2.8\times10^{16}\) cm and reaches velocities from \(\sim 50\) up to \(\sim 200\) km s\(^{-1}\), dominating the high-\(J\) CO wings out to \(\pm 250\) km s\(^{-1}\) [1007.1570] [1309.6206].

At sub-arcsecond scales, the central dense core mapped at 350 GHz shows that most of the detected emission arises within \(\sim 630\) AU \((\sim 0.7'')\) from the central star. Its expansion accelerates roughly linearly from \(\sim 3\) km s\(^{-1}\) in the innermost part to \(\sim 16\) km s\(^{-1}\) at 630 AU, with a best-fit gradient \(a\simeq 23\) km s\(^{-1}\) arcsec\(^{-1}\) in \(\Delta v(r)\simeq a\cdot r+b\) and intercept \(b\simeq 3\) km s\(^{-1}\) [1305.1731].

Two related kinematic patterns recur in the literature. First, the position-velocity cuts of the outflows show a linear increase of velocity with distance, \(v\propto r\), described as a linear “Hubble-flow” relation [1603.00199]. Second, the low-velocity molecular emission traces cavity walls, while the high-velocity emission concentrates near the lobe tips [1308.6332] [1603.00199]. These patterns are central to current shaping models.

## 3. Central stars, ionized core, and secular change

Optical spectroscopy identifies an unsettled central engine. The nuclear optical spectrum combines very broad C III and C IV emission, a forest of [Fe III] lines, He I \(\lambda 5876\), narrow H I lines, and a luminous near-infrared continuum coincident with a cool post-AGB star. This combination is interpreted as characteristic of an active symbiotic system, and the broad carbon lines match a WC8-type companion by the Crowther et al. criteria [1409.2531]. In the lobes, Balmer, He I, and [O III] are largely dust-scattered nuclear emission, whereas low-ionization lines such as [N I], [O I], [S II], and [N II] peak in shocked knots [1409.2531].

The compact H II region has been increasing in both flux density and size over several decades. At 22.49 GHz, seven VLA epochs from 1982.48 to 2007.52 show a monotonic flux rise from \(205\pm11\) mJy to \(517\pm10\) mJy, while the major axis grows from \(0.333\pm0.015''\) to \(0.499\pm0.002''\) and the minor axis from \(0.103\pm0.009''\) to \(0.185\pm0.001''\) [1306.2630]. The fitted electron-density law is
\[
n_{\rm e}(r)=(4.9\pm0.9)\times 10^{4}\,[r/1'']^{-2.4\pm0.2}\ {\rm cm}^{-3},
\]
and the frequency dependence of the major axis, \(\theta_{\rm maj}\propto \nu^{-0.43\pm0.04}\), is interpreted as the consequence of this gradient [1306.2630].

Millimeter radio recombination lines probe even smaller radii. In 2015, H \(41\alpha\), H \(39\alpha\), H \(31\alpha\), and H \(30\alpha\) were detected with FWHM values from \(47.6\pm0.8\) km s\(^{-1}\) down to \(37.1\) km s\(^{-1}\); relative to 1987, these lines are stronger by factors of \(2\)–\(3\) and broader by \(20\)–\(60\%\), indicating that the ionized wind now has larger expansion velocities and mass-loss rate than \(\sim 29\) years earlier [1704.01773]. A non-LTE model of the ionized core adopts \(T_e=1.65\times10^4\) K, \(n_e(r)=2.4\times10^7(r/45\,{\rm au})^{-2.0}\ {\rm cm}^{-3}\), and an expansion law rising from \(\sim 10\) km s\(^{-1}\) near 45 au to \(\sim 35\) km s\(^{-1}\) at large \(r\), with \(>90\%\) of the mm emission coming from \(\lesssim 110\) au [1704.01773].

The optical line ratios evolve secularly. Across the nebula, \([{\rm O\,III}]/H\beta\) and \({\rm He\,I}/H\beta\) have been steadily rising, \(H\alpha/H\beta\) has been steadily decreasing, \(H\gamma/H\beta\) remains nearly constant, and low-ionization line ratios formed in the shocked knots have been in decline in different ways at various locations [1409.2531]. These trends support a picture in which the ionizing output of the central source has increased while the broad \(H\alpha\) component associated with the symbiotic star has faded.

## 4. Molecular envelope, excitation, and circumstellar chemistry

CRL 618 has a rich molecular inventory extending from centimeter to far-infrared wavelengths. Mid-infrared TEXES/IRTF spectroscopy in the ranges \(778\)–\(784\) and \(1227\)–\(1249\) cm\(^{-1}\) identified more than 170 ro-vibrational lines of C\(_2\)H\(_2\), HCN, C\(_4\)H\(_2\), and C\(_6\)H\(_2\), but no unambiguous signature of C\(_8\)H\(_2\) [1012.0010]. The derived total column densities are
\[
N({\rm HCN})=2.5\times10^{17}\ {\rm cm}^{-2},\quad
N({\rm C_2H_2})=3.1\times10^{17}\ {\rm cm}^{-2},\quad
N({\rm C_4H_2})=2.1\times10^{17}\ {\rm cm}^{-2},
\]
\[
N({\rm C_6H_2})=9.3\times10^{16}\ {\rm cm}^{-2},\quad
N({\rm C_8H_2})<5\times10^{16}\ {\rm cm}^{-2},
\]
with rotational temperatures ranging from \(\simeq 100\) to \(350\) K and vibrational temperatures from \(\simeq 100\) to \(500\) K, demonstrating that the inner envelope is strongly out of local thermodynamic equilibrium [1012.0010]. The abundance ladder \( {\rm C_2H_2}>{\rm C_4H_2}>{\rm C_6H_2}\), together with the non-detection of C\(_8\)H\(_2\), is interpreted as the outcome of a UV-driven radical-neutral network in which stellar ultraviolet photons fragment C\(_2\)H\(_2\) to C\(_2\)H, which then reacts with C\(_2\)H\(_2\) to build longer chains until photodestruction outpaces growth [1012.0010].

High-\(J\) CO data add a complementary thermal picture. Herschel/HIFI observations detect \(^{12}\)CO and \(^{13}\)CO \(J=6\)–5, \(10\)–9, and \(16\)–15, with wide profiles and spectacular line wings [1007.1570] [1309.6206]. Reanalysis with a spatio-kinematical model indicates that the fast bipolar outflow is hotter than previously estimated, with \(T\sim 300\) K at \(10^{16}\) cm and \(\sim 100\) K at the outer edge; the dense central core has \(T\approx 120\) K and very low expansion velocity [1309.6206]. Earlier analysis already concluded that gas flowing at \(100\) km s\(^{-1}\) must have \(T_k\ge 200\) K and that the high temperature is evidence that the fast outflow has been accelerated by a shock and has not yet cooled down [1007.1570]. Within a comparative Herschel/HIFI sample, CRL 618 stands out as having a particularly warm fast wind with characteristic \(T_k\gtrsim 200\) K [1109.6145].

The dense core also shows a complex carbon- and nitrogen-bearing chemistry at 350 GHz. More than 100 isolated or nearly isolated transitions were detected, dominated by HC\(_3\)N and its \(^{13}\)C isotopologues, HCN and its isotopologues, and CH\(_2\)CHCN toward the innermost part [1305.1731]. The isotopic ratios in the core are \(^{12}{\rm C}/^{13}{\rm C}=9\pm4\) and \(^{14}{\rm N}/^{15}{\rm N}=150\pm50\), both lower than solar values [1305.1731].

At 20–25 GHz, the detected molecular spectrum is restricted to NH\(_3\) inversion absorption. Two-component rotational-diagram analysis gives \(T_{\rm ex}=28\pm10\) K and \(400\pm200\) K for the broad components and \(290\pm70\) K for the narrow absorption clump; no cyanopolyyne lines were detected at the survey sensitivity [2006.07062]. This suggests strong chemical stratification and, plausibly, efficient destruction or ionization of long carbon-chain species under the harder UV field and shocks.

A rare transient phenomenon is the 4765 MHz OH maser. Arecibo detected a narrow line with FWHM \(\sim 0.6\) km s\(^{-1}\), peak flux density \(\sim 145\) mJy, and \(V_{\rm LSR}\approx -59.8\) km s\(^{-1}\), about \(40\) km s\(^{-1}\) blueshifted from the systemic velocity; the line was absent in later epochs and is therefore interpreted as a variable maser [1906.08245]. Proposed origins include shock-induced H\(_2\)O dissociation after sublimation of icy objects and an oxygen-rich circumstellar region associated with a binary companion [1906.08245].

## 5. Magnetic fields and polarization structure

Dust polarimetry shows that CRL 618 hosts an ordered magnetic field, but the inferred geometry depends on angular scale. SMA observations at \(330\)–\(345\) GHz detected dust continuum polarized emission above \(4\sigma\) with peak polarized intensity \(9.6\) mJy beam\(^{-1}\) \((4.4\sigma)\), mean polarized intensity \(7.0\) mJy beam\(^{-1}\) \((3.2\sigma)\), and a mean fractional polarization \(p_{\rm mean}\simeq 0.3\%\) [1312.0272]. The field vectors, obtained by rotating the E-vectors by \(90^\circ\), form a coherent, slightly curved pattern opening to the east, with measured mean position angle \(\langle PA_B\rangle \simeq 96^\circ\pm5^\circ\) [1312.0272]. The CO \(J=3\)–2 outflow axis is \(PA_{\rm CO}\simeq 94^\circ\pm5^\circ\), so the field lies parallel to the outflow major axis within \(\simeq 2^\circ\) [1312.0272]. No equatorial toroidal component is detected at that resolution; the field is described as pure poloidal or dipole-like [1312.0272].

The polarization fraction decreases monotonically toward regions of high Stokes \(I\), an anti-correlation described approximately as \(p\propto I^{-0.5\pm0.1}\) [1312.0272]. The paper discusses beam-averaging of tangled field lines, loss of grain alignment in the dense core, and varying grain properties as possible causes.

JCMT POL-2 observations at \(850\,\mu{\rm m}\) probe much larger scales, with a native beam FWHM of \(14.1''\), corresponding to \(\sim 1.27\times10^4\) au at \(d=0.9\) kpc [2507.21829]. Over the ten central pixels, the mean magnetic-field position angle is \(\theta_B=(66^\circ\pm12^\circ)\) east of north; in the north/east periphery it is \((161^\circ\pm5^\circ)\), and in the south it is \(\simeq 33^\circ\) [2507.21829]. The measured polarization fraction spans \(\sim 0.5\)–\(6\%\), with a high-SNR mean of \(\sim 2\%\) in the central region [2507.21829]. The central \(\theta_B\approx 66^\circ\) is within \(\sim 23^\circ\) of the mean bullet-outflow axis at \(\sim 89^\circ\), and within \(\sim 4^\circ\) of the northeast extreme position angle, suggesting association with one cavity wall [2507.21829].

Taken together, the polarimetric studies do not support a single, scale-independent toroidal geometry. At SMA resolution, CRL 618 exemplifies a well-ordered poloidal field in a young source; at JCMT resolution, polarized emission may preferentially arise from material in the walls of the dust cavity opened by the bullets [1312.0272] [2507.21829]. A plausible implication is that the field morphology is being reorganized by outflow-envelope interaction rather than sampling a static large-scale configuration.

## 6. Shaping mechanisms and dynamical interpretations

Two main classes of shaping model have been advanced for CRL 618’s multipolar structure. One is the multi-directional bullet model. In this framework, five dense clumps are ejected in two epochs, with bullets 1–2 at \(t\approx -90\) yr and bullets 3–5 at \(t\approx -45\) yr, interacting with a toroidal dense core and a spherical AGB halo [1603.00199]. Each bullet is modeled as a \(200\) AU \(\times 200\) AU cylinder with \(T\approx 200\) K; bullets 1–2 have \(M_b\simeq 0.011\,M_\odot\), bullets 3–5 have \(\simeq 0.004\,M_\odot\), and the total bullet mass is \(M_{b,{\rm tot}}\approx 0.034\,M_\odot\) [1603.00199]. ZEUS-3D simulations including molecular and atomic cooling and simplified H\(_2\) chemistry reproduce five collimated lobes, a U-shaped low-velocity cavity wall, high-velocity CO emission at all five lobe tips, and the observed linear \(v\)-\(r\) relation in PV diagrams [1603.00199].

The corresponding observational basis is strong. SMA CO \(J=3\)–2 and HCN \(J=4\)–3 imaging resolves six distinct fast molecular outflow components aligned with the optical lobes. Their tip radial velocities range from \(110\) to \(165\) km s\(^{-1}\), their dynamical ages from \(34\pm13\) to \(109\pm43\) yr, and their estimated masses from \(0.45\times10^{-3}\) to \(2.44\times10^{-3}\,M_\odot\) [1308.6332]. The outflows fall into two age groups: inner bullets around \(45\pm25\) yr and outer bullets around \(105\pm40\) yr, supporting two separate, roughly simultaneous ejection events in multiple directions [1308.6332].

A second class is the asymmetric precessing jet model. Three-dimensional hydrodynamical simulations of a precessing jet launched from an orbiting companion, with alternation in which lobe is launched at each periastron passage, reproduce a four-finger, mirror-symmetric pattern comparable to HST images [1408.4699]. In the best-fit model, the secondary is on an eccentric orbit with \(e=0.5\), orbital period \(\tau_o=15.4\) yr, the disk axis precesses with half-angle \(\alpha=15^\circ\) and period \(\tau_p=61.6\) yr, and the jet speed decreases as \(v_{j0}(t)=400\ {\rm km\,s^{-1}}-(3\ {\rm km\,s^{-1}\,yr^{-1}})\,t\) with a periodic modulation [1408.4699]. After \(120\) yr, the simulated fingers have proper motions of \(150\)–\(335\) km s\(^{-1}\) and kinematic ages of \(\sim 79\)–\(114\) yr, again matching the observationally inferred century-scale ages [1408.4699].

The proposed launch mechanisms reflect this modeling diversity. For bullets, the literature discusses magneto-rotational explosions near the end of the AGB and nova-like explosions around a binary companion, the latter motivated in part by the unusually low \(^{12}{\rm C}/^{13}{\rm C}\sim 10\) and \(^{14}{\rm N}/^{15}{\rm N}\sim 150\) isotope ratios in the core [1603.00199] [1305.1731]. For the precessing-jet scenario, the key ingredient is the interaction between a Roche-lobe-filling companion, an accretion disk, orbital motion, and precession [1408.4699].

Rather than collapsing these models into a single mechanism, the current literature treats CRL 618 as a system in which binary interaction, episodic ejection, shocks, dense equatorial structure, and magnetic fields all plausibly participate in shaping. This is consistent with the broader empirical picture of a rapidly evolving source whose morphology, chemistry, and ionization state are all changing on humanly accessible timescales [1409.2531] [1506.05508].

Source: https://www.emergentmind.com/topics/crl-618