GoKawiilThe Helix Nebula (NGC 7293) is one of the closest and brightest planetary nebulae, and therefore a benchmark for resolving how the late-stage ejecta of stars couple to their surroundings. Using the Gaia EDR3/DR3 astrometric solution for the central star (WD 2226-210), we adopt a distance of d = 198.6 (+1.6/−1.8) pc (ref. 10). Imaging of the Helix has shown it to be highly complex. Its bright main nebula comprises an inner disc and a surrounding outer torus, embedded within a larger structure whose upstream side is truncated, consistent with interaction between the expanding asymptotic giant branch (AGB) ejecta and the ambient interstellar medium (ISM)11,12,13. The ionized nebula is threaded by thousands of dense cometary knots and associated molecular material, indicating that much of the ejected material remains in a clumpy, only partially processed phase11,14. Deep imaging and spectroscopy have also revealed a bow-shock feature in the faint outer halo, in the direction of the nebula’s motion through the local ISM12,15. Together, these properties make the Helix uniquely suited to place direct constraints on how fragmented stellar ejecta are dispersed and mixed into the ISM: an essential step in the recycling of mass, dust and newly synthesized elements in galaxies4,9.
The Helix Nebula was observed in the light of Hα, [N ii] and [O iii] with the partially built Modular Optical Telephoto Hyperspectral Robotic Array (MOTHRA). MOTHRA is an array of high-end telephoto lenses equipped with tiltable ultra-narrow interference filters, located at the El Sauce Observatory in Chile. Its design evolved from the Dragonfly Spectral Line Mapper at New Mexico Skies Observatory16,17. When completed, MOTHRA will have 1,140 lenses distributed over 30 mounts, and be optically equivalent to a 4.8 m f/0.08 refractor. The data described here are equivalent to roughly 20 minutes of on-source exposure time with the completed array.
The MOTHRA Hα image of the Helix is shown in Fig. 1. It shows many features that have not been seen in ionized gas before, such as extensions of the plumes in the northwest and southeast11,13 and turbulent and complex low surface brightness Hα emission in the southwest13. The most striking feature in the Hα image is a forest of arcs and partial arcs on the eastern side of the nebula. We identify at least 22 arcs on the eastern side, labelled 1–22 in Fig. 3 in order of increasing distance from the central white dwarf. Although most of the features appear to be new discoveries, several can be seen in previous GALEX and Hα images. Besides the large and complex feature 14 this includes arcs 3, 10 and 13, among others11,13,15. The arcs are undetected in [O iii] and faint in [N ii]; from the brightest region of arc 14 we measure [N ii]/Hα = 0.06 ± 0.01 and [O iii]/Hα < 0.015 (2σ). We also find faint arc-like features on the western side, at a similar distance from the white dwarf as the much brighter ones in the east.
Fig. 1: MOTHRA Hα imaging of the Helix Nebula. Full size image The MOTHRA continuum-subtracted Hα image is shown with an inverted grey scale, emphasizing faint outer features. In the bright central regions a combined Hubble Space Telescope and Kitt Peak 4 m image is superposed on the MOTHRA data11. The Hubble Space Telescope image was generated from data in the Advanced Camera for Surveys F502N ([O iii]) and F658N (Hα) filters. The arrow indicates the Gaia-determined direction of motion of the central white dwarf with respect to the ambient gas. The MOTHRA image shows many features at large (roughly greater than 1 pc) distances from the white dwarf that had not been detected in Hα before. The most striking of these are numerous bow shocks on the east side of the nebula, where AGB ejecta encounter the ambient ISM at supersonic relative velocities. Scale bar, 5′ = 0.29 pc. The colour Hubble Space Telescope/Kitt Peak image is reproduced from NASA, ESA, C. R. O’Dell (Vanderbilt University) and M. Meixner, P. McCullough and G. Bacon (Space Telescope Science Institute).
Following earlier studies we interpret the eastern features as bow shocks in which expanding stellar ejecta encounter the ISM at supersonic velocities13,15. The velocity of the Helix with respect to the ISM is roughly 36 km s−1 towards 95° east of north in the plane of the sky and roughly 27 km s−1 along the line of sight15,18, for a combined ISM velocity of v ISM ≈ 45 km s−1 with respect to the systemic velocity of the nebula. On the eastern side the shock velocity is the sum of v ISM and the expansion velocity of the ejecta, v shock, e ≈ |v ISM + v exp |. On the western side the ejecta encounter a turbulent postshock wake that has passed through, and mixed with, the planetary nebula, v shock, w ≈ |v wake − v exp |. The wake velocity is expected to be small with respect to the systemic velocity of the nebula. In the case of the well-studied AGB star Mira, the processed gas being shed from the bow shock into the immediate downstream tail lags the star by only roughly 10% (ref. 19); applying the same scaling to the Helix gives v wake = 0−10 km s−1 for the flow on the western side.
Using the MAPPINGS V code20 we derive shock velocities on the eastern side of v shock, e = 80−90 km s−1 from the [O ii]/Hα and [N ii]/Hα line ratios (Methods). Subtracting the 45 km s−1 ISM velocity gives an expansion velocity of the ejecta of v exp = 35−45 km s−1, consistent with previously measured Hα kinematics in the region of the brightest bow15. The implied velocity field around the Helix is shown in Fig. 2. Near the east–west axis the shock velocities are roughly 80 km s−1 in the east and roughly 35 km s−1 in the west. For these velocities, shock models predict Hα luminosities that are 1 to 2 orders of magnitude fainter in the west than in the east, consistent with the appearance of the bows on the two sides of the Helix (Methods).
Fig. 2: Velocities experienced by fragments. Full size image Schematic velocity field around the Helix, for a bulk velocity with respect to the ISM of v ISM = 45 km s−1 eastward, a postshock flow velocity of v wake = 5 km s−1 and a radial expansion velocity of the ejecta of v exp = 40 km s−1. The highest velocities are found on the east side, where we see the strong bow shocks.
The expansion velocity of v exp = 35–45 km s−1 indicates a dynamical age of the clumps of 20,000–30,000 years at r ≈ 1 pc, predating the formation of the planetary nebula roughly 12,000 years ago21. The material therefore most likely belongs to an older circumstellar envelope that was ejected during the late-AGB phase, and has now fragmented into many individual clumps. This interpretation is strengthened by the fact that the bows lie at approximately the same radius as the roughly 40′ outer WISE 12-μm halo, which has been associated with dust from an AGB wind13. Fast winds and outflows, with velocities that can exceed the canonical roughly 5−20 km s−1 range of the main AGB phase22, are commonly observed during the late-AGB and early post-AGB phases23,24. These flows are often bipolar rather than isotropic25, which may explain why there appears to be a preferred axis connecting the strong bows in the east to northeast of the Helix to the weak bows in the west to southwest.
We fit the bow morphologies with a range of functional forms: parabolas, hyperbolas, ellipses and Wilkinoids26. We adopt the parabolic fits as our fiducial model, as they provide a reasonable description of the data and retain the same functional form under projection27. As many of the structures do not show a complete bow, and some are broader and less sharply bounded than ideal thin-shell bow shocks, we do not attach direct dynamical meaning to the adopted fit family itself. Instead, we use the fits to extract geometric quantities (in particular the characteristic curvature scale), and use the variation between fit families as an estimate of the systematic uncertainty. The fitting procedure is detailed in the Methods, with the results shown in Fig. 3. Most of the 22 bows are reasonably well fit by a parabola, although the wings are often closer to hyperbolic27. The brightest bow (14) is more sharply peaked than the model curve; inspection of the region near the apex shows that it is broken up in a complex network of smaller shocks.
Fig. 3: Parabolic profile fits to bow shocks. Full size image Twenty-two complete and partial bow shocks are identified in the Hα image. They are fitted with parabolas, indicated with the red lines (Methods). Red dots indicate the foci of the parabolas; these correspond to the expected approximate locations of the objects that produce the shocks. The lack of Hα detections near the foci indicates that the objects producing the bows are largely neutral. The bows are numbered according to the distance of the apex to the central star. Two known features, the NE Object and the NE Arc11, are also marked.
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