Serendipitous Discovery of a Faint Planetary Nebula in the Massive Young LMC Cluster NGC 1866

TL;DR

We report the serendipitous discovery of Ka LMC 1, a faint planetary nebula in the massive young LMC cluster NGC 1866, identified in VLT/MUSE integral-field data. The PN’s location near the cluster core and a radial velocity of $+301.5$ km s$^{-1}$, consistent with the cluster, together with a cluster age of $200$ Myr implying a progenitor mass of about $3.9\,M_\odot$, strongly support cluster membership. Ka LMC 1 shows a classic ring morphology with a diameter of about $6''$ (≈ $1.3$ pc) and a dynamical age near $1.8\times 10^{4}$ yr, with an integrated spectrum rich in [N II] indicating a Type I PN; HST imaging reveals a blue central star, and comparison with post-AGB evolutionary tracks yields mixed agreement, suggesting ongoing refinements to late-stage stellar evolution models. The analysis points to substantial dust extinction within the nebula and hints that the central star may suffer little reddening if viewed nearly pole-on, motivating targeted follow-up observations to constrain PN evolution in clusters and inform stellar-velocity and nucleosynthesis studies.

Abstract

During an integral-field spectroscopic study of stars in the massive young open cluster NGC 1866 in the Large Magellanic Cloud, we serendipitously discovered a faint planetary nebula (PN). We designate it "Ka LMC 1," and find that its location near the cluster center, along with the agreement of its radial velocity with that of the cluster, imply a high probability of membership in NGC 1866. The 200 Myr age of the cluster indicates that the PN's progenitor star had an initial mass of about 3.9 Msun. The integrated spectrum of Ka LMC 1 shows strong emission lines of [N II], consistent with it being a "Type I" nitrogen-rich PN. The nebula exhibits a classical ring morphology, with a diameter of ~6", corresponding to an advanced expansion age of about 18,000 yr. Archival images of NGC 1866 obtained with the Hubble Space Telescope reveal a faint blue central star. Comparison of the star's luminosity with predictions from one set of theoretical post-asymptotic-giant-branch evolutionary tracks for single stars implies an age roughly consistent with the dynamical age of the PN, but the agreement with alternative modern tracks is much poorer. Analysis of the emission-line spectrum suggests considerable dust extinction within the nebula; however the central star possibly suffers little reddening because we may be viewing it nearly pole-on in a bipolar PN. Our accidental discovery was made using data that are not ideal for study of Ka LMC 1; we suggest several avenues of future targeted studies that would provide valuable and nearly unique new information for constraining models of late stellar evolution.

Figures (3)

• Figure 1: MUSE synthetic emission-line image of the LMC massive young cluster NGC 1866, showing the newly discovered PN Ka LMC 1---the red elliptical ring on the northwest side of the cluster with a major axis of $\sim$$6"$ (1.3 pc at the distance of the LMC). Color-coding is [N$\;$] $$6584 (red), H$$ (green), and [O$\;$] $$5007 (blue). All wavelengths shifted to the radial velocity of the LMC. Image dimensions are $60"\times60"$. The numerous green objects are Be stars, bright at H$$, in this young cluster that is rich in rapidly rotating upper-main-sequence stars.
• Figure 2: Monochromatic images of Ka LMC 1 after correction of stellar residuals at emission lines of [N$\;$] $6583$, [O$\;$] $5007$, H$$, H$$, and [S$\;$] $6716$, redshifted to the nebular radial velocity of +301.5 ${\rm km\,s^{-1}}$. Each frame has dimensions of $14\hbox{$.\!\!^{\prime\prime}$}6\times14\hbox{$.\!\!^{\prime\prime}$}6$. For reference, the upper-left frame shows the [N$\;$] image before correction for superposed field stars. Here the white artifacts have amplitudes between $-1146$ and $-4854$ flux units per pixel. The greyscale stretch extends from $-70$ to 3000 units (asinh scaling). Morphological features are indicated with colored circles: cyan = inner bubble, red = rim, blue = shell. The sky annulus for DAOPHOT background subtraction, used in creating the integrated spectrum of the nebula, is defined by the blue and magenta circles.
• Figure 3: Integrated spectrum of Ka LMC 1 within an annular aperture that encloses its bright rim. Flux density is plotted in units of $10^{-20}$erg cm$^{-2}$ s$^{-1}$ Å$^{-1}$. The top panel shows the brightest emission lines, and in the bottom panel the scale is expanded to show the weak lines. Selected emission lines are labeled in the top panel, and include H$$, H$$, [O$\;$] $4959,5007$, [N$\;$] $5754$, [O$\;$] $6300$, [N$\;$] $6548,6583$, [S$\;$] $6716,6731$, and [Ar III] $7136$. Wavelengths of a few lines of He$\;$, [N$\;$], and [O$\;$] that are weak or undetected are also marked.