XSHOOTER spectroscopy of the enigmatic planetary nebula Lin49 in the Small Magellanic Cloud

TL;DR

This study presents a detailed spectroscopic analysis of the planetary nebula Lin49 in the Small Magellanic Cloud, revealing its chemical composition, central star properties, and near-IR excess likely due to small particles or dense structures.

Contribution

It provides the first comprehensive modeling of Lin49's nebular and stellar properties, highlighting the near-IR excess and its possible origins in C60-rich planetary nebulae.

Findings

Lin49 is C-rich and metal-deficient with abundances matching AGB models.

The photoionisation model fits most spectral data but not the near-IR excess.

The near-IR excess may be due to small particles or high-density structures.

Abstract

We performed a detailed spectroscopic analysis of the fullerene C60-containing planetary nebula (PN) Lin49 in the Small Magellanic Cloud using XSHOOTER at the ESO VLT and the Spitzer/IRS instruments. We derived nebular abundances for nine elements. We used TLUSTY to derive photospheric parameters for the central star. Lin49 is C-rich and metal-deficient PN (Z~0.0006). The nebular abundances are in good agreement with Asymptotic Giant Branch nucleosynthesis models for stars with initial mass 1.25 Msun and metallicity Z = 0.001. Using the TLUSTY synthetic spectrum of the central star to define the heating and ionising source, we constructed the photoionisation model with CLOUDY that matches the observed spectral energy distribution (SED) and the line fluxes in the UV to far-IR wavelength ranges simultaneously. We could not fit the ~1-5 um SED using a model with 0.005-0.1 um-sized graphite grains and a constant hydrogen density shell owing to the prominent near-IR excess, while at other wavelengths the model fits the observed values reasonably well. We argue that the near-IR excess might indicate either (1) the presence of very small particles in the form of small carbon clusters, small graphite sheets, or fullerene precursors, or (2) the presence of a high-density structure surrounding the central star. We found that SMC C60 PNe show a near-IR excess component to lesser or greater degree. This suggests that these C60 PNe might maintain a structure nearby their central star.