GTC Spectroscopic Surveys of Planetary Nebulae in the Milky Way and M31

TL;DR

This work presents GTC spectroscopic surveys of planetary nebulae in the Milky Way and M31, obtaining high-quality optical–NIR spectra for 24 compact Galactic PNe and ~30 halo PNe in M31. By combining these data with archival UV and mid-IR observations, the authors perform and plan extensive photoionization modeling (e.g., with Cloudy) to derive central-star properties and accurate abundances, enabling robust calibrations of ionization correction factors and constraints on Galactic abundance gradients. The M31 halo PNe show near-solar oxygen abundances, supporting a scenario where disk-formed, metal-rich gas was displaced into the halo, with implications for halo assembly and interaction histories. Overall, the study advances AGB nucleosynthesis constraints, circumgalactic chemical evolution, and the link between stellar evolution and galaxy dynamics through multi-wavelength spectroscopy and modeling.

Abstract

We report spectroscopic surveys of planetary nebulae (PNe) in the Milky Way and Andromeda (M31), using the 10.4-m Gran Telescopio Canarias (GTC). The spectra are of high quality and cover the whole optical range, mostly from 3650 Å to beyond 1 micron, enabling detection of nebular emission lines critical for spectral analysis as well as photoionization modeling. We obtained GTC spectra of 24 compact (angular diameter <5 arcsec) PNe located in the Galactic disk, ~3-20 kpc from the Galactic centre, and can be used to constrain stellar evolution models and derive radial abundance gradients of the Milky Way. We have observed 30 PNe in the outer halo of M31 using the GTC. These halo PNe are uniformly metal-rich and probably all evolved from low-mass stars, consistent with the conjecture that they all formed from the metal-rich gas in M31 disk but displaced to their present locations due to galaxy interactions.

Figures (4)

• Figure 1: Spatial distribution of compact PNe (large, cyan dots) against the general distribution of Galactic PNe (small, black dots). Left: top-view of the Galactic disk. Right: galacocentric distance vs. distance from the Galactic plane. Images adopted from Stanghellini16.
• Figure 2: Left: GTC OSIRIS long-slit spectrum of Galactic planetary nebula PN G068.7$+$14.8; blue and red represent the R1000B and R1000R grisms, respectively. Right-top: $HST$ WFC3 F502N narrowband image of PN G068.7$+$14.8 in the [O iii] $\lambda$5007 emission line, adopted from Stanghellini16; the GTC long slit (white-dashed lines) with 1-arcsec width is overlaid. Right-bottom: Central star position of PN G068.7$+$14.8 (blue star) in H-R diagram, where model tracks of the H-burning post-AGB sequences calculated by Bertolami16 at $Z$ = 0.02 are overplotted, different line types representing different initial and final masses. The red curves are the isochrones for evolutionary ages ($\tau$ = 0, 5000, 10,000, 15,000, and 20,000 years) since the beginning of post-AGB defined at log$T_{\rm eff}$ = 3.85. The central star position of another Galactic compact nebula PN G048.5$+$04.2 (green star) is also presented for purpose of comparison of a different evolutionary status from that of PN G068.7$+$14.8. Top-inset: Zoom-in of the GTC spectrum showing the C iii and C iv lines possibly emitted from the central star, with a likely P-Cygni profile marked (Huang & Fang, in preparation).
• Figure 3: Left: Distribution of PNe in M31$+$M33. The PNe targeted by our GTC observations Fang13Fang15Fang18 are highlighted in red-filled circles and red asterisks. The inset shows the PNe in M33 in M31-centred coordinates. Right: GTC OSIRIS long-slit spectrum of a halo PN in M31 (marked by a red-dashed circle in the left panel) obtained with the R1000B (blue) and R1000R (red) grisms, with emission lines labeled.
• Figure 4: Radial distribution of oxygen abundances in M31 (figure adopted from Fang18). The M31 halo PNe previously targeted by our GTC observations are red-filled circles (see Figure \ref{['M31_PNe']}-left). The M31 outer-disk PNe (observed by Kwitter12, Balick13, and Corradi15) are black dots. Other literature samples of PNe, H ii regions and supergiants in M31 are over-plotted (see legend). The black-solid straight line is a linear fit to the disk PNe between 20 and 40 kpc Kwitter12. The red-dotted line marks the solar value. The horizontal black-dashed and dotted lines represent the mean metallicity and dispersion (also grey-shaded) of halo stars between 10 and 60 kpc Richardson09.