Hunting for Planetary Nebulae toward the Galactic Center

TL;DR

This study confirms two planetary nebulae near the Galactic center through near-infrared spectroscopy, revealing their properties, abundances, and likely location in the nuclear stellar disk, contributing to understanding stellar evolution in this region.

Contribution

First confirmation of planetary nebulae in the nuclear stellar disk near the Galactic center using multi-wavelength observations and abundance analysis.

Findings

Both objects are confirmed planetary nebulae based on spectral and abundance evidence.

Distances are estimated at approximately 8-9 kpc, placing them near Sgr A*.

These are likely the first planetary nebulae identified in the nuclear stellar disk.

Abstract

We present near-infrared (IR) spectra of two planetary nebula (PN) candidates in close lines of sight toward the Galactic center (GC) using the Gemini Near-Infrared Spectrograph (GNIRS) at Gemini North. High-resolution images from radio continuum and narrow-band IR observations reveal ringlike or barrel-shaped morphologies of these objects, and their mid-IR spectra from the Spitzer Space Telescope exhibit rich emission lines from highly-excited species such as [S IV], [Ne III], [Ne V], and [O IV]. We also derive elemental abundances using the Cloudy synthetic models, and find an excess amount of the $s$-process element Krypton in both targets, which supports their nature as PN. We estimate foreground extinction toward each object using near-IR hydrogen recombination lines, and find significant visual extinctions ($A_V > 20$). The distances inferred from the size versus surface brightness relation of other PNe are $9.0\pm1.6$ kpc and $7.6\pm1.6$ kpc for SSTGC 580183 and SSTGC 588220, respectively. These observed properties along with abundance patterns and their close proximity to Sgr A$^*$ (projected distances $<20$ pc) make it highly probable that these objects are the first confirmed PN objects in the nuclear stellar disk. The apparent scarcity of such objects resembles the extremely low rate of PN formation in old stellar systems, but is in line with the current rate of the sustained star formation activity in the Central Molecular Zone.