Photoionized Herbig-Haro objects in the Orion Nebula through deep high-spectral resolution spectroscopy II: HH204

TL;DR

This study uses high-resolution spectroscopy and imaging to analyze the physical and chemical properties of HH 204 in the Orion Nebula, revealing the effects of shocks and ionization on its composition.

Contribution

It provides a detailed kinematic and chemical analysis of HH 204, highlighting the importance of resolving kinematic components for accurate abundance measurements.

Findings

High-ionization emission from HH 204 includes shock-excited gas.

Oxygen abundance discrepancy is zero in HH 204.

Dust destruction in the bowshock increases elemental abundances.

Abstract

We analyze the physical conditions, chemical composition and other properties of the photoionized Herbig-Haro object HH~204 through Very Large Telescope (VLT) echelle spectroscopy and Hubble Space Telescope (\textit{HST}) imaging. We kinematically isolate the high-velocity emission of HH~204 from the emission of the background nebula and study the sub-arcsecond distribution of physical conditions and ionic abundances across the HH object. We find that low and intermediate-ionization emission arises exclusively from gas at photoionization equilibrium temperatures, whereas the weak high-ionization emission from HH~204 shows a significant contribution from higher temperature shock-excited gas. We derive separately the ionic abundances of HH~204, the emission of the Orion Nebula and the fainter Diffuse Blue Layer.In HH~204, the O$^{+}$ abundance determined from Collisional Excited Lines (CELs) matches the one based on Recombination Lines (RLs), while the O$^{2+}$ abundance is very low, so that the oxygen abundance discrepancy is zero. The ionic abundances of Ni and Fe in HH~204 have similar ionization and depletion patterns, with total abundances that are a factor of 3.5 higher than in the rest of the Orion Nebula due to dust destruction in the bowshock. We show that a failure to resolve the kinematic components in our spectra would lead to significant error in the determination of chemical abundances (for instance, 40\% underestimate of O), mainly due to incorrect estimation of the electron density.