Properties of the ionized gas in HH 202 - II. Results from echelle spectrophotometry with Ultraviolet Visual Echelle Spectrograph

Abstract

We present results of deep echelle spectrophotometry of the brightest knot of the Herbig–Haro object HH 202 in the Orion Nebula – HH 202‐S – using the Ultraviolet Visual Echelle Spectrograph in the spectral range from 3100 to 10 400 Å. The high spectral resolution of the observations has permitted to separate the component associated with the ambient gas from that associated with the gas flow. We derive electron densities and temperatures from different diagnostics for both components, as well as the chemical abundances of several ions and elements from collisionally excited lines, including the first determinations of Ca+ and Cr+ abundances in the Orion Nebula. We also calculate the He+, C2+, O+ and O2+ abundances from recombination lines. The difference between the O2+ abundances determined from collisionally excited and recombination lines – the so‐called abundance discrepancy factor – is 0.35 and 0.11 dex for the shock and nebular components, respectively. Assuming that the abundance discrepancy is produced by spatial variations in the electron temperature, we derive values of the temperature fluctuation parameter, t2, of 0.050 and 0.016 for the shock and nebular components, respectively. Interestingly, we obtain almost coincident t2 values for both components from the analysis of the intensity ratios of He i lines. We find significant departures from case B predictions in the Balmer and Paschen flux ratios of lines of high principal quantum number n. We analyse the ionization structure of HH 202‐S, finding enough evidence to conclude that the flow of HH 202‐S has compressed the ambient gas inside the nebula trapping the ionization front. We measure a strong increase of the total abundances of nickel and iron in the shock component, the abundance pattern and the results of photoionization models for both components are consistent with the partial destruction of dust after the passage of the shock wave in HH 202‐S.

Radiative rates for forbidden M1 and E2 transitions of astrophysical interest in doubly ionized iron-peak elements

“…Recent Hubble Space Telescope/Space Telescope Imaging Spectograph (HST/STIS) observations from the Weigelt blobs of Eta Carinae (η Car) have revealed several forbidden lines of Fe III and Ni III (Zethson et al 2012). Lines of doubly ionized species have also been detected in various galactic sources, such as Herbig-Haro objects in the Orion Nebula (Mesa-Delgado et al 2009) and extragalactic objects, including active galactic nuclei (Vestergaard & Wilkes 2001). Reliable radiative data are therefore essential for interpreting these observations and obtaining a diagnostic of the physical conditions in the astrophysical plasma.…”

Carbon and oxygen abundances from recombination lines in low-metallicity star-forming galaxies. Implications for chemical evolution★

“…According to Mesa-Delgado et al (2009) the fraction of O embedded in dust grains in the Orion Nebula is about 0.12 dex. Peimbert & Peimbert (2010) have estimated that the depletion of O increases with increasing O/H.…”

Emission Line Ratios of FE III as Astrophysical Plasma Diagnostics

“…Emission lines arising from transitions among the finestructure levels of the 3d 6 configuration of Fe III are widely observed in the optical spectra of astrophysical sources, including planetary nebulae, H II regions, and quasars (see, for example, Garstang et al 1978;Keenan et al 1993;Ryans et al 2003;Mesa-Delgado et al 2009 and references therein). These Fe III transitions are also important tracers of Fe abundance in the case of H II regions and lowly ionized planetary nebulae, where they are often the only ionization state of Fe detected in the optical band.…”