Fundamental Problems in Plasma Astrophysics: The Thermal Equilibrium of NGC 7009

Details Fundamental Problems in Plasma Astrophysics: The Thermal Equilibrium of NGC 7009 Fundamental Problems in Plasma Astrophysics: The Thermal Equilibrium of NGC 7009

Jul 1999

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999hst..prop.8114r&link_type=abstract

HST Proposal ID #8114

Physics

Hst Proposal Id #8114 Interstellar And Intergalactic Matter

Scientific paper

NGC 7009 has become the standard-bearer for a growing body of work that finds nebular abundances derived from recombination lines are up to factors of 5-15 larger than those obtained from the optical/UV collisionally excited lines. These can be forced into agreement at the higher value if large unexplained temperature fluctuations {t^2} exist. STIS long-slit observations will allow us to study the line-of-sight t^2 in addition to plane-of-the-sky variations, within the slit areas, thereby creating 3-dimensional t^2 information. Using a method we developed and recently applied to our Orion Nebula HST data, we will find t^2 in the O^++ and N^+ regions using the highly T_e-sensitive O III] 1663 and N II] 2142 lines; this provides information on how t^2 depends on level of ionization or excitation of the gas. We propose to complement the long-slit STIS spectra with WFPC2 images in several filters, most importantly in the 4364 A and 5008 A lines of O III, whose ratio can be used to diagnose T_e-fluctuations over the whole nebula on the microscopic scale {0.1''}. We have ISO IR spectroscopy of NGC 7009 to help further characterize electron densities and T_e macroscopically. Our goals are to understand the conditions within the nebula and the physical origin of the inferred t^2 and to find a prescription for determining reliable nebular abundances - fundamental to understanding the chemical evolution of the Galaxy.

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