VLA H92α and H115β Recombination Line Observations of the Galactic Center HiiRegions: The Sickle (G0.18−0.04) and the Pistol (G0.15−0.05)

Abstract

The Very Large Array has been used in the CnB and DnC configurations to observe the remarkable Galactic center sources, the Sickle and the Pistol, near l = 018, b = -004. These H II regions have an unusual morphology and may be physically associated with the linear nonthermal filaments at l = 018, which appear to intersect the sources. The H92α (8.31 GHz) and H115β (8.43 GHz) radio recombination lines arising from these sources have been observed with angular resolutions of 6'' and 9'', respectively. In addition, there is a probable detection of the helium line in the Pistol (Y+ ~ 14% ± 6%), while in the Sickle, no helium lines were detected with an upper limit of Y+ ~ 5% (3 σ). A complex velocity field has been observed in both sources. The LSR velocity in the Sickle varies from 0 to 75 km s-1, with an average velocity of ~35 km s-1; the average velocity of the Pistol is ~115 km s-1. The recombination line properties of the Sickle and the Pistol (FWHM line widths, line-to-continuum ratios, β-to-α ratio) are consistent with photoionization from hot stars. The β-to-α line ratio of ~0.35 ± 0.07 over most of the Sickle and Pistol does not differ significantly from the LTE value of 0.28. The average LTE electron temperature, T*e, for the Sickle (~5500 K) is similar to typical Galactic center H II regions, and T*e for the Pistol is somewhat lower (~3600 K). An additional H II component in the line profiles of three regions of the Sickle in which the nonthermal filaments are present suggests that an interaction between the ionized gas and the nonthermal filaments is occurring. The probable detection of He92α in the Pistol and the nondetection in the Sickle may be due to a difference in the radiation field of the ionizing sources.