The Sagittarius B2 Star-forming Region. III. High-Resolution H52 alpha and H66 alpha Observations of Sagittarius B2 Main

Abstract

The Galactic center star-forming region Sagittarius B2 Main [Sgr B2 (M)] has been observed with the Very Large Array in the continuum (7 mm and 1.3 cm) and in the radio recombination lines H52α, H66α, and He 66α. The H66α(∼0".25 resolution) and H52α(∼2".5 resolution) data provide high spatial resolution kinematics of the ionized gas associated with the massive stars. Both high spatial resolution and sensitivity to large-scale structures are essential to thoroughly sample the H II regions in Sgr B2 (M) where source sizes of the 36 H II regions range from unresolved (&lt;0".3) to extended (∼10"). H66α recombination line emission has been detected toward 23 of the 36 continuum sources in Sgr B2 (M). The mean velocity of the detected sources is 62±2 km s<SUP>-1</SUP>. He 66α line emission is detected toward nine of the 36 continuum sources. The 1.3 cm helium and hydrogen data are used together as a high-resolution probe of Y<SUP>+</SUP> values and variations within Sgr B2 (M). An average singly ionized helium abundance of &lt;Y<SUP>+</SUP>&gt; = 10.3%±1.2% is derived. Of the sources where a Y<SUP>+</SUP> value is determined, only Sgr B2 F has an anomalously low value (Y<SUP>+</SUP> ≤ 5%). <P />The recombination line data are examined in detail toward several of the most unusual sources. A very broad radio recombination line (ΔV<SUB>H66α</SUB> ∼ 64 km s<SUP>-1</SUP>, ΔV<SUB>H52α</SUB> ∼ 59 km s<SUP>-1</SUP>) is measured, and high electron temperatures [T<SUB>e(H66α)</SUB><SUP>*</SUP> = 26,400 K] are derived for Sgr B2 F. The broad lines and high temperatures may be due to a nonequilibrium environment at the edge of an expanding H II region. Assuming that the F and G H II regions have expanded in a dense molecular environment, they appear to be young, with an average age of &lt;τ&gt; = 0.7±0.3 x 10<SUP>4</SUP> yr. This lower limit on the age is consistent with the dynamical age of the nearby massive molecular outflow observed by Lis et al. A doubly peaked H66α line has been detected toward the ultracompact H II (UC HII) region Z10.24. This source may be either a very young (τ &lt; 100 yr) shell source or the ionized base of a molecular outflow. The small number of morphological cometary H II regions in Sgr B2 (&lt;5%) and the observed velocity gradients in those regions that do have a cometary morphology indicate that the moving star bow shock model cannot account for any of the H II regions in Sgr B2 (M).