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Submillimeter Array and Very Large Array Observations in the Hypercompact HII region G35.58-0.03

The formation of hypercompact (HC) HII regions is an important stage in massive star formation. Spectral line and continuum observations can explore its dynamic conditions. We present high angular resolution observations carried out with the Submillimeter Array (SMA) and the Very Large Array (VLA) toward the HC HII region G35.58-0.03. With the 1.3 mm SMA and 1.3 cm VLA, we detected a total of about 25 transitions of 8 different species and their isotopologues (CO, CH$_3$CN, SO$_2$, CH$_3$CCH, OCS, CS, H, and NH$_{3}$). G35.58-0.03 consists of an HC HII core with electron temperature $T_e^*\geqq5500$ K, emission measure EM $\approx 1.9\times10^{9}$ pc cm$^{-6}$, local volume electron density $n_e=3.3\times10^{5}$ cm$^{-3}$, and a same width of radio recombination line FWHM $\approx$ 43.2 km s$^{-1}$ for both H30$α$ and H38$β$ at its intrinsic core size $\sim$3714 AU. The H30$α$ line shows evidence of an ionized outflow driving a molecular outflow. Based on the derived Lyman continuum flux, there should be an early-type star equivalent to O6.5 located inside the HII region. From the continuum spectral energy distribution from 3.6 cm, 2.0 cm, 1.3 cm, 1.3 mm and 0.85 mm to 0.45 mm, we distinguished the free-free emission (25% $\sim$ 55%) from the warm dust component (75% $\sim$ 45%) at 1.3 mm. The molecular envelope shows evidence of infall and outflow with an infall rate 0.05 M$_{ \odot}$ yr$^{-1}$ and a mass loss rate $5.2\times10^{-3}$ ${\rm M_{ \odot} yr^{-1}}$. The derived momentum ($\sim0.05$ M$_{\odot}$ km s$^{-1}$) is consistent between the infalling and outflowing gas per year. It is suggested that the infall is predominant and the envelope mass of dense core is increasing rapidly, but the accretion in the inner part might already be halted.