Probing the Physical Conditions and Star Formation Processes in the Galactic H II Region S305

Abstract

Abstract We present multiscale and multiwavelength observations of the Galactic H ii region S305, which is excited by massive O8.5V and O9.5V stars. Infrared images reveal an extended sphere-like shell (extension ∼7.5 pc; at T d = 17.5–27 K) enclosing the S305 H ii region (size ∼5.5 pc; age ∼1.7 Myr). The extended structure observed in the Herschel temperature map indicates that the molecular environment of S305 is heated by the massive O-type stars. Regularly spaced molecular condensations and dust clumps are investigated toward the edges of the infrared shell, where the polycyclic aromatic hydrocarbon (PAH) and H2 emission is also observed. The molecular line data show a signature of an expanding shell of molecular gas in S305. Giant Metrewave Radio Telescope 610 and 1280 MHz continuum maps reveal overdensities of the ionized emission distributed around two O-type stars, which are surrounded by the horseshoe envelope (extension ∼2.3 pc). A molecular gas deficient region/cavity is identified toward the center of the horseshoe envelope, which is well traced with PAH, H2, molecular, and dust emission. The edges of the infrared shell are found to be located in the front of the horseshoe envelope. All these outcomes provide the observational evidence of the feedback of O-type stars in S305. Moreover, nonthermal radio emission is detected in S305 with an average spectral index α ∼ −0.45. The variations in α, ranging from −1.1 to 1.3, are explained due to soft synchrotron emission and either optically thicker thermal emission at high frequencies or a suppression of the low-frequency emission by the Razin–Tsytovich effect.