Starless cloud core L1517B in envelope expansion with core collapse

Various spectral emission lines from the star-forming molecular cloud core L1517B manifest red asymmetric double-peaked profiles with stronger red peaks and weaker blue peaks, in contrast to the oft-observed blue-skewed molecular spectral line profiles with blue peaks stronger than red peaks. Invoking a spherically symmetric general polytropic hydrodynamic shock model for the envelope expansion with a core collapse (EECC) phase, we show the radial flow velocity, mass density, and temperature structures of a self-similar evolution for L1517B in a dynamically consistent manner. By prescribing simple radial profiles of abundance distribution for pertinent molecules, we perform molecular excitation and radiative transfer calculations using the publicly available RATRAN code set for the spherically symmetric case. Emphatically, the spectral profiles of line emissions from the same molecules but for different line transitions as well as spectra of closely pertinent isotopologues strongly constrain the self-similar hydrodynamics of a cloud core with prescribed abundances. Our computational results show that the EECC model reproduces molecular spectral line profiles in sensible agreement with the observational data of the Institut de Radioastronomie Millimétrique (IRAM), Five College Radio Astronomical Observatory, and Effelsberg 100 m telescopes for L1517B. We also report the spatially resolved observations of the optically thick line HCO⁺(1 - 0) using the Purple Mountain Observatory 13.7 m telescope at Delingha in China and the relevant fitting results. Hyperfine line structures of NH₃ and N₂H⁺ transitions are also fitted to consistently reveal the dynamics of the central core collapse. As a consistent model check, radial profiles of 1.2mm and 850 μm dust continua observed by the IRAM 30 m telescope and the Submillimeter Common-User Bolometer Array, respectively, are also fitted numerically using the same EECC model that produces the molecular line profiles. L1517B is likely undergoing an EECC shock phase. For future observational tests, we also predict several molecular line profiles with spatial distributions, the radial profile of the sub-millimeter continuum at wavelength 450 μm, as well as the radial profiles of the column density and visual extinction for L1517B.