Far-Ultraviolet Observations of the Spica Nebula and the Interaction Zone

TL;DR

This study uses far-ultraviolet observations and radiative transfer simulations to analyze the dust scattering properties and structure of the Spica Nebula and its interaction zone, revealing dust parameters and emission origins.

Contribution

It provides new measurements of dust scattering parameters and the three-dimensional structure of the interaction zone around Spica using FUV data and Monte Carlo simulations.

Findings

Dust albedo is 0.38±0.06, asymmetry factor is 0.46±0.06.

Distance to the interaction zone is approximately 70 pc, with a thickness of about 40 pc.

C IV emission is widespread, while Si II* emission is confined to the H II region.

Abstract

We report the analysis results of far ultraviolet (FUV) observations, made for a broad region around $\alpha$ Vir (Spica) including the interaction zone of Loop I and the Local Bubble. The whole region was optically thin and a general correlation was seen between the FUV continuum intensity and the dust extinction, except in the neighborhood of the bright central star, indicating the dust scattering nature of the FUV continuum. We performed Monte-Carlo radiative transfer simulations to obtain the optical parameters related to the dust scattering as well as the geometrical structure of the region. The albedo and asymmetry factor were found to be 0.38$\pm$0.06 and 0.46$\pm$0.06, respectively, in good agreement with the Milky Way dust grain models. The distance to and the thickness of the interaction zone were estimated to be 70$^{+4}_{-8}$ pc and 40$^{+8}_{-10}$ pc, respectively. The diffuse FUV continuum in the northern region above Spica was mostly the result of scattering of the starlight from Spica, while that in the southern region was mainly due to the background stars. The \ion{C}{4} $\lambda\lambda$1548, 1551 emission was found throughout the whole region, in contrast to the \ion{Si}{2}* $\lambda$1532 emission which was bright only within the \ion{H}{2} region. This indicates that the \ion{C}{4} line arises mostly at the shell boundaries of the bubbles, with a larger portion likely from the Loop I than from the Local Bubble side, whereas the \ion{Si}{2}* line is from the photoionized Spica nebula.