Discovery of Raman-scattered lines in the massive luminous emission-line star LHA 115-S 18

TL;DR

This paper reports the first detection of Raman-scattered lines in the spectrum of the peculiar emission-line star LHA 115-S 18, providing new insights into its classification and physical properties.

Contribution

The discovery of Raman-scattered lines in LHA 115-S 18 is a novel finding that may help clarify its controversial classification.

Findings

First detection of Raman-scattered lines in a B[e] star

Identification of lines at 6825 Å and 7082 Å as Raman-scattered features

Potential implications for understanding the star's nature

Abstract

LHA 115-S 18 is a very peculiar emission-line star exhibiting the B[e] phenomenon. Located in the Small Magellanic Cloud, its spectrum shows features of an extremely wide range of excitation and ionization stages, extending from highly ionized atomic lines (Si IV, C IV, He II) in the UV and optical regions to molecular emission bands of CO and TiO in the optical and IR regions. The most distinguishing spectral characteristic of LHA 115-S 18 is the high variability detected in the He II {\lambda}4686 emission line, which can be a very conspicuous or completely invisible feature. In this work, we report on another peculiarity of LHA 115-S 18. From high-resolution optical spectra taken between 2000 and 2008, we discovered the appearance and strengthening of two emission features at {\lambda}6825 \AA, and {\lambda}7082 \AA,, which we identified as Raman-scattered lines. This is the first time these lines have been detected in the spectrum of a massive luminous B[e] star. As the classification of LHA 115-S 18 is highly controversial, we discuss how the discovery of the appearance of Raman-scattered lines in this peculiar star might help us to solve this puzzle.