Early Results from the Galactic O-Star Spectroscopic Survey: C III Emission Lines in Of Spectra

TL;DR

This paper introduces the Ofc spectral category characterized by C III emission lines in Galactic O stars, discusses their relation to other spectral features, and presents new extreme Of?p stars, enhancing understanding of stellar physics.

Contribution

It defines the new Ofc spectral category, explores its relation to other emission features, and reports discovery of two new extreme Of?p stars, expanding the classification and understanding of O star spectra.

Findings

Introduction of the Ofc spectral category with C III emission lines

Correlation of C III emission with spectral type O5 and cluster associations

Discovery of two new extreme Of?p stars, NGC 1624-2 and CPD -28°2561

Abstract

On the basis of an extensive new spectroscopic survey of Galactic O stars, we introduce the Ofc category, which consists of normal spectra with C III \lambda\lambda4647-4650-4652 emission lines of comparable intensity to those of the Of defining lines N III \lambda\lambda4634-4640-4642. The former feature is strongly peaked to spectral type O5, at all luminosity classes, but preferentially in some associations or clusters and not others. The relationships of this phenomenon to the selective C III \lambda5696 emission throughout the normal Of domain, and to the peculiar, variable Of?p category, for which strong C III \lambda\lambda4647-4650-4652 emission is a defining characteristic, are discussed. Magnetic fields have recently been detected on two members of the latter category. We also present two new extreme Of?p stars, NGC 1624-2 and CPD -28^{\circ}2561, bringing the number known in the Galaxy to five. Modeling of the behavior of these spectral features can be expected to better define the physical parameters of both normal and peculiar objects, as well as the atomic physics involved.