New X-ray Detections of WNL Stars

TL;DR

This study reports new X-ray detections of WNL stars WR 16 and WR 78, showing that X-ray emission occurs across all WN spectral types, with lower luminosities linked to slower wind speeds.

Contribution

It provides the first X-ray detections of WNL stars WR 16 and WR 78, expanding understanding of X-ray emission in late WN stars and its relation to stellar wind properties.

Findings

X-ray emission is present in WN stars across all spectral types.

WNL stars WR 16 and WR 78 show low X-ray luminosities.

X-ray luminosity correlates with wind kinetic energy.

Abstract

Previous studies have demonstrated that putatively single nitrogen-type Wolf-Rayet stars (WN stars) without known companions are X-ray sources. However, almost all WN star X-ray detections so far have been of earlier WN2 - WN6 spectral subtypes. Later WN7 - WN9 subtypes (also known as WNL stars) have proved more difficult to detect, an important exception being WR 79a (WN9ha). We present here new X-ray detections of the WNL stars WR 16 (WN8h) and WR 78 (WN7h). These new results, when combined with previous detections, demonstrate that X-ray emission is present in WN stars across the full range of spectral types, including later WNL stars. The two WN8 stars observed to date (WR 16 and WR 40) show unusually low X-ray luminosities (Lx) compared to other WN stars, and it is noteworthy that they also have the lowest terminal wind speeds (v_infty). Existing X-ray detections of about a dozen WN stars reveal a trend of increasing Lx with wind luminosity Lwind = (1/2) M_dot v_infty^2, suggesting that wind kinetic energy may play a key role in establishing X-ray luminosity levels in WN stars.