Hot and cool: two emission-line stars with constrasting behaviours in the same XMM-Newton field

TL;DR

This study presents X-ray spectral analysis of two stars with contrasting behaviors, revealing wind-shock emission in an O-type star and coronal activity in a dMe star, using XMM-Newton observations for the first time.

Contribution

First spectral analysis of HD155806 and 1RXSJ171502.4-333344, demonstrating different X-ray emission mechanisms in stars within the same field.

Findings

HD155806's X-ray spectrum is soft and consistent with wind-shock models.

1RXSJ171502.4-333344 shows coronal X-ray emission with high FIP element abundance.

Different stellar behaviors are confirmed through spectral properties.

Abstract

High-energy emissions are good indicators of peculiar behaviours in stars. We have therefore obtained an XMM-Newton observation of HD155806 and 1RXSJ171502.4-333344, and derived their spectral properties for the first time. The X-ray spectrum of HD155806 appears soft, even slightly softer than usual for O-type stars (as shown by a comparison with the O9 star HD155889 in the same XMM field). It is well-fitted with a two-component thermal model with low temperatures (0.2 and 0.6 keV), and it shows no overluminosity (log[LX/Lbol]=-6.75). The high-resolution spectrum, though noisy, reveals a few broad, symmetric X-ray lines (FWHM ~ 2500 km/s). The X-ray emission is compatible with the wind-shock model and therefore appears unaffected by the putative dense equatorial regions at the origin of the Oe classification. 1RXSJ171502.4-333344 is a nearby flaring source of moderate X-ray luminosity (log[LX/Lbol]=-3), with a soft thermal spectrum composed of narrow lines and presenting a larger abundance of elements (e.g. Ne) with a high first ionization potential (FIP) compared to lower-FIP elements. All the evidence indicates a coronal origin for the X-ray emission, in agreement with the dMe classification of this source.