Evolutionary status of the Of?p star HD 148937 and of its surrounding nebula NGC 6164/5

Abstract

Aims. The magnetic star HD 148937 is the only Galactic Of?p star surrounded by a nebula. The structure of this nebula is particularly complex and is composed, from the center out outwards, of a close bipolar ejecta nebula (NGC 6164/5), an ellipsoidal wind-blown shell, and a spherically symmetric Strömgren sphere. The exact formation process of this nebula and its precise relation to the star's evolution remain unknown. Methods. We analyzed infrared Spitzer IRS and far-infrared Herschel/PACS observations of the NGC 6164/5 nebula. The Herschel imaging allowed us to constrain the global morphology of the nebula. We also combined the infrared spectra with optical spectra of the central star to constrain its evolutionary status. We used these data to derive the abundances in the ejected material. To relate this information to the evolutionary status of the star, we also determined the fundamental parameters of HD 148937 using the CMFGEN atmosphere code. Results. The Hα image displays a bipolar or "8"-shaped ionized nebula, whilst the infrared images show dust to be more concentrated around the central object. We determine nebular abundance ratios of N/O = 1.06 close to the star, and N/O = 1.54 in the bright lobe constituting NGC 6164. Interestingly, the parts of the nebula located further from HD 148937 appear more enriched in stellar material than the part located closer to the star. Evolutionary tracks suggest that these ejecta have occured ∼ 1.2 − 1.3 and ∼ 0.6 Myrs ago, respectively. In addition, we derive abundances of argon for the nebula compatible with the solar values and we find a depletion of neon and sulfur. The combined analyses of the known kinematics and of the new abundances of the nebula suggest either a helical morphology for the nebula, possibly linked to the magnetic geometry, or the occurrence of a binary merger.

A remarkable change of the spectrum of the magnetic Of?p star HD 148937 reveals evidence of an eccentric, high-mass binary

“…As pointed out by Mahy et al (2017), HD 148937 is the only magnetic O-type star known to be surrounded by a nebula that can be attributed to mass loss. θ 1 Ori C and NGC 1624-2 have surrounding nebulosities, but they are H ii regions associated with the star-forming environments of these stars.…”

“…However, the Hipparcos parallax they used is only significant at 3σ confidence. Given V = 6.7 and E(B − V) = 0.61 (Mahy et al 2017), adopting the Hipparcos distance the implied absolute magnitude of the system is M V = −3.3, which is somewhat faint for an O star (and another 0.75 mag fainter if we correct for a roughly equal-brightness companion).…”

“…And it may well be that both binarity and magnetism contribute to the observed nebular characteristics, e.g. binarity as the origin, with the magnetic field sculpting the "figure 8" shape described by Mahy et al (2017). Moreover, wide-field imagery of the ISM and nebulosity adjacent to HD 148937 (see apod.nasa.gov/apod/ap160330.html and the analysis by Romero 2006) may be indicative of larger-scale and longer-timescale interactions of the system.…”

“…Sana et al (2014) further detected a faint (∆H = 5.39 ± 0.15) companion at 3.30±0.06 arcsec. Interestingly, none of the previous spectroscopic and imaging studies of the system (Nazé et al 2008(Nazé et al , 2010Wade et al 2012;Mahy et al 2017) have reported any direct evidence of binarity.…”

“…Finally, HD 148937 is distinguished from other magnetic O stars by its remarkable bipolar nebula, NGC 6164/5 (see Mahy et al 2017, and references therein). Recently, Mahy et al (2017) analyzed Spitzer and Herschel observations of the NGC 6164/5 nebula to constrain its global morphology, kinematics and abundances, and the evolutionary status of the central star. They concluded that the structure of the nebula is particularly complex and is composed of a close bipolar ejecta nebula, an ellipsoidal wind-blown shell, and a spherically symmetric Strömgren sphere.…”