Long-term spectroscopic monitoring of LBVs and LBV candidates

Abstract

We present results of a long-term spectroscopic monitoring program (since mid 2009) of Luminous Blue Variables with the new HERMES echelle spectrograph on the 1.2m Mercator telescope at La Palma (Spain). We investigate high-resolution (R = 80,000) optical spectra of two LBVs, P Cyg and HD 168607, the LBV candidates MWC 930 and HD 168625, and the LBV binary MWC 314. In P Cyg we observe flux changes in the violet wings of the Balmer Hα, Hβ, and He i lines between May and Sep 2009. The changes around 200 to 300 km s⁻¹ are caused by variable opacity at the base of the supersonic wind from the blue supergiant.

We observe in MWC 314 broad double-peaked metal emission lines with invariable radial velocities over time. On the other hand, we measure in the photospheric S ii λ5647 absorption line, with lower excitation energy of ~14 eV, an increase of the heliocentric radial velocity centroid from 37 km s⁻¹ to 70 km s⁻¹ between 5 and 10 Sep 2009 (and 43 km s⁻¹ on 6 Apr 2010). The increase of radial velocity of ~33 km s⁻¹ in only 5 days can confirm the binary nature of this LBV close to the Eddington luminosity limit.

A comparison with VLT-UVES and Keck-Hires spectra observed over the past 13 years reveals strong flux variability in the violet wing of the Hα emission line of HD 168625 and in the absorption portion of the Hβ line of HD 168607. In HD 168625 we observe Hα wind absorption at velocities exceeding 200 km s⁻¹ which develops between Apr and June 2010.