Combined spectroscopy and intensity interferometry to determine the distances of the blue supergiants P Cygni and Rigel

Abstract

In this paper we report on spatial intensity interferometry measurements within the Hα line on two stars: the Luminous Blue Variable supergiant P Cygni and the late-type B supergiant Rigel. The experimental setup was upgraded to allow simultaneous measurement of two polarization channels, instead of one in our previous setup, and the zero baseline correlation function on-sky to validate independent estimates obtained from the stellar spectrum and the instrumental spectral throughput. Combined with simultaneous spectra measurements and based on radiative transfer models calculated with the code CMFGEN, we were able to fit our measured visibility curves to extract the stellar distances. Our distance determinations for both P Cygni (1.61 ± 0.18 kpc) and Rigel (0.26 ± 0.02 kpc) agree very well with the values provided by astrometry with the Gaia and Hipparcos missions, respectively. This result for Rigel was obtained by adopting a stellar luminosity of L⋆ = 123000 L⊙, which is reported in the literature as being consistent with the Hipparcos distance to Rigel. However, due to the lack of consensus on Rigel’s luminosity, we also explore how the adoption of the stellar luminosity in our models affects our distance determination for Rigel. In conclusion, we support, in an independent way, the distance to Rigel as the one provided by the Hipparcos mission, when taking the luminosity of 123000 L⊙ at face value. This study is the first successful step towards extending the application of the Wind Momentum Luminosity Relation method for distance calibration from an LBV supergiant to a more normal late-type B supergiant.

Field and intensity correlations: the Siegert relation from stars to quantum emitters

“…This result is superimposed in Fig. 4b to the g (2) measurements performed on-sky between 3 August 2020 and 9 August 2020 [42]. Again, a good overlap is observed, illustrating the validity of the Siegert relation.…”

“…Combined to spatial interferometry, this can be used to determine the angular diameter of the source. This is especially interesting to perform such measurements on emission lines to characterize the extended atmosphere of the star, which can give access to other fundamental parameters such as its distance [42,46].…”

“…2 and described in detail in Ref. [42]. Very briefly, the light is first collected by a 1 m telescope at the C2PU facility on the Plateau de Calern site of Observatoire de la Côte d'Azur (OCA).…”

“…In association with spectral measurement and radiative transfer modeling, the distance of the star can be inferred, as shown in Refs. [42,46].…”

Intensity interferometry at Calern and beyond: progress report

“…9 Using an improved version of our instrument, this technique was reapplied to P Cyg and additionally to β Ori. 10 The long term goal of our group is to continue the progress made thus far towards an instrument which can perform interferometric measurements in visible wavelengths (B, V, R, I photometric bands) with sensitivity to stars brighter than at least magnitude 8 if installed onto major facilities.…”