Application limit of the photocentre displacement to fundamental stellar parameters of fast rotators -- Illustration on the edge-on fast rotator Regulus

TL;DR

This study investigates the effectiveness and limitations of using photocentre displacements to determine fundamental parameters of fast-rotating stars, exemplified by the edge-on star Regulus, through simulations and observational data comparison.

Contribution

It establishes the conditions under which photocentre displacements can reliably constrain stellar parameters of fast rotators, enhancing interferometric analysis methods.

Findings

Photocentres can constrain key stellar parameters for unresolved fast rotators.

For marginally resolved objects, photocentres mainly determine the rotation axis position angle.

The study defines physical criteria for applying photocentre analysis to fast rotator observations.

Abstract

Differential Interferometry allows to obtain the differential visibility and phase, in addition to the spectrum. The differential phase contains important information about the structure and motion of stellar photosphere such as stellar spots and non-radial pulsations, and particularly the rotation. Thus, this interferometric observable strongly helps to constrain the stellar fundamental parameters of fast rotators. The spectro-astrometry mainly uses the photocentre displacements, which is a first approximation of the differential phase, and is applicable only for unresolved or marginally objects. We study here the sensitivity of relevant stellar parameters to the simulated photocentres using the SCIROCCO code: a semi-analytical algorithm dedicated to fast rotators, applied to two theoretical modeling stars based on Achernar and Regulus, in order to classify the importance of these parameters and their impact on the modeling. We compare our simulations with published VLTI/AMBER data. This current work sets the limits of application of photocentre displacements to fast rotators, and under which conditions we can use the photocentres and/or the differential phase, through a pre-established physical criterion. To validate our theoretical study, we apply our method of analysis on observed data of the edge-on fast rotator Regulus. For unresolved targets, with a visibility $V\sim 1$, the photocentre can constrain the main stellar fundamental parameters of fast rotators, whereas from marginally resolved objects ($0.8 \leq V < 1$), mainly the rotation axis position angle ($\rm PA_{\rm rot}$) can be directly deduced from the vectorial photocentre displacement, which is very important for young cluster studies.