Ecole Doctorale Sciences Fondamentales et Appliquées
Université Nice Sophia Antipolis
UFR Sciences
Description of the thesis
These last years have brought an increasing number of astrophysical results using the
impressive capabilities of high angular resolution imaging permitted by the modern optical
arrays (CHARA, VLTI) and instruments combining from 4 to 6 telescopes. It opens not only
the road for the long term future of optical interferometry but generates new scientific
opportunities on the current facilities. In spite of a long-term effort of many stellar
astronomers, the current knowledge of accurate masses, radii and radiative parameters of
especially hotter stars and stars away from the main sequence is still less accurate than
what would be needed for really detailed tests of the stellar evolutionary models or a
calibration of the distance scale. The optical interferometry and spectro-interferometry
offers a new means to improve the situation, allowing deriving some basic physical
properties quite independently of the spectroscopic and photometric methods.
In a recent paper (Mourard et al., (2015), A&A 577, “Spectral and spatial imaging of the
Be+sdO binary ??Persei”) we have used the VEGA instrument in 4-telescope mode to
observe the complex system of ? Per. This has proven to be very efficient in terms of data
collection and the paradigm of limited (u,v) coverage has been broken.
The thesis project is to take benefit of these recent progresses to boost one of the engaged
observing program with Professor Harmanec in Praha on hierarchical systems for
accurate stellar parameters determination. Long term observations of ksi Tauri are now
finished and a paper will be published soon. This program has shown that we need
observations at various epochs of course but that it is much more efficient to collect the
maximum of constraints at a given epoch. Thus 4T mode will be perfect for that as it will
permit to collect enough data at a given epoch to strongly constrain our model fitting
tools.
In 2015 we secured observations of V1143 Cygni which is a nearby (d ? 40 pc) eclipsing
pair of similar F-type main-sequence stars and strongly eccentric (e ? 0.54) orbit with
7.64-day period. Although the apsidal period is long (of the order of 104 yr), the
configuration is good for apsidal motion studies. The apsidal motion has been thoroughly
studied by Gimenez & Margrave (1985) and absolute dimensions of the orbit and
components of the binary were published by Andersen et al. (1987), which are not so
different from those published by Wilson & Raichur 2008. Some authors (e.g. Albrecht et
al. 2007) reported discrepancy between the theoretical and the observed apsidal motion.
Moreover, this
nearby system can also serve as a verification of our combined methods (photometric +
spectroscopic + interferometric) for an independent estimate of systematic uncertainties.
In complement to this first object, we consider also investigating the case of V2368 Oph
(HD 156208) that was used as one of comparison stars in photometric observations of the
well-known eclipsing binary U Oph. Perryman & ESA (1997) reported that is also an
eclipsing binary, suggesting a tentative period of 7.70 d. The first detailed study of this
binary, now known as V2368 Oph, was published by Harmanec et al. (2011). They found
the true orbital period of 38.3 d and an eccentric orbit with e = 0.51 and derived also
orbital elements for both binary components. Both stellar eclipses of about 0.2 mag. deep
are observed. While the multiplicity of A-type stars is not unusual as it may reach 69±7%
(De Rosa et al. 2014), this particular eclipsing binary is composed of two A stars which
have both evolved away from the main sequence. We thus think V2368~Oph offers a
unique opportunity to study a `differential' evolution of A-type stars, once the masses of
both components will be determined with sufficient precision. A combined astrometric,
orbital and light-curve solution could help to achieve this goal.