- PhD thesis: Stars at a glance: with high-resolution spectroscopy and long-baseline interferometry
Supervisor: doc. Mgr. Miroslav Brož, Ph.D.
Consultant: prof. RNDr. Petr Harmanec, DrSc.
In the last years, compact stellar systems or systems with circumstellar matter can be studied with extensive datasets, including astrometry, photometry, spectroscopy, spectral-energy distribution (SED), interferometric visibility, closure phase, triple product, differential visibility, or differential phase. To interpret these sophisticated observations, a sufficiently complex and precise photo-dynamical models are needed. In this work, we suggest to study at least several stellar systems, using modelling tools like Xitau (Nemravová et al. 2016, Brož 2017), Pyshellspec (Mourard et al. 2018, Broz et al. 2021), PHOEBE 2 (Prša et al. 2020, Conroy et al. 2020), or their alternatives.
Extensive datasets already exists, e.g., for: δ Ori, ω CMa, V923 Aql, β Lyr, HD 152246, ξ Tau, or λ Tau. A brief description of these systems is attached below. In particular, there are new high-precision light curves from MOST, TESS, BRITE, interferometry from VEGA/CHARA, MIRC/CHARA, VLTI/AMBER, and spectroscopy from VLT/CRIRES, 2.2m FEROS, 3.6m HARPS, IUE, as found in the ESO and MAST archives. Consequently, a student should understand details of these observations, their uncertainties and systematics, even though the ultimate goal is a theoretical understanding of the systems.
Of course, it is necessary to continue with the development of our modelling tools (in Fortran, Python programming languages). For example, there are azimuthal asymmetries (spiral arms), or long-term temporal variability; it is useful to implement the subplex (Rowan 1990) or Powell's methods to improve a convergence; synthetic spectra have to be computed for non-standard chemical composition, e.g., with the Tlusty program (Hubený & Lanz 1995, Hubený & Lanz 2017); for hot star it may be necessary to use synthetic spectra for atmospheres with winds (FASTWIND or CMFGEN, Puls et al. 2020); alternatively, a Bayesian formulation is possible for problems studied previously by frequentist approach (or χ2).
For systems having spectroscopic observations with a full phase coverage, or interferometric observation with a sufficient (u,v)-plane coverage, the inverse problem can be solved by unconstrained methods (Doppler tomography, inverse Fourier transform). Ideally, they should agree with geometrically-constrained models, but the real world is not ideal. As a result, a student should be able to construct complex models (of stellar systems), which is useful general knowledge.
All systems include a hot star. δ Ori (HD 36486, HR 1852, HIP 25930) is a multiple system consisting of six gravitationally-bound stars (denoted Aa1, Aa2, Ab, B, C1, C2; Oplistilova et al. 2020). Component A consists of a detached eclipsing binary (P = 5.732436 d), and a more distant tertiary (P ~ 103 d). The ω CMa and V923 Aql systems are Be stars - exhibiting long-term changes in radial velocity, V/R ratio, the Balmer emission lines, brightness, and colour. ω CMa (28 CMa, HD 56139, HR 2749, HIP 35037) might be binary, as inferred from its period analysis, and V923 Aql (HD 183656, HR 7415, HIP 95929) is a confirmed binary, with P = 214.716 d (Wolf et al. 2021). The β Lyr system (HD 174638, HR 7106, HIP 92420) consists of six components in total. In the centre, there is a semidetached eclipsing binary with an optically thick accretion disk and thin circumstellar matter, creating strong emission lines (Broz et al. 2021). The remaining components are single stars. HD 152246 (HIP 82685) is a triple system consisting of a close binary (O-type stars) with a slightly eccentric orbit (e = 0.11) and period P = 6.0049 d, which revolves in a 470-day highly eccentric orbit (e = 0.87) with another massive and bright component (Nasseri et at. 2014). ξ Tauri (HD 21364, HR 1038, HIP 16083) is a hierarchical quadruple system. Three components are B-type main sequence dwarfs. Two of them form a detached eclipsing binary (P = 7.15 d), the third one has a period of 145 days, and the fourth one about fifty years (Nemravová et al. 2016). Finally, λ Tauri (HD 25204, HR 1239, HIP 18724) is one of the most compact and strongly interacting stellar systems. The inner pair - a semidetached binary - with a period of only 3.95 days, is orbited by the third component every 33.025 days (Berdyugin et al. 2018).
- Master thesis: Spectroscopic and photometric investigation of selected hot stars
Supervisor: prof. RNDr. Petr Harmanec, DrSc.
Massive hot stars are of general importance for astrophysics. Their nuclear evolution is relatively rapid so that they enrich the interstellar matter by heaviest chemical elements produced by the nuclear reactions in their interiors. Thank to their brightness, they are also well observable at large distances from us. Recent research indicates that the majority of hot stars originates as binaries or even muptiple systems. Understanding this fact could help to improve the general theory of the origin of stars. Finally, the formation and long-term variability of extended gaseous envelopes (veils) of Be stars remain uxplained as far as the mechanism of their origin and time variations is concerned. Therefore, a thorough study of long-term variability could help in finding the true physical cause of the origin of the envelopes.
- Bachalor thesis: An analysis and solution of the light curves of the massive triple star δ Orionis
Supervisor: prof. RNDr. Petr Harmanec, DrSc.
Delta Ori is a triple system consisting of an eclipsing binary with the orbital period of 5.73 d and with a mildly eccentric orbit and a slow apsidal motion, and a distant tertiary with an orbital period of the order of several hundreds of days. The optical spectrum is dominated by spectral lines of the primary and of the tertiary. The latter were erroneously misidentified for the lines of the secondary. which led to the determination of anomalous binary masses. The lines of the secondary are very weak and were only detected via a special technique of analysis. Thanks to collaboration with foreign colleagues, and to public data archives, the proponent of this Thesis has access to series of photometric observations of delta Ori from several different photometers on artificial Earth satellites. These observations seem to indicate that the light curve is affected by some rapid physical variations, in addition to binary eclipses. The nature of these variations has to be properly identified and their effect on the light-curve solution must be evaluated.