Deep imaging survey of the environment of Alpha Centauri - II. CCD imaging with the NTT-SUSI2 camera

Context: The nearby pair of solar-type stars Alpha Centauri is a favorable target for an imaging search for extrasolar planets. Indications exist that the gravitational mass of Alpha Cen B could be higher than its modeled mass, the difference being consistent with a substellar companion of a few tens of Jupiter masses. However, Alpha Centauri usually appears in star catalogues surrounded by a large void area, due to the strong diffused light. Aims: We searched for faint comoving companions to Alpha Cen located at angular distances of the order of a few tens of arcseconds, up to 2-3 arcmin. As a secondary objective, we built a catalogue of the detected background sources. Methods: In order to complement our adaptive optics search at small angular distances (Paper I), we used atmosphere limited CCD imaging from the NTT-SUSI2 instrument in the Bessel V, R, I, and Z bands. Results: We present the results of our search in the form of a catalogue of the detected objects inside a 5.5 arcmin box around this star. A total of 4313 sources down to mV~24 and mI~22 were detected from this wide-field survey. We extracted the infrared photometry of part of the detected sources from archive images of the 2MASS survey (JHK bands). We investigate briefly the nature of the detected sources, many of them presenting extremely red color indices (V-K > 14). Conclusions: We did not detect any companion to Alpha Centauri between 100 and 300 AU, down to a maximum mass of ~15 times Jupiter. We also mostly exclude the presence of a companion more massive than 30 MJup between 50 and 100 AU.Comment: Accepted for publication as a Research Note in A&

Abundance analysis of cool extreme helium star: LSS 3378

Abundance analysis of the cool extreme helium (EHe) star LSS 3378 is presented. The abundance analysis is done using LTE line formation and LTE model atmospheres constructed for EHe stars. The atmosphere of LSS 3378 shows evidence of H-burning, He-burning, and s-process nucleosynthesis. The derived abundances of iron-peak and alpha-elements indicate absence of selective fractionation or any other processes that can distort chemical composition of these elements. Hence, the Fe abundance (log epsilon(Fe) = 6.1) is adopted as an initial metallicity indicator. The measured abundances of LSS 3378 are compared with those of R Coronae Borealis (RCB) stars and with rest of the EHe stars as a group.Comment: 7 pages, 4 figures, 3 tables, MNRAS format, accepted for publication in MNRA

Nõukogude Liidu ja Venemaa käsitus mereõigusest: liberalism ja kohalik vastupanu

Väitekirja elektrooniline versioon ei sisalda publikatsiooneKäesoleva doktoritöö eesmärk on analüüsida Nõukogude ja Venemaa mereõiguse käsitust iseloomustavaid tunnuseid. Täpsemalt püütakse selgitada, kuidas mereõigust tänapäeva Venemaal kasutatakse ja mõistetakse ning kuidas seda mõisteti Nõukogude Liidus kui Venemaa õiguseellases riigis. Väitekirjas käsitletakse ajavahemikku alates 1960. aastate lõpust, mil NSV Liit sai mõjukaks mereriigiks, kuni 2022. aastani ning analüüsitakse Nõukogude ja Venemaa käsitust mereõigusest läbi aja ja ruumi prisma. Allpool on esitatud uurimisprojekti põhijäreldused, millest ilmneb Moskva keerukas ja puhuti vastuoluline lähenemine mereõigusele. Doktoritöö esimene järeldus ütleb, et vaatlusalusel perioodil sai Nõukogude Liidust liberaalset mereõiguse käsitust pooldav mereriik, ja sama kehtib jätkuvalt Venemaa kohta. Liberaalse käsituse keskmes on püüd kaitsta avamerevabadust ning vaba juurdepääsu merevaradele. Seda järeldust kinnitab Moskva hoiak 1982. aasta mereõiguse konventsiooni läbirääkimiste ajal ning samuti praegu toimuvatel läbirääkimistel, mille eesmärk on võtta vastu rahvusvaheliselt siduv dokument väljaspool riikide jurisdiktsiooni asuvate alade elurikkuse kaitse kohta. Moskva liberaalseid kalduvusi kinnitab ka Venemaa merepiiridel kehtestatud ühisrežiimide analüüs. Väitekirja teine järeldus lisab esimest täpsustavaid nüansse. Sellest ilmneb, et Nõukogude Liidu ja Venemaa käsitus mereõigusest ei ole järjekindlalt liberaalne. Kui Nõukogude Liit ja Venemaa on pidanud mõnda merd ajalooliselt endale kuuluvaks (see kehtib Arktika merede ning viimasel ajal ka Aasovi mere kohta), üritab Moskva mereõiguse konventsioonist mööda minnes seal endale rohkem õigusi saada. Selleks püüab Venemaa valitsus või selle doktriin õigustada ajaloolistel argumentidel põhinevaid erandrežiime. Samas on oluline märkida, et Moskva vastuseis mereõiguse konventsioonile on seni piirdunud vaid nende kahe piirkonnaga ning selle märkimisväärne laienemine ja üldiseks muutumine ei ole tõenäoline. 1982. aasta mereõiguse konventsiooni liberaalne vaim, mida Moskva aitas 1960. ja 1970. aastatel kujundada, vastab endiselt Venemaa huvidele merel.The object of this PhD thesis is the analysis of the specificities of the Soviet and Russian approach to the law of the sea. More precisely, it seeks to elucidate how the law of the sea is used and understood by contemporary Russia and how it was used and understood by its state-predecessor, the Soviet Union. Spanning from late 1960s, when the Soviet Union became a maritime power until 2022, this dissertation analyses the Soviet and Russian and Russian approach to the law of the sea both through the prism of time and space. The main conclusions of this research project are the following and reveal Moscow’s complex and at times ambiguous approach to the law of the sea. The first finding of this doctoral dissertation is that during the period examined, the Soviet Union has become and Russia remains a maritime power defending a liberal approach to the law of the sea centred around the defence of high seas’ freedoms and free access to the sea’s resources. This finding is supported by Moscow’s attitude during the negotiations of the Law of the Sea Convention of 1982 and the ongoing negotiations of the international legally binding instrument on biodiversity beyond national jurisdiction. Furthermore, the analysis of Russia’s joint regimes at its maritime borders confirms Moscow’s liberal tendencies. The second finding of this dissertation nuances the first. It demonstrates that the Soviet and Russian approach to the law of the sea is not consistently liberal. In seas that the Soviet Union and Russia have historically considered its own – namely in the Arctic and more recently in the Azov Sea – Moscow attempts to circumvent the Law of the Sea Convention in order to obtain more rights. To do so, using historical arguments, the Russian State or its doctrine try to promote exceptional regimes built on historical arguments. Nevertheless, it is important to note that Moscow’s contestation of the Law of the Sea Convention has so far been circumscribed to these two regions and is unlikely to significantly increase and become generalized. The 1982 Law of the Sea Convention’s liberal ethos, which Moscow helped shape throughout the 1960s and 1970s, still satisfies Russia’s interests at sea.https://www.ester.ee/record=b552436

Critical exponential tiltings for size-conditioned multitype Bienaym\'e--Galton--Watson trees

We consider here multitype Bienaym\'e--Galton--Watson trees, under the conditioning that the numbers of vertices of given type satisfy some linear relations. We prove that, under some smoothness conditions on the offspring distribution $\mathbf{\zeta}$, there exists a critical offspring distribution $\tilde{\mathbf{\zeta}}$ such that the trees with offspring distribution $\mathbf{\zeta}$ and $\tilde{\mathbf{\zeta}}$ have the same law under our conditioning. This allows us in a second time to characterize the local limit of such trees, as their size goes to infinity. Our main tool is a notion of exponential tilting for multitype Bienaym\'e--Galton--Watson trees.Comment: 18 page

New determination of abundances and stellar parameters for a set of weak G-band stars

Weak G-band (wGb) stars are very peculiar red giants almost devoided of carbon and often mildly enriched in lithium. Despite their very puzzling abundance patterns, very few detailed spectroscopic studies existed up to a few years ago, preventing any clear understanding of the wGb phenomenon. We recently proposed the first consistent analysis of published data for 28 wGb stars and identified them as descendants of early A-type to late B-type stars, without being able to conclude on their evolutionary status or the origin of their peculiar abundance pattern. We used newly obtained high-resolution and high SNR spectra for 19 wGb stars in the southern and northern hemisphere to homogeneously derive their fundamental parameters, metallicities, as well as the spectroscopic abundances for Li, C, N, O, Na, Sr, and Ba. We also computed dedicated stellar evolution models that we used to determine the masses and to investigate the evolutionary status and chemical history of the stars in our sample. We confirm that the wGb stars are stars in the mass range 3.2 to 4.2 M$_\odot$. We suggest that a large fraction could be mildly evolved stars on the SGB currently undergoing the 1st DUP, while a smaller number of stars are more probably in the core He burning phase at the clump. After analysing their abundance pattern, we confirm their strong N enrichment anti-correlated with large C depletion, characteristic of material fully processed through the CNO cycle to an extent not known in other evolved intermediate-mass stars. However, we demonstrate here that such a pattern is very unlikely due to self-enrichment. In the light of the current observational constraints, no solid self-consistent pollution scenario can be presented either, leaving the wGb puzzle largely unsolved.Comment: 19 pages , 14 figures, accepted for publication in Astronomy & Astrophysic

Gaia FGK Benchmark Stars: Effective temperatures and surface gravities

Large Galactic stellar surveys and new generations of stellar atmosphere models and spectral line formation computations need to be subjected to careful calibration and validation and to benchmark tests. We focus on cool stars and aim at establishing a sample of 34 Gaia FGK Benchmark Stars with a range of different metallicities. The goal was to determine the effective temperature and the surface gravity independently from spectroscopy and atmospheric models as far as possible. Fundamental determinations of Teff and logg were obtained in a systematic way from a compilation of angular diameter measurements and bolometric fluxes, and from a homogeneous mass determination based on stellar evolution models. The derived parameters were compared to recent spectroscopic and photometric determinations and to gravity estimates based on seismic data. Most of the adopted diameter measurements have formal uncertainties around 1%, which translate into uncertainties in effective temperature of 0.5%. The measurements of bolometric flux seem to be accurate to 5% or better, which contributes about 1% or less to the uncertainties in effective temperature. The comparisons of parameter determinations with the literature show in general good agreements with a few exceptions, most notably for the coolest stars and for metal-poor stars. The sample consists of 29 FGK-type stars and 5 M giants. Among the FGK stars, 21 have reliable parameters suitable for testing, validation, or calibration purposes. For four stars, future adjustments of the fundamental Teff are required, and for five stars the logg determination needs to be improved. Future extensions of the sample of Gaia FGK Benchmark Stars are required to fill gaps in parameter space, and we include a list of suggested candidates.Comment: Accepted by A&A; 34 pages (printer format), 14 tables, 13 figures; language correcte

The Zeta Herculis binary system revisited. Calibration and seismology

We have revisited the calibration of the visual binary system Zeta Herculis with the goal to give the seismological properties of the G0 IV sub-giant Zeta Her A. We have used the most recent physical and observational data. For the age we have obtained 3387 Myr, for the masses respectively 1.45 and 0.98 solar mass, for the initial helium mass fraction 0.243, for the initial mass ratio of heavy elements to hydrogen 0.0269 and for the mixing-length parameters respectively 0.92 and 0.90 using the Canuto & Mazitelli (1991, 1992) convection theory. Our results do not exclude that Zeta Her A is itself a binary sub-system; the mass of the hypothetical unseen companion would be smaller than 0.05 solar mass. The adiabatic oscillation spectrum of Zeta Her A is found to be a complicated superposition of acoustic and gravity modes; some of them have a dual character. This greatly complicates the classification of the non-radial modes. The echelle diagram used by the observers to extract the frequencies will work for ell=0, 2, 3. The large difference is found to be of the order of 42 mu Hz, in agreement with the Martic et al. (2001) seismic observations.Comment: 12 pages, A&A in pres