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Project Veselka: Vertical Stratification of Element Abundances

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Project Veselka: Vertical Stratification of Element Abundances Project VeSElkA: Vertical Stratification of Element Abundances in CP stars∗ V. R. Khalack1, F. LeBlanc1 1Département de Physique et d’Astronomie, Université de [56] suggested the possibility of abundance abnor- Moncton, Moncton, N.-B., Canada E1A 3E9 malities in the atmospheres of these stars. Several [email protected] decades later, this idea was confirmed by the results [email protected] of a differential coarse analysis using the curves of growth [7]. It should be noted, that the observed pe- culiar abundance of chemical species is only related to the stellar atmosphere and does not reflect the chemical composition of the entire star. A new research project on spectral analysis that aims to characterize the vertical stratification of element We turn our attention to two types of stars with abundances in stellar atmospheres of chemically pe- abundance anomalies: blue horizontal-branch (BHB) culiar (CP) stars is discussed in detail. Some results stars that burn helium in their core and hydrogen in on detection of vertical abundance stratification in a shell [44], and main sequence stars that burn hy- several slowly rotating main sequence CP stars are drogen in their core. BHB stars are found mostly presented and considered as an indicator of the effec- in globular clusters. Comprehensive surveys show tiveness of the atomic diffusion mechanism responsi- that the hot BHB stars have abundance anomalies ble for the observed peculiarities of chemical abun- as compared to the cool BHB stars in the same clus- dances. This study is carried out in the frame of ter [16, 19]. Hot BHB stars show enhanced abun- Project VeSElkA (Vertical Stratification of Elements dances of iron, magnesium, titanium and phosphor, Abundance) for which 34 slowly rotating CP stars and depleted helium [5, 6]. Surveys of rotational ve- have been observed with the ESPaDOnS spectropo- locities of BHB stars indicate that stars with Teff ≥ larimeter at CFHT. 11500 K possess modest rotation with Vsin i < 10 km s−1, while the cooler stars rotate more rapidly Key words: diffusion, line: profiles, stars: abun- on average [48, 54]. Slow rotation is an indicator of dances, atmospheres, chemically peculiar, magnetic a hydrodynamically stable atmosphere and the ob- field, rotation served abundance peculiarities as well as the brighter u-magnitudes (as compared to the theoretical pre- diction for u-magnitudes based on stellar evolution introduction models) of hot BHB stars [18] can be explained in the frame of atomic diffusion mechanism [43] at play in At certain stages of stellar evolution, some stars their atmospheres. Competition between the gravi- show peculiarity of spectral lines in their spectra that tational and radiative forces in a hydrodynamically argues in favour of an enhanced or depleted abun- stable atmosphere can cause accumulation or deple- dance of several chemical species in their stellar at- tion of chemical elements at certain optical depths mospheres with respect to the solar abundance. Ab- 2 and lead to a vertical stratification of element abun- normally strong lines of silicon in α CVn were first dances. Hui-Bon-Hoa et al. [23] have constructed reported by Antonia Maury [42], while the strong stellar atmosphere models of hot BHB stars with lines of ionised silicon and ionised strontium were vertical stratification of elements, that successfully arXiv:1506.01269v1 [astro-ph.SR] 3 Jun 2015 found by Annie Cannon [9] in some bright south- reproduced some of the anomalies mentioned above. ern stars in the process of determining their spec- Analysing the available high resolution spectra of tral class. The first systematic study and classifi- some hot BHB stars, Khalack et al. [26, 27] have cation of stars of spectral classes B-F with abnor- detected vertical stratification of the abundance of mally strong lines of various chemical species was several chemical elements, including iron. The stel- performed by Morgan [45, 46]. To explain the mys- lar atmosphere models of hot BHB stars calculated tery of the observed abnormally strong lines, Shapley self-consistently with abundance stratifications pre- dict that the iron abundance stratification decreases ∗Based on observations obtained at the Canada-France-Hawaii eff Telescope (CFHT), which is operated by the National Research as a function of T and becomes negligible for BHB Council of Canada, the Institut National des Sciences de l’Univers stars hotter than Teff ≃ 14000 K [37]. This theoreti- of the Centre National de la Recherche Scientifique of France, and cal result is consistent with the detected iron abun- the University of Hawaii. dance slopes found by Khalack et al. [26, 27] from 1 Advances in Astronomy and Space Physics spectral line analysis of hot BHB stars with different Ap stars (HD 133792 and HD 204411) [52]. Mean- effective temperatures. while the rare-earth elements (for example, Pr and A significant portion of upper main sequence stars Nd) are usually pushed into the upper atmosphere show abundance peculiarities of various chemical of Ap stars [40]. species and are commonly named chemically pecu- Detection of vertical stratification of element abun- liar (CP) stars [49]. George Preston [49] introduced dances in stellar atmospheres of stars is an indicator this collective term and divided the CP stars in four of the effectiveness of the atomic diffusion mecha- groups: CP1 metallic-line stars (type Am-Fm), CP2 nism that may be responsible for the observed abun- magnetic peculiar B and A-type stars (type Bp-Ap), dance peculiarities. The vertical stratification of el- CP3 mercury-manganese stars (type Hg-Mn) and ement abundances in a hydrodynamically stable at- CP4 helium weak stars. Following a large num- mosphere can be estimated through the analysis of ber of extensive studies of CP stars, new types of multiple line profiles that belong to the same ion of abundance peculiarities have been discovered and the studied element [24, 25] using the ZEEMAN2 an extension of Preston’s classification was proposed code [35], for instance. This approach has been [39, 58]. In this new scheme, the CP4 group includes successfully employed to study the vertical abun- only magnetic He-weak stars (Sr-Ti-Si branch), while dance stratification in the atmospheres of several the non-magnetic He-weak stars (P-Ga branch) are BHB [26, 27] and HgMn stars [61]. placed in the group CP5. The He-rich magnetic To search for the signatures and to study the abun- stars (usually having spectral class B2) are placed dance stratification of chemical species with optical in the group CP6. The CP7 group is reserved for depth in the atmospheres of CP stars, we have ini- the non-magnetic He-rich stars. In this classification tiated Project VeSElkA, which means rainbow in the odd numbers correspond to non-magnetic stars, ukrainian and stands for Vertical Stratification of while groups with the even numbers contain stars Element Abundances. Slowly rotating (Vsin i < 40 that possess a detectable magnetic field. More de- km/s) CP stars of the upper main sequence were se- tails about the history of discovery and study of CP lected for our study using the catalogue of Ap, HgMn stars can be found in [21]. and Am stars of [55]. This limitation for the rota- tional velocity is imposed with the aim to increase An extensive list of known and suspected Ap, the probability for a star to have a hydrodynami- HgMn and Am stars was recently published by Ren- cally stable atmosphere, where the atomic diffusion son & Manfroid [55], while a list of known magnetic mechanism can produce vertical stratification of ele- CP stars has been compiled by Bychkov et al. [8]. ment abundances. A low value of Vsin i also leads to Some upper main sequence CP stars show variability narrow and mostly unblended line profiles in the ob- of absorption line profiles in their spectra with the served spectra, which is beneficial for our abundance period of stellar rotation due to horizontal inhomoge- analysis. neous distributions of elements’ abundance in their stellar atmosphere [32]. One can see the periodic (days, months, years) variation of spectral line pro- files with the axial rotation of a CP star due to the observations and data reduction variation of contribution from different parts of the stellar atmosphere to these lines. Ap stars also show The selected slowly rotating CP stars and two nor- signatures of a strong magnetic field and its struc- mal main sequence stars (used as reference stars) ture correlates with the patches of overabundances have been observed during 2013-14 with ESPaDOnS (or underabundances) of certain elements [31, 57]. (Echelle SpectroPolarimetric Device for Observa- Observationally, it was shown that the properties of tions of Stars) at the CFHT (Canada-France-Hawaii the line profile variability due to abundance patches Telescope) employing the deep-depletion e2v device as well as the magnetic field structure of Ap stars Olapa. The instrument performances as well as the do not change during several decades [41, 50] and, optical characteristics of the spectropolarimetre are therefore, it is usually assumed that stellar atmo- described in [14].1 spheres of Ap stars are hydrodynamically stable. In ESPaDOnS acquires high resolution (R= 65000) a hydrodynamically stable atmosphere the presence Stokes IV spectra throughout the spectral range from of a magnetic field can intensify accumulation or de- 3700Å to 10500Å in a single exposure [13]. In order pletion of chemical elements at certain optical depths to be able to find convincing signatures of vertical [2, 53, 60]. From the analysis of line profiles of ionised stratification of element abundances from the spec- iron and ionised chromium in the spectra of β CrB, tral analysis, we require spectra with high signal- Ryabchikova et al. [51] have shown that iron and to-noise ratio (close to one thousand per bin in the chromium abundances increase towards the deeper spectral region around 5150Å).
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