Abstract. Based on high S/N spectra obtained at La Silla, Chile, and the Special Astrophysical Observatory, Russia, the abundances of He, C, O, Ne, Mg, Si, Ca, Fe, Sr, and Ba in 27 optically bright B5-B9 main-sequence stars were determined. NLTE effects were taken into account. A variety of abundance patterns is present in late B stars. Accurate surface abundances of the diffusion indicators O, Mg, Ca, Sr and Ba suggest that element stratification due to diffusion is common in the program stars. Models of stellar atmospheres which include meridional mixing can explain the observed anomalies. Although the program stars represent only a volume-limited sample of the solar neighbourhood this result is important for the cosmochemical evolution of the Galaxy: the surface abundances of the stars investigated do not necessarily reflect the chemical composition of the interstellar cloud they originated from. Furthermore, five program stars show narrow absorption lines in Ca II K which can be attributed to circumstellar gas. Neon serves as a trace element for the occurrence of weak stellar winds. Neon overabundances of some stars derived under the assumption of LTE suggest that such winds have been detected. In sharp contrast, the more realistic treatment of NLTE leads to solar neon abundances and thus reveals that weak stellar winds are absent in the program stars.
Abstract. β Pictoris (HR 2020) is the most prominent prototype of stars with circumstellar disks and has generated particular interest in the framework of young planetary systems. Given its spectral type A5, stellar activity is not expected. Nevertheless, resonance lines of C and O typical for a chromosphere and transition region have been unambiguously detected with FUSE. We present results from an XMM-Newton observation of β Pic and find evidence for X-ray emission. In particular, we detected an emission of O at 21.6 Å with the MOS detectors. These findings present a challenge for the development of both stellar activity and disk models. We discuss and investigate various models to explain the observed emission including the presence of a cool corona and a boundary layer.
Abstract.We have analysed a time series of high resolution spectra (R = 217 000) of the Ca K line of 9 stars which are candidates for the presence of circumstellar material from our previous studies. We have searched for variable narrow absorption components similar to those extensively studied in the case of β Pictoris. Our data show long-term variations in the spectra of HR 2550 and HR 3685 which can be attributed to the dynamics of circumstellar gas. About one third of the sample stars show variable line absorption but only β Pictoris seems to exhibit uniquely strong variations on short (nightly) timescales. In order to examine possible interstellar contributions we compared our results with interstellar data from the literature. The column densities of our absorption features are up to three orders of magnitude higher than those found for the Local Interstellar Cloud.
Abstract. We determined abundances of O, Ca, Fe, Ba and Y for a sample of dusty and dust-free A stars, taken from the list of Cheng et al. (1992). Five of the stars have an infrared-excess due to circumstellar dust. Ongoing accretion from their circumstellar surroundings might have modified the abundances in the photospheres of these stars, but our results clearly show, that there is no difference in the photospheric composition of the dusty and dust-free stars. Instead all of them show the typical diffusion pattern which diminishes towards larger rotational velocities.
Abstract. We have derived the mean broadening profile of the star V 102 in the region of the open cluster IC 4665 from high resolution spectroscopy. At a projected equatorial rotation velocity of v sin i = (105 ± 12) km s −1 we find strong deviation from classical rotation. We discuss several scenarios, the most plausible being strong differential rotation in latitudinal direction. For this scenario we find a difference in angular velocity of ∆Ω = 3.6 ± 0.8 rad d−1 (∆Ω/Ω = 0.42 ± 0.09). From the Hα line we derive a spectral type of A9 and support photometric measurements classifying IC 4665 V 102 as a non-member of IC 4665. At such early spectral type this is the strongest case of differential rotation observed so far. Together with three similar stars, IC 4665 V 102 seems to form a new class of objects that exhibit extreme latitudinal shear in a very shallow convective envelope.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.