C, N, O abundances and carbon isotope ratios in evolved stars of the open clusters Collinder 261 and NGC 6253

Context. Investigations of abundances of carbon and nitrogen in the atmospheres of evolved stars of open clusters may provide comprehensive information on chemical composition changes caused by stellar evolution. Aims. Our main aim is to increase the number of open clusters with determined carbon-to nitrogen and carbon isotope ratios. Methods. High-resolution spectra were analysed using a differential model atmosphere method. Abundances of carbon were derived using the C_2 Swan (0,1) band head at 5635.5 {\AA} (FEROS spectra) and the C_2 Swan (1,0) band head at 4737 {\AA} (UVES spectra). The wavelength interval 7980-8130 {\AA}, with strong CN features was analysed to determine nitrogen abundances and 12^C/13^C isotope ratios. The oxygen abundances were determined from the [Oi] line at 6300 {\AA}. Results. The average value of 12^C/13^C isotope ratios of Cr 261 is equal to 18 \pm 2 in four giants and to 12 \pm 1 in two clump stars; it is equal to 16 \pm 1 in four clump stars of the open cluster NGC 6253. The mean C/N ratios in Cr 261 and NGC 6253 are equal to 1.67 \pm 0.06 and 1.37 \pm 0.09, respectively. Conclusions. The 12^C/13^C and C/N values in Cr 261 and NGC 6253 within limits of uncertainties agree with the theoretical model of thermohaline-induced mixing as well as with the cool-bottom processing modelComment: 7 pages, 8 figure

The extent of mixing in stellar interiors: the open clusters Collinder 261 and Melotte 66

Context: Determining carbon and nitrogen abundances in red giants provides useful diagnostics to test mixing processes in stellar atmospheres. Aims: Our main aim is to determine carbon-to-nitrogen and carbon isotope ratios for evolved giants in the open clusters Collinder 261 and Melotte 66 and to compare the results with predictions of theoretical models. Methods: High-resolution spectra were analysed using a differential model atmosphere method. Abundances of carbon were derived using the C_2 Swan (0,1) band head at 5635.5 A. The wavelength interval 7940-8130 A, which contains CN features, was analysed to determine nitrogen abundances and carbon isotope ratios. The oxygen abundances were determined from the [O_I] line at 6300 A. Results: The mean values of the elemental abundances in Collinder 261, as determined from seven stars, are: [C/Fe]=-0.23 +- 0.02 (s.d.), [N/Fe]=0.18 +- 0.09, [O/Fe]=-0.03 +- 0.07. The mean 12^C/13^C ratio is 11 +- 2, considering four red clump stars and 18 for one star above the clump. The mean C/N ratios are 1.60 +- 0.30 and 1.74, respectively. For the five stars in Melotte 66 we obtained: [C/Fe]=-0.21 +- 0.07 (s.d.), [N/Fe]=0.17 +- 0.07, [O/Fe]=0.16 +- 0.04. The 12^C/13^C and C/N ratios are 8 +- 2 and 1.67 +- 0.21, respectively. Conclusions: The 12^C/13^C and C/N ratios of stars in the investigated open clusters were compared with the ratios predicted by stellar evolution models. The mean values of 12^C/13^C ratios in Collinder 261 and Melotte 66 agree well with models of thermohaline-induced extra-mixing for the corresponding stellar turn-off masses of about 1.1 - 1.2 Msun. The mean C/N ratios are not decreased as much as predicted by the model in which the thermohaline- and rotation-induced extra-mixing act together.Comment: 6 pages, 8 figure

Atomic and Molecular Data for Optical Stellar Spectroscopy

High-precision spectroscopy of large stellar samples plays a crucial role for several topical issues in astrophysics. Examples include studying the chemical structure and evolution of the Milky Way galaxy, tracing the origin of chemical elements, and characterizing planetary host stars. Data are accumulating from instruments that obtain high-quality spectra of stars in the ultraviolet, optical and infrared wavelength regions on a routine basis. These instruments are located at ground-based 2- to 10-m class telescopes around the world, in addition to the spectrographs with unique capabilities available at the Hubble Space Telescope. The interpretation of these spectra requires high-quality transition data for numerous species, in particular neutral and singly ionized atoms, and di- or triatomic molecules. We rely heavily on the continuous efforts of laboratory astrophysics groups that produce and improve the relevant experimental and theoretical atomic and molecular data. The compilation of the best available data is facilitated by databases and electronic infrastructures such as the NIST Atomic Spectra Database, the VALD database, or the Virtual Atomic and Molecular Data Centre (VAMDC). We illustrate the current status of atomic data for optical stellar spectra with the example of the Gaia-ESO Public Spectroscopic Survey. Data sources for 35 chemical elements were reviewed in an effort to construct a line list for a homogeneous abundance analysis of up to 100000 stars.Comment: Published 30 April 2015 in Physica Script

Stabilization zone structure in jet diffusion flames from liftoff toblowout

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/77310/1/AIAA-1989-153-230.pd

Chemical composition of evolved stars in the open cluster NGC 2506

In this study we present abundances of 12^C, 13^C, N, O and up to 26 other chemical elements in two first ascent giants and two core-helium-burning 'clump' stars of the open cluster NGC 2506. Abundances of carbon were derived using the C_2 Swan (0,1) band head at 5635.5 A. The wavelength interval 7940-8130 A, with strong CN features, was analysed in order to determine nitrogen abundances and carbon isotope ratios. The oxygen abundances were determined from the [O I] line at 6300 A. NGC 2506 was found to have a mean [Fe/H]=-0.24 +- 0.05 (standard deviation). Compared with the Sun and other dwarf stars of the Galactic disc, mean abundances in the investigated clump stars suggest that carbon is depleted by about 0.2 dex, nitrogen is overabundant by about 0.3 dex and other chemical elements have abundance ratios close to solar. The C/N and 12^C/13^C ratios are lowered to 1.25 +- 0.27 and 11 +- 3, respectively.Comment: MNRAS accepte