Browsing by Subject "chemical-compositions"

Now showing 1 - 4 of 4

CS 22964-161: A Double-Lined Carbon- And S-Process-Enhanced Metal-Poor Binary Star
(2008-04) Thompson, Ian B.; Ivans, , Inese I.; Bisterzo, Sara; Sneden, Christopher; Gallino, Roberto; Vauclair, Sylvie; Burley, Gregory S.; Shectman, Stephen A.; Preston, George W.; Sneden, Christopher
A detailed high-resolution spectroscopic analysis is presented for the carbon-rich low-metallicity Galactic halo object CS 22964-161. We have discovered that CS 22964-161 is a double-lined spectroscopic binary and have derived accurate orbital components for the system. From a model atmosphere analysis we show that both components are near the metal-poor main-sequence turnoff. Both stars are very enriched in carbon and in neutron-capture elements that can be created in the s-process, including lead. The primary star also possesses an abundance of lithium close to the value of the "Spite plateau.'' The simplest interpretation is that the binary members seen today were the recipients of these anomalous abundances from a third star that was losing mass as part of its AGB evolution. We compare the observed CS 22964-161 abundance set with nucleosynthesis predictions of AGB stars, discuss issues of envelope stability in the observed stars under mass transfer conditions, and consider the dynamical stability of the alleged original triple star. Finally, we consider the circumstances that permit survival of lithium, whatever its origin, in the spectrum of this extraordinary system.
The First Carbon-Enhanced Metal-Poor Star Found In The Sculptor Dwarf Spheroidal OCR Issue
(2015-02) Skuladottir, A.; Tolstoy, E.; Salvadori, S.; Hill, V.; Pettini, M.; Shetrone, M. D.; Starkenburg, E.; Shetrone, Matthew D.
The origin of carbon-enhanced metal-poor (CEMP) stars and their possible connection with the chemical elements produced by the first stellar generation is still highly debated. In contrast to the Galactic halo, not many CFMP stars have been found in the dwarf spheroidal galaxies around the Milky Way. Here we present detailed abundances from ESO VET/IVES high resolution spectroscopy for ET0097, the first CEMP star found in the Sculptor dwarf spheroidal, which is one of the best studied dwarf galaxies in the Local Group, This star has [Fe/H] = 2.03 +/- 0.10, [C/Fe] = 0.51 +/- 0.10 and [N/Fe]) = 1.18 +/- . 0.20, which is the first nitrogen measurement in this galaxy. The traditional definition of CEIVIP Stars is [C/Fe] >= 0.70, but taking into account that this luminous red giant branch star has undergone mixing, it was intrinsically less nitrogen enhanced and more carbon rich when it was formed, and so it falls under the definition of CEMP stars, as proposed by Aoki et al. (2007. ApJ, 655. 492) to account for this effect. By making corrections for this mixing. we conclude that the star had EC/Fel 0.8 during its earlier evolutionary stages. Apart from the enhanced C and N abundances, ET0097 shows no peculiarities in other elements lighter than Zn. and no enhancement of the heavier neutron capture elements (Ba, La, Ce, Ncl, Sm Eu, Dy). making this a CEMP-no star. However, the star does show signs of the weak hprocess. with an overabundance of the lighter neutron-capture elements (Sr, Y, Zr). To explain the abundance pattern observed in ET0097, we explore the possibility that this star was enriched by primordial stars. In addition to the detailed abundances for ET0097. we present estimates and upper limits for C abundances in 85 other stars in Sculptor derived from UN molecular lines. including 11 stars with [Fe/H] <= -2. Combining these limits with observations horn the literature, the fraction of CEMP-no stars in Sculptor seems to be significantly lower than in the Galactic halo.
Granulation In K-Type Dwarf Stars I. Spectroscopic Observations
(2008-12) Ramirez, I.; Prieto, C. Allende; Lambert, D. L.; Ramirez, I.; Prieto, C. Allende; Lambert, D. L.
Aims. We seek to detect and quantify the effects of surface convection (granulation) on the line spectra of K-dwarfs as a first step towards a rigorous testing of hydrodynamic models for their atmospheres. Methods. Very high-resolution (R similar or equal to 160 000-210 000), high signal-to-noise ratio (S/N greater than or similar to 300) spectra of nine bright K-dwarfs were obtained with the 2dcoude spectrograph on the 2.7m telescope at McDonald Observatory to determine wavelength shifts and asymmetries of Fe I lines. Spectra of the same stars acquired with the High Resolution Spectrograph (R similar or equal to 120, 000) on the 9.2m Hobby Eberly Telescope were used as radial velocity templates to calibrate the wavelength scale of the 2dcoude spectra. Results. The observed shapes and positions of Fe I lines reveal asymmetries and wavelength shifts that indicate the presence of granulation. In particular, line bisectors show characteristic C-shapes while line core wavelengths are blueshifted by an amount that increases with decreasing equivalent width (EW). On average, Fe I line bisectors have a span that ranges from nearly 0 for the weakest lines (residual core flux greater than or similar to 0.7) to about 75 m s(-1) for the strongest lines (residual core flux similar or equal to 0.3), while wavelength shifts range from about -150 m s(-1) in the weakest (EW similar or equal to 10m angstrom) lines to 0 in the strongest (EW similar or equal to 100m angstrom) features. A more detailed inspection of the bisectors and wavelength shifts reveals star-to-star differences that are likely associated with differences in stellar parameters, projected rotational velocity, and stellar activity. While the first two are understood and confirmed by our data, the relation to stellar activity, which is based on our finding that the largest departures from the expected behavior are seen in the most active stars, requires further investigation. For the inactive, slow projected rotational velocity stars, we detect, unequivocally, a plateau in the line-shifts at high EW values (EW greater than or similar to 100 m angstrom), a behavior that had been identified before only in the solar spectrum. The detection of this plateau allows us to determine the zero point of the convective blueshifts, which is useful to determine absolute radial velocities. Thus, we are able to measure such velocities with a mean uncertainty of about 60 m s(-1) for four of our sample stars.
Radial Velocities and Pulsation Ephemerides of 11 Field RR Lyrae Stars
(2011-06) For, Bi-Qing; Preston, George W.; Sneden, Christopher; For, Bi-Qing; Sneden, Christopher
We present new radial velocities (RVs), improved pulsation periods, and reference epochs of 11 field RR Lyrae ab-type variables: AS Vir, BS Aps, CD Vel, DT Hya, RV Oct, TY Gru, UV Oct, V1645 Sgr, WY Ant, XZ Aps, and Z Mic. This study is based on high-resolution spectra obtained with the echelle spectrograph of the 2.5 m du Pont telescope at Las Campanas Observatory. We obtained similar to 200 spectra per star (i.e., a total of similar to 2300 spectra), distributed more or less uniformly throughout their pulsation cycles. RV curves and photometric light curves phased to our new ephemerides are presented for all program stars. In a subsequent paper, we will use these spectra to derive stellar atmospheric parameters and chemical compositions throughout the pulsational cycles, based purely on spectroscopic constraints.