Abstract We have investigated the absorption shapes of atomic lines and H α in RR Lyrae stars. We used the database of high-resolution spectra gathered with the Las Campanas Observatory du Pont ...Telescope, analyzing a set of about 2700 short exposure spectra of 17 RRab and 5 RRc variables. To increase the signal-to-noise ratio of the spectra for each star, we first coadded spectra in small photometric phase bins, and then coadded metallic line profiles in velocity space. The resulting line absorption shapes vary with photometric phase in a consistent manner for all RRab stars, while exhibiting no obvious phase-related variations for the RRc stars. We interpret these line profile variations in terms of velocity gradients in the photospheric layers that produce absorption line profiles. The H α profiles are much broader, indicative of shock temperatures of order 100,000 K.
Neutron-Capture Elements in the Early Galaxy Sneden, Christopher; Cowan, John J.; Gallino, Roberto
Annual review of astronomy and astrophysics,
01/2008, Letnik:
46, Številka:
1
Journal Article
Recenzirano
The content of neutron-capture (trans-iron-peak) elements in the low-metallicity Galactic halo varies widely from star to star. The differences are both in bulk amount of the neutron-capture elements ...with respect to lighter ones and in element-to-element ratios among themselves. Several well-defined abundance distributions have emerged that reveal characteristic rapid and slow neutron-capture nucleosynthesis patterns. In this review we summarize these observed metal-poor star's abundances, contrasting them with the Solar-system values, comparing them to theoretical predictions, using them to assess the types of stars responsible for their specific anomalies, and speculating on the timing and nature of early Galactic nucleosynthesis.
We analyzed series of spectra obtained for 12 stable RRc stars observed with the echelle spectrograph of the du Pont telescope at Las Campanas Observatory and we analyzed the spectra of RRc Blazhko ...stars discussed by Govea et al. We derived model atmosphere parameters, Fe/H metallicities, and X/Fe abundance ratios for 12 species of 9 elements. We co-added all spectra obtained during the pulsation cycles to increase signal to noise and demonstrate that these spectra give results superior to those obtained by co-addition in small phase intervals. The RRc abundances are in good agreement with those derived for the RRab stars of Chadid et al. We used radial velocity (RV) measurements of metal lines and H to construct variations of velocity with phase, and center-of-mass velocities. We used these to construct RV templates for use in low- to medium-resolution RV surveys of RRc stars. Additionally, we calculated primary accelerations, radius variations, and metal and H velocity amplitudes, which we display as regressions against primary acceleration. We employ these results to compare the atmosphere structures of metal-poor RRc stars with their RRab counterparts. Finally, we use the RV data for our Blazhko stars and the Blazhko periods of Szczygie & Fabrycky to falsify the Blazhko oblique rotator hypothesis.
Abundance Variations within Globular Clusters Gratton, Raffaele; Sneden, Christopher; Carretta, Eugenio
Annual review of astronomy and astrophysics,
01/2004, Letnik:
42, Številka:
1
Journal Article
Recenzirano
▪ Abstract Abundance variations within globular clusters (GCs), and of GC stars with respect to field stars, are important diagnostics of a variety of physical phenomena, related to the evolution of ...individual stars, mass transfer in binary systems, and chemical evolution in high density environments. The broad astrophysical implications of GCs as building blocks of our knowledge of the Universe make a full understanding of their history and evolution basic in a variety of astrophysical fields. We review the current status of the research in this field, comparing the abundances in GCs with those obtained for field stars, discussing in depth the evidence for H-burning at high temperatures in GC stars, describing the process of self-enrichment in GCs with particular reference to the case of the most massive Galactic GC (ω Cen), and discussing various classes of cluster stars with abundance anomalies. Whereas the overall pattern might appear very complex at first sight, exciting new scenarios are opening where the interplay between GC dynamical and chemical properties are closely linked with each other.
ABSTRACT We have derived new, very accurate abundances of the Fe-group elements Sc through Zn (Z = 21−30) in the bright main-sequence turnoff star HD 84937 based on high-resolution spectra covering ...the visible and ultraviolet spectral regions. New or recent laboratory transition data for 14 species of seven elements have been used. Abundances from more than 600 lines of non-Fe species have been combined with about 550 Fe lines in HD 84937 to yield abundance ratios of high precision. The abundances have been determined from both neutral and ionized transitions, which generally are in agreement with each other. We find no substantial departures from the standard LTE Saha ionization balance in this Fe/H = −2.32 star. Noteworthy among the abundances are Co/Fe = +0.14 and Cu/Fe = −0.83, in agreement with past studies of abundance trends in this and other low-metallicity stars, and which has not been noted previously. A detailed examination of scandium, titanium, and vanadium abundances in large-sample spectroscopic surveys reveals that they are positively correlated in stars with Fe/H < −2; HD 84937 lies at the high end of this correlation. These trends constrain the synthesis mechanisms of Fe-group elements. We also examine the Galactic chemical evolution abundance trends of the Fe-group elements, including a new nucleosynthesis model with jet-like explosion effects.
This paper presents the detailed abundances and r-process classifications of 126 newly identified metal-poor stars as part of an ongoing collaboration, the R-Process Alliance. The stars were ...identified as metal-poor candidates from the RAdial Velocity Experiment (RAVE) and were followed up at high spectral resolution (R ∼ 31,500) with the 3.5 m telescope at Apache Point Observatory. The atmospheric parameters were determined spectroscopically from Fe i lines, taking into account non-LTE corrections and using differential abundances with respect to a set of standards. Of the 126 new stars, 124 have Fe/H < −1.5, 105 have Fe/H < −2.0, and 4 have Fe/H < −3.0. Nine new carbon-enhanced metal-poor stars have been discovered, three of which are enhanced in r-process elements. Abundances of neutron-capture elements reveal 60 new r-I stars (with +0.3 ≤ Eu/Fe ≤ +1.0 and Ba/Eu < 0) and 4 new r-II stars (with Eu/Fe > +1.0). Nineteen stars are found to exhibit a "limited-r" signature (Sr/Ba > +0.5, Ba/Eu < 0). For the r-II stars, the second- and third-peak main r-process patterns are consistent with the r-process signature in other metal-poor stars and the Sun. The abundances of the light, , and Fe-peak elements match those of typical Milky Way (MW) halo stars, except for one r-I star that has high Na and low Mg, characteristic of globular cluster stars. Parallaxes and proper motions from the second Gaia data release yield UVW space velocities for these stars that are consistent with membership in the MW halo. Intriguingly, all r-II and the majority of r-I stars have retrograde orbits, which may indicate an accretion origin.
Abstract
We have gathered optical-region spectra, derived model atmosphere parameters, and computed elemental abundances for 15 red giant stars in the open cluster NGC 7789. We focus on the light ...element group CNOLi that provides clues to evolutionary changes associated with internal fusion events and chemical mixing. We confirm and extend an early report that NGC 7789 stars 193 and 301 have anomalously large Li abundances, and that these values are apparently unconnected to any other elements’ abundances in these stars. A companion study of He
i
λ
10830 lines in both field stars and cluster members shows that star 301 has a strong He feature while star 193 does not. Possible explanations for the large Li abundances of these stars include helium flash-induced mixing events and binary interactions at some past or present times. In either case an internal eruption of energy could cause fresh synthesis of lithium via the Cameron-Fowler Berillyum transport mechanism. Rapid transport of lithium to the outer layers may have created significant chromospheric transient disturbances, producing enough helium ionization to allow for the strong
λ
10830 absorption in star 301.
The abundance of H2 in molecular clouds, relative to the commonly used tracer CO, has only been measured toward a few embedded stars, which may be surrounded by atypical gas. We present observations ...of near-infrared absorption by H2, CO, and dust toward stars behind molecular clouds, providing a representative sample of these molecules in cold molecular gas, primarily in the Taurus Molecular Cloud. We find 1.0 × 1021 cm−2, 1.5 × 1017 cm−2 (1.8 × 1017 including solid CO), and 6000. The measured ratio is consistent with that toward embedded stars in various molecular clouds, but both are less than that derived from millimeter-wave observations of CO and star counts. The difference apparently results from the higher directly measured ratio.
Dispersion among the light elements is common in globular clusters (GCs), while dispersion among heavier elements is less common. We present detection of r-process dispersion relative to Fe in 19 red ...giants of the metal-poor GC M92. Using spectra obtained with the Hydra multi-object spectrograph on the WIYN Telescope at Kitt Peak National Observatory, we derive differential abundances for 21 species of 19 elements. The Fe-group elements, plus Y and Zr, are homogeneous at a level of 0.07-0.16 dex. The heavy-elements La, Eu, and Ho exhibit clear star-to-star dispersion spanning 0.5-0.8 dex. The abundances of these elements are correlated with one another, and we demonstrate that they were produced by r-process nucleosynthesis. This r-process dispersion is not correlated with the dispersion in C, N, or Na in M92, indicating that r-process inhomogeneities were present in the gas throughout star formation. The r-process dispersion is similar to that previously observed in the metal-poor GC M15, but its origin in M15 or M92 is unknown at present.
We report the discovery of a new actinide-boost star, 2MASS J09544277+5246414, originally identified as a very bright (V = 10.1), extremely metal-poor (Fe/H = −2.99) K giant in the LAMOST survey, and ...found to be highly r-process-enhanced (r-II; Eu/Fe = +1.28), during the snapshot phase of the R-Process Alliance (RPA). Based on a high signal-to-noise ratio (S/N), high-resolution spectrum obtained with the Harlan J. Smith 2.7 m telescope, this star is the first confirmed actinide-boost star found by RPA efforts. With an enhancement of Th/Eu = +0.37, 2MASS J09544277+5246414 is also the most actinide-enhanced r-II star yet discovered, and only the sixth metal-poor star with a measured uranium abundance (U/Fe = +1.40). Using the Th/U chronometer, we estimate an age of 13.0 4.7 Gyr for this star. The unambiguous actinide-boost signature of this extremely metal-poor star, combined with additional r-process-enhanced and actinide-boost stars identified by the RPA, will provide strong constraints on the nature and origin of the r-process at early times.
PDF
