Detailed chemical abundance analysis of the thick disk star cluster Gaia 1

Star clusters, particularly those objects in the disk-bulge-halo interface are as of yet poorly charted, albeit carrying important information about the formation and the structure of the Milky Way. Here, we present a detailed chemical abundance study of the recently discovered object Gaia 1. Photometry has previously suggested it as an intermediate-age, moderately metal-rich system, although the exact values for its age and metallicity remained ambiguous in the literature. We measured detailed chemical abundances of 14 elements in four red giant members, from high-resolution (R=25000) spectra that firmly establish Gaia 1 as an object associated with the thick disk. The resulting mean Fe abundance is $-0.62\pm$0.03(stat.)$\pm$0.10(sys.) dex, which is more metal-poor than indicated by previous spectroscopy from the literature, but it is fully in line with values from isochrone fitting. We find that Gaia 1 is moderately enhanced in the $\alpha$-elements, which allowed us to consolidate its membership with the thick disk via chemical tagging. The cluster's Fe-peak and neutron-capture elements are similar to those found across the metal-rich disks, where the latter indicate some level of $s$-process activity. No significant spread in iron nor in other heavy elements was detected, whereas we find evidence of light-element variations in Na, Mg, and Al. Nonetheless, the traditional Na-O and Mg-Al (anti-)correlations, typically seen in old globular clusters, are not seen in our data. This confirms that Gaia 1 is rather a massive and luminous open cluster than a low-mass globular cluster. Finally, orbital computations of the target stars bolster our chemical findings of Gaia 1's present-day membership with the thick disk, even though it remains unclear, which mechanisms put it in that place.Comment: 11 pages, 11 figures, accepted for publication in Astronomy & Astrophysics. Some figure sizes reduce

A spectroscopic binary in the Hercules dwarf spheroidal galaxy

We present the radial velocity curve of a single-lined spectroscopic binary in the faint Hercules dwarf spheroidal (dSph) galaxy, based on 34 individual spectra covering more than two years of observations. This is the first time that orbital elements could be derived for a binary in a dSph. The system consists of a metal-poor red giant and a low-mass companion, possibly a white dwarf, with a 135-days period in a moderately eccentric ($e=0.18$) orbit. Its period and eccentricity are fully consistent with metal-poor binaries in the Galactic halo, while the projected semimajor axis is small, at $a_p$ sin$i$ = 38 R$_{sun}$. In fact, a very close orbit could inhibit the production of heavier elements through $s$-process nucleosynthesis, leading to the very low abundances of neutron-capture elements that are found in this star. We discuss the further implications for the chemical enrichment history of the Hercules dSph, but find no compelling binary scenario that could reasonably explain the full, peculiar abundance pattern of the Hercules dSph galaxy.Comment: 7 pages, 3 figures, accepted for publication in the Astrophysical Journa

Abundances and kinematics of carbon-enhanced metal-poor stars in the Galactic halo*; A new classification scheme based on Sr and Ba

Carbon-enhanced metal-poor (CEMP) stars span a wide range of stellar populations, from bona fide second-generation stars to later forming stars that provide excellent probes of, e.g., binary mass transfer. Here we analyse 11 metal-poor stars of which 10 are CEMP stars. Based on high signal-to-noise (SNR) X-Shooter spectra, we derive abundances of 20 elements (C, N, O, Na, Mg, Ca, Sc, Ti, Cr, Mn, Fe, Ni, Sr, Y, Ba, La, Ce, Pr, Nd, Eu). From the high SNR spectra, we trace the chemical contribution of the rare earth elements (REE) from various production sites, finding a preference for metal-poor low-mass AGB stars of 1.5Mo in CEMP-s stars, while CEMP-r/s stars may indicate a more massive AGB contribution (2-5Mo). A contribution from the r-process - possibly from neutron star mergers (NSM), is also detectable in the REE abundances, especially in the CEMP-r/s. Combining spectra with Gaia DR2 astrometric data indicates that all but one star in our sample (and most literature stars) belong to the Galactic halo. They exhibit a median orbital eccentricity of 0.7, and are found on both pro- and retrograde orbits. The orbital parameters of CEMP-no and CEMP4s stars are remarkably similar in the 98 stars we study. A special CEMP-no star, with very low Sr and Ba content, possesses the most eccentric orbit among the stars in our sample, passing close to the Galactic centre. Finally, we propose an improved scheme to sub-classify the CEMP stars, making use of the Sr$/$Ba ratio, which can also be used to separate very metal-poor stars from CEMP stars in 93 stars in the metallicity range $-4.2<$[Fe/H]$<-2$. The Sr/Ba ratio can also be used for distinguishing CEMP-s,-r/s and -no stars. The Sr/Ba ratio is also a powerful astro-nuclear indicator, as AGB stars exhibit very different Sr/Ba ratios, compared to fast rotating massive stars and NSM, and it is fairly unbiased by NLTE and 3D corrections.(abridged)Comment: 15 pages, 4 pages appendix, 11 figures, accepted for publication in A&

"Det er viktig for meg med tilbakemeldinger". Underveisvurdering fra et elevperspektiv

I masteravhandlingen har jeg utført en kvalitativ studie med fokus på underveisvurdering fra et elevperspektiv. Observasjon og gruppeintervju er benyttet som metode i 5. klasse, fra to forskjellige skoler. Målet har vært å belyse hvordan elevene oppfatter underveisvurderingen i sin hverdag. For å besvare dette har jeg benyttet tre forskningsspørsmål: «Hvordan bidrar underveisvurdering til at elevene vet hva de kan?», «Hvordan opplever elevene underveisvurderingen i sin skolehverdag?» og «Hvordan oppfatter elevene at underveisvurderingen bidrar til videre læring?». Resultatene viser at elevene vet hva de kan gjennom strategien to tanker og en stjerne, prøver i de ulike fagene, egenvurdering etter matematikkprøvene og til dels gjennom a-planen. Videre er det en refleksjonsforskjell blant elevene knyttet til hvordan underveisvurderingen bidrar til videre læring. Det trekkes også frem viktigheten med dialog i skriftlige tilbakemeldinger, og at elevene må forstå vurderingen. På begge skolene er det et forbedringspotensial i forhold til elevenes bevissthet rundt egen læring, og hvordan målene og forventningene blir benyttet i undervisningen. Det ligger i tillegg et større potensial i a-planen, enn det elevene formidler at de benytter. Når det kommer til elevenes opplevelse av underveisvurderingen, gir elevene et tips om å gi mer positive tilbakemeldinger som de kan lære ut av

An observational study of metal-poor stars

The outer atmosphere of the first generation of low-mass stars retain to a great extent the original composition of the interstellar medium at the time and place of their birth. Hence the earliest phases of Galactical chemical evolution and nucleosynthesis can be investigated by means of studying the old, metal-poor stars. A minority of these stars exhibit dramatic enhancements in their abundances of heavy neutron-capture elements and/or of carbon. The key question for Galactic chemical evolution models is whether these peculiarities reflect the composition of the natal clouds, or if they are due to later (post-birth) mass transfer of chemically processed material from a binary companion. If the latter is the case, these stars should all be members of binary systems. This thesis presents high-resolution elemental-abundance analysis for a sample of 23 very metal-poor (VMP; [Fe=H] < -2.0) stars, 12 of which are extremely metal-poor (EMP; [Fe/H] < -3.0), and 4 of which are ultra metal-poor (UMP; [Fe/H] < -4.0). The results of radial velocity monitoring of 17 r-process enhanced stars (r-I and r-II stars), 24 CEMP-no stars, 18 CEMP-s and four CEMP-r/s stars, are also presented. The stars, for which the abundance analysis were performed, were targeted to explore differences in the abundance ratios of Li, C, N, O, the alpha-elements, the iron-peak elements, and a number of neutron-capture elements. These are elements that constrain the possible astrophysical sites of element production. This sample has substantially increased the number of known carbon-enhanced metal-poor (CEMP) and nitrogen-enhanced metal-poor (NEMP) stars. The sample of stars include eight that are considered "normal" metal-poor stars, six CEMP-no stars, �ve CEMP-s stars, two CEMP-r stars, and two CEMP-r/s stars. One of the CEMP-r stars and one of the CEMP-r/s stars are possible NEMP stars. Lithium is detected for three of the six CEMP-no stars, all of which are Li-depleted with respect to the Li plateau for metal-poor dwarfs found by Spite & Spite. This suggests that whatever site(s) produced C either do not completely destroy lithium, or that Li has been astrated by early-generation stars and mixed with primordial Li in the gas that formed the stars observed at present. Carbon and nitrogen abundances for the CEMP stars reveal, for the majority, that a small degree of mixing has happened in their progenitor stars ([C/N] > 0). However, signs of a larger degree of mixing ([C/N] < 0) is found in some CEMP-no stars, but these stars are only found at the lowest metalicities ([Fe/H] < -3:4). CEMP-no stars with large enhancements in Na, Mg, and Al are also only found below this metallicity. This sample confirms the existence of two separate bands in the absolute carbon abundances of CEMP stars, as suggested by Spite et al. The derived abundances for the alpha-elements and iron-peak elements of the stars are similar to those found in previous large samples of metal-poor stars. Finally evidence for a 'floor' in the absolute Ba abundances of CEMP-no stars at A(Ba) ~ -2.0 is also presented. Binary frequencies of 18%, 17%, and 75% are found for the r-process enhanced, CEMPno and CEMP-s stars, respectively. These results show that the nucleosynthetic processes, responsible for the strong carbon excess in the CEMP-no stars, and the r-process element enhancement in the r-I and r-II stars, are unrelated to their binary population. Instead, the element excess was imprinted on the natal molecular clouds of these stars by an external, distant source. The high frequency of binary stars found for the CEMP-s stars however, demonstrate that the peculiar abundance pattern of these stars is coupled to the binary nature of the stars