arXiv subjects

Camilla Juul Hansen

Publications and source records attributed to Camilla Juul Hansen.

23 records · Page 2Linked to original sources

arXiv2017

Spectroscopic study of the elusive globular cluster ESO452-SC11 and its surroundings

Globular clusters (GCs) are amongst the oldest objects in the Galaxy and play a pivotal role in deciphering its early history. We present the first spectroscopic study of the GC ESO452-SC11 using the AAOmega spectrograph at medium resolution. Given the sparsity of this object and high degree of foreground contamination due to its location toward the bulge, few details are known for this cluster: there is no consensus of its age, metallicity, or its association with the disk or bulge. We identify 5 members based on radial velocity, metallicity, and position within the GC. Using spectral synthesis, accurate abundances of Fe and several $α$-, Fe-peak, neutron-capture elements (Si,Ca,Ti,Cr,Co,Ni,Sr,Eu) were measured. Two of the 5 cluster candidates are likely non-members, as they have deviant Fe abundances and [$α$/Fe] ratios. The mean radial velocity is 19$\pm$2 km s$^{-1}$ with a low dispersion of 2.8$\pm$3.4 km s$^{-1}$, in line with its low mass. The mean Fe-abundance from spectral fitting is $-0.88\pm0.03$, with a spread driven by observational errors. The $α$-elements of the GC candidates are marginally lower than expected for the bulge at similar metallicities. As spectra of hundreds of stars were collected in a 2 degree field around ESO452-SC11, detailed abundances in the surrounding field were measured. Most non-members have higher [$α$/Fe] ratios, typical of the nearby bulge population. Stars with measured Fe-peak abundances show a large scatter around Solar values, though with large uncertainties. Our study provides the first systematic measurement of Sr in a Galactic bulge GC. The Eu and Sr abundances of the GC candidates are consistent with a disk or bulge association. Our calculations place ESO452 on an elliptical orbit in the central 3 kpc of the bulge. We find no evidence of extratidal stars in our data. (Abridged)

astro-ph.GA↗

arXiv2016

Chemical abundances in a high velocity RR Lyrae star near the bulge

Low-mass, variable, high-velocity stars are interesting study cases for many aspects of Galactic structure and evolution. Until recently, the only known high- or hyper-velocity stars were young stars thought to originate from the Galactic centre. Wide-area surveys like APOGEE and BRAVA have found several low-mass stars in the bulge with Galactic rest-frame velocities larger than 350 km/s. In this study we present the first abundance analysis of a low-mass, RR Lyrae star, located close to the Galactic bulge, with a space motion of ~ -400 km/s. Using medium-resolution spectra, we derive abundances (including upper limits) of 11 elements. These allow us to chemically tag the star and discuss its origin, although our derived abundances and metallicity, at [Fe/H] =-0.9 dex, do not point toward one unambiguous answer. Based on the chemical tagging, we cannot exclude that it originated in the bulge. However, combining its retrograde orbit and the derived abundances suggests that the star was accelerated from the outskirts of the inner (or even outer) halo during many-body interactions. Other possible origins include the bulge itself, or the star could be stripped from a star cluster or the Sagittarius dwarf galaxy when it merged with the Milky Way.

astro-ph.SR↗

arXiv2012

Chemical tagging and the second r-process

Elements in the range 37 < Z < 47 provide key information on their formation process. Several studies have shown that some of these elements are formed by an r-process, that differs from the main r-process creating europium. Through a detailed abundance study of Rb - Ag I will show, by comparing these abundances to those of Ba and Eu, that their formation processes differ. The formation process of Pd and Ag deviates from the weak/main s-process as well as from the main r-process. Hence, Pd and Ag - and to some extend Zr - are created by a second/weak r-process. However, the characteristics and formation site of this process is not well understood. The abundance ratios of Rb/Zr help constrain the neutron number density of the formation site, while comparing the Pd and Ag abundances to yield predictions can provide limitations on the entropy and electron fraction of the formation environment. This study presents clues on the second r-process. Furthermore, the formation processes of the heavy elements might not differ in a clear cut way. Several of these neutron-capture processes might yield various amounts of heavy elements (e.g. Sr and Ba) at the same time or metallicity. This could possibly help explain the large star-to-star abundance scatter for these two elements below [Fe/H]= -2.5. Knowing their origin is important in the era of large surveys (e.g Gaia-ESO). Strontium and barium will, limited by resolution and signal-to-noise ratio, be the only detectable heavy elements in the most metal-poor stars. Hence, they will, depending on metallicity, be the main tracers of the weak and main s-/r-processes. Understanding the effects of stellar parameters, synthetic spectrum codes, model atmospheres, and NLTE on the Sr abundances are crucial to describe the chemical evolution of our Galaxy. I will present these effects for Sr.

astro-ph.SR↗

arXiv2012

Silver and palladium help unveil the nature of a second r-process

The rapid neutron-capture process, creating about half of the heaviest elements in the Solar System was believed to be unique. Many recent studies have shown that this does not include the formation of lighter elements (in particular 38 < Z < 48). Among those, palladium (Pd) and especially silver (Ag) are expected to be key indicators of a possible second r-process, but until recently they have been studied only in a few stars. Therefore we target Pd and Ag in a large sample of stars and compare these abundances to those of Sr, Y, Zr, Ba and Eu produced by the slow (s-) and rapid (r-) neutron-capture processes. Hereby we investigate the nature of the formation process of Ag and Pd. Through a homogeneous 1D LTE analysis of 71 stars we derive stellar abundances using the spectrum synthesis code MOOG, and MARCS model atmospheres. We calculate abundance ratio trends and compare the derived abundances to site-dependent yield predictions (low mass O-Ne-Mg cc SN, and parametrised high entropy winds), to extract characteristics of the second r-process. The abundance ratios of the heavy elements yield correlations and anti-correlations. These trends lead to clear indications of the existence of a second/weak r-process, responsible for the formation of Pd and Ag. By comparing to the model predictions, we find that the conditions under which this process takes place differ from the main r-process in needing lower neutron number densities, neutron-to-seed ratios, entropies and/or favour higher electron abundances. Our analysis confirms that Pd and Ag form via a r-process that differs from the main r-process, the main and weak s-processes, and charged particle freeze-outs. This process is efficiently working down to [Fe/H] = -3.3 (where our sample ends). Our results may indicate that a combination of these explosive sites is needed to explain the observationally-derived abundance patterns.

astro-ph.SR↗

arXiv2010

Stellar atmosphere parameters with MAx, a MAssive compression of x^2 for spectral fitting

MAx is a new tool to estimate parameters from stellar spectra. It is based on the maximum likelihood method, with the likelihood compressed in a way that the information stored in the spectral fluxes is conserved. The compressed data are given by the size of the number of parameters, rather than by the number of flux points. The optimum speed-up reached by the compression is the ratio of the data set to the number of parameters. The method has been tested on a sample of low-resolution spectra from the Sloan Extension for Galactic Understanding and Exploration (SEGUE) survey for the estimate of metallicity, effective temperature and surface gravity, with accuracies of 0.24 dex, 130K and 0.5 dex, respectively. Our stellar parameters and those recovered by the SEGUE Stellar Parameter Pipeline agree reasonably well. A small sample of high-resolution VLT-UVES spectra were also used to test the method and the results have been compared to a more classical approach. The speed and multi-resolution capability of MAx combined with its performance compared with other methods indicates that it will be a useful tool for the analysis of upcoming spectral surveys.

astro-ph.GA↗