P. North

Extremely metal-poor stars in classical dwarf spheroidal galaxies: Fornax, Sculptor, and Sextans

We present the results of a dedicated search for extremely metal-poor stars in the Fornax, Sculptor, and Sextans dSphs. Five stars were selected from two earlier VLT/Giraffe and HET/HRS surveys and subsequently followed up at high spectroscopic resolution with VLT/UVES. All of them turned out to have [Fe/H] less than or similar to -3 and three stars are below [Fe/H] similar to -3.5. This constitutes the first evidence that the classical dSphs Fornax and Sextans join Sculptor in containing extremely metal-poor stars and suggests that all of the classical dSphs contain extremely metal-poor stars. One giant in Sculptor at [Fe/H] = -3.96+/-0.06 is the most metal-poor star ever observed in an external galaxy. We carried out a detailed analysis of the chemical abundances of the alpha, iron peak, and the heavy elements, and we performed a comparison with the Milky Way halo and the ultra faint dwarf stellar populations. Carbon, barium, and strontium show distinct features characterized by the early stages of galaxy formation and can constrain the origin of their nucleosynthesis.

Detection of an extraordinarily large magnetic field in the unique ultra-cool Ap star HD 154708

We have discovered an extraordinarily large mean longitudinal magnetic field of 7.5 kG in the ultra-cool low mass Ap star HD 154708 using FORS 1 in spectropolarimetric mode. From UVES spectra, we have measured a mean magnetic field modulus of 24.5 kG. This is the second-largest mean magnetic field modulus ever measured in an Ap star. Furthermore, it is very likely that this star is one of the coolest and least massive among the Ap stars and is located in the H-R diagram in the same region in which rapidly oscillating Ap stars have been detected. We note that all known roAp stars have much smaller magnetic fields, by at least a factor of three.

A purely geometric distance to the binary star Atlas, a member of the Pleiades

We present radial velocity and new interferometric measurements of the double star Atlas, which permit, with the addition of published interferometric data, to precisely derive the orbital parameters of the binary system and the masses of the components. The derived semi-major axis, compared with its measured angular size, allows to determine a distance to Atlas of 132 ± 4 pc in a purely geometrical way. Under the assumption that the location of Atlas is representative of the average distance of the cluster, we confirm the distance value generally obtained through main sequence fitting, in contradiction with the early Hipparcos result (118.3 ± 3. 5p c).

The early days of the Sculptor dwarf spheroidal galaxy

We present the high-resolution spectroscopic study of five -3.9 <= [Fe/H] <= -2.5 stars in the Local Group dwarf spheroidal, Sculptor, thereby doubling the number of stars with comparable observations in this metallicity range. We carry out a detailed analysis of the chemical abundances of a, iron peak, and light and heavy elements, and draw comparisons with the Milky Way halo and the ultra-faint dwarf stellar populations. We show that the bulk of the Sculptor metal-poor stars follow the same trends in abundance ratios versus metallicity as the Milky Way stars. This suggests similar early conditions of star formation and a high degree of homogeneity of the interstellar medium. We find an outlier to this main regime, which seems to miss the products of the most massive of the Type II supernovae. In addition to its help in refining galaxy formation models, this star provides clues to the production of cobalt and zinc. Two of our sample stars have low odd-to-even barium isotope abundance ratios, suggestive of a fair proportion of s-process. We discuss the implication for the nucleosynthetic origin of the neutron capture elements.

Magnetically controlled circumstellar plasma in the helium-weak stars

We report the discovery of variable C uf769uf776 line profiles in the magnetic helium-weak stars HD 142301, HD 142990, and HD 144334, all of which are members of the Sco-Cen association, and the hot helium-weak star HD 175362. These stars display behavior that is similar to HD 5737 and HD 79158, two helium-weak stars for which magnetically confined plasma has previously been reported. The present study thus extends the C uf769uf776 survey to all of the stars for which strong radio emission has been detected. No C uf769uf776 absorption has been detected for two other Sco-Cen chemically peculiar stars, HD 144844 and 146001, nor has it been detected from archival spectra of HD 28843, HD 49333, or HD 124224. The reported detections are the first helium-weak stars that do not belong to thesn subclass that have been found to display such UV line profile variations. It appears that the restriction of this phenomenon to the extreme helium-weak subgroup was a selection effect and that the occurrence of magnetically trapped corotating plasma is more widespread among the upper main sequence magnetic spectrum variables. We show model calculations for the C uf769uf776 variability that clarify the plasmaspheric structure. Supporting Hα observations show no evidence of line profile variability for HD 142301 and HD 144334, while HD 142990 appears to have extremely weak emission at certain phases, and the profile of HD 124224 shows some changing asymmetry.

Properties and nature of Be stars ? 30. Reliable physical properties of a semi-detached B9.5e+G8III binary BR CMi = HD 61273 compared to those of other well studied semi-detached emission-line binaries

Reliable determination of the basic physical properties of hot emission-line binaries with Roche-lobe filling secondaries is important for developing the theory of mass exchange in binaries. It is a very hard task, however, which is complicated by the presence of circumstellar matter in these systems. So far, only a small number of systems with accurate values of component masses, radii, and other properties are known. Here, we report the first detailed study of a new representative of this class of binaries, BR CMi, based on the analysis of radial velocities and multichannel photometry from several observatories, and compare its physical properties with those for other well-studied systems. BR CMi is an ellipsoidal variable seen under an intermediate orbital inclination of ~51 degrees, and it has an orbital period of 12.919059(15) d and a circular orbit. We used the disentangled component spectra to estimate the effective temperatures 9500(200) K and 4655(50) K by comparing them with model spectra. They correspond to spectral types B9.5e and G8III. We also used the disentangled spectra of both binary components as templates for the 2-D cross-correlation to obtain accurate RVs and a reliable orbital solution. Some evidence of a secular period increase at a rate of 1.1+/-0.5 s per year was found. This, together with a very low mass ratio of 0.06 and a normal mass and radius of the mass gaining component, indicates that BR CMi is in a slow phase of the mass exchange after the mass-ratio reversal. It thus belongs to a still poorly populated subgroup of Be stars for which the origin of Balmer emission lines is safely explained as a consequence of mass transfer between the binary components.

What is the Milky Way outer halo made of? High resolution spectroscopy of distant red giants

In a framework where galaxies form hierarchically, extended stellar haloes are predicted to be an ubiquitous feature around Milky Way-like galaxies and to consist mainly of the shredded stellar component of smaller galactic systems. The type of accreted stellar systems are expected to vary according to the specific accretion and merging history of a given galaxy, and so is the fraction of stars formed in-situ versus accreted. Analysis of the chemical properties of Milky Way halo stars out to large Galactocentric radii can provide important insights into the properties of the environment in which the stars that contributed to the build-up of different regions of the Milky Way stellar halo formed. In this work we focus on the outer regions of the Milky Way stellar halo, by determining chemical abundances of halo stars with large present-day Galactocentric distances,

Properties and nature of Be stars 30. Reliable physical properties of a semi-detached B9.5e+G8III binary BR CMi = HD 61273 compared to those of other well studied semi-detached

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Extremely metal-poor stars in classical dwarf spheroidal galaxies: Fornax, Sculptor, and Sextans (vol 524, pg A58, 2010)

15 kpc. The data-set we acquired consists of high resolution HET/HRS, Magellan/MIKE and VLT/UVES spectra for 28 red giant branch stars covering a wide metallicity range,

Extremely metal-poor stars in classical dwarf spheroidal galaxies: Fornax, Sculptor, and Sextans (Corrigendum)

-3.1 lesssim