Context. The rapid neutron-capture process, which created about half of the heaviest elements in the solar system, is believed to have been unique. Many recent studies have shown that this uniqueness is not true for the formation of lighter elements, in particular those in the atomic number range 38 < Z < 48. Among these, 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. We therefore 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. Aims. We study the abundances of seven elements (Sr, Y, Zr, Pd, Ag, Ba, and Eu) to gain insight into the formation process of the elements and explore in depth the nature of the second r-process. Methods. By adopting a homogeneous one-dimensional local thermodynamic equilibrium (1D LTE) analysis of 71 stars, we derive stellar abundances using the spectral synthesis code MOOG, and the MARCS model atmospheres. We calculate abundance ratio trends and compare the derived abundances to site-dependent yield predictions (low-mass O-Ne-Mg core-collapse supernovae and parametrised high-entropy winds), to extract characteristics of the second r-process. Results. The seven elements are tracers of different (neutron-capture) processes, which in turn allows us to constrain the formation process(es) of Pd and Ag. The abundance ratios of the heavy elements are found to be correlated and anti-correlated. These trends lead to clear indications that a second/weak r-process, is responsible for the formation of Pd and Ag. On the basis of the comparison to the model predictions, we find that the conditions under which this process takes place differ from those for the main r-process in needing lower neutron number densities, lower neutron-to-seed ratios, and lower entropies, and/or higher electron abundances. Conclusions. Our analysis confirms that Pd and Ag form via a rapid neutron-capture process that differs from the main r-process, the main and weak s-processes, and charged particle freeze-outs. We find that this process is efficiently working down to the lowest metallicity sampled by our analysis ([Fe/H] = −3.3). Our results may indicate that a combination of these explosive sites is needed to explain the variety in the observationally derived abundance patterns.

Radial velocities measured from near-infrared spectra are a potentially powerful tool to search for planets around cool stars and sub-stellar objects. However, no technique currently exists that yields near-infrared radial velocity precision comparable to that routinely obtained in the visible. We are carrying out a near-infrared radial velocity planet search program targeting a sample of the lowestmass M dwarfs using the CRIRES instrument on the VLT. In this first paper in a planned series about the project, we describe a method for measuring high-precision relative radial velocities of these stars from K-band spectra. The method makes use of a glass cell filled with ammonia gas to calibrate the spectrograph response similar to the "iodine cell" technique that has been used very successfully in the visible. Stellar spectra are obtained through the ammonia cell and modeled as the product of a Doppler-shifted template spectrum of the object and a spectrum of the cell, convolved with a variable instrumental profile model. A complicating factor is that a significant number of telluric absorption lines are present in the spectral regions containing useful stellar and ammonia lines. The telluric lines are modeled simultaneously as well using spectrum synthesis with a time-resolved model of the atmosphere over the observatory. The free parameters in the complete model are the wavelength scale of the spectrum, the instrumental profile, adjustments to the water and methane abundances in the atmospheric model, telluric spectrum Doppler shift, and stellar Doppler shift. Tests of the method based on the analysis of hundreds of spectra obtained for late M dwarfs over six months demonstrate that precisions of ∼ 5 m s −1 are obtainable over long timescales, and precisions of better than 3 m s −1 can be obtained over timescales up to a week. The obtained precision is comparable to the predicted photon-limited errors, but primarily limited over long timescales by the imperfect modeling of the telluric lines.

Using the Fourier Transform Spectrometer at Lund Observatory, intensity calibrated spectra of singly ionized zirconium have been recorded and analyzed. Oscillator strengths for 263 Zr ii spectral lines in the region 2500-5400 Å have been derived by combining new experimental branching fractions with previously measured radiative lifetimes. The transitions combine 34 odd parity levels with 29 low metastable levels between 0 and 2.4 eV. The experimental branching fractions have been compared with theoretical values and the oscillator strengths with previously published data when available. The oscillator strengths have been employed to derive the solar photospheric Zr abundance based on both 1D and 3D model atmospheres. Based on the seven best and least perturbed Zr ii lines in the solar disk-center spectrum, we determine the solar Zr abundance to log ε Zr = 2.58 ± 0.02 when using a 3D hydrodynamical solar model atmosphere. The new value is in excellent agreement with the meteoritic Zr abundance.

We report absolute oscillator strengths for 119 Cr ii transitions in the wavelength region 2050−4850 Å. The transition probabilities have been derived by combining radiative lifetimes, measured with time-resolved laser induced fluorescence, and branching fractions from intensity calibrated Fouirer transform spectrometer data. New radiative lifetimes for the 3d 4 ( 5 D)4p 4 F, 4 D and 6 P terms are reported, adding up to a total of 25 energy levels with measured lifetimes used to derive this improved set of atomic data.

Context. In 1992 we began a precision radial velocity survey for planets around solar-like stars with the Coudé Echelle Spectrograph and the Long Camera (CES LC) at the 1.4 m telescope in La Silla (Chile) resulting in the discovery of the planet ι Hor b. We have continued the survey with the upgraded CES Very Long Camera (VLC) and the HARPS spectrographs, both at the 3.6 m telescope, until 2007. Aims. In this paper we present additional radial velocities for 31 stars of the original sample with higher precision. The observations cover a time span of up to 15 years and permit a search for Jupiter analogues. Methods. The survey was carried out with three different instruments/instrument configurations using the iodine absorption cell and the ThAr methods for wavelength calibration. We combine the data sets and perform a joint analysis for variability, trends, and periodicities. We compute Keplerian orbits for companions and detection limits in case of non-detections. Moreover, the HARPS radial velocities are analysed for correlations with activity indicators (CaII H&K and cross-correlation function shape). Results. We achieve a long-term RV precision of 15 m/s (CES+LC, 1992-1998, 9 m/s (CES+VLC, 1999, and 2.8 m/s (HARPS, 2003(HARPS, -2009, including archive data), respectively. This enables us to confirm the known planetary signals in ι Hor and HR 506 as well as the three known planets around HR 3259. A steady RV trend for Ind A can be explained by a planetary companion and calls for direct imaging campaigns. On the other hand, we find previously reported trends to be smaller for β Hyi and not present for α Men. The candidate planet Eri b was not detected despite our better precision. Also the planet announced for HR 4523 cannot be confirmed. Long-term trends in several of our stars are compatible with known stellar companions. We provide a spectroscopic orbital solution for the binary HR 2400 and refined solutions for the planets around HR 506 and ι Hor. For some other stars the variations could be attributed to stellar activity, as e.g. the magnetic cycle in the case of HR 8323. Conclusions. The occurrence of two Jupiter-mass planets in our sample is in line with the estimate of 10% for the frequency of giant planets with periods smaller than 10 yr around solar-like stars. We have not detected a Jupiter analogue, while the detections limits for circular orbits indicate at 5 AU a sensitivity for minimum mass of at least 1M Jup (2M Jup ) for 13% (61%) of the stars.

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Experimental branching fractions (BFs) of Mo II, ranging in wavelength from 1970 to 4370 Å, have been measured from intensity calibrated spectra recorded with the Lund UV Fourier transform spectrometer (FTS). Radiative lifetimes for 10 levels have been measured using the method of laser-induced fluorescence (LIF). Combining BFs with new as well as previously measured lifetimes, 16 in total, oscillator strengths of 91 lines were derived. Seven transitions are resonance lines involving the ground state. The BF results are compared with calculations made with the Cowan code and the f-values are compared with previously published data. Improved wavelengths from an ongoing term analysis are also reported.

Abstract. We have measured radiative lifetimes of ten Th II levels by using the laser-induced fluorescence technique and branching fractions with Fourier transform spectroscopy. By combining the new branching fractions with a total of 23 lifetimes, from the present work and from measurements by Simonsen et al. (1988), absolute oscillator strengths for 180 lines have been derived. Some of these new f -values reported are relevant for radioactive dating of stars.