Hampus Nilsson

New and improved experimental oscillator strengths in Zr II and the solar abundance of zirconium

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 angstrom 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 epsilon(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. (Less)

The FERRUM project: improved experimental oscillator strengths in CrII

We report absolute oscillator strengths for 119 Cr II transitions in the wavelength region 2050-4850 angstrom. 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)(D-5)4p F-4, D-4 and P-6 terms are reported, adding up to a total of 25 energy levels with measured lifetimes used to derive this improved set of atomic data.

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

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.

Experimental oscillator strengths in Th II

We have measured radiative lifetimes of ten Th II levels by using thelaser-induced fluorescence technique and branching fractions withFourier transform spectroscopy. By combining the new branching fractionswith a total of 23 lifetimes, from the present work and frommeasurements by Simonsen et al. (cite{Simonsen}), absolute oscillatorstrengths for 180 lines have been derived. Some of these new f-valuesreported are relevant for radioactive dating of stars.

Uncertainty of oscillator strengths derived from lifetimes and branching fractions

Abstract A widely used method for determination of transition probabilities and oscillator strengths is based on measurements of branching fractions and radiative lifetimes. In the present work the different sources of uncertainty in branching fraction measurements using Fourier transform spectroscopy and lifetime measurements using laser induced fluorescence are discussed. A detailed description is presented of how the uncertainties should be combined to provide a well-defined uncertainty of the derived quantity. Finally, an example shows how the individual uncertainties can be presented in an “uncertainty budget”.

THE PROPOSED GIANT PLANET ORBITING VB 10 DOES NOT EXIST

We present high-precision relative radial velocities of the very low mass star VB 10 that were obtained over a time span of 0.61 years as part of an ongoing search for planets around stars at the end of the main sequence. The radial velocities were measured from high-resolution near-infrared spectra obtained using the CRIRES instrument on the Very Large Telescope with an ammonia gas cell. The typical internal precision of the measurements is 10 m s(-1). These data do not exhibit significant variability and are essentially constant at a level consistent with the measurement uncertainties. Therefore, we do not detect the radial velocity variations of VB 10 expected due to the presence of an orbiting giant planet similar to that recently proposed by Pravdo & Shaklan based on apparent astrometric perturbations. In addition, we do not confirm the similar to 1 km s(-1) radial velocity variability of the star tentatively detected by Zapatero Osorio and colleagues with lower precision measurements. Our measurements rule out planets with M-p > 3 M-Jup and the orbital period and inclination suggested by Pravdo & Shaklan at better than 5 sigma confidence. We conclude that the planet detection claimed by Pravdo & Shaklan is spurious on the basis of this result. Although the outcome of this work is a non-detection, it illustrates the potential of using ammonia cell radial velocities to detect planets around very low mass stars.

The planet search programme at the ESO CES and HARPS - IV. The search for Jupiter analogues around solar-like stars

Context. In 1992 we began a precision radial velocity survey for planets around solar-like stars with the Coude Echelle Spectrograph and the Long Camera (CES LC) at the 1.4m telescope in La Silla (Chile) resulting in the discovery of the planet iota Hor b. We have continued the survey with the upgraded CES Very Long Camera (VLC) and the HARPS spectrographs, both at the 3.6m 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, 1999-2006), and 2.8 m/s (HARPS, 2003-2009, including archive data), respectively. This enables us to confirm the known planetary signals in iota Hor and HR 506 as well as the three known planets around HR 3259. A steady RV trend for epsilon 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 beta Hyi and not present for alpha Men. The candidate planet epsilon 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 iota 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. (Less)

Improved oscillator strengths and wavelengths for Os I and Ir I, and new results on early r-process nucleosynthesis ?

The radioactive decay of 238 Ua nd 232 Th has recently been used to determine ages for some of the oldest stars in the Universe. This has highlighted the need for accurate observational constraints on production models for the heaviest r-process elements which might serve as stable references, notably osmium and iridium. In order to provide a firmer basis for the observed abundances, we have performed laser-induced fluorescence measurements and Fourier Transform Spectroscopy to determine new radiative lifetimes and branching fractions for selected levels in Os I and Ir I. From these data, we determine new absolute oscillator strengths and improved wavelengths for 18 Os I and 4 Ir I lines. A reanalysis of VLT spectra of CS 31082-001 and new results for other stars with Os and Ir detections show that (i): the lines in the UV and 4260 A yield reliable Os abundances, while those at 4135, 4420 A are heavily aected by blending; (ii): the Os and Ir abundances are identical in all the stars; (iii): the heavy-element abundances in very metal-poor stars conform closely to the scaled solar r-process pattern throughout the range 56 Z 77; and (iv): neither Os or Ir nor any lighter species are suitable as reference elements for the radioactive decay of Th and U.

A Goddard High Resolution Spectrograph Atlas of Echelle Observations of the HgMn Star χ Lupi

Observations of the ultra-sharp-lined, chemically peculiar star chi Lupi taken by the Goddard High Resolution Spectrograph in echelle mode are presented. Thirty-six intervals of the spectral region between 1249 and 2688 Angstrom are covered with resolving powers in the range 75,000-93,000. Line identifications are provided, and the observed spectra are compared with synthetic spectra calculated using the SYNTHE program and associated line lists with changes to the line lists. The significance of these spectra for the chi Lupi Pathfinder Project and the closely related atomic physics effort is discussed in a companion paper.

Lifetime measurements and calculated transition probabilities in W III

New experimental lifetimes for two levels in doubly ionized tungsten, W III, have been obtained using the time-resolved laser-induced-fluorescence technique. Theoretical oscillator strengths obtained from a HFR calculation including the effects of core-polarization are reported for all transitions from levels below E 2, thus greatly extending our knowledge of the radiative data in W III. Good agreement is found between the calculated lifetimes and the experimental values from this and previous works. These new results fill a gap in the available data for this ion which is expected to play an important role in fusion reactors.