arXiv subjects

Johan Holmberg

Publications and source records attributed to Johan Holmberg.

12 recordsLinked to original sources

arXiv2014

QED effects in scattering processes involving atomic bound states: Radiative recombination

The standard S-matrix formulation cannot generally be used in the treatment of atomic scattering processes, involving bound-state QED effects, due to the special type of singularity that can here appear. This type of singularity can be handled by means of methods designed for structure calculations. It is essentially a consequence of the optical theorem that similar techniques can be applied also in scattering processes. The optical theorem for free particles gives a relation between the effective Hamiltonian and the cross section, a relation that is valid also when bound states are present. We have found that the method with the Covariant-evolution-operator/Green's operator that we have developed primarily for structure problems can here be applied in a rather straightforward manner. The new procedure is demonstrated for the case of radiative recombination.

quant-ph↗

arXiv2012

Energy-dependent perturbation theory: Possibility for improved tests of quantum-electrodynamics

Measurements of energy separations in highly charged ions can in many cases nowadays be performed with very high accuracy, an accuracy that sometimes cannot be matched by the corresponding theoretical calcula- tions. Furthermore, it has recently been demonstrated that there is a systematic deviation between experimental and theoretical results for the K- alpha lines of medium-heavy heliumlike ions. We have during a number of years been developing a general procedure for energy-dependent perturbative calcu- lations, which opens up a unique possibility of incorporating the energy- dependent QED perturbations into the all-order many-body perturbation expansion in a rigorous way. Such an expansion will yield several important effects, never before accounted for in this type of analysis, which is expected to increase the theoretical accuracy considerably. Calculation of some of these effects have been performed at our laboratory in Gothenburg, and numerical results are given. Further work along this line is now in progress. To what extent the improved procedure might explain the discrepancy found by Chantler et al. remains to be seen.

physics.atom-ph↗

arXiv2008

A comprehensive comparison of the Sun to other stars: searching for self-selection effects

If the origin of life and the evolution of observers on a planet is favoured by atypical properties of a planet's host star, we would expect our Sun to be atypical with respect to such properties. The Sun has been described by previous studies as both typical and atypical. In an effort to reduce this ambiguity and quantify how typical the Sun is, we identify eleven maximally-independent properties that have plausible correlations with habitability, and that have been observed by, or can be derived from, sufficiently large, currently available and representative stellar surveys. By comparing solar values for the eleven properties, to the resultant stellar distributions, we make the most comprehensive comparison of the Sun to other stars. The two most atypical properties of the Sun are its mass and orbit. The Sun is more massive than 95 -/+ 2% of nearby stars and its orbit around the Galaxy is less eccentric than 93 +/- 1% of FGK stars within 40 parsecs. Despite these apparently atypical properties, a chi^2 -analysis of the Sun's values for eleven properties, taken together, yields a solar chi^2 = 8.39 +/- 0.96. If a star is chosen at random, the probability that it will have a lower value (be more typical) than the Sun, with respect to the eleven properties analysed here, is only 29 +/- 11%. These values quantify, and are consistent with, the idea that the Sun is a typical star. If we have sampled all reasonable properties associated with habitability, our result suggests that there are no special requirements for a star to host a planet with life.

astro-ph↗

arXiv2006

Broadband UBVRI Photometry of Horizontal-Branch and Metal-Poor Candidates from the HK and Hamburg/ESO Surveys. I

We report broadband UBV and/or BVRI CCD photometry for a total of 1857 stars in the thick-disk and halo populations of the Galaxy. The majority of our targets were selected as candidate field horizontal-branch or other A-type stars (FHB/A, N = 576), or candidate low-metallicity stars (N = 1221), from the HK and Hamburg/ESO objective-prism surveys. Similar data for a small number of additional stars from other samples are also reported. These data are being used for several purposes. In the case of the FHB/A candidates they are used to accurately separate the lower-gravity FHB stars from various higher-gravity A-type stars, a subsample that includes the so-called Blue Metal Poor stars, halo and thick-disk blue stragglers, main-sequence A-type dwarfs, and Am and Ap stars. These data are also being used to derive photometric distance estimates to high-velocity hydrogen clouds in the Galaxy and for improved measurements of the mass of the Galaxy. Photometric data for the metal-poor candidates are being used to refine estimates of stellar metallicity for objects with available medium-resolution spectroscopy, to obtain distance estimates for kinematic analyses, and to establish initial estimates of effective temperature for analysis of high-resolution spectroscopy of the stars for which this information now exists.

astro-ph↗

arXiv2006

On the mass-to-light ratio of the local Galactic disc and the optical luminosity of the Galaxy

We measure the volume luminosity density and surface luminosity density generated by the Galactic disc, using accurate data on the local luminosity function and the disc's vertical structure. From the well measured volume mass density and surface mass density, we derive local volume and surface mass-to-light ratios for the Galactic disc, in the bands B, V and I. We obtain mass-to-light ratios for the local column of stellar matter of (M/L)_B = 1.4 +/- 0.2, (M/L)_V = 1.5 +/- 0.2 and (M/L)_I = 1.2 +/- 0.2. The dominant contributors to the surface luminosity in these bands are main sequence turn-off stars and giants. Our results on the colours and mass-to-light ratios for the ``Solar cylinder'' well agree with population synthesis predictions using Initial Mass Functions typical of the Solar Neighbourhood. Finally we infer the global luminosity of the Milky Way, which appears to be under-luminous by about 1-sigma with respect to the main locus of the Tully-Fisher relation, as observed for external galaxies.

astro-ph↗

arXiv2005

The colours of the Sun

We compile a sample of Sun-like stars with accurate effective temperatures, metallicities and colours (from the UV to the near-IR). A crucial improvement is that the effective temperature scale of the stars has recently been established as both accurate and precise through direct measurement of angular diameters obtained with stellar interferometers. We fit the colours as a function of effective temperature and metallicity, and derive colour estimates for the Sun in the Johnson/Cousins, Tycho, Stromgren, 2MASS and SDSS photometric systems. For (B-V)_Sun, we favour the ``red'' colour 0.64 versus the ``blue'' colour 0.62 of other recent papers, but both values are consistent within the errors; we ascribe the difference to the selection of Sun-like stars versus interpolation of wider colour-Teff-metallicity relations.

astro-ph↗

arXiv2005

The local density of matter mapped by Hipparcos

We determine the velocity distribution and space density of a volume complete sample of A and F stars, using parallaxes and proper motions from the Hipparcos satellite. We use these data to solve for the gravitational potential vertically in the local Galactic disc, by comparing the Hipparcos measured space density with predictions from various disc models. We derive an estimate of the local dynamical mass density of 0.102 +/- 0.010 solar masses per cubic parsec which may be compared to an estimate of 0.095 solar masses per cubic parsec in visible disc matter. Our estimate is found to be in reasonable agreement with other estimates by Creze et al. and Pham, also based on Hipparcos data. We conclude that there is no compelling evidence for significant amounts of dark matter in the disc.

astro-ph↗

arXiv2005

Kinematics of the Galactic Halo from Horizontal Branch stars in the Hamburg/ESO Survey

Large samples of Field Horizontal Branch (FHB) stars make excellent tracers of the Galactic halo; by studying their kinematics, one can infer important physical properties of our Galaxy. Here we present the results of a medium-resolution spectroscopic survey of 530 FHB stars selected from the Hamburg/ESO survey. The stars have a mean distance of ~7 kpc and thus probe the inner parts of the Milky Way halo. We measure radial velocities from the spectra in order to test the model of Sommer-Larsen et al., who suggested that the velocity ellipsoid of the halo changes from radially-dominated orbits to tangentially-dominated orbits as one proceeds from the inner to the outer halo. We find that the present data are unable to discriminate between this model and a more simple isothermal ellipsoid; we suggest that additional observations towards the Galactic centre might help to differentiate them.

astro-ph↗

arXiv2004

The local surface density of disc matter mapped by Hipparcos

We determine the surface density of matter in the disc of the Galaxy at the solar position using K giant stars. The local space density and luminosity function of the giants is determined using parallaxes from the Hipparcos satellite; for more distant giants, observed in a cone at the South Galactic Pole, distances are determined using intermediate band DDO photometry (which has been calibrated to the Hipparcos absolute magnitudes). From this sample, we determine the gravitational potential vertically of the local Galactic disc, by comparing the number of giant stars observed in the cone with the number expected for various models of the matter distribution in the disc. We derive an estimate of the dynamical disc mass surface density of 56 +/- 6 solar masses /pc^2, which may be compared to an estimate of 53 solar masses/pc^2 in visible matter. For all gravitating matter (disc + dark halo) we find the total density within 1.1 kpc of the disc midplane to be 74 +/- 6 solar masses/pc^2. As has been found by a number of studies, including our own, we find no compelling evidence for significant amounts of dark matter in the disc.

astro-ph↗

arXiv2002

White dwarfs and Galactic dark matter

We discuss the recent discovery by Oppenheimer et al (2001) of old, cool white dwarf stars, which may be the first direct detection of Galactic halo dark matter. We argue here that the contribution of more mundane white dwarfs of the stellar halo and thick disk would contribute sufficiently to explain the new high velocity white dwarfs without invoking putative white dwarfs of the dark halo. This by no means rules out that the dark matter has been found, but it does constrain the overall contribution by white dwarfs brighter than M_V ~ 16 to significantly less than 1% of the Galactic dark matter. This work confirms a similar study by Reyle et al (2001).

astro-ph↗

arXiv2001

The solar neighbourhood age-metallicity relation - does it exist?

We derive stellar ages, from evolutionary tracks, and metallicities, from Stromgren photometry, for a sample of 5828 dwarf and sub-dwarf stars from the Hipparcos Catalogue. This stellar disk sample is used to investigate the age-metallicity diagram in the solar neighbourhood. Such diagrams are often used to derive a so called age-metallicity relation. Because of the size of our sample, we are able to quantify the impact on such diagrams, and derived relations, due to different selection effects. Some of these effects are of a more subtle sort, giving rise to erroneous conclusions. In particular we show that [1] the age-metallicity diagram is well populated at all ages and especially that old, metal-rich stars do exist, [2] the scatter in metallicity at any given age is larger than the observational errors, [3] the exclusion of cooler dwarf stars from an age-metallicity sample preferentially excludes old, metal-rich stars, depleting the upper right-hand corner of the age-metallicity diagram, [4] the distance dependence found in the Edvardsson et al. sample by Garnett & Kobulnicky is an expected artifact due to the construction of the original sample. We conclude that, although some of it can be attributed to stellar migration in the galactic disk, a large part of the observed scatter is intrinsic to the formation processes of stars.

astro-ph↗

arXiv2000

The reality of old moving groups - the case of HR 1614

We prove the existence of the old and metal-rich moving group HR1614. This is done using the new Hipparcos parallaxes in combination with metallicities derived from Stromgren photometry, supported by dynamical simulations of the evolution of old moving groups in the galactic potential. A new selection criterion for this moving group is presented as well as a new, extended sample of probable member stars. In particular we find that the HR 1614 moving group has an age of about 2 Gyr (using Bertelli et al. 1994 isochrones) and a [Fe/H] = 0.19 +/- 0.06 dex. We also revisit and apply our new selection criterion to the samples in Eggen (1992) and Eggen (1998). It is found that, when binaries and stars with too low metallicity have been removed, 15 of his stars fulfill our criteria.

astro-ph↗