Alain Jorissen

HERMES: a high-resolution fibre-fed spectrograph for the Mercator telescope

The HERMES high-resolution spectrograph project aims at exploiting the specific potential of small but flexible telescopes in observational astrophysics. The optimised optical design of the spectrograph is based on the well-proven concept of white-pupil beam folding for high-resolution spectroscopy. In this contribution we present the complete project, including the spectrograph design and procurement details, the telescope adaptor and calibration unit, the detector system, as well as the optimised data-reduction pipeline. We present a detailed performance analysis to show that the spectrograph performs as specified both in optical quality and in total efficiency. With a spectral resolution of 85 000 (63 000 for the low-resolution fibre), a spectral coverage from 377 to 900 nm in a single exposure and a peak efficiency of 28%, HERMES proves to be an ideal instrument for building up time series of high-quality data of variable (stellar) phenomena.

Reprocessing the Hipparcos data of evolved stars III. Revised Hipparcos period-luminosity relationship for galactic long-period variable stars ?;??

We analyze the K band luminosities of a sample of galactic long-period variables using parallaxes measured by the Hipparcos mission. The parallaxes are in most cases re-computed from the Hipparcos Intermediate Astrometric Data using improved astrometric fits and chromaticity corrections. The K band magnitudes are taken from the literature and from measure- ments by COBE, and are corrected for interstellar and circumstellar extinction. The sample contains stars of several spectral types: M, S and C, and of several variability classes: Mira, semiregular SRa, and SRb. We find that the distribution of stars in the period-luminosity plane is independent of circumstellar chemistry, but that the dierent variability types have dierent P-L distributions. Both the Mira variables and the SRb variables have reasonably well-defined period-luminosity relationships, but with very dierent slopes. The SRa variables are distributed between the two classes, suggesting that they are a mixture of Miras and SRb, rather than a separate class of stars. New period-luminosity re- lationships are derived based on our revised Hipparcos parallaxes. The Miras show a similar period-luminosity relationship to that found for Large Magellanic Cloud Miras by Feast et al. (1989). The maximum absolute K magnitude of the sample is about 8:2 for both Miras and semi-regular stars, only slightly fainter than the expected AGB limit. We show that the stars with the longest periods (P> 400 d) have high mass loss rates and are almost all Mira variables.

Multiple Molecular Winds in Evolved Stars. I. A Survey of CO(2-1) and CO(3-2) Emission from 45 Nearby Asymptotic Giant Branch Stars

This paper describes observations of a new phenomenon in evolved mass-losing asymptotic giant branch (AGB) stars: the presence of two winds with different expansion velocities. CO(2-1) and CO(3-2) line emission was observed for 45 AGB stars at high velocity resolution and double winds found in 20% of the sample. Highly asymmetric lines were found in six other stars. The data tentatively suggest that double winds occur when the star undergoes a change (pulsational mode, chemical composition) and that the very narrow components represent the onset of a new phase of mass loss.

MESS (Mass-loss of Evolved StarS), a Herschel key program

MESS (Mass-loss of Evolved StarS) is a guaranteed time key program that uses the PACS and SPIRE instruments on board the Herschel space observatory to observe a representative sample of evolved stars, that include asymptotic giant branch (AGB) and post-AGB stars, planetary nebulae and red supergiants, as well as luminous blue variables, Wolf-Rayet stars and supernova remnants. In total, of order 150 objects are observed in imaging and about 50 objects inspectroscopy. This paper describes the target selection and target list, and the observing strategy. Key science projects are described, and illustrated using results obtained during Herschel’s science demonstration phase. Aperture photometry is given for the 70 AGB and post-AGB stars observed up to October 17, 2010, which constitutes the largest single uniform database of far-IR and sub-mm fluxes for late-type stars.

Discovery of three lead-rich stars

About half of the stable nuclei heavier than iron are believed to be synthesized during the late stages of evolution of stars with masses in the range 0.8–8 solar masses. These elements are then expelled into the interstellar medium through stellar winds after being ‘dredged up’ towards the surface of the stars. These processes occur when the star is in the ‘asymptotic giant branch’ (AGB) phase of its life. Nuclei (mainly iron) deep inside the star slowly capture neutrons and progressively build up heavier elements (the ‘s-process’). For AGB stars that formed early in the history of the Galaxy, and that therefore have very low abundances of elements heavier than helium (‘metals’), models predict that the s-process will accumulate synthesized material with atomic weights in the Pb–Bi region. Such stars will therefore have large overabundances of lead relative to other heavy elements. Here we report the discovery of large amounts of lead in three metal-poor stars (HD187861, HD196944 and HD224959). Our analysis shows that these stars are more enriched in lead than in any other element heavier than iron. The excellent agreement between the observed and predicted abundances reinforces our current understanding of the detailed operation of the s-process deep in the interiors of AGB stars.

Wind accretion in binary stars — II. Accretion rates

Smoothed particle hydrodynamics (SPH) is used to estimate accretion rates of mass, linear and angular momentum in a binary system where one component undergoes mass loss through a wind. Physical parameters are chosen such as to model the alleged binary precursors of barium stars, whose chemical peculiarities are believed to result from the accretion of the wind from a companion that was formerly on the asymptotic giant branch (AGB). The binary system modelled consists of a 3-M⊙ AGB star (losing mass at a rate 10-6 M⊙ yr-1) and a 1.5-M⊙ star on the main sequence, in a 3-au circular orbit. Three-dimensional simulations are performed for gases with polytropic indices γ = 1, 1.1 and 1.5, to bracket more realistic situations that would include radiative cooling. Mass accretion rates are found to depend on resolution, and we estimate typical values of 1-2 per cent for the γ = 1.5 case and 8 per cent for the other models. The highest resolution obtained (with 400000 particles) corresponds to an accretor of linear size ≈ 16 R⊙. Despite being (in the γ = 1.5 case) about 10 times smaller than theoretical estimates based on the Bondi-Hoyle prescription, the SPH accretion rates remain large enough to explain the pollution of barium stars. Uncertainties in the current SPH rates remain, however, owing to the simplified treatment of the wind acceleration mechanism, as well as to the absence of any cooling prescription and to the limited numerical resolution. Angular momentum transfer leads to significant spin-up of the accretor and can account for the rapid rotation of HD 165141, a barium star with a young white dwarf companion and a rotation rate unusually large among K giants. In the circular orbit modelled in this paper, hydrodynamic thrust and gravitational drag almost exactly compensate and so the net transfer of linear momentum is nearly zero. For small but finite eccentricities and the chosen set of parameters, the eccentricity tends to decrease.

The distribution of exoplanet masses

The present study derives the distribution of secondary masses M2 for the 67 exoplanets and very low-mass brown-dwarf companions of solar-type stars, known as of April 4, 2001. This distribution is related to the distribution of M2 sini through an integral equation of Abels type. Although a formal solution exists for this equation, it is known to be ill-conditioned, and is thus very sensitive to the statistical noise present in the input M2 sini distribution. To overcome this diculty, we present two robust, independent approaches: (i) the formal solution of the integral equation is numerically computed after performing an optimal smoothing of the input distribution and (ii) the Lucy-Richardson algorithm is used to invert the integral equation. Both approaches give consistent results. The resulting statistical distribution of exoplanet true masses reveals that there is no reason to ascribe the transition between giant planets and brown dwarfs to the threshold mass for deuterium ignition (about 13.6 MJ). The M2 distribution shows instead that most of the objects have M2 10 MJ, but there is a small tail with a few heavier candidates around 15 MJ.

On the age heterogeneity of the Pleiades, Hyades, and Sirius moving groups

Aims. We investigate the nature of the classical low-velocity structures in the local velocity field, i.e. the Pleiades, Hyades, and Sirius moving groups. After using a wavelet transform to locate them in velocity space, we study their relation with the open clusters kinematically associated with them. Methods. By directly comparing the location of moving group stars in parallax space to the isochrones of the embedded clusters, we check whether, within the observational errors on the parallax, all moving group stars could originate from the ongoing evaporation of the associated cluster. Results. We conclude that, in each moving group, the fraction of stars making up the velocity-space overdensity superimposed on the background is higher than the fraction of stars compatible with the isochrone of the associated cluster. These observations thus favour a dynamical (resonant) origin for the Pleiades, Hyades, and Sirius moving groups.

The Hyades stream: an evaporated cluster or an intrusion from the inner disk?

The nature of the Hyades stream, or Hyades moving group, is a long-standing question of Galactic Astronomy. While it has become widely recognized that the Hercules stream, an unbound group of stars lagging behind galactic rotation and moving outward in the galactic disk, is associated with the outer Lindblad resonance of the rotating galactic bar, there is still some debate about the nature of the more prominent low-velocity stream sharing the kinematics of the Hyades open cluster. Is this stream caused by additional non-axisymmetric perturbations of the galactic potential, such as transient or quasi-stationary spiral waves, or by the on-going evaporation of the Hyades cluster? Here, a simple observational test has been designed to determine whether the Hyades stream is primarily composed of coeval stars originating from the Hyades cluster, or of field stars. Using the Geneva-Copenhagen survey of F and G dwarfs, we compare the mass distribution and metallicity of the stream to those of field disk stars. If the Hyades stream is composed of stars trapped at resonance, its mass distribution should obey the present-day mass function (PDMF) of the disk, and its metallicity should reflect its origin in the inner regions of the Galaxy. On the other hand, if it is an evaporated cluster, we expect a different mass distribution, depending on the inital mass function (IMF) of the cluster, and on the proportion of evaporated stars as a function of mass. We find that extreme conditions have to be adopted for the selective evaporation and IMF of the cluster to make the observed mass distribution of the stream only roughly consistent (at a one-sigma level) with the coeval evaporated cluster scenario. The observed mass distribution is in much better agreement with the PDMF of the field. The peculiar metallicity of the stream is inconsistent with that of a field population from the solar neighbourhood trapped in the primordial cluster during its formation process and subsequently evaporated. These observations thus favour a resonant origin for the Hyades stream.

Asymmetric photoreactions as the origin of biomolecular homochirality: A critical review

The role of asymmetric photoreactions (occurringin space or on the primitive Earth) in the origin of biomolecularhomochirality is critically reviewed. A general description of thevarious possible ways for light to interact with chiral moleculesis first presented on the basis of a series expansion of thedielectric constant: natural, magnetic and magnetochiral circulardichroism are identified with the first three terms in thisdevelopment. Natural and magnetochiral circular dichroismmay cause, through asymmetric photolysis, an enantiomeric excessin a racemic mixture of chiral molecules irradiated, respectively,by circularly polarized ultraviolet light, or by unpolarizedultraviolet light in the presence of a magnetic fieldnon-perpendicular to the light beam. Terrestrial and extraterrestrialsites matching these conditions are then critically reviewed.Finally, we stress the possibility to arrive at the homochiralityof amino acids through a path involving D-ribose during RNA worldas an alternative to the usual scenarios operating directly onamino acids.