| 11. |
- Dolk, Linus, et al.
(författare)
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On the elemental abundance and isotopic mixture of mercury in HgMn stars
- 2003
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Ingår i: Astronomy & Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 402:1, s. 299-313
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Tidskriftsartikel (refereegranskat)abstract
- Optical region spectra of 31 HgMn stars have been studied for the abundance and isotope mixture of mercury. In the course of the investigation the lines Hg I lambda 4358 and Hg Ii lambda lambda3984,6149 have been studied, with abundances established for all three lines in several HgMn stars. The mercury isotope mixture has been determined from high resolution spectra of the lambda 3984 line.Possible signs of an ionization anomaly have been detected by the comparison of the abundance derived from the Hg I line and the Hg Ii lines in seven of the observed HgMn stars. A possible correlation of the mercury abundance with Teff has been detected. Possible signs of a weak anticorrelation of the manganese and mercury abundance in HgMn stars have been found, which could be interpreted as a sign of inhomogeneous surface distribution of these elements. For a number of the HgMn stars in this study the mercury abundance and isotope mixture are reported for the first time.
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| 12. |
- Dolk, Linus, et al.
(författare)
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The laboratory analysis of Bi II and its application to the Bi-rich HgMn star HR 7775
- 2002
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Ingår i: Astronomy & Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 388:2, s. 692-703
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Tidskriftsartikel (refereegranskat)abstract
- The bismuth spectrum emitted from a hollow cathode discharge has been recorded with a Fourier Transform Spectrometer (FTS). Accurate wavelengths have been determined for 104 lines, and several new energy levels have been found, while the accuracy of previously known level energies have been improved. The hyperfine structure of all observed lines has been analyzed, yielding hyperfine constants A and B for 56 levels. With the aid of the laboratory measurements the optical region spectrum of the HgMn star HR 7775 has been studied for all observable lines. The wavelengths and hfs constants established from the laboratory work have been combined with theoretical gf values to identify spectral lines and make an abundance estimation of bismuth. It has been established that bismuth is present in HR 7775 at an enhancement level of approximately 5 orders of magnitude relative to the meteoritic abundance, consistent with previous observations in the ultraviolet region of this star. Astrophysical gf values are presented for a number of lines.
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| 13. |
- Dolk, Linus, et al.
(författare)
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The presence of Nd and Pr in HgMn stars
- 2002
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Ingår i: Astronomy & Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 385:1, s. 111-130
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Tidskriftsartikel (refereegranskat)abstract
- Optical region spectra for a number of upper main sequence chemically peculiar (CP) stars have been observed to study singly and doubly ionized praseodymium and neodymium lines. In order to improve existing atomic data of these elements, laboratory measurements have been carried out with the Lund VUV Fourier Transform Spectrometer (FTS). From these measurements wavelengths and hyperfine structure (hfs) have been studied for selected , and lines of astrophysical interest. Radiative life times for some excited states of have been determined with the aid of laser spectroscopy at the Lund Laser Center (LLC) and have been combined with branching fractions measured in the laboratory to calculate gf values for some of the stronger optical lines of . With the aid of the derived gf values and laboratory measurements of the hfs, a praseodymium abundance was derived from selected lines in the spectrum of the Am star 32 Aqr. This abundance was used to derive astrophysical gf values for selected lines in 32 Aqr, and these gf values were used to get a praseodymium abundance for the HgMn star HR 7775. The praseodymium abundance in HR 7775 was then utilized to derive astrophysical gf values for all observable lines in this star. The neodymium abundance, derived from unblended lines of in HR 7775, has been utilized to establish astrophysical gf values for observed lines in the optical region of this star. Selected and lines have been identified and studied in a number of HgMn stars and three hot Am stars. The praseodymium and neodymium abundance change rapidly from an approximate 1-1.2 dex enhancement for the hot Am stars to 1.5-3 dex enhancement for the cool HgMn stars, indicating a well-defined boundary between the hot Am and HgMn stars in the vicinity of 10 500 K. The enhancement of praseodymium and neodymium in Am and HgMn stars may be explained by diffusive processes active in the stellar atmosphere, while the observed discontinuity might be explained by a thin hydrogen convection zone thought to be present for the Am stars, but absent in the HgMn stars. The absence of a convection zone would cause the diffused elements to gather higher in the atmosphere of HgMn stars compared to Am stars, and explain the observed increase in abundance.
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| 14. |
- Duggan, P, et al.
(författare)
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Gamma-ray bursts and X-ray melting of material to form chondrules and planets
- 2003
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Ingår i: Astronomy & Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 409:2, s. 9-12
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Tidskriftsartikel (refereegranskat)abstract
- Chondrules are millimeter sized objects of spherical to irregular shape that constitute the major component of chondritic meteorites that originate in the region between Mars and Jupiter and which fall to Earth. They appear to have solidified rapidly from molten or partially molten drops. The heat source that melted the chondrules remains uncertain. The intense radiation from a gamma-ray burst (GRB) is capable of melting material at distances up to 300 light years. These conditions were created in the laboratory for the first time when millimeter sized pellets were placed in a vacuum chamber in the white synchrotron beam at the European Synchrotron Radiation Facility (ESRF). The pellets were rapidly heated in the X-ray and gamma-ray furnace to above 1400 degreesC melted and cooled. This process heats from the inside unlike normal furnaces. The melted spherical samples were examined with a range of techniques and found to have microstructural properties similar to the chondrules that come from meteorites. This experiment demonstrates that GRBs can melt precursor material to form chondrules that may subsequently influence the formation of planets. This work extends the field of laboratory astrophysics to include high power synchrotron sources.
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| 15. |
- Eriksson, Mattias, et al.
(författare)
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Modeling the wind structure of AG Peg by fitting of C IV and N V resonance doublets
- 2004
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Ingår i: Astronomy & Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 422:3, s. 987-999
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Tidskriftsartikel (refereegranskat)abstract
- The latest outburst of AG Peg has lasted for 150 years, which makes itthe slowest nova eruption ever recorded. During the time of IUEobservations (1978-1995) line profiles and intensity ratios of the N Vand C IV doublet components changed remarkably, and we discuss plausiblereasons. One of them is radiative pumping of Fe II which is investigatedby studying the fluorescence lines from pumped levels. Three Fe IIchannels are pumped by C IV and one by N V. The pumping rates of thoseFe II channels as derived by the modeling agree well with the strengthsof the Fe II fluorescence lines seen in the spectra. We model the C IVand N V resonance doublets in IUE spectra recorded between 1978 and 1995in order to derive optical depths, expansion velocities, and theemissivities of the red giant wind, the white dwarf wind and theircollision region. The derived expansion velocities are ∼60 kms-1 for the red giant wind and ∼700 km s-1 forthe white dwarf wind. We also suggest a fast outflow from the system at∼150 km s-1. The expansion velocity is slightly higherfor N V than for C IV. Emission from the collision region stronglyaffects the profile of the N V and C IV resonance doublets indicatingits existence.
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| 16. |
- Feltzing, Sofia, et al.
(författare)
-
A new, cleaner colour-magnitude diagram for the metal-rich globular cluster NGC 6528. Velocity dispersion in the Bulge, age and proper motion of NGC 6528
- 2002
-
Ingår i: Astronomy & Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 385, s. 67-86
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Tidskriftsartikel (refereegranskat)abstract
- Using two epochs of HST/WFPC2 images of the metal-rich globular clusterNGC 6528 we derive the proper motions of the stars and use them toseparate the stars belonging to NGC 6528 from those of the Galacticbulge. The stellar sequences in the resulting colour-magnitude diagramfor the cluster are significantly better determined than in previouslypublished data. From comparison of the colour-magnitude diagram with thefiducial line for NGC 6553 from Zoccali et al. (cite{Zo01}) we concludethat the two globular clusters have the same age. Further, using alpha-enhanced stellar isochrones, NGC 6528 is found to have an age of 11+/-2 Gyr. This fitting of isochrones also give that the cluster is 7.2 kpcaway from us. From the measured velocities both the proper motion of thecluster and the velocity dispersion in the Galactic bulge are found. NGC6528 is found to have a proper motion relative to the Galactic bulge ofl > = 0.006 and b> = 0.044arcsec per century. Using stars with ~ 14 555 < 19(i.e. the red giant branch and horizontal branch) we find, for theGalactic bulge, sigma l= 0.33+/- 0.03 and sigma b=0.25+/-0.02 arcsec per century. This give sigma l/sigmab=1.32+/-0.16, consistent both with previous proper motionstudies of K giants in the Galactic bulge as well as with predictions bymodels of the kinematics of bulge stars. Based on observations with theNASA/ESA Hubble Space Telescope, obtained at the Space Telescope ScienceInstitute, which is operated by the Association of Universities forResearch in Astronomy, Inc. under NASA contract No. NAS5-26555. TablesA1, A2 and A3 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/385/67
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| 17. |
- Feltzing, Sofia, et al.
(författare)
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Signatures of SN Ia in the galactic thick disk. Observational evidence from alpha -elements in 67 dwarf stars in the solar neighbourhood
- 2003
-
Ingår i: Astronomy & Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 397:1, s. 1-4
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Tidskriftsartikel (refereegranskat)abstract
- We present the first results of a larger study into the stellarabundances and chemical trends in long-lived dwarf stars in the solarneighbourhood that belong to (based on their kinematics) the thin andthick galactic disk, respectively. We confirm that the trends of alpha-elements in the thin and thick disk are distinct (this has previouslybeen shown for Mg by Fuhrmann cite{Fuhrmann}, but e.g. Chen et al.cite{Chen} claimed the trends to follow smoothly upon each other). Wefind that the thick disk show the typical signature of contribution fromSN Ia (i.e. the ``knee'') to the enrichment of the interstellar gas outof which the later generations of thick disk stars formed. The trendstarts out as [Mg/Fe] ~ 0.35 at [Fe/H] ~ -0.7 and continue on this levelwith increasing [Fe/H] until -0.4 dex where a decline in [Mg/Fe] startsand steadily continues down to 0 dex at solar metallicity. The same istrue for the other alpha -elements (e.g. Si). Using ages from theliterature we find that the thick disk in the mean is older than thethin disk. Combining our results with other observational facts wesuggest that the most likely formation scenario for the thick disk is,still, a violent merger event. We also suggest that there might betentative evidence for diffusion of orbits in todays thin disk (based onkinematics in combination with elemental abundances). Based onobservations collected at the European Southern Observatory, La Silla,Chile, Proposals #65.L-0019(B) and 67.B-0108(B).
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| 18. |
- Feltzing, Sofia, et al.
(författare)
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The nature of super-metal-rich stars. Detailed abundance analysis of 8 super-metal-rich star candidates
- 2001
-
Ingår i: Astronomy & Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 367, s. 253-265
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Tidskriftsartikel (refereegranskat)abstract
- We provide detailed abundance analyses of 8 candidate super-metal-richstars. Five of them are confirmed to have [Fe/H]>0.2 dex, thegenerally-accepted limit for super-metal-richness. Furthermore, wederive abundances of several elements and find that the stars followtrends seen in previous studies of metal-rich stars. Ages are estimatedfrom isochrones and velocities calculated. We find that there do existvery metal-rich stars that are older than 10 Gyr. This is contrary towhat is found in several recent studies of the galactic age-metallicityrelation. This is tentative evidence that there might not exist aone-to-one relation between age and metallicity for all stars. This isnot surprising considering the current models of the independentevolution of the different galactic components. We also find that onestar, HD 182572, could with ~ 75% chance be a thick disk star with, forthe thick disk, an extremely high metallicity at 0.34 dex. This star is,intriguingly, also somewhat enhanced in the alpha -elements. Based onobservations obtained at the McDonald Observatory.}
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| 19. |
- Feltzing, Sofia, et al.
(författare)
-
The solar neighbourhood age-metallicity relation - Does it exist?
- 2001
-
Ingår i: Astronomy & Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 377, s. 911-924
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Tidskriftsartikel (refereegranskat)abstract
- We derive stellar ages, from evolutionary tracks, and metallicities,from Strömgren photometry, for a sample of 5828 dwarf and sub-dwarfstars from the Hipparcos Catalogue. This stellar disk sample is used toinvestigate the age-metallicity diagram in the solar neighbourhood. Suchdiagrams 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 onsuch diagrams, and derived relations, due to different selectioneffects. Some of these effects are of a more subtle sort, giving rise toerroneous conclusions. In particular we show that [1] theage-metallicity diagram is well populated at all ages and especiallythat old, metal-rich stars do exist, [2] the scatter in metallicity atany given age is larger than the observational errors, [3] the exclusionof cooler dwarf stars from an age-metallicity sample preferentiallyexcludes old, metal-rich stars, depleting the upper right-hand corner ofthe age-metallicity diagram, [4] the distance dependence found in theEdvardsson et al. sample by Garnett & Kobulnicky is an expectedartifact due to the construction of the original sample. We concludethat, although some of it can be attributed to stellar migration in thegalactic disk, a large part of the observed scatter is intrinsic to theformation processes of stars. Based on results from the ESA Hipparcossatellite. Full Table 1 is only available in electronic form at the CDSvia anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/377/911
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| 20. |
- Flicker, Ralf
(författare)
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Efficient first-order performance estimation for high-order adaptive optics systems
- 2003
-
Ingår i: Astronomy & Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 405:3, s. 1177-1189
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Tidskriftsartikel (refereegranskat)abstract
- It is shown how first-order performance estimation of high-orderadaptive optics (AO) systems may be efficiently implemented in a hybridnumerical simulation by the use of 1) sparse matrix techniques forwavefront reconstruction, 2) undersampled pupil-plane turbulence-inducedaberrations, and 3) analytical models that compensate - in the limit ofinfinite exposure time - for the errors introduced by undersampling. Asparse preconditioned conjugate gradient (PCG) method is applied forwavefront reconstruction, and it is seen that acceptable AO performancemay be achieved at a relative error tolerance of 0.01, at which thecomputational cost of the sparse PCG scales approximately asO(n1.2), where n is the number of actuators in the system.Estimations of adaptive optics performance for extremely high-ordersystems are presented, including multi-conjugate andlaser-guide-star-based systems. The scaling laws for AO performance withtelescope diameter D and turbulence outer scale L0 coupled with the useof laser guide stars are also investigated. It is shown that a single ora small number of laser guide stars (LGS) may still provide a usefullevel of compensation to telescopes with diameters in the range 30-100m, if L0 is on the order of or smaller than D. The deviations fromKolmogorov theory are also investigated for LGS AO. To the best of theauthors knowledge, results presented for a n=65 282 case represent thelargest multi-conjugate adaptive optics system simulated in full todate.
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