| 1. |
- Kotak, Rubina, et al.
(författare)
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Time-resolved optical spectroscopy of the pulsating DA white dwarf HS 0507+0434B : New constraints on mode identification and pulsation properties
- 2002
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Ingår i: Astronomy & Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 388:1, s. 219-234
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Tidskriftsartikel (refereegranskat)abstract
- We present a detailed analysis of time-resolved optical spectra of the ZZ Ceti white dwarf, HS 0507+0434B. Using the wavelength dependence of observed mode amplitudes, we deduce the spherical degree, , of the modes, most of which have . The presence of a large number of combination frequencies (linear sums or differences of the real modes) enabled us not only to test theoretical predictions but also to indirectly infer spherical and azimuthal degrees of real modes that had no observed splittings. In addition to the above, we measure line-of-sight velocities from our spectra. We find only marginal evidence for periodic modulation associated with the pulsation modes: at the frequency of the strongest mode in the lightcurve, we measure an amplitude of km s -1, which has a probability of 2% of being due to chance; for the other modes, we find lower values. Our velocity amplitudes and upper limits are smaller by a factor of two compared to the amplitudes found in ZZ Psc. We find that this is consistent with expectations based on the position of HS 0507+0434B in the instability strip. Combining all the available information from data such as ours is a first step towards constraining atmospheric properties in a convectionally unstable environment from an observational perspective.
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| 2. |
- Bensby, Thomas, et al.
(författare)
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A possible age-metallicity relation in the Galactic thick disk?
- 2004
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 1432-0746 .- 0004-6361. ; 421, s. 969-976
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Tidskriftsartikel (refereegranskat)abstract
- A sample of 229 nearby thick disk stars has been used to investigate theexistence of an age-metallicity relation (AMR) in the Galactic thickdisk. The results indicate that that there is indeed an age-metallicityrelation present in the thick disk. By dividing the stellar sample intosub-groups, separated by 0.1 dex in metallicity, we show that the medianage decreases by about 5-7 Gyr when going from [Fe/H] ≈ -0.8 to[Fe/H] ≈ -0.1. Combining our results with our newly publishedα-element trends for a local sample of thick disk stars that showsignatures from supernovae type Ia (SN Ia), we draw the conclusion thatthe time-scale for the peak of the SN Ia rate is of the order of 3-4 Gyrin the thick disk. The tentative evidence for a thick disk AMR that wepresent here also has implications for the thick disk formationscenario; star-formation must have been an ongoing process for severalbillion years. This appears to strengthen the hypothesis that the thickdisk originated from a merger event with a companion galaxy that puffedup a pre-existing thin disk.Based on data obtained with the Hipparcos satellite.
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| 3. |
- Bensby, Thomas, et al.
(författare)
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Elemental abundance trends in the Galactic thin and thick disks as traced by nearby F and G dwarf stars
- 2003
-
Ingår i: Astronomy & Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 410:2, s. 527-551
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Forskningsöversikt (refereegranskat)abstract
- Based on spectra from F and G dwarf stars, we present elementalabundance trends in the Galactic thin and thick disks in the metallicityregime -0.8< [Fe/H < +0.4. Our findings can besummarized as follows. 1) Both the thin and the thick disks show smoothand distinct abundance trends that, at sub-solar metallicities, areclearly separated. 2) For the alpha -elements the thick disk showssignatures of chemical enrichment from SNe type Ia. 3) The age of thethick disk sample is in the mean older than the thin disk sample. 4)Kinematically, there exist thick disk stars with super-solarmetallicities. Based on these findings, together with other constraintsfrom the literature, we discuss different formation scenarios for thethick disk. We suggest that the currently most likely formation scenariois a violent merger event or a close encounter with a companion galaxy.Based on kinematics the stellar sample was selected to contain starswith high probabilities of belonging either to the thin or to the thickGalactic disk. The total number of stars are 66 of which 21 belong tothe thick disk and 45 to the thin disk. The analysis is based onhigh-resolution spectra with high signal-to-noise (R 48 000 and S/Ngtrsim 150, respectively) recorded with the FEROS spectrograph on LaSilla, Chile. Abundances have been determined for four alpha -elements(Mg, Si, Ca, and Ti), for four even-nuclei iron peak elements (Cr, Fe,Ni, and Zn), and for the light elements Na and Al, from equivalent widthmeasurements of 30 000 spectral lines. An extensive investigation ofthe atomic parameters, log gf-values in particular, have been performedin order to achieve abundances that are trustworthy. Noteworthy is thatwe find for Ti good agreement between the abundances from Ti I and TiIi. Our solar Ti abundances are in concordance with the standardmeteoritic Ti abundanceBased on observations collected at the European Southern Observatory, LaSilla, Chile, Proposals #65.L-0019(B) and 67.B-0108(B).Full Tables eftab:linelist and eftab:abundances are onlyavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/410/527
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| 4. |
- Bensby, Thomas, et al.
(författare)
-
Oxygen trends in the Galactic thin and thick disks
- 2004
-
Ingår i: Astronomy & Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 415:1, s. 155-170
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Tidskriftsartikel (refereegranskat)abstract
- We present oxygen abundances for 72 F and G dwarf stars in the solarneighbourhood. Using the kinematics of the stars we divide them into twosub-samples with space velocities that are typical for the thick andthin disks, respectively. The metallicities of the stars range from[Fe/H] ≈ -0.9 to +0.4 and we use the derived oxygen abundances of thestars to: (1) perform a differential study of the oxygen trends in thethin and the thick disk; (2) to follow the trend of oxygen in the thindisk to the highest metallicities. We analyze the forbidden oxygen linesat 6300 Å and 6363 Å as well as the (NLTE afflicted) tripletlines around 7774 Å. For the forbidden line at 6300 Å wehave spectra of very high S/N (>400) and resolution (R ≳ 215000). This has enabled a very accurate modeling of the oxygen line andthe blending Ni lines. The high internal accuracy in our determinationof the oxygen abundances from this line is reflected in the very tighttrends we find for oxygen relative to iron. From these abundances we areable to draw the following major conclusions: (i) That the [O/Fe] trendat super-solar [Fe/H] continues downward which is in concordance withmodels of Galactic chemical evolution. This is not seen in previousstudies as it has not been possible to take the blending Ni lines in theforbidden oxygen line at 6300 Å properly into account; (ii) Thatthe oxygen trends in the thin and the thick disks are distinctlydifferent. This confirms and extends previous studies of the otherα-elements; (iii) That oxygen does not follow Mg at super-solarmetallicities; (iv) We also provide an empirical NLTE correction for theinfrared O I triplet that could be used for dwarf star spectra with aS/N such that only the triplet lines can be analyzed well, e.g. stars atlarge distances; (v) Finally, we find that Gratton et al. (1999)overestimate the NLTE corrections for the permitted oxygen triplet linesat ∼7774 Å for the parameter space that our stars span.Based on observations collected at the European Southern Observatory, LaSilla and Paranal, Chile, Proposals #65.L-0019, 67.B-0108, and69.B-0277.The full Table 4 is 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?/A+A/415/155
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| 5. |
- Bensby, Thomas, et al.
(författare)
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The distance scale of planetary nebulae
- 2001
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Ingår i: Astronomy & Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 374, s. 599-614
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Tidskriftsartikel (refereegranskat)abstract
- By collecting distances from the literature, a set of 73 planetarynebulae with mean distances of high accuracy is derived. This sample isused for recalibration of the mass-radius relationship, used by manystatistical distance methods. An attempt to correct for a statisticalpeculiarity, where errors in the distances influences the mass-radiusrelationship by increasing its slope, has been made for the first time.Distances to PNe in the Galactic Bulge, derived by this new method aswell as other statistical methods from the last decade, are then usedfor the evaluation of these methods as distance indicators. In order ofachieving a Bulge sample that is free from outliers we derive newcriteria for Bulge membership. These criteria are much more stringentthan those used hitherto, in the sense that they also discriminateagainst background objects. By splitting our Bulge sample in two, onewith optically thick (small) PNe and one with optically thin (large)PNe, we find that our calibration is of higher accuracy than most othercalibrations. Differences between the two subsamples, we believe, aredue to the incompleteness of the Bulge sample, as well as the dominanceof optical diameters in the ``thin'' sample and radio diameters in the``thick'' sample. Our final conclusion is that statistical methods givedistances that are at least as accurate as the ones obtained from manyindividual methods. Also, the ``long'' distance scale of Galactic PNe isconfirmed.
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| 6. |
- Biémont, E, et al.
(författare)
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Radiative lifetime and oscillator strength determinations in Sm III
- 2003
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Ingår i: Astronomy & Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 399:1, s. 343-349
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Tidskriftsartikel (refereegranskat)abstract
- Radiative lifetimes of 6 levels belonging to the4f5(6Hdeg,6Fdeg)5d configuration of Sm III have beenmeasured for the first time using a time-resolved laser-inducedfluorescence technique. Experimental data have been compared withsemi-empirical calculations. The agreement is excellent for 4 levelswith 6Fdeg as a parent term. Larger discrepancies(53 and 37) are observed for 2 levels with 6Hdegand 6Fdeg, respectively, as a parent term and areexplainable by strong mixing and cancellation effects occurring in theline strength calculations which are responsible of instabilitiesoccurring in the lifetimes considered as a function of J value withinthe term. Branching ratios for the lines depopulating the levels ofinterest have been measured by Fourier transform spectroscopy. A firstset of oscillator strengths has been deduced for this ion.
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| 7. |
- Castro-Tirado, A. J., et al.
(författare)
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GRB 030227 : The first multiwavelength afterglow of an INTEGRAL GRB
- 2003
-
Ingår i: Astronomy & Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 411:1, s. 315-319
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Tidskriftsartikel (refereegranskat)abstract
- We present multiwavelength observations of a gamma-ray burst detected byINTEGRAL (GRB 030227) between 5.3 hours and ~ 1.7days after the event. Here we report the discovery of a dim opticalafterglow (OA) that would not have been detected by many previoussearches due to its faintess (R ~ 23). This OA was seen to declinefollowing a power law decay with index alpha R = -0.95 +/-0.16. The spectral index beta_ opt/NIR yielded -1.25 +/- 0.14. Thesevalues may be explained by a relativistic expansion of a fireball (withp = 2.0) in the cooling regime. We also find evidence for inverseCompton scattering in X-rays.Based on observations with INTEGRAL, an ESA project with instruments andscience data centre funded by ESA member states (especially the PIcountries: Denmark, France, Germany, Italy, Switzerland, Spain), CzechRepublic and Poland, and with the participation of Russia and the USA.Also partially based on observations collected by the Gamma-Ray BurstCollaboration at ESO (GRACE) at the European Southern Observatory, Chile(ESO Large Programme 165.H-0464).
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| 8. |
- Cayrel, R., et al.
(författare)
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First stars V - Abundance patterns from C to Zn and supernova yields in the early Galaxy
- 2004
-
Ingår i: Astronomy & Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 416:3, s. 1117-1138
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Tidskriftsartikel (refereegranskat)abstract
- In the framework of the ESO Large Programme ``First Stars'', veryhigh-quality spectra of some 70 very metal-poor dwarfs and giants wereobtained with the ESO VLT and UVES spectrograph. These stars are likelyto have descended from the first generation(s) of stars formed after theBig Bang, and their detailed composition provides constraints on issuessuch as the nature of the first supernovae, the efficiency of mixingprocesses in the early Galaxy, the formation and evolution of the haloof the Galaxy, and the possible sources of reionization of the Universe.This paper presents the abundance analysis of an homogeneous sample of35 giants selected from the HK survey of Beers et al. (cite{BPS92},cite{Be99}), emphasizing stars of extremely low metallicity: 30 of our35 stars are in the range -4.1 <[Fe/H]< -2.7, and 22 stars have[Fe/H] < -3.0. Our new VLT/UVES spectra, at a resolving power ofR∼45 000 and with signal-to-noise ratios of 100-200 per pixel overthe wavelength range 330-1000 nm, are greatly superior to those of theclassic studies of McWilliam et al. (cite{MPS95}) and Ryan et al.(cite{RNB96}).The immediate objective of the work is to determine precise,comprehensive, and homogeneous element abundances for this large sampleof the most metal-poor giants presently known. In the analysis wecombine the spectral line modeling code ``Turbospectrum'' with OSMARCSmodel atmospheres, which treat continuum scattering correctly and thusallow proper interpretation of the blue regions of the spectra, wherescattering becomes important relative to continuous absorption (λ< 400 nm). We obtain detailed information on the trends of elementalabundance ratios and the star-to-star scatter around those trends,enabling us to separate the relative contributions of cosmic scatter andobservational/analysis errors.Abundances of 17 elements from C to Zn have been measured in all stars,including K and Zn, which have not previously been detected in starswith [Fe/H] < -3.0. Among the key results, we discuss the oxygenabundance (from the forbidden [OI] line), the different and sometimescomplex trends of the abundance ratios with metallicity, the very tightrelationship between the abundances of certain elements (e.g., Fe andCr), and the high [Zn/Fe] ratio in the most metal-poor stars. Within theerror bars, the trends of the abundance ratios with metallicity areconsistent with those found in earlier literature, but in many cases thescatter around the average trends is much smaller than found in earlierstudies, which were limited to lower-quality spectra. We find that thecosmic scatter in several element ratios may be as low as 0.05 dex.The evolution of the abundance trends and scatter with decliningmetallicity provides strong constraints on the yields of the firstsupernovae and their mixing into the early ISM. The abundance ratiosfound in our sample do not match the predicted yields frompair-instability hypernovae, but are consistent with element productionby supernovae with progenitor masses up to 100 M⊙.Moreover, the composition of the ejecta that have enriched the matterBased on observations obtained in the frame of the ESO programme ID165.N-0276(A).Full Tables 3 and 8 are 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/416/1117 This work hasmade use of the SIMBAD database.
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| 9. |
- Christlieb, N., et al.
(författare)
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The Hamburg/ESO R-process Enhanced Star survey (HERES). I. Project description, and discovery of two stars with strong enhancements of neutron-capture elements
- 2004
-
Ingår i: Astronomy & Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 428:3, s. 1027-1037
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Tidskriftsartikel (refereegranskat)abstract
- We report on a dedicated effort to identify and study metal-poor stars strongly enhanced in r-process elements ([r/Fe]>1 dex; hereafter r-IIstars), the Hamburg/ESO R-process Enhanced Star survey (HERES).Moderate-resolution (∼2 Å) follow-up spectroscopy has been obtained for metal-poor giant candidates selected from the Hamburg/ESO objective-prism survey (HES) as well as the HK survey to identify sharp-lined stars with [Fe/H]<-2.5 dex. For several hundred confirmed metal-poor giants brighter than B∼ 16.5 mag (most of them from theHES), ``snapshot'' spectra (R∼ 20 000; S/N ∼ 30 per pixel) are being obtained with VLT/UVES, with the main aim of finding the 2-3% r-II stars expected to be among them. These are studied in detail by means of higher resolution and higher S/N spectra. In this paper we describe a pilot study based on a set of 35 stars, including 23 from the HK survey,eight from the HES, and four comparison stars. We discovered two new r-II stars, CS 29497-004 ([Eu/Fe]=1.64± 0.22) and CS 29491-069([Eu/Fe]=1.08± 0.23). A first abundance analysis of CS 29497-004 yields that its abundances of Ba to Dy are on average enhanced by 1.5 dex with respect to iron and the Sun and match a scaled solar r-process pattern well, while Th is underabundant relative to that pattern by 0.3dex, which we attribute to radioactive decay. That is, CS 29497-004 seems not to belong to the class of r-process enhanced stars displaying an ``actinide boost'', like CS 31082-001 (Hill et al. 2002), or CS30306-132 (Honda et al. 2004b). The abundance pattern agrees well with predictions of the phenomenological model of Qian & Wasserburg.Based in large part on observations collected at the European Southern Observatory, Paranal, Chile (proposal number 68.B-0320).}
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| 10. |
- Depagne, E., et al.
(författare)
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First Stars. II. Elemental abundances in the extremely metal-poor star CS 22949--037. A diagnostic of early massive supernovae
- 2002
-
Ingår i: Astronomy & Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 390:1, s. 187-198
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Tidskriftsartikel (refereegranskat)abstract
- CS 22949-037 is one of the most metal-poor giants known ([Fe/H]~-4.0),and it exhibits large overabundances of carbon and nitrogen (Norris etal.). Using VLT-UVES spectra of unprecedented quality, regardingresolution and S/N ratio, covering a wide wavelength range (from lambda= 350 to 900 nm), we have determined abundances for 21 elements in thisstar over a wide range of atomic mass. The major new discovery is anexceptionally large oxygen enhancement, [O/Fe] = 1.97+/-0.1, as measuredfrom the [O I] line at 630.0 nm. We find an enhancement of [N/Fe] of2.56+/- 0.2, and a milder one of [C/Fe] = 1.17+/-0.1, similar to thosealready reported in the literature. This implies Zstar =0.01Zsun. We also find carbon isotopic ratios12C/13C =4+/-2.0 and 13C/14N=0.03 +0.035-0.015, close to the equilibrium valueof the CN cycle. Lithium is not detected. Na is strongly enhanced([Na/Fe] = +2.1 +/- 0.2), while S and K are not detected. Thesilicon-burning elements Cr and Mn are underabundant, while Co and Znare overabundant ([Zn/Fe]=+0.7). Zn is measured for the first time insuch an extremely metal-poor star. The abundances of the neutron-captureelements Sr, Y, and Ba are strongly decreasing with the atomic number ofthe element: [Sr/Fe] ~ +0.3, [Y/Fe] ~ -0.1, and [Ba/Fe] ~ -0.6. Amongpossible progenitors of CS 22949-037, we discuss the pair-instabilitysupernovae. Such very massive objects indeed produce large amounts ofoxygen, and have been found to be possible sources of primary nitrogen.However, the predicted odd/even effect is too large, and the predictedZn abundance much too low. Other scenarios are also discussed. Inparticular, the yields of a recent model (Z35Z) from Heger and Woosleyare shown to be in fair agreement with the observations. The onlydiscrepant prediction is the very low abundance of nitrogen, possiblycurable by taking into account other effects such as rotationallyinduced mixing. Alternatively, the absence of lithium in our star, andthe values of the isotopic ratios 12C/13C and13C/14N close to the equilibrium value of the CNcycle, suggest that the CNO abundances now observed might have beenaltered by nuclear processing in the star itself. A 30-40Msun supernova, with fallback, seems the most likelyprogenitor for CS 22949-037. Based on observations made with the ESOVery Large Telescope at Paranal Observatory, Chile (programme ID165.N-0276(A)).
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| 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
-
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
-
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
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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
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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)
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The solar neighbourhood age-metallicity relation - Does it exist?
- 2001
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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
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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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| 21. |
- Flicker, Ralf, et al.
(författare)
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Tilt anisoplanatism in laser-guide-star-based multiconjugate adaptive optics. Reconstruction of the long exposure point spread function from control loop data
- 2003
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Ingår i: Astronomy & Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 400:3, s. 1199-1207
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Tidskriftsartikel (refereegranskat)abstract
- A method is presented for estimating the long exposure point spreadfunction (PSF) degradation due to tilt anisoplanatism in alaser-guide-star-based multiconjugate adaptive optics systems fromcontrol loop data. The algorithm is tested in numerical Monte Carlosimulations of the separately driven low-order null-mode system, and isshown to be robust and accurate with less than 10 relative error inboth H and K bands down to a natural guide star (NGS) magnitude ofmR=21, for a symmetric asterism with three NGS on a 30 arcsec radius.The H band limiting magnitude of the null-mode system due to NGSsignal-to-noise ratio and servo-lag was estimated previously to mR=19.At this magnitude, the relative errors in the reconstructed PSF andStrehl are here found to be less than 5 and 1, suggesting that the PSFretrieval algorithm will be applicable and reliable for the full rangeof operating conditions of the null-mode system.
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| 22. |
- François, P., et al.
(författare)
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First Stars. III. A detailed elemental abundance study of four extremely metal-poor giant stars
- 2003
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Ingår i: Astronomy & Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 403:3, s. 1105-1114
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Tidskriftsartikel (refereegranskat)abstract
- This paper reports detailed abundance analyses for four extremelymetal-poor (XMP) giant stars with [Fe/H]<-3.8, based onhigh-resolution, high-S/N spectra from the ESO VLT (Kueyen/UVES) and LTEmodel atmosphere calculations. The derived [alpha /Fe] ratios in oursample exhibit a small dispersion, confirming previous findings in theliterature, i.e. a constant overabundance of the alpha -elements with avery small (if any) dependence on [Fe/H]. In particular, the very smallscatter we determine for [Si/Fe] suggests that this element shows aconstant overabundance at very low metallicity, a conclusion which couldnot have been derived from the widely scattered [Si/Fe] values reportedin the literature for less metal-poor stars. For the iron-peak elements,our precise abundances for the four XMP stars in our sample confirm thedecreasing trend of Cr and Mn with decreasing [Fe/H], as well as theincreasing trend for Co and the absence of any trend for Sc and Ni. Incontrast to the significant spread of the ratios [Sr/Fe] and [Ba/Fe], wefind [Sr/Ba] in our sample to be roughly solar, with a much lowerdispersion than previously found for stars in the range -3.5 < [Fe/H]< -2.5.Based on observations made with the ESO Very Large Telescope at ParanalObservatory, Chile (Large Programme ID 165.N-0276(A)).The complete version of Table 5 is only available in electronic form atthe CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.125.5) or viahttp:/ /cdsweb.u-strasbg.fr/cgi-bin/qcat?J /A+A/403/1105
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| 23. |
- Grundahl, F, et al.
(författare)
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Abundances of RGB stars in NGC 6752
- 2002
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Ingår i: Astronomy & Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 385, s. 14-17
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Tidskriftsartikel (refereegranskat)abstract
- Abundances of O, Fe, Al, Mg, Na and Li determined from high-resolutionand high signal-to-noise spectra of 21 red giant branch (RGB) stars inthe globular cluster NGC 6752 are presented. We demonstrate that theStrömgren c1 index correlates extremely well with themeasured NH, CH and CN indices and that variations in c1 aredue to differences in UV NH band strengths. As shown by Grundahl et al.(cite{grundahl02}), the RGB stars in all 20 globular clusters surveyedpossess large star-to-star variations in their c1 index,which we interpret as significant inhomogeneities in N abundances. Thewell known relations among O and Na and Mg and Al as well as with CN,CH, and NH are also present in these stars, some of which are fainterthan the RGB bump. We find that the Li (6708 Å) equivalent widthbecomes too small to be measured as the luminosity of the stars increaseabove the luminosity of the RGB bump. Taken together, these data enforcethe results of Gratton et al. (cite{gratton01}) for turnoff andsub-giant branch stars, and suggest that in this cluster the abundancecorrelations observed among the brighter giants are in place prior toappreciable RGB ascent and likely due to previous generation(s) ofstars, rather than due to mixing in the observed stars. Based onobservations obtained with the ESO Very Large Telescope UVESspectrograph for programme 65.L-0165(A). Based on observations obtainedwith the Danish 1.5 m telescope at the European Southern Observatory, LaSilla, Chile.
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| 24. |
- Gullberg, Dag, et al.
(författare)
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Determination of accurate stellar radial-velocity measures
- 2002
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Ingår i: Astronomy & Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 390:1, s. 383-395
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Tidskriftsartikel (refereegranskat)abstract
- Wavelength measurements in stellar spectra cannot readily be interpretedas true stellar motion on the sub-km s-1 accuracy level dueto the presence of many other effects, such as gravitational redshiftand stellar convection, which also produce line shifts. Following arecommendation by the IAU, the result of an accurate spectroscopicradial-velocity observation should therefore be given as the``barycentric radial-velocity measure'', i.e. the absolute spectralshift as measured by an observer at zero gravitational potential locatedat the solar-system barycentre. Standard procedures for reducingaccurate radial-velocity observations should be reviewed to take intoaccount this recommendation. We describe a procedure to determineaccurate barycentric radial-velocity measures of bright stars, based ondigital cross-correlation of spectra obtained with the ELODIEspectrometer (Observatoire de Haute-Provence) with a synthetic templateof Fe I lines. The absolute zero point of the radial-velocity measuresis linked to the wavelength scale of the Kurucz (1984) Solar Flux Atlasvia ELODIE observations of the Moon. Results are given for the Sun and42 stars, most of them members of the Hyades and Ursa Major clusters.The median internal standard error is 27 m s-1. The externalerror is estimated at around 120 m s-1, mainly reflecting theuncertainty in the wavelength scale of the Solar Flux Atlas. For the Sunwe find a radial-velocity measure of +257+/- 11 m s-1referring to the full-disk spectrum of the selected Fe I lines. Based onobservations made at Observatoire de Haute-Provence
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| 25. |
- Hartman, Henrik, et al.
(författare)
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Identification of emission lines in the low-ionization strontium filament near Eta Carinae
- 2004
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Ingår i: Astronomy & Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 419:1, s. 215-224
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Tidskriftsartikel (refereegranskat)abstract
- Spectral lines observed in the Sr-filament of eta Car in the wavelengthregion 2480-10140{AA}. The lines are sorted by wavelength in table 2 andsorted according to the periodic table in table 3. Both tables areavailable in postscript format as the file tables23.ps where the sub andsuperscripts for the atomic transitions are kept.(2 data files).
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