A high resolution (R = 110.000), very high S/N (>600) spectrum of the
metal-poor turnoff star G271-162 has been obtained in connection with the
commissioning of UVES at VLT/Kueyen. Using both 1D hydrostatic and 3D
hydrodynamical model atmospheres, the lithium isotope ratio has been estimated
from the LiI 670.8 nm line by means of spectral synthesis. The necessary
stellar line broadening (1D: macroturbulence + rotation, 3D: rotation) has been
determined from unblended KI, CaI and FeI lines. The 3D line profiles agree
very well with the observed profiles, including the characteristic line
asymmetries. Both the 1D and 3D analyses reveal a possible detection of 6Li in
G271-162, 6Li/7Li = 0.02 +-0.01 (one sigma). It is discussed if the smaller
amount of 6Li in G271-162 than in the similar halo star HD84937 could be due to
differences in stellar mass and/or metallicity or whether it may reflect an
intrinsic scatter of the Li isotope ratio in the ISM at a given metallicity.Comment: 5 pages with 6 figures. Accepted as a letter in A&

Asplund, M.

D'Odorico, S.

Hill, V.

Nissen, P. E.

English

arXiv

A high resolution (lambda /Delta lambda =~ 110 000), very high S/N (>~ 600) spectrum of the metal-poor turnoff star G 271-162 has been obtained in connection with the commissioning of UVES at VLT/Kueyen. Using both 1D hydrostatic and 3D hydrodynamical model atmospheres, the lithium isotope ratio has been estimated from the \ion{Li}{i},670.8 nm line by means of spectral synthesis. The necessary stellar line broadening (1D: macroturbulence + rotation, 3D: rotation) has been determined from unblended \ion{K}{i}, \ion{Ca}{i} and \ion{Fe}{i} lines. The 3D line profiles agree very well with the observed profiles, including the characteristic line asymmetries. Both the 1D and 3D analyses reveal a possible detection of \element[][6]{Li} in G 271-162, element [][6]{Li}/element [][7]{Li} = 0.02+/-0.01 (1sigma ). It is discussed if the smaller amount of \element[][6]{Li} in G 271-162 than in the similar halo star HD 84937 could be due to differences in stellar mass and/or metallicity or whether it may reflect an intrinsic scatter of \element[][6]{Li}/\element[][7]{Li} in the ISM at a given metallicity. Based on public data released from the UVES commissioning at the VLT/Kueyen telescope, ESO, Paranal, Chile

Nissen, P.E

Asplund, Martin

Hill, V

D'Odorico, S

The Australian National University

Astron. Astrophys. 357, L49–L52 (2000) ASTRONOMYANDASTROPHYSICSLetter to the EditorThe lithium isotope ratio in the metal-poor halo star G271-162from VLT/UVES observations?P.E. Nissen1, M. Asplund2, V. Hill 3, and S. D’Odorico31 University of Aarhus, Institute of Physics and Astronomy, 8000 Aarhus C, Denmark (pen@ifa.au.dk)2 Uppsala Astronomical Observatory, Box 515, 751 20, Sweden (martin@astro.uu.se)3 European Southern Observatory, Karl-Schwarzschild Strasse 2, 85748 Garching b. München, Germany (vhill@eso.org, sdodoric@eso.org)Received 16 March 2000/ Accepted 11 April 2000Abstract. A high resolution (λ/∆λ ' 110 000), very highS/N (>∼ 600) spectrum of the metal-poor turnoff star G 271-162 has been obtained in connection with the commissioning ofUVES at VLT/Kueyen. Using both 1D hydrostatic and 3D hy-drodynamical model atmospheres, the lithium isotope ratio hasbeen estimated from the Lii 670.8 nm line by means of spectralsynthesis. The necessary stellar line broadening (1D: macro-turbulence + rotation, 3D: rotation) has been determined fromunblended Ki, Cai and Fei lines. The 3D line profiles agreevery well with the observed profiles, including the character-istic line asymmetries. Both the 1D and 3D analyses reveal apossible detection of6Li in G 271-162,6Li/7Li = 0.02± 0.01(1σ). It is discussed if the smaller amount of6Li in G 271-162than in the similar halo star HD 84937 could be due to differ-ences in stellar mass and/or metallicity or whether it may reflectan intrinsic scatter of6Li/7Li in the ISM at a given metallicity.Key words: stars: abundances – stars: atmospheres – stars: fun-damental parameters – stars: individual: G271-162 – ISM: cos-mic rays1. IntroductionDue to the special status of6Li for astrophysics and cosmol-ogy, much work has been devoted to the search for this isotopein metal-poor stars ever since the first detection in HD 84937by Smith et al. (1993). The reason for this interest is threefold:i) The presence of6Li in the envelope of halo stars severelylimits the possible depletion of7Li, and thus allows a more ac-curate determination of the primordial7Li abundance (Ryan etal. 1999; Asplund & Carlsson 2000);ii) 6Li abundances pro-vide an additional test of theories for the production of the lightelements (Li, Be and B) by cosmic ray processes;iii) Since6Liis an even more fragile nuclei than7Li it is a sensitive probe ofpossible mixing events during the stellar life.Send offprint requests to: P.E. Nissen? Based on public data released from the UVES commissioning atthe VLT/Kueyen telescope, ESO, Paranal, ChileTo date6Li is claimed to have been detected in two metal-poor halo stars, HD 84937 and BD +26 3578, and two metal-poor disk stars, HD 68284 and HD 130551 (Smith et al. 1998;Hobbs & Thorburn 1997; Cayrel et al. 1999; Nissen et al. 1999).Clearly, an observational test of models for the formation andevolution of6Li and for the depletion of6Li in stellar envelopesrequires a much larger set of6Li data spanning a large metallic-ity range. With the advent of high-resolution spectrographs on8m-class telescopes this is now becoming feasible.The determinations of6Li abundances are based on the in-creased width and asymmetry of the Lii 670.8 nm doublet intro-duced by the isotope shift (0.16̊A = 7.1 km s−1) of 6Li. Sincethe line is not resolved, the derived6Li abundance dependson the adopted stellar line broadening as estimated from otherspectral lines. Until now all such analyses have relied on 1Dhydrostatic model atmospheres which cannot predict the inher-ent line asymmetries introduced by the convective motions inthe atmosphere and thus is a potential source of uncertainty.An attractive alternative is provided by the new generation of3D hydrodynamical model atmospheres (e.g. Stein & Nordlund1998; Asplund et al. 1999, 2000) which self-consistently com-pute the time-dependent convective velocity fields and thus areable to predict the convective line asymmetries.In the presentLetterwe analyze a high resolution, very highS/N spectrum of the metal-poor halo star G271-162 obtainedduring the commissioning of UVES on VLT/Kueyen using both1D and 3D model atmospheres. Thereby we also investigate theability of UVES to provide high quality spectra as needed inmany astrophysical studies.2. Observations and reductions of UVES spectraThe metal-poor star G 271-162 (BD−10 388) was chosen asone of the targets for the first commissioning of UVES, the UV-Visual Echelle Spectrograph (D’Odorico & Kaper, 2000) on theESO VLT UT2 (Kueyen) telescope. The aim was to test if UVEScan supply high quality spectra suitable for a determination ofthe lithium isotope ratio in stars – one of the most demandingproblems in observational stellar astrophysics. Being situatedat the turnoff point for halo stars in the HR diagram, G 271-LETTERL50 P.E. Nissen et al.: The lithium isotope ratio in the metal-poor halo star G271-162 from VLT/UVES observationsFig. 1. UVES spectrum of G 271-162 in the regions of the Lii 670.8and Ki 769.9 nm lines with nearby thorium emission lines inserted162 is very similar to HD 84937; hence, one would expect theLi isotope ratio to be about the same, i.e.6Li/7Li ' 0.06. Fur-thermore, G 271-162 represents a group of about 50 halo turnoffstars with10 <∼ V <∼ 11.5 that are too faint to be searched for6Li using high resolution spectrographs at 4m class telescopes.With UVES it should be possible to reach these stars and henceto increase the sample of stars with6Li determinations by anorder of magnitude.Four spectra of G 271-162 were obtained on October 11 -13, 1999, with exposure times ranging from 30 to 60 min. Thewidth of the entrance slit was set at 0.3 arcsec to get a resolu-tion close to 120 000. An image slicer is foreseen for this highresolution mode of UVES, but was not available during the firstcommissioning run. Instead, the star was continuously movedalong the slit within±5 arcsec corresponding to approximately50 pixels on the CCDs. This widening of the echelle ordersis essential in obtaining very high S/N. The lack of the imageslicer made the efficiency of UVES critically dependent on theseeing, but it is noted that on one night with excellent seeingon Paranal (0.35 arcsec) the efficiency of UVES was as high ascould be expected in the image slicer mode for average seeingconditions.The red arm of UVES was applied with the two CCDs (EEV4096 × 2048 and MIT-LL 4096 × 2048) covering the spectralregions 610 - 705 and 715 - 810 nm in 14 and 10 echelle orders,respectively. Cross disperser #3 was chosen to provide amplespace (' 150 pixels) between the orders for accurate measure-ment of the background.The spectra have been reduced using standard IRAF tasksfor order definition, background subtraction, FF division, ex-traction of orders, wavelength calibration and continuum nor-malization. The spectrum of a bright B-type star, HR 8858, wasobserved and used to remove telluric lines present in some spec-tral regions. After correction for radial velocity shifts the fourspectra were combined to one. Fig. 1 shows the very highS/Nand resolution obtained.The wavelength calibration is based on 30-40 thorium linesper echelle order, and the rms of a 2-dimensional 4th orderpolynomial fit between pixel and wavelength space is 1.1 mÅ.In addition, the Th lines can be used to check the resolution; theinstrument profile is well approximated with a Gaussian and noasymmetry is seen. The resolution as measured from the FWHMof the Th lines varies, however, along a given spectral orderfrom about 100 000 to 120 000. Thanks to the large number ofTh lines available, the variation could be mapped and the actualinstrumental resolution for a given stellar line determined andapplied in connection with the spectrum synthesis. It is notedthat this problem of resolution variations has been fixed duringthe second commissioning run of UVES.3. AnalysisThe analysis of the Lii feature (equivalent width 29.0 m̊A) hasbeen carried out using both 1D hydrostatic (Asplund et al.1997) and 3D hydrodynamical LTE model atmospheres (As-plund et al. 1999). According to Strömgren photometry thestellar parameters of G 271-162 areTeff = 6295 ± 70 K and[Fe/H] = −2.15 ± 0.2 while the uncertain Hipparcos par-allax suggests log = 3.7+0.4−0.7. G 271-162 closely resemblesHD 84937 for which the IR flux method (IRFM), the Hipparcosparallax and stellar spectroscopy implyTeff = 6330 ± 80 K,logg = 4.0 ± 0.1 and [Fe/H]= −2.25 ± 0.2, which we there-fore also adopt for G 271-162. It should be emphasized thoughthat the exact choice of parameters does not influence signifi-cantly the lithiumisotope ratiodeterminations although it doesof course affect theabsoluteabundances.When applying 1D model atmospheres to spectral synthe-sis additional line broadening besides the instrumental broad-ening is required. This broadening, which is mainly due tomacroturbulence, was approximated by a Gaussian function anddetermined from aχ2 fit of five unblended lines (Cai 612.2,Cai 616.2, Fei 623.0, Cai 643.9, and Ki 769.9 nm), all of sim-ilar strength as the Lii 670.8 nm line. In the case of theK i 769.9 nm resonance line the hyperfine structure and isotopicshift between39K and41K were taken into account althoughthe effect turned out to be rather negligible. A more sophis-ticated broadening like a radial-tangential profile did not im-prove the agreement with the observed profiles. In contrast, inhydrodynamical model atmospheres the self-consistent convec-tive motions account for all of the line broadening except forthe unknown rotational velocityvrotsini of the star. This param-et r was determined by fitting a disk-integration of the angle-dependent theoretical 3D line profiles to the observed profilesof the Ki, Cai and Fei lines.The comparison between theoretical and observed profileswas quantified using aχ2 analysis in an analogous way to pre-vious investigations (Nissen et al. 1999). Theχ2 was computedthroughχ2 =∑(Oi −Si)2/σ2, whereOi is the observed spec-tral point,Si the synthesis andσ = (S/N)−1 as estimated intwo adjacent continuum windows to the lines. When estimatingthe FWHM of the Gaussian broadening (1D) andvrotsini (3D),the abundance and the wavelength zeropoint of the observedspectrum were allowed to vary to optimize the fit (Figs. 2 and3). The most probable value corresponds to the minimum ofχ2 and∆χ2 = χ2 − χ2min = 1, 4 and 9 represent the 1-, 2-and 3σ confidence limits of the determinations. These broaden-LETTERP.E. Nissen et al.: The lithium isotope ratio in the metal-poor halo star G271-162 from VLT/UVES observations L51Table 1.Strömgren photometry from Schuster & Nissen (1988)ID V (b − y)0 m0 c0 βHD 84937 8.33 0.303 0.056 0.354 2.613G 271-162 10.35 0.306 0.055 0.355 2.602BD +26 3578 9.37 0.308 0.045 0.366 2.600Table 2.Stellar parameters and Li isotope ratiosID Teff [Fe/H] MV 6Li/7LiHD 84937 6315 K −2.14 3.58 0.059 ± 0.016G 271-162 6295 K −2.15 3.53 0.020 ± 0.010BD +26 3578 6280 K −2.60 3.08 0.050 ± 0.030ing parameters were subsequently used in a similarχ2 analysisof the Li i 670.8 nm line. The free parameters in the compari-son between the predicted and observed profile were, besidesthe6Li fraction, the total Li abundance and the wavelength ze-ropoint of the observed profile. It is emphasized that for eachvalue of6Li/7Li the other free parameters are optimized to getthe smallest possible value ofχ2.The 1D analysis of the Ki, Cai and Fei lines implies aFWHM = 5.9 ± 0.1 km s−1of the stellar line broadeningfunction. However, the detailed agreement with the observa-tions are far from perfect (Fig. 2). In particular, the distinct redasymmetry of all lines similar to the solar case (e.g. Asplundet al. 2000) is not accounted for. The result of theχ2 analy-sis of the Lii 670.8 nm doublet suggests a possible detection of6Li: 6Li/7Li = 0.02± 0.01 (1σ). The minimumχ2red = χ2/ν,whereν is the number of degrees of freedom in the fit, is satis-factory close to 1.The temperature inhomogeneities and velocity fields in the3D hydrodynamical model atmospheres introduce characteris-tic line asymmetries, which for the Sun perfectly describe theobserved asymmetries even on an absolute wavelength scale(Asplund et al. 2000). The existence of the predicted line asym-metries in the 3D analysis vastly improves the agreement withthe observed profiles for G 271-162 (Fig. 3); other lines showsimilarly encouraging correspondence. This provides additionalsupport for the realism of the 3D model when describing the realstellar photosphere, as well as giving a higher degree of con-fidence in the estimate of the lithium isotope ratio. The stellarrotation was determined to bevrotsini = 2.9 ± 0.1 km s−1,which in turn implies6Li/7Li = 0.02±0.01 (1σ) according toFig. 5, the same result as in the 1D analysis. Again, the minimumχ2red is close to 1 as it should.4. Discussion and conclusionsBoth the 1D and 3D analyses suggest a possible detection of6Liin G 271-162 at the level of6Li/7Li = 0.02 ± 0.01. Given theremaining uncertainties in the determination of the stellar linebroadening (e.g. NLTE effects) the detection should, however,be considered preliminary. The similarity between the 1D and3D results and the excellent agreement between the observedFig. 2. The predicted (solid line) and observed (diamonds)Cai 616.2 nm line together with the residual fluxes based on the 1Danalysis. The Gaussian broadening with which the theoretical spectrumhas been convolved include the combined effects of macroturbulence,rotation and instrumental resolution. Note the remaining systematicsignal in the residualsFig. 3. The predicted (solid line) and observed (diamonds)Cai 616.2 nm line together with the residual fluxes based on the 3Danalysis. The rotational broadening has been included through a disk-integration after which a Gaussian instrumental profile has been ap-plied. Note the excellent ability of the 3D profile to describe the ob-served red asymmetry of the line not possible with 1D modelsand predicted profiles in 3D, could be interpreted as supportfor a positive detection though. It also lends added confidenceto previous detections of6Li in metal-poor stars based on 1Dinvestigations (e.g. Smith et al. 1998; Nissen et al. 1999). Ev-idently, the convective line asymmetries are rather negligiblecompared to the isotopic shift of the6Li doublet.Regardless of whether6Li has been detected in G 271-162o not, the6Li/7Li ratio appears in any case to be smaller than inHD 84937 for which a weighted average of the results of Hobbs& Thorburn (1997), Smith et al. (1998) and Cayrel et al. (1999)is 6Li/7Li = 0.059 ± 0.016 (1σ). This is quite puzzling, be-cause the two stars have almost identical parameters accordingto theuvby-β photometry of Schuster & Nissen (1988). Table 1lists the measured indices for the three halo stars with6Li mea-surements after correction for a small reddening of G 271-162,E(b − y) = 0.020, as derived from the theβ - (b − y)0 cali-bration of Schuster & Nissen (1989). Table 2 shows the derivedparameters using the IRFM calibration ofTeff vs. (b − y)0 byAlonso et al. (1996a), the [Fe/H] calibration of Schuster & Nis-sen (1989) and theMV calibration of Nissen & Schuster (1991).The directly measured IRFM temperatures of HD 84937 andLETTERL52 P.E. Nissen et al.: The lithium isotope ratio in the metal-poor halo star G271-162 from VLT/UVES observationsFig. 4. The predicted (solid line) and observed (diamonds)Li i 670.8 nm lines together with the residual fluxes for the best fitwith 6Li/7Li = 0.02 in the 3D analysis. The Gaussian broadeningwith which the theoretical spectrum has been convolved include onlythe instrumental resolution while the rotational broadening has beenapplied following a full disk-integration of the 3D profilesFig. 5. The variation of∆χ2 in the 3D analysis of the Lii 670.8 nmline as a function of the relative abundance of6Li for different adoptedvrotsini: 2.7 (dotted), 2.9 (solid) and 3.1 km s−1 (dashed). The cor-responding estimates of6Li/7Li are0.027 ± 0.009, 0.023 ± 0.009and0.017 ± 0.010, respectively. The widths of Ki, Cai and Fei linesimply vrotsini = 2.9 ± 0.1 km s−1. The horizontal lines represent theformal1σ, 2σ and3σ confidence limitsBD +26 3578 are 6330 and 6310 (Alonso et al. 1996b) in excel-lent agreement with the values in Table 2, whereas G 271-162has not been measured. The absolute magnitude of HD 84937from the Hipparcos parallax (ESA, 1997) isMV = 3.82±0.19in good agreement with the photometric value within the quotederrors. For the two other stars the relative error of the Hippar-cos parallax is far too large to estimateMV with any reason-able accuracy. Finally, a preliminary abundance analysis showsthat HD 84937 and G 271-162 have the same metal abundanceswithin ±0.15 dex.Fig. 6 shows the location of the three stars in theMV-log Teffdiagram. Clearly, HD 84937 and G 271-162 have nearly thesame mass. This makes it difficult to explain the lower6Liabundance in G 271-162 as a depletion effect and raises theinteresting question of a possible intrinsic scatter of6Li/7Li inthe ISM at a given metallicity. In this connection we note thatsome models for the formation of the light elements by cosmicray processes in the early Galaxy predict a scatter of one or-der of magnitude in the abundance of6Li, Be and B relativeto Fe, e.g. the bimodal superbubble model of Parizot & DruryFig. 6. The position of HD 84937, BD+26 3578 and G 271-162 in theMV-log Teff diagram compared to evolutionary tracks from Vanden-Berg et al. (2000) with masses and metallicities indicated(1999) and the supernovae-driven chemical evolution model forthe Galactic halo by Suzuki et al. (1999).It is clear that the ability of UVES on VLT/Kueyen to gatherhigh quality, high resolution and very highS/N spectra of evenV ' 10.m5 stars as evident from the present study, has opened upthe possibility for a large survey of6Li/7Li for stars with a widerange of metallicities. An observing program with this exact aimusing UVES/VLT is currently in progress, which is expected tobe of great importance both for an improved understanding ofBig Bang nucleosynthesis, cosmic chemical evolution of thelight elements and stellar mixing events.Acknowledgements.We are greatly indebted to all the people involvedin the conception, construction and commissioning of the UVES in-strument, without whom this project would have been impossible.ReferencesAlonso A., Arribas S., Martı́nez-Roger C., 1996a, A&A 313, 873Alonso A., Arribas S., Martı́nez-Roger C., 1996b, A&AS 117, 227Asplund M., Gustafsson B., Kiselman D., Eriksson K., 1997, A&A318, 521Asplund M., Nordlund̊A., Trampedach R., Stein R.F., 1999, A&A 346,L17Asplund M., Carlsson M., 2000, submitted to A&AAsplund M., NordlundÅ., Trampedach R., Allende Prieto C., SteinR.F., 2000, submitted to A&ACayrel R., Spite M., Spite F., et al., 1999, A&A 343, 923D‘Odorico S., Kaper L., 2000, UVES User Manual, ESO Doc. no VLT-MAN-ESO-13200-1825ESA, 1997,The Hipparcos and Tycho Catalogues, ESA SP-1200Hobbs L.M., Thorburn J.A., 1997, ApJ 491, 772Nissen P.E., Lambert D.L., Primas F., Smith V.V., 1999, A&A 348, 211Nissen P.E., Schuster W.J., 1991, A&A 251, 457Parizot E, Drury L., 1999, A&A 349, 673Ryan S.G., Norris J.E., Beers T.C., 1999, ApJ 523, 654Schuster W.J., Nissen P.E., 1988, A&AS 73, 225Schuster W.J., Nissen P.E., 1989, A&A 221, 65Smith V.V., Lambert D.L., Nissen P.E., 1993, ApJ 408, 262Smith V.V., Lambert D.L., Nissen P.E., 1998, ApJ 506, 405Stein R.F., Nordlund̊A., 1998, ApJ 499, 914Suzuki T.K., Yoshii Y., Kajino T., 1999, ApJ 522, L125VandenBerg D.A., Swenson F.J., Rogers F.J., Iglesias C.A., AlexanderD.R., 2000, ApJ 532, 430LETTER

The lithium isotope ratio in the metal-poor halo star G271-162 from VLT/UVES observations

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The lithium isotope ratio in the metal-poor halo star G271-162 from VLT/UVES observations

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