Abnormal lithium abundance in several Ap-Bp stars
A. Shavrina, N.S. Polosukhina, N.A. Drake, D.O. Kudryavtsev, V.F. Gopka, V.A. Yushchenko, A.V.Yushchenko
aa r X i v : . [ a s t r o - ph . S R ] A p r Abnormal lithium abundance in several Ap-Bp starsA. Shavrina , N.S. Polosukhina , N.A. Drake , D.O. Kudryavtsev , V.F. Gopka , V.A.Yushchenko , A.V. Yushchenko c (cid:13) Main Astronomical Observatory NAN Ukraine, Crimean Astrophysical Observatory, Ukraine, Sankt-Peterburg University, Russia, Special Astrophysical Observatory, Russia, Odessa University, Ukraine, Sejong University, (South) Koreae-mail: [email protected] , [email protected] , [email protected] , [email protected] ,[email protected] , [email protected] , [email protected] Using observations of rapidly rotating roAp star HD 12098, obtained with 6m telescope of SAORAS in 2005-2008 in nine rotational phases (0.001-0.817), and code V. Tsymbal ROTATE weidentified 3 lithium spots on the surface of star, one of them coincided with the position of onemagnetic pole and two others are located near the other pole, symmetrically with respect to it.Perhaps magnetic field of the star has a more complex structure than the dipole [17]. Lithiumabundance in the spots is greater than cosmic one. It can be explained by diffusion of lithium inthe presence of a magnetic field or by production of lithium in the reactions of ”spallation” onthe surface of star near the magnetic poles (see, for example, [4]). Enhanced lithium abundanceswas also determined for other two slowly rotating Ap stars HR 465 and HD 965. The analysis isperformed using a synthetic spectrum method taking into account magnetic splitting of lines bythe SYNTHM code of S. Khan [5].
INTRODUCTION
Spectral observations of Ap-CP stars at BTA telescope (SAO RAS) with echelle spectrometer NES [10] areperformed under the project ”Lithium in CP stars”. It was found several stars with anomalous lithium content.The oscillating star HD 12098, first roAp star, opened recently on northern sky [3], was among these stars. Thespectrum of this star showed strong and variable Li I line 6708 ˚A, analysis of which revealed a high abundanceof lithium [1]. It has been shown the presence of lithium spots on the surface of the star [17], similar to tworoAp stars HD 83368 and HD 60435, for which lithium spots were previously detected on their surface [15]. In2008 three new phases have added to the observations and we made a new analysis based on 9 observed phases(0.001-0.817) with Tsymbal’s code ROTATE. Also the lithium blend 6708 ˚A was analyzed in the spectra oftwo slowly rotating Ap stars HR 465 and HD 965, for which we used the method of synthetic spectrum takinginto account the magnetic splitting of lines [5].
The star HD 12098
The method of synthetic spectrum on the base of model atmospheres was applied for analysis taking intoaccount the VALD list and some additional lines of rare earth elements (REE), calculated by the authors [16].Basing on Fe I/Fe II lines we selected the model atmosphere for HD 12098 from Kurucz’ grid: Teff = 7750 Kand lg g = 4.0. Using this effective temperature, evolutionary tracks from the work of Schaller et al. [14] andthe formula from the paper of Kochukhov and Bagnulo [6], we determined the stellar radius R = 1.71 R ⊙ , logg = 4.2, Vrot = 15.8 km/s, i = 55 o and vsini = 13 km/s. The profile of the lithium blend Li I 6708 ˚A werecalculated using the code SYNTHM of S.Khan [5] taking into account magnetic splitting. In the work of [17] amodel of the dipole magnetic field for HD 12098 was constructed on the base of Ryabchikova et al. [12] valuesof effective magnetic field for this star. With the code ROTATE of V.Tsymbal (see [15]) the observations ofthis star in 2005-2007 (phases 0.018 - 0.573, 6 phases) were analyzed. Lithium spots, that almost coincidedwith the locations of the poles of magnetic dipole, have been identified and the lithium abundance in them weredetermined. Observations of 2008 (DK) added 3 new phases and we have undertaken a new analysis by the codeROTATE basing on 9 observed phases (0.001-0.817). Instead of two lithium spots [17], new analysis showedthe presence of 3 lithium spots, one of which coincides with the position of one pole of the magnetic field, thetwo other spots are located near the other pole, symmetrically with respect to it. Possibly magnetic field ofthe star has a more complex structure than the dipole [17] and the magnetic field model must be recalculatedwith new observations. Table 1 shows longitude and latitude of the Li spots in equatorial coordinates, radiusof the each spot and the lithium abundance in the spot ǫ (Li). The observed profiles Li I 6708 ˚A line for the 91able 1: Lithium spots parametersN Longitude Latitude R ǫ (Li)1 30 o -20 o o o o o o -25 o o r e s i dua l i n t en s i t y , r wavelength, ACe III Pr IIII Sm III Li II Li II Ce III 0.0180.0820.1130.1520.5050.5730.6070.8170.001 Figure 1:
Observed (bold line) and model (7750/4.2) spectra of HD 12098 in the range 6705.5 – 6708.5 ˚A for 9 rotationalphases (on the right) rotational phases and model profiles (calculated with spot parameters of Table 1) are shown in Figure 1. Lithiumabundance in the spots is higher than cosmic. It can be explained by diffusion of lithium in the presence of amagnetic field or by production of lithium in the reactions of ”spallation” on the stellar surface near the polesof the magnetic field (see, for example, [4]).
The star HD 465
The star HR 465 (HD 9996) shows the variability of the spectrum with 23 years period ([11]). The spectraobservations were made by AYu in 2004 with eshelle spectrograph BOES ( http : )at 1.8m telescope at the Bogunsan observatory (BOAO) in Korea (R = 80000, S/N > B s in two phases, which were needed to model the profileLi I 6708 ˚A. These values were 1500-2000 G for BOES spectrum and 4500-5000 G for ELODIE one (Figs. 2, 3)The effective temperature and the gravity for the star HR 465 were identified in the work of V.Yushchenko et al.[18] on the base of equivalent widths of neutral and ionized iron lines: Teff = 11840 K, lg g = 4.3. To simulatethe BOES spectrum, which shows strong REE and lithium lines, we used model atmosphere from Kurucz’sgrid with close parameters 11750/4.0 [8], a list of lines VALD [7] and some additional REE lines, calculated bythe authors [16]. We determined the lithium abundance ǫ (Li) = 5.40, which is 2 orders exceeds the ”cosmic”lithium abundance, the abundance of cerium ǫ (Ce) = 6.99 and upper limit of samarium abundance ǫ (Sm) =6.55. The last two values are significantly (more than 5 orders of magnitude) higher than the solar abundance2 r e s i dua l i n t en s i t y , r wavelength, ACr III Fe III Fe IIINd II I obs4500 G5000 G5500 G 0.5 0.6 0.7 0.8 0.9 1 1.1 6146 6146.5 6147 6147.5 6148 6148.5 6149 6149.5 6150wavelength, ACr II I Cr II Fe III Nd III Fe IIIobs.dat1500 G2000 G1000 G Figure 2: a) Observed spectrum (bold line) in the phase of ”chrome” and model(11750/4.0) spectra of HR465 in the range 6147-6150 ˚A b) Observed spectrum (bold line) in the phase of ”REE” and model(11750/4.0)spectra of HR 465 in the range 6147-6150 ˚Aof cerium and samarium. Note that to describe the profile of the Fe II 6708.885 AA we have forced to use aresolution in the spectrum of R = 50000 instead of 80000 and the iron abundance ǫ (Fe) = 8.90, which is morethan one order of magnitude above solar (Fig. 4). In the Figure 4 we see the doubling of lithium and REElines, apparently due to the spots structure of the stellar surface. The star HD 965
The observed spectrum of magnetic peculiar star HD 965 of spectral class A5p (SIMBAD, http://simbad.u-strasbg.fr/simbad/)was obtained by AYu with eshelle spectrograph BOES at 1.8m telescope at the Bogunsan observatory (BOAO)of Korea in December 2004. HD 965 - is slowly rotating Ap star with strong magnetic fields: surface magneticfield Bs = 4400 G has been measured by Mathys et al. 1997 ([9]. The rotational period of the star is morethan 10 years [2]. Parameter set Teff = 7500, lg g = 4.0 and vsini = 3 km/s needed to calculate model profileof Li I 6708 ˚A blend were taken from [13]. The method of synthetic spectrum on the base of the model at-mosphere 7500/4.0 from Kurucz grid [8] taking into account the lines of VALD list and some additional REElines, calculated by the authors [16], was applied to simulate profiles of blend 6708 ˚A composed of the lithiumdoublet 6707.8 and Ce II 6708.1 AA lines. The magnetic splitting of lines was handled with Khan’s codeSYNTHM [5]. Comparing the observed spectrum of the star HD 965 with computed ones we determined theboundaries of the abundance of lithium and cerium, namely ǫ (Li) = 2.90 - 3.09, which are close to the ”cosmic”lithium abundance. Apparently, the strong magnetic field prevent the mixing and the destruction of the originallithium. The cerium abundance is in the range ǫ (Ce) = 3.85 - 4.27, which is almost three order greater thansolar. Note that the lines of both elements in spectrum of this star show spots structure apparently associatedwith two poles of the magnetic field. CONCLUSION
A new analysis of the spectra of roAp star HD 12098 recently discovered in the northern sky shows the presenceof 3 lithium spots, one of which coincides with the location of one magnetic pole, the two other spots are nearanother pole, symmetrical for him. Perhaps magnetic field of the star is more complicated, than the dipole([17]) and its model must be recalculated, taking into account new observations. Enhanced lithium abundancesin the spots on the surface of this rapidly rotating star and in the atmospheres of the two other slowly rotatingAp stars - HR 465 and HD 965 can be caused by physical processes that prevent mixing of stellar matter inthe atmospheres and preserve their original high levels, namely, the suppression of convection by magnetic fieldand coupled with diffusion of lithium, directed to the surface of stars. Another possibility is the real processesof lithium production in the surface layers of the atmosphere, such as the reactions of ”spallation” in whichaccelerated in the magnetic fields protons and alpha-particles destroy the heavier atoms C, N, O with formationof lithium. This possibility is shown in the theoretical work of Goriely [4] for Przybylski’s star (roAp star witha large excess of REE).
Acknowledgements
We thank V. Tsymbal and S. Khan for the opportunity to use their codes ROTATEand SYNTHM. 3 r e s i dua l i n t en s i t y , r wavelength, ACe III Pr IIII Sm III Li II Ce III Fe IIIobsm11750/4.0,lg N(Li)/N(H)=-6.60,lg N(Sm)/N(H)=-5.45 0.75 0.8 0.85 0.9 0.95 1 1.05 6706 6706.5 6707 6707.5 6708 6708.5 r e s i dua l i n t en s i t y , r wavelength, ACe III Pr IIII Sm III Li II Ce IIIobsmod 7500/4.0,lg(Nli/Nh)=-8.86,lg(Nce/Nh)=-7.84mod 7500/4.0,lg(Nli/Nh)=-9.06,lg(Nce/Nh)=-7.54mod 7500/4.0,lg(Nli/Nh)=-9.06,lg(Nce/Nh)=-7.84 Figure 3: a) Observed spectrum (bold line) in the phase ”REE” and model spectra (11750/4.0) of HR 465 inthe range 6705.5 - 6708.5 ˚A. b) Observed (bold line) and model (7500/4.0) spectra of HD 965 for 6705.5 -6708.5 ˚A for boundary abundances of Li and Ce (see figure).
REFERENCES [1]
Shavrina A.V., Polosukhina N.S., Drake N.A., Kudryavtsev D.O.
Anomalous abundance of lithium in the roApstar HD 12098 recently discovered in the northern hemisphere of the sky / / Astrophysics - 2008. - Bf 51, N 4. -P. 517 - 525.[2]
Elkin V. G., Kurtz D. W., Mathys G., Wade G. A., Romanyuk I. I., Kudryavtsev D. O., Smolkin S.
High timeresolution spectroscopy and magnetic variability of the cool Ap star HD965 // Monthly Notices of the RAS – 2005.– , N 3. – P. 1100 – 1104.[3]
Girish V., Seetha S., Martinez P., et al.
HD 12098 - A new northern hemisphere roAp star // Astron. and Astrophys.,– 2001. – , N 1. – P. 142 – 147.[4]
Goriely S.
Nucleosynthesis by accelerated particles to account for the surface composition of HD 101065 // Astron.and Astrophys. – 2007. – , N 2. – P. 619 – 626.[5]
Khan S.
The code of spectrum synthesis in atmospheres of Ap stars // J. Quant. Spectr. and Radiat. Transf. –2004. – , N 1. – P. 71 – 81.[6] Kochukhov O., Bagnulo S.
Evolutionary state of magnetic chemically peculiar stars // Astron. and Astrophys. –2006. – , N 2. – P. 763 – 775.[7]
Kupka F.,Piskunov N., Ryabchikova T.A. et al.
VALD-2: Progress of the Vienna Atomic Line Data Base // Astron.and Astrophys. Suppl. – 1999. – , N 1. – P. 119 – 133.[8]
R.L. Kurucz
Data Bank, –1993. – CD-ROM [9]
Mathys G., Hubrig S., Landstreet J. D., Lanz T., Manfroid J.
The mean magnetic field modulus of AP stars //Astron. and Astrophys. – 1997. – , N 3. – P. 881 – 885.[10]
Panchuk V., Piskunov N., Klochkova V. et al.
The spectral complex of the Nasmyth focus of the 6-m telescopeBTA // Preprint SAO, – 2002[11]
Preston G.W., Wolff S.C.
The Very Slow Spectrum, Magnetic, and Photometric Variations of HD 9996 // Astrophys.J. – 1970. – , N 4. – P. 1071 –1076.[12]
Ryabchikova T., Wade G., Auriere M. et al.
Rotational periods of four roAp stars // Astron. and Astrophys. –2005. – , N 1. – P. L 55 – L58. Ryabchikova T., Kochukhov O., and Bagnulo S.
Isotopic anomaly and stratification of Ca in magnetic Ap stars //Astron. and Astrophys. – 2008. – , N 3. – P. 811 – 823.[14]
Schaller G., Schaerer D., Meynet G., Maeder A.
New grids of stellar models from 0.8 to 120 solar masses at Z =0.020 and Z = 0.001 // Astron. and Astrophys. Suppl. Ser. – 1992. – . – P. 269 – 331.[15] Shavrina A.V., Polosukhina N.S., Zverko J. et al.
Lithium on the surface of cool magnetic CP stars. II. Spectrumanalysis of HD 83368 and HD 60435 with lithium spots // Astron. and Astrophys. – 2001. – , N 2. – P. 571 –578.[16]
Shavrina A.V., Polosukhina N.S., Pavlenko Ya. V. et al.
The spectrum of the roAp star HD 101065 (Przybylski’sstar) in the Li I 6708 A spectral region // Astron. and Astrophys. – 2003. – , N 3. – P. 707 – 713.[17]
Shavrina A.V. , Glagolevskij Yu.V., Drake N.A.et al.
Lithium spots in the Northern roAp star HD 12098 // OdessaAstron. Publ. –2008. – . – P. 111 – 114.[18] Yushchenko V., Gopka V., Yushchenko A., Shavrina A., Hubrig S., Musaev F.
Atmospheric Parameters and Chem-ical Composition of Peculiar Stars HR465, HD91375, and HD25354 // Odessa Astron. Publ. – 2008. – . – P. 153– 157.. – P. 153– 157.