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Physics of Magnetic Stars, 2007, pp. 325­334

Ca isotopic anomaly in the atmospheres of Ap stars
T. Ryab chikova
1 2 3 4

1,2

, O. Ko chukhov3 , S. Bagnulo

4

Institute of Astronomy, Russian Academy of Sciences, Pyatnitskaya 48, 119017 Moscow, Russia Department of Astronomy, University of Vienna, Turkenschanzstrasse 17, A-1180 Wien, Austria ¨ Department of Astronomy and Space Physics, Uppsala University Box 515, SE-751 20 Uppsala, Sweden European Southern Observatory, Casilla 19001, Santiago 19, Chile

Abstract. We present results of the Ca stratification analysis in the atmospheres of 21 magnetic chemically p eculiar (Ap) stars. This analysis was based on the sp ectral observations carried out with the UVES sp ectrograph attached to the 8m VLT telescop e. Ca was found to b e strongly stratified in all stars with different effective temp eratures and magnetic field strengths. This element is overabundant by 1­1.5 dex b elow log 5000 -1 and strongly depleted ab ove log 5000 = -1.5. Based on the overall Ca abundance distributions, we mo delled a profile of the IRtriplet Ca ii 8498 line. It shows a significant contribution of the heavy isotop es 46 Ca and 48 Ca, which represent less than 1% of the solar Ca isotopic mixture. In Ap stars with the relatively small surface magnetic fields ( 4 - 5 kG) the light 40 Ca isotop e is concentrated close to the photosphere, while the heavy isotop es are pushed towards the outer layers. Isotopic separation disapp ears in the atmospheres of stars with magnetic fields ab ove 6­7 kG. The observed overall Ca stratification and isotopic anomalies may b e explained by a combined action of the radiativelydriven diffusion and the light-induced drift. Key words: stars:atmospheres ­ stars: chemically p eculiar ­ stars: magnetic fields ­ stars: abundances ­ pro cess: diffusion ­ pro cess: light-induced drift

1

Intro duction

After the pioneering work by Michaud (1970) particle diffusion in stellar envelop es and atmospheres is considered as the main pro cess resp onsible for the atmospheric abundance anomalies in the p eculiar stars of the Upp er Main Sequence. Detailed diffusion calculations p erformed for a set of chemical elements in the atmospheres of magnetic p eculiar stars predicted an existence of abundance stratification. For a small numb er of elements, including Ca, an effect of the stratified element distribution on the sp ectral line profiles was demonstrated in early studies (Borsenb erger at al. 1981), but the absence of high-resolution, high signal-to-noise sp ectroscopic observations did not allow the direct comparison b etween the observations and diffusion calculations. This step was carried out by Bab el (1992), who calculated the Ca abundance distribution in the atmosphere of the magnetic star 53 Cam and showed that the unusual shap e of Ca ii K line ­ a sharp transition b etween the wide wings and extremely narrow core ­ is a result of a step-like Ca distribution with abundance decrease at log 5000 -1. Following Bab el, the step-function approximation of the abundance distribution was commonly employed in many stratification studies based on the
Based on observations collected at the European Southern Observatory, Paranal, Chile (ESO programme No. 68.D-0254)

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observed profiles of sp ectral lines (Wade et al. 2003; Ryab chikova et al. 2002; Ryab chikova et al. 2005; Ryab chikova et al. 2006). Ca was found to b e stratified the same way as in 53 Cam (enhanced concentration of Ca b elow log 5000 -1 and its depletion ab ove this level) in all stars for which stratification analysis have b een p erformed: CrB (Wade et al. 2003), Equ (Ryab chikova et al. 2002), HD 204411 (Ryab chikova et al. 2005), HD 133792 (Ko chukhov et al. 2006) and HD 144897 (Ryab chikova et al. 2006). Recently another Ca anomaly was detected, first in the sp ectra of HgMn stars by Castelli & Hubrig (2004) and then in Ap stars by Cowley & Hubrig (2005 - CH). These authors found a displacement of the lines of Ca ii IR triplet due to significant contribution of the heavy Ca isotop es. CH merely noted the fact of the presence of heavy isotop es, but they did not p erform any quantitative analysis. This was done by Ryab chikova, Ko chukhov & Bagnulo, and the preliminary results were published in a review pap er by Ryab chikova (2005). It was shown that the contribution of Ca heavy isotop es decreases with the increase of magnetic field strengths, and disapp ears when the field exceeds 3 kG. In present study we give a detailed analysis of the Ca stratification in the atmospheres of magnetic Ap stars of different temp eratures and magnetic field strengths with the application to a mo delling of the IR triplet Ca ii 8498 line.

2

Observations and data reduction.

Twenty-one slowly rotating Ap stars were chosen for the Ca stratification analysis. For all but two stars, HD 24712 and HD 66318, high-resolution, high signal-to-noise-ratio sp ectra were obtained with the UVES instrument at the ESO VLT in the context of program 68.D-0254. The UVES instrument is describ ed by Dekker et al. (2000). The observations were carried out using b oth available dichroic mo des. In b oth the blue arm and the red arm the slit width was set to 0.5 , for a sp ectral resolution of ab out 80 000. The slit was oriented along the parallactic angle, in order to minimize losses due to atmospheric disp ersion. Almost the full wavelength interval from 3030 to 10400 ° was observed A ° ° except for a few gaps, the largest of which was at 5760-5835 A and 8550-8650 A. In addition, there are several small gaps, ab out 1 nm each, due to the lack of overlapping b etween the ´ helle orders in ec the 860U setting. Sp ectra of HD 24712, HD 66318 and HD 61421 (Pro cyon) were obtained with the same setting and were extracted from ESO archive. Due to the gaps in sp ectral coverage, only one line of the Ca ii IR triplet, 8498.023 ° could b e observed in this UVES setting and is accessible A, for mo delling. The Ca IR triplet line studied overlaps with the hydrogen lines from the Paschen series. Due to the difficulties of continuum normalization at the edge of observed sp ectral region, we have employed theoretical sp ectrum synthesis to establish the correct continuum level. In this pro cedure observations around Ca ii 8498.023 ° were adjusted, so that the pseudo-continuum of the Paschen A line wings matches predictions of the theoretical sp ectrum synthesis. The list of the program stars is given in Table 1. In addition, two stars, HD 27411 (A3m) and Pro cyon (HD 61421), were used as standards for the Ca isotopic study.

3

Mo del atmosphere parameters

Fundamental parameters of the program stars are given in Table 1. For most stars effective temp eratures Teff and surface gravities log g were taken from the literature (last column of Table 1). For HD 965, HD 47103, HD 118022, and HD 134214 atmospheric parameters were derived using Str¨ omgren photometric indices (Hauck & Mermillio d 1998) with the calibrations by Mo on & Dworetsky (1985) and by Napiwotzki et al. (1993) implemented in the TEMPLOGG co de (Rogers 1995). For HD 75445, HD 176232, and HD 203932 effective temp eratures were slightly corrected by fitting H


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Table 1: Fundamental parameters of target stars. HD numb er 217522 122970 24712 134214 965 203932 137949 176232 75445 166473 29578 128898 116114 137909 47103 188041 66318 133792 118022 170973 144897 27411 61421 Teff (K) 6750 6930 7250 7315 7500 7550 7550 7650 7650 7700 7800 7900 8000 8000 8180 8800 9200 9400 9500 10750 11250 7650 6510 log g ve sin i Bs -1 ) (km s (kG) Program stars 2.5 1.5 5.5 2.5 5.6 2.3 2.0 3.1 3.0 4.4 5.3 1 1.0 5.0 2.0 1.5 3.0 3.0 0.0 8.6 2.5 5.6 12.5 1.5 2.5 6.2 2.5 5.4 0.0 16.3 0.0 3.6 0.0 15.5 0.0 1.1 10.0 3.0 8.0 0.0 3.0 8.8 Reference stars 18.5 0.0 3.5 0.0 Reference

4.30 4.10 4.30 4.45 4.00 4.34 4.30 4.00 4.00 4.20 4.20 4.20 4.10 4.30 3.50 4.00 4.25 3.70 4.00 3.50 3.70 4.00 3.96

Gelbmann (1998) Ryab chikova et al. (2000 ) Ryab chikova et al. (1997 ) this pap er this pap er Gelbmann et al (1997) Ryab chikova et al. (2004b) Ryab chikova et al. (2000 ) Ryab chikova et al. (2004b) Gelbmann et al. (2000) Ryab chikova et al. (2004b) Kupka et al. (1996) Ryab chikova et al. (2004b) Ryab chikova et al. (2004b) this pap er Ryab chikova et al. (2004a ) Bagnulo et al. (2003) Ko chukhov et al. (2006) this pap er Kato (2003) Ryab chikova et al. (2006 ) this pap er Allende Prieto et al. (2002)

profile. The mean surface magnetic fields B s were derived from the resolved and partially resolved Zeeman patterns. In all stars rotational velo cities were estimated by fitting line profiles of the magnetically insensitive Fe i 5434.5 and 5576.1 ° lines. Mo del atmospheres were calculated with the A ATLAS9 co de (Kurucz 1993).

4

Ca stratification analysis

Before p erforming careful study of the IR Ca ii 8498 line profile, we have to investigate Ca abundance distribution in Ap atmospheres. In all program stars Ca stratification was derived using a set of sp ectral lines in the optical region, for which no indication of the significant isotopic shifts exists. Atomic parameters of these lines, as well as the Ca ii 8498 line, are given in Table 2. Stratification analysis requires high accuracy not only for the oscillator strengths but also for the damping parameters, b ecause Ca has a tendency to b e concentrated close to the photospheric layers where the electron density is high. In particular, it is imp ortant for Ca ii lines. For Ca ii 3158, 3933, 8248, 8254, 8498 lines the Stark damping constants were taken from the pap er by Dimitrijevi´ & c Sahal-Br´ hot (1993), where semi-classical calculations as well as a compilation of the exp erimental ec data were presented. For the rest of the Ca lines the Stark damping constants calculated by Kurucz


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(1993) were used. The oscillator strengths were taken mostly from the lab oratory exp eriments, and they are verified by the recent NLTE analysis of calcium in late-typ e stars (Mashonkina et al. 2007). Because of the large range in effective temp eratures and magnetic field strengths, we could not use the same set of lines for all stars. Table 2: A list of sp ectral lines used for the stratification calculations. The columns give the ion identification, central wavelength, the excitation p otential (in eV) of the lower level, oscillator strength (log g f ), the Stark damping constant, and the reference for the oscillator strength. Ion Ca i Ca i Ca i Ca i Ca i Ca i Ca i Ca i Ca i Ca i Ca i Ca i Ca i Ca i Ca i Ca i Ca i Ca i Ca i Ca i Ca i Wavelength 3158.869 3933.655 4226.728 5021.138 5339.188 5857.451 5867.562 6122.217 6162.173 6163.755 6166.439 6169.042 6169.563 6449.808 6455.598 6456.875 6462.567 6471.662 8248.796 8254.721 8498.023 Ei (eV) 3.123 0.000 0.000 7.515 8.438 2.933 2.933 1.896 1.899 2.521 2.521 2.253 2.256 2.521 2.523 8.438 2.523 2.526 7.515 7.515 1.692 log g f 0.241 0.105 0.244 -1.207 -0.079 0.240 -1.57 -0.316 -0.090 -1.286 -1.142 -0.797 -0.478 -0.502 -1.340 0.410 0.262 -0.686 0.556 -0.398 -1.416 log St -4.90 -5.73 -6.03 -4.61 -3.70 -5.42 -4.70 -5.32 -5.32 -5.00 -5.00 -5.00 -4.99 -6.07 -6.07 -3.70 -6.07 -6.07 -4.60 -4.60 -5.70 Ref. Teo dosiou 1989 Teo dosiou 1989 Smith & Gallagher 1966 Seaton et al. 1994 Seaton et al. 1994 Smith 1988 Smith 1988 Smith & O'Neil 1975 Smith & O'Neil 1975 Smith & Raggett 1981 Smith & Raggett 1981 Smith & Raggett 1981 Smith & Raggett 1981 Smith & Raggett 1981 Smith 1988 Seaton et al. 1994 Smith & Raggett 1981 Smith & Raggett 1981 Seaton et al. 1994 Seaton et al. 1994 Teo dosiou 1989

i i i i

i

i i i

The Ca stratification analysis was p erformed using the step-function approximation of the abundance distribution (for details see Ryab chikova et al. 2005). In a few cases the step-function approximation can not provide an adequate description of the full set of sp ectral lines. The obvious reasons are the use of normal non-magnetic star atmosphere with homogeneous element distribution for a star with known abundance stratification, and a deviation of the abundance distribution from the simple step-function. In co oler stars the range of formation depth of the optical lines is different from the IR-triplet lines of interest, therefore Ca abundance in the upp er atmospheric layers derived from the optical lines may b e not accurate enough for the description of cores of IR lines. Also, continuum normalization in the IR lines region is indirectly based on the adopted effective temp eratures, which may intro duce significant uncertainty and sometimes lead to a p o or fit in the line wings. We start the analysis with the b est homogeneous Ca abundance derived from a chosen set of sp ectral lines, and then vary parameters of the step-function until the adequate fit to the observed line profiles is achieved. Magnetic sp ectral synthesis co de SYNTHMAG(Piskunov 1999; Ko chukhov 2006) was used in our calculations. Fig. 1 displays the results of the stratification analysis for HD 176232 (10 Aql), where synthetic profiles calculated with homogeneous Ca distribu-


CA ISOTOPIC ANOMALY IN THE ATMOSPHERES OF AP STARS

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tion log(C a/Ntot ) = -5.14 are shown by the dashed line while those calculated with the stratified Ca distribution are shown by the full line. The derived Ca distribution is given in Fig. 2 (right panel). The stratified Ca abundance yields two times smaller standard deviation compared to the homogeneous Ca distribution.

Figure 1: A comparison b etween the observed line profiles (dots) and calculations with the stratified Ca abundance distribution (full line) and with the homogeneous Ca abundance (dashed line) in HD 176232. The same pro cedure was applied to all stars included in our sample. Ca distributions in the atmospheres of several stars are shown in Fig. 3. They are all characterized by an abundance jump in the region -1.3 log 5000 -0.5, an 1­1.5 dex overabundance deep in the atmosphere and a strong Ca depletion ab ove log 5000 = -1.5. It is difficult to say if there is any dep endence on the effective temp erature and/or on the magnetic field strength.

5

Ca isotopic anomaly

Ca has six stable isotop es, 40, 42, 43, 44, 46, 48, and in the solar-system matter Ca mixture consists mainly of 40 Ca (96.9 % -- see Anders & Grevesse 1989). Table 3 gives wavelengths of all Ca isotop es following the isotopic shifts measured by N¨ ortersh¨ auser et al. (1998) as well as the isotopic fractional oscillator strengths corresp onding to the solar-system matter mixture. With the solar-matter isotopic mixture we calculated Ca ii 8498 line profile in the sp ectra of our reference stars Pro cyon and HD 27411 and compared them with the observations. Fig. 4 shows the results of this comparison. Although in the Pro cyon sp ectrum our LTE calculations cannot provide a very go o d fit, however, no wavelength shift was detected in b oth stars. At the same time, the observed profile of this line in the sp ectrum of our program star HD 217522 presented in Fig.4 has a complex structure and is clearly redshifted with the strongest comp onent b eing at the p osition of the heaviest Ca isotop e. The core of the IR Ca ii 8498 line is formed higher than any of the optical lines, except Ca ii 3933. For most stars the Ca ii 3933 line were not accounted in the stratification calculations,


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HD 176232 (10 Aql)
1

-4

HD 176232 (10 Aql)
-5 -6 -7 -8

Residual intensity

0.6

log(Ca/Ntot)

0.8

0.4

40

Ca

48

46-48

Ca

-9 -10

40

Ca distribution Ca distribution

0.2 8494

8496

8498 8500 Wavelength, A

8502

8504

-6

-4

log5000

-2

0

2

Figure 2: A comparison b etween the observed (filled circles) and synthetic line profiles of Ca ii 8498 (left panel), calculated with Ca distribution shown in the right panel. Synthetic sp ectrum calculations with the solar-matter Ca isotopic mixture are shown by full line, and those with Ca isotopic separation as indicated in the right panel are shown by dashed line. Ca distribution derived from the optical lines is shown by the solid blue line in the right panel, while isotopic separation are shown by red line (dashed + solid). For illustration purp ose two distributions are slightly shifted relative to each other.
-3 -4 -5
HD HD HD HD HD HD 217522, 134214, 75445, 137909, 166473, 66318, Teff=6750 Teff=7315 Teff=7650 Teff=8000 Teff=7700 Teff=9200 K, K, K, K, K, K, Bs<1.5 kG Bs=3.1 kG Bs=3.0 kG Bs=5.4 kG Bs=8.6 kG Bs=15.5 kG

log(Ca/Ntot)

-6 -7 -8 -9 -10

-6

-4

-2 log5000

0

2

Figure 3: Ca abundance distributions in the atmospheres of a few stars with different effective temp eratures and magnetic field strengths. Dashed line shows optical regions where 40 Ca is dominated, while the solid line indicates the regions of predominant 46 Ca and 48 Ca isotop es concentration. Solar Ca abundance is marked by dotted line.

therefore Ca abundance in the upp er atmosphere may b e rather uncertain, b ecause all other optical lines are not sensitive to abundance variations ab ove log 5000 = -2.0 to -2.5. The abundance in the upp er atmosphere is defined by the slop e of the abundance gradient in the jump region. If the Ap atmosphere is close to the normal ATLAS9 one (Kurucz 1993) adopted in our analysis, then Ca ii 8498 line should b e fitted with the Ca abundance distribution derived from optical lines. Our calculations show that while it is correct for the observed total intensity, in part of program stars we cannot fit the line cores, which are often redshifted. Fig. 2 (left panel, dashed line) shows a fit of synthetic sp ectrum calculated with the solar-matter Ca isotopic mixture and Ca abundance


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Table 3: Atomic data for the isotopic comp onents of Ca ii 8498. The fractional isotop e abundances corresp ond to the comp osition solar-system matter. , ° A 8498.023 8498.079 8498.106 8498.131 8498.179 8498.223 isotop e 40 42 43 44 46 48 log g f -1.43 -3.60 -4.29 -3.10 -5.81 -4.14

1.2

40

1

48

Ca II 8498.023 Ca II 8498.223

0.8

Procyon, Teff=6510 K

Residual intensity

0.6

0.4
HD 217522, Teff=6750 K

0.2
HD 24711, Teff=7650 K

0

-0.2 8495 8496 8497 8498 8499 8500 Wavelength, A 8501 8502

Figure 4: A comparison b etween the observed line profiles (dots) and the calculations with the solar-matter Ca isotopic mixture (full line) in the sp ectra of Pro cyon and Am star HD 27411. The observed sp ectrum of Ap star HD 217522 is in the middle. distribution (right panel) to the observed sp ectrum of HD 176232. One immediately notices that while the line wings are fitted rather satisfactory, the line core cannot b e fitted with the solar-matter Ca isotopic mixture. When we separate 40 Ca and 46 Ca, 48 Ca isotop es in the atmosphere as indicated in Fig. 2 (right panel), then we get a satisfactory agreement b etween the observed and calculated sp ectra (full line in the left panel of Fig. 2). Of course, it is a crude approximation, however it gives us direct evidence of the Ca isotopic separation in the atmospheres of Ap stars. This pro cedure was applied to all stars of our program. Fig. 5 gives an example of our fitting

Fe II 8499.606

Fe I 8496.984


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Ca II 8498.023

Ca II 8498.223

Fe I 8496.984

1.4

1.2

40

1

48

HD 217522, Teff=6750 K, Bs<1.5 kG

Residual intensity

0.8

HD 75445, Teff=7650 K, Bs=3.0 kG

0.6

HD 134214, Teff=7315 K, Bs=3.1 kG

0.4

HD 137909, Teff=8000 K, Bs=5.4 kG

0.2

HD 166473, Teff=7700 K, Bs=8.6 kG

0
HD 66318, Teff=9200 K, Bs=15.5 kG

-0.2 8496

8497

8498 8499 Wavelength, A

Fe II 8499.606

8500

8501

Figure 5: The same as in Fig. 2 (left panel) but for a set of program stars.

pro cedure for a subset of stars with different effective temp eratures and different magnetic field strengths, and the corresp onding Ca stratifications with the isotopic separation are shown in Fig. 3. In the stars with small to mo derate magnetic fields we clearly see a significant contribution of the heavy isotop es 46 Ca and 48 Ca, and this contribution decreases with the increase of the magnetic field strength. Even in HD 137909 ( CrB) with the mean magnetic mo dulus B s =5.4 kG one still needs a small contribution of 48 Ca, but under the assumption of very sp ecific Ca distribution shown in Fig. 3. We have to intro duce a rapid increase of Ca abundance in a thin upp er atmospheric layer ab ove log 5000 = -5. In principle, it do es not contradict the theoretical Ca diffusion calculations. Both Borsenb erger et al. (1981 -- Fig. 6) and Bab el (1992) obtained Ca abundance increase in the upp er layers after the abundance jump. However, NLTE treatment of the Ca lines formation is needed to investigate the upp er atmospheric layers. The same results were obtained for other stars with similar magnetic field strengths: HD 965, HD 137949, HD 29578.


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6

Discussion

If the overall distribution of Ca abundance in the atmospheres of Ap stars follows the predictions of the radiatively driven diffusion, our results on the isotopic separation favour the light-induced drift (LID) as the main pro cess resp onsible for this separation. Indeed, according to Atutov & Shalagin (1988) LID arises when the radiation field is anisotropic inside the line profile. Such an anisotropy takes place for a line of the trace isotop es, 46 Ca, 48 Ca for instance, in the solar-matter mixture, which is sitting in the wing of a strong line of the main isotop e 40 Ca, and the main isotop e should induce the drift velo city for other isotop es. If we have a trace isotop e's line in the red wing of the main isotop e's line, then the drift velo city is directed from towards the upp er atmosphere and the trace isotop es are pushed upwards. This is the case for the Ca isotopic structure. Zeeman splitting changes the line shap e and decreases the flux anisotropy for a trace isotop e's line. When magnetic field b ecomes strong enough, 4 - 5 kG, then the flux anisotropy disapp ears and the isotopic separation is ceasing. Therefore, the observed Ca isotopic anomaly in magnetic stars may b e qualitatively explained by the combined action of the radiatively-driven diffusion and light-induced drift.
Acknowledgements. This work was supported by the RAS Presidium Program "Origin and Evolution of Stars and Galaxies", by Austrian Science Fund (FWF-P17580N2) and by grant 11630102 from the Royal Swedish Academy of Sciences. TR acknowledges partial support from RFBR grant 06-02-16110a and the Leading Scientific School grant 162.2003.02.

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