Mercury and platinum abundances in mercury-manganese stars

a r X i v :a s t r o -p h /9805064v 1 6 M a y 1998

Mercury and platinum abundances in

mercury-manganese stars

C.M.Jomaron 1,,M.M.Dworetsky 1and

D.A.Bohlender 2

1

University College London,Gower Street,London,England WC1E 6BT 2

National Research Council of Canada,Herzberg Institute of Astrophysics,

5071W.Saanich Road,Victoria,BC,Canada V8X 4M6

Abstract.

We report new results for the elemental and isotopic abundances of the normally rare elements mercury and platinum in HgMn stars.Typical over-abundances can be 4dex or more.The isotopic patterns do not follow the fractionation model of White et al (1976).

Key words:stars:abundances —stars:chemically peculiar

1.HgMn stars

The HgMn stars correspond in T e?to the main-sequence between A0and B6(11000–16000K).Their abundance anomalies include both overabundances (e.g.,Mn,P,Ga,Sr,Hg,Pt)and underabundances (e.g.,He,Al,Ni,Co).They have no detectable ordered magnetic ?elds like the classical Ap stars (SrEuCr or Si types).While normal A0stars have a typical rms v sin i ～164km s ?1(Dworet-sky,1974),HgMn stars have typical v sin i of 10–20km s ?1;some are as low as 2–3km s ?1.Smith (1996)is recommended for a more detailed overview.

2.Data

We obtained high-resolution,high S/N optical spectra of Hg ii and Pt ii lines for several stars with the Gecko spectrograph (R ～105)on the Canada-France-Hawaii Telescope,and complete spectra for many more stars with the Hamilton ′Echelle Spectrograph (λλ3900–9000,R ～5×104)at Lick Observatory.

The models for the lines are based on isotopic and hyper?ne structure mea-surements from the literature (Engleman 1989;Dworetsky et al.1998)and on gf-values from Dworetsky et al (1984)and Dworetsky (1980).We used the spectrum-synthesis codes UCLSYN (Smith 1992)and BINSYN (Smalley 1996).Best ?ts for two stars,HR 7775and χLupi,are shown in Fig.1,and the results for those and several other stars are summarised in Tables 1–3,where abun-dances are given on the scale with log N (H)=12.

2

3.Hg i vs Hg ii

The strongest optical lines of Hg i in these stars derive from the3S to3P tran-sitions atλ4046,λ4358andλ5461.Although the?rst of these is a contaminant of the Pt iiλ4046feature,the other two are clearly present in several stars.

Table1shows the abundance of Hg i derived for?ve stars using the lines λ4358andλ5461and compares them with the abundance derived for Hg ii using the summed isotopes of theλ3984line(CFHT data).The agreement is excellent.

Table1.Equivalent widths and abundances of Hg in?ve HgMn stars Star

log A W(m?A)log A W(m?A)

6.2813 6.3071

28Her 5.80 4.5

5.904

6.2066

ιCrB 6.287.5

6.01

7.5 6.3346

Mercury and platinum abundances in mercury-manganese stars3

Table2.Percentage Hg composition by isotope

isotope

204

38.637.210.1 1.1<129.86

201

3.70.3?--23.10

199

1.60.2?--9.97

196

Table3.Percentage Pt composition by isotope

isotope

198

43325.3

195

1-32.9

192

would push99%of Hg into204Hg,normally a rare isotope.The parameter q is derived from the equation

logα

q=

(2)

[N A/N202]⊙

A similar equation based on196Pt may be de?ned for Pt isotopes.

5.Results

The fractionation hypothesis does not adequately represent the best observed structures for Hg and Pt.Fig.3shows the predicted pro?les(in HR7775)for Hg iiλ3984and Pt iiλ4061for a constant q as deduced from the abundance ratios of the two heaviest isotopes.There is clearly not enough of the light isotopes present.In the cases ofχLupi and28Her we?nd that the enhancements of the heavy isotope of Hg are so extreme that virtually all the Hg is in one isotope.

4

3983.80

3983.903984.00

3984.10

3984.20

Wavelength / ?

0.4

0.60.8

1.0

1.2

1.4

N o r m a l i s e d F l u x

204

202

200

198

196

199a

c b 201a

b Cr

Fe

HR 7775

χLupi

Figure https://www.wendangku.net/doc/0a8919083.html,parison of Hg ii pro?les in HR 7775and χLupi:λ3984

0.40 0.50 Wavelength ? 4046 / ? 0.2

0.4

0.60.81.0

1.2 N o r m a l i s e d F l u x

(a)

HR 7775χLupi

192

194

b 195

a 196

198

Hg I

0.60 0.70 0.80

Wavelength ? 4061 / ?

(b)

HR 7775

χLupi

192

194

195a

195b 196

198Mn Fe

Figure https://www.wendangku.net/doc/0a8919083.html,parison of Pt ii pro?les in HR 7775and χLupi:(a)λ4046;(b)λ4061

6.Discussion

Our results contradict what was previously a largely accepted concept,namely,that the strong Hg and Pt isotopic abundance anomalies could be parameterised in a fractionation model.This is clearly not the case in at least several stars.

One (unlikely)ad hoc explanation is that the original abundances in these stars were di?erent from cosmic.The fact that the Pt anomalies ‘shadow’the Hg may support this view.However,while surely involving radiative di?usion and gravitational settling,any complete theory needs to take into account other factors such as the detailed ?ux pro?le of the stellar atmosphere.These exotic

Mercury and platinum abundances in mercury-manganese stars

5

abundances are now established results in need of a theoretical explanation.

0.800.900.10 1.10 Wavelength ? 3983 / ? 0.20.4

0.60.81.0

N o r m a l i s e d F l u x

(a)

204

202

200

198

+196

199a c b 201

a

b Cr

Fe

0.60 0.70 0.80

Wavelength ? 4061 / ?

(b)

192

194

195a

195b 196198Mn Fe

Figure 3.Observed data versus synthetic fractionation pro?le for HR 7775:(a)Hg ii λ3984;(b)Pt ii λ4061

Acknowledgements.The authors gratefully acknowledge discussions with Rolf En-gleman,Jr,and contributions by students N.R.Crawley and D.J.Harman at UCL.Re-search on chemically peculiar stars at UCL is supported by PPARC grant GR/K58500and travel to telescopes by PPARC grant GR/K60107.

References

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