IRON
LINES
AND
SURFACE
GRAVITY
DETERMINATION
S. A r r l b a s and C . M a r t i n e z R o g e r ,
Instituto de Astrofisica de Canarias, 38200
Tenerife,
Spain.
-
La
FOR
τ
CETI
Laguna,
ABSTRACT.
W e present a spectroscopic analysis for the gravity d e t e r m i n a tion
of τ C e t i on t h e basis of a new and high q u a l i t y set of d a t a .
T h e iron
lines a r e s t u d i e d b y u s i n g t h e O x f o r d o s c i l l a t o r s t r e n g t h s .
The results are
compared with those obtained with "solar" oscillator strengths.
Non-LTE
e f f e c t s s e e m t o be i m p o r t a n t in this s t a r .
T h e F e l a t λ 5 2 6 . 9 5 5 n m l i n e is
used t o a p p l y t h e B l a c k w e l l a n d W i l l i s ( 1 9 7 7 ) m e t h o d t o d e t e r m i n e t h e
surface gravity.
T h e d e r i v e d l o g g is 4 . 7 0 ± 0 . 1 .
1.
OBSERVATIONS
AND
ANALYSIS
T h e o b s e r v a t i o n s w e r e m a d e in O c t o b e r 1 9 8 5 a t E S O using C A T + C E S a n d a
R e t i c o n d e t e c t o r w i t h a resolving p o w e r of 8 0 0 0 0 .
Reductions were carried
out using t h e E S O I H A P s y s t e m .
W h e e e possible t h e lines w e r e analysed
using t h e O x f o r d o s c i l l a t o r s t r e n g t h ( B l a c k w e l l e t a l . , 1 9 8 6 a , a n d r e f e r e n c e s
therein).
F o r high e x c i t a t i o n lines w h e r e no l a b o r a t o r y m e a s u r e m e n t s of
adequate precision are available, we have d e t e r m i n e d "solar" oscillator
s t r e n g t h s f r o m t h e s o l a r f l u x s p e c t r u m , in a s i m i l a r w a y t o S m i t h e t a l .
(1986).
The sources for the damping constant values are the works by
Simmons and Blackwell (1982) and Gurtovenko and Kondrashova (1980).
For
the d a m p e d line used to d e t e r m i n e t h e s u r f a c e gravity, ï ^ / N was o b t a i n e d
by a d j u s t i n g its p r o f i l e in t h e s o l a r f l u x s p e c t r u m ( K u r u c z e t a l . , 1 9 8 4 ) t o
the synthetic one generated with the Holweger and Müller (1974) solar model.
In o r d e r t o o b t a i n t h e v a l u e f o r T e f f w e have a p p l i e d t h e I n f r a r e d F l u x
M e t h o d (Blackwell et al., 1986 b and references t h e r e i n ) , obtaining a value
of 5 2 5 0 Κ w i t h a p r o b a b l e e r r o r of ± 50 K.
T h e e q u i v a l e n t w i d t h s w e r e i n t e r p r e t e d in t e r m s of log ( A g f ) values by
using an L T E c o d e .
For the entire analysis w e have used m o d e l atmospheres
g e n e r a t e d by t h e M A R C S suite of p r o g r a m s .
The M A R C S atmospheres are
l i n e - b l a n k e t e d a n d f l u x c o n s t a n t a t m o s p h e r e s o f t h e s a m e t y p e as t h o s e
published by B e l l et a l . ( 1 9 7 6 ) .
T h e a n a l y s i s t o d e t e r m i n e t h e m i c r o t u r b u l e n c e is a n a l o g o u s t o t h e o n e
used by S m i t h et a l . ( 1 9 8 6 and r e f e r e n c e s t h e r e i n ) .
Thus, the microturbulence
( ξ ) and the abundance ( A ) of an e l e m e n t are obtained simultaneously f r o m
t h e f u n c t i o n Α = Α ( ξ ) f o r a s e t o f l i n e s w i t h a w i d e r a n g e in e q u i v a l e n t w i d t h s .
445
G. Cayrel de Strobel and M. Spite (eds.), The Impact of Very High S/N Spectroscopy on Stellar Physics, 445-448.
©1988 by the IAU.
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446
ih Figures 1 a, b and 2 a, b we present the behaviour for the Fel lines in
Ceti and in the Sun, when their respective M A R C S models were used.
Figures b indicate the s c a t t e r in abundance as a function of £ .
W e can
observe that m i n i m a are sited at 1 . 1 2 and 1.19 Km s
for * C e t i and the
Sun respectively.
In Figures 1 a, b the lowest e x c i t a t i o n lines have not been
considered for reasons discussed b e l o w .
Figur« i . Dlaarana abundanca-atcreturbwlanca rar
fCati (a) far a aat af Iran llitM vita different
•avivaient ν 14th*. In U m lewer figure (b) U m
ecetter In U m abundance vereue alcreturbuleace la
•resented . In Ulla
flfura « m can ebeerve new U m
•inlaws la relatively wall defined at t - l . i a M " .
Thaaa figurée neve been obtained by aaa af U m
MAJtC* nodel with »arenetere Teff-»a»0«K. laf f-4.ao
! and (Fa/MJ— 0.40
0.0
2.
Fel
0.S
1.0
I.S
2.0
2.S
Figura a . Oaaa than in figura I far U m aun, ualna
U m aolar WülCS nodal, «ow U m valu* of 1 wblcb
produce* U m nlnlnua In abundanoa 1 · 1.10 tarn".
LINES
When the iron Ihnes with a c c u r a t e oscillator strength (low e x c i t a t i o n ) were
analysed a clear correlation b e t w e e n abundance and e x c i t a t i o n was found.
Lines with e x c i t a t i o n higher than 3 e V were analysed by use of "solar"
oscillator s t r e n g t h s .
In this c a s e , the absolute abundance is not directly
significant but the abundance relative to the Sun is reliable.
W e observed
also an important d i f f e r e n c e in the relative abundance value when comparing
with the lower e x c i t a t i o n lines.
These e f f e c t s are unlikely to be explained
by errors in the a t o m i c p a r a m e t e r s , equivalent widths or e f f e c t i v e temperature
taking into account the e s t i m a t e d errors for t h e s e m a g n i t u d e s .
An e x p l a n a tion for this could be n o n - L T E e f f e c t s in the solar photosphere.
A t h a y and
Lites ( 1 9 7 2 ) pointed out that such e f f e c t s could be important for the Fel and
Fell in the solar photosphere.
However, R u t t e n and Kostik ( 1 9 8 2 ) have found
that the H o l w e g e r and Müller ( 1 9 7 4 ) model c o r r e c t s these e f f e c t s as well as
t h e Bell et a l . ( 1 9 7 6 ) m o d e l .
A s also in this case reasonable changes in the
model p a r a m e t e r s do not explain the e f f e c t , we must think of n o n - L T E in
e x c i t a t i o n in
Ceti.
For all these reasons the value for the iron abundance
remains m o r e uncertain than e x p e c t e d from the quality of the a t o m i c and
observational data used h e r e .
Thus we consider
τ
log ( F e / H ) = 7 . 0 0 * 0 . 0 7
[Fe/H] = -0.53 ±0.07
and
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447
3.
GRAVITY
DETERMINATION
For the s u r f a c e gravity d e t e r m i n a t i o n we applied the Blackwell and Willis
( 1 9 7 7 ) m e t h o d to the Fel λ 5 2 6 . 9 5 5 line, which is strongly broadened by
collisions.
In this c a s e , abundance is obtained from lines with similar e x c i tation, in order t o minimize n o n - L T E e f f e c t s .
When the M A R C S solar models
are used, the best fit is found for log g = 3 . 9 4 .
O t h e r authors have also
found important d i f f e r e n c e s in the log g values when using different solar
models (Ruland e t a l . , 1 9 8 0 ) .
This shows the difficulty in absolute g d e t e r minations.
For x C e t i we found the best a d j u s t e m e n t with the M A R C S model
with log g = 4 . 2 0 (Figure 3 ) . When the log g value obtained for τ Ceti si
normalized to the proper solar o n e , we obtain log g = 4 . 7 0 ± 0 . 1 0 .
The
log g value obtained disagrees with the one obtained by Smith and
Drake ( 1 9 8 7 ) f r o m the calcium line λ 6 1 6 . 2 1 7 nm (log g = 4 . 5 0 ± 0 . 1 ) .
Partially, this can be explained by the d i f f e r e n c e in the e f f e c t i v e t e m p e r a t u r e
assumed.
On the other hand it is also n o t i c e a b l e that Bell et a l . ( 1 9 8 5 )
obtained a d i f f e r e n c e of 0.1 dex in the log g value for Arcturus when this
was obtained from the Cal λ 6 1 6 . 2 1 7 nm and from the Fel ^. 5 2 6 . 9 5 5 lines in
the s a m e sense as obtained h e r e .
Acknowledgements
We are grateful to the D e p a r t m e n t of A s t r o p h y s i c s , University of Oxford
for the use of its f a c i l i t i e s .
W e also thank D r . , G . Smith who gave us s o m e
results in advance to publication.
ri*tr« s . i M t fit for tte rtin g — m m . far
« i i n - H U n . μ ι éi
Ii
tm
M
üSnrSr
References
A t h a y , R . G . , and L i t e s , B . W . : 1 9 7 2 , A s t r o p h y s . J. 1 7 6 , 8 0 9 .
Bell, R . A . , Edwardsson, B . , G u s t a f s s o n , B.: 1 9 8 5 , M N R A S 212, 4 9 7 .
Bell, R . A . , Ericsson, K . , G u s t a f s s o n , B . , Nourld, Α . : 1 9 7 6 , A s t r o n . A s t r o p h y s .
Suppl. Ser. 3 4 , 2 2 9 .
B l a c k w e l l , D . E . , B o o t h , A . J . , Petford, A . D . , L e g g e t t , S . K . , Mountain, C M . ,
S e l b y , M . J . : 1 9 8 6 b , M N R A S 221, 4 2 7 .
B l a c k w e l l , D . E . , B o o t h , A . J . , Haddock, A . D . , Petford, A . D . , L e g g e t t , S . K . :
Downloaded from https:/www.cambridge.org/core. IP address: 88.99.165.207, on 14 Jun 2017 at 17:36:07, subject to the Cambridge Core terms of use,
available at https:/www.cambridge.org/core/terms. https://doi.org/10.1017/S0074180900035452
448
1 9 8 6 a , M N R A S 220,
549.
B l a c k w e l l , D . E . , W i l l i s , R . B . : 1 9 7 7 , M N R A S 180,
Gurtovenko, E.A., Kondrasova, N . N . : 1980, Solar
Holweger,
A s t r o n . A s t r o p h y s . 42, 4 0 7 .
H . , Müller, E.A.: 1974, Solar
P h y s . 39,
169.
Phys.
18,
347.
19.
Kurucz,
R . L . , F u r e l i d , I., B r a u l t , J . , T e s t e r m a n , L.: 1 9 8 4 , Solar F l u x A t l a s
from 269 to 1300 n m .
National Solar Observatory Atlas N o . 1 .
Ruland, F., Holv eger, H., G r i f f i n , R.R., Biehl, D.: 1980, Astron. Astrophys.
92, 7 0 .
R u t t e n , R . J . , K o s t i k , R . I . : 1 9 8 2 , A s t r o n . A s t r o p h y s . 115,
104.
S m i t h , G . , D r a k e , J.J.: 1987, (In Press).
S m i t h , G . , E d w a r d s s o n , B . , F r i s k , V . : 1 9 8 6 , A s t r o n . A s t r o p h y s . 165,
126.
S i m m o n , G . J . , B l a c k w e l l , D . E . : 1 9 8 2 , A s t r o n . A s t r o p h y s 112,
209.
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available at https:/www.cambridge.org/core/terms. https://doi.org/10.1017/S0074180900035452