Abstract. Precise longitudinal magnetic fields of Ap stars have been measured using circularly polarised spectra analyzed using the Least-Squares ...
Precise measurements of the longitudinal magnetic fields of chemically peculiar stars a
a
b
S . L . S . S h o r l i n , J . D . L a n d s t r e e t , G . A . W a d e , J . - F . D o n a t i
c
a
Department of Physics and Astronomy, The University of W estern Ontario, London, Ontario, Canada N6A 3K7 Astronomy Department, University of Toronto at Mississauga, Mississauga, Ontario, Canada L5L 1C6 c Observatoire Midi-Pyrenees, 14 Avenue Edouard Belin, 31400 Toulouse, Prance 6
A b s t r a c t . P re c ise l o n g i t u d i n a l m a g n e t i c fie ld s o f A p s t a r s h a v e b e e n m e a s u r e d u sin g circu larly p o la r is e d s p e c t r a a n a l y z e d u s in g t h e L e a st- S q u a re s D e c o n v o lu tio n (L S D ) p r o c e d u r e . T h e u n c e r t a i n t i e s a s s o c ia te d w i t h t h e s e m e a s u r e m e n t s a r e significantly b e t t e r t h a n t h o s e ty p ic ally achieved u s i n g o t h e r m e t h o d s , a n d effectively redefine t h e s t a n d a r d o f m e a s u r e m e n t s o f t h e l o n g i t u d i n a l m a g n e t i c f ie ld . N o significant l o n g i t u d i n a l fie ld h a s b e e n m e a s u r e d for a n y o f 2 3 A m a n d H g M n s t a r s , i n c l u d in g several s t a r s in w h ic h s t r o n g , c o m p le x m a g n e t i c f ie ld s h a v e p re v io u sly b e e n c la im e d . T h e im p lic a t io n i s t h a t e i t h e r so m e A m a n d H g M n s t a r s e x h i b i t m a g n e t i c f i e l d s w h ich a r e q u a l i t a tively different f r o m t h o s e d e t e c t e d in a n y o t h e r class o f s t a r s , o r t h a t t h e p re v io u s d e t e c t i o n s a r e s p u r i o u s . K e y w o r d s :
m a g n e t i c f i e l d s , A p a n d B p s t a r s
T h e ch em ically p e c u l i a r A a n d В t y p e s t a r s h a v e
3. M easurement of longitudinal magnetic fields
long b e e n classified i n t o t w o g e n e ra l g r o u p s : t h e
L e a s t - S q u a r e s D e c o n v o lu tio n (L S D )
m a g n e t i c a n d n o n m a g n e t i c s u b t y p e s . S t r o n g , o r d e r e d
c o r r e l a t i o n t e c h n iq u e w h ich t a k e s a d v a n t a g e o f the
1 . Introduction
is a cross-
m a g n e t ic f ie ld s h a v e b e e n found a n d m e a s u r e d in
sim ila r s h a p e s o f s p e c t r a l lines a n d t h e i r p o la risa tio n
t h e A p S r C r E u , A p Si, H e-weak S iS r T i a n d H e -stro n g
fe a tu r e s t o c o m p u t e a v e ra g e i n t e n s i t y a n d p o la risa -
s ta r s , w hile f ie ld s h a v e n o t ge n e ra lly b e e n d e t e c t e d
t i o n profiles from a ste lla r s p e c t r u m . T h e u s e of all
i n t h e A p H g M n , A m , H e-w eak P G a a n d
B o o
t h e lin es in a s p e c t r u m g r e a t l y i m p r o v e s t h e signal-
s ta r s . H owever, p r o b a b l e d e t e c t i o n s o f s t r o n g , com -
to -n o ise r a t i o ( S / N ) in t h e s t a r ' s d eco n vo lved Stokes
plex m a g n e t i c f ie ld s in several A m a n d A p H g M n s t a r s
V profile. T h e m e a n lo n g it u d in a l m a g n e t i c field B e
(M a th y s & L a n z , 1990, B i k m a e v et al., 1998, M a t h y s
m a y b e in ferred fro m t h e m e a n S to k e s V profile (e.g.
& H u brig, 1995, H u b r i g et al., 1999) h a ve r e c e n t ly
D o n a t i et al., 1997). H igh S/N p o l a r i s a t i o n profiles
been r e p o r t e d . T h e f i e l d s a r e su g g e ste d t o b e p e r h a p s
p r o v i d e o n e w i t h t h e o p p o r t u n i t y t o m e a s u r e t h e lon-
m o re like th o se o f a c t i v e l a t e - t y p e s t a r s t h a n t h o s e o f
g i t u d i n a l fie ld s o f m a g n e t i c s t a r s w i t h v e ry h igh p re-
t h e classical m a g n e t i c A a n d В s t a r s .
cision, a s well a s to set s t r i n g e n t u p p e r lim it s o n the field s o f n o n m a g n e t i c s t a r s . T h e t e c h n i q u e is most a p p lic a b le for s t a r s w i t h m a n y s p e c t r a l lines, which
2. Observations
in c r e a s e t h e effective S/N of t h e p o l a r i s a t i o n profile. C ircularly p o la r is e d (S to k e s
V ) a n d u n p o l a r i s e d
H ow ever, t h e te c h n iq u e is n o t lim it e d t o sharp-lined
(Stokes J ) s p e c t r a w e r e o b t a i n e d in F e b r u a r y 1997
s t a r s a n d m a y b e u se d over a b r o a d r a n g e o f vsini
a n d 1998, a n d J a n u a r y 1999 u s in g t h e M u S iC o S
( W a d e et al., 1999, Shorlin et al., 1999).
echelle s p e c t r o p o l a r i m e t e r a t t a c h e d to t h e 2 m B e r n a r d L yot Telesco p e a t P ic -d u -M id i o b servato ry. T h i s a p p a r a t u s allow s t h e a c q u isitio n of a stellar sp e c -
4. R esults
t r u m t h r o u g h o u t t h e r a n g e 450 to 660 n m w ith a r e solving pow er o f a b o u t 35000. O b se rv e d s t a r s in c lu d e
4 . 1 . M a g n e t i c A p s t a r s
1 4 m a g n e t i c A p s t a r s , 1 6 A m s t a r s , seven A p H g M n s t a r s a n d o n e
B o o s t a r . U p t o e igh te e n s p e c t r a w ere
o b t a i n e d for e a c h A p s t a r , w hile A m , H g M n a n d B o o s t a r s w e re o b se rv e d u p t o four t im e s eac h .
T h e l o n g i t u d i n a l fie ld m e a s u r e m e n t s o b t a i n e d for the cool A p s t a r 4 9 C a m , p h a s e d a c c o r d i n g to t h e 4.3 d r o t a t i o n a l p e r i o d a n d c o m p a r e d w i t h e a rlie r Balmer-
© Special Astrophysical Observatory of the Russian AS,2000
55
line Zeem an A n a ly z e r o b s e r v a tio n s , a r e sh ow n in Fig.l. T h e i m p r o v e m e n t in precision is e v id e n t. T h e uncertainty in o u r n e w m e a s u r e m e n t s r e p r e s e n t s m o r e than an o r d e r of m a g n i t u d e i m p r o v e m e n t over t h o s e made w ith t h e B a lm e r- lin e Z e e m a n a n a ly z e r. T h e reader is referred to W a d e et al. (1999) for t h e details of field m e a s u r e m e n t s for in d iv id u a l m a g n e t i c Ap stars. The d i s t r i b u t i o n o f s t a n d a r d e r r o r s a s s o c ia te d with lo n gitu d in al field m e a s u r e m e n t s for o u r s a m p le of magnetic Ap s t a r s is sh o w n in F i g u r e 2. T h i s d istribution im plies a m e d i a n B e u n c e r t a i n t y of 48 G a n d a standard d e v i a t i o n of 38 G . O n e - h u n d r e d fo u rteen of the 129 A p s t a r field m e a s u r e m e n t s h a v e u n c e r t a i n ties smaller t h a n 100 G . T h i s s h o u ld b e c o m p a r e d u n c e r t a i n t i e s of b e t w e e n 200 a n d with the ty p ic a l 300 G o b t a in e d u s i n g o t h e r m o d e r n lo n g it u d in a l field measurement t e c h n i q u e s . 4.2. A m a n d A p H g M n s t a r s N o significant l o n g i t u d i n a l m a g n e t i c field h a s b e e n detected for a n y o f t h e 2 3 A m o r H g M n s t a r s s t u d i e d here (see T a b le 1), i n c l u d i n g H D 291 73, H D 195479 and о Peg ( H D 214994) for w h ic h c o m p le x fields h a v e previously b e e n c la im e d b a s e d on field d e t e c t io n s obtained using o t h e r t e c h n i q u e s ( L a n z & M a t h y s , 1993, Mathys & L a n z , 1 990). T h e m e d i a n of t h e 1 sigm a u n c e r tainties a sso c ia te d w i t h B e m e a s u r e m e n t s for t h e A m stars is 17 G a n d t h e s t a n d a r d d e v ia tio n of t h e d istribution is 1 6 G . T h e m e d i a n s t a n d a r d e r r o r for t h e HgMn sta rs is 84 G a n d t h e s t a n d a r d d e v ia tio n is 46 G . The m e d i a n u n c e r t a i n t y is s o m e w h a t h ig h e r for the HgMn s t a r d u e to t h e lower line d e n s ity in t h e i r spectra a n d a fain ter a v e r a g e m a g n i t u d e , as well as perhaps u n c e r t a i n t i e s in t h e a b u n d a n c e t a b l e w h ic h was used to c r e a t e H g M n line m a s k s . Among t h e n o n - m a g n e t i c s t a r s t h e le a st successV ir, t h e single B o o s t a r o b se rve d . ful case was Its spectrum h a s v e ry few lines w h ic h a r e all q u i t e broad. LSD w as o n ly m a r g i n a l l y successful in e x t r a c t ing mean profiles for t h i s s t a r . Not only d o t h e s e r e s u l t s p r o v id e t h e b e s t fie ld measurements for i n d i v i d u a l A m a n d H g M n s t a r s , they furth erm ore p r o v i d e t h e s t r o n g e s t s t a t is t ic a l evidence for t h e a b s e n c e of significant o r d e r e d m a g n e t i c fie ld s in A m s t a r s . T h e s t r o n g e s t d ip o le fie ld s s t a t i s tically c o m p a tib le w i t h t h e A m s t a r d a t a (w ith p o lar strengths o f a r o u n d 50 G ) a r e w eaker t h a n t h e equipartition t h r e s h o l d (of o r d e r 100 G ) , a n d t h u s have m inim al i m p a c t o n t h e s t r u c t u r e a n d d y n a m i c s of their a t m o s p h e r e s .
s t a r s . Such p recisio n allows o n e to d e t e r m i n e t h e d e ta ile d s h a p e s o f fie ld v a r i a t i o n s a n d p recise r o t a t i o n a l
5. Conclusions
p e r i o d s for m a g n e t i c A p s t a r s . T h e d e t a ils o f t h e d et e r m i n a t i o n s a r e discussed b y W a d e e t al. (1 999).
LSD Stokes V profiles m a y be u se d to infer very
T h e n o n - d e t e c t i o n o f t h e l o n g i t u d i n a l fie ld s o f a n y
precise lo n g itu d in a l m a g n e t i c fields for m a g n e t i c Ap
of 23 A m or H g M n s t a r s is c e r t a i n l y significant. Since
56
very c o m p le x fields a r e d e t e c t e d in l a t e - t y p e s t a r s u s-
effectiveness of u n p o l a r i s e d t e c h n i q u e s . W e would
ing t h e L S D t e c h n i q u e ( D o n a t i , 1998), o u r r e s u l t s
th erefo re a d v ise a d e g r e e of c a u t i o n be a d o p t e d when
a p p e a r in c o n s is t e n t w ith t h e c la im s o f s t r o n g , com -
i n t e r p r e t i n g f i e l d d e t e c t i o n s o b t a i n e d u sin g these
p lex m a g n e t i c fie ld s in t h e p h o t o s p h e r e s o f t h e A m
m e t h o d s .
a n d H g M n s t a r s . I n p a r t i c u l a r , o u r r e s u l t s c o r r o b o r a t e n e i t h e r t h e d e t e c t i o n s o f f ie ld s in H D 2 9 1 7 3 a n d H D 195479 b y L a n z & M a t h y s (1993) n o r t h e d e t e c tio n in о P e g by M a t h y s & L a n z (1 9 9 0 ). T h e i r d et e c t io n s w ere b a s e d o n m e a s u r e m e n t s o f t h e s u b t l e Z e e m a n b r o a d e n i n g a n d d e s a t u r a t i o n o f unpolarised s p e c t r a l lines in t h e p r e s e n c e of a m a g n e t i c field. H owever, all o b s e r v a t io n s c a n p r o b a b l y b e re c o n ciled if w e a s s u m e t h a t t h e fie ld s o f t h e A m s t a r s a r e highly c o m p le x ( m o r e c o m p le x t h a n t h e so lar f i e l d , re s u ltin g in very efficient c a n c e l l a t i o n of t h e lo n g itu d in al f i e l d over t h e ste lla r surface) w h ile a t t h e s a m e t i m e very s t r o n g (several k G i n s t r e n g t h , i n o r d e r t o b e c o n siste n t w ith t h e o b s e r v a t io n s o f L a n z к M a t h y s (1993) a n d M a t h y s к L a n z ( 1 9 9 0 ) ) . O n t h e o t h e r h a n d , i t i s i m p o r t a n t t o k eep i n m i n d t h a t t h e d e t e c tio n of w eak m a g n e t i c fields u s in g u n p o l a r i s e d s p e c t r a i s s o m e w h a t a m b i g u o u s , a n d s u s c e p t ib le t o s y s t e m a tic e r r o r s ( p a r t i c u l a r l y d u e t o line b l e n d s ) . T h e re fo re o u r n e w o b s e rv a tio n s , w h ic h a r e b y th e m s e lv e s consiste n t w i t h t h e a b s e n c e o f fie ld s in A m s t a r s , m a y well p r o v id e i n s t e a d a n i n d e p e n d e n t e v a l u a t i o n o f t h e
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