6. 7. 8. 9. 10. II. 12. 13. 14. 15. 16. 17. 18. 19. 20.

L. L u u d , I z v . Akad. Nauk E s t . SSR, S e t . F i z . M a t . ~ 19, 177 ( 1 9 7 0 ) ~ A. Uns~id, PhysJk der Stern~tmosph~ron, Berlin (1955). J. M. Bridges and H. L. Kornblith, Astrophys. J., 592, 793 (i974). M. M. Phillips, Astrophys. J. Suppl. Ser., 399, 377 (1979). W. L. Wiese and J. R. Fuhr, J. Phys. Chem. Ref. Data, 4, No. 2 (1975) o J. R. Roberts, T. Andersen, and G. Srensen, Astrophys. J., 181, 587 (1973) o G. A. Kasobov and V. V. Veselov, Spectroscopic Tables for Low-Temperature Plasmas [in Russian], Atomizdat, Moscow (1973). L. Khyanni and M. Kipper, Publ. Tartuskoi Obs., 4_33, 51 (i975). R. Viotti, Astrophys. J., 204, 293 (1976). E. R. Mustel', Stellar Atmospheres [in Russian], Fizmatgiz, Moscow (1960). M. Hack, Struve Stellar Spectroscopy -- Normal Stars, Observatorio Astronomico Trieste (1969), p. 177. R. Faraggiana and M. Hack, Astron. Astrophys., 155, 55 (1971). K. Hunger, Z. Astrophys., 399, 273 (1956). O. Engvold, Phys. Scr., 16, 48 (1977). R. M~d~le, R. G r i f f i n , R. G r i f f i n , a n d H. H o l w e g e r , A s t r o n . A s t r o p h y s . S u p p l . S e t . , 19, 303 ( 1 9 7 5 ) .

PARAMETERS PECULIAR

OF THE ATMOSPHERES OF THE CHEMICALLY

STARS ~ And AND ~2 CVn

V. V. Leushin,

L. I. Snezhko,

and V. V. Sokolov

The hydrogen lines P13--P17 and the line H y have been investigated in the spectra of the Ap stars ~ And and ~2 CVn. From the hydrogen lines, the value log g = 3.5 was found for both stars. It is shown that for both stars the values of Tel f found by the infrared-flux method are close to the real values.

1.

Introduction

A p p l i c a t i o n t o Ap s t a r s o f t h e s t a n d a r d m e t h o d s o f d e t e r m i n i n g t h e p a r a m e t e r s o f the atmospheres encounters certain difficulties. The e x c e s s a b u n d a n c e o f r e a d i l y ionized elements and the additional absorption in the ultraviolet that they produce lead to a change in the structure of the atmosphere and thus to a change in the energy distribution i n t h e s p e c t r u m [1, 2 ] . The a d d i t i o n a l absorption in the visible part of the spectrum distorts the characteristics of the continuum; moreover, all these effects a r e v a r i a b l e i n t i m e on a c c o u n t o f t h e n o n u n i f o r m i t y o f t h e p h y s i c a l c o n d i t i o n s on t h e s u r f a c e o f r o t a t i n g Ap s t a r s . A l t h o u g h on t h e a v e r a g e t h e c o r r e s p o n d i n g characteristics o f t h e s p e c t r a o f Ap s t a r s d i f f e r l i t t l e from the characteristics for normal stars, for individual stars the different methods give very different results. As an e x a m p l e , o n e c a n g i v e t h e l i s t o f d e t e r m i n a t i o n s o f T _ ~ d u r i n g t h e l a s t 15 years for the well-studied s i l i c o n Ap s t a r ~2 CVn ( T a b l e 5 o f [ 3 ] ) . For this star, t h e v aol u e s o f T~_ ~I f o u n d f r o m t h e v i s i b l e p a r t o f t h e s p e c t r u m l i e i n t h e r a n g e f r o m 10 000 t o 14 000 K. A c c o r d i n g l y , t h e v a l u e s o f l o g g f o u n d f r o m t h e H. p r o f i l e lie i n t h e r a n g e f r o m l o g g = 4 f o r T e f f = 14 000 ~ t o l o g g = 3 f o r T e l f =Y9500 ~ [ 4 ] . It is obvious that such a large spread in the determinations of the basic parameters of the atmospheres makes it difficult to obtain a quantitative estimate of the peculiarities of the spectrum and thus makes it harder to understand the nature Of these peculiarities. The problem of determining the basic parameters of the atmospheres of Ap stars is solved by the infrared-flux method developed by Blackwell and Shallis [5, 6] to determine the effective temperature of stars. In this method, Tel f is found from the Special Astrophysical O b s e r v a t o r y , USSR Academy o f S c i e n c e s ; Rostov State universltyl Translated from Astrofizika, V o l . 2 0 , No. 3, p p . 4 2 9 - 4 3 5 , M a y - J u n e , 1 9 8 4 . Original article s u b m i t t e d May 2 0 , 1 9 8 3 ; accepted for publication F e b r u a r y 3, 1 9 8 4 .

230

0571-7132/84/2003-0230508.50

9 1984 Plenum Publishing

Corporation

01!. . . . .~

~. . . . . . .' . .

~ 0

"~-~ .....

I// 4

8

12

16

20 0

4

8

12

16

20

F i g . 1. T h e o r e t i c a l a n d o b s e r v e d Hy l i n e p r o f i l e s . n u m b e r s g i v e t h e v a l u e s o f l o g g a n d T e f f = 11 200~ a) a And, b ) ~2 CVn.

24

The

bolometric flux and monochromatic fluxes in the infrared region of the spectrum measured on the surface of the Earth. Calculations of model atmospheres show that the fluxes in the infrared region of the spectrum depend weakly on the chemical composition and the surface gravity in the atmospheres of the stars. Thus, the infrared-flux method gives a direct estimate of Tel f that, in contrast to the traditional indirect methods, depends weakly on the redistribution of the energy between the ultraviolet and the visible region of the spectrum that arises when the chemical composition of the atmosphere changes [3]. Taking determinations of Tel f by the infrared-flux method [3], we posed the problem of using the hydrogen lines to determine the parameters of the atmospheres for the Ap stars ~2 CVn (peculiarity type Si--Eu) and ~ And (peculiarity type Hg--Mn). The standard procedure for determining log g using the H_ profile, which is formed in a wide range of optical depths 10 -3 ~ z ~ 1.2, can encounte~ difficulties due to changes in the structure of the atmosphere by peculiarities of the chemical composition. Therefore, we also used observations of the lines of the Paschen series, which are formed in a much narrower interval of optical depths, 0.05 g T ~ 0.3, i.e., i n t h e same l a y e r s o f t h e a t m o s p h e r e i n which the metal spectrum in the visible region is formed. The c o m b i n e d s t u d y o f t h e Balmer and Paschen lines not only raises the reliability of the determination of the basic parameters of the atmosphere but also, because of the large difference between the d e p t h s o f t h e i r f o r m a t i o n , o p e n s up a p o s s i b i l i t y of detecting anomalies in the structure of the atmosphere. 2. Observational

Material

and its Analysis

Spectrograms of ~2 CVn and a And in the photographic region were obtained using the 122-cm telescope of the Crimean Astrophysical Observatory of the USSR Academy of Sciences with dispersion 15 ~/mm [7]. For the given problem, the spectrograms were reanalyzed, particular attention being paid to the determination of the continuum. Profiles of ~ obtained by averaging the results of analysis of four spectrograms are g i v e n i n F i g . 1, i n w h i c h t h e h o r i z o n t a l b a r shows t h e s p r e a d o f t h e i n d i v i d u a l determinations. The Hy p r o f i l e f o r u2 CVn e x h i b i t s a spread appreciably greater than that of And; this is a manifestation of the physical variability of the hydrogen lines of ~2 CVn [ 8 , 1 8 ] . Spectrograms in the near infrared were obtained in June and August 1978 w i t h t h e 6-m t e l e s c o p e a n d r e c i p r o c a l dispersion 28~/mm. S e n s i t i z e d Kodak NI e m u l s i o n was u s e d , a n d t h e s p e c t r a l resolution was - 0 . 8 A. All the photometric a n a l y s i s was d o n e w i t h a d i r e c t - i n t e n s i t y microphotometer of the Special Astrophysical Observatory. The s p e c t r a i n t h e r e g i o n ~A 8 4 0 0 - 8 8 0 0 A o b t a i n e d f r o m t h e d i f f e r e n t s p e c t r o g r a m s h a v e t h e same s p r e a d f o r t h e two s t a r s , so that physical variability of t h e P a s c h e n l i n e s o f ~2 CVn i s n o t r e v e a l e d . Spectra in the region of the lines P13--P17, normalized to the continuum drawn through the tops of the spectral details, a r e shown i n F i g . 2. These data were obtained by averaging the results of analysis of five s p e c t r o g r a m s f o r ~2 CVn a n d f o u r s p e c t r o g r a m s f o r ~ And. The c o m p o n e n t s o f t h e i n f r a r e d Ca I I t r i p l e t a r e v e r y weak a n d do n o t d i s t o r t the profiles of the Paschen lines. In the a 2 CVn spectrum, the details that can be identified with the components of the

231

r

1

I 9, V -~''

1

0.2

t 0,3 P17

Pie

Pts

P,~ -

F i g . 2. T h e o r e t i c a l and o b s e r v e d s p e c t r u m i n t h e region of the lines P14--P17. The numbers g i v e t h e v a l u e s o f l o g g and T e l f = 11 200 ~. The o p e n circles a r e f o r ~ And, t h e b l a c k c i r c l e s for ~2 CVn.

.~5

- 25

-15

-5

F i g . S. P r o f i l e o f t h e l i n e P13 i n t h e s p e c t r u m of: ~ And ( o p e n circles) and theoretical profile of the blend (PI3 + Ca II ~ $662).

i n f r a r e d Ca I I t r i p l e t h a v e e q u i v a l e n t w i d t h s W~ ~ 0 . 0 8 A; f o r s And t h e y h a v e v a l u e s WI ~ 0 . 0 5 A. T h e s e v a l u e s a r e much l e s s t h a n t h e e q u i v a l e n t w i d t h s o f t h e l i n e s o f the Paschen series. 3. R e s u l t s The i n f r a r e d - f l u x effective t e m p e r a t u r e s a r e T e l f = 10 830 ~ f o r e 2 CYn a n d T e l f = 11 300 ~ f o r a And [ 3 ] . I n [ 9 ] , m o d e l a t m o s p h e r e s w i t h a l l o w a n c e f o r t h e a n o m a l o u s chemical composition and the additional absorption in the ultraviolet f o r ~2 CYn g a v e t h e v a l u e T e l f = 11 5 0 0 ~ t h e p h a s e ~ = 0 a n d T e l f = 12 000~ i n t h e p h a s e ~ = 0 . 5 . Giving greater weight to the estimate of Tel f obtained by the infrared-flux m e t h o d , we t o o k t h e v a l u e T e l f = 11 200 ~ t o r e p r e s e n t t h e h y d r o g e n s p e c t r u m o f ~2 CVn a v e r a g e d o v e r t h e phases. The ~2 CVn s p e c t r u m e x h i b i t s e n h a n c e d S i I I l i n e s ; the change in their intensity with the phase of the period is small [10]. A quantitative estimate gives an e x c e s s a b u n d a n c e of s i l i c o n i n t h e a t m o s p h e r e o f ~2 CVn o f 1 . 5 - 2 o r d e r s o f m a g n i tude, this estimate depending weakly on the value taken for Tel f [11]. Since the silicon abundance strongly influences the structure o f t h e a t m o s p h e r e [2, 1 2 ] , t o interpret t h e e2 CVn s p e c t r u m we u s e d m o d e l a t m o s p h e r e s i n w h i c h t h e s i l i c o n abundance was 100 t i m e s t h e s o l a r . The m o d e l a t m o s p h e r e s f o r ~2 CVn w e r e c a l c u l a t e d using the p r o g r a m SAM 1 [13] w i t h t h e p a r a m e t e r s T e l f = 1 1 2 0 0 ~ l o g g = 4 a n d l o g g = 3o To interpret t h e a And s p e c t r u m , we u s e d M i h a l a s ' S b l a n k e t e d m o d e l s w i t h p a r a m e t e r s e e l f = 0.45, log g = 4, and log g = 3 [14]. The d e t a i l s of the program used to calculate the line spectrum are given in [15]. The b r o a d e n i n g o f H. was c a l c u l a t e d using the data of [16]; the broadening of the Paschen lines, using Griem's theory [17]. Since in the region of the observed lines P14--P18 there is a strong overlapping of the wings of the Paschen lines, f o r c o m p a r i s o n w i t h t h e o b s e r v a t i o n s we c a l c u l a t e d a synthetic spectrum taking into account the contribution to the absorption at the given wavelength of all these lines. To e s t i m a t e t h e c o n t r i b u t i o n of the components of the infrared Ca I I t r i p l e t , we c a l c u l a t e d t h e b l e n d (P13 + Ca I I t 8662); the parameters of the b r o a d e n i n g o f t h e l i n e Ca I I t 8662 w e r e t a k e n i n a c c o r d a n c e w i t h [ 1 7 ] . Figure 1 compares the observed and theoretical profiles of H~ I t can be s e e n t h a t i n t h e c a s e o f a And t h e o b s e r v e d Hy p r o f i l e can be s a t i s f a c t o r i l y represented by t h e t h e o r e t i c a l profiles, and the value of log g found using the wing of the line is log g ~ 3.3. I n t h e c a s e o f ~2 CYn, t h e a g r e e m e n t i s l e s s s a t i s f a c t o r y , since the central parts of the observed profile are excessively deep. It is not possible to i m p r o v e t h e a g r e e m e n t i n t h e c e n t r a l p a r t s o f t h e p r o f i l e by c h a n g i n g t h e v a l u e o f T e l f o r by g o i n g o v e r t o n o r m a l c h a m i c a l c o m p o s i t i o n , and t h e r e f o r e the excess depth o f t h e l i n e My c a n n o t b e e x p l a i n e d i n t h e f r a m e w o r k o f a s t a n d a r d h o m o g e n e o u s m [ d e l atmosphere. ~ h e v a l u e o f l o g g f o u n d f r o m t h e w i n g s o f Hy f o r ~2 CVn i s l o g g ~ 3 . 5 ~ and it is certainly l e s s t h a n 4. Figure 2 compares the observed hydrogen spectrum with the theoretical spectrum in the region of the lines P14--P17. The b r o k e n c u r v e shows t h e p o s i t i o n o f t h e t r u e c o n t i n u u m f o r t h e m o d e l w i t h l o g g = 3 ; it illustrates

232

the strong influence of the overlapping of the wings of the lines on the form of the line spectrum. Indeed, the overlapping of the wings leads to a weakening of the lines with increasing log g, whereas the unblended hydrogen lines become stronger at the same time. The value of log g found from the lines P14--PI7 for ~ And and ~2 CVn is log g ~ 3.5, the theoretical and observed profiles agreeing well. Figure 3 shows the observed PI3 profile for u And and the theoretical profile of the blend (PI3 + Ca II A 8662) for the model with log g = 3. The theoretical profile was calculated without allowance for the overlapping with the neighboring hydrogen lines and is broadened by folding with the instrumental profile to reproduce the component Ca II ~ 8662 in the scale of the figure. One can see that the contribution of the component of the Ca ~I triplet is small. Its equivalent width is W~ = 0.05 A, so that the profile of the'blend is determined solely by the hydrogen line. For effective t e m p e r a t u r e Tel f = I0 000 ~ the equivalent width of the Ca II ~ 8662 has the value WA = 0.15 A, which it three times greater than the detection limit W~ ~ 0.05 ~ of our spectrograms. On the other hand, for Tel f > 11 500 ~ the equivalent width W~ of the component Ca II ~ 8662 becomes less than the detection limit. Thus, the presence of the components of the Ca II triplet at the detection limit with W~ ~ 0.05-0.08 ~ shows that the values of Tel f determined by the infrared-flux method for u And and ~2 CVn are close to the real values. Overall, our investigation of the hydrogen spectrum shows that the atmospheres of And and ~2 CVn have surface gravities of log g ~ 3.5. In the case of u And, both the spectrum in the region of the lines PI4--PI7 as well as the H 7 profile can be satisfactorily represented by the theoretical results for a model atmosphere with normal chemlcal composition and Tel f = 11 200 ~ and log g ~ 3.5. In the case of ~2 CVn, the spectrum in the region of the lines P14--P17" and the wings of the line H~ can also be satisfactorily represented by the theoretical results for a model with a 100-fold silicon excess and the same values of Tel f and log g. However, the central parts of the H 7 profile of ~2 CVn cannot be reconciled with the theoretical results in the framework of a standard homogeneous model atmosphere. Study of the hydrogen spectrum confirms the Tel f values found for u And and ~ CVn by the infrared-flux method [3]. An additional confirmation of this is the presence in the spectra of the components of the Ca II triplet at the detection limit of our spectrograms. The reduced value of the surface gravity obtained here for two stars of different peculiarity types may be of interest for the problem of interpreting the anomalies in the spectra of Ap stars.

LITERATURE CITED 1. 2. 3. 4. 5~ 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18.

K. S t e p i ~ n and H. Muthsam, A s t r o n . A s t r o p h y s . , 9 2 , 171 ( 1 9 8 0 ) . V. V. L e u s h i n , V. V. S o k o l o v , and G. P. T o p i l ' s k a y a , A s t r o f i z i k a , 1 8 , 423 ( 1 9 8 2 ) . M. J . S h a l l i s and D. E. B l a c k w e l l , A s t r o n . A s t r o p h y s . , 7 9 , 48 ( 1 9 7 9 ) . I . M. K o p y l o v , R. N. K u m a i g o r o d s k a y a , L. I . S n e z h k o , V. V. S o k o l o v , and N. M. Chunakova, A s t r o n . Z h . , 5 5 , 1214 ( 1 9 7 8 ) . D. E. B l a c k w e l l and M. J . S h a l l i s , Mon. N o t . R. A s t r o n . S o c . , 180, 177 ( 1 9 7 7 ) . D. E. B l a c k w e l l , M. J . S h a l l i s , and M. J . S e l b y , Mon. N o t . R. A s t r o n . S o c . , 188, 847 (1979). V. V. L e u s h i n , I z v . Krym. A s t r o f i z . O b s . , 4 3 , 113 ( 1 9 7 1 ) . K. I . K o z l o v a , A s t r o f i z . I s s l e d . , 2 , 18 ( 1 9 7 0 ) . H. Muthsam and K. S t e p i e n , A s t r o n . A s t r o p h y s . , 8_66, 240 ( 1 9 8 0 ) . I . M. K o p y l o v and R. N. K u m a i g o r o d s k a y a , A s t r o f i z . I s s l e d . , 5 , 47 ( 1 9 7 3 ) . J . G. Cohen, A s t r o p h y s . J . , 159, 473 ( 1 9 7 0 ) . S. E. Strom and K. M. S t r o m , A s t r o p h y s . J . , 155, 17 ( 1 9 6 9 ) . S. W r i g h t and J . A r g y r o s , Commun. U n i v . London O b s . , No. 76 ( 1 9 7 5 ) . D. M i h a l a s , A s t r o p h y s . J . S u p p l . S e r . , 1 3 , 1 ( 1 9 6 6 ) . L. I . S n e z h k o , S o o b s h e h . SAO, No. 3, 3 ( 1 9 7 1 ) . C. R. V i d a l , J . C o o p e r , and E. W. S m i t h , A s t r o p h y s . J . S u p p l . S e r . , 2 5 , 37 ( 1 9 7 3 ) . H. H. Griem, P l a s m a S p e c t r o s c o p y , McGraw-Hill ( 1 9 6 4 ) . R. N. K u m a i g o r o d s k a y a , A s t r o f i z . I s s l e d . , 2 , 26 ( 1 9 7 0 ) .

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