ADVANCES IN UV ASTRONOMY BY THE IUE SATELLITE

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ADVANCES IN UV ASTRONOMY BY THE IUE SATELLITE

M. Grewing

To cite this version:

M. Grewing. ADVANCES IN UV ASTRONOMY BY THE IUE SATELLITE. Journal de Physique

Colloques, 1988, 49 (C1), pp.C1-273-C1-278. �10.1051/jphyscol:1988158�. �jpa-00227572�

JOURNAL DE PHYSIQUE

Colloque C1, Suppl6ment au n03, Tome 49, Mars 1988

ADVANCES I N UV ASTRONOMY BY T H E IUE SATELLITE

M. GREWING

Astronomisches Institut der Universitdt, Waldhauserstrasse 64, 0-7400 Tiibingen, F.R.G.

ABSTRACT

On 26 January 1978 t h e I n t e r n a t i o n a l U l t r a v i o l e t E x p l o r e r (IUE) s a t e l l i t e has been launched i n t o a geosynchronous o r b i t . Operated j o i n t l y by NASA, ESA, and t h e SERC, i t has s i n c e c o l l e c t e d r o u g h l y 50.000 UV s p e c t r a o f o b j e c t s o f w i d e l y d i f f e r e n t n a t u r e such as comets and p l a n e t s o r b i t i n g t h e Sun, n e a r b y c o o l l a t e - t y p e s t a r s , i n t e r s t e l l a r m a t t e r i n o u r own and nearby g a l a x i e s , l o c a l and d i s t a n t h o t e a r l y - t y p e s t a r s , e n t i r e g a l a x i e s , and d i s t a n t quasars. Even t h e r e c e n t supernova SN 1987a i n t h e L a r g e M a g e l l a n i c Cloud has been and c o n t i n u e s t o be a t a r g e t f o r t h i s s a t e l l i t e j u s t as many o t h e r sources i n t h e sky. A few examples have been s e l e c t e d t o h i g h l i g h t some o f t h e unique r e s u l t s t h a t have been achieved t h r o u g h o b s e r v a t i o n s w i t h t h e IUE.

The IUE S a t e l l i t e

On 26 January 1978 a Thor D e l t a 2914, launched f r o m Cape Caneveral, F l o r i d a , c a r r i e d t h e I n t e r n a t i o n a l U l t r a v i o l e t E x p l o r e r (IUE) S a t e l l i t e i n t o a h i g h E a r t h o r b i t . The s a t e l l i t e ' s a l t i t u d e o s c i l l a t e s between r o u g h l y 26.000 and 46.000 krn, c o r r e s p o n d i n g t o a 24 h o u r s geosynchronous o r b i t . Onboard t h e s p a c e c r a f t i s a R i t c h e y C h r e t i e n t y p e t e l e s c o p e w i t h a 45cm d i a m e t e r p r i m a r y m i r r o r .

I n t h e f o c a l p l a n e o f t h e t e l e s c o p e i s a f i n e - e r r o r - s e n s o r which uses an image d i s s e c t o r t u b e t o r e c o r d an o p t i c a l image of t h e s k y i n a 16 a r c m i n diameter f i e l d which i s t e l e m e t e r e d t o t h e ground t o a l l o w t h e astronomer t o v e r i f y t h e s a t e l l i t e ' s p o i n t i n g and t o s e l e c t an o b j e c t f o r onboard t r a c k i n g purposes.

A l s o i n t h e f o c a l p l a n e o f t h e t e l e s c o p e a r e t h e e n t r a n c e a p e r t u r e s o f a s h o r t - and a long-wavelength s p e c t r o g r a p h which c o v e r t h e 115-200 nm and 190-320 nm range, r e s p e c t i v e l y . These s p e c t r o g r a p h s can be o p e r a t e d i n b o t h a h i g h r e s o l u t i o n ( X / A X - ~ . ~

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l o 4 ) and a l o w r e s o l u t i o n (W A x - 3 0 0 ) mode. The h i g h r e s o l u t i o n i s a c h i e v e d by an E c h e l l e c o n f i g u r a t i o n , t h e l o w e r one b y r e p l a c i n g t h e E c h e l l e g r a t i n g by a f l a t f o l d i n g m i r r o r . F i g . 1 ( Boggess e t a l . 1978) shows t h e o v e r a l l s p a c e c r a f t c o n f i g u r a t i o n . F i g . 2 ( a l s o t a k e n from Boggess e t a l . 1978) shows t h e p r i n c i p l e o f t h e l a y o u t o f t h e spectrographs. It a l s o i l u s t r a t e s t h e l o c a t i o n o f t h e SEC V i d i c o n cameras which a r e used f o r i n t e g r a t i o n of t h e s p e c t r a b e f o r e t h e y a r e r e a d o u t aqd t e l e m e t e r e d t o t h e ground.

The concept o f t h e IUE i n s t r u m e n t t u r n e d o u t t o be v e r y v e r s a t i l e and s c i e n t i f i c a l l y e x t r e m e l y e f f i c i e n t because t h e astronomer can make a r e a l t i m e d e c i s i o n which wavelength band and which o f t h e two modes t o use on a p a r t i c u l a r t a r g e t .

The IUE s a t e l l i t e has been b u i l t j o i n t l y by NASA, ESA and t h e SERC, w i t h 2/3 o f t h e o r i g i n a l e f f o r t and t h e c u r r e n t use of t h e s a t e l l i t e b e i n g on t h e American s i d e , w h i l e ESA and t h e B r i t i s h SERC have b o t h c o n t r i b u t e d 1 / 6 o f t h e o r i g i n a l e f f o r t and European astronomers can now j o i n t l y u t i l i z e 1 / 3 o f t h e t o t a l o b s e r v i n g t i m e , T h i s happens i n p r a c t i c e t h r o u g h t h e European ground s t a t i o n a t V i l l a f r a n c a near Madrid/Spain.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1988158

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Fig.1: The IUE s p a c e c r a f t Fig.2: The IUE f o c a l p l a n e i n s t r u m e n t a t i o n

The IUE s a t e l l i t e has i n many ways become a m i l e s t o n e i n modern space astronomy, l a s t n o t l e a s t t h r o u g h i t s l o n g f u n c t i o n a l l i f e t i m e which has exceeded t h e o r i g i n a l d e s i g n l i f e by f a r and w i l l soon r e a c h a f u l l decade.

I n t h e f o l l o w i n g s e c t i o n s s e v e r a l s c i e n t i f i c problems w i l l b r i e f l y be addressed t o i l l u s t r a t e a r e a s i n astronomy where o b s e r v a t i o n s w i t h t h e IUE s a t e l l i t e have been p a r t i c u l a r l y f r u i t f u l . B e f o r e t o u c h i n g on i n d i v i d u a l t o p i c s , i t may

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however, be b e s t t o i l l u s t r a t e t h e o v e r a l l impact o f t h e m i s s i o n b y q u o t i n g a few s t a t i s t i c a l numbers: t h e IUE s a t e l l i t e has by now c o l l e c t e d more t h a n 50.000 s p e c t r a on more t h a n 10.000 d i f f e r e n t t a r g e t s which range f r o m s o l a r system o b j e c t s t o d i s t a n t quasars. T h i s huge amount o f d a t a has been and c o n t i n u e s t o be analysed by thousands o f astronomers around t h e g l o b e who e i t h e r work on d a t a which t h e y c o l l e c t e d themselves o r on d a t a r e t r i e v e d f r o m t h e IUE a r c h i v e . T h i s a r c h i v e c o n t a i n s a l l IUE o b s e r v a t i o n s t h a t have been r e l e a s e d , n o r m a l l y s i x months a f t e r t h e measurements have been taken.

F i g . 3: The d i s t r i b u t i o n o f t h e IUE t a r g e t s on a s k y map i n c l u d i n g a l l o b s e r v a t i o n s u n t i l 1986

( t a k e n f r o m G i a r e t t a e t a l . 1987).

COMETS

P r i o r t o t h e l a u n c h o f t h e IUE, o n l y few UV s p e c t r a o f comets were a v a i l a b l e . Today, a l a r g e d a t a base of such s p e c t r a e x i s t s c o n t a i n i n g d a t a f o r 27 o b j e c t s . One o f them i s comet P/Halley, and F i g . 4 t a k e n f r o m Festou and Feldman (1987) shows i t s UV spectrum.

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F i g . 4: The UV spectrum o f comet P / H a l l e y ( t a k e n f r o m Festou and F e l dman 1987).

As cometary m a t e r i a l i s t h o u g h t t o r e v e a l t h e p h y s i c a l and chemical c o n d i t i o n s i n t h e e a r l y days o f t h e s o l a r system, many a t t e m p t s have been made t o s t u d y t h e i r chemical composition. UV o b s e r v a t i o n s a r e p a r t i c u l a r l y u s e f u l i n t h i s c o n t e x t because t h e y add t o t h e l i s t o f m o l e c u l e s t h a t can be s t u d i e d q u a n t i t a t i v e l y and t h e r e b y h e l p t o r e v e a l t h e r e l a t i o n s between p a r e n t m o l e c u l e s and t h e i r d e r i v a t i v e s produced by s o l a r r a d i a f i o n and s o l a r w i n d i n t e r a c t i o n .

HOT EARLY-TYPE STARS

The h o t t e r t h e atmosphere o f a s t a r , t h e l a r g e r t h e f r a c t i o n o f t h e energy t h a t i t r a d 5 a t e s a t s h o r t e r t h a n o p t i c a l wavelengths, i.e. i n t h e u l t r a v i o l e t and extreme u l t r a v i o l e t ranges. T h i s a p p l i e s i n p a r t i c u l a r t o e a r l y - t y p e s t a r s w i t h s u r f a c e t e m p e r a t u r e s i n excess o f 10.000 K. As t h e b u l k o f t h e i r e m i s s i o n o c c u r s a t wavelengths which a r e n o t o b s e r v a b l e f r o m t h e ground, s p e c t r a c o l l e c t e d w i t h t h e IUE s a t e l l i t e o f f e r a unique o p p o r t u n i t y t o t e s t t h e o r i e s o f t h e i r atmospheres. Of p a r t i c u l a r i n t e r e s t i s t h e s t u d y o f so c a l l e d I P Cygni l i n e ' p r o f i l e s . These a r e e m i s s i o n l i n e s , u s u a l l y ones t h a t connect t o t h e ground s t a t e , which show an a b s o r p t i o n component i n t h e i r b l u e wing, i.e. a t s h o r t e r wavelengths. T h i s i s t a k e n as e v i d e n c e f o r s t e l l a r winds and one can i n d e e d d e t e r m i n e q u a n t i t a t i v e l y t h e r a t e a t which a s t a r i s l o s i n g m a t e r i a l . These r a t e s a r e sometimes so h i g h t h a t t h i s must have an e f f e c t on t h e t o t a l mass o f a s t a r and t h e r e b y on i t s e v o l u t i o n w i t h t i m e . F i g . 5 t a k e n from H u t c h i n g s e t a1

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⁽¹⁹⁸⁷⁾ shows examples o f P Cygni p r o f i l e s , whereas F i g . 6 t a k e n f r o m C a s s i n e l l i and Lamers (1987) i l l u s t r a t e s an e m p i r i c a l c o r r e l a t i o n between observed w i n d v e l o c i t i e s , expressed i n u n i t s o f t h e s t e l l a r escape v e l o c i t i e s , and t h e e f f e c t i v e t e m p e r a t u r e of t h e s t a r s . The e x i s t e n c e o f such a c o r r e l a t i o n argues f o r r a d i a t i o n p r e s s u r e b e i n g t h e c h i e f d r i v i n g mechanism f o r t h e winds i n e a r l y - t y p e s t a r s . F i g . 7, a l s o t a k e n f r o m C a s s i n e l l i and Lamers (19871, f i n a l l y summarises t h e mass l o s s r a t e s f o u n d f o r s t a r s w i t h d i f f e r e n t temperatures and l u m i n o s i t i e s . It p r o v e s t h e p o i n t t h a t a t l e a s t f o r some s t a r s dM/dt i s l a r g e enough t o have a n o t i c e a b l e e f f e c t on s t e l l a r e v o l u t i o n .

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0 ; lum~nos#ly clorr I or f - = lurnnnortty clorr P-II.

1.70 4.60 4.50 1.40 4.30 4.20 4.10 4.00 390 Log (T,ff)

Fig.6: The r e l a t i o n between t h e t e r m i n a l v e l o c i t y o f s t e l l a r winds n o r m a l i z e d t o t h e escape v e l o c i t y i n t h e photosphere and t h e e f f e c t i v e t e m p e r a t u r e of t h e s t a r s ( f r o m C a s s i n e l l i and Lamers 1987).

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Fig.7: Mass l o s s r a t e s i n t h e H e r t z s p r u n g - R u s s e l l diagram. L a r g e r c i r c l e s i n - d i c a t e a l a r g e r r a t e ( f r o m C a s s i n e l l i and Lamers 1987) COOL LATE-TYPE STARS

F o r t h e same reason g i v e n above, namely t h a t h o t s t a r s w i l l p r e d o m i n a n t l y r a d i a t e i n t h e UV, one would expect c o o l s t a r s t o be v e r y p o o r sources o f UV r a d i a t i o n . That t h i s need n o t be t r u e i s i l l u s t r a t e d a l r e a d y b y t h e Sun which e m i t s n o t i c e a b l e amounts o f r a d i a t i o n a t s h o r t e r wavelengths f r o m i t s o u t e r atmospheric l a y e r s , i t s chromosphere and i t s corona. UV o b s e r v a t i o n s a r e i n d e e d an e x c e l l e n t t o o l t o probe , t h e e x i s t e n c e of such extended atmospheres a l s o i n o t h e r l a t e t y p e s t a r s , and t h e IUE s a t e l l i t e has been used e x t e n s i v e l y t o s y s t e m a t i c a l l y e x p l o r e t h e i r occurence as a f u n c t i o n of t h e s t a r s ' s u r f a c e parameters which can be r e p r e s e n t e d by t h e s t a r s ' l o c a t i o n i n t h e Hertzsprung-Russell-diagram. F i g . 8 t a k e n f r o m Jordan and L i n s k y (1987) shows v a r i o u s d i v i d i n g 1 i n e s i n t h e H-R-diagram above o r below which c e r t a i n atmospheric phenomena occur.

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F i g . 8: A H e r t z s p r u n y - R u s s e l l - d i a g r a m i n which t h e a b s o l u t e magnitude o f s t a r s (MY) i s p l o t t e d versus t h e c o l o u r V-R which i s r e l a t e d t o t h e i r s p e c t r a l t y p e , The rough l o c a t i o n of boundaries i s g i v e n above o r below which c e r t a i n atmospheric phenomena occur.

As i n t h e case o f t h e Sun, t h e e x i s t e n c e o f a chromosphere and corona i n a s t a r i m p l i e s t h a t e s t a r i s l o s i n g m a t t e r , sometimes a t a v e r y l o w r a t e as i n t h e case o f t h e Sun M o l y r ) , sometimes a t a much h i g h e r r a t e which i s o f g r e a t importance f o r t h e s t a r ' s f u r t h e r e v o l u t i o n .

STARS I N AN ADVANCED EVOLUTIONARY STAGE

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PLANETARY NEBULAE

As s t a r s approach t h e end o f t h e a c t i v e l i f e t i m e , which we s h a l l d e f i n e h e r e as t h e t i m e where t h e y can b u r n n u c l e a r f u e l , t h e y seem t o be l o s i n g l a r g e amounts o f m a t t e r , l e a v i n g even s t a r s w i t h o r i g i n a l l y 4 o r more s o l a r masses w i t h a n end mass o f l e s s t h a n 1.5 MO. At l e a s t some o f t h i s mass l o s s , though p r o b a b l y o n l y a m i n o r f r a c t i o n o f it, m a n i f e s t s i t s e l f as a ' p l a n e t a r y n e b u l a i which s u r r o u n d s t h e c e n t r a l s t a r a t a t y p i c a l d i s t a n c e of 1017 cm. Both t h e c e n t r a l o b j e c t and t h e n e b u l a a r e i d e a l l y s u i t e d f o r o b s e r v a t i ns w i t h t h g IUE, t h e s t a r because i t has a h i g h s u r f a c e

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t e m p e r a t u r e of s e v e r a l 10 lf n o t 10 K, t h e n e b u l a because i t o n t a i n s a a r e f i e d plasma w i t h a d e n s i t y o f t y p i c a l 1 4 s e v e r a l 102 t o s e v e r a l 105 atoms/cm5 a t a t e m p e r a t u r e o f t h e o r d e r o f 10 K. Such plasmas e m i t s t r o n g i n t e r c o m b i n a t i o n and f o r b i d d e n l i n e s many o f which happen t o f a l l i n t o t h e wavelengths ranges covered by t h e IUE, and hence such o b s e r v a t i o n s a r e numerous. Examples a r e shown i n F i g . 9.

Of p a r t i c u l a r i m p o r t a n c e i s t h e f a c t t h a t t h e t h r e e Carbon i o n s C 11, C 111, and C I V a l l have t r a n s i t i o n s i n t h e IUE r a n g e whereas t h e y a r e unobservable a t o p t i c a l wavelengths. T h e i r r e l e v a n c e i s i m m e d i a t e l y c l e a r when one r e c a l l s t h a t Carbon p l a y s a k e y r o l e i n t h e n u c l e o s y t h e s i s o f t h e chemical elements i n t h e Universe.

F i g . 9: S e l e c t e d e m i s s i o n l i n e s seen i n t h e UV spectrum o f t h e p l a n e t a r y nebula NGC 7027 ( s e c t i o n s f r o m t h e o r i g i n a l two-dimensional image c o n t a i n i n g t h e E c h e l l e o r d e r s , and p l o t s of t h e e x t r a c t e d d a t a ) .

THE SUPERNOVA 1987A I N THE LMC

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INTERSTELLAR MEDIUM OBSERVATIONS

D i r e c t l y t i e d i n w i t h t h e d i s c u s s i o n o f t h e f i n a l stages o f s t e l l a r e v o l u t i o n i s t h e e x p l o s i o n o f a supernova which o c c u r s when a s t a r can no l o n g e r s u p p o r t i t s own w e i g h t by c r e a t i n g a l a r g e enough i n t e r n a l p r e s s u r e . T h i s happens when t h e energy l o s s processes, i n c l u d i n g n e u t r i n o l o s s e s , outweigh t h e energy g a i n f r o m f u s i o n r e a c t i o n s . A s t a r t h e n r e s o r t s t o g r a v i t a t i o n a l c o l l a p s e as s h o r t - t e r m source o f energy b e f o r e a n e u t r o n s t a r o r b l a c k h o l e has formed t h a t can a g a i n be p r e s s u r e s t a b i l i z e d .

Such an e v e n t was observed on 23 F e b r u a r y 1987 i n t h e L a r g e M a g e l l a n i c Cloud (LMC), one of t h e n e i g h b o r i n g systems t o o u r Galaxy a t a d i s t a n c e o f r o u g h l y 50 kpc. While t h e supernova e x p l o s i o n c r e a t e d an o p t i c a l l y e x t r e m e l y b r i g h t o b j e c t which s t a y e d v i s i b l e t o t h e naked eye f o r many months, t h e u l t r a v i o l e t i n t e n s i t y i n c r e a s e d i n i t i a l l y b u t t h e n f a d e d away r a p i d l y due t o t h e b u i l t - u p o f a b s o r b i n g l a y e r s . Only much l a t e r a r e t h e s e expexted t o become t r a n s p a r e n t again.

I t i s t o o e a r l y t o d i s c u s s h e r e i n any d e t a i l t h e o b s e r v a t i o n s of t h e supernova e v e n t p r o p e r as t h e o b s e r v a t i o n s and t h e i r a n a l y s i s s t i l l c o n t i n u e . We s h a l l i n s t e a d f o c u s o u r a t t e n t i o n on one p a r t i c u l a r aspect: t h e unique o p p o r t u n i t y of h a v i n g a b r i g h t background l i g h t source a t t h e d i s t a n c e o f t h e LMC which a l l o w s t o s t u d y i n d e t a i l t h e p r o p e r t i e s o f t h e i n t e r s t e l l a r m a t e r i a l which happens t o l i e a l o n g t h i s p a r t i c u l a r l i n e o f s i g h t i n t h e space between t h e supernova and us.

C1-278 JOURNAL

DE

PHYSIQUE

Analogous s t u d i e s u s i n g much f a i n t e r background l i g h t sources ( 1 0 t h magnitude s t a r s i n t h e L a r g e and Small M a g e l l a n i c Clouds) belong t o t h e p i o n e e r i n g o b s e r v a t i o n s c a r r i e d o u t by t h e IUE s a t e l l i t e ( Savage and de Boer 1979). They r e v e a l e d t h e e x i s t e n c e o f b o t h n e u t r a l and i o n i s e d m a t e r i a l a t l a r g e d i s t a n c e s f r o m t h e g a l a c t i c plane. W i t h t h e more t h a n 100 t i m e s b r i g h t e r supernova as a background t a r g e t , t h e s e s t u d i e s can now be performed w i t h a t o t a l l y unprecedented s i g n a l / n o i s e r a t i o , a l l o w i n g even t h e f a i n t e s t a b s o r p t i o n components t o be t r a c e d down t o t h e l i m i t s e t b y t h e r e s o l u t i o n o f t h e IUE spectrographs. Examples of such o b s e r v a t i o n s a r e shown i n F i g . 1 0 t a k e n f r o m Grewing e t a l . (1987). They r e v e a l t h e presence o f m u l t i p l e a b s o r p t i o n components a r i s i n g f r o m a number o f d i f f e r e n t atoms and ions.

F i g . 10: S e l e c t e d r e g i o n s f r o m t h e s p e c t r a l a t l a s of t h e SN1987A and v e l o c i t y p l o t s o f a few r e p r e s e n t a t i v e i n t e r s t e l l a r a b s o r p t i o n l i n e s (A=Mg I 2852, B=Mg I 1 2802, C=Fe I 1 2585, D=Zn I 1 2025, E=A1 I 1 1 1854, and F=C I V 1548).

CONCLUDING REMARKS

The o b s e r v a t i o n s o f t h e supernova SN1987a b y t h e IUE s a t e l l i t e h i g h l i g h t one aspect which has a l r e a d y been mentioned b e f o r e : t h e l o n g f u n c t i o n a l l i f e t i m e o f t h e IUE o b s e r v a t o r y . Wheras ground-based o b s e r v a t o r i e s n o r m a l l y l a s t f o r decades i f n o t c e n t u r i e s , s a t e l l i t e o b s e r v a t o r i e s g e n e r a l l y have a f i n i t e l i f e t i m e o f o n l y a f e w years, e i t h e r because t h e i r o r b i t decays o r because t h e onboard consumables r u n o u t . T h i s b e i n g t h e case, and t a k i n g i n t o account a l s o t h a t i t n o r m a l l y t a k e s a decade b e f o r e a space experiment t h a t has been c o n c i e v e d i s b u i l t and s t a r t s t o d e l i v e r data, i t i s no doubt e x t r e m e l y f u r t u n a t e t h a t t h i s supernova exploded w h i l e t h e IUE i s s t i l l as superb an i n s t r u m e n t as i t was 10 y e a r s ago when i t was p u t i n t o i t s o r b i t .

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