Ultraviolet absorption lines in cosmic ray heated H I region

Silk, J.

English

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ULTRAVIOLET ABSORPTION L I N E S  
I N  COSMIC RAY HEATED H I m G I O N S  
Joseph S i l k  
Princeton University Observatory 
5 
January 1970 
SUMMARY 
A s m a l l  but s ignif icant  f ract ion of the more abundant atoms i n  H I regions 
w i l l  be ionized by low energy cosmic rays, i f  such par t ic les  are indeed responsi- 
ble  fo r  heating and ionizing the neutral  hydrogen. Some of the higher ion s t a t e s  
i n  H I regions should be detectable by rocket or  satell i te spectroscopic observa- 
t ions of the  ultimate absorption l ines .  
tained f o r  ions such as C 111, N 11, N 111, S i  I11 a d  S I11 i n  low density H I 
regions. 
Equivalent widths of order 100 mA are ob- 
c 
https://ntrs.nasa.gov/search.jsp?R=19700016017 2020-03-12T02:35:19+00:00Z
- 2 -  
Observations of the i n t e r s t e l l a r  medium, primarily by the 21 cm l i ne  of neu- 
tralhydrogen and the  dispersion measures of pulsars, indicate tha t  the mean tem- 
perature and electron density i n  H 1 regions are  too high t o  be explained without 
invoking a nonthermal energy source such as subcosmic rays (Hayakawa, Nishimwa 
and Takaynagi 1961 ; Pikelner 1968; Spitzer and Tomasko 1968; Field, Goldsmith and 
Habing 1969) or  sof t  x-rays ( S i l k  and Werner 1969; Sunyaev 1969). 
The heating and ionization of H I regions by energetic par t ic les  has i n  f a c t  
been investigated i n  considerable d e t a i l  (Spitzer and Scott 1969; Goldsmith, 
Habing' and Field 1969). 
erget ic  par t ic les  on the  ionization equi l ibr ia  of the heavier atoms present i n  
H I regions. 
near future, many of these atoms should produce observable absorption l ines.  
Spitzer and Zabriskie (1959) have l i s t e d  the absorption l ines  formed i n  the most 
However no at tent ion has been given t o  the e f fec t  of en- 
With the  advent of space satell i te u l t rav io le t  spectroscopy i n  the 
abundant s t a t e s  of i n t e r s t e l l a r  atoms. 
In  the present note, I wish t o  point out t h a t  a s m a l l  but significant frac- 
t ion  of  the more abundant atoms i n  H I regions w i l l  be present i n  higher states 
of ionization, owing, t o  the effects  of energetic par t ic les ,  and w i l l  give rise to . 
detectable absorption l ines .  
t o  calculate the  ionization equilibria,  the resu l t s  of the c lass ica l  theory have 
These ion s t a t e s  are  l i s t e d  i n  Table I. In20rder 
been used fo r  the cross-section fo r  electron impact ionization, &i. The cross- 
sections f o r  ionization by proton and electron impact'are approximately equal ' 
at the  same velocity, provided tha t  the proton energy E exceeds about 0.2 MeV. 
For the  ionization of an ion with ionization poten t ia l  I and number of electrons 
i n  the outer shel l ,  a reduced cross-section may be defined.by (i 
1 
1 
- 3 -  
where IH is  the  hydrogen ionization potential ,  13.6 eV. 
approximate c lass ica l  theories indicate tha t  u. ( E )  should be a universal func- 
t ion,  the same for  a l l  elements i n  a l l  stages of ionization. More refined cal- 
culations show that u. does i n  fac t  d i f f e r  appreciably fo r  different  elements; 
however a mean ui may be define$ which i s  consistent wi%h a l l  of the  results t o  
within a factor  of about 2 (Seaton 1964). 
If E = meE/mpI, then 
1 
1 
Only radiative recombinations are considered, and it is  assumed tha t  the 
t h  recombinations are hydrogenic. Hence the recombination coefficient t o  the i 
s t a t e  of ionization of an atom i s  
ai = (i + 112aE , 
where a i s  the  t o t a l  recombination coefficient H f o r  hydrogen. One then obtains 
the  general resu l t  for the abundance of the  (i + 1 ) th  s t a t e  of ah ion X, 
ne/%* where y = 
The relative abundances of the more abundant ions which have t ransi t ions 
from the ground state a t  wavelengths longward of gl&A are  l i s t e d  i n  column 2 
of Table 1, normalized t o  10 fo r  hydrogen. 
wavelengths are l i s ted .  
6 In  column 3, the corresponding 
In  cases where there are several  l ines ,  only tha t  l i n e  
has been selected with the largest  osc i l la tor  strength f ;  other t ransi t ions of 
in te res t  are  indicated i n  parentheses. The corresponding f-values a re  l i s t e d  
i n  column 4, and are  taken from a recent compilation by Smith (1969). Columns 
5 and 6 contain the opt ica l  depths T~ at the  center of each l ine,  aad the cor- 
b 
responding equivalent widths fo r  two modelH I regions. 
tabulated by Spitzer (1968) has been used. 
cloud (Spitzer 1968) of density qj: = 10 
The curve of growth 
Case (a) represents a "standard" 
A m e 4  electron and s ize  7 pc. 
- 4 -  
density ne = 3 X lom3 (Spitzer and Scott 1969), and a radial velocity dispersion 
of 2 km/sec have been assumed. 
n = 2 x 
and Habing 1969). 
low density medium, and a dispersion in radial velocities is adopted of 8 Irm/sec 
along the line of sight. 
For case (b) the parameters % = 0.1 and 
cmm3 are chosen to represent the intercloud medium (Field, Goldsmith e 
A line of sight is considered which traverses 100 pc of this 
Because the absorption lines in the dominant states of the more abundant ions 
such as C 11, N I, A1 11, Si I1 a.nd S I1 are heavily saturated (cf. Spitzer and 
Zabriskie 1959), Table 1 indicates that the absorption lines produced in H I re- 
gions by C 111, N 11, Al 111, Si I11 and S I11 should be easily detectable with 
an ultraviolet spectrograph in space, with resolution comparable to that of 
ground-based instruments. Even the ions C IV, N I11 and S IV should be observ- 
able for H I regions with density % 5 1 
on y , and so will be a sensitive indicator of the degree of ionization of the 
H I region. The present calculation indicates that even though H I 'lstandardl' 
clouds produce strong absorption lines for the most abundant ion states, the l ow 
density, more highly- ionized H I regions tend to produce even broader features, 
because the lines are less heavily saturated. Low density H I regions dominate 
completely for those lines produced by the higher ion states. 
although the line strengths depend 
2 
Similar absorption lines for the higher ion states are, of course, predicted 
in H I1 regions where these same ions are collisionally ionized by thermal eiec- 
trons. 
in H I and H I1 regions from the Doppler shifts measured for individual lines 
because the lines characteristic of H I regions such as C 11, 0 I and N I will 
However it should be possible to distinguish between those lines formed 
t 
not be 
known H I or H I1 regions along the line of sight should also enable some sepa- 
ration to be made. It would be feasible to determine whether the intercloud 
produced ih H I1 regions. Comparison with the radial velocities of 
- 5 -  
medium i s  predominantly hot H I or low density H I1 by choosing a star a t  suf- 
f i c i en t ly  high galact ic  la t i tude  so -that the l i n e  of s ight  did not pass through 
any dense H I regions. The failure t o  detect  any of these l ines  i n  the in te r -  
s t e l l a r  medium would pose grave d i f f i cu l t i e s  f o r  the  hy-pothesis t ha t  H I regions 
are  heated by energetic par t ic les .  Analogous predictions of absorption l i n e  in- 
, 
t ens i t i e s  may be made fo r  H I regions heated by s o f t  x-rays; these w i l l  be pre- 
sented i n  a subsequent paper. 
I wish t o  acknowledge helpful conversations with Drs. W. H. Smith and L. 
Spitzer. T h i s  research has been supported i n  par t  by National Aeronautics and 
Space Administration grant NGL-31-001-007 to Princeton University. 
I O N  ABUNDANCES AND EQUIVUNT WIDTHS 
OF INTERsTElLLAR LINES 
c I1 
c I11 
C I V  
N I  
N 11 
N 111 
A 1  I1 
AI. I11 
S i  I1 
S i  I11 
S i  IV 
s I1 
s I11 
s r v  
A I  
A I1 - 
- _  
Abundance 
re la t ive  
6 t o  H = 10 
* 
Wavelength 
(A)  
1334.5 (1036.3) 
977 
I 548.2 (1 550.8) 
1199.6 r31 
(1135 f31, 964 t31) 
1084 (915.6) 
989.8 
I 670.8 
1854.7 (1862.8) 
1260.4 (1808, 1526.7 ' 
1304.4, 1193.3) 
1206.5 (1895.5) 
1393.7 (1402.7) 
1259.5 (1253.89 1250.5) 
1190.2 (1012.5) 
I 062.7 
1048.2 (1066.7) 
91 9.8 
*abs 
0.113 
0.28 
0.19 
0.14 
0.11 
0.1 
1.8 
0.6 
0.68 
1.43 
0.55 
0.01 2 
0.48 
0.59 
0.23 
0.009 
-I--"- 
---L---l_ 
*O 
(a> (b) 
4800 180 
12 28 
0.002 0.4 
1600 60 
16 40 
0.02 3.1 
605 22 
0.002 0.6 
2700 102 
10.3 26 
0.001 0.2 
27 0.1 
2.2 5 
0.01 0.2 
1 08 4 
0.08 0.2 - _I_ 
Equivalent 
Width (rnA) 
(4 (b)  
73 
31 
0.06 
62 
* 35 
0.3 
81 
0.06 
67 
38 
0.02 
46 
23 
0.2 
4 
1 
232 
144 
9.1 
1 94 
161 
89 
237 
61 
21 5 
173 
17 
52 
1 26 
13 
105 
11 
Whenever several ultimate l ines  appearr i n  a multiplet, with mutual separation 1 A 
or  less,  only the largest  waveiength i s  gLven, with the number of l ines  i n  the m u l -  
t i p l e t  indicated i n  square parentheses. \ 
- 7 -  
REFERENCES 
Field,  G. B. , Goldsmith, D. W. and Habing, H. J. 1969, Ap. J. (Letters),  m, L149. 
Goldsmith, D. W. , Habing, H. J. arzd Field, G. B. 1969, Ap. J. , a, 173. 
Hayakawa, S. , N i s h i m u r a ,  S. and Takaynagi, K. 1961, Pub. A s t r .  SOC. Japan, u, 184. 
Pikelner, S. 1968, Soviet A s t r . - A .  J., &, 737. 
Seaton, M. J. 1964, Planet. Space Sci. , E, 55. 
Silk,  J. and Werner, M. W. 1969, Ap. J., m, 185. 
Smith, W. H. 1969 (private communication). 
Spitzer, L. 1968, Diffuse Matter i n  Space (New York; Interscience). 
Spitzer, L. and Scott, E. H. 1969, Ap. J., a, 161. 
Spitzer, L. and Tomasko, M. G. 1968, Ap. J. , m, 971. 
Spitzer, L. and Zabriskie, F. 1959, P. A. S. P., & 412. 
Sunyaev, R. 1969, Astron. Zh. , $$, 929. 


Ultraviolet absorption lines in cosmic ray heated H I regions

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