Stellar model atmosphere abundance analysis to correlate microturbulent velocity parameters with iron to hydrogen ratio for G dwarf

Strom, S. E.

English

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1 
ON THE VARIATION O F  THE MICROTURBULENCE 
PARAMETER WITH CHEMICAL COMPOSITION 
S. E. S t rom 
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January 1968 
Smithsonian Institution 
As  t r op hys i c  a1 Ob s e rvato r y 
Cambridge, Massachusetts 021 38 
https://ntrs.nasa.gov/search.jsp?R=19680009879 2020-03-12T09:28:42+00:00Z
I ON THE VARIATION O F  THE MICROTURBULENCE 
PARAMETER WITH CHEMICAL COMPOSITION 
S. E. STROM 
Smithsonian Astrophysical Observatory 
Cambridge, Massachusetts 
Received 
ABSTRACT 
It  is  shown that the correlation found bv previous investigators between 
the value of the microturbulent velocity parameter  5 .-t 
ratio for  G dwarfs resul ts  f rom invalid assumptions implicit in the differen- 
t ial  curve-of-growth techniques used to der ive 
phere abundance analyses,  the deduced values of E L  will be very close to the 
and the i ron to hydrogen 
6 By using model a tmos-  t '  
~. 
mean value found for  s t a r s  for  approximately so la r  composition. 
Several  authors (e. g . ,  Wallerstein 1962, Aller and Greenstein 1960) have 
suggested on the basis  of differential curve-of -growth (DCOG) analyses that 
ex t reme subdwarf atmospheres a re  character ized by unusually small values 
of the turbulent velocity parameter  5 
evidence that the turbulent velocity dec reases  along with the iron-to-hydrogen 
ratio and thus, in a crude way, with age. This suggestion has led to the 
speculation that the lower values of et ( e ,  5 1 km/sec )  for  the most  metal-  
deficient subdwarfs a r e  related to the decrease  of chromospheric activity 
with age. 
a tmospheres ,  contradicts the results of previous investigations in that for  
two extreme subdwarfs, HD 19445 and HD 140283, they find, respectively, 
& - 2 km/sec  and 2 
- m G t h o r  gratefully acknowledges the surmort of Grant No. NGR 22-024-001 
f r o m  the National Aeronautics and Space Administration. 
Wallerstein (1962) has presented t' 
Recent work by Cohen and St rom (1968), based on detailed model 
5,L 3 km/sec.  
1 
In this note we present  arguments that demonstrate  that the lower values 
* ,  of 5, derived previously for  subdwarfs a r e  a resu l t  of the assumptions 
implicit  in the DCOG analyses,  and that the t rue  values of 5 a r e  likely to be 
close to those found for  normal  G dwarfs.  
t 
. 
Computation of s te l lar  abundances using DCOG techniques have long been 
popular because e r r o r s  in the assumed transit ion probabilities , atmospheric 
s t ructure ,  and thermodynamic state of the gas ,  a s  well a s  observational 
e r r o r s ,  tend to cancel if  two s tars  of fairly s imi la r  temperature,  gravity, 
and composition a r e  compared and i f  homogeneous plate mater ia l  i s  used. 
In the previous work on subdwarfs quoted above, the authors all performed 
DCOG analyses using the sun as the pr imary  abundance standard. 
temperatures  and gravi t ies  of the subdwarfs under analysis differed only 
slightly f rom the solar  values of these quantities, the DCOG technique would 
be expected to yield reasonable results.  However, the difference in composi- 
tion between a subdwarf and the sun produces a somewhat subtle but essent ia l  
change in the temperature  structure of the atmosphere.  
subdwarf has a lower metal-to-hydrogen ratio and hence a l e s se r  degree of 
blanketing, owing to the presence of spec t ra l  l ines.  
t u re s  of the sun and of a subdwarf of matching emergent  continuum flux in 
the visible a r e  compared in Figure 1 , where we sketch the qualitative 
behavior of the T(T) relation for the sun and the subdwarf. Note that, owing 
to the grea te r  line blanketing, the sun has a lower temperature  for  T 5 5 x 10 
The temperature  difference a t  the boundary can be  a s  grea t  a s  A T  
o r  approximately 300°K for  s t a r s  of near -so lar  effective temperature ,  
Since the 
In particular,  the 
The temperature  s t ruc -  
- 2  . - 0.05 Teff, b 
The lines on the weak-line part of the curve of growth a r e  formed 
pr imar i ly  in the optical depth region marked W in Figure 1 , whereas the 
l ines on the saturated par t  of the curve a r e  formed in region S. 
of Fe I appear in sufficient quantity in the subdwarfs to obtain reliable values 
of 5 f rom the curve of growth. Since the temperature  for  subdwarfs in region 
S is higher than is the case  for  the sun, the fraction of F e  that is  F e I  is  conse- 
quently lower relative to region W, where the so la r  and subdwarf tempera-  
t u re s  a r e  a lmost  the same.  
Only l ines 
t 
In both the so la r  and subdwarf cases ,  F e  is 
2 
. 
almost  entirely singly ionized. 
and subdwarf curves  of growth that resu l t  for  F e  I is shown in F igure  2. 
l inear  par t s  of the curve have been shifted to take account of the differing 
iron-to-hydrogen ratio. The difference in e lectron p res su re  caused by the 
differing compositions of the sun and the subdwarf has little effect on the 
shapes of the curves of growth, since we a r e  comparing the strengths of the 
saturated lines relative to those of the weak lines. 
A qualitative comparison between the so la r  
* .  The 
If one uses  the solar  curve of growth to analyze the subdwarf, one would 
deduce that t 
f r o m  the DCOG techniques. 
and Strom (1968) avoids this difficulty, however, and higher values of k t  a r e  
deduced. 
is lower in the subdwarf. This is precisely the resul t  obtained 
The model atmosphere approach used by Cohen 
An argument analogous to the one outlined above may explain why s t a r s  
having higher iron-to-hydrogen ratios than does the sun appear to have some- 
what higher values of 5,. 
temperature  to be lower, owing to the increase in the importance of line 
blanketing. 
growth a r e  somewhat stronger relative to the solar  curve of growth and lead 
to an  overest imate  of the turbulence. 
In these meta l - r ich  s t a r s  we expect the boundary 
Therefore,  the F e  I lines on the saturated par t  of the curve of 
Assignment of excitation temperatures  for  s t a r s  of differing composition 
may also resul t  owing to the differences in temperature  s t ruc ture  caused by 
line blanketing . 
This work suggests that the correlation between turbulence and composi- 
tion found previously may not be real. 
pr imari ly ,  if not entirely, f rom the fai lure  of the DCOG method of analysis 
to take account of the differential changes in atmospheric s t ructure  caused 
by line blanketing . 
The correlation appears to resul t  
3 
* .  
REFERENCES 
Aller,  L. H. ,  and Greenstein, J. L. 1960, Ap. J. Suppl.,>, 
Cohen, J. G . ,  and Strom, S. E. 1968, Ap. J. (in press ) .  
Wallerstein, G. 1962, Ap. J. Suppl.,6, 407. 
4 
139. 
. . 
T 
.- 
 IO-^ I d 2  lo-' I 
r 
Fig.  1 - The behavior of the temperature-optical  depth relations for  the sun 
and fo r  a typical subdwarf. 
formation a r e  indicated. 
The regions of weak (W) and saturated ( S )  line 
Fig.  2 - The qualitative behavior of the curve of growth for  F e  I for  the sun 
and f o r  a typical subdwarf. 
5 


On the variation of the microturbulence parameter with chemical composition

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