Over-ionization of potassium and sodium in late-type dwarf star

Kandel, R. S.

English

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'THE ELEC _2.ON PRESSURE IN THE ATMOSPHERES OF LATE-TYPE DWARFS"
Robert S. Kandel
; Goddard Institute for Space Studies, NASA
New York, New York
Some years ago Vardya and B_hm (])examined the profile
of the CaI resonance line at _4227A, in the spec'crum of the
M dwarf l[D 95735, and came to the conclusion that if the
effective temperature of the star is lower than 3400°K, un-
known sources of opacity may be present in its atmosphere.
\
This arises because the pressure-broadened profile predicted
_ by the lower-temperature models is far broader than the pro-
i file actually observed• The introduction of an additional
opacity changes the relation between gas pressure and optical
|epth, so that on the whole the line is formed in regions of
lower pressure and therefore is not so broad•
In the course of observations which I made some years ago
!_=[I_ at the Observatoire de Haute-Provenee, in France, at the
"__t' coud_
cmtelescopeI
r : _227 line in several late-type dwarfs, ranging in spectral
type from K7 to M3. The stars observed were:
Star
6 iCyg B
lID <_U230
l_ 95735
}_D+36°239 ".
lid 119850
Dispersion
(A/ms)
i0
10,20
4O
4O
40
used
https://ntrs.nasa.gov/search.jsp?R=19680018231 2020-03-23T22:52:19+00:00Z
The remarkable feature, as seen in Figure i, is that the
profile hardly varies among these stars; moreover the agree-
ment with the published profile of Vardya and B6hm is quite
good.
On the other hand, the profiles predicted by model atmos-
pheres were far broader than those actually observed, so that
the discrepancy suggested by Vardya and Bohm exists here too.
The models used have been described by me elsewhere (2) Here
I shall only remark that changing from a model in radiative
equilibrium, to one which goes over to an adiabatic at low
optical depth, has practically no effect on the predicted
emergent profile. The figures 2 and 3 illustrate the sense
of the discrepancy, which of course becomes more serious as
one goes to lower-effective-temperature models. A similar
discrepancy exists for the region of the CaII H and K lines
(figure 4), but here the continuum level cannot be defined
at all, so not much weight can be attached to this.
Now this disaccord also manifests itself in the same
sense for the Na D lines (figure 5) ; therefore, if it is to
be explained by an unknown absorber, it will mean that there
is little wavelength dependence involved. In the atmosphere
of a cool star, one way of obtaining such a nearly gray supple-
mentary absorption, would be to have an excess electron
pressure and thus a higher concentration of H-, the negative
hydrogen ion.
--2--
r It may seem that departures from local thermodynamic equil-
,rium (LTE) are unlikely in the dense atmospheres of late-type
dwarfs. However, it should be noted that the electron den-
sities are quite low (Ne_1012-1013); thus, provided that
collisions involving neutral particles, such as H atoms or H 2
molecules, are relatively ineffective in ionization or recom-
bination, a strong radiation field could quite easily dominate
the ionization equilibrium and lead to departures from the
Saha equation.
The question then is: which metals are suitable candidates
for over-ionization? Also, is the necessary radiation field
available? In order for over-ionization to have an effect,
the metal must not be already nearly totally ionized according
to LTE. In the case of the stars observed, near spectral
type Mo, this excludes such metals as potassium, sodium, and
calcium, since an overionization will only affect the neutral
population without changing the electron pressure. Examination
of the LTE equation of state for these atmospheres (see,eg.
(3)
Vardya ) shows that magnesium and silicon could furnish
many electrons if there were departures from LTE. However,
for these metals we cannot seriously expect such departures,
since the bound-free absorption of the neutrals will practically
cut off all the radiation in the relevant wavelengths (1400-
1700 A) (see, e.g. Gingerich (4)) . Thus it seems that the
suggested mechanism must be inoperative in these stars.
-3-
It may however come into play for cooler stars where
according to the Saha equation, the metals Ca, K and Na are
only partially ionized. These elements are not sufficiently
abundant for their bound-free absorption to affect the
radiation field. Furthermore, in the wavelengths region for
photoionization of these metals, especially K and Na, there
may be significant chromospheric radiation available, including
in particular the strong emission to be expected in the H and K
lines of MgII at 12800 A. (5) Since there will be in any case
very little ionization in the atmosphere, the blocking effect
of ionized lines can be neglected.
Thus we suggest that while it still seems necessary to
invoke unknown absorbants in the atmosphere of K6-M2 dwarfs,
and while it is of course likely that such absorbants will
continue to operate in cooler stars, one should take account
of a possible over-ionization of K and Na in the very cool_st
stars, leading to an excess electron pressure and higher
H- opacity.
-4-
This work was begun at the Observatoire de Paris-Meudon,
France. It was completed at the Goddard Institute for Space
Studies under the tenure of a NAS-NRC Resident Research Assoc-
iateship , supported by the National Aeronautics and Space
Administration. All of this support is gratefully acknowledged.
Cl)
(2)
(3)
(4)
(5)
REFERENCES
Vardya, M. S. and Bohm, K. H. 1965, M. N. 13___i, 89
Kandel, R. 1967. Ann. d'Astrophys. 30, 439
Vardya, M. S. 1966. M. N. 13____4,347
Gingerich, O. 1964. Proc. ist Harvard-Smithsonian Conf.
on Stellar Atmospheres. SAO Special Report N ° 167, p.108.
Kandel, R. 1967,Ann . d'Astrophys. 3-0, 999.
-5-
.4 .
ROBERT S. KANDEL
CAPTIONS FOR FIGURES:
"THE ELECTRON PRESSURE IN THE ATMOSPHERES OF LATE-TYPE DWARFS"
Fig 1" Smoothed observed profiles of the CaI g line at
_4227 A.
Fig. 2- Comparison of observed and computed profiles. The
curve numbered (1) is computed using the normal
abundance: A(Ca)/A(H) =6.5xi0 -6. For the other
curves this abundance has been multiplied by the
number shown.
Fig. 3: Same as Figure 2, but for a colder model: the dis-
agreement becomes stronger.
Fig. 4: The blend of the CaII K line and the A1 I resonance
line, between 3930 and 3945 A, in the spectrum of
61 Cyg B. Again, observed and computed profiles.
Fig. 5 : The Na D lines in the spectrum of 61 Cyg B and HD 88239,
compared with computed profiles for Oeff=l.16 and
Geff=l.25. Note that H. L. Johnson (Bol. Obs. Ton. y
Tac. 3, 305, 1964) assigns Oeff=l.23 to 61 Cyg B,
and PID 88230 is most probably cooler, so that a
strong disagreement exists for Na D.
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The electron pressure in the atmospheres of late-type dwarfs

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