The use of the equivalent width of Ha as a luminosity criterion for late-type stars is investigated. The method is found to be no more accurate than MK luminosity classifications. Two spectrograms are presented which indicate a change in the Ha equivalent width in 22 Vul over a three-year period

Weistrop, Donna

Caltech Authors - Main

EQUIVALENT WIDTH OF H-ALPHA IN 
LATE-TYPE STARS 
DONNA WEISTROP 
Mount Wilson and Palomar Observatories 
Carnegie Institution of Washington 
California Institute of Technology 
Received August 8, 1967 
The use of the equivalent width of Ha as a luminosity criterion for late-type 
stars is investigated. The method is found to be no more accurate than MK 
luminosity classifications. Two spectrograms are presented which indicate a 
change in the Ha equivalent width in 22 Vul over a three-year period. 
Among absolute magnitude criteria for late-type stars more accu- 
rate than MK luminosity classifications, one of the most successful 
has been the Wilson-Bappu effect. The use of the Ca n emission line 
to determine absolute magnitudes has some disadvantages, however. 
Its location near A 3900 is a major drawback in stars that have rela- 
tively weak fluxes in the ultraviolet. Measurement of the width of 
the emission against the central portion of a strong absorption line 
requires spectrograms of high spectral resolution and long exposure 
times. Ha, on the other hand, is located in the red, where a large 
fraction of the flux of late-type stars is found, and, in addition, ap- 
pears in absorption against this strong background. For the above, 
and other, reasons, the use of Ha as a luminosity criterion was in- 
vestigated by Kraft, Preston, and Wolff (1964). It was found that 
there indeed exists a relation between the ultraviolet absolute mag- 
nitude, Mv, and the half-width of Ha. 
In order to use giants and supergiants for galactic structure stud- 
ies, absolute magnitudes for stars of apparent magnitude 10 or 11 
must be obtained. Since dispersions of 5 to 10Â/mm are necessary 
to measure the half-width of Ha, the exposure time needed is pro- 
hibitively long. Therefore, an investigation was initiated to deter- 
mine whether the equivalent width of Ha, which can be determined 
from plates of smaller dispersion, might also be useful as a luminos- 
ity criterion. 
The spectrograms used in this study were taken with the 16-inch 
camera of the Mount Wilson 60-inch Cassegrain spectrograph; the 
546 
EQUIVALENT WIDTH OF H-ALPHA 547 
dispersion is approximately 30Â/mm, and Kodak Ila-E plates were 
used throughout. Values of the equivalent width of Ha, h, were ob- 
tained from tracings made on the Caltech microphotometer. 
The results of this study are given in Table I where Mv is taken 
from Kraft et al. ( 1964). For those stars for which more than one 
spectrogram has been obtained, a mean value of log h is listed. The 
probable error of an equivalent width determined from one plate is 
0.29 Â. Our results do not agree with those of Peat ( 1964 ), but con- 
siderable doubt has been cast on the individual equivalent widths he 
derives, owing to the width of his passband ( Price 1966 ). It was im- 
possible to compare the present results with those of Price ( 1966 ), 
owing to the small number of high-luminosity stars considered in his 
work. 
Figure 1 illustrates the relationship between ultraviolet absolute 
magnitude and log h. In spite of the scatter, a definite correlation 
between Mv and log h is found. A least-squares solution gives for 
the straight line the relation 
Mu = 2.38 - 17.45 log h. 
±0.61 ±2.89 (m.e. ) 
The probable error in Mv determined from one observation is 
±1.17 mag., considerably larger than the ±0.5 magnitude precision 
available from half-width measurements by Kraft et al. The size of 
the uncertainty indicates that the use of Ha equivalent widths as a 
luminosity criterion is no more accurate than MK luminosity classi- 
fications, and half-widths must be used to obtain improved absolute 
magnitudes, thus effectively eliminating this method from use in ga- 
lactic structure studies. 
There are indications from the data that the equivalent width of 
Ha changes in some stars. One of the most prominent cases is that of 
22 Vulpeculae, which shows a change of about 30% in equivalent 
width over a three-year period. Plate I reproduces two spectro- 
grams of 22 Vul, one taken in 1963, the other in 1966, which clearly 
illustrate the change in the hydrogen line. There is evidence that 
some other stars also show this effect, but the photometry is not ac- 
curate enough to provide definite proof. Further investigations 
should be undertaken to determine the extent of the Ha variations 
and the types of stars that display this phenomenon, since the possi- 
548 DONNA WEISTROP 
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550 DONNA WEISTROP 
Fig. 1 — Correlation between Mv and log h for the observed stars of known Mv. 
As noted, G-type stars are represented by dots and K-type stars by crosses. 
bility of changes in the width of Ha casts considerable doubt on the 
use of Ha as a luminosity criterion. 
I am indebted to Dr. R. P. Kraft for the use of the spectrograms 
taken by him in 1963. 
REFERENCES 
Kraft, R. P, Preston, G. W., and Wolff, S. C. 1964, Ap. J. 140, 235. 
Peat, D. W. 1964, M.N.R.A.S. 128, 435. 
Price, M. J. 1966, M.N.R.A.S. 133, 449. 


English

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Equivalent Width of Hα in Late-Type Stars

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