Non-Periodic Phenomena in Variable Stars
IAU Colloquium, Budapest, 1968
VARIATIONS IN delta SCUTI STARS
During the recent years, a few short period variables have been discovered on
or near the main sequence. Very often, these objects show peculiar variations
in light or radial velocity.
Many questions are raised concerning the structure of these stars, the nature
of the pulsation and their relation to other groups of similar objects.
A group of French astronomers just began to study these stars from the
theoretical and observational point of view.
Up to now, some observational work has been completed. The only reduced data
concern gamma Bootis and 14 Aurigae.
IS gamma BOOTIS A SPECTRUM VARIABLE STAR?
A. BAGLIN
Institut d'Astrophysique de Paris
F. PRADERIE M. N. PERRIN
Observatoire de Meudon Observatoire de Lyon
I. gamma Bootis has been recognised as a photometric variable for a long time.
It has been followed by several authors (Guthnick and Prager (1914), Guthnick
and Schneller (1943) Guthnick and Fischer (1940), Magalashvili and Kumsishvili
(1965)). But according to the Kukarkin and Parenago Catalogue, it does not belong
to a well defined type of variable. Sometimes it experiences periodic variations
and the period is very regular 0.2903137 days; sometimes it shows erratic
variations or even constancy.
Due to its spectral type -A7- and luminosity class -III- it is tempting to
classify the star as a delta Scuti or a dwarf cepheid.
It does not seem to be a spectroscopic binary but, as the rotational velocity
is large -135 km/sec, measured by Slettebak (1965) the binary motion, if any,
would be difficult to detect.
As to the velocity field which could be connected with the light variation,
no one has even reported (Miczaika 1952).
Simultaneous photometric and spectroscopic observations have been made on
several nights in April 1968.
II. The photometric device has the same characteristics as in Chevalier
et al. (1968), the same filter has been used (35 A centered on H beta). Two
comparison stars have been measured: HD 125 642 (A2, m_v = 5.98) and HD 130817
(F0, m_v = 5.98). The large brightness of gamma Boo does not allow to use
comparison stars of the same magnitude as the star itself. Due to the large
flux the photometer works in conditions not very far from saturation, and the
precision of the measurements is less for this star than for the comparison
star. The seeing conditions were good on only two nights: April 24th and 25th.
During these two nights no variation of the brightness of gamma Boo could be
detected (Table I); the fluctuations are everywhere within the errors amplitude.
The internal error per single measurement derived from the observations of the
comparison stars is 0.005 mag.
Table I
Photometric results
Date N_1 N_2 N_3 mean(m_1-m_2) mean(m_1-m_3)
4-20-68 25 9 14 0.044 3.215
4-21-68 8 3 5 0.039 3.218
4-24-68 34 18 16 0.029 3.206
4-25-68 41 21 20 0.030 3.205
Subscript 1 refers to the comparison star HD 125642
2 " " " " HD 130817
3 " " " " HD 127762 (gamma Boo)
The bar indicates the mean value during the interval of observations on the
corresponding night.
N is the number of measurements per star; columns 5 and 6 give magnitude
differences in the chosen pass-band.
III. Spectroscopic observations have been made at the 120 cm telescope
of the Observatoire de Haute Provence, with a prism spectrograph. The
dispersion is 77 A/mm at H_gamma - II aO backed plates have been used.
The large rotational velocity does not permit a reliable study based on
weak lines.
Kopylov (1959) has measured a few lines. Balmer lines seem to be normal,
but the equivalent width of K line: W_K = 5.3 A is large for the spectral
type.
Measurements of k index by Mc Namara and Augason (1962) and by Henry (1966)
give the same value: k = 0.792, corresponding to an equivalent width of 4.53 A.
This number is large as compared to the mean K line equivalent width for normal
stars of the same b-y. As far as we know the K line has never been tested for
variability in gamma Boo.
One aim of this preliminary programm was to obtain spectra with short enough
exposure times to avoid averaging effects over a large fraction of the period.
An unpredicted feature appeared: on two consecutive nights, the spectra
show large variations of the width of the hydrogen and the K lines within a
few hours (Fig. 1).
Equivalent widths of the lines H_delta, H_gamma and K are given in Table II.
Table II
Variations of the equivalent width of the K, H_gamma and H_delta lines in gamma Boo.
Date U. T. W_K W_H delta W_H gamma
4/20/68 24h 30m 4.3: 13.6:
4/20/68 27 07 3.5: 13.1: 11.8:
4/20/68 27 37 5.0 7.2 7.6
4/21/68 25 35 3.2 15.4 16.2
4/21/68 26 11 4.9 14.3 12.9
4/21/68 27 02 4.6 12.0 13.9
4/22/68 22 44 4.4 11.9 12.3
4/23/68 23 55 4.8 13.3 13.0
4/24/68 24 50 5.0 13.0 14.3
4/24/68 27 03 4.6 14.2 12.4
4/25/68 21 08 4.7 14.0 12.5
4/25/68 22 41 5.1 12.5 13.0
4/25/68 24 33 5.1 12.0 12.3
4/25/68 25 20 5.1 12.8 13.3
4/25/68 26 25 4.9 12.2 13.6
The symbol: indicates poor photometry (see Figure 1).
The main phenomenon is the decrease of the equivalent width of the
K line by a factor 1.5. At the time of the K line minimum, the equivalent
widths of H_gamma and H_delta appear once to be reduced (4-20-68, 25H 35).
On 4-20-8, 27H 37, the hydrogen lines are very weak.
During the following nights, no variations can be detected, taking into
account the rather bad precision of the photometry. Particularly, it is seen
in figure 1 that the spectrogram taken on 24-20-68, 27H 07 presents some
features due to irregular sweeping during the exposure time. So that one
cannot conclude from the first spectrograms of our series that there is an
inverse behaviour of hydrogen and calcium II lines.
IV - Weakening of K line can be attributed to a very sudden increase
of the surface temperature of the star or to a chromospheric activity. A sudden
heating of the very outer layers of a region of the star can produce an emission
core in the K line, which reduces the equivalent width, but cannot be seen
as an emission at the resolution of our spectra.
For a few known spectrum variable stars, important variations in k
index have been reported by Henry (1966) which are almost of the same order
of magnitude as in gamma Boo.
The question could be raised whether gamma Boo belongs to the Ap stars;
the Sr II lines should be studied in that perspective.
REFERENCES
Chevalier, C., Le Contel, J. M., Perrin, M. N., 1968, Astrophys. Letters Vol. II.
Guthnick, P. et Prager R., 1914, Veröff. der Sternw. Berlin-Babelsberg, 1, 45.
Guthnick, P. and Schneller, H., 1943 Astr. Nachr., 273, 274.
Guthnick, P. and Fischer, H., 1940, Astr. Nachr., 271, 80.
Henry, R. C., 1966, Princeton thesis (unpublished).
Kopylov, I. M., 1959, Izv. Krym. astr. Obs., 22, 189.
Miczaika, G. R., 1952, Z. Astrophys., 30, 134.
Magalashvili, N. L. et Koumsishvili, J. J., 1965, Abastumansk. astrofiz. Gora
Kanobili-bull. 32, 3.
McNamara, D. H, and Augason, G., 1962, Astrophys. J. 135, 64.
Slettebak, A., 1955, Astrophys. J. 121, 653