COMMISSION 27 OF THE I. A. U.
INFORMATION BULLETIN ON VARIABLE STARS
Number 1097
Konkoly Observatory
Budapest
1976 February 7
"ULTRA-VIOLET" BLAZHKO-EFFECT OF X Ari
The RRab star X Ari (P=0.651d) was observed photoelectrically
in UBV-system (ascending branches of light and colour curves) and
simultaneously spectroscopically (descending branches of velocity
curves) by Preston and Paczynski (1964).
During the analysis of metallic lines velocities we have
discovered that velocity amplitudes are extremely different on JD
2437985 (50 km/sec) and on JD 2437976 (more than 63 km/sec) as
one can see from the Figure. Observations during JD 2437918
repeat the velocity curve with maximal amplitude and those during
JD 2438010 show the curve having intermediate amplitude (nearer
to maximal one). One can suspect the Blazhko-effect for X Ari
from these observations.
Nevertheless there is no correlation between B, V and B-V
curves and metallic velocity curve variation. These light and
colour curves have almost the same amplitudes (to several
hundredth of a magnitude) during all nights.
However, we find some differences between U-B curves on
different nights, though these differences are not large. It is
possible to divide all U-B curves into two distinct groups by using
four criteria: according to phases U-B colours equal to +0.03
and +0.05 mag. on ascending branches, +0.15 mag. on descending
ones and U-B values at phase 0.0. We have found that the U-B
curves corresponding to large and to small amplitude metallic
velocity curves always belong to different groups. In particular,
U-B curves on the nights JD 2437918 and JD 2437976 (large metallic
velocity amplitudes) belong to the same group, whatever the
criterion is. The U-B curve on JD 2438010 (intermediate velocity
curve range) also belongs to the "large metallic DeltaV_r group" but
one on JD 2438017 (no velocity observations available) belongs
to the opposite "small metallic DeltaV_r group" where we also find
the U-B curve on JD 2437985 (small metallic velocity range directly
from spectroscopic observations). One can see that amplitudes
are the smallest ones on JD 2437985
and JD 2438017 due to systematically
bluer U-B values around the phase 0.0.
It is interesting to note that just
on JD 2437985(metallic DeltaV_r the smallest)
and on JD 2438017 we found the
[FIGURE 1]
brightest maxima of U light curves
(9.45 and 9.47 mag., respectively).
On JD 2437918 and JD.2437976 (metallic
DeltaV_r the largest) we found weaker U
maxima (9.49 mag., the same as in the
cases of JD 2437916 and JD 2438010).
Consequently, we can suppose we have three "moments" of large
metallic DeltaV_r (JD 2437918, 2437976, 2438010) and two "moments"'of
small amplitudes (JD 2437985, 2438017). Then, we can determine
the intervals between these dates: 58, 34, 32 days. One can consider
JD 2437945 as the "moment" of large metallic DeltaV_r (three U-B
criteria of four, and U=9.53 mag. in light maximum). If it is so,
we obtain two intervals instead of one equal to 58 days: 27d (JD
2437918-2437945) and 31d (JD 2437945- 2437976); these interval
values are in accordance with the already_obtained ones: 34d and
32d. So, now we can determine approximate value of the period of
Blazhko-effect of X Ari: 31 days as mean from four values (27, 31,
34, 32 days).
Thus, we discovered the existence of unusual Blazhko-effect
for X Ari which influences only U and U-B curves. So we can suppose
the existence of some high temperature processes (for example
shortwave recombination radiation produced by a shock wave
in higher layers of stellar atmosphere) whose effectiveness is
modulated with the period of about 31 days. P1e want to note that
hydrogen velocities which are strongly influenced by shock wave
kinematics in a higher stellar layers of X Ari, show the highest
velocities of star contraction just on JD 2437985 (the smallest.
metallic DeltaV_r). We can find only one determination of the hydrogen
velocity of expansion on this night but it is the highest one,
too, for this phase among all nights.
Prof. B.V. Kukarkin kindly communicated to me his idea that
the coincidence of the existence of unusual Blazhko-effect and
the very low metal abundance of X Ari may not be casual.,
Astronomical Council of the M.S. FROLOV
USSR Academy of Science
References:
Preston, G.W. and Paczynski, B., 1964, Astrophys.J., 140, 181 [BIBCODE 1964ApJ...140..181P ]
Frolov, M.S., 1966, Astronomical Circular, No. 365
[From IBVS 1125]
AMENDMENT
to the paper "Ultra-Violet Blazhko-Effect of X Ari" published in
IBVS No.1097
I have noticed some errors I made in Umax-magnitudes of X Ari
on the second page of my paper, however, all conclusions remain
unchanged. The correct magnitudes should be:
JD 2438017 - 9.45 mag, 2437918 - 9.50, 2437976 - 9.49,
2437916 - 9.50, 2438010 - 9.46, 2437945 - 9.48 (in order of
the text).
M. S. FROLOV