Full text of DOI:10.1134/1.1320499

Astronomy Reports, Vol. 44, No. 11, 2000, pp. 738–744. Translated from Astronomicheskiœ Zhurnal, Vol. 77, No. 11, 2000, pp. 834–841.
Original Russian Text Copyright © 2000 by Aliev, Ismailov.
Discovery of Rapid Spectral Variability in the Ap Star c Psc
S. G. Aliev and N. Z. Ismailov
Astrophysical Observatory, Shemakha, Azerbaijan
Received November 1, 1999
Abstract—Analysis of spectrograms with high spectral and temporal resolution taken during four nights in 1998 has
revealed spectral-line variability of the peculiar star HD 220825 with a quasi-period of 0.d0.12 ± 0d. 0005. The
observed variations have the form of wavelike oscillations whose amplitude decreases or increases during the
observations. The CrII, SiII, and TiII lines exhibit the strongest variability. The scale lengths of peculiar regions
are estimated to be from 5000 to 15000 km based on data for different lines. The results provide evidence for
the presence of real wavelike dynamical processes at the stellar surface that change the physical conditions in
the stellar atmosphere. © 2000 MAIK “Nauka/Interperiodica”.
1. INTRODUCTION
characteristics of the Coudé focus, spectrometer, and
CCD.
Systematic studies of short-period variability in
peculiar stars with strong magnetic fields are of consid-
erable interest for pulsation theory. Breger [1] pointed
out that short-term photometric variability in magnetic
stars could be associated with pulsations, as is the case
in many main-sequence stars. According to the data of
Percy [2], the peculiar star 21 Com exhibits photomet-
ric variability with a period of 30 min. Wood [3] also
observed short-period photometric variations in pecu-
liar stars. A number of authors have pointed out rapid
pulsed variability in Ap stars (see, e.g., [4–6]).
The optical design of the spectrometer was devel-
oped by Musaev [11] based on the Coudé spectrograph
of the 2-m telescope of the Shemakha Astrophysical
Observatory. The complex was assembled and was sub-
ject to preliminary testing at the 1-m telescope of the
Special Astrophysical Observatory of the Russian
Academy of Sciences [11]. Full-scale tests were per-
formed at the Shemakha Astrophysical Observatory.
The entire spectral interval covered by the CCD is
subdivided into 45 and 40 échelle orders, red and blue,
respectively. The instrument can simultaneously cover
the spectral regions near Hα in the 72nd order and near
Obviously, the short periods observed thus far in
some peculiar stars cannot be explained by spots
appearing on the visible side of the stellar disk as a λ4350 Å in the 110th order, with reciprocal linear dis-
result of axial rotation. Therefore, investigation of such persions of 4.8 and 3.1 Å/mm, respectively. For our
short-period variations makes it possible to study the program observations, we used the spectrum from the
physics of local regions associated with spots. The aim 72nd through the 112nd orders.
of the present paper is to analyze rapid variability in the
Given the optical parameters of the spectrometer com-
spectrum of χ Psc. HD 220825 (χ Psc) is a typical pecu-
plex, the resulting two-pixel spectral resolution was R =
liar Cr, Sr star that is known to exhibit periodic photo-
30000. For focal distances of the collimator and camera
metric and spectral variations (P = 0d. 5853) [7]. The
photometric observations of Rakos [8] and Genderen
[7] yield similar photometric periods (P = 0d. 5805 and
0d. 5853, respectively). Aliev [9] showed that the photo-
metric period coincides with the period of the spectral
variations. The star has a variable magnetic field [10].
equal to Fc = 5400 mm and Fk = 350 mm, respectively, and
the Coudé focal-plane scale of 2.86 arcsec mm–1, we
derived a normal width of d = 2.01 arcsec (0.73 mm) for
the spectrometer slit.
We used the solar spectrum in the spectral interval
considered to construct the dispersion curve. The stan-
dard deviation of individual reference lines from the
mean dispersion curve did not exceed ±0.005 Å. To
check for possible shifts during the observations, we
obtained an Ne-Fe comparison spectra before and after
2. OBSERVATIONS
We observed χ Psc with an échelle spectrometer each exposure. We reduced the spectrograms using
attached to the Coudé focus of the 2-m telescope of the standard techniques using a MS DOS program devel-
Shemakha Astrophysical Observatory of the Academy oped at the Special Astrophysical Observatory [12].
of Sciences of Azerbaijan. The spectrometer was This program computes equivalent widths, halfwidths,
equipped with a 530 × 580 CCD. Table 1 gives the main radial velocities, and other spectral-line parameters.
1063-7729/00/4411-0738$20.00 © 2000 MAIK “Nauka/Interperiodica”

DISCOVERY OF RAPID SPECTRAL VARIABILITY
739
Table 1. General parameters of the telescope-spectrograph-
We observed the two standard stars HR 8506 and
HR 0265 to measure their heliocentric radial velocities.
Table 2 gives the results for the 72nd, 74th, and 98th
orders. The first row of the table gives the numbers of
the spectral orders, the boundaries of the spectral inter-
vals covered by the CCD window, and the mean disper-
sions for the orders. The radial velocities Vr were
obtained by averaging four to six values in each order.
As can be seen, for fairly narrow spectral lines and sig-
nal-to-noise ratios of no less than S/N Ӷ 80–100, Vr can
be measured with an rms error of ±0.6–1.0 km/s. Low
signal-to-noise ratios increase the radial-velocity error to
5–6 km/s. The resulting radial velocities for the stan-
dard stars agree well with the data from [13], within the
quoted errors.
detector system
Parameter
Value
Aperture
2000 mm
Equivalent focus
Collimator diameter
Collimator focal length
Camera focal length
Camera diameter
Échelle grids:
72000 mm
150 mm
540 mm
350 mm
352 mm
line frequency
37.5 and 75 lines/mm
63.5°
blaze angle
size
200 × 300 mm
Our measurements showed that the error in the
equivalent width Wλ is determined mainly by the esti-
mated continuum level and the signal-to-noise ratio. In
our measurements, the rms error is about 5–10% of the
equivalent width. The error in the halfwidth is about
∆λ1/2 ±0.02–0.03 Å.
Working orders:
at 37.5 lines/mm
at 75 lines/mm
Spectral interval
70–140
35–70
λλ 3600–8000 Å
Length of spectrum at λ6563 Å 57 Å
Figure 1a shows a normalized section of the spec-
trum of the standard star HR 0265 near the Hα line for
data obtained on several nights. It is clear that all signif-
icant features agree well for six spectrograms obtained
on different nights, confirming the high stability of our
equipment. Figure 1b shows the dispersion of the rela-
tive intensity of the same spectral section as a function
of wavelength. We can see that the dispersion of the rel-
ative profile intensity of individual spectral sections
does not exceed 0.5%.
Dispersion at λ 6563 Å
Number of orders on CCD:
in the red
0.0984 Å/px
46
in the blue
40
Optimum slit width (at Hα)
Spectral resolution (at Hα)
CCD size
2′.′0
≈30 000
530 × 580 px
24 µm
Pixel size
The exposure times for χ Psc were 3–10 min,
depending on the seeing. The signal-to-noise ratio was
about 70–100 near the Hα line and about half this value
in the blue region of the spectrum. We obtained a total
of 34 spectrograms, with 15 spectrograms the first
night, 13 the second, and 3 more during each of the
remaining two nights. The first two observing runs
lasted more than 1.5 hours each.
Linear CCD size
12.72 × 13.92 mm
120 K
Operating temperature
Accumulated signal:
1-hour exposure
S/N = 10 (λ6563 Å, V = 9m. 4)
S/N = 60 (λ6563 Å, V = 5m. 0)
10m (S/N = 10, 2-hour expo-
sure)
10-min exposure
Limiting magnitude
3. RESULTS
Prism reflection angle
45°
Prism dispersion (at Hα)
150 Å/mm
We computed the equivalent widths, halfwidths,
depths, and radial velocities of spectral lines. The mea-
surements of individual lines revealed complex, multi-
component structure in the CrII, SiII, TiII, FeII, and
MgII lines. We obtained a mean halfwidth of 1.068 ±
0.01 Å for the MgII 4481 Å line, which corresponds
to a rotational velocity of vsini = 57.2 ± 0.7 km/s. If,
as implied by its spectral type, the star has a radius of
R = 3R᭪, the true rotational velocity at the equator is
Veq ~ 260 km/s and the inclination of the rotation axis
is i ~ 12°. Consequently, the star is observed almost
pole-on.
gen lines exhibit oscillations. The radial-velocity varia-
tions of the Hβ line are especially prominent, with an
amplitude that exceeds that of the Hα-line variations.
Other measured lines also exhibit similar variability,
including MgII 4481 Å, CrII 4836, 4848 Å, TiII 4571 Å,
SiII 5041, 5056 Å, FeII 5316, 4991 Å, etc. By way of
example, we show in Fig. 3 the time dependence of the
radial velocities for the CrII 4836 Å and SiII 5041 Å
lines for two different nights. As is evident from this
figure, the radial velocities of these lines exhibit wave-
like variations. During the first night (JD 2451057), the
Figure 2 shows the measured Wλ and Vr for Hα and
Hβ for two nights. This figure shows that both the
equivalent widths and the radial velocities of the hydro- radial-velocity amplitude decreased (Fig. 3a), whereas
ASTRONOMY REPORTS Vol. 44
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740
ALIEV, ISMAILOV
it increased during the second night (JD 2451058).
I/I0
1.2
A similar effect can be seen in the Hβ measurements in
Fig. 2.
(a)
1.0
0.8
0.6
0.4
0.2
0
Figure 4 shows the time dependences of the equiva-
lent widths of these same two lines for the same nights.
The variations of Wλ virtually repeat the radial-velocity
curve in Fig. 3. This provides evidence that we are
observing the effect of real short-term dynamical pro-
cesses that change the physical conditions in the stellar
atmosphere.
HR0265
6555
Figure 5 presents the radial velocities of the
CrII 4558 and 4836 Å lines folded with the photometric
6550
6560
(b)
6565
6570
Dispersion I/I0, %
0.5
period derived by van Genderen [7] (P = 0d. 5853). It is
clear that our measurements, which do not cover the
entire photometric period, nevertheless show variations
with this period. Our data show radial-velocity maxima
and a very unusual enhancement of the line equivalent
widths and halfwidths near phases 0.3–0.5. The radial
velocity Vr rises sharply from minimum to maximum
near phase 0.0. Consequently, the radial-velocity curve
is asymmetric with respect to its extrema. This provides
further support for the hypothesis that the angle i
between the stellar rotational axis and the line of sight
is small. The period in question could, indeed, plausi-
bly be a manifestation of the star’s axial rotation [9].
0.4
0.3
0.2
0.1
0
–0.1
6552
6557
6562
6567
λ, Å
Thus, our program-star observations covering 1.5–2 h
and consisting of 3–5 min exposures enabled variabil-
ity of the star with a period of 0d. 012 ± 0d. 0005 to be
Fig. 1. (a) Section of the spectrum of HR 0265 (G2III) near
H on various observing dates and (b) dispersion of the rel-
α
ative intensity.
10
(a)
(b)
10
5
5
0
0
–5
–5
–10
–15
–20
–10
–15
57.44
57.46
57.48
57.50
58.34 58.36 58.38 58.40 58.42 58.44
JD 2451000+
JD 2451000+
Fig. 2. Variations of equivalent widths (circles) and radial velocities (triangles) of the H (filled circles and triangles) and H (open
α
β
circles and triangles) lines in the spectrum of HD 220825 on two different nights.
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DISCOVERY OF RAPID SPECTRAL VARIABILITY
741
15
40
30
(a)
(b)
10
5
20
0
10
–5
–10
–15
0
–10
–20
–20
57.44
57.46
57.48
57.50 58.35
58.37
58.39
58.41
58.43
JD 2451000+
Fig. 3. Radial-velocity variations of the CrII 4836 Å (filled circles) and SiII 5041 Å (open circles) lines in the spectrum of
HD 220825 on two nights.
0.25
0.19
(a)
(b)
0.20
0.15
0.10
0.05
0.16
0.13
0.10
0.07
57.44
57.46
57.48
57.50 58.35
58.37
58.39
58.41
58.43
JD 2451000+
Fig. 4. Same as Fig. 3 for the line equivalent widths.
confidently detected. This period was observed rather observed for the most stable lines, such as FeII 5326 Å
and FeI 5382 Å. The largest scatter is observed for the
SiII, CrII, and TiII lines. We can also see from Fig. 7
that the total range of the radial-velocity variations for
all the elements is ±30 km/s. The maximum amplitude of
the Vr variations for individual elements varies from 5 to
15 km/s (see, e.g., Figs. 2 and 6). On the whole, the inter-
vals for the radial-velocity variations for different ele-
ments overlap. As noted above, the most stable lines have
the smallest radial velocities, and the more peculiar the
element, the higher the amplitude of its Vr variations.
Figure 8 shows the phase dependences of the equiva-
lent widths of the MgII 4481 Å, CrII 4558 Å, TiII 4571 Å,
CrII 4848 Å, FeII 4991 Å, FeII 5316 Å, and FeI 5383 Å
lines for a period of 0d.012. For two lines, trends repre-
sented by fourth-order polynomials are given. We can
see that the equivalent widths of the MgII 4481 Å line
show periodic variations. The equivalent widths of
other lines also vary with phase to various degrees,
although, for example, the FeII 5316 Å line shows
stably on both nights (Figs. 2–4).
Figure 6 shows the radial-velocity curves for the
MgII 4481 Å and CrII 4836, 4558 Å lines folded with
a period of P = 0d. 012. The plot includes all data
obtained during the four nights. It is clear that these
lines exhibit periodic variations with P = 0d. 012 over
20 days, i.e., throughout the observing season. Figure 7
shows a similar graph with data for the Hα, Hβ,
FeI 5383 Å, SiII 5041 Å, and TiII 4571 Å lines in addi-
tion to the lines shown in Fig. 6. Overall, these lines
also show periodic variations with P = 0d. 012. Since the
radial velocities of the different lines are shifted some-
what relative to each other, we see a shift in the radial-
velocity curves in Fig. 7 from minimum to maximum
values (see also Fig. 6). On the whole, it is clear from
Fig. 7 that the shifts for various elements are 10–12 km/s.
The lower envelope in Fig. 7 is better defined than the
upper envelope. The minimum radial velocities are nearly no equivalent-width variations.
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ALIEV, ISMAILOV
Table 2. Radial-velocity measurements for the two standard stars
72nd order
74th order
98th order
6542–6600 Å 0.099 Å/px 6323–6367 Å 0.076 Å/px 4823–4866 Å 0.073 Å/px
Date, UT
Vr, km/s
HR 8506
30.07.98
27.08.98
28.08.98
29.08.98
01.09.98
00h40m
23 59
21 41
21 50
22 30
–7.6 ± 6
–5.1 ± 2.1
–9.65 ± 1.0
–6.4 ± 1.4
–8 ± 1.8
–7.1 ± 3.1
–6.9 ± 1.0
–10.7 ± 1.4
–7.8 ± 0.6
–18 ± 0.9
–6 ± 4
–14.6 ± 1.8
–12.8 ± 3.3
–8.7 ± 2.6
–12 ± 1.2
HR 0265
30.07.98
28.08.98
28.08.98
29.08.98
01.09.98
02 05
00 35
23 15
22 10
01 30
–36 ± 4.4
–40 ± 1.3
–46.6 ± 1.1
–51 ± 2.1
49.9 ± 1.8
50 ± 0.6
–46.2 ± 0.8
–50.3 ± 0.6
–51.1 ± 1.5
–51.9 ± 0.6
–49 ± 3.1
–50.1 ± 2.4
–53.8 ± 1.8
–54.1 ± 2.4
–53.1 ± 3.5
Since the amplitude of the photometric variability of It is apparently not always possible to reliably detect
the star in the V band is 0m. 01 [7], the line equivalent
widths can be expected to vary by up to 10%. However,
the observed variations of Wλ exceed 50–100%. Espe-
such variations in individual spectral lines due to the
peculiar processes occurring on the star. Furthermore,
such short-term variations in the stellar spectrum might
be detectable only at certain axial-rotation phases and
cially strong equivalent-width variations are observed
under certain physical conditions.
for the lines of the peculiar elements CrII, SiII, TiII, and
Our results agree fairly well with the photometric
for the Balmer Hβ line.
period derived earlier [7]. At the same time, the stellar
spectrum exhibits pulsational variations with a period
4. DISCUSSION AND CONCLUSIONS
of 17 minutes, which remain stable over 20 days. We
observed simultaneous variations in both the equivalent
widths and halfwidths of spectral lines and in their radial
velocities, providing evidence for the action of real short-
term dynamical processes at the stellar surface.
Our observations of the Ap star χ Psc made with
high spectral and temporal resolution indicate rapid
spectral variability. The variability time scale is 0d. 012.
The full amplitude of the Doppler-shift variations
differs from one spectral line to another and varies from
5 to 50 km/s. Given the variation time scale, these
amplitudes imply that the sizes of active regions in the
atmosphere of χ Psc range from 5000 to 50000 km.
Note that these are typical sizes for sunspots.
Figures 2–4 demonstrate wavelike variations in Vr
and Wλ for the peculiar SiII and CrII lines and Hβ. The
time dependence of the variability amplitude also suggests
the presence of wavelike impulsive processes acting at the
star’s surface. Judging from these figures, the characteris-
tic times for the development and decay of these oscilla-
tions are from one to two hours. Rotation alone is too slow
a process to explain such rapid, transient oscillations. The
observed effect could be associated with a resonance
between certain oscillation processes. For example, simi-
lar oscillations on the Sun have speeds of the order of the
sound speed (10–30 km/s) [14].
Vr, km/s
15
5
–5
–15
–25
0
0.5
1.0
Phase
1.5
2.0
Borra and Landstreet [15] showed that, as a rule,
there is little evidence for irregular magnetic-field
oscillations, except for oscillations with amplitudes of
the order of 200 Oe observed in some Ap stars. There is
Fig. 5. Radial velocities of the CrII 4836 Å (filled circles)
and 4558 Å (open circles) lines folded with the photometric
period of P = 0d.5853 [7].
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DISCOVERY OF RAPID SPECTRAL VARIABILITY
Vr, km/s
743
15
10
5
0
–5
–10
–15
–20
–25
0
0.5
1.0
Phase
1.5
2.0
Fig. 6. Radial velocities of the MgII 4481 Å (filled triangles), CrII 4836 Å (filled circles), 4558 Å (open circles) lines folded with
the period P = 0d. 012.
Vr, km/s
40
30
20
10
0
–10
–20
–30
0
0.5
1.0
1.5
2.0
Phase
Fig. 7. Radial velocities of the H , H , MgII 4481 Å, CrII 4836, 4558 Å, FeI 5383 Å, SiII 5041 Å, and TiII 4571 Å lines folded with
α
β
the period P = 0d. 012.
Wλ, Å
0.65
0.55
0.45
0.35
0.25
0.15
0.05
MgII 4481 Å
CrII 4558 Å
TiII 4571 Å
CrII 4848 Å
FeII 4991 Å
FeII 5316 Å
FeI 5383 Å
–0.05
0
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0
Phase
Fig. 8. Equivalent widths of lines in the spectrum of χ Psc folded with the period P = 0d. 012. Fourth-order polynomial trends are
drawn for the MgII 4481 Å and FeII 5316 Å lines.
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ALIEV, ISMAILOV
little doubt that, for most of these stars, atmospheric
oscillations on long time scales can be explained using
inclined rotator models. However, our results have
shown the presence of shorter-term atmospheric varia-
tions in the atmospheres of Ap stars.
REFERENCES
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Our observational data lead us to the following con-
clusions.
(1) We have detected rapid variability of the spec-
trum of χ Psc with a quasi-period of 0d. 012 ± 0d. 0005.
This variability is observed both in the line radial veloc-
ities and in other line parameters.
9. S. G. Aliev, in Magnetic Ap Stars [in Russian] (Elm,
(2) The observed variations with a quasi-period of
Baku, 1975), p. 80.
P = 0d. 012 have the form of damped or amplified oscil-
lations. The amplitudes of these oscillations are compa-
rable to those of acoustic waves on the Sun.
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[Astron. Lett. 19, 315 (1993)].
12. G. A. Galazutdinov, Preprint No. 92 (SAO, 1992).
(3) On the whole, the variations of the spectral
parameters can be described by the photometrically
determined axial-rotation period. However, the radial-
13. T. G. Barnes, III, T. J. Moffet, and M. H. Slovak, Publ.
Astron. Soc. Pac. 98, 223 (1986).
velocity variations are significant whether they are 14. A. B. Severnyœ, in Some Problems of Solar Physics [in
Russian] (Nauka, Moscow, 1988), p. 102.
folded with the photometric period or with the period
we have inferred in this paper.
15. E. F. Borra and J. D. Landstreet, Astrophys. J., Suppl.
Ser. 42, 421 (1980).
(4) Based on the observed variations of the spectral-
line parameters, we estimate the scale lengths of active
formations at the stellar surface to be 5 to 50 thousand km.
Translated by A. Dambis
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