Non-Periodic Phenomena in Variable Stars
IAU Colloquium, Budapest, 1968
Of AND Be STARS
Introductory Report by
ARNE SLETTEBAK
Perkins Observatory, Ohio State and Ohio Wesleyan Universities,
Delaware, Ohio, USA
I. THE Of STARS
The Of stars are characterized by broad emission lines of N III 4634,
4641, and He II 4686, in addition to absorption lines shown by normal O-type
stars of corresponding spectral type. The presence of these emission features
can be understood in terms of selective fluorescence processes proposed by
Bowen (1935). Emission can also usually be found at H alpha and C III 5696 in
most Of stars, and at Si IV 4089 and 4116, Ni IV 4057, N V 4603 and 4619,
and He I 5876 in some Of stars (Underhill 1966).
Some 13 per cent of the O-stars listed in recent catalogues are of type Of,
according to Underhill (1966). No sharp distinction between the absorption-line
O-type stars and the Of stars exists however, as has been shown by Wilson
(1958), Underhill (1958a), and Kumajgorodskaya (1962).
The extent of the shell producing the emission in Of stars has been
estimated by Struve and Swings (1940), Swings (1942), Oke (1954), Underhill
(1958b), Hutchings (1968a), and others. Generally, the emission appears to
arise from a shell not more than one or two stellar radii in extent. The existence
of P-Cygni line profiles with very broad emission features in the spectra of Of
stars suggests that these objects have expanding atmospheres with considerable
internal motions. Wilson (1958) found for the Of star lambda Cephei band
widths for He II 4686 and N III 4634-41 of the same order as observed in
the spectra of Wolf-Rayet stars, indicating Doppler velocities in excess of
1000 km/sec. Radial velocity measurements by Hutchings (1968a) of two Of
stars suggest extended accelerating atmospheres with velocities reaching 600
km/sec and beyond.
Struve and Swings (1940, 1941), Oke (1954), Mannino and Humblet
(1955), Underhill (1959), Kumajgorodskaya (1960), Hutchings (1967a, 1968a),
and others have commented on the variability of the spectra of Of stars.
The emission features show pronounced variations, while variability of the
absorption lines has also been reported. Evidence of extremely rapid
variations was first given by Oke (1954), who found emission-line intensities
in the Of stars HD 34656 and HD 190429 N varying in a matter of hours.
Oke pointed out that the variations of the N III and He II line profiles take
two forms: (a) the main part of the line varies considerably in shape; (b) the
main lines are accompanied by emission and absorption satellites on the red
and violet sides which come and go from plate to plate. More recently,
Hutchings (1967a, 1968a) has found similar variations.
In October of 1966, an attempt was made to observe rapid variations,
of the kind reported by Oke, with the 72-inch Perkins reflector of the Ohio
State and Ohio Wesleyan Universities at the Lowell Observatory in Flagstaff,
Arizona. Spectrograms of dispersion 40 A/mm at H gamma on Kodak IIa-O emulsion
were taken during a series of consecutive nights of the Of stars lambda Cephei,
9 Sagittae, HD 34656, and HD 190429 N. The spectrograms were considerably
widened in order to aid the detection and measurement of the diffuse and
shallow emission features. Line profiles of the N III 4634-41 and He II 4686
features for the four stars are illustrated in Figures 1-4. These are intended
only to show observed line profile changes; no correction for underlying
absorption has been made, nor are radial velocity differences shown.
Although the spectrograms are of rather low dispersion, rapid variations in
the line profiles are evident. These may occur from night to night, as in the
spectra of lambda Cephei (Fig. 1) taken on the nights of Oct. 22nd and Oct. 23rd,
or within a few hours, as shown in the lambda Cephei spectra of Oct. 24th.
In addition to the rapid variations, the great widths of the emission bands
noted by Wilson (1958) is confirmed by the present material. Brief comments about
the aforementioned Of stars plus two additional Of stars follow:
lambda Cep. Spectrograms taken of this O6f star with the Perkins 69-inch
telescope (28 A/mm at H gamma) in Delaware, Ohio prior to those illustrated in
Fig. 1 also show interesting changes. On Sep. 6, 1952, N III 4641 was stronger
in emission than N III 4634 and He II 4686, had a violet satellite emission
feature. Eight nights later, on Sep. 14, 1952, the two N III lines were more
equal in intensity, while the He II satellite line was much weaker than
previously. Again, on Nov. 6, 1956, the emission spectrum was similar to
that just described for Sep. 6, 1952, while by Nov. 24, 1956, the N III emission
was much stronger and broader and the violet He II satellite line had
disappeared.
In addition to the changes in the emission lines illustrated in Fig. 1,
the absorption lines also show rapid changes at times. On Oct. 25.28, 1966,
for example, the He I 4471 and He II 4541 lines appeared rather sharp relative
to their usual diffuse appearance.
9 Sag. The N III 4641 emission was generally stronger than the 4634
component in this O8f star, as shown in Fig. 2, but on occasion (i.e., Oct.
26.19) they appeared nearly equal. Meanwhile, the He II 4686 emission also
showed rapid intensity variations and actually disappeared on the night of
Oct. 25.19.
HD 34656. This O7f star showed essentially equal N III 4634 and 4641
components except on the night of Oct. 24th, when the 4641 feature was the
stronger of the two as illustrated in Fig. 3. On the night of Oct. 27th, the
N III emission was markedly weaker than on the other nights. The He II 4686
line appears in absorption in this star and was much sharper on the nights
of Oct. 24th and 27th than on the other three nights. No corresponding
pronounced changes in other absorption lines were detected.
HD 190429 N. Variations in the emission line profiles of this O5f star
are shown in Fig. 4. Rapid changes in the absorption lines also occured: the
He II 4541 and 4200 lines appeared more diffuse on the spectrogram of Oct.
27.08 than on the other plates.
Fig. 1. Variations in the profiles of the N III 4634-41 and He II 4686
emission features in the spectrum of the O6f star lambda Cephei
during six consecutive nights.
29 CMa. Spectrograms of 20 A/mm dispersion taken with the Perkins 72-inch
telescope on the nights of March 18 and 21, 1968 show a violet satellite
emission line to the He II 4686 emission feature on the 21st, which was not
present on the 18th.
zeta Pup. Both N III 4634-41 and He II 4686 show a complex emission
structure on 20 A/mm spectrograms taken with the Perkins 72-inch telescope on
the nights of March 18 and 21, 1968, but no obvious differences were visible.
Fig. 2. Variations in the profiles of the N 111 4634-41 and He 11 4686 emission
features in the spectrum of the O8f star 9 Sagittae during five
consecutive nights.
Fig. 3. Variations in the profiles of the N III 4634-41 emission feature and
He II 4686 absorption line in the spectrum of the O7f star HD 34656
during five consecutive nights
Rapid variations of the kind illustrated in Figures 1-4 should be
studied with high-dispersion spectra over continuous time periods and include
both intensity and radial velocity measurements. If, as seems probable at
present, the variations are non-periodic, the picture brought to mind is one
of large-scale, turbulent atmospheric motions with velocities of hundreds of
kilometers per second. Whether or not axial rotation plays a role is not clear
but it probably does not play the dominant role since sharp-lined Of stars
(as would be expected from those seen pole-on) are non-existent or very rare
(Slettebak 1956). The analogy between Of stars and Be stars (where rotation
is the dominant line-broadening agent) which is sometimes made should therefore
probably not be carried too far.
What then is the nature of the Of stars and how are they related to normal
O-type stars? The picture is not yet clear. There is evidence in favor of these
objects being somewhat more luminous than absorption-line O-type stars of
corresponding type (Roman 1951; Slettebak 1956; Kumajgorodskaya 1962;
van den Bergh 1968) but no general agreement (Underhill 1955). The recent
suggestion by van den Bergh (1968) that Of stars of a given spectral type are
older and hence presumably more highly evolved than are absorption-line O stars
of the same spectral type is interesting in this connection and should be
investigated further.
II. THE Be STARS
Some 10 per cent of the brighter B-type stars show emission lines of
hydrogen (Merrill and Burwell 1933) and are designated as Be stars. Of these,
about 10 per cent are Ia supergiant stars (Abt and Golson 1966). The latter
have emission at H alpha (sometimes also at H beta), which usually appears as a P
Cygni-type line profile, and which is often variable. A detailed study of the
variations for three B-type supergiants was made by Underhill (1961), who
found non-periodic radial velocity changes and suggested that the variations
are due to atmospheric motions.
Fig. 4. Variations in the profiles of the N III 4634-41 and He II 4686
emission features in the spectrum of the O5f star HD 190429 N during
five consecutive nights.
The remaining 90 per cent of Be stars are of much lower luminosity,
apparently lying a magnitude or two above the main sequence, and are
characterized by very large line broadening. The use of the term "Be stars"
in the remainder of this paper shall refer only to these objects. Struve (1931)
was the first to suggest that the emission lines in Be stars arise in gaseous
rings ejected from rapidly-rotating stars at the limit of instability, and this
interpretation still seems valid today.
The spectra of Be stars typically show variations. Among a group of 40 of the
brightest Be stars observed by McLaughlin (1961) for many years, only 8 failed
to show convincing evidence of spectral changes. In the terminology of McLaughlin,
the spectrum variations are of three kinds: (1) appearance and disappearance
of a shell absorption spectrum; (2) E/C variation; (3) V/R variation.
An enormous literature regarding spectrum variation in Be stars exists and only
a few authors and papers can be cited in the following paragraphs.
Absorption shell spectrum variation. When the axis of rotation of a Be
star is oriented such that the equatorial ring is in or near the plane including
the observer, an absorption-line spectrum arising from the ring or shell is
visible in addition to the emission lines. Be stars with this orientation are
called "shell stars", and the shell spectra strengthen and weaken as their
shells come and go. Such changes in some of the brighter shell stars have been
studied intensively - by Struve, Merrill, McLaughlin, Underhill and others.
There is evidence for an "oscillation" of 8-10 years during which the
equatorial shell appears and disappears (Merrill, 1956), but some shell stars
(gamma Cas, zeta Tau) behave much more erratically. The mechanism which triggers
the formation of a shell remains an unsolved problem.
E/C Variation. Changes in the ratio of intensity of the emission lines
to the neighboring continuous spectrum are designated as E/C variations.
Such changes are usually associated with the appearance and disappearance
of the absorption shell spectra for shell stars and also reflect the coming and
going of the shell. For stars seen nearly pole-on, these E/C variations are the
only spectral evidence of the shell phase. In a study of 8 Be stars, Lacoarret
(1965) found E/C variations ranging between 3 and about 15 years.
Spectrograms taken with the Perkins 69-inch telescope of the shell star
Pleione in 1949 show a relatively strong absorption shell spectrum with weak
emission at H beta. Two years later, the absorption shell spectrum was much
weaker, while the emission was somewhat stronger. In 1957 the absorption
shell spectrum had disappeared, except for the cores of the Balmer lines,
while the emission at H beta was very strong. The appearance of the spectrum
in 1968 was again similar to that in 1957.
V/R Variation. In a typical Balmer-line profile shown by a Be star which is
oriented such that the equatorial shell is seen projected against at least
a portion of the photosphere of the star, a narrow central absorption
divides the wider emission into a violet and a red component. These two
emission components frequently show variations in relative intensity which
have been designated as V/R variations. In a study of 54 Be stars for which
spectrograms were available over a 24-year period, Copeland and Heard (1963)
found that two-thirds showed V/R variation.
The V/R variations often show apparent periodicities of several years.
McLaughlin (1963, 1966) found a period of about 10 years for the V/R variation
in 105 Tauri, and about 4.5 years for HD 20336 in the interval 1916-31.
The V/R variation behaved much more erratically for the latter star outside
the aforementioned time interval, however, at times stopping altogether.
A similar erratic behaviour was found for pi Aquarii, while beta^1 Monocerotis
started V/R variation with a period of about 12.5 years in 1924 after showing
no variation for 20 years prior to that year (McLaughlin 1958). Copeland and
Heard (1963) found periodic V/R variations in nearly half of their V/R
variables, with a mean period of 6.8 years. Although several have been proposed,
no model exists as yet which can satisfactorily explain these V/R variations
(McLaughlin 1961).
In addition to such long-period variations in V/R, much shorter variations
have been observed. Recently, Hutchings (1967b) has observed very rapid profile
variations in the spectrum of gamma Cas using photoelectric scanning techniques.
He finds significant changes in the H gamma profile in intervals of one
hour or less and suggests that a continuously changing velocity of expansion
covering most of the stellar surface is responsible (Hutchings 1968b).
An attempt to observe short-period variations in the spectra of two bright
Be stars was made by the writer in October of 1966. Using spectrograms
of dispersion 40 A/mm at H gamma taken on Kodak IIa-O emulsion with the
Perkins 72-inch reflector, 28 Tau and gamma Cas were each observed once and
sometimes twice during seven consecutive nights. No convincing changes
were observed in the spectrum of 28 Tau, which showed R slightly stronger
than V at H beta during the entire period of observation. The spectrum
of gamma Cas was also well behaved, for the most part, but a definite and
sudden change occurred during the night of Oct. 25th, as shown in Fig. 5.
Although V was usually somewhat stronger than R at H beta during the observation
period, the two emission components became nearly equal for a short time
on Oct. 25th before returning to normal again on the following night.
It should be emphasized that such relatively low-dispersion spectrograms
will only show the more obvious variations. Scanning techniques of the type
employed by Hutching (1967b) are probably required to bring out with
certainty the small-amplitude rapid variations. The existence of such sudden
and sporadic changes suggests again, as in the case of the Of stars, rapid
atmospheric motions. The Perkins observations suggest a difference in the
degree of atmospheric activity, however: the Be stars seem to act up only
on occasion whereas the Of atmospheres are apparently in a state of constant
turmoil.
Another way of attacking the problem of variability in the Be stars is
by looking for changes in the total light and colors of these objects. Although
gamma Cas showed changes of one magnitude or more during its shell phase, most
Be stars vary by much smaller amounts. Schmidt (1959) found variations in light
of, slightly more than 0.1 mag. for the shell star Omicron Andromedae and
concluded that it is a contact binary. Jackisch (1963) made UBV observations
of the Be stars 48 Per and 53 Per and found magnitude variations of 0.10 and
0.07, respectively, for the two stars. Feinstein (1968) observed 72 bright
southern Be stars in the UBV system over a three-year period. He found that
33 stars displayed variations in V larger than 0.06 mag. and that 21 changed
in U-B by more than 0.06 mag. The variations were found to be either progressive
or irregular, with some changes as large as 0.3 mag.
Fig. 5. Rapid V/R variations in HP in the spectrum of the Be star
gamma Cassiopeiae.
Recently, T. P. Roark has started a series of photometric observations
of selected Be stars with the Perkins 72-inch reflector to look for short-period
variations. He is measuring uvby colors on the Strömgren system, plus H alpha
spectrum scans with the Boyce scanner. Preliminary results show night to
night variations in H alpha and the Strömgren c_1 index for several of the stars,
with possible variations in (b-y).
The need for more observations is obvious. The most valuable information
would include simultaneous spectroscopic and photometric observations over
continuous periods of time. Only in this way can any periodicities be sorted
out from the random changes and the true causes of the variations finally be
understood.
REFERENCES
Abt, H. A. and Golson, J. C., 1966, Astrophys. J. 143, 306.
Bowen, I. S., 1935, Astrophys. J. 81, 1.
Copeland, J. A. and Heard, J. F., 1963, Pub. David Dunlap Obs., 11, 317.
Feinstein, A., 1968, Z. Astrophys., 68, 29.
Hutchings, J. B., 1967a, Observatory, 87, 273.
Hutchings, J. B., 1967b, Observatory, 87, 289.
Hutchings, J. B., 1968a, Monthly Notices R. astr. Soc., 141, 219 (in press).
Hutchings, J. B., 1968b, Monthly Notices R. astr. Soc., 141, 329 (in press).
Jackisch, G., 1963, Inf. Bul. Var. Stars no. 40. (IBVS N°.40)
Kumajgorodskaya, R. N., 1960, Izv. Krym. astrofiz. Obs., 24, 91.
Kumajgorodskaya, R. N., 1962, Izv. Krym. astrofiz. Obs., 28, 135.
Lacoarret, M., 1965, Ann. Astrophys. 28, 321.
Mannino, G. and Humblet, J., 1955, Ann. Astrophys., 18, 237.
McLaughlin, D. B., 1958, "Etoiles á raies d'émission", U. de Liége, p. 231.
McLaughlin, D. B., 1961, J. R. ast. Soc. Can., 55, 73.
McLaughlin, D. B., 1963, Astrophys. J., 137, 1085.
McLaughlin, D. B., 1966, Astrophys. J., 143, 285.
Merrill, P. W., 1956, Vistas in Astronomy. (Pergamon Press), Vol. 2, 1375.
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Oke, J. B., 1954, Astrophys. J., 120, 22.
Roman, N. G., 1951, Astrophys. J., 114, 492.
Schmidt, H., 1959, Z. Astrophys., 48, 249.
Slettebak, A., 1956, Astrophys. J., 124, 173.
Struve, O., 1931, Astrophys. J., 73, 94.
Struve, O. and Swings, P., 1940, Astrophys. J., 91, 546.
Swings, P., 1942, Astrophys. J., 95, 112.
Swings, P. and Struve, O., 1941, Publ. astr. Soc. Pacific, 53, 35.
Underhill, A. B., 1955, Publ. Dom. astrophys. Obs. Victoria, 10, 169.
Underhill, A. B., 1958a, "Etoiles á raies d'émission", U. de Liége, p. 17.
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Underhill, A. B., 1959, Publ. Dom. astrophys. Obs. Victoria, 11, 283.
Underhill, A. B., 1961, Publ. Dom. astrophys. Obs. Victoria, 11, 353.
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DISCUSSION
Bakos: How does rotational broadening effect line profiles?
Slettebak: At the present time, rotational broadening and line broadening due
to large-scale turbulent motions cannot be distinguished from a study of
individual line profiles. - Both mechanisms broaden lines in the same
way. Therefore it is impossible to asses the relative importance of these
two broadening agents in the Of stars. Although large-scale turbulence
probably dominates, rotation may also play a role in the line broadening
of the Of stars.
Detre: We have some long runs of photoelectric measures of omicron Andromedae.
We have obtained only some random fluctuations smaller in amplitude
than 0.02 which are not correlated with Schmidt's period. The star seems
to be not an eclipsing binary.
As I have mentioned in my Introductory Report, there are some recent
theoretical papers considering magnetic effects in Of and Be stars, e.g.,
the paper by Hazlehurst on the magnetic release of a ring. Have we
some observational evidence about the shape of the shell?
Slettebak: In his study of Of stars (1954) J. B. Oke concluded from the
spectroscopic data that the emission lines probably arise in regions
where the fairly strong absorption lines are formed and not more than
one or two stellar radii above the star. I do not know of observational
evidence regarding the actual shape of the emitting shell region, although
it seems unlikely that this is ring-shaped, as in the Be stars.
Bakos: gamma Cas is a visual binary with separation of about 20". Would be of
interest to obtain UBV magnitudes of this secondary?
Sahade: You mentioned the appearance of satellite lines in the spectrum of 29
CMa. Perhaps it is in order to remember that 29 CMa is a spectroscopic
binary and that there are variations in the absorption line intensities
probably due to the opacity of the gaseous streams in the system.
Milone: Are any of the Of stars magnetic variables?
Slettebak: I do not know of any measurement of magnetic fields in these objects.
Feast: There appears to be a much higher ratio of Of stars to normal
O stars amongst runaway stars than amongst non-runaway stars. This could
point to higher than average masses for the Of stars though the
difficulties - determining velocities of Of stars must be borne in mind.
Hutchings: Line radial velocity measurements of the Of stars HD 151804,
HD 152408 show a range in velocities, correlated with the excitation potential
of the lines. This suggests a spherically symmetrical expanding envelope
and strong line profiles computed with such a model match the observations,
with a small rotation - v sin i of the order of 50 km/sec.
Almár: Are there any curve of growth analyses of Of stars showing large
turbulent velocity ?
Slettebak: I do not know.