Non-Periodic Phenomena in Variable Stars
IAU Colloquium, Budapest, 1968
RY SGR DURING THE 1967-8 MINIMUM: A BLUE CONTINUUM AND
OTHER SPECTROSCOPIC AND PHOTOMETRIC OBSERVATIONS*
M. W. FEAST
Radcliffe Observatory, Pretoria, S. Africa
Apart from visual estimates of magnitudes, the amount of information
available on the R CrB type variables is surprisingly small. This is
particularly true of the changes that occur during light minima. The principal
published data are a very valuable series of spectra taken by Herbig (1949)
during the decline of R CrB itself to minimum in 1949, and an analysis
by Mrs. Payne-Gaposchkin (1963) of several coudé plates taken by Greenstein
during the 1960 minimum of the same star. There is evidently a considerable
need for rather detailed spectroscopic and photometric studies of individual
minima of the variables, especially as these may vary from star to star and
from one minimum to another. This is a preliminary report of observations
of RY Sgr during the present minimum.
At maximum light RY Sgr is very similar to R CrB itself, as is shown
by Danziger's (1965) high dispersion study. The star has been under visual
observation by de Kock for many years and he observed a decline in brightness
during July 1967. Since that time medium and low dispersion spectroscopy
and UBV photometry has been carried out with the Radcliffe 74-inch reflector
and the Cape 40-inch reflector as often as other work permitted. The present
minimum is the deepest recorded for 20 years and we are fortunate in having
secured a fairly good coverage of it. The star has not yet returned to maximum
brightness and a detailed analysis of the observations may take some while.
However a number of facts have emerged which seem to be of particular interest.
The initial drop of the star was very rapid, about 3.5m in 15 days. The
star was first observed spectroscopically at about 9.5m (V) during this decline
(that is about 3 magnitudes below maximum). It then showed a rich emission
spectrum which strengthened relative to the continuum as the star faded further.
At the first minimum (~ 10.3m) the spectrum (at 48 A/mm) showed some
350 emission lines on a fairly weak continuum. Most of the emission lines
correspond to lines seen in absorption at light maximum and are of Fe II,
Ti II, Sc II etc. There seems to be a qualitative correspondence to the emission
spectrum reported by Mrs. Payne-Gaposchkin in the initial phase of the
decline of R CrB itself in 1960 though a detailed comparison has not yet been
made. The continuum at this phase shows an interesting phenomenon which has not
apparently been reported before. From the red limit of ordinary photographic
plates to about 4000A, the continuum is weak and provides a relatively low
background to the emission lines. This could well be the residual stellar
continuum. By 4030A this continuum is quite weak and one would not expect
to see any continuum to shorter wavelengths even for an early type star.
However at about 4000A the continuum intensity increases considerably and
absorption lines are seen. The principal absorptions are H and K of Ca II.
These are not the normal broad stellar H and K lines seen at maximum light,
but much narrower lines showing structure suggestive of blending of several
discrete components. Narrow emission complicates the appearance but the
absorption components have predominantly negative velocity shifts
(up to ~ 200 km/sec).
Apparently these absorptions are formed in shells or streams moving
away from the stellar surface.
The increase in the continuum intensity shortward of 4000A together with
the absence of the normal wide stellar H and K absorptions makes it rather
difficult to explain the continuum in this region as normal (thermal) radiation
from the star. Rather it appears necessary to look for some interpretation
involving an atomic or a molecular emission continuum. In addition to this
continuum there is an indication of banded structure in the 3880A region.
These are almost certainly emission bands of CN. CN was first seen in
emission by Herbig in R CrB during the 1949 minimum.
The most likely kind of process for the continuum is perhaps free-bound
transitions (of electrons), and if we consider only transitions involving ground
states then the energy involved (i.e. 3.1 eV for a wavelength cut off of about
4000A) drastically limits the possible emitters. The predicted continuum which
appears most plausible as an identification is the electron attachment spectrum
of CN. The electron affinity of CN is 3.1 eV and the inverse process of the
one considered here (i.e. electron detachment of CN-) has been considered
by Branscomb and Pagel (1958) as a possible source of opacity in cool carbon
stars. Quantitative work on the shape of the continuum for a comparison
with theory might enable this suggested identification to be tested. A possible
contribution at shorter wavelengths (< 3900A) from the electron attachment
spectrum of C_2 should also be borne in mind.
After the steep fall to 10.3m the light curve goes through a substantial hump.
The chief spectroscopic characteristic of this hump is the increase of the normal
continuum with respect to the emission lines. If we follow Mrs. Gaposchkin's
interpretation of R CrB then this would correspond to a brightening of the
underlying star but it should be noted that the H and K absorptions still have
the abnormal appearance discussed above and the continuum in this region may
still be chiefly non-thermal (CN- ?). Quantitative work should decide whether
Mrs. Gaposchkin's explanation in terms of the interplay of independent
absorption and emission spectra holds in the present case. Certainly the
appearance of the spectrum is most peculiar at certain phases. For instance
near the top of the hump (~ 9^m) there is a practically continuous spectrum,
the only very strong absorptions being Mg II and C I (besides shell, Ca II).
Other emissions and absorptions are very weak. As the star fades again
the continuum drops relative to the emission. The non-thermal continuum now
seems to have gone and the continuum observed is probably a residual stellar
continuum. The sharp emission spectrum is now quite different from that
observed earlier, being qualitatively similar to that observed by Herbig during
the faint phase of R CrB in 1949. The main feature of this spectrum is
the weakness of Fe II and the strength of Ti II and Se II emissions.
Actually there is no abrupt change in the sharp emission spectrum but a gradual
smooth change from the earliest observations onwards. The overall strength
of this emission spectrum seems to fall off with time rather independent of
the magnitude of the star (e.g. independent of the occurrence of the hump).
This is similar to the falloff in the strength of the emission spectrum of R CrB
in 1960 (as reported by Mrs. Gaposchkin). In addition in RY Sgr there is at
the same time a gradual change in the relative intensities of the lines. The
changes effect adjacent lines and cannot therefore be attributed to the interplay
of a fading emission spectrum of fixed relative intensities with a fixed absorp-
tion spectrum showing changing reddening. The general behaviour is a reduction
of the relative intensities of lines of high excitation potential but the
detailed behaviour is rather complex.
These observations may well be consistent with Mrs. Gaposchkin's suggestion
that the emission lines originate in the outer parts of the star (a chromosphere)
from which the source of excitation has been cut off (by obscuration of the
central star or otherwise). Two points are worth noticing in this connection.
Firstly, because the emission spectrum is found in RY Sgr to be varying quite
rapidly with time, it is doubtful whether the physical conditions of the
chromosphere, even in the earliest phases, correspond to chromospheric
conditions at light maximum. This probably also applies to the chromosphere
parameters deduced by Mrs. Gaposchkin for R CrB. Secondly, the decay of
an emission region suddenly cut off from its source of excitation would appear
an ideal place for the production of an electron attachment spectrum, such
as that of CN was postulated earlier.
As RY Sgr fades towards minimum broad emission lines of H and K (Ca II),
3888 (He I) and the D lines were seen. The star faded to 13.6m (V) before going
into conjunction with the sun and was picked up at nearly the same magnitude
a few months later.
The recovery in brightness is characterized spectroscopically by the gradual
disappearance of the last vestiges of the emission line spectrum and a return
to essentially the normal maximum absorption spectrum (by ~ 9.5m (V)).
Photoelectrically the rise is seen to progress in a number of smooth waves
with a period of about 30 to 40 days. Similar waves in RY Sgr were suggested
by Jacchia (1933) from visual observations. These waves are seen in V, B -V
and U-B. No extensive photometry at maximum light has apparently been made but
visual observations indicate variations of amplitude ~ 0.5m and period ~39 days,
quite similar to those observed during the rise. It has been suggested that
these are cepheid like variations (i.e. pulsations). If this is so then the star
appears to be pulsating in a normal (maximum) fashion when at least 5 magnitudes
below maximum. This (together with the recovery of the normal spectrum well
below maximum) would suggest that, at least at this stage, the diminution
of light cannot be due to some phenomenon in the star itself but must be
caused by some form of obscuration above the stellar surface (e.g. O'Keefe's
(1939) hypothesis of graphite absorption).
The colour changes in the UBV system are remarkable. At the bottom of the
first rapid decline the star is quite blue (B-V ~ + 0.24, U-B ~ -0.60).
This is at least partly due to the effects of the (non-thermal) blue continuum.
The complex variations in the colours on the descending branch of the light
curve are probably largely caused by emission line effect. During the later
stages, as the star rises again and when the emission lines are probably not
affecting the colours to any great extent, the star is very red (reaching B-V
~ +1.48 U-B ~ +1.32) and is also performing loops both in the two colour
diagram and the V, B-V diagram. These results are best interpreted as due
to some form of intrinsic stellar variability together with heavy overlying
reddening and obscuration.
While RY Sgr was faint it was noticed to have a faint companion about 12"
distant V = 15.69 B-V = +0.79 U-B = +0.15. After allowing for a small amount
of reddening (cosecant law) this star lies close to the unreddened main
sequence relation in the two colour plot and fitting to the main sequence
indicates an absolute magnitude of about +5.1m (V). If the star is a physical
companion then the absolute magnitude of RY Sgr at maximum is about -4^m (V).
This would be quite an acceptable absolute magnitude and it would appear
worthwhile making a determined effort to decide if the companion is really
physical. Some direct photographs have been taken by Dr. P. J.Andrews to serve
as first epoch plates for the purpose.
REFERENCES
Branscomb, L. M. and Pagel, B. E. J., 1958, Mon. Not. R. astr. Soc., 118, 258.
Danziger, I. J., 1965, Mon. Not. R. astr. Soc., 130, 199.
Herbig, G. H., 1949, Astrophys. J., 110, 143.
Jacchia, L., 1933, Publ. Oss. astr. Univ. Bologna, 2, 173.
O'Keefe, J. A., 1939, Astrophys. J., 90, 294.
Payne-Gaposchkin, C., 1963, Astrophys. J., 138, 320.
* This discussion is based on observations in Pretoria and the Cape
chiefly by J. B. Alexander, P. J. Andrews, R. M. Catchpole, P. Corben,
D. H. P. Jones, R. P. de Kock, T. Lloyd Evans, J. W. Menzies, E. N.
Walker and the writer. Several of these workers are participating in a
study of the observations and it is hoped to publish a detailed joint
paper at some later date. I am very grateful to my colleagues for
allowing me to use the data for this preliminary discussion and for
valuable conversations.
DISCUSSION
Fernie: Have you made observations of any other southern R CrB stars?
Feast: A few spectroscopic observations were made some years ago mainly of
S Aps and W Men (a member of the Large Magellanic Cloud). It is intended
to carry out further spectroscopic, and possibly also photoelectric work.