Non-Periodic Phenomena in Variable Stars
					 IAU Colloquium, Budapest, 1968


		     EXTREMELY YOUNG STARS

		     Introductory Report by

			 W. WENZEL

   Institut für Sternphysik, Sternwarte Sonneberg, DDR

It is not so easy to give an Introductory Report on a topic like that, 
where during the last years new empirical material grew rapidly, a topic 
which is still too young for a consolidation of our knowledge. I must 
therefore concentrate on some partial questions which, according to my 
opinion, seem of some importance. When we speak now of extremely young 
variables we mean, of course, the evolutionary age (which is very short) 
and not the absolute age of the objects. In this way a B-type star of 
10^6 years is no longer young, contrary to a K- or M-type star of the 
same age. Let us therefore call those stars very young ones which during 
their evolution have not yet reached the age zero main sequence, that is 
to say the gravitationally contracting stars.
The astronomers knew stars of that kind for a long time. They were 
hidden in the groups "T Tauri", "RW Aurigae" and "Nebular Variables". 
During the last fifteen years there was an increasing tendency to make 
sure that the above mentioned stellar types are, at least partially, 
such contracting stars.
Recently quite a few models of contracting stars have been computed. 
In this summary it is not our task to go into details as to these 
theoretical models. To-day the evolutionary tracks as well as the 
corresponding speeds in the HR-Diagram are known, at least in principle. 
We use to compare these evolutionary tracks with the HR-Diagrams of very 
young clusters or associations and in general we can notice a fairly 
good agreement. As always, difficulties will arise not before the 
conditions are more closely examined and the numerous details of the 
observations have to be explained.
Now the most important detail exists in the fact that a high percentage 
of the contracting stars in a young cluster is variable, but, as it 
seems, in a completely irregular fashion. This is the reason why we are 
engaged in this Colloquium also with "extremely young stars".
Another criterion of the variable contracting stars is their peculiar 
spectrum. At low dispersion the presence, for instance, of the emission 
line H alpha is used in a certain number of surveys for the search of 
very young stars. The question which percentage of the contracting objects 
is, in a given time interval, really constant in light (for example in a 
young cluster) and in which way these invariable stars differ physically 
from the variables is at the moment still open.
The same problem exists as to the presence of spectral peculiarities. 
And the matter becomes still more complicated by the fact that in a 
certain number of objects the amplitude of variability is subject to 
long-term variations whereby such a star may appear in constant light 
for some years. The time scale of this phenomenon and its frequency have 
still to be investigated.
Some research on young aggregates concerns the distribution of stars 
with respect to the masses or luminosities. This is important for us 
only in so far as the type of irregular light variation of a contracting 
star should undoubtedly depend on its mass (that is on its luminosity or 
on its spectral type). A detailed examination of this relation together 
with a statistical investigation of the various types of variables might 
therefore serve as a test for the duration of the state of variability 
or the initial mass-function, respectively. This must be emphasized 
because the variability of many faint objects is mostly easier to 
observe than abnormalities in the spectrum.
Knowledge of the bolometric corrections is, of course, necessary. Above 
all we must take into consideration the recent papers on the infra-red excess 
in T Tauri stars and related objects as well as the still insufficient 
knowledge of the intrinsic colours in other spectral regions.
In order to find the bolometric correction, especially in the infrared, 
and to explain the mechanisms of the variability we must take into account 
the influence of the surrounding interstellar material. In this respect 
the observation that the intensity of the H alpha-emission in faint T Tauri 
stars is correlated with the strength of the interstellar extinction in 
the immediate surroundings of the object in question points to the existence 
of a rather extensive sphere of activity of the interstellar medium 
(Götz, 1967). On the other hand there are hints that the intensity of 
the H alpha-emission is related to peculiarities of the irregular variations. 
But, it is true, the observations of different authors are in this case not 
all in good agreement and furthermore too few in number.
Nearly all well-examined contracting variables are in direct connection with 
clouds of interstellar matter. It must be emphasized that the investigators of 
variable stars should be very careful in classifying an object as "irregular" 
or "of RW Aurigae-type" if only insufficient observations are available.
The comparison of the variability of T Tauri stars and related objects 
inside and outside of interstellar clouds will supply another contribution 
towards finding the mechanism of variability. For there is no doubt that the 
light variation is partially determined by the circumstellar shells or clouds. 
These envelopes on the other band should possess a physical connection to 
the above-mentioned spheres of activity of the interstellar matter. But we must 
not forget that in this case the real age of the respective variable plays a 
part, as one might expect that the connection with interstellar clouds on 
the average decreases with increasing age.
At present we know with some confidence that stellar formation in a certain 
region could last for some time (for instance Orion-associations). It is
therefore an important task to analyse the light curves with respect to the 
different ages of the variables.
The analysis of the light curves must, of course, be accompanied by 
investigations on the spectral variations. It is known that the spectra 
of contracting stars are rich in peculiarities, originating partly from 
the stars themselves, partly from the extensive atmospheres, shells and 
circumstellar clouds. But here we do not discuss the different components 
of these peculiarities. We have already mentioned the abnormal distribution 
of intensity in the continuous spectrum, brought about by the various 
additional superposed continua.
In this connection the question concerning the presence of solid particles 
in the circumstellar shells (especially in the variables of R Monocerotis-type)
is of importance. The variable absorption effect, produced by such 
particles, is now and then taken for the interpretation of the 
variability in other types of variables (R Coronae and other carbon 
stars). If clouds of solid particles also play a role with T Tauri stars 
and related types, then, because of the short time-scale involved, we 
might have important hints as to the evolution of the above-mentioned 
shells by investigating the different kinds of the irregular variations.
So far we have mentioned the light variations of contracting young stars 
without giving details of these variations and their peculiarities. We will 
do this now more extensively.
At first we must notice that we are accustomed to describe the light 
variation as "completely irregular", although we know that there are in 
some objects temporary or permanent quasi-periodical phenomena which 
might be characterized by a certain length of the cycles. The search for 
other periodic components in the fluctuations is difficult because only 
very few accurate continuous series of observations are available.
As we are concerned with a large range of masses and evolutionary ages 
and as we are nevertheless inclined to consider the variable young stars 
as a whole, we must combine a great number of different forms of 
variability under one and the same aspect. We have a considerable number 
of classification schemes. Some of them are built up according to 
photometric characteristics, others partly according to spectral 
differences. The scheme, recommended in 1964 by Commission 27 of the IAU 
and included in the second Supplement of the General Catalogue of 
Variable Stars, represents a compromise in this respect. These 
classification principles of the irregular light variation (as far as 
presumably young stars are concerned) are the following: Spectrum early, 
intermediate to late, or similar to T Tauri; variation rapid, slow, or 
characterized by flares; with or without relation to diffuse nebulae.
I consider this scheme only as a tentative. Allow me to give some 
reasons.
1. The large difference between the light curves of the types T Orionis 
and, for instance, RW Aurigae is not properly expressed. In case the 
spectra of the respective stars were unknown, we would classify both 
objects as "Ins", that is "irregular observed in the region of diffuse 
nebulae and producing light variations of 0.5m to 1.0m in the course of 
several hours or days". It is in this connection without importance that 
RW Aurigae itself has just a distance of 2 deg from the nearest dark cloud. 
I quote this star only because it is well known.
To recall these types please remember the light curves of T Orionis 
(Parenago, 1955), DD Serpentis (Meinunger, 1967) and RW Aurigae 
(Kholopov, 1962), obtained from photographic and visual observations by 
several authors. Photoelectric observations will follow.
2. Another problematic case is presented by the so-called Is-stars 
(rapid irregular variables apparently not connected with diffuse 
nebulae). Extensive investigations have shown the number of these 
objects to be scarce in reality and it would be best to examine each 
newly-discovered Is-star meticulously whether it is correctly classified 
or not. In particular this is necessary for all those Is-stars, which 
lie within real T-associations or in their surroundings, for one had 
obviously to attribute a special astrophysical importance to these 
variables in case they were genuine.
An additional question of importance is the difference between Inb- or 
InT-stars on one side and the slowly, irregularly variable giants 
(abbreviated in the new catalogues by the symbol L) on the other side 
if, in routine work, the spectrum or the position in nebular regions is 
not properly investigated. This may be illustrated by the light curves 
of T Tauri (Ahnert, 1956) and the S-star AD Cygni (Beyer, 1948). In this 
field much observational work is still to be done.
There is also involved the question for the early phases of contraction, 
namely the configurations with very large radii and low temperatures at 
the beginning of those evolutionary tracks which are fairly well known 
at present. We must admit that for the time being we cannot identify 
these early phases with objects observed in the sky. Let me quote in 
this connection, without going into details, the Herbig-Haro-objects and 
the variable star FU Orionis. The outburst of that star (Wachmann, 1938) 
was interpreted as the last stage of the dynamical contraction of an 
opaque protostar before the quasi-hydrostatic contraction begins, but 
there are wholly different models as well.
Let me add now a few words about our photoelectrically observed light 
curves.
For a rough empirical classification of variable stars in the 
contracting phase the light curves shown up till now might be sufficient 
(proto-types for instance RW Aurigae, T Orionis, BO Cephei or 
DD Serpentis, T Tauri, FU Orionis). However, in order to investigate the 
fluctuations in detail we need in addition photoelectric observations in 
several colours, simultaneously with spectral observations, if possible. 
The different kinds being manifold, a lot of work is waiting for the 
observers.


		     RW AURIGAE

The light variations are of an extremely complex nature. We find the 
following components (Wenzel 1966):
Waves of several hours' duration, amplitude some tenths of one magnitude;
Symmetrical outbursts, duration one to two hours, amplitude roughly 0.1m;
Unsymmetrical flares, presumably originating from the M-companion; 
Fluctuations of some hundredths of one magnitude, apparently caused by 
variations of the emission line intensity;
Quasi-periodic fluctuations, cycle length roughly three days, amplitude 
0.5 to 1 magnitude.
The colour-luminosity-diagram (l.c.) shows a large intrinsic scatter, as 
well as the two-colour-diagram (l.c.). Both these diagrams give the effect of 
the emission lines and the abnormal continua.


		    WW VULPECULAE

The star WW Vulpeculae seems to be of T Orionis-type (Fig. 1), characterized 
by unperiodic minima together with slow and short fluctuations of the 
normal light. It would be interesting to look for a T-association or an 
aggregate of faint H alpha-stars in the neighbourhood of this object with the
spectral type A. The diagrams V/B-V (Fig. 2) and U-B/B-V (Fig. 3) show a 
much smaller scatter than it is the case with RW Aurigae.
In the diagrams V/(B-V) the direction of the main sequence in the respective 
interval of B-V is shown by the straight line; in the diagrams (U-B)/(B-V) 
the arrow indicates the interstellar extinction, which has not been applied 
to the observations.

  

		     Fig. 1

  

		     Fig. 2

  

		     Fig. 3


		     SV CEPHEI 

Apparently SV Cephei shows a certain resemblance to WW Vulpeculae, 
although the long-term variation of the so-called "normal light" is much more 
marked (Fig. 4). The small changes of the colour indices U-B and B-V in a range 
of two magnitudes for V (Fig. 5, 6) are very striking. Besides, we notice that 
slow fluctuations of the "normal light" or of the "medium brightness" are 
significant also for other extremely young variables.


		     T TAURI

The proto-type star T Tauri has shown in the course of our observations 
very small fluctuations between 10.2 and 10.5 in V (Fig. 7). This 
variability consists of a slow component (0.2m in approximately 100d) 
and a more rapid component (0.1m in 1 to 5 days). The changes of B-V 
are very small (about 0.05m) (Fig. 8), the changes of U-B somewhat 
larger (Fig. 9). According to observations, made on old plates, the 
amplitude of variability in this star was in former times much larger 
than it is to-day, and amounted then to nearly four magnitudes.

  

		     Fig. 4

  

		     Fig. 5

  

		     Fig. 6

  

		     Fig. 7

  

		     Fig. 8

  

		     Fig. 9

SU Aurigae has a light variation resembling WW Vulpeculae (Fig. 10) to a 
certain degree. This similarity is also manifested in the diagrams V/B-V 
(Fig. 11) and U-B/B-V (Fig. 12). It is remarkable, however, that the 
mean spectral type of SU Aurigae is G2, compared with spectral type A 
for WW Vulpeculae, T Orionis or SV Cephei.

  

		     Fig. 10

  

		     Fig. 11

  

		     Fig. 12


		  RY TAURI AND CQ TAURI

Let me close now with two very strange diagrams V/B-V. RY Tauri (Fig. 13) 
has two tendencies: the star moves vertically in the diagram (B-V remaining 
nearly constant), or, especially near the maximum, perpendicular to the 
direction of the main sequence.
With CQ Tauri (Fig. 14) the movement of the object across the diagram 
V/B-V is clearly curved: while getting brighter the star becomes at first 
redder, and then turns more blue.
I hope it was possible with these examples of our photoelectric work to 
underline in some way the variety of different forms which these extremely 
young variables show. In this discussion I have laid stress upon some aspects 
of the variability in brightness which might be observed most easily also in 
relatively faint objects. We must admit, of course, that remarkable spectral 
changes take place in these stars, too. To discuss these phenomena and to 
treat the theoretical mechanisms of variability I must leave, however, to my 
colleagues more versed in these fields.

  

		     Fig. 13

  

		     Fig. 14


		     REFERENCES

Ahnert, P., 1956, Budapest Mitt. 3, No. 42.(CoKon N°.42)
Beyer, M., 1948, Erg. Astr. Nachr. 12, No. 2.
Götz, W., 1967, Veröff. Sternw. Sonneberg 7, No. 1. 
Kholopov, P. N., 1962, Perem. Zvezdy 10, No. 6. 
Meinunger, L., 1967, Mitt. veränderl. Sterne 3, 20.
Parenago, P. P., 1955, Trudy gos. astr. Inst. Sternberga 25, 216. 
Wachmann, A., 1938, Astrophys. J. 35, p. 81. 
Wenzel, W., 1966, Mitt. veränderl. Sterne 4, No. 4.