Non-Periodic Phenomena in Variable Stars
IAU Colloquium, Budapest, 1968
ON THE GROUP OF YOUNG STARS IN THE SOLAR VICINITY
M. A. ARAKELIAN
Byurakan Astrophysical Observatory, Armenia, USSR
The problem of the origin of flare stars in the immediate vicinity of the
sun is rather interesting, since the flare phenomenon is usually associated
with stars which are almost certainly young. Two points of view have been
expressed concerning the origin of these stars. The first one suggests that flare
stars in the solar vicinity are members of a stellar association which once
existed in the region where at present the sun is located [Ambarzumian
(1957), Haro and Chavira (1965)]. The other point of view suggests that UV
Cet stars are outcomers from the nearest stellar associations which exist now
(Herbig 1962). If the latter is thee case these stars should be distributed more
or less uniformly in some volume exceeding considerably that of a single
association.
In this report some data are presented, which seem to be in favour of
the first of the mentioned possibilities.
I. The Space Density of the Flare Stars in the Solar Vicinity. The method
of the stellar space density determination was proposed earlier by the author
(Arakelian, 1968), which permits to receive some rather reliable estimates using
the distances of a comparatively small number of the nearest stars. This method
was applied to the determination of the luminosity function of K and M dwarfs
with hydrogen emission lines (Arakelian, in press). The total density of dwarfs
with hydrogen emission is found to be 0.055 pc^-3, while the value of 0.118
pc^-3 was obtained for the total stellar space density (Arakelian, 1968b).
Thus, the space density of dwarfs with hydrogen emission is about the half
of that of all stars.
The dependence of the ratio of the number of known flare stars to the
number of dwarfs with hydrogen emission upon the absolute visual magnitude
was obtained and used for the determination of the luminosity function of
stars known as flaring variables. This relation is presented by the following
linear function
n/N = (0.04 + 0.01) M_v - (0.12 +- 0.09), (1)
where n is the number of flare stars and N is the number of dwarfs with
hydrogen emission. The luminosity function of flare stars and the luminosity
function of stars with hydrogen emission has been determined using relation
(1). The space density of flare stars in the solar vicinity is found to be 0.027
pc^-3. This value must obviously be considered as a lower limit, since it may
increase with the accumulation of observational data on flare stars.
Nevertheless, even the data we have at present demonstrate high density of flare
stars in the solar vicinity.
Supposing that the density of flare stars is constant up to sufficiently
large distances and using the obtained luminosity function we may estimate
the number of flare stars brighter than the arbitrary apparent magnitude
m_v in a given solid angle with its summit in the sun. Let us assume this angle
to be 16 square degrees in order to make some comparisons of these results
with that obtained with the 40"-Schmidt telescope of the Byurakan Observatory
in the course of searching for flare stars in the region of NGC 7023
(Mirzoyan and Parsamian 1969). We obtained by numerical integration
N_16(m_v) = 3.5*10^-9*10^0.6m_v (2)
The Byurakan observations show that the number of discovered flare
stars is much less, than it should be, provided that the space density of these
stars is constant and equal to its value in the solar vicinity. Indeed, we have
from (2) that N_16(17) = 56.
The mentioned searches were made by means of consecutive exposures
used firstly by G. Haro and his collaborators (1965). Five minute exposures
were used at Byurakan and their limiting apparent photographic magnitude
was m_pg = 18.5. Thus, the limiting apparent visual magnitude of flare
stars at Byurakan plates may be supposed to be 17.0m.
On the other hand, only 9 flare stars were discovered at Byurakan during
40 hours of observation in the region of NGC 7023. One flare star in this region
was discovered earlier by. L. Rosino and G. Romano. It is very probable,
that most of these flare stars are actually members of NGC 7023, and not
field stars. Nevertheless, even neglecting this possibility one can see that the
number of discovered flare stars is much less than their expected number.
One may suppose that this deficiency in discovered flare stars is due to the
small duration of observations. Therefore we discuss the mathematical expectation
of the number of the flares themselves. Let us assume the mean value of the
time interval between those flares which are accessible for photographic
observations to be 100 hours (this value is apparently overestimated).
Then the mathematical expectation of the number of flares of 56 stars during
40 hours will be of the expected value. This deviation may be explained only
by the assumption that the value of the mean space density of the flare stars
at large distances, used in the calculation of the expected number of flares,
has been highly overestimated.
Thus, the value of the space density of flare stars previously referred
to must be attributed to the immediate vicinity of the sun only. Their density
at larger distances is much lower.
2. Kinematical Properties of Dwarfs with Hydrogen Emission. Ten years
ago A. Vyssotsky and Dyer (1957) and W. Gliese (1958) showed that the space
velocity dispersion of dwarfs with hydrogen emission was nearly one half
of that of the dwarfs of the same type without emission in their spectra.
Intending to use a larger number of stars, we studied the kinematics of these
stars on the basis of radial velocities (Arakelian, 1958). The conclusion of the
mentioned authors has been confirmed. It has been shown, thereby, that a
K-term was present in the velocity of these stars amounting to several km/sec.
This property makes the group of dwarfs with hydrogen emission similar to
known stellar associations. Since the expansion of the latter was predicted by
Ambarzumian (1949) and verified for two associations by Blaauw (1952)
and E. Markarian (1953).
3. The Stellar Luminosity Function in the Immediate Vicinity of the
Sun. The stellar luminosity function was determined on the basis of stars
nearer than 20 pc (Arakelian, 1968). The behaviour of this function is somewhat
unusual in the region of faint stars, since two maxima are present at M_v = 13
and M_v = 15. This property has apparently been noted by other authors, but
it has been attributed to random fluctuations. However, it has been shown
(Arakelian, 1968) that the probability of such fluctuations is rather small.
The maximum at M_v = 15 and the partial densities for M_v > 15 are
almost entirely due to stars with hydrogen emission. Therefore, one can
suggest that the luminosity function obtained in my above mentioned paper,
is a superposition of that of the field stars and that of the members of some
group.
Thus, the data presented above seem to suggest the presence of a group
of genetically related young stars in the solar vicinity.
REFERENCE
1) Ambarzumian, V. A., 1949, Astr. Zu., 26, 3.
2) Ambarzumian, V. A., 1957, Non-Stable Stars, p. 9. Yerevan.
3) Arakelian, M. A., 1958, Izvestia of Academy of Sciences of Armenian SSR, 11.
No. 5. 79.
4) Arakelian, M. A., 1968, Doklady of Academy of Sciences of USSR, 179, 555.
5) Arakelian, M. A., 1968a in press.
6) Arakelian, M. A., 1968b, Astrophysika, 4.
7) Blaauw, A., 1952, No. 433. Bull. astr. Inst. Netherl.
8) Gliese, W., 1958, Z. Astrophys. 45. 293.
9) Haro, G. and Chavira, E., 1965. Vistas in Astronomy, 8, 89.
10) Herbig, G. H., 1962. Symposium on Stellar Evolution, p. 45, La Plata.
11) Markarian, B. E., 1953. Soobsch. Byurakan Observatory, No. 11, 3.
12) Mirzoyan, L. V., 1969. Parsamian, E. S., 1969. Report on the present conference. CoKon No. 65 19
13) Vyssotski, A. and Dyer, E., 1957. Astrophys. J. 125., 297.
DISCUSSION
Herbig: What is the kinematic age of this cloud of Me stars around the Sun
as inferred from the velocity dispersion?
Arakelian: The age of this group is rather low, of the order of 10^6 years. This
fact is indeed the difficulty of the proposed point of view, since there
are no T Tauri stars in the vicinity of the Sun.