COMMISSION 27 OF THE I. A. U.
           INFORMATION BULLETIN ON VARIABLE STARS
                        NUMBER 689

                                           Konkoly Observatory
                                           Budapest
                                           1972 June 27


MULTI-COLOUR PHOTOMETRY OF THE UVn-TYPE VARIABLE, NS ORIONIS, DURING AN
EXCEPTIONALLY LONG-LIVED OUTBURST


The flare star, NS Ori (=Ton.157) is an H-alpha emission-line object in the
Orion nebula aggregate (Haro 1953, 1968)^x. During a programme of photographic
UBVR photometry (Andrews 1970) NS Ori was noted as exceptionally bright on a
number of plates and the star appears to have undergone a major outburst
lasting about two months. Classified by Parenago (1954) from the
frequency, function of random observations of magnitude the star is given as
Class III (more frequently faint than bright) and the range is from m_pq=15.2
to 17.6 (Rosino and Cian 1962). NS Ori is characterized by varying emission
H-alpha strength and a conspicuous ultraviolet excess (Haro and Herbig 1955)
and is of particular photometric interest in that it appears below the main
sequence (Haro and Chavira 1969). The colorimetric behaviour of such objects
is exceedingly complex (See e.g. Sincheskul 1971) and they exhibit extreme
departure from theoretical pre-main sequence evolutionary tracks. Interpreted
as extremely young stars less massive than the sun and in the process of
gravitational contraction these objects present considerable problems, but
recent work suggests that they may be surrounded by infalling circumstellar
dust shells.

The light curve, two-colour and colour-magnitude diagrams for NS Ori (Fig. 1)
illustrates the persistent nature of the blue and ultraviolet excesses over a
wide range of apparent brightness. Seven nightly measures of UBVR are
available which, apart from measure No. 5, show U-B=-1.4+-0.3 and B-V=0.4+-0.1,
that is, approximately constant within the photoelectric accuracies. This
places NS Ori well above the normal U-B/B-V relation for main sequence stars,

                           [FIGURE 1]

                   Fig. 1. Outburst of NS Ori

                           [FIGURE 2]

   Fig. 2. Excursion of NS Ori in U-B/V-R diagram compared with mean data
   for 278 flare stars (vertical lines - See Text) and H-alpha emission
   stars in Orion.
   Emission Strength: Permanent, strong (filled circles); Variable,
   strong (triangles); Temporary or weak (open circles) according to
   Haro (1953).

and indeed, shows this star as an extreme case as regards the colour anomalies
of the flare stars and emission objects of the Orion aggregate (Andrews 1972).
See Fig. 2 in which the track of NS Ori is shown in relation to the U-B/V-R
distribution of Orion flare and emission stars. The presence of emission
H-alpha is seen to be an important spectroscopic criterion in establishing
which stars present the most anomalous U-B colours. In Fig. 2 the emission
stars are designated with symbols according to their relative emission-line
strength (Haro 1953), and the flare star distribution, excluding those in
which emission H-alpha has been observed, is depicted as vertical lines
representing the dispersion in U-B at constant V-R (in steps of 0.^m1). In the
colour-magnitude diagrams NS Ori remains well below the main sequence at both
maximum and minimum in B-V/V but moves redward with decreasing brightness in
V-R/V remaining essentially within the band defined by other flare and
emission stars, at all times elevated above the main sequence. The variable
exhibits the largest change in colour in V-R (about 1 magnitude) due to its
much smaller amplitude in the red spectral region. The latter result
emphasizes the importance of taking into consideration the variability of
emission stars in the evolutionary interpretation of the V-R/V diagram of young
aggregates (See e.g. Sedyakina 1971).


Armagh Observatory
15 June 1972

                                              A.D. ANDREWS

References:

Andrews, A.D., 1970, Bol. Obs. Tonantzintla y Tacubaya 5, No. 34, p. 195. [BIBCODE 1970BOTT....5..195A ]
Andrews, A.D., 1972, Unpublished.
Haro, G., 1953, Ap.J. 117, p. 73. [BIBCODE 1953ApJ...117...73H ]
Haro, G., 1968, Stars and Stellar Systems Vol. 7, Nebulae and Interstellar
   Matter (ed. Middlehurst, B.H. and Aller, L.H.) Univ. Chicago Press.
Haro, G. and Chavira, E., 1969, Bol. Obs. Tonantzintla y Tacubaya 5, No. 32,
   p. 59. [BIBCODE 1969BOTT....5...59H ]
Haro, G., and Herbig, G.H., 1955, Ibid, 2, No. 12, p. 33. [BIBCODE 1955BOTT....2l..33H ]
Parenago, P., 1954, Trudy Astr. Inst. Sternberg 25.  [BIBCODE 1954TrSht..25....3P ]
Rosino, L. and Cian, A., 1962, Contr. Oss. Astrof. Asiago N. 125. [BIBCODE 1962CoAsi.125....1R ]
Rosino, L. and Pigatto, L., 1969, Ibid. N. 231. [BIBCODE 1969CoAsi.231....1R ]
Sdyakina, A.N., 1971, Var. Stars Acad. Sci. U.S.S.R 18, No. 2. p. 213. [BIBCODE 1971PZ.....18..213S ]
Sincheskul, V.N., 1971, Ibid. 18, No. 2, p. 201. [BIBCODE 1971PZ.....18..201S ]

^x A single flare of amplitude 1.5m (p.g.) has been detected on the first of
two plates taken 2 hours apart (Rosino and Pigatto 1969) on 25/2/63, an event
apparently, mis-quoted in the Tonantzintla lists (Haro 1968) as an ultraviolet
flare of 2 magnitudes. NS Ori is also stated elsewhere as varying by greater
than 2 magnitudes in 19 hours (Haro 1953).