COMMISSION 27 OF THE I. A. U.
        INFORMATION BULLETIN ON VARIABLE STARS
                    NUMBER 613

                                    Konkoly Observatory
                                    Budapest
                                    1972 January 20


            THE COMING SHELL PHASE OF AX MONOCEROTIS


The 6.^m8 star AX Monocerotis = HD 45910 (1900.0: alpha=6^h25^m12^s, delta=
+5deg56'.1; 1972.0: alpha 6^h29^m03^s, delta=+5deg53'.4) has long been known to
have strikingly variable spectrum. For example, the hydrogen lines are at
times broad, shallow, and structureless; at other times they have a pronounced
structure similar to that of P Cygni or a very slow nova. The absorption
component of the P Cygni profile is occasionally double or even triple, with
very strong, deep and rather narrow components. Moreover, a metallic shell
spectrum appears at times as a transient feature.

Mrs. A.P. Cowley (1) showed that this metallic shell spectrum appears
periodically (although with variable duration and intensity) with the orbital
period of the system, 232.5 days. AX Monocerotis is a binary: the primary
component is something like B3:IV:ep, while the secondary, visible in the
visual region of the spectrum, is about K2:II:. The metallic shell spectrum
always appears roughly a quarter period before conjunction with the K star in
front. The metallic shell may be associated with gas streaming from the K
giant to the B component. In our model of the system, soon to be published,
the K star originally was the more massive component, expanded to the Roche
limit, has lost a considerable part of its mass, and is now near the end of the
rapid phase of mass loss. The material that flows from it first forms a disk
or ring around the B star and eventually is accreted by it. The P Cygni
profiles of the emission lines indicate that the process of accretion is not
quiet and leads to occasional outbursts or at least vivid surface activity. The
metallic shell is probably associated with the original stream rather than with
the gaseous structure enveloping the B star.

Systematic observations of AX Mon may verify this model and, if it is correct
in principle, they may furnish very valuable material about the mass transfer
in close binary systems and about the process of accretion of the material, as
well as about the interaction between the B star and the circumstellar disk or
ring around it.

A very favorable time for such observations is right now. Next conjunction with
the K star in front will be on May 29, 1972. According to Cowley's 1962
observations, the metallic shell lines should appear around phase 0.80 (April
13) and attain their maximum strength around May 13. This would be too late
for observations. However, in most cases the shell phases seem to have come
earlier. Boyarchuk and Pronik (2) observed the onset of the shell lines at
phase 0.65, which will be about March 9, and in the past it came as early as
phase 0.6 (February 26). Magalashvili and Kumsishvili (3) observed in four
cycles a rather pronounced decrease in the U magnitude of AX Monocerotis by
about 0.^m4, while the B and V magnitudes showed little change. This
phenomenon may be due to increased continuous absorption beyond the Balmer
limit. The decline began around phase 0.55 (corresponding to February 15 this
year), and the minima invariably occurred at phase 0.75 (April 2).

Spectroscopic and photometric observations as well as scanning the line
profiles for rapid changes may be very rewarding in the coming weeks.


MIROSLAV PLAVEC
President of Commission 42
University of California, Los Angeles

PETR HARMANEC
Astronomical Institute, Ondrejov,
Czechoslovakia

References:

(1) Cowley A.P., 1964, Ap.J. 139, 817. [BIBCODE 1964ApJ...139..817C ]
(2) Bojarcuk A.A., Pronik I.I., 1967, Izvestija Krymskoj Astrof. Obs. 37, 236. [BIBCODE 1967IzKry..37..236B ]
(3) Magalasvili I.L., Kunsisvili Ja. I., 1969 Bull. Abastumani No. 37, 3. [BIBCODE 1969AbaOB..37....3M ]