Australian Sky & Telescope — January 01, 2018

http://www.skyandtelescope.com.au 49

by Alan Plummer VARIABLE STARS

AAVSO web-based chart plotter (https://

http://www.aavso.org/apps/vsp/). Every

research paper on Z CMa I’ve ever read

uses AAVSO data, so I can assure you

that any observations you would like to

contribute will be appreciated.

■ ALAN PLUMMER observes from the

Blue Mountains west of Sydney, and

can be contacted at alan.plummer@

variablestarssouth.org

Z CMa and its

hidden companion

A binary with a ‘wild and unpredictable’ light curve.

W

hen we look into the
constellation of Canis Major
— past nearby Sirius and out
into the star fields of the Milky Way’s
Perseus Arm — we’re looking rimward,
away from the galactic centre. This field
is rich with young, open star clusters,
and bright and dark nebulae. At a
distance of around 3,500 light-years
we find the remains of a collapsing
molecular cloud rich in silicates, iron
and carbon, infalling onto a protostar
of around three solar masses. This
protostar is Z Canis Majoris. Typical
of these young objects (called FU
Orionis variables, or FUORS), it has an
accretion disk with polar jets.
But infrared observations reveal that
there is more to this system than meets
the eye. Only one hundred astronomical
units (a.u.) from the Z CMa primary is a
binary companion, an optically obscured
protostar of an uncertain mass (perhaps
16 to 38 solar masses) surrounded by a
dusty cocoon starting 20 a.u. out and of
50 a.u. radius. Protostars of this higher
mass are called EX Orionis stars (or
EXORS), and accretion disks and jets

X Z CMa is located

at 7h 3m 43.16s, –11°

33 ́ 06.2 ̋ (epoch

J2000). This chart

is approximately 1.5

degrees wide, and

comparison star

visual magnitudes are

shown with decimal

points omitted to avoid

confusion with faint

stars — so 72 denotes a

magnitude 7.2 star.

also are indicated. This star drives a jet

12 light-years long, clearly visible in sky

survey photographs.

The Z CMa system is bright, ranging

from 8th to 10th magnitude but with a

wild and unpredictable light curve driven

by accretion, mass ejection and variable

line-of-sight extinction implying a hole

in the EXOR’s cocoon. Most targeting

software will have Z CMa listed, or

it can be located with the aid of the