Sky & Telescope - February 2023 - 17

JOSEPH R. CALLINGHAM / ESO
researchers. Unlike the quick motion that was apparent in
dust plumes around other Wolf-Rayet binaries, the dust pattern
surrounding Apep was hardly moving, in contrast to the
measured speed of the gas in the system. Astronomers had
expected that any new or recently formed dust would move
in concert with the gas in which it was formed. Instead, Apep
apparently had two winds moving at different speeds.
Callingham's team proposed that what we're seeing is a
system in which the Wolf-Rayet star itself has both a slow
equatorial wind and a fast polar wind. Such an anisotropic
state can arise when a star is rotating at a near-critical
rate. This discovery has led to Apep being regarded as the
Milky Way's strongest candidate for one day exploding as a
gamma-ray burst.
Future Studies
Supernovae aside, questions remain about Wolf-Rayets and
their dust production. Thus far, astronomers have only
observed dust formation in carbon-rich binaries and not in
nitrogen-rich binaries. Yet modeling suggests that a carbonrich
environment is not a strict requirement for colliding
winds to create dust.
Also, not all WC binaries produce observable dust. Gamma
Velorum - which at magnitude 1.8 is the brightest WolfRayet
system in our night sky as well as the closest, lying
some 1,000 light-years from us - is a tight binary, yet it
displays none of the dust formation seen in other collidingwind
binaries.
Astronomers now think that Wolf-Rayets are a significant
source of dust in the interstellar medium, particularly in
galaxies where other prominent sources (such as aging asymptotic
giant branch stars) haven't yet had the time to make their
mark. Highlighting their importance, Ryan Lau (Institute of
Space & Astronautical Science, Japan Aerospace Exploration
Agency) and colleagues secured early science observation time
with the James Webb Space Telescope to further explore the
WOLF-RAYET HUNT Here are eight
WR stars worth seeking out, including
Gamma Velorum, which is the closest
known. The others on the list are all surrounded
by associated nebulae; some
may be sighted visually with filters, but
all are rewarding imaging targets. Several
entries may require Northern Hemisphere
observers to travel to lower latitudes in
order to bag them. The first three appear
in the evenings in February and March.
The WR numbering system comes
from the Sixth Catalogue of Galactic
Wolf-Rayet Stars, published in 1981. The
latest catalog of known WR stars in the
Milky Way, from which these data have
been compiled, is maintained at
https://is.gd/WRcatalog.
APEP This infrared composite image shows the stunning dust spiral
nicknamed Apep. The lower " star " is the unresolved Wolf-Rayet binary
that produces the coil of dust. The upper star is a companion star in the
triple system.
impact that these stars' dust has on the surrounding interstellar
medium, and how that shapes the next generation of
stars (image, page 14). Although they live fast and die young,
Wolf-Rayets' legacy may last for billions of years.
¢ Contributing Editor GREG BRYANT sometimes takes a long,
hard look at the closest Wolf-Rayet star, Gamma Velorum, on
clear nights from Sydney, Australia. His most recent S&T article
was " Open Cluster Exoplanets " (July 2021).
Wolf-Rayet Hunt
WR No.
6
7
11
18
23
75
134
136
Other Name(s)
EZ CMa,
Sh 2-308
NGC 2359
Gamma Velorum
NGC 3199
WR 23 Nebula
RCW 104
WR 134 Nebula
NGC 6888
HD
50896
56925
68273
89358
92809
147419
191765
192163
Right ascension and declination are for equinox 2000.0.
Mag(v)
6.9
11.4
1.8
10.8
9.1
10.9
8.0
7.4
RA
06h 54.2m
07h 18.5m
08h 09.5m
10h 17.0m
10h 41.6m
16h 24.4m
20h 10.2m
20h 12.1m
Dec.
-23° 56′
-13° 13′
-47° 20′
-57° 55′
-58° 46′
-51° 32′
+36° 11′
+38° 21′
sk yand tele scope .o r g * FEBRUARY 2023 17
https://www.is.gd/WRcatalog http://skyandtelescope.org
Sky & Telescope - February 2023

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