Sky & Telescope - June 2021 - 59

The polarization induced by aligned
dust grains is nearly 50% over most of
the nebula. This effect can be seen visually by inserting a variable polarizing
ﬁlter into the light path. Experiment
by slowly rotating the ﬁlter to different
positions and look for changes in the
nebula's brightness.

Minkowski's Butterﬂy (M 2-9), or the
Twin Jet Nebula, is one of the most dramatic bipolar PPNe imaged by the HST.
It displays a delicate axial symmetry on
both sides of the nucleus and a dusty,
pinched waist. Age estimates place it
at around 2,500 years, which implies a
distance of around 4,000 light-years.
Astronomers usually classify M 2-9
as a symbiotic nebula. In this scenario,
the binary central star consists of a hot
white dwarf and a Mira-type companion. The latter loses mass via its stellar
wind or else transfers mass to the white
dwarf when it overﬂows its Roche lobe.
The white dwarf ionizes this gas, thus
mimicking a young planetary nebula.
Balick and collaborators' recent model
of M 2-9 using hydrodynamic simulations showed that a conical spray blasts
out at an incredible 200 km/s (450,000
miles/hr) in the polar direction. The gas
interacts with the pre-existing AGB wind
that is slower and denser, forming and
shaping a variety of features.
The Butterﬂy's " proboscis " or outer
lobe extends 1′ in both directions, so
N4

S1
S2
S3

ANIMATION: Go to https://is.gd/butterﬂy_animation to see the Egg Nebula's
evolution between 1997 and 2010.

which is responsible for the bipolar nature of the outflow. RA: 17h 05.6m Dec: -10° 09′.
~60′ x 60′

HD 154600
90
N3
N2
N1

Even after observing the entire NGC
(7,500+ objects), Contributing Editor
STEVE GOTTLIEB still enjoys chasing
exotic targets like pre-planetary nebulae.

WT MINKOWSKI'S BUTTERFLY The central object in this pre-planetary nebula is a binary star,
Outer
lobe

and 1,400 years ago at distinct points in
the binary star's orbit.
This Butterﬂy ﬂoats in central
Ophiuchus: You'll ﬁnd it 4.6° northwest
of 4th-magnitude Nu (ν) Serpentis and
a little more than 1° west-northwest
of 5.4-magnitude HD 155078. My
ﬁrst observation of it was 35 years ago
through a 13.1-inch reﬂector at 214×.
M 2-9 appeared as a thin, 30″-long
streak, bisected by a 14.5-magnitude
central star. A narrowband ﬁlter produced a slight contrast improvement.
In my 18-inch scope at 435×, two
thin wings, each 15″ to 20″ in length,
extend north and south from the
central star. The wings appear detached
from the star, and the northern knot
N3 occasionally pops into view. With
exceptional seeing at 565× in my
24-inch scope, N3 morphed into a short
bar perpendicular to the wing.
Despite their small angular size,
pre-planetary or post-AGB nebulae are
among the most mysterious and alluring objects captured by the HST. I hope
the three I've highlighted here whet
your appetite for more.

Inner
lobe

17h 30m

17h 00m

M14

OPHIUCHUS
Core

TYC 5647-607-1
103

Inner
lobe

M 2-9

HD 155078

SERPENS
CAUDA ν
-15°

Outer
lobe

TYC 5648-1148-1
96

M 2-9

-10°

16h 30m

M10

-5°

HD 154638
83

M12

Star magnitudes

PHOTO: ESA / HUBBLE / N ASA / JUDY SCHMIDT; DIAG R A M: RO M A NO CORR A DI E T A L. / ASTR ONOMY
AND ASTR OPHYSICS 2011; DSS: POSS-II / STSCI / CA LTECH / PA LO M A R OBSERVATORY

Wings of a Cosmic Butterﬂy

the nebula's total size is 120″ × 12″. At
the outer tips are blobs of compressed
gas (N4 and S4; see the diagram below).
The bright inner lobe is hourglassshaped and nested with two separate
shells: an inner " bulb " and a thin outer
" sheath. " Two brighter knots, N3 and
S3, are near the tips of the inner lobes
and appear ﬂattened in HST images. In
Balick's model, the knots formed in situ
and were not ejected by the core.
A 2011 study by Romano Corradi
(Instituto de Astrofísica de Canarias)
and colleagues compared 12 years' worth
of observations and found dramatic
changes in the Butterﬂy's inner lobe. Hα
and O III images showed the knot pairs
S1/S2 and N1/N2 marching along the
walls of the lobe from east to west. The
knots rotate around the symmetry axis
(the period is nearly one century) within
a " lighthouse " jet from the central star.
This pattern points to a symbiotic binary
source, whose orbital period matches the
rotation period of the knots. The phenomenon results from a collimated jet of
high-velocity particles that shocks and
excites the bulb walls on impact.
The thin equatorial region has two
expanding, ring-shaped structures of
molecular gas and dust with diameters
of 2.5″ and 7″. A 2012 investigation
headed by Arancha Castro-Carrizo (of
the Institut de Radioastronomie Millimétrique in France) found these rings
move in different directions, suggesting
two mass-loss episodes occurred 900

2
3
4
5
6

-20°
sk yandtelescope.org * JUNE 2 021

https://www.is.gd/but-terfly_animation https://www.is.gd/but-terfly_animation https://www.is.gd/but-terfly_animation http://www.skyandtelescope.org

Sky & Telescope - June 2021

Table of Contents for the Digital Edition of Sky & Telescope - June 2021