Download Core-collapse supernovae and their massive progenitors

Crowther, Smartt: Meeting report
Stellar winds and evolution
Relative to lower mass stars, the evolution of
high-mass stars is complicated by (a) the metallicity dependence of their radiatively line-driven
stellar winds, causing weaker winds at low metallicity, and (b) their initial rotational velocities,
providing rotationally induced mixing within
their interiors. It is only within the past decade
that allowances for both effects have been made
within evolutionary models. O star winds are
driven primarily by CNO and Fe-peak elements,
with a predicted dependence of M· ∝ Z0.7, that is
supported by empirical mass-loss rates for stars
within the Milky Way, LMC and SMC (e.g. figure 1) drawn from the VLT FLAMES survey of
massive stars (Evans et al. 2005, 2006).
The agreement between predicted and empiriA&G • February 2007 • Vol. 48 Core-collapse
supernovae and
their massive
progenitors
In this summary of the 10 November 2006 RAS Specialist Discussion
Meeting, organizers Paul Crowther and Steve Smartt consider
aspects of massive stellar evolution, their dependence upon stellar
winds, progenitor models and explosion mechanisms. In addition,
observations of core-collapse supernovae were discussed alongside
the question of whether supernovae form significant quantities of
dust, of relevance to the detection of dust in star-forming galaxies in
the early universe.
1: Empirical wind momenta
(Dmom = M·v∞√R) for luminous
O stars in the Milky Way,
LMC and SMC (dwarfs: black
circles, giants: blue triangles,
supergiants: red squares)
from Mokiem et al. (2006) and
theoretical predictions (dotted
lines) for SMC metallicity from
Vink et al. Good agreement is
achieved after correction by
a factor of ~2 to allow for the
sensitivity of strong winds to
wind clumping (open symbols).
For the Milky Way and LMC,
the empirical metallicity
dependence (dM/dt ∝ Z0.8)
agrees well with theory
(dM/dt ∝ Z0.7) (Mokiem et al.
2007).
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log Dmom
M
assive stars form in star clusters within
star-forming galaxies, pollute the
interstellar medium and inject energy
and momentum via powerful stellar winds and
core-collapse supernovae. Direct detection of
massive stars via the UV continua and indirectly
via ionized H ii regions provides some of the
most stringent constraints upon the physical
properties of galaxies at high-redshift. Within
the past few years a direct connection has been
established between certain core-collapse supernovae (SNe) and gamma-ray bursts (GRBs), supporting the collapsar model in which the GRB
results from the death throes of a rapidly rotating carbon–oxygen (Wolf–Rayet) star.
The Initial Mass Function favours the formation
of low- and intermediate-mass stars over highmass stars. The division is usually set at 8 M⊙ – the
boundary between stars ultimately forming a CO
white dwarf or producing a Fe-core and under­
going a core-collapse SNe. Spectroscopically,
stars with initial masses of 8–25 M⊙ are B dwarfs
on the main sequence, or O dwarfs at higher initial mass, and are distinguished by the presence
of strong stellar winds from radiation pressure,
arising from their high surface temperatures and
luminosities. These strong winds have a major
effect upon the evolution of massive stars.
High-mass stars possess convective cores and
radiative envelopes, a situation reversed in the
Sun and low-mass stars. While there is energy
transport from the convective and radiative
regions, only the convective core participates in
nuclear reactions, unless hydrogen-rich material
is mixed downwards from the outer zones. Once
the core hydrogen is exhausted, the star leaves
the main sequence and becomes a blue supergiant, and ultimately a red supergiant (RSG) for
stars with initial mass up to perhaps 20–30 M⊙.
Observationally, there is an absence of luminous
RSGs, known as the Humphreys–Davidson limit,
such that initially more massive stars circumvent the RSG phase, pass through a Luminous
Blue Variable stage, before ending their life as
Wolf–Rayet stars.
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4.0
cal mass-loss rates of O stars is impressive, albeit
subject to a modest (factor of two) reduction in
empirical results from wind clumping in the Hαforming region. The origin of wind structure is
thought to be due to the intrinsic instability of
radiatively driven winds. However, establishing
robust clumping factors for O stars remains challenging. Indeed, far-UV diagnostics suggest massloss rates of up to a factor of 10 less than predicted
rates. Clumping aside, empirical mass-loss rates
support the theory of radiatively driven winds of
O-type stars, although the situation for B supergiants is less satisfactory. Optical Hα line profile
and radio free-free continua-derived empirical
4.5
5.0
5.5
log (L/LSun)
6.0
6.5
mass-loss rates fall substantially below theoretical predictions; correction for wind clumping
would make the comparison still worse.
The FLAMES survey also provides valuable
information on the initial rotational velocities of O stars. Evolutionary models have been
calculated for various initial rotational velocities. Rotation rates of Milky Way O stars are
well known, but their strong winds make the
stars spin down rapidly, while few Magellanic
Cloud measurements were available before the
FLAMES survey. A comparison between LMC
and SMC stars, drawn from a homogeneous
dataset, indicate higher rotational velocities for
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Crowther, Smartt: Meeting report
the SMC stars, typically 175 ± 100 km s–1 (Mokiem et al. 2006). Rotational velocities have not
been significantly affected by stellar winds, and
are significantly lower than 300 km s–1 presently
adopted in evolutionary models of Meynet and
Maeder (2000).
Although the majority of high-mass stars
undergo classical core-collapse during the red
supergiant phase, due to iron disintegration in
their cores, very massive (Super) Asymptotic
Giant Branch (SAGB) stars may explode as electron capture supernovae (Eldridge, this meeting).
Such stars, with initial masses of approximately
9 M⊙ undergo carbon burning and a thermal pulse
making the core grow until either the entire envelope is ejected (producing a massive white dwarf)
or encountering electron captures on 20Ne and
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Mg, triggering a supernova. Poelarends et al.
(2006) indicate up to 10% of core-collapse SNe
are due to SAGB stars at solar metallicity, with a
higher fraction in metal-poor environments.
Type II-P SNe (“P” stands for the plateau phase
in their lightcurve) are by far the most common
type of core-collapse SNe, with an expected
RSG progenitor, due to their extended, massive
H-rich envelopes, for which single-star evolutionary models suggest initial masses of up to
about 20 M⊙. Observationally, RSG progenitors
for a number of Type II-P SNe are confirmed
from pre-explosion images (e.g. figure 2). Type
IIL and IIb supernovae are less common, with
a relatively low-mass hydrogen envelope, and
denser circumstellar media. These result either
from somewhat more massive single stars (up to
approx 25 M⊙), or close binaries, with red, yellow or blue supergiant progenitors. Type IIn SNe
are rarer still, with a dense circumstellar environment, arising either from a single H-rich star
with a very dense wind (possibly Luminous Blue
Variables) or an interacting binary. It is not yet
established whether H-rich LBVs may explode
as a supernova, without first evolving to the blue
to the H-poor Wolf–Rayet (WR) stage.
At solar metallicity, stars initially more massive
than ~25 M⊙ end their lives as either a nitrogenrich (WN) or carbon-rich (WC) WR star. WN
and WC stars are believed to be the immediate
progenitors of a subset of Type Ib (H-poor) and
Type Ic (H and He-poor) supernovae, respectively, whose circumstellar environment matches
that of a WR star. Observationally, SN 2002ap
(Type Ic) so far provides the most stringent constraints upon a potential WR progenitor, revealing an upper limit of MB = –4 mag, in common
with a subset of Magellanic Cloud WC stars
(Eldridge, this meeting). The observed ratio of
Type Ib/c SN to Type II lies between expectations
for single, rotating stars and close binaries.
Wolf–Rayet stars possess higher wind densities
than other early-type stars, although they too
have been reduced in recent years due to wind
clumping, which is more readily measured than
for their O star cousins. Evolutionary models
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2: Composite INT image of M74 (NGC 628) showing the pre-explosion star of SN2003gd (top inset, Gemini)
and six months after the supernova (bottom inset, INT), for which a RSG progenitor of the type II-P SN
was deduced, based additionally upon HST WFPC2/ACS imaging. (Smartt et al. 2004) (ING).
for the Wolf–Rayet stage have typically assumed
metallicity independent mass-loss rates, which
both observational (Crowther et al. 2002) and
theoretical (Vink and de Koter 2005) evidence
now challenges. The metallicity dependence of
WN winds appears to be similar to O stars, with
a somewhat weaker dependence for WC stars
due to their high (primary) carbon and oxygen
abundances, of relevance to the observed ratio
of WC to WN stars predicted by evolutionary
models (e.g. Eldridge and Vink 2006).
Gamma-ray bursts
The explosion mechanism for core-collapse
supernovae is now receiving close attention due
to the clear link now established between some
long-duration gamma-ray bursts (GRBs) and
bright Type Ic supernovae. The two discoveries
that revolutionized this field were SN 1998bw/
GRB 980425 (Galama et al. 1998) – the first discovery of an explosion that produced both a GRB
and a SN – and SN 2003dh/GRB 030329 (Stanek
et al. 2003). In more distant GRBs, although the
afterglow is normally observed, the supernovae
are intrinsically much fainter and hence are
often undetected. This confirmed the collapsar
model, involving a compact, rotating hydrogendeficient massive progenitor, i.e. Wolf–Rayet
star (MacFadyen and Woosley 1999). Rotation
is critically important since the collapsar model
involves highly collimated jets produced along
the polar axes, arising from a dense, equatorial
accretion disc feeding the central black hole. The
dense stellar winds from WR stars hinder the
direct measurement of rotational velocities, but
polarimetry favours negligible deviation from
spherical symmetry in most solar metallicity
WR stars. An unsolved challenge to evolutionary models involves the requirement of a high
angular momentum within the Wolf–Rayet core
in the collapsar model. Evolutionary models
allowing for magnetic fields involve cores that
are efficiently spun down before collapsing, in
most models, either due to the shear between the
slowly rotating RSG envelope and core, or loss
of angular momentum during the WR phase as
a result of its high mass-loss rate. These permit
the observed rotational rates of young pulsars
(e.g. a period of 33 ms for the Crab pulsar, figure
3) to be reproduced. However, collapsars would
“The mission lifetime of SWIFT
combined with follow-up from
ground and space promises more
exciting discoveries and solutions”
A&G • February 2007 • Vol. 48
Crowther, Smartt: Meeting report
nova did occur but that it was extremely faint, of
similar magnitude to faint Type II SNe.
The location of long-duration GRBs in HST
images of their host galaxies suggests that they
are more likely to be associated with the brightest regions of star formation in irregular host
galaxies than normal Type II SNe (Fruchter et
al. 2006). The mission lifetime of SWIFT combined with follow-up from ground and space
promises more exciting discoveries and solutions
to the outstanding puzzles in this area linking
high-energy physics to the last stages of massive
stellar evolution.
Supernovae explosions
3: This image of the Crab nebula encompasses our knowledge of supernovae. A bright new star was
recorded in 1054 by Chinese astronomers, four times brighter than Venus at visible in the daylight. This
was the supernova that created the Crab nebula. The nebula is rich in heavy elements such as oxygen,
silicon, neon and iron. A spinning neutron star is visible at the core of the remnant, from the collapse of
massive star progenitor. Warm dust glowing at mid-infrared wavelengths has been detected and the
mass estimated at about 0.01 M⊙. This composite image of the Crab nebula (6 arcmin or 3.7 pc across)
uses data from WFPC2 aboard the Hubble Space Telescope, with blue, green and red images sensitive to
neutral oxygen, ionized sulphur and doubly ionized oxygen. (STScI-2005-37, NASA, ESA and J Hester)
require an order of magnitude shorter periods of
<2 ms (Woosley and Heger 2006).
At low metallicity, initially high rotational
velocities may be capable of avoiding the
extended envelope, leading to a near “chemically
homogeneous” evolution (Maeder 1987). The
mechanical mass-loss induced spin-down during
the He star may be avoided since low-metallicity
WR stars are believed to possess weak winds,
resulting in sufficient angular momentum in the
core upon core-collapse (Yoon and Langer 2005).
SMC O stars from the VLT FLAMES survey provide some observational evidence in support of
such extreme evolution (Mokiem et al. 2006).
Of course, only a tiny fraction of SNe produce a
GRB, with an apparent bias towards metal-poor
environments. This favours the above scenario,
with respect to alternative close binary models
that would not necessarily show a low-metallicity
bias. Langer and Norman (2006) have shown that
the observed statistics of GRBs could be explained
by considering all sufficiently high-mass stars at
low (<1/10 Z⊙) metallicity, i.e. those whose end
product is a black hole. In reality, it is perhaps the
A&G • February 2007 • Vol. 48 high rotation velocity tail of less metal-deficient
stars (<1/3 Z⊙) that provide GRB progenitors.
As such GRBs would trace the low-metallicity
star-formation history of the universe.
As the SNe are significantly fainter than the
burst afterglows, it is only after the power-law
afterglow fades (after about 10 days) that the
SNe are seen as late bumps or flux excesses. Until
recently, all long-duration GRBs that were close
enough for SN detection did indeed show Type Ic
features. However, the recent, very close events
GRB 060505 and GRB 060614 show no sign
of supernovae down to magnitudes of around
MR = –14 mag (Fynbo et al. 2006). Three possible reasons have been given. The most prosaic
is that the GRB was much more distant and its
spatial location within an apparent host galaxy is a chance coincidence, and hence the SN
would be too faint to be detected (Cobb et al.
2006). The second is that this could be a new
explosion mechanism and formation channel
for long-duration GRBs that does not involve
core-collapse of a massive, rotating progenitor
(Gehrels et al. 2006). The third is that a super-
Observations of the luminosity and the kinetic
energy of core-collapse supernovae are vital to
constrain the explosion models and determine if
there is any link between the explosion mechanism and mass of the star. There are peculiar Type
II-P SNe that have distinctly lower luminosities
and kinetic energies (measured by the expansion
velocity of the ejecta) than normal (Pastorello
et al. 2004). Explosion models of 8–9 M⊙ stars
have been made by Kitaura et al. (2006) in which
electron capture by an oxygen–neon–magnesium
core triggers collapse, for which a SAGB star is
a possible progenitor (Poelarends et al. 2006).
Another plausible model for these supernovae
are high-mass progenitors that form a blackhole, for which fall-back onto the compact object
reduces the overall observed energy (Zampieri et
al. 2003). The direct detection of the progenitor
star of SN 2005cs (8–12 M⊙), which is a faint
II-P explosion (Pastorello et al. 2006), suggests
the low-mass scenario is valid for this particular supernova. But the lightcurve model for
SN 2003Z, for example, suggests the high-mass
scenario is required to explain the velocity and
lightcurve evolution. A larger sample of events
and a detailed characterization of their physical
parameters is required to determine which of the
progenitor scenarios is the most likely.
The physical mechanism that produces a supernova explosion from the last stages of stellar
evolution has been studied for several decades.
The energy source of the explosion is the reservoir of gravitational potential energy locked up
in the Chandrasekhar-mass iron core which is
somehow converted into the kinetic, neutrino
and electromagnetic energy we can observe. The
first models employed a hydrodynamic bounce
of the collapsing mantle off the newly formed
proto-neutron star with the shock accelerated
by deposition of neutrino energy. However, both
1‑D and multi-D models incorporating the best
known neutrino physics have not resulted in
explosions consistently across a range of progenitor masses. In general, models for stars of
higher initial mass than 8–9 M⊙ fail to revive the
stalled shock with neutrino heating.
Burrows et al. (2006) have recently proposed
a new alternative in which the core reverberates
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Crowther, Smartt: Meeting report
and generates strong sound waves. They suggest
that such acoustic power could potentially transport energy to the mantle to drive the explosion.
The Burrows group has developed a 2‑D model
of the collapse of an 11 M⊙ star and followed
accretion onto the proto-neutron star beyond
500 ms after the bounce (figure 4). An acoustic oscillation arises at 200 ms and the power
generated by the oscillating core is enough to
drive the explosion. This mechanism was not
identified previously as models were stopped
at 200–300 ms and the oscillations in the core
were either removed or not followed fully in 2‑D,
which hence suppressed the acoustic vibrations.
Burrows (this meeting) suggests that hybrid
models of neutrino-driven and acoustic-driven
explosions may be able to explain core-collapse
across a wide range of progenitor masses. Their
models make definite predictions for the neutrino
and gravitational wave spectra that are emitted
and which are highly dependent on the physical parameters of the explosion. The acoustic
mechanism is by nature highly aspheric and is
the most promising model to explain the explosion of massive progenitors and the high space
velocities seen in the galactic pulsar population.
A critical test for the models will come when
next Local Group or Milky Way core-collapse
supernova explodes.
Cosmic dust production
When a massive star explodes as a supernova and
the ejecta expand and cool, they may be important sites for the formation of cosmic dust. The
detection and characterization of dust in the early
universe is important for studies of abundances in
damped Lyman-α systems, cooling of molecular
clouds and star formation, and the determination
of the star formation rate. It seems likely that a
rapid injection of dust in the universe occurred
during the first 1 Gyr; prime suspects are supernovae from massive stars (Todini and Ferrara 2001).
There has been little work on characterizing dust
formation in nearby SNe, but Spitzer Space Telescope results are begining to change the field.
Two groups with significant UK leadership are
studying nearby core-collapse SNe with Spitzer.
If core-collapse SNe are indeed important contributors to dust production at high-redshift,
then we should be measuring dust masses of
around 0.1–1 M⊙ in their ejecta. Sugerman et
al. (2006) claim to have detected mid-infrared
excesses consistent with cooling dust in the ejecta
of the Type II-P SN 2003gd (recall figure 2) during the period 499–678 days after outburst. Their
radiation transfer model predicts up to 0.02 M⊙
of dust has formed. In an earlier study, an enormous mass of dust of 0.1–0.15 M⊙ (Barlow et al.
2005) was estimated to have formed in an optically thick dust shell around SN 2002hh (another
II-P). However, Meikle et al. (2006) showed that
most of the strong mid-IR emission originated
in this cool, obscured star-formation region or
1.38
4: Simulation from Burrows et al. (2006) representing an isodensity plot (with a wedge excised) of the
exploding core for a 11 M⊙ model in which the sonic power is driven mostly by a core oscillation that is
excited by the violent accretion streams, powered by the energy of gravitational infall.
molecular cloud along the line-of-sight, such
that the mass of the pre-existing dust in the SN
circumstellar medium required to produce the
observed infrared echo is about 0.04 M⊙. Hence
there is no clear evidence of large amounts of
dust condensing in the ejecta of SN 2002hh. Further analysis of three other nearby II-P SNe by
Meikle (this meeting) also suggest low amounts
of dust in the ejecta (of order between 10–3 and
10–4 M⊙), which is not enough to make them cosmologically significant producers.
Both teams remark on the possibility that the
dust is significantly clumped and are working on
how to deal with this issue. In a further development, the molecule SiO has been observed in the
mid-infrared spectra of the Type II-P SN 2005af
in the 3.6–24 µm window with Spitzer (Kotak et
al. 2006). It is thought that this molecule is a key
stage in condensation to dust in the ejecta. It is
hoped that during the lifetime of Spitzer we may
learn if supernovae really are key to high-redshift
dust production. Cooler dust may be probed in
the sub-mm in the future with ALMA. ●
Dr Paul Crowther, University of Sheffield (Paul.
[email protected]); Prof. Steve Smartt,
Queen’s University Belfast ([email protected]).
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