Massive Stars
The most direct method of deriving empirical mass-loss rates for hot stars is
through analysis of the UV resonance transitions from dominant
metal ions. However, difficulties in deriving the wind
ionization balance using currently available (trace) ions means that
mass-loss rates remain uncertain. With FUSE, additional wind lines are
being observed, spanning a much wider range of species. From these data,
the degree of ionization can be determined accurately, so that
physical and wind properties can be measured with confidence. As part of
the VLT FLAMES Massive Star Large Program consortium, notably
Rohied Mokiem, I have studied massive stars in the Galaxy and
Magellanic Clouds - spanning a factor of 5 in metallicity - so that the variation of
mass-loss properties with metal content can be measured,
of importance in the study of high redshift galaxies.
SMC O stars indeed possess weaker winds than those at Solar composition,
with higher temperature for individual O subtypes.
With Danny Lennon and Nolan Walborn
I have studied the physical properties of Galactic B stars,
revealing good agreement with predicted wind strengths for B0-B0.5 Ia
supergiants, after allowance for wind clumping, but winds from B0.7-B3
Ia supergiants are overestimated. The bistability jump in B supergiants
- signalling a sudden division in wind to escape velocity -
was re-investigated but not confirmed.
With Laurent Drissen and Veronique Petit
I am monitoring an Luminous Blue Variable star in the
Magellanic galaxy NGC 2366 10,000,000 light years away. This star has a B
supergiant spectral type except that it is currently undergoing a giant
eruption, such
that one solar earth mass of material is being ejected every day! Such
high and constant mass-loss is exceptional even amongst massive stars,
although all are irregular and structured at a basic level. It is
believed that such intense episodes of mass-loss are responsible for
the removal of hydrogen envelopes in massive stars, causing evolution
to the Wolf-Rayet phase.
It is widely accepted that OB stars are driven by radiation
pressure, whilst the situation for the denser winds of Wolf-Rayet stars is
less clear. Recent evidence in support of radiation pressure as a source
of driving for WR stars has been derived from studies of LMC and Galactic
WC stars. These are the immediate precursors of Type Ic SN and possibly
`hypernovae' (with a connection to long duration Gamma
Ray Bursts). We find that carbon-rich WC stars stars in the LMC (with
early spectral types) have weaker winds than Galactic WC stars (later
spectral types). This difference naturally explains the
shift in spectral class to earlier type at lower metallicity. The
situation for nitrogen-rich WN stars is less straightforward since they
show a much wider range of wind properties.
Fundamental properties of stars can most readily be obtained from
analysis of eclipsing double-lined eclipsing binaries. We have shown
that WR20a in the open cluster Westerlund 2 has the highest masses of
any star known to date, with each component of the 4 day binary having
a mass in excess of 80 times the Solar mass, each shining a million times
brighter. Rarely, Wolf-Rayet stars are members of close binary
systems with compact objects. We have identified the only two unambiguous
cases of Wolf-Rayet plus black hole binaries from our surveys of IC10 and
NGC300 (see figure). These possess the highest stellar-mass black holes
known to date, and are plausible progenitors of binary black-holes.
See
'Farthest' star-mass black hole report from BBC Science News in Jan 2010. Westerlund 1, shown here, is a candidate for the
most massive young cluster in the Local Group, with
a population of WR stars, Red Supergiants and yellow hypergiants
with a central
density of 100,000 Solar masses per cubic parsec - see the ESO
Press Release, and so is the nearest contender for a young
Globular Cluster. A neutron star CXO J164710.2-455216 has recently
been detected in Chandra X-ray observations of Wd1 -- see the
Chandra Press Release. We have re-evaluated the initial masses of the most massive stars in
the
youngest, highest mass Milky Way (NGC 3603) and LMC (R136) clusters, from
which initial masses of 170 and 320 solar masses were obtained, together
with at least 185 solar masses for the Arches cluster. Monte Carlo
simulations suggest that each are consistent with an upper mass limit of
circa 300 solar masses, a factor of two higher than hitherto believed.
We are now investigating all the high mass stars in R136 with an
upcoming Hubble Space Telescope STIS programme (see details). Beyond R136, the central cluster
of 30 Doradus, we are undertaking the VLT-FLAMES Tarantula Survey, a multi-epoch optical
spectroscopic study of 800 massive stars in the 30 Doradus
region. We have also obtained Gemini GNIRS spectroscopy of massive stars within
the Galactic 1806-20 cluster, from which a downward revision to its
distance to 8.7 kpc was obtained (its kinematic distance has hitherto
been taken as 15 kpc). This revision is significant since it leads to
a factor of three reduction in the 2004 December outburst of the
magnetar (highly magnetized pulsar) SGR 1806-20 - providing it is
associated with the young massive cluster - which in turn indicates
a reduction in the contaminaton of BATSE short GRBs from giant
outbursts from magnetars. It is significant that both 1806-20 and
Westerlund 1 apparently host magnetars, with progenitor masses of
approx 50 Msun.
Finally, we are also investigating the chemical properties of
high-redshift Lyman break galaxies, such as MS1512-cB58 and
Q1307-BM1163, sampling an epoch when the universe was only a few Gyr old,
using the stellar wind signatures from massive OB stars in their
integrated spectra. Surprisingly, the metal content of such early
galaxies as indicated by oxygen, is already within a factor of a few
of the present Milky Way galaxy.
OB stars and Luminous Blue Variables
Recent Papers:
Wolf-Rayet stars
Recent Papers:
Young massive clusters
Recent Papers:
Starbursts in galaxies
Recent Papers:
Star Formation
Recent Papers:
The birth of massive stars remains a major puzzle for theorists and
observers alike. What we do know is that they are formed within Giant
Molecular Clouds (such as Orion). Once the hot star starts to `shine', it
illuminates its dense surrounding region - which is called an UltraCompact
HII region - the surrounding material prevents direct observation of the
star until the natal gas has been blown away, but one can study the birth
environment of massive stars at radio and mid-infrared wavelengths - as
shown here for two Galactic UCHII regions, based on observations with the
Midcourse Space Experiment. Near-IR spectroscopy from VLT/ISAAC has
provided robust classification of the ionizing star of the G23.96+0.15
UCHII region, only the second such case due to high interstellar and
circumstellar dut extinction.
New observations are also being made with
the Spitzer mid-IR
telescope, primarily throught the GLIMPSE survey, shown on the
right for RCW49, plus our own IRAC and IRS study of the W31 star forming
region.
CSPN
Recent Papers:
In addition to studies of massive stars, some low and intermediate mass
stars pass through a phase at the end of their (long) life when a
`Planetary Nebula' is being formed, in which late pulses remove surface
hydrogen layers, revealing a spectrum reminiscent of massive stars - a
subset of these Central Stars of Planetary Nebulae (CSPN) exhibit a
Wolf-Rayet spectrum. I have studied a variety of these with Orsola De
Marco - e.g. SwSt1. The image to the right shows a HST image of one such
[WC]-type CSPN, BD+30 3639 which has been the subject of a recent
comprehensive UV, optical and IR study.
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28-Jul-2010
