The Magellanic
Clouds | 
The Cluster NGC 1818
The Magellanic
Clouds | The Cluster NGC 1818
The LMC cluster team home page
As part of a large project investigating star formation, stellar evolution and the formation and evolution of stellar clusters in the Large Magellanic Cloud using the Hubble Space Telescope (Cycle 7 approved project 7307), a team of astronomers, primarily based at the Institute of Astronomy in Cambridge, has been investigating the population of a young, bright stellar cluster - NGC 1818 - in the outskirts of the Large Magellanic Cloud. The team members are:
As part of the project we analysed archive data from a previous cycle of one of our target cluster, NGC 1818. The Principal Investigator of the project providing the archive data was Jim Westphal, a paper describing the original results was published by Deidre Hunter et al, in 1997 in the Astrophysical Journal volume 478, pages 124-133 (Electronic ApJ reference [subscription required]).
During the analysis of the data, we realised that one of the objects in the field was unusually blue and bright. A quick calculation suggested that the object was consistent with being a very young white dwarf, probably about 100,000 years old, which had recently formed from a ultra--bright red giant, like the dozen other red giants seen in the field. We submitted a short letter discussing this and other results from our analysis to the Astrophysical Journal, Letters. A preprint is available from the XXX preprint servers; UK server, US server
Hubble Finds that Even Massive Stars Just Fade Away
Pinpointing the rapidly fading ember of a recently burned out
star, NASA's Hubble Space Telescope is giving astronomers a better estimate
on just how big a star can be before it ultimately explodes as a supernova.
Based on Hubble's detection of a rare, young white dwarf star, astronomers
conclude that its progenitor was a whopping 7.6 times the mass of our Sun.
Previously, astronomers had estimated that stars anywhere from 6 to 10
solar masses would not just quietly fade away as white dwarfs, but abruptly
self-destruct in torrential explosions.
This new lower limit will help astronomers refine theories of how galaxies
developed in the early universe, determine the rate at which supernovae
enrich interstellar space with heavy elements for building new generations
of stars
and planets, and estimate the number of neutron stars in space (neutron
stars are the crushed stellar cores resulting from supernovae).
Rebecca Elson, Steinn Sigurdsson, and Gerard Gilmore of Cambridge University,
and co-investigators, discovered the ultra-hot white dwarf during a
search in archival Hubble Wide Field Planetary Camera 2 pictures of
the young star cluster NGC 1818, located 164,000 light-years away in
the Large Magellanic Cloud -- a satellite galaxy of our Milky Way.
The trick was to identify a newly formed white dwarf that was still
exceptionally hot and bright immediately after the burnout and collapse of
its progenitor star. Such a dwarf would be so "young" -- relative to older
very faint dwarfs in the cluster -- it would allow a direct link back to the
most massive stars now present in the cluster. That's because the most
massive stars are the shortest lived, and so are first to burnout
as white dwarfs (if indeed they are too small to explode as supernovae).
Because the star cluster NGC 1818 is ten times larger than those found
closer to us within our own galaxy, chances were far better for catching the
young dwarf before it swiftly dimmed on its way to the "graveyard" of faint
dwarfs. Also, the cluster is only about 40 million years old, and so still
contains massive stars.
Hubble is ideally suited for hunting for white dwarfs so far away because
its exquisite resolution can pinpoint them among the cluster's crowded
stellar population, and can easily detect the blue light from the sizzling
50,000 degree Fahrenheit surface temperature of the young dwarf.
Once the candidate star was identified, a spectrum of the star was obtained
by staff at the Anglo-Australian Telescope. The spectrum indicates
that it is indeed a member of the cluster, with an excess of very blue
light that would be consistent with a young white dwarf.
Detailed modeling is
underway to understand its exact evolutionary state.
The results will be reported in the Astrophysical Journal Letters.
The cluster, NGC 1818, is thought to be 20-40 million years old, the uncertainty in the age is because of systematic differences in different possible stellar models, and not the data. It is likely that the cluster is close to 40 million years old; and very likely that all the stars in the cluster formed within 3 million years of each other. It is known that the more massive a star is, the quicker it evolves and dies; stars like the sun - with a mass of one solar mass - live for maybe 8-10 billion years, stars less massive than that last correspondingly longer, while stars that are 20 - 100 times more massive than our sun live only a few million years. We also know that the most massive stars end their lives as type II supernovae - the core of the star collapses at the end of its life, and forms a neutron star or possibly a black hole, with the energy released driving an enormous explosion that forces the rest of the star, and, critically, all the chemicals formed as the star burned its hydrogen fuel, out into space at thousands of kilometers per second. These explosions are one of the most violent events known in the universe, one was observed in a different part of the Large Magellanic Cloud in february 1987.
We also know that low mass stars, like our sun, die quietly, ending up as hot, compact white dwarfs, which cool passively over millions or billions of years. Such stars may undergo a beautiful but short lived "planetary nebulae" (see also here for pictures) phase before the white dwarf forms.
What we don't know is the critical mass at which stars stop going supernova, and form white dwarfs instead. We are pretty sure this mass is somewhere between 6 and 10 times the mass of the sun, but as high mass stars are rare, the actual cut-off mass determines critically both how many type II supernova there are (and hence how many pulsars and low mass black holes are formed), and also how much of the various "metals" - that is all the elements other than hydrogen, helium and lithium - are formed. All our gold, uranium, and much of our iron, silicon, carbon and oxygen comes from supernovae.
The most massive stars in NGC 1818 are thought to be about 7.6 times more massive than the sun. This is right in the range that we expect the critical mass for white dwarf formation to be, and if white dwarfs have started forming, the Hubble Space Telescope, with its superb resolution and sensitivity to blue and ultra-violet objects, should be able to see them. We have found one such object in 1818 and conjectured it might be a young white dwarf, in which case we have found a critical key marker for stellar evolution and constrained very strongly this important boundary on the fate of stars. Even if this particular object is not a white dwarf or a post-AGB star (which is essentially a white-dwarf-in-formation, the transition from a red giant to a white dwarf), it could be a background bright quasar, a foreground sdB dwarf halo star (very rare) or a peculiar blue dwarf from the Magellanic Cloud stars but not part of the cluster, even then the principle that we can detect these young white dwarfs in Magellanic clusters and use them to constrain stellar evolution is correct, and future observations seem certain to find some white dwarfs.
On march 5th 1998, a spectrum of the candidate object was obtained at the AAT. The director of the AAT, Brian Boyle, provided a Target of Opportunity override in order to catch the LMC in the current season (the LMC is a southern hemisphere early evening object this time of year, and will be a day time object until the autumn, so without this no ground based data could have been taken for several months). The first night observations were precluded by clouds. An 1800 second exposure was obtained at 10:24 UT covering a wavelength range of 3407 - 4992 Angstrom, with a pixel resolution of 1.5 Angstrom.
We thank Brian, Ray Stathakis, Helen Johnston at the AAT and Matt Burleigh of Leicester for taking the data.
A look at the spectrum confirmed that the object is real, blue and stellar. It is not a QSO or a blue background galaxy. The spectrum has not been fully analysed yet, and extensive modeling of the detailes of the spectrum will be necessary, but the raw spectrum show narrow, shallow Balmer lines and a large UV excess, indicating a hot object with a tenuous atmosphere. It is not a DA (hydrogen) white dwarf, but nor was it expected to be. Cross--correlating the spectrum with spectra of other stars in the cluster gives a velocity of the object relative to that of NGC 1818 of 0 +/- 20 km/sec, ie the object is not a foreground star, is most certainly a member of the LMC and is consistent with being a cluster member to the limits to the data currently available.
Current interpretation (in the absence of a fully reduced spectrum or detailed atmosphere models) is that the object is consistent with being a "post-AGB" star, that is the core of a burnt out star that used to be a (red) giant, and is now cooling down passively, with the spectrum possibly dominated by a tenuous remnant of its envelope. Alternatively its a member of the LMC field, which is significantly overluminous and exceptionally hot for some reasons.
Last updated 14/03/99
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