Star-birth myth 'busted' (w/ Podcast)
(麻豆淫院Org.com) -- An international team of researchers has debunked one of astronomy's long held beliefs about how stars are formed, using a set of galaxies found with CSIRO鈥檚 Parkes radio telescope.
When a cloud of interstellar gas collapses to form stars, the stars range from massive to minute.
Since the 1950s astronomers have thought that in a family of new-born stars the ratio of massive stars to lighter ones was always pretty much the same 鈥� for instance, that for every star 20 times more massive than the Sun or larger, you鈥檇 get 500 stars the mass of the Sun or less.
鈥淭his was a really useful idea. Unfortunately it seems not to be true,鈥� said team research leader Dr Gerhardt Meurer of Johns Hopkins University in Baltimore.
The different numbers of stars of different masses at birth is called the 鈥榠nitial mass function鈥� (IMF).
Most of the light we see from galaxies comes from the highest mass stars, while the total mass in stars is dominated by the lower mass stars.
By measuring the amount of light from a population of stars, and making some corrections for the stars鈥� ages, astronomers can use the IMF to estimate the total mass of that population of stars.
Results for different galaxies can be compared only if the IMF is the same everywhere, but Dr Meurer鈥檚 team has shown that this ratio of high-mass to low-mass newborn stars differs between galaxies.
For instance, small 'dwarf' galaxies form many more low-mass stars than expected.
To arrive at this finding, Dr Meurer鈥檚 team used galaxies from the HIPASS Survey (HI Parkes All Sky Survey) done with CSIRO鈥檚 Parkes radio telescope.
鈥淎ll of these galaxies were detected with the Parkes telescope because they contain substantial amounts of neutral hydrogen gas, the raw material for forming stars, and this emits radio waves,鈥� said CSIRO鈥檚 Dr Baerbel Koribalski, a member of Dr Meurer鈥檚 team.
Selecting galaxies on the basis of their neutral hydrogen gave a sample of galaxies of many different shapes and sizes, unbiased by their star formation history.
The astronomers measured two tracers of star formation, ultraviolet and H-alpha emissions, in 103 galaxies using NASA鈥檚 GALEX satellite and the 1.5-m CTIO optical telescope in Chile.
H-alpha emission traces the presence of very massive stars called O stars, which are born with masses more than 20 times that of the Sun.
The UV emission, traces both O stars and the less massive B stars 鈥� overall, stars more than three times the mass of the Sun.
Meurer鈥檚 team found that this ratio, of H-alpha to UV emission, varied from galaxy to galaxy, implying that the IMF also did, at least at its upper end.
Their work confirms tentative suggestions made first by Veronique Buat and collaborators in France in 1987, and then a more substantial study last year by Eric Hoversteen and Karl Glazebrook working out of Johns Hopkins and Swinburne Universities that suggested the same result.
鈥淭his is complicated work, and we鈥檝e necessarily had to take into account many factors that affect the ratio of H-alpha to UV emission, such as the fact that B stars live much longer than O stars,鈥� Dr Meurer said.
Dr Meurer鈥檚 team suggests the IMF seems to be sensitive to the physical conditions of the star-forming region, particularly gas pressure.
For instance, massive stars are most likely to form in high-pressure environments such as tightly bound star clusters.
The team鈥檚 results allow a better understanding of other recently observed phenomena that have been puzzling astronomers, such as variation of the ratio of H-alpha to ultraviolet light as a function of radius within some galaxies. This now makes sense as the stellar mix varying as the pressure drops with radius (just like the pressure varies with altitude on the Earth).
Importantly, the team also found that essentially all galaxies rich in neutral hydrogen seem to form stars.
鈥淭hat means surveys for neutral hydrogen with radio telescopes will find star-forming galaxies of all kinds,鈥� Dr Meurer said.
The Australian SKA Pathfinder, the next-generation radio telescope now being developed by CSIRO, will find neutral hydrogen gas in half a million galaxies, allowing a comprehensive examination of star-formation in the nearby universe.
Provided by CSIRO ( : )
