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| Paper IPM / Astronomy / 12434 |
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| Abstract: | |||||||
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We have conducted a near-infrared monitoring campaign at the UK Infrared
Telescope, of the Local Group spiral galaxy M33 (Triangulum). The main aim was
to identify stars in the very final stage of their evolution, and for which the
luminosity is more directly related to the birth mass than the more numerous
less-evolved giant stars that continue to increase in luminosity. In this second
paper of the series, we construct the birth mass function and hence derive the
star formation history. The star formation rate has varied between ?0.002 and
0.007 Msun yr-1 kpc-2. We give evidence of two epochs of a star formation rate
enhanced by a factor of a few - one that happened â¥6 Gyr ago and produced â¥80
per cent of the total mass in stars, and one around 250 Myr ago that lasted ?200
Myr and formed â¤6 per cent of the mass in stars. We construct radial and
azimuthal distributions in the image plane and in the galaxy plane for
populations associated with old first-ascent red giant branch (RGB) stars,
intermediate-age asymptotic giant branch (AGB) stars and young (massive) blue
and red supergiants. We find that the RGB stars follow an spheroidal
distribution, while younger stars follow a flat-disc distribution. The
intermediate-age population displays signs of a pseudo-bulge or possibly a bar.
The inner spiral arm pattern as recorded in mid-19th-century drawings is
confirmed. We interpret our findings as evidence for an old, pressure-supported
component and a younger disc formed 6 Gyr ago, with an accretion event occurring
250 Myr ago giving rise to the compact nucleus in M33. Our study provides
support for recent Padova stellar evolution models except that super-AGB stars
likely reach low temperatures and thus high mass-loss rates, supporting the
super-AGB nature of the progenitors of dust-enshrouded supernovae such as SN
2008S.
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