Note: More master thesis topics in Stellar Physics can
be found at here
Rotation rates of massive main sequence stars
Massive stars are generally rapid rotators. It is generally assumed that
the observed projected rotation rates of main sequence stars
(i.e. with stars performing their core hydrogen burning evolution)
reflect their initial rotation rates,
based on the short life times of these objects.
However, observed stars are never truly "just born", i.e., they have a non-zero age.
And the rotation rate of a massive star, even on the main sequence, where its structure
does not change much, can change as function of time due to various effects, such
that any observed distribution of (projected) rotational velocities can deviate from
the initial distribution. Such effects are in particular stellar winds, which can
drag out stellar angular momentum, winds coupled to a magnetic surface field,
and internal angular momentum transport. We can model all these effects
using a sophisticated stellar evolution computer code, and
thereby constrain the distribution function of initial rotation rates
based on observed distributions of projected rotational velocities.
This project has a strong relation to large recently finished and ongoing
observational projects in which we are involved.
Binaries in the FLAMES Survey of Massive Stars
The recently finished FLAMES Survey of Massive Stars allowed
for the first time, through the
ESO multi-object spectrograph FLAMES on the VLT, to obtain high quality
spectra for a large number of hot massive main sequence (core hydrogen burning) stars.
Next to rapidly rotating stars with indications for internal rotationally induced
mixing (in particular surface nitrogen enhancements),
this survey surprisingly found significant fractions of massive stars with
unexpected properties: very slow rotators with chemically enriched surfaces,
as well as evolved rapid rotators without signs of internal rotationally induced
mixing. Binary evolution models have been suggested as an explanation, an our group has produced
some models which could potentially explain such properties. Some of the stars
with unexpected properties likely are binaries based on observed radial velocity
shifts found in the multi epoch spectra. The task here is to investigate whether the
constraints on orbital period and mass ratio derived from the observed radial velocity
shifts are consistent, or in contradiction, with the binary evolution scenarios which
could explain these observations. Furthermore, it should be tested whether
the known binaries amongst the enriched rapid rotators
in the FLAMES sample are consistent with the idea of rotational mixing within the
single star picture.
Massive star isochrones: adding the effect of close binaries
Isochrones are lines in the Hertzsprung-Russell diagram connecting star of the same age.
They are a standard tool in astronomy to provide age dating of star clusters.
Close binary evolution, in particular through mass transfer from one star to its companion,
can lead to the so called rejuvenation phenomenon, i.e. the mass gainer grows in mass,
increases the mass of its convective core, and thereby brings fresh hydrogen into the
core. Additionally, the mass gainer is spun-up in the accretion process which may
trigger continuous internal mixing process which can further extend the life time of
this star. In a coeval star cluster population which contains close binary stars,
it is thus expected that isochrones are no longer one-dimensional structures. Aim of this
project is to investigate which error in the age dating of clusters can occur if
the binary effect is neglected (which is mostly so!).
Last updated May 5, 2014