| dc.description.abstract | Timescales for stellar evolution and star and planet formation are critical to provide constraints
on theories. The accuracy of these timescales, and therefore our ability to confidently
reject a given model, rely on the accuracy of the derived ages for star-forming-regions (SFRs). In
this study I have developed the new techniques and adopted or updated the existing techniques
necessary to derive precise age orders for a range of SFRs.
Deriving precise ages for SFRs requires precise distances and extinctions. I have applied a
new technique, 2 fitting (Naylor &Je ries, 2006), to derive a set of self-consistent and statistically
robust distances (and mean extinctions), with associated uncertainties for 12 SFRs. I have also
revised and formalised a widely used method of deriving individual extinctions, the Q-method
(Johnson &Morgan, 1953). These new data show that the largest remaining uncertainty in deriving
distances to SFRs is composition.
Deriving ages or age orders for pre-main-sequence (pre-MS) populations using pre-MS
theoretical isochrones has been shown to be unreliable at present (Naylor et al., 2002; Bonatto
et al., 2004; Pinsonneault et al., 2004), largely due to model dependencies and spreads within a
colour-magnitude diagram (CMD). Therefore, I have developed a technique to model the pre-MS,
generating empirical isochrones, which e ectively removes spreads in pre-MS populations in a
CMD.
The derived distances and extinctions have been applied to the empirical isochrones, enabling
the creation of an age ordered ladder in intrinsic colour and absolute magnitude. This
has been calibrated using ages for fiducial sequences and nominal ages assigned to the separable
groups, which are as follows:
1 Myr, NGC2244 and IC5146.
2 Myrs, NGC6530 and the ONC.
3 Myrs, Ori, Cep OB3b, NGC2264 and Ori.
45 Myrs, NGC2362 and IC348.
10 Myrs, NGC7160.
13 Myrs, h and Per.
20 Myrs, NGC1960.
40 Myrs, NGC2547.
2
Once assigned the nominal ages and age orders were combined with ancillary data to investigate
rotation rate and disc evolution. The general trends of rotation rate distribution evolution
and disc fraction changes with age confirmed existing estimates for the disc survival, and therefore
star-disc interaction through disc-locking, with a timescale of 5 Myrs. However, this study
also revealed some of the first evidence of local environment e ects. IC348 appears ‘out of sequence’
in both the rotation rate distribution and disc fraction. Specifically, IC348 has a larger disc
fraction than expected at its nominal age and exhibits a rotation rate distribution expected from a
much younger SFR (i.e. the ONC). This could be a consequence of the lower number density of
O stars (none exist in IC348) and therefore a lower density of UV flux, which acts to hasten disc
dissipation.
Finally, a potentially important feature of stellar populations in a CMD, the R-C gap was
identified. This separation in a CMD of the fully convective pre-MS and main-sequence (MS) stars
with radiative cores was found to vary as a function of age. As the R-C gap is also measurable in
colour it provides a distance independent age indicator. I have explained the underlying physics
of the R-C gap and discussed possible applications of this phase change of the stellar interior.
In addition, an overlap between the pre-MS and MS sections of the R-C gap was apparent in all
SFRs where the R-C gap could be unambiguously identified. This R-C gap overlap shows that
the studied SFRs must contain a spread in isochronal ages. However, the interpretation of this
spread is dependent on the underlying assumptions. If one assumes stars form by a robust slowstar-
formation (SSF) mechanism and isochronal ages represent the true age of a star, then these
spreads can be construed as true age spreads. Alternatively, if one adopts a rapid-star-formation
model (RSF), this spread can be explained as a variation in accretion histories of the constituent
stars. As found by Siess et al. (1999) and Tout et al. (1999) accretion can act to accelerate pre-MS
star evolution, meaning the isochronal age does not represent the true age of the star. This increases
the advantages of empirical isochrones and age ordering over the derivation of individual ages for
SFRs. Indeed, this R-C gap overlap could be used to ‘normalise out’ any spread in age or accretion
history and therefore increase the power of derived age orders. | en_GB |
