1,436 research outputs found
Rotating Stellar Models Can Account for the Extended Main Sequence Turnoffs in Intermediate Age Clusters
We show that the extended main sequence turnoffs seen in intermediate age
Large Magellanic Cloud (LMC) clusters, often attributed to age spreads of
several hundred Myr, may be easily accounted for by variable stellar rotation
in a coeval population. We compute synthetic photometry for grids of rotating
stellar evolution models and interpolate them to produce isochrones at a
variety of rotation rates and orientations. An extended main sequence turnoff
naturally appears in color-magnitude diagrams at ages just under 1 Gyr, peaks
in extent between ~1 and 1.5 Gyr, and gradually disappears by around 2 Gyr in
age. We then fit our interpolated isochrones by eye to four LMC clusters with
very extended main sequence turnoffs: NGC 1783, 1806, 1846, and 1987. In each
case, stellar populations with a single age and metallicity can comfortably
account for the observed extent of the turnoff region. The new stellar models
predict almost no correlation of turnoff color with rotational vsini: the red
edge of the turnoff is populated by a combination of slow rotators and edge-on
rapid rotators.Comment: 7 pages, 4 figures, 1 table, ApJ accepted. Conclusions unchange
Bayesian Ages for Early-Type Stars from Isochrones Including Rotation, and a Possible Old Age for the Hyades
We combine recently computed models of stellar evolution using a new
treatment of rotation with a Bayesian statistical framework to constrain the
ages and other properties of early-type stars. We find good agreement for
early-type stars and clusters with known young ages, including beta Pictoris,
the Pleiades, and the Ursa Majoris Moving Group. However, we derive a
substantially older age for the Hyades open cluster (750+/-100 Myr compared to
625+/-50 Myr). This older age results from both the increase in main-sequence
lifetime with stellar rotation and from the fact that rotating models near the
main-sequence turnoff are more luminous, overlapping with slightly more massive
(and shorter-lived) nonrotating ones. Our method uses a large grid of
nonrotating models to interpolate between a much sparser rotating grid, and
also includes a detailed calculation of synthetic magnitudes as a function of
orientation. We provide a web interface at www.bayesianstellarparameters.info
where the results of our analysis may be downloaded for individual early-type
(B-V<~0.25) Hipparcos stars. The web interface accepts user-supplied parameters
for a Gaussian metallicity prior and returns posterior probability
distributions on mass, age, and orientation.Comment: 11 pages, 6 figures, ApJ accepted. Error fixed: ages -> ~15% younger.
bayesianstellarparameters.info update
The Age and Age Spread of the Praesepe and Hyades Clusters: a Consistent, ~800 Myr Picture from Rotating Stellar Models
We fit the upper main sequence of the Praesepe and Hyades open clusters using
stellar models with and without rotation. When neglecting rotation, we find
that no single isochrone can fit the entire upper main sequence at the
clusters' spectroscopic metallicity: more massive stars appear, at high
significance, to be younger than less massive stars. This discrepancy is
consistent with earlier studies, but vanishes when including stellar rotation.
The entire upper main sequence of both clusters is very well-fit by a
distribution of 800 Myr-old stars with the spectroscopically measured
[Fe/H]=0.12. The increase over the consensus age of ~600-650 Myr is due both to
the revised Solar metallicity (from to ) and to the lengthening of main sequence lifetimes and increase in
luminosities with rapid rotation. Our results show that rotation can remove the
need for large age spreads in intermediate age clusters, and that these
clusters may be significantly older than is commonly accepted. A
Hyades/Praesepe age of ~800 Myr would also require a recalibration of
rotation/activity age indicators.Comment: 6 pages, 4 figures, ApJ accepted. Replaced with accepted version,
conclusions unchange
Optimal Fitting, Debiasing, and Cosmic Ray Rejection for Detectors Read Out Up-the-Ramp
This paper derives the optimal fit to a pixel's count rate in the case of an
ideal detector read out nondestructively in the presence of both read and
photon noise. The approach is general for any readout scheme, provides
closed-form expressions for all quantities, and has a computational cost that
is linear in the number of resultants (groups of reads). I also derive the bias
of the fit from estimating the covariance matrix and show how to remove it to
first order. The ramp-fitting algorithm I describe provides the value
of the fit of a line to the accumulated counts, enabling hypothesis testing for
cosmic ray hits using the entire ramp. I show that this approach can be
substantially more sensitive than one that only uses the difference between
sequential resultants, especially for long ramps and for jumps that occur in
the middle of a group of reads. It can also be implemented for a computational
cost that is linear in the number of resultants. I provide and describe a pure
Python implementation of these algorithms that can process a 10-resultant ramp
on a detector in 8 seconds with bias removal, or in
20 seconds including iterative cosmic ray detection and removal, on a
single core of a 2020 Macbook Air. This Python implementation, together with
tests and a tutorial notebook, are available at
https://github.com/t-brandt/fitramp.Comment: 30 pages, 9 figures. Python implementation available at
https://github.com/t-brandt/fitram
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