Inverting Color-Magnitude Diagrams to Access Precise Star Cluster Parameters: A New White Dwarf Age for the Hyades (original) (raw)
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Inverting color-magnitude diagrams to access precise star cluster parameters: a Bayesian approach
Astrophysical Journal, 2007
We have extended our Bayesian modeling of stellar clusters-which uses main-sequence stellar evolution models, a mapping between initial masses and white dwarf (WD) masses, WD cooling models, and WD atmospheres-to include binary stars, field stars, and two additional main-sequence stellar evolution models. As a critical test of our Bayesian modeling technique, we apply it to Hyades UBV photometry, with membership priors based on proper motions and radial velocities, where available. Under the assumption of a particular set of WD cooling models and atmosphere models, we estimate the age of the Hyades based on cooling WDs to be 648 ± 45 Myr, consistent with the best prior analysis of the cluster main-sequence turnoff (MSTO) age by Perryman et al. Since the faintest WDs have most likely evaporated from the Hyades, prior work provided only a lower limit to the cluster's WD age. Our result demonstrates the power of the bright WD technique for deriving ages and further demonstrates complete age consistency between WD cooling and MSTO ages for seven out of seven clusters analyzed to date, ranging from 150 Myr to 4 Gyr.
New Techniques to Determine Ages of Open Clusters Using White Dwarfs
Astrophysical Journal, 2005
Currently there are two main techniques for independently determining the ages of stellar populations: main sequence evolution theory (via cluster isochrones) and white dwarf cooling theory. Open clusters provide the ideal environment for the calibration of these two clocks. Because current techniques to derive cluster ages from white dwarfs are observationally challenging, we discuss the feasibility of determining white dwarf ages from the brighter white dwarfs alone. This would eliminate the requirement of observing the coolest (i.e., faintest) white dwarfs. We discuss our method for testing this new idea, as well as the required photometric precision and prior constraints on metallicity, distance, and reddening. We employ a new Bayesian statistical technique to obtain and interpret results.
The Astrophysical Journal, 2005
I explore the current ability of both white dwarf cooling theory and main sequence stellar evolution theory to accurately determine stellar population ages by comparing ages derived using both techniques for open clusters ranging from 0.1 to 4 Gyr. I find good agreement between white dwarf and main sequence evolutionary ages over the entire age range currently available for study. I also find that directly comparing main sequence turn-off ages to white dwarf ages is only weakly sensitive to realistic levels of errors in cluster distance, metallicity, and reddening. Additional detailed comparisons between white dwarf and main sequence ages have tremendous potential to refine and calibrate both of these important clocks, and I present new simulations of promising open cluster targets. The most demanding requirement for these white dwarf studies are very deep (V ≥ 25-28) cluster observations made necessary by the faintness of the oldest white dwarfs.
White dwarfs and the ages of Open clusters
Journal of Physics: Conference Series, 2009
Open clusters provide the ideal environment for the calibration of ages determined from main sequence evolutionary theory (via cluster isochrones) and ages determined from white dwarf cooling theory. In an effort to measure more precise cluster ages, our group has developed a new technique using Bayesian statistics. Here we will discuss new capabilities of the technique, as well as the first application to real data, using the Hyades as a test case. Because the faintest white dwarfs have likely evaporated from the Hyades, we also demonstrate the first successful application of the bright white dwarf technique for deriving ages from the bright cluster white dwarfs alone.
A comparative analysis of the observed white dwarf cooling sequence from globular clusters
Monthly Notices of the Royal Astronomical Society, 2016
We report our study of features at the observed red end of the white dwarf cooling sequences for three Galactic globular clusters: NGC 6397, 47 Tucanae and M 4. We use deep colourmagnitude diagrams constructed from archival Hubble Space Telescope (Advanced Camera for Surveys) to systematically investigate the blue turn at faint magnitudes and the age determinations for each cluster. We find that the age difference between NGC 6397 and 47 Tuc is 1.98 +0.44 −0.26 Gyr, consistent with the picture that metal-rich halo clusters were formed later than metal-poor halo clusters. We self-consistently include the effect of metallicity on the progenitor age and the initial-to-final mass relation. In contrast with previous investigations that invoked a single white dwarf mass for each cluster, the data show a spread of white dwarf masses that better reproduce the shape and location of the blue turn. This effect alone, however, does not completely reproduce the observational data-the blue turn retains some mystery. In this context, we discuss several other potential problems in the models. These include possible partial mixing of H and He in the atmosphere of white dwarf stars, the lack of a good physical description of the collision-induced absorption process and uncertainties in the opacities at low temperatures. The latter are already known to be significant in the description of the cool main sequence. Additionally, we find that the present-day local mass function of NGC 6397 is consistent with a top-heavy type, while 47 Tuc presents a bottom-heavy profile.
White Dwarfs in Open Clusters: Calibrating the Clock
We present an update of our on-going effort to improve the precision of white dwarf cosmochronology via careful analyses of white dwarf photometry in open clusters. To improve the precision of white dwarf and main sequence age analysis, we are developing a new interpretative scheme using a Bayesian statistical approach that matches observations to simulated stellar clusters. Here we present our first tests of the Bayesian approach with simulated stellar clusters with ages of 1, 2, and 4 billion years.
The age and colors of massive white dwarf stars
Astronomy and Astrophysics, 2007
Aims. We present evolutionary calculations and colors for massive white dwarfs with oxygen-neon cores for masses between 1.06 and 1.28 M ⊙ . The evolutionary stages computed cover the luminosity range from log(L/L ⊙ ) ≈ 0.5 down to −5.2. Methods. Our cooling sequences are based on evolutionary calculations that take into account the chemical composition expected from massive white dwarf progenitors that burned carbon in partially degenerate conditions. The use of detailed non-gray model atmospheres provides us with accurate outer boundary conditions for our evolving models at low effective temperatures. Results. We examine the cooling age, colors and magnitudes of our sequences. We find that massive white dwarfs are characterized by very short ages to such an extent that they reach the turn-off in their colors and become blue at ages well below 10 Gyr. Extensive tabulations for massive white dwarfs, accessible from our web site, are also presented.
Hubble Space Telescope Observations of the White Dwarf Cooling Sequence of M4
The Astrophysical Journal Supplement Series, 2004
We investigate in detail the white dwarf cooling sequence of the globular cluster Messier 4. In particular, we study the influence of various systematic uncertainties, both observational and theoretical, on the determination of the cluster age from the white dwarf cooling sequence. These include uncertainties in the distance to the cluster and the extinction along the line of sight, as well as the white dwarf mass, envelope, and core compositions and the white dwarf-main-sequence mass relation. We find that fitting to the full two-dimensional color-magnitude diagram offers a more robust method for age determination than the traditional method of fitting the onedimensional white dwarf luminosity function. After taking into account the various uncertainties, we find a best-fit age of 12.1 Gyr, with a 95% lower limit of 10.3 Gyr. We also perform fits using two other sets of cooling models from the literature. The models of Chabrier et al. yield an encouragingly similar result, although the models of Salaris et al. do not provide as good a fit. Our results support our previous determination of a delay between the formation of the Galactic halo and the onset of star formation in the Galactic disk. Fig. 1.-Proper motion displacements dx and dy (in units of HST pixels) are shown for the 6 yr baseline spanned by our observations. The coordinate system is centered on the bright cluster stars. All objects in the field are shown on this diagram. A simple proper motion cut of 0.5 pixels separates cluster members ( filled circles) from background (open circles).
A Bayesian Approach to Deriving Ages of Individual Field White Dwarfs
We apply a self-consistent and robust Bayesian statistical approach to determining the ages, dis- tances, and ZAMS masses of 28 field DA white dwarfs with ages of approximately 4 to 8 Gyrs. Our technique requires only quality optical and near-IR photometry to derive ages with < 15% uncertain- ties, generally with little sensitivity to our choice of modern initial-final mass relation. We find that age, distance, and ZAMS mass are correlated in a manner that is too complex to be captured by traditional error propagation techniques. We further find that the posterior distributions of age are often asymmetric, indicating that the standard approach to deriving WD ages can yield misleading results.
The White Dwarf Age of NGC 2477
Astrophysical Journal, 2010
We present deep photometric observations of the open cluster NGC 2477 using HST/WFPC2. By identifying seven cluster white dwarf candidates, we present an analysis of the white dwarf age of this cluster, using both the traditional method of fitting isochrones to the white dwarf cooling sequence, and by employing a new Bayesian statistical technique that has been developed by our group. This new method performs an objective, simultaneous model fit of the cluster and stellar parameters (namely age, metallicity, distance, reddening, as well as individual stellar masses, mass ratios, and cluster membership) to the photometry. Based on this analysis, we measure a white dwarf age of 1.035 +/- 0.054 +/- 0.087 Gyr (uncertainties represent the goodness of model fits and discrepancy among models, respectively), in good agreement with the cluster's main sequence turnoff age. This work is part of our ongoing work to calibrate main sequence turnoff and white dwarf ages using open clusters, and to improve the precision of cluster ages to the ~5% level.