A white dwarf cooling age of 8 Gyr for NGC 6791 from physical separation processes

NGC 6791 is a well studied open cluster1 that it is so close to us that can be imaged down to very faint luminosities. The main sequence turn-off age (~8 Gyr) and the age derived from the termination of the white dwarf cooling sequence (~6 Gyr) are significantly different. One possible explanation is that as white dwarfs cool, one of the ashes of helium burning, 22Ne, sinks in the deep interior of these stars. At lower temperatures, white dwarfs are expected to crystallise and phase separation of the main constituents of the core of a typical white dwarf, 12C and 16O, is expected to occur. This sequence of events is expected to introduce significant delays in the cooling times, but has not hitherto been proven. Here we report that, as theoretically anticipated, physical separation processes occur in the cores of white dwarfs, solving the age discrepancy for NGC 6791.Comment: 3 pages, 2 figures, published in Natur

The broadening of the main sequence in the open cluster M38

Our recent multi-band photometric study of the colour width of the lower main sequence of the open cluster M37 has revealed the presence of a sizeable initial chemical composition spread in the cluster. If initial chemical composition spreads are common amongst open clusters, this would have major implications for cluster formation models and the foundation of the chemical tagging technique. Here we present a study of the unevolved main sequence of the open cluster M38, employing Gaia DR3 photometry and astrometry, together with newly acquired Sloan photometry. We have analysed the distribution of the cluster’s lower main sequence stars with a differential colour-colour diagram made of combinations of Gaia and Sloan magnitudes, like in the study of M37. We employed synthetic stellar populations to reproduce the observed trend of M38 stars in this diagram, and found that the observed colour spreads can be explained simply by the combined effect of differential reddening across the face of the cluster and the presence of unresolved binaries. There is no need to include in the synthetic sample a spread of initial chemical composition as instead necessary to explain the main sequence of M37. Further photometric investigations like ours, as well as accurate differential spectroscopic analyses on large samples of open clusters, are necessary to understand whether chemical abundance spreads are common among the open cluster population

Ne22 distillation and the cooling sequence of the old metal-rich open cluster NGC 6791

Recent Monte Carlo plasma simulations to study in crystallizing carbon-oxygen (CO) white dwarfs (WDs) the phase separation of Ne22 (the most abundant metal after carbon and oxygen) have shown that, under the right conditions, a distillation process that transports Ne22 toward the WD centre is efficient and releases a considerable amount of gravitational energy that can lead to cooling delays of up to several Gyr. Here we present the first CO WD stellar evolution models that self-consistently include the effect of neon distillation, and cover the full range of CO WD masses, for a progenitor metallicity twice-solar appropriate for the old open cluster NGC 6791. The old age (about 8.5 Gyr) and high metallicity of this cluster -- hence the high neon content (about 3% by mass) in the cores of its WDs -- maximize the effect of neon distillation in the models to be compared with the observed cooling sequence. We discuss the effect of distillation on the internal chemical stratification and cooling time of the models, confirming that distillation causes cooling delays up to several Gyr, that depend in a non-monotonic way on the mass. We also show how our models produce luminosity functions (LFs) that can match the faint end of the observed WD LF in NGC 6791, for ages consistent with the range determined from a sample of cluster's eclipsing binary stars, and the main sequence turn-off. Without the inclusion of distillation the theoretical WD cooling sequences reach too faint magnitudes compared to the observations. We also propose James Webb Space Telescope observations that can independently demonstrate the efficiency of neon distillation in the interiors of NGC 6791 WDs, and help resolve the current uncertainty on the treatment of the electron conduction opacities for the hydrogen-helium envelope of the WD models

Signature of a chemical spread in the open cluster M37

Recent Gaia photometry of the open cluster M37 have disclosed the existence of an extended main-sequence turn off -- like in Magellanic clusters younger than about 2 Gyr -- and a main sequence that is broadened in colour beyond what is expected from the photometric errors, at magnitudes well below the region of the extended turn off, where neither age differences nor rotation rates (the candidates to explain the extended turn off phenomenon) are expected to play a role. Moreover, not even the contribution of unresolved binaries can fully explain the observed broadening. We investigated the reasons behind this broadening by making use of synthetic stellar populations and differential colour-colour diagrams using a combination of Gaia and Sloan filters. From our analysis we have concluded that the observed colour spread in the Gaia colour-magnitude diagram can be reproduced by a combination of either a metallicity spread Delta[Fe/H] ~ 0.15 plus a differential reddening across the face of the cluster spanning a total range DeltaE (B - V) ~ 0.06, or a spread of the initial helium mass fraction DeltaY ~ 0.10 plus a smaller range of reddening DeltaE (B - V) ~ 0.03. High-resolution differential abundance determinations of a sizeable sample of cluster stars are necessary to confirm or exclude the presence of a metal abundance spread. Our results raise the possibility that also individual open clusters, like globular clusters and massive star clusters, host stars born with different initial chemical compositions

Exploring the origin of the extended main sequence turn off in M37 through the white dwarf cooling sequence

We use new observations from the Canada-France-Hawaii Telescope to study the white dwarf cooling sequence of the open cluster M37, a cluster that displays an extended main sequence turn-off and, according to a recent photometric analysis, also a spread of initial chemical composition. By taking advantage of a first epoch collected in 1999 with the same telescope, we have been able to calculate proper motions for sources as faint as g ∼ 26 (about ∼ 6 magnitudes fainter than the Gaia limit), allowing us to separate cluster members from field stars. This has enabled us to isolate a sample of the white dwarf population of M37, reaching the end of the cooling sequence (at g ∼ 23.5). The here-derived atlas and calibrated catalogue of the sources in the field of view is publicly released as supplementary on-line material. Finally, we present an exhaustive comparison of the white dwarf luminosity function with theoretical models, which has allowed us to exclude the age-spread scenario as the main responsible for the extended turnoff seen in the cluster colour-magnitude-diagram

Photometry and astrometry with JWST - III. A NIRCam-Gaia DR3 analysis of the open cluster NGC 2506

In the third paper of this series aimed at developing the tools for analysing resolved stellar populations using the cameras on board of the James Webb Space Telescope (JWST), we present a detailed multiband study of the 2 Gyr Galactic open cluster NGC 2506. We employ public calibration data sets collected in multiple filters to: (i) derive improved effective Point Spread Functions (ePSFs) for 10 NIRCam filters; (ii) extract high-precision photometry and astrometry for stars in the cluster, approaching the main sequence (MS) lower mass of ∼0.1 M·; and (iii) take advantage of the synergy between JWST and Gaia DR3 to perform a comprehensive analysis of the cluster's global and local properties. We derived a MS binary fraction of ∼57.5 per cent, extending the Gaia limit (∼0.8 M·) to lower masses (∼0.4 M·) with JWST. We conducted a study on the mass functions (MFs) of NGC 2506, mapping the mass segregation with Gaia data, and extending MFs to lower masses with the JWST field. We also combined information on the derived MFs to infer an estimate of the cluster present-day total mass. Lastly, we investigated the presence of white dwarfs (WDs) and identified a strong candidate. However, to firmly establish its cluster membership, as well as that of four other WD candidates and of the majority of faint low-mass MS stars, further JWST equally deep observations will be required. We make publicly available catalogues, atlases, and the improved ePSFs

The HST large programme on NGC 6752 – III. Detection of the peak of the white dwarf luminosity function

We report on the white dwarf (WD) cooling sequence of the old globular cluster NGC 6752, which is chemically complex and hosts a blue horizontal branch. This is one of the last globular cluster WD cooling sequences accessible to imaging by the Hubble Space Telescope. Our photometry and completeness tests show that we have reached the peak of the luminosity function of the WD cooling sequence, at a magnitude mF606W = 29.4 ± 0.1, which is consistent with a formal age of ∼14 Gyr. This age is also consistent with the age from fits to the main-sequence turn-off (13–14 Gyr), reinforcing our conclusion that we observe the expected accumulation of WDs along the cooling sequence. © 2019 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Societ

Photometry and astrometry with JWST-I. NIRCam point spread functions and the first JWST colour-magnitude diagrams of a globular cluster

As the James Webb Space Telescope (JWST) has become fully operational, early release data are now available to begin building the tools and calibrations for precision point-source photometry and astrometry in crowded cluster environments. Here, we present our independent reduction of NIRCam imaging of the metal-poor globular cluster M 92, which were collected under Director's Discretionary Early Release Science programme ERS-1334. We derived empirical models of the point spread function (PSF) for filters F090W, F150W, F277W, and F444W, and find that these PSFs: (i) are generally undersampled (FWHM ∼2 pixel) in F150W and F444W and severely undersampled (FWHM ∼1 pixel) in F090W and F277W; (ii) have significant variation across the field of view, up to ∼15-20 per cent; and (iii) have temporal variations of ∼3-4 per cent across multi-epoch exposures. We deployed our PSFs to determine the photometric precision of NIRCam for stars in the crowded, central regions of M 92, measured to be at the ∼0.01 mag level. We use these data to construct the first JWST colour-magnitude diagrams of a globular cluster. Employing existing stellar models, we find that the data reach almost the bottom of the M 92 main sequence (∼0.1 M), and reveal 24 white dwarf candidate members of M 92 in the brightest portion of the white dwarf cooling sequence. The latter are confirmed through a cross-match with archival HST UV and optical data. We also detect the presence of multiple stellar populations along the low-mass main sequence of M 92

The Age of the Milky Way Inner Halo

The Milky Way galaxy is observed to have multiple components with distinct properties, such as the bulge, disk, and halo. Unraveling the assembly history of these populations provides a powerful test to the theory of galaxy formation and evolution, but is often restricted due to difficulties in measuring accurate stellar ages for low mass, hydrogen-burning stars. Unlike these progenitors, the "cinders" of stellar evolution, white dwarf stars, are remarkably simple objects and their fundamental properties can be measured with little ambiguity from spectroscopy. Here I report observations and analysis of newly formed white dwarf stars in the halo of the Milky Way, and a comparison to published analysis of white dwarfs in the well-studied 12.5 billion-year-old globular cluster Messier 4. From this, I measure the mass distribution of the remnants and invert the stellar evolution process to develop a new relation that links this final stellar mass to the mass of their immediate progenitors, and therefore to the age of the parent population. By applying this technique to a small sample of four nearby and kinematically-confirmed halo white dwarfs, I measure the age of local field halo stars to be 11.4 +/- 0.7 billion years. This age is directly tied to the globular cluster age scale, on which the oldest clusters formed 13.5 billion years ago. Future (spectroscopic) observations of newly formed white dwarfs in the Milky Way halo can be used to reduce the present uncertainty, and to probe relative differences between the formation time of the last clusters and the inner halo.Comment: Published in Nature, 2012, 486, 90. Second version corrects a missing reference (#10) in the third paragraph and Figure 1 captio

The HST Large Program on ω Centauri. V. Exploring the Ultracool Dwarf Population with Stellar Atmosphere and Evolutionary Modeling

Brown dwarfs can serve as both clocks and chemical tracers of the evolutionary history of the Milky Way due to their continuous cooling and high sensitivity of spectra to composition. We focus on brown dwarfs in globular clusters that host some of the oldest coeval populations in the galaxy. Currently, no brown dwarfs in globular clusters have been confirmed, but they are expected to be uncovered with advanced observational facilities such as the James Webb Space Telescope (JWST). In this paper we present a new set of stellar models specifically designed to investigate low-mass stars and brown dwarfs in ω Centauri - the largest known globular cluster. The parameters of our models were derived from iterative fits to Hubble Space Telescope photometry of the main-sequence members of the cluster. Despite the complex distribution of abundances and the presence of multiple main sequences in ω Centauri, we find that the modal color-magnitude distribution can be represented by a single stellar population with parameters determined in this study. The observed luminosity function is well represented by two distinct stellar populations having solar and enhanced helium mass fractions and a common initial mass function, in agreement with previous studies. Our analysis confirms that the abundances of individual chemical elements play a key role in determining the physical properties of low-mass cluster members. We use our models to draw predictions of brown dwarf colors and magnitudes in anticipated JWST NIRCam data, confirming that the beginning of the substellar sequence should be detected in ω Centauri in forthcoming observations