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Register a new userDiagnostics of Stellar Modelling from Spectroscopy and Photometry of Globular Clusters
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We conduct a series of comparisons between spectroscopic and photometric observations of globular clusters and stellar models to examine their predictive power. Data from medium-to-high resolution spectroscopic surveys of lithium allow us to investigate first dredge-up and extra mixing in two clusters well separated in metallicity. Abundances at first dredge-up are satisfactorily reproduced but there is preliminary evidence to suggest that the models overestimate the luminosity at which the surface composition first changes in the lowest-metallicity system. Our models also begin extra mixing at luminosities that are too high, demonstrating a significant discrepancy with observations at low metallicity. We model the abundance changes during extra mixing as a thermohaline process and determine that the usual diffusive form of this mechanism cannot simultaneously reproduce both the carbon and lithium observations. Hubble Space Telescope photometry provides turnoff and bump magnitudes in a large number of globular clusters and offers the opportunity to better test stellar modelling as function of metallicity. We directly compare the predicted main-sequence turn-off and bump magnitudes as well as the distance-independent parameter $Delta M_V ~^{rm{MSTO}}_{rm{bump}}$. We require 15 Gyr isochrones to match the main-sequence turn-off magnitude in some clusters and cannot match the bump in low-metallicity systems. Changes to the distance modulus, metallicity scale and bolometric corrections may impact on the direct comparisons but $Delta M_V ~^{rm{MSTO}}_{rm{bump}}$, which is also underestimated from the models, can only be improved through changes to the input physics. Overshooting at the base of the convective envelope with an efficiency that is metallicity dependent is required to reproduce the empirically determined value of $Delta M_V ~^{rm{MSTO}}_{rm{bump}}$.
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Recent work has revealed two classes of Globular Clusters (GCs), dubbed Type-I and Type-II. Type-II GCs are characterized by a blue- and a red- red giant branch composed of stars with different metallicities, often coupled with distinct abundances in the slow-neutron capture elements (s-elements). Here we continue the chemical tagging of Type-II GCs by adding the two least-massive clusters of this class, NGC1261 and NGC6934. Based on both spectroscopy and photometry, we find that red stars in NGC1261 are slightly enhanced in [Fe/H] by ~0.1 dex and confirm that red stars of NGC 6934 are enhanced in iron by ~0.2 dex. Neither NGC1261 nor NGC6934 show internal variations in the s-elements, which suggests a GC mass threshold for the occurrence of s-process enrichment. We found a significant correlation between the additional Fe locked in the red stars of Type-II GCs and the present-day mass of the cluster. Nevertheless, most Type II GCs retained a small fraction of Fe produced by SNe II, lower than the 2%; NGC6273, M54 and omega Centauri are remarkable exceptions. In the appendix, we infer for the first time chemical abundances of Lanthanum, assumed as representative of the s-elements, in M54, the GC located in the nucleus of the Sagittarius dwarf galaxy. Red-sequence stars are marginally enhanced in [La/Fe] by 0.10pm0.06 dex, in contrast with the large [La/Fe] spread of most Type II GCs. We suggest that different processes are responsible for the enrichment in iron and s-elements in Type-II GCs.
Evidence that the multiple populations (MPs) are common properties of globular clusters (GCs) is accumulated over the past decades from clusters in the Milky Way and in its satellites. This finding has revived GC research, and suggested that their formation at high redshift must have been a much-more complex phenomenon than imagined before. However, most information on MPs is limited to nearby GCs. The main limitation is that most studies on MPs rely on resolved stars, facing a major challenge to investigate the MP phenomenon in distant galaxies. Here we search for integrated colors of old GCs that are sensitive to the multiple-population phenomenon. To do this, we exploit integrated magnitudes of simulated GCs with MPs, and multi-band Hubble Space Telescope photometry of 56 Galactic GCs, where MPs are widely studied, and characterized as part of the UV Legacy Survey of Galactic GCs. We find that both integrated $C_{rm F275W,F336W,F438W}$ and $m_{rm F275W}-m_{rm F814W}$ colors strongly correlate with the iron abundance of the host GC. In second order, the pseudo two-color diagram built with these integrated colors is sensitive to the MP phenomenon. In particular, once removed the dependence from cluster metallicity, the color residuals depend on the maximum internal helium variation within GCs and on the fraction of second-generation stars. This diagram, which we define here for Galactic GCs, has the potential of detecting and characterizing MPs from integrated photometry of old GCs, thus providing the possibility to extend their investigation outside the Local Group.
Synthetic photometry is a great tool for studying globular clusters, especially for understanding the nature of their multiple populations. Our goal is to quantify the errors on synthetic photometry that are caused by uncertainties on stellar and observational/calibration parameters. These errors can be taken into account when building synthetic color-magnitude diagrams (CMDs) that are to be compared to observed CMDs. We have computed atmosphere models and synthetic spectra for two stars, Pollux and Procyon, that have stellar parameters typical of turn-off and bottom red giant branch stars in globular clusters. We then varied the effective temperature, surface gravity, microturbulence, the carbon, nitrogen, and oxygen abundances, and [Fe/H]. We quantified the effect on synthetic photometry in the following filters: Johnson UBVRI and HST F275W, F336W, F410M, F438W, F555W, F606W, and F814W. We estimated the effects of extinction, atmospheric correction, and of the Vega reference spectrum on the resulting photometry. We tested the ability of our models to reproduce the observed spectral energy distribution and observed photometry of the two stars. We show that variations are generally stronger in blue filters. Dispersions on synthetic colors due to uncertainties on stellar parameters vary between less than 0.01 and to 0.04 magnitude, depending on the choice of filters. Uncertainties on the zero points, the extinction law, or the atmospheric correction affect the resulting colors at a level of a few 0.01 magnitudes in a systematic way. The models reproduce the flux-calibrated spectral energy distribution of both stars well. Comparison between synthetic and observed UBVRI photometry shows a variable degree of (dis)agreement. The observed differences likely indicate that different calibration processes are performed to obtain respectively observed and synthetic photometry.
Sudden changes in the internal structure of stars, placed at the interface between convective and radiative regions, regions of partial ionisation, or between layers that have acquired different chemical composition as a result of nuclear burning, often produce specific signatures in the stars oscillation spectra. Through the study of these signatures one may gain information on the physical processes that shape the regions that produce them, including diffusion and chemical mixing beyond the convectively unstable regions, as well as information about the helium content of stars. In this talk, I will review important theoretical and observational efforts conducted over the years towards this goal. I will emphasise the potential offered by the study of acoustic, gravity, and mixed modes observed in stars of different mass and evolutionary stages, at a time when space-based data is allowing us to build on the knowledge gained from the study of the sun and white dwarfs, where these efforts have long been undertaken, extending the methods developed to stars across the HR diagramme.
We present Ca-CN-CH-NH photometry for the well-known globular cluster (GC) M3 (NGC 5272). We show new evidence for two M3 populations with distinctly different carbon and nitrogen abundances, seen in a sharp division between CN-weak and CN-strong red-giant branches (RGBs) in M3. The CN-strong population shows a C-N anticorrelation that is a natural consequence of the CN cycle, while the CN-weak population shows no or a weak C-N anticorrelation. Additionally, the CN-weak population exhibits an elongated spatial distribution that is likely linked to its fast rotation. Our derived metallicity reveals bimodal metallicity distributions in both populations, with $langle$[Fe/H]$rangleapprox-$1.60 and $-$1.45, which appear to be responsible for the discrete double RGB bumps in the CN-weak and the large $W^{1G}_{F275W-F814W}$ range. From this discovery, we propose that M3 consists of two GCs, namely the C1 (23%, $langle$[Fe/H]$rangleapprox-1.60$) and C2 (77%, $langle$[Fe/H]$rangleapprox-1.45$), each of which has its own C-N anticorrelation and structural and kinematical property, which is a strong indication of independent systems in M3. The fractions of the CN-weak population for both the C1 and C2 are high compared to Galactic GCs but they are in good agreement with GCs in Magellanic Clouds. It is believed that M3 is a merger remnant of the two GCs, most likely in a dwarf galaxy environment, and accreted to our Galaxy later in time. This is consistent with recent proposals of an ex-situ origin of M3.
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