Abstract
Recent work has revealed two classes of globular clusters (GCs), dubbed Type I and Type II. Type II GCs are characterized by both 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, NGC 1261 and NGC 6934. Based on both spectroscopy and photometry, we find red stars in NGC 1261 to be 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 NGC 1261 nor NGC 6934 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%; NGC 6273, M54, and
ω
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 0.10 ± 0.06 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.
Abstract We present the first spectroscopic estimates of the chemical abundance of M dwarf stars in a globular cluster (GC), namely 47 Tucanae. By exploiting NIRSpec on board the James Webb Space ...Telescope, we gathered low-resolution spectra for 28 stars with masses in the range ∼0.4–0.5 M ⊙ . The spectra are strongly affected by the H 2 O water vapor bands, which can be used as indicators of oxygen abundance. The spectral analysis reveals that the target stars feature a different O abundance, with a difference of ∼0.40 dex between the first and the most polluted second population. The observed range is similar to that observed among red giant stars. This result reinforces previous findings based on the analysis of photometric diagrams, including the “chromosome maps,” providing a first, and more direct, evidence of light element variations in the M dwarfs’ mass regime. The observation that the multiple populations, with their variations in light elements, exhibit the same patterns from the lower main sequence all the way to the red giant branch further strengthens the notion that multiple stellar populations in GCs formed in a series of bursts of star formation.
In a Globular Cluster (GC), the mass loss during the red giant branch (RGB) phase and the helium content are fundamental ingredients to constrain the horizontal-branch (HB) morphology. While many ...papers have been dedicated to the helium abundance in different stellar populations, small efforts have been made to disentangle the effects of mass loss and helium content. We exploit the nearby GC NGC 6121 (M4), which hosts two well-studied main stellar populations, to infer both helium and RGB mass loss. We combine multi-band Hubble Space Telescope photometry of RGB and main-sequence (MS) stars of M4 with synthetic spectra to constrain the relative helium content of its stellar populations. We find that the second-generation stars in M4 are enhanced in helium mass fraction by with respect to the remaining stars that have pristine helium content. We then infer the mass of the HB stars by searching for the best match between the observations and HB populations modeled assuming the helium abundance of each population estimated from the MS. By comparing the masses of stars along the HB, we constrain the mass loss of first- and second-generation stars in M4. We find that the mass lost by the helium-enriched population is ∼13% larger than the mass lost by the first-generation stars ( ). We discuss the possibility that this mass-loss difference depends on helium abundance, the different formation environment of the two generations, or a combination of both.
Abstract
The
Hubble
Space
Telescope
(
HST
) UV Legacy Survey of Galactic Globular Clusters (GCs) has investigated multiple stellar populations by means of the “chromosome map” (ChM) diagnostic ...tool that maximizes the separation between stars with different chemical compositions. One of the most challenging features revealed by ChM analysis is the apparent inhomogeneity among stars belonging to the first population, a phenomenon largely attributed to He variations. However, this explanation is not supported by uniformity in the
p
-capture elements of these stars. The
HST
survey has revealed that the GC NGC 3201 shows exceptionally wide coverage in the
parameter of the ChM. We present a chemical abundance analysis of 24 elements in 18 giants belonging to the first population of this GC and having a wide range in
. As far as the
p
-capture elements are concerned, the chemical abundances are typical of first-generation (1G) stars, as expected from the location of our targets in the ChM. Based on radial velocities and chemical abundance arguments, we find that the three stars with the lowest
values are binary candidates. This suggests that at least those stars could be explained with binarity. These results are consistent with evidence inferred from multiband photometry that evolved blue stragglers (BSs) populate the bluest part of the 1G sequence in the ChM. The remaining 15 spectroscopic targets show a small range in the overall metallicity by ∼0.10 dex, with stars at higher
values having higher absolute abundances. We suggest that a small variation in metals and binarity governs the color spread of the 1G in the ChM and that evolved BSs contribute to the bluest tail of the 1G sequence.
Abstract
A critical issue in the asymptotic giant branch (AGB) self-enrichment scenario for the formation of multiple populations in globular clusters (GCs) is the inability to reproduce the ...magnesium isotopic ratios, despite the model in principle can account for the depletion of magnesium. In this work, we analyse how the uncertainties on the various p-capture cross sections affect the results related to the magnesium content of the ejecta of AGB stars. The observed distribution of the magnesium isotopes and of the overall Mg-Al trend in M13 and NGC 6752 are successfully reproduced when the proton-capture rate by 25Mg at the temperatures ∼100 MK, in particular the 25Mg(p, γ)26Alm channel, is enhanced by a factor ∼3 with respect to the most recent experimental determinations. This assumption also allows us to reproduce the full extent of the Mg spread and the Mg–Si anticorrelation observed in NGC 2419. The uncertainties in the rate of the 25Mg(p, γ)26Alm reaction at the temperatures of interest here leave space for our assumption and we suggest that new experimental measurements are needed to settle this problem. We also discuss the competitive model based on the supermassive star nucleosynthesis.
ABSTRACT
We use images collected with the near-infrared camera (NIRCam) onboard the JWST and with the Hubble Space Telescope (HST) to investigate multiple populations at the bottom of the main ...sequence (MS) of 47 Tucanae. The mF115W versus mF115W − mF322W2 colour–magnitude diagram (CMD) from NIRCam shows that, below the knee, the MS stars span a wide colour range, where the majority of M-dwarfs exhibit blue colours, and a tail of stars are distributed towards the red. A similar pattern is observed from the mF160W versus mF110W − mF160W CMD from HST, and multiple populations of M-dwarfs are also visible in the optical mF606W versus mF606W − mF814W CMD. The NIRCam CMD shows a narrow sequence of faint MS stars with masses smaller than $0.1\, \mathcal {M}_{\odot }$. We introduce a chromosome map of M-dwarfs that reveals an extended first population and three main groups of second-population stars. By combining isochrones and synthetic spectra with appropriate chemical composition, we simulate colours and magnitudes of different stellar populations in the NIRCam filters (at metallicities Fe/H = −1.5 and Fe/H = −0.75) and identify the photometric bands that provide the most efficient diagrams to investigate the multiple populations in globular clusters. Models are compared with the observed CMDs of 47 Tucanae to constrain M-dwarfs’ chemical composition. Our analysis suggests that the oxygen range needed to reproduce the colours of first- and second-population M-dwarfs is similar to that inferred from spectroscopy of red giants, constraining the proposal that the chemical variations are due to mass transfer phenomena in proto-clusters.
Abstract
Multiple stellar populations (MPs) are a distinct characteristic of globular clusters (GCs). Their general properties have been widely studied among main-sequence, red giant branch (RGB), ...and horizontal branch (HB) stars, but a common framework is still missing at later evolutionary stages. We studied the MP phenomenon along the asymptotic giant branch (AGB) sequences in 58 GCs, observed with the Hubble Space Telescope in UV and optical filters. Using UV–optical color–magnitude diagrams, we selected the AGB members of each cluster and identified the AGB candidates of the metal-enhanced population in type II GCs. We studied the photometric properties of the AGB stars and compared them to theoretical models derived from synthetic spectral analysis. We observed the following features: (i) the spread of AGB stars in photometric indices sensitive to variations of light elements and helium is typically larger than that expected from photometric errors; (ii) the fraction of metal-enhanced stars in the AGB is lower than that in the RGB in most of the type II GCs; (iii) the fraction of 1G stars derived from the chromosome map of AGB stars in 15 GCs is larger than that of RGB stars; and (v) the AGB/HB frequency correlates with the average mass of the most helium-enriched population. These findings represent clear evidence of the presence of MPs along the AGB of Galactic GCs and indicate that a significant fraction of helium-enriched stars, which have lower mass in the HB, do not evolve to the AGB phase, leaving the HB sequence toward higher effective temperatures, as predicted by the AGB manqué scenario.
Abstract
We present a timing analysis of the 2015 outburst of the accreting millisecond X-ray pulsar SAX J1808.4−3658, using non-simultaneous XMM–Newton and NuSTAR observations. We estimate the ...pulsar spin frequency and update the system orbital solution. Combining the average spin frequency from the previous observed, we confirm the long-term spin-down at an average rate
$\dot{\nu }_{{\rm SD}}=1.5(2)\times 10^{-15}$
Hz s−1. We also discuss possible corrections to the spin-down rate accounting for mass accretion on to the compact object when the system is X-ray active. Finally, combining the updated ephemerides with those of the previous outbursts, we find a long-term orbital evolution compatible with a binary expansion at a mean rate
$\dot{P}_{{\rm orb}}=3.6(4)\times 10^{-12}$
s s−1, in agreement with previously reported values. This fast evolution is incompatible with an evolution driven by angular momentum losses caused by gravitational radiation under the hypothesis of conservative mass transfer. We discuss the observed orbital expansion in terms of non-conservative mass transfer and gravitational quadrupole coupling mechanism. We find that the latter can explain, under certain conditions, small fluctuations (of the order of few seconds) of the orbital period around a global parabolic trend. At the same time, a non-conservative mass transfer is required to explain the observed fast orbital evolution, which likely reflects ejection of a large fraction of mass from the inner Lagrangian point caused by the irradiation of the donor by the magnetodipole rotator during quiescence (radio-ejection model). This strong outflow may power tidal dissipation in the companion star and be responsible of the gravitational quadrupole change oscillations.
Abstract
Recent work has shown that near-infrared (NIR) Hubble Space Telescope (HST) photometry allows us to disentangle multiple populations (MPs) among M dwarfs of globular clusters (GCs) and to ...investigate this phenomenon in very-low-mass (VLM) stars. Here, we present the color–magnitude diagrams of nine GCs and the open cluster NGC 6791 in the F110W and F160W bands of HST, showing that the main sequences (MSs) below the knee are either broadened or split, thus providing evidence of MPs among VLM stars. In contrast, the MS of NGC 6791 is consistent with a single population. The color distribution of M dwarfs dramatically changes between different GCs, and the color width correlates with the cluster mass. We conclude that the MP ubiquity, variety, and dependence on GC mass are properties common to VLM and more-massive stars. We combined UV, optical, and NIR observations of NGC 2808 and NGC 6121 (M4) to identify MPs along with a wide range of stellar masses (∼0.2–0.8
⊙
), from the MS turnoff to the VLM regime, and measured, for the first time, their mass functions (MFs). We find that the fraction of MPs does not depend on the stellar mass and that their MFs have similar slopes. These findings indicate that the properties of MPs do not depend on stellar mass. In a scenario where the second generations formed in higher-density environments than the first generations, the possibility that the MPs formed with the same initial MF would suggest that it does not depend on the environment.
Context.
Asteroseismic investigations of solar-like oscillations in giant stars allow for the derivation of their masses and radii. For members of open clusters, this allows us to obtain an age ...estimate of the cluster that is expected to be identical to the age estimate given by the colour-magnitude diagram, but independent of the uncertainties that are present for that type of analysis. Thus, a more precise and accurate age estimate can be obtained.
Aims.
We aim to identify and measure the asteroseismic properties of oscillating giant members of the open cluster NGC 6866 and utilise them for a cluster age estimate. Model comparisons also allow constraints to be placed on the stellar physics. Here, we investigate the efficiency of convective-core overshoot during the main sequence evolution, which has a significant influence on the age estimations for these relatively massive giants. The effects of rotation and core overshoot are similar, but not identical, and so, we also investigated the potential of our measurements to distinguish between these effects.
Methods.
We identified six giant members of NGC 6866 via photometry, proper motions, and parallaxes from
Gaia
, and spectroscopic literature measurements. These were combined with asteroseismic measurements, which we derived using photometric data from the
Kepler
mission for five of the stars. Comparisons to stellar-model isochrones constrained the convective-core overshoot and enable a more precise and accurate age estimate than previously possible.
Results.
A significant amount of differential reddening is found for NGC 6866. Asteroseismology establishes the helium-core burning evolutionary phase for the giants, which have a mean mass of 2.8
M
⊙
. Their radii are significantly smaller than predicted by current 1D stellar models unless the amount of convective-core overshoot on the main sequence is reduced to
α
ov
≤ 0.1 ⋅
H
p
in the step-overshoot description. Our measurements also suggest that rotation has affected the evolution of the stars in NGC 6866 in a way that is consistent with 3D simulations, but not with current 1D stellar models. The age of NGC 6866 is estimated to be 0.43 ± 0.05 Gyr, which is significantly younger and more precise than most previous estimates.
Conclusions.
We derive a precise cluster age while constraining convective-core overshooting and the effects of rotation in the stellar models. A comparison to age estimates from machine learning methods of the same and similar giant stars uncovers potential biases for automated asteroseismic and non-asteroseismic age estimates of helium-core burning stars.
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