We study the relationship between the iron abundance (IA) in red giant branch (RGB) stars and their radial distribution (RD) in Galactic globular clusters (GCs). We relied on publicly available archival data on IA in red giants (RGs) of GCs. We built a sample of ten target GCs in which the number of these RGs exceeded one hundred stars. In each GC of the sample, we compared the RDs of two sub-samples of stars, more iron-rich (IR) and more iron-poor (IP) than the clusters' mean values of [Fe/H]. Their RDs turned out to be different at statistically significant confidence levels in NGC 104 (47 Tuc), NGC 1851, NGC 3201, and NGC 6752 in the sense that the IP RGs were more centrally concentrated than their IR counterparts. In 47 Tuc, the difference is significant at a higher confidence level within the PRAD of R = 8.0', where the IA increases by Delta[Fe/H] ~ 0.03 dex toward the cluster outskirts. In the latter three GCs, Delta[Fe/H] ~ 0.05 dex. Interestingly, the V magnitude of the RGB bump and the horizontal branch was recently shown to fade outward in 47 Tuc and was suggested to originate from a He abundance trend. The fading caused by the IA trend is similar to that observed for the RGB bump. Although the difference between the RDs of IP and IR RGs is statistically insignificant in other GCs, NGC 288 is the only GC of the sample, in which IR RGB stars are formally more centrally concentrated. Interestingly, three of the four GCs are highly concentrated. (Abridged)
Bifurcated patterns of blue straggler stars in their color--magnitude diagrams have atracted significant attention. This type of special (but rare) pattern of two distinct blue straggler sequences is commonly interpreted as evidence of cluster core-collapse-driven stellar collisions as an efficient formation mechanism. Here, we report the detection of a bifurcated blue straggler distribution in a young Large MagellanicCloud cluster, NGC 2173. Because of the cluster's low central stellar number density and its young age, dynamical analysis shows that stellar collisions alone cannot explain the …
COSMIC-LAB: Unexpected Results from High-resolution Spectra of AGB Stars in Globular Clusters
本论文旨在阐明恒星演化中研究最少的阶段之一：渐近巨星分支（AGB）。最近对银河系球状星团（GC）的研究结果表明，AGB阶段可能包含了关于母星团中奇异星（Beccari et al. 2006）和多种群（Campbell et al. 2013）演化历史的重要信息。本论文分析了由甚大望远镜（ESO）和2.2米望远镜（MPG）获得的四个银河系GC中AGB星的大样本高分辨率光谱。所获得的结果提供了一个以前未知的物理机制的证据，影响大多数AGB星的大气中的一些化学元素的中性物种：因为它，来自中性线的丰度被系统地低估，而那些从电离线测量保持不受影响。这样的行为完全对应于恒星大气中非局部热力学平衡（NLTE）条件下的预期。出于这个原因，在这项工作中，我们将其称为“NLTE效应”，但要注意的是，情况可能并非如此。事实上，目前的NLTE模型无法解释观测到的效应，这表明我们对NLTE的理解还不够，或者AGB大气中发生了一些更复杂的物理现象。在所有研究的GC中都发现了这种效应。它影响大多数（但不是全部）AGB星，在某些情况下，也影响一些RGB星。对于铁和钛线（即，提供最大数量的中性线和电离线的元素）。对检测到的现象的深入理解至关重要，因为它对正确确定GC化学和富集历史具有巨大影响（例如，它可以模拟虚假的铁扩散）。
Ultra-diffuse galaxies (UDGs) are unusual galaxies with low luminosities, similar to classical dwarf galaxies, but sizes up to $\sim\!5$ larger than expected for their mass. Some UDGs have large populations of globular clusters (GCs), something unexpected in galaxies with such low stellar density and mass. We have carried out a comprehensive study of GCs in both UDGs and classical dwarf galaxies at comparable stellar masses using HST observations of the Coma cluster. We present new imaging for 33 Dragonfly UDGs with the largest effective radii ($>2$ kpc), and additionally include 15 UDGs and 5…
We present a detailed analysis of the kinematic properties of the multiple populations (MPs) in the low-mass Galactic globular cluster NGC6362 based on a sample of about 500 member stars for which radial velocities (RVs), Fe and Na abundances have been homogeneously derived. At distances from the cluster center larger than about 0.5 $r_h$, we find that first (FG - Na-poor) and second generation (SG - Na-rich) stars show hints of different line-of-sight velocity dispersion profiles, with FG stars being dynamically hotter. This is the first time that differences in the velocity dispersion of MPs…
We use the Keck Cosmic Web Imager integral-field unit spectrograph to: 1) measure the global stellar population parameters for the ultra-diffuse galaxy (UDG) Dragonfly 44 (DF44) to much higher precision than previously possible for any UDG, and 2) for the first time measure spatially-resolved stellar population parameters of a UDG. We find that DF44 falls below the mass--metallicity relation established by canonical dwarf galaxies both in and beyond the Local Group. We measure a flat radial age gradient ($m_{\rm age} \sim +0.01_{-0.08}^{+0.07}$ log Gyr kpc$^{-1}$) and a flat-to-positive metall…
We use N-body simulations to revisit the globular cluster (GC) ``timing problem'' in the Fornax dwarf spheroidal (dSph). In agreement with earlier work, we find that, due to dynamical friction, GCs sink to the center of dark matter halos with a cuspy inner density profile but ``stall'' at roughly 1/3 of the core radius ($r_{\rm core}$) in halos with constant-density cores. The timescales to sink or stall depend strongly on the mass of the GC and on the initial orbital radius, but are essentially the same for either cuspy (NFW) or cored halos normalized to have the same total mass within $r_{\r…
Stars in massive star clusters exhibit intrinsic variations in some light elements (MPs) that are difficult to explain in a coherent formation scenario. In recent years, HST photometry has led to the characterisation of the global properties of th MPs in an unparalleled level of detail. In particular, the colour-colour diagrams known as chromosome maps have been proven to be very efficient at separating cluster stars with field-like metal abundance distribution (1P) from object with distinctive light-element abundance anti-correlations (2P). The unexpected wide colour ranges covered by the 1P …
MATLAS-42, A Globular Cluster-Rich Ultra-Diffuse Galaxy That Diverges from the "Failed Galaxy'' Formation Pathway
Swinburne University（斯威本科技大学）；Instituto de Astrofísica de Canarias（加那利群岛天体物理学研究所）；Universidad de La Laguna（拉古纳大学）；San José State University（圣何塞州立大学）；University of California Santa Cruz（加州大学圣克鲁兹分校）；Yale University（耶鲁大学）
Asymptotic giant branch (AGB) stars in the globular cluster NGC6752 have been found to exhibit some chemical peculiarities with respect to the red giant branch (RGB) stars. A discrepancy between [FeI/H] and [FeII/H] (not observed in RGB stars) has been detected adopting spectroscopic temperatures. Moreover, a possible lack of second-population stars along the AGB was claimed. The use of photometric temperatures based on (V-K) colors was proposed to erase this iron discrepancy. Also, ad hoc scenarios have been proposed to explain the absence of second-population AGB stars. Here we analyzed a sa…
Multiple populations along the asymptotic giant branch: a Gaia+APOGEE study of 22 Galactic globular clusters
American University of Sharjah（沙迦美国大学）；Istituto Nazionale di Astrofisica - Osservatorio Astronomico di Padova（意大利国家天体物理研究所帕多瓦天文台）；Dipartimento di Fisica e Astronomia “Galileo Galilei”, Univ. di Padova（帕多瓦大学伽利略·伽利莱物理与天文学系）；Research School of Astronomy and Astrophysics, Australian National University（澳大利亚国立大学天文与天体物理研究学院）；NSF’s NOIRLab（美国国家科学基金会NOIR实验室）；Space Telescope Science Institute（太空望远镜科学研究所）；Dipartimento di Tecnica e Gestione dei Sistemi Industriali, Università degli Studi di Padova（帕多瓦大学工业系统技术与管理系）
Historically, photometry has been largely used to identify stellar populations (MPs) in Globular Clusters (GCs) by using diagrams that are based on colours and magnitudes that are mostly sensitive to stars with different metallicities or different abundances of helium, carbon, nitrogen and oxygen. In particular, the pseudo two-colour diagram called chromosome map (ChM), allowed the identification and the characterization of MPs in about 70 GCs by using appropriate filters of the Hubble Space Telescope (HST) that are sensitive to the stellar content of He, C, N, O and Fe. We use here high-preci…
NGC 6441 is a bulge globular cluster with an unusual horizontal branch morphology and a rich population of RR Lyrae (RRL) and Type II Cepheid (T2C) variables that is unexpected for its relatively high metallicity. We present near-infrared (NIR, $JHK_s$) time-series observations of 42 RRL, 8 T2Cs, and 10 eclipsing binary candidate variables in NGC 6441. The multi-epoch observations were obtained using the FLAMINGOS-2 instrument on the 8-m Gemini South telescope. Multi-band data are used to investigate pulsation properties of RRL and T2C variables including their light curves, instability strip,…
Aims. We use the spectra of more than 30,000 red giant branch (RGB) stars in 25 globular clusters (GC), obtained within the MUSE survey of Galactic globular clusters, to calibrate the Ca II triplet (CaT) metallicity relation and derive metallicities for all individual stars. We investigate the overall metallicity distributions as well as those of the different populations within each cluster. Methods. The Ca II triplet in the near-infrared at 8498, 8542, and 8662 AA is visible in stars with spectral types between F and M and can be used to determine their metallicities. In this work, we calibr…
We report a possible discovery of a relic star cluster in the Sextans dwarf spheroidal galaxy. Using the \textit{hk} index ($\equiv$($Ca-b$)$-$($b-y$)) as a photometric metallicity indicator, we discriminate the metal-poor and metal-rich stars in the galaxy and find unexpected number density excess of metal-poor stars located 7'.7 ($\sim$190 pc in projected distance) away from the known galactic center. The $V-I$ color$-$magnitude diagram (CMD) for stars around the density excess reveals that both the main sequence and the giant branch are considerably narrower and redder than the bulk of fiel…
Blue Stragglers are stars located in an unexpected region of the color-magnitude diagram of a stellar population, as they appear bluer and more luminous than the stars in the turnoff region. They are ubiquitous, since they have been found among Milky Way field stars, in open and globular clusters, and also in other galaxies of the Local Group. Here we present a study on the blue straggler population of the old and metal-rich open cluster Collinder 261, based on Gaia DR2 data and on a multi-epoch radial velocity survey conducted with FLAMES@VLT. We also analyze the radial distribution of the bl…
Context. Sulfur (S) is one of the lesser-studied $α$-elements. Published investigations of its behavior have so far focused on local stars, and only a few clusters of the Milky Way have been considered to study this topic. We aim to study the S content of the globular cluster Ruprecht 106 -- which has never before been studied for this purpose, but is known to present low levels of the [$α$/Fe] abundance ratio -- and the open cluster Trumpler 5. The only star studied so far in Trumpler 5 shows an unexpectedly low abundance of S. Aims. With this work, we aim to provide the first S abundance in …
