Main-sequence Turnoff Stars Research Articles

Li Evolution among Stars of Low/Intermediate Mass: the Metal-deficient Open Cluster NGC 2204

We have analyzed high-dispersion spectra in the Li 6708 Å region for 167 stars within the anticenter cluster NGC 2204. From 105 probable members, abundance analysis of 45 evolved stars produces [Fe/H] = −0.40 ± 0.12, [Si/Fe] = 0.14 ± 0.12, [Ca/Fe] = 0.29 ± 0.07, and [Ni/Fe] = −0.12 ± 0.10, where quoted errors are standard deviations. With E(B − V) = 0.07 and (m − M)0 = 13.12, appropriate isochrones provide an excellent match from the main sequence through the tip of the giant branch for an age of 1.85 ± 0.05 Gyr. Li spectrum synthesis produces A(Li) below 1.4 at the base of the red giant branch to a detectable value of −0.4 at the tip. Six probable asymptotic giant branch stars and all but one red clump star have only Li upper limits. A rapidly rotating red giant is identified as a possible Li-rich giant, assuming it is a red clump star. Main-sequence turnoff stars have a well-defined A(Li) = 2.83 ± 0.03 (sem) down to the Li-dip wall at the predicted mass of 1.29 M ☉. Despite having the same isochronal age as the more metal-rich NGC 2506, the luminosity distribution of red giants reflects a younger morphology similar to NGC 7789, possibly indicating a deeper impact of metallicity on stellar structure and A(Li) than previously assumed. As in NGC 2506 and NGC 7789, the NGC 2204 turnoff exhibits a broad range of rotation speeds, making abundance estimation impossible for some stars. The place of the cluster within Galactic A(Li) evolution is discussed.

The wave-like disc oscillations of mono-age stellar populations in the Solar neighbourhood from Gaia DR3

ABSTRACT The North–South asymmetry in the number density and bulk velocity of stars in the Solar neighbourhood provides valuable insights into the formation and evolution of the Milky Way disc. Our objective is to investigate the wave-like disc oscillations of mono-age stellar populations in the Solar neighbourhood using data from Gaia Data Release 3. We have selected a comprehensive sample of main-sequence turn-off stars. The ages of these stars can be accurately determined using isochrone fitting methods. Our findings indicate that the North–South density and mean vertical velocity asymmetries remain consistent across all age groups. The uniformity of perturbations across all subsamples suggests that all populations are responding to the same external influence, which likely affects them irrespective of their age. Moreover, the fact that these perturbations appear consistently implies they could be either ongoing or recent. Regarding vertical velocity dispersions, we observe that older stars exhibit larger dispersions.

Chemical clocks and their time zones: understanding the [s/Mg]–age relation with birth radii

ABSTRACT The relative enrichment of s-process to α-elements ([s/α]) has been linked with age, providing a potentially useful avenue in exploring the Milky Way’s chemical evolution. However, the age–[s/α] relationship is non-universal, with dependencies on metallicity and current location in the Galaxy. In this work, we examine these chemical clock tracers across birth radii (${R}_\text{birth}$), recovering the inherent trends between the variables. We derive ${R}_\text{birth}$ and explore the [s/α]–age–${R}_\text{birth}$ relationship for 36 652 APOGEE DR17 red giant and 24 467 GALAH DR3 main-sequence turn-off and subgiant branch disc stars using [Ce/Mg], [Ba/Mg], and [Y/Mg]. We discover that the age–$\rm [{\it s}/Mg]$ relation is strongly dependent on birth location in the Milky Way, with stars born in the inner disc having the weakest correlation. This is congruent with the Galaxy’s initially weak, negative $\rm [{\it s}/Mg]$ radial gradient, which becomes positive and steep with time. We show that the non-universal relations of chemical clocks is caused by their fundamental trends with ${R}_\text{birth}$ over time, and suggest that the tight age–$\rm [{\it s}/Mg]$ relation obtained with solar-like stars is due to similar ${R}_\text{birth}$ for a given age. Our results are put into context with a Galactic chemical evolution model, where we demonstrate the need for data-driven nucleosynthetic yields.

Linking the Metallicity Enrichment History to the Star Formation History: An SFH-regulated Chemical Evolution Model and Its Implications for the Gas Cycling Process

The metallicity enrichment history (MEH) of a galaxy is determined by its star formation history (SFH) and the gas cycling process. In this paper, we construct a chemical evolution model that is regulated by the SFH of the system. In this SFH-regulated model, the evolution of all other variables, including the MEH, can be determined by the SFH. We test this model on six locally isolated dwarf galaxies covering three dwarf types that were observed by the Local Cosmology from Isolated Dwarfs (LCID) project. The SFHs and MEHs of these LCID galaxies are relatively reliable because they have been measured from deep color–magnitude diagrams that reach the main-sequence turnoff stars with good photometric accuracy. With simple assumptions of the star formation law and the mass-dependent outflows, our SFH-regulated model successfully reproduces the MEHs of all six LCID galaxies from their SFHs, with only one free parameter, the wind efficiency η ∼ 1.0, for all six galaxies. This model provides a physically motivated link that directly connects the SFH and MEH of a galaxy, which will be useful to accommodate into the state-of-the-art stellar population synthesis models to help relieve the nuisance of the heavy degeneracy between the ages and metallicities of the stellar populations.

Determination of dynamical ages of open clusters through the A+ parameter – II

ABSTRACT Blue straggler stars (BSS), one of the most massive members of star clusters, have been used for over a decade to investigate mass segregation and estimate the dynamical ages of globular clusters (GCs) and open clusters (OCs). This work is an extension of our previous study, in which we investigated a correlation between theoretically estimated dynamical ages and the observed $A^+_{\mathrm{rh}}$ values, which represent the sedimentation level of BSS with respect to the reference population. Here, we use the ML-MOC algorithm on Gaia EDR3 data to extend this analysis to 23 OCs. Using cluster properties and identified members, we estimate their dynamical and physical parameters. In order to estimate the $A^+_{\mathrm{rh}}$ values, we use the main sequence and main sequence turnoff stars as the reference population. OCs are observed to exhibit a wide range of degrees of dynamical evolution, ranging from dynamically young to late stages of intermediate dynamical age. Hence, we classify OCs into three distinct dynamical stages based on their relationship to $A^+_{\mathrm{rh}}$ and Nrelax. NGC 2682 and King 2 are discovered to be the most evolved OCs, like Family III GCs, while Berkeley 18 is the least evolved OC. Melotte 66 and Berkeley 31 are peculiar OCs because none of their dynamical and physical parameters correlate with their BSS segregation levels.

HALO7D. III. Chemical Abundances of Milky Way Halo Stars from Medium-resolution Spectra

The Halo Assembly in Lambda Cold Dark Matter: Observations in 7 Dimensions (HALO7D) survey measures the kinematics and chemical properties of stars in the Milky Way (MW) stellar halo to learn about the formation of our Galaxy. HALO7D consists of Keck II/DEIMOS spectroscopy and Hubble Space Telescope–measured proper motions of MW halo main-sequence turnoff stars in the four Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey fields. HALO7D consists of deep pencil beams, making it complementary to other contemporary wide-field surveys. We present the [Fe/H] and [α/Fe] abundances for 113 HALO7D stars in the Galactocentric radial range of ∼10–40 kpc along four separate pointings. Using the full 7D chemodynamical data (3D positions, 3D velocities, and abundances) of HALO7D, we measure the velocity anisotropy, β, of the halo velocity ellipsoid for each field and for different metallicity-binned subsamples. We find that two of the four fields have stars on very radial orbits, while the remaining two have stars on more isotropic orbits. Separating the stars into high-, mid-, and low-[Fe/H] bins at −2.2 and −1.1 dex for each field separately, we find differences in the anisotropies between the fields and between the bins; some fields appear dominated by radial orbits in all bins, while other fields show variation between the [Fe/H] bins. These chemodynamical differences are evidence that the HALO7D fields have different fractional contributions from the progenitors that built up the MW stellar halo. Our results highlight the additional information available on smaller spatial scales compared to results from a spherical average of the stellar halo.

StarHorse results for spectroscopic surveys and Gaia DR3: Chrono-chemical populations in the solar vicinity, the genuine thick disk, and young alpha-rich stars

The Gaia mission has provided an invaluable wealth of astrometric data for more than a billion stars in our Galaxy. The synergy between Gaia astrometry, photometry, and spectroscopic surveys gives us comprehensive information about the Milky Way. Using the Bayesian isochrone-fitting code StarHorse, we derive distances and extinctions for more than 10 million unique stars listed in both Gaia Data Release 3 and public spectroscopic surveys: 557 559 in GALAH+ DR3, 4 531 028 in LAMOST DR7 LRS, 347 535 in LAMOST DR7 MRS, 562 424 in APOGEE DR17, 471 490 in RAVE DR6, 249 991 in SDSS DR12 (optical spectra from BOSS and SEGUE), 67 562 in the Gaia-ESO DR5 survey, and 4 211 087 in the Gaia RVS part of the Gaia DR3 release. StarHorse can increase the precision of distance and extinction measurements where Gaia parallaxes alone would be uncertain. We used StarHorse for the first time to derive stellar ages for main-sequence turnoff and subgiant branch stars, around 2.5 million stars, with age uncertainties typically around 30%; the uncertainties drop to 15% for subgiant-branch-only stars, depending on the resolution of the survey. With the derived ages in hand, we investigated the chemical-age relations. In particular, the α and neutron-capture element ratios versus age in the solar neighbourhood show trends similar to previous works, validating our ages. We used the chemical abundances from local subgiant samples of GALAH DR3, APOGEE DR17, and LAMOST MRS DR7 to map groups with similar chemical compositions and StarHorse ages, using the dimensionality reduction technique t-SNE and the clustering algorithm HDBSCAN. We identify three distinct groups in all three samples, confirmed by their kinematic properties: the genuine chemical thick disk, the thin disk, and a considerable number of young alpha-rich stars (427) that are also a part of the delivered catalogues. We confirm that the genuine thick disk’s kinematics and age properties are radically different from those of the thin disk and compatible with high-redshift (z ≈ 2) star-forming disks with high dispersion velocities. We also find a few extra chemical populations in GALAH DR3 thanks to the availability of neutron-capture element information.

On the origin of UV-dim stars: a population of rapidly rotating shell stars?

ABSTRACT The importance of stellar rotation in setting the observed properties of young star clusters has become clearer over the past decade, with rotation being identified as the main cause of the observed extended main sequence turn-off (eMSTO) phenomenon and split main sequences. Additionally, young star clusters are observed to host large fractions of rapidly rotating Be stars, many of which are seen nearly equator-on through decretion discs that cause self-extinction (the so called ‘shell stars’). Recently, a new phenomenon has been reported in the ∼1.5 Gyr star cluster NGC 1783, where a fraction of the main-equence turn-off stars appears abnormally dim in the UV. We investigate the origin of these ‘UV-dim’ stars by comparing the UV colour–magnitude diagrams of NGC 1850 (∼100 Myr), NGC 1783 (∼1.5 Gyr), NGC 1978 (∼2 Gyr), and NGC 2121 (∼2.5 Gyr), massive star clusters in the Large Magellanic Cloud. While the younger clusters show a non-negligible fraction of UV-dim stars, we find a significant drop of such stars in the two older clusters. This is remarkable as clusters older than ∼2 Gyr do not have an eMSTO, thus a large populations of rapidly rotating stars, because their main-sequence turn-off stars are low enough in mass to slow down due to magnetic braking. We conclude that the UV-dim stars are likely rapidly rotating stars with decretion discs seen nearly equator-on (i.e. are shell stars) and discuss future observations that can confirm or refute our hypothesis.

Evidence for Population-dependent Vertical Motions and the Long-lived Nonsteady Lopsided Milky Way Warp

We present a Galactic disk vertical velocity analysis using OB type stars (OB stars), red clump (RC) stars, and main-sequence turnoff (MSTO) stars with different average age populations crossmatched with LAMOST DR5 and Gaia DR3. We show that the vertical velocities of the three populations clearly vary with the Galactocentric distance (R) and the younger stellar population has a stronger increasing trend in general. The bending and breathing modes indicated by the vertical motions are dependent on the populations and vary with spatial locations. These vertical motions may be due to the Galactic warp, or minor mergers, or nonequilibrium of the disk. Assuming the warp is the dominant component, we find that the amplitude of the warp (γ, Z ω ) of OB stars (younger population) is larger than that of RC stars (medium population) and the latter is also larger than that for MSTO stars (older population), which is in agreement with other independent analyses of stellar density distribution, and supports that the warp is a long-lived, nonsteady structure and is time evolving. This conclusion is robust whether the line of nodes ϕ w is fixed or is a free parameter (with ϕ w being around 3°−8.°5 as the best fit). Furthermore, we find that the warp is lopsided with asymmetries along the azimuthal angle (ϕ).

Spectroscopic follow-up of statistically selected extremely metal-poor star candidates from GALAH DR3

ABSTRACT The advent of large-scale stellar spectroscopic surveys naturally leads to the implementation of machine learning techniques to isolate, for example, small sub-samples of potentially interesting stars from the full data set. A recent example is the application of the t-SNE statistical method to ∼600 000 stellar spectra from the GALAH survey in order to identify a sample of candidate extremely metal-poor (EMP; [Fe/H] ≤ –3) stars. We report the outcome of low-resolution spectroscopic follow-up of 83 GALAH EMP candidates that lack any previous metallicity estimates. Overall, the statistical selection is found to be efficient (∼one-third of the candidates have [Fe/H] ≤ –2.75) with low contamination (<10 per cent have [Fe/H] > –2), and with a metallicity distribution function that is consistent with previous work. Five stars are found to have [Fe/H] ≤ –3.0, one of which is a main sequence turnoff star. Two other stars are revealed as likely carbon-enhanced metal-poor (CEMP) stars of type CEMP-s, and a known carbon star is re-identified. The results indicate that the statistical selection approach employed was successful, and therefore it can be applied to forthcoming even larger stellar spectroscopic surveys with the expectation of similar positive outcomes.

Photometric calibration in u-band using blue halo stars

ABSTRACT We develop a method to calibrate u-band photometry based on the observed colour of blue Galactic halo stars. The Galactic halo stars belong to an old stellar population of the Milky Way and have relatively low metallicity. The ‘blue tip’ of the halo population – the main sequence turn-off (MSTO) stars – is known to have a relatively uniform intrinsic edge u-g colour with only slow spatial variation. In SDSS data, the observed variation is correlated with Galactic Latitude, which we attribute to contamination by higher metallicity disc stars and fit with an empirical curve. This curve can then be used to calibrate u-band imaging if g-band imaging of matching depth is available. Our approach can be applied to single-field observations at |b| > 30°, and removes the need for standard star observations or overlap with calibrated u-band imaging. We include in our method the calibration of g-band data with ATLAS-Refcat2. We test our approach on stars in KiDS DR 4, ATLAS DR 4, and DECam imaging from the NOIRLab Source Catalog (NSC DR2), and compare our calibration with SDSS. For this process, we use synthetic magnitudes to derive the colour equations between these data sets, in order to improve zero-point accuracy. We find an improvement for all data sets, reaching a zero-point precision of 0.016 mag for KiDS (compared to the original 0.033 mag), 0.020 mag for ATLAS (originally 0.027 mag), and 0.016 mag for DECam (originally 0.041 mag). Thus, this method alone reaches the goal of 0.02 mag photometric precision in u-band for the Rubin Observatory’s Legacy Survey of Space and Time (LSST).

The GALAH survey: chemical clocks

ABSTRACT We present the first large-scale study that demonstrates how ages can be determined for large samples of stars through Galactic chemical evolution. Previous studies found that the elemental abundances of a star correlate directly with its age and metallicity. Using this knowledge, we derive ages for 214 577 stars in GALAH DR3 using only overall metallicities and chemical abundances. Stellar ages are estimated via the machine learning algorithm XGBoost for stars belonging to the Milky Way disc with metallicities in the range −1 < [Fe/H] < 0.5, using main-sequence turn-off stars as our training set. We find that stellar ages for the bulk of GALAH DR3 are precise to 1–2 Gyr using this method. With these ages, we replicate many recent results on the age-kinematic trends of the nearby disc, including the solar neighbourhood’s age–velocity dispersion relationship and the larger global velocity dispersion relations of the disc found using Gaia and GALAH. These results show that chemical abundance variations at a given birth radius are small, and that strong chemical tagging of stars directly to birth clusters may prove difficult with our current elemental abundance precision. Our results highlight the need to measure abundances for as many nucleosynthetic production sites as possible in order to estimate reliable ages from chemistry. Our methods open a new door into studies of the kinematic structure and evolution of the disc, as ages may potentially be estimated to a precision of 1–2 Gyr for a large fraction of stars in existing spectroscopic surveys.

Four-hundred Very Metal-poor Stars Studied with LAMOST and Subaru. II. Elemental Abundances

We present homogeneous abundance analysis of over 20 elements for 385 very metal-poor (VMP) stars based on the LAMOST survey and follow-up observations with the Subaru Telescope. It is the largest high-resolution VMP sample (including 363 new objects) studied by a single program, and the first attempt to accurately determine evolutionary stages for such a large sample based on Gaia parallaxes. The sample covers a wide metallicity range from [Fe/H] ≲ −1.7 down to [Fe/H] ∼ −4.3, including over 110 objects with [Fe/H] ≤ −3.0. The expanded coverage in evolutionary status makes it possible to define the abundance trends respectively for giants and turnoff stars. The newly obtained abundance data confirm most abundance trends found by previous studies, but also provide useful updates and new samples of outliers. The Li plateau is seen in main-sequence turnoff stars with −2.5 < [Fe/H] < −1.7 in our sample, whereas the average Li abundance is clearly lower at lower metallicity. Mg, Si, and Ca are overabundant with respect to Fe, showing decreasing trend with increasing metallicity. Comparisons with chemical evolution models indicate that the overabundance of Ti, Sc, and Co are not well reproduced by current theoretical predictions. Correlations are seen between Sc and α-elements, while Zn shows a detectable correlation only with Ti but not with other α-elements. The fraction of carbon-enhanced stars ([C/Fe] > 0.7) is in the range of 20%–30% for turnoff stars depending on the treatment of objects for which C abundance is not determined, which is much higher than that in giants (∼8%). Twelve Mg-poor stars ([Mg/Fe] < 0.0) have been identified in a wide metallicity range from [Fe/H] ∼ −3.8 through −1.7. Twelve Eu-rich stars ([Eu/Fe] > 1.0) have been discovered in −3.4 < [Fe/H] < −2.0, enlarging the sample of r-process-enhanced stars with relatively high metallicity.

Anisotropy of Halo Main Sequence Turnoff Stars Measured with New MMT Radial Velocities and Gaia Proper Motions

We measure the anisotropy of the Milky Way stellar halo traced by a dense sample of 18 < r < 21 mag F-type main sequence turnoff stars using Gaia eDR3 proper motions and new radial velocity measurements published here.

MUSE spectroscopic observations of the young massive cluster NGC 1850

Context. The double cluster NGC 1850 in the Large Magellanic Cloud is the nearest young massive cluster of the Local Group with a mass similar to those of Galactic globular clusters. Recent studies have revealed an extended morphology of its main-sequence turn-off (MSTO), which can be interpreted as a spread in either age or internal rotation. In spite of its proximity, an accurate spectroscopic determination of its chemical properties is still missing. Aims. We aim to investigate the general chemistry and the kinematics of this stellar system to test whether possible signs of multiple populations are observable in this cluster. Methods. We analysed the spectra obtained with MUSE in adaptive optics mode of 1167 stars in both components of this cluster (NGC 1850A and NGC 1850B). Thanks to this data set, we were able to measure accurate global metallicities, Ba abundances, and radial velocities for a sample of 38 red supergiants and a guess of the oxygen abundance in the brightest turn-off stars belonging to NGC 1850A. Results. We find an average metallicity of ⟨[M/H]⟩ = −0.31 ± 0.01, a mean Ba abundance of ⟨[Ba/Fe]⟩ = +0.40 ± 0.02, and a systemic radial velocity of ⟨vLOS⟩ = 251.1 ± 0.3 km s−1. The dispersion of the radial velocities suggests a dynamical mass of log(M/M⊙) = 4.84 ± 0.1, while no significant systemic rotation is detected. We detect a significant bimodality in O I line strength among the turn-off stars of NGC 1850A with ∼66% of stars with [O/Fe]∼ − 0.16 and the rest with no detectable line. The majority of O-weak stars preferentially populate the red side of the MSTO and show H lines in emission, suggesting that they are Be stars rotating close to their critical velocity. Among normal MSTO stars, red stars have broader line profiles than blue ones, on average, suggesting a correlation between colour and rotational velocity. Conclusions. The mean metallicity of this cluster lies at the metal-rich side of the metallicity distribution of the Large Magellanic Cloud following its age-metallicity relation. The Ba and O abundances agree with those measured in the bar of this galaxy. The correlation between line broadening and colour suggests that the observed colour spread among turn-off stars can be due to a wide range in rotational velocity covered by these stars.

Ages of Main-sequence Turnoff Stars from the GALAH Survey

Main-sequence turnoff (MSTO) stars are good tracers of Galactic populations since their ages can be reliably estimated from atmospheric parameters. Based on the GALAH survey, we use the Yale rotation evolution code to determine the ages of 2926 MSTO stars with a mean age uncertainty of ∼10% considering the variation of C and O abundances. The ages of CO-poor stars are systematically affected by ∼10% due to the C and O abundances, globally shifting to ∼0.5 Gyr older compared to the results using solar metal mixture. Of the stars with [Fe/H] ∼ 0.3–0.5 or [O/Fe] ≤ −0.25, many have fractional age differences of ≥20%, and even reach up to 36%. The age–metallicity relation appears to possibly indicate the existence of two distinct sequences: a young sequence of stars with ages mostly <7 Gyr, and a relatively older sequence of stars with ages mostly >7 Gyr, overlapping at 5 Gyr ≤ age ≤ 7 Gyr. Moreover, the trends of abundances-to-age ratios show two corresponding sequences, especially in the [O/Fe]–age plane. We also find that [Y/Mg] is a good chemical clock in disk populations. The young sequence and the old sequence cannot be separated based on chemistry or kinematics; therefore, stellar age is an important parameter to distinguish these two sequences in our sample.

The GALAH Survey: dependence of elemental abundances on age and metallicity for stars in the Galactic disc

ABSTRACT Using data from the GALAH survey, we explore the dependence of elemental abundances on stellar age and metallicity among Galactic disc stars. We find that the abundance of most elements can be predicted from age and [Fe/H] with an intrinsic scatter of about 0.03 dex. We discuss the possible causes for the existence of the abundance–age–metallicity relations. Using a stochastic chemical enrichment scheme that takes the volume of supernovae remnants into account, we show the intrinsic scatter is expected to be small, about 0.05 dex or even smaller if there is additional mixing in the ISM. Elemental abundances show trends with both age and metallicity and the relationship is well described by a simple model in which the dependence of abundance ([X/Fe]) on age and [Fe/H] are additively separable. Elements can be grouped based on the direction of their abundance gradient in the (age,[Fe/H]) plane and different groups can be roughly associated with three distinct nucleosynthetic production sites, the exploding massive stars, the exploding white dwarfs, and the AGB stars. However, the abundances of some elements, like Co, La, and Li, show large scatter for a given age and metallicity, suggesting processes other than simple Galactic chemical evolution are at play. We also compare the abundance trends of main-sequence turn-off (MSTO) stars against that of giants, whose ages were estimated using asteroseismic information from the K2 mission. For most elements, the trends of MSTO stars are similar to that of giants. The existence of abundance relations implies that we can estimate the age and birth radius of disc stars, which is important for studying the dynamic and chemical evolution of the Galaxy.

A black hole detected in the young massive LMC cluster NGC 1850

ABSTRACT We report on the detection of a black hole (NGC 1850 BH1) in the ∼100-Myr-old massive cluster NGC 1850 in the Large Magellanic Cloud. It is in a binary system with a main-sequence turn-off star (4.9 ± 0.4 M⊙), which is starting to fill its Roche lobe and is becoming distorted. Using 17 epochs of Very Large Telescope/Multi-Unit Spectroscopic Explorer observations, we detected radial velocity variations exceeding 300 km s−1 associated with the target star, linked to the ellipsoidal variations measured by the fourth phase of the Optical Gravitational Lensing Experiment in the optical bands. Under the assumption of a semidetached system, the simultaneous modelling of radial velocity and light curves constrains the orbital inclination of the binary to 38° ± 2°, resulting in a true mass of the unseen companion of $11.1_{-2.4}^{+2.1}\,{\rm M}_{\odot }$. This represents the first direct dynamical detection of a black hole in a young massive cluster, opening up the possibility of studying the initial mass function and the early dynamical evolution of such compact objects in high-density environments.

Determination of dynamical ages of open clusters through the A+ parameter – I

ABSTRACT The sedimentation level of blue straggler stars (BSS) has been shown to be a great tool to investigate the dynamical states of globular clusters (GCs). The area enclosed between the cumulative radial distributions of BSS and a reference population up to the half-mass radius of the clusters, $A^+_{\mathrm{rh}}$, is known to be a measure of the sedimentation of BSS in GCs. In this work, we calculate $A^+_{\mathrm{rh}}$ for 11 open clusters (OCs) using a combined list of main-sequence turn-off stars, sub-giant branch stars, and red giant branch stars as reference population. The BSS, the reference populations, and the cluster members are identified using the proper motions and parallaxes from the Gaia DR2 data. In a subset of clusters, the BSS are confirmed cluster members on the basis of radial velocity information available in the literature. Using the Pearson and Spearman rank correlation coefficients, we find weak correlations between the estimated values of $A^+_{\mathrm{rh}}$ and other markers of dynamical ages of the clusters, i.e. the number of central relaxations a cluster has experienced since its formation, and the structural parameters of the clusters. Based on statistical tests, we find that these correlations are similar to the corresponding correlations among the less evolved GCs, albeit within large errors.

A deep search for faint Chandra X-ray sources, radio sources, and optical counterparts in NGC 6752

ABSTRACT We report the results of a deep search for faint Chandra X-ray sources, radio sources, and optical counterparts in the nearby, core-collapsed globular cluster, NGC 6752. We combined new and archival Chandra imaging to detect 51 X-ray sources (12 of which are new) within the 1.9 arcmin half-light radius. Three radio sources in deep ATCA 5 and 9 GHz radio images match with Chandra sources. We have searched for optical identifications for the expanded Chandra source list using deep Hubble Space Telescope photometry in B435, R625, H α, UV275, and U336. Among the entire sample of 51 Chandra sources, we identify 18 cataclysmic variables (CVs), 9 chromospherically active binaries (ABs), 3 red giants (RGs), 3 galaxies (GLXs), and 6 active galactic nuclei (AGNs). Three of the sources are associated with millisecond pulsars (MSPs). As in our previous study of NGC 6752, we find that the brightest CVs appear to be more centrally concentrated than the faintest CVs, although the effect is no longer statistically significant as a consequence of the inclusion in the faint group of two intermediate brightness CVs. This possible difference in the radial distributions of the bright and faint CV groups appears to indicate that mass segregation has separated them. We note that photometric incompleteness in the crowded central region of the cluster may also play a role. Both groups of CVs have an inferred mass above that of the main-sequence turnoff stars. We discuss the implications for the masses of the CV components.