Extended Main-sequence Turnoff Research Articles

Study of 26 Galactic Open Clusters with Extended Main-sequence Turnoffs

Recent studies indicate that some Galactic open clusters (OCs) exhibit extended main-sequence turnoff (eMSTO) in their color–magnitude diagrams (CMDs). However, the number of Galactic OCs with eMSTO structures detected so far is limited, and the reasons for their formation are still unclear. This work identifies 26 Galactic OCs with undiscovered eMSTOs and investigates the causes of these features. Stellar population types and fundamental parameters of cluster samples are acquired using CMD fitting methods. Among them, the results of 11 OCs are reliable as the observed CMDs are well-reproduced. We propose the crucial role of stellar binarity and confirm the importance of stellar rotation in reproducing eMSTO morphologies. The results also show that the impact of age spread is important, as it can adequately explain the structure of young OCs and fit the observed CMDs of intermediate-age OCs better.

UOCS-XII. A study of open cluster NGC 6940 using UVIT/AstroSat cluster properties and exotic populations

ABSTRACT We study an open cluster NGC 6940 using AstroSat/UVIT (Ultraviolet Imaging Telescope) data and other archival data. This is an intermediate age cluster (∼1 Gyr), located at about 770 pc distance, harbouring several exotic populations apart from normal single and binary stars. We identify members of this cluster using a machine learning algorithm, ML-MOC and identify 492 members, including one blue straggler star (BSS), two yellow straggler stars (YSS), 11 blue lurker (BL) candidates, and two red clump (RC) stars. The cluster shows the effect of mass segregation, with massive stars segregated the most into the cluster, followed by the equal-mass binary members and the single low-mass stars. We report the presence of an extended main-sequence turn-off (eMSTO) feature in this cluster and suggest that the age spread may be a contributing factor behind it. However, the effect of stellar rotation, and the dust absorption needs to be examined more comprehensively with a larger fraction of MSTO stars. All the sixteen sources mentioned above have a counterpart in the UVIT/F169M filter. In order to characterize them, we construct multi-wavelength spectral energy distributions (SEDs) of 14 of these objects having no nearby sources within 3 arcsec. The BSS is successfully fitted with a single-component SED. We find that three BLs, two YSS, and one RC star have UV excess greater than 50 per cent and successfully fit two-component SEDs having hot companions. The parameters derived from the SEDs imply that the hot companions of BLs and the RC star are low-mass and normal-mass white dwarfs, whereas the hot companions of YSS are likely to be subdwarf B stars. We suggest that at least 6 out of 14 stars (∼42 per cent) are formed via mass transfer and/or merger pathways.

Extended main-sequence turnoff and red clump in intermediate-age star clusters: A study of NGC 419

With the goal of untangling the origin of extended main-sequence turnoffs (eMSTOs) and extended red clumps (eRCs) in star clusters, in this work we present the study of the intermediate-age cluster NGC 419, situated along the Bridge of the Small Magellanic Cloud. To this aim, we analyzed multi-epoch, high angular resolution observations acquired with the Hubble Space Telescope for this dynamically young cluster, which enabled the determination of precise proper motions and therefore the assessment of the cluster membership for each individual star in the field of view. With this unprecedented information at hand, we first studied the radial distribution of kinematically selected member stars in different eMSTO subregions. The absence of segregation supports the rotation scenario as the cause for the turnoff color extension and disfavors the presence of a prolonged period of star formation in the cluster. A similar analysis on the eRC of NGC 419 confirms the absence of segregation, providing further evidence against an age spread, which is at odds with previous investigations. Even so, the currently available evolutionary models including stellar rotation fail at reproducing the two photometric features simultaneously. We argue that either shortcomings in these models or a different origin for the red clump feature, such as a nonstandard differential mass loss along the red giant branch phase, are the only way to reconcile our observational findings with theoretical expectations.

Hubble Space Telescope survey of Magellanic Cloud star clusters: UV-dim stars in young clusters

ABSTRACT Young and intermediate-age star clusters of both Magellanic Clouds exhibit complex colour–magnitude diagrams. In addition to the extended main-sequence turn-offs (eMSTOs), commonly observed in star clusters younger than ∼2 Gyr, the clusters younger than ∼800 Myr exhibit split main sequences (MSs). These comprise a blue MS, composed of stars with low rotation rates, and a red MS, which hosts fast-rotating stars. While it is widely accepted that stellar populations with different rotation rates are responsible for the eMSTOs and split MSs, their formation and evolution are still debated. A recent investigation of the ∼1.7-Gyr-old cluster NGC 1783 detected a group of eMSTO stars extremely dim in ultraviolet (UV) bands. Here, we use multiband Hubble Space Telescope photometry to investigate five star clusters younger than ∼200 Myr, including NGC 1805, NGC 1818, NGC 1850, and NGC 2164 in the Large Magellanic Cloud, and the Small Magellanic Cloud cluster NGC 330. We discover a group of bright MS stars in each cluster that are significantly dim in the F225W and F275W bands, similar to what is observed in NGC 1783. Our result suggests that UV-dim stars are common in young clusters. The evidence that most of them populate the blue MS indicates that they are slow rotators. As a by-product, we show that the star clusters NGC 1850 and BHRT 5b exhibit different proper motions, thus corroborating the evidence that they are not gravitationally bound.

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

ABSTRACT 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 mag 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). Here, the derived atlas and calibrated catalogue of the sources in the field of view is publicly released as supplementary online 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 turn-off seen in the cluster colour–magnitude diagram.

Hubble Space Telescope survey of Magellanic Cloud star clusters

In the past few years, we have undertaken an extensive investigation of star clusters and their stellar populations in the Large and Small Magellanic Clouds (LMC and SMC) based on archival images collected with the Hubble Space Telescope. We present photometry and astrometry of stars in 101 fields observed with the Wide Field Channel of the Advanced Camera for Surveys and the Ultraviolet and Visual Channel and the Near-Infrared Channel of Wide Field Camera 3. These fields comprise 113 star clusters. We provide differential-reddening maps for those clusters with significant reddening variations across the field of view. We illustrate various scientific outcomes that arise from the early inspection of the photometric catalogs. In particular, we provide new insights into the extended main-sequence turnoff (eMSTO) phenomenon: (i) We detected eMSTOs in two clusters, KMHK 361 and NGC 265, which had no previous evidence of multiple populations. This finding corroborates the conclusion that the eMSTO is a widespread phenomenon among clusters younger than ∼2 Gyr. (ii) The homogeneous color-magnitude diagrams (CMDs) of 19 LMC clusters reveal that the distribution of stars along the eMSTO depends on cluster age. (iii) We discovered a new feature along the eMSTO of NGC 1783, which consists of a distinct group of stars on the red side of the eMSTO in CMDs composed of UV filters. Furthermore, we derived the proper motions of stars in the fields of view of clusters with multi-epoch images. Proper motions allowed us to separate the bulk of bright field stars from cluster members and investigate the internal kinematics of stellar populations in various LMC and SMC fields. As an example, we analyze the field around NGC 346 to disentangle the motions of its stellar populations, including NGC 364 and BS 90, young and pre-main-sequence stars in the star-forming region associated with NGC 346, and young and old field stellar populations of the SMC. Based on these results and the fields around five additional clusters, we find that young SMC stars exhibit elongated proper-motion distributions that point toward the LMC, thus providing new evidence for a kinematic connection between the LMC and SMC.

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.

The Role of Binarity and Stellar Rotation in the Split Main Sequence of NGC 2422

In addition to the extended main-sequence turnoffs widely found in young and intermediate-age (∼600 Myr to 2 Gyr old) star clusters, some younger clusters even exhibit split main sequences (MSs). Different stellar rotation rates are proposed to account for the bifurcated MS pattern, with red and blue MSs (rMS and bMS) populated by fast and slowly rotating stars, respectively. Using photometry from Gaia Early Data Release 3, we report a Galactic open cluster with a bifurcated MS, NGC 2422 (∼90 Myr). We exclude the possibilities that the bifurcated MS pattern is caused by photometric noise or differential reddening. We aim to examine if stellar rotation can account for the split MS. We use spectra observed with the Canada–France–Hawaii Telescope and the Southern African Large Telescope and directly measured , the projected rotational velocities, for stars populating the bMS and rMS. We find that their values are weakly correlated with their loci in the color–magnitude diagram because of contamination caused by a large fraction of rMS stars with low projected rotational velocities. Based on the spectral energy distribution fitting method, we suggest that these slowly rotating stars at the rMS may hide a binary companion, which breaks the expected –color correlation. Future time-domain studies focusing on whether these slowly rotating stars are radial velocity variables are crucial to test the roles of stellar rotation and binarity in generating the split MSs.

Rotational Variation Allows for Narrow Age Spread in the Extended Main-sequence Turnoff of Massive Cluster NGC 1846

The color–magnitude diagrams (CMDs) of intermediate-age star clusters (≲2 Gyr) are much more complex than those predicted by coeval, nonrotating stellar evolution models. Their observed extended main-sequence turnoffs (eMSTOs) could result from variations in stellar age, stellar rotation, or both. The physical interpretation of eMSTOs is largely based on the complex mapping between stellar models—themselves functions of mass, rotation, orientation, and binarity—and the CMD. In this paper, we compute continuous probability densities in three-dimensional color, magnitude, and space for individual stars in a cluster’s eMSTO, based on a rotating stellar evolution model. These densities enable the rigorous inference of cluster properties from a stellar model, or, alternatively, constraints on the stellar model from the cluster’s CMD. We use the MIST stellar evolution models to jointly infer the age dispersion, the rotational distribution, and the binary fraction of the Large Magellanic Cloud cluster NGC 1846. We derive an age dispersion of ∼70–80 Myr, approximately half the earlier estimates due to nonrotating models. This finding agrees with the conjecture that rotational variation is largely responsible for eMSTOs. However, MIST models do not provide a satisfactory fit to all stars in the cluster and achieve their best agreement at an unrealistically high binary fraction. The lack of agreement near the main-sequence turnoff suggests specific physical changes to the stellar evolution models, including a lower mass for the Kraft break and potentially enhanced main-sequence lifespans for rapidly rotating stars.

NGC1818 unveils the origin of the extended main-sequence turn-off in young Magellanic Clouds clusters

The origin of young star clusters represents a major challenge for modern stellar astrophysics. While stellar rotation partially explains the colour spread observed along main-sequence turn-offs, i.e. where stars leave the main-sequence after the exhaustion of hydrogen in their core, and the multiple main sequences in the colour-magnitude diagrams of stellar systems younger than approximately 2 Gyr, it appears that an age difference may still be required to fulfill the observational constraints. Here we introduce an alternative approach that exploits the main-sequence turn-on, i.e. the point alongside the colour-magnitude diagram where pre-main-sequence stars join the main-sequence, to disentangle between the effects of stellar rotation and age to assess the presence, or lack thereof, of prolonged star formation in the approximately 40-Myr-old cluster NGC1818. Our results provide evidence for a fast star formation, confined within 8 Myr, thus excluding age differences as responsible for the extended main-sequence turn-offs, and leading the way to alternative observational perspectives in the exploration of stellar populations in young clusters.

On the precision of full-spectrum fitting of simple stellar populations – IV. A systematic comparison with results from colour–magnitude diagrams

ABSTRACT In this fourth paper of a series on the precision of ages of stellar populations obtained through the full-spectrum fitting technique, we present a first systematic analysis that compares the age, metallicity, and reddening of star clusters obtained from resolved and unresolved data [namely colour–magnitude diagrams (CMDs) and integrated-light spectroscopy] using the same sets of isochrones. We investigate the results obtained with both Padova isochrones and MIST isochrones. We find that there generally is a good agreement between the ages derived from CMDs and integrated spectra. However, for metallicity and reddening, the agreement between results from analyses of CMD and integrated spectra is significantly worse. Our results also show that the ages derived with Padova isochrones match those derived using MIST isochrones, both with the full-spectrum fitting technique and the CMD fitting method. However, the metallicity derived using Padova isochrones does not match that derived using MIST isochrones using the CMD method. We examine the ability of the full-spectrum fitting technique in detecting age spreads in clusters that feature the extended main-sequence turnoff (eMSTO) phenomenon using two-population fits. We find that three out of five eMSTO clusters in our sample are best fitted with one single age, suggesting that eMSTOs do not necessarily translate to detectable age spreads in integrated-light studies.

A detailed study of stellar populations of three star clusters with extended main-sequence turnoffs

ABSTRACT This paper investigates the extended main-sequence turnoffs (eMSTOs) and stellar populations of three clusters with different ages using Gaia Data Release 2 (DR2) data in detail. The colour–magnitude diagrams (CMDs) of three Galactic clusters show broad main sequences, eMSTOs, blue stragglers, and red giant clumps. We explore the effects of age spread, binaries, and rotating stars on the morphology of the entire CMD, paying particular attention to the effects of binary stars and rotating stars on eMSTOs. Some synthetic CMDs are built on the basis of the advanced stellar population synthesis (ASPS) model to study the observed CMDs and search for the best-fitting stellar population models. The age of the youngest stars, age spread, metallicity, colour excess, distance modulus, binary fraction, and rotating star fraction of clusters are determined. The results show that both binaries and rotating stars are responsible for the eMSTO of intermediate-age cluster LP 585, but binary stars affect more than rotating stars on the eMSTO morphologies of the youngest (M11) and oldest (NGC 6819) clusters in our samples. It suggests that M11 (∼0.3 Gyr) is possibly a composite stellar population of rotating and binary stars, while clusters LP 585 (∼1.1 Gyr) and NGC 6819 (∼2.0 Gyr) are simple stellar populations of rotating and binary stars.

Stellar Population Astrophysics (SPA) with the TNG

Stock 2 is a little-studied open cluster that shows an extended main-sequence turnoff (eMSTO). In order to investigate this phenomenon and characterise the cluster itself, we performed high-resolution spectroscopy in the framework of the Stellar Population Astrophysics project. We employed the High Accuracy Radial velocity Planet Searcher in the Northern hemisphere spectrograph at the Telescopio Nazionale Galileo (TNG). We completed our observations with additional spectra taken with the Catania Astrophysical Observatory Spectropolarimeter. We observed 46 stars (dwarfs and giants). They represent by far the largest sample collected for this cluster to date. We provide the stellar parameters, the extinction, and the radial and projected rotational velocities for most of the stars. Chemical abundances for 21 species with atomic numbers up to 56 have also been derived. We note a differential reddening in the cluster field with an average value of 0.27 mag. This appears the main cause for the observed eMSTO because it cannot be explained as the result of different rotational velocities, as found in other clusters. We estimate an age for Stock 2 of 450 ± 150 Ma, which corresponds to a MSTO stellar mass of ≈2.8 M⊙. The cluster mean radial velocity is about 8.0 km s−1. We find a solar-like metallicity for the cluster, [Fe/H] = −0.07 ± 0.06, compatible with its galactocentric distance. The chemical abundances of main-sequence stars and giants are compatible within the errors. The exceptions are barium and strontium, which are clearly overabundant in giants, and cobalt, which is only marginally overabundant. Finally, the chemical composition of Stock 2 is fully compatible with the composition observed in other open clusters of the Galactic thin-disc population.

Photometric and Kinematic Study of the Open Clusters SAI 44 and SAI 45

Abstract We carry out a detailed photometric and kinematic study of the poorly studied sparse open clusters SAI 44 and SAI 45 using ground-based BVR c I c data supplemented by archival data from Gaia eDR3 and Pan-STARRS. The stellar memberships are determined using a statistical method based on Gaia eDR3 kinematic data, and we found 204 members in SAI 44 while only 74 members are identified in SAI 45. The average distances to SAI 44 and SAI 45 are calculated to be 3670 ± 184 and 1668 ± 47 pc. The logarithmic age of the clusters are determined to be 8.82 ± 0.10 and 9.07 ± 0.10 yr for SAI 44 and SAI 45, respectively. The color–magnitude diagram of SAI 45 hosts an extended main-sequence turnoff (eMSTO). The apparent age spread is found to be similar to the apparent age spread predicted on the basis of the age spread and cluster age relation predicted by rotation models. This indicates that eMSTO is a stellar evolution rather than star formation phenomenon in SAI 45. We conclude that eMSTO in SAI 45 is mainly caused by the different rotation rates of stars as the SYCLIST synthetic population with different rotation rates was able to reproduce the observed eMSTO, and stars in the red part of the eMSTO were preferentially concentrated in the inner region, which again hints at different rotations being the reason for the extension in the upper MS. This finding supports the theory attributing the origin of eMSTO to the different rotations of eMSTO stars. The mass function slopes are obtained as −2.24 ± 0.66 and −2.58 ± 3.20 in the mass rages 2.426–0.990 M ⊙ and 2.167–1.202 M ⊙ for SAI 44 and SAI 45, respectively. SAI 44 exhibits the signature of mass segregation while we found weak evidence of mass segregation in SAI 45 possibly due to tidal stripping. The dynamical relaxation times of these clusters indicate that both clusters are in a dynamically relaxed state. Using the AD-diagram method, the apex coordinates are found to be ( 69 .° 79 ± 0 .° 11 , − 30 .° 82 ± 0 .° 15 ) for SAI 44 and (− 56 .° 22 ± 0 .° 13 , − 56 .° 62 ± 0 .° 13 ) for SAI 45. The average space velocity components of the clusters SAI 44 and SAI 45 are calculated in units of km s−1 as (−15.14 ± 3.90, −19.43 ± 4.41, −20.85 ± 4.57) and (28.13 ± 5.30, −9.78 ± 3.13, −19.59 ± 4.43), respectively.

Short-period Variability in the Globular Cluster NGC 419 and the SMC Field

Abstract Delta Scuti (δ Sct) stars have been extensively studied in our Galaxy but far less in extragalactic systems. Here we study the population of δ Sct variables in NGC 419, an intermediate-age globular cluster of the Small Magellanic Cloud (SMC), using g, r, and i Gemini-S/Gemini Multi-Object Spectrograph (GMOS) time-series observations. Our goal is to study the role of such variables in the cluster extended main-sequence turnoff (MSTO). We report the discovery of 54 δ Sct stars and three eclipsing binaries in the NGC 419 field. We find only a handful of the δ Sct stars at the MSTO of NGC 419, while the majority are fainter, indicating that the cluster is younger (≲1.2 Gyr) than previously thought. Considering their radial distribution, we identify only six δ Sct stars as probable members of NGC 419, while the 48 remaining are likely δ Sct stars of the SMC field. Cluster δ Sct stars appear close to the red edge of the MSTO, supporting the idea that the extended MSTO has its origin in an age spread. The 48 field δ Sct stars represent the largest detection of δ Sct stars made in the SMC. The period distribution of these newly detected δ Sct stars (0.04 day ≲ P ≲ 0.15 day) is similar to that detected in other systems. The amplitude distribution (0.05 mag ≲ Δr ≲ 0.60 mag) is likely biased because of the lack of low-amplitude stars. We finally use the δ Sct stars to calculate distances using different period–luminosity relations. The average distance moduli obtained are 18.76 ± 0.14 mag for NGC 419 and 18.86 ± 0.11 mag for the SMC field, which agree with previous measurements.

Multiple populations of Hβ emission line stars in the Large Magellanic Cloud cluster NGC 1971

We revisited the young Large Magellanic Cloud star cluster NGC 1971 with the aim of providing additional clues to our understanding of its observed extended main-sequence turnoff (eMSTO), a feature commonly seen in young star clusters that has recently been argued to be caused by a real age spread similar to the cluster age (∼160 Myr). We combined accurate Washington and Strömgren photometry of stars with high membership probability to explore the nature of this eMSTO. From different ad hoc defined pseudo-colors, we found that bluer and redder stars distributed throughout the eMSTO do not show any inhomogeneities of light- and heavy-element abundances. These blue and red stars are split into two clearly different groups only when the Washington M magnitudes are employed, which delimites the number of spectral features that cause the appearance of the eMSTO. We speculate that Be stars populate the eMSTO of NGC 1971 because (i) Hβ contributes to the M passband, (ii) Hβ emissions are common features of Be stars, and (iii) the Washington M and T1 magnitudes are tightly correlated; the latter measuring the observed contribution of Hα emission line in Be stars, which is in turn correlated with Hβ emissions. This is the first observational result to our knowledge that indicates that Hβ emissions are the origin of eMSTOs observed in young star clusters. Our results certainly open new possibilities of studying eMSTO from photometric systems with passbands centered at features commonly seen in Be stars.

Study of color-magnitude diagram of star cluster SL 506

We report a study of the color-magnitude diagram (CMD) of globular cluster SL 506, which shows extended main sequence turn-off (eMSTO). The CMD is derived from deep F450W (B) and F555W (V) images of Hubble Space Telescope/Wide Field Planetary Camera 2 (HST/WFPC2). We reproduce the CMD via simple stellar population including the combination of rotational stars and binaries, and composite stellar population of non-rotating single stars. We derive the parameters of the star cluster, including age, age spread, metallicity, color excess, distance modulus, binary fraction, and rotating star fraction, by comparing the observed and synthetic CMDs. The results show that the eMSTO region of the cluster can be reproduced by both age spread and a combination of rotating and binary stars, but the blue stragglers, color and magnitude distributions can not be reproduced well by age spread. This implies that SL 506 is a simple population with rotating, single and binary stars.

A study of stellar population of intermediate-age globular cluster NGC 2249

The Large Magellanic Cloud (LMC) star cluster NGC 2249 has been found to exhibit an extended main-sequence turn-off (eMSTO) in the color-magnitude diagram (CMD) derived from the data of the Hubble Space Telescope (HST). Such CMDs can potentially be explained via a spread of stellar age or rotation rate of single-aged stars, leading to a confusion of its stellar population type. In this work, we use different combinations of age spread (A), rotation (R), and binarity (B) to study the stellar population of this cluster, via a deep HST CMD. Because binaries are commonly observed in star clusters, three models of this study take binaries into account. We finally derive the best-fit stellar population parameters, i.e., metallicity (Z = 0.008), age (0.7–1.0Gyr), reddening (E(B-I) = 0.52–0.55 mag), distance modulus (18.54–18.57mag), binary fraction (50%), rotating star fraction (0–50%), and star formation history mode for NGC 2249. We find that both age spread (A + B or A + R + B) and rotating stars (R + B) can reproduce the main CMD features well, while rotation alone does not reproduce the detailed eMSTO morphology. This suggests that an age spread of 200Myrs is needed to form the eMSTO of cluster NGC 2249, even if stellar rotation is taken into account. Dynamical simulation could be useful for revealing the origin of the eMSTO.

When Does the Onset of Multiple Stellar Populations in Star Clusters Occur? III. No Evidence of Significant Chemical Variations in Main-sequence Stars of NGC 419

Abstract Recent studies have revealed that the onset age for the presence of multiple stellar populations (MPs) in star clusters seems to correspond to the disappearance of the extended main-sequence turnoff in young clusters, a pattern associated with stellar rotations. A speculative suggestion is that MPs might be caused by the magnetic brake, a stellar evolutionary effect linked to rotation. In this work, we use the young massive cluster NGC 419 as a testbed. We examined if its magnetically baked MS stars would exhibit MPs. Using the deep ultraviolet and visible images observed through the Hubble Space Telescope, combined with a specific color index that is sensitive to the nitrogen (N) abundance, we examined if its late G- and K-type MS stars are affected by N variation. Our analysis reports that the morphology of its GK-type MS is most likely a simple stellar population, and only a negligible probability, which indicates a N variation up to 0.4 dex is present. We therefore conclude that there is no significant N variation among its GK-type MS stars. The absence of a significant chemical variation among the late-type MS stars indicates that MPs might not be a specific pattern of magnetically braked stars.

Effects of Close Binary Evolution on the Main-sequence Morphology of Young Star Clusters

Abstract Star clusters are the building blocks of galaxies. They are composed of stars of nearly equal age and chemical composition, allowing us to use them as chronometers and as testbeds for gauging stellar evolution. It has become clear recently that massive stars are formed preferentially in close binaries, in which mass transfer will drastically change the evolution of the stars. This is expected to leave a significant imprint in the distribution of cluster stars in the Hertzsprung–Russell diagram. Our results, based on a dense model grid of more than 50,000 detailed binary-evolution calculations, show several distinct, coeval main-sequence (MS) components, most notably an extended MS turnoff region, and a group of near-critical rotating stars that is spread over a large luminosity range on the red side of the classical MS. We comprehensively demonstrate the time evolution of the features in an animation, and we derive analytic expressions to describe these features. We find quantitative agreement with results based on recent photometric and spectroscopic observations. We conclude that while other factors may also be at play, binary evolution has a major impact on the MS morphology of young star clusters.