Theoretical Isochrones Research Articles

JWST imaging of the closest globular clusters

We present observations of the two closest globular clusters, NGC 6121 and NGC 6397, taken with the NIRISS detector of JWST. The combination of our new JWST data with archival Hubble Space Telescope (HST) images allows us to compute proper motions, disentangle cluster members from field objects, and probe the main sequence (MS) of the clusters down to <0.1 M⊙ as well as the brighter part of the white-dwarf sequence. We show that theoretical isochrones fall short in modeling the low-mass MS and discuss possible explanations for the observed discrepancies. Our analysis suggests that the lowest-mass members of both clusters are significantly more metal-rich and oxygen-poor than their higher-mass counterparts. It is unclear whether the difference is caused by a genuine mass-dependent chemical heterogeneity, low-temperature atmospheric processes altering the observed abundances, or systematic shortcomings in the models. We computed the present-day local luminosity and mass functions of the two clusters; our data reveal a strong flattening of the mass function indicative of a significant preferential loss of low-mass stars in agreement with previous dynamical models for these two clusters. We have made our NIRISS astro-photometric catalogs and stacked images publicly available to the community.

Variable Stars in M31 Stellar Clusters from the Panchromatic Hubble Andromeda Treasury

Variable stars in stellar clusters can offer key constraints on stellar evolution and pulsation models, utilizing estimates of host cluster properties to constrain stellar physical parameters. We present a catalog of 86 luminous (F814W < 19) variable stars in M31 clusters identified by mining the archival Panchromatic Hubble Andromeda Treasury (PHAT) survey using a combination of statistical analysis of sparse PHAT light curves and difference imaging. We determine the evolutionary phases and initial masses of these variable stars by matching them with theoretical isochrones generated using host cluster properties from the literature. We calculate the probability of PHAT photometry being blended due to the highly crowded nature of cluster environments for each cluster-variable star, using these probabilities to inform our level of confidence in the derived properties of each star. Our 86 cluster-variable stars have initial masses between 0.8 and 67 M ⊙. Their evolutionary phases span the main sequence, more evolved hydrogen- and helium-burning phases, and the post–asymptotic giant branch. We identify numerous candidate variable star types: RV Tauri variables, red supergiants, and slowly pulsating B-type supergiants, along with Wolf–Rayet stars, α Cygni and Mira variables, a classical Cepheid, and a possible superasymptotic giant. We characterize 12 cluster-variable stars at higher confidence based on their difference image quality and lower blending probability. Ours is the first systematic study of variable stars in extragalactic stellar clusters leveraging the superior resolution of the Hubble Space Telescope and demonstrating the unique power of stellar clusters in constraining the fundamental properties of variable stars.

The Absolute Age of NGC 3201 Derived from Detached Eclipsing Binaries and the Hess Diagram

We estimate the absolute age of the globular cluster NGC 3201 using 10,000 sets of theoretical isochrones constructed through Monte Carlo simulation using the Dartmouth Stellar Evolution Program. These isochrones take into consideration the uncertainty introduced by the choice of stellar evolution parameters. We fit isochrones with three detached eclipsing binaries and obtained an age independent of distance. We also fit isochrones with differential reddening corrected Hubble Space Telescope photometry data utilizing two different Hess diagram-based fitting methods. Results from three different methods analyzing two different types of data agree to within 1σ, and we find the absolute age of NGC 3201 = 11.85 ± 0.74 Gyr. We also perform a variable importance analysis to study the uncertainty contribution from individual parameters, and we find the distance is the dominant source of uncertainty in photometry-based analysis, while total metallicity, helium abundance, α-element abundance, mixing length, and treatment of helium diffusion are an important source of uncertainties for all three methods.

Isochrone fitting of the open cluster M67 in the era of Gaia and improved model physics

ABSTRACT The Gaia mission has provided highly accurate observations that have significantly reduced the scatter in the colour–magnitude diagrams of open clusters. As a result of the improved isochrone sequence of the open cluster M67, we have created new stellar models that avoid commonly used simplifications in 1D stellar modelling, such as mass-independent core overshooting and a constant mixing length parameter. This has enabled us to deliver a precise isochrone specifically designed for M67, available for download. We follow a commonly used qualitative approach to adjust the input physics to match the well-defined colour–magnitude sequence, and we test the model-predicted masses against a known eclipsing binary system at the main sequence turnoff of the cluster. Despite using improvements in photometry and stellar physics we cannot match the masses of both binary components with the same theoretical isochrone. A $\chi ^{2}$-based isochrone fitting approach using our preferred input physics results in a cluster age of $3.95^{+ 0.16}_{- 0.15}$ Gyr.

Asteroseismology of the young open cluster NGC 2516

Context. Asteroseismic modelling of isolated stars presents significant challenges due to the difficulty in accurately determining stellar parameters, particularly the stellar age. These challenges can be overcome by observing stars in open clusters whose coeval members share an initial chemical composition. The light curves from the all-sky survey by the Transiting Exoplanet Survey Satellite (TESS) allow us to investigate and analyse stellar variations in clusters with an unprecedented level of detail for the first time. Aims. We aim to detect gravity-mode oscillations in the early-type main-sequence members of the young open cluster NGC 2516 to deduce their internal rotation rates. Methods. We selected the 301 member stars with no more than mild contamination as our sample. We analysed the full-frame image light curves, which provide nearly continuous observations in the first and third years of TESS monitoring. We also collected high-resolution spectra using the Fiber-fed Extended Range Optical Spectrograph for the g-mode pulsators, with the aim of assessing the Gaia effective temperatures and gravities and preparing for future seismic modelling. Results. By fitting the theoretical isochrones to the colour-magnitude diagram of a cluster, we determined an age of 102 ± 15 Myr and inferred that the extinction at 550 nm (A0) is 0.53 ± 0.04 mag. We identified 147 stars with surface-brightness modulations: 24 with gravity (g-)mode pulsations (γ Doradus or slowly pulsating B-type stars) and 35 with pressure (p-)mode pulsations (δ Sct stars). When sorted by colour index, the amplitude spectra of the δ Sct stars show a distinct ordering and reveal a discernible frequency-temperature relationship. The near-core rotation rates, measured from period spacing patterns in two slowly pulsating B-type (SPB) stars and nine γ Dor stars, reach up to 3 d−1. This is at the high end of the values found from Kepler data of field stars of similar variability type. The γ Dor stars of NGC 2516 have internal rotation rates as high as 50% of their critical value, whereas the SPB stars exhibit rotation rates close to their critical rate. Although the B-type stars are rotating rapidly, we did not find long-term brightness and colour variations in the mid-infrared, which suggests that there are no disc or shell formation events in our sample. We also discussed the results of our spectroscopic observations for the g-mode pulsators.

The hierarchical formation of 30 Doradus as seen by JWST

The 30 Doradus region in the Large Magellanic Cloud (LMC) is the most energetic star-forming region in the Local Group. It is powered by the feedback from the massive stars in R 136, the 1–2 Myr old central massive cluster. 30 Doradus has therefore long been regarded as a laboratory for studying star and star cluster formation under conditions reminiscent of the early Universe. We use JWST NIRCam observations to analyse how star formation proceeds in the region. Using selections based on theoretical isochrones on colour-magnitude diagrams, we identify populations of different ages. We select pre-main-sequence (PMS) stars and young stellar objects that show excess emission from warm dust or emission lines. Studying the spatial distribution of the different populations, we find that the youngest PMS stars with ages &lt;0.5 Myr are located in an elongated structure that stretches towards the north-east from the central cluster. The same structure is found in the sources that show an infrared excess, appears to be overlapping with cold molecular gas, and covers previously investigated sites of ongoing star formation. Pre-main-sequence stars with ages between 1 and 4 Myr and upper main-sequence stars are concentrated in the centre of R 136, while older stars are more uniformly distributed across the field and likely belong to the LMC field population. Nonetheless, we find stars with excess emission from on dust or emission lines as far as 100 pc from the centre, indicating extended recent star formation. We interpret the elongated structure formed by the youngest PMS stars to be an indication of the still-ongoing hierarchical assembly of the R 136 cluster. Additionally, the lower density of old PMS stars with emission due to ongoing accretion in the central region suggests that feedback from the R 136 stars is effective in disrupting the disks of PMS stars.

Cluster Ages to Reconstruct the Milky Way Assembly (CARMA)

We present CARMA, the Cluster Ages to Reconstruct the Milky Way Assembly project, the aim of which is to determine precise and accurate age measurements for the entire system of known Galactic globular clusters (GCs) and to use them to trace the most significant merger events experienced by the Milky Way. The strength of CARMA relies on the use of homogeneous photometry, theoretical isochrones, and statistical methods, which will enable us to define a systematic uncertainty-free chronological scale for the complete sample of Milky Way GCs. In this paper, we describe the CARMA framework in detail, and present a first application on a sample of six metal-rich GCs, with the aim being to unequivocally elucidate the debated origin of NGC 6388 and NGC 6441. Our results demonstrate that this pair of clusters is coeval with another four systems that have a clear in situ origin. Moreover, their location in the age–metallicity plane matches that occupied by in situ field stars. The accurate age comparison enabled by the CARMA methodology rules out the possibility that NGC 6388 and NGC 6441 were accreted as part of a past merger event.

The Absolute Age of M92

The absolute age of a simple stellar population is of fundamental interest for a wide range of applications but is difficult to measure in practice, as it requires an understanding of the uncertainties in a variety of stellar evolution processes as well as the uncertainty in the distance, reddening, and composition. As a result, most studies focus only on the relative age by assuming that stellar evolution calculations are accurate and using age determinations techniques that are relatively independent of distance and reddening. Here, we construct 20,000 sets of theoretical isochrones through Monte Carlo simulation using the Dartmouth Stellar Evolution Program to measure the absolute age of the globular cluster M92. For each model, we vary a range of input physics used in the stellar evolution models, including opacities, nuclear reaction rates, diffusion coefficients, atmospheric boundary conditions, helium abundance, and treatment of convection. We also explore variations in the distance and reddening as well as its overall metallicity and α enhancement. We generate simulated Hess diagrams around the main-sequence turn-off region from each set of isochrones and use a Voronoi binning method to fit the diagrams to Hubble Space Telescope Advanced Camera for Surveys data. We find the age of M92 to be 13.80 ± 0.75 Gyr. The 5.4% error in the absolute age is dominated by the uncertainty in the distance to M92 (∼80% of the error budget); of the remaining parameters, only the total metallicity, α element abundance, and treatment of helium diffusion contribute significantly to the total error.

Low-density star cluster formation: discovery of a young faint fuzzy on the outskirts of the low-mass spiral galaxy NGC 247

ABSTRACT The classical globular clusters found in all galaxy types have half-light radii of rh ∼ 2–4 pc, which have been tied to formation in the dense cores of giant molecular clouds. Some old star clusters have larger sizes, and it is unclear if these represent a fundamentally different mode of low-density star cluster formation. We report the discovery of a rare, young ‘faint fuzzy’ star cluster, NGC 247-SC1, on the outskirts of the low-mass spiral galaxy NGC 247 in the nearby Sculptor group, and measure its radial velocity using Keck spectroscopy. We use Hubble Space Telescope imaging to measure the cluster half-light radius of rh ≃ 12 pc and a luminosity of LV ≃ 4 × 105L⊙. We produce a colour–magnitude diagram of cluster stars and compare to theoretical isochrones, finding an age of ≃300 Myr, a metallicity of [Z/H] ∼ −0.6 and an inferred mass of M⋆ ≃ 9 × 104M⊙. The narrow width of blue-loop star magnitudes implies an age spread of ≲50 Myr, while no old red-giant branch stars are found, so SC1 is consistent with hosting a single stellar population, modulo several unexplained bright ‘red straggler’ stars. SC1 appears to be surrounded by tidal debris, at the end of an ∼2 kpc long stellar filament that also hosts two low-mass, low-density clusters of a similar age. We explore a link between the formation of these unusual clusters and an external perturbation of their host galaxy, illuminating a possible channel by which some clusters are born with large sizes.

Synthetic Color–Magnitude Diagrams for the Globular Cluster 47 Tucanae

Globular clusters contain the largest coeval populations of stars in the Milky Way, making them prime targets for stellar evolution studies. We present new theoretical isochrones for the brightest monometallic globular cluster 47 Tucanae, based on the spectroscopic measurements of the chemical composition in literature. Calculated colors are compared to existing photometric observations with Advanced Camera for Surveys on the Hubble Space Telescope. We find that the adopted abundances are consistent with the blue tail of the observed color distribution, implying a possible systematic offset between spectroscopic and photometric abundances of globular clusters. Additionally, we make color–magnitude predictions for future observations with the Near Infrared Camera on the James Webb Space Telescope in bands corresponding to the ongoing observational campaigns.

Chemical composition of the young massive cluster NGC 1569-B

Context.The chemical composition of young massive clusters (YMCs) provides stellar population information on their host galaxy. As potential precursors of globular clusters (GCs), their properties can help us understand the origins of GCs and their evolution.Aims.We present a detailed chemical abundance analysis of the YMC NGC 1569-B. The host galaxy, NGC 1569, is a dwarf irregular starburst galaxy at a distance of 3.36 ± 0.20 Mpc. We derive the abundances of theα, Fe-peak, and heavy elements.Methods.We determined the abundance ratios from the analysis of an optical integrated-light (IL) spectrum of NGC 1569-B, obtained with the HIRES echelle spectrograph on the Keck I telescope. We considered different red-to-blue supergiant ratios (NRSG/NBSG), namely: the ratio obtained from a theoretical isochrone (NRSG/NBSG = 1.24), the ratio obtained from a resolved colour–magnitude diagram of the YMC (NRSG/NBSG = 1.53), and the ratio that minimises theχ2when comparing our model spectra with the observations (NRSG/NBSG = 1.90). We adopted the latter ratio for our resulting chemical abundances.Results.The derived iron abundance is sub-solar with [Fe/H] = −0.74 ± 0.05. In relation to the scaled solar composition, we find enhancedα-element abundances, [⟨Mg, Si, Ca, Ti⟩/Fe]= + 0.25 ± 0.11, with a particularly high Ti abundance of +0.49 ± 0.05. Other super-solar elements include [Cr/Fe] = +0.50 ± 0.11, [Sc/Fe] = +0.78 ± 0.20, and [Ba/Fe] = +1.28 ± 0.14, while other Fe-peak elements are close to scaled solar abundances: ([Mn/Fe] = −0.22 ± 0.12 and [Ni/Fe] = +0.13 ± 0.11).Conclusions.The composition of NGC 1569-B resembles the stellar populations of the YMC NGC 1705-1, located in a blue compact dwarf galaxy. The two YMCs agree with regard toα-elements and the majority of the Fe-peak elements, except for Sc and Ba, which are extremely super-solar in NGC 1569-B – and higher than in any YMC studied so far. The blue part of the optical spectrum of a young population is still a very challenging wavelength region to analyse using IL spectroscopic studies. This is due to the uncertain contribution to the light from blue supergiant stars, which can be difficult to disentangle from turn-off stars, even when resolved photometry is available. We suggest that the comparison of model fits at different wavelengths offers a route to determining the red-to-blue supergiant ratio from IL spectroscopy.

A Gaia based analysis of open cluster Berkeley 27

In this paper, we have used the Gaia’s Data Release-3 (DR3) data to study an intermediate-age open cluster Berkeley 27 (Be 27). A total of 131 most probable cluster members are picked within the cluster’s radius based on the membership probability (>80%). The cluster’s radius was estimated as 3.74 arcmin. The mean proper motion (PM) of Be 27 was determined to be (μαcosδ, μδ)= (−1.076±0.008, 0.152±0.007) mas yr−1. The blue straggler stars (BSS) of the cluster were found to be located in the central region. Theoretical isochrones of metallicity Zmetal = 0.008 were compared to the color-magnitude diagram (CMD) of Be 27. As a result, a heliocentric distance of 4.8 ± 0.2 kpc and log (age) = 9.36 ± 0.03 were determined for Be 27. The Galactic orbits are derived using the Galactic potential model which demonstrate that Be 27 follows a circular path around the Galactic center. The cluster does not seem to be affected much by the tidal forces from the Galactic thin disk.

Probing the innermost region of the AU Microscopii debris disc

Context. AU Mic is a young and nearby M-dwarf star harbouring a circumstellar debris disc and one recently discovered planet on an eight-day orbit. Large-scale structures within the disc were also discovered and are moving outwards at high velocity. Aims. We aim to study this system with the highest spatial resolution in order to probe the innermost regions and to search for additional low-mass companions or set detection limits. Methods. The star was observed with two different high-angular resolution techniques probing complementary spatial scales. We obtained new Ks-band sparse aperture masking observations with VLT/SPHERE, which we combined with data from VLT/NACO, VLTI/PIONIER and VLTI/GRAVITY. Results. We did not detect additional close companions within the separation range 0.02–7 au from the parent star. We determined magnitude upper limits for companions of H ~ 9.8 mag within 0.02-0.5 au, Ks ~ 11.2 mag within 0.4–2.4 au, and L ~ 10.7 mag within 0.7–7 au. Using theoretical isochrones, we converted these magnitudes into upper limits on the mass of ~17 Mjup, ~12 Mjup, and ~9 Mjup, respectively. The PIONIER observations also allowed us to determine the angular diameter of AU Mic, θLD = 0.825 ± 0.033stat ± 0.038sys mas, which converts to a linear radius R = 0.862 ± 0.052 R⊙ when combined with the Gaia parallax. Conclusions. We did not detect the newly discovered planets orbiting AU Mic (M &lt; 0.2 Mjup), but we derived upper limit masses for the innermost region of AU Mic. We do not have any detection with a significance beyond 3σ, the most significant signal with PIONIER being 2.9σ and that with SPHERE being 1.6σ. We applied the pyMESS2 code to estimate the detection probability of companions by combining radial velocities, multi-band SPHERE imaging, and our interferometric detection maps. We show that 99% of the companions down to ~0.5 Mjup can be detected within 0.02 au or 1 Mjup down to 0.2 au. The low-mass planets orbiting at ≲0.11 au (≲11 mas) from the star will not be directly detectable with the current adaptive optics (AO) and interferometric instruments because of its close orbit and very high contrast (~10−10 K). It will also be below the angular resolution and contrast limit of the next Extremely Large Telescope Infrared (ELT IR) imaging instruments.

PARSEC V2.0: Stellar tracks and isochrones of low- and intermediate-mass stars with rotation

We present a new comprehensive collection of stellar evolutionary tracks and isochrones for rotating low- and intermediate-mass stars assembled with the updated version of PARSEC V2.0. This version includes our recent calibration of the extra mixing from overshooting and rotation, as well as several improvements regarding the nuclear reaction network, treatment of convective zones, mass loss, and other physical input parameters. The initial mass of the stellar models covers the range 0.09 M⊙ to 14 M⊙ for six sets of initial metallicity, from Z = 0.004 to Z = 0.017. Rotation is considered for stars above ∼1 M⊙ with a smooth transition between non-rotating and extremely fast-rotating models, based on the initial mass. For stars more massive than ∼1.3 M⊙, the full rotation range, from low to critical, is considered. We adopt the solar-scaled chemical mixtures with Z⊙ = 0.01524. All the evolutionary phases from the pre-main-sequence to the first few thermal pulses on the asymptotic giant branch or central C exhaustion are considered. The corresponding theoretical isochrones are further derived with TRILEGAL code and are converted into several photometric systems, taking different inclination angles into account. Besides magnitudes, they also offer many other stellar observables in line with the data being provided by current large surveys. The new collection is fully integrated in a user-friendly web-interface for the benefit of easily performing stellar population studies.

A Blind All-sky Search for Star Clusters in Gaia EDR3: 886 Clusters within 1.2 kpc of the Sun

Although previous searches for star clusters have been very successful, many clusters are likely still omitted, especially at high-Galactic-latitude regions. In this work, based on the astrometry of Gaia EDR3, we searched nearby (ϖ > 0.8 mas) all-sky regions, obtaining 886 star clusters, of which 270 candidates have not been cataloged before. At the same time, we have presented the physical parameters of the clusters by fitting theoretical isochrones to their optical magnitudes. More halo members and expanding structures in many star clusters were also found. Most of the new objects are young clusters that are less than 100 million years old. Our work greatly increased the sample size and physical parameters of star clusters in the solar neighborhood, in particular, 46 clusters are newly found with ∣b∣ > 20°, which represents a nearly threefold increase in the number of clusters at high-Galactic-latitude regions.

Benchmarking mesa isochrones against the Hyades single star sequence

Abstract Based on GAIA Early Data Release 3 (EDR3), we revisit and update our sample of bonafide single stars in the Hyades open cluster. The small observational uncertainties in parallax and photometry of EDR3 result in a tightly defined stellar sequence, which is ideal for the testing and calibration of theoretical stellar evolutionary tracks and isochrones. We benchmark the solar-scaled mesa evolutionary models against the single star sequence. We find that the non-rotating mesa models for [Fe/H] = +0.25 provide a good fit for stars with masses above 0.85, and very low mass stars below 0.25 M⊙. For stars with masses between 0.25 and 0.85 M⊙, the models systematically under predict the observed stellar luminosity. One potential limitation of the models for partially convective stars more massive than 0.35 M⊙ is the prescription of (superadiabatic) convection with the mixing-length theory parameter αML tuned to match the Solar model. Below 0.35 M⊙, the increased scatter in the stellar sequence might be a manifestation of the convective kissing instability, which is driven by variations in the 3He nuclear energy production rate due to instabilities at the convective core to envelope boundary. For a Hyades-like stellar population, the application of solar-scaled models to subsolar mass stars could result in a significant underestimate of the age, or an overestimate of the metallicity. We suggest that future grids of solar-scaled evolutionary stellar models could be complemented by Hyades-scaled models in the mass range 0.25 to 0.85 M⊙.

Chemical Abundances of Young Massive Clusters in NGC 1313∗ ∗ Based on observations made with ESO telescopes at the La Silla Paranal Observatory under program ID 084.B-0468(A).

We analyze spectroscopic observations of five young massive clusters (YMCs) in the barred spiral galaxy NGC 1313 to obtain detailed abundances from their integrated light. Our sample of YMCs was observed with the X-shooter spectrograph on the Very Large Telescope. We make use of theoretical isochrones to generate synthetic integrated-light spectra, iterating on the individual elemental abundances until converging on the best fit to the observations. We measure abundance ratios for [Ca/Fe], [Ti/Fe], [Mg/Fe], [Cr/Fe], and [Ni/Fe]. We estimate an Fe abundance gradient of −0.124 ± 0.034 dex kpc−1, and a slightly shallower α gradient of −0.093 ± 0.009 dex kpc−1. This is in contrast to previous metallicity studies that focused on the gas-phase abundances, which have found NGC 1313 to be the highest-mass barred galaxy known not to have a radial abundance gradient. We propose that the gradient discrepancy between the different studies originates from the metallicity calibrations used to study the gas-phase abundances. We also observe an age–metallicity trend that supports a scenario of constant star formation throughout the galaxy, with a possible burst in star formation in the southwest region where YMC NGC 1313–379 is located.

Optical observations of star clusters NGC 1513 and NGC 4147; white dwarf WD 1145$$+$$017 and K band imaging of star-forming region Sh 2-61 with the 3.6-m Devasthal optical telescope

The $UBVRI$ CCD photometric data of open star cluster NGC 1513 are obtained with the 3.6-m Indo-Belgian Devasthal optical telescope (DOT). Analyses of the GAIA EDR3 astrometric data have identified 106 possible cluster members. The mean proper motion of the cluster is estimated as $\mu_{\alpha}Cos{\delta}=1.29\pm0.02$ and $\mu_{\delta}=-3.74\pm0.02$ mas yr$^{-1}$. Estimated values of reddening $E(B-V)$ and distance to the NGC 1513 are 0.65$\pm$0.03 mag and 1.33$\pm$0.1 kpc respectively. An age of $225\pm25$ Myr is assigned to the cluster by comparing theoretical isochrones with deep observed cluster sequence. Using observations taken with the 3.6-m DOT, values of distance and age of the galactic globular cluster NGC 4147 are estimated as $18.2\pm0.2$ Kpc and $14\pm2$ Gyr respectively. The optical observations of planetary transit around white dwarf WD1145+017 and $K$-band imaging of star-forming region Sharpless Sh 2-61 demonstrate observing capability of 3.6-m DOT. Optical and near-infrared observations of celestial objects and events are being carried out routinely with the 3.6-m DOT. They indicate that the performance of the telescope is at par with those of other similar telescopes located elsewhere in the world. We therefore state that this observing facility augurs well for multi-wavelength astronomy including study of astrophysical jets.

The young Galactic cluster NGC 225: binary stars’ content and total mass estimate

Abstract Galactic star clusters are known to harbour a significant amount of binary stars, yet their role in the dynamical evolution of the cluster as a whole is not comprehensively understood. We investigated the influence of binary stars on the total mass estimate for the case of the moderately populated Galactic star cluster NGC 225. The analysis of multi-epoch radial velocities of the 29 brightest cluster members, obtained over two observational campaigns, in 1990-1991 and in 2019-2020, yields a value of binary fraction of α = 0.52 (15 stars out of 29). Using theoretical isochrones and Monte Carlo simulations we found that the cluster mass increases at least 1.23 times when binaries are properly taken into account. By combining Gaia EDR3 photometric data with our spectroscopic observations, we derived estimates of NGC 225 fundamental parameters foras follows: mean radial velocity &amp;lt;Vr &amp;gt; = − 9.8 ± 0.7 km s−1, log (τ)=8.0-8.2 dex, distance D = 676 ± 22 pc, and colour excess E(B − V) = 0.29 ± 0.01 mag.

High-contrast, high-angular resolution view of the GJ 367 exoplanet system

ABSTRACT We search for additional companions in the GJ 367 exoplanet system and aim to better constrain its age and evolutionary status. We analyse high-contrast direct imaging observations obtained with HST/NICMOS, VLT/NACO, and VLT/SPHERE. We investigate and critically discuss conflicting age indicators based on theoretical isochrones and models for Galactic dynamics. A comparison of GAIA EDR3 parallax and photometric measurements with theoretical isochrones suggests a young age ≤60 Myr for GJ 367. The star’s Galactic kinematics exclude membership to any nearby young moving group or stellar stream. Its highly eccentric Galactic orbit, however, is atypical for a young star. Age estimates considering Galactic dynamical evolution are most consistent with an age of 1–8 Gyr. We find no evidence for a significant mid-infrared excess in the WISE bands, suggesting the absence of warm dust in the GJ 367 system. The direct imaging data provide significantly improved detection limits compared to previous studies. At 530 mas (5 au) separation, the SPHERE data achieve a 5σ contrast of 2.6 × 10−6. The data exclude the presence of a stellar companion at projected separations ≥0.4 au. At projected separations ≥5 au we can exclude substellar companions with a mass ≥1.5 MJup for an age of 50 Myr, and ≥20 MJup for an age of 5 Gyr. By applying the stellar parameters corresponding to the 50 Myr isochrone, we derive a bulk density of ρplanet = 6.2 g cm−3 for GJ 367 b, which is 25 per cent smaller than a previous estimate.