A photometric investigation of the young open cluster Trumpler 15

Deep and extensive CCD photometric observations $UBV(RI)_{C}H_{\alpha}$ were carried out in the area of the open cluster NGC 3293. The new data set allows to see the entire cluster sequence down to $M_{V} \approx +4.5$, revealing that stars with $M_{V} < -2$ are evolving off the main sequence; stars with $-2 < M_{V} < +2$ are located on the main sequence and stars with $M_{V} > +2$ are placed above it. According to our analysis, the cluster distance is $d = 2750 \pm 250 pc$ ($V_{0}-M_{V} = 12.2 \pm 0.2$) and its nuclear age is $8 \pm 1 Myr$. NGC 3293 contains an important fraction of pre--main sequence (PMS) stars distributed along a parallel band to the ZAMS with masses from 1 to $2.5 \cal M_{\sun}$ and a mean contraction age of $10 Myr$. This last value does not differ too much from the nuclear age estimate. Actually, if we take into account the many factors that may affect the PMS star positions onto the colour--magnitude diagram, both ages can be perfectly reconciled. The star formation rate, on the other hand, suggests that NGC 3293 stars formed surely in one single event, therefore favouring a coeval process of star formation. Besides, using the $H_{\alpha}$ data, we detected nineteen stars with signs of having $H_{\alpha}$ emission in the region of NGC 3293, giving another indication that the star formation process is still active in the region. The computed initial mass function for the cluster has a slope value $x = 1.2 \pm 0.2$, a bit flatter than the typical slope for field stars and similar to the values found for other young open clusters.

We present CCD VIKC photometry down to V≈ 21 mag in the field of the rich open cluster NGC 2194, which is projected towards the Galactic anticentre direction. We measured V magnitudes and V−I colours for a total of 2515 stars in a field of 13.6 × 13.6 arcmin2. These data are supplemented with CCD photometry in the C, M and T1 filters of the Washington system and photoelectric CMT1T2 photometry of 20 red giant candidates. Based on the best fits of isochrones computed by the Geneva and Padova groups to the (V, V−I) colour — magnitude diagram, we derive a colour excess E(V−I) = 0.75, a cluster distance of 3.2 kpc and an age of 400 Myr. Five independent Washington abundance indices yield a mean cluster metallicity of [Fe/H]=−0.27 ± 0.06, which is compatible with the existence of both a radial and Z gradient in the Galactic disc. NGC 2194 is a relatively young open cluster, whose deficiency in metal content can be accounted for by the fact that it was born not only far from the Galactic centre in the outer disc, but also at a relatively high Z value.

Context. Usually, a loose stellar distribution can be classified as an OB stellar group, an association, or a young open cluster. We compare data with the typical OB association Vul OB1. Aims. Here, we discuss the nature of Bochum 1, a typical example of an object affected by the above classification problem. Methods. Field-decontaminated 2MASS photometry is used to analyse colour–magnitude diagrams (CMDs) and stellar radial density profiles (RDPs) of the structures present in the region of Bochum 1. Results. The field-decontaminated CMD of Bochum 1 shows main sequence (MS) and pre-main sequence (PMS) stars. We report two new small angular-size, compact young clusters and one embedded cluster in the area of Bochum 1. Vul OB1 harbours the young open cluster NGC 6823 and the very compact embedded cluster Cr 404. The Vul OB1 association includes the H II region Sh2-86, and its stellar content is younger (≈3 Myr) than that of Bochum 1 (≈9 Myr), which shows no gas emission. Bochum 1 harbours one of the newly found compact clusters as its core. The RDP of Bochum 1 is irregular and cannot be fitted by a King-like profile, which suggests significant erosion or dispersion of stars from a primordial cluster. Similarly to Bochum 1, the decontaminated CMD of NGC 6823 presents conspicuous MS and PMS sequences. Taken separately, RDPs of MS and PMS stars follow a King-like profile. The core shows an excess density of MS stars that mimics the profile of a post-core collapse cluster. At such a young age, it can be explained by an excess of stars formed in the prominent core. Conclusions. The present study suggests that Bochum 1 is a star cluster fossil remain that might be dynamically evolving into an OB association. Bochum 1 may be a missing link connecting early star cluster dissolution with the formation of low-mass OB associations.

Context . While large samples of nearby M dwarfs are becoming available, it is usually difficult to estimate some intrinsic physical parameters for isolated field M dwarfs, especially their ages, radii, and masses. These parameters can be estimated through a comparison with model stellar isochrones and/or by studying eclipsing binary systems. Aims . Our goal is to gather and analyze a large sample of variable M dwarfs that are members of nearby young open clusters and have known reddenings, distances, ages, and metallicities. This distilled sample would be useful for the comparison with theoretical models and also for the identification of eclipsing binaries with substellar companions (brown dwarfs or giant planets) for future follow-up. Methods . We selected two dozen benchmark young and nearby open clusters (mostly with 40 &lt; age &lt; 132 Myr and 400 &lt; D &lt; 700 pc, respectively). We then combined the optical data from Gaia Data Release 3 (DR3) with near-IR JHK s −band photometry from the VISTA Variables in the Vía Láctea eXtended survey (VVVX) for these target clusters using the proper motions in order to select members within the cluster radii. Gaia and VVVX both provide a wide areal coverage that is ideally suited for mapping these nearby clusters, which are extended on the sky. We then produced optical and near-IR color–magnitude and color-color diagrams and applied appropriate color cuts to choose the M-type dwarfs at the end of the main sequence. The reddening and extinction for all clusters were estimated using the J − K s color distributions of the cluster M dwarfs. We also compared them with the PARSEC theoretical isochrones adjusted to the respective cluster metallicities, ages, extinctions, and distances. Results . Finally, we selected variable M stars according to Gaia DR3 (using phot_variable_flag=VARIABLE) and present a catalog of 318 variable M-dwarf star members of two dozen young nearby Galactic open clusters. The cross-validation with the Gaia DR3 parallaxes supports the cluster membership for all these sources. We also present a few examples to validate the sample of variable cluster M dwarfs. Conclusions . This well-characterized catalog of M-dwarf members of benchmark open clusters not only constitutes a prime sample to search for transiting substellar companions, but also offers a training dataset for machine-learning applications aimed at selecting future similar targets.

Radial and rotational velocities, chromospheric activity and lithium abundances are presented for an X-ray-selected sample of stars in the young (220 Myr) open cluster NGC 6475. Low-mass members of the cluster have been identified on the basis of photometric and spectroscopic criteria. The observations show that the rapid spin-down seen amongst solar-type stars in the Pleiades is incomplete at 220 Myr, as there are F, G and K stars in NGC 6475 with υ sin i > 10 km s−1. Peak rotation rates for G stars are 12–14 km s−1 and are robust to uncertain inclination angles, because the magnetic activity of these stars is less than the saturation value observed for fast rotators (υ sin i > 15 km s−1) in the Pleiades. Two mid-K stars are found with υ sin i ∼25 km s−1 and with saturated magnetic activity levels, indicating that spin-down time-scales are mass dependent, increasing from ∼ 20 Myr for early-G stars to > 75 Myr for mid-K stars. We have compared our rotation data, for the most rapidly rotating G and K stars in NGC 6475 and in young open clusters, with published rotational evolution models incorporating magnetic dynamo saturation at high rotation rates ( > Ωsat). Models with interior angular momentum transport are unable to simultaneously fit the rapidly rotating stars of the α Per, Pleiades and NGC 6475 clusters. Likely solutions are a plausible increase in the age of the Pleiades to 100 Myr, the incorporation of centrifugal wind driving in the angular momentum loss treatment, or ad hoc differences in initial conditions. If an increase in the age of the Pleiades is assumed, we find that a mass dependent Ωsat is required, such that dynamo saturation occurs for G and K stars at a Rossby number of ∼ 0.4. This value is incompatible with the faster saturation thresholds inferred from coronal X-rays, but Ωsat can be increased if solid body rotation is enforced or centrifugal wind driving is included. Comparisons with such models are favourable for G stars but less so for K stars. This may result from neglecting the evolving moment of inertia at early ages. All the cool stars exhibit Lii 6708-Å absorption features. When compared to Li in the Pleiades and Hyades, it is evident that the G and K stars in the Hyades have experienced substantial Li depletion whilst on the main sequence, between 220 and 600 Myr, and probably at earlier times on the main sequence as well. There is marginal scatter of about 0.1 dex in the Li abundances of G and early K stars, possibly increasing to 1.0 dex in a few cooler objects. The data are insufficient to allow us to decide whether there is a mechanism by which to eliminate the scatter in Li abundances of the Pleiades K stars before they reach the Hyades age, or whether the difference reflects different cluster initial conditions. Interpretation is complicated by the X-ray selection bias, favouring the observation of the more rapidly rotating, Li-rich stars. Metallicity may also play a role, but our spectral ⋆⁥E-mail: djj@st-and.ac.uk (DJJ); rdj@astro.keele.ac.uk (RDJ) †Present address: School of Physics and Astronomy, University of St. Andrews, North Haugh, St. Andrews, Fife KY16 9SS. synthesis estimate of [Fe/H]= +0.110 ± 0.034 is comparable to the Pleiades and Hyades values.

The luminosity function (LF) of young open cluster stars has been studied for decades to obtain a slope of the initial mass function (IMF), which is important for understanding of star formation. Young open clusters contain a lot of pre-main-sequence (PMS) stars the positions of which in the H—R diagram deviate significantly from the main sequence and consequently the PMS massluminosity relation (MLR) differs from the ZAMS one. Such features of the MLR and related details of young open cluster LFs they produce are the subject of this poster. Three peaks have been studied: (1) the main peak (H-peak)—generated by PMS stars settling on the main sequence; (2) the R-peak—generated by inversion of the radius on the stellar birthline; the D-peak—generated by changes in the MLR and associated with deuterium burning. The evolution of these features with cluster age and possible application for young open cluster study are discussed.

The current status of the problem of anomalous increases in the relative number of Be stars in young open stellar clusters is summarized in this article. The relative content of Be stars is shown to increase with the age of the clusters and reaches a maximum for stars in spectral classes B0-B3 when the age of the clusters is 12-20 Myr. In some clusters that are rich in B stars, the relative content exceeds 40%. It is shown that in young clusters with ages less than 9 Myr, Be stars are very rare or not observed at all. Models of the evolution of rotating massive stars indicates that the Be phenomenon may occur toward the end of the lifetime of B stars in the main sequence because of an increase in the ratio of the rotation velocity of a star to the critical velocity, but this cannot explain the presence of more than 40% Be stars in some young open stellar clusters. In this review, considerable attention is devoted to massive binary systems in different stages of the evolution of the binary system. The transfer of angular momentum during exchanges of mass in systems with different periods is a noteworthy alternative to the evolution of angular momentum in single stars. Some examples are shown of binary systems of different types which are or may be encountered in open stellar clusters.

We examine the effects of different assumptions about the initial conditions, angular momentum loss law, and angular momentum transport on the angular momentum evolution of 0.5-1.2 solar mass stars. We first perform a parameter variation study to test the sensitivity of the surface rotation rate as a function of mass and age to changes in the initial conditions and input physics. We then check to see if the distribution of initial conditions for a given physical scenario is consistent for open clusters of different ages. The behavior of the rapid rotators is highly sensitive to the saturation threshold for angular momentum loss (ωcrit), above which angular momentum loss scales linearly with the rotation rate. Very high values for ωcrit suppress rapid rotation prior to the main sequence, and very low values permit rapid rotation to survive for too long. For solid-body (SB) and differential rotation (DR) models, higher mass models rotate more rapidly than lower mass models for the same initial conditions and ωcrit. DR models differ from SB models in both the direct effect of core-envelope decoupling and a change in the calibration of the angular momentum loss law needed to reproduce the solar rotation at the age of the Sun; the effects of both are discussed. Slow rotation in young clusters can be achieved with modest disk lifetimes (3-10 Myr) for the DR models and longer disk lifetimes for the SB models (10 or more Myr). In addition, the slowly rotating DR models spin down during the early main sequence more than the slowly rotating SB models do. When compared with the cluster data, the observed mass dependence of the rapid rotator phenomenon can be reproduced only with a mass-dependent ωcrit for both the SB and DR models. A scaling of ωcrit inversely proportional to the convective overturn timescale can reproduce the observed mass-dependent spindown. The observed spindown of the slow rotators in the young open clusters is in better agreement with the DR than the SB models. We also discuss observational tests to distinguish different classes of models using low-mass stars and rotation periods in open clusters.

We perform a new analysis of the extant ROSAT and XMM-Newton X-ray surveys of the southern open cluster Blanco 1, utilizing new BVIc photometric and proper motion data sets. In our study, we match optical counterparts to 47 X-ray sources associated with Blanco 1 cluster members, six of which were listed in previous X-ray studies as cluster nonmembers. Our new catalog of optical counterparts to X-ray sources clearly traces out the Blanco 1 main sequence in a color-magnitude diagram, extending from early G to mid-M spectral types. Additionally, we derive new X-ray luminosities as well as ratios of X-ray to bolometric luminosities for confirmed cluster members. We compare these X-ray properties to other young open clusters, including the coeval Pleiades cluster, to investigate the relationship between age and X-ray activity. We find that stars in Blanco 1 generally exhibit X-ray properties similar to those of other open clusters, namely increasing Lx /L bol with reducing mass for earlier-type stars, and a saturation limit of Lx /L bol at a magnitude of 10–3 for stars with V – Ic 1.25. More generally, the X-ray detected stars in Blanco 1 have X-ray emission magnitudes that agree with the overall trends seen in the other young clusters. We observe that X-ray emission decays as a function of age and the rate of this decay is mass dependent. Specifically, for higher mass stars, the trend is Skumanich like (i.e., Lx /L bol age–1/2); however, as one goes to lower masses the magnitude of X-ray emission becomes less of a function of age. In fact, for the lowest mass stars (M-type), there is no observable reduction in X-ray production during the first ~1 Gyr of their lives. However, due to a lack of sensitivity to low X-ray fluxes, there may exist M-type stars that have less than saturated levels of X-ray flux which are not included in our study. In a direct comparison of Blanco 1 to the Pleiades open cluster, members of both clusters have similar X-ray characteristics; however, there does appear to be some discrepancies in the distribution of Lx /L bol as a function of color that may be related to scatter seen in the Pleiades CMD. Moreover, previous comparisons of this nature for Blanco 1 were not possible due to the reliance on photographic photometry. This is where the power of precise, homogeneous, and standardized CCD photometry allows for a high fidelity, detailed study of the X-ray properties of stars in Blanco 1, as well as a thorough comparison of Blanco 1 to other well-studied open clusters.

We present current results from the ongoing WIYN Open Cluster Study radial-velocity survey for 1410 stars in the young (150 Myr) open cluster M35 (NGC 2168) and establish a benchmark for initial conditions in young open clusters. We find for periods ≲ 1000 days a minimum binary frequency of 0.36 – 0.51. We also analyze the spatial, period and eccentricity distributions of the binary systems and find that the period and eccentricity distributions are well approximated by scaled field distributions from Duquennoy &amp; Mayor (1991). With our large sample size and long baseline, we have a unique understanding of the binary population in this young cluster, making it ideal for defining initial conditions for dynamical simulations.

view Abstract Citations (168) References (99) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Evolution of Rotation and Activity in Young Open Clusters: IC 2391 Patten, Brian M. ; Simon, Theodore Abstract We have undertaken a program of ground- and space-based observations to measure photometric rotation periods and X-ray luminosities for late-type stars in the young open cluster IC 2391. At an age of ∼30 Myr, IC 2391 is an ideal cluster in which to observe conditions at the zero-age main sequence (ZAMS) since the solar-type stars within this cluster have not been on the main sequence long enough to undergo significant magnetic braking. For late-type stars, the ZAMS represents the endpoint of pre-main-sequence evolution, wherein gravitational contraction and interactions with circumstellar accretion disks dominate the angular momentum evolution of the star. The ZAMS also represents the beginning of the main-sequence life of the star, wherein its subsequent evolution is dominated by magnetic braking. Knowledge of ZAMS stellar parameters, such as rotation rates and activity levels, is therefore crucial for accurate theoretical model predictions. Our survey of IC 2391 X-ray sources has revealed a number of new candidates for cluster membership, most of these being late-type stars. Among the solar-type stars in IC 2391, we find a factor of ∼20 spread in the distribution of rotation periods and also in the distribution of X-ray luminosities. We therefore conclude that stars arrive on the ZAMS with a wide range of rotation rates and activity levels. When we compare IC 2391 with older clusters, such as the Pleiades and the Hyades, we find there is an overall decline in the median rotation rate and X-ray luminosity with age, but the dispersion in X-ray luminosity, as a fraction of the median value, actually grows larger. This is because the young, rapidly rotating stars lie along a plateau of magnetic saturation, where LX has a weak dependence on rotation period, while for the older, more slowly rotating stars, the weaker LX has a strong dependence on rotation period. We find an abrupt turnover in the Rossby diagram near NR = 0.3, which suggests there is a fundamental change in the nature of dynamo activity for rapidly rotating stars. Publication: The Astrophysical Journal Supplement Series Pub Date: October 1996 DOI: 10.1086/192346 Bibcode: 1996ApJS..106..489P Keywords: GALAXY: OPEN CLUSTERS AND ASSOCIATIONS: INDIVIDUAL ALPHANUMERIC: IC 2391; STARS: CORONAE; STARS: EVOLUTION; STARS: LATE-TYPE; STARS: ROTATION; X-RAYS: STARS full text sources ADS | data products SIMBAD (117)

The very young open cluster (OC) NGC2244 in the Rosette Nebula was studied with fieldstar- decontaminated Two-Micron All-Sky Survey (2MASS) photometry, which shows the main-sequence (MS) stars and an abundant pre-MS (PMS) population. Fundamental and structural parameters were derived with colour–magnitude diagrams (CMDs), stellar radial density profiles (RDPs) and mass functions (MFs). Most previous studies centred NGC2244 close to the bright K0V star 12Monocerotis, which is not a cluster member. Instead, the nearinfrared RDP indicates a pronounced core near the O5 star HD46150.We derive an age within 1–6 Myr, an absorption AV =1.7±0.2, a distance from the Sun dʘ =1.6±0.2 kpc (≈1.5 kpc outside the solar circle), an MF slope χ = 0.91 ± 0.13 and a total (MS+PMS) stellar mass of ~625Mʘ. Its RDP is characterized by the core and cluster radii Rc ≈5.6 arcmin (≈2.6 pc) and RRDP ≈ 10 arcmin (≈4.7 pc), respectively. Departure from dynamical equilibrium is suggested by the abnormally large core radius and the marked central stellar excess. We also investigate the elusive neighbouring OC NGC2239, which is low mass (mMS+PMS ≈ 301Mʘ), young (5 ± 4Myr) rather absorbed (AV = 3.4 ± 0.2), and located in the background of NGC2244 at dʘ = 3.9 ± 0.4 kpc. Its RDP follows a King-like function of Rc ≈ 0.5 arcmin ≈ 0.5 pc and RRDP ≈ 5.0 arcmin ≈ 5.6 pc. The MF slope, χ = 1.24 ± 0.06, is essentially Salpeter’s initial mass function. NGC2244 is probably doomed to dissolution in a few 10 7 yr. Widefield extractions and field-star decontamination increase the stellar statistics and enhance both CMDs and RDPs, which is essential for faint and bright star clusters.

We investigate the nature of 4 young and low-mass open clusters (OCs) located in the $2^{nd}$ and $3^{rd}$ quadrants with near-IR 2MASS photometry (errors $<0.1$ mag). After field decontamination, the colour-magnitude diagrams (CMDs) display similar morphologies: a poorly-populated main sequence (MS) and a dominant fraction of pre-MS (PMS) stars somewhat affected by differential reddening. Pismis 5, vdB 80 and BDSB 96 have MS ages within $5\pm4$ Myr, while the MS of NGC 1931 is $10\pm3$ Myr old. However, non-instantaneous star formation is implied by the wider ($\sim20$ Myr) PMS age spread. The cluster masses derived from MS + PMS stars are low, within $\sim60-180 \ms$, with mass functions (MFs) significantly flatter than Salpeter's initial mass function (IMF). Distances from the Sun are within $1.0-2.4$ kpc, and the visual absorptions are in the range $\aV=1.0-2.0$. From the stellar radial density profiles (RDPs), we find that they are small ($\rc\la0.48$ pc, $\rl\la5.8$ pc), especially Pismis 5 with $\rc\approx0.2$ pc and $\rl\approx1.8$ pc. Except for the irregular and cuspy inner regions of NGC 1931 and Pismis 5, the stellar RDPs follow a King-like profile. At $\sim10$ Myr, central cusps - which in old clusters appear to be related to advanced dynamical evolution - are probably associated with a star-formation and/or molecular cloud fragmentation effect. Despite the flat MFs, vdB 80 and BDSB 96 appear to be typical young, low-mass OCs. NGC 1931 and especially Pismis 5, with irregular RDPs, low cluster mass and flat MFs, do not appear to be in dynamical equilibrium. Both may be evolving into OB associations and/or doomed to dissolution in a few $10^7$ yr.

Accurate stellar ages are essential for our understanding of the star formation history of the Milky Way and Galactic chemical evolution, as well as to constrain exoplanet formation models. Gyrochronology, a relationship between stellar rotation and age, appears to offer a reliable age indicator for main-sequence (MS) stars over the mass range of approximately 0.6–1.3 M ⊙. Those stars lose their angular momentum due to magnetic braking and as a result their rotation speeds decrease with age. Although current gyrochronology relations have been fairly well tested for young MS stars with masses greater than 1 M ⊙, primarily in young open clusters, insufficient tests exist for older and lower mass MS stars. Binary stars offer the potential to expand and fill in the range of ages and metallicity over which gyrochronology can be empirically tested. In this paper, we demonstrate a Monte Carlo approach to evaluate gyrochronology models using binary stars. As examples, we used five previously published wide binary pairs. We also demonstrate a Monte Carlo approach to assess the precision and accuracy of ages derived from each gyrochronology model. For the traditional Skumanich models, the age uncertainties are σ age/age = 15%–20% for stars with B − V = 0.65 and σ age/age = 5%–10% for stars with B − V = 1.5 and rotation period P ≤ 20 days.

Previous results in the literature have found the young inner-disc open cluster NGC 6705 to be mildly α-enhanced. We examined this possibility via an independent chemical abundance analysis for 11 red-giant members of NGC 6705. The analysis is based on near-infrared APOGEE spectra and relies on LTE calculations using spherical model atmospheres and radiative transfer. We find a mean cluster metallicity of $\rm [Fe/H] = +0.13 \pm 0.04$, indicating that NGC 6705 is metal-rich, as may be expected for a young inner-disc cluster. The mean α-element abundance relative to iron is $\rm \langle [\alpha /Fe]\rangle =-0.03 \pm 0.05$, which is not at odds with expectations from general Galactic abundance trends. NGC 6705 also provides important probes for studying stellar mixing, given its turn-off mass of M ∼ 3.3 M⊙. Its red giants have low 12C abundances ([12C/Fe] = −0.16) and enhanced 14N abundances ([14N/Fe] = +0.51), which are key signatures of the first dredge-up on the red giant branch. An additional signature of dredge-up was found in the Na abundances, which are enhanced by [Na/Fe] = +0.29, with a very small non-LTE correction. The 16O and Al abundances are found to be near-solar. All of the derived mixing-sensitive abundances are in agreement with stellar models of approximately 3.3 M⊙ evolving along the red giant branch and onto the red clump. As found in young open clusters with similar metallicities, NGC 6705 exhibits a mild excess in the s-process element cerium with $\rm [Ce/Fe] = +0.13\pm 0.07$.