Stars In NGC Research Articles

Ultraluminous X-ray sources with He star companions

ABSTRACT Ultraluminous X-ray sources (ULXs) are non-nuclear point-like objects observed with extremely high X-ray luminosity that exceeds the Eddington limit of a $\rm 10\, M_\odot$ black hole. A fraction of ULXs has been confirmed to contain neutron star (NS) accretors due to the discovery of their X-ray pulsations. The donors detected in NS ULXs are usually luminous massive stars because of the observational biases. Recently, the He donor star in NGC 247 ULX-1 has been identified, which is the first evidence of a He donor star in ULXs. In this paper, we employed the stellar evolution code mesa (Modules for Experiments in Stellar Astrophysics) to investigate the formation of ULXs through the NS+He star channel, in which a He star transfers its He-rich material onto the surface of an NS via Roche lobe overflow. We evolved a large number of NS+He star systems and provided the parameter space for the production of ULXs. We found that the initial NS+He star systems should have $\sim 0.7\!-\!2.6 \, \mathrm{M}_\odot$ He star and $\sim 0.1\!-\!2500\, \mathrm{d}$ orbital period for producing ULXs, eventually evolving into intermediate-mass binary pulsars. According to binary population synthesis calculations, we estimated that the Galactic rate of NS ULXs with He donor stars is in the range of $\sim 1.6\!-\!4.0\times 10^{-4}\, {\rm yr}^{-1}$, and that there exist $\sim 7-20$ detectable NS ULXs with He donor stars in the Galaxy.

Anomalously low-mass core-He-burning star in NGC 6819 as a post-common-envelope phase product

Precise masses of red giant stars enable a robust inference of their ages, but there are cases where these age estimates are very precise but also very inaccurate. Examples are core-helium-burning (CHeB) stars that have lost more mass than predicted by standard single-star evolutionary models. Members of star clusters in the database represent a unique opportunity to identify such stars because they combine exquisite asteroseismic constraints with independent age information (members of a star cluster share a similar age and chemical composition). We focus on the single metal-rich ($Z Li-rich low-mass CHeB star KIC4937011, which is a member of the open cluster NGC 6819 (turn-off mass of $ i.e. an age of $ 2.4$ Gyr). This star has a lower mass by $ than expected for its age and metallicity, which might be explained by binary interactions or mass loss along the red giant branch (RGB). To infer formation scenarios for this object, we performed a Bayesian analysis by combining the binary stellar evolutionary framework binary\_c v2.2.3 with the dynamic nested-sampling approach contained in the dynesty v2.1.1 package. We find that this star probably is the result of a common-envelope evolution (CEE) phase during the RGB stage of the primary star in which the low-mass ($<0.71$ main-sequence companion does not survive. The mass of the primary star at the zero-age main sequence is in the range with a log-orbital period in the range days )$. During the CEE phase, $ of material is ejected from the system, and the final star reaches the CHeB stage after helium flashes as if it were a single star with a mass of $ which is what we observe today. Although the proposed scenario is consistent with photometric and spectroscopic observations, a quantitative comparison with detailed stellar evolution calculations is needed to quantify the systematic skewness of the radius, luminosity, and effective temperature distributions towards higher values than observations.

Origin of extended main-sequence turn-off in open cluster NGC 2355

ABSTRACT The presence of extended Main-Sequence Turn-Off (eMSTO) in the open clusters has been attributed to various factors, such as spread in rotation rates, binary stars, and dust-like extinction from stellar excretion discs. We present a comprehensive analysis of the eMSTO in the open cluster NGC 2355. Using spectra from the Gaia–European Southern Observatory (ESO) archives, we find that the stars in the red part of the eMSTO have a higher mean v sin i value of 135.3 $\pm$ 4.6 km s$^{-1}$ compared to the stars in the blue part that have an average v sin i equal to 81.3 $\pm$ 5.6 km s$^{-1}$. This suggests that the eMSTO in NGC 2355 is possibly caused by the spread in rotation rates of stars. We do not find any substantial evidence of the dust-like extinction from the eMSTO stars using ultraviolet data from the Swift survey. The estimated synchronization time for low-mass ratio close binaries in the blue part of the eMSTO suggests that they would be mostly slow-rotating if present. However, the stars in the blue part of the eMSTO are preferentially located in the outer region of the cluster indicating that they may lack low-mass ratio close binaries. The spread in rotation rates of eMSTO stars in NGC 2355 is most likely caused by the star-disc interaction mechanism. The stars in the lower main sequence beyond the eMSTO region of NGC 2355 are slow rotating (mean v sin i = 26.5 $\pm$ 1.3 km s$^{-1}$) possibly due to the magnetic braking of their rotations.

Pulsating hydrogen-deficient white dwarfs and pre-white dwarfs observed with TESS

Context.Significant advances have been achieved through the latest improvements in the photometric observations accomplished by the recent space missions, which substantially boost the study of pulsating stars via asteroseismology. The TESS mission has already proven to be of particular relevance for pulsating white dwarf and pre-white dwarf stars.Aims.We report a detailed asteroseismic analysis of the pulsating PG 1159 star NGC 246 (TIC 3905338), which is the central star of the planetary nebula NGC 246, based on high-precision photometric data gathered by the TESS space mission.Methods.We reduced TESS observations of NGC 246 and performed a detailed asteroseismic analysis using fully evolutionary PG 1159 models computed accounting for the complete prior evolution of their progenitors. We constrained the mass of this star by comparing the measured mean period spacing with the average of the computed period spacings of the models, and we also employed the observed individual periods to search for a seismic stellar model.Results.We extracted a total of 17 periodicities from the TESS light curves from the two sectors where NGC 246 was observed. All the oscillation frequencies are associated withg-mode pulsations, with periods spanning from ∼1460 to ∼1823 s. We found a constant period spacing of ΔΠ = 12.9 s, which allowed us to deduce that the stellar mass is higher than ∼0.87 M⊙if the period spacing is assumed to be associated withℓ = 1 modes, and that the stellar mass is ∼0.568 M⊙if it is associated withℓ = 2 modes. The less massive models are more consistent with the distance constraint fromGaiaparallax. Although we were not able to find a unique asteroseismic model for this star, the period-to-period fit analyses suggest a high stellar mass (≳0.74M⊙) when the observed periods are associated with modes withℓ = 1 only, and both a high and an intermediate stellar mass (≳0.74 M⊙and ∼0.57 M⊙, respectively) when the observed periods are associated with modes with a mixture ofℓ = 1, 2.

Variable Stars in the 50BiN Open Cluster Survey. III. NGC 884

Open clusters are the basic building blocks that serve as a laboratory for the study of young stellar populations in the Milky Way. Variable stars in open clusters provide a unique way to accurately probe the internal structure, temporal and dynamical evolutionary stages of individual stars and the host cluster. The most powerful tool for such studies is time-domain photometric observations. This paper follows the route of our previous work, concentrating on a photometric search for variable stars in NGC 884. The target cluster is the companion of NGC 869, forming the well-known double cluster system that is gravitationally bound. From the observation run in 2016 November, a total of 9247 B-band CCD images and 8218 V-band CCD images were obtained. We detected a total of 15 stars with variability in visual brightness, including five Be stars, three eclipsing binaries, and seven of unknown types. Two new variable stars were discovered in this work. We also compared the variable star content of NGC 884 with its companion NGC 869.

UVIT/AstroSat Investigation of a Low-luminous Blue Straggler Star in NGC 362: Detection of an Extremely Low-mass White Dwarf as Companion

In the present study, we identified an extremely low-mass white dwarf as a companion to a low luminous blue straggler star within the Galactic globular cluster NGC 362. To conduct the analysis, we utilized data obtained from various sources, including AstroSat’s Ultra Violet Imaging Telescope, UVOT, and the 2.2 m ESO telescope. By examining the spectral energy distribution of the blue straggler star candidate, we successfully identified an extremely low-mass white dwarf as its binary companion. We determined the effective temperature, radius, and luminosity for both, the extremely low-mass white dwarf and the blue straggler star candidate. Specifically, the extremely low-mass white dwarf exhibits an effective temperature of 15000 K, a radius of 0.17 R⊙, a luminosity of 1.40 L⊙, and a mass range of 0.19-0.20 M⊙. Furthermore, the position of the straggler star within the cluster suggests their formation via the Case A/B mass-transfer mechanism in a low-density environment.

The young massive SMC cluster NGC 330 seen by MUSE

Context. The origin of the initial rotation rates of stars, and how a star’s surface rotational velocity changes during the evolution, either by internal angular momentum transport or due to interactions with a binary companion, remain open questions in stellar astrophysics. Aims. Here, we aim to derive the physical parameters and study the distribution of (projected) rotational velocities of B-type stars in the ∼35 Myr-old, massive cluster NGC 330 in the Small Magellanic Cloud. NGC 330 is in an age range where the number of post-interaction binaries is predicted to be high near the cluster turnoff (TO). Methods. We developed a simultaneous photometric and spectroscopic grid-fitting method adjusting atmosphere models on multiband Hubble Space Telescope (HST) photometry and Multi Unit Spectroscopic Explorer (MUSE) spectroscopy. This allowed us to homogeneously constrain the physical parameters of over 250 B and Be stars (i.e., B-type stars with emission lines), brighter than mF814W = 18.8 mag. Results. The rotational velocities of Be stars in NGC 330 are significantly higher than the ones of B-type stars. The rotational velocities vary as a function of the star’s position in the color-magnitude diagram, qualitatively following predictions of binary population synthesis. A comparison to younger clusters shows that stars in NGC 330 rotate more rapidly on average. Conclusions. The rotational velocities of the ∼35 Myr old population in NGC 330 quantitatively agree with predictions for a stellar population that underwent significant binary interactions: the majority of the B-type stars could be single stars or primaries in pre-interaction binaries. The rapidly spinning Be stars could be mass and angular momentum gainers in previous interactions, while those Be stars close to the TO may be spun-up single stars. The slowly rotating, apparently single stars above the TO could be merger products. The different v sin i characteristics of NGC 330 compared to younger populations can be understood in this framework.

CCD UBV and Gaia DR3 Analyses of the Open Clusters King 6 and NGC 1605

A detailed analysis of ground-based CCD UBV photometry and space-based Gaia Data Release 3 (DR3) data for the open clusters King 6 and NGC 1605 was performed. Using the pyUPMASK algorithm on Gaia astrometric data to estimate cluster membership probabilities, we have identified 112 stars in King 6 and 160 stars in NGC 1605 as the statistically most likely members of each cluster. We calculated reddening and metallicity separately using UBV two-color diagrams to estimate parameter values via independent methods. The color excess E(B − V) and photometric metallicity [Fe/H] for King 6 are 0.515 ± 0.030 mag and 0.02 ± 0.20 dex, respectively. For NGC 1605, they are 0.840 ± 0.054 mag and 0.01 ± 0.20 dex, respectively. With reddening and metallicity kept constant, we have estimated the distances and cluster ages by fitting PARSEC isochrones to color–magnitude diagrams based on the Gaia and UBV data. The photometric distances are 723 ± 34 pc for King 6 and 3054 ± 243 pc for NGC 1605. The cluster ages are 200 ± 20 Myr and 400 ± 50 Myr for King 6 and NGC 1605, respectively. The mass function slopes were found to be 1.29 ± 0.18 and 1.63 ± 0.36 for King 6 and NGC 1605, respectively. These values are in good agreement with the value of Salpeter. The relaxation times were estimated as 5.8 Myr for King 6 and 60 Myr for NGC 1605. These indicate that both clusters are dynamically relaxed since these times are less than the estimated cluster ages. A Galactic orbit analysis shows that both clusters formed outside the solar circle and are members of the young thin-disk population.

Asteroseismic age estimate of the open cluster NGC 6866 using Kepler and Gaia

Context. Asteroseismic investigations of solar-like oscillations in giant stars allow for the derivation of their masses and radii. For members of open clusters, this allows us to obtain an age estimate of the cluster that is expected to be identical to the age estimate given by the colour-magnitude diagram, but independent of the uncertainties that are present for that type of analysis. Thus, a more precise and accurate age estimate can be obtained. Aims. We aim to identify and measure the asteroseismic properties of oscillating giant members of the open cluster NGC 6866 and utilise them for a cluster age estimate. Model comparisons also allow constraints to be placed on the stellar physics. Here, we investigate the efficiency of convective-core overshoot during the main sequence evolution, which has a significant influence on the age estimations for these relatively massive giants. The effects of rotation and core overshoot are similar, but not identical, and so, we also investigated the potential of our measurements to distinguish between these effects. Methods. We identified six giant members of NGC 6866 via photometry, proper motions, and parallaxes from Gaia, and spectroscopic literature measurements. These were combined with asteroseismic measurements, which we derived using photometric data from the Kepler mission for five of the stars. Comparisons to stellar-model isochrones constrained the convective-core overshoot and enable a more precise and accurate age estimate than previously possible. Results. A significant amount of differential reddening is found for NGC 6866. Asteroseismology establishes the helium-core burning evolutionary phase for the giants, which have a mean mass of 2.8 M⊙. Their radii are significantly smaller than predicted by current 1D stellar models unless the amount of convective-core overshoot on the main sequence is reduced to αov ≤ 0.1 ⋅ Hp in the step-overshoot description. Our measurements also suggest that rotation has affected the evolution of the stars in NGC 6866 in a way that is consistent with 3D simulations, but not with current 1D stellar models. The age of NGC 6866 is estimated to be 0.43 ± 0.05 Gyr, which is significantly younger and more precise than most previous estimates. Conclusions. We derive a precise cluster age while constraining convective-core overshooting and the effects of rotation in the stellar models. A comparison to age estimates from machine learning methods of the same and similar giant stars uncovers potential biases for automated asteroseismic and non-asteroseismic age estimates of helium-core burning stars.

Photometric determination of the mass accretion rates of pre-main-sequence stars

Aims. We study the properties of low-mass stars recently formed in the field of the NGC 376 cluster in the Small Magellanic Cloud (SMC). Methods. Using photometric observations acquired with the Hubble Space Telescope (HST) in the V, I, and Hα bands, we identify 244 candidate pre-main-sequence (PMS) stars showing Hα excess emission at the 5σ level and with Hα equivalent width of 20 Å or more. We derive physical parameters for all PMS stars, including masses, ages, and mass accretion rates. We compare the effective mass accretion rate of stars in NGC 376 to that of objects in the NGC 346 cluster, with NGC 346 featuring similar metallicity but higher total mass and gas density. Results. We find a median age of 28 Myr for this population (with 25th and 75th percentiles at about 20 and 40 Myr, respectively), in excellent agreement with previous studies of massive stars in the same field. The PMS stars are rather uniformly distributed across the field, whereas massive stars are more clustered. The spatial distribution of PMS objects is compatible with them having formed in the centre of the cluster and then migrating outwards. We find that in NGC 376 the mass accretion rate is systematically lower than in NGC 346 for stars of the same mass and age. This indicates that, in addition to metallicity, there are other environmental factors affecting the rate of mass accretion onto PMS stars. Our observations suggest that the gas density in the star-forming region might play a role.

Investigating kinematics and dynamics of three open clusters towards Galactic anticentre

ABSTRACT We present the intracluster kinematics and dynamics of three open clusters: NGC 1193, NGC 2355, and King 12 by incorporating kinematical and photometric data from Gaia DR3, as well as a ground-based telescope. After selecting cluster members based on proper motion data, clusters’ fundamental and structural parameters are investigated. We found the clusters at distances of 4.45, 1.97, and 3.34 kpc from the Sun in the direction of the Galactic anticentre. The luminosity function of the cluster NGC 1193 is flat, whereas it advances towards the fainter ends of the other two clusters. We observed a dip in the luminosity function of King 12. The mass function slopes for all three clusters differ from the solar neighbourhood reported by Salpeter, with NGC 1193 and NGC 2355 being flatter and King 12 having a higher value than the Salpeter value. The intra-cluster kinematics depict that stars in King 12 are moving outwards due to tidal forces from the Galactic disc, which we confirmed by plotting the cluster’s orbit in the Galaxy. Stars in NGC 2355 are moving with smaller relative velocities and have zero mean relative motion, which signifies that the cluster is neither contracting nor evaporating. The Galactic orbits of NGC 1193 suggest that it is orbiting farther from the Galactic disc, and so is less impacted by the Galactic tidal forces.

Investigation of open cluster NGC 7789 using GAIA EDR3 data

We present the membership determination of the NGC 7789 open cluster located at Cassiopeia. Based on the astrometric data from Gaia Early Data Release 3 (positions, parallaxes, and proper motions) are considered to derive cluster members of the open cluster. The Hierarchical Density-Based Spatial Clustering of Applications with Noise and Gaussian Mixture Model techniques in determining the frequency and degree of a star’s membership in a cluster were being utilized in the study. We calculate the parallax and distance of the NGC 7789 cluster center to be 0.1994 ± 0.11 mas and 18637 ± 333110 pc respectively. The cluster center is calculated to be α = 359° 16' 56.44'', δ = 56° 42''38.46 ' and proper motion to be µα = −1.386 ± 0.68 mas/yr and µδ = −1.052 ± 0.37 mas/yr. We also utilized the Automated Stellar Cluster Analysis tool and obtained: log(age) = ~9.20, distance modulus (m-M)v =0.6516, reddening E(B V)=0.26±0.28, and metallicity [Fe/H] = −0.26, respectively. The stars in NGC 7789 show very little movement on proper motion and might be caused by fainter observations.

The cosmic DANCe of Perseus

Context. Star-forming regions are excellent benchmarks for testing and validating theories of star formation and stellar evolution. The Perseus star-forming region, being one of the youngest (< 10 Myr), closest (280−320 pc), and most studied in the literature, is a fundamental benchmark. Aims. We aim to study the membership, phase-space structure, mass, and energy (kinetic plus potential) distribution of the Perseus star-forming region using public catalogues (Gaia, APOGEE, 2MASS, and Pan-STARRS). Methods. We used Bayesian methodologies that account for extinction to identify the Perseus physical groups in the phase-space, retrieve their candidate members, derive their properties (age, mass, 3D positions, 3D velocities, and energy), and attempt to reconstruct their origin. Results. We identify 1052 candidate members in seven physical groups (one of them new) with ages between 3 and 10 Myr, dynamical super-virial states, and large fractions of energetically unbounded stars. Their mass distributions are broadly compatible with that of Chabrier for masses ≳0.1 M⊙ and do not show hints of over-abundance of low-mass stars in NGC 1333 with respect to IC 348. These groups’ ages, spatial structure, and kinematics are compatible with at least three generations of stars. Future work is still needed to clarify if the formation of the youngest was triggered by the oldest. Conclusions. The exquisite Gaia data complemented with public archives and mined with comprehensive Bayesian methodologies allow us to identify 31% more members than previous studies, discover a new physical group (Gorgophone: 7 Myr, 191 members, and 145 M⊙), and confirm that the spatial, kinematic, and energy distributions of these groups support the hierarchical star formation scenario.

Self-Enrichment in Globular Clusters: The Crucial Role Played by Oxygen

Results from photometry and high-resolution spectroscopy have demonstrated the existence of multiple populations in globular clusters, where one or more stellar generations of stars coexist with the original population. We study the possibility that the new generation(s) of stars formed from the gas lost by intermediate mass (4–8 M⊙) stars during the asymptotic giant branch (AGB) evolution, possibly after dilution with residual pristine gas in the cluster. To this aim, we confront the chemistry of the AGB ejecta with the distribution of the chemical composition of stars in NGC 6402. We find that a satisfactory consistency between the observational evidence and the self-enrichment by AGBs hypothesis is reached if the mass-loss rates experienced for the latter stars is ∼10 times smaller than found for the solar metallicity counterparts of the same mass. We also comment on the importance of the knowledge of the oxygen abundance as a key indicator of the extent of the nucleosynthesis at which the gas from which the stars belonging to the second generation of the clusters formed, and the degree of dilution with pristine gas.

High-precision abundances of first-population stars in NGC 2808: confirmation of a metallicity spread

Photometric investigations have revealed that Galactic globular clusters (GCs) exhibit internal metallicity variations amongst the so-called first-population stars, which until now were considered to have a homogeneous initial chemical composition. This is not fully supported by the sparse spectroscopic evidence, which so far gives conflicting results. Here, we present a high-resolution re-analysis of five stars in the Galactic GC NGC 2808 taken from the literature. Target stars are bright red giants with nearly identical atmospheric parameters belonging to the first population according to their identification in the chromosome map of the cluster, and we measured precise differential abundances for Fe, Si, Ca, Ti, and Ni to the ∼0.03 dex level. Thanks to the very small uncertainties associated with the differential atmospheric parameters and abundance measurements, we find that target stars span a range of iron abundance equal to 0.25 ± 0.06 dex. The individual elemental abundances are highly correlated with the positions of the stars along the extended sequence described by first-population objects in the cluster chromosome map: bluer stars have a lower iron content. This agrees with inferences from the photometric analysis. The differential abundances of all other elements also show statistically significant ranges that point to intrinsic abundance spreads. The Si, Ca, Ti, and Ni variations are highly correlated with iron variations and the total abundance spreads for all elements are consistent within the error bars. This suggests a scenario in which short-lived massive stars exploding as supernovae contributed to the self-enrichment of the gas in the natal cloud while star formation was still ongoing.

Search for LBVs in the Local Volume galaxies: study of two stars in NGC 1156

ABSTRACT We continue the search for luminous blue variables (LBVs) in Local Volume galaxies in order to study their fundamental parameters. In this paper, we report the discovery of two new LBVs in the dwarf irregular galaxy NGC 1156. Both stars exhibit spectral variability simultaneously with strong brightness variations: ΔRc = 0.84 ± 0.23m for J025941.21+251412.2 and ΔRc = 2.59 ± 0.10m for J025941.54+251421.8. The bolometric luminosities of the stars are in the range of LBol ≈ (0.8–1.6) × 106 L⊙. These values are corrected for reddening AV ≈ 0.9 and are given for the distance to the galaxy D = 7.0 ± 0.4 Mpc, which we have measured by the tip of the red-giant branch method. Both stars are above the Humphreys–Davidson limit in the region of relatively low temperatures, Teff ≲ 10 kK, on the temperature–luminosity diagram. J025941.54+251421.8 had a temperature below the hydrogen ionization threshold at maximum brightness, exhibiting behaviour very similar to that of the known LBV R71 during its 2012 outburst. We have estimated the masses of the detected LBVs and studied the properties of their stellar environment. We discuss our results within the framework of both a single star and a binary system evolution scenario for LBVs.

Stellar wind properties of the nearly complete sample of O stars in the low metallicity young star cluster NGC 346 in the SMC galaxy

Context. Massive stars are among the main cosmic engines driving the evolution of star-forming galaxies. Their powerful ionising radiation and stellar winds inject a large amount of energy in the interstellar medium. Furthermore, mass-loss (Ṁ) through radiatively driven winds plays a key role in the evolution of massive stars. Even so, the wind mass-loss prescriptions used in stellar evolution models, population synthesis, and stellar feedback models often disagree with mass-loss rates empirically measured from the UV spectra of low metallicity massive stars. Aims. The most massive young star cluster in the low metallicity Small Magellanic Cloud galaxy is NGC 346. This cluster contains more than half of all O stars discovered in this galaxy so far. A similar age, metallicity (Z), and extinction, the O stars in the NGC 346 cluster are uniquely suited for a comparative study of stellar winds in O stars of different subtypes. We aim to use a sample of O stars within NGC 346 to study stellar winds at low metallicity. Methods. We mapped the central 1′ of NGC 346 with the long-slit UV observations performed by the Space Telescope Imaging Spectrograph (STIS) on board of the Hubble Space Telescope and complemented these new datasets with archival observations. Multi-epoch observations allowed for the detection of wind variability. The UV dataset was supplemented by optical spectroscopy and photometry. The resulting spectra were analysed using a non-local thermal equilibrium model atmosphere code (PoWR) to determine wind parameters and ionising fluxes. Results. The effective mapping technique allowed us to obtain a mosaic of almost the full extent of the cluster and resolve stars in its core. Among hundreds of extracted stellar spectra, 21 belong to O stars. Nine of them are classified as O stars for the first time. We analyse, in detail, the UV spectra of 19 O stars (with a further two needing to be analysed in a later paper due to the complexity of the wind lines as a result of multiplicity). This more than triples the number of O stars in the core of NGC 346 with constrained wind properties. We show that the most commonly used theoretical mass-loss recipes for O stars over-predict mass-loss rates. We find that the empirical scaling between mass-loss rates (Ṁ) and luminosity (L), Ṁ ∝ L2.4, is steeper than theoretically expected by the most commonly used recipes. In agreement with the most recent theoretical predictions, we find within Ṁ ∝ Zα that α is dependent upon L. Only the most luminous stars dominate the ionisation feedback, while the weak stellar winds of O stars in NGC 346 and the lack of previous supernova explosions in this cluster restrict the kinetic energy input.

A chemical analysis of seven red giants of the Galactic cluster NGC 4349

ABSTRACT In this study, we undertake a comprehensive investigation of seven red giant stars of NGC 4349 (#005, #009, #079, #127, #168, #174, and #203) and one non-member star (#053). Accurate abundances of 19 chemical species and 12C/13C isotopic ratio are determined using high-signal-to-noise FEROS spectra and standard spectroscopic analysis. Seven of the giant stars in our sample present a mean iron abundance of [Fe/H] = −0.16 ± 0.04, while the star NGC 4349−053 is classified as a non-member of the cluster, based on the membership analysis with the upmask code. Also, we estimate an age of 282 ± 32 Myr and E(B − V) = 0.40 ±0.02, after fitting an isochrone for the metallicity of NGC 4349 and with the photometric and astrometric data sets from the literature. Furthermore, we carry out for the first time a chemical composition study for star $\#079$, so far neglected in the literature. This red giant star is in a binary system and presents chemical abundances compatible with the open cluster. However, its parallax and proper-motion components suggest otherwise. Finally, we identify NGC 4349 as a young open cluster with slightly enriched s-process elements ([s/Fe] = +0.16 ± 0.05 dex). In the context of the so-called chemical clocks, NGC 4349 presents a concordance with some observed relationships for H-burning elements and α- and s-process elements.

Towards an understanding of YSO variability: a multiwavelength analysis of bursting, dipping, and symmetrically varying light curves of disc-bearing YSOs

ABSTRACT Using simultaneous optical and infrared light curves of disc-bearing young stars in NGC 2264, we perform the first multiwavelength structure function study of YSOs. We find that dippers have larger variability amplitudes than bursters and symmetric variables at all time-scales longer than a few hours. By analysing optical-infrared colour time-series, we also find that the variability in the bursters is systematically less chromatic at all time-scales than the other variability types. We propose a model of YSO variability in which symmetric, bursting, and dipping behaviour is observed in systems viewed at low, intermediate, and high inclinations, respectively. We argue that the relatively short thermal time-scale for the disc can explain the fact that the infrared light curves for bursters are more symmetric than their optical counterparts, as the disc reprocesses the light from all rotational phases. From this model, we find that the accretion variability on to these YSOs roughly follows a random-walk.

The in situ origin of the globular cluster NGC 6388 from abundances of Sc, V, and Zn of a large sample of stars

Chemical tagging of globular clusters (GCs) is often done using abundances of α-elements. The iron-peak elements Sc, V, and in particular Zn were proposed as an alternative to α-elements to tag accreted GCs in the metal-rich regime, where the dwarf galaxy Sagittarius and its GCs show peculiarly marked under-abundances of these heavier species with respect to Milky Way stars. A handful of stars in NGC 6388 was used to suggest an accreted origin for this GC, contradicting the results from dynamics. We tested the efficiency of the iron-peak method by using large samples of stars in NGC 6388, compared to thousands of field stars in the disc and the bulge of the Milky Way. Our abundance ratios of Sc (185 stars) and V (35 stars) for NGC 6388 are within about 1.5σ from the average for the field stars with a similar metallicity, and they are in perfect agreement for Zn (31 stars), claimed to be the most sensitive element concerning the accretion pattern. Moreover, the chemo-dynamical plots, coupled to the bifurcated age-metallicity relation of GCs in the Galaxy, clearly rule out any association of NGC 6388 to the groups of accreted GCs. Using a large set of GC abundances from the literature, we also show that the new method with Sc, V, and Zn seems to be efficient in picking up GCs related to the Sagittarius dwarf galaxy. Whether this is also generally true for accreted GCs seems to be less evident, and it should be verified with larger and homogeneous samples of stars both in the field and in GCs.