Identified Cluster Members Research Articles

Constraining Free–Free Emission and Photoevaporative Mass-loss Rates for Known Proplyds and New VLA–identified Candidate Proplyds in NGC 1977

We present NSF's Karl G. Jansky Very Large Array observations covering the NGC 1977 region at 3.0, 6.4, and 15.0 GHz. We search for compact radio sources and detect continuum emission from 34 NGC 1977 cluster members and 37 background objects. Of the 34 radio-detected cluster members, 3 are associated with known proplyds in NGC 1977, 22 are associated with additional young stellar objects in NGC 1977, and 9 are newly identified cluster members. We examine the radio spectral energy distributions, circular polarization, and variability of the detected NGC 1977 sources and identify 10 new candidate proplyds whose radio fluxes are dominated by optically thin free–free emission. We use measurements of free–free emission to calculate the mass-loss rates of known proplyds and new candidate proplyds in NGC 1977, and find values ∼10−9 to 10−8 M ⊙ yr−1, which are lower than the mass-loss rates measured toward proplyds in the Orion Nebula Cluster but consistent with the mass-loss rates predicted by external photoevaporation models for spatially extended disks that are irradiated by the typical external ultraviolet (UV) fields encountered in NGC 1977. Finally, we show that photoevaporative disk winds in NGC 1977 may be illuminated by internal or external sources of ionization, depending on their positions within the cluster. This study provides new constraints on disk properties in a clustered star-forming region with a weaker UV environment than the Orion Nebula Cluster but a stronger UV environment than low-mass star-forming regions like Taurus. Such intermediate UV environments represent the typical conditions of Galactic star and planet formation.

EXPLORING OPEN STAR CLUSTER MEMBERSHIPS WITH N-BODY SIMULATIONS AND MACHINE LEARNING

This work explores the application of supervised machine learning algorithms on N-body simulations to analyze the membership of open star clusters. The simulations used in this study are based on the Plummer model, clusters formed with constant star-formation efficiency (SFE) per free-fall time. We use simulations with different SFE and initial random realization. The random forest model was trained using simulations based on a 15% SFE over a time period of 20-100 million years. Subsequently, the model was tested on other N-body simulations with SFEs ranging from 17% to 25%, demonstrating consistently high classification accuracy throughout the dynamic evolution of the tested simulations. The model successfully identified cluster members with minimal deviations despite variations in SFE. Additionally, the algorithm maintained robustness against noise and initial conditions. Most of the errors observed in the model were false positives (FP), often located within a 2 Jacobi radius, suggesting gravitational binding to the cluster's center. This framework and learning strategy exhibit effectiveness and hold promise for further application in analyzing mock observations obtained from N-body simulations. Future work will focus on extending this method to more realistic observational scenarios.

Dynamical analysis of clusters of galaxies from cosmological simulations

Context. Studies of cluster mass and velocity anisotropy profiles are useful tests of dark matter models and of the assembly history of clusters of galaxies. These studies might be affected by unknown systematics caused by projection effects. Aims. We aim to test observational methods for the determination of mass and velocity anisotropy profiles of clusters of galaxies. Particularly, we focus on the MAMPOSSt technique Methods. We used results from two semi-analytic models of galaxy formation, coupled with high-resolution N-body cosmological simulations, the DLB07 catalog, and the FIRE catalog based on the new GAlaxy Evolution and Assembly model. We tested the reliability of the Jeans equation in recovering the true mass profile when full projected phase-space information is available. We examined the reliability of the MAMPOSSt method in estimating the true mass and velocity anisotropy profiles of the simulated halos when only projected phase-space information is available, as in observations. Results. The spherical Jeans equation provides a reliable tool to determine cluster mass profiles, both when considering the whole population of cluster galaxies, and when considering subsamples of tracers separated by galaxy color; the exception to this is for the central region, where deviations may be attributed to dynamical friction effects or galaxy mergers. The results are equally good for prolate and oblate clusters. Using only projected phase-space information, MAMPOSSt provides estimates of the mass profile with a standard deviation of 35–69% and a negative bias of 7–17%, nearly independent of radius, which we attribute to the presence of interlopers in the projected samples. The bias changes sign; that is, the mass is over-estimated, for prolate clusters with their major axis aligned along the line of sight. The MAMPOSSt method measures the velocity anisotropy profiles accurately in the inner cluster regions and there is a slight overestimate in the outer regions for the whole sample of observationally identified cluster members, and, separately, for red and blue galaxies.

The GOGREEN survey: Internal dynamics of clusters of galaxies at redshift 0.9–1.4

Context.The study of galaxy cluster mass profiles (M(r)) provides constraints on the nature of dark matter and on physical processes affecting the mass distribution. The study of galaxy cluster velocity anisotropy profiles (β(r)) informs the orbits of galaxies in clusters, which are related to their evolution. The combination of mass profiles and velocity anisotropy profiles allows us to determine the pseudo phase-space density profiles (Q(r)); numerical simulations predict that these profiles follow a simple power law in cluster-centric distance.Aims.We determine the mass, velocity anisotropy, and pseudo phase-space density profiles of clusters of galaxies at the highest redshifts investigated in detail to date.Methods.We exploited the combination of the GOGREEN and GCLASS spectroscopic data-sets for 14 clusters with massM200 ≥ 1014 M⊙at redshifts 0.9 ≤ z ≤ 1.4. We constructed anensemblecluster by stacking 581 spectroscopically identified cluster members with stellar massM⋆ ≥ 109.5 M⊙. We used the MAMPOSSt method to constrain severalM(r) andβ(r) models, and we then inverted the Jeans equation to determine theensembleclusterβ(r) in a non-parametric way. Finally, we combined the results of theM(r) andβ(r) analysis to determineQ(r) for theensemblecluster.Results.The concentrationc200of theensemblecluster mass profile is in excellent agreement with predictions from Λ cold dark matter (ΛCDM) cosmological numerical simulations, and with previous determinations for clusters of similar mass and at similar redshifts, obtained from gravitational lensing and X-ray data. We see no significant difference between the total mass density and either the galaxy number density distributions or the stellar mass distribution. Star-forming galaxies are spatially significantly less concentrated than quiescent galaxies. The orbits of cluster galaxies are isotropic near the center and more radial outside. Star-forming galaxies and galaxies of low stellar mass tend to move on more radially elongated orbits than quiescent galaxies and galaxies of high stellar mass. The profileQ(r), determined using either the total mass or the number density profile, is very close to the power-law behavior predicted by numerical simulations.Conclusions.The internal dynamics of clusters at the highest redshift probed in detail to date are very similar to those of lower-redshift clusters, and in excellent agreement with predictions of numerical simulations. The clusters in our sample have already reached a high degree of dynamical relaxation.

Exploring open cluster properties with Gaia and LAMOST

Context. In Gaia DR2, an unprecedented high level of precision has been reached at sub-milliarcsecond for astrometry and millimagnitudes for photometry. Using cluster members identified with the astrometry and photometry in Gaia DR2, we can obtain a reliable determination of cluster properties. However, because of the shortcomings of Gaia spectroscopic observations in dealing with densely crowded cluster regions, the RVs and metallicity values for cluster member stars from Gaia DR2 are still lacking. It is necessary to combine the Gaia data with the data from large spectroscopic surveys, such as LAMOST, APOGEE, GALAH, and Gaia-ESO. Aims. In this study our aim is to improve the cluster properties by combining the LAMOST spectra. In particular, we provide the list of cluster members with spectroscopic parameters as an add-value catalog in LAMOST DR5, which can be used to perform a detailed study for a better understanding of the stellar properties, by using their spectra and fundamental properties from the host cluster. Methods. We cross-matched the spectroscopic catalog in LAMOST DR5 with the identified cluster members in Cantat-Gaudin et al. (2018, A&A, 618, A93). We then used members with spectroscopic parameters to derive statistical properties of open clusters. Results. We obtained a list of 8811 members with spectroscopic parameters and a catalog of 295 cluster properties. The provided cluster properties include astrometric parameters, spectroscopic parameters, derived kinematic and orbital parameters, and isochrone fitting results. In addition, we study the radial and vertical metallicity gradient and age-metallicity relation with the compiled open clusters as tracers, finding slopes of −0.053 ± 0.004 dex kpc−1, −0.252 ± 0.039 dex kpc−1, and 0.022 ± 0.008 dex Gyr−1, respectively. The slopes of the metallicity distribution relation for young clusters (0.1 Gyr < Age < 2 Gyr) and the age-metallicity relation for clusters within 6 Gyr are both consistent with the literature results. In order to fully study the chemical evolution history in the disk, more spectroscopic observations for old and distant open clusters are needed for further investigation.

Near-infrared Variability of Low-mass Stars in IC 1396A and Tr 37

Abstract We have monitored nearly a square degree in IC 1396A/Tr 37 over 21 epochs extending over 2014–2016 for sources variable in the JHK bands. In our data, 65% ± 8% of previously identified cluster members show variations, compared with ≲0.3% of field stars. We identify 119 members of Tr 37 on the basis of variability, forming an unbiased sample down to the brown dwarf regime. The K-band luminosity function in Tr 37 is similar to that of IC 348 but shifted to somewhat brighter values, implying that the K- and M-type members of Tr 37 are younger than those in IC 348. We introduce methods to classify the causes of variability, based on behavior in the color–color and color–magnitude diagrams. Accretion hot spots cause larger variations at J than at K with substantial scatter in the diagrams; there are at least a dozen, with the most active resembling EXors. Eleven sources are probably dominated by intervention of dust clumps in their circumstellar disks, with color behavior indicating the presence of grains larger than for interstellar dust, presumably due to grain growth in their disks. Thirteen sources have larger variations at K than at J or H. For 11 of them, the temperature fitted to the variable component is very close to 2000 K, suggesting that the changes in output are caused by turbulence at the inner rim of the circumstellar disk exposing previously protected populations of grains.

The Arches cluster revisited

The Arches is one of the youngest, densest and most massive clusters in the Galaxy. As such it provides a unique insight into the lifecycle of the most massive stars known and the formation and survival of such stellar aggregates in the extreme conditions of the Galactic Centre. In a previous study we presented an initial stellar census for the Arches and in this work we expand upon this, providing new and revised classifications for ∼30% of the 105 spectroscopically identified cluster members as well as distinguishing potential massive runaways. The results of this survey emphasise the homogeneity and co-evality of the Arches and confirm the absence of H-free Wolf-Rayets of WC sub-type and predicted luminosities. The increased depth of our complete dataset also provides significantly better constraints on the main sequence population; with the identification of O9.5 V stars for the first time we now spectroscopically sample stars with initial masses ranging from ∼16 M⊙ to ≥120 M⊙. Indeed, following from our expanded stellar census we might expect ≳50 stars within the Arches to have been born with masses ≳60 M⊙, while all 105 spectroscopically confirmed cluster members are massive enough to leave relativistic remnants upon their demise. Moreover the well defined observational properties of the main sequence cohort will be critical to the construction of an extinction law appropriate for the Galactic Centre and consequently the quantitative analysis of the Arches population and subsequent determination of the cluster initial mass function.

A Comprehensive Study of 94 Open Clusters Based on the Data from IPHAS, GAIA DR2, and Other Sky Surveys

We determine the color excesses, photometric distances, ages, astrometric parallaxes and proper motions for 94 open clusters in the northern part of the Milky Way. We estimate the color excesses and photometric distances based on the data from IPHAS photometric survey of the northern Galactic plane using individual total-to-selective extinction ratios R_r=A_r/E_(r-i) for each cluster computed via the color-difference method based on IPHAS r, i, and H_alpha-band, 2MASS J, H, and K_s-band, WISE W1-band, and Pan-STARRS i, z, and y-band data. The inferred R_r values vary significantly from cluster to cluster spanning the R_r=3.1--5.2 interval with a mean and standard deviation equal to <R_r>=3.99 and sigma R_r=0.34, respectively. We identified cluster members using (1) absolute proper motions determined from individual-epoch positions of stars retrieved from IPHAS, 2MASS, URAT1, ALLWISE, UCAC5, and Gaia DR1 catalogs and positions of stars on individual Palomar Sky Survey plates reconstructed based on the data provided in USNO-B1.0 catalog and (2) absolute proper motions provided in Gaia DR2 catalog, and computed the average Gaia DR2 trigonometric parallaxes and proper motions of the clusters. The mean formal error of the inferred astrometric parallaxes of clusters is of about 7microarcseconds, however, a comparison of astrometric and photometric parallaxes of our cluster sample implies that Gaia DR2 parallaxes are, on the average, systematically underestimated by 45 +/-9 microarcseconds. This result agrees with estimates obtained by other authors using other objects. At the same time, we find our photometric distance scale to be correct within the quoted errors (the inferred correction factor is equal to unity to within a standard error of 0.025).

Protoplanetary Disk Properties in the Orion Nebula Cluster: Initial Results from Deep, High-resolution ALMA Observations

Abstract We present Atacama Large Millimeter Array 850 μm continuum observations of the Orion Nebula Cluster that provide the highest angular resolution (∼0.″1 ≈ 40 au) and deepest sensitivity (∼0.1 mJy) of the region to date. We mosaicked a field containing ∼225 optical or near-IR-identified young stars, ∼60 of which are also optically identified “proplyds.” We detect continuum emission at 850 μm toward ∼80% of the proplyd sample, and ∼50% of the larger sample of previously identified cluster members. Detected objects have fluxes of ∼0.5–80 mJy. We remove submillimeter flux due to free–free emission in some objects, leaving a sample of sources detected in dust emission. Under standard assumptions of isothermal, optically thin disks, submillimeter fluxes correspond to dust masses of ∼0.5–80 Earth masses. We measure the distribution of disk sizes, and find that disks in this region are particularly compact. Such compact disks are likely to be significantly optically thick. The distributions of submillimeter flux and inferred disk size indicate smaller, lower-flux disks than in lower-density star-forming regions of similar age. Measured disk flux is correlated weakly with stellar mass, contrary to studies in other star-forming regions that found steeper correlations. We find a correlation between disk flux and distance from the massive star θ 1 Ori C, suggesting that disk properties in this region are influenced strongly by the rich cluster environment.

Abell 315: reconciling cluster mass estimates from kinematics, X-ray, and lensing

Determination of cluster masses is a fundamental tool for cosmology. Comparing mass estimates obtained by different probes allows to understand possible systematic uncertainties. The cluster Abell 315 is an interesting test case, since it has been claimed to be underluminous in X-ray for its mass (determined via kinematics and weak lensing). We have undertaken new spectroscopic observations with the aim of improving the cluster mass estimate, using the distribution of galaxies in projected phase space. We identified cluster members in our new spectroscopic sample. We estimated the cluster mass from the projected phase-space distribution of cluster members using the MAMPOSSt method. In doing this estimate we took into account the presence of substructures that we were able to identify. We identify several cluster substructures. The main two have an overlapping spatial distribution, suggesting a (past or ongoing) collision along the line-of-sight. After accounting for the presence of substructures, the mass estimate of Abell 315 from kinematics is reduced by a factor 4, down to M200=0.8 (-0.4,+0.6) x 10^14 Msun. We also find evidence that the cluster mass concentration is unusually low, c200=r200/r-2 <~ 1. Using our new estimate of c200 we revise the weak lensing mass estimate down to M200=1.8 (-0.9,+1.7) x 10^14 Msun. Our new mass estimates are in agreement with that derived from the cluster X-ray luminosity via a scaling relation, M200=0.9+-0.2 x 10^14 Msun. Abell 315 no longer belongs to the class of X-ray underluminous clusters. Its mass estimate was inflated by the presence of an undetected subcluster in collision with the main cluster. Whether the presence of undetected line-of-sight structures can be a general explanation for all X-ray underluminous clusters remains to be explored using a statistically significant sample.

RELATIVE PROPER MOTIONS IN THE RHO OPHIUCHI CLUSTER

Abstract : Near-infrared images optimized for astrometry have been obtained for four fields in the high-density L 1688 cloud core over a 12 year period. The targeted regions include deeply embedded young stellar objects (YSOs) and very low luminosity objects too faint and/or heavily veiled for spectroscopy. Relative proper motions in R.A. and decl. were computed for 111 sources and again for a subset of 65 YSOs, resulting in a mean proper motion of (0,0) for each field. Assuming each field has the same mean proper motion, YSOs in the four fields were combined to yield estimates of the velocity dispersions in R.A. and decl. that are consistent with 1.0 km s1. These values appear to be independent of the evolutionary state of the YSOs. The observed velocity dispersions are consistent with the dispersion in radial velocity derived for optically visible YSOs at the periphery of the cloud core and are consistent with virial equilibrium. The higher velocity dispersion of the YSOs in the plane of the sky relative to that of dense cores may be a consequence of stellar encounters due to dense cores and filaments fragmenting to form small groups of stars or the global collapse of the L 1688 cloud core. An analysis of the differential magnitudes of objects over the 12 year baseline has not only confirmed the near-infrared variability for 29 YSOs established by prior studies, but has also identified 18 new variability candidates. Four of these have not been previously identified as YSOs and may be newly identified cluster members.

The progenitors of magnetic white dwarfs in open clusters

According to the fossil-field hypothesis magnetic fields are remnants of the previous stages of evolution. However, population synthesis calculations are unable to reproduce the magnetic white dwarf (MWD) sample without binary interaction or inclusion of a population of progenitor with unobservable small-scale fields. One necessary ingredient in population synthesis is the initial-to-final-mass relation (IFMR) which describes the mass-loss processes during the stellar evolution. When white dwarfs are members of open clusters, their evolutionary histories can be assessed through the use of cluster properties. In this work, we assess the cluster membership by correlating the proper-motion of MWDs with the cluster proper-motion and by analyzing the candidates spectroscopically with our magnetic model spectra in order to estimate the effective temperature and radii. We identified SDSS J085523.87+164059.0 to be a proper-motion member of Praesepe. We also included the data of the formerly identified cluster members NGC 6819-8, WD 0836+201 and estimated the mass, cooling age and the progenitor masses of the three probable MWD members of open clusters. According to our analysis, the newly identified cluster member SDSS J085523.87+164059.0 is an ultra-massive MWD of mass 1.12 $\pm$ 0.11 Msolar. We increase the sample of MWDs with known progenitor masses to ten, with the rest of the data coming from the common proper motion binaries. Our investigations show that, when effects of the magnetic fields are included in the diagnostics, the estimated properties of these cluster MWDs do not show evidence for deviations from the IFMR. Furthermore, we estimate the precision of the magnetic diagnostics which would be necessary to determine quantitatively whether magnetism has any effect on the mass-loss.

FLAMINGOS NEAR INFRARED SURVEY OF THE SERPENS CLOUD MAIN CORE

We obtained JHK images and low-resolution JH spectra in the embedded young cluster in the Serpens cloud Main core (also known as Serpens North). We determined spectral types for 15 previously identified cluster members (for 5 of them for the first time), 1 new candidate, and 11 stars that appear to be field interlopers. Extinction, for which we derived an analytical expression, was obtained by taking SpT and near-IR excess into account. The location on the Hertzsprung-Russell diagram indicates that we probed a low-mass population of the cloud (0.05 - 1.5 Msol), including 1 - 3 brown dwarfs. We used our individually determined photospheric parameters to analyze the ISO and the Spitzer determined spectral energy distribution classes. The latter were correlated with the age and location of the sources in the cloud. We find that most flat objects from our study (4 out of 5) have SEDs consistent with reddened classical T Tau stars; however, when comparing to the thick disk SEDs of lower mass M-type objects, we find that the flat ones show more excess, perhaps indicating an earlier evolutionary stage. We determined a median age for the cluster to be 1 Myr for distance of 380 pc, and 3 Myr for a less likely distance of 260 pc. The core of the cluster is on average younger than the rest of the cluster. We do not find objects with disks past 5 Myr. We do find diskless, X-ray bright objects younger than 1 Myr, as was also noted in the study of Winston et al (2009). We find two groups of young objects associated with dark filaments, indicating that star formation was not always confined to the core.

Cluster radius and sampling radius in the determination of cluster membership probabilities

We analyze the dependence of the membership probabilities obtained from kinematical variables on the radius of the field of view around open clusters (the sampling radius, Rs). From simulated data, we show that the best discrimination between cluster members and non-members is obtained when the sampling radius is very close to the cluster radius. At higher Rs values more field stars tend to be erroneously assigned as cluster members. From real data of two open clusters (NGC 2323 and NGC 2311) we obtain that the number of identified cluster members always increases with increasing Rs. However, there is a threshold Rs value above which the identified cluster members are severely contaminated by field stars and the effectiveness of membership determination is relatively small. This optimal sampling radius is \sim 14 arcmin for NGC 2323 and \sim 13 arcmin for NGC 2311. We discuss the reasons for such behavior and the relationship between cluster radius and optimal sampling radius. We suggest that, independently of the method used to estimate membership probabilities, several tests using different sampling radius should be performed in order to evaluate the existence of possible biases.

IFU observations of the core of Abell 2218

We present a study of the morphologies and stellar content of the galaxies in the central ∼ 74”×64” region of the galaxy cluster Abell 2218 (see Fig. 1). The spectroscopic data were obtained with the integral field unit PMAS (Roth et al. 2005) in the PPAK mode (Verheijen et al. 2004; Kelz et al. 2006), at the 3.5 m telescope of the Calar Alto Observatory. The covered wavelength range was 4650–8000 Å, with a spectral resolution FWHM∼ 10Å. These data were combined with with deep HST/ACS F475W, F555W, F625W and F850LP imaging, and additional data from the literature.In Fig. 2 we show the rest-frame B-V vs. MV colour-magnitude diagram for the identified cluster members. The data are segregated according to an objective morphological classification based on the Sérsic index. Contrary to previous results (e.g. Ziegler et al. 2001; Smail et al. 2001) there is a clear indication that the cluster population is not dominated by early-type galaxies, and that there exists an almost parity between early- and late-type objects. In addition, the late-type galaxies spread over a wider range of colours and luminosities with respect to the early-types. This indicates that early-type objects are more massive and have older stellar populations, while late-type galaxies are less massive and exhibit a wider range of stellar population properties.The results agree with the two-steps scenario for the evolution of galaxies in clusters (e.g. Poggianti 2003), which explains the co-existence of a primordial population of early-type galaxies formed at early epochs, with an additional population of late-type galaxies that have been captured by the cluster, infalling, suffering a short enhancement of the star formation that is later quenched by the interaction with the environment, and that afterwards evolved passively, becoming redder and fainter. Finally, galaxy-galaxy interactions and dry merging processes were responsible for the building up of new massive spheroidal galaxies.Full details are given in Sánchez et al. (2007).

A fossil galaxy cluster: an X-ray and optical study of RX J1416.4+2315

We present a detailed X-ray and optical study of a distant fossil system RX J1416.4+2315 (z=0.13), combining Chandra and XMM-Newton observations, optical photometry and spectroscopy. X-ray emitting hot gas imaged by both the Chandra and XMM-Newton shows a globally relaxed spatial distribution, supporting the idea that fossil groups are old galaxy systems with no recent mergers. However, the diffuse X-ray emission shows signs of asymmetries in the core of the system. With a mean gas temperature of ~ 4.0 keV and total gravitational mass of 3.1 x 10^14 solar mass, within the virial radius, this is better described as a fossil galaxy cluster rather than a fossil group. The temperature profile shows no sign of a significant cooler core despite a cooling time dropping to 5 Gyr within the resolved core. We find a mass concentration parameter c_200 ~ 11 which is relatively high for a cluster of this mass, indicative of an early formation epoch. Using the spectroscopically identified cluster members we present the galaxy luminosity function for this fossil system. We measure the velocity dispersion of the galaxies to be ~ 700 km/s based on 18 confirmed members. The dynamical mass is nearly twice the total gravitational mass derived from the X-ray analysis. The measured R-band mass-to-light ratio, within the virial radius, is ~ 440 M/L (solar) which is not unusual for clusters of galaxies. The central giant elliptical galaxy has discy isophotes and spectral features typical of elliptical galaxies.

Properties of X-ray Flares on Young Stars

AbstractThe interest in the giant flares found on young stars results from the fact that their energetics is vastly different from solar flares. The recent ROSAT observations of young clusters yield a good basis for a systematic study of these flares.We have investigated 18 very deep pointed observations of star forming regions in IC348, Chamaeleon, ρ Ophiuchi, and Orion. To study the age dependence of the flare properties we also investigated the observations of the young clusters IC2391, Pleiades, and Hyades. In this way we have evaluated the X-ray emission of several hundred young stars with ages ranging from about 106 years to 7 ń 108 years. We have found 36 large flares on identified cluster members and have determined the flare properties with a simple flare model.The flares show the same relation between the total X-ray flare energy and the quiescent X-ray luminosity as the dM star flares. The total X-ray flare energies decrease smoothly from the very high values found for very young stars to solar values with increasing cluster age. Our results indicate that the large flares on young stars are similar to scaled-up solar-like activity rather than to accretion processes.