A New Nearby Candidate Star Cluster in Ophiuchus atd ≃ 170 pc

We present a novel view on the morphology and dynamical state of ten prominent, nearby (≤500 pc), and young (∼30–300 Myr) open star clusters withGaiaDR2:α Per, Blanco 1, IC 2602, IC 2391, Messier 39, NGC 2451A, NGC 2516, NGC 2547, Platais 9, and the Pleiades. We introduce a pioneering member-identification method that is informed by cluster bulk velocities and deconvolves the spatial distribution with a mixture of Gaussians. Our approach enables inferring the true spatial distribution of the clusters by effectively filtering field star contaminants while at the same time mitigating the effect of positional errors along the line of sight. This first application of the method reveals vast stellar coronae that extend for ≳100 pc and surround the cluster cores, which are comparatively tiny and compact. The coronae and cores form intertwined, coeval, and comoving extended cluster populations, each encompassing tens of thousands of cubic parsec and stretching across tens of degrees on the sky. Our analysis shows that the coronae are gravitationally unbound but largely comprise the bulk of the stellar mass of the populations. Most systems are in a highly dynamic state, showing evidence of expansion and sometimes simultaneous contraction along different spatial axes. The velocity field of the extended populations for the cluster cores appears asymmetric but is aligned along a spatial axis unique to each cluster. The overall spatial distribution and the kinematic signature of the populations are largely consistent with the differential rotation pattern of the Milky Way. This finding underlines the important role of global Galactic dynamics in the fate of stellar systems. Our results highlight the complexity of the Milky Way’s open cluster population and call for a new perspective on the characterization and dynamical state of open clusters.

We investigated the prospects for systematic searches of white dwarfs at low Galactic latitudes, using the VLT Survey Telescope (VST) H$\alpha$ Photometric Survey of the Galactic plane and Bulge (VPHAS+). We targeted 17 white dwarf candidates along sightlines of known open clusters, aiming to identify potential cluster members. We confirmed all the 17 white dwarf candidates from blue/optical spectroscopy, and we suggest five of them to be likely cluster members. We estimated progenitor ages and masses for the candidate cluster members, and compared our findings to those for other cluster white dwarfs. A white dwarf in NGC 3532 is the most massive known cluster member (1.13 M$_{\odot}$), likely with an oxygen-neon core, for which we estimate an $8.8_{-4.3}^{+1.2}$ M$_{\odot}$ progenitor, close to the mass-divide between white dwarf and neutron star progenitors. A cluster member in Ruprecht 131 is a magnetic white dwarf, whose progenitor mass exceeded 2-3 M$_{\odot}$. We stress that wider searches, and improved cluster distances and ages derived from data of the ESA Gaia mission, will advance the understanding of the mass-loss processes for low- to intermediate-mass stars.

The Galactic open clusters, in particular the oldest members, serve as excellent probes of the structure and evolution of the Galactic disk. Individual clusters provide excellent tests of stellar and dynamical evolution. Cluster spatial and age distributions provide insight into the processes of cluster formation and destruction that have allowed substantial numbers of old open clusters to survive. Spectroscopic and photometric data for the old clusters yield kinematic, abundance, and age information that clarifies the relationship between the old open cluster population and other Galactic populations. New samples of old open clusters, which span a large range in distance and age, are used to define disk abundance gradients and the cluster age-metallicity relationship, and they point to a complex history of chemical enrichment and mixing in the disk.

How Gaia sheds light on the Milky Way star cluster population

Based on our compiled catalogue of fundamental astrophysical parameters for 593 open clusters, we analyze the relations between the chemical composition, spatial positions, Galactic orbital elements, age, and other physical parameters of open star clusters. We show that the population of open clusters is heterogeneous and is divided into two groups differing by their mean parameters, properties, and origin. One group includes the Galactic clusters formed mainly from the interstellar matter of the thin disk with nearly solar metallicities ([Fe/H] > -0.2) and having almost circular orbits a short distance away from the Galactic plane, i.e., typical of the field stars of the Galactic thin disk. The second group includes the peculiar clusters formed through the interaction of extragalactic objects (such as high--velocity clouds, globular clusters, or dwarf galaxies) with the interstellar matter of the thin disk, which, as a result, derived abnormally low (for field thin-disk stars) metallicities and/or Galactic orbits typical of objects of the older Galactic subsystems.

We study the evolution and observability of young, compact star clusters near the Galactic center, such as the Arches and Quintuplet systems. The clusters are modeled by integrating the equations of motion of all stars while accounting for the internal evolution of stars and binaries, as well as the effect of the Galactic tidal field. We find that clusters within 150 pc of the Galactic center dissolve within ~55 Myr, but their projected densities drop below the background density in the direction of the Galactic center within only a few megayears, effectively making these clusters undetectable after that time. Detailed observations of the Arches cluster, taken at face value, suggest that its mass function is unusually flat and that the cluster contains an overabundance of stars more massive than 20 M☉. Our dynamical analysis, however, shows that the observed characteristics of the Arches cluster are consistent with a perfectly normal initial mass function. The observed anomalies are then caused by a combination of observational selection effects and the dynamical evolution of the cluster. We calibrate the current parameters of the Arches cluster using a normal initial mass function and conclude that the cluster is more massive than 40,000 M☉, has a half-mass radius of about 0.35 pc, and is located between 50 and 90 pc from the Galactic center.

Technology is now allowing for the investigation of star clusters outside of the Milky Way. As attention turns to the extragalactic star clusters, a perception that the system of star clusters in the Milky Way is well understood may grow, resulting in the neglect of these important objects. In this review, the status of our understanding of the Milky Way's open star cluster population will be discussed. Specifically, I will attempt to illustrate not only the important information that can and must be learned from these nearby star clusters, but also the degree to which our understanding of the Galactic open clusters remains incomplete.

Context. Open clusters (OCs) are popular tracers of the structure and evolutionary history of the Galactic disc. The OC population is often considered to be complete within 1.8 kpc of the Sun. The recent Gaia Data Release 2 (DR2) allows the latter claim to be challenged. Aims. We perform a systematic search for new OCs in the direction of Perseus using precise and accurate astrometry from Gaia DR2. Methods. We implemented a coarse-to-fine search method. First, we exploited spatial proximity using a fast density-aware partitioning of the sky via a k-d tree in the spatial domain of Galactic coordinates, (l, b). Secondly, we employed a Gaussian mixture model in the proper motion space to tag fields quickly around OC candidates. Thirdly, we applied an unsupervised membership assignment method, UPMASK, to scrutinise the candidates. We visually inspected colour-magnitude diagrams to validate the detected objects. Finally, we performed a diagnostic to quantify the significance of each identified over-density in proper motion and in parallax space. Results. We report the discovery of 41 new stellar clusters. This represents an increment of at least 20% of the previously known OC population in this volume of the Milky Way. We also report on the clear identification of NGC 886, an object previously considered an asterism. This study challenges the previous claim of a near-complete sample of OCs up to 1.8 kpc. Our results reveal that this claim requires revision, and a complete census of nearby OCs is yet to be found.

Far-infrared and radio continuum maps have been made of the central 6' of the inner-galaxy HII regions G30.8-0.0 (in the W43 complex) and G25.4-0.2, along with radio and molecular line measurements at selected positions. The purpose of this study is an effort to understand star formation in the molecular ring at 5 kpc in galactic radius. Measurements at several far infrared wavelengths allow the dust temperature structures and total far infrared fluxes to be determined. Comparison of the radio and infrared maps shows a close relationship between the ionized gas and the infrared-emitting material. There is evidence that parts of G30.8 are substantially affected by extinction, even at far-infrared wavelengths. Using radio recombination line and CO line data for G25.4-0.2, the distance ambiguity for this source is resolved. The large distance previously ascribed to the entire complex is found to apply to only one of the two main components. The confusion in distance determination is found to result from an extraordinary near-superposition of two bright HII regions. Using the revised distances of 4.3 kpc for G25.4SE and 12 kpc for G25.4NW, it is found that the latter, which is apparently the fainter of the two sources, is actually the more luminous. The ratio of total luminosity to ionizing luminosity is very similar to that of HII regions in the solar circle. Assuming a coeval population of ionizing stars, a normal initial mass function is indicated.

The study of the cosmic near-infrared background (CIB) light after subtraction of resolved sources can push the limits of current observations and yield information on galaxies and quasars in the early universe. Spatial fluctuations of the CIB exhibit a clustering excess at angular scales ∼1° whose origin has not been conclusively identified, but disentangling the relative contribution from low- and high-redshift sources is not trivial. We explore the likelihood that this signal is dominated by emission from galaxies and accreting black holes (BHs) in the early Universe. We find that, the measured fluctuation signal is too large to be produced by galaxies at redshifts z > 8, which only contribute ∼0.01–0.05 nW m−2 sr−1 to the CIB. Additionally, if the first small mass galaxies have a normal initial mass function, the light of their ageing stars (fossils) integrated over cosmic time contributes a comparable amount to the CIB as their pre-reionization progenitors. In order to produce the observed level of CIB fluctuation without violating constraints from galaxy counts and the electron optical depth of the IGM, minihaloes at z > 12 must form preferably top-heavy stars with efficiency f* ≳ 0.1 and at the same time maintain a very low escape fraction of ionizing radiation, fesc < 0.1 per cent. If instead the CIB fluctuations are produced by high-z BHs, one requires vigorous accretion in the early universe reaching ρacc ≳ 105 M⊙ Mpc−3 by z ≃ 10. This growth must stop by z ∼ 6 and be significantly obscured not to overproduce the soft cosmic X-ray background and its observed coherence with the CIB. We therefore find the range of suitable high-z explanations to be narrow, but could possibly be widened by including additional physics and evolution at those epochs.

We have obtained radial gradients in the spectral features of the λ4000-Å break (D4000) and Mg2 for a sample of 11 central cluster galaxies (CCGs): eight in clusters with cooling flows and three in clusters without. After careful removal of the emission lines found within the D4000 and Mg2 bandpasses for some objects, the new data strongly confirm the correlations between line-strength indices and the cooling flow phenomenon found in our earlier study. We find that such correlations depend on the presence and characteristics of emission lines in the inner regions of the CCGs. The nuclear indices are correlated with the mass deposition rate (M.) only when emission lines are found in the central regions of the galaxies. The central D4000 and Mg2 indices in cooling flow galaxies without emission lines are completely consistent with the indices measured in CCGs in clusters without cooling flows. CCGs in cooling flow clusters exhibit a clear sequence in the D4000-Mg2 plane, with a neat segregation depending on emission-line type and blue morphology. This sequence can be modelled, using stellar population models with a normal initial mass function (IMF), by a recent (∼ 0.1 Gyr old) burst of star formation, although model uncertainties do not allow us to completely discard continuous star formation or a series of bursts over the last few Gyr. In CCGs with emission lines, the gradients in the spectral indices are flat or positive inside the emission-line regions, suggesting the presence of young stars. Outside the emission-line regions, and in cooling flow galaxies without emission lines, gradients are negative and consistent with those measured in CCGs in clusters without cooling flows and giant elliptical galaxies. Index gradients measured exclusively in the emission-line region correlate with M.. Using the same population models we have estimated the radial profiles of the mass transformed into new stars. The derived profiles are remarkably parallel to the expected radial behaviour of the mass deposition rate derived from X-ray observations. Moreover, a large fraction ( probably most ) of the cooling flow gas accreted into the emission-line region is converted into stars. In the light of these new data, we discuss the evolutionary sequence suggested by McNamara, in which radio-triggered star formation bursts take place several times during the lifetime of the cooling flow. We conclude that this scenario is consistent with the available observations.

Recent studies of heavy r-process elements in low-[Fe/H] halo stars have suggested that an initial population of metal-free very massive stars (VMSs) may be required to provide early Fe enrichment without coproducing heavy r nuclei. We find similar abundance trends in α-elements (which should be copiously produced by VMSs) but not in other elements such as carbon (which should not), in agreement with this hypothesis. We then combine the corresponding level of prompt initial enrichment with models of VMS nucleosynthetic yields and spectra to estimate the corresponding ionizing fluxes. The result suggests that there may have been enough VMS activity to reionize the universe. The unusually hard spectrum of VMSs would imply a different reionization history from canonical models. He II could have been reionized at high redshift only to recombine as a subsequent generation of stars formed with a normal initial mass function.

view Abstract Citations (29) References (45) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS A Photometrical and Dynamical Study of the Helix Galaxy NGC 2685 Peletier, Reynier F. ; Christodoulou, Dimitris M. Abstract We present optical and near-infrared surface photometry of the Helix galaxy NGC 2685 and its two rings. We detected the inner ("polar") ring in the near-infrared J-band and found that its colors are consistent with a normal initial mass function and an age of at most 5-6 Gyr. The inner ring is almost as red as the central galaxy suggesting that it is a long- lived structure. The central galaxy may be either an S0 or an E6/E7 galaxy and has a reddened nucleus. We have constructed gas-dynamical models of the inner self-gravitating ring in a variety of potentials and searched for long-lived models of both rings. These models indicate that the overall potential is either generally oblate-like triaxial or prolate-like triaxial in shape. Based on an analysis of the physical mechanism which can produce such a two-ring structure, we believe that the prolate-like triaxial model is the best current model. This model predicts that both rings achieve very long-lived states within only ~20 inner-ring rotation periods after the accretion of gas into the galaxy's potential. This time scale is in good agreement with the determined upper limit for the age of the inner ring. Publication: The Astronomical Journal Pub Date: April 1993 DOI: 10.1086/116517 Bibcode: 1993AJ....105.1378P Keywords: Astronomical Photometry; Elliptical Galaxies; Galactic Nuclei; Galactic Structure; Gas Dynamics; Ring Structures; Astronomical Models; Gravitational Effects; Near Infrared Radiation; Oblate Spheroids; Prolate Spheroids; Astrophysics; GALAXIES: INDIVIDUAL: NGC 2685; GALAXIES: ELLIPTICAL AND LENTICULAR; GALAXIES: NUCLEI; GALAXIES: KINEMATICS AND DYNAMICS full text sources ADS | data products SIMBAD (2) NED (1)

Numerical hydrodynamical calculations are used to study the effects of the onset of star formation on the residual gas in a primordial low-mass Local-Group dwarf spheroidal galaxy in the size range 0.3-1.0 kpc. It is demonstrated that photoionization in the presence of a moderate gas-density gradient can be responsible for gas ejection on a time-scale of a few times 10 to the 7th yr. The results indicate that, given a normal initial mass function, many protodwarf galaxies may have been dispersed by the onset of star formation.

view Abstract Citations (48) References (29) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Infrared and Radio Emission from S0 Galaxies Bally, John ; Thronson, Harley A., Jr. Abstract We have analyzed the 12-100 micron infrared emission from early-type S0 (lenticular) galaxies using the Infrared Astronomical Satellite (IRAS) database. Out of a sample of 74 galaxies, about 30% have been detected in all four IRAS bands and about 80% in at least one band with a signal-to-noise ratio in excess of 3. The distribution of S0 galaxies in the 12 micron/25 micron vs 60 micron/100 micron color-color diagram is similar to the distribution of late-type spiral galaxies, indicating that S0 galaxies have interstellar media that might support ongoing star formation. We identify three different locations for S0 galaxies in a plot of nonthermal continuum versus far-infrared emission: (1) galaxies that follow the infrared/radio relation found for spirals and that may support star formation with a normal initial mass function (IMF); (2) galaxies with excess radio-continuum emission that may contain an active nucleus and extensive nonthermal emission from jets or radio lobes; and (3) a small number of lenticulars with a slight excess of infrared emission, relative to their radio emission. We do not find significant evidence for star formation with a distorted IMF in which only low-mass stars are forming, which is a possible explanation for sources in location (3). Many of the most radio-luminous galaxies are either not detected or are very faint at infrared wavelengths. Although at the high end of the infrared brightness scale some radio emission may be produced as a result of the birth of massive stars and their eventual demise in supernova explosions, most of the strong radio emitters probably resemble the radio-loud elliptical galaxies in which the radio power is ultimately produced by an active nucleus. The absence of strong radio sources among late-type galaxies suggests that the nuclear source of nonthermal emission is smothered by the interstellar medium in these systems. The S0 galaxies are the dividing line between galaxies that produce their radio emission by an active nucleus and those that produce their emission primarily as a by-product of the formation of massive stars and/or a cool interstellar medium. Publication: The Astronomical Journal Pub Date: January 1989 DOI: 10.1086/114957 Bibcode: 1989AJ.....97...69B Keywords: Disk Galaxies; Extragalactic Radio Sources; Galactic Evolution; Infrared Sources (Astronomy); Interstellar Matter; Color-Color Diagram; Infrared Astronomy Satellite; Signal To Noise Ratios; Star Formation; Astrophysics; RADIO SOURCES: GALAXIES; INFRARED: SOURCES; GALAXIES: GENERAL full text sources ADS | data products NED (72) SIMBAD (69)