Yu. N. Efremov

The early expansion of cluster cores

The observed properties of young star clusters, such as the core radius and luminosity profile, change rapidly during the early evolution of the clusters. Here we present observations of six young clusters in M51 where we derive their sizes using Hubble Space Telescope (HST) imaging and ages using deep Gemini-North spectroscopy. We find evidence for a rapid expansion of the cluster cores during the first 20 Myr of their evolution. We confirm this trend by including data from the literature of both Galactic and extragalactic embedded and young clusters, and possible mechanisms (rapid gas removal, stellar evolutionary mass loss and internal dynamical heating) are discussed. We explore the implications of this result, focussing on the fact that clusters were more concentrated in the past, implying that their stellar densities were much higher and relaxation times (trelax) correspondingly shorter. Thus, when estimating if a particular cluster is dynamically relaxed (i.e. when determining if a cluster’s mass segregation is due to primordial or dynamical processes), the current relaxation time is only an upper limit, with trelax likely being significantly shorter in the past.

Hierarchical star formation in M33: fundamental properties of the star-forming regions

Star formation within galaxies appears on multiple scales, from spiral structure, to OB associations, to individual star clusters, and often substructure within these clusters. This multitude of scales calls for objective methods to find and classify star-forming regions, regardless of spatial size. To this end, we present an analysis of star-forming groups in the local group spiral galaxy M33, based on a new implementation of the minimum spanning tree method. Unlike previous studies which limited themselves to a single spatial scale, we study star-forming structures from the effective resolution limit (∼~20 pc) to kpc scales. Once the groups are identified, we study their properties, for example, size and luminosity distributions, and compare them with studies of young star clusters and giant molecular clouds (GMCs). We find evidence for a continuum of star-forming group sizes, which extends into the star cluster spatial scale regime. We do not find a characteristic scale for OB associations, unlike that found in previous studies, and we suggest that the appearance of such a scale was caused by spatial resolution and selection effects. The luminosity function of the groups is found to be well represented by a power law with an index, −2, the same as has been found for the luminosity and mass functions (MFs) of young star clusters, as well as the MF of GMCs. Additionally, the groups follow a similar mass–radius relation as GMCs. The size distribution of the groups is best described by a lognormal distribution, the peak of which is controlled by the spatial scale probed and the minimum number of sources used to define a group.We show that within a hierarchical distribution, if a scale is selected to find structure, the resulting size distribution will have a lognormal distribution. We find an abrupt drop of the number of groups outside a galactic radius of ~4 kpc (although individual high-mass stars are found beyond this limit), suggesting a change in the structure of the star-forming interstellar medium, possibly reflected in the lack of GMCs beyond this radius. Finally, we find that the spatial distribution of HII regions, GMCs, and star-forming groups are all highly correlated.

Resolved photometry of extragalactic young massive star clusters

Aims. We present colour–magnitude diagrams (CMDs) of young massive star clusters in several galaxies located well beyond the Local Group. The richness of these clusters allows us to obtain large samples of post-main sequence stars and test how well the observed CMDs are reproduced by canonical stellar isochrones. Methods. We use imaging of seven clusters in the galaxies NGC 1313, NGC 1569, NGC 1705, NGC 5236 and NGC 7793 obtained with the Advanced Camera for Surveys on board the Hubble Space Telescope and carry out PSF-fitting photometry of individual stars in the clusters. The clusters have ages in the range ∼(5−50) × 10 6 years and masses of ∼10 5 M� –10 6 M� . Although crowding prevents us from obtaining photometry in the inner regions of the clusters, we are still able to measure up to 30–100 supergiant stars in each of the richest clusters. The resulting CMDs and luminosity functions are compared with photometry of artificially generated clusters, designed to reproduce the photometric errors and completeness as realistically as possible. Results. In agreement with previous studies, our CMDs show no clear gap between the H-burning main sequence and the Heburning supergiant stars, contrary to predictions by common stellar isochrones. In general, the isochrones also fail to match the observed number ratios of red-to-blue supergiant stars, although the difficulty of separating blue supergiants from the main sequence complicates this comparison. In several cases we observe a large spread (1–2 mag) in the luminosities of the supergiant stars that cannot be accounted for by observational errors. We find that this spread can be reproduced by including an age spread of ∼(10−30) × 10 6 years in the models. However, age spreads cannot fully account for the observed morphology of the CMDs and other processes, such as the evolution of interacting binary stars, may also play a role. Conclusions. Colour–magnitude diagrams can be successfully obtained for massive star clusters out to distances of at least 4–5 Mpc. Comparing such CMDs with models based on canonical isochrones we find several areas of disagreement. One interesting possibility is that an age spread of up to ∼30 Myr may be present in some clusters. The data presented here may provide useful constraints on models for single and/or binary stellar evolution.

Star complexes and associations in the Andromeda galaxy

Large-scaleU andB plates obtained with the 2 m Ritchey-Chrétien telescope of the Rozhen Observatory (Bulgaria) were searched for new resolved star groups and for independent delineation of the boundaries of the known ones in M31. We detected 210 groups as real O-associations the mean diameter of which is 80 pc. Many of Hodges open clusters are also reclassified as O-associations. The majority of van den Berghs OB-associations were recognized as star complexes and their mean diameter is 650 pc. Almost all O-associations are located inside the star complexes. A dozen of new star complexes (mainly around the dark lanes between OB78 and OB22) and numerous groups presumably not containing O-stars were found out. The nature of these groups has to be object of further investigations. Young star groups closer than 3 kpc to the center of M31 were not identified.

Cepheids in LMC Clusters and the Period-Age Relation

We have made a new comparison of the positions of Cepheids and clusters in the LMC and constructed a new empirical period-age relation taking into account all available data on Cepheids in the LMC bar provided by the OGLE project. The most probable relation is logT=8.50−0.65 logP, in reasonably good agreement with theoretical expectations. Numerous Cepheids in rich clusters of the LMC provide the best data for comparing theories of stellar evolution and pulsation and the dynamical evolution of clusters with observations. These data suggest that stars undergoing their first crossing of the instability strip are first-overtone pulsators, though the converse is true of only a small fraction of first-overtone stars. Several rich clusters with suitable ages have no Cepheids—a fact that is not understood and requires verification. Differences in the concentration of Cepheids toward their cluster centers probably reflect the fact that the clusters are at different stages of their dynamical evolution, with the Cepheids in cluster coronas being ejected from the cluster cores during dynamical interactions between stars.

The Car–Sgr arm as outlined by superclouds and the grand design of the Galaxy

It is found that H I superclouds are regularly spaced along the very long Car-Sgr arm, the mutual distances being mostly 0.1 and 0.2 in units of the Solar distance to the centre. Such regularity is instrinsic to the grand design galaxies with density-wave spiral arms. A higher density of old stars and star clusters within the Car-Sgr arm is observed, implying a strong density wave. A symmetric second arm should exist in a grand design galaxy yet this arm is well seen only in the I1 quadrant, being behind the Per arm. The positions of most superclouds, the giant H I1 regions and GMCs over the Galaxy are compatible with a four-arm spiral structure with two less pronounced additional arms midway between the two principal arms; the nearby Per arm is one of t,he secondary arms. The pitch angle of such a grand design spiral pattern is 10°-120. If this pattern does exist, the long segments of the (urn6 have none tlie above-mentioned spiraltracers. At any rate the Car-Sgr arm is certainly mud1 brighter and more regular than all the others. The most probable aim class of the Galaxy is 9-12: it is multiarm or grand design galaxy.

6-m telescope spectroscopic observations of the bubble complex in NGC 6946

We describe the results of a long-slit spectroscopic study of an unusual star complex in the nearby spiral galaxy NGC 6946 using the SAO 6 m telescope and the Keck 10 m telescope. The complex resembles a circular bubble 600 pc in diameter with a young super star cluster (SSC) near the center. The kinematics of ionized gas is studied through H emission with several slit positions. Position{velocity diagrams show two distinct features with high speed motions. One is an irregularly shaped region to the east of the SSC, 270 pc in size, in which most of the H emission is blue shifted by 120 km s 1 , and another is a 350 pc shell centered on the SSC with positive and negative velocity shifts of 60 km s 1 .B almer and Hei absorption lines in the SSC give an age of 12{13 Myr, which is consistent with the photometric age but signicantly older than the kinematic ages of the high speed regions. The energetics of the SSC and its interaction with the environment are considered. The expansion energies exceed 10 52 ergs, but the power outputs from winds and supernova in the SSC are large enough to account for this. The intensities of Balmer, (N ii), and (S ii) emission lines within and around the complex indicate that shock excitation makes a signicant contribution to the emission from the most energetic region.

Classical Cepheids and the spiral structure of the milky way

We use data on space distribution of the currently most complete sample of Cepheids with reliable distances (565 stars), located within ~5 kpc from the Sun, to study the spiral pattern of the Milky Way galaxy. We estimate the pitch angle as 9°−10°; the most accurate estimate, i = 9.5° ± 0.1°, was obtained assuming the existence of a global four-armed spiral pattern; the solar phase angle in the spiral pattern is χ⊙ = −121° ± 3°. Comparing positions of the spiral arms delineated by classical Cepheids and galactic masers, with the age difference of these objects in mind, we estimate the rotation angular speed of the spiral pattern to be ΩP = 25.2 ± 0.5 km s−1kpc−1.

Regularities in the distribution of star/gas complexes in the spiral arms of our galaxy and M31

The fragmentation of gaseous spiral arms in the outer Galaxy into superclouds has been studied using recently published data on the HI distribution in the Galactic disk. Regular chains of superclouds have been found or confirmed in the Cygnus (Outer) and Carina arms, with the spacings between the superclouds being concentrated near 0.1 and 0.2 of the solar Galactocentric distance. The star complexes in the northwestern arm of the galaxy M31 are spaced, on average, 1.2 kpc apart, with the most distinct chain of complexes being located in the arm region where Beck et al. (1989) detected a strong and wavy (along the arm) magnetic field. Its wavelength turns out to be related to the spacing between the complexes. In this arm, the HII regions lie inside the star complexes, which, in turn, are located inside the gas-dust lane. In contrast, the southwestern arm of M31 is split into a gas-dust lane and a dense stellar arm, which is not fragmented into star complexes. Here, the HII regions are located along the boundary between the gas-dust and stellar components of the arm; other evidence for the presence of a spiral shock wave triggering star formation is also observed, which is probably attributable to the large pitch angle of this segment of the southwestern arm. It may be suggested that the shock wave rapidly leads to star formation everywhere in this arm, while in the northwestern arm, where the shock wave is absent, star formation begins in the superclouds formed along the arm by the magneto-gravitational instability. This is how the chains of star complexes in the northwestern arm of M31 and, obviously, the chains of superclouds in the Carina and Cygnus arms of our Galaxy have been formed. The detection of a regularmagnetic field in the corresponding segments of these arms can be predicted.

Ionized and neutral gas in the peculiar star/cluster complex in NGC 6946

The characteristics of ionized and H I gas in the peculiar star/cluster complex in NGC 6946, obtained with the 6-m telescope (BTA) Special Astrophysical Observatory Russian Academy of Sciences (RAS), the Gemini North telescope, and the Westerbork Synthesis Radio Telescope, are presented. The complex is unusual as hosting a super star cluster, the most massive known in an apparently non-interacting giant galaxy. It contains a number of smaller clusters and is bordered by a sharp C-shaped rim. We found that the complex is additionally unusual in having peculiar gas kinematics. The velocity field of the ionized gas reveals a deep oval minimum, similar to 300 pc in size, centred 7 arcsec east of the supercluster. The V-r of the ionized gas in the dip centre is 100 km s(-1) lower than in its surroundings, and emission lines within the dip appear to be shock-excited. This dip is near the centre of an H I hole and a semi-ring of H II regions. The H I (and less certainly, H II) velocity fields reveal expansion, with the velocity reaching similar to 30 km s(-1) at a distance about 300 pc from the centre of expansion, which is near the deep minimum position. The superstar cluster is at the western rim of the minimum. The sharp western rim of the whole complex is plausibly a manifestation of a regular dust arc along the complex edge. Different hypotheses about the complex and the V-r depressions origins are discussed, including a high-velocity H I cloud/dark minihalo impact, a blue compact dwarf galaxy merging, and a gas outflow due to release of energy from the supercluster stars.