Stefan Schmeja

The structures of embedded clusters in the Perseus, Serpens and Ophiuchus molecular clouds

The young stellar population data of the Perseus, Ophiuchus and Serpens molecular clouds are obtained from the Spitzer Cores to Discs (c2d) legacy survey in order to investigate the spatial structure of embedded clusters using the nearest-neighbour (NN) and minimum-spanning tree method. We identify the embedded clusters in these clouds as density enhancements and analyse the clustering parameter Q with respect to source luminosity and evolutionary stage. This analysis shows that the older Class 2/3 objects are more centrally condensed than the younger Class 0/1 protostars, indicating that clusters evolve from an initial hierarchical configuration to a centrally condensed one. Only IC 348 and the Serpens core, the older clusters in the sample, show signs of mass segregation (indicated by the dependence of Q on the source magnitude), pointing to a significant effect of dynamical interactions after a few Myr. The structure of a cluster may also be linked to the turbulent energy in the natal cloud as the most centrally condensed cluster is found in the cloud with the lowest Mach number and vice versa. In general, these results agree well with theoretical scenarios of star cluster formation by gravoturbulent fragmentation.

Evolving structures of star-forming clusters

Context. Understanding the formation and evolution of young star clusters requires quantitative statistical measures of their structure. Aims. We investigate the structures of observed and modelled star-forming clusters. By considering the different evolutionary classes in the observations and the temporal evolution in models of gravoturbulent fragmentation, we study the temporal evolution of the cluster structures. Methods. We apply different statistical methods, in particular the normalised mean correlation length and the minimum spanning tree technique. We refine the normalisation of the clustering parameters by defining the area using the normalised convex hull of the objects and investigate the effect of two-dimensional projection of three-dimensional clusters. We introduce a new measure ξ for the elongation of a cluster. It is defined as the ratio of the cluster radius determined by an enclosing circle to the cluster radius derived from the normalised convex hull. Results. The mean separation of young stars increases with the evolutionary class, reflecting the expansion of the cluster. The clustering parameters of the model clusters correspond in many cases well to those from observed ones, especially when the ξ values are similar. No correlation of the clustering parameters with the turbulent environment of the molecular cloud is found, indicating that possible influences of the environment on the clustering behaviour are quickly smoothed out by the stellar velocity dispersion. The temporal evolution of the clustering parameters shows that the star cluster builds up from several subclusters and evolves to a more centrally concentrated cluster, while the cluster expands slower than new stars are formed.

Protostellar mass accretion rates from gravoturbulent fragmentation

We analyse protostellar mass accretion rates u M from numerical models of star formation based on gravoturbulent fragmentation, considering a large number of different environments. To within one order of magnitude, u M ≈ MJ/τff with MJ being the mean thermal Jeans mass and τff the corresponding free-fall time. However, mass accretion rates are highly time- variant, with a sharp peak shortly after the formation of the protostellar core. We present an empirical exponential fit formula to describe the time evolution of the mass accretion and discuss the resulting fit parameters. There is a positive correlation between the peak accretion rate and the final mass of the protostar. We also investigate the relation of u M with the turbulent flow velocity as well as with the driving wavenumbers in different environments. We then compare our results with other theoretical models of star formation and with observational data.

The peculiar variable V838 Mon

V838 Mon underwent, after a first nova-like outburst in January and a usual decline, a second outburst after one month, and a third weak one again a month later. Moreover, a very small increase of the temperature at the beginning of April gives us a hint about a physical process with a period of one month. We obtained a BV R IC time-sequence and modelled the photometric behaviour of the object. This leads us to the conclusion that the interstellar foreground extinction has to be 0.6 mag E(B − V ) 0.8 mag and that the quasi-photosphere had persistently unusually low temperatures for nova-like systems. The photometry was used to follow the dramatic changes of the expansion. While the appearing 10-µm excess can be well described by the heating of material ejected during this event, the IRAS emission near the location of the progenitor originates most likely from dust, which were formed during the previous evolution of the object. Assuming that the light echoes are coming from circumstellar material, the distance is 640 to 680 pc ‐ smaller than the 790 pc given in Munari et al. (2002). In our opinion V838 Mon and V4332 Sgr are manifestations of a new class of eruptive variables. We do not count M31RV to be in this class.

Planetary nebula or symbiotic Mira? Near infrared colours mark the difference

Nebulae around symbiotic Miras look very much like genuine planetary nebulae, although they are formed in a slightly different way. We present near infrared photometry of known and suspected symbiotic nebulae obtained with the Deep Near Infrared Southern Sky Survey (DENIS). We demonstrate that the near infrared colours are an excellent tool to distinguish symbiotic from genuine planetary nebulae. In particular we find that the bipolar planetary nebulae M 2-9 and Mz 3 are in fact symbiotic Miras. Further observations on prototype symbiotic Miras prove that the proposed classification scheme works generally.

Global survey of star clusters in the Milky Way: III. 139 new open clusters at high Galactic latitudes

Context. An earlier analysis of the Milky Way Star Cluster (MWSC) catalogue revealed an apparent lack of old (t & 1 Gyr) open clusters in the solar neighbourhood (d. 1 kpc). Aims. To fill this gap we undertook a search for hitherto unknown star clusters a ssuming that the missing old clusters reside at high Galactic latitudes|b| > 20 ◦ . Methods. We were looking for stellar density enhancements using a star count algorithm on the 2MASS point source catalogue. To increase the contrast between potential clusters and the field, we applie d filters in colour-magnitude space according to typical colour-magnitude diagrams of nearby old open clusters. The subsequent comparison with lists of known objects allowed us to select so far unknown cluster candidates. For verification they were process ed with the standard pipeline used within the MWSC survey for computing cluster membership probabilities and for the determination of structural, kinematic and astrophysical parameters. Results. In total we discovered 782 density enhancements, 522 of which were classified as real objects. Among them 139 are new open clusters with ages 8.3 < log(t [yr]) < 9.7, distances d < 3 kpc and distances from the Galactic plane 0.3 < Z < 1 kpc. This new sample has increased the total number of known high latitude open clusters by about 150%. Nevertheless, we still observe a lack of older nearby clusters up to 1 kpc from the Sun. This volume is expected to still contain about 60 unknown clusters that probably escaped our detection algorithm, which fails to detect sparse overdensitie s with large angular size.

Identifying star clusters in a field: A comparison of different algorithms

Star clusters are often hard to find, as they may lie in a dense field of background objects or, because in the case of embedded clusters, they are surrounded by a more dispersed population of young stars. This paper discusses four algorithms that have been developed to identify clusters as stellar density enhancements in a field, namely stellar density maps from star counts, the nearest neighbour method and the Voronoi tessellation, and the separation of minimum spanning trees. These methods are tested and compared to each other by applying them to artificial clusters of different sizes and morphologies. While distinct centrally concentrated clusters are detected by all methods, clusters with low overdensity or highly hierarchical structure are only reliably detected by methods with inherent smoothing (star counts and nearest neighbour method). Furthermore, the algorithms differ strongly in computation time and additional parameters they provide. Therefore, the method to choose primarily depends on the size and character of the investigated area and the purpose of the study (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Old star clusters in the FSR catalogue

We investigate the old star clusters in the sample of cluster candidates from the Froebrich, Scholz & Raftery (FSR) list. Based on photometry from the Two-Micron All-Sky Survey, we generated decontaminated colour–magnitude and colour–colour diagrams to select a sample of 269 old stellar clusters. This sample contains 63 known globular clusters, 174 known open clusters and 32 so far unclassified objects. Isochrone fitting has been used to homogeneously calculate the age, distance and reddening to all clusters. The mean age of the open clusters in our sample is 1 Gyr. The positions of these clusters in the Galactic plane show that 80 per cent of open clusters older than 1 Gyr have a Galactocentric distance of more than 7 kpc. The scaleheight for the old open clusters above the Galactic plane is 375 pc, more than three times as large as 115 pc, which we obtain for the younger open clusters in our sample. We find that the mean optical extinction towards the open clusters in the disc of the Galaxy is 0.70 mag kpc −1 . The FSR sample has a strong selection bias towards objects with an apparent core radius of 30–50 arcsec and there is an unexplained paucity of old open clusters in the Galactic longitude range of 120 ◦ < l < 180 ◦ .

THE CLUSTERING BEHAVIOR OF PRE-MAIN-SEQUENCE STARS IN NGC 346 IN THE SMALL MAGELLANIC CLOUD

We present evidence that the star-forming region NGC 346/N66 in the Small Magellanic Cloud is the product of hierarchical star formation, probably from more than one star formation event. We investigate the spatial distribution and clustering behavior of the pre-main-sequence (PMS) stellar population in the region, using data obtained with Hubble Space Telescopes Advanced Camera for Surveys. By applying the nearest neighbor and minimum spanning tree methods on the rich sample of PMS stars previously discovered in the region, we identify 10 individual PMS clusters in the area and quantify their structures. The clusters show a wide range of morphologies from hierarchical multipeak configurations to centrally condensed clusters. However, only about 40% of the PMS stars belong to the identified clusters. The central association NGC 346 is identified as the largest stellar concentration, which cannot be resolved into subclusters. Several PMS clusters are aligned along filaments of higher stellar density pointing away from the central part of the region. The PMS density peaks in the association coincide with the peaks of [O III] and 8 μm emission. While more massive stars seem to be concentrated in the central association when considering the entire area, we find no evidence for mass segregation within the system itself.

The spatial distribution of substellar objects in IC 348 and the Orion Trapezium cluster

Context. Some theoretical scenarios suggest the formation of brown dwarfs as ejected stellar embryos in star-forming clusters. Such a formation mechanism can result in different spatial distributions of stars and substellar objects. Aims. We aim to investigate the spatial structure of stellar and substellar objects in two well-sampled and nearby embedded clusters, namely IC 348 and the Orion Trapezium cluster (OTC), to test this hypothesis. Methods. Deep near-infrared K-band data that are complete enough to sample the substellar population in IC 348 and OTC are obtained from the literature. The spatial distribution of the K-band point sources is analysed using the minimum spanning tree (MST) method. The Q parameter and the spanning trees are evaluated for stellar and substellar objects as a function of cluster core radius Rc. Results. The stellar population in both IC 348 and OTC display a clustered distribution, whereas the substellar population is distributed homogeneously in space within twice the cluster core radius. Although the substellar objects do not appear to be bound by the cluster potential well, they are still within the limits of the cluster and not significantly displaced from their birth sites. Conclusions. The spatially homogeneous distribution of substellar objects is best explained by assuming higher initial velocities, distributed in a random manner and going through multiple interactions. The overall spatial coincidence of these objects with the cluster locations can be understood if these objects are nevertheless travelling slowly enough to feel the gravitational influence of the cluster. The observations support the scenario of substellar objects forming as “ejected stellar embryos”. Higher ejection velocities are necessary, but net spatial displacements may not be needed to explain the observational data.