Dimitrios A. Gouliermis

Mass segregation in young Magellanic Cloud star clusters: Four clusters observed with HST

We present the results of our investigation on the phenomenon of mass segregation in young star clusters in the Magellanic Clouds. HST/WFPC2 observations on NGC 1818, NGC 2004 and NGC 2100 in the Large Magellanic Cloud and NGC 330 in the Small Magellanic Cloud have been used for the application of diagnostic tools for mass segregation: i) the radial density profiles of the clusters for various mass groups and ii) their mass functions (MFs) at various radii around their centres. All four clusters are found to be mass segregated, but each one in a different manner. Specifically not all the clusters in the sample show the same dependence of their density profiles on the selected magnitude range, with NGC 1818 giving evidence of a strong relation and NGC 330 showing only a hint of the phenomenon. NGC 2004 did not show any significant signature of mass segregation in its density profiles either. The MFs radial dependence provides clear proof of the phenomenon for NGC 1818, NGC 2100 and NGC 2004, while for NGC 330 it gives only indications. An investigation of the constraints introduced by the application of both diagnostic tools is presented. We also discuss the problems related to the construction of a reliable MF for a cluster and their impact on the investigation of the phenomenon of mass segregation. We find that the MFs of these clusters as they were constructed with two methods are comparable to Salpeters IMF. A discussion is given on the dynamical status of the clusters and a test is applied on the equipartition among several mass groups in them. Both showed that the observed mass segregation in the clusters is of primordial nature.

A NEW METHOD FOR THE ASSESSMENT OF AGE AND AGE SPREAD OF PRE-MAIN-SEQUENCE STARS IN YOUNG STELLAR ASSOCIATIONS OF THE MAGELLANIC CLOUDS* , **

We present a new method for the evaluation of the age and age spread among pre-main-sequence (PMS) stars in star-forming regions in the Magellanic Clouds, accounting simultaneously for photometric errors, unresolved binarity, differential extinction, stellar variability, accretion, and crowding. The application of the method is performed with the statistical construction of synthetic color-magnitude diagrams (CMDs) using isochrones from two families of PMS evolutionary models. We convert each isochrone into two-dimensional probability distributions of artificial PMS stars in the CMD by applying the aforementioned biases that dislocate these stars from their original CMD positions. A maximum-likelihood technique is then applied to derive the probability for each observed star to have a certain age as well as the best age for the entire cluster. We apply our method to the photometric catalog of ~2000 PMS stars in the young association LH 95 in the Large Magellanic Cloud, based on the deepest HST/ACS imaging ever performed toward this galaxy, with a detection limit of V ~ 28, corresponding to M ~ 0.2 M . We assume the initial mass function and reddening distribution for the system, as they have been previously derived by us. Our treatment shows that the age determination is very sensitive to the considered grid of evolutionary models and the assumed binary fraction. The age of LH 95 is found to vary from 2.8 Myr to 4.4 Myr, depending on these factors. We evaluate the accuracy of our age estimation and find that the method is fairly accurate in the PMS regime, while the precision of the measurement of the age is lower at higher luminosities. Our analysis allows us to disentangle a real age spread from the apparent CMD broadening caused by the physical and observational biases. We find that LH 95 hosts an age spread that is represented well by a Gaussian distribution with an FWHM of the order of 2.8-4.4 Myr depending on the model and binary fraction. We detect a dependence of the average age of the system with the stellar mass. This dependence does not appear to have any physical meaning, being rather due to imperfections of the PMS evolutionary models, which tend to predict lower ages for the intermediate-mass and higher ages for the low-mass stars.

THE COMPLETE INITIAL MASS FUNCTION DOWN TO THE SUBSOLAR REGIME IN THE LARGE MAGELLANIC CLOUD WITH HUBBLE SPACE TELESCOPE ACS OBSERVATIONS

In this photometric study of the stellar association LH 95 in the Large Magellanic Cloud (LMC) we focus on the pre-main-sequence (PMS) population in order to construct, for the first time, the subsolar initial mass function (IMF) in the LMC. The basis for this investigation consists of the deepest photometry ever performed in the LMC with the Advanced Camera for Surveys (ACS) onboard the Hubble Space Telescope. We improve our catalog of point sources obtained with ACS (Paper I). We carry out a Monte Carlo technique to subtract the contribution of the general field of LMC. We isolate the central region in the observed area of the association, as the part characterized by the highest concentration of PMS stars. We analyze the reddening distribution of the system in order to obtain the mean value of extinction and we study the mass function of its field-subtracted population. For this purpose, we introduce a new set of evolutionary models, derived from the calculations on the evolution of PMS stars by Siess and collaborators. We use these models with our observations of LH 95 to derive the IMF of the system. This mass function is reliably constructed for stars with masses down to 0.43 M ☉, the lowest mass ever observed within reasonable completeness in the Magellanic Clouds. Consequently, its construction offers an outstanding improvement in our understanding of the low-mass star formation in the LMC. The system IMF of LH 95 shows a definite change in its slope for masses M 1 M ☉, where it becomes more shallow. In general, the shape of this IMF agrees very well with a multiple power law, as the typical Galactic IMF, down to the subsolar regime. The change in the slope (the knee) of our IMF at ~1 M ☉ also agrees with the average Galactic IMF. As far as the slope of this system IMF is concerned, it is found to be somewhat more shallow than the corresponding classical Galactic IMF in the subsolar regime, probably due to unresolved binarity, while for stars with M 1 M ☉ it becomes slightly steeper. We do not find significant differences in the shape of the overall IMF of LH 95 from that of each of the three individual subclusters of the association. This clearly suggests that the IMF of LH 95 is not subject to local variability.

Age spread in W3 main: Large Binocular Telescope/LUCI near-infrared spectroscopy of the massive stellar content

We present near-infrared multi-object spectroscopy and JHK(s) imaging of the massive stellar content of the Galactic star-forming region W3 Main, obtained with LUCI at the Large Binocular Telescope. We confirm 15 OB stars in W3 Main and derive spectral types between O5V and B4V from their absorption line spectra. Three massive young stellar objects are identified by their emission line spectra and near-infrared excess. The color-color diagram of the detected sources allows a detailed investigation of the slope of the near-infrared extinction law toward W3 Main. Analysis of the Hertzsprung-Russell diagram suggests that the Nishiyama extinction law fits the stellar population of W3 Main best (E(J -H)/ E(H - K-s) = 1.76 and RKs = 1.44). From our spectrophotometric analysis of the massive stars and the nature of their surrounding H II regions, we derive the evolutionary sequence of W3 Main and we find evidence of an age spread of at least 2-3 Myr. While the most massive star (IRS2) is already evolved, indications for high-mass pre-main-sequence evolution are found for another star (IRS N1), deeply embedded in an ultracompact H II (UCH II) region, in line with the different evolutionary phases observed in the corresponding H II regions. We derive a stellar mass of W3 Main of (4 +/- 1) x 10(3) M-circle dot by extrapolating from the number of OB stars using a Kroupa initial mass function and correcting for our spectroscopic incompleteness. We have detected the photospheres of OB stars from the more evolved diffuse H II region to the much younger UCH II regions, suggesting that these stars have finished their formation and cleared away their circumstellar disks very fast. Only in the hyper-compact H II region (IRS5) do the early-type stars seem to be still surrounded by circumstellar material.

PHAT stellar cluster survey. I. Year 1 catalog and integrated photometry

The Panchromatic Hubble Andromeda Treasury (PHAT) survey is an ongoing Hubble Space Telescope (HST) multi-cycle program to obtain high spatial resolution imaging of one-third of the M31 disk at ultraviolet through near-infrared wavelengths. In this paper, we present the first installment of the PHAT stellar cluster catalog. When completed, the PHAT cluster catalog will be among the largest and most comprehensive surveys of resolved star clusters in any galaxy. The exquisite spatial resolution achieved with HST has allowed us to identify hundreds of new clusters that were previously inaccessible with existing ground-based surveys. We identify 601 clusters in the Year 1 sample, representing more than a factor of four increase over previous catalogs within the current survey area (390 arcmin2). This work presents results derived from the first ~25% of the survey data; we estimate that the final sample will include ~2500 clusters. For the Year 1 objects, we present a catalog with positions, radii, and six-band integrated photometry. Along with a general characterization of the cluster luminosities and colors, we discuss the cluster luminosity function, the cluster size distributions, and highlight a number of individually interesting clusters found in the Year 1 search.

The Complex Distribution of Recently Formed Stars. Bimodal Stellar Clustering in the Star-Forming Region NGC 346.

We present a detailed stellar clustering analysis with the a pplication of the two-point correlation function on distinct young stellar ensembles. Our aim is to understand how stellar systems are assembled at the earliest stages of their format ion. Our object of interest is the star-forming region NGC 346 in the Small Magellanic Cloud. It is a young stellar system wellrevealed from its natal environment, comprising complete samples of pre‐main-sequence and upper main-sequence stars, very close to their formation. We apply a comprehensive characterization of the autocorrelation function for both cent rally condensed stellar clusters and self-similar stellar distributions through numerical sim ulations of stellar ensembles. We interpret the observed autocorrelation function of NGC 346 on the basis of these simulations. We find that it can be best explained as the combination of two d istinct stellar clustering designs, a centrally concentrated, dominant at the central pa rt of the star-forming region, and an extended self-similar distribution of stars across the c omplete observed field. The cluster component, similar to non-truncated young star clusters, is determined to have a core radius of � 2.5 pc and a density profile index of � 2.3. The extended fractal component is found with our simulations to have a fractal dimension of � 2.3, identical to that found for the interstellar medium, in agreement to hierarchy induced by turbulence. This suggests that the stellar clustering at a time very near to birth behaves in a complex manner. It is the combined result of the star formation process regulated by turbulence and the early dynamical evolution induced by the gravitational potential of condensed stellar cluste rs.

LEGACY EXTRAGALACTIC UV SURVEY (LEGUS) WITH THE HUBBLE SPACE TELESCOPE. I. SURVEY DESCRIPTION

The Legacy ExtraGalactic UV Survey (LEGUS) is a Cycle 21 Treasury program on the Hubble Space Telescope aimed at the investigation of star formation and its relation with galactic environment in nearby galaxies, from the scales of individual stars to those of ~kiloparsec-size clustered structures. Five-band imaging from the near-ultraviolet to the I band with the Wide-Field Camera 3 (WFC3), plus parallel optical imaging with the Advanced Camera for Surveys (ACS), is being collected for selected pointings of 50 galaxies within the local 12 Mpc. The filters used for the observations with the WFC3 are F275W(λ2704 A), F336W(λ3355 A), F438W(λ4325 A), F555W(λ5308 A), and F814W(λ8024 A); the parallel observations with the ACS use the filters F435W(λ4328 A), F606W(λ5921 A), and F814W(λ8057 A). The multiband images are yielding accurate recent (lesssim50 Myr) star formation histories from resolved massive stars and the extinction-corrected ages and masses of star clusters and associations. The extensive inventories of massive stars and clustered systems will be used to investigate the spatial and temporal evolution of star formation within galaxies. This will, in turn, inform theories of galaxy evolution and improve the understanding of the physical underpinning of the gas-star formation relation and the nature of star formation at high redshift. This paper describes the survey, its goals and observational strategy, and the initial scientific results. Because LEGUS will provide a reference survey and a foundation for future observations with the James Webb Space Telescope and with ALMA, a large number of data products are planned for delivery to the community.

THE PANCHROMATIC HUBBLE ANDROMEDA TREASURY V: AGES AND MASSES OF THE YEAR 1 STELLAR CLUSTERS

We present ages and masses for 601 star clusters in M31 from the analysis of the six filter integrated light measurements from near ultraviolet to near infrared wavelengths, made as part of the Panchromatic Hubble Andromeda Treasury (PHAT). We derive the ages and masses using a probabilistic technique, which accounts for the effects of stochastic sampling of the stellar initial mass function. Tests on synthetic data show that this method, in conjunction with the exquisite sensitivity of the PHAT observations and their broad wavelength baseline, provides robust age and mass recovery for clusters ranging from � 10 2 2×10 6 M⊙. We find that the cluster age distribution is consistent with being uniform over the past 100 Myr, which suggests a weak effect of cluster disruption within M31. The age distribution of older (> 100 Myr) clusters fall towards old ages, consistent with a power-law decline of index 1, likely from a combination of fading and disruption of the clusters. We find that the mass distribution of the whole sample can be well-described by a single power-law with a spectral index of 1.9±0.1 over the range of 10 3 3×10 5 M⊙. However, if we subdivide the sample by galactocentric radius, we find that the age distributions remain unchanged. However, the mass spectral index varies significantly, showing best fit values between 2.2 and 1.8, with the shallower slope in the highest star formation intensity regions. We explore the robustness of our study to potential systematics and conclude that the cluster mass function may vary with respect to environment. Subject headings: Galaxies: Individual (M31), Star clusters — Methods: data analysis, statistical — Techniques: photometric

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.

A comprehensive comparative test of seven widely used spectral synthesis models against multi-band photometry of young massive-star clusters

We test the predictions of spectral synthesis models based on seven different massive-star prescriptions against Legacy ExtraGalactic UV Survey (LEGUS) observations of eight young massive clusters ...