The lower mass function of the young open cluster Blanco 1: from 30 M_(Jup) to 3 M_☉

Aims. We performed a deep wide field optical survey of the young (~100−150 Myr) open cluster Blanco 1 to study its low mass population well down into the brown dwarf regime and estimate its mass function over the whole cluster mass range. Methods. The survey covers 2.3 square degrees in the I and z-bands down to I ≃ z ≃ 24 with the CFH12K camera. Considering two different cluster ages (100 and 150 Myr), we selected cluster member candidates on the basis of their location in the (I, I − z) CMD relative to the isochrones, and estimated the contamination by foreground late-type field dwarfs using statistical arguments, infrared photometry and low-resolution optical spectroscopy. Results. We find that our survey should contain about 57% of the cluster members in the 0.03−0.6 M_☉ mass range, including 30–40 brown dwarfs. The candidate’s radial distribution presents evidence that mass segregation has already occured in the cluster. We took it into account to estimate the cluster mass function across the stellar/substellar boundary. We find that, between 0.03 M_☉ and 0.6 M_☉, the cluster mass distribution does not depend much on its exact age, and is well represented by a single power-law, with an index α = 0.69 ± 0.15. Over the whole mass domain, from 0.03 M_☉ to 3 M_☉, the mass function is better fitted by a log-normal function with m_0 = 0.36 ± 0.07 M_☉ and σ = 0.58 ± 0.06. Conclusions. Comparison between the Blanco 1 mass function, other young open clusters’ MF, and the galactic disc MF suggests that the IMF, from the substellar domain to the higher mass part, does not depend much on initial conditions. We discuss the implications of this result on theories developed to date to explain the origin of the mass distribution

The lower mass function of young open clusters

We report new estimates for the lower mass function of 5 young open clusters spanning an age range from 80 to 150 Myr. In all studied clusters, the mass function across the stellar/substellar boundary (~0.072 Mo) and up to 0.4 Mo is consistent with a power-law with an exponent alpha of -0.5 +/- 0.1, i.e., dN/dM ~ M**(-0.5).Comment: 8 pages, 4 figure

Extended Red Emission and the evolution of carbonaceaous nanograins in NGC 7023

Extended Red Emission (ERE) was recently attributed to the photo-luminescence of either doubly ionized Polycyclic Aromatic Hydrocarbons (PAH$^{++}$), or charged PAH dimers. We analysed the visible and mid-infrared (mid-IR) dust emission in the North-West and South photo-dissociation regions of the reflection nebula NGC 7023.Using a blind signal separation method, we extracted the map of ERE from images obtained with the Hubble Space Telescope, and at the Canada France Hawaii Telescope. We compared the extracted ERE image to the distribution maps of the mid-IR emission of Very Small Grains (VSGs), neutral and ionized PAHs (PAH$^0$ and PAH$^+$) obtained with the Spitzer Space Telescope and the Infrared Space Observatory. ERE is dominant in transition regions where VSGs are being photo-evaporated to form free PAH molecules, and is not observed in regions dominated by PAH$^+$. Its carrier makes a minor contribution to the mid-IR emission spectrum. These results suggest that the ERE carrier is a transition species formed during the destruction of VSGs. Singly ionized PAH dimers appear as good candidates but PAH$^{++}$ molecules seem to be excluded.Comment: Accepted for publication in A&

Dynamical Analysis of Nearby ClustErs. Automated astrometry from the ground: precision proper motions over wide field

The kinematic properties of the different classes of objects in a given association hold important clues about its member's history, and offer a unique opportunity to test the predictions of the various models of stellar formation and evolution. DANCe (standing for Dynamical Analysis of Nearby ClustErs) is a survey program aimed at deriving a comprehensive and homogeneous census of the stellar and substellar content of a number of nearby (<1kpc) young (<500Myr) associations. Whenever possible, members will be identified based on their kinematics properties, ensuring little contamination from background and foreground sources. Otherwise, the dynamics of previously confirmed members will be studied using the proper motion measurements. We present here the method used to derive precise proper motion measurements, using the Pleiades cluster as a test bench. Combining deep wide field multi-epoch panchromatic images obtained at various obervatories over up to 14 years, we derive accurate proper motions for the sources present in the field of the survey. The datasets cover ~80 square degrees, centered around the Seven Sisters. Using new tools, we have computed a catalog of 6116907 unique sources, including proper motion measurements for 3577478 of them. The catalogue covers the magnitude range between i=12~24mag, achieving a proper motion accuracy <1mas/yr for sources as faint as i=22.5mag. We estimate that our final accuracy reaches 0.3mas/yr in the best cases, depending on magnitude, observing history, and the presence of reference extragalactic sources for the anchoring onto the ICRS.Comment: Accepted for publication in A&

Mass-detection of a matter concentration projected near the cluster Abell 1942: Dark clump or high-redshift cluster?

A weak-lensing analysis of wide-field $V$- and $I$-band images centered on the cluster Abell 1942 has uncovered a mass concentration $\sim 7$ arcminutes South of the cluster center. A statistical analysis shows that the detections are highly significant. No strong concentration of bright galaxies is seen at the position of the mass concentration, though a slight galaxy number overdensity and a weak extended X-ray source are present about 1' away from its center. From the spatial dependence of the tangential alignment around the center of the mass concentration, we inferred a lower bound on the mass inside a sphere of radius $0.5 h^{-1}$\ts Mpc of $1\times 10^{14}h^{-1}M_\odot$, much higher than crude mass estimates based on X-ray data. No firm conclusion can be inferred about the nature of the clump. If it were a high-redshift cluster, the weak X-ray flux would indicate that it had an untypically low X-ray luminosity for its mass; if the X-ray emission were physically unrelated to the mass concentration, this conclusion would be even stronger. The search for massive halos by weak lensing enables us for the first time to select halos based on their mass properties only and to detect new types of objects, e.g., dark halos. The mass concentration in the field of A1942 may be the first example of such a halo.Comment: Sumitted to A&A Main Journal. 15 pages, 11 figures. 75 Kb gzipped tar file. Figures with images not included, but available on ftp.iap.fr /pub/from_users/mellier/A1942: a1942darkclump.ps.gz (2.1 Mb

Brown dwarfs in the Pleiades cluster : clues to the substellar mass function

We present the results of a 6.4 square degrees imaging survey of the Pleiades cluster in the I and Z-bands. The survey extends up to 3 degrees from the cluster center and is 90% complete down to I~22. It covers a mass range from 0.03 Msun to 0.48 Msun and yields 40 brown dwarf candidates (BDCs) of which 29 are new. The spatial distribution of BDCs is fitted by a King profile in order to estimate the cluster substellar core radius. The Pleiades mass function is then derived accross the stellar-substellar boundary and we find that, between 0.03 Msun and 0.48 Msun, it is well represented by a single power-law, dN/dM \propto M^{-alpha}, with an index alpha=0.60 +/- 0.11. Over a larger mass domain, however, from 0.03 Msun to 10 Msun, the mass function is better fitted by a log-normal function. We estimate that brown dwarfs represent about 25% of the cluster population which nevertheless makes up less than 1.5% of the cluster mass. The early dynamical evolution of the cluster appears to have had little effect on its present mass distribution at an age of 120 Myr. Comparison between the Pleiades mass function and the Galactic field mass function suggests that apparent differences may be mostly due to unresolved binary systems.Comment: 13 pages, 9 pages, accepted in A&