440 research outputs found
The lower mass function of the young open cluster Blanco 1: from 30 M_(Jup) to 3 M_â
Aims. We performed a deep wide field optical survey of the young (~100â150 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 â z â 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 â 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 0.03â0.6 M_â mass range, including 30â40 brown dwarfs. The candidateâs 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_â
and 0.6 M_â, 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_â to 3 M_â, the mass function is better fitted by a log-normal function with m_0 = 0.36 ± 0.07 M_â and Ï = 0.58 ± 0.06.
Conclusions. Comparison between the Blanco 1 mass function, other young open clustersâ 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 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 - and -band images centered on
the cluster Abell 1942 has uncovered a mass concentration 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 \ts Mpc of , 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&
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