211 research outputs found
Young and embedded clusters in Cygnus-X: evidence for building up the IMF?
We provide a new view on the Cygnus-X north complex by accessing for the
first time the low mass content of young stellar populations in the region.
CFHT/WIRCam camera was used to perform a deep near-IR survey of this complex,
sampling stellar masses down to ~0.1 M. Several analysis tools,
including a extinction treatment developed in this work, were employed to
identify and uniformly characterise a dozen unstudied young star clusters in
the area. Investigation of their mass distributions in low-mass domain revealed
a relatively uniform log-normal IMF with a characteristic mass of 0.320.08
M and mass dispersion of 0.400.06. In the high mass regime, their
derived slopes showed that while the youngest clusters (age < 4 Myr) presented
slightly shallower values with respect to the Salpeter's, our older clusters (4
Myr < age < 18 Myr) showed IMF compliant values and a slightly denser stellar
population. Although possibly evidencing a deviation from an 'universal' IMF,
these results also supports a scenario where these gas dominated young clusters
gradually 'build up' their IMF by accreting low-mass stars formed in their
vicinity during their first ~3 Myr, before the gas expulsion phase, emerging at
the age of ~4 Myr with a fully fledged IMF. Finally, the derived distances to
these clusters confirmed the existence of at least 3 different star forming
regions throughout Cygnus-X north complex, at distances of 500-900 pc, 1.4-1.7
kpc and 3.0 kpc, and revealed evidence of a possible interaction between some
of these stellar populations and the Cygnus-OB2 association.Comment: 20 pages, 19 figures. Contains an appendix with 10 extra figure
The 0.03-10Mo mass function of young open clusters
We report the present day mass functions (PDMFs) of 3 young open clusters
over a mass range from 30 Jupiter masses to 10~\msunn. The PDMFs of the 3
clusters are remarkably similar, suggesting little impact of specific
conditions (stellar density, metallicity, early dynamical evolution) on the
mass distribution. Functional forms are provided to allow quantitative
comparison with MFs derived in other environments.Comment: 6 pages, to appear in "IMF@50", Corbelli, Palla, Zinnecker ed
The relation between accretion rates and the initial mass function in hydrodynamical simulations of star formation
We analyse a hydrodynamical simulation of star formation. Sink particles in
the simulations which represent stars show episodic growth, which is presumably
accretion from a core that can be regularly replenished in response to the
fluctuating conditions in the local environment. The accretion rates follow
, as expected from accretion in a gas-dominated
potential, but with substantial variations over-laid on this. The growth times
follow an exponential distribution which is tapered at long times due to the
finite length of the simulation. The initial collapse masses have an
approximately lognormal distribution with already an onset of a power-law at
large masses. The sink particle mass function can be reproduced with a
non-linear stochastic process, with fluctuating accretion rates , a distribution of seed masses and a distribution of growth times. All
three factors contribute equally to the form of the final sink mass function.
We find that the upper power law tail of the IMF is unrelated to Bondi-Hoyle
accretion.Comment: 13 pages, 13 figures, MNRAS accepte
A deep, wide-field search for substellar members in NGC 2264
We report the first results of our ongoing campaign to discover the first
brown dwarfs (BD) in NGC 2264, a young (3 Myr), populous star forming region
for which our optical studies have revealed a very high density of potential
candidates - 236 in 1 deg - from the substellar limit down to at least
20 M for zero reddening. Candidate BD were first selected
using wide field () band imaging with CFH12K, by reference to current
theoretical isochrones. Subsequently, 79 (33%) of the sample were found
to have near-infrared 2MASS photometry ( 0.3 mag. or better),
yielding dereddened magnitudes and allowing further investigation by comparison
with the location of NextGen and DUSTY isochrones in colour-colour and
colour-magnitude diagrams involving various combinations of ,, and
. We discuss the status and potential substellarity of a number of
relatively unreddened (A 5) likely low-mass members in our
sample, but in spite of the depth of our observations in , we are as yet
unable to unambiguously identify substellar candidates using only 2MASS data.
Nevertheless, there are excellent arguments for considering two faint (observed
18.4 and 21.2) objects as cluster candidates with masses
respectively at or rather below the hydrogen burning limit. More current
candidates could be proven to be cluster members with masses around 0.1
M {\it via} gravity-sensitive spectroscopy, and deeper near-infrared
imaging will surely reveal a hitherto unknown population of young brown dwarfs
in this region, accessible to the next generation of deep near-infrared
surveys.Comment: 10 pages, 12 figures, accepted by 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&
Substellar multiplicity in the Hyades cluster
We present the first high-angular resolution survey for multiple systems
among very low-mass stars and brown dwarfs in the Hyades open cluster. Using
the Keck\,II adaptive optics system, we observed a complete sample of 16
objects with estimated masses 0.1 Msun. We have identified three
close binaries with projected separation 0.11", or 5 AU. A
number of wide, mostly faint candidate companions are also detected in our
images, most of which are revealed as unrelated background sources based on
astrometric and/or photometric considerations. The derived multiplicity
frequency, 19+13/-6 % over the 2-350 AU range, and the rarity of systems wider
than 10 AU are both consistent with observations of field very low-mass
objects. In the limited 3-50 AU separation range, the companion frequency is
essentially constant from brown dwarfs to solar-type stars in the Hyades
cluster, which is also in line with our current knowledge for field stars.
Combining the binaries discovered in this surveys with those already known in
the Pleiades cluster reveals that very low-mass binaries in open clusters, as
well as in star-forming regions, are skewed toward lower mass ratios () than are their field counterparts, a result that
cannot be accounted for by selection effects. Although the possibility of
severe systematic errors in model-based mass estimates for very low-mass stars
cannot be completely excluded, it is unlikely to explain this difference. We
speculate that this trend indicates that surveys among very low-mass field
stars may have missed a substantial population of intermediate mass ratio
systems, implying that these systems are more common and more diverse than
previously thought.Comment: Accepted for publication in Astronomy & Astrophysics; 11 pages, 6
figure
A photometric and astrometric investigation of the brown dwarfs in Blanco 1
We present the results of a photometric and astrometric study of the low mass
stellar and substellar population of the young open cluster Blanco 1. We have
exploited J band data, obtained recently with the Wide Field Camera (WFCAM) on
the United Kingdom InfraRed Telescope (UKIRT), and 10 year old I and z band
optical imaging from CFH12k and Canada France Hawaii Telescope (CFHT), to
identify 44 candidate low mass stellar and substellar members, in an area of 2
sq. degrees, on the basis of their colours and proper motions. This sample
includes five sources which are newly discovered. We also confirm the lowest
mass candidate member of Blanco 1 unearthed so far (29MJup). We determine the
cluster mass function to have a slope of alpha=+0.93, assuming it to have a
power law form. This is high, but nearly consistent with previous studies of
the cluster (to within the errors), and also that of its much better studied
northern hemisphere analogue, the Pleiades.Comment: 8 Pages, 5 Figures, 2 Tables and 1 Appendix. Accepted for publication
in MNRA
The low-mass population of the Rho Ophiuchi molecular cloud
Star formation theories are currently divergent regarding the fundamental
physical processes that dominate the substellar regime. Observations of nearby
young open clusters allow the brown dwarf (BD) population to be characterised
down to the planetary mass regime, which ultimately must be accommodated by a
successful theory. We hope to uncover the low-mass population of the Rho
Ophiuchi molecular cloud and investigate the properties of the newly found
brown dwarfs. We use near-IR deep images (reaching completeness limits of
approximately 20.5 mag in J, and 18.9 mag in H and Ks) taken with the Wide
Field IR Camera (WIRCam) at the Canada France Hawaii Telescope (CFHT) to
identify candidate members of Rho Oph in the substellar regime. A spectroscopic
follow-up of a small sample of the candidates allows us to assess their
spectral type, and subsequently their temperature and membership. We select 110
candidate members of the Rho Ophiuchi molecular cloud, from which 80 have not
previously been associated with the cloud. We observed a small sample of these
and spectroscopically confirm six new brown dwarfs with spectral types ranging
from M6.5 to M8.25
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
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