33 research outputs found
Constraints on the formation mechanism of the planetary mass companion of 2MASS 1207334-393254
In this paper we discuss the nature and the possible formation scenarios of
the companion of the brown dwarf 2MASS 1207334-393254. We initially discuss the
basic physical properties of this object and conclude that, although from its
absolute mass (), it is a planetary object, in terms of its mass
ratio and of its separation with respect to the primary brown dwarf, it
is consistent with the statistical properties of binaries with higher primary
mass. We then explore the possible formation mechanism for this object. We show
that the standard planet formation mechanism of core accretion is far too slow
to form this object within 10 Myr, the observed age of the system. On the other
hand, the alternative mechanism of gravitational instability (proposed both in
the context of planet and of binary formation) may, in principle, work and form
a system with the observed properties.Comment: 5 pages, MNRAS in pres
The dependence of the sub-stellar IMF on the initial conditions for star formation
Abridged: We have undertaken a series of hydrodynamical simulations of
multiple star formation in small turbulent molecular clouds. Our goal is to
determine the sensitivity of the properties of the resulting stars and brown
dwarfs to variations in the initial conditions imposed. In this paper we report
on the results obtained by applying two different initial turbulent velocity
fields. The slope of the turbulent power-law spectrum alpha is set to -3 in
half of the calculations and to -5 in the other half. We find that, whereas the
stellar mass function seems to only be weakly dependent on the value of alpha,
the sub-stellar mass function turns out to be more sensitive to the initial
slope of the velocity field. We argue that, since the role of turbulence is to
create substructure from which gravitational instabilities may grow, variations
in other initial conditions that also determine the fragmentation process are
likely to affect the shape of the sub-stellar mass function as well. The
absence of many planetary mass `free-floaters' in our simulations, especially
in the mass range 1-10 MJ, suggests that, if these objects are abundant, they
are likely to form by similar mechanisms to those thought to operate in
quiescent accretion discs, instead of via instabilities in gravitationally
unstable discs. Finally, we find that multiple and single stars share
comparable kinematical properties, both populations being able to attain
velocities in the range 1-10 km/s. From these values we draw the conclusion
that only low-mass star-forming regions such as Taurus-Auriga or Ophiuchus,
where the escape speed is low, might have suffered some depletion of its single
and binary stellar population.Comment: 13 pages, 6 figures, accepted by MNRA
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
A Hubble Space Telescope ACS Search for Brown Dwarf Binaries in the Pleiades Open Cluster
We present the results of a high-resolution imaging survey for brown dwarf
binaries in the Pleiades open cluster. The observations were carried out with
the Advance Camera for Surveys onboard the Hubble Space Telescope. Our sample
consists of 15 bona-fide brown dwarfs. We confirm 2 binaries and detect their
orbital motion, but we did not resolve any new binary candidates in the
separation range between 5.4AU and 1700AU and masses in the range
0.035--0.065~Msun. Together with the results of our previous study (Martin et
al., 2003), we can derive a visual binary frequency of 13.3\%
for separations greater than 7~AU masses between 0.055--0.065~M_{\sun} and
mass ratios between 0.45--0.91.0. The other observed properties of
Pleiades brown dwarf binaries (distributions of separation and mass ratio)
appear to be similar to their older counterparts in the field.Comment: 29 pages, 7 figures, 6 tables, accepted for publication in Ap
On the properties of young multiple stars
Abridged/ We present numerical results on the properties of young binary and
multiple stellar systems. Our analysis is based on a series of SPH + Nbody
simulations of the fragmentation of small molecular clouds, that fully resolve
the opacity limit for fragmentation. We have produced a statistically
significant number of stable multiple systems, with components separations in
the range 1-1000 AU. At the end of the hydrodynamical evolution (0.5 Myr) we
find that ~60% of stars and brown dwarfs are members of multiples systems, with
about a third of these being low mass, weakly bound outliers in wide eccentric
orbits. Our results imply that in the stellar regime most stars are in
multiples (~80%) and that this fraction is an increasing function of primary
mass. After Nbody integration to 10.5 Myr, the percentage of bound objects has
dropped to ~40%, as most very low mass stars and brown dwarfs have been
released to the field. Brown dwarfs are never found to be very close companions
to stars (brown dwarf desert at very small separations), but one case exists of
a brown dwarf companion at intermediate separations (10 AU). Our simulations
can accommodate the existence of brown dwarf companions at large separations,
but only if the primaries of these systems are themselves multiples. We have
compared the outcome of our simulations with the properties of real stellar
systems as deduced from the IR CM diagram of the Praesepe cluster and from
spectroscopic and high-resolution imaging surveys of young clusters and the
field.Comment: 14 pages, 4 figures, 1 table, accepted by MNRA
17 new very low-mass members in Taurus. The brown dwarf deficit revisited
Recent studies of the substellar population in the Taurus cloud have revealed
a deficit of brown dwarfs (BD) compared to the Trapezium cluster population
(Briceno et al 1998; Luhman 2000; Luhman et al 2003a; Luhman 2004). However,
these works have concentrated on the highest stellar density regions of the
Taurus cloud. We have performed a large scale optical survey of this region,
covering a total area of 30 deg^2, and encompassing the densest part of the
cloud as well as their surroundings, down to a mass detection limits of 15
Jupiter Masses (MJ). In this paper, we present the optical spectroscopic
follow-up observations of 97 photometrically selected potential new low-mass
Taurus members, of which 27 are strong late-M (SpT < M4V) candidates. These
observations reveal 5 new very low mass (VLM) Taurus members and 12 new BDs.
Combining our observations with previously published results, we derive an
updated substellar to stellar ratio in Taurus of Rss =0.23 +/- 0.05. This ratio
now appears consistent with the value previously derived in the Trapezium
cluster under similar assumptions of 0.26 +/- 0.04. We find strong indication
that the relative numbers of BDs with respect to stars is decreased by a factor
2 in the central regions of the aggregates with respect to the more distributed
population. Our findings are best explained in the context of the
embryo-ejection model where brown dwarfs originate from dynamical interactions
in small N unstable multiple systems.Comment: 20 pages, 15 figure
A young binary Brown Dwarf in the R-CrA star formation region
We present imaging and spectroscopic observations with HST (WFPC2, ACS/HRC
and STIS), VLT (FORS2) and Keck (HIRES) of the dM8 ultra-cool dwarf DENIS-P
J185950.9-370632, located in the R-CrA region. The presence of lithium
absorption at 670.8 nm and the strong Halpha emission indicate a young age and
a sub-stellar mass. Our diffraction-limited images resolve a companion at the
separation limit of HST/ACS (~0.06"). The 2.1 mJy flux in the LW2 filter
(5.0-8.5 microns) of the Infrared Space Observatory (see 1999A&A...350..883)
likely corresponds to an infrared excess, suggesting the presence of
circumstellar material. Proper motion and photometric measurements, as well as
the Halpha activity, confirm membership in the R-CrA star forming region. If
confirmed by further observations, DENIS-P J185950.9-370632 would be the first
accreting sub-stellar multiple system observed to date.Comment: 18 pages, 11 figures, 3 tables Version with full resolution figures
available on: ftp://ftp.mpe.mpg.de/people/hbouy/publications/DENIS1859.ps.g
Dwarfs after Mergers? The case of NGC520, NGC772, Arp141, NGC3226/7, NGC3656 and Arp299
We present results from a survey of dwarf galaxy candidates in the vicinity
of strongly interacting galaxies. The goal of the survey was a test of the
hypothesis that massive condensations of stars and HI in tidal tails of large
interacting galaxies may be a significant source of independent,
self-gravitating dwarf galaxies. These so called tidal dwarf galaxies (TDG) can
be expected to resemble the blue luminous knots found in tidal tails, but also
might appear as redder, evolved systems if formed much before they are being
observed. For the present study, a homogeneous subsample of 6 fields was
selected from the catalog of extended objects in fields around 15 strongly
interacting galaxies by Deeg et al. (1998). Criteria for the subsample were:
similar redshifts of the central interacting galaxies, and photometric
completeness of the extended objects in V and R. The number density of these
TDG candidates was compared with expected background galaxy densities. Within
the statistical errors, background galaxies account for most, if not all of the
extended objects. There is no evidence for a substantial locally formed dwarf
galaxy population. Thus, we conclude that field galaxy-galaxy interactions are
likely to result in the formation of only a few long-lived TDG.Comment: 9 pages, 4 figures (1 colour), 3 tables. Accepted by A&
The Most Metal-Poor Quadruple System of Subdwarfs G89-14
The system of subdwarfs G89-14 is one of the most metal-poor multiple stars
with an atmospheric metal abundance . Speckle
interferometry at the 6-m BTA telescope has revealed that G89-14 consists of
four components. Measurements of the magnitude difference between the
components and published data have allowed their masses to be estimated:
, , , and . The ratio of the orbital periods
of the subsystems has been obtained, 0.52 yr : 3 000 yr : 650 000 yr
(1:5769:1250000), indicative of a high degree of hierarchy of G89-14 and its
internal dynamical stability. The calculated Galactic orbital elements and the
low metallicity of the quadruple system suggest that it belongs to the Galactic
halo.Comment: 5 pages, 3 figure