487 research outputs found
The substellar mass function in the central region of the open cluster Praesepe from deep LBT observations
Studies of the mass function (MF) of open clusters of different ages allow us
to probe the efficiency with which brown dwarfs (BDs) are evaporated from
clusters to populate the field. Surveys in old clusters (age > 100 Myr) do not
suffer so severely from several problems encountered in young clusters, such as
intra-cluster extinction and large uncertainties in BD models. Here we present
the results of a deep photometric survey to study the MF of the old open
cluster Praesepe (age 590 Myr and distance 190 pc), down to a 5 sigma detection
limit at i~25.6 mag (~40M_Jup). We identify 62 cluster member candidates, of
which 40 are substellar, from comparison with predictions from a dusty
atmosphere model. The MF rises from the substellar boundary until ~60M_Jup and
then declines. This is quite different from the form inferred for other open
clusters older than 50 Myr, but seems to be similar to those found in very
young open cluster, whose MFs peak at ~10M_Jup. Either Praesepe really does
have a different MF from other clusters or they had similar initial MFs but
have differed in their dynamical evolution. We further have identified six
foreground T dwarf candidates towards Praesepe, which require follow-up
spectroscopy to confirm their nature.Comment: 8 pages, 5 figures, to appear in the online proceedings of the Cool
Stars 16 conferenc
White dwarf masses derived from planetary nebulae modelling
We compare the mass distribution of central stars of planetary nebulae (CSPN)
with those of their progeny, white dwarfs (WD). We use a dynamical method to
measure masses with an uncertainty of 0.02 M. The CSPN mass
distribution is sharply peaked at . The WD distribution peaks
at lower masses () and shows a much broader range of masses.
Some of the difference can be explained if the early post-AGB evolution is
faster than predicted by the Bl\"ocker tracks. Between 30 and 50 per cent of WD
may avoid the PN phase because of too low mass. However, the discrepancy cannot
be fully resolved and WD mass distributions may have been broadened by
observational or model uncertainties.Comment: 4 pages, accepted for A&A Letter
Search for free-floating planetary-mass objects in the Pleiades
(Abridged) We aim at identifying the least massive population of the solar
metallicity, young (120 Myr), nearby (133.5 pc) Pleiades star cluster with the
ultimate goal of understanding the physical properties of intermediate-age,
free-floating, low-mass brown dwarfs and giant planetary-mass objects, and
deriving the cluster substellar mass function across the deuterium-burning mass
limit at ~0.012 Msol. We performed a deep photometric and astrometric J- and
H-band survey covering an area of ~0.8 deg^2. The images with completeness and
limiting magnitudes of J,H ~ 20.2 and ~ 21.5 mag were acquired ~9 yr apart
(proper motion precision of +/-6 mas/yr). J- and H-band data were complemented
with Z, K, and mid-infrared magnitudes up to 4.6 micron coming from UKIDSS,
WISE, and follow-up observations of our own. Pleiades member candidates were
selected to have proper motions compatible with that of the cluster, and colors
following the known Pleiades sequence in the interval J = 15.5-8.8 mag, and
Z_UKIDSS - J > 2.3 mag or Z nondetections for J > 18.8 mag. We found a neat
sequence of astrometric and photometric Pleiades substellar member candidates
in the intervals J = 15.5-21.2 mag and ~0.072-0.008 Msol. The faintest objects
show very red near- and mid-infrared colors exceeding those of field
high-gravity dwarfs by >0.5 mag. The Pleiades photometric sequence does not
show any color turn-over because of the presence of photospheric methane
absorption down to J = 20.3 mag, which is about 1 mag fainter than predicted by
the color-computed models. Pleiades brown dwarfs have a proper motion
dispersion of 6.4-7.5 mas/yr and are dynamically relaxed at the age of the
cluster. The Pleiades mass function extends down to the deuterium burning-mass
threshold, with a slope fairly similar to that of other young star clusters and
stellar associations.Comment: Accepted for publication in A&A. 16 page
Ligand design and preparation, photophysical properties, and device performance of an encapsulated-type pseudo-tris(heteroleptic) iridium(iii) emitter
The organic molecule 2-(1-phenyl-1-(pyridin-2-yl)ethyl)-6-(3-(1-phenyl-1-(pyridin-2-yl)ethyl)phenyl)pyridine (H3L) has been designed, prepared, and employed to synthesize the encapsulated-type pseudo-tris(heteroleptic) iridium(III) derivative Ir(Îș6-fac-C,CâČ,Câł-fac-N,NâČ,Nâł-L). Its formation takes place as a result of the coordination of the heterocycles to the iridium center and the ortho-CH bond activation of the phenyl groups. Dimer [Ir(ÎŒ-Cl)(η4-COD)]2 is suitable for the preparation of this compound of class [Ir(9h)] (9h = 9-electron donor hexadentate ligand), but Ir(acac)3 is a more appropriate starting material. Reactions were carried out in 1-phenylethanol. In contrast to the latter, 2-ethoxyethanol promotes the metal carbonylation, inhibiting the full coordination of H3L. Complex Ir(Îș6-fac-C,CâČ,Câł-fac-N,NâČ,Nâł-L) is a phosphorescent emitter upon photoexcitation, which has been employed to fabricate four yellow emitting devices with 1931 CIE (x:y) ⌠(0.52:0.48) and a maximum wavelength at 576 nm. These devices display luminous efficacies, external quantum efficiencies, and power efficacies at 600 cd mâ2, which lie in the ranges 21.4â31.3 cd Aâ1, 7.8â11.3%, and 10.2â14.1 lm W1â, respectively, depending on the device configuration
A Constraint on brown dwarf formation via ejection: radial variation of the stellar and substellar mass function of the young open cluster IC2391
Using the Wide Field Imager (WFI) at the ESO 2.2m telescope at La Silla and
the CPAPIR camera at the CTIO 1.5m telescope at Cerro Tololo, we have performed
an extensive, multiband photometric survey of the open cluster IC2391 (D~146pc,
age~50Myr, solar metallicity). Here we present the results from our photometric
survey and from a spectroscopic follow-up of the central part of the survey.Comment: 4 pages, 3 figures, to appear in the proceedings of the Cool Stars 15
conferenc
Brown dwarfs and very low mass stars in the Praesepe open cluster: a dynamically unevolved mass function?
[Abridged] In this paper, we present the results of a photometric survey to
identify low mass and brown dwarf members of the old open cluster Praesepe (age
of 590[+150][-120]Myr and distance of 190[+6.0][-5.8]pc) and use this to infer
its mass function which we compare with that of other clusters. We have
performed an optical (Ic-band) and near-infrared (J and Ks-band) photometric
survey of Praesepe with a spatial coverage of 3.1deg^2. With 5sigma detection
limits of Ic=23.4 and J=20.0, our survey is sensitive to objects with masses
from about 0.6 to 0.05Msol. The mass function of Praesepe rises from 0.6Msol
down to 0.1Msol and then turns-over at ~0.1Msol. The rise observed is in
agreement with the mass function derived by previous studies, including a
survey based on proper motion and photometry. Comparing our mass function with
that for another open cluster with a similar age, the Hyades (age ~ 600Myr), we
see a significant difference. Possible reasons are that dynamical evaporation
has not influenced the Hyades and Praesepe in the same way, or that the
clusters did not have the same initial mass function, or that dynamical
interactions have modified the evolution of one or both clusters. Although a
difference in the binary fractions of the clusters could cause the observed
(i.e. system) mass functions to differ, measurements in the literature give no
evidence for a significant difference in the binary fractions of the two
clusters. Of our cluster candidates, six have masses predicted to be equal to
or below the stellar/substellar boundary at 0.072Msol.Comment: 11 pages, 11 figures, accepted for publication in A&A. Higher
resolution of Figures 2-3-4-5 in A&A published version. Revised version
corrected for Englis
New Insights into the Problem of the Surface Gravity Distribution of Cool DA White Dwarfs
We review at length the longstanding problem in the spectroscopic analysis of
cool hydrogen-line (DA) white dwarfs (Teff < 13,000 K) where gravities are
significantly higher than those found in hotter DA stars. The first solution
that has been proposed for this problem is a mild and systematic helium
contamination from convective mixing that would mimic the high gravities. We
constrain this scenario by determining the helium abundances in six cool DA
white dwarfs using high-resolution spectra from the Keck I 10-m telescope. We
obtain no detections, with upper limits as low as He/H = 0.04 in some cases.
This allows us to put this scenario to rest for good. We also extend our model
grid to lower temperatures using improved Stark profiles with non-ideal gas
effects from Tremblay & Bergeron and find that the gravity distribution of cool
objects remains suspiciously high. Finally, we find that photometric masses
are, on average, in agreement with expected values, and that the high-log g
problem is so far unique to the spectroscopic approach.Comment: 44 pages, 14 figures, accepted for publication in the Astrophysical
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