585 research outputs found
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
Spectroscopy of brown dwarf candidates in IC 348 and the determination of its substellar IMF down to planetary masses
Context. Brown dwarfs represent a sizable fraction of the stellar content of
our Galaxy and populate the transition between the stellar and planetary mass
regime. There is however no agreement on the processes responsible for their
formation. Aims. We have conducted a large survey of the young, nearby cluster
IC 348, to uncover its low-mass brown dwarf population and study the cluster
properties in the substellar regime. Methods. Deep optical and near-IR images
taken with MegaCam and WIRCam at the Canada-France-Hawaii Telescope (CFHT) were
used to select photometric candidate members. A spectroscopic follow-up of a
large fraction of the candidates was conducted to assess their youth and
membership. Results. We confirmed spectroscopically 16 new members of the IC
348 cluster, including 13 brown dwarfs, contributing significantly to the
substellar census of the cluster, where only 30 brown dwarfs were previously
known. Five of the new members have a L0 spectral type, the latest-type objects
found to date in this cluster. At 3 Myr, evolutionary models estimate these
brown dwarfs to have a mass of ~13 Jupiter masses. Combining the new members
with previous census of the cluster, we constructed the IMF complete down to 13
Jupiter masses. Conclusions. The IMF of IC 348 is well fitted by a log-normal
function, and we do not see evidence for variations of the mass function down
to planetary masses when compared to other young clusters.Comment: Accepted to A&A (8 November 2012
Discovery of an extended debris disk around the F2V star HD 15745
Using the Advanced Camera for Surveys aboard the Hubble Space Telescope, we
have discovered dust-scattered light from the debris disk surrounding the F2V
star HD 15745. The circumstellar disk is detected between 2.0" and 7.5" radius,
corresponding to 128 - 480 AU radius. The circumstellar disk morphology is
asymmetric about the star, resembling a fan, and consistent with forward
scattering grains in an optically thin disk with an inclination of ~67 degrees
to our line of sight. The spectral energy distribution and scattered light
morphology can be approximated with a model disk composed of silicate grains
between 60 and 450 AU radius, with a total dust mass of 10E-7 M_sun (0.03
M_earth) representing a narrow grain size distribution (1 - 10 micron).
Galactic space motions are similar to the Castor Moving Group with an age of
~10E+8 yr, although future work is required to determine the age of HD 15745
using other indicators.Comment: 7 pages, 4 figures, ApJ Letters, in pres
Is stellar multiplicity universal? Tight stellar binaries in the Orion nebula Cluster
We present a survey for the tightest visual binaries among 0.3–2 M⊙ members of the Orion nebula Cluster (ONC). Among 42 targets, we discovered 13 new 0.025–0.15 arcsec companions. Accounting for the Branch bias, we find a companion star fraction (CSF) in the 10–60 au range of 21
+8−5
per cent, consistent with that observed in other star-forming regions (SFRs) and twice as high as among field stars; this excess is found with a high level of confidence. Since our sample is dominated by disc-bearing targets, this indicates that disc disruption by close binaries is inefficient, or has not yet taken place, in the ONC. The resulting separation distribution in the ONC drops sharply outside 60 au. These findings are consistent with a scenario in which the initial multiplicity properties, set by the star formation process itself, are identical in the ONC and in other SFRs and subsequently altered by the cluster’s dynamical evolution. This implies that the fragmentation process does not depend on the global properties of a molecular cloud, but on the local properties of prestellar cores, and that the latter are self-regulated to be nearly identical in a wide range of environments. These results, however, raise anew the question of the origin of field stars as the tight binaries we have discovered will not be destroyed as the ONC dissolves into the Galactic field. It thus appears that most field stars formed in regions that differ from well-studied SFRs in the solar neighbourhood, possibly due to changes in core fragmentation on Gyr time-scales
Medium-separation binaries do not affect the first steps of planet formation
The first steps of planet formation are marked by the growth and
crystallization of sub-micrometer-sized dust grains accompanied by dust
settling toward the disk midplane. In this paper we explore whether the first
steps of planet formation are affected by the presence of medium-separation
stellar companions. We selected two large samples of disks around single and
binary T Tauri stars in Taurus that are thought to have only a modest age
spread of a few Myr. The companions of our binary sample are at projected
separations between 10 and 450 AU with masses down to about 0.1 solar masses.
We used the strength and shape of the 10 micron silicate emission feature as a
proxy for grain growth and for crystallization respectively. The degree of dust
settling was evaluated from the ratio of fluxes at two different mid-infrared
wavelengths. We find no statistically significant difference between the
distribution of 10 micron silicate emission features from single and binary
systems. In addition, the distribution of disk flaring is indistinguishable
between the single and binary system samples. These results show that the first
steps of planet formation are not affected by the presence of a companion at
tens of AU.Comment: To appear in the Astrophysical Journa
Mid-Infrared Observations of Class I/Flat-Spectrum Systems in Six Nearby Molecular Clouds
We have obtained new mid-infrared observations of 65 Class I/Flat-Spectrum
(F.S.) objects in the Perseus, Taurus, Chamaeleon I/II, Rho Ophiuchi, and
Serpens dark clouds. We detected 45/48 (94%) of the single sources, 16/16
(100%) of the primary components, and 12/16 (75%) of the secondary/triple
components of the binary/multiple objects surveyed. The composite spectral
energy distributions (SEDs) for all of our sample sources are either Class I or
F.S., and, in 15/16 multiple systems, at least one of the individual components
displays a Class I or F.S. spectral index. However, the occurrence of mixed
pairings, such as F.S. with Class I, F.S. with Class II, and, in one case, F.S.
with Class III, is surprisingly frequent. Such behaviour is not consistent with
that of multiple systems among T Tauri stars (TTS), where the companion of a
classical TTS also tends to be a classical TTS, although other mixed pairings
have been previously observed among Class II objects. Based on an analysis of
the spectral indices of the individual binary components, there appears to be a
higher proportion of mixed Class I/F.S. systems (65-80%) than that of mixed
Classical/Weak-Lined TTS (25-40%), demonstrating that the envelopes of Class I/
F.S. systems are rapidly evolving during this evolutionary phase. We report the
discovery of a steep spectral index secondary companion to ISO-ChaI 97,
detected for the first time via our mid-infrared observations. In our previous
near- infrared imaging survey of binary/multiple Class I/F.S. sources, ISO-ChaI
97 appeared to be single. With a spectral index of Alpha >= 3.9, the secondary
component of this system is a member of a rare class of very steep spectral
index objects, those with Alpha > 3. Only three such objects have previously
been reported, all of which are either Class 0 or Class I.Comment: 31 pages, 4 figures, 6 table
Keck Adaptive Optics Imaging of Nearby Young Stars: Detection of Close Multiple Systems
Using adaptive optics on the Keck II 10-meter telescope on Mauna Kea, we have
surveyed 24 of the nearest young stars known in search of close companions. Our
sample includes members of the MBM 12 and TW Hydrae young associations and the
classical T Tauri binary UY Aurigae in the Taurus star-forming region. We
present relative photometry and accurate astrometry for 10 close multiple
systems. The multiplicity frequency in the TW Hydrae and MBM 12 groups are high
in comparison to other young regions, though the significance of this result is
low because of the small number statistics. We resolve S 18 into a triple
system including a tight 63 mas (projected separation of 17 AU at a distance of
275 pc) binary for the first time, with a hierarchical configuration
reminiscent of VW Chamaeleontis and T Tauri. Another tight binary in our sample
-- TWA 5Aab (54 mas or 3 AU at 55 pc) -- offers the prospect of dynamical mass
measurement using astrometric observations within a few years, and thus could
be important for testing pre-main sequence evolutionary models. Our
observations confirm with 9-sigma confidence that the brown dwarf TWA 5B is
bound to TWA 5A. We find that the flux ratio of UY Aur has changed
dramatically, by more than a magnitude in the H-band, possibly as a result of
variable extinction. With a smaller flux ratio, the system may once again
become detectable as an optical binary, as it was at the time of its discovery
in 1944. Taken together, our results demonstrate that adaptive optics on large
telescopes is a powerful tool for detecting tight companions, and thus
exploring the frequency and configurations of close multiple systems.Comment: accepted for publication in The Astronomical Journa
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