442 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
Gemini Planet Imager observational calibrations V: astrometry and distortion
We present the results of both laboratory and on sky astrometric characterization of the Gemini Planet Imager (GPI). This characterization includes measurement of the pixel scale* of the integral field spectrograph (IFS), the position of the detector with respect to north, and optical distortion. Two of these three quantities (pixel scale and distortion) were measured in the laboratory using two transparent grids of spots, one with a square pattern and the other with a random pattern. The pixel scale in the laboratory was also estimate using small movements of the artificial star unit (ASU) in the GPI adaptive optics system. On sky, the pixel scale and the north angle are determined using a number of known binary or multiple systems and Solar System objects, a subsample of which had concurrent measurements at Keck Observatory. Our current estimate of the GPI pixel scale is 14.14 ± 0.01 millarcseconds/pixel, and the north angle is -1.00 ± 0.03°. Distortion is shown to be small, with an average positional residual of 0.26 pixels over the field of view, and is corrected using a 5th order polynomial. We also present results from Monte Carlo simulations of the GPI Exoplanet Survey (GPIES) assuming GPI achieves ~1 milliarcsecond relative astrometric precision. We find that with this precision, we will be able to constrain the eccentricities of all detected planets, and possibly determine the underlying eccentricity distribution of widely separated Jovians
High-contrast imaging in the Hyades with snapshot LOCI
To image faint substellar companions obscured by the stellar halo and
speckles, scattered light from the bright primary star must be removed in
hardware or software. We apply the "locally-optimized combination of images"
(LOCI) algorithm to 1-minute Keck Observatory snapshots of GKM dwarfs in the
Hyades using source diversity to determine the most likely PSF. We obtain a
mean contrast of 10^{-2} at 0.01", 10^{-4} at <1", and 10^{-5} at 5". New brown
dwarf and low-mass stellar companions to Hyades primaries are found in a third
of the 84 targeted systems. This campaign shows the efficacy of LOCI on
snapshot imaging as well as on bright wide binaries with off-axis LOCI,
reaching contrasts sufficient for imaging 625-Myr late-L/early-T dwarfs purely
in post-processing.Comment: 12 pages, 12 figures, to appear in SPIE Astronomy 2012, paper
8447-16
Astrometric Monitoring of the HR 8799 Planets: Orbit Constraints from Self-Consistent Measurements
We present new astrometric measurements from our ongoing monitoring campaign
of the HR 8799 directly imaged planetary system. These new data points were
obtained with NIRC2 on the W.M. Keck II 10 meter telescope between 2009 and
2014. In addition, we present updated astrometry from previously published
observations in 2007 and 2008. All data were reduced using the SOSIE algorithm,
which accounts for systematic biases present in previously published
observations. This allows us to construct a self-consistent data set derived
entirely from NIRC2 data alone. From this dataset, we detect acceleration for
two of the planets (HR 8799b and e) at 3. We also assess possible
orbital parameters for each of the four planets independently. We find no
statistically significant difference in the allowed inclinations of the
planets. Fitting the astrometry while forcing coplanarity also returns
consistent to within 1 of the best fit values, suggesting that if
inclination offsets of 20 are present, they are not detectable
with current data. Our orbital fits also favor low eccentricities, consistent
with predictions from dynamical modeling. We also find period distributions
consistent to within 1 with a 1:2:4:8 resonance between all planets.
This analysis demonstrates the importance of minimizing astrometric systematics
when fitting for solutions to highly undersampled orbits.Comment: 18 pages, 11 figures. Accepted for publication in A
A Search for Companions to Brown Dwarfs in the Taurus and Chamaeleon Star Forming Regions
We present the results of a search for companions to young brown dwarfs in
the Taurus and Chamaeleon I star forming regions (1/2-3 Myr). We have used
WFPC2 on board HST to obtain F791W and F850LP images of 47 members of these
regions that have spectral types of M6-L0 (0.01-0.1 Msun). An additional
late-type member of Taurus, FU Tau (M7.25+M9.25), was also observed with
adaptive optics at Keck Observatory. We have applied PSF subtraction to the
primaries and have searched the resulting images for objects that have colors
and magnitudes that are indicative of young low-mass objects. Through this
process, we have identified promising candidate companions to 2MASS
J04414489+2301513 (rho=0.105"/15 AU), 2MASS J04221332+1934392 (rho=0.05"/7 AU),
and ISO 217 (rho=0.03"/5 AU). We reported the discovery of the first candidate
in a previous study, showing that it has a similar proper motion as the primary
through a comparison of astrometry measured with WFPC2 and Gemini adaptive
optics. We have collected an additional epoch of data with Gemini that further
supports that result. By combining our survey with previous high-resolution
imaging in Taurus, Chamaeleon, and Upper Sco (10 Myr), we measure binary
fractions of 14/93 = 0.15+0.05/-0.03 for M4-M6 (0.1-0.3 Msun) and 4/108 =
0.04+0.03/-0.01 for >M6 (10 AU. Given the youth
and low density of these three regions, the lower binary fraction at later
types is probably primordial rather than due to dynamical interactions among
association members. The widest low-mass binaries (>100 AU) also appear to be
more common in Taurus and Chamaeleon than in the field, which suggests that the
widest low-mass binaries are disrupted by dynamical interactions at >10 Myr, or
that field brown dwarfs have been born predominantly in denser clusters where
wide systems are disrupted or inhibited from forming.Comment: Astrophysical Journal, in pres
Direct Detection and Orbit Analysis of the Exoplanets HR 8799 bcd from Archival 2005 Keck/NIRC2 Data
We present previously unpublished July 2005 -band coronagraphic data of
the young, planet-hosting star HR 8799 from the newly-released Keck/NIRC2
archive. Despite poor observing conditions, we detect three of the planetary
companions (HR 8799 bcd), two of them (HR 8799 bc) without advanced image
processing. Comparing these data with previously published 1998-2011 astrometry
and that from re-reduced October 2010 Keck data constrains the orbits of the
planets. Analyzing the planets' astrometry separately, HR 8799 d's orbit is
likely inclined at least 25 from face-on and the others may be on in
inclined orbits. For semimajor axis ratios consistent with a 4:2:1 mean-motion
resonance, our analysis yields precise values for HR 8799 bcd's orbital
parameters and strictly constrains the planets' eccentricities to be less than
0.18--0.3. However, we find no acceptable orbital solutions with this resonance
that place the planets in face-on orbits; HR 8799 d shows the largest deviation
from such orbits. Moreover, few orbits make HR 8799 d coplanar with b and c,
whereas dynamical stability analyses used to constrain the planets' masses
typically assume coplanar and/or face-on orbits. This paper illustrates the
significant science gain enabled with the release of the NIRC2 archive.Comment: 7 pages, 3 figures, ApJ Letters in pres
The International Deep Planet Survey II: The frequency of directly imaged giant exoplanets with stellar mass
Radial velocity and transit methods are effective for the study of short
orbital period exoplanets but they hardly probe objects at large separations
for which direct imaging can be used. We carried out the international deep
planet survey of 292 young nearby stars to search for giant exoplanets and
determine their frequency. We developed a pipeline for a uniform processing of
all the data that we have recorded with NIRC2/Keck II, NIRI/Gemini North,
NICI/Gemini South, and NACO/VLT for 14 years. The pipeline first applies
cosmetic corrections and then reduces the speckle intensity to enhance the
contrast in the images. The main result of the international deep planet survey
is the discovery of the HR 8799 exoplanets. We also detected 59 visual multiple
systems including 16 new binary stars and 2 new triple stellar systems, as well
as 2,279 point-like sources. We used Monte Carlo simulations and the Bayesian
theorem to determine that 1.05[+2.80-0.70]% of stars harbor at least one giant
planet between 0.5 and 14M_J and between 20 and 300 AU. This result is obtained
assuming uniform distributions of planet masses and semi-major axes. If we
consider power law distributions as measured for close-in planets instead, the
derived frequency is 2.30[+5.95-1.55]%, recalling the strong impact of
assumptions on Monte Carlo output distributions. We also find no evidence that
the derived frequency depends on the mass of the hosting star, whereas it does
for close-in planets. The international deep planet survey provides a database
of confirmed background sources that may be useful for other exoplanet direct
imaging surveys. It also puts new constraints on the number of stars with at
least one giant planet reducing by a factor of two the frequencies derived by
almost all previous works.Comment: 83 pages, 13 figures, 15 Tables, accepted in A&
- …