51 research outputs found
Giant planet migration, disk evolution, and the origin of transitional disks
We present models of giant planet migration in evolving protoplanetary disks.
Our disks evolve subject to viscous transport of angular momentum and
photoevaporation, while planets undergo Type II migration. We use a Monte Carlo
approach, running large numbers of models with a range in initial conditions.
We find that relatively simple models can reproduce both the observed radial
distribution of extra-solar giant planets, and the lifetimes and accretion
histories of protoplanetary disks. The use of state-of-the-art photoevaporation
models results in a degree of coupling between planet formation and disk
clearing, which has not been found previously. Some accretion across planetary
orbits is necessary if planets are to survive at radii <~1.5AU, and if planets
of Jupiter mass or greater are to survive in our models they must be able to
form at late times, when the disk surface density in the formation region is
low. Our model forms two different types of "transitional" disks, embedded
planets and clearing disks, which show markedly different properties. We find
that the observable properties of these systems are broadly consistent with
current observations, and highlight useful observational diagnostics. We
predict that young transition disks are more likely to contain embedded giant
planets, while older transition disks are more likely to be undergoing disk
clearing.Comment: 13 pages, 9 figures. Accepted for publication in Ap
Mapping the Shores of the Brown Dwarf Desert III: Young Moving Groups
We present the results of an aperture masking interferometry survey for
substellar companions around 67 members of the young (~8-200Myr) nearby
(~5-86pc) AB Doradus, Beta Pictoris, Hercules-Lyra, TW Hya, and
Tucana-Horologium stellar associations. Observations were made at near infrared
wavelengths between 1.2-3.8 microns using the adaptive optics facilities of the
Keck II, VLT UT4, and Palomar Hale Telescopes. Typical contrast ratios of
~100-200 were achieved at angular separations between ~40-320mas, with our
survey being 100% complete for companions with masses below 0.25\msolar across
this range. We report the discovery of a \msolar companion to
HIP14807, as well as the detections and orbits of previously known stellar
companions to HD16760, HD113449, and HD160934. We show that the companion to
HD16760 is in a face-on orbit, resulting in an upward revision of its mass from
\mjupiter to \msolar. No substellar
companions were detected around any of our sample members, despite our ability
to detect companions with masses below 80\mjupiter for 50 of our targets: of
these, our sensitivity extended down to 40\mjupiter around 30 targets, with a
subset of 22 subject to the still more stringent limit of 20\mjupiter. A
statistical analysis of our non-detection of substellar companions allows us to
place constraints on their frequency around ~0.2-1.5\msolar stars. In
particular, considering companion mass distributions that have been proposed in
the literature, we obtain an upper limit estimate of ~9-11% for the frequency
of 20-80\mjupiter companions between 3-30AU at 95% confidence, assuming that
their semimajor axes are distributed according to in this range.Comment: Accepted by Ap
The First Planets: the Critical Metallicity for Planet Formation
A rapidly growing body of observational results suggests that planet
formation takes place preferentially at high metallicity. In the core accretion
model of planet formation this is expected because heavy elements are needed to
form the dust grains which settle into the midplane of the protoplanetary disk
and coagulate to form the planetesimals from which planetary cores are
assembled. As well, there is observational evidence that the lifetimes of
circumstellar disks are shorter at lower metallicities, likely due to greater
susceptibility to photoevaporation. Here we estimate the minimum metallicity
for planet formation, by comparing the timescale for dust grain growth and
settling to that for disk photoevaporation. For a wide range of circumstellar
disk models and dust grain properties, we find that the critical metallicity
above which planets can form is a function of the distance r at which the
planet orbits its host star. With the iron abundance relative to that of the
Sun [Fe/H] as a proxy for the metallicity, we estimate a lower limit for the
critical abundance for planet formation of [Fe/H]_crit ~ -1.5 + log(r/1 AU),
where an astronomical unit (AU) is the distance between the Earth and the Sun.
This prediction is in agreement with the available observational data, and
carries implications for the properties of the first planets and for the
emergence of life in the early Universe. In particular, it implies that the
first Earth-like planets likely formed from circumstellar disks with
metallicities Z > 0.1 Z_Sun. If planets are found to orbit stars with
metallicities below the critical metallicity, this may be a strong challenge to
the core accretion model.Comment: 12 pages, 5 figures; accepted for publication in Ap
Mapping the Shores of the Brown Dwarf Desert II: Multiple Star Formation in Taurus-Auriga
We have conducted a high-resolution imaging study of the Taurus-Auriga
star-forming region in order to characterize the primordial outcome of multiple
star formation and the extent of the brown dwarf desert. Our survey identified
16 new binary companions to primary stars with masses of 0.25-2.5 Msun, raising
the total number of binary pairs (including components of high-order multiples)
with separations of 3--5000 AU to 90. We find that ~2/3--3/4 of all Taurus
members are multiple systems of two or more stars, while the other ~1/4--1/3
appear to have formed as single stars; the distribution of high-order
multiplicity suggests that fragmentation into a wide binary has no impact on
the subsequent probability that either component will fragment again. The
separation distribution for solar-type stars (0.7--2.5 Msun) is nearly log-flat
over separations of 3--5000 AU, but lower-mass stars (0.25--0.7 Msun) show a
paucity of binary companions with separations of >200 AU. Across this full mass
range, companion masses are well described with a linear-flat function; all
system mass ratios (q=M_B/M_A) are equally probable, apparently including
substellar companions. Our results are broadly consistent with the two expected
modes of binary formation (freefall fragmentation on large scales and disk
fragmentation on small scales), but the distributions provide some clues as to
the epochs at which the companions are likely to form.Comment: Accepted to ApJ; 32 pages, 7 figures, 6 tables in emulateapj forma
On-Sky Demonstration of a Linear Band-limited Mask with Application to Visual Binary Stars
We have designed and built the first band-limited coronagraphic mask used for
ground-based high-contrast imaging observations. The mask resides in the focal
plane of the near-infrared camera PHARO at the Palomar Hale telescope and
receives a well-corrected beam from an extreme adaptive optics system. Its
performance on-sky with single stars is comparable to current state-of-the-art
instruments: contrast levels of or better at 0.8" in after
post-processing, depending on how well non-common-path errors are calibrated.
However, given the mask's linear geometry, we are able to conduct additional
unique science observations. Since the mask does not suffer from pointing
errors down its long axis, it can suppress the light from two different stars
simultaneously, such as the individual components of a spatially resolved
binary star system, and search for faint tertiary companions. In this paper, we
present the design of the mask, the science motivation for targeting binary
stars, and our preliminary results, including the detection of a candidate
M-dwarf tertiary companion orbiting the visual binary star HIP 48337, which we
are continuing to monitor with astrometry to determine its association.Comment: Accepted to Ap
The NIRSPEC Ultracool Dwarf Radial Velocity Survey
We report the results of an infrared Doppler survey designed to detect brown
dwarf and giant planetary companions to a magnitude-limited sample of ultracool
dwarfs. Using the NIRSPEC spectrograph on the Keck II telescope, we obtained
approximately 600 radial velocity measurements over a period of six years for a
sample of 59 late-M and L dwarfs spanning spectral types M8/L0 to L6. A
subsample of 46 of our targets have been observed on three or more epochs. We
rely on telluric CH4 absorption features in the Earth's atmosphere as a
simultaneous wavelength reference and exploit the rich set of CO absorption
features found in the K-band spectra of cool stars and brown dwarfs to measure
radial velocities and projected rotational velocities. For a bright, slowly
rotating M dwarf standard we demonstrate a radial velocity precision of 50 m/s,
and for slowly rotating L dwarfs we achieve a typical radial velocity precision
of approximately 200 m/s. This precision is sufficient for the detection of
close-in giant planetary companions to mid-L dwarfs as well as more equal mass
spectroscopic binary systems with small separations (a<2 AU). We present an
orbital solution for the subdwarf binary LSR1610-0040 as well as an improved
solution for the M/T binary 2M0320-04. We also combine our radial velocity
measurements with distance estimates and proper motions from the literature to
estimate the dispersion of the space velocities of the objects in our sample.
Using a kinematic age estimate we conclude that our UCDs have an age of
5.0+0.7-0.6 Gyr, similar to that of nearby sun-like stars. We simulate the
efficiency with which we detect spectroscopic binaries and find that the rate
of tight (a<1 AU) binaries in our sample is 2.5+8.6-1.6%, consistent with
recent estimates in the literature of a tight binary fraction of 3-4%.
(abridged)Comment: 39 pages, 20 figures. Accepted for publication in Ap
LkCa 15: A Young Exoplanet Caught at Formation?
Young and directly imaged exoplanets offer critical tests of planet-formation
models that are not matched by RV surveys of mature stars. These targets have
been extremely elusive to date, with no exoplanets younger than 10--20 Myr and
only a handful of direct-imaged exoplanets at all ages. We report the direct
imaging discovery of a likely (proto)planet around the young (~2 Myr) solar
analog LkCa 15, located inside a known gap in the protoplanetary disk (a
"transitional disk"). Our observations use non-redundant aperture masking
interferometry at 3 epochs to reveal a faint and relatively blue point source
($M_K'=9.1+/-0.2, K'-L'=0.98+/-0.22), flanked by approximately co-orbital
emission that is red and resolved into at least two sources (M_L'=7.5+/-0.2,
K'-L'=2.7+/-0.3; M_L'=7.4+/-0.2, K'-L'=1.94+/-0.16). We propose that the most
likely geometry consists of a newly-formed (proto)planet that is surrounded by
dusty material. The nominal estimated mass is ~6 M_{Jup} according to the 1 Myr
hot-start models. However, we argue based on its luminosity, color, and the
presence of circumplanetary material that the planet has likely been caught at
its epoch of assembly, and hence this mass is an upper limit due to its extreme
youth and flux contributed by accretion. The projected separations (71.9 +/-
1.6 mas, 100.7 +/- 1.9 mas, and 88.2 +/- 1.8 mas) and deprojected orbital radii
(16, 21, and 19 AU) correspond to the center of the disk gap, but are too close
to the primary star for a circular orbit to account for the observed inner edge
of the outer disk, so an alternate explanation (i.e., additional planets or an
eccentric orbit) is likely required. This discovery is the first direct
evidence that at least some transitional disks do indeed host newly-formed (or
forming) exoplanetary systems, and the observed properties provide crucial
insight into the gas giant formation process.Comment: Accepted to ApJ; 15 pages, 4 figures, 2 tabls in emulateapj forma
Resolved Images of Large Cavities in Protoplanetary Transition Disks
Circumstellar disks are thought to experience a rapid "transition" phase in
their evolution that can have a considerable impact on the formation and early
development of planetary systems. We present new and archival high angular
resolution (0.3" = 40-75 AU) Submillimeter Array (SMA) observations of the 880
micron dust continuum emission from 12 such transition disks in nearby
star-forming regions. In each case, we directly resolve a dust-depleted disk
cavity around the central star. Using radiative transfer calculations, we
interpret these dust disk structures in a homogeneous, parametric model
framework by reproducing their SMA visibilities and SEDs. The cavities in these
disks are large (R_cav = 15-73 AU) and substantially depleted of small
(~um-sized) dust grains, although their mass contents are still uncertain. The
structures of the remnant material at larger radii are comparable to normal
disks. We demonstrate that these large cavities are common among the
millimeter-bright disk population, comprising at least 20% of the disks in the
bright half of the millimeter luminosity (disk mass) distribution. Utilizing
these results, we assess some of the physical mechanisms proposed to account
for transition disk structures. As has been shown before, photoevaporation
models do not produce the large cavity sizes, accretion rates, and disk masses
representative of this sample. It would be difficult to achieve a sufficient
decrease of the dust optical depths in these cavities by particle growth alone:
substantial growth (to meter sizes or beyond) must occur in large (tens of AU)
regions of low turbulence without also producing an abundance of small
particles. Given those challenges, we suggest instead that the observations are
most commensurate with dynamical clearing due to tidal interactions with
low-mass companions --young brown dwarfs or giant planets on long-period
orbits.Comment: ApJ, in pres
The Expanding Business of the Entrepreneurial University: Job Creation
This chapter explores the role of universities in job creation. It does this by taking two approaches. The first is to look at how the university sees its role as expanding from traditional first and second mission activities to encompass third mission activities including industry engagement and how this supports job creation and economic development. The second approach is to examine how new jobs are created in a geographic region or country, and the role that the university can play in support of this. Typical third mission activities such as incubators, technology transfer, and science parks are also examined; including the role of government support and incentives
- …