171 research outputs found
The Atomic and Molecular Content of Disks Around Very Low-mass Stars and Brown Dwarfs
There is growing observational evidence that disk evolution is stellar-mass
dependent. Here, we show that these dependencies extend to the atomic and
molecular content of disk atmospheres. We analyze a unique dataset of
high-resolution Spitzer/IRS spectra from 8 very low-mass star and brown dwarf
disks. We report the first detections of Ne+, H2, CO2, and tentative detections
of H2O toward these faint and low-mass disks. Two of our [NeII] 12.81 micron
emission lines likely trace the hot (>5,000 K) disk surface irradiated by X-ray
photons from the central stellar/sub-stellar object. The H2 S(2) and S(1)
fluxes are consistent with arising below the fully or partially ionized surface
traced by the [NeII] emission, in gas at about 600 K. We confirm the higher
C2H2/HCN flux and column density ratio in brown dwarf disks previously noted
from low-resolution IRS spectra. Our high-resolution spectra also show that the
HCN/H2O fluxes of brown dwarf disks are on average higher than those of T Tauri
disks. Our LTE modeling hints that this difference extends to column density
ratios if H2O lines trace warm > 600 K disk gas. These trends suggest that the
inner regions of brown dwarf disks have a lower O/C ratio than those of T Tauri
disks which may result from a more efficient formation of non-migrating icy
planetesimals. A O/C=1, as inferred from our analysis, would have profound
implications on the bulk composition of rocky planets that can form around very
low-mass stars and brown dwarfs.Comment: Accepted to Ap
The Exoplanet Population Observation Simulator. I - The Inner Edges of Planetary Systems
The Kepler survey provides a statistical census of planetary systems out to
the habitable zone. Because most planets are non-transiting, orbital
architectures are best estimated using simulated observations of ensemble
populations. Here, we introduce EPOS, the Exoplanet Population Observation
Simulator, to estimate the prevalence and orbital architectures of multi-planet
systems based on the latest Kepler data release, DR25. We estimate that at
least 42% of sun-like stars have nearly coplanar planetary systems with 7 or
more exoplanets. The fraction of stars with at least one planet within 1 au
could be as high as 100% depending on assumptions about the distribution of
single transiting planets. We estimate an occurrence rate of planets in the
habitable zone around sun-like stars of eta_earth=36+-14%. The innermost
planets in multi-planet systems are clustered around an orbital period of 10
days (0.1 au), reminiscent of the protoplanetary disk inner edge or could be
explained by a planet trap at that location. Only a small fraction of planetary
systems have the innermost planet at long orbital periods, with fewer than ~8%
and ~3% having no planet interior to the orbit of Mercury and Venus,
respectively. These results reinforce the view that the solar system is not a
typical planetary system, but an outlier among the distribution of known
exoplanetary systems. We predict that at least half of the habitable zone
exoplanets are accompanied by (non-transiting) planets at shorter orbital
periods, hence knowledge of a close-in exoplanet could be used as a way to
optimize the search for Earth-size planets in the Habitable Zone with future
direct imaging missions.Comment: Accepted in AAS journals, code available on githu
The Dispersal of Protoplanetary Disks
Protoplanetary disks are the sites of planet formation, and the evolution and
eventual dispersal of these disks strongly influences the formation of
planetary systems. Disk evolution during the planet-forming epoch is driven by
accretion and mass-loss due to winds, and in typical environments
photoevaporation by high-energy radiation from the central star is likely to
dominate final gas disk dispersal. We present a critical review of current
theoretical models, and discuss the observations that are used to test these
models and inform our understanding of the underlying physics. We also discuss
the role disk dispersal plays in shaping planetary systems, considering its
influence on both the process(es) of planet formation and the architectures of
planetary systems. We conclude by presenting a schematic picture of
protoplanetary disk evolution and dispersal, and discussing prospects for
future work.Comment: 23 pages, 6 figures. Refereed review chapter, accepted for
publication in Protostars & Planets VI, University of Arizona Press (2014),
eds. H.Beuther, C.Dullemond, Th.Henning, R.Klesse
CLIcK: a Continuum and Line fItting Kit for circumstellar disks
Infrared spectroscopy with medium to high spectral resolution is essential to
characterize the gas content of circumstellar disks. Unfortunately, conducting
continuum and line radiative transfer of thermochemical disk models is too
time-consuming to carry out large parameter studies. Simpler approaches using a
slab model to fit continuum-subtracted spectra require the identification of
either the global or local continuum. Continuum subtraction, particularly when
covering a broad wavelength range, is challenging but critical in rich
molecular spectra as hot (several hundreds K) molecular emission lines can also
produce a pseudo continuum. In this work, we present CLIcK, a flexible tool to
simultaneously fit the continuum and line emission. The DDN01 continuum model
(Dullemond et al. 2001) and a plane-parallel slab of gas in local thermodynamic
equilibrium are adopted to simulate the continuum and line emission
respectively, both of them are fast enough for homogeneous studies of large
disk samples. We applied CLIcK to fit the observed water spectrum of the AA Tau
disk and obtained water vapor properties that are consistent with literature
results. We also demonstrate that CLIcK properly retrieves the input parameters
used to simulate the water spectrum of a circumstellar disk. CLIcK will be a
versatile tool for the interpretation of future James Webb Space Telescope
spectra.Comment: Accepted for publication in A&A, 9 pages, 9 figure
A candidate planetary-mass object with a photoevaporating disk in Orion
In this work, we report the discovery of a candidate planetary-mass object
with a photoevaporating protoplanetary disk, Proplyd 133-353, which is near the
massive star Ori C at the center of the Orion Nebula Cluster
(ONC). The object was known to have extended emission pointing away from
Ori C, indicating ongoing external photoevaporation. Our
near-infrared spectroscopic data suggests that the central source of Proplyd
133-353 is substellar (M9.5), might have a mass probably less than 13
Jupiter mass and an age younger than 0.5 Myr. Proplyd 133-353 shows a similar
ratio of X-ray luminosity to stellar luminosity to other young stars in the ONC
with a similar stellar luminosity, and has a similar proper motion to the mean
one of confirmed ONC members. We propose that Proplyd 133-353 was formed in a
very low-mass dusty cloud near Ori C as a second-generation of
star formation, which can explain both its young age and the presence of its
disk.Comment: 6 pages, 4 figures. Accepted for publication in ApJ
The Formation of Brown Dwarfs: Observations
We review the current state of observational work on the formation of brown
dwarfs, focusing on their initial mass function, velocity and spatial
distributions at birth, multiplicity, accretion, and circumstellar disks. The
available measurements of these various properties are consistent with a common
formation mechanism for brown dwarfs and stars. In particular, the existence of
widely separated binary brown dwarfs and a probable isolated proto-brown dwarf
indicate that some substellar objects are able to form in the same manner as
stars through unperturbed cloud fragmentation. Additional mechanisms such as
ejection and photoevaporation may play a role in the birth of some brown
dwarfs, but there is no observational evidence to date to suggest that they are
the key elements that make it possible for substellar bodies to form.Comment: Protostars and Planets V, in pres
Earths in Other Solar Systems N-body simulations: the Role of Orbital Damping in Reproducing the Kepler Planetary Systems
The population of exoplanetary systems detected by Kepler provides
opportunities to refine our understanding of planet formation. Unraveling the
conditions needed to produce the observed exoplanets will sallow us to make
informed predictions as to where habitable worlds exist within the galaxy. In
this paper, we examine using N-body simulations how the properties of planetary
systems are determined during the final stages of assembly. While accretion is
a chaotic process, trends in the ensemble properties of planetary systems
provide a memory of the initial distribution of solid mass around a star prior
to accretion. We also use EPOS, the Exoplanet Population Observation Simulator,
to account for detection biases and show that different accretion scenarios can
be distinguished from observations of the Kepler systems. We show that the
period of the innermost planet, the ratio of orbital periods of adjacent
planets, and masses of the planets are determined by the total mass and radial
distribution of embryos and planetesimals at the beginning of accretion. In
general, some amount of orbital damping, either via planetesimals or gas,
during accretion is needed to match the whole population of exoplanets.
Surprisingly, all simulated planetary systems have planets that are similar in
size, showing that the "peas in a pod" pattern can be consistent with both a
giant impact scenario and a planet migration scenario. The inclusion of
material at distances larger than what Kepler observes has a profound impact on
the observed planetary architectures, and thus on the formation and delivery of
volatiles to possible habitable worlds.Comment: Resubmitted to ApJ. Planet formation models available online at
http://eos-nexus.org/genesis-database
High-resolution Spectroscopy of [Ne II] Emission from TW Hya
We present high-resolution echelle spectra of [Ne II] 12.81 micron emission
from the classical T Tauri star (CTTS) TW Hya obtained with MICHELLE on Gemini
North. The line is centered at the stellar radial velocity and has an intrinsic
FWHM of 21\pm 4 km/s. The line width is broader than other narrow emission
lines typically associated with the disk around TW Hya. If formed in a disk,
the line broadening could result from turbulence in a warm disk atmosphere,
Keplerian rotation at an average distance of 0.1 AU from the star, or a
photoevaporative flow from the optically-thin region of the disk. We place
upper limits on the [Ne II] emission flux from the CTTSs DP Tau and BP Tau.Comment: Accepted by ApJ. 18 pages, including 2 figures and 2 table
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