38 research outputs found
Planetesimal formation by the streaming instability in a photoevaporating disk
Recent years have seen growing interest in the streaming instability as a
candidate mechanism to produce planetesimals. However, these investigations
have been limited to small-scale simulations. We now present the results of a
global protoplanetary disk evolution model that incorporates planetesimal
formation by the streaming instability, along with viscous accretion,
photoevaporation by EUV, FUV, and X-ray photons, dust evolution, the water ice
line, and stratified turbulence. Our simulations produce massive (60-130
) planetesimal belts beyond 100 au and up to of
planetesimals in the middle regions (3-100 au). Our most comprehensive model
forms 8 of planetesimals inside 3 au, where they can give rise to
terrestrial planets. The planetesimal mass formed in the inner disk depends
critically on the timing of the formation of an inner cavity in the disk by
high-energy photons. Our results show that the combination of photoevaporation
and the streaming instability are efficient at converting the solid component
of protoplanetary disks into planetesimals. Our model, however, does not form
enough early planetesimals in the inner and middle regions of the disk to give
rise to giant planets and super-Earths with gaseous envelopes. Additional
processes such as particle pileups and mass loss driven by MHD winds may be
needed to drive the formation of early planetesimal generations in the planet
forming regions of protoplanetary disks.Comment: 20 pages, 12 figures; accepted to Ap
Protostellar disks subject to infall: a one-dimensional inviscid model and comparison with ALMA observations
A new one-dimensional, inviscid, and vertically integrated disk model with
prescribed infall is presented. The flow is computed using a second-order
shock-capturing scheme. Included are vertical infall, radial infall at the
outer radial boundary, radiative cooling, stellar irradiation, and heat
addition at the disk-surface shock. Simulation parameters are chosen to target
the L1527 IRS disk which has been observed using ALMA (Atacama Large Millimeter
Array). The results give an outer envelope of radial infall and which encounters a radial shock at the
centrifugal radius () across which the radial velocity is greatly
reduced and the gas temperature rises from a pre-shock value of K
to K over a spatially thin region calculated using a separate
shock structure code. At , the azimuthal velocity
transitions from being to being nearly Keplerian. These results
qualitatively agree with recent ALMA observations which indicate a radial shock
where SO is sublimated as well as a transition from a region
to a Keplerian inner disk. However, in one set of observations, the
position-velocity map of cyclic-CH, together with a certain ballistic
maximum velocity relation, has suggested that the radial shock coincides with a
ballistic centrifugal barrier, which places the shock at , i.e, inward of , rather than outward as given
by our simulations. It is argued that radial velocity plots from previous
magnetic rotating-collapse simulations also indicate that the radial shock is
located outward of . The discrepancy with observations is
analyzed and discussed, but remains unresolved.Comment: Originally, we incorrectly took Semenov etal. opacities to be m
per gm of dust rather than gas. Thus our opacities were too low by a factor
of 100. Making the correction reduced the temperature across the shock but
left velocities and densities nearly unchanged. To account for SO sublimation
in L1527 observed by ALMA, we performed a separate 1D shock calculation
including non-LTE effect
Debris Disks in the Scorpius-Centaurus OB Association Resolved by ALMA
We present a CO(2-1) and 1240 um continuum survey of 23 debris disks with
spectral types B9-G1, observed at an angular resolution of 0.5-1 arcsec with
the Atacama Large Millimeter/Submillimeter Array (ALMA). The sample was
selected for large infrared excess and age ~10 Myr, to characterize the
prevalence of molecular gas emission in young debris disks. We identify three
CO-rich debris disks, plus two additional tentative (3-sigma) CO detections.
Twenty disks were detected in the continuum at the >3-sigma level. For the 12
disks in the sample that are spatially resolved by our observations, we perform
an independent analysis of the interferometric continuum visibilities to
constrain the basic dust disk geometry, as well as a simultaneous analysis of
the visibilities and broad-band spectral energy distribution to constrain the
characteristic grain size and disk mass. The gas-rich debris disks exhibit
preferentially larger outer radii in their dust disks, and a higher prevalence
of characteristic grain sizes smaller than the blowout size. The gas-rich disks
do not exhibit preferentially larger dust masses, contrary to expectations for
a scenario in which a higher cometary destruction rate would be expected to
result in a larger mass of both CO and dust. The three debris disks in our
sample with strong CO detections are all around A stars: the conditions in
disks around intermediate-mass stars appear to be the most conducive to the
survival or formation of CO.Comment: 16 pages, 6 figures, accepted for publication in Ap
Understanding the origin of the [OI] low-velocity component from T Tauri stars
The formation time, masses, and location of planets are strongly impacted by the physical mechanisms that disperse protoplanetary disks and the timescale over which protoplanetary material is cleared out. Accretion of matter onto the central star, protostellar winds/jets, magnetic disk winds, and photoevaporative winds operate concurrently. Hence, disentangling their relative contribution to disk dispersal requires identifying diagnostics that trace different star–disk environments. Here, we analyze the low-velocity component (LVC) of the oxygen optical forbidden lines, which is found to be blueshifted by a few km s−1 with respect to the stellar velocity. We find that the [O i] LVC profiles are different from those of [Ne ii] at 12.81μm and CO at 4.7μm lines pointing to different origins for these gas lines. We report a correlation between the luminosity of the [O i] LVC and the accretion luminosity Lacc. We do not find any correlation with the X-ray luminosity, while we find that the higher is the stellar far-UV (FUV) luminosity, the higher is the luminosity of the [O i] LVC. In addition, we show that the [O i] λ6300/λ5577 ratio is low (ranging between 1 and 8). These findings favor an origin of the [O i] LVC in a region where OH is photodissociated by stellar FUV photons and argue against thermal emission from an X-ray-heated layer. Detailed modeling of two spectra with the highest S/N and resolution shows that there are two components within the LVC: a broad, centrally peaked component that can be attributed to gas arising in a warm disk surface in Keplerian rotation (with FWHM between ∼40 and ∼60 km s−1), and a narrow component (with FWHM ∼ 10 km s−1 and small blueshifts of ∼2 km s−1) that may arise in a cool (1000 K) molecular wind
Kinematic Links and the Coevolution of MHD Winds, Jets, and Inner Disks from a High-resolution Optical [OI] Survey
We present a survey of optical [O I] emission at 6300 Å toward 65 T Tauri stars at the spectral resolution of ∼7 km s−1 . Past work identified a highly blueshifted velocity component (HVC) tracing microjets and a less blueshifted low-velocity component (LVC) attributed to winds. We focus here on the LVC kinematics to investigate links between winds, jets, accretion, and disk dispersal. We track the behavior of four types of LVC components: a broad and a narrow component (“BC” and “NC,” respectively) in LVCs that are decomposed into two Gaussians which typically have an HVC, and single-Gaussian LVC profiles separated into those that have an HVC (“SCJ”) and those that do not (“SC”). The LVC centroid velocities and line widths correlate with the HVC EW and accretion luminosity, suggesting that LVC/winds and HVC/jets are kinematically linked and connected to accretion. The deprojected HVC velocity correlates with accretion luminosity, showing that faster jets come with higher accretion. BC and NC kinematics correlate, and their blueshifts are maximum at ∼35°, suggesting a conical wind geometry with this semi-opening angle. Only SCs include n13–31 up to ∼3, and their properties correlate with this infrared index, showing that [O I] emission recedes to larger radii as the inner dust is depleted, tracing less dense/hot gas and a decrease in wind velocity. Altogether, these findings support a scenario where optically thick, accreting inner disks launch radially extended MHD disk winds that feed jets, and where inner disk winds recede to larger radii and jets disappear in concert with dust depletion
A New Look at T Tauri Star Forbidden Lines: MHD Driven Winds from the Inner Disk
Magnetohydrodynamic (MHD) and photoevaporative winds are thought to play an
important role in the evolution and dispersal of planet-forming disks. We
report the first high-resolution (6\kms) analysis of [S II]
4068, [O I] 5577, and [O I] 6300 lines from a sample
of 48 T Tauri stars. Following Simon et al. (2016), we decompose them into
three kinematic components: a high-velocity component (HVC) associated with
jets, and a low-velocity narrow (LVC-NC) and broad (LVC-BC) components. We
confirm previous findings that many LVCs are blueshifted by more than 1.5
kms thus most likely trace a slow disk wind. We further show that the
profiles of individual components are similar in the three lines. We find that
most LVC-BC and NC line ratios are explained by thermally excited gas with
temperatures between 5,00010,000 K and electron densities
cm. The HVC ratios are better reproduced by shock
models with a pre-shock H number density of cm.
Using these physical properties, we estimate for the LVC and for the HVC. In
agreement with previous work, the mass carried out in jets is modest compared
to the accretion rate. With the likely assumption that the NC wind height is
larger than the BC, the LVC-BC is found
to be higher than the LVC-NC. These results suggest that most of the mass loss
occurs close to the central star, within a few au, through an MHD driven wind.
Depending on the wind height, MHD winds might play a major role in the
evolution of the disk mass.Comment: 45 pages, 23 figures, and 7 tables, accepted by Ap
A High-resolution Optical Survey of Upper Sco: Evidence for Coevolution of Accretion and Disk Winds
Magnetohydrodynamic (MHD) and photoevaporative winds are thought to play an
important role in the evolution and dispersal of planet-forming disks. Here, we
analyze high-resolution ( 7 kms) optical spectra from a
sample of 115 T Tauri stars in the Myr Upper Sco association and
focus on the [O I]6300 and H lines to trace disk winds and
accretion, respectively. Our sample covers a large range in spectral type and
we divide it into Warm (G0-M3) and Cool (later than M3) to facilitate
comparison with younger regions. We detect the [O I]6300 line in 45
out of 87 upper sco sources with protoplanetary disks and 32 out of 45 are
accreting based on H profiles and equivalent widths. All [O I]
6300 Upper Sco profiles have a low-velocity (centroid
kms, LVC) emission and most (36/45) can be fit by a single Gaussian
(SC). The SC distribution of centroid velocities and FWHMs is consistent with
MHD disk winds. We also find that the Upper Sco sample follows the same
accretion luminosityLVC [O I]6300 luminosity relation and the same
anti-correlation between SC FWHM and WISE W3-W4 spectral index as the younger
samples. These results indicate that accretion and disk winds coevolve and
that, as inner disks clear out, wind emission arises further away from the
star. Finally, our large spectral range coverage reveals that Cool stars have
larger FWHMs normalized by stellar mass than Warm stars indicating that [O
I]6300 emission arises closer in towards lower mass/lower luminosity
stars.Comment: 47 pages, 32 figures; ApJ accepte