58 research outputs found
The dry and carbon poor inner disk of TW Hya: evidence for a massive icy dust trap
Gas giants accrete their envelopes from the gas and dust of proto-planetary
disks, so it is important to determine the composition of the inner few AU,
where most giant planets are expected to form. We aim to constrain the
elemental carbon and oxygen abundance in the inner disk (2.3 AU) of TW Hya
and compare with the outer disk ( AU) where carbon and oxygen appear
underabundant by a factor of 50. Archival infrared observations of TW Hya
are compared with a detailed thermo-chemical model, DALI. The inner disk gas
mass and elemental C and O abundances are varied to fit the infrared CO, H
and HO line fluxes. Best fitting models have an inner disk that has a gas
mass of with C/H and O/H
. The elemental oxygen and carbon abundances of the
inner disk are times underabundant compared to the ISM and are
consistent with those found in the outer disk. The uniformly low volatile
abundances imply that the inner disk is not enriched by ices on drifting bodies
that evaporate. This indicates that drifting grains are stopped in a dust trap
outside the water ice line. Such a dust trap would also form a cavity as seen
in high resolution sub-millimeter continuum observations. If CO is the major
carbon carrier in the ices, dust needs to be trapped efficiently outside the CO
ice line of 20 AU. This would imply that the shallow sub-millimeter rings
in the TW Hya disk outside of 20 AU correspond to very efficient dust traps.
The more likely scenario is that more than 98\% of the CO has been converted
into less volatile species, e.g. CO and CHOH. A giant planet forming in
the inner disk would be accreting gas with low carbon and oxygen abundances as
well as very little icy dust, potentially leading to a planet atmosphere with
strongly substellar C/H and O/H ratios.Comment: 6 pages, 3 figures, accepted to A&A letter
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
Probing planet formation and disk substructures in the inner disk of Herbig Ae stars with CO rovibrational emission
Context. CO rovibrational lines are efficient probes of warm molecular gas and can give unique insights into the inner 10 AU of proto-planetary disks, effectively complementing ALMA observations. Recent studies find a relation between the ratio of lines originating from the second and first vibrationally excited state, denoted as v2∕v1, and the Keplerian velocity or emitting radius of CO. Counterintuitively, in disks around Herbig Ae stars the vibrational excitation is low when CO lines come from close to the star, and high when lines only probe gas at large radii (more than 5 AU). The v2∕v1 ratio is also counterintuitively anti-correlated with the near-infrared (NIR) excess, which probes hot and warm dust in the inner disk.
Aims. We aim to find explanations for the observed trends between CO vibrational ratio, emitting radii and NIR excess, and to identify their implications in terms of the physical and chemical structure of inner disks around Herbig stars.
Methods. First, slab model explorations in local thermal equilibrium (LTE) and non-LTE are used to identify the essential parameter space regions that can produce the observed CO emission. Second, we explore a grid of thermo-chemical models using the DALI code, varying gas-to-dust ratio and inner disk radius. Line flux, line ratios, and emitting radii are extracted from the simulated lines in the same way as the observations and directly compared to the data.
Results. Broad CO lines with low vibrational ratios are best explained by a warm (400–1300 K) inner disk surface with gas-to-dust ratios below 1000 (N_(CO) 10¹⁸ cm⁻²) at the cavity wall. In all cases, the CO gas must be close to thermalization with the dust (T_(gas) ~ T_(dust)).
Conclusions. The high gas-to-dust ratios needed to explain high v2∕v1 in narrow CO lines for a subset of group I disks can be naturally interpreted as due to the dust traps that are proposed to explain millimeter dust cavities. The dust trap and the low gas surface density inside the cavity are consistent with the presence of one or more massive planets. The difference between group I disks with low and high NIR excess can be explained by gap opening mechanisms that do or do not create an efficient dust trap, respectively. The broad lines seen in most group II objects indicate a very flat disk in addition to inner disk substructures within 10 AU that can be related to the substructures recently observed with ALMA. We provide simulated ELT-METIS images to directly test these scenarios in the future
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
Ro-vibrational Spectroscopy of CI Tau -- Evidence of a Multi-Component Eccentric Disk Induced by a Planet
CI Tau is currently the only T Tauri star with an inner protoplanetary disk
that hosts a planet, CI Tau b, that has been detected by a radial velocity
survey. This provides the unique opportunity to study disk features that were
imprinted by that planet. We present multi-epoch spectroscopic data, taken with
NASA IRTF in 2022, of the CO and hydrogen Pf line emissions
spanning 9 consecutive nights, which is the proposed orbital period of CI Tau
b. We find that the star's accretion rate varied according to that 9~d period,
indicative of companion driven accretion. Analysis of the CO emission
lines reveals that the disk can be described with an inner and outer component
spanning orbital radii 0.05-0.13~au and 0.15-1.5~au, respectively. Both
components have eccentricities of about 0.05 and arguments of periapses that
are oppositely aligned. We present a proof-of-concept hydrodynamic simulation
that shows a massive companion on a similarly eccentric orbit can recreate a
similar disk structure. Our results allude to such a companion being located
around an orbital distance of 0.14~au. However, this planet's orbital
parameters may be inconsistent with those of CI Tau b whose high eccentricity
is likely not compatible with the low disk eccentricities inferred by our
model.Comment: Accepted to A
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 (∆v ∼ 6 km s−1 ) analysis of [S II] λ4068, [O I] λ5577, and [O I] λ6300 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 km s−1 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 5,000−10,000 K and electron densities ∼ 107 −108 cm−3 . The HVC ratios are better reproduced by shock models with a pre-shock H number density of ∼ 106 − 107 cm−3 . Using these physical properties, we estimate M˙ wind/M˙ acc for the LVC and M˙ jet/M˙ acc 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 M˙ wind/M˙ acc 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
Evolution of protoplanetary disks from their taxonomy in scattered light: Group I vs. Group II
High-resolution imaging reveals a large morphological variety of
protoplanetary disks. To date, no constraints on their global evolution have
been found from this census. An evolutionary classification of disks was
proposed based on their IR spectral energy distribution, with the Group I
sources showing a prominent cold component ascribed to an earlier stage of
evolution than Group II. Disk evolution can be constrained from the comparison
of disks with different properties. A first attempt of disk taxonomy is now
possible thanks to the increasing number of high-resolution images of Herbig
Ae/Be stars becoming available. Near-IR images of six Group II disks in
scattered light were obtained with VLT/NACO in Polarimetric Differential
Imaging, which is the most efficient technique to image the light scattered by
the disk material close to the stars. We compare the stellar/disk properties of
this sample with those of well-studied Group I sources available from the
literature. Three Group II disks are detected. The brightness distribution in
the disk of HD163296 indicates the presence of a persistent ring-like structure
with a possible connection with the CO snowline. A rather compact (less than
100 AU) disk is detected around HD142666 and AK Sco. A taxonomic analysis of 17
Herbig Ae/Be sources reveals that the difference between Group I and Group II
is due to the presence or absence of a large disk cavity (larger than 5 AU).
There is no evidence supporting the evolution from Group I to Group II. Group
II are not evolved version of the Group I. Within the Group II disks, very
different geometries (both self-shadowed and compact) exist. HD163296 could be
the primordial version of a typical Group I. Other Group II, like AK Sco and
HD142666, could be smaller counterpart of Group I unable to open cavities as
large as those of Group I.Comment: 16 pages, 7 figures, published by A&
Probing planet formation and disk substructures in the inner disk of Herbig Ae stars with CO rovibrational emission
Context. CO rovibrational lines are efficient probes of warm molecular gas and can give unique insights into the inner 10 AU of proto-planetary disks, effectively complementing ALMA observations. Recent studies find a relation between the ratio of lines originating from the second and first vibrationally excited state, denoted as v2∕v1, and the Keplerian velocity or emitting radius of CO. Counterintuitively, in disks around Herbig Ae stars the vibrational excitation is low when CO lines come from close to the star, and high when lines only probe gas at large radii (more than 5 AU). The v2∕v1 ratio is also counterintuitively anti-correlated with the near-infrared (NIR) excess, which probes hot and warm dust in the inner disk.
Aims. We aim to find explanations for the observed trends between CO vibrational ratio, emitting radii and NIR excess, and to identify their implications in terms of the physical and chemical structure of inner disks around Herbig stars.
Methods. First, slab model explorations in local thermal equilibrium (LTE) and non-LTE are used to identify the essential parameter space regions that can produce the observed CO emission. Second, we explore a grid of thermo-chemical models using the DALI code, varying gas-to-dust ratio and inner disk radius. Line flux, line ratios, and emitting radii are extracted from the simulated lines in the same way as the observations and directly compared to the data.
Results. Broad CO lines with low vibrational ratios are best explained by a warm (400–1300 K) inner disk surface with gas-to-dust ratios below 1000 (N_(CO) 10¹⁸ cm⁻²) at the cavity wall. In all cases, the CO gas must be close to thermalization with the dust (T_(gas) ~ T_(dust)).
Conclusions. The high gas-to-dust ratios needed to explain high v2∕v1 in narrow CO lines for a subset of group I disks can be naturally interpreted as due to the dust traps that are proposed to explain millimeter dust cavities. The dust trap and the low gas surface density inside the cavity are consistent with the presence of one or more massive planets. The difference between group I disks with low and high NIR excess can be explained by gap opening mechanisms that do or do not create an efficient dust trap, respectively. The broad lines seen in most group II objects indicate a very flat disk in addition to inner disk substructures within 10 AU that can be related to the substructures recently observed with ALMA. We provide simulated ELT-METIS images to directly test these scenarios in the future
The Effect of Dust Evolution and Traps on Inner Disk Water Enrichment
Substructures in protoplanetary disks can act as dust traps that shape the radial distribution of pebbles. By blocking the passage of pebbles, the presence of gaps in disks may have a profound effect on pebble delivery into the inner disk, crucial for the formation of inner planets via pebble accretion. This process can also affect the delivery of volatiles (such as H2O) and their abundance within the water snow line region (within a few au). In this study, we aim to understand what effect the presence of gaps in the outer gas disk may have on water vapor enrichment in the inner disk. Building on previous work, we employ a volatile-inclusive disk evolution model that considers an evolving ice-bearing drifting dust population, sensitive to dust traps, which loses its icy content to sublimation upon reaching the snow line. We find that the vapor abundance in the inner disk is strongly affected by the fragmentation velocity (vf) and turbulence, which control how intense vapor enrichment from pebble delivery is, if present, and how long it may last. Generally, for disks with low to moderate turbulence (α ≤ 1 × 10−3) and a range of vf, radial locations and gap depths (especially those of the innermost gaps) can significantly alter enrichment. Shallow inner gaps may continuously leak material from beyond it, despite the presence of additional deep outer gaps. We finally find that for realistic vf (≤10 m s−1), the presence of gaps is more important than planetesimal formation beyond the snow line in regulating pebble and volatile delivery into the inner disk
- …