Over million years of evolution, gas dust and ice in protoplanetary disks can be chemically reprocessed. There is evidence that the gas-phase carbon and oxygen abundances are subsolar in disks ...belonging to nearby star forming regions. These findings have a major impact on the composition of the primary atmosphere of giant planets (but it may also be valid for super-Earths and sub-Neptunes) as they accrete their gaseous envelopes from the surrounding material in the disk. In this study, we performed a thermochemical modeling analysis with the aim of testing how reliable and robust are the estimates of elemental abundance ratios based on (sub)millimeter observations of molecular lines. We created a grid of disk models for the following different elemental abundance ratios: C/O, N/O, and S/O, and we computed the line flux of a set of carbon-nitrogen and sulphur-bearing species, namely CN, HCN, NO, C
2
H, c–C
3
H
2
, H
2
CO, HC
3
N, CH
3
CN, CS, SO, H
2
S, and H
2
CS, which have been detected with present (sub)millimeter facilities such as ALMA and NOEMA. We find that the line fluxes, once normalized to the flux of the
13
CO
J
= 2−1 line, are sensitive to the elemental abundance ratios. On the other hand, the stellar and disk physical parameters have only a minor effect on the line flux ratios. Our results demonstrate that a simultaneous analysis of multiple molecular transitions is a valid approach to constrain the elemental abundance ratio in protoplanetary disks.
The protoplanetary system HD 169142 is one of the few cases where a potential candidate protoplanet has recently been detected by direct imaging in the near-infrared. To study the interaction between ...the protoplanet and the disk itself, observations of the gas and dust surface density structure are needed. This paper reports new ALMA observations of the dust continuum at 1.3 mm, 12CO, 13CO, and C18O J = 2−1 emission from the system HD 169142 (which is observed almost face-on) at an angular resolution of ~\hbox{$0\farcs3 \,\times\, 0\farcs2$}0 .̋ 3 × 0 .̋ 2 (~35 × 20 au). The dust continuum emission reveals a double-ring structure with an inner ring between \hbox{$0\farcs17{-}0\farcs28$}0 .̋ 17−0 .̋ 28 (~20−35 au) and an outer ring between \hbox{$0\farcs48{-}0\farcs64$}0 .̋ 48−0 .̋ 64 (~56−83 au). The size and position of the inner ring is in good agreement with previous polarimetric observations in the near-infrared and is consistent with dust trapping by a massive planet. No dust emission is detected inside the inner dust cavity (R ≲ 20 au) or within the dust gap (~35−56 au) down to the noise level. In contrast, the channel maps of the J = 2−1 line of the three CO isotopologs reveal gas inside the dust cavity and dust gap. The gaseous disk is also much larger than the compact dust emission; it extends to ~1\hbox{$\farcs5$}.̋ 5 (~180 au) in radius. This difference and the sharp drop of the continuum emission at large radii point to radial drift of large dust grains (>μm size). Using the thermo-chemical disk code dali, we modeled the continuum and the CO isotopolog emission to quantitatively measure the gas and dust surface densities. The resulting gas surface density is reduced by a factor of ~30−40 inward of the dust gap. The gas and dust distribution indicate that two giant planets shape the disk structure through dynamical clearing (dust cavity and gap) and dust trapping (double-ring dust distribution).
We present the analysis of 34 new VLT/X-Shooter spectra of young stellar objects in the Chamaeleon I star-forming region, together with four more spectra of stars in Taurus and two in Chamaeleon II. ...The broad wavelength coverage and accurate flux calibration of our spectra allow us to estimate stellar and accretion parameters for our targets by fitting the photospheric and accretion continuum emission from the Balmer continuum down to ~700 nm. The dependence of accretion on stellar properties for this sample is consistent with previous results from the literature. The accretion rates for transitional disks are consistent with those of full disks in the same region. The spread of mass accretion rates at any given stellar mass is found to be smaller than in many studies, but is larger than that derived in the Lupus clouds using similar data and techniques. Differences in the stellar mass range and in the environmental conditions between our sample and that of Lupus may account for the discrepancy in scatter between Chamaeleon I and Lupus. Complete samples in Chamaeleon I and Lupus are needed to determine whether the difference in scatter of accretion rates and the lack of evolutionary trends are not influenced by sample selection.
Planet-forming disks are not isolated systems. Their interaction with the surrounding medium affects their mass budget and chemical content. In the context of the ALMA-DOT program, we obtained ...high-resolution maps of assorted lines from six disks that are still partly embedded in their natal envelope. In this work, we examine the SO and SO2 emission that is detected from four sources: DG Tau, HL Tau, IRAS 04302+2247, and T Tau. The comparison with CO, HCO+, and CS maps reveals that the SO and SO2 emission originates at the intersection between extended streamers and the planet-forming disk. Two targets, DG Tau and HL Tau, offer clear cases of inflowing material inducing an accretion shock on the disk material. The measured rotational temperatures and radial velocities are consistent with this view. In contrast to younger Class 0 sources, these shocks are confined to the specific disk region impacted by the streamer. In HL Tau, the known accreting streamer induces a shock in the disk outskirts, and the released SO and SO2 molecules spiral toward the star in a few hundred years. These results suggest that shocks induced by late accreting material may be common in the disks of young star-forming regions with possible consequences for the chemical composition and mass content of the disk. They also highlight the importance of SO and SO2 line observations in probing accretion shocks from a larger sample.
Abstract
The composition of giant planets is imprinted by their migration history and the compositional structure of their hosting disks. Studies in recent literature have investigated how the ...abundances of C and O can constrain the formation pathways of giant planets forming within few tens of au from a star. New ALMA observations, however, suggest planet-forming regions possibly extending to hundreds of au. We explore the implications of these wider formation environments through
n
-body simulations of growing and migrating giant planets embedded in planetesimal disks, coupled with a compositional model of the protoplanetary disk where volatiles are inherited from the molecular cloud and refractories are calibrated against extrasolar and Solar System data. We find that the C/O ratio provides limited insight on the formation pathways of giant planets that undergo large-scale migration. This limitation can be overcome, however, thanks to nitrogen and sulfur. Jointly using the C/N, N/O, and C/O ratios breaks any degeneracy in the formation and migration tracks of giant planets. The use of elemental ratios normalized to the respective stellar ratios supplies additional information on the nature of giant planets, thanks to the relative volatility of O, C, and N in disks. When the planetary metallicity is dominated by the accretion of solids C/N* > C/O* > N/O* (* denoting this normalized scale), otherwise N/O* > C/O* > C/N*. The S/N ratio provides an additional independent probe into the metallicity of giant planets and their accretion of solids.
Aims. The composition of planetary solids and gases is largely rooted in the processing of volatile elements in protoplanetary disks. To shed light on the key processes, we carry out a comparative ...analysis of the gas-phase carbon abundance in two systems with a similar age and disk mass, but different central stars: HD 100546 and TW Hya. Methods. We combine our recent detections of C super(0) in these disks with observations of other carbon reservoirs (CO, C super(+), C sub(2) H) and gas-mass and warm-gas tracers (HD, O super(0)), as well as spatially resolved ALMA observations and the spectral energy distribution. The disks are modelled with the DALI 2D physical-chemical code. Stellar abundances for HD 100546 are derived from archival spectra. Results. Upper limits on HD emission from HD 100546 place an upper limit on the total disk mass of < or =0.1 M sub(?). The gas-phase carbon abundance in the atmosphere of this warm Herbig disk is, at most, moderately depleted compared to the interstellar medium, with C/H sub(gas)= (0.1?1.5) x 10 super(-4). HD 100546 itself is a lambda Bootis star, with solar abundances of C and O but a strong depletion of rock-forming elements. In the gas of the T Tauri disk TW Hya, both C and O are strongly underabundant, with C/H sub(gas)= (0.2?5.0) x 10 super(-6) and C / O > 1. We discuss evidence that the gas-phase C and O abundances are high in the warm inner regions of both disks. Our analytical model, including vertical mixing and a grain size distribution, reproduces the observed C/H sub(gas) in the outer disk of TW Hya and allows to make predictions for other systems.
We present the initial result of a large spectroscopic survey aimed at measuring the timescale of mass accretion in young, pre-main-sequence stars in the spectral type range K0–M5. Using multi-object ...spectroscopy with VIMOS at the VLT we identified the fraction of accreting stars in a number of young stellar clusters and associations of the ages of between 1–30 Myr. The fraction of accreting stars decreases from ~60% at 1.5–2 Myr to ~2% at 10 Myr. No accreting stars are found after 10 Myr at a sensitivity limit of 10-11 $M_{\odot}\,{\rm yr}^{-1}$. We compared the fraction of stars showing ongoing accretion ($f_{\rm acc}$) to the fraction of stars with near-to-mid infrared excess ($f_{\rm IRAC}$). In most cases we find $f_{\rm acc}$ < $f_{\rm IRAC}$, i.e. , mass accretion appears to cease (or drop below detectable level) earlier than the dust is dissipated in the inner disk. At 5 Myr, 95% of the stellar population has stopped accreting material at a rate of ${\ga}10^{-11}$ $M_{\odot}\,{\rm yr}^{-1}$, while ~20% of the stars show near-infrared excess emission. Assuming an exponential decay, we measure a mass accretion timescale ($\tau_{\rm acc}$) of 2.3 Myr, compared to a near-to-mid infrared excess timescale ($\tau_{\rm IRAC}$) of 3 Myr. Planet formation and/or migration, in the inner disk might be a viable mechanism to halt further accretion onto the central star on such a short timescale.
Context.
The role of bipolar jets in the formation of stars, and in particular how they are launched, is still not well understood.
Aims.
We probe the protostellar jet launching mechanism using ...high-resolution observations of the near-infrared (IR) Fe
II
λ
1.53,1.64
μ
m emission lines.
Methods.
We consider the case of the bipolar jet from Classical T Tauri star, DO Tau, and investigate the jet morphology and kinematics close to the star (within 140 au) using AO-assisted IFU observations from GEMINI/NIFS.
Results.
We find that the brighter, blueshifted jet is collimated very quickly after it is launched. This early collimation requires the presence of magnetic fields. We confirm velocity asymmetries between the two lobes of the bipolar jet, and also confirm no time variability in the asymmetry over a 20-year interval. This sustained asymmetry is in accordance with recent simulations of magnetised disc winds. We examine the data for signatures of jet rotation. We report an upper limit on differences in radial velocity of 6.3 and 8.7 km s
−1
for the blue- and redshifted jets, respectively. Interpreting this as an upper limit on jet rotation implies that any steady, axisymmetric magneto-centrifugal model of jet launching is constrained to a launch radius in the disc plane of
r
0
< 0.5 and 0.3 au for the blue- and redshifted jets, respectively. This supports an X-wind or narrow disc-wind model. However, the result pertains only to the observed high-velocity Fe
II
emission, and does not rule out a wider flow launched from a wider radius. We report the detection of small-amplitude jet axis wiggling in both lobes. We rule out orbital motion of the jet source as the cause. Precession can better account for the observations but requires double the precession angle, and a different phase for the counter-jet. Such non-solid body precession could arise from an inclined massive Jupiter companion, or a warping instability induced by launching a magnetic disc wind.
Conclusions.
Overall, our observations are consistent with an origin of the DO Tau jets from the inner regions of the disc.
Gaia-ESO Survey observations of the young Gamma Velorum cluster led to the discovery of two kinematically distinct populations, Gamma Vel A and B, respectively, with population B extended over ...several square degrees in the Vela OB2 association. Using the Gaia DR2 data for a sample of high-probability cluster members, we find that the two populations differ not only kinematically, but are also located at different distances along the line of sight, with the main cluster Gamma Vel A being closer. A combined fit of the two populations yields ϖA = 2.895 ± 0.008 mas and ϖB = 2.608 ± 0.017 mas, with intrinsic dispersions of 0.038 ± 0.011 mas and 0.091 ± 0.016 mas, respectively. This translates into distances of 345.4+1.0+12.4−1.0−11.5 345.4 − 1.0 − 11.5 + 1.0 + 12.4 $ 345.4_{-1.0-11.5}^{+1.0+12.4} $ pc and 383.4+2.5+15.3−2.5−14.2 383.4 − 2.5 − 14.2 + 2.5 + 15.3 $ 383.4_{-2.5-14.2}^{+2.5+15.3} $ pc, respectively, showing that Gamma Vel A is closer than Gamma Vel B by ~38 pc. We find that the two clusters are nearly coeval, and that Gamma Vel B is expanding. We suggest that Gamma Vel A and B are two independent clusters located along the same line of sight.
Context.
Hydrogen deuteride (HD) rotational line emission can provide reliable protoplanetary disc gas mass measurements, but this molecule is difficult to observe and detections have been limited to ...three T Tauri discs. No new data have been available since the
Herschel
Space Observatory mission ended in 2013.
Aims.
We set out to obtain new disc gas mass constraints by analysing upper limits on HD 1–0 emission in
Herschel
/PACS archival data from the DIGIT key programme.
Methods.
With a focus on the Herbig Ae/Be discs, whose stars are more luminous than T Tauris, we determined upper limits for HD in data previously analysed for its line detections. We studied the significance of these limits with a grid of models run with the DALI physical-chemical code, customised to include deuterium chemistry.
Results.
Nearly all the discs are constrained to
M
gas
≤ 0.1
M
⊙
, ruling out global gravitational instability. A strong constraint is obtained for the HD 163296 disc mass,
M
gas
≤ 0.067
M
⊙
, implying Δ
g/d
≤ 100. This HD-based mass limit is towards the low end of CO-based mass estimates for the disc, highlighting the large uncertainty in using only CO and suggesting that gas-phase CO depletion in HD 163296 is at most a factor of a few. The
M
gas
limits for HD 163296 and HD 100546, both bright discs with massive candidate protoplanetary systems, suggest disc-to-planet mass conversion efficiencies of
M
p
/(
M
gas
+
M
p
) ≈ 10–40% for present-day values. Near-future observations with SOFIA/HIRMES will be able to detect HD in the brightest Herbig Ae/Be discs within 150 pc with ≈ 10 h integration time.
PDF
