70 research outputs found
ALMA and Herschel Observations of the Prototype Dusty and Polluted White Dwarf G29-38
ALMA Cycle 0 and Herschel PACS observations are reported for the prototype,
nearest, and brightest example of a dusty and polluted white dwarf, G29-38.
These long wavelength programs attempted to detect an outlying, parent
population of bodies at 1-100 AU, from which originates the disrupted
planetesimal debris that is observed within 0.01 AU and which exhibits L_IR/L =
0.039. No associated emission sources were detected in any of the data down to
L_IR/L ~ 1e-4, generally ruling out cold dust masses greater than 1e24 - 1e25 g
for reasonable grain sizes and properties in orbital regions corresponding to
evolved versions of both asteroid and Kuiper belt analogs. Overall, these null
detections are consistent with models of long-term collisional evolution in
planetesimal disks, and the source regions for the disrupted parent bodies at
stars like G29-38 may only be salient in exceptional circumstances, such as a
recent instability. A larger sample of polluted white dwarfs, targeted with the
full ALMA array, has the potential to unambiguously identify the parent
source(s) of their planetary debris.Comment: 8 pages, 5 figures and 1 table. Accepted to MNRA
Five steps in the evolution from protoplanetary to debris disk
The protoplanetary disks of Herbig Ae stars eventually dissipate leaving a
tenuous debris disk comprised of planetesimals and dust, as well as possibly
gas and planets. This paper uses the properties of 10-20Myr A star debris disks
to consider the protoplanetary to debris disk transition. The physical
distinction between these two classes is argued to rest on the presence of
primordial gas in sufficient quantities to dominate the motion of small dust
grains (not the secondary nature of the dust or its level of stirring). This
motivates an observational classification based on the dust spectrum,
empirically defined so that A star debris disks require fractional excesses <3
at 12um and <2000 at 70um. We also propose a hypothesis to test, that the main
sequence planet/planetesimal structures are already in place (but obscured)
during the protoplanetary disk phase. This may be only weakly true if planetary
architectures change until frozen during disk dispersal, or completely false if
planets and planetesimals form during disk dispersal. Five steps in the
transition are discussed: (i) carving an inner hole to form a transition disk;
(ii) depletion of mm-sized dust in outer disk, noting the importance of
determining whether this mass ends up in planetesimals or is collisionally
depleted; (iii) final clearing of inner regions, noting that many mechanisms
replenish moderate hot dust levels at later phases, and likely also operate in
protoplanetary disks; (iv) disappearence of gas, noting recent discoveries of
primordial and secondary gas in debris disks that highlight our ignorance and
its impending enlightenment by ALMA; (v) formation of ring-like planetesimal
structures, noting these are shaped by interactions with planets, and that the
location of planetesimals in protoplanetary disks may be unrelated to the dust
concentrations therein that are set by gas interactions.The authors are grateful for
support from the European Union through ERC grant
number 279973.This is the author accepted manuscript. The final version is available via Springer at http://link.springer.com/article/10.1007/s10509-015-2315-6/fulltext.html
Gas absorption towards the η Tel debris disc: winds or clouds?
η Telescopii is an ∼23 Myr old A-type star surrounded by an edge-on debris disc hypothesized to harbour gas. Recent analysis of far- and near-ultraviolet spectroscopic observations of η Tel found absorption features at ∼−23 and ∼−18 km s−1 in several atomic lines, attributed to circumstellar and interstellar gas, respectively. In this work, we put the circumstellar origin of the gas to a test by analysing high-resolution optical spectroscopy of η Tel and of three other stars with a similar line of sight as η Tel: HD 181327, HD 180575, and ρ Tel. We found absorption features at ∼−23 and ∼−18 km s−1 in the Ca II H&K lines, and at ∼−23 km s−1 in the Na I D1&D2 doublet in η Tel, in agreement with previous findings in the ultraviolet. However, we also found absorption features at ∼−23 km s−1 in the Ca II K lines of the three other stars analysed. This strongly implies that the absorption lines previously attributed to circumstellar gas are more likely due to an interstellar cloud traversing the line of sight of η Tel instead
Formation of giant planets around intermediate-mass stars
To understand giant planet formation, we need to focus on host stars close to
, where the occurrence rate of these planets is the highest. In this initial study, we carry out pebble-driven core accretion planet formation modelling to investigate the trends and optimal conditions for the formation of giant planets around host stars in the range of
. We find that giant planets are more likely to form in systems with a larger initial disc radius; higher disc gas accretion rate; pebbles of ∼millimeter in size; and birth location of the embryo at a moderate radial distance of ∼10 au. We also conduct a population synthesis study of our model and find that the frequency of giant planets and super-Earths decreases with increasing stellar mass. This contrasts the observational peak at
, stressing the need for strong assumptions on stellar mass dependencies in this range. Investigating the combined effect of stellar mass dependent disc masses, sizes, and lifetimes in the context of planet population synthesis studies is a promising avenue to alleviate this discrepancy. The hot-Jupiter occurrence rate in our models is
around
– similar to RV observations around Sun-like stars, but drastically decreases for higher mass stars
Size-selective accretion of dust onto CPDs: Low CPD masses and filtration of larger grains
The major satellites of Jupiter and Saturn are believed to have formed in circumplanetary discs, which orbit forming giant protoplanets. Gas and dust in CPDs have different distributions and affect each other by drag, which varies with grain size. Yet simulations of multiple dust grain sizes with separate dynamics have not been done before. We seek to assess how much dust of each grain size there is in circumplanetary discs. We run multifluid 3D hydrodynamical simulations including gas and four discrete grain sizes of dust from 1μm to 1 mm, representing a continuous distribution. We consider a 1 Mjup protoplanet embedded in a protoplanetary disc around a 1 M⊙ star. Our results show a truncated MRN distribution at smaller grain sizes, which starts to tail off by a = 100μm and is near zero at 1 mm. Large dust grains, which hold most of the dust mass, have very inefficient accretion to the CPD, due to dust filtration. Therefore CPDs’ dust masses must be small, with mass ratio ∼ a few × 10¯⁶ to the protoplanet. These masses and the corresponding millimetre opacities are in line with CPD fluxes observed to date
ALMA observations of the narrow HR 4796A debris ring
The young A0V star HR 4796A is host to a bright and narrow ring of dust, thought to originate in collisions between planetesimals within a belt analogous to the Solar system’s Edgeworth–Kuiper belt. Here we present high spatial resolution 880 μm continuum images from the Atacama Large Millimeter Array. The 80 au radius dust ring is resolved radially with a characteristic width of 10 au, consistent with the narrow profile seen in scattered light. Our modelling consistently finds that the disc is also vertically resolved with a similar extent. However, this extent is less than the beam size, and a disc that is dynamically very cold (i.e. vertically thin) provides a better theoretical explanation for the narrow scattered light profile, so we remain cautious about this conclusion. We do not detect 12CO J=3–2 emission, concluding that unless the disc is dynamically cold the CO+CO2 ice content of the planetesimals is of order a few per cent or less. We consider the range of semi-major axes and masses of an interior planet supposed to cause the ring’s eccentricity, finding that such a planet should be more massive than Neptune and orbit beyond 40 au. Independent of our ALMA observations, we note a conflict between mid-IR pericentre-glow and scattered light imaging interpretations, concluding that models where the spatial dust density and grain size vary around the ring should be explored
Unlocking the secrets of the midplane gas and dust distribution in the young hybrid disc HD 141569
Context. HD 141569 is a pre-main sequence star with a disc uniquely placed between protoplanetary and debris discs, similar to the older "hybrid" type discs.
Aims: This work aims to place the mass and spatial structure of the disc midplane in the context of the debris, hybrid and protoplanetary discs.
Methods: We observed HD 141569 with ALMA in 1.3 mm continuum and 13CO (2-1). This is the first detection and image of the optically thin gas emission from the midplane of this disc.
Results: In continuum emission, we detect a combination of an unresolved central peak and a ring of millimetre emission at 220 ± 10 au, slightly interior to one of the rings discovered in scattered light. The minimum dust mass of the ring is 0.13 ± 0.02 M⊕ while the unresolved millimetre peak at the stellar location is predominantly thermal emission due to a minimum of 1.2 ± 0.2 M⊕ of dust. 13CO is distributed asymmetrically around the stellar position with a peak at 1ʺ&dotbelow;1 distance and a PA of -33°. The gas is detected as far as 220 ± 10 au, a radial separation the same as that of the mm ring. Assuming optically thin emission and standard ISM abundances, we used our 13CO data to derive the gas mass in the disc of (6.0 ± 0.9) × 10-4M⊙. Comparison to published 12CO data shows that 12CO is optically thick, explaining why estimates based on 12CO underestimated the gas mass
Exocometary gas structure, origin and physical properties around β Pictoris through ALMA CO multitransition observations
Recent ALMA observations unveiled the structure of CO gas in the 23 Myr-old
Pictoris planetary system, a component that has been discovered in many
similarly young debris disks. We here present ALMA CO J=2-1 observations, at an
improved spectro-spatial resolution and sensitivity compared to previous CO
J=3-2 observations. We find that 1) the CO clump is radially broad, favouring
the resonant migration over the giant impact scenario for its dynamical origin,
2) the CO disk is vertically tilted compared to the main dust disk, at an angle
consistent with the scattered light warp. We then use position-velocity
diagrams to trace Keplerian radii in the orbital plane of the disk. Assuming a
perfectly edge-on geometry, this shows a CO scale height increasing with radius
as , and an electron density (derived from CO line ratios through
NLTE analysis) in agreement with thermodynamical models. Furthermore, we show
how observations of optically thin line ratios can solve the primordial versus
secondary origin dichotomy in gas-bearing debris disks. As shown for
Pictoris, subthermal (NLTE) CO excitation is symptomatic of H densities
that are insufficient to shield CO from photodissociation over the system's
lifetime. This means that replenishment from exocometary volatiles must be
taking place, proving the secondary origin of the disk. In this scenario,
assuming steady state production/destruction of CO gas, we derive the CO+CO
ice abundance by mass in Pic's exocomets to be at most 6%,
consistent with comets in our own Solar System and in the coeval HD181327
system.LM acknowledges support by STFC and ESO through graduate studentships and, together with MCW and QK, by the European Union through ERC grant number 279973. Work of OP is funded by the Royal Society Dorothy Hodgkin Fellowship, and AMH gratefully acknowledges support from NSF grant AST-1412647.This is the final version of the article. It first appeared from Oxford University Press via https://doi.org/10.1093/mnras/stw241
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