28 research outputs found
Stellar winds can affect gas dynamics in debris disks and create observable belt winds
Context: Gas is now detected in many extrasolar systems around mature stars
aged between 10 Myr to 1 Gyr with planetesimal belts. Gas in these
mature disks is thought to be released from planetesimals and has been modelled
using a viscous disk approach. At low densities, this may not be a good
assumption as the gas could be blown out by the stellar wind instead.
Methods: We developed an analytical model for A to M stars that can follow
the evolution of gas outflows and target when the transition occurs between a
disk or a wind. The crucial criterion is the gas density for which gas
particles stop being protected from stellar wind protons impacting at high
velocities on radial trajectories.
Results: We find that: 1) Belts of radial width with gas densities
cm would create a wind rather
than a disk, which would explain the recent outflowing gas detection in NO Lup.
2) The properties of this belt wind can be used to measure stellar wind
properties such as their densities and velocities. 3) Debris disks with low
fractional luminosities are more likely to create gas winds, which could be
observed with current facilities.
Conclusions: The systems containing low gas masses such as Fomalhaut or TWA 7
or more generally, debris disks with fractional luminosities or stellar luminosity (A0V or earlier) would rather create gas outflows (or belt winds) than
gas disks. Gas observed to be outflowing at high velocity in the young system
NO Lup could be an example of such belt winds. The detection of these gas winds
is possible with ALMA (CO and CO could be good wind tracers) and would
allow us to constrain the stellar wind properties of main-sequence stars, which
are otherwise difficult to measure.Comment: 18 pages, 13 figures, abstract shortened, accepted for publication in
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An ALMA Survey of M-dwarfs in the Beta Pictoris Moving Group with Two New Debris Disc Detections
Previous surveys in the far-infrared have found very few, if any, M-dwarf
debris discs among their samples. It has been questioned whether M-dwarf discs
are simply less common than earlier types, or whether the low detection rate
derives from the wavelengths and sensitivities available to those studies. The
highly sensitive, long wavelength Atacama Large Millimetre/submillimetre Array
can shed light on the problem. This paper presents a survey of M-dwarf stars in
the young and nearby Beta Pictoris Moving Group with ALMA at Band 7
(880\,m). From the observational sample we detect two new sub-mm excesses
that likely constitute unresolved debris discs around GJ\,2006\,A and
AT\,Mic\,A and model distributions of the disc fractional luminosities and
temperatures. From the science sample of 36 M-dwarfs including AU\,Mic we find
a disc detection rate of 4/36 or 11.1\% that rises to
23.1\% when adjusted for completeness. We conclude that this
detection rate is consistent with the detection rate of discs around G and K
type stars and that the disc properties are also likely consistent with earlier
type stars. We additionally conclude that M-dwarf stars are not less likely to
host debris discs, but instead their detection requires longer wavelength and
higher sensitivity observations than have previously been employed.Comment: Accepted to MNRA
Inner edges of planetesimal belts: collisionally eroded or truncated?
The radial structure of debris discs can encode important information about
their dynamical and collisional history. In this paper we present a 3-phase
analytical model to analyse the collisional evolution of solids in debris
discs, focusing on their joint radial and temporal dependence. Consistent with
previous models, we find that as the largest planetesimals reach collisional
equilibrium in the inner regions, the surface density of dust and solids
becomes proportional to within a certain critical radius. We
present simple equations to estimate the critical radius and surface density of
dust as a function of the maximum planetesimal size and initial surface density
in solids (and vice versa). We apply this model to ALMA observations of 7 wide
debris discs. We use both parametric and non-parametric modelling to test if
their inner edges are shallow and consistent with collisional evolution. We
find that 4 out of 7 have inner edges consistent with collisional evolution.
Three of these would require small maximum planetesimal sizes below 10 km, with
HR 8799's disc potentially lacking solids larger than a few centimeters. The
remaining systems have inner edges that are much sharper, which requires
maximum planetesimal sizes km. Their sharp inner edges suggest they
could have been truncated by planets, which JWST could detect. In the context
of our model, we find that the 7 discs require surface densities below a
Minimum Mass Solar Nebula, avoiding the so-called disc mass problem. Finally,
during the modelling of HD 107146 we discover that its wide gap is split into
two narrower ones, which could be due to two low-mass planets formed within the
disc.Comment: Accepted for publication in MNRAS, 21 pages, 11 figure
A Deep Search for Five Molecules in the 49 Ceti Debris Disk
Surprisingly strong CO emission has been observed from more than a dozen debris disks around nearby main-sequence stars. The origin of this CO is unclear, in particular whether it is left over from the protoplanetary disk phase or is second-generation material released from collisions between icy bodies like debris dust. The primary unexplored avenue for distinguishing the origin of the material is understanding its molecular composition. Here we present a deep search for five molecules (CN, HCN, HCO+, SiO, and CH3OH) in the debris disk around 49 Ceti. We take advantage of the high sensitivity of the Atacama Large Millimeter/submillimeter Array at Band 7 to integrate for 3.2 hr at modest spatial (1″) and spectral (0.8 km s-1) resolution. Our search yields stringent upper limits on the flux of all surveyed molecular lines, which imply abundances relative to CO that are orders of magnitude lower than those observed in protoplanetary disks and solar system comets, and also those predicted in outgassing models of second-generation material. However, if C I shielding is responsible for extending the lifetime of any CO produced in second-generation collisions as proposed by Kral et al., then the line ratios do not reflect true ice phase chemical abundances but rather imply that CO is shielded by its own photodissociation product, C I, and other molecules are rapidly photodissociated by the stellar and interstellar radiation field
Herschel Observations of Disks around Late-type Stars
A set of twenty late-type (K5-M5) stars were observed with the Herschel Space Observatory at 100 and 160 microns with the goal of searching for far-infrared excesses indicative of the presence of circumstellar disks. Out of this sample, four stars (TYC 7443-1102-1, TYC 9340-437-1, GJ 784 and GJ 707) have infrared excesses above their stellar photospheres at either 100 or 160 μm or both. At 100 microns TYC 9340-437-1 is spatially resolved with a shape that suggests it is surrounded by a face-on disk. The 100 μm excess flux associated with GJ 707 is marginal at around 3σ. The excess flux associated with GJ 784 is most likely due to a background galaxy as the dust radius estimated from the spectral energy fit implies that any associated dust disk should have been resolved in the Herschel images but is not. TYC 7443-1102-1 has been observed with ALMA which resolves the emission at its location into two distinct sources making the Herschel excess most likely also due to a background galaxy. It is worth noting that this star is in the 23 Myr old β Pic association. With a disk luminosity on the order of 10⁻³ L_*, this system is an ideal follow-up target for high-contrast imaging and ALMA
Dust Populations in the Iconic Vega Planetary System Resolved by ALMA
The Vega planetary system hosts the archetype of extrasolar Kuiper belts, and
is rich in dust from the sub-au region out to 100's of au, suggesting intense
dynamical activity. We present ALMA mm observations that detect and resolve the
outer dust belt from the star for the first time. The interferometric
visibilities show that the belt can be fit by a Gaussian model or by power-law
models with a steep inner edge (at 60-80 au). The belt is very broad, extending
out to at least 150-200 au. We strongly detect the star and set a stringent
upper limit to warm dust emission previously detected in the infrared. We
discuss three scenarios that could explain the architecture of Vega's planetary
system, including the new {ALMA} constraints: no outer planets, a chain of
low-mass planets, and a single giant planet. The planet-less scenario is only
feasible if the outer belt was born with the observed sharp inner edge. If
instead the inner edge is currently being truncated by a planet, then the
planet must be 6 M and at au to have cleared
its chaotic zone within the system age. In the planet chain scenario, outward
planet migration and inward scattering of planetesimals could produce the hot
and warm dust observed in the inner regions of the system. In the single giant
planet scenario, an asteroid belt could be responsible for the warm dust, and
mean motion resonances with the planet could put asteroids on star-grazing
orbits, producing the hot dust.Comment: 18 pages, 3 figures, Accepted for publication in Ap