2,100 research outputs found
Neutron Star Planets: Atmospheric processes and habitability
Of the roughly 3000 neutron stars known, only a handful have sub-stellar
companions. The most famous of these are the low-mass planets around the
millisecond pulsar B1257+12. New evidence indicates that observational biases
could still hide a wide variety of planetary systems around most neutron stars.
We consider the environment and physical processes relevant to neutron star
planets, in particular the effect of X-ray irradiation and the relativistic
pulsar wind on the planetary atmosphere. We discuss the survival time of planet
atmospheres and the planetary surface conditions around different classes of
neutron stars, and define a neutron star habitable zone. Depending on as-yet
poorly constrained aspects of the pulsar wind, both Super-Earths around
B1257+12 could lie within its habitable zone.Comment: Submitted to A&
Spirals in protoplanetary disks from photon travel time
Spiral structures are a common feature in scattered-light images of
protoplanetary disks, and of great interest as possible tracers of the presence
of planets. However, other mechanisms have been put foward to explain them,
including self-gravity, disk-envelope interactions, and dead zone boundaries.
These mechanisms explain many spirals very well, but are unable to easily
account for very loosely wound spirals and single spiral arms. We study the
effect of light travel time on the shape of a shadow cast by a clump orbiting
close (within au) of the central star, where there can be
significant orbital motion during the light travel time from the clump to the
outer disk and then to the sky plane. This delay in light rays reaching the sky
plane gives rise to a variety of spiral- and arc-shaped shadows, which we
describe with a general fitting formula for a flared, inclined disk.Comment: Accepted for publication in A&A Letters. Videos available at
dl.dropboxusercontent.com/u/3526708/spiralmovies.zi
Near-infrared emission from sublimating dust in collisionally active debris disks
Hot exozodiacal dust is thought to be responsible for excess near-infrared
(NIR) emission emanating from the innermost parts of some debris disks. The
origin of this dust, however, is still a matter of debate. We test whether hot
exozodiacal dust can be supplied from an exterior parent belt by
Poynting-Robertson (P-R) drag, paying special attention to the pile-up of dust
that occurs due to the interplay of P-R drag and dust sublimation.
Specifically, we investigate whether pile-ups still occur when collisions are
taken into account, and if they can explain the observed NIR excess. We compute
the steady-state distribution of dust in the inner disk by solving the
continuity equation. First, we derive an analytical solution under a number of
simplifying assumptions. Second, we develop a numerical debris disk model that
for the first time treats the complex interaction of collisions, P-R drag, and
sublimation in a self-consistent way. From the resulting dust distributions we
generate thermal emission spectra and compare these to observed excess NIR
fluxes. We confirm that P-R drag always supplies a small amount of dust to the
sublimation zone, but find that a fully consistent treatment yields a maximum
amount of dust that is about 7 times lower than that given by analytical
estimates. The NIR excess due this material is much smaller (<10^-3 for A-type
stars with parent belts at >1 AU) than the values derived from interferometric
observations (~10^-2). Pile-up of dust still occurs when collisions are
considered, but its effect on the NIR flux is insignificant. Finally, the
cross-section in the innermost regions is clearly dominated by barely bound
grains.Comment: 18 pages, 10 figures, A&A accepte
The lunar phases of dust grains orbiting Fomalhaut
Optical images of the nearby star Fomalhaut show a ring of dust orbiting the
central star. This dust is in many respects expected to be similar to the
zodiacal dust in the solar system. The ring displays a clear brightness
asymmetry, attributed to asymmetric scattering of the central starlight by the
circumstellar dust grains. Recent measurements show that the bright side of the
Fomalhaut ring is oriented away from us. This implies that the grains in this
system scatter most of the light in the backward direction, in sharp contrast
to the forward-scattering nature of the grains in the solar system. In this
letter, we show that grains considerably larger than those dominating the solar
system zodiacal dust cloud provide a natural explanation for the apparent
backward scattering behavior. In fact, we see the phases of the dust grains in
the same way as we can observe the phases of the Moon and other large solar
system bodies. We outline how the theory of the scattering behavior of
planetesimals can be used to explain the Fomalhaut dust properties. This
indicates that the Fomalhaut dust ring is dominated by very large grains. The
material orbiting Fomalhaut, which is at the transition between dust and
planetesimals, can, with respect to their optical behavior, best be described
as micro-asteroids.Comment: Accepted for publication in A&
Searching for a dusty cometary belt around TRAPPIST-1 with ALMA
Low-mass stars might offer today the best opportunities to detect and characterize planetary systems, especially those harbouring close-in low-mass temperate planets. Among those stars, TRAPPIST-1 is exceptional since it has seven Earth-sized planets, of which three could sustain liquid water on their surfaces. Here we present new and deep ALMA observations of TRAPPIST-1 to look for an exo-Kuiper belt which can provide clues about the formation and architecture of this system. Our observations at 0.88 mm did not detect dust emission, but can place an upper limit of 23 µJy if the belt is smaller than 4 au, and 0.15 mJy if resolved and 100 au in radius. These limits correspond to low dust masses of ̃10-5 to 10-2 M⊕, which are expected after 8 Gyr of collisional evolution unless the system was born with a >20 M⊕ belt of 100 km-sized planetesimals beyond 40 au or suffered a dynamical instability. This 20 M⊕ mass upper limit is comparable to the combined mass in TRAPPIST-1 planets, thus it is possible that most of the available solid mass in this system was used to form the known planets. A similar analysis of the ALMA data on Proxima Cen leads us to conclude that a belt born with a mass ≳1 M⊕ in 100 km-sized planetesimals could explain its putative outer belt at 30 au. We recommend that future characterizations of debris discs around low-mass stars should focus on nearby and young systems if possible
The methanol lines and hot core of OMC2-FIR4, an intermediate-mass protostar, with Herschel/HIFI
In contrast with numerous studies on the physical and chemical structure of low- and high-mass protostars, much less is known about their intermediate-mass counterparts, a class of objects that could help to elucidate the mechanisms of star formation on both ends of the mass range. We present the first results from a rich HIFI spectral dataset on an intermediate-mass protostar, OMC2-FIR4, obtained in the CHESS (Chemical HErschel Survey of Star forming regions) key programme. The more than 100 methanol lines detected between 554 and 961 GHz cover a range in upper level energy of 40 to 540 K. Our physical interpretation focusses on the hot core, but likely the cold envelope and shocked regions also play a role in reality, because an analysis of the line profiles suggests the presence of multiple emission components. An upper limit of 10^(-6) is placed on the methanol abundance in the hot core, using a population diagram, large-scale source model and other considerations. This value is consistent with abundances previously seen in low-mass hot cores. Furthermore, the highest energy lines at the highest frequencies display asymmetric profiles, which may arise from infall around the hot core
Measuring the atomic composition of planetary building blocks
Volatile molecules are critical to terrestrial planetary habitability, yet
difficult to observe directly where planets form at the midplanes of
protoplanetary disks. It is unclear whether the inner 1 AU of disks are
volatile-poor or if this region is resupplied with ice-rich dust from colder
disk regions. Dust traps at radial pressure maxima bounding disk gaps can cut
off the inner disk from such volatile reservoirs. However, the trap retention
efficiency and atomic composition of trapped dust have not been measured.
We present a new technique to measure the absolute atomic abundances in gas
accreting onto T Tauri stars and infer the bulk atomic composition and
distribution of midplane solids retained in the disk around the young star TW
Hya. We identify line emission from gas-phase material inside the dust
sublimation rim of TW Hya. Gaussian decomposition of the strongest H Paschen
lines isolates the inner disk hydrogen emission. We measure several key
elemental abundances, relative to hydrogen, using a chemical photoionization
model and infer dust retention in the disk. With a 1D transport model, we
determine approximate radial locations and retention efficiencies of dust traps
for different elements. Volatile and refractory elements are depleted from TW
Hya's hot gas by factors of ~10^2 and up to 10^5, respectively. Dust traps
beyond the CO and N2 snowline cumulatively sequester 96% of the total dust,
while the trap at 2 AU retains 3% of the initial dust mass. The high depletions
of Si, Mg, and Ca are explained by a third trap at 0.3 AU. TW Hya has a
significant volatile reservoir rich in C- and N-ices in its outer ring
structure. However, the lack of C resupply may leave the terrestrial
planet-forming region dry and carbon-poor. Any planets that form within the
silicate dust trap at 0.3 AU may resemble Earth in terms of the degree of their
volatile depletion.Comment: Accepted for publication in A&A Letters. 6 pages, 3 figures, plus
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