29 research outputs found
A quantification of hydrodynamical effects on protoplanetary dust growth
Context. The growth process of dust particles in protoplanetary disks can be
modeled via numerical dust coagulation codes. In this approach, physical
effects that dominate the dust growth process often must be implemented in a
parameterized form. Due to a lack of these parameterizations, existing studies
of dust coagulation have ignored the effects a hydrodynamical gas flow can have
on grain growth, even though it is often argued that the flow could
significantly contribute either positively or negatively to the growth process.
Aims. We intend to provide a quantification of hydrodynamical effects on the
growth of dust particles, such that these effects can be parameterized and
implemented in a dust coagulation code.
Methods. We numerically integrate the trajectories of small dust particles in
the flow of disk gas around a proto-planetesimal, sampling a large parameter
space in proto-planetesimal radii, headwind velocities, and dust stopping
times.
Results. The gas flow deflects most particles away from the
proto-planetesimal, such that its effective collisional cross section, and
therefore the mass accretion rate, is reduced. The gas flow however also
reduces the impact velocity of small dust particles onto a proto-planetesimal.
This can be beneficial for its growth, since large impact velocities are known
to lead to erosion. We also demonstrate why such a gas flow does not return
collisional debris to the surface of a proto-planetesimal.
Conclusions. We predict that a laminar hydrodynamical flow around a
proto-planetesimal will have a significant effect on its growth. However, we
cannot easily predict which result, the reduction of the impact velocity or the
sweep-up cross section, will be more important. Therefore, we provide
parameterizations ready for implementation into a dust coagulation code.Comment: 9 pages, 6 figures; accepted for publication in A&A; v2 matches the
manuscript sent to the publisher (very minor changes
Breaking through: The effects of a velocity distribution on barriers to dust growth
It is unknown how far dust growth can proceed by coagulation. Obstacles to
collisional growth are the fragmentation and bouncing barriers. However, in all
previous simulations of the dust-size evolution in protoplanetary disks, only
the mean collision velocity has been considered, neglecting that a small but
possibly important fraction of the collisions will occur at both much lower and
higher velocities. We study the effect of the probability distribution of
impact velocities on the collisional dust growth barriers. Assuming a
Maxwellian velocity distribution for colliding particles to determine the
fraction of sticking, bouncing, and fragmentation, we implement this in a
dust-size evolution code. We also calculate the probability of growing through
the barriers and the growth timescale in these regimes. We find that the
collisional growth barriers are not as sharp as previously thought. With the
existence of low-velocity collisions, a small fraction of the particles manage
to grow to masses orders of magnitude above the main population. A particle
velocity distribution softens the fragmentation barrier and removes the
bouncing barrier. It broadens the size distribution in a natural way, allowing
the largest particles to become the first seeds that initiate sweep-up growth
towards planetesimal sizes.Comment: 4 pages, 3 figures. Accepted for publication as a Letter in Astronomy
and Astrophysic
Using Galactic Cepheids to verify Gaia parallaxes
Context. The Gaia satellite will measure highly accurate absolute parallaxes
of hundreds of millions of stars by comparing the parallactic displacements in
the two fields of view of the optical instrument. The requirements on the
stability of the 'basic angle' between the two fields are correspondingly
strict, and possible variations (on the microarcsec level) are therefore
monitored by an on-board metrology system. Nevertheless, since even very small
periodic variations of the basic angle might cause a global offset of the
measured parallaxes, it is important to find independent verification methods.
Aims. We investigate the potential use of Galactic Cepheids as standard candles
for verifying the Gaia parallax zero point. Methods. We simulate the complete
population of Galactic Cepheids and their observations by Gaia. Using the
simulated data, simultaneous fits are made of the parameters of the
period-luminosity relation and a global parallax zero point. Results. The total
number of Galactic Cepheids is estimated at about 20 000, of which nearly half
could be observed by Gaia. In the most favourable circumstances, including
negligible intrinsic scatter and extinction errors, the determined parallax
zero point has an uncertainty of 0.2 microarcsec. With more realistic
assumptions the uncertainty is several times larger, and the result is very
sensitive to errors in the applied extinction corrections. Conclusions. The use
of Galactic Cepheids alone will not be sufficient to determine a possible
parallax zero-point error to the full potential systematic accuracy of Gaia.
The global verification of Gaia parallaxes will most likely depend on a
combination of many different methods, including this one.Comment: 7 pages, 6 figures. Accepted for publication in Astronomy and
Astrophysic
Building the cosmic distance scale: from Hipparcos to Gaia
Hipparcos, the first ever experiment of global astrometry, was launched by
ESA in 1989 and its results published in 1997 (Perryman et al., Astron.
Astrophys. 323, L49, 1997; Perryman & ESA (eds), The Hipparcos and Tycho
catalogues, ESA SP-1200, 1997). A new reduction was later performed using an
improved satellite attitude reconstruction leading to an improved accuracy for
stars brighter than 9th magnitude (van Leeuwen & Fantino, Astron. Astrophys.
439, 791, 2005; van Leeuwen, Astron. Astrophys. 474, 653, 2007).
The Hipparcos Catalogue provided an extended dataset of very accurate
astrometric data (positions, trigonometric parallaxes and proper motions),
enlarging by two orders of magnitude the quantity and quality of distance
determinations and luminosity calibrations. The availability of more than 20000
stars with a trigonometric parallax known to better than 10% opened the way to
a drastic revision of our 3-D knowledge of the solar neighbourhood and to a
renewal of the calibration of many distance indicators and age estimations. The
prospects opened by Gaia, the next ESA cornerstone, planned for launch in June
2013 (Perryman et al., Astron. Astrophys. 369, 339, 2001), are still much more
dramatic: a billion objects with systematic and quasi simultaneous astrometric,
spectrophotometric and spectroscopic observations, about 150 million stars with
expected distances to better than 10%, all over the Galaxy. All stellar
distance indicators, in very large numbers, will be directly measured,
providing a direct calibration of their luminosity and making possible detailed
studies of the impacts of various effects linked to chemical element
abundances, age or cluster membership. With the help of simulations of the data
expected from Gaia, obtained from the mission simulator developed by DPAC, we
will illustrate what Gaia can provide with some selected examples.Comment: 16 pages, 16 figures, Conference "The Fundamental Cosmic Distance
scale: State of the Art and the Gaia perspective, 3-6 May 2011, INAF,
Osservatorio Astronomico di Capodimonte, Naples. Accepted for publication in
Astrophysics & Space Scienc
Evidence for the formation of comet 67P/Churyumov-Gerasimenko through gravitational collapse of a bound clump of pebbles
The processes that led to the formation of the planetary bodies in the Solar System are still not fully understood. Using the results obtained with the comprehensive suite of instruments on-board ESA’s Rosetta mission, we present evidence that comet 67P/Churyumov-Gerasimenko likely formed through the gentle gravitational collapse of a bound clump of mm-sized dust aggregates (“pebbles”), intermixed with microscopic ice particles. This formation scenario leads to a cometary make-up that is simultaneously compatible with the global porosity, homogeneity, tensile strength, thermal inertia, vertical temperature profiles, sizes and porosities of emitted dust, and the steep increase in water-vapour production rate with decreasing heliocentric distance, measured by the instruments on-board the Rosetta spacecraft and the Philae lander. Our findings suggest that the pebbles observed to be abundant in protoplanetary discs around young stars provide the building material for comets and other minor bodies
The Large Magellanic Cloud and the Distance Scale
The Magellanic Clouds, especially the Large Magellanic Cloud, are places
where multiple distance indicators can be compared with each other in a
straight-forward manner at considerable precision. We here review the distances
derived from Cepheids, Red Variables, RR Lyraes, Red Clump Stars and Eclipsing
Binaries, and show that the results from these distance indicators generally
agree to within their errors, and the distance modulus to the Large Magellanic
Cloud appears to be defined to 3% with a mean value of 18.48 mag, corresponding
to 49.7 Kpc. The utility of the Magellanic Clouds in constructing and testing
the distance scale will remain as we move into the era of Gaia.Comment: 23 pages, accepted for publication in Astrophysics and Space Science.
From a presentation at the conference The Fundamental Cosmic Distance Scale:
State of the Art and the Gaia Perspective, Naples, May 201
Planetesimal formation by sweep-up: How the bouncing barrier can be beneficial to growth
The formation of planetesimals is often accredited to collisional sticking of
dust grains. The exact process is unknown, as collisions between larger
aggregates tend to lead to fragmentation or bouncing rather than sticking.
Recent laboratory experiments have however made great progress in the
understanding and mapping of the complex physics involved in dust collisions.
We want to study the possibility of planetesimal formation using the results
from the latest laboratory experiments, particularly by including the
fragmentation with mass transfer effect, which might lead to growth even at
high impact velocities. We present a new experimentally and physically
motivated dust collision model capable of predicting the outcome of a collision
between two particles of arbitrary masses and velocities. It is used together
with a continuum dust-size evolution code that is both fast in terms of
execution time and able to resolve the dust well at all sizes, allowing for all
types of interactions to be studied without biases. We find that for the
general dust population, bouncing collisions prevent the growth above
millimeter-sizes. However, if a small number of cm-sized particles are
introduced, for example due to vertical mixing or radial drift, they can act as
a catalyst and start to sweep up the smaller particles. At a distance of 3 AU,
100-meter-sized bodies are formed on a timescale of 1 Myr. We conclude that
direct growth of planetesimals might be a possibility thanks to a combination
of the existence of a bouncing barrier and the fragmentation with mass transfer
effect. The bouncing barrier is here even beneficial, as it prevents the growth
of too many large particles that would otherwise only fragment among each
other, and creates a reservoir of small particles that can be swept up by
larger bodies. However, for this process to work, a few seeds of cm in size or
larger have to be introduced.Comment: 17 pages, 13 figures. Accepted for publication in Astronomy and
Astrophysic
Modeling dust growth in protoplanetary disks: The breakthrough case
Context. Dust coagulation in protoplanetary disks is one of the initial steps toward planet formation. Simple toy models are often not sufficient to cover the complexity of the coagulation process, and a number of numerical approaches are therefore used, among which integration of the Smoluchowski equation and various versions of the Monte Carlo algorithm are the most popular.
Aims. Recent progress in understanding the processes involved in dust coagulation have caused a need for benchmarking and comparison of various physical aspects of the coagulation process. In this paper, we directly compare the Smoluchowski and Monte Carlo approaches to show their advantages and disadvantages.
Methods. We focus on the mechanism of planetesimal formation via sweep-up growth, which is a new and important aspect of the current planet formation theory. We use realistic test cases that implement a distribution in dust collision velocities. This allows a single collision between two grains to have a wide range of possible outcomes but also requires a very high numerical accuracy.
Results. For most coagulation problems, we find a general agreement between the two approaches. However, for the sweep-up growth driven by the “lucky” breakthrough mechanism, the methods exhibit very different resolution dependencies. With too few mass bins, the Smoluchowski algorithm tends to overestimate the growth rate and the probability of breakthrough. The Monte Carlo method is less dependent on the number of particles in the growth timescale aspect but tends to underestimate the breakthrough chance due to its limited dynamic mass range.
Conclusions. We find that the Smoluchowski approach, which is generally better for the breakthrough studies, is sensitive to low mass resolutions in the high-mass, low-number tail that is important in this scenario. To study the low number density features, a new modulation function has to be introduced to the interaction probabilities. As the minimum resolution needed for breakthrough studies depends strongly on setup, verification has to be performed on a case by case basis
Planetesimal formation via sweep-up growth at the inner edge of dead zones
Context. The early stages of planet formation are still not well understood. Coagulation models have revealed numerous obstacles to the dust growth, such as the bouncing, fragmentation, and radial drift barriers. Gas drag causes rapid loss, and turbulence leads to generally destructive collisions between the dust aggregates.
Aims. We study the interplay between dust coagulation and drift to determine the conditions in protoplanetary disk that support the formation of planetesimals. We focus on planetesimal formation via sweep-up and investigate whether it can take place in a realistic protoplanetary disk.
Methods. We have developed a new numerical model that resolves the spatial distribution of dust in the radial and vertical dimensions. The model uses representative particles approach to follow the dust evolution in a protoplanetary disk. The coagulation and fragmentation of solids is taken into account in the Monte Carlo method. A collision model adopting the mass transfer effect, which can occur for different-sized dust aggregate collisions, is implemented. We focus on a protoplanetary disk that includes a pressure bump caused by a steep decline of turbulent viscosity around the snow line.
Results. Our results show that high enough resolution of the vertical disk structure in dust coagulation codes is needed to obtain adequately short growth timescales, especially in the case of a low turbulence region. We find that a sharp radial variation in the turbulence strength at the inner edge of dead zone promotes planetesimal formation in several ways. It provides a pressure bump that efficiently prevents the dust from drifting inwards. It also causes a radial variation in the size of aggregates at which growth barriers occur, favoring the growth of large aggregates by sweeping up of small particles. In our model, by employing an ad hoc α viscosity change near the snow line, it is possible to grow planetesimals by incremental growth on timescales of approximately 105 years