110 research outputs found
Contrast sensitivities in the Gaia Data Release 2
The source detection sensitivity of Gaia is reduced near sources. To
characterise this contrast sensitivity is important for understanding the
completeness of the Gaia data products, in particular when evaluating source
confusion in less well resolved surveys, such as in photometric monitoring for
transits. Here, we statistically evaluate the catalog source density to
determine the Gaia Data Release 2 source detection sensitivity as a function of
angular separation and brightness ratio from a bright source. The contrast
sensitivity from 0.4 arcsec out to 12 arcsec ranges in DG = 0-14 mag. We find
the derived contrast sensitivity to be robust with respect to target
brightness, colour, source density, and Gaia scan coverage.Comment: Accepted to A&A. 3 pages, two figure
Tidal disruption versus planetesimal collisions as possible origins for the dispersing dust cloud around Fomalhaut
Recent analysis suggests that the faint optical point source observed around
Fomalhaut from 2004-2014 (Fomalhaut b) is gradually fading and expanding,
supporting the case that it may be a dispersing dust cloud resulting from the
sudden disruption of a planetesimal. These types of disruptions may arise from
catastrophic collisions of planetesimals, which are perturbed from their
original orbits in the Fomalhaut dust ring by nearby giant planets. However,
disruptions can also occur when the planetesimals pass within the tidal
disruption field of the planet(s) that perturbed them in the first place,
similar to the Shoemaker-Levy event observed in the Solar System. Given that a
gravitationally focusing giant planet has a much larger interaction
cross-section than a planetesimal, tidal disruption events can match or
outnumber planetesimal collision events in realistic regions of parameter
space. Intriguingly, the Fomalhaut dust cloud offers an opportunity to directly
distinguish between these scenarios. A tidal disruption scenario leads to a
very specific prediction of ephemerides for the planet causing the event. At a
most probable mass of 66 Mearth, a semi-major axis of 117 AU, and a system age
of 400-500 Myr, this planet would be readily detectable with the James Webb
Space Telescope. The presence or absence of this planet at the specific,
predicted position is therefore a distinctive indicator of whether the
dispersing cloud originated from a collision of two planetesimals or from the
disruption of a planetesimal in the tidal field of a giant planet.Comment: 6 pages, 3 figures, accepted for publication in A&
On the unusual gas composition in the Beta Pictoris debris disk
The metallic gas associated with the Beta Pic debris disk is not believed to
be primordial, but arises from the destruction of dust grains. Recent
observations have shown that carbon and oxygen in this gas are exceptionally
overabundant compared to other elements, by some 400 times. We study the origin
of this enrichment under two opposing hypothesis, preferential production,
where the gas is produced with the observed unusual abundance (as may happen if
gas is produced by photo-desorption from C/O-rich icy grains), and preferential
depletion, where the gas evolves to the observed state from an original solar
abundance (if outgassing occurs under high-speed collisions) under a number of
dynamical processes. We include ... ... We find ... ...Comment: accepted to ApJ, revised text is highlighted in red color, the old
figure 2 is removed, other figures are revise
Accretion Disks around Young Stars: Lifetimes, Disk Locking and Variability
We report the findings of a comprehensive study of disk accretion and related
phenomena in four of the nearest young stellar associations spanning 6-30
million years in age, an epoch that may coincide with the late stages of planet
formation. We have obtained ~650 multi-epoch high-resolution optical spectra of
100 low-mass stars that are likely members of the eta Chamaeleontis (~6 Myr),
TW Hydrae (~8 Myr), beta Pictoris (~12 Myr) and Tucanae-Horologium (~30 Myr)
groups. Our data were collected over 12 nights between 2004 December - 2005
July on the Magellan Clay 6.5m telescope. Based on H line profiles,
along with a variety of other emission lines, we find clear evidence of
on-going accretion in three out of 11 eta Cha stars and two out of 32 TW Hydrae
members. None of the 57 beta Pic or Tuc-Hor members shows measurable signs of
accretion. Together, these results imply significant evolution of the disk
accretion process within the first several Myr of a low-mass star's life. While
a few disks can continue to accrete for up to ~10 Myr, our findings suggest
that disks accreting for beyond that timescale are rather rare. This result
provides an indirect constraint on the timescale for gas dissipation in inner
disks and, in turn, on gas giant planet formation. All accretors in our sample
are slow rotators, whereas non-accretors cover a large range in rotational
velocities. This may hint at rotational braking by disks at ages up to ~8 Myr.
Our multi-epoch spectra confirm that emission-line variability is common even
in somewhat older T Tauri stars, among which accretors tend to show
particularly strong variations. Thus, our results indicate that accretion and
wind activity undergo significant and sustained variations throughout the
lifetime of accretion disks.Comment: to appear in The Astrophysical Journa
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