393 research outputs found
ALMA observations of the molecular gas in the debris disk of the 30 Myr old star HD 21997
The 30 Myr old A3-type star HD 21997 is one of the two known debris dust
disks having a measurable amount of cold molecular gas. With the goal of
understanding the physical state, origin, and evolution of the gas in young
debris disks, we obtained CO line observations with the Atacama Large
Millimeter/submillimeter Array (ALMA). Here we report on the detection of 12CO
and 13CO in the J=2-1 and J=3-2 transitions and C18O in the J=2-1 line. The gas
exhibits a Keplerian velocity curve, one of the few direct measurements of
Keplerian rotation in young debris disks. The measured CO brightness
distribution could be reproduced by a simple star+disk system, whose parameters
are r_in < 26 AU, r_out = 138 +/- 20 AU, M_*=1.8 +0.5 -0.2 M_Sun, and i = 32.6
+/- 3.1 degrees. The total CO mass, as calculated from the optically thin C18O
line, is about (4-8) x 10^-2 M_Earth, while the CO line ratios suggest a
radiation temperature on the order of 6-9 K. Comparing our results with those
obtained for the dust component of the HD 21997 disk from the ALMA continuum
observations by Mo\'or et al., we conclude that comparable amounts of CO gas
and dust are present in the disk. Interestingly, the gas and dust in the HD
21997 system are not co-located, indicating a dust-free inner gas disk within
55 AU of the star. We explore two possible scenarios for the origin of the gas.
A secondary origin, which involves gas production from colliding or active
planetesimals, would require unreasonably high gas production rates and would
not explain why the gas and dust are not co-located. We propose that HD 21997
is a hybrid system where secondary debris dust and primordial gas coexist. HD
21997, whose age exceeds both the model predictions for disk clearing and the
ages of the oldest T Tauri-like or transitional gas disks in the literature,
may be a key object linking the primordial and the debris phases of disk
evolution.Comment: 8 pages, 4 figures, accepted for publication in Ap
Near-infrared spectroscopy of EX Lupi in outburst
EX Lup is the prototype of the EXor class of young eruptive stars: objects
showing repetitive brightenings due to increased accretion from the
circumstellar disk to the star. In this paper, we report on medium-resolution
near-infrared spectroscopy of EX\,Lup taken during its extreme outburst in
2008, as well as numerical modeling with the aim of determining the physical
conditions around the star. We detect emission lines from atomic hydrogen,
helium, and metals, as well as first overtone bandhead emission from carbon
monoxide. Our results indicate that the emission lines are originating from gas
located in a dust-free region within ~ 0.2 AU of the star. The profile of the
CO bandhead indicates that the CO gas has a temperature of 2500 K, and is
located in the inner edge of the disk or in the outer parts of funnel flows.
The atomic metals are probably co-located with the CO. Some metallic lines are
fluorescently excited, suggesting direct exposure to ultraviolet photons. The
Brackett series indicates emission from hot (10000 K) and optically thin gas.
The hydrogen lines display a strong spectro-astrometric signal, suggesting that
the hydrogen emission is probably not coming from an equatorial boundary layer;
a funnel flow or disk wind origin is more likely. This picture is broadly
consistent with the standard magnetospheric accretion model usually assumed for
normally accreting T Tauri stars. Our results also set constraints on the
eruption mechanism, supporting a model where material piles up around the
corotation radius and episodically falls onto the star.Comment: 12 pages, 8 figures, 1 table, accepted for publication in Ap
Khovanov homology is an unknot-detector
We prove that a knot is the unknot if and only if its reduced Khovanov
cohomology has rank 1. The proof has two steps. We show first that there is a
spectral sequence beginning with the reduced Khovanov cohomology and abutting
to a knot homology defined using singular instantons. We then show that the
latter homology is isomorphic to the instanton Floer homology of the sutured
knot complement: an invariant that is already known to detect the unknot.Comment: 124 pages, 13 figure
Effects of Temperature–Climate Patterns on the Production of Some Competitive Species on Grounds of Modelling
Climate change has serious effects on the setting
up and the operation of natural ecosystems. Small increase
in temperature could cause rise in the amount of some
species or potential disappearance of others. During our
researches, the dispersion of the species and biomass
production of a theoretical ecosystem were examined on
the effect of the temperature–climate change. The answers
of the ecosystems which are given to the climate change
could be described by means of global climate modelling
and dynamic vegetation models. The examination of the
operation of the ecosystems is only possible in huge centres
on supercomputers because of the number and the
complexity of the calculation. The number of the calculation
could be decreased to the level of a PC by considering
the temperature and the reproduction during modelling a
theoretical ecosystem, and several important theoretical
questions could be answered
Anion emission from water molecules colliding with positive ions: Identification of binary and many-body processes
It is shown that negative ions are ejected from gas-phase water molecules
when bombarded with positive ions at keV energies typical of solar-wind
velocities. This finding is relevant for studies of planetary and cometary
atmospheres, as well as for radiolysis and radiobiology. Emission of both H-
and heavier (O- and OH-) anions, with a larger yield for H-, was observed in
6.6-keV 16O+ + H2O collisions. The ex-perimental setup allowed separate
identification of anions formed in collisions with many-body dynamics from
those created in hard, binary collisions. Most of the ani-ons are emitted with
low kinetic energy due to many-body processes. Model calcu-lations show that
both nucleus-nucleus interactions and electronic excitations con-tribute to the
observed large anion emission yield.Comment: 5 pages, 4 figure
Mid-infrared interferometric variability of DG Tau: implications for the inner-disk structure
Context. DG Tau is a low-mass pre-main sequence star, whose strongly
accreting protoplanetary disk exhibits a so-far enigmatic behavior: its
mid-infrared thermal emission is strongly time-variable, even turning the 10
m silicate feature from emission to absorption temporarily. Aims. We look
for the reason for the spectral variability at high spatial resolution and at
multiple epochs. Methods. We study the temporal variability of the mid-infrared
interferometric signal, observed with the VLTI/MIDI instrument at six epochs
between 2011 and 2014. We fit a geometric disk model to the observed
interferometric signal to obtain spatial information about the disk. We also
model the mid-infrared spectra by template fitting to characterize the profile
and time dependence of the silicate emission. We use physically motivated
radiative transfer modeling to interpret the mid-infrared interferometric
spectra. Results. The inner disk (r<1-3 au) spectra exhibit a 10 m
absorption feature related to amorphous silicate grains. The outer disk (r>1-3
au) spectra show a crystalline silicate feature in emission, similar to the
spectra of comet Hale-Bopp. The striking difference between the inner and outer
disk spectral feature is highly unusual among T Tauri stars. The mid-infrared
variability is dominated by the outer disk. The strength of the silicate
feature changed by more than a factor of two. Between 2011 and 2014 the
half-light radius of the mid-infrared-emitting region decreased from 1.15 to
0.7 au. Conclusions. For the origin of the absorption we discuss four possible
explanations: a cold obscuring envelope, an accretion heated inner disk, a
temperature inversion on the disk surface and a misaligned inner geometry. The
silicate emission in the outer disk can be explained by dusty material high
above the disk plane, whose mass can change with time, possibly due to
turbulence in the disk.Comment: 16 pages, 13 figure
Discovery of molecular gas around HD 131835 in an APEX molecular line survey of bright debris disks
Debris disks are considered to be gas-poor, but recent observations revealed
molecular or atomic gas in several 10-40 Myr old systems. We used the APEX and
IRAM 30m radiotelescopes to search for CO gas in 20 bright debris disks. In one
case, around the 16 Myr old A-type star HD 131835, we discovered a new
gas-bearing debris disk, where the CO 3-2 transition was successfully detected.
No other individual system exhibited a measurable CO signal. Our Herschel Space
Observatory far-infrared images of HD 131835 marginally resolved the disk both
at 70 and 100m, with a characteristic radius of ~170 au. While in stellar
properties HD 131835 resembles Pic, its dust disk properties are
similar to those of the most massive young debris disks. With the detection of
gas in HD 131835 the number of known debris disks with CO content has increased
to four, all of them encircling young (40 Myr) A-type stars. Based on
statistics within 125 pc, we suggest that the presence of detectable amount of
gas in the most massive debris disks around young A-type stars is a common
phenomenon. Our current data cannot conclude on the origin of gas in HD 131835.
If the gas is secondary, arising from the disruption of planetesimals, then HD
131835 is a comparably young and in terms of its disk more massive analogue of
the Pic system. However, it is also possible that this system similarly
to HD 21997 possesses a hybrid disk, where the gas material is predominantly
primordial, while the dust grains are mostly derived from planetesimals.Comment: Accepted for publication in ApJ, 18 pages, 9 figures, 5 table
The 2008 outburst of EX Lup - silicate crystals in motion
EX Lup is the prototype of the EXor class of eruptive young stars. These
objects show optical outbursts which are thought to be related to runaway
accretion onto the star. In a previous study we observed in-situ crystal
formation in the disk of EX Lup during its latest outburst in 2008, making the
object an ideal laboratory to investigate circumstellar crystal formation and
transport. This outburst was monitored by a campaign of ground-based and
Spitzer Space Telescope observations. Here we modeled the spectral energy
distribution of EX Lup in the outburst from optical to millimeter wavelengths
with a 2D radiative transfer code. Our results showed that the shape of the SED
at optical wavelengths was more consistent with a single temperature blackbody
than a temperature distribution. We also found that this single temperature
component emitted 80-100 % of the total accretion luminosity. We concluded that
a thermal instability, the most widely accepted model of EXor outbursts, was
likely not the triggering mechanism of the 2008 outburst of EX Lup. Our
mid-infrared Spitzer spectra revealed that the strength of all crystalline
bands between 8 and 30 um increased right after the end of the outburst. Six
months later, however, the crystallinity in the 10 um silicate feature complex
decreased. Our modeling of the mid-infrared spectral evolution of EXLup showed
that, although vertical mixing should be stronger during the outburst than in
the quiescent phase, fast radial transport of crystals (e.g., by stellar/disk
wind) was required to reproduce the observed mid-infrared spectra.Comment: Accepted for publication in ApJ, 37 pages, 11 figures, 2 table
Teste de aceitabilidade mercadológica de duas cultivares de soja especiais para alimentação humana.
Fundamental Vibrational Transition of CO During the Outburst of EX Lupi in 2008
We report monitoring observations of the T Tauri star EX Lupi during its outburst in 2008 in the CO fundamental
band at 4.6–5.0 μm. The observations were carried out at the Very Large Telescope and the Subaru Telescope at six
epochs from 2008 April to August, covering the plateau of the outburst and the fading phase to a quiescent state.
The line flux of CO emission declines with the visual brightness of the star and the continuum flux at 5 μm, but
composed of two subcomponents that decay with different rates. The narrow-line emission (50 kms^(−1) in FWHM) is
near the systemic velocity of EX Lupi. These emission lines appear exclusively in v =1–0. The line widths translate
to a characteristic orbiting radius of 0.4 AU. The broad-line component (FWZI ~ 150 km s^(−1)) is highly excited up
to v ≤ 6. The line flux of the component decreases faster than the narrow-line emission. Simple modeling of the
line profiles implies that the broad-line emitting gas is orbiting around the star at 0.04–0.4 AU. The excitation state, the decay speed of the line flux, and the line profile indicate that the broad-line emission component is physically distinct from the narrow-line emission component, and more tightly related to the outburst event
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