185 research outputs found
A companion candidate in the gap of the T Cha transitional disk
T Cha is a young star surrounded by a cold disk. The presence of a gap within
its disk, inferred from fitting to the spectral energy distribution, has
suggested on-going planetary formation. We observed T Cha in L' and K_s with
NAOS-CONICA, the adaptive optics system at the VLT, using sparse aperture
masking. We detected a source in the L' data at a separation of 62+-7 mas,
position angle of 78+-1 degrees, and a contrast of delta L' = 5.1+-0.2 mag. The
object is not detected in the Ks band data, which show a 3-sigma contrast limit
of 5.2 mag at the position of the detected L' source. For a distance of 108 pc,
the detected companion candidate is located at 6.7 AU from the primary, well
within the disk gap. If T Cha and the companion candidate are bound, the
comparison of the L' and Ks photometry with evolutionary tracks shows that the
photometry is inconsistent with any unextincted photosphere at the age and
distance of T Cha. The detected object shows a very red Ks-L' color for which a
possible explanation would be a significant amount of dust around it. This
would imply that the companion candidate is young, which would strengthen the
case for a physical companion, and moreover that the object would be in the
substellar regime, according to the Ks upper limit. Another exciting
possibility would be that this companion is a recently formed planet within the
disk. Additional observations are mandatory to confirm that the object is bound
and to properly characterize it.Comment: 4 pages, 4 figures; accepted for publication by A&
Multifractal Scaling, Geometrical Diversity, and Hierarchical Structure in the Cool Interstellar Medium
Multifractal scaling (MFS) refers to structures that can be described as a
collection of interwoven fractal subsets which exhibit power-law spatial
scaling behavior with a range of scaling exponents (concentration, or
singularity, strengths) and dimensions. The existence of MFS implies an
underlying multiplicative (or hierarchical, or cascade) process. Panoramic
column density images of several nearby star- forming cloud complexes,
constructed from IRAS data and justified in an appendix, are shown to exhibit
such multifractal scaling, which we interpret as indirect but quantitative
evidence for nested hierarchical structure. The relation between the dimensions
of the subsets and their concentration strengths (the "multifractal spectrum'')
appears to satisfactorily order the observed regions in terms of the mixture of
geometries present: strong point-like concentrations, line- like filaments or
fronts, and space-filling diffuse structures. This multifractal spectrum is a
global property of the regions studied, and does not rely on any operational
definition of "clouds.'' The range of forms of the multifractal spectrum among
the regions studied implies that the column density structures do not form a
universality class, in contrast to indications for velocity and passive scalar
fields in incompressible turbulence, providing another indication that the
physics of highly compressible interstellar gas dynamics differs fundamentally
from incompressible turbulence. (Abstract truncated)Comment: 27 pages, (LaTeX), 13 figures, 1 table, submitted to Astrophysical
Journa
Discovery of a Luminous Quasar in the Nearby Universe
In the course of the Pico dos Dias survey (PDS), we identified the stellar
like object PDS456 at coordinates alpha = 17h 28m 19.796s, delta = -14deg 15'
55.87'' (epoch 2000), with a relatively nearby (z = 0.184) and bright (B =
14.69) quasar. Its position at Galactic coordinates l_II = 10.4deg, b_II =
+11.2deg, near the bulge of the Galaxy, may explain why it was not detected
before. The optical spectrum of PDS456 is typical of a luminous quasar, showing
a broad (FWHM ~ 4000 km/s) H_\beta line, very intense FeII lines and a weak
[OIII]\lambda5007 line. PDS456 is associated to the infrared source IRAS
17254-1413 with a 60 \mum infrared luminosity L_{60} = 3.8 x 10^{45} erg/s. The
relatively flat slopes in the infrared (\alpha(25,60) = -0.33 and \alpha(12,25)
= -0.78) and a flat power index in the optical (F_{\nu} \propto \nu^{-0.72})
may indicate a low dust content. A good match between the position of PDS456
and the position of the X-ray source RXS J172819.3-141600 implies an X-ray
luminosity L_x = 2.8 x 10^{44} erg/s. The good correlation between the strength
of the emission lines in the optical and the X-ray luminosity, as well as the
steep optical to X-ray index estimated (\alpha_{ox} = -1.64) suggest that
PDS456 is radio quiet. A radio survey previously performed in this region
yields an upper limit for radio power at ~ 5 GHz of ~ 2.6 x 10^{30} erg/s/Hz.
We estimate the Galactic reddening in this line-of-sight to be A_B \simeq 2.0,
implying an absolute magnitude M_B = -26.7 (using H_0 = 75 km s^{-1} Mpc^{-1}
and q_0 = 0). In the optical, PDS456 is therefore 1.3 times more luminous than
3C 273 and the most luminous quasar in the nearby (z \leq 0.3) Universe.Comment: 12 pages, LaTeX (aasms4.sty) + 3 figures; accepted for publication in
the Astrophysical Journal Letter
Editorial : Mammalian responses to climate change : from organisms to communities
No abstract available.http://frontiersin.org/Ecology_and_Evolutiondm2022Mammal Research InstituteZoology and Entomolog
The Fractal Density Structure in Supersonic Isothermal Turbulence: Solenoidal versus Compressive Energy Injection
In a systematic study, we compare the density statistics in high resolution
numerical experiments of supersonic isothermal turbulence, driven by the
usually adopted solenoidal (divergence-free) forcing and by compressive
(curl-free) forcing. We find that for the same rms Mach number, compressive
forcing produces much stronger density enhancements and larger voids compared
to solenoidal forcing. Consequently, the Fourier spectra of density
fluctuations are significantly steeper. This result is confirmed using the
Delta-variance analysis, which yields power-law exponents beta~3.4 for
compressive forcing and beta~2.8 for solenoidal forcing. We obtain fractal
dimension estimates from the density spectra and Delta-variance scaling, and by
using the box counting, mass size and perimeter area methods applied to the
volumetric data, projections and slices of our turbulent density fields. Our
results suggest that compressive forcing yields fractal dimensions
significantly smaller compared to solenoidal forcing. However, the actual
values depend sensitively on the adopted method, with the most reliable
estimates based on the Delta-variance, or equivalently, on Fourier spectra.
Using these methods, we obtain D~2.3 for compressive and D~2.6 for solenoidal
forcing, which is within the range of fractal dimension estimates inferred from
observations (D~2.0-2.7). The velocity dispersion to size relations for both
solenoidal and compressive forcing obtained from velocity spectra follow a
power law with exponents in the range 0.4-0.5, in good agreement with previous
studies.Comment: 17 pages, 11 figures, ApJ in press, minor changes to language,
simulation movies available at
http://www.ita.uni-heidelberg.de/~chfeder/videos.shtml?lang=e
Spectroscopic Study of IRAS 19285+0517(PDS 100): A Rapidly Rotating Li-Rich K Giant
We report on photometry and high-resolution spectroscopy for IRAS 19285+0517.
The spectral energy distribution based on visible and near-IR photometry and
far-IR fluxes shows that the star is surrounded by dust at a temperature of
250 K. Spectral line analysis shows that the star is a K
giant with a projected rotational velocity = 9 2 km s.
We determined the atmospheric parameters: = 4500 K, log =
2.5, = 1.5 km s, and [Fe/H] = 0.14 dex. The LTE abundance
analysis shows that the star is Li-rich (log (Li) = 2.50.15),
but with essentially normal C, N, and O, and metal abundances. Spectral
synthesis of molecular CN lines yields the carbon isotopic ratio
C/C = 9 3, a signature of post-main sequence evolution and
dredge-up on the RGB. Analysis of the Li resonance line at 6707 \AA for
different ratios Li/Li shows that the Li profile can be fitted best
with a predicted profile for pure Li. Far-IR excess, large Li abundance,
and rapid rotation suggest that a planet has been swallowed or, perhaps, that
an instability in the RGB outer layers triggered a sudden enrichment of Li and
caused mass-loss.Comment: To appear in AJ; 40 pages, 9 figure
A 2MASS Analysis of the Stability of Southern Bok Globules
We used near-infrared 2MASS data to construct visual extinction maps of a
sample of Southern Bok globules utilizing the NICE method. We derived radial
extinction profiles of dense cores identified in the globules and analyzed
their stability against gravitational collapse with isothermal Bonnor-Ebert
spheres. The frequency distribution of the stability parameter xi_max of these
cores shows that a large number of them are located in stable states, followed
by an abrupt decrease of cores in unstable states. This decrease is steeper for
globules with associated IRAS point sources than for starless globules.
Moreover, globules in stable states have a Bonnor-Ebert temperature of T = 15
+- 6 K, while the group of critical plus unstable globules has a different
temperature of T = 10 +- 3 K. Distances were estimated to all the globules
studied in this work and the spectral class of the IRAS sources was calculated.
No variations were found in the stability parameters of the cores and the
spectral class of their associated IRAS sources. On the basis of 13CO J = 1-0
molecular line observations, we identified and modeled a blue-assymetric line
profile toward a globule of the sample, obtaining an upper limit infall speed
of 0.25 km/s.Comment: 53 pages, 15 figures, accepted for publication in Ap
Efficacy of different cooling methods for capture-induced hyperthermia in antelope
The capture of wild animals is a stressful event which may cause a capture-induced
hyperthermia, resulting in morbidity or mortality. We investigated whether various cooling
techniques were effective at lowering the body temperature of hyperthermic animals. To
achieve this, we implanted miniature temperature-sensitive data loggers into the abdomens
of 12 blesbok (Damaliscus pygargus phillipsi ). Five animals were cooled by dousing with
water of different temperatures (4°C, 17°C, 28°C) and fanning after dousing with 28°C water.
Seven animals were cooled by ice-packs, a fine mist spray of 28°C water, intravenous (IV)
infusion of one litre of 4°C saline solution or 28°C water-dousing. The body temperature after
capture was significantly elevated to as high as 41°C to 42°C. Water-dousing interventions
significantly decreased minimum body temperature but there was no difference in the
minimumbody temperature reached or the magnitude of cooling between the different water
temperatures or by the addition of fanning. The ice-packs also lowered body temperature,
whereas mist spraying did not.The use of ice packs and dousing with water between 4°C and
28°C were the most effective techniques to reduce capture-induced hyperthermia in
blesbok.Water-dousing,when done appropriately, is the most practical and effective method
to cool an animal with capture-induced hyperthermia.This study was funded by the National Research
Foundation, South Africa.http://www.sawma.co.za/am201
How free-ranging ungulates with differing water dependencies cope with seasonal variation in temperature and aridity
Large mammals respond to seasonal changes in temperature and precipitation by behavioural and physiological flexibility. These responses are likely to differ between species with differing water dependencies. We used biologgers to contrast the seasonal differences in activity patterns, microclimate selection, distance to potential water source and body temperature of the water-independent gemsbok (Oryx gazella gazella) and water-dependent blue wildebeest (Connochaetes taurinus), free-living in the arid Kalahari region of Botswana. Gemsbok were more active nocturnally during the hot seasons than in the cold-dry season, while wildebeest showed no seasonal difference in their nocturnal activity level. Both species similarly selected shaded microclimates during the heat of the day, particularly during the hot seasons. Wildebeest were further than 10 km from surface water 30% or more of the time, while gemsbok were frequently recorded >20 km from potential water sources. In general, both species showed similar body temperature variation with high maximum 24-h body temperature when conditions were hot and low minimum 24-h body temperatures when conditions were dry, resulting in the largest amplitude of 24-h body temperature rhythm during the hot-dry period. Wildebeest thus coped almost as well as gemsbok with the fairly typical seasonal conditions that occurred during our study period. They do need to access surface water and may travel long distances to do so when local water sources become depleted during drought conditions. Thus, perennial water sources should be provided judiciously and only where essential
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