2,731 research outputs found
The curious case of the companion: evidence for cold accretion onto a dwarf satellite near the isolated elliptical NGC 7796
The isolated elliptical (IE) NGC 7796 is accompanied by an interesting
early-type dwarf galaxy, named NGC7796-DW1. It exhibits a tidal tail, very boxy
isophotes, and multiple nuclei or regions (A, B, and C) that are bluer than the
bulk population of the galaxy, indicating a younger age. These properties are
suggestive of a dwarf-dwarf merger remnant. We use the Multi-Unit Spectroscopic
Explorer (MUSE) at the VLT to investigate NGC 7796-DW1. We extract
characteristic spectra to which we apply the STARLIGHT population synthesis
software to obtain ages and metallicities of the various population components
of the galaxy. The galaxy's main body is old and metal-poor. A surprising
result is the extended line emission in the galaxy, forming a ring-like
structure with a projected diameter of 2.2 kpc. The line ratios fall into the
regime of HII-regions, although OB-stellar populations cannot be identified by
spectral signatures. Nucleus A is a relatively old (7 Gyr or older) and
metal-poor super star cluster, most probably the nucleus of the dwarf, now
displaced. The star-forming regions B and C show younger and distinctly more
metal-rich components. The emission line ratios of regions B and C indicate an
almost solar oxygen abundance, if compared with radiation models of HII
regions. NGC7796-DW1 occupies a particular role in the group of transition-type
galaxies with respect to its origin and current evolutionary state, being the
companion of an IE. The dwarf-dwarf merger scenario is excluded because of the
missing metal-rich merger component. A viable alternative is gas accretion from
a reservoir of cold, metal-rich gas. NGC7796 has to provide this gas within its
X-ray bright halo. As illustrated by NGC7796-DW1, cold accretion may be a
general solution to the problem of extended star formation histories in
transition dwarf galaxies. (abridged)Comment: comments: 13 pages, 8 figures, accepted for publication in Astronomy
& Astrophysic
A thermodynamical fiber bundle model for the fracture of disordered materials
We investigate a disordered version of a thermodynamic fiber bundle model
proposed by Selinger, Wang, Gelbart, and Ben-Shaul a few years ago. For simple
forms of disorder, the model is analytically tractable and displays some new
features. At either constant stress or constant strain, there is a non
monotonic increase of the fraction of broken fibers as a function of
temperature. Moreover, the same values of some macroscopic quantities as stress
and strain may correspond to different microscopic cofigurations, which can be
essential for determining the thermal activation time of the fracture. We argue
that different microscopic states may be characterized by an experimentally
accessible analog of the Edwards-Anderson parameter. At zero temperature, we
recover the behavior of the irreversible fiber bundle model.Comment: 18 pages, 10 figure
Increased HCO production in the outer disk around HD 163296
Three formaldehyde lines were observed (HCO 3--2, HCO
3--2, and HCO 3--2) in the protoplanetary disk
around the Herbig Ae star HD 163296 with ALMA at 0.5 arcsecond (60 AU) spatial
resolution. HCO 3--2 was readily detected via imaging, while
the weaker HCO 3--2 and HCO 3--2 lines
required matched filter analysis to detect. HCO is present throughout most
of the gaseous disk, extending out to 550 AU. An apparent 50 AU inner radius of
the HCO emission is likely caused by an optically thick dust continuum. The
HCO radial intensity profile shows a peak at 100 AU and a secondary bump at
around 300 AU, suggesting increased production in the outer disk. Different
parameterizations of the HCO abundance were compared to the observed
visibilities with minimization, using either a characteristic
temperature, a characteristic radius or a radial power law index to describe
the HCO chemistry. Similar models were applied to ALMA Science Verification
data of CO. In all modeling scenarios, fits to the HCO data show an
increased abundance in the outer disk. The overall best-fit HCO model shows
a factor of two enhancement beyond a radius of 27020 AU, with an inner
abundance of . The HCO emitting region has a lower
limit on the kinetic temperature of K. The CO modeling suggests
an order of magnitude depletion in the outer disk and an abundance of in the inner disk. The increase in HCO outer disk emission
could be a result of hydrogenation of CO ices on dust grains that are then
sublimated via thermal desorption or UV photodesorption, or more efficient
gas-phase production beyond about 300 AU if CO is photodisocciated in this
region
DCO, DCN and ND reveal three different deuteration regimes in the disk around the Herbig Ae star HD163296
The formation pathways of deuterated species trace different regions of
protoplanetary disks and may shed light into their physical structure. We aim
to constrain the radial extent of main deuterated species; we are particularly
interested in spatially characterizing the high and low temperature pathways
for enhancing deuteration of these species. We observed the disk surrounding
the Herbig Ae star HD 163296 using ALMA in Band 6 and obtained resolved
spectral imaging data of DCO (=3-2), DCN (=3-2) and ND
(=3-2). We model the radial emission profiles of DCO, DCN and
ND, assuming their emission is optically thin, using a parametric model
of their abundances and radial excitation temperature estimates. DCO can be
described by a three-region model, with constant-abundance rings centered at 70
AU, 150 AU and 260 AU. The DCN radial profile peaks at about ~60 AU and
ND is seen in a ring at ~160 AU. Simple models of both molecules using
constant abundances reproduce the data. Assuming reasonable average excitation
temperatures for the whole disk, their disk-averaged column densities (and
deuterium fractionation ratios) are 1.6-2.6 cm
(0.04-0.07), 2.9-5.2 cm (0.02) and 1.6-2.5 cm (0.34-0.45) for DCO, DCN and ND, respectively.
Our simple best-fit models show a correlation between the radial location of
the first two rings in DCO and the DCN and ND abundance
distributions that can be interpreted as the high and low temperature
deuteration pathways regimes. The origin of the third DCO ring at 260 AU is
unknown but may be due to a local decrease of ultraviolet opacity allowing the
photodesorption of CO or due to thermal desorption of CO as a consequence of
radial drift and settlement of dust grains
Sooting propensity of dimethyl carbonate, soot reactivity and characterization
Oxygenated compounds have gained interest in the last few years because they represent an attractive alternative as additive to diesel fuel for reducing soot emissions. Although dimethyl carbonate (DMC) seems to be a good option, studies about its propensity to form soot, as well as the knowledge of the characteristics of this soot are still missing. For that reason, this paper focuses on the potential of DMC to form soot, as well as on the reactivity and characterization of this soot. Results from pyrolysis experiments performed in an atmospheric pressure flow reactor at different temperatures (1075-1475 K) and inlet DMC concentrations (approximately 33, 333 and 50, 000 ppm) show that both soot and gas yields are affected by the pyrolysis temperature, while an increase in the inlet DMC concentration only affects slightly the soot yield, without notable influence on the gas yield. DMC shows a very low tendency to produce soot because the CO/CO2 formation is favoured and thus few carbon atoms are available for soot formation. A chemical kinetic model developed, without incorporating soot particles dynamics, can predict well the gas-phase trends. The comparison of the soot amount profile obtained with the PAH amount profile determined by the model suggests a good first approach toward a model including soot formation. The soot reactivity study toward O2 (500 ppm) and NO (2000 ppm) at 1475 K, as well as its characterization, show that the higher the temperature and the inlet DMC concentration of soot formation, the lower the reactivity of the soot
A segmented period-luminosity relation for nearby extragalactic Scuti stars
The period-luminosity relations (PLR) of Milky Way Scuti (
Sct) stars have been described to the present day by a linear relation.
However, when studying extragalactic systems such as the Magellanic Clouds and
several dwarf galaxies, we notice for the first time a non-linear behaviour in
the PLR of Sct stars. Using the largest sample of
extragalactic Sct stars from data available in the literature
mainly based on OGLE and SuperMACHO survey in the Large Magellanic Cloud
(LMC) we obtain that the best fit to the period-luminosity () plane is
given by the following piecewise linear relation with a break at (or d) for shorter periods (sp) and longer
periods (lp) than the break-point:
Geometric or depth effects in the LMC, metallicity dependence, or different
pulsation modes are discarded as possible causes of this segmented PLR seen in
extragalactic Sct stars. The origin of the segmented relation at days remains unexplained based on the current data.Comment: 9 pages, 3 figures, 1 table. Accepted for publication into The
Astrophysical Journal Letter
ALMA detection of the dusty object silhouetted against the S0 galaxy NGC 3269 in the Antlia cluster
Context. An intriguing silhouette of a small dust patch can be seen against the disk of the S0 galaxy NGC 3269 in the Antlia cluster in optical images. The images do not provide any clue as to whether the patch is a local Jupiter mass-scale cloudlet or a large extragalactic dust complex.Aims. We aim to resolve the nature of this object: is it a small Galactic cloudlet or an extragalactic dust complex?Methods. ALMA and APEX spectroscopy and Gemini GMOS long-slit spectroscopy were used to measure the velocity of the patch and the NGC 3269 disk radial velocity curve.Results. A weak 16 2.5 km s(-1) wide (CO)-C-12(2-1) T-MB 19 +/- 2.5. mK line in a 2 .('') . '' 2 by 2 .('') .'' 12 beam associated with the object was detected with ALMA. The observed heliocentric velocity, V-r,V- hel=3878 +/- 5.0 km s(-1), immediately establishes the extragalactic nature of the object. The patch velocity is consistent with the velocity of the nucleus of NGC 3269, but not with the radial velocity of the NGC 3269 disk of the galaxy at its position. The similar to 4 '' angular size of the patch corresponds to a linear size of similar to 1 kpc at the galaxy's Hubble distance of 50.7 Mpc. The mass estimated from the (CO)-C-12(2-1) emission is similar to 1.4x10(6)(d/50.7 Mpc)M-2(circle dot), while the attenuation derived from the optical spectrum implies a dust mass of similar to 2.6x10(4)(d/50.7 Mpc)M-2(circle dot). The derived attenuation ratio A ' (B)/(A ' (B)-A ' (R)) of 1.6 +/- 0.11 is substantially lower than the corresponding value for the mean Milky Way extinction curve for point sources (2.3).Conclusions. We established the extragalactic nature of the patch, but its origin remains elusive. One possibility is that the dust patch is left over from the removal of interstellar matter in NGC 3269 through the interaction with its neighbour, NGC 3268.Peer reviewe
Experimental harvesting of fish populations drives genetically based shifts in body size and maturation
Size-selective harvesting in commercial fisheries can induce rapid changes in biological traits. While experimental and wild harvested populations often exhibit clear shifts in body size and maturation associated with fishing pressure, the relative contributions of genetic and environmental factors to these shifts remain uncertain and have been much debated. To date, observations of so-called fisheries-induced evolution (FIE) have been based solely on phenotypic measures, such as size data. Genetic data are hitherto lacking. Here, we quantify genetic versus environmental change in response to size-selective harvesting for small and large body size in guppies (Poecilia reticulata) across three generations of selection. We document for the first time significant changes at individual genetic loci, some of which have previously been associated with body size. In contrast, variation at neutral microsatellite markers was unaffected by selection, providing direct genetic evidence for rapid evolution induced by size-selective harvesting. These findings demonstrate FIE in an experimental system, with major implications for the sustainability of harvested populations, as well as impacts on size-structured communities and ecosystem processes. These findings highlight the need for scientists and managers to reconsider the capacity of harvested stocks to adapt to, and recover from, harvesting and predation. © 2013 The Ecological Society of America
Magnetic Anisotropy Variations and Non-Equilibrium Tunneling in a Cobalt Nanoparticle
We present detailed measurements of the discrete electron-tunneling level
spectrum within nanometer-scale cobalt particles as a function of magnetic
field and gate voltage, in this way probing individual quantum many-body
eigenstates inside ferromagnetic samples. Variations among the observed levels
indicate that different quantum states within one particle are subject to
different magnetic anisotropy energies. Gate-voltage studies demonstrate that
the low-energy tunneling spectrum is affected dramatically by the presence of
non-equilibrium spin excitations
Magnetic-field dependence of energy levels in ultrasmall metal grains
We present a theory of mesoscopic fluctuations of g tensors and avoided
crossing energies in a small metal grain. The model, based on random matrix
theory, contains both the orbital and spin contributions to the g tensor. The
two contributions can be experimentally separated for weak spin-orbit coupling
while they merge in the strong coupling limit. For intermediate coupling,
substantial correlations are found between g factors of neighboring levels.Comment: 9 pages, 5 figure
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