698 research outputs found
Chemical evolution and depletion pattern in Damped Lyman-alpha (DLA) systems
In this paper we point out a previously unnoticed anticorrelation between the
observed abundance ratio [X/Zn] (where Zn is assumed to be undepleted and X
stands for the refractories Fe, Cr and Ni) and metal column density
([Zn/H]+log(N{HI})) in DLAs. We suggest that this trend is an unambiguous sign
of dust depletion, since metal column density is a measure of the amount of
dust along the line of sight. Assuming that DLAs are (proto-)galactic disks and
using detailed chemical evolution models with metallicity dependent yields we
study chemical evolution and dust depletion patterns for alpha and iron-peak
elements in DLAs. When observational constraints on the metal column density of
DLAs are taken into account (as suggested in Boisse et al. 1998) we find that
our models reproduce fairly well the observed mild redshift evolution of the
abundances of 8 elements (Al, Si, S, Cr, Mn, Fe, Zn and Ni) as well as the
observed scatter at a given redshift. By considering the aforementioned
dependence of abundance ratios on metal column density, we further explore the
general dust depletion pattern in DLAs, comparing to our model results and to a
solar reference pattern. We suggest that further measurements of the key
elements, i.e. Zn, S and Mn, will help to gain more insight into the nature of
DLAs. In any case, the presently uncertain nucleosynthesis of Zn in massive
stars (on which a large part of these conclusions is based) should be carefully
scrutinised.Comment: 12 pages, 4 figures, Astronomy and Astrophysics, in pres
Abundance gradients and their evolution in the Milky Way disk
Based on a simple, but fairly successful, model of the chemical evolution of the Milky Way disk, we study the evolution of the abundances of the elements He, C, N, O, Ne, Mg, Al, Si, S, Ar and Fe. We use metallicity dependent yields for massive stars with and without mass loss. We find that most observed abundance profiles are correctly reproduced by massive star yields, but C and N require supplementary sources. We argue that massive, mass losing stars can totally account for the abundance profile of C, while intermediate mass stars are the main source of N; in both cases, some conflict with corresponding data on extragalactic HII regions arises, at least if current observations in the Galaxy are taken at face value. The observed behaviour of Al is marginally compatible with current massive star yields, which probably overestimate the ``odd-even'' effect. We also find that the adopted ``inside-out'' formation scheme for the Milky Way disk produce abundance profiles steeper in the past. The corresponding abundance scatter is smaller in the inner disk than in the outer regions for a given interval of Galactic age
Millimetre continuum observations of comet C/2009 P1 (Garradd)
Little is known about the physical properties of the nuclei of Oort cloud
comets. Measuring the thermal emission of a nucleus is one of the few means for
deriving its size and constraining some of its thermal properties. We attempted
to measure the nucleus size of the Oort cloud comet C/2009 P1 (Garradd). We
used the Plateau de Bure Interferometer to measure the millimetric thermal
emission of this comet at 157 GHz (1.9 mm) and 266 GHz (1.1 mm). Whereas the
observations at 266 GHz were not usable due to bad atmospheric conditions, we
derived a 3-sigma upper limit on the comet continuum emission of 0.41 mJy at
157 GHz. Using a thermal model for a spherical nucleus with standard thermal
parameters, we found an upper limit of 5.6 km for the radius. The dust
contribution to our signal is estimated to be negligible. Given the water
production rates measured for this comet and our upper limit, we estimated that
Garradd was very active, with an active fraction of its nucleus larger than
50%.Comment: Accepted for publication in Astronomy & Astrophysics. 5 pages, 2
figure
The properties of the Malin 1 galaxy giant disk: A panchromatic view from the NGVS and GUViCS surveys
Low surface brightness galaxies (LSBGs) represent a significant percentage of
local galaxies but their formation and evolution remain elusive. They may hold
crucial information for our understanding of many key issues (i.e., census of
baryonic and dark matter, star formation in the low density regime, mass
function). The most massive examples - the so called giant LSBGs - can be as
massive as the Milky Way, but with this mass being distributed in a much larger
disk. Malin 1 is an iconic giant LSBG, perhaps the largest disk galaxy known.
We attempt to bring new insights on its structure and evolution on the basis of
new images covering a wide range in wavelength. We have computed surface
brightness profiles (and average surface brightnesses in 16 regions of
interest), in six photometric bands (FUV, NUV, u, g, i, z). We compared these
data to various models, testing a variety of assumptions concerning the
formation and evolution of Malin 1. We find that the surface brightness and
color profiles can be reproduced by a long and quiet star-formation history due
to the low surface density; no significant event, such as a collision, is
necessary. Such quiet star formation across the giant disk is obtained in a
disk model calibrated for the Milky Way, but with an angular momentum
approximately 20 times larger. Signs of small variations of the star-formation
history are indicated by the diversity of ages found when different regions
within the galaxy are intercompared.For the first time, panchromatic images of
Malin 1 are used to constrain the stellar populations and the history of this
iconic example among giant LSBGs. Based on our model, the extreme disk of Malin
1 is found to have a long history of relatively low star formation (about 2
Msun/yr). Our model allows us to make predictions on its stellar mass and
metallicity.Comment: Accepted in Astronomy and Astrophysic
Hydrogen Isocyanide in Comet 73P/Schwassmann-Wachmann (Fragment B)
We present a sensitive 3-sigma upper limit of 1.1% for the HNC/HCN abundance
ratio in comet 73P/Schwassmann-Wachmann (Fragment B), obtained on May 10-11,
2006 using Caltech Submillimeter Observatory (CSO). This limit is a factor of
~7 lower than the values measured previously in moderately active comets at 1
AU from the Sun. Comet 73P/Schwassmann-Wachmann was depleted in most volatile
species, except of HCN. The low HNC/HCN ratio thus argues against HNC
production from polymers produced from HCN. However, thermal degradation of
macromolecules, or polymers, produced from ammonia and carbon compounds, such
as acetylene, methane, or ethane appears a plausible explanation for the
observed variations of the HNC/HCN ratio in moderately active comets, including
the very low ratio in comet 73P/Schwassmann-Wachmann reported here. Similar
polymers have been invoked previously to explain anomalous 14N/15N ratios
measured in cometary CN.Comment: 6 pages, 5 figures, 2 table
Imaging of crystalline and amorphous surface regions using time-of-flight secondary-ion mass spectrometry (ToF-SIMS): application to pharmaceutical materials
The structure of a material, in particular the extremes of crystalline and amorphous forms, significantly impacts material performance in numerous sectors such as semiconductors, energy storage, and pharmaceutical products, which are investigated in this paper. To characterize the spatial distribution for crystalline−amorphous forms at the uppermost molecular surface layer, we performed time-of-flight secondary-ion mass spectroscopy (ToF-SIMS) measurements for quench-cooled amorphous and recrystallized samples of the drugs indomethacin, felodipine, and acetaminophen. Polarized light microscopy was used to localize crystallinity induced in the samples under controlled conditions. Principal component analysis was used to identify the subtle changes in the ToF-SIMS spectra indicative of the amorphous and crystalline forms for each drug. The indicators of amorphous and crystalline surfaces were common in type across the three drugs, and could be explained in general terms of crystal packing and intermolecular bonding, leading to intramolecular chain scission in the formation of secondary ions. Less intramolecular scission occurred in the amorphous form, resulting in a greater intensity of molecular and dimer secondary ions. To test the generality of amorphous−crystalline differentiation using ToF-SIMS, a different recrystallization method was investigated where acetaminophen single crystals were recrystallized from supersaturated solutions. The findings indicated that the ability to assign the crystalline/amorphous state of the sample using ToF-SIMS was insensitive to the recrystallization method. This demonstrates ToF-SIMS capabilities of detecting and mapping ordered crystalline and disordered amorphous molecular materials forms at micron spatial resolution in the uppermost surface of a material
Towards understanding the relation between the gas and the attenuation in galaxies at kpc scales
[abridged]
Aims. The aim of the present paper is to provide new and more detailed
relations at the kpc scale between the gas surface density and the face-on
optical depth directly calibrated on galaxies, in order to compute the
attenuation not only for semi-analytic models but also observationally as new
and upcoming radio observatories are able to trace gas ever farther in the
Universe.
Methods. We have selected a sample of 4 nearby resolved galaxies and a sample
of 27 unresolved galaxies from the Herschel Reference Survey and the Very
Nearby Galaxies Survey, for which we have a large set of multi-wavelength data
from the FUV to the FIR including metallicity gradients for resolved galaxies,
along with radio HI and CO observations. For each pixel in resolved galaxies
and for each galaxy in the unresolved sample, we compute the face-on optical
depth from the attenuation determined with the CIGALE SED fitting code and an
assumed geometry. We determine the gas surface density from HI and CO
observations with a metallicity-dependent XCO factor.
Results. We provide new, simple to use, relations to determine the face-on
optical depth from the gas surface density, taking the metallicity into
account, which proves to be crucial for a proper estimate. The method used to
determine the gas surface density or the face-on optical depth has little
impact on the relations except for galaxies that have an inclination over 50d.
Finally, we provide detailed instructions on how to compute the attenuation
practically from the gas surface density taking into account possible
information on the metallicity.
Conclusions. Examination of the influence of these new relations on simulated
FUV and IR luminosity functions shows a clear impact compared to older oft-used
relations, which in turn could affect the conclusions drawn from studies based
on large scale cosmological simulations.Comment: 24 pages, 21 figures, accepted for publication in A&
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