658 research outputs found
Phase transformations in 40-60-GPa shocked gneisses from the Haughton Crater (Canada): An Analytical Transmission Electron Microscopy (ATEM) study
In order to better understand phase transformations, chemical migration, and isotopic disequilibrium in highly shocked rocks, we have performed a microprobe and an ATEM study on gneisses shocked up to 60 GPa from the Haughton Crater. This study reveals the following chemical and structural characteristics: (1) SiO2 dominant areas are formed by a mixture of pure SiO2 polycrystalline quartz identified by electron diffraction pattern and chemical analysis and a silica-rich amorphous phase containing minor amounts of aluminium, potassium, and iron; (2) Areas with biotitelike composition are formed by less than 200-nm grains of iron-rich spinels embedded in a silica-rich amorphous phase that is very similar to the one described above; (3) Layers with feldsparlike composition are constituted by 100-200-nm-sized alumina-rich grains (the indexation of the crystalline structure is under progress) and the silica-rich amorphous phase; (4) Zones characterized by the unusual Al/Si ratio close to 1 are formed by spinel grains (200-nm-sized) embedded in the same silica-rich amorphous phase; and (5) The fracturated sillimanites contain domains with a lamellar structure, defined by the intercalation of 100-nm-wide lamellae of mullite crystals and of a silica-rich amorphous phase. These mullite crystals preserved the crystallographical orientation of the preshock sillimanite. All compositional domains, identified at the microprobe scale, can thus be explained by a mixture in different proportion between the following phases: (1) a silica-rich amorphous phase, with minor Al and K; (2) quartz crystals; (3) spinel crystals and alumina-rich crystals; (4) sillimanite; and (5) mullite. Such mixtures of amorphous phases and crystals in different proportions explain disturbed isotope systems in these rocks and chemical heterogeneities observed on the microprobe
Confidence limits of evolutionary synthesis models III. On time-integrated quantities
Evolutionary synthesis models are a fundamental tool to interpret the
properties of observed stellar systems. In order to achieve a meaningful
comparison between models and real data, it is necessary to calibrate the
models themselves, i.e. to evaluate the dispersion due to the discreteness of
star formation as well as the possible model errors. In this paper we show that
linear interpolations in the log M - log t_k plane, that are customary in the
evaluation of isochrones in evolutionary synthesis codes, produce unphysical
results. We also show that some of the methods used in the calculation of
time-integrated quantities (kinetic energy, and total ejected masses of
different elements) may produce unrealistic results. We propose alternative
solutions to solve both problems. Moreover, we have quantified the expected
dispersion of these quantities due to stochastic effects in stellar
populations. As a particular result, we show that the dispersion in the 14N/12C
ratio increases with time.Comment: 11 pages, 8 figures, accepted by A&
On the effective temperature scale of O stars
We rediscuss the temperature of O dwarfs based on new non-LTE line blanketed
atmosphere models including stellar winds computed with the CMFGEN code of
Hillier & Miller (1998). Compared to the latest calibration of Vacca et al.
(1996), the inclusion of line blanketing leads to lower effective temperatures,
typically by 4000 to 1500 K for O3 to O9.5 dwarf stars. The dependence of the
Teff-scale on stellar and model parameters - such as mass loss,
microturbulence, and metallicity - is explored, and model predictions are
compared to optical observations of O stars. Even for an SMC metallicity we
find a non-negligible effect of line blanketing on the Teff-scale. The
temperature reduction implies downward revisions of luminosities by 0.1 dex and
Lyman continuum fluxes Q0 by approximately 40% for dwarfs of a given spectral
type.Comment: 6 pages, 4 figures. To be published in A&
New constraints on dust emission and UV attenuation of z=6.5-7.5 galaxies from millimeter observations
We have targeted two recently discovered Lyman break galaxies (LBGs) to
search for dust continuum and [CII] 158 micron line emission. The strongly
lensed z~6.8 LBG A1703-zD1 behind the galaxy cluster Abell 1703, and the
spectroscopically confirmed z=7.508 LBG z8-GND-5296 in the GOODS-N field have
been observed with the Plateau de Bure interferometer (PdBI) at 1.2mm. These
observations have been combined with those of three z>6.5 Lya emitters (named
HCM6A, Himiko, and IOK-1), for which deep measurements were recently obtained
with the PdBI and ALMA. [CII] is undetected in both galaxies, providing a deep
upper limit for Abell1703-zD1, comparable to recent ALMA non-detections. Dust
continuum emission from Abell1703-zD1 and z8-GND-5296 is not detected with an
rms of 0.12 and 0.16 mJy/beam. From these non-detections we derive upper limits
on their IR luminosity and star formation rate, dust mass, and UV attenuation.
Thanks to strong gravitational lensing the limit for Abell1703-zD1 is probing
the sub-LIRG regime ( Lsun) and very low dust
masses ( Msun). We find that all five galaxies are
compatible with the Calzetti IRX- relation, their UV attenuation is
compatible with several indirect estimates from other methods (the UV slope,
extrapolation of the attenuation measured from the IR/UV ratio at lower
redshift, and SED fits), and the dust-to-stellar mass ratio is not incompatible
with that of galaxies from z=0 to 3. For their stellar mass the high-z galaxies
studied here have an attenuation below the one expected from the mean relation
of low redshift (z<1.5) galaxies. More and deeper (sub)-mm data are clearly
needed to directly determine the UV attenuation and dust content of the
dominant population of high-z star-forming galaxies and to establish more
firmly their dependence on stellar mass, redshift, and other properties.Comment: 10 pages, 7 figures. Minor revisions. Accepted for publication in A&
Very High-Redshift Lensed Galaxies
We review in this paper the main results recently obtained on the
identification and study of very high-z galaxies usinglensing clusters as
natural gravitational telescopes. We present in detail our pilot survey with
ISAAC/VLT, aimed at the detection of z>7 sources. Evolutionary synthesis models
for extremely metal-poor and PopIII starbursts have been used to derive the
observational properties expected for these high-z galaxies, such as expected
magnitudes and colors, line fluxes for the main emission lines, etc. These
models have allowed to define fairly robust selection criteria to find z~7-10
galaxies based on broad-band near-IR photometry in combination with the
traditional Lyman drop-out technique. The first results issued from our
photometric and spectroscopic survey are discussed, in particular the
preliminary confirmation rate, and the global properties of our high-z
candidates, including the latest results on the possible z=10.0 candidate
A1835-1916. The search efficiency should be significantly improved by the
future near-IR multi-object ground-based and space facilities. However, strong
lensing clusters remain a factor of ~5-10 more efficient than blank fields in
this redshift domain, within the FOV of a few arcminutes around the cluster
core, for the typical depth required for this survey project.Comment: 14 pages, 7 figures, Proceedings of IAU Symposium No. 225: The Impact
of Gravitational Lensing on Cosmology, Y. Mellier and G. Meylan, Ed
No temperature fluctuations in the giant HII region H 1013
While collisionally excited lines in HII regions allow one to easily probe
the chemical composition of the interstellar medium in galaxies, the possible
presence of important temperature fluctuations casts some doubt on the derived
abundances. To provide new insights into this question, we have carried out a
detailed study of a giant HII region, H 1013, located in the galaxy M101, for
which many observational data exist and which has been claimed to harbour
temperature fluctuations at a level of t^2 = 0.03-0.06. We have first
complemented the already available optical observational datasets with a
mid-infrared spectrum obtained with the Spitzer Space Telescope. Combined with
optical data, this spectrum provides unprecedented information on the
temperature structure of this giant HII region. A preliminary analysis based on
empirical temperature diagnostics suggests that temperature fluctuations should
be quite weak. We have then performed a detailed modelling using the pyCloudy
package based on the photoionization code Cloudy. We have been able to produce
photoionization models constrained by the observed Hb surface brightness
distribution and by the known properties of the ionizing stellar population
than can account for most of the line ratios within their uncertainties. Since
the observational constraints are both strong and numerous, this argues against
the presence of significant temperature fluctuations in H 1013. The oxygen
abundance of our best model is 12 + log O/H = 8.57, as opposed to the values of
8.73 and 8.93 advocated by Esteban et al. (2009) and Bresolin (2007),
respectively, based on the significant temperature fluctuations they derived.
However, our model is not able to reproduce the intensities of the oxygen
recombination lines . This cannot be attributed to observational uncertainties
and requires an explanation other than temperature fluctuations.Comment: accepted in Astronomy & Astrophysic
ISAAC/VLT observations of a lensed galaxy at z=10.0
We report the first likely spectroscopic confirmation of a z 10.0 galaxy from
our ongoing search for distant galaxies with ISAAC/VLT. Galaxy candidates at z
>~ 7 are selected from ultra-deep JHKs images in the core of gravitational
lensing clusters for which deep optical imaging is also available, including
HST data. The object reported here, found behind Abell 1835, exhibits a faint
emission line detected in the J band, leading to z=10.0 when identified as
Ly-a, in excellent agreement with the photometric redshift determination.
Redshifts z < 7 are very unlikely for various reasons we discuss. The object is
located on the critical lines corresponding to z=9 to 11. The magnification
factor \mu ranges from 25 to 100. For this object we estimate SFR(Ly-a)
(0.8-2.2) Msun/yr and SFR(UV) (47-75) Msun/yr, both uncorrected for lensing.
The steep UV slope indicates a young object with negligible dust extinction.
SED fits with young low-metallicity stellar population models yield (adopting
mu=25) a lensing corrected stellar mass of M*~8.e+6 Msun, and luminosities of
2.e+10 Lsun, corresponding to a dark matter halo of a mass of typically M_tot>~
5.e+8 Msun. In general our observations show that under excellent conditions
and using strong gravitational lensing direct observations of galaxies close to
the ``dark ages'' are feasible with ground-based 8-10m class telescopes.Comment: To be published in A&A, Vol. 416, p. L35. Press release information,
additional figures and information available at http://obswww.unige.ch/sfr
and http://webast.ast.obs-mip.fr/galaxie
HST's view of the youngest massive stars in the Magellanic Clouds
Accurate physical parameters of newborn massive stars are essential
ingredients to shed light on their formation, which is still an unsolved
problem. The rare class of compact H II regions in the Magellanic Clouds (MCs),
termed ``high-excitation blobs'' (HEBs), presents a unique opportunity to
acquire this information. These objects (~ 4" to 10", ~ 1 to 3 pc, in diameter)
harbor the youngest massive stars of the OB association/molecular cloud
complexes in the MCs accessible through high-resolution near-IR and optical
techniques. We present a brief overview of the results obtained with HST mainly
on two HEBs, one in the LMC (N159-5) and the other in the SMC (N81).Comment: 5 pages, to appear in the Proceedings of the 41st ESLAB Symposium
"The Impact of HST on European Astronomy", 29 May to 1 June 2007, ESTEC,
Noordwijk, Netherlands; eds. Guido De Marchi and Duccio Macchett
Starburst99: Synthesis Models for Galaxies with Active Star Formation
Starburst99 is a comprehensive set of model predictions for
spectrophotometric and related properties of galaxies with active star
formation. The models are an improved and extended version of the data set
previously published by Leitherer & Heckman (1995). We have upgraded our code
by implementing the latest set of stellar evolution models of the Geneva group
and the model atmosphere grid compiled by Lejeune et al. (1997). Several
predictions which were not included in the previous publication are shown here
for the first time. The models are presented in a homogeneous way for five
metallicities between Z = 0.040 and 0.001 and three choices of the initial mass
function. The age coverage is 10^6 to 10^9 yr. We also show the spectral energy
distributions which are used to compute colors and other quantities. The full
data set is available for retrieval at
http://www.stsci.edu/science/starburst99/. This website allows users to run
specific models with non-standard parameters as well. We also make the source
code available to the community.Comment: 32 pages, LaTeX. All the Figures and the summary Table are located at
http://www.stsci.edu/science/starburst99/, ApJ accepte
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