3,112 research outputs found
Discovery of distant RR Lyrae stars in the Milky Way using DECam
We report the discovery of distant RR Lyrae stars, including the most distant
known in the Milky Way, using data taken in the band with the Dark Energy
Camera as part of the High cadence Transient Survey (HiTS; 2014 campaign). We
detect a total of 173 RR Lyrae stars over a ~120 deg^2 area, including both
known RR Lyrae and new detections. The heliocentric distances d_H of the full
sample range from 9 to >200 kpc, with 18 of them beyond 90 kpc. We identify
three sub-groups of RR Lyrae as members of known systems: the Sextans dwarf
spheroidal galaxy, for which we report 46 new discoveries, and the ultra-faint
dwarf galaxies Leo IV and Leo V. Following an MCMC methodology, we fit
spherical and ellipsoidal profiles of the form rho(R) ~ R^n to the radial
density distribution of RR Lyrae in the Galactic halo. The best fit corresponds
to the spherical case, for which we obtain a simple power-law index of
n=-4.17^{+0.18}_{-0.20}, consistent with recent studies made with samples
covering shorter distances. The pulsational properties of the outermost RR
Lyrae in the sample (d_H>90 kpc) differ from the ones in the halo population at
closer distances. The distribution of the stars in a Period-Amplitude diagram
suggest they belong to Oosterhoff-intermediate or Oosterhoff II groups, similar
to what is found in the ultra-faint dwarf satellites around the Milky Way. The
new distant stars discovered represent an important addition to the few
existing tracers of the Milky Way potential in the outer halo.Comment: Accepted for publication in The Astrophysical Journa
Serendipitous discovery of RR Lyrae stars in the Leo V ultra-faint galaxy
During the analysis of RR Lyrae stars discovered in the High cadence
Transient Survey (HiTS) taken with the Dark Energy Camera at the 4-m telescope
at Cerro Tololo Inter-American Observatory, we found a group of three very
distant, fundamental mode pulsator RR Lyrae (type ab). The location of these
stars agrees with them belonging to the Leo V ultra-faint satellite galaxy, for
which no variable stars have been reported to date. The heliocentric distance
derived for Leo V based on these stars is 173 +/- 5 kpc. The pulsational
properties (amplitudes and periods) of these stars locate them within the locus
of the Oosterhoff II group, similar to most other ultra-faint galaxies with
known RR Lyrae stars. This serendipitous discovery shows that distant RR Lyrae
stars may be used to search for unknown faint stellar systems in the outskirts
of the Milky Way.Comment: Accepted in ApJ Letter
INCONEL 718 SINGLE AND MULTIPASS MODELLING OF HOT FORGING
10International audienceA better understanding of the competition between several mechanisms (dynamic recovery, dynamic recrystallization and plasticity hardening) is crucial for aircraft engine manufacturers. The aim of this paper is to improve the microstructure and therefore the mechanical properties of a nickel based superalloy used for rotating forged pieces. A nickel superalloy microstructure is the result of several successive hot forging processes: multipass processes, with intermediate dwell time and quenching. In this paper, an original three dimensional approach able to simulate these processes is proposed. The specific role of the different steps of the processes is analysed. In this approach, several forging thermo-mechanical parameters are taken into account: the working temperature, the strain rate, the final strain, the interpass time, etc. At high forging temperature, the studied INCONEL 718 presents an austenitic matrix γ (face centred cubic) assumed to be in a single phase. This approach proposes a sequential coupling of two models, one devoted to deformation and the other to recrystallization. Such a coupling enables the estimation of the effect of deformation and of different recrystallization types on mechanical behaviour and on micro-structural evolution. The approach is performed at the grain scale and takes into account the whole thermo-mechanical cycle with a focus on the dynamic behaviour. The first polycrystalline model is based on the plasticity mechanisms at the grains scale. The framework corresponds to finite transformations (large lattice rotations and small elastic strains). The model is implemented in ABAQUS and CAST3M finite element codes. The second model is based on the recrystallization theory and uses a 3D cellular automaton. It describes dynamic recrystallization phenomena such as nucleation-growth and static or post-dynamic recrystallization. Such recrystallization mechanisms were observed during interpass time or during the successive heatings depending on the thermo-mechanical paths used in multipass forging. Dislocation densities are the internal variables common to the two models. The simulations are performed on a 3D Representative Elementary Volume (aggregate) obtained from Electron Back Scattering mapping. Numerical results are compared to experimental microstructures
Early-Time Ultraviolet and Optical Hubble Space Telescope Spectroscopy of the Type II Supernova 2022wsp
We report early-time ultraviolet (UV) and optical spectroscopy of the young,
nearby Type II supernova (SN) 2022wsp obtained by the Hubble Space Telescope
(HST)/STIS at about 10 and 20 days after the explosion. The SN 2022wsp UV
spectra are compared to those of other well-observed Type II/IIP SNe, including
the recently studied Type IIP SN 2021yja. Both SNe exhibit rapid cooling and
similar evolution during early phases, indicating a common behavior among SNe
II. Radiative-transfer modeling of the spectra of SN 2022wsp with the TARDIS
code indicates a steep radial density profile in the outer layer of the ejecta,
a supersolar metallicity, and a relatively high total extinction of E(B-V) =
0.35 mag. The early-time evolution of the photospheric velocity and temperature
derived from the modeling agree with the behavior observed from other
previously studied cases. The strong suppression of hydrogen Balmer lines in
the spectra suggests interaction with a pre-existing circumstellar environment
could be occurring at early times. In the SN 2022wsp spectra, the absorption
component of the Mg II P Cygni profile displays a double-trough feature on day
+10 that disappears by day +20. The shape is well reproduced by the model
without fine-tuning the parameters, suggesting that the secondary blueward dip
is a metal transition that originates in the SN ejecta.Comment: Submitted to ApJ Letters on 4/11/202
UBVRIz Light Curves of 51 Type II Supernovae
We present a compilation of UBV RIz light curves of 51 type II supernovae
discovered during the course of four different surveys during 1986 to 2003: the
Cerro Tololo Supernova Survey, the Calan/Tololo Supernova Program (C&T), the
Supernova Optical and Infrared Survey (SOIRS), and the Carnegie Type II
Supernova Survey (CATS). The photometry is based on template-subtracted images
to eliminate any potential host galaxy light contamination, and calibrated from
foreground stars. This work presents these photometric data, studies the color
evolution using different bands, and explores the relation between the
magnitude at maximum brightness and the brightness decline parameter (s) from
maximum light through the end of the recombination phase. This parameter is
found to be shallower for redder bands and appears to have the best correlation
in the B band. In addition, it also correlates with the plateau duration, being
thus shorter (longer) for larger (smaller) s values.Comment: 110 pages, 9 Figures, 6 Tables, accepted in A
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On the Origin of SN 2016hil—A Type II Supernova in the Remote Outskirts of an Elliptical Host
Type II supernovae (SNe) stem from the core collapse of massive (>8 M ⊙) stars. Due to their short lifespan, we expect a very low rate of such events in elliptical hosts, where the star formation rate is low, and which are mostly comprised of an old stellar population. SN 2016hil (iPTF16hil) is an SN II located in the extreme outskirts of an elliptical galaxy at z = 0.0608 (projected distance 27.2 kpc). It was detected near peak (M_r ~ −17 mag) 9 days after the last non-detection. The event has some potentially peculiar properties: it presented an apparently double-peaked light curve, and its spectra suggest low metallicity content (Z < 0.4 Z ⊙). We place a tentative upper limit on the mass of a potential faint host at log M/M⊙ = 7.27^(+0.43)_(-0.24) using deep optical imaging from Keck/LRIS. In light of this, we discuss the possibility of the progenitor forming locally and other more exotic formation scenarios such as a merger or common-envelope evolution causing a time-delayed explosion. Further observations of the explosion site in the UV are needed in order to distinguish between the cases. Regardless of the origin of the transient, observing a population of such seemingly hostless SNe II could have many uses, including an estimate the amount of faint galaxies in a given volume, and tests of the prediction of a time-delayed population of core-collapse SNe in locations otherwise unfavorable for the detection of such events
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