75 research outputs found
Spatial field reconstruction with INLA: Application to simulated galaxies
Aims. Monte Carlo Radiative Transfer (MCRT) simulations are a powerful tool
for understanding the role of dust in astrophysical systems and its influence
on observations. However, due to the strong coupling of the radiation field and
medium across the whole computational domain, the problem is non-local and
non-linear and such simulations are computationally expensive in case of
realistic 3D inhomogeneous dust distributions. We explore a novel technique for
post-processing MCRT output to reduce the total computational run time by
enhancing the output of computationally less expensive simulations of
lower-quality.
Methods. We combine principal component analysis (PCA) and non-negative
matrix factorization (NMF) as dimensionality reduction techniques together with
Gaussian Markov random fields and the Integrated nested Laplace approximation
(INLA), an approximate method for Bayesian inference, to detect and reconstruct
the non-random spatial structure in the images of lower signal-to-noise or with
missing data.
Results. We test our methodology using synthetic observations of a galaxy
from the SKIRT Auriga project - a suite of high resolution magneto-hydrodynamic
Milky Way-sized galaxies simulated in cosmological environment by 'zoom-in'
technique. With this approach, we are able to reproduce high photon number
reference images times faster with median residuals below .Comment: To be published in Numerical methods and codes of Astronomy and
Astrophysic
Searching for light echoes due to circumstellar matter in SNe Ia spectra
We present an analytical model for light echoes (LEs) coming from circumstellar material (CSM) around Type Ia Supernovae (SNe Ia). Using this model we find two spectral signatures at 4100 Å and 6200 Å that are useful to identify LEs during the Lira law phase (between 35 and 80 days after maximum light) coming from nearby CSM at distances of 0.010.25 pc. We analyze a sample of 89 SNe Ia divided into two groups according to their B V decline rate during the Lira law phase, and search for LEs from CSM interaction in the group of SNe with steeper slopes by comparing their spectra with our LE model. We find that a model with LEs + pure extinction from interstellar material (ISM) fits the observed spectra better than a pure ISM extinction model that is constant in time, but we find that a decreasing extinction alone explains the observations better without the need of LEs, possibly implying dust sublimation due to the radiation from the SN.Facultad de Ciencias Astronómicas y GeofísicasInstituto de Astrofísica de La Plat
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
EmulART: Emulating Radiative Transfer - A pilot study on autoencoder based dimensionality reduction for radiative transfer models
Dust is a major component of the interstellar medium. Through scattering,
absorption and thermal re-emission, it can profoundly alter astrophysical
observations. Models for dust composition and distribution are necessary to
better understand and curb their impact on observations. A new approach for
serial and computationally inexpensive production of such models is here
presented. Traditionally these models are studied with the help of radiative
transfer modelling, a critical tool to understand the impact of dust
attenuation and reddening on the observed properties of galaxies and active
galactic nuclei. Such simulations present, however, an approximately linear
computational cost increase with the desired information resolution. Our new
efficient model generator proposes a denoising variational autoencoder (or
alternatively PCA), for spectral compression, combined with an approximate
Bayesian method for spatial inference, to emulate high information radiative
transfer models from low information models. For a simple spherical dust shell
model with anisotropic illumination, our proposed approach successfully
emulates the reference simulation starting from less than 1% of the
information. Our emulations of the model at different viewing angles present
median residuals below 15% across the spectral dimension, and below 48% across
spatial and spectral dimensions. EmulART infers estimates for ~85% of
information missing from the input, all within a total running time of around
20 minutes, estimated to be 6x faster than the present target high information
resolution simulations, and up to 50x faster when applied to more complicated
simulations.Comment: 85 pages, 33 figures, 9 table
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
Supernova 2010ev: A reddened high velocity gradient type Ia supernova
Aims. We present and study the spectroscopic and photometric evolution of the
type Ia supernova (SN Ia) 2010ev. Methods. We obtain and analyze multi-band
optical light curves and optical-near-infrared spectroscopy at low and medium
resolution spanning from -7 days to +300 days from the B-band maximum. Results.
A photometric analysis shows that SN 2010ev is a SN Ia of normal brightness
with a light curve shape of and a stretch s =
suffering significant reddening. From photometric and
spectroscopic analysis, we deduce a color excess of
and a reddening law of . Spectroscopically, SN 2010ev
belongs to the broad-line SN Ia group, showing stronger than average Si II
{\lambda}6355 absorption features. We also find that SN 2010ev is a
high-velocity gradient SN, with a value of km s d.
The photometric and spectral comparison with other supernovae shows that SN
2010ev has similar colors and velocities to SN 2002bo and SN 2002dj. The
analysis of the nebular spectra indicates that the [Fe II] {\lambda}7155 and
[Ni II] {\lambda}7378 lines are redshifted, as expected for a high velocity
gradient supernova. All these common intrinsic and extrinsic properties of the
high velocity gradient (HVG) group are different from the low velocity gradient
(LVG) normal SN Ia population and suggest significant variety in SN Ia
explosions.Comment: 16 pages, 13 figures, 7 tables. Accepted to A&
Searching for light echoes due to circumstellar matter in SNe Ia spectra
We present an analytical model for light echoes (LEs) coming from circumstellar material (CSM) around Type Ia Supernovae (SNe Ia). Using this model we find two spectral signatures at 4100 Å and 6200 Å that are useful to identify LEs during the Lira law phase (between 35 and 80 days after maximum light) coming from nearby CSM at distances of 0.010.25 pc. We analyze a sample of 89 SNe Ia divided into two groups according to their B V decline rate during the Lira law phase, and search for LEs from CSM interaction in the group of SNe with steeper slopes by comparing their spectra with our LE model. We find that a model with LEs + pure extinction from interstellar material (ISM) fits the observed spectra better than a pure ISM extinction model that is constant in time, but we find that a decreasing extinction alone explains the observations better without the need of LEs, possibly implying dust sublimation due to the radiation from the SN.Facultad de Ciencias Astronómicas y GeofísicasInstituto de Astrofísica de La Plat
Progenitor properties of type II supernovae: fitting to hydrodynamical models using Markov chain Monte Carlo methods
Context. The progenitor and explosion properties of type II supernovae (SNe II) are fundamental to understanding the evolution of massive stars. Particular attention has been paid to the initial masses of their progenitors, but despite the efforts made, the range of initial masses is still uncertain. Direct imaging of progenitors in pre-explosion archival images suggests an upper initial mass cutoff of ∼18 M⊙. However, this is in tension with previous studies in which progenitor masses inferred by light-curve modelling tend to favour high-mass solutions. Moreover, it has been argued that light-curve modelling alone cannot provide a unique solution for the progenitor and explosion properties of SNe II.
Aims. We develop a robust method which helps us to constrain the physical parameters of SNe II by simultaneously fitting their bolometric light curve and the evolution of the photospheric velocity to hydrodynamical models using statistical inference techniques.
Methods. We created pre-supernova red supergiant models using the stellar evolution code MESA, varying the initial progenitor mass. We then processed the explosion of these progenitors through hydrodynamical simulations, where we changed the explosion energy and the synthesised nickel mass together with its spatial distribution within the ejecta. We compared the results to observations using Markov chain Monte Carlo methods.
Results. We apply this method to a well-studied set of SNe with an observed progenitor in pre-explosion images and compare with results in the literature. Progenitor mass constraints are found to be consistent between our results and those derived by pre-SN imaging and the analysis of late-time spectral modelling.
Conclusions. We have developed a robust method to infer progenitor and explosion properties of SN II progenitors which is consistent with other methods in the literature. Our results show that hydrodynamical modelling can be used to accurately constrain the physical properties of SNe II. This study is the starting point for a further analysis of a large sample of hydrogen-rich SNe.Facultad de Ciencias Astronómicas y GeofísicasInstituto de Astrofísica de La Plat
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