46 research outputs found
Statistical techniques for the detection and analysis of solar explosive events
Solar explosive events are commonly explained as small scale magnetic
reconnection events, although unambiguous confirmation of this scenario remains
elusive due to the lack of spatial resolution and of the statistical analysis
of large enough samples of this type of events. In this work, we propose a
sound statistical treatment of data cubes consisting of a temporal sequence of
long slit spectra of the solar atmosphere. The analysis comprises all the
stages from the explosive event detection to its characterization and the
subsequent sample study. We have designed two complementary approaches based on
the combination of standard statistical techniques (Robust Principal Component
Analysis in one approach and wavelet decomposition and Independent Component
Analysis in the second) in order to obtain least biased samples. These
techniques are implemented in the spirit of letting the data speak for
themselves. The analysis is carried out for two spectral lines: the C IV line
at 1548.2 angstroms and the Ne VIII line at 770.4 angstroms. We find
significant differences between the characteristics of the line profiles
emitted in the proximities of two active regions, and in the quiet Sun, most
visible in the relative importance of a separate population of red shifted
profiles. We also find a higher frequency of explosive events near the active
regions, and in the C IV line. The distribution of the explosive events
characteristics is interpreted in the light of recent numerical simulations.
Finally, we point out several regions of the parameter space where the
reconnection model has to be refined in order to explain the observations.Comment: Accepted for publication in Astronomy and Astrophysics (in Section 9.
The Sun) on 18/01/2011. 17 pages, 22 Figure
Properties of ultra-cool dwarfs with Gaia. An assessment of the accuracy for the temperature determination
We aimed to assess the accuracy of the Gaia teff and logg estimates as
derived with current models and observations. We assessed the validity of
several inference techniques for deriving the physical parameters of ultra-cool
dwarf stars. We used synthetic spectra derived from ultra-cool dwarf models to
construct (train) the regression models. We derived the intrinsic uncertainties
of the best inference models and assessed their validity by comparing the
estimated parameters with the values derived in the bibliography for a sample
of ultra-cool dwarf stars observed from the ground. We estimated the total
number of ultra-cool dwarfs per spectral subtype, and obtained values that can
be summarised (in orders of magnitude) as 400000 objects in the M5-L0 range,
600 objects between L0 and L5, 30 objects between L5 and T0, and 10 objects
between T0 and T8. A bright ultra-cool dwarf (with teff=2500 K and \logg=3.5
will be detected by Gaia out to approximately 220 pc, while for teff=1500 K
(spectral type L5) and the same surface gravity, this maximum distance reduces
to 10-20 pc. The RMSE of the prediction deduced from ground-based spectra of
ultra-cool dwarfs simulated at the Gaia spectral range and resolution, and for
a Gaia magnitude G=20 is 213 K and 266 K for the models based on k-nearest
neighbours and Gaussian process regression, respectively. These are total
errors in the sense that they include the internal and external errors, with
the latter caused by the inability of the synthetic spectral models (used for
the construction of the regression models) to exactly reproduce the observed
spectra, and by the large uncertainties in the current calibrations of spectral
types and effective temperatures.Comment: 18 pages, 17 figures, accepted by Astronomy & Astrophysic
RR Lyrae stars as standard candles in the Gaia Data Release 2 Era
We present results from the analysis of 401 RR Lyrae stars (RRLs) belonging
to the field of the Milky Way (MW). For a fraction of them multi-band (,
, ) photometry, metal abundances, extinction values and
pulsation periods are available in the literature and accurate trigonometric
parallaxes measured by the Gaia mission alongside Gaia -band time-series
photometry have become available with the Gaia second data release (DR2) on
2018 April 25. Using a Bayesian fitting approach we derive new near-,
mid-infrared period-absolute magnitude-metallicity () relations and new
absolute magnitude-metallicity relations in the visual ()
and bands (), based on the Gaia DR2 parallaxes. We find
the dependence of luminosity on metallicity to be higher than usually found in
the literature, irrespective of the passband considered. Running the adopted
Bayesian model on a simulated dataset we show that the high metallicity
dependence is not caused by the method, but likely arises from the actual
distribution of the data and the presence of a zero-point offset in the Gaia
parallaxes. We infer a zero-point offset of mas, with the Gaia DR2
parallaxes being systematically smaller. We find the RR Lyrae absolute
magnitude in the , , and bands at metallicity of
[Fe/H]= dex and period of P = 0.5238 days, based on Gaia DR2 parallaxes
to be mag, mag, mag and mag, respectively.Comment: 18 pages, 21 figures, 4 tables. Accepted for publication in MNRA
Gaia's Cepheids and RR Lyrae Stars and Luminosity Calibrations Based on Tycho-Gaia Astrometric Solution
Gaia Data Release 1 contains parallaxes for more than 700 Galactic Cepheids
and RR Lyrae stars, computed as part of the Tycho-Gaia Astrometric Solution
(TGAS). We have used TGAS parallaxes, along with literature () photometry and spectroscopy, to calibrate the zero point
of the Period-Luminosity and Period-Wesenheit relations of classical and type
II Cepheids, and the near-infrared Period-Luminosity,
Period-Luminosity-Metallicity and optical Luminosity-Metallicity relations of
RR Lyrae stars. In this contribution we briefly summarise results obtained by
fitting these basic relations adopting different techniques that operate either
in parallax or distance (absolute magnitude) space.Comment: 5 pages, 4 figures, proceedings for the 22nd Los Alamos Stellar
Pulsation Conference Series Meeting "Wide field variability surveys: a
21st-century perspective", held in San Pedro de Atacama, Chile, Nov. 28 -
Dec. 2, 201
The variability processing and analysis of the Gaia mission
We present the variability processing and analysis that is foreseen for the
Gaia mission within Coordination Unit 7 (CU7) of the Gaia Data Processing and
Analysis Consortium (DPAC). A top level description of the tasks is given.Comment: 4 pages, 1 figure. To be published in the proceedings of the
GREAT-ITN conference "The Milky Way Unravelled by Gaia: GREAT Science from
the Gaia Data Releases", 1-5 December 2014, University of Barcelona, Spain,
EAS Publications Series, eds Nicholas Walton, Francesca Figueras, and
Caroline Soubira
Identification and characterization of ultracool dwarfs using J-PLUS
In this presentation we describe the different methodological approaches followed to identify and characterize ultra-cool dwarfs in the J-PLUS Science Verification Data Release 2
A brief review of the effect of wildfires on rockfall occurrence
Wildfires and rockfalls are among the major hazards in forested mountainous regions across Europe. Understanding processes and conditions that lead to rockfalls during and after a wildfire in different geological contexts is, therefore, of great relevance. The increase of rockfalls associated with the occurrence of wildfires is connected to several factors, not only in the detached area but also in the propagation and affected area. Wildfires cause changes in the mechanical properties of rocks and discontinuities as well as the loss of protective capacity from vegetation, complemented by the effect induced by firefighting activities and by extreme temperatures that may deteriorate the installed protective measures. After the occurrence of a wildfire, there is an increase in the frequency and intensity of rockfalls in the burned area, causing a major impact of rockfalls on road networks and inhabited areas. Additionally, the rockfall risk perception is usually increased due to the removal of vegetation by wildfires, exposing both rock blocks and the rock mass. In this review, the main factors that influence the occurrence of rockfalls after a wildfire are briefly reviewed
Cygnus OB2 DANCe: A high-precision proper motion study of the Cygnus OB2 association
This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society. © 2016 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.We present a high-precision proper motion study of 873 X-ray and spectroscopically selected stars in the massive OB association Cygnus OB2 as part of the DANCe project. These were calculated from images spanning a 15 yr baseline and have typical precisions <1 mas yr−1. We calculate the velocity dispersion in the two axes to be σα(c) = 13.0+0.8 −0.7 and σδ (c) = 9.1+0.5 −0.5 km s−1, using a two-component, two-dimensional model that takes into account the uncertainties on the measurements. This gives a three-dimensional velocity dispersion of σ3D = 17.8 ± 0.6 km s−1 implying a virial mass significantly larger than the observed stellar mass, confirming that the association is gravitationally unbound. The association appears to be dynamically unevolved, as evidenced by considerable kinematic substructure, non-isotropic velocity dispersions and a lack of energy equipartition. The proper motions show no evidence for a global expansion pattern, with approximately the same amount of kinetic energy in expansion as there is in contraction, which argues against the association being an expanded star cluster disrupted by process such as residual gas expulsion or tidal heating. The kinematic substructures, which appear to be close to virial equilibrium and have typical masses of 40– 400 M, also do not appear to have been affected by the expulsion of the residual gas. We conclude that Cyg OB2 was most likely born highly substructured and globally unbound, with the individual subgroups born in (or close to) virial equilibrium, and that the OB association has not experienced significant dynamical evolution since then.Peer reviewe