37 research outputs found
First systematic high-precision survey of bright supernovae
Rapid variability before and near the maximum brightness of supernovae has the potential to provide a better understanding of nearly every aspect of supernovae, from the physics of the explosion up to their progenitors and the circumstellar environment. Thanks to modern time-domain optical surveys, which are discovering supernovae in the early stage of their evolution, we have the unique opportunity to capture their intraday behavior before maximum. We present high-cadence photometric monitoring (on the order of seconds-minutes) of the optical light curves of three Type Ia and two Type II SNe over several nights before and near maximum light, using the fast imagers available on the 2.3 m Aristarchos telescope at Helmos Observatory and the 1.2 m telescope at Kryoneri Observatory in Greece. We applied differential aperture photometry techniques using optimal apertures and we present reconstructed light curves after implementing a seeing correction and the Trend Filtering Algorithm (TFA, Kovács et al. 2005, MNRAS, 356, 557). TFA yielded the best results, achieving a typical precision between 0.01 and 0.04 mag. We did not detect significant bumps with amplitudes greater than 0.05 mag in any of the SNe targets in the VR-, R-, and I-bands light curves obtained. We measured the intraday slope for each light curve, which ranges between −0.37−0.36 mag day−1 in broadband VR, −0.19−0.31 mag day−1 in R band, and −0.13−0.10 mag day−1 in I band. We used SNe light curve fitting templates for SN 2018gv, SN 2018hgc and SN 2018hhn to photometrically classify the light curves and to calculate the time of maximum. We provide values for the maximum of SN 2018zd after applying a low-order polynomial fit and SN 2018hhn for the first time. We conclude that optimal aperture photometry in combination with TFA provides the highest-precision light curves for SNe that are relatively well separated from the centers of their host galaxies. This work aims to inspire the use of ground-based, high-cadence and high-precision photometry to study SNe with the purpose of revealing clues and properties of the explosion environment of both core-collapse and Type Ia supernovae, the explosion mechanisms, binary star interaction and progenitor channels. We suggest monitoring early supernovae light curves in hotter (bluer) bands with a cadence of hours as a promising way of investigating the post-explosion photometric behavior of the progenitor stars
The VLT-FLAMES Tarantula survey: III. A very massive star in apparent isolation from the massive cluster R136
Gamma rays from dark matter annihilation in the Draco and observability at ARGO
The CACTUS experiment recently observed a gamma ray excess above 50 GeV from
the direction of the Draco dwarf spheroidal galaxy. Considering that Draco is
dark matter dominated the gamma rays may be generated through dark matter
annihilation in the Draco halo. In the framework of the minimal supersymmetric
extension of the standard model we explore the parameter space to account for
the gamma ray signals at CACTUS. We find that the neutralino mass is
constrained to be approximately in the range between 100 GeV ~ 400 GeV and a
sharp central cuspy of the dark halo profile in Draco is necessary to explain
the CACTUS results. We then discuss further constraints on the supersymmetric
parameter space by observations at the ground based ARGO detector. It is found
that the parameter space can be strongly constrained by ARGO if no excess from
Draco is observed above 100 GeV.Comment: 15 pages, 4 figure
Annihilation vs. Decay: Constraining dark matter properties from a gamma-ray detection
Most proposed dark matter candidates are stable and are produced thermally in
the early Universe. However, there is also the possibility of unstable (but
long-lived) dark matter, produced thermally or otherwise. We propose a strategy
to distinguish between dark matter annihilation and/or decay in the case that a
clear signal is detected in gamma-ray observations of Milky Way dwarf
spheroidal galaxies with gamma-ray experiments. The sole measurement of the
energy spectrum of an indirect signal would render the discrimination between
these cases impossible. We show that by examining the dependence of the
intensity and energy spectrum on the angular distribution of the emission, the
origin could be identified as decay, annihilation, or both. In addition, once
the type of signal is established, we show how these measurements could help to
extract information about the dark matter properties, including mass,
annihilation cross section, lifetime, dominant annihilation and decay channels,
and the presence of substructure. Although an application of the approach
presented here would likely be feasible with current experiments only for very
optimistic dark matter scenarios, the improved sensitivity of upcoming
experiments could enable this technique to be used to study a wider range of
dark matter models.Comment: 29 pp, 8 figs; replaced to match published version (minor changes and
some new references
The Large Magellanic Cloud and the Distance Scale
The Magellanic Clouds, especially the Large Magellanic Cloud, are places
where multiple distance indicators can be compared with each other in a
straight-forward manner at considerable precision. We here review the distances
derived from Cepheids, Red Variables, RR Lyraes, Red Clump Stars and Eclipsing
Binaries, and show that the results from these distance indicators generally
agree to within their errors, and the distance modulus to the Large Magellanic
Cloud appears to be defined to 3% with a mean value of 18.48 mag, corresponding
to 49.7 Kpc. The utility of the Magellanic Clouds in constructing and testing
the distance scale will remain as we move into the era of Gaia.Comment: 23 pages, accepted for publication in Astrophysics and Space Science.
From a presentation at the conference The Fundamental Cosmic Distance Scale:
State of the Art and the Gaia Perspective, Naples, May 201
A lower bound on the mass of Dark Matter particles
We discuss the bounds on the mass of Dark Matter (DM) particles, coming from
the analysis of DM phase-space distribution in dwarf spheroidal galaxies
(dSphs). After reviewing the existing approaches, we choose two methods to
derive such a bound. The first one depends on the information about the current
phase space distribution of DM particles only, while the second one uses both
the initial and final distributions. We discuss the recent data on dSphs as
well as astronomical uncertainties in relevant parameters. As an application,
we present lower bounds on the mass of DM particles, coming from various dSphs,
using both methods. The model-independent bound holds for any type of fermionic
DM. Stronger, model-dependent bounds are quoted for several DM models (thermal
relics, non-resonantly and resonantly produced sterile neutrinos, etc.). The
latter bounds rely on the assumption that baryonic feedback cannot
significantly increase the maximum of a distribution function of DM particles.
For the scenario in which all the DM is made of sterile neutrinos produced via
non-resonant mixing with the active neutrinos (NRP) this gives m_nrp > 1.7 keV.
Combining these results in their most conservative form with the X-ray bounds
of DM decay lines, we conclude that the NRP scenario remains allowed in a very
narrow parameter window only. This conclusion is independent of the results of
the Lyman-alpha analysis. The DM model in which sterile neutrinos are
resonantly produced in the presence of lepton asymmetry remains viable. Within
the minimal neutrino extension of the Standard Model (the nuMSM), both mass and
the mixing angle of the DM sterile neutrino are bounded from above and below,
which suggests the possibility for its experimental search.Comment: 20 pages, published in JCA
Low-mass pre--main-sequence stars in the Magellanic Clouds
[Abridged] The stellar Initial Mass Function (IMF) suggests that sub-solar
stars form in very large numbers. Most attractive places for catching low-mass
star formation in the act are young stellar clusters and associations, still
(half-)embedded in star-forming regions. The low-mass stars in such regions are
still in their pre--main-sequence (PMS) evolutionary phase. The peculiar nature
of these objects and the contamination of their samples by the evolved
populations of the Galactic disk impose demanding observational techniques for
the detection of complete numbers of PMS stars in the Milky Way. The Magellanic
Clouds, the companion galaxies to our own, demonstrate an exceptional star
formation activity. The low extinction and stellar field contamination in
star-forming regions of these galaxies imply a more efficient detection of
low-mass PMS stars than in the Milky Way, but their distance from us make the
application of special detection techniques unfeasible. Nonetheless, imaging
with the Hubble Space Telescope yield the discovery of solar and sub-solar PMS
stars in the Magellanic Clouds from photometry alone. Unprecedented numbers of
such objects are identified as the low-mass stellar content of their
star-forming regions, changing completely our picture of young stellar systems
outside the Milky Way, and extending the extragalactic stellar IMF below the
persisting threshold of a few solar masses. This review presents the recent
developments in the investigation of PMS stars in the Magellanic Clouds, with
special focus on the limitations by single-epoch photometry that can only be
circumvented by the detailed study of the observable behavior of these stars in
the color-magnitude diagram. The achieved characterization of the low-mass PMS
stars in the Magellanic Clouds allowed thus a more comprehensive understanding
of the star formation process in our neighboring galaxies.Comment: Review paper, 26 pages (in LaTeX style for Springer journals), 4
figures. Accepted for publication in Space Science Review
Photometric variability of ob-type stars as a new window on massive stars
We present the first systematic study of 4646 spectroscopically confirmed early-type massive stars in the Small Magellanic Cloud (SMC), using variability as a tool to confine the physics of OB-type massive stars. We report the discovery of ∼100 massive eclipsing systems which are useful for the accurate determination of the fundamental parameters of massive stars and we evaluate the frequency of multiplicity. In addition, we explore the occurrence of the Oe/Be phenomenon and provide a large number of candidate non-radial pulsators, which can be further studied via asteroseismology. The results of this work (Kourniotis et al. 2014) will contribute to a better understanding of the role of metallicity in triggering processes associated to matter ejections and/or disk formation, which in turn affect mass loss and stellar rotation. Copyright © International Astronomical Union 2015