Collapse scenarios in magnetized star-forming regions

Estudiante: Juárez-Rodríguez, Carmen
Supervisada por: Josep Miquel Girart Medina; Palau, A.

Star-forming regions are located at the densest parts of molecular clouds. To be able to observe the embedded dense cores, long-wavelength high-angular resolution observations are needed. In this work we have used observational data from the Plateau de Bure (PdBI) and the Submillimeter Array (SMA) radio-interferometers.…
Estudiante: Juárez-Rodríguez, Carmen
Supervisada por: Josep Miquel Girart Medina; Palau, A.

Star-forming regions are located at the densest parts of molecular clouds. To be able to observe the embedded dense cores, long-wavelength high-angular resolution observations are needed. In this work we have used observational data from the Plateau de Bure (PdBI) and the Submillimeter Array (SMA) radio-interferometers. In addition, we have used the single-dish IRAM 30m telescope which recovers extended emission. From the obtained data we have studied the thermal dust continuum emission and the molecular line emission from their rotational electronic transitions. The dust continuum emission allows us to study the structure and physical properties of the cores such as mass, column density, etc. As the dust continuum emission is polarized, it also provides magnetic field information. On the other hand, the molecular line emission provides the kinematic information of the gas; we can also derive the column density, mass and other properties associated to the gas.
To carry out the goal of this thesis we have studied three regions in different physical and chemical conditions: an isolated pre-stellar core in a quiescent magnetized environment in the Pipe nebula, a high-mass star-forming region in a more evolved evolutionary state called NGC 6334 V, and a highly-fragmented lower-mass region called L1287.

Collapse scenarios in magnetized star-forming regions

Estudiante: Carmen Juárez Rodríguez
Supervisada por: Josep M. Girart; Aina Palau

Turbulence, magnetic fields and gravity driven flows are important for the formation of new stars. Although magnetic fields have been proven to be important in the formation of stars, only a few works have been done combining magnetic field and kinematic information.  Such studies are important to analyze…
Estudiante: Carmen Juárez Rodríguez
Supervisada por: Josep M. Girart; Aina Palau

Turbulence, magnetic fields and gravity driven flows are important for the formation of new stars. Although magnetic fields have been proven to be important in the formation of stars, only a few works have been done combining magnetic field and kinematic information.  Such studies are important to analyze both gravity and gas dynamics and be able to compare them with the magnetic field. In this thesis we will combine dust polarization studies with kinematic analysis towards different star-forming regions. The aim of this thesis has been to study the physical properties of dense cores (at scales < 0.1 pc) from molecular line and dust emission, and to study the role of the magnetic field in their dynamic evolution. For this, we have used millimeter and submillimeter observational data.

The studies have been performed towards 3 different star-forming regions. The pre-stellar core FeSt 1-457, located in an isolated and magnetized environment in the Pipe nebula. The high-mass star-forming region NGC6334V, in a more advanced evolutionary state and in an environment surrounded by other massive star-forming regions. And L1287, a lower-mass region but with similar characteristics to NGC6334V, with presence of high-velocity gas and several centimeter and infrared sources.

The studies of the pre-stellar core FeSt1-457 and the massive region NGC6334V, show how the magnetic field has been overcome by gravity and is not enough to avoid the gravitational collapse. In addition, NGC6334V and the lower-mass region L1287 present very similar scenarios with the material converging from large scales (~0.1 pc) to the potential wells of both regions at smaller scales (~0.02 pc) through two dense gas flows separated by 2-3 km/s. In a similar scenario, FeSt1-457 is located just in the region where two dense gas structures, separated by 3 km/s, appear to converge.

Precipitation measurements with polarimetric GNSS Radio Occultations

PhD Thesis started at the end of 2012
In 2009, the Spanish Ministry of Science and Innovation approved a proposal to modify the Global Positioning System (GPS) receiver and to allocate a Polarimetric (Pol) Radio Occultation (RO) antenna in the Spanish PAZ satellite. PAZ became an opportunity to test the new Pol-RO concept, which aims…

In 2009, the Spanish Ministry of Science and Innovation approved a proposal to modify the Global Positioning System (GPS) receiver and to allocate a Polarimetric
(Pol) Radio Occultation (RO) antenna in the Spanish PAZ satellite. PAZ became an opportunity to test the new Pol-RO concept, which aims to capture ROs using a
two orthogonal linear polarization antenna. The experiment has been named Radio Occultations and Heavy Precipitation with PAZ (ROHP-PAZ). The objective is to
measure the phase difference between the horizontal and the vertical components of the incoming electromagnetic field that is induced by heavy precipitation flattened
raindrops. This effect, widely studied in the weather radar community, will be measured from space using GNSS signals for the first time with PAZ, which is
planned to be launched in 2017 (date yet to be confirmed).

The main objective of this new concept is to enhance the RO capabilities by providing vertical precipitation information along with the current standard RO
thermodynamic products (i. e. temperature, pressure and moisture). Until now, no other observing system has been able to provide simultaneous thermodynamic and
precipitation information under extreme conditions. The high vertical resolution, global coverage and all-weather capability properties of the RO observations com-
bined with vertical indication of precipitation intensity can be of great value for heavy rain characterization, and therefore for climate and weather forecasting and
research.

Within this context, the theoretical background for the technique, its feasibility and applications have been assessed in this dissertation. The theoretical basis
has been developed combining electromagnetic propagation theory and cloud and precipitation microphysics. Very detailed forward scattering simulations at L-band
have been obtained in order to relate the microphysical parameters with the expected Pol-RO observables. Feasibility has been addressed using coincident (in
space and time) RO profiles and space-based precipitation observations. Such simultaneous observations allow for the characterization of actual RO measurements
according to the coincident precipitation information, and allow us to obtain, for example, the noise level under precipitating scenarios. Finally, the applications have
been investigated through realistic end-to-end simulations of the Pol-RO observations, which provide the anticipated Pol-RO products for different precipitation
situations, regions, and seasons.

Before the launch of the satellite, a field campaign has been conducted with the aim of starting the characterization of the polarimetric measurements. The engi-
neering model of the PAZ antenna was placed at the top of a mountain peak in order to capture, for the first time, linear polarimetric GNSS signals at low graz-
ing angles. Although the geometry and the scenario are different from those that PAZ will be studying from space, this campaign has been useful to start identifying
the hardware internal effects and unexpected precipitation features that will affect the Pol-RO observations. These effects have been incorporated into the simulations,
providing valuable feedback to obtain more realistic Pol-RO products.

These exercises yielded several relevant results. The noise level analysis from actual RO observations sensing precipitation scenarios has allowed us to set a de-
tectability threshold for the technique, indicating that a high percentage of moderate to heavy precipitation events will be detected with PAZ. Nevertheless, the inte-
grated nature of the Pol-RO observable does not allow us to distinguish between the contributions from the rain’s intensity and extension, leaving an ambiguity in the
provided product. In an attempt to solve such ambiguity, a tomographic approach has been proposed, which has yielded promising theoretical results. Moreover, it
has been shown how the Pol-RO observables can be linked to physical precipitation parameters, such as the along-ray averaged rain rate, in a probabilistic way.
The end-to-end simulation has also revealed that the ionosphere will induce a non-negligible depolarization that will require calibration.

Besides providing feedback for the simulations, the data from the field campaign have also shown the first observational evidence that precipitation and other hy-
drometeors have a noticeable effect on the GNSS polarimetric signals. These effects have been compared with the simulations, showing agreement within an order of
magnitude. The collocated data has also been used to show the potential applications of Pol-ROs products. Comparison of model outputs with RO retrievals, in the presence of heavy rain, has shown discrepancies that will need further investigation, and Pol-RO data appears to be a well-fitted dataset for such studies.

Precipitation measurements with Polarimetric Radio Occultations

The new Polarimetric Radio Occultation technique will be tested from space for the first time aboard the Spanish satellite PAZ. This concept aims to capture Radio Occultations using a two orthogonal linear polarization antenna, with the objective to measure the phase difference between the horizontal…

The new Polarimetric Radio Occultation technique will be tested from space for the first time aboard the Spanish satellite PAZ. This concept aims to capture Radio Occultations using a two orthogonal linear polarization antenna, with the objective to measure the phase difference between the horizontal and the vertical components of the incoming electromagnetic field that is induced when heavy precipitation flattened raindrops are present in the ray-path. The theoretical background for the technique, its feasibility and applications have been assessed in this dissertation. Forward scattering simulations at L-band have been obtained combining electromagnetic propagation theory and cloud and precipitation microphysics. The feasibility has been proven using coincident (in space and time) RO profiles and space-based precipitation observations, indicating that a high percentage of moderate to heavy precipitation events will be detected with PAZ. The applications have been investigated through realistic end-to-end simulations. Using them, it has been shown how the Pol-RO observables can be linked to physical precipitation parameters, and some potential applications have been proposed. Furthermore, the ionosphere has been identified as a potential threat that will require calibration.

The Sun as a laboratory of particle physics

Estudiante: Vinyoles, N.
Supervisada por: Aldo Serenelli ; Jordi Isern Vilaboy

The Sun is by far the most studied and best-known star. The solar struc- ture, revealed by helioseismology and solar neutrinos, is well determined, and accurate solar models give information about the past, present and the future of the Sun. These solar models, or Standard Solar Models (SSM), are…
Estudiante: Vinyoles, N.
Supervisada por: Aldo Serenelli ; Jordi Isern Vilaboy

The Sun is by far the most studied and best-known star. The solar struc- ture, revealed by helioseismology and solar neutrinos, is well determined, and accurate solar models give information about the past, present and the future of the Sun. These solar models, or Standard Solar Models (SSM), are useful for describing the solar interior, and at the same time provide a deeper knowledge on other disciplines, such as stellar structure and evolution, particle physics and even non-standard particle physics. In fact, the large density of the solar core, its temperature and sheer size allow studying physics in environments that are hard to reproduce in earth-based experiments. Consequently, the Sun is a powerful laboratory to test non-standard particle physics. In particular, the Sun offers very interesting possibilities for studying weakly interacting light particles that arise from extensions of the Standard Model of particles to address some of the most pressing open questions in fundamental physics, such as the nature of dark matter. The main goal of this thesis is to use solar models to study the impact of different types of weakly interacting particles on the solar structure. Then, based on the structural changes they produce, the goal is to set the most restrictive bounds to properties of these particles using solar data from helioseismology and neutrinos. In order to pursue this goal, it is important to have realistic solar models that reproduce, as best as possible, the available observations. Motivated by this fact, this thesis presents a new generation of SSMs that includes recent updates on some important nuclear reaction rates and a more con- sistent treatment of the equation of state. Models also include updated errors computed using Monte-Carlo simulations and a novel and flexible treatment of opacity uncertainties based on opacity kernels, required in the light of recent theoretical and experimental works on radiative opacity. In fact, radiative opacities are proposed as one of the possible solutions to the solar abundance problem, that is the conflict between helioseismic predictions from the models and observations when new releases of so- lar composition are used to construct the SSM, in contrast to the good agreement obtained with the older compositions based on more simplistic one-dimensional model atmospheres. Therefore, to have an understand- ing of the current status of the radiative opacities and the corresponding uncertainties is important. For that reason, an exhaustive study of the available radiative opacity tables is presented in this thesis in the context of SSMs. Current uncertainties in the solar composition and opacities can be over- come for studies of particle physics that do not depend on a detailed knowledge of the solar interior composition. For this purpose, the Best Fit Model, a solar model that better reproduces the observations using realistic evolutionary inputs, is introduced. Finally, a new statistical analysis that combines helioseismology and solar neutrino observations (the 8B and 7Be fluxes) is presented, and it is used to place upper limits to the properties of non standard weakly interacting particles, and in particular, to axions, hidden photons and minicharged particles. This statistical analysis is based on the Best Fit Model and it includes theoretical and observational errors, accounts for tensions be- tween input parameters of solar models and can be easily extended to include other observational constraints. For the fist time, constraints on the properties of these particles are placed by using a method that com- bines both helioseismology and solar neutrino observations. Additionally, the fact that Best Fit Models are the basis of the statistical analysis results in more robust bounds independent on the solar abundance problem. The bounds obtained are: for the axion-photon coupling constant g10 < 4.1 at 3 CL, for the product of the kinetic mixing and mass of hidden photons, χm < 1.8·10E-12 V at 3 CL and for the chage of the minicharged particles, ε = 2.2·10−14 at 2 CL for mf = 0−25 eV. For all the cases, the results are the most restrictive solar bounds, being a factor of 2 better for axions and hidden photons. Moreover, the results obtained for hidden photons and minicharged particles are globally the most restrictive bounds.

The Sun as a laboratory of particle physics

Estudiante: Núria Vinyoles Vergés

The main goal of this thesis is to use solar models to study the impact of different types of weakly interacting particles on the solar structure. Then, based on the structural changes they produce, the goal is to set the most restrictive bounds to properties of these particles using solar data from…
Estudiante: Núria Vinyoles Vergés

The main goal of this thesis is to use solar models to study the impact of different types of weakly interacting particles on the solar structure. Then, based on the structural changes they produce, the goal is to set the most restrictive bounds to properties of these particles using solar data from helioseismology and neutrinos. With that purpose, a new statistical analysis that combines helioseismology and solar neutrino observations is presented and it is used to place upper limits to the properties of non standard weakly interacting particles, and in particular, to axions, hidden photons and minicharged particles. The results are the most restrictive solar bounds, being approximately a factor two better than previous ones. Moreover, the results obtained for hidden photons and minicharged particles are globally the most restrictive bounds.

Cosmology with galaxy surveys

Estudiante: Pujol, Arnau

One of the fundamental goals of Cosmology is to understand the matter and energy content of the Universe, and the way its contents determine the growth of fluctuations and the evolution of the observed structure. In order to tackle these questions, we need to study the overall structure and evolution of the universe and its constituents. We thus need to sample large enough volumes as to be meaningful and representative. We also need to probe different epochs to learn about its evolution. Observationally, these requirements are only met by big surveys that probe wide areas and a large fraction of the local Universe and that are deep enough to sample structures at high redshift. In the last few years, the number of surveys has been impressive and impossible to review here in detail. The most useful of them in terms of cosmological and large-scale structure applications have been the detailed redshift surveys of the local universe. The Two-Degree Field Galaxy Redshift Survey (2dFGRS; Colless et al 2001) and the Sloan Digital Sky Survey (SDSS; York et al 2000) have measured many hundreds of thousands of galaxy spectra. This has resulted in a major improvement in our knowledge of the galaxy population through, for example, the galaxy luminosity function (e.g., Norberg et al 2002; Blanton et al 2003), the galaxy power spectrum (e.g. Cole et al 2005; Tegmark et al 2004a), galaxy clustering (e.g., Norberg et al 2001; Gaztañaga 2002; Zehavi et al 2004, Cabre & Gaztanaga 2008), clusters of galaxies (e.g., de Propis et al 2002; Bahcall et al 2003), the cosmic star formation history (e.g., Baldry et al 2002; Glazebrook et al 2003), galaxy biasing (e.g., Verde et al 2002; Tegmark et al 2004a, Gaztañaga et al 2005), weak lensing (e.g., Fischer et al 2000; Gaztañaga 2003; Hirata et al, 2004), strong lensing (e.g., Inada et al 2003).... In this project we want to focus in biasing, ie how light from galaxies trace the underlaying dark matter (DM) distribution. The goal is to study how to best measure (or model) biasing evolution for current and upcoming galaxy surveys, such as SDSS, DES and PAU. We need to characterize biasing as a function of cosmic time and galaxy properties such as spectral type, color, morphology, magnitude or spatial environment. The biasing parameters can be linked to models of galaxy formation and can also be used to provide new clues on the nature of DM and DE (i.e. through new standard rulers or the evolution of the growth factor). The study can be done by measuring redshift space distortions (due to peculiar velocity of galaxies), the higher order clustering of galaxies and also by the comparison of the galaxy distribution to weak lensing mass reconstruction. We plan to use the N-body simulation run in our group (ie see MICE: www.ice.cat/mice) which include both the dark matter distribution and mock galaxy distribution form a variety of prescriptions and models of galaxy formation.
Galaxy surveys are an important tool for cosmology. The distribution of galaxies allow us to study the formation of structures and their evolution, which are needed ingredients to study the evolution and content of the Universe. However, according to the standard model of cosmology, the so-called ΛCDM model,…
Estudiante: Pujol, Arnau

Galaxy surveys are an important tool for cosmology. The distribution of galaxies allow us to study the formation of structures and their evolution, which are needed ingredients to study the evolution and content of the Universe. However, according to the standard model of cosmology, the so-called ΛCDM model, most of the matter is made of dark matter, which gravitates but does not interact with light. Hence, the galaxies that we observe from our telescopes only represent a small fraction of the total mass of the Universe. Because of this, we need to understand the connection between galaxies and dark matter in order to infer the total mass distribution of the Universe from galaxy surveys.  At large scales, galaxies trace the matter distribution. In particular, the galaxy density fluctuations at large scales are proportional to the underlying matter fluctuations by a factor that is called galaxy bias. This factor allows us to infer the total matter distribution from the distribution of galaxies, and hence knowledge of galaxy bias has a very important impact on our cosmological studies. This PhD thesis is focused on the study of galaxy and halo bias at large scales.  There are several techniques to study galaxy bias, in this thesis we focus on two of them. The first technique uses the fact that galaxy bias can be modelled from a galaxy formation model. One of the most common models is the Halo Occupation Distribution (HOD) model, that assumes that galaxies populate dark matter haloes depending only on the halo mass. With this hypothesis, and assuming a halo bias model, we can relate galaxy clustering with matter clustering and halo occupation. However, this hypothesis is not always accurate enough. We use the Millennium Simulation to study galaxy and halo bias, the halo mass dependence of halo bias, and its effects on galaxy bias prediction. We also study the local density dependence of halo bias, and we show that density constrains much more bias than mass. Another technique to study galaxy bias is by using weak gravitational lensing to directly measure mass in observations. Weak gravitational lensing is the field that studies the weak image distortions of galaxies due to the light deflections produced by the presence of a foreground mass distribution. Theses distortions can then be used to infer the total mass (baryonic and dark) distribution at large scales. We develop and study a new method to measure galaxy bias from the combination of weak lensing and galaxy density fields. The method consists on reconstructing the weak lensing maps from the distribution of the foreground galaxies. Bias is then measured from the correlations between the reconstructed and real weak lensing fields. We test the different systematics of the method and the regimes where this method is consistent with other methods to measure linear bias.

Cosmological models of the early- and late- universe with bradion and tachion fields

Estudiante: N. Myrzakulov

Foreign adviser of this thesis defended at Eurasian National university, Astana
Estudiante: N. Myrzakulov

Foreign adviser of this thesis defended at Eurasian National university, Astana

Producing simulated catalogues for next generation galaxy surveys

Estudiante: Izard, A.
Supervisada por: Pablo Fosalba Vela; Martin Crocce

El objetivo de la tesis es profundizar en el estudio del modelo de formación de estructuras a gran escala en el universo usando las observaciones de los nuevos cartografiados de galaxias. Con este objetivo, durante el proyecto de tesis se trabajará en el desarrollo de herramientas analíticas y numéricas que permitan, en un primer lugar, modelar adecuadamente los observables con el nivel de complejidad y detalle que se ajuste a las propiedades de los datos futuros, y en un segundo término, la explotación óptima de dichos datos para obtener cotas de alta precisión sobre los parámetros cosmológicos básicos. En concreto se espera centrar la actividad en dos de las áreas mas activas en la actualidad en la cosmología observacional y que potencialmente pueden aportar mas información sobre el proceso de formación de estructuras: la abundancia de cúmulos de galaxias, la distribución (o clustering) de galaxias y el efecto de lente débil (weak lensing) debido a las grandes estructuras del universo. El programa de tesis se desarrollará en el contexto de la participación activa del grupo receptor, el grupo de Astrofísica extragaláctica y Cosmología, en grandes cartografiados de galaxias como son el Dark Energy Survey (DES), Physics of the Accelerating Universe (PAU) y la misión espacial EUCLID.
Current and future galaxy surveys will be able to map the large-scale structure of the Universe with unprecedented detail and measure cosmological parameters with exquisite precision. In order to develop the science cases and the analysis pipelines, it is necessary an accurate modelling of…
Estudiante: Izard, A.
Supervisada por: Pablo Fosalba Vela; Martin Crocce

Current and future galaxy surveys will be able to map the large-scale structure of the Universe with unprecedented detail and measure cosmological parameters with exquisite precision. In order to develop the science cases and the analysis pipelines, it is necessary an accurate modelling of the non-linear gravitational evolution. This thesis presents a methodology for producing accurate mock catalogues, much faster than conventional methods (2-3 orders of magnitude) and incorporating past light cone effects. First, we present the optimization of a quasi N-body method in the compromise between accuracy and computational cost. We studied how variations in the code parameter space have and impact on the accuracy of observables such as the halo abundance and distribution and matter clustering. We propose optimal parameter configurations for achieving high accuracy as compared to exact N-body simulations and we explore different calibration techniques to match even better two-point halo clustering statistics. The next step is mimicking the geometry of real astrophysical observations, in which distant objects are seen in the past light cone. We introduce ICE-COLA, a simulation code developed for this thesis that implements the production of light cone catalogues on-the-fly. The user can generate three different kinds of data types. The first contains all the information of the phase-space matter distribution while the others store high-level data catalogues ready to use to model galaxy surveys. This enables large compression factors of ∼ 2 orders of magnitude in the data volume to be stored. In particular, the code can generate halo catalogues in the light cone and pixelated two-dimensional projected matter density maps in spherical concentric shells around the observer. Using ICE-COLA we produce large light cone simulations and perform an extensive validation of the catalogues. We introduce a novel methodology to model weak gravitational lensing with an approximate method and we show that we can resolve most of the scales probed by current weak lensing experiments. Finally we extend the results to halo mock catalogues with weak lensing quantities, which represents a key step forward modelling galaxy clustering and weak lensing observables consistently in a quasi N-body approach.

Observation and interpretation of type IIb supernova explosions

Estudiante: Antonia Morales Garoffolo
Supervisada por: Nancy Elias de la Rosa; Jordi Isern Vilaboy

p { margin-bottom: 0.08in; } Las supernovas (SNs) juegan un importante papel en muchos campos de la física moderna, desde la cosmología hasta la física nuclear. En particular, al devolver al medio interestellar elementos pesados ​​sintetizados durante toda la vida de la estrella y la explosión (nucleosíntesis explosiva) hace de las SNs los principales contribuyentes a la evolución química de las galaxias. La cantidad y composición de este material depende de la física de la explosion y de la estructura de la estrella antes de explotar. Hay dos clases principales de SNs: Termonuclear (SNIa -- recientes estudios han mostrado una importante diversidad entre estos objetos, una vez se consideraban altamente homogéneos), procedentes de la explosión de una enana blanca (WR) por la acreción de masa de una estrella compañera, y de Colapso Gravitatorio (CC-SNs – heterogéneas debido a las diferentes configuraciones de la estrella progenitora en el momento de la explosión; hay varios tipos como II-P, II-L, IIn, IIb, Ib, Ic) que proceden del colapso del núcleo de una estrella masiva con masa(ZAMS) > 8 masas solares.
Recientemente, el Institut de Ciencies de L'Espai (ICE/IEEC-CSIC) ha ayudado en la construcción y gestión del Observatorio del Montsec (OAdM), ubicado al sur de los Pirineos. Este observatorio alberga un telescopio de 80 cm que será controlado de forma remota desde el instituto. Así pues, la estrategia básica de este proyecto es la creación de un programa de seguimiento de la evolución de SNs cercanas desde el observatorio del Montsec con la finalidad de gestionar un estudio coordinado del estudio físico de los diferentes tipos de supernovas y de la determinación de la contribución de éstas al enriquecimiento químico. A su vez, se desarrollarán programas y modelos adecuados para interpretar adecuadamente los datos.
De manera complementaria, se trabajará activamente en dos grandes colaboraciones internacionales para el estudio de supernovas usando los telescopios del Observatorio Europeo Austral (ESO): ESO-New Technology Telescope (NTT) and Telescopio Nazionale Galileo (TNG) long term program, y PESSTO - Public ESO Spectroscopic Survey for Transient Objects. Ambas colaboraciones se fundirán el próximo año en una sola y están formadas por numerosos institutos, principalmente europeos. Estos proyectos se centran en la obtención de datos de calidad en una amplia gama de longitudes de onda que luego serán comparados con los modelos teóricos, desarrollados por institutos dentro de la colaboración.
Core-collapse supernovae (CC-SNe) explosions represent the final demise of massive stars. Among the various types, there is a group of relatively infrequent CC-SNe termed type IIb, which appear to be hybrids between normal type II SNe (those characterised by H emission) and type Ib (those that lack H…
Estudiante: Antonia Morales Garoffolo
Supervisada por: Nancy Elias de la Rosa; Jordi Isern Vilaboy

Core-collapse supernovae (CC-SNe) explosions represent the final demise of massive stars. Among the various types, there is a group of relatively infrequent CC-SNe termed type IIb, which appear to be hybrids between normal type II SNe (those characterised by H emission) and type Ib (those that lack H features in their spectra but exhibit prominent He\,{\sc i} lines). The nature of the stellar progenitors leading to type IIb SNe is currently unknown, although two channels are contemplated: single massive stars that have lost part of their outer envelope as a consequence of stellar winds, and massive stars that shed mass by Roche-Lobe overflow to a companion. The latter is in fact the favoured scenario for most of the objects observed up to now. In the majority of cases, when there are no direct progenitor detections, some hints about type IIb SN progenitors (e.g., initial mass) can be derived indirectly from the objects' light curves (LCs) and spectra. Motivated by the relatively
few well-sampled
observational datasets that
exist up to date for type IIb SNe and the unknowns on their progenitors, we carried out extensive observations (mainly in the optical domain) for the young type IIb SNe 2011fu and 2013df. Both these SNe are particularly interesting because they show a first LC peak caused by shock breakout, followed by a secondary $^{56}$Ni-decay-powered maximum. The analysis of the data for SNe 2011fu and 2013df points to precursors that seem to have been stars with large radii (of the order of 100~R$_{\odot}$), with low mass hydrogen envelopes (tenths of
M$_{\odot})$, and relatively low initial masses ($12\textendash18$~M$_{\odot}$), which could have formed part of interacting binary systems.  The nature of a third SN IIb candidate, OGLE-2013-SN-100, proved to be enigmatic.
OGLE-2013-SN-100, shows a first peak in the LC, and other  characteristics somewhat similar to those of type IIb SNe. However, after a deeper analysis, we conclude OGLE-2013-SN-100 is likely not a SN of type IIb. We provide an alternative possible explanation for this object, which implies a combination of a SN explosion and interaction of its ejecta with circumstellar-material.  SNe~2011fu and 2013df were included in a larger sample of type IIb SNe to carry out a comparative study of their observables and environment. Regarding the host galaxies,  90~\% of the objects are located in giant ($r<-18$~mag) hosts. In addition, the SNe are about equally split in low star formation and high star formation rate spiral galaxies. Concerning the SN ultra-violet (UV), optical, and near-infrared (NIR) LCs, we find a dispersion in both shape and brightness. Particularly, a few objects show a sharp declining early phase in the UV and double-peaked optical-NIR LCs.
However, the absence of a first LC peak, in some of the cases, may be due to lack of early observations.
In addition, we found dispersion in the evolution of the colour indices of the SNe, making the colour comparison method not suitable to estimate extinction toward a type IIb SN. In the optical domain, the study of the (secondary) peak brightness in the \textit{R} band shows that low luminosity events could be uncommon and the average
brightness of the sample is $\sim -17.5$~mag. As for the spectral properties, the SNe that show an early spike in their LCs exhibit blue, shallow-lined early-time spectra and arise from extended progenitors ($R\sim 100$~R$_{\odot}$). Additionally, while there is an overall resemblance of the measured ejecta velocities, there is also dispersion of equivalent widths, nebular line luminosities and ratios among all the objects that could indicate differences in the ionisation state of the ejecta and mixing. All in all, we find heterogeineity in the studied observables of the sample of type IIb SNe, which reflects the variety of their explosion parameters and progenitor properties.

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