Found 109 talks archived in Stars
The initial mass function describes the distribution of masses for a population of stars and substellar objects when they are born. It defines the evolution of a population of stars and provides constrains on the star formation theory. The determination of the initial mass function in the substellar regime is still an open question in Astrophysics. Brown dwarfs do not have enough mass to sustain hydrogen fusion. As a consequence, mass and age are degenerate for these objects. An older high mass object may be indistinguishable from a younger low mass object. In my PhD thesis, through the characterization of brown dwarfs using several observational methods, I work towards solving the general problem of constraining the substellar initial mass function.
In my first project, I calculated trigonometric parallaxes of a sample of six cool brown dwarfs. I determined the luminosity for our objects and I found that one of them might be a brown dwarf binary. In my second project, I confirmed the youth of seven brown dwarfs (ages between 1 and 150 Myr) using spectroscopic data.In the last project of this PhD thesis, I aimed to refine the brown dwarf binary fraction using spectroscopic data in the optical and in the near infrared for 22 brown dwarfs. I found six new brown dwarf binary candidates, two of them were previously known.
The determination of distances, ages and the refinement of the brown dwarf binary fraction in this PhD thesis contribute to the determination of the initial mass function. In the next years, the Gaia satellite, the James Webb Space Telescope and the E-ELT will provide new data, allowing the discovery of new brown dwarf binaries, the constraining of atmospheric and evolutionary models, and the refinement of the initial mass function.
It is often assumed that when stars reach their Eddington limit, strong outflows are initiated, and that this happens only for extreme stellar
masses. I will show that in realistic models of stars up to 500 Msun, the Eddington limit is not reached at the stellar surface. Instead, I will argue that the Eddington limit is exceeded inside the stellar envelope, in hydrogen-rich stars above about 1 ... 30 Msun, and in Wolf-Rayet stars above 7 Msun, with drastic effects for their structure and stability. I will discuss the observational evidence for this, and outline evolutionary consequences.
All the elements from carbon to uranium present in the Solar System were produced by hundreds to thousands of stars belonging to different stellar generations that evolved and died during the presolar evolution of the Galaxy. Using the abundances of radioactive nuclei inferred from meteoritic analysis we can date the last of these stellar additions. We have found that the last contribution of elements such as carbon and slow neutron-capture elements to the Solar System from an asymptotic giant branch star occurred 15-30 Myr before the formation of the Sun. This provides us with an upper limit of the time when the precursor material of the Solar System became isolated from the bulk of the galactic material. Interestingly, it compares well to the lifetime of high-mass molecular clouds suggesting that the Sun was born in a very large family of stars.
In this talk we will present our most recent numerical and observational results on the formation, evolution, and X-ray emission from hot bubbles in nebulae around evolved stars. Our studies include hot bubbles around massive and low-mass stars, e.g., Wolf-Rayet nebulae and planetary nebulae. Our results show that the diffuse X-ray emission from these hot bubbles is a dynamic process that involves mixing of nebular material into the hot bubble due to hydrodynamical instabilities, photoevaporation, thermal conduction, and dust cooling. The formation of these hot bubbles is governed by the evolution of the stellar wind parameters, and its properties can be used to study stellar evolution.
The origins of neutron(n)-capture elements (atomic number Z > 30) have historically been discerned from the interpretation of stellar spectra. However, in the last decade nebular spectroscopy has been demonstrated to be a potentially powerful new tool to study the nucleosynthesis of n-capture elements. In this talk, I will discuss exciting new advances made in this field with near-infrared and optical observations of planetary nebulae, and atomic data investigations that enable the analysis of spectroscopic data.
The stellar initial mass function (IMF) is usually assumed to be a probability density distribution function. Recent data appear to question this interpretation though, and I will discuss alternative applications and results concerning the possibly true nature of the IMF. Empirical evidence has emerged that the IMF becomes top-heavy in intense star bursts and at low metallicity. Related to the IMF are binary star distribution functions, and these evolve through dynamical processes in embedded star clusters. The insights gained from these considerations lead to a mathematically computable method for calculating stellar populations in galaxies, with possibly important implications for the matter cycle in galaxies. It turns out that the galaxy-wide IMF, the IGIMF, becomes increasingly top-heavy with increasing galaxy-wide star formation rate, while at the same time the binary fraction in the galactic field decreases.
A comprehensive understanding of sub-stellar objects (brown dwarfs and extrasolar giant planets) and their population characteristics (e.g. IMF, formation history) is only possible through the robust interpretation of ultra-cool objects spectroscopy. However, the physics of ultra-cool atmospheres is complicated by a variety of challenging ingredients (dust properties, non-equilibrium chemistry, molecular opacities). Moreover, while hydrogen-burning stars stabilize on the stellar main-sequence, sub-stellar objects continuously cool down (since they lack an internal source of energy) and evolve towards later spectral types. Their atmospheric parameters are a strong function of age. In this talk I will present the spectroscopic analysis of a large sample of L and T dwarfs, complementing the spectroscopic data with astrometry from the PARSEC program, in order to constrain the sub-stellar initial mass function and formation history. I will then describe our new effort to identify and characterize a large sample of benchmark systems, combining Gaia capabilities with large area near-infrared surveys such as UKIDSS, SDSS, and VVV, in order to calibrate effectively the theoretical models.
I will talk about our current understanding of globular cluster (GC) formation and what we have yet to learn about them. I will particularly focus on the chemical and dynamical properties of the neglected GC NGC4372, which I studied for the first time with high-resolution spectroscopic observations.
Its chemical abundances revealed it as a typical representative of the old, metal-poor halo group. More interesting, however, are its structural and kinematic properties as the cluster has an unusually high intrinsic rotation for its metallicity and appears to be rotationally flattened. I will discuss what
rotating GCs tell us about their early evolution.
MASTER-Kislovodsk auto-detection system discovered a faint transient in the Andromeda galaxy on January 13th 2015. It was originally identified as a classical nova and received designation M31N 2015-01a. Further observations showed discrepancies with the spectra and lightcurves typical for the classical novae. The transient was re-identified as a likely stellar merger (aka Luminous Red Nova (LRN)), similar to V838Mon. In this presentation I will deliver a short overview of our current understanding of this class of objects and a summary of the current state of the ongoing observing campaign of the M31 LRN. Recent results will be discussed with a particular emphasis on the contributions made possible by GTC and other observing facilities at Observatorio Roque de los Muchachos. At the final part of the presentation I will touch on follow up observations once M31 is available for observations again.
With the aim of testing the relation between supernova (SN) rate and star formation rate, we conducted a SN search in a sample of local starburst galaxies (SBs) where both star formation rates and extinction are extremely high. The search was performed in the near-infrared, where the bias due to extinction is reduced using HAWK-I on the VLT. We discovered six SNe, in excellent agreement with expectations, when considering that, even in our search, about 60% of events remain hidden in the nuclear regions due to a combination of reduced search efficiency and very high extinction.
In addition I will present my plans for next months at IAC for the "Starbursts and EMIR project". I will participate in the commissioning of the instrument at La Palma, collaborating in the development of the ETC and I will compile a catalog of starbursts for EMIR with the aim to study their imprint in the cosmic evolution of galaxies.
- An ionised bubble before the epoch of re-ionisationDr. José Miguel Rodriguez EspinosaTuesday April 14, 2020 - 12:30 (Aula)
- TBD (the Amanar project: under the same sky)Dr. Sandra Benítez HerreraTuesday April 28, 2020 - 12:30 (Aula)