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A Practical Overview of Stellar Spectroscopy and Abundance Analysis

presented by

Laia Casamiquela

I will present a brief and practical overview of stellar spectroscopy  and abundance analysis using high resolution spectroscopic data. First,  I will briefly explain some basic concepts and ingredients to understand  how to derive atmospheric parameters and chemical abundances from a  stellar spectrum. Then I will illustrate the process doing a real time  analysis of real spectra with the code iSpec (Blanco-Cuaresma et al. 2014).

Prebiotic molecules show chemical similarities with biologically relevant molecules, such as amino acids, nucleobases, sugars and peptide chains, and are thought to be involved in their formation.  The interstellar presence of prebiotic molecules has led to the idea that biotic molecules on Earth may have derived from these interstellar molecules.

In this seminar, observations of prebiotics towards the young sun-like protobinary IRAS 16293-2422 are presented, showing that prebiotic molecules likely were present at the earliest formational stages of our Solar system. Solid-state formation pathways of these molecules are investigated in the laboratory and show that much of the prebiotic inventory can derive from reactions on icy dust grains.

The James Webb Space Telescope (JWST )will open a
new area in the domain of exoplanet atmosphere
characterizations  and  will  require  accurate  
models  to  interpret the observations.  In this context,
we propose a protocol to compare various atmospheric
codes,  to identify and discuss the significant
differences in the results and to help the codes evolve
to become as consistent as possible.  We applied this
protocol on 3 forward models and one retrieval.  We updated
them to account for the major differences and we are now
able to identify the remaining differences observable with
the JWST.

More than one-third of the 4000+ planet candidates found by NASA’s Kepler spacecraft are associated with target stars that have more than one planet candidate, and such “multis” account for the vast majority of candidates that have been verified as true planets.The large number of multis tells us that flat multiplanet systems like our Solar System are common. Virtually all of the candidate planetary systems are stable, as tested by numerical integrations that assume a physically motivated mass-radius relationship. Statistical studies performed on these candidate systems reveal a great deal about the architecture of planetary systems, including the typical spacing of orbits and flatness. The characteristics of several of the most interesting confirmed Kepler & K2 multi-planet systems will also be discussed.

The leading evolutionary model for the outer solar system, an orbital instability between the solar system’s giant planets, has been shown to greatly disturb the orbits of the young terrestrial planets. Undesirable outcomes such as over-excited orbits, ejections and collisions can be avoided if the instability occurs before the inner planets are fully formed. Such a scenario also has the advantage of limiting the mass and formation time of Mars when it occurs within several million years (Myr) of gas disk dissipation. The dynamical effects of the instability cause many small embryos and planetesimals to scatter away from the forming Mars, and lead to heavy mass depletion in the Asteroid Belt. We present new simulations of this scenario that demonstrate its ability to accurately reproduce the eccentricity, inclination and resonant structures of the Asteroid Belt. Furthermore, we perform simulations using an integration scheme which accounts for the fragmentation of colliding bodies. The final terrestrial systems formed in these simulations provide a better match to the actual planets’ compact mass distribution and dynamically cold orbits. An early instability scenario is thus very successful at simultaneously replicating the dynamical state of both the inner and outer solar system.

The classical picture of protoplanetary discs forming smooth, continuous structures of gas and dust has been challenged by the growing number of spatially resolved observations. These observations indicate that radial discontinuities and large-scale asymmetries may be common features of the emission of protoplanetary discs, and they are often interpreted as signatures of the presence of (hidden) planets. They stress the need to better understand how disc-planets interactions generally, and planetary migration more specifically, impact the dust’s thermal emission in protoplanetary discs. In this talk, I will report our recent and ongoing efforts in predicting the dust’s radio emission in protoplanetary discs due to the presence and migration of massive gap-opening planets, via two-fluid (gas+dust) hydrodynamical simulations post-processed with radiative transfer calculations. I will show how these predictions apply to the discs around AB Aurigae and MWC 758.

In its 11th year on orbit, the Fermi LAT continues to discover GeV gamma-rays from
about 24 pulsars per year. The most sensitive way to find them is to use rotation ephemerides
to « phase fold » the gamma photons, to then look whether the resulting
phase histogram is flat or not. We have done this for over a thousand radio pulsars,
using ephemerides provided by the Nancay, Jodrell Bank, and Parkes radio telescopes.
I will present sixteen gamma-ray pulsars we found, and explain how they enhance the sample
of over 220 pulsars we already had. I will discuss possible causes of the gamma-ray
« deathline » near a spindown power of Edot ~ few 1E33 erg/s.
Finally, I will say some words about extrapolating the observed gamma-ray pulsar
population to estimate the contribution of unresolved pulsars to the diffuse background.

Dealing with large amount of data  is  a new problematic task in astrophysics. One may distinguish the management of these data (astroinformatics) and their scientific use (astrostatistics) even if the border is rather fuzzy. Dimensionality reduction  in both the number of observations and the number of variables (observables) is necessary for an easier physical understanding. This is the purpose of classification which has been traditionally eye-based and essentially still is but this becomes not possible anymore. In this talk, I present a general overview of machine learning approaches for unsupervised classification, with applications to stars (chemical abundances) and galaxies (spectra).