Research :: PhD Topics for 2014 :: Stars

Topics under the thematic line Towards a comprehensive study of starsavailable for the 2014 Call (9 PhD Topics):

  • 2014/201 – Determination of stellar parameters for M-dwarf stars: the NIR approach
  • 2014/202 – On using stellar pulsations to constrain atmospheric models of strongly magnetic stars
  • 2014/203 – Galactic stellar populations and planet formation with the Gaia-ESO Survey data
  • 2014/204 – Impact of convection in pulsating intermediate-mass stars: exploring different formulations for the convective flux
  • 2014/205 – A New Generation of Stellar Models with Adjustable Surface Convection
  • 2014/206 – Spectral analysis of δ-Scuti stars
  • 2014/207 – The activity and evolution of low-mass Young Stellar Objects
  • 2014/208 – Probing the interior of red giant stars: a new tool for the asteroseismic characterization of large samples of stellar pulsators
  • 2014/209 – Accretion versus outflow regions around Young Stellar Objects

For details, please see below the abstract and advisors for each topic.

2014/201 – Determination of stellar parameters for M-dwarf stars: the NIR approach

Advisors: Sérgio Sousa (CAUP) and Nuno C. Santos (CAUP)

This project is focused on the spectroscopic analysis of near-infra-red (NIR) spectra of M-dwarf stars with the goal of deriving precise and homogeneous stellar parameters. The knowledge of correct stellar parameters of planet-hosts, in particular their radii, is essential for the derivation of the properties of the discovered planets. The goals of the present project are thus of great importance for the full success of future space missions like TESS, CHEOPS, and PLATO.

With this in mind, the major goal of the proposed project is to adapt current methodologies, well established for the analysis of high resolution optical spectra of FGK dwarfs, to be used with near-IR high resolution spectra that will soon be available from new instruments such as SpiRou, CARMENES, and CRIRES+. In this context the student is expected to implement an automatic code to estimate stellar parameters directly from the measurement of line equivalent widths, and to apply it to M stars hosting planets. Besides the determination of precise stellar (and thus planetary) properties, the student will then further explore possible correlations between the properties of the stars and the presence of the planets, which can give important clues for planet formation models.

Notes: This is a closed topic.

2014/202 – On using stellar pulsations to constrain atmospheric models of strongly magnetic stars

Advisors: Margarida Cunha (CAUP)

The PhD project proposed here aims at developing a theoretical tool capable of reproducing the pulsational velocity field in the outer layers of chemically peculiar, strongly magnetic stars, as well as the associated radial velocity, as determined by the observer. That tool will be used to infer the 3D pulsational velocity field in the atmospheres of particular stars, based on observational data. The comparison between theoretical models and time-resolved, high-resolution spectroscopic data, obtained with instruments such as UVES and HARPS (both at ESO), will provide information on the geometrical depth at which particular chemical elements accumulate in these stars. That information will be used to test state–of-the-art atmospheric models. Finally, the student will be involved in the preparation for the optimal exploitation of the data to be acquired by the recently approved ESA mission PLATO2.0, in what concerns one of its additional science goals, namely, the study of classical pulsators.

The outcome of this project is expected to constitute an important step forward towards testing and improving model atmospheres, the modelling of microscopic diffusion, and the correct exploitation of asteroseismic data on chemically peculiar, magnetic stars.

Notes: This is a closed topic.

2014/203 – Galactic stellar populations and planet formation with the Gaia-ESO Survey data

AdvisersVardan Adibekyan (CAUP) and Sérgio Sousa (CAUP)

The on-going Gaia-ESO Survey is a very ambitious project which uses ESO/VLT to obtain detailed spectra of about 100000 stars from all the Galactic stellar populations. We propose to use the data from this survey (for which we have privileged access) to study the chemical and kinematic properties of stellar populations. The goal is to understand the formation and evolution of the distinct populations that we can identify with the data. In addition, we propose to investigate the observed planet hosting stars to understand possible links between the different stellar populations and planet formation processes. This latter study should help us to better estimate the frequency of the different type of planets for each stellar population and in our Galaxy in general.

Notes: This is also offered as topic 105, under the thematic line on Planets.

2014/204 – Impact of convection in pulsating intermediate-mass stars: exploring different formulations for the convective flux

Advisors: Antonio García-Hernández (CAUP)  and Mário João P. F. G. Monteiro(CAUP)

Convection remains one of the most important open issues in stellar physics. The standard approach has been used to calculate global stellar evolution with relative success, but precise seismic data for a large sample of stars has shown the inadequacies of the common approach to include convective energy transport in stellar models. This project aims at extending the formulation used to estimate the convective flux in 1D models, including some of the alternatives available in the literature as particular cases. The goal is to explore the parameter space in such formulae and its impact on the seismic properties of models for intermediate-mass pulsation stars that have been observed by CoRoT and Kepler. In order to do so priority will be given to the study of δScuti stars with oscillations data, through the implementation of new formulations in the MESA code for stellar evolution (Paxton et al. 2011). The goal is characterise the range of alternative fluxes that better represent the seismic data and are compatible with the mode identification from spectroscopy.

2014/205 – A New Generation of Stellar Models with Adjustable Surface Convection

Advisors: Mário João P. F. G. Monteiro (CAUP) and Michael Bazot (CAUP)

Helioseismology and asteroseismology are powerful observational methods that allow probing the interiors of stars. Measuring oscillation frequencies allows one to constrain theoretical stellar models, i.e. estimate the mass of a star, its age, initial chemical com- position or other related physical characteristics. The use of observational constraints sensitive to its interior is of the uttermost importance since i) some fundamental characteristics (e.g. the mass or the age) are mostly determined by the central physical state of the star, ii) the physics of the surface layers are complex and our theoretical models are much cruder for these regions (and hence unlikely to reproduce observations de- pending mostly on them). This second issue is at the heart of a long-standing problem in helio- and asteroseismology. Even though the stellar oscillations are sensitive to the stellar interior, they are still affected by the surface layers. It has long been recognized that there exist systematic differences between computed frequencies, based on our best solar models, and observed solar frequencies.

It is this issue we want to investigate in this program. Since it is believed that these differences arise from an improper treatment of the upper layers of stars, we want to test alternative physical models that might reinstate an agreement between theory and observations. This would in turn be extremely useful in order to calibrate properly theoretical models using solar data and then apply them to other stars. It has often been conjectured that the current modeling of convection could be the most important factor leading to the existing disagreement. Therefore, we will explore some parametric models for convection that will allow fine-tuning of the efficiency of the process, as well as its characteristic length scales. To that effect, we will use Bayesian computational methodologies in order to explore the multi-dimensional spaces of parameter underlying these convection models.

The outcome of the project, through the development of the theoretical insights into convection models relevant for stellar physics, is expected to have a major impact on how high precision space data (from CoRoT, Kepler and PLATO in the future) is exploited to provide effective constraints of the physics of stars and the characterization of planet hosts.

2014/206 – Spectral analysis of δ-Scuti stars

Advisors: Michael Bazot (CAUP) and António García-Hernández (CAUP)

Asteroseismology is a powerful technique that allows to obtain data sensitive to the stellar interior. It is extremely useful since many important physical characteristics of stars (e.g. their mass, their age) depend strongly on their innermost regions. The seismic quantities used to constrain theoretical stellar models are the eigenfrequencies of the various pulsation modes of the stars. These modes are extracted from time series representing the variations of either the radial velocity or luminosity of stars. The number of pulsation modes, their typical frequency range, their amplitude and even their form (harmonic signals, damped harmonic signals) vary from one star to the other. With the advent of the space missions CoRoT and Kepler, very long time series have offered the possibility to measure oscillation frequencies with great precision.

A common problem in asteroseismology is the study of the so-called classical pulsators and in particular delta-Scutis. These stars are known to have very stable harmonic pulsation modes, which have been observed for a very long time using ground-based facilities. However, the study of space-mission data led to the extraction of a large number of modes, sometimes several hundreds, many of low amplitude, whose physical interpretation is difficult. The goal of this project is to reanalyze some of these time series using various optimization techniques for spectral analysis. In particular, we will rely on the progresses offered by the use of sparse signal modelling (i.e. considering that the signal is zero almost everywhere except at some discrete frequencies) in order to explore the possibility that different solutions to the problem of the extraction of the frequency from time series of classical pulsators exist.

The outcome of the project is expected to have a major impact on one of the most puzzling problem uncovered by the asteroseismology space missions (from CoRoT, Kepler and PLATO in the future) and will help to better understand the physics of classical pulsators.

2014/207 – The activity and evolution of low-mass Young Stellar Objects

Advisors: Jorge Filipe Gameiro (CAUP) and Silvia Alencar (UFMG)

Understanding the formation of stars and the evolution of the Young Stellar Objects (YSO) is an important goal in Astrophysics. The formation of planetary system and the origin of our solar system is directly coupled to star formation and evolution of circumstellar disks in YSO. The mass- loss processes (outflows/winds) and their connection to the mass-accretion are fundamental during this protostellar phase and still not well understood. The project includes observations that will come from the large Gaia-ESO spectroscopic survey and focus on the study of YSO: (a) derive stellar parameters and characterize the activity in several star formation regions, (b) determine the mass and age through stellar evolution models and study the pre-main-sequence evolutionary stage and disk evolution from a statistical point of view, (c) Analysis of high- resolution spectra of active YSO to study inner disk region and compare with current accretion/winds models and get better estimates of accretion and mass-loss rate. The proposed studies are mainly observational and involve also the exploitation of multiwavelength observations in ground-based large telescopes.

Notes: This is topic requiring a mixed fellowship, with up to one year to be spent in Minas Gerais. This is a closed topic.

2014/208 – Probing the interior of red giant stars: a new tool for the asteroseismic characterization of large samples of stellar pulsators

Advisors: Margarida Cunha (CAUP), Pedro Avelino (CAUP) and Dennis Stello (Univ. Sydney)

As a result of the launch of the CoRoT and Kepler satellites, the astronomical community has, today, exquisite asteroseismic data on thousands of red giant stars. The analysis of just a fraction of these data has already led to a number of very exciting new results, published in high-impact journals, such as Nature and Science. Moreover, given the recent approval by ESA of the mission PLATO2.0 (launch around 2023), it is expected than in a decade the number of red giants with detected oscillations will increase by orders of magnitude. To exploit this large observational sample of stars, further development of asteroseismic theoretical tools is required. In particular, it is imperative that we have tools capable of modelling the pulsations in large grids of red giant models, covering a wide parameter space (in mass, age, metallicity), in a reasonable amount of time. In this context, the main goal of this project is to develop a new linear adiabatic pulsation code, significantly more efficient than those currently available, that may be applied to large grids of red giant models, necessary to fully exploit the available data. Along with the characterization of the red giant populations based on the data already available, the proposed tool will constitute a magnificent step forward towards the preparation for the exploitation of the data that will be acquired by the PLATO2.0 mission.

Notes: This is a topic requiring a mixed fellowship, with up to one year to be spent in Sydney.

2014/209 – Accretion versus outflow regions around Young Stellar Objects

Advisors: João Lima (CAUP), Christophe Sauty (Observatoire de Paris), Véronique Cayatte (Observatoire de Paris)

The evolution of young stellar objects (YSO’s) is critically dependent on the balance between the accretion of matter coming from their surrounding disks and the collimated jets outflowing around their poles. The magnetic field plays a crucial role in both channelling these flows of plasma and controlling their dynamics and energetics.

Modelling the overall stellar formation on all scales from the global collapse and jet feedback down to the inner disk star interaction is presently out of reach. Thus, we concentrate on the central magnetospheric accretion connected to the stellar outflow in class II objects like T Tauri stars.

T Tauri stars are well observed objects. In such evolved protostars, observational evidences suggest that jets originate in the inner regions of the accretion disk or even closer in the magnetosphere of the central source. Thus it is crucial to understand the connection between these two regions, one controlling accretion and the other responsible for the jet. The results might have an impact in understanding the new observations of jets close to the source coming from instruments like ALMA.

In this project we aim firstly, at modelling the structure of the magnetosphere of the YSO in which accretion takes place with very simple MHD analytical models. Secondly, we aim at blending such models with existing self-similar MHD models for jets around these objects. Finally we propose to extend the the analytical approach presented here to a numerical one, using existing numerical codes like PLUTO. The advantage of such numerical approach is to enable the study of time dependent solutions.

Notes: This is a closed topic. It requires a mixed fellowship, with up to one year to be spent in Paris.