Research :: PhD Topics for 2019 :: Stars

Topics under the thematic line Towards a comprehensive study of starsavailable for the 2019 Call (5 PhD Topics):

  • 2019/224 - Galactic archaeology of solar-type stars with the NASA TESS mission
  • 2019/225 - New insights into stellar physics from the NASA-TESS survey on A-type stars
  • 2019/226 - Probing inside stellar cores with ultra-precise, space-borne data
  • 2019/227 - Role of the magnetic field in the formation and evolution of star-forming hub-filament
  • 2019/228 - The physics of high-mass star formation: Zoom-in on MaCProS

For details, please see below the abstract and advisors for each topic. Prospective candidates are welcome to contact directly the proposers of the topics for inquiries and further details.

2019/224 - Galactic archaeology of solar-type stars with the NASA TESS mission

Advisors: Tiago Campante (IA U.Porto), Vardan Adibekyan (IA U.Porto), Aldo Serenelli (ICE-CSIC & IEEC, Bellaterra)

Abstract: The Transiting Exoplanet Survey Satellite (TESS) is a NASA space mission, launched in April 2018, that will perform an all-sky survey for planets transiting bright nearby stars. Furthermore, TESS’s excellent photometric precision will enable asteroseismology, the study of stars by the observation of their natural, resonant oscillations. Asteroseismology is proving to be particularly relevant for the study of solar-type stars (i.e., low-mass, main-sequence stars and cool subgiants), in great part due to the exquisite photometric data previously made available by NASA’s Keplerspace telescope and, more recently, by the repurposed K2 mission. In extending the legacy of Kepler/K2, one will perform an ensemble asteroseismic study of bright solar-type stars that reside in the solar neighborhood, making use of data collected by TESS. The proposed research will provide a well-characterized sample of benchmark solar-type stars to be used in studies of the chemical evolution of the solar neighborhood, which in turn will impact on Galactic archaeology studies. Specifically, one will aim at constraining the relation between age and elemental abundances for nearby field stars. Calibration of this relation with the accuracy and precision offered by asteroseismology is crucial for a better understanding of the chemical history of the Galaxy, and offers clues to the degree to which different stellar populations in the disk have mixed. This relation can then be used to estimate the ages of stars with no asteroseismic measurements but with precise abundance determinations (e.g., using the high-resolution ESPRESSO spectrograph at the VLT), thus allowing to significantly expand the above sample beyond the solar neighborhood.

University: Porto

Note: The student will spend up to 12 months at the Instituto de Ciencias del Espacio (ICE-CSIC), Bellaterra, Spain working under the supervision of Dr. Aldo Serenelli.

2019/225 - New insights into stellar physics from the NASA-TESS survey on A-type stars

Advisors: Margarida Cunha (IA U.Porto), Daniel Holdsworth (University of Central Lancashire)

Abstract: The study of stellar pulsations – or Asteroseismology- provides the only means to directly probe the interior of a star and constrain its physical and dynamical properties. Among the different types of stars exhibiting stellar pulsations, the Ap stars are unique for their strong magnetic fields and for their chemical peculiarity. These properties make them laboratories for studying the physical processes taking place inside stars that are currently most challenging to our understanding, such as macroscopic mixing, microscopic diffusion, and magnetic fields. The high-precision photometric time-series that the NASA TESS mission will acquire on several hundred Ap stars offer a key opportunity to make definitive tests to the modelling of the physical processes mentioned before. To explore that opportunity, the project proposes an end-to-end approach where the student will start by analyzing the data from this satellite and then use the results of that analysis to infer information about the properties of the stars through modelling. The ultimate goal is to take advantage of the TESS data to raise to a new level the modelling of one of the richest classes of stellar pulsators, as well as our understanding of key physical mechanisms operating inside stars.

University: Porto

Type: This topic may correspond to a mixed fellowship with up to 1 year abroad.

2019/226 - EnsemProbing inside stellar cores with ultra-precise, space-borne data

Advisors: Margarida Cunha (IA U.Porto), Benoît Mosser (LESIA - Observatoire de Pari), Mathieu Vrard (IA U.Porto)

Abstract: As a result of the launch of the CoRoT and Keplersatellites, the astronomical community has, today, exquisite asteroseismic data on thousands of stars. Moreover, in a decade this number will increase by orders of magnitude as a result of the launch, in 2026, of the ESA mission PLATO. A large fraction of these stars exhibits variability associated to the presence of gravity waves or waves of mixed nature (gravity and acoustic). These waves offer a unique opportunity to probe the conditions in stellar cores. The goal of this project is to explore the opportunity offered by the ultra-precise space-borne data made available byKeplerto constrain physical processes that shape the chemical gradients in stellar cores and that are, thus, decisive for stellar evolution. This will be achieved through the development and application of diagnostic tools aimed specifically at extracting the information contained in gravity and mixed-mode pulsations. By doing so, the project will also make a key contribution to the preparation of the future PLATO mission.

University: Porto

Type: This topic may correspond to a mixed fellowship with up to 1 year abroad.

2019/227 - Role of the magnetic field in the formation and evolution of star-forming hub-filament

Advisors: Doris Arzoumanian (IA U.Porto), Nanda Kumar (IA U.Porto), Sylvain Bontemps (Observatory of Bordeaux)

Abstract: Understanding how stars form is one of the most important and wide research topics in astrophysics. Low mass stars, like our Sun, may host planets where life could emerge, and the most massive stars govern the physics and chemical enrichment of the interstellar medium of galaxies. Although the evolutionary sequence of solar type stars is relatively well described today, the physics of the early stages of star formation is still largely unknown, especially that of high-mass stars.

Recent observations of the interstellar medium (the gas and dust filling the space between stars) have revealed the impressive organization of the matter into complex networks of filaments. The densest filaments are now identified as the main birthplaces of stars and the hubs formed by the intersection of several filaments host embedded clusters of stars from low to high masses.Much progress has been accomplished in recent years on the characterization of the density and velocity structures of filaments and hubs. However, the structure of the magnetic field and its role in their formation and evolution is still unexplored.

The goal of the proposed PhD project is to unveil the detailed structure of the magnetic field in star-forming hub-filament complexes.

The PhD candidate is expected to analyse observations of thermal dust emission in total and polarized intensity to study the density and magnetic field structures of star-forming hub-filament systems. The data will be obtained with state-of-the-art international telescopes. The candidate will be involved in writing the observational proposals, carrying out the observations, and reducing and analysing the data. The observational results will be compared to theoretical models and numerical simulations to infer the role of the magnetic field in the physics at play in hub-filament systems and its interplay with turbulence and gravity leading to fragmentation and formation of stars.

The outcome of the PhD project will provide new insights on the role of the magnetic field in the formation of hub-filament systems and their star-formation activity. These results may also be a founding block to understand the global star formation process in galaxies.

The candidate will be based at IA (Porto University) and integrated to the activities of the star-formation group of CAUP. To reach the proposed scientific objectives, the PhD thesis will be co-supervised by an expert in high-mass star-formation studies from the observatory in Bordeaux (France). Several visits to Bordeaux will be planed. Close interactions with theorists in Japan are also foreseen.

Thanks tothe proposed project, the candidate will gain deep knowledge in the physics of the interstellar medium and star-formation studies, building strong skills in observations and developing a key sense in theoretical interpretation. The expertise acquired during the PhD will provide key opportunities for future research in star-formation studies, especially in polarization astronomy, which will be highly valued in the coming years with the numerous polarization instruments newly installed on telescopes or planned for the near future.

University: Porto

2019/228 - The physics of high-mass star formation: Zoom-in on MaCProS

Advisors: Nanda Kumar (IA U.Porto), Pedro Palmeirim (IA U.Porto), Doris Arzoumanian (IA U.Porto)

Abstract: In the early 70’s, theoretical considerations of the radiation feedback on gravitational collapse led to the prediction of an upper limit on the stellar mass that can form via accretion process. The subsequent three decades has gone by in much debate in an effort to resolve this problem, however, without success. It has also led us to identify other feedback processes that might set an upper limit on the stellar mass, namely, fragmentation, stellar winds, ionisation pressure and magnetic fields. Some theoreticians have now settled upon a resolution of these issues and agree that there is no upper limit on the maximum stellar mass. However, observational studies have so far been unable to test, confront or have a say on how nature has produced the most massive stars that one finds in our Milky Way galaxy, or, if there is an upper limit on the stellar mass that can form via accretion.

Palmeirim & Kumar (2019, in preparation) have assembled the most massive and compact proto-and-pre-stellar cores (dubbed as MaCProS) based on a careful analysis of the physical properties of the star forming cores such as surface density, luminosity, mass, temperature and so on. This analysis is based on exploiting a wealth of new information from the Milky Way Galactic Plane legacy surveys Herschel Hi-Gal and ATLASGAL. This is unlike the brightness limited samples that are currently popular.

The goal of this thesis project is to conduct a wide variety of infrared and sub-millimetre high-spatial resolution observations to zoom-in on the best MaCProS. Dedicated tests will be conducted to examine them to enable our understanding of the physics of high-mass star-formation. The approach here will especially focus on tests devised in the light of the filament-hub paradigm of star formation and specially aimed to probe sub-100s of AUs in physical size. Observations using ALMA, SOFIA, JWST and VLTI are foreseen.

The candidate student should demonstrate a solid understanding of undergraduate level physics, chemistry and mathematics, a flair to study star formation, a willingness to undertake scientific adventures behind computer displays and on top of the worlds high-rise mountain observatories. During the PhD, the student will specialise in reduction and analysis of sub-millimetre and infrared data, both from single telescopes and interferometric arrays.  Several collaborative visits to the UK, Spain, and Germany are also expected.

University: Porto