Stellar evolution and stellar populations

Is the Milky Way a 2- or a 4-armed spiral?
G. Carraro

The distribution of young sources in the Galaxy shows a clear 4-armed structure, while NIR observations indicate only 2 major arms. For dynamical studies, it is essential to know if the perturbation in the Galactic disk is 2-or 4-armed i.e. whether or not the Sagittarius arm is a major arm. Star-counts cannot be used to estimate the
stellar surface density in the arm due to the variable and patchy extinction toward the center. It is proposed to measure radial velocities of _ about 2000 early-type stars in the direction of the Galactic Center with B< 15.5  and heliocentric distances up to 3.5 kpc i.e. across the Sagittarius arm. These stars were identified on Objective Prism plates and cross-identified with the 2MASS survey to yield accurate positions. The radial velocity variation of the stars
as a function of distance is not disturbed by Galactic rotation and will uniquely show if the mass associated to the Sagittarius arm is high enough to course a significant velocity perturbation i.e. if it is a major arm.
The PhD project requires measuring radial velocities of the sample stars, and their distances. In addition metal abundances will also be measured  to provide an estimate of at actual radial metallicity gradient in the inner disk. Much theory will be also developed to compare the derived radial velocity distribution with different models of the Galactic potential.


Blue straggler stars in Galactic open clusters
G. Carraro

A sample of 708 spectra has been collected of blue straggler star  candidates in 10 Galactic open clusters, spanning a wide range of ages, with the aim of testing observational evidence that open clusters host the largest BSS population among stellar systems and provide crucially needed observational constraints to BSS formation models. While this property of open clusters hints to a different formation scenario of BSS in them, it has to be interpreted with great caution because usually their colour-magnitude diagrams  are heavily contaminated by common galactic disc stars. A detailed membership study is therefore mandatory to assess their real population of BSS, while the derived variability will provide the required information on the properties of the BSS. Preliminary membership is already available from spectra that cover 3 epochs. Additional 34 hrs have been recently granted to cover 3 additional epochs for these 700 stars, to consolidate memberships and derive hints on different binary species. This PhD project requires radial velocity measurements, membership assessment and period derivation for binary stars. Comparison with models of stragglers formation will be also an important contribution.

Binary neutron star mergers and the origin of short gamma-ray bursts
R. Ciolfi

Binary neutron star (BNS) mergers are the ultimate multimessenger events. Besides being among the most promising sources of gravitational waves, they are likely accompanied by a variety of electromagnetic counterparts across the whole spectrum, from gamma-rays to radio. In particular, they are thought to be the central engine powering short gamma-ray bursts (SGRBs), one of the most powerful explosions observed in the universe.
The project will focus on general relativistic magnetohydrodynamics simulations of BNS mergers (e.g. https://arxiv.org/pdf/1701.08738.pdf), with the aim of providing new evidence in favour or disfavour of the SGRB/BNS connection. The PhD student will run BNS merger simulations employing the public Einstein Toolkit infrastructure and our private code WhiskyMHD, analyze the results in detail (also visualizing them in 2D/3D), and actively participate in their interpretation. Depending on the student's skills and interests, a significant involvement in code development is possible as well.

A 4-dimensional view of the Magellanic Clouds System

L. Girardi, 
P. Marigo
The VMC survey is mapping the entire Magellanic Clouds system with the most efficient telescope presently operating in the near-infrared, namely the ESO VISTA 4m telescope. Details about the survey can be found at arxiv.org/abs/1406.7170.  The deep images in the Y, J, and Ks passbands are allowing us to determine the spatially-resolved star formation history (SFH) with unprecedented accuracy, and to construct a detailed 3D map of the system (see Rubele et al. papers in ADS). The present SFH analysis is being entirely performed at the Padova Astronomical Observatory, using the data divided in small regions of the sky. The primary scope of the PhD project is to develop a new formalism that will allow us to analyse the all data simultaneously, deriving a consistent 4D model (i.e. the 3D spatial distribution+age) for the stars in the Magellanic Clouds. The process also foresees using the deep HST data we have from approved HST proposals (both past and ongoing). The PhD student will acquire basic skills in stellar evolution, stellar populations, and data analysis via optimization algorithms. He will also be inserted in a successful international collaboration (including researchers in Australia, Argentina, Belgium, Chile, Germany and UK) that will last long past the survey observations are completed.

The Specific Production Method to Decode the Star Formation History in Resolved Stellar Populations
L. Greggio

The study of resolved stellar populations in galaxies is a powerful tool to investigate on the galaxy formation process and on the history of star formation in the universe. Stellar counts in different parts of the color-magnitude diagrams (CMD) can be directly translated into initial mass of the parent stellar populations at specific ages, using appropriate conversion factors given by the stellar evolution theory.  In this thesis project,  the student will derive these conversion factors  by computing and analysing a large number of simulated of CMDs of simple stellar populations, spanning a wide range of metallicites and with ages up to the Hubble time.  Various  combinations of photometric filters will be tested, to find those which are most effective  to derive  ages and metallicity distributions of composite stellar populations. The dependence of the coefficients on other input pameters, (e.g.  Initial Mass Function, evolutionary sequences, alpha-to-Iron abundance ratios), will also be evaluated.  The dataset will yield the specific production coefficients  with which the star formation history of a composite system can be reconstructed from star counts in diagnostic regions on the CMD, and their uncertainty related to the input parameters. This work will provide a new method to study the star formation history in stellar systems, which is currently performed by constructing blind fits  of synthetic CMDs. This alternative method will be particularly efficient for the analysis of huge data sets,  as those  delivered by wide field observations and future very large telescopes (TMT, E-ELT). The thesis project will also include the validation of the new method on literature results.

Galactic archeology in the era of the large surveys
S. Lucatello

Galactic Archaeology is the study of presently observable objects and phenomena to gain insight on the events which took place at the epoch of Galaxy assembly. The understanding of the formation of the Milky Way and of its components is a fundamental step in the comprehension of the processes involved in the formation of spiral galaxies, which is of particular relevance given the relatively common nature of galaxies like ours. Stellar populations can trace, through their chemistry and kynematics, the chemo-dynamical evolution of galaxies, and of processes that lead to their formation. In this context the study of the resolved populations in the Milky Way and in the Local Group provide an ideal laboratory.
Chemical tagging, that is the identification of spatially separated stars which share the same chemical composition, complemented by dynamical information, can be used to recognize the signature of the original birth-place of the studied objects. This approach allows to gather key information in reconstructing the star-forming aggregates and accreted galaxies that contributed to the formation of the galaxy hosting the population under analysis.
 The student will work with data from several of the large spectroscopic stellar surveys (e.g. APOGEE1/2, GES, RAVE, GES, Gaia), deriving information about their composition and spatial motions to identify groups of stars born together. The aim is to disentangle the populations accreted from those formed in-situ, estimating, for instance, the contribution of of partially or completely dissolved Globular Clusters to the building up of the Halo and Bulge, or of disrupted Open Clusters to the disk etc. During this work the student will develop not only expertise directly related to the astrophysical topics involved but also considerable skills related to data science and big data, given the size of the databases s/he will be working with.

Simulating the nursery of stars in the era of Gaia
M. Mapelli

Understanding the formation of star clusters (SCs) is one of the most pressing challenges of contemporary astrophysics. In the local Universe, most  stars (~70-90 per cent) are believed to form in SCs. Despite the importance and ubiquity of SCs, their formation is still an enigma. Ongoing optical and near-infrared ground-based surveys (e.g. the Gaia-ESO Survey and the Vista Magellanic Cloud Survey) and the Gaia mission are about to provide a wealth of new data about young SCs in the local Universe. Thus, theoretical models must be ready to meet the challenge posed by the new observations. The aim of this thesis is to investigate the formation and evolution of young SCs by means of innovative hydro-dynamical simulations. The student will compare the results of the simulations with the data from the Gaia ESO Survey and other ongoing surveys. We will make predictions for the Gaia mission. The simulations will adopt a completely new methodology, to account for stellar dynamics, for stellar evolution, and for the hydro-dynamics of the parent molecular gas.

 More details on our group and on these topics can be found at web.pd.astro.it/mapelli/group.html

Demography of compact-object binaries in the era of gravitational wave astronomy
Michela Mapelli

On September 14 2015, gravitational waves (GWs) were detected for the first time, about 100 years after Einstein's prediction. GW150914, the first event observed by LIGO, is associated with the merger of a double black hole binary (BHB). This is the first confirmation that BHBs exist, and opens a new perspective to study the formation and evolution of BHBs. SEVN, our new population-synthesis code for N-body simulations, successfully predicted the existence of events like GW150914. The aim of this project, is to use the SEVN code  to put constraints on the formation scenarios of current and forthcoming GW detections, and to understand the demographics of double-compact object binaries in different environments. More details on our group and on these topics can be found at web.pd.astro.it/mapelli/group.html


Extracting star cluster dynamics from raw data with deep learning
M. Mapelli - M. Pasquato

A wealth of astronomical data has recently been obtained for Milky Way star clusters (both open and globular) in different sources and spectral windows. This data is currently being compared to theoretical predictions to obtain insight in star cluster dynamics, attempting to answer questions such as: do star clusters contain intermediate-mass black holes (IMBHs)? What is the retention fraction of dark remnants?
What is the binary fraction and binary binding energy distribution? How it evolved over time? Which clusters underwent core-collapse?
Which star clusters are potential sources of gravitational radiation? Our ability to devise theoretical models and make predictions
currently drives the data acquisition and analysis process, and limits the amount of information we can extract from the available data.
A data-driven approach based on machine learning would instead allow us to reduce scientific questions to classification and regression problems, where the input is either raw observational data or mock data from simulations. The student will help me make this happen. They will work on:
- running star cluster simulations using a variety of tools (direct N-body and Montecarlo codes)
- making mock observations (e.g. HST-like images) out of simulation snapshots
- training a convolutional neural network (e.g. using Tensorflow) on said images to classify them according to the relevant scientific question (e.g. are they IMBH hosts or not?)
- eventually applying the classifier to actual data (e.g. HST images) This is an ambitious project for ambitious students who want to work on high-risk high-reward science, with a focus on acquiring skills that are in growing demand in Astronomy (Python coding skills, handling big data from large scale simulations, machine learning) and the opportunity to be part of a literal paradigm shift.

Calibrating models of asymptotic giant branch stars with HST data
P. Marigo, L. Girardi

AGB stars contribute a large fraction of the light emitted from galaxies, especially in the near-infrared, and even in optical wavelengths for young galaxies observed at high redshifts. Calibrating stellar models of the AGB is therefore of utmost importance for studies of galaxy formation and evolution. This calibration is now possible using the combined photometry of several millions of resolved stars in galaxies up to distances of 4 Mpc, coming from three HST surveys: the ACS Nearby Galaxy Survey Treasury in the optical, a snapshot survey that observed the same galaxies with the WFC3 camera in the near infrared, and the multi-cycle Panchromatic Hubble Andromeda Treasury, which mapped a large fraction of the Andromeda galaxy in 6 filters from the UV to the near infrared. The PhD project consists in modelling such data using the most up-to-date models of stellar evolution. Close interaction with the Seattle group led by Julianne Dalcanton (the PI of ANGST and PHAT) is foreseen. The PhD student will also be inserted in the STARKEY team lead by P. Marigo and recently funded with a Consolidator Grant by the European Research Council.

What causes the multiple populations in Magellanic Cloud star clusters?
P. Marigo, 
L. Girardi

Intermediate-age star clusters in the Magellanic Clouds provide the most striking examples of multiple star forming events in star clusters. We have high-quality HST data for 19 star clusters in both Magellanic Clouds obtained in HST cycles 18 and 20. The data allow us, for the first time, to measure the signatures of multiple populations - especially the multiple turn-offs in the dual red clumps of He-burning stars - in a complete and well-characterized sample of intermediate age clusters, spanning a wide range of masses and concentrations. This will hopefully lead to a better understanding of the factors driving the formation of multiple populations. These database also constitutes an invaluable resort for the calibration of stellar evolutionary models. We welcome PhD applicants to work with this dataset. The project would be done in close collaboration with Paul Goudfrooij's group at the Space Telescope Science Institute in Baltimore.

A search for binaries with massive companions in the core of the closest globular cluster
A.Milone, L. Bedin

We propose a PhD thesis on accurate astrometric measurements. These measurements aim at  constraining the number of binaries with massive companions (black holes, neutron stars, or white dwarfs) in the core of M4 by measuring the "wobble" of the   bright (main-sequence) companion around the center of mass of the pair. The wobble   will be measured by leveraging the high-precision spatial resolution of WFC3/UVIS. The goal is to constrain the total number of binaries with massive companions and periods   between 0.5 months < P < ~15 years. Mass segregation predicts these binaries should  be strongly concentrated in the core and we will measure their radial distribution.  Moreover, as a by-product, the PhD student can also identify and measure the mass of a central black hole, if this cluster happens to host one. This proposal will complement other ongoing investigations on the binary population in M4, and will allow us to construct a dynamical model of the cluster to help us understand why its core is not collapsed.                                                                                                                                                                Stellar-mass black holes, neutron stars, and white dwarfs must form as a result of   normal stellar evolution, and these observations are designed to leave no chance of    failing to see their influence on their binary companions.                          
The PhD student that will be involved in this project, will take advantage of our expertise  in high-accuracy astrometry on HST images, and she/he will be expected to improve these techniques  and will develop the analyses procedures  to extract the information on the binary content of this cluster.

Near-field cosmology with globular clusters
A. Milone, A. F. Marino

How did the globular clusters formed in the early Universe?
What is their role in the assembly of the Galactic Halo?
Which is their contribution to the re-ionization of the Universe?
This is a thesis project to address such hot topics by investigating stellar populations in globular clusters.

In recent years, our group has developed new revolutionary tools for characterizing multiple populations in star clusters.
These tools, which are based on the synergy between spectroscopy and multi-band photometry have dramatically changed our view of globular clusters and resulted in the most-important and most-cited papers ofthe field (e.g. Milone et al.2017, MNRAS, 467, 3636).

The candidate will exploit these innovative tools to
i)  analyze UV, visual and NIR photometry from the Hubble SpaceT elescope and from the major wide-field ground-based facilities.
ii)  and/or study high- resolution spectra collected with the VLT andother major telescopes.

This project will provide new insights on the processes that occurred at high redshift and resulted in the formation of multiple populations and in the assembly of the Galaxy.

This thesis is complementary to the project 'GALFOR' funded by theEuropean Research Council. The candidate will be involved in an international team, which includes the major experts of the field (see progetti.dfa.unipd.it/GALFOR/ for details).

Multiple populations in young Magellanic Clouds clusters
A. Milone

The discovery of multiple populations in young and intermediate-age clusters has been one of the major findings in the field of stellar populations of the last decade (e.g. Milone et al. 2009, A&A, 497, 755).
Their origin is one of the most-intriguing open issues of stellar astrophysics and provides new constraints on the assembly of the galaxies and on star formation and evolution.

The Hubble Space Telescope has recently collected for us an unique dataset to investigate, for the first time, the multiple-population phenomenon in a large sample of young clusters (GO-14710, PI. Milone).
The candidate will investigate this dataset together with additional HST archive images by using the most-advanced techniques of data reduction and analysis.
Results will provide a major breakthrough in our understanding of the multiple-population phenomenon.

This thesis is complementary to the project 'GALFOR' funded by the
European Research Council. The candidate will be involved in an
international team, which includes the major experts of the field (see
progetti.dfa.unipd.it/GALFOR/ for details).

Multiple stellar populations in star clusters
G. Piotto, D. Nardiello

The recent discovery (by our group) of multiple evolutionary sequences in Galactic globular clusters has seriously challenged the paradigm of star clusters hosting a simple stellar population, formed at the same epoch, from the same material. These new findings may have vast implications on star formation processes in the early Universe, and on population synthesis models used to interpret the light from galaxies at cosmological distances. This is a lively debated argument in the present astronomical literature. Our research project is mainly based on observations with the Hubble Space Telescope, and largest ground-based facilities. The PhD student will be inserted in a large international collaboration (of which the group in Padova is the leader) and work on the analysis of data from: 1) already approved and new proposed Hubble Space Telescope programs, and 2) approved ESO/VLT and LBT  programs both with wide field imagers and high resolution spectrographs. The PhD student will gain an expertise on high precision photometry and astrometry on HST data and/or on the best techniques to exploit high-resolution spectroscopic data. The possibility of formation stages at the Space Telescope Science Institute, and other European and USA universities is offered


Galactic disk evolution as traced by open clusters
A. Vallenari, L. Athanassoula

Open clusters are regarded as tracers of the evolution of our disk, of its age and metallicity distribution.
However to rely on them we need to understand how they interact with the disk environment. Indeed,
 the Milky Way shows a variety of morphological and dynamical substructures that are expected to affect the evolution of star clusters. The
effect of interactions with giant molecular clouds and spiral arm passages on the cluster dissolution time-scale  and orbits has emerged from dynamical simulations.
Star clusters steadily loose stars through and form long tidal tails. A detailed understanding of the formation, morphology and dynamics of tidal tails resulting from star clusters in galaxies are of great interest to observations especially in the Milky Way. Another effect is the possibility that spiral arms and the bar perturb their orbits inducing a migration from their original orbit.
The goal of the Thesis is to simulate the evolution of a cluster in the Milky Way disk, using a code already developed by L. Athanassoula.
The PhD student is expected to spent 50% of the time in Marseille to work with L. Athanassoula, one of the leaders in the field.
The results will be compared with the data of the second data release of Gaia, that will be available in 2018, and of the data coming from spectroscopic surveys(WEAVE, ESO-Gaia Survey...) in which Padova is deeply involved. The PhD student will be part of an international environment, with frequent international contacts. The project will lead to a PHD title of Padova and Marseille Universities

Metallicity of open clusters and field stars from the Gaia ESO Survey
A. Vallenari , G. Carraro

The Gaia ESO survey operating with FLAMES at the VLT will observe high resolution spectra of open clusters and field stars in the disk, bulge, halo of our Galaxy. These observations will allow to derive the radial velocities and chemical abundances of the stars down to V=19. Coupling these data with the positions and distances of the stars derived by Gaia will allow to derive the characteristics of the stellar population in the Galaxy and to put constrains on the process of the Galaxy formation (mergers vs. in situ formation).
The thesis will focus on the analysis and discussion of the detailed chemical abundances elements in field and open clusters to better constrain the formation process of the Galactic components.

The Ultra-Faint galaxies problem
S. Zaggia

The “missing satellites” problem is one of the greatest challenges to the ΛCDM paradigm. Too few satellite galaxies are observed with respect to the dark-matter haloes predicted by ΛCDM simulations. A possible solution is that a few of the dark-matter haloes started to form stars before the epoch of re-ionisation and the feedback from these galaxies has suppressed star formation in 90% of the remaining dark-matter haloes, that are still lurking, dark, starless, around the Milky Way. These early-forming dwarf galaxies are “fossils” of the epoch before re-ionisation, the “Dark Ages” and can be recognised by their old ages and low metallicities.
The recently discovered class of ultra-faint dwarf galaxies holds the promise to contain several of these “fossils”. Our group has started an observing campaign which will use MUSE, X-Xhoother and FORS2 data for an in-depth study of some of the most promising ultra-faint candidates. The PhD student will learn how to extract accurate metallicities and abundance for these galaxies together with radial velocities and accurate photometry with the final aim at understanding the real nature of these galaxies. The possibility of formation stages at Observatoire the Paris is also offered.

The Structure of the Galaxy in the Gaia-ESO Survey
S. Zaggia, G. Carraro, A. Vallenari

The Gaia-ESO is a 5 years public spectroscopic survey, almost completed and released, which used the FLAMES-UVES instrument of the VLT to target >100000 stars, systematically covering all major components of the Milky Way, from halo to star forming regions, providing the first homogeneous overview of the distributions of kinematics and elemental abundances. This alone will revolutionize knowledge of Galactic and stellar evolution: when combined with Gaia astrometry the survey will quantify the formation history and evolution of young, mature and ancient Galactic populations.
The FLAMES spectra have already allowed to: quantify individual elemental abundances in each star of the sample; yield precise radial velocities; follow the formation, evolution and dissolution of open clusters as they populate the disc, and provide a legacy dataset that adds enormous value to the Gaia mission and ongoing ESO imaging surveys.
The PhD project offered is aimed at studying kinematic gradients and abundance-phase-space structure in poorly explored regions of the Galaxy combining the Gaia-ESO data (already released) with the first fully kinematic release of the Gaia astrometry for a 4-D kinematic and chemical analysis. The PhD student will learn how to analyze and combine large databases of data, and to interpret the results in the framework of the Milky Way stellar populations history of formation.