TOSCA - Topical Overview on Star Cluster Astrophysics

28 Oct 2024, 09:00 → 31 Oct 2024, 18:50 UTC

Siena, Santa Chiara Lab

Giovanni Morlino (Istituto Nazionale di Astrofisica (INAF)), Stefano Menchiari (INAF - Osservatorio Astrofisico di Arcetri), Giada Peron (Istituto Nazionale di Astrofisica (INAF)), Gaia Verna (Università di Siena - INFN PIsa)

The Topical Overview on Star Cluster Astrophysics (TOSCA) workshop aims at creating a common environment for different astrophysical communities to have a joint and comprehensive discussion on the physics of star clusters. 

Star Clusters are among the most studied celestial objects. They are the fundamental building blocks of galaxies and represent crucial laboratories for understanding a wide range of astrophysical problems. 

In recent times, star clusters have been proposed as cosmic ray factories, as suggested by the detection of some of the youngest and most massive star clusters in the high- and very high-energy gamma-ray band. However, we still lack of a comprehensive understanding of particle acceleration and propagation in those objects. While the acceleration is probably driven by the power injected through stellar winds and supernova explosions, the particle propagation is affected by stellar feedback which shapes the surrounding medium. The presence of freshly accelerated particles provides a new component that may modify the way stellar cluster feedback onto the interstellar medium.

Scientific topics will include:

• Gamma-ray observations
• Particle acceleration and propagation
• Stellar feedback
• Stellar wind physics
• Population properties

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This event has received fundings from INAF minigrant 2024 initiative entitled "Probing Young Massive Stellar Cluster as Cosmic Ray Factories" (P.I. S. Menchiari) and further INAF financial support from the yearly call for fundings for school and congress organization.

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IMPORTANT: The registration is restricted to invited/accepted contributors. If you want to submit an abstract for evaluation is not necessary to register, but you need to have an indico account. Please create one for the abstract submission. 

 

Monday 28 October

13:00 → 14:00 Registration

14:00 → 14:20 Workshop introduction

Convener: Giovanni Morlino (Istituto Nazionale di Astrofisica (INAF))

14:20 → 18:20 Gamma-ray observations

Convener: Gaia Verna (Università di Siena - INFN PIsa)

14:20 Review Gamma Rays

Speaker: Dr Luigi Tibaldo (IRAP)

15:00 Recent results on Stellar clusters as detected by HESS

Speaker: Lars Mohrmann

15:30 Young massive clusters with Fermi-LAT

Speaker: Giada Peron (Istituto Nazionale di Astrofisica (INAF))

16:00 Coffe break

16:20 Overview of the Cygnus OB2 region in gamma-rays

Speaker: ruizhi yang

16:50 Population studies of SCs derived from Fermi-LAT observations

Speaker: Ava Webber (Clemson University)

17:20 Prospect for gamma-ray detection of Galactic stellar clusters

Speaker: Alison Mitchell (Erlangen Centre for Astroparticle Physics, Friedrich-Alexander-Universität Erlangen-Nürnberg)

17:50 Aluminium-26 from massive star clusters: theory and observations

Speaker: Roland Diehl

18:20 → 20:30 Welcome Reception

Tuesday 29 October

09:00 → 13:10 Particle acceleration and propagation

Convener: Elena Amato (Istituto Nazionale di Astrofisica (INAF))

09:00 Review Cosmic Rays

Speaker: Stefano Gabici

09:40 -

Speaker: Lucia Härer

10:10 Deciphering the Cygnus region

The Cygnus region has become a gamma-ray source of prime interest since the detection of ultra-high energy photons by LHAASO. This likely indicates the presence of a hadronic source of PeV cosmic rays in the region, although the accelerator has not been yet identified. In this talk, I will summarize our knowledge of this star-forming complex and, using large-scale hydrodynamic simulations, I will critically assess possible acceleration mechanisms, in particular related to the Cygnus OB2 association. I will show that wind-wind interactions are not efficient enough to enhance particle acceleration, and that a cluster wind termination shock is not expected to form. Therefore, it seems that the gamma-ray spectrum can only be understood by considering recent supernovae explosions in the region.

Speaker: Thibault Vieu (Max-Planck-Institut für Kernphysik)

10:40 -

Speaker: Prof. Pasquale Blasi

11:10 Coffee break

11:40 Superbubbles as Galactic PeVatrons: The Potential Role of Rapid Second-Order Fermi Acceleration

Superbubbles are now one of the few remaining potential PeVatron candidates in the Galaxy, based on the identification of the Cygnus Cocoon as a LHAASO identified PeVatron and Westerlund 1 as a very powerful TeV gamma-ray source with a large shell-like morphology. At the moment, it is not clear how superbubbles accelerate particles to PeV energies. Recent focus has been on the role of the collective cluster wind termination shock which could accelerate particles through first order Fermi acceleration. However, it is not clear whether the termination shock is powerful enough or even always existent. A termination shock may explain the shell-like morphology of Westerlund 1, but then requires a leptonic origin of the gamma-ray emission.
In this talk I will argue that second-order Fermi acceleration in the tenuous region outside the termination may be fast enough to accelerate particles to the PeV domain. What is required is a large Alfvén velocity (V>500 km/s), caused by the tenuous density, and a high level of magnetic-field turbulence. For the latter cosmic-ray containment estimates based on gamma-ray observations provide evidence.

Speaker: Jacco VInk

12:10 Cosmic rays from Galactic star clusters

In this contribution, I will discuss about a model for the production of cosmic rays from massive, young star clusters present in the Galaxy. I will show that cosmic-ray acceleration at the shocks associated with fast stellar winds of star clusters can produce significant contribution in cosmic rays at energies ~ 10^16 - 10^18 eV. When combined with the contribution from regular supernova remnants in the Galaxy and an extra-galactic component, cosmic-rays from star clusters can reasonably explain the observed all-particle spectrum and composition of cosmic rays.

Speaker: Satyendra Thoudam

12:40 Multimessanger astrophysics from SCs

Speaker: Silvia Celli (Sapienza University of Rome & INFN)

13:10 → 14:40 Lunch

14:40 → 15:10 Contributed posters - Flash Talks

14:40 Isolated massive star candidates in nearby star forming galaxies

The debate on whether star formation of massive stars always occurs clustered or could also happen in isolation is still open. Although small in numbers, massive stars strongly affect the environment around them: they can stop or trigger star formation, reshape the distribution of the gas around them and enrich the ISM due to supernova events or stellar winds. Thus, understanding how frequently high-mass stars can form in isolation in sparse density environments becomes extremely important. So far this has mostly been studied in only three Local Group galaxies: the Milky Way and the Magellanic Clouds. All three galaxies show a fraction of seemingly isolated massive stars that do not appear to be part of even very low-mass clusters nor can be explained as runaways or walkaways.
In order to shed light onto this open question, we are undertaking a systematic survey of other star-forming galaxies in the Local Volume to address this question with better statistics, using high-resolution photometry from two UV-optical Hubble Space Telescope legacy surveys, GULP and LEGUS. In this poster presentation, I will mainly focus on the spiral galaxy NGC 4242 and compare our findings to the Local Group.

Speaker: Pietro Facchini (Zentrum für Astronomie der Universität Heidelberg, Astronomisches Rechen-Institut)

14:45 Gamma-ray signatures of particle acceleration at stellar wind termination shocks up to PeV energies

Young massive stellar clusters have recently brought attention as PeVatrons candidates, to explain the knee of the cosmic ray spectrum and how protons can be accelerated to such energy scale in galactic sources. The new detector LHAASO is the first to probe well the photon detection band >0.1 PeV, that can correspond to multi-PeV hadronic cosmic rays. Thus, it enables the use of its gamma-ray data to constrain the galactic particle acceleration models and parameters, and to identify the contribution from the different categories of galactic accelerators to the observed cosmic ray flux, especially in the PeV domain.

To that extent, we model the escape and the transport of cosmic rays from their accelerator to molecular clouds, where a lot of p-p interactions producing gamma rays occur. We are focusing on the case where the source is a young massive star cluster, hence the particles are accelerated in stellar wind termination shocks before escaping. We try to determine in a semi-analytical approach the parameters needed (distance between cloud and source, time, slope of injection, number of stars, etc) to produce an excess in the gamma-ray flux corresponding to PeV cosmic rays, that could be detectable by LHAASO. This enables to constrain the subspace of the parameter space for which a detectable excess could exist, and therefore constrains the subset of systems (cluster+cloud) that could produce such an excess. Then, the goal is to find such systems and compare predictions of the models for the gamma-ray flux to LHAASO data in order to determine more precisely different acceleration parameters, such as the wind termination shock efficiency or the injection spectrum in the interstellar medium. Another goal is to try to explain some of the dark PeVatrons seen by LHAASO with systems star cluster+cloud.

Speaker: Alexandre INVENTAR (APC Laboratory)

14:50 Young massive star clusters as Galactic PeVatrons

Speaker: Alberto Bonollo (Istituto Nazionale di Astrofisica (INAF))

14:55 Constraining the Diffusion Coefficient and Cosmic-Ray Acceleration Efficiency using Gamma-ray Emission from the Star-Forming Region RCW 38

We report a detailed study of gamma-ray emission near the young Milky Way star cluster (0.5 Myr old) in the star-forming region RCW 38. Using 15 years of data from the Fermi-LAT, we find a significant (σ>22) detection coincident with the cluster, producing a total gamma-ray luminosity of L = (2.66±0.92) x 10^34 erg s^-1 adopting a power-law spectral model (Γ=2.34±0.04) in the 0.1-500 GeV band. Using an empirical relationship and Starburst99, we estimate the total wind power to be 7×10^36 erg s^−1. This corresponds to a CR acceleration efficiency of η_CR = 0.4 for a diffusion coefficient consistent with the local interstellar medium of D = 10^28 cm^2 s^−1. Alternatively, the gamma-ray luminosity could also account for a lower acceleration efficiency of 0.1 if the diffusion coefficient in the star-forming region is smaller D = 2.5 x 10^27 cm^2 s^−1. In addition, we perform a Chandra X-ray analysis of the region to compare the hot-gas pressure from the CR pressure and find the former is four orders of magnitude greater, suggesting that the CR pressure is not dynamically important relative to the stellar wind feedback. As RCW 38 is too young for supernovae to have occurred, the high CR acceleration efficiency in RCW 38 demonstrates that stellar winds may be an important source of Galactic cosmic rays.

Speaker: Paarmita Pandey (The Ohio State University)

15:00 Creating a Hydrodynamic simulation of Cygnus OB2

The Cygnus X star-forming region has been of great interest to the high-energy astrophysics community due to the diffuse gamma-ray emission detected by Fermi, HAWC and LHASSO in recent years. At the heart of this region lies the OB association Cygnus OB2, with tens of powerful O stars and 3 Wolf-Rayet stars. It has been argued that efficient stellar wind interactions in the vicinity of massive star clusters create favourable conditions for particle acceleration up to very high energies, which could potentially explain the observed diffuse gamma-ray emission in this region. However the core of Cygnus OB2 is rather extended, which puts into question the appropriateness of simplified spherical models and calls for a more detailed investigation of the wind-wind interaction given the peculiarity of the region.

In this poster, we describe a large-scale hydrodynamic simulation of a massive star cluster whose stellar population mimics that of the Cygnus OB2 association, as a collaborative project between research groups specialising in both particle acceleration theory and winds of massive stars. The main-sequence stars are first simulated during 1.6 Myr, until a quasi-stationary state is reached. At this time, the three Wolf-Rayet stars observed in Cygnus OB2 are added to the simulation, which continues to 2 Myr. Using a high-resolution grid in the centre of the domain, we can resolve the most massive stars individually, which allows us to probe the kinetic structures at small (parsec) scales. We find that, although the cluster excavates a spherical “superbubble” cavity, the stellar population is too loosely distributed to blow a large-scale cluster wind termination shock, and that collective effects from wind-wind interactions are much less efficient than usually assumed. This challenges our understanding of the ultra-high energy emission observed from the region. This work includes detailed treatment of the massive stellar population, incorporating Gaia astrometric data, empirically determined mass loss and terminal wind velocities for the most powerful stars, and stellar evolution. In this poster we will emphasise how these considerations directly affect results of interest to the high-energy astrophysics community.

Speaker: Cormac Larkin (MPI Kernphysik/ARI)

15:05 Protostellar Jets as Particle Accelerators. The case of HH 80-81.

Massive stars are capable to accelerate particles due to their powerful winds ejected during the main sequence and post-main sequence evolutionary stages. However, recent studies on massive young stellar objects with Fermi-LAT have demonstrated that collimated jets, created by the protostars while they are still accreting mass, can produce a significant amount of accelerated particles even though during the pre-main sequence stages.

In this context, we have studied the particle acceleration in IRAS 18162-2048, a massive protostar with ~20 M☉ that powers the longest collimated jet in our galaxy. The jet is located in a very dense environment, surrounded by a recently reported protostellar cluster with many medium and low-mass protostars. The main knots of the jet (HH 80, HH 81, and HH 80N) have been detected in radio and X-ray wavelengths, emitting non-thermal emission. In this work, we have associated a Fermi-LAT 4FGL source with the protostellar jet based on positional arguments.

The study of the high-energy spectrum of the source, spanning from 300 MeV to ~1 GeV, suggests a soft particle distribution consistent with diffusive shock acceleration. We test both leptonic and hadronic models, finding them consistent with the kinetic energy of the outflow jet. We also perform a morphological analysis, finding interesting correlations between the emission and the molecular clouds in the region.

In our poster, we will present the detection of gamma-ray emission originated by a protostar in a star-forming region. The proven capability of massive stellar objects for accelerating particles since their forming stages is an interesting starting point for analyzing cosmic ray production within massive star-forming clusters, as we expect many of these protostars in the densest regions of the active forming clusters.

Speaker: Javier Méndez-Gallego (IAA-CSIC)

15:10 → 19:10 Stellar feedback

Convener: Dr Stefano Menchiari (Instituto Astrofísica Andalucía (CSIC))

15:10 Review Stellar Feedback

Speaker: Anna Rosen (San Diego State University)

15:50 Stellar wind bubbles

Speaker: Lachlan Lanchaster

16:20 Simulations of stellar feedback

Speaker: Ahmad Ali

16:50 Coffee break

17:10 Observations of HII regions

Speaker: Loren Anderson

17:40 Dynamic of HII regions

Speaker: Christopher Matzner (University of Toronto)

18:10 Cosmic-ray feedback

Speaker: Philipp Girichidis

18:40 Cosmic ray-driven astrochemistry

Speaker: Brandt Gaches

20:30 → 23:00 Social Dinner

Wednesday 30 October

09:00 → 13:10 Stellar wind physics

Conveners: Prof. Jorick Vink, Niccolo' Bucciantini (Istituto Nazionale di Astrofisica (INAF))

09:00 Review Stellar Wind Physics -1

Speaker: Jon Sundqvist

09:35 Review: Wolf-Rayet winds

Speaker: Andreas Sander

10:10 Winds from very massive stars

Speaker: Gautham Sabhahit

10:40 Colliding Winds and non-Thermal emission

Speaker: Julian Pittard

11:10 Coffee break

11:40 Stellar wind simulations and bow shocks

Speaker: Jonathan Mackey

12:10 X-rays from the Winds of Massive Stars

Speaker: Stan Owocki

12:40 Stellar Physics and the Chemical Enrichment of the ISM from Massive Stars

Speaker: Marco Limongi (Istituto Nazionale di Astrofisica (INAF))

13:10 → 13:20 * Group Picture *

13:20 → 18:20 Free afternoon

Thursday 31 October

09:00 → 13:10 Population properties

Conveners: Giuseppe Germano Sacco (Istituto Nazionale di Astrofisica (INAF)), Nick Wright

09:00 Review Population properties

Speaker: Elena Sabbi

09:40 Why are some massive stars found in clusters and some in OB associations?

Speaker: Jesús Maíz Apellániz

10:10 Massive stars in OB associations

Speaker: Sara Berlanas

10:40 A census of OB stars within 1 kpc and the star formation and core collapse supernova rates of the Milky Way

O- and B-type stars constitute valuable tools across many areas of astronomy. They are significant sources of stellar feedback, through which they enrich the interstellar medium in new chemical elements, but also contribute to and hinder the emergence of future generations of stars. Being short-lived, they tend to remain near their birth environment, and thereby are vital tracers of the position and motion of the Milky Way spiral arms. Furthermore, they are crucial to study star formation and early evolution, as well as stellar multiplicity.

It is thus important to map out the distribution of OB stars across the Galaxy. Data from Gaia DR3 offers an opportunity to produce an updated census of their population. In this work, we have applied an improved, flexible SED fitting tool to identify and characterize OB stars in the solar neighborhood. With this tool we have identified about 25,000 O- and B-type stars (hotter than 10,000 K) within 1 kpc of the Sun, with a completeness of 90-95 % across all magnitudes. This list of OB stars typically include fainter stars non-included in similar catalogues, and reach higher completeness particularly in the solar neighborhood (< 300 pc).

Several overdensities on this map correspond to well-known regions such as Orion, Sco-Cen, Vela OB2, Cepheus and Circinus, hinting at the presence of OB associations and/or massive star clusters. These overdensities thereby constitute potential sources of stellar feedback, driving particle acceleration in the surrounding medium, and also future targets for detection of gravitational waves. Since we used our census of OB stars to provide a new estimation of the star formation and supernova rates within the local Milky Way, we have confirmed this by determining a rate of about 20 core-collapse supernova explosions per million year within 1 kpc.

Speaker: Alexis Quintana (University of Alicante)

11:10 Coffee break

11:40 Massive stars ejected from clusters

Speaker: Sally Oey

12:10 X-ray observations of massive star clusters

Speaker: Mario Giuseppe Guarcello (Istituto Nazionale di Astrofisica (INAF))

12:40 The life cycle of star clusters in low-metallicity dwarf galaxies

The earliest known proto-globular clusters (GCs) detected with the James
Webb Space Telescope were compact (effective radii ~parsecs) and
extremely dense, ideal to harbour energetic massive stars and possibly
intermediate mass black holes. The detailed conditions and time-scales
of star formation and stellar feedback during the earliest stages of
galaxy assembly are however still unclear. This is both due to the
limiting resolution (parsecs) and sensitivity in even the most optimal
gravitationally lensed detections; and the lack of complementary
high-fidelity simulations able to capture the key astrophysical
processes of clustered star formation on small, sub-parsec scales.

In the GRIFFIN project (Galaxy Realizations Including Feedback From
INdividual massive stars) we examine the formation and evolution of
resolved star clusters up to the GC-mass range using high-resolution
(sub-parsec, star-by-star) hydrodynamical simulations. Our
low-metallicity dwarf galaxy simulations account for the radiation,
stellar winds and supernovae of individual stars. I will briefly discuss
how massive star clusters form hierarchically and rapidly over
time-scales of less than 10 Myr, yet they are able to self-enrich
through stellar winds. Recently, we supplemented the methodology with a
regularised integrator to accurately solve the stellar gravitational
dynamics on small spatial scales. This was shown to be critically
important for the modelling of more realistic star cluster life cycles
from formation until disruption in the tidal field of the host galaxy. I
will conclude with future avenues toward unravelling the cosmic origin
of GCs.

Speaker: Natalia Lahen

13:10 → 14:40 Lunch

14:40 → 18:50 Discussion

Convener: Giovanni Morlino (Istituto Nazionale di Astrofisica (INAF))