Stellar Astrophysics Research Group @IA: 2024 in Review

Our activity during 2024 was distributed across three main research lines: Sun and Heliosphere, Stellar Physics, and Stellar Populations.

Sun and Heliosphere research line. This research line focuses on studying the solar atmosphere as well as the influence of solar activity on the heliosphere and the Earth's atmosphere, also known as Space Weather. We continued to participate and collaborate in several national and international projects. Group members lead the Portuguese participation in the SWATNet MSCA Innovative Training Network, are actively contributing to the European Solar Telescope, are developing a coupled thermosphere-ionosphere model for the Atlantic mid-latitude regions in a partnership with MIT, and are investigating the consequences of the rapid secular drift of the northern magnetic dip pole. Furthermore, we have been contributing to the definition of the scientific priorities of the PoET telescope concerning solar and stellar physics.

Stellar Physics research line. This research line focuses on understanding the physical processes that take place in stars, from the stellar interior to the surface. The exploitation of asteroseismic data from TESS (NASA) is being carried out in the context of the TESS Asteroseismic Science Consortium (TASC), in which our team is strongly involved, with representation on the Steering Committee and co-leadership of one of its working groups. Furthermore, our team continues to be strongly involved in the preparation of the stellar science component of PLATO (ESA), with responsibilities in the design, implementation, and validation of sections of the Stellar Analysis System (SAS) pipeline, leading work packages in the framework of both the PLATO Science Management (PSM) and the PLATO Data Center (PDC). The team also continued to actively contribute to the Ariel (ESA) consortium, in the context of which we coordinate a working group responsible for determining the fundamental parameters of the mission's target stars.

Our research also included studies of low-mass stellar and substellar populations in young clusters and star-forming regions in the Milky Way, with the goal of understanding the main formation channels of brown dwarfs and free-floating planetary-mass objects, together with their potential dependence on the star-forming environment.

Stellar Populations research line. This research line focuses on the precise characterization of solar-type and red-giant stars, which provides valuable information that can be readily applied to several areas of research, including Galactic archaeology and Galactic chemical evolution. We continued to be actively engaged in several international consortia, including ANDES@ELT, the Maunakea Spectroscopic Explorer (MSE), ESPRESSO@VLT, and NIRPS@ESO 3.6-m Telescope. We highlight the team's involvement in ANDES@ELT, where we play a role in the development of the scientific priorities and the definition of top level requirements for the instrumentation.

Scientific highlights for 2024*

Highlight #1. Automated sunspot detection and solar wind forecasting

The implementation of automated methods for sunspot detection is essential to achieve better objectivity, efficiency, and accuracy in identifying sunspots and analyzing their morphological properties. A sought-for application is the contouring of sunspots. In Bourgeois et al. (2024, Solar Physics, 299, 10), the authors constructed sunspot contours from Solar Dynamics Observatory (SDO)/Helioseismic and Magnetic Imager (HMI) intensity images by means of an automated method based on the application of mathematical morphology (see Fig. 1). The method was applied to high-resolution data (SDO intensitygrams), with a good agreement obtained between the measured sunspot areas and those provided by two standard reference catalogs. The method appears to be robust for sunspot identification, and the analysis suggests its application to more complex and irregular-shaped solar structures, such as polarity inversion lines inside delta-sunspots.

Figure 1: Sunspot contours overlaid on an SDO/HMI intensity image. From Bourgeois et al. (2024).

Solar wind forecasting is a core component of Space Weather, a field that has been the target of many novel machine-learning approaches. The continuous monitoring of the Sun has provided an ever-growing ensemble of observations, facilitating the development of forecasting models that predict solar wind properties on Earth and other celestial objects within the solar system. This enables us to prepare for and mitigate the effects of solar wind-related events on Earth and space. The performance of some simulation-based solar wind models, however, depends heavily on the quality of the initial guesses used as initial conditions. In Barros et al. (2024, Engineering Applications of Artificial Intelligence, 133, 108266), the authors aimed at improving the accuracy of these initial conditions by employing a Recurrent Neural Network model. The study's findings confirmed that Recurrent Neural Networks can generate better initial guesses for the simulations, resulting in faster and more stable simulations.

We note that both these works were led by PhD students in our group.

Highlight #2. Advances in observational and theoretical asteroseismology

Fueled by space photometry, asteroseismology is vastly benefiting the study of cool main-sequence stars, which exhibit convection-driven solar-like oscillations. Even so, the tiny oscillation amplitudes in K dwarfs continue to pose a challenge to space-based asteroseismology. A viable alternative is offered by the lower stellar noise in Doppler observations. In Campante et al. (2024, Astronomy & Astrophysics, 683, L16), the authors presented the definite detection of solar-like oscillations in the bright K5 dwarf ε Indi based on time-intensive observations collected with the ESPRESSO spectrograph at the VLT, thus making it the coolest seismic dwarf ever observed. The peak amplitude of radial modes is 2.6±0.5 cm/s, or a mere 14% of the solar value (see Fig. 2). Measured mode amplitudes are ∼2 times lower than predicted from a nominal L/M scaling relation and favor a scaling closer to (L/M)^1.5 below 5500 K, carrying important implications for our understanding of the coupling efficiency between pulsations and near-surface convection in K dwarfs. This detection conclusively shows that precise asteroseismology of cool dwarfs is possible down to at least the mid-K regime using next-generation spectrographs on large-aperture telescopes, effectively opening up a new domain in observational asteroseismology. A press release was issued by IA accompanying the publication of this article.

Figure 2: Infographics comparing ε Indi with the Sun. Credit: Paulo Pereira (IA).

Sharp structural variations induce specific signatures on stellar pulsations that can be studied to infer localized information on the stratification of a star. This information is key to improve our understanding of the physical processes that lead to these structural variations and how to model them. In Cunha et al. (2024, Astronomy & Astrophysics, 687, A100), the authors revisited and extended the analysis of the signatures of different types of buoyancy glitches in gravity-mode and mixed-mode pulsators presented in earlier works, including glitches with step-like, Gaussian-like, and Dirac-δ-like shapes. In particular, they provided analytical expressions for the perturbations to the periods and showed that these can be reliably used in place of the expressions provided for the period spacings, with the advantage that the use of the new expressions does not require modes with consecutive radial orders to be observed. They further discussed the impact on the glitch signature of considering a glitch in the inner and outer half of the g-mode cavity, emphasizing the break of symmetry that takes place in the case of mixed-mode pulsators.

Highlight #3. Towards improved stellar models

Modeling of chemical transport mechanisms is crucial for the accurate characterization of stars. Atomic diffusion is one such process, being commonly included in stellar models. However, it is usually neglected for F-type or more massive stars because it produces surface abundance variations that are unrealistic. Additional mechanisms to counteract atomic diffusion must therefore be considered. It has been demonstrated that turbulent mixing can prevent excessive variation in surface abundances, while it can be calibrated to mimic the effect of radiative acceleration on iron. In Moedas et al. (2024, Astronomy & Astrophysics, 684, A113), the authors evaluated the effect of calibrated turbulent mixing on the characterization of a sample of F-type stars and how stellar parameter estimates compare with those obtained when chemical transport mechanisms are neglected. They found a greater dispersion in the inferred values of mass, radius, and age for the more massive stars in their sample due to the absence of atomic diffusion in one of the two model grids used. This work ultimately shows that a proper modeling of microscopic transport processes is crucial for the accurate estimation of stellar fundamental parameters, which is not only true for G-type stars but also for F-type stars. This work was led by a PhD student in our group.

Highlight #4. Exploitation of James Webb Space Telescope (JWST) data

Two group members have succeeded in getting JWST programs approved as PI/Co-PI: Sílvia Vicente as PI during General Observer (GO) Cycle 2; and Koraljka Mužić as Co-PI during GO Cycles 1 and 3. As a result, exploitation of JWST data by group members has seen its first published results in 2024.

Most low-mass stars form in stellar clusters that also contain massive stars, which are sources of far-ultraviolet (FUV) radiation. Theoretical models predict that this FUV radiation produces photodissociation regions (PDRs) on the surfaces of protoplanetary disks around low-mass stars, which affects planet formation within the disks. In Berné et al. (2024, Science, 383, 988), which includes group member Sílvia Vicente, the authors reported James Webb Space Telescope and Atacama Large Millimeter Array observations of a FUV-irradiated protoplanetary disk in the Orion Nebula (see Fig. 3). They quantified the mass-loss rate induced by the FUV irradiation and found that it is sufficient to remove gas from the disk in less than a million years. This is rapid enough to affect giant planet formation in the disk. A research note (Portuguese only) was issued by IA accompanying the publication of this article.

Figure 3: Hubble image of the Orion Nebula and zoom-in on the d203-506 protoplanetary system with JWST. Credit: NASA/STScI/Rice Univ./C. O'Dell et al./O. Berné, I. Schrotter, PDRs4All.

The discovery and characterization of free-floating planetary-mass objects (FFPMOs) is fundamental to our understanding of star and planet formation. In Langeveld et al. (2024, The Astronomical Journal, 168, 179), which includes group members Koraljka Mužić and Daniel Capela, the authors reported results from an extremely deep spectroscopic survey of the young star cluster NGC 1333 using Near-InfraRed Imager and Slitless Spectrograph (NIRISS) wide field slitless spectroscopy on the James Webb Space Telescope. They discovered six new candidates with L-dwarf spectral types that are plausible planetary-mass members of NGC 1333, with estimated masses between 5 and 15 M_Jup. They did not find any objects later than mid-L spectral type (≲ 4 M_Jup). The paucity of Jupiter-mass objects, despite the survey's unprecedented sensitivity, suggests that their observations reached the lowest-mass objects that formed like stars in NGC 1333. Their findings put the fraction of FFPMOs in NGC 1333 at ∼10% of the number of cluster members, significantly more than expected from the typical log-normal stellar mass function. A research note (Portuguese only) was issued by IA accompanying the publication of this article.

Highlight #5. Hosts of 3 international events on Asteroseismology and Space Weather

We organized the 8th TESS/15th Kepler Asteroseismic Science Consortium Workshop (TASC8/KASC15) in Porto. TASC8/KASC15 served as a platform for the comprehensive review and discussion of the latest findings in the field of asteroseismology, with the focus being placed on new ways of improving the physics in stellar models (aspects such as convection, angular momentum transport, magnetic fields, and mixing were thoroughly addressed). There were a total of 214 registered participants, of which 189 on-site and 25 online. Conference proceedings were published here.

The conference was followed by a Doctoral School, again organized by members of our research group. The Porto Summer School on Asteroseismology (PSSA) brought together 50 students and 13 invited lecturers. Students from institutes from all over the world attended the School, namely, from Europe (33), Oceania (6), North America (5), Asia (5), and Africa (1). School proceedings were published here.

Furthermore, we co-organized the European Space Weather Week 2024 (ESWW 2024) in Coimbra. The European Space Weather Week is the main annual event in the European Space Weather and Space Climate calendar. It began as a forum for the European Space Weather community and has since grown into an international event with global attendance. There were more than 600 registered participants at the event.

Happy 2025!!!

* When applicable, text from the corresponding journal abstract is used (and adapted).