Resumen:

Using interferometric radio observations (particularly long baseline interferometry - VLBI), it is possible to accurately constrain the mass of young stars in binary and multiple stellar systems. This provides strong constraints for stellar evolution models since the location of the stars in the HR diagram can then be compared directly with the tracks for the appropriate masses. In this talk, I will present several young star mass measurements based on VLBI observations and discuss the implications for evolutionary models. I will particularly focus on intermediate mass stars (M > 2 Sun) where constraints are rare. I will also briefly discuss mass constrains for long period multiple systems based on conventional radio interferometry, which are highly complementary of VLBI measurements by providing access to a different region of the parameter space.

Resumen:

Chemically peculiar Ap and Bp stars host strong large-scale magnetic fields in the range of $200$~G up to $30$~kG, which are often considered to be the origin of fossil magnetic fields. We assess the evolution of such fossil fields during the star formation process and the pre-main sequence evolution of intermediate stars, considering fully convective models, models including a transition to a radiative protostar and models with a radiative core. We also examine the implications of the interaction between the fossil field and the core dynamo. We employ analytic and semi-analytic calculations combined with current observational constraints. For fully convective models, we show that magnetic field decay via convection can be expected to be very efficient for realistic parameters of turbulent resistivities. Based on the observed magnetic field strength - density relation, as well as the expected amount of flux loss due to ambipolar diffusion, it appears unlikely that convection could be suppressed via strong enough magnetic fields. On the other hand, a transition from a convective to a radiative core could very naturally explain the survival of a significant amount of flux, along with the presence of a critical mass. We show that in some cases, the interaction of a fossil field with a core dynamo may further lead to changes in the surface magnetic field structure. In the future, it will be important to understand in more detail how the accretion rate evolves as a function of time during the formation of intermediate-mass protostars, including its impact on the protostellar structure.

Resumen:

The solar neighborhood is populated with Nearby ($\textless 200$ pc) Young ($\textless 100$ Myr) stellar co-Moving Groups (NYMG; e.g. Torres et al., 2006; Gagné et al., 2018; Kerr et al., 2021). Although their origin is a matter of debate, their ages could be one order of magnitude smaller than their dynamical times around the Galaxy which supports the idea of these groups as remnants of stellar populations dispersing in the galactic disc.\\ The NYMGs are ideal candidates for the observational study of the Initial Mass Function (IMF) because of their proximity and low extinction, and because their youth guarantees that only their very massive stars have evolved out of the main sequence. Then, their mass distribution equals the IMF in most of the stellar and sub-stellar mass ranges. However, only a few determinations of the IMF of these groups have been done so far (e.g. Gagné et al., 2017) due to the observational challenge of their wide spatial distribution. \\ In this talk, I will present a robust determination of the IMF of 5 NYMGs in the mass range $0.04 \textless M/M_\odot \textless 5$. The selection of members was made with a clustering code based on the DBSCAN algorithm (Ester et al., 1996) for the detection of NYMGs using data from Gaia DR3 (Gaia Collaboration et al., 2022) astrometry and photometry. I will present the validation process of this tool which allowed us to quantify the fraction of completeness and contamination from old stars in each of the detected NYMGs. Finally, I will discuss how the resulting IMF compares with previous determinations for the NYMGs and the solar neighborhood, and its interpretation in the context of the dispersion of the NYMGs.

Resumen:

The Canis Major Association OB1/R1 (CMa) is an interesting region to study the effects of supernovae (SN) in star formation. Evidence indicates that at least three such events affected the environment and evolution processes of young stellar objects in the CMa. This ongoing work aims to unpuzzle the star formation history of CMa and improve the survey of young low-mass stars in the region, especially toward the star FZ CMa. Probably this stellar population is mixed, lying between two clusters in different age ranges: one group associated with the star Z CMa ($\leq$5 Myr) and the other associated with the star GU CMa ($\geq$10 Myr). The survey is still incomplete, although it is crucial to investigate the possibility of triggering and/or inhibiting star formation by SN or even the early destruction of circumstellar disks. To characterize this population, we analyzed optical spectra (R $\sim$ 4400), acquired with the Gemini South telescope in the multi-object mode, searching for typical features of this evolutionary phase, such as Li absorption (6708 \AA) and H$\alpha$ emission. We have over a hundred candidate stars available, with $\sim$ 20\% of them being counterparts of X-ray sources. We present the spectral classification and estimates of physical parameters of these stars derived from spectral analysis. By using the equivalent width of the H$\alpha$ line, we have distinguished in our sample the Classical T Tauri (CTT), which are disk-bearing stars, from the Weak-line T Tauri (WTT) that do not have an active accretion disk. Previous results based on infrared sources indicate a low fraction of disk-bearing stars compared to other star-forming regions at the same age. In addition, we explored multi-band photometric data by constructing different color-color diagrams. These data have been compared to evolutionary models to identify color excesses that are related to the accretion process.

Resumen:

Our previous works, based on X-ray and infrared data, we found that Canis Major OB1 (CMa OB1) is a Galactic stellar association having a very intriguing star-formation scenario, which probably had at least three star formation episodes. In order to better understand the association stellar population and its star formation history, I recently conducted a study using a clustering code that employs five-dimensional data from the Gaia DR2 catalog to identify physical groups in the vicinity of CMa OB1 and to obtain their astrometric parameters. Additionally, we estimated the age of these groups using two different isochrone-fitting methods. With this, we confirm the existence of 15 star groups with distances between 570 and 1650 pc, of which 10 are previously known clusters and five are new open cluster candidates. We identified that four younger groups (< 20 Myr), CMa05, CMa06, CMa07 and CMa08, are CMa OB1 contents. While CMa06 coincides with the star formation region very well studied by our group, CMa R1, CMa08, one of the new candidate clusters, may be the progenitor cluster of runaway stars that indicate the existence of different star formation episodes in the association. Since the youngest group, CMa06, is still immersed in the remaining material of the molecular cloud associated with the Sh 2-262 nebula, it continues to form stars, on the other hand CMa05, CMa07, and CMa08 seem to be in more evolved stages of evolution, with no recent star-forming activity. The results of this work fit well in a monolithic scenario of star formation, with a common formation mechanism, and having suffered multiple episodes of star formation. This suggests that the hierarchical model alone, which explains the populations of other parts of the same association, is not sufficient to explain its whole formation history.

Resumen:

The main processes that trigger the collapse and fragmentation of giant molecular clouds (GMCs) in star-forming regions are poorly understood. Collisions between giant molecular clouds (GMCs) have been proposed as a mechanism to compress gas to trigger the birth of massive star clusters and associations and thus be potentially relevant to the formation of most stars in galaxies. The spatial distributions of dense gas cores within protoclusters generated by such collisions may encode important diagnostic information about the process and the environmental properties of the parent clouds. To investigate this, we carried out 3D numerical simulations of magnetized turbulent GMCs. Non-colliding cases were also considered. The projected 2D mass surface density structures of the clouds, including cases after applying simulated ALMA observations, were analyzed. In particular, dense cores were identified with the dendrogram method, and the Minimum Spanning Tree (MST) of the cores was computed. We compared the results from clouds with different initial magnetic (B-) field strengths, ranging from 10 to 50 micro G. The number and mass fraction of dense cores are suppressed in the more strongly magnetized and non-colliding cases. We consider various MST statistics, including the Q parameter, how these evolve during the simulations, and their potential ability to diagnose magnetic and collisional conditions. We also examine mass segregation, including detailed properties of the most massive cores, which relates to the question of whether or not massive stars tend to form in more central locations in protoclusters. Finally, we study the properties of the filaments within and around the protoclusters, including their widths, mass per unit lengths, energy balance, and magnetic field strengths and orientations.