Resumen:

We are in the monumental era of large optical and infrared spectroscopic surveys (e.g. Gaia, SDSS, LAMOST, Gaia-ESO, GALAH, WHT-WEAVE). Fiber spectroscopy allows now to obtain high quality spectra for hundreds of stars at a time, and the use of infrared detectors opens the field for the study of young stars in star forming regions within our reach. The pre-main sequence phase of stellar evolution is not as well characterized as the main-sequence or the red-giant branch. Young star spectra are affected by dust extinction from the parental cloud gas, by the presence of circumstellar material and diverse effects associated with accretion and variability. Using spectra from the SDSS APOGEE-2 and Milky Way Mapper surveys, we are building up one of the largest systematic census of pre-main sequence stars (see e.g. Román-Zúñiga et al. 2023, Kounkel et al. 2023). One of our main goals is to increase the precision in the parameter determination for young stars. For this goal we participate in the development of tools (e.g. neural networks: APOGEE Net - Sprague et al 2022, unsupervised model fitting: TONALLI- Adame et al. 2023) aiming to reduce model dependent discrepancies and degeneracies in the $\mathrm{T_{eff}}$, $\log{g}$, [Fe/H] space. We are also applying methodologies to understand and reduce the effects of extinction and disks in spectral classification (Zepeda et al i.p.) and to determine precise metal abundances with BACCHUS (Lopez-Valdivia et al i.p.) We discuss these methodologies, show our current efforts and results in the construction of better young star catalogs, and we will share our plans to use them along with kinematic data (e.g. combining APOGEE radial velocities with proper motions from Gaia), to understand the evolution of nearby young star clusters. We will also discuss strategies and details to be considered (e.g. model dependencies) in current big data treatments.

Resumen:

T Tauri stars (TTS) are young, low-mass stars ($< 2M_{\odot}$) in pre-main sequence phase, considered precursors to planetary systems. Their youth is robustly indicated by the presence of LiI absorption line in their optical spectra. TTS can be divided into two categories: classical TTS (CTTS) displaying $H_{\alpha}$ line emission due to a circumstellar disk, and weak-line TTS (WTTS) where the $H_{\alpha}$ line is relatively weak and attributed to chromospheric activity. The Chinese Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) provides a large amount of spectra in the wavelength range of 3700 to 9000 \AA, with approximately 10 million low-resolution spectra ($R\approx1800$) available in DR8. In this study we present the results of detecting and classifying TTS in the Orion Star Forming Complex using machine learning (ML) techniques and LAMOST spectra. We used over 5000 kinematic candidates selected by using proper motion and parallax data from the GAIA DR3 database. Our main training sample consisted of 342 TTS from the Ori OB1a substellar association, as reported by Hernandez et al. (2023). To ensure the robustness of the classifiers, we also included a set of non-TTS (field stars) in the training sample. Automatic equivalent width measurements were performed on 18 spectral features, including $H_{\alpha}$ and LiI lines, as well as TiO and VO bands. We implemented two ML classifiers: a binary model to classify whether a target is a TTS or not, and a multiclass model to assign a spectral type to the detected TTS. We employed the Monte Carlo method for error propagation to determine probabilities during the classification process. To assess the classifiers' performance, we used the F1 score, which yielded a value of 0.91. In a preliminary classification of 766 stars, we successfully identified 249 TTS. These TTS candidates were visually inspected, confirming an accuracy of 0.94.

Resumen:

Flares and superflares are eruptive events related to magnetic reconnection in the star’s magnetosphere where, a high burst of energy and electromagnetic radiation is released. Flares can affect the formation and early evolution of planets close to their host star, especially during their formation around Young Stellar Objects (YSOs). In this work, we present the most numerous survey of flare detection on YSOs up to date, with the majority of them classified as T Tauri Stars (TTSs). Our sample belongs to the Taurus region at $\sim$140 parsecs with an age range of $\sim$1-15 Myr. We studied flare characteristics such as duration, peak amplitude, bolometric luminosity and energy released. We examined 643 light curves using TESS observations with a 2 minutes cadence in sectors 43 and 44. As a result, we detected 1704 flares over a sample of 346 stars, representing almost 67\% of the known low-mass members of Taurus. Also, we estimated the mean bolometric flare energy and amplitude for stars with multiple flares. Our measurements of flares in the optical range were related with the luminosity in Rx and UV bands, finding correlations between them. From LAMOST and FAST spectra, we measured H$\alpha$ equivalent widths of the sampled stars and later classified them by the presence of accretion disk. Additionally, we calculated the R$_{HK}$ factor and compared the chromospheric activity with the flare features alongside the age, mass, and type of TTSs.

Resumen:

The transport of angular momentum and matter is essential for the evolution of astrophysical disks, particularly protostellar disks. Recently, several works investigated the role of non-ideal MHD effects in those objects and their impact on angular momentum transport. On the other hand, the damping of Alfvén waves was already proposed as a viable mechanism for disk heating, increasing its ionization fraction and, consequently, the region of the disk subject to Magneto-Rotational Instability. In this work, we analyze the role of the resonant damping of surface Alfvén waves in protostellar disks with a local approximation, which allows us to describe the radial shear associated with the differential rotation as a linear velocity. We consider the Alfvén waves propagation and dissipation/amplification in the presence of both viscosity and dissipative non-ideal MHD effects. In general, we find that the effectiveness of these processes is independent of the dissipation coefficients, as already obtained in previous works, and, depending on the propagation properties of the wave, can lead to damping or the development of instability. We also quickly compared the damping rate obtained with our mechanism and the turbulent damping, already studied in the literature as a source for non-thermal disk heating. We found that, under reasonable conditions, the resonant damping is more effective than the turbulent damping in heating the disk, indicating that the proposed mechanism can decrease the dead zone extent. We are now performing a more robust numerical analysis to fully describe the effect of the resonant damping of surface Alfvén waves over the ionization fraction and transport of angular momentum in those disks.

Resumen:

Classical Be stars are a subset of main-sequence B-type stars associated with viscous Keplerian decretion disks responsible for their emission lines. In addition, fast rotation is a fundamental ingredient for the existence of Be stars, as they are the fastest among non-degenerate stars. To date, three scenarios try to explain the spin-up mechanism and the existence of Be stars: Be stars are born as fast rotators; the spin-up would occur during the evolution of a binary system; or they would start as slow rotators, increasing their rotation throughout their evolution. Even though all scenarios may play a role in the origin of Be stars, discriminating the relative prevalence of each scenario is a key step towards a better understanding of these objects. In this talk, I will present the analysis of BVRI+Halpha photometry of NGC\,330, a young stellar cluster in the Small Magellanic Cloud (SMC), observed at the 4-m SOAR telescope (Chile) using the SOAR Adaptive Optics Module Imager (SAMI). Thanks to the depth and the high angular resolution (0.30''-0.40'') of the images, the entire massive stellar population up to early A-type stars are disentangled and resolved at SMC distances. Alongside well-consolidated models of B+Be stars (BeATLAS), SAMI observations provide an excellent dataset to search, identify and classify Be stars. With supervised machine learning classification models, a Be/(Be+B) fraction of approximately 50\% was found for stars up to V=18.5 mag, and 20\% for the whole B spectral type range, a considerably larger fraction of Be candidates than previously reported from high-resolution spectroscopic works in the literature ($\approx$30\%). Combined with the analysis of further young open clusters in the Magellanic Clouds, this survey will provide crucial evidence for understanding the Be phenomenon as a function of metallicity and age, putting critical observational constraints on the origin of these massive stellar objects.

Resumen:

In this study, we analyzed a sample of 24 Be stars located in the northern galactic region. Utilizing the IRAF software, we measured crucial emission line fluxes, including Br-Alpha, Pf-Gamma, and Hu 14 in the L band of the spectrum. These measurements enabled us to construct the observational Lenorzer diagram, providing valuable insights into the star group. To gain further understanding, we employed the HDUST code to generate a grid of BeAtlas stellar models. By investigating correlations among five parameters in these disk models, we revealed a positive association between n-Sigma (parameter potential law-surface density), n-M (parameter potential law-Mass of Star), and Sigma-M parameters. This observation suggests that disks with higher or lower Sigma density are linked to stronger or weaker recombination processes, respectively. Consequently, we could classify disks into optically thick or thin regions based on this information. Interestingly, the parameter associated with disk inclination does not appear to be decisive in model selection. This finding led to a restriction of parameters in the models present in Be star disks. In conclusion, our oral presentation offers a comprehensive approach to understanding and characterizing circumstellar disks in Be stars, providing relevant information for future research on these dynamic astronomical systems.

Resumen:

In the search for a more efficient modeling of complex chemistry in numerical simulations new studies have adopted a new method known as post-processing routines. A commonly used approach to simulate chemistry, particularly useful when considering demanding simulations where on-the-fly (OTF) chemistry models mean more computational resources which cannot be approached. This method enables the simulation of chemistry over the raw simulation’s dynamic history, alleviating the use of computational resources compared to OTF chemistry while accurately capturing the evolution of chemical species. In this study we aim to validate the efficiency of post-processing routines presented in Bovino et al. 2019 (ApJ, 887, 224). By employing the chemistry package KROME to analyze and recover the abundances of different chemical species over the simulation data obtained from Bate 2019 (MNRAS, 484, 2341), specifically, investigating the solar metallicity scenario. The primary objective of this project is to comprehensively study the evolution of complex chemistry in circumstellar and protoplanetary discs. By meticulously tracing the evolution of the simulations' discs and analyzing the chemistry on different samples, we can accurately replicate the dynamical and chemical evolution before and after the formation of stars in the simulation. Through this research, we anticipate gaining deeper insights into complex chemical processes that shape the early stages of protoplanetary systems.

Resumen:

NGC 6357, a star-forming region located at a distance of approximately $1.7$ kpc from the Sun, is composed of giant molecular clouds as well as three prominent star clusters. It also hosts three HII regions, very massive stars and thousands of YSOs in different evolutionary stages. We present VVVX PMs for $1681$ young stars in this region and observe that they cluster around two mean PM values, giving rise to two kinematically different populations of stars. One of them is spatially related to dusty regions and its components have projected movements along the filaments of the area. These suggests that it is a younger population and could be related to a triggered star formation process developed in the molecular material of the region and powered by the expansion of the ionized gas. In addition, due to accretion episodes and because of the presence of cool spots on their surface, YSOs are known to be highly variable in terms of brightness. Using VVVX data, with a baseline of approximately eight years, we studied their flux changes: a unique follow-up of these stars in this region. Using the $K_s$-band, we can overcome its extreme levels of extinction and compile, for the first time, a catalogue of $775$ light curves associated to young stars. Each of them has been classified according to the $Q$ and $M$ values, related to the physical process that dominates the variability. We linked these categories to the different kinematical populations and observe that hints of an age difference can also be observed through the different classes.