Observational and theoretical perspective of massive star formation

Abstract

In this thesis, we are aimed to better understand the massive star formation process paying special attention to the role of the magnetic field. To do this, we will carry out a multi-scale analysis with a double approach, theoretical and observational.

a) The role of the magnetic field in the fragmentation process: the case of G14.225-0.506

In this first work, we study the fragmentation of an infrared dark cloud that has a filamentary structure and two hubs. We will pay special attention to the magnetic field present in the environment of the hubs and we will try to relate it to the different levels of fragmentation that these hubs present. In order to carry out this study, we present the result of the CSO observations at 350 um, towards both hubs, North and South, in the infrared dark cloud G14.225-0.506 (from here G14.2). We also show the analysis of polarization and intensity gradient making use of the method developed by Koch et al. 2012, Koch et al. 2012b.

In the N-hub we find a magnetic field with a uniform distribution along the east-west direction. However, in the southern hub the B-field shows a bimodal distribution. The intensity-gradient in the N-hub shows a single local minimum. In the S-hub, the intensity gradient reveals two minima reflecting the bimodal distribution of the magnetic field where each component points to each of the minima of the intensity gradient.

Analysis of the maps |delta| and Sigma_B in the N-hub indicates that, in the vicinity of the hub, gravity dominates the magnetic field. We have also estimated the intensity of the magnetic field finding higher values in the N-hub than in the S-hub. This supports the idea that the different levels of fragmentation exhibited by hubs depend on differences in magnetic field.

b) Modeling the accretion disk around the high-mass protostar GGD 27-MM1

In this second project, we descend to accretion disk scales to understand how massive stars form and evolve. In this work we have used ALMA observations at a wavelength of 1.14~mm with very high angular resolution that resolve the disk around the massive star GGD27-MM1. Motivated by the similarity of this system to those found in low mass, we have modeled the emission of the disk using the models developed by D'Alessio et al. 2006 for low mass stars. The main objective is to investigate whether the assumptions that are valid for disks around low-mass stars could be extrapolated to the case of massive stars.

As a result we have found a very massive disk of about ~5 Msun which represents around 25 % of the stellar mass. This mass is consistent with the calculated dynamic mass. The disk has a radius of ~170 au with a 49º of inclination. We have compared the temperature and density structure obtained with our model with potential functions and show that the GGD~27--MM1 system is a potential template for future similar studies in other high-mass protostars. Specifically, we have found a flared disk with a maximum scale height of ~13 au and a temperature profile that goes from ~150 K on the outside of the disk to ~ 1400 K on the inner edge of the disk. Analysis of the Toomre parameter Q evaluated at the midplane temperature of the disk indicates that the disk is stable up to a radius 100 au. This work shows that D'Alessio's models can be used as a first approximation and also obtain various observational predictions.

En esta tesis, pretendemos contribuir a explicar el proceso de formación de estrellas masivas prestando especial atención al papel del campo magnético. Para ello, realizaremos un análisis multiescala con una doble aproximación, teórica y observacional. Por un lado investigaremos el proceso de acrecimiento a través de un disco de acrecimiento entorno a una estrella de alta masa.

a) El papel del campo magnético en el proceso de fragmentación: el caso de G14.225-0.506

En este primer trabajo, estudiamos la fragmentación de una nube oscura infrarroja que presenta una estructura filamentaria y dos hubs. Prestaremos especial atención al campo magnético presente en el entorno de los hubs y trataremos de relacionarlo con los distintos niveles de fragmentación que presentan estos hubs. Para poder llevar a cabo este estudio presentamos el resultado de las observaciones del CSO a 350 masas solares, hacia ambos hubs, Norte y Sur, en la nube oscura infrarroja G14.225-0.506 (desde aquí G14.2). También mostramos el análisis de la polarización y gradiente de intensidad haciendo uso del método desarrollado por (Koch et al. 2012, Koch et al. 2012b).

b) El modelado del disco de acrecimiento entorno a la protoestrella masiva GGD 27-MM1

En este segundo proyecto, descendemos hasta escalas de discos de acrecimiento para entender como las estrellas masivas se forman y evolucionan. En este trabajo hemos usado observaciones de ALMA a una longitud de onda de 1.14 mm con muy alta resolución angular (aproximadamente 40 mili-segundos de arco) que resuelven el disco entorno a la estrella masiva GGD27-MM1. El disco se dispone perpendicularmente al espectacular chorro de 10 pc de longitud que emana de la fuente, conocido como HH80-81. Motivados por el parecido de este sistema a los que encontramos en baja masa, hemos modelado la emisión del disco haciendo uso de los modelos desarrollados por D'Alessio et al. 2006 para estrellas de baja masa.