The star-forming core ahead of HH 80N: studying the interaction with a parsec scale jet

Abstract

The region GGD 27, located in Sagittarius at a distance of 1.7 kpc, is an active star forming region. The most well-known observational signature of this region, the HH 80/81/80N jet, is one of the largest collimated jet system known so far, spanning over a total length of about 5 pc.

Ahead of the Herbig-Haro object HH 80N, assumed to be the northern head of the jet, there is a dense core of 0.3 pc in size and mass of 20 M (HH 80N core) that exhibits peculiar chemistry, possibly induced by the UV radiation incoming from the HH object. In addition, the HH 80N core has star forming signatures, namely a bipolar molecular outflow and supersonic infall velocity (0.6 km s(-1)), which differs from what standard contracting core models predict. This rises the question whether the HH 80/81/80N jet has triggered or sped up the star formation process in this core. VLA cm continuum observations carried out as a preliminary part of this thesis show that the jet could expand further away than HH 80N. This makes possible the cause-and-effect relationship between the jet and the star forming signatures of the core. In this thesis we study the possible influence of the HH 80/81/80N jet on the properties of the HH 80N core in several approaches.

We first derive the physical and chemical properties of the large envelope of the HH 80N core using data obtained with the BIMA and IRAM 30m telescopes. All the detected BIMA molecular species arise in a ring-like structure contracting at 0.6 km s(-1). This morphology is likely the result of strong molecular depletion occurring at the inner part of the core (r < 0.1 pc), when the gas reaches densities > 10(5) cm(-3), as derived from a multi-transition analysis of CS and SO lines observed with IRAM 30m. Despite the overall morphology and kinematic similarity between the different species, there is significant molecular differentiation along the ring-like structure. Part of this differentiation may be caused by the UV irradiation of the nearby HH 80N object that illuminates the part of the core facing HH 80N, which results in an abundance enhancement of some species.

Given the peculiar kinematic properties of the HH 80N core, we analyze the continuum emission of the core using self-consistent standard models of protostellar collapse to see if the dynamical evolution of this core departs from the predictions of classic models of star formation.

We find that a young protostar (IRS1) surrounded by a slowly rotating collapsing envelope plus a circumstellar disk provide a good fit to our data observed at several bands. However, this model implies that the core has a static envelope at radii > 1.5 x10(4) AU, apparently in contradiction with the contracting ring-like structure that has 10(6) AU of radius. Besides, the continuum maps at 350 m and 3.5 mm reveal additional clumps independent from the main continuum peak at IRS 1. Therefore, we propose a scenario where IRS1 has infall motions with no connection with the kinematics of the large envelope of the HH 80N core. The later is possibly affected by the HH 80/81/80N jet and exhibits fragmentation that may lead to the formation of several protostars.

In order to disentangle the gas motions belonging to the protostellar collapse of IRS1 from those associated to large scale kinematics we analyze high-density tracers (NH(3), HN(13)C, NH(2)D), observed at high angular resolution with VLA and PdBI, aiming to recover kinematic information in parts of the HH 80N core where other molecules are depleted. The results question the scenario of the contracting molecular ring. Instead, the velocity gradients seen with high density tracers suggest that part of the core is being swept by the HH 80/81/80N jet in the process of dissipating energy (e.g. though generation of magneto hydrodynamic waves in the medium). Future simulations of the events occurring in the HH 80N region are paramount to confirm whether the formation of IRS1 was caused by the interaction of the HH 80N core with the HH 80/81/80N outflow.

La font amb formació estel_lar per davant d'HH 80N: estudi de la seva interacció amb un jet a escala de parsecs Josep-Maria Masqué Saumell - Universitat de Barcelona

L'objecte Herbig-Haro HH 80N representa l'extrem nord del sistema HH 80/81/80N, el jet estel.lar més llarg i col.limat conegut fins ara, que es troba a 1.7 kpc de distància a la regió de Sagitari. Observacions al voltant d'HH 80N revelen que a 0.3 pc de l'objecte HH hi ha un nucli dens (font d'HH 80N) de 0.3 pc de grandària i de massa 20 M, que mostra una química i cinemàtica peculiars i, a més, té un protoestel embegut al seu interior (IRS1).

Donades aquestes característiques existeix la possibilitat de que les propietats de la font d'HH 80N hagin estat alterades pel jet d'HH 80/81/80N. L'objectiu d'aquesta tesi és l'estudi de la influència dels jets protoestel_lars centrant-nos en el cas particular de la regió d'HH 80N.

Primer, trobem que la radiació ultraviolada provinent d'HH 80N és capaç d'alterar la química de la part de la font més exposada a l'objecte HH. Segon, malgrat una possible influència dinàmica del jet a la font d'HH 80N, el procés de col.lapse d'IRS1 es pot explicar dins el marc clàssic de formació estel.lar. No obstant, trobem que gran part del material de la font d'HH 80N constitueix un embolcall independent d'IRS1, el qual es fragmenta en vàries condensacions de gas. Possiblement, el gas d'aquest embolcall extens ha estat parcialment escombrat per ones magneto-hidrodinàmiques produïdes pel pas del jet. Aquest tipus d'interacció podria provocar inestabilitats a la font d'HH 80N donant lloc al naixement induït d'estels. Concloem que futures simulacions magneto-hidrodinàmiques són necessàries per confirmar que la formació d'IRS1 ha estat provocada per la interacció del jet HH 80/81/80N amb la font d'HH