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An astrochemical study of the early evolutionary stages of sub-stellar mass objects

Periodic Reporting for period 1 - ABDES (An astrochemical study of the early evolutionary stages of sub-stellar mass objects)

Berichtszeitraum: 2015-09-01 bis 2017-08-31

Over the last decade, extensive observational and theoretical effort has been invested to understand the early stages of evolution of Sun-like stars. Similar studies are still very sparse for stars at the very low-mass end, with masses less than one-tenth of the mass of the Sun. Often termed as the least massive known ‘stars’, brown dwarfs (BDs) are sub-stellar objects that have insufficient mass to burn hydrogen in their cores and shine like stars. Yet, these faint astronomical objects are found to be as numerous as Sun-like stars in various star-forming regions, thereby making them an equally vital constituent of our Galaxy. Any research on star formation in the Milky Way is thus incomplete without gaining a comprehensive knowledge on the early stages of evolution of the least massive but ubiquitous component of the Galaxy.

My MSCA-IF project, named ABDES, was an extensive observational and theoretical investigation of the diverse properties of BDs during their early Class I stage of evolution. To conduct this study, I have combined high-quality multi-wavelength observations using high-class telescopes and instruments, and new, state-of-the-art physical and chemical models developed exclusively for early-stage BDs. This has allowed a successful completion of the different tasks in the project.

The objectives and methodologies of the project were:

a) to combine multi-wavelength observations with state-of-the-art models and conduct an in-depth study of the internal physical and chemical structure, kinematics, dynamics, and chemical composition in Class I BDs;
b) to investigate the fundamental role played by accretion and outflow processes in shaping the evolution of Class I BDs using optical/near-infrared spectra and images.

This survey has looked into the analogies in the various characteristics of BDs and Sun-like stars during their early evolutionary stages. The fundamental questions that the project aimed to answer were:

(i) Is the initial chemical composition and the internal physical and chemical structure of Class I BDs similar to Class I Sun-like stars?
(ii) Are the accretion and outflow processes during the early evolutionary phases as intense for BDs as observed for Sun-like stars?
(iii) Is there a continuity in the formation and early evolutionary processes across the stellar/sub-stellar boundary?

The ABDES project has delivered the first of its kind results on the fundamental properties of sub-stellar objects during their initial formation stages. It has provided the first detailed derivation of the physical and chemical composition of an early-stage BD, and has advanced our understanding of star formation in the Galaxy at the sub-stellar mass end.
The preparatory work for the project included building a statistically large sample of Class I BDs, and obtaining multi-wavelength observations. At the beginning of the project, I had identified a sample of 24 bona fide Class I BDs in various star-forming regions. I then secured a number of multi-wavelength observing data sets to fulfill the various tasks of the project: (i) Sub-mm/mm continuum and molecular line observations to map the physical+chemical structure; (ii) Optical/near-infrared spectroscopy, spectro-imaging, and narrow-band imaging to study the accretion and outflow properties. Our results have shown that the sub-mm/mm continuum and molecular line detection, deep imaging of jets, and the spectral features related to accretion/outflow activity are not elusive but are detectable in Class I BDs.

The main highlights from the studies conducted in the Orion, Serpens, and Ophiuchus regions are described below.

1) The most striking discovery has been the recent detection of a spectacular extended jet driven by a Class I BD, named M1701117 (attached Fig. 1). The results on this new jet are published in Riaz et al. 2017, ApJ, 844, 47. The jet, named HH 1165) has been termed as the first sub-stellar analog of a parsec length protostellar HH jet. It has been features in several Astronomy public magazines and on several Astronomy news websites. Our results have also shown for the first time that the accretion and outflow rates for the Class I BDs are within the range of low-mass protostars, and are also comparatively higher than Class II BDs, as expected form their earlier evolutionary stage.

2) Our molecular gas line observations for Class I BDs have revealed emission in several of the well-known chemical tracers observed in low-mass protostars. This project has highlighted for the first time the molecular species that can trace the inner most, dense regions towards the inner disk of the Class I BDs and has identified the tracers that can survive in the gas phase towards the dense core. The chemical structure for Class I BDs shows some surprising similarities with pre-stellar dense cores, which show a depletion in the abundance profile for most species. We have found a trend of decreasing molecular abundances with decreasing bolometric luminosities, which reflects both the scaled-down physical structure of the Class I BDs as well as the differences in the chemical evolution of the system.

The results from the project have been published in peer-review papers [Riaz et al. 2015a, MNRAS, 446, 2550; Riaz et al. 2015b, ApJ, 815, L31; Riaz et al. 2016, ApJ, 831, 2; Riaz et al. 2017, ApJ, 844, 47] and two additional papers are submitted (Riaz et al. 2017b,c). I have also provided Open Access to all my publications through the repository, e.g. astrophysics arXiv (http://arxiv.org/astro-ph/). The newly discovered jet HH 1165 received press releases at MPE and NOAO, and was also promoted to the general public through the Sky & Telescope, Earth & Sky, Astronomy magazines, and several public Astronomy news websites. I have also presented the results at international conferences (EWASS 2017 in Prague, ESO conference in Chile, Cool stars conference in Uppsala).
A unique feature of the project is the first detailed derivation of the internal physical+chemical structure of proto-BDs. We have state-of-the-art tools available that have allowed us to build a physical+chemical radiative transfer model exclusively for proto-BDs. We are planning interferometric observations with ALMA that can emphasize the emission from the small-scale structures, and will also be important to investigate the depletion and enhancements of molecular species predicted by the models in the different components of proto-BDs. ALMA will be ideal to confirm the predictions from the state-of-the-art models, and to obtain a more robust parameterization of the internal structure of the proto-BDs.

The ABDES project has provided the first accurate measurements of the accretion and out- flow activity rates for a large sample of Class I BDs. We plan to propose for near-infrared observations with the JWST space telescope in late 2017, which will be essential to measure the collimation and kinematics of the jet close to the driving source, and study the shock physics and excitation conditions in the proto-BD jets. We are also developing state-of-the-art models of a high-velocity jet enclosed by a low-velocity outflow driven by a Class I BD. Combining the high-sensitivity JWST observations with these new models will forge new methodologies in studying the jet launching and propagation mechanisms in Class I BDs.
Figure 1. The newly discovered jet HH 1165 is the first large-scale jet driven by a Class I BD.