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Massive star formation with Interferometers: Research with ALMA

Final Report Summary - MIRA (Massive star formation with Interferometers: Research with ALMA)

Massive stars – stars with masses larger than 8 Mo – are rare in our Galaxy. However, they dominate the energetics and feedback of galaxies since they inject vast amounts of energy in the form of winds, ultraviolet (UV) radiation and supernovae explosions throughout their lives. Massive stars are known to modify the dynamical, thermal and chemical properties of the interstellar medium (ISM), shaping the structure of galaxies and largely impacting on their evolution. In addition, they are a key source of heavy elements, enriching the ISM for future generations of stars and their solar systems.

Despite the importance of massive stars, the physical processes involved in massive star formation remain poorly understood. In this project (MIRA), I have used molecules and their chemistry to enhance our understanding of the physical mechanisms that lead to formation of massive stars, key to determine their role in galaxy formation and evolution.

For this purpose, I have covered observationally all stages of early massive star birth from the initial conditions in filamentary Infrared-Dark Clouds (or IRDCs), to the later stages represented by massive and chemically active, hot molecular cores. In this project, the physical structure and gas dynamics of massive star forming regions have been probed from parsec-scales down to spatial scales of <0.1 pc. In addition, I have taken advantage of new broad bandwidth receivers available at single-dish and interferometric telescopes, and of models of the chemistry in star forming regions, to infer a complete picture of the physical processes involved in massive star formation. The research objectives of the MIRA project aimed at:
- a) Establishing the formation mechanisms of filamentary IRDCs. I have explored one possible scenario given by cloud-cloud collisions in the Galactic disk. Once IRDCs are formed, I have also investigated the internal structure of IRDCs and whether clumps gather mass by gas accretion (or infall) along the clouds’ axis.
- b) Determining the level of fragmentation in cold IRDC clumps, believed to be at their earliest stages of evolution and expected to split into smaller (<0.1pc-size) cores. I have studied the chemical composition of IRDC cores and model their evolution from their cold to hot phase.
- c) Resolving the internal physical structure in hot molecular cores to unveil dust holes, disks, ionized winds and/or low-mass companions within them.
For research objective a), I have carried out large-scale, broad bandwidth observations toward a sample of filamentary IRDCs with little star formation activity. I have mapped the emission from both low-density molecular tracers, such as 13CO and C18O, and from species known to probe the interaction of large-scale shocks (expected from cloud-cloud collisions) such as SiO and CH3OH.
The maps from the low-density tracers 13CO and C18O have revealed that the molecular gas in IRDCs is distributed in several velocity-coherent, filamentary structures separated in velocity space by a few km s-1. My analysis of the internal motions of these structures shows for the first time the presence of a common velocity gradient, suggesting that the structures retain the motions of the cloud’s initial gravitational collapse. No infall signatures are found in the IRDCs, in contrast to previous studies toward more evolved clouds. These results have important implications for theoretical models of molecular cloud formation and have been published in Jimenez-Serra et al. (2014, MNRAS, 439, 1996).
In addition, the emission from SiO and CH3OH toward my sample of IRDCs is found to be widespread over several parsec-scales, which could be an indication of large-scale shock interactions between the velocity-coherent filaments. This would imply that once the filaments are formed, the cloud’s global gravitational collapse forces the filaments to merge one onto another, inducing a large-scale shock that triggers massive star and star cluster formation (Jimenez-Serra et al. 2015, in prep.).
For research objective b), I have used a sample of eight, very cold IRDC clumps that have been imaged by myself at high-angular resolution with the Submillimeter Array (SMA) and the Atacama Large Millimeter Array (ALMA). Thanks to the broad bandwidth of these observations, not only I have resolved the internal structure of the clumps into cores but I have also established their chemical inventory.
The SMA and ALMA images reveal a clear correlation between the clumps’ dust temperature and their level of fragmentation, IRDC core distribution, core sizes and morphologies (i.e. diffuse or compact) and core chemical composition. From this, I have concluded that clumps initially present a relatively diffuse structure with little fragmentation and, as the clumps evolve, fragmentation increases and the individual cores accrete mass yielding more massive and more compact objects. In addition, the chemical modelling of the observed IRDC cores reveals that the cores’ increasing chemical complexity with time is a consequence of the progressive desorption of ices from dust grains with protostellar heating. This is the first time that all evolutionary stages in IRDC cores are tracked from their cold to their hot stage within the same sample. These results will be reported shortly in Jimenez-Serra et al. (2015, in prep.).
For research objective c), I have obtained ALMA observing time to image at very high-angular resolution (<0.1”) the kinematics of the ionized gas at the innermost regions around the young stellar object (YSO) Monoceros R2 IRS2. This source presents hydrogen radio recombination line emission affected by non-LTE amplification (masers). The project will be executed in mid-2015 and will reveal with unprecedented detail the formation mechanisms of ionized winds in massive YSOs.
Finally, as part of my training at the European Southern Observatory (ESO), I have also contributed to the development of science cases for the construction of the Band 2 receivers of ALMA. In particular, I have shown that ALMA Band 2 will be key in the detection of the simplest amino acid, glycine, toward Solar-type System precursors (see Jimenez-Serra et al. 2014, ApJ, 787, L33). This contribution will appear in the White Paper of the ALMA Band 2 receivers (Fuller et al. 2015, in prep.) and it has attracted some attention among the Square Kilometer Array (SKA) community. I have written several technical documents and contributions for the “Cradle of Life” SKA Working Group (one SKA Science Use Case, and contributions to SKA conference chapter proceedings and to the Spanish White Paper of the SKA).

Note: with regard to the publications as listed in Template A - all publications that indicate NO open access, a free copy can be obtained from the website linked.