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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - GESTATE (testinG massivE STar formATion modEis)

Teaser

Summary

High-mass stars control the dynamics and chemistry of our Galaxy. They are crucially responsible for processing higher elements. The formation of very massive stars - similar to any other star such as Sun -take place through gravitational collapse of dense clouds of gas and dust - however, at a much rapid pace and violent conditions. It involves high accretion rates, very dense accretion flows quickly fragmenting to secondary pre-stellar cores and an early onset of nuclear fusion. How do such extreme conditions influence the physics of high-mass star formation is a fore-front question in modern astrophysics. Theoretical studies and numerical simulations deliberate scenarios of this physics - yet to be tested in detail by observations. The objectives of this project were to conduct specific observations, tailored to test front-line scenarios of high-mass star formation.

A forming star is known to gain mass through episodic accretion events, rather than accretion flows (constant influx of material). Episodic accretion leads to the variable brightness well-known in low-mass young stars, including the extreme cases of FU-Ori type bursts. Such variations are hitherto unknown in high-mass stars. Our objective is to search for this variability using the VISTA VVV survey data.

Accretion flows forming high-mass stars are extremely dense, therefore, quickly become Jeans unstable and fragment into secondary stellar seeds. This seed competes with the primary, to gain mass from the common reservoir, setting an upper limit on how massive a star can become. Our aim is to examine the physics of this scenario using detailed observations of prototypical high-mass systems.

A high-mass protostar is subject to an enormous level of internal energy derived from Kelvin-Helmholtz contraction and early nuclear burning. It is also fed with high entropy material from the disk. It has been suggested that the high entropy drives the massive protostars to bloat/puff up during most of its formation phase as it adjusts itself. A bloated massive star will then have a cooler photosphere, albeit being luminous. We want to search for signatures of the bloated cooler and luminous photospheres in young massive stars.

Work performed

Semester 1: Telescope time proposals were prepared and submitted. The process of job application was made, that has led to successful post MC IEF employment in Portugal with a promotion to the level of Associate Professor. I took up the unforeseen task of undergraduate teaching as an opportunity at the University of Hertfordshire. Adaptive optics (AO) assisted integral field spectroscopic (IFU) data of high-mass proto-binary systems was reduced. IFU data reduction methods are quite complex, as, 2D spectral data is calibrated and combined to produce a 3D cube and with high astrometric precision while using the AO data.

Semester 2: The VISTA VVV data was used to identify high amplitude variable objects from the entire survey area. Spectral energy distributions were analysed to identify the sub-sample of young high-mass stars. Analysis of light curves and colours were made. This work was published in ApJ as lead author. ALMA proposal to cycle 4 call was submitted as PI and succeeded.

Semester 3: I undertook partial data analysis of the ALMA data to search for disks around O-stars (an EU level project with multi-country collaboration), shared by different sub-teams. I attended a special training course on XCLASS, which is a module especially developed for ALMA data when numerous unidentified emission lines are found and continuum fitting is a challenge for line forests. Proposals to test the bloated star scenario was submitted and succeeded.

Semester 4: The discovery of high-amplitude variable high-mass young stars led us to question the general nature of variability in high-mass stars. To do so we envisaged creating a new user-friendly VVV light curve database. The paw print photometry was merged with efficient noise and artefact removal and applied to the entire survey data resulting in a database that is being tested. A non-expert user will be able to query for source lists to obtain quick light curves.

Two new students were trained during the period, apart from the existing doctoral student at the University of Porto. I reviewed three articles for international journals, attended three international conferences, fostered new network and collaborations, delivered one conference talk, three seminars and presided over a PhD jury as the lead expert.

Final results

The discovery of highly variable high-mass young stars was hitherto unknown. It opens a field of investigation to connect the low and high mass accretion scenarios. Our VLT-SINFONI observations uncover near-equal high-mass stars embedded in common envelopes, one younger than the other. ALMA observations of these targets are being analysed. The combined picture emerging from this study can explain the classical near-equal binarity in massive stars as a formation rather than dynamical effect - at the same time, supports fragmentation induced starvation setting the upper limits on the stellar mass. To test for bloating in high-mass stars, we have successfully obtained infrared spectroscopic observations of a large sample of young massive stars and they are being analysed. These results are well beyond the state-of-the-art. Several high-impact research publications will only be ready and published within a time frame of another year - owing to the range of observations and techniques from the infrared to the radio wavelengths. The new light curve database for the VISTA VVV ESO survey will be an important gateway of utility to the VVV public survey consortium that will lead to increased exploitation of the survey.

Mobility from Portugal to the UK, experience of undergraduate teaching and living in a university town led to a cultural and generational shock that took a while to stabilise - similar to a forming high-mass star bombarded with high-entropy gas. This shock led me to profound realisations, emerging from reflection and study. They have made positive changes in me, an unforeseen effect of mobility, the influence of which lies beyond the workplace and research topic. I believe cultural shock was an effect of moving from an economically weaker, conservative Catholic country to an economically superior multi-religious, cosmopolitan, Protestant country that is the stamp of liberalism and democracy. I intend to summarise/publish the detailed analysis of my reflection elsewhere at a later time which shall be appropriately acknowledged.