High-Contrast Imaging of Massive Stars
Astrophysical Background
The formation of massive stars remains a hot topic in astronomy today. Many theories exist, describing in detail the formation processes of these crucial objects in the Universe. It is believed that massive stars are the first objects to appear after the Big bang, therefore responsible for the state of our present Universe. The first stars were enormous, ultra-heavy stars that evolved over very short timescales and ended their lives in highly energetic supernovae. These supernovae perturbed the interstellar environment over large scales and induced the collapse of molecular gas which formed other stars that, after some time, exploded and perturbed their environment which then formed other stars. This process repeated itself (and is still going on, but at a slower rate) to form the current Universe. Dynamical and gravitational processes are responsible for the formation of galaxies but we will not be discussing it here. Massive stars are generally defined as having masses > 8 M⊙ and with formation timescales of about 105 years.
The latest observational results show that 80 to 100% of massive stars are at least in a binary system but it remains a mystery as to why they are found that way. The most promising theory for the formation of massive stars (and their companions) is disk fragmentation theory. This description starts with the massive protostar accreting gas from the surrounding environment into a disk. The rotation of the star is responsible for this disk and the disk itself rotates. This dynamical movement prevents the fall of all the material into the star but since the material is accreting into the disk, then the mass within the disk increases.
Now, if the accretion rate of the disk > accretion rate of the star, then the disk will reach a point where it will be more massive than the central object. This gives rise to local perturbation within the disk which creates smaller and fainter stars in the disk. Once the main star reachers the main sequence, its radiation pressure chases the remaining gas and disk away and we are able to see the now-formed companions. Dynamical interaction between the central star and its companions can push the smaller stars away from the system or merge them with the main star.
The Problem
Observing the companions is a difficult endeavour. Numerous constraints hinder our ability to see them: their rareness, large distances, short formation timescales and the fact that they evolve in the obscured environments of star formation. It is important for us to be able to detect and characterise the companions of massive stars as we would be able to know the initial conditions of their formation and eventually the formation of the central star. We therefore need to detect and characterise the end products.
Goals
We aim to detect faint and low-mass companions around massive stars in the Carina region. This region is a massive nearby star forming region where many massive stars are created. It is massive enough to provide us with a statistically significant, uniformly selected sample and a well populated upper IMF. We wish to obtain the multiplicity properties of all O and WR type stars, which totals in 91 massive stars. To achieve this goal, we are using the SPHERE instrument on board the VLT Unit telescope 3. We use this instrument in IRDIS/IFS mode, which are two sub-instruments on SPHERE. IFS (FOV = 1.73”) enables us to detect and characterise the companions at close separations of the main star and IRDIS (FOV = 11”) helps to discriminate between real companions and foreground or background stars on the image. The data obtained is reduced and analysed using the Vortex Image Processing (VIP) software package, which is a high-contrast imaging module library. Using these techniques enable us to detect and characterise the companions of massive stars in the Carina region and therefore help us infer their multiplicity properties and constrain the theories of massive star formation.
References
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I.A. Bonnell & M.R. Bate, Star formation through gravitational collapse and competitive accretion, 2006, MNRAS 370, 488
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K. Kratter, The Formation and Evolution of Massive Binaries, Four Decades of Research on Massive Stars, ASP Conference Series, Vol. 465, 2012
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C. Gomez-Gonzalez et al., VIP: Vortex Image Processing Package For High-Contrast Direct Imaging, ApJ, 154:7, 2017
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H. Zinnecker & H.W.Yorke, Toward Understanding Massive Star Formation, ARAA, Vol. 45, 2007
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H. Sana et al., Southern Massive Stars at High Angular Resolution: Observational Campaign and Companion Detection, ApJS 215, 15, 2014