Abigail J. Frost

Career Stage
Postdoctoral Researcher
Poster Abstract

Protostellar disks play key roles for low-mass stars. They are involved in the accretion process, whereby the forming protostar is fed material by the disk, and disks are thought to be the nurseries of planets. As different processes occur within the disks, they form various substructures such as inner holes, rings and gaps. Massive stars (greater than 8 solar masses) form from the same clouds of dust and gas as low-mass stars and affect the Universe at all scales through supernovae, black hole merger events and through their production of heavy elements. Despite their importance, the formation of massive stars is poorly understood as they are deeply embedded, distant and rare, which introduces more observational challenges. While disks have recently been found around massive protostars, massive disk evolution is yet to be constrained. In my poster slides, I present our recent work which uses a multi-scale analysis to constrain the characteristics of massive stellar disks. The method was applied to a sample of massive young stellar objects (MYSOs), allowing the characteristics to be directly compared. We find that all the massive protostars are surrounded by disks, and detect inner holes and gap-like substructures within 75% of the sample. In order to attribute these geometrical characteristics to a potential evolutionary sequence, we combine our results with a spectral analysis which found that different ages of MYSO show different spectral features. We find that all the disks with substructure as classified as more evolved sources, implying that the substructures seen are a result of disk evolution as they are for low-mass stars. Given that the timescales of massive star formation are orders of magnitude faster than for low-mass stars, this implies that the process associated with planet formation and disk dispersal may be able to occur earlier than previously thought.

Plain text summary
In order to answer the question ‘how do you build a star?’ I use a cartoon I made which shows how we currently think star formation occurs. All stars form within parsec-scale regions of dust and gas called molecular clouds. Denser, cool pockets of the cloud ~10000au in radius (dense cores or Class 0 sources) attract mass under gravity. Eventually the core reaches a critical mass and collapse. As the core shrinks, its opacity increases and it heats up. It’s angular velocity also increases to conserve angular momentum, spreading out material in the plane of rotation. Much like a ball of pizza dough spreading out flat as it is thrown, this leads to the formation of a disk. Outflows launch from the centrally concentrated centre (protostar), also to help conserve angular momentum. Surrounding natal core (envelope) material falls towards the central source. This process takes 10,000-100,000yrs with the emission of this Class I source peaking at higher. After 100,000-1 million years, the surrounding envelope dissipates, leaving behind a Class II source with a disk and outflows. Disk material feeds the central protostar, producing spikes in emission. The outflows and accretion subside as the source evolves (Class III). The disk goes through a number of changes, potentially clearing its inner regions. As the disk is not uniformly dense, pockets of mass can grow, creating planetesimals. Eventually after ~10 million years, the central object starts fusing hydrogen into helium. The remaining disk is blown away, leaving a main sequence star and any planets which were able to form.

On slide 2 I describe that for low-mass stars like our Sun, these Classes have been observed. However for massive stars, this formation process is not clear as massive protostars are deeply embedded, rare and form much quicker than low-mass stars. Molecular clouds and cold massive cores have been observed and recently embedded systems similar to Class I low-mass sources have been detected around massive young stellar objects (MYSOs). However, how the disks around MYSOs evolve is yet to be constrained.

Slide 3 shows another cartoon I developed, illustrating how we used different observations to probe different regions of MYSOs; interferometric data between 7-13 micron at 0.01” resolution, imaging data at ~20 micron at 0.1” resolution and spectral energy distributions (SEDs) which describe the emission of the source from micron to millimetre wavelengths. By fitting a single radiative transfer model to all three datasets simultaneously, the geometric characteristics of the MYSOs are obtained at multiple scales. The SEDs provide no spatial information, but allow the entire environment to be probed when combined with high resolution data. Radial profiles of the real and model images are affected by emission outflow cavity walls in the envelope. The interferometric data trace the smallest scales. It was sensitive to cavity emission, and due to the additional constraints from the imaging data, this emission could be separated from disk emission, allowing the disks to be studied. Variation is present within the disks, with 75% of the disks displaying some substructure. Dust has been cleared past the sublimation radius, leaving inner holes, in a large portion of the disks. One source also displays a gap-like structure, reminiscent of the Class III disks of low-mass sources.

In slide 4, I describe how we attribute the substructures in these disks to an evolutionary sequence. Cooper (2013) studied the spectra of MYSO and found that bluer, hotter (and therefore more evolved) sources show some spectral features, while redder, younger sources show others. They define three evolutionary using H2, H I and fluorescent Fe II as diagnostics. H2 is strong in young Type I sources and decreases with Type as outflow activity decreases, while HI emission appears and increases as the protostar gets hotter. The eldest sources emit fl. Fe II as they start producing UV radiation. We classified the MYSOs from our geometrical study by these Types. All the Type III sources have substructure in their disk, none is seen in the Type I disks and its split for Type II, implying these are associated with more evolved sources. A strong correlation (0.92) exists between the size of the inner holes of the MYSOs and their luminosity, implying that photoevaporation, which is a potential cause of clearing for low-mass sources, might be the culprit. Ultimately, our findings imply that MYSO disks evolve like low-mass disks, despite the faster timescales involved. This constitutes an important first step in investigating disk evolution for massive forming stars.
Poster Title
Unveiling the evolution of disks around massive forming stars