SAM MORRELL


ASTROPHYSICIST

My Research

I have a wide variety of interests within astrophysics that include the physics driving the evolution of low-mass stars, the nature of young stellar objects and the coupling between young stellar systems and how this effects the occurance, evolution and characterisation of exoplanets and their atmospheres.

Understanding the Characteristics and Evolution of Low-mass Stars

My recent research has involved understanding the fundamental properties of low-mass stars. Despite the fact that these stars the key ingredients to the modern slew of exoplanet discovery and characterisation missions, they are still remarkably poorly understood. Using cutting edge observations from the second data release of the Gaia mission, and a whole slew of photometry from a variety of all-sky surveys, we are working on trying to glean insights into what we do and do not understand about the physics that drives them.

Of particular interest to me is understanding the  apparent radius inflation in this class of star when compared directly to a model of equal luminosity. This discrepancy is commonly noted in M dwarf stars at all stages of evolution, but there still remain some important open questions:

  • How large is the radius discrepancy between these stars and the models?
  • Is the amount of inflation in some way linked to convective inhibition, driven by stellar magnetic fields?
  • Can the the measured inflation be described entirely by systematics due to star spots and photospheric opacities?
  • If real, what is the underlying physics driving this divergence from models?
  • Can we compensate for the effect and provide the community with more accurate stellar radii for the purposes of stellar evolution and exoplanet characterisation?
  • What is the true effect of this discrepancy on star- and planet-formation timescales?

Notable Publications

The Evolution of Young Stellar Objects

Advances in stellar evolution theory have proven to be some of the cornerstones upon which modern astronomy is built. However, there is still much that is not known about the exact nature and timescales over which stars form and evolve. Despite the many studies that have focussed on stellar clusters, our current work has provided glimpses at how truly lacking our census of young stellar objects (YSOs) really is. Crucially, how can the traditional model of star formation in giant molecular clouds (GMCs) fit with the apparently distributed nature of star formation that can be observed throughout the Milky Way.

I am currently interested in better understanding young stellar systems and their evolution towards the main sequence. In particular , I have previously worked on using observations to probe the dynamics of these systems: how the mass is transferred between the accretion disc and the star and how this interaction affects both.

Of particular importance of this research is understanding the protoplanetary environment. What the conditions are for planetesimals within the disc and what physical processing go into conglomoration from micrometre sized dust grains into planetesimals and whether this can explain how the metre size barrier is broken.

Notable Publications

Interstellar Dust or Protoplanetary Grain

Throughout my MPhys project I worked as part of a team, under the supervision of Tim Naylor, in the Astrophysics group at the University of Exeter. The research entailed studying interstellar dust grains and the way they interact with incident light. The observational manifestation of this effect, called extinction, is a dimming and reddening of the light. It has been firmly established in the literature that this effect is strongly wavelength dependent, with blurer incident light being subjected to more extinction than redder light. As much of the astronomical work that harnesses the work of large number statistics utilises the colours and brightness of stars in individual bands, this poses a serious problem for all manner of astronomical experiments. Our research involved modifying existing models that explain this effect in diffuse regions, such as the mean interstellar medium (ISM), to explain the anomalous extinction laws in dense regions; such as those of star forming regions.

Why is Dust a Problem?

Nearly everywhere you look within the Milky Way, our host galaxy, there are almost imperceptible dust grains. Because they have been well studied and we believe we understand them quite well, they pose little trouble for modern astrophysics. However, the problem comes in denser regions such as gas clouds. Due to the much cooler temperature, a result of the shielding effect of the cloud, it's thought that dust grains have a fundamentally different composition and size distribution to their well studied counterparts in the ISM. This does indeed appear to be the case.

Hence, extinction poses a problem for several different regions. First, much effort is currently being used to study these regions, as they present tantilising snapshots of the formative stages of stellar evolution. Additionally, the large samples that are currently being drawn from Gaia DR2, which are only set to increase in size when LSST comes online, are subject to a mean extinction that is proportional to their distance from the observer. Studies of these two diverse populations of grain are the key to unlocking improved accuracy of stellar fluxes and colours, as well as the life cycles of and physics governing the evolution of the dust grains themselves. It is for both of these reasons that the research topic is of great interest.