Solving the Ubiquitous Problem of Stellar Radii

Lead Research Organisation: University of Exeter
Department Name: Physics

Abstract

The student will primarily use optical and infrared data to study populations of recently formed stars in the Milky Way to better understand the physics of star formation and to study the planet-forming discs which surround them. We will use GAIA to study the medium-term variability of the stars, in addition to follow-up ground-based studies. We will use wide-field survey data to study population statistics.

Publications

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Carter A (2020) Detection of Na, K, and H2O in the hazy atmosphere of WASP-6b in Monthly Notices of the Royal Astronomical Society

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Contreras Peña C (2019) Determining the recurrence time-scale of long-lasting YSO outbursts in Monthly Notices of the Royal Astronomical Society

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Hillenbrand L (2018) Gaia 17bpi: An FU Ori-type Outburst in The Astrophysical Journal

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Hillenbrand L (2018) Gaia 17bpi: An FU Ori-type Outburst in The Astrophysical Journal

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Morrell S (2019) Exploring the M-dwarf Luminosity-Temperature-Radius relationships using Gaia DR2 in Monthly Notices of the Royal Astronomical Society

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Morrell S (2018) How to build a radio telescope in Astronomy & Geophysics

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Morrell S (2020) Erratum: Exploring the M-dwarf Luminosity-Temperature-Radius relationships using Gaia DR2 in Monthly Notices of the Royal Astronomical Society

Studentship Projects

Project Reference Relationship Related To Start End Student Name
ST/N504063/1 01/10/2015 31/03/2021
1635907 Studentship ST/N504063/1 01/10/2015 31/03/2019 Samuel Arthur Morrell
 
Description This award has allowed me to develop new techniques and tools to measure the sizes of stars. For stars around the size of our Sun and smaller this is an especially pressing concern, as they are the most abundant in the Galaxy, and are the key targets of current and upcoming extrasolar planet discovery campaigns. Through this award I have confirmed that our best models are currently unable to describe how the smallest stars, known as M-dwarfs, evolve. This work first verified that this was the case for young stars in the Pleiades and Praesepe stellar clusters by showing that no model at a single age could explain describe all of the stars in the cluster. I further studied this problem on older stars, around the age of our Sun, by compiling the largest dataset of radius measurements of M-dwarf stars currently in existence (15,274 stars). This allowed me to discover that magnetism, an effect throught to explain why stars appear larger than models, is unable to explain the radius of these stars, as well as yielding insights about their magnetic fields and starspots on their surfaces.

This work also yielded relationships between temperature-radius and luminosity-radius relationships for these stars. Through this award, I performed a feasibility study for future work by applying this methodology to well studied exoplanet hosting stars, ranging in size from M-dwarfs to about twice the size of our Sun. My methodology yielded measurements of radius more accurate and more precise than existing methods. These measurements have already made their way into publications from the exoplanet community. This work demonstrated that this technique is an important tool for understanding the incoming slew of exoplanet discoveries, and detailed the next steps to further improving the measurements it provides.
Exploitation Route The most important output of this work is the methodology that was developed throughout the project. This method is able to determine the radius of even traditionally difficult to measure stars to a better accuracy and precision than has previously been possible. In addition, measurements can be accomplished rapidly with readily available archive data, making these measurements quick and inexpensive to perform.

It is this method that made it possible to produce the catalogue of >15,000 radius measurements, which is several orders of magnitude larger than previously available. The temperature-radius and luminosity-radius relationships produced from this work are also important for both better understanding exoplanets around M-dwarfs, as the knowledge of these planets is inextricably tied to how well we understand their host stars, and for improving stellar models. Both the catalogue and relationships have been released for free to the community in a variety of computer readable formats.

The feasbility study performed upon exoplanet host funding for this award demonstrate a promising avenue for this research, and are likely to form part of future research funding proposals.
Sectors Other

 
Title Spectroscopic and photometric observations of M dwarf stars in the Pleiades and Praesepe 
Description This dataset contains spectroscopic and photometric observations of a sample of stars within the Pleiades and Praesepe open clusters. This dataset has contributed a large amount to our research on the evolution of low-mass stars within the clusters. This dataset is notable for robust flux calibration, meaning that we can compare directly to models of stars. Because of this, we have been provided insight into the physics of M dwarf stars that has lead to the genesis of two publications that are currently in preparation. 
Type Of Material Database/Collection of data 
Year Produced 2017 
Provided To Others? No  
Impact This dataset has allowed us to make direct comparisons to models of stars and note the differences between them. From this we have been able to study the differences in physics between real observations and the model simulations of the same stars. From these studies we have been able to determine and confirm that these kind of stars are larger than model predictions, which has a large impact on many areas of astronomy, such as stellar and plentary evolution, exoplanet discovery and characterisation, and exoclimatology. 
 
Title Stellar radii for late-K and early-M dwarf stars 
Description Using optical / near-infrared / mid infrared photometry and gemetric distances from Gaia DR2, we have been able to assemble a catalogue of 15 365 stars for which we have determined the radii using a method and piece of software that we developed for this project. 
Type Of Material Database/Collection of data 
Year Produced 2019 
Provided To Others? No  
Impact This catalogue of stars has allowed us to perform extensive studies of the properties of main sequence stars. First we have been able to study the correlations between both radius and activity to help determine what physics is driving the radius inflation of main sequence M-dwarfs. This can then feedback on and inform the next generation of stellar models - which are crucial for many different fields of astronomy. In the more short term, this database can be used by exoplanet scientists to determine the radii of their exoplanet host stars, and hence the exoplanets which orbit them. As the stellar models under perform in this respect, accurate radii are invaludable to the current slew of exoplanet characterisation surveys that are currently hunting for Earth-like planets around this type of star. 
 
Title Photgrid 
Description This software is a suite of tools that was written for this project and facilitaties the fitting of photometry in order to yield the radius and temperature of a star. This works with distances, which are now provided in abundance by the geometric distances from Gaia Data Release 2. Using this software we have been able to accurately fit the radii of between M and A spectral types. The technique generalises well to many distances and is tollerant to poor quality photometry, as well lacking photometry in one or several of the fit bands. Othe pieces of software in the package have been developed to pre-process stellar atmosphere models to produce grids of synthetic photometry with which to fit, as well as tools to help post-process and analyse results files output by the code. 
Type Of Technology Software 
Year Produced 2018 
Impact Based on our testing, this software is able to accurately determine the radii of relatively faint stars, inaccessible to other methods of radius determination. This permits us characterisation of large samples of stars, given accurate stellar atmospheres. In this case, the software has been applied to studying the M-dwarf inflation problem. We have also applied to characterising exoplanet host stars with great success. We have shown that this software can be used to obtain accurate radii M-dwarf, which are the prime targets for the next generation of exoplanet hunting and characterisation campaigns. This software or technique could be applied to this in order to make exoplanet host parameters more accurate than current means. In addition, the software has permitted us to better understand M-dwarf stars which will feedback into and inform the work of the stellar modelling community when developing the next generation of models. 
 
Description The Astronomical Big Data Revolution - Digital Taunton, Taunton, UK 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Regional
Primary Audience Industry/Business
Results and Impact This talk was aimed at local businesses in Somerset and the surrounding areas in the South West of the UK. My talk was about the groudnbreaking science we were able to do with the groundbreaking new datasets that were becoming available in astronomy. The implication being that if we could harness the power of big data, so could the local businesses. Following the talk, I had conversations with a number of staff from local businesses that were curious about big data, machine learning, and some of the techniques that were used in this work, with the aim of potentially using it themselves.
Year(s) Of Engagement Activity 2019
 
Description The Astronomical Big Data Revolution - TechExeter, Exeter, UK 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Regional
Primary Audience Industry/Business
Results and Impact This activity was a talk about the work carried out as part of this grant, presented to business leaders from around Devon and other parts of the South West of the UK. The talk focussed on how big data is being applied to long-standing problems in astronomy. The crux of the talk and ensuing discussion was that if these techniques can be useful to astronomy, they can also be useful for businesses. As a result of this talk, I was invited to do a further talk for more businesses at a similar DigitalTaunton event.
Year(s) Of Engagement Activity 2019