Mark Hollands
Research
I am primarily concerned with the study of white dwarfs. This type of stellar remnant represents the end-state for the 95%+ of main-sequence stars with masses below 8 Solar-masses, which will not undergo a core-collapse supernova. Instead, while in the giant-star phase of evolution, the outer 50% of the star will be ejected in an intense stellar wind. The remaining 50%, no longer producing pressure via nuclear fusion in the stellar interior, is unable to contend with the intense gravitational forces. The core thus collapses until the quantum-phenomenon known as electron-degeneracy pressure can balance gravity. The result is a hot (100,000 K), immensely dense, Earth-sized object -- the white dwarf.
The interior of a typical white dwarf is mostly (99%) a mix of carbon-oxygen ions in a sea of degenerate electrons. Around this, is an envelope of helium (about 1% of the mass), and usually, a final shroud of hydrogen (0.01% of the mass). Our telescopes of course can only directly see the outer-most layers and what they're made of. Therefore the interior composition is constrained from our theoretical understanding of how stars evolve, but also from observations of pulsating white dwarfs where waves travelling through the interior make it to the surface, providing information on the density of the central carbon-oxygen core.
The interior of a typical white dwarf is mostly (99%) a mix of carbon-oxygen ions in a sea of degenerate electrons. Around this, is an envelope of helium (about 1% of the mass), and usually, a final shroud of hydrogen (0.01% of the mass). Our telescopes of course can only directly see the outer-most layers and what they're made of. Therefore the interior composition is constrained from our theoretical understanding of how stars evolve, but also from observations of pulsating white dwarfs where waves travelling through the interior make it to the surface, providing information on the density of the central carbon-oxygen core.
Because white dwarfs do not fuse-nuclei in their cores, they instead radiate away their internal energy, at first cooling rapidly, and then much more slowly the temperature of the white dwarf outer layers decrease. The oldest white dwarfs known to the Milky Way can have cooled from an initial 100000 K to a relatively cool 3000 K (only half the temperature at the surface of the Sun). Because the processes that affect white dwarf cooling rates are fairly well understood, the coolest (and thus oldest) white dwarfs provide firm constraints on the age of the Galactic disk.
First author publications (full ADS list here)
- Alkali metals in white dwarf atmospheres as tracers of ancient planetary crusts
M. A. Hollands, P.-E. Tremblay, B.T. Gänsicke, D. Koester, N. P. Gentile-Fusillo Nature Astronomy (2021) - An ultra-massive white dwarf with a mixed hydrogen-carbon atmosphere as a likely merger remnant
M. A. Hollands, P.-E. Tremblay, B.T. Gänsicke, M.E. Camisassa, D. Koester, A. Aungwerojwit, P. Chote, A. H. Córsico, V. Dhillon, N. P. Gentile-Fusillo, M. J. Hoskin, P. Izquierdo, T. R. Marsh, D. Steeghs, Nature Astronomy (2020) - The Gaia 20 pc white dwarf sample
M. A. Hollands, P.-E. Tremblay, B.T. Gänsicke, N. P. Gentile-Fusillo, S. Toonen, MNRAS 480, 3942 (2018) - Cool DZ white dwarfs II: Compositions and evolution of old remnant planetary systems
M. A. Hollands, B.T. Gänsicke, D. Koester, MNRAS 477, 93 (2018) - Cool DZ white dwarfs I: Identification and spectral analysis
M. A. Hollands, D. Koester, V. Alekseev, E.L. Herbert, B.T. Gänsicke, MNRAS 467, 4970 (2017) - The incidence of magnetic fields in cool DZ white dwarfs
M. A. Hollands, B.T. Gänsicke, D. Koester, MNRAS 450, 681 (2015)
Talks Given
- Alkali metals in white dwarf atmospheres as tracers of ancient planetary crusts - online seminar (Sheffield, 2021)
- The detection of lithium in cool white dwarf atmospheres - presented at White Dwarfs from Physics to Astrophysics (online KITP meeting, 2021)
- Alkali metals in white dwarf atmospheres as tracers of ancient planetary crusts - online seminar (UFRGS, 2021)
- A white dwarf of spectral type DAQ - presented at White Dwarfs as Probes of Fundamental Physics and Tracers of Planetary, Stellar, and Galactic Evolution IAU symposium 357 (Hilo, 2019)
- Spectroscopy of partially burnt runaway stars - presented at Stars on the Run II (Potsdam, 2019)
- A white dwarf with an usual composition as a possible product of a binary merger - presented at The Beginning and Ends of Double White Dwarfs (Copenhagen, 2019)
- Partially burnt remnants of SNIax in the Milky Way - seminar (Southampton, 2019)
- Partially burnt remnants of SNIax in the Milky Way - presented at The Supernova - Supernova Remnant Connection (RAS meeting, London, 2019)
- Revisiting the white dwarf local sample in the era of Gaia - presented at the 21st European white dwarf workshop (Austin, 2018)
- Remnant planetary systems around the oldest white dwarfs - seminar (Caltech, 2017)
- Chemistry and evolution of the oldest white dwarf planetary systems - presented at Astrochemsitry VII IAU symposium 332 (Puerto Varas, 2017)
- Chemistry and evolution of the oldest white dwarf planetary systems - presented at Planetary Systems Beyond the Main Sequence II (Technion, 2017)
- Chemistry and evolution of the oldest white dwarf planetary systems - seminar (Institute of Astronomy, Cambridge, 2016)
- A large sample of cool, strongly-polluted DZ white dwarfs - presented at the 20th European white dwarf workshop (Warwick, 2016)
- Metals as tracers of magnetism in old white dwarfs - presented at the Stellar End Products meeting (ESO Garching, 2015)
- Planetary systems as tracers of magnetism in old white dwarfs - presented at the workshop: Mysteries of the Suns magnetic field III: Understanding stellar activity (Warwick, 2015)
- Ancient planetary systems around white dwarfs - presented at the 19th European white dwarf workshop (Montreal, 2014)
Write to:
Mark Hollands
Department of Physics,
University of Warwick,
Coventry CV4 7AL
UK
Contact details:
E-Mail: M.Hollands.1 AT warwick.ac.uk