Recent research

Here are some brief descriptions of research I have been working on, together with links to the relevant published articles or preprints.

Exoplanets turn the knobs on radio star transmissions

posted Oct 20, 2019, 3:37 PM by Jeremy Drake

Many postings on this site deal in one way or another with the energetic radiation from the hot, multi-million degree outer atmospheres - the "corona" - of stars like the Sun. Heated by the dissipation of magnetic energy generated by an interior dynamo, the most conspicuous manifestation of stellar coronae is their X-ray emission. The X-ray emission is a natural consequence of the interaction of fast-moving electrons and ions in an ionized gas at such high temperatures. Electron impacts with ions "excite" electrons still bound to ions into higher energy stats which then spontaneously decay, emitting the energy in the form of energetics photons - X-rays - at discrete energies. But an electron simply moving in the electric field of an ion, or in the magnetic field of the star, can also lead to photon emission over a very large range of energies, including the radio. Stars like the Sun are, in fact, routinely detected at radio wavelengths.

Light travelling through plasma is subject to refraction and bending - just like through other substances such as glass or water. The effects are generally negligible in stellar coronae but become significant at radio frequencies. Exoplanets close in to their parent stars have the potential to affect the radio
propagation because they act as a barrier to the stellar wind plasma and change its flow and density structure as the plasma streams past. University of Massachusetts Lowell professor, Ofer Cohen, lead a study published in the Astrophysical Journal to examine this effect using 3D numerical simulations and determine whether it has any observable signature.  We found quite large modulations of up to a factor of 2 in the synthetic stellar radio emission in the 10-100 MHz range could be induced by a planet as it moved on its orbit. The intensity modulations were sensitive to both the strength and polarity of the planetary magnetic field.

Several studies in the past have examined the possibility of detecting radio emission from exoplanets themselves, but the predicted signal tends to lie mostly at very low frequencies less than a few MHz that do not pass through the Earth's ionosphere. Our study indicates that radio observations from the ground at higher frequencies could instead be used to investigate the magnetic properties of exoplanets. The magnetic field of a planet is particularly interesting as a probe of interior planetary structure and also for the protection it affords from stellar winds and coronal mass ejections that can erode planetary atmospheres.

20 years of Chandra: stars and planetary systems

posted Oct 12, 2019, 7:33 PM by Jeremy Drake   [ updated Nov 22, 2019, 5:11 AM ]

While working in astrophysics is generally a "jolly good gig", spillover into everyday personal life is pretty much a given, leaving you always paddling around in it to some extent. This year is the 20th anniversary of the launch of the Chandra X-ray Observatory, and at the Chandra X-ray Center we decided to write a science e-book to celebrate Chandra's accomplishments. Allocated the "all things stars and planets" chapter, I allotted a solid three weeks to get things together, just before a family summer holiday in Sicily... an utterly foolish underestimate of the actual effort required to assimilate 20 years' worth of results on an incredibly diverse range of astrophysics almost as rich as the Sicilian cuisine that was inevitably to feed it.  And spillover was well past the knees and rising by the time we landed in Catania, and I ended up swimming in it much more than in the Meditteranean. 

Anyway, to quote the abstract...
Beginning with a tour of the X-ray solar system zoo, including the stunning pulsating X-ray aurorae of Jupiter, we then move on to the hot million-degree outer atmospheres of stars like our own Sun, whose X-ray emission is driven by an internal magnetic dynamo. The same emission processes are also vigorously present in the youngest stars, and we highlight some Chandra observations and results on nascent stellar and planetary systems. Chandra surveys and high-resolution spectroscopy of massive stars have provided a new window on the means by which they scavenge X-ray emission from their radiatively-driven winds, sometimes modulating this output by strong underlying stellar magnetic fields. We touch upon the evanescent X-radiation from intermediate-mass stars before arriving at the inevitable evolutionary endpoints of all but massive stars, first in energized X-ray emitting planetary nebulae, then in the slowly cooling, soft-X-ray emitting photospheres of white dwarfs. We conclude with white dwarfs in close binary systems, rejuvenated by interaction with a companion and where accretion gives play to a new range of energetic behavior even more spectacular and cataclysmic than the coruscant astrophysical road down which they have travelled.

The chapter is packaged up into a rather long preprint available here.

Spin doctor prescribes new dynamo treatment

posted Oct 12, 2019, 1:52 PM by Jeremy Drake

There are many subfields of astrophysics in which a rudimentary understanding of the physics involved has been established for many years, but for which the real details of what is really going on remains elusive or uncertain. The Coronal Heating Problem is one of these: how does the Sun heat its outer atmosphere to temperatures exceeding a million degrees? During the late '70s and following the first results of a survey of stars in X-rays by the Einstein Satellite, it became clear that the magnetic field was the key. But the details remain elusive: are magnetic waves responsible, or is it all through magnetic reconnection - the energy released when magnetic fields are stretched and stressed and snap back to a more relaxed state?  

The magnetic dynamo responsible for generating the magnetic field in the first place is another: where exactly is the dynamo operating and how exactly do stellar rotation and convection interplay to make it happen as we observe it? The dominant source of the solar magnetic field is thought to be the "tachocline" - the interface between the convective envelope and radiative core that are observed through helioseismology to be spinning at different rates and are then a source of magnetic shear.  

The size of the radiative core in stars like the Sun shrinks toward lower masses, and at masses of about 1/3 that of the Sun it disappears altogether.  These stars should then have magnetic dynamos that differ from that in more massive stars. One way of probing this is to use X-rays emitted by hot coronal gas as a proxy for the rate of generation of magnetic field. Since stellar spin is ultimately the driver of differential rotation and dynamo activity, how does X-ray emission behave in fully-convective stars vs those with a radiative core that are spinning at different rates? An earlier post detailed evidence from four fully-convective stars showing that X-ray emission vs rotation rate appears the same in the two groups of stars.  In a much more extensive survey lead by spin doctor Nick Wright from Keele University and published in the Monthly Notices of the Royal Astronomical Society, we found further evidence that the radiative core makes little difference: stars either side of the fully-convective boundary behave very similarly. The implication is that stellar dynamos are not dominated by the tachocline, but instead by the differential rotation distributed through the lower convection zone. 

Keeping a lid on it

posted Oct 11, 2019, 12:43 PM by Jeremy Drake   [ updated Oct 12, 2019, 1:55 PM ]

In a post from late 2016, I discussed the magnetic energy build-up on the surfaces of stars like the Sun, as the magnetic fields are twisted and stretched by the turbulence and flows beneath the stellar surface where they are anchored. This energy is released from time to time as the fields interact and snap back into less stressed states, resulting in phenomena such as flares and coronal mass ejections (CMEs).

CMEs are extremely difficult to detect on stars - see the posting on a monster CME from the demon star (Algol) for a very rare exception. This is a problem because CME impacts on exoplanets can erode their atmospheres and it is important to know how severe this effect is. If the relationship between X-ray flares and CMEs on the Sun is extrapolated to the most magnetically active stars there is an energy problem: the CMEs would require about 10% of the total stellar energy budget from nuclear fusion, which is not physically possible.  So how do CMEs behave on active stars and do we avoid the energy catastrophe?

From the research described in the 2016 post, it appeared that CMEs might be suppressed in active stars by their strong overlying magnetic fields. Extensive supercomputer CME simulations lead by SAO postdoc Julian Alvarado-Gómez and published in the Astrophysical Journal found that this is indeed the case - the field on an active star is essentially a magnetic lid that keeps CMEs contained. Up to a point that is: if they were given enough potential energy at the beginning of the simulation CMEs could still break through the lid. The suppression then keeps the lid on run-of-the-mill CMEs with energies similar to just about all the events ever observed on the Sun. Monster CMEs, though, can still escape and potentially ravage any planets in their way.

The Revolution Revolution!

posted Oct 11, 2019, 11:10 AM by Jeremy Drake   [ updated Oct 11, 2019, 11:13 AM ]

The magnetic field generating capability of stars like the Sun is driven by rotation acting in concert with convection. Convective motions in a rotating fluid sphere - which is essentially what a star is - lead to a pattern of differential rotation, where different regions of the stellar convective envelope are rotating at different rates. It is the shear between adjacent layers that stretches and wraps the magnetic field, amplifying it in the process. Magnetic field in a stellar interior is buoyant and rises to the surface, producing an array of interesting physical phenomena that have been reported in several previous posts. Examples include stellar winds, coronal mass ejections, and X-ray emission

Stellar winds take away angular momentum from the star, slowing its spin. The Sun, whose equatorial rotation period is about 25 days, for example, would have been spinning with a rotation period of only a couple of days or so when it was born. We can see stellar spin-down at work by observing the rotation periods of stars in open clusters in which all cluster members are essentially the same age.  These rotation periods are distributed in a puzzling bimodal way, with some stars remaining as rapid rotators for many millions of years longer than others. Angular momentum evolution theory has struggled for decades to explain this. 

Smithsonian Astrophysical Observatory and Harvard Institue for Applied Computational Science scientist Cecilia Garraffo has lead a study published in the 2018 July 20 Astrophysical Journal that can now finally explain this rotation behaviour.  The answer appears to lie in the distribution of the magnetic field over the surface of the star. If the field is disorganized and split into many components, or ``multipolar", the wind is closed down and angular momentum loss greatly reduced compared with the case of magnetic field organized into a single dipole.  Observations indicate that magnetic field complexity of Sun-like stars is originally quite high when they are born but decreases rapidly as they spin down. This growth of dipolar field applies a strong magnetic brake that rapidly slows the star's spin, leading to two populations: the remaining fast rotators and a population of slower rotators as observed.  A revolution in the understanding of stellar revolution.

The planets with an electric heater turned to high

posted Nov 4, 2018, 9:19 AM by Jeremy Drake   [ updated Nov 4, 2018, 9:22 AM ]

We now know that planets are common around M dwarfs, and the 100 billion or so M dwarfs in our own Galaxy mean the chance of there being "habitable" worlds with liquid water on their surface should be extremely high.  But liquid water is not the only consideration for habitability. The magnetic fields of M dwarfs and their associated energetic radiation - both in the form of a plasma "wind" and UV to X-ray emission - are generally much stronger in comparison to their visible light than for solar-like stars. As a consequence, close-in planets around M dwarfs endure a distinctly more hostile space environment than the Earth does ,and an environment that might strip planet's atmosphere and surface water. 

The wind from a star like the Sun - including M dwarfs - is not just plasma; it also carries with it magnetic field that originates within the star in a magnetic dynamo driven by the star's rotation. When encountering a planet, the moving magnetic field and stellar wind plasma generate electric currents within the planetary ionosphere called "Birkeland currents", after Norwegian explorer and physicist Kristian Birkeland who first proposed them.  The ionosphere of a planet is the very outer atmosphere where ionization by the Sun's UV, EUV and X-ray light creates a significant number of ions and free electrons. In the Earth's case, the ionosphere is the region between 60 and 1000 km or so in altitude.  The currents that flow heat up the ionosphere in a process called "Joule heating", just like in an electric heater.  Enough energy is injected by Joule heating to expand and make a significant difference to the extent of the Earth's very outer atmosphere.

University of Massachusetts researcher, Prof. Ofer Cohen, lead a study published in the Astrophysical Journal Letters to estimate the heat injected into the the ionospheres of the planets around the M dwarf TRAPPIST-1.  The TRAPPIST-1 planets experience stellar wind conditions up to 100,000 times stronger than Earth, and Cohen's team found that, if the TRAPPIST-1 planets have managed to retain an atmosphere, the Joule heating rate is commensurately elevated by enormous amounts.  The net effect will be to increase the planet's atmospheric loss rate, likely further jeopardising their capability to harbour and sustain life over billion year timescales.

Finding Life in... Outer Space?!

posted Mar 23, 2018, 10:56 AM by Jeremy Drake

Back in 2013, I proposed a three part film series to the Smithsonian Channel on "Life in the Cosmos" - an attempt to look into all the astrophysics involved in the origin and evolution of life.  They liked the proposal and looked for partners and funding. A couple of false starts and a year or two later and the scope was down to two films. We ended up making one that was filmed last year. The second morphed into an edit of a BBC-commissioned film featuring the great Stephen Hawking - I think his last film appearance.  Ok, maybe just cut to the PR blurb...

“I wanted to look at a different slant to the story of the discovery of exoplanets,” said Drake, “including more of the astrophysics that goes into it and the complex science rationale behind the possibility of life developing somewhere other than Earth.” 

“Finding Life in Outer Space” travels across incredible tracts of space and time exploring the greatest mystery of the universe - why life exists. The show follows the scientists responsible for some of the biggest breakthroughs in understanding the origins of life and uncovers how cutting-edge observations and experiments are transforming the view of how the universe works. 

Hydrothermal geyser pools in the Atacama Desert in Chile reveal secrets about the first steps life took on the primeval Earth, and high in the Arctic Circle, the northernmost science facility on the planet keeps an eye on the magnetic shield that protects humans from devastation by the solar wind. 

The film explores U.S laboratories where the secrets of astrochemistry are recreated, and looks to England where they are closing the gap between chemistry and biology by simulating conditions on Earth four billion years ago. Telling a story of creation on an amazing scale and with incredible consequence, “Finding Life in Outer Space” shows that science may be pointing to one fantastic conclusion - we are not alone.

X-rays vs Protons: Dawn of Creation

posted Mar 23, 2018, 10:25 AM by Jeremy Drake   [ updated Mar 23, 2018, 10:56 AM ]

Planets are formed in the remnant "protoplanetary disk" of gas surrounding a newborn star. The process is one of hierarchical merging, of dust grains to form planetesimals, and planetesimals to form protoplanets that are then large enough to accrete material from the disk by their own gravity. Planets also interact with the gas in the disk, which can pull it closer or push it further away from its host star. Natal planetary characteristics and their resulting orbits then depend critically on the properties of the disk and how long it hangs around.

If it were not for energetic radiation from the host star, the gas disk would hang around for many millions of years. Instead, X-rays and fast protons heat the very outer layers and drive off mass in a disk wind.  They also weakly ionize the gas, whose charged particles interact through electric and magnetic forces and make the gas viscous. The viscous gas slows itself down in its orbit and gradually spirals into the central star - see the posting on an observation of this at work. Within 10 million years the gas disk is gone.

Stellar X-rays had long been thought the principle driver of these processes, until an estimate of energetic proton fluxes suggested instead that they were responsible. This estimate assumed protons travel in straight lines from their acceleration sites in the star's flares and coronal mass ejections. But charged particles follow curved magnetic field lines, and young stars have strong magnetic fields - several hundred times stronger that the Sun's.  We simulated the trajectories of energetic protons within the magnetosphere and wind environment of a young "T Tauri" star.  We found most of the protons get trapped by the strong stellar magnetic field.  Those that escape follow the magnetic field in particular trajectories, hitting the protoplanetary disk in specific places in a mottled pattern. They cannot dominate the global disk ionization in this way, but disk models will need to be extended to understand the effects of the strong, localized bombardment that can dominate the ionization in specific places where the magnetic field intercepts the disk. This work was lead by SAO visiting scientist Federico Fraschetti, and was published in the 2018 February 1 edition of the Astrophysical Journal.

The Ring of Darkness

posted Feb 26, 2018, 10:11 AM by Jeremy Drake

The speed with which stars orbit around the centre of a galaxy - the ``rotation curve'' - tell us about the distribution of mass within the galaxy.  One of the most remarkable discoveries made in astronomy employed this technique of examining the orbital speeds of stars in galaxies that are viewed nearly edge-on.  The data indicated that most of the mass of a typical galaxy lies in an extended halo beyond the visible stars and gas .  This otherwise unseen mass was dubbed ``dark matter".   

The importance of stellar kinematics for understanding galaxies gives the impression that our own Milky Way rotation curve would be well-defined and comprehensively observed.  Instead, the kinematics of the Galaxy outside the Solar Circle - the orbit of the Sun about the Galactic centre -  is quite poorly known. University of Hertfordshire PhD student Amy Harris set out to show that stars slightly more massive than the Sun, of spectral type A to F, can be uniquely exploited to remedy the situation.  These types of star can be efficiently selected from photometric surveys.  Being relatively bright, they can be seen to large distances, and being relatively young they still reflect the kinematics of Galactic rotation.  

Amy used Multiple Mirror Telescope observations of a sample of about 800 A/F stars to probe a narrow 1 degree sightline across our Galaxy and found a surprising increase in orbital speed with increasing distance from the Galactic centre. The results support earlier suggestions that the Milky Way has ring of dark matter lying between 13 and 18 kpc from the centre. These are just the first pilot study results and Amy's method is now set to be deployed to test these findings in much more extensive upcoming surveys.  Amy's work was published in the 2018 January 5 edition of the Monthly Notices of the Royal Astronomical Society.

A star of the beat generation

posted Dec 2, 2017, 1:50 PM by Jeremy Drake

Stars like the Sun generate copious magnetic fields that induce all sorts of interesting behaviour at the stellar surface.  Sunspots betraying bundles of emerging magnetic field, and the diffuse white halo of light scattered off the magnetically-driven solar wind seen around the Sun during a total eclipse, are good examples of this. The magnetic field is generated by the combination of the Sun's rotation and the convection that characterises the outer 30% or so of its radius. The magnetic field is always changing and goes through a cycle in which the polarity of the fieldthe "north" and "south" of the magnetreverses and changes back again over a period of about 22 years.  Similar cyclic magnetic behaviour is seen in about 60% of Sun-like stars.

Iota Horologii is a star like the Sun but much younger, with an age of "only" about 600 million years. It came to prominence when found to host a planet with a mass of 2.5 Jupiters in an orbit similar to that of Earth.  ι Hor rotates four times faster than the Sun because it has not had time to lose as much angular momentum through its wind. This faster rotation drives a stronger magnetic dynamo. Earlier studies had found a much shorter magnetic cycle than the Sun's with a period of only 1.6 yearsin fact the shortest known to date. As part of a much wider investigation of this intriguing exoplanet host, SAO Postdoctoral Scholar Julian Alvarado-Goméz studied the light from ionized calcium atoms in the star's upper atmosphere, or "chromosphere."  The chromosphere is heated by energy dissipated from the surface magnetic field and so can be used to trace the magnetic cycle. In a paper published in the 2017 October 10 edition of the Monthly Notices of the Royal Astronomical Society, Alvarado-Goméz and colleagues find the cycle is actually two cycles, with periods of 1.4 and 2 years, superimposed and "beating" against each other to make an apparent 1.6 year average. Still to be done is to understand exactly how and why magnetic cycles on the Sun and stars arise at all.

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