Recent research

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

#BLACKLIVESMATTER #SHUTDOWNSTEM #SHUTDOWNACADEMIA

posted Jun 13, 2020, 9:34 AM by Jeremy Drake   [ updated Jun 13, 2020, 9:34 AM ]


Six of one, half a dozen of the other

posted Mar 16, 2020, 7:22 PM by Jeremy Drake

A couple of earlier postings have discussed the stellar phenomenon known as "coronal mass ejections" (CMEs). CMEs are the result of the sudden release of magnetic energy in the outer atmosphere, or corona, of a star that flings out plasma at speeds of hundreds of kilometers per second. The first direct visible light observation of a CME was of course of one on the Sun, made on 1971 December 14 by the seventh Orbiting Solar Observatory (OSO-7) satellite.

Detecting CMEs on other stars has proved much more problematic and extremely difficult. This is an important problem because some stars can generate magnetic energy at a rate 10,000 times higher than the Sun and CMEs can have a dramatic impact on the atmospheres of exoplanets. This is especially problematic for close-in planets in the habitable zones around stars less massive than the Sun.

We recently analysed a good CME candidate event seen during an X-ray observation of the infamous "Demon Star", Algol, and have since been searching through the literature for other CME candidates. In a study lead by Smithsonian Astrophysical Observatory postdoc Sofia Moschou and published in the 2019 June 1 edition of the Astrophysical Journal, we found 12 of them. While the inferred CME masses as a function of associated flare energy generally were similar or below the extrapolated mean for solar events, the CME kinetic energies were consistently lower than the analogous solar extrapolation by average factors of about 100. These results suggest that the CMEs associated with very energetic flares on magnetically active stars are more limited in terms of the ejecta velocity than the ejecta mass, possibly because of the restraining influence of strong overlying magnetic fields and drag within the stellar wind. One glimmer of potentially good news is that the lower CME velocities present a more optimistic scenario for the effects of CME impacts on exoplanets.

Fugitive Plasma Caught Speeding on Yellow Giant Star

posted Mar 15, 2020, 8:13 AM by Jeremy Drake   [ updated Mar 16, 2020, 7:01 PM ]

Coronal mass ejections on the Sun are the result of the release of stored magnetic energy that flings out up to 100 billion tonnes of hot plasma at speeds of millions of kilometers an hour. They are the most powerful magnetic phenomena that occur in the solar system. Since stars can exhibit magnetic activity levels up to ten thousand times higher than the Sun, the obvious questions are: do they have coronal mass ejections and what do they look like?

It is extremely difficult to detect CMEs on other stars. A rare example of a possible event on the "Demon Star", Algol, was described in an
earlier post. One potential method is to use high-resolution X-ray spectroscopy to try and detect the Doppler shift of plasma as it is ejected from the star.

Using time-resolved X-ray spectroscopy of a stellar flare on the active giant star HR 9024 obtained with the Chandra X-ray Observatory, a team lead by Palermo Observatory scientist Costanza Argiroffi detected Doppler shifts that indicate both upward and downward motions of multi-million-degree plasma amounting to 100–400 km per second, combined with a later blueshift corresponding to an upward motion of 90 km per second.

The first motions are consistent with the behaviour of plasma rapidly heated by a stellar flare and confined by magnetic fields, but we interpreted the later upward motion as a huge coronal mass ejection. We estimated the mass of the ejected plasma to be about a quadrillion tonnes - similar to the mass of the entire atmosphere of the Earth and a hundred thousand times more massive than typical very large solar coronal mass ejections. The study was published on 2019 May 27 in Nature Astronomy

Breezing through the Space Environment of Barnard’s Star b

posted Mar 5, 2020, 6:18 AM by Jeremy Drake   [ updated Apr 22, 2020, 11:51 AM ]

The discovery of a planet orbiting the nearby - at 6 light years - red dwarf called Barnard's Star was a major step in our growing understanding that planetary systems around stars are the norm rather than the exception. Named after Yerkes Observatory astronomer E. E. Barnard, who first noted its high "proper motion" across the sky, the star is the fourth nearest known star to the Sun, after the three members of the Alpha Centauri system.  It's planet, Barnard’s Star b, orbits at a distance similar to that of Mercury around the Sun, but just outside of the supposed "habitable zone" of the faint red dwarf where liquid water is thought sustainable. Nevertheless, the relatively low magnetic activity level and attendant atmosphere-removing X-ray and EUV flux of Barnard's star has raised questions as to whether an atmosphere might exist on the planet.

Stellar winds are also known to be destroyers of planetary atmospheres - the loss of water from early Mars due to solar wind erosion being a good example. We therefore decided to examine the likely "space weather" around Barnard's Star. Supercomputer magnetohydrodynamic stellar wind models were simulated using Barnard's Star stellar parameters and likely surface magnetic field configuration. The results were described in a paper jointly lead by Smithsonian Astrophysical Observatory scientists Julian Alvarado-Gomex and Cecilia Garraffo and published in the 2019 April edition of the Astrophysical Journal.

Barnard’s Star b experiences less intense wind pressure than the much more close-in planet Proxima b, and the planets of the TRAPPIST-1 system, with space weather conditions not too much more extreme than experienced by Earth. The milder wind conditions are more a result of its much greater orbital distance rather than in differences in the surface magnetic field strengths of Proxima and Barnard's Star. Can Barnard's Star b still retain an atmosphere? Possibly. However, it should be recalled that the planetary system is old at possibly up to 10 Gyr, or twice the age of the solar system. Barnard's star would have been much more active in its first billion years or so and it is during that era that the atmosphere of Barnard's Star b would truly have been under assault from its host star.

Proton Beams of Death

posted Mar 4, 2020, 7:42 PM by Jeremy Drake   [ updated Apr 24, 2020, 6:29 AM ]

The most numerous stars in the Universe are those of the lowest masses - the M dwarfs. They are so dim compared with stars like our own Sun that their so-called habitable zones, where any planets might habour water in liquid form, are very close in. Unfortunately, this proximity is potentially hazardous for any planets with ambitions of engendering life. M dwarfs emit proportionally much more of their power in energetic radiation, such as X-rays and extreme ultraviolet light, in addition to winds, coronal mass ejections and energetic particles, than more massive stars like the Sun does. This radiation can scour and evaporate the atmospheres of close-in planets, possibly leaving behind only barren, rocky cores.

Energetic particles - mostly protons - accelerated in flares or in shockwaves from coronal mass ejections could be especially dangerous, causing showers of secondary ionizing particles in an atmosphere and destroying UV-protecting ozone. It is very difficult to asses exactly what the threat of energetic protons is because they do not travel in straight lines, like X-rays. They are instead guided and deflected by the magnetic field carried out from the star by its plasma wind through interplanetary space.  

In a paper lead by University of Arizona and Smithsonian Astrophysical Observatory scientist Federico Fraschetti and published in the 2019 March edition of the Astrophysical Journal, we decided to try and work out where energetic protons from the star end up. A supercomputer magnetohydrodynamic model of the stellar wind and turbulent magnetic field of a TRAPPIST-1-like system was created, and we then fired protons into this and mapped the results. We found that particles are strongly focused toward the equatorial planetary orbital plane, potentially bombarding any planets with a proton flux up to a million times more intense than experienced by the present-day Earth.  Proton beams of death?

The Optics of Superflares

posted Mar 4, 2020, 8:20 AM by Jeremy Drake   [ updated Mar 4, 2020, 8:21 AM ]

Stellar flares are explosive events in the outer atmospheres of stars powered by the sudden release of stored magnetic energy generated in the interior. The analogy of suddenly releasing a stretched elastic band is often drawn: the elastic is magnetic field that is stretched and stressed by the roiling convection of plasma beneath the stellar surface, or photosphere.  As the magnetic field snaps back into a lower energy configuration it accelerates electrons and protons that impact the photosphere, heating and "evaporating" plasma into the corona where it reaches temperatures of millions of K and radiates in X-rays. 

The Kepler spacecraft can detect the impact of flare-accelerated particles on the stellar photosphere because of a local increase in the photospheric temperature at the flare footpoints - from 6000 K to 10,000 K for a flare on a Sun-like star for example.  X-ray satellites, like ESA's XMM-Newton, can detect the flare in X-rays.  Both satellites observed the young Pleiades cluster at the same time back in 2015 February with the aim of detecting flares and understanding the way energy is distributed between the hot X-ray emitting gas and the optical emission of the flare footpoints. 

The Pleiades is a cluster of stars 136 parsecs away formed about 100 million years ago - very young stars in the context of the 4.6 billion-year-old Sun. As detailed in earlier postings, young stars rotate rapidly - up to once every day or so - engendering strong magnetic fields and flaring, with flare energies up to 10,000 times more energetic than seen on the Sun. Solar flares typically release 50 or more times more energy in visible light than in X-rays, and this large difference is key for understanding the physics and energy requirements of flaring.  What happens then in flares many times more energetic, such as on the young Pleiades Sun-like stars? 

A paper lead by Palermo Observatory scientist Mario Guarcello and published in Astronomy and Astrophysics found that the Kepler visible flare signature typically corresponded to energies 2-3 times higher than seen in the X-rays with XMM-Newton, a much smaller difference than in lower energy solar flares. This suggests that more energetic flares are less efficient in evaporating plasma into the corona, although much work still remains to understand the details of why.

Mind the Gap

posted Mar 3, 2020, 7:08 PM by Jeremy Drake   [ updated Mar 3, 2020, 7:15 PM ]

One of the most challenging puzzles in stellar evolution that emerged in the 1970s and early 1980s was the "cataclysmic variable period gap". Cataclysmic variables (CVs) are interacting binary stars comprising a white dwarf accreting from a more normal companion star. As the population of known CVs grew, it became clear that systems with periods between 2 and 3.12 hr were rarely observed compared with those with shorter and longer periods.

CV orbits generally shrink over time as angular momentum is lost to the stellar wind of the secondary companion. It was realised that if this angular momentum loss abruptly changed at a period of about 3 hours, then the accretion would stop: voila, the CV would detach and not be observed as an accreting object, explaining the period gap. The theory was called "interrupted magnetic breaking".

The underlying motivation for a fairly abrupt angular momentum loss reduction stems from arguments that magnetic dynamo action in Sun-like stars that powers stellar winds and angular momentum loss occurs at the interface between the convection zone and the radiative interior — the “tachocline”. For typical CVs, the orbital period of three hours corresponds to the secondary becoming fully-convective - the tachocline disappears, along with winds and angular momentum loss. However, an earlier post describes work that indicates no such change in magnetic activity in fully convective stars occurs: the tachocline is apparently not important: interrupted magnetic braking theory is broken.

A paper lead by Smithsonian and Harvard researcher Cecilia Garraffo published in the 2018 November 20 edition of the Astrophysical Journal rescues the theory by explaining the stellar wind and angular momentum loss reduction instead in terms of the growing complexity of the surface magnetic field. As noted in an earlier post, a more complex field shuts down the wind and drastically reduces the magnetic braking. Interrupted magnetic braking is mended.

The Radio Refraction Mind-bender

posted Mar 2, 2020, 2:15 PM by Jeremy Drake

Refraction is a physical phenomenon that spans the entire electromagnetic spectrum.  It is typically seen in optical terms as the bending of light as it passes between materials with different optical properties - glass and air for the example of eyeglasses.  Radio waves experience refraction too, especially when passing through a plasma, like that of the solar corona and wind.  The plasma refractive index  - the ratio of the speed of light in a vacuum to the speed of the radio waves in the plasma - increases with decreasing radio frequency, such that refraction is more important for low radio frequencies than high frequencies.  It also increases with the plasma density, and there is a critical density at which radio waves can no longer penetrate when they are essentially reflected instead.  The radio "photosphere" of a star, or its apparent emitting surface, then actually increases in size as radio frequency decreases.

Radio frequencies provide powerful probes of the solar corona and wind and can be observed directly from the ground, unlike the ultraviolet and X-rays. However, at "low" frequencies - of the order of a GHz and below - refraction must be taken into account in order to infer the true location or shape of the emitting source as the radio waves are bent in their path to the telescope. This is highly non-trivial as it requires detailed 3D knowledge of the plasma density. This can generally only be achieved in computer simulations. 

A paper lead by Smithsonian Astrophysical Observatory postdoc Sofia Moschou and published in the 2018 November 1 edition of the Astrophysical Journal presents a new sophisticated synthetic radio imaging tool that calculates and visualizes the bremsstrahlung radio emission from the corona and wind, following the path of radio waves from each tiny piece of space within the 3D simulation as they propagate outward along curved trajectories. Test model results are in good agreement with radio observations of the solar disk. The development promises to be a valuable new means of interpreting and understanding radio observations of the Sun. But it is in observations of stars, whose disks cannot be resolved spatially, where it could be even more important and simulations are the only means to infer what is actually going on.

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.

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