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Stellar News Feed Archive

Orbital Decay of X-ray Binaries Sunday, December 1, 2013 - 12:07

The authors investigate the orbital decay of two black hole X-ray binaries. Combined with previous data, they were able to measure how fast the orbit is decaying and make predictions about the exact cause of this orbital decay. These systems are some of the best to look for this orbital decay because of their extreme masses compared to other systems. By obtaining spectra to create a radial velocity curve, the authors were able to add an additional O-C point with a larger baseline than before. This larger baseline is important because it allows time for small changes to build up.

Read the full story on Astrobites

 

A Fiery Drama of Star Birth and Death Wednesday, November 27, 2013 - 10:49

The Large Magellanic Cloud is one of the closest galaxies to our own. Astronomers have now used the power of ESO’s Very Large Telescope to explore one of its lesser known regions. This new image shows clouds of gas and dust where hot new stars are being born and are sculpting their surroundings into odd shapes. But the image also shows the effects of stellar death — filaments created by a supernova explosion.

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Something cataclysmic in the Kepler field Tuesday, November 26, 2013 - 15:15

BOKS 45906 was identified as having colors consistent with other CVs in a pre-launch survey of the Kepler spacecraft’s field of view, and was also seen to go through an outburst period, increasing in brightness by a factor of ~25 for five days.  Ramsay et al. were looking to study CVs in the Kepler field, so they included it as one of their targets for further monitoring during the Kepler mission. In the end, they were able to use three years of Kepler data, as well as observations from the Isaac Newton TelescopeSwift, and the Hale 200″ Telescope, to learn more.

Read the full story on Astrobites

Imaging the circumstellar environment of the young T Tauri star SU Aurigae Wednesday, November 20, 2013 - 09:54

The circumstellar environments of classical T Tauri stars are challenging to directly image because of their high star-to-disk contrast ratio. One method to overcome this is by using imaging polarimetry where scattered and consequently polarised starlight from the star's circumstellar disk can be separated from the unpolarised light of the central star. We present images of the circumstellar environment of SU Aur, a classical T Tauri star at the transition of T Tauri to Herbig stars. The images directly show that the disk extends out to ~500 au with an inclination angle of $\sim$ 50$^\circ$. Using interpretive models, we derived very small grains in the surface layers of its disk, with a very steep size- and surface-density distribution. Additionally, we resolved a large and extended nebulosity in our images that is most likely a remnant of the prenatal molecular cloud. The position angle of the disk, determined directly from our images, rules out a polar outflow or jet as the cause of this large-scale nebulosity.

Authors: S. V. Jeffers, M. Min, H. Canovas, M. Rodenhuis, and C. U. Keller

Read the paper on arXiv

BOKS 45906: a CV with an orbital period of 56.6 min in the Kepler field? Tuesday, November 19, 2013 - 08:55

BOKS 45906 was found to be a blue source in the Burrell-Optical-Kepler-Survey which showed a 3 mag outburst lasting ~5 d. We present the Kepler light curve of this source which covers nearly 3 years. We find that it is in a faint optical state for approximately half the time and shows a series of outbursts separated by distinct dips in flux. Using data with 1 min sampling, we find clear evidence that in its low state BOKS 45906 shows a flux variability on a period of 56.5574+/-0.0014 min and a semi-amplitude of ~3 percent. Since we can phase all the 1 min cadence data on a common ephemeris using this period, it is probable that 56.56 min is the binary orbital period. Optical spectra of BOKS 45906 show the presence of Balmer lines in emission indicating it is not an AM CVn (pure Helium) binary. Swift data show that it is a weak X-ray source and is weakly detected in the bluest of the UVOT filters. We conclude that BOKS 45906 is a cataclysmic variable with a period shorter than the `period-bounce' systems and therefore BOKS 45906 could be the first helium-rich cataclysmic variable detected in the Kepler field.

Authors: Gavin Ramsay (Armagh Observatory), Steve B. Howell, Matt A. Wood, Alan Smale, Thomas Barclay, Sally A. Seebode, Dawn Gelino, Martin Still, John K. Cannizzo

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Radial velocity variations in the young eruptive star EX Lup Monday, November 18, 2013 - 23:37

EX Lup-type objects (EXors) are low-mass pre-main sequence objects characterized by outbursts attributed to highly enhanced disk accretion. The trigger mechanism of EXor outbursts is still debated. One theory requires a close (sub)stellar companion that perturbs the inner disk and triggers the onset of the outburst. Here, we study the radial velocity (RV) variations of EX Lup, the prototype of EXors. We conducted a 5-year RV survey with HARPS and FEROS.

We discuss two possibilities to explain the RV data: a geometry with two accretion columns rotating with the star, and a single accretion flow synchronized with the orbital motion of the hypothetical companion. In the companion scenario, the companion's mass would fall into the brown dwarf desert, which, together with the unusually small separation would make EX Lup a unique binary system, with interesting implications on the physical mechanisms responsible for triggering the outburst.

Authors:  Á. Kóspál, M. Mohler-Fischer, A. Sicilia-Aguilar, P. Ábrahám, M. Curé, Th. Henning, Cs. Kiss, R. Launhardt, A. Moór, A. Müller

Read the full abstract and pre-print paper on arXiv

A review of pulsating stars from the ASAS data Monday, November 18, 2013 - 10:28

The All-Sky Automated Survey (ASAS) appeared to be extremely useful in establishing the census of bright variable stars in the sky. A short review of the characteristics of the ASAS data and discoveries based on these data and related to pulsating stars is presented here by an enthusiastic user of the ASAS data.

Author:  Andrzej Pigulski

Read the pre-print from arXiv

Hubble views an old and mysterious cluster Thursday, November 14, 2013 - 11:44

The NASA/ESA Hubble Space Telescope has captured the best ever image of the globular cluster Messier 15, a gathering of very old stars that orbits the centre of the Milky Way. Astronomers studying the cluster with Hubble in 2002 found there to be something dark and mysterious lurking at its heart. It could either be a collection of dark neutron stars, or an intermediate-mass black hole. Of the two possibilities it is more likely that Messier 15 harbours a black hole at its centre.

Read the press release

 

Photometric evolution of Nova Del 2013 (V339 Del) during the optically thick phase Wednesday, November 13, 2013 - 09:33

We present and discuss the BVRI photometric evolution of Nova Del 2013 from the time of discovery, which occurred a few days before maximum brightness, to day +77, when the optically thick phase was over.

 

Authors: U. Munari, A. Henden, S. Dallaporta, G. Cherini

 

Read the pre-print on arXiv

Towards a better understanding of the distance scale from RR Lyrae variable stars: A case study for the inner halo globular cluster NGC 6723 Monday, November 11, 2013 - 10:59

Understanding the formation and evolution of our Galaxy has always been one of the key quests in modern astrophysics for decades (e.g. Freeman & Bland-Hawthorn 2002). Since RRLs are easily identifiable and they can provide a powerful means to probe the chemical compositions and dynamical properties of the old stellar populations, RRLs in the Galactic globular cluster (GC) systems or in the field are of particular importance to address the question of the early history of our Galaxy (see, for example, Smith 1995). Also, being a primary distance indicator, the distance to RRLs can be accurately measured and RRLs can help to yield important insights into the structure of our Galaxy. Recent studies by Drake et al. (2013) or Pietrukowicz et al. (2012) are excellent examples of making use of RRLs to understand the substructures in the Galactic halo expected from the theory of the hierarchical structure formation, and to delineate the bar structure in the central part of our Galaxy.

Authors: Jae-Woo Lee, Mercedes Lopez-Morales, Kyeong-Soo Hong, Young-Woon Kang, Brian L. Pohl, Alistair Walker

Read the abstract on arXiv

Read the pre=print on arXiv

A Study of the Unusual Z Cam Systems IW Andromedae and V513 Cassiopeia Thursday, November 7, 2013 - 22:29

The Z Cam stars IW And and V513 Cas are unusual in having outbursts following their standstills in contrast to the usual Z Cam behavior of quiescence following standstills. In order to gain further understanding of these little-studied systems, we obtained spectra correlated with photometry from the AAVSO throughout a 3-4 month interval in 2011. In addition, time-resolved spectra were obtained in 2012 that provided orbital periods of 3.7 hrs for IW And and 5.2 hrs for V513 Cas. The photometry of V513 Cas revealed a regular pattern of standstills and outbursts with little time at quiescence, while IW And underwent many excursions from quiescence to outburst to short standstills. The spectra of IW And are similar to normal dwarf novae, with strong Balmer emission at quiescence and absorption at outburst. In contrast, V513 Cas shows a much flatter/redder spectrum near outburst with strong HeII emission and prominent emission cores in the Balmer lines. Part of this continuum difference may be due to reddening effects. While our attempts to model the outburst and standstill states of IW And indicate a mass accretion rate near  3×10−9 solar masses per year, we could find no obvious reason why these systems behave differently following standstill compared to normal Z Cam stars.

Authors: Paula Szkody, Meagan Albright, Albert P. Linnell, Mark E. Everett, Russet McMillan, Gabrelle Saurage, Joseph Huehnerhoff, Steve B. Howell, Mike Simonsen, Nick Hunt-Walker

Read the pre-print on arXiv

Multi-Wavelength Photometry of the T Tauri Binary V582 Mon (KH 15D): A New Epoch of Occultations Thursday, November 7, 2013 - 09:52

We present multi-wavelength (VRIJHK) observations of KH 15D obtained in 2012/13, as well as a master table of standard photometry spanning the years 1967 to 2013. The system is a close, eccentric T Tauri binary embedded in an inclined precessing circumbinary (CB) ring. The most recent data show the continued rise of star B with respect to the trailing edge of the occulting horizon as the system's maximum brightness steadily increases. The wealth of data in time and wavelength domains allows us to track the long-term CCD color evolution of KH 15D. We find that the V-I behavior is consistent with direct and scattered light from the composite color of two stars with slightly different temperatures. There is no evidence for any reddening or bluing associated with extinction or scattering by ISM-sized dust grains. Furthermore, we probe the system's faint phase behavior at near-infrared wavelengths in order to investigate extinction properties of the ring and signatures of a possible shepherding planet sometimes invoked to confine the CB ring at ~ 5 AU. The wavelength independence of eclipse depth at second contact is consistent with the ring material being fully opaque to 2.2 microns. The color-magnitude diagrams demonstrate excess flux in J and H at low light levels, which may be due to the presence of a hot, young Jupiter-mass planet.

Authors: Diana Windemuth, William Herbst

Read the abstract and pre-print on arXiv

Mystery of 'Hot, Young Stars' in Ancient Red Galaxies --Solved! Thursday, November 7, 2013 - 09:13

New evidence from NASA's Wide-field Infrared Survey Explorer (WISE) and Galaxy Evolution Explorer (GALEX) missions provide support for the "inside-out" theory of galaxy evolution, which holds that star formation starts at the core of the galaxy and spreads outward. Like tree rings, inner and outer portions of a galaxy's disk are a historical record. Two NASA missions find evidence that star formation bursts started in galaxy centers and spread outward. Unexplainedultraviolet light might come from a late phase in the lives of older stars .

Read the full press release

Read the technical paper on arXiv

Symbiotic stars in X-rays Tuesday, November 5, 2013 - 08:10

Until recently, symbiotic binary systems in which a white dwarf accretes from a red giant were thought to be mainly a soft X-ray population. Here we describe the detection with the X-ray Telescope (XRT) on the Swift satellite of nine white dwarf symbiotics that were not previously known to be X-ray sources and one that had previously been detected as a supersoft X-ray source. The nine new X-ray detections were the result of a survey of 41 symbiotic stars, and they increase the number of symbiotic stars known to be X-ray sources by approximately 30%. The Swift/XRT telescope detected all of the new X-ray sources at energies greater than 2 keV. Their X-ray spectra are consistent with thermal emission and fall naturally into three distinct groups. The first group contains those sources with a single, highly absorbed hard component that we identify as probably coming from an accretion-disk boundary layer. The second group is composed of those sources with a single, soft X-ray spectral component that probably originates in a region where low-velocity shocks produce X-ray emission, i.e., a colliding-wind region.

Authors:  G. J. M. Luna, J. L. Sokoloski, K. Mukai, T. Nelson

Read the full abstract an pre-print at arXiv

Face to phase with RU Lupi Monday, November 4, 2013 - 09:42

We present new results on the classical T Tauri star RU Lupi based on three observing runs collecting high-resolution spectra, complementary NIR spectra, multicolour photometric data, and X-ray observations. The photospheric absorption lines are weakened. This veiling becomes extremely strong on occasion, and we show that this effect is due to narrow emission lines that fill in the photospheric lines. The blue-shifted wings in the optical emission lines of He I, attributed to a stellar wind, are remarkably stable in equivalent width. In contrast, the red-shifted wings change dramatically in strength depending on rotational phase. From the pattern of variability we infer that these wings originate in accreting gas close to the star, and that the accretion funnels are bent and trail the hot spot. Forbidden emission lines are very stable over the entire observing period and originate in the disk wind. A system of narrow blue-shifted absorption features seen in lines of Ca II and Na I can be traced to a disk wind as well. Slightly blue-shifted emission components are present in the forbidden lines and might be related to a wide angle molecular disk wind.

Authors: Gösta F. Gahm, Henricus C. Stempels, Frederick M. Walter, Peter P. Petrov, Gregory J. Herczeg

Read the pre-print on arXiv

Could a Milky Way Supernova Be Visible from Earth in Next 50 Years? Friday, November 1, 2013 - 09:02

Astronomers at The Ohio State University have calculated the odds that, sometime during the next 50 years, a supernova occurring in our home galaxy will be visible from Earth.

The good news: they’ve calculated the odds to be nearly 100 percent that such a supernova would be visible to telescopes in the form of infrared radiation.

The bad news: the odds are much lower—dipping to 20 percent or less—that the shining stellar spectacle would be visible to the naked eye in the nighttime sky.

Yet, all this is great news to astronomers, who, unlike the rest of us, have high-powered infrared cameras to point at the sky at a moment’s notice. For them, this study suggests that they have a solid chance of doing something that’s never been done before: detect a supernova fast enough to witness what happens at the very beginning of a star’s demise. A massive star “goes supernova” at the moment when it’s used up all its nuclear fuel and its core collapses, just before it explodes violently and throws off most of its mass into space.

“We see all these stars go supernova in other galaxies, and we don’t fully understand how it happens. We think we know, we say we know, but that’s not actually 100 percent true,” said Christopher Kochanek, professor of astronomy at Ohio State and the Ohio Eminent Scholar in Observational Cosmology. "Today, technologies have advanced to the point that we can learn enormously more about supernovae if we can catch the next one in our galaxy and study it with all our available tools.

Read the full press release

Read the pre-print on arXiv

Nova Aquilae 1918 (V603 Aql) Faded by 0.44 mag/century from 1938-2013 Monday, October 28, 2013 - 09:14

We present the light curve of the old nova V603 Aql (Nova Aql 1918) from 1898-1918 and 1934-2013 using 22,722 archival magnitudes. All of our magnitudes are either in, or accurately transformed into, the Johnson B and V magnitude systems. This is vital because offsets in old sequences and the visual-to-V transformation make for errors from 0.1-1.0 magnitude if not corrected. Our V603 Aql light curve is the first time that this has been done for any nova. Our goal was to see the evolution of the mass accretion rate on the century time scale, and to test the long-standing prediction of the Hibernation model that old novae should be fading significantly in the century after their eruption is long over. The 1918 nova eruption was completely finished by 1938 when the nova decline stopped, and when the star had faded to fainter than its pre-nova brightness of B=11.43±0.03 mag. We find that the nova light from 1938-2013 was significantly fading, with this being seen consistently in three independent data sets (the Sonneberg plates in B, the AAVSO V light curve, and the non-AAVSO V light curve). We find that V603 Aql is declining in brightness at an average rate of 0.44±0.04 mag per century since 1938. This work provides remarkable confirmation of an important prediction of the Hibernation model. However, our result does not uniquely point to the Hibernation model because other models of novae evolution are now making similar predictions.

Authors:  Christopher B. Johnson (LSU), Bradley E. Schaefer (LSU), Peter Kroll (Sonneberg Observatory), Arne Henden (AAVSO)

Read the pre-print on arXiv

Amplitude Variations in Pulsating Red Supergiants Thursday, October 24, 2013 - 08:55

We have used long-term AAVSO visual observations and Fourier and wavelet analysis to identify periods and study long-term amplitude variations in 44 red supergiants. Of these, 12 stars had data which were too sparse and/or had low amplitude and/or were without conspicuous peaks in the Fourier spectrum; 6 stars had only long (2500-4000 days) periods without significant amplitude variation. The other 26 stars had one or two periods, either "short" (hundreds of days) or "long" (thousands of days), whose amplitudes varied by up to a factor of 8, but more typically 2-4. The median timescale of the amplitude variation was 18 periods. We interpret the shorter periods as due to pulsation, and the longer periods as analogous to the "long secondary periods" found in pulsating red giants.

Authors:  John R. Percy and Viraja C. Khatu

Read the pre-print paper from arXiv

Origin of W UMa-type contact binaries - age and orbital evolution Tuesday, October 22, 2013 - 15:49

Recently, our understanding of the origin of W UMa-type contact binaries has become clearer. Initial masses of their components were successfully estimated by Yildiz and Dougan using a new method mainly based on observational properties of overluminous secondary components. In this paper, we continue to discuss the results and make computations for age and orbital evolution of these binaries. It is shown that the secondary mass, according to its luminosity, also successfully predicts the observed radius. While the current mass of the primary component is determined by initial masses, the current secondary mass is also a function of initial angular momentum. We develop methods to compute the age of A- and W-subtype W UMa-type contact binaries {in terms of} initial masses and mass according to the luminosity of the secondaries. 

Author:  Mutlu Yildiz

Read the full abstract and pre-print paper on arXiv

Astronomers Shed New Light on the Rarest and Brightest Exploding Stars Thursday, October 17, 2013 - 13:52

The research is published on Thursday (17 October) in Nature Magazine – one of the world’s most prestigious science publications. It proposes that the most luminous supernovae – exploding stars – are powered by small and incredibly dense neutron stars, with gigantic magnetic fields that spin hundreds of times a second.

Scientists at Queen’s Astrophysics Research Center observed two super-luminous supernovae – two of the Universe’s brightest exploding stars – for more than a year. Contrary to existing theories, which suggested that the brightest supernovae are caused by super-massive stars exploding, their findings suggest that their origins may be better explained by a type of explosion within the star’s core which creates a smaller but extremely dense and rapidly spinning magnetic star.

Read the full press release

Read the abstract and pre-print on arXiv

Multi-Longitude Observation Campaign of KV Cancri: an RR Lyrae star with irregular Blazhko modulations Thursday, October 17, 2013 - 10:20

We present the results of multi-longitude observations of KV Cancri, an RR Lyrae star showing an irregular Blazhko effect. With a pulsation period of 0.50208 day, the times of light curve maxima are delayed by 6 minutes per day. This daily delay regularly leads to long periods of time without maximum light curve observations for a given site. To cope with this observing time window problem, we have organized a multi-longitude observation campaign including a telescope of the AAVSONet. From the observed light curves, 92 pulsation maxima have been measured covering about six Blazhko periods. The Fourier analysis of magnitudes at maximum light has revealed a main Blazhko period of 77.6 days and also a secondary period of 40.5 days. A Fourier analysis of (O-C) values did not show the secondary Blazhko period. The frequency spectrum of the complete light curve, from a Fourier analysis and successive pre-whitening with PERIOD04, has shown triplet structures around the two Blazhko modulation frequencies but with slightly different periods (77.8 and 42.4 days). The second Blazhko frequency is statistically not a harmonic of the main Blazhko frequency. Besides the two Blazhko modulations KV Cnc presents other particularities like irregularities from Blazhko cycle to cycle and very fast magnitude variations which can reach a maximum of 2.5 magnitudes per hour over a period of 15 minutes. This campaign shows that regular observations by amateur astronomers remain important. Indeed such a detailed characterization of the Blazhko effect could not be obtained from large-scale surveys, as cooperative long time-series observations are needed.

Authors: Pierre de Ponthiere, Michel Bonnardeau, Franz-Joseph Hambsch, Tom Krajci, Kenneth Menzies, Richard Sabo

Read the paper on arXiv

Variable Stars and Galactic Structure Wednesday, October 16, 2013 - 16:16

Variable stars have a unique part to play in Galactic astronomy. Among the most important of these variables are the Cepheids (types I and II), the RR Lyraes and the Miras (O- and C-rich). The current status of the basic calibration of these stars in their roles as distance, structure and population indicators is outlined and some examples of recent applications of these stars to Galactic and extragalactic problems is reviewed. The expected impact of Gaia on this type of work is discussed and the need for complementary ground based observations, particularly large scale near-infrared photometry, is stressed.

Authors:  Michael W. Feast, Patricia A. Whitelock

Read the pre-print paper on arXiv

How The Largest Star Known Is Tearing Itself Apart Wednesday, October 16, 2013 - 09:43

When astronomers studied the images of Westerlund 1 they spotted something truly unique. Around one of the stars, known as W26, they saw a huge cloud of glowing hydrogen gas, shown as green in this new image. Such glowing clouds are ionized, meaning that the electrons have been stripped away from the atoms of hydrogen gas.

Clouds of this type are rarely found around massive stars and are even rarer around red supergiant stars such as W26 -- this is the first ionized nebula ever discovered around such a star. W26 itself would be too cool to make the gas glow; the astronomers speculate that the source of the ionizing radiation may be either hot blue stars elsewhere in the cluster, or possibly a fainter, but much hotter, companion star to W26. The fact that the nebula is ionized will make it considerable easier to study in the future than if it were not ionized.

On investigating the star W26 in more detail the researchers realized that the star is probably the largest star ever discovered, with a radius 1,500 times larger than the Sun and is also one of the most luminous red supergiants known. Such large and luminous massive stars are believed to be highly evolved, all of which suggests that W26 is coming towards the end of its life and will eventually explode as a supernova.

The nebula observed around W26 is very similar to the nebula surrounding SN 1987A, the remnant of a star that exploded as a supernova in 1987. SN 1987A was the closest observed supernova to Earth since 1604 and as such it gave astronomers a chance to better study the properties of these explosions. Studying objects like the new nebula around W26 will help astronomers to understand the mass loss processes around these massive stars, which eventually lead to their explosive demise.

Read the pre-print on arXiv

Read the press release at ESO

Dr. George H. Herbig (1920–2013) Monday, October 14, 2013 - 07:58

Dr. George H. Herbig, astronomer emeritus at the University of Hawaii at Manoa and a member of the prestigious National Academy of Sciences, has died at the age of 93. He joined the faculty of the UH Institute for Astronomy in 1987 after a long and distinguished career at the Lick Observatory, now part of the University of California, Santa Cruz, and he attained emeritus status at UHM in 2001.

He has been widely acclaimed for his pioneering studies of star formation and the properties and evolution of young stars. His contributions laid the foundation for much of what we know about the birth and early development of stars.

Dr. Herbig’s work on young stars is so fundamental and comprehensive that he is widely seen as the father of the field of star formation studies. He revolutionized the field by identifying and characterizing the physical features of stars that are so young they did not exist when our earliest human ancestors walked the Earth. He recognized that the T Tauri stars, as they are called, have roughly the same mass as our sun, but have much stronger versions of many of the sun’s features, such as magnetic activity, spectral emissions, and lithium content.

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The planets around NN Ser: still there Tuesday, October 8, 2013 - 11:05

We present 25 new eclipse times of the white dwarf binary NN Ser taken with the high-speed camera ULTRACAM on the WHT and NTT, the RISE camera on the Liverpool Telescope, and HAWK-I on the VLT to test the two-planet model proposed to explain variations in its eclipse times measured over the last 25 years. The planetary model survives the test with flying colours, correctly predicting a progressive lag in eclipse times of 36 seconds that has set in since 2010 compared to the previous 8 years of precise times. Allowing both orbits to be eccentric, we find orbital periods of 7.9 +/- 0.5 yr and 15.3 +/- 0.3 yr, and masses of 2.3 +/- 0.5 Mjup and 7.3 +/- 0.3 Mjup. We also find dynamically long-lived orbits consistent with the data, associated with 2:1 and 5:2 period ratios. The data scatter by 0.07 seconds relative to the best-fit model, by some margin the most precise of any of the proposed eclipsing compact object planet hosts. Despite the high precision, degeneracy in the orbit fits prevents a significant measurement of a period change of the binary and of N-body effects. Finally, we point out a major flaw with a previous dynamical stability analysis of NN Ser, and by extension, with a number of analyses of similar systems.

Authors:  T.R. Marsh, S.G. Parsons, M.C.P. Bours, S.P. Littlefair, C.M. Copperwheat, V.S. Dhillon, E. Breedt, C. Caceres, M.R. Schreiber

Read the paper and abstract on arXiv

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