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Phobos

Editor’s note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we repost astrobites content here at AAS Nova once a week. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org!

Title: On the Impact Origin of Phobos and Deimos I: Thermodynamic and Physical Aspects
Author: Ryuki Hyodo, Hidenori Genda, Sébastien Charnoz, and Pascal Rosenblatt
First Author’s Institution: Tokyo Institute of Technology, Japan
Status: Accepted to ApJ, open access

Where Did Phobos and Deimos Come From?

Phobos and Deimos, Mars’ two small moons, were initially believed to be the result of interplanetary kidnapping. Many moons in our solar system appear to be captured objects, and the featureless reflectance spectra of Phobos and Deimos hint that they might be D-type asteroids. However, captured objects tend to have highly eccentric orbits, and both Phobos and Deimos orbit Mars in a nearly circular fashion. More recently, it has been proposed that both moons are the result of a massive impact 4.3 billion years ago — instead of having been captured from interplanetary space, they could have coalesced from the debris disk generated by the impact. Past research has shown that the masses and orbits of Phobos and Deimos can be explained by this method. This theory could also explain the presence of Borealis basin, an extended low-altitude region spanning Mars’ north pole, which can be seen in Figure 1.

Figure 1. Topographical map of Mars. Borealis basin is the low-lying (blue) region in the northern hemisphere. It encompasses many officially-named regions, such as Vastitas Borealis and Utopia Planitia. [Adapted from this image, which is made from data from the Mars Orbiter Laser Altimeter aboard Mars Global Surveyor]

In this paper, the authors use smoothed particle hydrodynamics (SPH) simulations to learn more about the thermodynamical and structural properties of the debris disk generated in the proposed impact. SPH is a common method used when simulating astrophysical fluids, or systems with a large number of particles that can be treated as a fluid, like stars in colliding galaxies. In SPH (which has been detailed in previous Astrobites like this one and this one), the properties at a given location within a fluid are extracted by weighting the properties of the particles near that point using a smoothing function (sometimes called a smoothing kernel), which is often simply a Gaussian.

Building Baby Moons

To generate a debris disk, the authors modeled a collision between young Mars and an impactor with 3% the mass of Mars. The first 20 hours of their simulation are shown in Figure 2.

Figure 2. Snapshots from the first 20 hours after the simulated impact. Top row: Positions of particles over time. Red points are Mars particles, yellow are particles that fall on to Mars, white are disk particles, and cyan are particles that escape the system. Bottom row: Temperature of the particles. Shock heating in the moments after impact liquefies much of the material. [Hyodo et al. 2017]

The material ejected in the collision is so hot that it becomes molten, but quickly cools into roughly 1.5-meter solid droplets. The initially eccentric orbits of the disk particles precess over the next 30–40 years, resulting in a collisional torus of material around the planet. The collisions within this torus heat the material once more, again rendering it molten. Most of the material cools into 100-micron-sized droplets, but a small fraction of the silicates in the disk vaporize and condense into 0.1-micron-sized particles that can coat the larger particles. Although the solid-to-gas transition is inefficient, the process of gas-to-solid condensation generates the fine silicate particles that could be responsible for the observed, asteroid-like spectral properties.

Figure 3. The cumulative fraction of Mars-originating disk particles as a function of the depth below Mars’ surface from which they originated. Beyond 4 Mars radii (solid line), there is a higher percentage of particles originating from > 50 km below the surface than in the disk as a whole (dashed line). [Hyodo et al. 2017]

Another important finding from this work is that regardless of the angle of the impact, the disk contains material from both the impactor and young Mars. Figure 3 shows the distribution of disk particles as a function of how far below Mars’ surface they originated. Regardless of the impact angle, the disk as a whole contains at least 35% Martian material by mass. In the outer disk, beyond 4 Mars radii, this fraction rises to ~70%. What’s more, the Martian material largely comes from the mantle, about 50–150 km below the surface. This means that although the angle of impact and the radial distance at which the moons form will determine how much Mars material they contain, all formation scenarios lead to the moons being composed of a mixture of the impactor and Martian mantle. Although we currently have rovers scratching the surface, our best hope of learning about the material beneath Mars’ crust could be by studying its moons.

Know Before You Go: How Can This Help Future Mars Missions?

Now that we have some idea what to expect if Phobos and Deimos were formed from a debris disk, how can we use this knowledge? These results will be valuable for the planning of future Mars-system sample-return missions, like JAXA’s planned Martian Moon eXploration (MMX) mission, which is set to launch in the early 2020s. MMX is slated to make close observations of both Phobos and Deimos before collecting a sample from one of the moons and returning it to Earth. Performing simulations like these in advance of future sample return missions will help scientists interpret their findings to learn about the origin of these two moons as well as the interior of Mars itself.

About the author, Kerrin Hensley:

I am a third year graduate student at Boston University, where I study the upper atmospheres and ionospheres of Venus and Mars. I’m especially interested in how the ionospheres of these planets change as the Sun proceeds through its solar activity cycle and what this can tell us about the ionospheres of planets around other stars. Outside of grad school, you can find me rock climbing, drawing, or exploring Boston.

cloudy exoplanet

Editor’s note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we repost astrobites content here at AAS Nova once a week. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org!

Title: A Cloudiness Index for Transiting Exoplanets Based on the Sodium and Potassium Lines: Tentative Evidence for Hotter Atmospheres Being Less Cloudy at Visible Wavelengths
Author: K. Heng
Author’s Institution: Center for Space and Habitability, University of Bern
Status: Published in ApJL, open access

Background

Most astronomers hate clouds, except for those who study them. Not only do clouds on the sky ruin observing nights, clouds on other planets obscure everything underneath, too. When observing extra-solar planets during transit, different atoms, molecules, and particles absorb light at different specific wavelengths, making the apparent radii of the planets change when viewed in different colors; this is known as transmission spectroscopy. Although it is a powerful tool to infer the atmospheric composition of exoplanets, these clouds often hinder our efforts to understand such alien worlds. With clouds present at high altitude, only the atmosphere above the clouds can be seen, and it is too thin to provide any spectral features (see this popular example). It turns out that a useful strategy for dealing with a problem is to avoid it. Valuable space telescope time could be saved if we could filter cloud-free objects from ground-based measurements. Today’s paper presents an index to quantify the degree of cloudiness for transiting planets.

Aim

The author first revisits how previous studies used the slope of transmission spectra caused by scattering (blue light is scattered more than red) to distinguish cloudy and dense atmospheres. Such dense atmospheres are more packed (with smaller scale height) and can produce flat spectra that are compatible with cloud-free hydrogen-dominated atmospheres. The author demonstrates that the scattering cross sections of gaseous molecules (e.g. hydrogen, nitrogen) and aerosols or condensates (cloud particles) have the same wavelength dependence. Measuring the spectral slope alone does not resolve the ambiguity between clouds and atmospheric composition. With this motivation, the author proceeds to find a cloudiness index that does not depend on the spectral slope.

Methods

Sodium has large cross sections so it can produce a prominent spectral line without being abundant. It is, in fact, the first extrasolar atmospheric detection on HD 209458b. In a clear, cloud free atmosphere, the difference in transit radii between the line center and wing of sodium can be theoretically calculated. By measuring the actual difference in transit radii between the line center and wing (Δ Robs), the author constructs a dimensionless index (C) for the degree of cloudiness as the ratio of Δ R and Δ Robs. For an entirely cloud-free atmosphere, Δ R equals to Δ Robs and C = 1. Very cloudy atmospheres have C Gt 1. This cloudiness index is independent of the spectral slope, with the caveat that it is limited to planets with sodium or potassium line detections.

 

Figure 1: Cloudiness index plotted against equilibrium temperature (top), surface gravity (middle), planetary mass (bottom). The labels “W6,” “W17,” “W31,” “W39,” “H1,” “H12,” and “HD189” refer to WASP-6b, WASP-17b, WASP-31b, WASP-39b, HAT-P-1b, HAT-P-12b, and HD 189733b, respectively [Heng 2016].

Application to Data

Figure 1 plots the cloudiness index (C) versus equilibrium temperature, gravity, and mass of the planets. The uncertainty of the cloudiness basically stems from estimating the scale height with the equilibrium temperature in the calculation, which is larger for colder planets. An interesting trend of decreasing C with increasing equilibrium temperature (a proxy for stellar flux) is seen in the top panel. If the trend is real, it implies that more irradiated planets tend to be less cloudy. Future measurements of sodium lines at higher resolutions and for a larger sample will confirm or debunk the trend. If the trend holds, the author suggests that we can weed out cloudy objects for the detailed survey of JWST, to avoid spending lots of time (and money) for uninformative, featureless spectra.

About the author, Shang-Min Tsai:

I am a 3rd year PhD student at the University of Bern and part of the Exoplanets & Exoclimes Group led by Prof. Kevin Heng. We are developing various open-source tools to study exoplanets. I work on modelling of atmospheric chemistry and dynamics. When I am not coding or debugging, I enjoy basketball and playing board games.

LUVOIR

Editor’s note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we repost astrobites content here at AAS Nova once a week. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org!

Title: A statistical comparative planetology approach to the hunt for habitable exoplanets and life beyond the Solar System
Authors: Jacob L. Bean, Dorian S. Abbot and Eliza M.-R. Kempton [see disclaimer below] First Author’s Institution: University of Chicago
Status: Published in ApJL, open access

The discovery of the first exoplanet around a main sequence star in 1995 brought a great deal of attention to the search for life beyond our solar system (i.e., The Big Question). Since then, we have found thousands of extrasolar planets. As humans, our first instinct is to comb over this huge sample to find the best candidates for habitability (such as TRAPPIST-1 and Proxima b), and then painstakingly characterize them to check if they really are habitable. But this process is very resource-expensive and it hasn’t really paid off thus far. What if there are other, more effective ways to answer The Big Question?

The Problem with the Systems Science Approach

People are already planning and designing space telescopes whose main scientific aim is the search for habitable environments on exoplanets, such as LUVOIR and HabEx. As such, these instruments carry the weight of a world upon them. According to the authors of today’s paper, we cannot simply bring multiple detailed characterizations to bear on a handful of promising candidates; in order to be able to effectively answer The Big Question, we need to adopt a statistical approach.

Determining if an exoplanet is habitable or not goes way beyond simply pinpointing its position relative to a star: it is based on a combination of empirical inference and theoretical modeling. The objective of what the authors call the systems science approach is to identify such planets and make claims about the possibility that they harbor life.

The problem is that this framework requires considerable resources (observational and computational) to yield robust answers. In the end, it may not even reveal a definitive signature of life in your small sample of habitable planet candidates!

Lessons from Kepler and Friends

Inspired by research performed with Kepler data, Jacob Bean and collaborators propose the statistical comparative planetology approach. Its premise lies in utilising the diversity of planets in large samples to obtain information about their nature based on comparative studies. This approach has successfully been applied to identify false positive rates in Kepler data, as well as to study the water content of transiting planets. These results are robust in that they only depend on simple physical models, and they allow the identification of outliers, which reveal potential model weaknesses.

One of the examples the authors describe in today’s paper aims to test the habitable zone concept using carbon dioxide abundances. Planets that receive more irradiation from their star need less CO2 to maintain accommodating temperatures via the greenhouse effect, while those that receive less irradiation need more carbon dioxide. The idea of the test is to measure CO2 content on planets inside habitability zones and see if they are compatible with the amount expected for planets with accommodating temperatures (see the figure below). These measurements don’t need to be extremely precise given that the uncertainties in the physical properties of the planets is offset by the large sample size.

 

The habitable zone concept (blue curve) assumes a decreasing amount of CO2 as stellar irradiation increases. The black points represent hypothetical planets scrambled away from the blue curve based on the expected uncertainties of a statistical inference. The habitable zone concept can be tested if we are able to perform this kind of bulk analysis of a large sample of planets.

A similar test can be performed for water content on potentially habitable planets: if they are closer to the inner edge of the habitable zone, they should have lost all their water due to a runaway greenhouse effect, while those near the outer edge of the zone should have little water because it has frozen out. Any deviation from this means we are getting habitability wrong. Again, this test does not require a lot of precision, but does need a large number of planets.

The authors make it clear that the statistical approach is not the single best way to study habitability. For instance, this method still requires detailed analysis of a few planets to identify key diagnostics for habitability, and it is limited to planets around M-dwarf stars, whose habitable zones are the easiest to observe. However, comparative planetology still looks like a very pragmatic way to circumvent the unknowns and progress towards answering The Big Question.

Disclaimer: the leading author of today’s paper, Jacob L. Bean, was my research supervisor in 2016, but I am not in any form involved with this particular publication.

About the author, Leonardo dos Santos:

Leo is a graduate student of Astronomy at University of São Paulo, Brazil. His current research consists of studying physical and chemical properties of stars similar to the Sun, hunting for exoplanets and developing astronomical software. Not to be confused with the constellation.

Illustration of a dark body in the distant outer reaches of the solar system.

Editor’s note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we repost astrobites content here at AAS Nova once a week. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org!

Title: OSSOS VI. Striking Biases in the detection of large semimajor axis Trans-Neptunian Objects
Authors: Shankman C., Kavelaars J. J. et al.
First Author’s Institution: University of Victoria, Canada
Status: Accepted to AJ, open access

Dubbed one of the “Wildest Alien Planet Discoveries of 2016,” the prospect of Planet Nine has been truly exciting for all planet lovers alike. The line of evidence for this elusive planet lurking in the outer reaches of our solar system has been called into question, however, by the study discussed in today’s bite.

The Case for Planet Nine

The hypothesized Planet Nine is an icy and bigger analogue of Earth (about ten times as massive, and almost the size of Neptune or Uranus) on an orbit with a period of 15,000 years, and an aphelion (farthest approach from the Sun) ~1,200x as far away as the Earth from the Sun. This far-off region in which Planet Nine spends most of its time belongs to the Kuiper belt, and it is observed to be populated by mini planet-like icy bodies known as trans-Neptunian objects (TNOs) that have been left over as relics of the early formation of our solar system.

Figure 1. The closely spaced orbits of six of the most distant previously detected TNOs, as originally identified in 2016, points towards the existence of a ninth planet whose gravity affects these movements. (Source: via Caltech)

Since the early 2000s, researchers have noted that the long-period TNOs are not uniformly distributed in space. Instead, a number of these TNOs are found on orbits that are quite eccentric, with large semi-major axes, and they are clustered together in two distinct bundles relative to other TNOs. To explain this, scientists recently performed simulations and suggested the presence of an additional planet, a.k.a. Planet Nine — which, if it exists, could have perturbed the original orbits of these TNOs and clumped them together in space (see Fig. 1). With the discovery of more such TNOs and the understanding of their exact paths around the Sun, the idea of clustering — and hence of Planet Nine — can be tested.

So, Are There Any Clusters?

Over the last four years (2013-17), a large mapping survey known as the Outer Solar System Origins Survey (OSSOS) has used the Canada-France-Hawaii Telescope to look at eight different patches of the sky, taking a tremendous leap in the direction of finding more TNOs (see Fig. 2). During its tenure, the survey discovered more than 800 new TNOs, eight of which possess the long-period properties that could show evidence for or against the existence and effects of Planet Nine.

With the help of these eight “special” TNO discoveries, Cory Shankman (an OSSOS researcher) and his team try to shed some light on the matter by performing an independent test of the idea that a long-period massive planet is shaping the outer solar system. Cory and his team report, in today’s paper, that the eight OSSOS detections have orbits oriented across a wide range of angles, and are statistically consistent with those drawn from a random distribution. Additionally, one of the observations sits at right angles to the previously proposed clusters, thereby significantly reducing the odds that a tight spatial clustering in the orbits of long-period Kuiper-belt objects exists.

Figure 2. A top-down view of the Solar System depicting the schematic of OSSOS pointings (solid gray), orbits of four of the detected long-period TNOs (red, green, cyan, yellow), and Neptune’s orbit (blue); Figure 5 in the original paper.

As part of their analysis, the team also simulated the orbits of a uniform spread of tens of thousands of distant TNOs to test which bodies would be visible to their survey and found areas of the sky, like the dense star fields of our galaxy, where certain orientations of TNO orbits become very hard to detect. Based on their findings, the OSSOS team argued that detection biases resulting from a combination of atmospheric variations (affecting the depth of observation) and ease of discovery in sparse parts of the sky could have led to false indications of clustering in previously published studies, which in turn weakens the case for the existence of Planet Nine.

The Verdict

According to the team, the OSSOS survey was not designed to exclusively look for long-period TNOs, and hence their results cannot definitively prove, or rule out, the existence of Planet Nine, but they “substantially weaken the evidence that has been used to justify the need for an additional very massive planet in our solar system”. If the previous data was indeed unbiased, incorporating these new objects in the original simulations might lend more confidence to the hypothesis. However, the final word will perhaps come from a direct detection of more long-period TNOs through a dedicated survey, or of Planet Nine itself, if it exists, as scientists continue to work hard on pushing the boundaries of how far out we can look.

About the author, Bhawna Motwani:

Third year grad student doing computational research in stellar evolution and planet formation at Caltech. In my leisure time, I like to go Latin dancing, enjoy live theatre/music, and take on cultural and culinary pursuits.

artist's superhabitable planet

Editor’s note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we repost astrobites content here at AAS Nova once a week. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org!

Title: Natural and Artificial Spectral Edges in Exoplanets
Authors: Manasvi Lingam, Abraham Loeb
First Author’s Institution: Harvard-Smithsonian Center for Astrophysics
Status: Submitted to ApJL, open access

The explosion in the number of discovered exoplanets — especially some interesting systems with terrestrial planets in the habitable zone — has attracted a lot of attention. We are moving one step closer to the ultimate question: are we alone? Today’s paper looks at certain distinctive spectral features that could be caused by “extraterrestrial plants”, or even crazier: advanced civilizations.

The Sky Is Blue and All the Leaves Are Green

Ever wondered why most plants look green? The first answer you might get is because of chlorophyll, the green pigments responsible for photosynthesis. Plants carry out photosynthesis to convert water and CO2 into sugar and oxygen, using energy from the Sun. But one might further ask: why is chlorophyll green? Well, chlorophyll absorbs light primarily in the range from ~450 nm (blue) to ~650 nm (red). It operates in the visible spectrum range but is not as efficient in green light. So in visible light, the photons at green wavelength are reflected the most, producing the color that we see. This causes the small bump of the leaf reflectance near 500 nm (0.5 μm) in Figure 1. Note the sharp jump of reflectance starting around 0.7 μm and going into the infrared. This so-called “red edge” can be a useful feature for detecting vegetation on planets, since few substances in nature have such high reflectivity in that wavelength range. The strength of the red-edge feature is used on Earth to monitor the growth of vegetation (such as crops). Imagine if our eyes were a little more sensitive toward red; we would see the world very differently with plants turning red (and much brighter)!

Figure 1. The reflectance R for silicon-based solar cells (black) and plants (red), shown as a function of wavelength λ. The peaks in the reflectivity in the UV region of silicon and at 0.7 μm of plants are the distinct “spectral edges”. [Lingam & Loeb 2017]

We Need More Energy!

In today’s paper, the authors also boldly explore the possible “artificial spectral edges” — that is, advanced civilizations modifying the planet surface such that it changes the observable spectra as well. It is conceivable to assume that advanced civilizations would come up with a method to handle energy crises. One possible way is to harness a significant amount of energy from the star by constructing large arrays of solar cells. This is particularly relevant for tidally-locked planets around M-stars, such as Proxima b, where the dayside is permanently illuminated. The solar cells are made of semiconductors (typically silicon), which have an energy gap between the valence band and the conduction band. Photons with energies less than the band gap are scattered, causing high reflectance, similar to plants but at a shorter wavelength in UV. The authors explored a hypothetical scenario in which planets are covered with mega-scale arrays of solar cells, showing the reflectance for silicon-based solar cells in Figure 1.

tidally locked exoplanet

Figure 2. Schematic illustration of terraforming on tidally locked exoplanets. Photovoltaic arrays on the day side are used to harness stellar energy, which is redistributed as heat and light on the night side. [Lingam & Loeb 2017]

Another similarity between natural vegetation and solar cells is that, on tidally locked planets, they most likely are only situated on the day side (see the schematic in Figure 2). Therefore, as the fraction of vegetation or solar cells varies during the orbit, the changes of photometric flux in different wavelengths could be analyzed to characterize the spectral features. The authors calculated the change in the reflected light contrast to be within the sensitivity of future telescopes, like WFIRST (10-3 ppm) and LUVOIR (10-4 ppm), provided that (i) the coverage is large enough, (ii) the viewing angle is favorable, and (iii) the cloud cover is limited.

Of course, this is not saying we are going to find extraterrestrial life tomorrow, but it is helpful to keep in mind the possible information hidden in the reflected light. After all, the last thing we want is to see the sign, yet miss it.

About the author, Shang-Min Tsai:

I am a 3rd year PhD student at the University of Bern and part of the Exoplanets & Exoclimes Group led by Prof. Kevin Heng. We are developing various open-source tools to study exoplanets. I work on modelling of atmospheric chemistry and dynamics. When I am not coding or debugging, I enjoy basketball and playing board games.

Editor’s note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we repost astrobites content here at AAS Nova once a week. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org!

Title: iPTF17cw: An engine-driven supernova candidate discovered independent of a gamma-ray trigger
Authors: A. Corsi et al.
First Author’s Institution: Texas Tech University
Status: Submitted to ApJ, open access

Using the intermediate Palomar Transient Factory (iPTF), a broadlined type Ic supernova (BL-Ic SN) was accidentally uncovered during follow up observations of the newest gravitational wave in town — GW170104.  Further investigation of iPTF17cw suggests it is the first discovery of a candidate relativistic BL-Ic SN found independently of a gamma-ray trigger. Today’s bite presents the discovery, classification and follow-up observations of this fascinating supernova.

Coincidence?

On the 4th of January 2017, the ripples in space-time created by two merging black holes were detected by the Laser Interferometer Gravitational-Wave Observatory (LIGO). This detection resulted in a call to scientists around the world for follow-up observations, with the aim of locating an elusive electromagnetic (EM) counterpart. Observations undertaken by the extensive iPTF, a fully automated survey measuring a region almost 40 times the size of the full moon, revealed a candidate event.

Figure 2: The location of iPTF17cw, indicated by the star, superimposed on the likely origin of GW170104 as determined by LIGO. Black contours indicate the 90% credible region. [Corsi et al.]

Dubbed iPTF17cw, the event originated outside of the 90% localisation area of the gravitational wave, shown above in Figure 1, allowing scientists to quickly rule out the two events as being related.

Follow-Up Observations

With the gravitational-wave scenario ruled out, scientists moved towards the event being a supernova. At the time of its discovery this supernova had a magnitude of R = 19.5 mag, but it was not visible during observations of the same field taken in December. Therefore, several follow-up observations of the source were made to work out what had caused this burst of energy.

Figure 3: Light curves created after several follow up observations of iPTF17cw. Several similar supernovae light curves are also plotted. [Corsi et al.]

The authors note several interesting features in their observations including the detection of crucial narrow emission lines, such as those created by hydrogen, allowing for a redshift of z = 0.093 to be calculated. Several broadline features were also observed, causing the team to classify the supernova as a type Ic SN. Type Ic SNe are a subclass of SN described as engine-driven, meaning they’re commonly associated with Gamma Ray Bursts (GRBs).

In the Gap

GRBs are short-lived, energetic explosions of gamma-rays which are the brightest known events in the universe. Their origin is not well understood, but the most likely cause is a star collapsing to form a neutron star or black hole — a supernova! GRBs are relativistic because they eject particles at speeds comparable to the speed of light; as a result, they fade on timescales of a few days, whereas the afterglow of a SN is known for its longevity, shining brightly for hundreds of years.

The link between GRBs and BL-Ic SN has been established for over twenty years, yet it is still uncertain why some supernovae emit jets of energetic particles. This jet-like behaviour is observed in the X-ray and gamma-ray regime, meaning beams must be directed towards Earth to be observed. Moreover, a small portion of SNe discovered to have relativistic jets lack GRB components. It is unclear whether these events could be a new population of events that exist somewhere in the gap between SNe and GRBs, or if we’ve just missed key observations due to technological barriers. To establish whether iPTF17cw is relativistic and associated with a GRB, the team moved towards multi-wavelength observations of the surrounding area.

Multi-Wavelength Observations

Using the Very Large Array (VLA), the authors observed of iPTF17cw over a 3-month period, yielding a faint point-like radio source at 6 GHz. Because of its point-like nature, it’s unlikely to be caused by star formation occurring in the host galaxy.

Figure 4: VLA observations of iPTF17cw, red point-like source in the centre, at 6 GHz. [Corsi et al.]

The team then investigated the X-ray regime with the Swift Satellite and Chandra X-ray observatory to search for counterparts, but they detected X-rays only with Chandra. This was not a confident detection, yet its location is consistent with the optical and radio counterparts of the burst. Next, the authors searched the Fermi and Swift catalogues to find possible gamma-ray counterparts. A candidate match, a gamma-ray burst lasting 30s called GRB 161228, was uncovered.

Analysis of multi-wavelength data suggests that GRB 161228 and iPTF17cw are likely to be related. The rate of Fermi GRBs falling within the region of GRB 161228 was estimated to be 0.05 per month, meaning a chance coincidence of the events occurring but not being related has a probability of around 5%.

Is This a New Kind of SN?

Most of the conclusions drawn for this SN rely heavily on comparisons with previous events, such as similarities in the light curves, shown in Figure 3. There are clear agreements between the engine-driven SN 1998bw and relativistic SN 2009bb, which further suggest iPTF17cw and GRB 161228 are related.

Figure 4: VLA observations of iPTF17cw at 14 GHz. No detection was found. [Corsi et al.]

Finally, this SN was not detected at 14 GHz radio frequencies, suggesting that the SN is relativistic because it faded very quickly at this more energetic frequency. That would put this SN in a rare category: relativistic and discovered independently of gamma-rays. Follow-up observations with the VLA are crucial to confirm iPTF17cw’s relativistic nature by confirming that the SN has also faded at lower frequencies.

Thanks to the iPTF many more BL-Ic SNe are now being discovered, and a greater sample will greatly improve our understanding of this weird phenomenon of engine-driven SNe. In fact, the team expects to collect a sample of BL-Ic SNe in a year as large as the sample that has been collected over the last five years. It is only a matter of time before scientists conclude if a GRB counterpart is required, or if we have an extra category of events existing in the SN–GRB gap.

About the author, Amber Hornsby:

First year postgraduate researcher based in the Astronomy Instrumentation Group at Cardiff University. Currently I am working on detectors for future observations of the Cosmic Microwave Background. Other interests include coffee, Star Trek and pizza.

chirp

Editor’s note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we repost astrobites content here at AAS Nova once a week. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org!

Title: Electromagnetic Chirps from Neutron Star-Black Hole Mergers
Authors: Jeremy Schnittman, Tito Dal Canton, Jordan Camp, David Tsang, and Bernard Kelly
First Author’s Institution: NASA Goddard Space Flight Center and Joint Space-Science Institute
Status: Submitted to ApJ, open access

One of the biggest scientific accomplishments of the last few years was the discovery of gravitational waves by the LIGO Collaboration, which you can read about on Astrobites here. There is, of course, still plenty of work to be done in this field. For example, no experiment has definitively detected an electromagnetic counterpart (which would give off radiation somewhere in the electromagnetic spectrum) to a gravitational wave, although the Fermi Gamma-ray Burst Monitor may have seen hints of one. Detecting such a component would be scientifically interesting for many reasons. The authors of today’s paper give us two such reasons: first, LIGO currently only has a rudimentary ability to localize where in the sky gravitational waves are coming from. Identifying the specific galaxy that produced the gravitational wave would allow us to constrain certain astrophysical models. Second, we could possibly combine an electromagnetic (EM) counterpart with a sub-threshold gravitational-wave signal (one that is not statistically significant on its own) to glean more information about astrophysical events.

Some models of blacks holes merging with neutron stars call for short gamma-ray bursts (GRBs; the brightest EM explosions observed anywhere in the universe) to be produced by the merger. This is the same process that produces some gravitational waves. For some of these GRBs, there is a “precursor” gamma-ray flare a few seconds before the peak of the GRB emission. This may seem like only a short amount of time, but since the black hole and neutron star are orbiting each other very rapidly, this event would occur hundreds of orbits before the merger actually occurs. The environment here includes some of the most extreme forces in the known universe, and all the particles involved are extremely relativistic (traveling close to the speed of light). Therefore, the light curve for the precursor flare that eventually reaches the observer on Earth will be affected by phenomena such as relativistic Doppler beaming and gravitational lensing. The former makes the light appear at a different luminosity than it actually is and the latter causes the light to bend on its way to us. The authors explain that all the physics combines to give off an electromagnetic “chirp”, similar to the “chirps” that gravitational waves give off. Therefore, it is conceivable that algorithms similar to those used by the LIGO collaboration could be used to search for the electromagnetic chirps.

The authors used a Monte Carlo radiation transport code to calculate the light curves and the resulting spectra on Earth of a neutron-star–black-hole merger. Free parameters included the masses of the neutron star and black hole, along with the separation between them. Figure 1 shows what the thermal emission coming from the surface of the neutron star would like like over time, from the point of view of an observer looking at it edge-on (see the caption for details). They note that the inclination angle of the observer does affect their results, with the Einstein ring — a signature of gravitational lensing — only being visible at high angles. However, at smaller angles a modulation from the relativistic beaming is still present.

Figure 1: An illustration showing what the neutron star/black hole system would look like to an edge-on observer at different times. In a) the ring is caused by gravitational lensing effects; b) is the point of maximum blueshift; c) shows a weaker gravitational lensing effect; d) is the point of maximum redshift.

As the system gives off gravitational radiation, the distance between the black hole and the neutron star decreases with the size of the orbit. This causes the frequency and amplitude of the light curve described above to increase, and a “chirp” — just like in the gravitational-wave signal — is observed. See the figure below for an illustration of the inspiral. The different features of the light curve (from the gravitational lensing and the beaming) dominate at different times here, and from this the black hole mass can be determined. If the light curve is precise enough, the neutron star’s radius and equation of state could even be determined.

Figure 2: The electromagnetic modulation for the inspiral of a neutron star/black hole binary merger (initial separation 50 solar masses), with the zoomed in portions corresponding to the beginning and the end.

The authors do note that the luminosity range in which the EM “chirp” could be detected using satellites such as Fermi GBM is fairly small: if it is too bright, a fireball will occur that would mask the chirp. They end the paper by observing that there are still chirps that could be detected with current technologies, and that future gravitational-wave observatories such as LISA could potentially work as a trigger for electromagnetic experiments to target their observations.

About the author, Kelly Malone:

I am a fourth year physics graduate student at Penn State University studying gamma-ray astrophysics. I previously received bachelor’s degrees in physics and astronomy from UMass Amherst in 2013.

brown dwarf

Editor’s note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we repost astrobites content here at AAS Nova once a week. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org!

Title: The First Brown Dwarf Discovered by the Backyard Worlds: Planet 9 Citizen Science Project
Authors: Marc J. Kuchner, Jacqueline K. Faherty, Adam C. Schneider et al.
First Author’s Institution: NASA Goddard Space Flight Center
Status: Published in ApJL, open access

Not everyone can be a star. Brown dwarfs, for example, have failed at their attempt. These objects have masses below the necessary amount to reach pressure and temperature high enough to burn hydrogen into helium in their cores and thus earn the classification “star”. It’s not very long since we’ve learned of their existence. They were proposed in the 1960s by Dr. Shiv S. Kumar, but the first one was only observed many years later, in 1988 — and we are not even sure it is in fact a brown dwarf! We’ve only reached a substantial number of known brown dwarfs with the advent of infrared sky surveys, such as the Two Micron All Sky Survey (2MASS) and the Wide-field Infrared Survey Explorer (WISE).

Discovering and characterising cold brown dwarfs in the solar neighbourhood is one of the primary science goals for WISE. There are two ways of doing that: 1) identifying objects with the colours of cold brown dwarfs; 2) identifying objects with significant proper motion. Brown dwarfs are relatively faint objects, so they need to be nearby to be detected. We can detect the movement of such nearby targets against background stars, which are so distant that they appear to be fixed on the sky. This movement is called proper motion. As the signal-to-noise ratio is not very good for such faint objects, the second method is the preferred one. However, single exposure WISE images are not deep enough to find most brown dwarfs. This is where today’s paper enters. The authors have launched a citizen science project called “Backyard Worlds: Planet 9” to search for high proper motion objects, including brown dwarfs and possible planets orbiting beyond Pluto, in the WISE co-add images. Co-add images are simply a sum of the single exposures images taking into account corrections to possible shifts between them. This increases signal-to-noise ratio and helps to detect faint targets. In today’s paper, the authors report the first new substellar discovery of their project: a brown dwarf in the solar neighbourhood, which was identified only six days after the project was launched!

Citizen Science: A Promising Approach

The idea behind citizen science is to engage numerous volunteers to tackle research problems that would otherwise be impractical or even impossible to accomplish. The Zooniverse community hosts lots of such projects, in disciplines ranging from climate science to history. Citizen science projects have made some remarkable discoveries in astronomy, such as KIC 8462852 (aka “Tabby’s Star”, “Boyajian’s star” or “WTF star”).

In “Backyard Worlds: Planet 9”, volunteers are asked to examine short animations composed of difference images constructed from time-resolved WISE co-adds. The difference images are obtained by subtracting the median of two subsequent images from the image to be analysed. This way, if an object does not significantly move, it will disappear from the analysed image with the subtraction, leaving only moving objects to be detected. The images are also divided into tiles small enough to be analysed on a laptop or cell phone screen. The classification task consists of viewing one animation, which is composed of four images, and identifying candidates for two types of moving objects: “movers” and “dipoles”. Movers are fast moving sources, that travel more than their apparent width over the course of WISE’s 4.5 year baseline. Dipoles are slower-moving sources that travel less than their apparent width, so that there will be a negative image right next to a positive image, since the subtraction of the object’s flux will only be partial. An online tutorial is provided to show how to identify such objects and distinguish them from artifacts such as partially subtracted stars or galaxies, and cosmic rays.

The Discovery: WISEA 1101+5400

Figure 1: Two co-adds of WISE data separated by 5 years showing how WISEA 1101+5400 has moved. The region shown is 2.0” x 1.6” in size. [Kuchner et al. 2017]

Five users reported a dipole on a set of images, which can be seen here, the first report taking place only six days after the project was launched. The object, called WISEA 1101+5400, can be seen in Figure 1. This source would be undetectable in single exposure images, but in these co-adds it is visible and obviously moving. Follow-up spectra were obtained using the SpeX spectrograph on the 3-m NASA Infrared Telescope Facility (IRTF). The average spectrum is shown in Figure 2. Both the object’s colours and the obtained spectra are consistent with a field T dwarf, a type of brown dwarf.

Figure 2: In black, the spectrum for WISEA 1101+5400. A field T5.5 brown dwarf, SDSS J0325+0425, is shown in red for comparison. Atomic and molecular opacity sources that define the T-dwarf spectral class are indicated. [Kuchner et al. 2017]

Assuming WISEA 1101+5400 is the worst case scenario, i.e. about as faint an object as this survey is able to detect and with the minimum detectable proper motion, the authors estimate that “Backyard Worlds: Planet 9” has the potential to discover about a hundred new brown dwarfs. If WISEA 1101+5400 is not the worst case scenario, but objects even fainter or with lower proper motion can be found, this number could go up.

Although the discovery of only one brown dwarf might not seem worthy of celebration, this discovery demonstrates the ability of citizen scientists to identify moving objects much fainter than the WISE single exposure limit. It is yet more proof that science could use the help of enthusiasts. So if you’re not doing anything now, why not head over to https://www.zooniverse.org/ and help a scientist?

About the author, Ingrid Pelisoli:

I am a third year PhD student at Universidade Federal do Rio Grande do Sul, in Brazil, and currently a visiting academic at the University of Warwick, UK. I study white dwarf stars and (try to) use what we learn about them to understand more about the structure and evolution of our Galaxy. When I am not sciencing, I like to binge-watch sci-fi and fantasy series, eat pizza, and drink beer.

long-period exoplanet

Editor’s note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we repost astrobites content here at AAS Nova once a week. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org!

Title: Orbits for the Impatient: A Bayesian Rejection Sampling Method for Quickly Fitting the Orbits of Long-Period Exoplanets
Authors: Sarah Blunt, Eric L. Nielsen, Robert J. De Rosa, et al.
First Author’s Institution: Brown University
Status: Published in ApJ, open access

Discoveries of exoplanets happen quite often these days — so much so that the discovery alone is not enough to satisfy collective scientific curiosity. Discovery with direct imaging, in particular, does not usually reveal much about the planet, other than its existence. However, unlike the transit method and radial velocity measurements, direct imaging allows us to observe exoplanets with very long periods, which is an under-sampled population among currently known exoplanets. Still, this double-edged method of measurement cannot give us full orbital parameters of the planetary system. Long-period exoplanets cannot be easily observed by any other method but direct imaging, so the question arises — how can we find the orbital properties of this planetary population with the measurements we have?

orbits

A visualization of the OFTI method sampling, scaling and rotating a randomly selected orbit of the fitted exoplanet. In the lowest image, the red lines are the accepted orbits while the gray lines show the rejected orbits. [Blunt et al. 2017]

The authors of today’s paper use a new rejection sampling method to quickly find the orbits of these exoplanets, called Orbits for the Impatient (OFTI). This method generates random orbital fits from astrometric measurements, then scales and rotates the orbits, and then rejects orbits too unlikely. A visualization of this process is shown in the figure to the left.

This method uses astrometric observations and their uncertainties with prior probability density functions to produce posterior probability density functions of generated orbits. The main process of a rejection sampling method goes like this: the code generates random sets of orbital parameters, calculates a probability for each value, then rejects values with lower probabilities. The rejection process in OFTI is determined by comparing the generated probability to a selected number in (0,1). If the generated probability is greater than the random variable, then the orbit is accepted. This process repeats until any desired numbers of orbits have been selected.

Usually, algorithms such as Metropolis-Hastings MCMC are used for orbital fitting problems. However, this method takes far less time than an MCMC approach. The OFTI trials are independent, so the fitting and rejection-sampling can be done several times without incurring a bias in fitting. Running OFTI for several successive trials gives an unbiased estimate of the orbit up to 100 times faster than traditional Metropolis-Hasting MCMC fitting.

You may wonder how this method manages to run quickly without compromising the accuracy of its results. The answer to this musing is, of course, clever computational and statistical tricks. OFTI uses vectorized arrays rather than iterative loops when possible, and it is specifically designed to run multiple trials in parallel. Since there is an associated error with the astrometric measurements that OFTI uses to generate orbits, it first calculates the minimum χ2 value of all orbits tested during an initial run. Then it subtracts the minimum χ2 value from all other generated χ2 values. This way, orbits with an artificially high χ2 are not unfairly flat-out rejected. OFTI also confines the inclination and mass based on prior measurements, then uses the maximum, minimum and standard deviation of the array to change the range of values for these parameters, which prevents the generation of obviously unlikely orbits.

In this paper, the authors use this fitting method to find orbital parameters for 10 directly imaged exoplanets and other objects, including brown dwarfs and low-mass stars. The objects have at least two measured epochs of astrometry each; however in these cases, the orbits have not yet been measured because the measurements only cover a short range of the objects’ orbits. Using OFTI, the authors were able to successfully solve for the orbits of each of these substellar objects. The fitting for one of these objects, GJ 504 b, the current coldest imaged exoplanet, is shown in the figure below.

GJ 504 b

The orbit sampling of the planet GJ 504 b around star GJ 504 A. The 100 most probable orbits are colored accordingly. The right section of the image shows the measurements made of the object in black, and the red line shows the minimum orbit. [Blunt et al. 2017]

The most obvious application of this new process is long-period exoplanets, but the authors also solve for the orbits of a variety of other systems, including trinary stars and brown-dwarf systems. OFTI is also very useful in planning follow-up observations of targets. This method is incredibly useful, not only to planetary scientists but also to all kinds of stellar specialists. Impatient scientists can now use this method to achieve quick and accurate results — which are, quite frankly, the best kind of results.

About the author, Mara Zimmerman:

Mara is working on her PhD in Astronomy at the University of Wyoming. She has done research with binary stars, including “heartbeat” stars, and currently works on modeling debris disks.

IceCube

Editor’s note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we repost astrobites content here at AAS Nova once a week. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org!

Title: Evidence against star-forming galaxies as the dominant source of IceCube neutrinos
Authors: Keith Bechtol et al.
First Author’s Institution: Wisconsin IceCube Particle Astrophysics Center and University of Wisconsin-Madison
Status: Published in ApJ, open access

The IceCube Neutrino Observatory is a giant telescope embedded deep within the South Pole ice — a trap waiting to detect elusive astrophysical neutrinos. These neutrinos have traveled from violent, often explosive astrophysical sources all the way to the Earth without interacting with another particle. Many of these neutrinos will pass straight through the Earth and out the other side without leaving a trace. However, the strings of detectors inserted a kilometer deep into the Antarctic ice that make up IceCube (Fig. 1) can detect the radiation produced by rare neutrino interactions and, in fact, the IceCube collaboration announced the first ever detection of astrophysical neutrinos back in 2013. (Check out this astrobite on their discovery!) The origin of these particles, however, remains a mystery.

Fig. 1. The IceCube telescope consists of thousands of detectors attached along strings embedded within a cubic kilometer of Antarctic ice.

Active galactic nuclei and gamma-ray bursts were early favorites for the origin of the IceCube neutrinos, but one by one potential sources have been ruled out. Today’s paper eliminates another top contender, further deepening the mystery.

Star-forming galaxies (SFGs) host constant collisions between energetic cosmic rays and interstellar gas. These collisions produce unstable particles called pions, which live for tiny fractions of a second before decaying into more sturdy particles like neutrinos. Along with these neutrinos comes a similar amount of energetic gamma-rays. Observations of the gamma-rays can therefore support or, as the case may be, undermine the argument that SFGs produce the IceCube neutrinos.

Gamma-rays observed by the Fermi Large Area Telescope can be divided into two categories — those that can be traced back to a particular source, and those that make up a diffuse fog of gamma-rays with no particular known origin. Many of the diffuse gamma-rays come from point sources too faint to be individually resolved. In order for SFGs to be a valid source of IceCube neutrinos, they must produce a significant fraction of these diffuse gamma-rays. However, recent studies of the diffuse gamma-ray background suggest that blazars make up at least 72%, leaving little room for SFG gamma-rays.

In today’s paper, Keith Bechtol and his collaborators predict the maximum number of neutrinos that could be produced by SFGs without violating the limits on the gamma-ray emission. Figure 2 shows that even with the maximum allowed gamma-ray emission, SFGs just are not going to produce enough neutrinos to account for everything we are seeing in IceCube.

Fig. 2. The constraints on the gamma-rays emitted by star forming galaxies (red line) limit the neutrinos these galaxies can produce (black line). The IceCube neutrinos represented by the black points lie significantly above the black line, still unexplained. [Bechtol et al. 2017]

So, one more promising source is eliminated, narrowing down the options, and pushing more and more unexpected possibilities to the forefront. Other recent work on the origin of IceCube neutrinos suggests that perhaps radio galaxies can explain both the IceCube neutrinos and the remaining piece of the diffuse gamma-ray emission. Others claim that we have acted too quickly in ruling out SFGs — perhaps more complex modeling of the particle interactions within these galaxies will allow for some of the gamma-rays that must be produced to be quickly absorbed, escaping our detection. Continued analysis and larger datasets will someday reveal the answer to this mystery. It is already exciting, however, that we are beyond the realm of expected answers. New observations always reveal the unexpected.

About the author, Nora Shipp:

I am a 2nd year grad student at the University of Chicago. I work on combining simulations and observations to learn about the Milky Way and dark matter.

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