Features RSS

CI Tau

Blistering hot, giant planets zip around many stars similar to the Sun. New observations of one such planet — likely surrounded by a set of three gas-giant siblings — is now raising questions about the formation of giant planets.

Birthing a Hot Jupiter

Hot Jupiters — gas giants orbiting their hosts at radii of < 0.1 AU — are thought to orbit around 1% of main-sequence solar-type stars. In spite of the many hot Jupiters we’ve discovered, however, we still don’t fully understand how these toasty giants form. Are they born and evolve in situ, alongside their host? Or do they develop further out in the early protoplanetary disk of gas and dust surrounding a young star, and migrate inward later in their lifetimes? What role might outer gas-giant siblings play in this process?

CI Tau continuum observations

Synthesized image of the CI Tau continuum observations, revealing three annular gaps between 10 and 100 AU. The inset shows a 0.35”-wide zoom on the innermost gap, imaged with a finer resolution. [Clarke et al. 2018]

One challenge to answering these questions is we’ve primarily observed older hot Jupiters, rather than those still in the process of forming. But the protoplanetary disk surrounding the young star CI Tau hosts a nascent hot Jupiter — and new observations reveal evidence for additional gas giants in the disk. What can we learn from this system?

A Unique Disk

The Atacama Large Millimeter/submillimeter Array (ALMA) is progressively building its reputation as an imager of the detailed structure within young protoplanetary disks; you might recall one of its first releases, the spectacular image of the concentric gaps and rings of the disk surrounding HL Tau.

In a new study led by Cathie Clarke (University of Cambridge’s Institute of Astronomy, UK), this groundbreaking telescope has been used to image another young star system, CI Tau, which hosts the first hot-Jupiter candidate found still within a protoplanetary disk.

Though CI Tau is roughly the same mass and luminosity as the Sun, it’s only about 2 million years old. The presence of the 11.3-Jupiter-mass giant planet in a close-in orbit in the disk therefore shows that whatever the formation mechanism for hot Jupiters, they must arrive at their close orbits very rapidly!

New Planets, New Challenges

CI Tau model

Synthetic image of the CI Tau continuum emission produced from the authors’ gas and dust hydrodynamical simulation containing three planets. The authors’ models well reproduce the observations of the system. [Clarke et al. 2018]

The hot Jupiter isn’t CI Tau’s only source of intrigue, though: Clarke and collaborators demonstrate that this disk likely hosts another three gas-giant planets of 0.75, 0.15, and 0.4 Jupiter masses, orbiting at 14, 43, and 108 AU. The authors reach this conclusion by fitting detailed dynamical models of the dust and gas to the ALMA data as well as a wealth of supplementary data for the well-studied system.

If confirmed, the presence of these outer planets poses a significant challenge to our understanding of planet formation. Based on current models, the outermost planets shouldn’t have been able to accrete enough material from the outer disk in just 2 million years to reach their current sizes. And even if this mystery could be explained, another exists: the planets’ growth past a certain point should have triggered rapid inward migration.

Though the new observations of CI Tau have raised as many questions as answers, they solidify the association between close-in hot Jupiters and gas giants on wider orbits. As ALMA continues to build a census of young protoplanetary disks, we can hope that future observations will shed further light on the process of building giant planets.

Citation

“High-resolution Millimeter Imaging of the CI Tau Protoplanetary Disk: A Massive Ensemble of Protoplanets from 0.1 to 100 au,” C. J. Clarke et al 2018 ApJL 866 L6. doi:10.3847/2041-8213/aae36b

neutron-star merger

High-energy radiation released during the merger of two neutron stars last year has left astronomers puzzled. Could a burst of gamma rays from 2015 help us to piece together a coherent picture of both explosions?

A Burst Alone?

When two neutron stars collided last August, forming a distinctive gravitational-wave signal and a burst of radiation detected by telescopes around the world, scientists knew that these observations would change our understanding of short gamma-ray bursts (GRBs). Though we’d previously observed thousands of GRBs, GRB 170817A was the first to have such a broad range of complementary observations — both in gravitational waves and across the electromagnetic spectrum — providing insight into its origin.

GRB energetics

Total isotropic-equivalent energies for Fermi-detected gamma-ray bursts with known redshifts. GRB 170817A (pink star) is a factor of ~1,000 dimmer than typical short GRBs (orange points). GRB 170817A and GRB 150101B (green star) are two of the closest detected short GRBs. [Adapted from Burns et al. 2018]

But it quickly became evident that GRB 170817A was not your typical GRB. For starters, this burst was unusually weak, appearing 1,000 times less luminous than a typical short GRB. Additionally, the behavior of this burst was unusual: instead of having only a single component, the ~2-second explosion exhibited two distinct components — first a short, hard (higher-energy) spike, and then a longer, soft (lower-energy) tail.

The peculiarities of GRB 170817A prompted a slew of models explaining its unusual appearance. Ultimately, the question is: can our interpretations of GRB 170817A safely be applied to the general population of gamma-ray bursts? Or must we assume that GRB 170817A is a unique event, not representative of the general population?

New analysis of a GRB from 2015 — presented in a recent study led by Eric Burns (NASA Goddard SFC) — may help to answer this question.

A Matter of Angles

What does a burst from 2015 have to do with the curious case of GRB 170817A? Burns and collaborators have demonstrated that this 2015 burst, GRB 150101B, exhibited the same strange behavior as GRB 170817A: its emission can be broken down into two components consisting of a short, hard spike, followed by a long, soft tail. Unlike GRB 170817A, however, GRB 150101B is not underluminous — and it lasted less than a tenth of the time.

GRB 150101B

Fermi count rates in different energy ranges showing the short hard spike and the longer soft tail in GRB 150101B. The short hard spike is visible above 50 keV (top and middle panels). The soft tail is visible in the 10–50 keV channel (bottom panel). [Burns et al. 2018]

Intriguingly, these similarities and differences can all be explained by a single model. Burns and collaborators propose that GRB 150101B and GRB 170817A exhibit the exact same two-component behavior, and their differences in luminosity and duration can be explained by quirks of special relativity.

High-speed outflows such as these will have different apparent luminosities and durations depending on whether we view them along their axis or slightly from the side. Burns and collaborators demonstrate that these the two bursts could easily have the same profile — but GRB 150101B was viewed nearly on-axis, whereas GRB 170817A was viewed from an angle.

If this is true, then perhaps more GRBs have hard spikes and soft tails similar to these two; the tails may just be difficult to detect in more distant bursts. While more work remains to be done, the recognition that GRB 170817A may not be unique is an important one for understanding both its behavior and that of other short GRBs.

Citation

“Fermi GBM Observations of GRB 150101B: A Second Nearby Event with a Short Hard Spike and a Soft Tail,” E. Burns et al 2018 ApJL 863 L34. doi:10.3847/2041-8213/aad813

galactic outflow

New observations by the Atacama Large Millimeter/submillimeter Array (ALMA) provide a close look at a galaxy that may be in the process of shutting down its star formation.

Transitioning a Galaxy

galaxy types

We think that galaxies transition from blue spirals actively forming stars (left) to red, quiescent ellipticals (right). [Hubble/Galaxy Zoo]

Though we know much more about the processes of galaxy formation and evolution than we did even a decade ago, many key points still elude us. One particular puzzle is that of how star formation ends in a galaxy. We think that galaxies eventually transition from bright, blue, star-forming disks into red and quiescent ellipticals — but what causes star formation in a galaxy to shut down during this transition?

Since galaxies form stars out of cold gas, we could assume that star formation stops only when the cold gas supply is depleted. But observations suggest that star formation can shut down across a galaxy much more quickly than the timescale for using up the gas supply — sometimes turning off within just a few tens of millions of years. Such a rapid shutdown is termed “violent quenching”.

SDSS J1341–0321

Top: Hubble images of SDSS J1341–0321. Bottom: Contours show the location of the galaxy’s molecular gas: all CO J(2 → 1) molecular gas (left), just the gas moving rapidly toward us (middle) and just the gas moving rapidly away from us (right). [Geach et al. 2018]

Options for Violent Quenching

What mechanisms could suddenly prevent cold gas from contracting into stars in the disk of a galaxy? The most efficient approach is the rapid removal or destruction of the molecular gas.

In one common picture of rapid gas removal, powerful jets emitted from the supermassive black hole in a galaxy’s active nucleus (AGN) play a key role. In this model, the jets blow out the galaxy’s molecular gas on short timescales — and in so doing, they both clear out gas available for star formation, and also propel metal-enriched gases into the circumgalactic medium.

But new observations have challenged the picture of AGN-driven outflows as the standard violent-quenching mechanism. In a recent study led by Jim Geach (University of Hertfordshire, UK), a team of scientists presents a new view of a quenching galaxy that doesn’t seem to have an AGN.

Look to the Stars

Using ALMA, Geach and collaborators trace the molecular gas in SDSS J1341–0321, a massive and compact galaxy thought to have recently undergone a major merger and now showing signs of early-stage quenching. Despite this galaxy hosting no evidence of an active nucleus, the ALMA observations reveal an outflow of cool gas moving at a speedy 1,000 km/s relative to the stars.

Antennae Galaxies

The Antennae Galaxies are an example of a starburst galaxy with rapid star-formation activity driven by a recent merger. [NASA, ESA, and the Hubble Heritage Team (STScI/AURA)-ESA/Hubble Collaboration]

Geach and collaborators suggest that this outflow is violent quenching in another form: a powerful stellar outflow currently expelling around 300 solar masses of gas per year. They argue that this outflow was launched within the last 5 million years from a central starburst — a region of compact, vigorous star formation — triggered by SDSS J1341–0321’s recent merger. In this model, the stars themselves blow out all the gas — and once the gas is gone, star formation will turn off and the galaxy will appear red and quiescent.

If this model correctly describes SDSS J1341–0321, the next question is whether similar stellar outflows could account for violent quenching in other compact, massive galaxies across the universe. While we don’t yet know the answer, it seems likely that future high-resolution observations — perhaps also made with ALMA — will help us to find out!

Citation

“Violent Quenching: Molecular Gas Blown to 1000 km s−1 during a Major Merger,” J. E. Geach et al 2018 ApJL 864 L1. doi:10.3847/2041-8213/aad8b6

Acoustic waves in a star with a planet

Aldebaran, the brightest star in the constellation Taurus, was one of the first stars suspected to harbor an exoplanet. The presence of its planetary companion, Aldebaran b, was confirmed in 2015, and the decades of data preceding the discovery might harbor a few more surprises.

p-mode oscillations

A model of acoustic oscillations, also called p-modes, which can be used to infer fundamental properties of stars through asteroseismology. The vertical extent of the oscillation has been exaggerated by a factor of 1,000. [NASA/MSFC]

A Recognizable Target

Like hundreds of other exoplanets, Aldebaran b was discovered via the radial velocity method, in which the tug of a planet causes a detectable shift in the wavelengths of absorption lines in its parent star’s spectrum.

Radial velocity measurements can reveal more than just the presence of planetary companions, however; periodic oscillations of the star itself can be hidden in the radial velocity signal. These oscillations depend on the fundamental parameters of the star: mass, radius, surface gravity, and effective temperature.

To search for stellar oscillations of Aldebaran, a team of astronomers led by Will Farr (University of Birmingham, UK) delved into more than three decades of historical radial velocity measurements.

Farr et al. 2018 Fig. 3

Radial velocity measurements from the Hertzsprung SONG Telescope. The long-period planetary signal is superimposed upon the shorter-period p-mode signal. Click to enlarge. [Farr et al. 2018]

Digging Through the Data

The authors fit to the data a Keplerian model — to confirm the previously discovered planetary signal — and a Continuous Auto-Regressive Moving Average (CARMA) model — to search for stellar oscillations. In addition to verifying the presence of Aldebaran b, Farr and collaborators found evidence for stellar oscillations with maximum power at a frequency of 2.2 microhertz — well within the typical range for a red giant.

The authors followed up on these findings with high-cadence radial velocity observations from the Hertzsprung SONG Telescope and photometry from K2, the revived version of the Kepler Space Telescope. Analysis of both these datasets showed evidence of stellar oscillations consistent with those seen in the more irregularly sampled historical data.

Farr et al. 2018 Fig. 6

A model of the stellar irradiance received at Aldebaran b’s orbital distance over the course of Aldebaran’s main-sequence lifetime. [Farr et al. 2018]

Getting to Know Aldebaran

Farr and collaborators were able to estimate the orbital parameters of Aldebaran b and the mass and age of its parent star. The authors find that Aldebaran has a mass of 1.16 solar masses and an age of 6.4 billion years. This suggests that while Aldebaran is currently more than 500 times more luminous than the Sun, it likely matched the Sun’s output early in its life.

At an orbital distance of 1.5 AU, Aldebaran b probably enjoyed stellar irradiance similar to modern-day Earth in the distant past — about 4.4 billion years ago. However, Aldebaran b is at least 5.8 Jupiter masses (and may be massive enough to be a brown dwarf, depending on the inclination of its orbit), so it’s unlikely to have ever hosted life as we know it.

As Farr and collaborators have shown, it’s possible to extract accurate stellar and orbital parameters from irregularly sampled radial velocity data — which is plentiful thanks to radial-velocity surveys searching for exoplanets. With this technique, we may soon know of thousands of planets around red giant stars!

Citation

“Aldebaran b’s Temperate Past Uncovered in Planet Search Data,” Will M. Farr et al 2018 ApJL 865 L20. doi:10.3847/2041-8213/aadfde

inhomogeneous universe

The universe is expanding — but we’re still not sure how quickly! With past measurements of this expansion rate causing yielding conflict and debate, a new study investigates whether we can resolve the evident tension.

Conflicting Measurements

Hubble constant

Estimated values of the Hubble constant, 2001–2018. Data marked with circles show local, distance-ladder-calibrated measurements; data marked with squares indicate global measurements from the CMB and baryon-acoustic oscillations. Click to enlarge. [Kintpuash]

The first sign that the universe around us is expanding was found in the late 1920s, when astronomers first recorded evidence that distant galaxies appear to be moving away from us at ever faster rates, the more distant they are. This led to the development of the Hubble constant (H0), a value used to quantify this observed rate of expansion — and its value has been debated ever since.

Though we’ve come a long way since our initial, imprecise measurements of H0, today the two primary methods of measuring the Hubble constant remain in tension:

  1. Local measurements can be made by determining the distances and recession speeds of visible objects in the universe; Type Ia supernova surveys provide the standard candles needed for these measurements. Using this approach, scientists obtain an H0 value around us of ~74 (km/s)/Mpc.
  2. Global measurements are made by estimating the Hubble constant from measurements of the cosmic microwave background (CMB), relic radiation from the Big Bang. By fitting a multi-parameter model to Planck-mission observations of the CMB, scientists obtain a slower expansion estimate of ~68 (km/s)/Mpc for H0.

Inhomogeneity to the Rescue?

This discrepancy of nearly 9% between the two measurements — which cannot be brought into agreement by the measurements’ error bars — remains puzzling. Is one or the other group of astronomers making a mistake, or underestimating their errors? Could there be new physics at play in the cosmological model used to interpret the CMB results?

inhomogeneous universe

Expansion rate (left) and density (right) of a simulated inhomogeneous anisotropic universe. [Macpherson et al. 2018]

Some scientists have proposed an alternative explanation: what if the global expansion rate of the universe is not the same as the local rate? One possibility is that we live in a local void, an underdense region of the universe that expands faster than does the universe overall.

To determine whether the tension between the two types of H0 measurements can be explained by such an inhomogeneous universe, a team of scientists led by Hayley Macpherson (Monash University) has explored the behavior of a simulated universe.

A Simulated Universe

Local deviations in H0

The global measurement (Planck measurement; blue solid line) and local measurement (Riess et al. measurement; red solid line) of the Hubble constant can’t be brought into agreement by local deviations in the Hubble constant due to inhomogeneities (blue data showing distribution of various local spheres). [Macpherson et al. 2018]

Macpherson and collaborators simulated the growth of large-scale cosmological structures using numerical relativity. Starting with an inhomogeneous universe, the authors evolved random density fluctuations of the universe from its birth to today, and then investigated what effect these inhomogeneities have on local measurements of the Hubble constant.

The authors find that, in their simulated universe, local measurements of the Hubble constant differ by less than 1% compared to the global value. An inhomogeneous universe therefore cannot explain the nearly 9% difference we measure between the CMB-inferred global and supernova-measured local values of the Hubble constant.

What’s next? It’s back to the drawing board — the mystery of our expanding universe continues to elude us. Here’s hoping that high-precision measurements from future surveys will help us to further refine our understanding!

Citation

“The Trouble with Hubble: Local versus Global Expansion Rates in Inhomogeneous Cosmological Simulations with Numerical Relativity,” Hayley J. Macpherson et al 2018 ApJL 865 L4. doi:10.3847/2041-8213/aadf8c

image of AT2018cow's location

The recent discovery of an unusual transient event, nicknamed “the Cow”, has set the community of transient astronomers abuzz (amoo?). What do we know about this odd event so far?

Thinking Outside the Box

AT2018cow

The location of AT2018cow: a post-discovery image (top left), a pre-discovery reference image (top right), a subtracted difference image (bottom left), and a Pan-STARRS multi-color image (bottom right). [Prentice et al. 2018]

Once upon a time, supernovae seemed somewhat well characterized. But with the advent of today’s large, wide-field transient surveys that scan the visible sky every few nights, it seems like we’re now constantly discovering new supernova-like events that don’t quite fit into previous, neatly defined categories.

Among the large variety of new classes of transients uncovered by these surveys are supernova-like events whose lightcurves rise and fall much faster than standard supernovae. One example is AT2017gfo, the first confirmed kilonova, which was paired with the neutron-star merger first detected in gravitational waves in August 2017. Additional examples of these rapidly evolving transients span a wide range of peak absolute magnitudes (from –15 to –22 magnitude) and rise times (~1–10 days), making them difficult to explain through a single scenario.

AT2018cow light curves

ATLAS, Liverpool Telescope, GROND, and Swift light curves of AT2018cow. [Adapted from Prentice et al. 2018]

Now astronomers have found one more unusual, luminous, and fast-evolving transient: AT2018cow. In a new study, a team of astronomers led by Simon Prentice (Queen’s University Belfast, UK) has presented the discovery and initial analysis of the first 18 days of this event.

An Unusual Transient

The Cow was first discovered with ATLAS, a twin 0.5-m telescope system located in Hawaii, on the night of 16 June 2018. Post-discovery monitoring of the Cow with various telescopes spanning optical, near-infrared, and ultraviolet wavelengths reveals a variety of odd properties.

The Cow’s peak luminosity was remarkably high: ~1.77 x 10^44 erg/s, or about 10–100 times brighter than a typical supernova. It reached the peak very quickly, brightening by more than 5 mag in just 3.3 days, while typical supernovae have rise times of perhaps 10–20 days. In addition, the Cow had a high peak blackbody temperature (~27,000 K), low estimated ejecta mass (just 0.1–0.4 solar mass), and relatively featureless and non-evolving spectra.

Magnetar from a Collision?

magnetar

Artist’s impression of a strongly magnetized neutron star. [NASA/Penn State University/Casey Reed]

The combination of the Cow’s odd properties eliminates a number of more common progenitor explanations, such as supernova shock breakout. The authors do explore one scenario that could produce properties similar to the Cow’s, however: the formation of a magnetar — a strongly magnetized neutron star — from the merger of a binary neutron star system. Such a model, Prentice and collaborators say, would predict a transient with a peak luminosity, decline rate, and effective temperature that are all consistent with those of the Cow.

How can we confirm this picture? The next step will be to compare additional observations of AT2018cow in radio and X-ray wavelengths — which were made simultaneously with those reported here in near-infared through ultraviolet — to the magnetar models to see if the models also match those observations. If so, we may have an explanation for this unusual transient.  

Citation

“The Cow: Discovery of a Luminous, Hot, and Rapidly Evolving Transient,” S. J. Prentice et al 2018 ApJL 865 L3. doi:10.3847/2041-8213/aadd90

Great Red Spot

A camera on the Juno spacecraft has returned stunning high-resolution images of Jupiter’s Great Red Spot. What can we learn about the properties of this long-lived storm?

A Dramatic Storm

Great Red Spot Hubble

The Great Red Spot, as imaged by Hubble in 2017. [NASA/ESA/A. Simon (GSFC)]

Jupiter’s Great Red Spot, an exceptionally long-lived storm churning south of Jupiter’s equator, has been observed continuously for nearly two centuries. Though this atmospheric vortex is the largest and longest-lived of any planet in our solar system, our observations suggest that the Great Red Spot is gradually shrinking: the major axis of the ellipse was ~21° in longitude 40 years ago, and only ~14° in the last few years. Some studies suggest the Great Red Spot may even vanish within the next 20 years.

Our current understanding of the morphology of this storm comes primarily from detailed observations by spacecraft since 1979 — first by the two Voyager spacecraft as they flew by, then by the Galileo orbiter, and then by the Hubble Space Telescope. These past observations have ranged in resolution from about 15 to 150 km per pixel. Now, since the 2016 arrival of the Juno spacecraft in orbit around Jupiter, there’s a new player in town: JunoCam.

cloud-top morphologies

Identification of different Great-Red-Spot features and winds. See article text for label descriptions. [Adapted from Sánchez-Lavega et al. 2018]

JunoCam: Public Outreach and Science

JunoCam is a visible-light camera with a 58° field of view. The camera scans as the spacecraft rotates, producing images with resolution down to 7 km per pixel in some areas! JunoCam’s remarkable photos of Jupiter’s atmospheric patterns — taken as Juno skims just thousands of kilometers above Jupiter’s cloud tops — have certainly drawn the public eye. But though JunoCam’s primary intent is as a tool for public engagement, its images can serve a scientific purpose as well.

In a new study, a team of scientists led by Agustín Sánchez-Lavega (University of the Basque Country, Spain) have used the unprecedented detail of JunoCam’s observations to examine the various cloud morphologies inside the Great Red Spot.

Rich Dynamics

Great Red Spot features

Close views of the five features the authors identify within the Great Red Spot cloud tops. Click to enlarge. [Adapted from Sánchez-Lavega et al. 2018]

Sánchez-Lavega and collaborators identify five particular morphologies within the cloud tops of the Great Red Spot:

  1. Compact cloud clusters
    Several groups of compact clouds resemble altocumulus clouds observed on Earth. These may suggest condensation of ammonia.
  2. Mesoscale waves
    Interfering trains of wave packets indicate stable conditions in this region.
  3. Spiraling vortices
    A large eddy of ~500 km in radius suggest a region of intense horizontal wind shear.
  4. Central turbulent nucleus
    The red nucleus of the Great Red Spot spans ~5,200 km in length (that’s about 40% of Earth’s diameter) and ~3,150 km in width.
  5. Large dark thin filaments
    Undulating dark gray filaments 2,000–7,000 km in length circulate at high speeds around the outer park of the vortex. These may be darker aerosols or represent areas with different altitudes.

The team’s measurements of the overall wind field in the Great Red Spot demonstrate that though the Spot may be dramatically shrinking, its wind field has shown little change over 40 years of observation. The rich variety of morphologies we’re seeing therefore likely represents just the top of a dynamical system with a much deeper circulation.

We can’t wait to see what else JunoCam reveals during the Juno mission!

Citation

“The Rich Dynamics of Jupiter’s Great Red Spot from JunoCam: Juno Images,” A. Sánchez-Lavega et al 2018 AJ 156 162. doi:10.3847/1538-3881/aada81

black-hole snapshots

In 2006 an ambitious project was begun: creating the world’s largest telescope with the goal of imaging the shadow of a black hole. But how will we analyze the images this project produces?

A Planet-Sized Telescope

EHT participating telescopes

The locations of the participating telescopes of the Event Horizon Telescope (EHT) and the Global mm-VLBI Array (GMVA) as of March 2017. Jointly, these telescopes plan to image the shadow of the event horizon of the supermassive black hole at the center of the Milky Way. [ESO/O. Furtak]

The Event Horizon Telescope (EHT) is composed of radio observatories around the world. These observatories combine their data using very-long-baseline interferometry to create a virtual telescope that has an effective diameter of the entire planet!

The EHT, researchers hope, will have the power to peer in millimeter emission down to the very horizon of an accreting black hole — specifically, Sgr A*, the supermassive black hole in the Milky Way’s center — to learn about black-hole physics and general relativity in the depths of this monster’s gravitational pull.

Today, the EHT is closer than ever to its goal, as the project continues to increase its resolving power and sensitivity as more telescopes join the system. Another important aspect of this project exists, however: the ability to analyze and characterize the images it produces in a meaningful way.

PCA decomposition example

A simple example of using principal component analysis to decompose a set of images into independent eigenimages. The example images (top row) are snapshots from a simple model of a Gaussian spot moving on a circular path. The first four components of the principal component analysis decomposition — the four leading eigenimages — are shown in the bottom row, labeled with their corresponding eigenvalues. [Adapted from Medeiros et al. 2018]

Recently, a team of scientists led by Lia Medeiros (University of Arizona, University of California Santa Barbara) has demonstrated that a novel approach — principal component analysis — may be a useful tool in this process.

Principal Components

Principal component analysis is a clever mathematical approach that allows the user to convert a complicated set of observations of variables into their “principal components”. This process — commonly used in traditional statistical applications like economics and finance — can simplify the amount of information present in the observations and help identify variability.

Medeiros and collaborators demonstrate that a time sequence of simulated EHT observations — produced from high-fidelity general-relativistic magnetohydrodynamic simulations of a black hole — can be decomposed using principal component analysis into a sum of independent “eigenimages”. These eigenimages provide a means of compressing the information in the snapshots: most snapshots can be reproduced by summing just a few dozen of the leading eigenimages.

image reconstruction

A typical snapshot from a simulation (top), followed by three different reconstructions of the snapshot from the leading 10, 40, and 100 eigenimages. [Adapted from Medeiros et al. 2018]

Exploring Steady and Variable Flow

How is this useful? If images from simulations of a black hole can be represented by sums of eigenimages, so can the actual observations produced by the EHT. By comparing the two sets of observations — real and simulated — to each other within this eigenimage framework, we’ll be able to better understand the components of what we’re observing. In addition, the mathematics of principal component analysis allow for this to work even with sparse interferometric data, as is expected with EHT observations.

Furthermore, recognizing images that aren’t represented well by the leading eigenimages is equally important. These outlier images can be indicative of flaring or otherwise variable phenomena around the black hole, and identifying moments in which this occurs will help us to better understand the physics of accretion flows around black holes.

So keep an eye out for the first images from the EHT, expected soon — there’s a good chance that principal component analysis will be helping us to make sense of them!

Citation

“Principal Component Analysis as a Tool for Characterizing Black Hole Images and Variability,” Lia Medeiros et al 2018 ApJ 864 7. doi:10.3847/1538-4357/aad37a

Solar corona

The hot, tenuous solar corona is visible during a total solar eclipse, and astronomers have long studied the structure and dynamics of the ghostly coronal streamers. Now, a special observing campaign has allowed us to see the corona in unprecedented detail.

STEREO sees a CME

NASA’s STEREO has observed the solar atmosphere and solar wind — including coronal mass ejections like the one pictured here — for over a decade. [NASA/STEREO]

A STEREO View

Despite the wealth of knowledge we’ve amassed about our nearest star, there is still a lot we don’t know about the corona — the uppermost region of the solar atmosphere. Previous observations of the outer corona have indicated that the region is smooth and lacking in small-scale structure — but is it really?

To learn more about the outer corona, a team led by Craig DeForest (Southwest Research Institute) analyzed images from a special observing campaign by NASA’s Solar Terrestrial Relations Observatory-A (STEREO-A). In a departure from its typical observing mode, STEREO-A increased its imaging cadence by a factor of four and exposure time by a factor of six. Using careful image-processing techniques, DeForest and collaborators extracted hidden details from the STEREO-A images.

DeForest et al. 2018 Fig. 2

Left: An unprocessed image from the STEREO-A campaign. Right: The same frame with stray light and the F-corona (sunlight scattered off of dust grains) removed. Click to enlarge. [DeForest et al. 2018]

An Eye for Detail

The authors found that the seemingly smooth outer corona is made up of small-scale filamentary substructures that are visible down to the resolution limit of the instrument — corresponding to approximately 20,000 km.

These dense, narrow filaments might arise due to the dynamics of the corona itself, but it’s also possible that each filament can be traced back to an individual granule on the solar photosphere. This has exciting implications for future observations, which may be able to capture changes in the corona that correspond to changes in the granulation pattern on the solar surface.

The observed density variations also have consequences for how the solar wind is generated. The boundary between the solar atmosphere and the solar wind is often taken to be the Alfvén surface, where the radial velocity of the solar plasma exceeds the Alfvén speed — the speed at which hydrodynamic waves travel in a magnetized plasma. Because the Alfvén speed is a function of the plasma density, these extreme density variations over small spatial scales imply that the Alfvén surface is less of a discrete level in the solar atmosphere and more of a broad zone over which the coronal plasma gradually disconnects from the Sun and begins to flow outward as the solar wind. 

DeForest et al. 2018 Fig. 7

Top: An image of the corona in polar coordinates. Bottom: The same region, highlighting the locations of sharp brightness gradients. Click to enlarge. [DeForest et al. 2018]

Ready for Its Close-up

Luckily, we’ll have the opportunity to examine the intricate structure in the outer corona up close. DeForest and collaborators predict that NASA’s Parker Solar Probe, which will travel through the outer corona, will be able to discern the small-scale structures they discovered in the STEREO images. The spacecraft may observe changes in the density of the coronal plasma of up to an order of magnitude over the span of just ten minutes.

We won’t have to wait long to find out — launched in August 2018, Parker Solar Probe will have its first close encounter with the Sun in November 2018. By 2025, the spacecraft will be sampling the coronal plasma less than 10 solar radii above the solar photosphere.

Citation

C. E. DeForest et al 2018 ApJ 862 18. doi:10.3847/1538-4357/aac8e3

astropy

One of the greatest misconceptions about astronomy as a profession is that we all sit alone in front of a telescope eyepiece every night, gazing at the stars. In reality, today’s observational astronomy is collaborative — and it takes the form of ones and zeros on a computer.

RGB images with astropy

Two different RGB images of the region newar the Hickson 88 group, both produced with Astropy from Sloan Digital Sky Survey data. The top image uses default plot parameters; the bottom has parameters set to show a greater dynamical range. [The Astropy Collaboration et al. 2018]

A Computer-Driven World

Before the days of photography, the field of astronomy did rely on lone professionals who observed the heavens through their telescopes; after that, astronomers exposed film plates to gather data. Today, astronomy is a largely computer-driven field: observations are made by telescopes that often aren’t in the same location as the astronomers, and the images the telescopes take are stored as files full of data.

Modern observational astronomers need the coding skills to process these data and turn them into images and tables. They need to use computers to fit models to the data to better understand what they’re seeing. They need to present their results via complex plots and graphs — which are again produced using code.

As a result of this reality for astronomers, the handling of astronomical data has become in large part a community-driven, collaborative process; when good ideas are shared, each individual astronomer can spend less time reinventing the wheel. It’s in this spirit that the Astropy project was first developed. In a recent publication, the Astropy collaboration has now detailed the current status of this project.

Pooling Resources

Astropy commits

Plot of the total number of commits (contributions consisting of changes or additions) to the Astropy core package over time. [The Astropy Collaboration et al. 2018]

Many astronomers conduct their work in Python, a freely available, general-purpose programming language. Often, chunks of code that are useful to one astronomer are also useful to another — for instance, code that defines specific astronomical constants, or a module that reduces data in a certain way. Astropy is an open-source and open-development community library for such pieces of generally useful Python code for astronomy.

The Astropy project was started in 2011. Since then, the package has been used in hundreds of projects, and its scope has grown considerably. Anyone is able to contribute to this body of code, and it continues to be actively developed — as of version 2.0, the Astropy package contained over 212,244 lines of code contributed by 232 unique contributors.

Status of Astropy

In their recent publication, the authors describe some of the features currently contained in the Astropy core package — like support for coordinate transformations, reading and writing astronomical files, manipulating quantities with units attached, and modeling and visualizing data. 

example of coordinate systems in figures

Spitzer data providing another example of a figure made using an Astropy subpackage, which allows for the overlay of multiple coordinate systems and customization of which ticks and labels are shown on each axis. [Beerer et al. 2010]

The Astropy collaboration also discusses their plans for the future of the project: in addition to planned changes and additions to the core package, the next major release will also include an overhaul of the Astropy educational and learning materials, designed to make it easier for new users to start taking advantage of the resources in the Astropy package.

Critical efforts like the Astropy project not only provide and develop software tools essential to modern academic research, but they also help lower the barrier to entry for the next generation of professional astronomy researchers. With such support in a collaborative community, we can only imagine what modern astronomy will look like a few generations in the future!

Citation

“The Astropy Project: Building an Open-Science Project and Status of the v2.0 Core Package,” The Astropy Collaboration et al 2018 AJ 156 123. doi:10.3847/1538-3881/aabc4f

1 … 72 73 74 75 76 … 111