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NGC 4258

In December, AAS Nova Editor Susanna Kohler had the opportunity to fly aboard the NASA/DLR Stratospheric Observatory for Infrared Astronomy (SOFIA). This week we’re taking a look at that flight, as well as some of the recent science the observatory produced and published in an ApJ Letters Focus Issue.

One of SOFIA’s great strengths is that the instruments mounted on this flying telescope can be easily swapped out, allowing for a broad range of infrared observations. Three of SOFIA’s instruments are featured in science recently published in the ApJ Letters Focus Issue: the Far Infrared Field-Imaging Line Spectrometer (FIFI-LS), the High-Resolution Airborne Wideband Camera Plus (HAWC+), and the Echelon-Cross-Echelle Spectrograph (EXES).

FIFI-LS

The FIFI-LS instrument mounted on the SOFIA telescope. [NASA/SOFIA/USRA/Greg Perryman]

Meet FIFI-LS

FIFI-LS is a German-built instrument that can record spectra for each pixel of its field of view simultaneously, exploring objects in two far-infrared channels: 51–120 µm and 115–203 µm. Many astronomically interesting emission lines fall into these ranges — particularly those that trace the formation of massive stars and the properties of the interstellar medium.

By simultaneously capturing both images and spectra, FIFI-LS is able to deeply probe the composition and physical properties (like pressure and temperature) of heavily dust-obscured, star-forming regions in our own galaxy, as well as those in nearby external galaxies and galactic nuclei.

Some Recent FIFI-LS Science

In a study led by Gerold Busch (University of Cologne, Germany), scientists detail the first detection with FIFI-LS of a nearby luminous AGN, or active galactic nucleus. Despite its relative nearness, this galaxy is still roughly 500 million light-years away, making it the most distant object ever studied with SOFIA.

The team compares FIFI-LS’s spatially resolved observations of the infrared emission line [CII] in the galaxy to optical observations of Hα, an emission line known to trace star formation. By demonstrating that the two different types of emission occur in the same places in the galaxy, the team shows that [CII] emission can be used as a powerful diagnostic tool for tracing star formation even in distant galaxies — and even when those galaxies host luminous active nuclei.

••••••

two views of M51

Left: FIFI-LS image of [CII] emission from M51. Right: X-ray, optical, and infrared composite image of M51. The deficit of [CII] emission from the upper companion galaxy suggests it has a much lower star formation rate. [Left: Adapted from Pineda et al. 2018; Right: X-ray: NASA/CXC/SAO; Optical: Detlef Hartmann; Infrared: NASA/JPL-Caltech]

A publication led by Jorge Pineda (Jet Propulsion Laboratory) details a SOFIA-produced map of [CII] emission in the spectacular grand design galaxy M51 and the small companion galaxy M51b with which it is merging. The map reveals a deficit of [CII] emission in the companion galaxy, suggesting this small galaxy isn’t forming stars at the same rate as its larger cousin.

••••••

The molecular cloud BYF 73 is currently collapsing in on itself, making it a promising target in which to watch the formation of massive stars. In a study led by Rebecca Pitts (University of Florida), scientists have gathered multi-wavelength observations of this nursery, including mid-infrared data from FIFI-LS. The observations reveal the presence of eight very young (around just 7,000 years old), very massive protostars (the largest is ~240 times the mass of the Sun) embedded in the center of the cloud — providing an excellent opportunity to learn about the early stages of massive star formation.

••••••

galactic center

Three-color image of the galactic center indicating the position of the luminous HII region Sgr B1. HII regions are shown in blue. Click to enlarge. [Simpson et al. 2018]

The extreme conditions in the center of our galaxy may provide an interesting environment for star formation. Theory suggests that as streams of gas whip around the supermassive black hole at the galactic center, Sgr A*, the gas becomes compressed, causing stars to form. But some observations don’t quite fit with this theory.

In a study led by Janet Simpson (SETI Institute), scientists use FIFI-LS observations to explore one such puzzle: the luminous HII region Sgr B1. Sgr B1 is predicted to have passed by Sgr A* around 1.5 million years ago, yet FIFI-LS’s measurements show that the stars in this region are closer to 4 million years old. This suggests the stars actually formed in a separate cluster — now dispersed — that had an earlier star-forming passage by Sgr A*.

••••••

NGC 4258 jets

A zoomed-out (left) and zoomed-in (right) view of NGC 4258’s center, with contours of the [CII] emission superimposed on false-color representations of Hubble data. The [CII] emission is associated with the shocks and turbulence in the galaxy’s jets, which are marked by the line ending in two circles. [Appleton et al. 2018]

[CII] emission doesn’t just trace star formation! In a study led by Phil Appleton (IPAC/Caltech), scientists have used FIFI-LS’s observations of the active galaxy NGC 4258 to show that [CII] emission is also associated with warm molecular gas and soft X-ray hotspots, both created by shocks and turbulence in the speeding jets launched from the center of an active galaxy. These observations demonstrate that we can use [CII] emission to learn about how these energetic outflows interact with their environments.

Citation

ApJL Focus Issue:
Focus on New Results from SOFIA

FIFI-LS articles:
“The Close AGN Reference Survey (CARS): SOFIA Detects Spatially Resolved [C ii] Emission in the Luminous AGN HE 0433-1028,” G. Busch et al. 2018 ApJL 866 L9. doi:10.3847/2041-8213/aae25d
“A SOFIA Survey of [C ii] in the Galaxy M51. I. [C ii] as a Tracer of Star Formation,” Jorge L. Pineda et al. 2018 ApJL 869 L30. doi:10.3847/2041-8213/aaf1ad
“Gemini, SOFIA, and ATCA Reveal Very Young, Massive Protostars in the Collapsing Molecular Cloud BYF 73,” Rebecca L. Pitts et al. 2018 ApJL 867 L7. doi:10.3847/2041-8213/aae6ce
“SOFIA FIFI-LS Observations of Sgr B1: Ionization Structure and Sources of Excitation,” Janet P. Simpson et al. 2018 ApJL 867 L13. doi:10.3847/2041-8213/aae8e4
“Jet-related Excitation of the [C ii] Emission in the Active Galaxy NGC 4258 with SOFIA,” P. N. Appleton et al. 2018 ApJ 869 61. doi:10.3847/1538-4357/aaed2a

SOFIA's GREAT instrument

In December, AAS Nova Editor Susanna Kohler had the opportunity to fly aboard the NASA/DLR Stratospheric Observatory for Infrared Astronomy (SOFIA) with the German Receiver for Astronomy at Terahertz Frequencies (GREAT) instrument. This week we’re taking a look at that flight, as well as some of the recent science the observatory produced and published in an ApJ Letters Focus Issue.

boarding SOFIA

The SOFIA team and a handful of invited guests board the plane before a night in the stratosphere. [AAS Nova/S. Kohler]

It was 6 pm and I was boarding a plane for a 10-hour flight — but there were plenty of signs that this wasn’t your typical redeye.

For starters, I’d completed rigorous safety training earlier that day that included instructions on how to rappel down from the escape hatch of the cockpit.

Another sign was the catering (or lack thereof): no little bags of peanuts or lukewarm trays of airplane food. We each brought our own snacks, and we would be eating them cold — the microwave perched above the mini-fridge was off-limits tonight. The instrument currently mounted on the telescope was sensitive to potential leaked radiation from the microwave; no one wanted to be that guy whose nuked burrito ruined the night’s data.

My travel companions were another indicator that this was no normal flight: there were only 26 other people aboard, most of whom were dressed in flight jumpsuits adorned with mission patches. All of us were bundled up against the slow chill of the 60°F cabin and wearing communication headsets.

SOFIA cabin

SOFIA’s cabin, facing the rear of the plane. The educator consoles in the foreground allow guests to follow along with the observations. The mission directors sit at the next set of consoles. At the rear of the plane is the solid bulkhead separating the telescope cavity; the GREAT instrument is visible just in front of this, mounted on the back of the telescope. [AAS Nova/S. Kohler]

We were aboard SOFIA — the Stratospheric Observatory for Infrared Astronomy — and we were getting ready for a night of flying through the sky, doing infrared astronomy from the stratosphere.

Auspicious Beginnings

Takeoff was surprisingly quick: a short taxi and rapid climb into the air. The strangeness of the steep ascent was intensified by my seat: I’d been offered a spot at a console in front of a panel of computers, facing the tail of the plane. I’ve never taken off facing backwards before; I was grateful for the shoulder harness that kept me securely in my seat!

Onboard as an invited science writer, I had the great fortune of multiple guides on my flight; in addition to communication manager Nicholas Veronico, science outreach lead Randolf Klein flew with me that evening. As we listened in on our headsets to the chatter between the various team members, Randolf acted as my SOFIA translator, clarifying what was happening and what we could expect.

flight path

Screen capture from flightaware.com of our flight path for the night.

There’s Science to Be Done!

We would be observing with the German Receiver for Astronomy at Terahertz Frequencies (GREAT) instrument, and the goals for the night were clearly laid out. We’d begin with a short leg as we flew away from the coast and out over the Pacific, during which the door would open and the telescope would be initialized.

During our next, 130-minute leg, we would be roughly paralleling California’s coastline, heading as far north as the Oregon border. GREAT — a high-resolution spectrometer — had just been swapped onto SOFIA, so we would use this leg to point southwest at Mars, a known target that could be used to properly align the instrument for the remainder of its flights during this rotation.

In the next leg, we’d turn southward and point the telescope toward the evolved star U Orionis. On this 37-minute segment, we’d capture spectra to study the physical processes responsible for exciting water masers — emission sources that work like naturally occurring lasers — in the star’s outer shells.

Orion A

The Orion A star-forming region, as imaged by Herschel. OMC2 FIR4, the specific region we were exploring with SOFIA tonight, is circled in red. [ESA/Herschel/Ph. André, D. Polychroni, A. Roy, V. Könyves, N. Schneider]

Next we would swing in the direction of Hawaii and fly roughly three-quarters of the way out to the islands, completing a 125-minute leg observing a specific molecule, deuterated hydroxyl (OD), in a stellar nursery in the Orion A molecular cloud. From these observations, scientists hope to explore the origins of some of the simplest molecules found in our universe.

Lastly, we’d turn back to the northeast and fly a final long leg home, pointing at the W3 massive star-forming region in Cassiopeia to better understand the emission we see in the HII regions that surround young stars.

Suspense in the Stratosphere

The goals may have been clear, but astronomers know that observing never goes exactly as planned.

For us, the first challenge came early on: shortly into the telescope initialization leg, we hit sudden, strong turbulence. From the chatter in our headsets and the telescope’s view plotted on the console in front of me — which suddenly contained wildly swinging stars rather than the stable targets of moments before — it was clear that something had gone wrong.

Randolf explained: the turbulence had knocked the telescope into safe mode, and if the team couldn’t quickly reset the scope and complete the instrument alignment before our current leg ended, the entire night’s mission was at risk.

Thankfully, SOFIA’s team is used to handling challenges under time pressure. Before the leg was up, both the telescope and instrument were set up and aligned, and we all breathed a sigh of relief as the plane turned southwest onto our first science leg.

A Team Effort

Watching the data stream in on the screens in front of me that night was an exciting experience. Several of the GREAT team members were armed with laptops and were analyzing the data as fast as it arrived; since spectra can be hard to gauge by eye, real-time computer processing was a must to determine whether the team was getting what they needed.

SOFIA telescope operators

The telescope operators hard at work during our flight. [AAS Nova/S. Kohler]

Even more fascinating than seeing the data, however, was watching how the different subgroups of the SOFIA team worked together throughout the night to obtain the observations. The onboard team included three pilots, two mission directors, two telescope operators, and nearly a dozen scientists — both instrument and data specialists — associated with GREAT.

Constant adjustments were needed during the flight: if the pilots were notified of altitude constraints from the FAA, this would be relayed to the mission directors, who might modify the flight plan or observing times; if the scientists weren’t happy with the data they analyzed, they might make requests of the telescope operators or the instrument specialists. Meanwhile, the mission directors kept the team on track with regular announcements about how much time was left in each leg.

This complex stream of communication between groups took place over many radio channels and in multiple languages, and I found the apparent ease with which the team navigated it oddly beautiful and humbling.

Window into Our Universe

SOFIA post flight

AAS Editor Susanna Kohler after a night flying aboard SOFIA. [AAS Nova/S. Kohler]

As the final observing leg wrapped up, I headed up to the flight deck for a rare opportunity to sit in a 747 cockpit during landing. Chatting with the three pilots — who had more than 300 SOFIA flights under their collective belts — I could tell that they enjoyed being a part of the team. Flying planes is pretty cool, but flying planes for science? That’s something else.

Our runway lights flicked on, guiding our way to a landing in the quiet California desert. As I looked up through the cockpit glass at the night sky, reflecting on my remarkable flight experience, one thing was clear: SOFIA is an extraordinary window into our universe.

*****

SOFIA mission patch

A successful flight! [AAS Nova/S. Kohler]

Check back tomorrow to learn about some of the recent science conducted with SOFIA.

Further Reading

SOFIA

In December, AAS Nova Editor Susanna Kohler had the opportunity to fly aboard the NASA/DLR Stratospheric Observatory for Infrared Astronomy (SOFIA) with the German Receiver for Astronomy at Terahertz Frequencies (GREAT) instrument. This week we’re taking a look at that flight, as well as some of the recent science the observatory produced and published in an ApJ Letters Focus Issue.

What’s more exciting than jetting through the stratosphere over the Pacific Ocean? Doing so with an opening the size of a garage door gaping in the side of your airplane — while observing the universe! Such is the bizarre experience of flying aboard the Stratospheric Observatory for Infrared Astronomy, or SOFIA.

Unusual Plane for an Unusual Payload

SOFIA on the ground

It’s a beautiful day to tour SOFIA! [AAS Nova/S. Kohler]

More than a year ago, I walked onto the tarmac at NASA’s Armstrong Flight Research Center in Palmdale, California, and caught my first glimpse of SOFIA. I was visiting to tour the observatory and its support facilities at the invitation of the SOFIA program.

The Boeing 747SP gleamed in the sunlight, looking oddly stubby compared to its more familiar commercial-jetliner cousins. Of course, its short body is the least unusual thing about SOFIA; the giant, 18-by-13.5-foot door cut near the tail on its port side is unusual as well — not to mention the telescope pointed out of it.

NASA purchased the plane from United Airlines in 1997 and developed SOFIA as a joint project with the German Aerospace Center (DLR). The goal? To convert the plane into a flying infrared observatory vastly more capable than the venerable Kuiper Airborne Observatory (KAO) it was replacing.

atmospheric opacity

Opacity of the Earth’s atmosphere to different wavelengths of light. SOFIA nominally observes from 0.3 µm to 1.6 mm, a window that is largely difficult to access with ground-based observatories due to the high atmospheric opacity. Click to enlarge. [NASA]

Challenge of Observing an Infrared Universe

Infrared light is a powerful tool for observing the universe. Not only do many objects shine in infrared — more than half of all starlight is emitted at infrared wavelengths! — but we can also use infrared light to probe environments obscured by gas and dust. Infrared astronomy teaches us about everything from stellar birth to celestial magnetic fields, newly forming solar systems, and even black holes lurking at the centers of galaxies.

Infrared observations are foiled by water vapor in Earth’s atmosphere, which is why most infrared telescopes are located in space. But once a telescope is in space, it’s difficult to make repairs or upgrades. SOFIA is a neat solution to this problem: the observatory is able to climb higher than 41,000 feet — above 99% of the Earth’s infrared-blocking atmosphere. After a night of taking data from the stratosphere, however, SOFIA returns to the ground, where it can receive repairs or upgrades as needed.

HAWC+

This view toward the rear of SOFIA’s pressurized cabin shows an instrument, HAWC+, mounted on the back of SOFIA’s telescope. SOFIA’s instruments can be swapped out many times each year to ensure a wide variety of observations. [NASA]

Even better, SOFIA scientists aren’t constrained to a single instrument mounted on the telescope. SOFIA’s instruments — which include cameras, spectrometers, and polarimeters — are interchangeable, and they’re swapped out 25–30 times each year. This allows the observatory to make a variety of measurements across the infrared spectrum, with a versatility completely unlike any space telescope.

Engineering a Flying Telescope

As I boarded SOFIA, it was immediately obvious that the interior had been completely redesigned since the plane’s time with United. Instead of rows of cramped seats, SOFIA’s cabin contained workstations with computer monitors and communication ports. In places, the interior walls were missing the usual plastic facade, leaving the guts of the plane visible. Most prominent of all, the rear of the plane was sealed off by a solid bulkhead with complex machinery jutting through it.

SOFIA cutaway view

A cutaway view of SOFIA labeling the observatory’s primary components. Click to enlarge. [SOFIA]

SOFIA’s 2.7-m telescope mirror (three times the diameter of the KAO’s 0.9-m mirror) is just behind this wall in the depressurized rear of the plane, where it points out the open door during flight. The business end of the telescope assembly extends through the bulkhead and into the pressurized cabin; the chosen instrument for the current flight is mounted here, where the scientists in the cabin can access it.

You’ve probably experienced for yourself the turbulence that comes from flying on an airplane. How is SOFIA able to make steady observations of sources mid-flight? As my guides, SOFIA team members Zaheer Ali and Jason Disbrow, walked me through the observatory, they explained some of the remarkable engineering involved.

SOFIA telescope stabilization

SOFIA educational schematic detailing how the telescope is stabilized. [NASA/SOFIA/USRA/ASP/L. Proudfit]

SOFIA’s telescope and instrument are not attached directly to the structure of the plane; instead, they are mounted to the bulkhead via a moving gimbal system. Rubber bladders, gyroscopes, and a spherical shell of pressurized oil all work together to buffer the plane’s motion and allow the telescope to float, locked on its target. While the plane may move around the telescope, the telescope itself remains stable.

Planning a Science Flight

After exploring SOFIA, I was escorted back through the hangar to the building that houses SOFIA’s roughly 80 onsite staff members — from software experts to aerospace engineers to scientists. The other half of SOFIA’s team is located at the SOFIA Program Office at NASA’s Ames Research Center about 350 miles to the north.

While meeting with members of SOFIA’s operational staff, I learned more about the complexities of operating a flying telescope. SOFIA’s altitude coordinate can be controlled by tilting the telescope up or down, but its azimuth coordinate is set by the direction the plane is flying. This necessitates intense in-flight coordination to successfully lock on to sources.

2017 SOFIA flight paths

Map showing SOFIA’s 2017 flight paths. Though SOFIA is primarily based out of Palmdale, CA, the observatory also spends some time each year deployed to New Zealand to observe from the southern hemisphere. [NASA/SOFIA/Kassandra Bell]

What’s more, SOFIA’s outings require careful pre-flight planning. After observing proposals for SOFIA are approved, they are painstakingly pieced together: the target observations must form complementary legs of flights roughly 10 hours long, starting and ending in Palmdale. Further adding to the challenge, each flight plan must also avoid restricted air space and be flexible enough that pilots can cooperate with any other Federal Aviation Administration (FAA) constraints that arise.

An Opportunity to Fly

By the end of my tour, I was hooked on SOFIA’s story: a crazy idea with significant technical challenges had somehow been made into a successful reality. Now I desperately wanted to experience SOFIA during a science flight, to better understand how this was possible.

I was in luck. A year later, I was aboard SOFIA again — but this time, I was seeing the science in action.

Check back tomorrow to read the story of my flight!

Further Reading

Interstellar Boundary Explorer (IBEX)

The Interstellar Boundary Explorer (IBEX) has been observing the ebb and flow of energetic neutral atoms within our solar system for the past decade. How has the flux of these particles changed in that time?

Desai et al. 2019 Fig. 1

Global maps of the ENA flux at five energies, for four time periods. Click to enlarge. [Desai et al. 2019]

Studying the Solar System Bubble

Our solar system is shielded from the interstellar medium by a bubble of solar-wind plasma called the heliosphere. We can study the properties of the heliosphere by monitoring the flux of energetic neutral atoms (ENAs), which form when speedy solar-wind ions steal electrons from incoming neutral interstellar atoms and lose their positive charge.

No longer confined to move along the magnetic field of the solar wind, these neutral atoms can return to the inner solar system, where they are detected by Earth-orbiting spacecraft like IBEX. Over the past decade, IBEX has given us a three-dimensional view of the ENAs within our solar system. The solar wind has certainly changed a lot in that time — how have the ENAs responded?

Desai et al. 2019 Fig. 4

Projected global maps of the spectral index for four energy ranges over the nine years of IBEX observations. The spectral index clearly increases in the northern hemisphere in the highest energy bands. The ribbon feature has been masked. Click to enlarge. [Desai et al. 2019]

Mapping the Heliosphere

A team led by Mihir Desai (Southwest Research Institute and University of Texas at San Antonio) analyzed nine years of IBEX observations to learn how the flux of ENAs has changed over time. They focused on the globally distributed flux, which represents the primary ENA population, as opposed to the so-called “ribbon feature”, a strange, arc-like feature thought to trace ENAs produced through a different mechanism.

By comparing the flux of ENAs in adjacent energy bands, the authors constructed maps of the ENA spectral indices — a measure of how the ENA flux is distributed over different energies — over the whole sky. They found that the evolution of the spectral indices over the years depends on both the energy of the ENAs and their latitude. Particularly striking was the increase in the spectral indices in the northern hemisphere at the highest energies (1.7–4.3 keV), which was caused by a larger decrease in the flux of the highest energy ENAs.

By comparing the evolution of the ENA spectral indices to the speed of the solar wind during the same time period, the authors find that the behavior of the ENA flux is linked to changes in the solar wind speed with a delay of 2–3 years. This finding solidifies the connection between the solar wind parameters and the ENA flux.

Desai et al. 2019 Fig. 11

Reconstructed solar wind speed from 1985 to 2018. The solar wind speed exhibits more latitudinal variation during solar minimum (e.g. 2009) than solar maximum (e.g. 2014). [Desai et al. 2019]

Continued Evolution

How will the ENA flux evolve in the future? Based on the observed solar wind speeds and the expected time delay, the authors predict that the ENA spectral indices will continue to evolve.

More specifically, because the northern hemisphere solar wind speed increased from 2014 to 2018, the authors expect the ENA spectral index to decrease in the northern hemisphere, especially at the higher energies that they’ve found to be more responsive to solar wind changes. In the southern hemisphere, they expect the spectral indices to rise and fall with the more variable solar wind speed at those latitudes.

IBEX has enough fuel to support its continued operations for several decades — so we can expect to learn much more about the ENAs in our solar system in the future!

Citation

“Temporal Evolution of the Latitude and Energy Dependence of the Energetic Neutral Atom Spectral Indices Measured by the Interstellar Boundary Explorer (IBEX) Over the First Nine Years,” M. I. Desai et al 2019 ApJ  875 2. doi:10.3847/1538-4357/ab0f37

'Oumuamua

More than a year has passed since the discovery of 1I/2017 ’Oumuamua, a bizarre body that burst onto the scene and then disappeared into the distance as quickly as it had arrived. During ‘Oumuamua’s visit, astronomers gathered some 800+ observations from telescopes around the world, which together reveal a strange light curve that raises more questions than answers.

A few things are agreed upon. ‘Oumuamua’s orbit indicates it originated outside of our solar system, making it the first visiting interstellar body we’ve witnessed. Its shape appears to be highly elongated, suggesting it’s more cigar-shaped than spherical. And its light curve reveals a periodicity of roughly 8 hours, potentially indicating the speed at which this odd body rotates.

But many unsolved questions remain. What are ‘Oumuamua’s structure, composition, and shape? Where did it come from? How was it launched onto its journey to our solar system?

Solar Push for a Fluffy Body?

One of ‘Oumuamua’s biggest mysteries relates to the discovery late last year that this asteroid wasn’t moving just under the influence of gravity; instead, ‘Oumuamua was experiencing a mysterious additional acceleration away from the Sun.

light sail

Artist’s impression of an artificial light sail, a thin spacecraft that can be propelled by radiation pressure. [Josh Spradling / The Planetary Society]

What could cause ‘Oumuamua’s added boost? Some scientists have suggested that radiation pressure — the push from solar photons hitting the object — could speed it up enough to explain observations. But for this to work, the asteroid would need an enormous surface-area-to-mass ratio.

One study suggested this could be achieved if ‘Oumuamua took the form of a giant light sail less than a millimeter thick (naturally reinvigorating the “is it aliens?” debate). But a recent study by Amaya Moro-Martín (Space Telescope Science Institute) suggests there might be another way: ‘Oumuamua could have a more ordinary shape, but an exceedingly low density.

cosmic dust

Image through a microscope of a porous interplanetary dust particle. Could ‘Oumuamua be an extremely low-density aggregate of icy dust? [Donald E. Brownlee (U. of Washington) and Elmar Jessberger (Institute for Planetology, Germany)]

The least dense manmade solid, aerographene, has a density on the order of 10-4 g/cm3, or ~10% the density of air. Moro-Martín suggests that an object with a tenth of this density, ~10-5 g/cm3, could get enough of a boost from the Sun to explain our observations of ‘Oumuamua.

Given this low density value, is this scenario actually likely? It turns out that fluffy, porous materials occur naturally in space, in the form of aggregates of icy dust particles. If the icy-aggregate explanation for ‘Oumuamua is correct, then the asteroid could have formed in the outer reaches of a nearby protoplanetary disk — and this could open a new window onto the study of the building blocks of planets around young stars. 

comet 67P/Churyumov-Gerasimenko

Image of comet 67P/Churyumov-Gerasimenko outgassing as it is heated by the Sun. Could similar processes be occurring on ‘Oumuamua? [ESA/Rosetta/MPS for OSIRIS Team]

Added Nudge from Migrating Jets?

There’s a more mundane explanation for ‘Oumuamua’s anomalous acceleration than radiation pressure, however: outgassing, which occurs as volatiles heat beneath a body’s surface and evaporate. This process is commonly seen in the jetted tails of comets, but there are some problems with using it to explain ‘Oumuamua’s motion.

First, no outgassing was observed from ‘Oumuamua; in fact, Spitzer observations placed strict limits on the amount of carbon-based material that could be evaporating from it. Second, calculations show that traditional comet-like outgassing would create torques that would spin ‘Oumuamua up rapidly, causing it to fly apart.

A new study may have found a way around these problems, however. A publication led by Darryl Seligman (Yale University) suggests that ‘Oumuamua may have been accelerated by outgassing not from a fixed point, but from migrating jets that follow the warmth, tracking the side of the asteroid closest to the Sun. Instead of spinning out of control, ‘Oumuamua’s motion might then resemble a pendulum, gently rocking back and forth to produce the ~8-hr period seen in the light curve.

'Oumuamua light curve

Light curves (left) and periodograms (right) for actual ‘Oumuamua observations (top row) and synthetic observations for three of the authors’ outgassing models, using three different aspect ratios for the body (next three rows). The bottom row reflects a flat photometry. Click to enlarge. [Seligman et al. 2019]

What about the Spitzer constraints on the visible evaporation? Seligman and collaborators suggest that the outgassing was primarily in the form of water vapor rather than carbon-based material. This could occur if the body had an unusually carbon-poor composition compared to a typical comet.

Ready for the Next Visitor

Could one of these explanations solve the mystery of ‘Oumuamua’s odd acceleration? Or could the true answer be a combination of proposed scenarios? With ‘Oumuamua long gone, we can’t be sure until we spot another interstellar visitor like it — but you can bet we’ll be prepared next time!

Citation

“Could 1I/’Oumuamua be an Icy Fractal Aggregate?,” Amaya Moro-Martín 2019 ApJL 872 L32. doi:10.3847/2041-8213/ab05df
“On the Anomalous Acceleration of 1I/2017 U1 ‘Oumuamua,” Darryl Seligman et al 2019 ApJL 876 L26. doi:10.3847/2041-8213/ab0bb5

superluminous supernova

What causes the bizarre, extragalactic fast radio bursts we’ve detected over the last decade? A new study of an unusually bright supernova may have found the key to answering this question.

Clues from a Repeating Burst

FRB 121102 host

The host of FRB 121102 is placed in context in this Gemini image. [Gemini Observatory/AURA/NSF/NRC]

When a mysterious millisecond radio pulse of extragalactic origin — a fast radio burst (FRB) — was recently found to repeat, it gave astronomers a rare chance to hunt down this source’s host galaxy. FRB 121102 was isolated to a star-forming, low-metallicity dwarf galaxy located roughly 3 billion light-years away, and a dimmer, persistent radio source was discovered in the same region that produced the bursts.

This localization lent support to one theory for the origin of FRB 121102 (and possibly other FRBs): that the bursts are powered by a magnetized neutron star born decades ago in a superluminous supernova.

Magnetar Culprit?

Superluminous supernovae are a type of stellar explosion at least ten times more powerful than standard supernovae. These supernovae may shine extra bright due to the birth of a neutron star with extremely strong magnetic fields — a magnetar — that spins on millisecond timescales, emitting radiation and winds as its magnetic fields decay and further powering the explosion.

magnetar

Artist’s impression of a magnetar — an extremely magnetized neutron star — in a young star cluster. [ESO/L. Calçada]

In the superluminous-supernova explanation for FRBs, the magnetar born in the stellar explosion could, even a decade later, power brief radio bursts. In addition, it would generating a glowing nebula visible to us as a persistent radio source.

A team of scientists has now sought to test this picture by examining known superluminous supernovae and searching for signs of co-located persistent or bursting radio sources. In a recent publication led by Tarraneh Eftekhari (Harvard-Smithsonian Center for Astrophysics), they detail their first success.

Radio Source Found

Using the Very Large Array, Eftekhari and collaborators discovered a persistent radio source coincident with the superluminous supernova PTF10hgi, an explosion that went off 7.5 years ago, roughly 1.5 billion light-years away. This is the first time a radio source of any kind has ever been associated with a superluminous supernova, providing an important link between these explosions and other phenomena.

PTF10hgi

Left: Radio continuum map from VLA 6-GHz observations of PTF10hgi. Right: Near-ultraviolet image of the host galaxy of PTF10hgi from Hubble, with radio contours overlaid. The small red circle shows the optical position of PTF10hgi. [Eftekhari et al. 2019]

The authors test a variety of different potential origins for the radio emission seen, like star formation activity, an active galactic nucleus, and a supernova blast wave. Though none of these scenarios can yet be ruled out with certainty, Eftekhari and collaborators show that each of them is highly unlikely.

Instead, the radio emission is extremely reminiscent of the persistent radio source associated with FRB 121102. The authors show that all of the observations are consistent with a magnetar central engine powering a glowing nebula embedded in the supernova ejecta. And while a bursting radio source wasn’t found coincident with the supernova, the authors’ observations were only 40 minutes long; a longer observation time may yet discover a co-located FRB.

The authors detail future observations that should be able to rule out alternative origins for the radio emission and strengthen the case for a superluminous-supernova-born magnetar as a source of fast radio bursts. In the meantime, it’s exciting to watch as the pieces of this puzzle start to come together!

Citation

“A Radio Source Coincident with the Superluminous Supernova PTF10hgi: Evidence for a Central Engine and an Analog of the Repeating FRB 121102?,” T. Eftekhari et al 2019 ApJL 876 L10. doi:10.3847/2041-8213/ab18a5

brown dwarf clouds

Nearby, cool dwarfs offer us the unique opportunity to learn more about the atmospheres of planetary-mass bodies. In a new study, Hubble observations shed some light on the clouds of one such object.

Looking for Variability

Ross 458C

A false-color image of the Ross 458 system. The bright source in the top left is the binary star system; the planetary-mass brown dwarf companion Ross 458C is circled on the bottom right. [UKIDSS]

As the list of known planet-like bodies beyond our solar system grows, we continue to find new ways to study and understand these objects. One area of increasing interest and capability is the study of atmospheres.

When we directly image planet-like bodies using high-resolution telescopes, we can look for variability in the light from the body over time. Regular, periodic variability can indicate the presence of clouds in that object’s atmosphere — and observations of the variability can reveal information about the cloud structure and composition.

An Atlas of Clouds

To this end, a large Hubble Space Telescope Treasury program was begun in 2015: Cloud Atlas. The program’s aim is to use high-precision, time-resolved Hubble photometry and spectroscopy to explore the atmospheres and clouds of exoplanets, planetary-mass brown dwarfs, and more massive brown dwarfs — bodies with typical temperatures between 800 K and 1,700 K.

There are two major steps to the Cloud Atlas project:

  1. Construct a broad sample of observed planets and brown dwarfs showing variability. This will ultimately allow us to compare the cloud maps between different objects and determine how cloud properties depend on things like a body’s temperature and size.
  2. Conduct detailed studies of the most interesting objects in this sample, using longer follow-up observations that cover complete rotations of the bodies.

In a recent publication led by Elena Manjavacas (W.M. Keck Observatory; The University of Arizona), the Cloud Atlas team describes their detailed observations of the planetary-mass brown dwarf Ross 458C as part of the latter half of this project.

Ross 458C variability

Observations of Ross 458C’s light curve over time in three different bands (white, top; narrow J-band, center; narrow H-band, bottom) reveal a distinct periodic variation fit by a sine function. [Adapted from Manjavacas et al. 2019]

Clouds on a Planetary-Mass Companion

Ross 458C is a T8 brown dwarf that orbits a binary system less than 40 light-years away. This cool object lies near the blurry line between brown dwarf and planet, representing the coldest dwarf yet to undergo detailed cloud studies.

Manjavacas and collaborators’ long observations of Ross 458C over seven consecutive Hubble orbits — during which 77 spectra were captured — revealed variability in the dwarf’s spectrum at a level of 2.62 ± 0.2% over the wavelength range of 1.1–1.64 µm. This modulation suggests the dwarf’s rotation period is somewhere around 6.75 hours.

By exploring how the dwarf’s variability changes across different wavelengths, the authors hope to be able to understand what its clouds are made up of and how the cloud structure is distributed in the atmosphere. Current best estimates from modeling suggest that these may be heterogeneous sulfide clouds (very different from Earth’s clouds of water!) — which could also explain the reddish color of Ross 458C.

The discovery of Ross 458C’s rotational modulations is important in and of itself: it suggests that clouds are typical even in the atmospheres of the coolest dwarfs. We can look forward to plenty of follow-up work further exploring the atmosphere of a planetary-mass companion with this convenient laboratory.

Citation

“Cloud Atlas: Rotational Spectral Modulations and Potential Sulfide Clouds in the Planetary-mass, Late T-type Companion Ross 458C,” Elena Manjavacas et al 2019 ApJL 875 L15. doi:10.3847/2041-8213/ab13b9

Atacama Large Millimeter/submillimeter Array

Interferometric arrays collect massive amounts of information, leaving astronomers with a happy problem: too much data! How can we handle mountains of data in an efficient way?

Murchison Widefield Array

One of many tiles comprising the Murchison Widefield Array (MWA). Radio interferometric arrays like MWA generate vast amounts of data. [Dr. John Goldsmith/Celestial Visions]

Too Much of a Good Thing?

Astronomers have come a long way from the early days of manually cataloging stars and sketching sunspots by hand. Even though today’s data sets are larger and more complex, many astronomers still manually calibrate and process their data.

This hands-on data processing won’t always be feasible, though; interferometry — the process of linking together tens to thousands of telescopes or antennae to produce images with ever-finer angular resolution — generates far more data than humans could hope to handle manually. Just one minute’s worth of data from the Murchison Widefield Array (MWA), a radio interferometer made up of 4,096 antennae, yields roughly 10,000 images!

With the number of interferometers increasing, we’ll need to be smart about how we process all that data to minimize computing hours while maximizing the quality of the output. Among the many detectors requiring novel data-processing techniques is the planned Square Kilometer Array (SKA), which will comprise a million antennae and 2,000 radio telescopes. How can we get a handle on all this data without getting too hands-on?

Mondal et al. 2019 Fig. 2

An illustration of how increasing numbers of detectors are included in the model of the target for self-calibration. The first step includes only the blue detectors near the center, and subsequent steps add the red, teal, black, and yellow detectors to increase the complexity of the model. [Mondal et al. 2019]

Dealing with Data Pileup

To tackle this problem, a team led by Surajit Mondal (Tata Institute of Fundamental Research, India) developed an automated processing pipeline for interferometric data — the Automated Imaging Routine for Compact Arrays for the Radio Sun (AIRCARS). They focused on processing solar radio images, which need to capture a huge dynamic range — from extremely bright active regions to faint, wispy filaments.

One of the challenges in radio interferometry is removing the effects of instrumental artifacts and the plasma in Earth’s atmosphere. Most radio interferometry data are corrected with a self-calibration process that treats the instrumental artifacts and the brightness of the target as free parameters and iteratively minimizes the difference between the observations and a model of the target.

AIRCARS works especially well when applied to a compact array — one with many detectors clustered in the center and fewer near the outskirts. This configuration allows the pipeline to start with relatively little information about the target from just a few central detectors and gradually build a complex model of the target to be used in its self-calibration routine.

Mondal et al. 2019 Fig. 5

An example of the improvement of the dynamic range of MWA images through the self-calibration process. The number of iterations increases from top to bottom and left to right. The dashed circle indicates the location of the Sun’s disk. Click to enlarge. [Mondal et al. 2019]

AIRCARS in Our Future

In their tests on MWA data, the authors find that AIRCARS is capable of capturing a dynamic range up to 100,000:1 — a huge improvement over previous processing methods.

Mondal and collaborators note that AIRCARS can be configured to attain the maximum possible dynamic range without constraints on computing time, or to accept user-imposed time limits to rapidly process large amounts of data, depending on the user’s computational requirements.

Because the pipeline needs no human supervision, astronomers can take a step back from processing the vast amount of incoming data and focus instead on the exciting science we can do with interferometry.

Citation

“Unsupervised Generation of High Dynamic Range Solar Images: A Novel Algorithm for Self-calibration of Interferometry Data,” Surajit Mondal et al 2019 ApJ 875 97. doi:10.3847/1538-4357/ab0a01

buckyball

From a jumble of confusing clues in Hubble observations of interstellar space, scientists have picked out evidence of a celebrity molecule: ionized Buckminsterfullerene, or buckyballs.

Sorting Out Diffuse Signals

What makes up the tenuous gas and dust that pervades our galaxy, filling the space between stars? What kinds of complex molecules can form naturally in our universe, outside of the potentially contrived conditions of Earth-side laboratories? Where might these molecules form, and how are they distributed throughout space?

interstellar sightlines

Hubble spectra of seven heavily-reddened interstellar sightlines (top seven black lines) and four unreddened standard stars (bottom four lines). The red line at the top indicates a laboratory spectrum for C60+. Positions of the four absorption features associated with C60+ are marked with vertical dashed lines. Click to enlarge. [Cordiner et al. 2019]

These are among the many open questions regarding the chemistry of our universe. One particular, longstanding puzzle for astronomers is the cause of what’s known as “diffuse interstellar bands”: hundreds of broad absorption features that appear in optical to near-infrared spectra of reddened stars.

These features are not caused by the stars themselves, so they must be due to absorption of light by the diffuse interstellar medium (ISM) between us and the stars. But the jumble of hundreds of features — and the unknown conditions under which they are produced — has made it incredibly challenging to identify the individual molecules present in the diffuse ISM.

A new study led by Martin Cordiner (NASA Goddard SFC; Catholic University of America) presents observations from the Hubble Space Telescope — thus avoiding the additional complication of absorption features from the Earth’s atmosphere — that explore these diffuse interstellar bands further. Hubble’s sightlines toward 11 stars provide confirmation of one special molecule within this jumble: Buckminsterfullerene.

buckyball

Model of the structure of a buckyball. [Mstroek]

A Celebrity Molecule

The C60+ ion, formally known as Buckminsterfullerene and informally known as a “buckyball”, is an enormous molecule consisting of 60 carbon atoms arranged in a soccer-ball shape. Previously, the largest known molecules definitively detected in the diffuse interstellar medium contained no more than three atoms heavier than hydrogen — so the detection of buckyballs represents a dramatic increase in the known size limit!

Cordiner and collaborators use a novel scanning technique to obtain ultra-high signal-to-noise spectra of seven stars that are significantly reddened by obscuring ISM and four stars that are not. They then search for absorption signals at four wavelengths — 9348, 9365, 9428, and 9577 Å — predicted by laboratory experiments to be associated with C60+.

mean spectra

Mean spectra for the observed sightlines for reddened (black, top) and unreddened (gray, bottom) stars, around four predicted absorption features for C60+. The laboratory comparison spectra for C60+ are overlaid as red lines. [Cordiner et al. 2019]

The authors find obtain reliable detections of the three strongest of these absorption lines in the spectra toward the seven reddened stars, and find no sign of this absorption in the four unobscured stars. The 9348 Å absorption was not detected, but as this is predicted to be a very weak feature, this result is not surprising. The relative strengths of the three detected lines also fit with laboratory predictions.

The consistency of Cordiner and collaborators’ results with prediction provides the strongest confirmation yet of the presence of buckyballs in the diffuse ISM. This detection may help us to characterize other components of the diffuse ISM and better understand the conditions under which complex molecules exist in the extreme, low-density environment of interstellar space.

Citation

“Confirming Interstellar C60+ Using the Hubble Space Telescope,” M. A. Cordiner et al 2019 ApJL 875 L28. doi:10.3847/2041-8213/ab14e5

binary neutron star

When two compact objects — neutron stars or black holes — merge, will they emit light? A recent study looks at a neglected factor that could affect the answer: electric charge.

Dark or Light?

neutron-star merger

Artist’s impression of two merging neutron stars producing a gamma-ray burst. [National Science Foundation/LIGO/Sonoma State University/A. Simonnet]

Most theories agree that a compact binary containing a neutron star can emit light when it merges. This is because these systems contain lots of neutron-rich matter that can then radiate in the final stages of merger, in the form of gamma-ray bursts, kilonovae, and afterglows.

But what about compact binaries containing two black holes? Or so-called “plunging” black-hole–neutron-star mergers in which the neutron star plunges directly into the black hole before it can be disrupted? Are these mergers all doomed to darkness?

Possible Charge

Not according to Bing Zhang, a scientist at University of Nevada Las Vegas. Recently, Zhang proposed that black holes might carry electric charge in a surrounding magnetosphere. As charged black holes spiral around and around each other during a merger, they could generate electromagnetic radiation: a characteristic signal that rises sharply just before merger.

Now Zhang is back with a generalized model for the merger of charged compact objects, which also explores possible signatures from electrically charged neutron stars. In a new study, he works out the details and reports on where we might be able to detect these signals.

Searching for a Signal

All compact binaries containing a neutron star should emit radiation from electric charge, since neutron stars are definitely charged — they’re essentially spinning magnets. But for most systems containing a neutron star, Zhang demonstrates, the radiation associated with the object’s charge will be non-detectable, since it’s so much dimmer than other electromagnetic signatures from merger (like a gamma-ray burst).

Crab pulsar

The Crab pulsar is a highly magnetized, spinning neutron star that powers the Crab nebula seen in this composite image. [X-ray: NASA/CXC/SAO/F.Seward; Optical: NASA/ESA/ASU/J.Hester & A.Loll; Infrared: NASA/JPL-Caltech/Univ. Minn./R.Gehrz]

There’s hope, though, in the scenario of a plunging neutron-star–black-hole merger. If the neutron star is less than 20% the size of the black hole, it can be consumed whole, preventing any of the typical electromagnetic signatures from occurring. In this case, the radiation from the charged, inspiralling neutron star is the only electromagnetic signal present.

If the neutron star in such a system has a magnetic field similar to that of the Crab pulsar — possible in young star clusters — the charge signal can reach detectable levels, according to Zhang’s calculations. In fact, it’s possible that we could observe such a signal as a fast radio burst, the mysterious millisecond radio bursts that we’ve seen originating from beyond our galaxy.

Looking Ahead

Many unknowns are still present in this picture. How is the electric radiation converted into observable emission? How commonly do we expect plunging neutron-star–black-hole mergers to occur as described? Will we be able to link radiation from charged mergers to a gravitational-wave chirp?

One thing is for certain: if we can, indeed, observe the light from charge in a compact-binary merger, this would provide an exciting new opportunity to further probe these distant, exotic systems.

Citation

“Charged Compact Binary Coalescence Signal and Electromagnetic Counterpart of Plunging Black Hole–Neutron Star Mergers,” Bing Zhang 2019 ApJL 873 L9. doi:10.3847/2041-8213/ab0ae8

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