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habitable-zone planets

As of last week, the count of confirmed exoplanets officially exceeds 4,000 — and while we’ve learned a lot about planet formation from this wealth of data, it’s also prompted new questions. Could the recent detection of two intriguing new planets shed light on one of these open puzzles?

radius gap

In 2017, a team of scientists led by B.J. Fulton identified a gap in the distribution of radii of small Kepler-discovered planets. [NASA/Ames/Caltech/University of Hawaii (B. J. Fulton]

Mind the Gap

Our growing exoplanet statistics recently revealed a curious trait: there’s a gap in the radius distribution of planets slightly larger than Earth. Rocky super-Earth planets of up to ~1.5 Earth radii are relatively common, as are gaseous mini-Neptunes in the range of ~2–4 Earth radii. But we’ve detected very few planets in between these sizes.

What’s the cause of this odd deficit? One theory is that high-energy radiation emitted by stars early in their lifetimes erodes the atmospheres of planets that are too close in, stripping them of their expansive shells of gas and leaving behind only their dense, rocky cores. Planets that lie further out or start with a thicker shell may be spared this fate, retaining some of their gas for a significantly larger, fluffier construction.

Disentangling Factors

HD 15337 lightcurve

Folded light curves for HD 15337 showing the transits of planets b (top) and c (bottom). [Gandolfi et al. 2019]

This theory can be difficult to test, however, due to the large number of intertwined variables. The super-Earths and mini-Neptunes we’ve observed lie at varying distances from their host stars — but they also orbit around different types of stars with very different radiation histories. It’s hard to tell what role these various factors play in the planets’ evolution.

But a recent discovery from the Transiting Exoplanet Survey Satellite (TESS) may help simplify this picture. With more than 750 planet-candidate detections so far, TESS is rapidly adding to our exoplanet statistics — and two TESS-discovered planets around HD 15337 may be especially useful for better understanding the radius gap.

A Non-Identical Pair

In a publication led by Davide Gandolfi (University of Turin, Italy), a team of scientists carefully analyzes the TESS light curves for HD 15337, as well as archival spectroscopic data from the High Accuracy Radial velocity Planet Searcher. They show that there is evidence for the presence of two planets — HD 15337 b and c — that have similar masses: ~7.5 and ~8.1 Earth masses, respectively.

But while HD 15337 b appears to be a close-in (period of 4.8 days), rocky super-Earth with radius of 1.6 Earth radii and density of 9.3 g/cm3, HD 15337 c lies further out (period of 17.2 days) and is a fluffy mini-Neptune, with a radius of 2.4 Earth radii and a density of 3.2 g/cm3.

atmospheric erosion

Artist’s impression of a star’s high-energy radiation evaporating the atmosphere of its planet. [NASA’s Goddard SFC]

Since these two planets orbit the same star, it seems likely that their different orbital radii are what led to their places on either side of the radius gap. Using a planet atmospheric evolution algorithm, Gandolfi and collaborators show that the properties of the two planets can be produced by high-energy radiation from HD 15337 early in the system’s lifetime.

As our observational statistics for exoplanets continue to grow, it’s exciting to see how these continued discoveries can both raise and address new questions of planet formation and evolution. Who knows what else we’ll learn as detections continue to pile up!

Citation

“The Transiting Multi-planet System HD15337: Two Nearly Equal-mass Planets Straddling the Radius Gap,” Davide Gandolfi et al 2019 ApJL 876 L24. doi:10.3847/2041-8213/ab17d9

NGC 6397

Unusually blue and bright stars may not have only themselves to thank for their uniqueness. A new study looks at one way these unconventional objects might form in clusters … with a little help from a friend. 

Cluster Stand-Outs

blue stragglers on an HR diagram

Sketch of a Hertzsprung–Russell diagram for a star cluster. Blue stragglers lie above the main-sequence turnoff point for the rest of the stars in the cluster. [RicHard-59]

A stellar cluster is a group of stars that were all born together and should evolve in a consistent way. According to stellar evolution theory, for a given cluster, the stars of the cluster should fall onto a well-defined track on a Herzsprung–Russell (H–R) diagram — a plot of stellar brightness vs. color — with the stars’ location on the track dependent only on their initial mass.

But a few stars defy this logic. These so-called “blue stragglers” seem to have been left behind as their fellow cluster inhabitants evolved without them; on the H–R diagram, blue stragglers lie alone above the main-sequence turnoff point, shining brighter and bluer than they should be.

Just a Little Boost?

What causes these unorthodox stars? The simplest explanation is that they are main-sequence stars that belatedly received a bump in their mass. Theorists favor two possible formation channels:

  1. Mass transfer from an evolved donor onto a main-sequence star in a binary, which increases the main-sequence star’s mass and consequently causes it to become brighter and hotter.
  2. Collision and merger of two main-sequence stars, which forms a new, more massive main-sequence star that is brighter and hotter than usual.

But these two channels can only explain some observed blue stragglers; other systems — like WOCS ID 7782, a binary consisting of two blue stragglers in a 10-day orbit — are unlikely to have formed in either of these ways.

formation channel for blue stragglers

Schematic detailing the authors’ proposed scenario for the formation of WOCS 7782, in which a binary pair of main-sequence stars have material fed onto them by an evolved outer tertiary companion. [Portegies Zwart & Leigh 2019]

With WOCS ID 7782 in mind, scientists Simon Portegies Zwart (Leiden University) and Nathan Leigh (American Museum of Natural History; Stony Brook University; and University of Concepción, Chile) have now proposed an alternative formation channel.

A Third Star in the Mix

Portegies Zwart and Leigh’s model relies on one important element: a third star. In their proposed scenario, two main-sequence stars in a close binary are orbited by a giant, evolved companion star. As this evolved star ages and overflows its Roche lobe, gas flows from it onto the main-sequence binary, increasing the masses of the two inner stars.

Snapshot from one of the authors’ simulated triple systems. The binary system at left is being fed by gas from the outer tertiary companion on the right. [Portegies Zwart & Leigh 2019]

The authors use simulations to show that the final result of this process can be a close binary with two similar-mass blue stragglers, just as seen in WOCS ID 7782. In this scenario, the outer companion eventually evolves into a hard-to-spot white dwarf on a wide orbit with a period of more than ~5.8 years.

In addition to potentially explaining WOCS ID 7782, Portegies Zwart and Leigh’s model can produce a number of other masses, geometries, and configurations for blue-straggler systems, depending on the initial masses and separations of the binary and the outer companion. This formation scenario — which relies on just a little help from a friend — may therefore go a long way toward explaining the formation of the blue-straggler systems that have stumped us before now.

Citation

“A Triple Origin for Twin Blue Stragglers in Close Binaries,” Simon Portegies Zwart and Nathan W. C. Leigh 2019 ApJL 876 L33. doi:10.3847/2041-8213/ab1b75

active galactic nucleus

How does life arise on exoplanets? What environments are necessary for it to survive? What conditions pose threats to life? These are some of the many questions of astrobiology, the study of life beyond our solar system.

While much research explores how stellar radiation influences whether planets can form or sustain life, fewer studies examine other sources of radiation. Today’s study explores extreme sources: the violent and bright centers of active galaxies.

Extreme Sources

active galactic nuclei

Active galactic nuclei emit powerful high-energy radiation as they accrete gas and dust. [ESA/NASA, the AVO project and Paolo Padovani]

Active galactic nuclei, or AGN, contain enormous supermassive black holes that rapidly accrete gas and dust, emitting harsh high-energy radiation in the process. Could this radiation seriously hamper the formation and evolution of extraterrestrial life on nearby planets?

A recent study by Harvard University scientists Manasvi Lingam, Idan Ginsburg, and Shmuel Bialy suggests the opposite may be true: the radiation from AGN has the potential to aid (Earth-like) life’s chances of formation and survival.

The Cons

Let’s start with the downside of having an enormous source of high-energy radiation lurking nearby: as we’ve all experienced via the occasional sunburn, too much ultraviolet (UV) radiation can have harmful effects for life. On the extreme end, an excess of UV radiation can inhibit photosynthesis — a process on which the vast majority of Earth-based life relies for survival — and can damage DNA and other biomolecules.

But how much is too much? Lingam and collaborators conduct a series of calculations to show that an AGN doesn’t have much of an impact on the vast majority of planets in a galaxy. If the Milky Way had an active nucleus, the danger zone for potential extinction via UV radiation would extend to just ~100 light-years from the AGN — a tiny distance on the scale of our 100,000-light-year galaxy.

astrobiology

Artist’s impression of radiation as a driver of the chemistry of early life. [NASA]

The Pros

But UV light isn’t all bad! In fact, UV radiation is a necessary ingredient for the prebiotic chemical reactions that led to the synthesis of biomolecular building blocks on Earth. Lingam and collaborators show that, within a certain distance of the AGN (out to ~150 light-years in our Milky-Way-size active-galaxy example), UV radiation from the accreting black hole could actually jump-start these chemical reactions and eventually lead to the formation of life.

What’s more, the authors demonstrate that the visible light from AGN can power photosynthesis on nearby planets — which could be particularly useful for free-floating planets that don’t have a host star to provide that light. The zone for which this holds true is broad: out to more than 1,100 light-years, for a Milky-Way-size active galaxy. 

The Upshot

AGN influence

Author estimates for the distances at which an AGN could enable prebiotic chemistry (dO, blue line), facilitate potential extinction events (dB, red line), and permit photosynthesis (dP, green line) as a function of the black hole mass. Solid and dashed lines indicate two different levels of AGN power. [Lingam et al. 2019]

Ultimately, Lingam and collaborators find that AGN won’t influence life formation and survival either way for the vast majority of planets in a Milky-Way-size galaxy; beyond ~3,000 light-years from the galactic center, planets won’t notice the radiation from the AGN.

But in other types of galaxies — like those with especially large supermassive black holes, or compact dwarf galaxies with high stellar densities at their center — AGN could play a significant role in sparking life and helping it to stay alive.

Citation

“Active Galactic Nuclei: Boon or Bane for Biota?,” Manasvi Lingam et al 2019 ApJ 877 62. doi:10.3847/1538-4357/ab1b2f

MAVEN orbiting Mars

When solar ultraviolet and X-ray photons collide with atoms and molecules in Mars’s atmosphere, they form a layer of plasma called an ionosphere. That’s what happens on the sunlit side, at least. What’s going on in Mars’s shadow? 

Mars ionosphere cartoon

A cartoon depicting the interaction of the solar wind with Mars’s atmosphere, as well as likely regions for atmospheric escape. [NASA/GSFC]

Planetary Plasma

Even though there are no solar photons striking Mars’s atmosphere at night, plasma is still present — but it’s not immediately clear where it comes from. Does it come from bombardment by galactic cosmic rays or trapped solar-wind particles, or is it transported from the sunlit side by winds?

And once the plasma has been produced, what happens to it? Is it lost when electrons and ions reunite to form neutrals, or does it escape the planet’s atmosphere entirely?

One way to assess the sources and sinks of plasma is by calculating the rates of production by electron-impact ionization — when energetic electrons ionize neutrals through collisions — and loss by dissociative recombination — when molecular ions capture an electron and are split apart. If the rates are equal, those two processes dominate. If not, other processes must play a role.

Cui et al. 2019 Fig. 1

From left to right, the densities of the major ion and neutral species, neutral (black) and electron (red) temperatures, and the average electron intensity. Click to enlarge. [Adapted from Cui et al. 2019]

MAVEN on a Mission 

Evaluating whether or not the two rates are equal requires neutral and ion densities, electron temperatures, and a spectrum of incident energetic electrons. Luckily, NASA’s Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft, which has been orbiting Mars since 2014, collects all that information and more.

Normally, MAVEN comes within 150 km of Mars’s surface, but it occasionally drops its closest approach to 125 km. These so-called Deep Dip campaigns, of which there have been nine, give scientists a close look at the densest plasma in the ionosphere. In this study, a team led by Jun Cui (Sun Yat-sen University, Chinese Academy of Sciences, and National Astronomical Observatories, China) analyzed data from two Deep Dip orbits in 2015 and 2016.

Using the in-situ measurements made along each orbit, Cui and collaborators calculated the rate at which CO2 — the dominant neutral species — is ionized by electron impacts and the rate at which O2+, NO+, and HCO+ — the three dominant ion species — dissociatively recombine.

Cui et al. 2019 Fig. 4

Comparison of the dissociative recombination and electron-impact ionization rates for the two orbits. Open circles represent calculations made with individual measurements, while closed squares indicate average values for each altitude bin. The starred points have been corrected for instrumental effects. Click to enlarge. [Cui et al. 2019]

A Complex Nightside Picture

At low altitudes (below 140 km for the midnight orbit and 180 km for the dawn orbit), the authors found that the electron-impact ionization rate agrees with the dissociative recombination rate, which indicates that sources of plasma other than electron-impact ionization don’t play a major role at these altitudes.

At high altitudes, however, the rate of electron-impact ionization is higher than the rate of dissociative recombination, which is a sign that there is another important plasma loss process happening at those altitudes. It’s possible that magnetic pressure gradients at those altitudes encourage ions to escape down Mars’s magnetotail.

Last month, MAVEN finished its two-month aerobraking campaign, during which the spacecraft altitude dipped as low as ~125 km to use atmospheric drag to change its orbit, giving scientists a long look at Mars’s ionosphere. Expect more atmospheric news from MAVEN in the future!

Citation

“Evaluating Local Ionization Balance in the Nightside Martian Upper Atmosphere during MAVEN Deep Dip Campaigns,” J. Cui et al 2019 ApJL 876 L12. doi:10.3847/2041-8213/ab1b34

M82

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).

HAWC+

The HAWC+ instrument mounted on the SOFIA telescope. [NASA]

Meet HAWC+

HAWC+ is a one-of-a-kind instrument: it’s the only currently operating astronomical camera that takes images in far-infrared light. HAWC+ observes in the 50-μm to 240-μm range at high angular resolution, affording us a detailed look at low-temperature phenomena, like the early stages of star and planet formation.

In addition to the camera, HAWC+ also includes a polarimeter, which allows the instrument to measure the alignment of incoming light waves produced by dust emission. By observing this far-infrared polarization, HAWC+ can produce detailed maps of otherwise invisible celestial magnetic fields. The insight gained with HAWC+ spans an incredible range of astronomical sources, from nearby star-forming regions to the large-scale environments surrounding other galaxies.

Some Recent HAWC+ Science

Cygnus A core

Artist’s conception of Cygnus A, surrounded by the torus of dust and debris with jets launching from its center. Magnetic fields are illustrated trapping dust near the supermassive black hole at the galaxy’s core. [NASA/SOFIA/Lynette Cook]

Cygnus A is the closest and most powerful radio-loud active galactic nucleus. At its heart, a supermassive black hole is actively accreting material, producing enormous jets — but this core is difficult to learn about, because it is heavily shrouded by dust.

In a recent study led by Enrique Lopez-Rodriguez (SOFIA Science Center; National Astronomical Observatory of Japan), a team of scientists has used HAWC+ to observe the polarized infrared emission from aligned dust grains in the dusty torus surrounding Cygnus A’s core. Lopez-Rodriguez and collaborators find that a coherent dusty and magnetic field structure dominates the infrared emission around the nucleus, suggesting that magnetic fields confine the torus and funnel the dust in to accrete onto the supermassive black hole.

••••••

Messier 82 and NGC 253 are two nearby starburst galaxies — galaxies with a high rate of star formation. Such galaxies often have strong outflowing galactic winds, which are thought to contribute to the enrichment of the intergalactic medium with both heavy elements and magnetic fields.

A study led by Terry Jay Jones (University of Minnesota) uses HAWC+ to map out the magnetic field geometry in the disk and central regions of these two galaxies. M82 shows the most spectacular results, revealing clear evidence for a massive polar outflow that drags the magnetic field vertically away from the disk along with entrained gas and dust.

••••••

lensed starburst galaxy

SOFIA/HAWC+ 89 μm detection of the gravitationally lensed starburst galaxy J1429-0028. Right: false-color composite image of J1429-0028 from Hubble and Keck. [Ma et al. 2018]

A study led by Jingzhe Ma (University of California, Irvine) presents the HAWC+ detection of the distant, gravitationally lensed starburst galaxy HATLAS J1429-0028. This beautiful system consists of an edge-on foreground disk galaxy and a nearly complete Einstein ring of an ultraluminous infrared background galaxy. What causes this background galaxy to shine so brightly in infrared wavelengths? The HAWC+ observations suggest it’s not due to emission from an active galactic nucleus; instead, this galaxy is likely powered purely by star formation.

••••••

G 9 region

The G 9 region, as represented by the Digital Palomar Observatory Sky Survey. The cyan polygon represents the SOFIA HAWC+ coverage of the filamentary dark cloud GF 9. The yellow diamond marks the YSO GF 9-2. [Clemens et al. 2018]

In a recent study examining the geometry of magnetic fields surrounding sites of massive star formation, Dan Clemens (Boston University) and collaborators obtained HAWC+ observations of a young stellar object (YSO) embedded in a molecular cloud. The polarimetric measurements of HAWC+ revealed the magnetic field configuration around the YSO, the dense core that hosts it, and the clumpy filamentary dark cloud that surrounds it, GF 9.

Surprisingly, the observations show a remarkably uniform magnetic field threading the entire region, from the outer, diffuse cloud edge all the way down to the smallest scales of the YSO surroundings. These results contradict some models of how cores and YSOs form, providing important information that will help us better understand this process.

Citation

ApJL Focus issue:
Focus on New Results from SOFIA

HAWC+ articles:
“The Highly Polarized Dusty Emission Core of Cygnus A,” Enrique Lopez-Rodriguez et al. 2018 ApJL 861 L23. doi:10.3847/2041-8213/aacff5
“SOFIA Far-infrared Imaging Polarimetry of M82 and NGC 253: Exploring the Supergalactic Wind,” Terry Jay Jones et al. 2019 ApJL 870 L9. doi:10.3847/2041-8213/aaf8b9
“SOFIA/HAWC+ Detection of a Gravitationally Lensed Starburst Galaxy at z = 1.03,” Jingzhe Ma et al. 2018 ApJ 864 60. doi:10.3847/1538-4357/aad4a0
“Magnetic Field Uniformity Across the GF 9-2 YSO, L1082C Dense Core, and GF 9 Filamentary Dark Cloud,” Dan P. Clemens et al. 2018 ApJ 867 79. doi:10.3847/1538-4357/aae2af

young stellar objects

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).

EXES

The EXES instrument mounted on the SOFIA telescope. [NASA/SOFIA/EXES/Matthew Richter]

Meet EXES

EXES is used for high-resolution spectroscopy at mid-infrared wavelengths — from 4.5 to 28.3 µm — to study molecular gas in dense, quiescent clouds and protostellar disks. EXES uses a special coarsely-ruled aluminum reflection grating to spread light into a spectrum, allowing scientists to identify specific spectral lines associated with emission from different molecules.

The instrument’s high spectral resolution enables the study of molecular hydrogen, water vapor, and methane from sources like molecular clouds, protoplanetary disks, interstellar shocks, circumstellar shells, and planetary atmospheres. For many sources, EXES is able to achieve comparable sensitivity even to space-based observatories like Spitzer.

Some Recent EXES Science

Becklin-Neugebauer object location

Image from the Subaru telescope showing the location of the Becklin-Neugebauer object in Orion. [NAOJ/Subaru Telescope]

A young, massive star dubbed the Becklin-Neugebauer object is irrationally speeding through the Orion nebular cluster at a relative speed of ~30 km/s! One proposed explanation for this object’s unusual velocity is that it was caught in a three-body dynamical interaction inside a nebula, during which it was violently ejected.

If true, we could expect that the Becklin-Neugebauer object might have dragged some of the hot, dense molecular gas along with it when it was ejected. A team of scientists led by Nick Indriolo (Space Telescope Science Institute) used EXES to search for signs of hot water molecules moving along with the Becklin-Neugebauer object, and came up empty-handed — adding one more perplexing clue to the mystery of this strange source.

••••••

Hot molecular cores are compact regions of dense gas that represent an intermediary stage of massive star formation; once a protostar forms in a collapsing cloud, it heats its surroundings and drives an outflow of evaporating material.

A study led by Andrew Barr (Leiden University, the Netherlands) explores the composition of the hot molecular core AFGL 2591 using EXES infrared observations. The authors detect carbon monosulfide (CS), a molecule that can be used to probe the physical conditions deep in the innermost parts of the hot core near the base of the outflow.

••••••

young stellar object

Hubble image of a nearby Young Stellar Object, V1331Cyg. [ESA/NASA/Hubble/K. Stapelfeldt/B. Stecklum/A. Choudhary]

In another look at sulfur chemistry in massive star formation, Ryan Dungee (Institute for Astronomy, University of Hawaii) and collaborators observed warm sulfur dioxide gas (SO2) near the massive young stellar object (YSO) MonR2 IRS3, a collapsing protostar still embedded in a molecular cloud. The high resolution of EXES’s observations allowed the authors to identify the most likely source of the gas: sublimating ices in the hot core close to the massive young stellar object. These observations help us to understand the underlying chemistry of the birth of massive stars.

••••••

Does Jupiter’s moon Europa host plumes of water erupting from its surface? So suggest Hubble images from 2012 and recently re-analyzed data from NASA’s Galileo spacecraft. To test this theory, a team led by William Sparks (SETI Institute and Space Telescope Science Institute) used SOFIA/EXES to search for direct evidence of the presence of water vapor erupting from Europa’s surface.

Europa

Composite image of Europa from Galileo and Voyager, superimposed on Hubble data that suggests the presence of plumes of water vapor at roughly the 7 o’clock position off Europa’s limb. [NASA/ESA/W. Sparks (STScI)/USGS Astrogeology Science Center]

The result? If plumes are indeed present on Europa, they can’t be carrying much water vapor. EXES saw no evidence of plumes, placing an upper limit on the amount of water ejected from the moon in this way during SOFIA’s observations. This limit is lower than the amount of water implied by the previous Hubble observations — leaving yet another mystery unsolved and deepening the question of whether Europa has what it takes to support life.

Citation

ApJL Focus issue:
Focus on New Results from SOFIA

EXES articles:
“High Spectral Resolution Observations toward Orion BN at 6 μm: No Evidence for Hot Water,” Nick Indriolo et al. 2018 ApJL 865 L18. doi:10.3847/2041-8213/aae1ff
“Infrared Detection of Abundant CS in the Hot Core AFGL 2591 at High Spectral Resolution with SOFIA/EXES ,” Andrew G. Barr et al. 2018 ApJL 868 L2. doi:10.3847/2041-8213/aaeb23
“High-resolution SOFIA/EXES Spectroscopy of SO2 Gas in the Massive Young Stellar Object MonR2 IRS3: Implications for the Sulfur Budget,” Ryan Dungee et al. 2018 ApJL 868 L10. doi:10.3847/2041-8213/aaeda9
“A Search for Water Vapor Plumes on Europa using SOFIA,” W. B. Sparks et al. 2019 ApJL 871 L5. doi:10.3847/2041-8213/aafb0a

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

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