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Exoplanet K2-18b

Whether or not a planet lies in its star’s habitable zone is commonly used to gauge its ability to host life. But what about non-habitable-zone planets that have sources of heat besides starlight?

Warming Up in the Zone

A star’s traditional habitable zone marks the range of distances at which an orbiting planet receives enough heat from its star to host liquid water on its surface. Since water (or another liquid) is generally considered a necessary ingredient for life to arise and survive, stellar habitable zones represent convenient boundaries within which to search for life beyond our solar system.

Habitable zones

Schematic showing how the traditional habitable zone’s location and width changes around different types of stars. [NASA]

But we already know that habitable zones don’t tell the whole story. Plenty of planets that lie within their stars’ habitable zones aren’t livable — perhaps because they’re inhospitable gas giants, or because they have the wrong type of atmosphere, or because they’re routinely blasted by energetic stellar flares from their host.

Could the opposite be true as well, however? Could planets outside of a star’s habitable zone be capable of supporting life? A new study by scientists Manasvi Lingam (Harvard University; Florida Institute of Technology) and Abraham Loeb (Harvard University) explores this possibility. 

Stars Aren’t Everything

surface temperatures over time

Surface temperature as a function of age in Myr for a world with radioisotope abundances 1,000x that of Earth, for three different planet masses. The blue, green, and brown horizontal lines bound the temperature range in which liquid water, ammonia, and ethane can exist, respectively. [Lingam & Loeb 2020]

External heating from starlight is not the only way to keep a planet warm enough for surface liquids, Lingam and Loeb argue. There are additional processes that can instead heat a planet’s surface from the inside — in particular, radioactive decay and primordial heat from the planet’s formation. How powerful would these processes need to be for a planet to maintain liquid on its surface long enough for life to arise and evolve, even without the added heat from starlight?

To be inclusive of life forms that may be different from Earth’s, Lingam and Loeb choose to explore three different liquids in their models: water, ammonia, and ethane. The authors investigate the radioactive heat flux from both long-lived and short-lived isotopes, as well as the typical heat flux released as a world cools after its formation.

Radioactive Worlds

Lingam and Loeb find that a rocky super-Earth with a tenuous atmosphere would need radioactive isotope abundances roughly 1,000 times higher than that of Earth to host long-lived water oceans without the help of starlight. Long-lived ethane oceans are easier to achieve, requiring only 100 times Earth’s radioisotope abundances.

neutron star merger

Artist’s impression of the collision and merger of two neutron stars. [NSF/LIGO/Sonoma State University/A. Simonnet]

Are these high concentrations feasible? Worlds in the dense inner regions of the galactic bulge (where radioisotope-producing neutron-star mergers are more common) or in gas-poor environments are expected to exhibit higher radioisotope abundances. These higher concentrations may be enough to generate the heat needed to sustain liquid on the planets’ surfaces.

Since the number of planets outside of stellar habitable zones is likely orders of magnitude larger than the number inside them, the chance for life on non-habitable-zone worlds opens a wealth of possibilities. Keep an eye out in the future — the James Webb Space Telescope may be able to detect the infrared signatures of some of these internally heated worlds!

Citation

“On the Habitable Lifetime of Terrestrial Worlds with High Radionuclide Abundances,” Manasvi Lingam and Abraham Loeb 2020 ApJL 889 L20. doi:10.3847/2041-8213/ab68e5

TESS

We can only really see what’s going on at the surface of a star. However, the motions within stellar interiors show themselves as subtle variations in the star’s brightness, and with the dense observations of planet-finding missions, we can pick up these variations at very fine levels.

Brightness Changes

stellar oscillations

Asteroseismology uses different oscillation modes of a star to probe its internal structure and properties. [Tosaka]

Several things can be responsible for a star’s changes in brightness, such as planets or companion stars, but stars can also brighten and dim intrinsically. The outer layers of a star can oscillate as the star cools and heats up, while convective processes can cause different oscillations in the body and at the surface of a star. By studying these oscillations — in a process called asteroseismology — we can learn more about stellar properties and internal structures.

Asteroseismology studies require stellar observations taken regularly over a long time. Luckily for astronomers, those are the same sort of observations required for the transit planet-finding method! Over the last decade, missions like Kepler and CoRoT have provided data that’s been used both to find planets and to characterize the insides of stars.

HR diagram with frequency for stars in sample

A Hertzspring-Russell (HR) diagram showing stars’ maximum oscillation frequency (instead of brightness) versus their temperature. The gray dots are all Hipparcos stars brighter than sixth magnitude, while the red dots are the red giant stars used in this study. The Sun is shown at the lower-left corner of the diagram. The tracks show evolutionary models for stars with the masses indicated (in solar-masses) [Aguirre et al. 2020].

The Transiting Exoplanet Survey Satellite (TESS) is carrying out an all-sky survey searching for planets. It covers the sky in sectors, taking images of its entire field of view every thirty minutes. The thirty-minute cadence is well-suited to characterize red giant stars’ oscillations, and TESS is expected to turn up roughly 500,000 red giant stars whose oscillations can be studied.

So it makes sense to set up a framework to analyse TESS data in the context of stellar oscillations. Motivated by this, a group of scientists led by Víctor Silva Aguirre (Aarhus University, Denmark) carried out the first analysis of red giant star oscillations using TESS data.

TESS-ting for Frequencies

Aguirre and collaborators considered stars observed in TESS sectors 1 and 2 (not necessarily both) for their sample. They then used the Hipparcos astrometry mission catalog to identify red giant stars whose oscillations could be seen in TESS data. The maximum frequency of oscillations is set by a star’s temperature, mass, and brightness.

Assuming a mass of 1.2 solar masses for red giant stars (a reasonable assumption), the maximum frequency could be expected to fall between 30 and 220 µHz. From the available stars, the authors selected the twenty-five brightest stars for their oscillation analysis.

We’ve Got the Power (Frequency Spectrum)!

Frequency power spectra for red giant stars

Representative power spectra obtained from the red giant light curves. The left panels have the spectra in log–log space, with the maximum frequency distinguished by the vertical arrow. The right panels show a close-up of the power spectra showing frequencies associated with particular oscillations. The separation between those dominant frequencies is shown using the red vertical lines with the horizontal arrow showing the average separation [Adapted from Aguirre et al. 2020].

From the light curves of each star, Aguirre and collaborators created power frequency spectra, which show the power in the various frequencies that make up a signal. For stars, such power spectra can be used to identify oscillations that could be associated with internal processes.

By combining the oscillation frequency information with astrometry data from the Gaia mission, Aguirre and collaborators showed that they could measure the properties of these red giants to high precision levels — stellar radii can be determined to a precision of a few percent, masses to 5%–10%, and ages to around 20%.

With the incoming flood of TESS and Gaia data, we will almost certainly be able to characterize red giant stars better than we have in the past. And this is to say nothing of what will come out of TESS’s extended mission, beginning July 2020!

Citation

“Detection and Characterization of Oscillating Red Giants: First Results from the TESS Satellite,” Víctor Silva Aguirre et al 2020 ApJL 889 L34. https://doi.org/10.3847/2041-8213/ab6443

Collinder 261

Though stars within the same cluster all typically form around the same time, they don’t all evolve in the same way. A recent study has carefully explored a population of particularly unusual, straggling stars in the old open cluster Collinder 261.

blue stragglers in NGC 6362

This Hubble image of the globular cluster NGC 6362 reveals a number of stars that appear younger and bluer than their companions: so-called blue stragglers. [ESA/Hubble & NASA]

Why So Blue?

A stellar cluster is typically born in a burst of star formation that creates member stars from the same source material. After the stars form, the cluster ages over cosmic time, its individual stars evolving according to their masses. Bright, blue, massive stars have short lifespans, evolving quickly off the main sequence; dim, red, low-mass stars live much longer and evolve slowly. This difference causes clusters to become progressively redder as they age.

For particularly old clusters, we would not expect to see any bright blue stars, as these should have all aged off the main sequence already. And yet, again and again, we find handfuls of these bright blue stars — in the Milky Way’s globular and open clusters, and even in other nearby galaxies. How do these so-called blue stragglers arise?

Oh, to Be Young Again

formation of blue stragglers

Two possible formation channels for blue stragglers: two stars collide (top), or a star gains mass from a binary companion (bottom). [NASA/ESA]

Since blue-straggler stars are more massive and brighter than expected for their host cluster, we think they must have gained that mass more recently. There are two proposed rejuvenation scenarios that could create blue stragglers:

  1. Two stars collide and merge to form one massive star.
  2. A star gains mass from a close-binary companion.

By studying populations of blue stragglers and exploring these possibilities, we have the potential opportunity to learn about about cluster dynamics and histories, and about binary systems. But blue stragglers tend to lie near the very crowded centers of galaxies — which makes it difficult to observe individual stars and be certain of their membership in the cluster.

Led by Maria Rain (University of Padua, Italy), a team of scientists has now met this challenge using the precise stellar measurements of the Gaia mission in conjunction with spectroscopy from an instrument on ESO’s Very Large Telescope in Chile. The team conducted one of the most detailed studies of a blue straggler population, exploring the open cluster Collinder 261.

Stars of a Different Color

proper motions for Collinder 261

Proper motions for Collinder 261. Black circles are cluster members; blue circles are blue straggler candidates, and orange circles are yellow straggler candidates. [Adapted from Rain et al. 2020]

Aged at 7 to 9.3 billion years, Collinder 261 is one of the oldest open clusters of the Milky Way. Rain and collaborators used Gaia data describing the colors and brightnesses, the proper motions, and the parallaxes of stars in Collinder 261’s field to identify 53 blue straggler candidates and one potential yellow straggler — an evolved blue straggler — in the cluster. The authors then followed up 10 of these stars with spectroscopic measurements, determining that at least five of them are members of close binary systems.

While these data are not yet enough to draw firm conclusions about the origin of blue stragglers, it should be possible to spectroscopically follow up the remaining candidate stars to learn more. This study provides a particularly detailed exploration of these odd straggling stars, which we can hope to build on in the near future.

Citation

“A study of the blue straggler population of the old open cluster Collinder 261,” M. J. Rain et al 2020 AJ 159 59. doi:10.3847/1538-3881/ab5f0b

galaxy collision

Thanks to the Laser Interferometer Gravitational-wave Observatory (LIGO), we now know that black holes in our distant universe sometimes find each other in a dramatic inspiral and collision, releasing a burst of gravitational-wave emission that we can detect here on Earth.

But what happened earlier in these black holes’ lives to bring them to this point? A new study explores the possibility that LIGO’s black holes once lay at the centers of very small galaxies — until those galaxies collided.

black hole merger

Simulated image of two merging black holes, viewed face-on. LIGO has announced the detection of ten of these events so far. [SXS Lensing]

Central Lurkers

Since we discovered the first wiggles in spacetime signifying the distant merger of two black holes, LIGO has announced around ten confident detections of gravitational waves from black hole–black hole collisions — with the prospect of many more discoveries in the future.

But how did these black holes find each other? A team of scientists led by Christopher Conselice (University of Nottingham, UK) has proposed a picture that hinges on the central black holes we believe lie at the heart of most, if not all, galaxies.

The team proposes that very low-mass dwarf galaxies contain central black holes of less than 100 solar masses. The mergers of pairs of these tiny galaxies ultimately lead to the inspirals and mergers of their central black holes — possibly accounting for the majority of LIGO’s detections of black hole–black hole collisions. 

Testing Feasibility

Conselice and collaborators test this scenario by breaking it down into multiple steps.

black hole mass vs. galaxy mass

The relationship between black hole mass and host galaxy stellar mass (black solid line; blue dashed lines show uncertainties), extrapolated down to low masses. Red lines indicate the masses of LIGO-detected black holes. [Conselice et al. 2020]

  1. Can you get central black holes of the right mass?
    We’ve observed a relationship between galaxy mass and central black hole mass. By extrapolating this relationship to low masses, we find that ultradwarf galaxies can have central black holes of less than 100 solar masses — consistent with the LIGO-observed black holes of 10–70 solar masses.
  2. Will these ultradwarf galaxies merge frequently enough?
    Mergers of galaxies occurred more frequently in the early universe than they do today. Cosmological models indicate that galaxies don’t merge frequently enough today to reproduce LIGO’s observations — but at a redshift of z ~ 1.5 or higher, ultradwarf galaxies could merge often enough to match LIGO-measured gravitational-wave event rates.
  3. Will the black holes collide fast enough after the galaxies merge?
    If the galaxies merged at a redshift of z > 1.5, the central black holes would have to sink to the middle of the merger, inspiral, and collide on timescales of 6–8 billion years to match LIGO observations. This is feasible if the black holes are embedded in a massive star cluster at the galaxy center.

A Future Hunt for Hosts

Conselice and collaborators’ calculations show that merging ultradwarf galaxies in the distant universe could, conceivably, account for LIGO’s black hole–black hole merger detections.

Fornax dwarf

An example of a dwarf spheroidal galaxy. The smallest dwarfs are far too faint to detect at high redshifts with current technology. [ESO/Digitized Sky Survey 2]

In the future, we can hope to test this theory by better pinpointing the hosts of gravitational-wave events. If we find that the black-hole collisions all originate from bright, massive galaxies, then the ultradwarf-merger theory is out. But if we can’t spot the hosts, this might be because they’re ultradwarfs that are too small and faint to detect.

The field of gravitational-wave astronomy is still only just coming of age, and theoretical work like this study shows how just how much we can hope to learn in the future!

Citation

“LIGO/Virgo Sources from Merging Black Holes in Ultradwarf Galaxies,” Christopher J. Conselice et al 2020 ApJ 890 8. doi:10.3847/1538-4357/ab5dad

SunPy

Python, one of the foremost high-level programming languages, has played a growing role in the analysis of astronomical data. With the recent release of a new software package, SunPy, it’s now easier than ever for solar physicists to use Python as well.

SDO/AIA

An example of a SunPy-generated Map visualization using data from the Solar Dynamics Observatory’s AIA instrument. The bottom panel shows a zoomed-in view from the top panel, focusing on an erupting flare. [Adapted from The SunPy Community et al. 2020]

Juggling Ones and Zeros

The modern era of astronomy relies heavily on computer software to advance our understanding of the universe. Long gone are the days of sketching what we see through telescope eyepieces; now astronomers receive their telescope observations in the form of files full of data that must be carefully analyzed using complex code bases.

Preferences for one programming language over another evolve over time as our needs evolve — and Python is currently a rising star. Major companies like Google, Wikipedia, and Facebook all make use of Python, and astronomers are increasingly adopting Python for their data analysis in place of past staples like Fortran and IDL.

A Shared Enterprise

The field of solar physics is driven by publicly available observations of the Sun that stream in on a constant basis from a number of ground- and space-based observatories. As solar physics, like the rest of astronomy, is a largely collaborative field, it makes sense to share the software tools that are used to analyze this common data.

To this end, a group of solar physicists has come together to produce SunPy, a community-developed free and open-source software package that consists of tools for analyzing solar and heliospheric data. In a recent article, the SunPy team has detailed this Python-based package and the overarching SunPy project, which develops and maintains the package and supports the ecosystem surrounding it.

SunPy codebase

Growth of the SunPy codebase over time — both the total lines of code (solid line) and comments (dashed line). The dip after version 0.9 is the result of a major code reorganization. [The SunPy Community et al. 2020]

What Can SunPy Do For You?

Looking to explore some solar data? The SunPy software package is freely accessible, and its first official stable release was issued last year. As of version 1.0, SunPy consisted of nearly 50,000 lines of code, comments, and documentation that support a large set of common tasks in the analysis of solar data.

Here are just a few things you can do using the SunPy package:

  • Query and download data from many different solar missions and instruments via a general, standard, and consistent interface.
  • Load and visualize time series data — measurements of how, say, a particular type of flux from a region changes over time — and images.
  • Perform transformations between the variety of coordinate systems commonly used to describe events and features both on the Sun and within the heliosphere.
SunPy Gallery

SunPy comes with a detailed user’s guide and example gallery to assist users. [SunPy]

Looking Ahead

SunPy will be supported with two new releases planned per year. Future development already on the books includes support for generic spectra, multidimensional data sets, and a standardized approach to metadata.

The SunPy team hopes to grow community involvement and establish financial support in the future, in order to further expand SunPy development. In the meantime, the SunPy project’s team of volunteer developers have done an admirable job of building a powerful set of shared tools for the solar physics community.

Citation

“The SunPy Project: Open Source Development and Status of the Version 1.0 Core Package,” The SunPy Community et al 2020 ApJ 890 68. doi:10.3847/1538-4357/ab4f7a

FU Orionis

Some young stars seem to spend a brief portion of their lives undergoing dramatic, flaring outbursts. A new study has used the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile to get the closest look yet at one of these systems — possibly identifying the cause of the flares.

flaring star

Artist’s impression of a young star throwing a temper tantrum as it suddenly increases its accretion rate and flares. [Caltech/T. Pyle (IPAC)]

Young Stellar Temper Tantrums

FU Orionis (FU Ori, for short) objects are young, pre-main-sequence stars that grow suddenly brighter — by several magnitudes! — over the span of perhaps a year. These flaring states can last on the order of decades, and they’re thought to be related to a period of increased accretion onto the star during its early years. A star may gain a significant portion of its final mass during these events.

Beyond this general picture, there’s much we don’t understand about how or why this increase in accretion occurs. Does every star undergo a FU Ori phase early in its lifetime, accumulating extra mass in spurts and brightening each time it does? Or do only some stars behave this way? What causes the change in accretion rate? What ends this phase?

The first step to answering some of these questions is to obtain high-resolution images of FU Ori objects so that we can better explore their structure and behavior. In a new study, a team of scientists led by Sebastián Pérez (University of Santiago, Chile; University of Chile) has used ALMA to capture a detailed look at the archetypal FU Ori system for which these objects were named.

Signs of Interaction

ALMA FU Ori continuum

ALMA continuum observations show the dust of the two disks surrounding the binary stars of FU Orionis. Each disk is about 11 AU in radius. [Pérez et al. 2020]

FU Orionis is a binary pair of young stars that lies roughly 1,360 light-years away in the constellation of Orion. Pérez and collaborators’ ALMA observations of the system resolved, for the first time, the disks of accreting dust surrounding each of the young stars. Modeling of these disks allowed the authors to infer that they are roughly 11 AU in radius and their separation is perhaps 250 AU.

Observations of the gas in the disks reveal its kinematics, demonstrating that the rotation of each disk is somewhat asymmetric and skewed. The authors propose that this indicates some sort of close encounter for the disks — perhaps the flyby of another star within this crowded star-forming region, or possibly even the direct interaction of the two disks of the binary with each other.

An elongated arc of gas may connect the two components, further strengthening the argument that the two disks are interacting. And close passes between the disks of a binary or perturbations from a flyby could easily increase the accretion rate onto the stars, fueling the FU Ori outburst that we now observe.

Several other FU Ori systems are in known binaries, providing additional targets that we can follow up with to test whether disk interactions can truly explain these objects’ sudden, dramatic flares. Meanwhile, ALMA continues to play an important role in helping us to explore how stars form and evolve.

Citation

“Resolving the FU Orionis System with ALMA: Interacting Twin Disks?,” Sebastián Pérez et al 2020 ApJ 889 59. doi:10.3847/1538-4357/ab5c1b

Artist’s impression of high redshift galaxy

Studying star formation in the early universe can give us clues about what the universe was like when the earliest massive galaxies were forming. How efficiently were these first galaxies making stars only a billion years after the Big Bang?

Lighting Up the Universe

The universe wasn’t always a treasure trove of galaxies. Not long after the Big Bang, it consisted largely of opaque neutral hydrogen, and the only photons present were either from the cosmic microwave background or emitted during electron transitions in hydrogen atoms. Between redshifts of z = 20 and z = 6 (i.e., between 150 million and 1 billion years after the Big Bang), the first galaxies formed and their stars ionized the hydrogen, allowing light to travel freely through the universe. 

This sounds very neat and straightforward, but the specifics of this process are hazy (note the very large time window!). What sort of stars did the ionizing? Did it happen all at once or more slowly? How long did it take? Pinning down the star formation rates of galaxies at that transition redshift of z ~ 6 can help us answer some of these questions.

Spectra and line maps of HZ10 and LBG-1

Left: spectra of galaxies HZ10 and LBG-1 highlighting the CO and carbon lines studied. The red line in the HZ10 spectrum is the Gaussian fit to the detected CO. The blue histogram is the CO emission, scaled down by a factor of 40. Right: integrated line maps of HZ10 and LBG-1 with contours showing CO (white lines) and carbon (black lines). The maps are created by integrating the individual spectra that compose the map, collapsing the data from three dimensions to two. The maps are scaled based on the carbon detections. [Pavesi et al. 2019]

The star formation rate of a galaxy is governed by many factors, not least of which are inflows and outflows of gas and the makeup of the gas contained within the galaxy. Measuring flows in early galaxies is beyond the capability of current instruments, but a lot can be learned by studying the cold, star-formation-fueling molecular gas that lies within galaxies.

And that’s precisely what a group of scientists led by Riccardo Pavesi (Cornell University) did. Using observations taken by the Very Large Array (VLA) and the Atacama Large Millimeter/submillimeter Array (ALMA), Pavesi and collaborators studied molecular features in a sample of galaxies at z = 5–6.

Reading the Lines

For this study, Pavesi and collaborators leverage particular emission lines associated with carbon monoxide (CO), carbon, and nitrogen. This emission can appear in long-wavelength observations when the galaxies in question are very distant. The authors also study the overall radiation emanating from dust in their target galaxies.

A notable finding from the analysis is the highest redshift detection of CO emission, which comes from a galaxy at z ~ 5.7. However, this galaxy, HZ10, also stands out for another reason: it appears to contain a large reservoir of gas, perfect for making stars.

Ratio of molecular gas mass to stellar mass versus redshift for galaxies between z = 0 and z = 6 based on CO measurements

The ratio of molecular gas mass to stellar mass versus redshift for galaxies located at 0 < z < 6, with the calculated masses based on CO measurements. Two galaxies from this study, HZ10 and LBG-1 are highlighted at the high redshift end of the plot, between z = 5 and z = 6. The gray line is a quadratic relation between mass ratio and redshift. The gray points are also HZ10 and LBG-1 with their mass ratios based on a different interpretation of the CO measurements. [Pavesi et al. 2019]

Star-ting and Stopping

The presence of unused star-making material so early in the universe is significant. Previous measurements of star formation at z > 5 found that early galaxies formed stars very efficiently — but because these studies probed only the most luminous of galaxies, it was unclear whether this result would hold for more typical galaxies at this redshift.

HZ10’s large gas reservoir indicates that this “normal” galaxy has a much lower star-formation efficiency than the brighter galaxies we’ve previously studied at this redshift; it actually shares more characteristics with galaxies at z ~ 3. HZ10 offers some of the first evidence that galaxies with lower star formation efficiencies exist at z > 3.

As with most new discoveries, we need larger samples and high quality observations to better understand this — stay tuned!

Citation

“Low Star Formation Efficiency in Typical Galaxies at z = 5–6,” Riccardo Pavesi et al 2019 ApJ 882 168. https://doi.org/10.3847/1538-4357/ab3a46

Omega Centauri

The globular cluster Omega Centauri makes for an impressive sight — millions of stars gravitationally bound into a beautiful sphere, its core alight from the glow of densely packed bodies. A recent study has unveiled a new discovery at the heart of this cluster: five long-anticipated pulsars.

What Lies At the Core

globular clusters

Image of two globular clusters (can you spot them? Look carefully!) in the Milky Way. Omega Centauri is brighter and more massive than either of these — or any other Milky Way globular cluster. [ESO/D. Minniti/VVV Team]

Located just 17,000 light-years away, Omega Centauri is an intriguing object of study. Though we know of more than 200 globular clusters — compact spheres of old stars — that lie in the outer regions of the Milky Way, Omega Centauri is the most massive and the most luminous. Its properties have led scientists to speculate that this cluster was once a dwarf galaxy that was captured by the Milky Way and had its outer stars stripped away.

Omega Centauri’s large mass and unusual formation history open two interesting possibilities:

  1. The cluster might contain massive black holes.
    Theory predicts that the conditions at the center of massive stellar clusters are ripe for collisions that drive the growth of intermediate-mass black holes with perhaps hundreds to tens of thousands of solar masses.
  2. The cluster core might show evidence of dark matter annihilation.
    If Omega Centauri was once a dark-matter-dominated dwarf galaxy, then its relatively close distance makes it an excellent target to search for dark matter annihilating at its center.
pulsar

Artist’s illustration of a pulsar, a fast-spinning, magnetized neutron star. [NASA]

Pulsars as Probes

How can we explore these possibilities? Our best bet would be to study the motions and signatures of radio pulsars — rapidly rotating, magnetized neutron stars — in the cluster’s core. The motions of these dense objects would provide information about the core’s dynamics, potentially revealing the gravitational influence of lurking massive black holes. The signatures of the pulsars’ emission could also tell us about the interstellar medium of the cluster, constraining particle dark matter annihilation models.

But though radio pulsars are common in the cores of other globular clusters, they’ve remained elusive in Omega Centauri. A tantalizing hint came in 2010 with the discovery of a gamma-ray source in the cluster’s core — but years of searching for pulsed radio emission from this location turned up nothing.

A Population Found

pulse profiles

The pulse profiles of the five newly discovered millisecond radio pulsars in Omega Centauri’s core. Click to enlarge. [Dai et al. 2020]

A new study led by Shi Dai (CSIRO Australia Telescope National Facility) has now used the high sensitivity of the Ultra-Wideband Low receiver on the Parkes radio telescope in Australia to up the search intensity. The result? The team found five faint millisecond pulsars hiding in Omega Centauri’s core.

These newly discovered pulsars have spin periods that range from 4.1 to 6.8 milliseconds. While four of them are isolated objects, the fifth lies in an eclipsing binary system with a very low-mass star, orbiting once every 2.1 hours.

These pulsars may just be the tip of the iceberg: Dai and collaborators found additional compact sources in deep radio continuum images of Omega Centauri’s center, suggesting there may be more pulsars awaiting discovery. In the meantime, tracking the five pulsars now known will give us an excellent opportunity to probe the properties of this massive, bright cluster and learn more about its secrets.

Citation

“Discovery of Millisecond Pulsars in the Globular Cluster Omega Centauri,” Shi Dai et al 2020 ApJL 888 L18. doi:10.3847/2041-8213/ab621a

Parker Solar Probe

What might we learn about the Sun if we could fly a spacecraft close enough to dip down and skim through its atmosphere? Thanks to the Parker Solar Probe, we don’t have to speculate!

The Parker Solar Probe (PSP) is a telescope designed to orbit the Sun at least 24 times, dipping closer and closer to our star’s surface over its mission lifetime. Its first few orbits have already been completed at a distance of about 35.7 solar radii from the Sun’s center. Just this past month, PSP used the gravitational pull of Venus to drop its orbit to 27.8 solar radii — and by 2024, after several such maneuvers, PSP will be flying just 8.86 solar radii (that’s less than 4 million miles) from the Sun’s surface, soaring through the Sun’s tenuous outer atmosphere.

Solar corona

A view of the solar corona during the 2015 total solar eclipse in Svalbard, Norway. [S. Habbal, M. Druckmüller and P. Aniol]

This innovative spacecraft will bring us our closest look yet at the magnetic structure and heating of this outer atmosphere — the Sun’s corona — and give us the chance to better explore the solar wind, the stream of energetic particles that flows off of the Sun and pervades our solar system.

Though PSP’s orbit still has a lot further to drop, it’s already flying closer to the Sun than any other spacecraft ever has! This means we’ve already been able to do some remarkable science in the year and a half since its mission began. A new special issue of the Astrophysical Journal Supplement Series now presents roughly 50 studies detailing the findings from PSP’s first two orbits around the Sun.

A few broad categories of topics explored among these articles are:

  • switchbacks

    Illustration of magnetic switchbacks in the solar wind, first discovered by Parker Solar Probe. Click to view an animation. [NASA’s Goddard Space Flight Center/Conceptual Image Lab/Adriana Manrique Gutierrez]

    Switchbacks

    On large scales, the solar wind looks like a smooth flow of particles streaming radially outward from the Sun. But on scales close to the Sun’s surface, this flow is much more complex. PSP has measured a phenomenon termed “switchbacks” — rapid reversals in the direction of the magnetic field that governs the solar wind flow. Several articles detail what PSP has revealed about this phenomenon.

  • Plasma physics

    The high time and frequency resolutions of PSP’s instruments allow the probe to capture unprecedented observations of different plasma phenomena in the ionized gas close in around the Sun. PSP’s first two orbits have produced data on various wave modes, electron holes, magnetic reconnection, radio bursts, microinstabilities, plasma turbulence, and more. Several articles in this issue are devoted to analysis of these detections.

  • Small energetic-particle events

    Some solar activity can rapidly accelerate particles to enormous speeds. Since such energetic-particle events can pose a serious hazard to spacecraft and astronauts, we want to better understand what triggers them and how the particles are accelerated. PSP detected a large number of small energetic-particle events associated with various phenomena — and several articles in this issue detail what we’ve learned from these observations.

  • Plasma structures

    filament eruption

    A huge variety of plasma structures — like this erupting filament — can be witnessed on the Sun. [NASA’s Goddard SFC]

    When plasma is ejected from the Sun, it erupts into space in a variety of structures. PSP carries a camera system that has imaged the complex features of smaller plasma structures; in this special issue, these observations are analyzed and even combined with data from other Sun-watching spacecraft to build three-dimensional views of the structures. From this, we can better understand how magnetic fields govern the geometry and motions of the ionized gas emitted from the Sun.

  • Dust-free zone

    Though dust pervades our solar system, theory predicts that close to the Sun, the high temperatures should prevent dust from existing. Several articles describe PSP’s observations that suggest thinning dust levels; data from PSP’s future travels even closer to the Sun will hopefully confirm the presence of a dust-free zone and determine where, exactly, it lies.

solar cycle

We’re likely at a solar minimum right now in between two activity cycles, as shown here in the predictions made from sunspot observations over the last several cycles. The Sun should become progressively more active over the course of PSP’s mission lifetime. Click to enlarge. [David Hathaway, NASA, Marshall Space Flight Center]

These observations are just the start of what we can hope to learn from the Parker Solar Probe. We should expect to see many updates to our understanding of the corona and the solar wind as PSP explores regions closer to the Sun, as solar activity increases (we’re currently at a solar cycle minimum), and as in-flight calibrations of the PSP instruments continue. Stay tuned!

Citation

Special ApJS Issue on Parker Solar Probe

“Introduction,” Marcia Neugebauer 2020 ApJS 246 19. doi:10.3847/1538-4365/ab67cf

Comet 2I/Borisov

What do we know about the second object to visit us from another stellar system? Detailed Hubble images have given us plenty to consider!

Extrasolar Guests Among Us

'Oumuamua

Artist’s interpretation of interstellar asteroid 1I/’Oumuamua. [M. Kornmesser/ESO]

When asteroid 1I/‘Oumuamua tore through our solar system last year, it was a one-of-a-kind event: the first time we had observed an object from another stellar system pass through our own. But nature likes to keep us on our toes — and it wasn’t long before the next interstellar object paid us a visit.

In October of 2019, comet 2I/Borisov was first spotted. The body swung through perihelion — the point closest to the Sun — in December and then sped along on its way back out through our solar system. It’s expected to reach the distance of Jupiter in July of 2020, and the distance of Saturn by March of 2021.

Apples and Oranges, Asteroids and Comets

Our two interstellar visitors thus far, however, are surprisingly dissimilar. Unlike ‘Oumuamua, Borisov doesn’t have an obviously elongated, tumbling shape. And though observations of ‘Oumuamua showed it to be entirely inactive, Borisov has the appearance of a typical solar system comet: it has a prominent coma — a cloud of gas and dust — around it, and its spectrum contains weak lines that demonstrate ongoing outgassing.

As comet Borisov approached perihelion, a team of scientists led by David Jewitt (UC Los Angeles) imaged it with the Hubble Space Telescope, capturing this body at a distance of just 2.4 AU from the Sun. These observations provide us with a stunningly detailed look at an interstellar object.

A Comet Under Scrutiny

What have we learned from these new data?

  • comet 2I/Borisov

    Composite Hubble image of comet Borisov taken on 12 October 2019 shows the coma and extensive tail of dust trailing from the comet. The 5” scale bar corresponds to a distance of 10,000 km. [Jewitt et al. 2020]

    Size
    Jewitt and collaborators use three independent constraints — the comet’s surface brightness, the rate of its acceleration from forces other than gravity, and its gas production — to measure the size of the comet’s nucleus. They show that this body is likely much smaller than originally thought: just 200–500 meters in radius, as opposed to the 2–16 km estimated from initial observations.
  • Density
    The authors show that the above constraints also dictate a minimum density for the comet nucleus of at least 25 kg/m3 (for comparison, Earth’s density is 5,500 kg/m3). This rules out extreme low-density models for this body like some of those proposed to explain ‘Oumuamua’s unexpected acceleration (which have proposed densities of as little as 0.01 kg/m3).
  • Spin-up
    The small radius for Borisov renders this body susceptible to being spun up by torques from asymmetric outgassing. Jewitt and collaborators show that in the ~0.6 years Borisov will spend heating up within 3 AU of the Sun — and, consequently, outgassing as its water sublimates — the spin of the comet could change significantly, perhaps even causing the body to break up entirely! We can keep an eye out for spin changes as we observe it in the future.

There’s still plenty to learn as we continue to track this newest interstellar object — and we can surely expect more visitors to drop in for future observations!

Citation

“The Nucleus of Interstellar Comet 2I/Borisov,” David Jewitt et al 2020 ApJL 888 L23. doi:10.3847/2041-8213/ab621b

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