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How Old Are Globular Clusters?

Some 150 globular clusters are associated with the Milky Way, great collections of stars inhabiting the galactic halo. Their stars have long been assumed to be ancient, making the question of life there intriguing: If life caught hold in these tightly packed clusters early in the universe’s evolution, could ancient civilizations have formed that might persist even today? I know of only one planet that has yet been found in a globular cluster, but we’re obviously early in the game, and planets have been discovered in open clusters, which are much less densely packed.

Just how little we know about globular clusters, though, is made apparent by the work of Elizabeth Stanway (University of Warwick), whose new paper argues that such clusters could be billions of years younger than we have thought. Working with JJ Eldridge (University of Auckland), Stanway invokes a model called Binary Population and Spectral Synthesis (BPASS). In play here is the evolution of binary stars within globular clusters, a line of research that has been put to work in prior studies of young stellar populations in the Milky Way and elsewhere.

Image: The globular cluster Terzan 1. Clusters like these have been thought to contain some of the oldest known stars. New research puts their age into question. Credit: Judy Schmidt/ESA/NASA.

A significant problem in observing globular clusters is described in the paper. Having discussed using the spatially resolved light of individual stars to determine stellar properties, the authors note:

However, in the majority of old stellar populations, such detailed investigation of resolved stellar properties is impossible due to a lack of depth or angular resolution, and the integrated light of the unresolved stars must instead be used to constrain the properties of the population as a whole. In this scenario observable characteristics of the source spectral energy distribution (SED), including photometric colours and spectroscopic emission lines or indices, are compared to those determined for models of known age and composition.

And here’s the crux, which is why the age of such significant galactic features as these remains in doubt:

Either a best fitting template or a relation calibrated on such templates is then used to characterise the population. As a result, such analyses are strongly dependent on the properties of the template stellar population models.

Out of this understanding has sprung a new generation of models for the study of galactic evolution. But as the authors point out, binary interactions can have marked effects on a star’s development, and current research indicates that multiple star systems are ubiquitous in globular clusters, making single star models problematic when conclusions are being drawn from the integrated light of the entire cluster. And it turns out that the model populations that best fit observations are substantially younger than those derived from older spectral models.

Thus the utility of BPASS, which the authors have upgraded from a version described in an earlier paper. The BPASS upgrade is targeted at refining age estimates for older populations of stars. The researchers examine the elements in the spectra of binary stars, looking at systems where the larger star expands into a giant while the smaller star strips away its atmosphere. In this model, both stars are assumed to have formed at the same time as the globular cluster.

From the paper:

…incorporating binary stellar evolution pathways, together with the most up-to-date stellar evolution and atmosphere models for single stars, into stellar population synthesis models can make a significant difference in the interpretation of their integrated light properties. The lifetimes of stars in different mass ranges can be modified, by mass transfer and mixing, as can their temperatures and gravities. Mass transfer onto a secondary can produce more massive, and therefore brighter, stars at late ages, even if their stellar atmospheres are typical of cool red giants. A population can also incorporate stellar types simply not found in the absence of binary evolution.

Image: Binary star evolution within a globular cluster. Credit & copyright: Mark A. Garlick/University of Warwick.

Given the dependence of the age estimates of older star populations on evolutionary models, it’s helpful to see that the BPASS work can reproduce observed values in globular clusters and galaxies. And as the model is continuing to be refined, it produces this result:

Model fits to photometry and spectroscopic indices yield a consistently younger fit, often at slightly higher metallicity, than fits to older calibrations, when new stellar atmosphere models and binary stellar evolution pathways are included.

And this:

At its most basic level, this means that we are able to reproduce the photometry of mature, quiescent galaxies and clusters at younger ages than previous model sets (i.e. ∼ 5 − 8 Gyr, rather than 10-14 Gyr).

Stellar interactions, then, may tell the tale, perhaps adjusting our estimates of cluster age by billions of years. Still ancient, these vast ‘cities of stars’ pose huge questions — how stable are planetary orbits, for example, given the interactions between such tightly packed stars? But the vision of places like Lagash, the planet in Isaac Asimov’s “Nightfall” (called Kalgash in the later novel), its skies always ablaze with stars, keeps the science fiction fan in me speculating. What would it be like to be on a planet deep within something as splendid as the image below?

Image: Thousands and thousands of brilliant stars make up this globular cluster, Messier 53, captured with crystal clarity in this image from the NASA/ESA Hubble Space Telescope. Bound tightly by gravity, the cluster is roughly spherical and becomes denser towards its heart. These enormous sparkling spheres are by no means rare, and over 150 exist in the Milky Way alone, including Messier 53. It lies on the outer edges of the galaxy, where many other globular clusters are found, almost equally distant from both the centre of our galaxy and the Sun. Although they are relatively common, the famous astronomer William Herschel, not at all known for his poetic nature, once described a globular cluster as “one of the most beautiful objects I remember to have seen in the heavens.” This picture was put together from visible and infrared exposures taken with the Wide Field Channel of Hubble’s Advanced Camera for Surveys.

Our understanding of globular clusters is, of course, a work in progress. Says Stanway:.

“It’s important to note that there is still a lot of work to do – in particular looking at those very nearby systems where we can resolve individual stars rather than just considering the integrated light of a cluster – but this is an interesting and intriguing result. If true, it changes our picture of the early stages of galaxy evolution and where the stars that have ended up in today’s massive galaxies, such as the Milky Way, may have formed. We aim to follow up this research in future, exploring both improvements in modelling and the observable predictions which arise from them.”

The paper is Stanway & Eldridge, “Reevaluating Old Stellar Populations,” in press at Monthly Notices of the Royal Astronomical Society (preprint).



Things always get interesting when the American Astronomical Society meets, which it is now doing in Denver, in sessions that will run until June 7. There should be no shortage of topics emerging from the meeting, but the first that caught my eye was a different approach to the putative world some are calling Planet Nine. Teasing out the existence of a planet at the outer edges of the Solar System has involved looking at gravitational interactions among objects that we do know about, and extrapolating the presence of a far more massive body.

But the methodology may be flawed, if new work from Ann-Marie Madigan and colleagues at the University of Colorado Boulder is correct. At a press briefing at the AAS meeting, the team presented its view that objects like Sedna, an outlier that takes more than 11,000 years to complete an orbit around the Sun, should be considered in relation to other so-called ‘detached bodies.’ Almost 13 billion kilometers out, Sedna is one of a collection of such objects that appear in some ways to be in another category from the more conventional inner worlds.

Image: An artist’s rendering of Sedna, which looks reddish in color in telescope images. Credit: NASA/JPL-Caltech.

Sedna and its ilk come nowhere near the larger planets of our system, and their orbits may tell a tale. As this CU-Boulder news release explains, it was an undergraduate student named Jacob Fleisig who began to model a significant pattern known as ‘inclination instability’ that Madigan had previously described in the literature. Fleisig’s computer modeling illustrates how inclination instability can ease Sedna’s orbit from oval to circular over time.

In the model, accumulating gravitational forces drive growth in the orbital inclinations of objects in eccentric orbits. From the new work:

…secular (orbit-averaged) gravitational torques between orbits in the disk drive exponential growth of their inclinations. As the orbits’ inclinations grow, they tilt in the same way with respect to the disk plane. This leads to clustering in their angles of pericenter and the initially thin disk expands into a cone shape. Concurrently, the orbital eccentricities decrease and perihelion distances increase.

If such a mechanism is at work in our own system, we would expect it to occur between minor planets originally scattered to large orbital eccentricities via interactions with the giant planets. Such objects then become gravitationally detached from those planets. The authors believe this mechanism can explain the orbits of high perihelia objects like Sedna. Current studies posit anywhere from 1 to 10 Earth masses of cometary material existing at hundreds of AU from the Sun. The authors see such objects being originally scattered by the giant planets and their orbits decoupled by perturbations from cluster gas and nearby stars.

Perhaps this is all we need to explain the orbits of detached objects. There would be no need for a ‘Planet Nine’ at the edge of the Solar System. Instead, the detached objects achieve their present orbits through a series of small-scale interactions.

“There are so many of these bodies out there. What does their collective gravity do?” asks Madigan. “We can solve a lot of these problems by just taking into account that question.”

A great deal of work is ahead as the authors apply findings from their current computer simulations — “focused on the linear phase of the inclination instability in an idealized set-up” — to the outer Solar System. The paper notes their intent to model the gravitational influences of the giant planets and to focus on individual minor planets — especially those whose orbits become retrograde — instead of averaging their results for many hypothetical objects.

The paper is Madigan et al., “On the Dynamics of the Inclination Instability,” submitted to The Astrophysical Journal (preprint).



Breakthrough Starshot Sail RFP

Breakthrough Starshot held an ‘industry day’ on Wednesday May 23rd devoted to its lightsail project to take nanocraft to another star, framing the release of a Request for Proposals during its early concepts and analysis phase. The RFP focuses on the sail itself, investigating sail materials and stability under thrust. Step A proposals are due June 22, step B proposals on July 10, with finalists to be notified and contracts awarded this summer. The intent of the RFP is laid out in documents and slides from the meeting that Breakthrough has now placed online.

From the RFP itself:

The scope of this RFP addresses the Technology Development phase – to explore LightSail concepts, materials, fabrication and measurement methods, with accompanying analysis and simulation that creates advances toward a viable path to a scalable and ultimately deployable LightSail.

We’ve been talking about Breakthrough Starshot in these pages for a long time, as a search through the archives will reveal. The intention is to send gram-scale probes commonly referred to as ‘starchips’ attached to sails on the scale of meters to a nearby star to investigate its planets, with the highly interesting Proxima Centauri b an obvious target given that it is also the closest star to the Sun. Propelling the sail will be a gigawatt-scale ground-based laser. 20 years of flight time sets up the flyby, with data transmission returning images of the star system.

$100 million in research and development is to be spent over the next 5 years to determine the feasibility of both laser and sail. Overall, the five-year technology development period will be managed by three groups: Starshot’s sail committee, a photon engine committee and a systems engineering committee. After that, the goal from years 6 to 11 is to build a low-power prototype for space testing. After that comes a full scale laser system (called the ‘photon engine’) over the next 20 years, with launch of an interstellar mission occurring in approximately 30 years.

It’s an ambitious schedule, to be sure, but the early conceptual steps are now being taken, with initial sail work investigating candidate materials and sail stability. Keeping a sail stable under the high accelerations induced by the laser array is obviously critical and has already involved discussions and papers on different sail configurations, while the equally critical issue of sail materials is likewise considered in the RFP, which is Phase 1 of the sail’s development.

Phase 2 of the technology development program for the lightsail will validate lightsail materials and stable designs by proof of concept laboratory demonstrations, while Phase 3 takes us into laboratory testing of scalable prototypes. All of this points toward the eventual goal of a prototype mission that would launch nanocraft to a target here in the Solar System.

With this RFP focusing on Phase 1, Breakthrough Starshot seeks proposals for quantitative models that can produce testable predictions of sail performance, mathematical models defining the necessary conditions for sail stability, experimental methods for lightsail material fabrication and precision measurements to ‘validate optical, thermal and mechanical stability of materials.’ Here is the RFP statement of key issues involved in identifying candidate materials and designs:

  • Design of a reflector consistent with the mission requirement of achieving 0.2c for ~1g payload and LightSail area of >1 m2
  • Design of passive, adaptive or active features that enable or enhance stability, damage resistance, thermal robustness, and durability under deformation
  • Assessment of candidate materials (including thin-films, micro/nanopatterned structures, 2D materials) for thermal/mechanical stability
  • Development of measurement techniques and protocols for LightSail material properties (absorption, reflectivity, temperature, stress state, etc.)
  • Identification of materials for which scale-up and manufacture at the >1 m2 scale is feasible
  • Materials that facilitate integration of the LightSail with the Starchip
  • Development of the next generation Starchip scale spacecraft with a path towards incorporation into the Nanocraft

The second objective of the RFP is to identify and assess optimally-shaped designs for a stable sail that can withstand the temperatures and accelerations involved in pushing nanoncraft to 20 percent of the speed of light in a matter of minutes. The RFP notes the key issues here as:

  • Validation via, e.g., multi-physics simulation (optical, mechanical, thermal, etc.) of LightSail durability and dynamic stability and sensitivity to Photon Engine laser propulsion beam geometry and ground demonstrations
  • Evaluation of spacecraft stability in the context of a LightSail integrated with a Starchip payload
  • Development of optimization-based tools for evaluating LightSail designs matched to corresponding laser beam profiles.
  • Defining a roadmap for test and verification, including:
    o Measurement techniques for thin membranes at a small (<1 cm2) scale
    o Developing diagnostics and instrumentation needed for LightSail stability

You can download the RFP from the Breakthrough Starshot site for bidding information; note that multiple awards are anticipated. A layout of the proposal process and requirements is provided there, with submission information. The procurement is a two-step process with initial (short) white paper proposal evaluated by the Starshot lightsail committee and experts in beamed propulsion, and a second round in which finalists are invited to make final proposals. Contract negotiations will then be performed by the Starshot lightsail committee.



Dawn at Ceres: Imagery from a Changing Orbit

I’m looking forward to the buildup as New Horizons gets ever closer to Kuiper Belt Object MU69 and whatever surprises will attend the flyby. But the ongoing operations of the Dawn spacecraft orbiting Ceres equally command the attention. The image below is one of the first images Dawn has returned in more than a year, a stark view of surface features taken on May 16 of this year. The altitude here is 440 kilometers — for scale, the large crater near the horizon is about 35 kilometers wide. The foreground crater is about 120 kilometers from that crater, within a jumbled landscape suggestive of ancient terrain underlying the more recent impact.

Image: On the way to its lowest-ever and final orbit, NASA’s Dawn spacecraft is observing Ceres and returning new compositional data (infrared spectra) and images of the dwarf planet’s surface, such as this dramatic image of Ceres’ limb. Dawn has returned many limb images of Ceres in the course of its mission. These images offer complementary perspective to the images generally obtained by imaging the surface directly beneath the spacecraft. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA.

As with Pluto/Charon, so with Ceres — we’ve named many surface features, and can thus say that the image, a view of terrain at 23 degrees north latitude, 350 degrees east longitude, is in the neighborhood of mountainous terrain that includes Kwanzaa Tholus. A tholus is a type of small mountain — we now have six named tholi and montes, somewhat bigger mountains, in this region of the dwarf world.

Backing out the view in the image below, you can see Kwanzaa Tholus marked at image center, though its gradual rise makes it difficult to pick out. Here we’re looking at a 2017 mosaic taken by Dawn in its high-altitude mapping orbit at about 1470 kilometers above the surface.

Image: Scientists say Kwanzaa Tholus may have once been as prominent as Ahuna Mons, the tallest and most noticeable mountain on Ceres. Ahuna Mons is likely a cryovolcano, a volcano formed by the gradual accumulation of thick, slowly flowing icy materials. Because ice is not strong enough to preserve an elevated structure for extended periods, cryovolcanoes on Ceres are expected to gradually collapse over tens of millions of years. This means Kwanzaa Tholus and other tholi in that area could be degraded mountains, which also formed from cryovolcanic activity. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA.

But back to the present, for Dawn is drawing closer to Ceres as it moves into its final orbit, which it will reach in early June. This orbit will take the spacecraft as close as 50 kilometers above the surface, so we can expect the spectacular imagery to continue. It’s worth noting how complex this orbital adjustment is, involving examination of more than 45,000 possible trajectories before engineers chose the final plan. Along with the images, Dawn will be collecting gamma ray and neutron spectra (using the GRaND gamma ray and neutron detector instrument) by way of gaining insights into the chemical makeup of Ceres’ upper crust.

“The team is eagerly awaiting the detailed composition and high-resolution imaging from the new, up-close examination,” said Dawn’s Principal Investigator Carol Raymond of NASA’s Jet Propulsion Laboratory, Pasadena, California. “These new high-resolution data allow us to test theories formulated from the previous data sets and discover new features of this fascinating dwarf planet.”

We’re now going into what is known as XM07, or extended mission orbit 7, a designation that, as mission director and chief engineer Marc Rayman wryly observes, “illustrates the team’s flair for the dramatic.” Since XM05 the spacecraft’s orbit has been elliptical, for Dawn was not designed to operate at low altitude, and we’re dealing with a spacecraft whose reaction wheels have failed, making controlling its orientation an issue. Dawn must fire its small hydrazine-fueled thrusters to control its orientation in space. In his JPL blog, Rayman describes the orbit:

Although the elliptical orbits introduce many new technical challenges for the team, Dawn still takes a spiral route from each orbit to the next, just as it did earlier at Ceres and at Vesta when the orbits were circular. In essence, the ion engine smoothly shrinks the starting ellipse until the new ellipse is the size needed. These trajectories are very complicated to plan and to execute, but with the expert piloting of the experienced team, the maneuvering is going very well.

Image: Mission director and chief engineer Marc Rayman, whose mission updates are indispensable for anyone following Dawn. Both Dawn and New Horizons have done a superb job in keeping mission activities visible to the public. Credit: JPL.

The XM07 ellipse will take Dawn from less than 50 kilometers back out to 4,000 kilometers, with each revolution lasting 27 hours and 13 minutes. Rayman notes that XM07 will place extreme demands on Dawn’s photography because of the high speed of the spacecraft this near to the surface and the pointing problems involved in capturing a specific target. For more on this and a full description of XM07 and its consequences, I’ll send you to Rayman’s blog, which in any case should be on your radar as an informative and witty look deep inside the mission.

Image: This view of Juling Crater was constructed from pictures Dawn took from XM06 at an altitude of 385 kilometers. The science team has presented other views of this 20-kilometer crater, including last month, when Rayman and company described the discovery that the amount of ice on the shadowed northern wall changed over six months in 2016. Ceres is not a static world. When Dawn dives down lower in June, it will obtain sharper images than this (at other locations). Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA.



Galactic Habitability and Sgr A*

Yesterday I looked at evidence for oxygen in a galaxy so distant that we are seeing it as it was a mere 500 million years after the Big Bang. It’s an intriguing find, because that means there was an even earlier generation of stars that lived and died, seeding the cosmos with elements heavier than hydrogen and helium. It’s hard to imagine the vast tracts of time since populated with stars and, inevitably, planets without speculating on where and when life developed.

But as we continue to speculate, we should also look at the factors that could shape emerging life in galaxies like our own. Tying in neatly with yesterday’s post comes a paper from Amedeo Balbi (Università degli Studi di Roma “Tor Vergata”), working with colleague Francesco Tombesi. The authors are interested in questions of habitability not in terms of habitable zones in stellar systems but rather habitable zones in entire galaxies. For we know that at the center of our Milky Way lurks the supermassive black hole Sgr A*, whose effects must be considered.

Such black holes are known to produce vast amounts of ionizing radiation in the highly visible form of quasars or active galactic nuclei (AGN). The atmospheric loss and biological damage inflicted on a rocky planet as it is exposed to intense X-ray and extreme ultraviolet radiation can be extreme, and such conditions would have marked our own galaxy’s AGN phase.

The concern here is to examine how ionizing radiation can impact habitability by exposing planetary surfaces to high-energy fluxes, while also degrading planetary atmospheres. We’ve looked at these issues now and again on this site, with particular regard to red dwarf stars, the most common class of star in the galaxy but also a type prone to flare activity particularly when young. We now consider whether there are regions in our galaxy that would be less likely to be habitable because of the effects of Sgr A*, which was not always as quiet as it is now.

Image: Centaurus A is one of the active galactic nuclei closest to Earth. It emits strong radio emission and produces a relativistic jet. Credit: ESO/WFI (Optical); MPIfR/ESO/APEX/A.Weiss et al. (Submillimetre); NASA/CXC/CfA/R.Kraft et al. (X-ray).

AGNs represent a class of galaxies that appear in a wide range of shapes and spectral features. Some of that spectral variation may, according to one theory, involve our viewing angle and the obscuring effects of dust. The peak of Sgr A*’s active phase is thought to have occurred less than 8 billion years ago and to have lasted between 107 and 109 years. The paper examines how this activity would affect the habitability of the Milky Way.

Image: An artist impression of the quasar ULAS J1120+0641. Credit: ESO/M. Kornmesser.

The researchers compared the XUV flux at various distances from galactic center to the dosage that would prove lethal for organisms on Earth, producing ‘critical fluxes’ for complex life as well as for prokaryotes (some radiation-resistant terrestrial prokaryotes can survive high radiation doses). How far from Sgr A* would a planet have to be to be exposed to a critical flux?

The issue is complicated by the possible response of local organisms, which might evolve to cope with increased radiation under varying environmental conditions, and it is also true that radiation doses lower than lethality could spur biological mutations. But given what they assume to be plausible values for a lethal absorbed dose of ionizing radiation, the authors believe that complex life would have been in jeopardy during Sgr A*’s peak active phase at distances as large as 10 kpc [32,600 light years] from galactic center. By comparison, the Sun lies about 8 kpc from the center of the Milky Way, having formed well after the Sgr A* peak.

From the paper:

This may not have prevented the appearance of life per se, since prokaryotes could have survived to higher fluxes. However, we point out that the biological effect of the ionizing radiation from Sgr A* would have been in addition to that of any other source of ionizing radiation, for example from the host star, and would add to the loss of a large fraction of the atmosphere. Even if some organisms might have survived by developing radiation-resistance or finding protected niches, the global effect on the biosphere would have been significant.

Let me circle back around to the question of atmosphere loss, also critical in assessing life’s chances in the era of Sgr A* peak activity. Assuming that the torus of the AGN would have been co-aligned with the galactic plane, the authors produce the chart below, showing the total amount of atmospheric mass lost at the end of the AGN phase of Sgr A* by a planet with the same density as Earth, as a function of the distance from the galactic center.

Image: This is Figure 1 from the paper. Caption: The total mass lost at the end of the AGN phase of Sgr A* by a terrestrial planet at distance D from the galactic center, in units of the atmosphere mass of present day Earth. Each curve was computed assuming a value for the efficiency of hydrodynamic escape of either ε = 0.1 or ε = 0.6. An optical depth τ = 1 corresponds to locations close to the galactic plane (maximum attenuation by the AGN torus) while τ = 0 corresponds to high galactic latitudes (no attenuation). Credit: Amedeo and Tombesi.

The conclusion is stark: Rocky planets in the galactic bulge would have been exposed to enough XUV radiation during peak Sgr A* activity to lose a significant fraction of their atmosphere. The mass loss for distances from the galactic plane of 0.5 kpc or less could be comparable to the atmosphere of Earth today. It would take significant volcanism and outgassing to regenerate an atmosphere sufficiently to repair such a loss.

…our results imply that the inner region of the Milky Way might have remained uninhabitable until the end of the AGN phase of its central black hole, and possibly thereafter. This has important consequences in assessing the likelihood of ancient life in the Galaxy, and should be taken into account in future studies of the Galactic habitable zone. It also suggests further investigations on the relation between supermassive black holes in galactic cores and planetary habitability.

The paper is Balbi and Tombesi, “The habitability of the Milky Way during the active phase of its central supermassive black hole,” Scientific Reports 7, article #: 16626 (2017). Full text.



Star Formation at ‘Cosmic Dawn’

When life first arose in the universe is a question to which we have no answer. A key problem here is that without knowing how rare — or common — life’s emergence is, we can’t draw conclusions about where (or when) to find it. One thing that is accessible to us, though, is information about when stars began the process of producing the elements beyond hydrogen and helium that are constituents of our own living systems. And on that score, we have interesting news from an international team of scientists about extremely old galaxies.

Led by Takuya Hashimoto and Akio Inoue (Osaka Sangyo University), the researchers have gone to work on a galaxy known as MACS1149-JD1, using data acquired from the Atacama Large Millimetre/Submillimetre Array (ALMA) and the European Southern Observatory’s Very Large Telescope (VLT). The team’s paper in Nature confirms that the galaxy is some 13.28 billion light years away. Thus we see it as it appeared when the universe was about 500 million years old. Another galaxy, GN-z11, has been measured at 13.4 billion light years using data from the Hubble Space Telescope.

But the Hubble data offer a less precise measurement than Hashimoto and Inoue found, because the distance to MACS1149-JD1 is based on two independent emission lines from atoms of hydrogen and oxygen, and therein lies a tale. This is the most distant known source of oxygen ever detected. It implies that this galaxy contains stars that formed using elements produced in a still earlier generation of stars whose death produced heavier elements. Here’s what second author Nicolas Laporte (University College London) has to say on this:

“This is an exciting discovery as this galaxy is seen at a time when the Universe was only 500 million years old and yet it already has a population of mature stars. We are therefore able to use this galaxy to probe into an earlier, completely uncharted, period of cosmic history.”

Image: Galaxy MACS1149-JD1 located 13.28 billion light-years away imaged with the NASA/ESA Hubble Space Telescope. The first zoom shows how the galaxy was observed with the ESO VLT centred on a rectangular slit. The final zoom shows the Hubble image with contours of ionized oxygen detected by ALMA. Credit: ALMA (ESO/NAOJ/NRAO), NASA/ESA Hubble Space Telescope, Hashimoto et al.

But let’s pause for a moment to remind ourselves of how tricky it is to calculate distances for objects at the edge of the visible universe. Here I’m quoting from the National Astronomical Observatory of Japan’s website on the matter:

…the distances to astronomical object beyond about a couple billion light-years are estimated using redshift — a change in the wavelength of light from the object. The relation between the redshift and the distance depends on the history of the Universe: how fast (or slow) the Universe has been expanding. This is determined by models (based on physics) and certain characteristic parameters. These parameters are still being determined, so their estimated values change from year to year. Also, there are different ways to define distances (co-moving distances, luminosity distances, …), which give different values. Because of the complexity, sometimes different astronomers find different distances for the same object.

With that in mind, we can proceed to Laporte, who detected hydrogen emissions from MACS1149-JD1 using the Very Large Telescope, producing an independent confirmation of the distance Hashimoto and Inoue had deduced from their ALMA data (both using, in NAOJ’s words, ‘cosmological parameters given in the most recent published results by the major collaborations’). In the ALMA work, the signal of ionized oxygen had been stretched from the infrared down to microwave wavelengths by the continuing expansion of spacetime.

Both Laporte and the Hashimoto/Inoue team have run up a history of detecting the most distant known sources, with Laporte responsible for an earlier detection of oxygen at 13.2 billion light years. What we wind up with the new paper is evidence for star formation at an even earlier period, some 250 million years after the Big Bang, meaning that galaxies existed at times earlier than we can currently detect them. Co-author Richard Ellis (University College London) notes the implications for life:

“Determining when cosmic dawn occurred is akin to the `Holy Grail’ of cosmology and galaxy formation. With MACS1149-JD1, we have managed to probe history beyond the limits of when we can actually detect galaxies with current facilities. There is renewed optimism we are getting closer and closer to witnessing directly the birth of starlight. Since we are all made of processed stellar material, this is really finding our own origins.”

Are these likewise the origins of other living things in the cosmos? My assumption is that the answer is yes, but without confirmatory data, that thought can be no more than a speculation.

The paper is Hashimoto et al., “The onset of star formation 250 million years after the Big Bang,” Nature 557 (2018), 392-395 (abstract).



Pluto: A Cometary Formation Model

The ongoing work of mining New Horizons’ abundant data from the outer system continues at a brisk pace. But missions occur in context, and we also have discoveries made at comet 67P/Churyumov-Gerasimenko by the European Space Agency’s Rosetta probe to bring to bear. The question that occupies Christopher Glein and Hunter Waite (both at SwRI) is how to explain the chemistry New Horizons found at Pluto and what it can tell us about Pluto’s formation.

At the heart of their new paper in Icarus is the question of Pluto’s molecular nitrogen (N2), which plays a role on that world similar to methane on Titan, water on Earth and CO2 on Mars. All are volatiles, meaning they can move between gaseous and condensed forms at the temperature of the planet in question. We’ve learned that solid N2 is the most abundant surface ice visible to spectroscopy on Pluto, as witness the spectacular example of Sputnik Planitia.

Image: NASA’s New Horizons spacecraft captured this image of Sputnik Planitia — a glacial expanse rich in nitrogen, carbon monoxide and methane ices — that forms the left lobe of a heart-shaped feature on Pluto’s surface. SwRI scientists studied the dwarf planet’s nitrogen and carbon monoxide composition to develop a new theory for its formation. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute.

Anyone glued to their screen during New Horizons’ 2015 flyby will recall the surprise generated by Sputnik Planitia’s youthful terrain, now believed to be the result of the flow of solid N2. Scientists also found that nitrogen can sublimate at Pluto’s surface, so we have a volatile cycle that causes surface pitting and frosts, one that likewise accounts for the existence of Pluto’s atmosphere. Given the importance of nitrogen, the issue of its origin looms large because of what it can tell us about Pluto’s formation, and by extension that of other outer system objects.

Here the crossover with the Rosetta data is helpful. What Glein and Waite are suggesting is that Pluto may owe its existence to comets, accumulating the N2 observed by New Horizons through accretion. Says Glein, with a nod to the nitrogen-rich ice of Sputnik Planitia:

“We’ve developed what we call ‘the giant comet’ cosmochemical model of Pluto formation. We found an intriguing consistency between the estimated amount of nitrogen inside the glacier and the amount that would be expected if Pluto was formed by the agglomeration of roughly a billion comets or other Kuiper Belt objects similar in chemical composition to 67P, the comet explored by Rosetta.”

Image: New Horizons not only showed humanity what Pluto looks like, but also provided information on the composition of Pluto’s atmosphere and surface. These maps — assembled using data from the Ralph instrument — indicate regions rich in methane (CH4), nitrogen (N2), carbon monoxide (CO) and water (H2O) ices. Sputnik Planitia shows an especially strong signature of nitrogen near the equator. SwRI scientists combined these data with Rosetta’s comet 67P data to develop a proposed “giant comet” model for Pluto formation. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute.

Whether Pluto formed from the accretion of cometary ices or from materials with a chemical composition closer to that of the solar nebula remains an open question, with the paper addressing both possibilities and finding both to be consistent with the data. Tangled up with the question is the issue of carbon monoxide (CO) and the low CO/N2 ratios observed at Pluto. The problem is that these ratios should be much higher than what we find in Pluto’s atmosphere. How to explain the missing carbon monoxide?

There are implications here for what may still lie beneath the icy world’s surface. From the paper:

…we have performed aqueous geochemical calculations showing the great thermodynamic instability of CO dissolved in cold (liquid) water, even for a restricted metastable equilibrium system… The destruction of CO to formate or carbonate species… would be strongly favored if Pluto has or had a subsurface ocean. This mechanism can be applied to the cometary model, but not to the solar model as the CO/H2O ratio is too large in the latter. Hence, the cometary model seems preferable, with more options to reconcile the CO/N2 ratio.

And implications for Pluto’s continuing surface activity are also present:

We note that the burial and aqueous destruction hypotheses for missing CO are not necessarily mutually exclusive. A major implication of these processes is that the observed composition of Pluto cannot be completely primitive, even if its N2 is indeed primordial. This resonates with the dynamic geology seen by New Horizons (Moore et al., 2016).

Numerous questions remain to be answered, but the authors propose an evolutionary scenario that begins with Pluto accumulating cometary nitrogen and CO, with subsequent interactions accounting for loss of the CO and outgassing of N2. This takes into account the surface accumulation of the latter at Sputnik Planitia and cometary ‘resupply’ of a small amount of CO that mixes with surface nitrogen. This is one of various possible scenarios, but it is consistent with the data the paper examines and points to future work.

Among many questions remaining, the issue of how much N2 might be inside Pluto looms large, as does its distribution in the crust, rocky core and possible liquid water ocean. We’d also like to know whether the abundance of N2 found at comet 67P is representative of the cometary population at large and of larger icy Kuiper Belt Objects.

And among ten questions posed by the authors in their conclusion, this one is significant: Why do we lack any detection of CO2 on Pluto while finding it on Triton, and is there a consistent way of resolving the question with regard to the lack of CO on both bodies? This gets us into the broad issue of the mechanisms that provide volatiles to worlds like these. Check the paper’s conclusion for a useful wrap-up of future directions to be taken with the New Horizons data, and the areas where new data will be required.

The paper is Glein and Waite, “Primordial N2 provides a cosmochemical explanation for the existence of Sputnik Planitia, Pluto,” Icarus Vol. 313 (October, 2018), pp. 79-92 (abstract / preprint).



Administrative Item

Some of you may have noticed a blip in the comments moderation over the past 24 hours. I think all messages have now come through, but a software upgrade on my server is the culprit. Things seem to have gone back to normal now.

On an unrelated matter, I won’t be able to get off a post today or tomorrow. On Tuesday, I’ll have some interesting information about Breakthrough Starshot. [I had originally promised this for Monday, having forgotten about the US holiday].



TESS: The View into the Galactic Plane

I want to be sure to get the first image from TESS, the Transiting Exoplanet Survey Satellite, into Centauri Dreams, given the importance of the mission and the high hopes riding on it as the next step in exoplanet exploration. Now we move from the Kepler statistical survey methodology to a look at bright, nearby stars, and plenty of them. TESS will cover an area of sky far larger than the amount of sky we see in this image, which looks out along the plane of the galaxy from a perspective that matches southern skies on Earth.

Image: This test image from one of the four cameras aboard the Transiting Exoplanet Survey Satellite (TESS) captures a swath of the southern sky along the plane of our galaxy. TESS is expected to cover more than 400 times the amount of sky shown in this image when using all four of its cameras during science operations. Credits: NASA/MIT/TESS.

Showing some 200,000 stars, the image is centered on the southern constellation Centaurus, with a bit of the Coalsack Nebula at the upper right and, if you look along the bottom edge, the bright star Beta Centauri readily visible. Here I add the usual caution that Beta Centauri has nothing to do with Alpha Centauri — it is a separate place and interesting in its own right. About 400 light years out, this is, like Alpha Centauri, a triple system, and with Alpha Centauri, it serves as one of the pointer stars to the lovely asterism known as the Southern Cross.

While a triple system, Beta Centauri’s component stars are nothing like the G-class Centauri A, K-class Centauri B and M-dwarf Proxima Centauri. At Beta Centauri we have a close binary consisting of two B-class stars orbiting each other over a period of 357 days, both stars thought to be variables now evolving off the main sequence. Orbiting the binary is Beta Centauri B, another B-class star with an orbital period of between 125 and 220 years [see comments to this post for the revision I’ve made to the orbital period of this star].

This first TESS image is a test of the spacecraft’s four cameras, with a science-quality image expected to be released in June. Meanwhile, the spacecraft is easing into its highly elliptical orbit, with a final thruster burn scheduled for May 30. Science operations should commence in mid-June once orbital adjustments and camera calibrations are completed.

Image: An illustration of TESS as it passed the Moon during its lunar flyby. This provided a gravitational boost that placed TESS on course for its final working orbit. Credit: NASA’s Goddard Space Flight Center.

This first story on an operational TESS gives me echoes of New Horizons, which we followed here from launch to Pluto and now on to MU69 and whatever KBO may be next. Some people caution against anthropomorphizing machines, a human tendency when we deal with our pets, applying human perspectives to creatures considerably different than ourselves. To me the distinction is meaningless. We apply our passions and values to the things that matter to us, and New Horizons and TESS fit that bill for me. I recognize that spacecraft are not ‘creatures’ but my own values of commitment and purpose ride with these machines that can feel neither.



Is Asteroid 2015 BZ509 from another Stellar System?

It’s conceivable that getting humans to an interstellar object may not involve journeying all the way to another star. We’ve learned that wandering asteroids and comets move between stars, as the case of ‘Oumuamua demonstrated, and early research offers the possibility that such objects exist in large numbers. Now we have (514107) 2015 BZ509, which is conceivably an interloper into our system of another sort. Two researchers believe that this asteroid near the orbit of Jupiter is not just passing through, but a captured object from another stellar system.

A comparison with Triton seems apt. One of the most compelling pieces of evidence that Neptune’s largest moon is actually a Kuiper Belt Object is its retrograde orbit. We see the same thing with 2015 BZ509, and for Fathi Namouni (Observatoire de la Côte d’Azur) and Helena Morais (Universidade Estadual Paulista), that sends a clear message. The researchers have offered their work in a new paper in Monthly Notices of the Royal Astronomical Society.

“How the asteroid came to move in this way while sharing Jupiter’s orbit has until now been a mystery,” explains lead author Namouni. “If 2015 BZ509 were a native of our system, it should have had the same original direction as all of the other planets and asteroids, inherited from the cloud of gas and dust that formed them.”

Image: Images of 2015 BZ509 obtained at the Large Binocular Telescope Observatory (LBTO) that established its retrograde co-orbital nature. The bright stars and the asteroid (circled in yellow) appear black and the sky white in this negative image. Credit: C. Veillet / Large Binocular Telescope Observatory.

Namouni and Morais used computer simulations to track the errant asteroid back in time, arguing that 2015 BZ509 has moved this way since the birth of the Solar System some 4.5 billion years ago, an indication that it could not have formed there originally. The case also relies on the fact that the Sun formed in a tightly packed star cluster where movement of objects ejected by gravitational forces within their own system into orbits around other stars would not have been uncommon. Thus this rogue asteroid may well contain information about planet formation and evolution as well as telling us more about the Sun’s original siblings.

So what exactly do we know about this object? 2015 BZ509 is in a resonant, co-orbital motion with Jupiter and represents the first discovery of a retrograde co-orbital asteroid with Jupiter or any other planet. Its orbital eccentricity of 0.3805 takes it inside and then outside of Jupiter’s orbit at its closest approaches (176 million kilometers). The orbital period is 11.65 years and the inclination is 163 degrees, an evidently stable orbit if a complicated one.

Image: Stellar nursery NGC 604 (NASA/HST), where star systems are closely packed and asteroid exchange is thought to be possible. Asteroid (514107) 2015 BZ 509 may have emigrated from its parent star and settled around the Sun in a similar environment. Credit: NASA / Hubble Heritage Team (AURA/STScI).

I’ve long speculated in these pages about interstellar missions that operate not by making fast jumps to other stellar systems but by moving incrementally over thousands of years, taking advantage of the fact that the Oort Cloud extends tens of thousands of AU, if not more, from the Sun. If similar debris were found around nearby stars, a patient species evolving new technologies and biologies as it progressed could simply ‘walk across’ to the next system.

Now we’re learning that we may be able to study interstellar materials without going nearly as far. We’re nowhere near ready to send humans into Jupiter space, but if this work is confirmed, then targets like (514107) 2015 BZ 509 will inevitably receive future visitors. We have so much to learn about how common objects like these are, but the researchers believe the number could be high.

In the passage from the paper below, the term ‘polar corridor’ refers to objects pushed into polar inclinations all the way out to the Oort Cloud. The ‘clones’ refer to the computer simulations Namuni and Marais conducted, simulating the evolution of one million clones of 2015 BZ 509:

The one million clone simulation provides further evidence that there are currently more extrasolar asteroids in the Solar system. In effect, if more objects were captured along [with] 2015 BZ509 by Jupiter early in the Solar system’s history, the less stable orbits must have left the co-orbital region by way of chaotic diffusion into the polar corridor. This occurs because the N-body problem is time-reversible and unstable clones of 2015 BZ509 that are followed into the future exit the co-orbital region and end up in the polar corridor. The prominent presence of the polar corridor in the simulation over the age of the Solar System mainly in the trans-Neptunian region implies that it is currently populated by extrasolar asteroids.

Make no mistake, most of the digital clones the researchers put into motion fell victim to gravitational forces that would have prevented the kind of orbital resonance we see with this object. But a few did remain in stable configurations all the way back to the earliest days of the Solar System. How strong a case this makes for an interstellar origin is still in doubt. Scott Tramaine (Institute for Advanced Study), for example, questions whether gravitational nudges coupled with a collision could account for 2015 BZ509, with no interstellar involvement at all.

For more on Tremaine’s doubts and a look at the possible involvement of the still undiscovered Planet Nine, see Lee Billings’ essay in Scientific American. Billings also speculates about robotic mission possibilities. It’s clear that 2015 BZ509 is going to remain newsworthy — and controversial — as we try to build our census of interstellar objects near the Sun.

The paper is Namouni & H. Morais, “An interstellar origin for Jupiter’s retrograde co-orbital asteroid,” Monthly Notices of the Royal Astronomical Society: Letters (2018). Abstract.