by Paul Gilster | Feb 22, 2012 | Exoplanetary Science |
I love the way Zachory Berta (Harvard-Smithsonian Center for Astrophysics) describes his studies of the transiting super-Earth GJ1214b. Referring to his team’s analysis of the planet’s atmosphere, Berta says “We’re using Hubble to measure the infrared color of sunset on this world.” And indeed they have done just this, discovering a spectrum that is featureless over a wide range of wavelengths, allowing them to deduce that the planet’s atmosphere is thick and steamy. The conclusion most consistent with the data is a dense atmosphere of water vapor.
Discovered in 2009 by the MEarth project, GJ1214b has a radius 2.7 times Earth’s and a mass 6.5 times that of our planet. It’s proven to be a great catch, because its host star, an M-dwarf in the constellation Ophiuchus, offers up a large 1.4 percent transit depth — this refers to the fractional change in brightness as the planet transits its star. Transiting gas giants, for example, usually have transit depths somewhere around 1 percent, while the largest transit depth yet recorded belongs to HD 189733b, a ‘hot Jupiter’ in Vulpecula that shows a depth of approximately 3 percent.
Image: This artist’s impression shows how the super-Earth around the nearby star GJ1214 may look. Discovered by the MEarth project and investigated further by the HARPS spectrograph on ESO’s 3.6-metre telescope at La Silla, GJ1214b has now been the subject of close analysis using the Hubble Space Telescope, allowing researchers to learn not only about the nature of its atmosphere but also its internal composition. Credit: ESO/L. Calçada.
GJ1214b is also near enough (13 parsecs, or about 40 light years) that Berta’s follow-up studies on its atmosphere using the Hubble Space Telescope’s WFC3 instrument have proven fruitful. What the researchers are looking at is the tiny fraction of the star’s light that passes through its upper atmosphere before reaching us. The variations of the transit depth as a function of wavelength comprise the planet’s transmission spectrum, which the team is able to use to examine the mean molecular weight of the planet’s atmosphere. Earlier work on GJ1214b’s atmosphere had been unable to distinguish between water in the atmosphere and a worldwide haze, but the new work takes us a good way farther and into the planet’s internal structure.
The reason: A dense atmosphere rules out models that explain the radius of GJ1214b by the presence of a low-density gas layer. The researchers take this to its logical conclusion:
Such a constraint on GJ1214b’s upper atmosphere serves as a boundary condition for models of bulk composition and structure of the rest of the planet. It suggests GJ1214b contains a substantial fraction of water throughout the interior of the planet in order to obviate the need for a completely H/He- or H2-dominated envelope to explain the planet’s large radius. A high bulk volatile content would point to GJ1214b forming beyond the snow line and migrating inward, although any such statements about GJ1214b’s past are subject to large uncertainties in the atmospheric mass loss history…
If you examine the density of this planet (known because we have good data on its mass and size), you get about 2 grams per cubic centimeter, a figure that suggests GJ1214b has much more water than the Earth and a good deal less rock (Earth’s average density is 5.5 g/cm3). We can deduce a fascinating internal structure, one in which, as Berta says, “The high temperatures and high pressures would form exotic materials like ‘hot ice’ or ‘superfluid water’ – substances that are completely alien to our everyday experience.”
We wind up with a waterworld enveloped in a thick atmosphere, one orbiting its primary every 38 hours at a distance of some 2 million kilometers, with an estimated temperature of 230 degrees Celsius. I was delighted to see that Berta and colleagues have investigated the possibility of exomoons around this planet, looking for “shallow transit-shaped dimmings or brightenings offset from the planet’s transit light curve.” The team believes no moon would survive further than 8 planetary radii from GJ1214b, and indeed they find no evidence for Ganymede-size moons or greater, but the paper is quick to note that:
Due to the many possible configurations of transiting exomoons and the large gaps in our WFC3 light curve, our non-detection of moons does not by itself place strict limits on the presence of exo-moons around GJ1214b.
All of which is true, but what a pleasure to see the hunt for exomoons continuing to heat up.
The paper is Berta et al., “The Flat Transmission Spectrum of the Super-Earth GJ1214b from Wide Field Camera 3 on the Hubble Space Telescope,” accepted for publication in The Astrophysical Journal (preprint).
by Paul Gilster | Feb 21, 2012 | Deep Sky Astronomy & Telescopes |
HLX-1 (Hyper-Luminous X-ray source 1) is thought to be a black hole, one that’s a welcome discovery for astronomers trying to puzzle out the mysteries of black hole formation. Located roughly 290 million light years from Earth and situated toward the edge of a galaxy called ESO 243-49, this black hole looks to be some 20,000 times the mass of the Sun, which makes it mid-sized when compared with the supermassive black holes at the center of many galaxies. The latter can have masses up to billions of times more than the Sun — the black hole at the center of our own galaxy is thought to comprise about four million solar masses.
Just how a supermassive black hole forms remains a subject for speculation, but study of HLX-1 is giving us clues that point in the direction of a series of mergers of small and mid-sized black holes. For it turns out that HLX-1, discovered by Sean Farrell (Sydney Institute for Astronomy in Australia and University of Leicester, UK) and team at X-ray wavelengths, shows evidence for a cluster of young, hot stars surrounding the black hole itself. Working in ultraviolet, visible and infrared light using the Hubble instrument as well as in X-rays using the Swift satellite, the team found that the accretion disk alone could not explain the emissions they were studying.
All of this leads to an interesting supposition about the black hole’s origins, as Farrell notes:
“The fact that there’s a very young cluster of stars indicates that the intermediate-mass black hole may have originated as the central black hole in a very low-mass dwarf galaxy. The dwarf galaxy was then swallowed by the more massive galaxy.”
Image: This spectacular edge-on galaxy, called ESO 243-49, is home to an intermediate-mass black hole that may have been purloined from a cannibalised dwarf galaxy. The black hole, with an estimated mass of 20,000 Suns, lies above the galactic plane. This is an unlikely place for such a massive back hole to exist, unless it belonged to a small galaxy that was gravitationally torn apart by ESO 243-49. The circle identifies a unique X-ray source that pinpoints the black hole. Credit: NASA, ESA, and S. Farrell (University of Sydney, Australia and University of Leicester, UK).
The cluster of young stars appears to be about 250 light years across and encircles the black hole. We may be looking, then, at the remains of a galaxy that has been effectively destroyed by its collision with another galaxy, while the black hole at the dwarf’s center interacted with enough gaseous material to form the new stars. Farrell and Mathieu Servillat (Harvard-Smithsonian Center for Astrophysics) peg the cluster’s age at less than 200 million years, meaning that most of these stars would have formed after the dwarf’s collision with the larger galaxy. Says Servillat:
“This black hole is unique in that it’s the only intermediate-mass black hole we’ve found so far. Its rarity suggests that these black holes are only visible for a short time.”
The paper on this work discusses the process in greater detail:
Tidally stripping a dwarf galaxy during a merger event could remove a large fraction of the mass from the dwarf galaxy, with star formation triggered as a result of the tidal interactions. This could result in the observed IMBH [intermediate mass black hole] embedded in the remnant of the nuclear bulge and surrounded by a young, high metallicity, stellar population. It has been proposed that such accreted dwarf galaxies may explain the origin of some globular clusters, with the remnant cluster appearing more like a classical globular cluster as its stellar population ages.
HLX-1’s X-ray signature is still relatively bright (which is how Farrell found it in 2009 using the European Space Agency’s XMM-Newton X-ray space telescope). But as it depletes the supply of gas around it, the black hole’s X-ray signature will weaken. Its ultimate fate may indeed be to spiral into the center of ESO 243-49, to merge with the supermassive black hole there. Consider intermediate-sized black holes like this one the ‘missing link’ between stellar mass black holes and their supermassive counterparts at galactic center. The nuclei of dwarf galaxies (along with globular clusters) have previously been suggested as the likely environments for their formation, ideas which are given powerful support through the study of what’s happening in ESO 243-49.
The paper is Farrell et al., “A Young Massive Stellar Population Around the Intermediate Mass Black Hole ESO 243-49 HLX-1,” accepted for publication in The Astrophysical Journal Letters (preprint).
by Paul Gilster | Feb 20, 2012 | Outer Solar System |
How likely are we to find other planets in the universe that are as habitable as Earth? One key to the puzzle has long been thought to be the presence of Jupiter in our own Solar System. In fact, the presence of the giant planet has become a player in the so-called ‘rare Earth’ argument that sees Jupiter as just one factor that makes our Solar System unique. Put a gas giant in the proper position in any solar system and, so the argument goes, dangerous objects from the outer system will be deflected, protecting the inner planets and allowing life to flourish. The issue gets a hard look from Jonathan Horner (University of New South Wales) and Barrie Jones (The Open University, UK) in a paper delivered in Canberra in September of 2011.
Jupiter as protector has a certain appeal. Voyager, Galileo and other probes have shown us a massive planet that is otherwise cold and forbidding, but a world with enough mass to have huge effects on other objects in the Solar System. Horner and Jones perform a series of dynamical studies to see just how potent this effect is, noting that when the question was first studied, long-period comets were the objects most thought of in terms of Earth-crossing orbits. Jupiter’s effect on these seemed clear — a significant fraction of them would be ejected from the system entirely by its influence, keeping life-threatening impacts to a much smaller number.
But our picture of the Solar System has changed dramatically in the years since, and we now believe that long-period comets are only a small part of the total picture. The ‘impact flux,’ those objects hitting the Earth, also includes near-Earth asteroids and short-period comets. Near-Earth asteroids come from the inner Solar System as well as the main belt, and it may be that some are the remains of short-period comets. Given the numbers of NEAs we’re finding, some researchers suggest that they may constitute as much as 75 percent of potential impactors.
Image: NASA’s Cassini spacecraft took this true color mosaic of Jupiter while on its way to Saturn. The smallest visible features are approximately 60 kilometers (37 miles) across. Although Cassini’s camera can see more colors than humans can, Jupiter’s colors in this new view appear very close to the way the human eye would see them. Credit: NASA.
Short-period comets are likewise a danger, their orbital periods short enough that we can observe their return, with many of them having periods of around five or six years. With origins in the Centaurs (Jupiter family comets) as well as the Edgeworth/Kuiper Belt, the Jovian and Neptunian Trojans and perhaps the inner Oort Cloud, these objects are also thought to constitute a major part of the impact threat. The authors’ dynamical simulations tell an interesting tale about the role of all three impact scenarios. Long-period comets are indeed deflected by Jupiter in its present orbit, but the paper argues that they constitute only about 5 percent of the total threat. It turns out that the interactions of the other two populations present a more complicated picture:
In each case, the impact rate from such objects is markedly lower for planetary systems that include a massive Jupiter (such as our own) than for those that have a Saturn-mass (or slightly smaller) planet at the same location. However, for masses lower than ~0.15 times that of our Jupiter, the impact flux experienced by an Earth-like planet falls dramatically in both cases, such that the impact rate were no Jupiter present (or only a very low-mass planet occupied Jupiter’s orbit) would actually be lower than that for the scenarios involving our Jupiter. As such, it seems that Jupiter can easily be at least as much, if not more, of a foe than it is a friend.
So much for the protective Jupiter motif. What happens if we make Jupiter’s orbit more eccentric than it is now? The impact flux from these simulations turns out to be greater, though ‘not punishingly so,’ to use the authors’ words. Orbital eccentricity seems to be of secondary importance compared to mass in determining the impact flux in the host system. What does turn up — and this is with increased orbital inclination rather than eccentricity of the orbit — is a greater than 50 percent depletion of the asteroid belt on relatively short time-scales (107 years) for all but the least massive ‘Jupiter’ tested (0.01 MJ). Systems like this would, after that time, contain a much depleted asteroid belt, posing a correspondingly lesser threat to the inner system.
The finding is clearly stated: “The simple notion that giant planets are required to ensure a sufficiently benign impact regime for potentially habitable worlds to be truly habitable is clearly therefore not valid.” What level of impact flux is best suited for the development of life is a separate question. Here the authors have to punt, noting that planets like the Earth should form inside the so-called ‘snow line,’ where the only water present would be found trapped in hydrated silicates. These would be dry worlds that need an external source for their oceans.
If the bulk of Earth’s water was delivered from comets from the outer system, then the role of Jupiter may have been significant. But the paper notes that the hydration of the Earth probably occurred during the migration of the outer planets, when there would have been destabilization and redistribution of the Solar System’s population of small bodies as well. This paper picks up in the post-migration era and is not designed to study the hydration question.
As to life itself, the following passage is interesting:
Once Earth-like planets have been hydrated, the role of impacts will clearly shift from having import in the delivery of volatiles to otherwise dry worlds to directly affecting the course of the development of life. Since the development of life on our planet, a significant number of ‘mass extinctions’ have occurred, in which the great majority of organisms have been extinguished. Although many of these are currently believed to [have] been caused by factors other than impacts, at least a few are thought to have been at least partially the result of collisions between the Earth and small bodies. At first glance, it seems reasonable to assume that the most promising conditions for life to develop, once a host planet has received sufficient hydration, would be those featuring the lowest impact rate (i.e., those with the least massive giant planets, or no giant planets at all, or very massive giant planets). However, it could equally be argued that at least some impact flux is necessary in order to trigger occasional mass extinctions — without the mass extinction that wiped out the dinosaurs, for example, it is debatable whether we would currently be here, debating the importance of such extinctions!
We’re left with a complex picture regarding the role of giant planets, one with clear implications for exoplanet studies. Each system needs to be analyzed carefully in terms of the complicated impact scenario, and it is certainly not enough to base everything on the existence of a gas giant in a particular orbit. Given the number of interesting targets we’re likely to find with Kepler and CoRoT and later searches of more nearby stars, prioritizing systems for astrobiological investigation is going to be important. The configuration of gas giants will be one factor in making these decisions, but Horner and Jones remind us that the variables of the impact flux are wide enough to allow life-bearing planets to exist in a wide variety of solar systems.
The paper is Horner and Jones, “Quantifying Jupiter’s influence on the Earth’s impact flux: Implications for planetary habitability” (preprint). Thanks to Dave Moore and Antonio Tavani for the reference.
by Paul Gilster | Feb 17, 2012 | Exoplanetary Science |
The dreams of Alpha Centauri I used to have as a boy all focused on visual effects. After all, the distance between Centauri A and B ranges from 11.4 to 36.0 AU. What would it be like to have a second star in our Solar System, one that occasionally closed to a little more than Saturn’s distance from the Sun? What would a day be like with two stars, and even more, what would night be like with a star that close lighting up the landscape? I also wondered about how much effect a second star would have on the planets in our system, curious as I was about gravitational effects and even the possible repercussions for weather and seasonal change.
Image: The Alpha Centauri star system and other objects near it in the sky. Image copyright Akira Fujii / David Malin Images.
You can imagine, then, that Duncan Forgan’s new paper hit close to home. Forgan (University of Edinburgh) has taken discussions of habitability around Centauri B to a new level by analyzing the effect of Centauri A on habitability using latitudinal energy balance models that allow him to study how small changes in the properties of a planet can affect the overall climate there. Such models have been useful in studying things like climate variability due to orbital eccentricity and other factors, and Forgan puts them to work to chart the effect of a binary companion.
Alpha Centauri in the Last Fifteen Years
Before I get into the results of the habitability study, though, I want to go through some of the more recent work on Alpha Centauri, all summarized carefully in the Forgan paper. Indeed, I point you to this paper with great assurance that if you are interested in the Centauri stars, you’ll find a useful bibliography and summary here that will quickly get you up to speed (though the bibliography would be better if it listed paper titles along with the rest of the citations). Let’s run through some of the more salient work — in most cases I’ll skip the authors and citations in this discussion, knowing that Forgan’s work containing all of these is freely available at the arXiv site.
Centauri A and B, being high metallicity stars, are presumably prime candidates for circumstellar disks with a high solid material component, making the building blocks of planets readily available, and deepening the spectral lines for improved precision in radial velocity studies. Another useful factor for observations is that the binary is inclined by only 11 degrees with respect to our line of sight, an important fact because it means that any planets we discover through RV methods will yield a mass that is fairly accurate, assuming that the planets around these stars have formed in the same orbital plane. Without such knowledge, the mass figures from RV studies vary widely depending on assumptions about the target system’s inclination.
Studies on planet formation have shown that both Centauri A and B should be capable of forming terrestrial planets even when the perturbations caused by the binary companion are taken into account. Early studies on this question have found that the planetesimal disks seem to be stable out to about 3 AU of the parent stars, assuming a reasonable inclination of the disk relative to the binary plane, meaning something less than 60 degrees. More recent work by Thébault and colleagues has shown that the later stages of accretion may not be efficient because the binary companion can inhibit the growth of larger objects outside 0.75 AU (Cen A) and 0.5 AU (Cen B).
What does this mean? Most likely that the formation of gas giants is unlikely here (a finding that squares with previous radial velocity surveys), while if we can get past the problem of forming larger planetesimals referred to above, Earth-mass planets should be able to form in the habitable zone of Centauri B, assuming an eccentricity of no more than 0.3. A 2009 study I’m not familiar with by Michtchenko & Porto de Mello makes the case that any terrestrial planets that do form in Centauri B’s habitable zone should be dynamically stable despite perturbations from Centauri A under certain conditions of eccentricity and orbital inclination, but planets with inclinations to the orbital plane larger than about 35 percent should experience strong instability.
So where is the habitable zone around Centauri B? Kasting and team used a model that assumed Earth-mass planets with similar atmospheric composition and found a habitable zone ranging from 0.5 to 0.9 AU, although this 1993 study did not include the perturbing influence of Centauri A. But Forgan notes this with regard to the light reaching Centauri B planets:
If main sequence relations for the luminosity of each object are assumed, the insolation experienced by planets in the habitable zone of ? Cen B due to ? Cen A would be no more than a few percent of the total insolation of the ? Cen AB system at the binary’s periastron, and around one tenth of a percent at apastron. This insolation can be diminished further by eclipses of ? Cen A by ? Cen B, the duration of which is estimated to be of order a few Earth days.
Tuning the Model for Centauri B
Kasting was using a global radiative balance model (GRBM), but he and other researchers later deployed latitudinal energy balance models (LEBMs) of the kind Forgan uses in his new study, the latter being more complex and incorporating assumptions about latitude and season and other properties that would be temperature dependent. Forgan adjusts the model to include the effects of the binary (neglecting the distant M-dwarf Proxima Centauri). From the paper:
A planet in global radiative balance is not in general in local radiative balance, and by extension habitability is not a discrete concept (i.e. either habitable or uninhabitable), but a continuous one, where a certain fraction of the planet’s surface will be habitable at any given time. In the LEBM, the evolution of the planet’s temperature T (?) is described by a diffusion equation made nonlinear by the addition of the heating and cooling terms, as well as an albedo which makes a rapid transition from low to high as temperature decreases past the freezing point of water. As a result, small changes in the properties of a planet can strongly affect the resultant climate.
The latitudinal energy balance model, then, seems the best approach for asking how the perturbations caused by Centauri A might affect planets in the habitable zone of Centauri B.
So what does Forgan find? It turns out that calculating the habitable zone of Centauri B’s inner and outer boundaries can be roughly correct if we leave Centauri A out of the picture — the dimensions of the habitable zone remain more or less the same. But adding Centauri A does create oscillations in the planet’s climate that happen when Centauri A is at its closest to Centauri B. The temperature variations caused by Centauri A are no more than several K, and could alter the fraction of habitable surface on planets at the habitable zone boundaries by about 3 percent, a figure made flexible depending on the size of oceans or planetary obliquity.
The paper goes on to note the possible effect on life (science fiction writers take note):
It is reasonable to speculate that if life were to exist on planets around ? Cen B, that they may develop two circadian rhythms (cf Breus et al. 1995) corresponding to both the length of day around the primary, and the period of the secondary’s orbit (approx 70 years). Altering the available habitat by a few percent may also in?uence migration patterns and population evolution.
Small changes over time, though, can lead to big results, as Forgan goes on to remind us:
While we have demonstrated that the temperature ?uctuations for planets around ? Cen B due to ? Cen A are relatively small, the consequences of a periodic temperature forcing of a few K to long term climate evolution cannot be fully understood from this work. To fully appreciate the impact on (for example) ocean circulation and carbonate-silicate cycles requires further investigation with more advanced climate models.
Simulations and Their Limitations
The paper analyzes the results of the simulation for different classes of planets, from fully habitable worlds to uninhabitable hot planets, uninhabitable snowball planets, and two other classes — eccentric transient planets and binary transient planets — that are both partially habitable, with the habitability oscillating according to either the planet’s orbit around Centauri B (eccentric transients) or the period of Centauri A (binary transients). Forgan is careful to comment on the limitations of the LEBM model, which is not sensitive to long-term climate processes, and he notes that adding clouds and a carbonate-silicate cycle into the mix would potentially extend the outer edge of the habitable zone. Another limitation: The model is not sensitive to planets with extremely slow rotations.
Nevertheless, previous work with such modeling has shown its effectiveness, and the picture of the potential Centauri B system that emerges is one in which habitable worlds could well flourish. On the latter score, one other note:
The inner edge of the habitable zone is less well-de?ned than the outer edge – atmospheric changes could allow liquid water above 373 K, and the runaway greenhouse effect may become important at temperatures nearer 350 K (Spiegel et al. 2008 and references within). In any case, the outer edge is likely to be more interesting from an astrobiological standpoint, as current and future instrumentation will be more capable of prob[ing] spectral features of planets at larger semi-major axes (see e.g. Kaltenegger & Selsis 2010).
What’s needed now, of course, are radial velocity results from the ongoing studies of Alpha Centauri, which will begin to tell us whether or not rocky terrestrial worlds actually exist there. This is tricky work — radial velocity methods are much happier with huge gas giants in close orbits than with small rocky planets, which demand a much longer analysis. The paper is Forgan, “Oscillations in the Habitable Zone around Alpha Centauri B,” accepted for publication in Monthly Notices of the Royal Astronomical Society (preprint) and highly recommended to anyone with an interest in planets around our nearest stellar system.
by Paul Gilster | Feb 16, 2012 | Deep Sky Astronomy & Telescopes |
Luminous Blue Variables are large, bright stars that give rise to periodic eruptions, like the so-called “Great Eruption” of Eta Carinae that was first noted in 1837 and continued to be observed for an additional 21 years. Things must have been lively around the companion star thought to orbit in the nebula around Eta Carinae, for the LBV blew off about 20 solar masses in this era, mimicking a supernova as it became the second brightest star in the sky. We’ve witnessed similar ‘supernova impostor’ events in other galaxies, but at 7500 light years, the Eta Carinae system is relatively nearby, allowing close study by Hubble and other telescopes.
What brings Eta Carinae’s 1837 event back into the news is the use of so-called ‘light echoes’ to study what happened at a time when astronomy was in a much earlier state. Armin Rest (Space Telescope Science Institute) notes how useful the work is turning out to be:
“When the eruption was seen on Earth 170 years ago, there were no cameras capable of recording the event. Everything astronomers have known to date about Eta Carinae’s outburst is from eyewitness accounts. Modern observations with science instruments were made years after the eruption actually happened. It’s as if nature has left behind a surveillance tape of the event, which we are now just beginning to watch. We can trace it year by year to see how the outburst changed.”
Rest is referring to light bouncing off dust clouds, echoing the cataclysm of 170 years ago so that astronomers can study it with the latest technology. The light echo technique has been employed before in the study of supernovae, culling information about the speed and temperature of the material ejected from the star. You may recall, too, that the anomalous object called Hanny’s Voorwerp, discovered by a Dutch school teacher using the Galaxy Zoo project, turned out to be a gas cloud illuminated by a beam of intense optical and ultraviolet emission from the center of a nearby galaxy, an event researchers dubbed the ‘first quasar light echo.’
Image: The color image at left shows the Carina Nebula, with the massive double-star system Eta Carinae near the top of the image. The star system, about 120 times more massive than the Sun, produced a spectacular outburst that was seen on Earth from 1837 to 1858. But some of the light from the eruption took an indirect path and is just now reaching our planet. The light bounced off dust clouds (the boxed region about 100 light-years away at the bottom of the image) and was rerouted to Earth, a phenomenon called a light echo. The three black-and-white images at right show light from the eruption illuminating dust clouds near the doomed star system as it moves through them. The effect is like shining a flashlight on different regions of a vast cavern. Credit: NASA, NOAO, and A. Rest (Space Telescope Science Institute, Baltimore, Md.).
In the case of Eta Carinae, the echo showed up when Rest and colleagues compared visible-light observations of the system with earlier observations from the Cerro Tololo Inter-American Observatory (CTIO) in Chile. An intense spectroscopic follow-up allowed the astronomers to measure the speed of the outflow (about 195 kilometers per second), and to determine its temperature (5000 K). Among Luminous Blue Variables, Eta Carinae seems unusual in that the outflow from the central region is cooler than observed in other erupting stars, a fact we’ll return to in a moment.
Having this kind of cosmic play-back gives us all kinds of interesting possibilities, including the fact that we already know that a year after its 1843 peak in brightness (which is what the current work measures), another brightening was observed in 1844. Rest’s team is thus awaiting the light echo of that event, which should be observable in about six months and will offer a more complete view of the eruption. Massive Eta Carinae, thought likely to explode as a supernova within the next million years, has much to teach us about the behavior of Luminous Blue Variables. “This star really seems to be an oddball,” Rest adds. “Now we have to go back to the models and see what has to change to actually produce what we are measuring.”
A ‘Lesser Eruption’ occurring around 1890 was observed spectroscopically and shows a strikingly different light spectrum, indicating to the authors of the paper on this work that two distinct physical processes may have been involved in the two events. Whatever the case, Eta Carinae is extraordinarily useful to astronomers because LBV giant eruptions are a rarity, with only the Great Eruption of this star and a giant eruption of the star P Cygni in the 17th Century recorded in our galaxy in the last 400 years. All other supernova ‘impostor’ events have been extragalactic. The challenges posed by this intriguing star are made clear in the paper:
? Car’s Great Eruption has been considered the prototype of the extragalactic SN imposters or ? Car analogues, even though it is actually an extreme case in terms of radiated energy (1049:3 erg), kinetic energy (>1050 erg), and its decade-long duration. The spectra of the light echo indicates now that it is not only extreme, but a different and rather unique object. It is dif?cult to see how strong emission lines could be avoided in an opaque wind where the continuum photosphere is determined by a change in opacity, and its temperature and broad absorption lines are more consistent with the opaque cooling photosphere of an explosion. The cause that triggered such an explosion and the mass-loss without destroying the star is still unknown, but predictions from future radiative transfer simulations trying to explain ? Car and its Great Eruption can now be matched to these spectral observations. Other alternative models that were proposed, e.g. the ones that use mass accretion from the companion star during periapsis passage as a trigger for the eruption, can be either veri?ed or dismissed.
The work points to the conclusion that Eta Carinae is too cool to qualify as the same kind of supernova impostor observed in other galaxies because such stars are thought to be far hotter. This article in Nature quotes Augusto Damineli (University of São Paulo) as saying the findings have surprised everyone. “All well educated astronomers would have bet that they would find the spectrum of a 7,000-kelvin star.” Rest and colleagues are now looking for further Eta Carinae light echoes in different parts of the sky to build up a fuller picture of the eruption.
The paper is Rest et al., “Light echoes reveal an unexpectedly cool ? Carinae during its 19th-century Great Eruption,” Nature 482, 375–378 (16 February 2012). Abstract / Preprint.
