Do Stars Fall Quietly into Black Holes, or Crash into Something Utterly Unknown?

This artist's impression shows a star crossing the event horizon of a supermassive black hole located in the center of a galaxy. The black hole is so large and massive that tidal effects on the star are negligible, and the star is swallowed whole. The effects of gravitational lensing distorting the light of the star are not shown here. Credit: Mark A. Garlick/CfA.  High Resolution (jpg) - Low Resolution (jpg)

This is the first in a sequence of two artist's impressions that shows a huge, massive sphere in the center of a galaxy, rather than a supermassive black hole. Here a star moves towards and then smashes into the hard surface of the sphere, flinging out debris. The impact heats up the site of the collision. Credit: Mark A. Garlick/CfA. High Resolution (jpg) - Low Resolution (jpg)

In this second artist's impression a huge sphere in the center of a galaxy is shown after a star has collided with it. Enormous amounts of heat and a dramatic increase in the brightness of the sphere are generated by this event. The lack of observation of such flares from the center of galaxies means that this hypothetical scenario is almost completely ruled out. Credit: Mark A. Garlick/CfA. High Resolution (jpg) - Low Resolution (jpg)

Cambridge, MA -Astronomers at The University of Texas at Austin and Harvard University have put a basic principle of black holes to the test, showing that matter completely vanishes when pulled in. Their results constitute another successful test for Albert Einstein's General Theory of Relativity.

Most scientists agree that black holes, cosmic entities of such great gravity that nothing can escape their grip, are surrounded by a so-called event horizon. Once matter or energy gets close enough to the black hole, it cannot escape — it will be pulled in. Though widely believed, the existence of event horizons has not been proved.

"Our whole point here is to turn this idea of an event horizon into an experimental science, and find out if event horizons really do exist or not," said Pawan Kumar, a professor of astrophysics at The University of Texas at Austin.

Supermassive black holes are thought to lie at the heart of almost all galaxies. But some theorists suggest that there’s something else there instead — not a black hole, but an even stranger supermassive object that has somehow managed to avoid gravitational collapse to a singularity surrounded by an event horizon. The idea is based on modified theories of General Relativity, Einstein’s theory of gravity.

While a singularity has no surface area, the noncollapsed object would have a hard surface. So material being pulled closer — a star, for instance — would not actually fall into a black hole, but hit this hard surface and be destroyed.

Kumar, his graduate student Wenbin Lu, and Ramesh Narayan, a theorist from the Harvard-Smithsonian Center for Astrophysics, have come up with a test to determine which idea is correct.
"Our motive is not so much to establish that there is a hard surface," Kumar said, "but to push the boundary of knowledge and find concrete evidence that really, there is an event horizon around black holes."

The team figured out what a telescope would see when a star hit the hard surface of a supermassive object at the center of a nearby galaxy: The star’s gas would envelope the object, shining for months, perhaps even years.

Once they knew what to look for, the team figured out how often this should be seen in the nearby universe, if the hard-surface theory is true.

"We estimated the rate of stars falling onto supermassive black holes," Lu said. "Nearly every galaxy has one. We only considered the most massive ones, which weigh about 100 million solar masses or more. There are about a million of them within a few billion light-years of Earth."

They then searched a recent archive of telescope observations. Pan-STARRS, a 1.8-meter telescope in Hawaii, recently completed a project to survey half of the northern hemisphere sky. The telescope scanned the area repeatedly during a period of 3.5 years, looking for "transients" — things that glow for a while and then fade. Their goal was to find transients with the expected light signature of a star falling toward a supermassive object and hitting a hard surface.

"Given the rate of stars falling onto black holes and the number density of black holes in the nearby universe, we calculated how many such transients Pan-STARRS should have detected over a period of operation of 3.5 years. It turns out it should have detected more than 10 of them, if the hard-surface theory is true," Lu said.

They did not find any.

"Our work implies that some, and perhaps all, black holes have event horizons and that material really does disappear from the observable universe when pulled into these exotic objects, as we’ve expected for decades," Narayan said. "General Relativity has passed another critical test."

Now the team is proposing to improve the test with an even larger telescope: the 8.4-meter Large Synoptic Survey Telescope (LSST, now under construction in Chile). Like Pan-STARRS, LSST will make repeated surveys of the sky over time, revealing transients — but with much greater sensitivity.
This research has been published in the June issue of the journal Monthly Notices of the Royal Astronomical Society.

Headquartered in Cambridge, Mass., the Harvard-Smithsonian Center for Astrophysics (CfA) is a joint collaboration between the Smithsonian Astrophysical Observatory and the Harvard College Observatory. CfA scientists, organized into six research divisions, study the origin, evolution and ultimate fate of the universe.

For more information, contact:

Rebecca Johnson
UT Austin Astronomy Program
512-475-6763
rjohnson@astro.as.utexas.edu

Peter Edmonds
Harvard-Smithsonian Center for Astrophysics
+1 617-571-7279
pedmonds@cfa.harvard.edu

Source:  Harvard-Smithsonian Center for Astrophysics

New Method of Searching for Fifth Force

The orbits of two stars, S0-2 and S0-38 located near the Milky Way’s supermassive black hole will be used to test Einstein’s theory of General Relativity and potentially generate new gravitational models. Image Credit: . SAKAI/A.GHEZ/W.M. Keck Observatory/ UCLA Galactic Center Group. 

W. M. Keck Observatory Data Leads To First Of Its Kind Test of Einstein’s Theory of General Relativity

A UCLA-led team has discovered a new way of probing the hypothetical fifth force of nature using two decades of observations at W. M. Keck Observatory, the world’s most scientifically productive ground-based telescope.

There are four known forces in the universe: electromagnetic force, strong nuclear force, weak nuclear force, and gravitational force. Physicists know how to make the first three work together, but gravity is the odd one out. For decades, there have been theories that a fifth force ties gravity to the others, but no one has been able to prove it thus far.

“This is really exciting. It’s taken us 20 years to get here, but now our work on studying stars at the center of our galaxy is opening up a new method of looking at how gravity works,” said Andrea Ghez, Director of the UCLA Galactic Center Group and co-author of the study.

The research is published in the current issue of Physical Review Letters.

Ghez and her co-workers analyzed extremely sharp images of the center of our galaxy taken with Keck Observatory’s adaptive optics (AO). Ghez used this cutting-edge system to track the orbits of stars near the supermassive black hole located at the center of the Milky Way. Their stellar path, driven by gravity created from the supermassive black hole, could give clues to the fifth force.

“By watching the stars move over 20 years using very precise measurements taken from Keck Observatory data, you can see and put constraints on how gravity works. If gravitation is driven by something other than Einstein’s theory of General Relativity, you’ll see small variations in the orbital paths of the stars,” said Ghez.

Pictured above: UCLA Professor of Astrophysics and Galactic Center Group Director Andrea Ghez, a Keck Observatory astronomer and recipient of the 2015 Bakerian Medal. Image Credit: Kyle Alexander

This is the first time the fifth force theory has been tested in a strong gravitational field such as the one created by the supermassive black hole at the center of the Milky Way. Historically, measurements of our solar system’s gravity created by our sun have been used to try and detect the fifth force, but that has proven difficult because its gravitational field is relatively weak.

“It’s exciting that we can do this because we can ask a very fundamental question – how does gravity work?” said Ghez. “Einstein’s theory describes it beautifully well, but there’s lots of evidence showing the theory has holes. The mere existence of supermassive black holes tells us that our current theories of how the universe works are inadequate to explain what a black hole is.”

Ghez and her team, including lead author Aurelien Hees and co-author Tuan Do, both of UCLA, are looking forward to summer of 2018. That is when the star S0-2 will be at its closest distance to our galaxy’s supermassive black hole. This will allow the team to witness the star being pulled at maximum gravitational strength – a point where any deviations to Einstein’s theory is expected to be the greatest.

About Adaptive Optics

W. M. Keck Observatory is a distinguished leader in the field of adaptive optics (AO), a breakthrough technology that removes the distortions caused by the turbulence in the Earth’s atmosphere. Keck Observatory pioneered the astronomical use of both natural guide star (NGS) and laser guide star adaptive optics (LGS AO) and our current systems now deliver images three to four times sharper than the Hubble Space Telescope. AO has imaged the four massive planets orbiting the star HR8799, measured the mass of the giant black hole at the center of our Milky Way Galaxy, discovered new supernovae in distant galaxies, and identified the specific stars that were their progenitors.

About W. M. Keck Observatory

The W. M. Keck Observatory operates the largest, most scientifically productive telescopes on Earth. The two, 10-meter optical/infrared telescopes on the summit of Maunakea on the Island of Hawaii feature a suite of advanced instruments including imagers, multi-object spectrographs, high-resolution spectrographs, integral-field spectrometers, and world-leading laser guide star adaptive optics systems. The Observatory is a private 501(c) 3 non-profit organization and a scientific partnership of the California Institute of Technology, the University of California, and NASA.

 Source: W.M. Keck Observatory

Newly discovered fast-growing galaxies could solve cosmic riddle – and show ancient cosmic merger

Figure 1: Artist's impression of a quasar and neighboring merging galaxy. The galaxies observed by Decarli and collaborators are so distant that no detailed images are possible at present. This combination of images of nearby counterparts gives an impression of how they might look in more detail. Image: MPIA using material from the NASA/ESA Hubble Space Telescope

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Additional figures and download

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Astronomers have discovered a new kind of galaxy in the early universe, less than a billion years after the Big Bang. These galaxies are forming stars more than a hundred times faster than our own Milky Way. The discovery could explain an earlier finding: a population of suprisingly massive galaxies at a time 1.5 billion years after the Big Bang, which would require such hyper-productive precursors to grow their hundreds of billions of stars. The observations also show what appears to be the earliest image of galaxies merging. The results, by a group of astronomers led by Roberto Decarli of the Max Planck Institute for Astronomy, have been published in the 25 May issue of the journal Nature.

When a group of astronomers discovered unusually massive galaxies in the early universe a few years ago, the sheer size of these galaxies, with hundreds of billions of stars, posed a puzzle. The galaxies are so distant, we see them as they were a mere 1.5 billion years after the Big Bang, when the universe was about 10% its present age. How were they able to form so many stars, in such a comparatively short time?

Now, a serendipitous discovery by a group of astronomers led by Roberto Decarli from the Max Planck Institute for Astronomy is pointing to a possible solution to the mystery: a population of hyper-productive galaxies in the very early universe, at a time less than a billion years after the Big Bang.

Roberto Decarli says: "We were looking for something different: for star formation activity in the host galaxies of quasars. But what we found, in four separate cases, were neighboring galaxies that were forming stars at a furious pace, producing a hundred solar masses' worth of new stars per year." Quasars constitute a brief phase of galaxy evolution, powered by the infall of matter onto a supermassive black hole at the center of a galaxy.

Fabian Walter, leader of the observation program using the ALMA observatory in Chile that led to the discovery, says: "Very likely it is not a coincidence to find these productive galaxies close to bright quasars. Quasars are thought to form in regions of the universe where the large-scale density of matter is much higher than average. Those same conditions should also be conducive to galaxies forming new stars at a greatly increased rate."

Whether or not these newly discovered galaxies can indeed be the precursors of their more massive, later kin, and thus solve the cosmic puzzle, will depend on how common they are in the universe. That is a question for follow-up observations planned by Decarli and his colleagues.

The ALMA observations also showed what appears to be the earliest known example of two galaxies undergoing a merger. In addition to forming new stars, mergers are another major mechanism of galaxy growth – and the new observations provide the first direct evidence that such mergers have been taking place even at the earliest stages of galaxy evolution, less than a billion years after the Big Bang. press@nature.com

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Background Information

The results described here have been published as Decarli et al., "Rapidly star-forming galaxies adjacent to quasars at z>6" in the May 25, 2017 edition of the journal Nature.

Accredited journalists can obtain a copy of the paper in the press area of the nature.com website or by contacting [press@nature.com].

The MPIA researchers involved are:

Roberto Decarli, Fabian Walter, Bram Venemans, Emanuele Farina, Chiara Mazzucchelli, and Hans-Walter Rix

in collaboration with:

Eduardo Bañados (Carnegie Observatories, Pasadena), Frank Bertoldi (University of Bonn), Chris Carilli (NRAO and Cavendish Laboratory, Cambridge), Xiaohui Fan (University of Arizona), Dominik Riechers (Cornell University), Michael A. Strauss (Princeton University), Ran Wang (Peking University), and Y. Yang (Korea Astronomy and Space Science Institute).

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Science Contact

Decarli, Roberto
Roberto Decarli
Phone: (+49|0) 6221 528-368
Email: decarli@mpia.de
Links: Personal homepage



Public Information Officer

Markus Pössel

Public Information Officer
Phone:(+49|0) 6221 528-261
Email: pr@mpia.de

Source: Max Planck Institute for Astronomy

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In-depth description:

A group of astronomers led by Roberto Decarli at the Max Planck Institute has discovered surprisingly productive galaxies in the very early universe. These galaxies, which we see as they were less than a billion years after the Big Bang, produce more than hundred solar masses worth of stars every year – and could be the key to explaining a population of somewhat later unusually massive galaxies that other astronomers had discovered in the early universe, about 1.5 billion years after the Big Bang. Those later massive galaxies posed a particular kind of puzzle: While less than a billion years old themselves, they contain numerous reddish stars almost as old as these galaxies themselves, indicating that they must have been forming stars at a high rate for almost all of their existence.

Understanding cosmic history

On the one hand, the history of the universe as a whole is simpler than the history of Earth's human inhabitants. Cosmological history directly follows simple fundamental laws, namely the laws of physics. On the other hand, this ups the ante for cosmologists: They should be able to explain in terms of physical processes how the universe has reached its present state from a fairly boring, almost homogeneous beginning directly after the Big Bang, 13.8 billion years ago.

There are several key classes of objects whose properties and evolution need explaining. First of all, there is dark matter, which does not interact with light and other forms of electromagnetic radiation at all. Over the past 13.8 billion years, dark matter has clumped together under its own gravity, forming the gigantic filaments of the cosmic web, the backdrop or framework of cosmic history. On smaller scales, dark matter has formed loose, almost spherical associations known as halos. Gas collecting in those halos has formed galaxies: collections of between hundreds of thousands and hundreds of billions of stars, suffused with (mostly hydrogen) gas.

To the best of current astronomical knowledge, every massive galaxy contains a supermassive black hole in its central regions, with masses between a few hundred thousand and a few billion times the mass of the Sun. (The central black hole of our own galaxy has a mass of 4 million solar masses.)

When sufficient amounts of matter fall into such a supermassive black hole, it turns into a quasar: directly before falling into the black hole, matter collects in a swirling disk; this "accretion disk" is heated up as more and more infalling matter deposits its energy; the extreme temperature of the disk (think "incandescent light bulb") and additional effects make the quasar into one of the brightest objects in the universe, as bright as all the stars of a large galaxy combined.

In addition to stars, and rare and transient phenomena like quasars, there is intergalactic gas – again, mostly hydrogen, both in the galaxies themselves and filling the void between galaxies, and between the filaments of the cosmic web.

Cosmic history on display

Cosmic history describes the formation and the evolution of these objects, including their interactions. How and when did galaxies form their stars? Is intergalactic gas funneled into galaxies, providing new raw material for star formation? Does quasar activity hinder or encourage star formation? Is star formation the same throughout history, or did galaxies become less productive, or more productive, over time? By now, the field of cosmic historiography can provide at least some answers. Open questions are pursued using modeling, simulations, and observations – including recent massive surveys that enable statistics with samples of hundreds of thousands of objects.

Astronomical distances are so large that it takes the light of distant objects an impressive time to reach us here on Earth. That provides astronomers with a cross section of cosmic history. For instance, we see the Andromeda galaxy as it was 2.5 million years ago, since Andromeda's light has taken 2.5 million years to reach us. Other galaxies, we see as they were billions of years ago.

Thus, while we cannot follow the entire history of any single object, astronomical observations do show us the different stages of cosmic history. Assuming that at least on average, no location within the universe is markedly different from any other – for instance, that we will find the same numbers of galaxies, or quasars, with the same average properties –, we can observe distant objects as they once were, and draw conclusions about our own past.

An unusual population of massive galaxies

Cosmology must take the many observations that represent different epochs of cosmic history and weave them into a consistent physical narrative: Objects that have been found in one particular epoch must have formed in some earlier epoch. One example is the discovery of a substantial population of very massive galaxies, each with hundreds of billions of stars and a total mass of hundreds of billions of solar masses, in an epoch around 1.5 billion years after the Big Bang (z ∼ 4) by Caroline Straatman (then Leiden University, now at MPIA) and collaborators in 2014.

Once this observation has been made, it needs to be explained. For there to be galaxies that rich in stars at a time of 1.5 billion years after the Big Bang, when the universe was a bit more than 10% its present age, the precursors of these galaxies must have formed stars at an enormous rate at earlier epochs.

But do we see evidence for such actively star-forming galaxies in the very early universe?

A serendipitous discovery

The new results by Roberto Decarli and collaborators described here have shed new light on this question – albeit serendipitously, as the astronomers' initial aim had been somewhat different. Using the ALMA observatory, they were looking for very distant star-forming host galaxies of quasars. Since quasars are galactic nuclei, each is embedded in what is known as its host galaxy. There have long been questions about the interaction of quasars with their host galaxies – do they, for instance, inhibit star formation in the galaxy surrounding them?

More generally, what are the properties of these host galaxies – and are they related to the fact that the galaxy is hosting a quasar? To address such questions, Decarli and his colleagues studied known quasars so distant they represent the first billion years of cosmic history – and in targeting these quasars, they looked specifically for emission associated with star-forming activity.

Signs of star formation activity

Star formation involves gas clouds collapsing under their own gravity. If gravity is strong enough to compress the central regions to such high densities, and heat them to such high temperatures, that nuclear fusion sets in, turning hydrogen nuclei (protons) into helium. The result is, by definition, a star: an object bound by its own gravity, with nuclear fusion in its core region, shining brightly as the energy liberated during the fusion processes is transported outwards. But in order to reach these high densities, and such an advanced state of collapse, the cloud needs to cool down during the collapse.

That is surprisingly difficult: Hydrogen molecule, it turns out, are not very efficient in radiating away heat in the form of light. Most of the cooling-down is mediated by a kind of atom that occurs only very rarely in such collapsing clouds, but is able to radiate energy very efficiently: carbon. There are typically only three carbon atoms for each 100,000 hydrogen atoms in a modern-day star-forming environment, but in particular in its singly ionized form, with one electron having broken free from the atom, carbon is a highly efficient radiator, shining brightly in a very narrow frequency range known among astronomers as the [CII] line.

(The square brackets indicate that this is a line that is only visible under the rarified conditions of outer space – in laboratory experiments at higher gas density, the atoms in question are more likely to lose their energy by colliding with other atoms, before they can radiate [CII] light.)

Starforming regions are the main source of [CII] light in galaxies. Conversely, by measuring the amount of [CII] light emitted by a galaxy, one can estimate the rate at which that galaxy is forming new stars.

Distant star formation with ALMA

For close-up objects, the [CII] line has a wavelength of 158 μm, in the far infrared range of the spectrum. Unfortunately, the Earth's atmosphere is virtually opaque for light at that wavelength, and observations of this kind can only be made by airborne or space observatory, most recently SOFIA and Herschel.

For very distant objects, though, there is an additional effect that makes ground-based observations possible. For an observer on Earth, the light of very distant objects is stretched by the so-called cosmological redshift, an effect of the expansion of the universe. For the galaxies and quasars that Decarli and his colleagues were aiming at, light is stretched by a factor of about seven (corresponding to a z value z ~ 6), bringing the line into the millimeter wave regime, which is observable using ground-based telescopes like ALMA. That allows for high-resolution, sensitive observations.

ALMA is a telescope array composed of about 50 high-precision antennas, operated by an international consortium in the Atacama desert in Chile, and represents a significant increase in sensitivity over previous such observatories. Before the present study, [CII] studies on high redshift ('high-z') quasar host galaxies had only been done in small samples (with up to four quasars per study). With ALMA, bigger samples became feasible: Decarli and his colleagues obtained sensitive [CII] data for 25 galaxies.

Not the galaxies they were looking for

And for four of these targets, the astronomers were in for a surprise. Yes, there were quasars in those images, but there were galaxies as well. Not the quasars' host galaxies, but companion galaxies, each a little offset from the quasar target. And these were galaxies that were shining brightly in [CII], evidently forming more than a hundred solar masses' worth of stars per year. In galactic terms, that is quite a lot. Our home galaxy, for instance, forms no more than one solar mass per year. The other galaxies astronomers had previously found in this period of the early universe had star formation rates between one and ten solar masses per year.

The objects observed by Decarli and colleagues are so distant that we see them as they were a bit more than 900 million years after the Big Bang (z ∼ 6). But at that rate of forming new stars, these galaxies could indeed be the precursors of the star-rich galaxies found by Straatman and her colleagues at 1.5 billion years after the Big Bang (z ∼ 4).

The group around Decarli found a missing piece of the puzzle of cosmic history: A population of young, vigorously star-forming galaxies at a time 900 million years after the Big Bang. If this type of galaxy is sufficiently common, it could explain the unexpectedly star-rich galaxies about 600 million years later.

Quasars, overdensities and star formation

In all probability, finding these galaxies so close to quasars is no coincidence. The details will need to be examined much more thoroughly, including additional observations, but one general correlation suggests itself: In order to explain how the black holes driving quasars were able to amass a billion solar masses that early in the history of the universe, these quasars should be located in the highest-density regions of the universe at that time. It is plausible that the same overdense environment was conducive to the formation of the newly found, quickly star-forming galaxies as well. Thus, one would be more likely to find these galaxies in the neighbourhood of quasars.

Either alternatively or in addition, it is possible that the quasar's activity encouraged the nearby galaxy to form more stars, for instance by pushing on that galaxy's gas from the outside, setting off more local cloud collapses than would otherwise have happened. If these newly discovered active galaxies are representative of a more widespread population of vigorously star-forming galaxies in the very early universe, occurring even in the many regions where there are no quasars (albeit more rarely), they would be sufficient to account for the massive, evolved galaxies discovered by Straatman and collaborators.

The first known merger?

One of the four objects, the quasar with the catalogue number PJ308-21, is particularly interesting. Its star-forming companion galaxy is comparatively close to the quasar, and appears to be stretched out into a long shape towards the quasar. This kind of deformation is to be expected if the companion galaxy is interacting with the quasar host galaxy.

 This kind of interaction, each galaxy distorted with tidal forces of the other galaxy's gravity, commonly is the prelude to the merger of these galaxies, resulting in the formation of a larger single galaxy. In the current models of galaxy evolution, this is a key mechanism for how galaxies have grown in the course of cosmic history. If the new observation indeed shows a galaxy merger, it would be the earliest known such merger.

 All in all, the newly discovered population has shown us one piece of the cosmic narrative, namely how the somewhat later, star-rich galaxies formed. It is also pointing astronomers in a specific direction to find out more about the history of the early universe, namely towards an investigation of the role of overdensities, and of possible interactions, in the formation of the quasars and their companions.

   
Further steps

Next, Decarli and his colleagues will need to fully characterize their newly discovered sources:

Since these galaxies do not show obvious signs of accreting central black holes, which would outshine the faint stellar emission of the host galaxy, and which might influence star-formation in the galaxy, these newly discovered galaxies are ideal laboratory to study the first stages of the formation of massive galaxies. What kinds of stars do they contain, and in what proportion? What is their total mass, and how many stars have already been formed in these galaxies? What are the properties of the gas between the stars in these galaxies, the interstellar medium – how dense is it, what is its temperature, what fraction of it is ionized? And are these galaxies indeed only found very close to quasars, or do they exist in other environments, as well?

Answering these questions will require a whole battery of telescopes: from ALMA via the Hubble Space Telescope and the Spitzer Space Telescope to various ground-based telescopes and, in the immediate future, the James Webb Space Telescope. But by analyzing the data from these telescopes, with their different specializations and strengths, astronomers should be able to write a detailed version of this particular chapter of cosmic history: how the earliest massive galaxies came into being.

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Star Forming Filaments

A false-color image map of the gas density in the Musca star-forming filament (the highest densities are shown in red). New theoretical work on the structure of these long filaments proposes several kinds of star-forming zones along the length and successfully reproduces many of the features seen in filaments like this one in Musca. Credit: Kainulainen, 2016

Interstellar molecular clouds are often seen to be elongated and "filamentary" in shape, and come in a wide range of sizes. In molecular clouds, where stars form, the filamentary structure is thought to play an important role in star formation as the matter collapses to form protostars. Filamentary clouds are detected because the dust they contain obscures the optical light of background stars while emitting at infrared and submillimeter wavelengths. Observations of some filaments indicate that they are themselves composed of bundles of closely spaced fibers with distinct physical properties.

Computer simulations are able to reproduce some of these filamentary structures, and astronomers generally agree that turbulence in the gas combined with gravitational collapse can lead to filaments and protostars within them, but the exact ways in which filaments form, make stars, and finally dissipate are not understood. The number of new stars that develop, for example, varies widely between filaments for reasons that are not known.

The usual model for a star forming filament is a cylinder whose density increases towards the axis according to a specific profile, but which otherwise is uniform along its length. CfA astronomer Phil Myers has developed a variant of this model in which the filament has a star-forming zone along its length where the density and diameter are higher, with three generic profiles to describe their shapes. Besides being a more realistic description of a filament's structure, the different density profiles develop different strength gravitational "wells" naturally leading to different numbers of stars forming within them.

Myers compares the star formation properties of these three kinds of zones with the properties of observed star formation filaments, with excellent results. The filament in the molecular cloud in Musca has relatively little star formation, and can be reasonably well explained with one of the three profiles indicative of an early stage of evolution. A small cluster of young stars in the Corona Australis constellation fits a second model that has evolved for longer, while Ophiuchus hosts a filament that may be near the end of its star forming lifetime and resembles the third type. The three profiles so far seem able to account for the full range of conditions. The new results are an important step in bringing more sophistication and realism to the theory of star forming filaments. Future work will probe the specific processes that fragment the various star-forming zones into their stars.

Reference(s): 

"Star-forming Filament Models," Philip C. Myers, ApJ 838, 10, 2017.

Source:  Smithsonian Astrophysical Observatory (SAO)

Collapsing Star Gives Birth to a Black Hole

N6946-BH1

Credits:  NASA, ESA, and C. Kochanek (OSU)

Released images

Release Video

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Astronomers have watched as a massive, dying star was likely reborn as a black hole. It took the combined power of the Large Binocular Telescope (LBT), and NASA's Hubble and Spitzer space telescopes to go looking for remnants of the vanquished star, only to find that it disappeared out of sight.

It went out with a whimper instead of a bang.

The star, which was 25 times as massive as our sun, should have exploded in a very bright supernova. Instead, it fizzled out—and then left behind a black hole.

"Massive fails" like this one in a nearby galaxy could explain why astronomers rarely see supernovae from the most massive stars, said Christopher Kochanek, professor of astronomy at The Ohio State University and the Ohio Eminent Scholar in Observational Cosmology.

As many as 30 percent of such stars, it seems, may quietly collapse into black holes — no supernova required.

"The typical view is that a star can form a black hole only after it goes supernova," Kochanek explained. "If a star can fall short of a supernova and still make a black hole, that would help to explain why we don’t see supernovae from the most massive stars."

He leads a team of astronomers who published their latest results in the Monthly Notices of the Royal Astronomical Society.

Among the galaxies they've been watching is NGC 6946, a spiral galaxy 22 million light-years away that is nicknamed the "Fireworks Galaxy" because supernovae frequently happen there — indeed, SN 2017eaw, discovered on May 14th, is shining near maximum brightness now. Starting in 2009, one particular star, named N6946-BH1, began to brighten weakly. By 2015, it appeared to have winked out of existence.

After the LBT survey for failed supernovas turned up the star, astronomers aimed the Hubble and Spitzer space telescopes to see if it was still there but merely dimmed. They also used Spitzer to search for any infrared radiation emanating from the spot. That would have been a sign that the star was still present, but perhaps just hidden behind a dust cloud.

All the tests came up negative. The star was no longer there. By a careful process of elimination, the researchers eventually concluded that the star must have become a black hole.

It's too early in the project to know for sure how often stars experience massive fails, but Scott Adams, a former Ohio State student who recently earned his Ph.D. doing this work, was able to make a preliminary estimate.

"N6946-BH1 is the only likely failed supernova that we found in the first seven years of our survey. During this period, six normal supernovae have occurred within the galaxies we've been monitoring, suggesting that 10 to 30 percent of massive stars die as failed supernovae," he said.

"This is just the fraction that would explain the very problem that motivated us to start the survey, that is, that there are fewer observed supernovae than should be occurring if all massive stars die that way."

To study co-author Krzysztof Stanek, the really interesting part of the discovery is the implications it holds for the origins of very massive black holes — the kind that the LIGO experiment detected via gravitational waves. (LIGO is the Laser Interferometer Gravitational-Wave Observatory.)

It doesn't necessarily make sense, said Stanek, professor of astronomy at Ohio State, that a massive star could undergo a supernova — a process which entails blowing off much of its outer layers — and still have enough mass left over to form a massive black hole on the scale of those that LIGO detected.

"I suspect it's much easier to make a very massive black hole if there is no supernova," he concluded.

Adams is now an astrophysicist at Caltech. Other co-authors were Ohio State doctoral student Jill Gerke and University of Oklahoma astronomer Xinyu Dai. Their research was supported by the National Science Foundation.

NASA's Jet Propulsion Laboratory in Pasadena, California, manages the Spitzer Space Telescope mission for NASA's Science Mission Directorate, Washington, D.C. Science operations are conducted at the Spitzer Science Center at Caltech in Pasadena, California. Spacecraft operations are based at Lockheed Martin Space Systems Company, Littleton, Colorado. Data are archived at the Infrared Science Archive housed at the Infrared Processing and Analysis Center at Caltech. Caltech manages JPL for NASA.

The Large Binocular Telescope is an international collaboration among institutions in the United Sates, Italy and Germany.

The Hubble Space Telescope is a project of international cooperation between NASA and ESA (European Space Agency). NASA’s Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope. The Space Telescope Science Institute in Baltimore, Maryland, conducts Hubble science operations. STScI is operated for NASA by the Association of Universities for Research in Astronomy, Inc., in Washington, D.C.

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Related Links 

This site is not responsible for content found on external links

• The science paper by S. Adams et al.
• NASA's Hubble Portal
• NASA's Spitzer Portal
• Caltech's Spitzer Portal
• Ohio State University's Release

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Contacts

Christopher Kochanek / Krzysztof Stanek
Ohio State University, Columbus, Ohio
614-292-5954 / 614-292-3433
kochanek.1@osu.edu / stanek.32@osu.edu

Scott Adams
Caltech, Pasadena, California
626-395-8676
smadams@caltech.edu

Pam Frost Gorder
Ohio State University, Columbus, Ohio
614-292-9475
gorder.1@osu.edu

Elizabeth Landau
Jet Propulsion Laboratory, Pasadena, California
818-354-6425
elizabeth.r.landau@jpl.nasa.gov

Ray Villard
Space Telescope Science Institute, Baltimore, Maryland
410-338-4514
villard@stsci.edu

Source: HubbleSite

VLA Reveals New Object Near Supermassive Black Hole in Famous Galaxy

Artist's conception of newly-discovered secondary supermassive black hole orbiting the main, 
central supermassive black hole of galaxy Cygnus A. 
Credit: Bill Saxton, NRAO/AUI/NSF

VLA radio images (orange) of central region of Cygnus A, overlaid on Hubble Space Telescope image, from 1989 and 2015. 
Animated GIF. Credit: Perley, et al., NRAO/AUI/NSF, NASA

VLA radio image (orange) of central region of Cygnus A, overlaid on Hubble Space Telescope image, from 1989. 

Credit: Perley, et al., NRAO/AUI/NSF, NASA

2015 VLA radio image (orange) of Cygnus A, overlaid on Hubble Space Telescope image. 

Credit: Perley, et al., NRAO/AUI/NSF, NASA

1989 VLA radio image of the central region of Cygnus 

A. Credit: Perley, et al., NRAO/AUI/NSF

2015 VLA radio image of the central region of Cygnus A. 

Credit: Perley, et al., NRAO/AUI/NSF

Pointing the National Science Foundation’s Very Large Array (VLA) at a famous galaxy for the first time in two decades, a team of astronomers got a big surprise, finding that a bright new object had appeared near the galaxy’s core. The object, the scientists concluded, is either a very rare type of supernova explosion or, more likely, an outburst from a second supermassive black hole closely orbiting the galaxy’s primary, central supermassive black hole.

The astronomers observed Cygnus A, a well-known and often-studied galaxy discovered by radio-astronomy pioneer Grote Reber in 1939. The radio discovery was matched to a visible-light image in 1951, and the galaxy, some 800 million light-years from Earth, was an early target of the VLA after its completion in the early 1980s. Detailed images from the VLA published in 1984 produced major advances in scientists’ understanding of the superfast “jets” of subatomic particles propelled into intergalactic space by the gravitational energy of supermassive black holes at the cores of galaxies.

“This new object may have much to tell us about the history of this galaxy,” said Daniel Perley, of the Astrophysics Research Institute of Liverpool John Moores University in the U.K., lead author of a paper in the Astrophysical Journal announcing the discovery.

“The VLA images of Cygnus A from the 1980s marked the state of the observational capability at that time,” said Rick Perley, of the National Radio Astronomy Observatory (NRAO). “Because of that, we didn’t look at Cygnus A again until 1996, when new VLA electronics had provided a new range of radio frequencies for our observations.” The new object does not appear in the images made then.

“However, the VLA’s upgrade that was completed in 2012 made it a much more powerful telescope, so we wanted to have a look at Cygnus A using the VLA’s new capabilities,” Perley said.

Daniel and Rick Perley, along with Vivek Dhawan, and Chris Carilli, both of NRAO, began the new observations in 2015, and continued them in 2016.

“To our surprise, we found a prominent new feature near the galaxy’s nucleus that did not appear in any previous published images. This new feature is bright enough that we definitely would have seen it in the earlier images if nothing had changed,” said Rick Perley. “That means it must have turned on sometime between 1996 and now,” he added.

The scientists then observed Cygnus A with the Very Long Baseline Array (VLBA) in November of 2016, clearly detecting the new object. A faint infrared object also is seen at the same location in Hubble Space Telescope and Keck observations, originally made between 1994 and 2002. The infrared astronomers, from Lawrence Livermore National Laboratory, had attributed the object to a dense group of stars, but the dramatic radio brightening is forcing a new analysis.

What is the new object? Based on its characteristics, the astronomers concluded it must be either a supernova explosion or an outburst from a second supermassive black hole near the galaxy’s center. While they want to watch the object’s future behavior to make sure, they pointed out that the object has remained too bright for too long to be consistent with any known type of supernova.

“Because of this extraordinary brightness, we consider the supernova explanation unlikely,” Dhawan said.

While the new object definitely is separate from Cygnus A’s central supermassive black hole, by about 1500 light-years, it has many of the characteristics of a supermassive black hole that is rapidly feeding on surrounding material.

“We think we’ve found a second supermassive black hole in this galaxy, indicating that it has merged with another galaxy in the astronomically-recent past,” Carilli said. “These two would be one of the closest pairs of supermassive black holes ever discovered, likely themselves to merge in the future.”

The astronomers suggested that the second black hole has become visible to the VLA in recent years because it has encountered a new source of material to devour. That material, they said, could either be gas disrupted by the galaxies’ merger or a star that passed close enough to the secondary black hole to be shredded by its powerful gravity.

“Further observations will help us resolve some of these questions. In addition, if this is a secondary black hole, we may be able to find others in similar galaxies,” Daniel Perley said.

Rick Perley was one of the astronomers who made the original Cygnus A observations with the VLA in the 1980s. Daniel Perley is his son, now also a research astronomer.

“Daniel was only two years old when I first observed Cygnus A with the VLA,” Rick said. As a high school student in Socorro, New Mexico, Daniel used VLA data for an award-winning science fair project that took him to the international level of competition, then went on to earn a doctoral degree in astronomy.

Also at the time of those first VLA observations of Cygnus A, Carilli and Dhawan were office mates as graduate students at MIT.

Carilli, now NRAO’s Chief Scientist, was Rick’s graduate student while working as a predoctoral fellow at NRAO. His doctoral dissertation was on detailed analysis of 1980s VLA images of Cygnus A.

The National Radio Astronomy Observatory is a facility of the National Science Foundation, operated under cooperative agreement by Associated Universities, Inc.

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Media Contact:

Dave Finley, Public Information Officer
(575) 835-7302
dfinley@nrao.edu

Source:  National Radio Astronomy Observatory (NRAO)

Understanding Star Formation in the Nucleus of Galaxy IC 342

A BIMA-SONG radio map of the IC 342 central molecular zone; dots indicate locations of SOFIA/GREAT observations.

Credits: Röllig et al.

An international team of researchers used NASA’s Stratospheric Observatory for Infrared Astronomy, SOFIA, to make maps of the ring of molecular clouds that encircles the nucleus of galaxy IC 342. 

The maps determined the proportion of hot gas surrounding young stars as well as cooler gas available for future star formation. The SOFIA maps indicate that most of the gas in the central zone of IC 342, like the gas in a similar region of our Milky Way Galaxy, is heated by already-formed stars, and relatively little is in dormant clouds of raw material.

At a distance of about 13 million light years, galaxy IC 342 is considered relatively nearby. It is about the same size and type as our Milky Way Galaxy, and oriented face-on so we can see its entire disk in an undistorted perspective. Like our galaxy, IC 342 has a ring of dense molecular gas clouds surrounding its nucleus in which star formation is occurring. However, IC 342 is located behind dense interstellar dust clouds in the plane of the Milky Way, making it difficult to study by optical telescopes.

The team of researchers from Germany and the Netherlands, led by Markus Röllig of the University of Cologne, Germany, used the German Receiver for Astronomy at Terahertz frequencies, GREAT, onboard SOFIA to scan the center of IC 342 at far-infrared wavelengths to penetrate the intervening dust clouds. Röllig’s group mapped the strengths of two far-infrared spectral lines – one line, at a wavelength of 158 microns, is emitted by ionized carbon, and the other, at 205 microns, is emitted by ionized nitrogen.

The 158-micron line is produced both by cold interstellar gas that is the raw material for new stars, and also by hot gas illuminated by stars that have already finished forming. The 205-micron spectral line is only emitted by the hot gas around already-formed young stars. Comparison of the strengths of the two  spectral lines allows researchers to determine of the amount of warm gas versus cool gas in the clouds.

Röllig’s team found that most of the ionized gas in IC 342’s central molecular zone (CMZ) is in clouds heated by fully formed stars rather than in cooler gas found farther out in the zone, like the situation in the Milky Way’s CMZ. The team’s research was published in Astronomy and Astrophysics, volume 591.

“SOFIA and its powerful GREAT instrument allowed us to map star formation in the center of IC 342 in unprecedented detail,” said Markus Röllig of the University of Cologne, Germany, “These measurements are not possible from ground-based telescopes or existing space telescopes.”

Researchers previously used SOFIA’s GREAT spectrometer for a corresponding study of the Milky Way’s CMZ. That research, published in 2015 by principal investigator W.D. Langer, et. al, appeared in the journal Astronomy & Astrophysics 576, A1; an overview of that study can be found here. 

SOFIA is a Boeing 747SP jetliner modified to carry a 100-inch diameter telescope. It is a joint project of NASA and the German Aerospace Center, DLR. NASA’s Ames Research Center in California’s Silicon Valley manages the SOFIA program, science and mission operations in cooperation with the Universities Space Research Association headquartered in Columbia, Maryland, and the German SOFIA Institute (DSI) at the University of Stuttgart. The aircraft is based at NASA’s Armstrong Flight Research Center's Hangar 703, in Palmdale, California.

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For More Information

For more information about SOFIA, visit: http://www.nasa.gov/sofia • http://www.dlr.de/en/sofia

For information about SOFIA's science mission and scientific instruments, visit:  http://www.sofia.usra.edu • http://www.dsi.uni-stuttgart.de/index.en.html

Media Point of Contact

Nicholas A. Veronico
NVeronico@sofia.usra.edu • SOFIA Science Center
NASA Ames Research Center, Moffett Field, California

Editor: Kassandra Bell

Source: NASA/Galaxies

First radio detection of lonely planet disk shows similarities between stars and planet-like objects

Artists' impression of the gas and dust disk around the planet-like object OTS44. First radio observations indicate that OTS44 has formed in the same way as a young star. Image: Johan Olofsson (U Valparaiso & MPIA) 

First radio observations of the lonely, planet-like object OTS44 reveal a dusty protoplanetary disk that is very similar to disks around young stars. This is unexpected, given that models of star and planet formation predict that formation from a collapsing cloud, forming a central object with surrounding disk, should not be possible for such low-mass objects. Apparently, stars and planet-like objects are more similar than previously thought. The finding, by an international team led by Amelia Bayo and including several astronomers from the Max Planck Institute for Astronomy, has been published in Astrophysical Journal Letters.

A new study of the lonely, planet-like object OTS44 has provided evidence that this object has formed in a similar way as ordinary stars and brown dwarfs – a surprising result that challenges current models of star and planet formation. The study by a group of astronomers, led by Amelia Bayo of the University of Valparaiso and involving several astronomers from the Max Planck Institute for Astronomy, used the ALMA observatory in Chile to detect dust from the disk surrounding OTS44.

This detection yielded mass estimates for the dust contained in the disk, which place OTS44 in a row with stars and brown dwarfs (that is, failed stars with too little mass for sustained nuclear fusion): All these objects, it seems, have rather similar properties, including a similar ratio between the mass of dust in the disk and the mass of the central object. The findings supplement earlier research that found OTS44 is still growing by drawing matter from its disk onto itself – another tell-tale similarity between the object and young stars.

Similarities with young stars

Taken together, this is compelling evidence that OTS44 formed in the same way as stars and brown dwarfs, namely by the collapse of a cloud of gas and dust. But going by current models of star and planet formation, it should not be possible for an object as low-mass as OTS44 to form in this way. An alternative way, the formation of multiple objects in one go, with low-mass objects like OTS44 among them, is contradicted by the observations, which show no such companion objects anywhere near OTS44.

The strength of the radiation received from the dust at millimetre wavelength also suggests the presence of large, millimetre sized dust grains. This, too, is surprising. Under the conditions in the disk of a low-mass object, dust is not expected to clump together to reach this size (or beyond). Instead, the OTS44 dust grains appear to be growing – and might even be on the way of forming a mini-moon around the object; another similarity with stars and their planetary systems.

Amelia Bayo (University of Valparaiso), who led this research effort, says: “The more we know about OTS44, the greater its similarities with a young star. But its mass is so low that theory tells us it cannot have formed like a star!”

Thomas Henning of the Max Planck Institute for Astronomy adds: “It is amazing how an observatory like ALMA allows us to see half an Earth mass worth of dust orbiting an object with ten times the mass of Jupiter at a distance of 500 light-years. But the new data also shows the limit of our understanding. Clearly, there is still a lot to learn about the formation of low-mass astronomical objects!”

Background information

The work described here has been published as A. Bayo et al., "First millimeter detection of the disk around a young, isolated, planetary-mass object" in the May 18, 2017 edition of the Astrophysical Journal Letters.

Link to the article

The MPIA researchers involved are:

Viki Joergens, Yao Liu (also Purple Mountain Observatory, Nanjing, China), Johan Oloffson (also Universidad de Valparaíso), Thomas Henning, and Henrik Beuther in collaboration with Amelia Bayo (first author; Universidad de Valparaíso [UV]), Robert Brauer (University of Kiel), Javier Arancibia (UV), Paola Pinilla (University of Arizona), Sebastian Wolf, Jan Philipp Ruge (both University of Kiel), Antonella Natta (Dublin Institute for Advanced Studies and INAF-Osservatorio Astrofisico di Arcetri), Katharine G. Johnson (University of Leeds), Mickael Bonnefoy (IPAG Grenoble), and Gael Chauvin (IPAG Grenoble and Unidad Mixta Internacional Franco-Chilena de Astronomía, Santiago).

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In-depth description: First radio detection of lonely planet disk shows similarities between stars and planet-like objects

A new study of the lonely, planet-like object OTS44 has provided evidence that this object has formed in a similar way as ordinary stars and brown dwarfs – a surprising result that challenges current models of star and planet formation. The study by a group of astronomers, led by Amelia Bayo of the University of Valparaiso and involving several astronomers from the Max Planck Institute for Astronomy, used the ALMA observatory in Chile to detect dust from the disk surrounding OTS44.

From collapsing clouds to stars

Stars are formed when part of a giant cloud of gas collapses under its own gravity. But not every such collapse results in a star. The key criterion is one of mass: If the resulting object has sufficient mass, its gravity is strong enough to compress the central regions to such high densities, and heat them to such high temperatures, that nuclear fusion sets in, turning hydrogen nuclei (protons) into helium. 

The result is, by definition, a star: an object bound by its own gravity, with nuclear fusion in its core region, shining brightly as the energy liberated during the fusion processes is transported outwards.

Initially, the newly born star is surrounded by the remnants of the collapsed cloud. But in the natural course of collapsing, both the star and the cloud have begun to rotate at an appreciable rate. The rotation serves to flatten the material surrounding the young star, forming what is known as a protoplanetary disk of gas and dust. True to its name, this is where planets begin to form: The dust clumps to larger and larger grains and pebbles, increasing in size until, finally, the resulting objects are large enough to join together under the influence of its own gravity, forming solid planets thousands or even tens of thousands kilometers in diameter like our Earth, or collecting appreciable amounts of the surrounding gas to form gas giants, like Jupiter in our solar system.

If the object resulting from the collapse of the initial cloud has between 0.072 and 0.012 times the mass of the Sun – which corresponds to between 75 and 13 times the mass of Jupiter – what emerges is called a brown dwarf: a failed star, with some intermittent fusion reactions of deuterium (heavy hydrogen, consisting of one proton and one neutron) in the core regions, but no sustained, long-lasting phase of hydrogen fusion.

The strange case of OTS44

Can collapse produce even lighter objects, with similar masses as that of planets? A thorough analysis of the object OTS44, published in 2013 by a group of astronomers led by Viki Joergens from the Max Planck Institute for Astronomy (MPIA), presented strong evidence that this is indeed the case. OTS44 is a mere two million years old – in terms of stellar or planetary time-scales a newborn baby. The object has an estimated 12 Jupiter masses and is floating through space without a close companion. It is part of the Chamaeleon star forming region in the Southern constellation Chamaeleon, a little over 500 light-years from Earth, where numerous new stars are in the process of being born from collapsing clouds of gas and dust.

Just like a young star, OTS44 is surrounded by a disk of gas and dust, one of only four known low-mass objects (with about a dozen Jupiter masses or less) known to harbour a disk. Most conspicuously, OTS44 is still in the process of growing – that is, drawing material from the disk onto itself at a substantial rate. The disk itself is quite substantial; both this disk and the infalling material (accretion) are telltale signs of the standard mode of star formation – an indication that there is no fundamental difference between the formation of low-mass objects such as OTS44 and the formation of ordinary stars. OTS44 probably has the lowest mass of all objects where both a disk and infalling material have been detected.

Brown dwarf vs. planet-like object

We have so far avoided calling OTS44 either a brown dwarf or something else. In fact, nomenclature varies: Some astronomers call every object that has formed by direct collapse and is not a star a brown dwarf; by this criterion, only objects that form in disks around a central object can be planets. There is an alternative definition that hinges on the fact that an object like OTS44 does not have sufficient mass for a significant episode of deuterium fusion, and does not qualify as a brown dwarf on that account. We will compromise by referring to OTS44 as a planet-like object.

While the case of OTS44 shows that even planet-like objects can form by collapse, the details are anything but clear. For the formation of comparatively low-mass objects, be they very light stars, or brown dwarfs, or lonely planets, there are two main possibilities – but both are problematic in the case of OTS44. The first possibility is a direct collapse by a small isolated cloud. But going by our current knowledge, such a direct collapse should not be able to form such a planetary-mass object directly.

Much more likely is the alternative, namely that OTS44 could have formed as part of a larger collapsing cloud, when the collapsing regions fragmented, producing several objects, including OTS44, instead of a single larger body. But this does not mesh well with the observations. OTS44 is not now part of any multiple system. And even if we assume it was somehow ejected from such a system, OTS44 is still very young, and could not have moved far from its birth system – and that birth system would not have had time to dissolve completely into separate stars and/or brown dwarfs. But there is only a single object within 10,000 astronomical units (10,000 times the average Sun-Earth-distance) of OTS44, where the siblings of OTS44 could reasonably be expected, and there are no signs that this object was part of a collapsing, fragmenting cloud.

Tracking dust with ALMA

Clearly, there is more to be learned. That is what motivated a group of researchers led by Amelia Bayo (University of Valparaiso, Chile) to find out more about OTS44. The group includes a number of researchers from the Max Planck Institute for Astronomy (MPIA), as well as several former MPIA astronomers. Amelia Bayo was herself a postdoctoral researcher at MPIA before moving on to the University of Valparaiso, and in science, the international stations of an astronomer’s career often result in collaboration networks – in this case, a strategic collaboration between astronomers at the Universidad de Valparaiso in Chile and the MPIA's Planet and Star Formation Department led by Thomas Henning. The two institutions have an additional link: the Universidad de Valparaiso hosts an astronomical Max Planck Tandem Group, which commenced work in early 2017. With such tandem groups, the Max Planck Society fosters international cooperation with specific excellent research institutions.

In this particular case, the group gathered by Bayo for observing OTS44 included several members with the necessary skills and experience to make full use of the ALMA observatory: a constellation of 50 radio antennae for detecting millimeter and submillimeter radiation, operated by an international consortium and located in the Atacama desert in Chile.

The astronomers applied for ALMA time to observe the disk of OTS44 at millimeter wavelengths. Millimeter wavelengths are particularly suited to detect dust grains, which are present in protoplanetary disks (and account for one percent or more of the disk mass; these mass estimates are a subject of ongoing research). At least in the disks around more massive objects, these dust grains are the seeds of planet formation.

Dust mass and a surprisingly universal relation

For millimeter waves, the disk is optically thin, in other words: observations show the millimeter radiation from all the dust in the disk. (In an optically thick disk, we would only see radiation from the surface layers; the lower layers would be obscured by the upper layers.) This allowed the astronomers to estimate the total amount of dust in the disk – although the result still depends on the disk temperature. Temperature estimates for such disks, given the measured overall luminosity, give values between 5.5 Kelvin and 20 Kelvin for the OTS44 disk. This leads to estimates for the dust mass between 0.07 times the mass of the Earth (for the highest temperature estimate) and 0.64 Earth masses (for the lowest temperature).

These mass estimates confirm the similarity between stars and lower-mass objects: Systematic studies had shown earlier that for young stars and brown dwarfs, there is an approximate relationship between the mass of the central object and the mass of the dust in the surrounding disk. Inserting the data points for OTS44, the lonely planet-like object fits very well into the overall picture – indicating that the same overall mechanism is involved in all these cases, putting all central objects from about a hundredth to a few solar masses onto the same footing.

Dust grains of unusual size

Another interesting consequence stems from the fact that the disk is emitting significant amounts of millimeter radiation in the first place. This indicates the presence of certain amounts of grains of dust that are about a millimeter in size. Going by the current theories of planet formation, this is surprising: such larger dust grains should not have been able to form in a disk around such a low-mass object. In such a disk, the dust grains orbit the central mass like so many microscopic planets, following the laws first found by Johannes Kepler in the early 17th century. The gas of the disk, on the other hand, has internal pressure, which makes it rotation somewhat slower. The “head wind” felt by dust grains as they move through the slower gas should slow down the smaller grains, making them drift inwards before they finally fall onto the central object. There are arguments that these detrimental effects are particularly strong in lower-mass objects. From these calculations, it follows that the dust grains in the disk should have vanished when they were somewhat smaller – and should not have had the time to clump to form the observed millimetre-size grains.

Once the millimetre-size grains are there, the situation becomes less problematic – with their larger size, these grains do not feel the head wind as acutely as their smaller kin. But the presence of these larger grains poses a puzzle – and hints at the intriguing possibility that lonely planets might even be able to grow even larger dust grains, and may be even go as far as forming downright miniature moons, in their surrounding disks.

Similarities with young stars

All, in all, the new results make OTS44 look more and more similar to a young star, surrounded as it is by a disk, given the earlier evidence that it is still growing by incorporating material from that disk, and now with the new evidence that the ratio of the dust mass to the mass of the central object follows the same relation as for brown dwarfs and stars.

Evidently, the current models that preclude low-mass objects from forming in this particular way, via the collapse of a cloud of gas, are missing something. Observations like these new ones for OTS44 can be hoped to point us in the right direction for what that missing something might be, and thus towards a better understanding of the formation of low-mass objects in the universe.

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Science Contact

Prof. Dr. Thomas Henning
Director - Max Planck Institute for Astronomy; Professor at the University of Heidelberg
Phone:+49 6221 528-200
Email: henning@mpia-hd.mpg.de
Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg
Personal homepage - MPIA Heidelberg



Public Information Officer

Markus Pössel
Public Information Officer
Phone:(+49|0) 6221 528-261
Email: pr@mpia.de

Source: Max Planck Institute for Astronomy