Astronomy

Within the Solar System, most of our astrobiological research is aimed at Mars, which is considered to be the next-most habitable body beyond Earth. However, future efforts are aimed at exploring icy satellites in the outer Solar System that could also be habitable (like Europa, Enceladus, Titan, and more). This dichotomy between terrestrial (rocky) planets that orbit within their a system’s Habitable Zones (HZ) and icy moons that orbit farther from their parent stars is expected to inform future extrasolar planet surveys and astrobiology research.

In fact, some believe that exomoons may play a vital role in the habitability of exoplanets and could also be a good place to look for life beyond the Solar System. In a new study, a team of researchers investigated how the orbit of exomoons around their parent bodies could lead to (and place limits on) tidal heating – where gravitational interaction leads to geological activity and heating in the interior. This, in turn, could help exoplanet-hunters and astrobiologists determine which exomoons are more likely to be habitable.

The research was conducted by graduate student Armen Tokadjian and Professor Anthony L. Piro from the University of Southern California (USC) and The Observatories of the Carnegie Institution for Science. The paper that describes their findings (“Tidal Heating of Exomoons in Resonance and Implications for Detection“) recently appeared online and has been submitted for publication in the Astronomical Journal. Their analysis was inspired largely by the presence of multiplanet moon systems in the Solar System, such as those that orbit Jupiter, Saturn, Uranus, and Neptune.

Illustration of Jupiter and the Galilean satellites. Credit: NASA

In many cases, these icy moons are believed to have interior oceans resulting from tidal heating, where gravitational interaction with a larger planet leads to geological action in the interior. This, in turn, allows for liquid oceans to exist due to the presence of hydrothermal vents at at the core-mantle boundary. The heat and chemicals these vents release into the oceans could make these “Ocean Worlds” potentially habitable – something scientists have been hoping to investigate for decades. As Tokadjian explained to Universe Today via email:

“In terms of astrobiology, tidal heating may boost the surface temperature of a moon to a range where liquid water can exist. Thus even systems outside the habitable zone may warrant further astrobiological studies. For example, Europa hosts a liquid ocean due to tidal interactions with Jupiter, although it lies outside the Solar System’s ice line.”

Considering how plentiful “Ocean Worlds” are in the Solar System, it is likely that similar planets and multi-moon systems can be found throughout our galaxy. As Piro explained to Universe Today via email, the presence of exomoons has a lot of important implications for life, including:

• Large moons like our own can stabilize the planet’s axial tilt, so the planet has regular seasons
• Tidal interactions can prevent planets from tidally locking with their host star, impacting the climate
• Moons can tidally heat a planet, helping it maintain a molten core, which has many geological implications
• When a gaseous planet is in the habitable zone of a star, the moon itself can host life (think of Endor or Pandora)

In recent decades, geologists and astrobiologists have theorized that the formation of the Moon (ca. 4.5 billion years ago) played a major role in the emergence of life. Our planetary magnetic field is the result of its molten outer core rotating around a solid inner core and in the opposite direction of the planet’s own rotation. The presence of this magnetic field shields Earth from harmful radiation and is what allowed our atmosphere to remain stable over time – and not slowly stripped away by solar wind (which was the case with Mars).

An amazingly active Io, Jupiter’s “pizza moon,” shows multiple volcanoes and hot spots in this photo taken with Juno’s infrared camera. Credit: NASA/JPL-Caltech /SwRI/ASI/INAF/JIRAM/Roman Tkachenko

In short, the interactions between a planet and its satellites can affect the habitability of both. As Tokadjian and Piro showed in a previous paper using two candidate exoplanets as an example (Kepler-1708 b-i and Kepler-1625 b-i), the presence of exomoons can even be used to explore the interior of exoplanets. In the case of multi-moon systems, said Tokadjian and Piro, the amount of tidal heating depends on several factors. As Piro illustrated:

“As a planet raises tides on a moon, some of the energy stored by the deformation is transferred into heating the moon. This process is dependent on many factors, including the interior structure and size of the moon, the mass of the planet, planet-moon separation, and the moon’s orbital eccentricity. In a multi-moon system, the eccentricity can be excited to relatively high values if the moons are in resonance, leading to significant tidal heating.”

“In Armen’s work, he nicely shows, in analogy to the tidal heating we see for Io around Jupiter, that resonant interactions between multiple moons can efficiently heat exomoons. By ‘resonant,’ we mean the case where the periods of moons obey some integer multiple (like 2 to 1 or 3 to 2) so that their orbits gravitationally ‘kick’ each other regularly.”

In their paper, Tokadjian and Piro considered moons in a 2:1 orbital resonance around planets of varying size and type (i.e., smaller rocky planets to Neptune-like gas giants and Super-Jupiters). According to their results, the largest tidal heating will occur in moons that orbit rocky Earth-like planets with an orbital period of two to four days. In this case, the tidal luminosity was over 1000 times that of Io, and the tidal temperature reached 480 K (~207 °C; 404 °F).

Artist’s impression of the view from a hypothetical moon around an exoplanet orbiting a triple star system. Credit: NASA

These findings could have drastic implications for future exoplanet and astrobiology surveys, which are expanding to include the search for exomoons. While missions like Kepler have detected many exomoon candidates, none have been confirmed since exomoons are incredibly difficult to detect using conventional methods and current instruments. As Tokadjian explained, tidal heating could offer new methods for exomoon detection:

“First, we have the secondary eclipse method, which is when a planet and its moon move behind a star resulting in a dip in stellar flux observed. If the moon is significantly heated, this secondary dip will be deeper than what is expected from the planet alone. Second, a heated moon will likely expel volatiles like sodium and potassium through volcanism much like the case of Io. Detecting sodium and potassium signatures in the atmospheres of exoplanets can be a clue for exomoon origin.”

In the coming years, next-generation telescopes like the James Webb (which will be releasing its first images on July 12th) will rely on their combination of advanced optics, IR imaging, and spectrometers to detect chemical signatures from exoplanet atmospheres. Other instruments like the ESO’s Extremely Large Telescope (ELT) will rely on adaptive optics that will allow for Direct Imaging of exoplanets. The ability to detect chemical signatures of exomoons will greatly increase their ability to find potential signs of life!

Further Reading: arXiv

The post Tidal Heating Could Make Exomoons Much More Habitable (and Detectable) appeared first on Universe Today.

Making a 3D map of our galaxy would be easier if some stars behaved long enough to get good distances to them. However, red supergiants are the frisky kids on the block when it comes to pinning down their exact locations. That’s because they appear to dance around, which makes pinpointing their place in space difficult. That wobble is a feature, not a bug of these massive old stars and scientists want to understand why.

So, as with other challenging objects in the galaxy, astronomers have turned to computer models to figure out why. In addition, they are using Gaia mission position measurements to get a handle on why red supergiants appear to dance.

Artist’s impression of the red supergiant star Betelgeuse as it was revealed with ESO’s Very Large Telescope. It shows a boiling surface and material shed by the star as it ages. Credit: ESO/L.Calçada Understanding Red Supergiants

The population of red supergiants has several common characteristics. These are stars at least eight times the mass of the Sun, and they’re enormous. A typical one is at least 700 to 1,000 times the solar diameter. At 3500 K, they’re much cooler than our ~6000 K star, although measuring those temperatures is tricky. They are super bright in infrared light, but dimmer in visible light than other stars. They also vary in their brightness which (for some of them) may be related to that dancing motion. More on that in a moment.

If the Sun was a red supergiant, Earth wouldn’t be around. That’s because the star’s atmosphere would have reached out to Mars and swallowed our planet up. The best-known examples of these stellar behemoths are Betelgeuse and Antares. Red supergiants exist throughout the galaxy. There’s a population of them you can see at night in a nearby cluster called Chi Persei. It’s part of the well-known “Double Cluster”.

The Structure of Red Supergiants

So, we have this population of stars that don’t behave as expected and don’t lend themselves to easy measurements. Why is that? They’ve expanded so much that they end up with a very low surface gravity. Because of that, their convective cells (the structures that carry heat from inside to the surface) get pretty large. One cell covers as much as 20-30 percent of the radius of the star. That actually “interrupts” the brightness of the star.

The convection not only moves heat from the inside out, but also helps the star eject material into nearby space. And, we’re not talking small poofs of gas and plasma, either. A red supergiant can send a billion times more mass to space than the Sun does. All that action makes the star appear frothy and like its surface is boiling madly. In essence, it makes the star’s position appear to dance in the sky.

Red Supergiants in the Grand Scheme of Things

Red supergiant material becomes part of the chemical “inventory” of galaxies. The elements these stars create go on to become new stars and worlds. So, it helps to get a good understanding of how these stars lose their mass throughout their lives. It’s all part of understanding stellar evolution in the Milky Way and its impact on the cosmic environment. That’s why astronomers want to trace the total mass that these aging stars blow out to space. They also measure the stellar wind velocity and calculate the geometry of the cloud of “star stuff” that envelopes a red supergiant.

Now, what does this have to do with the dancing action? Well, the boiling of the convection cells and the buildup of a shell of material around the star adds to its variability. That is, it affects its brightness over time.

One way that astronomers use to determine a star’s exact position is by using its “photo-center”. That’s the center of light of the star. If the star varies in brightness (for whatever reason), that photo-center shifts. It won’t match the barycenter. (That’s the common center of gravity between the star and the rest of its system. It is a component in distance measurements.) In essence, the photo-center varies as the star’s brightness changes. Combined with the action of the huge convection cells, the star appears to dance in space.

A video of the simulation of a red supergiant surface shows that the constantly changing photosphere of the star (big image) leads to changes in the apparent position of the star’s center (small image at lower left). Credit: A. Chiavassa, Tl Grassi, et al. The Dance Changes the Distance Estimate

The red supergiant “position problem” attracted Andrea Chiavassa (Laboratoire Lagrange, the Exzellenzcluster ORIGINS, and the Max Planck Institute for Astrophysics). She and astronomer Rolf Kudritzki (Munich University of Observatory and the Institute of Hawai’i) and a science team created simulations of the boiling surfaces and variability of red supergiant brightness.

“The synthetic maps show extremely irregular surfaces, where the largest structures evolve on timescales of months or even years, while smaller structures evolve over the course of several weeks,” said Chiavassa. “This means that the position of the star is expected to change as a function of time.”

The team compared their model to stars in Chi Persei. That cluster was measured by the Gaia satellite, so the positions of most of its stars are very precise. Well, all but the red supergiants. “We found that the position uncertainties of red supergiants are much larger than for other stars. This confirms that their surface structures change dramatically with time as predicted by our calculations”, explained Kudritzki.

This change in observable position provides a solution to understanding the shifting positions of red supergiants. That, in turn, presents difficulties in measuring exact distances to many of these stars. The current model also gives clues to the evolution of these objects. But, knowing what’s causing the stars to dance offers a path to a solution when calculating their distances. Future models will help astronomers refine those distances, and provide more insight into what’s happening to these stars as they age.

For More Information

Dancing Pattern of Red Supergiants on the Sky

Probing Red Supergiant Dynamics Through Photo-center Displacements measured by Gaia

The post Red Supergiant Stars Bubble and Froth so Much That Their Position in the Sky Seems to Dance Around appeared first on Universe Today.

The United States Space Force has activated a new unit that is tasked with providing "critical intelligence on threat systems, foreign intentions and activities in the space domain."

Stars form inside massive clouds of gas and dust called molecular clouds. The Nebular Hypothesis explains how that happens. According to that hypothesis, dense cores inside those clouds of hydrogen collapse due to instability and form stars. The Nebular Hypothesis is much more detailed than that short version, but that’s the basic idea.

The problem is that it only explains how single stars form. But about half of the Milky Way’s stars are binary pairs or multiple stars. The Nebular Hypothesis doesn’t clearly explain how those stars form.

Most stars about the same mass as our Sun or larger aren’t single stars. Most are members of multiple star systems, especially binary stars. While the nebular theory explains how single stars form, there are competing theories for how multiple stars form.

First of all, after a molecular cloud collapses into a star, it forms a rotating disk of gas and dust around the young protostar, called a circumstellar disk. One theory explaining how multiple stars form says that a pair or more of young protostars are fragments of a parent disk that was once much larger. Another theory says that the young protostars form independently, then one captures the other in an orbital arrangement.

T Tauri stars are less than 10 million years old and represent the type of young stars found in stellar nurseries like the Orion Cloud Complex. It shows the disc surrounding the young star, out of which planets will eventually form. The researchers behind this new study examined the dense cores that form young stars like this to find differences between cores that formed multiple stars and those that formed single stars like our Sun. Image Credit: ALMA (ESO/NAOJ/NRAO)

But when stars form inside a molecular cloud, it begins with a dense core inside the cloud. That core initiates the gravitational collapse that gathers enough gas in one place to form a star. The question is, what’s different about some of those cores that cause multiple stars to form versus single stars?

That’s what astronomers at Hawaii’s James Clerk Maxwell Telescope (JCMT) wanted to understand.

The JCMT is a 15-meter radio telescope at Mauna Kea Observatory in Hawaii. The telescope’s submillimeter observations allow it to observe the molecular clouds where stars are born. The researchers used it to observe the Orion Molecular Cloud Complex (OMCC), the closest active stellar nursery to Earth, which is still about 1500 light-years away. The OMCC contains two giant molecular clouds (GMCs), Orion A and Orion B. They also used observations from ALMA and Japan’s Nobeyama Telescope.

The team watched multiple star systems forming in the Orion Complex and made important discoveries about the process. They presented their findings in a paper published in The Astrophysical Journal. The paper is “ALMA Survey of Orion Planck Galactic Cold Clumps (ALMASOP): How Do Dense Core Properties Affect the Multiplicity of Protostars?” The first author is Qiuyi Luo, a Ph.D. student at Shanghai Astronomical Observatory.

“During the transition phase from a prestellar to a protostellar cloud core, one or several protostars can form within a single gas core,” the paper stars. “The detailed physical processes of this transition, however, remain unclear.”

For this study, the team of researchers collected observations of 43 protostellar cores in the Orion molecular cloud complex with the JCMT. Then they used the powerful ALMA telescope to examine the interior structure of the cores.

This image shows the G205.46-14.56 clump located in the Orion Molecular Cloud Complex. The yellow contours show the dense cores discovered by JCMT, and the zoomed-in pictures show the 1.3mm continuum emission of ALMA observation. These observations give insight into the formation of various stellar systems in dense cores. Image Credit: Qiuyi Luo et al. 2022.

The research shows that about 30% of the 43 dense cores form binary or multiple stars, and the remainder forms only single stars. The astronomers measured and estimated the sizes and masses of the cores. They found that binary/multiple cores have higher densities and masses, although the sizes of all the cores aren’t much different.

This figure from the study shows the exemplar core G196.92-10.37. (a) is a JCMT image with a Spitzer image superimposed on it. The yellow circle is the zoomed-in region in (b.) (b) shows continuum contour levels. (c) shows ALMA data and also indicates that the core is forming three stars: A, B, and C. Image Credit: Qiuyi Luo et al. 2022.

“This is understandable,” said first author Qiuyi Luo. “Denser cores are much easier to fragment due to the perturbations caused by self-gravity inside molecular cores.”

From there, the team turned to Japan’s 45-meter Nobayama radio telescope. They observed what’s known as the N2H+ J=1-0 molecular line in all 43 dense cores. N2H+ is diazenylium, one of the first ions ever found in interstellar clouds. This molecular line is easily observed through Earth’s atmosphere with fine precision. Astronomers use it to map the density and velocity of the gas in molecular clouds.

Those observations showed that dense cores that form multiple stars are more turbulent than cores that form single stars.

This figure from the study shows the Mach number for gas in the dense cores as measured with the N2H+ line. Higher Mach numbers mean more turbulence, and this figure shows that binary and multiple star cores are more turbulent than cores forming single stars. Image Credit: Qiuyi Luo et al. 2022.

“These Nobeyama observations provide a good measurement of turbulence levels in dense cores. Our findings indicate that binary/multiple stars tend to form in more turbulent cores,” said Prof. Ken’ichi Tatematsu, who led the Nobeyama observations.

Lead author Qiuyi Luo summarized the study’s findings in a press release. “In a word, we found that binary/multiple stars tend to form in denser and more turbulent molecular cores in this study.”

This figure from the study shows the gas velocity in two of the dense cores. Blue indicates lower velocity, and red indicates higher velocity. The arrows show the directions of the local increasing velocity gradients, with the lengths indicating their magnitudes. The top core, labelled in orange, is a binary core, and the bottom core, labelled in black, is a single core. Image Credit: Qiuyi Luo et al. 2022.

Co-author Sheng-Yuan Liu added, “The JCMT has proven to be a great tool for uncovering these stellar nurseries for ALMA follow-ups. With ALMA providing unprecedented sensitivity and resolution so that we can do similar studies toward a much sample of larger dense cores for a more thorough understanding of star formation.”

The researchers also found that the stars in each binary or multiple arrangement are usually at very different evolutionary stages. The more evolved protostars are generally further from the center of the dense cores than their younger counterparts. This indicates that as stars evolve, they migrate out of their natal cores.

This study shows some differences between cores that form single stars versus cores that form binary and multiple stars. But it’s only the beginning: there’s much more to learn and many more questions.

One of the questions is what role do magnetic fields play in star formation? Star-forming clouds can be highly magnetized. Magnetic fields from the interstellar medium thread their way through star-forming clouds, and astronomers know that magnetic fields can affect the star formation rate. Do they play a role in determining if a single star forms versus multiple stars?

This figure is from a separate study that simulated the effect of magnetic fields on star-forming regions. The left is a simulated star-forming region without a magnetic field, right is with a magnetic field. Each white circle is a protostar, and red indicates gas moving at high velocities. Without magnetism, the mass collapses into a central region with less outflowing gas. With magnetism, the protostars are more spread out, and more gas is escaping. This seems to indicate that magnetic fields inhibit the formation of dense structures. Image Credit: Krumholz and Federrath 2019.

“We have yet to look at the effect of magnetic fields in our analysis,” said corresponding author Tie Liu, who was also the lead for the ALMA observations. “Magnetic field may suppress the fragmentation in dense cores, so we are excited to focus the next stage of our research on this area using the JCMT.”

The authors point out that the low sample size hampers their results. Forty-three dense cores may not be enough data to draw conclusions from, especially because they’re all from the same molecular cloud. The study was also limited by the resolution of the various observatories and telescopes used in the study.

“Our results could be further tested using future higher spatial and spectral resolution observations toward a more complete dense core sample in various molecular clouds that are in widely different environments,” they conclude.

More:

• Press Release: JCMT Astronomers find that denser and more turbulent environments tend to form multiple stars
• New Research: ALMA Survey of Orion Planck Galactic Cold Clumps (ALMASOP): How Do Dense Core Properties Affect the Multiplicity of Protostars?
• Related Research: The Role of Magnetic Fields in Setting the Star Formation Rate and the Initial Mass Function
• Universe Today: Nearby Supernovae Exploded Just a few Million Years Ago, Leading to a Wave of Star Formation Around the Sun

The post This is How You Get Multiple Star Systems appeared first on Universe Today.

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