Astronomy

Is there a known relation to patterns of star formation in a molecular cloud?

Is there a known relation to patterns of star formation in a molecular cloud?


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I was wondering if there was a known pattern to what mass of stars can form in a molecular cloud depending on what had formed before them? Having read that when blue giants form and go supernova in a relatively short period compared to other stars main sequence time scales and when these massive blue giant go supernova the force of their explosion can create new star formation activity as the supernova pushes on the molecular clouds gas structure. This made me wonder if there was a known chain reaction of specific masses of stars that will form or even without the blue giants forming if there was a known pattern of what stars will form given enough density in the molecular cloud and if the chain reaction of forming stars has a known pattern?


The secret life of the Orion Nebula

One of the most well-known astronomical objects in the sky is the Orion Nebula and this image depicts the wider region of the Orion Molecular Cloud Complex that is home to it. Image credit: © Patrick Gilliland / Royal Museums Greenwich. Star formation is basically a simple process: You take a very cold cloud consisting of hydrogen gas and a sprinkling of dust and leave the system to get on with it. Then, within the space of a few million years, the sufficiently cold regions will collapse under their own gravity and form new stars.

Reality is a bit more complicated. A particular feature is that there seem to be two types of star formation. In conventional, smaller molecular clouds, only one or a few stars form &mdash until the gas has dispersed over a period of three million years or so. Larger clouds survive around ten times longer. Whole star clusters are born simultaneously in these clouds and very massive suns are formed.

Why is it that so many stars are created during these approximately 30 million years? In astronomical terms, this period is quite short. Most attempts at an explanation are based on a kind of chain reaction in which the formation of the first stars in the cloud triggers the formation of further stars. Supernova explosions of the most massive (and therefore shortest-lived) stars which have just formed could be one explanation, as their shock waves compress the cloud material and thus create the seeds for new stars.

Amelia Stutz and Andrew Gould from the Max Planck Institute for Astronomy in Heidelberg are pursuing a different approach and bringing gravity and magnetic fields into play. To test their idea, they undertook a detailed investigation of the Orion Nebula, 1300 light-years away. The bright red gas cloud with the complex pattern is one of the best-known celestial objects.

The starting point for Stutz and Gould’s considerations are maps of the mass distribution in a structure known as an “integral-shaped filament” because of its form &mdash it resembles that of a curved integral sign &mdash and which includes the Orion Nebula in the central section of the filament. The Heidelberg-based researchers also drew on studies of the magnetic fields in and around this object. Birthplace of the stars: The integral-shaped filament, the two star clusters above the filament, and cloud L1641 in the south can be seen on these images of the Orion A star formation region. The picture on the left shows a density map compiled with data from ESA’s Herschel Space Observatory, the one on the right an infrared image taken by NASA’s Wide-field Infrared Survey Explorer (WISE). The photo in the centre is a combination of both images. Image credits: © A. M. Stutz / MPIA. The data show that magnetic fields and gravitation have approximately the same effect on the filament. Taking this as their basis, the two astronomers developed a scenario in which the filament is a flexible structure undulating to and fro. The usual models of star formation, on the other hand, are based on gas clouds which collapse under their own gravitation.

Important proof for the new idea is the distribution of protostars and infant suns in and around the filament. Protostars are the precursors of suns: they contract even further until their nuclei have reached densities and temperatures which are high enough for nuclear fusion reactions to start in a big way. This is the point at which a star is born.

Protostars are light enough to be dragged along when the filament undulates backwards and forwards. Infant stars, in contrast, are much more compact and are simply left behind by the filament or launched into the surrounding space as if fired from a slingshot. The model can thus explain what the observation data actually show: protostars are to be found only along the dense spine of the filament infant stars, on the other hand, are found mainly outside the filament.

This scenario has the potential for a new mechanism which could explain the formation of whole star clusters on (in astronomical terms) short timescales. The observed positions of the star clusters suggest that the integral-shaped filament originally extended much further towards the north than it does today. Over millions of years, one star cluster after another seems to have formed, starting from the north. And each finished star cluster has scattered the gas-dust mixture surrounding it as time has passed.

This is why we now see three star clusters in and around the filament: the oldest cluster is furthest away from the northern tip of the filament the second one is closer and is still surrounded by filament remnants the third one, in the centre of the integral-shaped filament, is just in the process of growing.

The interaction of magnetic fields and gravity allows certain types of instabilities, some of which are familiar from plasma physics, and which could lead to the formation of one star cluster after another. This hypothesis is based on observational data for the integral-shaped filament. It is not a mature model for a new mode of star formation, however. Theoreticians have first to carry out appropriate simulations and astronomers have to make further observations.

Only when this preparatory work is complete will it be clear whether the molecular cloud in Orion represents a special case. Or whether the birth of star clusters in a medley of magnetically trapped filaments is the usual route to forming whole clusters of new stars in space within a short period.


Contents

Within the Milky Way, molecular gas clouds account for less than one percent of the volume of the interstellar medium (ISM), yet it is also the densest part of the medium, comprising roughly half of the total gas mass interior to the Sun's galactic orbit. The bulk of the molecular gas is contained in a ring between 3.5 and 7.5 kiloparsecs (11,000 and 24,000 light-years) from the center of the Milky Way (the Sun is about 8.5 kiloparsecs from the center). [3] Large scale CO maps of the galaxy show that the position of this gas correlates with the spiral arms of the galaxy. [4] That molecular gas occurs predominantly in the spiral arms suggests that molecular clouds must form and dissociate on a timescale shorter than 10 million years—the time it takes for material to pass through the arm region. [5]

Vertically to the plane of the galaxy, the molecular gas inhabits the narrow midplane of the galactic disc with a characteristic scale height, Z, of approximately 50 to 75 parsecs, much thinner than the warm atomic (Z from 130 to 400 parsecs) and warm ionized (Z around 1000 parsecs) gaseous components of the ISM. [7] The exception to the ionized-gas distribution are H II regions, which are bubbles of hot ionized gas created in molecular clouds by the intense radiation given off by young massive stars and as such they have approximately the same vertical distribution as the molecular gas.

This distribution of molecular gas is averaged out over large distances however, the small scale distribution of the gas is highly irregular with most of it concentrated in discrete clouds and cloud complexes. [3]

Giant molecular clouds Edit

A vast assemblage of molecular gas that has more than 10 thousand times the mass of the Sun [9] is called a giant molecular cloud (GMC). GMCs are around 15 to 600 light-years in diameter (5 to 200 parsecs) and typical masses of 10 thousand to 10 million solar masses. [10] Whereas the average density in the solar vicinity is one particle per cubic centimetre, the average density of a GMC is a hundred to a thousand times as great. Although the Sun is much more dense than a GMC, the volume of a GMC is so great that it contains much more mass than the Sun. The substructure of a GMC is a complex pattern of filaments, sheets, bubbles, and irregular clumps. [5]

Filaments are truly ubiquitous in the molecular cloud. Dense molecular filaments will fragment into gravitationally bound cores, most of which will evolve into stars. Continuous accretion of gas, geometrical bending, and magnetic fields may control the detailed fragmentation manner of the filaments. In supercritical filaments observations have revealed quasi-periodic chains of dense cores with spacing of 0.15 parsec comparable to the filament inner width. [11]

The densest parts of the filaments and clumps are called "molecular cores", while the densest molecular cores are called "dense molecular cores" and have densities in excess of 10 4 to 10 6 particles per cubic centimeter. Observationally, typical molecular cores are traced with CO and dense molecular cores are traced with ammonia. The concentration of dust within molecular cores is normally sufficient to block light from background stars so that they appear in silhouette as dark nebulae. [12]

GMCs are so large that "local" ones can cover a significant fraction of a constellation thus they are often referred to by the name of that constellation, e.g. the Orion Molecular Cloud (OMC) or the Taurus Molecular Cloud (TMC). These local GMCs are arrayed in a ring in the neighborhood of the Sun coinciding with the Gould Belt. [13] The most massive collection of molecular clouds in the galaxy forms an asymmetrical ring about the galactic center at a radius of 120 parsecs the largest component of this ring is the Sagittarius B2 complex. The Sagittarius region is chemically rich and is often used as an exemplar by astronomers searching for new molecules in interstellar space. [14]

Small molecular clouds Edit

Isolated gravitationally-bound small molecular clouds with masses less than a few hundred times that of the Sun are called Bok globules. The densest parts of small molecular clouds are equivalent to the molecular cores found in GMCs and are often included in the same studies.

High-latitude diffuse molecular clouds Edit

In 1984 IRAS identified a new type of diffuse molecular cloud. [16] These were diffuse filamentary clouds that are visible at high galactic latitudes. These clouds have a typical density of 30 particles per cubic centimeter. [17]

Star formation Edit

The formation of stars occurs exclusively within molecular clouds. This is a natural consequence of their low temperatures and high densities, because the gravitational force acting to collapse the cloud must exceed the internal pressures that are acting "outward" to prevent a collapse. There is observed evidence that the large, star-forming clouds are confined to a large degree by their own gravity (like stars, planets, and galaxies) rather than by external pressure. The evidence comes from the fact that the "turbulent" velocities inferred from CO linewidth scale in the same manner as the orbital velocity (a virial relation).

Physics Edit

The physics of molecular clouds is poorly understood and much debated. Their internal motions are governed by turbulence in a cold, magnetized gas, for which the turbulent motions are highly supersonic but comparable to the speeds of magnetic disturbances. This state is thought to lose energy rapidly, requiring either an overall collapse or a steady reinjection of energy. At the same time, the clouds are known to be disrupted by some process—most likely the effects of massive stars—before a significant fraction of their mass has become stars.

Molecular clouds, and especially GMCs, are often the home of astronomical masers.


Gazing into Magnetized Interstellar Clouds to Understand How Stars Are Born

To make a star, the universe needs three major ingredients: gravity, turbulence, and a magnetic field. Along with a mixture of gas and dust, the interplay of these three forces manages to achieve the conditions necessary to create the twinkling, bright balls of light we’re all so familiar with gazing at in the night sky. Astronomers have been probing the galaxy for years in order to understand how each factor plays a role when a star is born. Now, one piece of the puzzle just became a bit clearer.

In a paper published in Nature Astronomy, BU astronomer Thushara Pillai finds that magnetic fields within molecular clouds—formations made of cosmic dust and gas—help feed young, growing stars. Pillai’s research helps solve some of the mysteries about how stars come to be and to what extent molecular clouds in interstellar space play a role.

By using high-resolution telescope images of polarized infrared light, Pillai is able to pinpoint where the magnetic field surrounding the group of stars becomes weaker, changes direction, and is pulled back toward the cluster along with the surrounding particles and dust.

These charged particles flow along the field, almost like a conveyor belt, into the cluster of stars. It’s believed that many stars form at the intersection of filaments, vast patterns of cold cosmic gas threaded through every molecular cloud in the interstellar medium—the region of space beyond the bubble of heat and charged particles that emanate from our sun, called the heliosphere.

“The change in direction of the magnetic field…that is what is new about this science,” says Pillai, a College of Arts & Sciences senior research scientist at BU’s Institute for Astrophysical Research. She made the discovery by observing star formation in Serpens South, a nearby cluster of about 60 stars, only about 1,400 light-years away from Earth, that was discovered in 2008. The stars in the cluster are relatively young. Their proximity to Earth and young age provided Pillai with an ideal vantage point for observing the conditions of the molecular cloud surrounding the young stars.

Once stars form within a molecular cloud, the environment is altered in a way that makes it impossible to tell what things were like when the process began. “By capturing filaments like Serpens South in a very young state, we capture them in a very unique moment,” Pillai says.

Magnetic fields in the universe are invisible to most telescopes—plus, the interstellar magnetic field is about 10,000 times weaker than Earth’s magnetic field, making measuring magnetic strength extra difficult. By using data from NASA’s Stratospheric Observatory for Infrared Astronomy (SOFIA)—a specially outfitted Boeing 747 airplane that flies into the stratosphere and observes the universe with an onboard, high-powered camera called HAWC+ that is capable of capturing infrared wavelengths of light invisible to the eye—Pillai, along with BU astronomer Dan Clemens and their collaborators can see dust grains as they align perpendicular to the fields, allowing them to infer the strength and direction of the magnetic field. This is a level of detail that couldn’t be achieved with a ground-based telescope, Pillai explains. She compared data from HAWC+ to different observations captured by a telescope based on land.

Clemens, a CAS professor and chair of astronomy, says the combination of observations from SOFIA and ground telescopes creates a powerful new tool for revealing the vital details of how molecular clouds play a role in star formation.

Pillai captured this stunning image of Serpens South by layering four different telescope wavelengths that expose different aspects of the region at the same time. The color image, originally obtained by the Spitzer Space Telescope, revealed the young stars and the dusty molecular cloud out of which they are born. This image was overlaid with data from SOFIA, revealing the dazzling, threadlike black structure that illustrates the direction of the magnetic field. The bright spots, which look almost like green beads on a celestial string, are the young stars, called protostars, that formed at the intersection of the filament. With this full picture, Pillai and her team can see that stars much like our own sun prefer to form in dense star clusters at these intersections.

“We have reached a point with the SOFIA telescope where we can resolve the structure of the cloud in a way that we can see where the magnetic field actually starts to become weak, pulled in by the tremendous gravitational attraction of the cluster,” says Pillai. The flows of gas traveling toward the cluster center are so dense that they carry and warp the magnetic fields, eventually compressing together so densely that the region collapses under the influence of its own gravity, raising the temperature so high that atoms fuse and give birth to a new star.

This research was supported by NASA, the German Space Center, the Universities Space Research Association, the National Science Foundation, the Bonn-Cologne Graduate School, the Brazilian National Council for Scientific and Technological Development, and Fundação de Amparo à Pesquisa do Estado de Minas Gerais.


The laws of star formation challenged

An international team led by researchers at CNRS, Université Grenoble Alpes and the French Alternative Energies and Atomic Energy Commission (CEA) has challenged currently held ideas about star formation. The unprecedented resolution of the observations obtained using the Atacama Large Millimetre/Submillimetre Array (ALMA) enabled them to measure the quantity of high-mass star-forming cores in a remote, very active region of our Galaxy, and show that there is a higher proportion of them there than expected. Published in Nature Astronomy, the findings could challenge the widespread assumption that the mass distribution of a population of star-forming cores is identical to that of the stars they spawn.

In space, hidden behind the dusty veils of nebulae, clouds of gas clump together and collapse, forming the structures from which stars are born: star-forming cores. These cluster together, accumulate matter and fragment, eventually giving rise to a cluster of young stars of various masses, whose distribution was described by Edwin Salpeter as an astrophysical law in 1955.

Astronomers had already noticed that the ratio of massive objects to non-massive objects was the same in clusters of star-forming cores as in clusters of newly-formed stars. This suggested that the mass distribution of stars at birth, known as the IMF1, was simply the result of the mass distribution of the cores from which they formed, known as the CMF2. However, this conclusion resulted from the study of the molecular clouds closest to our Solar System, which are not very dense and therefore not very representative of the diversity of such clouds in the Galaxy. Is the relationship between the CMF and the IMF universal? What do we observe when we look at denser, more distant clouds?

These were the questions asked by researchers at the Grenoble Institute of Planetology and Astrophysics (CNRS/Université Grenoble Alpes) and the Astrophysics, Instrumentation and Modelling Laboratory, (CNRS/CEA/Université Paris Diderot)3 when they started to observe the active star-formation region W43-MM1, whose structure is far more typical of molecular clouds in our Galaxy than those observed previously. Thanks to the unprecedented sensitivity and spatial resolution of the ALMA antenna array in Chile, the researchers were able to establish a statistically robust core distribution over an unmatched range of masses, from solar-type stars to stars 100 times more massive. To their surprise, the distribution did not obey Salpeter's 1955 law.

It turned out that, in the W43-MM1 cloud, there was an overabundance of massive cores, while less massive cores were under-represented. These findings call into question not only the relationship between the CMF and the IMF, but even the supposedly universal nature of the IMF. The mass distribution of young stars may not be the same everywhere in our Galaxy, contrary to what is currently assumed. If this turns out to be the case, the scientific community will be forced to re-examine its calculations about star formation and, eventually, any estimates that depend on the number of massive stars, such as the chemical enrichment of the interstellar medium, the numbers of black holes and supernovae, etc.

The teams will continue their work with ALMA within a consortium of around forty researchers. Their aim is to study 15 regions similar to W43-MM1 in order to compare their CMFs and ascertain whether the characteristics of this cloud can be generalised.

© ESO/ALMA/F. Motte/T. Nony/F. Louvet/Nature Astronomy.

The active star-formation region W43-MM1, as observed using the world's largest millimetre interferometer, ALMA. The high number of star formation sites, known as cores and here identified by ellipses, are evidence of the intense star formation activity in this region.


The Formation of Massive Stars

Editor’s note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites the original can be viewed at astrobites.org.

Title: The Role of Outflows, Radiation Pressure, and Magnetic Fields in Massive Star Formation
Authors: Anna Rosen and Mark Krumholz
First Author’s Institution: Center for Astrophysics | Harvard & Smithsonian
Status: Accepted to ApJ

This newly formed star at the heart of the Orion Nebula is blowing a bubble that’s preventing further star formation around it. [NASA/SOFIA/Pabst et al.]

Pushing the Boundaries

The fact that massive stars are so rare is reflective of a more general problem with star formation: its inefficiency. Estimates of star formation efficiencies are as low as 33%. As massive stars begin to form, they launch powerful molecular outflows from their poles. These jets can interact with the surrounding molecular cloud and eject large quantities of material. This, in combination with other feedback mechanisms, limits the star’s ability to accrete material, ultimately limiting its final mass. Knowing the upper limit of just how massive a star can be is incredibly valuable, for it allows us to set the upper boundary of the initial mass function. This function models the initial distribution of stellar masses for a given population of stars, and it is impossible to simulate the evolution of a stellar population without one. This is where massive stars are important, for they are the dominant source of radiative feedback and energy injection into the ISM through supernovae. So, to help determine these upper mass limits, we must simulate the processes that inhibit star formation in as much detail as possible.

Massively Magnetic Outflows Radiation Pressure (Games)

What does an MMORPG like EVE Online have in common with a radiative magnetohydrodynamic simulation? An insane amount of calculations. As the name implies, such a simulation models radiative transfer in addition to magneto-fluid dynamics. The simulation models stellar radiation fields and collimated outflows (the flow is parallel everywhere) for every star, and also factors in the indirect radiative feedback from dust, magnetic fields, and supersonic turbulence. The authors ran three main simulations: TurbRad (radiative feedback only), TurbRad+OF (adds collimated outflows), and TurbRad+OFB (adds magnetic fields).

Figure 1: Density plots for the authors’ three simulations, with the most massive star shown in the center of each panel. [Rosen & Krumholz 2020]

In Figure 1, after the stellar mass of the protostellar core exceeds 30 solar masses, we see several pressure-dominated bubbles expanding away from the star (this is most noticeable in the middle row TurbRad+OF simulation). This process is known as the “flashlight effect”, where thick material is beamed away from the poles, causing low-density bubbles to expand outwards.

Go With The Flow

Over time, strong entrained outflows begin to break through the protostellar core and eject large quantities of material, as can be seen in Figure 2.

Figure 2: Projected y–z densities of the entrained outflows. [Rosen & Krumholz 2020]

Don’t Forget the B Field

Figure 3: The star formation efficiencies for the total stellar population (top) and the primary, most massive star (bottom) as functions of simulation time for the three different simulations. [Rosen & Krumholz 2020]

In such an involved phenomena like star formation, there are many nuances. Magnetic fields slow the growth rate of stars by helping to prevent the core from fragmenting, however there are several non-ideal effects (such as the Hall effect) that could theoretically impact the star formation process. These non-ideal effects were not considered, although it is unknown whether such effects have any noticeable impact on star formation efficiencies.

A Joint Effort

This comprehensive series of simulations, one of the first to account for so many factors, demonstrates the role of outflows, magnetic fields and radiation pressure in limiting the formation of massive stars and reducing the overall star formation efficiency. This study shows that feedback from outflows dominates the feedback from radiation pressure, and that magnetic fields further inhibit star formation. Importantly, both outflows and magnetic fields are needed to reproduce the low efficiencies obtained from observations.

About the author, Mitchell Cavanagh:

Mitchell is a PhD student in astrophysics at the University of Western Australia. His research is focused on the applications of machine learning to the study of galaxy formation and evolution. Outside of research, he is an avid bookworm and enjoys gaming, languages, and code jams.


Why is massive star formation quenched in galaxy centers?

A study led by a researcher at the IAC and published today in Nature Astronomy points to the role of the magnetic field as responsible for decelerating the formation of massive stars in the center of galaxies. Without this process the Big Bang would be questioned.

The current cosmological model to explain our universe, the "Big Bang" model, aims to describe all the phenomena we observe, which includes the galaxies and their evolution from earliest times to the present day. One of the major problems faced by the standard form of this model is that it has predicted a star formation rate -speed at which new stars are born- which is far too big. All the star forming material in galaxies should have been turned into stars when the universe had only a fraction of its present age, 13,8 billion years. However, over half the galaxies we see, mainly the spirals, are very actively forming stars right now. This discrepancy between theoretical prediction and observation has forced to look much more closely at processes which can slow down the rate of star formation during the lifetimes of galaxies, collectively known as "star formation quenching." Without quenching the standard Big Bang model fails to predict the universe as we know it.

There have been a number of mechanisms proposed for quenching, for example "feedback" from supernovae or active galactic nuclei which breaks up the star forming clouds and reduces the star formation rate, but the measurement and verification of yet other possible processes is of great importance. One of this mechanisms has just been published in Nature Astronomy led by the Instituto de Astrofísica de Canarias (IAC) researcher, Fatemeh Tabatabaei. The study points to magnetic fields and cosmic rays as responsible for massive stars forming slowly.

Studying in great detail the star formation parameters of the central region of the spiral galaxy NGC 1097, they concluded that the presence of a relatively large magnetic field is acting as a quenching agent, due to a magnetic field that exerts a pressure within a gas cloud which can slow down or stop its tendency to collapse and form stars. But the results have gone further, because researchers have shown that this mechanism is in fact working around the center of NGC 1097. They combined observations in the visible and the near infrared from the Hubble Space Telescope with radio observations from the Very Large Array and the Submillimeter Array to explore the effect of the turbulence, stellar radiation, and magnetic field on massive star formation in the galaxy's nuclear ring. This ring contains a number of clearly distinct zones where stars are forming inside huge molecular cloud complexes. The principal result they obtained was an inverse relation between the star formation rate in a given molecular cloud and the magnetic field within it: the larger the field the slower is the star formation rate.

"To do this, we made a specific separation of the magnetic field and its energy from other sources of energy in the interstellar medium, which are the thermal energy, and the general non-thermal but non-magnetic energy" explains Fatemeh Tabatabaei. "Only by combining the high quality observations at very different wavelengths could we do this and when we separated these energy sources the effect of the magnetic field was surprisingly clear." Almudena Prieto, another of the authors adds in the same sense: "although I have been working on the central zone of NGC 1097 at optical and infrared wavelengths for some time, only when we took into account the magnetic field could we realize its relevance in decreasing the rate at which stars are formed."

This result has several interesting consequences and throws light on several types of interrelated astrophysical puzzles. Firstly, as the magnetic field does not allow very large molecular clouds to collapse and form stars, star formation can occur only after the clouds break up into smaller clouds. This means that this region will have a higher fraction of low-mass stars than in other zones of the galaxy. The tendency of very massive galaxies to contain a high fraction of low-mass stars at their centers is a recent discovery, and is still in some ways controversial, but is reinforced by the work reported here. Also of interest is the fact that the presence of supermassive black holes in the centers of galaxies does tend to enhance the nuclear magnetic field, so that this quenching mechanism should be most effective in the bulges of galaxies.


The cosmic commute towards star and planet formation

The molecular gas in galaxies is organised into a hierarchy of structures. The molecular material in giant molecular gas clouds travels along intricate networks of filamentary gas lanes towards the congested centres of gas and dust where it is compressed into stars and planets, much like the millions of people commuting to cities for work around the world.

To better understand this process, a team of astronomers led by Jonathan Henshaw at Max Planck Institute for Astronomy (MPIA) have measured the motion of gas flowing from galaxy scales down to the scales of the gas clumps within which individual stars form. Their results show that the gas flowing through each scale is dynamically interconnected: while star and planet formation occurs on the smallest scales, this process is controlled by a cascade of matter flows that begin on galactic scales. These results are published today in the scientific journal Nature Astronomy.

The molecular gas in galaxies is set into motion by physical mechanisms such as galactic rotation, supernova explosions, magnetic fields, turbulence, and gravity, shaping the structure of the gas. Understanding how these motions directly impact star and planet formation is difficult, because it requires quantifying gas motion over a huge range in spatial scale, and then linking this motion to the physical structures we observe. Modern astrophysical facilities now routinely map huge areas of the sky, with some maps containing millions of pixels, each with hundreds to thousands of independent velocity measurements. As a result, measuring these motions is both scientifically and technologically challenging.

In order to address these challenges, an international team of researchers led by Jonathan Henshaw at the MPIA in Heidelberg set out to measure gas motions throughout a variety of different environments using observations of the gas in the Milky Way and a nearby galaxy. They detect these motions by measuring the apparent change in the frequency of light emitted by molecules caused by the relative motion between the source of the light and the observer a phenomenon known as the Doppler effect. By applying novel software designed by Henshaw and Ph.D. student Manuel Riener (a co-author on the paper also at MPIA), the team were able to analyse millions of measurements. "This method allowed us to visualise the interstellar medium in a new way," says Henshaw.

The researchers found that cold molecular gas motions appear to fluctuate in velocity, reminiscent in appearance of waves on the surface of the ocean. These fluctuations represent gas motion. "The fluctuations themselves weren't particularly surprising, we know that the gas is moving," says Henshaw. Steve Longmore, co-author of the paper, based at Liverpool John Moores University, adds, "What surprised us was how similar the velocity structure of these different regions appeared. It didn't matter if we were looking at an entire galaxy or an individual cloud within our own galaxy, the structure is more or less the same."

To better understand the nature of the gas flows, the team selected several regions for close examination, using advanced statistical techniques to look for differences between the fluctuations. By combining a variety of different measurements, the researchers were able to determine how the velocity fluctuations depend on the spatial scale.

"A neat feature of our analysis techniques is that they are sensitive to periodicity," explains Henshaw. "If there are repeating patterns in your data, such as equally spaced giant molecular clouds along a spiral arm, we can directly identify the scale on which the pattern repeats." The team identified three filamentary gas lanes, which, despite tracing vastly different scales, all seemed to show structure that was roughly equidistantly spaced along their crests, like beads on a string, whether it was giant molecular clouds along a spiral arm or tiny "cores" forming stars along a filament.

The team discovered that the velocity fluctuations associated with equidistantly spaced structure all showed a distinctive pattern. "The fluctuations look like waves oscillating along the crests of the filaments, they have a well-defined amplitude and wavelength," says Henshaw adding, "The periodic spacing of the giant molecular clouds on large-scales or individual star-forming cores on small-scales is probably the result of their parent filaments becoming gravitationally unstable. We believe that these oscillatory flows are the signature of gas streaming along spiral arms or converging towards the density peaks, supplying new fuel for star formation."

In contrast, the team found that the velocity fluctuations measured throughout giant molecular clouds, on scales intermediate between entire clouds and the tiny cores within them, show no obvious characteristic scale. Diederik Kruijssen, co-author of the paper based at Heidelberg University explains: "The density and velocity structures that we see in giant molecular clouds are 'scale-free', because the turbulent gas flows generating these structures form a chaotic cascade, revealing ever smaller fluctuations as you zoom in -- much like a Romanesco broccoli, or a snowflake. This scale-free behaviour takes place between two well-defined extremes: the large scale of the entire cloud, and the small scale of the cores forming individual stars. We now find that these extremes have well-defined characteristic sizes, but in between them chaos rules."

"Picture the giant molecular clouds as equally-spaced mega-cities connected by highways," says Henshaw. "From a birds eye view, the structure of these cities, and the cars and people moving through them, appears chaotic and disordered. However, when we zoom in on individual roads, we see people who have travelled from far and wide entering their individual office buildings in an orderly fashion. The office buildings represent the dense and cold gas cores from which stars and planets are born."


The perilous process of star birth

This image shows shock waves produced by an explosion that happened when several young stars were ejected from the Orion Molecular Cloud – a known birthplace of stars – about 500 years ago. The colors represent a relative Doppler shifting of the millimeter-wavelength light emitted by carbon monoxide gas. Composite image via ALMA (ESO/NAOJ/NRAO)/ Gemini South/ J. Bally/H. Drass et al.

One of the most famous stellar nurseries in our galaxy can be seen by astronomers in the direction to the constellation Orion the Hunter. There, within an enormous cloud of interstellar gas and dust, stars are being born, ultimately getting dense enough and hot enough inside to spark thermonuclear fusion, the process by which stars shine. It’s been known for a long time that star death can be an explosive process, and that supernovae, or gigantic star explosions, are associated with the death of stars. But, as the image above shows, star birth can unleash violent and explosive events, too. This image was released on April 7, 2017. It’s from the ALMA telescope in Chile. The National Radio Astronomy Observatory explained what is represents in a statement:

Around 500 years ago, a pair of adolescent protostars had a perilously close encounter that blasted their stellar nursery apart.

Astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA) have examined the widely scattered debris from this explosive event, gaining new insights into the sometimes-fierce relationship among sibling stars.

Shortly after starting to form some 100,000 years ago, several protostars in the Orion Molecular Cloud 1 (OMC-1), a dense and active star factory about 1,500 light-years from Earth just behind the Orion Nebula, latched onto each other gravitationally and gradually drew closer.

Eventually, two of these stars either grazed each other or collided, triggering a powerful eruption that launched other nearby protostars and hundreds of giant streamers of dust and gas into interstellar space at speeds greater than 150 kilometers per second [about 93 miles per second]. This cataclysmic interaction released as much energy as our sun emits over the course of 10 million years.

John Bally with the University of Colorado led this research, now published in the peer-reviewed Astrophysical Journal. He said:

What we see in this once-calm stellar nursery is a cosmic version of a 4th of July fireworks display, with giant streamers rocketing off in all directions.

According to the researchers, explosions like this one are probably relatively short-lived in contrast to supernova explosions. The remnants of these explosions – like the one you see in the image above – may last only centuries. But, Bally said:

Though fleeting, protostellar explosions may be relatively common. By destroying their parent cloud, as we see in OMC-1, such explosions may also help to regulate the pace of star formation in these giant molecular clouds.

Bally and his team observed this feature previously with the Gemini South telescope in Chile. These earlier images, taken in the near infrared, reveal the remarkable structure of the streamers, which extend nearly a light-year from end to end.

Hints of the explosive nature of this outflow were first uncovered in 2009 with the Submillimeter Array in Hawaii. The new ALMA data, however, provide greater clarity.

The Orion Molecular Cloud Complex is an enormous cloud of interstellar gas and dust within the Milky Way Galaxy. A major component of this cloud is seen here in the southern half of the constellation Orion. Read more about this image from Fred Espenak at AstroPixels.

Bottom line: An explosion within the Orion Molecular Cloud (OMC-1) – a known birthplace of stars in our galaxy – dramatically shows that star birth can unleash violent and explosive events.


Background information

This study titled “Ubiquitous velocity fluctuations throughout the molecular interstellar medium” is published today in the journal Nature Astronomy. Besides the main author Jonathan D. Henshaw, 25 people from 21 research institutes from 8 countries are involved in the publication. Of these, Jonathan D. Henshaw, Manuel Riener, Eva Schinnerer, Henrik Beuther and Thomas Henning are conducting research at MPIA.

The scientists use data from the following observatories: Atacama Large Millimeter/submillimeter Array (ALMA), Morita Atacama Compact Array, Five College Radio Astronomy Observatory (FCRAO), Institut de Radioastronomie Millimétrique (IRAM) Plateau de Bure Interferometer, Mopra Radio Telescope, Herschel Space Observatory


This is a Binary Star in the Process of Formation

About 460 light years away lies the Rho Ophiuchi cloud complex. It’s a molecular cloud—an active star-forming region—and it’s one of the closest ones. R. Ophiuchi is a dark nebula, a region so thick with dust that the visible light from stars is almost completely obscured.

But scientists working with ALMA have pin-pointed a pair of young proto-stars inside all that dust, doing the busy work of becoming active stars.

The binary system of stars is called IRAS 16293-2422, and there’s a third star that’s also part of the system. It’s a widely-studied binary pair, partly because observations have revealed the presence of complex organic molecules, including a simple sugar, in the gas around the stars. That showed scientists that the building blocks for life can be present in the material that solar systems form out of.

But this study was about the pair of proto-stars themselves, rather than the building blocks of life. The authors set out to learn more about the morphology and the kinematics of this young system

Lead author of this new work is María José Maureira from Germany’s Max Planck Institute for Extraterrestrial Physics (MPE), an international powerhouse of scientific research. The study’s title is “Orbital and mass constraints of the young binary system IRAS 16293-2422 A.” It will be published in The Astrophysical Journal.

Dark nebulae like Rho Ophiuchi are difficult to study. Grains of interstellar dust block visible light. Astronomers have to observe them in radio or infrared. In this new study, the researchers used ALMA (Atacama Large Millimeter/sub-millimeter Array) to probe through all the light-blocking dust. As ALMA’s name makes clear, it observes wavelengths of about one millimeter, between infrared and radio. ALMA is an interferometer, combining 66 high-precision antenna as one telescope, giving it a high angular resolution.

This photo of the ALMA antennas on the Chajnantor Plateau in Chile, more than 16,000 feet (5000 meters) above sea level, was taken a few days before the start of ALMA Early Science and shows only one cluster of the 66 dishes. ALMA views the sky in “submillimeter” light, a slice of the spectrum invisible to the human eye that lies between infrared and radio waves. Credit: ALMA (ESO/NAOJ/NRAO)/W. Garnier (ALMA)

Inside the Rho Ophiuchi dark nebula lies a binary system named IRAS 16293-2422 A. Though it’s already a widely-studied object, previous studies produced some conflicting results. Different observations at different wavelengths showed multiple compact sources of radiation at different locations. The thick dust was making things difficult.

This study went further than previous studies. ALMA allowed the team of researchers to pinpoint the sources within the dark nebula. Astronomers already knew about the presence of what’s known as Protostar-B in the system, a well-known object. But their observations also revealed the two compact point sources of radiation, A1 and A2, in more detail than ever before.

“The small disks are probably still being fed and growing!”

Paola Caselli, STudy Co-Author and director at MPE, and head of the Center for Astrochemical Studies.

In thier paper, the authors write “Here, we present ALMA Band 3 continuum observations with a resolution of 0.046” (6.5 au) that reveal for
the first time two compact sources at wavelengths tracing dust thermal emission, coincident with the location of the cm compact sources A1 and A2, thus confirming IRAS 16293 A1-A2 as a binary.”

“Our observations confirm the location of the two close proto-stars and reveal that each is surrounded by a very small dust disk. Both, in turn, are embedded in a large amount of material showing complex patterns,” said lead author Maureira in a press release.

Detailed view of the binary proto-star system with a size comparison to our solar system. The separation between the sources A1 and A2 is roughly the diameter of the Pluto orbit. The size of the disk around A1 (unresolved) is about the diameter of the asteroid belt. The size of the disk around A2 is about the diameter of the Saturn orbit. Image Credit: © MPE

Both young stars are similar in mass to the Sun. A1 is just under one solar mass, while A2 is about 1.4 solar masses. Each one is embedded in its own dust disk. A2’s dust disk is somewhat larger than A1’s, and appears at an angle relative to the orientation of the Rho Ophiuchi cloud structure. That’s an unusual detail, and points to some chaos in the system. The already known Protostar B has a disk that’s face-on from our vantage point, adding to the chaotic nature.

The team had 30 years of data on this system at their disposal. They added their new observations to all of that data, and came up with some new conclusions. The two proto-stars are orbiting each other every 360 years, at an orbit that’s similar to Pluto’s full extent in our Solar System.

“This is the first time that we were able to derive the full orbital parameters of a binary system at this early stage of star formation,” said co-author Jaime Pineda, also from MPE, who contributed to the modelling.

“With these results we are finally able to dive into one of the most embedded and youngest proto-stellar systems, unveiling its dynamical structure and complex morphology, where we clearly see filamentary material connecting the circumstellar disks to the surrounding region and likely to the cirbumbinary disk. The small disks are probably still being fed and growing!” emphasizes Paola Caselli, director at MPE and head of the Center for Astrochemical Studies.

This figure from the study helps explain some of the results. These are high angular resolution images of the IRAS 16293 system from ALMA. It shows the triple nature of the system. Point source B is some distance away from the binary pair. It’s embedded in its own disk of dust, seen face-on, that’s about 40 AU across. The A1 and A2 point sources are revealed as distinct objects, separated by 54 AU, and each in its own disk. The image also shows the extended dust structure enveloping both point sources. Image Credit: Maureira et al, 2020.

As part of their observations the team also looked at organic molecules. By observing the spectral lines, they were able to gain additional information on the binary pair. That helped them more fully characterize the motion of the gas around both stars.

“This was only possible thanks to the great sensitivity of ALMA and the observations of molecules which uniquely trace these dense regions. Molecules send us signals at very specific frequencies, and, following changes of such frequencies across the region (due to internal motions) one can reconstruct the complex kinematics of the system. This is the power of astrochemistry.”

The team identified spectral lines for different molecules in the gas. By mapping them and their velocities, they could construct the kinematics in the system. From left to right are velocity maps for Carbon Sulfide, Isocyanic Acid, Formamide, and Formic Acid. Image Credit: Maureira et al, 2020.

The authors sum up their findings in the paper’s conclusion. “The range of protostellar masses inferred from the orbital analysis and the gas kinematics are consistently higher than previous estimations using lower resolution observations…”

They also point out that the system is likely gravitationally bound. “…the binary system A and single source B are also likely bound, forming a triple hierarchical system.” The authors say that further observations and simulations will “…help to further constrain the dynamics and individual masses of this deeply embedded triple system.”