# Binary Star Initial Speed

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I am writing a 2D simulation for binary stars orbiting each other. Currently, the initial position of the stars are on opposite sides of the screen with the left star having a velocity up the page and the right star having an equivalent velocity down the page.

The question I have is there a formula for calculating what that initial speed should be so that the stars remain in a stable orbit? Optimally, the formula would take into account the mass of the stars (which are equivalent), the distance apart, and G.

My apologies if this question would be better directed elsewhere or if I overlooked the answer in my own research.

You will always get a "stable" orbit if the stars have less than escape velocity relative to each other. (unless you are modelling the stars as having non-zero radii so they can collide) The stars will enter into elliptical orbits around a barycentre.

But I guess you want a circular orbit. For a circular orbit the speed $$v$$ is given by

$$v^2=GM/r$$

where r is the orbit radius (from the centre of mass), G is Newtons gravitational constant, and M is the reduced mass $$M= frac{m_1m_2}{m_1+m_2}$$ for stars of mass $$m_1$$ and $$m_2$$.

You will find it convenient to use astronomical units (not SI units), so distance in AU, time in years, mass in "solar masses" In these units $$G=(2pi)^{2}$$ and you avoid difficulties with very large and very small values. (in these units the speed of the Earth is $$2pi$$ AU/year which makes sense)

To generalise from James K's answer, which gives the condition for a circular orbit…

The condition for the binary to remain bound is that the total energy of the system, which is the sum of the potential energy $$V$$ and the kinetic energy $$T$$ (as evaluated in a centre-of-momentum frame) is less than zero.

$$T+V < 0$$

Considering the system as two point masses obeying Newtonian gravity, the gravitational potential energy $$V$$ is given by:

$$V = -frac{G m_1 m_2}{left| vec{r_2} - vec{r_1} ight|}$$

where $$m_1$$ and $$m_2$$ are the masses, $$vec{r_1}$$ and $$vec{r_2}$$ are the position vectors of the masses, and $$G$$ is the gravitational constant.

The kinetic energy is given by:

$$T = frac{1}{2} m_1 left| vec{v_1} ight|^2 + frac{1}{2} m_2 left| vec{v_2} ight|^2$$

Where $$vec{v_1} = dot{vec{r_1}}$$ and $$vec{v_2} = dot{vec{r_2}}$$ are the velocity vectors of the two masses.

Using the definition of the centre-of-momentum frame $$m_1 vec{v_1} + m_2 vec{v_2} = vec{0}$$, and expressing in terms of the relative positions and velocities

$$r = left| vec{r_2} - vec{r_1} ight| v = left| vec{v_2} - vec{v_1} ight|$$

and the reduced mass

$$mu = frac{m_1 m_2}{m_1 + m_2}$$

the condition can be written:

$$frac{1}{2} mu v^2 - frac{G m_1 m_2}{r} < 0$$

Which can be rearranged to give

$$v^2 < frac{2 G left( m_1 + m_2 ight)}{r}$$

An easy way to initialize a simulation is to initialise the primary stationary at the origin, pick the secondary's position and velocity to match this condition, then subtract the centre-of-mass velocity from the individual velocities to ensure your system doesn't go wandering off the screen.

What about cases not meeting the condition? If the total energy is exactly zero (i.e. replace the less-than sign with equality), the orbit will be parabolic. If the energy exceeds zero, the orbit will be hyperbolic.

If the relative velocity and relative position vectors are co-linear (or the velocity vector is zero) then the motion will be linear: if the total energy is less than zero then the masses will collide, if the total energy is greater than zero and the velocities directed outwards then they will escape to infinity.

## Where do hypervelocity stars come from?

700 km/s, which is more than 3 times the Solar velocity! This star is moving so quickly that its velocity is high enough to escape the Milky Way. The existence of such stars, deemed hypervelocity stars (HVSs), was predicted almost 20 years earlier by Hills (1988).

The so-called Hills mechanism ejects stars at high speeds from the center of the Galaxy after a binary stellar system gravitationally interacts with the supermassive black hole at the center of the Galaxy. In such a three-body interaction, one star can be ejected at very high speeds while the other remains in the central region of the Galaxy on a highly eccentric orbit. A number of highly eccentric short-period stars are observed in the Galactic center, which suggests associated HVSs may exist. Since the initial discovery of an HVS in 2005, many more have been discovered (see this astrobite). A small warning: the exact definition of HVSs can vary throughout the literature. In this post, stars traveling away from the Galactic center with velocities high enough to have become unbound from the central black hole are referred to as HVSs.

While it is very likely that the Hills Mechanism does create HVSs, it is unclear whether all HVSs are created via this method. Curious if there is a supplemental method to produce HVSs, the authors of today’s paper examine the production of HVSs via supernovae explosions.

How many HVSs are produced by supernovae?
Zubovas et al. examine the production rate and spatial distribution of HVSs from the ejection of a binary companion during a supernovae explosion. Most stars, especially most large stars, are part of a binary system. If one of the stars is massive enough to undergo a core collapse supernovae, the ejection of its envelope will push its companion outward. In some cases, this outward force will be strong enough not only to unbind the binary, but to send the second star whizzing out of the Galaxy.

This study expands upon previous studies of supernovae production of HVSs by performing a detailed Monte Carlo analysis. The Monte Carlo analysis determines the production rate and spatial distribution of HVSs by randomly sampling from predicted initial distributions and applying expected physical models. The initial parameters include inputs such as the expected number and spatial orientation of binary systems in the Galactic center, the expected spatial separation of the stars within those systems, and how much of the ejecta energy is imparted onto the companion star.

While the initial parameter distributions are physically motivated, it is important to note that these are underlying assumptions which strongly affect the results. In a simple example, if the binary fraction is actually 2 times lower than the input assumption, a Monte Carlo simulation will predict twice as many HVSs as there actually are. The physical models of supernovae explosions are also rather uncertain, which could strongly affect the Monte Carlo predictions.

Zubovas et al. figure 2a. This figure shows a cumulative distribution of the velocities for stars ejected from the system. Only a small fraction of ejected stars have velocities high enough to escape the Galactic potential (

The authors find that more than 93% of the time, supernovae explosions disrupt the binary system, ejecting the secondary however, most ejected secondary stars in the Galactic center remain bound to the central supermassive black hole. Based on different initial model parameters, the ejection rate of binary companions from the Galactic center could be between

4 – 25%. This corresponds to ejecting a star around once every 4 – 22 million years.

Zubovas et al. find that over 100 million years the highest speed HVS produced via supernovae explosions is

500 – 700 km/s, indicating that this ejection method cannot explain the fastest known HVSs (

750 km/s). The predicted spatial distribution of HVSs formed by supernovae, like the observed population of HVSs, is not spherically symmetric. This is because the initial simulated population was anisotropic, and this quantity is preserved, which is also true for the Hills Mechanism. Overall, the predicted production rate of HVSs via supernovae is comparable to the production rate predicted for the Hills mechanism. This suggests the observed population of HVSs may have two progenitor populations.

## Found: fastest eclipsing binary

Observations made with a new instrument developed for use at the 2.1-meter (84-inch) telescope at the National Science Foundation’s Kitt Peak National Observatory have led to the discovery of the fastest eclipsing white dwarf binary yet known. Clocking in with an orbital period of only 6.91 minutes, the rapidly orbiting stars are expected to be one of the strongest sources of gravitational waves detectable with LISA, the future space-based gravitational wave detector.

### The Dense “Afterlives” of Stars

After expanding into a red giant at the end of its life, a star like the Sun will eventually evolve into a dense white dwarf, an object with a mass like that of the Sun squashed down to a size comparable to Earth. Similarly, as binary stars evolve, they can engulf their companion in the red giant phase and spiral close together, eventually leaving behind a close white dwarf binary. White dwarf binaries with very tight orbits are expected to be strong sources of gravitational wave radiation. Although anticipated to be relatively common, such systems have proven elusive, with only a few identified to date.

### Record-setting White Dwarf Binary

A new survey of the night sky, currently underway at Palomar Observatory and Kitt Peak National Observatory, is changing this situation.

Each night, Caltech’s Zwicky Transient Facility (ZTF), a survey that uses the 48-inch telescope at Palomar Observatory, scans the sky for objects that move, blink, or otherwise vary in brightness. Promising candidates are followed up with a new instrument, the Kitt Peak 84-inch Electron Multiplying Demonstrator (KPED), at the Kitt Peak 2.1-meter telescope to identify short period eclipsing binaries. KPED is designed to measure with speed and sensitivity the changing brightness of celestial sources.

This approach has led to the discovery of ZTF J1539+5027 (or J1539 for short), a white dwarf eclipsing binary with the shortest period known to date, a mere 6.91 minutes. The stars orbit so close together that the entire system could fit within the diameter of the planet Saturn.

“As the dimmer star passes in front of the brighter one, it blocks most of the light, resulting in the seven-minute blinking pattern we see in the ZTF data,” explains Caltech graduate student Kevin Burdge, lead author of the paper reporting the discovery, which appears in today’s issue of the journal Nature .

### A Strong Source of Gravitational Waves

Closely orbiting white dwarfs are predicted to spiral together closer and faster, as the system loses energy by emitting gravitational waves. J1539’s orbit is so tight that its orbital period is predicted to become measurably shorter after only a few years. Burdge’s team was able to confirm the prediction from general relativity of a shrinking orbit, by comparing their new results with archival data acquired over the past ten years.

J1539 is a rare gem. It is one of only a few known sources of gravitational waves—ripples in space and time—that will be detected by the future European space mission LISA (Laser Interferometer Space Antenna), which is expected to launch in 2034. LISA, in which NASA plays a role, will be similar to the National Science Foundation’s ground-based LIGO (Laser Interferometer Gravitational-wave Observatory), which made history in 2015 by making the first direct detection of gravitational waves from a pair of colliding black holes. LISA will detect gravitational waves from space at lower frequencies. J1539 is well matched to LISA the 4.8 mHz gravitational wave frequency of J1539 is close to the peak of LISA’s sensitivity.

### Discoveries Continue for Historic Telescope

Kitt Peak’s 2.1-meter telescope, the second major telescope to be constructed at the site, has been in continuous operation since 1964. Its history includes many important discoveries in astrophysics, such as the Lyman-alpha forest in quasar spectra, the first gravitational lens by a galaxy, the first pulsating white dwarf, and the first comprehensive study of the binary frequency of stars like the Sun. The latest result continues its venerable track record.

Lori Allen, Director of Kitt Peak National Observatory and Acting Director of NOAO says, “We’re thrilled to see that our 2.1-meter telescope, now more than 50 years old, remains a powerful platform for discovery.”

“These wonderful observations are further proof that cutting-edge science can be done on modest-sized telescopes like the 2.1-meter in the modern era,” adds Chris Davis, NSF Program Officer for NOAO.

As remarkable as it is, J1539 was discovered with only a small portion of the data expected from ZTF. It was found in the ZTF team’s initial analysis of 10 million sources, whereas the project will eventually study more than a billion stars.

“Only months after coming online, ZTF astronomers have detected white dwarfs orbiting each other at a record pace,” says NSF Assistant Director for Mathematical and Physical Sciences, Anne Kinney. “It’s a discovery that will greatly improve our understanding of these systems, and it’s a taste of surprises yet to come.”

“General relativistic orbital decay in a seven-minute-orbital-period eclipsing binary system,” Burdge et al. 2019, Nature , https://www.nature.com/articles/s41586-019-1403-0

Kitt Peak National Observatory (KPNO) is part of the National Optical Astronomy Observatory (NOAO), which is operated by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with the National Science Foundation (NSF). NSF is an independent federal agency created by Congress in 1950 to promote the progress of science. NSF supports basic research and people to create knowledge that transforms the future. The NOAO community is honored to be permitted to conduct astronomical research on Iolkam Du’ag (Kitt Peak), a mountain with particular significance to the Tohono O’odham Nation.

Nice report! So a known system has erupted. That's nice, too. But I thought most of my nuclei were fabricated in a supernova, rather than in a regular nova, at least it sounds more impressive :^) Yes, it happened long ago, but not necessarily far away (astronomically speaking).

Thanks Bob for this excellent guide to observing the new nova in Cassiopeia. Following your finder charts and those from the AAVSO website link I was able to locate the object without difficulty last night. The nova was clear to see with my 7" Maksutov at 76x despite observing conditions being far from ideal with thin cloud and the gibbous Moon high up. I was also successful in observing with binoculars, with 10x70 the nova was an easy spot and I just managed to glimpse it in 8.5x44. Looking forward to tracking brightness changes over future observations.

Thanks very much Bob! I saw the nova through 10x42 image-stabilized binoculars at the beginning of dawn, around 5:30 am PDT, the past three mornings. Saturday I was "pretty sure", yesterday I was confident I had found the nova, and this morning it was a snap. Your chart and directions make it very easy to find. Visually the nova is entirely unremarkable, just one of a wide pair of stars around 8th magnitude. As with so many things in the sky, you need to understand what you're looking at to appreciate it.

I'm impressed that Mr. Nakamura would notice a 9.6-magnitude star where none had been before. Do you know if he found it by visually inspecting his images, or did he use software?

## How can binary stars orbit each other so fast?

By: The Editors of Sky & Telescope July 24, 2006 0

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In a News Note (S&T: November 2004, page 16) you described WR 20a, a binary star in Carina with components of 83 and 82 solar masses and an orbital period of 3.686 days. How can two huge balls of gas move so fast?

Grab any introductory astronomy textbook and turn to the chapter on binary stars. There you’ll find some simple formulas relating the two stars’ masses to their separation and orbital period. These equations come from Isaac Newton’s theory of universal gravitation and Johannes Kepler’s laws of orbital motion, which have served astronomers well for more than three centuries.

When I plug in the masses and period given in our News Note, I find that the stars are about two-thirds as far apart as Mercury and the Sun, and that they whirl around each other with a relative speed of more than 700 kilometers per second (11/2 million miles per hour).

That is very fast, and it may strike you as outlandish, but it’s completely consistent with orbital mechanics. It’s also within the realm of our own experience. We’re actually zipping along at about 240 km per second right now as our solar system circles the galactic center!

The components of WR 20a are Wolf-Rayet stars, extremely hot, luminous bodies about 20 times larger than the Sun and only a hundredth as dense. Even whirling around as they do, they have no trouble holding themselves together. Such is the awesome power of 80-plus Suns’ worth of gravitating mass.

## Binary Star Initial Speed - Astronomy

Context: The collapsar model for long gamma-ray bursts requires a rapidly rotating Wolf-Rayet star as progenitor.
Aims: We test the idea of producing rapidly rotating Wolf-Rayet stars in massive close binaries through mass accretion and consecutive quasi-chemically homogeneous evolution - the latter had previously been shown to provide collapsars below a certain metallicity threshold.
Methods: We use a 1D hydrodynamic binary evolution code to simulate the evolution of a 16+15 M ☉ binary model with an initial orbital period of 5 days and SMC metallicity (Z=0.004). Internal differential rotation, rotationally induced mixing and magnetic fields are included in both components, as well as non-conservative mass and angular momentum transfer, and tidal spin-orbit coupling.
Results: The considered binary system undergoes early Case B mass transfer. The mass donor becomes a helium star and dies as a type Ib/c supernova. The mass gainer is spun-up, and internal magnetic fields efficiently transport accreted angular momentum into the stellar core. The orbital widening prevents subsequent tidal synchronization, and the mass gainer rejuvenates and evolves quasi-chemically homogeneously thereafter. The mass donor explodes 7 Myr before the collapse of the mass gainer. Assuming the binary to be broken-up by the supernova kick, the potential gamma-ray burst progenitor would become a runaway star with a space velocity of 27 km s -1 , traveling about 200 pc during its remaining lifetime.
Conclusions: .The binary channel presented here does not, as such, provide a new physical model for collapsar production, as the resulting stellar models are almost identical to quasi-chemically homogeneously evolving rapidly rotating single stars. However, it may provide a means for massive stars to obtain the required high rotation rates. Moreover, it suggests that a possibly large fraction of long gamma-ray bursts occurs in runaway stars.

## The Sun is surrounded by millions of binary stars!

The Sun swings around its galactic orbit alone, a solo voyage through space.

But for half the stars in the Milky Way that's not the case. They exist with a companion, a duo traveling the cosmos together. Bound by gravity, these binary stars come in a bewildering variety of characteristics and in many ways are the key to understanding the cosmos.

That's due to the Gaia observatory and a dedicated team of astronomers who culled the enormous stockpile of data it generated.

An artist’s map of the Milky Way galaxy, with a circle 3,000 light years in radius centered on the Sun. Credit: NASA / JPL-Caltech / Robert Hurt (SSC-Caltech) / Phil Plait (annotation)

Gaia is a European Space Agency mission that orbits the Sun in a stable position about 1.5 million kilometers from Earth. Its mission is to measure the positions, motions, colors, and other characteristics of over nearly two billion stars in the galaxy. That's it. It doesn't specify what kind of star to look at (other than it has to be bright enough to see) or drill down to answer a specific question.

Instead, the idea is to create a vast database of stars and their physical parameters so that astronomers, imaginative as they are, can then parse it to find answers the question they have.

One question is, how many binary stars are there that we can find using Gaia data? And once we have that in hand, what can we learn about them and stars in general from the data?

And that's what the team of astronomers did. To find binaries, they looked for pairs of stars that were both the same distance from Earth (using parallax, the apparent change in position of a star over time as Gaia orbits the Sun) and moving in the same direction at the same speed — what we call proper motion. All stars we can see orbit the center of the Milky Way, and over time their position in the sky changes due to that motion. Gaia can measure extremely small changes in a star's position, yielding pretty decent measurements for ones out to 3,000 light years.

To make sure they had true binaries and not chance alignments between stars at different distances from us, they went through a series of data filters, culling the list. For example, two stars that were too far apart to be bound to each other via gravity were dropped, as were stars in clusters or part of a triple-star system.

In the end they can't eliminate chance alignments entirely, but they can show statistically that they are very low in number in their final catalog.

Hubble image of one of the closest binary stars to the Sun: Sirius A (center) and its white dwarf companion B (to the lower left) A is roughly 10,000 times brighter. Credit: NASA, ESA, H. Bond (STScI), and M. Barstow (University of Leicester

They found 1.26 million stars that have a 90% chance of being bound binaries, or, if they up the chance to 99%, 1.1 million stars. Either way, well over half a million binary systems within 3,000 light years of the Sun.

That right away is an amazing result. The nearest star system, Alpha Centauri, is over 4 light years away. That makes it seem like there aren't many stars close to the Sun, but that number goes up with the cube of the distance you're looking at — remember, the volume of a sphere is 4/3 x pi x radius 3 , and it's that radius cubed that dominates. Look out 40 light years and there will be a thousand stars. Within 400 light years there are a million.

By 3,000 light years there should be about 400 million stars * ! Most are too faint for Gaia to see, though. But of the ones it can, over a million are in committed relationships with another star.

The two stars of the binary system [BHB2007] 11 are in the process of forming, drawing material from the disk surrounding both via a pair of filaments, wound up due to the motion of the stars around each other. Credit: ALMA (ESO/NAOJ/NRAO), Alves et al.

The stats: Of these binaries, 900,000 systems had both stars in them being stars like the Sun, fusing hydrogen into helium in their cores, what we call main sequence stars. 16,000 were in a system with one main sequence star and one white dwarf, the hot core of a star exposed to space after the star dies. 1,400 were in a white dwarf/white dwarf system (usually called a double degenerate system), 10,000 systems had one star being a giant — nearing the end of its life, and swollen up hugely — and 130 were both giant stars. 13,000 were systems with subgiant stars, where the star is just started to use up its fuel and is on the path to swelling up to a giant.

These numbers are immensely useful to astronomers. Stars in binaries form together from the same gas cloud, so they have the same age and composition. That helps eliminate confounding factors when trying to figure out other characteristics. Their orbit around each other gives you their masses. White dwarfs have a well understood cooling rate, so we can get their ages as well (the faintest they saw in the Gaia data were over 10 billion years old, a large fraction of the age of the Universe). Observing the subgiants and giants in these systems shows us how they evolve over time, too.

Artwork depicting a second sun, a binary companion to the Sun that may have existed billions of years ago. Credit: M. Weiss

It's probably just a coincidence that the Sun isn't in a binary. Some stars are, some aren't. We've found many exoplanets in binary systems, orbiting one star or both, so it's not like single stars are the only ones with planets. But the more we know about binary stars the more informed our understanding of the way planets form, too.

And, to be honest, we don't know for sure the Sun was always solo. It's possible it was once part of a binary system. So again, learning more about them will help us understand the Sun better, which means understanding ourselves better.

Binary or non-binary, they're all stars, each worth learning about and understanding them as they are. Perhaps there's a more general lesson in there, too.

* The galaxy is a flat disk, so around this distance the geometry gets more complicated than using the distance cubed and the number increases more like distance squared.

## White Dwarf Explosions: The Mild Kind

Let’s consider the following system of two stars: one has become a white dwarf and the other is gradually transferring material onto it. As fresh hydrogen from the outer layers of its companion accumulates on the surface of the hot white dwarf, it begins to build up a layer of hydrogen. As more and more hydrogen accumulates and heats up on the surface of the degenerate star, the new layer eventually reaches a temperature that causes fusion to begin in a sudden, explosive way, blasting much of the new material away.

In this way, the white dwarf quickly (but only briefly) becomes quite bright, hundreds or thousands of times its previous luminosity. To observers before the invention of the telescope, it seemed that a new star suddenly appeared, and they called it a nova. [1] Novae fade away in a few months to a few years.

Hundreds of novae have been observed, each occurring in a binary star system and each later showing a shell of expelled material. A number of stars have more than one nova episode, as more material from its neighboring star accumulates on the white dwarf and the whole process repeats. As long as the episodes do not increase the mass of the white dwarf beyond the Chandrasekhar limit (by transferring too much mass too quickly), the dense white dwarf itself remains pretty much unaffected by the explosions on its surface.

Introduction
1. The zoo of binary stars Henri M. J. Boffin
2. Statistics of binary and multiple stars M. Moe
3. Gaia and LSST: their importance in binary star research L. Eyer, Nami Mowlavi, Isabelle Lecoeur-Taibi, Lorenzo Rimoldini, Berry Holl, Marc Audard, Simon Hodgkin, Dafydd W. Evans, Lukasz Wyrzykowsi, George Seabroke, Andrej Prša, and Dimitri Pourbaix
4. Population synthesis of binary stars R. G. Izzard and G. M. Halabi
5. Low- and intermediate-mass star evolution: open problems M. Salaris
6. The symbiotic stars U. Munari
7. Binary post-AGB stars as tracers of stellar evolution H. van Winckel
8. The importance of binarity in the formation and evolution of planetary nebulae D. Jones
9. Massive star evolution: binaries as two single stars C. Georgy and S. Ekström
10. Binarity at high masses H. Sana
11. Luminous blue variables: their formation and instability in the context of binary interactions A. Mehner
12. Type Ia supernovae: where are they coming from and where will they lead us? F. Patat and N. Hallakoun
13. Binary interactions and gamma-ray bursts N. R. Tanvir
14. Binaries as sources of gravitational waves G. Nelemans
15. The impact of binaries on the stellar initial mass function P. Kroupa and T. Jerabkova
16. The formation of binary stars: insights from theory and observation C. J. Clarke
17. The Maxwell's demon of star clusters M. Mapelli
18. Alternative stellar evolution pathways R. D. Mathieu and E. M. Leiner
19. Clocks and scales: playing with the physics of blue stragglers F. R. Ferraro and B. Lanzoni
20. Binaries at very low metallicity S. Lucatello
21. Population and spectral synthesis: it doesn't work without binaries J. J. Eldridge and E. R. Stanway.

Giacomo Beccari, European Southern Observatory, Garching
Giacomo Beccari is a staff astronomer at the European Southern Observatory, Garching. He is a former winner of the Levi-Montalcini Prize and co-author of the Ecology of Blue Straggler Stars (2014).

Henri M. J. Boffin, European Southern Observatory, Garching
Henri M. J. Boffin is a staff astronomer at the European Southern Observatory, Garching. Recently he has shown the importance of binary stars in explaining planetary nebulae, including discovering the binary star of Fleming 1. He pioneered the use of optical interferometry to study mass transfer in symbiotic stars.

## New Studies Give Boost to Binary Star Formation Theory

Using the new capabilities of the upgraded Karl G. Jansky Very Large Array (VLA), scientists have discovered previously-unseen binary companions to a pair of very young protostars. The discovery gives strong support for one of the competing explanations for how double-star systems form.

Astronomers know that about half of all Sun-like stars are members of double or multiple-star systems, but have debated over how such systems are formed.

“The only way to resolve the debate is to observe very young stellar systems and catch them in the act of formation,” said John Tobin, of the National Radio Astronomy Observatory (NRAO). “That’s what we’ve done with the stars we observed, and we got valuable new clues from them,” he added.

Their new clues support the idea that double-star systems form when a disk of gas and dust whirling around one young star fragments, forming another new star in orbit with the first. Young stars that still are gathering matter from their surroundings form such disks, along with jet-like outflows rapidly propelling material in narrow beams perpendicular to the disk.

When Tobin and an international team of astronomers studied gas-enshrouded young stars roughly 1,000 light-years from Earth, they found that two had previously-unseen companions in the plane where their disks would be expected, perpendicular to the direction of the outflows from the systems. One of the systems also clearly had a disk surrounding both young stars.

“This fits the theoretical model of companions forming from fragmentation in the disk,” Tobin said. “This configuration would not be required by alternative explanations,” he added.

The new observations add to a growing body of evidence supporting the disk-fragmentation idea. In 2006, a different VLA observing team found an orbiting pair of young stars, each of which was surrounded by a disk of material. The two disks, they found, were aligned with each other in the same plane. Last year, Tobin and his colleagues found a large circumstellar disk forming around a protostar in the initial phases of star formation. This showed that disks are present early in the star formation process, a necessity for binary pairs to form through disk fragmentation.

“Our new findings, combined with the earlier data, make disk fragmentation the strongest explanation for how close multiple star systems are formed,” said Leslie Looney of NRAO and the University of Illinois.

“The increased sensitivity of the VLA, produced by a decade-long upgrade project completed in 2012, made the new discovery possible,” Claire Chandler of NRAO said.

The new capability was particularly valuable at the VLA’s highest frequency band, from 40-50 GHz, where dust in the disks surrounding young stars emits radio waves. The astronomers observed the young stars during 2012 with the VLA and with the Combined Array for Research in Millimeter-wave Astronomy (CARMA) in California.

## Watch the video: Ποία νύχτα; Ποίο αστέρι; Ποίο όνειρο; (June 2022).

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