How Do Stars Get Kicked Out of Globular Clusters?

Hubble image of Messier 54, a globular cluster located in the Sagittarius Dwarf Galaxy. Credit: ESA/Hubble & NASA

Globular clusters are densely-packed collections of stars bound together gravitationally in roughly-shaped spheres. They contain hundreds of thousands of stars. Some might contain millions of stars.

Sometimes globular clusters (GCs) kick stars out of their gravitational group. How does that work?

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Stars Getting Kicked out of the Milky Way can Help us map its Dark Matter Halo

Artist concept showing a hypervelocity star escaping our galaxy. Credit: NASA, ESA, and G. Bacon (STScI)

Dark matter is notoriously difficult to study. It’s essentially invisible to astronomers since it can’t be seen directly. So astronomers rely on effects such as the gravitational lensing of light to map its presence in the universe. That method works well for other galaxies, but not so well for our own. To map dark matter in the Milky Way, we rely mostly on the motions of stars in our galaxy. Since dark matter attracts regular matter gravitationally, the method works well for areas of the galaxy where there are stars. Unfortunately, most of the stars lie along the galactic plane, making it difficult to map dark matter above and below that plane. But a recent study proposes a way to map more of our galaxy’s dark matter using runaway stars.

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Astronomers Discover Hundreds of High-Velocity Stars, Many on Their Way Out of the Milky Way

Credit: NAOC/Kong Xiao

Within our galaxy, there are thousands of stars that orbit the center of the Milky Way at high velocities. On occasion, some of them pick up so much speed that they break free of our galaxy and become intergalactic objects. Because of the extreme dynamical and astrophysical processes involved, astronomers are most interested in studying these stars – especially those that are able to achieve escape velocity and leave our galaxy.

However, an international team of astronomers led from the National Astronomical Observatories of China (NAOC) recently announced the discovery of 591 high-velocity stars. Based on data provided by the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) and the ESA’s Gaia Observatory, they indicated that 43 of these stars are fast enough to escape the Milky Way someday.

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This Star has been Kicked Out of the Milky Way. It Knows What It Did.

Researchers from the University of Michigan confirm that a runaway star was ejected from the Milky Way's disk rather than the galactic core. Image Credit: Kohei Hattori
Researchers from the University of Michigan confirm that a runaway star was ejected from the Milky Way's disk rather than the galactic core. Image Credit: Kohei Hattori

Every once in a while, the Milky Way ejects a star. The evicted star is typically ejected from the chaotic area at the center of the galaxy, where our Super Massive Black Hole (SMBH) lives. But at least one of them was ejected from the comparatively calm galactic disk, a discovery that has astronomers rethinking this whole star ejection phenomenon.

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The Milky Way Could Be Spreading Life From Star to Star

Using information from Gaia's second data release, a team of scientists have made refined estimates of the Milky Way's mass. Credit: ESA/Gaia/DPAC

For almost two centuries, scientists have theorized that life may be distributed throughout the Universe by meteoroids, asteroids, planetoids, and other astronomical objects. This theory, known as Panspermia, is based on the idea that microorganisms and the chemical precursors of life are able to survive being transported from one star system to the next.

Expanding on this theory, a team of researchers from the Harvard Smithsonian Center for Astrophysics (CfA) conducted a study that considered whether panspermia could be possible on a galactic scale. According to the model they created, they determined that the entire Milky Way (and even other galaxies) could be exchanging the components necessary for life.

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Gaia Sees Stars Out in Deep Space, Flying Between Galaxies

An artist's conception of a hypervelocity star that has escaped the Milky Way. Credit: NASA

In December of 2013, the European Space Agency (ESA) launched the Gaia mission. Since that time, this space observatory has been busy observing over 1 billion astronomical objects in our galaxy and beyond – including stars, planets, comets, asteroids, quasars, etc. – all for the sake of creating the largest and most precise 3D space catalog ever made.

The ESA has also issued two data releases since then, both of which have led to some groundbreaking discoveries. The latest comes from the Leiden Observatory, where a team of astronomers used Gaia data to track what they thought were high-velocity stars being kicked out of the Milky Way, but which actually appeared to be moving into our galaxy.

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Determining the Mass of the Milky Way Using Hypervelocity Stars

An artist's conception of a hypervelocity star that has escaped the Milky Way. Credit: NASA

For centuries, astronomers have been looking beyond our Solar System to learn more about the Milky Way Galaxy. And yet, there are still many things about it that elude us, such as knowing its precise mass. Determining this is important to understanding the history of galaxy formation and the evolution of our Universe. As such, astronomers have attempted various techniques for measuring the true mass of the Milky Way.

So far, none of these methods have been particularly successful. However, a new study by a team of researchers from the Harvard-Smithsonian Center for Astrophysics proposed a new and interesting way to determine how much mass is in the Milky Way. By using hypervelocity stars (HVSs) that have been ejected from the center of the galaxy as a reference point, they claim that we can constrain the mass of our galaxy.

Their study, titled “Constraining Milky Way Mass with Hypervelocity Stars“, was recently published in the journal Astronomy and Astrophysics. The study was produced by Dr. Giacomo Fragione, an astrophysicist at the University of Rome, and Professor Abraham Loeb – the Frank B. Baird, Jr. Professor of Science, the Chair of the Astronomy Department, and the Director of the Institute for Theory and Computation at Harvard University.

Stars speeding through the Galaxy. Credit: ESA

To be clear, determining the mass of the Milky Way Galaxy is no simple task. On the one hand, observations are difficult because the Solar System lies deep within the disk of the galaxy itself. But at the same time, there’s also the mass of our galaxy’s dark matter halo, which is difficult to measure since it is not “luminous”, and therefore invisible to conventional methods of detection.

Current estimates of the galaxy’s total mass are based on the motions of tidal streamers of gas and globular clusters, which are both influenced by the gravitational mass of the galaxy. But so far, these measurements have produced mass estimates that range from one to several trillion solar-masses. As Professor Loeb explained to Universe Today via email, precisely measuring the mass of the Milky Way is of great importance to astronomers:

“The Milky Way provides a laboratory for testing the standard cosmological model. This model predicts that the number of satellite galaxies of the Milky Way depends sensitively on its mass. When comparing the predictions to the census of known satellite galaxies, it is essential to know the Milky Way mass. Moreover, the total mass calibrates the amount of invisible (dark) matter and sets the depth of the gravitational potential well and implies how fast should stars move for them to escape to intergalactic space.”

For the sake of their study, Prof. Loeb and Dr. Fragione therefore chose to take a novel approach, which involved modeling the motions of HVSs to determine the mass of our galaxy. More than 20 HVSs have been discovered within our galaxy so far, which travel at speeds of up to 700 km/s (435 mi/s) and are located at distances of about 100 to 50,000 light-years from the galactic center.

Artist’s conception of a hyperveloctiy star heading out from a spiral galaxy (similar to the Milky Way) and moving into dark matter nearby. Credit: Ben Bromley, University of Utah

These stars are thought to have been ejected from the center of our galaxy thanks to the interactions of binary stars with the supermassive black hole (SMBH) at the center of our galaxy – aka. Sagittarius A*. While their exact cause is still the subject of debate, the orbits of HVSs can be calculated since they are completely determined by the gravitational field of the galaxy.

As they explain in their study, the researchers used the asymmetry in the radial velocity distribution of stars in the galactic halo to determine the galaxy’s gravitational potential. The velocity of these halo stars is dependent on the potential escape speed of HVSs, provided that the time it takes for the HVSs to complete a single orbit is shorter than the lifetime of the halo stars.

From this, they were able to discriminate between different models for the Milky Way and the gravitational force it exerts. By adopting the nominal travel time of these observed HVSs – which they calculated to about 330 million years, about the same as the average lifetime of halo stars – they were able to derive gravitational estimates for the Milky Way which allowed for estimates on its overall mass.

“By calibrating the minimum speed of unbound stars, we find that the Milky Way mass is in the range of 1.2-1.9 trillions solar masses,” said Loeb. While still subject to a range, this latest estimate is a significant improvement over previous estimates. What’s more, these estimates are consistent our current cosmological models that attempt to account for all visible matter in the Universe, as well as dark matter and dark energy – the Lambda-CDM model.

Distribution of dark matter when the Universe was about 3 billion years old, obtained from a numerical simulation of galaxy formation. Credit: VIRGO Consortium/Alexandre Amblard/ESA

“The inferred Milky Way mass is in the range expected within the standard cosmological model,” said Leob, “where the amount of dark matter is about five times larger than that of ordinary (luminous) matter.”

Based on this breakdown, it can be said that normal matter in our galaxy – i.e. stars, planets, dust and gas – accounts for between 240 and 380 billion Solar Masses. So not only does this latest study provide more precise mass constraints for our galaxy, it could also help us to determine exactly how many star systems are out there – current estimates say that the Milky Way has between 200 to 400 billion stars and 100 billion planets.

Beyond that, this study is also significant to the study of cosmic formation and evolution. By placing more precise estimates on our galaxy’s mass, ones which are consistent with the current breakdown of normal matter and dark matter, cosmologists will be able to construct more accurate accounts of how our Universe came to be. One step clsoer to understanding the Universe on the grandest of scales!

Further Reading: Harvard Smithsonian CfA, Astronomy and Astrophysics

Chinese Astronomers Spot Two New Hypervelocity Stars

An artist's conception of a hypervelocity star that has escaped the Milky Way. Credit: NASA

Most stars in our galaxy behave predictably, orbiting around the center of the Milky Way at speeds of about 100 km/s (62 mi/s). But some stars achieve velocities that are significantly greater, to the point that they are even able to escape the gravitational pull of the galaxy. These are known as hypervelocity stars (HVS), a rare type of star that is believed to be the result of interactions with a supermassive black hole (SMBH).

The existence of HVS is something that astronomers first theorized in the late 1980s, and only 20 have been identified so far. But thanks to a new study by a team of Chinese astronomers, two new hypervelocity stars have been added to that list. These stars, which have been designated LAMOST-HVS2 and LAMOST-HVS3, travel at speeds of up to 1,000 km/s (620 mi/s) and are thought to have originated in the center of our galaxy.

The study which describes the team’s findings, titled “Discovery of Two New Hypervelocity Stars From the LAMOST Spectroscopic Surveys“, recently appeared online. Led by Yang Huang of the South-Western Institute for Astronomy Research at Yunnan University in Kunming, China, the team relied on data from Large Sky Area Multi-Object Fibre Spectroscopic Telescope (LAMOST) to detect these two new hypervelocity stars.

Footprint of the LAMOST pilot survey and the first three years’ general survey. Credit: LAMOST

Astronomers estimates that only 1000 HVS exist within the Milky Way. Given that there are as many as 200 billion stars in our galaxy, that’s just 0.0000005 % of the galactic population. While these stars are thought to originate in the center of our galaxy – supposedly as a result of interaction with our SMBH, Sagittarius A* – they manage to travel pretty far, sometimes even escaping our galaxy altogether.

It is for this very reason that astronomers are so interested in HVS. Given their speed, and the vast distances they can cover, tracking them and creating a database of their movements could provide constraints on the shape of the dark matter halo of our galaxy. Hence why Dr. Huang and his colleagues began sifting through LAMOST data to find evidence of new HVS.

Located in Hebei Province, northwestern China, the LAMOST observatory is operated by the Chinese Academy of Sciences. Over the course of five years, this observatory conducted a spectroscopic survey of 10 million stars in the Milky Way, as well as millions of galaxies. In June of 2017, LAMOST released its third Data Release (DR3), which included spectra obtained during the pilot survey and its first three years’ of regular surveys.

Containing high-quality spectra of 4.66 million stars and the stellar parameters of an additional 3.17 million, DR3 is currently the largest public spectral set and stellar parameter catalogue in the world. Already, LAMOST data had been used to identify one hypervelocity star, a B1IV/V-type (main sequence blue subgiant/subdwarf) star that was 11 Solar Masses, 13490 times as bright as our Sun, and had an effective temperature of 26,000 K (25,727 °C; 46,340 °F).

Artist’s impression of hypervelocity stars (HVSs) speeding through the Galaxy. Credit: ESA

This HVS was designated LAMOST-HSV1, in honor of the observatory. After detecting two new HVSs in the LAMOST data, these stars were designated as LAMOST-HSV2 and LAMOST-HSV3. Interestingly enough, these newly-discovered HVSs are also main sequence blue subdwarfs – or a B2V-type and B7V-type star, respectively.

Whereas HSV2 is 7.3 Solar Masses, is 2399 times as luminous as our Sun, and has an effective temperature of 20,600 K (20,327 °C; 36,620 °F), HSV3 is 3.9 Solar Masses, is 309 times as luminous as the Sun, and has an effective temperature of 14,000 K (24,740 °C; 44,564 °F). The researchers also considered the possible origins of all three HVSs based on their spatial positions and flight times.

In addition to considering that they originated in the center of the Milky Way, they also consider alternate possibilities. As they state in their study:

“The three HVSs are all spatially associated with known young stellar structures near the GC, which supports a GC origin for them. However, two of them, i.e. LAMOST-HVS1 and 2, have life times smaller than their flight times, indicating that they do not have enough time to travel from the GC to the current positions unless they are blue stragglers (as in the case of HVS HE 0437-5439). The third one (LAMOST-HVS3) has a life time larger than its flight time and thus does not have this problem.

In other words, the origins of these stars is still something of a mystery. Beyond the idea that they were sped up by interacting with the SMBH at the center of our galaxy, the team also considered other possibilities that have suggested over the years.

Artist’s impression of the ESA’s Gaia spacecraft, looking into the heart of the Milky Way  Galaxy. Credit: ESA/ATG medialab/ESO/S. Brunier

As they state in these study, these “include the tidal debris of an accreted and disrupted dwarf galaxy (Abadi et al. 2009), the surviving companion stars of Type Ia supernova (SNe Ia) explosions (Wang & Han 2009), the result of dynamical interaction between multiple stars (e.g, Gvaramadze et al. 2009), and the runaways ejected from the Large Magellanic Cloud (LMC), assuming that the latter hosts a MBH (Boubert et al. 2016).”

In the future, Huang and his colleagues indicate that their study will benefit from additional information that will be provided by the ESA’s Gaia mission, which they claim will shed additional light on how HVS behave and where they come from. As they state in their conclusions:

“The upcoming accurate proper motion measurements by Gaia should provide a direct constraint on their origins. Finally, we expect more HVSs to be discovered by the ongoing LAMOST spectroscopic surveys and thus to provide further constraint on the nature and ejection mechanisms of HVSs.”

Further Reading: arXiv

‘Lopsided’ Supernova Could Be Responsible for Rogue Hypervelocity Stars

Tauris argues that a lopsided supernova explosion may be the source of certain hypervelocity stars (image credit: IsiacDaGraca).

Hypervelocity stars have been observed traversing the Galaxy at extreme velocities (700 km/s), but the mechanisms that give rise to such phenomena are still debated.  Astronomer Thomas M. Tauris argues that lopsided supernova explosions can eject lower-mass Solar stars from the Galaxy at speeds up to 1280 km/s.   “[This mechanism] can account for the majority (if not all) of the detected G/K-dwarf hypervelocity candidates,” he said.

Several mechanisms have been proposed as the source for hypervelocity stars, and the hypotheses can vary as a function of stellar type.  A simplified summary of the hypothesis Tauris favors begins with a higher-mass star in a tight binary system, which finally undergoes a core-collapse supernova explosion.  The close proximity of the stars in the system partly ensures that the orbital velocities are exceedingly large.  The binary system is disrupted by the supernova explosion, which is lopsided (asymmetric) and imparts a significant kick to the emerging neutron star.  The remnants of supernovae with massive progenitors are neutron stars or potentially a more exotic object (i.e., black hole).

Conversely, Tauris noted that the aforementioned binary origin cannot easily explain the observed velocities of all higher-mass hypervelocity stars, namely the B-stars, which are often linked to an ejection mechanism from a binary interaction with the supermassive black hole at the Milky Way’s center.  Others have proposed that interactions between multiple stars near the centers of star clusters can give rise to certain hypervelocity candidates.

Certain astronomers argue that hypervelocity stars can stem from interactions in dense star clusters (image credit: Hubble)
Some astronomers argue that certain hypervelocity stars can stem from interactions in dense star clusters (image credit: NASA, ESA, and E. Sabbi (ESA/STScI))

There are several potential compact objects (neutron stars) which feature extreme velocities, such as B2011+38, B2224+65, IGR J11014-6103, and B1508+55, with the latter possibly exhibiting a velocity of 1100 km/s.  However, Tauris ends by noting that, “a firm identification of a hypervelocity star being ejected from a binary via a supernova is still missing, although a candidate exists (HD 271791) that’s being debated.”

Tauris is affiliated with the Argelander-Institut für Astronomie and Max-Planck-Institut für Radioastronomie. His findings will be published in the forthcoming March issue of the Monthly Notices of the Royal Astronomical Society.

The interested reader can find a preprint of Tauris’ study on arXiv.  Surveys of hypervelocity stars were published by Brown et al. 2014 and Palladino et al. 2014.

Hypervelocity Neutron Stars Crashing Into White Dwarfs — A Scenario for the Loneliest Supernovae?

University of Warwick researchers explain mystery of the loneliest supernovas. Compact binary star systems that have been thrown far from their host galaxy when one star of that pair became a neutron star, go through a second trauma when the remaining white dwarf star is eventually pulled onto the neutron star Credit: Artist’s impression is free to use with story but must include this credit: © Mark A. Garlick / space-art.co.uk / University of Warwick

It’s hard to comprehend the vast emptiness of space. Especially when we detect odd signatures, such as luminous explosions that are neither as bright nor as long as traditional supernovae, originating in the unfathomable emptiness.

But a team of astronomers is now beginning to understand these so-called calcium-rich transients, often referred to as the Universe’s loneliest supernovae, hypothesizing that they’re created by collisions between white dwarf stars and neutron stars — both of which have been thrown out of their galaxy.

“One of the weirdest aspects is that they seem to explode in unusual places. For example, if you look at a galaxy, you expect any explosions to roughly be in line with the underlying light you see from that galaxy, since that is where the stars are” said lead author Joseph Lyman from the University of Warwick in a press release. “However, a large fraction of these are exploding at huge distances from their galaxies, where the number of stellar systems is miniscule.”

The team guessed there could be very faint dwarf galaxies, hiding beneath the limit of detection, but found nothing with our best telescopes, namely the Very Large Telescope in Chile and the Hubble Space Telescope.

“So the question becomes, how did the get there?” pondered Lyman. Roughly a third of these events occur at least 65 thousand light-years away from a potential host galaxy.

We’ve discovered dozens of so-called hypervelocity stars — single stars that escape their home galaxy, traveling rapidly throughout intergalactic space — and even one runaway globular cluster. Nature clearly has a way of kicking systems out of an entire galaxy, likely by an interaction with the supermassive black hole lurking in the center of that galaxy.

So it’s viable that the source of these supernovae was first kicked out of its host galaxy. But the second puzzle wondered what type of system could have caused such an odd explosion.

Previous studies show that calcium comprises up to half of the material thrown off in these transients, compared to only a tiny fraction in normal supernovae. It remained unclear how to explain such a calcium-rich system.

So the research team compared their data to short-duration gamma ray bursts, which are also seen to explode in remote locations with no coincident galaxy detected. We think these enigmatic bursts occur when two neutron stars collide, or when a neutron star merges with a black hole.

Alas, the research team discovered that if a neutron star collided with a white dwarf, the explosion would not only provide enough energy to generate the low luminosity of the calcium rich transients, but it would also produce calcium rich material.

“What we therefore propose is these are systems that have been ejected from their galaxy,” said Lyman. “A good candidate in this scenario is a white dwarf and a neutron star in a binary system. The neutron star is formed when a massive star goes supernova. The mechanism of the supernova explosion causes the neutron star to be ‘kicked’ to very high velocities (100s of km/s). This high velocity system can then escape its galaxy, and if the binary system survives the kick, the white dwarf and neutron star will merge causing the explosive transient.”

Any merger should also produce high-energy gamma-ray bursts, motivating further observations of any new examples.

The paper has been published today in the journal Monthly Notices of the Royal Astronomical Society and is available online.