Determining the Mass of the Milky Way Using Hypervelocity Stars

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

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

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?

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.

“With a Little Help From Their Friends,” Magnetars Form in Binary Systems, New Study Suggests

Astronomy is a discipline of extremes. We’re constantly searching for the most powerful, the most explosive, and the most energetic objects in the Universe. Magnetars — extremely dense and highly magnetic neutron stars — are no exception to the rule. They’re the strongest known magnets in the Universe, millions of times more powerful than the strongest magnets on Earth.

But their origin has eluded astronomers for 35 years. Now, an international team of astronomers think they’ve found the partner star of a magnetar for the first time, an observation that suggests magnetars form in binary star systems.

When the core of a massive star runs out of energy, it collapses to form an incredibly dense neutron star or black hole. Meanwhile the outer layers of the star blow away in a stupendously powerful explosion, known as a supernova. A teaspoon of “neutron star stuff” would have a mass of about a billion tonnes, and a few cups would outweigh Mount Everest.

Magnetars are an unusual form of neutron stars with powerful magnetic fields. While there are roughly a dozen known magnetars in the Milky Way, one stands out as being the most peculiar. CXOU J164710.2-455216 — located 16,000 light-years away in the young star cluster Westerlund 1 — is unlike any other magnetar because astronomers can’t see how it formed in the first place.

Astronomers estimate that this magnetar must have been born in the explosive death of a star about 40 times the mass of the Sun. “But this presents its own problem, since stars this massive are expected to collapse to form black holes after their deaths, not neutron stars,” said Simon Clark, lead author on the paper, in a press release. “We did not understand how it could have become a magnetar.”

So astronomers went back to the drawing board. The most promising solution suggested that the magnetar formed through the interactions of two massive stars orbiting one another. Once the more massive star began to run out of fuel, it transferred mass to the less massive companion, causing it to rotate more and more rapidly — a crucial ingredient to creating ultra-strong magnetic fields.

In turn, the companion star became so massive that it shed a large amount of its recently gained mass. This caused it “to shrink to low enough levels that a magnetar was born instead of a black hole — a game of stellar pass-the-parcel with cosmic consequences” said coauthor Francisco Najarro from the Centro de Astrobiología in Spain.

This image of the young star cluster Westerlund 1 was taken with the Wide Field Imager on the MPG/ESO 2.2-metre telescope at ESO’s La Silla Observatory in Chile. Although most stars in the cluster are hot blue supergiants, they appear reddish in this image as they are seen through interstellar dust and gas. European astronomers have for the first time demonstrated that the magnetar in this cluster — an unusual type of neutron star with an extremely strong magnetic field — probably was formed as part of a binary star system. The discovery of the magnetar’s former companion (Westerlund 1-5) elsewhere in the cluster helps solve the mystery of how a star that started off so massive could become a magnetar, rather than collapse into a black hole. Credit: ESO
This image shows both the magnetar and its former binary companion, which has been kicked far away. Image Credit: ESO

There was only one slight problem: no companion star had been found. So Clark and colleagues set out to search for a star in other parts of the cluster. They used ESO’s Very Large Telescope to hunt for a hypervelocity star — an object escaping the cluster at an incredible speed — that might have been kicked out of orbit by the supernova explosion that formed the magnetar.

One star, known as Westerlund 1-5, matched their prediction.

“Not only does this star have the high velocity expected if it is recoiling from a supernova explosion, but the combination of its low mass, high luminosity and carbon-rich composition appear impossible to replicate in a single star — a smoking gun that shows it must have originally formed with a binary companion,” said coauthor Ben Ritchie from Open University.

The discovery suggests that double star systems may be essential for forming these enigmatic stars.

The paper has been published in Astronomy & Astrophysics, and is available for download here.

How do Hypervelocity Stars End up Breaking The Speed Limit?

The Sun is racing through the Galaxy at a speed that is 30 times greater than a space shuttle in orbit (clocking in at 220 km/s with respect to the galactic center). Most stars within the Milky Way travel at a relatively similar speed. But certain stars are definitely breaking the stellar speed limit. About one in a billion stars travel at a speed roughly 3 times greater than our Sun – so fast that they can easily escape the galaxy entirely!

We have discovered dozens of these so-called hypervelocity stars. But how exactly do these stars reach such high speeds? Astronomers from the University of Leicester may have found the answer.

The first clue comes in observing hypervelocity stars, where we can note their speed and direction. From these two measurements, we can trace these stars backward in order to find their origin. Results show that most hypervelocity stars begin moving quickly in the Galactic Center.

We now have a rough idea of where these stars gain their speed, but not how they reach such high velocities. Astronomers think two processes are likely to kick stars to such great speeds. The first process involves an interaction with the supermassive black hole (Sgr A*) at the center of our Galaxy. When a binary star system wanders too close to Sgr A*, one star is likely to be captured, while the other star is likely to be flung away from the black hole at an alarming rate.

The second process involves a supernova explosion in a binary system. Dr. Kastytis Zubovas, lead author on the paper summarized here, told Universe Today, “Supernova explosions in binary systems disrupt those systems and allow the remaining star to fly away, sometimes with enough velocity to escape the Galaxy.”

There is, however, one caveat. Binary stars in the center of our Galaxy will both be orbiting each other and orbiting Sgr A*. They will have two velocities associated with them. “If the velocity of the star around the binary’s center of mass happens to line up closely with the velocity of the center of mass around the supermassive black hole, the combined velocity may be large enough to escape the Galaxy altogether,” explained Zubovas.

In this case, we can’t sit around and wait to observe a supernova explosion breaking up a binary system. We would have to be very lucky to catch that! Instead, astronomers rely on computer modeling to recreate the physics of such an event. They set up multiple calculations in order to determine the statistical probability that the event will occur, and check if the results match observations.

Astronomers from the University of Leicester did just this. Their model includes multiple input parameters, such as the number of binaries, their initial locations, and their orbital parameters. It then calculates when a star might undergo a supernova explosion, and depending on the position of the two stars at that time, the final velocity of the remaining star.

The probability that a supernova disrupts a binary system is greater than 93%. But does the secondary star then escape from the galactic center? Yes, 4 – 25% of the time. Zubovas described, “Even though this is a very rare occurrence, we may expect several tens of such stars to be created over 100 million years.” The final results suggest that this model ejects stars with rates high enough to match the observed number of hypervelocity stars.

Not only do the number of hypervelocity stars match observations but also their distribution throughout space. “Hypervelocity stars produced by our supernova disruption method are not evenly distributed on the sky,” said Dr. Graham Wynn, a co-author on the paper. “They follow a pattern which retains an imprint of the stellar disk they formed in. Observed hypervelocity stars are seen to follow a pattern much like this.”

In the end, the model was very successful at describing the observed properties of hypervelocity stars. Future research will include a more detailed model that will allow astronomers to understand the ultimate fate of hypervelocity stars, the effect that supernova explosions have on their surroundings, and the galactic center itself.

It’s likely that both scenarios – binary systems interacting with the supermassive black hole and one undergoing a supernova explosion – form hypervelocity stars.  Studying both will continue to answer questions about how these speedy stars form.

The results will be published in the Astrophysical Journal (preprint available here)