# Is there a way to estimate the age of M dwarf stars?

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Wolf 1061 (only 13.8 ly away) was recently found to have three rocky planets one of which is in the habitable zone. It was stated on the english wikipedia site that it has a very stable light curve and was not found to have "any significant activity such as sunspots or flares." Would this imply that this star is very old? The expected lifetime of this M dwarf with a mass estimated to be 0.25 solar mass could potentially reach a trillion years.

Really difficult.

Very young ($<50$ Myr) M dwarfs may still exhibit lithium in their photospheres. Any older and it will have been depleted.

For older dwarfs you then move on to looking at rotation and activity. Both decline with age, but in an M dwarf as cool as this, the decay timescale is many Gyr, so it is only weakly constraining and not very well calibrated for such low mass stars. A very slow rotation rate or very low level of magnetic activity might indicate it was older than 5 Gyr.

The lack of flares and the lack of light curve modulation by starspots may indicate that it has spun down, is magnetically inactive and is therefore older than 5 Gyr ( I cannot immediately locate the source of this Wikipedia claim). The ratio of X-ray to bolometric luminosity would be more definitive and could probably be derived from the ROSAT all-sky survey (or a constraining upper limit would be found).

A useful paper is by Stelzer et al. (2013). http://arxiv.org/abs/1302.1061 She finds various activity diagnostics for nearby stars, including this one. Its projected rotation velocity is 1.5 km/s, which suggests a rotation period of $sim 10$ days for a star of this size. This could not be less constraining on the age! The X-ray to bolometric flux ratio is $10^{-4.45}$ which suggests it is neither very young or very old - say between 1 and 5 Gyr for a star of this mass.

Another possibility is to look at the Galactic space motion and position. Older stars tend to have higher velocities, especially perpendicular to the Galactic plane, and also tend to be found further from the Galactic plane. The velocities for this object do not look large, suggesting it is not very old, ie younger than perhaps 10 Gyr and likely a young disk object with age of $<5$ Gyr.

You can also look at the metallicity. This is difficult to measure in M-dwarfs. Maldonado et al. (2015) give [Fe/H]$=-0.05 pm 0.03$. This is very close to the solar value and does not constrain the age beyond suggesting the star is younger than 10Gyr.

Beyond this, it is basically impossible. Even if you had a very precise distance and could place the star on a HR diagram, low mass M dwarfs evolve so little in luminosity and temperature over many Gyr that this offers no constraint.

So my conclusion - the age is between 1 and 5 Gyr, with the probability peaking somewhere in the middle.

## Starship Asterisk*

### RAS: Students Map Milky Way with Dwarf Stars

Post by bystander » Wed Mar 16, 2016 6:24 pm

Two astronomy students from Leiden University have mapped the entire Milky Way galaxy in dwarf stars for the first time. They show that there are a total of 58 billion dwarf stars, of which seven per cent reside in the outer regions of our Galaxy. This result is the most comprehensive model ever for the distribution of these stars. .

The Milky Way, the galaxy we live in, consists of a prominent, relatively flat disc with closely spaced bright stars, and a halo, a sphere of stars with a much lower density around it. Astronomers assume that the halo is the remnant of the first galaxies that fused together to form our Galaxy.

To find out exactly what the Milky Way looks like, astronomers have previously made maps using counts of the stars in the night sky. Leiden Astronomy students Isabel van Vledder and Dieuwertje van der Vlugt used the same technique in their research. Rather than studying bright stars, the two students used Hubble Space Telescope data from 274 dwarf stars, which were serendipitously observed by the orbiting observatory while it was looking for the most distant galaxies in the early Universe. The particular type of star they looked at were red dwarfs of spectral class M. .

### Re: RAS: Students Map Milky Way with Dwarf Stars

Post by Ann » Thu Mar 17, 2016 1:44 am

If the Milky Way contains 100 billion stars and 58 billion of them are M-type dwarfs, then the M-type dwarfs make up less than 60% of all stars in our galaxy. If this scenario is true, we should ask ourselves why M-type stars appear to be considerably more common in our part of the galaxy than in the galaxy as a whole. Finally, we should perhaps be a little surprised that our galaxy contains rather fewer stars than most astronomers had expected.

But if the Milky Way contains 400 billion stars and only 58 billion of them are M-type dwarfs, then the M-type dwarfs make up only about 15% of all stars in our galaxy, a rather ridiculously low figure.

And if it is true that there are only 58 billion M-type stars in the Milky Way, and they still make up 80% of all stars in our galaxy, then there are only a little more than 70 billion stars in the Milky Way, which is a lower number than almost any astronomer had anticipated.

The new work appears in "The Size and Shape of the Milky Way Disk and Halo from M- type Brown Dwarfs in the BoRG survey", Isabel van Vledder, Dieuwertje van der Vlugt, B.W. Holwerda, M. A. Kenworthy, R. J. Bouwens, and M. Trenti., Monthly Notices of the Royal Astronomical Society, Oxford University Press, in press.

There is no such thing as an M-type brown dwarf.

What do the rest of you here at Starship Asterisk* think about the claim that the total number of M-type dwarf stars in the Milky Way is 58 billion? Chris?

## 'Superflares' may make it hard for life to begin around dwarf stars

These powerful stellar eruptions pour out huge amounts of UV radiation.

Powerful stellar eruptions could pose a serious challenge to the origin and evolution of life around the universe, a new study suggests.

Such outbursts throw off large amounts of ultraviolet (UV) radiation, which is not only directly harmful to life as we know it but can also strip away the atmospheres of relatively close-orbiting planets. These issues are especially pronounced for worlds circling red dwarfs, small and dim stars that make up about 75% of the Milky Way galaxy's stellar population.

For starters, red dwarfs are more active than sunlike stars, especially when they're young. And, because each red dwarf is so dim, its "habitable zone" &mdash the range of orbital distances where liquid water could be stable on a world's surface &mdash is much closer-in than for a star such as our sun.

The new study helps flesh out this skeletal outline. Researchers calculated the likely UV emissions generated by red-dwarf superflares, as well as the radiation loads absorbed by rocky planets that might reside in the small stars' habitable zones.

"We found planets orbiting young stars may experience life-prohibiting levels of UV radiation, although some micro-organisms might survive," study lead author Ward Howard, a doctoral student in the Department of Physics and Astronomy at the University of North Carolina (UNC), Chapel Hill, said in a statement.

Howard and his colleagues measured the temperatures of 42 superflares emitted by 27 red dwarfs. They did so by analyzing observations made simultaneously by the Evryscope, an array of small telescopes at the Cerro Tololo Inter-American Observatory in Chile, and NASA's Transiting Exoplanet Survey Satellite, which has been hunting for alien worlds from Earth orbit since 2018.

These observations were obtained every 2 minutes, allowing the scientists to get a detailed temperature profile across the brief life of the red-dwarf superflares, which typically emit most of their UV radiation during a 10- to 15-minute-long peak. Temperature is strongly correlated with UV emission, so the researchers were then able to estimate the radiation loads imposed by the outbursts.

The new information could aid a variety of other astrobiological investigations going forward, team members said.

&ldquoLonger term, these results may inform the choice of planetary systems to be observed by NASA's James Webb Space Telescope based on the system's flaring activity," study co-author Nicholas Law, an associate professor of physics and astronomy at UNC-Chapel Hill and the Evryscope principal investigator, said in the same statement.

The new study continues the team's ongoing investigation into red-dwarf flaring and its potential impacts on life. For example, a 2018 paper led by Howard suggested that superflares have dimmed the astrobiological potential of Proxima b, a rocky, Earth-size world that orbits in the habitable zone of the red dwarf Proxima Centauri, the sun's nearest stellar neighbor.

The new study has been accepted for publication in The Astrophysical Journal. You can read a preprint of it for free at arXiv.org.

Mike Wall is the author of "Out There" (Grand Central Publishing, 2018 illustrated by Karl Tate), a book about the search for alien life. Follow him on Twitter @michaeldwall. Follow us on Twitter @Spacedotcom or Facebook.

## How Habitable Are Planets That Orbit Red Dwarfs – The Most Common Type of Stars in the Galaxy?

A research study using data from NASA’s Chandra X-ray Observatory and Hubble Space Telescope gives new insight into an important question: how habitable are planets that orbit the most common type of stars in the Galaxy? The target of the new study is Barnard’s Star, which is one of the closest stars to Earth at a distance of just 6 light-years. Barnard’s Star is a red dwarf, a small star that slowly burns through its fuel supply and can last much longer than medium-sized stars like our Sun. It is about 10 billion years old, making it twice the age of the Sun.

The authors used Barnard’s Star as a case study to learn how flares from an old red dwarf might affect any planets orbiting it. The artist’s illustration t the top of this page depicts an old red dwarf like Barnard’s Star (right) and an orbiting, rocky planet (left).

Credit: X-ray light curve: NASA/CXC/University of Colorado/K. France et al. UV light curve: NASA/STScI

The research team’s Chandra observations of Barnard’s Star taken in June 2019 uncovered one X-ray flare and their Hubble observations taken in March 2019 revealed two ultraviolet high-energy flares (shown in the graphic above). Both observations were about seven hours long and both plots show X-ray or ultraviolet brightness extending down to zero. Based on the length of the flares and of the observations, the authors concluded that Barnard’s Star unleashes potentially destructive flares about 25% of the time.

The team then studied what these results mean for rocky planets orbiting in the habitable zone — where liquid water could exist on their surface — around an old red dwarf like Barnard’s Star. Any atmosphere formed early in the life of a habitable-zone planet was likely to have been eroded away by high-energy radiation from the star during its volatile youth. Later on, however, planet atmospheres might regenerate as the star becomes less active with age. This regeneration process may occur by gases released by impacts of solid material or gases being released by volcanic processes.

However, the onslaught of powerful flares like those reported here, repeatedly occurring over hundreds of millions of years, may erode any regenerated atmospheres on rocky planets in the habitable zone. The illustration shows the atmosphere of the rocky planet being swept away to the left by energetic radiation from flares produced by the red dwarf. This would reduce the chance of these worlds supporting life. The team is currently studying high-energy radiation from many more red dwarfs to determine whether Barnard’s Star is typical.

Reference: “The High-energy Radiation Environment around a 10 Gyr M Dwarf: Habitable at Last?” by Kevin France, Girish Duvvuri, Hilary Egan, Tommi Koskinen, David J. Wilson, Allison Youngblood, Cynthia S. Froning, Alexander Brown, Julián D. Alvarado-Gómez, Zachory K. Berta-Thompson, Jeremy J. Drake, Cecilia Garraffo, Lisa Kaltenegger, Adam F. Kowalski, Jeffrey L. Linsky, R. O. Parke Loyd, Pablo J. D. Mauas, Yamila Miguel, J. Sebastian Pineda, Sarah Rugheimer, P. Christian Schneider, Feng Tian and Mariela Vieytes, 30 October 2020, The Astronomical Journal.
DOI: 10.3847/1538-3881/abb465

A paper describing these results, led by Kevin France of the University of Colorado at Boulder, appears in the October 30, 2020 issue of The Astronomical Journal. NASA’s Marshall Space Flight Center manages the Chandra program. The Smithsonian Astrophysical Observatory’s Chandra X-ray Center controls science and flight operations from Cambridge and Burlington, Massachusetts.

## Contents

### Early theorizing Edit

The objects now called "brown dwarfs" were theorized by Shiv S. Kumar in the 1960s to exist and were originally called black dwarfs, [9] a classification for dark substellar objects floating freely in space that were not massive enough to sustain hydrogen fusion. However: (a) the term black dwarf was already in use to refer to a cold white dwarf (b) red dwarfs fuse hydrogen and (c) these objects may be luminous at visible wavelengths early in their lives. Because of this, alternative names for these objects were proposed, including planetar [ check spelling ] and substar. In 1975, Jill Tarter suggested the term "brown dwarf", using "brown" as an approximate color. [6] [10] [11]

The term "black dwarf" still refers to a white dwarf that has cooled to the point that it no longer emits significant amounts of light. However, the time required for even the lowest-mass white dwarf to cool to this temperature is calculated to be longer than the current age of the universe hence such objects are expected to not yet exist.

Early theories concerning the nature of the lowest-mass stars and the hydrogen-burning limit suggested that a population I object with a mass less than 0.07 solar masses ( M ) or a population II object less than 0.09 M would never go through normal stellar evolution and would become a completely degenerate star. [12] The first self-consistent calculation of the hydrogen-burning minimum mass confirmed a value between 0.07 and 0.08 solar masses for population I objects. [13] [14]

### Deuterium fusion Edit

The discovery of deuterium burning down to 0.013 solar masses and the impact of dust formation in the cool outer atmospheres of brown dwarfs in the late 1980s brought these theories into question. However, such objects were hard to find because they emit almost no visible light. Their strongest emissions are in the infrared (IR) spectrum, and ground-based IR detectors were too imprecise at that time to readily identify any brown dwarfs.

Since then, numerous searches by various methods have sought these objects. These methods included multi-color imaging surveys around field stars, imaging surveys for faint companions of main-sequence dwarfs and white dwarfs, surveys of young star clusters, and radial velocity monitoring for close companions.

### GD 165B and class "L" Edit

For many years, efforts to discover brown dwarfs were fruitless. In 1988, however, a faint companion to a star known as GD 165 was found in an infrared search of white dwarfs. The spectrum of the companion GD 165B was very red and enigmatic, showing none of the features expected of a low-mass red dwarf. It became clear that GD 165B would need to be classified as a much cooler object than the latest M dwarfs then known. GD 165B remained unique for almost a decade until the advent of the Two Micron All-Sky Survey (2MASS) which discovered many objects with similar colors and spectral features.

Today, GD 165B is recognized as the prototype of a class of objects now called "L dwarfs". [15] [16]

Although the discovery of the coolest dwarf was highly significant at the time, it was debated whether GD 165B would be classified as a brown dwarf or simply a very-low-mass star, because observationally it is very difficult to distinguish between the two. [ citation needed ]

Soon after the discovery of GD 165B, other brown-dwarf candidates were reported. Most failed to live up to their candidacy, however, because the absence of lithium showed them to be stellar objects. True stars burn their lithium within a little over 100 Myr, whereas brown dwarfs (which can, confusingly, have temperatures and luminosities similar to true stars) will not. Hence, the detection of lithium in the atmosphere of an object older than 100 Myr ensures that it is a brown dwarf.

### Gliese 229B and class "T" – the methane dwarfs Edit

The first class "T" Brown Dwarf was discovered in 1994 by Caltech astronomers Shrinivas Kulkarni, Tadashi Nakajima, Keith Matthews, and Rebecca Oppenheimer, [17] and Johns Hopkins scientists Sam Durrance and David Golimowski. It was confirmed in 1995 as a substellar companion to Gliese 229. Gliese 229b is one of the first two instances of clear evidence for a brown dwarf, along with Teide 1. Confirmed in 1995, both were identified by the presence of the 670.8 nm lithium line. The latter was found to have a temperature and luminosity well below the stellar range.

Its near-infrared spectrum clearly exhibited a methane absorption band at 2 micrometres, a feature that had previously only been observed in the atmospheres of giant planets and that of Saturn's moon Titan. Methane absorption is not expected at any temperature of a main-sequence star. This discovery helped to establish yet another spectral class even cooler than L dwarfs, known as "T dwarfs", for which Gliese 229B is the prototype.

### Teide 1 – the first class "M" brown dwarf Edit

The first confirmed class "M" brown dwarf was discovered by Spanish astrophysicists Rafael Rebolo (head of team), María Rosa Zapatero Osorio, and Eduardo Martín in 1994. [18] This object, found in the Pleiades open cluster, received the name Teide 1. The discovery article was submitted to Nature in May 1995, and published on 14 September 1995. [19] [20] Nature highlighted "Brown dwarfs discovered, official" in the front page of that issue.

Teide 1 was discovered in images collected by the IAC team on 6 January 1994 using the 80 cm telescope (IAC 80) at Teide Observatory and its spectrum was first recorded in December 1994 using the 4.2 m William Herschel Telescope at Roque de los Muchachos Observatory (La Palma). The distance, chemical composition, and age of Teide 1 could be established because of its membership in the young Pleiades star cluster. Using the most advanced stellar and substellar evolution models at that moment, the team estimated for Teide 1 a mass of 55 ± 15 M J, [21] which is below the stellar-mass limit. The object became a reference in subsequent young brown dwarf related works.

In theory, a brown dwarf below 65 M J is unable to burn lithium by thermonuclear fusion at any time during its evolution. This fact is one of the lithium test principles used to judge the substellar nature of low-luminosity and low-surface-temperature astronomical bodies.

High-quality spectral data acquired by the Keck 1 telescope in November 1995 showed that Teide 1 still had the initial lithium abundance of the original molecular cloud from which Pleiades stars formed, proving the lack of thermonuclear fusion in its core. These observations confirmed that Teide 1 is a brown dwarf, as well as the efficiency of the spectroscopic lithium test.

For some time, Teide 1 was the smallest known object outside the Solar System that had been identified by direct observation. Since then, over 1,800 brown dwarfs have been identified, [22] even some very close to Earth like Epsilon Indi Ba and Bb, a pair of brown dwarfs gravitationally bound to a Sun-like star 12 light-years from the Sun, and Luhman 16, a binary system of brown dwarfs at 6.5 light-years from the Sun.

## Dating the stars: most accurate red giant age yet

Scientists identify stars leftover from cosmic collision.

Researchers have successfully dated some of our galaxy’s oldest stars back to a cosmic collision, using data on their oscillations and chemical composition.

The team, led by Josefina Montalbán of the University of Birmingham, UK, investigated the age of some red giant stars that were originally part of a satellite dwarf galaxy called Gaia-Enceladus, which collided with the Milky Way 11.5 billion years ago.

In their study, published in Nature Astronomy, the researchers surveyed 100 red giant stars and found that the Gaia-Enceladus stars were all similar in age or slightly younger than the other stars that began life in the Milky Way. This builds on the existing theory that the Milky Way had already started making stars before it merged with Gaia-Enceladus.

“The chemical composition, location and motion of the stars we can observe today in the Milky Way contain precious information about their origin,” says Montalbán.

“As we increase our knowledge of how and when these stars were formed, we can start to better understand how the merger of Gaia-Enceladus with the Milky Way affected the evolution of our Galaxy.”

As part of their analysis, they used a technique called asteroseismology, which measures relative frequency and amplitudes of the natural modes of oscillations of stars. This gives information about the size and internal structure of stars, which then helps estimate star age.

They combined this data with spectroscopy – a technique that measures light and radiation produced by matter – to identify the chemical composition of the stars, which also reveals information about age.

“We have shown the huge potential of asteroseismology in combination with spectroscopy to deliver precise, accurate relative ages for individual, very old, stars,” says co-author Andrea Miglio of the University of Bologna, Italy.

“Taken together, these measurements contribute to sharpen our view on the early years of our Galaxy and promise a bright future for Galactic archeoastronomy.”

### Deborah Devis

Dr Deborah Devis is a science journalist at The Royal Institution of Australia.

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Astronomical theory has lately run into a series of problems. The most troublesome problem is the most recent calculation of the so-called Hubble constant using the repaired Hubble Space Telescope.

Astronomical theory has lately run into a series of nasty problems. The most troublesome problem is the most recent calculation of the so-called Hubble constant using the repaired Hubble Space Telescope. This constant is used to calculate the expansion rate of the universe.1 Based on measurements of 20 Cepheid variable stars in the Virgo Cluster of galaxies, the Hubble constant was measured at 80 kilometres per second per megaparsec (km s -1 Mpc -1 ).2 , 3 Assuming the Big Bang theory for the origin of the universe, the above expansion rate corresponds to an ‘age’ of the universe of 8 to 12 billion years, depending upon how much ‘dark matter’ is in the universe. This more sophisticated measurement agrees with other less precise recent measurements. Another group of astronomers led by Allan Sandage have claimed and consistently measured the Hubble constant at about 50 km s -1 Mpc -1 .4 This would make the universe about 14 to 20 billion years old.5 Several astronomers recently argued that astronomical theories would best fit a Hubble constant of 30 km s -1 Mpc -1 .6

The newer, younger age contradicts the age of globular clusters, dense groupings of stars in a galaxy, that are thought to be 16 billion years old. Thus, astronomers are presented with the paradox that the objects in the universe may be much older than the universe itself. It also is

‘. . . a blow for the Big Bang account of the beginning of the Universe, although not necessarily a fatal one.’7

Another recent report throws confusion on the postulated dark matter in the universe.8 Dark matter is needed by old age theorists to account for rapid stellar speeds in galaxies. If there is enough of this dark matter, much more than visible matter, then the universe would also be ‘closed’. Assuming the Big Bang model, a closed universe would eventually collapse back onto itself, if there was enough dark matter. It was hoped that this dark matter would be mostly in the form of small stars called red dwarfs. New Hubble Space Telescope measurements now indicate there are hardly any of these red dwarf stars. So cosmologists must rely more on some type of exotic matter, which has so far been undetected. A further problem is that the red dwarfs they did detect are believed to weigh in at 20 % of the sun’s mass, which is contrary to popular models of star formation. One of these red dwarfs was seen to produce a flare, an event supposedly reserved only for more massive stars.9

Closer to home, astronomers are finally concluding after 25 years of measurements that the missing solar neutrinos are really missing. Four different detectors can account for only 30ñ50% of the neutrinos that theoretical models of solar fusion say must exist. This paradox

‘. . . refutes the basic logic of the reaction chain that powers the sun by the fusion of protons into heavy elements.’10

More specifically, in the proton-proton fusion reaction boron-8 must be made from beryllium-7, but hardly any neutrinos of beryllium-7 are detected while plenty of boron-8 neutrinos are detected. Although indicating fusion reactions in the sun, the missing neutrinos point to some glaring theoretical problems in understanding our own sun, not to speak of distant stars.

[Ed. note: while the above argument on neutrinos was cogent given the information then available, subsequent information has superseded it, meaning that the shortfall problem seems to have been solved. Therefore creationists should no longer use this as an ‘ age’ argument—see more detail.]

Apparent velocities greater than the speed of light have been claimed before in radio-emitting components in some distant quasars and active galactic nuclei. These claims have been uncertain because of the extreme distances surmised for these exotic objects. Now, however, it is claimed that apparent velocities greater than the speed of light have been detected within our own Milky Way Galaxy.11 Quasars are in the news again. A super-massive black hole at the centre of a galaxy is thought to provide the tremendous energy for a quasar. However, a recent report at the annual meeting of the American Astronomical Society indicates that only four out of the 15 quasars surveyed by the Hubble Space Telescope are associated with galaxies.12 Geoffrey Burbidge believes the ramifications of this discovery are far reaching and challenge the paradigm that quasars are huge black holes. In another development, some physicists are attacking the very existence of black holes:-

‘But a handful of physicists who have offered their work at recent meetings and in upcoming publications think black hole seekers are pursuing a chimera, something like the ether of the 19th century.’13

These reports from the most widely read journals in the world indicate the field of astronomy has quite a number of severe theoretical problems.

## Repeating ‘Fast Radio Burst’ Found to Originate from Extremely Distant Dwarf Galaxy

The globally distributed dishes of the European VLBI Network are linked with each other and the 305-m William E. Gordon Telescope at the Arecibo Observatory in Puerto Rico. Together they have localized FRB121102’s exact position within its host galaxy. Image credit: Danielle Futselaar, www.artsource.nl.

Fast radio bursts (FRBs) are powerful, rarely detected bursts of energy from space.

Astrophysicists estimate that there are between 2,000 and 10,000 FRBs occurring in the sky every day.

These events have durations of milliseconds and exhibit the characteristic dispersion sweep of radio pulsars. They emit as much energy in one millisecond as the Sun emits in 10,000 years, but the physical phenomenon that causes them is unknown.

The first FRB was discovered in 2007, although it was actually observed some six years earlier, in archival data from a pulsar survey of the Magellanic Clouds.

There are now 18 known FRBs. All were detected using single-dish radio telescopes that are unable to narrow down the object’s location with enough precision to allow other observatories to identify its host environment.

Unlike all the others, however, FRB 121102, discovered in November of 2012 at the Arecibo Observatory in Puerto Rico, has recurred numerous times — a pattern first detected in late 2015 by Dominion Radio Astrophysical Observatory astronomer Paul Scholz.

Rising just ahead of the winter constellation Orion, FRB 121102 has a home in the constellation Auriga.

“There’s a patch of the sky from which we’re getting this signal — and the patch of the sky is arc minutes in diameter. In that patch are hundreds of sources. Lots of stars, lots of galaxies, lots of stuff,” said team member Dr. Shami Chatterjee, an astronomer at Cornell University.

The repeating bursts from FRB 121102 allowed astronomers to watch for it in 2016 using NSF’s Karl G. Jansky Very Large Array (VLA). In 83 hours of observing time over six months in 2016, they detected nine bursts.

Using the precise VLA position, the team used the Gemini North telescope on Maunakea in Hawai’i to make a visible-light image that identified a faint dwarf galaxy at the location of the bursts.

Spectroscopic data from Gemini also enabled the astronomers to determine that the dwarf galaxy is more than 3 billion light-years from Earth.

“While the exact cause of the high-energy bursts remains unclear, the fact that this particular FRB comes from a distant dwarf galaxy represents a huge advance in our understanding of these events,” Dr. Chatterjee said.

“The host galaxy for this FRB appears to be a very humble and unassuming dwarf galaxy, which is less than 1% of the mass or our Milky Way galaxy,” said team member Dr. Shriharsh Tendulkar, an astronomer at McGill University.

“That’s surprising. One would generally expect most FRBs to come from large galaxies which have the largest numbers of stars and neutron stars — remnants of massive stars.”

“This dwarf galaxy has fewer stars, but is forming stars at a high rate, which may suggest that FRBs are linked to young neutron stars.”

“There are also two other classes of extreme events — long duration gamma-ray bursts and superluminous supernovae — that frequently occur in dwarf galaxies, as well.”

“This discovery may hint at links between FRBs and those two kinds of events.”

Gemini composite image of the field around FRB 121102 (indicated). The dwarf host galaxy was imaged, and spectroscopy performed, using the Gemini North telescope. Image credit: Gemini Observatory / AURA / NSF / NRC.

In addition to detecting the bursts from FRB 121102, the VLA observations also revealed an ongoing, persistent source of weaker radio emission in the same region.

Next, the team used the European VLBI Network (EVN), along with the William E. Gordon Telescope of the Arecibo Observatory, and NSF’s Very Long Baseline Array (VLBA) to determine the object’s position with even greater accuracy.

“These ultra-high precision observations showed that the bursts and the persistent source must be within 100 light-years of each other,” said team member Dr. Jason Hessels, of the Netherlands Institute for Radio Astronomy and the University of Amsterdam.

The next big question is ‘what powers FRB 121102?’ “We think it may be a magnetar – a newborn neutron star with a huge magnetic field, inside a supernova remnant or a pulsar wind nebula – somehow producing these prodigious pulses,” Dr. Chatterjee said.

“Or, it may be an active galactic nucleus of a dwarf galaxy. That would be novel.”

“Or, it may be a combination of those two ideas – explaining why what we’re seeing may be somewhat rare.”

The characterization of the host galaxy was published in the Astrophysical Journal Letters, and accompanied the team’s results on a campaign to precisely locate FRB 121102, published in the journal Nature.

S. Chatterjee et al. 2017. A direct localization of a fast radio burst and its host. Nature 541, 58-61 doi: 10.1038/nature20797

S.P. Tendulkar et al. 2017. The Host Galaxy and Redshift of the Repeating Fast Radio Burst FRB 121102. ApJL 834, L7 doi: 10.3847/2041-8213/834/2/L7

B. Marcote et al. 2017. The Repeating Fast Radio Burst FRB 121102 as Seen on Milliarcsecond Angular Scales. ApJL 834, L8 doi: 10.3847/2041-8213/834/2/L8

## Is There a way to Detect Strange Quark Stars, Even Though They Look Almost Exactly Like White Dwarfs?

The world we see around us is built around quarks. They form the nuclei of the atoms and molecules that comprise us and our world. While there are six types of quarks, regular matter contains only two: up quarks and down quarks. Protons contain two ups and a down, while neutrons contain two downs and an up. On Earth, the other four types are only seen when created in particle accelerators. But some of them could also appear naturally in dense objects such as neutron stars.

Neutron star vs a quark star. Credit: CXC/M. Weiss

The standard model for neutron stars holds that neutrons remain largely intact within their interior. Thus, a neutron star is like a huge atomic nucleus held together by gravity rather than the strong nuclear force. But we don’t fully understand how neutrons interact at extreme temperatures and densities. It’s possible that within a neutron star the neutrons break down into a soup of quarks, forming what is known as a quark star. Quark stars would look like neutron stars but would be slightly smaller.

If quark stars exist, then it’s possible that high-energy up and down quarks could collide to create strange quarks. And this is where things could get, well, a bit strange. Strange quarks are much heavier than up and down quarks, so strange quarks would tend to form a new type of nucleon known as strangelets. A simple strangelet would consist of an up, down, and a strange quark. Because strangelets are much denser than protons and neutrons, contact between the two would rip apart the protons and neutrons to create more strangelets. Essentially, if strange matter comes into contact with regular matter it doesn’t take long for it to be converted to strange matter. You could have everything from strange stars to strange planets.

Strange quarks can appear in regular nucleons. Credit: APS/Alan Stonebraker

While strange matter is an interesting idea, it isn’t a popular one. To begin with, if strange quark matter forms in some neutron stars, it should form in all of them, causing them to collapse. But we see lots of neutron stars that are too large to be strange quarks. There’s also the fact that strange quarks can appear within regular protons and neutrons. For example, although a proton is “made” of two up quarks and a down quark, that’s really only an average. Quantum fluctuations mean that strange quarks can appear for short periods of time. But they aren’t stable and don’t convert nucleons into strange matter. So if strange matter does exist, it likely only exists within large and dense objects.

Still, it is worth looking for strange matter objects in the universe, and recently a study has found a few candidates. The study searched for a type of object known as strange dwarfs. These hypothetical objects have a mass similar to a white dwarf, but instead of being made of regular matter in a degenerate state, they are made of strange quark matter. As a result, they would be much smaller than white dwarfs.

The mass-radius relation for white dwarfs. Credit: Brian Koberlein

To find these objects, the team looked at data from the Montreal White Dwarf Database (MWDD), which has data on more than 50,000 white dwarfs. For about 40,000 of them, the database lists both the mass and surface gravity of the white dwarfs. The mass of a white dwarf can be determined by the Doppler shift of its light as it orbits a companion star, or by gravitational lensing, while the surface gravity can be measured by the gravitational redshift of its light.

If you know the mass and surface gravity of a star, you can easily calculate its radius. The team did this and then compared them to the mass and radius relation for white dwarfs. Most of them followed the relation, but 8 of the stars didn’t. They were much smaller in size and matched predictions for a quark dwarf.

The data of this work isn’t strong enough to prove these objects are strange dwarfs, but they are worth further study. Something is strange about them, and it would be good to determine whether that’s due to strange quarks or something else.

Reference: Abudushataer Kuerban, et al. “Searching for Strange Quark Matter Objects Among White Dwarfs.” arXiv preprint arXiv:2012.05748 (2020).

## The nearest star to us may be too hellish for life. Here’s why.

Proxima Centauri has a potentially habitable exoplanet. But the temperamental star may have fried that “potentially” away.

Proxima b, the closest exoplanet to our Solar System, has been a focal point of scientific study since it was first confirmed (in 2016). This terrestrial planet (aka rocky) orbits Proxima Centauri, an M-type (red dwarf) star located 4.2 light-years beyond our Solar System – and is a part of the Alpha Centauri system. In addition to its proximity and rocky composition, it is also located within its parent star’s habitable zone (HZ).

Until a mission can be sent to this planet (such as Breakthrough Starshot), astrobiologists are forced to postulate about the possibility that life could exist there. Unfortunately, an international campaign that monitored Proxima Centauri for months using nine space- and ground-based telescopes recently spotted an extreme flare coming from the star, one which would have rendered Proxima b uninhabitable.

The campaign was led by Meredith A. MacGregor, an assistant professor of astrophysics from the University of Colorado Boulder, and included members from the Carnegie Institution for Science, Sydney Institute for Astronomy (SIfA), CSIRO Astronomy and Space Science, Space Telescope Science Institute (STScI), the Harvard-Smithsonian Center for Astrophysics (CfA), and multiple universities.

M-type stars like Proxima Centauri are a class of low-mass, low-luminosity stars that are known to be variable and unstable compared to other classes. In particular, these stars are prone to flare-ups, which occur when there’s a shift in their magnetic fields that accelerates electrons to near light-speed (NLS). These electrons interact with the star’s plasma, causing an eruption that produces emissions across the entire electromagnetic (EM) spectrum.

To determine how much Proxima Centauri flares, the research team observed the star for 40 hours over the course of several months in 2019. This included the Australian Square Kilometre Array Pathfinder (ASKAP), Atacama Large Millimeter/submillimeter Array (ALMA), Hubble Space Telescope (HST), Transiting Exoplanet Survey Satellite (TESS), and the du Pont Telescope.

These telescopes recorded a massive flare on May 1st, 2019, capturing the event as it produced a wide-EM spectrum of radiation and tracing its timing and energy in unprecedented detail. As MacGregor explained in a recent Carnegie Science press release:

“The star went from normal to 14,000 times brighter when seen in ultraviolet wavelengths over the span of a few seconds… If there was life on the planet nearest to Proxima Centauri, it would have to look very different than anything on Earth. A human being on this planet would have a bad time.”

Since red dwarfs are rather dim compared to other types of stars, flare-ups are not likely to produce much in the way of visible light. Ordinarily, astronomers consider themselves lucky if they can observe flares of this kind with just two instruments. This campaign was the first time that astronomers were able to get multi-wavelength coverage of a stellar flare, which allowed them to observe the huge surges in ultraviolet and millimeter-wave radiation.

The team’s findings, which appeared in The Astrophysical Journal Letters on April 21st, constitute one of the most in-depth anatomies of a flare from any star in our galaxy. In the future, these signals could help researchers gather more information about how stars generate flares, which could have immense implications for exoplanet and habitability studies. Unfortunately, it does not bode well for planets like Proxima b.

This research is the latest in a series of papers and studies conducted since Proxima b was discovered that indicate how the system is not suitable for life. As the closest exoplanet to Earth, and located in the star’s HZ, Proxima b is the most likely candidate for follow-up observations and astrobiological surveys. But according to this latest study, the flares it emits would have likely rendered the planet sterile a long time ago. As Weinberger explained:

“Proxima Centauri is of similar age to the Sun, so it’s been blasting its planets with high energy flares for billions of years. Studying these extreme flares with multiple observatories lets us understand what its planets have endured and how they might have changed. Now we know these very different observatories operating at very different wavelengths can see the same fast, energetic impulse.”

Beyond Proxima Centauri, the findings could also have implications for all planets that orbit within the HZs of red dwarf stars. M-type dwarfs are the most common type of stars in our galaxy and account for about 70% of stars in the entire Universe. Of the over 4,375 exoplanets that have been confirmed to date, a statistically significant number of the “Earth-like” planets have been found orbiting M-type dwarfs.

This has led many astronomers to speculate that the best place to find potentially habitable rocky planets is in red dwarf star systems. For this to be true, most of these stars would have to be significantly less active than Proxima Centauri. On a more positive note, the research suggests that our closest stellar neighbor could have more surprises in store for astronomers, like previously unknown types of flares that demonstrate exotic physics.

This research was conducted with support provided by the NASA Goddard Space Center.

1. Derward

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2. Apophis

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3. Cleit

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4. Marsyas

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