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A New Class for Tau Scorpii

Many classes of stars are named for an early, distinguished member of a certain type of stars. For example, Cepheid variables take their namesake from the periodic variable Delta Cephei, first recognized by John Goodricke, although Eta Aquillae, another Cepheid, was recognized as a periodic variable with the same period just before Delta Cephei. Since the time of Goodricke’s discovery, many more classes of objects have been discovered from T Tauri, to W Ursa Majoris, to Delta Scorpii.

But sometimes, stars must wait before more members of their class are discovered. Tau Scorpiiis a massive B0 star and one of the rare high mass stars for which magnetic fields have been measured. To distinguish it even further, studies have shown that its magnetic field is unusually complex, being much more tangled than most stars and not showing distinct dipoles. Additionally, this unusual star has been shown to have weaker stellar winds (and consequently, mass loss rates) than most B0 type stars, as well as spectral features that are simultaneously characteristic of stars on the main sequence and young giants. Meanwhile, the star is believed to be only a few million years old. A first step towards characterizing such odd objects is to find more. Fortunately, astronomers have discovered two more stars similar to Tau Scorpii.

The two new stars, HD 66665 and HD 63425, were first recognized as unusual from their spectra, taken by the Canada-France-Hawaii Telescope. Using these spectra, the team, led by Véronique Petit at West Chester University, recognized that these stars had the same peculiar winds as Tau Scorpii. While Petit’s group could not completely constrain the mass loss rates, they did place an upper limit on both, establishing that they too shared the “weak wind problem” in which the expected mass loss rate for such stars was roughly 20 times higher. This prompted the team to investigate each star for magnetic fields.

Although the team wasn’t able to fully analyze the magnetic fields during their observing run to determine just how unusual they were, the team did establish both stars did have magnetic fields present and that they were similar in strength to that of Tau Scorpii. These two pieces of information has led the team to conclude that HD 66665 and HD 63425, along with Tau Scorpii, constitute a new class of stars. Additional confirmation could come from similar conclusions on the age of the analogues.

Petit’s team doesn’t speculate as to the nature of this emerging class in this paper. However, an earlier work of which Petit was a co-author, examined Tau Scorpii specifically. In it, the team examined whether the unusual field was a “frozen in” fossil from formation, or actively produced by an unusual dynamo inside the star. Fields produced by dynamos require large portions of the interior of the star undergoing convection. Models of massive stars predict that convection is likely to be limited in such stars. Another key component is rotation. Tau Scorpii is an extremely slow rotator, so the team concluded that a dynamo is unlikely in this case. As such, the fossil-field theory was more likely. Further investigation of HD 66665 and HD 63425 will certainly be necessary to further compare these stars to Tau Scorpii.

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Magnetic Fields on O-Class Stars

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The primary method by which astronomers can measure magnetic field strength on stars is the Zeeman effect. This effect is the splitting of spectral lines into two due to the magnetic field’s effect on the quantum structure of the orbitals. For massive O-class stars, their spectra are largely featureless in the visual portion of the spectra due to an insufficient number of atoms with electrons in the necessary orbitals to undergo transitions which can produce visual spectral lines. Thus, determining whether or not these stars have magnetic fields has been a unique challenge. A new paper from researchers at the University of Amsterdam, led by Roald Schnerr, looks for evidence of these fields in the form of synchrotron radiation.

Synchrotron radiation is a form of light produced when relativistic, charged particles move through a magnetic field. The light emitted can be generated in any portion of the spectra from radio to gamma rays, depending on the strength of the field. Astronomically, this was first detected in 1956 by Geoffrey Burbidge in the jets of M87 and has since been used to explain emission in planetary magnetospheres, supernovae, near black holes, and around pulsars.

This form of energy distinguishes itself from other forms of light in two main fashions. The first is that it is highly polarized. This property is generated by the electric and magnetic components always being in the same planes and can be studied with filters that only allow light with its fields in appropriate planes to pass. The second is that the radiation created is “non-thermal”. In other words, it doesn’t match the distribution of wavelengths generated by a blackbody.

Models of massive, O-class stars suggest they should contain magnetic fields. Some evidence has seemed to confirm this. Previous studies have also shown that the stellar winds from some of these stars varies with timescales similar to the rotation rates of the stars which could be interpreted as winds being slowed on some faces by the magnetic field as it swept by.

Schnerr’s team attempted to bolster the evidence for magnetic fields by detecting the non-thermal radiation from these stars. The team selected 5 stars which have been shown to have strongly variable winds, some with cyclic variations and used the Westerbork Synthesis Radio Telescope, in the Netherlands to search for non-blackbody signals. The radio range was selected due to the predicted magnetic field strength.

Ultimately, only three of the five selected targets could be observed with the chosen telescope and only one of those, ξ Persei, showed evidence of a non-thermal spectrum. But while this strengthens the case for magnetic fields on the star, it raises another question: From where do the relativistic particles originate? Although O-class stars have strong stellar winds, their speeds are well studied and well below the necessary velocity.

One clue could come from the fact that ξ Persei is a “runaway star”. These stars have velocities and plunge through the interstellar medium at 30-200 km/sec. The team suggests that a bow shock created by this motion could result in sufficiently high velocities. Whether or not ξ Per has such a bow shock is something that could be determined with additional observations.

While this research provides some interesting clues to the nature of these magnetic fields on these stars, it still relies on a small sample. This technique can certainly be expanded to a larger number of stars in the future and may help astronomers better constrain their models of stellar workings.

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Most Massive Star Discovered: Over 300 Suns at Birth!

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Often, writing about astronomy tends to mirror the job of those writing for the Guinness Book of World Records – just when you think a record is practically unbeatable, somebody else appears to show up the previous record-holder. This is surely the case with the stellar heavyweight (er, “heavymass”) R 136a1, which has been shown by data taken using the European Southern Observatory’s Very Large Telescope and the Hubble Space Telescope to tip the stellar scales at 265 times the mass of our Sun. What’s even more impressive is that R 136a1 has lost mass over the course of its lifetime, and likely was about 320 solar masses at birth. That deserves a “Yikes!”

R 136a1 lies in a cluster of young, massive stars with hot surface temperatures that is located inside the Tarantula Nebula. The Tarantula Nebula is nested inside the Large Magellanic Cloud, one of the Milky Way’s closest galactic neighbors, 165,000 light-years away. The cluster is called RMC 136a (or more commonly referred to as R136), and in addition to the whopper that is R 136a1, there are three other stars with masses at birth in the 150 solar mass range.

Extremely massive stars like R 136a1 were previously thought to be unable to form, posing a challenge to stellar physicists as to just how this behemoth came about. It’s possible that it formed by itself in the relatively dense gas and dust of the R136 cluster, or that multiple smaller stars merged to create the larger star at some point early on in its lifetime.

If breaking the mass record weren’t enough, R136a1 also happens to be the most luminous star ever discovered, with an output of energy that is over 10 million times that of the Sun. If you want to learn more about how astronomers determine the mass and luminosity of stars, here is an excellent and thorough introduction to the subject.

To validate the models used in determining the mass and luminosity of the stars in R136, the team of astronomers led by Paul Crowther, Professor of Astrophysics at the University of Sheffield, used the VLT to examine NGC 3603, a closer stellar nursery. NGC 3603 is only 22,000 light years away, and two of the stars in that cluster are in a binary system, which allowed the team to measure their masses.

A comparison of the smallest stars (red dwarfs), Sun-like stars, blue dwarfs, and the most massive star ever discovered, R 136a1. Image Credit: ESO/M. Kornmesser

We are lucky to have observed this extremely massive star, as the rule for the most massive stars is, “Live fast, die young.” The more massive a star is, the faster it churns through the fuel that powers its increased luminosity. Our Sun, which has a medium amount of mass in relation to the two extremes, will last for around for about 10 billion years. Smaller, red dwarf stars can last trillions of years, while large stars on the scale of R 136a1 only glimmer in all of their brilliance for millions of years.

What will happen to R 136a1 at the end of its life? Stars with a mass of over 150 Suns ultimately explode in a light show of staggering proportions generated by what’s called a pair-instability supernova. For more on this phenomenon, check out this article from Universe Today from last year.

Source: ESO press release

A nod and a snarky wink to Genevieve Valentine

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New Movie Reveals Birth of Super-Suns

A two-year look at “proplyds,” or protoplanetary disks in the constellation Orion has provided astronomers with a new high-resolution time-lapse movie that reveals the process of how massive star form. The birth of the largest stars has been mysterious, in part, because massive stars are rare and tend to spend their youth enshrouded by dust and gas hiding them from view. “We know how these stars die, but not how they are born,” said Lincoln Greenhill, a principal investigator for team using radio images a thousand times sharper and more detailed than any previously obtained.

Using the Very Long Baseline Array (VLBA) as a powerful “zoom lens, astronomers studied a massive young protostar called Source I (pronounced “eye”) in Orion. The youthful cluster cannot be seen with traditional telescopes because of the surrounding gas and dust, but this new look shows that massive stars form like their smaller siblings, with disk accretion and magnetic fields playing crucial roles.

The team observed Source I at monthly intervals over two years and then assembled the individual images into a time-lapse movie. Click here to watch the movie.

The VLBA detected thousands of silicon monoxide gas clouds called masers – naturally occurring laser-like beacons often associated with star formation. Some masers were as close to the protostar as Jupiter is to our Sun, which is also a record. Many of the masers existed long enough for their motions to be tracked across the sky and along our line of sight, yielding their 3-d motions through space.

“Source I is the richest source of masers in the Galaxy, that we know of,” said Lynn Matthews, lead author of the new work, who is now a researcher at the MIT Haystack Observatory. “Without the masers, we couldn’t track the gas motions in such detail so close to this massive star, and would be relatively blind to its formation.”

“In astronomy, it’s rare to see changes over the course of a human lifetime. With this new movie, we can see changes over just a few months as gas clumps swarm around this young protostar,” added Smithsonian astronomer and co-author Ciriaco Goddi.

The resulting movie reveals signs of a rotating accretion disk, where gas is swirling closer and closer to the protostar at the center. It also shows material flowing outward perpendicular to the disk in two large V’s – actually the edges of cone-shaped streams of gas. Such outflows foster star formation by carrying angular momentum away from the system.

Intriguingly, the outflow streams appear to curve as they leave the disk. “The bending path of these masers provides key evidence that magnetic fields may be influencing gas motions very close to the protostar,” pointed out Claire Chandler of NRAO, a co-principal investigator of the study.

Magnetic field lines are familiar from their effect on iron filings sprinkled around a bar magnet, outlining loops extending from one pole of the magnet to the other. In the case of Source I and other massive protostars, magnetic field lines may extend outward into space, wrapping in a helix that is shaped much like Twizzlers candy. Outflowing gas streams along those field lines.

“Magnetic fields are supposed to be weak and unimportant to the birth process for massive stars,” said Matthews. “But masers would not travel along gentle arcs unless they experience some sort of force – probably a magnetic force.”

The data don’t show whether the magnetic field arises in the star or in the accretion disk. Future observations by the Expanded Very Large Array (E-VLA) and the Atacama Large Millimeter Array (ALMA) may be able to distinguish between competing hypotheses. The team plans to look for other fingerprints of magnetic fields around Source I.

“Our two-year movie is just the beginning,” said Smithsonian astronomer and co-principal investigator Elizabeth Humphreys.

Source: Harvard Smithsonian