Recurrent Novae, Light Echoes, and the Mystery of T Pyxidis

Some of the most violent events in our Universe were the topic of discussion this morning at the 222nd meeting of the American Astronomical Society in Indianapolis, Indiana as researchers revealed recent observations of light echoes seen as the result of stellar explosions.

A light echo occurs when we see dust and ejected material illuminated by a brilliant nova. A similar phenomenon results in what is termed as a reflection nebula. A star is said to go nova when a white dwarf star siphons off material from a companion star. This accumulated hydrogen builds up under terrific pressure, sparking a brief outburst of nuclear fusion.

A very special and rare case is a class of cataclysmic variables known as recurrent novae. Less than dozen of these types of stars are known of in our galaxy, and the most famous and bizarre case is that of T Pyxidis.

Located in the southern constellation of Pyxis, T Pyxidis generally hovers around +15th magnitude, a faint target even in a large backyard telescope. It has been prone, however, to great outbursts approaching naked eye brightness roughly every 20 years to magnitude +6.4. That’s a change in brightness almost 4,000-fold.

But the mystery has only deepened surrounding this star. Eight outbursts were monitored by astronomers from 1890 to 1966, and then… nothing. For decades, T Pyxidis was silent. Speculation shifted from when T Pyxidis would pop to why this star was suddenly undergoing a lengthy phase of silence.

Could models for recurrent novae be in need of an overhaul?

T Pyxidis finally answered astronomers’ questions in 2011, undergoing its first outburst in 45 years. And this time, they had the Hubble Space Telescope on hand to witness the event.

Light curve of the 2011 eruption of T Pyxidis. (Credit: AAVSO).
Light curve of the 2011 eruption of T Pyxidis. (Credit: AAVSO).

In fact, Hubble had just been refurbished during the final visit of the space shuttle Atlantis to the orbiting observatory in 2009 on STS-125 with the installation of its Wide Field Camera 3, which was used to monitor the outburst of T Pyxidis.

The Hubble observation of the light echo provided some surprises for astronomers as well.

“We fully expected this to be a spherical shell,” Said Columbia University’s Arlin Crotts, referring to the ejecta in the vicinity of the star. “This observation shows it is a disk, and it is populated with fast-moving ejecta from previous outbursts.”

Indeed, this discovery raises some exciting possibilities, such as providing researchers with the ability to map the anatomy of previous outbursts from the star as the light echo evolves and illuminates the 3-D interior of the disk like a Chinese lantern. The disk is inclined about 30 degrees to our line of sight, and researchers suggest that the companion star may play a role in the molding of its structure from a sphere into a disk. The disk of material surrounding T Pyxidis is huge, about 1 light year across. This results in an apparent ring diameter of 6 arc seconds (about 1/8th the apparent size of Jupiter at opposition) as seen from our Earthly vantage point.

Paradoxically, light echoes can appear to move at superluminal speeds. This illusion is a result of the geometry of the path that the light takes to reach the observer, crossing similar distances but arriving at different times.

And speaking of distance, measurement of the light echoes has given astronomers another surprise. T Pyxidis is located about 15,500 light years distant, at the higher 10% end of the previous 6,500-16,000 light year estimated range. This means that T Pyxidis is an intrinsically bright object, and its outbursts are even more energetic than thought.

Light echoes have been studied surrounding other novae, but this has been the first time that scientists have been able to map them extensively in 3 dimensions.

An artist's conception of the disk of material surrounding T Pyxidis. (Credit: ESA/NASA & A. Feild STScl/AURA).
An artist’s conception of the disk of material surrounding T Pyxidis. (Credit: ESA/NASA & A. Feild STScl/AURA).

“We’ve all seen how light from fireworks shells during the grand finale will light up the smoke and soot from the shells earlier in the show,” said team member Stephen Lawrence of Hofstra University. “In an analogous way, we’re using light from T Pyx’s latest outburst and its propagation at the speed of light to dissect its fireworks displays from decades past.”

Researchers also told Universe Today of the role which amateur astronomers have played in monitoring these outbursts. Only so much “scope time” exists, very little of which can be allocated exclusively to the study of  light echoes. Amateurs and members of the American Association of Variable Star Observers (AAVSO) are often the first to alert the pros that an outburst is underway. A famous example of this occurred in 2010, when Florida-based backyard observer Barbara Harris was the first to spot an outburst from recurrent novae U Scorpii.

And although T Pyxidis may now be dormant for the next few decades, there are several other recurrent novae worth continued scrutiny:

Name Max brightness Right Ascension Declination Last Eruption Period(years)
U Scorpii +7.5 16H 22’ 31” -17° 52’ 43” 2010 10
T Pyxidis +6.4 9H 04’ 42” -32° 22’ 48” 2011 20
RS Ophiuchi +4.8 17H 50’ 13” -6° 42’ 28” 2006 10-20
T Coronae Borealis +2.5 15H 59’ 30” 25° 55’ 13” 1946 80?
WZ Sagittae +7.0 20H 07’ 37” +17° 42’ 15” 2001 30

 

Clearly, recurrent novae have a tale to tell us of the role they play in the cosmos. Congrats to Lawrence and team on the discovery… keep an eye out from future fireworks from this rare class of star!

Read the original NASA press release and more on T Pyxidis here.

 

Anarchic Star Formation Found In Dust Cloud

If you think that breaking all the rules is cool, then you’ll appreciate one of the latest observations submitted by the Danish 1.54 meter telescope housed at ESO’s La Silla Observatory in Chile. In this thought-provoking image, you’ll see what kind of mayhem occurs when stars are forged within an interstellar nebula.

Towards the center of the Milky Way in the direction of the constellation of Sagittarius, and approximately 5000 light-years from our solar system, an expansive cloud of gas and dust await. By comparison with other nebulae in the region, this small patch of cosmic fog known as NGC 6559 isn’t as splashy as its nearby companion nebula – the Lagoon (Messier 8). Maybe you’ve seen it with your own eyes and maybe you haven’t. Either way, it is now coming to light for all of us in this incredible image.

Comprised of mainly hydrogen, this ethereal mist is the perfect breeding ground for stellar creation. As areas contained within the cloud gather enough matter, they collapse upon themselves to form new stars. These neophyte stellar objects then energize the surrounding hydrogen gas which remains around them, releasing huge amounts of high energy ultraviolet light. However, it doesn’t stop there. The hydrogen atoms then merge into the mix, creating helium atoms whose energy causes the stars to shine. Brilliant? You bet. The gas then re-emits the energy and something amazing happens… an emission nebula is created.

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This zoom starts with a broad view of the Milky Way. We head in towards the centre, where stars and the pink regions marking star formation nurseries are concentrated. We see the huge gas cloud of the Lagoon Nebula (Messier 8) but finally settle on the smaller nebula NGC 6559. The colourful closing image comes from the Danish 1.54-metre telescope located at ESO’s La Silla Observatory in Chile. Credit: ESO/Nick Risinger (skysurvey.org)/S. Guisard. Music: movetwo

In the center of the image, you can see the vibrant red ribbon of the emission nebula, but that’s not the only thing contained within NGC 6559. Here swarms of solid dust particles also exist. Consisting of tiny bits of heavier elements, such as carbon, iron and silicon, these minute “mirrors” scatter the light in multiple directions. This action causes NGC 6559 to be something more than it first appears to be… now it is also a reflection nebula. It appears to be blue thanks to the magic of a principle known as Rayleigh scattering – where the light is projected more efficiently in shorter wavelengths.

Don’t stop there. NGC 6559 has a dark side, too. Contained within the cloud are sectors where dust totally obscures the light being projected behind them. In the image, these appear as bruises and dark veins seen to the bottom left-hand side and right-hand side. In order to observe what they cloak, astronomers require the use of longer wavelengths of light – ones which wouldn’t be absorbed. If you look closely, you’ll also see a myriad of saffron stars, their coloration and magnitude also effected by the maelstrom of dust.

It’s an incredible portrait of the bedlam which exists inside this very unusual interstellar cloud…

Original Story Source: ESO News Release.

Stunning New Look at Reflection Nebula Messier 78

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Here’s another “Hidden Treasure” from the European Southern Observatory, from the astrophotography competition where amateurs create images from unused ESO data. In this new image of Messier 78, brilliant starlight ricochets off dust particles in the nebula, illuminating it with scattered blue light and creating what is called a reflection nebula. Almost like fog around a street light, a reflection nebula shines only with the light from an embedded source that illuminates the dust. This image was taken with the Wide Field Imager on the MPG/ESO 2.2-metre telescope at the La Silla Observatory in Chile. Comparing this image with others previously taken of Messier 78 shows that remarkably, this object has changed significantly in the last ten years.

This beautiful image was the overall winner of ESO’s Hidden Treasures 2010 astrophotography competition created by Igor Chekalin, who won with his image of this stunning object.

Messier 78 can easily be observed with a small telescope, being one of the brightest reflection nebulae in the sky. It lies about 1350 light-years away in the constellation of Orion (The Hunter) and can be found northeast of the easternmost star of Orion’s belt.

For those of you who want to take a look on your own:
Right Ascension: 05:46.7
Declination: +00:03
Distance: 1.6 (kly)
Visual Brightness: Magnitude 8.3

This image contains many other striking features apart from the glowing nebula. A thick band of obscuring dust stretches across the image from the upper left to the lower right, blocking the light from background stars. In the bottom right corner, many curious pink structures are also visible, which are created by jets of material being ejected from stars that have recently formed and are still buried deep in dust clouds.

Two bright stars, HD 38563A and HD 38563B, are the main powerhouses behind Messier 78. However, the nebula is home to many more stars, including a collection of about 45 low mass, young stars (less than 10 million years old) in which the cores are still too cool for hydrogen fusion to start, known as T Tauri stars. Studying T Tauri stars is important for understanding the early stages of star formation and how planetary systems are created.

Messier 78 region taken in 2006 (below) with the 4-metre Mayall telescope at Kitt Peak, Arizona with a new image from ESO.Credit: ESO/T. A. Rector/University of Alaska Anchorage, H. Schweiker/WIYN and NOAO/AURA/NSF and Igor Chekalin

But this object has changed significantly in the last ten years. In February 2004 the experienced amateur observer Jay McNeil took an image of this region with a 75 mm telescope and was surprised to see a bright nebula — the prominent fan shaped feature near the bottom of this picture — where nothing was seen on most earlier images. This object is now known as McNeil’s Nebula and it appears to be a highly variable reflection nebula around a young star.

This color picture was created from many monochrome exposures taken through blue, yellow/green and red filters, supplemented by exposures through an H-alpha filter that shows light from glowing hydrogen gas. The total exposure times were 9, 9, 17.5 and 15.5 minutes per filter, respectively.

Source: ESO