Big Eyes on a Big Star

Mass is constantly being recycled in the universe. One of the most common ways recycling is achieved is through stellar mass-loss. All stars exhibit some form of mass loss. Smaller stars can be quite stingy and hold onto their mass by burning away slowly. Other massive stars can lose prodigious amounts of mass each year. The rate at which a star loses mass depends on its initial mass, age, spectral type, luminosity class, rotation rate, evolutionary stage, even the proximity of a companion star.

The first generation of stars consisted mostly of hydrogen and helium. These stars then seeded the interstellar medium with heavier elements, shedding material via massive winds, planetary nebulae and supernova explosions.

This material then became the building blocks for the next generation of stars and planets. Understanding mass loss is essential for following the evolution of single stars, binaries, star clusters, and galaxies.

However, mass loss is among our weakest areas in understanding fundamental stellar processes. In large part, this is due to lack of detailed, direct observations of stellar photospheres and the mass-loss process. High-resolution optical interferometry with telescope arrays is beginning to provide these data, ushering in a promising a new era in mass-loss studies.

The angular resolution that a telescope can achieve is proportional to its diameter. The larger the telescope, the better its resolution. The purpose of astronomical interferometry is to mix signals from a collection of telescopes to produce images having the same angular resolution as an instrument the size of the entire collection.

The Very Large Telescope Array (VLT)

Recently, a team of French astronomers captured one of the sharpest color images ever made. The team members are Jean-Baptiste Le Bouquin and Antoine M�rand (ESO), Sylvestre Lacour and St�phanie Renard (LAOG, CNRS, Grenoble, France), and Eric Thi�baut (AIRI, Observatoire de Lyon, France).

The image was taken over several consecutive nights with ESO's Very Large Telescope Interferometer (VLTI), a virtual telescope about 100 meters across. The result is an amazing image that reveals a spherical molecular shell around an evolved Mira star, T Leporis, 500 light-years away!

Image showing T Leporis' atmospheric layers and the size of the star relative to the diameter of the Earth's orbit.
(Credit: ESO)

Miras are giant pulsating variable stars. Evolved stars like this have used up most of their nuclear fuel, causing them to swell up to enormous radii and become unstable. This internal instability causes them to expand and contract more or less regularly. T Lep pulsates with a period of 380 days, and in the process loses the equivalent of the Earth�s mass every year to space. In the end, all that is left is a white dwarf, the ash of the core, surrounded by the gases lost to space from the star�s wind and tenuous grip on its outer layers. We see these stellar remnants as planetary nebula elsewhere in the galaxy. Our Sun will become a Mira in a few billion years, engulfing the inner planets in its final blaze of glory.

Mira stars are among the biggest factories of molecules and dust in the Universe. Since the molecules and dust are formed in the outer layers of a Mira's atmosphere, astronomers would like to be able to see these layers to better understand the processes of mass loss and dust formation. Until now, this has been all but impossible due to the distance to even the nearest stars. The angular size of even giant swollen stars like T Lep were too small to image from Earth.

�T Leporis looks so small from the Earth that only an interferometric facility, such as the VLTI at Paranal, can take an image of it. VLTI can resolve stars 15 times smaller than those resolved by the Hubble Space Telescope,� says Le Bouquin.

�We were able to construct an amazing image, and reveal the onion-like structure of the atmosphere of a giant star at a late stage of its life for the first time,� says team member, Antoine M�rand. �Numerical models and indirect data have allowed us to imagine the appearance of the star before, but it is quite astounding that we can now see it, and in color.

Obtaining images like these was one of the main motivations for building the Very Large Telescope Interferometer. We have now truly entered the era of stellar imaging.�

400 years after Galileo turned his tiny telescope towards the sky, we can now actually see stars atmospheres directly, using some of astronomy's most amazing new tools of discovery.

Wild Stars Day One Part 2

Okay, so we still don't find many stars in the CV 'period gap' with orbital periods of 2-3 hours, so in ten years since the last Wild Stars conference, and with all the new CVs discovered and measured in that time, this is a real phenomena.

We find a lot of stars piling up around three hours period and many of these are accreting at very high rates. The secondaries are losing prodigious amounts of mass to their cannibalistic white dwarf companions.

The leading theory explains this gap as the point where magnetic braking ceases and these binaries abruptly stop accreting matter from the secondary to the white dwarf. As these binaries continue to lose orbital energy through gravitational wave propagation they evolve through the period gap from 3 hours to two hours. At this point they've spiraled in close enough for the secondary to fill its Roche lobe and accretion starts up again.

From orbital period (Porb) 2 hours and less the only way the system loses orbital energy is through gravity waves. Typically these stars have low accretion rates and it takes a long time for them to build up enough material in the disk to go into outburst. So the secondaries are not losing mass very quickly, and we'd expect to see another spike in the population of CVs at shorter orbital periods.

One problem is the fact that these systems tend to be quite faint in quiescence, so they are harder to find than their bright actively outbursting friends at the longer Porbs. With the recent results of the Sloan Digital Sky Survey (SDSS) we've uncovered more ad more of these short period low accretion rate CVs, but there is still some debate about whether we have actually begun to pin down the actual spacial density of these objects. There aren't that many faint, short period CVs close to us, so they must indeed be rare objects.

CV theories also predict a minimum Porb of about 65 minutes. There may be something wrong with our models though, because observationally, the minimum seems to be closer to 80 minutes. Either we have a whole bunch of highly evolved, low accreting faint stars out there at 23rd magnitude (beyond the limit of SDSS) or there is something lacking in our understanding of the physics at this minimum threshold for CV period evolution.

To be honest, not much of this is new, or cutting edge astrophysics anymore. A lot of these same issues were being discussed ten years ago. We may have more observations and phenomenology, but we don't seem to have made any significant progress in our understanding of these Wild Stars.

That is a topic I want to interrogate Steve Howell, one of the local organizers, about tomorrow. I'll let you know what he thinks.

Wild Stars -Day One

Much of variable stars research is related to stellar evolution. We have a pretty good handle on how single stars evolve over billions of years. They are born in clouds of dust and gas, contract due to gravity until they reach a critical limit at which nuclear processes begin converting hydrogen to helium and heavier elements. At this point a star is born.

The most important factor in the evolutionary track of a star is its initial mass. Giant stars burn up their fuel quickly and die spectacularly. Dwarf stars live for tens of billions of years, miserly using up their fuel while putting out a conservative amount of energy.

A majority of stars follow an evolutionary path that eventually causes them to swell to hundreds of times their original radius, throwing off layers of their outer atmosphere via stellar winds and from their losing their gravitational grip on the outer layers of their swollen stellar atmosphere. These stars eventually become planetary nebula with white dwarfs at their centers.

These degenerate white dwarf stars are fascinating objects. They do not produce new energy, like stars. They are the remaining ash of the core of evolved stars, slowly cooling through time from their original very high temperatures. They are small and extremely dense, planet sized objects with approximately the mass of our sun squeezed into their small frames.

Things get a lot more complicated when two or more stars in orbit around each other are involved. If the stars' orbits are wide enough, each star may be able to follow its normal evolutionary path for billions of years. However, cataclysmic variables are stellar pairs, typically containing a white dwarf and a swollen M dwarf, in orbit around each other so close that the orbital period can be measured in hours. Mass is exchanged from the secondary to the white dwarf via an accretion disk. This interaction has a profound effect on the evolution of the stars involved.

Some of these white dwarfs have extremely strong magnetic fields. The accretion process is interrupted in part in intermediate polars, or completely in polars (AM Her stars). No real accretion disk is formed. Instead the mass transferred in polars slams down onto a small area at the magnetic pole of the white dwarf, or goes into strange orbits following the magnetic field lines in an intermediate polar.


Fine. We can and do observe all these properties of these binary systems now, along with the outbursts and high and low states of activity CVs are well known for. But, the burning question of the day today was "How do they get this way?"

Where do these pairs come from? Are they born as normal stars in unremarkable circumstances that somehow evolve into wild pairs of objects orbiting each other so closely they are exchanging material? How long does this take? How do they lose their orbital energy, and what is that energy converted into? Even more perplexing is, where do the magnetic fields in CVs come from?

Above: The common envelope explanation for the evolution of CVs from a pair of main sequence stars, to a pair with a Giant Branch star and main sequence dwarf to eventually becoming a white dwarf and main sequence dwarf pair (CV).

The generally accepted explanation is called the Common Envelope evolution scheme. As a pair of stars evolves, changes in the mass of one or both stars affects the orbital characteristics of the pair, and they lose energy and begin to spiral in towards each other. At a critical point in this process one star evolves to the point that it fills its Roche lobe and the pair becomes involved in a cloud of dust and gas shared by both called a 'common envelope'. We believe CVs evolve out of this phase into the semi-detached systems we see as dwarf novae and magnetic CVs.

Just how this happens is still not well understood, and how either star acquires a mega-Gauss magnetic field in the process is an even less understood process. The mystery was framed and discussed in a couple very interesting papers given in the afternoon session. James Liebert pointed out the fact that the Sloan Digital Sky Survey has found over 1200 close pairs containing a white dwarf and an M dwarf, precisely the kinds of pairs we believe are the progenitors of CVs, yet none of them have been found to contain a white dwarf with a strong magnetic field.

Nearly 25% of CVs are magnetic systems so where do they come from if not these pre-CV pairs? In fact, all highly magnetic white dwarfs appears as either single stars or components of CV binaries.

Christopher Tout proposed in the following paper that highly magnetic white dwarfs must be formed as a result of the common envelope phase of binary evolution. He went further to suggest that the single white dwarfs with the highest magnetic fields are the result of a pair of stars merging into one highly magnetic white dwarf from the common envelope phase. And the magnetic CVs we observe, polars and intermediate polars, are the result of systems that almost merge before eveolving into magnetic CVs.

There are also fundamental questions about the evolution of CV pairs. Do these stars continue to spiral in towards each other, reaching shorter and shorter periods? How does accretion and mass loss affect this evolution? How do we explain the well known 'period gap' where there are almost no actively accreting systems with orbital periods between 2 and 3 hours?

A graphical demonstration of the period gap. The vertical axis is the number of known CVs. The horizontal axis is the period in hours (top) or fractions of a day (bottom).

What is so special about this orbital period? What shuts off the accretion process at 3 hours yet lets it re-engage at less than 2 hours. What is the actual period minimum for CVs? Is it 65 minutes, as theory predicts, or is 80 minutes, as observations seem to imply?

There are lots of questions. I hope to get at least some of the answers this week. Stay tuned, and we'll find out together.

The images used in this article are from space artist Mark A. Garlick. Visit him on the web at www.space-art.co.uk and www.markgarlick.com

Wild Stars Pictorial Review

Steve Howell, head cowboy, and coiner of the famous acronym TOADs (tremendous outburst amplitude dwarf novae) welcomes everyone to the conference and explains where the bathrooms are.

His other main task for this conference seems to be getting everyone who wants one, a receipt for their expense reports. Poor Steve.

His poster on magnetic CVs has an awesome visualization. I'll try to get permission to reproduce it here. It is way cool....err, I mean hot.




Chritsian Knigge opens the paper session by reviewing what we know about the secondaries in CVs and their role in the evolution of these systems.

Check out the visualization of that bloated, star spotted, crazy looking secondary. Wild stars indeed!

More interesting is the fact that CV donor stars are larger and cooler than individual main sequence stars of equal mass. Observing these secondary properties may tell us a lot about the evolutionary track of these systems. Fascinating stuff presented very well. Two Simothumbs up for this one.

What's that? You say you don't understand magnetic braking? Don't worry, I'm in a room full of PhDs who will talk about it all day, but they don't understand it either!


Hands down winner of the animated visualizations for the conference thus far definitely goes to D.V. Bisikalo from the Russian Academy of Sciences, Moscow. His illustrations of accretion and the outflow of material into the envelope around the binary were fascinating to watch and quite detailed. The parameters and science used to achieve these results may be unrealistic, but the animations were glorious! Not only that, but watching the accretion overflow, I had an 'aha moment' for something I've been working on regarding Z Cam outbursts.




For those of you who remember my blog on 'Dusty Toads', here is one of the authors, Don Hoard, talking about surprising dusty environments around cataclysmic variables. They went hunting for information about the red secondary star of WZ Sge with the Spitzer Space Telescope and found so much dust they couldn't observe the secondary! A surprising result that may lead to, well, who knows?

I'll be interviewing Steve and Don about their dusty toads and where this new result may lead CV research.

On a personal note: it has been a lot of fun meeting the people and associating the names with the faces. I met several Japanese observers and important contributors to CVnet-Akira Arai, Hiroshima University, Izumi Hachisu, University of Tokyo, Akira Imada, Kagoshima University, Daisaku Nogami, Kwasan Observatory, Kyoto University. I also got to meet and talk with astronomers using AAVSO data for their papers here at this conference or elsewhere: Brad Schaefer, Louisiana State University, Christian Knigge, University of Southampton.

AAVSO was well represented with Arne Henden giving a poster presentation with hundreds of AAVSO light curves and Paula Szkody talking about pulsating white dwarfs in SDSS CVs.

Boris Gaensicke, who I met for the first time in Cambridge, UK last spring, seems to have his fingers in so many pies here it is quite remarkable. He is listed as a co-author or principle investigator on at least 40% (UNSCIENTIFIC SIMO-ESTIMATE) of the papers being presented.

And on a personal basis, I had the pleasure of meeting Kurtis Williams, of Professor Astronomy's Astronomy Blog .
He has been kind enough to support the AAVSO Writers Bureau with his blogs and is an all around nice guy who it is my pleasure to have met finally.

It's been a good time so far. More later.

Simostronomy on Slacker Astronomy pt. 2

It took us a while, but the second part of my discussion with Michael and Doug on Slacker Astronomy is now online.

You can download the podcast here.

In this episode, we talk about where research in cataclysmic variables is going, what astronomers are expecting to find and some of the surprises we've found along the way. I was fortunate to get three astronomers from the Wild Stars in the Old West II conference, Christopher Tout, Paula Szkody and Boris Gaensicke, to give up their coffee break time to let me interview them for this show.

My thanks to Michael and Doug for having me on the show, and to Paula, Christopher and Boris for giving me something interesting to contribute.

Boris Gaensicke / Christopher Tout /Paula Szkody

So check it out:

Podcast: Simo-Slacker Interviews Pt. II

The Frozen Dome

Normally, if I heard or read the words 'Dome C' I would think they referred to the third dome in a cluster of structures at some observatory. Recently, I've come to learn that Dome C is also the name for one of the coldest places on earth, one of several summits on the Antarctic Ice Shelf.

Ironically, there is still an astronomical link. Dome C is considered to be one of the best potential sites for a new observatory on the face of our planet. For one thing, the Sun never gets higher than 38 degrees above the horizon, so there is a lot of night time for observing from the south polar region. Even better, there is almost no infrared sky glow, the air is extremely dry, there is almost no aerosol or dust, and no light pollution. The Antarctic Plateau is the largest desert on Earth, so there is very little precipitation and a very high percentage of cloud-free time. Surprisingly, the wind is also quite mild at Dome C, averaging a mere 6 mph in winter. That is a good thing, considering the average annual temperature is -55C, with lows of -80C and balmy highs in the -25C range. Who needs wind chill when it's that cold?

Most importantly, the seeing is typically 2.5 times better at Dome C than at the best existing observatories. Star images taken through a telescope at Dome C would be 2.5 times sharper and 6 times brighter.

The image on the left is a simulation of a star field as observed from the best existing observatory sites; the image in the middle is the same star field as observed from Dome C. To see as many stars from a mid-latitude observatory, you would need to build a telescope 2.5 times bigger, which would cost ten times as much, and would give the image on the right, which makes the stars look brighter but doesn't improve the sharpness of the image.

Image and text from 'Exceptional astronomical seeing conditions above Dome C in Antarctica', by
by Jon S. Lawrence, Michael C. B. Ashley, Andrei Tokovinin, and Tony Travouillon, published in Nature, 16 September 2004.

Three interesting papers have been released to the pre-print server arXiv.org describing the PILOT program (the Pathfinder for an International Large Optical Telescope), a proposed observatory on Dome C in Antarctica. The first paper presents an overview of the instrumentation suite and its expected performance, a summary of the key science goals and a discussion of the future of Antarctic astronomy.

Paper 2 describes a series of projects dealing with the distant Universe. One potential project that caught my eye is the search for pair-instability supernovae (PISNe) and gamma-ray burst afterglows. These could be our best glimpses into stars formed in the very early days of the Universe. PISNe are predicted to be the product of super massive stars formed in the early history of the Universe. These stars were formed before there were any heavier elements, so their unique chemical composition and masses resulted in a different kind of final disruption of the supernovae progenitors in this era. The light curves of these PISNe are predicted to be have slower rise times and to stay bright for much longer than SN closer to home. This is pretty cutting edge astrophysics, seeing as how no PISN has ever been found.

PILOT could also examine some of the first evolved galaxies and galaxy clusters to inform us of the processes in the evolution of structure in the Universe. They also propose a large-area weak-lensing survey and a program to obtain supernovae infrared light-curves to examine the nature and evolution of dark energy and dark matter.

The ability to do infra-red astronomy from the planet's surface makes PILOT a good match and essentially the only competition for the James Webb Space Telescope in the coming decade.

Paper 3 presents a series of projects dealing with the nearby Universe. Several projects are proposed that examine stellar populations in nearby galaxies and stellar clusters, to gain insight into the formation and evolution of younger galaxies and stars.

Other projects will investigate the formation processes of stellar and planetary systems. Three projects in the field of exoplanet science are proposed. These include a search for free-floating low-mass planets and dwarfs, a program of follow-up observations of gravitational microlensing events, and a study of infrared light-curves for previously discovered exoplanets.

Free-floating low-mass planets; now there is a category of interesting objects. The plan is to examine nearby star clusters to search for planets not associated with stars down to several Jupiter masses. Why would astronomers be so interested in free floating planets? Because typically, exoplanets light is difficult or impossible to disentangle from the light of their accompanying star. If we can find exoplanets free of the overpowering glare of their host stars we can study the chemical composition and atmospheric properties of these planets.

And finally a study of coronal mass ejections from the Sun, and a monitoring program searching for small-scale Low Earth Orbit satellite debris items are also proposed.

The opportunities to do exciting, results-oriented science exploration and discovery from Antarctica is is almost as mind-numbing as the night time temperatures resident astronomers and technicians will have to bear to perform the work.

Constructing, operating and maintaining a telescope at the bottom of the world under these conditions will be another great story. Now that I know about Dome C and PILOT, I'll keep an ear to the ground and let you know when there are new developments.

The Bug Nebula

I don't do a lot of 'eye-candy' posts about pictures, but this one really caught my eye as I scanned my Google reader at lunch time today. I've blogged about this before, but this is just an awesome picture from the Hubble Space Telescope.

One of the final stages of stellar evolution of a Sun-like star results in nebulae like this one, NGC 6302, also known as the Bug Nebula. This glowing, expanding shell of ionized gas is known as a planetary nebula.

NGC 6302 The 'Bug Nebula'

Credit: NASA, ESA, and the Hubble SM4 ERO Team

At the end of the star's life, during the red giant phase, the outer layers of the star are expelled via pulsations and strong stellar winds. This is not a nice breeze on a cool day. We're talking winds up to 600,000 miles per hour. Eventually all that is left in the center is the ash of the previous star's core, a white dwarf. The hot white dwarf emits ultraviolet radiation that ionizes the ejected outer layers of the star. This energized shell radiates as a planetary nebula.

They are a relatively short-lived phenomenon, lasting a few tens of thousands of years, as compared to a typical stellar lifetime of several billion years.

So, what does this thing look like through a telescope on the ground? Here is a drawing done by Scott Mellish at the eyepiece of a 56cm Dobsonian telescope, at 314x,  from Australia.

The Furor Over FUOrs

FU Orionis and its associated nebula. Image credit: ESO

In 1937, an ordinary 16th magnitude star in the constellation Orion began to brighten steadily. Thinking it was a nova, astronomers were astounded when the star just kept getting brighter and brighter over the course of a year. Most novae burst forth suddenly and then begin to fade within weeks. But this star, now glowing at 9th magnitude, refused to fade. Adding to the puzzle, astronomers could see there was a gaseous nebula nearby shining from the reflected light of this mysterious star, now named FU Orionis. What was this new kind of star?

FU Ori has remained in this high state, around 10th magnitude ever since. This was a from of stellar variability never seen before. Since there were no other examples of this kind of variable star astronomers were forced to learn what they could from the only known example, or wait for another event to provide more clues.

Finally, more than 30 years later, FU Ori-like behavior appeared again in 1970 when the star now known as V1057 Cyg increased in brightness by 5.5 magnitudes over 390 days. Then in 1974, a 3rd example was discovered when V1515 Cyg rose from 17th magnitude to 12th magnitude over an interval lasting years. Astronomers began piecing the puzzle together from these clues.

FU Orionis stars are pre-main sequence stars in the early stages of stellar development. They have only just formed from clouds of dust and gas in interstellar space, which occur in active star- forming regions. They are all associated with reflection nebulae, which become visible as the star brightens.

This artist's concept shows a young stellar object 

and the whirling accretion disk surrounding it.  

NASA/JPL-Caltech

 

Astronomers are interested in these systems because FUOrs may provide us with clues to the early history of stars and the formation of planetary systems. At this early stage of evolution, a YSO is surrounded by an accretion disk, and matter is falling onto the outer regions of the disk from the surrounding interstellar cloud. Thermal instabilities, most likely in the inner portions of the accretion disk, initiate an outburst and the young star increases its luminosity. Our Sun probably went through similar events as it was developing.

One of the major challenges in studying FU Orionis stars is the relatively small number of known examples. Although approximately 20 FU Orionis candidates have been identi?ed, only a handful of these stars have been observed to rise from their pre-outburst state to their eruptive state.

Now, in the last year, several new FUOrs have been discovered. In November 2009, two newly discovered objects were announced in Central Bureau Electronic Telegrams (CBET) #2033. Patrick Wils, John Greaves and the Catalina Real-time Transient Survey (CRTS) collaboration had discovered them in CRTS images.

The first of these objects appears to coincide with the infrared source IRAS 06068-0641.  Discovered by the CRTS on Nov. 10, it had been continuously brightening from at least early 2005 (when it was mag 14.8 on unfiltered CCD images) to its present mag 12.6. A faint cometary reflection nebula was visible to the east.  A spectrum taken with the SMARTS 1.5-m telescope at Cerro Tololo, on Nov. 17, confirmed it to be a young stellar object.  The object lies inside a dark nebula to the south of the Monocerotis R2 association, and is likely related to it.  

Also inside this dark nebula, a second object, coincident with IRAS 06068-0643, had been varying between mag 15 and 20 over the past few years, reminiscent of UX-Ori-type objects with very deep fades.  This second object is also associated with a variable cometary reflection nebula, extending to the north.  The spectrum of this object also shows H_alpha and the strong Ca II infrared triplet in emission. 

Light curves, spectra and images can be found here.

In August 2010, two new eruptive, pre-main sequence stars were discovered in Cygnus.  The first object was an outburst of the star HBC 722. The object was reported to have risen by 3.3 magnitudes from May 13 to August 16, 2010. Spectroscopy reported by U. Munari et al in ATel #2808, Aug 23, 2010 support this object's classification as an FU Ori star. Munari and his team reported the object at 14.04V on Aug 21, 2010.

The second object, coincident with the infrared source IRAS 20496+4354, was discovered by K. Itagaki (Yamagata, Japan) on August 23, 2010 and reported in CBET 2426.  The object appears very faint (magnitude 20) in a DSS image taken in 1990.  Subsequent spectroscopy and photometry of this object by U. Munari showed that this object also has the characteristics of an FU Ori star. Munari reported the object at 14.91V on August 26, 2010. 
 
Both these objects are now the subjects of an AAVSO observing campaign announced October 1, 2010 in AAVSO Alert Notice 425. Dr. Colin Aspin (U. Hawai'i) has requested the help of AAVSO observers in performing long-term photometric monitoring of these two new YSOs in Cygnus. AAVSO observations will be used to help calibrate optical and near-infrared spectroscopy to be obtained during the next year. 

Since these stars are newly discovered, very little is known about their behavior. Their classification as FU Ori variables is based on spectroscopy by U. Munari et al. Establishing a good light curve and maintaining it, over the next several years, will be crucial to understanding these stars. This kind of long-term monitoring is one of the things at which amateur astronomers excel.

November 10, 2010, results presenting rare pre and post outburst observations from the Palomar Transient Factory (PTF) show that HBS722 is a bona fide FU Ori type star that was a classical T-Tauri star before eruption, providing strong evidence that FU Orionis eruptions represent periods of enhanced disk accretion and outflow, likely triggered by instabilities in the accretion disk. 

Another paper, released the next day, also based on observations from the PTF, shows IRAS 20496+4354 brightened by more than 5 magnitudes, reaching 13.5R in September 2010. Near-infrared spectra appear quite similar to a spectrum of McNeil's Nebula/V1647 Ori, a FUOr which has undergone several brightenings in recent decades.

So after a very slow start, discoveries of new YSOs and our understanding of the dusty disk environments around them are starting to heat up. With new tools and new examples to study we are peering into the the early stages of stellar and planetary formation and finding some of our models have been pretty close to the truth. We expect to find more and similar objects as new all-sky surveys begin to cover the sky, but these objects will still be relatively rare and therefore interesting, because this period in a star's evolution is short-lived and only takes place in the active star forming regions of galaxies.


Images of HBC722 and  IRAS 20496+4354 from  
Discovery of possible FU-Ori and UX-Ori type objects
Wils, P., Greaves, J. and the CRTS collaboration, Nov 18th 2009.
http://crts.caltech.edu/CSS091110.html

J-E-T-S, Jets, Jets, Jets!

Bipolar jet from a young stellar object (YSO). Credit: Gemini Observatory, artwork by Lynette Cook

It seems oddly appropriate to be writing about astrophysical jets on Thanksgiving Day, when the New York football Jets will be featured on television. In the most recent issue of Science, Carlos Carrasco-Gonzalez and collaborators write about how their observations of radio emissions from young stellar objects (YSOs) shed light one of the unsolved problems in astrophysics; what are the mechanisms that form the streams of plasma known as polar jets? Although we are still early in the game, Carrasco-Gonzalez et al have moved us closer to the goal line with their discovery.

Astronomers see polar jets in many places in the Universe. The largest polar jets are those seen in active galaxies such as quasars. They are also found in gamma-ray bursters, cataclysmic variable stars, X-ray binaries and protostars in the process of becoming main sequence stars. All these objects have several features in common: a central gravitational source, such as a black hole or white dwarf, an accretion disk, diffuse matter orbiting around the central mass, and a strong magnetic field.

Relativistic jet from an AGN.
Credit: Pearson Education, Inc.

When matter is emitted at speeds approaching the speed of light, these jets are called relativistic jets. These are normally the jets produced by supermassive black holes in active galaxies. These jets emit energy in the form of radio waves produced by electrons as they spiral around magnetic fields, a process called synchrotron emission. Extremely distant active galactic nuclei (AGN) have been mapped out in great detail using radio interferometers like the Very Large Array in New Mexico. These emissions can be used to estimate the direction and intensity of AGNs magnetic fields, but other basic information, such as the velocity and amount of mass loss, are not well known.

On the other hand, astronomers know a great deal about the polar jets emitted by young stars through the emission lines in their spectra. The density, temperature and radial velocity of nearby stellar jets can be measured very well. The only thing missing from the recipe is the strength of the magnetic field. Ironically, this is the one thing that we can measure well in distant AGN. It seemed unlikely that stellar jets would produce synchrotron emissions since the temperatures in these jets are usually only a few thousand degrees. The exciting news from Carrasco-Gonzalez et al is that jets from young stars do emit synchrotron radiation, which allowed them to measure the strength and direction of the magnetic field in the massive Herbig-Haro object, HH 80-81, a protostar 10 times as massive and 17,000 times more luminous than our Sun.

Finally obtaining data related to the intensity and orientation of the magnetic field lines in YSO's and their similarity to the characteristics of AGN suggests we may be that much closer to understanding the common origin of all astrophysical jets. Yet another thing to be thankful for on this day.

Elephant Trunks in Space

I love it when astronomers come down to earth and create terms we can all understand to explain strange astronomical phenomena that very few of us understand. My new favorite- Elephant Trunks.

Massive stars are born in dense molecular clouds in the universe. As the nuclear furnace inside them begins to burn, they generate huge quantities of radiation which heats and ionizes the surrounding gases. The pressure and wind from the star forces these regions to expand.

This doesn't happen in a perfectly symmetrical manner, but instead, the expanding gases form clumps and bubbles. Some of the denser clumps may actually be the seeds for elongated structures astronomers now refer to as 'elephant trunks.' You've seen pictures of these elongated structures, like the famous Hubble Space Telescope image of the 'Pillars of Creation.'


Looking at this image it's easy to see the elephant trunk shapes. There is even a nebula called the Elephant Trunk Nebula.

Image credit: Gerhard Bachmayer

Several papers have come out recently describing attempts to model this process using computer simulations. Search results from astro-ph at arXiv brings up some very interesting papers, if you're into stellar evolution...or circus animals.

The Angular Momentum of Condensations Within Elephant Trunks

Star Formation in the Eagle Nebula

Properties of protostars in the Elephant Trunk globule IC 1396A

And my favorite mixed metaphor and acronym paper of all time-

The Eagle's EGGs: fertile or sterile?

The Eagle here refers to the Eagle Nebula. EGGs are Evaporating Gaseous Globules, and the authors want to know if the EGGs are sterile or fertile, in other words, will stars be born here? C'mon, this is fun stuff!

All you really need to know is there are still no flying monkeys in space, but we now have elephant trunks. They come in all sizes and shapes. And what do astronomers find in these trunks?

The only thing cuter than baby elephants, baby stars.