Supernovae Alphabet Soup

SN 2011fe aka PTF11kly
Image: Wikipedia

The International Astronomical Union (IAU) is the sole body responsible for the official naming of astronomical objects. So if you have a problem with the way things in the Universe are named, you now know where to send your email and letters of protest.

Before we get into this, a quick grammar note. When we discuss more than one supernova, they are called supernovae (super- no- vee), not supernovas. The same holds true for more than one nova. They are novae (no- vee). Please don't write and ask me about Novas. Those are old Chevrolets, not stars.

Fortunately, the naming convention used for supernovae is pretty simple and straightforward. The name is formed by combining the prefix SN, for supernova, the year of discovery and a one- or two-letter designation. The first 26 supernovae of the year get an upper case letter from A to Z (SN 1987A). After that, we start over with pairs of lower-case letters, starting with aa, ab, and so on (SN 2005ap).

Of course there are exceptions, there are always exceptions. That's one of the things about astronomical nomenclature that is maddening, but I digress...

Four important historical supernovae are known simply by the year they occurred- SN 1006, SN 1054, SN 1572 (more commonly referred to as Tycho's Nova), and SN 1604 (also known as Kepler's Star).

One reason I'm bringing this subject up now is that we are ending the year, so we are approaching the time where we reset the naming schema for 2012 and the first supernova of the new year will get named SN 2012A. With the annual number of discoveries rising each year to well over 500, it is always a bit surprising how long it takes for that first one of the year to get named. So each year we hold an unofficial contest here on Simostronomy to see who will discover the first SN of the new year.

One of the reasons it usually doesn't occur on the first day of the year is that supernova discoveries have to be officially confirmed spectroscopically before they get an official IAU designation. When someone discovers a possible supernova it gets reported to the IAU and then listed on the CBAT Transient Objects Confirmation Page. If it is a possible SN it gets a temporary designation of PSN (possible supernova) followed by its coordinates (PSN J01560719+1738468).

Only after someone has taken a spectrum confirming it is a supernova does it get a name with the year and letter combination. This can take several days, so it is unlikely a SN discovered on January 1 will be named until later in the week or the second week of the month. If it were discovered on December 23rd and confirmed on the 1st of January it would still get a name from the previous year.

This time lag will not be acceptable in the near future, with surveys like LSST coming on line. Astronomers will want immediate notification of discoveries of all types of transient objects including supernovae, so what has happened is new groups searching for SNe have begun to make up their own names.

The Catalina Real Time Survey is one such group. They are discovering dozens of possible supernovae that don't always get official IAU designations. Their discoveries are all named CSS (Catalina Sky Survey) followed by the date in yymmdd format and then the rough coordinates, like this CSS111227:104742+021815. Crazy, huh?

ROTSE, the Robotic Optical Transient Search Experiment, also discovers SNe and gives them their own designation in the form of ROTSE3 (the third iteration of this experiment) followed by coordinates, such as ROTSE3 J133033.0-313427.

And there is the Palomar Transient Factory which names its discoveries with the prefix PTF of course, such as PTF11kly, the nearest supernovae in decades, visible with small telescopes in M101. This SN eventually received an IAU designation, SN 2011fe, but that just created more confusion, since now it is known variously by both names in the literature.

Somehow managing to keep it all together amidst the confusion, David Bishop maintains the Latest Supernova Website where you can see discovery images and keep track of your favorite supernovae and related news. There is an excellent article about David and how his website evolved from simple beginnings.

So if you're asking WTF? about the latest SNe the on the WWW the URL that will lead you through the ABC's is definitely http://www.rochesterastronomy.org/supernova.html.

Got that? Good, there will be a quiz later...

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 identified, 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

The Infrared Astronomical Satellite (IRAS)

The Infrared Astronomical Satellite (IRAS) was the first-ever space-based observatory to perform a surveyof the entire sky at infrared wavelengths. Launched on January 25, 1983, its mission lasted ten months. The telescope was a joint project of the United States (NASA), the Netherlands (NIVR), and the United Kingdom (SERC).

It discovered about 350,000 sources, many of which are still awaiting identification. About 75,000 of those are believed to be starburst galaxies, still in their star-formation stage. Many other sources are young stars with disks of dust around them, possibly the early stage of a planetary system formation.

UX Ori type variables

Artist drawing of the dusty environment around a young stellar object. 

Planets are believed to form in these environments when stars are relatively young. 

Image credit: NASA

Named after the prototype of the class, UX Ori type variables (commonly called UXOrs) are intermediate mass, pre-main sequence Herbig Ae/Be stars. The light curves of these stars show Algol-like fadings of 1 magnitude or more in random intervals of days to weeks. These fadings are accompanied by a change in color and are thought to be caused by non-uniform dust structure around the star.

AAVSO 3000 day light curve of UX Ori

Antares

Antares, alpha Scorpii, is the brightest star in the constellation Scorpius and the 16th brightest star in the sky. Antares is a class M supergiant star. What does that mean? Well, with a radius of about 800 times the Sun; if it were placed in the center of our solar system, its outer surface would lie between the orbits of Mars and Jupiter.

Antares is big...really big!

Antares is approximately 600 light-years distant. It is a type LC "slow irregular variable" star, whose apparent magnitude slowly varies from +0.88 to +1.16. Its absolute magnitude is -5.28 Mv. Its visual luminosity is about 10,000 times that of the Sun, but because the star radiates a considerable part of its energy in the infrared part of the spectrum, the bolometric luminosity equals roughly 65,000 times that of the Sun.

The mass of the star is calculated to be 15 to 18 solar masses, which isn't very massive considering its bloated dimensions, so Antares has a very low average density.

Antares has a hot blue companion star, Antares B, of spectral type B2.5. It is normally difficult to see in small telescopes due to Antares' glare, but it can be picked out from the glare in 6 inch or larger telescopes on good nights. I've seen it in telescopes on still nights and to me it looks green, but this is probably a contrast effect. The orbit is poorly known, with an estimated period of ~878 years.

What Antares and its companion star might look like from a planet orbiting at a safe distance. 

Copyright Don Dixon 1997

Apparent Magnitude and Absolute Magnitude

The apparent magnitude of a celestial body is the measure of its brightness as seen by an observer on Earth, in the absence of the atmosphere.

The Apparent Magnitude of some familiar objects:
The Sun  -26.73
Full Moon -12.6
Maximum brightness of Venus -4.6
Maximum brightness of Mars -2.9
Maximum brightness of Jupiter -2.9
Sirius, the brightest star in the night sky -1.47
The bright star Vega 0.0
Approximate faint limit of a naked eye observer under ideal conditions 6.5
Faintest stars visible in 9x50 binoculars 9.5
Faintest stars visible in my 12" telescope from an urban site under an average moonless sky 15.7
Faintest stars visible to HST in visual wavelengths 30

Naturally, if all stars were the same brightness, the closer stars would be brighter and the further stars would be fainter. In fact, you could measure how far away a star was just by measuring how bright it appears.

But things in Nature are never that simple. In our Universe there are small, average stars very close by to Earth that appear quite bright, and humongous, brilliant stars very far away that appear just as bright or brighter as seen from Earth. In order to compare apples to apples, as it were, we need a system that evens the playing field. That is where absolute magnitude comes in to play.

In astronomy, absolute magnitude measures an object's actual intrinsic brightness. The absolute magnitude equals the apparent magnitude an object would have if it were at a standard distance (10 parsecs, or 1 Astronomical Unit, depending on object type) away from the observer. This allows the true brightnesses of objects to be compared without regard to distance.

American Association of Variable Star Observers (AAVSO)

The American Association of Variable Star Observers (AAVSO) is an international non-profit organization whose mission is: to observe and analyze variable stars; to collect and archive observations for worldwide access; to forge strong collaborations between amateur and professional astronomers; and to promote scientific research and education using variable star data.

The AAVSO website is a wealth of information on variable stars, their types, characteristics and the research being done on them. You can plot light curves of stars in the AAVSO database, or just check the most recent data in the quick look files. 

 Light curve of the unusual star KR Aurigae

Since the founding in 1911, the people of the AAVSO have propelled the field of variable stars forward. That is the true strength and genius of the AAVSO. The members and observers who have propelled variable star science and astronomy forward by their participation and collaborations.

Simopedia: the mad plan

Now that I've had more time to think about it, the plan for Simopedia is coming together. Basically, I'll do an occasional post on a subject and tag it with the Simopedia tag. In the end, I hope to have enough of these 'definitions' to use as reference link outs from blogs on current events or news for terms and ideas previously covered in the Simopedia blogs.

So they serve a few purposes. 1- They can serve as reference, 2- They are still informative, and hopefully interesting enough to read in their own right, 3- They give me topics to write about on days when I have time to write but there aren't necessarily any fascinating or noteworthy stellar astronomy current events or news stories, 4- I'm learning a lot as I discover more about each of the topics, which in turn should make me better at disseminating stellar astronomy information to you, my readers.

For now, I'm still writing about subjects beginning with the letter A. I have a few more I think would be good to include, and then we'll move on to the exciting world of the letter B.

Astrometry

Astrometry is the branch of astronomy concerned with the precise measurements of the positions and movements of stars and other celestial bodies.

Astrometry has been important in history for maritime navigation, since navigators used to calculate their position on Earth upon the observation of stars. (Yes, Jimmy, back in the olden days before GPS satellites!)

Today, astrometry is still important for keeping time. The international time standard is the Coordinated Universal Time (UTC), which is the atomic time synchronized to Earth's rotation by means of exact observations.

Astrometry dates back at least to the Greek astronomer Hipparcos in the 2nd century B.C. He compiled the first catalogue of stars and also invented the brightness scale (magnitude) we still use today.

From 1989 to 1993, the European Space Agency's Hipparcos satellite performed astrometric measurements resulting in a catalogue of positions accurate to 20-30 milliarcseconds for over a million stars.

Modern astrometry was founded by the German mathematician and astronomer, Friedrich Bessel. Bessel was the first to measure the distance to a star as a result of measuring its parallax. He gave the distance to 61 Cygni as 10.3 light years, which is very close to the currently accepted distance of 11.4 light years.

Although once thought of as an esoteric field with little useful application for the future, information obtained by astrometric measurements is now very important in today's research.

Today, astronomers use astrometric techniques for tracking near-Earth objects, and to detect extrasolar planets by measuring the slight wiggle in the parent star's motion on the sky. NASA's planned Space Interferometry Mission will utilize astrometric techniques to detect gas giants around other stars, and perhaps even terrestrial planets nearby.

Argelander

Today I’d like to introduce you to one of the famous historical figures in astronomy, Friedrich Wilhelm August Argelander. You just know he’s important because he has four names! Not only that, the crater Argelander on the Moon and the asteroid 1551 Argelander are named for him.

Argelander was important to astronomy in ways he could never have imagined. A lot of the things we know today, and many of the things we are still seeking answers to, Argelander was already working on in the 19th century. Friedrich Wilhelm August Argelander was a pioneer in astronomy.

Argelander was born March 22nd, 1799 in Memel, which at that time was in the kingdom of Prussia, now Lithuania. His father was a wealthy Finnish merchant and his mother was German. Not a lot is known about his early years, but he eventually came to study under the famous German mathematician and astronomer, Friedrich Bessel, and in 1822 he obtained a PhD at Königsberg University, famous for its School of Mathematics.

Argelander began his professional career in 1820 as an assistant in Bessel's Königsberg Observatory. A few years later, Bessel helped Argelander land a job as director of the Turku observatory in Finland and in 1828 he became professor of astronomy at the university there. Tragically, the observatory, and most of the university buildings, burned down in 1827 and Argelander began the design and construction of a new observatory in Helsinki, where the university was relocated. The new observatory was completed in 1832.

1836 he was appointed professor of astronomy at Bonn, where King Friedrich Wilhelm IV built Argelander an impressive new observatory. As it happens, the king and Argelander were actually old friends. In 1806, following Prussia's defeat by Napoleon, Friedrich Wilhelm, then the crown prince, had sought refuge in the Argelander home in Memel, East Prussia. It pays to have a rich father and friends in high places!

Argelander was very interested in the positions and motions of the stars, and in the direction the Sun and solar system were traveling through the stars. In 1837 he published his first results in a book, "About the Proper Motion of the Solar System", in which Argelander had come to the conclusion that he did not have enough data for the exact answers to his questions. This provided him with the incentive to begin mapping the exact positions of the stars in the Northern sky from 1852 on in Bonn, Germany, a monumental task before the use of photographic plates in astronomy.

Argelander's name is best known for this compilation, called the Bonner Dorchmusterung, the largest and most comprehensive of all the pre-photographic star catalogs. Under Bessel he had begun a survey of the sky from 15°S to 45°N. This was extended at Bonn to an area from 90°N to 2°S and when finally completed eleven years later, in 1863, it listed the positions of 324,198 stars down to ninth magnitude. Again, they did all this work without the use of photography…

Also in 1863, Argelander founded an international organization of astronomers named the Astronomische Gesellschaft, now the second oldest astronomical society after the Royal Astronomical Society.

Being a variable star enthusiast, I became interested in Argelander because he is generally considered the father of variable star astronomy. He was the first astronomer to begin a careful study of variable stars. One of them, epsilon Aurigae, is still a fascinating and challenging mystery to astronomers today.

At the time, only a handful of variables were known, and he was responsible for introducing the modern system of naming them using the capital letters R-Z. It was believed that variability was a rare phenomenon and that this would provide plenty of names for the variables yet to be discovered. In a few years this proved inadequate and the naming system was extended to double letters, and then a numbering system. Today, tens of thousands of variable stars are cataloged, with more being discovered all the time.

Argelander loved the stars, especially variable stars, and was one of the all-time great observers of the heavens. In 1844 he published "An Appeal to the Friends of Astronomy" in ‘Schumacher's Astronomical Year Book.’ This was translated to English and reprinted by Annie Jump Cannon, in Popular Astronomy in 1912.

“Therefore do I lay these hitherto sorely neglected variables most pressingly on the heart of all lovers of the starry heavens. May you become so grateful for the pleasure which has so often rewarded your looking upward, which has constantly been offered you anew, that you will contribute your little mite towards the more exact knowledge of these stars!

May you increase your enjoyment by combining the useful and the pleasant, while you perform an important part towards the increase of human knowledge, and help to investigate the eternal laws which announce in endless distance the almighty power and wisdom of the Creator! Let no one, who feels the desire and the strength to reach this goal, be deterred by the words of this paper.

The observations may seem long and difficult on paper, but are in execution very simple, and may be so modified by each one's individuality as to become his own, and will become so bound up with his own experiences that, unconsciously as it were, they will soon be as essentials.

As elsewhere, so the old saying holds here, "Well begun is half done," and I am thoroughly convinced that whoever carries on these observations for a few weeks, will find so much interest therein that he will never cease. I have one request, which is this, that the observations shall be made known each year. Observations buried in a desk are no observations. Should they be entrusted to me for reduction, or even for publication, I will undertake it with joy and thanks, and will also answer all questions with care and with the greatest pleasure.”

Yea, it’s a little wordy and flowery, but it’s obviously written by someone who loves observing and studying the stars.

The “Argelander Step Method” is a visual method of estimating the magnitude of a variable star. It involves comparing the variable with a comparison star of known constant magnitude, and assigning a step value that reflects the brightness of the variable as distinguished from that of the comparison star. Estimates of the form ‘A(3)V, V(1)B’ are the result, and the magnitude of the variable (V) can be calculated from the known magnitudes of the comparisons (A and B). This is very similar in practice to methods still used today by visual observers of variable stars.

Argelander died February 17th, 1875. But the body of astronomical knowledge that stands on the shoulders of this giant continues today, in the cataloging of the position and motion of the stars within our galaxy, and the study of variable stars, from supernovae and Cepheids used to determine the distances to far away galaxies to the transits of extra solar planets across the faces of stars. In 2006, the three astronomical institutes of the Bonn University were merged and renamed as the Argelander-Institut für Astronomie.

The lunar impact crater Argelander

Algol

Algol- Beta Persei, also known as the "Demon Star", is 93 light years away in the constellation of Perseus.

Algol, was the first eclipsing binary to be discovered in 1669 and is the prototypical star of its class. Usually magnitude 2.1 at maximum, it dips to 3.4 every two days, 20 hours and 49 minutes. The entire eclipse lasts about 10 hours and is visible to the naked eye. There is also a secondary eclipse when the brighter star occults the fainter secondary. This secondary eclipse can only be detected photoelectrically.

The light curve of Algol demonstrates the geometry of the system. Eclipsing binaries are among the most important kinds of stars, as they provide us with information on stellar masses and dimensions.

But Algol is equally famed for the "Algol paradox." The less massive star is already a subgiant, and the star with much greater mass is still on the main-sequence. This seems paradoxical because the component stars of any binary are thought to have formed at approximately the same time and should have similar ages. So the more massive star, rather than the less massive one, should have evolved fastest and left the main sequence.

The paradox is resolved by the fact that in many binary stars, material can be exchanged between the two stars, disturbing the normal process of stellar evolution. The originally more massive star will reach the next stage in its evolution despite having lost much of its mass to its companion.

Accretion

Accretion- The term accretion describes the growth of a massive object by gravitationally attracting more matter. This is commonly done through the formation of an accretion disk of gaseous matter. We find accretion disks around smaller stars or stellar remnants in a close binary, or around black holes in the centers of galaxies.

Accretion disks form in non-magnetic cataclysmic variable binaries

Accretion also refers to the collision and sticking of microscopic dust and ice particles in protoplanetary discs and protoplanet systems, leading to planetesimals which gravitationally accrete more small particles and other planetesimals.

Images courtesy Mark A. Garlick

http://www.space-art.co.uk/index.php

Pleas do not use without permission from the artist

Simopedia: A New Simo-series of Blogs

This year I'm starting a new series of blogs called Simopedia. Simply put, I plan to work my way through the alphabet, in no particular order, selecting various stellar topics and categorizing them by letter. Hopefully, in a year or two we will have an alphabet soup of interesting and fun facts- Simopedia.