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Sunday, May 29, 2016

Type Ia Supernovae, Recurrent Novae and Classical Novae

A Possible Symbiotic Relationship Between Type Ia Supernovae, Recurrent Novae, and Classical Novae: Do These Events Occur in the Same or Different Stellar Masses?

Astronomers are trying to determine the answer to the question of whether there is a direct relationship between Type Ia Supernovae (SNeIa), Classical Novae (CNe) and Recurrent Novae (RNe). Some evidence has been found to support a distinct relationship between catastrophic SNe events and RNe outbursts. There is evidence that a tipping of the scale into a full-fledged Type Ia SN may be related to RNe, and though some evidence seems to point to this, all the particulars are not out regarding the three types. Do the three events occur within the same stellar mass, or are they separate events occurring in separate stellar masses. 

Image of Las Cumbres Observatory Global Telescope Network
Las Cumbres Observatory Global Telescope Network
The results of a study, led by Ben Dilday, a postdoctoral researcher in physics at UC Santa Barbara and at Las Cumbres Observatory Global Telescope Network (LCOGT), are surprising because previous indirect –– but strong –– evidence had pointed to the merger of two white dwarf stars as the originators of other Type Ia supernovae. One interesting subject is the planetary nebula NGC 5189. It has a unique, torqued-spiral shape, with peculiar patterns of knots around it's periphery, possibly due to a wobbling in the stars' rotation, or, possibly, to the presence of a binary white dwarf system at its center.

Symbiotic binaries are systems containing both a white dwarf (WD) and a red giant component. Symbiotic novae are those systems in which thermonuclear eruptions occur on the WD component. These are to be distinguished from events driven by accretion disk instabilities analogous to dwarf novae eruptions in cataclysmic variable outbursts. Another class of symbiotic system is that in which the WD is extremely luminous and it seems likely that quiescent nuclear burning is ongoing on the accreting WD. A fundamental question is the secular evolution of the WD. Do the repeated outbursts or quiescent burning in these accreting systems cause the WD to gain or lose mass? If it is gaining mass, can it eventually reach the Chandrasekhar Limit and become a supernova (a Type Ia SN if it can hide the hydrogen and helium in the system)? In order to better understand these systems, a new study has begun of the evolution of Thermonuclear Runaways (TNRs) in the accreted envelopes of WDs using a variety of initial WD masses, luminosities, and mass accretion rates. Astrophysicists have put into use a 1-D hydro code, NOVA, which includes the new convective algorithm of Arnett, Meakin, and Young, the Hix and Thielemann nuclear reaction solver, the Iliadis reaction rate library, the Timmes equation of state, and the OPAL opacities. It is reported that (1) the WD grows in mass for all simulations so that canonical 'steady burning' does not occur, and, (2) that only a small fraction of the accreted matter is ejected in some (but not all) simulations. They have also found that the accreting systems, before thermonuclear runaway, are too cool to be seen in X-ray searches for Type Ia SN progenitors.

Image of SN PTF 11kx Imaged by BJ Fulton Las Cumbres Global Telescope Network
SN PTF 11kx Imaged by BJ Fulton
Las Cumbres Global Telescope Network
The case of SN PTF 11kx, discovered by the Palomar Transient Factory, showed this relationship beyond further doubt. Astronomers could discern that the supernova was surrounded by shells of hydrogen gas that had likely been blown off in previous nova eruptions, decades before the supernova occurred. Novae are more frequent but weak explosions not catastrophic to the star. While similar shells of material had been seen before in a handful of Type Ia supernovae, their origin was debated and they had never before been firmly linked to novae - some doubted that the material was near to the supernova at all. SN PTF 11kx proved differently. The surrounding gas was moving too slowly to be from the supernova event itself, but too fast to be from a typical stellar wind. Lars Bildsten, director of UC Santa Barbara's Kavli Institute for Theoretical Physics, hypothesized that it was material shot out from a previous nova eruption, which had been slowed as it collided with the wind from the red giant star. UCSB graduate student Kevin Moore showed this hypothesis to be plausible, and would lead to gas moving at speeds seen in the observations. Adding credence to the theory was the fact that the material moved at two different speeds -- faster-moving interior material to slower-moving exterior -- exactly as expected. The outer material had been slowing for decades, while the inner material had less time to slow. But if this was the case, the fast-moving supernova ejecta should have eventually collided with the nova material. About two months after the explosion, this is exactly what happened. New observations showed that the supernova ejecta was colliding with the interior shell of material. It was impossible to doubt that this gas was nearby the supernova. "This was the most exciting supernova I've ever studied," said Dilday, "For several months, almost every new observation showed something we'd never seen before."

This research has made use of the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration; The Palomar Transient Factory at the California Institute of Technology; Baltic Astronomy, Vol 21, pages 76 - 87, 201arXiv:1211.6145v1 [astro-ph.SR]
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Friday, May 20, 2016

Digital Shoebox

A few weeks ago I found the equivalent of a shoebox full of old snapshots. As I was looking through an old backup of a filesystem from a computer I had long since parted with I ran across a directory, simply titled "CCD", circa 2003. A quick glance at the files of this lost directory I immediately knew what I had: these files, all FITS, and JPEG format, were astronomical image files, a reminder from the not so distant past when we first discovered the excitement of capturing night sky wonders with a CCD camera.

Image of Mars - August 2003 - Image by Muir Evenden
Mars - August 2003 - Image by Muir Evenden
Before the CCD revolution, Mike Petrasko and I tried our hand at imaging using traditional photographic processes by taking images on film, mostly wide field views of the Milky Way, Orion, and other areas that fascinated us. Our friend and colleague Harry Hammond took this to the next level with prime focus photography, a much more demanding endeavor (please refer to Harry's article Old Dogs and New Tricks). When CCD's became available (and more affordable!) to the amateur market it was something we embraced wholeheartedly: here was a way to get "prime-focus" quality images but without the long hours spent in front of a guiding eyepiece. Once I had a job well paying enough to afford the equipment, you can bet I did not hesitate to start the adventure, starting with a Celestron 8 on a Losmandy G11 mount and a kit camera CCD and quickly evolving into a Celestron 14 on a Software Bisque Paramount and an Apogee camera.

Image of NGC 6946 - August 2003 - Image by Muir Evenden
NGC 6946 - August 2003 - Image by Muir Evenden
So what I had here in our digital shoebox was a moment in time when we were getting our feet wet and honing our chops in the realm of CCD's and computers, and looking at these images now I think we can be rightly proud of our efforts. Of course compared to the achievements of what amateurs have been able to accomplish with the equipment available today our images may seem downright pedestrian, but that's OK as we don't expect to win a contest with these pictures but simply revel in the pleasure knowing that we captured this light from galaxies, nebulae, and planets, socking it away for future enjoyment. And what do you know, the future just arrived!
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Sunday, May 15, 2016

Students Observe the Mercury Transit

On Monday, May 9, 2016, close to 100 students from grades 7 thru 12 walked into the Kohout-Dingley Observatory located on the Sacred Heart High School campus to safely view the rare transit of the planet Mercury crossing the disk of the Sun. This was the first use of the new Baader solar filter manufactured by AstroZap recently purchased specifically for the event. The session, in partnership with staff of Insight Observatory, hosted entire classes throughout the course of the school day as well small groups of students visiting during their lunch session. All observers were treated not only to the Mercury transit event but were able to observe groups of sunspots as well.

Image of Sacred Heart Students Observing the Mercury Transit Photo by Joe Masi
Sacred Heart Students Observing the Mercury Transit
Photo by Joe Masi
The early morning hours of May, 9th brought a potential promise of clear observing conditions for the transit. However, just as I was leaving for the observatory to setup for the day, clouds started quickly rolling in. I checked in with the transit's live coverage feed from Sky and Telescope Magazine and J. Kelly Beatty, one of the publication's Cheif Editors, was reporting the same weather conditions from Cambridge, Massachusetts. Once I arrived at the school's campus around 8:10 am located in Kingston, Massachusetts, I noticed the clouds were starting break up and scatter a bit. Joe Masi, a science teacher at the school, assisted me with opening the facility that houses the 11" Celestron Schmidt-Cassegrain Telescope. As the sky proceeded to clear up, we locked the solar filter onto place on the telescope and proceeded to slew the telescope towards the Sun. Remember, NEVER look directly at the Sun find it. As I focused the telescope I immediately detected a defined sharp black dot on the Sun's disk along with a few small groupings of sunspots. Mr. Masi wanted to get a quick peek before his 9:00 am class started. He quickly spotted the tiny planet as well.

A little after 9:00 am, Mr. Masi returned to the observatory with his first class. As the students in groups of 2 or 3 took turns viewing the Mercury transit upstairs at the telescope, the rest of the class below learned about the event taking place with a display set up by Insight Observatory on the first floor. The science teachers were discussing what was actually transpiring as well by using the display as an aide. While Mr. Masi's first class was enjoying the event, he suggested that the students image the transit on their smartphones. Some of the students attempted this throughout the day (as I did myself). However, it was very painstaking trying to hold the device steady enough to get an image in decent focus. A student from the first class, Nolan D., had the most successful image of the day. The morning classes that visited the observatory were treated to excellent views of the transit as the sky completely cleared up. However, the students that stopped by during their lunch period were challenged by fast-moving clouds. As they patiently waited, they were able to get a quick glimpse.

Image of Mercury Transit at 9:15 am EDT - Image by Nolan D.
Mercury Transit at 9:15 am EDT - Image by Nolan D.
After the last class left around 2:10 pm, I sat in the observing chair at the telescope and took in the last part of this rare event. I watched the silhouette of the planet slowly disappear at the sun's limb exactly at 2:41 pm EDT. It was a great astronomy education experience and pleasure sharing the transit of Mercury with the Sacred Heart students. Some of them are already looking forward to the next one in November of 2019.

A special thanks to Frank Lopez of the Stellarvision Astronomy and Science Shop located in Tuscon, Arizona for his prompt delivery of the AstroZap Solar Filter allowing us to observe this special event.
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