March 31, 2020

19.06.10: A New Breed of Supernova?



Every student of Astrophysics 101 soon learns that there are two main types of supernovae; Type 1a, which occur when a white dwarf star accretes matter from a bloated companion, passes the Chandrasekhar Limit and explodes, and Type II, when a star 8 times the mass of our Sun or larger reaches the end of its fusion burning life and promptly explodes… but are these snapshots of the final phases of stellar evolution really that neat and tidy? Recently, evidence has been mounting that there may be other sub-branches to the supernova tale, and not just the two flavors and the sub-categories that we learned in school. The first round of evidence comes from a team at the Harvard-Smithsonian Center for Astrophysics and their study of supernova 2005E. This blast occurred in the galactic halo of the galaxy NGC 1032 in the constellation Cetus, not your typical supernova breeding territory. Supernovae are usually seen in rich star forming regions, not in metal poor outer galactic suburbs. This event was a fizzle, ejecting only 300 times the mass of Jupiter into its nearby environs.

The mystery deepened as a team from Hiroshima University released their results of a study of another supernova, 2005cz. Located in the elliptical galaxy NGC 4589, this eruption was also only 20% as bright as models predict, showing that while the initial mass may have been just above what was required for a Type II supernova, it beared none of the classic hallmarks of either species of events. Both of these supernovae, along with 6 others recorded, show a high concentration of calcium in their spectra, a hint that they may not be related to either of the previously known types.

So, what’s going on? Do we need to re-write all those old astrophysical texts? It’s unlikely that a progenitor star migrated all the way to a galactic halo region in its short life span simply to explode. A possible scenario could be a pair of binary white dwarfs (or do you say dwarves?) in a tight orbit, with one stealing the helium shell of another and bursting. Spectra taken of both events seem to support this scenario… this mystery may have a tie-in with the seeming lack of “Type 1A’s in waiting” mentioned in this space in an article on a recent survey of nearby galaxies… will this hybrid style of supernova become known as “Type III” or “Type 2.5”?

12.05.10- White Dwarf Lite?

The Kepler space telescope may have bagged an unexpected prize during its hunt for exo-planets. Along with five published exoplanets illustrated above, Kepler snared two potentially bizarre objects. Dubbed KOI (Kepler Objects of Interest) -81 and 74, these companions actually appear dimmer passing behind the parent star rather than in front of it. This suggests a bright luminous object(s) with an Earth-like diameter but much more massive… a white dwarf? Possibly, but the objects seem to be physically too large to fit this class of objects. White dwarfs have an upper limit of about 1.4 solar masses, also known famously as the Chandrasekhar limit. Recently, scientist Jason Rowe of NASA Ames research center has been able to directly measure the masses of these companions by measuring the way the companions physically warp, or distort the bodies of their primary companions. The result; these stars are in the realm of 0.1 solar masses, which would make them some the lightest white dwarfs known. Obviously, this also becomes a problem because although small and luminous, KOI-81 and -74 probably aren’t supported solely by electron degeneracy pressure that characterizes standard classical white dwarfs. The situation just got stranger and stranger… were these objects large super-heated planets or light white dwarfs?

Enter an international team of astronomers meeting at Kavli Institute in Peking (Beijing) China. Using an innovative technique known as Doppler boosting, they were able to pinpoint the mystery objects mass at 0.2 solar masses, on the low end but still in the realm of a white dwarf. This makes even more sense if one considers a white dwarf accreting mass from a primary companion, ala a Type 1A supernovae candidate…(hey, didn’t we write in this space last week about the lack of these beasties?)   Doppler boosting works in terms of catching subtle fluctuations in the brightening of an approaching object as measured by photons received over a given unit of time and dimming as it recedes…altogether a complicated affair, considering this must be untangled from a flurry of other signals. This unexpected find illustrates that surreptitious discoveries are often the norm in astronomy, if only someone is willing to look for them!

Astro-Challenge:When will T Pyxidis Finally Pop?

Earlier this year, the astronomical community was wowed by the eruption of the star U Scorpii. As reported last year in this space, U Sco is a recurrent nova, a flare star that undergoes outburst at irregular intervals. Less than 10 recurrent novae have been identified. The initial action was caught by two Florida based amateurs, and demonstrates that hands on, observational astronomy is still alive and well even in the modern age of astronomical automation. This week, as the waning gibbous moon slides out of the evening sky, I’d like to turn your attention to another of these rare beasts; T Pyxidis. Located in the constellation Pyxis, the Mariner’s Compass, this is one of those unimaginative southern hemisphere constellations thrust upon us in the 18th century. Visually unremarkable, it contains a handful of deep sky objects and clears the horizon sufficiently in the spring evenings for observers in the southern United States to perform routine observations. T Pyxidis itself is a binary system consisting of a white dwarf cannibalizing a sun-like star. When enough in falling matter accumulates, T Pyx flares up from its normal barely detectable magnitude +15.5 to +7.0, almost naked eye visibility. This has happened at roughly 20 year intervals in the years 1890, 1902, 1920, 1944, 1966…and then T Pyx fell silent. We are currently 44 years and counting for an outburst, and this is definitely a star worth continuous scrutiny. The light curve is that of a slow nova, rapidly brightening over a couple of nights, fluctuating at its peak brightness for about a month, and then fading out over proceeding months. T Pyx is a prime candidate for a galactic Type Ia supernova, and at a distance of 3260 light years, could put on quite a show. Of course, said final act could occur tonight, or 10 million years from now; but this current lull makes you think; there has to be a lot of material accreting up there! Its coordinates are;

R.A: 09h 04m 41.5s

Dec: -32 22m 47.5s

And for an uber-cool finder chart that Sky & Telescope produced a few years back, follow this link… (hint: for use in the field, take the chart and  invert the colors in Paint or Photoshop!)

Good luck, and with a little patience, YOU could be the next amateur to catch T Pyx in the act!

The Astro-term for this week is Chandrasekhar Limit. This is the mass limit of a body in which electron degeneracy pressure can push outward against gravitational collapse. First calculated by Indian astrophysicist Subrahmanyan Chandrasekhar in 1930, this mass is usually given as 1.44 solar masses. Below this limit, a white dwarf and an accompanying planetary nebula will occur; above this mass limit, a core collapse supernova will occur, leaving behind a pulsar or a black hole. T Pyxidis has to be very near its very own Chandrasekhar Limit, with the amount of in-falling mass it is accumulating… will it pop in our lifetime?