Recurrent Nova M31N-2008-12a

On July 15th, 2015 I wrote a blog entry about RNe M31N-2008-12a, a recurrent nova in our neighboring galaxy Messier 31, the Andromeda Galaxy LINK.  Well its fall now, and either in the midst of or past what should be the expected annual or six month outburst for this ever increasing White Dwarf as it feeds mass from it's Red Giant Star companion.  This table will show the expected time frames when this binary WD (White Dwarf) and RGC (Red Giant Companion) are expected to have an outburst. The -a after the year is when the first outburst is expected, based on data, though never observed or confirmed by observation yet. The -b are the second or annual outburst that have been observed and recorded since 2008 and also confirmed via archival data from 1992, 1993 and 2001.

You can see here, that based on data gathered by observations made by Chandra, the Kiso
Schmidt Telescope, the Okayama Telescope, the MiyakiArgenteus Observatory, all in Japan; the Xingming Observatory, China; the OndĖ‡rejov Observatory, Czech Republic; Montsec Observatory, Spain; and the Kitt Peak Observatory, USA, that the observed time between measured outbursts is 351 ± 13 days though the team led by M.J. Darnley believes the true value to be half of this. Thus the reason for the -a listings.  This is based on observations of eruptions that were identified starting in 2008, with eruptions having a recurrent period of 347 +/- 10 days (Darnley et al. 2016a).  When archival X-ray detections of eruptions in 1992, 1993 and 2001 (Henze et al. 2014) are factored in, the evidence is very strong that the real recurrence period is more likely 174 +/- 10 days or half of what it is currently. One reason we may not see these is Andromeda, M31 is close to the Sun from March through May.  If the second occurrence occurs during this time frame, and it seems it most likely would, the Sun is most likely blocking us from seeing the second outburst each year. It does indicate though that the white dwarf is close, really close to the Chandrasekhar limit of 1.4 solar masses. That means a Type Ia supernova or a direct collapse into a neutron star if neon is present in the white dwarf.

Novae are powered by thermonuclear runaway occurring at the base of an acreted layer at the surface of a white dwarf (WD) star.  The white dwarf star is formed when a star, between the size of our Sun to 8 or perhaps 9 times the mass of our Sun, ends it life by going through the Red Giant branch of its life. The star ran out of hydrogen to burn and so contracted due to gravity. As fusion again begins to burn, this causes radiation pressure and the star now expands. The outer layers of hydrogen in the star redden and the star assumes more of a red appearance, thus due to the expansion and growth, becomes a Red Giant or RG. That contraction allowed hydrogen around the helium core to ignite It sheds its outer layers via the process As a star like our Sun burns through its hydrogen layer, it begins to contract as the hydrogen fuel is used up.  This usually is around 8 to 10 billion years into the stars life. This image reflects this process.

ESO/M. Kornmesse LINK

At this point, the helium core of the red giant becomes highly dense to the point that the core cannot expand anymore. As the hydrogen envelope around the core continues to bring heat, the core as being degenerate now cannot respond and temperature becomes high enough that the helium nuclei begin to fuse into carbon via the Triple Alpha process.

Image Credit: Borb/CC

The carbon can then fuse with helium nuclei to become oxygen and neon.  As stated for many of these red-giant stars, the core will not change its size nor will it cool.  The most common red giants are stars on the red-giant branch (RGB).  RGB stars are still fusing hydrogen into helium in a shell surrounding an inert helium core. Other red giants are the red-clump stars in the cool half of the horizontal branch, fusing helium into carbon in their cores via the triple-alpha process (see above); Finally we have the asymptotic-giant-branch (AGB) stars with a helium burning shell outside a degenerate carbon–oxygen core, and a hydrogen burning shell just beyond that. Stars less than about 10 times the mass of the Sun become asymptotic-giant branch stars - red giants with inert, degenerate carbon/oxygen cores, which are fusing helium in the shell around the core. This helium fusion causes the star to become unstable and the envelope is ejected as a planetary nebula.  This LINK provides a good overall view of the process of converting a Sun like star into a Red Giant.

A cataclysmic variable or CV novae occur in close binary systems, perhaps roughly the size of the Earth-Moon system, maybe slightly larger or smaller.    The White Dwarf or WD accretes material for its companion, usually via an accretion disk around the WD. The companion can be a Sun like star to a Red Giint Star. Here our WD has already gone through the stages of a stellar like star like our Sun, evolved, ascended up the Red-Giant branch and thrown off its envelope with only the white dwarf (WD) left behind.  The companion in M31N 2008-12a is now a Red-Giant itself, have evolved to that point and via a stellar wind is sharing or accretting mater unto the white dwarf. As the material builds, it reaches a point where an the carbon and oxygen begin to fuse so quickly that this causes an eruption. That eruption  Since this CV novae has such a short recurrence period, this infers that the WD in M31N 2008-12a must be a high mass, perhaps the highest mass WD yet found in a CV system.   The unveiling of the super soft source or SSS X-ray emission after each eruption of just six days, again points to a low ejected mass combined with the short duration of the SSS of about 19 days, further supports the notion that this is a high mass WD.  As a result the work of Tang et al. (2014) and Kato et al. (2015) indicates that the mass of this WD will reach the Chandrasekhar mass of 1.4 solar masses in less than 1 million years (1 Myr).

Whether this system ends up as a SN Type Ia or ends up as a neutron star will be determined by the composition of the WD. An oxygen neon white dwarf will result in a neutron star when the WD reaches the Chandrasekhar mass of 1.4 solar masses, and the WD will collapse right into the neutron star.  If the WD is composed of carbon and oxygen then it will result in a SN Type Ia when it reaches the Chandrasekhar mass of 1.4 solar masses.  The work of Hillman et al. (2016) shows that a carbon oxygen white dwarf can grow from its initial formation mass of < 1.1 solar masses to the Chandrasekhar mass of 1.4 via a long series of hydrogen flares (novae) interspersed with helium flashes or He-novae with little to no tuning of the system parameters or accretion rate. This has significantly strengthened the case for novae contributing to the SN Ia progenitor population.

The companion to the WD based on compelling evidence from the spectral lien development, to the coronal emission lines, to the Raman emission band and the photometric behavior of each eruption point to red giant with wind accretion in this system.  Further, the distance from the RGC (Red Giant Companion) to the WD is most likely short, resulting in an accretion disk that survives each eruption, allowing for the WD to accrete mass very shortly after the eruption.  The accretion disk must be particularly massive, with a Red Giant donor with a high mass accretion rate and accretion disk luminosity,. With at least one annual and most likely two eruptions every year occurring in M31N 2008-12a since at least the early 1990's, astronomers have raced to find the missing eruptions. This has led to the Liverpool Telescope or LT led by Steele et al.,  observing and discovering a shell like elliptical nebula centered on the nova.  It is large, very large, measuring 130 pc along the major axis and 90 pc across the minor axis.

To show how large that is, we know that a parsec is roughly 3.26 light years or 19 million miles (31 trillion km).  At 130 pc that means  2.47 billion miles or the distance from Earth to the planet Neptune. The minor axis would be 1.71 billion miles or the distance from the Sun to Uranus. Darnley et al. (2015) proposed this extended nebulosity is the super-remnant of many thousands of past eruptions from M31N 2008-12a.  Hydrodynamic simulations of the formation of such a phenomena are underway and support this hypothesis.  Ten orbits of the Hubble Space Telescope Cycle 24 time was awarded to study the structure and possible formation pathway of the super remnant and this was to have taken place in December of 2016.

The key remaining determination is the composition of the WD itself. Is the WD composed of Oxygen and Neon, leading to the formation of a neutron star when the Chandrasekhar mass limit of 1.4 is reached.  Again, if the WD is composed of Oxygen and Carbon then the final destination will result in a SNe Type Ia when the Chandrasekhar mass limit of 1.4 is achieved. Regardless, M31N 2008-12a is the leading candidate for a pre-explosion SN Ia progenitor.  If confirmed, the super-remnant nebula that is around M31N 2008-12a may exist around all RNe (recurrent novae) and may be a glaring sign that points to a SNe Ia and thus lead us to an avenue of identifying the progenitor type of Type Ia supernova, or a collapse neutron star WD scenerio.

M31N 2008-12a has been compared to RN V745 Sco, a Recurrent Novae in our galaxy perhaps M31N 2008-12a closest cousin.  RN V745 Sco has a RG companion and like M31N 2008-12a is fueled by wind accretion from its RG companion.  Thus it is easy to compare the two.  RN V745 Sco has undergone observed/detected eruptions in 1937, 1989 and most recently 2014. This results in a period of 25 years between eruptions.  It is assumed based on the fact that peak luminosity reaching only 10th magnitude that some eruptions may have been missed on RN V745 Sco.

RN V745 Sco has a faster SSS turn-off and the shortest SSS phase of any observed nova so far. It appears that the SSS phase only lasts until day 10 for RN V745 Sco vs day 18 for M31N 2008-12a and shows that less matter is ejected in RN V745 Sco than M31N 2008-12a and that a smaller amount of hydrogen is needed in RN V745 Sco to trigger an eruption, making the mass of the WD there more massive than the WD in M31N 2008-12a.  However the WD in M31N 2008-12a would be accreting more mass during its eruptions and moving more rapidly toward the Chandrasekhar mass of 1.4.   Bottom line though the WD in RN V745 Sco is most likely gaining mass and is yet another good candidate for a SN Type Ia progenitor.

V745 Sco and M31N 2008-12a are two extreme RNe that share several observational characteristics. They are:

  • Both objects low mass ejecta appear to interact strongly with the stellar wind from the RG companion, slowing significantly in the process and producing high temperature shocks. 
  • Their SSS spectra extend to high temperatures and appear to feature strong, variable emission lines. 
  • While the WD in V745 Sco may be more massive, the WD in M31N 2008-12a appears to have the higher accretion rate, providing the unique opportunity to observe at least one eruption per year, most likely two.  
  • Assuming that V745 Sco has a 25 year cycle in eruptions, the next eruption in V745 Sco will occur around 2039, which means that astronomers will have studied M31N 2008-12a sufficiently to provide detail predictions on the variations on the eruption properties of V745 Sco. 

So, I have quoted a lot from the articles and will now attempt a layman's explanation of why M31N 2008-12a is so important in our understanding of Recurrent Nova (RNe) and progenitor of Type Ia SNe.  Discovered in 2008 with an eruption from the WD in the system, this RNe has erupted every year since 2008. Archival evidence points to eruptions in 1992, 1993 and 2001, while pointing to a huge elliptical nebula around the WD that measures from the Earth to Neptune on the minor axis and from the Sun to Uranus in its major axis. Here is that visually:

That is a significant size nebula which has resulted from thousands of eruptions and the wind coming off the Red Giant companion.  A large accretion disk exists centered around the WD with material coming from the companion Red Giant from the wind put off by the RGC.  This mass accretes unto the WD until it erupts, at least once a year, most likely twice a year. The accretion disk survives the eruption and the WD in M31N 2008-12a rapidly begins to acquire mass, hurtling it to a cataclysmic day of reckoning sometime within the next 1 million years. This nebula may be a leading reoccurring feature of RNe that are progenitors to a Type Ia SN explosion as we can see in M31N 2008-12a's cousin, RN V745 Sco.  M31N 2008-12a is the best identified progenitor for a Type Ia SN in Messier 31, the Andromeda Galaxy that has been identified to this point. Whether M31N 2008-12a ends up as a Type Ia SN or becoming a Neutron Star will depend on its composition. If the WD in M31N 2008-12a is an Oxygen Neon composition, we will end up when the WD reaches 1.4 solar masses, with a neutron star. If the WD is a Carbon Oxygen composition, then when the WD reaches 1.4 solar masses, we will end up with a SN Type Ia supernova. 

Either way, the science behind and the journey of identifying the evolution of this white dwarf, and its final destination is perhaps the most enjoyable aspect of what is occurring here.  Would I love in some future fall day, at new moon or other, to point my 24" or 17.5" up at Messier 31, the Andromeda Galaxy and observe a Type Ia Supernova going off in the location where M31N 2008-12a exists? Absolutely. However, a million years is very short time period in terms of Astronomical Units and measurement, in a human lifespan, that is forever. It may be up to some future astronomer and perhaps an amateur astronomer to discover this system when and if it goes Supernova. However it is fun to point to the sky to the general location, to take a peak even if you can't see it in your telescope at mag. 17 to 21, but knowing what is going on, what has gone on, what will most likely occur should make this object enjoyable when it does go off. 

I've made two sets of drawings digitally to show the system and what could occur. Realize no details are meant to be exact and the images are made for my blog, are owned and copyrighted by me. So here we go. 

Above is M31N 2008-12a with the Red Giant companion feeding material to the White Dwarf via a solar wind and a very large nebula and accretion disk forming around the White Dwarf. 

Above shows the White Dwarf, having accreted mass, going into eruption and shedding that mass from the accretion disk. Some of that matter will remain increase the mass of the White Dwarf and sending it further on its way to the Chandrasekhar limit of 1.4 solar masses. 

Here the WD is now in full eruption/outburst and the brightness dims the nebula and accretion disk which does survive. 

The eruption begins to fade and the system begins toward "normal." 

Back where we stated. 

Some future day we head to what appears to be another eruption . . . 

In reality, the White Dwarf in M31N 2008-12a has exceed the Chandreasakar limit of 1.4 solar masses and being composed of Oxygen and Carbon, the White Dwarf explodes into a Type Ia SN. 

Here is the second set of images. 

Here is the Red Giant with the White Dwarf accretion material and I have tried to make a wind visible in the image. 

Here is how I have made the eruption appear off the White Dwarf. 

Again, one day the White Dwarf, composed of Oxygen and Carbon exceeds that Chandrasekhar limit of 1.4 solar masses and explodes into a SN Type Ia. 

The SN Type Ia occurs 

For affect, and realize the SN Type Ia would have faded I am sure by this time, but I wanted to show the companion Red Giant being hurled away from the SN Type Ia with material (X-ray) that was stripped off of its surface out in front and a debris shadow. The Red Giant companion would be heading in a direction to the upper right or lower left with the material and debris shadow in this case heading to the lower right. 

I published before what I believed was the location but using Simbad and the tools there I believe I have a much closer location.  Here are those images.

1. The purple cross hairs show in the Andromeda Galaxy, M31 the location of M31N 2008 12a or RX J0045.4+4154 as it is known on charts. 

2. This next image sows a similar view but now we are starting to move in on the objects location. 

3. Here I use an online free tool called Greenshot to insert some lines to show some possible star hoping points to get in close to M31N 2008 12a.

4. Stars and hops 1 & 2 are the two larger stars below and to the left of M31N 2008 12a in the image right above. Now I have zoomed in even more, identifying M31N 2008 12a's location and provided a star hop to the general area. 

4a. Another view of the star hop to get closer in to this object.  Note the double stars (I haven't looked them to confirm if they are a double pair) and then the hops to the two stars, 2 and 3 and then you can go to 4 and 5 and down or work your way up from 3.  Your choice!

5. Below you can see the close in section and location of M31N 2008 12a.  You can see the triangle of brighter stars to the upper left, two stars in a somewhat off angle line below and slightly to the left, and a bright star with two faint stars above it to the right and slightly down.  In 20,000 years or so if math and science are correct, we will observe a Type Ia Supernova if this is a Carbon Oxygen White Dwarf, or a White Dwarf Collapse to a Neutron Star if this is a Neon Oxygen White Dwarf.  So far I think it points to a Carbon Oxygen WD as no signs of Neon have been detected in the Spectra but we may not be picking it up, so all bets are off to the final object that is created from this. 

6. Here I have put in some green lines that angle and could help someone get closer in to where this object is if they are wanting to do that. 

There is a lot more on this object and if you search the net, you'll find it.  I intend to keep following the object and see what science learns from observing it as it is fascinating to me. 

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