Binary Stars Orbiting Each Other INSIDE a Planetary Nebula

Planetary nebulae are a fascinating astronomical phenomena, even if the name is a bit misleading. Rather than being associated with planets, these glowing shells of gas and dust are formed when stars enter the final phases of their lifespan and throw off their outer layers. In many cases, this process and the subsequent structure of the nebula is the result of the star interacting with a nearby companion star.

Recently, while examining the planetary nebula M3-1, an international team of astronomers noted something rather interesting. After observing the nebula’s central star, which is actually a binary system, they noticed that the pair had an incredibly short orbital period – i.e. the stars orbit each other once every 3 hours and 5 minutes. Based on this behavior, the pair are likely to merge and trigger a nova explosion.

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Hubble’s 1923 Nova in Andromeda Erupts Again!

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On December 11, 1923, Edwin Hubble discovered a nova in the Andromeda galaxy. Novae occurring in our Milky Way’s sister galaxy have proven to be not that uncommon, as there have been over 800 novae detected in M31 in the last 100 years. Hubble’s 1923 discovery became known as M31N 1923-12c, the third nova discovered in December of 1923.

Fast forward to January 21, 2012, and another nova has been discovered in M31, already the second novae seen in January 2012. K. Nishiyama and F. Kabashima reported the discovery and it has been given the designation, PNV J00423804+4108417. A day later, a spectrum was taken with the 9.2m Hobby-Eberly Telescope using the Marcario Low-Resolution Spectrograph, confirming the new nova in M31, and that it is a member of the He/N spectroscopic class.

What’s even more interesting, however, is that the new nova likely comes from the same progenitor as Hubble’s 1923 nova!

Artist's rendition of the recurrent nova RS Oph Credit: David Hardy/PPARC

Classical novae are a subclass of cataclysmic variable stars. They are semi-detached binary systems where an evolved, late-type star fills its Roche lobe and transfers mass to its white dwarf companion. If the mass accretion rate onto the white dwarf is sufficiently low, it allows this gas to pile up and become degenerate. Eventually, after thousands to tens of thousands of years, a thermonuclear runaway ensues in this highly pressurized layer of gas, leading to a nova eruption. These eruptions can reach an absolute magnitude as bright as about MV -10, making them among the most luminous explosions in the Universe. Their high luminosities and rates, about 50 per year in a galaxy like M31, make novae very useful to astronomers exploring the properties of close binaries in extragalactic stellar populations.

Comparing its position with the approximately 900 novae in W. Pietsch’s M31 nova catalog revealed that PNV J00423804+4108417 was located about six arc seconds from the cataloged position of M31N 1923-12c, the nova discovered by Edwin Hubble on December 11, 1923. Given that the positions of M31 novae from early photographic surveys were typically reported to a precision of only ten arc seconds, and that He/N spectra are often associated with recurrent novae, astronomers considered the possibility that M31N 1923-12c and PNV J00423804+4108417 represented two outbursts arising from the same nova progenitor. To explore this possibility further, F. Schweizer (Carnegie Observatories) located Hubble’s original plate in the Carnegie Observatories archives and performed an eyeball comparison of the position of Hubble’s nova with that of PNV J00423804+4108417, finding them to match within ~1.5″. You can see the images for yourself here.

Edwin Hubble

After digitally scanning the Hubble plate and comparing the position of the nova relative to those of three nearby USNO reference stars, analysis revealed that M31N 1923-12c was located
at R.A. = 00 42 38.06; Decl. = 41 08 41.0 (J2000). Hubble’s M31N 1923-12c and this year’s PNV J00423804+4108417 are the same object!

88 years and a handful of days later, PNV J00423804+4108417 represents the second recorded outburst of the recurrent nova M31N 1923-12c. Like the telescope named for him, Hubble’s legacy to astronomy and astrophysics continues to grow to this very day. Way to go, Edwin.

This blog post adapted fromĀ Astronomer’s Telegram #3914
M31N 1923-12c is a recurrent nova in M31
Authors: A. W. Shafter (SDSU), M. J. Darnley, M. F. Bode (Liverpool JMU, UK), R. Ciardullo (PSU), F. Schweizer (Carnegie Observatories)

Will V445 Puppis Become a Ia Supernova?

As the “V” in the designation of V445 Puppis indicates, this star was a variable star located in the constellation of Puppis. It was a fairly ordinary periodic variable, although with a rather complex light curve, but still showing a distinct periodicity of about fifteen and a half hours. It wasn’t especially bright, yet something seemed to tug at my memory regarding the star’s name as I scanned through articles to write on. Just over a year ago, Nancy wrote a post on V445 Puppis stating it’s a supernova just waiting to happen. A new article challenges this claim.

In December of 2000, V445 Puppis underwent an unusual nova. It was first noticed on December 30th, but archival records showed the eruption began in early November of that year and reached a peak brightness on November 29th. The system was known to be a binary star system with a shared envelope in which the primary star was a white dwarf and thus, a nova was the most readily available explanation.

However, this wasn’t a normal nova. Spectroscopic observations early the next year showed the ejecta lacked the helium emission seen in classical novae in which hydrogen piles up on a white dwarf surface until it undergoes fusion into helium. Instead, astronomers saw lines of iron, calcium, carbon, sodium, and oxygen expanding at nearly 1,000 km/sec. This fit better with a proposed type of explosion where, instead of hydrogen collecting on the dwarf’s surface, it was helium and the eruption seen was a helium flash in which it was helium that underwent fusion. Slowly the star faded, and debris from the eruption cooled to form dust. Today, the star itself is completely obscured in the visible portion of the spectrum.

The 2009 paper by Woudt, Steeghs, and Karowska that Nancy cited, suggested accretion might continue until the white dwarf passed the Chandrasekhar limit and exploded as a type Ia supernova. However, the authors of the new paper, led by V. P. Goranskij at Moscow University, say that this 2000 detonation has effectively ruled out that possibility because an explosion of that magnitude would likely destroy the envelope of the donor star. Their evidence for this is the very same structure Woudt noted in his paper (shown above).

While the structure looks to be bipolar in nature, other observations have suggested that there is an additional component along the line of sight and that the structure is more of a doughnut shape. In this case, the total amount of material lost is higher than originally anticipated and must have come from from the envelope of the companion star. Additionally, observations in wavelengths able to pierce the dust have been unable to resolve a strong stellar source which suggests that the donor star’s envelope has been largely blown away as well. Additionally, this large and rapid loss of mass from the system may have broken the gravitational bond between the two stars and allowed the giant star to be ejected from the system, which would also preclude the possibility of a supernova in the future.

The conclusion is that V445 Puppis is not a candidate for a supernova of any type in the future. It’s own premature fireworks have likely destroyed whatever chance it may have had for an even grander show in the future.