New White Dwarf Mass Limit

[T. McGrath]

For the last 88 years we have used Subrahmanyan Chandrasekhar's calculations to determine the maximum mass of a white dwarf.  As a result of that mass limit a peak brightness was derived and the Standard Candle was born.  However, those calculations were made based upon certain assumptions, namely that the white dwarf was not rotating and had no magnetic field.  More recent discoveries (specifically SN 2003fg, SN 2006gz, SN 2007if, and SN2009d) have demonstrated that those assumptions made in 1930 may not hold true in some cases.  The white dwarf mass prior to SN 2007if's deflagration, for example,  was estimated to be 2.4 M_☉.

In 2013 a paper that was published by Upasana Das and Banibrata Mukhopadhyay, from the Indian Institute of Science, proposes a new white dwarf limit with the assumption that the white dwarf is highly magnetized.  Arguing that the outward pressure of the magnetic field of the white dwarf would partially counteract the gravitational pull inward, thus allowing the white dwarf to accumulate additional mass, beyond Chandrasekhar's Limit, before deflagration.  As a result of the high nickel content in the spectra of these superluminous Type Ia SN, they are suggesting that the progenitors were generating a very strong magnetic field and that allowed them to accumulate additional mass.  Furthermore, they also suggest that magnetars, which are currently presumed to be neutron stars with strong magnetic fields, be re-examined as potentially highly magnetic white dwarfs.  They calculate the new limit for highly magnetic white dwarfs to be 2.58M_☉.

Banibrata Mukhopadhyay more recently published a more comprehensive paper, taking into account various rotational speeds as well as varying levels of magnetic fields, and now estimates the white dwarf maximum limit to be between 2.3 and 2.8 M_☉.

This would seem to imply that our Standard Candle for measuring distance is not so "standard" after all.  In many cases we can use other data about the SN to help determine its absolute magnitude, such as the rate and composition of its ejecta, but absent that data we can no longer assume that Type Ia SN will always result in an absolute magnitude of M_B -19.46 at peak brightness simply based upon its light curve.

Sources:
New Mass Limit of White Dwarfs - International Journal of Modern Physics D, Volume 22, Issue 12, October 2013 (free preprint)
Nearby Supernova Factory Observation of SN 2007if: First Total Mass Measurement of a Super-Chandrasekhar-Mass Progenitor - The Astrophyisical Journal, Volume 713, Number 2, March 2010 (free issue)
Significantly Super-Chandrasekhar Limiting Mass White Dwarfs as Progenitors for Peculiar Over-Luminous Type Ia Supernovae - arXiv :  1509.09008, September 2015
The Evolution and Fate of Super-Chandrasekhar Mass White Dwarf Merger Remnants - Monthly Notices of the Royal Astronomical Society, Volume 463, Issue 4, December 2016 (free preprint)
The Type Ia Supernova SNLS-03D3bb from a Super-Chandrasekhar-Mass White Dwarf Star - Lawrence Berkeley National Laboratory, April 2008 (open access)

 

Edited February 2, 2018 by T. McGrath

[mathematic]

Standard candles start with Cepheid variable stars.  For long distances Type IA supernovae.

[T. McGrath]

On 2/2/2018 at 1:00 PM, mathematic said:

Standard candles start with Cepheid variable stars.  For long distances Type IA supernovae.

It can only be a standard candle when the absolute magnitude is known and does not change with each event.  During the 1990s, and earlier, we erroneously thought all Type Ia SN had an absolute magnitude of M_B -19.46 because of Chandrasekhar's work in 1930.  We know today that Type Ia SN have an absolute magnitude range  of  -14.2 < M_B < -20.    Which means that they are not the standard candle everyone assumed they were 20 years ago.

Edwin Hubble used Cepheid variable stars as his standard candle in 1927 when he calculated the age of the universe was only two billion years old.  Cepheid variables only work as a standard candle if you have the right classical Cepheid variable.  If you happen upon a Type II Cepheid variable, or an anomalous Cepheid variable, or a double-mode Cepheid variable, or a RR Lyrae variable then they are not the standard candle you desire them to be.

Expanding the Chandrasekhar Limit has other implications as well.  A 2.8 M_⊙ white dwarf would be more massive than the most massive neutron star yet observed.  What was once thought to only be neutron stars with masses greater than 1.44 M_⊙, may actually be rapidly rotating and/or highly magnetic white dwarfs instead.

The only accurate means of measuring cosmological distances is parallax.  There is no standard candle in astronomy.

[pavelcherepan]

Interesting. If that were true, it would have large implications in general cosmology as lambda-CDM model was based on observation of Ia supernovae. This is a big can of worms to be dealt with.

[T. McGrath]

11 hours ago, pavelcherepan said:

Interesting. If that were true, it would have large implications in general cosmology as lambda-CDM model was based on observation of Ia supernovae. This is a big can of worms to be dealt with.

As I stated in the OP, with sufficient data we can distinguish between sub- and super-Chandrasekhar Type Ia SN.  For example, the rate at which the ejecta travels for all sub-Chandrasekhar Type Ia SN (a.k.a. Type Iax SN since 2013) is less than 10,000 km/s, and the super-Chandrasekhar Type Ia SN all have high concentrations of nickel in their spectra.  In the case of SN 2007if, more than a solar mass of nickel was observed in its spectra, which is what prompted their high estimate of the progenitor's mass.  There are additional indicators that allow us to separate the sub- and super-Chandrasekhar Type Ia SN from each other, but it does require that additional data.

We can no longer just assume it is a Type Ia SN merely because of its light curve.  More data is required before we can have any certainty about its absolute magnitude.

Quote

We estimate that in a given volume there are SNe Iax for every 100 SNe Ia, and for every 1 M _☉ of iron generated by SNe Ia at z = 0, SNe Iax generate ~0.036 M _☉.

If between 18% and 48% of the Type Ia SN prior to 2013 were misclassified and should really be Type Iax SN, or sub-Chandrasekhar Type Ia SN with an absolute magnitude range between -14.2 < M_B < -18.9, then we do need to look at the SN data that was collected to determine whether the lambda-CDM model needs correcting.  A great many assumptions about the absolute magnitude has been made in the past regarding SN (particularly at z > 1) based solely upon its light curve.  We need the data that demonstrates a Type Ia SN is not a sub- or super-Chandrasekhar Type Ia SN, otherwise it calls into question the calculated distances.

Source:
Type Iax Supernovae: A New Class of Stellar Explosion - The Astrophysical Journal, Volume 767, Number 1, March 2013 (free issue)

 

 

Edited February 7, 2018 by T. McGrath