Cosmic Expansion: A Close Look at a ‘Standard Candle’

Astronomy relies on so-called ‘standard candles’ to make crucial measurements about distance. Cepheid variables, for example, perhaps the most famous stars in this category, were examined by Henrietta Swan Leavitt in 1908 as part of her study of variable stars in the Magellanic clouds, revealing the relationship between this type of star’s period and luminosity. Edwin Hubble would use distance calculations based on this relationship to estimate how far what was then called the ‘Andromeda Nebula’ was from our galaxy, revealing the true nature of the ‘nebula.’

In recent times, astronomers have used type Ia supernovae in much the same way, for comparing a source’s intrinsic brightness with what is observed in the sky likewise determines distance. The most commonly described type Ia supernovae model occurs in binary systems where one of the stars is a white dwarf, and the assumption among astronomers has been that this category of supernova produces a consistent peak luminosity that can be used to measure interstellar, and intergalactic, distances.

It was through the study of type Ia supernovae that the idea of dark energy arose to explain the apparent acceleration of the universe’s expansion, but we can also point to our methods for measuring the Hubble constant, which helps us gauge the current expansion rate of the cosmos.

Given the importance of standard candles to astronomy, we have to get them right. Now we have new work out of the Max Planck Institute for Astronomy in Heidelberg. A team led by Maria Bergemann draws our assumptions about these supernovae into question, and that could cause a reassessment of the rate of cosmic expansion. At issue: Are all type 1a supernovae the same?

Bergemann’s work on stellar atmospheres has, since 2005, focused on new models to examine the spectral lines observed there, the crucial measurements that lead to data on a star’s temperature, surface pressure and chemical composition. Computer simulations of convection within a star and the interactions of plasma with the star’s radiation have been producing and reinforcing so-called Non-LTE models that assume no local thermal equilibrium, leading to new ways to explore chemical abundances that alter our previous findings on some elements.

The team at MPIA has zeroed in on the element manganese using observational data in the near-ultraviolet, and extending the analysis beyond single stars to work with the combined light of numerous stars in a stellar cluster, which allows the examination of other galaxies. It takes a supernova explosion to produce manganese, and different types of supernova produce iron and manganese in different ratios. Thus a massive star going supernova, a ‘core collapse supernova,’ produces manganese and iron differently than a type 1a supernova.

Image: By examining the abundance of the element manganese, a group of astronomers has revised our best estimates for the processes behind supernovae of type Ia. Credit: R. Hurt/Caltech-JPL, Composition: MPIA graphics department.

Working with a core of 42 stars within the Milky Way, the team has essentially been reconstructing the evolution of iron and manganese as produced through type Ia supernova explosions. The researchers used iron abundance as an indicator of each star’s age relative to the others; these findings allow them to track the history of manganese in the Milky Way. What they are uncovering is that the ratio of manganese to iron has been constant over the age of our galaxy. The same constant ratio between manganese and iron is found in other galaxies of the Local Group, emerging as what appears to be a universal chemical constant.

This is a result that differs from earlier findings. Previous manganese measurements used the older LTE model, one assuming that stars are perfect spheres, with pressure and gravitational force in equilibrium. Such work helped reinforce the idea that type Ia supernovae most often occurred with a white dwarf drawing material from a giant companion. The data in Bergemann’s work, using Non-LTE models (No Local Thermal Equilibrium), are drawn from ESO’s Very Large Telescope and the Keck Observatory. A different conclusion emerges about how type Ia occurs.

The assumption has been that these supernovae happen when a white dwarf orbiting a giant star pulls hydrogen onto its own surface and becomes unstable, having hit the limiting mass discovered by Subrahmanian Chandrasekhar in 1930 (the “Chandrasekhar limit”). This limiting mass means that the total mass of the exploding star is the same from one Type Ia supernova to another, which governs the brightness of the supernova and produces our ‘standard candle.’ The 2011 Nobel Prize in Physics for Saul Perlmutter, Brian Schmidt, and Adam Riess comes out of using Type Ia as distance markers, with readings showing that the expansion of the universe is accelerating, out of which we get ‘dark energy.’

But the work of Bergemann and team shows that other ways to produce a type Ia supernova may better fit the manganese/iron ratio results. These mechanisms may appear the same as the white dwarf/red giant scenario, but because they operate differently, their brightness varies. Two white dwarfs may orbit each other, producing a merger with resulting explosion, or double detonations can occur in some cases as matter accretes onto the white dwarf, with a second explosion in the carbon-oxygen core. In both cases, we are exploring a different scenario than the standard type Ia.

The problem: These alternative supernovae scenarios do not necessarily follow the standard candle model. Double detonation explosions do not require a star to reach the Chandrasekhar mass limit. Explosions below this limit will not be as bright as the standard type Ia scenario, meaning the well-defined intrinsic brightness we are looking for in these events is not a reliable measure. And it appears that the constant ratio of manganese to iron that the researchers have found implies that non-standard type Ia supernovae are not the exception but the rule. As many as three out of four type 1a supernovae may be of this sort.

This is the third paper in a series that is designed to provide observational constraints on the origin of elements and their evolution within the galaxy. The paper notes that the evolution of manganese relative to iron is “a powerful probe of the epoch when SNe Ia started contributing to the chemical enrichment and, therefore, of star formation of the galactic populations.” The models of non-local thermodynamic equilibrium produce a fundamentally different result from earlier modeling and raise questions about the reliability of at least some type Ia measurements to gauge distance.

Given existing discrepancies between the Hubble constant as measured by type Ia supernovae and other methods, Bergemann and team have nudged the cosmological consensus in a sensitive place, showing the need to re-examine our standard candles, not all of which may be standard. Upcoming observational data from the gravitational wave detector LISA (due for a launch in the 2030s) may offer a check on the prevalence of white dwarf binaries that could confirm or refute this work. Even sooner, we should have the next data release (DR3) of ESA’s Gaia mission as a valuable reference.

The paper is Eitner et al., “Observational constraints on the origin of the elements III. Evidence for the dominant role of sub-Chandrasekhar SN Ia in the chemical evolution of Mn and Fe in the Galaxy,” in press at Astronomy & Astrophysics (preprint).