Gaia and the 14000 (white) dwarfs astrobites gas 0095 download

Figure 2: Gaia’s Hertzsprung-Russell diagram which groups stars via their brightness and colour into distinct regimes. Whereas most of the stars we observe sit on the so-called main sequence, today we’re interested in the stars which sit to the lower left of the diagram – the white dwarfs. Credit: ESA/Gaia/DPAC

Throughout its five-year mission, Gaia is carefully monitoring the precise position of over a billion stars, whilst also having the photometric capabilities to determine the brightness of a star as a function of colour. This is important as the colour of a star informs astronomers its age and temperature. Devised in the early 20th century, this colour-magnitude information is often summarised in a Hertzsprung-Russell diagram which is shown in Figure 2.

Alongside regions like the main sequence, where most ordinary stars like our Sun sit, and the giant branch, containing (yes, you guessed it) the giant stars; we have a smaller population of stars known as white dwarfs. A white dwarf is a dense stellar core which is left behind when a medium-mass star, between 0.5-8 times the mass of the Sun, ends its life.

The Hertzsprung-Russell diagram summarising the new Gaia data is particularly exciting for white dwarf enthusiasts everywhere because it is direct evidence for something not previously observed – two distinct populations of white dwarfs. Such grouping is known as bifurcation and is clearly evident in Figure 2 (if you open up the picture in a new window and zoom in). However, the authors have gone further than just pointing this out, they’ve recreated it using models. Models, models, models

Two main differences observed in the properties of white dwarfs are determined by whether it has a hydrogen-rich, or helium-rich atmosphere, where hydrogen-rich white dwarfs are redder than their helium-rich counterparts. Thus, the authors also investigated whether this could explain the bifurcation.

Although close inspection of Figure 3, which compares the observed colour-magnitude diagram from Gaia (Figure 3, panel 1) to simulations (Figure 3, panels 2 & 3), reveals slight bifurcation in the simulated population of stars formed via single burst of star formation (Figure 3, panel 3), it isn’t distinctive enough to stay once added to the simulated population of stars formed at a constant rate (Figure 3, panel 2). So, the atmospheric composition does not lead to the bifurcation of white dwarf stars….but what about mass? A mass-ive success

Dubbed the Gaia model, the authors derive the mass distribution of a subset of their 14,000 white dwarfs using the Gaia data to predict what colour the white dwarfs should be if observed by the Sloan Digital Sky Survey (SDSS). A second model, called the disc model, is formed from the synthetic white dwarf created earlier. To look for similarities between the Gaia and disc model in real SDSS data, the authors randomly sample each data set and create another colour-magnitude plot.

Figure 5: The mass distribution of the Gaia white dwarfs – there are clearly two groups of white dwarfs here. The red line indicates the best fit data required to estimate what the SDSS magnitude should be. The bottom half of Figure 5 from paper.

The second peak in this plot, the high-mass peak, is particularly interesting because for white dwarfs to be that heavy, they have to have formed via the merging of two nearby white dwarfs. The number of stars in the merger peak also match up to previous predictions for the number of white dwarfs that form through mergers. However, these more massive white dwarfs are intrinsically dimmer than their lighter counterparts, meaning most surveys will be biased towards observing the lighter, brighter white dwarfs and the numbers of previously known massive white dwarfs will be reduced as a consequence.

So, it’s been exactly a month since the second Gaia catalogue was released and we have already learned a tremendous amount about a small population of stars within our galaxy, by increasing our white dwarf catalogue from 250 objects to almost 14,000. With its unique and extensive data set, it’s hard to predict what astronomers will uncover next, but clearly, the Gaia dataset will be revealing the secrets of the (local) universe for years to come.