Title: VVV-WIT-08 : the giant star that blinked
Authors: Leigh C. Smith, Sergey E. Koposov, Philip W. Lucas et al.
First Author’s Institution: Indian Institute of Astrophysics
Status: Accepted in MNRAS [open access]
It was a normal December day back in 2011. Stars were shining, exoplanets were transiting and astronomers were astronomying(?). However, this was a very special month for VVV-WIT-08 – a giant star located in the bulge of the Milky Way. It was during this month that this star decided to blink! Over the next four months, the brightness of this star dropped by a whopping 97%. It then rose back to its original value in another four months and hasn’t changed ever since. The authors of today’s paper analyse this mysterious stellar blink and try to identify its cause.
Catching the blink
The authors found this mysterious star while searching for variable stars using data from the Vista Variables in the Via Lactea (VVV) survey. The VVV survey operates from the Paranal Observatory in Chile, and has been monitoring the near-infrared brightness of more than a billion stars in the Milky Way since 2010. The authors searched the VVV database to identify sources whose brightness in the Ks photometric band changed by more than 4 magnitudes, and the blinking weirdo VVV-WIT-08 popped right out! Figure 1 shows the Ks band lightcurve of this star, which clearly shows the smooth decline and rise in brightness. The authors augment their Ks band data with additional optical wavelength (V and I-band) observations from OGLE – another survey that monitors stars in our Galaxy. The V and I-band observations are also shown in Figure 1, and show a similar decline and rise in brightness. The authors note that the amplitude of the decline is the same in all three photometric bands.
What is the star?
The authors collected multi-band brightnesses from multiple telescopes spanning the optical to infrared wavelengths. They use these measurements to construct a spectral energy distribution (SED, brightness vs wavelength) of this star. Detailed modeling of the SED suggests that the star is a ~1 MSun star with a temperature of ~3600 K and an age of 8.3 billion years (for comparison, the Sun has a temperature of ~6000 K and an age of 4.6 billion years). Although the mass of the star is very similar to the Sun, its luminosity is a thousand times brighter. Overall, these properties are consistent with an evolved, cool giant star in the bulge of our Galaxy.
But why did it blink?
Short answer – we do not know yet! The authors analyse several possible causes that could give rise to such variability. First, weird variable stars known as R-Coronae-Borealis (RCB) stars are known to show random dips in their brightness. However, the amplitude of the dips of RCB stars change dramatically with wavelength, unlike for VVV-WIT-08. Second, young stellar objects can exhibit high amplitude variability. However, the authors obtained a spectrum of VVV-WIT-08 and it does not show any YSO-like features. This leaves them with a third likely option – occultation by an object that passes in front of the star. The authors find that the light curve of VVV-WIT-08 is well explained by a model in which an opaque, elliptical object that is bound to the star passes in front of it. As we do not see any radiation from this object in the lightcurve, it has to be really faint. These properties suggest that the occulter could be a circumstellar disk around an object that is bound to the giant star.
The authors thus postulate that VVV-WIT-08 could be part of a population of binary stars where the primary star is a cool giant and the companion star hosts an opaque circumstellar disk. This companion star could be a main-sequence star, a white dwarf, or a neutron-star or a black hole. However, these explanations also have their shortcomings. For instance, disks around main-sequence stars are not opaque enough. Disks around white dwarfs are opaque, but they are not large enough to explain such drastic declines in brightness. Black hole or neutron star disks are pretty large, but they also emit radiation in X-rays that we do not quite see. It is also possible that the disk around the companion star is formed from material that escapes the surface of the giant star. It is all up for grabs!
The key to identifying the precise mechanism responsible for the “blinking”-variability is to identify more blinkers. Existing IR surveys such as Palomar Gattini-IR and upcoming surveys such as the Vera Rubin Observatory could be instrumental in solving this mystery.
Astrobite edited by Huei Sears
Featured image credit : Philip Lucas