Andrew Allan
To better understand massive stars in low metallicity environments, we used the VLT’s ESPRESSO and X-Shooter instruments to observe the dwarf galaxy PHL293B. Excitingly, we find the sudden disappearance of the dominant stellar signature from our 2019 spectra, in particular the broad H lines with P~Cygni profiles that have been associated with a massive luminous blue variable (LBV) star. We compute radiative transfer models using CMFGEN that fit the observed spectrum of the LBV and are consistent with ground-based and archival Hubble Space Telescope photometry. Our models show that during before disappearing, the star had a luminosity of 2.5-3.5x1e6 Lsun, a mass-loss rate of 0.005 and 0.02 Msun/yr and wind velocity of 1000 km/s, indicating an extremely eruptive state. We consider two main hypotheses for the absence of the broad emission components from the spectra obtained since 2011. One possibility is that we are seeing the end of an LBV eruption of a surviving star, with a mild drop in luminosity, a shift to hotter effective temperatures, and some dust obscuration. Alternatively, the LBV could have collapsed to a massive black hole without the production of a bright supernova.
We utilise CMFGEN (Hillier & miller 1998), a radiative transfer code which models the transport of light through a star creating synthetic spectra for the star before its disappearance. We compare these synthetic spectra to a spectrum of PHL293B taken pre-disappearance in 2002. By considering the best-fitting models, we predict the following properties for the star: A mass loss rate between 0.005-0.02 solar masses each year, a luminosity between 2.5 and 3.5 million times that of the Sun, and an approximate wind velocity of 1000 km/s. These properties indicate that the LBV was undergoing an eruption before it disappeared.
We offer three potential explanations for the stars disappearance, the first of which being the star has survived. In this case, a drop in luminosity due to the end of the eruption between 2001-2011 combined with obscuration by dust could lead to a surviving star being no longer visible in optical wavelengths. The dust would form from material ejected during the strong eruption. A comparison of the X-Shooter 2009 and 2019 observations allows us to rule out hot dust (hotter than 1500 K) as these probe near-infrared wavelengths. However, mid-infrared observations are needed to rule out cooler dust.
An alternative explanation, is the collapse to a black-hole without a bright transient. In this case, the eruption would signal the end of the star's life. Using the evolutionary models of Georgy et al. 2013 and Groh et al. 2019b, we predict an initial stellar mass between 85–120 times the solar value. They also suggest that the black-hole could acquire between 40-90 solar masses through fallback. This hypothesis is particularly exciting as collapse to a black-hole without a bright supernova has been observed only once before for a 25 solar-mass star in N6946-BH1 (Adams et al. 2017).
The final hypothesis is an undetected type-IIn supernova as first proposed by Burke et al. 2020. This may have occurred between 1995-1998 as no photometry of PHL293b was taken during this period. If true, the inferred stars signature present in earlier observations would instead be the result of supernova ejecta crashing into dense circumstellar material. We consider the prior two hypotheses more likely as the supernova explanation requires that a potentially prolonged interaction went undetected. Furthermore, non-detections set upper limits for both the X-ray and Radio luminosities below that expected for Type II supernova.
We require more observations before we can rule out any of the proposed hypotheses. Fortunately, we will get the chance to re-observe PHL293b using the Hubble Space Telescope (HST) in the next year. Combining past HST observations of PHL293b with synthetic photometry of our models, we predict significant variation in apparent magnitudes in the HST filters due to the stars disappearance. Therefore, our future HST observation should hopefully be capable of confirming and explaining the stars disappearance.
If you wish to read more about our work, please consider our research paper: https://arxiv.org/pdf/2003.02242 and the ESO press release: https://www.eso.org/public/news/eso2010/ .