C/Oratio as function of [Fe/H] with the stars containing good carbon and oxygen values from the GALAH survey. Black line originates from the data shown in Fig. 1 and marks the mean C/O ratio from all stars in our sample. The number of the stars shown here is less than was used to derive the [C/H] and [O/H] value in Fig. 1 because stars with good carbon value might not have a good oxygen value at the same time. As [Fe/H] increases, so does the C/O ratio. This implies that more oxygen can be bound with carbon to form CO and CO₂ for increasing [Fe/H].