We often state that HOYS has as one of its aims to investigate how the accretion rates change on the young stars. But so far we have not really done any detailed investigation on this. However, we have started this now with a small student project.
One of the potential tracers for the mass accretion rates is the emission strength in the Hydrogen Alpha line (Balmer Alpha) in the red part of the spectrum. Our data collection includes images taken with narrow band Halpha filters, and thus we can start with some simple analysis. For starters we only selected young stars in two target regions from one observatory that has done regular observations in Halpha as well as the R and I-band. We then simply determine the median brightness in each of these filters and show the magnitude differences in R-I against R-Ha in the above plot.
The R-I colour along the x-axis is a measure of the spectral type, and thus to some extend the mass of the star. Reddening due to dust also plays a role and needs to be carefully considered at some point. Along the y-axis, the R-Ha colour is a measure of the strength of the Hydrogen Alpha emission line. Positive values mean the line is strong in emission, negative values mean the line is strong in absorption. The latter should not really happen. Zero R-Ha colour basically means these are ‘normal’ stellar photospheres without strong Balmer lines.
We can see that a large number of sources have no emission lines. There is a slight trend of R-Ha with R-I colour. Basically R-Ha increases by about 0.05mag between R-I=0.2 and 0.6mag. This is normal and caused by the change in colour of the stars. We can also see a number of strong Halpha emitters. All of which seem to be very red (R-I>0.6mag). This is probably a sign of them having higher reddening due to circumstellar disk material. Objects with a lot of that material are also more likely to be strong accretors.
Some of the negative R-Ha values are most likely caused by general variability of the source. We now need to see how we can improve plot to account for this. Then we will investigate how the emission line strength varies over time (it does!) and we will include data from other regions and telescopes for a better statistical picture. Watch this space 🙂