This week we actually have a look at three different light curves. They are of the stars V505Ori, V510Ori, and TX Ori. These are all situated in the target area Sigma Ori (target 005). They are young variable stars, which are part of a HST program that will take ultra-violet spectra of them and other young stars. We aim to provide ground based auxiliary photometry for the objects to put the UV spectra into context.

As you can see in the light curves, all three stars are extremely variable in all filters, by up to two magnitudes on very short time scales. It is hence vital to know the exact brightness and colour at the time of the HST observations to correctly interpret the data. There will of course be other follow up with e.g. the TESS telescope. But TESS does not deliver colour information. Colours can only be obtained with this ground based data, in particular if there is no one to one correlation between brightness and colour – which can be the case for young stars.

Below is again the detailed description for our call to follow those targets over the next winter in aid of this project – this was send out earlier to members via our newsletter.


Priority imaging of SigmaOri

HOYS is taking part in providing ground based follow up for the ULLYSES legacy program of the HST. This program will use 1000 HST orbits to observe spectra of young stars in the ultra-violet – an ability Astronomy will loose when the HST is retired.

All target stars for the program can be found on the project website. Three of these targets are already in one of our fields – the SigmaOri cluster (target 005). These are the objects SO518 (V505 Ori), SO583 (TX Ori), and SO1153 (V510 Ori). We would like to encourage everyone to observe those targets each night in all filters (B,V,R,I,Ha and U if you have) to provide optical broad band photometry for those objects. These are needed to put the UV spectra into context as all those sources are highly variable.

Please judge from the coordinates for the objects (listed below) and your field of view if you need more than one frame to catch all of them. They easily fit into a one degree field of view around our nominal SigmaOri coordinates, but if you have a smaller field you might want to take several images. You should, however, submit all these images for target 005.

On nights when the HST is not taking spectra of the sources, one observation (or two) per night is sufficient (please stack if you take more than one frame per filter). The day before, the day after and the day(s) when the spectra are taken by the HST, please take as many images as you can and submit them all individually (if they are deep enough for the objects to be visible), so we are able to accurately measure the brightness and variability of the sources when the spectra are taken. The predicted HST observing times for the objects should always be listed in this document. Currently, the three targets are predicted to be observed early December for a period of 2-4days. But please check if those dates change.

Target RA DEC Spectral Type M* log(dm/dt) AV V
SO518 5h38m27.26s -2d45m09.7s K6.0 0.75 -8.54 0.0 14.16
SO583 5h38m33.69s -2d44m14.1s K4.5 1.09 -8.13 0.0 12.06
SO1153 5h39m39.83s -2d31m21.9s K5.5 0.88 -8.38 0.2 14.30

 Table1: Target Coordinates and apparent V magnitudes.