Alert Notice 773: EX Lup outburst coverage requested

March 19, 2022

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Dr. Christian Knigge (University of Southampton) requests AAVSO observers' assistance in a new long-term campaign on the Young Stellar Object EX Lup, which is going into outburst. Photometry and spectroscopy are requested.

Dr. Knigge writes: "The young stellar object EX Lup is apparently just going into outburst (see Zhou et al. 2022, ATel #15271).

"These outbursts *might* be due to the same disk instability that is responsible for dwarf nova eruptions, but nobody knows for sure. One way to test this would be by observing the colour evolution all the way around the outburst. This has apparently never been done properly -- usually all of these sorts of outbursts are only covered once near peak.

"The outbursts last a long time -- quite possibly a year or so (so of course there are seasonal gaps). But the nice thing is that if we could just get multi-colour snapshots on a ~daily basis, starting right now, that would be immensely valuable. It's a very bright object -- around 8-12 mags, depending on state, so it's 'easy'.

"If observers who can look at this could get multi-colour photometry between now and when the outburst is over, this might allow a test of the theory for these outbursts. B and V would be fine, but if people also have access to more extreme bands -- e.g. U / R / I -- that would also be valuable, since it extends the baseline. But these should be *extra* -- getting a consistent long-term set in two colours is preferable to a mish-mash."

"A single snapshot per night per observer is definitely enough to be extremely valuable. The variations should be slow enough for that. If it's not too much trouble, something like 2-3 observations over the course of a night would be great, of course.

"It might also be worth noting that getting long (multi-hour) light curves at high time-resolution every now and then would be valuable as well, for different reasons -- it would allow us to study how the flickering in these systems evolves over an outburst. Based on comparisons to compact binaries like CVs, one might expect the flickering amplitude to scale with brightness."

Observers are requested to begin coverage immediately and continue until further notice - at least until the outburst is well over. Visual observations are welcome to supplement the light curve. Photometry reported to the AAVSO International Database (AID) by F.-J. Hambsch (Mol, Belgium, his remote observing site Atacama, Chile) shows EX Lup on 2022 March 19.3989 UT at 11.872 V +/- 0.006; 19.3999, 12.399 B +/- 0.007; 19.4006, 11.223 R +/- 0.005; 19.4023, 11.916 U +/- 0.018; 19.4033, 10.415 I +/- 0.007.

"Spectroscopy would be super-interesting and is requested: one spectrum per week, covering the rise, plateau, and decline phases, with perhaps a little more frequent coverage during the rise and a little less later on (when evolution slows down).

Earlier major outbursts of EX Lup recorded in the AID occurred in August 1955 (lasting ~237 days) and in January 2008 (lasting at least 204 days - EX Lup was discovered rising to outburst upon emerging from its seasonal gap). Several smaller outbursts are also seen (use the AAVSO Light Curve Generator to see the long term light curve).

Coordinates (2000.0): R.A. 16 03 05.48  Dec. -40 18 25.1 (from VSX page for EX Lup)
Range: 8.5 - 14.3 V
Variability Type: EXOR

Charts with comparison stars for EX Lup may be created using the AAVSO Variable Star Plotter (VSP).

Observations should be reported to the AAVSO International Database or AAVSO Spectroscopy Database using the name EX LUP.

This AAVSO Alert Notice was compiled by Elizabeth O. Waagen using material provided by Dr. Knigge.


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