This star, not always develops humps in the light curve (or are less obvious)
I don't know if this behavior it's normal in other humpers.
Alfredo Glez-Herrera (GZN)
Its not uncommon for LPV's that display humps to not show them every cycle. Previous studies have shown a wide range in frequency of humps for different stars ranging from every cycle to a hump in every 7th cycle.
S UMi does not appear in Frank Schorr's list of LPV's with humps. Furthermore its mean light curve developed by Karlsson does not show a conspicuous hump feature although there does appear to be a subtle gradient change in the ascending portion of the light curve.
Using VSTAR I get the light curve and the phase plot attached.
On the other hand, since may, the bright varies in the range of 8.5v - 8.8v.
I think that the humps are more evident than in the previous light curves. Could be S UMi a good candidate to the "Hump Club" ?
Alfredo Glez-Herrera (GZN)
Good work Alfredo!
There definitely appears to be an interesting feature in the form of a discontinuity in the ascending branch of the light curve particularly around mid 2015. It's not unusual that these humps are not shown in every cycle. Also it's GCVS period of 330 days is relatively long which seems to be a pre-requisite for showing this sort of behaviour more clearly.
So I agree that S UMi may be a good candidate for the "Hump Club"! Continued close monitoring is required to see if we can detect further events.
I used VStar to look at S Umi's light curve back to 1903 in the International Database. The first "hump" event I can clearly see in the light curve (LC) is around JD 2419635 (August 1912) with the following cycle being even more pronounced. I see "humps" on S Umi's LC all the way until upto the last full cycle. However like many of the Miras that show this behavior, "the hump" or "stall" doesn't occur on every cycle. It might make an interesting study to see if there is a pattern as to what cycles the "humps" occur!
I've add S Umi to my "LPV Humps" list on the LPV Section. I like the name "Hump Club"!
Thanks Alfredo for pointing this LPV out. I hope you keep observing it and keep an eye on it!
I'm very interested to know what is the mechanism that produces the humps.
It is something extrinsic, such as a close companion or a cloud of dust as the type RCB stars? or perhaps it is something internal in the star as huge sunspot groups (which would explain why the humps are not always repeated in every cycle).
Do you know if someone has made spectrographic analysis before, during, and after the phase hump?
Definitely a kind of stars that deserve special attention.
There seems to be various opinions as to the mechanism that produces the "humps" or standstills on many Mira stars. The opinions range from multiple pulsation modes to atmospheric effects such as shocks. I have not seen mentioned in the literature I've read things like a cloud of dust or large sunspot groups. Mostly I think these humps tend to be discounted and that possible all Miras go through a phase where they show humps on at least some of their cycles. (I would think that this in itself might be interesting!) One study/paper said that 38% of Miras show humps.
I don't remember anywhere in the papers I've read where spectrum was taken before, during and after a hump event. This is something I would like to do at some point but doing this has some high hurdles to over come!
Another thing that I find somewhat interesting is that the hump event shows up in filter B, V, Rc and Ic and apparently with no time offsets. I don't have enough knowledge to know if this important or not. To see this go to the LC generator and look for the last 500 days in B, V, Rc and Ic for S Ori.
I just find these humps events as something interesting to follow and observe and maybe these observations might help explain the behavior someday!
Here are some more Mira stars with humps or shoulders, according to recent studies by Swedish amateur astronomers:
GS Cyg, V750 Cyg, EH Gem, V393 Her, V358 Lac, V389 Lac, DT Ori, IU Peg.
Thank you very much for this new list of Miras that show "hump" behavior! I'll take a look at them quickly. Would it be ok to add these stars to my list of Miras that show "hump" behavior here on the LPV Forum?
That is OK and would be much appreciated. The stars mentioned belong to a set of 50 fairly bright but neglected Mira stars that have now been studied by Swedish amateurs for a couple of years. A paper describing the main results will be published in JAAVSO in the near future.
On the whole, observing neglected bright Mira stars is a branch of astronomy where amateurs can contribute a lot. It is a bit surprising that many Mira stars with an average maximum visual magnitude around 13 or 14 have remained more or less unstudied, even if they were discovered many decades ago.
Changes in the magnitude range of S Ursae Minoris. Bright minimum the last 8 cycles? Taking a look at the AAVSO light curve, the changes are evident (one magnitude brighter)