In this image of V537 CrA seems to be in outburst. The image set was taken on June, 22nd, 2025 but processed only three days ago with Siril, which uses the green channel for photometry. A light curve of some reported magnitudes to AAVSO during the last few month show that V537 CrA magnitude hovers around 10.5 magnitude in the V channel. Would you check and, if possible, take some additional images? I will be at the field on next saturday and I will take new images.
Hello Celso,
V0537 CrA is a red variable (it was previously classified as MISC and most of them are actually semirregular or irregular red giants) and, as such, it is not an outbursting object.
You just shared a Google drive link and the permissions are not set to public so I can’t see your image, but you mentioned that the star is around 10.5 in V in the AID, which is normal. You did not mention how bright you found it though.
If you don’t use a V filter, keep in mind that your observations can’t be compared with the V data, and red stars tend to be much brighter if you observe unfiltered.
You said you extracted the green channel for photometry. And what was the result? Had you observed the star previously to compare both results? That would let us know how different your results are from V.
In any case, an outburst from this star is not likely.
Cheers,
Sebastian
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Hello Sebastian,
I took this V537 CrA star from the AAVSO Target Tool which listed it as a cataclysmic star, and I saw it has a light curve around 10th magnitude. I estimated the magnitude as 8.49 using Siril which uses the green channel for photometry. I checked and changed the sharing of the image so anyone with the link can see the image. I will review previous images I took a couple of months before to see if there is a different magnitude. I has only a few reports in the AAVSO database.
Greetings.
Celso.
Sebastian, I have just processed some images of V537 CrA dated Aug. 30th, 1924. Siril estimated a magnitude of 10.2553+/-0.0056, or (10.2553-10.698+10.7)=10.2573. Comparison star was TYC7900-1233-1 (AAVSO mag. 10.7, Siril 10.6980) and check star was TYC7900-2155-1 (AAVSO 12.0, Siril 11.9427). I will take some images next saturday.
Greetings.
Hi Celso,
V0537 CrA is not a cataclysmic variable.
Can you send a screenshot of the Target Tool listing it as such?
Are you sure the green channel is being reduced? In any case, I don’t know how large the difference between V and that magnitude might be but non-standard photometry of red variables is tricky.
Any visual observer taking a look at the star right now might help checking if it is brighter or not.
PS: Thread moved to the Alerts forum so interested people will see it more easily.
(It seems I am not allowed to post images)
[https://drive.google.com/file/d/1BNL8OgbhDt8oRCGYcgrbZ85t5nG116vZ/view?usp=sharing]
This V537 CrA star is supervised by the YSO section. I presumed it is a cataclysmic variable. The images are real, no matter the classification and no matter the channel, R, G, B or RGB, it is brighter than other 10th magnitude stars in the image. Now Siril says V537 CrA is at the center of the image. The Astrometry.net plate solve agrees. If the estimate is wrong, there must be an explanation.
Hi Celso,
YSOs are Young Stellar Objects, cataclysmic variables are not YSOs. They are studied by the CV section.
V0537 CrA is not a YSO either, but I can understand that, since it was classified as MISC before, it might have been included there.
But it doesn’t belong there, it is not young, it is old, a red giant.
When you say “no matter the channel”, well, it does matter, since a red star will be much brighter in the R channel.
One thing you could do is to measure another red giant in the field with the same filters to see what magnitude you get.
There is one: HD 167526 at 18 17 29.21 -39 04 59.0 (J2000.0).
It is a 9th-10th mag. star. Provided your system does not saturate at that magnitude (what is the brighter limit for you?)
Since I have been doing spectral classifications of red stars by the hundred of late from GAIA3 spectra, I had a look at these stars. V537 CrA is a nice M6III, while I make nearby HD 167526 to be M5III. Houk gives M4III for the HD star, but it could well have been quite faint on her blue-sensitive plates (not a big difference in any case). Neither is a cv or YSO, as has been noted.
\Brian
If it is a YSO then it’s a WTTS but as I said to Celso (private communication) if it isnt a YSO then it’s a regular red variable, as you say. It is, however, in a starforming area so that might be why the YSO designation was given originally.
The GAIA3 spectra are quite distinctly giants, not dwarfs, as weak T Tauri stars would appear. Also the luminosities inferred by the GAIA distances are that of ordinary giants, nearly 100x more luminous that near-main-sequence stars.
\Brian
Hi to all,
It is too much instructive and comprehensive the explanations on what type of star V537 CrA is, however, there is still an explanation due for the increasing in brightness. In the image, there is an obvious and undeniable brightness difference between the magnitude 10 equivalent stars and V537 CrA in all the channels that requires explanation. What it could be? If it is a stable old giant red star, it is unlikely to have that kind of brightness increase and their light curve shows no jumps in two years (https://drive.google.com/file/d/1MCZx0jxbMZ4bFPs_ovO3rnaCpu5udRcC/view?usp=sharing).
Celso, you were assuming that the star jumped in brightness, but we didn’t have another observation with the same setup. This was your only datapoint. If it was standard photometry, I would agree there was a brightening, but under these circumstances, we needed to get external confirmation.
Actually, there is a way to see if there was a brightening: checking ASAS-SN data. Your observation is from June 22 so the brightening should have been detected and be seen in the online light curve.
Here we have the ASAS-SN light curve.
There is no brightening. The star was slowly heading for a maximum at g= 11.33 on June 22 (JD 2460648)
This is the complete ASAS-SN light curve to see the behaviour over the last decade.
And also to see the star g-V color, which will let us estimate an approximate V mag. of 10.6 on June 22.
So, you should check how your photometry is being made and if TG is actually the correct way to report it. Non-standard photometry of red stars is tricky.
Cheers,
Sebastian
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Thank you Sebastian. My first message was asking for a confirmation if it was an outburst or a correction if it was not. I will check my procedures and equipment again. On August 2024 I took images of V537 CrA with a smaller 80mm f5.6 refractor and used the same procedure, the magnitude estimation was 10.26. This time I used a C8 2000mm f6.3, the only difference. But what is hard to understand for me is why the only “bad” estimation is for V537 and not for the comparison and check stars. Comparison star 10.7 (TYC-7900-12331-1) is 10.7 in Siril, 11.0 in Simbad; HD167526 has the estimated magnitude of 8.0778 but check star TYC7900-2155-1 in AAVSO chart is 12.0, Siril gives 11.7179 and Simbad 11.79.
Hi Celso.
During August 2024 the star slowly brightened form 11.3 to 11.2 g, so its brightness was similar within 0.1 mag.
The differences between the comp stars and the variable are to be expected because comp stars are not red, and if your equipment is sensitive to the red, the variable will be brighter.
This assumption is confirmed when you mention that HD 167526 was magnitude 8.08, when it is actually much fainter in V.
Cheers,
Sebastian
OK, now I understand. Just a final answer, my camera is a ASI533MCPro and I use the Optolong UV/IR filter. I have yet to calibrate with the Standard Fields. I also have a 642-800 nm filter and a 850-1000nm filter. Are they of any use for photometry? Thank you.
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