Green Star (YSO Candidate)

And probably the last piece of the puzzle:

AT 2019bca outburst of candidate CV ASASSN-17an, discovery date 2019-02-18

So this would be the second recorded outburst of this likely CV, but probably not periodically because then it would have also had an outburst around 2021-03 (and have another one around 2023-04)

I am not sure that I am as confident in that the r-band is definitely indicative of a flare-up episode. But, here are the relevant g-band single exposures.


Perhaps I conclude too fast based on only the flip GIF by eye (seems so obvious to me, ofcourse I have no formal training to deduce magnitudes :slight_smile: )
Can’t say I see any such big difference in the g-band exposures though

Thanks for the g-band GIF! I quite expected the g band 2017 to be a lot brighter too… :thinking:

The single exposure where it brightens has other objects brightening too. Bad seeing that night, maybe?

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Yes was going to mention that. Does also feel that there is a bigger increase in the target though than in the surrounding objects. Could be that the longer exposure time revealed this disparity?

yeah I noticed that, but the increase in target looks to be much more than with the surrounding objects, relatively

One of my instutitions think this is actually a planetary nebula but however without an infra-red source, it could just be a star with a weird colour.

There is an infrared source. Its not particularly red in WISE though and shows variability which is inconsistent with a PN.

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Does feel like a PN more than a CV

Howso? Aside from the green color (which may be explained by a single exposure issue), its textbook CV.

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Eh I meant CV not PN. I was going to say I thought it might be an anomalous cepheid as these have been found to maybe be linked with blue stragglers which to me seems to make the greenness a step less absolutely dumbfounding.

Agreed. I think I said that in the r-band data post. The PSF extracted flux on the individual exposures should resolve that concern and make the question quantitative. Anyone can grab those using the CAT files released in DECaPS plus the zpcal file (I haven’t had a chance yet). We are trying to make it even easier to grab that information with a merged individual detections table… but that will still be in process for ~ two weeks.


GAIA DR3 lightcurve / photometry points

What I take from this is that it does look variable to me, with sometimes a short peak

Dumb question but are these mags related to colours? Sorry I’m just an astronomy tourist…

Colors are differences in magnitude between different bands (so G-RP, BP-RP etc).

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Ok but the legacy names the data points G mag, BP mag and RP mag, not G-RP, BP-RP etc.?

Mag measures brightness, on a negative log scale, with 0 being about the brightest star in the sky, and 5 being the faintest you can see in a dark site with unaided eyes. There’s a factor of about 100 in those 5 mags of difference.

Since it’s a log scale, if you take the mag in one band, say G band, and subtract another band from it, say RP, then that difference corresponds to a ratio of brightnesses. (So for astronomers, a “color” is a technical term, a difference of magnitudes in different bands. By convention, the bluer band comes first, so that a small color is “blue” and a large color is “red”.) If you took the same star and moved it further and further away, it would have larger and larger observed mags, but the difference in mags – its color – would remain constant.

Then there’s the magical art of taking observations in mulitple bands and turning them into RGB color images, which is a different thing entirely :slight_smile:


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Yes thank you

I’m somewhat familiar with the use of mag, its just the G, RP & BP part that’s confusing to me (used to grizy / W1-W4 etc.), and the fact the coloured dots on the GAIA lightcurve chart are resembled in RGB I was wondering if that represents to bluest to reddest wavelength like normally gri is mapped to blue-green-red?

And thus showing that the object isn’t intrinsically ‘greener’ but has peaks of increased magnitude every now and then

Yeah, the Gaia mags are confusing, because G is a broader filter that contains RP and BP (plot from this page: Gaia (E)DR3 passbands - Gaia - Cosmos):


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ok… :+1: