SN 2018zd

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SN 2018zd

Postby Peter Somogyi » Fri Mar 09, 2018 10:12 am

Hi,

Yesterday, with lucky imaging between clouds, I've caught SN 2018zd, must be around 14 Vmag. Could have done much better with normal sky.
It's located perfectly for northern observers, ideal for Alpy, and can be shot all over the night! (clouds more becoming constant prevented me).
Only 2 of my 9 x 20 minute exposures got good snr, others possibly got taken through varying veils.

See:
http://www.astronomerstelegram.org/?read=11379
http://www.rochesterastronomy.org/sn2018/sn2018zd.html
https://wis-tns.weizmann.ac.il/object/2018zd

It took me a longer time to find it in the wide field view of my f/4 system (300/1200 Newton + Alpy 600 + Lodestar X2 + ATIK 414 EXm):
sn2018zd_guide_image.jpg
2.5 sec exposure
sn2018zd_guide_image.jpg (79.2 KiB) Viewed 2564 times


Although ATEL#11379 classifies it as type IIn, Gelato is uncertain for my specturm (I've given NGC 2146 velocity value from simbad: 891 without any sign):
sn2018zd_gelato_offers.jpg
sn2018zd_gelato_offers.jpg (132.18 KiB) Viewed 2564 times

Browsing through the offers, my human look with a few days age prefers a IIp one (despite ATEL's IIn), because only they have such a prominent peak around 4684A (the 2nd peak at 4869A almost vanished on my spectrum):
sn2018zd_gelato_2009bw_IIP.jpg
sn2018zd_gelato_2009bw_IIP.jpg (87.41 KiB) Viewed 2564 times

It's relative intensity, 9 x 20 minute (out of only 1-2 are good SNR, others like a half and less), ATIK 414 EXm, Alpy 600, 300/1200 Newton, 18 micron slit.
The narrow peak at 7102A may be a noise, had weak signal towards the red.
I believe it may me important to re-observe quickly, having so close to peculiar.

Cheers,
Peter
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Re: SN 2018zd

Postby etienne bertrand » Fri Mar 09, 2018 1:29 pm

Hello Peter,

Goods works, nice spectra !
Regards.
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Re: SN 2018zd

Postby Robin Leadbeater » Fri Mar 09, 2018 1:37 pm

Hi Peter,

Yes I have been watching this one too. The initial classification of Ia-CMS in TNS by the ZTF team (who use a spectrograph with a similar low resolution to my ALPY 200) was obviously wrong and shows why it is important not to just believe what programs like GELATO and SNID say. It is clearly a type II but it was discovered very early (Patrick Wiggins took an image a few hours before which does not show it.) As the Global SN project team say on TNS, it is too early to be sure of the sub type. A spectrum at maximum and the light curve will tell us

Cheers
Robin
LHIRES III #29 ATIK314 ALPY 600/200 ATIK428 Star Analyser 100/200 C11 EQ6
http://www.threehillsobservatory.co.uk
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Re: SN 2018zd

Postby Peter Somogyi » Fri Mar 09, 2018 6:17 pm

Thank you Etienne and Robin for the comments!
Seeing the Rochester link update, yesterday it came up to 13.8 Vmag (V filter).
- Peter
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Re: SN 2018zd

Postby Peter Somogyi » Mon Mar 12, 2018 10:45 pm

Hello,

Another shot just from now (10 x 600 s, clear sky but interrupted by clouds, 300/1200 Newton, Alpy 600, ATIK 414, 18 micron slit):
sn2018zd_old_vs_new.png
sn2018zd_old_vs_new.png (84.22 KiB) Viewed 2346 times

Old (2018.03.08) elevated by 0.5 in PlotSpectra (both normalized for 5000-6000A).
Lot of changes in 4 days:
- HeII 4686 vanished completely
- tiny Balmer P-cygni profiles appear (very roughly at 2-3000 km/s near H-alpha, question where is the 0 point)
Gelato still claims "Type II uncertain", best offered fit for now is SN 1998S type IIn with 17.5 - 21 days.

Cheers,
Peter
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Re: SN 2018zd

Postby Robin Leadbeater » Mon Mar 12, 2018 11:44 pm

Hi Peter,

We see ngc2146 approximately face on so the redshift of of 0.0028 for ngc2146 is probably a good figure for the supernova RV (~900km/s. You could try recording the galaxy spectrum. The supernova H alpha emission peak should line up with the galaxy H alpha line).

~3-4000 km/s ejecta velocity is quite a bit lower than the type II supernovae I have recorded (8-11000 km/s) but I think this varies quite a bit depending on the mass of the star.

If you post the fits file, I could run it through SNID and see what that says

Cheers
Robin
LHIRES III #29 ATIK314 ALPY 600/200 ATIK428 Star Analyser 100/200 C11 EQ6
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Re: SN 2018zd

Postby Peter Somogyi » Tue Mar 13, 2018 12:48 am

Hi Robin,

Thank you for the look, attaching both my fits in zip here, since I am just learning SNs any help appreciated:
sn2018zd_0312.zip
(18.99 KiB) Downloaded 120 times

By Gelato, The SN 1998S type IIn has a similarily low velocity, however that's much narrower. Here I think the absorptions are wider and must be well resolved by the Alpy (R~670 average) at the H-alpha, with a rounded shape (not a sharp one). By Simbad, NGC2146 has 891 km/s redshift, so no need to spend another hours for the host galaxy, I guess.

In IRAF, setting 0 point to fixed 6562.7A (no redshift considered) generates this graph:
sn2018zd_ha_20180312_6562_7.png
sn2018zd_ha_20180312_6562_7.png (4 KiB) Viewed 2335 times

So I can read a roughly 2500 km/s for which if i add the host's 891 km/s (as a positive redshift I assume), then getting a total of 3400 km/s (rounded).

Clear skies,
Peter
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Re: SN 2018zd

Postby Forrest Sims » Tue Mar 13, 2018 3:55 am

Hi,

Here is my contribution from last night to add to Peter's data. The humidity was very high and there were some thin clouds wafting though at times during the exposures. I took 8 600sec exposures of the target. I had a very difficult time finding a Reference star as all candidates other than HD043378 were too far off in air mass from the target. But with HD043378 being so bright I had to take only 10 sec images of which I took 7 in order not to saturate. The calibration looks good to the red side of about 3925Å.

Woody
Attachments
sn2018zd_20180312_219_Forrest Sims.png
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Re: SN 2018zd

Postby Peter Somogyi » Tue Mar 13, 2018 5:53 pm

Hi Woody,

You seem to have good SNR in the red, yet I wouldn't dare to tell for your spectrum that any P-Cyg appeared.
Maybe happened in the 0.5 days difference? (2018-03-12.219 vs 12.765)
Hope someone continues and confirms (poor forecast here).

- Peter
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Re: SN 2018zd

Postby Robin Leadbeater » Tue Mar 13, 2018 7:02 pm

Hi Peter,

I am also learning as I go along ;-) I ran your spectra through SNID. From the earlier spectrum it confirms a type IIn with good confidence, for example a good fit to 1998s 13 days before maximum.

asdb_sn2018zd_20180308_841_SNID.png
asdb_sn2018zd_20180308_841_SNID.png (43.93 KiB) Viewed 2270 times


It was not sure about the second spectrum though. One of the better visual fits is to a supernova impostor (LBV 2003hy)

asdb_sn2018zd_20180312_765_SNID.png
asdb_sn2018zd_20180312_765_SNID.png (43.72 KiB) Viewed 2270 times


I looked up LBV 2003hy and found a reference that suggests that, although it was initially classified as an LBV, later measurements suggested it was in fact probably a type IIn supernova

http://adsabs.harvard.edu/abs/2005ASPC..332...86M

Cheers
Robin
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