GAIA transient GAIA21dxg / TNS AT2021wui

Hamish Barker
Posts: 226
Joined: Thu Jun 20, 2019 12:11 am

GAIA transient GAIA21dxg / TNS AT2021wui

Post by Hamish Barker »

https://www.wis-tns.org/object/2021wui

I saw this object originally listed as type unknown at magnitude 10 on the rochester astronomy "bright supernovae" page
https://www.rochesterastronomy.org/snimages/

I took a spectrum quite low in the sky, and Terry Bohlsen has taken a better one:
at2021wui_20210825_046_Hamish Barker_and_TBohlsen.png

However, now the Rochester list indicates it as type CV, but no details of spectrum or classification report are included.

The object was notified from the GAIA alerts team as having brightened 0.3 mag in 2 years.

Here is the the light curve for the coordinates which I have retreived from the ASAS-SN survey:
at2021wui-excelplot1000days.png
zoomed in a bit
at2021wui-asassn-lightcurve-1000days.png
It's seems similar to a k2iii spectrum but the continuum falls of more slowly compared to the k2iii. There seems to be absorbtion at Hbeta but modest emission at Halpha.

Does anyone have suggestions as to what it is likely to be, and why should there be emission at Ha but absorbtion at Hb?

Cheers,
Hamish
Hamish Barker
Posts: 226
Joined: Thu Jun 20, 2019 12:11 am

Re: GAIA transient GAIA21dxg / TNS AT2021wui

Post by Hamish Barker »

Dr. Ángel R. López-Sánchez ( Senior Lecturer,
Australian Astronomical Optics &
Department of Physics and Astronomy
Macquarie University, Sydney, Australia)
has had a look at this and sent me the following commentary:
I’ve had a quick look to your [HB Note: Angel was actually looking at Terry Bohlsen's] spectra: it’s definitely H-alpha and very probably [N II] 6583 in emission, likely each line has at least a couple of components (but the resolution of your spectra does not allow to check that, you need high resolution).

Here is the screenshot of my quick check, and the results of a double Gaussian fit:

Gaussian fit 1: SNR = 16.13 RMS = 3.440e-02 center = 6556.293 ± 0.344 flux = 1.056 ± 0.065 EW = -1.06 ± 0.07 FWHM = 14.60 ± 1.14 A = 667.8 ± 52.1 km/s
Gaussian fit 2: SNR = 11.58 RMS = 1.248e-01 center = 6583.134 ± 2.060 flux = 1.677 ± 0.145 EW = -1.68 ± 0.14 FWHM = 42.36 ± 4.39 A = 1937.2 ±200.7 km/s


The broad of the emission lines, particularly the [N II] 6583 emission, suggests that there is some expanding ionised material (a bubble) around the object or ejected from the object.

The high [NII]/Ha ratio, [NII]/Ha = 1.6, indicates that the ionisation is not coming from the photoinization from massive stars/hot stars, but from shocks (as it happens around active galactic nuclei, AGNs, or around other stellar energetic sources).
at2021wui_20210825_533_TBohlsen_Ha.png
Terry Bohlsen
Posts: 191
Joined: Thu Sep 29, 2011 7:40 am

Re: GAIA transient GAIA21dxg / TNS AT2021wui

Post by Terry Bohlsen »

I have taken a few nights data of this interesting object. The graphs below are from the same night using my LISA at R=1600 and Lospec of R=5000.
There seems to be Ha emission that is blue shifted about 280 km/s. Hb is in absorption and shifted 170km/s to the red.
Any suggestions about a mechanism?
at2021wui_20210828_425_TBohlsenHa.png
at2021wui_20210828_425_TBohlsenHa.png
Attachments
at2021wui_20210828_425_TBohlsenHb.png
Terry Bohlsen
Armidale NSW
Australia
Robin Leadbeater
Posts: 1926
Joined: Mon Sep 26, 2011 4:41 pm
Contact:

Re: GAIA transient GAIA21dxg / TNS AT2021wui

Post by Robin Leadbeater »

Are you sure your K spectral classification is correct? According to SIMBAD this is CD-37 11550, a B0.5V star, listed as LS 4145 in "MK SPECTRAL TYPES FOR OB+ STARS IN THE SOUTHERN MILKY WAY"
http://articles.adsabs.harvard.edu/pdf/ ... ..89..293V
Could it be highly reddened by IS extinction ? (spectral classification should based on the lines in the spectrum not the shape of the continuum)
for example it also appears in this series of papers according to SIMBAD
https://simbad.u-strasbg.fr/simbad/sim- ... ..15...53K

Cheers
Robin
LHIRES III #29 ATIK314 ALPY 600/200 ATIK428 Star Analyser 100/200 C11 EQ6
http://www.threehillsobservatory.co.uk
Robin Leadbeater
Posts: 1926
Joined: Mon Sep 26, 2011 4:41 pm
Contact:

Re: GAIA transient GAIA21dxg / TNS AT2021wui

Post by Robin Leadbeater »

SIMBAD gives B-V as 0.86 but B-V for a B0.5V star should be -0.29 from
http://www.pas.rochester.edu/~emamajek/ ... s_Teff.txt

Try dereddening your spectrum by 1.15 and see what it looks like.

Given the extinction, the V mag and the parallax from SIMBAD you could also estimate the absolute magnitude and see if it matches that expected for a B0.5v star

Cheers
Robin
LHIRES III #29 ATIK314 ALPY 600/200 ATIK428 Star Analyser 100/200 C11 EQ6
http://www.threehillsobservatory.co.uk
Terry Bohlsen
Posts: 191
Joined: Thu Sep 29, 2011 7:40 am

Re: GAIA transient GAIA21dxg / TNS AT2021wui

Post by Terry Bohlsen »

Hi Robin
This certainly makes sense. Looking at my noisy LISA spectrum and comparing it to a B2 standard after rectifying the continuum it fits better than a K star.
So this makes the star a Be star maybe that is slowly going into emission?
at2021wui(normcont)TBohlsen.png
Terry Bohlsen
Armidale NSW
Australia
Hamish Barker
Posts: 226
Joined: Thu Jun 20, 2019 12:11 am

Re: GAIA transient GAIA21dxg / TNS AT2021wui

Post by Hamish Barker »

nice one Robin! that does seem to be a reasonable explanation.

I didn't know or think to click on the simbad (or other) links on the TNS page.

So, could the Be emission be sufficient to explain the circa 17% increase in flux? I see only a little pimple of Halpha. Assuming that and say Hbeta were previously deep absorbtions, could that add up to 17% increase? Does the additional flux from what is presumably the disc of the Be stars increase not only at Halpha (or other lines), but also in the continuum?

I found this nice paper with a model an example star with fairly similar (6 month increase then decrease) photometric behaviour:
https://ui.adsabs.harvard.edu/abs/2013A ... S/abstract

And this one which corellated data from the KELT survey with data from the BeSS database. Well well, all roads lead to, perhaps not Rome but rather, BeSS!
https://iopscience.iop.org/article/10.3 ... ajaa6396f2
Robin Leadbeater
Posts: 1926
Joined: Mon Sep 26, 2011 4:41 pm
Contact:

Re: GAIA transient GAIA21dxg / TNS AT2021wui

Post by Robin Leadbeater »

Hi Hamish,

It might be a Be star but there are lots of potential causes for H alpha emission in stars besides Be star discs. As suggested by Ángel R. López-Sánchez the line profiles from high resolution spectra might give a clue. One thing we can do though is check the luminosity to see how it compares with the suggested main sequence spectral classification.

If the catalogue B0.5v classification is correct then expected absolute magnitude Mv is -3.5

From SIMBAD the apparent magnitude mv = 10.6. We then need to correct this for IS extinction
from the B,V in SIMBAD and the assumed B0.5v classification E(B-V) = 1.15.
(This high figure is not unreasonable as the total galactic extinction in that direction is extremely high at E(B-V) = 5.7 !)
https://irsa.ipac.caltech.edu/applications/DUST/
The extinction at V is ~3.1 * E(B-V) = 3.6 so the extinction corrected mv = 7.0

The distance from gaia parallax is 2300pc so Mv ~-4.8.

So perhaps more luminous than the -3.5 expected for a main sequence star. For comparison the blue supergiant P Cygni (B1i) which shows H and He emission lines from stellar wind is Mv -6.3
LHIRES III #29 ATIK314 ALPY 600/200 ATIK428 Star Analyser 100/200 C11 EQ6
http://www.threehillsobservatory.co.uk
Robin Leadbeater
Posts: 1926
Joined: Mon Sep 26, 2011 4:41 pm
Contact:

Re: GAIA transient GAIA21dxg / TNS AT2021wui

Post by Robin Leadbeater »

Here is Terry Bohlsen's spectrum dereddened by 1.15 compared with Pickles B1v.
AT2021wui_dered-0p15_TB.png
AT2021wui_dered-0p15_TB.png (21.45 KiB) Viewed 8036 times
Whatever it is, it is certainly not a K2iii star so to avoid any ongoing confusion I recommend amending your comment on the IAU TNS website.

Cheers
Robin
LHIRES III #29 ATIK314 ALPY 600/200 ATIK428 Star Analyser 100/200 C11 EQ6
http://www.threehillsobservatory.co.uk
Robin Leadbeater
Posts: 1926
Joined: Mon Sep 26, 2011 4:41 pm
Contact:

Re: GAIA transient GAIA21dxg / TNS AT2021wui

Post by Robin Leadbeater »

Note also that the suggested line at 6583 is [NII] not NII. (The square brackets are important, this is a forbidden transition)

it is important that any observations we submit to professional sites where there is no gatekeeper like IAU TNS are as accurate as possible. Our continued access may depend on it

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