AAVSO: American Association of Variable Star Observers

Proposal #200

Proposer (43063) Cameron McEwing (tech.mechanic@gmail.com) obscode: MCJC
Assigned To(3663) Dirk Terrell
Date SubmittedMarch 4, 2021

Name: Cameron McEwing (affiliation AAVSO, Variable Stars South)
Code: MCJC
Observation Use: Scientific as per Alert Notice 732.
Use of Data: Submit photometry, using VPhot, to AAVSO database
Can I have the images sent directly to my VPhot account.
Target Star: ASASSN-21co

I would like to observe this long period eclipsing binary in response to Alert Notice 732 using AAVSO Net southern hemisphere telescopes with a daily cadence. Observations should continue until at least a week after the eclipse finishes. This would be around 87 days after Feb 12, 2021. I am happy to share the observing work with other observers who may request AAVSO Net to monitor this object. As very little is known about this object, I believe this is a valuable use of AAVSO Net time and will add to our understanding of the nature of ASASSN-21co eclipses.

Magnitude Range: 12.35 – 11.7 mag
Comp Stars: from X26320AL or X26320AN chart – 136, 128, 121, 119 will confirm once I process the first images through VPhot
Observation Cadence: 1 per day if possible
Filters Required: B, V, R, I
Number of Exposures per Run: two per filter
Exposure Times: Onsite best guess, review after initial images.
Start Date: Mar 4, 2021
End Date: 87 days after Feb 12, 2021

The science use case for this object is detailed in Alert Notice 732...

Dr. Stella Kafka (AAVSO) requests observers' assistance in monitoring the long period eclipsing binary ASASSN-21co.

ASAS-SN data show that an eclipse started between 2021 February 10 and 12. The ASAS-SN light curve in V and g and relevant photometry may be seen here:

An expanded version of the g light curve and its photometry may be seen here:

Archival data from KELT show a suspected period of 11.9 years, with eclipse duration of ~80 days. ASASSN-21co is V~11.7 and g~12.5 at maximum. Very little else is known about this system.

Observations are requested to begin immediately in order to capture early features of the eclipse. Color photometry (all filters) is requested, as is spectroscopy of all resolutions. Visual observations are welcome.

Please continue to monitor ASASSN-21co at least until after it has returned to maximum. Continued coverage may be requested after the eclipse is over, so please be sure to subscribe to the forums for this campaign to receive updates.

Observations submitted to the AAVSO International Database are as follows:

2021 Mar 01.74370 UT, 10.315 I +-/-0.005 (G. Myers, Hillsborough, CA);
01.74297, 10.320 I +-/-0.005 (Myers);
01.74221, 11.431 R +-/-0.006 (Myers);
01.74183, 11.435 R +-/-0.006 (Myers);
01.74073, 12.344 V +-/-0.004 (Myers);
01.73929, 12.347 V +-/-0.004 (Myers);
01.73769, 14.038 B +-/-0.008 (Myers);
01.73625, 14.032 B +-/-0.008 (Myers);

Coordinates (2000.0): 18 17 51.54 -58 07 49.3 (from VSX page for ASASSN-21co)

Target RA (H.HH) Dec (D.DD) Magnitude Telescope Observation Frequency Expiration Date Proprietary Term
ASASSN-21co 18.297650 -58.13036 12.35–11.7 BSM_Berry 1 Sept. 1, 2021 1 Year


(43063) Cameron McEwing — March 4, 2021, 12:07 a.m.

I didn't mean to set a proprietary term, the data should be open to all users. Observation frequency should be once per day if possible. Telescope choice should depend on availability and suitability for this target. I'm not sure what to set for the expiration date. The observations should continue until around 87 (eclipse duration plus one week) after the start of the eclipse on Feb 12. Cameron :-)

(43063) Cameron McEwing — March 4, 2021, 2:59 a.m.

Also as I am new to AAVSO, if you need a reference please feel free to contact Ken Menzies.

(43063) Cameron McEwing — March 4, 2021, 10 p.m.

From the ASAS-SN data (see link at bottom) it looks like the eclipse is already well underway, with a drop of half a magnitude (in g) already (very exciting). I thought a daily cadence looked about right as this would give about 80 points across the eclipse period, and I wasn't sure if a higher cadence was practical. I also wasn't sure how long after the eclipse I should go, and I'm guessing there is a possibility that the eclipse duration is not well established yet.

In terms of exposures per night, I put down two as I assumed I might need more than one to help with SNR, but I'm guessing this would depend on the chosen exposure, telescope, and also the magnitude range of the comp stars used.

I wasn't sure about which VSP chart numbers to use, as these would depend on the FOV of the chosen telescope. And as far as comp stars go I thought 132, 128, 123, and 119 from photometric sequence/chart X26322EL (see link below) looked promising as they had BV indexes in the range 0.66-0.83 (but this is without knowing the actual color of the target. No estimate in VSX and I couldn't find it in SIMBAD either). As one of these stars will end up being the check star, is it worth requesting more comps for this field?

I also had a look at the ASAS-SN phase plot that Sebastian put in VSX (see link at bottom), to get an idea of the magnitude range and behavior of this system. It looks like there may be a secondary eclipse that is slightly offset from 0.5 phase, which is interesting.

Link to ASAS-SN light curve ... https://asas-sn.osu.edu/sky-patrol/coordinate/bda80996-59f6-407a-bef8-c0a9f7d00ce9
Link to ASAS-SN phase plot ... https://www.aavso.org/vsx_docs/2215574/133/ASASSN-21co.gif
Photometric Sequence/Chart ... https://app.aavso.org/vsp/photometry/?star=ASASSN-21co&scale=E&orientation=visual&type=chart&fov=30.0&maglimit=16.5&resolution=150&north=down&east=right

(43063) Cameron McEwing — March 5, 2021, 2:26 a.m.

Measurements taken by MGW on a CDK17 0.43m at Siding Spring on March 2, 2021 gave the following BVRI values…

(untransformed? [Transformed=0 in LGC])
B = 14.008
V = 12.345
R = 11.441
I = 10.33

Measurements by HMB (VVS) 8 hours earlier show similar values.

This gives a BV colour index estimate of 1.663 which is reddish. If this is accurate, it may be worth using a similarly red star like 121 (000-BNX-188, B-V= 1.184) in sequence/chart X26322EL as a check star for this target.

LCG plots over the last 5 days (Feb 28-Mar 4) seem to have remained reasonably flat in BVRI, which is not following the downward trend of the ASAS-SN g-band data prior to this (Feb 12 - Feb 27). This is a little puzzling, and needs some thought.

[HMB used 123 (000-BNX-189) as a check, and 119 (000-BNX-187) as a comp.
MGW used 136 (000-BNX-192) as a check, and 128 (000-BNX-190) as a comp.]

Link to ASAS-SN data for ASASSN-21co … https://asas-sn.osu.edu/sky-patrol/coordinate/bda80996-59f6-407a-bef8-c0a9f7d00ce9

(43063) Cameron McEwing — March 5, 2021, 3:25 a.m.

Oops, I missed Gordon's original data in the alert notice. This gives a similar BV color index of 1.690.

(43063) Cameron McEwing — March 5, 2021, 3:52 a.m.

I had a look at the latest ASAS-SN data for ASASSN-21co and this shows the lightcurve has flattened out and possibly started to brighten again, which is surprising and agrees with the observed flattening in the data from AAVSO observers. This is really intriguing and interesting behavior.

Link to latest ASAS-SN data for ASASSN-21co ... https://asas-sn.osu.edu/sky-patrol/coordinate/8cc5012f-1b59-498c-a748-2845fb681423

(4726) Kenneth Menzies — March 5, 2021, 5:10 p.m.

Committed to Berry and S

Comments on this proposal are closed.