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SAO 58521.

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SAO 58521: ( centre of M37 )

 

I wonder if anyone's noticed the varying magnitude of this orange star: usually around 9.0 mag, but on occasions at least a magnitude brighter and dominating the cluster. Has a variable been missed here because it's staring us in the face or considered to be in deep sky tourist territory? I'm waiting for hands to go up!

Until now, no proof of variability
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Interesting. It is not included in the deeper photometric surveys of the cluster because it is very bright.

There are discrepancies about its spectral type. It was classified as M1III in 1926ApJ....64..225A but as K0III + A5V in 1966AJ.....71..736U. SIMBAD lists F8 from 1967ApJS...14..359W but this comes from a 1954 paper by Lindblad.
Since the 1966 paper already mentions the other spectral types, it looks like the composite spectrum plus reddening is more likely.
If the star was a red giant it would be easier to support its variability.

Photometric information from WEBDA and GCPD (three differtent sources):
V   B-V   U-B
9.16    +1.53    +1.19
9.208    +1.626    +1.350
9.192    +1.657    +1.303

TASS lists V= 9.17.
Tycho-2 says V= 9.21.

From the available photometry, there is no apparent variability, with the star being nearly constant around V= 9.19.
It is not easy to estimate the brightness of a star in an open cluster visually.
But there is no continuous coverage nut just isolated observations so more data are needed.

Cheers,
Sebastian

 

  Update on SAO 58521
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I'm now making estimates of SAO 58521 whenever conditions permit. There's no doubt that this star is varying with an amplitude of around one magnitude within a cycle of just a few days. I need a run of clear nights, so as to establish what type of variable we have here. Watch this space; or, better still, participate.

Variability proof
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Changing from being constant (from photometric surveys) to show a full magnitude amplitude would really be a surprise. V data would be needed to support this.

Cheers,
Sebastian
 

SAO 58521
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That star doesn't seem to be in VSX.  What are you using for comp stars?

Jim Roe [Roe]

The comparison stars.
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Both my comparison stars are, necessarily, within M37.

 

GSC 2410 :1293 = 9.7mag.

GSC 2410 : 316 = 10.5mag.

 

I'm using chart 10548CRC, which, of course, bears no official title...yet.

Tycho variability?
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Sebastian Otero wrote:

Photometric information from WEBDA and GCPD (three differtent sources):
V   B-V   U-B
9.16    +1.53    +1.19
9.208    +1.626    +1.350
9.192    +1.657    +1.303

TASS lists V= 9.17.
Tycho-2 says V= 9.21.

From the available photometry, there is no apparent variability, with the star being nearly constant around V= 9.19.

Hmmm, My downloaded Tycho2 db (2001?) shows this as a very red star (TYC2410-1320-1, Vt mag=9.432,  Bt-Vt = +2.1 )

Why the discrepancy, or is there some real variability in the record?

Mike LMK

Vt and Bt are not Johnson V and B
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Mike,
You can't compare Vt and Bt with V and B.
I gave the transformed values.

Bill,
about your comparison stars (GCPD data).

GSC 02410-01293 = 9.7mag.  This (actually V= 9.63) is a combined magnitude for that star that has V= 9.94 and a 22" companion with V= 11.17. So if you resolve them, the magnitude is 9.94.

GSC 02410-00316 = 10.5mag.  Also a wrong choice because this is an even closer double. Your star has V= 10.65 and has a 17" companion to the North with V= 10.73. The combined magnitude is V= 9.94.

So you are using two 10 mag. stars to estimate a 9.2 mag. star. You need brighter comparison stars and you need them to be free from contamination by nearby stars.

I recommend the following stars:

HD 39136 V= 8.79; B-V= 0.10 (GCPD)
HD 38876 V= 8.86; B-V= 0.18 (Tycho-2)
HD 39491 V= 9.33; B-V= 0.06 (Tycho-2)

You will need to come and go to an adjacent field but it is better to extrapolate values from fainter comparison stars.

Cheers,
Sebastian

SAO 58521
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This is smack in the middle of the cluster. Are you able to isolate this star from the cluttered field so that small differences in focus seeing or position of the measurement aperture don't affect the photometry? Please excuse the ignorant question. But I have not imaged the cluster with the idea of doing photometry on an individual star. Even at 3000 mm FL, M37 is very dense around that star.

B Walter, WBY

See Attached DSS Image. I can count at least 4 interfering stars, one quite bright.

Further update on SAO 58521
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Sebastian; Hi! Thanks for your continuing interest in this I'd like to leave the criticism about the two comparison stars aside for the moment: we'll come back to this presently. I don't want a fog of technicalities to get in the way of what's actually going on. Sufficient to say that the cited magnitudes look about right to me.

 

The recent Siberian High of chill conditions and clear skies have given us an unaccustomed respite here. Last night – April 2nd; 0.8694 I'd come to the conclusion that we had a Cepheid- like object on our hands, and I went to the scope expecting to see it on a rising branch at around 9.5mag. I was astonished to find it faint, at about 10.5mag ( similar to my 2nd comparison star). It then brightened to 9.4mag, in the space of 30 minutes:I could almost see it rising! Thereafter, it did very little in the remaining time available.

 

What to make of this? I conjecture, It can only be an eclipsing binary of short period, perhaps even a contact pair. We need spectroscopic analysis...but, more to the point, we need more visual observers. Thus, my original call to attention in mid February. What divers databases have is beside the point. What do we actually see?

 

The floor is open.

 

Bill Wilson.

 

A difficult task
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Hi, Bill,

The "fog of technicalities" you mention are the key to get a reliable brightness estimate. That fog is not going to get in the way of what's actually going on but is going to show you what is not going on.

You talk about spectroscopic observations but we already mention there are some in the literature, the more recent one being K0III+A5V. The spectrum is not consistent with a cepheid. It is also not consistent with a short period binary rising from 10.5 to 9.4 in 30 minutes (A 1.1 mag. amplitude is larger than possible for EW's even taking ellipsoidal variability into account). It is important to put everything in context. What we know about the star and what we observe.

I think Brad is right and making brightness estimates in such a crowded field is not a good idea. In the past I tried to do that in NGC 4755 and NGC 3293 and realized how difficult it was.
You are making conclusions using wrong values for your comparison stars, that are surrounded by other stars and which in turn are both much fainter than the target. All of the observing circumstances (also the lack of similar magnitude stars in the field) make brightness estimates very difficult.

Cheers,
Sebastian
 

Signing off.
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Hi Sebastian,

 

The jury should adjourn now. It's not my intention to be the sole defender of a proposition based on my observations, but rather to encourage other members to have a closer look at M37 and its central star. No doubt this thread will have stirred sufficient interest. We do have 600 enthusiasts to hand, after all.

 

It's time to rest the case, and I've many other stars to occupy my time. The last word is yours; of course.

 

“Cheers”

 

Bill.

Need VSX entry
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Regardless of difficulties of finding comp stars, or nearby contaminating neighbors (unless one or more of them are variable too!) tracking a one magnitude amplitude variable like this visually should be easy and obvious.

I will put it on my program list, but need an AUID for it, please.

Mahalo,

Mike LMK

Observations
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Ok, guys, an AUID has been assigned so now you can submit observations for this star.

Cheers,
Sebastian

Post Script.
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Hi Sebastian,

 

Thanks for facilitating the auditing of SAO 58521. I hope this hasn't been all too irritating for you! I wouldn't have been lobbying like this if I'd been in any doubt about the reality of this star's behaviour...Hadn’t even dared to highlight the maximum of February 25th until now: but there it must stand amongst my other estimates. I'm hoping, incidentally, not to be the sole contributor to that page!

 

Best wishes,

 

Bill.

PSF Fitting
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This is one case that proabably calls for some CCD photometry via point spread function (psf) fitting.  But that technique which is good for crowded fields, was very time comsuming when I dabbled with it in IRAF 15 years or so ago.  It was also somewhat of an art at the time that required experience to get good at.   I don't know if things have changed or improved in psf fitting software since then.

Good luck with this star.

....Tim

Point Spread Function.
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Hi Tim,

 

Thanks for your good wishes. I looked up “Point Spread Function” in “Google”. This might be a powerful tool in a professional investigation on what we have here. I hope to tempt the interest of professional astrophysicists once my observations have been endorsed by other observers. Everything depends on this. Without confirmation, all we will be left with is just another “fisherman's yarn” and it will be “dead in the water” – to pile on yet more metaphors!

 

What's needed now, is this collaboration... and more clear skies. The aim is to ennoble SAO 58521 To full variable star status: V - what ever it is - Aurigae.

 

All the best,

 

Bill.

An emerging pattern.
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Hi Mike,

 

Welcome to the observing team! This baby deserves rechecks, even during a session: see my estimates of last evening – April 6th. Here, there was 0.6 mag rise during the hour!

 

We're getting warm. This has to be eclipse phenomena: but there's something else here too. All very intriguing. Fortunately, the apparition has several more weeks to run.

 

Bill.

Little if any variation
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Well, I have observed this star 3 times over the past 12 days, and it has been within .05 mag V (9.75-9.8) every time, which is within my normal working accuracy range. Based on these limited observations, I would have to say there is no apparent variation.

I have been very careful to use the same equipment, technique (high power, large defocus to remove color appearance - visual rods only) and same comp stars 9.96, 9.33 Tycho 2 with the Bessell corrections.

Of course, I could be just catching it at the "wrong" time and missing these rapid fluctuations which have been reported. So more observations are in order. It is getting low in the season for me at my latitude, though.

I should point out due to the very bright red color, Purkinje effect, at low powers the star appears several tenths brighter. This is a tricky star to consistently and accurately estimate visually!

Mike LMK

I think I would try this
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I think I would try this star, if weather permits, by CCD.

For crowded fields it is possible to do PSF photometry with IRIS software.

Taking Stock.
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Hello Everyone,

 

Losing Auriga now in the evening twilight. It seems we're going to have to wait for the next apparition to close in on SAO 58521 and really nail it all down. What's needed is a ten hour session, taking estimates every 20 minutes – perhaps even several of these! End of the year might offer this opportunity; though that's asking a lot of the weather.

 

I'm in little doubt about my own observations: though I would have liked early confirmation. I'm sure that will come in time. Trust the trickle of interest will be maintained to the autumn - (sorry “fall”).

 

There's a further matter here. It's been pointed out to me that SAO 58521 is a double star – WDS 05523+3233 ( Guide 8). Sure enough, a nearby companion is shown on our AAVSO charts. Using chart 11385I – a 10' chart – I measured this directly. I get 7”.5 ; PA 072° ; the companion looks to be about 10.8 mag. This does not agree with the WDS entry, and nor have I ever noticed it. Can anyone throw light on this facet of the SAO58521 story?

 

Bill Wilson.

Small variations seen
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I've observed this star on 4 separate occaisons over a 3 week span. I do notice a small variation of 0.1 magnitude visually. This is a small amount to detect visually, but I am being careful to use the same comps and methods/equipment.

I should note that these variations are all against the star TYC 2410-1293-1 (v=9.96 Tycho2+Bessell corrections). Its also possible that SAO 58521 is in fact constant, and this Tycho star is varying slightly, or even both STARS vary, or a close companion with combined light is varying!

As Bill says, this star and this comp need more in-depth observations.

Mike LMK

AAVSO 49 Bay State Rd. Cambridge, MA 02138 aavso@aavso.org 617-354-0484