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SZ Hya (RRAB/BL Star) ) Brightening in V?

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msheald
SZ Hya (RRAB/BL Star) ) Brightening in V?

Hello!

    I just reduced last night's images, and it appears that V is brightening.

    I followed this star several years ago and just started to follow it again. Looking at past measurements, I did not see any where the V magnitude is brighter than the I magnitude, but that happened with last night's run.

    Could this be real or due to my equipment/technique? I would appreciate folks' thoughts on evaluating this.

    Thank you and best regards.

Mike

hambsch
hambsch's picture
SZ Hya

Hi Mike,

I could take it up in my observing proegram for a couple of nights.

Josch.

sah
sah's picture
SZ Hya (RRAB/BL Star) ) Brightening in V?

I have been observing SZ Hya for about 40 years.  This star has a strong Blazhko effect with a Blazgko period of about 26 days. Throughout this Blazhko cycle, the maximum magnitude can vary by a half magnitude or more and the phase of the maximum can shift by an hour or so. The brightness and phase of the minimum also varies.

 

The shape of the light curve seems to change from RRab to the RRc and back during the Blazhko cycle. These type of stars are referred to as dual mode or RRd type stars. A sample light curve from 2010 is attached.

SAM

 

File upload: 
hambsch
hambsch's picture
SZ Hya

Hi SAM,

as you know I am following many Blazhko effect RR Lyr stars and several papers have been already published in JAVSO.

Your sample LC shows a strong BL effect as you also mention, but you only took images in one filter. Mike did observe in BVI filters and I will also do so. THis gives more information about the physical properties of the star than just one single filter observations.

Anyway, I got interested and will observe it and send data to the AAVSO.

Josch

msheald
Thank you, all, for your

Thank you, all, for your comments. I am encouraged that my technique was OK (at least at first blush!) Best regards.

Mike

msheald
Cloudy! I missed the peak the

Cloudy! I missed the peak the last two nights, so I could not see if V got brighter than I. It may clear tonight. Best regards.

Mike

hambsch
hambsch's picture
First observations

Hi,

first observations from two nights ago are uploaded. I also missed the peak. Last night I was clouded out at least for SZ Hya. It might be though as if I band is brighter than V band. from my observations. Will keep it on my agenda though.

Josch

 

HQA
HQA's picture
SZ Hya

SZ Hya is classified as an A7 star, though its spectral type will change throughout its pulsational cycle.  At A7, the typical (B-V) color is about 0.2mag, and (V-Ic) ~1.2x(B-V), so I'd expect (V-Ic) ~ 0.3mag.  That means Ic should, in general, be about 0.3mag brighter than V.  That pretty well matches what we see in the LCGv2 plot.  If you saw V brighter than Ic, then usually the only way that can happen is if the star was much hotter (say, B7) at some phase.  I don't think that is normal, if both V and Ic are taken at the same time.  if V is taken on one day, and Ic on the next, then all bets are off.  Also, the rising phase of an RR Lyr is very fast, so you can, sometimes, take Ic and then follow it with a V measure, and in the interim, the star has brightened by 0.3mag or so and V shows as being brighter.  Another way of getting erroneous colors is if you use a comp star with incorrect published photometry.

So beware of pulsating stars - to get accurate colors, the exposures have to be nearly simultaneous.

Arne

msheald
Interesting! Thank you, Arne.

Interesting! Thank you, Arne.

    I took 2 minute integrations in B,V, and I, alternating one after the other rather than in series. The magnitude peak is in the middle of my observing session now, so, hopefully, I'll get more complete time series over the next couple of nights since the clouds should clear. Best regards.

Mike

HQA
HQA's picture
SZ Hya

Hi Mike,

SZ Hya's period is 12.9hrs, with a rise to maximum of about 2 hrs.  With an amplitude of 1.4mag in V, that means it changes 0.012mag per minute during the rise.  If you are taking 2 minute exposures with some dead time between them, then you could see a 0.05-0.06mag fainter V than would be the case if the V and Ic exposures were truly simultaneous.  So whether Ic or V is brighter depends on the phase of the star, and what you mean by "brighter" (not a quantative term).  This is why pulsating stars can be difficult to do correctly, and why exposure timing is so critical.  If the star were strictly repeating from cycle to cycle, you could do one cycle in V and the next in Ic.  In this case, you have the complication of a Blazhko modulation, so cycles don't repeat.  That makes for fun light curves, but complicates the analysis!

Enjoy the star!  If you still have concerns, it would be best to post a light curve that shows the issue.

Arne

msheald
SZ HYA Error Identified!

Hello! I found the error!

    It looks like I analyzed B images using V comps/check, so the mags got raised several tenths of a magnitude.

    When I went back to the uploaded file, the observtion times of B and V were identical - not so in the calibrated images. When I deleted those images and recalibrated and re-analyzed everything, the results fit nicely with the expected results and the data that others had uploaded.

    I want to thank everyone for their help - for Arne's guidance and for everyone who added SZ HYA to their observing regimen for these last seveal nights so that I could check my results. I appreciate it, and I've learned a lot about tracking down errors when results don't fit.

    Thank you again and best regards.

Mike

hambsch
hambsch's picture
Comp stars

Hi Mike,

you should not use different comp stars for the different filters. Comp stars should be the same for all filters. I would recommend looking at LesvePhotometry, a free photometry program I use all the time. It is based on a databas for variables an comp stars and you will never misinterpret your images as the ITS header is used for filters, expoures etc.

It only needs the full version of PinPoint.

The link is:http://www.dppobservatory.net/AstroPrograms/LesvePhotometryDownloadPage.php

I will keep the star in my program.

Josch

msheald
Comp Stars for SZ Hya

Hello! I used the same comp stars. However, it looks like I selected the B filter in MPO Canopus for my V images, so it used the B mags for the comps instead of the V magnitudes, which resulted in the highter magnitudes for the V images. The B and I images were analyzed with the same comp stars but with the appropriate B and I data. Best regards.

Mike

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