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HD 5501 - Eclipsing Binary (2006-09-14 note)

[ written 2006-09-14, graphs updated with extra points on 2006-10-05 ]

Photometric studies of HD 5501 are being conducted as a follow -up to recent discovery of this peculiar eclipsing binary by Mais, Gray & Richards (2006).  The notes below record ongoing progress of the study. 

• Introduction

• Methodology

• Measurements

• Results

• Discussion / Future Work

Introduction 
HD 5501 (= BD+59 154 = BSD 8-397) is a 9th magnitude star in the constellation of Cassiopeia.   It has recently been discovered to be an eclipsing binary with a period of 7.53 days with a primary eclipse of 0.45 magnitude and a secondary of 0.3 magnitude. The system is constantly varying suggesting an ellipsoidal variable with the stars in or near contact, perhaps with one or both Roche lobes filled. (The Discovery and Initial Characterization of a New Eclipsing Binary with Peculiar Properties,   Mais, D. E.; Gray, R.O.; Richards, D., Society for Astronomical Science 2006).

My contribution to the initial characterization of this new Eclipsing Binary was through software support to the project, specifically the provision of my Variable Magnitude Analysis (VMA) program which enabled fast, automated magnitude determination of images taken by Dale Mais over the course of nearly a year.

As a follow up to this original work, I've made my own observations of HD 5501 to confirm the star's period  and potentially provide additional insights into the character of this eclipsing binary.

HD 5501 (Cassiopeia)

Annotated CCD Image 30 sec exposure, 2x2 binning, C Filter  2006-04-23 21:49h UT (#116018)

Methodology
Photometry was conducted with a Meade LX200 8" operating a F/6.7 using an ST- 7e camera, 2x2 binned pixels and Bessell BVR  filters.  

Initial work began with with locating the HD 5501 and establishing a standard target position that would cover HD 5501 together with suitable comparison stars (frame size 16.5 x 11 arc minutes).    After a little experimentation a position 3.9 arc min S, 3.0 arc min W of HD 5501 was chosen, which enabled the following comparison stars to be accessed.

Note : Bt, Vt magnitudes in table above are from Tycho catalog. 
B, V, R magnitudes are transforms from Bt, Vt using Arne Henden's best fit transform equations [also documented at Tass-Survey.org reference ]

HD 5501 and Reference Stars

Annotated CCD Image 30 sec exposure, 2x2 binning, C Filter  2006-04-23 21:49h UT (#116018)

Two comparison stars were used (HIP 4429 and HD236584). Ideally these stars should have colours which are more similar to HD 5501 in order to minimise secondary extinction  effects,  however the first choice reference star (HD 5513) was found to be variable, or at least apparently so,  and could not be used as a valid comparison or control star. GSC 4017:2008 was used as the control star. 

Images were obtained as sets of 3 images in each filter bands (initially B & V bands, but subsequently B, V & R bands). Exposures in B, V, and R filters were 60s, 30s, and 30s respectively and were chosen to provide reasonable Signal -to-Noise, without saturating the stars and without entering non-linear range of the CCD chip.   Image reduction was carried out using CCDSOFT using appropriate  Dark and Flat support frames. 

Magnitude determinations was performed using specially written scripts (VMA routines with my AIS program) and collated using an Excel Workbook.  At the present time the comparison star magnitudes/colour values have not be separately checked and there has not yet been any attempt to fully transform the magnitude results to Standard BVR magnitudes. Nevertheless the relative magnitudes obtained so far are still considered to be useful.

For each image the magnitude of HD5501 and control star was calculated using the equation

  M _[star] = mi_[star] +  (  M_[comp1]  - mi_[comp1]  +  M_[comp2] - mi_[comp2]  ) / 2

where mi is Instrumental Magntitude and M[comp] is the comparison star Magnitude in Filter Band

For each set of 3 images in each band the average magnitude value was calculated after first rejecting frames where Signal To Noise was particularly low or anomalous related to passing cloud.    Skies at my observing site are not particularly photometric and it can be a challenge to preserve sufficient data whilst maintaining data quality.

Measurements

Individual image measurements are collated within an Excel Workbook and plotted.  Average magnitude measurements in B, V & R bands are shown in the three graphs below:

Initial monitoring led to a suspicion that observations on 2006-07-18 (V 0.15m below 'normal') formed part of an eclipse event. However it was not until a run of good weather between 2006-09-02 and 2006-09-10 that a more complete set of data could be collected which confirmed the variability of the star and illuminated its pattern.

A full assessment of error bars associated with measurement is still to be conducted, however initial assessment indicates the variation in HD 5501's brightness is significantly larger than the stochastic and systematic variation seen in the control star. However there is some concern that the control star variation isn't more flat.   This might indicate a small variation in either one of the Comparison Stars (C1 or C2)  or the control star (K3).
[ This variation has been subsequently investigated ]

Differences are noted between the expected and the measured colour indices for HD 5501 and control star. These are tabulated below :

A variety of possible explanations have been identified, which require further investigation. However a consistent feature is that the above stars are calculated as being brighter and redder than expected (this may be significant in formulating an explanation and appropriate correction)

• magnitudes used for Comparison Stars might be wrong 

• expected magnitudes/Colour Indices of HD5501 and the control star might be wrong. 

• colour differences have not been accounted for

• data is not transformed to Standard Magnitude

Results

Results of magnitude and colour determinations of HD 5501 to the present date are presented in the following graphs

The above graphs indicate a variability in brightness with time, but with a relative uniformity in colour.  

The graphs below show the magnitude and colour determinations folded using the previously proposed period of 7.53 days. 

An alternative folding of the data is shown below where a period of 3.675 days is used (exactly half the previously proposed period)

During the first 3 months of the study it was noted that the offset star HD 5513 (8.3 arc min from HD 5501) shows possible brightness variations of between 0.1 and 0.25m. [Plots for HD 5513 are shown separately ]

Discussion / Future Steps

Photometry data recorded to date indicate a variability in HD5501's brightness with time, with a relative uniformity in colour,  which is typical of an eclipsing variable.  Initial assessment involving folding the data suggests that the brightness variability is consistent with the previously proposed period of 7.53 days, with a primary and secondary eclipses of around 0.3 - 0.4m depth. However the data could be also be folded with 3.675 days with just a single significant eclipse of 0.4m depth

Further monitoring will be performed to further characterize HD 5501's variability.

At the same time further monitoring will be performed on the offset star HD 5513.in order to confirm it variability and if appropriate characterize the variability.

David Richards (Sept 2006)

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HD 5501 - Comparison Star QC (2006-10-12)

The above note on HD 5501 described a concern over the variability seen in the calculated brightness of the control star (K3). Ideally the control star would be approximately flat indicating that the comparison star(s) were not varying and could be considered sound reference stars.  Although the control star variation (+/- 0.05m) was much less than the variation shown by HD 5501 (+/- 0.15m), it may potnetially indicate a source of reduced measurement accuracy.    The control star variation would appear to indicate a potential variability in either the control star (K3) or one of the two comparison stars used  (C1 or C2/K2).  [  It had already been noticed that the original first choice comparison star (HD 5513) was showing a clear variation, which required it to be rejected as a valid comparison star ]

To investigate where the problem lay, each star (C1, C2/K2, K3) was used in turn as a comparison star to calculate a V Band magnitude for the other two stars.  The three resulting graphs (shown below) highlight that the largest variation in calculated check star magnitudes and accompanying strongest correlation between the calculated magnitudes of the check stars is seen when C1 is used as the comparison star - this strongly suggests that it is the C1 star (HIP 4429) which is showing variable behavior, responsible for the variation in control star magnitude. 

On the one hand this removes a further star from list of offset stars that can be used as comparison star, but on the other points to a second 'new' variable star found during this study.

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HD 5501 - Re-Analysis (2006-12-30)

Following a QC of earlier used comparison stars it was decided that a reanalysis of HD 5501 was required in order to remove the affect of the C1 star variability that was potentially reducing the accuracy of earlier magnitude estimates.  This time the C2/K2 reference star (HD 236584) was used as the sole comparison star, with K3 star acting as the control star again.

New graphs are shown below (with data up to end 2006)

Principal conclusions on HD 5501 from earlier note (2006-09-14) are unchanged.  Some concern remains over the calculated colour indices, however this is mainly considered to be due to uncertainty in the colour indices of the comparison star used, and does affect the conclusions on presence of variability, period & eclipse depth.

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HD 5513 - Suspected New Eclipsing Variable

As part of photometric studies of HD 5501 it was found that HD 5513 shows brightness variations of between 0.15 and 0.30m. Further monitoring is being undertaken to see if variation can be confirmed or otherwise explained and if there is a regular period or pattern. 

As of 2006-12-30, the data is providing a light curve consistent with an eclipsing binary with a period of approximately 1.198 days.  The transit appears to be 4.3 to 6.0 hours long (15-21% of period) with a depth of approximately 0.3m.

In the analysis below the C2 reference star (HD 236584) was used as the sole comparison star, with K3 star acting as the control star.

Image showing HD 5513

Annotated CCD Image 30 sec exposure, 2x2 binning, C Filter  2006-04-23 21:49h UT (#116018)

Based on apparent period of  1.1988 days estimated from observation through period 2006-06-26 to 2006-10-24, a minimum was predicted for approximately 22-45 - 23:00h UT on 2006-10-28, and a monitoring R & V images were acquired covering a 2 hours either side of this predicted time. The results shown below, confirm a minimum close to the predicted time, and help support the previous period estimate.

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HIP 4429 - Suspected Variable

As part of photometric studies of HD 5501 it was found that HIP 4429 shows brightness variations of ~ 0.15m. Initial analysis suggests a short period variation overlaid on a longer period variation.  Further monitoring is being undertaken to see if variation can be confirmed and if there is a regular period or pattern. 

In the analysis below the C2 reference star (HD 236584) was used as the sole comparison star, with K3 star acting as the control star.

Image showing HIP 4429

Annotated CCD Image 30 sec exposure, 2x2 binning, C Filter  2006-04-23 21:49h UT (#116018)

HIP 4429 - V Band magnitude variation for 3 specific nights of observations (2006-10-28, 2006-11-06, 2006-12-19)

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