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Request for observations of Iota Virginis

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#1 mccarthymark

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Posted 03 June 2020 - 09:00 PM

I'm working on an observing project of pairs visible in my 20-inch where change in their orbital position might be noticed in the next ~30 years.  I maintain a list of "Short Period" pairs generated from SkyTools 3 Pro on my laptop, and observe from that list when seeing permits.  The search criteria are multiple systems with periods of less than 60 years and separations of between 0.2" and 15".  Since I do not have the ability to take accurate measures, I simply make a sketch of what I see in relation to other field stars.  In the years hence, I hope to notice the change by comparing my sketches.

 

Iota Virginis was included in the list generated by SkyTools, showing it is 4.09/+8, PA 80*, sep. 0.45", and I made an observation the night of 25 May 2020:

 

20-inch, 1067x & apodizing mask: "!! Blue and red! Overlapping disks.  The blue brighter star is in front, red is behind it.  Clear notching, some diffraction but clear.  No other stars in view.  Color not so vivid, pale blue/red.  Notched elongation.  Check spectra."

 

My sketches (at the bottom of this post) show a clear snowman, and it really jumped out at me as being an obvious and colorful pair, which made me make the excited !! note.  

 

When it came time over the following week to write up my observations, I found Iota Vir was not in the WDS.  I went to SIMBAD and was confused by the papers (I'm not a scientist), but did note it was spoken of as an astrometric binary (meaning only the primary has been observed and it binarity is inferred by it's movement in the sky), and that the B star had never been detected.

 

I emailed a friend who is better at interpreting scientific papers, and he provided a link to a 2018 study which said in part:

 

"Based on a direct combination of the Hipparcos data with astrometric ground-based observational catalogues having epochs between 1938 and 1999 the preliminary orbits and component masses are calculated for 6 binaries with no previous orbit calculation: ι Vir (HIP 69701) with period of 55 years, photocentric semi-major axis of 200 mas, relative semi-major axis of 830 mas and a dwarf secondary of 0.6 solar masses."

 

My friend pointed out the major axis would be 400 mas or 0.4”, which is close to the SkyTools value.  Also the B star's magnitude is estimated but thought to be >8th magnitude.

 

I was out again the night of 2 June 2020, and under worse seeing and transparency conditions, recorded the following:

 

8-inch 667x: swimming in seeing but can see a red bump emerging behind the blue-white orb in the diffraction.  Disks bloated due to seeing
8-inch 1067x: stars bloated but it's clear, blue in front of a red orb.  Red seems only about one or two delta mag
20-inch 533x: seeing too poor to get good focus
20-inch 533x + apodizing mask: yellow, seems elongated in the same direction as what I saw in 8-inch
20-inch 667x + ap mask: deeper yellow-orange peanut in diffraction
20-inch 1067x + ap mask: blue star + red as peanut in diffraction.

 

I used similar magnitude Mu Vir as a standard star to compare at all powers with the masks, and I only saw it as round (bloated, but round) and without a discoloration effect such as red bumps etc.

 

When I disengaged the servo motors and let Iota Vir drift, I estimated a PA of the red star, B, being between SSE and due south of the blue-white A, which is in the northern quadrant.

 

Have I observed the apparently never before seen B star?  I would greatly appreciate observations by others, particularly those able to make measurements, as some way of confirmation.

Attached Thumbnails

  • Iota vir (2).jpg

Edited by mccarthymark, 03 June 2020 - 09:08 PM.

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#2 fred1871

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Posted 04 June 2020 - 12:30 AM

Blue and red made me wonder if Iota Vir was low in the sky when you observed it; not least because the star is listed as spectral type F7, which suggests a yellowish colour, certainly not the blue white of Spectral type B stars or perhaps A.

 

Then I had a look at the article you referenced. A delta-m (brightness difference) of 4 magnitudes, possibly more. And a major semi-axis of ~200mas (0.2"). So it won't get much wider than that.

 

Rayleigh for a 20-inch is ~0.27", and for notching that suggests about 0.22" (0.8R). For an 8-inch, Rayleigh is 0.68" , with 0.5R being 0.34". If the delta-m is 4mags, even that degree of elongation being visible is unlikely with an 8-inch. So that fits for a single star with elongation (and colours) due to atmospheric dispersion, again suggesting a low altitude.

 

I had a look at the list of attempts to get a measure of Iota Vir with speckle interferometry using large scopes.

Brian Mason got no resolution in 2001 with a 4-metre scope; result listed as <0.03"

Andrei Tokovinin got no resolution in 2009 with a different 4-metre scope; result listed as <0.04".

In between, attempts were made with the CHARA Interferometric Array, in 2007 and 2008, with a huge baseline, used at several lengths, which gave no resolution, the results in each attempt given as <0.008".

 

The calculated orbit in the linked paper does not suggest high eccentricity, that would give large variations in separation, so CHARA would be expected to get a result (resolution) anywhere in the orbit.

 

Preliminary impression is that resolution (assuming it's not a bogus double) is not possible with modest telescopes. Also, that the derived orbital parameters need more data for a better assessment, as the suggested orbit seems unlikely. The period they derived - of 55 years - will be given from Kepler's 3rd Law from the assumed major semi-axis, the assumed masses (from "A's" spectrum and delta-m of stars), and the likely  good accuracy of the parallax which allows the orbit scale to be derived.... etc  So the period, like the form of orbit, will depend on uncertainties. And note the size and rather random condition of the residuals marked on the orbit as calculated.


Edited by fred1871, 04 June 2020 - 09:37 AM.

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#3 mccarthymark

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Posted 04 June 2020 - 01:27 AM

Hi Fred,

 

Once I read up on the star after my first observation, I realized the data was against a positive observation -- the delta mag, and so on.  But this is precisely why I requested observations through telescopes, so others could check and report what they see.

 

It was viewed both nights at meridian from my location, 37.58 deg. N.

 

I thought perhaps it might be an atmospheric effect, which is why the second night I observed nearby Mu Vir as described above; a 3.88 Vmag and F2V -- not an exact match to Iota but close.  I saw Mu as round through all powers & masks, no weird colors.

 

The first night I only used 1067x and saw the colors.  The second night I noticed it was mainly yellow or yellow-orange at lower powers until I was above 1000x when it was more distinctly two colors which I interpreted as pale red & pale blue.

 

In none of the observations was it separated; best was snowman or peanut.  B's mag is estimated in the paper to be greater than 8th; assuming it is closer to 6th, then a snowman is certainly possible.  I wondered whether I could see it in the 8-inch the second night because I had previously seen it with 20-inch and knew what to expect or look for.

 

Thanks for the information on the speckle; I certainly don't argue.  But I do encourage those who are interested to have a look.  Especially if you're in a southerly latitude where the star is higher in the sky.


Edited by mccarthymark, 04 June 2020 - 01:30 AM.

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#4 mccarthymark

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Posted 10 June 2020 - 03:53 PM

After some more research and talking with others, I can conclude my observations above are spurious.  The culprit is most certainly atmospheric dispersion.

 

Keeping in mind that I knew nothing of this star before observing it, and what I was seeing seemed to match the 0.4" separation given by SkyTools, and that (for whatever reason) my control star Mu Vir did not exhibit any distortion to my eye, I hope I can be forgiven.

 

As Fred noted the weight of the research shows the companion is not detected.  The B star, if it exists, is likely so faint (6 delta mag or more) as to be out of reach of my telescope.  

 

Someone pointed out to me that atmospheric dispersion would produce the effect I described -- a double image, with colors.  He noted that the double image caused by atmospheric dispersion is always orientated perpendicular to the horizon.  And indeed I noticed as a N-S to NNW-SSE position angle in my observations at 8-inch when Iota Vir was at meridian.  One way to test for atmospheric dispersion is to observe the star to the east and then to the west of meridian; one should notice the double image "position angle" would change direction accordingly, unlike a true binary which would keep it's PA through its diurnal motion through the sky.  I haven't had the chance to inspect Iota Vir in this way, but I believe it would show exactly this phenomenon.

 

I am embarrassed by my posts above, but I can salvage some lessons from the experience:

 

1) I've learned a new observing "quality control" technique I can apply in the future

 

2) ">8" and "+8" in scientific papers and SkyTools mean "fainter than" 8th mag, not "brighter than"

 

3) Put more stock in the scientific data: if they're saying it's not really possible it probably isn't (but I'll still observe anyway, just to see).


Edited by mccarthymark, 10 June 2020 - 03:55 PM.

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#5 payner

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Posted 10 June 2020 - 05:00 PM

Do not feel embarrassed. You are learning.  We learn through reading, studying, asking questions and trial and error by stepping out of the boat.

Clear skies,


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#6 nerich

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Posted 10 June 2020 - 05:06 PM

 

I am embarrassed by my posts above, but I can salvage some lessons from the experience:

 

1) I've learned a new observing "quality control" technique I can apply in the future

 

2) ">8" and "+8" in scientific papers and SkyTools mean "fainter than" 8th mag, not "brighter than"

 

3) Put more stock in the scientific data: if they're saying it's not really possible it probably isn't (but I'll still observe anyway, just to see).

 

No need for embarrassment. I've also had my fair share of false observations. A few years ago, being new to double stars and never having paid much attention to star magnitudes anyway, I thought I saw a 12th-magnitude companion less than two arc seconds from a 5th-magnitude primary, with 80mm of aperture! Of course I saw something (an artifact of poor seeing), but I had no way of knowing that it couldn't be that faint component until I brought it to the forum and asked. 

With each observation, I learn a little more about what's easy, what's hard, and what's impossible. There are some super skilled observers here, and that was intimidating at first. But I've found that this forum is one of the friendlier and more supportive corners of CN. Any time I receive a gentle correction from a fellow observer, it's always in the spirit of good will and learning.

So keep reporting! We'll all keep learning from each other. 


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#7 fred1871

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Posted 10 June 2020 - 10:43 PM

Have you worked out what allowed the atmospheric dispersion to be so obvious, given that you said Iota Vir was viewed on the meridian? My first thought, as I mentioned, had been atmospheric dispersion which is why I asked about the altitude of the star at the time, atmospheric dispersion usually being a result of low altitude. (Yes, I should have spelled it out).

 

Discovering the cause is helpful for you in the future, to avoid the same issue again, and a possible help to other observers. It's surprising, given a meridian altitude of around 46 degrees for Iota Vir at your location, to get atmospheric dispersion - Summary: By "discovering the cause" I mean "how does atmospheric dispersion show so strongly at that altitude"? I expect that the very high magnifications contributed to its visibility. Was that the only factor?


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#8 Cotts

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Posted 12 June 2020 - 09:29 PM

I have clearly seen Atmospheric Dispersion on stars 45º above the horizon....  

 

Dave



#9 fred1871

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Posted 13 June 2020 - 03:00 AM

Unexpected to see such a striking version of it at that elevation. Hence Mark's startlement and some other comments. Dave, what size scope and power were you using?



#10 Cotts

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Posted 13 June 2020 - 09:32 AM

Unexpected to see such a striking version of it at that elevation. Hence Mark's startlement and some other comments. Dave, what size scope and power were you using?

This was from my double star measures using 8-inch MakCas and video lucky imaging.  Even at elevations of 45º or more the stars were oval in the direction perpendicular to the ground.  Only slightly oval, mind you, but oval nevertheless.  I was shooting at 3 metres focal length with an image scale of around 0.15" per pixel......My centroid-finding software gave eccentricity data for my stellar images with 1.000 being perfectly circular.   I cannot recall the values I got but numbers in the 0.95 range come to mind...

 

 

When i first saw this phenomenon I thought it was an equipment problem so I did all the usual tests of collimation and camera orientation and so forth to eliminate astigmatism.   Turns out the ellipses showed up with two different cameras and on at least 3 different telescopes.... 

 

I knew about the dispersion at low altitudes and avoided trying to image  below 45º.  I was surprised when I still got oval images..

 

These small elliptical 'distortions' would probably not have been seen visually as the images i was examining were stacked integrations of many hundreds of individual frames..... I cannot even imagine what eyepiece magnification would duplicate the degree to which my process 'enlarged' the stars - I was looking at a pair of stars on the order of 5" separation such that the pair filled my computer screen..... 

 

Dave




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