double stars – Cosmic Reflections

UCAC4 379-071051: A Possible New Double Star Discovery

Shadow path of UCAC4 379-071051 during its occultation by asteroid 2392 Jonathan Murray on April 18, 2025 UT

On April 18, 2025 UT, I observed an occultation of the 11.9-magnitude star (m_v) UCAC4 379-071051 in the constellation Libra by the 17.7-magnitude asteroid (m_v , at the time of observation) 2392 Jonathan Murray.

As you can see in the light curve below, I observed a two-dip event. Since the magnitude drops are unequal, that rules out an asteroid satellite.

2392 Jonathan Murray apparently covered up the brighter component of a heretofore unknown double star (either binary or optical), followed by a brief interlude before the fainter component was covered up.

The magnitude drop when the fainter star was occulted was 0.44. When the brighter star was occulted, the magnitude drop only goes down as far as the limiting magnitude of my sky in the images, which was around 14th magnitude (Tucson is not as dark as it once was). Using the magnitude drop when the fainter component was being covered by the asteroid (the only reliable magnitude drop we have), and knowing the magnitudes of the two components must sum to 11.9 magnitude, we can calculate that the magnitude of the two components are:

Primary Component (1st star occulted): 12.34

Secondary Component (2nd star occulted): 13.09

Occultation analysis expert Dave Gault (Australia) used my light curve and knowledge of the asteroid’s size and motion (and all the other factors that need to be considered) to provide the following preliminary double star solutions (there was not enough information for a single solution):

Double Star Solution #1
Separation = 36.9 mas
Position Angle = 103.6°

Double Star Solution #2
Separation = 37.0 mas
Position Angle = 113.8°

These two solutions are quite close to one another. Averaging the two separations, we get 36.95 milliarcseconds. Gaia DR3 indicates that the parallax of this star is 1.323 milliarcseconds. That’s a distance of almost 756 parsecs or 2,500 light years. At that distance, these two stars have an apparent separation (in the plane of the sky, so a minimum) of 28 AU, or a little less than the distance between the Sun and Neptune in our own solar system. This is not unreasonable for a true binary star system, but, of course, the fainter star could be many light years further away than the brighter star (which is presumably the component Gaia measured in determining the parallax). In that case, this would just be a chance alignment of two stars at different distances but not physically associated with one another.

High-resolution spectroscopic observations of UCAC4 379-071051 over time could determine whether or not this is a true binary star system. Astrometric measurements over time with whatever supercedes Gaia (likely) or analysis of occultations of this star by other asteroids (unlikely) could also determine whether or not these stars are a true binary star system.

Confounding factors in the double star solutions include (1) We don’t know the exact size of asteroid 2392 Jonathan Murray (Neowise gives a diameter of 6.5 miles); (2) We don’t yet know the shape or orientation (at event time) of this asteroid (my occultation observation was the first time this asteroid has been observed to occult a star); (3) We don’t know the orientation of my single observation chord (what part of the asteroid crossed the two stars).

As you can see, there’s a lot to consider in trying to interpret this atypical (though not all that unusual) “double dip” occultation event. Of course, the very first thing we did was to rule out any terrestrial cause of the second smaller dip (clouds, for example), and we had to also rule out any equipment anomalies that could have caused the second smaller dip (CCD anomalies, for example). After convincing ourselves that this was a real event, we proceeded with the analysis. A big thank you to Dave Gault and Dave Herald in Australia for their work and expertise in analyzing this data!

TYC 5134-1820-1: A New Double Star Discovery

Shadow path of TYC 5134-01820-1 occulted by asteroid 1330 Spiridonia – June 26, 2023 UT

On 26 June 2023 UT, Vince Sempronio near Benson, Arizona and David Oesper near Tucson, Arizona observed an occultation of the 12.2-magnitude* star Tycho 5134-1820-1 in the constellation Aquila by the 15.1-magnitude asteroid 1330 Spiridonia. The predicted magnitude drop should have been around 2.9 magnitudes (15.1^m-12.2^m) by both observers, but I observed only about a 0.2-magnitude drop, and Vince a 1.5 magnitude-drop. After expert analysis by David Gault and David Herald in Australia, it was determined that we had discovered a new double star!

Observer locations for the June 26, 2023 occultation event

Fortuitously, Vince had observed 1330 Spiridonia covering up only the primary (brightest) component, and I had observed 1330 Spiridonia covering up only the secondary component. Both of us made our observations with 8-inch telescopes.

Vince Sempronio’s light curve (11.2 seconds, 70 data points)

David Oesper’s light curve (60.7 seconds, 455 data points)

The double star solution from our observations gives the following:

G magnitude of the primary component: 12.4

G magnitude of the secondary component: 13.9

Separation: 59.7 milliarcseconds (0.0597 arcseconds)

Position Angle: 141.8° (eastward from north)

Follow up observations over time will be needed to determine whether this is an optical double (chance alignment) or a true binary system. The distance to TYC 5134-1820-1 is currently estimated to be between 2,689 and 2,883 light years (SIMBAD). Definitely not in the neighborhood.

Even though double stars are common in our galaxy (and everywhere else in the universe), and understanding that our observations represent only the tiniest contribution to scientific knowledge, there is satisfaction in knowing that we discovered something not known by anyone else before. Besides, you never know when a discovery such as this will draw attention to an unusual and astrophysically-interesting system.

In conclusion, here is but one example showing that observations of stellar occultations by the minor planets of our solar system presents an exquisite method of discovering very close double (and possibly binary) stars, not assayable by any other technique.

*Gaia G magnitude

Double Star Discovery: TYC 724-273-1

On 20 Oct 2021 UT, I observed the star TYC 724-273-1 in the constellation Orion being covered up by the asteroid 444 Gyptis. The star disappeared at 5:31:53.856 UT and reappeared at 5:32:10.506, a duration of 16.65 seconds.

The published apparent visual magnitude of this star is 11.5 and the published apparent visual magnitude of 444 Gyptis at the time of the event is 12.5.

The combined magnitude (m_c) of star + asteroid just before (and after) the occultation event is given by

$m_{c}=m_{o}-2.5\log_{10}\left (10^{0.4(m_{o}-m_{*})}+1 \right )$

where m_o is the magnitude of the asteroid
and m_* is the magnitude of the star

This gives us a combined magnitude of 11.14 just before the occultation.

While the asteroid is covering up the star, you should only see the asteroid, so the magnitude should decrease from 11.14 to 12.5, a magnitude drop of 1.36 magnitudes.

Much to my surprise, I observed a magnitude drop of only 0.54.

Is it possible that 444 Gyptis only covered up one component of a previously undiscovered double star? That idea is bolstered by the fact that the event occurred 14.8 seconds earlier than predicted, a full 3.7σ early.

Entertaining the double-star idea, our task is to determine the magnitudes of the two blended stars and which one got covered up. Let us call the magnitudes of the two components m_*1 and m_*2, with m_*1 being the component that got covered. We already know that m_*1 + m_*2 must equal m_* = 11.5. We also know that the observed magnitude drop of the m_*1 plus the unobserved magnitude drop that the m_*2 star would have had must equal the expected magnitude drop of 1.36. This gives us enough information to calculate m_*1 and m_*2 individually.

$m_{*1} = -\log_{10}\left (10^{-\left (m_{c}+\Delta m_{obs} \right )/2.5}-10^{-0.4m_{o}} \right )/0.4$

$m_{*2} = -\log_{10}\left (10^{-\left (m_{*}/2.5\right )}-10^{-0.4m_{*1}} \right )/0.4$

where m_o is the magnitude of the asteroid
     and m_* is the magnitude of the star
     and m_c is the magnitude of the star + asteroid
     and m_*1 is the magnitude of the occulted star component
     and m_*2 is the magnitude of the unocculted star component
     and Δm_obs is the observed magnitude drop

This gives us a magnitude of 12.36 for the occulted component and 12.15 for the unocculted component. Thus we can see that I observed the fainter component of the double star being occulted by asteroid 444 Gyptis.

Finally, we can do an extra check to make sure that the magnitudes of the two star components plus the asteroid equals the combined magnitude of 11.14 we expected right before the occultation occurred.

$m_{c}=-2.5\log_{10}\left (10^{-0.4m_{*1}}+10^{-0.4m_{*2}}+10^{-0.4m_{o}} \right )$

Here’s a little SAS program I wrote to do the calculations.

data magdrop;
   format mstar mastr mcomb pdelm odelm mstr1 mstr2 mtot 5.2;
   mstar = 11.5;
   mastr = 12.5;
   odelm = 0.54;
   x = 0.4*(mastr - mstar);
   mcomb = mastr - 2.5*log10(10**x + 1);
   pdelm = mastr - mcomb;
   mstr1 = log10(10**((mcomb+odelm)/-2.5) - 10**(-0.4*mastr))/-0.4;
   mstr2 = log10(10**(mstar/-2.5) - 10**(-0.4*mstr1))/-0.4;
   mtot = -2.5*log10(10**(-0.4*mstr1)+10**(-0.4*mstr2)+10**(-0.4*mastr));
   file print;
   put 'Published Magnitude of Occulted Star = ' mstar;
   put 'Magnitude of Asteroid = ' mastr;
   put 'Combined Magnitude Right Before Occultation = ' mcomb;
   put 'Predicted Magnitude Drop = ' pdelm;
   put 'Observed Magnitude Drop = ' odelm;
   if (odelm/pdelm > 0.5 and mstr1 > mstr2) or
      (odelm/pdelm < 0.5 and mstr1 < mstr2) then do;
      put 'Magnitude of Star Component Occulted = ' mstr2;
      put 'Magnitude of Star Component Not Occulted = ' mstr1;
   end;
   else do;
      put 'Magnitude of Star Component Occulted = ' mstr1;
      put 'Magnitude of Star Component Not Occulted = ' mstr2;
   end;
   put 'Total Magnitude of Both Star Components + Asteroid = ' mtot;
run;

Published Magnitude of Occulted Star = 11.50                                                      
Magnitude of Asteroid = 12.50                                                                     
Combined Magnitude Right Before Occultation = 11.14                                               
Predicted Magnitude Drop = 1.36                                                                   
Observed Magnitude Drop = 0.54                                                                    
Magnitude of Star Component Occulted = 12.36                                                      
Magnitude of Star Component Not Occulted = 12.15                                                  
Total Magnitude of Both Star Components + Asteroid = 11.14