Colorful binary star systems for small telescopes: Part 2

I previously discussed my ongoing attempt to develop an automated method of getting the physical properties of binary/multiple star systems visible to amateur astronomers. In Part 1 I got as far as getting basic observables and IDs for the Primary stars in Bob King's article,"Colored Double Stars, Real and Imagined" by Bob King (Sky & Telescope, December 14 2016).

The next step, described here, is to use the the Washington Double Star Catalog to identify and list the probably companions of each Primary. The WDS is an ongoing project that summarizes observations of bright visual double or multiple stars. Their aim is to determine which, if any, are physically associated in a gravitationally bound stellar systems as opposed to being chance line of sight superpositions. As such the WDS project makes use of multiple historical observations, measured distances (by parallax) and proper motions, the results of which are summarized in their catalog.

To cut a long story short I've written some bash and python scripts that download the latest WDS catalog from Vizier, then takes the outputs from the scripts described in Part 1 and combines that with the WDS catalog to come up with a list of Primary, Secondary, (Tertiary etc) stellar IDs based on some WDS-related selection criteria.

I was surprised to find that even for many visually bright double stars their exact status, as genuine binary or multiple star systems or as chance line-of-sight superpositions, is not currently 100% known. In addition, for some "classic" doubles, the latest information suggests they are likely not genuine binaries.

To illustrate this I'll continue to show examples based on Bob King's Colored Double Stars.

Firstly, we'll select components where the WDS catalog notes that there is evidence consistent with them being members of the same physical stellar system.

Then for purposes of comparison we'll separately select all WDS possible components but exclude those where there is evidence is against them being associated.

Filtering based on positive evidence for multiplicity

Here we are looking for double/multiple stellar systems where the available WDS info suggests they're real gravitationally bounds systems. To do this we select components with already determined actual orbits, or statistically similar parallax and/or proper motions (i.e. they're at close to the same distance and/or are moving in the sky in the same way). These correspond to the WDS 'Note' column entries 'C', 'O', 'T', 'V' or 'Z'.

python3 process_wds_ids.py king_processed.fits.gz king_wds_postv_ids.html \
  --wds-detail=king_wds_postv_detail.html --filter=positive
[...output trimmed for blog...]
0 input targets with no WDS info: []
8 input targets where positive filtering removed all components: ['1 Ari', 
   'iota Tri = 6 Tri', 'eta Per', '32 Eri', 'rho Ori', 'iota Ori', 'gamma Lep', 
   '24 Com']

This produced an HTML table of input star name and output component WDS IDs (king_wds_postv_ids.html), along with an optional separate table listing select information from the WDS catalog for each selected component (king_wds_postv_detail.html) which is shown below:

WDS	Comp	Obs2	pa2	sep2	mag1	mag2	SpType	Notes
00491+5749	AB	2016	325	13.4	3.52	7.36	G1V+M	NO P
02039+4220	BC	2010	96	0.2	5.3	6.5	B8V+A0V	NO
05154+3241	Ca,Cb	1999	100	2.0	7.33	14.1	F2V+DA1.3	NV
07166-2319	BC	1999	165	999.9	5.84	6.76	A5m+F0	NV
08467+2846		2016	308	31.3	4.13	5.99	G7.5IIIa	NV
14514+1906	AB	2017	300	5.6	4.76	6.95	G8V+K5V	NO
17146+1423	AB	2017	104	4.8	3.48	5.4	M5Ib-II	NO
18015+2136		2017	256	6.5	4.85	5.2	A5IIIn	NV
18448+3736	AD	2017	150	43.8	4.34	5.62	F0IVv	NZ V
19307+2758	AB	2017	54	34.6	3.19	4.68	K3II+B9.5	NZ
19307+2758	Aa,Ac	2008	101	0.4	3.37	5.16	K3III+B0V	NO
20136+4644	Aa,Ab	1985	111	0.0	3.93	0.0	K4Ib+B3V	NO
20210-1447	AB	2012	267	205.4	3.15	6.08	F8V+A0	NV
20210-1447	Aa,Ab	2014	42	0.0	3.1	4.9	F8V+A0	NO
20210-1447	Ba,Bb	2015	59	0.5	6.16	9.14	A0III	NO
20467+1607	AB	2017	266	8.9	4.36	5.03	K1IV+F7V	NO Z
22292+5825	AC	2017	192	40.7	4.21	6.11	F5Iab+B7	NZ

In addition to the 8 cases where the script noted there were no doubles with information that made them 'likely' physical companions, a look at the king_wds_postv_detail.html table shows some of the remaining objects aren't great visual doubles systems either.

• gamma And: For WDS 02039+4220, the likely double is components B & C, i.e. not including gamma And itself! The two B & C components are only separated by 0.2 arcseconds. As we don't all have the Hubble Space Telescope this is effectively a spectroscopic binary system and not useful visual double.
• 145 CMa: The same problem arises for J07166-2319 (called h3945 CMa in Bob King's list), where again the likely system is components B & C, not including the bright "primary" itself.
• 14 Aur: For WDS 05154+3241 the only likely physical system is a spectroscopic binary system of component C. So once again the "primary" is not part of a likely physical binary systsem with any of its visually close neighbors. The second component of the Ca,Cb pair is a white dwarf, which is interesting, but at 14th magnitude is far too faint to see with a small amateur scope.
• 31 Cyg: A similar situation arises for WDS 20136+4644, where in this case it is only the primary itself that survives filtering because it too is a spectroscopic binary.

The final thing to note is the systems where the likely companion to the primary is not the closest companion: zeta Lyr A & D and delta Cep A & C.

Filtering based on negative evidence for multiplicity

In this case we accept all WDS components except those where the evidence suggests they're not related, i.e. not part of the same physical system. The script remove components with statistically different parallax and/or proper motions, or are otherwise noted in the WDS as being of dubious validity. These correspond to the WDS 'Note' column entries 'S', 'U', 'X', and 'Y'.

python3 process_wds_ids.py king_processed.fits.gz king_wds_negtv_ids.html \
    --wds-detail=king_wds_negtv_detail.html --filter=negative
Processing 22 targets from king_processed.fits.gz
[...output trimmed for blog...]
0 input targets with no WDS info: []
0 input targets where negative filtering removed all components: []

This results in information overload, as too many candidate components that currently lack sufficient information to be rejected end up passing through the filter.

WDS	Comp	Obs2	pa2	sep2	mag1	mag2	SpType	Notes
00491+5749	AB	2016	325	13.4	3.52	7.36	G1V+M	NO P
00491+5749	BD	2000	1	172.2	7.36	12.8	K7V	N P
01501+2217		2016	165	2.9	6.33	7.21	G3III	N
02039+4220	A,BC	2016	63	9.4	2.31	5.02	K3IIb	N
02039+4220	BC	2010	96	0.2	5.3	6.5	B8V+A0V	NO
02124+3018		2016	69	3.7	5.26	6.67	G0III	N
02507+5554	AB	2012	295	31.4	3.76	8.5	M3Ib-IIa	N
02507+5554	AE	2012	297	242.9	3.76	9.24	M3Ib-IIa	N
02507+5554	CD	2012	116	5.1	11.61	12.7	OB-	N
02507+5554	CG	2012	230	15.3	11.61	14.0	OB-	N
03543-0257	AB	2017	349	6.9	4.8	5.89	G8III+A2V	N
03543-0257	AC	2003	5	165.9	4.8	10.5	G8III	N
05133+0252	AB	2015	62	6.9	4.62	8.5	K2II	N
05154+3241	AD	2010	322	179.7	5.03	10.75	A9IV	N
05154+3241	BC	2014	210	22.7	10.9	7.33	+F2V	N
05354-0555	AB	2012	141	11.6	2.77	7.73	O9III	N
05354-0555	BC	2014	94	40.3	7.73	9.81	B4	N
05445-2227	AB	2012	350	95.0	3.64	6.28	F6V+K2V	N
05445-2227	BC	1999	8	112.1	6.28	11.37	K2V	NL
07166-2319	BC	1999	165	999.9	5.84	6.76	A5m+F0	NV
08467+2846		2016	308	31.3	4.13	5.99	G7.5IIIa	NV
12351+1823		2016	272	20.4	5.11	6.33	K2III	N
14514+1906	AB	2017	300	5.6	4.76	6.95	G8V+K5V	NO
17146+1423	AB	2017	104	4.8	3.48	5.4	M5Ib-II	NO
18015+2136		2017	256	6.5	4.85	5.2	A5IIIn	NV
18448+3736	AD	2017	150	43.8	4.34	5.62	F0IVv	NZ V
19307+2758	AB	2017	54	34.6	3.19	4.68	K3II+B9.5	NZ
20136+4644	AC	2016	173	108.6	3.93	6.97	K2II	N
20136+4644	AD	2016	322	336.7	3.93	4.83	K2II	N
20136+4644	CH	2014	62	60.6	6.97	12.6	B5V	N
20136+4644	CI	2015	136	60.2	6.97	12.26	B5V	N
20136+4644	DC	2003	150	431.8	4.83	6.97	A5IIIn	N
20136+4644	FJ	2015	217	4.2	13.9	15.1		N R
20136+4644	HK	2015	262	8.9	11.74	10.87		N K
20210-1447	AB	2012	267	205.4	3.15	6.08	F8V+A0	NV
20210-1447	AC	2012	133	226.1	3.15	8.83	F8V+A0	N
20210-1447	BC	2000	111	396.7	6.08	8.83	A0III	N
20210-1447	DE	2000	321	3.9	13.7	14.4		N
20467+1607	AB	2017	266	8.9	4.36	5.03	K1IV+F7V	NO Z
22292+5825	AC	2017	192	40.7	4.21	6.11	F5Iab+B7	NZ
22292+5825	DE	2008	23	1.4	13.9	14.0		N

I've included the table for completeness, although I can't recommend using this form of filtering. A lot of faint components with large angular separations are listed, which simply aren't interesting from an amateur astronomical point-of-view.

What next?

This post is already very long and much delayed, so I'll put off the next stage of the process until Part 3. In that post we'll take our improved list of stars that includes the primary and likely companion IDs ("king_wds_postv_ids.html" above) and run that through the script from Part 1 that queries Simbad to get the observable properties: positions, magnitudes, parallaxes, proper motions, effective temperatures and spectral types for the stars. Once we have the observables we'll finally be ready to calculate the derived properties we're interested in: distance, luminosity, radius, and rough stellar masses.

Acknowledgements:

This research has made use of the Washington Double Star Catalog maintained at the U.S. Naval Observatory.