Southern Doubles, Stars and Variables
SOUTHERN ASTRONOMICAL DELIGHTS : Presented by Andrew James
|PAGE DS 012
10 Mar 2009
|RA : 12h
Dec : -30° to -90°
|Constellations : Hya, Ant, Cen, Vel, Car, Cru,
Mus, Cha, Oct.
Best Observed : Feb - Jul (Text Ordered by RA)
|RA : 12h
Dec : -30° to -90°
SELECTED SOUTHERN DOUBLES and VARIABLES
ALPHA CRUCIS' COMPONENTS WDS Name 1st LAST 1st LAST 1st LAST Mags. Spec. pmRA pmDec. DUN 252 AB 1826 1994 05.4" 04.0" 114 114 A=1.25 B=1.15 mag. B1V -0.36 ± -012 DUN 252 AC 1826 1991 87.2" 90.0" 200 202 A=1.25 C=4.80 mag. B4IV -042 ± -012 DUN 252 BC 1859 1947 89.8" 90.6" 205 205 B=2.3 C=6.0 mag.? B1V -0.36 ± -012
Δ252 AB and AC have similar proper motions and are certainly related. However in 1997 Token found a further three nearby stars as possible components were found by speckle interferometry. These are now catalogued as TOK 7 Ca, TOK 7 Cb and TOK 7 Cc, whose designations were added by the United States Naval Observatory (U.S.N.O.) on the 1st November 2001. It is uncertain if these star could eer be seen visually as the main star are overwhelmed these components brightness. There 1997 positions were made as follows;
ALPHA CRUCIS "NEW" COMPANIONS WDS Name WDS Desig. Sep. P.A. TOK 7 Ca 12266-6306 2.2 052 TOK 7 Cb 12266-6306 4.9 214 TOK 7 Cc 12266-6306 6.7 119
The “WDS 2001 Notes” gives;
“DUN 252 /α1 is a spectroscopic binary and shares common proper motion with HR 4729 and 4731.” It is currently thought there are at least five (5) components in the system. Both the coordinates and proper motions are similar being obtained from ACRS, PPM, IRS, FK5 catalogs.
Acrux is catalogued in the WDS (and elsewhere) as Dunlop 252 (Δ252), and this appears in Megastar and many other sources. However, this is perplexingly wrong. James Dunlop in his double star catalog (Dunlop, J.; “XVII. Approximate Places of Double Stars in the Southern Hemisphere, observed at Parramatta in New South Wales.”; Memoirs. Astronomical Society of London, 3, 257; (1829)) is;
Δ122 and Δ123 (12266-6306) are given for Alpha (α) Crucis - with DUN 122 for the AB pair and DUN 123 for the AB-C pair. At first I was fooled by this in Megastar Versions 3.0, and 4.0.31 and 5.0.5, as if you type either Δ252, Δ122 or Δ123 you still get Alpha Crucis. But if you look at the data box you still get Δ252. It turns out the Megastar cannot identify the pair in question, it finds the last star listed in that catalog you are looking up. Hence Δ122 and Δ123 don’t actually exist! DUN 252 is also used in Sky Catalog 2000.0. Amazingly, Astronomical Objects for Southern Telescopes (AOST2) fail to mention the catalog number as Δ252 for Alpha Crucis. (Note : This is object No.522 in AOST2 - coincidently this is 252 if you switch the first two numbers!)
Furthermore, Dunlop’s pairs appear in Right Ascension order. Hence, Δ124 is γ Cru (Gacrux) and Δ125 is Beta Crucis (Mimosa and NEVER Becrux!) Here is an example. According to James Dunlop from the same source given above, DUN 252 is actually the Bode star 29 Tucanae, whose 1827 position is 23h 34m 00s -65° 22' (23h 43m 37s -64° 24'). Dunlop says this is an orange sf 6 and 8 magnitude stars a position angle of 27.4° and the separation of 3.4' - a wide double indeed! 29 Tuc is located 2°NW of 4.5 magnitude Epsilon (epsilon;) Tucanae. This particular pair also doesn’t appear in the WDS for obvious reasons. I also checked in Dunlop’s 629 deep-sky object to look for DUN 255, and find it is an “elliptical nebula” no where near Acrux.
The source of this error is likely the Index of Double Stars (IDS) of 1963, and is the first reference I could find with this problem. Worst the usage has now become quite universal and the error appears in many many sources and has stayed so for many decades.
Again this highlights many problems with southern stars that have been perpetuated over the years. This interesting problem, in my opinion should be corrected in the WDS in the near future.
γ Cru / Gamma Crucis / Δ124 / Gacrux (12312-5707) at 1.67 magnitude is the 23rd brightest star in the sky, An orange-red colour, Gamma Crucis in the northern part of the Cross and is left with the unfortunate proper name of Gacrux. This both ugly and inelegant name still makes me wince, and was first given it by Elijah H. Burritt in the 1840’s. When viewed with binoculars or a small telescope, the bluish-white 6.39 mag companion (Hartung in AOST1 says white) makes the pair Δ124 AB, and this contrasts nicely with orange-red primary. For some reason, the WSD 2001 gives it a slightly lower 6.45 magnitude, though much of the earlier data present the view that the magnitude was 6.71v. The component is presently a very easy 125.4 arcsec away (1991), and it seems that this distance over time continues to increase by an amazing about 39.2 arcsec per century. From this we can estimate that in 2002, the separation is about 129.7 arcsec, and 130.1 in 2003. Also the PA since Dunlop discovered the pair in PA has also decreased by 17° or about 0.8° per decade. Today’s PA is about 26° (2002).
The brightest star in this multiple is Δ124 A is catalogued as HIP 61084 / PPM 341058 / SAO 240019 / HD 108903. This star displays a significant proper motion of -264.33±0.47mas.yr-1 in Dec and 27.94±0.54mas.yr-1, so it is heading almost due south from its position and roughly towards Acrux. It parallax is given as 37.09±0.67mas, which gives the distance to Gacrux as 26.96±0.49pc. or 87.93±1.60ly. Spectrally the primary is M3.5 III, and the B-V is 1.516, meaning it is a cool giant star. γ Cru A itself is a variable star that has been designated 34G Cru or as NSV 5672, which varies irregularly between 1.60V and 1.67V magnitude. The prism image is noticeable revealing several prominent dark lines that have also been discussed by Hartung in ASOT1 (1968). This the first star I ever looked at with a spectroscope, and I could see the main lines Hartung was talking about immediately - two in the red and orange, two in the yellow and green, a strong green band, and (several lines in the violet. This is a great example of the spectra of stars, and worthy to someone who has not seen this before.
Δ124 B is of spectral class A3V, displaying a B-V of 0.167. Listed as T8654:3421:1 and not in the Hipparcos catalogue, the measures however, are still as telling. All three parameters - pm RA, pmDec and Parallax are well outside the primary’s data. I.e. pmRA; 10.20±2.80mas, pmDec; -15.90±2.10, while the parallax is 4.20±2.80 - suggesting distance of 238pc. or 776ly. There is also significant agreement in the PPM star catalogue, so both these stars are certainly unrelated.
Another star is classed in the pair, and this is Δ124 C. Dunlop discovered this deep yellow star in 1825, but it was not measured until H.C. Russell did so twice in 1879 - and it hasn’t been measured since! At 9.5 magnitude it lies (according to the WDS01 at 155.1 arcsec from the primary along PA 82°. Looking at the Tycho position of some ten years ago (1992), the only catalogue available with this star, the position is now more like ~234° along PA 78°. The Tycho data also gives a significantly different proper motion and parallax; being pmRA; 103.30±13.5, pmDec 23.70±14.5 and the parallax of 19.40±14.00. Although these errors are quite large, they are in different directions, suggesting this star is unrelated to either Δ124 A or Δ124 B. Although the star hasn’t had a spectrum obtained as yet, the B-V of 0.66 tells us it must be either a late G-type star or early K-type star.
So although all three stars are not physically connected, the ‘AB’ pair has a wonderful colour contrast, that is especially attractive with smaller instruments, with the third star adding to the star-spangled field. This is a wide and easy double, easily seen in apertures as low as 5cm, and still colourful in larger apertures.
β Mus / Beta Musca / “Bemusca”/ R 207 / Syd 1-207 / HIP 62322 / SAO252019 (12463-6806) rates somewhere in the Top 10 of southern pairs, and is a “must see” for all southern amateurs. Culminating on the 12th May at 9pm and 27th March at midnight, Beta Musca is the most northerly star of the main trapezium-shaped body of the ‘Southern Fly’, and appears 1.3°N along PA 40°, from α Mus / Alpha Musca (12372-6908). A renown binary system, R 207 was first seen by H.C. Russell at Sydney Observatory mid-evening on the 15th April 1878. In 1882, Russell claimed in “New Double Stars found while measuring Herschel’s Cape Stars in 1882”, that β Musca was, “...this is one of the closest doubles I know.” Measures made during the 1880’s on this star were prolific - twice by Russell himself, and eight times by J.A.Pollock. H.C. Russell was well aware of the significance of this pair, and had the fortune to find R 207 having its fastest orbital motion in the orbit during its closest approach (periastron) sometime in the early 1870’s - later determined from the observed orbital motion as 1872.290 - 10th April 1872. As Herschel had missed this obvious but difficult pair, it was natural to assume that the star had begun to significantly widened since Herschel’s observations.
Magnitudes are 3.6 and 3.9, with the combined magnitude being 3.04, but the “Sixth Orbit Catalog” gives 3.04 and 3.52. PA is presently about 45° (2002), with the separation is c.1.3 arcsec. Presently both stars can be easily seen in 20cm, but remains difficult in anything less than 15cm.
AOST2 says about Beta Muscae;
“...the stars reached a separation of 1.4″ in about 1950 and now seem to be gradually closing again; 12.5cm will separate the stars on a night of good seeing. This is a long-period binary, and a fine object in a starry field.”
The Washington Double Star Catalogue 1996 (WDS96) states in the notes that the period is 383.12 yr., a=1.74 arc sec, while the motion is direct. Component ‘A’ has also been observed to have a variable radial velocity, so it might be also binary itself. The "6th Catalog of Binary Stars" (2002)) gives the orbital elements as;
|Elements :||P = 383.120 yr||T = 1872.29||a = 1.735||e = 0.526||i = 61.30°||ω = 98.32°||Ω = 161.81°|
The most reliable data to date was first published by F. Mourao (1963) and appeared in (Bul.Astr.Obs. R.Bel., 5,143 (1964)). All the orbital elements remain uncertain, as only a quarter of orbit has been observed, and this is all just after periastron - the most critical position for determining orbital elements. I have used the latest data to draw the apparent orbit. Closest approach was sometime in the beginning of 1872.150 then widened until its first maxima were reached in the mid 1960’s. Presently the pair is slowly decreasing in separation, and this will continue until 2 029AD, where the separation reaches 1.25 arcsec - the second minima in the orbit. Again the orbit increases in separation, reaching the greatest apparent separation of 1.715 arc ec in 2 165AD. About the same time, observations of β Mus will increase in precision, and the true period, etc. of the system will be properly known. Over the next hundred years or so, roughly after about 2 165AD, the separation and position angle will change more rapidly. By 2 258AD, the separation again reaches the minimum separation of 0.38 arcsec, making it difficult pair in most amateur telescopes below 30cm, and even larger if the seeing is poor.
Orientation of the true orbit is inclined 8.3° to the east-west line, with ‘P’ and ‘A’ being the date of the respective periastron and apastron of the orbit. (See the Figure Above)
Hipparcos found the distance as 95.41±5.56pc (311.2±18.1 ly) from the Sun, using the measured parallax of 10.48±0.65mas. The results here seem questionable. For example, the parallax measured in the Tycho catalogue gives the distance as 39±4pc (25.60±6.10mas), more the two-and-a-half times larger than Hipparcos. Using the dynamical parallax, the theoretical distance of the system derived from the magnitudes and orbital data, finds the distance of 50.6pc or 164.9 ly. I again calculated this using the ‘full’ Baize-Romani method (first determined in 1945) and obtained 51pc. Such divergence in the values makes the mass and size of the orbit as doubtful. From the available data, I think that the Hipparcos distance is likely too large, and it is certainly much closer than the quoted error. It is possible that the proximity of the two stars might be causing an incorrect measurement. Hipparcos had significant problems in measuring pairs between about 1.5 and 0.2 arcsec due to the configuration of the optical system. β Musca, because of the stars brightness, lies on the boundary of this region. Overall, I think the 51pc. distance is likely closest to the truth. Compared to the Solar System, Beta Muscae’s true orbital dimensions are huge. If the Hipparcos distance of 95.41pc. is correct, then the dimensions of the true, then at the time of periastron passage the two stars are some 42.4 A.U. apart, this increases to the furthest apastron distance of nearly 200 A.U. - five times the orbit of Pluto! If the correct distance is closer at 50.6pc, then the proportion of the orbital size is smaller, dimensioning periastron at 22.3 A.U. and apastron as 104 A.U.
Proper motions suggests the motion of travel is moving further south each century by some -40.40 arcsec in RA and -10.32 arcsec in declination, with Beta Musca is moving way from us at about +42kms-1. Spectral class of both main sequence stars is B2V and B2.5V, implying effective surface temperatures of 24 400K and 20 950K, respectively. The measured B-V for both star’s combined light is -0.178, reflecting its visual blue colour. Absolute magnitudes (Mabs) are -1.3 and -1.0, respectively, making the two stars 275 LL⊚ and 208 LL⊚ times the luminosity of the Sun. Using this data, and applying the results from the dynamical parallax, produces the minimum value for the total combined solar mass of 4.6 ML⊚, and individual masses of 2.33 ML⊚ and 2.28 ML⊚, respectively. Furthermore, the minimum radii of each star are about 3.0 RL⊚ and 2.6 RL⊚, roughly four million kilometres across. Limitations on mass, radius and absolute magnitude using evolution theory place possible value about 30% to 40% higher than the ones quoted above.
In all, this is an interesting system and challenging for those with moderate apertures. Due to the brightness of the two stars, cleanly separating the two is highly dependant on the seeing. Using 20cm (C8) at 225X and 333X, I have observed the pair on more than three or four dozen times, seeing the two Airy disks clearly separated perhaps only 40% of the time. (Incidentally, I have tried without success to observe this pair using the 30cm (11.75-inch) Refractor at Sydney Observatory in 1977 and 1978. This was mainly limited by the seeing, however!)
β Cru / Δ125 / I 362AB / Beta (β) Crucis / Mimosa (12477-5941) is a blue supergiant star and rank as the 20th naked-eye star in the sky having the diameter some thirteen times larger than the Sun. Beta Crucis "A" is a Beta Cepheid-type variable star, some of which appear also in the nearby Jewel Box / NGC 4755 which is about 1°SE of Mimosa. Observation of Beta Cepheid’s are often found to vary in periods anywhere between 0.1 to 0.7 days, displaying only small changes in luminosity by between 0.1 and 0.3 magnitudes. Spectral classes are often found between B0 to B3, and this applies across all the seven main categories of luminosity classes. β Crucis is of B0.5III-IV spectral type. Common bright examples of field Beta Cepheids included nearby to the Jewel Box - δ Crucis λ Crucis and β Centauri (See NSP 16 at Planetary Nebula Homepage),and β Canis Majoris.
Beta Crucis varies between 1.28V and 1.31V magnitude (0.034V in recent estimates) over 0.2365072 days. It also shows seven different harmonic periods combining to producing the main "P0" period. These other periods are;
β Crucis has a nearby companion that is also known as Δ125 AC, is about 2.5'SW or 361.7 arcsec (2002 AD) away along PA 23°. It appears as a deep ruby red of about 8.6 magnitude star EsB-365. Easily seen even in 7.5 cm telescopes, and admired by Sir John Herschel, it truly came to prominence within the Espin Birmingham 1900’s catalogue because of its remarkable spectra. The displayed colour is a testament to both its grandfather-like age and remarkably cool temperature. EsB-365 is technically known as a carbon star, mainly as the spectral lines of Carbon and Nitrogen are quite prominent, while the other common lines are either faint or are not seen at all. The positions have decreased by 12.4 arcsec in 124 years since last measured, while the PA has remanded fixed at 23°. This 10.0 arcsec per century suggest that in the sky both stars will join together in the year 4 550 AD. About the year 4 050 AD and 5 050AD, this will become one of the best colour contrasting pairs in the entire sky. It is still almost certain that these two stars are not associated.
A second companion is I 362 AB. which is another white star of 11.4 magnitude and extends about 44.3 arcseconds along PA 322° from the primary.The positions of the primary and this star have not moved since Innes discovery in 1901.
PLEASE NOTE: The name for MIMOSA lately has the name BECRUX now used in modern star atlases and software, including Astronomy 1999. This was introduced in 1993 by American observers. In no circumstances should this name be used nor promoted!
Comment: Nearby is the Jewel Box / NGC 4755. This magnificent open star cluster, the was discovered by James Dunlop (Δ301) that John Herschel named the Jewel Box NGC 4755, which is both east, and slightly south of β Crucis. NGC 4755 is a small “A” shaped asterism and is different than most open cluster because of its 6′ size and numerous bright stars. This is a nice cluster in a starry field whose distance is some 2.4 kpc. (See NGC 4755 Text on this Homepage.)
μ Cru / Δ126 / Mu Crucis (12546-5711) is located 2.7°N, and a little east, of β Cru. This broad, bright bluish pair has no common name. (Mucrux might be a suitable name.) Discovered by Dunlop in 1826, and listed as Δ126 in his catalogue, this beautiful blue duo is located in a star-studded field. It is certainly easy to resolve in small telescopes. The magnitudes are 4.03 and 5.10, while the separation is a wide 34.7″ at PA 17°. The spectral class are B2IIV-V and B5Vne, respectively. Looking at the Hipparcos data some credence adds that this pair is likely attached, as the proper motions and distances are quite similar. I.e. Distances are μ1 = 116±7pc (or 361±24ly.) and μ2 = 111±7pc (or 381±25ly.), Common proper motions; π1 = RA:-30.45±0.37mas and Dec:-13.55±0.41mas. And μ1 = RA:-32.45±0.37mas and Dec:-10.93±0.40mas. This gives the mean distance to μ Cru as 113.5pc or 370ly. If this is so, then the two stars are separated in space by some 0.0192pc or 0.063ly. suggesting the period maybe as long 30 000 years. The second star, μ2 Crucis, is Gamma Cassiopeia-type (GCAS) variable, changing by 0.19 mag in an unknown period. Little has changed positions since discovery, and the twelve other observations have shown little additional data. This pair is a good test for 7x50’s binoculars that can just separate the duo, but likely during poor seeing, this might be a bit more difficult.
Δ127 (12598-5555) is one of the nicest pairs I have seen and I have taken great delight in observing this pair numerous times in the last thirty odd years of double star observing. Puzzlingly missed in both the AOST’s and most other noted deep-sky descriptions, like because it is overshadowed by the brighter gems in nearby Crux and Centaurus. This pair cqan be found in mid-southern Centaurus and the north-eastern corner of Crux. It is easily located some 1.4°ENE (PA 61°) of the magnificent Mu Crucis (Δ125). Dunlop discovered the pair sometime in 1825, although his first measure appears as 1826 in the WDS01. Δ127 appears as two distinct blue stars of 8.27v and 8.98v magnitude and is presently aligned along PA 126°. Easily seen in 7.5cm, the separation has slowly widened from 13.0 arcsec to its present 16.8 arcsec (2002). Dunlop listed the pair as “Anonym.” and gave the 1827 position as RA: 12h 49m 00s Dec: -54° 57', (1827) precessed to 12h 59.2m -55° 53' (2000), and identifies the 7th and 8th magnitude star about 5'W of the positions given for Δ127; in the WDS01. Although Dunlop’s separation is given as 10 arcsec, it is likely that this is an error by him, as the measures since then make it impossible for it to have been this close. The WDS01 seems to be aligning itself to use the Herschel measures that the observation was made in 1835 for the separation and using Dunlop’s 1826 PA of 51°sf. (129°). Is this really an error in the WDS01?
Looking at the general observational data, Δ127 A is HIP 63422/ PPM 341501/ SAO 240455, which Hipparcos gives as the 8.27 mag. as stated above. Also the parallax is 13.54±3.02, the spectral class is B7IV/V and the B-V is 0.040. Δ127 B is HIP 63424 / PPM 341502 / SAO 240457 and gives the fainter magnitude of 8.98. Parallax is stated as 14.15±7.02mas, suggesting the mean distance of 70.7pc. Dun 127 B’s spectral class of B9 matches near to the B-V of 0.110.
Also from these data, both the parallaxes and proper motions seem very similar, and we can conclude that these two stars have a very high probability of being gravitationally attached. The Tycho data matches the Hipparcos data quite well except for the cpm which are significantly different. Tycho gives the magnitudes as 8.21 and 8.90, while the PPM says 8.5 and 9.1. Overall the Δm;’s are 0.71, 0.69 and 0.6, and although not listed in the WDS02’s Deltam catalogue, the mean of the Hipparcos-Tycho value of 0.70 seems about right. The Guide Star Catalogue lists the primary twice as GSC 8652:1908 and GSC 8652:192. It is likely the measuring machine has read the individual stars once as the ‘A’ component and again as the combined ‘AB’ component.
Overall, Δ127 is contained in a field of many 10th and 11th magnitude stars, which I thought seem to be winding NE to SW across the field. This is a lovely wide pair.
I have identified four faint pairs in the field using the 20cm where the limiting magnitude was about 13.8. These pairs are not listed as pairs, as such, but they can be seen in the 18'x18' STScI image surrounding Δ127. (See Attached Image)
Δ127 Pair 1 (12599-5548) is a north-south 11th and 12th magnitude pair (PA of 6°) and 18 arcsec apart. Both have the respective magnitudes of 11.3 and 12.6 in the GSC matching fairly well with my estimates. Pair 1’s position can be identified easily because it is in the same field about 7.4'N of Δ127.
Δ127 Pair 2 (12594-5550) appears 6.0'NW (PA 325°) of Δ127. I estimated the magnitudes as 11th and 13th and the separation of around 15 arcsec. Using the GSC star data, the magnitude of the primary is GSC 8652:263 at 12.4 magnitude while the secondary star GSC 8652:1538 is 13.3 magnitude. Positional differences is about 17.7 arcsec along PA 80°. It can be identified as the pair is the most eastern of a brood of nine 10th to 12th magnitude stars and also being closest to my Pair 1. I saw both stars displaying just a hint yellowish and slightly dingy-red.
Δ127 Pair 3 (12596-5558) is the second brightest in my listing and lies 4.0'SSW of Δ127. The primary is 11th (GSC says 11.2) magnitude and the companion is 12th (GSC says 12.8, though I estimate it is slightly brighter than this. Pair 3 is 19.6 arcsec apart along PA 246° (WSW). The bright star is GSC 8652:581 and I thought it a tad bluish. I saw no colour in its “companion”.
Δ127 Pair 4 (13007-5603) lies some 11'SE (PA 137°) from Δ127 and is the brightest in my listing. Pair 4 A is GSC 8652:116 and is given as a 9.9 magnitude star - the most northerly star of a 2' triangle of c.9th magnitude stars. The Pair 4 B companion is listed as a 12.6 magnitude "Non-Star" even though it appears on the STScI image. I thought it was slightly brighter than this, and certainly brighter than seen in the image. I calculated the distance apart as 15.7 arcsec along PA 314° (NW). This seems about right.
Δ127 Pair 5 (13009-5550) is another pair I first noticed on the STScI image. The GSC gives this as an 11.0 magnitude "Non-Star" though it is clearly visible. I estimate the companion must be at least 13.5 magnitude and the separation must be about 5" to 6"arcsec aligned toward PA 260°. I haven’t tried to observe this star, but it should be visible in 20cm, and definitely in 25cm or 30cm.
Several other pairs are in this field, though they are of little interest as they are a little too wide. In the 26mm Plössel with 20cm appears at the SW edge of the field (some 18' away from Δ127) is a obvious wide unlisted 9.7 and 11.1 magnitude pair separated by about 36″ around PA 45°. The primary is deep-yellow in colour and its companion seems white.
1. STScI IMAGE :
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|“Southern Astronomical Delights”
10 Mar 2009