Southern Doubles, Stars and Variables
SOUTHERN ASTRONOMICAL DELIGHTS : Presented by Andrew James
|PAGE DS 020
10 Mar 2009
|RA : 20h
Dec : -30° to -90°
Constellations : Sgr, Mic, Tel, Ind, Pav, Oct.
Best Observed : Jul - Nov (Text Ordered by RA)
|RA : 20h
Dec : -30° to -90°
SELECTED SOUTHERN DOUBLES and VARIABLES
It is also noted that the WDS96 also oddly gives the magnitudes as 7.2v and 7.4v when the stars obviously seem fainter and as seen in all the other sources. These values are closer to the component’s combined magnitude, whose average here agrees well with the calculated 7.3 magnitude. However the proper motions (pmRA, pmDec.; A:+039,-087 B: +034-099) are consistent with the pair being attached. If they are attached, the period is likely to be long. This is a pretty double star in a fairly uninteresting field. Spectral class is F6/8+F.
Based on the mean parallax of 9.70±6.6, whose errors and size agrees well for each individual stars, the distance is about 102pc. Rough calculation on the parameters of this system infer that the minimum separation must be more than 700 A.U. (1.1x1010 km.) giving an orbital period more than 10 800 years. Absolute magnitudes (Mv) are calculated as +2.8 and +2.9. Assuming the calculated mass sum (ΣM⊚) is 3.1 M⊚, then masses for the two stars are roughly 1.6 M⊚ and 1.5 M⊚ (or at least in that proportion.)
I 411 (20161-6210) lies either 5.9° (PA 319°) from β Pav or 4.0° (PA 10°) from yellow 3.6 magnitude, δ Pav. Discovered by Innes in 1901, the separation of 0.8 to 0.9 arcsec has changed little but the position angle shows a slow decrease in 282° to 274° (1991) - a difference of -8°.
Listed as 8.2 and 9.4v (8.57V/ 9.79V) this light yellow pair is an interesting but tough, and is visible in 20cm. I have tried to separate the two cleanly in 20cm (C8) at 333x and 400x. but found he stars only elongated in moderate seeing. Under better conditions I believe I could see these two stars with at least a small gap.
Hipparcos measured the 1991 the position angle as 274° with the accurate separation of 0.859±0.004 arcsec. Presently the proper motions stated in the WDS03 [A:+048-076, B:+048-076. Hipparcos gives 44.15±1.20 and -75.08±1.19 mas.yr-1. The c.p.m. being 87.1mas/yr-1 along PA 150°] strongly suggest these stars are both gravitational bound in a true binary system. Spectral class of G0V also matches the visual colours.
Using the obtained parallax of 3.81±1.52 mas, the distance calculates to 262±124pc (856±106ly.) The minimum for the true separation of about 223AU (3.3x1010 km) giving a period c.1 700 years. Based on the measured ‘V' magnitudes, the estimated ΣM⊚ is 3.9 and divides roughly into 2.2M⊚ and 1.7M⊚. These figures are fine, but the spectral class does not match well with the absolute magnitudes (Mv) of +1.4 and +2.7. (This becomes worst when using the ‘v’ magnitudes of 8.2 and 9.4.) For all solar-like stars the Mv’s should be about +4. Furthermore, the colour and spectral class is further matched by the Hipparcos measured the B-V of 0.621±0.015. It is likely that the parallax is wrong here.
If the value of c.12mas is adopted for the parallax will correct for this discrepancy. If this latter result is true, then the minimum period reduced to 400yr. (66 AU apart), the ΣM⊚ is 2.3 and divides roughly into 1.2M⊚ and 0.9M⊚. Here the Mv’s will be 4.0 and 5.1, respectively. The problem with I 411 is that Hipparcos photometrically measured for the single star as just HIP 99896 / SAO254761 / PPM 364498. Although resolved, the parallax of the singular value will be prone to some greater error than if estimated for the two stars as individuals. This will be an interesting pair to watch is the ensuingcenturies.
"I 411-Pair 1" (20188-6154) - When coming across I 411 I also noticed another very wide pair that lies in the same field some 25'NE (PA 51°). What surprised me was the colour contrast between the light blue or bluish star and a deep yellow companion - that in 20cm at 50x looked quite startling. When I looked for this as a pair in the WDS, I could not find any reference. To me it looks like some lost wide Dunlop or John Herschel double. Magnitudes are 6.6 and 8.7, with the separation is 108 arcsec (1.8') along the eastern PA of 83°. Spectral classes are A0V and G8III.
It is unlikely that these stars are attached as the parallaxes are different (8.30mas against 1.62mas) Looking at the proper motions, the brighter ‘primary’ is moving almost due south relative to the ‘secondary’. Looking at the general motion the pair, as seen in Dunlop's or Herschel's time, would have found the PA to be about 90° and perhaps 104 arcsec apart. This would certainly fall within their double star criteria.
“I 411-Pair 1” would be easy in any small telescope - even though such telescopes would be unlikely to be able resolve I 411.
"I 411-Pair 2" (20144-6216) is some 13.5'WSW (PA 240°) from I 424 is the reddish (6.74V) magnitude HIP 99758 / SAO 254758 / PPM 364486. B-V is +1.738 and the Spectral class is M3S. It also has a wide 9.6 magnitude red companion [B-V: +1.771] T 9090:753:1, which contrasts oddly with this reddish ‘primary’. Separation is 103 arcsec (1.4') with the PA being 307°. It is likely these two are not attached as the merge proper motion regarding these stars are quite different. In all though, the reddish duo certainly adds to this magical field.
Note: Another 13'S of "I 411 -Pair 2" is the orange 7.1 magnitude HIP 99723 /SAO 254755 /PPM 364484 Mag(v): 7.13 B-V: +1.109 Spect: K0III.
Δ231 (20366-7104) is stated by Dunlop to be Bode star 74 Pavonis. This wide blueish-white and light yellow pair of 7.1 and 8.9 magnitude (6.83V / 8.86V) is separated by 49.0 arcsec along PA 286°. Since discovery in 1825 or 1826 by Dunlop, the width of the two stars continues to diminish from 64.4 to 49.8 arcsec or ΔSep of -14.1° (1991) at an approximate rate of 0.85 arcsec per decade. A position angle of 286° shows a change of only -2° in the same period. 3.3°ENE (PA 61°) from 4.0 mag. ε Pav / Epsilon Pav or 4.9°S PA189° from β Pav. Both stars have differing proper motions and parallaxes making this certainly an optical pair. Easy even in 5cm.
If the motions continue to move in a linear fashion, then the star will be closest in about 2550A.D.
Two brightish stars appear in the same field. Some 10'SW (PA228°) is the orange 6.4 magnitude star, HIP 101570. The second is 7.1 magnitude star is HIP 101894 5.9'ESE and near the galaxy IC 5024.
IC 5024 / ESO 74-3 / PGC 65160 (20402-7106) lies in the same field as Δ231 some 17'E (PA 98°). The galaxy is listed as 14.9 magnitude and appears as a small wisp of nebulosity some 45"x40" looking like a smallish PNe. Catalogues list it as barred spiral of the SB(s)c: type though nothing like this is seen in the telescope. IC 5024 is visible in 20cm or 25cm, though it is not easy with the proximity of another 7.1 magnitude star, HIP 101894 / SAO 257834 / PPM 374734 (20389-7105), just 5.9' away to the ESE. This is a orangery-red M3III star contrasts nicely with the pair and the galaxy. 5.4'SSE of HIP 101894 is another faint galaxy that is listed as 16.0 magnitude. This is MAC 2039-7111 whose size is given in the Mitchell Galaxy Catalogue as 0.9'x0.7' along PA 145°. I have never seen it but larger apertures, perhaps 40cm and above, should see this as a faint haze.
R 323 (20412-6242). This is a faint yellow Russell pair in Pavo that was discovered by him in 1880. It can be found some 3.6° (PA 352°) from 3.4 magnitude Beta Pavonis or 1.2°W (PA 258°) away from RMK 26 (20516-6226). The general field is easily identified because there is another orange 6.2 magnitude star (HIP 101965 / SAO 254844 / PPM 364703)some 15.4' (PA 215°) to the southwest.
H.C. Russell discovered this pair in the early evening of the 7th September (1872.680) and did not measure the pair properly until the early morning (2pm) on the 13th August (1880.612) when R323 was the meridian. On this night the measured the mean position angle as 321.564° (321.5°) and a mean separation of 2.72 arcsec, giving both stars as “9,10”.
Magnitudes are listed as (9.54V and 10.21V) this yellowish and white pair. Observations give the separation as 2.9 arcsec along PA 318°, which has increased since discovery by +5° and +0.2 arcsec. Little is known about the pair though it is likely these stars might be connected, as past observers have measured the system some twelve times over the years.
Little information is known about R 323, as the true parallax and proper motions have so far proved fairly elusive. From the motion so far exhibited in the measures continues this connection will likely to become known in the next century or two. R 323 can be seen in small apertures, but would likely favour any telescope larger than 10.5cm. Spectral class is F5V.
Alpha (α) Cygni / Deneb / Arided / 50 Cyg / H 73 (20414+4517) appears as the brightest star in the ‘Northern Cross’ of Cygnus has Deneb ends its tail. Rising omly to a maximum height of only about 10° from the skyline of Sydney, Australia. The ancient natural philosopher Eratosthenes termed the constellation as a hen, though it was Aratos that named it as a “quickly flying swan”. Deneb’s name is a corruption of the Arabic name Al Dhanab, which literally means the “hen's tail”. The German observer, Rosemund in the 15th Century named it Uropygium - meaning the ‘Pope’s nose’ or ‘Parson’s nose’ after the part of the roast chicken eaten at the dinner table - depending if your religion was either Catholic or Protestant! This shimmering pure white star is the 19th brightest in the nighttime sky - having the visual magnitude of 1.25. In true brightness it is extremely luminous. The liberate energy is more than a million times that of the Sun and is only outshone by a few stars - second only to Rigel. For many years it distance was given as 1 500ly., though the latest data suggests it maybe as close as 1 000 ly.
Deneb is also the wide pair H 73 which was discovered by William Herschel. Magnitudes are 1.2 and 11.7, respectively, while the separation is a wide 75.4 arcsec along PA 106°. Both stars have remained relatively fixed since discovery, and it is likely that this star is merely just in the field.
Deneb is the brightest example of the ACYG’s and this is followed by others examples like Mu Normae, Mu Sagittarii, Epsilon (2) Orionis and Omicron (2) Centauri. All variables in the category are true monsters, that some have referred not as supergiants but hypergiants. (All are luminosity class Ia.) These variable supergiants have spectral types between B0.5 and A3, and all display characteristic emission spectra. They are among the brightest stars in the galaxy, whose absolute magnitudes commonly exceed -7.5! Some fifty-two are known (1998), with another nine being suspected ACYG’s. Primary periods for the very poorly observed light curves range between 8 days and 46 days, though the longest suspected period may be last about 180 days.
This first-magnitude star has the mass somwhere between 12 M⊚ to 14 M⊚ - being about half the value of the earliest estimates from the 1960’s. Like most heavyweights, the general stellar parameters are equally mind-boggling. Its diameter seventy times the diameter of the Sun (14 R⊚), equivalent to 97 million kilometres and is slightly smaller than the distance between Venus and the Sun! Deneb’s effective temperature is 9 160K, while the -8 absolute magnitude is equivalent to a candle-power several million times more luminous than our Sun! The spectral class is A2Iae, whose ‘supergiant’ luminosity class was first decided by Keenan and Morgan in 1951. Closest in kinship is Orion’s first-magnitude blue Rigel, which is slightly less luminous but higher surface temperature. The spectral class of Rigel being B8I.
Deneb varies between 1.21 and 1.29 magnitudes over numerous semi-periods averaging some 11.7 days. Some have even considered this star as the prototype example of the ACYG Class. Remarkably, other changes have been observed with Deneb since its variability was discovered in 1887. This includes a slow and continuous rise in the brightness of the Hydrogen-Epsilon (Hε) line when compared to the nearby Calcium II line some 0.16nm away. This particular observation also appears in "Burnham’s Celestial Handbook"; Vol.2 , p.757, and much of the discussion on this page by Burnham is taken from the paper by Liller, M.H. and Liller, W., “Spectral Changes in Deneb, 1887-1964.”; AJ.,142, p.1028-1032 (1964) It seems that this trend is continuing, and a more recent detailed discussion on this can be found in Albayak B."A Spectral Analysis of Deneb (A2 Iab)"; PASP, 111, 917 (1999)) Deneb, like V370 Car, maybe illuminating the majority of the nearby nebulae. In Deneb’s case, this is the grand NGC 7000 "North American Nebula" (20580:+4429:), IC 5067-70 (20510:+4400:) some 3.5°E, and the IC 5068 Complex (20503+4231) 2.8°SW.
Our view of supergiants is pictured as just the scaled-up version of ordinary stars, but this view is in someway flawed. In fact, all supergiants are quite unstable and nearly all of them display either variations in brightness, radial velocity or colour. As early as 1951, Buscombe, and later Groth in 1960, established the existence of significant atmospheric motions, which Huang and Struve (1960) interpreted as solar like prominences appearing from Deneb’s photosphere. To explain these variations remains uncertain, but more recent explanations for the pulsation mechanism are likely to be strong shock waves traversing across the stellar photosphere. What we maybe observing are ‘starquakes’ rumbling across the surface.
This picture is quite different to the solar photosphere, which is more or less a surface more akin to a saucepan of boiling water, with the solar wind radiating outward from the surface. Lucy, L.B. (A.J., 206, 499 (1976)) was first to suggest the supergiant’s variability could originate from a system of “multiple non-radial pulsations” One prediction was that at least sixteen separate components were hidden in Deneb’s spectrum, causing the surface to bob up and down in amplitudes anywhere between the velocity of ±0.29 and ±1.02 kms-1 - something akin to a cork bobbing up and down in the ‘ocean’ of the star’s photosphere. Furthermore, the transverse waves are travelling about 15 kms-1 (54 000 kph) across Deneb’s surface, faster in fact than the local velocity of sound of 13.7kms-1. So violent are the shock waves, that significant mass loss is being induced away from the star.
Astronomers biggest woes are that all ACYG’s are uniformly quite distant, with Deneb likely being the closest at 490pc. (1 600 ly). Recent studies of GP Vel and Mu Sgr have been investigated because they are eclipsing binaries, allowing improved estimations of mass, but overall, the available data remains poor.
The most extraordinary thing with these stars is that we maybe seeing real stellar evolution in action.
“One of the more intriguing results [Hayashi and Cameron (1962). that during the late stages of helium-burning the star moved rapidly [moves] across the Hertzsprung gap, changing from early type A to M in some 20 000 years, averaging [a loss of] 0.3K per year. It is entirely possible the examination of spectrograms...over a long time, evolutionary changes could be detected...”
“Perhaps the most attractive hypothesis is that, like the Sun, Deneb has a definite cycle of activity with the period of at least one hundred years and, more probably, several hundred years. To differentiate between these... will require many more years’ of observations... [the] B-V colour changes at a rate of 0.3K. It is entirely possible produce detectable changes in ten or twenty years.”
Other ACYG variables are V369 Car, which is some 45'NW from the second, V370 Car that is 20'E from another bright open cluster NGC 3293. Together these three star are in fact highly luminous and are interesting in that they are associated with prominent nebula or open clusters.
Note 1: much of this section is adapted from the forthcoming article “White Monster Variables - Deneb, V370 and V369 Carinae” which is yet unpublished.
Note 2 : NGC 3324 contains the 5.5 magnitude variable V370 Car, which appears in the in the article ‘The Jewel Box’. It is the central star that is illuminating of the nebulosity surrounding the cluster of NGC 3324. (See “Other Clusters” NGC 3324)
Δ232 / μ2 Oct / Mu (2) Octanis (20417-7521) is a deep yellow and yellow pair that lies some 49' due south of the slightly brighter Mu (2) Oct. Interestingly Dunlop calls it “anonym” and does not identify the star Mu (2) Octanis. Furthermore, Mu (1)
DUN 232 is one of only two pairs in Octans in AOST1, which Hartung says;
“...but there seems to be slow direct motion and the stars maybe physically connected. It is an easy object, and the more northerly of the stars constituting Mu Octanis.”
Little, J. L; "Ten O'clock High"; Southern Sky; Sep/Oct 1994 p.40, says of this pair;
“...a pleasant little double available to 6cm 'scopes. Both stars are tinged yellow...a fairly wide double...so use low magnification to keep them together.”
Δ232’s magnitudes are 7.1v and 7.6v (6.51V, 7.07V) and the available Delta-m data by Pickering (1912) and Wallenquist (1948) finds the visual photometric difference of 0.51 magnitude. Separation is 16.8 arcsec along position angle 17° (1992). Since discovery in 1826 the system had slowly decrease from 18.5 arcsec while the position angle show prograde motion from 14° to 17°. There is some significant proper motions (A: +156,-162, B: +165,-172) show that these two yellow stars are very likely connected. Spectral class is G5III + G5. This pair will be interesting to watch in the coming decades.
ESO 47-7 / Fair 552 / PGC 65218 (20421-7516) is some 5'NNE is a very small and faint 15.5 magnitude galaxy covering about 20 x18 arcsec, even though the catalogued size is more like 0.9'x0.7'. Little is known about this galaxy, and it is difficult to see in apertures below 40cm to 50cm because of the proximity of the Dunlop pair. An occultation bar here would certainly be an advantage. It can be readily identified, however, as there is a faint duo of 11th and 13th stars some 5 arcsec apart that is not listed as any known pair. Little is known about the galaxy itself, though it is thought to be a barred spiral and exhibits a relatively high velocity of 15 400 kms-1. If this distance is true, then ESO 47-7 is far more distance than the regular galaxies that amateurs frequently observe. Using a Hubble Constant of 65kps-1.Mpc-1, the distance calculates out as 236Mpc or 770 million light-years.
β Pav / Beta (β) Pavonis / HIP 102395 / SAO 254862 / PPM 364757 (20450-6612) is a rich blue 3.4 magnitude star located in western Pavo and is roughly half-way between 4.2 magnitude γ Pav and δ Pav. Hipparcos measured the parallax (π) as 23.71±0.63 mas, calculating the true distance of 42.17±1.12pc. or 137.6±3.66ly. Based on the measured parameters of spectral class (A5IV) and brightness (3.42v and the B-V of +0.163), the absolute magnitude (Mv) of β Pav is +0.27 - some 70 times the luminosity of the Sun. Surface temperature is 8 170K while the derived diameter is some 5.8 million kilometres or 8.4 times the solar diameter. From the general measured motions, the common proper motion (cpm) is 43.68 mas.yr-1 and moving relative to the fixed stars WNW along position angle 346°.
RMK 26 (20516-6226) is a two bright and fabulous white jewels that is one of the best in the southern skies. Listed as 5.7v and 6.0v (6.23V and 6.58V) magnitude, these two great telescopic gems can be located in eastern Pavo, in a fairly lonely part of sky, RMK 26 appears to the naked-eye as a 5.1 magnitude star some 3.9°N (PA 11°) of Beta Pavonis. E.J. Hartung in AOST1 describes it both as a “fine pair” and “...an attractive pair for small apertures;” and I certainly concur. RMK 26 is visible in apertures as small as 7.5cm using medium magnification, however it is certainly more breathtaking in 15cm or 20cm when you can see some clear space between the two stars.
This was among the first ten double star I every observed and was one of the closest my first target list. Using 76mm f/11 reflector, under reasonable seeing conditions, in 1972, I remember ‘pumping’ the telescope magnification (150x) and clearly splitting the duo quite cleanly. I also remember how thrilled I became in seeing these two components with my own eyes. In subsequent years I have observed this same pair on several dozen occasions. Using apertures of two different 10.5cm, 12.5cm, 15cm, two 20cm telescopes and 30cm; and I have become quite familiar with this lovely pair - but for some reason it was never as good as the first time I saw split it. The last time I saw it was using 10.5cm Maksutov on the night of the 30th October 2003, under good seeing and transparency, I could visually tell that there was no difference in the either the colours nor the relative positions.
When Rümker found this pair in 1835 his measures gave the separation of 2.7 arsec along position angle 101°. Some 57 measures till 1993 has shown the position angle is slowly decreasing (101° to 83°; PA=18°) while the separation continues to slowly diminish from about 2.7-2.8 arcsec to 2.4 arceds. Both stars have similar proper motions that suggesting the these are gravitationally attached.
Using the available data, Hipparcos find the combined parallax of 13.67±1.16mas suggesting the distance of 73.15±6.25pc (238.6±20.4ly.) Using the given magnitudes, separation and assuming that this is the true distance of the duo, it can be calculated the separation between the two stars is about 175 A.U. (2.1x1010km.) Combined mass (ΣM⊚ is 4.4, dividing by the absolutes magnitudes (Mv= +1.7 and 1.8, respectively) for the given spectral and luminosity class (A2-3IV-V), gives the individual masses as 2.3M⊚ and 2.1M⊚. A minimum period for this system is more than 10 000 years. As Hartung does suggest in AOST2, this is “...a long period binary”.
After calculating these very general parameters for the two components the mind begins (just!) to understand the physical nature of double stars and the reality of binary star motion. It is interesting to sit back and think of the fixed stars, which in our human lifespan, and in the vast majority of cases, is exactly how the stars appear. Yet after a few centuries (perhaps four or five generations) we have come to realise that the motion among the double stars are real. From the careful measurement of the position of these stars - by dedicated people who have long left this world behind, we can piece together the individual stories of these alien stellar worlds.
The mind wanders into imagining; What the sky would be like from an Earth-like planet orbiting one of these suns? How wonderful would it be to see you own sun producing night and day but to also see a slow moving star, brighter than the our own Moon, transfixed into one part of the sky. In the centuries that would have to pass, this bright companion star would slowly move against the background, revealing to the people inhabiting our Earth-like planet that this was part of your world and not just one among the myriads of other stars in the local neighbourhood and the Milky Way.
A truly nice pair that is not to be missed!
U Pav (20555-6243) is a Mira variable that lies 32'SE (PA 134°) from RMK 26. U Pav’s magnitudes vary between 8.6V to less than 12th over the period of 289.7 days. (Epoch: 2417139). Knowledge about the general parameters for the light changes is quite poor. This reddish star of spectral class M4e.
|“Southern Astronomical Delights”
10 Mar 2009