Southern Doubles, Stars and Variables
SOUTHERN ASTRONOMICAL DELIGHTS : Presented by Andrew James
|PAGE DS 001
10 Mar 2009
|RA : 01h
Dec : -30° to -90°
Constellations : Scl, For, Phe, Eri, Tuc, Hyi, Oct
Best Observed : Sep - Jan (Text Ordered by RA)
|RA : 01h
Dec : -30° to -90°
SELECTED SOUTHERN DOUBLES and VARIABLES
ζ Phe / Zeta Phoenicis /
RMK 2 & RST 1205 / HIP 5348 / SAO 232306 / HD 6882
John Herschel once described this visual system as “a very beautiful system, in a starry field”, and it is difficult not to disagree with him. AOST2 describes;
“This beautiful white pair ornaments [in] a field with a few stars.”
H.C. Russell’s Identification
Fig. 1 RMK2 AB-C :
The primary is the pair Zeta Phe Aa but is listed only as Zeta Phe in the GVSC4 - without any normal designations usually given to variable stars. It is the brighter A component in this pair that is the eclipsing variable / spectroscopic binary.
Observations for this detached (EA / DM) eclipsing binary are difficult to make because of the proximity to the ‘B’ star. Magnitudes vary between 3.92 V and 4.42 V, over the period of 1.6697671 days equal to 01 day 16 hours 04.5 min (JDE 2441643.689). It rises in brightness in 0.20037 days - almost exactly four hours. Both are main sequence star with the given spectral classifications of B6V and B9V.
Little is known about this eclipsing binary and the system’s physical data is a little scant. Hartung’s AOST2 states that the eclipsing binary has the total mass of about 9M⊚, which is close to the most recent calculations placed on the total mass. Individual masses are respectively, 6M⊚ and 3M⊚ suns.
Distance has been previously estimated to be about 67pc or 218 ly from the Sun. Hipparcos gives 11.66±0.77mas for the parallax implying a distance of 85.76±5.81pc or 279.8±19.0ly. The system is receding from us at +18kms-1.
The wider pair, of the pair RMK 2 (AB-C system) in the multiple is likely physically associated. All the visual stars appear bluish, white and white.
κ Tuc / Kappa Tucanae / HJ 3423 (01158-6853) is a lovely colour contrasting pair some 4°N of the Small Magellanic Cloud and 2°ENE of Lambda Tuc (See Δ2) Innes says the stars are ‘yellow, purple or bluish’;, yet if you read both AOST1 and 2, the pair’s colours are described as “yellow and orange.” I very much fooled for some time with these colours and I could not figure out who was describing what. Hartung in AOST is referring to the wide pair I.e. HJ 3423AB and the companion pair I 27CD than just HJ 3432AB alone. The text in both of these editions is slightly ambiguous. Innes is comparing the colour of HJ 3432AB - the pair Kappa itself. AOST says the main pair is “...beautiful in a moonlit field” I tried this and have no disagreement! It is certainly very attractive.
Δ3 / R Scl / R Sculptoris (01270-3233) is a remarkable object that was discovered by Benjamin Apthorp Gould in 1878 and which Innes later knew of the variability of this star. Although not even really a optical double star and therefore is not listed in the WDS 01. Dunlop says in (Ref.1) of this star;
“A very singular star of the 7th magnitude, of an uncommon red purple colour, very dusky and ill-defined; 3 obs(ervations) on this star; a small star preceding, and another following.”
Dunlop’s position when precession is taken into account find the co-ordinates as 01h 27m 41s -32° 37'. This is certainly the ‘star’ Dunlop was referring too with today’s position merely 10.2'N (PA347.6°).
Dunlop’s paper refers to a “small star preceding” (west) and this is 9.4 magnitude HIP6719/ SAO193317/ HD8833/ PPM277033 (01265-3233) that lies some 6.4' away at PA 263°. Spectral class of this yellow star is F8. Another star following is obvious deep yellow 10.2 magnitude PPM277055/ T7002:1474:1 (01277-3232). The Tycho-2 data gives the parallax as 65.70±24.90mas. An unconfirmed double star of R Scl, and near Δ3’s position, is LDS 2199 in Sculptor, which would not have been seen by Dunlop. Although this star is separated in 1955 by 10 arcsec along PA 234°, this 8.0 and 19.0 magnitude pair is of little interest to the amateur.
R Scl is presently the 14th brightest SRB type variable star known which varies between 9.1p to 12.9p over some 370 days. (363.1days in some texts) The spectra of the Carbon star is C6,5 EA(NP). Burnham’s describes it in the older type of as N3p. R Scl also appears in the RASNZ’s variable star map No.70.
R Scl is also listed by Brian Skiff. “A Revised Catalogue of the Espin-Birmingham Red Stars” (1998) as EsB 26, with the following details.
EB 26* / HD 8879 / R Scl
RA: 01h 26m 58.0s Dec:-32° 32′ 36″
5.8 / 3.9 / C6,5
EB 26 / HR 423
Spec also C5,4 and C6,4. photom also V = 6.4 / B-V = 4.8 (ApJ,167,521).
1. The astronomer Benjamin Apthorp Gould (1824-1896) has an interesting history. Gould was the first American to receive a PhD in astronomy and the founder of The Astronomical Journal, which printed its first edition in 1849.
I 714 (01362-5731) is a faint pair in the same field as the 1st magnitude Achernar some 21'SW (PA 214°). Magnitudes are 9.6 and 9.8v (10.15V and 10.20V) with the separation at last measure (1991) was 1.2 arcsec along position angle 17°. Since discovery by Innes in 1912, the two stars continue to show prograde motion that has continued to decreased from 30°. Separation, however, has stayed relatively fixed. I 714 looks certain to be gravitationally attached as they share the sane proper motions. The glare of Achernar in the field does make this seem more difficult than it really is, but it is easily visible in 15cm, and likely in 10.5cm will care. Spectral class is given as G6V.
α Eri / Achernar / Alpha
Eridani (01377-5714) is the 9th brightest star in the sky and is
an obvious vivid bluish or bluish-white star (B-V of -0.158) that
lies at the very end of the constellation of Eridanus - The River,
being placed merely about 40'N of the northern Hydrus border. At
+0.45 visual magnitude it is one of the most prominent of the
southern first-magnitude stars and remains circumpolar from latitudes
south of about Sydney, Australia (-34°) and remains invisible
for many northern observers. Furthermore it can be used, whatever the
time of night, to find with the naked-eye the South Celestial Pole -
whose position is marked by the mid-point of an imaginary line
between Beta Centauri and Achernar itself - and during November (29th
at 9pm), Achernar from Sydney reaches its culmination some 24°
south of the local zenith.
From antiquity, Eridanus is said to have been two separate river tributaries which could be probably termed Eridanus Borealis and Eridanus Australis. The northern stream flows east-west from Cetus to the star Cursa, ending quite near the bright variable star ο Cet / Mira / Omicron Ceti / 68 Cet / HIP 10826 (02193-0259). The second stream meanders both southerly and eastwards from Cetus, ending with 2.9 magnitude starθ Eri / Theta Eridani / Acamar / HIP 13847 (02593-1018) - the ‘old’ Achernar. It was much later did the constellation become extended into the modern Eridanus by adding another 17° further south. Perhaps this was when the newly aware southern explorers became familiar to Achernar’s existence, who then immediately placed this star to the constellation. In the last eight thousand years, Alpha Eridani remains completely below the horizon from all areas within Europe, North America and the Middle-East.
ALPHA ERIDANI /
NAME 1: HIP 7588
NAME 2: SAO 232481
NAME 3: PPM 331199
NAME 4: HD 10144
DESIG : (01377-5714)
R.A. : 01h 37m 42.750s
Dec. : -57° 14′ 12.00″
Mag.v : +0.44
Mag.U ; 0.445
B-V : -0.158±0.007
U-B : -0.064±
Sp. : B3Vep to A4III
Mabs : -2.8
Lum. : 1 150 L⊚
π : 22.68±0.57 mas
Dist..: 44.09±1.11 pc.
Dist..: 143.8±3.61 ly.
pmRA : +88.02±0.62
pmDec : -40.08±0.51
cpm. : 96.715 mas.yr-1
cpmPA : @ 114°
Rv. : 16±3kms-1
Temp. : 18 900±500K
Mass : 9.9±2.0 M⊚
Size. : 2.5x1.6 mas.
Rad. : 3.8x5.9 R⊚
Dia. : 7.6x10.8 D⊚
Rot. : 225 kms-1
Source: Hipparcos, Tycho, Various
Achernar is listed as HIP 7588 / PPM 331199 / SAO 232481 / HD 10144 whose exact position is placed at R.A.; 01h 37m 42.750s and Dec.: -57° 14′ 12.00″. Using the available data this 0.45v B3 star lies at the distance of 44.09±1.11pc. or 143.8±3.61 ly - taken from the Hipparcos parallax of 22.68±0.57 (A&A.,323, 49 (1997)) (Acamar, mentioned above is a little farther away at 161 light-years.) This calculates an absolute magnitude (Mv) of -2.8 giving the stellar, via the Mass-Luminosity Relationship (MLR), of some 12±2M⊚. A few sources contend if the MV is lower at around -1.6, that then this would reduce both the mass, radius and surface temperature, giving the more realistic stellar mass of around 10.0M⊚. This in turn would make the star's luminosity about 1 100 times that of the Sun.
Using the current proper HIP motions of pmRA; +88.02±0.62 pmDec; -40.08±0.51 (A&A., 323, 49 (1997)), Achernar is moving SE along PA 114° with the common proper motion (cpm) being 96.715 mas.yr-1). This direction is towards the yellowish 2.8 mag. α Hyi / Alpha Hyi / HIP 9236 / SAO 248474/ PPM 352685 (01588-6134) some 5.1° away. A radial velocity of moderate quality is presently +16kms-1, and differing by +3kms-1 from the first derived results found in 1928 then later in 1953.
Achernar presently lies in the upper setion of the Main Sequence with the the spectral class ranging over the last fifty years between between B3Vp and B5IV (n=16), with the most recent estimations giving either B3Vep (emission lines and peculiarities) or just B3Vp. Surface temperature is 18 900±500K - though there does seem some problems with a more precise value due to mass loss from the Be emission-line star. This also influences the true physical size - indicating Achernar actually is towards the lower values for its surface temperature. It is likely the star is being partly masked by the circumstellar material that is continuously spewed into space from Achernar’s gaseous surface. Some astronomers think this material appears as a belt of hot gas orbiting the star’s equator.
According to an article by Domiciano De Souza, A., et al.; “The spinning-top Be star Achernar from VLTI-VINCI.”; A&A.,407, 47 (2003), that was also reported in ‘News Notes’ Sept 2003 S&T (“The Flattest Star”; S&T, 106, 3, p.20.); the star itself shows some unusual characteristics due to the very rapid rotation. Observations were taken using the ESO’s Very Large Telescope Interferometer (VLTI) that produced measures of the physical diameter along differing position angles. Although the rapid rotation of 225 kms.-1 for Achernar was known previously from the broadening of the spectral lines, these observers found that the star was far more ovoid and showing more oblateness than previously thought. These measures indicate the size 2.5 × 1.6 mas, equivalent to, at the given distance, of 3.8R⊚ by 5.9R⊚ - a diameter 7.6 by 11.8 times that of the Sun. The direction of the widest axis is orientated along PA 48°.
These same authors also point out that the star seems to be on the very limit of possible rotational velocity - and anymore, and the star would be endangered by exceeding the theoretical 300kms-1 and would then tear itself to apart. Also high rotational velocity also means magnetic activity manifesting itself as “starspots” across the surface. No variable activity has been seen with Achernar, and it is not listed in the GVSC4 nor the NSV catalogues.
According to James
Kaler Achernar might be an example of the variable stars classed
as the rare Lambda Eri’s - being a
sub-class of the Beta Cepheids. BCEP’s
are caused by either minor stellar pulsations or surface variations
Note: Achernar’s Colour Values are; U=0.445 B-V=-0.017 U-V=-0.064.
λ Eri / Lambda Eri / HIP 23972 (05091-0845) itself is a β Cep type variable star whose brightness changes between 4.22V to 4.34V in the period of 0.701538 days. Its spectral class is B2IVe, lying at the distance of 538±327pc. or 1 750 ly. derived from the parallax of 1.86±0.88mas. Calculations show the is a luminous star with an absolute magnitude of about −4.4 was mass is in the order of 18.4 M⊚. λ Eri can be found 1.4° WSW of Rigel / β Orionis or 3.7°S of β Eri; also known as Cursa. Regarding λ Eri’s position, this star lies is at the opposite end of the celestial river of Eridanus.
Δ4 / DUN 4 (01388-5327) lies in Eridanus is some 3.8° due north of the first magnitude star Achernar or 2.8° N due north of Δ5. Dunlop’s position when precessed finds good agreement with the coordinates being 01h 38s 54.5m -53° 25' (2000) This light yellow pair is a slightly fainter version of p Eridani / Δ5 system and is oddly not given a mentioned in either AOST1 or 2. Visual magnitudes are 7.1 and 8.6 (7.11V and 8.64V) is an interesting moderate bright pair whose combined spectral classes are given as F5IV-V that agrees well with the colours of my own visual observations. Δ4 has shown a moderate decline in separation (15.8 to 10.1 arcsec.) and position angle (107° to 104°), which will likely in the future will gain more interest towards the end of the 21st Century. This is really good and interesting pair for 7.5cm small telescopes and easily visible in 10.5cm. No doubt this is likely a binary with a substantial period. A pretty pair which is quite attractive for amateur eyes and remains among my favourites.
Ross Gould in “Constellation of the Month - Eridanus, Southern Half”; Southern Cross - Dec. 1998;.
“...Dunlop 4 [is] an easy pair of mags 7.1 and 8.4 at 10.4 arcsec separation. The primary is yellowish, and there are two fainter stars offset from the pair, helping the effect.”
Δ5 / p Eridani / 6 Eri / h.3453 (01398-5612) ranks as one of the best southern double stars. Even though it is far from the Milky Way, Δ5 is easy to find as it is some 1.1°N (PA 16°) from Achernar / Alpha Eridani at the end of the river. The stars to the naked-eye appear as 5.3 mag star; and telescopically both 5.8 and 5.9 magnitudes, respectively.
Dunlop described the pair p Eridani as; “Very nearly equal. Pretty
Furthermore, he says of the pair; (Ref 1. pg.259)
“A beautiful double star; both stars white; the preceding a little dusky. I cannot say which of the stars is large; perhaps following, if there be any difference. The distance is about equal to one diameter of the following star, which I estimate at about 2½ seconds.”
Since discovery in December 1825, this pair has slowly widened from 2.5 arc sec at PA 343.1°, to 11.5 arcsec at PA is 190.4° (2001). By the time of John Herschel‘s mean observations in 1835.144, the separation was 3.68 arcsec and the PA has changed by some 40° to 301.7°. Next in the measures were by Jacobs in 1846 (276.3° at 4.21 arcsec) and 1856 (259.6° at 4.61 arcsec). By the time of the H.C. Russell’s three main observations 1870 (253.4° and 5.46 arcsec), 1878 (236.8° and 6.09 arcsec) and 1880 (234.7° and 6.30 arcsec) the pair continued to widen in a straight line. Russell said in the paper “New Double Stars, and Measures of some of those found by Sir John Herschel” Royal Society of NSW (7th September 1881) about p Eri’s orbit, that;
“...as I endeavoured to show you last year, in reference to p Eridani, in the supposed orbit of which, as the observations accumulated, the ellipse had gradually to be increased, until in the end the most probable curve, if I may so express myself, were shown to be a straight line, or, in other words, the motion which was supposed to prove it a binary is found to be probably due to proper and not orbital motion.”
Russell was certain that the orbit was in fact due to proper motion, and he published in the Sydney Morning Herald that he has determined that the two stars were merely close together in the sky and not associated as a binary. Another article appeared in Nature p.589 on 19th April 1883.
“THE BINARY STAR p ERIDANI - In Communication to the Royal Society of New South Wales in June, 1980, Mr. Russell, the director of the Observatory at Sydney, suggested, from the measures made since 1856, including his own up to 1880, that this object might not be a binary star at all, but merely afforded an instance of one star passing before another by reason of its proper motion. This opinion is repeated in the volume of double-star results obtained at Sydney, published last year. “In fact,” observes Mr. Russell, “a straight line accords better with the observations made subsequent to Herschel’s than an ellipse, and it would appear that the changes are due to simply to proper motion ; of this I think there cannot be any doubt....” The question has just been very fully and carefully considered by Mr. Downing, of the Royal Observatory, Greenwich, who arrives at an opposite conclusion to that of Mr. Russell, and considers “there is not sufficient evidence to justify us in asserting that p Eridani is other than a binary star.” We entirely agree with Mr. Downing in his opinions. If we only compare the measures made by Jacob in 1845.46, with those of Russell and Tebbutt, 1878.80, we get the following expressions :-
showing differences from Herschel’s mean measures, epoch 1834.996, of -51 in position, and +082 in distance, which is too large to be tolerated. This star has been occasionally miscalled 6 Eridani, which would imply that it was one of Flamsteed’s stars. Flamsteed, it is true, has a star which he calls 6 Eridani, and which is B.A.C. 926 ; the binary is B.A.C. 521. The letter p was attached to a star by Lacaillé in the catalogue at the end of his Cælum Australe Stelliferum. The No.6 is merely borrowed from Bode.”
Russell main thrust in his argument which assumes that both Dunlop’s and Herschel’s observations were both under-measured - mainly due to their relatively poor equipment, of poor quality, and so, were simply reject. His support for this was based on the re-measures of all John Herschel pairs south of -34° declination, in which he summarises;
“In very many cases considerable differences between h.’s (John Herschel) observations with the reflector and mine have been found; but a complete list of them has not been made, because the reflector observations so often differ from those of h. made with his equatorial that it did not appear worthwhile.”
In 1880 only seven southern binaries were known (twenty-seven in the entire sky), and that p Eridani’s rejection reduced this number to six. Eventually, after the observations and orbit analysis by R.T.A. Innes in 1911 from the Union Observatory, Johannesburg, South Africa reinstated p Eridani binary star status. From the four observations, the mean PA was 216.9° at 8.3 arcsec. Gore was the first to publish the first orbit in 1912, stating that it was a binary’s orbital period was 302 years - predicting that apastron would occur during 1975 and that the rate of change would be 2° every three years. Innes also says of p Eridani;
“Assuming that the mass of this system is equal to that of the sun, its parallax would be 0.160 arcsec (20 light years); or if it is assumed that its luminosity is equal to the sun’s, it parallax would be 0.09 arcsec (36 light years.)”
Several orbital elements were published as more observations were
added, but the next analyses gave significantly shorter periods. For
example, B. Dawson in 1919 found the short period of about 219 years.
Next was W.J. Luyten and E.G. Ebbinghausen in 1934, who found the
period of 251 years and orbital eccentricity of 0.80. This was
followed by the more modern T.S. van Albada’s (1957), whose elements were much used in
the 1960s and 1970s, and gave a much longer period of 483.66 years
and an eccentricity of 0.554.
Presently, the orbital period of this retrograde pair is estimated to be 484 years, whose semi-major axis is 7.8 arcsec. Periastron for the system in 1813 and just after Dunlop’s observations. Then the positions were fairly close but they have continue to widen. Furthest separation is presently expected to be sometime during 2048, if you use the properly round period values of 483.7 years. If the full 483.66 value is used, the van Albada’s elements, this is more likely closer to 2055. At this time, the separation will be about 11.82 arcsec.
If the present elements are adopted, periastron was in 1826, and
will again occur in 2310 AD. Closest approach is 3.40 arcsec.
Incidentally, the companion passed through the descending node of the
orbit in 1989.
This is one of the nicest binary systems in the sky.
χ Eridani / Chi Eri / HJ 3473 (01560-5137) is a close pair with a significant difference in magnitude. It lies some 3.2°E of Phi Eri, and since discovery, the separation of the pair has slowly diminished from 12.0 arcsec in 1899 to 4.9 arcsec (1956) while the PA has increased from 196° to 204°. Projecting the values suggests the separation should now be widening on the opposite side of the primary, and should be about 0.8 arcsec and about PA 40° (2002). Stellar magnitudes are 3.7v and 10.7v. The pair is said to be yellow and white. As time moves on this pair has become more difficult. According to Hartung in 1968, 25cm is needed to see the companion, and this is when the projected separation was 3.4 arcsec. If we consider these changes, possibly 30cm is now required to see it. With the hexagonal diaphragm it might be now possible to see them in as little as 20cm. However, doing all this in 1994 with 20cm (C8), I failed to resolve this pair. To again meet the conditions that Hartung observed under the 3.4 arcsec separation will occur in about 2020 AD. This is likely a bound system and will be interesting to watch in the coming years. If attached, the period is certainly fairly long.
|“Southern Astronomical Delights”
10 Mar 2009