He2-73 in Musca (Surrounding Objects)

Last page here featured the planetary nebula of He2-73. This month explores the surrounding region.

The following objects can be found in a moderately starry field, using moderate to low power, about 40′S of the Crux-Musca border. I originally centred on this field in 20cm using the 26mm Plossel (78× with 32 arcmin field); deliberately just looking for the pair COO 130, as mentioned in both AOST1&2. Later I investigated the other four objects nearby, then centring at the fields mid-position at 11h 50.4m -65° 04′. Here you can hold all in one field: TZ Mus, IC 2966, He2-80, B2730 and my target pair! Furthermore, half a field southwards, using moderate powers, there are also two faint Cepheids — UU Mus and TZ Mus (See descriptions below).

He2-73 Field

Figure 1. Field Chart : Surrounding Objects of He2-73
This shows the 1°×1° field of He2-73 in Musca and surrounding objects in NSP 22. The figure also shows the nebula IC 2966 and the motion of the high proper motion star L145-141 between 2010 AD and 3000 AD.

Nearby Double Stars

COO 130 / HIP 57851 / SAO 251617 / HD 103079 (11519-6512) was discovered by S.I. Bailey in 1894, during routine observations to produce the extensive Cordova Observatory (CD) star catalogue beginning in the 1890s. It is really surprising that neither John Herschel nor H.C. Russell found this system in their extensive double star surveys, yet it is very remotely possible that the star appeared single between 1835 and 1880. COO 130 lies almost 20′ due east of He2-73, and the field is marked with three stars in a bent line at the south-eastern edge of the field at low powers. These field stars have respective visual magnitudes, from south to north, of 7.5, 7.5 and 9.5, and form an oddly-shaped triangular figure.

COO 130 comprises 5.2v and 7.4v magnitude (4.97V & 7.25V) stars with 1.6 arcsec separation and position angle of 159°, as last measured by the Hipparcos satellite in 1991. From the available data, these stars are very slowly diminishing in separation, thus making future amateur observations more difficult.

Resolution is possible in 15cm telescopes but this becomes much easier in 20cm. AOST2 does claim this is … just possible … in 7.5cm, but my own four different observations, between 1978 and 1981, using the 7.5cm f/10 reflector and 10.8cm refractor, could not resolve the pair even with good seeing. Using 20cm, these stars are much easier to split, and I saw the colours as bluish and white, or perhaps blue and white, which is about right when compared to the given B4V spectral class for the primary.

This same double star is featured in AOST2 as object #483, where a comment on the connection of the system states:

the angle is slowly increasing but separation has not changed… Physical connection between the stars seems likely.

This is an example of a bright prominent double star that observers have neglected likely just because it is not among either the prominent Russell, Herschel or Dunlop pairs. Equal proper motions are -029 and -001 mas for R.A. and Dec., respectively.

Selected Measures

PA   Sep  Year
162  2.1  1894
159  1.8  1940
159  1.6  1991

B 2730 (11478-6519) is a faint pair within the same field as the He2-73, some 12.6′SSE, first discovered by W.H. van den Bos while, presumably, measuring nearby COO 130. This 9.5v and 11.5 magnitude bluish and colourless pair is easily visible in dark skies, using either 10.5cm apertures or 7.5cm with some diligence. B 2730 was last measured in 1987, where the separation was measured as 6.4 arcsec along the easterly position angle of 104°.

Nearby Variable Stars

TX Mus (11471-6524) is another variable 7.5′SW further on from B 2730 (above). This 12.4 mag RR Lyrae variable of type AB. It is positioned some 3′N of 9th mag star PPM 778644/ HD 102466 (11472- 6527). The small rise in brightness from 12.4 to 12.8 takes 2 hours 16 minutes, being 20% (M-m/D) of the short 0.473226 day period. Discovery of this period was is based on Julian Date (JDE) of 2424260.264±0.003 of 19th April 1925.
This period was recently updated by German astronomer, Joachim Hubscher, in the Information Bulletin on Variable Star (IBVS.) No. 5802 on 25th October 2007. He determines that the minima are now set from the 10th June 2007 at 22h 00m 29s UT (JDE 2454262.417). Below TX Mus finds its small 4′ sized asterism of about twelve stars of 12th magnitude all shaped in an arc. Little else is known about this variable.

UU Mus / HIP 57884 (11523-6524) (Appears on map U450) is a 9.5 mag star some 12.0′SSE (PA=160°) from COO 130, and both are shown in Uranometria 2000.0. UU Mus varies between 9.13−10.28 over 11.63641 days. The rise of 1.15 magnitudes takes approximately 4.305 days, where the spectrum changes from G0p to the hotter F7. Epoch 2436208.270. B−V=0.982 Plx=0.00285±0.00127.

TZ Mus (11509-6508) is fainter than UU Muscae, although it was discovered first. It lies some 6.9′NW through PA 306° from COO 130. Brightness varies between 11.16V and 12.12V (Δ=0.96) magnitude over the period of 4.944885 days. Epoch 2424259.37. This yellow-orange star can be usefully checked, because the wide Pair 1 (11515-6504), to the NE of TZ Mus, is about 5.3′ away. The white 10.0 magnitude star is PPM 778689, and I estimated the magnitudes as 10 and 11.5, separated by about 30 arcsec at PA 20°. No one so far has listed this faint pair in any of the double star catalogues.

BC Mus / GSC 8981:2285 (11475 -6443) is merely 1.6̶N from the Centaurus/ Musca border. This 12.1 mag semi-detached eclipsing binary can be seen in a moderate field, with the PNe He2-73 due south and at the bottom of the field. IC 2966 is on the eastern edge of the same field with BC Mus located on the NW edge. [Actually 20′NE of IC 2966 or 26′NNW (PA 345°).] Alternatively, use the bluish 7.0 mag HIP 57451/ SAO 251582/ HD 102370/ BD-64 564/ PPM 358966 (11466-6446) some 6.8′NE (PA 70°.) The target star is marked by a roughly 1.4′ spaced equilateral triangle of stars, whose apex points due south, with the blue-coloured star to the east being the variable.
BC Mus varies between 12.2p and 14.5p over 3.511171 days. Based on the maximum on the 11th March 1927 (JDE 2424951.363), the ratio of the primary eclipse to the period is 12%, being some 0.42134052 days. Seeing this deep 2.3 magnitude drop in brightness for an eclipsing binary is unusual, and to catch the magnitude drop of the eclipse, you will certainly have to inspect the field several times. Incidentally, about 10′ to 15′NE from this triangle, is a small void almost empty of stars. Also the surrounding fields are scattered with many stars below 10th magnitude.

Bright Nebula : IC 2966

IC 2966 / VdBH 56 / Bran 374 (11502-6452) (U450) appears is a faint small 3 arcmin reflection / bright emission nebula that is sometimes referred just as nebulosity around a star. In 20cm, I spent some time searching for this object in dark skies and frankly returned without much success. The stated position s merely 20.2′NW at PA 333° of COO 130, and is shown in both Uranometria 2000.0 and Sky Atlas 2000.0. It was discovered by R.H. Frost using a 24-inch refractor at the Arequipa station in the late 1890s (possibly 1898) and catalogued as Frost 791 (Ann. Harv. Coll. Obs.; Vol.lx, p.179 Nebula found by Photography). Later the object was entered into the Index Catalogue (1908) described as: pL, bM — pretty large, bright in middle. IC 2966 is really much smaller than this description suggests, and the latest observations limit this size to 3.0 arcmin across.

IC 2968

Figure 2. IC 2966 Nebula in Musca
Image size: 12.5′×12.5′ from ALADIN previewer (Red: AAO/R/DSS2; Blue: SER/I/ DSS2; Green: average. Red/blue image modified by Silvering, reducing saturation).
Note: L145-141 / HIP 57367 will pass through this nebula in 2640 AD.

Classed as 2 I in Sky Catalogue 2000.0, and classed 1 — Bright (1 Brightest — 2 Faintest) of Intermediate I photographic colour (scaled from Red-Grey- Blue), they give the visual magnitude of the nebula as 11.49v (10.9B) with the general spectral class of B0.5. Illuminating the nebulosity is the star PPM 778678 / CSI -64-14771 (11507-6455), placed a further 2.4′E at PA 114° of the central nebula position. Given as 10.0 mag (PPM says 9.5), this star to my eyes appears distinctly red in the telescope. (Note: Another star is 11.1 magnitude GSC 8981:293 on the northern edge the nebula's boundary.) Data in the Tycho catalogue from the Hipparcos satellite give a parallax of 8.90 mas and, if this is the star brightening the nebula, then distance is roughly 110 pc. According to Auke Slotegraafs Deep Sky Observers Companion, Van den Bergh and Herbest noted that there is little difference in the appearance in either blue or red astronomical plates — only that the maximum diameter is 2.4 arcmin in red and 2.6 arcmin in blue.

Looking at the object, the photographic images, and the data on this object, it is possible that the brightness of the nebulosity has changed in the last hundred years or so, as most stars that illuminate reflection nebulae are more commonly blue, I.e. Like the Pleiades. It is possible that significant obscuration is being brought into play here, hiding much of the nebulous light.

Nearby High Proper Motion Star L145-141

L145-141 / GJ 440 / HIP 57367 / WD1142-645 (11458- 6450) in north-western Musca is one of the closer stars to the Sun. Its main interest is due to the very high proper motion, which is almost equal in prominence with the far more famous southern gem, α Centauri. L145-141, in fact, has the 27th highest proper motion known, and is the most southerly of the Top 100 of all the quickest moving stars in the sky. It is catalogued as Gliese-Jones’s GJ 440 or, alternatively, W. J. Luytens designation of L145-141. This faint 11.5v mag. star is easily found within a lovely starry field. [Note: L145-141, in Uranometria 2000.0 Chart 450, is shown as a tiny dot above the ( of the character string printed on the chart. It is not the brighter star above the (, but the one left of it!] When centred on the nearby 7th mag HIP 57451, L145-141 lies 7.6′SW away along PA 234°, almost mirroring the position of BC Mus and HIP 57451. The actual star is the southern of two 11th magnitude stars, separated by 1.3 arcmin. As earlier seen in NSP 22; Figure 3 shows the 1° field locating both this star and the variable BC Mus.

L145-141s Field

Figure 3. L145-141 / HIP 57367
Image of Surrounding Field and Observed Proper Motion : HIP 57367 is moving across the field right to left. In 2011, the position is about half the distance again between the 16 yr. (1998-1982) difference blue star to orange star in image. Image source and format details as for Figure 2.

General proper motions were originally obtained for this region in the late 1920s, with the data published in 1930. A little later, the proximity of the star, based on the measured parallax of 0.195 arcsec, was found to be 5.12 pc. or 16.7 ly. This happens to be about twice the current distance of Sirius. Hipparcos later improved the distance to the closer 4.617(44)±0.050pc or 15.07±0.16 ly. from the huge parallax of 0.21640±0.00211 arcsec, making this the 37th closest star to the Sun. More recent observations by Henry, T.J. et al. (1997) during the RECONS observation program of nearby stars, found the slightly improved value of 0.21657±0.00201 arcsec, and this was determined by the weighted means of both the Hipparcos and Yale Parallax Catalog. Note: These data are also available from the Astronomisches Rechen-Institut (AIR) in Heidelberg using the ARICNS (ARI Data Base for Nearby Stars).

From this distance, the absolute magnitude (Mv) is 13.18, being some nine magnitudes or 2,100 times less luminous than the Sun. In 1986, the quoted 11.44v magnitude was found using UBV photometry, but more recent observations by Henry et al. (1997) find 11.50V, with the B−V magnitude being the whitish value of 0.196.

Observed proper motions for L145-141 have since proven to be very rapid, being similar in value to α Centauri, (See NSP 17) with the values in R.A. +2665.17±0.198 mas. and Dec. -346.83±0.191 mas. This is equivalent to 2.665 arcsec per year or 1° every 1,350 years. The vector changes in R.A. by 4.442′ or 0.074° and Dec. by 0.578′ or 0.009° each century, calculating the motion, in PA terms, as moving towards PA 97.4°, so that it is travelling almost due east. Such strong easterly motion is highly unusual as the majority of these stars are going the opposite way. I.e. directly opposite the direction of even Alpha and Proxima Centauri! There is likely some significance to this, as to the cause and stellar origin of the star with this prodigious velocity.

Presently, the proper motion is towards the emission nebula IC 2966 (11502-6452), which it will reach in 2640 AD, passing about 2′S from its centre in 2670 AD. Eventually, L145-141 will also pass close to the position of the planetary IC 4191 (13088-6739), (see NSP 20) though by then the PNe nebulosity may already be totally gone.

Spectral data reveal a stellar white dwarf whose spectral class was first tentatively given by Zakhozhaj, V.A. (1979) as A8VII. This was later changed to DC, indicating distinct lack of emission lines or just showing a continuous spectrum. This was again updated to the more modern DQ6, which suggests the element carbon is present. The 6 in this instance gauges the surface temperature which, calculated by the equation 50,400/T, is 8400K, making its nearest sister star in temperature Procyon B in Canis Minor. The DQ6 classification is also similar to the earlier A8 spectrum, suggesting the surface temperature is more like 8,200K. To date, only UV carbon lines have been detected. An estimated mass for L145-141 is about 0.5M⊙ and size can be calculated as about 13,000 km across.

L145-141 is currently fourth closest of all the known white dwarfs, the first two of course being the companions to Sirius and Procyon. Although this star is not some PNN, it is likely that not too long ago, at least in astronomical terms, this was once a PNe central star. The original nebulosity has vanished and has now long since been forgotten.

Two Pairs Near L145-141

In addition to the pair near TZ Mus, there are two more pairs in this field. Pair 2, of similar PA and separation, is about 3′ NE of L145-14, but these are about a magnitude fainter than the high-proper-motion star. Pair 3 is 5.5′ S. This wider 9.6v and 9.8v pair points towards HIP 57451.

References L145-141

  1. Hoberg, J.B., et al., A new look at the local white dwarf population., Astron.J., 135, 1225 (2008).
  2. Subasavage, et al., The solar neighborhood. XXI. Parallax Double results from the CTIOPI 0.9 M program: 20 new members of the 25 parsec white dwarf sample. Astron.J., 137, 4547 (2009).
  3. Zakhozhaj V.A.; Catalogue of nearest stars until 10pc.; Vestnik Kharkovskogo Universiteta (VKha), 190, 52 (1979).


Last Update : 30th September 2012

Southern Astronomical Delights © (2012)

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