OPEN STAR CLUSTERS : 9 of 11

(1) The HYADES : HORNS of the BULL
An Important Astrophysical Star Cluster

The Hyades / Mel 25 / C 0424+157 (04269+1552) is a very large open star cluster that lies in the constellation of Taurus. It is shaped as the familiar ‘V’, being the famed horns of the bull.

Our knowledge of most of its star’s motion and distance means Aldebaran is not a member of the cluster, but lies in the foreground some half the distance from the centre of the Hyades.

For astrophysicists, the Hyades remains one of the most important of the open star cluster in the entire sky, being known as one of the closest clusters to the Sun. With the close distance of about 45 parsecs (45 pc.) or 150 light years, makes this stellar group as the best known ‘celestial laboratory’ when studying open clusters in minute detail. The Hyades also contains many binary stars, and we have determined some details of the range of stellar masses, which in turn has provided accurate estimations of the luminosities of the components without the usually large and unknowing dimming effect of interstellar absorption by the Milky Way along our line-of-sight. The Hyades, component stars are reddened by only 0.010 magnitudes. Many of the Hyades stars are also solar-like, whose metallicity (Z) is +0.17, with the Sun being nominally +0.0122. Metallicity (Z) is often expressed from the Iron to Hydrogen ratio or [Fe/H] finds −0.14±0.08, against the solar value of 0.00.^★

^★ Metals in Red Giant Branch (RGB) stars are much debated among stellar evolutionists. They are known to be affect the stated radii and effective temperatures (T_eff.) This Z or [Fe/H] value is actually approximate, mainly because of the differences in helium content. Here stellar models can be made with other values. I.e. Piau (2011) uses 0.0126.

Figure. 2. The Hyades : Brightest Stars This shows the brightest star visible down to 8th magnitude. Stars are labelled with Bayer Letter and Flamsteed Number or just the Flamsteed Number. Only Aldebaran and Ain have given names. Many of these stars appear in the Table 1 below.

This cluster has about four-hundred members in all. It covers about 4 parsecs across, whose spherical volume encompasses about 67 cubic parsecs. Density is therefore about six stars per cubic parsec. The inner core of the cluster contains some 140 stars which appear within the field of 6°. Its whole associated region finds this number increases to perhaps one thousand stars in true membership.

Figure. 3. The Hyades Colour-Magnitude Diagram This is a colour-magnitude diagram adopted from the stellar magnitude and colour data from Micela, et al. (1988)

Taurus OB1 is the association component of the cluster, whose distance is calculated to be about 200 to 400 parsecs from the Sun. Investigation of the cluster properties suggests that the open cluster is older than the association, something that has been not found to be an unusual relationship. The open cluster, however, is known to be the nuclei of the looser association. The explanation for this lies in the fact that the open cluster must contain the largest mass. The association was in or near the cloud but for the open cluster the stars were created within in the densest parts of the nebula. It is thought that an association will form later only because the compression time distilling the stars happens radially from the inside to outside within the nebula. An example of this is the expanding hole of the Rosetta Nebula / NGC 2237-2238 (06317+0504) which is a very rich star forming region in equatorial constellation of Monoceros. Such investigations into this simple hypotheses continues today. See the Image : Rosetta Nebula

Distance of the Hyades

In the recent past the use of trigonometric parallaxes was the principal investigation tool. These values were obtained by transit circle or meridian circle. The first estimations found that the Hyades lay at some 3.25±0.08 (as the distance modulus) corresponding to the distance of 44.7 parsecs or 146 light-years. Several estimations towards the late-1980s reveal there is close correlation between several independent values. One result obtained in 1988 found the distance modulus as 3.31±0.06 (m-M)H being equivalent to the distance of 45.6±0.9pc. or 150±1.3ly. Using all the proper motions, the traverse velocities and obtained direct data from the binaries in the cluster have been determined on all the identified cluster stars. Knowing the mean ratios of the proper motions and traverse velocities, can then produce a reasonable estimation of true distance. True membership of the cluster is determined by the components common proper motion (cpm), being the location of the merging or convergent point in the sky — the place where all the cluster stars appear to be heading. Investigations have proven that most stars within 7° of sky are all travelling in roughly the same direction and velocity, collectively merging towards a point in space some 35° west of the 1st magnitude star, Aldebaran.

Figure. 4 : The Hyades Cluster : Proper Motion and Merge Point This show the motion of some of the Hyades stars towards the merge point, in which the stars are travelling. The star Aldebaran is a foreground star and is not associated with the Hyades cluster.

Currently the age of the Hyades cluster is estimated to be about 625 million years (2002) or approximately one-eighth the age of our sun while the Hyades OB-association is about 650 to 700 million years. WEBDA presently give the age as log (8.896) or 787 million years. These were all derived from the cluster’s CMD and the stars they hold, and are mostly estimated by the rate in which the stars of various masses should evolve based on current stellar evolution theory.

The Hyades distance is generally considered most important in determining the galactic scales near the Sun, and forms one of the first ‘stepping stones’ to find distance of galaxies, quasars and the scale of the universe.

BRIGHTEST VISIBLE HYAD STARS

Figure. 5. The Hyades Colour-Magnitude Diagram : Bright Stars This is a colour-magnitude diagram adopted from the tabled Hipparcos data above. It clearly shows the top of the main sequence of the cluster, with most being white A-type stars. The most luminous are the four main K0 spectral types. I have excluded the non-members of Aldebaran and π Tau. Orange star shining at 5th magnitude is 75 Tau, and seems to suggest it may not be a Hyades member. Compared to the Sun, with the absolute magnitude of about +4.2, would appear below the bottom edge of the figure. All these plotted stars above are more luminous than the Sun.

Hyades References

1. Micela, et al., “Einstein survey of stars in the Hyades : Optical Properties and X-ray Luminosities.”, . AJ., 325, 798 (1988)
2. Perryman, M.A.C., et al., “The Hyades: Distance, Structure, Dynamics, and Age.”, A&A., 331, 81 (1998)
3. Raboud, D., Mermilliod, J.-C., “Investigation of the Pleiades ClusteMCAr: IV. The Radial Structure.”, A&A., 101, 329 (1998)
4. Richichi, A., Roccatagliata, V.,“Aldebaran’s angular diameter: How well do we know it?”, A&A., 433, 305 (2005)
5. Robichon, N., et al. ”Open Clusters with Hipparcos: I. Mean Astrometric Parameters.”, A&A., 345, 471 (1999)

Aldebaran

“Aldebaran” Enya “The Celts” (1986)

Aldebaran / α Tau / 87 Tau / HIP 21412 (04359+1630) is the deep orange or orange-yellow coloured first magnitude star, placed at the most southerly tip of the Bull’s horn. According to Richard Allen (1899), the given name of Aldebaran comes from an Arabic word meaning “to follow”. Its lesser used name is Parillicium. This star greatly predominates over the fainter stars of the Hyades, as the brightest star in Taurus and the 14th brightest in the sky. In legend, Aldebaran in mythological is said to be the blood procured from nearby Orion the Hunter, after Taurus had charged and pierced his flesh.

Aldebaran is of the closer bright stars to the Sun, whose distance is 20.43±0.32 parsecs, 20.43^+0.32_-0.33 pc. (formal) or 66.65±1.06 ly.; from the HIP2 parallax of 48.94±0.77 mas. Proper motions for Aldebaran are significant, being pmRA.; +62.78±0.89 mas.yr^-1 and pmDec.; −189.35±0.58 mas.yr^-1, therefore finding the common proper motion of +199.49 mas.yr^-1. This is equivalent to about one arcsec per year or 0.333° per millennium. Traverse star velocity is 18.9 kms^-1, obtained from the radial velocity of +54.11±0.04 km.sec^-1.

James Kaler (1989, 2009) calculates the mass at about 1.7M⊙, though this figure is wrapped in a great amount of uncertainty. Diameter is said to be 35.8 million kilometers.....

Aldebaran has an absolute magnitude of −0.57±0.03, but with bolometric correction, is −1.57 making the luminosity 362 times Sol. Some alternative sources find −0.6 and 518±32, respectively, using the older parallax of 50.09±0.95 mas. I.e. Piau, et al. (2011) Table 1. finds the M_bol as −2.04±0.06, with the bolometric correction of 2.51.

Surface temperature is a coolish 4,051K matching well with the often tabulated data of 4,000K. SIMBAD lists twenty measured temperatures, whose average is 3,913±57K. Spectral type is K5III, with given precise magnitudes are 0.985V, 2.468B, 0.697U or 0.10R. Known as an LC-type variable star, its does in fact show slightly inconsistency, having an irregular amplitude varying by some 0.2 magnitudes. This change, however, is not noticeable to the naked eye.

One of the best observational opportunities is the fairly regular lunar occultations, which has had many historically events, some being even prior to the telescope. Johnson (1885) wrote a letter to the Royal Astronomical Society on these older occultations. One event he suggests occurred was from Athens on 11th March 509 AD, being recorded in a Greek manuscript examined by Bullialdus. Another possible event was in 547 AD. Yet another was seen to the naked-eye by Copernicus in March 1497 followed by another by D. Fabricius on 4th March 1607. In England, Bevis (1737) observed the close approach of Aldebaran to Moon, while Graham (1739) saw both its disappearance and reappearance, whose occultation lasted 01h 02m 53s.

Lunar occultations have been used for many decades for the determination of stellar diameters. Smalley (2005) finds 19.96±0.03 mas as the mean diameter of, then compensating for limb-darkened, increases the apparent size to 20.58±0.03 mas. At distance, obtained by the HIP2 trigonometric parallax (2007), determines the true diameter as 44.2±0.9 million km.

Surface gravity (g), is an acceleration expressed in solar units by g_☉ = G×M_☉ /R_☉²). For Arcturus it is log (1.18±0.18) or 15.1±1.5 m.s^-1, being the useful measure of the physical conditions of the stellar atmosphere, but is also related photospheric pressure and density. [Sun = log(4.4374±0.0005) or 27,377 km.s^-1, Sirius = log(8.0) or 100 million, Betelgeuse = log(−0.6) or 4.0 Jupiter = log(3.4) or 2,500. Earth an body drops at the acceleration of 9.80665 m.s^-1 with the surface escape velocity as 11.2 km.s^-1. Surface gravity here is translated as log(2.992). Note here too, that the surface gravity between Earth and Betelgeuse is very different, and although the mass of this red giant far exceeds the Earth, the thin density of the atmosphere means that escaping from Betelgeuse is slightly easier. (At the centre of stars and planets it is zero gravity, while placed above the surface of the Earth the acceleration value become smaller!)

In astrophysics, the surface gravity is an important parameter that is related to the physical properties of the stars and an important influences of calculating and analysing stellar abundances. It is also crucially part in creating all useful stellar models, often reflecting energy or flux distributions and the profiles of spectral lines. Results are only indirect estimations, whose direct measurement can only be obtained for stars within spectroscopic binary systems.

Gatewood (2008) has recently investigated Aldebaran by astrometry in the hope for finding possible faint companions.

Aldebaran References

1. Bevis, J., “Observation of the Moon’s Transit by Aldebaran, April 3, 1736. Made at London by John Bevis, M.D.”, Phil.Trans., 40, 90 (1737)
2. Brandt, J.C., “St. Helena, Edmond Halley, the Discovery of Stellar Proper Motion, and the Mystery of Aldebaran.”, J.Ast.His.&Heritage, 13, (2), 149 (2010)
3. Gatewood, G., “Astrometric Studies of Aldebaran, Arcturus, Vega, The Hyades, and Other Regions, , Astron.J., 136, 452, (2008)
4. Graham, G., “An Occultation of Aldebaran by the Moon, Dec. 12. 1738. p.m. Observed in Fleetstreet with a Reflecting Telescope of 15 Inches in Length.”, Phil.Trans., 41, 632 (1739)
5. Piau, L., et al., “Surface convection and red-giant radius measurements”, A&A., 526, 100 (2011)
   + Table 1. “Observational and physical parameters of our selected sample of giant and subgiant stars with interferometrically measured angular diameters.“
6. Richichi, A., Roccatagliata, V.,“Aldebaran’s angular diameter: How well do we know it?”, A&A., 433, 305 (2005)
7. Sato, B., et al., “A planetary companion to the Hyades giant ε Tauri.”, AJ., 661, 527 (2007)
8. Smalley, B., “Teff and log g determinations”, Mem.S.A.It.Sup., 8, 130 (2005)

Ain / ε Tauri

Ain / ε Tau / 74 Tau / HIP 20889 / Mel 25 70 (04286+1911). This 3.53V magnitude star is the top-point star of the Hyades that marks the tip of the A-shaped asterism. Ain appears light orange being K0III spectral class and the B−V of 1.016.

Distance is 45.97±0.51 pc. or 146.7±1.6, though the more recent published more precise result finds 47.529±1.852 pc. At either distance, the absolute magnitude (M_V is still around +0.3. Proper motions are pmRA.; +106.19±0.38 mas and pmDec. −37.84±0.30 mas., making the common proper motion (cpm) as 112.73±0.48 mas. in PA 196.65°. Using the radial velocity of 38.86±0.13 km.sec^-1, the traverse velocity (Vμ) becomes 11.65 km.sec^-1, making the true star velocity (V★) of 40.56 km.sec^-1.

Calculation of stellar parameters for ε Tau was all made by Sato et al. (2007), finding the mass as 2.7±0.1M☉, and whose surface temperature (T_eff) is 4901K. They calculate the luminosity as 97±8 L☉/L★, and the metallicity [Fe/H] of +0.17 from the observed FeI and FeII lines in the spectra. Stellar diameter is 17.9 million kilometres or 13.7R±0.6R☉ with the pedestrian rotational velocity (v.sin i) of 2.5 km.s^-1. Surface gravity or log g is 2.64±0.07 (g = 436 km.s^-2), being much higher than Aldebaran. Data in Simbad (2013) gives the following averaged results; T_eff= 4,862±355K (n=21), log g= 2.7±0.4 (n=20) and [Fe/H]= +0.16±0.25 (n=21)

I gratefully used all these data to check my own calculator to see what results I could obtain — especially in light of the puzzling differences found with the higher mass calculation of Aldebaran (above) as compared to standard sources. The Table below is surprisingly in quite good agreement.

Note: General catalogued temperature for K0III stars is 4853K. Also the bolometric correction is −0.38 for K0III stars, and correcting the absolute magnitude finds this value as −0.11. As noted below, the period of the companion I calculated as 1.817 years or 663.7 days.

Exoplanet : *eps Tau b

Ain has an exoplanet or extra-solar planet candidate known as *eps Tau b / HD 28305b that was discovered by Sato, et al. (2007) using the spectrograph attached to the Grubb-Parson’s 1.88-metre at National Okayama Astrophysical Observatory (NOAO)^[1],[2] in Japan, atop Mount Chikurin-Ji. (372m.) Period for the companion is 594.90 days, and orbits some 1.93 AU from Ain, which at this distance is a parallax of 4.23 mas. Using my Astro Calculator inputs and this 4.23 mas projected semi-major axis (a), finds the period, using the technique by Allen et al. (2000), as 1.817 years or 663.7 days! True separation is 1.93AU, as per the calculator, though using the calculation technique of Allen et al. (2000) gives the larger projected separation or <a> as 2.71AU. Not at all bad I think!

Orbital eccentricity is 0.151±0.023. Mass from the spectroscopic velocity curve is about 7.6±0.2 M_Jup. The tiny stellar doppler shift, or wobble, has the small semi-amplitude (K₁) of 95.9±1.9 m.sec^-1, while ω is 94.4±7.4°.

Great importance continues to be attached to the discovery of eps Tau B, suggesting that planets are formed in star clusters and with the age of 625±25 Myr. As Sato et al. (2007) say:

“This timescale is the most secure upper limit ever set on giant planet formation, because uncertainty in the age of field stars is typically on the order of Gyr. The planet directly provides a constraint on the timescale of giant planet formation independent of observations of protoplanetary disks around young stars.… The discovery demonstrates that G and K giants in open clusters are the most appropriate for planet searches around massive stars by radial velocity techniques, while those on the main sequence are unsuitable for this purpose because of the lack of absorption lines in their spectra.”

They conclude (pg.531):

“Well-determined ages of open clusters enable us to trace the evolution of planets and life. For example, at age 600 Myr, which corresponds to that of Hyades, oceans and the first primitive life forms appeared on the Earth. Habitable zones around early-type stars are about 10 AU corresponding to an angular separation of 0.200 at the distance of Hyades, and those around solar-type stars are at 0.0200 or about 1.0 AU). Earth-like planets in these habitable zones can be good targets for future space coronagraphy and interferometry missions. Nearby open clusters with well-determined ages may provide us live candidates for quests for life.”

Ain References

1. Allen, C., Poveda, A., Herrera, M.A., “Catalogue of wide binaries”, A&A.,, 356, 529 (2000)
2. Sato, B., et al., “A planetary companion to the Hyades giant ε Tauri.”, AJ., 661, 527 (2007)
3. van Belle, G.T., von Braun, K., “Directly determined linear radii and effective temperatures of exoplanet host stars.”, AJ., 694, 1085 (2009)

δ¹ Tauri

δ¹ Tau / 61 Tau / HIP 20455 / Mel 25 41 (04229+1733) is similar to Ain / ε Tau. It shines at 3.76V magnitude, and appears light orange of K0III spectral class with B−V of 0.983.

Distance is 47.71±1.32 pc. or 155.6±4.3.

Last Update : 19th April 2017

Southern Astronomical Delights © (2017)

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