################################################################ June 2020 Red supergiant stars for summer photoelectric photometry (PEP) The three lines are 1) variable, 2) comparison, 3) check Magnitudes given are V / B-V / V-I Included notes are mostly from James Kaler, stellar evolution expert Tom Calderwood, AAVSO PEP section head tjc@cantordust.net ################################################################ Stars alf Sco ------------- V 16:29:24.5 -26:25:55 0.983 / 1.826 / 2.9 C 16:24:06.2 -20:02:15 HR 6104 4.5 / 1.010 / 0.99 K 16:07:24.3 -20:52:07 HD 144608 4.316 / 0.838 / 0.85 BM Sco ------------- V 17:40:58.6 -32:12:52 5.36 / 1.6 / 1.7 C 17:52:19.8 -34:25:01 HD 162391 5.84 / 1.13 / 1.09 K 17:53:23.5 -34:53:43 HD 162587 5.6 / 1.14 / none available (close double 0.23 asec) V1488 Cyg = omi-2 Cyg ---------- V 20:15:28.3 47:42:51 3.9 / 1.52 / 1.45 C 20:01:21.6 50:06:17 HD 190147 5.058 / 1.122 / 1.14 K 20:13:18.0 46:48:57 HD 192514 4.82 / 0.100 / 0.19 NSV 13518 = xi Cyg = ksi Cyg ----------- V 21:04:55.9 43:55:40 3.733 / 1.652 / 1.63 C 21:29:27.0 46:32:26 HD 204771 5.237 / 0.977 / 0.95 K 21:33:58.9 45:35:31 HD 205435 4.007 / 0.887 / 0.94 V381 Cep ------------ V 21 19 15.7 +58 37 25 5.674 / 1.335 / 1.08 C 21 17 14.2 +55 47 53 HD 202987 5.981 / 1.450 / 1.45 K 21 20 33.4 +60 45 23 HD 203574 6.1 / 0.995 / 0.98 eps Peg -------------- V 21:44:11.2 09:52:30 2.387 / 1.524 / 1.42 C 22:10:22.0 14:37:48 HD 210461 6.33 / 1.08 / 1.04 K 21:56:56.4 12:04:36 HD 208565 5.541 / 0.053 / 0.07 zet Cep ------------- V 22:10:51.3 58:12:05 3.354 / 1.566 / 1.58 C 22:12:02.0 60:45:33 HD 210939 5.355 / 1.169 / 1.14 K 22:11:48.8 56:50:22 HD 210855 5.253 /0.508 / 0.61 ###################################################################### Notes alf Sco Its great distance of 550 light years (second Hipparcos reduction) reveals that it is truly luminous, to the eye almost 10,000 times brighter than the Sun. Because it is cool, only about 3600 degrees Kelvin at its surface, it radiates a considerable amount of its light in the invisible infrared. When that is taken into account, the star becomes some 60,000 times brighter than the Sun (with considerable uncertainty). A low temperature coupled with high luminosity tells us that the star must be huge, luminosity and temperature giving a radius of about 3 Astronomical Units. It is so big that astronomers can easily detect and measure the size of its apparent disk, which gives an even bigger radius of 3.4 AU, 65 percent the size of the orbit of Jupiter. The difference is caused by uncertainties in distance, temperature, the state of pulsation, and the actual location of the mass-losing surface, as the star is slowly evaporating under a fierce wind that has encased it in a gas cloud, or nebula, that shines by light scattered from the ultraluminous star within. The amount of dimming by interstellar dust is not well known. If as high as half a magnitude, Antares could be as bright as 90,000 Suns, pretty much taking care of the radius discrepancy. Buried within the wind is a fifth magnitude (5.5) hot class B (B2.5) companion star (only 3 seconds of arc away) that hides within Antares' bright glare. The two are separated by roughly 550 AU and take perhaps 2500 years to orbit each other. The companion hollows out a small ionized region within the wind, and although blue-white, has the reputation of appearing green as a result of a contrast effect with its brilliant reddish mate. Antares, with an uncertain mass of 15 to 18 solar masses, probably does not have much time left to it. It is massive enough someday to develop an iron core and eventually to explode as a brilliant supernova. ---------------------------------------------------------------------- BM Sco From "VLTI/AMBER spectro-interferometry of the late-type supergiants V766 Cen (=HR 5171 A), σ Oph, BM Sco, and HD 206859" Wittkowski et al, 2017 BM Sco is generally classified as a K1-2.5 Ib super-giant (Simbad adopted K2 Ib). However, our luminosity of logL/L=3.5±0.3 places this source into a transitional zone between red giants and RSGs corresponding to evolutionarytracks of initial mass 5–9M. McDonald et al. (2012) placed this sources at an even lower luminosity of logL/L=3.0. This position in the HR diagram is more consistent with a classification as K3 III by Houk (1982). The measured effective temperature of 3890±310 K is consistent with the spectral classification. BM Sco is also among the source of Levesque et al. (2005) and our fundamental properties are consistent with theirs. We have found a significant uncorrelated flux fraction of∼25% from an overresolved component, most likely pointing to a large back-ground dust shell. This is consistent with McDonald et al. (2012) who list BM Sco as one of relatively few giants with infrared ex-cess and evidence for circumstellar material. BM Sco also shows extended CO features in the visibility that may be slightly be-yond model predictions. These features are not typical for a star of this mass range, but would be more typical for a higher luminosity and thus higher mass star. This makes BM Sco an interesting target for follow-up observations. These features might be caused by an interaction with a close unseen companion. We can also not exclude that adopted values for the distance or for the calculation of the bolometric flux may be erroneous, and that BM Sco in fact is a higher luminosity and higher mass star. ---------------------------------------------------------------------- omi2 Cyg OMI-2 CYG (Omicron-2 = 32 Cygni). Stellar astronomy is filled with both confusion and coincidences, the "Omicrons" of Cygnus providing a full measure of each. Fourth magnitude (3.79) Omicron-1 (31 Cygni in Flamsteed parlance) and also-fourth magnitude (3.98) Omicron-2 (32 Cygni) make a line-of- sight wide "double" to the east of Deneb. Tucked next to Omicron-1 is fainter fifth magnitude (4.83) 30 Cygni. Unfortunately, some authors have referred to the close pair 30 and 31 as Omicron-1 and Omicron-2, while others have called 30, 31, and 32 Omi-1, Omi-2, and Omi-3. Worse, since 30 Cygni seems to be related to 31, it is sometimes also called 31 Cygni D, even though its motion through space shows that the two do not physically belong together. We stick to the most common definition of 31 and 32 as Omi-1 and Omi- 2. More intriguing Omi-1 and Omi-2 (31 and 32), though not related at all, are almost identical kinds of stars, both long-period "Algol-type" eclipsing binaries with super- large class K primaries orbited by hot, dimmer class B dwarf secondaries much like Haedus I (Zeta Aurigae). Every 3.143 years, Omi-2, 1100 light years away, takes a slight dip as the class K (K2) supergiant just barely grazes a class B (B3) dwarf, producing a dimming of a mere 0.06 magnitudes (about six percent). In fact, for this type of system, Omi-2 sets a record for the weakest eclipse. Only if the primary star is huge can such long-period eclipses have any probability of being seen. The temperature of the supergiant is problematic, the measures ranging from 3900 to 3500 Kelvin. Adopting the more realistic upper value to account for infrared radiation (and ignoring the fainter B star) gives a luminosity of 11,100 times that of the Sun and a diameter of 230 times solar, close to two Astronomical Units. Direct measure of angular diameter agrees. Analysis of the orbit gives a mass of 5 solar for the class B star and 9.7 for the prominent supergiant. Less than 20 million years old, its fate is either to explode or to create a rare neon- magnesium white dwarf. Slightly metal-deficient compared to the Sun, this ponderously rotating star spins with a projected equatorial speed of just 3.4 kilometers per second. If that is the true rotational velocity (that is, if the rotation axis is perpendicular to the line of sight), the star would take an amazing nine years to make a full rotation. Stars like Almaaz, Omi-1 Cygni, and Omi-2 Cygni are of great importance, as during much of the eclipse, the light from the class B star shines through the large star's hugely distended "chromosphere," the tenuous layer above its actual surface, backlighting it and allowing its detailed study. ---------------------------------------------------------------------- xi Cyg XI CYG (Xi Cygni). Xi Cygni (Xi the 14th letter of the Greek alphabet) is one whopper of a star, moreover one with an unusual spectral class. Back to basics first. Xi shines at the bright end of fourth magnitude (3.72) within Cygnus, the Swan, a few degrees to the east of much brighter Deneb, the two stars bookending the famed North America Nebula. Though orange class K giants are quite common, cooler class K supergiants (this one K4.5) are not. The relative faintness to the eye comes from the star's large (and somewhat uncertain) distance, measured at 1200 light years. There are two quite diverse temperature determinations, 4007 and 3490 Kelvin. Since the warmer value fits better with that expected for the class, it will be adopted here. Using the temperature to estimate the amount of infrared radiation, and of course the distance, the star is found to shine with the luminosity of 9400 Suns, which gives a radius of 200 solar, or 0.94 Astronomical Units. Direct measure of angular diameter coupled with distance gives 1.04 AU, essentially the same value, both showing that the star is as big as the orbit of the Earth. An extended atmosphere makes the star 30 percent bigger. As a stable supergiant, Xi Cyg has to be supported through the internal fusion of helium into carbon and oxygen, its mass between 8 and 10 times that of the Sun depending on the exact state of evolution. Three other characteristics vie for attention. Though clearly a star that belongs to our part of the Galaxy, Xi Cyg is metal- deficient, having only 35 percent the iron content (relative to hydrogen) of the Sun. And despite its great distance, there is no measurable interstellar dust absorption along the line of sight, the star appearing in a sort of clear Galactic "window." The equatorial rotation speed has also been measured at 3.4 kilometers per second, though that is a lower limit as the axial tilt is not known. If the star's axis is perpendicular to the line of sight, Xi Cyg takes eight years to make a full rotation! Having begun life as a hot class B dwarf (around class B1) only 30 or so million years ago, Xi Cyg is clearly on its way out. Right at the boundary of masses that will become either massive white dwarfs (below 8 or 10 solar) or blow up as "core-collapse" supernovae (above that limit), its fate is uncertain. However it ends, since the star is still fusing its internal helium, we still have some time left to admire it. ---------------------------------------------------------------------- eps Pegasus Physically, Enif is a coolish, orange class K (K2) supergiant with a temperature of 4460 Kelvin. From its distance of 670 light years, we calculate a total luminosity 6700 times that of the Sun, as befits a supergiant. Also befitting its great status is its diameter, which from direct measures of angular diameter and its luminosity and temperature is 150 times that of the Sun, large enough to take the star halfway to the orbit of the planet Venus. If Enif were our star, it would appear 40° across in our sky, about the angular extent of the entire constellation of Pegasus itself. As a supergiant, Enif is both dying and massive. It seems to have a mass around 10 times that of the Sun and is now either fusing its helium into carbon and oxygen or is about to start. Like Betelgeuse, it might either explode as a supernova or turn into a heavy, rare neon-oxygen white dwarf that has shrunk to less than the size of Earth. Even with all these great qualities, however, Enif still seems like an ordinary (if such there be) supergiant. Two characteristics set it apart. It seems to be part of a family of three very similar supergiants, the other two the Alpha and Beta stars of nearby Aquarius, Sadalmelik and Sadalsuud. The triplets, all at roughly the same luminosity and distance (Sadalmelik at 760 light years, Sadalsuud at 610) may have been born together in the same extended group, and over the past 15 or so million years of their existence have drifted well over 100 light years apart. Most intriguing, however, is Enif's possible erratic and violent behavior. In 1972, an observer in Florida saw Enif as bright as Altair in Aquila, five times brighter than normal, whereupon it faded. For over 10 minutes it appeared to pop some kind of enormous flare, one vastly brighter than the small ones that occur frequently on the Sun. Such events are rare -- only two dozen or so are known -- and not well documented, nor is there any theory for them. We apparently do not understand supergiants -- or any other kind of star for that matter -- quite as well as we think we do. ---------------------------------------------------------------------- zeta Cep Zeta is a magnificent star in its own right, a class K (K1.5) lesser supergiant with a temperature of 4310 Kelvin. Parallax (second Hipparcos reduction) gives a distance of 836 light years (give or take 20). Along with Delta Cep, Zeta appears to be part of the Cepheus OB6 association (centered at 880 light years), an expanding group of massive stars that share a common birthplace. After a 0.22 magnitude correction for dimming by interstellar dust (derived from that of Delta Cephei, with which it is apparently related), we find that it shines with a luminosity 5900 times that of the Sun from a surface swollen through stellar evolution to 140 times the solar size, making it two-thirds the size of Earth's orbit. Direct measures of angular diameter give a similar value, 145 times solar, showing that the various parameters are in good shape. Rather metal-rich (the iron-to-hydrogen 1.6 times solar), the star is also losing mass at a (very rough) modest millionth of a solar mass per year. If now evolving toward internal helium fusion, Zeta would have to be some 9 the mass of the Sun to have its current temperature and luminosity, while if already fusing helium to carbon the mass could be lower, a "mere" 7 times solar. Now about 50 million years old, Zeta began life at the hotter end of the range of class B dwarfs. At the edge of the 8- to-10 solar mass limit at which stars develop iron cores and then explode as supernovae, its most likely fate is to produce a massive white dwarf (perhaps one made of neon instead of carbon) near the 1.4 solar limit at which such dense remnants can survive. If there IS a companion, and it is close enough to feed sufficient matter to the white-dwarf-to-be, it is marginally possible that the limit could be overflowed, resulting in the white dwarf's collapse and again a supernova.