Observers' Forum

This is the place where you can share observing experiences, make suggestions, ask for advice, and sound off on any topic likely to interest other observers. Material for inclusion in Observers' Forum should be sent to the editor:

John Isles, 11105 Tremont Lane, Plymouth, MI 48170 e-mail: jisles@voyager.net

In this Observers' Forum we are pleased to have interesting contributions from Jerry Knowles and Tom Williams. I've also included two items I came across on the Internet.

Enthralled
Jerry Knowles (KS), Rhode Island

  One dazzling wonder earth imparts    is not of the earth, but far beyond---  where the Hunter with his faithful Dogs    still tries to catch the trembling Hare,  and the hero, Perseus, exults    to see Andromeda twist free.  That lady's filmy gown shines bright    with a diamond pendant, most finely cut.  Now turn nine hours away.  You'll see    a Lion out to catch a Crab.  Nearby the Twins try a shuffle step,    and Seven Sisters dance up a storm  ---oh, what a feat---on the back of a Bull.    Look out, here comes a Charioteer!  If the Bull didn't get you, the chariot's near.    Can you believe this universe?  

Some Out-of-Place Variable Stars
John Isles (ILS), Michigan

Seiichi Yoshida recently posted an interesting note in the e-mail group VSNET. It used to be that variable stars were named according to the constellations in which they fell. There are, however, a few cases where this isn't so, usually because the star falls close to a constellation boundary and the coordinates originally reported were inaccurate. T Leonis, for example, is actually located just across the border of the neighboring constellation Virgo. The star has so long been known as T Leonis that I sincerely hope there's no question of it ever being renamed something like V1793 Virginis! Well, Seiichi did a search of the GCVS (through the 74th Namelist of new variable stars) and found quite a few stars that are not named in their true constellations. It's of no practical value, but here---just for fun---is his list. I'm not sure if it's complete.

SX ANT is in Pyx  FZ APS is in TrA  V634 ARA is in Aps  BG AUR is in Tau  VY CAP is in Aqr  V349 CAS is in Per  R CEP is in UMi  BQ COM is in Boo  V1523 CYG is in Cep  SU CVN is in UMa  WX ERI is in Tau  XY HER is in Dra  IP HYA is in Cen  RR HYI is in Oct  T LAC is in Peg  T LEO is in Vir  HK LUP is in Sco  FI NOR is in TrA  CT PER is in Cas  VV PYX is in Pup  V384 SCO is in Sgr  CZ SCT is in Aql  MX SGE is in Aql  V2315 SGE is in Sgr  V1024 SGR is in Sct  V1049 SGR is in Sct  V1050 SGR is in Sct  V3443 SGR is in CrA  ER TAU is in Aur  ES TAU is in Aur  BM VUL is in Peg

Not to Be Confused, Please!
Bob Wing (WN), Ohio

[In the AAVSO's Internet discussion group there was an exchange a few months back about the similar (and similarly named) stars S Vul, V Vul, and SV Vul. Bob Wing rounded out the story as follows. Ed.]

In following the interesting discussion about S Vul and SV Vul, I was reminded of a very similar star with a very similar name, the RV Tauri star V Vul. Here is a summary of the characteristics of the three stars, taken from the 4th edition of the GCVS (1985-90):

       Star            Var. Type       V range         Period        Spectral Type  S Vul           DCep (?)        8.69-9.42       68.464 d        G0-K2(M1)  V Vul           RVa             8.05-9.53       75.7            G4e-K3(M2)  SV Vul  	DCep            6.72-7.79       45.0121       F7 Iab-K0 Iab   

The spectral types are given for both maximum and minimum light. These are all quite similar, but with interesting differences. For the first two, no luminosity class is given, and a double type is given for minimum light. A type like K2(M1) means that the atomic lines indicate a type of K2, but that TiO bands are present in the spectrum at a strength corresponding to type M1. A normal K2 star is too warm to show any TiO absorption at all. This situation can arise in an extended atmosphere---the atomic lines are formed in the relatively warm photosphere, while the TiO bands are impressed upon the spectrum when the light passes through a cooler outer envelope. Such double types are common in RV Tauri stars, but not in cepheids.

The periods and spectral ranges of S Vul and V Vul are so similar that they almost HAVE to be the same kind of star. There seems to be no doubt that V Vul is an RVa variable, and so one is forced to wonder if S Vul could be an RVa as well. SV Vul, on the other hand, is certainly a cepheid---its shorter period, earlier spectral type, supergiant classification, and lack of TiO absorption at minimum are all consistent with being a long-period cepheid.

Observationally, RV Tauri stars are usually recognized by having light curves with alternating deep and shallow minima, and the period quoted is the time between successive deep minima (i.e., there are two minima per cycle). In cepheids, the minima are all the same depth, and there is only one per cycle. Unfortunately, there are borderline cases. In some RV Tauri stars the two minima are nearly the same depth, and in some others, the shallow minimum is SO shallow that it could be missed. Yesterday Ole Klinting reported that he had looked at his extensive observations of S Vul and found that it is sufficiently regular to be considered a cepheid. There is also an extensive literature on this star (thanks, Brian [Skiff], for posting the results of your SIMBAD literature search); in particular, Heiser (PASP 108, 603, 1996) gives photoelectric APT light curves in B and V. It sure looks like a cepheid!

The real test of whether a star is a cepheid or an RV Tauri star must be based on its mass, age, and stage of evolution. Classical cepheids are young, massive stars (probably 5-15 solar masses) that have recently left the main sequence and are evolving through the instability strip in the HR diagram, on their way to becoming red supergiants (although some of them, just to complicate things, may be crossing the instability strip for the second time). RV Tauri stars, on the other hand, are old, low-mass stars (probably 1-2 solar masses), usually considered to be on the asymptotic giant branch (AGB), which means they have already been through the red giant stage, and then the horizontal branch stage, and are now working their way back to the red giant stage with a different interior structure.

Strange, isn't it, that two such different kinds of stars can have such similar pulsation characteristics. Sometimes the light curve is not enough to make the distinction. One approach to the problem is to do a full-fledged abundance analysis---classical cepheids should have normal (metal-rich) compositions, whereas AGB stars are usually metal-poor and often show enhancements of particular elements that they have produced in their own interiors and mixed to the surface.

Another approach is to compute evolutionary tracks and the pulsative properties of the models. And it seems that this has been done. In the same paper by Heiser, he cites theoretical work and shows that S Vul has the right period and the right position in the HR diagram to be a 13 solar mass star on its second crossing of the diagram. If S Vul can be successfully modeled as a cepheid, I guess it's a cepheid.

But then, what about V Vul? Could it too be a cepheid, but one that has the light curve of an RV Tauri star?

I'd better stop.

As for the NAMES of these three stars, well, that's just weird.

Variable Stars and Unidentified Flying Objects
Tom Williams (WLM), Texas

Readers of the Observers' Forum may be interested in a sideline that has developed for AAVSO Vice President Bill Dillon. A member of the Fort Bend Astronomical Society in Houston, Texas, Bill has, for several years, participated with a small group of FBAS members in the search for previously unknown asteroids. Using CCD imaging techniques, the group has made a number of independent discoveries of new asteroids.

Recently, Bill and his group responded to a request for help from astronomers at the Arecibo Radio Observatory, Puerto Rico. The help needed was to improve the orbital elements of the asteroid 2000 EH26 so it could be studied with the Arecibo telescope. Since the asteroid is an exceptionally fast-moving object, it is difficult to pin down its orbit with sufficient accuracy to permit the Arecibo telescope to track it through space.

Bill and the FBAS group took short CCD exposures of the predicted position of the faint object; each exposure was no more than 30 seconds. They then added ten such exposures together to find useable images of the asteroid. The asteroid did not appear on any one of the exposures but was visible when the images were stacked. The resulting position determination allowed astronomers at Arecibo to carry out successful radar studies of 2000 EH26 to determine its size, spin, and the nature of its surface. The FBAS astronomers' feat was described by science writer Carlos Byars in the Sunday, June 2, 2000 edition of the Houston Chronicle.

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