Subject:
|
Re: The "geography" of local space
|
Newsgroups:
|
lugnet.space
|
Date:
|
Sat, 20 Nov 1999 08:58:19 GMT
|
Viewed:
|
743 times
|
| |
|
|
This thread just keeps going and going!
Mr L F Braun wrote:
> "John J. Ladasky Jr." wrote:
>
> > Hi, Tim,
> >
> > Tim Courtney wrote:
> >
> > > Yeah. I'd just need maps of the Sol system, Alpha Centauri, and Barnard's
> > > Star and how they relate to each other as far as distances go for
> > > Zacktron. Most other stuff is unexplored space to the Z universe. But a
> > > useful tool.
> >
> > Maps? You mean, with planets? Wouldn't we all like to have that! I can provide
> > brief summaries of what we know about these three star systems -- though you
> > probably already know all you need to know about Sol.
> >
> > -----
> >
> > We don't know yet whether such planets actually exist! In recent years, you may
> > have been hearing in the news that planets have been discovered orbiting other
> > stars. ...
>
> Apparently they actually got occlusion readings from a near star in the
> last week--by "near" I'm talking ~40pc. I can find the article and the
> star name, but it's one of the HDs (that narrows it down to several hundred
> thousand objects....;) ).
Right. Lindsay is referring to HD 209458. Catchy name, huh? 8^)
The Doppler shift research team that I mentioned in an earlier post predicted that HD
209458 has a planetary companion. The object is predicted to be about two-thirds the
mass of Jupiter, and orbiting quite close to its parent star, with a period of only 3.5
days. They have always forwarded their finds to a second research group that does
astrometry -- very precise measurements of the *intensity* of stars. The Doppler
measurements are done by color.
Consider that, from our vantage point on Earth, we can see Mercury and Venus cross in
front of our own Sun regularly. That is because our Sun and all of its planets are more
or less in the same plane. When one of these planets makes a transit in front of the
Sun, the amount of sunlight that reaches us is moderately reduced.
The Doppler people have asked the astrometry people to attempt to confirm their
predictions for every candidate planet that they have found. For this to work, the orbit
of the extrasolar planet around its parent star must more or less intersect the Earth,
analogous to the situation with our Sun and Mercury or Venus. Most systems are not so
cooperative, but HD 209458 appears to be. At the exact time predicted by the Doppler
team, the light from the parent star was observed to dim by about two percent, and then
brighten again. The entire event took about two hours.
If you have access to the standard USENET newsgroup hierarchy, you can surf over to
sci.astro and read all about the HD 209458 announcement -- including a (modestly)
critical thread started by Yours Truly...
http://x38.deja.com/viewthread.xp?AN=549231033&search=thread&svcclass=dnyr&ST=PS&CONTEXT=943084904.17301511&HIT_CONTEXT=943084904.17301511&HIT_NUM=7&recnum=%3c80ring$qri$1@nnrp1.deja.com%3e%231/1&group=sci.astro&frpage=getdoc.xp&back=clarinet
> > I won't swamp you with the details of my search through the catalogs for nearby
> > stars that are most likely to allow human habitation (or other life like us). But
> > none of the very nearest stars are that great. The Alpha Centauri system is a
> > possibility, however there is the problem of orbital stability. After our own
> > solar system and Alpha Centauri, the three next closest candidates are Tau Ceti
> > (distance = 3.647 parsecs), Eta Cassiopeiae A (5.953 pc), and HD 191408 (6.052
> > pc). You would therefore need a MUCH bigger map to encompass just five plausible
> > systems for life as we know it.
>
> Hmmm...what's happened to E Eridani, the 3pc K2 star, and the 61 Cygni system? If I'm
> not mistaken E Eridani was the original target of Project Ozma and is believed (as oh
> so many are) to have an exceedingly small companion, on the order of 5% of the star's
> mass orbiting at approximately 8AU.
O.K., you made me go back and look at these stars...
-----
I rejected epsilon Eridani on the basis of its listing as a "variable star" in the
Hipparcos notes. But, looking more closely at that variability, it's nothing like the
"flare star" behavior of, say, Proxima Centauri. The notes list it as a
"micro-variable," meaning that the peak-to-peak variability is under 0.03 units. That's
only three percent! Its spectral type is K2, so it's in my (admittedly arbitrary) range
for habitability -- F5 to K4. Some interesting additional notes read as follows: "Period
of rotation 11.10d, cyclic variation 5 year." O.K., put it back on the list!
Now, what's this about a companion that's a full five percent of the stellar mass? I
have a reference to a list of stars that the Doppler team has examined, for which they
have found NO planets:
http://www.obspm.fr/encycl/list_no.html
e Eri, a.k.a. HD 22049, a.k.a. HIP 16537, is on this list. They state confidently that
the stars listed have no bodies orbiting them that are larger than three Jupiter masses.
Now, the Doppler method of measurement is also limited by the angle of inclination of the
orbiting body. So, if the object in question was orbiting in a plane nearly perpenicular
to our line of sight, it's possible that we would miss it. However, if it was orbiting
in a nearly perpendicular plane, and it was five percent of the mass of its parent
(roughly 33.5 Jupiter masses!), it would be REALLY easy to observe a wobble in the proper
motion of the star. To my knowledge, this hasn't been seen.
-----
Now, how about 61 Cygni? For starters, it's a double star system. The two stars are
spectral types K5 and K7. As I mentioned above, I placed my cutoff, somewhat arbitrarily
perhaps, at spectral type K4.
Synonyms for 61 Cygni A include HD 201091 and HIP 104214. This star is listed as a
"variable of BY Draconis type." Now, I don't know the details about this type of
variable star. Do you? However, the visual magnitude of 61 Cyg A was observed to vary
from +5.16 to +5.85 by Hipparcos. Other variable measurements are references. This is
nearly a two-fold variation in intensity. That didn't sound too hospitable to life to
me.
61 Cygni B, a.k.a. HD 201092, a.k.a. HIP 104217, is much more stable in its output.
However, it has a spectral type of K7, as I mentioned, so it wasn't included on my list.
A K7 star is about 1/10 as bright as our own Sun, so a planet that would receive an
equivalent amount of light as Earth would have to be located about 100 times closer than
the Earth is to the Sun. That's tidal-locking distance, I would guess.
I haven't been able to find references to the separation of 61 Cyg A and B, nor their
orbital period, etc. These factors can only complicate the possibility of life or living
in this system.
-----
More to come!
--
John J. Ladasky Jr., Ph.D.
Department of Structural Biology
Stanford University Medical Center
Stanford, CA 94305
--
|
|
Message has 1 Reply:
 | | Re: The "geography" of local space
|
| (...) Hey, it's a good thread. (...) Dangnab frickin' frackin' million-and-fifty-two catalogues... (...) Nice incitation effect there. It does seem that an intensity change of over 1.5% ought to be detectable again! Given (as you said) that a (...) (25 years ago, 22-Nov-99, to lugnet.space)
|
Message is in Reply To:
| | Re: The "geography" of local space
|
| (...) Apparently they actually got occlusion readings from a near star in the last week--by "near" I'm talking ~40pc. I can find the article and the star name, but it's one of the HDs (that narrows it down to several hundred thousand objects....;) (...) (25 years ago, 17-Nov-99, to lugnet.space)
|
34 Messages in This Thread:
 
- Entire Thread on One Page:
- Nested:
All | Brief | Compact | Dots
Linear:
All | Brief | Compact
This Message and its Replies on One Page:
- Nested:
All | Brief | Compact | Dots
Linear:
All | Brief | Compact
|
|
|
|
