Astronomers Measure Distance to Well-Known Star

Mike Dworetsky wrote in message
...


"Steve Willner" wrote in message
...
In article ,
"greywolf42" writes:

Unless the pair is an eclipsing binary, it is not possible to
determine the orbital inclination using interferometry -- or
direct measurement of resolvable binaries. One can only
get a projected ellipse.

One also gets the _time_ at which the stars are at each position on
the projected ellipse. This gives the inclination. Someone else
explained how in this newsgroup a few months ago, but it might be
better to look at a textbook that includes diagrams.

That would be great -- but it's not possible. Whoever it was, was blowing
smoke.

The distance between the two bodies (even if it could be known) cannot
determine the distance of the binary to Earth.

It can if radial velocities are available. The radial velocities
plus the inclination give the distance between the stars in physical
units (AU, for example). The interferometry gives the same distance
in angular units (arcseconds), and the ratio gives the distance.

The article mentions that it is difficult to get radial velocities for
this binary due to the high rotational velocities of the stars,

Rotation does broaden the stellar signature. But this is simply further
muddying of the data. You still can't get the inclination from radial
velocity.

but also says
that recent attempts to determine velocities (by Pierre North) are
encouraging. IOW, the problem is difficult but not impossible.
Once the velocities are measured, a definite answer to the question of the
distance can be obtained.

But you can't determine the velocities without first determining the orbital
inclination -- which you don't have.

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greywolf42
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Odysseus wrote in message
...
David Knisely wrote:

Actually, there have been other indications of inaccuracies in the
Hipparcos
data from other sources, so prior to this announcement, it was at least
suspected. Now, it appears to be confirmed that at least for this star,
the
Hipparcos distance was slightly in error. For good overall accuracy,
multiple
measurements from multiple independent sources is the best way to go.
Clear
skies to you.

It seems to me that Hipparcos was only very slightly off; I don't see
this result as showing a very dramatic discrepancy.

100% of the reported shift is pretty dramatic.

The catalogue
says the parallax for Atlas (HIP 17847) is 8.57 +/- 1.03 mas;

Not bad for a physical resolution of 3 mas.

according to my quick calculation this converts to a distance
anywhere between 340 and 433 light-years. So the new figure of
434-446 LY is barely beyond the end of the standard-error bar from
the Hipparcos data.

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greywolf42
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"greywolf42" writes:
Steve Willner wrote in message
It can if radial velocities are available. The radial velocities
plus the inclination give the distance between the stars in physical
units (AU, for example). The interferometry gives the same distance
in angular units (arcseconds), and the ratio gives the distance.

The problem is that you can't know the radial velocities without first
knowing the inclination. Which you don't have.

You must be working from an erroneous definition of radial velocity.
In astronomy, the commonly understood definition of radial velocity is
along the line of sight between the observer and target. Such
velocities are easily detected by standard Doppler techniques, if a
spectral line is detectable.

CM

Craig Markwardt wrote in message
news

"greywolf42" writes:
Steve Willner wrote in message
It can if radial velocities are available. The radial velocities
plus the inclination give the distance between the stars in physical
units (AU, for example). The interferometry gives the same distance
in angular units (arcseconds), and the ratio gives the distance.

The problem is that you can't know the radial velocities without first
knowing the inclination. Which you don't have.

You must be working from an erroneous definition of radial velocity.
In astronomy, the commonly understood definition of radial velocity is
along the line of sight between the observer and target. Such
velocities are easily detected by standard Doppler techniques, if a
spectral line is detectable.

Sorry, in my response I had read 'orbital velocity'. Radial velocity is
insufficient to determine the orbital parameters such as orbital
inclination.

The point being that one cannot determine distance from interferometry --
unless you get lucky and have an eclipsing binary (which automatically tells
you the orbital inclination).

--
greywolf42
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In article ,
"greywolf42" writes:
Sorry, in my response I had read 'orbital velocity'. Radial velocity is
insufficient to determine the orbital parameters such as orbital
inclination.

Radial velocity can determine some of the orbital parameters but not
all. Inclination is indeed one that cannot be determined.

The point being that one cannot determine distance from interferometry --
unless you get lucky and have an eclipsing binary (which automatically tells
you the orbital inclination).

This is incorrect, as has now been explained to you at least twice in
recent months. The combination of a visual orbit, which gives
inclination as well as other orbital parameters, and a radial
velocity orbit, which gives the physical orbit size, can indeed
determine distance. If it still isn't obvious to you, I suggest you
consult a textbook for details.

--
Steve Willner Phone 617-495-7123
Cambridge, MA 02138 USA
(Please email your reply if you want to be sure I see it; include a
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Steve Willner wrote in message
...
In article ,
"greywolf42" writes:
Sorry, in my response I had read 'orbital velocity'. Radial velocity is
insufficient to determine the orbital parameters such as orbital
inclination.

Radial velocity can determine some of the orbital parameters but not
all. Inclination is indeed one that cannot be determined.

Yep.

The point being that one cannot determine distance from
interferometry -- unless you get lucky and have an
eclipsing binary (which automatically tells
you the orbital inclination).

This is incorrect, as has now been explained to you at least twice in
recent months. The combination of a visual orbit, which gives
inclination as well as other orbital parameters,

We don't have 'visual orbits' in the interferometery-only observation under
discussion.

and a radial velocity orbit, which gives the physical orbit size,

One needs inclination to get the physical orbit size.

can indeed
determine distance. If it still isn't obvious to you, I suggest you
consult a textbook for details.

I'm well aware of the limitations, thanks.

Let's recap the statement that started all this...

From the PR flack sheet:
"The new results come from careful observation of the orbit of Atlas and its
companion -- a binary relationship.... Using data from the Mount Wilson
stellar interferometer, ... and the Palomar Testbed Interferometer..., the
team determined a precise orbit of the binary."

If -- as the flak sheet stated -- the data was only from two
interferometers, then we don't have a 'visual orbit.' Now, it could be that
the flak sheet simply didn't mention visual observations, and only mentioned
the interferometer observations.

--
greywolf42
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In article ,
"greywolf42" writes:
We don't have 'visual orbits' in the interferometery-only observation under
discussion.

Just what do you think the interferometer observations give, if not a
visual orbit? Do you agree that a "visual orbit" is derived from
measurements of separation and position angle as a function of time?
Which of these do you think is missing?

--
Steve Willner Phone 617-495-7123
Cambridge, MA 02138 USA
(Please email your reply if you want to be sure I see it; include a
valid Reply-To address to receive an acknowledgement. Commercial
email may be sent to your ISP.)

"g" == greywolf42 writes:

g Let's recap the statement that started all this...

g From the PR flack sheet: "The new results come from careful
g observation of the orbit of Atlas and its companion -- a binary
g relationship.... Using data from the Mount Wilson stellar
g interferometer, ... and the Palomar Testbed Interferometer..., the
g team determined a precise orbit of the binary."

g If -- as the flak sheet stated -- the data was only from two
g interferometers, then we don't have a 'visual orbit.' Now, it
g could be that the flak sheet simply didn't mention visual
g observations, and only mentioned the interferometer observations.

I'm going to ignore the excessive nit-picking about whether an
interferometric image can be considered a "visual" image. If your
point is to point out that press releases sometimes simplify
explanations to the point that they can be erroneous, well, that's a
point that's been made here many times. Depending upon the
institution, sometimes the astronomers have little control over the
press release. In all cases, one is trying to balance between trying
to provide an accurate explanation as well as provide analogies or
explanations that do not "scare" off people. (How far do you think a
press release would get if it talked about the Fourier transform
imaging implicit in all interferometers?) Frankly, I don't like
either situation, but picking on this particular PR wouldn't seem to
rectify the situation.

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