Re: Milky Way MACHO detected

From: Robert J. Bradbury (
Date: Sat Dec 08 2001 - 07:29:03 MST

Fiddlesticks (or why I hate press releases...)

The "real" articles behind this are in Nature this week.

Alcock, C. et al, "Direct detection of a microlens in the Milky Way"

Comment by Andy Gould:
"Astronomy: Gravitational microlens in motion"

Starting with the LLNL Press release (provided by Damien)...
[You would think *they* could get it right given that this
is where the team is located!]

> By fusing microlensing light data, high-resolution images and
> spectroscopy, researchers can finally view a complete picture
> of a MACHO (Massive Compact Halo Object) by measuring
> its mass, distance and velocity.

The key here is that they are actually using both the Hubble and
a spectrometer on the ESO VLT to do this work (that is good because
if one wants to detect a Matrioshka Brain/Dyson Shell the spectra
will be critical). However the star and the lens are still too
close together to separate the spectra so interpretation is
"complicated". (To my uneducated eye, I'll accept the spectra
appears to be a star.)

> This demonstrates that
> precision brightness measurements and extensive follow-up
> will allow astronomers to characterize a significant fraction of
> the Milky Way's dark matter.

Hmmmm,.... What the article actually says:

"We note that the detection of one such event in our current sample of
13-18 events is completely consistent with the expected number of
Milky Way events associated with known populations. We have completed
HST analysis of 17 of these events, and find no other plausible candidates
for lensing by Milky Way stars."

1 out of 17 isn't, IMO, a "significant fraction". Bottom line -- for most
of the events characterized so far they are just as clueless as they were
5 years ago. The Lens magnitude (V = 22.6) is quite a bit under the limit
of the Hubble (V >= 26) so I would expect that they could detect faint
stars causing lensing out to significantly greater distances. They did not.

> which turned out to be a red star in the Milky Way.

They postulate it is an M4-5 red dwarf with an absolute magnitude of 13.6.
> The observation makes it possible to determine the mass of the
> MACHO and its distance from the Earth. In this case, the
> MACHO is a small star with a mass between 5 percent and 10
> percent of the mass of the sun at a distance of 600 light-years.

Cough. There are some pretty significant conflicts in the distance/star
type conclusions. Using parallax, they compute the distance at 200 pc
(652 ly), but using stellar type and brightness they get 650 +/- 190 pc
(or 2120 ly). It seems they feel the brightness measurements are less
accurate because they cannot yet measure the metalicity of the lens
(which effects brightness computations).

>From the ESA Press Release:

> Dark Matter
> ESA/Hubble
> Hubble Spots Dark Matter?
> Astronomers believe they may have finally solved one of the
> greatest mysteries in astronomy; or maybe they just found
> another star. Using the Hubble Space Telescope, the team
> captured an image of a red dwarf star using a technique
> called microlensing; another star in front acted like a
> gravitational lens to focus the red dwarf's light to make it
> visible on Earth. Red dwarf stars like this could make up a
> portion of what astronomers call "Dark Matter".
> Original Source: ESA News Release
> Other Coverage: SpaceDaily - BBC News - -
> Spaceflight Now
> Similar Stories: Astronomy

Actually the star is visible without the lens (if you
have a large enough telescope). It is also generally accepted
that there the total mass of red dwarf stars is unlikely to make
up a meaningful fraction of the Dark Matter. It seems all we have
so far is general agreement between accepted stellar populations
(which do not include any fraction of non-visible Dysonized stars)
and the fraction of the microlensing events caused by such visible stars.


This archive was generated by hypermail 2b30 : Sat May 11 2002 - 17:44:24 MDT