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Centro de Radioastronomía y Astrofísica, UNAM
Campus Morelia
Proper Motions of Stars and Gas in our
Galaxy
Luis F. Rodríguez
Centro de Radioastronomía y Astrofísica, UNAM,
Campus Morelia
Radio results of proper motions of stars and gas and in our Galaxy that
allow in some cases the determination of masses and distances and in
others a much deeper understanding of the origin, age and evolution of
the source studied.
Will not include extragalactic results, neither results involving maser
lines. The proper motions discussed here are obtained from continuum
(thermal and nonthermal) observations of galactic sources, moving at
nonrelativistic speeds.
The radial velocity of a source
can be obtained from a single
measurement and the application
of the Doppler effect. In contrast,
the plane-of-the-sky velocity
requires of at least two
measurements, as separated in
time as possible.
The effect is usually very small…
• A body moving at the respectable speed of
1,000 km/s in the pane of the sky, and
located at the center of the Milky Way (at
8.5 kpc), will take 40 years to move one arc
second.
Definition of proper motion
STELLAR MASSES
• Most of the information on stellar masses comes from the
study of orbital motions in binary systems, using Kepler´s
third law:
4 2 a 3
mM 
2
G P
STELLAR MASSES
• That is, if we know a, the semimajor axis
of the orbit, and P, the orbital period, we can
find m+M, the sum of the masses of the two
stars. But, how can we study very young
stars that form inside clouds with high
obscuration and that cannot be studied with
the usual optical and infrared techniques?
• Fortunately, some young stars have radio
emission that allows this type of studies.
RADIO OBSERVATIONS
• Remarkably, protostars can be tracked at
radio wavelengths due to three processes:
1. Gyrosynchrotron from active stellar
magnetosphere
2. Free-free emission from ionized outflows
3. Thermal emission from circumstellar disks
No extinction. However, processes (2) and (3)
produce extended sources. These emissions can
or cannot be present.
L1551
Ha + [SII]
Devine et al. (1999)
L1551 IRS5
VLA-A 2 cm
Proper Motions
• The large, lineal motions are due to the
relative motion between the Sun and the
object and they coincide with what is
expected.
• However, there is also relative motion
between the two components, suggesting
orbital motions.
Total proper motions
“Relative” proper motions
From the observations and making the
following (reasonable) assumptions:
•
•
•
•
Plane of orbit near plane of the sky.
Circular orbit.
=> M+m = 1.2 Msun; P = 260 years
If in the main sequence, the luminosity of this
system should be like 1 solar luminosty, but it
actually has like 30 solar luminosity.
• This confirms that, as expected, forming stars
have a large luminosity excess, most probably as a
result of accretion.
IRAS 16293-2422, VLA-A, 3.5 cm, un
sistema triple
VLBA
Very Long Baseline Interferometry
• You can get amazing positional precision,
0.0001” and better.
• Not always can be applied, the source has to be
very compact and relatively intense (implying
nonthermal processes).
• With VLBI you can measure the subtle effect of
the geometric parallax, that can provide accurate
distances.
Stellar parallax
As the Earth moves in its
orbit around the Sun, the
nearby stars seem to
change their position with
respect to the remote,
“fixed” stars.
d=1/p
d = distance to star in
parsecs
p = parallax angle of the
star in arc seconds.
You detect the combination of the elliptical motion
of the parallax plus the lineal proper motion due to
relative motion.
T Tauri: prototype of a stellar class.
Distance = 149.0 +- 0.8 parsec, the best precision
achieved for this type of stars (Loinard et al. 2006).
Hipparcos
Why can we do better than astrometric
satellites for this type of stars?
Young stars are highly obscured in the optical
and infrared or, if detected, they are
associated with nebulosity that difficults the
astrometry.
Not all the stars are in bound
orbits…
• In the Orion Nebula we found two young
massive stars that appear to have been
ejected from the same point some 500 years
ago (the same age of the University of
Valencia).
BN moves to the NW at
27+-1 km s-1.
I moves to the SE at
12+-2 km s-1.
Encounters of three or more bodies can
produce the formation of close binaries or
even mergers, with the ejection of one star
and possibly of gas (Bally & Zinnecker
2005).
Reipurth (2000)
As a matter of fact, surrounding
the BNM/KL region, there is a
well known molecular outflow
with an upper limit of 1,000
years for its age.
It is conceivable that the
molecular outflow and the
ejection of BN and I happened
in the same event.
The energy in the outflow is
about 4X1047 ergs, that can be
provided by the formation of a
close binary system or even a
merger.
Possibility of mergers is very
important because it may
provide alternate way to form
massive stars.
Nebular expansion
• Herbig-Haro objects
• Bulk expansion of planetary nebulae and
compact H II regions.
Radio HH objects can be seen to move away from central source.
Velocities are indicators of the mass of the central source; one of
the few ways to establish if we are dealing or not with massive
stars.
Finally, can you measure the expansion of relatively
extended objects like planetary nebulae and HII regions?
Yes, measure “bulk” motion.
You can get the distance to these
nebulae
• 1. With spectroscopy, find expansion
velocity.
• 2. Measure angular expansion in the plane
of the sky.
• 3. Use relation between distance, velocity,
and proper motion to find distance.
• A recent example:
M2-43
Guzmán et al,
(2006)
VLA observations
Distance = 6.0 +1.5 kpc, largest
distance
determined with
this technique.
Most of the error
comes from the
modeling, not from
the observations.
Conclusions
• As a result of great improvements in
angular resolution and positional accuracy,
astrometry undergoes a renaissance.
• Approach that takes advantage of old
archival data that can be compared with
more recent observations.
• Addresses key parameters in astronomy
such as mass, distance, and age.