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The distance to nearby star-forming regions
New science enabled by micro-arcsecond astrometry
Socorro, July 21, 2009
Laurent Loinard
Centro de Radioastronomía y Astrofísica, UNAM
Morelia, Michoacán
Main Collaborators:
Rosy Torres (UNAM)
Luis F. Rodriguez (UNAM )
Amy Mioduszewski (NRAO)
The Very Long Baseline Array (VLBA)
Brewster
Washington
North Liberty
Iowa
Hancock
New Hampshire
Owens Valley
California
Mauna Kea
Hawaii
St. Croix
Virgin Islands
Kitt Peak
Arizona
Los Alamos
New Mexico
Pie Town
New Mexico
Fort Davis
Texas
All observations presented here are phased-referenced and at 3.6 cm
Absolute astrometry
VLBA (at 3.6 cm):
o Angular resolution: 1 milli-arcsecond (mas)
o Absolute astrometry precision: 50 micro-arcseconds (as)
o Phase calibration (and, therefore, astrometry) is relative to quasars
Perfect instrument for astrometry
Possible origins of motion for Galactic astronomical sources:
o
Trigonometric parallax (  = 1/d )
o
Proper motion

of large scale origin (Galactic rotation, cluster dynamics, etc..)

of small-scale origin (e.g. orbital motions in multiple systems)
What the VLBA can do for you…
+
=

d(pc)
(mas)
 (mas yr -1)

1
2
1000 500
2000 1000
5
10
20
50
100
200
500
1,000
2,000
5,000
10,000
200
100
50
20
10
5
2
1
0.5
0.2
0.1
400
200
100
40
20
10
4
2
1
0.4
0.2
(Proper motions calculated for v = 10 km/s)
Recall: the absolute astrometry precision of the VLBA is better 0.05 mas
Rationale (I)
While main sequence stellar evolution is reasonably well-understood, our
comprehension of the birth and pre-main sequence evolution of stars is
significantly poorer.
o Accurate observational data are needed to constrain the models.
o Distance errors dominate the error budget
Rationale (II)
Much of what we know about star-formation is based on observations of
just a handful of nearby sites of star-formation (Taurus, Ophiuchus, Orion,
etc.)
Improving the distance estimate for those few regions will have a significant
impact on star-formation studies.
Those regions are within 1 few hundred parsecs, well within the reach of the
VLBA.
Those regions contain adequate non-thermal sources.
The nearest star-forming regions: ``The Gould belt’’
Measure the distance to several young stars in nearby
star-forming regions to obtain an accurate estimate of the
distances to those star-forming sites.
Up until now, 4+ regions have been studied: Taurus, Ophiuchus, Perseus,
Orion, and a bit of Serpens.
T Tauri
Loinard et al. 2007
d = 147.6 +/- 0.6 pc -- 0.5% precision…
Other sources in Taurus
Star
Distance (pc)
T Tau
147.6 +/- 0.6
Hubble 4
128.5 +/- 0.6
HDE283572
130.8 +/- 0.5
HP Tau
161.2 +/- 0.9
V773 Tau
134.2 +/- 3.7
d = 141 pc
d = 128.5 +/- 0.6 pc
d = 130.8+/- 0.5 pc
Torres et al. 2007, 2009a, 2009b
Taurus (more interesting…)
East: 160 pc
3D Structure
L1495: 130 pc
South: 145 pc
Torres et al. 2008
Taurus (more interesting…)
Ballesteros-Paredes et al. (1999)
CO data: Ungerechts & Thaddeus (1988)
With this orientation nearly along the Galactic center-Galactic anticenter direction, tidal
forces due to the Galactic potential oppose gravity and cloud collapse (Ballesteros-Paredes
2009). This may help explain the low star-forming efficiency of Taurus.
What does that tell us about early stellar evolution?
HP Tau
(A small coeval group)
(Torres et al. 2009)
-ophiuchus
• Debate: is it at 120 pc (as claimed recently) or at 160 pc (as traditionally thought)??
Knude & Hog 1999
-ophiuchus
• Debate: is it at 120 pc (as claimed recently) or at 160 pc (as traditionally thought)??
• So far, two stars observed (S1 and DoAr21) - more to come…
d = 121.89 -5.32+5.83 pc
d = 116.90 -6.41+7.20 pc
Loinard et al. 2008
Ophiuchus mean distance…
d = 120 +/- 4 pc
But…
4 pc
Haro1-14c: 111 +/- 19 pc
Encrenaz et al. 1975
I16293: 178 +/- 30 pc
Schaefer et al. 2008
Imai et al. 2007
Orion
Perseus
Hirota et al. 2008
d = 235 +/- 18 pc
The nearest star-forming regions: Gould’s belt
141 +/- 10 pc
235 +/- 18 pc
120 +/- 4 pc
From 200 to 400 pc…
414 +/- 7 pc
Conclusions
• The VLBA can measure the distance to star-forming regions in the
Gould Belt to very good precision.
• Taurus is, on average, at about 141 pc (but with significant depth).
• Ophiuchus is at 120 pc, Orion is at 414 pc, Perseus is at 235 pc.
•We are working on Serpens.
Perspectives?
East: 160 pc
L1495: 130 pc
South: 145 pc
Torres et al. 2008
What is we had 50 stars rather than 5?
3D Structure
Perspectives (I)
With 10 times more sensitivity (after the VLBA sensitivity upgrade is complete), it should become
possible to truly map out the distribution of star-forming regions around the Sun. Of course, better
sensitivity also implies closer calibrators, and better calibration
E.g.: Taurus (of course), Orion is much larger than the Orion Nebula, It is not clear if all of
Perseus is at the same distance (NGC1333 vs. IC 348), Ophiuchus streamers, etc.. (Note that these
regions tend to be heavily obscured, so optical experiments are unlikely to improve significantly the
situation: the VLBA results will be used for decades to comoe).
Obtain a list of more than a hundred stars with well-measured parameters (luminosity, age,
spectral type, etc.)
100 pc
Perspectives (I)
With 10 times more sensitivity (after the VLBA sensitivity upgrade is complete), it should become
possible to truly map out the distribution of star-forming regions around the Sun. Of course, better
sensitivity also implies closer calibrators, and better calibration
E.g.: Taurus (of course), Orion is much larger than the Orion Nebula, It is not clear if all of
Perseus is at the same distance (NGC1333 vs. IC 348), Ophiuchus streamers, etc.. (Note that these
regions tend to be heavily obscured, so optical experiments are unlikely to improve significantly the
situation: the VLBA results will be used for decades to comoe).
Obtain a list of more than a hundred stars with well-measured parameters (luminosity, age,
spectral type, etc.)
Perspectives (II)
If there is sufficient fraction of binaries, get well-measured masses
However… we have observed pretty much all the known non-thermal emitters in star-forming
regions. Need first to identify new sources
Solution: a multi-beam VLBA search thanks to the new software correlator.
Thanks!
Back to T Tauri : a triple system
T Tau N
Duchene et al. 2002
The bright compact radio source detected at the
VLBA is T Tau Sb.
What about orbital motions?
Orbital motions in the Sa/Sb system
IR Data
VLBA Data
Recent Keck observations: Courtesy Gail Shaeffer
Orbital motions: V773 Tau
Duchene et al. 2003
Source V773Tau (a spectroscopic binary in a quadruple system)
First six VLBA observations (of 18)
Very accurate mass determinations of
ALL the members of that system
Source V773Tau (a spectroscopic binary in a quadruple system)
Relation between orbital phase and flux