Diapositiva 1

1
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)
2
The Very Long Baseline Array (VLBA)
All observations presented here are
phased-referenced and at 3.6 cm
3
Absolute astrometry

• VLBA (at 3.6 cm)
• Angular resolution 1 milli-arcsecond (mas)
• Absolute astrometry precision 50
  micro-arcseconds (?as)
• Phase calibration (and, therefore, astrometry)
  is relative to quasars

Perfect instrument for astrometry

• Possible origins of motion for Galactic
  astronomical sources
• Trigonometric parallax ( ? 1/d )
• Proper motion
• of large scale origin (Galactic
  rotation, cluster dynamics, etc..)
• of small-scale origin (e.g.
  orbital motions in multiple systems)

4
What the VLBA can do for you


?
?

• d(pc) 1 2
  5 10 20 50 100
  200 500 1,000 2,000
  5,000 10,000
• (mas) 1000 500 200
  100 50 20 10 5
  2 1 0.5
  0.2 0.1
• ? (mas yr -1) 2000 1000 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
5
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.

• Accurate observational data are needed to
  constrain the models.
• Distance errors dominate the error budget

6
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.
7
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.
8
T Tauri
Loinard et al. 2007
d 147.6 /- 0.6 pc -- 0.5 precision
9
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
10
Taurus (more interesting)
3D Structure
L1495 130 pc
East 160 pc
South 145 pc
Torres et al. 2008
11
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.
12
What does that tell us about early stellar
evolution?
HP Tau (A small coeval group) (Torres et al. 2009)
13
?-ophiuchus

• Debate is it at 120 pc (as claimed recently) or
  at 160 pc (as traditionally thought)??

Knude Hog 1999
14
?-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 -5.325.83  pc
d 116.90 -6.417.20  pc
Loinard et al. 2008
15
Ophiuchus mean distance
d 120 /- 4 pc
16
(No Transcript)
17
But
4 pc
Haro1-14c 111 /- 19 pc
Encrenaz et al. 1975 Schaefer et al. 2008 Imai et
al. 2007
I16293 178 /- 30 pc
18
Orion
19
Perseus
Hirota et al. 2008
d 235 /- 18 pc
20
The nearest star-forming regions Goulds belt
141 /- 10 pc
414 /- 7 pc
120 /- 4 pc
235 /- 18 pc
From 200 to 400 pc
21
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.

22
Perspectives?
L1495 130 pc
East 160 pc
South 145 pc
Torres et al. 2008
23
What is we had 50 stars rather than 5?
3D Structure
24
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.)
25
100 pc
26
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.)
27
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.
28
Thanks!
29
(No Transcript)
30
Back to T Tauri a triple system
T Tau N
T Tau Sa
T Tau Sb
Duchene et al. 2002
The bright compact radio source detected at the
VLBA is T Tau Sb.
What about orbital motions?
31
Orbital motions in the Sa/Sb system
IR Data
VLBA Data
Recent Keck observations Courtesy Gail Shaeffer
32
Orbital motions V773 Tau
Duchene et al. 2003
33
Source V773Tau (a spectroscopic binary in a
quadruple system)
Very accurate mass determinations of ALL the
members of that system
First six VLBA observations (of 18)
34
Source V773Tau (a spectroscopic binary in a
quadruple system)
Relation between orbital phase and flux