Observing Binary Stars

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Observing Binary Stars

• Claudio Melo
• European Southern Observatory

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Some definitions

• Seeing the quality of observing conditions
  induced by turbulence in Earth's atmosphere,
  which blurs the images of astronomical objects.
  
• Seeing is measured in arcsec
• 1 arc sec car headlights 100km
• Angular size of the Moon Angular size of the
  Sun 30 arc min
  
• Telescope resolution The ability of a telescope
  to distinguish fine detail on planets or to see a
  close double star as two separate images is
  called the resolving power.

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Some definitions

• Difraction limit of the VLT (8.2m) as a function
  of the wavelength
  
• Human eye resolving power of 1 arcmin
• Seeing at 0.5mm around 0.3'' (very good night) to
  2.0'' (poor night). On average 0.8-1.1''
  
• Thus, due to the atmosphere, the images are
  seeing limited, even if the telescope can do
  better!

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Visual Binaries

• Two stars gravitationally bound orbiting around a
  common centre of mass in a eliptical motion which
  can be detected and resolved using some imaging
  devices (photographic plate, CCDs).

• Typical angular separation many times l/D
• Tens to a few hundred AUs
• aCen B is 23 AUs apart from aCen B

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High angular resolution thechniquesHow to beat
the atmosphere

• Speckle interferometry Idea is to observe an
  object faster than the time-scale in which the
  atmosphere changes, i.e., 10ms (in the optical)
  and 100ms (in the infra-red) ? Diffraction limited

• Only good for very bright objects due to the very
  short exposure times ? determined by the read-out
  noise
  
• Dmk4 for d l/D
• Analyze the data in the Fourier space
• More about it on Elliots talk

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High angular resolution thechniquesHow to beat
the atmosphere
Wave front distorted by the atmosphere

• Adapative optics A deformable mirror changes its
  shape in order to compensate for the effects of
  the atmosphere.

Deformable mirror
Real-time calculator
Instrument focus
Wave front analyzer
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High angular resolution thechniquesHow to beat
the atmosphere
Wave front distorted by the atmosphere
Deformable mirror
T-Tau
Real-time calculator
Wave front analyzer
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High contrast imaging
30 MJup
5 MJup
J18 (10.2)Ks17.4(10.2)sep4.8
equ. 30MJup. (10 Myrs) J20.2(12.4)Ks19.4(12.2
)sep2.8
equ.3-5 MJup (10 Myrs)

• Sensitive to a few Jup. mass planets, wide
  (50AUs) sep.
  
• (Chauvin et al.,2003)

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Astrometric binaries

• Since L M3, the photocenter of a binary system
  is much closer to the primary star than the
  barycenter. Thus, in addition to the proper
  motion of the binary system, small jitters will
  appear corresponding to the orbital motion of the
  primary.
• Sirius and Procyon were the first astrometric
  binaries found in 1844 by Bessel.
  
• Hard to find binaries ? need long-term
  astrometric campaings and high accuracy
  
• Hipparcos (120000 stars) ? 2-4mas down to V12.0
  (Complete to V9.0)
  
• Next generation GAIA ? V16 (complete V15),
  5x107 targets and accuracy of 10mas
  
• Planetary case PRIMA (Martins talk)
• Also many other talks

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Spectroscopic binaries

• Close multiple systems (Pdays) that can only be
  detected by the periodic wobble of the spectral
  lines in the combined spectrum.
  
• SB1 Only one set of lines is seen
• SB2 (shown below) Two sets of lines are seens in
  the composite spectrum

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Radial Velocity

• Cross-correlation Function (CCF) is the result of
  the convolution of the object spectrum by a
  template

?
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The CCF

• Almost all lines participate in the convolution
• The CCF is a sort of mean spectral line
• Sensitivy to global phenomenona affecting all
  lines (rotation, radial velocity, metallicity,
  spots)
  
• Can achive good results with low S/N spectra (for
  late-type stars)

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Radial Velocity Curve Orbital Solution

• The geometry of the problem and radial velocity
  curve fitting
  

• a semi-major axis
• e eccentricity
• T periatron passage
• w longitude of periastron
• W longitude of the ascending node
• q true anomaly at time t
• K semiaplitude of the velocity curve
• P orbital period

Line of sight
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Examples of Orbital Solution
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Finding Extra-Solar Planets
First extra-solar planet
T-Tauri SB23rd companion
Whats the main difference in these radial
velocity curves for the planetary and stellar
case?
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Radial Velocity Accuracy

• Planets Ka few tents of m/s
• Stars Kbroad range of values but always km/s

• We have to be able to control the sprectrograph
  shifts through the night related to changes in
  the temperature and atmospheric pressure
  (ThAr)50m/s per mbar or 0.3 degrees
• Keep the zero-point (good calibrations)

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HARPS The state-of-the-art of RV measurements
ThAr simultaneous Calibrations fiber
Object fibers
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Binary frequency among the solar-type stars

• Duquennoy Mayor (1991)
• Sample 164 stars (F7-G9) in the solar
  neighborhood
  
• Radial velocity measurements collected during 13
  yrs. with CORAVEL

60 of the solar-type stars in solar
neighborhood are in multiple systems!
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...and among the T-Tauri stars?

• Near IR high-angular resolution (e.g. Speckle,
  direct imaging, lunar occultation)
  
• Typical resolving power 0.013"-13.0" (depending
  on the ?mk)log Porb ? 3.5 - 7.5 days

The binary rate among the T-Tauri is 2x higher!
Binarity is established very soon 106 yrs ?
stellar formation
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Questions

• Is the PMS binary frequency distribution 2x
  higher everywhere?? PMS Binary frequency 100
  
• Or it's only the shape of the distribution which
  changes in time?

The key is to measure the binary frequency over
an orbital period interval as large as that
explored by Duquennoy Mayor (1991)
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(No Transcript)
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Spectroscopic Campaign

• Sample of 65 stars already observed with
  high-angular-resolution techniques
  
• Ghez et al. (1993) ? ?-OphGhez et al. (1997) ?
  Lup/Cha/CrA
  
• GOAL measure the SB frequency of a sample for
  which the visual binary frequency is already
  known
  
• 3 years radial velocity survey performed at
  ESO/La Silla with the 1.2m Swiss Euler
  Telescope/CORALIE and 1.5-m ESO/FEROS

give the companion fraction per star
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Results 1

• 0 ? log Porb ? 1
• 1 triple system.csf0.04?0.03 (MS0.02?0.02)
• 1 ? log Porb ? 2
• 1 triple system, 1 binary.csf0.07?0.03
  (MS0.04?0.02)
  
• 4.5 ? log Porb ? 6.0
• csf0.28?0.05 (MS0.15?0.05)
• 6.0 ? log Porb ? 7.6
• csf0.24?0.04 (MS0.11?0.04)

Melo (2001)
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Results 2

• No excess among the SB
• 2x more binaries among the VB
• SB frequency that found by Mathieu et al. (1992)

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Summary

• 60 of field solar-type stars are in multiple
  systems (Duquennoy Mayor 1991)
  
• Among the T-Tauri stars Visual BF of 1-2x MS
  stars (Patience Duchêne 2001)
  
• The T-Tauri SB frequency MS SB 10
• Oph seems to have a SB frequency higher than 10,
  however the results are not statistically
  significant

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Patience Duchêne (2001)
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Patience Duchêne (2001)