The distances to AM CVn, HP Lib,

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The distances to AM CVn, HP Lib, CR Boo, V803
Cen and GP Com
Paul Groot, Radboud University
Nijmegen
Fritz Benedict, Barbara McArthur (UTexas
Austin) Gijs Nelemans, Luisa Morales-Rueda, Gijs
Roelofs (RU) Tom Marsh (Warwick), Danny Steeghs
(CfA)
Paul Groot, Nijmegen Workshop on AM CVn stars 2005
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The very short version of the talk
dAM CVn 606 -95135 pc
dHP Lib 197 -1213 pc
dCR Boo 337 -3543 pc
dV803 Cen 347 -2731 pc
dGP Com 75 -22 pc
Paul Groot, Nijmegen Workshop on AM CVn stars 2005
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Overview

• Why do we want distances?
• How do we get distances?
• How is it done in practice
• Results
• Comparisons
• Derived parameters

Paul Groot, Nijmegen Workshop on AM CVn stars 2005
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Distances are crucial

• Sets the luminosity
• Sets the mass-transfer rate
• Enables the calculation of space densities
• Allows to establish population memberships

Paul Groot, Nijmegen Workshop on AM CVn stars 2005
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How to get them

• As always one of the most difficult tasks
• Model fitting (secondaries, disks, SEDs)
• Ground-based parallaxes
• Space-based parallaxes

Paul Groot, Nijmegen Workshop on AM CVn stars 2005
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Currently most accurate method

• HST Fine Guidance Sensor parallaxes

Paul Groot, Nijmegen Workshop on AM CVn stars 2005
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HST Fine Guidance Sensors

• One of the Guide Sensors can be used to measure
  accurate
• positions.
• It is a white light interferometer
• Yields parallaxes with a precision of 0.2
  milli-arcseconds
• Sensitive to V 16

Paul Groot, Nijmegen Workshop on AM CVn stars 2005
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AM CVn observations

• Obtained observations of AM CVn, HP Lib, CR
  Boo, V803 Cen
• and GP Com
• Observations consisted of 5 pairs of visits
  spanning 1.5 years
• (2.5 years for GP Com)
• Observations yield parallax and proper motion

parallax
proper motion
Paul Groot, Nijmegen Workshop on AM CVn stars 2005
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AM CVn observations

• Observations were obtained at maximum and
  minimum parallax
• Paired because of possible brightness changes,
  separated by
• two weeks (happened once).
• A total of 50 HST orbits (5x5x2).
• On average 3-4 visits per object and reference
  stars (3-5)

GP Com!
Paul Groot, Nijmegen Workshop on AM CVn stars 2005
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AM CVn Results

• After correction for spectroscopic parallax of
  reference stars
• After correction of the Lutz-Kelker bias (which
  underestimates
• the parallax)
• pAM CVn 1.65 /- 0.30 mas µ AM CVn
  34.25 0.88 mas/yr
• p HP Lib 5.07 /- 0.33 mas µ HP
  Lib 33.59 1.54 mas/yr
• pCR Boo 2.97 /- 0.34 mas µ CR Boo
  38.80 1.78 mas/yr
• pV803 Cen 2.88 /- 0.24 mas µV803
  Cen 9.94 2.98 mas/yr
• pGP Com 13.34 /- 0.33 mas µ GP Com
  352.36 12.79 mas/yr

Paul Groot, Nijmegen Workshop on AM CVn stars 2005
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Correction for AV

• Done on the basis of the spectra of the
  reference stars,
• combined with the colours of the objects V, J,
  K

Paul Groot, Nijmegen Workshop on AM CVn stars 2005
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Lutz-Kelker bias
Errors are on the parallax, not the distance.

Since the volume sampled is much larger at higher
distances (smaller parallaxes), there will be
more objects that scatter into the parallax bin
from lower parallaxes then from higher
parallaxes so you will underestimate the distance
from this.
Paul Groot, Nijmegen Workshop on AM CVn stars 2005
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AM CVn distances
AM CVn HP Lib CR Boo
V803 Cen GP Com pabs (mas) 1.65(30)
5.07(33) 2.97(34) 2.88(24)
13.34(33) µrel (mas/yr) 34.25(88)
33.59(1.54) 38.80(1.78) 9.94(2.98)
352.36(12.79) Pos. Angle 67.0(1.7)
314(14) -79.9(3.7) 248(11)
-84.7(3.1) m-M 8.9(9)
6.5(3) 7.6(6) 7.7(4)
4.37(12) V 14.0(1)
13.6(1) 14.5(1) 13.5(1)
15.9(1) Av 0.05(2)
0.34(6) 0.03(4) 0.31(9)
0.02(11) LK Corr -0.27
-0.04 -0.11 -0.06
-0.01 Mv 4.8(9)
7.0(3) 6.5(6)
5.4(4) 11.3(2)
Paul Groot, Nijmegen Workshop on AM CVn stars 2005
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Comparisons
AM CVn Ground based parallax 235 pc (Dahn,
priv.comm.) If that were
correct p 4.25 mas. Compared
to 1.65 0.30 this is 8.7 sigma away!
288 50 pc (Nasser, Solheim Seminoff
2001) 420 20 pc (El-Khoury
Wickramisinghe 2000) HP Lib 188 50 pc (
NSS 2001) CR Boo 469 50 pc (NSS 2001), 206
15 pc (EW 2000) V803 Cen 405 50 pc (NSS
2001) GP Com 68 7 pc (Thorstensen 2004
ground based par) The GP Com value gives
confidence in CE 315 distance done in the same
way by Thorstensen 77 8 pc (priv.com.)
Paul Groot, Nijmegen Workshop on AM CVn stars 2005
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AM CVn Abs. Magnitudes
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.
To get to Lbol and M

• For Mass-radius relation use Eggleton relation
• Calculate RL1 from period, Mwd and q ( 0.1)
• Get Lbol from MV through bolometric correction
  BC

Paul Groot, Nijmegen Workshop on AM CVn stars 2005
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The bolometric correction
T (AM CVn, HP Lib, CR Boo, V803 Cen) 25000 K
BC -2.5 T GP Com 10000 K BC -0.4
Paul Groot, Nijmegen Workshop on AM CVn stars 2005
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.
Accretion luminosities and M
AM CVn HP Lib CR Boo V803
Cen GP Com
L (erg/s) 3.63.0 x1034 4.91.5 x1033
7.64.5 x1033 2.10.8 x1034 1.30.2 x1031
(M?/yr) 1.41.3 x10-8 1.90.5 x10-9
2.81.5 x10-9 7.83.1 x10-9 4.80.8
x10-12
But this is highly dependent on Mwd en BC. E.g.
for AM CVn if Mwd 1.0 Msun
4.6 x10-9 Msun / yr for AM CVn if BC
-2.0 8.8 x10-9 Msun / yr
Paul Groot, Nijmegen Workshop on AM CVn stars 2005
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Paul Groot, Nijmegen Workshop on AM CVn stars 2005
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Paul Groot, Nijmegen Workshop on AM CVn stars 2005