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The Origins of Hot Subdwarf Stars as Illuminated by Composite-
Spectrum Binaries
The Origins of Hot Subdwarf Stars as Illuminated by Composite-
Spectrum Binaries
Michele A. Stark
(Penn State University)
Advisor: Richard A. Wade
Thursday Jan 13, 2005 AAS 205th Meeting, San Diego Session 170 – White Dwarfs and Hot Subdwarfs
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Plan of AttackPlan of Attack
I. What a hot subdwarf is
II. Why hot subdwarfs are cool
III. How hot subdwarfs gets hot
IV. How we are cool:a. 2MASS
b. Spectroscopy
c. Nature of the companions
V. What does it all mean?
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I. Hot Subdwarf ClassificationI. Hot Subdwarf ClassificationSubdwarf B
(sdB)Subdwarf O
(sdO)
Teff (K) ~20,000–30,000 > 30,000
Main Spectral Feature
Hydrogen Helium
L & log g Dispersion
Small Large
Evolutionary Paths to T, L, log g
Single Multiple
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II. Why hot subdwarfs are cool…II. Why hot subdwarfs are cool…• Galactic field Extended
Horizontal Branch (EHB) stars
Mtot = ~0.5 M
Menv < ~0.01 M
• Source of “UV-upturn” (or “UVX”) seen in old stellar populations
• EHB stars have been imaged in UVX galaxies
Brown et al. 2000, ApJ, 532, 308
M32 STIS, UV
25”
25”
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III. How a hot subdwarf gets hot:Binary Formation Scenario
III. How a hot subdwarf gets hot:Binary Formation Scenario
•Current scenario for Galactic field sdB formation
•Idea: Binary interactions significant mass loss(i.e., Mengel et al. 1976; Han et al. 2002, 2003)
•Consequence: nearly all sdB should be found in “close” binary systems
•Known Binary Companions: white dwarfs (P ~ hours 30 days)
handful of very low mass dM (M ~ 0.08 M)
GK-type stars (“long period,” >years = unknown)
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IV. How we are coolIV. How we are cool• Idea:Idea:
Use late-type companions to better understand sdBs (L, d, population, etc.)
• How?:How?: 2MASS sdBs w/ unresolved late-type companions
(J [1.2 m], H [1.65 m], and Ks [2.2 m])
Spectroscopically study the companions
• Goals & Future Significance: Identify nature & physical parameters of the
companions Determine physical parameters of sdBs in composites Provide limits for binary formation channels
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IVa. 2MASS CompositesIVa. 2MASS Composites• Two distinct
“clumps” in 2MASS (J-KS) vs. (J-H) color-color space: “Single” colored Composite colored Composite
“Single”
Blue
Red
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IVa. 2MASS CompositesIVa. 2MASS Composites• Companions:
Late F, G, K only No dM or early F
(most pronounced in the best photometry sample, (Q) < 0.1)
• Magnitude Limited: 60±4% “single” (357) 40±4% composite (237)
• “Volume” Limited: 75±4% “single” (309) 25±4% composite (104)
“Single”
Composite
Blue Red
All
(Q)< 0.2
(Q)< 0.1
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IVb. Spectroscopic Sub-SamplesIVb. Spectroscopic Sub-Samples
• Hot Subdwarfs 21 single 53 composite
• V< 13.5
• J-KS > +0.5
Single
Composite
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IVb. Spectroscopic Sub-SamplesIVb. Spectroscopic Sub-Samples
• “Standards” 84 stars Pop I, MS and SG MV from HIP
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IVb. Spectra ObtainedIVb. Spectra Obtained• KPNO 2.1m GoldCam, McDonald 2.7m LCS• ~4600–8900 Å, ~1.3 Å/pix
Single
Standard
Composite
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IVc. Refining Model GridIVc. Refining Model Grid
• Fit standard star parameters
• Use fits to interpolate grid
• Compare grid to observed hot subdwarf composites
StandardsModel
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IVc. Refining Model GridIVc. Refining Model Grid
• Take interpolated EWs/colors and predict the EWs/colors if diluted by a sdB
• Unable to flux calibrate used Kurucz flux distributions to compute dilutions at different wavelengths
• Compare predicted diluted EWs/colors to observed composites…
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IVc. Individual FitsIVc. Individual Fits
•Best fit comp:(B-V)c0.95
MV,c6.25~K2
•Best fit sdB:(B-V)sdB-0.22
MV,sdB4.10
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IVc. Individual FitsIVc. Individual Fits•Same sdB, different comp.(B-V)sdB ~ -0.24
MV,sdB ~ 4.3
(B-V)c ~ +0.7, +0.9
MV,C ~ 5.3, 6.0
(~F7 V)
(~K1 V)
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IVc. Individual FitsIVc. Individual Fits•Same companion, different sdB(B-V)c ~ +0.9
MV,C ~ 6.0
(B-V)sdB~-0.22,-0.26
MV,sdB ~ 4.1, 4.7
(cool/bright sdB)
(hot/faint sdB)
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IV. Preliminary Fit SummaryIV. Preliminary Fit Summary• Majority of
companions are best fit with main sequence
• A few appear to be subgiants
• Cover the range of subdwarf temperatures (no temperature bias or trends evident)
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IV. Preliminary Fit SummaryIV. Preliminary Fit Summary• Work remains to be done over next ~6
months: Refine model grids and model-matching
technique Investigate possible reddening effects Verify results do not change for terminal-age
EHB
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V. What does it all mean?V. What does it all mean?•Goals & Future Importance:
Identify nature & physical parameters of the companionsDetermine physical parameters of sdBs in compositesProvide limits for binary formation processes
Refine binary formation rates for Algols, SN Ia, CV, etc.
Refine population synthesis models for UV-upturn
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V. What does it all mean?V. What does it all mean?•2MASS Results:
Fraction of composite sdB in current samples: • 40% (magnitude limited)• 25% (volume limited)
There are no companions of dM type or earlier than ~mid-F type in current sdB samples
•(Preliminary) Spectroscopy Results:Most late-type companions are fit by main sequence starsThere are a handful of subgiants
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Special Thanks to:Special Thanks to:
G. B. Berriman
T. Bogdanovic
J. Ding
R. Ganguly
K. A. Herrmann
K. T. Lewis
A. Narayanan
NASA-GSRPNOAO Graduate Travel SupportPennsylvania Space Grant
ConsortiumSigma Xi Grants-in-Aid of
Research Zaccheus Daniel Foundation for
Astronomical Science