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POST COMMON ENVELOPE BINARIES
FROM THE
SLOAN DIGITAL SKY SURVEY
Alberto Rebassa-Mansergas
Supervisor : Dr. Boris Gaensicke
Co-supervisor : Dr. Pablo Rodríguez-Gil
Working with:
Dr. Linda Schmidtobreick
Dr. Matthias Schreiber
INTRODUCTION
PCEBs = wide MS binaries + CE phase
Friction within the envelope leads to a rapid decrease of the binary orbit
E and J extracted from the orbit ejects the CE
WDMS binaries = WD + MS
(no CE)
PCEBs are the progenitors of the following fascinating systems:
- double degenerates
- gamma ray bursts
- super soft sources
- black-hole candidates
- CVs and X-Ray binaries
- milisecond pulsars
- supernova Type Ia
Population models are available
clear lack of observational
constraints
We need to establish a large sample of one type of close compact binaries
PCEBs consisting of a WD and a MS are the best systems because they are:
- numerous (population studies are feasible)
- well understood in terms of single star evolution
- nearby and easily accessible with 2-8m telescopes
- no mass transfer systems
18 PCEBs identified from RVs
SDSS
~1500 WDMS
stellar parameters (WD + MS)
IDENTIFYING PCEBs IN THE SDSS
WD clearly visible in the blue
The MS dominates the red
IDENTIFYING PCEBs IN THE SDSS
WD clearly visible in the blue
The MS dominates the red
Na λλ 8183.27,8194.81 doublet
Hα emission (if present)
~10% of the spectroscopic SDSS objects are observed more than once
RV variations will identify such a system as a strong PCEB candidate
SDSSJ0246+0041 display an
extremely large radial velocity
variation
Hα emission =
Gaussian
+
parabola
Na doublet =
double-Gaussian
fixed separation
+
parabola
18 strong PCEB candidates imply ~15% in our WDMS sample. However:
- In most cases only two spectra are available
- The low spectral resolution of SDSS limit the detection of significant radial
velocity change to ~10 km/s
- Na doublet will smear in binaries with extremely short orbital periods
PCEB fraction among the SDSS WDMS might be higher than predicted,
probably in agreement with the ~20% obtained by the population models
Follow-up with higher spectral resolution will be necessary to confirm
this hypothesis
STELLAR PARAMETERS
Decompose the WDMS into its WD and MS components
M-dwarf templates, a grid of observed WD templates and a grid of WD model spectra
Two steps
(1) Fit the WDMS spectra model
M-dwarf Sp
the flux scaling factor between
the M star template and the
observed spectrum
(2) M-dwarf template subtracted
Residual line profiles in the WD
fitted with the grid of WD models
WD Teff and log(g)
the flux scaling factor between the WD
model and the WD observed spectrum
WD Mass from Bergeron et al's (1995) tables
Teff and log(g) obtained from the fit to the whole spectrum were select
the “hot” or “cold” solutions from the line profiles
Histograms consistent in broad terms
with other authors:
- WD mass peaks at 0.6 solar masses
- The most common Sp are M3-M4
- The most frequent Teff are between
10000-20000 k.
- log(g) peaks at log(g) = 8
Distances estimated from the best-fit flux scaling factors of the two spectral
components:
For the WD:
For the M-star:
It is necessary to assume a radius for the secondary star
This requires a spectral type-radius relation for M stars
Problem! Lack of observational work
Compile Sp and R from the literature
empirical Sp-R relation for M stars
“Average” relation irrespective of
- ages
- metallicities
- activity levels
The Sp-R relation is compared to:
- Theoretical models
- Directly measured radii from
eclipsing binaries and
interferometry
- Directly measured radii from
eclipsing WDMS binaries
(RR Cae, NN Ser, DE CVn,
RXJ2130.6+4710,
EC 13471-1258)
2/3 of the systems have d(sec) ≈ d(wd) within their errors. However, there is a clear
trend for outliers where d(sec) > d(wd)
- Systematic problems in the WD fits?
- A relationship with close binarity?
- Problems in determining the Sp of the secondary star?
- Problems in the Sp-R relation?
Could magnetic activity affect the Spectral type of the secondary?
We assume that the secondary star appears hotter that it should for
its given mass. This implies a change of 1-2 Sp subclasses, and hence
a change in the Teff and the radius.
Could magnetic activity affect the Spectral type of the secondary?
We assume that the secondary star appears hotter that it should for
its given mass. This implies a change of 1-2 Sp subclasses, and hence
a change in the Teff and the radius.
CONCLUSIONS
- We have identified 18 PCEBs and PCEB candidates among a sample of 101
WDMS for which repeat SDSS spectroscopic observations are available.
- From the SDSS spectra we determine Sp of the companions, Teff, M, log(g) of
the WDs, as well as distance estimates to the systems. Even though some of
the stellar parameters obtained from our decomposing/fitting technique differ
from those obtained from other authors, our results agree in broad terms.
- In about 1/3 of the WDMS studied, the SDSS spectra suggest that the secondaries
have Sp types too early for their masses. This behaviour could be explained by
magnetic activity if covering a significant fraction of the star by cool dark spots will
raise the temperature of the inner spots regions.
- The fraction of PCEBs among the WDMS population is ~15%, However, our data
suggest a higher fraction, probably in agreement with the results obtained from
population models.
SUPPORTING MATERIAL
Hα emission radial velocities
Na doublet radial velocities.
ORBITAL PERIODS OF THE PCEBs
We assume i = 90 degrees and also that the radial velocities sample the
maximum quadrature of the radial velocity amplitud.
Thus we get absolute maximum periods of the PCEBs, which range between
0.46d – 7880d. The actual periods are likely to be susbtancially shorter,
especially for those systems where only two SDSS spectra are available
and the phase sampling is correspondingly poor.
Comparison with Raymond et al. (2003)
Comparison with Silvestry et al. (2006)
2/3 of the systems have d(sec) ≈ d(wd) within their errors. However, there is a clear
trend for outliers where d(sec) > d(wd). We considered:
- Systematic problems in the WD fits?
- A relationship with close binarity?
- Problems in determining the Sp of the secondary star?
- Problems in the Sp-R relation?
For Sp later than M3 the theoretical Sp-R relation is not sufficient enough to
shift the outliers. For Sp earlier than 2.5 the theoretical relation exacerbates
the d(sec) > d(wd) problem.