POST COMMON ENVELOPE BINARIES FROM THE

SLOAN DIGITAL SKY SURVEY

Alberto Rebassa-Mansergas

Supervisor : Dr. Boris GaensickeCo-supervisor : Dr. Pablo Rodríguez-Gil

Working with:

Dr. Linda SchmidtobreickDr. Matthias Schreiber

INTRODUCTION

PCEBs = wide MS binaries + CE phase

Friction within the envelope leads to a rapid decrease of the binary orbitE 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

SDSS ~1500 WDMS

stellar parameters (WD + MS)

18 PCEBs identified from RVs

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

IDENTIFYING PCEBs IN THE SDSS

SDSSJ0246+0041 display anextremely 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 confirmthis 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 themaximum quadrature of the radial velocity amplitud.

Thus we get absolute maximum periods of the PCEBs, which range between0.46d – 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 toshift the outliers. For Sp earlier than 2.5 the theoretical relation exacerbatesthe d(sec) > d(wd) problem.