Symbiotic binaries and GALAH: a fresh look to known systems and a search for new ones
Symbiotic binaries (SyB) are composed by a red giant (RG) and a degenerate
companion, normally a white dwarf (WD) and in a very few cases a neutron
star. In most SyB the WD burns steadily at the surface (Teff ~ 10^5 K and L
~ 10^4 Lsun), ionizing the wind from the RG and giving rise to a rich
emission line spectrum. While the burning phase last for 10^2 - 10^3 yrs,
the switch-off period during which the WD refuels by accreting from the RG
companion lasts for much longer, with a population proportion
SyB-quiet/SyB-loud >> 1. During the quiet phase, a SyB is detectable by the
hard X-rays emitted during accretion and by feeble emission lines noticeable
only on high resolution spectra like those collected by GALAH. In known
SyB, the WD are massive, some approaching the Chandrasekhar limit as
supported by their spectroscopic orbits and by the short time-interval
between successive nova outbursts. The massive WD and the large
mass-reservoir available for transfer from the RG companion, make SyB a
promising channel to Supernovae type Ia, as originally proposed and modelled
by Munari and Renzini (1992, ApJLett 397, L87).
Our proposal has a few goals:
-) revisit known systems for a fresher look. The resolving power of GALAH spectra is at the high end of available normal observations, and this will allow a finer investigation of circumstellar gas kinematics and extent of the ionization bounded HII region, pollution of the RG from nuclear processed material blown/ejected from the WD, detection of bipolar jets from compressed/colliding winds, etc. A cursory inspection of available GALAH has found spectra for some known SyB never before observed at high resolution;
-) search for new SyB-quiet systems. GALAH red giants with emission lines suitable of coming from an accretion disk or stream, will be cross-checked for X-ray and UV emission (not expected from isolated RGs) accessing archives of space mission like Swift, GALEX, etc;
-) known SyB-loud systems have been proved from near-IR data to belong
to the Bulge population (Whitelock and Munari 1992, A&A 255, 171).
It will be relevant to test if the SyB-quiet systems have a
similar tendency (either by spectral properties and by Galactic
kinematics from GALAH+Gaia DR2);
-) if statistical significance will allow, to derive an estimate for the relative numbers of SyB-quiet and SyB-loud system. If they are proved to both belong to the same Galactic population, it will be possible to scale the observed space density of SyB-loud to SyB-quiet systems, providing an estimate of the total SyB population in the Galaxy. Convolving this with the lifetime of the SyB phase in the evolution of a binary and comparing with the SN Ia rate, an assessment of the feasibility of the SyB-channel to SN Ia will be obtained.
