The curious case of an old, accreting T Tauri binary system and a new probe for accretion in Herbig Ae/Be stars

In the first part of the talk, I will discuss the discovery of a rare class of T Tauri binary system with

accretion disc older than the disc dissipation time scale, thereby having implications for planet

formation. PDS 11 has been identified as an isolated, high galactic latitude classical T Tauri binary

system with the components being of M2 spectral type. The lithium doublet LiI λ6708, a signature

of youth, is present in the spectrum of PDS 11A, but not in PDS 11B. From the application of

lithium depletion boundary age-dating method and a comparison with the LiI λ6708 equivalent

width distribution of moving groups, we estimated an age of 10 – 15 Myr for PDS 11A.

Comparison with pre-main sequence evolutionary models indicates that PDS 11A is a 0.4 solar

mass T Tauri star at a distance of 114 – 131 pc. PDS 11 system does not appear to be associated

with any known star forming regions or moving groups. PDS 11 is a new addition, after TWA 30

and LDS 5606, to the interesting class of old, dusty, wide binary classical T Tauri systems in which

both components are actively accreting.

The second part of the talk is on our optical/infrared spectroscopic survey program of a selected

sample of Herbig Ae/Be (HAeBe) stars with 2-m Himalayan Chandra Telescope (HCT), Hanle. For

this study we have extensively used our in-house TIFR Near Infrared Spectrometer and Imager

(TIRSPEC), mounted on HCT. An interesting result of this near-simultaneous optical, near-IR

spectroscopic campaign is the proposition of using OI λ8446 as an accretion indicator in HAeBe

stars. This result follows from our demonstration that Lyman beta fluorescence is the dominant

excitation mechanism for the production of OI emission lines in HAeBe stars.

In addition to the studies of these Galactic sources, I will also discuss our recent work on the

discovery of a rare S-type symbiotic binary in the Large Magellanic Cloud. RP870 is the ninth

symbiotic system discovered in the LMC and is unique in having an O-type main sequence star in symbiotic relationship with an M0 giant.