SCUBA Observations of Dust around Lindroos Stars: Evidence for a Substantial Submillimetre Disc Population

We have observed 22 young stars from the Lindroos sample (Lindroos 1986) at 850 μm with SCUBA on the JCMT to search for evidence of dust discs. Stars in this sample are the less massive companions of B-type primaries and have well defined ages that are 10 − 170 Myr; i.e., they are about to, or have recently arrived on the main sequence. Dust was detected around three of these stars (HD74067, HD112412 and HD99803B). The emission around HD74067 is centrally peaked and is approximately symmetrically distributed out to ∼ 70 arcsec from the star. This emission either arises from a two component disc, one circumstellar and the other circumbinary with dust masses of 0.3 and > 27M⊕ respectively, or an unrelated background object. The other two detections we attribute to circumsecondary discs with masses of 0.04 and 0.3 M⊕; we were also able to show that a circumprimary disc is present around HD112413 with a similar mass to that around the companion HD112412. Cross-correlation of our sample with the IRAS catalogs only showed evidence for dust emission at 25 μm and 60 μm toward one star (HD1438); none of the sub-mm detections were evident in the far-IR data implying that these discs are cold (< 40 K assuming β = 1). Our sub-mm detections are some of the first of dust discs surrounding evolved stars that were not detected by IRAS or ISO and imply that 9-14% of stars could harbour previously undetected dust discs that await discovery in unbiased sub-mm surveys. If these discs are protoplanetary remnants, rather than secondary debris discs, dust lifetime arguments show that they must be devoid of small < 0.1 mm grains. Thus it may be possible to determine the origin of these discs from their spectral energy distributions once these have been better defined. The low inferred dust masses for this sample support the picture that protoplanetary dust discs are depleted to the levels of the brightest debris discs (∼ 1M⊕) within 10 Myr, although if the extended emission of HD74067 is associated with the star, this would indicate that > 10M⊕ of circumbinary material can persist until ∼ 60 Myr and would also support the theory that T Tauri discs in binary systems are replenished by circumbinary envelopes.