Optical, infrared and millimetre-wave properties of Vega-like systems

Vega-like stars are main-sequence stars that exhibit excess infrared emission due to dust grains which are believed to be distributed in circumstellar discs. To facilitate modelling of the properties of the discs and of their constituent grains, we have assembled a new observational database for a large sample of candidate Vega-like systems, which comprises UBVRI optical photometry, JHKLL'M near-IR photometry, mid-IR spectra, and mm/submm-wave photometry. Although our sample of stars are all believed to be main-sequence, it is noteworthy that most have large IR excesses compared with the ‘prototype’ Vega-like stars, and they may therefore be somewhat younger. The excess fluxes are, in a few cases, greater than can be explained by emission from pure radiation-reprocessing discs, even in the optically thick case, suggesting additional contributions by extended envelopes and/or flux generated by viscous dissipation in a flared accretion disc. The dereddened optical colours are, for the whole sample, entirely consistent with photospheric emission from main-sequence stars. The near-IR photometry revealed that 9 out of 23 Vega-like sources exhibit excess near-IR emission with colours resembling those of Herbig Ae/Be systems, while the rest show only photospheric emission at near-IR wavelengths. Our mid-IR spectra reveal that most of the discs show combinations of silicate and unidentified infrared (UIR) emission features, demonstrating the presence of both oxygen- and carbon-rich dust species. Our new mm/submm photometry indicates that most of the sample display small mm-wave spectral indices, implying that they possess grains that are much larger than those found in the interstellar medium, with implications for past or on-going grain growth within the discs. Fractional excess luminosities, Lir/L*, were calculated from the optical photometry. for some of our sources, values ∼ 10−5 – 10−3 were found, similar to those of the prototype Vega-like systems. Others gave Lir/L* > 0.25, the maximum value for a flat passively reprocessing disc. Two sources gave Lir/L* > 0.5, the maximum for a flared reprocessing disc. These large values are comparable to those of Herbig Ae/Be and T Tauri stars, and suggest that some of the excess IR emission from these sources is due to disc accretion or the presence of circumstellar envelopes.