Other
Scientific paper
Aug 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996phdt........41j&link_type=abstract
PhD Thesis, University of Wisconsin-Madison, USA, 1996.
Other
4
Scientific paper
This work is an observational study of disks around low-mass, pre--main-sequence binary stars. Its purpose is to study the extent of binary-disk interactions and to determine whether binaries modify the structure of their associated disks. We present 800 micron continuum photometry of pre--main-sequence binary stars with projected separations ap < 150 AU in the Scorpius-Ophiuchus and Taurus-Auriga star-forming regions. Combining our observations with published 1300 micron continuum photometry, we find that binaries with 1 < ap < 50--100 AU have lower submillimeter continuum fluxes than wider binaries or single stars with a confidence level of greater than 99%, implying reduced disk masses. Thus, binary companions with separations less than 50--100 AU significantly influence the nature of associated disks. A simple model suggests that large gaps in disks with surface densities typical of wide-binary or single-star disks can reduce submillimeter fluxes to levels consistent with the observed limits. This model shows that the present submillimeter flux upper limits do not necessarily imply a large reduction in disk surface densities outside of cleared gaps. IRAS 60 micron fluxes show that most binaries have at least one circumstellar disk, with typical lower limits of Mdisk = 10-5 Modot. Thus, circumstellar disk surface densities are no more than two orders of magnitude smaller than those of typical disks around single stars. Our upper limits on submillimeter fluxes place upper limits of 0.005 Modot on circumbinary disk masses among binaries with 1 < ap < 50--100 AU; however, circumbinary disks are found around some binaries with separations less than a few AU null. We then present λ = 1.3 and 3 mm aperture synthesis imaging of the multiple T Tauri system UZ Tauri. UZ Tau is a hierarchical quadruple composed of a sub-AU spectroscopic binary, UZ Tau E, 530 AU distant from a 50 AU binary, UZ Tau W null. Both dust and gas emission from the 50 AU binary are at least a factor of four lower than from the spectroscopic binary. Since UZ Tau E and W have similar stellar masses, luminosities, and ages, we conclude that the mass of dust and gas associated with UZ Tau W is reduced solely by the influence of a companion with a separation comparable to a typical disk radius. The disk emission from UZ Tau E is similar to that from single T Tauri stars. In a 1 arcsec-resolution aperture synthesis map, CO (2 --> 1) emission is elongated with a size of 300 AU; a disk model fit to the continuum spectral energy distribution yields a disk mass of 0.06 Modot, larger than the ``minimum mass solar nebula''. In contrast, no CO emission is detected from UZ Tau W, and its 1.3 mm continuum emission is unresolved in a 1 arcsec (FWHM) beam (corresponding to a 70 AU radius). The small extent of the emission and dynamical considerations imply that the 50 AU binary cannot be surrounded by any appreciable circumbinary disk; its mm-wave emission is from circumstellar disks around one or both components with masses in the range of 0.002--0.04 Modot. Finally, we present a study of disk clearing by pre--main-sequence binaries with separations less than 1 AU null. Several binaries have spectral energy distributions (SEDs) with little or no infrared excess at λ = 1--5 microns, requiring that their inner disks be optically thin. In each case the inferred size of the cleared region is comparable to a few times the binary semimajor axis as predicted by theories of binary-disk interactions. Other binaries show large near- and mid-infrared excesses, with little evidence of cleared regions in their disks. The infrared excesses in these binaries can be reproduced by a model in which a gap is cleared by the binary but is partially filled with very low-surface-density dust; this hot, optically-thin dust also reproduces the 9.8 micron silicate emission features observed in two of these systems. Thus, all binaries studied show SEDs which are consistent with the presence of partially- or wholly-cleared regions in their disks at the locations predicted by theories of binary-disk interactions, though in some cases disk clearing is not required by the SED structure. The common presence of near-infrared excesses indicates that circumstellar material either is not rapidly depleted in close binaries or is replenished from outside the binary orbit. Many of the binaries also have circumbinary disks that have sufficient mass to drive significant orbital evolution during the binaries' pre--main-sequence lifetimes.
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