Observations of Circumstellar Disks at Centimeter Wavelengths

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Session 39 -- Interferometry II
Oral presentation, Tuesday, 2:00-6:30, Zellerbach Playhouse Room

[39.05] Observations of Circumstellar Disks at Centimeter Wavelengths

A. W. Grossman, L. G. Mundy, J. P. McMullin (Univ. of Maryland)

Observations of circumstellar dust at centimeter wavelengths provides unique information about conditions in the proto-solar nebula because of the low dust opacity at these wavelengths and the high spatial resolution capability of existing radio interferometers.

We present results of a search for centimeter wavelength dust emission from the five embedded young stellar objects: NGC 1333 IRAS 4A, IRAS 4B, IRAS 05338--0624, IRAS 05375--0731, and S86FIRS1, and the three T~Tauri stars: RY Tau, GG Tau, and DL Tau. Continuum observations were obtained with the NRAO Very Large Array at wavelengths of 1.3, 2.0, and 3.6 cm.

Emission was detected from all five of the embedded objects. In three of these, this emission is identified as arising from ionized gas, probably associated with stellar winds. In the remaining two cases, NGC 1333 IRAS~4A and 4B, dust emission is suspected to be responsible for between 20 and 100 percent of the $\lambda$=1.3 cm flux.

Emission was detected from only one of the T~Tauri stars, RY~Tau. It is presently unclear if this emission arises from ionized gas or dust. Using standard circumstellar disk models for the dust emission associated with T~Tauri stars, the upper limits to the centimeter wavelength fluxes for DL~Tau and GG~Tau can be used to constrain the properties of the circumstellar dust. Assuming a power law form for the dust emissivity ($\epsilon\propto\lambda^{-\beta}$), the upper limits require that $\beta$, as measured from millimeter to centimeter wavelengths, is greater than 0.8 for DL~Tau and greater than 1.6 for GG~Tau. These values of $\beta$ are considerably larger than values measured at sub-millimeter wavelengths in the same sources ($\beta\sim0.0$). If the dust emissivity is described as a two part power law corresponding to two different values of $\beta$, as is suggested for dust in the interstellar medium, the GG~Tau data indicate that the value of $\beta$ changes around $\lambda$=2 mm. This change occurs at an order of magnitude greater wavelength than that proposed for interstellar dust and may reflect the growth of grain size in the circumstellar environment.

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