A 3.3mm Survey of Circumstellar Material in rho Ophiuchus

Abstract

Millimetre observations of pre-main sequence stars probe the cold dust in the outer regions of the forming star system. It is in these outer regions that planets are thought to form out of the gas and dust within the circumstellar disk. Therefore, by observing young stellar objects at millimetre wavelengths we can determine the characteristics of circumstellar material, and hence, we can infer the possibility of a system forming planets. In order to characterise the circumstellar environment of pre-main sequence stars, a 3.3mm survey of continuum dust emission was performed using the OVRO interferometer array. A sample of targets in the nearby star forming region rho Ophiuchus were selected to encompass the entire range of pre-main sequence evolution, from starless clumps to Class III T Tauri stars. These data were then used to constrain properties of the circumstellar material in these young systems, in particular the dust opacity index, beta, and the mass of the material. The value of beta is used to study whether grain growth of the dust is occurring in these systems, which is a key stage in some planet formation models. Calculating the mass of the system can indicate whether enough circumstellar material is present to form planets. We also combine these new millimeter observations with literature data to create spectral energy distributions (SEDs), which are then fit in order to determine a best-fit model of the system using the models of Robitaille et al. (2007). We find that the dust opacity index evolves from a value similar to that of the ISM for the starless clumps beta ~ 2.3, to one closer to beta ~ 0.2 for the Class II objects. This low beta could be accounted for by the inclusion of larger grains within the disk, indicating the possibility of grain growth through the evolutionary stages of a young system. For the Class I and Class II sources we find similar trends in beta to those found in Andrews & Williams (2007a). An estimate of circumstellar mass assuming a constant dust opacity index was made using the flux of each source. Carrying out this calculation yielded a mean mass of 0.033Msun, with almost 50% of the sources having a mass > 0.01Msun. A more in depth estimate of mass was also performed using the value newly calculated value of the dust opacity index. The mass estimates indicated many of the sources observed by this survey have the mass of dust available in the system to form planets.