We discuss the properties of an accretion disk around a star with parameters typical of classical T Tauri stars (CTTS), and with the average accretion rate for these disks. The disk is assumed steady and geometrically thin. The turbulent viscosity coefficient is expressed using the alpha prescription and the main heating mechanisms considered are viscous dissipation and irradiation by the central star. The energy is transported by radiation, turbulent conduction and convection. We find that irradiation from the central star is the main heating agent of the disk, except in the innermost regions, R less than 2 AU. The irradiation increases the temperature of the outer disk relative to the purely viscous case. As a consequence, the outer disk (R larger than 5 AU) becomes less dense, optically thin and almost vertically isothermal, with a temperature distribution T proportional to R^{-1/2}. The decrease in surface density at the outer disk, decreases the disk mass by a factor of 4 respect to a purely viscous case. In addition, irradiation tends to make the outer disk regions stable against gravitational instabilities.Comment: 41 pages, 14 postscript figures, LaTeX, accepted by Ap

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We construct detailed vertical structure models of irradiated accretion disks around T Tauri stars with interstellar medium dust uniformly mixed with gas. The dependence of the structure and emission properties on mass accretion rate, viscosity parameter, and disk radius is explored using these models. The theoretical spectral energy distributions (SEDs) and images for all inclinations are compared with observations of the entire population of classical T Tauri stars (CTTSs) and class I objects in Taurus. In particular, we Ðnd that the median near-infrared Ñuxes can be explained within the errors with the most recent values for the median accretion rates for CTTSs. We further show that the majority of the class I sources in Taurus cannot be class II sources viewed edge-on because they are too luminous and their colors would be consistent with disks seen only in a narrow range of inclinations. Our models appear to be too geometrically thick at large radii, as suggested by (1) larger far-infrared disk emission than in the typical SEDs of T Tauri stars, (2) wider dark dust lanes in the model images than in the images of HH 30 and HK Tau/c, and (3) a larger predicted number of stars extincted by edge-on disks than consistent with current surveys. The large thickness of the model is a consequence of the assumption that dust and gas are well mixed, suggesting that some degree of dust settling may be required to explain the observations.

Using high angular resolution ($0B25-0B05) Very Large Array (VLA) observations made at 3.6 cm, 1.3 cm, and 7 mm during the period 1991-2004, we report the detection of large proper motions in the components of the radio continuum jet associated with the high-mass young stellar object (YSO) HW2 in the star-forming region Cepheus A. The relative proper motions observed for the two main components of the outflow, moving away from the central source in nearly opposite directions, are of the order of 140 mas yr À1 , or $480 km s À1 at a distance of 725 pc. The proper motions observed in the northeast and southwest lobes are not completely antiparallel, and the central elongated source seems to be changing orientation. We discuss possible scenarios to account for these and other observed characteristics. We also report the detection of a 7 mm compact continuum condensation of emission near the center of the thermal radio continuum jet, which we propose as the location of the exciting star.

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Planet formation is believed to occur in the disks of gas and dust that surround young solar-type stars 1 . Most stars, however, form in multiple systems 2-5 , where the presence of a close companion could affect the structure of the disk 6-8 and perhaps interfere with planet formation. It has been difficult to investigate this because of the resolution needed. Here we report interferometric observations (at a wavelength of 7 mm) of the core of the star-forming region L1551. We have achieved a linear resolution of seven astronomical units (less than the diameter of Jupiter's orbit). The core of L1551 contains two distinct disks, with a separation of 45 AU; these appear to be associated with a binary system. Both disks are spatially resolved, with semi-major axes of about 10 AU, which is about a factor of ten smaller than disks around isolated stars 9-12 . The disk masses are of order 0.05 solar masses, which could be enough to form planetary systems like our own.L1551 is a molecular cloud in Taurus that is known to be undergoing intensive star formation, although restricted to lowmass stars (that is, with masses comparable to or smaller than that of the Sun). The embedded infrared source L1551 IRS5 (ref. 13) is believed to be associated with a very young stellar source. Its bolometric luminosity is ϳ30 solar luminosities 14 and it is embedded in a dense envelope of molecular gas and dust that extends over ϳ10 3 -10 4 AU (refs 15, 16). The continuum millimetre emission, when observed with angular resolutions of ϳ1 arcsec, has been interpreted as originating from heated dust in a protoplanetary disk with dimensions of about 100 AU (refs 17, 18).In the centimetre wavelengths, there is compact radio continuum emission with elongated morphology 19,20 that aligns with the largescale bipolar molecular outflow 21 . This centimetre radiation is known to be free-free emission that originates in the ionized outflowing gas 22 . However, when observed with subarc second angular resolution, the core of the centimetre emission from L1551 IRS5 breaks into two compact components separated by 0.3 arcsec, which have been interpreted as being either a protobinary system 19 or the inner ionized edges of a gas and dust toroid around a single star 20 . As the millimetre continuum emission, tracing the dust, has been interpreted as coming from a single disk, the singlestar interpretation has been favoured recently. However, there are several observations that suggest that two independent outflow systems emanate from L1551 IRS5 (refs 23-25), favouring a binary nature for the source. Furthermore, observations at 2.7-mm (ref. 26) with angular resolution of about 0.5 arcsec have been used to argue for the presence of two unresolved disks in L1551 IRS5.Our observations were made with the Very Large Array of the National Radio Astronomy Observatory in its highest angular resolution A configuration. The 7-mm observations were made in 1997 January 10, and the 3.6-cm observations were made in 1996 December 10. As the observations are separated b...