J. Cantó

Accretion disks around young objects. I. The Detailed vertical structure

We discuss the properties of an accretion disk around a star with parameters typical of classical T Tauri stars (CTTSs) 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 α 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 5 AU) becomes less dense, optically thin, and almost vertically isothermal, with a temperature distribution T ∝ R-1/2. The decrease in surface density at the outer disk decreases the disk mass by a factor of 4 with respect to a purely viscous case. In addition, irradiation tends to make the outer disk regions stable against gravitational instabilities.

Accretion Disks Around Young Objects. II. Tests of Well-Mixed Models with Ism Dust

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 find that the median near-infrared fluxes 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.

Stellar jets with intrinsically variable sources

Models for high Mach number, isothermal, pressure-matched jets ejected from sources with variable velocities are presented. It is found that the relaxation of the assumption of a time-independent source allows the complex position-velocity diagrams and multiple bow shock structures observed in some stellar jets to be explained in a straightforward way. Analytic and numerical models are presented for jets with variable velocity sources. A comparison with previously published observations of the HH 46/47 jet is performed which shows how the velocity variations observed along a jet can be used to reconstruct the past time variability of the source. This technique is promising for future studies of the mechanism by which stellar jets are produced. 41 refs.

Compact protoplanetary disks around the stars of a young binary system

Planet formation is believed to occur in the disks of gas and dust that surround young solar-type stars. Most stars, however, form in multiple systems, where the presence of a close companion could affect the structure of the disk 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 Jupiters 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. The disk masses are of order 0.05 solar masses, which could be enough to form planetary systems like our own.

The Hot, Diffuse Gas in a Dense Cluster of Massive Stars

We present an analytic model describing the cluster wind flow that results from the multiple interaction of the stellar winds produced by the stars of a dense cluster of massive stars. The analytic solution (obtained by matching an inner and an outer solution at the radius of the stellar cluster) can have asymptotically subsonic or supersonic behavior, the latter possibility being appropriate for the case of a cluster surrounded by a low-pressure environment. We also present a three-dimensional numerical simulation of such a cluster wind. We find that the behavior of the mean flow computed from the numerical simulation quite closely follows the flow properties deduced from the analytic model. Finally, we discuss the observational properties of the cluster wind produced by dense clusters such as the Arches cluster close to the center of our Galaxy. In particular, we predict that the X-ray emission from the intracluster gas in this stellar cluster could be detectable.

Large Proper Motions in the Jet of the High-Mass YSO Cepheus A HW2

Using high angular resolution (~025-005) 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-1, or ~480 km s-1 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.

The triple radio continuum source in serpens : the birth of a Herbig-Haro system ?

VLA observations of the triple radio continuum source in Serpens have revealed its extraordinary characteristics. While it is associated with a star-forming region, its outer components exhibit nonthermal spectra and large proper motions. Here, we present the results of high-sensitivity, multifrequency VLA radio continuum observations of this source

Evidence for Evolution of the Outflow Collimation in Very Young Stellar Objects

We present Very Long Baseline Array proper-motion measurements of water masers toward two young stellar objects (YSOs) of the W75 N star-forming region. We find that these two objects are remarkable for having a similar spectral type, being separated by 07 (corresponding to 1400 AU), and sharing the same environment, but with a strikingly different outflow ejection geometry. One source has a collimated, jetlike outflow at a 2000 AU scale, while the other has a shell outflow at a 160 AU scale expanding in multiple directions with respect to a central compact radio continuum source. This result reveals that outflow collimation is not only a consequence of ambient conditions but is something intrinsic to the individual evolution of stars and brings to light the possibility of noncollimated outflows in the earliest stages of YSOs.

Shadows behind Neutral Clumps in Photoionized Regions

Shadows behind neutral clumps in photoionized regions can have a neutral core surrounded by gas that is photoionized by the diffuse flux produced by the nebula. We present a simple analytic model describing the configuration of such shadows. We also present numerical gasdynamic simulations of the relaxation to the final, steady state. These models have clear applications (e.g., to the cometary knots in the Helix Nebula) but can also be applied in other astrophysical contexts.

Detection of radio continuum emission from Herbig-Haro objects 1 and 2 and from their central exciting source

The region in Orion containing HH 1 and HH 2 was observed with the VLA at 20, 6, and 2 cm on several occasions from 1981 to 1984. At lower resolution, four continuum sources were detected. Two of these sources coincide positionally with HH 1 and HH 2. At 6 cm and higher resolution, HH 1 is resolved into at least two components. The emission is probably bremsstrahlung originating in the same region where the visible line emission is produced. This is the first detection of radio continuum from classic Herbig-Haro objects. At a position closely centered between HH 1 and HH 2, an object that can be interpreted as the energy source of the system was detected. The central source spectrum is S(nu) of about nu to the alpha power, where alpha = 0.4 + or - 0.2, suggesting a stellar wind. Finally, the fourth radio continuum source coincides positionally with an H2O maser and is probably excited by an independent star. There is evidence of time variability in its radio flux. No emission was detected from the Cohen-Schwartz (1979) star at the 0.1 mJy level.