What is a Protostar? A protostar can be defined as a Young Stellar Object (YSO) in the process of accreting the bulk of the material it will contain when it reaches the main sequence

• The kinematics of protostellar regions are very complex including motions like:
  • Infall
  • Rotation
  • Outflow
  • Turbulence
• Definitive identification of a protostar requires the direct observation of collapsing or infalling molecular gas.
• Without observational constraints on the velocity, temperature, and density conditions in a collapsing protostar, critical tests of even the most rudimentary star formation theories are not possible.

• In an optically thick transition, an infalling core is expected to exhibit the ``classic'', self-absorbed, double-peaked, asymmetric blue line profile.
• Amount of asymmetry depends on optical depth of transition. For optically thin transition, line profile would be symmetric with respect to the rest velocity of the cloud.

 

The Classic Asymmetric Blue Infall Signature

 

Velocity Centroid

 

Expected Centroid Velocity Map of a Core that has pure rotation

In association with Chris Walker and Alan Boss (Carnegie Institution of Washington), I have developed a radiative transfer code which, for the first time, predicts the emergent spectra from nonspherical, collapsing protostellar objects [Walker, Narayanan, and Boss 1994, Narayanan 1997(thesis), Narayanan, Walker and Buckley 1998, Narayanan and Walker 1998]. Our code is written to accept as input, the solutions of 3-D hydrodynamic collapse simulations (e.g. Boss 1993), or the semi-analytical solutions for rotating collapse such as that presented by Terebey, Shu and Cassen (TSC 1984). These models suggest it is possible to identify infalling gas motions even in the presence of rotation and outflow. We used the radiative transfer code to predict CS and HCO+ emission from single and binary protostellar systems. Using velocity centroid maps, we predicted the ``blue-bulge'' signature of infall. The blue-bulge is a morphological appearance of the centroid velocity map, where the infall velocities dominate over rotational velocities in the innermost regions of a flattened cloud core. The figure below shows the centroid velocity maps derived for the HCO+ 4-3 transition from our theoretically derived model profiles to a protobinary (separation of the protobinary 14 AU). The figure shows the appearance of the centroid velocity map at five time steps of collapse. The free-fall time (t_ff) is 38, 000 years in this simulation. At early times, the core is characterized by pure rotation motions (as seen in the figure above). At later times, because the infall velocities are starting to dominate over the rotational velocities, especially towards the center of the collapse, the blue-shifted velocity contours are seen to "bulge" over to the red-shifted half of the rotational axis. And hence the blue-bulge!

Blue-Bulge Signature

Accretion processes and molecular outflows seem to be an integral part of star formation. Molecular outflows are believed to be driven by accretion in the protostellar disk. Due to the small physical scales and large obscuration, accretion processes cannot presently be observed easily. However, because of the believed accretion-outflow connection, the more extended molecular outflows can provide a convenient time-history of accretion activity onto the underlying YSO. I have written a 3-dimensional outflow code and a radiative transfer code which permits us to estimate the timescale, mass, momentum, and energy associated with multiple episodes of outflow activity. (Narayanan and Walker, 1996).

The "STANDARD" theory of star formation, if you will, posits that stars are born deep within molecular clouds, when a molecular cloud core is formed as a density enhancement over the surrounding ambient cloud. This core then grows under the control of ambipolar diffusion. When the core becomes gravitationally unstable it collapses under its self-gravity (Shu, Adams and Lizano 1987). How well does this picture of quasistatic growth under quiescent conditions really match up with the observed reality of violent conditions in star formation regions? In fact, stars rarely seem to form in isolation. A significant fraction, possibly even most of the stars, are expected to form in groups and clusters (Lada & Lada 1991).

As part of his PhD thesis at UMass, Chris Devries is working with me in understanding the process of triggered star formation, where external agents such as winds or supornovae shocks from evolved stars, or outflows from nearby protstars form the trigger for the onset of collapse. We are collaborating with Harri Vanhala at the Carnegie Institute of Washington, in the prediction of emergent line profiles and maps of molecular transitions from numerical models of collapse.