A Star is ÒBorn,Ó and then How Will it Move?

 

Notes: This document was prepared by Alyssa A. Goodman on 10/22/04, to guide a discussion in Harvard University Astronomy 98.) The accompanying homework assignment is also online, as is an additional list of relevant links.

 

What is the surrounding gravitational potential, then & in the future?[1]

 

From ÒnearbyÓ gas?

From ÒnearbyÓ co-eval and extant stars?

 

To answer these:

 

What is mass & velocity distribution of gas and stars (ÒnowÓ & in the past/future)?

 

Gas: Molecular Spectral-Line Maps + Dust Maps

Stars: Proper Motions + Radial Velocities (+ Ages)

 

In the long runÉ

 

The gravitational potential of the Galaxy takes over, and stars are scrambled & scattered to a higher and higher velocity dispersion. (See Wielen 1977 & references thereto.)

 

Astronomical Constraints, Inputs, Terminology & Techniques

 

   Stars:

 

Star Clusters

Globular Cluster: Tightly bound cluster of stars that formed ~together, long ago (ages ~age of the Universe)

Open Cluster: Less tightly bound cluster of stars that formed together and will dissolve in ~10Õs of Myr to 1 Gyr (typically in 100Õs of Myr)

Young (Embedded) Clusters: Very young (<10 Myr-old) groups of young stars that form together.  Future binding depends on cluster mass & velocity, seems that 90% dissolve before ~10 Myr (Lada & Lada 2003).  When they unbind but are still seen to move together, theyÕre known as a Òstellar association.Ó

 

Astrometry

A star at 100 pc distance moving 1 km s-1 across the sky will move 2 milliarcsec (mas) in one year.  Note that this number scales linearly with distance, or velocity.  So, for example, a motion of 1000 km s-1 at the Galactic Center (10 kpc) is 20 mas yr-1.   IR adaptive optics or speckle interferometry allows position determinations of ~1 mas yr-1, for bright enough sources.  Optical accuracies are roughly similar.  Radio interferometric accuracy can be better, but requires (very) bright compact emission.

 

Also, Indirect Proper Motion: Curvature of outflows can give motion of a young star (e.g. PV Ceph, Goodman & Arce 2004).

 

Radial Velocity

The accuracy of radial velocity measurements depends on how many spectral lines are used in a fit of a stars Doppler velocity.  In planet-finding algorithms, thousands of lines are used, and accuracies for the host starÕs velocity are ~a few m s-1, but typical radial velocity measurements give accuracies 10-100 times less good.  Still, this is plenty good for measuring starsÕ motions.  The difficulty with this method for young stars is that their spectra do not often show sharp enough lines in the IR to get a good velocity.  Optically-visible stars are typically older (less embedded).

 

   Gas:

 

Direct ÒProper MotionÓ:

Only possible where masers, resolvable with VLBI techniques are present.  Otherwise, not feasible.

 

Radial Velocity:

Spectral-line measurements give distibution of gas velocity along the line of sight, but weighted by optical depth and abundance of the tracer species.

 

Physical (Numerical) Calculations of Relevance:

 

Stars: N-body simulations (small systems or large)

 

  Gas: Magnetohydrodynamic simulations (e.g. Ostriker, Stone & Gammie 2001).

 

  Stars+Gas: Gas simulations with ÒsinkÓ particles (e.g. Bate, Bonnell & Bromm 2002).

 

 

 

 

 

Link to this document in MSWD format

 

 

 

 

 

 

 


[1] Note this ignores effects of gas (ram) pressure, and assumes stars move ballistically, which is not true for the youngest stars that carry gaseous structures with them.