VLA Expansion Project Science Working Group:
Stars and the Sun
(17 November 1998)

Summary by M. Rupen.

Chaired by Tim Bastian
Also present:
  Peter Barnes, Chris Carilli, Barry Clark, Bob Hjellming, Amy Mioduszewski,
  Rick Perley, Michael Rupen

Several major themes were discussed.  These included the stars across
the HR diagram, disks and jets, energy release, and the heliosphere.

1- Superluminal Jets spilled over from the galactive group
  Follow-up: Bob Hjellming

  * Barry suggests a major theme: Disks and Jets in the Universe
    + Disks and jets always seem to go together (Falcke model):
      - protostars    - superluminals    - AGN
    + a good propaganda picture:
         Cygnus A
         NGC 4258
         GRS 1915+105
      all together in one image

  * A few numbers (from Bob's email):
    apparent size= 170mas * v_app/c * t(days)/d(kpc)
             v_app currently seen is between 0.1 and 2 c
             t(days) is  0 to 10's or 100's of days
             d       is  Galactic (1-15 kpc)
  * Will be resolved by A+ within days of initial jet ejection 
    (7mm beam in A+ is 10mas -> 10kpc source at 0.3c in 2 days)
    n.b. GRS 1915 or GRO J1655 are 1c -> at 10kpc resolved in under 1 day!
    n.b. some die very rapidly (e.g. GRS 1915+105)
    [A is factor 6 worse]
  * First "complete" images of such sources 
    + recover all the flux, at high resolution
    + much more reliable astrometry and mapping ("Which blob did you say
      that was?")
    + better time coverage (responds more quickly than VLBI can)
  * VLA upgrade offers RESOLUTION,  imp. for mapping (A+ and high
                                      frequencies, to resolve luminal-ish
                                      sources at the Galactic center)
                   RAPID RESPONSE,  imp. for transients and response to X-ray
        GOOD SNAPSHOT UV-COVERAGE,  imp. for (1) mapping sources which
                                      move during the observations, and
                                      (2) getting good maps quickly & often
  * Tie-ins:  X-ray and high-energy satellites
              MMA (maybe???), for getting the mass-loss rates of the
                companion which feeds the beastie
              "tying the disk to the outflow"
              only hope of imaging black hole systems on the scale of their
                accretion disks
              "mapping the accretion zones in quasars"
              "ins and outs of BH binaries"

  * There are lots of "pretty pictures": 1915+105 etc.

  * A few worries:
    + What *specifically* will one learn?  What new physics do we get out of
      Some possibilities:
        - image the acceleration region in jets (cf. SS433)
        - measure jet lateral expansions (would have to be nearby?)
        - track magnetic fields from inception (what does this mean?)
        - learn whether the jet velocity is indeed a direct indication
          of the mass of the central object

2- Imaging novae (thermonuclear explosions on white dwarf)
  Follow-up: Bob Hjellming
  * Only one thermal nova imaged so far (Nova Cyg 1992, MERLIN + VLA/HST)
    Plus there are some non-thermal ones (e.g. CI Cam) which are also
      X-ray transients  [maybe should not be in "nova" category, but we are
      looking for themes here...]
  * A few numbers (from Bob's email -- though I get 210 rather than 35mas!):
    apparent size= 210mas * v/1000km/s * t(yr)/d(kpc)
             v       is 100s to 1000s of km/s
             t       is a few to 10 years
             d       is  Galactic (1-15 kpc)
  * Will be resolved by A+ within a year of the initial explosion
    A is a factor 6 worse
  * VLA upgrade offers RESOLUTION,  imp. for mapping (A+ and high
                                      frequencies, to resolve luminal-ish
                                      sources at the Galactic center
                   RAPID RESPONSE,  imp. for transient sources (CI Cam)
  * Tie-ins:  X-ray and high-energy satellites -- may account for a
                significant fraction (1/3??) of their transients
              "the only non-man-made thermonuclear explosions that can be 
                studied in nature"  (OK, so we ignore supernovae...)

3- Imaging stellar chromospheres, coronae, and winds
  Follow-up: Tim Bastian (also Richard Simon)
  * chief limitation to observing stars at radio wavelengths has
    been SENSITIVITY  -- the core upgrade will allow several times
    10^4 stars to be detected. Richard Simon will provide estimates
    on detectability across the HR diagram
  ==> the radio emitting properties of whole populations of stars will
      be accessible for the first time

  How many stars will we see?  Check our Richard Simon's page on
      Thermal Emission from Normal Stars (18Nov98)

  a- Stellar Winds
  Follow-up: Tim Bastian
  * MS stars:
    With the improved sensitivity one should be able to detect thermal
    (free-free) emission from the winds of early main sequence stars,
    of order 1.5x10^{-10} V_infty nu_9^{-1/2} D_kpc^1.5 solar
      mass per year for a assumed flux density of 5 microJy (where 
      V_infty is the terminal speed in km/s, nu_9 is the frequency 
      in GHz, and D is the distance in kpc). 
    (1) among other mechanisms, such winds have been invoked to modify 
      the abundance of Lithium-7 in stars -- there is a delicate
      balance between turbulence, diffusion, rotation, and mass loss which
      determines whether lithium is mixed to deeper layers in the star
      where it is destroyed, whether it is exposed on the surface. 
      This needs to be looked at carefully before we can make any
    (2) basic stellar evolution -- winds are one of the big outstanding
      uncertainties; having a handle on the mass loss rate for
      garden-variety stars could help enormously
  * OB/WR stars:
    - separate thermal wind emission from synch. (which comes from 
      wind-termination shocks)
    - might be able to see these in M31
  * VLA upgrade offers SENSITIVITY
            A+  offers RESOLUTION
  * Tie-ins: fundamental astronomy
  * Possible problems:
    - How about the MMA?  Won't it do this better?
      Barry: MMA may be confused by dust emission, whereas we can see the
        free-free which gives the wind -- i.e. separate the disk and the
        outflow (sound familiar??? :)

  b- Origins of Stellar Activity
    Follow-up: Tim Bastian 
  * Meant to be a Grand Theme: YSOs, comparing other stars to the sun, 
    learning about the origin of the solar system (meteors etc. suggest high
    energy bombardment early on in the solar system, so we should see
    the corresponding stellar activity in young stars) 
  * So far, one can do only pathological cases; upgrade should allow
    detections of tens of thousands of stars
  * open clusters - to date, only small numbers of stars in open
    clusters such as the Pleiades, alpha Per, or the Hyades have been 
    detected. With the enormous sensitivity of the upgraded VLA,
    studies of open clusters of various ages will enable stellar
    activity as a function of mass, age, rotation, to be studied in detail.

  c- Energy release in solar and stellar coronae
    Follow-up: Tim Bastian
  * Spectroscopic Imaging of the Sun
    + Broad BW ratios --> track type III-like dm bursts on the sun during
      flares --> see when and where the energy is released by reconnection
              (can be done with single-dishes now, but no mapping)
    + 13cm is especially interesting
    + VLA upgrade offers BROAD BANDWIDTHS
  * Tie-ins: "magnetic reconnections in astrophysical contexts"
             "details of energy release on the Sun"
  * Extend to stars
    - Type II bursts (MHD waves at ~1000km/s) and Type III
      (superthermal, fraction of c) bursts on other stars, esp. RS CVn 
      and dMe's (although Arecibo will not be sitting still!)

4- Imaging the Dynamic Heliosphere (!)
  Follow-up: Tim Bastian
  * Sensitivity improvement --> huge numbers of background sources
    provide a dense grid to probe the foreground solar wind
    --> much more complete maps of the scattering medium (heliosphere) --
  * VLA upgrade offers SENSITIVITY (more sources)
                       LONGER BASELINES (spectrum goes from Kolmogorov on
                         VLBI scales, to much flatter on VLA scales --
                         the energy is presumably being put in somewhere in
                         the "break" region, which is given by A+)

5- Topics requiring more thought (might turn into Big Questions)
  *** How many sources will we get in the primary beam?  Which wavelength
    offers the most sources?  Can we use this to
      + find extreme scattering events (one out of each 1-2e4 sources at
        any given time)
      + map scattering inside the Galaxy
    Follow-up: Rick Perley

  * Planetary nebulae -- is it interesting to count them all over the galaxy?
    "like a SN, only more boring" (bgc)
    Follow-up: no-one

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