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					In Praise of IRAS

        Terrestrial planet
      formation and other
      strange phenomena
            Virtues of IRAS

• Mining the IRAS catalog 24(!) years after
  launch, with colleagues:

• Carl Melis, Joseph Rhee(UCLA)
• Inseok Song (IPAC => Univ. Georgia)
 Why bother with IRAS when we
 now have Spitzer which is much
  more sensitive and has much
    better spatial resolution?

 With Spitzer, 1000s of stars are pointed
 at during its lifetime, but IRAS covered
100 times more sky. This conveys some
  advantages; for example, for the very
rare nearby star with very high LIR/Lbol,(=
  “tau”) IRAS is the better search tool.
   Because IRAS was first it
skimmed a lot of IR-bright cream

Spitzer stellar campaigns primarily focus on
extensive observation of fainter stars in
regions of already known interest, (e.g., the
h and chi Persei clusters [Thayne Currie]),
so that the rare, very infrared-bright field
sources may be missed.
• We have correlated the IRAS catalog with
  the Hipparcos and Tycho catalogs, which
  are also all-sky surveys.

• In this way rare, lonely (isolated), infrared
  bright stars can be found that otherwise
  will be missed by Spitzer and sometimes
  were missed in earlier studies that
  correlated the IRAS catalog with older
  stellar catalogs.
 The dustiest not-so-young stars
           near Earth

    We have been investigating two classes of
   extraordinary field stars first detected by IRAS:

 Main sequence stars that likely are windows into
    formation processes for terrestrial planets.

A (still) growing collection of first ascent giant stars
          with properties never previously seen.
The Zodiacal Light
The COBE Sky (8–200 µm)
Scattered Sunlight & Warm Zodiacal Dust




                          T ≈ 250 K
             T ≈ 6000 K




                             Leinert & Gruen 1990
“Vega-like” debris disks

Particle lifetimes << age of star.
   So, particles are “second
 generational”, the debris from
     larger, unseen objects.
HST ACS planet search
                            HST Fomalhaut detection -- consistent with sub-mm maps




             Hubble Space Telescope




                                           JCMT SCUBA 450 micron map (Wyatt & Dent 2002)
   Kuiper Belt vs asteroid belt
• The dust at almost all Vega-like stars is sufficiently
  cold to be orbiting with semi-major axes of 50 AU or
  more from the central star. Thus, the debris disks are
  almost always to be considered (young) analogs of
  the Sun’s Kuiper Belt.
• Until 2005, among the 100+ main sequence stars
  with far-IR excess, only one example of warm dust
  signifying a potential asteroid belt analog had been
  reliably established – at the A-type star zeta Lep, of
  age ~100 Myr (Jura & Chen). LIR/LBol ~10-4
• Absence of warm dust is true even for stars with ages
  as young as tens of Myr. Thus, dust in the terrestrial
  region dissipates very quickly.
Two unusually dusty sun-like
                    stars:

HD 23514: Pleiades member

BD+20 307: main sequence
                 field star
Beautiful new Spitzer data on
         BD+20 307



  Alycia Weinberger, Inseok Song, Eric Becklin
Pleiad HD 23514
Comparison of LIR/LBol in Sun’s zodiacal cloud and
  in analogous regions at stars with warm dust

     Name        Sp. Type    LIR/LBol      Age
                                          (Myr)
  Lep             A3       0.65 x 10-4   300
 HD 23514          F6V         0.02       100
 BD+20 307         G0V         0.04       ???
  Crv             F2V       5 x 10-4     600
 HD 72905          G1.5      1 x 10-4     400
 HD 69830          K0V       2 x 10-4     2,000
 Zodiacal dust     G2V         10-7       4,600
   Era of heavy bombardment in
         early solar system
• Until ~600 Myr following the formation of the Sun, the
  bombardment rate in the early solar system was
  sporadically heavier than at present by factors up to
  1000.
• At BD+20 307, which is ~1,000,000 times dustier
  than the present solar system, the current
  bombardment rate might be incredibly large!
       Zodiacal dust properties

• In the solar system, the typical zodiacal dust
  particle is 30-100 microns in size. As a
  consequence of the Poynting-Robertson (PR)
  effect, such particles will spiral into the Sun in a
  time (~105 yrs) short compared to the collision
  time (~107 yrs). Thus, the zodiacal particles are
  not whittled down to smaller sizes by collisions
  with other particles.
 At HD 23514 and BD+20 307, the strong
silicate emission features indicate the dust
    particles are of micron size (due to a
            collisional cascade?).

 As a result, at these stars, PR lifetimes
  from <~1 AU, are very short. How can
there be so much dust in such tight orbits
          around the two stars?
      Very recent collision of two
        planet-mass objects??


• To account for the estimated dust mass at BD+20
  307, one must pulverize a 300 km diameter object
  (e.g., Davida, the 5th largest asteroid) into micron-size
  particles.

• Perhaps something analogous to the collision
  postulated to explain Earth’s moon has occurred
  recently within a planetary system at BD+20 307.
    When World’s collide?
 Initially, BD+20 307 might have been
          regarded as a “miracle”
 But discovery of a Pleiades star with a
  comparably large tau, has changed
BD+20 307 from a miracle into a statistic.
  We can now estimate how common
  collisions of young planets may be.
 By comparing the number of adolescent-
 age (70-700 Myr old), solar-type, stars at
which IRAS could have detected the “very-
  dusty” phenomenon, with the number of
such stars that were actually found by IRAS
to be very dusty, i.e., two, we estimate that
 about one star in 1000 is very dusty and,
     thus, the lifetime of the very-dusty
phenomenon at a typical solar-like star is a
              few 100,000 years.
But the lifetime of the observed, micron-size
  particles is orders of magnitude shorter.
    To interpret our observations, we
considered a model of colliding “planetary
                embryos”:

    Dust particles => planetesimals =>
      planetary embros => planets

    C. Agnor and E. Asphaug (UCSC)
considered collisions of large bodies in the
 late stages of planetary formation in the
          terrestrial planet zone.
 To make the terrestrial planets, requires a
  minimum of many hundreds of planetary
     embryos of dimensions >~1000 km.
Embryo collisions can result in coalescence
or, alternatively, fragmentation of the smaller
 embryo into smaller objects along with the
           ejection of copious debris.
   A typical large fragment size is ~100 m.
 Collisions of embryos continues for as long
                 as a few 100 Myr,
          i.e., to the age of HD 23514
                   3.5
N(a)da  N O a            da

         4  3 3.5
M~       3
            a a da ~ a1/ 2
(a) ~  N(a)a da ~  a a da ~ a
              2         2 3.5       1/ 2



       tc  P / ~ a       1/ 2
 Thus, the smallest particles (~1 um) collide
the fastest, for HD 23514 ~50 years at 1 AU.

    And the lifetime of the large, 100 m,
      fragments will be ~500,000 yrs.

    Thus, catastrophic disruption of large
 planetary embryos (mass estimate to follow
next) can supply material for a sufficient time
to account for the event lifetime indicated by
       the observations of HD 23514
A collisional cascade will whittle
 particles down until, when “a” ~
0.5 m, radiation pressure blows
   them away from HD 23514.

  Are there any loss mechanisms that can
    compete with collisions followed by
             radiative blowout?
    The PR effect that dominates
    the orbits of the zodiacal dust
   in our solar system is too slow
     (~1000 yrs) to compete with
        collisions at HD 23514
But stellar wind drag might be sufficiently fast
 to compete -- if so, then a “sweeper planet”
    might be necessary to account for the
      absence of hot dust at HD 23514.
To determine how rapidly mass is lost
 due to either collisions and radiative
 blowout or stellar wind drag one can
estimate the minimum dust mass (Mmin)
  needed to intercept 2% of the light
        emitted by HD 23514.
M min  16aR / 3
               2
         Mmin ~ few x 1022 g,

  with mass loss rate ~ 1013 g/s

In a few 100,000 years, the total mass lost will
    be ~1026 g -- the mass of Earth’s moon.

Or for an average density of a few g/cm3, an
        object with radius ~2000 km.
  Not possible to distinguish from
    our data and this model if most
solar-like stars undergo very dusty
 episodes, or if only a (significant)
  minority do so, but perhaps with
   catastrophic conversion of even
  more mass to dust particles than
   indicated on the previous slide.
How old is BD+20 307?

     ~300 Myr (Song et al 2005)

  Based on lithium content, Galactic
 space motion (UVW), and upper limit
        to ROSAT X-ray flux
   Michael Muno & I received
Chandra X-ray time to measure
                the X-ray flux.

Gregory Henry joined us to measure the
    rotation period of BD+20 307 from
      photometric observations with an
       automated telescope at Fairborn
    Observatory (in southeast Arizona)
Both the Chandra-measured
   X-ray flux (just below the
ROSAT upper limit) and the
  rotation period (~3.5 days)
 indicated a youthful star, of
      age a few 100 Myr.
All looked consistent and
       “reasonable”

        But then…
   In late October 2007, Alycia
 Weinberger made the surprising
discovery, from observations on 3
successive days, that BD+20 307
  is ~3.5 day period double-line
       spectroscopic binary!
  This was surprising because the 2004
epoch, Song et al echelle spectrum showed
         no suggestion of binarity
  So, Frank Fekel and Mike
 Williamson joined our team
  and conducted a 45 day
   spectroscopic campaign
earlier this year (at Fairborn
Observatory) to measure the
            system.

      Here are their results:
    Where do things stand now?

        Because of the short orbital period (in
     conjunction with the [weak] 6708 A lithium
   absorption line strength), we can be confident
       that the orbital and rotation periods are
     synchronized and that neither the rotation
period, nor any activity indicators that depend on
it, (e.g. X-ray flux), tell us anything about the age
                     of BD+20 307
Instead, for an age estimate,
we must use other indicators,
such as lithium line strength,
Galactic space motion (UVW),
metallicity, and placement on
       the HR diagram
In summary, BD+20 307 is
probably a few Gyr old, and
possibly considerably older.

 Can we be witnessing the aftermath of a
collision of two terrestrial planets orbiting
              such an old star!?
Is there an unseen faint star or
    substellar object whose
     gravitational field has
 destabilized the orbits of two
        rocky planets??

     Some suggestion in the radial
   velocity data of the presence of a
    3rd object of considerable mass
    Whatever its age, if the
 massive amounts of dust do
 point toward the presence of
terrestrial planets, then BD+20
307 is the first known example
of planets of any mass in orbit
  around a close binary star.
Gemini/Michelle spectrum of HD
           23514
To finish, now a brief
    discussion of a
   remarkable, but
     almost totally
  neglected, bright,
isolated, stellar IRAS
        source:

    BP Piscium
       A little bit of history…




First mention of BP Psc:
object #202 in Stephenson’s
(1986) list of high-latitude
H-alpha emission-line stars
   15-20 years ago, various astronomers
  were investigating high Galactic latitude
  IRAS sources to determine if any might
    be nearby T Tauri stars (rather than,
  say, galaxies). For example, this is the
   way the first members of the TW Hya
        Association were identified.


Thus, was BP Psc “rediscovered”
 In 1996, my then student Richard Webb
      (whose Ph. D. thesis first firmly
established the TW Hya Association as a
substantial group of very nearby T Tauri
   stars), my former Ph. D. student Joel
  Kastner, Thierry Forveille (Grenoble),
and I obtained optical and radio line data
  on BP Psc (Galactic latitude -57 deg).
Before 2006, BP Psc never was a
 subject of serious investigation
When included as just one star in large surveys it
has been generally classified as a pre-main
sequence star but with a wide range of spectral
types. But, in the absence of a measured proper
motion or distance, even classification as a distant,
luminous, post-AGB star seemed conceivable to us
in 1996…

Thus, BP Psc was relegated to a back burner…

Then, in 2006 we recognized that the Tycho proper
motion, ~50 mas/yr, ruled out such a distant star.
  Thus began an extensive
   observational campaign
  Carl Melis, Joseph Rhee, Eric Becklin
  (UCLA), Inseok Song (Gemini), David
    Meier (NRAO), Alycia Weinberger
 (Carnegie), Bruce Macintosh, Christian
  Marois (LLNL), James Graham, Geoff
Marcy (Berkeley), David Wilner (Harvard),
Thierry Forveille (Grenoble), Joel Kastner
  (RIT), Travis Barman (Lowell), Patrick
 Palmer (Chicago), Mike Bessell (ANU)
TW Hya




         Weinberger at al 2002
Already christened “HH 999”
by Bo Reipurth (& these data
are not yet even published)!

   Will this designation have to be
rescinded if BP Psc is actually a post-
         main sequence star!?
Private Communication:
   Bo Reipurth to BZ:




“Yes, rescinded”
                How old is BP Psc?
                     How far away?
                Where was it born?
          Why so much warm dust?
   What is the mass accretion rate?
Does it have any “fellow travelers”?
 Comparison of gravity-sensitive
lines in our HIRES spectra of BP
  Psc with lines from late-G and
  early-K type dwarfs, subgiants,
   and first-ascent giants, imply
that, most likely, BP Psc has the
gravity of a first-ascent giant star.
  Is BP Psc the first known example of a first
  ascent giant star with a substantial orbiting
   gaseous disk, rapid accretion of gas, and
 associated jets and HH objects? If so, then
 what strange phenomenon has occasioned
its massive rapidly accreting disk of dust and
      gas? A shredded companion star or
   substellar object? Are planets likely now
   forming in the dusty CO disk? (a gazillion
        years after BP Psc itself formed)
  W UMa binaries and “hot
   Jupiters” are two known
    classes of close binary
systems where the secondary
   could be engulfed by the
 evolving primary as it leaves
      the main sequence.
   We believe that the IRAS
catalog reveals a suite of such
  common envelope objects,
 which, apparently are often
  (always?) accompanied by
     dusty excretion disks.
     One such dusty giant HD
233517, was modeled previously
  by Mike Jura (UCLA) as a cold
excretion disk. However, we are
 finding multiple examples in the
 IRAS catalog of giant stars with
 much warmer dust; presumably
  such stars are closer in time to
     the actual merger event.
         TYC 4144 329 2
mid-F type -- dusty secondary of
visual binary with ordinary K-type
   primary (~60 pc from Earth)
     Trig Parallax of BP Psc
         wanted urgently!


If pre-main sequence, then distance will be
   <~100 pc. If post main sequence, then
       distance will be a few 100 pc.
        Galactic Space Motions
Youthful Group    U      V      W    age     distance
                        (km/s)       (Myr)   (pc)
•   TW Hydrae     -11   -18     -5    8      ~60
•   Tucana/Hor    -11   -21      0   30      ~45
•    Pictoris    -11   -16     -9   12      ~40
•   AB Doradus     -8   -27    -14   70?     ~30
•    Cha         -12   -19    -10    8       97
•   Carina-Near   -26   -18     -2   200     ~30

•   BP Psc        -12   -23    +5    20      81
•   BP Psc        -14   -27    +2    10      104
•   BP Psc        -19   -33    -2    5       139
•   BP Psc        -21   -36    -4    3       157
       Kinematic mass vs
       evolutionary mass
  M(kin) = R(Vkep)2/G = R(VP)2/G(sini)2

           R = radius of CO disk
       Vkep = Keplerian velocity at R
    VP = projected line of sight velocity
i = disk inclination angle (i = 0 is face-on)
 Kinematic mass = 0.61 M(sun)
• Evolutionary mass (from Baraffe et al):

• 1.26 M(sun)       (for an age of 10 Myr)

• The ratio of the kinematic & evolutionary
  masses hardly varies for ages between 3 &
  20 Myr (both are ~proportional to distance)
As some of you were born
    *after* IRAS flew,

IRAS swept almost the entire sky at 4
           wavelengths:

     12, 25, 60 and 100 microns
        Gas & Dust masses
• Dust mass (Md) from submillimeter flux:

           Md = SD2/kB(Td)

  D = distance to star
  S = flux
  k = opacity (cm2/g)
  B
   Dust mass = 0.7 Earth
masses (for cold dust at 36 K)
The mass of H2 depends on the uncertain
optical depth of the 12CO lines. We have an
upper limit on the brightness of the 13CO, J =
2-1 line which implies that the 12CO optical
depth is <14. Thus, in the outer regions of
the CO disk,

    6 < M(gas)/M(dust) < 100
  In the past two years, five more stars of
   age >~100 Myr with warm dust in the
   terrestrial region have been identified

With Spitzer, Beichman et al 2005 found an ~1 Gyr old K-type star (HD
   69830) with LIR/LBol ~10-4 and silicate emission features seen in the
   wavelength range accessible to Spitzer’s IRS. Three Neptunes
   present! (Note: excess emission at 25 micron was marginally
   detected by IRAS!)

From old IRAS data, we identified two solar-mass, adolescent stars -- a
   Pleiad and a field star BD+20 307. Follow-up of the latter at Keck and
   at Gemini revealed a huge LIR/LBol (4%) and evidence for micron-size
   crystalline and amorphous silicate particles.
Scenario
    Solar system time scales and ages
          of young nearby stars
• Formation of Jupiter         < 10 Myr
• Formation of Earth’s core    ~ 30 Myr
• Era of heavy bombardment
  in inner solar system        ~ 600 Myr

•    Cha cluster              8 Myr
•   TW Hydrae Assoc.           8 Myr
•    Pictoris moving group    12 Myr
•   Tucana/Horologium Assoc.   30 Myr
•   AB Dor moving group        70 Myr
•   Carina-Near moving group   200 Myr
      So why devote any time to
            “StHa 202”?
• In the early 90’s, a few researchers became
  interested in “isolated,” high-b T Tauri stars
   – Various such TTS identified in the 80’s
       • First & still most famous: TW Hya (Rucinski & Krautter 1982)
       • de la Reza et al., Stephenson identified more
• Existence of such stars raised fundamental
  questions:
   – Where were they born?
   – How old/distant are they?
       • Do they still retain dusty/gaseous disks?
       • Are they closer than Tau/Oph/Cha TTS (i.e., d < 140 pc)?
           – If so, opportunity to study star & planet formation “at close range”
Is BP Psc one of the nearest, oldest, classical,
T Tauri stars and the closest known example of
      bipolar jets and HH objects, but with
    uniquely(?) low lithium abundance and a
unique(?) IRS spectrum, along with a variety of
  other lesser problems (not mentioned in this
                       talk)

                      Or
    A decade of wondering…
   Is BP Psc an evolved star?
• Optical emission line spectrum reminiscent of
  symbiotic stars
   – Dusty disks/envelopes common
      • Rotating, gaseous disk in such a system would be a first!
• Infrared SED reminiscent of post-asymptotic giant
  branch stars
   – Many examples known at high latitude
   – Some post-AGB (binary) systems known to harbor dusty
     disks
      • Again, a rotating molecular disk would be quite a find!
• Where is the associated young group?

				
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posted:8/5/2011
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