Accretion Signatures from Massive Young Stellar Objects

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Accretion Signatures from Massive Young Stellar
Objects
Bob Blum Cerro Tololo, La Serena, Chile
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Collaborators

• Peter Conti (JILA, University of Colorado)
• Augusto Damineli (IAG, University of São Paulo)
• Elysandra Figuerêdo (IAG, University of São
  Paulo)
• Cássio Barbosa (IAG, University of São Paulo)

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What is an MYSO?!

• A Hot Star at the moment of its birth''
• search for evidence of its formation process
• time scales
• complementary to other wavelength regimes NIR
  gives opportunity to see star and disk
• Which evolutionary stage (Churchwell 2002)?
• Pre-stellar Core (PSC) - SED peaks at 200 um
• Hot core (HC) - Embedded protostar, high
  accretion rate, but yet not a NIR source. SED
  peaks at 100 um.
• UCHII? - Bulk radiated at 100 um, can have NIR
  source

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Observational Properties

• Stand out in near infrared colors
• excess emission in color - color plot
• over luminous in NIR color - magnitude diagram
• Red, rising, near infrared spectrum (1-2.5 um)
• Veiled or weak photospheric lines
• strong nebular lines (H, He), atomic lines (Fe,
  Mg)
• Molecular lines (H2, CO), in-flow, out-flow,
  disk?
• Excess emission at longer wavelengths, associated
  with UCHII regions

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Where are we looking?

• Galactic Giant HII Regions
• This implies multiple O stars
• Allows exploration of cluster SF mode
• Galactic Ultra Compact HII Regions
• Herbig Be Stars
• JHK imaging K-band spectroscopy

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Some Recent Work

• Herbig AeBe Stars
• Ishii et al. 2001, ApJ, 121, 3191. K-band
  spectroscopy of IRAS AeBe precursors. Low res
  spectra. CO emission rates similar to AeBe. H2
  found only in the most heavily enshrouded. NIR
  spectra similar to those found in high mass SF
  regions.
• Fuente et al. 2003, ApJ, 598, L39. IRAM
  observations. Find less massive disks than for Ae
  stars - dissipation.

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High Resolution

• Success of low mass YSO model CO emission Carr
  (1989), Carr et al. (1994), Chandler et al.
  (1995, S106), Najita et al. (1996)
• High mass YSOs
• Scoville et al. 1983, ApJ, 275, 201
• Kraus et al. 2000, AA, 362, 158
• Bik Thi 2004, AA, 427, L13 (Bik Thesis)
• Blum et al. 2004, ApJ, 617, 1167
• 20-25 CO emission objects, many more MYSOs

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The Luminosity Problem

• Strong excess in NIR due to reprocessing.
• Objects are many times more luminous than K-band
  photosphere would indicate.
• Models give JHK excess of 3-4 mag for late O star
  (Hillenbrand et al. 1992) seen face on.
• Difficult to obtain stellar properties.
• Some are UCHII regions (W31).
• Use MIR and longer wavelengths (e.g. Barbosa et
  al. 2003, NGC 3576) to characterize
  Luminosity-Mass

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CO profiles

• Keplerian rotation, geometrically thin disk
• Power law T(r) and ?(r)?distributions
• T(i) 5000 K
• n 1010 cm-3
• Fit ri/ro, vsin(i) at ri, Vo, NCO
• Emission is typically optically thin

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What does the CO profile tell us?

• Velocity extent of blue wing, red peak - gives
  vsin(i) at inner radius. Best determined
  parameter.
• Inclination, mass, and radius degenerate.
• v2sin2(i) GM sin2(i)
• For approximate masses, inner CO emission radius
  is less than 1 AU.

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Narrow CO Profiles

• Some (too many?) sources show narrow CO profiles
• BN, Scoville et al. (1983)
• MCW349, Kraus et al. (2000)
• Bik Thi (2004)
• NGC3576 48
• Geometry may play a role - this is bad, more
  parameters.

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HII emission

• More complex than simple fits for CO
• Br?, Br?, He I
• Spatially resolved
• Requires ionization structure
• Multiple components stellar wind, disk wind, HII
  region...

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He I
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Molecular Outflow?
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Is there a mass limit on accretion?

• Lower mass HAeBe stars seem to show clear disk
  evolution/dissipation inversely proportional to
  stellar mass. (Hollenbach, Yorke, Lizano)
• HBe stars and MYSOs (early B, late O) show clear
  accretion signatures but, these are the second
  rank hot (massive) stars.
• What about first rank O stars?

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W51 RS7
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W51 RS7
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W51 RS7

• Find kO5-6 type.
• Disk signature, but around MYSO or companion?
• High resolution spectrum needed.
• MYSOs are short lived. Expect fewer examples at
  higher masses.

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Conclusions

• Evidence suggests more massive stars have shorter
  dissipation time scales.
• (at least one) First rank stars have composite
  spectra.
• What is the nature of the CO emission?
• Keplerian rotation. M17, other sources show clear
  disk signature at different inclination angles.
• BN, NGC3576 48, others show geometry is
  important or other mechanism(s) at play