Globules in Planetary Nebulae Review of some recent advances on the nature and origin of globules P. J. Huggins (NYU)

Overview We focus on classical cometary globules as in the Helix Nebula Many new advances: Hubble Helix project; HST imaging of other PN High resolution imaging of molecules Theory: shadowing, photo-evaporation, etc. We address here a few key questions

NOAO HST Helix globules

What are globules ? We already know, indirectly: dense knots of neutral gas Now we can see the gas at high resolution: resolve individual globules H2 v = 1-0 S(1) line 2.12 micron CO J = 1-0 line at 2.6 mm

H2H2 Speck et al 2002 Cox et al – ISO Huggins et al NTT Hubble Helix Nicmos

CO Huggins et al IRAM

Globules are dense neutral knots Make up most/all of the molecular envelope Planet-like masses: from CO ~ M o Main H 2 emission surface excited (PDR) Molecules (CO and H 2 ) in the tails Very quiescent - quasi static

Are Helix globules unique? No Globules are seen by HST in other nearby PNe: Dumbbell (NGC 6853) Ring (NGC 6720) These are the next nearest PNe with massive neutral envelopes Probably all evolved PNe with neutral envelopes are composed of globules HST O’Dell et al. 2002

Are globules primordial? Dyson et al. (1989) proposed formation in high density contrast instabilities in AGB atmospheres If correct: Should be proto-globules in AGB and PN envelopes In principle could be detected Searches for proto-globules (Huggins & Mauron 2002) Technique: dust scattered light NGC 7027 (HST) I RC (VLT) Basic idea: these are likely Helix precursors

Are globules primordial? test NGC7027 data smooth M g = M g =

Are globules primordial? Envelopes of NGC 7027 & IRC are smooth: no proto-globules of mass M o or more Unless these are atypical precursors, e.g., Wrong chemistry Wrong time etc. We rule out primordial origin in high density-contrast stellar ejecta

Origins: recent evolution Globules are etched from amorphous complexes into final form by the: stellar uv radiation Evidence I: Direct observation of shadowing [N II]/H  Henry et al. 1999

Evidence II: morphology alignment clustering Molecular observations consistent with this scenario Main uncertainty is role of wind dynamics in tail formation. Data scarce (Meaburn et al. 1998) ACS H  Hub Helix

Origin of the globules Idea that globules form from instabilities in the envelope is an old one (Capriotti 1973) But how exactly ? O’Dell et al. (2002) have drawn attention to filaments in IC 4406 as possible precursors

Huggins & Mauron (2002) propose that globules form during the energetic interactions in the proto-PN and young PN phases Cite the smooth-fragmented transition in the archetype NGC 7027 NGC 7027 – HST wide V NGC W/814W

Origin of the globules: NGC 7027 The fragments are connected- filamentary Bursting or explosive structure needs further theoretical work Suggests a unified model in which global shaping and globules share a common origin NGC HST 555W/814W

Globules in PNe: Summary Globules are neutral condensations They have planet-like masses They are probably not primordial They are likely instabilities that develop early Sculpted by uv radiation and shadowing We need Better kinematics to study the tails Realistic simulations of fragmentation