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

H2

Speck et al 2002

Cox et al. 1998 – ISO

Huggins et al. 2002 -NTT

Hubble Helix 2003 - Nicmos

COHuggins et al. 2002 - IRAM

Globules are dense neutral knots

Make up most/all of the molecular envelope

Planet-like masses: from CO ~ 10-5 Mo

Main H2 emission surface excited (PDR)

Molecules (CO and H2) 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 +10216 (VLT) Basic idea: these are likely Helix precursors

Are globules primordial? test

NGC7027 data smooth Mg = 3 10-5

Mg = 3 10-5 Mg = 3 10-5 Mg = 3 10-6

Are globules primordial?

Envelopes of NGC 7027 & IRC+10216 are smooth: no proto-globules of mass 10-5 Mo 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 Molecularobservations consistentwith thisscenario

Main uncertainty is role of wind dynamics in tail formation. Data scarce (Meaburn et al. 1998)

ACS HHub 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 7027 - 555W/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 7027 - 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