Determination of the Physical Conditions of the Knots in the Helix Nebula from Optical and Infrared Observations

2.6 MB·English

by  C. R. O'Dell

Submittedto theAstronomicalJournal
PreprinttypesetusingLATEXstyleemulateapjv.10/09/06
DETERMINATION OF THE PHYSICAL CONDITIONS OF THE KNOTS IN THE HELIX NEBULA FROM
OPTICAL AND INFRARED OBSERVATIONS1
C. R. O’Dell
DepartmentofPhysicsandAstronomy,VanderbiltUniversity,Box1807-B,Nashville,TN37235
W. J. Henney
CentrodeRadioastronom´ıayAstrof´ısica,UniversidadNacionalAuto´nomadeM´exico,ApartadoPostal3-72,58090Morelia,
Michaoaca´n, M´exico
and
7 G. J. Ferland
0 DepartmentofPhysicsandAstronomy,UniversityofKentucky, Lexington, KY40506
0 (Received 2006 November13)
2 Submitted to the Astronomical Journal

n ABSTRACT
a
We use new Hubble Space Telescope and archived images to clarify the nature of the ubiquitous
J
knots in the Helix Nebula, which are variously estimated to contain a significant to majority fraction
3
of the material ejected by its central star.
2
We employ published farinfraredspectrophotometryand existing 2.12µm imagesto establishthat

1 the population distribution of the lowest ro-vibrational states of H2 is close to the distribution of a
v gas in local thermodynamic equilibrium (LTE) at 988±119 K. In addition, we present calculations
6 that showthat the weaknessofthe H2 0-0S(7) line is not a reasonfor makingthe unlikely-to-be true
3 assumption that H2 emission is caused by shock excitation.
6 We derive a total flux from the nebula in H lines and compare this with the power available from
2
1 the central star for producing this radiation. We establish that neither soft X-rays nor 912–1100 ˚A
0 radiation has enough energy to power the H radiation, only the stellar extreme ultraviolet radiation
2
7 shortward of 912 ˚A does. Advection of material from the cold regions of the knots produces an
0 extensive zone where both atomic and molecular hydrogen are found, allowing the H to directly be
2
/
h heated by Lyman continuum radiation, thus providing a mechanism that will probably explain the
p excitation temperature and surface brightness of the 2.12 µm cusps and tails.
- Newimagesoftheknot378-801intheH 2.12µmlinerevealthatthe2.12µmcuspliesimmediately
2
o
inside the ionized atomic gas zone. This property is shared by material in the “tail’ region. The H
r 2
t 2.12µmemissionofthecuspconfirmspreviousassumptions,whilethetail’spropertyfirmlyestablishes
s that the “tail” structure is an ionizationbounded radiationshadow behind the optically thick core of
a
theknot. Thenew2.12µmimagetogetherwitharchivedHubbleimagesisusedtoestablishapattern
:
v of decreasing surface brightness and increasing size of the knots with increasing stellar distance.
Xi Although the contrast against the background is greater in 2.12 µm than in the optical lines, the
higher resolution and signal of optical images remains the most powerful technique for searching for
r
a knots.
A unique new image of a transitional region of the nebula’s inner disk in the HeII 4686 ˚A line fails
to show any emission from knots that might have been found in the He++ core of the nebula. We
also re-examined high signal-to-noise ratio ground-based telescope images of this same inner region
and found no evidence of structures that could be related to knots.
Subject headings: Planetary Nebulae:individual(Helix Nebula, NGC 7293)
1. INTRODUCTION lated structures. They are an important component of
the mass lost by their host stars, for the characteristic
The dense knotsthatpopulate the closestbrightplan-
etarynebulaNGC7293(theHelixNebula)mustplayan massofindividualknotshasbeenreportedas≥10−5M⊙
importantrolein masslossfromhighlyevolvedinterme- (fromCO emission,Huggins etal.2002),5.6×10−5 M⊙
(from the dust optical depth determination by Meaburn
diate mass stars and therefore in the nature of enrich-
et al. (1992), adjusted for the improved distance), and
ment of the interstellar medium (ISM) by these stars.
It is likely that similar dense condensations are ubiqui- about3.8×10−5M⊙ (O’Dell&Burkert1997,againfrom
the dust optical depth but with better spatial resolu-
tous among the planetary nebulae (O’Dell et al. 2002)
tion), and their number has been variously estimated to
as the closest five planetary nebulae show similar or re-
be from 3500 (O’Dell & Handron 1996) from optical ob-
Electronicaddress: [email protected] servationstomuchlargernumbers(23,000Meixneretal.
1 Based on observations with the NASA/ESA Hubble Space 2005,henceforth MX05; 20,000–40,000Hora et al. 2006,
Telescope,obtainedattheSpaceTelescopeScienceInstitute,which henceforthH06)frominfraredimaging. Therefore,these
is operated by the Association of Universities for Research in As-
condensationscontainasignificantfractiontoamajority
tronomy,Inc.,underNASAContractNo. NAS5-26555.
2 O’Dell et al.
of all the material ejected. It is an extremely important Sun, the 1.5′′ chord of the bright cusp surrounding the
point to understand if the ISM is being seeded by these neutral central core of 378-801 is 4.9×1015 cm. O’Dell
knots and if they survive long enough to be important & Burkert (1997) estimate that the peak density in the
in the general properties of the ISM and also the pro- ionized cusp is about 1200 cm−3 and the centraldensity
cess of formationof new stars. To understand those late of the core, derived from the optical depth in dust, is
phases, long after the knots have escaped the ionizing 4.8×105 cm−3, a number similar to the H density of
2
environment of their central stars, one must understand ≥105 cm−3 necessary to produce the thermalized popu-
their characteristics soon after their formation-which is lationdistributionfoundfortheJstateswithinthev =0
the subject of this study. levels of the electronic (X 1Σ+) ground state by Cox98.
g
There has been a burst of interest in the Helix Neb- Cox98determinedthattwosampleregionsofknotswere
ula and its knots beginning with the lower resolution closeto a populationdistributionofabout900K,a sim-
groundbased study of Meaburn et al. (1992) and the ilarresultisfound byananalysis(§ 4.2)ofnew observa-
Hubble Space Telescope (HST) images at better than tions (H06) of different regions of knots.
0.1′′ resolution(O’Dell&Handron1996,O’Dell&Burk- As was arguedin O’Dell & Handron (1996),the knots
ert 1997) in the optical window. The entire nebula has are neutral condensations ionized on the side facing the
been imaged in the H2 v=1-0S(1) 2.12 µm line at scales centralstar. Lo´pez-Mart´ınetal. (2001)haveshownthat
and resolutions of about 4′′ (Speck et al. 2002), and theearlyapparentdiscrepancybetweentheobservedand
1.7′′/pixel (H06), while Huggins et al. 2002) have stud- predicted surface brightness of the bright cusps is re-
iedonesmallregionat1.2′′ resolution,andtheNIC3de- solvedonceoneconsidersthedynamicnatureofthe flow
tector of the NICMOS instrument of the HST has been from the cusp ionization front, which depresses the re-
used by Meixner et al. (2004, MX05) to sample several combination emission from the ionized gas. The central
outer regions at about 0.2′′ resolution. A lower reso- cores are molecular, being visible in CO (Huggins et al.
lution ( 2′′) study in the longer wavelength 0-0 rovibra- 2002), and producing the multiple velocity components
tionallineshasimagedtheentirenebulawiththeSpitzer oneseesinthelowspatialresolution-highvelocityresolu-
SpaceTelescope (H06),extending a similarinvestigation tionCO studies (Young etal. 1999). The H emissionis
2
by Cox et al. (1998,henceforth Cox98) at 6′′/pixel with produced in a thin layer of material immediately behind
the Infrared Space Observatory. Radio observations of theionizedcusp(Hugginsetal. 2002). OHB00showthat
theCO(Hugginsetal. 2002,Youngetal. 1999)andHI the opticalstructurewithinthe ionizedcuspcanonly be
(Rodr´ıguezet al. 2002)emissionhave even lower spatial explained if the materialis heated on a timescale that is
resolution, but, the high spectral resolution allows one longer than the time for cool material to flow from the
to see emission from individual knots. ionization front across the width of the cusp. This slow
The three dimensionalmodelfor the Helix Nebula has heating rate means that forbidden lines are seen only
also evolved during this time. We now know that the furtherawayfromthe ionizationfront(becausethese re-
inner part of the nebula is a thick disk of 500′′ diam- quireenergeticelectronsto causetheir collisionalexcita-
eter seen at an angle of about 23◦ from the plane of tion)whereasrecombinationlineslikeHαarisepreferen-
the sky (O’Dell et al. 2004, henceforth OMM04). This tiallyfromthecoolregionsclosertothe ionizationfront.
disk has a centralcoreof highionizationmaterialtraced A successionof papers (Burkert& O’Dell 1998;OHB00,
by He II emission (4686 ˚A), and a series of progressively OHF05 ) attempting to simultaneously accountfor both
lowerionizationzonesuntilitsionizationfrontisreached. the ionizationandflowof materialhaveproduceda gen-
The more easily visible lower ionization portions of the eral model where cool material in the molecular central
inner-disk form the inner-ring of the nebula. There are core flows towards the ionization front, is slowly heated,
polarplumesofmaterialperpendiculartothisinnerdisk and upon passing through the ionization front is more
extending out to at least 940′′ (OMM04) to both the rapidlyheatedandaccelerated. Atfirstexaminationthe
northwest and southeast. There is an apparent irregu- mostrecentmodelslooksatisfactory. However,themod-
lar outer-ring which Meaburn et al. (2005, henceforth els fail because the theoretically expected zone of 900 K
M05) argue is a thin layer of material on the surface of gas has much too low a column density to account for
the perpendicular plumes, whereas OMM04) and O’Dell the observed surface brightness in H (OHF05). These
2
(2005)arguethatthisisduetoalargerringlyingalmost H observationsmustbetellingusaboutsomethingover-
2
perpendicular to the inner disk. looked in previous models. As we discuss in § 4.4, this
The nature of the knots has attractedconsiderable at- missing ingredient seems to be that the earlier models
tention. O’Dell & Burkert (1997) determined the prop- were for a static structure, whereas the knots are actu-
erties using HST WFPC2 emission line images in Hα, ally in a state of active flow.
[N II], and [O III], while O’Dell et al. (2000, henceforth This paper reports on workintended to clarify the na-
OHB00)analyzedHSTslitlessspectraofthebrightknot ture of the knots in the Helix Nebula. New observations
378-801 in Hα and [N II], an investigation extended in with all three of the imaging instruments on the HST
a study (O’Dell et al, henceforth OHF05) with better were made and are described in § 2, then analyzed in
slitless images in the same lines and also the [O I] line § 3. A new and more refined theoretical model is pre-
at 6300 ˚A. We will adopt the position based designa- sented in § 4, while these and other recent observations
tion system described in O’Dell & Burkert (1997) and and models are discussed in § 5.
the trigonometric parallax distance of 219 pc from Har-
2. OBSERVATIONS
ris et al. (2007). The object 378-801 is the best studied
of the knots and the primary target for the program re- In this investigation we draw on both our own new
ported upon in this paper. At 219 pc distance from the observationsmadewiththe HST andpublishedobserva-
tions made with a variety of telescopes. These observa-
Knots in the Helix Nebula 3
tions range from the optical through the infrared. The images in the four offset pointings were combined
using tasks within the STSDAS package. No attempt at
2.1. New Observations absolutecalibrationwasmadebecauseoftheF658Nfilter
requiring additional observations with the F660N filter,
The new HST observations were made during eight
whichisdominatedby[NII](O’Dell2004). Theresulting
orbits over the period 2006 May 22-24 as program GO imageforF658NisshowninFigure3. Originallyof0.05′′
10628(co-authorO’Dellas PrinciplalInvestigator). The
pixels, it has been averagedinto 2x2 samples in order to
Helix Nebula is sufficiently large that we could simul-
increasethesignaltonoiseratio. Sincethefinestfeatures
taneously observe it with the three operating imaging are larger than the size of the resultant pixels (0.1′′) no
instruments, the WFPC2 (Holtzman et al. 1995), the
loss of detail was incurred. The F502N image is much
NICMOS (Thompson et al. 1998), and the ACS (Gon-
lowersignalandisnotpresentedhere,althoughportions
zaga,S.etal. 2005). Bycarefulselectionofthepointing
of it are reproduced and discussed in § 3.4.
andorientationofthespacecraft,wewereabletosample
threeregionsthatareusefulforunderstandingthe knots
2.1.3. WFPC2 Images in the He II filter of a Southwest
and their structure. In each case the new observations Field
areeitheruniqueorofsubstantiallylongerexposuretime
Sixteen exposures of 1100 s each were made with the
thanprevioussimilarobservations. Theplacementofthe
WFPC2 F469N filter that isolates the He II recombina-
fields of view are shown in Figure 1.
tion line at 4686 ˚A. The images from the four pointings
werecombinedusingtheSTSDASpackageofdithertasks
2.1.1. NICMOS Images in H2 of the Knot 378-801 and the results are shown in Figure 4. For comparison,
The NICMOS observations were made with the NIC3 a matching section of the ACS mosaic that was derived
camera (256x256 pixels, each about 0.2′′/pixel), with as part of program GO 9700 (OMM04) is also shown.
eightexposuresinboththeF212N(isolatingthe2.12µm
H line and the underlying continuum) and the F215N 2.2. Recently Published Observations
2
(isolating primarily the underlying continuum as there
Therearethreerecentpapersthatcontainobservations
arenostronglinesinthisregion)filters. Eachofthe six-
pertinent to our discussion of the nature of the knots
teen exposures was 1280 s duration. The pointing was
in the Helix Nebula. These include two space infrared
changedinafourpositionpattern,withstepsof5′′. The
observations and one groundbased study.
On-the-Flystandardprocessingimageproductswereour
startingpoint. TheseimageswerecombinedusingIRAF 2.2.1. Spitzer Space Telescope Infrared Images and Spectra
2andtasksfromtheHSTdataprocessingSTSDASpack-
In a recent paper H06 present the results of an exten-
age provided by the Space Telescope Science Institute
sive observationalstudy of the Helix Nebula. They have
(STScI). The method of calibration used in OHF05 was
imaged the object out to the northeast-arc (OMM04)
adopted,wherethe nebularandinstrumentalcontinuum
with the IRAC camera (Fazio et al. 2004) in four broad
was subtracted using the signal from the F215N filter.
filters centered on 3.6 µm, 4.5 µm, 5.8 µm, and 8.0 µm.
The resultant image is shown in Figure 2 along with
These filters are dominated by emission from rovibra-
aligned WFPC2 optical emission line images from pro-
tional lines within the v = 0 ground electronic state,
grams GO 5086 and 5311. The knot 378-801 is located
although an atomic continnum must be present in ad-
in the low central region of each image and details of its
dition to a few collisionally excited forbidden and re-
brightcuspand tailare analyzedin § 3.1 and§ 3.2. It is
combination emission lines. The resolution of these im-
notable that all of the H cusps have corresponding Hα
2 ages is about 2′′, so that one cannot resolve structure
and[NII]counterpartsandthatallofthe Hα and[N II]
within the cusps, but one can see structure along radial
cusps have H counterparts on these well exposed, high
2 lines passing through the bright cusps and their much
resolution images.
longertails. Spectra wereobtainedwiththe IRS(Houck
et al. 2004) at three positions, two falling in the outer-
2.1.2. ACS Images of a Southsoutheast Field Previously
ring at locations north and southwest of the central star
Observed with NICMOS in H2
and the third location (used for background subtrac-
A field to the southsoutheast of the central star and tion)fallingdirectlyonthe fainternortheast-arcfeature.
falling into the outer-ring portion of the Helix Nebula They present calibrated fluxes for rovibrational lines of
was imaged with the ACS camera. Eight exposures of H from 0-0 S(7) at 5.51 µm out to 0-0 S(1) at 17.0
2
nominally 1200 s each were made in both the F658N µm. They also present a groundbased image in a filter
filter(whichpassestheHαand[NII]6583˚Alinesequally centered on the 2.12 µm H line which is of comparable
2
well)andtheF502Nfilter(dominatedbythe[OIII]5007 spatial resolution but wider field of view that the Speck
˚A line). The field of view shown in Figure 1 overlaps etal. (2002)study. Thisimageisnotfluxcalibratedbut
only slightly with the ACS mosaic built up during the appearstogofainterthantheSpecketal. (2002)image.
GO 9700 survey with the same filter pairs (OMM04).
The signal to noise ratio was much higher than in the 2.2.2. HST GO 9700 images in H2
GO 9700surveysince thatstudy usedtotalexposuresof In the program GO 9700 study that produced a con-
about 850 s in both of filters. tinuous mosaic of ACS images MX05 also obtained par-
allelNIC3 images in the F212Nfilter at six science posi-
2 IRAF is distributed by the National Optical Astronomy Ob- tions andone sky position. Eachpositionwasactually a
servatories,whichisoperatedbytheAssociationofUniversitiesfor
double exposure of two pointings, which alloweda slight
ResearchinAstronomy,Inc.undercooperativeagreementwiththe
NationalSciencefoundation. overlap of the NIC3 fields. The total exposure at each
4 O’Dell et al.
GO 10628 NIC3
F212N−F215N
GO 10628
WFPC2−F469N
GPOO 9S720−0F−2N12ICN3
FG5O02 1N0 6+2 F86−5A8CNS
Fig. 1.—This1500′′x1650′′ imageoftheHelixNebulainHα+[NII]fromOMM04showsthefieldscoveredbythethreeimageprograms
ofGO10628andalsothetwoNIC3fieldsinprogramGO9700thatoverlapwiththeGO10628ACSfield.
Knots in the Helix Nebula 5
Fig. 2.—Each image inthis mosaicdepicts the same 47.5′′x44.8′′region targeted withthe NICMOSNIC3 camera. The H2 image Jis
fromGO10628,asdescribedinthetext,whiletheopticalemissionlineimagesarefromWFPC2programsGO5086and5311. Thevertical
axisispointedtowardsapositionangle(PA)of35◦. TheNIC3imageshavebeenscaledtothe0.0996′′/pixelscaleoftheWFPC2camera.
6 O’Dell et al.
Fig. 3.—Thishighcontrast215′′x213′′ imageoftheACSregionshowninFigure1hastheverticalaxispointedtowardsPA=350◦. It
demonstrates thatbrightcuspknots arefoundinthe outer-ringofthenebula, inaddition totheinner-ringknots thatdominate thefield
coveredbytheGO9700mosaic.
position was about 750 s and double that in the regions discussed in § 3.4, our ACS field overlaps with much of
of overlap. The method of calibration was different as the MX05 position 2 field, allowing a more meaningful
only F212N images were obtained (the few F175W im- comparison of optical and infrared images of this region
ageswerenotusefulforcalibrationoftheF212Nimages). than was possible in the MX05 study.
Itwasassumedthattheskyimagesincludedalltheback-
ground signal that need to be subtracted, which means 3. ANALYSISOFTHEOBSERVATIONS
that it did not subtract nebular continuum. The NIC3
imagesareunder-sampled(the pixelsizeof0.2′′ isabout ThesenewimagesoftheHelixNebulawiththreediffer-
ent HST cameras provide new information on a number
thesameasthetelescope’sresolutionatthiswavelength)
of subjects related to the nature and formation of the
as in our new observations of the field around 378-801.
knots. The NICMOS H image allows a discussion of
Whencomparingthedata,oneshouldnotethatthemax- 2
the cusp and tail structure, the He II image places con-
imumeffective exposuretime is about1500s forthe GO
straints on the location of the knots to the southwest,
9700F212Nimages,while in ourstudy itis 10,240s. As
and the ACS images allow a more complete comparison
Knots in the Helix Nebula 7
Fig. 4.— The left panel shows a 167′′x 204′′ WFPC2 F469N image with north as the vertical axis and is the result of combining 16
ditheredimages. Forcomparison,therighthandpanelshowsthesamesectionofthenebulafromamosaicofACSimages(OMM04)with
Hα+[NII]asred,anaverageofHα+[NII]and[OIII]asgreen,and[OIII]asblue. NoindicationoffeaturesshowingHeII4686˚Aemission
isseen.
ofearlyH imagestothe southandsoutheastwithcom-
2
parable resolution optical emission line images.
[OIII]
3.1. Stratification in the Bright Cusp of 378-801 1.00
y
The new NIC3 H 2.12 µm line observations t
2 i
were scaled to the same pixel size as the WFPC2 ns 0.75
(0.0996′′/pixel)usingbilinearinterpolationandcarefully te
n
aligned. Althoughthisregionhasbeenimagednumerous I
d 0.50
times in this line, this is clearly the best image in terms e
z
of both its resolution and signal. The three primary op- li H2
a
tical line images and the new H image are shown in m 0.25
2 r
Figure 2. In order to make a quantitative analysis, the No [NII]
imageswererotatedandasamplethreepixelswidealong 0.00
the axis of the cusp-core-tail was made. The results are
Hα
shown in Figure 5, where the peak value of the H 2.12
2
µm and [NII] emission is normalized to unity as are the 20 40 60
Distance Across the Cusp (WFPC2 Pixels)
outer portions of the [OIII] profiles.
Fig. 5.—Thenormalizedintensityofasamplealongtheaxisof
The optical line results are similar to those found
knot 378-801 is shown for three optical lines and the H2 2.12 µm
by O’Dell & Burkert (1997), Burkert & O’Dell (1998), infraredline. The2.12µm imagehas lower resolution(about two
O’Dell, Henney, & Burkert (2000) and OHF05 in that pixels)andacomparisonoftheappearanceinthedifferentlinesis
the ionization occurring furthest from the knots ioniza- discussedinthetext.
tion front ([O III]) is weak and extended, this intrinsic
low brightness allows one to see the core of the knot in more emission in that line in the sheath of ionized gas
silhouette against the background nebular emission in surrounding the shadow of the knot. The full width at
this line. Although the extinction peaks in the core of halfmaximum(FWHM)oftheHαandthe[NII]images
the knot, it extends to about 5′′ from the bright cusp. are0.48′′ andsincetheFWHMofthestarsinthefieldof
The [N II] emission is strongand displaced (0.05′′) away view is 0.25′′, quadratic subtraction of this instrumental
fromtheionizationfrontwithrespecttotheHαemission. component leaves an intrinsic line width in those emis-
One sees extinction in Hα from the core out to almost sionlinesof0.41′′. TheFWHMoftheH lineis0.61′′and
2
the same distance as in [O III]. The lack of apparent ex- thenearbystar’sis0.42′′,leavinganintrinsicFWHMfor
tinction in [N II] must be due to there being relatively H of0.44′′ withapeakdisplaced0.11′′ towardsthecore
2
8 O’Dell et al.
of the knot from the Hα peak. The FWHM corresponds
to a length of 1.40×1015 cm and the displacement to
3.5×1014 cm. The ionized line characteristics are sim-
1.00 [OIII] H2 [NII]
iliar to those found in the previous studies, but we have
y
added here the important characteristicof the small but t
i
s
certain displacement of the broader H2. The earlier H2 en 0.75
studieslackedtheresolutiontodeterminethischaracter- t
n
I
istic, or in the case of MX05, lacked the high resolution
d 0.50
optical lines necessary for the comparison. e
z
37T8-h8e01caislib1.r8a×ted10p−e4aekrgsucrmfa−c2es−br1isgrh−t1nienssthineHthe2c.1u2spµmof mali 0.25 Hα
2 r
line. This agrees well with the peak value of 1.0×10−4 No
erg cm−2s−1sr−1 found by Huggins et al. (2002), where 0.00
they used the calibration of Speck et al. (2002) and uti-
lizeda spatialresolutionthat wouldnothave recognized
the narrowness of the peak. 20 40 60
Distance Across the Tail (WFPC2 Pixels)
3.2. Stratification in the Tail of 378-801 Fig. 6.—Across-sectionofatailregionofknot378-901isshown,
withtheradiationineachlinenormalizedtounity. Thedetailsof
The well defined tail in 378-801 is primarily formed thesampleandinterpretationarepresentedinthetext.
by a shadow in the ionizing Lyman Continuum (LyC)
radiationcast by the optically thick core, with illumina-
tion occurring by diffuse (recombination) LyC photons
and direct radiation grazing the edge of the core. The
firstordertheory describingthis situationwaspresented 1.00 [NII] Intensity
by Canto´ et al. (1998) and applied to the tails of the ty optical depth
i
Helix Nebula and the shadows behind the Orion Nebula s ([OIII])x5
n 0.75
proplyds soonafter (O’Dell 2000). OHF05 discussedthe te
n
structure in tail of 378-801 within the light of this the- I

ory and its next order refinements (Wood et al. 2004), ed 0.50
z
but were unable to explain the details of what was be- i
l
ing seen. H2 in the tails was first detected by Walsh & ma 0.25
r
Ageorges (2003) and we are able to establish with our o
N
new observations where this emission arises.
0.00
We present in Figure 6 results from traces across the
tail of 378-801 extending from 3.1′′ to 6.0′′ behind the
peak of the cusp in Hα. This region does not extend 20 40 60
as far as the partially obscured knot lying on the east Distance Across the Tail (WFPC2 Pixels)
side of the tail with its cusp 8.3′′ beyond 378-801’s Hα Fig. 7.—Thenormalizedsurfacebrightnessofthetailsampleof
cusp. One sees thatthere is a welldefined signatureof a Figure 6 is shown together with the optical depth (multiplied by
five) determined fromthe [O III] profile. The similardistribution
limbbrightenedsheathinbothH 2.12µmandHα,with
2 of each argues that the [N II] emission is caused by scattering of
the peaks of the H2 emission occuring inside the Hα, as nebular[NII]emission,asdiscussedinthetext.
expected if the Hα is associated with a local ionization
front. Since the gas ionized by diffuse radiation should [NII]brightnessarguesthatthe[NII]isactuallynebular
bemuchcoolerthanthedirectlyilluminatednebulargas, or cusp light scattered by the only marginally optically
the Hα emissivity would be high and the ionized sheath thick (peak valueτ=0.2 columnofdust). This feature is
is well defined. These observations establish that condi- easy to see because the expected low electron tempera-
tionsinthetaildoallowanionizationfronttoform,while ture of the sheath’s ionization front would suppress the
Canto´etal. (1998)andO’Dell(2000)hadexpectedthat collisionally excited [N II]. If our intepretation of [N II]
the shadowed region may be fully ionized. It is not sur- is correct, then there should be a similar component of
prising that no [O III] emission is seen, rather, that the scattered Hα radiation. This may be what somewhat
dustinthetail,concentratedtothemiddleofthetail(as fills-in the region between the two limb-brightened com-
noted by OHF05), causes extinction of the background ponents(inadditiontothelowlevelofsurfacebrightness
nebular light. The apparentquandary, noted in OHF05, expectedwhenexaminingathinshell). Inthecusp[NII]
is that the [N II] emission appears to come from inside is stronger than Hα emission whereas in this part of the
the ionizedsheathofthe tail. We alreadynotedthatthe nebulatheoppositeistrue. Thispointarguesthatmuch
HαandH structureindicatesthatanionizationbound- of the scattered [N II] emission arises from the nearby
2
ary occurs at the edge of the radiation shadow, so there bright cusp, rather than the surrounding nebula.
is an apparent contradiction in finding ionized nitrogen The distribution along the tail core is discussed in de-
emission originating inside an ionization front. tail in OHF05 (their § 4.1.2) and only a few comments
This contradiction is removed by comparison of the need be added. The question is complex because the
[N II] emission and the optical depth, as determined by knotsseemtooriginatenearthenebularionizationfront,
the [O III] image. Figure 7 shows a comparison of the then are shaped by the radiation field as the ionization
dust optical depth and the [N II] intensity. The sim- front expands beyond them (O’Dell et al. 2002). This
ilarity of the distribution of the optical depth and the means that material in the shadowed region will have
Knots in the Helix Nebula 9
never seen direct ionizing radiation and could represent lines.
pre-knot material from the planetary nebula’s photon
dominated region (PDR). The other source of tail ma- 3.4. Comparison of the ACS Images in the
Southsoutheast with earlier NICMOS H Images
terial could be neutral gas accelerated outwards by the 2
rocket effect (e.g. Mellema et al. 1998). Unfortunately, As noted in § 2.1.2, our ACS field overlapped with
thehighvelocityresolutionstudyofCObyHugginsetal. one of the double-pointings made with NIC3 in F212N
(2002)doesnotreallyilluminate thequestion. Theiran- as part of program GO 9700 (MX05). MX05 compared
gular resolution was a Gaussian beam of 7.9′′ × 3.8′′, their F212N images with the corresponding five fields in
with the long axis aligned almost along the axis of the OMM04, where the resolution was about 1′′and ground-
tail of 378-801. Since there was a strong CO component based images were used because the HST ACS mosaic
coming from the partially shadowed knot lying 8.3′′ be- did not extend out this far. This factor of five difference
yond 378-801’s Hα bright cusp, this means that there in resolution made it difficult to draw firm conclusions
is a not a clear resolution of the CO emission from the about differences and similarities of appearance. They
core of 378-801 and the partially shadowed knot. As a did,however,concludethateventheirshort(750s)over-
result,onecannothope tointerpretthe smalldifferences lappingdoubleexposuresweresufficienttoestablishthat
inthepositionofthepeakemissionatdifferentvelocities the knot cusps were more visible in H . In § 2.1.1 we
2
as core-tail differences. This could be done with higher showedthatinthe vicinity of378-801that the knots are
spatial resolution CO observations. equally visible in both the ionization cusps and the H
2
cusps.
3.3. An Unsuccessful Search for He II emission in the InFigure8weseeacomparisonoftheGO9700F212N
Southwest Knots
(H )imageswithournewACSimages. Thereisanexcel-
2
Figure 4 shows our deep HeII images alongside the lent correlation of appearance, although the contrast of
same field covered at comparable resolution with the the H emissionabove the essentially zero nebular back-
2
ACS in Hα+[N II] and [O III]. A detailed comparisonof groundishigherthaninF658N(Hα+[NII])andasusual
the two images indicates that there is no case of a HeII the knots are only easily seen in the F502N ([O III])
feature corresponding to a Hα+[N II] or [O III] feature, when the knot is in the foreground and can be seen in
noranyHeIIonlyfeatures. ThepartoftheWFPC2field extinctionagainstthe backgroundnebular emission. Af-
closest to the central star is 144′′ distance. The profile terconsideringtheflexibilityofdisplayofthehighsignal
of the HeII core of the central disk, to which the knots to noise Hα+[N II] images, it is difficult to support the
inthis partofthe nebula belong(OMM04)derivedfrom conclusion that H 2.12 µm images are a better way of
2
awidefieldofview HeIIimage(O’Dell1998)showsthat searching for knot cusps, except for any regions where
the core is down almost to 50% of its peak emission at there is high obscuration.
thisdistance. Ifanyknotsactuallyoccurwithintheneb-
ula’sHeIIcore,wewouldexpectthatinthesimplestknot 3.5. Variation in the H2 Cusps with Distance from the
Central Star
models,wewouldseeaHeIIcuspoutsidethe[OIII]zone
ofeachknotandthis is notthe case. However,the basic Throughthe new H 2.12µm NIC3 images in the cur-
2
model is not that simple. rent program (GO 10628) and the earlier MX05 study
The detailed models of OHB00 and OHF05 show that with shorter exposures, there is a larger and hopefully
thenormalprogressionofionizationstatesinthecuspare representative sample of resolved H cusps over a wide
2
preserved,that is, closest to the ionization front there is rangeofstellardistances(φ). To lookforsystematic dif-
an Heo+H+ zone, outside of which there is a He++H+ ferences, we have identified isolated cusps in each of the
zone, and outside of that a He+++H+. In a nearly con- fieldsavailable. Weselectedthethreeclosestknotsinthe
stant electron temperature nebula the innermost zone is GO 10628 field, three in both of MX05’s positions one
best traced by the [N II] emission, the next zone by the and two, and two in MX05’s positions three and four,
[O III] emission, and the outermost zone by the HeII noisolatedcuspsbeingavailableinMX05’spositionfive.
emission. Things are not so simple in the case of the In each case we measured the surface brightness at the
knots. As the gas flows through the cusp it is heated peak of the H cusp, the approximate chord across the
2
only slowly, so that the collisionally excited [N II] emis- knot’s center, the width of the H cusp, and determined
2
sionpeaksfurtherout,wherethegasishotter,morethan φ. TheGO10628andMX05positions1-4hadaverageφ
makingupforthelowerfractionofN+ ions. Bythetime values of 139′′, 290′′, 278′′, 375′′, and 464′′ respectively.
that the second zone is reachedthe density has dropped Theaveragesurfacebrightnesses(inergcm−2s−1sr−1)of
significantly and the [O III] emission is broadand weak. the cusp peaks were 1.37×10−4, 7.5×10−5, 8.5×10−5,
One would expect to find a HeII zone associated with a 4.9×10−5, and 4.5× 10−5. The average cusp widths
knot only If the knot lies within the nebula’s HeII core. were 0.6′′, 0.3′′, 0.5′′, 0.8′′, and 1.1′′. The average chord
This HeII zone would be quite weak because the density values were 1.5′′, 2.5′′, 2.2′′, 2.4′′, and 2.7′′.
of the knot’s gas would have been greatly decreasedthis The most pronounced change in knot characteristic is
far out. Moreover, the gas has probably nearly reached thecusppeaksurfacebrightness,droppingaboutlinearly
the temperature of the nebula and these higher temper- withφ,withthecuspwidthgrowingslowlyandthechord
atures suppress the emission of this recombination line. width more steadily with φ. The physical interpretation
This means that it will be hard to actually detect by of these patterns awaits a better understanding of indi-
theirHeIIemissionanyknotswithintheHeIIcoreofthe vidual knots. It should be pointed out that the relative
nebula. Probably the strongest evidence that no knots physicaldistancesfromthestarsislikelytobeincreasing
exist in the HeII core lies in the fact that we don’t see more rapidly than the relative values of φ. This is be-
anyobjectsinextinctioninanyofthe observedemission cause in the 3-D model of the Helix Nebula in OMM04,
10 O’Dell et al.
Fig. 8.— This figure shows the appearance in the 88′′ wide field of overlapping ACS (GO 10628) and NIC3 (GO 9700) images, with
northup. TheH2cuspsoftheknotsaremuchmorevisiblethanthecombinedHα+[NII]ionizedcusps. However,closeexaminationshows
an ionized gas feature corresponding to each H2 feature. This is not true for the [O III] F502N features, which require that the knot be
locatedintheforeground. Thefaintcircularfeaturesareartifactsoftheimagingsystem.
themaindiskisinclinedatanangleabout23◦ outofthe isfactorily explains the knots. The initial descrepancy
planeofthesky,withthenorthwestsideclosertotheob- between the cusp surface brightness and the simplest
server, while the outer-disk is inclined about 53◦ out of photoionizationmodel (O’Dell & Handron1996)was re-
the plane of the sky, with the southsoutheast side closer solvedby Lo´pez-Mart´ınet al. (2001)when it was shown
to the observer. Objects associated with the inner-disk thattheadvection-dominatednatureoftheflowthrough
would have a distance multiplication factor of 1.09 and theknotionizationfrontsleadstoatotalrateofrecombi-
those in the outer-disk a multiplication factor of 1.66. nationsintheionizedgasthatissignificantlybelowwhat
The objects in the GO 10628 NIC3 field are almost cer- is predictedfromnaive modelsofionizationequilibrium.
tainly associated with the inner-disk. MX05’s positions The peculiar photoionization structure of Hα and [N II]
1 and2 couldbelong witheither system(accurateradial emission can also be understood in similar terms—the
velocitieswoulddeterminethis)andtheirpositions3and heating timescale of the ionized gas is comparable with
4 are almost certainly associated with the outer-disk. the dynamic timescales for flow awayfrom the knot sur-
face, leading to resolvable temperature gradients, which
3.6. Comparison of the Structure of the Knots in 2.12
stronglyaffect the relative distribution of recombination
µm and in Hα
lineandcollisionallineemission. Themostrefinedmodel
Figure2demonstratestheremarkablesimilarityofap- is that of OHF05, which included both the effects of the
pearance of the knots in Hα and in our F212N (2.12 radiation field and also the hydrodynamic expansion of
µm) images and we discussed the quantitative proper- the knot’s ionization front. It is probably accurate to
ties of 378-801 in § 3.1 and § 3.2. We have investigated say that the photoionized portions of the knots are now
the similarities ofthe knots by selectingthe nine objects adequately understood, or at least that the models are
(including 378-801) within the NIC3 field of view that broadly consistent with the best observations.
are sufficiently isolated to allow a goodbackgroundsub- The structure in the tails is only beginning to be un-
traction. The peak surface brightness in each cusp was derstood. To the first order, the tails are the effects of
derivedin both 2.12 µm and Hα for a sample 3 WFPC2 radiation shadows in the dominant ionizing species, the
pixelswide(0.3′′). Thepeaksurfacebrightnessinthetail LyC photons (Canto´ et al. 1998, O’Dell 2000). With
of each object was determined in a sample 11 WFPC2 this paper (§ 3.2) we have now determined that the well
pixels wide (1.1′′) across the tail, the closer end of the observed tails are ionization bounded, with H sheaths
2
samplebeing20WFPC2pixels(2.0′′)displacedfromthe inside the zone of ionized gas that occurs at the edge
tip of the bright cusp. We used the 2.12 µm calibration of the LyC shadow. The inner part of the tail is dense
described in § 2.1.1 and the Hα calibration of O’Dell & enough in dust to scatter surrounding nebular light, al-
Doi (1999), expressing the surface brightnesses in units though the origin of this material as arising from the
of photons cm−2 s−1 sr−1. The average cusp surface originalprocess that forms the knots or as material that
brightness ratio in 2.12 µm/Hα was 5.5±1.0. The av- is moving back from the knot remains uncertain.
erage surface brightness ratio for the tail as compared The greatest quandary surrounds the explanation of
with the cusp was 0.23±0.08 in 2.12 µm and 0.17±0.05 the H zone that is observedimmediately inside the ion-
2
in Hα. This means that in the cusp the H2 2.12 µm izedcuspsoftheknots. Theapproximatelocationofthis
line alone is putting out more than five times as many zoneofobservedH is qualitatively whereone wouldex-
2
photons as in Hα and that the contrastbetween the tail pect it. For reasons given below, it is almost certainly
and cusp may be slightly higher (the ratio is lower) in not excited by shocks. Other models (e.g. Natta & Hol-
Hα than in 2.12 µm. The remarkable similarity of the lenbach1998)argue that the heating is by absorptionof
knots in a recombination line that follows photoioniza- soft X-rays and others that the excitation mechanism is
tionofatomichydrogenandemissioninamolecularline probably fluorescence, where non-ionizing photons from
heretoforeassumedtobetheresultofradiativepumping the stellarcontinuumexcite moleculesto the B 1Σ+ and
u
between electronic states is discussed in § 4.5. C1Π electronicstates,whichthendecay,producingthe
u
4. DISCUSSION populations of the ground electronic state that give rise
to the observed infrared lines. Within the core of the
Our understanding of the physics of the knots has
knot the density is sufficiently high and the tempera-
evolvedwithabetterunderstandingofamodelthatsat-