FUSE Observations of Nebular O VI Emission from NGC 6543
a r X i v :a s t r o -p h /0411428v 1 15 N o v 2004
Draft version February 2,2008
Preprint typeset using L A T E X style emulateapj v.6/22/04
FUSE OBSERVATIONS OF NEBULAR O VI EMISSION FROM NGC 6543?
Robert A.Gruendl,You-Hua Chu
Astronomy Department,University of Illinois at Urbana-Champaign
1002West Green Street,Urbana,IL 61801
and
Mart ′?n A.Guerrero
Instituto de Astrof′?sica de Andaluc ′?a,CSIC
Apartado Correos 3004,E-18080,Granada,Spain
(Received;Accepted)
Draft version February 2,2008
ABSTRACT
NGC 6543is one of the few planetary nebulae (PNe)whose X-ray emission has been shown to be extended and originate from hot interior https://www.wendangku.net/doc/59897119.html,ing FUSE observations we have now detected nebular O VI emission from NGC 6543.Its central star,with an e?ective temperature of ～50,000K,is too cool to photoionize O V ,so the O VI ions must have been produced by thermal collisions at the interface between the hot interior gas and the cool nebular shell.We modeled the O VI emission incorporating thermal conduction,but ?nd that simplistic assumptions for the AGB and fast wind mass loss rates overproduce X-ray emission and O VI emission.We have therefore adopted the pressure of the interior hot gas for the interface layer and ?nd that expected O VI emission to be comparable to the observations.
Subject headings:conduction —planetary nebulae:individual (NGC 6543)—ultraviolet:ISM
1.INTRODUCTION
Planetary nebulae (PNe)are formed through dynamic interactions between the current fast stellar wind and previous asymptotic giant branch (AGB)wind (Kwok 1983;Frank,Balick,&Riley 1990);thus,the interior of a PN is ?lled with shocked fast wind.This hot interior gas and the cool nebular shell form a contact disconti-nuity where heat conduction (Spitzer 1962)is expected to occur.The resulting mass evaporation from the dense nebular shell into the hot interior lowers the tempera-ture and raises the density of the hot gas (Weaver et al.1977),signi?cantly increasing the X-ray emissivity.Hy-drodynamic models of PNe with heat conduction pre-dict X-ray emission that should be easily detectable with modern X-ray observatories (Zhekov &Perinotto 1996).Chandra and XMM-Newton have indeed detected dif-fuse X-ray emission from PNe,with plasma temper-atures of 1?3×106K and X-ray luminosities of L X =3?100×1031erg s ?1(Guerrero,Chu,&Gruendl 2004).The limb-brightened X-ray morphology and the low plasma temperatures are qualitatively consis-tent with the predictions of models with heat conduc-tion;however,the observed L X are generally too low.For example,the observed L X of NGC 6543,1×1032ergs s ?1(Chu et al.2001),is an order of magnitude lower than that modeled by Zhekov &Perinotto (1998).Similar discrepancies between observed and modeled L X have been seen in Wolf-Rayet bubbles (e.g.,NGC 6888,Garc′?a-Segura,Langer,&Mac Low 1996)and super-bubbles (e.g.,M17,Dunne et al.2003),suggesting that
?BASED
ON OBSERVATIONS MADE WITH THE NASA-CNES-CSA F AR ULTRAVIOLET SPECTROSCOPIC EX-PLORER.FUSE IS OPERATED FOR NASA BY THE JOHNS HOPKINS UNIVERSITY UNDER NASA CONTRACT NAS 5-32985.
Electronic address:gruendl@https://www.wendangku.net/doc/59897119.html,,chu@https://www.wendangku.net/doc/59897119.html, Electronic address:mar@iaa.es
this problem may be associated with thermal conduction
per se.
Thermal conduction has been assumed in theoretical models but not constrained empirically through obser-vation because interfaces at a few ×105K require di?-cult UV observations.Traditionally interfaces have been studied using spectral lines of C IV ,N V ,and O VI ;however,these species can be photoionized by hot stars (especially PN central stars with e?ective temperatures >100,000K)and the detection of these narrow absorp-tion lines can be hampered by the stellar P Cygni pro?le and confused by interstellar absorption lines.
We have chosen to study the interface layer in the PN NGC 6543using O VI emission because the phys-ical properties of its interior hot gas have been estab-lished by Chandra observations (Chu et al.2001)and its central star is too cool to produce O VI by photoion-ization (T e?～50,000K,Zweigle et al.1997).Far Ul-traviolet Spectroscopic Explorer (FUSE )observations of NGC 6543have been made and O VI emission is indeed detected.In this paper we describe the observations and data processing in §2,present the results in §3,and dis-cuss their implications in §4.
2.FUSE OBSERVATIONS AND REDUCTION
We have observed NGC 6543with FUSE to search for nebular O VI emission.The FUSE observatory has four spectrographs operating simultaneously to cover the 905–1187?A wavelength range with high resolution,λ/δλ?20,000.Details of the design and performance of the FUSE spectrographs are described by Moos et al.(2000)and Sahnow et al.(2000).
Three observations of NGC 6543were obtained in the time-tag mode using the HIRS aperture (1.′′25×20′′):“N-Cav”samples the northern edge of the central cavity,“N-Ext”the northern extension of the cavity,and “OFF”a region outside the nebula at 150′′north of the central star
2Gruendl et
al. Fig. 1.—Hubble Space Telescope Hαimage superposed with
X-ray
contours extracted from a smoothed Chandra ACIS-S image of NGC6543(Chu et al.2001).The approximate location of the FUSE HIRS apertures are marked by rectangles.
(see Fig.1for their locations).Additional observations of the central star,available in the archive and referred to as CS,used the MDRS aperture(4′′×20′′)and were acquired in the histogram mode.
Due to?exure of the instrument,only the LiF1de-tector segment remains?xed with respect to a position on the sky throughout an orbit.Thus,we only ana-lyzed observations obtained with the LiF1a detector seg-ment.The individual exposures were reprocessed using the FUSE calibration pipeline software,CALFUSE(ver-sion2.4.0),to extract a1-dimensional spectrum for each exposure.The observations of the central star obtained in histogram mode were also processed using the CAL-FUSE pipeline with its default settings.
The diagnostics produced by the CALFUSE pipeline were examined to ensure that the spectral extraction win-dow was properly centered,and to exclude burst/?are events and periods when the central star drifted into the HIRS aperture.Spectral drift between exposures was re-moved by cross-correlating the individual exposures rel-ative to the longest exposure.The typical o?set found was generally less than1pixel(～0.0067?A)and never more than5pixels.The individual exposures for each position were then weighted by their exposure times and combined.The coadded spectra for the N-Cav,N-Ext, OFF,and CS positions had total e?ective exposure times of14.4,13.7,37.3,and2.3ks,respectively.
3.RESULTS
The four FUSE spectra of NGC6543are presented in Figure2.The CS spectrum shows very broad P Cygni pro?le of the stellar O VI lines with numerous interstel-lar absorption lines of H2and low ionization ions.The OFF position shows only airglow lines of Lyβand O I. In contrast,the spectra at the N-Cav and N-Ext show prominent narrow emission lines of C IIλλ1036,1037, broad emission lines of O VIλλ1032,1038,and a broad unidenti?ed emission line at1034.432?A,superposed on continuum emission that has a spectral shape similar to that of the CS.
Fig.2.—FUSE spectra in the wavelength range around the O VI doublet.The aperture location is marked in the upper left corner of each panel.The dashed lines mark the rest wavelengths(V hel=0 km s?1)for Lyβ,and O VIλλ1032,1038lines.Absorption features of H2,C II and O I are identi?ed for the CS spectrum,emission features are identi?ed for the N-Cav spectrum,and airglow features are marked for the OFF spectrum.
TABLE1
Summary of O VI Emission Line Detections
Intrinsic Obs.Line Centroid FWHM Intensity
[km s?1]a[km s?1][erg cm?2s?1]
N-Cavλ1032?83.156.0 2.3+0.8
?0.9
×10?13
λ1038?79.945.9 1.2+0.4
?0.5
×10?13
N-Extλ1032?88.967.1 1.1+0.4
?0.4
×10?13
λ1038?88.651.4 6.7+2.3
?2.1
×10?14
a All velocities are in the heliocentric frame and assume
rest wavelength for the O VI transitions of1031.9261and
1037.6167?A(Morton2003).
The continuum emission in the nebular spectra is most likely stellar emission scattered by dust in the nebular gas.To remove this contamination,we have scaled the stellar spectrum by factors of0.018and0.0075and sub-tracted it from the nebular spectra of N-Cav and N-Ext, respectively.An expanded view of the O VI lines in Fig-ure3shows that the continuum contamination is e?ec-tively removed and the weaker O VIλ1038line becomes more prominent.We determine the centroid velocity and FWHM of each line by?tting a Gaussian pro?le,but measure the line intensity by direct summation.The O VIλ1038line?ux and shape should be regarded with some skepticism because the emission line is close to a strong C II absorption line and is in a spectral region where the P Cygni pro?le from the central star may be oversubtracted.The results of these measurements are summarized in Table1.
To convert from the observed to the intrinsic inten-sity of the O VI emission from NGC6543,we adopt E(1034?A?V)?10E(B?V),or A1034?13.2E(B?V)
FUSE Detection of O VI from NGC6543
3
Fig.3.—Detailed FUSE spectra for the O VIλλ1032,1038lines
from the N-Cav(top),and the N-Ext(middle).For comparison
the bottom panels show[O III]λ5007nebular emission from a
long-slit spectroscopic observation4.′′5north of the central star.
The bottom left panel shows the echellogram and the bottom right
shows the line pro?le extracted from the position marked by the
horizontal dashed lines on the echellogram.The systemic velocity
of NGC6543is marked by a dashed vertical line in each panel.
determined from the parameterized far-UV extinction
curve of Sasseen et al.(2002).The optical extinction of
NGC6543has been determined by Wesson&Liu(2004)
using the Balmer decrement as determined from HST
WFPC2images in the Hαand Hβlines.It is shown that
the extinction is fairly uniform across NGC6543and the
average logarithmic extinction at Hβis c(Hβ)=0.1.As
c(Hβ)=1.46E(B?V),the extinction at1034?A is A1034
=0.90±0.33,where the uncertainty is based on the obser-
vational scatter seen in far-UV extinction measurements
(Sasseen et al.2002).The intrinsic O VI line intensity is
therefore2.3+0.8
?0.9times the observed intensity.
In Figure3we further compare the velocity pro?le of the O VI lines to that of the[O III]λ5007line from an east-west oriented long-slit echelle spectroscopic obser-vation taken at a position4.′′5north of the central star. The brightest emission component in the[O III]line im-age has low velocities and corresponds to the dense enve-lope of the nebular shell,and the faint expanding“blis-ter”near the center of the line image corresponds to the northern extension of the central cavity.The heliocen-tric velocity of the O VI emission is similar to that of the expanding blister in the[O III]line indicating that they are physically associated.
4.DISCUSSION
FUSE observations of NGC6543and NGC7009 (Iping,Sonneborn,&Chu2002)detect,for the?rst time,O VI emission from PNe.Since the central star of NGC6543is too cool to photoionize O V,the O VI ions must be produced by thermal collisions in gas at temperatures of a few×105K at the interface layer be-tween the cool nebular shell and the hot gas in the PN interior.Therefore the intensity of O VI in NGC6543 can be used to investigate the physical processes at the interface.
Are the FUSE observations of O VI from NGC6543consistent with that expected from thermal conduction in such an interface layer?To model the expected O VI emission,we assume that the PN shell is a pressure-driven bubble and the radiative losses of the hot interior gas have a negligible e?ect on the dynamics.We further assume a constant AGB mass loss,resulting in a radial density pro?le∝radius?2for the circumstellar medium. The dynamics of such a bubble can be solved analytically (e.g.,Garc′?a-Segura&Mac Low1995)and is character-ized by a pressure and age which can be expressed in terms of observed quantities such as the fast stellar wind terminal velocity and mass loss rate,and the nebular shell size and expansion velocity.To calculate the tem-perature and density structure at the interface layer,ra-diative losses and thermal conduction are considered by numerically solving the continuity and energy equations (i.e.,Equations(42)and(43)of Weaver et al.1977) which rely directly upon the pressure and age found from the analytic solution.
The central star of NGC6543has a fast wind ter-minal velocity of1750km s?1and a mass loss rate of4.0×10?8M⊙yr?1(Perinotto,Cerruti-Sola&Lamers 1989).Adopting a distance of1.0±0.3kpc(Reed et al. 1999),the radii of N-Cav and N-Ext are0.02and0.04 pc,respectively.The expansion velocity of NGC6543’s nebular shell near the nebular minor axis is～20km s?1 (Miranda&Solf1992),which is appropriate for the N-Cav position.The[O III]emission associated with the N-Ext shows a velocity o?set of～30km s?1from the systemic velocity of NGC6543(see Fig.3);for an incli-nation angle of35?(Miranda&Solf1992)the expansion velocity will be～40km s?1.
Use of these observational parameters results in an an-alytic solution with a nebular age of～1000yrs and a pressure in the central cavity of～3×10?7dyne cm?2. If we compare these values with independent estimates we?nd that the age is reasonable for NGC6543(e.g., Reed et al.1999)but the pressure is roughly a factor of 10higher than indicated by observations of both the cool nebular shell and the hot gas interior.Consequently this simple model would over-produce L X as did the models of Zhekov&Perinotto(1998).Likewise,using the method outlined later,we?nd the expected O VI emission to be～10times higher than observed.Both of these dis-crepencies arise from the incorrect model prediction of pressure within the central cavity,and are likely caused by incorrect assumptions about the fast and AGB wind mass loss rates.
To obtain an alternative estimate of the O VI emission we consider the observed pressure in the nebular shell and hot interior.In the nebular shell Zhang et al.(2004)?nd an electron density of～6300cm?3and tempera-ture of6800K,and Robertson-Tessi&Garnett(2004)?nd～10000cm?3and～8000K,indicating a pressure of1.2–2.2×10?8dyne cm?2.For the interior hot gas Chu et al.(2001)?nd that the X-ray emitting gas has a plasma temperature of～1.6×106K and electron density of～50??1/2,where?is the?lling factor.For?=0.5,the pressure in the central cavity is～3×10?8dyne cm?2, similar to the pressure in the nebular shell.We adopt the pressure obtained for the hot gas in the continuity and energy equations to determine the temperature and density structure of the interface layer.The resulting
4Gruendl et al.
Fig. 4.—Results of the model calculation for the N-Cav ob-
servation of NGC6543showing pro?les for the temperature(top),
electron density(middle),O VI emissivity(bottom)as a function
of distance from the inner wall of the nebular shell.
temperature and density pro?le applicable to the N-Cav observations are plotted as a function of distance from
the inner wall of the nebular shell in Figure4.
The O VI density can be determined from the temper-
ature and density pro?le by assuming ionization equi-
librium and adopting a nebular oxygen abundance rel-
ative to hydrogen of5.6×10?4(Aller&Czyzak1983;
Bernard-Salas,et al.2003).The O VIλ1032,1038line intensity,I O VI,in ergs s?1cm?2can be derived from equations4.4and4.11of Spitzer(1978)and expressed
as:
I O VI=n e n O VI hν8.629×10?6
g j
e?χ/kT
V