Abell 48 - a rare WN-type central star of a planetary nebula

1.5 MB·English

by  H. Todt

Mon.Not.R.Astron.Soc.000,1–12(2002) Printed17January2013 (MNLATEXstylefilev2.2)
Abell 48 - a rare WN-type central star of a planetary
nebula(cid:63)
H. Todt1†, A.Y. Kniazev2,3,4 V.V. Gvaramadze4,5, W.-R. Hamann1,
D. Buckley3, L. Crause2, S. M. Crawford2,3, A. A. S. Gulbis2,3, C. Hettlage2,3,
3 E. Hooper6, T.-O. Husser7, P. Kotze2,3, N. Loaring2,3, K. H. Nordsieck6,
1
0 D. O’Donoghue3, T. Pickering2,3, S. Potter2, E. Romero-Colmenero2,3,
2 P. Vaisanen2,3, T. Williams8, M. Wolf6

n
1University of Potsdam, Institute of Physics and Astronomy, 14476 Potsdam, Germany
a
2South African Astronomical Observatory, PO Box 9, 7935 Observatory, Cape Town, South Africa
J
3Southern African Large Telescope Foundation, PO Box 9, 7935 Observatory, Cape Town, South Africa
6 4Sternberg Astronomical Institute, Lomonosov Moscow State University, Moscow, Russia
1 5Isaac Newton Institute of Chile, Moscow Branch, Russia
6Department of Astronomy, University of Wisconsin-Madison, 475 N. Charter St., Madison, WI 53706, USA
] 7Institut fu¨r Astrophysik Georg-August-Universit¨at, Friedrich-Hund-Platz 1, 37077 Gottingen, Germany
R
8Department of Physics and Astronomy, Rutgers University, 136 Frelinghuysen Road, Piscataway, NJ 08854, USA
S
.
h
p Versionfrom 17January2013
-
o
r
t ABSTRACT
s
Aconsiderablefractionofthecentralstarsofplanetarynebulæ(CSPNe)arehydrogen-
a
[ deficient.AlmostalloftheseH-deficientcentralstars(CSs)displayspectrawithstrong
carbon and helium lines. Most of them exhibit emission line spectra resembling those

2 ofmassiveWCstars.ThereforethesestarsareclassedasCSPNeofspectraltype[WC].
v
Recently,quantitativespectralanalysisoftwoemission-lineCSs,PB8andIC4663,re-
4
vealedthatthesestarsdonotbelongtothe[WC]class.InsteadPB8hasbeenclassified
4
as[WN/WC]typeandIC4663as[WN]type.Inthisworkwereportthespectroscopic
9
identification of another rare [WN] star, the CS of Abell48. We performed a spectral
1
. analysis of Abell48 with the Potsdam Wolf-Rayet (PoWR) models for expanding at-
1
mospheres. We find that the expanding atmosphere of Abell48 is mainly composed of
0
helium (85 per cent by mass), hydrogen (10 per cent), and nitrogen (5 per cent). The
3
residual hydrogen and the enhanced nitrogen abundance make this object different
1
from the other [WN] star IC4663. We discuss the possible origin of this atmospheric
:
v composition.
i
X
Key words: stars: abundances stars: AGB and post-AGB stars: mass-loss
r stars: Wolf Rayet planetary nebulae: general planetary nebulae: individual:
a
PN G029.0+00.4
1 INTRODUCTION a planetary nebula (PN) around the CS. While most of the
low mass stars stay hydrogen-rich at the surface through-
During their evolution, low mass stars in a certain mass
out their life, a fraction of about 20 per cent of CSs show a
range become central stars of planetary nebulæ (CSPNe).
hydrogen-deficient surface composition. Almost all of these
Central stars (CSs) are post-AGB stars with effective tem-
H-deficient CSs exhibit spectra with strong carbon and he-
peratures above 25kK, hot enough to ionize circum-stellar
lium lines, indicating that their surface is composed pre-
material, which was shed off during the AGB phase. The
dominantly by these elements (e.g. Go´rny & Tylenda 2000;
ionized circum-stellar gas becomes visible in the optical as
Werner & Heber 1991).
About half of the H-deficient CSs show emission line
(cid:63) Based on observations obtained with the South African
spectra resembling those of massive WC stars, i.e. carbon-
Large Telescope (SALT), commissioning programme
2010-1-RSA OTH-001andprogramme2011-3-RSA OTH-002. rich Wolf-Rayet (WR) stars with strong stellar winds. In
† E-mail:[email protected] analogy,theCSswithbrightcarbonemissionlinesareclas-
(cid:13)c 2002RAS
2 H. Todt et al.
sified as [WC] stars, where the brackets should distinguish
Table 1.ThecentralstarofAbell48.
them from their massive counterparts.
Recently,quantitativespectralanalysisoftwoCSswith RA(J2000) 18h42m46s.91
broad emission lines revealed that they do not belong to Dec.(J2000) −03◦13(cid:48)17(cid:48).(cid:48)2
l 29◦.0784
the [WC] class. The first one was PB8 (Todt et al. 2010a),
b 0◦.4543
whichshouldbeclassifiedas[WN/WC]type,andtheother
B (mag) 19.40±0.35
onewasIC4663(Miszalskietal.2012),whichshowsa[WN]
R(mag) 16.67±0.34
typespectrum.Inbothcasestheatmospheresofthesestars
I (mag) 15.50±0.05
are mainly composed of helium. The atmosphere of PB8 J (mag) 13.51±0.03
also contains hydrogen and traces of carbon, nitrogen, and H (mag) 12.83±0.03
oxygen,whileIC4886hasanalmostpureheliumatmosphere Ks (mag) 12.33±0.03
with only traces of nitrogen. [3.6](mag) 11.69±0.06
For some other CSs the membership to the [WN] class [4.5](mag) 11.25±0.10
wasdiscussed,butcouldnotbeproven(Todtetal.2010b). [5.8](mag) 11.06±0.09
[8.0](mag) 11.04±0.16
DePew et al. (2011) mentioned the CS of Abell48, which
shows a WN-type spectrum, but it was not clear whether
this is really a CSPN and not a massive WN star with a
resolvedintheIRonlywiththeadventoftheSpitzer Space
ring nebula. In this paper, we present a quantitative anal-
Telescope (Phillips & Ramos-Larios 2008; Wachter et al.
ysis of the optical spectrum of the CS of Abell48, which
2010). Figure 1 includes the Spitzer 8 and 24µm images of
reveals indeed a WN-like atmospheric composition. We will
Abell48 obtained with the Infrared Array Camera (IRAC;
also present arguments for the low-mass nature of Abell48.
Fazio et al. 2004) and the Multiband Imaging Photometer
The paper is organized as follows: Section 2 gives an
for Spitzer (MIPS; Rieke et al. 2004) within the framework
overview of previous observations of Abell48. In Section 3
of the Galactic Legacy Infrared Mid-Plane Survey Extraor-
wedescribeourobservationsandthedatareduction.Ashort
dinaire (GLIMPSE; Benjamin et al. 2003) and the 24 and
descriptionofourmethodsisgiveninSection4.Thespectral
70Micron Survey of the Inner Galactic Disk with MIPS
analysis is presented in Section 5. Section 6 deals with the
(MIPSGAL; Carey et al. 2009), respectively. In the 8µm
nebula, while the results and implications are discussed in
image the nebula shows a bright inner shell and a diffuse
Section 7.
outerhalowithanangularextentequaltothatoftheouter
optical shell, while at 24µm its morphology is very similar
to that of the radio nebula.
2 ABELL48 To our knowledge, there was not any significant study
ofAbell48untilrecently.TheinterestinAbell48hasarisen
Abell48 was discovered by Abell (1955) on the original
after Wachter et al. (2010) carried out optical spectroscopy
plates of the Palomar Sky Survey. In the red photograph
of its CS and classified it as WN6. Based on this result,
fromthissurvey(presentedinfig.1inAbell1966),Abell48
DePew et al. (2011) and Bojicic et al. (2012) have specu-
appears as a slightly elliptical double-ringed nebula with a
lated that Abell48 might be another member of the [WN]
major axis diameter of about 40(cid:48)(cid:48) (see also the left panel
or [WN/WC] class (Todt et al. 2010a), but there has not
of Fig. 1 for the Digitized Sky Survey II (DSS-II) red band
been a detailed study yet.
(McLean et al. 2000) image of the nebula). Abell (1955,
Table1givesthecoordinatesandphotometryoftheCS
1966) classed the nebula as PN (listed in the discovery pa-
of Abell48. The B and R magnitudes are from the Guide
pers under numbers 36 and 48, and named in the SIMBAD
Star Catalogue (GSC2.2)2, the I magnitude is from the
database PN A55 36 and PN A66 48, respectively), while
DENIS (Deep Near Infrared Survey of the Southern Sky)
itsCShasnotbeenclassified.Sincethattime,thePNclas-
database (DENIS Consortium 2005), the J, H, K magni-
sificationofAbell48wasgenerallyacceptedandthenebula s
tudesarefromthe2MASS(TwoMicronAllSkySurvey)All-
was included in the Strasbourg-ESO Catalogue of Galac-
SkyCatalogofPointSources(Cutrietal.2003),andtheco-
tic Planetary Nebulae (Acker et al. 1992) under the name
ordinatesandtheIRACmagnitudesarefromtheGLIMPSE
PN G029.0+00.4.
Source Catalog (I + II + 3D)3.
Abell48isastrongradiosource.Withthe1.4GHzflux
of 159±15mJy (Condon & Kaplan 1998), it is one of the
brightest objects among the PNe covered by the NRAO
VLA Sky Survey (NVSS; Condon et al. 1998). The circu- 3 OBSERVATIONS AND DATA REDUCTION
lar shell of Abell48 was clearly resolved in the Multi-Array
Abell48 was observed within the framework of our ongo-
Galactic Plane Imaging Survey (MAGPIS; Helfand et al.
ingprogrammeofspectroscopicfollow-upobservationsded-
2006), carried out with the Very Large Array (VLA)1. In
icated to evolved massive star candidates that were re-
the MAGPIS 20cm image Abell48 appears as a thick shell
vealedthroughthedetectionoftheirIRcircumstellarnebu-
of the same size as the optical nebula (see Fig. 1).
lae(Gvaramadzeetal.2009,2010a,b,c,2011,2012;Stringfel-
Abell48 was also detected in several major mid- and
low et al. 2011, 2012; Burgemeister et al. 2013). Although
far-infrared (IR) surveys, while its ring-like shell has been
Abell48wasnotincludedinour(Gvaramadzeetal.2010a)
1 TheauthorsofthissurveyclassedAbell48asahigh-probability 2 http://vizier.u-strasbg.fr/viz-bin/VizieR?-source=I/271
supernovaremnantcandidate. 3 http://cdsarc.u-strasbg.fr/viz-bin/Cat?II/293
(cid:13)c 2002RAS,MNRAS000,1–12
Abell48 - a rare WN-type CSPN 3
N
E
20"
Figure1. Fromlefttoright:DSS-IIredband,Spitzer8and24µm,andtheVLA20cmimagesofAbell48anditscentralstar(marked
byacircleintheDSS-IIand8µmimages).Theorientationandthescaleoftheimagesarethesame.
list of compact nebulae with central stars (because it was 4 METHODS
identifiedintheSIMBADdatabaseasaPN),wedecidedto
4.1 Spectral modelling
observe it to check whether its CS is an ordinary WN star
(as suggested by Wachter et al. 2010) or we deal with an For the analysis of the spectrum we used the Potsdam
extremely rare object – a [WN]-type CSPN. Wolf-Rayet(PoWR)modelsforexpandingatmospheres(see
Gra¨feneretal.2002;Hamann&Gra¨fener2004).ThePoWR
SpectralobservationsofAbell48wereobtainedwiththe code solves the non-LTE radiative transfer problem in a
Southern African Large Telescope (SALT; Buckley et al. spherically expanding atmosphere simultaneously with the
2006; O’Donoghue & et al. 2006) on 2011 August 25 dur- statisticalequilibriumequationswhileaccountingforenergy
ingthePerformanceVerificationphaseoftheRobertStobie conservation.Iron-grouplineblanketingistreatedbymeans
Spectrograph (RSS; Burgh et al. 2003; Kobulnicky et al. of the superlevel approach. Wind inhomogeneities in first-
2003), and one more spectrum was taken on 2012 April 12. orderapproximationaretakenintoaccount,assumingsmall-
The long-slit spectroscopy mode of the RSS was used, with scale clumps. In this work, we do not calculate hydrody-
a slit width of 1(cid:48).(cid:48)25 and a position angle of 0◦ for all ob- namically self-consistent models, as in Gra¨fener & Hamann
servations. The seeing was stable in the range of 1(cid:48).(cid:48)6 to (2005), but assume that the velocity field follows a β-law
2(cid:48).(cid:48)0.Weutilizedabinningfactorof2,togiveafinalspatial with β=1 with the mass-loss rate as a free parameter. Our
sampling of 0(cid:48).(cid:48)258 pixel−1. The Volume Phase Holographic computationsincludecomplexatomicmodelsforhydrogen,
(VPH) grating GR900 was used in these observations to helium, carbon, nitrogen, oxygen, and the iron-group ele-
cover a total spectral range of 4300–7300˚A with a final re- ments. Within our group, this code has been extensively
ciprocal dispersion of ∼0.97˚A pixel−1 and a spectral reso- usedforquantitativespectralanalysesofmassiveWRstars
lution FWHM of 5–6˚A. Three exposures were taken, each (e.g. Gra¨fener et al. 2002; Liermann et al. 2010) and WR-
with15minutes.SpectraoftheXecomparisonarcwereob- type CSPN (e.g. Todt et al. 2010a).
tainedtocalibratethewavelengthscale.Spectrophotometric
standard stars were observed during twilight.
4.2 Spectral fitting
RSS has three mosaiced 2048×4096 CCDs as a detec-
The typical emission-line spectra of Wolf-Rayet stars are
tor. First, data for each CCD were overscan subtracted,
predominantly formed by recombination processes in their
trimmed, gain and cross-talk corrected, and finally mo-
dense stellar winds. Therefore the continuum-normalized
saiced.ThisprimarydatareductionwasdonewiththeSALT
spectrum shows a useful scale-invariance: for a given stellar
science pipeline (Crawford et al. 2010). Second, primary
temperature T and chemical composition, the equivalent
corrected and mosaiced long-slit data were reduced in the ∗
widths of the emission lines depend only on the ratio be-
way described in Kniazev et al. (2008b). Finally, all two-
tweenthevolumeemissionmeasureofthewindandthearea
dimensionalspectrawereaveraged.Thespectrumofthesur-
ofthestellarsurface,toafirstapproximation.Anequivalent
roundingnebulawassubtractedusingtheIRAFbackground
quantity, introduced by Schmutz, Hamann, & Wessolowski
task and a one-dimensional spectrum of the CS was ex-
(1989), is the transformed radius
tractedwiththeapalltask.Thetwo-dimensionalspectrum
of the surrounding nebula was averaged along the slit. (cid:20) v (cid:46) M˙√D (cid:21)2/3
R =R ∞ . (1)
t ∗ 2500kms−1 10−4M yr−1
SALTisatelescopewithavariablepupil,anditsillumi- (cid:12)
nating beam changes continuously during the observation. DifferentcombinationsofstellarradiiR andmass-lossrates
∗
Thismakesanabsolutefluxcalibrationimpossible,evenus- M˙ canthusleadtonearlythesameemission-linestrengths.
ingspectrophotometricstandardstarsorphotometricstan- In the form given here, the invariance also includes the
dards.Thereforethespectrophotometricstandardstarswere micro-clumping parameter D, which is defined as the den-
usedonlyforarelativefluxcalibration,andtheabsoluteflux sitycontrastbetweenwindclumpsandasmoothwindofthe
level was scaled to the photometric R magnitude. same mass-loss rate. Consequently, mass-loss rates derived
(cid:13)c 2002RAS,MNRAS000,1–12
4 H. Todt et al.
empirically from fitting the emission-line spectrum depend
ontheadoptedvalueofD.Thelattercanbeconstrainedby 2 3
fittingtheextendedelectronscatteringwingsofstrongemis-
sion lines (e.g. Hillier 1991; Hamann & Koesterke 1998). 1.2
25
10
4
5 ANALYSIS 1.0
)
5.1 Stellar parameters R
/
In order to find the model that best matches to the obser- Rg (t
vation, we choose a systematic approach: We begin with a o 1
l0.8
firstdeterminationoflogT andlogR withthehelpofthe
∗ t
alreadyexistingmodelgridformassiveWNstarsofthelate
subtypes (WNL)4 (Hamann & Gra¨fener 2004), which was 2
recently updated with improved atomic data. 3
For each of the models in this grid, we calculated the 0.6
reduced χ2
ν 25 4 10
10
χ2ν = N1 (cid:88)N (fiobs−σ2fimod)2. (2) 4.7 4.8 4.9 5.0
i=1 log (T / K)
*
fobs denotes the by-eye rectified observed spectrum and Figure2.Contoursofthereducedχ2 forourgridof[WN]mod-
ν
fmod the continuum normalized synthetic spectrum of the els. The open circles indicate the calculated models. Between
model, each with N data points. We consider R and T as these data points the contour lines are interpolated. Note that
t ∗
the fitting parameters. We adjusted the uncertainty σ so, a ∆χ2ν = 1 corresponds to a 1σ confidence interval. Thus, the
that the reduced χ2 of the best fitting model equals unity, contourwithχ2ν =2indicatesthe1σ uncertaintyoftheparame-
corresponding to aνmean deviation of about 20 per cent. tersRt andT∗.Ourfinalmodelisindicatedbytheredsquare.
The best fitting model is the one with logT /K=4.85 and
∗
logR /R =0.9.
t (cid:12)
The models of this grid had been calculated with a
bolometricluminosityoflogL/L =5.3andadensitycon-
(cid:12)
trast of D = 4, as appropriate for many massive WN stars
2.30
(Hamannetal.2006).However,inSection7wepresentargu- 1.0
mentsthatAbell48isinfactaPN.Thereforewecalculated
a grid of models with parameters that are more appropri-
2.2
ate for CSPNe, i.e. for the stellar luminosity and mass of )

L = 6000L(cid:12) and M = 0.6M(cid:12) (see e.g. Scho¨nberner et al. R 2.45
2005; Miller Bertolami & Althaus 2007). / 0.8
Rt
latedMaosreeroiveesr,ofwmithodoeulrsfiwristthedstiiffmeraetnetocfhRetmaincadlTa∗buwnedcaanlccues- og ( 2.7
l
todeterminethemostappropriateatmosphericcomposition. 2.8
The best fit is achieved with the abundances presented in 3.3
3.8
Table3,asdescribedinSection5.2.Theseabundanceswere
0.6
used for our [WN] grid.
Since there is no UV observation available that would
2.2
contain spectral lines with P Cygni profiles, we had to 2.45 2.7
deduce the wind terminal velocity, v from the width of
∞
the optical emission lines. The best fit is achieved with 4.8 4.9 5.0
v = 1000±200kms−1. Line broadening by microturbu- log (T / K)
∞ *
lence is also included in our models. From the shape of the
Figure3. Contoursoftheratiobetweenthelinepeakheightsof
line profiles we estimated the microturbulence to be about
Niv7100/Niii4643(greendashedlines),Niv7100/Nv4933
100kms−1.
(bluefilledellipse),andHeii5412/Hei5876(blacksolidlines).
We repeated our χ2-fit analysis for this grid of [WN]
ν The thick contours represent the measured values, the thinner
models to get the uncertainties in Rt and T∗ (see Fig. 2). lines the 1σ uncertainty. The other symbols are the same as in
The1σ-uncertaintiescanbeobtainedfromthecontourwith Fig.2.
χ2 = 2, i.e. where the value of χ2 has increased by 1 com-
ν ν
paredtothebest-fittingmodel,forwhichχ2 =1.Thebest-
ν
fitting model from this [WN] grid, has T = 71+12kK and
∗ − 9
4 http://www.astro.physik.uni-potsdam.de
(cid:13)c 2002RAS,MNRAS000,1–12
Abell48 - a rare WN-type CSPN 5
5.2 Element abundances
Table 2. Ratiosbetweenthepeakheights:measuredratios,in-
ferredquantities,andratiosfromthefinalmodel.
In an iterative process with the determination of T and
∗
R , we varied the abundances of He, H, C, N, and O. As
Lineratio Observed Finalmodel t
thereisnoUV-spectrumoftheCSofAbell48availablethat
Heiiλ5412/Heiλ5876 2.45±0.25 2.2
would help to determine the abundances of the iron-group
Nivλ7120/Niiiλ4634 3.3±0.5 2.3 elements, we kept latter fixed to solar values.
Nivλ7120/Nvλλ4933/43 2.8±0.3 2.1 Helium.Fromthestrengthsoftheheliumemissionlines,
especially Heii 4-3, it is already obvious that the wind is
mostly composed of helium.
Hydrogen. None of our models is able to reproduce the
Hα/Heii and Hβ/Heii blends together with the unblended
logR /R =0.9±0.2, where the given error margin corre-
t (cid:12) Heii7-4linesimultaneouslyasobserved(cf.Fig.6).Hβ fits
spondstoadoublingofthedeviationbetweenmodelandob-
best with a hydrogen mass fraction of 20 per cent, while
servation, as measured by the reduced χ2 (see Equation 2).
ν the Hα/Heii blend matches the observation with about 3
Forafurtherrefinement,wecalculatedtheline-strength
per cent of hydrogen. As a compromise we adopt a value of
ratiosbetweensignificantlinesofHeii/Hei,Niv/Niii,and
10 per cent of hydrogen. A better resolved spectrum with
Niv/Nvforthisgridandcomparedthemtotheobservation
ahighersignal-to-noiseratio(S/N)wouldhelptopindown
(cf. Table 2). We used line ratios instead of absolute line
the exact hydrogen abundance.
strengthstodiminishtheinfluenceofchemicalabundances.
Nitrogen. The nitrogen lines in the spectrum of the CS
In practice, it is often difficult to measure the equivalent
of Abell48 are much stronger than in WN spectra of stars
widths of lines correctly, e.g. due to line blends. Therefore
with similar parameters. We had to increase the nitrogen
we used the peak height as a measure of the line strength.
abundance to 5 per cent by mass to match the observed
From our optical observation we estimate the uncer-
line strength of the Nv λλ4933/4944 multiplet and most
taintyinthenormalizedcontinuumtobeoftheorderof10
of the other nitrogen lines. The Niv λ7125 line and the
per cent in the region of the Niiiλ4643 line and 5 per cent
Niv/Nvblendat4600˚Aareunfortunatelynotwellmatched
attheotherdiagnosticlines.Weconsiderthisastheuncer-
by our model. Models with only X = 3 per cent provide
taintyinthepeakmeasurement,whichwethenusetoinfer N
a much better fit to the Niiiλ4634 line, while the strength
a 10 per cent uncertainty in the measured Heii/Hei and
of the Niv λ7125 line is best reproduced by models with
Niv/Nvpeakheightratioanda15percentuncertaintyin
X = 7 per cent. Generally, it is hard to find a sufficient
the measured Niv/Niii peak height ratio. N
fit for spectral lines of three different ions from the same
In Figure 3 we show contours of the Niv/Niii and
element.Thismightbeanindicationforaninhomogeneous
Heii/Hei line peak ratios. The bold lines correspond to
wind.
the observed values. These contours have two intersections.
Carbon. In the absence of other carbon lines within
As a further criterion we now consider the Niv/Nv ratio.
the range of our observed spectrum we have to rely on the
Unfortunately, all models of the grid give a smaller value
Civ λ5800 line only. This line can be best fitted with a
than observed, but the two models encircled by the shaded
modelthathasacarbonabundanceof0.3percentbymass.
area in the upper left corner of the diagram come closest.
Thegivenuncertaintyof±0.1percentaccountsonlyforthe
After manual inspection of the spectral fits we finally de-
uncertainty of the continuum normalization
cided for the final model with logR /R = 0.85+0.15 and
t (cid:12) −0.13 Oxygen. There are no strong oxygen lines in the spec-
T =70+4kK, indicatedbythe redsquare inFigure 3.The
∗ −5 tral range of our observation. A model with solar oxygen
given uncertainties were estimated from the 1σ contours in
abundance of X = 0.6 per cent by mass predicts a weak
this figure. O
emissionlinefromOiiiλ5592withalinestrengththatisal-
Remarkably, the final parameters derived under as-
mostbelowthedetectionlimit.Ahigheroxygenabundance
sumption of a CS luminosity hardly differ from those ob-
would not be compatible with the observation.
tainedabovefromthemassivestargrid.Thisdemonstrates
The final line fit is shown in Fig. 4 and the model pa-
thatthescale-invarianceformodelswithsameR holdsover
t rameters are compiled in Table 3.
a wide parameter range.
Together with the normalized spectrum we obtained
the synthetic spectral energy distribution (SED) in abso-
lute units from the PoWR models. The synthetic SED was 6 THE NEBULA OF ABELL48
fittedtotheflux-calibratedspectrumandphotometricmea-
6.1 Position–velocity diagram along the slit
surements by adjusting the distance and the reddening pa-
rameterE (cf.Fig.5).Thephotometricmagnitudesfor In Fig. 7 we present the Hα intensity and radial velocity
B−V
the Johnson and 2MASS measurements were converted to distributions along the slit, and the DSS-II red band image
fluxes with help of the calibrations by Holberg & Bergeron of Abell48 for comparison. The extension of the nebula in
(2006), the IRAC measurements with help of the calibra- the Hα line is very similar to that in the DSS-II image. Hα
tions by Cohen et al. (2003). emissionappearseverywherewithinthenebulaanditspeaks
The best SED-fit was obtained with a color excess of are clearly correlated with the CS and the shell’s position.
E = 2.10mag and the standard total-to-selective ab- We measured the diameter of the nebula of 43(cid:48)(cid:48) at a ten
B−V
sorptionratioR =3.1(Fitzpatrick1999).Weestimateda per cent level of the peak intensity in the Hα line. At the
V
distance of d = 1.9kpc towards Abell48 with the adopted distanceofAbell48of1.9kpc(seeSections5.1and7.3)the
stellar luminosity of 6000L . radius of the nebula is about 0.2pc.
(cid:12)
(cid:13)c 2002RAS,MNRAS000,1–12
6 H. Todt et al.
6
5
zed flux 34 H + HeII 10-4γ N IVN V HeII 9-4 NV 4-3 NIII C IV HeII 4-3 N IV N V N IV H + HeII 8-4β NV 7-6
ali
m V
or 2 N I
N
1
10x
0
4300 4400 4500 4600 4700 4800 4900 5000
6
ormalized flux 2345 N V N IVN V HeII 7-4 OIII N IV CIV HeINaI i.s. He II 18...26 - 5 N IVHe II 17-5N IVHeII 16-5N IVHeII 15-5 N VHeII 14-5 H + HeII 6-4α HeII 13-5 N V HeII 12-5 HeINIV HeII 11-5
N
1
0
5000 5250 5500 5750 6000 6250 6500 6750 7000 7250
λ / Ao
Figure4.OpticalspectrumoftheCSofAbell48:Observation(bluesolid)vs.syntheticspectrum(reddashed).Theobservedspectrum
hassomegapsandgetsmuchnoisiertowardsshorterwavelengths.
-14
B R I J H K
S
]
o-1A-15 IRAC

-2m
c

-1s
erg -16
[
fλ
g
o
l
-17 2MASS
3.6 3.8 4.0 4.2 4.4 4.6 4.8
log λ[Ao]
Figure 5.ObservedfluxdistributionofAbell48(blue/noisy)inabsoluteunits,includingthecalibratedspectrumandthephotometric
measurements(openbluesquares,seeTable1),comparedtotheemergentfluxofthemodel(red/smoothline).Themodelfluxhasbeen
reddenedandscaledtothedistanceaccordingtotheparametersgiveninTable3.
We calculated the line-of-sight velocity distribution velocity estimates where used which satisfy the criteria
(v ) along the slit using the method and programs de- S/N > 3 and σ < 10kms−1.
hel v
scribed in Zasov et al. (2000). To exclude systematic shifts
originating from the known RSS flexure, the closest bright Fig.7showsthatvhel alongtheslitspansarageof≈0
night sky line [Oi] λ6363˚A was used. Finally, only those to50kms−1.ThisrangeisverysimilartothatfoundforPNe
(see e.g., Torres-Peimbert et al. 2009). We measured v =
hel
50.4±4.2kms−1 intheareawhichisclosesttotheposition
(cid:13)c 2002RAS,MNRAS000,1–12
Abell48 - a rare WN-type CSPN 7
5
ed flux 34 HeII 8-4Hβ HeII 7-4 HeII 14-5
z
ali
m 2
Nor II 6-4Hα
1 He
0
4850 4900 5400 5450 6500 6550 6600
λ / Ao
Figure 6. Test for the hydrogen abundance in the CS of Abell48. The observed profiles (blue/thick solid line) are compared with
models(red/thinlines)fordifferenthydrogenabundance,butotherwisethesameparametersasofourbest-fitmodel(seeTable3).The
testedhydrogenmassfractionsarezero(dottedlines),10percent(thinsolidlines)and20percent(dashedlines),respectively.
Table 3.Abell48:Stellarandwindparameters. Table 4.LineintensitiesofthenebulaAbell48.
T∗ 70+−45 kK λ0(˚A)Ion F(λ)/F(Hβ) I(λ)/I(Hβ)
v∞ 1000±200 kms−1 4340Hγ 0.170±0.009 0.451±0.024
logRt 0.85+−00..1153 R(cid:12) 4363[Oiii] 0.014±0.005 0.034±0.013
logM˙ −6.4±0.2 M(cid:12)yr−1 44896519H[Oβiii] 11..010807±±00..003310 11..000005±±00..003236
R∗ 0.54±0.07 R(cid:12) 5007[Oiii] 4.050±0.094 3.165±0.077
D 4 (densitycontrast)
5755[Nii] 0.018±0.002 0.004±0.001
EB−V 2.10±0.05 mag 5876Hei 0.974±0.024 0.206±0.005
vrad 50±4 kms−1 6364[Oi] 0.042±0.003 0.005±0.000
H 10±10 percentbymass 6548[Nii] 3.103±0.118 0.282±0.012
He 85±10 percentbymass
6563Hα 32.555±0.721 2.906±0.074
C 0.3±0.1 percentbymass 6584[Nii] 8.831±0.215 0.770±0.021
N 5.0±2.0 percentbymass 6678Hei 0.616±0.015 0.048±0.001
O (cid:54)0.6 percentbymass 6717[Sii] 0.761±0.018 0.057±0.002
Fe 1.4×10−3 percentbymass 6731[Sii] 0.915±0.021 0.068±0.002
7065Hei 0.434±0.012 0.022±0.001
7136[Ariii] 2.117±0.051 0.102±0.003
of the CS and suggest that this velocity corresponds to the 7237[Ariv] 0.057±0.003 0.003±0.000
velocity of the mass center. With our spectral resolution 7281Hei 0.170±0.010 0.007±0.000
(FWHM=5.5±0.5˚A) for the range of the Hα line, we are 7320[Oii] 0.174±0.014 0.007±0.001
7330[Oii] 0.156±0.014 0.006±0.001
not able to measure the expansion velocity directly.
C(Hβ)dex 3.16±0.03
EB−V mag 2.15±0.01
6.2 Physical conditions and chemical abundances
The spectrum of the nebula of Abell48 has never been
studied before, because it is relatively faint and highly red- niqueofplasmadiagnosticsinthewaydescribedindetailin
dened.AnobjectiveprismspectrumwastakenbySanduleak Kniazevetal.(2008b).TheelectrontemperaturesT (Oiii),
e
(1976), where only the Hα line was detected. DePew et al. T (Nii),werecalculateddirectlyusingtheweakaurorallines
e
(2011) studied the CS of Abell48 with the Siding Spring ofoxygen[Oiii]λ4363andnitrogen[Nii]λ5755.Bothtem-
Observatory 2.3m telescope, but they did not publish any peratures and the number density n (Sii) are given in Ta-
e
spectrum. Even with SALT we were not able to detect any ble 5. From the detected emission lines we were able to de-
useful signal in the spectral range below 4300˚A. Our final terminethetotalelementabundancesforO,N,Ar,andHe.
1D spectrum of the nebula is shown in Fig. 8. TheseresultsareshowninTable5togetherwithsolarabun-
Emissionlinesinthespectrumofthesurroundingneb- dances (Asplund et al. 2009).
ulaweremeasuredapplyingtheMIDASprogramsdescribed The derived abundances of the α-elements O and Ar
in detail in Kniazev et al. (2004, 2005). Table 4 lists the arelowerthansolarbyabout−0.3dex.Incontrast,Heand
measured relative intensities of all detected emission lines N show abundances close to the solar values.
relative to Hβ (F(λ)/F(Hβ)), the ratios corrected for the The IR fluxes at wavelengths longer than 8µm, which
extinction (I(λ)/I(Hβ)), and the derived extinction coeffi- havebeenmeasuredbyMidcourseSpaceExperiment(MSX)
cient C(Hβ). The latter corresponds to E of 2.15 mag, satellite(Priceetal.2001)andtheWide-field Infrared Sur-
B−V
whichisverysimilartothevaluefoundindependentlyfrom vey Explorer (WISE; Wright et al. 2010), show a strong ex-
the spectrum of the CS (see Table 3). cess compared to the predicted stellar continuum. This ex-
Thespectrumofthenebulawasinterpretedbythetech- cesscouldbeduetothelowresolutionoftheseinstruments,
(cid:13)c 2002RAS,MNRAS000,1–12
8 H. Todt et al.
N Table 5.ElementalabundancesinthenebulaofAbell48.
Quantity Abell48 Suna
E Te(OIII)/K 11870±1640
Te(NII)/K 7200±750
ne(SII)/cm−3 1000±130
O+/H+(×105) 19.97±5.89
O++/H+(×105) 6.56±2.74
O/H(×105) 26.53±6.49
12+log(O/H) 8.42±0.11 8.69
N+/H+(×107) 416.9±151.6
ICF(N) 1.55
N/H(×105) 6.48±2.36
12+log(N/H) 7.81±0.16 7.83
log(N/O) −0.61±0.19 −0.86
Ar++/H+(×107) 6.06±2.53
ICF(Ar) 1.21
Ar/H(×107) 7.33±3.05
400 12+log(Ar/H) 5.86±0.18 6.40
log(Ar/O) −2.56±0.21 −2.29
α)
H He/H 0.133±0.005
I(200 12+log(He/H) 11.12±0.02 10.93
a SolarabundancesfromAsplundetal.(2009)
N S
0
7.1 Abell48 – a PN or a ring nebula around a
100 massive WR star?
TheanalysisofthespectrumofthenebulaofAbell48gives
us indications that this object is most likely a PN, and not
-1ms 50 a nebula around a massive star:
k
/ Diagnostic diagrams are very often used to classify emis-
vhel sion line sources and separate them from each other on
the base of the most important line ratios (see e.g., Kni-
0
azev et al. 2008a; Frew & Parker 2010, and references
therein). The most frequently used diagnostic diagram is
-20 0 20 log(F(Hα)/F([Nii] λλ6548,6584)) versus log(F(Hα)/F([Sii]
r/arcsec λλ6717,6731)). For Abell48 these ratios are 0.44 vs. 1.37.
With these values, Abell48 is located in the middle of the
Figure 7. Hα intensity and radial velocity distribution along
region occupied by PNe and far away from the place where
theslit.Upperpanel:DSS-IIredbandimageofAbell48withthe
Hii regions and WR shells are typically found.
positionofthe1(cid:48).(cid:48)25slitshownbydashedlines.Middlepanel:The
The density of the Abell48 nebula and the diagnostic
relativefluxoftheHα-linealongtheslitaftercontinuumsubtrac-
diagram log([Sii] λ6717/λ6731)) versus log(F(Hα)/F([Nii]
tion.N-Sdirectionoftheslitisshown.Theverticallineatr=0
correspondstothepositionoftheCS.Theregionr±2(cid:48)(cid:48)isshown λλ6548,6584)) also puts this source into the region of PNe
withvertical,dashedlinestomarkthearea,wheretheaverageve- (cf.Figure9),butnotofWRshells,whichallhavedensities
locityv =50.4±4.2kms−1wascalculated.Bottompanel:The similar to those of Hii regions (Esteban et al. 1992, 1994;
hel
velocity profile of the Hα line, corrected by using the night-sky Garnett & Chu 1994; Greiner et al. 1999; Stock et al. 2011;
line[Oi]λ6363˚A.Thehorizontallineindicatesthecalculatedve- Ferna´ndez-Mart´ın et al. 2012).
locity50.4kms−1,thathastobeclosetotheheliocentricvelocity
oftheCS.
7.2 Electron-scattering line wings
whichresultsinconfusionbetweenthestellarsourceandthe Strong emission lines in WR-type spectra show extended
nebular emission. line wings which can be attributed to line photons that are
redistributedbyscatteringonfreeelectronsinthewind.The
strength of these wings is sensitive to the degree of wind
inhomogeneities, the so-called clumping (see Section 4.2).
7 DISCUSSION
The spectra of massive WN stars are typically consistent
Asdiscussedintheintroduction,thereweredoubtswhether with a clumping density contrast of D = 4 ... 10 (Hillier
Abell48isaPNwithalow-masscentralstaroraringnebula 1991; Hamann & Koesterke 1998; Hamann et al. 2006).
aroundamassivestar.Nowwediscussseveralargumentsin In the spectrum of the CS of Abell48 these electron-
favour of the PN status of this object. scatteringlinewingsareveryweakandbarelyvisible,while
(cid:13)c 2002RAS,MNRAS000,1–12
Abell48 - a rare WN-type CSPN 9
20 III]
ounts Hγ Hβ [OIII] [OIII] [NII] HeI [OI] Hα [NII]HeI[SII] HeI[Ar [ArIV]HeI[OII]
c
ve 10
ati
el
R
0
30x
4500 5000 5500 6000 6500 7000 7500
λ / Ao
Figure 8. One-dimensionalreducedspectrumofthenebula.Mostofthedetectedstrongemissionlinesofthenebulaaremarked.All
detectedandmeasuredlinesforthenebulaarelistedinTable4.Thelowerspectrumisscaledby1/30toshowtherelativeintensitiesof
stronglines.
in contrast the massive WN models display significantly massive WN star of logL/L (cid:38) 5.3 (Hamann et al. 2006),
(cid:12)
stronger wings. butwithintherangeofluminositiesderivedforCSPNe(e.g.
However,themodelswhichwehavecalculatedforcen- MillerBertolami&Althaus2007).Weadoptthetypicallu-
tral star parameters, i.e. with logL/L = 3.7 and M = minosity of CSPNe of L = 6000L (see Section 5), which
(cid:12) (cid:12)
0.6M (cf. Section 5), show only weak electron-scattering leadstoadistancetoAbell48ofd=1.9kpc.Notethatthe
(cid:12)
linewingswiththesamevaluesfortheclumpingcontrastas luminosity of Abell48 and its R and M˙ (given in Table 3)
∗
for massive WN stars, consistent with the observation. We canbescaledtodifferentdistancesas∝d2,∝d,and∝d3/2,
consider this as an evidence for the low-mass nature of the respectively (Schmutz et al. 1989).
CS of Abell48. Alternatively,iftheCSofAbell48wereamassiveWN
star, then it should be located well beyond the Sagittarius
Arm. Indeed, if using the mean K -band absolute magni-
s
7.3 Location and luminosity of the CS of Abell48 tude of massive WN5-6 stars from Crowther et al. (2006),
M =−4.41magandtheK -bandextinctionderivedfrom
The best fit to the SED of the CS of Abell48 was achieved Ks s
theSEDfittingandlineratiosinthespectrumofthenebula,
with a color excess E =2.10±0.05 mag (which agrees
B−V one gets a distance modulus of 15.94mag, which translates
wellwithE =2.15±0.01magderivedfromthelinera-
B−V intoadistanceof15.4kpc.Anevenlargerdistancewouldbe
tiosinthespectrumofthenebula;seeSection6.2),implying
derived,ifoneestimatesA fromtheintrinsicJ−K color
anextinctioninthevisualofA ≈7mag.Thelattertrans- Ks s
V ofWN5-6starsof0.18maggiveninCrowtheretal.(2006).
lates into A ≈ 0.8mag, if one adopts the extinction law
Ks In this case, one gets A = 0.66mag and d = 16.4kpc.
fromRieke&Lebofsky(1985).Thesevaluesshouldbecom- Ks
These two distance estimates imply that Abell48 would be
pared with the full Galactic V and K -band extinctions in
s located either in the Perseus or the Outer Arm. This loca-
the direction of Abell48, which according to Schlegel et al.
tion,however,isincontradictionwiththemoderateextinc-
(1998)5 are ≈ 34 and 4mag, respectively. Thus, one can
tiontowardsAbell48inviewofthepresenceofanextinction
expect that Abell48 is located at a factor of ∼ 5 smaller
wall at ≈6kpc (e.g. Negueruela et al. 2011).
distance, d, than the line-of-sight depth of the Galaxy of
≈ 20kpc (e.g. Churchwell et al. 2009), i.e. at d ≈ 4kpc.
Similar distance estimate (d ≈ 3.9kpc) could be derived if
7.4 Abell48 as a runaway
one assumes an average of 1.8mag of visual extinction per
kpc in the Galactic plane. The short distance to Abell48 is also supported by the de-
These distance estimates are supported by a study of tection of a significant proper motion for its CS: µαcosδ =
the distribution of interstellar extinction towards l ≈ 29◦, −13.3±4.8masyr−1 andµδ =−14.8±4.8masyr−1 (Ro¨ser
which shows that A grows from ≈0.3 to 1.0mag between et al. 2010). To convert this proper motion and the radial
Ks
2.5 and 4kpc, then reaches a values of ≈1.5 mag at 6kpc, velocityofAbell48(seeTable3)intothepeculiartransverse
and then abruptly increases to ≈2.5mag between 6 and and radial velocities, we used the Galactic constants R0 =
7kpc(Negueruelaetal.2011).SinceAKs towardsAbell48is 8.0kpc and Θ0 =240kms−1 (Reid et al. 2009) and the so-
≈0.8mag, one has that this object should be located in the lar peculiar motion (U(cid:12),V(cid:12),W(cid:12)) = (11.1,12.2,7.3)kms−1
Scutum-Centaurusarmatd<4kpc(seefig.12inNegueru- (Scho¨nrich et al. 2010). The derived velocity components
ela et al. 2011) and that its luminosity of logL/L (cid:54) 4.3 and the total space velocity are given in Table 6 and im-
(cid:12)
is much smaller than the lowest plausible luminosity for a ply that Abell48 is a runaway system (e.g. Blaauw 1961).
For the error calculation, only the errors of the proper mo-
tionandradialvelocitymeasurementswereconsidered.The
5 Seealsohttp://irsa.ipac.caltech.edu/applications/DUST/ peculiar radial velocity of Abell48 contributes only slightly
(cid:13)c 2002RAS,MNRAS000,1–12
10 H. Todt et al.
1.4 WR nebulae
G2.4+1.4 (WR102)
MR26 (WR 23)
S308 (WR 6)
31])1.2 NRCGWC3110949 ( W(WRR 7 158))
67 L69.8+1.74 (WR131)
II HII regions
S1.0
([I
/ Abell48
7])
II 6710.8 PNe IC4663
S
([I0.6 PB8

0.4
0 1 2
log(I(Hα)/I([SII]))
Figure 9. Diagnostic diagram Hα/([Sii] λλ6717,6731) inten-
sityratioversus[Sii]λλ6717/6731ratioasameasureofelectron
density versus excitation for gaseous nebulae. Shown are the lo- Figure 10. 2◦ ×2◦ MSX 8.3µm image of the Galactic plane
cations for Hii regions as well as the positions of WR nebulae (shown by a dashed line) centered at l =29◦,b=0◦.5, with the
from (Esteban et al. 1992). Most PNe lie on the indicated strip position of Abell48 indicated by a circle. The inset shows the
from(Riesgo-Tirado&L´opez2002).Alsolabeledarethepositions IRAC 8µm image of Abell48. The arrows on both images show
of the [WN] and [WN/WC] stars Abell48, IC4663, and PB8 to thedirectionofmotionofAbell48.Theorientationoftheimages
demonstratetheirCSPNestatus. isthesame.
Table 6.Abell48:Peculiartransverse(inGalacticcoordinates), Table 7. Measured line ratios from the spectrum of the CS of
radial and total space velocities, and height over the Galactic Abell48forspectralsubtypeclassification.
planefordifferentadopteddistances.
lineratio peak/continuum
d vl vb vr vpec dsinb Nv4604/Niii4640 2.0
[kpc] [kms−1] [kms−1] [kms−1] [kms−1] [pc] Civ5808/Heii5411 0.7
1.9 −149±43 53±43 37±4 162±42 15 Civ5808/Hei5878 1.7
4.0 −296±91 104±91 -8±4 314±91 32
15.4 −970±350 378±350 74±4 1043±349 122
oxygen abundances are roughly solar. Therefore, we sug-
gesttoassignAbell48tothe[WN]class,becomingthusthe
to its total space velocity, v (Table 6). These estimates second known member of this spectral class after IC4663
pec
show that the larger d the larger v and correspondingly (Miszalski et al. 2012).
pec
thelowerthelikelihoodthattheCSofAbell48canbeaccel- TodeterminethedetailedsubtypeofAbell48,weapply
eratedtosuchhighvelocity.Consequently,onecanconclude the 3D classification scheme by Smith et al. (1996). The
that the short distance to Abell48 is more likely. measured line ratios (see Table 2 and 7) suggest that the
The runaway status of Abell48 is consistent with the star belongs to the [WN5] subtype.
orientationofitspeculiarvelocitywithrespecttotheGalac- In this connection, we note that none of the massive
tic plane. Fig.10 shows the MSX 8.3µm image of a 2◦×2◦ WNEstarspossessacompactcircumstellarnebula(Gvara-
region of the Galactic plane (centered at l = 29◦,b = 0◦.5). madze et al. 2009, 2010a, and references therein), which
One can see that Abell48 is moving in the “correct” direc- provides one more evidence in support of the PN status of
tion, i.e. away from the Galactic plane. On the other hand, Abell48.
the large margin of error in v leaves the possibility that
b
Abell48 is moving almost parallel to the Galactic plane,
7.6 Evolutionary status
which in turn is consistent with the small height above the
Galactic plane (cf. Table 6). Spectroscopically,Abell48withitsstrongheliumandnitro-
gen emission lines belongs clearly to the [WN] class. How-
ever, Abell48 seems to be different in some respect from
7.5 Classification
the other [WN] star IC4663. While Miszalski et al. (2012)
Based on the arguments above we conclude that Abell48 is could not find any residual hydrogen in IC4663, the pres-
a PN with a low-mass central star. enceofsomehydrogenisindicatedbyouropticalspectrum
The abundance pattern, that our spectral analysis re- of the CS of Abell48. Such residual hydrogen would rule
vealed,correspondstoaWN-typecomposition,whereishe- outamergerscenariowithasequenceHe-WD+He-WD→
lium dominates, nitrogen is enhanced and the carbon and [WN]→O(He)asproposedbyReindletal.(inprep.)forthe
(cid:13)c 2002RAS,MNRAS000,1–12