New Galactic supernova remnants discovered with IPHAS

2.2 MB·English

by  L. Sabin

Mon.Not.R.Astron.Soc.000,1–14(2012) Printed29January2013 (MNLATEXstylefilev2.2)
New Galactic supernova remnants discovered with IPHAS
L. Sabin1,2⋆, Q.A. Parker3,4,5, M.E. Contreras1, L. Olgu´ın6, D.J. Frew3,4, M. Stupar3,4,5,
3 R. Va´zquez1, N.J. Wright7, R.L.M Corradi8,9, R.A.H Morris10
1 1InstitutodeAstronom´ıa,UniversidadNacionalAuto´nomadeMe´xico,Apdo.Postal877,22800Ensenada,B.C,Mexico.
0 2InstitutodeAstonom´ıayMeteorolog´ıa,DepartamentodeF´ısica,CUCEI,UniversidaddeGuadalajara,Av.Vallarta2602,C.P.44130,Guadalajara,Jal.,Mexico
2 3MacquarieUniversityResearchCentreinAstronomy,Astrophysics&Astrophotonics,Sydney,NSW2109,Australia
n 4DepartmentofPhysicsandAstronomy,MacquarieUniversity,Sydney,NSW2109,Australia
a 5AustralianAstronomicalObservatory,POBox296,Epping,NSW1710,Australia
J 6Depto.deInvestigacio´nenF´ısica,UniversidaddeSonora,Blvd.RosalesEsq.L.D.Colosio,Edif.3H,83190Hermosillo,Sonora,Mexico
7 7Harvard-SmithsonianCenterforAstrophysics,60GardenStreet,Cambridge,MA02138,USA
2 8InstitutodeAstrof´ısicadeCanarias,E-38200Tenerife,Spain
9DepartamentodeAstrof´ısica,UniversidaddeLaLaguna,E-38200Tenerife,Spain
] 10AstrophysicsGroup,DepartmentofPhysics,BristolUniversity,TyndallAvenue,Bristol,BS81TL,UK
R
S
.
h
Received2012June11.Accepted2013January25.
p
-
o
r ABSTRACT
t
s Aspartofa systematic searchprogrammeofa 10-degreewide strip ofthe NorthernGalac-
a tic plane we present preliminary evidence for the discovery of four (and possibly five)
[
new supernova remnants (SNRs). The pilot search area covered the 19–20 hour right as-

1 cension zone sampling from +20 to +55 degrees in declination using binned mosaic im-
v ages from the INT Photometric Hα Survey (IPHAS). The optical identification of the can-
6 didate SNRs was based mainly on their filamentary and arc-like emission morphologies,
1 their apparently coherent, even if fractured structure and clear disconnection from any dif-
4 fuse neighbouring HII region type nebulosity. Follow-up optical spectroscopy was under-
6
taken, sampling carefully across prominent features of these faint sources. The resulting
.
1 spectra revealed typical emission line ratios for shock excited nebulae which are charac-
0 teristic of SNRs, which, along with the latest diagnostic diagrams, strongly support the
3 likelySNR natureof thesesources:G038.7-1.3(IPHASX J190640.5+042819);G067.6+0.9
1 (IPHASX J195744.9+305306); G066.0-0.0 (IPHASX J195749.2+290259) and G065.8-0.5
:
v (IPHASX J195920.4+283740). A fifth possible younger, higher density nebula SNR can-
i didate, G067.8+0.5 (IPHASX J200002.4+305035)was discovered ∼5 arcmins to the west
X
ofIPHASXJ195744.9+305306,andwarrantsfurtherstudy.Amulti-wavelengthcross-check
r fromavailablearchiveddataintheregionsofthesecandidateswasalsoperformedwithafo-
a
cusonpossibleradiocounterparts.Aclosepositionalmatchbetweenpreviouslyunrecognised
radiostructuresatseveralfrequenciesandacrossvariouscomponentsoftheHαopticalimage
datawasfoundforallSNR candidates.ThislendsfurtherdirectsupportfortheSNR nature
of these objects. Evolved SNRs may have very weak and/orhighly fragmentedradio emis-
sion which could explain why they had not been previously recognised but the association
becomesclearincombinationwiththeopticalemission.
Keywords: HαSurvey–ISM:Supernovaremnants.
1 INTRODUCTION enrich the interstellar medium (ISM) in heavy elements (such as
oxygen,ironandnickel)andothernucleosynthesisproducts.These
Supernova remnants(SNRs) areamong themajor contributors to
ejecta play an important role in the observed chemical composi-
thechemicalregenerationoftheGalaxy.Indeed,followingtheex-
tionofthelocalISMandthestellarsystemsthatmaysubsequently
plosion of their high mass progenitor stars in the case of core-
be formed from it. SNRs are also responsible for the release of
collapse supernovae (∼80%of allsupernovae) ortheType-Iabi-
largeamountsofenergy,generatingshockwavesthathighlightor
nary route (∼20-25% of all supernovae, Bazinetal. 2009), they
“shape”thesurroundinginhomogeneousISMwiththeconcurrent
production of hot, dense and bright ionised zones. The detection
andstudyofthoserapidlyevolvingremnants(afewthousandyears
⋆ E-mail:[email protected](LS)
2 L. Sabinet al.
old)arecrucialfortheunderstandingnotonlyofstellarevolution ofextendedopticalemissionwithanHαsurfacebrightnessdown
ofhighmassstarsinthecaseofcore-collapsesupernovabutalso, to≃2×10−17ergcm−2s−1arcsec−2(≃3Rayleighs).Thismakes
moreglobally,fortheunderstandingofGalacticabundance gradi- the survey sensitive to evolved, low surface brightness nebulae
ents,kinematicsandchemistry. suchasplanetarynebulae(PNe),includingthoseinteractingwith
The most recent and complete compilation of SNRs in the the ISM (Sabinetal. 2010; Wareingetal. 2006), symbiotic stars
GalaxyhasbeenassembledbyGreen(2009)whichcurrentlycom- (Corradietal. 2010), Wolf-Rayet (WR) shells (Stock&Barlow
prises274recognisedandverifiedobjects.However,accordingto 2010), Herbig-Haro objects, proplyd-like objects (Wrightetal.
Tammannetal.(1994)weshouldexpectaround1000SNRstobe 2012),andotherkindsofresolvednebulaeaswellasSNRs.
visible at any one time inour Galaxy. Thisdiscrepancy could be Thisisnot thefirst timeadeep, narrow-band optical survey
accountedforbythelack(anddifficulty)ofdetectionatbothends has been used to search for new SNRs as previously mentioned.
ofSNRevolutionwheneithertheSNRistoocompact/distanttobe Stuparetal.(2007,2008,2011)presentedthediscoveryofasignif-
seenorwhenitisonthevergeof completelydissipatinginto the icantnumberofnewopticallyidentifiedandconfirmedSNRsbased
ISM.ThemajorityoftheSNRshavebeenhistoricallydetectedvia on data from Southern Galactic plane Hα Survey (Parkeretal.
radio observations and the subsequent determination of the non- 2005). The authors also published a catalogue of newly uncov-
thermal natureof theemission wherethetypicallyobserved neg- eredopticalcounterparts for24known GalacticSNRspreviously
ative spectral index implies synchrotron emission. More recently identifiedonlyfromradioorX-rayobservations(Stupar&Parker
optical detectionsof new GalacticSNRshavebeenpossible (e.g. 2011). Boumisetal. (2005, 2008, 2009) also presented the first
Stuparetal.2008)evenintheabsenceofpreviousradioevidence optical detections and spectroscopic confirmation for a number
due tothe advent of high resolution, high sensitivity Hαsurveys of SNRs based on CCD data. While also in the northern sky
such as those described by Parker et al. (2005) and Drew et al. Fesen&Milisavljevic(2010)discoveredopticalfilamentsforSNR
(2005). G159.6+7.3whichhadnopreviousradiodetection.
Green (2004) highlighted the most important radio surveys Inthispaperwereportthepreliminaryresultsofasystematic
traditionallyusedforuncoveringSNRs.TheseincludetheEffels- searchfornewSNRsfromtheIPHASsurvey.Radioobservations
berg2.7GHzsurvey(Furstetal.1990)witharesolutionof ≃4.3 (acrossalargefrequencyrange)arewidelyusedtoidentifyandim-
arcmin, the Sydney University Molonglo Sky Survey (SUMSS) ageSNRssoinadditiontothecharacteroftheionisedopticalemis-
at 843 MHz (Bocketal. 1999) with a resolution of 43×43 arc- sionpresented,thedetectionofradioemissionfromtheexpanding
seconds cosec δ,theParkes-MIT-NRAO (PMN)radio continuum SNR shell is another important tracer, especially if it is possible
surveyat4.85GHz(Griffith&Wright1993)witharesolutionof to measure a negative spectral index to indicate synchrotron ra-
∼ 5arcminfortheSouthernsurveyand∼3arcminfortheNorth- diation. Weexamined archival data from the Westerbrock North-
ern counterpart undertaken at the same frequency on the Green- ernSkySurvey(WENSS)at325MHz(Rengelinketal.1997),the
banktelescope(GB87;Condonetal.1994)andfinallytheNRAO NRAOVLASkySurvey(NVSS)at1.4GHz(Condonetal.1998)
VLA Sky Survey (NVSS) at 1.4 GHz (Condonetal. 1998) with andtheGreenBank(87GB)4.85GHzsurvey(Gregoryetal.1996)
a resolution of 45 arcseconds. These surveys are able to distin- tolookforradiocounterpartstoourIPHAStargets.Furthermore,
guishSNRsoveravarietyofangularscalesdepending onthere- thehighenergy releasedbytheinitialsupernova explosion (typi-
spectivebeamsizes.ForareviewofSNRsinthePMNsurveysee callyabout1051 ergsfollowingBurke&Graham-Smith2002)re-
Stupar,Filipovic´,Parker&etal.(2007). sults in the generation of X-ray emission which can be detected
Small,andthereforeyoung(ordistant),SNRsmaynotbeop- by X-ray space telescopes such as the previous ROSAT and cur-
ticallyrecognisedduetothepresenceofhighlyvariableextinction rentChandraandXMMNewtonspaceobservatories.Archivaldata
atlowGalacticlatitudeswheremostSNRsareexpectedtoreside. fromthesesurveyshavealsobeeninterrogatedtolookforpossible
OtherkindsofSNRsthatarelikelytobemissedarethosehighly X-raycounterparts,bothdiffuseandcompact.Asearchforinfrared
evolved SNRswithlow surface brightness and largeangular size counterparts was also undertaken using most of the available IR
which are usually also highly fragmented. Indeed, radio surveys surveys (e.g. WISE, GLIMPSE) but this did not return any con-
are generally not very sensitive to extremely evolved SNRs (e.g. vincingcorrespondence.
Stupar&Parker 2011, Stuparetal. 2008) where the detected ra- Thepaperisorganisedasfollows.In§2wepresentthenew
diofragmentsareoftenunrecognised. Furthermore,intheoptical IPHASopticaldetectionsoftheSNRcandidates.In§3wepresent
regime,thetypicallylowemissionlevelsfromGalacticSNRsmake thespectroscopic follow-upofthecandidates. In§4somemulti-
themdifficulttoidentifywhileconfusion withtheubiquitous HII wavelength,principallyradiocounterpartstotheopticalimagesare
regionsisalsoaproblem.Theseselectioneffectshaveresultedina discussed,whilein§5wepresentadiscussionandourconclusions.
seriousbiasintheglobalstudyofSNRsandtheirproperties.
Aspartofasystematicefforttohelpaddressthiswearetak-
ing advantage of the recent, deep narrowband INT Photometric
2 IPHASDETECTIONSOFNEWSNRCANDIDATES
Hα Survey of the Northern Galactic plane (IPHAS; Drewetal.
2005, Gonzalez-Solaresetal. 2008) to look for “previously hid- Thisinitialsearchfornew,opticallydetectableSNRsconcentrated
den” but optically detectable SNRsin a similarway to those un- ontherightascensionrangefrom19h–20h.Thisisthefirstmajor
coveredbyStuparetal.(2008)intheSouth.IPHAS,whichstarted IPHASzonewhichhasbeensystematicallyandcarefullyscanned
in 2003, is now essentially complete. It used a wide-field CCD fornewemissionnebulaeofallkinds,includingPNeandHIIre-
camerawithr,iandHαfilterstoprovide anaccuratephotomet- gions.ThissearchformedamajorcomponentofthethesisofSabin
ricsurvey. IPHAScovers1800 deg2 of thenorthern plane across 2008.Aseparatepaperdedicatedtothefirst100confirmedPNere-
a±5degreelatitudestripandwasinitiatedasthedirectcounter- sultingfromthissearchisalsointrain(Sabinetal.2013,inprep.).
parttotheAAO/UKSTHαsurveyoftheSouthernGalacticplane Note that the full IPHAS sky area (1800 deg2) has been divided
(Parkeretal. 2005). The survey has a nominal resolution of 0.33 into2deg2 Hα-r(continuum removed) image mosaicsthat facil-
arcsec.pix−1andthenarrow-bandcomponentallowsthedetection itatedetection andrecognition of extended emissionnebulae. We
IPHAS Galacticsupernovaremnants 3
usedmosaicscalefactorsof15×15pixels(5arcsec) and5×5
Table1.Estimatedcentralpositionsandangularsemiminorandmajoraxes
pixels(1.7arcsec).Thefirstbinninglevelwaschosentoenablethe forthenewSNRcandidatesfoundviavisualinspectionoftheIPHASmo-
detection of low surface brightness objects (down to the IPHAS saicsinthe19–20hourRAzone.TheRA/DECpositionsareJ2000.
limit) (Sabinetal. 2010) over large angular scales while the sec-
ondwasusedtodetectintermediatesizenebulae,i.esmallerthan
≃15–20arcsecindiameter. IPHASXID GalacticID RA(J2000) DEC(J2000) Angularsize
TheSNRcandidateswereselectedbasedontheiropticalmor- hms 0’” (arcmin)
phologicalcharacteristics.Theywereconsideredasplausiblecan- J190640.5+042819 G038.7-1.3 19:06:40.5 +04:28:18.8 18.6×31.8
didatesiftheycouldbeidentifiedasdetachedbutcoherentfilamen- J195744.9+305306 G067.6+0.9 19:57:44.9 +30:53:05.6 45.2×49.2
tarystructuresorasdiscreteroundshellsorarcsintheIPHASHα-r J195749.2+290259 G066.0-0.0 19:57:49.2 +29:02:58.8 24.6×31.0
mosaicimages.TheHαBalmeremissionlineisanexcellenttracer J195920.4+283740 G065.8-0.5 19:59:20.4 +28:37:40.0 6.2×10.4
ofionisedgaswhilethecontinuumsubtractedimagesenhancethe J200002.4+305035 G067.8+0.5 20:00:02.4 +30:50:35.0 7.2×5.4
contrastevenfurther.Alreadycataloguedsources,includingknown
SNRs are also scrutinised as additional, previously unrecognised
emission components to these sources may become evident. For
Table 2. Summary details of the spectroscopic observations of the SNR
examplemostknownSNRswereuncovered onlyfromradiodata
candidates.Theformalwavelengthrangesofthespectraareincluded
butwecannow alsoreveal optical counterparts tomanyof these
remnantsforthefirsttime(Stupar&Parker2011).
IPHASXID Telescope Date λcoverage Exposure
Spectroscopic validation of newly identified optical compo- A˚ (seconds)
nents of perhaps widely dispersed SNR candidates is important
fortheirproperidentificationbutthiscanbedifficulttorealizein J190640.5+042819 SPM2.1m 07/06/2010 ≃4330–7530 1800
crowded zones such as the Galactic centre where several SNRs J195744.9+305306 SPM2.1m 21/08/2011 ≃4330–7530 1800
J195749.2+290259 SPM2.1m 22/08/2011 ≃4330–7530 1800
(and HII regions) can overlap (Green 2009, their figure 3). Our
J195920.4+283740 INT2.5m 18/04/2008 ≃3500–8960 3×1200
careful search over the preliminary RA zone resulted in a set of
J200002.4+305035 SPM2.1m 22/08/2011 ≃4330–7530 1800
fourorpossiblyevenfivecandidateSNRsbeinguncovered.These
are described individually below with summary details presented
inTable1andnamedaccordingtothenomenclatureadoptedforall
ture of the”duplicated” structure which may represent some evi-
IPHASextendedobjects(IPHASXJHHMMSS.s±DDMMSS).
dence of symmetrical mass ejection. Further study is needed but
OurfirstcandidateSNRIPHASXJ190640.5+042819wasse-
this is beyond the scope of this paper. The structure of IPHASX
lected due to its well defined semi-circular emission of radius
J195749.2+290259isagaintypicalofoptical(andradio)SNRsand
∼9.3′×15.9′togetherwithsomeelongatedtailsoffilamentaryfea-
wenotethesimilaritybetweenthisobjectandthewellknownSNR
turesparticularlytotheSouth.Thesemorphologicalstructuresclas-
IC443whichalsoexhibitsfaintfilament(G189.1+3.0; seeFesen
sifieditasagoodSNRcandidate(Fig.1–Top).
1984).
ThesecondcandidateisIPHASXJ195744.9+305306(Fig.2–
The last selected SNR candidate is IPHASX
Top)whichhastheappearanceofafracturedovalringwithsome
J195920.4+283740 which reveals an elongated and closed shell
internalemissionspursemanating fromtheEasternandNorthern
structure at faint isophotes over a region of radius 186×312 arc-
edges.Ithasaprojectedangularradiiof∼22.6′×24.6′.Thereare
seconds (Fig.5–Top). Thenebulaisgloballyvery faint except for
several HIIregionsinthegeneral vicinityof thiscandidateSNR,
theenhancededgeseenintheEasternside.
including Sh2-98and Sh2-97 whicharefound within ∼0.5deg
butthesearewellseparatedfromtheSNRcandidate.However,the
most northerly faint rim of the SNR appears somewhat detached
from the main shell and comes within 4 arcminutes of the outer 3 SPECTROSCOPICOBSERVATIONSAND
rimofthecircularHIIregiondirectlytotheNorthsosomeover- CONFIRMATORYRESULTS
lappingofemissionherecannotbecompletelyruledout.Thereis
As part of our general programme of IPHAS follow-up spec-
alsoaknownSNRsG067.7+01.8,located∼1degreewest.Clearly,
troscopy, we obtained low resolution confirmatory spectra of
thisgeneralareahostssomelargenebulae.Nevertheless,themain
these candidate SNRs. Our first SNR spectrum was of IPHASX
structureremainsclearlydetachedfromanyofthesurroundingHII
J195920.4+283740 takenonthe2.5mIsaacNewtonTelescopeon
regionsandwecanconsiderIPHASXJ195744.9+305306asasin-
LaPalma(CanaryIslands)inApril2008whiletheremainingcan-
gleobject.
didateswereobservedusingthe2.1-mtelescopeattheSanPedro
Acoherent,completelyseparate,ellipticalemissionstructure MartirObservatory(OAN-SPM1)betweenJune2010andAugust
was also found ∼5′ to the east of IPHASX J195744.9+305306
2011.Asummaryofthesespectroscopicobservationsaregivenin
(Fig.3–Top)whichwelabelIPHASXJ200002.4+305035 andwas
Table2whilethespectraarepresented inFig.6and theresulting
includedinourinvestigationasitcouldbeayounger,morecom-
emissionlinemeasurementsareshowninTable3.
pactSNRorperhapsevenanevolvedPN.
Onthe2.5mINTweusedthelowresolutionspectrographIDS
WealsoidentifiedIPHASXJ195749.2+290259(Fig.4)which
withtheEEV10detectorandR300Vgrating.Thiscoverstherange
has a filamentary partial shell morphology with a bright emis- ≃3500A˚-8960A˚ witharesolutionof≃1.8A˚ thoughthedatabe-
sion arc to the North. A fainter apparent morphological duplica- low3600A˚ isofnovalueduetotheatmosphericcut-off.Theslit
tionofthisstructure ∼5′ totheNorth-Westisalsoevident which
must be related given their strong similarity in appearance. Such
similar morphological repetition is also seen in parts of the well 1 The Observatorio Astrono´mico Nacional at the Sierra de San Pedro
known Vela SNR.We cannot probe deeper into a detailed expla- Ma´rtir(OAN-SPM)isanational facility operated bytheInstitutodeAs-
nation at this stage as we lack the data to fully establish the na- tronom´ıaoftheUniversidadNacionalAuto´nomadeMe´xico.
4 L. Sabinet al.
lengthwas3.3arcminutesandtheseeingduringtheobservations Animmediateconsequenceofsuchfaintnebulosityistheinability
variedbetween∼1.8and∼2.2arcseconds.Theexposuretimefor toeasilydetectothercommondiagnosticemissionspeciessuchas
the observation of IPHASX J195920.4+283740, which is a rela- theFelines(Fesen&Hurford1996).
tivelyfaintnebulawassetto3×1200sonaportionofitsbrighter Anotherissuerelatedtothefaintnessofourtargetsisthedis-
Easternedge.WenotethedatabluewardofHβinthespectrumdid crimination between true SNRs and other morphologically simi-
notshowanyemissionlinesduetothelowS/N. lar nebulae such as Wolf-Rayet (WR) shells, symmetric HII re-
Fortheremaining2.1-mSanPedroMartirtelescopeobservations gions or even low excitation zones in PNe, as these nebulae can
we used a Boller & Chivens spectrograph and Thomson & Mar- exhibitsimilaremission-lineratiosinsomecases.Anoverallbody
coni CCDs(2048×2048 pixels of size 14µm). An East-West slit of evidence is used to aid in the identification of likely SNRs
orientationwasadopted withthe5arcminutelengthslitcarefully andtoremovecontaminants.Thiscomprisestheopticalmorphol-
positionedonthemostprominentnebulararcsorfilamentstogive ogy, spectroscopic signature, local environment, presence of X-
betterS/Nbutalsochosen toallow sufficientskyforbackground ray sources internal to the nebulae and any corroborating radio
subtraction.Incaseswhere,despiteourbestefforts,itwasnotpos- emission. While other shock-excited nebulae such as supershells
sibletoextractsufficientdecentskywealsotookbackgroundspec- (Hunter 1994; Skeltonetal. 1999) and giant outflows associated
tra in relatively”blank” areas located in the immediate surround- withHerbig-Haroobjects(Reipurthetal.1997;Maderetal.1999)
ings of the nebula. This was particularly the case for IPHASX canhavesimilaremissionline-ratiostoSNRs,thesmall(lessthan
J195744.9+305306 (B&Cpositions).Theobservationswereob- thedegree)angularsizesofourcandidates,theirfilamentarymor-
tainedundervarying seeingconditionstypicallyaround2arcsec- phologies and the lack of star-forming activity in their vicinities
onds and were made with the 400 line/mm grating. This gave a stronglymilitatesagainstthesealternativeinterpretations.
spectralcoverageof≃4330A˚–7530A˚ witharesolutionof≃5A˚
sufficienttoencompassmostopticalemissionlinesofinterest.The
3.1 Newemissionlinediagnosticdiagrams
databluewardofHβ inthesespectradidnotexhibitanyemission
linesduetothelowS/Nandextinction.Thedatareductionofall The ability to separate out different classes of astrophysical ob-
spectrawasdonewiththeusualIRAFroutines.Theextinctioncor- jectaccordingtotheirobservedemission-linesratiosiswellestab-
rectionappliedtoourspectrawasperformedusingtheFitzpatrick lished,e.g.thewell-knownSabbadin,Minello&Bianchini(1977)
andMassaextinctioncurve(Fitzpatrick&Massa2007)forRV = ‘SMB’ Hα/[N II] versus Hα/[S II] diagnostic diagram or the
3.1. Baldwin,Phillips&Terlevich(1981)‘BPT’[OIII]λ5007/Hβ ver-
Inordertoderivetheerrorsonthemeasurementsweconsider sus[SII]/Hαdiagnosticdiagram.Wehavebeenrefiningandupdat-
mainlytwosourcesoferror.ThefirstisrelatedtotheCCDreadout ingthesediagnosticdiagramsusingcarefullyvettedspectroscopic
noiseand photon noise relativetotheskyand objects. Thiserror datafromtheliterature,supplementedwithourownflux-calibrated
isderivedviatheestimationofthevariance(σ2)ateachpointof spectroscopy forasignificant sampleof objectsof various kinds.
the spectrum (i.e. at each wavelength). The ”variance spectrum” Thenew figurespresented here forthefirsttimearebetterpopu-
obtained is processed the same way as for regular data spectra latedversionsofthediagramspresentedbyFrew&Parker(2010).
in IRAF assuming a non-correlated error approximation (Taylor ThesenewplotsincludemanymoreGalacticSNRsandothertypes
1997).Thereforethestatisticalerrorscorrespondingtoeachspec- ofnebulaethanpreviousefforts.Byonlyutilisingthemostreliable
trallineareextractedandregistered.Thesecondsourceoferrorwe data and identifications, a clearer picture of the loci of different
consideredhereislinkedtothefluxcalibrationandthedata were object typesisestablishedcompared towhat hasbeen previously
processedfollowingViironenetal.(2011). available.
The usual Hα, [NII]λλ6548,6583 and [SII]λλ6716,6731 In this paper we therefore base our SNR spectroscopic con-
emissionlineswereusedtoidentifythenatureofthenebulaebased firmations on their observed strong emission line-ratios used in
onthecriteriadefinedbyFesenetal.(1985)[hereafterF85]andre- conjunctionwiththesepowerful,updateddiagnosticdiagramscre-
finedmostrecentlybyFrew&Parker(2010).Themostconspicu- ated by one of us (DJF), based on an extensive database of
ousopticalfeaturesforthediscriminationofSNRsaretherelative spectroscopic measurements. The line-ratio data for the Galactic
strengthof[SII]linescomparedtoHα.Indeedalarge([SII]6716+ SNRswastaken from ourown unpublished spectroscopy as well
[SII]6731)/Hαratio>0.4–0.5(seeF85)isakeyprobefortheoc- as numerous literature sources, with notable contributions from
currenceoftheshockcharacteristicsoftheturbulentandenergetic Fesen&Kirshner (1980), Fesenetal. (1982, 1985), Blairetal.
environmentofSNRswhichisnotgenerallyfoundinHIIregions (1991),andBoumisetal.(2005,andreferencestherein).TheLMC
orPNe. SNR flux data is mostly from Payneetal. (2008, and references
The [SII]ratioisused as adiagnostic to determine electron therein) and the line fluxes for Galactic PNe are taken from the
densitiesinionizedplasmasandformostSNRsthisratioislikely sourcessummarisedinFrewetal.(2012).TheHIIregiondataare
tobeinthelowdensitylimitwitharatioof∼1.4.Inaddition,the mostly taken fromthe references given inFrew&Parker(2010),
presence ofrelativelystrong[NII]lines(with0.5<([NII]6548+ supplemented with unpublished data utilized by Boissayetal.
[NII]6583)/Hα<1.5 following F85) as well as the presence of (2012). The labelled domains include approximately 95% of the
moderate[OI]emission(λλ6300,6363)arealsogoodindicatorsof objectsofthatclasslistedinourdatabase.Afullerdescriptionof
alikelySNRnature.However,thelattercancauseproblemsinlow- theconstructionandcontentofthesenewdiagnosticdiagramswill
resolution spectraasthese linesarealsoquite strong inthenight be presented in their entirety in an upcoming paper (Frew et al.,
skymakingtheirdeconvolutionfromtheirbrightskylineequiva- 2013, in preparation, F13 hereafter). Our new sources are over-
lentsvery difficultunless thereisastrong velocitycomponent in plotted on these figures which demonstrates clearly how they lie
theSNR[OI]lines.Thisisespeciallygermaneiftheextendedneb- inthezonespopulatedbyconfirmedSNRs(seeFigure7andFig-
ulosity extends along the entire length of the slit. Our main lim- ure8).
itationthough istheinherent faintness of thetargetseven inHα, WeemphasisethatevolvedGalacticSNRshavecompositions
whichalsoexplainstheirnon-detectioninpreviousopticalsurveys. dominated by swept-up ISM, and show a clear abundance gra-
IPHAS Galacticsupernovaremnants 5
dient in the Galaxy (Binetteetal. 1982; F85); the most metal- uesof41and11(scaledtoHβ=100),supportingtheSNRclassifi-
rich examples in the inner Galaxy tend to plot at the lower- cation(seeFigure6andTable3).However,theobservedlineratios
left of the SNR domain in the Hα/[N II] versus Hα/[S II] di- fromthespectraforpointingAareclosertothoseexpectedfora
agram (Figure 7). Since the IPHAS candidates also plot here, HIIregion(Table3).Notethatpointing‘A’fallsonanebularcon-
this suggests that they are metal rich, which is consistent with densationinternaltotheproposedmainSNRshellandcouldsim-
the Galactic sight-lines for these objects. At any plausible dis- plybeanunrelatedcompactHIIregion,as,unlikethetwointernal
tance, they are located interior to the Solar Circle. This diag- emission spursto theNorth, thisblob does not appear connected
nostic plot also illustrates how the Galactic objects tend to plot to the main arcuate shell. Furthermore, the spectral characteris-
separately from the Magellanic Cloud remnants, a consequence ticsof thisregion (slitpositionA) aredifferent tothose obtained
of the lower nitrogen abundance in the latter (Meaburnetal. fromslitpositionsBandCwithinsteadaratherlow[SII]/Hαratio
2010; Leonidakietal. 2012). We also note that many of the (0.24) which does not advocate for a shock regime and very dif-
youngest core-collapse Galactic SNRs (e.g. Fesen&Kirshner ferent[NII]/Hαratios,addingfurtherweighttoitsidentificationas
1982;Leibowitz&Danziger1983;Winkler&Kirshner1985)are adiscreteHIIregion.Nevertheless,givenwehavenoindependent
chemically stratified, and show clear nitrogen overabundances estimateofthedistancesofthesecomponentswecannotcategori-
in many of their filaments (Chevalier&Kirshner (1978, 1979); callydiscardalinkbetweenthisputativecompactHIIregionand
MacAlpineetal.(1994,1996);Isenseeetal.2012). the surrounding main SNR shell. Additional information such as
TheindividualspectroscopicdetailsofeachIPHAScandidate distance estimationsand thekinematics of both structures arere-
aredescribedinthefollowingsubsections. quiredtofullyestablishanypossibleconnection.
The information brought by the extinction data would posi-
tion the filaments/structures related to pointing A and B at rela-
3.2 IPHASXJ190640.5+042819 tivelyclosedistancefromeachotherwithrespectivec(Hβ)values
of0.82and0.86.PointingC,withc(Hβ)=1.26,wouldthenappear
Forthiscandidate(refertotheimagesinFigure1)ournewoptical
asaindependentelementbutwecannotdiscardalinktoIPHASX
spectrumgivesratiosof[NII]/Hα=1.48(eliminatinganypossible
J195744.9+305306sincepointings‘B’and‘C’sharethesameSNR
confusion with a HII region) and [SII]/Hα=1.15, clearly placing
spectroscopicclassification.
theionisedgasintheshockregime.Placingtheseemissionlinera-
InsummarySNRcandidateIPHASXJ195744.9+305306falls
tiosintheF13andF85diagnosticdiagrams, indicatesthesource
inageneralareaofHIIregionsincludingaprobablycompact HII
liessquarelyintheregionoccupiedbySNRs.Theforbiddenoxy-
region(pointingA)superposedonthemainSNRshell.HIIregions
gen emission lines[OI]λλ6300,6364 which arerelatively strong
projectedonSNRsarenotunexpectedandourcombinedimaging
in SNR (contrary to HII regions and PNe) are also present and
andspectroscopicdatastillprovidestrongevidencethatIPHASX
were measured with extinction-corrected fluxes of 34.1 and 34.5
J195744.9+305306(G067.6+0.9)isatrueSNR.
respectively(forHβ=100andHα=286).The[OI]λ6300lineflux
shouldbe3×the[OI]λ6364fluxaccordingtothetheoreticalratio
of Einstein’sA coefficients. Theobserved values betray sky sub-
3.4 IPHASXJ200002.4+305035
tractionproblemsduetothepresenceofthesameoxygenlinesin
thesky-backgroundthatmakesthemdifficulttoextractproperlyas The spectral characteristics exhibited from pointing D (object
thenebularemissionfilledmostoftheslit.Althoughweproceeded shown in Figure 3) including from its location in the F13 dia-
with great care when removing the sky, there is clearly signifi- gram,andaquitehighbutnotconclusive[SII]/Hαratio,thatthis
cant uncertainty inthefluxmeasurement of theselines. IPHASX apparently coherent, oval nebulae may be an additional compact
J190640.5+042819 is our first confirmed SNR which can then SNRwhichwetentativelydenoteasIPHASXJ200002.4+305035
be named according to the standard Galactic SNR nomenclature orG067.8+0.5. It islocated 5′ totheEastof themoreextensive
(GD.d±D.d)asSNRG038.7-1.3. nebulosity linked to IPHASX J195744.9+305306 and is clearly
detached from it. The spectrograph slit was placed E-W across
the middle of the Western enhanced edge of the optical nebula.
3.3 IPHASXJ195744.9+305306 The absence of the Hβ line prevents us from deriving the ex-
tinction. Note that it isunrelated tothe nearby compact (FWHM
The large projected optical angular extent of IPHASX
∼1.62 arcseconds) PN candidate IPHAS J195956.42+304823.8
J195744.9+305306 (∼24.6′×22.6′- see Figure 2) led us to
(2MASSJ19595642+3048238) reportedbyViironenetal.(2009).
obtain several different pointings during the spectroscopic obser-
IPHASXJ200002.4+305035 alsoshowsthehighestelectronden-
vations so as to obtain a more representative sample of spectra sityofallthetargetswithne−=620±74cm−3whichmaybeanin-
across different possible components of this large source. Those
dicationofyouth.Furtherstudyofthisemissionnebulaisrequired
arenotedasA,BandCinthetoppanelofFigure2.Thereisan
toclarifyitsnaturebutanSNRidentificationisnotunwarrantedat
additional pointing D which was made on the faint, oval shaped,
thisstagefromtheopticalimageryandspectroscopyalone.
apparently detached nebula ∼5′ to the east of the main structure
andapproximately8′×6′ indiameter(Figure3).Thisturnsoutto
beaninterestingsourceinitsownrightandanotherpossibleSNR
3.5 IPHASXJ195749.2+290259
candidate(seebelow).
Figure7indicatesthatthespectrafrompointingsBandpar- ThespectroscopicanalysisofthiscandidateshowninFigure4,in-
ticularlyCprovidelineratioscompatiblewithanSNRoriginwith dicatesalikelySNRidentification(inthiscasethenebulaisnamed
a clear signature of shock-heated emission with [SII]/Hα ratios G066.0-0.0) either using the diagnostic diagrams from F13 (Fig-
of 0.57 and 0.99 respectively (Figure 8, Table 3)- see also the ure 7) or the conditions fixed by F85. The observed diagnostic
diagram by Phillips&Cuesta (1999). Pointing B also shows the emissionlineratiosof[SII]/Hα=0.83(clearlyintheshockregime
[OI]λλ6300,6364 emission lines with respective dereddened val- following F85), [NII]/Hα=1.9 (clearly eliminating a HII region
6 L. Sabinet al.
identification)and[SII]6717/6731=1.4indicatinganelectronden-
sityclosetothelowdensitylimit,allfitwellwithinthedefinition
ofanSNRfollowingF85.Wenoticethat,similarlytoG067.6+0.9
(IPHASXJ195744.9+305306: pointingsAandC),wedidnotde-
tectanyclear[OI]emission(seeFigure6).
3.6 IPHASXJ195920.4+283740
ThenebulapresentedinFigure5,iswellconstrainedspectroscop-
ically to fall within the SNR regime using the criteria of F85 (
[SII]/Hα=0.90and[NII]/Hα=1.25)andF13(seeFigure7).How-
ever, anew heliumrichemission point source possiblyrelatedto
a WR star has been identified close to the enhanced bow-shaped
easternemissioncomponentbyCorradietal.(2010),anddenoted
asIPHASJ195935.55+283830.3.Possibleassociationofthenebu-
laewiththisunusualstarwouldthrowdoubtonitsSNRnatureand
mightimplythattheshellresults,atleastpartly,toapastejection
from a massive star. We note that the elongated and asymmetric
filamentarymorphology ofIPHASXJ195920.4+283740 doesnot
show the ring-like structure(s) generally displayed by WR nebu-
lae(Stock&Barlow2010,Marston1995),whichlendssomesup-
port to an SNR classification. However, the IPHAS nebula does
not display the typical SNR filamentary morphology either. The
SMBlog(Hα/[NII])versuslog(Hα/[SII])diagnosticdiagramaf-
terFrew&Parker(2010)isusefulindiscriminatingSNRandWR
shellemission-linesignaturesastheytendtooccupydifferentloci
inthediagram.InthecaseofIPHASXJ195920.4+283740,theob-
ject plots well inside the SNR area and away from the WR shell
/ HII region domain. Nevertheless, the presence of a helium rich
starsoclose tothe enhanced easternedge of thenebula issuspi-
cioussuchthatanon-SNRorigincannotbecompletelyruledout,
despitetheindicativeemission-lineratiosandcorroboratingradio
signatures(seelater).
Figure1.Thetoppanelpresentsabinned(5arcsecond/pixels)Hαimageof
4 MULTI-WAVELENGTHDATA IPHASXJ190640.5+042819fromtheIPHASmosaicsoverlaidwith6cm
contoursfromthe87GBsurvey.Contoursgofrom0.01to0.14Jy/beam
4.1 G038.7-1.3–IPHASXJ190640.5+042819
inintervalsof0.02Jy/beam.ThereisanexcellentmatchbetweentheHα
andtheradiodata.Thepeakfluxhas90mJy.Thebottomimagepresents
TheregionofskycontainingthisSNRwascoveredbytworadio
theNVSS1.4GHz(21cm)radio image. Thepositional matchwithHα
telescopes, the 64 m Parkes telescope as part of the PMN 6 cm
andthe6cmemissionisobviousontheeasternside.Duetotheimproved
survey(Griffith&Wright1993)andthe91mNRAOGreenBank
resolutionof∼45arcseconds,thewholeSNRshell(delimitedbythewhite
telescope and included within the 87 GB survey (Condonetal.
arrows)canbenoticedwhichenables anupdatedSNRIDasG038.7-1.3
1989). Radio emission is clearly seen and follows quite well the basedonthegeometric centre oftheshell.TheNVSSradio contours go
opticalnebularmorphologyasshowninthetoppanelofFigure1. from0.001to0.05Jy/beaminintervalsof0.005Jy/beam.Thepositionof
TheyellowcontoursoverlaytheHαgreyscaleimageandshowthe thespectrographslitisindicated.
4.85GHz(6cm)radiofluxfromthe87GBsurveyastheresolu-
tionofthistelescopeatthisfrequency(∼3.5′)issomewhatbetter
thaninPMNsurvey(∼5′).Theradiodataseemstoindicateamuch from0.001to0.05Jy/beaminintervalsof0.005Jy/beam.Theinte-
largerextenttotheSNRthanisevidentfromtheopticaldataalone. gratedfluxforthisregionprovidesvaluesof90mJyfor87GBand
Notethat thisradioemission had not been previously recognised 109.2mJy and 119.9 mJy (21 cm) for the two NVSS frequencies
as being part of any coherent structure so we make the associa- available.Unfortunatelyitisproblematictoconcludeifthesemore
tionhereforthefirsttime.Theradiocontoursgofrom0.01to0.14 compactNVSSemissionpeaksarerelatedtotheSNRorarefrom
Jy/beaminintervalsof0.02Jy/beam. aseparateGalacticmJysourceorevenabackgroundextragalactic
TheSNRisalsoseenat1.4GHz(21cm)withintheNVSSsur- double-lobedradiosource.Evenifthisdouble-peakedsourceisun-
vey(Condonetal.1998)andshowninthebottompanelofFigure1. relatedtotheSNRtheNVSSimagestillshowsacomplete,though
Comparison with the 87 GB and PMN data shows that both the fragmentedSNRshellof∼25′indiameter.Arrowsareaddedtothe
6cmand21cmsurveysexhibitstrongemissionpeak(s)atthesame figuretofacilitaterecognition.ThisenablesanupdatedSNRIDof
location(RA=19h07m20s,DEC=4o32’).However,theNVSSim- G038.7-1.3basedonthenewgeometriccentreoftheshell(notewe
age has much better resolution (45 arcseconds) and resolves two retaintheoriginalIPHASopticalIDbuttheRA/DECoftheshell
compactemissionpeaksatthesamelocation.Theradiocontoursgo centre is approximately 19h 06m 35s +04o 35’ 00”). Due to the
IPHAS Galacticsupernovaremnants 7
C
A
B
Figure 3. Top panel: The Hα-r image mosaic of IPHASX
J200002.4+305035 with 15× pixel binning overlaid with NVSS con-
tours in green ranging from 0.0015 to 0.004 Jy/beam. The E-W slit
locationofthis‘D’pointingwasplacedmid-wayuptheWesternenhanced
edge of the shell. This detached nebula is at ∼5′ East of IPHASX
J195744.9+305306. Wenotethatthe NVSS1.4GHzemission ismainly
coincident with the bright optical interacting front on the West. In the
bottompanelthe87GB6cmemission,overlaidasyellowcontoursranging
from0.015to0.05Jy/beam,isdistributedoverthewholenebulaincluding
out to the faint opposing optical emission rim directly to the West. The
Figure 2. Top panel: The Hα-r image mosaic of IPHASX E-Wslitpositionisindicated.
J195744.9+305306 with 15×pixel binning overlaid with NVSS radio
contours ingreen ranging from0.0008to0.0027Jy/beam. The redE-W
barsindicatethelocationoftheslitsforthedifferentspectralpointingsA,B
andCwithlengthsof5′.Thenebulaisinagenerallycomplexenvironment
whichmakesithardertodefinethereallimits ofthenewSNRinHαat
theNorthernfainterrim(sampledbyslitC).Thediagonaldarkstreakisan
imageartefact(possiblyanasteroidtrail).Bottompanel:Asabovebutnow ferent frequencies) would be another clear proof to support their
withthelowerresolution6cm87GBradiosurveydataoverlaidinyellow SNRstatus.Howeverthiswasnot possiblewiththeexistingdata
contours whichrange from0.004to0.02Jy/beam. Asinthe caseofthe
due tothenon-compatibility between the radiosurveys (different
21cmNVSSdatathe6cm87GBradioemissionfollowstheedgesofthe
resolutions/beam sizes) and because of the generally fragmented
SNRveryclosely andespecially alongtheSouthEastpartofthenebula
natureoftheremnants.Newanddeeperradioobservationsarere-
wheretheopticalemissionisstrongest.Theoptical-radiooverlapisnotas
quiredtoobtaindecentintegratedradiofluxesfromwhichreliable
completeasinthecaseoftheSNRIPHASXJ190640.5+042819.
spectralindicescouldbeobtainedandhencetodemonstratetheex-
istenceofnon-thermalradioemission.Thisisbeyondthescopeof
lowresolutionat6cmfromthe87GBsurveywecouldnotmatch thispaper.
emission withthe NVSS shell at 21 cm apart from at the South- AsingleXraysourceisfoundwithintheconfinesoftheSNR,
Eastedge.WealsonotethatReichetal.(1984)detectedonlyone ROSAT1RXSJ190709.4+043100. Similarlytopulsars,aconnec-
source(similartothe6cm87GBsurveydata)attheSNRlocation tioncouldbemadeviathelocalisationofX-raysourcesinthecen-
usingtheEffelsberg100mdishwitharesolution∼4.3′. tralpartoftheSNRbutthissourceisnotlocatedclosetothegeo-
Weemphasize, andthisisalsovalidfortheremainingradio metriccentremakinganydirectlinkunlikely.Furthermore,thereis
analysisoftheotherobjects,thatmeasuringavalidspectralindex noindicationofanyextendedX-rayemissioninternaltotheshell
forourSNRcandidates(i.e.obtainingreliableradiofluxes atdif- whichcouldindicateaPlerion.
8 L. Sabinet al.
Figure 4. Image of new IPHAS SNR IPHASX J195749.2+290259. We
showtheopticalIPHASHαimageoverlaidwithyellow6cm87GBcon-
tourswhichrangefrom0.0014to0.14Jy/beam.Thereisaclearcorrespon-
dence between the optical and radio emission particularly on the North-
Westrim.Inthiscasewedidnotfindanyclearlyassociated1.4GHzemis-
sion.
AgaintheE-Wslitpositionismarked.
4.2 G067.6+0.9–IPHASXJ195744.9+305306
ThetoppanelinFigure2showstheHα-rimagemosaicofIPHASX Figure 5. Top panel: Full resolution Hα mosaic of potential new SNR
J195744.9+305306with15×pixelbinningoverlaidwithNVSSra- IPHASX J195920.4+283740 with the 21 cm NVSS (1.4 GHz) contours
diocontoursrangingfrom0.0008to0.0027Jy/beam.Thishigher emissionoverlaid.Thebottompanel,asabovewithnowthe6cm87GB
resolutionNVSS21cmemissiondirectlyandclearlyoverlapswith radioimage(4.85GHz)contoursemissionoverlaid.Thosetwomapsindi-
the strongest optical emission to the SSE. The complex environ- catesthatwhiletheeasternenhancededgeismappedclearlybythe1.4GHz
emission,the4.85GHzradioemissionisperfectlycoincidentwiththefull
mentsurroundingthenebulamakesithardertodefinethereallim-
opticalimagewhichmaypartiallyreflectthelowerresolutionofthe87GB
its of the new SNR in Hα. The red E-W bars indicate the loca-
data.Aheliumenrichedemissionlinestarislocatedclosebutinternalto
tionoftheslitsforthedifferentspectralpointingsA,BandCwith
theeasternarcofthenebulae. Spectrograph slitposition markedE-Was
lengthsof5′.ThebottompanelofFigure2isthesameexceptwith
before.
the6cm87GBradioemissionoverlaidascontoursrangingfrom
0.004to0.02Jy/beam.Heretheradioemissionappearscoherently
distributedalongtheSouthernrimofIPHASXJ195744.9+305306. onagrey-scale15×pixelbinnedIPHAS25×20′Hαimageinthe
Therearealsosomesmaller,sparseemissionblobsoutsidetheneb- toppanelofFigure3duetoitshigherresolution.Thebottompanel
ula which are probably unrelated. Detection of radio emission at isasabove with87 GB data overlaidas yellow contours ranging
two frequencies across components of the optical emission lends from0.015to0.05Jy/beam. Whilethe6cm87GBemissionap-
strongsupporttoalikelySNRidentificationwhencombinedwith pearstoentirelycovertheopticalnebulaincludingaweakexten-
theindicativespectroscopic signatures.Agroup ofX-raysources siontotheEasternside,thehigherresolutionNVSSdataismore
has been identified within the boundaries of the proposed new localizedandcoincidesmostcloselywiththeWesternenhancement
SNR. Except for one located in the North-East (detected with oftheovalshell(apossiblebow-shock?).
the ROSAT all-sky survey catalogue of optically bright OB-type
stars,Berghoeferetal.1996)andrelatedtothedouble/multiplestar
HR7640, themajorityoftheseX-raysourcesareconcentrated at 4.4 G066.0-0.0–IPHASXJ195749.2+290259
∼11.5′WestfromthegeometriccentreoftheSNRandaremostly
A multi-wavelength investigation of archival data reveals radio
ROSATbrightsources(Vogesetal.1999)unrelatedtotheSNR.
emissionat6cmfromthe87BGsurvey.Figure4showstheIPHAS
Hαgrey-scaleimageoverlaidwithyellow6cmcontoursranging
from0.0014to0.14Jy/beamthatappearstofollowtheedgesofthe
4.3 G067.8+0.5–IPHASXJ200002.4+305035
SNRespeciallytotheNorth–EastandNorth–Westthoughitisalso
ThepossiblenewSNRPHASXJ200002.4+305035hasclearradio somewhatfragmented.Aninterestingpointisthelocationonly≃
detectionsinboththeNVSSand87GB.WepresenttheNVSSdata 31′totheSouth–EastofanotherofournewIPHASidentifiedSNRs
overlaidasgreencontoursrangingfrom0.0015to0.0040Jy/beam G065.8-0.5(IPHASXJ195920.4+283740)furtherdiscussedbelow.
IPHAS Galacticsupernovaremnants 9
TheE-Wslitpositionofthespectroscopicobservationisindicated [NII]/Hα∼0.62.However,wedeterminethatthiscompactnebular
bythewhitehorizontalbar. region,whichislocatedinternaltothemainnebulashellandtar-
getedaspointing‘A’islikelytobeanunrelatedHIIregion.Thesec-
ondisIPHASXJ2000002.4+305035.Inthiscasethespectrumstill
4.5 G065.8-0.5–IPHASXJ195920.4+283740 displaysan[NII]/Hα> 0.5whichisatypicalofHIIregions.The
[OI]6300and6364A˚ emissionoftenseeninopticalremnantsare
For this SNR candidate an excellent match between the known
difficulttounambiguouslyidentifyinmostcasesduetolowspec-
NVSSradiosourceNVSS195938+283832(27.0mJyat1.4GHz)
tralresolutionsemployedmeaningthattheselinescannotbeeasily
and thebright Easternenhanced arcuate rimof thispossible bow
resolvedorsubtractedproperlyfromthestrongnightskylinesof
shockwasfound(seetoppanel ofFigure5).Thereisalsoavery
thesamespeciesduetothelackofsignificantlinevelocitydiffer-
goodcoincidencebetweenthe4.85GHz6cm87GBradioemis-
ence. Consequently, among the group of likely SNRs, only three
sionand theentireoptical nebulaextent including thefainterop-
show[OI]6300A˚ and/or[OI]6364A˚ withanycertaintythoughthe
posingrimtotheWest(seebottompanelofFigure5).Whentaken
lineintensitiesthemselvesareonlyindicativeandarenotrecorded
togetherwiththeopticalemissionlineratios,whichplacetheneb-
inthecanonical3:1ratio.
ulaintheSNRregimeofthevariousdiagnosticdiagrams,anSNR
identification seems likely. However, the situation is complicated It is also important to emphasise that shock emission is not
by possibleassociation withahelium enriched emission-line star inherent solely to SNRs and can result from stellar winds and
(Corradietal.2010)closetobutinternaltotheEasternarc traced interactions such as those produced by young-stellar objects and
bytheopticalandNVSSdata.Moreinvestigationisneededtoas- some PNe and WR stars. However, in general WR shells do not
certainthetruenatureofthisnebula. show strong [SII] emission, and largely plot in separate regions
of Sabbadin,Minello&Bianchini (1977) (SMB) type diagnostic
plots.
In order to further corroborate our results we also per-
5 DISCUSSIONANDCONCLUSIONS
formed a multi-wavelength investigation based on searches of
Wereportthediscoveryoffourorpossiblyfivenew,likelySNRs online archives including all available radio data from the vari-
followingasystematicandcarefulvisualsearchofthecontinuum ouscurrent on-line surveys. Radio emission, mainly at4.85 GHz
removed Hα mosaics of the IPHAS survey over the 19-20 hour (6 cm) from the 87 GB radio survey, was found in all cases
RA zone. These small-to-medium angular size Galactic nebulae with a decent positional and structural match to components of
and SNR candidates (i.e. with major axes ranging from ∼6–50′) the optical morphology. In most cases there were also detections
were selected primarily on the basis of theiroptical morphology. from the higher resolution 1.44 GHz (21 cm) NVSS survey data
PlausiblecandidateSNRsneedtoexhibit coherent filamentaryor which provides clearer and more direct associations. These radio
shell-like characteristics which are common features of currently matchesareparticularlygoodinthecasesofG038.7-1.3(IPHASX
knownopticallyidentifiedSNRs.Therelativelysmallangularsize J190640.5+042819), G065.8-0.5 (IPHASX J195920.4+283740)
of these objects compared with the degree-size SNRs found for andG067.8+0.5(IPHASXJ200002.4+305035)whichsupportsthe
many Galactic remnants (e.g. referto on-line catalogue of Green SNR nature of those objects. The remaining two SNRs display
– http://www.mrao.cam.ac.uk/surveys/snrs/snrs.data.html) implies morediscontinuous radioemissionbutstillwithcomponents that
thatthesenewlydiscoverednebulaecouldbemoredistantorper- follow the most intense regions of optical emission. Full overlap
haps younger, assuming they have not had the time to expand to betweentheradioandopticaldataisnotanecessaryconditionto
the dissipation phase. In the latter case a connection between an demonstrate association, particularly in the case of faint nebulae
SNRand ayoung pulsar might be realisedif they originatefrom andpossiblelocaldustanddensityvariationscanaffectthevisibil-
core-collapseevents. ityofopticalemissionacrossthetrueextentoftheSNR.
Ourspectroscopicfollow-upofthesecandidatesallowedusto Another source of confusion with possible SNR identifica-
identifythemallaslikelySNRsusingthestandarddiagnosticemis- tions of optical nebulae resides in the diffuse ionised gas (DIG)
sionlineratiosdefinedbyFesenetal.(1985)andthenewandmore emission (or warm interstellar medium, WIM) as its morphol-
comprehensive diagrams presented firstby Frew&Parker(2010) ogy can be confused with our equally faint SNR candidates.
andwithupdatedandimprovedversionsofthesefigurespresented Since the DIG is generally photo-ionised, as are HII regions
hereforthefirsttime.Aprobleminthistypeofinvestigationlies (Domgorgen&Mathis 1994), the separation of the DIG with
inthegenerally faintoptical emissionandlow surfacebrightness shock-ionised objects such as SNRscan be easilydone from our
ofsuchevolvedSNRswhichmakestheirspectroscopicinvestiga- opticalspectroscopy usingthelimitingconditions forbothstates:
tiondifficulton2mclasstelescopes.Nevertheless,inthisprelim- [SII]λλ6716,6731/Hα ≃ 0.4 and [NII]λλ6548,6583/Hα ≃ 0.5
inarystudy,theprimarycombinationofopticalimageryandspec- (seeforexampleGalarzaetal.1999,theirfigure8).Asmentioned
troscopy and their locations in the emission line diagnostic plots previously,ourline-ratiosclearlyindicatethatexceptforregion‘A’
lendstrongsupporttotheSNRoriginoftheexaminedcandidates. in IPHASX J195744.9+305306 (which is indeed likely an unre-
In several cases this has been further corroborated by the recog- latedHIIregion)andtheambiguousIPHASXJ2000002.4+305035,
nitionofpreviouslyun-cataloguedradioemissionusuallyatmore all the targets show shock-ionisation characteristics of SNRs (
thanonefrequencyfromarchivaldatawhichisclearlyassociated [SII]/Hα>∼0.4and[NII]/Hα>∼0.5.ADIGclassificationcan
withtheopticalnebulosities. thereforebeconfidentlydiscardedforthesesources.
ThespectroscopicdatashowtypicalSNRdiagnosticlinesex- Figure 6 and Table 3 indicate a relatively strong
hibitedbyallcandidatesandindicatesthepresenceofshockswith [OIII]5007A˚ which is known to be a good indicator of higher
[SII]/Hαlineratiosgreaterthan0.4–0.5.Theobserved[NII]/Hα shockspeed.Raymond(1979)showedthatthepresenceofthisline
lineratiosarealsogenerallyhigh(> 1.2)whichruleoutanypos- iscoincidentwithshockspeedgreaterthan80km/s.Thistrendis
sible confusion with HII regions. There are two exceptions. The confirmedwhencomparingourresultswiththetheoreticalmodels
firstisforpointing‘A’ofIPHASXJ195744.9+305306 whichhas by Allenetal. (2008) which indicate shock speeds between 200
10 L. Sabinet al.
Figure6.BlueandredspectralregionsofinterestforthefiveIPHAStargets.Themainemissionlinesarelabelledandthefluxesarenormalized.Thespectra
havebeencroppedforabetterdisplayofthenebularlines buttheareaofthegapisemptyoflinesorareinsidethenoiselevel.WerecallthatIPHASX
J200002.4+305035alsodesignatesthepointingD.
and1000 km/sintheSNRwhere[OIII]5007A˚is“well”detected. tificationsinthefuture.Consideringthereturnfromasingle1-hour
This does not apply to the C slit-position of J195744+305306 IPHASRAzonewemightexpectdozensofnewGalacticSNRcan-
wherethe[OIII]5007A˚ lineishardlydetectableandmostprobably didatestobeuncoveredbasedontheiropticalemissionsignatures.
mixed withnoise. Those high values would help to constrain the Thiswouldaddsignificantlytotheknown Galacticpopulationof
evolutionary phase of the SNR observed, as although we are not evolvedSNRs.
dealingwithSNRsintheirultimatephaseofdissipationintheISM
wearenotdealingwithyoungoneseither.
ACKNOWLEDGEMENTS
OurresultsonthefirstSNRsidentifiedfromIPHASarepre-
liminary, and each nebula warrantsadditional follow-up. Ouron- WethankboththeanonymousrefereeandP.Boumisforvaluable
goingsearchfornewSNRcandidatesisbasedoncarefulandsys- comments and suggestions that have contributed to significantly
tematic examination of the IPHAS survey image mosaics of the improvingthequalityofthepaper.Thisprojecthasbeensupported
GalacticplaneandishighlylikelytoprovidemanymoreSNRiden- byPAPIIT-UNAMgrantIN109509.Thispapermakesuseofdata