The ongoing pursuit of R Coronae Borealis stars: ASAS-3 survey strikes again

3.8 MB·English

by  P. Tisserand

Astronomy&Astrophysicsmanuscriptno.ASAS-RCB-South (cid:13)c ESO2013
January16,2013
The ongoing pursuit of R Coronae Borealis stars: ASAS-3 survey
strikes again.
P.Tisserand1,2,G.C.Clayton3,D.L.Welch4,B.Pilecki5,6,L.Wyrzykowski6,7,andD.Kilkenny8
1 ResearchSchoolofAstronomyandAstrophysics,AustralianNationalUniversity,CotterRd,WestonCreek,ACT2611,Australia
2 ARCCentreofExcellenceforAll-skyAstrophysics(CAASTRO)
3 DepartmentofPhysics&Astronomy,LouisianaStateUniversity,BatonRouge,LA70803,USA
4 DepartmentofPhysics&Astronomy,McMasterUniversity,Hamilton,Ontario,L8S4M1,Canada
5 UniversidaddeConcepcio´n,DepartamentodeAstronom´ıa,Casilla160-C,Concepcio´n,Chile
3 6 WarsawUniversityAstronomicalObservatory,Al.Ujazdowskie4,00-478Warszawa,Poland
1 7 InstituteofAstronomy,UniversityofCambridge,MadingleyRoad,CambridgeCB30HA,England
0 8 DepartmentofPhysics,UniversityoftheWesternCape,PrivateBagX17,Bellville7535,SouthAfrica
2
Received;Accepted
n
a
J Abstract

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Context.RCoronaeBorealisstars(RCBs)arerare,hydrogen-deficient,carbon-richsupergiantvariablestarsthatarelikelytheevolved
1
mergerproductsofpairsofCOandHewhitedwarfs.Only55RCBstarshavebeenfoundinourgalaxyandtheirdistributiononthe

sky is weighted heavily by microlensing survey field positions. A less-biased wide-area survey would provide the ability to test
]
R competingevolutionaryscenarios,understandthepopulationorpopulationsthatproduceRCBsandconstrainttheirformationrate.
Aims.TheASAS-3surveymonitoredtheskysouthofdeclination+28degbetween2000and2010toalimitingmagnitudeofV=
S
14.WesearchedASAS-3forRCBvariablesusinganumberofdifferentmethodstoensurethattheprobabilityofRCBdetectionwas
.
h ashighaspossibleandtoreduceselectionbiasesbasedonluminosity,temperature,dustproductionactivityandshellbrightness.
p Methods.Candidateswhoselightcurveswerevisuallyinspectedwerepre-selectedbasedontheirinfrared(IR)excessesduetowarm
- dust in their circumstellar shells using the WISE and/or 2MASS catalogues. Criteria on light curve variability were also applied
o whennecessarytominimizethenumberofobjects.Initially,wesearchedforRCBstarsamongtheASAS-3ACVS1.1variable star
r catalogue,thenamongtheentireASAS-3southsourcecatalogue,andfinallydirectlyinterrogatedthelightcurvedatabaseforobjects
t
s thatwerenotcataloguedineitherofthose.Wethenacquiredspectraof104starstodeterminetheirrealnatureusingtheSSO/WiFeS
a spectrograph.
[ Results.Wereport 21newly-discovered RCBstarsand2new DYPerstars.Twopreviouslysuspected RCBcandidates werealso

spectroscopically confirmed. Ourmethodsallowedustoextendourdetectionefficiencytofaintermagnitudes thatwouldnot have
4
beeneasilyaccessibletodiscoverytechniquesbasedonlightcurvevariability.Theoveralldetectionefficiencyisabout90%forRCBs
v
withmaximumlightbrighterthanV ∼13.
5
Conclusions.Withthesenewdiscoveries,76RCBsarenowknowninourGalaxyand22intheMagellanicClouds.Thisgrowing
7
sampleisofgreatvaluetoconstrainthepeculiaranddisparateatmospherecompositionofRCBs.Mostimportantly,weshowthat
4
the spatial distribution and apparent magnitudes of Galactic RCB stars is consistent with RCBs being part of the Galactic bulge
2
population.
.
1
Keywords.Stars:carbon-stars:AGBandpost-AGB-supergiants
1
2
1
: 1. Introduction limit, i.e., the merger of an He-WD with a CO-WD, a new
v
classofstarsmaybecreated.R CoronaeBorealisstars(RCBs)
i
X The mergerof two white dwarfs (WDs) is a rare astrophysical arethefavouredcandidates(Webbink1984;Jefferyetal.2011;
r phenomenonthatconcludesthelifeofaclosebinarysystem.It Shenetal.2012;Staffetal.2012).
a isthesourceofaveryinterestingnewclassofstarsorphenom-
RCBs are hydrogen-deficient and carbon-rich supergiant
enathataremuchstudiednowadays.ThemergeroftwoHelium-
stars.Theyarerare,withonly53knowninourgalaxy(Clayton
WDs, a low mass system (M . 0.6M ), results in hot sub-
Tot ⊙ 1996;Tisserandetal.2008;Clayton2012),andtheiratmosphere
dwarf type (sdB and sdO) stars, that explain the excess of UV
is made of 99% helium. The scarcity and unusual atmospheric
emission observed in elliptical galaxies (Podsiadlowskietal.
composition of RCBs suggest that they correspond either to a
2008). At the other end of the mass scale, the merger of two
brief phase of stellar evolution or an uncommon evolutionary
CO-WDs(M &1.4M )canproduceoneofthemostpowerful
Tot ⊙ path.Twomajorevolutionaryscenarioshavebeensuggested to
cataclysmiceventsknowninourUniverse,aTypeIaSupernova
explaintheirorigin:theDoubleDegenerate(DD)scenario,cor-
(Simetal. 2010; Pakmoretal. 2012) orresultin a neutronstar
respondingtothemergeroftwoWDs,andthefinalheliumShell
aftera gravitationalcollapse.Inthemiddlerange,whentheto-
Flash (FF) scenario (Ibenetal. 1996; Renzini 1990) which in-
talmassofthesystemdoesnotexceedtheChandrasekharmass
volvestheexpansionofastar,onthevergeofbecomingawhite
dwarf,to supergiantsize (a late thermalpulse).The DD model
Send offprint requests to: Patrick Tisserand; e-mail: hasbeenstronglysupportedbytheobservationsofan18Oover-
[email protected] abundance in hydrogen-deficientcarbon (HdC) and cool RCB
1
P.Tisserandetal.:NewRCoronaeBorealisstarsdiscoveredintheASAS-3southdataset.
stars (Claytonetal. 2007; Garc´ıa-Herna´ndezetal. 2010), 500 suchsignaturesamongtheASAS-3lightcurvesofpre-selected
timeshigherthanthe 18O solarabundance(Anders&Grevesse objects presenting a near-IR excess using the 2MASS magni-
1989), but also the surface abundance anomalies for a few el- tudes.WeappliedthisclassicalsearchingmethodtotheASAS-
ements, fluorine in particular (Pandeyetal. 2008; Jefferyetal. 3SouthACVS1.1variablestarscatalogueandthentotheentire
2011)andtheevolutionarytimescale(Saio&Jeffery2002).On ASAS-3southsourcecatalogue.Second,wevisuallyinspected
the other hand, nebulae were detected around four RCB stars thelightcurvesofallobjectsselectedbyTisserand(2012a,b)for
(Claytonetal. 2011), which indicates that a fraction of RCBs theirwarmshells.Thedetailsoftheseanalysesarediscribedin
couldstillresultfromtheFFscenario. section 3 and a comparisonof theirresults is discussed in sec-
RCB stars are of spectral type K to F with an ef- tion 9. The spectroscopicconfirmationof 23 new RCB stars is
fective temperature ranging mostly between 4000 and 8000 presentedinsection4,andeachstarisdiscussedindividuallyin
K (Asplundetal. 2000; Tisserand 2012a), with a few ex- section8.Adiscussionabouttheirmaximumbrightness,spatial
ceptions being hotter than 10000 K (V348 Sgr, DY Cen, distributionandspectralenergydistributioncanbefoundrespec-
MV Sgr and HV 2671) (DeMarcoetal. 2002). Interestingly, tivelyinsections 5,6and7.
KameswaraRaoetal. (2012) have shown recently that a mem-
WenotealsothatwesearchedforDYPerstars,whichmight
ber of this latter group, DY Cen, is in a binary system. This is
bethecoolercounterpartofRCB stars. We foundtwonewDY
the first star classified as an RCB star that is found to have a
Per stars amongthe entire ASAS-3 south surveydataset. More
companion.TheirstudyandthepeculiarabundanceofDYCen
details on DY Per stars and the analysis can be found in sec-
indicates that is has certainly not followed the same evolution
tion3.4.
path than classical RCB, but has possibly gone though a true
common-envelopesysteminthepast100years.
It is necessary to increase the known sample of RCB stars
to answer our questions about their origin and population.
With an absolute magnitude ranging from −5 6 M 6 −3.5 2. Observationaldata
V
(Alcocketal. 2001;Tisserandetal. 2009) anda brightcircum-
stellarshell,itispossiblenowadaystodetectthevastmajorityof The All Sky AutomatedSurvey(ASAS) (Pojmanski1997) has
RCBstarsintheMilkyWayandtheMagellanicCloudsbyusing beenmonitoringtheentiresouthernskyandpartofthenorthern
the datasets accumulated by ongoing surveys. In the favoured sky (δ < +28◦) since October 2000, using large field of view
DD scenario, such a volume-limited search would allow us to CCD cameras and a wide-band V filter. Sources brighter than
determinetherateofHe-WD +CO-WDmergersandtherefore V∼14 mag were catalogued(i.e. ∼ 107 objects).The system is
give constraints on population synthesis models (Ruiteretal. located at Las Campanas Observatory.The light curve of each
2009). Indeed, current models estimate a He-WD + CO-WD individualobjectcanbedownloadedfromtheASAS-3website:
merger birthrate between ∼ 10−3 and ∼ 5×10−3 per year http://www.astrouw.edu.pl/asas/. The CCD resolution is about
(Nelemansetal. 2001; Ruiteretal. 2009), which would corre- 14.8′′/pixelandthereforethe astrometricaccuracyisaround3-
spondtoabout100to 500RCB starsexistingnowadaysin our 5′′forbrightstars,butcouldbeupto15′′forfainterones.With
Galaxy,usinganRCBphaselifetimeofabout105years,aspre- suchresolutionthephotometryincrowdedfieldsisuncertain.
dicted by theoretical evolution models made by Saio&Jeffery
Spectroscopicfollow-upof104RCB candidates,24known
(2002). Another interesting outcome of our endeavour to in-
RCBstarsand6knownHdCstarswasperformedwiththeWide
crease the number of known RCB stars is that we can con-
Field Spectrograph(WiFeS) instrument(Dopitaetal. 2007) at-
strainsimulationsofmergingevents.Indeed,thedetailedstudy
tached to the 2.3 m telescope at Siding Spring Observatory of
of RCBs’ peculiarand diverseatmosphericcompositionwould
the Australian National University. WiFeS is an integral field
give further information to He-WD + CO-WD merger mod-
spectrographpermanentlymounted at the Nasmyth A focus. It
els with nucleosynthesis (Jefferyetal. 2011; Staffetal. 2012),
providesa25′′×38′′ fieldwith0.5arcsecsamplingalongeach
whichcouldinturnimprovemodelsofhighermassWD merg-
oftwenty-five38′′×1′′ slitlets.Thevisiblewavelengthinterval
ersthatcancausetransienttypeIaSupernovaeevents.
isdividedbyadichroicataround600nmfeedingtwoessentially
The spectroscopic follow-up of candidates is the final step
similarspectrographs.Observationsarepresentedwitha2-pixel
required to confirm them as RCB stars, as one needs to con-
resolution of 2 Å. The spectra were obtained during seven ob-
firm the hydrogen deficiency and high C/O ratio of the atmo-
servationalruns:27-29November2009,14-17July2010,9-13
sphere and also, in most cases, the low isotopic 13C/12C ratio
December 2010, 20-25 July 2011, 4-7 June 2012, 23-25 July
andhigh18O/16Oratio.Twodifferenttechniquesareusedtose-
2012and1-3August2012.Theyarepresentedinfigures11,12,
lect candidates. One can select objects that possess a hot cir-
14,15,16and17.
cumstellar shell using IR datasets (Tisserand 2012a), or apply
algorithmsto detectaperiodicfast andlarge declinesin bright- Additionalspectra were obtained with the Reticon detector
nessamongmillionsoflightcurvesobtainedinthevisibleregion and Unit spectrographon the 1.9m telescope at the Sutherland
ofthespectrum(Alcocketal.2001;Tisserandetal.2004,2008; site of the South African Astronomical Observatory (SAAO).
Milleretal.2012),orboth(Tisserandetal.2011). Allspectrawereobtainedwithgrating6whichhasaresolution
We have searched for new RCB stars among the entire of∼2.5Å,andausefulrangeofabout3600-5400Åattheangle
ASAS-3southsurveydatasetusingbothlightvariationsandIR setting used. The spectra were taken between 1989 and 1994.
excess techniques to increase our overall detection efficiency. They have been extracted, flat-field corrected, sky-subtracted
The survey has monitored millions of stars with a declination and wavelength calibrated. A flux calibration was done for
lowerthan+28◦,sinceOctober2000.Thesurveycharacteristics the spectra using the spectrophotometricstandard LTT7379 al-
are describedin section2. RCBs are mainlyknownto undergo thoughduetotheslitandvariableseeing,thedataarenotpho-
unpredictable,fast,andlargephotometricdeclines(upto9mag tometric.EightspectrawereobtainedofV532Oph.Thesewere
over a few weeks) due to carbon clouds forming close to the combinedbytakinga medianofallthespectra.Allspectraare
lineofsightthatobscurethephotosphere.First,wesearchedfor plottedinfigure13.
2
P.Tisserandetal.:NewRCoronaeBorealisstarsdiscoveredintheASAS-3southdataset.
Figure1:Galactic distribution(l,b)ofRCB stars intheMilkyWay centeredat(0,0).TheblackdotsareRCB stars knownbefore
thisanalysis,theredonesarenewlydiscovered.ThebluelinerepresentsthelimitofobservationoftheASAS-3southsurveywith
adeclinationof+28◦.
3. Analysis readyknowntowardstheGalacticBulge(Zaniewskietal.2005;
Tisserandetal. 2008, 2011), only 2 (EROS2-CG-RCB-3 and
Many largescale surveysare actually underwayand new RCB
OGLE-GC-RCB-2) were bright enough to be catalogued and
starscanbefoundamongthemillionsoflightcurvesproduced,
monitoredby the ASAS-3 survey,each with a maximummag-
althoughthe aperiodiclargebrightnessvariationsof RCB stars
nitudeof V ∼ 13.6mag.The formerstar hasonlyten mea-
makethemdifficulttofindandacautiousapproachisnecessary. max
surementsonitslightcurve,whilethelatterhasmorethan800.
A significant RCB star sample also needs to be gathered and
However, we note that the decline observed in the OGLE-III
characterised before more automatic searches can be made as
dataset (Tisserandetal. 2011, Figure 5), around HJD 2453500
thelightcurvesaresodiverse.Furthermore,theASAS-3dataset
days, is not clearly detectable in the ASAS-3 light curve of
is affected by systematic problemsthat result in some artificial
OGLE-GC-RCB-2duetothelimitingmagnitudeofthesurvey,
declineswhichmaybemisleading,aswellastheannualgapin
andthereforethisRCB starwouldnothavebeenfounddespite
theobservations,andvariationsofthesurveydepth.
itslargenumberofmeasurements.
Forourinitialapproach,wedecidedtoapplyaseriesofse-
To increase our detection efficiency and compare the pros
lection criteria to keep interesting candidates for spectroscopic
and cons of different methods we decided also to visually in-
follow-up. The selection criteria were defined to optimise the
specttheASAS-3 lightcurvesof allobjectslisted in Tisserand
numberofnewRCBsfoundbutalsotominimisethequantityof
(2012a), where no selection criteria based on the light curve
light curves for visual inspection. First, as the vast majority of
were used, but only 2MASS (Skrutskieetal. 2006) and WISE
RCBspossessawarmcircumstellardustshell,wekeptonlyob-
(Wrightetal.2010)IRcolourstoselectobjectswithhot,bright
jectsthatpresentanIRexcessusingthe2MASSJ,HandKmag-
circumstellar shells. The related analysis is discussed in sec-
nitudes (see Figure 3). Second, we selected objects that show
tion3.3andlabelledCandDinTable2.
large amplitude brightness variations and/or show a rapid de-
clinerateinluminosity.Thesecriteriadifferslightlydepending
on the ASAS-3 catalogues being analysed. They are described 3.1.SearchforRCBstarsintheACVS1.1catalogue
insections3.1and3.2. (AnalysisA)
This classical analysis was performed in two steps, named
analysesAandBinTable2.Initially,wefocusedontheASAS- This catalogue is a concatenation of five different catalogues
3variablestarscatalogue,ACVS1.1.We thendecidedtodirect of variable stars as detected by the ASAS collaboration:
oureffortstowardstheentireASAS-3 sourcestarcatalogue,as first with declinations < 0◦ and various ranges of right
we foundthat some previouslydiscoveredRCB stars were not ascension, 0h − 6h(Pojmanski 2002), 6h − 12h (Pojmanski
listed in ACVS1.1. The details of both analyses are described 2003), 12h − 18h(Pojmanski&Maciejewski 2004), 18h − 24h
below. (Pojmanski&Maciejewski2005),andwithdeclinations<+28◦
(Pojmanskietal.2005).ACVS1.1isavailableontheASASWeb
The limiting magnitude of the ASAS-3 survey is about
V ∼ 14 mag. We estimated that we can detect RCB stars page1andcontains50124entries.Foreachobjectlistedsuchpa-
lim
with a maximum magnitude no fainter than VRCB,max ∼ 13 rametersofvariabilityasperiodicity,amplitudeofvariationVamp
mag,whichcorrespondstomaximumdistancesof∼40and∼20 andmaximumbrightness,Vmax,aregiven,aswellastheIRAS
and2MASSphotometry.
kpc, respectively for RCBs with absolute magnitudes M of -
V
5 and -3.5 mag (Tisserandetal. 2009). We also expected that We used 2MASS colours and the Vamp parameter to select
most RCBs located in the Galactic Bulge and/or close to the stars for visual inspection. We selected all objects that present
Galactic plane, where the density of known RCBs is highest an IR excess as describedin Figure 3, butadded a supplemen-
(Tisserandetal. 2008), would not be in the ASAS-3 catalogue tary constraint on the amplitude of variation of Vamp > 1 mag
for two reasons: first, RCBs would be fainter due to high in- forobjectswith H−K < 0.2.A totalof 1685objectswere se-
terstellar extinction; second, the photometry and source detec- lected.Aftervisualinspectionoftheirlightcurvestosearchfor
tion become complicated with the high density of stars and
the low CCD resolution of ∼15′′/pixel. Of the 19 RCBs al- 1 ASASWebpage:http://www.astrouw.edu.pl/asas/?page=catalogues
3
P.Tisserandetal.:NewRCoronaeBorealisstarsdiscoveredintheASAS-3southdataset.
typical RCB declines in brightness, 17 stars were selected for
spectroscopicfollow-up.Therejectedobjectsweremostlyperi-
odicvariablestars,suchasMirasandeclipsingbinarysystems.
Seven stars presented unmistakable RCB-type declines in
brightnessand, indeed, their respective spectra confirmed their
truenatureasRCB stars. Theyarelisted in Table2andnamed
ASAS-RCB-1 to -7. Anotherstar, named ASAS-RCB-8, has a
lightcurvewitharelativelyslowdecline,butitsspectrumshows
thatitisa warmRCB star. ThesenewRCB starsarediscussed
individuallyinsection8.Theirspectraandlightcurvesarepre-
sentedinFigures11and6to9respectively.
ThreeoftheseRCBs,ASAS-RCB-3,-4and-6,werealsore-
centlyreportedandspectroscopicallyconfirmedbyMilleretal.
(2012). They analysed the light curves of each object listed in
the ACVS1.1 catalogue using a machine-learned algorithm to
classifyobjectsandappliedahardcutontheprobabilitythatan
objectbelongstotheRCB class.Theyfolloweduptheircandi-
datesspectroscopicallyandfoundfourRCBstars,thethreethat
have just been mentioned plus ASAS-RCB-9. We did not find
ASAS-RCB-9inAnalysisAbecausethe2MASSIRmagnitudes
Figure2:DistributionoftheVmagnitudeofobjectslistedinthe
givenintheACVS1.1cataloguearenotcorrect,certainlydueto
entireASAS-3southsourcecatalogueusedinthisstudy.
a crossmatching issue. Milleretal. (2012) technique is a very
interesting method to discover rare objects such as RCB stars,
butitwouldbenefitfrommoretrainingwithalargersampleof
knownRCBstars’lightcurves.ThenewlyfoundRCBstarsre- were applied to assure the validity of each object. We treated
portedherecouldbeusedtoimprovethealgorithmandincrease eachentryindividually,andduplicateswereremovedduringthe
its detection efficiency. As we discuss below, other RCB stars finalstagesoftheanalysis.
can still be found in the ASAS-3 dataset and the Milleretal. First, we kept only stars brighter than V < 13 mag as we
(2012)algorithmcouldbeofgreatuseinfindingRCBstarsthat found it difficult to recognise RCB photometric declines for
areatthelimitofdetectability. fainterobjects,knowingthattheASAS-3limitingmagnitudeis
Surprisingly,despiteoursuccessinrevealingeightnewRCB V ∼ 14 mag.The brightnessdistributionof all sourcescata-
lim
stars,wedidnotrediscoverall29previouslyknownbrightRCB logued is presented in Figure 2. After the magnitude cut, a to-
starsthatcouldhavepotentiallybeenlistedintheACVS1.1cat- talof6038078objectsremained.Second,wematchedeachob-
alogue.Indeed,11of the29 knownRCB stars werenotrecov- ject with the 2MASS catalogue using a progressive matching
ered. This is easily explainedforfourof the stars: Y Mus, DY radius technique, from 5′′to 15′′. Only 2541 ASAS-3 objects
Cen, V739 Sgr and MV Sgr, as they did not show any bright- did not have an association with the 2MASS catalogue. Third,
nessvariationintheirrespectiveASAS-3lightcurves.However, we applied the same selection criteria that were applied to the
this is not the case for 7 other known RCBs, namely SU Tau, ACVS1.1catalogue,selectingallobjectswithanIRexcess,ex-
UX Ant,UW Cen, V348Sgr,GU Sgr,RY SgrandV532Oph. cept that this time we added a strict cut at H−K > 0.2 (see
They were not found even though they exhibited large ampli- Figure 3). About 67000 objects passed this last selection and
tude declinesorrecoveryphaseson theiroverallASAS-3 light theirrespectiveASAS-3lightcurves,spanning10years(2000-
curves and had significant IR excesses. In fact, those 7 RCBs 2010), were downloaded through the ASAS-3 Web interface2
are not even in the ACVS1.1 catalogue. This is due to the fact usinganautomaticscript.Usingmagnitudescalculatedfromthe
thattheACVS1.1cataloguecontainsonlystarswherevariability smallestphotometricaperture,wecalculatedforeachobjectthe
occurredduringthe first few yearsof ASAS-3 data, and there- total amplitudeof variation V , based on fourmeasurements
amp
forestarswithlargebrightnessvariationslaterinthesurveyhave at both maximumand minimum brightness, and the maximum
been missed. Table 1 illustrates this by showing the difference rate of decline in brightness, D , based on five consecutive
max
between the maximum amplitude listed in the ACVS1.1 cata- measurements.We were carefulto rejectlightcurveswith sys-
logue and the one taking into account the entire ASAS-3 light tematicphotometricissuesthatcanmimicalargedecline.These
curves.Thisoutcomeindicatesthattheremaystillbeanumber wererecognisableastheyoccurredonmanylightcurvesduring
of undiscovered RCB stars in the entire ASAS-3 south source thesamenight.Thenumberoffastdeclinesoccurringpernight
starcatalogue.Theanalysisofthiscatalogueisdescribedbelow. wasthereforecountedoveralllightcurvesanddatafromnights
presenting 10 or more fast declines were rejected. Finally, we
visuallyinspected5264objectswithV >1.5magandanad-
3.2.SearchforRCBstarsintheentireASAS-3southsource amp
ditional1710objectswithD > 0.02mag.day−1.Fromthese
starcatalogue(AnalysisB) max
candidates,60objectswerechosenforspectroscopicfollow-up.
About 12 million stars were catalogued and monitored by the This method enabled us to confirm 4 new RCB stars, named
ASAS-3 south survey. Thanks to the ASAS collaboration, we ASAS-RCB-9to-12.TheyarelistedinTable2andtheirspectra
obtained the source star catalogue, which contains the coordi- andlightcurvesarepresentedinFigures11and7respectively.
nates of each object, as well as a V magnitude truncated to its Ofthe28knownRCBstarslocatedintheskyareamonitored
integer value, calculated from the 30 brightest measurements. bytheASAS-3southsurvey(strictlybelowdec< +28◦,R CrB
Thiscataloguecontainsallobjectslocatedsouthofthecelestial is therefore not counted), we succeeded this time in rediscov-
equatorwithmorethan200measurements,andnorthernobjects,
up to +28◦, with at least 100 measurements. These constraints 2 ASASWebinterface:http://www.astrouw.edu.pl/asas/?page=aasc
4
P.Tisserandetal.:NewRCoronaeBorealisstarsdiscoveredintheASAS-3southdataset.
ering23 ofthem with this technique.Asmentionedin the pre-
viousanalysis, V739Sgr,DY Cen,MV SgrandY Musdonot
present any photometric declines and could not therefore have
beenfound.(WenotethatYMuswouldnothavebeendetected
regardlessasitpresentsonlyaweakIRexcess.)Thelastknown
RCB starnotrediscoveredisV348Sgrsimplybecauseitisnot
listed in the ASAS-3 south source catalogue that we used, de-
spitethefactthatitslightcurveisavailablefromtheASASWeb
siteandpresentsmultipledeclines.Ithasnotbeenlistedbecause
only 95 measurements were counted, which is lower than the
thresholdof200usedtoformthesourcecatalogue.
Among the three strong RCB candidates suggested by
Tisserand (2012a), onlytwo were selected by the analysesthat
we have just described: ASAS-RCB-7 with Analysis A and
ASAS-RCB-9 with Analysis B. The remaining suggested can-
didate is V581 CrA. Its ASAS-3 light curve is available from
theinteractiveASASWebservicewithmorethan200measure-
ments. It was not found by both analysis, despite the fact that
it has gone throughlarge photometricvariations(see Figure 8)
and its 2MASS colours indicate an IR excess that would have
beenselectedbyourcriteria.We obtainedaspectrumandcon-
firmed its membership of the RCB class of stars. We named it
ASAS-RCB-13. We did not find an entry for that star, nor in
theACVS1.1catalogue,norintheentireASAS-3southsource
catalogue. Many of its photometric measurements are fainter Figure3:J−HvsH−Kdiagramsusingthe2MASSmagnitudes
thanV∼14mag,andtherefore,notallofthempassedourqual- ofallobjectslistedintheASAS-3ACVS1.1variablestarscata-
ity requierements.The totalnumberof qualifiedmeasurements logue(blackdots).Theredlinesshowtheselectioncriteriaused
dropped below the threshold to select ASAS-RCB-13 in the inAnalysisA.ThebluepointscorrespondtotheRCBstarsthat
ASAS-3sourcecatalogue. were previouslyknown,the red pointsto the 23 newly discov-
The fact thatV348 Sgrand ASAS-RCB-13 are missing in- eredRCBstars.Thegreenlinesrepresenttheselectionboxused
dicatedtousthatotherRCBstarsmayyetberevealedfromthe to findnewDY Perstars. Thegreenpointscorrespondtothe 2
ASAS-3 survey. Despite the success of our method of finding newDYPerstars.
new RCB stars, we may also have missed some becauseof the
2MASS association method we used to comply with ASAS-3
astrometricuncertainty(upto15arcseconds),orbecauseofthe
ourobservationalruns3.Ofthe18remaining,8showclearfea-
strict limit applied on the amplitude and rate of decline, or the
tures due to the C and CN molecules and weak abundanceof
limit on the number of measurements used to validate the ob- 2
13Cat4744and6260Å.TheyarenamedASAS-RCB-14to-21
jectsthatweresubsequentlyanalysed.Finally,wenotethatsome
andarelistedinTable 2.TheirspectraarepresentedinFigure11
RCBs may also have remained faint duringthe period covered
andlightcurvesinFigures8and9.
andcouldhaverecoveredtheirbrightnessafter2010,andsome
None of these 8 new RCB stars would have been selected
RCBs may have been in a faint phase on the reference images
by the classical methods described in sections 3.1 and 3.2, for
usedtocreatetheASAS-3sourcecatalogue.
threereasons.First,themaximumtotalamplitudevariationob-
servedforallofthemislowerthantheselectioncriterionapplied
3.3.SearchforRCBstarsusingtheTisserand(2012a,b) of1.5mag.Second,fivepresentamaximumluminosityfainter
catalogue(AnalysesCandD) than V ∼ 13. Finally, the light curves of five of them, namely
ASAS-RCB-14,-15,-19,-20and-21,possesstoofewmeasure-
The analyses described above have a limiting magnitude of mentstobeselectedasvalidobjectsintheentireASAS-3source
V ∼13,butwithastrongdecreaseindetectionefficiencyforob- catalogueweanalysed.Indeed,theseobectsonlyhavebetween
jectsfainterthanV ∼12.TocompleteoursearchoftheASAS-3 10 and 50 measurements. More photometric follow-up will be
south catalogue, we decided to download and visually inspect needed for these RCBs in order to detect more aperiodic fast
theASAS-3lightcurvesofeachobjectlistedinthecatalogueof declinesandtodefinitelyconfirmtheirnature.
RCB candidate stars that was created from the 2MASS survey Ofthe12newRCBsthatwerediscoveredbyclassicalmeth-
and the WISE preliminary data release (Tisserand 2012a), and ods(ASAS-RCB-1to-12),onlytwowerenotfoundbyournew
expanded using the WISE final all-sky data release (Tisserand method: ASAS-RCB-1 and -8, the two warmest RCBs of the
2012b). The selection was made in two steps. First, all WISE series. It is also worth emphasizing that ASAS-RCB-13 and -
selected objects were cross-matched with the entire ASAS-3 21 were found only because they were listed in the Tisserand
source catalogue and the light curves of the matching candi- (2012a)catalogue.NoneofthemwaslistedintheASAS-3south
dateswerevisuallyinspected(AnalysisC).Second,incasesome sourcecatalogueweused,demonstratingthedifficultyofsearch-
RCBswerenotincludedintheASAS-3sourcecatalogue(aswas ingforRCBstarsinthatdataset.
foundto be the case with ASAS-RCB-13), we interrogatedthe
ASAS-3databasetodownloadthelightcurvesofallremaining 3 NamelyASASJ143647-6404.5,J173401-2951.8,J174317-1824.0
WISE objects selected (Analysis D). Overall, 22 objects were andJ183631-2059.2.ThelaststarwasclassifiedasanMtypestarand
selectedforspectroscopicfollow-up,but4weretoofaintduring isthereforemostlikelyaMiravariablestar(Houk1967).
5
P.Tisserandetal.:NewRCoronaeBorealisstarsdiscoveredintheASAS-3southdataset.
Clearly the release of the post-2010 data of the ASAS-3 the amplitude of variation, V > 1.5 mag. A total of 12421
amp
south survey and of course those from the ASAS-3 north sur- lightcurveswerevisuallyinspectedtosearchforthetypicalDY
veywillbethesourceofnewRCBdiscoveries.Dedicatedalgo- Per symmetric decline and 10 candidateswere kept forfurther
rithmssuchastheonedevelopedbyMilleretal.(2012)willbe photometricfollow-up.Ofthese10candidates,weconfirmed2
very useful, especially if they combine their search with near- new DY Per stars. Theirspectra show C and CN features and
2
andmid-IRbroadbandphotometry. clear 13C enhancementat λ ∼ 4744Å.Theyare named ASAS-
DYPer-1and-2andarelistedinTable2.ASAS-DYPer-1iseven
brighterby at least 1.2 mag in K than the prototypeDY Persei
3.4.SearchforDYPerstarsintheentireASAS-3south
(seeTable3).Theirrespectivelightcurvesandspectraare pre-
sourcestarcatalogue
sented in Figures 10 and 15. We present the classification of
OnlytwoDYPerstarswereknowninourGalaxy:theprototype the other8 candidatesin Table 5: 5 show a spectrum with TiO
DYPersei(Zacˇsetal.2007;Yakovinaetal.2009)andEROS2- featuressimilartoMiratypestars,2shownebula-likeemission
CG-RCB-2(Tisserandetal.2008).Recently,Milleretal.(2012) lines, and the eighth is a hot supergiant star with very narrow
havereportedfourpossiblenewonesandSmirnova(2012)dis- hydrogenlines.
cussed in details the reasons why two bright cool N-type car-
bonstars, V1983CygandV2074Cyg,shouldalso beascribed
to this type of stars. DY Per stars are known to be cooler than 4. Spectroscopicconfirmation
typical RCB stars (with V−I ∼ 2 mag, T ∼ 3000− 3500
eff RCB stars are of spectral type K to F with an effective tem-
K).UnlikeRCBswhichtypicallyhavesharpdeclinesandslow
perature ranging mostly between 4000 and 8000 K (Tisserand
recoveries,DY Per stars decline symmetricallywith the fading
2012a),withafewexceptionsbeinghotterthan10000K(V348
andrecoveryhappeningatsimilarrates.Someexamplesofthese
Sgr, DY Cen, MV Sgr and HV 2671) (DeMarcoetal. 2002).
brightness variations can be found in Alcocketal. (2001) and
The standard RCB atmospheric composition can be found in
Tisserandetal.(2004,2009),where13MagellanicDYPerstars
Asplundetal. (1997, Table 1). The main characteristics of the
werereported.Soszyn´skietal.(2009)havealsopresentedalist
spectrumarehydrogendeficiency,withH/Herangingmostlybe-
ofMagellanicDYPercandidateswiththeirrespectiveOGLE-III
tween 10−6 and 10−4 (V854 Cen is the exception with H/He∼
light curves. Many of them are interesting and will need spec-
10−2); carbonrichnessequivalentto C/He∼ 1%; and N enrich-
troscopicfollow-upforconfirmation.SincesofewDYPerstars
ment,withC/N∼1.3(Asplundetal.2000).Acompletesummary
areknown,thecharacterisationofthisclassofstarsisnot com-
and discussion of the usual abundances found in RCB stars in
plete.DYPerstarsareknowntobecoolcarbon-richsupergiant
thecontextofhotandcoldWD mergerscenarioscanbefound
stars,withapossiblehydrogendeficiency(Keenan&Barnbaum
inJefferyetal.(2011).
1997;Zacˇsetal.2007)andanisotopic13C/12Cratiohigherthan
HdCstarsarealsohydrogen-deficientandcarbon-richstars,
RCBs.Theyaresurroundedbycircumstellardustshellsandthey
but there are no photometric declines on record. Only 6 HdC
form new dust as seen in light curve variations. It is shown
starsarecurrentlyknownandtheireffectivetemperaturerangeis
in Tisserandetal. (2009) that there exists a relation between
between5500and7000K(Tisserand2012a).Mid-infrareddata
the maximum depth of the optical brightness variation and the
indicatethatthereis nosignof theexistenceofa circumstellar
brightnessoftheshellinmid-IR.
shell and therefore they do not produce carbon dust clouds as
DY Perstars mightbethe coolercounterpartof RCB stars,
RCBstarsdo.TheexceptionisHD175893whichhasacircum-
andtheremaythereforebeanevolutionaryconnectionbetween
stellardustshell(T ∼ 500K)buthasneverbeenseento un-
both classes of stars. A DY Per star’s circumstellar dust shell eff
dergoadeclinephase(Tisserand2012a,Figure12).HD175893
is on average hotter (T ∼ 800 − 1500 K) than an RCB’s
eff couldthereforeplayanimportantroleintheunderstandingofan
shell(Tisserandetal.2009).Itisinterestingtonotethatasharp
evolutionarylinkbetweenRCBandHdCstars.
peakwas observedat ∼11.3µm in the mid-IRspectrumofDY
Theopticalspectraof24knownRCBstars,3hotRCBsand
Persei itself, obtained by Garc´ıa-Herna´ndezetal. (2011b) with
the6knownHdCstarsarepresentedrespectivelyinFigures12
the SpitzerInfraredSpectrograph,indicatingthe possible pres-
and13,16and17.TheywereobtainedbytheSSO/2.3m/WiFeS
ence of polycyclic aromatic hydrocarbons (PAHs) in its shell,
and the SAAO/1.9m/Reticon/Unit spectrographs which cover
whicharenotobservedinthemajorityofRCBstars4.Thepres-
two different wavelength ranges, respectively 3400 to 9700 Å
enceofPAHsinDYPerseineedstobeconfirmedwithspectro-
scopicobservationatwavelengthsshorterthan10µm. and 3600 to 5400 Å. A summary and further details of these
spectracanbefoundinSection2andTable1.
From the DY Per characteristics that have just been men-
tioned, it is clear that DY Per stars are also very similar to or- TheopticalspectrumofanRCBstardependsonitstemper-
dinary carbon stars with an N type spectrum and further stud- atureandalsoonitsbrightnessphase.Atmaximumbrightness,
ieswillbeneededtoinvestigateiftheyareindeedtwodifferent aclassicalRCBstarwillshoweasilyidentifiablefeaturesdueto
typesofobject. theC2andCNmolecules(Bessell&Wood1983)andnosignof
We searched for new DY Per stars in the ASAS-3 south hydrogen(HαorCH),whileahotRCBstarpresentsaspectrum
dataset by inspecting visually a sample of objects selected in with many emission lines (Figure 16). During a decline phase,
a J−H versusH−K 2MASS colourdiagram(see the selected carbon features are still recognisable but a few emission lines
areainFigure3).DYPerstarsareknowntolooksimilartoor- start to emerge, with the NaI (D) lines being the more promi-
dinary carbon stars in broadband photometry.For objects with nentone(seethespectraofESAql,ASAS-RCB-13andASAS-
0.3 6 H−K 6 0.5, we added a supplementary constraint on RCB-19 for examples). A detailed discussion of RCB spectra
during a decline phase can be found in Clayton (1996). The
4 Two RCB stars, one hot and one warm, DY Cen and V854 Cen, absence of enhanced 13C is also a characteristic of RCB stars
showPAHfeaturesintheirrespectivespectra(Garc´ıa-Herna´ndezetal. (Pollardetal. 1994), even if few RCB stars do have detectable
2011b,a).ThesearealsothetwoRCBstarswiththehighestmeasured 13C: V CrA, V854 Cen, VZ Sgr, and UX Ant have measured
hydrogenabundance(Asplundetal.1998). 12C/13C <25 (Kilkenny&Marang1989; Rao&Lambert2008;
6
P.Tisserandetal.:NewRCoronaeBorealisstarsdiscoveredintheASAS-3southdataset.
Hemaetal. 2012). We attempted to estimate the abundance of 5. Maximummagnitude
13C using the 13C12C band-headat 4744 Å, and also the 13CN
Themedianofthemaximumbrightnessfoundforthenewlyde-
bandat6260Å(LloydEvans1991).ForallnewRCBstarswith
tectedRCBstarsisV ∼ 12.5.Thatisonly1.5magnitude
max,med
clearcarbonfeaturesintheirspectrum,itwaspossibletodeter-
brighterthan the magnitude limit of the ASAS-3 south survey.
minethelowcontentof13Cintheiratmosphere.Asmostofthe
These magnitudes are listed in Table 4. We cannot be certain
newRCBstarsfoundarelocatedintheGalacticbulge,thisstudy
thattherealmaximumopticalbrightnesswasreachedduringthe
wasmadedifficultbythestronginterstellarreddeningthatlimits
ASAS-3surveyfor9RCBs asitlastedlessthan20days.They
theamountoffluxinthebluepartofthespectrum.
areASAS-RCB-2, -7,-12,-13,-14,-15,-19,-20and-21.Five
of these (ASAS-RCB-14. -15, -19, -20 and -21) have very few
Overall, we systematically followed up spectroscopically measurementsintheirlightcurves.AfurthergroupoffiveRCBs
104stars:23ofwhichwererevealedtobeRCBstarsand2oth- (ASAS-RCB-2,-12,-15,-19and-20)presentahighIRexcess,
ers to be DY Per stars. Their spectra are presented in Figures J−K > 4.5(seetable3andfigure3),whichindicatesthatthey
11,14and15.Theclassificationoftheremaining81objectsis areinaphaseofhighdustproductionactivity.
givenin Tables5 and6. The two main families of rejected ob- AcomparisonofthemaximumASAS-3Vmagnitudesofthe
jectsaretheMiravariablesandTTauristars.Miraspresentlarge 31knownGalacticRCB starslistedinTable 1andthe23new
photometricvariationsandaspectrumrichinoxygen.Weclas- RCBsisshowninFigure4,beforeandaftercorrectingforinter-
sifiedthemusingtheMgiantspectralibraryprovidedbyBessell stellar extinction.The interstellarextinctionA was calculated
V
(1991). T Tauri stars present hydrogen-richspectra with emis- using the Schlegeletal. (1998) E(B−V) reddening map with
sionlinesoveralargerangeoftemperatures(9rejectedobjects 4-pixelsinterpolationanda coefficientR = 3.1.Interestingly,
V
are classified as Orion type objects in the SIMBAD database). theapparentmaximumbrightnessofthenewRCBsisfainteron
Nine other stars presenting carbon features were not consid- averagethan the already knownRCBs. Aftercorrectionforin-
ered as RCB stars. They were rejected because they presented terstellarextinction,however,thisdifferenceisstronglyreduced
clearHα emissionin threecases and/orhighabundanceof 13C as the new RCBs are affected on average by a stronger inter-
for the 6 others. The latter group passed the selection criteria stellar reddening(see Figure 4, top-right).Overallthe Galactic
for spectroscopic follow-up because the IR selection criterion, RCBs’ maximummagnitudepeaksat V ∼ 9.8 magwhich
ASAS
H−K > 0.7mag,wasintentionallydefinedso asnottobetoo iswithintherangeexpectedforRCBstarslocatedatanaverage
restrictive. They are located in the continuation of the classi- distanceofD ∼ 8.2kpc5,thedistanceoftheGalactic centre
GC
cal giant branch evolutionary track in the J−H vs H−K dia- (Natafetal.2012).Indeed,withanabsolutemagnituderanging
graminsidetheselectioncriteriaareaofDYPerstarsshown in between −5 6 M 6 −3.5 (see Tisserandetal. 2009), RCBs
V
Figure3.Theirlightcurvesshowvariationsupto2mag,butwith locatedatthatdistancewouldhaveanapparentVmagnitudebe-
noclearsignsofafastdecline.Astheyallpresentlargephoto- tween9.6and11.1mag.
metric oscillations of ∼0.8 mag amplitude and their spectra do
notshowclearsignsofthepresenceofhydrogen,theyshouldbe
consideredasDYPerstarcandidates. 6. DistributionofGalacticRCBstars
The small number of known RCB stars and the biases af-
Stars that were consideredas RCB candidatesin the litera- fecting their discovery in different surveys have prevented us
turewerealsofollowedup(Clayton1996;Harrisonetal.1997; from obtaining a clear picture of their true spatial distribution.
Clayton2012).WeconfirmedIRAS1813.5-2419andV391Sct, Iben&Tutukov (1985) reported a scale height of h ∼ 400 pc
butrejectedfiveotherstars:BLCirandGMSerareMiras,with assumingM =−5andconcludedthatRCBsarepartofanold
bol
the spectral type M6 and M7 respectively;V1773 Oph is an F disk-likedistribution.However,Cottrell&Lawson(1998)noted
star;V1860SgrisagiantG6IIIstar;andV2331SgrisanSstar thattheHipparcosvelocitydispersionofRCBstarsissimilarto
thatpresentsweakTiOandCNfeatures,butnoZrOband-head. that of other cool hydrogen-deficientcarbon stars and extreme
heliumstars,suggestingthatRCBstarsmighthaveabulge-like
Among the 23 new RCB stars, 4 present a spectrum with distribution. Drilling (1986) and Zaniewskietal. (2005) came
manymetallic absorptionlines,particularlyC I, indicatingthat to a similar conclusion. Recently, Tisserandetal. (2008) also
their effective temperature is relatively high, T > 7500 K. found that the majority of Galactic RCB stars seem to be con-
eff
They are ASAS-RCB -8, -10, -12 and V391 Sct. For the other centratedin the bulgebutwith the surprisingpeculiarityofbe-
newRCBs,onecanusethetripletabsorptionlinesduetoionised ing distributed in a thin disk structure (61 < hRCB < 246 pc,
bulge
calcium (λ ∼ 8498, 8543 and 8662 Å). Indeed, Richer (1971) 95%c.l.).IntheLargeMagellanicCloud,RCBsaredistributed
showsthattheintensityoftheselinesisagoodindicatorofcar- mostlyalongthebar(Tisserandetal.2009,Figure2).
bonstarstemperature;thecoolerthetemperature,theweakerthe The growing number of known RCBs allows us to form a
lines.Thiscouldbeanimportanttoolasmostofthespectraare clearerpictureoftheirGalacticdistribution.Indeed,theirmax-
stronglyreddened,butitshouldbeusedwithcautionasweare imumapparentmagnitude(Section5)andtheirspatialdistribu-
notcertainthatthisobservationisapplicabletothewarmerand tion(Figure1)indicatethatGalacticRCBstarsdoinfactahave
specific carbonstars thatare RCBs. In the spectra presented in bulge-likedistribution.
Figures11and14,onecanseparatethe19remainingRCBstars
into3groupsofincreasingtemperaturebasedonthestrengthof
7. Spectralenergydistribution(SED)
theCa-IItripletabsorptionlines: Weak:ASAS-RCB-4, -5,-13,
-14,-19;Moderate:ASAS-RCB-2,-6,-7,-15,-16,-17,-18and We attempted to estimate the photosphere temperature for the
-21 ; Strong: ASAS-RCB-1, -3, -9, -11, -20 and IRAS 18135- newly found RCBs by fitting simple blackbodies to the series
2419. We also note that the 2 DY Per stars do not present any ofoptical,infraredandmid-infraredmeasurementscollectedby
Ca-IItripletabsorptionlines,indicatingthattheyareindeedcool
carbonstars(seeFigure15). 5 Itcorrespondstoadistancemodulusof14.6mag.
7
P.Tisserandetal.:NewRCoronaeBorealisstarsdiscoveredintheASAS-3southdataset.
Figure4:Top-left:DistributionofthemaximumASAS-3VmagnitudeobservedfortheknownGalacticRCBstars(red,seeTable1)
and the 23 new RCB stars (blue, see Table 4). Bottom-left: Same distribution but corrected for interstellar extinction using the
Schlegeletal.(1998)reddeningmapandR =3.1.(ASAS-RCB-9wasnotplottedhereasweconsidereditsreddeningvaluetobe
V
overestimatedandnotvalid.)Top-right:ReddeningE(B−V)forthetwolistsofobjects.
the following surveys: DENIS (Epchteinetal. 1994), 2MASS 1. If δJ = J − J > 0.2 mag: the epoch during the
2MASS DENIS
(Skrutskieetal. 2006), WISE (Wrightetal. 2010) and IRAS 2MASS measurements was considered to have been taken
(Helou&Walker1988).TheobservedmaximumASASVmag- undera higher dust extinction phase than the DENIS mea-
nitudeandtheIRDENISand2MASSmagnitudesthatweused surement.OnlythethreeuncorrectedDENISmagnitudes(I,
arelistedrespectivelyinTables4and 3.RCBstarphotospheres JandK)wereused.
areknowntohaveatemperaturebetween4000and8000K,with 2. If δJ = J −J < −0.2 mag:the epoch during the
2MASS DENIS
afewhotRCBshavingatemperaturehigherthan10000K,while DENISmeasurementswasconsideredtohavebeentakenun-
DYPerphotosphereshaveatemperaturebetween3000and3500 derahigherdustextinctionphasethanthe2MASSmeasure-
K.Thespectralenergydistribution(SED)of34knownRCBsare ment.The2MASSJ,HandKmagnitudeswereusedaswell
presentedinTisserand(2012a)andsomehavehadtheireffective astheDENISImagnitudecorrectedbyacarbonextinction
temperaturedeterminedfromspectralsynthesisanalysesapplied law(Groenewegen1995).ThatisIcorr = I −Acarbon,
DENIS DENIS I
to high resolution spectra (see Asplundetal. 2000). RCBs are withAcarbon =1.66×Acarbon =1.66×δJ.
I J
warmerthanclassical carbon-richstars, whichhavea tempera- 3. If J and J agrees within ±0.2 mag, only the
2MASS DENIS
turerangingmostlybetween2000and3000K.Classicalcarbon- 2MASS J, H and K magnitudes were used as well as the
richstars’ SED aredescribedin detailbyBergeatetal.(2001). uncorrectedDENISImagnitude.
ThereconstructedSED of thenewly discoveredRCBs are pre- 4. IfnoDENISmeasurementsexist,onlythe2MASSJ,Hand
sented in Figure 5. Three major issues prevented us from ob- Kmagnitudeswereused.
tainingstrongconstraintsonphotospheretemperatures:theun-
From Figure 5, it appears clearly that the DENIS and
synchronisedmeasurements,highinterstellarextinction andthe
2MASS measurements of, respectively, ASAS-RCB-12 and
unknownrealmaximumbrightnessforsomeoftheRCBs.
ASAS-RCB-1,-7,-15,-19,-20,IRAS1813.5-2419andASAS-
DYPer-1wereobtainedduringa declinephaseofluminonisity,
Indeed, except for ASAS-RCB-2, -10 and -16, the DENIS whendustcloudsobscuredthephotosphere.We cannotcorrect
and2MASSepochs(listedinTable3)werealltakenbeforethe forthis effect and thereforedid not use these magnitudesto fit
startoftheASAS-3survey.WedonotthereforeknowtheRCBs’ theSED.
luminosityphase(declineormaximum?)atthetimeoftheseIR The interstellar extinction correction was applied for all
observations.Thisdemonstratesthenecessityoflong-termmon- optical and infrared measurements following the reddening
itoring.Nevertheless,we attemptedto overcomethislimitation E(B−V) values listed in Table 4. The relative extinction co-
by using either the DENIS J and H or the 2MASS J, H and K efficients A/A , A /A , A /A and A /A determined by
I V J V H V K V
magnitudestoconstructtheSEDsaccordingtofourrules: Cardellietal. (1989, Table 3) were used. Their values are re-
8
P.Tisserandetal.:NewRCoronaeBorealisstarsdiscoveredintheASAS-3southdataset.
spectively0.479,0.282,0.19and0.114.TheE(B−V)reddening alogueofStephenson(1973).Ithasalsorecentlybeenfound
valuelistedforASAS-RCB-9isoverestimatedandnotvalid.We tobeanRCBstarbyMilleretal.(2012).
fittedittobe∼1.0magbyassumingthephotospheretemperature – ASAS-RCB-4:AlsoknownasGVOph,ithasrecentlybeen
tobe5000K. foundtobeanRCBstarbyMilleretal.(2012).
Asalreadydiscussedinsection5,wenotethatitisuncertain – ASAS-RCB-5: The only newly discovered RCB that has
whetherornottherealmaximumopticalbrightnesswasreached been monitored during phase 2 of the ASAS survey. The I
duringtheASAS-3surveyfor9RCBs.Therefore,themaximum bandlightcurvecoversatimerangebetweenHJD∼2450500
ASASVmagnitudesusedintheSEDofASAS-RCB-2,-7,-12, and2451500,andshowsonedecline.Itreachesamaximum
-13,-14,-15,-19,-20and-21shouldbe consideredasa lower magnitudeofI ∼10mag.
max
limit. – ASAS-RCB-6: Identified as a variable star by Luyten
TheresultsareshowninFigure5.Ourbestattempttofittwo (1932).Milleretal.(2012)hasalsofoundrecentlythatitis
blackbodiesovertheSEDusingonlyconvincingmeasurements indeed an RCB star. Interestingly, its spectrum shows fea-
is also presented. Overall, the circumstellar shell temperatures turesduetoC butnoneduetoCN,andweakCa-IIabsorp-
2
arewellconstrained,thankstotheWISEand/orIRASsurveys. tionlinesindicatingaloweffectivetemperature.
Evidenceofshellbrightnessvariationisalsovisible,especially – ASAS-RCB-7:Also knownasV653 Sco, itwas misclassi-
for ASAS-RCB-7, -11 and IRAS 1813.5-2419. The errors on fied as a Mira star in the GeneralCatalog of VariableStars
theshellfittedtemperaturesareintheorderof50K.Thephoto- (GCVS).It wasreportedas a variablestarforthe first time
spheretemperaturedependsstronglyontheextinctioncorrection byKukarkinetal. (1971) witha totalamplitudelargerthan
appliedandonwhetherornotthemeasurementswerereallyob- 2mag.
tainedduringa maximumbrightnessphase.As theirrespective – ASAS-RCB-8: Only one decline was observed during the
spectrashow,ASAS-RCB-8,-10anV391Sctwerefoundtobe 10-yearlongASAS-3survey.With0.003mag.day−1,thede-
hotRCBs(T >7500K).SixRCBs,i.e.ASAS-RCB-4,-5,-6, cline rate is about 10 times slower than to classical RCB
eff
-11, -16 and -18, appearto have a relatively cool photosphere, stars.ItsspectrumissimilartothewarmRCBstarUXAnt.
with 4000 < T < 5000 K, while another three, i.e. ASAS- – ASAS-RCB-9:AlsoknownasIONor,itwasrecentlyiden-
eff
RCB-2, -3 and -17,seem warmerwith 5000 < T < 6000K. tifiedasanRCBstarbyMilleretal.(2012).Itwasmisclassi-
eff
Also,asexpected,thetemperatureofbothDYPerstarsliesbe- fiedasaMirastarintheGCVS.WenotethatASAS-RCB-9
tween3000and4000K.Thephotospheretemperaturesforthese wasidentifiedasaTycho-2starhavinganinfraredexcessin
14starsareconsideredtobeafairestimate,butweunderlinethat theMSXPointSourceCatalogue(Clarkeetal.2005).With
theyareonlyindicativeandshouldbeusedwithcaution.Wedid V ∼10.8mag,itisthebrightestofallthenewRCBstars
max
notsucceedindeterminingaconsistenttemperatureestimatefor reportedinthisarticle.
theelevenremainingRCB stars, i.e.ASAS-RCB-1, -7,-9,-12, – ASAS-RCB-10: Its ASAS-3 light curve shows only one
-13,-14,-15,-19,-20,-21andIRAS1813.5-2419. large and rapid decline of ∼2.5 mag around MJD∼53700
Interestingly,wefoundthatASAS-RCB-18wasdetectedby days.ItsspectrumissimilartothewarmRCBstarUXAnt,
the IRAS survey at ∼65 µm. This measurement suggests that butaffectedbystrongdustreddening.
there exists a second circumstellar shell of cooler temperature, – ASAS-RCB-11: Also known as CGCS 3965, it was clas-
∼80±30K,whichmaybetheremnantofanolderevolutionnary sified for the first time as a carbon star in the cool carbon
phase. starscatalogueofStephenson(1973).ItsASAS-3lightcurve
showsfourdeclinephasesin10yearsandsomephotometric
oscillationatmaximumbrightness.
8. Individualstars
– ASAS-RCB-12: It was recently recognized as a probable
RCB star by Kazarovets&Arkhipova(2011). It has a high
Inthissection,wediscusseachnewlydiscoveredRCBandDY
IRexcess,withJ−K > 4mag,indicatingthepresenceofa
Per star. Their DENIS and 2MASS IR magnitudes are listed
brighthotcircumstellarshell.FromitsASAS-3lightcurve,
in Table 3 and their recorded ASAS-3 maximum V magni-
ASAS-RCB-12 seems to have been in a decline phase for
tudesaregiveninTable4withanestimationoftheirreddening.
morethan5yearsandslowlyrecoveredtomaximumbright-
Unfortunately,theDENISand2MASSsurveyswerecompleted
nessin∼2010.ItsspectrumshowsstrongfeaturesduetoC
atthetimewhentheASAS-3southsurveystartedandtherefore 2
butonlyveryweakfeaturesduetoCN.
we do not knowthe brightnessphase of the 23 new RCB stars
– ASAS-RCB-13:Also knownas V581CrA, itwas listed as
atthoseepochs.Interestingly,wenotethat6outof the23new
avariablestarbyKukarkinetal.(1968).Itwasidentifiedas
RCBstarswerealreadycataloguedascarbonstarsinthelitera-
a probable RCB star by Tisserand (2012a) because it was
ture.TheyareASAS-RCB-1,-3,-11,-16,-20and-21.
found to be misclassified as a Mira variable in the GCVS,
– ASAS-RCB-1: Also known as V409 Nor, it was classified whichdidnotmatchitsWISEmid-infraredcolour.Itsspec-
forthefirsttimeasacarbonstarinthecoolcarbonstarscat- trumshowsweakCNfeaturescomparedtotheC2.
alogueofStephenson(1973).Itwasrecentlyrecognizedasa – ASAS-RCB-14:Has alsobeendetectedbytheIRAS satel-
probableRCBstarbyKazarovets&Samus(2011).Itsspec- lite.ItsASAS-3lightcurvehasonlyasmallnumberofmea-
trumshowsstrongCa-IIabsorptionlinesindicatingthatitis surementsaround MJD∼53150 days, when ASAS-RCB-14
awarmRCBstar.Onlyonedeclineof∼2magisobservable was recovering its brightness. It reaches a maximum mag-
initsASAS-3lightcurve. nitudeofVmax ∼ 13.2beforeundergoingdeclineforalong
– ASAS-RCB-2: Hasa high IR excess, with J−K > 4 mag, period,indicatingaphaseofhighdustproductionactivity.
indicating the presence of a bright hot circumstellar shell. – ASAS-RCB-15: Has been identified as a variable star by
It remained fainter than 15th magnitude for more than 3.5 Terzanetal. (1997). We foundthat it has a high IR excess,
years,between2001and2004. with J−K > 4 mag, indicating the presence of a bright
– ASAS-RCB-3:AlsoknownasCGCS3744,itwasclassified hot circumstellar shell. Its ASAS-3 light curve has only a
forthefirsttimeasacarbonstarinthecoolcarbonstarscat- small number of measurements showing a recovery phase
9
P.Tisserandetal.:NewRCoronaeBorealisstarsdiscoveredintheASAS-3southdataset.
Table 1: Known Galactic RCB stars: ASAS-3 id and variabilityas observedin the ASAS-3 surveybetween December2000 and
October2009
Name ASAS-3id Numberof Maximumamplitude(mag) Maximum Spectrum
declines ASAS-3 ACVS1.1 brightness date
observed (2000-2009) (V ) V (mag) SSO/WiFeS SAAO/Reticon
amp max
Galactic
SUTau J054904+1904.3 3 4.5 9.8
UXAnt J105709-3723.9 3 2.8 12.0 13-12-2010 09-02-1989
UWCen J124317-5431.7 2 4.2 9.3 30-07-1991
YMus J130548-6530.8 none 10.25 17-07-2010 16-06-1990
DYCen J132534-5414.7 none 13.2 17-07-2010
V854Cen J143450-3933.5 9 3.9 3.91 7.0 23-07-2011 18-06-1990
SAps J150924-7203.8 3 4.5 4.61 9.7 17-07-2010 04-08-1989
RCrB J154834+2809.4 1 7.1 7.51 5.8
RTNor J162419-5920.6 1 2.0 0.19 10.1 23-07-2011 17-05-1990
RZNor J163242-5315.6 4 3.2 0.63 10.3 a 04-08-1991
V517Oph J171520-2905.6 8 3.0 1.30 11.4 23-07-2011 29-06-1989
V2552Oph J172315-2252.0 3 3.1 2.23 10.9 17-07-2010
23-07-2011
V532Oph J173243-2151.6 3 3.0 11.8
OGLE-GC-RCB-2 J175357-2931.7 1 0.7 0.83 12.6 b
EROS2-CG-RCB-3 J175829-3051.3 none 14.0 c
V1783Sgr J180450-3243.2 1 3.3 2.61 10.7 16-07-2010 31-08-1994
WXCrA J180850-3719.7 2 3.5 1.94 10.5 17-07-2010 17-08-1993
V739Sgr J181311-3016.2 none 12.9 16-07-2010 13-06-1990
V3795Sgr J181325-2546.9 1 1.2 1.20 11.0 16-07-2010 10-06-1994
VZSgr J181509-2942.5 2 4.3 1.12 10.3 17-07-2010 10-06-1994
RSTel J181851-4632.9 4 3.4 0.22 9.85 16-07-2010 10-06-1994
GUSgr J182416-2415.4 2 4.2 10.4 23-07-2011 11-06-1994
V348Sgr J184020-2254.5 6 3.0 12.2 15-07-2010
MVSgr J184432-2057.2 none 13.5 15-07-2010
FHSct J184515-0925.5 3 2.0 1.02 12.5 24-07-2011 29-06-1989
VCrA J184732-3809.6 1 2.8 0.54 9.7 23-07-2011 11-06-1994
SVSge J190812+1737.7 2 2.3 2.62 10.5 24-07-2011
V1157Sgr J191012-2029.7 9 2.8 2.17 11.5 15-07-2010 30-08-1994
24-07-2012
RYSgr J191633-3331.3 2 6.0 6.3 15-07-2010 17-06-1990
ESAql J193222-0011.5 10 3.2 2.05 11.65 24-07-2011
UAqr J220320-1637.6 2 3.8 2.41 11.2 15-07-2010 16-06-1990
aRZNorwasinadeclinephaseduringourobservation.Itisworthmentioningthatthereexistsanearby
Astar(V∼15mag).ThepresenceofthisbluestarexplainstheunexpectedchangeofcolourinRZNorduringadecline.
bSeeTisserandetal.(2011).cSeeTisserandetal.(2008).
Note1:DYPer,XXCam,ZUMI,NSV11154,V482CygandUVCasareoutsidetheskyareamonitoredbytheASAS-3southsurvey.
Note2:The17followingknownGalacticRCBsaretoofainttohavebeencataloguedandmonitoredbytheASAS-3southsurvey:
MACHO-135.27132.51,-301.45783.9,-308.38099.66and-401.48170.2237fromZaniewskietal.(2005),
EROS2-CG-RCB-1and-4to-14fromTisserandetal.(2008)andOGLE-GC-RCB-1fromTisserandetal.(2011).
followedbyadeclinearoundMJD∼54550days.Itreachesa selectedthisstarforspectroscopicfollow-upbecauseofthe
maximum magnitudeof V ∼ 14.3mag. ASAS-RCB-15 fast decline observed at MJD∼53600 days. It was selected
max
demonstratesthesuccessofthisstudyinextendingthelimits in the first place because of its mid IR WISE colours and
ofdetectability. 2MASS IR excess indicating the presence of a hotcircum-
– ASAS-RCB-16: Also knownas CGCS 3790,it was classi- stellar shell. It has also been detected by the IRAS survey.
fiedforthefirsttimeasacarbonstarinthecoolcarbonstars Themeasurementat∼65µmsuggeststhatthereexistsacold
catalogue of Stephenson (1973). It was also identified as a remnantaroundASAS-RCB-18thatmayberelatedtoanold
variable star by Plaut (1968). We can be confident that we evolutionaryphase.A∼80±30Kblackbodywasfittedtothis
measureditsrealmaximummagnitude,V ∼12.8mag,as star(seeFigure5).
max
itlastedabout1000days. – ASAS-RCB-19:Also known as NSV 11664, it was identi-
– ASAS-RCB-17:Wasalreadyidentifiedasavariablestarby fied as a variable star by Hoffmeister (1966). It has a high
Terzanetal.(1982).Onefastdeclineof∼1.2magisobserv- IRexcess,withJ−K > 4mag,indicatingthepresenceofa
ableonitsASAS-3lightcurveatMJD∼52700days.Wecan brighthotcircumstellarshell.
beconfidentthatwemeasureditsrealmaximummagnitude, – ASAS-RCB-20:AlsoknownasPTAql,itwasalreadyiden-
V ∼13.1mag,asitlastedabout700days. tifiedasacarbonstarbyGuglielmoetal.(1993).Itwasmis-
max
– ASAS-RCB-18:ItsASAS-3lightcurvepresentsfewvaria- classifiedas aMira starinthe GCVS. We notethatithasa
tionswithanamplitudeof∼1mag,butremainsoverallrela- high IR excess, with J−K > 4.5 mag, indicating the pres-
tivelystableasitdoesnotundergolargedeclinephases.We
10