Revealing evolved massive stars with Spitzer, WISE and SALT

0.58 MB·

by  A. Kniazev

Revealing evolved massive stars with Spitzer, WISE
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and SALT
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n
a
J
2 A. Kniazev∗†
1 SouthAfricanAstronomicalObservatory,POBox9,7935,SouthAfrica
SouthernAfricanLargeTelescopeFoundation,POBox9,7935,POBox9,7935,SouthAfrica
]
R E-mail: [email protected]
S
V. Gvaramadze
.
h
SternbergAstronomicalInstitute,LomonosovMoscowStateUniversity,Moscow119992,Russia
p
SpaceResearchInstitute,RussianAcademyofSciences,Profsoyuznaya84/32,117997Moscow,
-
o Russia
r
t E-mail: [email protected]
s
a
[
Wepresenttheresultsofopticalspectroscopicobservationsof54candidateevolvedmassivestars

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revealedthroughthedetectionofmid-infrarednebulaeofvariousshapessurroundingthemwith
v
9 theSpitzerSpaceTelescopeandWide-fieldInfraredSurveyExplorer.Theseobservations,carried
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out with the Southern African Large Telescope (SALT) in 2010–2015, led to the discovery of
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2 abouttwo dozensemission-linestars, of which15 starswe classify ascandidateluminousblue
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variables (cLBVs). Spectroscopic and photometric monitoring revealed significant changes in
.
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thespectraandbrightnessoffournewlyidentifiedcLBVs, meaningthattheyarenewmembers
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6 of the class of bona fide LBVs. We present an updated list of the Galactic bona fide LBVs.
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Currently, this list contains eighteen stars, of which more than 70 per cent are associated with
:
v circumstellarnebulae.Wealsodiscoveredaveryrare[WN]star–thecentralstaroftheplanetary
i
X nebulaAbell48,andaWN3starinaclose,eccentricbinarysystemwithanO6VstarintheLarge
r MagellanicCloud–thefirst-everextragalacticmassivestaridentifiedviadetectionofacircular
a
shellaroundit. MostoftheremainingtargetsaretentativelyclassifiedasOB,AandMstars.
SALTScienceConference2015-SSC2015-
1-5June,2015
StellenboschInstituteofAdvancedStudy,SouthAfrica
∗Speaker.
†ThispaperusesobservationsmadewiththeSouthernAfricanLargeTelescope(SALT).
(cid:13)c Copyrightownedbytheauthor(s)underthetermsoftheCreativeCommons
Attribution-NonCommercial-NoDerivatives4.0InternationalLicense(CCBY-NC-ND4.0). http://pos.sissa.it/
RevealingevolvedmassivestarswithSpitzer,WISEandSALT A.Kniazev
1. Introduction
Stars more massive than ≥20M experience the short-lived luminous blue variable (LBV)
⊙
stage [4], whichamongother evolutionary stages ofmassivestars isthemostinteresting inobser-
vational manifestations and perhaps the most important in the evolutionary sense [20, 25, 34, 8].
During this stage a massive star exhibits irregular spectroscopic and photometric variability on
time-scales from years to decades or longer, which is reflected in changes of the stellar type from
late O/early B supergiants to A/F-type ones (see e.g. [29, 7]) and changes in the brightness by
several magnitudes. At the brightness maximum, LBVs could be confused with supernovae (e.g.
[6, 35]), and it is believed that some LBVs could be the direct progenitors of supernovae (e.g.
[24, 8]). The LBV stars experience episodes of enhanced, sometimes eruptive, mass loss, so that
mostofthem(see[3,22]andTable2)aresurroundedbycompact(∼0.1−1pcindiameter)shells
withawiderangeofmorphologies (e.g. [27,39,12]).
TheLBVphenomenon isstillill-understood, whichismostly because theLBVstarsarevery
rare objects. The recent census of Galactic confirmed and candidate LBVs (cLBVs) presented in
[36] lists only 13 and 25 stars, respectively. The discovery of additional LBVs would, therefore,
be of great importance for understanding their evolutionary status and their connection to other
massivetransientstars,aswellasforunveilingthedrivingmechanism(s)oftheLBVphenomenon.
Detection ofLBV-likeshells can beconsidered asanindication that their associated stars are
massive and evolved, and therefore could be used for the selection of candidate massive stars for
follow-up spectroscopy. Becauseofthehugeinterstellar extinction intheGalacticplane, themost
effective channel forthedetection ofcircumstellar shellsisthrough imagingwithmoderninfrared
(IR) telescopes. Application of this approach using the Spitzer Space Telescope and Wide-field
InfraredSurveyExplorer(WISE)resultedinthediscoveryofhundredsofsuchshellswhosecentral
stars could be LBVsor other types of evolved massive stars ([12, 37, 26, 13]). Indeed, follow-up
opticalandIRspectroscopy ofthesecentralstarsledtothediscoveryofdozensofnewcLBV,blue
supergiant and Wolf-Rayet (WR) stars in the Milky Way ([9, 10, 11, 12, 37, 38, 14, 30, 31, 2, 5,
15, 16, 17, 22, 18, 19, 23]). Because of reddening many of the central stars are very dim in the
optical,whichmakesinevitabletheuseof8–10-mclasstelescopesliketheSouthernAfricanLarge
Telescope (SALT).Herewereporttheresults ofoptical spectroscopy withtheSALTof54central
starsofcompactmid-IRnebulae discovered withSpitzerandWISE.
2. Observations
The SALT observations were carried out in 2010–2015 with the Robert Stobie Spectrograph
(RSS; [1]) in the long-slit mode. In most cases, the spectra covered the range of 4200−7300 Å.
The primary reduction of the data was done with SALTscience pipeline. After that, the bias and
gain corrected and mosaicked long-slit data werereduced in the waydescribed in[21]. Examples
of 1D finally reduced spectra of adozen emission-line stars are shown inFigure 1, whileFigure2
presents the mid-IR images of circumstellar nebulae associated with these stars. The list of all
observed targetsisgiveninTable1.
TosearchforpossiblespectroscopicandphotometricvariabilityofthenewlyidentifiedcLBVs,
weobtainedadditionalspectrawiththeSALTandperformedphotometricmonitoringofthesestars
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RevealingevolvedmassivestarswithSpitzer,WISEandSALT A.Kniazev
Figure1: TheobservedSALTspectraofadozenemission-linestarsfromoursampleofcandidateevolved
massivestarsrevealedwithSpitzerandWISE.
withthe76-cmtelescope oftheSouthAfricanAstronomical Observatory.
3. Results
We carried out optical SALT spectroscopy of 54 candidate evolved massive stars. The first
resultsofourobservingprogramwerepresentedin[14,32,16,17,18,22,19]. Table1summarizes
the (mostly preliminary) spectral classification of the observed targets. We detected about two
dozenofemission-linestars,ofwhich15starswereclassifiedascLBVs. Subsequentspectroscopic
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RevealingevolvedmassivestarswithSpitzer,WISEandSALT A.Kniazev
31:50.0 J104823-593226 44:00.0 J114418-624520/MN1 J115443-631331
11:00.0
-59:32:00.0 30.0
12:00.0
10.0
20.0 -62:45:00.0 13:00.0
30.0 30.0 -63:14:00.0
40.0
50.0 46:00.0 15:00.0
30.0
28.0 24.0 10:48:20.0 16.0 30.0 20.0 10.0 11:44:002.00.0 11:55:00.0 40.0 54:20.0
J131004-631130/MN7 16:30.0 J131043-631745/MN8 04:00.0 J154842-550742/MN30
10:30.0 05:00.0
17:00.0
-63:11:00.0 06:00.0
30.0 07:00.0
30.0
08:00.0
-63:18:00.0
12:00.0 09:00.0
30.0
30.0 -55:10:00.0
20.0 10.0 13:10:00.0 09:50.0 13:11:00.0 50.0 40.0 10:30.0 20.0 15:49:00.0 40.0 48:20.0
51:00.0 J161517-505219 J164316-460042/MN46 J164937-453559/MN48
-45:59:00.0 33:00.0
30.0 30.0 34:00.0
-46:00:00.0
-50:52:00.0 30.0 35:00.0
-45:36:00.0
30.0 01:00.0
30.0 37:00.0
53:00.0 02:00.0 38:00.0
30.0
30.0 25.0 16:15:20.0 15.0 10.0 05.0 30.0 20.0 10.0 16:43:00.0 16:50:00.0 50.0 40.0 30.0 20.0 49:10.0
26:30.0 J174359-302838/MN64 J184159-051539/MN84 40.0 J190624+082201/MN101
27:00.0 30.0
15:00.0
30.0 20.0
28:00.0 30.0 10.0
30.0
8:22:00.0
29:00.0
-5:16:00.0 50.0
30.0
40.0
-30:30:00.0
30.0 30.0
15.0 10.0 05.0 17:44:00.0 55.0 43:50.0 06.0 04.0 02.0 18:42:00.0 58.0 56.0 41:54.0 28.0 26.0 19:06:24.0 22.0
Figure 2: Mid-IRimagesof circumstellarnebulae aroundstars whose spectra are shown in Figure1. All
butoneofthese nebulaewere discoveredwith Spitzerat 24m m. ThenebulaaroundJ115443-631331was
discoveredwithWISEat22m m. Thenebulashownintherightpanelofthesecondrowcontainstwostars
nearthegeometricalcentre. Oneofthem(markedbyawhitecircle)isthecLBVJ154842-550742/MN30.
Thesecondone(markedbyablackcircle)istheWC9starJ154842-550755.Thecoordinatesareinunitsof
RA(J2000)andDec.(J2000)onthehorizontalandverticalscales,respectively.
and photometric monitoring of these stars allowed us to confirm the LBV status of four of them
(see Tables1 and [17, 22, 19, 23]). Figure3 shows the evolution of the spectrum of Wray16-137
(oneofthefournewlyidentifiedGalacticbonafideLBVs)in2011–2014. OnecanseethattheHeI
emission lines have almost disappeared, while numerous FeII emissions have become prominent.
These changes along with significant brightness increase of the star (by about 1 mag during three
years)indicate thatcurrently Wray16-137experiences anSDor-likeoutburst.
Wealso discovered arare WN-type central starof aplanetary nebula (Abell48), which isthe
secondknownexampleof[WN]stars[32]. ThankstothehighangularresolutionofSpitzerimages
(6 arcsec at 24m m), we detected a new circular shell in the Large Magellanic Cloud. Follow up
spectroscopy of its central star withSALT(and several other telescopes) resulted inthe discovery
of a new WR star in a close, eccentric binary system with an O6V star [16]. The majority of the
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RevealingevolvedmassivestarswithSpitzer,WISEandSALT A.Kniazev
Table1:Theobservedcentralstarsofmid-IRnebulaeandtheir(mostlypreliminary)spectralclassification.
ThestarswhosespectraandnebulaearepresentedinFigures1and2arestarred.
Object Name Type Object Name Type
(1) (2) (3) (1) (2) (3)
J045304-692352 BAT993a WN3b+O6Va J153856-563722 MN25 OB
J052848-705105 OB J154527-535602 MN26 OB
J052412-683011 LHA120N OB J154842-550742∗ MN30 cLBVi
J071810-265124 CD−264148 OB J154842-550755 WC9j
J091714-495502 M J161132-512906 MN40 OB
J100122-550046 OB J161517-505219∗ cLBVk
J103638-580012 OB J163239-494213 MN44 LBVl
J104823-593226∗ HD93795 A J164316-460042∗ MN46 cLBVm
J110340-592559 HD96042 OB J164937-453559∗ MN48 LBVn
J111428-611820 Wray15-780 OB J170723-395651 MN50 Mo
J114418-624520∗ MN1 cLBVb J171307-384734 CD−3811646 OB
J115443-631331∗ cLBV J172031-330949 WS2 cLBVp
J120058-631259 MN2 OB J173753-302311 OB
J124626-632427 WRAY17-56 PNc J173918-312424 MN59 OBq
J131004-631130∗ MN7 cLBV J174359-302838∗ MN64 cLBVr
J131028-621331 OB J174627-302001 OB
J131043-631745∗ MN8 cLBVd J180433-210326 HD313642 A
J131933-623844 MN10 OBe J180612-211745 MN74 OB
J132647-615924 M J180823-221939 ALS4684 OB
J133628-634538 WS1 LBVf J182721-132209 OB
J133654-632552 OB J183217-091614 OBs
J135015-614855 Wray16-137 LBVg J183528-064415 OB
J140707-652934 CPD−642731 O J184159-051539∗ MN84 cLBVt
J143111-610202 MN14 OB J184246-031317 [WN5]u
J151342-585318 MN17 OB J185404+033544 M
J151641-582226 MN18 B1Iah J190421+060001 MN100 OBv
J151959-572415 MN19 OB J190624+082201∗ MN101 cLBVw
Notes: a [16]; b classified as an Oe/WN in [37]; c originally classified as a PN in [28]; d classified as
an Oe/WN in [37]; e originally classified as an OB in [38]; f [14, 22]; g [17]; h [18]; i classified as a
Be/B[e]/LBVin[38]; j originallyclassifiedasaWC9in[38];k classifiedasastarintransitionfromAGB
toPNin[33]; l [19];m [12]; n [23]; o classifiedasaM1Iin[37]; p [14]; q originallyclassifiedasanOB
in [37]; r classified as a Be in [37] and as an OB in [38]; s classified as a BA in [38]; t classified as a
Be/B[e]/LBVin[37]andasacLBVin[31];u originallyclassifiedasaWN6in[37]andthenre-classified
asa[WN5]in[32];vclassifiedasaFGin[38];w classifiedasB[e]/LBVin[38]andasacLBVin[31].
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RevealingevolvedmassivestarswithSpitzer,WISEandSALT A.Kniazev
Figure 3: Evolution of the (normalized) spectrum of the new Galactic bona fide LBV Wray 16-137 in
2011–2014(adoptedfrom[17]).
Table 2: Currentcensusof theGalactic bonafideLBVs. Theobjectswith detectedcircumstellarnebulae
arestarred.
HRCar∗ h Car∗ AGCar∗ Wray15-751∗
[GKM2012]WS1∗ Wray16-137∗ [GKF2010]MN44∗ Cl*Westerlund1W243
[GKF2010]MN48∗ HD160529 GCIRS34W [MMC2010]LBVG0.120−0.048∗
qF362 HD168607 MWC930∗ G24.73+0.69∗
AFGL2298∗ PCyg∗
remaining targetsweretentatively classifiedasOB,AandMstars.
Finally,wepresentinTable2thecurrentcensusoftheGalacticbonafideLBVs(eighteenstars
intotal). Theobjectswithdetectedcircumstellarnebulaearestarred. Asfollowsfromthetable,72
per cent of the Galactic confirmed LBVsare associated withnebulae. This provides further proof
that the detection of compact mid-IR shells is a powerful tool for identifying (candidate) LBVs.
Searches for new mid-IR nebulae continue, with further discoveries of LBVs and other related
starsanticipated.
Acknowledgments
TheobservationsreportedinthispaperwereobtainedwiththeSALTprograms2010-1-RSA_OTH-001,
2011-3-RSA_OTH-002,2013-1-RSA_OTH-014,2013-2-RSA_OTH-003and2015-1-SCI-017. AYK
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RevealingevolvedmassivestarswithSpitzer,WISEandSALT A.Kniazev
acknowledges support from the National Research Foundation (NRF) of South Africa. VVG ac-
knowledgestheRussianScienceFoundationgrant14-12-01096. Thisworkwaspartiallysupported
by the Russian Foundation for Basic Research grant 16-02-00148. Weare grateful to referees for
usefulsuggestions onthemanuscript.
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