Large proper motion of the Thorne-\.Zytkow object candidate HV 2112 reveals its likely nature as foreground Galactic S-star

0.36 MB·

by  Thomas J. Maccarone

Mon.Not.R.Astron.Soc.000,1–5() Printed1March2016 (MNLATEXstylefilev2.2)
˙
Large proper motion of the Thorne-Zytkow object
candidate HV 2112 reveals its likely nature as foreground
Galactic S-star
6 Thomas J. Maccarone
1
Department of Physics, Box 41051, Science Building, Texas Tech University, Lubbock TX, 79409-1051, USA
0
email:[email protected]
2
Selma E. de Mink
b
e Astronomical Institute “Anton Pannekoek”, University of Amsterdam, Postbus 94249, 1090 GE Amsterdam, The Netherlands
F email:[email protected]

9
2

]
R ABSTRACT
S UsingtheSouthernProperMotion(SPM)catalog,weshowthatthecandidateThorne-
. Z˙ytkow object HV 2112 has a proper motion implying a space velocity of about
h
p 3000kms−1if the object is located at the distance of the Small Magellanic Cloud.
- The proper motion is statistically different from that of the SMC at approximately
o 4σ in SPM, although the result can drop to about 3σ significance by including the
r
UCAC4 data and considering systematic uncertainties in addition to the statistical
t
s ones. Assuming the measurement is robust, this proper motion is sufficient to exclude
a
itsproposedmembershipoftheSmallMagellanicCloudandtoargueinsteadthatitis
[
likely to be a foreground star in the Milky Way halo. The smaller distance and there-

2 forelowerbrightnessargueagainstitsproposednatureasaThorne-Z˙ytkowobject(the
v
hypothesizedstar-likeobjectformedwhenanormalstarandaneutronstarmerge)or
5
a super-Asymptotic Giant Branch (AGB) star. Instead we propose a binary scenario
5
where this star is the companion of a former massive AGB star, which polluted the
4
object with via its stellar wind, i.e. a special case of an extrinsic S star. Our new
5
0 scenario solves two additional problems with the two existing scenarios for its nature
. as Thorne-Z˙ytkow object or present-day super-AGB star. The puzzling high ratio of
1
the strength of calcium to iron absorption lines is unexpected for SMC supergiants,
0
but is fully consistent with the expectations for halo abundances. Secondly, its strong
6
1 variabilitycannowbeexplainednaturallyasamanifestationoftheMiraphenomenon.
: We discuss further observational tests that could distinguish between the foreground
v
and SMC scenarios in advance of the improved proper motion measurements likely to
i
X come from Gaia.
r Key words: astrometry – stars:abundances – stars: chemically peculiar –
a
stars:individual:HV 2112
1 INTRODUCTION Object” (TZ˙O), the hypothesized red-giant like stellar ob-
jects with neutron stars inside their cores.
The nature of the star HV 2112, located in the same part TZ˙Os,firstmodeledbyThorne&Z˙ytkow(1975,1977),
of the sky as the Small Magellanic Cloud (SMC), has re- may form as a result of unstable mass transfer in a massive
cently become the center of a vivid debate. Levesque et al. X-ray binary after the neutron star is engulfed in the enve-
(2014) showed that it exhibits anomalous spectral features lopeofitscompanionstar(Taametal.1978).Alternatively,
suggestingenhancedabundancesofmolybdenum,rubidium, these objects may form when the supernova kick of newly
lithium, calcium and possibly also potassium when com- formed neutron star accidentally sends it towards and into
paredtootherSMCsupergiants.Thehighinferredbolomet- itscompanion(Leonardetal.1994).WhetherTZ˙Osactually
ricluminosityandtheenhancedabundancesofthefirstthree formisamatterofdebate.Fryeretal(1996)andChevallier
elementsleadtotheintriguingsuggestionbyLevesqueetal. (1993) argued that the neutron star would accrete enough
(2014) that HV 2112 could be a massive “Thorne-Z˙ytkow material to collapse to a black hole before forming a stable
(cid:13)c RAS
2 Maccarone & De Mink
TZ˙O. Biehle (1991, 1994) and Cannon (1993) predict that 2 PROPER MOTION MEASUREMENTS AND
massive TZOs, if stable, are powered by nuclear burning THEIR IMPLICATIONS
through the exotic interrupted rapid proton (irp)-process.
The star HV 2112 has had its proper motion measured as
Therefore they may be identified by enhanced abundances
part of the Southern Proper Motion survey. This is a sys-
of rare proton-rich elements, including Mo and Rb. Also
tematic astrometric study with a baseline of approximately
enhancements of Li have been predicted (Podsiadlowski et
fourdecadescoveringnearlytheentiresouthernskydownto
al. 1995). The predictions triggered several (proposed) ob-
approximately V =17.5 (Girard et al. 2011). For HV 2112,
serving campaigns hoping to identify massive TZOs (van
thefirstepochwasobtainedin1972andthesecondin2007.
Paradijs et al. 1995, Kuchner et al. 2002, Levesque et al.
Thestarshowsapropermotionof2.8±2.3mas/yearinright
2014).SofarHV2112isthemostpromisingcandidate,even
ascension and -9.8±2.3 mas/year in declination.
thoughtheabundancesofRbandMoarenotashighasex-
pectedandtheenhancementofCaandKwasnotpredicted. While the star is strongly variable, there are two rea-
sons to believe that the proper motion measurements from
Tout et al. (2014) discuss an alternative explanation SPM4.0arereliable.First,inbothepochs,itwasamongthe
whereHV2112isa“superasymptoticgiantbranch(AGB) brightest stars in its general vicinity – within two arcmin-
star”. This is the late evolutionary phase of stars with ini- utes, only one star is brighter in the second epoch V band
tialmassaround8M ,massiveenoughtoignitecarbon,but CCDdata,andonlytwostarsarebrighterinthefirstepoch
(cid:12)
still low enough to experience thermal pulses and dredge B band photometry. Additionally, within 2’, there are only
up episodes typical for AGB stars. Super AGB stars have 8starsbrighterthanB =16,sotheprobabilityoffindinga
higher core masses than typical red giants, giving them lu- star within 10” which is even 10% as bright as HV 2112 is
minositiesashighasredsupergiantsandTZ˙Osintheirearly about10−4.Second,withthe35yearbaseline,thetotalas-
phases(Siess2010).SimilartoAGBstarstheycan,inprin- trometricshiftmeasuredisabout0.35arcseconds.Thisshift
ciple, produce Mo and Rb through the slow neutron cap- issufficientlylargethatitwouldrequireextremefinetuning
ture process (Karakas & Lattanzio 2014) and Li through to produce an astrometric shift due to the combination of
the Cameron & Fowler (1971) mechanism. Enhancement of blending and source variability without the second object
Ca or K is, however, not predicted. beingapparentintheimages,andasshown,theprobability
of such a star being that nearby is of order 10−4.
InbothscenariosHV2112isassumedtobememberof The UCAC4 catalog (Zacharias et al. 2013) often gives
the SMC. This view is supported by its projected location anadditionalusefulsecondepochforpropermotionstothat
intheskyandthekinematicproperties:themeasuredradial fromtheSPM,althoughUCAC4andSPMmakeuseofthe
velocity (RV) is similar to that of the SMC (Neugent et al. same first epoch data. For this object, however, the second
2010). However, as Levesque et al. (2014) state, the possi- epoch from UCAC caught this variable star in a relatively
bility that it may be a halo giant with a similar RV cannot faintstate,sothatthepositionaluncertaintyisseveraltimes
be ruled out, but it would require a novel explanation for aslargeasintheSPMdata.TheUCAC4propermotiones-
the peculiar spectral features. timateis1.8±2.9mas/yrinrightascensionand−3.3±2.7
mas/yrindeclination.Ifonetakesaweightedaverageofthe
In this paper, we report the detection of a very large twomeasurements,thenpropermotionis2.4±1.8mas/year
propermotionbasedondatafromtheSouthernProperMo- in right ascension and −6.8±1.8 mas/year in declination.
tion survey (Girard et al. 2011). If confirmed, the proper This differs from zero at roughly 3.8σ, and from the -1.1
motionexcludesmembershipintheSMC,makingHV2112 mas/yr in declination of the SMC (Kallivayalil et al. 2013)
aforegroundMilkywaystar(likelyagiantduetoitspulsa- by3.2σ.ThedeviationfromthepropermotionoftheSMC
tional properties). A location in the Milky Way would rule in right ascension, which is -0.8 mas/yr (Kallivayalil et al.
out both proposed scenarios, the TZ˙O-nature and sAGB- 2013) is statistically insignificant, being about 1.7σ. Av-
nature of HV 2112. eraging SPM and UCAC using weights determined solely
bytheirstatisticaluncertaintiesisaconservativeapproach,
Weproposeanewscenariowhichinvolvespollutionby since any systematic uncertainties should be far more seri-
the wind of a former massive AGB companion star. In this ous for the fainter UCAC4 epoch than for the SPM data,
manner, the enhancement of molybdenum, rubidium, and andtheSPMmeasurementindicatesalargerpropermotion
possiblylithiumcomesfromtheprocessesthatwouldenrich than does the UCAC4 measurement.
amassiveAGBstar.Bynowtheformercompanionhaslost Wehavealsocheckedforsystematicsintheastrometry
itsentireenvelope.Itsremnant,mostlikelyamassivewhite of the field by taking the Hipparcos proper motion mea-
dwarf,isexpectedtobestillaroundinawideorbit.HV2112 surements for stars within 2 degrees of HV 2112 and cross-
in this scenario is the initially less massive secondary star. correlating them against the SPM 4.0 measurements. We
This scenario is similar to the scenario proposed by Iben have found that there is no systematic offset at a level of
& Renzini (1983) to explain extrinsic S stars as well as Ba about 0.5 mas/year, and that the differences between the
and CH stars (McClure 1984). One of the very attractive SPMandHipparcosmeasurementsarecomparabletothose
features of this scenario (and any scenario were it is placed expected from the stated statistical errors.
intheGalactichalo)isthenaturalexplanationprovidedfor We have test whether the internal errors for the SPM
the Ca (and K) features. These elements appear enhanced data are comparable to what is stated in the SPM cata-
whencomparedtoSMCgiants,buttheyarenormalforthe logue. Using the TOPCAT package, we have taken all the
Galactic halo where [Ca/Fe] enhancements are typical. We SPM stars within 1.5 degrees of HV 2112. We have then
discuss various testable predictions of this scenario. filtered to include only stars whose 2MASS colours (Cutri
(cid:13)c RAS,MNRAS000,1–5
HV 2112: an extrinsic S star in our Galaxy 3
1. 2.
v1014rgb
-15
Rb
-10 Mo
-5 main sequence main sequence wind accretion
primary secondary massive AGB onto secondary
as/yr 0 3. 4.
m
E / 5
pmD 10 RbMo LiMRbo
15
white dwarf enriched secondary white dwarf TZO-impostor
20
25 Figure 2. Cartoon of the alternative evolutionary scenario pro-
posedinvolvingawidebinarysystemlocatedintheGalactichalo
30
(1).TheprimarystarevolvestobecomeamassiveAGBstarand
-30 -20 -10 0 10 20 30 40 50
pollutesitslowmasscompanionwithitsenrichedwind(2),leav-
pmRA / mas/yr
ing behind a white dwarf after shedding its entire envelope (3).
Figure 1. The proper motions of stars within 2 degrees of Eventually, the enriched secondary becomes an AGB star itself
HV 2112, which have magnitudes from V=10 to V = 14 and showing the, now diluted, abundance patterns of enrichment in
J −K colors consistent with the giant branch for metal poor additiontoLimadeinsitu.Thecartoonsuggeststhatthewhite
stars (from J −K=0.78 to 1.35). HV 2112 has the black circle dwarf remnant of the primary is now accreting the wind of the
around it in the figure. It can be seen to be an outlier in the “TZO-impostor”.Thisisapossibility,butatpresentwehaveno
propermotiondiagram,withthemainlocusofpointstowardthe strongevidenceprovingordisprovingthishypothesis.
center being the SMC stars, and the outliers being foreground
stars.
3 A FORMATION SCENARIO FOR HV 2112
etal.2003)placethemononeofthegiantbranchesforthe At a distance of a few kpc, the luminosity of this object is
SMCandwhichhaveV between10and14(i.e.amagnitude likely to be ∼103L , rather than the 105L it would have
(cid:12) (cid:12)
rangeforwhichweexpecttheerrorstobesimilartothosefor in the SMC. Furthermore, is has been classified as a Mira-
HV 2112), and then examined only those stars with proper like variable from photometry (Samus et al. 2012), and its
motions of 5 masec/year or less. We find that for these 199 emission lines in its bright phases are also consistent with
stars,thestandarddeviationinthevaluesofthepropermo- Mira phenomenology (Levesque et al. 2014). The hypothe-
tions in both dimensions is about 90% of the mean values sis of a Mira star also fits well with the inferred luminosity
of the uncertainties on the proper motions from SPM 4.0. for the closer distance. These points were already noted in
Theexpectation,ifallthestarshadmeasurementuncertain- Levesqueetal.(2014),butrejectedonthebasisofthethen-
ties of the mean uncertainty, and there were no interloper likely association with the SMC. Levesque et al. (2014) did
stars,wouldbethatthestandarddeviationwouldbeabout briefly,butpresciently,notethatfuturekinematicsobserva-
75% of the 1σ uncertainty (the expectation is not exactly tions could potentially place the object in the Milky Way,
1σ because the more extreme outliers will be eliminated by andthatiftheydid,thiswouldleadtoadifferentinterpreta-
the 5 mas/year cut). It may then be that the measurement tionthantheThorne-Z˙ytkowobjectinterpretationpursued
uncertainties are underestimated by about 20%, and that intheirpaper.Likewise,thesuperasymptoticgiantbranch
the significance of the proper motion measurement relative (sAGB)starinterpretationconsideredbyToutetal.(2014)
to the SMC proper motion is at only the 3.0σ level, but alsocannotexplaintheobjectifitislocatedintheGalactic
this gives us additional confidence that there are no major foreground due to brightness considerations.
systematic uncertainties in SPM 4.0 that bias our results. On the other hand, the abundances may be explained
Any statistically significant measurement of a proper by a scenario in which matter is transferred from a mas-
motion in data with the precision of SPM 4.0 immediately sive AGB primary star to a binary companion. Eventually,
providesastrongargumentagainstalocationofthestarin thesecondarywillevolveoffthemainsequenceandbecome
the Small Magellanic Cloud, because the statistical uncer- anAGBstaritself,albeitofsignificantlylowermass,allow-
taintiesonthepropermotionsarelargerthantheescapeve- ing such an object still to exist in the old population in the
locityfromtheSMC.Thepropermotioninδof10mas/year halo.ThesestarsaregenerallyreferredtoasextrinsicSstars
atadistanceof62.1kpc(Graczyketal2014)yieldsaspace (e.g. Iben & Renzini 1983), following an initial purely phe-
velocity of 3100 km/sec, well above the escape velocity of nomenologicalclassificationof Sstars (Merrill1922),which
the SMC. If one instead takes this to be the velocity of a were later found, in a mnemonically fortunate coincidence
halostarintheMilkyWayhalo,andassumesa150km/sec to be rich in s-process elements (Smith & Lambert 1986).
speed (a reasonable value given the star’s radial velocity of The primary must be massive enough to produce substan-
157 km/sec – Levesque et al. 2014), then it is likely to be tial enhancements of molybdenum and rubidium. This cer-
locatedatadistanceofabout3kpc,althoughthestatistical tainly is expected for super-AGB stars (Lau et al. 2011;
errors on the proper motion measurement allow for a wide Tout et al. 2014), but is likely to happen down to even 4
rangeofpossibledistances.Boththespacevelocity,andthe M (Lugaroetal.2003),andisclearlyseeninsomeSstars
(cid:12)
height, probably a few kpc below the Galactic Plane, given (e.g.Lambertetal.1995showthatRbabundancesincrease
the Galactic latitude of 45 degrees, suggest that this is a withincreasings-processelementalabundances,whileAllen
halo star rather than a disk star in the Milky Way. & Barbuy 2006 show an excess of molbydenum in most of
(cid:13)c RAS,MNRAS000,1–5
4 Maccarone & De Mink
the members of a large sample of barium stars1.). The ex- for such a star to have a pulsational period of about 600
trinsic S stars form through the binary evolution channel dayscausedbythesamepulsationmechanismastheclassi-
above, while the intrinsic S stars are formed by dredge-up calMiras,eventhoughitsradiuswouldbemuchlargerthan
ofs-processelements,andshowenhancementofTc.TheTc a classical Mira.
signature indicates that the material has been placed into Ontheotherhand,theradialvelocityamplitudeofthe
the stellar atmosphere on a timescale of order or less than pulsationsshouldbequiteabitlargeriftheobjectisatSMC
the radioactive decay time of Tc. distancesthanifitisaforegroundGalacticMiravariable.In
The placement of the star in the Milky Way halo also essence,onecanapplytheBaade-Wesselinkmethod(Baade
helpstoexplainthecalciumabundanceanomalythatcould 1926;Wesselink1946)toestimatetheradiusandhencedis-
not be explained by the super-AGB star scenario discussed tance to the object. At SMC distances, the radius of the
inToutetal.(2014)norbytheTZOscenarioofLevesqueet star should be about 1500 R , so a variation of 40% about
(cid:12)
al. (2014). Levesque et al. (2014) did not have a calibrated themeanradiusisavariationof600R .Thepulsationsof
(cid:12)
estimateoftheCaabundanceofthestar,butdidshowthat Miras in general, and HV 2112 in particular are such that
the ratio of strengths of Ca lines to Fe lines was a factor of most of the variation takes place in a relatively small range
about 3 stronger than that for red supergiants in the SMC. ofpulsephase.Ifweassumethatsuchasizevariationtakes
Thisisindicativeofa[Ca/Fe]ratioabout3.Kobayashietal. placein0.2timesthepulseperiod,or120days,thenweex-
(2006)showsthathalostarsinthesolarneighborhoodwith pectavelocityofexpansionofabout50km/sec.Thisspeed
[Fe/H]<−1 tend to have [Ca/Fe] of about 0.4, which indi- is not large enough to be inherently problematic for either
catesacalciumtoironabundanceratioabout2.5.Themost scenario discussed here, but does provide a means for esti-
metal rich (and presumably youngest) SMC giants have ra- matingthestellarradius,andhencethedistancetothestar
tiosofαelementabundancestoironabundancesquitesimi- if a radial velocity monitoring campaign is undertaken.
lartosolar(Mucciarelli2014).Relativelylittlediscussionex-
istsaboutthepotassiumexcessinHV2112,butitisofsim-
ilarsizetotheCaexcessjudgingfromtheplotsinLevesque 4.1 Population estimate
et al. (2014), and this, also, can be explained well in terms
We can make a crude estimate of the number of extrinsic
of empirical halo abundances (Kobayashi et al. 2006).
S stars in the Galaxy with a massive AGB progenitors as
Lithium is a fragile element and is easily destroyed at
partoftestingtheplausibilityofthescenario.Sstarsmake
temperatures in excess of about 2.5x106K. Even a modest
about4%ofgiantsinthetemperaturerangewhereMstars
depth of mixing can lead to efficient depletion (Stancliffe
are normally found, and extrinsic S stars make about 1/3
2009). In principle, Lithium can be in the AGB phase of
of S stars (van Eck & Jorissen 2000). Extrinsic S stars are
the primary star (Cameron & Fowler 1971) and accreted
thus about 1% of the number of stars that appear to be M
onto the secondary. However, all Lithium originating from giantsincolorspace.TheSMCspansabout3×10−4 ofthe
the primary has most likely been destroyed when HV 2112 solid angle of the sky. The Milky Way has ∼ 1011 stars, of
evolved to become an AGB star and developed a deep con- which about 109 are in the halo, and about 107 are halo
vectiveenvelope.Thepresentdaylithiumabundanceismore
M giants. There should then be approximately 30 extrinsic
likelytheresultofproductioninsitu,inHV2112itself.Low
S stars in the Milky Way halo projected against the SMC.
mass AGB stars can produce lithium as a result of thermal
If we further assume that only the extrinsic S stars whose
halinemixing(Stancliffe2010).Analternativepossibilityis
progenitors were more massive than 4 M can have the re-
(cid:12)
thatthewhitedwarfleftbehindaccretesenoughmaterialto
quired abundances of rubidium and molybdenum, we must
produce a substantial nova rate. Evidence for lithium pro-
correctforthefactthatonly11%ofthestarsbetween1and
duction has been found in the classical nova V339 (Tajitsu
10 M (i.e. the ones that produce AGB stars without un-
(cid:12)
etal.2015).ThiscouldpollutetheenvelopeofHV2112.The
dergoingsupernovae)areabove4M (andhenceareheavy
(cid:12)
model predictions remain uncertain, but lithium is not un-
enough for production of these elements). We then expect
common in symbiotic Mira’s. For example V407 Cyg shows
a few extrinsic S stars with massive enough progenitors in
a strong lithium excess (Tatarnikova et al. 2003).
the SMC field. Since we additionally require that they be
in an AGB state themselves to explain the Mira variations
andtheLiproduction,theexpectednumberfallsbyafactor
of about 30 (Girardi et al. 2010), so our expected number
4 THE VARIABILITY OF THE SYSTEM AND
of objects is about 0.1. The scenario we propose thus quite
ITS IMPLICATIONS
plausiblyplacesasourceofthetypeweinvokeinalocation
Radialpulsationshaveperiodswhichscalewiththedynam- projected against the SMC, while also not placing so many
ical timescale of the star – that is, P ∝ ρ−1/2, where P is suchobjectsprojectedagainsttheSMCthatwewouldhave
the period of oscillation, and ρ is the density of the star. expected to have found many of them already.
Miravariableshavepulsationperiodofabout140-700days.
Their masses tend to be ≈1M and their radii tend to be
(cid:12)
about 300 R . The typical variations in radius are a factor
(cid:12) 5 ADDITIONAL TESTABLE PREDICTIONS
of about 1.5 (e.g. Mahler et al. 1997). Because the mass of
OF THE SCENARIO
the Thorne-Z˙ytkow object could be quite large, it is viable
The scenario makes several additional testable predictions.
First and foremost, the object should be in a wide binary
1 Thebariumstarsarethoughttobeanearlierevolutionarystate system. Orbital periods of extrinsic S stars are generally at
oftheextrinsicSstars least 600 days (Jorissen & Mayor 1992). The white dwarf
(cid:13)c RAS,MNRAS000,1–5
HV 2112: an extrinsic S star in our Galaxy 5
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