Interaction Between The Broad-lined Type Ic Supernova 2012ap and Carriers of Diffuse Interstellar Bands

0.34 MB·

by  D. Milisavljevic

Accepted to The Astrophysical JournalLetters on January4,2014
PreprinttypesetusingLATEXstyleemulateapjv.5/2/11
INTERACTION BETWEEN THE BROAD-LINED TYPE Ic SUPERNOVA 2012ap
AND CARRIERS OF DIFFUSE INTERSTELLAR BANDS
Dan Milisavljevic1,†, Raffaella Margutti1, Kyle N. Crabtree1, Jonathan B. Foster2, Alicia M. Soderberg1,
Robert A. Fesen3, Jerod T. Parrent3,4, Nathan E. Sanders1, Maria R. Drout1, Atish Kamble1,
Sayan Chakraborti1, Timothy E. Pickering5,6, S. Bradley Cenko7,8, Jeffrey M. Silverman9,
Alexei V. Filippenko8, Robert P. Kirshner1, Paolo Mazzali10,11,12, Keiichi Maeda13,14,
G. H. Marion9, Jozsef Vinko9,15, and J. Craig Wheeler9
4 Accepted to The Astrophysical Journal Letterson January 4, 2014
1
0 ABSTRACT
2 The diffuse interstellar bands (DIBs) are absorption features observed in optical and near-infrared
spectra that are thought to be associated with carbon-rich polyatomic molecules in interstellar gas.
n
a However,becausethecentralwavelengthsofthesebandsdonotcorrespondwithelectronictransitions
J of any known atomic or molecular species, their nature has remained uncertain since their discovery
almost a century ago. Here we report on unusually strong DIBs in optical spectra of the broad-
3
1 lined Type Ic supernova SN2012ap that exhibit changes in equivalent width over short (. 30 days)
timescales. The 4428˚A and 6283˚A DIB features get weaker with time, whereas the 5780˚A feature

] shows a marginal increase. These nonuniform changes suggest that the supernova is interacting with
E
anearbysourceoftheDIBsandthattheDIBcarrierspossesshighionizationpotentials,suchassmall
H cations or charged fullerenes. We conclude that moderate-resolution spectra of supernovae with DIB
. absorptions obtained within weeks of outburst could reveal unique information about the mass-loss
h
environmentoftheirprogenitorsystemsandprovidenewconstraintsonthepropertiesofDIBcarriers.
p
Subject headings: astrochemistry — molecular processes — ISM: lines and bands — ISM: molecules
-
o — supernovae: general — supernovae: individual (SN2012ap)
r
t
s
a 1. INTRODUCTION maximumintensity[FWHM] <1˚A)andweak(lessthan
[ 5%belowthe continuum),withcentralwavelengthsthat
One of the long unsolved problems in optical and in-
do not correspond with any known atomic or molecu-
1 frared astronomy is the nature of the diffuse interstellar
larspecies(Herbig1995;Hobbs et al.2009;Geballe et al.
v bands (DIBs). The DIBs represent more than 400 ab-
2011). TheywerefirstnoticedinstellarspectrabyHeger
1 sorption features observed in optical and near-infrared
(1922). Merrill(1934)subsequently uncoveredanumber
9 spectra that are typically narrow (full width at half-
of DIBs as ubiquitous interstellar features and their na-
9
2 1Harvard-Smithsonian Center for Astrophysics, 60 Garden ture has been an enduring subject of speculation.
. St.,Cambridge,MA02138 It is now well established that sources (or “carriers”)
1 2Yale Center for Astronomy and Astrophysics, Yale Univer- of the DIBs are found in the interstellar medium (ISM).
0 sity,NewHaven,CT06520, USA DIB features remain stationary in spectroscopic bina-
4 3Department of Physics & Astronomy, Dartmouth College,
ries,andthere areroughcorrelationsbetweenextinction
1 6127WilderLab,Hanover,NH03755, USA
: 4Las Cumbres Observatory Global Telescope Network, and Na ID column density with the intensity of DIB
v Goleta, CA,USA features (Herbig 1995). Searches for DIBs in circum-
i 5Southern African Large Telescope, PO Box 9, Observatory stellar shells have generally reported null detections or
X 7935, CapeTown,SouthAfrica
6Space Telescope Science Institute, 3700 San Martin Drive, results that cannot distinguish whether the absorption
r Baltimore,Maryland21218,USA arises in circumstellar material or the intervening ISM
a 7AstrophysicsScienceDivision,NASAGoddardSpaceFlight (Snow & Wallerstein 1972; Luna et al. 2008).
Center,MailCode661,Greenbelt, MD20771,USA Merrill (1934) was the first to suspect dust grains
8 Department of Astronomy,UniversityofCalifornia,Berke-
and/or molecules as possible carriers of the DIBs. Af-
ley,CA94720-3411, USA
9University of Texas at Austin, 1 University Station C1400, ter nearly a century of observational, theoretical, and
Austin,TX,78712-0259, USA experimental work, these two original suggestions have
10Astrophysics Research Institute, Liverpool John Moores remainedthe primarycandidates,occasionallyswapping
University,LiverpoolL35RF,UnitedKingdom
11Max-Planck-Institut fu¨r Astrophysik, Karl-Schwarzschild- in popularity (see Sarre 2006, and references therein).
Strasse1,85748Garching,Germany Current research favors multiple carriers produced by a
12INAF - Osservatorio Astronomico di Padova, Vicolo mixoffairlylargeandcomplexcarbon-based(“organic”)
dell’Osservatorio5,I-35122,Padova, Italy
13Department of Astronomy, Kyoto University polyatomic molecules composed of cosmically abundant
Kitashirakawa-Oiwake-cho,Sakyo-ku, Kyoto606-8502, Japan elementssuchasH,C,O,andN.Therehasbeenconsid-
14Kavli Institute for the Physics and Mathematics of the erableinvestigationofpolycyclic aromatichydrocarbons
Universe (WPI), Todai Institutes for Advanced Study, Univer- (PAHs) as DIB carriers, but as yet no firm associations
sity of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8583,
between PAH species and DIB features have been found
Japan
15DepartmentofOpticsandQuantumElectronics,University (see Cox 2011 for a recent review).
ofSzeged, Domter9,6720,Szeged, Hungary Insights into the chemical and physical properties of
†email: [email protected] DIBcarriershascomefromstudyoftheirbehaviorindif-
2 Milisavljevic et al.
Figure 1. Optical spectra of the broad-lined Type Ic SN2012ap during its first ∼ 40d after explosion. Three prominent DIBs around
4428˚A,5780˚A,and6283˚Aarehighlightedwithdashedlines. SALTandKeckdatahavea4–6˚AFWHMresolution. MMTdatahave7˚A
FWHM.Symbols showtelluricabsorptions intheSALT spectra thathave notbeen corrected. Spectra have beencorrected foraredshift
of z = 0.0121 measured from narrow HII regionlines of [OIII] λλ4959, 5007, Hα, and [NII] λλ6548, 6583 observed near the location of
thesupernova.
ferent interstellar environments, especially extragalactic spectra obtained with the 10m Southern African Large
ones. Most studies have focused on nearby star systems Telescope(SALT)usingthe RobertStobieSpectrograph
including the Magellanic Clouds and M31 (Cox et al. (RSS; Burgh et al. 2003), the 10m Keck-I telescope us-
2007;Cordiner et al.2008). Outsideoflimitedworkwith ing the Low Resolution Imaging Spectrometer (LRIS;
quasars (e.g., Ellison et al. 2008), only supernovae (SN) Oke et al.1995),andtheMMT6.5mtelescopeusingthe
havebeenbrightenoughtoprobeDIBsbeyondtheLocal Blue Channel spectrograph (Schmidt et al. 1989). The
Group (see, e.g., Cox & Patat 2008). In general, extra- spectra shown in Figure 1 are part of a larger dataset
galacticstudieshaveshownthatDIBcarrierabundances (Milisavljevic et al., in prep.).
canbe similar to Galactic values, though systematic dif- Unlike SN2008D, which transitioned to a Type Ib
ferences sometimes exist. SN exhibiting conspicuous He I, spectra of SN2012ap
In this Letter we report on recent observations of obtained weeks later continued to show broad fea-
a broad-lined Type Ic SN that exhibits some of the tures associated with ejecta traveling ∼ 2×104kms−1.
strongest DIBs ever detected in an extragalactic source. Milisavljevic et al.(2012) reportedthatthese laterspec-
These absorptions undergo changes in intensity over rel- tra were similar to those observed in broad-lined SN Ic
atively short timescales in a manner that suggests that such as SN1998bw and SN2002ap ∼ 1–2 weeks after
the SN explosion interacted with local carriers of DIBs. maximum light (see Fig. 1). Further examination shows
We conclude that moderate-resolutionspectra of SN ob- that the later spectra of SN2012ap also resemble those
tainedshortlyafteroutburstmayprovideanewandpow- ofSN2009bb,aSNIb/cthathadasubstantialrelativis-
erful probe of DIBs and offer clues about the progenitor ticoutflowpoweredbyacentralengine(Soderberg et al.
systems of these explosions. 2010; Pignata et al. 2011).
2. RESULTS 2.2. Strong DIB Features
2.1. Discovery and Classification Superimposed on the broad-lined Type Ic features of
SN2012ap was first detected by the Lick Obser- SN2012ap are conspicuous absorptions with equivalent
vatory Supernova Search at coordinates α(2000.0) = widths (EWs) & 1˚A associated with DIBs at the rest
05h00m13s.72andδ(2000.0)=−03◦20′51′.′2intheface-on wavelength of the host galaxy. The DIB features are
galaxy NGC 1729 (d ≈ 43.1 Mpc; Springob et al. 2009) strongest at 4428˚A, 5780˚A, and 6283˚A, which are the
on Feb. 10.23 UT (Jewett et al. 2012). The SN is lo- wavelengths of well-known DIBs typically seen in stellar
cated in the outskirts of the host galaxy some 7.1kpc in spectra (Herbig 1995). In Figure 2 we display enlarged
projection from the nucleus in a region with no obvious regionsaroundthesefeatures. Notshownisanotherpos-
star formation. sible DIB detection near 6203˚A that may be contami-
ThefirstreportsofopticalspectraofSN2012apclassi- nated by an OH telluric line at an observed wavelength
fieditasaTypeIb/cSNsimilartoSN2008Dnotlongaf- of 6280˚A.
ter explosion (Xu et al. 2012). This prompted extensive The central wavelengths of these DIBs do not change
follow-upobservationsbyourgroupthatincludedoptical with time, but the intensities do exhibit measurable
SN Interaction with a Carrier of DIBs 3
Figure 2. EnlargedDIBabsorptionfeaturesofSN2012ap. Theleftandmiddlecolumnsshowselectepochstoillustratepossiblechanges
inEW,andtherightcolumnshowsallEWmeasurements ofnarrowabsorptions. PhaseiswithrespecttoB-bandmaximumonFeb18.2
UT. We removed the global shape of the underlying continuum by smoothing the spectra with a running boxcar of width 50˚A and then
subtracting thesmoothed version. TheEWof theNaIDlinesofthe hostgalaxy shows nomeasurablechange. Tofurther illustratethat
thechanges arenotrelatedtoinstrumentalsetups,spectrafromthesamethespectrographusingidenticalconfigurations areplotted.
changes that are not uniform across different features Photons may modify or destroy carrier material via ion-
(Fig. 2, rightcolumn). The EWof DIB λ4428decreased izationand/ordissociation. Ifextremelynearby,the for-
by0.77±0.25˚Aover∼10daysandDIBλ6283decreased wardblast wave initiated by the explosion and traveling
by 0.49±0.28˚A over ∼ 30 days. The DIB λ5780 fea- with velocity ∼ 0.4c (Chakraborti et al., in prep.) will
ture, on the other hand, shows a weak but measurable disrupt molecules and dust grains within a ∼0.01pc ra-
increase of . 0.2˚A over ∼ 10 days. The Na ID line dius in the first 30 days.
at rest with respect to the host shows negligible change.
The Na ID line associated with foreground Milky Way 3.2. Physical Constraints on the DIB Carriers
extinction shows no change, as expected.
SN2012appeakedintheB bandonFeb18.2UT(Mil-
3. DISCUSSION isavljevic et al., in prep.), implying that the SN flux in-
creasedandthendecreasedatopticalwavelengthsasthe
3.1. SN Interaction with DIB Carriers
intensities of DIB λ4428 and λ6283 became weaker and
The DIB absorptionsseen in the spectra of SN2012ap DIB λ5780 became slightly stronger. This behavior is
areamongthestrongestextragalacticdetectionseverre- consistent with active interaction wherein separate DIB
ported. DetectionsofextragalacticDIBsatthisdistance carriers differing in robustness and/or location are af-
are rare and thus interesting as they allow one to com- fected by the SN independently.
pare Galactic ISM chemical properties with extragalac- Usingthetimeevolutionoftheblackbodytemperature
tic ones. However, what is unique and most informative andtotalluminosityderivedfromphotometrydata(Mil-
about the spectra of SN2012ap is that the DIB absorp- isavljevic et al., in prep.), we estimated the UV flux in
tionstrengthschange with timeandthatthe changes are the 5–50eV spectral range as a function of distance and
not uniform across different DIB features (see Fig. 2). timefromSN2012ap. Toestimatethelifetimesofmolec-
Various types of interaction between the SN and DIB ularmaterialinthisradiationfield,weapproximatedthe
carrier material may explain the observed changes (see, photoabsorption cross sections in this frequency range
e.g., Patat et al. 2010). We favor the scenario that the for small neutral molecules (Gallagher et al. 1988) and
carrier material is nearby and the SN is actively inter- PAHs (Verstraete et al. 1990; Jochims et al. 1996), cal-
acting with it. This interaction can take many forms. culated the photoabsorption rate, and assumed that all
4 Milisavljevic et al.
Figure 3. Model spectrum of known DIB absorption features compared to early-time spectra of SN 2012ap, SN 2009bb (Pignata etal.
2011), and SN 2008D (Modjazetal. 2009). SN 2009bb and SN 2008D have been corrected for redshifts of z = 0.010 and z =
0.007, respectively. All SN exhibit conspicuous absorptions having central wavelengths of well-known DIBs highlighted with verti-
cal dashed lines. Time is with respect to maximum light. The model DIB spectrum was created from a catalog retrieved online at
http://leonid.arc.nasa.gov/DIBcatalog.html.
absorption events lead to ionization or dissociation. Be- aroundSN2012ap. Using the photoabsorptioncrosssec-
cause these frequencies are above the peak of the black- tion of C as a representative case (Berkowitz 1999),
60
body curve, the absorption rates are highly sensitive to we estimate that neutral fullerenes (IP ≈ 7eV) near
theionizationpotential(IP)ofamolecule,andtheshape SN2012ap will be rapidly ionized, but fullerene cations
and size of its cross section. (IP ∼ 11eV) should have lifetimes of order days. The
The inferredlifetimes vary by severalordersof magni- fact that the observed changes in EW of these DIB fea-
tude, but within a distance of ∼ 0.01pc, at peak lumi- tures occur on the timescale of days in such an intense
nosityallbutthesmallestneutralmoleculesareexpected UV field suggests the carriersare fairly robustto ioniza-
to have lifetimes much less than one day. Within this tion and dissociation (particularly DIB λ5780), consis-
distance, the population of most neutral species will be tent with small cations or charged fullerenes.
rapidly depleted unless their formation from the break-
down of larger material is even more rapid. Cations, 3.3. Implications of a DIB–SN Subtype Correlation
owingtotheirhigherIPs,areestimatedtohavelifetimes
Two other core-collapse SN in the literature exhibit
on the order of days under the same conditions.
conspicuous DIBs in low-resolution spectra, and we ex-
In this context, it is interesting to note that the
amined their archival data: the Type Ib SN2008D
timescale for the increase in DIB λ5780 is comparable
with spectra published by Modjaz et al. (2009), and
to that of the decay in DIB λ4428, possibly suggesting
the broad-lined Type Ib/c SN2009bb published by
thatthe DIBλ5780carrieris aphotoproductofthe DIB
Pignata et al. (2011). Figure 3 shows early-time spec-
λ4428 carrier. In constrast, the decay in DIB λ6283 oc-
traoftheseobjects,withconspicuousDIBfeatureshigh-
curs over a longer timescale, suggesting the carrier is ei-
lighted. Although the relatively low spectral resolutions
ther more photostable or is more extended. The ratio of
andlimitedtemporalsamplingpreventdetailedanalyses
thestrengthofDIBλ5780toDIBλ5797(thelatterisnot
of these additional objects, the archival spectra suggest
detectedtowardSN2012ap)is positivelycorrelatedwith
thatsomeDIBfeaturesseenintheseotherSNhaveboth
increasing UV radiation environments (Vos et al. 2011).
narrow and broad components and that they may vary
The increase in strength of DIB λ5780 in these observa-
as they do SN2012ap.
tions suggests that this trend continues to very extreme
All three SN exhibited broad spectral features asso-
UV environments. ciated with ejecta moving at high velocities (& 2 ×
FullereneshavebeenproposedasDIBcarriers,andare 104kms−1) within weeks of explosion and all were ob-
significantlymorestableagainstdissociationbyUVradi-
served to have a color excess E(B −V) & 0.5 mag that
ationthansmallermolecules,typicallyrequiringenergies
implies substantial extinction (Soderberg et al. 2008;
of more than 10eV for dissociation (Diaz-Tendero et al.
Modjaz et al. 2009; Pignata et al. 2011; Milisavljevic et
2003). This increased dissociation energy might allow
al., in prep.). SN2012ap and SN2009bb share similar
fullerenes to survive longer in the radiationenvironment
explosion parameters of estimated ejecta mass (∼ 2–
SN Interaction with a Carrier of DIBs 5
4M⊙), 56Ni mass (∼ 0.2M⊙), and explosion kinetic en- lar disk could be an important factor in explaining why
ergy (∼1.5×1052erg). On the other hand, SN2008Dis strong DIB detections like those reported here are rare.
differentinthatitsbroadlinesdisappearedwithinweeks Finally, we note that varying strength in narrow ab-
as it transitioned to a SN Ib and its explosion energy sorption lines attributable to interaction between a SN
(∼ 1.5–6 ×1051erg; Soderberg et al. 2008; Tanaka et al. and a local environment has recently been recognized in
2009) is lower than those of SN2012apand SN2009bb. a growing number of cases, with significant implications
Chance alignments between DIB carrier-rich molecu- forthenatureoftheprogenitorsystems(e.g.,Patat et al.
lar clouds and these SN are possible. However, given 2007; Blondin et al. 2009; Dilday et al. 2012). However,
that the three SN with conspicuousDIB absorptions ex- those reports have been for Na ID, Ca II, Hα, He I,
amined in the literature are spectroscopically similar, it and Fe II lines with line-of-sight blueshifted velocities
may be that the SN progenitor systems are related to of . 100kms−1 originating from circumstellar material
the sources of the DIBs. If true, the carrier material re- aroundTypeIaSN.Thisisnotthesameaswhatisbeing
sponsible for the observed DIB absorptions in these SN observed in the core-collapse SN2012ap, where the DIB
should lie fairly close to the explosion site and could be features are near zero velocity and are associatedwith a
associated with mass loss from the progenitor star. carrier material having radically different physical prop-
Mass loss in massive stars is influenced by a num- erties.
ber of factors including the strength of their winds, ro-
4. CONCLUSIONS
tation, the presence of a binary companion, possible
eruptive mass-loss episodes, and environmental metal- The broad-lined Type Ic SN2012ap exhibits DIB ab-
licity (Chiosi & Maeder 1986; Humphreys & Davidson sorptions that are among the strongest ever detected
1994; Nugis & Lamers 2000). To investigate what role in an extragalactic object. The DIB features centered
metallicity might play in linking the three SN, the rel- around 4428˚A, 5780˚A, and 6283˚A undergo changes in
ative strengths of narrow lines from coincident host- EW over relatively short timescales (t < 30 days) in-
galaxy emission at the site of SN2012ap were measured dicativeof interactionbetween the SN andDIB carriers.
using the method described by Sanders et al. (2012). Similar absorptions observed in archival spectra of two
From the N2 diagnostic of Pettini & Pagel (2004), we additional SN suggests that SN2012ap may belong to
measure an oxygen abundance log(O/H) + 12 = 8.79 a subset of energetic SN Ib/c that exhibit changes in
with uncertainty 0.06 dex. Adopting a solar metallic- conspicuous DIB absorption features. If true, this cor-
ity of log(O/H)⊙ + 12 = 8.69 (Asplund et al. 2005), relation is consistent with the DIB carrier-rich material
our measurement indicates that SN2012ap exploded in being located close to the explosion, fairly resistent to
an environment of around solar metallicity that lies in thestrongUVfield,andpotentiallyassociatedwithmass
between the metallicity estimates of SN2009bb (1.7– loss of the progenitor star.
3.5Z⊙; Levesque et al. 2010) and SN2008D (0.5–1Z⊙; Our data with 4–7 ˚A resolution that monitored the
Soderberg et al. 2008). Considering broad-lined SN Ic spectral evolution of SN2012ap during its rise and fall
are typically found in environments of subsolar metal- in flux was on the cusp of detection for this uniquely
licity (Kelly & Kirshner 2012; Sanders et al. 2012), the strongsourceofDIBabsorptions. OnlythebroadestDIB
metallicity of these three SN is somewhat anomalous. featuresknownto haveFWHM widths ofapproximately
However, these objects were discovered by surveys tar- 2−12 ˚A were observed in our data set. Thus, multi-
geting high-mass metal-rich galaxies, so this weak trend epoch observations of SN with spectral resolutions of ≤
may be influenced by an observational bias. 1 ˚A beginning within days of explosion could uncover
A handful of reports connect strong DIB features ob-
the presence of a larger family of DIB features. Such
served in a narrow subset of mass-losing stars with
observations would be much more sensitive to possible
circumstellar shells (e.g., Tug & Schmidt-Kaler 1981; variations in Na I absorption strength, as well as detect
Cohen & Jones 1987). The circumstellar material is
possible subtle changes in the velocities of the NaI/DIB
often nitrogen-rich and the strength of the associated
features. Observedinthisway,SNwithDIBabsorptions
DIB features may vary (Heydari-Malayeriet al. 1993).
have the potential to reveal unique information about
Le Bertre & Lequeux(1993)identifiedWolf-Rayet(WR)
mass-loss environment of their progenitor systems and
stars of the WN subtype and luminous blue variable
probe DIB carriers in new ways that can bring us closer
(LBV) stars enriched in nitrogen as candidate objects
to understanding the their nature.
with circumstellar shells containing DIB carriers, and
proposed that nitrogen could act either as a constituent
of the DIB carriersor as a catalyst for their production. We thank ananonymousreferee for a helpful, detailed
It is intriguing that families of WR and LBV stars and critical reading of the paper. T. Snow kindly pro-
may be associated with DIB features. WR stars vided comments on an early draft of the paper. P.
are suspected progenitors of SN Ib/c (Gaskell et al. Massey provided insightful comments. G. Pignata, S.
1986), and have been implicated for SN2008D and Valenti, D. Malesani, and G. Leloudas shared archival
SN2009bb (Soderberg et al. 2008; Modjaz et al. 2009; spectra that were examined. Many of the observations
Soderberg et al. 2010; Pignata et al. 2011). Although reported in this paper were obtained with the South-
LBVs are not widely believed to be the direct progen- ern African Large Telescope. Additional data presented
itors of SN Ib/c, WR stars can evolve from a prior LBV herein were obtained at the W. M. Keck Observatory,
phase (Conti 1976). These stars exhibit varying degrees which is operated as a scientific partnership among the
ofasymmetricmassloss(see,e.g.,Nota et al.1995),thus CaliforniaInstituteofTechnology,theUniversityofCali-
an observer’s line of sight with respect to a circumstel- fornia,andNASA;theobservatorywasmadepossibleby
the generous financial support of the W. M. Keck Foun-
6 Milisavljevic et al.
dation. A. Miller, P. Nugent, and A. Morganhelped ob- Gaskell,C.M.,Cappellaro,E.,Dinerstein,H.L.,etal.1986,
taintheKeckobservations. Someobservationsalsocame ApJ,306,L77
fromtheMMTObservatory,ajointfacilityoftheSmith- Geballe,T.R.,Najarro,F.,Figer,D.F.,Schlegelmilch,B.W.,&
deLaFuente, D.2011,Nature,479,200
sonian Institution and the University of Arizona. Sup-
Heger,M.L.1922,LickObservatoryBulletin,10,141
portwasprovidedbytheDavidandLucilePackardFoun- Herbig,G.H.1995,ARA&A,33,19
dation Fellowship for Science and Engineering awarded Heydari-Malayeri,M.,Grebel,E.K.,Melnick,J.,&Jorda,L.
toA.M.S.J.M.S.issupportedbyanNSFAstronomyand 1993,A&A,278,11
AstrophysicsPostdoctoralFellowshipunderawardAST- Hobbs,L.M.,York,D.G.,Thorburn,J.A.,etal.2009,
ApJ,705,32
1302771. T.E.P. thanks the National Research Founda-
Humphreys,R.M.,&Davidson,K.1994, PASP,106,1025
tion of South Africa. R.P.K. and J.C.W. are grateful Jewett, L.,Cenko, S.B.,Li,W.,etal.2012,CentralBureau
for NSF grants AST-1211196 and AST-1109801,respec- ElectronicTelegrams,3037,1
tively. A.V.F.andS.B.C.acknowledgegeneroussupport Jochims,H.W.,Baumgartel,H.,&Lench,S.1996,A&A,314,
fromGaryandCynthiaBengier,theRichardandRhoda 1003
Kelly,P.L.,&Kirshner,R.P.2012,ApJ,759,107
Goldman Fund, the Christopher R. Redlich Fund, the
LeBertre,T.,&Lequeux, J.1993,A&A,274,909
TABASGO Foundation, and NSF grant AST-1211916. Levesque,E.M.,Soderberg,A.M.,Foley,R.J.,etal.2010,
K.N.C. has been supported by a CfA Postdoctoral Fel- ApJ,709,L26
lowship from the Smithsonian Astophysical Observa- Luna,R.,Cox,N.L.J.,Satorre,M.A.,etal.2008,
tory. This paper made extensive use of the SUSPECT A&A,480,133
database (http://www.nhn.ou.edu/∼suspect/). Merrill,P.W.1934, PASP,46,206
Milisavljevic,D.,Fesen,R.,Soderberg,A.,etal.2012, Central
BureauElectronicTelegrams,3037,2
Modjaz,M.,Li,W.,Butler,N.,etal.2009, ApJ,702,226
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