Extended CO emission in the field of the light echo of V838 Mon

0.29 MB·English

by  T. Kamiński

Astronomy&Astrophysicsmanuscriptno.kaminski08 c ESO2008
(cid:13)
February2,2008
Extended CO emission in the field of the light echo of V838
⋆
Monocerotis
T.Kamin´ski
DepartmentforAstrophysics,N.CopernicusAstronomicalCenter,Rabian´ska8,87-100Torun´,Poland
8 e-mail:[email protected]
0
0 Received;accepted
2
ABSTRACT
n
a Context.V838Moneruptedatthebeginningof2002becominganextremelyluminousstarwithL=106L .Theoutburstwasfollowed
J bythemostspectacularlightechothatrevealedthatthestarisimmersedinadiffuseanddustymedium,p⊙lausiblyinterstellarinnature.

0 LowangularresolutionobservationsofthestaranditsclosestvicinityinthelowestCOrotationaltransitionsrevealedamolecular
1 emissionfromthedirectionofV838Mon.TheoriginofthisCOemissionhasnotbeenestablished.
Aims.ThemainaimofthispaperistobetterconstrainthenatureoftheCOemission.Inparticular,weinvestigatetheideathatthe

] molecularemissionoriginatesinthematerialresponsiblefortheopticallightecho.
h Methods.Weperformedobservationsof13positionswithinthelightechointhetwolowestrotationaltransitionsof12COusingthe
p
IRAM30mtelescope.
o- Results.EmissioninCOJ=1–0andJ=2–1wasdetectedinthreepositions.InthreeotherpositionsonlyweakJ=1 0lineswere
found.ThelinesappearattwodifferentvelocitiesV =53.3kms 1andV =48.5kms 1,andbothcomponentsar−everynarrow
r LSR − LSR −
t withFWHM 1kms 1.
s ≈ −
Conclusions.ThemolecularemissionfromthedirectionofV838Monisextendedandhasaverycomplexdistribution.Weidentify
a
[ theemissionasarisingfromdiffuseinterstellarclouds.Aroughestimateofthemassofthemolecularmatterinthoseregionsgivesa
fewtensofsolarmasses.Theradialvelocityoftheemissionat53.3kms 1suggestthattheCO-bearinggasandtheechoingdustare
−
1 collocatedinthesameinterstellarcloud.
v
9 Keywords.radiolines:ISM–ISM:clouds–ISM:molecules–stars:individual:V838Mon–stars:peculiar
8
6
1 1. Introduction Crauseetal. (2003, 2005), Kipperetal. (2004), and Tylenda
1. (2005).
V838Monisastarthatbrightenedsignificantlyatthebeginning
0 Althoughthere is greatinterestin V838Mon amongastro-
of 2002becominga veryluminousobjectwith a luminosityof
8 physicists and many efforts have been made to gain a deeper
0 106 L . The outburst lasted for 3 months and was character-
: izedb⊙yarathercomplexlightcu∼rvewiththreedistinctmaxima understandingofthe object,its natureremainsunclear.A wide
v rangeofmechanismshavebeenproposedtoexplaintheoutburst
seenintheoptical.SeveralbroadP-Cygniprofilesobserveddur-
i including (i) thermonuclear models: a nova-like (Munarietal.
X ingthebrighteningimplythattheeventwasaccompaniedbyan
2002) or a He-shell flash (Lawlor 2005; Munari&Henden
outflowwithterminalvelocitiesofseveralhundredkms 1 (e.g.
r − 2005), and (ii) merging events: a stellar merger of stars with
a Kipperetal. 2004). The end of the eruption was characterized
massesof8M and0.3M (Soker&Tylenda2003,2007),and
bya verysteep decline with a totaldropin brightnessin theV
a giantswallow⊙ingplanets⊙(Retter&Marom2003;Retteretal.
bandofabout8magwithinonemonth.Unlikenovae,theobject
2006).Itseemsthatthemostpromisingisthestellarmergersce-
evolved from a hot stage with an F-type spectrum to progres-
nario(Tylenda&Soker2006;Soker&Tylenda2007).
sivelylowerandlowertemperaturesandeventuallyitturnedinto
a supergiant cooler than M10 (Evansetal. 2003). Named the SinceFebruary2002alightechoofV838Monhasbeenob-
firstLsupergiant,thestarhasexhibitedanextraordinarymolec- served(Hendenetal.2002).Itsspectacularevolution,especially
ularspectrumstudiedextensivelyinoptical(e.g.Pavlenkoetal. well documented by the series of the Hubble Space Telescope
2006)andinfrared(e.g.Lynchetal.2004).Evenfiveyearsafter (HST) images (Bondetal. 2003; Bond 2007), revealed that
theeruption,theobjectcontinuestoevolve-afterfouryearsofa V838 Mon is immersed in a diffuse and dusty medium with a
relativelyconstantphotometricbrightness,aneclipse-likeevent very complexspatial distribution.There is no agreementabout
has been observed in the B and V light curves (Munarietal. the origin of this matter. An analysis of the light echo images
2007a). Detailed descriptions of the spectral and photomet- performedbyTylenda(2004)showsthatthedustyregionhasa
ric behavior of V838 Mon can be found in: Munarietal. rather complex distribution and, most probably, is of interstel-
(2002), Kimeswengeretal. (2002), Wisniewskietal. (2003), lar origin(see also Crauseetal. 2005; Tylendaetal. 2005). On
theotherhand,therearesuggestionsthatthemattermighthave
been ejected by the object itself in the past (Bondetal. 2003;
Sendoffprintrequeststo:T.Kamin´ski
Bond2007).
⋆ Based on observations carried out with the IRAM 30-meter tele-
scope.IRAMissupportedbyINSU/CNRS(France),MPG(Germany), Alightechoanaloguewasdiscoveredintheinfraredwiththe
andIGN(Spain). SpitzerSpaceTelescope(Banerjeeetal. 2006).Theanalysisof
2 T.Kamin´ski:COemissioninthefieldofthelightecho
Table 1. Positions observed in CO J =1–0 and 2–1 in Sept.
2006.
position ∆α ∆δ int.time σ (1 0) σ (2–1)
rms − rms
[ ] [ ] [min] [mK] [mK]
′′ ′′
V838Mon 0 0 353.6 14.1 46.3
Off1 24 0 66.3 27.3 82.1
Off2 –24 0 55.2 30.7 83.4
Off3 0 24 55.2 33.4 95.8
Off4 0 –24 55.2 30.4 88.4
Off5 48 0 55.2 41.2 154.0
Off6 –48 0 44.2 42.7 130.3
Off7 0 48 55.3 33.9 106.1
Off8 0 –48 55.2 33.2 102.3
Off9 72 0 22.1 46.9 149.2
Off10 –72 0 33.1 36.2 117.3
Off11 –24 24 33.2 51.4 143.9
Off12 24 –24 33.1 50.4 165.7
theinfraredemissiongivesafewtenstoafewhundredsofsolar
massesasanorderofmagnitudeestimateforthemassofthegas
Fig.1. IRAM and KOSMA beams (HPBWs) overlayed on
associated with the emitting dust. This result is a strong argu-
the HST/ACS light echo image from 10 September 2006 ob-
mentsupportingtheideaofaninterstellaroriginoftheechoing
tained with the F814W filter (image downloaded via MAST,
matter.
http://archive.stsci.edu).Thetwolargestcircles(green)mark
Kamin´skietal. (2007a,b) discovered emission in the 12CO
the KOSMA telescope beams at 230 GHz and 345 GHz cen-
(2–1) and (3–2) transitions at the position of V838 Mon. The
teredatthestarpositionandtheyrepresentthelowangularres-
emissionwasdetectedinsidethelargeKOSMA-telescopebeams
olution observations in CO carried out in 2005 and 2006. The
withhalfpowerwidths(HPBWs)of130 and82 .Thedetected
′′ ′′ 13positionsobservedwiththeIRAM30mtelescopeon27-28
linesareverynarrowandappearatLSRvelocityof53.3kms 1.
− September2006arerepresentedbythesetofsmallercirclesthat
The data analysis performed in Kamin´skietal. (2007b) sug-
correspondto the beams at 115GHz (21 , blue) and 230 GHz
gest that the emission is extended,but on the basis of the low- ′′
(11 , red). These points with their coordinates are listed in
resolution observations it was not certain. Moreover, in obser- ′′
Table 1. The beams drawn with a solid line mark those posi-
vations performedin three epochs Kamin´ski et al. foundsmall
tionswhereCOemissionwasdetected,whilethebeamsdrawn
butpossiblyrealvariationsintheintensityoftheCO(2–1)line.
with a dashed line mark observed positions with no detection.
Dueto the unknowncontributiontothe intensitychangesfrom
The axis show the offset scales with respect to the V838 Mon
antenna-pointingerrors,thefindingisalsoveryuncertain.
position,whichismarkedwithastarsymbol.
Deguchietal.(2007)observedtheregionaroundV838Mon
inthe CO (1–0)transitionanddetecteda narrowemissionline
30 north from the star position. This detection together with
′′ The observationswere obtained with the IRAM 30 m tele-
the observationsreported in Kamin´skietal. (2007b) show that
scope at Pico Veleta, Spain, on 27-28 September 2006. The
someformofmolecularmattermustexistclosetothedirection
half-powerbeam widths of the telescope are 21.4 and 10.7 at
ofV838Mon.Itsnaturehasnotbeensufficientlyestablished,but ′′ ′′
115GHzand230GHz,respectively.Thespatialgridoftheob-
itistemptingtolinktheCOemittinggaswiththedustymedium
servationswasarrangedsothatthedistancebetweentwoneigh-
illuminatedbytheeruptionofV838Mon.
boringpositionsisclosetothetwicethebeam-widthat230GHz,
In the current paper, we present follow-up observations of
andtheindividualmeasurementsobtainedintheCO(2–1)tran-
thefieldaroundV838MonintheCO(1–0)and(2–1)rotational
sitioncanbeconsideredasindependent.
transitionswithamuchbetterangularresolutionthantheobser-
Asfrontendsweusedfour(singlepixel)heterodyneSISre-
vationsreportedinKamin´skietal.(2007a,b).Theobservational
ceivers(A100,B100,A230,andB230)simultaneously.Theob-
details are provided in Sect. 2, while the data are described in
servationswerecarriedoutinsinglesidebandmode,withrejec-
Sect.3.InSect.4,wediscusstheresults,inparticular,weinves-
tion factors of the image sideband set to 22 dB and 17 dB for
tigatethe possible originof the CO emission foundin the field
the observations at 115 GHz and 230 GHz, respectively. As a
around V838 Mon. Sect. 5 contains final conclusions and em-
backendweusedthespectrometricarrayVESPA.Allthespec-
phasizes the need for future observationsof the echo region in
trawereacquiredwitharesolutionof78.1kHzandwithaband-
moleculartransitions.
width of 107 MHz. The frequency resolution corresponds to a
resolution in velocity units of 0.20 km s 1 at 115 GHz, and
−
0.10kms 1at230GHz.
2. Observationsanddatareduction −
All the observations were performed using a frequency
We observed 13 positions within the light echo of V838 Mon, switchingtechnique.Thefrequencyswitchthrowwas15.6MHz
includingthestarposition.TheyarelistedinTable1andmarked (i.e.frequencywasswitchedby 7.8MHz),whichisequivalent
onthe opticallightechoimagein Fig. 1. Each of thepositions toavelocitythrowof40.7kms±1 at115GHz,and20.3kms 1
− −
was observed in the two lowest rotational transitions of 12CO, at230GHz.Pointingandfocusingwerecarriedoutonthestrong
i.e.J = 1–0at2.6mm(115.271GHz)andJ =2–1at1.3mm continuumsource0528+134every 2h.Theresultingpointing
∼
(230.538GHz). accuracyofourobservationsisbetterthan4 .
′′
T.Kamin´ski:COemissioninthefieldofthelightecho 3
0.2
The data were calibrated using a chopper wheel method
V838 Mon (1−0) (Kutner&Ulich 1981) giving spectra in the antenna tempera-
(2−1) ture,T ,i.e.correctedforatmosphericattenuation,ohmiclosses,
0.1 A∗
rearward spillover and scattering. The calibration at the IRAM
30misknowntobeveryaccurateandshouldbebetterthan10%
forobservationsat115GHz.
0
The wavy baselines typical for frequency switching obser-
vations were reduced by subtracting sinusoidal functions; in a
fewcasessubtractionofhighorderpolynomialswasnecessary.
Allbadchannelswereremoved,spectrafromtworeceiverswith
Off2 (−24, 0)
orthogonalpolarizationswere coadded,and the resulting spec-
0.1 trawerefoldedusingthestandardshift-and-addmethod.Finally,
theobservationswereconvertedtothemainbeamtemperature,
T ,usingforwardandmainbeamefficiencies(F andB re-
mb eff eff
0 spectively) of 0.95 and 0.74 at 115 GHz, and of 0.91 and 0.52
at230GHz.Allthedatareductionandanalysiswasperformed
usingtheGILDAS1softwarepackage.
AlthoughinthispapertheT scaleisused,onecaneasily
mb
Off4 (0,−24) convertthis intensity scale to ordinary flux density units using
the conversion factor S /T of 4.91 Jy K 1 for both the data
0.1 ν mb −
setsobtainedat115GHzand230GHz.Allthevelocitiesinthis
paperaregivenwithrespecttothelocalstandardofrest(LSR).
0 Totalintegrationtimes(asumoftheintegrationtimesfrom
two orthogonally polarized receivers) and noise levels of the
folded spectra are given in Table 1. The noise levels are given
asastandarddeviation(σ ,inthemainbeamtemperature)of
rms
alinearfittothefoldedspectrumexcludingallemissionfeatures
2 Off3 (0, 24)
andtheiraliases.
]
K
[b 1
m 3. Results
T
EmissionintheCO(1–0)transitionwasdetectedat6positions:
0
Off2, Off3, Off4, Off7, Off8, and at the V838 Mon position;at
threeofthosepositions,namelyatOff3,Off7,andOff8,alsoCO
12
Off7 (0, 48) (2–1)emissionisclearlypresent.Thebeamswithapositivede-
10
tection are shown with a solid line on Fig. 1. The spectra with
8
thedetectionsareshownonFig.2andalltheemissionlinesare
6 characterizedinTable2intermsoftheircentralvelocities,full
4 widthsathalfmaximum,intensitiesofthepeak,andintegrated
2 intensities. The values given in Table 2 are results of a single
Gaussiansfittothelineprofiles.Allthereducedspectraaredis-
0
1.2 playedinFigs.A.1andA.2intheonlineAppendixA.
1 The spectra displayed in Fig. 2 require a technical com-
Off8 (0,−48)
0.8 ment. Since frequency switching was employed as the observ-
0.6 ingmethod,the telluric(mesospheric)emissionofCO wasnot
0.4 removed from the spectra (see Thumetal. 1995, for more de-
0.2 tailes about telluric CO emission in frequency switching ob-
0 servations). The telluric emission appears at the LSR veloc-
−0.2 ity of the mesosphere in the time of the observations, which
−0.4 in our case was typically 8.8 km s−1. After folding proce-
∼
dure the telluric CO affects the spectra by the emission and its
30 35 40 45 50 55 60 65 70 75
two aliases, i.e. absorption-like features shifted by 7.8 MHz
V [km s−1] with respectto the emission feature.Unfortunately,a±t the Off8
LSR
(0, 48)positioncelestialemissionappearsat(unexpected)ve-
Fig.2. Spectra with detected emission in 12CO J = 1–0 (blue) loci−ty of 48.45 km s 1, which closely coincides with the posi-
−
and J = 2–1(red).Themeasuredparametersoftheseemission tion of the absorption alias of the CO (1–0) telluric emission
linesaregiveninTable2.Thestrong’absorption’featurepresent at 49.34 km s 1. Luckily, the emission line at 48.45 km s 1 is
− −
inthespectraoftheJ =1–0transitionat49kms−1isanaliasof notaffectedbythemesosphecicaliasonthespectrumbeforethe
amesosphericCOemission.IntheJ =2–1spectrathe’absorp- foldingprocedure.ThereforeonFig.2wepresentthespectrum
tion’featuresarealiasesoftheinterstellaremission(theyappear beforefoldingandallthemeasurementsgiveninTable2forthe
symmetricallyonbothsidesoftheemission).Sincethealiasof CO(1–0)lineat48.45kms 1 wereperformedontheunfolded
−
mesosphericemissionaffectstheprofileofinterstellarCO(1–0)
featureattheOff8positioninthefoldedspectrogram,weshow 1 Seehttp://www.iram.fr/IRAMFR/GILDAS
theunfoldedspectrumforthisposition.
4 T.Kamin´ski:COemissioninthefieldofthelightecho
Table2.ParametersobtainedfromsingleGaussiansfittedtothespectra.
12CO(1 0) 12CO(2–1)
position ∆α ∆δ V FWHM T− peak I V FWHM T peak I
LSR mb CO LSR mb CO
[ ] [ ] [kms 1] [kms 1] [K] [Kkms 1] [kms 1] [kms 1] [K] [Kkms 1]
′′ ′′ − − − − − −
V838Mon 0 0 53.4 1.1 0.10 0.13
Off2 –24 0 53.2 1.8 0.12 0.24
Off3 0 24 53.2 0.9 2.22 2.17 53.2 0.9 1.03 0.97
Off4 0 –24 53.3 1.7 0.12 0.22
Off7 0 48 53.3 1.0 9.47 9.77 53.3a 1.1a 12.70a 14.23a
Off8 0 –48 48.5b 0.8b 0.61b 0.54b 48.5 0.8 0.50 0.41
a theprofileisclearlynotasingleGaussian
b measurementsobtainedattheunfoldedspectrum
spectrum,whichhashoweverapoorernoisecharacteristicsthan loss before 2002, it would produce emission features certainly
theunfoldedonebyafactorof √2. broader and more complex than the ones we see in the two
In the following,we considerthe detected CO (1–0)emis- CO transitions (e.g. Teyssieretal. 2006). Similarly, the emis-
sion in two groups, namely weak (T < 0.15 K at the peak) sion found at the star position cannot be identified with the
mb
and strong features (with peak much greater than 0.15 K). matter lost during and after the 2002 eruption. Indeed, numer-
Weak emission is found at Off2, Off4, and at the star position. ousP-Cygniprofilesobservedduringtheoutburstrevealedmat-
The emission at all of these positions appears at a velocity of ter expelled with velocities of several hundred km s−1 (e.g.
53.3 km s 1 and does not have any detectable counterpart in Kipperetal. 2004) and the continuous outflow observed in
−
t∼he corresponding CO (2–1) spectra. Emission in CO (2–1) is V838 Mon since the outburst has a terminal velocity of about
notseen,mostprobablyduetohighernoiseinthe230GHzdata. 150kms−1(Munarietal.2007b).Ifanyofthislostmatterradi-
The strong emission of CO (1–0) is found at the three po- atesintheCOrotationaltransitions,theemissionshouldappear
sitions along the north-south direction: Off3, Off7, and Off8. asaverybroadfeaturewithFWHM>150kms−1.
EspeciallyprominentistheemissionatOff3andOff7.Thelines The narrowness of the detected∼CO lines indicates that the
both appear at radial velocity of 53.2 km s 1. The emission molecularemissionarisesinaninterstellarmedium.According
−
foundattheOff8position,ashasa∼lreadybeennoted,appearsat totheclassificationinvanDishoeketal.(1993)andonthebasis
theunexpectedradialvelocity48.45kms 1.Allthethreestrong ofline-widthmeasurements(Table2)wecanclassifythemolec-
−
CO(1–0)featureshavecounterpartsinthe(2–1)spectra.Ratios ularregionsweseeintheCOemissionasdiffuseclouds.Inlight
ofthoselinesarediscussedinSect.4.2. ofthe findingofAfs¸ar&Bond(2007), namelythatV838Mon
AscanbeseeninTable2,allthedetectedlinesareverynar- is a member of a B-stars association, the narrow CO emission
row.Theweakest CO (1–0)featuresappearsomewhatbroader canbeattributedtoaninterstellarmediumwithinthecluster.It
thanthestronglines.Notethatthefeaturefoundatthestarpo- maybethematterremainingafterthedissipationofmostofthe
sition,wheretheintegrationtimewasmuchlongerthanatOff2 parent molecular material from which the cluster had formed,
andOff4,isonlyslightlybroaderthatthestronglines,henceone muchlikethematterseeninthePleiadescluster.Wecannotrule
canstatethatthebroadeningoftheweakfeaturesisaneffectof out,however,thepossibilitythatweseeforegroundand/orback-
a lowersignalto noise ratioof the spectra.In general,the pro- groundcloudswith respect to the location of the cluster or the
filesofthedetectedlinesareverywelldescribedbyaGaussian staritself.Theradiallocationofthecloudsisdiscussedinmore
shape, except the most prominent line in our data, i.e. the CO detailinSect.4.3.
(2–1)featureatOff7,whichhasaslightlyasymmetricpeak. Using the well known XCO-factor method we can estimate
themassofmolecularmatterinsidethecloud.AsfoundinLiszt
(2007), the method gives satisfactory results even for very dif-
4. Discussion
fuse clouds,i.e. thosewith verysmallcolumndensitiesofCO.
Themassofmolecularhydrogencanbeexpressedas:
The IRAM observations reported here confirm the suggestion
made in Kamin´skietal. (2007b) that the CO emission discov-
M =X Ωd2m I , (1)
eredinthedirectionofV838Moninthelowangularresolution H CO H2 CO
KOSMAdata,isextendedandnotdirectlyrelatedtotheobject whereX istheconversionfactorforthecolumndensityofH
CO 2
thateruptedin2002.Indeed,themeasurementsindicatethatthe tointegratedintensityoftheCO (1–0)emission,Ωisthesolid
CO emissionhas a verycomplexspatialdistributionandis not angleofthe emittingregion,d isthe distanceto thecloud,and
onlylimitedtothepositionofV838Mon.Inthefollowingdis-
ICO = R TmbdV is the mainbeamintegratedintensity.Here we
cussionwe putsome constraintson theoriginofthemolecular
takeX =2.8(inunitsof1020cm 2K 1 km 1 s,Bloemenetal.
emission.Inparticular,weinvestigatethepossibilityofaphys- CO − − −
1986)althoughvaluesashighas6canbefoundinliteraturefor
ical connectionof the CO emitting matter with the dusty envi-
clouds in the outer Galaxy (e.g. Kamin´skietal. (2007b) found
ronmentrevealedbythelightecho.
X =5.4 for the dark clouds identified in the vicinity of V838
CO
Mon).WecanrewriteEq.(1)as:
4.1.Originoftheemission:circumstellarvs.interstellar
M =0.4Ωd2I M , (2)
H CO
Thestrongestargumentagainstcircumstellaroriginof theCO- ⊙
bearing gas is the narrowness of the emission lines. If the ex- whereΩisexpressedinarcmin2,dinkpc,andI inKkms 1.
CO −
tended molecular regionoriginatesas a result of a stellar mass The solid angle of a single beam at 115 GHz is approximately
T.Kamin´ski:COemissioninthefieldofthelightecho 5
0.144arcmin2.Weareinterestedinthetotalmassofthemolec- thatappearsat48.5kms 1maybeconsideredasemergingfrom
−
ularmatterradiatinginCO(1–0)inthesixpositions.Withthe a foregroundmolecularcloudwith respectto the SiO maser, if
sum of all the integrated intensities of 13.1 K km s 1 (see onetrustsinthekinematicaldistances.
−
∼
Table 2) and the distance to the star of 6.1 kpc (Sparksetal.
2007) we get a total mass of about 28 M . The CO emission
can be more extended than it appears in⊙our spatially limited 4.4.Associationwiththelightechomaterial?
measurements,sothetotalmassofmolecularmatterisprobably
even higher. Moreover,the above value does not accountfor a The CO emission is located in the field of the light echo. The
massofatomicgas,whichcancontributeconsiderablytotheto- questionthatnaturallyarisesiswhetherthemolecularemission
talmassofadiffusecloud.Thisroughestimateshows,however, is physically connectedwith the light echo material. The issue
that the mass of the sampled regions is already quite high and isworthwhiletoconsider,especiallyinthecontextofdiscussion
cannotbeinterpretedasmatterexpelledbythestar. abouttheoriginoftheechoingdust(seeSect.1).
Toanswerto theabovequestionthe relativelocationofthe
dustandtheradiatingmoleculesalongthelineofsightmustbe
4.2.Spatialcomplexityofthemoleculargas
known.Thedistancetotheechomaterialhasbeenestablishedby
a geometricanalysisof the echo evolution.As can be foundin
The CO (2–1) emission found in our data exhibit various in-
Bond(2007,seeFig.3therein),thereflectingregionsseeninthe
tensities with respect to the strength of the corresponding CO
echoshouldbelocated withinseveralpc fromthestar, so their
(1–0)emissiondetectedat the sameposition.The ratiosof the
heliocentricdistanceshouldbeabout6kpc(Sparksetal.2007).
integratedintensitiesoftheCO(1 0)to(2–1)lines, ,are
10:21
− R The distance to the CO-bearing gas can be only poorly con-
2.22, 0.69, and 1.31 for the positions Off3, Off7, and Off8, re-
strained. Kinematical distance to the componentat 53 km s 1,
spectively.Forahomogenousdistributionofinterstellargasone −
well correlated spatially with the echo on the plane of the sky,
wouldexpecttheCO(1 0)linetobestronger,sinceitwasob-
− is 7kpc,butduetostreamingmotionstherealdistancecanbe
served with the larger beam. The wide variety in the values of
∼
1 kpc higher or lower. Thus, the association of the molecular
inourdatacanbeinterpretedasaresultofahighlycom-
10:21 ∼
R gaswiththedustcannotberuledout.
plex distribution of the emitting gas. In other words, that can
be an effectof differentbeamfilling factorsforobservationsat ThecurrentunderstandingofV838Monseemstofavorsce-
115GHzand230GHz.Sharpinhomogeneitiesmustthenoccur narioswherethestarisconsideredasaveryyoungobject.Ifso,
onthe planeofthe sky atangularscales atleastcomparableto itshouldexhibitsystemic velocityveryclose tothe velocityof
ourbeam-sizeat230GHz,i.e.atrangesoforder10 .Thiscor- thelocalinterstellarmedium.Ifthestarhasthesamevelocityas
′′
respondstospatialscalesof 0.3pcatadistanceof6kpc.Such theSiOmaser,thenthelocalinterstellarmedium,includingthe
asmallscalestructureiscom∼monlyobservedinCOmapsofdif- lightechomaterial,wouldhavea velocityof54kms 1. Asal-
−
fuse molecularclouds(see Sect. 4.1). Alternatively,the variety readynotedintheprecedingsection,thisvalueagreesverywell
of valuescanbeexplainedbysomewhatunusualpopula- withthevelocityofmostoftheCOemission.Itsuggeststhatthe
10:21
R
tionofCOlevelsinthemolecularmedium,butthisseemstobe dustandmoleculargasarelocatedinthesamecloud.However,
unlikely. thissuggestionshouldbetreadedcautiously,since,inthecaseof
V838Mon,themasercanhavedifferentnaturethaninotherSiO
stellarsources(Deguchietal.2007),andconsequentlyitsradial
4.3.Thetwovelocitycomponents velocitycanbedifferentthantherealvelocityofthestar.
ThekinematicalcharacteristicsoftheCOemission(seeTable2) Furthermore,in the case of commonorigin of the dust and
indicates that the strong emission found in Off3 and Off7, to- CO-bearinggas,onemightexpectthatthereissomeoverallcor-
gether with the weak emission appearing in Off2, Off4, and in relation between the dust and CO distribution. Our data have
the star position, are all physically related. The emission fea- still too low angular resolution and too poor coverage of the
turesfoundatthesepositionsappearatnearlythesamevelocity fieldtolookforanymeaningfulcorrelationsinthebothdistribu-
of 53.3 0.1 km s 1 (1σ). Moreover, the emission sampled in tions,butsomegeneralremarkscanbemade.Ascanbeseenin
−
theCO±(1–0)data,isspatiallycontinuousandformsoneregion Fig.1,theCOemittingregionextendsbasicallyalongthenorth-
elongatedinthenorth-southdirection.Thus,theemissionlines southdirection,similarlytothemostprominentreflectionsseen
canbeinterpretedasemergingfromthesamediffusecloud.The in the light echo in the epoch close to the radio observations.
centralvelocityofthismolecularregionisveryclosetotheve- Theopticalechoishowevermoreextendedintheeast-westdi-
locityoftheSiOmaseremissionobservedfromthedirectionof rection. One should remember that the echo on a single epoch
V838Mon(54kms 1, Deguchietal.2005),whichisoftencon- image shows only a thin part of the whole dusty environment
−
sideredastheradialvelocityofthestaritself.Ifso,theemitting (e.g.Tylenda2004),whiletheCOemissionprobesthetotalcol-
molecularmatterseeninourdatashouldresideveryclosetothe umndensityofthecloudalongthelineofsight.Moreappropri-
star(seealsoSect.4.4). atewouldbe,though,tocomparethemapofmolecularemission
withtheopticalpicturessummedoverthetimeoftheobservable
The emission detected in the Off8 position is the only one
centeredat48.5km s 1. None ofthe spectracontainsan emis- evolutionoftheecho.
−
sioncomponentatintermediatevelocitiesbetween48.5kms 1 Insummary,thecurrentdatadonotallowustoconclusively
−
and53kms 1, hencethereisnotransitionzonebetweenthose verifywhethertheCOemissionisassociatedwiththelightecho
−
twokinematicregionsintheareaofourmeasurements.Thisfur- material,buttheymakesuchapossibilityveryprobable.Tover-
ther indicates that the portions of gas emitting at the two dis- ify theidea further,a directmeasurementof the radialvelocity
tinctvelocitiesarenotphysicallyconnectedandtheyformsepa- of the echoinggasor morereliable constraintson the systemic
ratemolecularcomplexes.Assumingstandardrotationlawofthe velocityofV838Monareneeded.ACOmapwithagoodsam-
GalaxyinthedirectionofV838Mon,thegasatlowerradialve- plingandcoveringaregionlargerthanthesizeofthelightecho
locityshouldreside 1kpcclosertotheobserver.Theemission wouldbeveryhelpfulaswell.
∼
6 T.Kamin´ski:COemissioninthefieldofthelightecho
5. Concludingremarks Retter,A.,&Marom,A.2003,MNRAS,345,L25
Retter,A.,Zhang,B.,Siess,L.,&Levinson,A.2006,MNRAS,370,1573
Wepresentobservationstowards13positionsinthefieldofthe Soker,N.,&Tylenda,R.2003,ApJ,582,L105
lightechoofV838MonintheCO (1–0)and(2–1)transitions. Soker,N.,&Tylenda,R.2007,inTheNatureofV838MonanditsLightEcho,
Themeasurementsrevealanextendedmolecularregionaround eds.R.L.M.Corradi&U.Munari,ASPConf.Ser.,363,280
Sparks, W. B., Bond, H. E., Cracraft, M., et al. 2007, ArXiv e-prints, 711,
thestarattwodistinctradialvelocities.TheCOemittingregion
arXiv:0711.1495
iselongatedinthenorth-southdirectionandexhibitsaverycom- Teyssier,D.,Hernandez,R.,Bujarrabal,V.,Yoshida,H.,&Phillips,T.G.2006,
plexdistributionontheplaneofthesky.WeidentifytheCOlines A&A,450,167
asemergingfromdiffuseinterstellarclouds.Nomolecularemis- Thum,C,Sievers,A.,Navarro,S.,Brunswig,W.,Pen˜alver,J.1995,IRAMTech.
Report228/95.
sionthatcanbeassociatedwiththestaritselfwasdetected.
Tylenda,R.2004,A&A,414,223
Thepossibleassociationofthemolecularemissionwiththe
Tylenda,R.2005,A&A,436,1009
lightechomaterialhasbeeninvestigated.AlthoughtheCOemis- Tylenda,R.,&Soker,N.2006,A&A,451,223
sion appears in the field of the light echo, its detailed spatial Tylenda,R.,Soker,N.,&Szczerba,R.2005,A&A,441,1099
distribution correlates only weakly with the light echo image. Wisniewski,J.P.,Morrison,N.D.,Bjorkman,K.S.,etal.2003,ApJ,588,486
Ontheotherhand,thevelocityoftheCOemissionagreesvery
wellwiththevelocityoftheSiOmaserdiscoveredfromthedi-
rection of V838 Mon, making the collocation of the dust and
CO-bearinggasprobable.
Tomoredeeplyinvestigatetheoriginofthemolecularemis-
sioninthefieldoftheecho,moreextendedmapoftheregionin
differentmolecular transitions is needed. A fully sampled map
oftheechoingregionwouldbehelpfultodrawmoreconclusive
statements about the suggested connection with the light echo
material.
Acknowledgements. TheauthorthanksR.Tylendaandtheanonymousreferee
for their useful comments and suggestions towards improving the paper. The
authorisalsogratefultoM.Pułeckaforherhelpinobtainingtheobservations
attheIRAM30mtelescope.Theresearchreportedinthispaperwassupported
fromagrantno.N20300432/0448financedbythePolishMinistryofScience
andHigherEducation.
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T.Kamin´ski:COemissioninthefieldofthelightecho,OnlineMaterialp1
AppendixA: Presentationofallthespectra
T.Kamin´ski:COemissioninthefieldofthelightecho,OnlineMaterialp2
Fig.A.1.Thefoldedspectrain12COJ = 1 0foralltheobserved13positions.The’absoprtion’featureat 49kms 1isanalias
−
− ∼
ofthetelluricCOemission.Thespectrawithpositvedetectionsareplottedwithcolorlines.
T.Kamin´ski:COemissioninthefieldofthelightecho,OnlineMaterialp3
Fig.A.2.Thefoldedspectrain12CO J = 2 1foralltheobserved13positions.Thespectrawith positivedetectionareplotted
−
withcolorlines.