Decrease of the organic deuteration during the evolution of Sun-like protostars: the case of SVS13-A

0.34 MB·

by  E. Bianchi

Mon.Not.R.Astron.Soc.000,1–13(2017) Printed31January2017 (MNLATEXstylefilev2.2)
Decrease of the organic deuteration during the evolution of Sun-like
protostars: the case of SVS13-A
7
1 E. Bianchi1,2∗ , C. Codella1, C. Ceccarelli3,4,1, F. Fontani1, L. Testi5,1, R. Bachiller6,
0 B. Lefloch3,4, L. Podio1, V. Taquet7
2
1INAF-OsservatorioAstrofisicodiArcetri,L.goE.Fermi5,Firenze,50125,Italy
n
2DipartimentodiFisicaeAstronomia,Universita` degliStudidiFirenze,Italy
a
J 3Univ.GrenobleAlpes,IPAG,F-38000Grenoble,France
4CNRS,IPAG,F-38000Grenoble,France
0
5ESO,KarlSchwarzschildstr.2,D–85748GarchingbeiMuenchen,Germany
3 6IGN,ObservatorioAstrono´micoNacional,CalleAlfonsoXIII,28004Madrid,Spain
7LeidenObservatory,LeidenUniversity,POBox9513,2300–RA,Leiden,TheNetherlands
]
R
S
Accepteddate.Receiveddate;inoriginalformdate
.
h
p
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ABSTRACT
o
r
t Wepresenttheresultsofformaldehydeandmethanoldeuterationmeasurementstowards
s
a theClassIlow-massprotostarSVS13-A,intheframeworkoftheIRAM30-mASAI(Astro-
[ chemicalSurveysAtIRAM)project.Wedetectedemissionlinesofformaldehyde,methanol,

andtheirdeuteratedforms(HDCO,D CO,CHD OH,CH OD)withE upto276K.The
1 2 2 3 up
v formaldehydeanalysisindicatesTkin ∼15–30K,nH2 >106cm−3,andasizeofabout1200
6 AUsuggestinganoriginintheprotostellarenvelope.Formethanolwefindtwocomponents:
5 (i) a high temperature (Tkin ∼ 80 K) and very dense (> 108 cm−3) gas from a hot corino
6 (radius ≃ 35 AU), and (ii) a colder (T 6 70 K) and more extended (radius ≃ 350 AU)
kin
8 region.
0 Thedeuteriumfractionationis 9×10−2 forHDCO,4 ×10−3 forD CO, and2–7 ×
2
1. 10−3forCH2DOH,uptotwoordersofmagnitudelowerthanthevaluesmeasuredinClass0
0 sources.Wederivealsoformaldehydedeuterationintheoutflow:4×10−3,inagreementwith
7 whatfoundintheL1157–B1protostellarshock.Finally,weestimate[CH DOH]/[CH OD]≃
2 3
1 2.ThedecreaseofdeuterationintheClassIsourceSVS13-AwithrespecttoClass0sources
: canbeexplainedbygas-phaseprocesses.Alternatively,alowerdeuterationcouldbetheeffect
v
i ofagradualcollapseoflessdeuteratedexternalshellsoftheprotostellarevelope.Thepresent
X measurementsfillinthegapbetweenprestellarcoresandprotoplanetarydisksinthecontext
r oforganicsdeuterationmeasurements.
a
Key words: Molecular data – Stars: formation– radio lines: ISM – submillimetre: ISM –
ISM:molecules
1 INTRODUCTION through gas phase chemistry in prestellar cores (Roberts & Mil-
lar 2000b). The picture is different for formaldehyde as well as
Deuteriumfractionationistheprocessthatenrichestheamountof
for methanol around protostars which are mostly formed via ac-
deuterium withrespect tohydrogen inmolecules. WhiletheD/H
tivegrainsurfacechemistry(e.g.Tielens1983).DeuteratedH CO
elementary abundance ratio is ∼ 1.6 × 10−5 (Linsky 2007), for 2
andCH OHarethenstoredinthegrainmantlestobeeventually
moleculesthisratiocanbedefinitelyhigherandcanbeaprecious 3
released into the gas phase once the protostar is formed and the
tool to understand the chemical evolution of interstellar gas (see
grainmantlesareheatedandsuccessivelyevaporated(e.g.Cecca-
e.g. Ceccarelli et al. 2014, and references therein). In particular,
rellietal.1998,2007;Pariseetal.2002,2004,2006)orsputtered
duringtheprocessleadingtotheformationofaSun-likestar,large
by protostellar shocks (Codella et al. 2012; Fontani et al. 2014).
deuterationofformaldehydeandmethanolisobservedincoldand
Asaconsequence,D/Hcanbeusedasfossilrecordofthephysical
dense prestellar cores (e.g. Bacmann et al. 2003, Caselli & Cec-
conditionsatthemomentoftheicywaterandorganicsformation
carelli2012,andreferencestherein).Formaldehydecanbeformed
(e.g.Taquetetal.2012,2013,2014).
While deuterated molecules have been detected towards the
∗ E-mail:[email protected] earlystagesoftheSun-likestarformation(i.e.prestellarcoresand
2 E. Bianchiet al.
Class0objects)aswellasintheSolarSystem(seee.g.Ceccarelli between2012and2014,usingthebroad-bandEMIRreceivers.In
etal.2014,andreferencestherein),nocleardetectionhasbeenob- particular the observed bands are at 3 mm (80–116 GHz), 2 mm
tainedforintermediateevolutionaryphases(ClassIandIIobjects). (129–173 GHz), and 1.3 mm (200–276 GHz). The observations
A handful of measurements of deuterium fractionation inClass I were acquired in wobbler switching mode, with a throw of 180′′
sources exists (i.e.Roberts &Millar 2007) but they refer only to towards the coordinates of SVS13-A, namely α = 03h 29m
J2000
fewtransitionssamplinglargeregions(upto58′′)wellbeyondthe 10s.42,δ =+31◦16′0′.′3Thepointingwascheckedbyobserv-
J2000
protostellarsystem.Inaddition,Loinardetal.(2002)reportedmea- ingnearbyplanetsorcontinuumsourcesandwasfoundtobeaccu-
surements of double deuterated formaldehyde in star-forming re- ratetowithin2′′–3′′.ThetelescopeHPBWsrangebetween≃9′′
gionswithboththeSEST(SwedishESOSubmillimetertelescope) at276GHzto≃30′′at80GHz.Thedatareductionwasperformed
andIRAMsingledishessuggestingadecreasewiththeevolution- usingtheGILDAS–CLASS2package.Calibrationuncertaintiesare
arystage.Watanabeetal.(2012)reportedthedeuteriumfractiona- stimatedtobe≃10%at3mmand∼20%atlowerwavelengths.
tionmeasurementstowardRCrAIRS7B,alow-massprotostarin Notethatsomelines(seeSect.3)observedat2mmand3mm(i.e.
theClass0/Itransitionalstage.TheydetectedH CO,measuringa withaHPBW>20′′)areaffectedbyemissionatOFFpositionob-
2
lowerD/H(∼0.05)comparedtodeuterationmeasuredinClass0 servedinwobblermode.Lineintensitieshavebeenconvertedfrom
objects.However,inthiscasethelowdeuteriumfractionationratios antenna temperature to main beam temperature (T ), using the
MB
donotdirectlysuggestanevolutionarytrend.Thealteredchemical mainbeamefficienciesreportedintheIRAM30-mwebsite3.
compositionoftheenvelopeofRCrAIRS7Bcanbearesultofthe
heating of the protostar parent core by the external UV radiation
fromthenearbyHerbigAestarRCrA.
3 RESULTS
SystematicobservationsofD/HinClassIobjectsaretherefore
required to understand how the deuterium fractionation evolves 3.1 Lineidentification
from prestellar cores to protoplanetary disks. In this context we
Line identification has been performed using a package devel-
presentastudyofformaldehydeandmethanoldeuterationtowards
opedatIPAGwichallowstoidentifylinesinthecollectedASAI
theClassIlow-massprotostarSVS13-A.
spectral survey using the Jet Propulsor Laboratory (JPL4, Pick-
ettetal.1998)andCologneDatabaseforMolecularSpectroscopy
(CDMS5;Mu¨lleretal.2001,2005)moleculardatabases.Wedou-
1.1 TheSVS13starformingregion
blecheckedthelineidentificationswiththeGILDASWeedspack-
TheSVS13starformingregionislocatedintheNGC1333cloud age (Maret et al. 2011). We detected several lines of H13CO,
2
inPerseusatadistanceof 235 pc(Hirotaetal. 2008). Itisasso- HDCO,D CO,13CH OHandCH DOH(seeTables1and2).Ex-
2 3 2
ciatedwithaYoungStellarObjects(YSOs)cluster,dominated,in amplesofthedetectedlineprofilesinT scaleareshowninFig-
MB
themillimeterbytwoobjects,labelledA,andB,respectively,sep- ure1.Thepeakvelocitiesofthedetectedlinesarebetween+8km
aratedby∼15′′.(seee.g.Chinietal.1997;Bachilleretal.1998; s−1 and+9kms−1,beingconsistent,once considered thefitun-
Looneyetal.2000;Chenetal.2009;Tobinetal.2016,andrefer- certainties,withthesystemicvelocityofbothAandBcomponent
encestherein).Interestingly,SVS13-AandSVS13-Bareassociated of SVS13(+8.6 km s−1, Chen et al. 2009; Lo´pez-Sepulcre et al.
withtwodifferentevolutionarystages.Ontheonehand,SVS13-B 2015).WefittedthelineswithaGaussianfunction,andexcluded
isaClass0protostarwithLbol≃1.0Lsun(e.g.Tobinetal.2016) fromtheanalysisthoselineswith|vpeak -vsys| >0.6km/splau-
driving a well collimated SiO jet (Bachiller et al. 1998). On the siblyaffectedbylineblending.Weselectfortheanalysisonlythe
other hand, SVS13-A is definitely more luminous (≃ 32.5 Lsun, lineswithasignaltonoise(S/N)higherthan4σ.Thespectralpa-
Tobinet al.2016) and isassociated withanextended outflow (> rametersofthedetectedlines,aswellastheresultsfromtheGaus-
0.07 pc, Leflochet al. 1998, Codella et al. 1999) as well as with sianfits,arepresentedinTables1and2,wherewereportthefre-
thewell-knownchainofHerbig-Haro(HH)objects7-11(Reipurth quencyofeachtransition(GHz),thetelescopeHPBW(′′),theex-
et al. 1993). In addition, SVS13-A has a low Lsubmm/Lbol ratio citationenergiesoftheupperlevelEup(K),theSµ2product(D2),
(∼0.8%) andahighbolometric temperature(Tbol ∼188K, To- thelinerms(mK),thepeaktemperature(mK),thepeakvelocities
bin et al. 2016). Thus, although still deeply embedded in a large (kms−1),thelinefullwidthathalfmaximum(FWHM)(kms−1)
scaleenvelope(Leflochetal.1998),SVS13-Aisconsideredamore andthevelocityintegratedlineintensityI (mKkms−1).
int
evolvedprotostar,alreadyenteredintheClassIstage.Forallthese
reasons, SVS13-A is an almost unique laboratory to investigate
howdeuterationchangefromtheClass0totheClassIphases.In 3.2 Formaldehydeisotopologues
Sect.2theIRAM30-mobservationsaredescribed, inSect.3we
WereportthedetectionofseverallinesofH COanditsisotopo-
report theresults,whileinSect.4wedevelop theanalysisof the 2
logues H13CO, HDCO and D CO. The measured intensity ratio
data;Sect.5isfortheconclusions. 2 2
between thelow energy transitionsof H CO and H13CO(ase.g.
2 2
the31,3–21,2 atEup=32K),is∼25,avaluewellbelowtheme-
dianvaluefortheinterstellarmediumof12C/13C∼68(Milamet
2 OBSERVATIONS al.2005).ThisindicatesthattheobservedH COtransitionsareop-
2
ticallythick.ThereforeweuseH13COtoderivetheformaldehyde
Theobservationsof SVS13-AwerecarriedoutwithIRAM30-m 2
deuteration.
telescope near Pico Veleta (Spain), in the framework of the As-
trochemicalSurveysAtIRAM1 (ASAI)LargeProgram.Thedata
consistofanunbiasedspectralsurveyacquiredduringseveralruns 2 http://www.iram.fr/IRAMFR/GILDAS
3 http://www.iram.es/IRAMES/mainWiki/Iram30mEfficiencies
4 https://spec.jpl.nasa.gov/
1 www.oan.es/asai 5 http://www.astro.uni-koeln.de/cdms/
3
Wedetected7linesofH13CO,5linesofHDCOand5linesof linesofCH DOHwithexcitationenergiesinthe20–276Krange.
2 2
D CO,withexcitationenergies,E ,inthe10–45Krange.Exam- Examplesofthedetectedlineprofilesformethanolisotopologues
2 up
plesofthedetectedlineprofilesareshowninFigure1;thedetected areshown inFigure1.Thespectralparametersandtheresultsof
transitionsandtheobservationalparametersaredisplayedinTable thegaussianfitareshowninTables1and2.Thelineprofilesare
1andTable2.Thelinesprofilesareclosetoagaussianshapeand broaderthanforformaldehydeisotopologues,withaFWHMupto
thepeak velocitiesareclose tothe systemic source velocity with 5.4kms−1.Noneoftheobservedprofilesshowabsorptionfeatures
valuesbetween+7.8kms−1and+9.0kms−1whiletheFWHMis due tothe wobbler contamination, pointing toan emittingregion
between0.9and2.5kms−1.ThreelinesofH13CO(withfrequen- smallerthanformaldehyde.
2
cies137.45 GHz, 141.98 GHz, and 146.64 GHz) and one lineof Interestingly, we detect two different transitions of both
HDCO(withfrequency134.2848) aredetectedinthe2mmband CHD OHandCH ODwithE between33Kand77K(seeTa-
2 3 up
andtheyareaffectedbycontaminationofemissionintheoffposi- ble 1 and Figure 3). The peak velocities are consistent with the
tions(seeSect.2fordetailsontheobservingtechniques),consis- systemic source velocity and the FWHMs are in agreement with
tentlywiththeanalysisreportedbyLo´pez-Sepulcreetal.(2015), thoseofthelinesfromtheothermethanolisotopologues.
using ASAI spectra. Thecontaminated linescorrespond toasize
of the telescope HPBW > 16′′. In these cases the measured in-
tensities will be treated as lower limits in the rotational diagram
3.4 Summaryoftheresults
analysis(seeSect.4.2).FortheD CO,onlyonelineisdetectedat
2
2mmbutitdoesnotshowanyabsorptionfeatureduetothewob- Insummary,thebulkofmethanolandformaldehydeisotopologues
bler contamination. Thiscan bean indication of a morecompact linesaredetectedinthe1mmband.Forthisreason,thetempera-
regionemittinginD COwithrespecttothatofHDCOemission. tureestimatefromtherotationaldiagramanalysis(seeSection4.2)
2
A similar behaviour has been observed in a different context by is not affected by the beam dilution. The 30-m HPBW is ∼10′′
Fuenteetal.(2015) towardstheintermediate-massClass0proto- at1mm,whichensuresthattheemissioniscomingfromSVS13-
starNGC7129FIRS2.Theydetected,usinginterferometricobser- A withno contamination fromSVS13-B(theseparation between
vations,anintenseandcompactD COcomponentassociatedwith SVS13-Aandthecompanion protostar is∼15′′).Thelinescoll-
2
the hot core. On the other hand Ceccarelli et al. (2001) detected lectedinthe2and3mmbandscouldbecontaminatedbytheemis-
in thelow-mass Class0 protostar IRAS16293-2422, an extended sionfromSVS13-B,becausetheHPBWislarger,buttheyareonly
D2CO emission (up to ∼5000 AU), associated with the external ahandfuloflines.
envelope. The present data do not allow us to draw reliable con- Interestingly,theformaldehydeprofilesshowlinewingsthat
clusions on the relative size of the two deuterated formaldehyde suggestemissionduetotheextendedoutflowdrivenbySVS13-A
isotopologues.However,inthecaseofSVS13-Aamorecompact (>0.07pc,Leflochetal.1998,Codellaetal.1999).
sizeissuggestedbythebroaderlineprofilesofD COwithrespect
2
toHDCO(seeFigure2).InFigure2weshowthedistributionofthe
linewidthsofthedetectedHDCOlinesinhatchedblueandD CO
2
linesincyano.ThebulkoftheHDCOlineshasaFWHMbetween 4 DISCUSSION
1.5and2.0kms−1whilefortheD COthepeakofthedistribution
2
is in the 2.0–2.5 km s−1 range. A further discussion on this will 4.1 LVGanalysis
bedoneinSection4followingtheresultsoftherotationaldiagram Weanalysed the H 13CO and 13CH OH observed lines with the
2 3
analysis. non-LTELargeVelocityGradient(LVG)approachusingthemodel
Interestingly, three lines of low excitation (Eup < 35 K) of described in Ceccarelli et al. (2003). For methanol we used the
H123CO (with frequencies 212.81 GHz, 206.13 GHz and 219.91 CH3OH-H2 collisional coefficients provided by the BASECOL
GHz)andalltheHDCOlines(exceptforthelineinthe2mmband) database (Dubernet et al. 2013). Inthe caseof formaldehyde, we
show weak (∼ 30 mK) wings clearly indicating emission due to consideredonlytheorthoform,forwhichtheH CO-H collisional
2 2
outflowsthatweanalyseseparatelyfromthemainlinecomponent. coefficients(Troscomptetal.2009a)areavailable.Weassumeda
BoltzmanndistributionfortheH ,usingforthemethanolanalysis
2
thestatisticalortho-to-pararatioof3.Inthecaseofformaldehyde
3.3 Methanolisotopologues
weassumedaortho-to-pararatioclosetozerofollowingTroscompt
Similarlytoformaldehyde,thedetectedlinesofCH OHareopti- et al. (2009b). We ran grids of models varying the kinetic tem-
3
callythick.Weverifieditthroughthemeasuredratiobetweenthe perature, T , (from 10 to 200 K), the H density, n , (from
kin 2 H2
insensities of CH3OH and 13CH3OH (as e.g. the 51,5–41,4 ++ 104 to 1010 cm−3), the H213CO column density, N(13H2CO),
at Eup = 49 K) that is∼ 2. For thisreason also in thiscase, we (from 1011 to 1013 cm−2), and the 13CH3OH column density,
use 13CH3OH to calculate methanol column density. In the case N(13CH3OH), (from 1016 to 1018 cm−2), while the emitting
of methanol, theprocess of lineidentificationwas morecomplex size,θ ,wasleftasfreeparameter.
s
than formaldheyde. This is due to the very rich spectra observed In the case of formaldehyde, the best fit was obtained with
withASAItowardsSVS13-Awithaconsequentchallenginglines N(13H2CO)=5.5×1012cm−2andθs=5′′±1′′:Figure4(upper
identification for a complex molecule such as CH OH. In addic- panel)showstheχ2 contourplotasafunctionofthetemperature
3 r
tion tothe criteriasummarized in Section3.1, wefurther require andH densityusingthesevalues.Thetemperaturescorresponding
2
FWHM>2kms−1todiscardanypossiblefalseidentification.For tothebestfitsolutionareT =20-25Kandthedensityarequite
kin
transitionswithmultiplecomponents(e.g.13CH3OH21,1–11,0and highnH2≃0.2–2×107cm−3,suggestingtobeclosetoLTE.Fig-
2 –1 –)weselectonlythelinesforwhichthedifferentcompo- ure4(lowerpanel)shows,forthebestfitsolution,theratiobetween
1,1 1,0
nent intensities are close to the expected LTE (Local Thermodi- themeasuredlinesintensitiesandtheLVGmodelpredictions,asa
namicEquilibrium)relativeintensities. functionofthelineupperlevelenergy.Thedetectedtransitionsare
Wereportthedetectionof18transitionsof13CH OHand27 predicted to be optically thin (opacities between 0.03 and 0.06).
3
4 E. Bianchiet al.
Figure1.ExamplesoflineprofilesinTMBscale(notcorrectedforthebeamdilution):speciesandtransitionsarereported.Theverticaldashedlinestandsfor
theambientLSRvelocity(+8.6Kms−1,Chenetal.2009)
Followingthissuggestion,weconsideredseparatelythelineswith
higher excitations (E > 40 K) observed with similar HPBWs
up
(between9′′and15′′).Thesolutionwiththelowestχ2corresponds
r
to N(13CH3OH) = 9 × 1016 cm−2 and an emitting size of θs
= 0′.′3 ± 0′.′1, i.e. a radius of 35 AU (see Figure 5). The best fit
solution corresponds to a temperature of T = 80 K and very
kin
high-densities,n >108 cm−3.Thelineopacitiesvaryfrom0.8
H2
to2.5,beingthusmoderatelyopticallythin.Allthesevaluessug-
gest that the emission detected at high excitations is dominated
by a hot corino, an environment which is typically very abun-
dant in methanol, due to thermal evaporation of the dust mantles
(e.g.Caselli&Ceccarelli2012).Interestingly,theoccurrenceofa
hot-corinoaroundSVS13-Ahasbeenrecentlysuggestedbyhigh-
excitationHDOlines,alsoobservedintheASAIcontextandindi-
catingaT largerthan150Konsmallerspatialscales(aradius
kin
∼25AU;Codellaetal.2016).
Theanalysisoftheremaining4lines(observedwithHPBWs
largerthan15′′)isnotstraightforwardgiventhe4transitionshave
Figure2.Distribution ofthelinewidth (FWHM)oftheobservedHDCO
almost the same E (20 – 28 K). The LVG approach suggests
andD2COlines.CyanoisforD2COandbluehatchedisforHDCO. typicalsolutionswituhpcolumndensitiesofN(13CH3OH)∼1015
cm−2,temperatures670K,densitiesatleast106cm−3,andsizes
≃ 2′′ – 4′′. The line opacities in this case range from 0.007 to
TheLVGanalysisclearlysupportstheassociationofformaldehyde 0.02, being thus optically thin. The lower densities and the more
withtheprotostellarenvelopewithasizeof∼1200AU. extended emitting size suggest that we are sampling a more ex-
Differentisthecaseof13CH3OHforwhichtheLVGmodel tendedregion(aradius∼350AU)aroundtheprotostarwherethe
does not converge towards a solution suggesting we are mixing temperatureisstillhighenoughtoallowthemethanolmoleculesto
emissionfromdifferentregions,possiblyduetodifferentHPBWs. bereleasedfromgrainmantles.
5
Table1.Listoftransitionsandlineproperties(inTMBscale)oftheHDCO,D2COandCH2DOHemissiondetectedtowardsSVS13-A
Transition νa HPBW Eupa Sµ2a rms Tpeakb Vpeakb FWHMb Iintb
(GHz) (′′) (K) (D2) (mK) (mK) (kms−1) (kms−1) (mKkms−1)
Deuteratedspecies
HDCO21,1–11,0 134.2848 18 18 8 17 158(5) +8.31(0.05) 1.1(0.1) 188(17)
HDCO31,2–21,1 201.3414 12 27 14 19 334(22) +8.43(0.03) 1.6(0.1) 561(21)
HDCO41,4–31,3 246.9246 10 38 20 17 312(22) +8.50(0.03) 1.9(0.1) 619(18)
HDCO40,4–30,3 256.5854 10 31 22 10 376(20) +8.54(0.01) 1.9(0.0) 777(11)
HDCO41,3–31,2 268.2920 9 40 20 21 207(21) +8.55(0.05) 2.0(0.1) 451(23)
p-D2CO31,3–21,2 166.1028 15 21 14 11 33(9) +8.79(0.21) 2.2(0.5) 79(15)
p-D2CO41,4–31,3 221.1918 11 32 20 16 92(7) +8.74(0.09) 1.8(0.2) 178(17)
o-D2CO40,4–30,3 231.4103 11 28 43 11 194(12) +8.88(0.03) 1.9(0.1) 381(12)
o-D2CO42,2–32,1 236.1024 10 50 33 13 56(7) +8.95(0.13) 2.4(0.3) 144(16)
p-D2CO41,3–31,2 245.5329 10 35 20 11 55(7) +8.85(0.11) 2.4(0.3) 139(13)
CH2DOH20,2–10,1e1 89.2753 28 20 1 3 12(2) +8.65(0.37) 4.2(1.1) 51(9)
CH2DOH61,5–60,6e0 99.6721 25 50 7 2 15(2) +8.09(0.17) 4.1(0.5) 68(6)
CH2DOH71,6–70,7e0 105.0370 23 65 8 3 17(3) +8.66(0.18) 3.1(0.4) 57(6)
CH2DOH31,2–21,1e1 135.4529 18 29 2 8 30(6) +8.48(0.22) 3.0(0.5) 98(15)
CH2DOH31,3–40,4e1 161.6025 15 29 1 9 36(7) +8.71(0.19) 2.7(0.4) 103(15)
CH2DOH51,5–41,4o1 221.2730 11 55 4 17 57(7) +8.31(0.19) 3.2(0.4) 195(22)
CH2DOH50,5–40,4e1 222.7415 11 46 4 10 75(8) +8.45(0.10) 4.3(0.2) 342(15)
CH2DOH52,3–41,4e0 223.0711 11 48 3 8 59(9) +7.98(0.11) 4.5(0.2) 284(13)
CH2DOH53,3–43,2o1c 223.1535 11 87 2 10 56(7) +8.34(0.13) 3.8(0.3) 223(15)
CH2DOH53,2–43,1o1c 223.1536
CH2DOH52,3–42,2e1 223.3155 11 59 3 12 70(8) +8.12(0.10) 3.0(0.2) 228(15)
CH2DOH54,2–41,1e0c 223.6162 11 95 1 10 47(11) +8.25(0.14) 3.5(0.3) 174(14)
CH2DOH54,1–40,0e0c 223.6162
CH2DOH51,4–41,3e1 225.6677 11 49 4 11 56(11) +8.18(0.15) 4.0(0.3) 237(17)
CH2DOH51,4–41,3e0 226.8183 11 37 3 10 51(10) +8.08(0.14) 3.8(0.3) 203(14)
CH2DOH152,13–151,14e0 228.2461 11 276 20 29 89(12) +7.84(0.20) 3.0(0.4) 285(37)
CH2DOH92,7–91,8e0 231.9692 11 113 11 14 99(8) +8.72(0.09) 3.7(0.2) 390(19)
CH2DOH82,6–81,7e0 234.4710 10 94 10 13 67(8) +9.17(0.14) 4.1(0.3) 293(19)
CH2DOH72,5–71,6e0 237.2499 10 76 8 12 64(12) +8.31(0.13) 3.9(0.3) 266(18)
CH2DOH71,6–62,4o1 244.5884 10 83 2 16 61(10) +8.09(0.18) 3.6(0.6) 231(26)
CH2DOH32,1–31,2e0 247.6258 10 29 2 9 48(8) +8.26(0.13) 3.7(0.3) 189(13)
CH2DOH32,2–31,3e0 255.6478 10 29 2 9 58(8) +8.61(0.12) 5.3(0.4) 331(16)
CH2DOH41,4–30,3e0 256.7316 10 25 3 9 61(8) +8.32(0.11) 4.3(0.2) 278(14)
CH2DOH42,3–41,4e0 258.3371 10 38 3 14 60(6) +8.33(0.16) 4.7(0.4) 302(21)
CH2DOH52,4–51,5e0 261.6874 9 48 4 17 45(9) +8.09(0.26) 4.3(0.5) 205(24)
CH2DOH130,13–121,12e0 262.5969 9 194 5 17 54(12) +8.51(0.21) 3.8(0.4) 219(23)
CH2DOH61,6–51,5e0 264.0177 9 48 4 14 64(7) +8.40(0.12) 2.8(0.3) 192(17)
CH2DOH72,6–71,7e0 270.2999 9 76 6 14 55(13) +8.35(0.17) 4.2(0.4) 243(19)
CH2DOH61,5–51,4e1 270.7346 9 62 4 16 60(10) +8.21(0.16) 3.5(0.3) 222(20)
CHD2OH50–40e1 207.771 11 48 4 14 43(9) +8.69(0.20) 2.7(0.4) 125(17)
CHD2OH53−–43−e1c 207.868 11 77 2 11 35(10) +7.21(0.24) 4.1(0.5) 153(17)
CHD2OH53−–43−e1c 207.869
CH3OD51+–41+ 223.3086 11 39 3 20 32(10) +8.53(0.41) 3.2(0.9) 108(27)
CH3OD50+–40+ 226.5387 11 33 4 18 48(10) +8.87(0.28) 4.6(0.6) 232(28)
aFrequenciesandspectroscopicparametersofHDCOandD2COhavebeenextractedfromtheCologneDatabaseforMolecularSpectroscopy(Mu¨lleretal.
2005).ThoseofCH2DOHareextractedfromtheJetPropulsionLaboratorydatabase(Pickettetal.1998).bTheerrorsinbracketsarethegaussianfit
uncertainties.cThelinescannotbedistinguishedwiththepresentspectralresolution.
4.2 RotationalDiagramanalysis tributionofalltheenergylevels,isdescribedbyaBoltzmanntem-
perature, that is the rotational temperature T . The upper level
rot
columndensitycanbewrittenas:
The LVG analysis previously described suggests LTE conditions
and opticallythinlines.Asaconsequence weused therotational
diagramanalysistodeterminethetemperatureandthecolumnden-
sityofformaldehydeandmethanol isotopologuesthroughamore 8πkν2 1
directapproach.Foragivenmolecule,therelativepopulationdis- Nu = hc3A η Z TmbdV (1)
ul bf
6 E. Bianchiet al.
Table2.Listoftransitionsandlineproperties(inTMBscale)oftheH123COand13CH3OHemissiontowardsSVS13-A
Transition νa HPBW Eupa Sµ2a rms Tpeakb Vpeakb FWHMb Iintb
(GHz) (′′) (K) (D2) (mK) (mK) (kms−1) (kms−1) (mKkms−1)
Isotopologues
o-H123CO21,2–11,1 137.4500 18 22 24 10 63(4) +8.50(0.08) 1.0(0.2) 64(10)
p-H123CO20,2–10,1 141.9837 17 10 11 10 54(10) +8.46(0.10) 1.1(0.2) 62(10)
o-H123CO21,1–11,0 146.6357 17 22 24 14 105(4) +8.39(0.06) 0.9(0.1) 98(13)
o-H123CO31,3–21,2 206.1316 12 32 44 13 124(15) +8.58(0.06) 2.5(0.2) 332(18)
p-H123CO30,3–20,2 212.8112 12 20 16 10 70(6) +7.76(0.14) 2.0(0.3) 235(65)
o-H123CO31,2–21,1 219.9085 11 33 43 10 134(12) +8.90(0.04) 2.2(0.1) 359(17)
o-H123CO41,4–31,3 274.7621 10 45 31 21 102(16) +8.34(0.11) 2.5(0.3) 270(25)
13CH3OH20,2–10,1 94.4110 26 20 2 2 10(2) +8.61(0.21) 3.9(0.4) 42(4)
13CH3OH21,1–11,0 94.4205 26 28 1 2 11(1) +9.19(0.16) 2.4(0.4) 27(4)
13CH3OH21,1–11,0– 95.2087 26 21 1 2 17(2) +7.56(0.10) 3.5(0.3) 65(4)
13CH3OH11,0–10,1 165.5661 15 23 1 8 76(8) +8.58(0.09) 3.6(0.2) 289(15)
13CH3OH71,6–70,7 166.5695 15 84 6 5 34(5) +8.64(0.13) 3.5(0.4) 125(10)
13CH3OH8−1,8–70,7 221.2852 11 87 5 11 49(11) +8.05(0.16) 3.5(0.3) 180(16)
13CH3OH51,5–41,4++ 234.0116 11 48 4 10 66(13) +8.87(0.11) 4.1(0.3) 284(16)
13CH3OH50,5–40,4 235.8812 10 47 4 13 70(8) +9.11(0.12) 3.3(0.3) 245(17)
13CH3OH5−1,5–4−1,4 235.9382 10 40 4 10 52(10) +8.84(0.15) 5.0(0.3) 275(17)
13CH3OH103,7–102,8-+ 254.5094 10 175 9 8 54(7) +8.31(0.09) 3.7(0.2) 215(10)
13CH3OH83,5–82,6-+ 254.8418 10 132 7 7 52(7) +8.29(0.09) 4.0(0.2) 218(11)
13CH3OH73,4–72,5-+ 254.9594 10 113 6 11 52(7) +8.25(0.14) 4.3(0.3) 238(15)
13CH3OH63,3–62,4-+ 255.0510 10 98 5 10 71(11) +8.63(0.11) 4.7(0.2) 353(16)
13CH3OH83,6–82,7+- 255.2656 10 132 7 6 47(6) +8.30(0.09) 3.9(0.2) 193(9)
13CH3OH93,7–92,8+- 255.3559 10 152 8 7 51(7) +8.52(0.09) 3.8(0.2) 208(11)
13CH3OH103,8–102,9+- 255.4970 10 175 9 9 46(9) +8.43(0.14) 4.1(0.3) 202(13)
13CH3OH52,3–41,3 263.1133 9 56 4 10 60(10) +8.31(0.12) 4.4(0.3) 278(15)
13CH3OH9−1,9–80,8 268.6354 9 107 6 13 55(13) +7.99(0.16) 4.1(0.4) 236(18)
aFrequenciesandspectroscopicparametersofH123COand13CH3OHhavebeenextractedfromtheCologneDatabaseforMolecularSpectroscopy(Mu¨ller
etal.2005).Upperlevelenergiesrefertothecorrespondinggroundstateofeachsymmetry.bTheerrorsinbracketsarethegaussianfituncertainties.
wherekistheBoltzmannconstant,ν isthefrequencyofthe ForHDCOwedetectedlinewingswithvelocitiesupto∼±3
transition,histhePlankconstant, cisthelightspeed, Aul isthe kms−1 withrespecttothesystemicsourcevelocity.Thislowve-
Einsteincoefficient,η 6isthebeam-fillingfactorandtheintegral locityemissionislikelyprobingambientmaterialswept-upbythe
bf
istheintegratedlineintensities. outflowassociatedwithSVS13-A(Leflochetal.1998).Wederived
N isrelatedtotherotationaltemperatureT ,asfollow: thetemperatureandcolumndensityofthisoutflowcomponentus-
u rot
ingtheresidualintensitiesaftersubtractingthegaussianfitofthe
N E
lngu =lnNtot−lnQ(Trot)− kTup (2) ambient component and then we analysed them separately. From
u rot therotationaldiagramanalysisweobtainedforboththeblue-and
wheregu isthegeneracyoftheupperlevel,Ntot isthetotal thered-shiftedemission,aTrot∼12K.Alsointhiscase,theTrot
columndensityofthemolecule,Q(T )isthepartitionfunctionat valueisnotaffectedbybeamdilutionbecausethelinescomefrom
rot
therotationaltemperatureandEupistheenergyoftheupperlevel. the1.3mmband.ThelowTrot valueisagainanindicationofan
AsafirststepweassumedasizefillingthesmallerIRAM30- extended emission, in agreement with the well-studied extended
mbeam,i.e.10′′,avalueconsistentwiththecontinuumemissionat outflowdrivenbySVS13-A(Leflochetal.1998).Weassumedalso
1.25mmobservedwithIRAM30-mradiotelescopebyLeflochet inthiscaseanarbitrarysourcesizeof10′′,obtainingTrot =12±
al.(1998).NotehoweverthattheTrotestimatedoesnotdependon 7 K and Ntot = 9± 15 × 1011cm−2 (HDCO blue wing), Trot =
thesourcesizeassumptionbecausealmostallthelineshavebeen 12±8KandNtot=6±12×1011cm−2(HDCOredwing).Inthe
observed with a beam of ∼ 10′′ and then suffer the same beam caseofH123CO,duetolinecontamination,wedetectedbluewings
dilution.TherotationaldiagramanalysisshowslowvaluesofT , onlyfortwolines;byassumingthesamerotationaltemperatureof
rot
around20K,consistent withtheLVGresultsandconsistent with the HDCO wings, we obtained a column density of Ntot ∼ 9 ×
an association with the extended molecular envelope around the 1010cm−2.
protostar.WeobtainedTrot=23±4KandcolumndensityNtot= ForH13COandHDCOwedetectbothparaandorthotransi-
25±6×1011 cm−2 (H123CO),Trot =15±2KandNtot =9± tions(seeT2ables1and2).Onceconsideredbothspeciesinasin-
3×1012cm−2(HDCO),andTrot=28±6Kandcolumndensity glerotationdiagram,thedistributiondoesnotshowanysignificant
Ntot=13±3×1011cm−2(D2CO),seeFigure6. scatterfromthelinearfit.Consideringthepoorstatistic(2paraand
5 ortho transitions for H13CO; 3 para and 2 ortho transitions for
2
HDCO)andtheuncertaintiesofthelineintensities,thisisconsis-
6 ηbf =θs2×(θs2+θb2)−1;θsandθbarethesourceandthebeamsizes tentwiththeo/pstatisticalvaluesatthehigh-temperaturelimit(3:1
(assumedtobebothacircularGaussian). forH123COand2:1forD2CO).
7
Figure3.TentativedetectionsofemissionduetoCHD2OH(upperpanel)
andCH3OD(lowerpanel)transitions.Transitionsandupperlevelenergies
arereported.RedcurvesarefortheGaussianfit.Notethatthemiddle-upper
Figure4. Upperpanel:The1σand2σcontourplotofχ2obtainedcon-
panelreportsemissionduetotwodifferenttransitions(seetheredvertical
sidering thenon-LTEmodelpredicted andobserved intensities ofallthe
bars).
detctedortho13H2COlines.ThebestfitisobtainedwithN(13H2CO)=
5.5×1012cm−2,θs=5′′,Tkin=20KandnH2>7×106cm−3.Lower
panel:Ratiobetweentheobservedlineintensitieswiththosepredictedby
For the methanol analysis one rotational temperature is not
thebestfitmodelasafunctionoflineupperlevelenergyEup.
abletofittherotationaldiagramsof13CH OHandCH DOH,sup-
3 2
portingtheoccurrenceoftwoemittingcomponentsassociatedwith
different excitation conditions, as already suggested by the LVG
4.3 Methanolandformaldehydedeuteration
analysis.
Abetterfitisobtainedusingtwoslopes(seeFigure7;again Weusethecolumndensitiesderivedfromtherotationdiagramsto
asafirststepassumingasourcesizeof10′′): derive the D/H ratio for formaldehyde and methanol. In order to
(i) one with a low Trot (15 ± 3 K for 13CH3OH and 27 ± properlymeasuretheD/H,thecolumndensitiesarederivedassum-
ingforeachspecies,thesourcesizesuggestedbytheLVGanalysis:
8 K for CH DOH) for the lines with E < 50 K. The column
d=e1n1si±ties5a×re21N01t3octm=−128f±or7CH×21D0O13Hc;m−2upfor13CH3OHandNtot 5E′′upfo<rf5o0rmKaladnedhy0d′.′e3ifsoortompoetlhoagnuoels,li∼nes3′w′ fitohrEmuepth>ano5l0lKin.esAwsaitlh-
readydiscussinSection3,itwasnotpossibletodirectlymeasure
(ii) onewithahigher Trot (99±13Kfor13CH3OHand190 thecolumndensityofthemainisotopologueofH2COandCH3OH
because the lines are optically thick. For this reason we derived
8d±e±n7s62itKi×esf1oa0rre1C4NHcmt2oD−tO2=Hf4o)r±fCo1rHt×2hDe1Ol0inH1e4.scmw−ith2Efourp13>CH503OKH.TahnedcNoltuomt=n athnedf1o3rmCHal3dOehHydceolaunmdnmdeethnasnitoielsc,oalsusmumnidnegnsaiti1e2sCf/r1o3mCtrhaetioH12o3fC8O6
(Milametal.2005)atthegalactocentricdistanceofSVS13-A.
These two excitation regimes are in agreement with what We report the obtained D/H ratios in Table 3. To be consis-
found withtheLVGapproach: ahot corinoand amoreextended tent,weassumedfortheD-speciestheT derivedfromthe13C-
rot
region associated with a lower temperature. The higher T val- isotopologues.Inanycasethefollowingconclusionsdonotchange
rot
uesobtainedforboth13CH OHandCH DOHwithrespecttothe ifweassumeforallthemoleculesthecorrespondingT .
3 2 rot
formaldehyde isotopologues suggest again that the origin of the ForH2COwemeasuredaD/Hof9±4×10−2.Wecancom-
emissionisnottheextendedenvelopebutthehotcorino. parethisvaluewithmeasurementsofdeuteratedformaldehydein
8 E. Bianchiet al.
25.0
24.5 H213CO source size = 10"
T= 23(4) K
24.0 N= 25(6) 1011 cm−2
23.5
)
gup23.0
/up
N 22.5
n(
l
22.0 blue wing
T= 12 K assumed
21.5 N= 9 1010 cm−2
21.0
20.5
5 10 15 20 25 30 35 40 45 50
Eup(K)
28
27 HDCO source size = 10"
26 T= 15(2) K
N= 9(3) 1012 cm−2
25
)
gup24
N/up23 blue wing
n( T= 12(7) K
l
22 N= 9(15) 1011 cm−2
21 red wing
T= 12(8) K
20 N= 6(12) 1011 cm−2
19
10 15 20 25 30 35 40 45 50
Eup(K)
Figure5. Upperpanel:The1σ(inred)and2σ(inblue)contourplotofχ2
obtainedconsideringthenon-LTEmodelpredictedandobservedintensities
thedetcted13CH3COlineswithEup>40K.Thebestfitisobtainedwith 24.0
3N×(131C01H03cmO−H3).=Lo9we×rp1a0n16el:cmRa−ti2o,bθestw=e0e′n.′3th,eTokibnse=rv8e0dlKineanidntennHsi2tie>s 23.5 D2CO source size = 10"
withthosepredictedbythebestfitmodelasafunctionoflineupperlevel T= 28(6) K
energyEup.Circlesreferto13CH3COAtransitionswhilestarsrefertoE 23.0 N= 13(3) 1011 cm−2
transitions.
22.5
)
up
g
/up22.0
Class0sourcesperformed by Pariseetal. (2006), using dataob- N
n(
tainedwiththesameantenna(IRAM30-m)andaconsistentbeam l 21.5
sampling.ThevaluemeasuredtowardsSVS13-Aisclosetotheav-
eragevaluereportedfortheClass0sources,whichisD/H∼0.12. 21.0
For the double deuterated formaldehyde we obtained a D/H
valueof4±1×10−3.Ifwecomparethisvaluewiththatreported 20.5
by Pariseet al. (2006), wecan note that it isdefinitely lower, by
20.0
15 20 25 30 35 40 45 50 55
atleastoneorderofmagnitude,suggestingthattheD/Hisindeed
Eup(K)
lowerinthemoreevolvedClassIobjects,likeSVS13-A,withre-
specttotheClass0sources.
5×1T0h−e3D,/aHvvalaulueeafgoarinthleowDe2rCoOfwatitlhearsetsopnecetotordtehreoHfDmCagOn,itiusd∼e pFaignuelr)ean6d. RDo2tCatOio(nlodwiaegrrpaamnsel)f.oArnHe12m3CitOting(urpepgeiornpsainzeel)o,fH10D′′CiOsa(smsuimddelde
thatthosereportedbyPariseetal.(2006)fortheClass0sources. (seetext).TheparametersNu,gu,andEupare,respectively,thecolumn
density, the degeneracy and the energy (with respect to the ground state
Thisestimateisevenmorereliablebecauseitisindependentfrom
ofeachsymmetry)oftheupperlevel.Thederivedvaluesoftherotational
H13CO.
2 temperaturearereported.Arrowsareforthelinesaffectedbywobblercon-
Finally,wederivedtheD/Hratioalsofortheoutflowinggas. tamination(seeSection3.2)andthusconsideredaslowerlimits.
Inthiscase,weassumedanextendedcomponentwithasourcesize
of 10′′,obtaining avalueof 4±6×10−3 for theHDCO inthe
9
29 102
SVS13-A
13CH3OH source size = 10" IRAS 4a
28 IRAS 4b
T= 15(3) K IRAS 2
27 N= 18(7) x 1013 cm−2 OD] ICReApSh e1u6s2E93
n(N/g)upup26 TN== 949(1(1) 3x) 1K014 cm−2 DOH]/[CH3101 Statistical value OOWMr3io(CHn2 2FIROIRc)42
l 25 H2
C
[
100 SVS13-A
24
23
0 50 100 150 200 100 101 102 103 104 105
Eup(K) Luminosity (Lbol)
29 Figure 8. Adapted from Ratajczak et al. (2011). The figure show the
[CH2DOH]/[CH3OD]ratioasafunctionoftheprotostarluminosity.The
CH2DOH source size = 10" horizontaldashedlinereferstothevaluepredictedbygrainchemistrymod-
28
els(Charnleyetal.1997).
T= 27(8) K
27 N= 11(5) x 1013 cm−2
N/g)upup26 TN== 1 89(02(7) 6x) 1K014 cm−2 1uc3msCin−Hg23tfhOoeHrrColtoHawt3ioOennDael,rpgaasysrtutirmtainoinsnigtfiuotnhncestis(oasnimzseefr∼soomu3r′R′ceaantsadijzcTezraaokntde=tTa1rl2o.t(K2o0)f1at1nh)de.
n( 25
l
24
4.4 The[CH DOH]/[CH OD]ratio
2 3
23 Finally, we used the detection of CH OD to derive a measure of
3
the[CH DOH]/[CH OD]ratio,andthustestthepredictionsofthe
2 3
220 50 100 150 200 250 300 currenttheoryofmethanoldeuteration.Basically,accordingtothe
Eup(K) grainchemistrystatisticalmodelsofCharnleyetal.(1997)andOs-
amuraetal.(2004)theratioofthesinglydeuteretedisotopologues
Figure7.Rotation diagrams for13CH3OH(upperpanel)andCH2DOH CH2DOH and CH3OD formed on the mantles should always be
(lowerpanel)assumingtwoemittingcomponents.Anemittingregionsize 3.However,thisisnotconfirmedbythefewmeasurementsinstar
of10′′isassumed(seetext).TheparametersNu,gu,andEupare,respec- formingregions.
tively,thecolumndensity,thedegeneracyandtheenergy(withrespectto Figure9(fromRatajczaketal.2011andreferencetherein)re-
thegroundstateofeachsymmetry)oftheupperlevel.Thederivedvalues portsthesofarmeasuredratiosasafunctionofthebolometriclu-
oftherotationaltemperaturearereported.
minosity,includingbothlow-andhigh-massstarformingregions.
The[CH DOH]/[CH OD]ratioalwaysdiffersfromthestatistical
2 3
valuesuggestingaweaktrend:theabundanceratioissubstantially
bluewingand3±6×10−3fortheHDCOintheredwing.These lowerinmassivehotcoresthanin(low-mass)hot-corinos(aswell
measurementsareinagreementwiththatmeasuredintheshocked asinintermediate-massprotostars),bytypicallyoneorderofmag-
regionassociatedwiththeL1157protostellaroutflowbyCodellaet nitude. In particular, in low mass protostars, CH OD is found to
3
al.(2012),thatreportedavalueof5–8×10−3usingIRAM30-m belessabundant thanCH2DOH,bymorethanafactor 10(Rata-
data. jczaketal.2011).Unlessthepredictionforthemethanolformation
ThederivedD/HratioforCH DOHwithrespecttoCH OH ondustgrainshastoberevised,thesemeasurementsaresuggest-
2 3
is indicated in Table 3. To calculate this ratio, we derived the ingthattheratioisalteredbygas-phasereactionsatworkoncethe
CH DOH column density assuming the same T of 13CH OH, deuteratedmethanolmoleculesarereleasedbythedustmantles.
2 rot 3
obtaining D/H ∼ 2 × 10−3, for the lines with excitation ener- The present work allows us to provide a little piece of in-
gies Eup < 50 K, and D/H ∼ 7 ± 1 × 10−3 for the lines with formation to this general context. For SVS13-A we obtained
E > 50 K. These values are two orders of magnitude below [CH DOH]/[CH OD] in the 2.0 – 2.5 range (see the magenta
up 2 3
the D/H reported in Parise et al. (2006), supporting that also the point in Figure 9), comparing the column density estimated
methanol deuteration for the Class I object SVS13-A is dramati- from the CH OD 5 –4 line and the column density from a
3 1+ 1+
callydecreasedwithrespecttoClass0objects. CH DOH line with similar energy (4 –4 e0). Our measure-
2 2,3 1,4
We give an estimate of the CHD OH and CH OD column mentseemstoquestionthepreviousconclusionsonachangeofthe
2 3
densitiesusingthetentativedetectedtwolines,whichcanbeused [CH DOH]/[CH OD]ratioasafunctionoftheprotostellarlumi-
2 3
as lower limits for the following analysis. We derived a value of nosity.Ontheotherhand,itsuggestsanevolutionwithtimegoing
Ntot ∼ 1 × 1016 cm−2 for CHD2OH and Ntot ∼ 6 × 1014 fromClass0toClassI,withCH2DOHmoreefficientlydestroyed
10 E. Bianchiet al.
than CH OD. To conclude, it is clear that we need further mea- addition, thedecrease of methanol deuteration occurs when most
3
surementstoproperlyinvestigareanypossibledependenceontime of themethanol isalready destroyed withabundances lower than
and/orluminosity. 1×10−8.Althoughformaldehydespendsmoretimeinthewarm
gasdueitslowerbindingenergy,itdoesnotshowanysignificant
decreaseofitsdeuteration(Charnleyetal.1997,Roberts&Millar
4.5 Deuteriumfractionationoforganics:fromClass0to
2007).
ClassI
Case2:Asecondpossibilityisthatthedecreaseofdeuteration
ThepresentresultsstronglysupportthatbothH COandCH OH isduetothegradualcollapseoftheexternalshellsoftheprotostel-
2 3
deuterationdecreaseswhenaprotostarleavestheClass0stageto larenvelope.Thedeuteriumchemistryisverysensitivetophysical
enterintheClassIphase.Figure9showstheD/Hratiomeasured (density,temperature)andchemical(COabundance,H2ortho/para
fororganicmoleculesatdifferentstagesoftheSun-likestarform- ratio) parameters (see Flower et al. 2006). Icy formaldehyde and
ingprocess,fromprestellarcorestoprotoplanetarydisks(thetime methanol deuterations increase with the density and with the de-
increasesfromthelefttotherightalongthex-axis).Thepresentob- creasingtemperatureduringtheformationofprestellarcores(see
servationforSVS13-AcanbeproperlycomparedwiththatofClass Taquetet al.2012b, 2013). Taquetet al.(2014) thereforeshowed
0objects,derivedbysamplingsimilarspatialscalesaroundthepro- that the deuteration of formaldehyde and methanol ices can de-
tostar.Themethanolandformaldehydedeuterationmeasurements creasebytwoordersofmagnitudefromthecentretotheexternal
ofSVS13-A,fillinthegapbetweenClass0objectsandprotoplan- partofprestellarcores,theexactvaluesdependingonthestructure
etarydisks,associatedwithClassII-IIIobjects. of the core and its history. In the subsequent protostellar phase,
For HDCO, the average value measured in Class 0 sources theshellsarethengraduallyaccretedfromthecentertotheouter
(Pariseetal.2006)isD/H∼0.12,consistentwiththevaluemea- partinaninside-outfashionduringthecorecollapse.Themethanol
suredinSVS13-A,whichisD/H∼8.6±3.5×10−2.Completely deuteration observed inthe early Class0 phase would reflect the
differentisthecaseofD CO,whichshowsanincreasegoingfrom materialatthecenteroftheprestellarcorewhereastheolderClass
2
prestellarcores(averagevalueD/H∼0.045,Bacmannetal.2003) Iphasereflectsthematerialcomingfromtheexternalcoreshells.
toClass0sources(D/H∼0.15,Pariseetal.2006)andthenastrong An instructive view has been reported by Codella et al. (2012),
decrease in SVS13-A (D/H = 3.8 ± 1.1 × 10−3. A similar be- who analysed the H2CO and CH3OH deuteration in the shocked
haviourisobservedforthemethanoldeuterationthatincreasefrom region L1157-B1, located relativelyfar (0.08 pc) from the proto-
avalueofD/H∼0.1inprestellarcores(Bizzocchietal.2014)to stardrivingtheshocksintheoutflow,andthussamplinganouter
D/H ∼ 0.52 inClass0 (Pariseet al.2006) and then significantly regionprobably associated witha(pre-stellar) densitylower than
decreaseinSVS13-AtoD/H=(1.5–7.1)×10−3. thatwheretheprotostarissuccessivelyborn.TheD/Hderivedfor
Inconclusion,theoverallcomparisonshowsacleartrendgo- L1157-B1 areindeed lower than what found for thestandard hot
ingfromtheprestellarcorestotheClass0objectsandtotheClass corinoIRAS16293-2422, i.e.theinner100AUoftheprotostellar
I source. The deuterium fractionation of organics increase going core.Inotherwords,H2COandCH3OHdeuterationcanbeused
from prestellar cores to Class 0 sources and then decreases up to measure the density at the moment of the ices formation, be-
to two orders of magnitude going from Class 0 protostars to the forethestartof thestarformingprocess: thehigher theD/H,the
moreevolvedphases.Inprotoplanetarydisksthefewavailableor- higherthedensity.InthecaseofSVS13-AtheD/Hforformalde-
ganicsmeasurements refertoDCN/HCN(Oberget al.2012) and hyde in the outflow, sampling a region definitely more extended
DCO+/HCO+ (van Dishoeck et al 2006, Guilloteau et al. 2006) thantheprotostellarhighdensitycocoon,isindeedsupportingthis
andareinagreementwiththedecreasingtrendwithvaluesbetween scenario(seeFigure10).Thedecreasebytwoordersofmagnitude
0.035and0.004.NotethattheprestellarcoresandtheClass0pro- from Class0 to ClassI protostars observed for the D2CO/H2CO
tostarsarenotorderedinagethusanytrendwhithintheclassesis andCH2DOH/CH3OHratiosisingoodagreementwiththemodel
notsignificant(asinCeccarellietal.2014). predictions by Taquet et al. (2014) within an order of magnitude
Why does D/H decrease from Class 0 to Class I protostars? althoughthemodelsstilltendtounderpredicttheabsoluteratios.It
Formaldehydeandmethanolobservedaroundembeddedprotostars should be noted that the decrease of formaldehyde and methanol
have been mostly formed at the surface of interstellar grains and deuterations with the evolutionary stage of the protostar are not
havebeenthenevaporatedthermallywhenthetemperatureexceeds necessarily accompanied by a decrease of water deuteration. As
their temperature of sublimation. Two possibilities can therefore watericeismostlyformedinmolecular cloudsbeforetheforma-
besuggested. Thedecrease ofD/HfromClass0toClassIcould tion of prestellar cores, its deuteration only weakly varies within
be due to (1) warm gas phase chemistry after the evaporation of prestellarcores.Thisscenariocanthereforesimultaneouslyexplain
formaldehydeormethanol;(2)alowerdeuterationoficyformalde- thedecreaseofdeuterationofformaldehydeandmethanolobserved
hydeandmethanolinClassIthaninClass0. inthisworkand,inaddition,theconstantdeuterationofwaterob-
Case1:Warmgasphasechemistrycandecreasethedeuterium servedtowardsSVS13-AbyCodellaetal.(2016).
fractionationofformaldehydeandmethanolthroughion-neutralre-
actions.Charnleyetal.(1997)showedthattheCH DOH/CH OH
2 3
decreasesdramaticallybytwoordersofmagnitudeattimeslonger
5 CONCLUSIONS
than3×105yr,becauseofelectronicrecombinationsthatdestroy
moreefficientlyCH2DOHthanCH3OH.Thetimescaleof3×105 WestudiedtheformaldehydeandmethanoldeuterationintheClass
yrisconsistentwiththetypicallifetimeofClassIprotostars(0.2- IobjectSVS13-AwiththeIRAM30-mantennaintheframework
0.5Myr;Evansetal.2009).However,therearetwoproblemswith oftheASAIlargeprogramconsistingofanunbiasedspectralsur-
thispicture:(i)revisedmodelsbyOsamuraetal.(2004)suggests veyat1.3,2and3mmtowardsthesource.Theaimofthisproject
longertimescales(upto106 yr),and(ii)thedynamicaltimescale wastounderstandhowthedeuteriumfractionationoforganicslike
ofthematerialinthehotcorinoenvelopeinsidethecentrifigualra- H COandCH OHchangeinaClassIobject,SVS13-A,withre-
2 3
diuscould be lower (1× 104-1 ×105 yr; Visser et al.2009). In specttotheClass0sources.Thebulkofthedetectedlinesarein