Edge-on T Tauri stars

1.4 MB·

by  Immo Appenzeller

Astronomy&Astrophysicsmanuscriptno.2217 February2,2008
(DOI:willbeinsertedbyhandlater)
⋆
Edge-on T Tauri stars
ImmoAppenzeller1,ClaudeBertout2,andOtmarStahl1
1 Landessternwarte,Ko¨nigstuhl,D69117Heidelberg,Germany
2 Institutd’Astrophysique,98bis,bl.Arago,F75014Paris,France
5
0 ReceivedMarch30,2004/AcceptedJanuary8,2005
0
2 Abstract.UsingtheUechellespectrographattheESOVLTweobtainedtwo-dimensionalhigh-resolution(R=50000)
spectraoftheedge-ondiskobjectsHH30 ,HKTauB,andHVTauC.Forcomparisonpurposeswealsoobservedwiththesame
n ∗
a equipmentboththeclassicalTTauristarHLTauandtheactivelate-typestarLDN1551-9.Thespectraofallthreeobserved
J edge-ondisksconsistofaTTauriemissionandabsorptionlinespectrumwithsuperimposedjetemissionlines.Analysisofthe
6 spectraconfirmedthatthedisksarecompletelyopaqueatvisiblewavelengthsandthatlightfromthecentralobjectsreachesus
2 onlyviascatteringlayersaboveandbelowthediskplanes.ThecentralobjectsofourtargetswerefoundtobenormalTTauri
starsshowing moderate but different amounts of veilingof their photospheric spectra, indicating different accretion ratesor

2 evolutionarystages.WesuggestthatallclassicalTTauristars(CTTSs)showthisobservedmorphologywhenviewededge-on.
v Partofthejetemissionfromedge-onsystemsisdirectlyvisibletousintheforbiddenlinesaswellasinHαandHe,afinding
2 whichcontradictsthepresentparadigmofapuremagnetospheric accretionoriginfortheformationofhydrogenandhelium
8
emissionlinesinmoderatelyactiveCTTSs.FromacomparisonwiththoseTaurus-AurigaCTTSsforwhichtheinclinationis
5
reliablyknown,weconcludethattheviewangleofCTTSsystemsisoneofthekeyparametersgoverningapparentHαemission
1
strengthintheTTauriclass.WediscussthevariouspossibleformationregionsfortheNaDlinesandshowthatprofilessimilar
0
toobservedonescanbeformedatthebaseofthediskwind.
5
0
/ Keywords.Stars:formation-Stars:pre-mainsequence-ISM:jetsandoutflows-Planetarysystems:proto-planetarydisks-
h
Line:formation
p
-
o
r 1. Introduction size and mass as the collapse proceeds, while feeding mass
t
s to the centralstar. However,thecomputeddiskmass andsize
a The development of high angular resolution imaging tech-
as the star approaches the main sequence both appear larger
:
v niques in the optical, infrared, and millimetric bands has led than observed, possibly because the role of the proto-stellar
i to the discoveryoverthe last decade of a numberof resolved
X jet, of MHD instabilities, andofnon-axissymmetriceffectsin
disks surrounding pre-main sequence stars (see the review
carryingawayangularmomentumoftheinfallingmatterisnot
r
by Me´nard&Bertout 2002). These observations have con-
a takenintoaccountinthesesimulations.
firmed the long held belief, based on indirect evidence, that
That the disk should grow radially because of out-
young, solar type stars are often associated with disks (e.g.
ward angular momentum transport as its evolution pro-
Appenzelleretal.1984;Bertoutetal.1988).
ceeds appears nevertheless plausible, as already noted
Whilethecircumstellardisksareknowntocontainthedust
by Lynden-Bell&Pringle (1974) and more recently by
andgasneededforplanetarysystemstodevelop,theirstructure
Hartmannetal. (1998). Itis perhapsnotsurprising,therefore,
and evolution remain elusive. The relationship between these
thatthelargestcircumstellardisksareobservedaroundyoung
disks and the ubiquitously observed proto-stellar jets is a re-
solar type stars that have become visible in the near-infrared
lated question that also remains unsolved. A disk/jet connec-
and optical ranges, the T Tauri stars or Class II objects in
tionis wellestablished(e.g.Cabritetal. 1990),buttheorigin
theevolutionaryschemeforsolar-masspre-mainsequenceob-
of the jet and its driving mechanism remain topics of intense
jects originally devised by Adamsetal. (1987) and extended
debate(seethereviewbyCabrit2002).
byAndre´etal.(1993).
Current models of gravitational collapse
Class II stars are surrounded by actively accreting disks
(Yorke&Bodenheimer 1999) predict that a quasi-hydrostatic
(Bertout et al. 1988). The accretion onto the star is thought
accretion disk forms around the stellar embryo and grows in
to be proceeding along the magnetic field lines of the stellar
Sendoffprintrequeststo:I.Appenzeller magnetosphere,whichdisruptstheinnerdiskregions.ClassII
⋆ BasedonobservationsobtainedwithUattheESOVeryLarge objects also harbor jets, although less powerful ones than in
Telescope,Paranal,Chile(proposalNo.70.C-0041(A)). previousevolutionaryphases,suchthattheyareoftenreferred
2 I.Appenzelleretal.:Edge-onTTauristars
toasmicro-jets.BecauseClassIIobjectsarevisibleintheopti- The comparison object HL Tau is listed as a CTTS in
calrange,wecanusethepowerfultechniqueofhigh-resolution the PMS star catalog of Herbig and Rao (1972). Its opti-
opticalspectroscopytostudythem. calspectrum,showingstrongandbroademission linessuper-
This technique has already been fruitfully used decades imposed on a veiled late-type photospheric absorption spec-
ago for the brightest young stellar objects (e.g. Mundt trum corresponding to K7, has been described, e.g., by
≈
1984) and, more recently, for several Orion proplyds (e.g. Herbig&KameswaraRao (1972), Cohen&Kuhi(1979), and
Henney&O’Dell1999),thespectraofwhicharestronglyaf- Basri&Batalha (1990). As in the case of our program ob-
fectedbydiskphoto-evaporationcausedbytheintenseUVra- jects,atopticalwavelengthswereceiveonlyindirectlightthat
diationfield fromtheTrapeziumstars. Butnohigh-resolution is scattered towards us by the reflection nebula of HL Tau
spectroscopy of the recently imaged edge-on disks surround- (Stapelfeldtetal.1995),althoughthecentralobjectisdirectly
ing nearby Class II objects has been available so far1. From visible at IR wavelength (Closeetal. 1997). Radio observa-
theobservedstrongpolarizationitisknownthatonlyscattered tionsofHLTauhavebeeninterpretedasevidenceforthepres-
lightisreachingusfromsuchobjects.Hence,theseobjectsare ence of a massive dust and gas disk observed at an inclina-
muchfainterthanordinaryT Tauristars atthe same distance, tion of about 67◦ (Beckwithetal. 1990; Hayashietal. 1993;
andlargetelescopesareneededtoobservethemwithhighres- Closeetal. 1997), surrounded by an extended envelope. The
olutionopticalspectroscopy. star is also known to drive a prominent jet directed approxi-
In order to learn more about the nature and evolutionary matelyperpendiculartotheproposeddisk(Mundtetal.1990;
stage of the centralobjects of these disks and to deriveinfor- Cabritetal.1996).AllemissionlinesofHLTauarebroadand
mation about the physical structure and properties of the ob- the forbidden lines are known to show complex, blue-shifted
served proto-stellar disk-jet systems, we used the VLT U lineprofiles(Appenzeller1983;Hamann1994).
spectrograph to study the resolved edge-on disks surround- Our spectrum of the comparison object HL Tau confirms
ing the the central object of HH30 (in the following referred the spectral properties described in the literature cited above.
to as “HH30 ”) and the young stellar objects HV Tau C and A detailed discussion of HL Tau is outside the scope of the
∗
HK Tau B. For comparisonwe also includedin our U ob- presentpaper;however,sinceourhighresolutionspectrumpro-
serving program the prominent classical T Tau star (CTTS) vides some improvedinformationon this object, our new ob-
HLTauandthepresumednon-emissionPMSstarLDN1551- servationswillbepresentedbelowtogetherwithourresultson
9. theedge-ondisks.
The second comparison star, LDN 1551-9, located about
HH 30 , the central object of HH30, was discov-
∗
7 NE of HL Tau, was identified (and classified as a K6 star)
ered by Mundt&Fried (1983). Subsequently HH 30 and ′
duringa searchforPMSstarsin inthe directionoftheLynds
HH 30 were studied by many different authors from
∗
Dark Cloud 1551 by Feigelson&Kriss (1983). Although no
the ground (Mundtetal. 1987, 1990; Cohen&Jones 1987;
lineemissionwasdetected,Favataetal.(2003)suggestedthat
Graham&Heyer 1990; Reipurthetal. 1993; Kenyonetal.
LDN 1551-9 is a PMS star of the Taurus star formation re-
1998) and with the HST (Burrowsetal. 1996; Woodetal.
giononthebasisofitsX-rayflux.OurUspectrumofLDN
1998; Watson&Stapelfeldt2004). Froma low-resolutionop-
1551-9showsaphotosphericabsorptionspectrumcorrespond-
ticalspectrumKenyonetal.(1998)estimatedthespectraltype
ing to aboutM0 with narrowemission lines of Ca H and K
ofHH30 tobearoundM0.
∗
andanHαabsorptionlinethatispartiallyfilledinbyemission.
HK Tau B was identified as an edge-on disk by
However,noLi(1)6708Åabsorptionisdetectable(EW<5
Stapelfeldtetal.(1998)fromHSTandbyKoresko(1998)from
mÅ).Theradialvelocityofthestardiffersbyabout10kms 1
ground-based adaptive optics high-resolution images. A de- −
fromtheCOvelocityofLDN1551andthetypicalradialveloc-
taileddescriptionoftheobservedpropertiesofthisobjectcan
ity oftheTaurusPMSstars(cf.Table 2), sothatweconclude
befoundinDucheˆneetal.(2003).
thatLDN1551-9isanactivelate-typestar,butverylikelynota
HV Tau C was discovered as the third component of the
PMSstar.Nevertheless,itsspectraltypeandthenarrowabsorp-
HV Tausystem bySimonetal.(1992).After forbiddenemis-
tion lines, showingno detectable rotationalbroadening,make
sion lines were observed in HV Tau C by Magazzu&Martin
this star well suited as a comparisonstar forour programob-
(1994), HV Tau C was included (as HH 233) in the Reipurth
jects.
(1999) catalog of Herbig-Haro objects. The true nature of
The paper is organized as follows: Section 2 presents the
HV Tau C was clarified by Woitas&Leinert (1998) who
observational details; in Section 3 we discuss the observed
showedthattheobservedIRfluxcouldnotbeexplainedbyan
spectra; in Section 4 we discuss possible explanationsfor the
HHnebulabutinsteadrequiredthepresenceofastellarcentral
observed spectral properties; and Section 5 presents our con-
source.Finally,HVTauCwasidentifiedasanedge-ondiskby
clusions.
Monin&Bouvier (2000) using ground-basedAO images and
Stapelfeldtetal.(2003)usingHSTimaging.
2. Observationsanddatareduction
1 Whilethisworkwasbeingrevised,White&Hillenbrand(2004)
Our data are based on observations carried out in service
published a spectroscopic study of a large sample of Taurus-Auriga
YSOs,including thetargetsstudiedhere. Ourlongslitspectracom- observing mode in December 2002 and January 2003 with
plementthesedatabyprovidingadditionalinformationontheorigin U, the Ultraviolet and Visual Echelle Spectrograph at the
ofemissionlinesinedge-onstars. Nasmyth platform B of ESO’s VLT UT2 (Kueyen) on Cerro
I.Appenzelleretal.:Edge-onTTauristars 3
Table1.Sometechnicaldetailsoftheobservedspectra
Object pos.angle nr.expos. tot.time(s)
HH30 120 ( disk) 6 14056
∗ ◦
k
HH30 30 ( disk) 4 10600
∗ ◦
⊥
HKTauB 135 ( disk) 3 8460
◦
⊥
HKTauB 45 ( disk) 3 8460
◦
k
HVTauC 109 ( disk) 3 8640
◦
k
HLTau 140 ( disk) 1 300
◦
k
LDN1551-9 0 1 300
◦
slitprojectedontoanimageofHKTauAandBinFig.1,while
furtherinformationontheobservationsislistedinTable1.
Theobservationaldatawerereducedandtwo-dimensional
spectra extracted using mostly standard ESO pipeline soft-
ware for U. An exceptionwas the order-mergerprocedure
where the ESO software did not produce satisfactory results,
mainlybecausethe verynoisyedgesoftheechelleordersde-
teriorated the S/N in overlapping regions; therefore, the or-
der merging was carried out using software developed at the
Fig.1.HST archiveimageof HK Tau A andB with the posi- LSW Heidelberg(Stahletal. 1999). All spectral frames were
tionsandextentoftheprojectedUred-channelspectrograph convertedtothesame(heliocentric)wavelengthscale.Forthe
slitsusedforthisstudy.Theslitlengthis11′.′8.Northisupand main targets, where several frames were available, we con-
Easttotheleft. structed mean spectra by calculating the median of the indi-
vidualspectra.
Inadditiontothetwo-dimensionalspectrawederivedsev-
Paranal,Chile.ForallobservationsthestandardsettingDIC1, eral sets of one-dimensional spectra by integrating along the
390+580,wasused.Theobservedwavelengthrangeextended directionperpendiculartodispersionwithdifferentintegration
from3280to4490Åinthebluechannelandfrom4726to6722 limits. As discussed in detail in the following sections, these
Åintheredchannel,exceptforagapfrom5708to5817Ådue one-dimensional spectra were used to obtain information on
to the space betweenthe two CCDs of the detectormosaic of thecentralstarsandonvarioussubcomponentsoftheobserved
theredchannel.Theprojectedslitwidthwas0.8,resultingin objects.Sinceallobjectsarestronglyreddened,thecontinuum
′′
a measured FWHM spectral resolution of about 50000. The S/Nwasinallcases< 1attheblueendofthespectralrange,
medianFWHMseeingfortheobservationswas0.83,butcon- butreachedvaluesup to 60 atthe redlimit. Emissionlines
′′ ≈
ditionsvariedsignificantlybetweentheindividualframeswith couldgenerallybedetectedandevaluatedforλ>3600Å.
extremaof0.53and1.38.
′′ ′′
The total integration times (between 5 minutes and 3.9 3. Theobservedspectra
hours) were split into individual exposures < 50 minutes.
Spectra were obtained for the program objects HH30 and 3.1.Basicspectralproperties
∗
HK Tau B with two differentposition angles of the projected
In ourU spectra allthree edge-ondisksshow similar gen-
spectrograph slit, with the slit oriented either parallel or per-
eral spectral properties. Moreover, apart from the absence or
pendicular to the disk plane. In the case of HV Tau C only a
weakness of permitted metallic emission lines (prominent in
spectrum with the slit parallelto the disk was obtained,since
HLTau),allouredge-ondiskshavespectrawhichqualitatively
withanorientationperpendiculartothedisktheslitwouldhave
resemble thatof HL Tau. Inparticular,all spectra includeab-
passedtooclosetothebrightmaincomponentHVTauA/Bof
sorptionlines of a late-type photosphere(with a spectraltype
this triple system. In the case of the comparison object LDN
around M0) and emission lines typical of CTTSs. Obviously,
1551-9 (expected to be an unresolved star), the standard NS
all observed edge-on disks contain CTTSs as central objects.
slit orientation was used. The slit was oriented parallel to the
Quantitatively,however,theedge-ondisksdoshowsomedis-
PA direction 140 for the comparison object HL Tau, i.e. ap-
◦ tinctdifferenceswhencomparedtootherTTauristars:
proximatelyparalleltotheextensionofthedustandCOdisks
reported to be present at the position of HL Tau and perpen- i) all three edge-on objects have exceptionally strong forbid-
diculartothe directionofthe HLTau jet(seee.g.Closeetal. denlinesrelativetothecontinuumflux;
1997;Mundtetal.1990). ii) as demonstrated in Table 3, the permitted emission lines
The pixelscalesandslit lengthswere, respectively,0.246 arenarrowerthaninHLTauandotherCTTSsandare(al-
′′
pixel 1 and7.6intheblueand0.182pixel 1 and11.8inthe most)undisplacedrelativetothephotosphericspectrum(cf.
− ′′ ′′ − ′′
red arm. As an example we present the contoursof the “red” Table2).
4 I.Appenzelleretal.:Edge-onTTauristars
Table2.Observedheliocentricradialvelocitiesofthephotosphericabsorptionspectra(v ),oftheHαemissionlinepeaks(v ),
α
the He emission line peaks (v ), the forbidden line peaks (v ), the sharp absorp∗tion (or, in the case of LDN 1551-9,
HeI forbidden
sharpemission)featuresoftheresonancelinesofNaandCa(v andv ),andthevelocityoftheCOemissionfromthe
Na D H&K
−
correspondingdarkclouds(v ).FortheforbiddenlinesofHL Tauthevelocitiesoftheblueandthe redpeaksofthedouble-
CO
peakedprofilearegiven.Allvaluesareheliocentricvelocitiesinkms 1.Wherepossible,statisticalmeanerrorsareincluded.If
−
noerrorislistedthem.e.isabout2kms 1.
−
Object Sp.Type v v v v v v v
α HeI forbidden Na D H&K CO
HH30 K7 +21.5∗ 2.0 +20.3 +20.4 +19.8 0.3 +20.6− 1.3 +19.0
∗
± ± ±
HLTau K7 +20.7 2.2 +99.6 +31.9 173.5 0.5 +20.7 1.0 +22.4 +19.0
± − ± ±
+15.4 1.2
±
HKTauB M1 +17.1 0.2 +12.5 +21.2 +12.1 1.0 +18.8 2.0 +17.9 +18.0
± ± ±
HVTauC M0 +20.3 1.2 +12.5 +24.7 +12.2 2.4 +16.8 0.5 +17.5 +18.7
± ± ±
LDN1551-9 M0 +31.0 0.5 +29.6 +19.0
±
Table 3. Observed average FWHM (in km s 1) of the ob-
−
servedemission lines (exceptfor the complexforbiddenlines
ofHLTau,forwhichwegivethefullwidthatzerointensity).
Ifnoerrorislistedthem.e.is 10%ofthemeasuredvalue.
≤
Object Hα He Na(D) CaH&K Forb.Lines
HH30 50 35 33 ? 25 1
∗
±
HLTau 267 170 149 130 300
≈
HKTauB 37 34 34 25 33 3
±
HVTauC 96 58 36 43 51 1
±
Fig.2.Sectionofthephotosphericspectrumoftwooftheedge-
As an example we note that the FWHM of the Hα emission on disks and of the two comparisonobjects. The four spectra
lines of our edge-on objects is < 100 km s−1 in all three have been normalizedto the same continuumlevel of 1.0. To
cases (with a mean of 61 13 km s−1), while for normal avoidoverlapping,thezerolevelshavebeenshiftedvertically
±
CTTS this value is in the range 100 - 500 km s−1 (see e.g., by0.5,1.0,1.5,and2.0,respectively,forHKTauB,HVTauC,
Hamann&Persson 1992). In contrast to many other CTTSs, HLTau,andHH30 .The6460-6467ÅsectionoftheHH30
∗
our edge-on disks show no, or only very weak, displaced or spectrum was affected by instrumental defects and therefore
broad forbidden-line components, and all emission lines ex- omitted.
hibit simpler and more symmetric profiles than normally ob-
servedinCTTSs. Table 4.Observedequivalentwidths(inÅ)ofselectedstrong
emissionlines.
AsshownbyFig.2,thespectraofthethreeedge-onobjects
areveiled.Theamountofveiling–definedbytheratio[excess
flux]/[photosphericflux]andestimatedfromtherestintensities Object Hα [O]λ6364 [S]λ6716
ofthestrongestphotosphericabsorptionlinesandbycompar- HH30 454 15 100.2 1.5 104.5 0.2
∗
± ± ±
ingthelinestrengthswiththoseobservedinLDN1551-9inthe HLTau 78 0.2 1.3 0.2 4.5 0.2
± ± ±
redspectralrange(around6400Å)–isabout0.2forHKTauB HKTauB 9.6 0.2 0.4 0.1 0.9 0.2
± ± ±
andapproximately0.5forHH30 andHVTauC.Thedifferent HVTauC 49.2 0.2 7.8 0.1 6.3 0.1
∗ ± ± ±
amountsofveilingmayindicatethatthecentralobjectsofour
targetscovera rangeof PMS evolutionarystagesor at least a
rangeofaccretionrates.
3.2.Thephotosphericabsorptionspectra
For all threeedge-onobjectsthe veilingis weaker thanin
HLTau,whichagainindicateseithermoremodestdiskaccre- Asalreadynoted,allouredge-ondiskspectracontainabsorp-
tion rates or a more advanced evolutionary stage of the cen- tionlineswithlineprofilesandrelativelinestrengthsthatagree
tral stars. Given the moderate veilings of our target stars, the well with those of late-type stellar photospheres. From com-
largeequivalentwidthsoftheiremissionlines(Table4)isun- paringtherelativelinestrengthswithMKstandardstarspectra
expected. observedatasimilarspectralresolutionandbyevaluatingthe
I.Appenzelleretal.:Edge-onTTauristars 5
Fig.4.Contourdiagramsofthe2-DspectrumofHH30 show-
∗
ing the [S] 6717 Å line with the slit oriented parallel to the
jet.Betweentwocontourstheintensitychangesbyafactorof
3.16(= √10).Thezero-pointofthevelocityscalecorresponds
totheobject’sadoptedsystemicvelocityof+20.1kms 1.
−
supporttothenormalCTTScharacterofthecentralobjectsof
theedge-ondisks.
3.3.Windandjetemissionlines
In T Tauri stars the winds and outflows, which often are col-
limated towards the rotation axes and then called “jets”, are
Fig.3.Contourdiagramsofthe2-DspectraofHH30 (a)with
∗ known to produce extended emission regions dominated by
the slit oriented parallel to the jet, and (b) with the slit paral-
low-ionization forbidden emission lines. In the spectra of all
lel to the disk, showing the Hα and the adjacent [N] lines.
ourthreeedge-ondisksandinthespectrumofHLTau,allex-
Betweentwocontourstheintensitychangesbyafactorof3.16 pected lines of [N], [O], [S], and [Fe] were clearly de-
(= √10). tected. HH30 , HV Tau C and HL Tau also showed [N]λλ
∗
5198,5200emissionwhile[O]λλ3726,3729and[O]λ5577
could be reliably detected only in HH30 and HV Tau C.
∗
shapes(butnottheabsolutestrength,whichmaybeaffectedby Therelativestrengthofthe[O]doubletcomponentsindicate
veiling)oftheTiO-Bands,weestimatedtheapproximatespec- rather high electron densities for the volume producing these
traltypeslistedinTable2.Theluminosityclassesappeartobe lines(n >105forHH30 and 104forHVTauC).
e ∗
≈
aboutIVbutcannotbe determinedpreciselyfromourspectra As illustrated by Figs. 3, 4, and 7, extended jet emis-
since they show veiling effects and since we have a sufficient sion was observed in the two spectra which were taken with
continuumS/N onlyinthe redwherefewdiagnosticlinesare the spectrographslit oriented perpendicularto the (projected)
available. Because of the limited wavelengthrangeusable for diskplane(i.e.thespectraPA=300 ofHH30 andPA=1350 of
∗
classification,thespectraltypeslistedinTable2cannotbere- HKTauB).Ontheotherhand,extendedforbidden-lineemis-
gardedasmoreaccuratethanthosederivedfromlowerresolu- sion was not detected in any of the other spectra, which all
tionspectra. weretakenwiththeslitparalleltothe(projected)diskplanes.
As again illustrated by Fig. 2, the photospheric line pro- Infact,theforbidden-lineemissionregionwasunresolved,i.e.
filesofallourthreeedge-onobjectsarenarrow.Inthecaseof the FWHM extent of the emission region was not larger than
HK Tau B, the low veiling allows us to derive an upper limit thatoftheseeingprofile,inallouredge-ondiskspectrataken
for a possible rotational broadening of v sini < 10 km s 1 witha disk-parallelslit. Thisfindingisconsistentwiththere-
−
×
fromtheweakphotosphericlines.Theprofilesobservedinthe sult of Burrowsetal. (1996), who found for the HH30 jet a
spectraofHH30 andHVTauCareconsistentwithsuchlow width at the base 20 AU, which is below the resolution of
∗
≤
v sinivaluesaswell.But,sinceonlytheintrinsicallybroad ourground-basedobservations.Thecontinuumemissioninthe
×
strong lines could be detected in these spectra, the observa- disk-parallelspectraalwaysshowedanextensionsignificantly
tionalupperlimitforrotationalbroadeningisashighas20km larger than the forbidden-line emission. Obviously, most the
s 1.Theselimitsdemonstratethatnorotationisobservedtobe forbidden-line flux that entered the spectrograph slit reached
−
higherthannormallyexpectedforCTTSs,whichgivesfurther usdirectlyfromanunresolvedemissionregionabovethedisk
6 I.Appenzelleretal.:Edge-onTTauristars
Fig.5.Profileofthe[S]6717ÅlineofHH30 ,plottedattwo
∗
different vertical scales (differing by a factor 10) to show the
twodifferentcomponents.Thezero-pointofthevelocityscale
correspondstotheobject’ssystemicvelocity.
whilethecontinuumemittedbythecentralstarisscatteredto-
wardsusbyanextendedandresolveddustyregion.
As pointed out (e.g.) by Takamietal. (2003) line emis-
sion regionsslightly off-set from the center of the continuum
emission can in theory also be produced by unresolved, very
closebinarycomponents.However,forouredge-onobjectajet
origin of the observed forbidden-line emission appears much
moreplausible.
Foreachofthethreeedge-onprogramobjectsallforbidden
lines (including the [Fe] lines) were found to show identi-
calorqualitativelysimilarlineprofilesintheone-dimensional Fig.6. Profile of the Hα line of our three targets and of the
mean spectra averaged over the two slit orientations. On the comparisonCTTSHLTau.Thezero-pointofthevelocityscale
otherhand,theseprofilesdifferbetweenthedifferenttargets. corresponds to the objects’ systemic velocity. The sharp ab-
sorptionatabout+50kms 1 isanartefact.
−
3.3.1. HH30
∗
InthecaseofHH30 theforbidden-lineprofilesaredominated that any tilt, if present, is very small. Since the HH30 jet is
∗
by narrow cores with a heliocentric peak radial velocity of knownto be slightly curved(cf. Burrowsetal. 1996), the ob-
+19.8 0.3 km s 1. This value is (within the error limits) in servedvelocityshiftsmaysimplyreflectsmalldeviationsfrom
−
agreem±entwith the weighted mean of all velocities measured alinearflow.
intheintegratedspectrumofHH30 (20.1kms 1),whichwas Thenarrow(jet)lineprofilecomponentisalsovisibleinHα
∗ −
adopted as the systemic velocity of HH30 . Underlying the (cf.Fig.3and6)andintheHeemissionlines.Asillustrated
∗
narrowcores,theforbidden-lineprofilesincludemuchweaker byFig.3(andFig.6),theHαlineprofilealsoincludesavery
and redshifted broadcomponents(FWHM > 100 km s−1, see broadcomponent(extendingtoatleast±340kms−1 fromthe
Fig.5). linecenter)atthepositionofthecontinuumsource(butnotin
The resolved extended emission in the direction of the thejet).Moreover,intheareacoveredbyourspectrographslit
prominent HH30 jet (i.e. towards PA 300, cf. Burrowsetal. thejetandthecounterjetshownearlythesameHαflux,while
1996) shows only the narrow profile≈components, which are theforbidden-linefluxisverydifferent.
slightlyredshiftedbyabout3.0kms 1 (seeFigs. 3&4).The
−
weakerandlessextendedemissiontowardsPA 2100 (usually
≈ 3.3.2. HKTauB
referredto as the“counter-jet”)showssomewhatbroaderline
profiles that are blue-shifted by about 13.0 km s 1 relative to As illustrated by Fig. 7, our HK Tau B spectrum, taken with
−
ouradoptedsystemicvelocity.Ifthesesmallvelocityshiftsare the slit perpendicular to the disk, shows extended forbidden-
duetoatiltofthejet,theywouldindicatethat–contrarytothe line emission directed towards PA 315 . In this direction the
◦
expectationfromthemorphology–thecounterjetisapproach- slit passes unfortunately close to the position of the brighter
ing us, while the north-northeastemission region is receding. star HK Tau A, which is knownto show exceptionallystrong
However, the small differences in the systemic velocity show Hα emission (Kenyonetal. 1998). Therefore, the Hα emis-
I.Appenzelleretal.:Edge-onTTauristars 7
Fig.8.Lineprofilesofthe[N]6583(solidline)and[S]6717
Å lines in the spectrum of HV Tau C. The zero-point of the
velocityscalecorrespondstotheobject’sadoptedsystemicve-
locity.
nouncedatthe[N]lines,while[S]λ6716ispracticallysym-
metric.AsillustratedbyFig.8,thebaseofallforbiddenlines
ofHVTauChasaboutthesameredshift.Hencetheasymme-
tryappearstobecausedbyashiftoftheemissionpeaktowards
shorterwavelengths.Apossibleexplanationoftheasymmetry
couldbethepresenceoftwo profilecomponents,asobserved
inHLTau(Fig.9),butwithavelocityseparationthatissmall
comparedtothelinewidthinthecaseofthe(almost)edge-on
diskobjectHVTauC.
Fig.7.Contourdiagramsofthe2-DspectraofHKTauBshow-
ing the region of the [S] 6717 Å line, (a) with the slit ori-
entedparalleltothejet,and(b)withtheslitparalleltothedisk. 3.3.4. HLTau
Between two contours the intensity changes by a factor of 2.
Our(presumablynon-edge-on)comparisonobject,HLTau,has
Thezero-pointofthevelocityscalecorrespondstotheobject’s
been known to have blue-shifted and, in some cases, double-
systemicvelocity.
peaked forbidden-line profiles (Appenzeller 1983; Hamann
1994;Hirthetal.1997).Thisisconfirmedbyournewobserva-
tionswithimprovedaccuracy.AsshownbyFig.9andTable2,
sionfromtheHKTauBjetiscontaminatedbystraylightfrom one forbidden line component is blue-shifted by 194 km s 1
−
the Hα line of HK Tau A. HK Tau A may also contribute to relativeto the photosphericspectrum,whilethe othercompo-
the observed forbidden line flux observed below the disk of nentshowsalmostthesameradialvelocityasboththeabsorp-
HKTauB,buttheobservedintensitydistributionperpendicu- tionspectrumandthesharpresonancelineabsorptioncompo-
larto thedispersiondirectionrulesoutanypossibilitythatall nentsofHL Tau.Appenzelleretal. (1984)andotherspointed
theextendedemissionisduetostraylightfromHKTauA.The out that such profiles are not unexpected for disk-jet systems
observed jet emission of HK Tau B is again dominated by a seen at intermediate inclinations. As illustrated by Fig. 9, the
narrowandalmostunshifted(∆v 5kms−1)linecomponent, relative strength of the two profile components varies greatly
≤
whileanunderlyingbroadercomponentcouldnotbedetected. between the different forbidden lines, indicating that the two
However,sincetheforbiddenlinesareweakerinHKTauBand components originate in different volumes, (as also observed
since the photospheric spectrum (less veiled) is more promi- insomeotherCTTSs,seeHirthetal.1997).Thisisconfirmed
nent, a broad component as weak as in the case of HH30∗ by our two-dimensionalHL Tau spectrum(taken with the slit
wouldnotbedetectableinourHKTauBspectra. paralleltotheprojecteddisk),wheretheemissionregionpro-
ducingthe blue-shiftedpeak is unresolved,and thusprobably
originatesin a directly observednarrow jet intersected by the
3.3.3. HVTauC
slit, while the region producing the red peak is extended and
In HV Tau C the forbidden lines are somewhat broader than showsaboutthesameangularextensionasthecontinuum.
inHH30 andHKTauB(seeTable3),andsomelineprofiles Itseemsplausiblethatthelow-velocityemissionoriginates
∗
appear slightly asymmetric, with the red wing slightly more in a rarefied extended emission region, as proposed for other
extendedthanthebluewing.Aslightasymmetryismostpro- CTTSswithsimilarprofilesbyKwan&Tademaru(1988).The
8 I.Appenzelleretal.:Edge-onTTauristars
Fig.9.Lineprofilesofthe[N]6583,[O]6300,and[S]6717
linesinthespectrumofHLTau.Theabscissagivesthehelio-
centricradialvelocity
slightblue-shift( 5kms 1 relativetothephotosphericspec-
−
≈
trum)couldbecausedeitherbyaslowoutfloworbyscattering
in a slowly movingmedium.Thatthe two componentsof the
[S]lineareofsimilarstrengthinFig.9,whilethe[S]λ6731
profilepublishedbyHirthetal.(1997)isdominatedbyamuch
stronger high-velocity component is probably due to our slit
orientationapproximatelyperpendiculartothejet,whichsup-
pressesmuchofthejetemission.
3.4.Circumstellarspectralfeatures
Fig.10. Na (D) resonance line profiles in the spectra of HH
TheCTTSdisksareknowntobeessentiallycomposedofrela-
30 ,HVTauC,HKTauB,andHLTau.
tivelydensecoolanddustygas,soweexpectedtoseespectro- ∗
scopicsignaturesforthedisksmainlyintheprofilesofthelow-
ionization resonance doublets of Ca (H & K) and Na (D).
Theobservedfeatureswereinallcaseswellseparatedfromthe
corresponding telluric lines because of the high resolution of wards us by matter above and below the disk plane. A more
the U spectra and the significantsystemic radial velocities detaileddiscussionoftheoriginofthisemissionwillbegiven
ofallobservedobjects. inSection4.
All three edge-on objects and HL Tau show the Ca and In addition to the emission we foundat all Na lines, and
Naresonancedoubletsinemission,althoughthelinesaresur- at all Ca resonance lines observed with sufficient S/N, very
prisingly weak in HH30 . In the edge-on disks the resonance narrowabsorptioncomponentswith radialvelocitiesagreeing
∗
lineemissionprofilesaregenerallynarrowwithFWHMvalues well with those of the photospheric spectra (cf. Table 2). For
similar to those of the forbidden lines and the He emission allthreeedge-onobjectstheprofilesoftheseabsorptioncom-
lines(cf.Table3andFig.10).AnexceptionmaybeCaH& ponentsdonotdeviatesignificantlyfromtheinstrumentalpro-
K ofHH30 ,whereweakbroaderemissionlinesappeartobe file (FWHM 6 km s 1). In the case of HL Tau the width of
∗ −
≈
present. However, the S/N of our spectra did not allow us to the absorption component corresponds to about twice the in-
reliably measure the widths of these very weak features. The strumental profile. For HH 30 , HK Tau B (and HL Tau) the
∗
non-edge-onsystemHLTaushowsmuchbroaderandstronger narrowNaabsorptioncomponentsreachwellbelowthelevel
(relative to the continuum)Ca and Na resonance emission. oftheadjacentcontinuumandalmosttozerointensity,indicat-
In Fig. 10 we compare the Na (1) emission profiles of the ingthattheabsorptionaffectspracticallyalllightreachingusat
edge-ondiskswiththoseofHLTau.Theregionfromwhichthe thiswavelength.Hence,theabsorptionmustatleastinpartbe
resonancelineemissionisreceivedhasaboutthesameangular ofinterstellarorcircumstellarorigin.Becauseoftheabsorption
extension as the continuum in all objects, so that the directly strengthandtheexcellentagreementoftheobservedvelocities
visiblepartsofthejetsdonotseemtocontributetoresonance withtheobjects’systemicvelocities,acircumstellaroriginap-
line emission. Most of the Na emission must originate near pears most plausible. Absorption could, in fact, take place in
thestar,whetherinthecentralregionofthediskoratthebase thesamecooldustymatterthatscattersthelightfromthecen-
of the jet, and is (like the continuum) obviously scattered to- tralobjectsandtheinnerdiskstowardsus.
I.Appenzelleretal.:Edge-onTTauristars 9
Incase theabsorbingmatterwererelatedtothedisks,our Table5.PropertiesofTaurus-AurigaCTTSswithknownincli-
2-D spectra taken with the slit parallelto the disk might well nationanglesand simultaneousEW(Hα) andveilingdetermi-
containinformationonthediskrotation.Iftheabsorbingmat- nations.Detailsandreferencesaregiveninthetext.
terwereorbitingat100AUaroundacentralobjectofonesolar
mass we would expect an orbital velocity of about 3 km s 1,
−
whichiswithinreachofUspectra.Therefore,we checked
(1) (2) (3) (4) (5) (6) (7) (8)
our2-Dspectraforapossibletiltofthesharpabsorptioncom-
Star HBC P vsini R i EW(Hα) v
ponent, but we found no evidence for rotation. The most ac- rot
[d] [km/s] [R ] [o] [Å]
curate measurement(NaD of HV Tau C) resulted in a (non- ⊙
BPTau 32 7.6 7.8 2.2 32 48.6 0.5
significant) tilt of 0.48 0.26 km s 1 arcsec 1. Since seeing
− − DETau 33 7.6 10 1.78 57 66.5 0.9
±
tends to smear possible rotational signatures in our ground- TTau 35 2.8 20.1 4.2 15 63.1 0.1
basedspectra,ourresultdoesnotruleoutrotationaldisksve- DFTau 36 8.5 16.1 3.45 50 61 1.8
locitiesofafewkms 1asestimatedabove. DGTau 37 6.3 21.7 2.8 58 60.5 2
−
DKTau 45 8.4 11.4 2.7 44 16.7 0.4
GGTau 54 10.3 10.2 2.5 56 40.2 0.3
4. Interpretationanddiscussion GKTau 57 4.65 18.7 2.37 46 34 0.2
DLTau 58 9.4 16 2.92 78 110.2 1.9
4.1.Forbiddenlineemission
AATau 63 8.2 11.3 1.94 69 14.2 0.3
DNTau 65 6.0 8.1 1.93 30 15.7 0.1
Aspointedoutabove,inallthreeedge-onobjectsthereisclear
RWAur 80 5.3 17.2 3.00 37 45.9 2.0
evidencethatthenarrowforbiddenlinecomponentsareemit-
tedbyjetflowsthatareseendirectlyaboveandbelowthedisks.
Thesmalllinewidthofthenarrowforbidden-linecomponents
and the small velocity shifts are readily explained by the fact TheHαandHelinessharemanysimilaritieswiththeforbid-
thatintheedge-onobjectsthegasinthejetandcounterjetis denlinesinedge-onobjects,andcanbeunderstoodinthesame
movingperpendiculartotheline-of-sightwithinafewdegrees. way.Inparticular,thenarrowHαandHeemissioncorescan
Theweaker,broad,andshiftedcomponentsobservedfromthe be understoodas jet emission seen aboveand belowthe disk,
sameextendedareafromwhichweobservethecontinuumflux so they provide direct evidence that the jet contributes to the
canpossiblybeexplainedbyadditionallightfromthejetsscat- formationofstrongpermittedemissionlinesintheseobjects.
teredtowardsusaboveandbelowthediskbythesamematerial Sincewehavenotedabovethestrongsimilaritiesbetween
that scatters the light from the central star. As the jet plasma edge-on stars and other CTTSs, the finding that the jet con-
moves supersonically with a significant line-of-sight velocity tributes to Hα suggests that emission from the CTTS outflow
componentrelativetothescatteringmedium,withmostofthe is an essential ingredient of Hα emission even in moderately
jet material moving away from the scattering layer, such ra- veiledCTTSs.Thiscontrastswithcurrentmagnetosphericac-
diation is expected to show significant line broadening and a cretionmodels,and suggeststhatthe view angleunderwhich
velocityshift,aswasindeedobserved. a given CTTS is seen partly determines the properties of its
The weakness of the forbidden-line emission from the emissionlines.
counter-jet of HH 30∗, in spite of strong Hα emission, may In order to test this new hypothesis,we now investigate a
indicate that this radiation is produced in a relatively dense cross-section of Taurus-Auriga CTTSs for which (a) precise
medium. Either the counter jet is moving into a much denser photometricperiodsandradialvelocitiesand(b)simultaneous
environment, which could also explain the smaller extent, or determinationsofHαequivalentwidthsandveilingareknown.
we see in Hα light coming from a different emission region ThissampleisgiveninTable5,wherethestarnameandHerbig
thatisscatteredinourdirection. and Bell catalog number are given in Columns 1 and 2. The
photometric periods and radial velocity measurements, origi-
4.2.WindcontributiontoHαandHelines natingfromacompilationoftheliteraturebyJ.Bouvier(priv.
comm.), are given in Column 3 and 4 respectively. The pho-
Currentmodelsfor CTTSs attribute Hα emission to the mag- tospheric radius, derived as in Strometal. (1989), is listed in
netospheric accretion region in all but the most extreme Column 5. The view angle inferredfrom the previousquanti-
CTTSs (Muzerolleetal. 1998, 2001). Before the magneto- ties is givenin Column6, while Columns7 and8 list the Hα
spheric accretion paradigm became consensual, a number of equivalentwidthsandveilingvaluesintheredspectralrange,
other possible origins for Balmer line emission had been asdeterminedbyAlencar&Basri(2000)fromhigh-resolution
discussed in the literature, for example, strong winds (e.g., Hamiltonspectrograms.Tothese 12stars, weaddthe3 edge-
Kuhi 1964; Hartmannetal. 1982), deep chromospheres(e.g., onTTSs,forwhichtheinclinationangleiscloseto90 .Their
◦
Calvetetal. 1984), or disk boundary layers (Basri&Bertout EWsaregiveninTable4.
1989). Because they can form in a large range of conditions, In order to meaningfully compare the Hα equivalent
itisparticularlydifficulttodisentanglethecontributionstothe widths,wecomputedtheirvalueatzeroveilingusingthepro-
Balmer lines from all possible emission regions. As we have ceduregiveninAlencar&Basri(2000).Bydoingthis,wepre-
seenabove,theparticulargeometryofedge-onobjectsallows sumably eliminated the effect of varyingmass accretion rates
us to isolate the contributionof the jet to forbiddenemission. indifferentstarsontheirlineemissionstrengths.Wethencon-
10 I.Appenzelleretal.:Edge-onTTauristars
observedwithinaprojectedangleof 45 ofthepoledirection.
◦
±
Toconnecttheseobservationalfactstotheactualscatteringge-
ometry requires some knowledge of disk geometry, distribu-
tion of the scattering matter, and of the scattering asymmetry
parameterofthedustgrainsinvolved.
4.3.1. Scatteringfromanextendedenvelope?
The spectroscopic results described above obviously contain
informationonthescatteringgeometry.Oneofourmainfind-
ingshasbeentheexceptionallynarrowNaresonanceemission
linescompared,e.g.,toHLTau.AspointedoutinSection3.4,
these lines were not detected in the parts of the jets observed
directlyaboveandbelowthedisks,whichmeansthattheouter
regionsofthejetsdonotsignificantlycontributetotheselines.
Moreover,theangularextentoftheareafromwhichtheselines
Fig.11.HistogramshowingtheCTTSHαequivalentwidthat
are observed seems to correspond closely to that of the stel-
zeroveilingaveragedoverthestarsincludedin3binsofequal
larcontinuum.ThustheNaemissionapparentlyoriginatesin
cosiintervalsspanningthefullrangeofviewangles.Asimilar
the central part of the edge-on systems in the vicinity of the
resultisfoundwhendistributingthestarsover5bins.
central stars. Since significant emission in neutral resonance
lines is expected from volumes of cool intermediate-density
gas, we expect the innermost regions of the system to be the
mainsourceofNaresonancelineemission.
structedahistogramoftheaveragecorrectedEW(Hα)inequal
cosiintervals.Fig.11showstheresultfora 3-binhistogram, Intheco-rotatingmagnetosphericaccretionmodelsgener-
with bins containing 6, 5, and 4 stars, respectively. A strong ally assumedtoday,the innerdisk,the accretionflow,andthe
correlationof the averagecorrectedEW(Hα) with view angle base of the jet rotate approximatelywith the Keplerian circu-
is apparent,readily understoodin the frameworkof the inter- lar velocity of the inner edge of the disk, so that we expect
pretationdiscussed above;asthe view angleincreases,the jet line emission from these regions to show rotationally broad-
contributionto the Hα flux becomesmore andmore apparent enedprofiles.Narrowprofilesareexpectedonlyiftheseregions
asaseparateemissionregion,thusdrivingtheequivalentwidth areviewedperpendiculartotherotationalvelocities,i.e.,essen-
value up.Note that the correlationis notdue to the particular tially pole-on. Obviously this would be the case if the matter
distribution used in Fig. 11, as we found that the correlation aboveandbelowthedisk,whichscatterstheradiationintoour
remainsstrongevenwith5bins. direction, is located close to the disk symmetry axis. Such a
While a detailed investigation of inclination effects on geometrywouldalsobeconsistentwiththenarrownessofpho-
CTTSspectrallinesisbeyondthescopeofthisworkonedge- tosphericlinesobserved.
on stars and will be the topic of a forthcomingpaper,Fig. 11 Scattering by matter vertically above and below the disk
clearly demonstrates that the view angle of CTTSs is one of center is to be expected if isotropically scattering dust grains
theparametersdeterminingtheiremissionproperties.Thetwo are involved and if (as predicted by protostellar model com-
parameters now known to play a role in CTTS line emission putations)thedisksaresurroundedbydusty,butopticallythin
are the mass accretion rate and the view angle of the system. remnantprotostellar envelopes,or if the disk’s matter density
ThedispersionofEW(Hα)withinthesinglebinsofFig.11in- decreasesslowlyintheverticaldirection.Ifthescatteringma-
dicatesthat(atleast)athirdyetunknownparametermustalso terialisofprotostellaroriginandisstillinfreefall,thiswould
playaroleinthecomplexCTTSlineemissionprocess.Tocon- resultinasmallblueshiftofthescatteredlight.Howeveratthe
cludethis section,we note thatsince the equivalentwidthsof verticaldistancesconsideredherethecorrespondingvelocities
Hαandforbiddenlinesvarywithinclination,physicalproper- would be no more than a few km s 1 and would thus not be
−
ties thatareusuallyderivedfromthese quantities,such asthe detectableinourspectrograms.
windmass-lossrate,mustbeviewedwithsomecaution. Thesuggestionthatedge-ondiskscouldbeeffectivelyseen
closetopole-onnotonlyappearscounterintuitivebutalsoap-
4.3.Otherpermittedlines:NaDemission parentlycontradictscurrentscatteringlightmodels.Inthelast
fewyears,edge-onTTauridiskshavebeenmodeledassuming
The interpretation of other permitted lines observed from the α-disksirradiatedbytheircentralstars(D’Alessioetal.1998,
spatiallyextendedscatteringregionaboveandbelowthedisks 1999, 2001) or using models with assumed parameterizedra-
is more difficult. Using the HST images and the known dis- dial and vertical density distributions (see e.g. Woodetal.
tanceoftheTaurusassociation(139pc,Bertoutetal.1999),we 1998; Watson&Stapelfeldt 2004). Theory, as well as obser-
findthatmostofthelightreachingusisscatteredtowardsusat vations, suggests that grain growth does occur in the T Tauri
heightsof about15 AU (HK Tau B), 35 AU (HV Tau C) and disks(seee.g.D’Alessioetal.2001).Diskmattercouldthere-
55AU(HH30 ).Inallthreecasesmostofthescatteredlightis fore show much stronger forward scattering properties than
∗