Table Of ContentAccepted to ApJ:January4,2008
PreprinttypesetusingLATEXstyleemulateapjv.10/09/06
QUIESCENT H EMISSION FROM PRE-MAIN SEQUENCE STARS IN CHAMAELEON I
2
Jeffrey S. Bary1, David A. Weintraub2, Sonali J. Shukla2, Jarron M. Leisenring1 and Joel H. Kastner3
1DepartmentofAstronomy,UniversityofVirginia,P.O.Box400325, Charlottesville,VA22904-4325
2DepartmentofPhysics&Astronomy,VanderbiltUniversity,P.O.Box1807StationB,Nashville,TN37235and
3CarlsonCenter forImagingScience, RIT,54LombMemorialDrive,Rochester, NY14623
Accepted to ApJ:January 4, 2008
ABSTRACT
We report the discovery of quiescent emission from molecular hydrogen gas located in the circum-
8 stellar disks of six pre-main sequence stars, including two weak-line TTS, and one Herbig AeBe star,
0 in the Chamaeleon I star forming region. For two of these stars, we also place upper limits on the
0 2→1 S(1)/1→0 S(1) line ratios of ∼ 0.4 and 0.5. Of the 11 pre-main sequence sources now known
2
to be sourcesof quiescentnear-infraredhydrogenemission, four possesstransitionaldisks,which sug-
n gests that detectable levels of H emission and the presence of inner disk holes are correlated. These
2
a H detections demonstrate thatthese inner holesare notcompletely devoidofgas,inagreementwith
2
J the presence of observable accretion signatures for all four of these stars and the recent detections
7 of [Ne ii] emission from three of them. The overlap in [Ne ii] and H detections hints at a possible
2
1 correlation between these two features and suggests a shared excitation mechanism of high energy
photons. Our models, combined with the kinematic information from the H lines, locate the bulk of
2
] the emitting gasata few tens ofAU fromthe stars. We alsofind a correlationbetweenH detections
h 2
andthosetargetswhichpossessthelargestHαequivalentwidths, suggestingalink betweenaccretion
p
- activity and quiescent H2 emission. We conclude that quiescent H2 emission from relatively hot gas
o within the disks of TTS is most likely related to on-going accretion activity, the production of UV
r photons and/or X-rays, and the evolutionary status of the dust grain populations in the inner disks.
t
s Subject headings: infrared: stars — solar system: formation — stars: pre-main sequence — stars:
a
[ individual (CS Cha, HD 97408,Sz 33, CV Cha, GM Aur, Glass I) — stars: proto-
planetary disks — ISM: molecules
1
v
5 1. INTRODUCTION of warm dust grains in the inner disks to be 3-6 Myr
6 (Lada et al. 2000; Haisch et al. 2000, 2001). Similarly,
Thereservoirsofgasanddustfoundorbitingmostsun-
7 observations of cold (T ∼ 10-100K) gas molecules (e.g.,
likestarswithagesontheorderofamillionyears—also
2 CO, HCN+, etc) from the outer regions of the disks
knownasTTauristars(TTS)—arenowsubjects ofin-
.
1 tense study, as they likely hold the secrets to the forma- (Beckwith & Sargent 1993; Zuckerman et al. 1995) tend
0 tion history of the solar systemand other planetary sys- to support the disk lifetime estimates drawn from near-
8 infrared surveys.
tems. The evolution of the observable characteristics of
0 More recently, new disk fraction and lifetime sur-
these planet forming disks and the underlying phenom-
: veys of TTS in nearby star forming regions have been
v ena responsible for their detectability may provide the
conducted utilizing the Infrared Array Camera (IRAC)
i necessary insight to further our understanding of planet
X and Multiband Imaging Photometer (MIPS) aboard the
forming processes and to determine the likelihood of the
r formation of planetary systems around sun-like stars. Spitzer Space Telescope. These instruments are capa-
a ble of detecting emission from dust grains (T ∼ 100-
The existence of circumstellar disks accompanying
300 K) located at intermediate disk radii, a region of
younglowmassstarspreviouslyhasbeeninferredbythe
these disks to which the previous photometric surveys
presence of detectable emission from trace constituents
were insensitive. Cieza et al. (2007) took a census of
of these disks (i.e., dust and gaseous molecules other
230 sources identified as weak-line TTS – weakly or
than H ). The earliest attempts to determine the frac-
2
non-accreting TTS possessing either a passive, optically
tion of stars that possess disks in young star form-
thin disk or no circumstellar material – in the Lupus,
ing clusters focused on their near-infrared continuum
Ophiuchus, and Perseus molecular clouds. They find a
(Strom et al. 1989) and submillimeter continuum emis-
higherdiskfrequencyforthesourceslocatedinthehigh-
sion (Weintraub et al. 1989; Beckwith et al. 1990). Cir-
est extinction regions of the clouds, suggesting that the
cumstellar disks possessing warm (T ∼ 1000 K) micron-
wTTS at the edges of the formation regions are slightly
sizeddustgrainswithin∼1AUofthepre-mainsequence
older and more evolved. Although they find ∼ 20%
stars produce near-infrared emission in excess of that
of these possess some circumstellar material, ∼ 50%
predictedforthestellarphotosphere,whilecircumstellar
of what appear to be the youngest stars in the sam-
disks possessing cold, micron-sized dust grains at radii
ple show no evidence of disks. These results, which
of several tens of AU produce excess radiation at mid-
are consistent with other Spitzer surveys of low mass
infrared and submillimeter wavelengths. From surveys
star forming regions (Furlan et al. 2006; Geers et al.
such as these, astronomers have determined the lifetime
2006; Kessler-Silacci et al. 2006; Padgett et al. 2006;
Electronicaddress: [email protected],[email protected],[email protected])t,.edleua,[email protected],ljuhkdpec,[email protected]
2 Bary et al.
on the existence of detectable emission from cool dust halfofmaximum(FWHM)onlyslightlylargerthanthose
grains,thatdisklifetimes arelessthantenmillionyears, reported for the previous four detections. In the case of
which roughly agrees with the conclusions about disk DG Tau, Takami et al. (2004) reported the detection of
lifetimes drawn from near-infrared surveys. Given the an H emission line blueshifted by 15 km s−1 and pos-
2
chaoticenvironmentnearthese youngstarsandthephe- sessing a FWHM of ∼ 35 km s−1. The authors also
nomena (i.e., accretion, photoevaporation, stellar and show that the emitting gas is extended along the axis
diskwinds,closeencounterswithotherclustermembers) ofa strongoutflow previouslyobservedfromthis source.
that conspire to destroy these disks, the lifetimes mea- Therefore,theemittinggasfromthissourceismostlikely
sured from these surveys can be explained by disk dis- associatedwiththeoutflowinggasdistinguishingthisde-
sipation models including all or some of these processes tectionfromtheothers. Inadditiontothesedetectionsof
(Hollenbach et al. 2000). rovibrationalemission, Sheret et al.(2003), Bitner et al.
Despite the growing consensus that disk lifetimes are (2007) and Martin-Za¨ıdiet al. (2007) have detected H
2
only a few million years that has been drawn from sev- emission from the mid-infrared pure rotational transi-
eraldifferenttechniquesforstudyingcircumstellardisks, tionsfromtheHerbigAeBestars,ABAurandHD97048,
astronomers must remain cautious when inferring the with a similarly quiescent kinematic signature.
presence or absence of disks based upon the detectabil- Here, we present five more detections of H near-
2
ity of disk tracers (i.e., dust and CO emission) because infraredemissionfrombothclassicalandweak-lineTTS,
the majority of the disk mass is composed of molecu- an infrared companion (IRC), and a detection from a
lar hydrogen (H ). As these disks evolve according to Herbig AeBe star in the Chamaeleon I star forming re-
2
the core accretion model of planet formation, the small gion. In addition, we place an upper limit on emission
micron-sized dust grains accumulate into larger bodies, from a second line of H at 2.2477 µm for two of these
2
whichwillchangethewavelengthdependenceofthedust sources. We review the possible stimulation mechanisms
emissivitiesandthe collectivesurfaceareaofdust grains with respect to recent X-ray and UV irradiated disk
emitting at wavelengths in the near-infrared. Since a models. We find that two of our past detections and
planetesimalmustreachamassof≃10earthmassesbe- one of the present detections possess transitional disks
fore it can gravitationally collect H gas from the disk, — disks with large inner disk holes devoid of emission
2
themaincomponentofthediskmustpersist,whileplan- from small warm dust grains. Two of these transitional
etesimals accrete into km-sized objects and disk tracers disksourcesalsopossessdetectableemissionfromamid-
disappear, in order for gas giant planets to be the result infrared [Ne ii] emission line predicted to be a tracer of
ofthecoreaccretionscenarioofplanetformation. There- high energy photons and a signature of photoevapora-
fore, in an accretionally-evolved system in which small tion, a mechanism thought to be potentially responsible
dustgrainshavecoagulatedintolargerplanetesimals,an forthepresenceoftheinnerdiskholesofthetransitional
evolutionary stage should exist during which both the disks. Kinematic information from the H emission fea-
2
near-infraredand mid-infrared excesses will have dimin- tures suggest that the emitting gas extends to within a
ishedsubstantiallywhilethemassivegaseouscomponent few AU of each source, demonstrating for the sources
remains present. with inner disk holes that gas remains despite the lack
In previous work, Weintraub et al. (2000), Bary et al. ofdustemission. Weestimatethelocationoftheregions
(2002),andBary et al.(2003)searchedavarietyofTTS, within the disks where potential excitation photons de-
predominantly wTTS, in the nearby Taurus-Auriga, ρ posittheirenergiesandfindagreementwiththe location
Ophiuchus, and TW Hya Association star forming re- of emitting gas determined from the velocity dispersion
gions for emission signatures from H persisting in such of the line. Finally, we relate the detection of quiescent
2
accretionally evolved circumstellar disks. Using various H emission to accretion activity and, possibly, the evo-
2
high-resolution,near-infraredspectrometers,narrowline lutionary status of the dust grain population.
emission centered at the v = 1 → 0 S(1) transition rest
wavelengthof 2.1218µm wasdetected fromseveralTTS 2. OBSERVATIONS
stars; some were known to possess circumstellar disks We obtained high spectral resolution (R ≃ 60,000)
(TW Hya, GG TauA, and LkCa 15), and one was previ- near-infrared spectra of thirteen TTS (Table 1) in the
ously thought to be devoidof any circumstellar material ChamaeleonI starforming regionon2003January7-18,
(DoAr21). Thedetectionofquiescentemissionfromthe UT,usingthePhoenixspectrometer(Hinkle et al.1998)
diskofawTTSwithnootherdetectabledisktracerssug- in queue-mode on the Gemini South 8-m telescope atop
gests that the gaseous component may indeed persist as Cerro Pachon in Chile. Phoenix utilizes a 256 × 1024
the disk evolves. However, we estimate the mass of hot pixel section of a 1024 × 1024 pixel InSb Aladdin II de-
H2 gas producing the observed features in these sources tector. Most of our spectral observations were centered
to be in the range 10−12-10−10 M⊙. For at least three at 2.1218 µm and were made using a 14′′ long, 0′.′17
of these four sources, a substantial reservoir of H2 gas (2 pixel) wide slit, which result in nominal instrumen-
exists in their disks suggestingthat the near-infraredH2 tal spatial and velocity resolutions of 0′.′05 and ∼ 5 km
emission feature, is itself only a tracer of the disk gas. s−1,respectively. Fortheobservationsofthetwofaintest
In the time since these detections were made, several sources,Sz33andCHX20E,andfollow-upobservations
other authors searching for the same emission signature centered at 2.2477 µm of CS Cha and HD 97048, we
alsohavedetectedH2 gas(Itoh et al.2003;Takami et al. used the larger, 0′.′34 (4 pixel) wide slit, resulting in a
2004; Ramsay Howat & Greaves 2007). In two of the velocity resolution of ∼ 6 km s−1. Spectra centered at
three additional sources toward which H2 was detected 2.1218 µm provided a spectral coverage from 2.1170 to
(LkHα 264 and ECHA J0843.3-7905),the emission lines 2.1262 µm, while those centered at 2.2477 µm covered
were similarly centered at 2.1218 µm, with full-width at 2.2426 to 2.2534 µm. The spectral region centered at
Quiescent H Emission 3
2
region. We define quiescent H emission as emission at
2
TABLE 1 low velocities relative to and centered on the stellar rest
Cha I SourceList
velocity. These detections nearly double the total num-
ber of TTS known to have quiescent H emission at this
Source Classa STpyepcterbal Hα(˚AE)Wc (LeroggsLs−x1d) Avewavelength. Fullspectraofboththede2tectionsandnon-
detections obtained with Phoenix are presented in Fig-
ure1. Foreachsourcewithdetectableemission,withthe
CSCha cTTS K4 -13.3,-20,-65 30.0 0.85
exceptionofthe binarysystem,GlassI,the emitting gas
HD97408 AeBe B9.5 -30.0 29.0 1.30
Sz33 cTTS M0 -9.5 28.9 1.45isdeterminedtobewithinasingleGaussian-shapedspec-
GlassIa wTTS K4 -4.0 30.1 1.92tralbeamwitha full-widthof∼1′′ placingthe emission
GlassIb IRC G5 -9.3 ··· ··· within∼80AUofeachofthesources(ChaI;D∼160pc)
CVCha cTTS G8 -60.5 30.1 1.67
. In the spectralimages of Glass Ia and Ib, and the sub-
Sz41 wTTS K0-K1 -2.7 30.2 1.17
LHα332-17 cTTS G2 -17.2 29.9 2.35sequent spectral extractions of Glass I, we find that the
CHX20E wTTS K4.5 0.0 29.4 0.31emission from this source extends between the compo-
Sz30 wTTS M0 -5.2 30.1 1.18nents of the binary, to the south of the primary, but not
HD97300 AeBe B9 0 29.2 1.80
north of the secondary (see §5.5).
Ced110IRS2 wTTS G2 >-1.2 30.7 3.39
WWCha cTTS K5 -67.4 29.8 3.20 All spectra have been shifted to the local standard of
rest(LSR)velocityframe. AssumingaV ∼2kms−1
LSR
a Class of stars taken from Feigelsonetal. (1993), Hamann&PerssonfortheChaIsources(James et al.2006),wefindthatin
(1992b), and Nattaetal. (2000).b Spectral types taken fromall cases, the detected emission lines are centered at the
Feigelson&Kriss (1989), Gauvin&Strom (1992), Hamann&Persson
(1992a), and Hamann&Persson (1992b).c Hα EWs fromrest wavelength of the v =1 →0 S(1) transition within
Appenzelleretal. (1983), Feigelson&Kriss (1989), Gauvin&Stromthe estimatedmeasurementerror(±1 kms−1). Inmost
(1992), Walter (1992), and Luhman (2004).d Log Lx values takencases, we find that the telluric correctionwas very effec-
from Feigelsonetal. (1993).e Visual extinction values taken fromtive, removing a significant number of telluric features.
Gauvin&Strom(1992).
The remaining absorption lines are expected to be asso-
the wavelength for the v = 1 → 0 S(1) H transi- ciated with the stellar photospheres of the target stars.
2
tion includes three bright telluric OH lines, at 2.11766, As wasfound inpreviousdetections ofH by Bary et al.
2
2.12325, and 2.12497 µm, while the spectra centered at (2002) and Bary et al. (2003), the H features are re-
2
the wavelength for the v = 2 → 1 S(1) transition con- solvedat this resolution. The FWHM for these lines fall
tains two OH lines at 2.24599 and 2.25192 µm. These within the range of ∼ 12 ≤ v ≤ 18 km s−1 for the five
OH lines were used to provide an absolute wavelength TTS and slightly higher, ∼ 20.4 km s−1 for the Herbig
calibration for each observation. AeBestar. TheH equivalentwidthsandextinctioncor-
2
Using integration times of 900 s for our programstars rected line fluxes are presented in Table 2 for the detec-
(Table 1), we obtained signal-to-noise ratios (SNR) in tionsandone‘possible’detection(WWChawhosespec-
the continua for all sources of ≥ 50, with the exception trum is presented with the non-detections in Figure 1),
ofthecontinuumforSz30,forwhichtheSNRwaslower. along with the 3σ upper limits for the five sources that
An integration time of 900 s also was used for the two were clearly non-detections. The line fluxes were dered-
observations of CS Cha and HD 97048, centered on the denedusing A values fromGauvin & Strom(1992)and
v
2.2477 µm H line. Our observing program included re- a standard A ∼ λ−1.84 extinction law adopted from
2 λ
peated 120 s observations of telluric calibration sources Martin & Whittet (1990).
– HR 3685 (A2IV), HR 5571 (B2III), HR 4798 (B2V), In observations centered on the v = 2 → 1 S(1) rovi-
HR 5231 (B2.5IV), and HR 1956 (B7IV) – possessing brationaltransitionofH at2.2477µmoftwo2.1218µm
2
few, if any, photospheric features in this K−band por- line emission sources, CS Cha and HD 97048, we did
tion of their spectra. Similar observations of HR 5231 not detect line emission. The upper limits to the v =
and HR 5762 (A2IV) centered at 2.2477 µm used as 2 → 1 S(1) line fluxes for CS Cha and HD 97048 are
telluric calibrators were made with shorter integrations 6.5×10−16 and4.4×10−15ergscm−2 s−1,respectively.
times of 60 s and 40 s, respectively. Flat-field images
wereobtainedusingtheGeminiFacilityCalibrationUnit 4. DISCUSSION
Quartz Halogen lamp (Ramsay-Howatet al. 2000). Ob- The lack of bulk motion of emitting H gas along the
2
servationsweremadebynoddingthetelescope5′.′0along line of sight and with respect to the systemic velocity of
theslit,producingimagepairsthatwerethensubtracted the stars is interpreted as evidence that the gas is not
toremoveskybackgroundanddarkcurrent. Thespectra entrained in a fast moving wind or outflow associated
wereextractedfrom0′.′25northto0′.′25southofthecen- with any of these young sources. Additionally, the lack
tral row of pixels of the spectralbeam, correspondingto of extended emission beyond the full-width of the spec-
the FWHM of the spectral beam. The target star spec- tral beam indicates that the emitting gas is confined to
tra were then ratioed with one of the telluric calibration regions within ∼ 80 AU of the stars — with the excep-
sources in order to remove telluric absorption features. tionof binary Glass Ia and Ib, in which the emissionex-
tends between the companions. Therefore, we conclude
3. RESULTS that, in all cases except Glass I, the emitting H is qui-
2
We have detected quiescent 2.1218 µm line emission escent gas residing in the circumstellar disks associated
from H from four additional TTS (CS Cha, Sz 33, withthesesources,andisnotshocked,spatiallyextended
2
CVCha,andGlassIa),oneIRC(GlassIb,thesecondary gas, as has been observed toward other Class I sources
componenttooneoftheTTSdetections),andoneHerbig and TTS (e.g., T Tauri, DG Tau; Herbst et al. (1996),
AeBe star (HD 97048)in the ChamaeleonI star forming Takami et al. (2004), Beck (2007)). The Glass I binary
4 Bary et al.
edgesofthe Gaussian-shapedfeatures,referredto as the
TABLE 2 half-width at zero intenisty (HWZI), to estimate the in-
Cha I H2 Detections andNon-Detections nermostradiusatwhichthedetectedH gasresides. As-
2
suming an inclination angle of 45◦, the main sequence
Source Kmaga LineFluxb Eq. Width FWHMmass corresponding to the spectral type of each source,
(ergscm−2 s−1) (˚A) (kms−1)
andestimatingthemaximumorbitalvelocityofgasfrom
the HWZI, we calculated the inner radii for gas in the
Detections
disks of CS Cha and Sz 33 to be 2.5 and 0.9 AU, re-
CSCha 8.26 2.1±0.1×10−15 0.10 12.6
spectively, while for the more massive stars, CV Cha
HD97408 6.27 8.6±0.4×10−15 0.07 20.4
Sz33 9.24 6.9±0.3×10−16 0.08 12.7and HD 97048, we find 3.2 and 5.5 AU. We note that
GlassIa 8.58 2.3±0.1×10−15 0.43 12.1these radii represent the innermost regions of the disks
GlassIb 7.35 6.7±0.3×10−15 0.05 17.6from which the emission arises and accounts for only a
CVCha 6.79 2.0±0.2×10−15 0.02 12.6fraction of the overall line emission. We conclude that
PossibleDetection the bulk of the emitting gas resides outside of these disk
WWCha 6.13 ∼3.7×10−15 0.02 11.8
radii.
Non-Detections
Sz41 7.88 <1.3×10−15 ··· ··· The results we have found for the distances from the
LHα332-17 6.26 <3.0×10−15 ··· ··· stars at which the hot H2 emission is generated is sim-
CHX20E 9.19 <3.9×10−16 ··· ··· ilar to that found by Bary et al. (2003), who found
Sz30 8.55 <3.1×10−15 ··· ··· FWHM of 9-14 km s−1 for several K type stars, plac-
HD97300 7.19 <1.9×10−15 ··· ··· ing the bulk of the emitting gas at about 10 AU,
Ced110IRS2 6.43 <6.8×10−15 ··· ···
by Ramsay Howat & Greaves (2007), who found a line
widthof∼18kms−1forasingleM3star(ECHAJ0843.3-
a K-bandmagnitudestakenfromLawsonetal.(1996)withtheexception
of Glass Ia and Ibfound inChellietal. (1988).b The uncertainties inth7e905)andbyBitner et al.(2007),whofoundalinewidth
linefluxesweredeterminedbaseduponthecontinuumnoiselevelsandthoef 8.5 km s−1 for a single AeBe star (AB Aur). In all of
integratedlinestrengths. these cases, the H gas emitting in rovibrationalor pure
2
rotational lines is not found close to the star (inside of
1 AU) nor at great distances from the stars, suggesting
that processes exciting this emission is most effective at
intermediate distancesof a few to a few tens of AUfrom
these stars.
Assuming that the emitting H gas is optically thin,
2
we have used Equation 2 from Bary et al. (2003), the
measured line fluxes (see Table 2), and a distance of
160 pc to Chamaeleon I, to calculate the masses of
‘hot’ H gas producing the line emission. We find that
2
these masses fall within the range 10−11 − 10−10 M⊙,
in agreement with previous estimates (Bary et al. 2003;
Ramsay Howat & Greaves 2007). On their own, these
masses do little for informing us about the total mass of
gas in the system or the total disk mass unless we un-
derstand the physical conditions of the gas and/or the
mechanism responsible for stimulating the emission. We
will address this problem in both §4.1 and §4.3.
Fig. 1.— In the left frame, we plot the spectra for sources
containingH2emissionlinefeatures. Thespectrahavebeentelluric 4.1. v =2→1 S(1)/v=1→0 S(1)
correctedandshiftedintotheVLSRreferenceframe. Wefindthat
spectral features center on the rest wavelength of 2.1218 µm for The v = 2 → 1 S(1)/v = 1 → 0 S(1) ratio has of-
the v = 1 → 0 S(1) transition of H2 in the stellar rest frame, ten been used to distinguish between two commonly ob-
to within errors for the accepted VLSR of Cha I members. In served mechanisms for stimulating H emission: shock
the right frame, we plot the spectra of the non-detections. These 2
heating and UV fluorescence. The value of this ra-
spectra have also been telluric corrected and shifted in the VLSR
referenceframeofthesources. Wenotetheinclusionofthespectra tio for a parcel of shock heated H2 gas has been ob-
of WW Cha with the non-detections, but list it as a ‘possible’ served to be ∼ 0.1, while a value of ∼ 0.5 has been
detectioninTable2duetothepresenceofa2.4σfeaturesimilarly
observed for UV fluorescent H emission from low den-
centeredat2.1218µm. 2
sity gas (n ≤ 104 cm−3) (Black & van Dishoeck 1987;
H
Gatley et al. 1987; Dinerstein et al. 1988). However,
appears to be an exceptional source with its extended Burton et al. (1990) found UV fluorescent H emission
2
emission. from clumpy photodissociation regions with shock-like
The small, but resolvedFWHM measured for the qui- values for the 2→1/1→0 line ratio and n ≥ 105 cm−3,
H
escent emission features must be broadenedby the Kep- suggesting that the value of 0.1 for the ratio is not
lerianrotationof the gas as it orbits the central sources, unique for shock heated gas. In addition to these two
since thermal broadening would require temperatures processes, the response of H gas irradiated by X-rays
2
> 6000 K. Using the same technique Carr et al. (2001) has been studied by several groups who conclude X-
andNajita et al.(2003)used to estimate the inner trun- rays are capable of stimulating H emission in a vari-
2
cationradiusofthe gasdiskusingCO emissionfeatures, ety of astrophysical environments, including circumstel-
we used the velocity shift of H gas at the outermost lar disks (Gredel & Dalgarno 1995; Maloney et al. 1996;
2
Quiescent H Emission 5
2
Tine et al.1997). Due to the collisionalnature ofthe X- presentwithinthedisksofthesesystems,whilethecooler
ray ionization process for stimulating H emission, this dust grains responsible for the longer wavelength excess
2
mechanism also has been predicted to populate the ex- persist in these disks. Therefore, the observed dips in
cited rovibrational states of the H molecules such that the SEDs of TTS have been interpreted as evidence of
2
the v = 2 → 1 S(1)/v = 1 → 0 S(1) ratio also has a holesorclearingsintheinnermostregionsofthesedisks.
value of ∼ 0.1. Therefore, given the apparent degener- These holes may arise due to the coagulation of small
acy of the line ratio, differentiating between these three dust grains into planetesimals or the presence of a mas-
mechanismswillalsorequiretheknowledgeofthedensity sive planet that dynamically cleared the inner regions
and kinematics of the emitting gas. of the disks (Skrutskie et al. 1990; Najita et al. 2007).
Nomura et al. (2007) present a detailed disk model Additionally, disk photoevaporation models, in which
that incorporates irradiation of the disk by X-rays and disk material is heated and ‘boiled’ away by highly en-
UV photons associated with the central source and in- ergetic X-ray and UV photons, also have been offered
cludes dust grain growth and setting towards the mid- as a phenomena responsible for the clearing of the in-
plane. Using this disk model, Nomura et al. (2007) cal- ner holes in these transitional disks (Clarke et al. 2001;
culate the level populations of H in the model disk for Alexander et al. 2006; McCabe et al. 2006).
2
a range of dust grain sizes and predict values for the For a few such transitional disk sources with inner
v = 2 → 1 S(1)/v = 1 → 0 S(1) H line ratio for three holes extending out to several tens of AUs, such as
2
stimulationscenarios: a)X-rayandUV,b)onlyUV,and GMAurandCSCha,tinyamountsofsmalldustgrains,
c)onlyX-ray. ThevaluesNomura et al.(2007)calculate ∼ 0.02 and 5 × 10−5 lunar masses, respectively, located
for the line ratios differ significantly from the nominal within ∼1 AU, have been detected (Calvet et al. 2005;
values mentioned above and depend on sizes of the dust Espaillat et al. 2007). Somewhat surprisingly, several of
grains(seeFigure17ofNomura et al.(2007)). Theyalso thesetransitionaldiskspossesswell-knownaccretionsig-
find that as the dust grains grow and the grain opac- natures (i.e., U−band excess, strong H i recombination
ity decreases,UVpumping shouldbecome the dominant lines, etc.) suggesting that gas must remain within the
stimulation mechanism over X-ray ionization. Accord- disks and the inner holes, despite the lack of detectable
ing to the findings of Nomura et al. (2007), an accurate dust emission. Chiang & Murray-Clay (2007) show that
measurement of the line ratio will determine the mecha- the accretion activity in these sources may be driven by
nism responsible for stimulating the H emission as well coronal X-rays activating the magneto-rotational insta-
2
asindicatetheevolutionarystatusofthedustgrainpop- bilityinthegasresidingatthedustdiskrimandresiding
ulation. within the inner disk hole.
The upper limits of the 2.2477 µm line observations
mentionedin§3canbe usedtodetermineanupperlimit 4.2.1. H2 Emission from Transitional Disks
on the 2→1/1→0 line ratio for CS Cha and HD 97048.
All sources toward which we have searched for H
2
Unfortunately, with upper limits for the ratio of 0.4 and
emission (Weintraub et al. 2000; Bary et al. 2002, 2003)
0.5 for CS Cha and HD 97048, respectively, these ob-
known to possess transitional disks have been found to
servations lack the sensitivity in both cases to test the
produce detectable levels of quiescent H emission at
2
predictions of Nomura et al. (2007) and do not rule out
2.1218 µm, including TW Hya, LkCa 15, CS Cha, and
the ‘only UV’ (scenario b) case.
GM Aur (Weintraub et al. (2005)); Bary et al. 2008,
Simultaneous observations of both lines made with a
in prep). In each case, kinematic information from
large aperture telescope and high resolution spectrom-
the emission lines, assuming a similar velocity disper-
eter (R ≥ 60,000) will be ideal for accurately measur-
sion as found for the other H detections and for the
2
ing this line ratio. The simultaneity of these observa- unresolved feature measured for the disk of TW Hya1
tions are important if the H emission is variable, as
2 (Weintraub et al. 2000), places H gas within the opti-
2
non-simultaneousobservationswouldthenlikelyproduce
cally thin inner disk hole for eachof these three sources.
inaccurate measurements of the line ratio. Indeed, the Rettig et al.(2004)detectedv =1→0rovibrationalCO
H emission is likely to be variable if the X-ray and UV
2 emission from the disk of TW Hya, but did not detect
fluxes, both known to be produced by highly variable the v = 2 → 1 CO band. Rettig et al. (2004) suggest
phenomena in these young systems, are responsible for
thatthe absenceof‘hot’COgasindicatesthatthe inner
stimulating the H molecules.
2 0.5AUofthediskhasbeencleared. Arotationaltemper-
4.2. Transitional Disks atureof∼430Kleadstheseauthorstoconcludethatthe
COemissionthey detectedinthis systemarisesinwarm
In the infrared, the shapes of the spectral energy dis- gasoccupyinga verynarrowannulus in the disk ranging
tributions(SEDs)ofcTTSsystemsaredescribedas‘flat’ between0.5and1AU.Morerecently,Salyk et al.(2007)
due to emission from circumstellar dust grains in ex- detected warm CO emission from the disks of TW Hya
cess of the Rayleigh-Jeans tail of the blackbody curve and GM Aur, further confirming the presence of molec-
observed for the stellar photospheres of main sequence ular gas in the inner regions of these transitional disks.
stars. However, infrared photometric surveys of many Several of the other six sources detected in H are clas-
2
TTS in nearby star forming regions, similar to those sified as borderline c/wTTS or wTTS, including Sz 33
discussed in §1, also identified dips in the 1-10 µm re-
gions of the otherwise flat SEDs associated with some 1 The detection of H2 emission from TW Hya is reported as
of their targets (Strom et al. 1989; Skrutskie et al.1990; an unresolved detection in CSHELL spectra taken at the IRTF.
Gauvin & Strom 1992). These findings suggest that the The FWHM of the spectral feature was incorrectly quoted as
< 6 km s−1 or the 1σ width. The FWHM of the unresolved line
small warm dust grains responsible for the short wave- isactuallymeasuredtohaveavalueof∼13.5kms−1,theinstru-
length infrared excesses in most cTTS disks are not mentalresolutionoftheinstrument.
6 Bary et al.
and DoAr 21 (Bary et al. 2002), which also makes them be associated with emitting gas produced via the UV
viable transitional disk candidates. Given that we es- fluorescence and X-ray stimulation mechanisms.
timate the innermost radius for the location of the gas Utilizing a standard TTS disk model (Glassgold et al.
producing the H emission in these sources to be near 1997), H cross-sections found in Yan et al. (1998), and
2 2
∼ 1 AU, we conclude that these inner disk holes are not ignoring the contribution of dust and other trace gas
completelydevoidofgas,inagreementwiththedetection components, to first order, we estimate where the UV
and measurementofaccretionactivity in severalsources photons and X-rays are absorbed and are likely to stim-
possessing transitional disks. ulate H molecules in an atypical TTS disk devoid of
2
We must consider whether this apparent correlation smalldust grains by calculating a rangeof opticaldepth
of H emission with the presence of transitional disks surfaces for X-rays and UV photons with different en-
2
could be the result of observational bias. Specifically, ergies impinging upon a disk with varying angles of in-
is H emission easier to detect toward transitional disk cidence. The physical depth into the disk that each of
2
sourcesbecausetheirnear-infraredcontinuumfluxlevels these photons penetrates is determined by
are lower and thus do not overwhelm the H line emis-
2
sion? If all cTTS generate some amount of quiescent H2 rτ r −q
τ = κ dr, (1)
emission, but sources with flat spectra and a resulting ν
Z (cid:18)r (cid:19)
higher near-infrared continuum flux make detection of rinner 0
these narrow and weak spectral features more difficult, whereτ istheopticaldepth,r istheradiusatwhich
inner
then the brightest K-band sources should be among the thephotonsenterthedisk,r isthepenetrationdepthof
τ
hardest toward which to detect quiescent H2 emission. thephotonforthegivenopticaldepth,r0=1AU,qisthe
HD 97048, Glass Ib, and CV Cha, all of which are de- power-lawindex(heretakentohaveavalueof2.75),and
tected in H2 emission, however, are among the bright- κν is the opacity of the H2 for photons with frequency
est K-band sources in our sample. Therefore, we sug- ν.The photons are assumed to enter the disk at 0.35 AU
gest that sources possessing transitional disks are pref- from the star, the innermost radius for which the model
erentially detected in H2 line emission because of physi- applies. Aftercalculatingthepenetrationdepths(rτ)for
cal conditions and phenomena intrinsic to these objects, a range of energies and optical depths, plots of the op-
rather than observational selection. tical depth surfaces were constructed (Figures 2 & 3) to
As discussed in §4.1, grain sizes have a notable effect illustratethemostlikelyregionsofthedisksinwhichthe
on the cross-sections for UV and X-ray photons; conse- photonswouldbe absorbed. The plots consistofdensity
quently, as disks evolve and dust grains grow larger, the contours derived from the Glassgold et al. (1997) disk
locations in a disk to which the photons penetrate and modeloverlaidwiththe opticaldepthsurfaces. Thethin
deposit their energies change (Nomura et al. 2007). As black lines represent density contours derived from the
a result, the TTS that are sources of quiescent H2 emis- disk model, while the bold black lines represent the op-
sionmaybethosethathavedisksthatpermitphotonsto tical depth surfaces. In Figure 2, we outline the surface
penetratetoregionswherethedensitiespermitsuccessful correspondingtotheuppermostboundaryofpenetration
stimulation of H2 emission. These disks would preferen- byeitherUV photonsorX-rays(i.e.,theτ =1.0surface
tially be those with optically thin inner regions and op- for a UV photon with energy 15.4 eV, corresponding to
tically thick outer regions, i.e., transitional disk sources. a wavelength of ∼ 805 ˚A). This is the lowest energy for
For sources that are both X-ray and UV sources, a real which the cross-section of an H molecule can be deter-
2
dependence of the stimulation of quiescent H2 emission mined using the models of Yan et al. (1998), and comes
on the size distribution of the dust grain population ar- closest to matching the photon energies used in the UV
gues for UV fluorescence as the dominant stimulation fluorescence models of Black and van Dishoeck (1987).
mechanism, as UV cross-sections are much more depen- InFigure3,twoopticaldepthsurfacesareplottedwith
dent on the sizes of the dust grains. the upper bold contour representing for τ = 0.2 for UV
photonswiththesameenergyasinFigure2andthesec-
4.3. The H Emitting Region
2 ond contour at lower disk elevation representing τ = 1.0
In this section, we address the question of the loca- for X-ray photons with an energy of 1 keV. Given the
tion of the emitting H gas stimulated by X-rays and/or greater penetration depths associated with the X-rays,
2
UV photons, in the context of a transitional disk. We the τ = 1 opticaldepth surface acts as the lower bound-
predict the regions of such disks in which the competing ary for the penetration depth of the high energy pho-
stimulationmechanismswillbe mosteffective by finding tons. As illustrated by the plot, some of the X-rays
areas of the disk bounded by optical depth surfaces as- are capable of penetrating into regions of the disk in
sociatedwithenergeticphotons. We thencomparethese whichthe densities exceedthe criticaldensity, ρ (where
c
regionswiththosedeterminedfromthekinematicsofthe ρ =10−16 gcm−3 or3×107 H cm−3;4thmodelcurve
c 2
emission features. from the midplane) for the formation of the 2.1218 µm
emission feature. Although some TTS are observed to
4.3.1. UV & X-ray Optical Depth Surfaces
produce X-rays with energies as high as 10 keV, we do
Previous spectroscopic data (Bary et al. 2003) and notinclude themonthisplotsincethey wouldpenetrate
those presented in this paper suggest that the emitting to even greater depths and densities far exceeding ρ .
c
gas extends from ≥ 1 AU out to intermediate disk radii We interpretthe regionlying betweenthese twosurfaces
of 10-30 AU, with the bulk of the emitting gas resid- and above the critical density contour as location in the
ing at intermediate disk radii. We seek to determine if disk where the photons are most likely to deposit their
any agreement may exist between these empirically de- energiesandwheretheH emissionismostlikelytoorig-
2
termined radii for the gas emission and those likely to inate. Using the constraints placed upon the location of
Quiescent H Emission 7
2
the emitting region by the spectral information, we de- earlier conclusions about the most likely location of the
duce that the bulk of the emission comes from a region majority of the H emission.
2
located 10-30 AU from the star and 5-15 AU above the Having located the most likely region for the emitting
disk midplane. H gas, we next determine how much of the disk mass
2
resides in the regions confined by the optical depth sur-
faces. Byintegratingthedensityprofileofthediskmodel
provided in Glassgold et al. (2000),
router 2π zupper r −q −z2
Mdisk = ρo e2H2rdrdφdz.
Zrinner Z0 Zzlower (cid:18)r0(cid:19)
(2)
where r and r and z and z are the
inner outer lower upper
limits of integration for the radial and vertical extents
of the portion of the disk to be considered, ρ is the
0
density of the disk at a radius of 1 AU and is equal to
1.4×10−9 g cm−3, andH is the verticalscale height pa-
rameter given by H (r/r )1.25, where H = 5 × 1011 cm
0 0 0
and r = 1 AU, we determine the mass of H contained
0 2
inthe relevantregionsoutlinedbythe opticaldepthsur-
faces. Using Equation 2 and setting the inner (r )
inner
and outer (r ) radiallimits to 10 and 30 AU, respec-
outer
tively, and approximating the vertical limits, z and
lower
Fig. 2.— An optical depth surface (τ = 1) for UV photons
withanenergyof15.4eVisplottedfrom0.35to∼45AUinbold zupper, from the optical depth plots to be 5 and 10 AU,
blackcontours. Theoptical depthsurfaceisoverlaidatop density respectively, we find a mass of 2.8 × 10−5 M⊙. This
contours (thin contours) generated usingthediskmodelofGlass- mass is more than enough to account for the observed
greopldreestenatls.a(2d0e0n0s)i.tyTohfeρde=ns1it0y−1c3ongtocumr−n3e.arWesitththiencdriesaksimngideplleavnae- eamniuspsipoenr.reFgoironcoemxtpeanrdisinong,froonmly150.8≤×z ≤10−159 AMU⊙2.liSeisncine
tion above the disk midplane, the values of the density contours
decreaselogarithmically,endingwithρ=10−19gcm−3. Theopti- neither stimulation mechanism is 100% efficient, a mass
caldepthsurfaceroughlycorrespondstothedistanceintothedisk larger than that contained in the horizontal layer from
thatphotons withawavelength of805 ˚Acouldpenetrate. 10 to 15 AU should be expected to reside in the region
ofthe disk wherethe emissionoriginates. Therefore,the
mass determined for the region 5 to 15 AU above the
disk midplane suggests that the emitting gas likely ex-
tends downward,towards the midplane of the disk, with
a minimum verticalheight on the order of 5 AU at radii
extending from a few to a few tens of AU for X-ray/UV
stimulated H emission in an accretionally-evolved disk
2
associated with a TTS.
In our calculations of the optical surfaces for UV and
X-ray photons, we have assumed that the gaseous com-
ponentofthediskhasbeenunaffectedbythe‘disappear-
ance’ of the micron-sized dust particles. If the gaseous
component of the disk were to become more tenuous as
thediskevolves,theresultantdensitystructurewouldal-
lowX-raysandUV photonsto penetrate toevengreater
depths into the disk thansuggestedby the opticaldepth
surfacesshowninFigures2-6andthecriticaldensitycon-
tour would move closer to the disk midplane and closer
tothestar. Inthiscase,whichissimilartothatexpected
for a transitional disk, the emission could arise from gas
Fig. 3.— The τ = 0.2 optical depth surface for 15.4 eV UV inregionsofthedisklocatedmuchclosertothemidplane
photons is plotted from 0.35 to ∼ 45 AU as a bold black line and closer to the star, and from gas at even larger disk
atop the disk density contours. The second bold black contour radii.
at lower diskelevation represents the τ = 1 optical depth surface
However, in the case of an evacuated inner disk, but
for1keVX-rays. Thefourthcurvefromthemidplanecorresponds
to10−16 gcm−3 or3×107 H2 cm−3 whichisthecriticaldensity a dust-filled outer disk, the opacities will increase in re-
for the v =1→0 S(1) transition of H2. It appears that most of gions of the disk still populated by dust grains. Con-
theX-rayswillpenetrate todensitiesnH2 >10−16 gcm−3. sequently, the shapes of the optical depth surfaces will
change from those we have calculated. We note that
These calculations of optical depth surfaces demon- 2 Masses determined for the gas in the region of the midplane
strate that the UV and X-ray photons most likely will (0AU)to5AUcanbefoundinTable3,butarenotincludedinthis
discussionbecausethemajorityofthismassliesbelowthecritical
deposit their energies in the upper levels of the disks at density contour, where no emissionfrom the v =1→0 S(1) can
intermediate distances from the sourcesand confirm our beproduced.
8 Bary et al.
amassaccretionrateof1.2×10−8 M⊙ yr−1 forCSCha
TABLE 3 from the U-band excess, while Johns-Krull et al. (2000)
Masses in H2 SourceRegions estimated it be 1.6 × 10−7 M⊙ yr−1 — despite contin-
uum measurements that the disk possesses an optically
rinner→router zlower→zupper H2 Mass thin inner region.
(AU) (AU) (M⊙)
UsingSpitzerIRSspectraandMIPSphotometryat24
and 70 µm, Espaillat et al. (2007) modeled the SED for
10→30 0→5 6.4×10−3
10→30 5→10 2.8×10−5 CSChaandfoundabest-fitmodelthatincludesaninner
10→30 10→15 5.8×10−9 hole with a radius of ∼ 43 AU, with a small amount of
hotdustgrainslocatedbetween0.1and1.0AU.Basedon
the extent of these inner holes is observed to vary from theEspaillatdiskmodel,thevariableHαmeasurements,
sourcetosourceandthattheseholeshaveobservedsizes the U-band excess, the strong [Ne ii] emission, and our
ranging from a few AU to ∼ 40 AU, suggesting slightly detection of H2 emission from the CS Cha disk, we sug-
different conditions for determining the location of the gest that CS Cha is an X-ray bright, actively accreting
emitting gas for individual sources. TTS possessinga transitionaldisk with a large optically
Although the optical depth surfaces calculated in this thin inner hole, containing little dust and a significant
section provide insight into the location of the emitting amountofgas. Thedustgrainpopulationislikelydomi-
gas from an accretionally evolved disk, a calculation of natedbylargegrains,hencethelownear-infraredexcess,
thesesamesurfacesemployingtheirradiateddiskmodels whilethegascontributessubstantiallytothevariableac-
ofNomura et al.(2007)andtransitionaldisks,including cretion rate.
a variety of cross-sectionsthat incorporatethe evolution
5.2. CV Cha
of the size of dust grains will improve upon this investi-
gation and make them applicable to a broad variety of LHα 332-21 is a binary cTTS (Reipurth & Zinnecker
accretionally-evolvedcircumstellar disks. 1993),composedofCVChaandCWCha,havingalarge
separationof11′.′7 (Ghez et al.1997). We observedonly
4.4. H2 & [Ne ii] CV Cha, the primary component, and have detected H2
The results of several investigations (Herczeg et al. emission from this source. CV Cha has an Hα EW of
2007; Lahuis et al. 2007; Espaillat et al. 2007; 60.5 ˚A (Gauvin & Strom 1992) and exhibits weak con-
Pascucci et al. 2006; Bary et al. 2008) have found tinuumveilinginhighresolutionopticalspectraobtained
four sources possessing [Ne ii] at 12.81 µm that also with UVES on the VLT (Stempels & Piskunov 2003).
possess transitional disks. Of the four transitional ExtendedUVemissionhasbeendetectedinIUEobserva-
sources possessing [Ne ii] emission, three (TW Hya, tionsofthissourcebyBrooks et al.(2001). Withaspec-
CS Cha, and GM Aur) also possess quiescent H traltypeofG8V(Rydgren1980;Takami et al.2003)and
2
emission, while the fourth (DM Tau) has not yet been aLogLx of30.1ergss−1 (Feigelson et al.1993),CVCha
observed as part of any H survey. The strong overlap is one of most massive sources in our survey and one of
2
between the transitional disk sources possessing both the strongest X-ray emitters in the Chamaeleon I star
[Ne ii] and H emission suggests a correlation exists forming region. This source is not known to possess a
2
between these two emission features and may be related transitional disk and, due to a large Hα EW and the
to the nature of the transitional disks. In fact, because presence of the aforementioned continuum veiling, ap-
of the high initial ionization potential of 20.6 eV for [Ne pears to be an actively accreting source. It appears that
ii] emission (Glassgold et al. 2007), this line probably this source possesses the necessary attributes, in that it
traces the same high energy photons we theorize are is a source of both UV and X-ray photons, to stimulate
responsible for stimulating the quiescent H2 emission in the observed quiescent H2 emission.
the near-infrared. Furthermore, the FWHM measured
for both the [Ne ii] and H in TW Hya suggest that the 5.3. Sz 33
2
emission from these two features arises at similar radii Sz 33 has been classified as both a
in the circumstellar disks. wTTS (Feigelson & Kriss 1989) and a cTTS
5. INDIVIDUALSOURCES (Feigelson & Lawson 2004) with a value of 9.5 ˚A
forits Hα EW, placingit amongstthe other transitional
5.1. CS Cha
objects, “stradling” the line between a cTTS and wTTS
CS Cha was classified by Hartigan (1993) as a cTTS, (Gauvin & Strom 1992). G´omez & Persi (2002) present
based on a measured Hα EW of 65 ˚A, though Ap- a low resolution (R ∼ 500) spectrum of Sz 33 that
penzeller et al. (1983; 13.3 ˚A) and Luhman (2004; contains Paβ and Brγ emission features, indicating
20 ˚A) found smaller values. The observed variabil- that this source is still actively accreting. Given the
similarity of its ambiguous classification as a cTTS
ity of Hα for CS Cha was previously suspected by
and/or a wTTS to other sources with detectable H
Henize & Mendoza (1973). Gauvin & Strom (1992) and 2
emission in our survey that possess optically thin inner
Persi et al. (2000), using IRAS and ISOCAM, respec-
disk regions, we suggest that the disk associated with
tively, found that CS Cha has little or no near-infrared
Sz 33 also may be a transitional disk.
excessbut substantialmid- to far-infraredexcesses. The
magnitude and variability of the Hα EW measurements
5.4. HD 97048
for CS Cha strongly suggest that this source is still ac-
tively accreting with a variable accretion rate — in sup- HD 97048 is classified as a B9.5V Herbig AeBe star,
port of this conclusion, Espaillat et al. (2007) estimated oneoftwoHerbigAeBe starsthatwereobservedaspart
Quiescent H Emission 9
2
ofthissurvey(HD97300wastheother). Ourdetectionof tures centered on both sources are slightly larger than
quiescentnear-infraredH emissionand the detection of those measured for the extended gas, with FWHM of
2
purerotationalH emissionbyMartin-Za¨ıdiet al.(2007) 12.6 and 17.6 km s−1 found for the primary and the
2
fromthissourceplacesitinthecompanyofABAur,an- secondary, respectively, and 10.3 and 9.1 km s−1 mea-
other Herbig AeBe star, for which pure rotational H sured for gas located 0′.′8 south the secondary and gas
2
emission has been detected by Bitner et al. (2007). In- extending 0′.′45 to the south of the primary component,
frared emission from this source is thought to originate respectively. The difference in the FWHM measured for
from the surface layer of a passively heated disk. Ex- thecomponentsofthebinarysuggestthatwearedetect-
tended mid-infrared emission is dominated by a strong ingemissionfromdiskswithdifferentinclinationsand/or
continuum produced by warm dust grains in the imme- central sources with different masses. For the extended
diate vicinity of the star and solid state emission bands gas,with its locationlikely to be ina circumbinarydisk,
from silicates and carbonaceous materials (Meeus et al. littlebroadeningoftheselineswillbecontributedbythe
2001; Natta et al. 2001; Prusti et al. 1994). The spec- Keplerian motion. Assuming the standard thermal ex-
trumrises rapidlylongwardof2.2 µmandpeaks around citation temperatures for H of 2000 K, the FWHM of
2
60 µm (Hillenbrand et al. 1992; van Boekel et al. 2004) the line is expected to be ∼ 7 km s−1, slightly smaller
andissimilartotheSEDsofTTSpossessingtransitional than the FWHM measured for the extended emission
disks. Anextendeddiskrangingfrom360to720AUwas features. The measured FWHM correspond to unrea-
observed with the Advanced Camera for Surveys on the sonablyhightemperaturesgreaterthan3000K,perhaps,
Hubble Space Telescope. Ca ii, Na i, and O i are all ob- suggestingthe presence of some turbulent broadening in
servedinabsorptionsuggestingthatthesourceisneither the extended gas emission.
actively accreting nor generating a bipolar outflow. De- Although it is not currently possible to reconcile the
spite the absence of the accretion activity necessary for broaderthan expected FWHM of the extended emission
the production of UV photons in the low mass counter- features with their origin in a circumbinary disk, it ap-
parts, copious amounts of UV radiation intrinsic to the pears we can rule out the association of this emission
SED of an early-type star as well as its detection as an with outflow activity. Since we find a larger FWHM for
X-raysourcedemonstratethepresenceofthe underlying the lines centered on the components of the binary, and
mechanisms for high energy photon production we be- the FWHM is similar to those detected for the other de-
lievemayberesponsibleforstimulatingtheH emission. tections in this survey, we infer that these features are
2
associated with the circumstellar disks of the individual
5.5. Glass Ia & Ib
components.
Glass Ia and Ib form a close binary system with
a separation of 2′.′4, or a minimum physical separa- 5.6. X-rays vs. Hα EWs
tion of ∼ 380 AU. The secondary component is classi- InFigure7,wepresentaplotofLogL versusHαEW
x
fied as an Infrared Companion (IRC) based upon the forallpublishedH detections andnon-detectionstode-
2
2.2 µm flux ratio (F2.2(optical primary)/F2.2(IR com- termine if any correlation exists between these two pa-
panion)) (Natta et al. 2000) of 0.32. Hartigan (1993) rameters and the presence or absence of detectable qui-
and Natta et al. (2000) independently have categorized escent H emission. If H emission is generated by X-
2 2
Glass Ia/Ibas a wTTS with a measuredHα EW of 4 ˚A. rays produced in coronae and by accretion activity on
Lommen et al. (2007) did not detect 3.3 mm continuum the surfaces of these stars, we expect a clear correla-
emission from Glass I, suggesting that there are little tiontoexistbetweensourceswithdetectableH emission
2
to no mm-sized dust grains in the circumstellar and cir- and their X-ray luminosities. Figure 7, however, shows
cumbinary environment. that sources with a variety of X-ray fluxes, including a
The detection of H emission from Glass I is quite few sources without detectable X-ray fluxes, are among
2
unique amongst our other detections since it represents those with observed quiescent H emission. Assuming
2
the only source for which we have detected spatially re- that large intervening columns of circumstellar gas are
solvedemission. Wehavedeterminedthatalloftheemis- not responsible for extinguishing the intrinsic X-ray lu-
sion whether centered on the primary, the secondary, or minositiesoftheapparentlyX-rayquietsources,thelack
in the extended emission, is centered on the stellar rest of any correlation between H detections and the pres-
2
velocity with a dispersion of only 1.5 km s−1, very near ence andmagnitude ofa X-rayflux suggeststhat X-rays
the estimated measurement error. Therefore, we find may not be responsible for stimulating the H emission.
2
that the emitting H gas in the system displays a com- Onthe otherhand, thereappearsto be somecorrelation
2
mon and systemic velocity. between Hα EW strengths and H detections as all but
2
When near-infrared H emission is found to be spa- a few sources in the region of the plot with Hα EWs
2
tially extended in the vicinity of a young star, the kine- greater than 10 ˚A (the rough boundary between cTTS
matics of the glowing gas typically revealthat it is asso- and wTTS), produce detectable levels of H emission.
2
ciated with shocks in an outflow. The lack of dispersion This correlation between Hα and H emission links ac-
2
in the centralvelocities of the extendedH emissionfea- cretion activity with quiescent H emission and are sim-
2 2
tures observed from the Glass Ia and Ib binary system, ilar to the findings of Carmona et al. (2007). Since ac-
however,argueagainstthe outflowscenarioandindicate cretionactivity in TTS is responsible for the production
that this gas is associated with circumstellar and cir- of the UV photons necessary to produce fluorescent H
2
cumbinary disks in this binary system. Gas orbiting a emission, this correlation suggests that UV fluorescence
2 M⊙ binary with a radius greater than 1700 AU will islikely the dominantstimulationmechanisminsome,if
not produce a detectable velocity shift at this resolu- not all of the detections.
tion. We do find that the FWHM of the emission fea- We now turnour attentionto the transitionaldisk ob-
10 Bary et al.
jects in Figure 7 denoted by the large asterisks overlaid
upon the filled circles. Each of these sources (LkCa 15,
CS Cha, and TW Hya) has been shown to possess an
innerdiskholethatiseitherdevoidofdustgrainsorpos-
sesses an evolved population of large dust grains. In ad-
ditionto these three transitionaldisk sources,HD 97048
has a similar infrared SED, as noted in §5.4, suggesting
thatittoopossessesalargeinnerdiskhole. Ifweconsider
thattheotherwTTS,Sz33andDoAr21,arelikelytran-
sitionaldisk objects,and,for the moment,ignorethe bi-
narysourcesGGTauandGlassIduetotheircomplexity,
onlythreeoftheH detections(CVCha,ECHAJ0843.3-
2
7905, and LkHα 264) now appear as cTTS with non-
transitional disks. However, the observations required
to construct infrared SEDs do not currently exist in the
literature. Future observations will be required to fur-
ther testthis correlation. Basedupon the correlationwe
havefoundbetweentransitionalobjectsandquiescentH Fig. 4.— Log Lx versus Hα EW values for all sources with
2 publishedobservationsofH2emission. Thedetectionsaredenoted
emission, we conclude that objects detected in quiescent with filled circles and non-detections with open circles. Objects
H emission and designated as cTTS also may harbor with known transitional disks are denoted by an asterisk. The
2 horizontaldashedlinedesignatesthecanonicaldividinglineat10˚A
transitional disks and that stimulation mechanisms re-
separatingcTTS(above)fromwTTS(below). Arrowspointingto
sponsible for producing the quiescent H emission are
2 the left designate the sources which were non-detections in X-ray
likely related to both accretion activity and the evolu- surveys. Detections arelabeledwithsourcenames.
tionary status of dust grains within their inner disks.
In addition to the possible impact of the evolutionary
status of dust grains, the presence of UV and/or X-ray narrow line widths, we used the HWZI and the FWHM
photons,andaccretionactivityontheproductionofqui- velocities of these Gaussian-shaped emission features to
escent H2 emission, variability may also play a role in estimate the innermost radii to which the gas extends
the detectability of these H2 emission signatures. As and the radii at which the bulk of the emitting gas re-
in the case of most optical and infrared emission lines sides . We find that the innermost radii for all of the
observed from TTS, the H2 spectral features may fluc- stars is on the order of ∼ 1 AU. We further find that
tuate on similar timescales. We expect this variability the radii about which the bulk of the emission is cen-
tohaveanegativeimpactonourdetectionrateandmay tered is on the order of ∼ 10 AU. For the binary source
explainwhysomesourcesthatsharefavorablecharacter- (GlassI),H emissionextendsbetweenbothcomponents
2
istics with sources that are detections do not appear to and to the south of the primary. Although the extended
producedetectablelevelsofH2emission. Therefore,vari- emission suggests that the gas could be entrained in an
ability may explainthe lack ofH2 detections for some of outflow, the lack of a detectable velocity gradient along
the borderline cTTS seen in Figure 7. In order to deter- the would-be axis of the outflow makes this an unlikely
mine if this is indeed the case, multi-epoch observations scenario. However,acircumbinarydiskoriginfortheex-
ofsomeofthe detectionswillbe requiredtomeasurethe tendedemissionisalsodifficulttoreconcilewiththelarge
magnitudes and timescales for any variability that may FWHM of the emission lines measured for the extended
be associated with these sources. Simultaneous obser- emission.
vations of the 2.1218 µm line as well as other accretion The initial goals for this survey was to determine if
indicators also would be helpful in linking the emission wTTS lacking detectable emission from standard disk
with UV activity associated with accretion hot spots on tracers still harbor protoplanetary disks capable of or
the stars. in the process of forming planetary systems. Our survey
revealedtwosuchstars,makingatotalofthree,including
6. CONCLUSIONS DoAr 21 discoveredby us in a previous survey.
Aspartofahighresolutionnear-infraredspectroscopic The range of H line fluxes
2
survey for H emission from TTS in nearby star form- (6.9×10−16 ≤ F ≤ 8.6×10−15 ergs cm−2 s−1)
2 2.12
ing regions, we have made new detections of emission and masses (10−10 ≤ MH2 ≤ 10−11 M⊙) measured for
features centered on the 2.1218 µm v = 1 → 0 S(1) theChamaeleonIsourceswithdetectedH lineemission
2
transition of H in the spectra of two cTTS (CS Cha were found to agree with all previous detections of
2
and CV Cha), two wTTS (Glass Ia and Sz 33), one IRC quiescent H emission. These trace amounts of gas
2
(Glass Ib), and one Herbig AeBe star (HD 97048) lo- do not directly inform us about the total disk masses
cated in the Chamaeleon I dark cloud. The detection of harbored by these systems, though almost certainly the
H we report toward HD 97048 is the first detection of mass of H detected directly through line emission at
2 2
near-infrared emission from quiescent H toward a Her- 2.1218 µm represents only a small fraction of the total
2
big AeBe star. In every case, the central velocity of mass in these disks. Our disk models suggest that the
the emitting gas is found to be coincident with the sys- total disk masses could be 102 to 107 times greater than
temic velocity of the source. Assuming a circumstellar the mass measured directly in this single emission line.
disk origin for these emission lines, an assumption well- Assuming that shock excitation is not the most likely
supported by the central velocities of these lines falling mechanismforstimulating narrowline emissionfromH
2
at the rest velocities of the respective stars and by the gaswhichisnotDopplershiftedfromthesystemicveloc-
