Far-ultraviolet Observations of the Taurus-Perseus-Auriga Complex

1.4 MB·English

by  Tae-Ho Lim

Draftversion January24,2013
PreprinttypesetusingLATEXstyleemulateapjv.12/16/11
FAR-ULTRAVIOLET OBSERVATIONS OF THE TAURUS-PERSEUS-AURIGA COMPLEX
Tae-Ho Lim1
KoreaAdvanced InstituteofScienceandTechnology(KAIST)
Kyoung-Wook Min1
KoreaAdvancedInstitute ofScienceandTechnology (KAIST)
3 and
1 Kwang-Il Seon2
0
KoreaAstronomyandSpaceScienceInstitute(KASI)
2
Draft version January 24, 2013

n
a ABSTRACT
J
Wehaveconstructedafar-ultraviolet(FUV)continuummapoftheTaurus-Auriga-Perseuscomplex,

3 oneofthelargestlocalassociationsofdarkclouds,bymergingthetwodatasetsofGALEXandFIMS,
2 which made observations at similar wavelengths. The FUV intensity varies significantly across the
whole region, but the diffuse FUV continuum is dominated by dust scattering of stellar photons. A
] diffuse FUV backgroundof∼1000CUis observed,partofwhichmaybe attributable to the scattered
A
photons of foreground FUV light, located in front of the thick clouds. The fluorescent emission of
G molecular hydrogen constitutes ∼10 % of the total FUV intensity throughout the region, generally
. proportional to the local continuum level. We have developed a Monte Carlo radiative transfer code
h
and applied it to the present clouds complex to obtain the optical properties of dust grains and
p
the geometrical structures of the clouds. The albedo and the phase function asymmetry factor were
o- estimated to be 0.42+−00..0055, and 0.47+−00..1217, respectively, in accordance with theoretical estimations as
r well as recent observations. The distance and thickness of the four prominent clouds in this complex
t
s wereestimatedusingasingleslabmodelappliedindividuallytoeachcloud. Theresultsobtainedwere
a ingoodagreementwiththosefromotherobservationsinthecaseoftheTauruscloud,asitsgeometrical
[ structure is rather simple. For other clouds, which were observed to have multiple components, the

results gave distances and thicknesses encompassing all components of each cloud. The distance and
1
v thickness estimations were not crucially sensitive to the exact values of the albedo and the phase
3 function asymmetryfactor,while the locations ofthe brightfield starsrelativeto the clouds as initial
3 photon sources seem to be the most important factor in the process of fitting.
4 Subject headings: ISM: clouds — ISM: dust, extinction — ISM: individual (Taurus, Auriga and
5 Perseus complex) — ISM: structure — ultraviolet: ISM
.
1
0
1. INTRODUCTION wholecomplexmaybephysicallyassociatedwiththePer
3
OB2stellar association,centeredat(l, b)=(160.5,-14.7)
1 The immense complex of the Taurus, Perseus, and
◦
with a diameter of ∼ 20 , which is the second closest
: Auriga (TPA) clouds is known to be one of the
v OB association to the Sun with a distance of ∼300 pc
largest local associations of dark nebula. According to
i on average (Belikov et al. 2002). Recent supernova ex-
X Ungerechts et al. (1987), the total masses of the whole
plosions in the Per OB2 associationare believed to have
r complexis ∼2×105 solarmass. The TPAcomplexcon- created an expanding HI supershell that is moving into
a tains star-forming cloudlets (TMC1, TMC2, NGC 1333,
thesurroundinginterstellarmedium(Sancisi et al.1974;
andIC348,forexample),abrightemissionnebula(NGC
Hartmann & Burton 1997).
1499,alsoknownasCaliforniaNebula), andstellarasso-
The TPA complex may consist of multiple layers of
ciations (Per OB2, for example). Low-mass star forma-
molecular gas located at various distances. The Taurus
tionprocessesarebelievedtobetakingplaceintheeast-
cloud is believed to spread between 100 pc and 200 pc
ern section of the complex (Taurus and Auriga), where
(Ungerechts et al. 1987), with a large part of it blocked
mostly low-mass T Tauri type stars are observed, while
by the cloud itself (Elias 1978; Kenyon et al.1994). The
both low- and high-mass stars are being formed in the
most distant part of the Taurus molecular cloud com-
westernsectionofthe Perseuscomplex(the PerOB2as-
plex may be interacting with a supershell blown by the
sociation, the open cluster NGC 1333, and so on). The
Per OB2 association (Olano et al. 1987). Cernis (1990)
argued that the cloud in the region of NGC 1333, one
[email protected] of the star forming regions located in and around the
1Korea Advanced Institute of Science and Technology Perseuscloud,consistsoftwolayers,andestimatedtheir
(KAIST), 373-1 Guseong-dong, Yuseong-gu, Daejeon, Korea
distances to be ∼170 pc and ∼230 pc based on an ex-
305-701, RepublicofKorea
2KoreaAstronomyandSpace ScienceInstitute (KASI), 61-1 tinction analysis. The Auriga dark cloud, located at
Hwaam-dong,Yuseong-gu,Daejeon,Korea305-348,Republicof a distance similar to that of the Taurus cloud, is usu-
Korea
2
allyregardedas partofthe Taurus-Aurigacomplex,and the results of the FUV observations made for the TPA
was suggested to overlap with the California cloud at region and study the clouds geometries, though the re-
its farthest side (Ungerechts et al.1987). The California sultsmightstillbeverysimple. WeperformMonteCarlo
cloud seems to be the most distant cloud in this region: simulations of dust scattering and compare the results
Lada et al. (2009) argued that the cloud is located at with the observed diffuse FUV emission map. In fact, it
∼450 pc from the Sun. was revealed in the study of the Taurus region that the
Theinterstellarradiationfield(ISRF),composedofra- continuum emission is closely related to dust extinction
diations originating directly from stellar sources as well (Lee et al.2006). Therehavebeenseveralreportsonthe
as the diffuse background, plays an important role in structures of the clouds in this region, mostly based on
the physical and chemical processes of the interstellar star counting methods, extinction studies, and compar-
medium(ISM).Forexample,far-ultraviolet(FUV)ISRF isons with the distances of the associated stars and neb-
photons may ionize atoms, dissociate molecules, or heat ulae. We show that the present method of dust scatter-
gases by ejecting electrons from dust grains or by di- ing simulations gives consistent estimates regarding the
rectly exciting atoms and molecules. The ISRF itself clouds’ geometries in this region. We also demonstrate
is also affected by interactions with the ISM. In fact, that the averagealbedo value obtained for this region is
most of the diffuse FUV background radiation is be- in good agreement with a theoretical estimation.
lieved to originate from the scattering of starlight by
interstellar dust (Bowyer 1991; Seon 2011). The ef- 2. DATAREDUCTION
fect of dust scattering is well observed in reflection neb-
We employed two data sets for the present study.
ulae and dark clouds as well as in the diffuse galac-
The first is from observations made by the FUV Imag-
tic light (DGL). Hence, the observations of these ob-
ing Spectrograph (FIMS) flown aboard the Korean mi-
jects have been used not only to trace interstellar dust,
crosatelliteSTSAT-1, whichwaslaunchedonSeptember
but also to obtain the optical properties of dust itself,
27, 2003and operatedfor a year and a half. The second
such as the albedo (a) and the phase function asym-
consists of FUV archival data of the Galaxy Evolution
metry factor (g). In the case of the DGL, dust scat-
Explorer (GALEX). FIMS is an imaging spectrograph
tering was observed to be moderately reflective in the
optimized for the observation of diffuse emissions with
forward direction (Lillie et al. 1976; Joubert et al. 1983;
two wavelength channels, a short wavelength channel
Hurwitz et al. 1991; Murthy et al. 1991, 1994, 1995; covering λλ 900–1150˚A, and a long wavelength channel
Witt et al.1997),withtheestimatedvaluesofaandgin
coveringλλ1350–1700˚A.Bothchannelshavearesolving
generalagreementwiththoseofthetheoreticalmodelfor
power of λ/∆λ∼ 550. We analyzed only the long wave-
carbonaceous-silicategrains,i.e.,0.40-0.60and0.55-0.65,
length channel data in the present study since the short
respectively, at UV wavelengths (Weingartner & Draine
wavelength channel was contaminated by many airglow
2001). For reflection nebulae (Murthy et al. 1993; Witt
lines. The fieldofview ofthe longwavelengthchannelis
1992; Gordon et al. 1994; Gibson et al. 2003) and indi- ◦ ′ ′
7.4 ×4.3,witha5–10 imagingresolutionalongtheslit,
vidual clouds (Witt et al. 1990; Mattila 1970; Hurwitz
andthespectralhalf-energylinewidth,averagedoverthe
1994),however,theresultswerevariedanddependenton
angular field, is 3.2 ˚A. The instrument, its in-orbit per-
the individual targets, probably due to uncertainties in
formance,andthedataanalysisproceduresaredescribed
thescatteringgeometryofthetargetcloudsaswellasthe
in Edelstein et al. (2006a, 2006b).
star systems that were the dominant sources of photons
◦
The FIMS data set that covers the TPA area of 154
(Draine2003;Bowyer1991). Calzetti et al.(1995)deter-
◦ ◦ ◦
– 180 in Galactic longitude and -28 – -2 in Galac-
mined a and g to be 0.7-0.8 and ∼0.75 around 1600 ˚A,
tic latitude is composed of 38 observations made during
respectively, for dust toward IC 43, while Gibson et al.
the sky survey toward this region. The total exposure
(2003) estimated a and g to be ∼0.22 and ∼0.74, re-
time is around 3.6 s per pixel, on average. Only the
spectively,forwavelengthsbelow2200˚AforthePleiades data from 1360 ˚A to 1680 ˚A, excluding the intense O I
nebula. For the Ophiuchus cloud, Sujatha et al. (2005)
airglow line at 1356 ˚A, was used for the present anal-
found a and g to be ∼0.40 and ∼0.55, respectively, at
ysis. The FIMS continuum image of the diffuse emis-
∼1100 ˚A, while Lee et al. (2008) obtained ∼0.36 and
sion was constructed by spatially binning the data with
∼0.52 for a and g, respectively, for 1370 ˚A–1670 ˚A. a square bin of 0.4◦ x 0.4◦ size, after removing pixels
The main goalin this line of researchhas been the es- thathad morethan 9,000ContinuumUnit (photons s−1
timation of the optical properties of dust grains. The cm−1sr−1˚A−1,hereafterCU)astheseweresuspectedto
geometry of the clouds and the distribution of stellar
be contaminated by bright stars. The 9000CU criterion
photon sources involved, however, were also assumed
was chosen as the data of GALEX, which intentionally
to influence the intensity of light scattered by dust in
avoided the observation of bright stars, did not exceed
the clouds, thereby providing constraints on the geo-
9000 CU in this region. Figure 1(a) shows the result-
metrical structures of the clouds themselves. For exam-
ing FIMS image, where the regions of missing data are
ple, Sujatha et al. (2005) assumed two extended sheet-
shownin black. While the map shows the expected gen-
like structures for their study on the Ophiuchus cloud.
eralfeatures of the TPA region,including the dark Tau-
Shalima et al. (2006) also used the morphology of a
ruscloudandthebrightPerOB2association,wecanalso
plane-parallel dust sheet in their scattering simulation
seethatthebrightregionsofthePerOB2associationare
of the Orionnebula. Although these assumed clouds ge-
veryextendedacrossthescanningdirection. Infact,this
ometries were highly idealistic, the estimated distances
extended appearance is due to instrumental scattering
were more or less reasonable when compared to results
alongthe slit. Furthermore,some areasof the FIMS im-
based on other observations. In this paper, we examine
age were made with extremely short exposures. Hence,
3
FIMS FUV Continuum GALEX FUV Continuum
-5.0 -5.0
-10.0 -10.0
Galactic latitude -15.0-20.0 Galactic latitude -15.0-20.0
-25.0 -25.0
175.0 170.0 165.0 160.0 155.0 175.0 170.0 165.0 160.0 155.0
Galactic longitude Galactic longitude
0 1496 2999 4501 6004 7500 0 1496 2999 4501 6004 7500
Fig.1.—TheFIMScontinuum image(a)andtheGALEXFUVimage(b).
we merged the GALEX FUV data with the FIMS data
to remedy these shortcomings, as the wavelength range
of the GALEX data is similar to that of the FIMS, i.e.,
1350 – 1750 ˚A.
From the GALEX data release website, we extracted
the All Sky Surveys imaging data (AIS of GR4/GR5
Data Release) of the TPA region, and plotted the im-
◦ ◦
age with 0.4 x 0.4 bins in Figure 1(b). We compared
the intensities of the two observations pixel by pixel to
check for possible biases that might exist between them.
We limited our comparison to only the regions in which
theFIMSintensitywaslessthan6000CU,toexcludethe
brightpixels affected by the instrumental scatteringand
the pixels where FIMS exposure time was greater than
2.5 s. In Figure 2, we show the resulting scatter plot, in
which we can see a remarkable correlation between the
two data sets. The linear fitting between the two inten-
sities is given by IGALEX = 1.01×IFIMS +185.50 in
CU.The standarddeviationofthe dataaboutthe linear
Fig.2.— A pixel-to-pixel scatter plot for the intensities of the
fit is ∼ 550CU. Since we believe GALEX data are more
TPAregionobservedbyFIMSandGALEXshowingaremarkable
reliablethanFIMSdataintheregionwheretheGALEX agreement. Theredsolidlineisalinearfitofthedatapoints.
dataareavailablebecauseofinstrumentalscatteringand
short exposure times of FIMS, we used GALEX data as dense cloud of the Taurus-Auriga complex in the high-
the basis ofthe mergedmapandfilled the regionswhere longitude half, where the dust extinction level is high.
GALEX did not observe with the FIMS data scaled ac- The existence of a dense cloud, however, does not nec-
cordingtotheabovelinearfittingfunction. Theresulting essarily imply that FUV intensity will be low there. For
merged map is shown in Figure 3(a). example,the Californiacloud near the Per OB2 associa-
tionisrichindustbutFUVintensityisstillbrightthere,
3. RESULTS a feature related to their respective distances, to be dis-
cussed later. In the dust extinction and CO emission
3.1. Examination of the FUV continuum map
maps,showninFigure3(c)andFigure3(d),respectively,
Figure 3 shows images of the TPA region constructed a common voidor cavity is seen surroundedby the dark
at different wavelengths: (a) the FUV map obtained by clouds of the TPA complex. The void is coincident with
merging the FIMS and GALEX data, (b) Hα emission the brightest region in the Hα emission map of Figure
map(Finkbeiner2003),(c)Schlegel,Finkbeiner&Davis 3(b) inthe vicinity ofthe centerofthe PerOB2associa-
(SFD) Dust Survey map (Schlegel et al. 1998), and (d) tion. Thoughnotnoticeablehere,adustemissionringis
theCOemissionmap(Dame et al.2001). TheFUVcon- seen around the cavity according to sub-millimeter and
tinuummapofFigure3(a)showsbrightenhancementin far-infraredmaps (Watson et al. 2005). In fact, the void
the regions of the Per OB2 association and the Pleiades regionwasidentifiedtobe anHI supershellwithadiam-
cluster, as well as dark regions coincident with the Tau- eter of ∼100 pc, which is believed to have been formed
rusandAurigaclouds. Hence,the FUV flux isbrighton by the interaction of the Per OB2 association with the
the low-longitude half where Hα emission is also bright, ambient interstellar medium (Sancisi et al. 1974; Heiles
while FUV flux seems to be heavily absorbed by the 1984). The bright spot south of the Taurus cloud is the
4
FUV Continuum
H-alpha emssion
-5.0 -5.0 (TICad 4p1o0l)e Nebula
Galactic latitude -10.0-15.0-20.0 A Pleiades C Galactic latitude -10.0-15.0-20.0 Califo(NrnGiaC N1e4b9u9)la
-25.0 B -25.0 Per OB2 Association
175.0 170.0 165.0 160.0 155.0
175.0 170.0 165.0 160.0 155.0
Galactic longitude
Galactic longitude
0 1496 2999 4501 6004 7500
0 1 4 18 72 288
CO emission
Dust Extinction
-5.0 -5.0 California Cloud
Galactic latitude -10.0-15.0-20.0 Dust void Galactic latitude -10.0-15.0-20.0 TauAruursig Cal oCuloduHdI Super sPhePerle lBr A
-25.0 -25.0 Perseus Cloud
175.0 170.0 165.0 160.0 155.0 175.0 170.0 165.0 160.0 155.0
Galactic longitude Galactic longitude
0.08 0.089 0.12 0.26 0.81 3 0 2.7 11 25 44 68
Fig.3.—(a) FUVintensity (inCU)overlaidbythe dustextinction contours, withthicker contours representing higherdust extinction
levels,(b)HαemissionmapshowninRayleighs,(c)SFDdustextinctionmap,E(B-V),and(d)COemissionmap,showninKkm/s. The
locationsA,B,andCinpanel (a)arethosewithdistinctphysicalcharacteristics anddiscussedinthetext.
reflection nebula around the Pleiades cluster. interesting feature seen in Figure 4 is that FUV inten-
We haveselectedthreedistinct subregionsasshownin sitydoesnotdecreasebelow1000CUevenataveryhigh
Figure 3(a) and made a pixel-to-pixel plot of the FUV extinction level, part of which we may attribute to the
intensityagainstdustextinction,withdifferentcolorsac- scatteredphotonsoftheforegroundFUVlight,locatedin
cordingtothesubregions. SubregionAincludestheTau- frontofthethickclouds(Lee et al.2006). Hurwitz et al.
ruscloudwhichisopticallythickwithlowFUVintensity, (1991)alsoobservedasimilardiffuse backgroundfor the
◦ ◦
andsubregionBisopticallythinwithalittlehigherFUV Galactic latitudes 10 <|b|<20 .
intensity. Subregion C is a region of high FUV intensity
3.2. Monte Carlo simulation of dust scattering
that includes both optically thin and thick areas. The
resultisshowninFigure4. First,wenotethatregionA, As shown in Figure 4, the scatter plot of FUV in-
which includes the thick Taurus cloud, indeed shows a tensity against the dust extinction level shows compli-
general anti-correlation between the FUV intensity and cated behaviors that depend on local extinction levels,
the dust extinction level, implying that FUV is heavily fluctuations of local radiation fields as well as mixing of
absorbedbythedenseTauruscloud. Ontheotherhand, the foreground and background FUV photons. On the
the result for region B shows a positive correlation, in otherhand,suchcomplicatedbehaviorsmightalsomake
agreementwiththegeneralnotionthatFUVintensityin- it possible to extract information regarding the struc-
creaseswithdustextinctionlevelintheopticallythinre- ture of the clouds, especially their relative locations, by
gion (Hurwitz 1994; Luhman et al. 1996). Indeed, most comparing the observed FUV intensities with those of
ofthe datapointsinregionBcorrespondtosmallvalues dust-scatteringsimulationsthatincorporatelocalphoton
of color excess, below ∼0.5. On the other hand, region sources and an assumed distribution of clouds. In fact,
C shows severe fluctuations, obviously due to the non- similar methods have been previously applied to several
uniform strong radiation fields of the Per OB2 associa- cloud systems (Gordon et al. 2001; Shalima et al. 2006;
tionoverawiderangeofthecolorexcessvalues. Another Lee et al. 2008; Sujatha et al. 2005, 2007).
Our main purpose in the present study is to under-
5
We willdiscusslaterthe dependence ofthe opticalprop-
erties of dust on the models of the two-photon effects.
Finally, we have subtracted uniformly an isotropic FUV
emission of ∼300 CU (Bowyer 1991) although this value
was obtained for the high Galactic latitude region. The
resulting FUV map is shown in Figure 6(a).
We employed a Monte Carlo Radiative Transfer tech-
nique, in which photons were allowed to have multiple
scatterings (Witt 1977; Wood et al. 1999; Seon 2009,
2012). At each scattering, after moving a mean free
path distance determined by the optical depth, the
photons were randomly scattered into an angle with
a probability given by the Henyey-Greenstein function
(Henyey & Greenstein1941). Weadoptedthepeeling-off
techniquetoachievesimulationefficiency,i.e.,regardless
of the actual scattering angle determined randomly, the
probability that the photon would be scattered in the
directiontowardtheSunwascalculatedforeachscatter-
ing, and the corresponding fraction of the intensity was
Fig.4.—AscatterplotoftheFUVintensityagainstdustextinc-
added up for the final image (Yusef-Zadeh et al. 1984).
tion: the blue, orange, and blackcolors correspond to the regions
A,B,andCinFigure3(a),respectively. We did not consider any wavelength changes upon scat-
tering. Thefreeparameterstobedeterminedbythesim-
standthroughsimulationsthe dustscatteringproperties ulation were the albedo, the asymmetry factor, and the
and the morphological structures in this broad region. distributionofdust,whiletheinitialphotonsourceswere
More specifically, we seek to understand the distances chosen to be the bright stars in the TPA region. There
and thicknesses of the TPA clouds, which includes both are,however,infinite numbers ofpossible choices for the
thick clouds and strong radiation fields of OB associ- three-dimensional spatial distribution of dust under the
ations, based on the observed FUV emission features. giveninformationofintegratedextinctionalongthelines
Hence, it is necessary to identify and remove contribu- ofsight,whichweadoptedfromtheSFDdustextinction
tions other than the dust-scattered emission from the map (Schlegel et al. 1998). Therefore, we assumed, for
observed FUV image. In addition to dust-scattering of convenience that the dust was distributed uniformly in-
stellar photons, diffuse interstellar FUV emissions can side a slab whose thickness and distance from the Sun
have three different origins including: i) ion line emis- were to be determined. The E(B-V) values of the SFD
sion coming from hot gas of 104.5− 105 K; ii) H2 flu- mapwereconvertedto opticaldepths at1565˚A,a repre-
orescent emission originating from the outer regions of sentative FUV wavelength of the present study, with an
molecular clouds irradiated by interstellar UV photons; assumed RV value of 3.1.
and iii) two-photon effects which are mostly relevant to The simulation box consisted of a 400 × 400 × 400
ionizedregions. OftheotherFUVsources,ionlineemis- pixel, with each pixel equivalent to 1 pc. The Sun was
sions should be negligible in the present region of thick located at the center of the front face of the simulation
clouds. On the other hand, we expect significant H2 flu- box. A total of 88307 stars was employed as the radia-
orescentemissionsinthisregionwhereboththickclouds tion sources, consisting of 18212 TD-1 stars and 70095
and strong radiation fields exist. Based on the spectral Hipparcosstars. Theluminositiesofthestarswerecalcu-
observations of FIMS, Figure 5 indeed shows that H2 lated using the observed FUV stellar fluxes for the TD-
fluorescent emission is detectable throughout the whole 1 stars and Castelli’s stellar models for the Hipparcos
TPA region. The contribution from the H2 fluorescent stars, based on the listed spectral types, with extinc-
emissionlines is estimated to be ∼10.8%, ∼11.4%, and tion effects taken into account (Thompson et al. 1978;
∼9.5 % of the total FUV intensity for the regions A, B, Perrymanet al. 1997; Castelli et al. 2003). As a first
and C, respectively,and ∼10%for the whole region. We step, we assumed a single dust slab model for the entire
have accordingly subtracted 10 % of the total intensity TPA region, which was to be improved at the second
from the map shown in Figure 3(a) to account for the step by considering individual clouds separately, using
contribution from H2 fluorescent emission. Estimation the values of the albedo and the asymmetry factor ob-
of the two-photon effect can be a little tricky since Hα tained in the first step. Simulations were carried out for
emission can also originate from regions other than ion- thefollowingparameterranges: thealbedovalues varied
ized regions, and Hα photons may also be scattered in from 0.20 to 0.60 with 0.01 steps, the asymmetry factor
a manner similar to the FUV continuum photons. Fur- varied from 0.0 to 0.80 with 0.01 steps, the distance to
thermore, Reynolds (1990) suggested a two-photon con- the front face varied from 10 pc to 390 pc with 10 pc
tinuum emission of ∼60 CU for 1 Rayleigh of Hα line in steps, and the thickness of the slab varied from 10 pc to
the case of a diffuse ionized gas, while Seon (2011) esti- 390 pc with 10 pc steps. The best-fit parameters were
matedittobe∼26CUfor1RayleighofHαemissionofa determinedbycomparingthesimulationresultswiththe
coolingionizedgas. Hence,wehaveexcludedtheCalifor- observed FUV map using the chi-square minimization.
nia nebula region, where Hα intensity is especially high, Figure 6(b) shows the simulated map with the best-fit
in our simulation and applied the formula of the FUV parameters: 0.42 for the albedo, 0.47 for the asymmetry
continuum intensity of 60 CU for 1 Rayleigh intensity factor, 50 pc for the distance to the front face, and 220
of Hα throughout the region as a simple approximation.
6
Fig.5.—SpectraofthewholeTPAregionandsubregionsA,B,andC,inwhichthefluorescentemissionlinesofmolecularhydrogenare
indicatedwithverticalredbars. Thedashedbluelinesrepresentsimplemodelsforthecontinuum emissions.
-5.0 -5.0
Galactic latitude -10.0-15.0-20.0 Galactic latitude -10.0-15.0-20.0
-25.0 -25.0
175.0 170.0 165.0 160.0 155.0 175.0 170.0 165.0 160.0 155.0
Galactic longitude Galactic longitude
0 1496 2999 4501 6004 7500 0 1496 2999 4501 6004 7500
Fig.6.— (a) Dust scattered FUV map constructed from the observations and (b) the Monte Carlo simulation results. The California
nebula regionis excluded fromour dust scattering simulations as its Hα intensity isextremely highand the associated two-photon effect
isdifficulttoestimate.
TABLE 1
Distance to the frontface andthe thickness of the fourselected
clouds, obtainedbytreatingeach cloud individually.
clouds Taurus Perseus Auriga California
Distancetothefrontface(pc) 120+−2800 70+−3200 140+−2900 170+−1100
Thickness(pc) 110+−2500 230+−3400 a 190+−7200 a 320+−5300 a
Distancetothecenter (pc) ∼175 ∼185 ∼235 ∼330
aThelargethicknessesofthePerseus,California,andAurigacloudsmaybedue
tomultiplelayers. RefertoSection4fortheirdiscussions.
7
Fig.7.—AscatterplotofthesimulatedFUVintensityagainsttheobservedFUVintensityfortheregionsoftheTaurus,Perseus,Auriga,
andCaliforniaclouds. Panel a)shows theresultobtained usingasingledustslabmodelandpanel b)shows theresultobtained withthe
individualcloudstreatedseparately. Thenavy,skyblue,yellow,andorangecolorscorrespondtotheregionsoftheTaurus,Perseus,Auriga,
andCaliforniaclouds,respectively.
Fig. 8.—1-σconfidencecontoursofthedistancetothefrontfaceandthethickness,plottedforthefourcloudsidentifiedinFigure3(d).
The cross symbol indicates the best-fit values. For this simulation, fixed values of the albedo (a = 0.42) and the asymmetry factor (g =
0.47)wereused.
8
pc for the thickness. The error ranges of the parameter with all these observations. For the Perseus cloud, the
values are mainly due to the statistical fluctuations of present simulation estimates a distance to the front face
the FIMS data with low exposure time: the 1-σ ranges and a thickness of 70+30 pc and 230+30 pc, respectively.
−20 −40
of the albedo and the asymmetry factor are 0.37 to 0.47 The scatter plot of the simulated vs. the observed FUV
and 0.20 to 0.58, respectively. Nevertheless, the esti- intensity in Figure 7(b) shows severely scattered data
mated albedo of 0.42 is close to the theoretical model points for the Perseus cloud, indicating that the model
value of ∼0.40, while the asymmetry factor is rather cloud does not adequately describe its actual structure.
small compared to the theoretical expectation of ∼0.65 In fact, the data points corresponding to the observa-
(Weingartner & Draine 2001). Jo et al. (2012) obtained tion are spread over a wide range of intensity, which we
0.43 and 0.45 for the albedo and the asymmetry factor, believe comes from the assumption of a single layer for
respectively, in their simulation for the Orion-Eridanus the cloud. Hence, we have divided the Perseus cloud
Superbubble. The simulated map shown in Figure 6(b) into two sub-regions and fit them separately. We name
looks similar to the observed FUV map of Figure 6(a), Per A as the western part of the cloud and Per B as
but we note that the region around the California neb- the eastern part, as shown in Figure 3(d), and these in-
ula is less bright and the Taurus region is less dark in cludethetworegionsNGC1333andIC348,respectively.
the simulated map than in the observed map. Indeed, With the same fixed values of the albedo (a = 0.42)and
the intensity varies less over the whole TPA region in the asymmetry factor (g = 0.47) as used in the previous
the simulated map than in the observed map. The rea- simulations, we find a distance to the front face and a
son for the reduced variability could be the assumption thickness of the Per A region of 100+30 pc and 180+30
−30 −50
ofasingleslabmodelforthe entireregion. Forexample, pc, respectively, and a distance to the front face and a
the reduction of intensity in the region around the Cali- thickness of the Per B region of ∼320 pc and ∼30 pc
fornia nebula was compensated by an increase in inten- within +10 pc error range, respectively. The scatter plot
sity in the Taurus region as the reduced chi-square was −10
is now improvedconsiderably,as shown in Figure 9. For
to be minimized with a single dust slab model. Hence,
the western portion of the Perseus cloud, Cernis (1990)
it should be meaningful for the individual clouds to be
obtained ∼170 pc and ∼230 pc for the distances of the
treated separately with their own distances and thick-
two layers in this region, while Hirota et al. (2008), us-
nesses. With the values of the albedo (a = 0.42) and
ing the parallax observation of the 22 GHz H O maser,
2
the asymmetry factor (g = 0.47) determined from the
directly measured the distance of NGC 1333to be ∼240
single slab model, we carried out simulations separately
pc. The ratherthick cloudobtainedinthe presentsimu-
fortheindividualcloudsofTaurus,Perseus,Auriga,and
lationforPerAmayreflectthe multi-layernatureofthe
California. FortheCaliforniacloud,weenlargedthesim-
cloud suggested by Cernis (1990). The distance to the
ulationboxto400pc×400pc×800pcasthecloudwas
eastern portion of the Perseus cloud containing IC 348
found to extend beyond 400 pc. Relative to the single
was estimated to be largerthan 300 pc (Herbig & Jones
slab model, the pixel-to-pixel scatter plot of the simu-
1983). It is interesting that the thickness was estimated
lated FUV intensity against the observedFUV intensity
tobe rathersmallforPerBinthepresentsimulation. It
shows significant improvement, as can be seen in Figure
wasseenthatthe intensityofthe scatteredemissionwas
7. Table 1showsthe improveddistancesandthicknesses
veryhighwhenthe cloudwasplacedatadistancebelow
of the TPA clouds, and Figure 8 shows contour plots
300 pc. We believe the emission observed in the Per B
for 1-σ error ranges. These best-fit values are in good
region comes from backward scattering reflected by the
agreement with the previous estimations based on other
cloud itself.
methods,aswillbediscussedinthefollowingsection. In
For the Auriga cloud, the distance to its front face
the casesof the Taurus andAuriga clouds, the distances
and its thickness are estimated in the present study
to the front faces seem to have rather large error ranges to be 140+20 pc and 190+70 pc, respectively, and for
in the lower limit. Unfortunately, very few bright stars −90 −20
the California cloud, they are 170+10 pc and 320+50
exist in front of these clouds that can constrain well the −10 −30
distances to their front faces. pc, respectively. Eklof (1959) suggested that the Au-
riga cloud has a two-layered structure near the eastern
4. DISCUSSION end of the California cloud, with the layers located at
We haveestimatedthe locationsofthe fourprominent ∼115-135 pc and ∼270-380 pc, based on an extinction
clouds in the TPA region by comparing the observed study. Ungerechts et al.(1987)determineda distanceto
FUV images with the results of the Monte Carlo sim- the California cloud of ∼350 pc while Lada et al. (2009)
ulations of dust scattering. For the Taurus cloud, we suggested that the cloud is located at ∼450 pc from the
obtained a distance to the front face and a thickness Sun. Straizys (2001) argued that the California cloud
of the cloud of 120+20 pc and 110+20 pc, respectively. regionistwo-layered,withthe firstlayerat∼160pc and
−80 −50
There have been many reports on the physical structure the second layer at ∼350 pc. With the present single-
of the Taurus cloud. Elias (1978) estimated the mean slabmodel,itisimpossibletoreproducesuchmulti-layer
distance to the Taurus complex to be ∼135 pc, based structures. Instead, we obtained single thick clouds, en-
on the distances to the stars associated with the Tau- compassing two layers of the clouds. We note that the
rus cloud. Ungerechts et al. (1987) suggested that the estimated value of ∼330 pc to the center of the Cali-
cloud spreads between 100 pc and 200 pc, based on the fornia cloud is larger than the distance to the Per OB2
results of star counting, the distances of the reflection association (∼300 pc on average, Belikov et al. (2002)).
nebulae, and the color excess of the field stars. Straizys Hence, the California cloud, being located behind the
(2001) also reported that extinction begins to increase Per OB2 association which is a candidate source for the
sharply at 130-180 pc. The present result agrees well bright FUV emission seen in this region, does not block
9
Fig.9.—AscatterplotofthesimulatedFUVintensityagainsttheobservedFUVintensityforthetwosubregionsAandBofthePerseus
cloud,markedinFigure3(d): (a)whenthewholesystemofthePerseuscloudistreatedasasinglelayer,and(b)whenthetwosub-regions
aretreated individually. The green and purple data points correspond to the regions of A, where NGC 1333 islocated, and B, where IC
348islocated, respectively.
600
500
c) 400
p
e (
g
an 300
al r
di
a 200
R
100
0
Taurus Auriga Per A Per B California
Clouds
This paper
Gottlieb & Upson (1969)
Racine (1968)
Elias (1978)
Preibisch & Smith (1997)
first & second layer, Eklof (1959)
Kenyon et al. (1994)
Ungerechts & Thaddeus (1987)
Wouterloot & Habing (1985)
Hirota et al. (2008)
first & second layer, Cernis (1990)
Strom et al. (1974)
Lada et al. (2009)
first & second layer, Ungerer et al. (1985)
Ripepi et al. (2002)
first & second layer, Straizys et al. (2001)
Fig.10.— Distance estimations of the four clouds based on the present Monte Carlo simulations of diffuse FUV emissions, compared
withpreviousobservations.
10
the emission from the association. Figure 10 shows a tances to the respective front face remained the same.
summaryofourdistanceestimationsforthe four clouds, The thickness decreased by ∼10 % for the Auriga and
along with previous observations for the corresponding the California clouds as well as Per A, and ∼30 % for
clouds. the Tauruscloud region,whichis darkestof alland thus
We have plotted in Figure 11 the velocity-clipped CO most affected. The reason for the decrease in thickness
emission maps for the ranges given in the Local Stan- is that reduction in FUV intensity was accomplished by
dardofRest(LSR) frame: (a)from-21.8km/s to +30.2 increasingtheshieldingeffectinthecaseoftheseclouds.
km/s, (b) from -10.7 km/s to -0.3 km/s, (c) from +0.3 On the other hand, the thickness of Per B increased by
km/s to +2.9 km/s, and (d) from +4.2 km/s to +10.7 ∼20 pc, reflecting the nature of backward scattering in
km/s. These maps arereproducedfromthe CO sky sur- the case of Per B. Nevertheless, these new values are all
veydata by Dame et al.(2001). While the differences in within the 1-sigma range of the results obtained for the
theLSRvelocitiesdonotnecessarilyimplytheirrelative nominal case.
distances, they may give insightful clues to the physical
associations of the corresponding clouds. For example, 5. CONCLUSIONS
the Californiacloudis seeninFigure 11(b) andno other We have constructed a FUV continuum map of the
clouds are seen in that velocity range. In Figure 11(d), TPA complex using FIMS and GALEX data. The mor-
however, we can see indications of the Taurus, Auriga, phological features seen in the complex are well under-
andPerseusclouds,butnotoftheCaliforniacloud. Such stood in terms of dust scattering, as demonstrated by
agroupingmayindicatethattheCaliforniacloudisphys- Monte Carlo simulations. The following are the main
icallynotconnectedtootherclouds,whiletherestofthe findings of the present study:
clouds system may be more or less associated. Further-
more, parts of the Auriga and Perseus clouds are also 1. ThediffuseFUVemissionseeninthecomplexorig-
seen in Figure 11(c). Hence, the fact that these clouds inates mostly from scattering of stellar photons by
areseeninawiderangeoftheLSRvelocitiesmaybecon- dust grains.
sistent with them having multi-layeredstructures. Bally
(2008) also noted that the large velocity gradient ob- 2. The FUV intensity of 1000 CU is observed at a
served in the Perseus cloud may be due to the nature of very high extinction level, which we regard as dif-
multi-components superimposed along the line of sight. fuse background and attribute to scattered FUV
The error ranges of the estimated optical properties photons located in the foreground to the thick
are a little large, especially in the value of the asym- clouds.
metry factor, where the reason was attributed earlier to
the statistical fluctuations of the FIMS data with low 3. Molecular hydrogen fluorescent emission consti-
exposure time. Nevertheless, the best fit values of the tutes ∼10%of the totalFUV intensity throughout
distancestothe respectivefrontface andthe thicknesses the region.
of the clouds do not seem to change much when even
4. We have derived the following scattering parame-
the upper or lower limits of these error ranges are used
ters for this region based on the Monte Carlo Ra-
for fitting. For example, the changes are mostly ∼10 pc
diative Transfer (MCRT) simulations: albedo (a)
for the distances to the front faces and ∼30 pc for the
thicknessesoftheclouds. Themostsensitiveparameters = 0.42+−00..0055 andasymmetry factor(g) = 0.47+−00..1217.
for fitting are instead the locations of the field stars as These values agree well with those obtained pre-
the photon sources relative to the clouds, not the opti- viouslyfor the Orion-EridanusSuperbubble region
cal properties of dust. We have also made a number of as well as theoretical estimations.
assumptions in the present study to obtain the optical
5. Wehaveestimatedthedistancestothefourpromi-
properties of dust and the geometrical structures of the
nent clouds in this complex, which are in good
clouds in the TPA region. First, we used a conversion
agreementwiththoseestimatedobservationallyus-
formula for the FUV continuum intensity of 60 CU for
ingothermethods. Thethicknessofeachcloudwas
1 Rayleigh intensity of Hα to estimate the two-photon
also reasonably determined when it has a simple
effects,whichholdsonlyforthe caseofahotdiffuse ion-
structure such as the Taurus cloud. The present
ized gas whose temperature is ∼8000K (Seon 2011). To
single slab model, however, was not able to repro-
see the effect of the choice of the conversion factor, we
duce multi-layered clouds, but gives rather thick
ranthesameMonteCarlosimulationbybutinsteadsub-
clouds, instead.
tractingthetwo-photoneffectwithaconversionfactorof
30 CU for 1 Rayleigh of Hα in one case and even with
6. The geometrical structures of the clouds are less
no subtraction in another case. The results were a =
sensitive to the exact values of the albedo and the
0.43, g = 0.49 for the former case, and a = 0.44, g =
asymmetry factor. Instead, the locations of the
0.52 for the latter case. All these values are within the
1-sigma range of the nominal case, a = 0.42+0.05 and g fieldstarsrelativetothecloudsarethemainfactors
−0.05 that constrain the distance and thickness of the
= 0.47+−00..1217, implying that the choice of the conversion clouds.
factordoesnotalterthe conclusionsmadeinthe present
study. Next,wesubtracted300CUasanisotropicback-
groundoftheFUVemission,whichwasobtainedforhigh
Galactic regions. When we subtracted 500 CU instead, FIMS/SPEAR is a joint project of KAIST and KASI
only the thicknesses of the clouds changedwhile the dis- (Korea) and UC Berkeley (USA), funded by the Ko-
rea MOST and NASA grant NAG5-5355. This research