OPACOS: OVRO POST-AGB CO

44 Pages·2012·1.47 MB·English

ToappearinApJS
PreprinttypesetusingLATEXstyleemulateapjv.11/12/01
OPACOS: OVRO POST-AGB CO(1 0) EMISSION SURVEY. I. DATA AND DERIVED NEBULAR
−
PARAMETERS
C. Sa´nchez Contreras1 and R. Sahai2
To appear in ApJS
ABSTRACT
We have performed interferometric observations of the 12CO(J=1 0) emission in a sample of 27 ob-
−
jectsspanningdifferentevolutionarystagesfromthelateAsymptoticGiantBranch(late-AGB),through
thepost-AGB(pAGB)phase,andtothePlanetaryNebula(PN)stage,butdominatedbypAGBobjects
andyoungPNs( 81%). Inthispaper(firstinaseries)wepresentourmapsandmainnebularproperties
≥
derived for the whole sample. Observations were performed with the millimeter wavelength array of the
OwensValleyRadioObservatory(OVRO).Theangularresolutionobtainedinoursurveyrangesbetween
2′.′3 and 10′.′7. The 13CO and C18O (J=1–0) transitions as well as the 2.6mm continuum emission have
been also observed in several objects. The detection statistics in the 12CO, 13CO, C18O transitions and
2.6mm continuum is 89%, 83%, 0%, and 37%, respectively. We report first detections of 12CO(J=1 0)
−
emission in 13 targets and confirm emission from several previous marginal detections. The molecular
envelope probed by 12CO(J=1 0) emission is extended for 18 (out of 24) sources; envelope asymme-
−
tries and/or velocity gradients are found in most extended objects. Our data have been used to derive
accurate target coordinates, systemic velocities and to characterize the envelope size, morphology, and
kinematics. We also provide an estimate of the total molecular mass and the fraction of it contained in
fast flows, lower limits to the linear momentum and to the isotopic 12C/13C ratio, as well as the AGB
mass-loss rate and time-scale for sources with extended CO emission.
Subject headings: stars: AGB and post-AGB, stars: mass loss, circumstellar matter, ISM: jets and
outflows, planetary nebulae: general
1. introduction exciting yet least understood problem in these late evolu-
tionary stages.
Planetary Nebulae (PN) are glowing shells of ionized
Before HST imaging revealed the wide range of com-
gas and dust ejected in the latest evolutionary stages of
plexmorphologiesinPPNsandPNs,thefavoredmodelfor
intermediate mass stars ( 1-8M ). PN evolve from the
∼ ⊙ explainingthebreakfromsphericalsymmetryinPNmor-
circumstellar envelopes around Asymptotic Giant Branch
phologies involved the interaction between a fast isotropic
(AGB) stars through a short-lived ( 103yr) and fasci-
∼ pAGB wind with an equatorially dense AGB CSE – the
nating evolutionary stage designated as the post-AGB
so-called “generalised interacting-stellar-winds” (GISW)
(pAGB)orpre-planetarynebula(PPN)phase(seee.g.the
model (Calvet & Peimbert 1983; Balick 1987). Although
review paper by van Winckel 2003). Spherical, slowly ex-
theGISWparadigmhasbeensuccessfulforexplainingthe
panding(V 15km s−1)circumstellarenvelopes(CSEs)
exp∼ large-scale shapes of round, elliptical, and bilobed PNs,
result from the intense mass-loss process during the AGB
this model, by itself, cannot account for the complicated
phase. In the pAGB phase, the mass-loss rate decreases
structuralandkinematicpatternsdisplayedbymostPPNs
dramatically and the central star, which evolves quickly
(seee.g.thereviewpaperonPNshapingbyBalick&Frank
towards higher effective temperatures at roughly constant
2002). AssuggestedbySahai&Trauger(1998),thesefea-
luminosity, is surrounded by a detached, expanding shell
tures could be explained by underlying jets or collimated,
of gas and dust. The AGB-to-PN transformation ends
fast winds (which may be episodic, have multi-directed
when the central star is hot enough to ionize most or all
components, and/or exhibit time-variable directionality)
the circumstellar material.
actively shaping the AGB CSEs. In spite of the grow-
PPNs and PNs display spectacular and varied nebu-
ing evidence of the effects of collimated, fast pAGB winds
lar morphologies with elongated lobes expanding at high
sculpting the AGB CSEs from the inside, the problem of
speeds (&100km s−1). Explaining the dazzling variety of
explaining the morphology and dynamics of PPNs and
morphologiesobserved,whichincludenotonlyaxisymmet-
young PNs (yPNs) still persists and the mechanism that
ric(ellipticalandbipolar)shellsbutalsomultipolarstruc-
couldpowerandcollimatepAGBejectionsremainsamys-
tures, multiple co-axial shells, highly collimated (jet-like
tery.
or knotty) point symmetric or corkscrew-like features, etc
Together with PPNs, observationally recognized by
(e.g. Meixner et al. 1999; Sahai et al. 2007a; Ueta et al.
prominent aspherical nebulosities surrounding a central
2007; Si´odmiak et al. 2008; Sahai et al. 2011a; Lagadec
pAGB star, there is a second class of pAGB objects with
et al. 2011, and references therein), is probably the most
1 Centro de Astrobiolog´ıa, INTA-CSIC. Postal address: European Space Astronomy Centre (ESAC), P.O. Box 78, E-28691 Villanueva de la
Can˜ada,Madrid,Spain
2 JetPropulsionLaboratory,MS183-900,CaliforniaInstituteofTechnology,Pasadena,CA91109,USA
1
2 S´anchez Contreras & Sahai
pAGB central stars, strong evidence for medium-sized represent the remnant AGB CSE left after fast collimated
( 50 AU) disks, and without discernible nebulosities (van outflows have excavated diametrically opposed holes in a
∼
Winckel 1999). This latter class, called disk-prominent spherical envelope, or they could result from equatorially
pAGB objects or dpAGB objects (Sahai et al. 2011b), enhanced mass-loss during the late-AGB phase. Part of
is not represented in our sample. A third small class the low velocity molecular emission in PPNs may also
of objects appear to show well developed PPN-like mor- arise in a compact rotating disk around the star; these
phologies and fast outflows but have AGB central stars disks have been proposed to be the main agents for the
(e.g.OH231.8+4.2; Bujarrabal et al. 2002, and references launchandcollimationofpAGBwinds(Soker2002;Frank
therein) – a few examples of such objects are included in & Blackman 2004, and references therein). To date, the
our sample. The classification of these objects as early presence of a compact disk in slow Keplerian rotation has
PPNs is not unreasonable, however, the discrepancy be- been confirmed in one PPN, the Red Rectangle (Bujarra-
tween the evolutionary status of their central stars (AGB bal et al. 2003).
mass-lossing stars) and their surrounding nebulosities re- OurCOsurveyismotivatedwiththeaimofbuildingup
mains to be understood. One possibility is that these ob- a large sample of pAGB objects (but also late-AGB stars
jectshostinteractingbinaryormultiplesystemscomposed and young PNs) with CO detections and inteferometric
of the primary mass-lossing AGB star and stellar or sub- COmaps. Thisisneededforanimprovedcharacterization
stellar companions. Thus this object class would include, of their molecular envelopes and making progress in our
e.g.,sourceslikeOH231.8+4.2,withamainsequencecom- limited understanding of the PPN/PN shaping process.
panion(S´anchezContrerasetal.2004c),andsymbioticbi- Interferometric observations have several advantages
nary stars, with white dwarf companions (e.g. Corradi et over single-dish studies. In addition to the obvious ad-
al. 2000). vantageofthehigherangularresolutionachieved,interfer-
Wehavecarriedoutoptical/NIRimagingsurveysaswell ometric techniques filter out extended emission from the
as an optical spectroscopic survey searching for system- interstellar medium (ISM), which often adversely affects
atic departures from sphericity and fast outflows amongst single-dishspectra(e.g.Heskeetal.1990). Onelimitation
lateAGBandearlypAGBstars, withthegoalofprobing, of interferometric observations is the lack of information
in their infancy, the physical processes that produce as- at zero spacing, which imposes a limit on the extent of
phericity (e.g. Sahai et al. 2007a; S´anchez Contreras et al. thelargestnebularstructuretowhichtheinterferometeris
2006b;S´anchezContrerasetal.2008). Thispaperpresents sensitive. Thelackofshort-spacingresultsinapartialloss
a complementary study at millimeter-wavelengths of the of the flux from extended structures with uniform surface
CO(andcontinuum)emissioninasampleofsuchobjects. brightness if these have large angular sizes compared with
TheseCOdataprobethecool,densemoleculargas,which thefringespacingscorrespondingtotheshortestbaselines.
comprisesthemostmassivecomponentoftheirenvelopes. The vast majority of CO surveys of evolved stars have
Molecular envelopes in PPNs often show two differ- beenperformed so far using single-dish telescopes (Knapp
ent kinematic components, slow and fast, which are re- &Morris1985;Likkeletal.1991;Loupetal.1993;Nyman
sponsible for the intense-narrow core and the weak-broad et al. 1992; Olofsson et al. 1993; Bujarrabal et al. 2001;
wings observed in the emission profile of CO rotational Kemper et al. 2003; Ramstedt et al. 2008, etc). Among
lines (Olofsson 1996; Bujarrabal et al. 2001, and refer- these surveys, that by Bujarrabal et al. (2001) contains
ences therein). PPN studies based on high-angular res- the largest proportion of PPNs. A major result from this
olutionCOmapsshowthatthefastmolecularcomponent workisthatfastmolecularoutflowsinmostPPNshavefar
correspondstomassive( 0.1-1M )bipolaroutflows(e.g. too much linear momentum to be powered by dust radia-
⊙
∼
Cox et al. 2000; Alcolea et al. 2001; Castro-Carrizo et al. tionpressure,whichisthewinddrivingmechanismduring
2002; Huggins et al. 2004; Meixner et al. 2004; Castro- the AGB phase. This observational result invalidates the
Carrizoetal.2005;S´anchezContrerasetal.2006a;Alcolea radiation-driven wind assumption of the GISW model.
et al. 2008; Castro-Carrizo et al. 2010). These outflows ThefirstinterferometricCOsurveyofevolvedstarswas
are thought to result from the acceleration of the AGB thatbyNerietal.(1998),whichcontainedmediumresolu-
wind (dense and slow) by shock interaction with under- tion (&10′′ at 3mm) maps of the 12CO J=1–0 and J=2–1
lying collimated pAGB ejections (fast but relatively tenu- transitions on a sample of 46 objects classified as AGB
ous). The fast, bipolar CO outflows of PPNs often follow ( 75%) and pAGB ( 25%) stars. More recently, Fong
aso-called“Hubble-flow”kinematics,i.e.radialexpansion e∼t al. (2006) presente∼d a small 12CO(J=1 0) imaging
−
with speeds increasing linearly with the distance from the survey of 8 evolved stars (2 AGBs, 5 PPNs, and 1 PN)
nucleus, suggesting that they have been accelerated in a with a mean angular resolution 5′.′6. A systematic study
∼
brief (.100yr) event (see references above). of CSEs around 46 AGB stars and 9 pAGB objects has
The slow molecular component in PPNs/yPNs is nor- been carried out recently; results of a sub-sample of 16
mallyattributedtotheremnantAGBCSE.InsomePPNs, objects from this survey are reported in Castro-Carrizo et
this remnant is observed as an extended, round halo ex- al. (2010).
panding at low velocity surrounding the bipolar outflows In addition to the aforementioned surveys, a few
(e.g., as in the PPN CRL618, S´anchez Contreras et al. amongst the most extended, strongest CO emitters, and
2004a). Inmany PPNs, part of theslow component arises “interesting”PPNshavebeenmappedusinginterferomet-
in a large ( 1016cm) and massive (&0.1M ) toroidal ric techniques (see references provided before in this sec-
⊙
≈
structure expanding orthogonally to the lobes (e.g. Zwei- tion). Detailed studies of individual objects are extremely
gleetal.1997;S´anchezContreras&Sahai2004b;Huggins valuable, however, the yet small number of PPNs with
2007). Theoriginofthesedensetoriisunclear: theycould published high-angular resolution maps comprehensively
OPACOS. I 3
analyzed (to our knowledge <15) prevents generalization Likkeletal.1992;Imaietal.2007;Yungetal.2011). Itis
of their results. Moreover, these individual works are per- believedthatintheseobjectstheinteractionofcollimated,
formedbyindependentgroupsusingdifferinganalysisand fastoutflowswiththeAGBCSEhasstarted 50-100years
∼
interpretativetechniques,whichisanimportantlimitation ago.
for statistical studies. Also, whether “interesting” means Our sample is characterized by low far infrared (IR)
“representative” or, rather, is an euphemism for “pecu- fluxes and, therefore, weak CO emission (given the rela-
liar” is unclear in some cases. In summary, surveys of tionship between the CO and far-IR emission; Bujarrabal
largesamplesofpAGBobjectsobservedinuniformcondi- et al. 1992). The IRAS 60µm fluxes of our targets are
tions and similarly analyzed are needed to derive general in the range [8.6-463]Jy, with 74% of the sample having
conclusions. f <100Jy and with a median value of f 43Jy (Fig.1,
60 60
∼
This is the first paper in a series resulted from our left panel). Our targets fill in the gaps in the IRAS two-
SNAPshot CO1-0 emission survey of evolved stars with color diagram where pAGB stars and PNs are expected
themillimeterwavelengtharrayoftheOwensValleyRadio to be located and that were scarcely populated by object
Observatory (OVRO) referred to as “OPACOS”: OVRO samples inearlier surveystudies (Fig.1, right panel). The
Post-AGB CO(1 0) emission Survey. Here, we present distribution of our targets in the NIR color-color diagram
−
thedata,mainobservationalresults,andderiveimportant (Fig.2)showsclearH K excessesforafractionofthem,
−
envelope parameters. In a forthcoming paper, correlation which are located above the blackbody line, indicative of
of such CO-derived parameters with other stellar and en- the presence of warm dust near the star.
velope properties obtained from multiwavelength data are Considering our target selection criteria (see above), we
investigated and discussed. donotexpectoursampletobestronglybiasedtowardany
of the properties listed in Table1, except maybe to O-rich
observations and data reduction
2.
sources. However, we add a cautionary note for readers
2.1. The sample who wish to generalize some of the results from this work:
(1) our sample size is still modest, and (2) the various
The objects observed in this work are listed in Table 1,
classes of objects may or may not be represented in their
where we provide the evolutionary class, the spectral type
correct proportions.
of the central star, the morphology of the optical and/or
Finally, our original sample also included two objects,
NIR nebula, the chemistry, the 12µm to 25µm IRAS flux
IRAS05506+2414 and IRAS19520+2759, considered to
ratio (f /f ), the 60µmIRAS flux (f ), and the distance
12 25 60 be pAGB candidates in our earliest surveys based on
to the source. References for these and other properties
their PPN-like IRAS colors and OH maser emission (see
are given for individual sources in Section 3.2. For the
e.g. S´anchez Contreras et al. 2008). Our 12CO(J=1 0)
optical/NIR morphology, we have adopted the primary −
OVRO maps as well as additional datasets favor a YSO
classification system by Sahai et al. (2007a), which estab-
nature in both cases (Sahai et al. 2008, Palau et al.in
lishes four main classes of nebular shapes: bipolar (B),
preparation) and, therefore, these sources will not be dis-
Multipolar (M), elongated (E), and irregular (I). Objects
cussed in this paper.
with star-like appearance in the HST images are denoted
as stellar (S). For the C-rich AGB star IRAS23166+1655
2.2. Interferometric mapping with OVRO
(AFGL3068),wehaveaddedaspecialcategory“spiral”to
indicate the shape of the envelope pattern observed in the Interferometric mapping of the 12CO(J=1 0) transi-
−
HST images (see 3.2.24 and Mauron & Huggins 2006). tionfor27objectswascarriedoutusingthesix10.4-man-
§
There are three programme objects with no optical/NIR tennaemillimeterarrayoftheOwensValleyRadioObser-
counterparts, indicative of very thick dust envelopes. vatory(OVRO),whichisnowpartofTheCombinedArray
Thesourcesinoursampleweremainlyselectedfromour for Research in Millimeter-wave Astronomy (CARMA3).
large list of candidate PPNs, based on their IRAS colors Observations were performed as part of a CO SNAP-shot
and fluxes and, for OH/IR stars, on the strength of their survey program in different runs between 2002 and 2004.
OH maser emission (for details see Sahai et al. 2007a). A log of the observations is provided in Table2, with the
Many of these have been confirmed to be bonafide PPNs list of sources, coordinates of the tracking center, obser-
based on our optical/NIR imaging/spectroscopic studies vation dates, array configurations, the half-power beam
(e.g. Sahai et al. 2007a; S´anchez Contreras et al. 2006b; width (HPBW), the orientation of the clean beam ma-
S´anchez Contreras et al. 2008). The spectral energy dis- jor axis, gain, passband and flux calibrators, and baseline
tributions(SEDs)ofourtargetsshowalackofwarmdust ranges (in the uv plane).
(f <f ) indicating a recent cessation of the heavy AGB For most sources, the digital spectral line correla-
12 25
mass-lossprocess,whichisbelievedtosignalthebeginning tor was configured to provide a total bandwidth of
ofpAGBevolution. Thecomplex,asphericalmorphologies 128MHz ( 330km s−1) with a channel spacing of 1MHz
of most objects in our sample as seen in high-angular res- ( 2.6km s∼−1). In some cases, a different configu-
∼
olution optical/NIR images indicate that the mechanism ration was used, providing a smaller bandwidth of
responsible for the breaking up of the spherical symmetry 90MHz ( 235km s−1). These bandwidths cover the
has been, and may still be, at work in these sources. Our full width∼of the 12CO profiles expect for two objects,
sample also includes two members of a particularly inter- IRAS22036+5306 and IRAS19374+2359, which were dis-
esting subclass of PPNs referred to as ‘Water Fountain’ covered to have exceptionally broad (>300km s−1) emis-
(labeled “wf”, Table1). These objects have high-velocity sion wings. For some targets, we performed simultaneous
jets (50-150km s−1) traced by H O maser emission (e.g. observations of the 13CO and C18O (J=1–0) transitions.
2
3 http://www.mmarray.org
4 S´anchez Contreras & Sahai
For13CO(J=1 0),thebandwidthandspectralresolution gets observed in this transition.
are similar to th−at of 12CO(J=1 0). For C18O(J=1 0), Our 12CO(J=1 0) maps are presented in Figure Set3
− − −
the units of the cross-correlator were set to bandwidths (availableintheelectroniceditionofthepaper). Theelec-
of 32MHz ( 85km s−1) with channel spacing of 1MHz tronically available material is presented in a format simi-
(2.7km s−1)∼. The 2.6mm continuum emission was ob- lar to that shown in Figure 3.1. The 12CO and 13CO line
served simultaneously using the dual-channel analog con- profiles for circumstellar detections are shown in Figs.4
tinuumcorrelator. Ourcontinuummapshaveabandwidth and 5. The line profiles from some of the targets are con-
of 3GHz since one of the four 1GHz-wide bands of the taminatedbynarrowabsorptionoremissionfeaturesfrom
continuum correlator, which contained the 12CO emission interstellarcloudsthatarealsopatentintheinterferomet-
line, has not been used to generate the final maps. ric maps.
Data calibration was performed using the MMA soft- Line parameters derived from the CO emission are pre-
ware package (Scoville et al. 1993). Gain-calibration was sented in Table3. In this table, (a) the center or systemic
done against nearby quasars that were observed at regu- velocity, V , has been obtained by fitting a symmet-
LSR
lar time intervals of 15-20minutes before and after each ric function to the profile, (b) the line full width at half
∼
target observation. Bright quasars were also observed at maximum (FWHM) and the full width at zero intensity
the begining and end of the track for passband calibra- level (FWZI) have been measured directly on our spec-
tion. Absolute flux calibration was obtained by observing tra and have typical errors of one half and one full chan-
planets. Quasars were also used as secondary flux cali- nel, i.e. 1.3 and 2.6 km s−1, respectively, (c) the param-
brators after carefully examining their flux history. Flux eter I is the peak surface brightness measured on the
CO
calibration errors could be of up to 20-30%. 2.6km s−1-widevelocity-channelmapstowardsthetarget,
In one case, the yPN IRAS19255+2123, the relatively and (d) the parameter I dV is the CO flux spec-
CO
intensecontinuumemissionhadtobesubtractedfromthe trallyandspatiallyintegrRatedoverthelineprofileandthe
original 12CO and 13CO(J=1 0) visibilities to produce CO-emitting region, respectively. As discussed in 4.1.1,
− §
purelineemissionmaps; thiswasdoneusingtheMIRIAD interferometric flux losses are unlikely to affect most of
task uvlin. our maps, except for IRAS23166+1655, for which our
Reconstruction of the maps from the visibilities was 12CO(J=1 0)interferometricdatarecoveronly20-30%of
−
done using standard tasks of the Multichannel Image Re- thetotalsingle-dishflux,andmaybeforIRAS22036+5306
construction,ImageAnalysisandDisplay(MIRIAD)soft- and IRAS22568+6141.
ware. After Fourier transforming the measured visibilities Our measurements of the 2.6mm continuum flux are
with robust weighting, data were cleaned and maps re- providedinTable4togetherwithAKARIfluxesat65and
stored. 90µm(f andf ). For2.6mmcontinuumnon-detections,
65 90
the rms noise (1σ) is given within brackets. Continuum
results emission is detected in 10 out of 27 targets, including the
3.
PN IRAS19234+1627, which is a CO non-detection. In
We have searched for CO emission in a total of Table4 we also list the values of the spectral slope, α,
27 evolved stars (Tables 1 and 2). Circumstellar adopting a frequency power law for the continuum flux
12CO(J=1 0) emission has been detected in 24 targets S να. The parameters α and α represent the con-
(Table3), i−.e. all objects except for IRAS19134+2131, tiνnu∝um spectral slope betwe6e5n 2.6mm90 and 65µm and be-
whichbelongstothewater-fountainsubclassofPPNs,the tween2.6mmand90µm,respectively. Fornon-detections,
PN IRAS19234+1627, and the yPN IRAS20462+3416. lower limits to the spectral slope are computed adopting
For these sources, which exhibit the lowest f60 fluxes in 3σ upper limits to the 2.6mm fluxes. The continuum-
our sample (<16Jy), we provide new upper limits to the emittingregionisextendedin4outof10sources,namely,
intensity of the 12CO emission. IRAS19234+1627, IRAS19548+3035, IRAS22568+6141,
We find 13 first-time detections in the 12CO(J=1 0) and IRAS23166+1655.
−
line, some of which were preliminarily reported by us
in S´anchez Contreras & Sahai (2004c). Out of these 13
3.1. Envelope parametrization
sources, 11 are first-time detections of any CO transition;
the other 2, IRAS18276 1431 and IRAS18348 0256, We have used our maps to characterize the CO and
− −
had been detected previously in the J=2–1 line (Heske 2.6mm-continuum emitting envelopes. In Table5 we list
et al. 1990). We included in our survey some sources the main envelope parameters obtained as follows. First,
where 12CO(J=1 0) emission had been searched for in theMIRIADtaskimfithasbeenusedtofita2Delliptical
−
the past. Amongst these, we have confirmed the pres- Gaussiantothemaps. Thisprocedureyieldstheposition,
ence of circumstellar 12CO emission in IRAS03206+6521, major and minor axes (at half the maximum intensity),
IRAS18560+0638, IRAS19255+2123, IRAS20000+3239, and orientation of the fitted ellipse and their errors, the
and IRAS22177+5936, with marginal (.3σ) detections latterbeingmainlydependentonthequalityofthefitand
prior to these observations (see references in 3.2). For the S/N in the maps. By comparing the output parame-
sources with previous 12CO(J=1 0) circumste§llar (>3σ) ters of the fit with the shape and orientation of the clean
−
detections,wehaveimprovedthespatialresolutionor/and beam, the algorithm determines whether a given source is
sensitivity of older data sets. extended or point-like.
For18sourceswehavealsoobservedthe13CO(J=1 0) For extended sources (18 of 24 in our sample), imfit
−
transition, out of which 15 are detected (including performs the beam-deconvolution of the image and yields
one marginal detection in IRAS19036+1407). No beam-deconvolved parameters such as the major and mi-
C18O(J=1 0)detectionsarereportedamongstthe3tar- noraxesoftheenvelopeathalfthemaximumintensity(θ
a
−
OPACOS. I 5
and θ ), and the orientation of θ measured from North adopting a λ−1 extinction power law.
b a
to East (i.e. its position angle, PA). We have computed
the geometric mean of θ and θ , which we refer to as 3.2.1. IRAS03206+6521 (OH138.0+7.2, RAFGL 5093)
a b
half-maximum diameter, θ1/2, as well as the asymmetry ThisMiravariable(Groenewegenetal.1999)appearsto
parameter a/b θa/θb. In Table5, we list these beam- bepoint-likeinourHST surveyofcandidatePPNs(Sahai
≡
deconvolved parameters together with the R.A. and Decl. et al. 2007a). It shows a strong 9.7µm silicon dust feature
offsets of the peak intensity relative to the coordinates of (Heske et al. 1990) and SiO and OH maser emission (Ny-
the phase tracking center (in Table2); errors to θ1/2, a/b manetal.1998;Omontetal.1993,andreferencestherein),
and PA are given within parenthesis. We also provide the indicative of O-rich chemistry. The double-peaked OH
velocity range of the velocity-integrated CO maps used to maser line suggests a molecular shell expanding at about
derive the envelope parameters as well as the maximum 9-10km s−1 (Heskeetal.1990;Engels&Jim´enez-Esteban
envelope extent (θmax), measured down to a 1σ level and 2007). No spectral type of the central star is available in
deconvolved taking into account the size of the beam. For the literature: based on variability and O-rich chemistry,
envelopes with large-scale asymmetries, θmax represents it is likely to be a late M-type.
the geometric mean of the corresponding values along the 12CO(J=1 0) single-dish spectra have been previously
major and minor axes. reported(Lou−petal.1993;Groenewegenetal.1999). Our
Considering the typical S/N in our data, the angular maps show a point-like 12CO(J=1 0) emission source
size of the smallest structure that will appear extended in with a line profile centered at VLSR−= 36.5 0.7km s−1
our maps (i.e. that will be observed with a half-maximum and full width similar to that spasnynsed −by the±OH maser,
diameterlargerthanthebeam)isθ1/2 0.25 HPBW.For 16km s−1 (Figs.3.1 and 4). The 12CO profile and total
thisandlargersizes,imfitperformsth∼ebea×mdeconvolu- l∼ine flux are comparable (within the errors) to the single-
tion of the maps and derives beam-deconvolved envelope dish spectrum, therefore major flux losses are not present
parameters. Forsmallersizes,however,thealgorithmindi- in our interferometric data.
catesthatthesourceispoint-like; thisenablesplacingup- Taking into account the galactic coordinates and radial
perlimitstoθ1/2<0.25 HPBWforsuchpoint-likesources velocity of IRAS03206+6521 (l=137.97◦, b=+07.26◦, and
×
(also given in Table5). VLSR= 36.5km s−1), we derive a value of the kinematic
sys −
We stress that all the envelope parameters presented in distance of d =3.6kpc. For this object, there is an es-
k
Table5 and discussed in the remaining of this paper have timate of the OH maser phase lag distance, d=3.4kpc
beenderivedfromtheoriginalmapsafterbeamdeconvolu- (Herman et al. 1986), which we adopt in this paper.
tion; therefore, theinfluenceofthebeamshapeandorien- [There is another estimate of the distance obtained by
tation, which remains patent in the original maps (shown comparing the linear and angular sizes of the OH shell,
in Figure Set 3), has been removed in such parameters. d=6.4 3.6kpc. Thisgeometricdistanceis,however,quite
±
uncertain because the linear size of the OH shell is in-
3.2. Individual sources directly estimaded from a model and the angular size
In this section, we discuss with some detail our targets of the envelope has large errorbars (0′.′4 0′.′15); Chap-
±
with 12CO(J=1 0) or 2.6mm continuum detections. We man et al. (1984)]. The bolometric flux of this object
first provide bac−kground information on the optical/NIR is Fbol 1250L⊙kpc−2, from which we derive a value of
∼
nebular morphology, the spectral type of the central star, dL=2.2kpc. For d 3kpc, the visual ISM extinction to-
∼
chemistry,andpreviousmolecularemissiondetections(in- wardsthesourceisAV 0.7magand,therefore,theintrin-
cluding masers). Then we describe our data from OPA- sic (de-reddened) bolom∼etric flux is Fbol 1280L⊙kpc−2,
COS and the main envelope characteristics deduced from whichimpliesatotalluminosityof1.2 10∼4L⊙atd=3kpc.
×
our maps. The nature of the 2.6mm continuum emis- This is at the high end of the range of typical luminosities
sion is concisely discussed for each source (SED plots and for pAGB objects.
modeling will be presented in paper II). Finally, we dis-
3.2.2. IRAS18055 1833 (V* AX Sgr)
cuss the distance d adopted for each object based on dif- −
ferent estimates in the literature (if available) and from This semiregular variable appears to be a bright point-
this work. The latter includes (i) the kinematic distance like source in our HST survey of candidate PPNs (Sahai
(d ) determined from the target radial velocity and the et al. 2007a). Its spectral type is G8Ia (G8-M2 as per
k
∼
galactic coordinates (l,b) by assuming a simple galactic the Variable Stars Catalogue). It has also been classified
rotation law and adopting a value for the A Oort con- asayellowsupergiant(Mv= 7.2to 8mag,whichiscon-
stant of 14.4km s−1kpc−1 and a galactocentric radius of sistentwithluminosityclass−I),soth−ereissomepossibility
8.5kpc (Kerr & Lynden-Bell 1986) and (ii) the “luminos- that its progenitor was a massive (&8M ) star. Based on
⊙
ity” distance (d ) estimated by adopting an intrinsic to- the presence of silicate dust features in the mid-IR and
L
tal luminosity of 6000L , typical of pAGB objects (e.g. its classification as 2.SE7 by Sloan et al. (2003) this is an
⊙
Bloecker 1995), to be compared with the bolometric flux O-rich source. No SiO or OH maser emission has been
(F ) computed by integrating the source SED. The vi- detected so far (Nyman et al. 1998; Omont et al. 1993).
bol
sual ISM extinction (A ) towards the target is also esti- No CO emission detection has been reported for this
V
mated using the numerical algorithm provided by Hakkila object before: 12CO(J=2 1) emission was unsucces-
−
et al. (1997), or the IRSA Galactic Dust Reddening and fully searched for by Omont et al. (1993). Our obser-
Extinction (GDR&E) calculator4 for upper limits to A . vations show a 12CO(J=1 0) emission line with sharp
V
ThisvalueofA isusedtocalculatethede-reddenedF edges centered at V +2−5km s−1 and rather affected
V bol LSR
∼
4 http://irsa.ipac.caltech.edu/applications/DUST/
6 S´anchez Contreras & Sahai
by ISM contamination (Figs.3.2 and 4). The relatively range V =[+22:+27]km s−1. A similar triple-peak pro-
LSR
large expansion velocity inferred from the CO line core file is observed in the 13CO(J=1 0) single-dish spectrum
(FWHM=FWZI 25km s−1)supportsahigh-massnature of CRL2688 by Quintana-Lacaci−et al. (2007). As shown
∼
in this case. The velocity-channel maps show structured by these authors, a bipolar wind with a low inclination
emittingregions;someofthesestructuresarepartiallydue (with respect to the plane of the sky) leads to wings like
to the presence of absorption and emission by intervening those observed in IRAS18135 1456, that is, narrow and
−
or nearby interstellar clumps. The velocity-integrated CO detached from the central line core.
map shows an extended source with a beam-deconvolved Our velocity-integrated 12CO map, excluding channels
half-maximumsizeof7′.′5 3′.′9androughlyorientedalong withISMcontamination,hasadeconvolvedhalf-maximum
the EW direction. Our×data indicate a velocity gra- size of 3′.′1 1′.′8 with its major axis roughly oriented
∼ ×
dient approximately in the perpendicular direction: the along the EW direction. The centroids of the red- and
blue- and red-wing emission clumps are offset by 1′′ to blue-velocity integrated maps at V =[+14.3:+11.7] and
LSR
the north and south, respectively (Fig. 3.2; bottom±right [ 14.3: 11.7]km s−1, i.e. adjacent to the line core, are
− −
panel). The elongation and velocity gradient found could offset to the South and North, respectively (see Fig3.3,
indicate an equatorially dense expanding torus, however, bottom-right panel). The elongation of the velocity-
theoriginofthevelocitygradientandasymmetryobserved integrated intensity map orthogonal to the NS velocity
cannot be firmly established as predominantly circumstel- gradient is consistent with the bulk of the low-velocity
lar (but, rather, may be an artifact due to the strong ISM emission arising in an expanding equatorial torus. The
contamination)and,therefore,weconsiderthisresultvery rms noise in our maps and ISM contamination of the cir-
tentative. cumstellar profile does not enable to reliably locate the
The distance to IRAS18055 1833 is extremely un- emission from the weak, broad wings of the profile (be-
certain. Given the low galac−tic longitude and lati- yond 14.3km s−1) and, therefore, the presence of a fast
tude of this source (l=11.6◦, b=0.7◦), the values of the bipola±rflow,suggestedbythetriple-peakedprofile,cannot
near and far kinematic distance are unreliable due to be confirmed.
non-circular motions close to the galactic center. The Wedetect2.6mmcontinuumemissionfromapoint-like
bolometric flux of this object is F 5400L kpc−2, source with a total flux 11mJy, consistent with thermal
bol ⊙
∼ ∼
from which we derive a value of d =1.05kpc. For 100K dust emission according to the slope of the SED
L
≈
d 1kpc, the visual ISM extinction towards the source from the IR to the mm-wavelength spectral region.
∼
is A =0.5mag and, therefore, the intrinsic (de-reddened) The distance to IRAS18135 1456 is uncertain. Given
V
bolometric flux is F =L 6400L kpc−2. For a larger its low galactic longitude (l=−15.7◦), the values of the
bol ⊙
∼
value of d, e.g. d (near)=3.3kpc, we expect A =2mag near and far kinematic distance are unreliable due to
k V
andF 1.1 104L kpc−2,implyingaratherhighlumi- non-circular motions close to the galactic center. We
bol ⊙
nosity of∼ 10×5L at such a distance, which is more typi- adopt here d=d =2.5kpc, taking into account the bolo-
⊙ L
cal of a m≈assive supergiant star. Here we adopt d=2kpc, metric flux of this object F 980L kpc−2. For such
bol ⊙
∼
which is intermediate to d and d (near). a distance the visual ISM extinction is A =1.5-2.0mag
L k V
and, thus, the intrinsic (de-reddened) bolometric flux
is F 1020L kpc−2, which implies an insignificantly
3.2.3. IRAS18135 1456 (OH 15.7, RAFGL 5458) bol ⊙
∼
− smaller value of the distance, d 2.4kpc.
L
Thisnon-variableOH/IRstar(vanderVeenetal.1989) ∼
appears as a point-like source in our HST survey of can-
3.2.4. IRAS18167 1209 (OH 18.5+1.4)
didate PPNs (Sahai et al. 2007a; Si´odmiak et al. 2008) −
and in 8µm subarcsec-resolution images (Lagadec et al. This OH/IR star appears as a point-like object in our
2011). SiO, H O, and OH masers have been detected, optical HST survey of candidate PPNs (Sahai et al.
2
which,togetherwiththepresenceof9.7µmsilicateabsorp- 2007a). The central star has a spectrum consistent with
tion,indicatesanO-richchemistry(Nakashima&Deguchi anF7classificationanditissurroundedbyacompactHα-
2007; Omont et al. 1993; Deguchi et al. 2007, and refer- emitting region (S´anchez Contreras et al. 2008). It shows
ences therein). The stellar spectral type is unknown, but strong OH maser emission with a double-peaked profile
sinceCObandheadsappearinabsorptionintheNIRspec- in the LSR range [+164.6:+187.8]km s−1; no SiO maser
trum of this object implying a photospheric temperature emission has been detected (van Langevelde et al. 1990;
of 3000-5000K (Oudmaijer et al. 1995), it may be G5- Nyman et al. 1998; Engels & Jim´enez-Esteban 2007).
K0∼(assuming it’s a supergiant). The double-peaked OH 12CO(J=1 0) and (J=2–1) single-dish spectra have
−
maser emission profile is consistent with shell expansion been reported by Winnberg et al. (1991): the emission
at 15km s−1 (Engels & Jim´enez-Esteban 2007). is centered at V =+176km s−1 and its profile is con-
LSR
N∼o CO emission has been previously reported for this sistent with expansion at 12km s−1. Our data show a
object. Our data (Figs.3.3 and 4) show a 12CO(J=1 0) weak, spatially unresolved∼12CO(J=1 0) emission source
emission line centered at VLSR=0 1km s−1, and wit−h a around V =+176km s−1 (Figs.3.4−and 4). Our mea-
sys ± LSR
peculiar triple-peak profile: there is a relatively intense surementofthelineflux(Table3)issmallerthanthatob-
central emission component with a full width similar to tained from single-dish observations ( 0.5 0.25Jy). The
∼ ±
that spanned by the OH masers; at both red and blue flux discrepancy may be attributed, at least partially, to
sidesofthelinecore(approximatelyatVLSR 22km s−1), the large flux error bars in both datasets (of up to 50%).
sys ±
there are two narrow emission features indicative of faster Although we cannot rule it out completely, we think our
motions. The red-shifted narrow feature is significantly data, with relatively low resolution (HPBW 5′′), are un-
∼
altered by strong ISM contamination, especially in the likely to be affected by significant interferometric flux
OPACOS. I 7
losses, which would impy the presence of a CO envelope are relatively uncertain due to the ISM contamination of
with an angular diameter &18′′. The presence of such a the circumstellar profile and moderate angular resolution.
large envelope is improbable because, as for most PPNs, Given the flux of the 12CO(J=1 0) line derived from
−
theouterradiusoftheCOenvelopeisunlikelytobelarger our data ( 1.5Jy), this line should have been detected in
than few 1017cm (Neri et al. 1998), implying an upper Knapp et∼al. (1989) and Likkel et al. (1991) single-dish
×
limit to the angular radius of the CO-emitting nebula of studies. Wedonotknowthereasonforthisinconsistency,
less than 1′′ given the probable large distance to this but large errors in the absolute flux calibration, pointing,
∼
object (see next paragraph). and/or strong ISM contamination in the single-dish data
As discussed by Winnberg et al. (1991), considering its couldexplainthefluxdiscrepancy. Inanycase, fluxlosses
galactic coordinates (l=18.52◦, b=+1.41◦), the radial ve- are unlikely to affect our data. We detect 13CO(J=1 0)
locity of IRAS18167 1209, VLSR=+176km s−1, is close emission from this source for the first time. The pro−file,
− sys
to the maximum allowable velocity for circular motions althoughnoisy,isconsistentwithadouble-peakshapecen-
and, therefore, it is probably located near the tangent teredatafewkm s−1redwardsfromthe12COline(Fig.5).
point,i.e.d 7-8kpc. Sincethebolometricfluxofthisob- A compact source of 2.6mm continuum emission is de-
k
ject is F ∼100L kpc−2, we derive a very similar value tected towards IRAS18276 1431. As discussed in de-
bol ⊙
∼ −
for d =7.7kpc. For d 7-8kpc, the visual ISM extinction tail by S´anchez Contreras et al. (2007), a population of
L
∼
is A =1.5-3.5mag and the intrinsic (de-reddened) bolo- big dust grains (radius &4mm) with temperatures 20-
V
metric flux is F 100-140L kpc−2, which consistently 150Kcanexplaintheobservedmillimeterandsubmill∼ime-
bol ⊙
∼
implies a similar value for d of 6.5-7.6kpc. No reliable ter flux.
L
OH maser phase-lag distance has been determined in this OH maser phase-lag measurements indicate a distance
case because of its weak variability. We adopt a distance to IRAS18276 1431 in the range d=2-5.4kpc (Bowers et
−
to IRAS18167 1209 of d 7kpc. al. 1983; Herman & Habing 1985). The near-kinematic
− ∼
distance is d =4.5kpc (Le Bertre et al. 1989). In this pa-
k
per, we adopt an intermediate value of d=3kpc, which is
3.2.5. IRAS18276 1431 (OH 17.7-2.0, AFGL 5497)
− also close to the value of d derived from the bolometric
L
This young O-rich PPN has been imaged in our flux of this object, F 870L kpc−2. For d 3kpc, the
bol ⊙
optical/HST and NIR/Adaptive Optics (AO) surveys visual ISM extinction is∼A =1.0-2.0mag and∼, therefore,
V
of candidate PPNs (Sahai et al. 2007a; S´anchez Con- the intrinsic (de-reddened) bolometric flux is F 900-
bol
treras et al. 2006b). It appears as a bipolar object 915L kpc−2, which consistently implies a similar∼value
⊙
with searchlight-beams and arcs, and has been studied of d 2.6kpc.
L
in detail by S´anchez Contreras et al. (2007). The spec- ∼
tral type of the central star is earlier than K5 (Le
∼ 3.2.6. IRAS 18348 0526 (OH 26.5+0.6, RAFGL 2205,
Bertre et al. 1989). The progressive disappearance of −
V* V437 Sct)
H O maser emission is consistent with a recent drop of
2
the mass-loss rate from 10−5M yr−1 to well below This is an O-rich Mira-type variable near the tip of the
⊙
10−7M yr−1 (Engels 2≈002). The nondetection of SiO AGBphase. ItwasobservedbutnotdetectedinourHST
⊙
≈
masers in IRAS18276 1431 is also consistent with its opticalsurveyofcandidatePPNs(Sahaietal.2007a). The
−
pAGBnature(Nymanetal.1998). StrongOHmaseremis- source has very bright 2MASS and MSX counterparts. It
sion is observed in a dense, equatorial region elongated in shows strong 9.7 and 18µm silicate features and water
the direction of PA=110◦, i.e. approximately perpendicu- ice far-IR bands (Sylvester et al. 1999). It has a spec-
lar to the bipolar lobes (Bains et al. 2003). The bipolar tral type of M (as per SIMBAD database). OH and SiO
lobes are marginally resolved in sub-arcsec resolution 8- masers are detected. The OH maser emission arises in
12µm images by Lagadec et al. (2011). an elongated shell of radius 2′.′5 with its major axis ori-
Single-dish12CO(J=2 1)emissionhasbeenpreviously entedalongPA=95◦ (Herma∼netal.1985;Bowers&John-
reported by Heske et al. (−1990). The 12CO(J=1 0) tran- ston 1990; Etoka & Diamond 2010). SiO maser emission,
−
sition was observed by Knapp et al. (1989) and Likkel mapped with the VLBA, reveals a rather chaotic, asym-
et al. (1991) but undetected with rms 0.2Jy. Data metric ring-like distribution with a characteristic radius
from this survey, therefore, represent the∼first detection 0′.′01 and maser plumes roughly oriented along the NS
of 12CO(J=1 0) emission from this target. Our CO ∼direction (Cotton et al. 2008).
maps from OP−ACOS (Fig.3.5) were presented and ana- Single-dishspectraof12CO(J=1 0)andhigher-J tran-
−
lyzedpriortothispaperinS´anchezContrerasetal.(2007); sitions have been previously reported by, e.g., Heske et
here, we briefly summarize these results. We find an ex- al. (1990) and Justtanont et al. (1996): strong galac-
tended CO envelope with a deconvolved half-maximum tic contamination of the single-dish 12CO(J=1 0) pro-
sizeof5′′ 3′′;thissizesimilartothatofthehaloobserved file is noticed. Interferometric 12CO(J=1 0) m−aps with
in the NI×R (S´anchez Contreras et al. 2007). The long-axis 9′.′8 7′′ resolution also exist (Fong et al. 2−002). We find
of the CO envelope appears to be roughly oriented along 12C×O emission over a full velocity range of 24km s−1,
∼
theEWdirection(afterbeam-deconvolutionofthemaps), similar to that spanned by the OH masers (Figs.3.6 and
thatis,similarlytotheequatoriallydensestructureprobed 4). The 12CO profile has a hint of asymmetry, the blue
byOHmaseremissionandvisibleasadarkwaistseparat- sideisweaker, maybeduetoself-absorptionorabsorption
ing the lobes in the optical and NIR images. Therefore, a by intervening ISM components. The upper limit to the
significant part of the CO emission probably arises in an 12CO(J=1 0) line flux published by Heske et al. (1990)
−
equatorially dense toroidal structure. We note, however, from single-dish observations, .0.5Jy, is consistent with
that the envelope parameters deduced from our CO maps our measurement, 0.6Jy, within 20-30% absolute flux
∼
8 S´anchez Contreras & Sahai
errors. This indicates that there are no significant flux ThisOH/IRobjectwasimagedinouroptical/HST and
lossesinourinterferometricdata. The12CO(J=1 0)flux NIR/AO surveys of candidate PPNs and shows a central
measured by Fong et al. (2002) with a 9′′ beam−is also bipolar shape (oriented along PA 60◦) surrounded by
0.6Jy. ∼ an extended halo traced out to a r∼ad−ius of 3′′ with cen-
∼The object appears extended in our 12CO(J=1 0) trosymmetricarc-likefeatures(Sahaietal.2∼007a;S´anchez
−
maps (Fig.3.6): the deconvolved, half-maximum size of Contrerasetal.2006b). One,possiblytwo,secondary(mi-
the velocity-integrated CO map is 4′.′3 3′.′8, with its long nor) lobes with collimated shapes (oriented along PA 75-
axis oriented roughly along the EW ×direction, i.e. par- 80◦) emanate from the central region5. The central∼star
allel to the major axis of the OH shell and dusty envi- hasbeenassignedaM1Ispectraltype(Su´arezetal.2006).
ronment probed by mid-IR emission (Bowers & Johnston A double-peaked OH maser emission profile indicates
1990; Chesneauetal. 2005). This resultis consistent with a dense molecular shell expanding with V 14km s−1
exp
∼
CO probing an equatorially dense toroidal structure. The (Engels & Jim´enez-Esteban 2007). No SiO or H O maser
2
deconvolved, half-maximum size of the CO envelope mea- emission has been reported (G´omez et al. 1990).
sured in our data is smaller than that measured by Fong No CO detection has been previously reported for this
et al. (2002), 8′.′5 5′.′5, in their lower (HPBW 9′′) res- object. Our data show 12CO(J=1 0) emission with a
∼ × ∼ −
olution maps. This discrepancy is unlikely to result from sligthly asymmetric profile over a total velocity range a
interferometric flux losses or the sensitivity limit in our little larger than the OH maser line (Figs.3.7 and 4). The
mapsgiventhegoodagreementbetweenthelinefluxmea- COemissionarisesinanelongatedenvelopewithadecon-
sured by us and that obtained from single-dish data and volved half-maximum size of 3′.′5 1′.′2 and roughly ori-
∼ ×
the interferometric maps by Fong et al. (2002) themselves ented along the NS direction. We identify a velocity gra-
(seepreviousparagraph). ThedifferentsizeoftheCOen- dientalongthesameaxis: thered-shiftedandblue-shifted
velopederivedfrommapswithdifferentangularresolution emission regions are systematically displaced towards the
may reflectthetwo distinct mass-loss regimes presumably North and South, respectively. The similar orientation of
undergone by this object (see Justtanont et al. 1996). the long axis of the CO envelope and the direction along
We find a velocity gradient in the nebula along the NS which the velocity gradient is observed suggest that the
direction: the red- (blue-) shifted emission is systemati- COemissionprobesabipolarmolecularoutflow. Thisout-
cally offset towards the North (South), respectively. Al- flowmaybetracingadistinct(third)bipolarejectionthan
thoughwecannotruleoutsuchagradientbeinganartifact those probed by the optical HST images. 13CO(J=1 0)
−
produced by ISM absorption of the circumstellar profile, emission is also detected (Fig.5). The envelope traced
the remarkable symmetry of the countours of the velocity by this transition, which is also extended, has a decon-
map around the CO emission peak favors a circumstellar volved half-maximun size of 5′.′6 4′.′2 and similar orien-
origin. Infact,anexpandingtoruswithitssymmetryaxis tation (within the errors) to∼the 12×CO-envelope.
projected along the NS direction and tilted with respect The near and far kinematic distances to
to the line of sight in such a way that the back (receding) IRAS18420 0512 taking into account its galactic co-
and front (approaching) sides of the torus are offset to ordinates a−nd radial velocity (l=27.57◦, b= 0.85◦,
the north and south, respectively, could satisfactorily ex- VLSR=+105km s−1) are d (near)=6 and d (far)−=9kpc.
sys k k
plainboththeEW-elongationoftheCOenvelopeandthe As discussed by de Jong (1983) and Herman et al. (1985),
NS-velocity gradient observed. Alternatively, the veloc- since the radial velocity is close to the maximum veloc-
ity gradient may indicate the presence of a compact bipo- ity allowed in that direction, IRAS18420 0512 could be
lar outflow running along the NS direction (i.e. orthogo- at the tangent point (d 7.5kpc). The −bolometric flux
nallytothetorus),co-linearwiththeasymmetricdistribu- of this object is F 1∼60L kpc−2 from which we de-
bol ⊙
tion of the SiO maser plumes. Higher-angular resolution rive d =6.1kpc. At s∼uch a distance (6-7kpc), the visual
L
CO mapping is needed to unambiguously characterize the ISM extinction in the direction to IRAS18420 0512 is
structureandkinematicsofthemolecularenvelopeinthis A =2-4mag and, thus, the dereddened bolome−tric flux
V
compact object. is F 190-230L kpc−2, which yields a consistent value
bol ⊙
We report the detection of weak 13CO(J=1 0) emis- forthe∼distanceofd =5-5.6kpc. Weadoptavalueforthe
sion in the range VLSR=[+15:+35]km s−1 (Fig.−5). distance to this objeLct of d=6kpc.
We adopt a distance to this object of d=1.1kpc, which
is the average of two independent estimates of the OH
3.2.8. IRAS18460 0151 (OH31.0-0.2)
maser phase lag distance (d=1.4 and 0.9kpc, respectively −
ThisOH/IRstarshowsH Omaseremissionspreadover
van Langevelde et al. 1990; Bowers & Johnston 1990). 2
The bolometric flux of this object is F 6300L kpc−2, a very wide velocity range (&200km s−1) and likely be-
bol ⊙
∼ longstotheclassof“waterfountain”PPNs(Deguchietal.
which yields a value for the luminosity distance of
2007, and references therein). Although no 2MASS coun-
d 1kpc. For d 1kpc, the visual ISM extinction is
L
∼ ∼ terpartcouldbefoundforthisobject,ithasabrightMSX
A =0.1-1.0mag (Hakkila et al. 1997) and, therefore, the
V
match and it is also identified at wavelengths 3.5-5.8µm
intrinsic (de-reddened) bolometric flux is not significantly
higher, F 6300-6600L kpc−2. in the Spitzer/GLIMPSE survey (Deguchi et al. 2007).
bol ⊙
∼ The rising Mid-IR spectrum toward longer wavelengths
points to a thick dust envelope, which is spatially unre-
3.2.7. IRAS 18420 0512 (OH 27.5-0.9) solved in 8-12µm subarcsec-resolution images (Lagadec et
− al. 2011). In addition to the very fast outflow probed by
5TheopticalnebulaofIRAS18420 0512iserroneouslyclassifiedaselongated(E)inTable3ofSahaietal.(2007a). Thecorrectmorphological
−
classificationshouldbebipolarwithminorlobes(B,ml)andassuchislabeledinourTable1.
OPACOS. I 9
the H O masers, the double-peaked OH maser emission larger than the OH maser profile, V [0:+43]km s−1.
2 LSR
profile (with peaks at V =+110 and +140km s−1) in- There are ISM absorption and emissi∼on clumps near
LSR
dicates a dense shell expanding at V 15km s−1 (En- IRAS18560+0638, which results in a triple-peaked CO
exp
∼
gels & Jim´enez-Esteban 2007). The optical counterpart profile with very sharp edges. Due to ISM contamination,
to IRAS18460 0151, which is expected to be very faint, it is very difficult to obtain an accurate value for the CO
−
cannotbeunambigouslyidentifiedinouroptical/HST im- flux (the value in Table3 is probably a lower limit). The
ages6,whichshowseveralfaint,point-likeobjectsnearthe 12CO(J=1 0) intensity at the line peak from single-dish
−
MSX and OH maser source. data (references above), 0.8 0.4Jy, and from OPACOS
±
No CO detection has been previously reported toward (0.45Jy) are comparable taking into account the men-
this object. Our data show 12CO(J=1 0) emission over tioned flux uncertainties. Therefore, and considering also
−
a total velocity range similar to that of the OH maser therelativelylargebeaminourmapsofIRAS18560+0638,
line (Figs.3.8 and 4). There is strong ISM contamina- it is unlikely that significant flux losses affect our interfer-
tion of the maps near the source but at velocities out- ometric data.
side the circumstellar profile (at V [+103:+108] and The velocity-integrated CO map, using only channels
LSR
90km s−1). The CO envelope is m∼arginally extended with weak or no ISM contamination, as well as the in-
∼withadeconvolvedhalf-maximumsizeof2′.′1 0′.′5andori- dividual velocity-channel maps indicate an extended CO
entedalongPA 50◦. Wefindatentativevelo×citygradient envelope,withadeconvolvedhalf-maximumsizeof 3′′-5′′
along the NS d∼irection: the red- and blue-shifted emis- and oriented roughly along PA 30◦. A velocity∼gradi-
≈−
sion are sistematically displaced towards the North and ent is found along this axis: the red- (blue-) shifted emis-
South, respectively. Since this source is only marginally sion is displaced towards the North (South). We detect
resolved, the errors in the envelope parameters derived, 13CO(J=1 0) emission for the first time in this object.
inparticulartheCOenvelopeorientationandasymmetry, The line is−centered at V =+20km s−1 with a full ve-
LSR
areexpectedtobelargerthantheaverage. Therefore, the locity extent comparable to that of the 12CO line (Fig.5).
relative orientation of the CO envelope and the tentative Our velocity-integrated 13CO map also suggests an ex-
gradient found is uncertain and we cannot elucidate the tended envelope with a deconvolved half-maximum size of
structure(s) responsible for the asymmetries observed. 5′.′2 2′.′5with its long axis oriented similarly to the12CO-
×
As discussed, e.g., by de Jong (1983) and Deguchi envelope. The alignment (within errors) of the velocity
et al. (2007), the radial velocity of IRAS18460 0151 gradientandthelongaxisofthe12CO-and13CO-envelope
(VLSR=+126km s−1) is close to the maximum ve−locity suggest a bipolar outflow in that direction. However, due
sys
allowedinthatdirection(l=31.01◦,b= 0.22◦)and,there- to the strong ISM contamination of the 12CO maps cand
fore,thisobjectisprobablyatthetang−entpoint,implying also because of the moderate S/N and angular resolution
a distance of d 7kpc, which we adopt. The bolomet- of the 13CO(J=1 0) dataset, we consider this resul ten-
ric flux of thisko∼bject is F 600L kpc−2. At a dis- tative. −
bol ⊙
tanceof 7kpc,thevisualISM∼extinctioninthedirection The distance to IRAS18560+0638 is estimated to be
to IRAS∼18420 0512 is A 3.5mag and, therefore, the d=1-2kpc from OH maser phase lag measurements (van
V
dereddened bo−lometric flux∼is F 690L kpc−2. This Langevelde et al. 1990). This value is in excellent agree-
bol ⊙
implies a total luminosity of the s∼ource L=3 104L at ment with our estimate of the near kinematic distance,
⊙
d=7kpc, which is at the high end of the typic×al luminos- dk=1.4kpc(l=39.71◦,b=+1.49◦,andVLSR=+20km s−1),
ity range for pAGB objects. The luminosity distance for which we adopt in this paper. At a distance of 1-
∼
this object is d =3kpc. 2kpc, the visual ISM extinction in the direction to
L
IRAS18560+0638 is A 0.1-1.4mag and, therefore, the
V
3.2.9. IRAS18560+0638 (OH39.7+1.5, V1366 Aql, dereddened bolometric fl∼ux is Fbol 9050-104L⊙kpc−2.
AFGL 2290) At d=1.4kpc the total luminosity is∼L 1.8 104L⊙.
∼ ×
This Mira-type variable star does not have an optical
3.2.10. IRAS19024+0044 (OH35.21 2.65)
counterpartdowntoalimitof 20mintheF photographic −
∼
band of The Guide Star Catalog, Version 2.3.2 (GSC2.3). This object is a young O-rich PPN as deduced from the
A very deep 9.7µm silicate feature indicates an optically detailed multiwavelength study by Sahai et al. (2005b),
thick envelope with O-rich chemistry (Heske et al. 1990). which included our 12CO(J=1 0) maps from OPACOS.
−
OH, H O, and SiO masers have been detected (Omont et HST optical images show a multipolar nebula of size
2
al. 1993; Deguchi et al. 2007; Nyman et al. 1998). The 3′.′7 2′.′3, with at least six lobes emanating from the
∼ ×
H O maser profile varies with time in a unique manner, central source. A central compact region shows Hα emis-
2
exhibitingadouble-peakedprofilesimilartothe1612MHz sionwithverybroadwings(FWZI 2400km s−1)andaP-
OH maser line during the bright phase of the star and a cygnilikeprofile,indicatingthepre∼senceofa 100km s−1
≈
singlelineclosetothestellarradialvelocitywhilethestar outflow (S´anchez Contreras et al. 2008). The central star,
was dim (Engels et al. 1997). whichisobscuredbyadustyequatorialstructureintheop-
12CO(J=2 1) emission was reported by Heske et al. ticalimages,hasaspectraltypeG0-5(Sahaietal.2005b).
−
(1990) with a profile severely affected by ISM contamina- OH maser emission suggests the presence of a dense ex-
tion. Heskeetal.(1990)andNerietal.(1998)publisheda panding envelope at V =13-14km s−1 (Sevenster 2002).
exp
marginaldetectionof12CO(J=1 0)withtheIRAM30m H O maser emission is not detected (Su´arez et al. 2007).
2
radiotelescope. Our data (Figs.3−.9 and 4) show circum- 12CO(J=1 0) emission was first detected in this ob-
−
stellar CO emission over a total velocity range slightly ject as part of OPACOS and reported by us in Sahai et
6 TakenaspartofourHST imagingsurveyprogramGO9463.
10 S´anchez Contreras & Sahai
al. (2005b). Likkel et al. (1991) sought for 12CO(J=1 0) ble5). The orientation of the 2.6mm continuum envelope
−
with the 30m IRAM telescope but emission was not de- is the same as that of the optical, IR, and radio nebulae.
tected down to a 1σ level of 0.13Jy. According to the The origin of the mm-wave continuum emission is proba-
∼
peak line intensity measured by us ( 0.7Jy), the line bly free-free emission from the ionized nebula.
should have been detected by these ∼authors, however, Since the VLSR of IRAS19234+1627 is unknown, we
sys
pointing errors in the single-dish observations, or flux cal- are unable to estimate d . From the bolometric flux of
k
ibration uncertainties and/or ISM contamination of the the source, F =60L kpc−2, we derive a large value of
bol ⊙
profile in both datasets may be the origin of the observed the luminosity distance of d =10kpc. At a distance of
L
discrepancy. In spite of these uncertainties and, since we 10kpc, the visual ISM extinction is A <6.5mag, which
V
∼
measure more flux than the rms of the single-dish data, leadstoadereddenedF <70L andalowerlimittothe
bol ⊙
we conclude that flux losses are improbable. distance of d >9.2kpc. Here we adopt d=9.5kpc.
L
Our data (Figs.3.10 and 4) show a relatively broad
12CO(J=1 0) profile centered at VLSR=+50km s−1 and 3.2.12. IRAS19255+2123 (OH56.1+2.1, K3-35)
−
affected by ISM contamination in several channels (e.g. at
ThisisaveryyoungO-richPNcharacterizedbyabipo-
V =+37.7 and +50.7km s−1). The velocity-integrated
LSR lar radio continuum emission morphology (Aaquist 1993;
CO map is spatially unresolved, however, the emission
Mirandaetal.2001). ThecharacteristicS-shapemorphol-
from the individual channels is centered at slighly differ-
ogy of the radio lobes can be successfully reproduced by
entoffsetsthatliewithina 2′.′5-longregionorientedalong
PA +45◦, i.e. well aligned∼with the optical lobes. There- a precessing jet, evolving in a dense circumstellar medium
∼ (Vel´azquez et al. 2007). Our optical/HST and NIR/AO
fore, the CO nebula probed by our maps is marginally
images show bipolar nebula with a well defined point-
extended, with a value of θ 1′′. The velocity gradi-
1/2≈ symmetric structure (Sahai et al. 2007a; S´anchez Contr-
entfound(seeFig. 3.10,bottom-rightpanel)isconsistent
erasetal.2006b). Ground-basedopticalimagesandR I
with the presence of a molecular bipolar outflow. −
colour map are consistent with the presence of a dense
Here we revise our value for the distance to
structure in the equatorial plane of the nebula (Miranda
IRAS19024+004 as estimated in Sahai et al. (2005b).
et al. 2000). Both OH and H O maser emission are ob-
We adopt d=10kpc, which is the far d (l=35.21◦, 2
k served, suggesting that IRAS19255+2123 departed from
b= 02.65◦, and V =+50km s−1). The bolometric flux
isF− =260L kpcL−S2R,whichwouldimplyaverylowlumi- the PPN phase only a few decades ago (Miranda et al.
bol ⊙ 2001; Tafoya et al. 2007). The H O maser emission is
nosity of L 3500L for the near d =3.7kpc. For the far 2
d thelumi∼nosity, L⊙ 2.6 104L ,iskatthehighendofthe foundintheequatorialregionsandatthetipsofthelobes
k ∼ × ⊙ (Miranda et al. 2001); SiO maser emission is not detected
typicalrangeforpAGBobjectsandinexcellentagreement
(Jewelletal.1991). FromthepresenceofHeIIemissionin
with the independent estimate of the luminosity obtained
the optical spectrum, a temperature of T 60,000K has
fromthestrengthoftheluminosity-dependentOI7771-5˚A eff≥
been inferred for the central star (de Gregorio-Monsalvo
IR triplet (S´anchez Contreras et al. 2008). The compact-
et al. 2004; Miranda et al. 2000).
ness of the optical and CO nebula also supports a large Single-dish 12CO(J=2 1) observations have been re-
distance to this object7. At a distance of 10kpc, a value −
∼ ported by Tafoya et al. (2007), who detected an in-
eotf AalV.∼(129.297m),agwihsicdherlievaeddsutsoinag tdheereadldgoenrietdhmFby=H2a7k0kLila tense, narrow ISM feature at VLSR∼+10km s−1 plus a
bol ⊙ weak, broad component observed over a velocity range of
and a total luminosity of L∼2.7×104L⊙. VLSR [0:+40]km s−1 that is most likely associated with
∼
IRAS19255+2123. A tentative (3σ) detection of circum-
3.2.11. IRAS19234+1627 (PN G051.5+00.2) stellar 12CO(J=1 0) emission is also reported by these
−
Ground-based optical imaging and spectroscopy indi- authors.
cates a PN nature for this IRAS source (Kerber et al. Our data confirm the presence of circumstel-
1996). HST opticalimagesshowaroughlyelliptical,limb- lar 12CO(J=1 0) emission from this object with
brightenednebulaofangulardimensions 11′′ 8′′withits a relatively br−oad profile, over the velocity range
longaxisorientedatPA 15◦ (Sahaieta∼l.201×1a). Afaint V =[+10.9:+47.3]km s−1 (Figs.3.12 and 4). At
LSR
point-like source is obse∼rved at the nebula center, which V =+10.9km s−1, we identify a 12CO emission clump
LSR
is very likely the central star. Radio emission maps and east of IRAS19255+2123 that is most likely the main
IR images from the Spitzer/GLIMPSE survey (Urquhart contributor to the narrow ISM emission component found
et al. 2009) show a nebular morphology similar to that in at this velocity in previous single-dish spectra. This ISM
the optical. feature is less intense in our interferometric data than in
Our maps show several CO emission clumps in the ve- single-dish spectra, suggesting that the extended emission
locity range V =[+51:+66]km s−1 (Fig.3.11). None of fromtheISMcloudispartiallyfilteredoutbytheinterfer-
LSR
the clumps coincide with the location of the central star ometer. The flux of the broad, circumstellar component
ofIRAS19234+1627orthe2.6mmcontinuumsourceand, of the CO profile measured by us and by Tafoya et al.
therefore,thedetectedCOemissionislikelytohaveanin- (2007) from their single-dish spectra are in agreement,
terstellar origin. which implies that there are no interferometric flux losses
Intense continuum emission at 2.6mm is detected, of the circumstellar CO emission in our data.
F =70mJy. The continuum source is extended and Our velocity-integrated CO map shows an extended
2.6mm
has a deconvolved half-maximum size of 5′.′5 4′.′7 (Ta- sourcewithadeconvolvedsizeof8′.′4 3′.′9andwithitslong
∼ × ×
7 TypicallinearsizesofPPNsarediscussedin 4.1.1. SeealsoTable7.
§
8 Thiselongationisunlikelytobeanartifactofthecontinuumsubtraction( 2.2)sinceitisobservedintheoriginalline+continuummaps.
§