A SOFIA FORCAST Grism Study of the Mineralogy of Dust in the Winds of Proto-planetary Nebulae: RV Tauri Stars and SRd Variables
R. A. Arneson, R. D. Gehrz, C. E. Woodward, L. A. Helton, D. Shenoy, A. Evans, L. D. Keller, K. H. Hinkle, M. Jura, T. Lebzelter, C. M. Lisse, M. T. Rushton, J. Mizrachi
DDraft version October 13, 2018
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A SOFIA FORCAST GRISM STUDY OF THE MINERALOGY OF DUST IN THE WINDS OFPROTO-PLANETARY NEBULAE: RV TAURI STARS AND SRd VARIABLES
R. A. Arneson, R. D. Gehrz, C. E. Woodward, L. A. Helton, D. Shenoy, A. Evans, L. D. Keller, K. H. Hinkle, M. Jura, ∗ T. Lebzelter, C. M. Lisse, M. T. Rushton, andJ. Mizrachi Minnesota Institute for Astrophysics, School of Physics and Astronomy, University of Minnesota, 106 Pleasant StreetS.E., Minneapolis, MN 55455, USA USRA-SOFIA Science Center, NASA Ames Research Center, Moffett Field, CA 94035, USA Astrophysics Group, Lennard Jones Laboratory, Keele University, Keele, Staffordshire ST5, 5BG, UK Department of Physics and Astronomy, 264 Center for Natural Sciences, Ithaca College, Ithaca, NY 14850, USA National Optical Astronomy Observatory, P.O. Box 26732, Tucson, AZ 85726, USA Department of Physics and Astronomy, University of California, Los Angeles, CA 90095, USA Institute for Astrophysics (IfA), University of Vienna, T¨urkenschanzstrasse 17, 1180 Vienna, Austria Solar System Exploration Branch, Space Department, Johns Hopkins University Applied Physics Laboratory, Laurel,MD 20723, USA Astronomical Institute of the Romanian Academy, Str. Cutitul de Argint 5, Bucharest, Romania, 040557 Biomedical Engineering Department, Stony Brook University, Stony Brook, NY 11794, USA (Received December 7, 2016; Revised May 25, 2017; Accepted May 26, 2017)
Submitted to ApJABSTRACTWe present a SOFIA FORCAST grism spectroscopic survey to examine the mineralogy of thecircumstellar dust in a sample of post-asymptotic giant branch yellow supergiants that are believedto be the precursors of planetary nebulae. Our mineralogical model of each star indicates the presenceof both carbon rich and oxygen rich dust species–contrary to simple dredge-up models–with a majorityof the dust in the form of amorphous carbon and graphite. The oxygen rich dust is primarily in theform of amorphous silicates. The spectra do not exhibit any prominent crystalline silicate emissionfeatures. For most of the systems, our analysis suggests that the grains are relatively large and haveundergone significant processing, supporting the hypothesis that the dust is confined to a Kepleriandisk and that we are viewing the heavily processed, central regions of the disk from a nearly face-onorientation. These results help to determine the physical properties of the post-AGB circumstellarenvironment and to constrain models of post-AGB mass loss and planetary nebula formation.
Keywords: astrochemistry — binaries: general — stars: AGB and post-AGB — stars:circumstellar matter — stars: evolution
Corresponding author: Ryan [email protected] ∗ Michael Jura died on 30 January, 2016 while this manuscript was being drafted. He participated in writing theproposals to gather the data, and was aware of the importance of the results at the time of his death. a r X i v : . [ a s t r o - ph . S R ] J un Arneson et al. INTRODUCTIONRV Tauri and yellow semi-regular (SRd) vari-ables are two classes of post-asymptotic giantbranch (post-AGB) stars that lie along the hor-izontal track on the Hertzsprung-Russell (H-R)diagram linking AGB stars to planetary nebu-lae (PNe). They are thought to be the imme-diate precursors of PNe and have been termed“proto-planetary nebulae” (PPNe) . RV Tauristars are characterized by semi-regular, bimodalvariability (possibly resulting from interactionwith a binary companion; Waelkens & Waters1993; Percy 1993; Fokin 1994), a high mass-lossrate, and often a prominent infrared (IR) excess.SRd variables are similar to RV Tauri stars inmany respects but are probably single star sys-tems, as indicated by the absence of regular pul-sations (Percy & Ursprung 2006).RV Tauri stars are a loosely defined subclassof Population II Cepheid variables named af-ter the prototype RV Tau. They are definedas luminous (I-II) mid-F to K supergiants witha typical mass of ∼ . (cid:12) (Tuchman et al.1993; Fokin 1994) that show alternating deepand shallow minima in their light curves (Pre-ston et al. 1963). They have formal periods(defined as the time between successive deepminimia) between 30 and 150 days, but cycle-to-cycle variability is common, and the ampli-tudes may reach up to 4 magnitudes in V (Percy1993). RV Tauri stars are divided into two pho-tometric classes (‘a’ and ‘b’) based on their lightcurves (Kukarkin 1958). The RVa class containsconstant mean magnitude stars, and the RVbclass contains stars that have a varying meanmagnitude with a period of 600 to 1500 days.There are several possible explanations for these The phrase “proto-planetary” is also widely usedby the exoplanetary and planet formation communitiesto refer to dusty disks around young stars. Also notethat in the literature the terms preplanetary or proto-planetary nebulae have been used interchangeably withthe term post-AGB objects. light variations. One explanation for the alter-nating minima is that there is a resonance be-tween the fundamental period and the first over-tone (Takeuti & Petersen 1983; Shenton et al.1992; Tuchman et al. 1993; Fokin 1994). An-other possibility is that the light variations aredue to a geometrical projection effect where thepulsating star is periodically obscured by a cir-cumbinary disk (Van Winckel et al. 1999; Maaset al. 2002).Preston et al. (1963) classified the RV Tauristars into three spectroscopic classes (‘A’, ‘B’,‘C’). RVA stars are spectral type G–K, and showstrong absorption lines and normal CN or CHbands while TiO bands sometimes appear atphotometric minima. RVB stars are generallyhotter spectral types, weaker lined, and showenhanced CN and CH bands. RVC stars are alsoweak lined but show normal CN and CH molec-ular bands. There is no correlation between thephotometric and spectroscopic classes.It has long been known (Gehrz & Woolf 1970;Gehrz 1972; Gehrz & Ney 1972) that some RVTauri stars (e.g. AC Her, U Mon, R Sct, R Sge)show very strong thermal IR emission from cir-cumstellar dust. Observations by the InfraredAstronomical Satellite (IRAS) confirmed theseprevious detections and increased the samplesize. IRAS detected considerable cool, circum-stellar dust around many of the RV Tauri stars,which has been interpreted as being due tostrong, dusty mass-loss during AGB evolution(Jura 1986). From CO observations, Alcolea& Bujarrabal (1991) suggest a mass loss rateof 4 × − to 7 × − M (cid:12) yr − within thelast 10 to 10 years for most of the RV Tauristars. Because of their position on the H-R di-agram, variability, high mass-loss rate and rar-ity (about 110 are known), it is generally be-lieved that RV Tauri stars represent a relativelyshort-term, unstable transitional phase betweenthe AGB and PNe phases of solar-mass stars.If RV Tauri stars are assumed to be evolving ust in the Winds of Proto-planetary Nebulae µ m range but a larger IR excessfor λ (cid:38) µ m (Bujarrabal et al. 1988; Alcolea& Bujarrabal 1991). The lack of a near-IR ex-cess and the presence of a large mid- and far-IR excess is evidence for a thick and extendeddust envelope that is relatively cool. The detec-tion of SiO around R Sct could be an indicationof on-going, weak mass loss (Bujarrabal et al.1989). Alcolea & Bujarrabal (1991) estimatethat ∼ CO and CO J = 2 → → CO J = 3 → → → → − x) Fe SiO ) and pyroxene(Mg − x Fe x SiO ) is formed. In carbon-rich gas,carbon rich dust particles such as SiC, amor- Arneson et al. phous carbon, and possible polycyclic aromatichydrocarbons (PAHs) are formed.The amount of crystalline grain material com-pared to amorphous grain material is gener-ally low, ∼ − M (cid:12) yr − )(Cami et al. 1998; Sylvester et al. 1999; So-gawa & Kozasa 1999; Suh 2002). However, bothcrystalline and amorphous grains have been de-tected simultaneously in stellar outflows, bothin the present study as well as in others. Gielenet al. (2008, 2009) showed that dust processingin circumstellar disk environments is conduciveto creating large, crystalline grains. As the diskis subjected to the hard radiation and stellarwind from the central source, the dust crystal-lization fraction increases (Gielen et al. 2011)and the disk dissipates (Kastner et al. 2004;Gezer et al. 2015; Kastner et al. 2016; Lisse et al.2017). Thus, the IR excess associated with thedusty disk diminishes as the system ages andtransitions to a PN.In this work we present 5–40 µ m IR spec-tra on a diverse sample of RV Tauri and SRdvariables from a grism spectroscopic study ofsuspected proto-planetary nebula precursorswith the Faint Object infraRed CAmera forthe SOFIA Telescope (FORCAST; Herter et al.2012) instrument on board the NASA Strato-spheric Observatory for Infrared Astronomy(SOFIA; Becklin et al. 2007; Gehrz et al. 2009;Young et al. 2012). With this rich data set, weproduce spectroscopic sampling of these objectsin the mid-IR. By modeling the emission we candetermine the source of the IR-excess, identifythe dust species present and quantify funda-mental dust properties, such as the grain sizedistribution and dust temperature. These pa-rameters help to determine the physical proper-ties of the post-AGB circumstellar environment and to constrain models of post-AGB mass lossand planetary nebula formation.In Section 2 we summarize the stars observedby our program. An overview of the observa-tions and data reduction strategies is given inSection 3. Section 4 contains the construction ofthe spectral energy distributions and the spec-tral decomposition model we used to measurethe mineralogy of the program stars. The re-sults of our model, a discussion of our resultsand our conclusions are presented in Sections5–7. PROGRAM STARSWe have selected a sample of RV Tauri and re-lated SRd stars based upon: 1) their availabil-ity for SOFIA flights from Palmdale, CA andChristchurch, NZ, 2) diversity of their IR spec-tral energy distributions (SEDs), 3) our abilityto obtain a signal-to-noise ratio compatible withour science objectives in a reasonable integra-tion time.The properties of the 18 RV Tauri and SRdvariables presented in this work are summarizedin Table 1. TX Per is sometimes categorized asan RV Tauri star and sometimes as an SRd vari-able. We concur with Percy & Coffey (2005),which refers to TX Per as being a “mild” RVTauri as the consecutive minima are very sim-ilar in depth, and categorize TX Per as an RVTauri variable.The mineralogy of many of these systems hasbeen studied previously (Molster et al. 2002c;Deroo et al. 2006; Gielen et al. 2007, 2011;Blommaert et al. 2014; Hillen et al. 2015).These studies have mostly focused on crystallinesilicates. Most of the studies found evidencefor large, crystalline dust grains indicative ofhighly processed material. Some of the studiessuggest that the crystalline and amorphous sil-icates are at different temperatures suggestingthat the two species are spatially separated andhave different formation histories. ust in the Winds of Proto-planetary Nebulae Table 1.
Properties of the RV Tauri and SRd Variables in this Survey
Name Type Spectral Type Period (d) a [Fe / H] PC c SC c SED d T eff (K) Binarity e Chemical Type f Ref.TW Cam RV F8IbG8Ib 87 -0.40 a A Disk 4800 1UY CMa RV G0 114 -0.50 a B 5500 2o Cen SRd G3Ia0 200 3RU Cen RV A7IbG2pe 65 -1.10 a B Disk 6000 Y 4, 5SX Cen RV F5G3/5Vp 33 -0.30 b B Disk 6250 Y 4, 5SU Gem RV F5M3 50 0.00 b A Disk 5250 6AC Her RV F2pIbK4e 75 -0.90 a B Disk 5900 Y O 7V441 Her SRd F2Ibe 70 Disk Y O 8, 9U Mon RV F8IbeK0pIb 91 -0.50 b A Disk 5000 Y O 1, 10CT Ori RV F9 136 -0.60 a B Disk 5500 10, 11TV Per SRd K0 358 12TX Per RV Gp(M2)K0e(M2) 78 -0.60 a A 4250 6AR Pup RV F0IF8I 76 0.40 b B Disk 6000 O 10, 13R Sge RV G0IbG8Ib 71 0.10 b A Disk 5100 13AI Sco RV G0K2 71 -0.30 b A Disk 5300 C? 2, 10R Sct RV G0IaeK2p(M3)Ibe 147 -0.20 a A Uncertain 4500 1RV Tau RV G2IaeM2Ia 79 -0.40 b A Disk 4500 C 1V Vul RV G4eK3(M2) 76 0.10 a A Disk 4500 2, 6 a Pulsation period in days b The estimated initial metallicity obtained via the Zn or S abundance (Gezer et al. 2015) c Photometric class (PC) and spectroscopic class (SC) d Spectral energy distribution classification from Gezer et al. (2015) e Y indicates confirmed binarity based on radial velocity measurements. Confirming binarity using this method is difficult because the photo-spheres of these variables have large amplitude radial pulsations. f Stellar chemical type from He et al. (2014) and references therein
References —(1) Giridhar et al. (2000); (2) Giridhar et al. (2005); (3) O’Connell (1961); (4) Maas et al. (2002); (5) Maas et al. (2005); (6) Rao& Giridhar (2014): (7) Giridhar et al. (1998); (8) Waters et al. (1993); (9) de Ruyter et al. (2006); (10) Kiss et al. (2007); (11) Gonzalez et al.(1997b); (12) Payne-Gaposchkin (1952); (13) Gonzalez et al. (1997a) OBSERVATIONS AND DATAREDUCTIONThe targets were observed with SOFIA dur-ing Guest Investigator (GI) Cycles 2, 3, and 4.Descriptions of the SOFIA Observatory and itsscience instrument (SI) suite have been given byBecklin et al. (2007), Gehrz et al. (2009), andYoung et al. (2012). All of the targets in oursurvey were observed using FORCAST.FORCAST is a dual-channel mid-IR cameraand spectrograph operating between 5–40 µ m.Each channel consists of a 256 ×
256 pixel ar- ray that yields a 3.4 (cid:48) × (cid:48) field-of-view with asquare plate scale of 0.768 (cid:48)(cid:48) , after distortion cor-rection. The Short Wave Camera (SWC) usesa Si:As blocked-impurity band (BIB) array op-timized for λ < µ m, while the Long WaveCamera’s (LWC) Si:Sb BIB array is optmizedfor λ > µ m. Observations can be madethrough either of the two channels individuallyor, by use of a dichroic mirror, with both chan-nels simultaneously across the entire range. Allof the observations presented in this work weretaken in the single channel, long-slit mode. Weutilized FORCAST’s suite of grisms, which pro- Arneson et al. vided low spectral resolution (R ≈ µ m range. The following grisms wereused for our observations: G1 covering 4.9–8.0 µ m, G3 covering 8.4–13.7 µ m, G5 covering 17.6–27.7 µ m, and G6 covering 28.7–37.1 µ m. All ofthe observations were taken in the “nod matchchop” mode (C2N) which used a chop throw of30 (cid:48)(cid:48) , a chop angle of either 0 ◦ or 30 ◦ , and nodithering.The data were reduced by the SOFIA Sci-ence Center using FORCAST Redux v1.5.0 andv1.2.0 pipeline versions (Clarke et al. 2015) andreleased to the authors as level 3 results. Westacked the spectra when there were multipleobservations of a given target. We did not useany of the data points between 9.19–10.0 µ m asthese are strongly affected by telluric ozone ab-sorption. The spectra were smoothed with a 3point un-weighted boxcar to emphasize spectralfeatures. SPECTRAL ENERGY DISTRIBUTIONSOF THE SURVEY OBJECTSWe present the IR SEDs of the survey objectsin Figure 1. As can be seen in Figure 2, all of theprogram stars except for TX Per were observedwith the G1, G3, and G5 grisms, and only 7of the 18 stars were also observed with the G6grism. TX Per was fainter than expected and wewere only able to obtain the G1 grism spectrum(4.9–8.0 µ m). We were unable to model thespectral energy distribution of TX Per with sucha limited wavelength range. The continuumnormalized SOFIA grism spectra are shown inFigure 2. We have also gathered archival broad-band IR photometry for comparison with theSOFIA spectra. Photometry from the Two Mi-cron All-Sky Survey (2MASS; Skrutskie et al.2006) at 1.25, 1.65, and 2.17 µ m, the AKARIsatellite (Murakami et al. 2007) at 9 and 18 µ m,the Wide-field Infrared Survey Explorer (WISE;Wright et al. 2010) at 3.4, 4.6, 12 and 22 µ m,and the Infrared Astronomical Satellite (IRAS;Neugebauer et al. 1984) at 12, 25, 60, and 100 µ m were all taken from the NASA/IPAC In-frared Science Archive (IRSA; Berriman 2008)database. Photometry from the Herschel (Pil-bratt 2003) Photoconductor Array Camera andSpectrometer (PACS; Poglitsch et al. 2010) at70, 100, and 160 µ m and Spectral and Photo-metric Imaging Receiver (SPIRE; Griffin et al.2010) at 250, 350, and 500 µ m were obtainedby using aperture photometry after sky back-ground subtraction. The archival photometrydata points were not used in any of the leastsquares fitting routines and are only plotted tovisualize the SED of each star.4.1. Dust Species
Previous studies have shown that the mostcommon dust species present in circumstellarenvironments are amorphous and crystalline sil-icates with olivine and pyroxene stoichiometries(Molster et al. 2002a,b,c; Gielen et al. 2011).Amorphous olivine has very prominent broadfeatures around 9.8 and 18 µ m. These featuresarise from the Si-O stretching and O-Si-O bend-ing modes. Amorphous pyroxene shows a 10 µ m feature similar to that of amorphous olivine,but shifted towards shorter wavelengths. Crys-talline forsterite has prominent emission fea-tures at 11.2, 23.7, and 33.7 µ m. Many of thestars in our sample are known to be oxygen rich(see Table 1) however an increased abundance ofcarbon dredged up as the star evolves is possible(Iben 1981; Chan & Kwok 1990). Models sug-gest that stars with main sequence masses lessthan 1.5 M (cid:12) do not experience third dredge-up. Therefore, the surface composition of thesestars is fixed by the first dredge-up and red gi-ant branch extra-mixing, and they are oxygenrich AGB stars. For main sequence stars withmasses over 6 M (cid:12) the stars undergo shallowthird dredge-up episodes. Stars in the inter-mediate main sequence mass range undergo re-peated third dredge-up episodes that bring car-bon to the surface to become carbon stars (for areview see Straniero et al. 2006; Karakas & Lat- ust in the Winds of Proto-planetary Nebulae years with carbonstars undergoing several pulses after the car-bon abundance first exceeds the oxygen abun-dance (Straniero et al. 1997). The interpulsetime exceeds the expansion time for circum-stellar shells and as expected nearly all normalAGB stars have shells of the same compositionas the star. Carbon rich AGB stars are expectedto have circumstellar shells dominated by amor-phous carbon or graphite grains with some sil-icon carbide (SiC) possibly present (Suh 2000;Speck et al. 2005, 2009). Amorphous carbondoes not have prominent IR features but con-tributes to the dust continuum emission, how-ever, graphitic carbon and silicon carbide haveemission features at 11.53 µ m and in the 10–13 µ m region, respectively. The strong deple-tion of iron in the photospheres of RV Tauristars suggests that metallic iron may be presentin the circumstellar environment. Iron grainscan form at temperatures 50–100 K lower thansilicates and are stable in O-rich environmentsabove 700 K (Kemper et al. 2002). Like amor-phous carbon, metallic iron lacks prominent IRfeatures and contributes to the overall dust con-tinuum. Therefore, the dust species we includedin our model are crystalline forsterite, amor-phous olivine, amorphous pyroxene, amorphouscarbon, silicon carbide, graphite, and metalliciron. We only considered the magnesium-richcrystalline species of olivine (forsterite) as ithas been found that the iron content of circum-stellar crystalline olivine around evolved starsis lower than 10% (Tielens et al. 1998; Mol-ster et al. 2002c). In addition, we only includedamorphous enstatite and pyroxene with an ironcontent of 0% as these species were found to bebetter fits to the spectra than the same specieswith an iron content of 50%. We tried includingMg-righ crystalline olivine, Mg-rich crystallineenstatite, crystalline bronzite, crystalline fay-alite, iron oxide, amorphous alumina, and amor- phous silica in our model, however none of thesedust species were significantly present. We dis-cuss the addition of some of these species morein section 6.4.Mass absorption coefficients for the differentdust species are calculated from optical con-stants using a homogeneous sphere approxima-tion (Min et al. 2005). Although the continuousdistribution of ellipsoids approximation (CDE;Bohren & Huffman 1983) is widely used, it isonly valid in the Rayleigh limit for small grainsizes. Because we are interested in the grainsize distribution we did not use the CDE ap-proximation. The details of the different opti-cal constants that we used can be found in Table2. In cases where the refractive index was re-ported for the three crystallographic directions,we assumed even distributions of each orienta-tion. There are many laboratory measurementsof IR optical constants available, correspond-ing to different material compositions, crystalstructures, annealing temperatures, measure-ment environments, grain sizes, and grain orien-tations. These different measurements producespectra with similar global features but withunique differences. While the minerals we havechosen to use in our model may result in differ-ent relative abundances compared to those de-rived in previous studies, we are less concernedwith making comparisons with those works andmore concerned with drawing comparisons be-tween the program stars in the present study.4.2. Spectral Decomposition Model
To identify the minerals present and to quan-tify the grain size distributions, we fit the ob-served SOFIA spectra with synthetic spectra ofvarious mineral species. The synthetic spec-tra were calculated from the optical constantsof each mineral. The conversion from labo-ratory measured optical constants of dust tomass absorption coefficients is not straightfor-ward. Several factors affect the observed emis-sion features, including the chemical composi-
Arneson et al.
Table 2.
Dust Species and Properties Used in this Work
Dust Species Composition Structure Density (g/cm ) Grain Size ( µ m) ReferenceForsterite Mg SiO C 3.27 0.1 Koike et al. (2003)Olivine Mg SiO A 3.71 0.1, 2.0 Dorschner et al. (1995)Pyroxene MgSiO A 3.20 0.1, 2.0 Dorschner et al. (1995)Carbon Pyrolized at 400 ◦ C A 1.435 0.1, 2.0 Jaeger et al. (1998b)Silicon Carbide α -SiC C 3.26 0.1, 2.0 Pegourie (1988)Graphite C 2.24 0.1, 2.0 Draine & Lee (1984)Metallic Iron Fe C 7.87 0.1, 2.0 Pollack et al. (1994) Note —The mineral structure is denoted as either amorphous (A) or crystalline (C). tion of the dust, the grain size, and the grainshape (Min et al. 2003, 2005). We constructeda basic model to fit the full FORCAST wave-length range. Given that we see the silicatefeatures in emission, we assume that the dustfeatures are in an optically thin part of the diskand, therefore, we approximate the spectrum asa linear combination of dust absorption profiles.The emission model is given by λF λ ∝ (cid:88) i c i µ i ( λ ) × (cid:88) j a j λB λ ( T j ) , (1)where µ i ( λ )(cm − ) is the absorption coeffi-cient of dust component i and c i gives the vol-ume fraction of that dust component, B λ ( T j )(W sr − m − ) denotes the Planck function attemperature T j and a j is the scaling factor forthe j th Planck function. The absorption coef-ficient is related to the mass absorption coef-ficient (opacity) by µ = κρ where κ (cm g − )is the mass absorption coefficient (opacity) and ρ (g cm − ) is the density of the dust component.The formulation of our model assumes that thestellar contribution to the SED in this range isnegligible. We further assume that all of thedust in a population is in thermal equilibriumwith all of the other dust species, regardlessof particle sizes or the ability to absorb andre-emit starlight. We first used a least squaresminimization to fit the Planck functions to theFORCAST continuum. We fit the functions to the entire FORCAST wavelength range avail-able except for the 8 − . µ m range, whichis dominated by silicate emission. Two Planckfunctions were used for all of the spectral mod-els except for o Cen and V Vul where onlya single Planck function was needed to fit theunderlying continuum. The best fitting Planckfunction parameters are shown in Figure 1 andsummarized in Table 3, however, the sum of thePlanck scaling factors, a j , have been normalizedto unity in order to represent the fraction ofdust at each temperature T j . The best fittingPlanck functions are plotted with the FOR-CAST spectra in Figure 1. It is probably notrealistic to model the dust as a single temper-ature component or even as a two-temperaturecomponent–a temperature gradient is probablymore realistic–but in order to keep the num-ber of parameters at a minimum we only use amaximum of two Planck functions in our model.After finding the best fitting Planck functions,the best fitting dust fraction coefficients, c i ,were found by a non-negative least squares min-imization of the model to the entire observedFORCAST wavelength range. The reduced χ of the spectrum is given by χ = N − M N (cid:80) i =1 (cid:12)(cid:12)(cid:12) F model ( λ i ) − F obs ( λ i ) σ i (cid:12)(cid:12)(cid:12) , (2)where N is the number of wavelength points, M the number of fit parameters, F model ( λ i ) is ust in the Winds of Proto-planetary Nebulae F obs ( λ i )is the observed flux at a given wavelength, and σ i the absolute error of the observed flux at eachwavelength λ i . The reduced χ values are sum-marized in Table 3.Errors on the dust fraction coefficients werecalculated from 5000 realizations of a MonteCarlo simulation with Gaussian noise distribu-tions. We omitted a mineral species from thefit if the error, σ c i , on c i is greater than thevalue of c i itself. Figure 4 illustrates the distri-butions of the best fitting dust fraction coeffi-cients, c i , and the covariance between the co-efficients (Foreman-Mackey 2016). Most of thecoefficients show little to no correlation with theexception of the carbon, graphite, and metalliciron species which are strongly anti-correlatedin most cases. Even though this model is onlyan approximation, it gives a good fit overall tothe observed spectra (see Figure 3). A full ra-diative transfer model that could account for atemperature gradient in the disk that may pro-duce a better fitting result is beyond the scopeof this work.4.3. Grain Size Distribution
To study the grain size distribution, we usedtwo dust grain sizes in our model with radiiof 0.1 and 2.0 µ m. These sizes were chosenbased on the work of Bouwman et al. (2001) andHonda et al. (2004) which found that, in the 10 µ m spectral region, 0.1 µ m grains sufficientlydescribe grains with a < µ m while 1 . − . µ m grains sufficiently describe grains with a > µ m. Larger sized grains were not consid-ered as the emission features from larger grainsbecome too weak to distinguish from the con-tinuum emission. Because the grains with radiiof 2.0 µ m are in the Mie scattering regime, weused the python module pymiecoated (Leinonen2012) to calculate the mass absorption coeffi-cients for the 2.0 µ m grains. pymiecoated com-putes the scattering properties of single- and dual-layered spheres in the Mie regime using theresults of Bohren & Huffman (1983) and the op-tical constants of bulk materials. The mass ab-sorption coefficient for crystalline forsterite wastaken directly from Koike et al. (2003). Becausewe did not have the optical constants for thismineral we were unable to calculate the massabsorption coefficient for larger sized grains inthe Mie scattering regime. RESULTSThe results of the spectral decompositionmodeling are shown in Figure 3. Details ofindividual sources are provided in Table 3. Thespectroscopic features present in these sourcesspan a broad range of dust properties and char-acteristics. All of the RV Tauri stars in our sam-ple, with the exceptions of UY CMa, TX Per,and R Sct have been reported as disk sources(Gezer et al. 2015). Two sources, o Cen andV Vul display a simple blackbody continuum.However, model fits demonstrate that they bothalso exhibit a weak IR excess (see Figure 1 (c)and (r)), suggesting that these systems may bein the final stages of dissipation. The remain-ing RV Tauri stars all exhibit emission fromcarbon-rich minerals with varying degrees ofamorphous and crystalline silicates.Two of our SRd sources, V441 Her and TVPer, show prominent silicate features. Thepresence of these features suggests that thesesources, too, may have dusty disks akin to thedisks of the RV Tauri stars. Alternatively, thesefeatures might also arise from normal dusty out-flows. In the case of V441 Her, the 10 µ m fea-ture is strong while the 20 µ m feature is weak, asis expected for relatively fresh and unprocessedamorphous silicates (Nuth & Hecht 1990). Incontrast, TV Per exhibits strong 10 and 20 µ msilicate emission, suggestive of prolonged expo-sure to hard radiation. We discuss this morefully in Section 6.0 Arneson et al.
Table 3 . RV Tauri and SRd Star Mineralogy
Star χ Mineral c i σ c i V f σ V f T (K) F T T (K) F T TW Cam 0.26 Graphite-small 3.75 0.15 0.58 2.6E-2 1365 ±
105 0.019 ± ± ± ±
201 0.010 ± ± ± Cen 0.32 Iron-small 7.01 0.48 0.49 3.8E-2 3780 ±
193 1.0 - -Graphite-small 5.96 0.49 0.42 3.7E-2Graphite-large 1.17 0.06 0.08 5.5E-3Pyroxene-small 0.17 0.03 0.01 2.1E-3RU Cen 0.23 Graphite-small 3.53 0.20 0.45 2.8E-2 535 ±
16 0.010 ± ± ± ±
14 0.042 ± ± ± ±
83 0.023 ± ± ± ± ± ± ± ±
68 0.019 ± ± ± Table 3 continued ust in the Winds of Proto-planetary Nebulae Table 3 (continued)
Star χ Mineral c i σ c i V f σ V f T (K) F T T (K) F T Olivine-small 0.44 0.08 0.06 1.0E-2Pyroxene-small 0.23 0.04 0.03 5.2E-3SiC-small 0.21 0.04 0.03 5.3E-3U Mon 0.50 Carbon-small 3.55 0.50 0.30 6.3E-2 772 ±
10 0.020 ± ± ± ±
13 0.041 ± ± ± ±
12 0.002 ± ± ± ± ± ± ± ±
29 0.026 ± ± ± ±
74 0.030 ± ± ± ±
44 0.017 ± ±
18 0.983 ± ±
10 0.052 ± ± ± Table 3 continued Arneson et al.
Table 3 (continued)
Star χ Mineral c i σ c i V f σ V f T (K) F T T (K) F T Carbon-small 3.21 0.21 0.37 2.5E-2Graphite-large 0.92 0.06 0.11 7.1E-3Pyroxene-small 0.47 0.03 0.05 3.3E-3SiC-small 0.38 0.03 0.04 3.9E-3V Vul 0.36 Iron-small 8.06 0.55 0.55 4.1E-2 678 ± Note — c i is the average best fit coefficient of a given mineral species. σ c i is the standard deviation of the average best fit coefficient, V f is the volume fraction, σ (V f ) is the error in the volume fraction, T , are the blackbody dust temperatures, and F T , is the fraction ofdust at those temperatures, respectively. ‘small’ refers to 0.1 µ m spherical grains, ‘large’ designates 2.0 µ m spherical grains. ust in the Winds of Proto-planetary Nebulae λ ( µm ) -14 -13 -12 -11 -10 λ F λ ( W m ) TW Cam B λ (1365 K) B λ (332 K)Total Continuum (a) TW Cam λ ( µm ) -14 -13 -12 -11 -10 λ F λ ( W m ) UY CMa B λ (1251 K) B λ (337 K)Total Continuum (b) UY CMa λ ( µm ) -14 -13 -12 -11 -10 λ F λ ( W m ) o Cen B λ (3780 K) (c) o Cen λ ( µm ) -14 -13 -12 -11 -10 λ F λ ( W m ) RU Cen B λ (535 K) B λ (203 K)Total Continuum (d) RU Cen Arneson et al. λ ( µm ) -14 -13 -12 -11 -10 λ F λ ( W m ) SX Cen B λ (715 K) B λ (244 K)Total Continuum (e) SX Cen λ ( µm ) -14 -13 -12 -11 -10 λ F λ ( W m ) SU Gem B λ (1444 K) B λ (349 K)Total Continuum (f) SU Gem λ ( µm ) -14 -13 -12 -11 -10 λ F λ ( W m ) AC Her B λ (489 K) B λ (207 K)Total Continuum (g) AC Her λ ( µm ) -14 -13 -12 -11 -10 λ F λ ( W m ) V441 Her B λ (1607 K) B λ (363 K)Total Continuum (h) V441 Her ust in the Winds of Proto-planetary Nebulae λ ( µm ) -14 -13 -12 -11 -10 λ F λ ( W m ) U Mon B λ (772 K) B λ (254 K)Total Continuum (i) U Mon λ ( µm ) -14 -13 -12 -11 -10 λ F λ ( W m ) CT Ori B λ (684 K) B λ (258 K)Total Continuum (j) CT Ori λ ( µm ) -14 -13 -12 -11 -10 λ F λ ( W m ) TV Per B λ (2340 K) B λ (297 K)Total Continuum (k) TV Per λ ( µm ) -14 -13 -12 -11 -10 λ F λ ( W m ) TX Per B λ (1534 K) (l) TX Per Arneson et al. λ ( µm ) -14 -13 -12 -11 -10 λ F λ ( W m ) AR Pup B λ (734 K) B λ (280 K)Total Continuum (m) AR Pup λ ( µm ) -14 -13 -12 -11 -10 λ F λ ( W m ) R Sge B λ (862 K) B λ (270 K)Total Continuum (n) R Sge λ ( µm ) -14 -13 -12 -11 -10 λ F λ ( W m ) AI Sco B λ (1049 K) B λ (322 K)Total Continuum (o) AI Sco λ ( µm ) -14 -13 -12 -11 -10 λ F λ ( W m ) R Sct B λ (1857 K) B λ (158 K)Total Continuum (p) R Sct ust in the Winds of Proto-planetary Nebulae λ ( µm ) -14 -13 -12 -11 -10 λ F λ ( W m ) RV Tau B λ (760 K) B λ (256 K)Total Continuum (q) RV Tau λ ( µm ) -14 -13 -12 -11 -10 λ F λ ( W m ) V Vul B λ (678 K) (r) V Vul Figure 1.
The observed SOFIA FORCAST spectrum (green curve) of our sample of stars is plottedtogether with the archival photometry from 2MASS (blue pentagons), MSX (blue triangles), AKARI (lightblue squares), WISE (yellow circles), IRAS (orange crosses), Herschel (red diamonds), and the best fittingPlanck functions (black dotted and dashed curves). WISE photometry upper limits are depicted as orangedownward arrows. Arneson et al. λ ( µm ) C o n t i nuu m N o r m a li ze d λ F λ ( W m ) TW Cam (a) TW Cam λ ( µm ) C o n t i nuu m N o r m a li ze d λ F λ ( W m ) UY CMa (b) UY CMa λ ( µm ) C o n t i nuu m N o r m a li ze d λ F λ ( W m ) o Cen (c) o Cen λ ( µm ) C o n t i nuu m N o r m a li ze d λ F λ ( W m ) RU Cen (d) RU Cen ust in the Winds of Proto-planetary Nebulae λ ( µm ) C o n t i nuu m N o r m a li ze d λ F λ ( W m ) SX Cen (e) SX Cen λ ( µm ) C o n t i nuu m N o r m a li ze d λ F λ ( W m ) SU Gem (f) SU Gem λ ( µm ) C o n t i nuu m N o r m a li ze d λ F λ ( W m ) AC Her (g) AC Her λ ( µm ) C o n t i nuu m N o r m a li ze d λ F λ ( W m ) V441 Her (h) V441 Her Arneson et al. λ ( µm ) C o n t i nuu m N o r m a li ze d λ F λ ( W m ) U Mon (i) U Mon λ ( µm ) C o n t i nuu m N o r m a li ze d λ F λ ( W m ) CT Ori (j) CT Ori λ ( µm ) C o n t i nuu m N o r m a li ze d λ F λ ( W m ) TV Per (k) TV Per λ ( µm ) C o n t i nuu m N o r m a li ze d λ F λ ( W m ) TX Per (l) TX Per ust in the Winds of Proto-planetary Nebulae λ ( µm ) C o n t i nuu m N o r m a li ze d λ F λ ( W m ) AR Pup (m) AR Pup λ ( µm ) C o n t i nuu m N o r m a li ze d λ F λ ( W m ) R Sge (n) R Sge λ ( µm ) C o n t i nuu m N o r m a li ze d λ F λ ( W m ) AI Sco (o) AI Sco λ ( µm ) C o n t i nuu m N o r m a li ze d λ F λ ( W m ) R Sct (p) R Sct Arneson et al. λ ( µm ) C o n t i nuu m N o r m a li ze d λ F λ ( W m ) RV Tau (q) RV Tau λ ( µm ) C o n t i nuu m N o r m a li ze d λ F λ ( W m ) V Vul (r) V Vul
Figure 2.
Continuum normalized SOFIA FORCAST spectrum (green curve) after dividing by the bestfitting continuum of our sample of stars showing the WISE archival photometry (yellow points) and telluricozone region (gray band). ust in the Winds of Proto-planetary Nebulae λ F λ ( W m )
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TW Cam
Pyroxene − smallCarbon − largeSiC − smallGraphite − smallGraphite − largeTotalFORCAST λ ( µm ) R e s i du a l (a) TW Cam λ F λ ( W m )
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UY CMa
Pyroxene − smallCarbon − largeGraphite − smallGraphite − largeForsterite − smallTotalFORCAST λ ( µm ) R e s i du a l (b) UY CMa λ F λ ( W m )
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Pyroxene − smallGraphite − smallGraphite − largeIron − smallTotalFORCAST λ ( µm ) R e s i du a l (c) o Cen λ F λ ( W m )
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RU Cen
Pyroxene − smallCarbon − largeSiC − smallGraphite − smallGraphite − largeForsterite − smallTotalFORCAST λ ( µm ) R e s i du a l (d) RU Cen Arneson et al. λ F λ ( W m )
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SX Cen
Olivine − largePyroxene − smallPyroxene − largeCarbon − largeGraphite − smallGraphite − largeTotalFORCAST λ ( µm ) R e s i du a l (e) SX Cen λ F λ ( W m )
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SU Gem
Pyroxene − smallCarbon − largeSiC − smallGraphite − smallGraphite − largeTotalFORCAST λ ( µm ) R e s i du a l (f) SU Gem λ F λ ( W m )
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AC Her
Pyroxene − smallCarbon − largeSiC − smallGraphite − smallGraphite − largeForsterite − smallTotalFORCAST λ ( µm ) R e s i du a l (g) AC Her λ F λ ( W m )
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V441 Her
Olivine − smallPyroxene − smallCarbon − smallSiC − smallGraphite − smallGraphite − largeTotalFORCAST λ ( µm ) R e s i du a l (h) V441 Her ust in the Winds of Proto-planetary Nebulae λ F λ ( W m )
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U Mon
Olivine − smallPyroxene − smallCarbon − smallSiC − smallGraphite − smallGraphite − largeIron − largeTotalFORCAST λ ( µm ) R e s i du a l (i) U Mon λ F λ ( W m )
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CT Ori
Pyroxene − smallCarbon − largeSiC − smallGraphite − smallGraphite − largeTotalFORCAST λ ( µm ) R e s i du a l (j) CT Ori λ F λ ( W m )
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TV Per
Olivine − smallPyroxene − smallCarbon − smallSiC − smallGraphite − smallGraphite − largeIron − largeTotalFORCAST λ ( µm ) R e s i du a l (k) TV Per λ F λ ( W m )
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AR Pup
Olivine − largePyroxene − smallCarbon − smallGraphite − smallGraphite − largeTotalFORCAST λ ( µm ) R e s i du a l (l) AR Pup Arneson et al. λ F λ ( W m )
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R Sge
Pyroxene − smallCarbon − smallSiC − smallGraphite − smallGraphite − largeTotalFORCAST λ ( µm ) R e s i du a l (m) R Sge λ F λ ( W m )
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AI Sco
Pyroxene − smallCarbon − largeSiC − smallGraphite − smallGraphite − largeTotalFORCAST λ ( µm ) R e s i du a l (n) AI Sco λ F λ ( W m )
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R Sct
Pyroxene − smallCarbon − largeGraphite − smallGraphite − largeTotalFORCAST λ ( µm ) R e s i du a l (o) R Sct λ F λ ( W m )
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RV Tau
Pyroxene − smallCarbon − smallSiC − smallGraphite − smallGraphite − largeTotalFORCAST λ ( µm ) R e s i du a l (p) RV Tau ust in the Winds of Proto-planetary Nebulae λ F λ ( W m )
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V Vul
Pyroxene − smallPyroxene − largeSiC − largeGraphite − smallGraphite − largeIron − smallTotalFORCAST λ ( µm ) R e s i du a l (q) V Vul Figure 3.
Best model fits for our sample of stars, showing the contribution of the different mineral species.
Top : the observed SOFIA FORCAST spectra and 1 σ errors (black points) are plotted together with the bestmodel fit (red curve) and the mineral species (colored curves). The data points between 9.19–10.0 µ m havebeen removed as these are strongly affected by telluric ozone absorption. Bottom : the normalized residualspectra after dividing by the best model of the observed spectra. Arneson et al. DISCUSSIONThe 10 and 20 µ m emission features can beused to quantify the grain size and age of thecircumstellar dust (van Boekel et al. 2003, 2005;Juh´asz et al. 2010). The peak-to-continuumratio of the 10 µ m feature can be used as ameasure of the amount of grain growth be-cause larger grains will produce a less promi-nent feature. In addition, the continuum sub-tracted 10/20 µ m flux ratio has been shown todecrease monotonically with increased process-ing and therefore can be used to indicate theage of the circumstellar silicates (Nuth & Hecht1990). Older, more processed grains will havea lower 10/20 µ m ratio. A plot of these tworatios for all our program stars can be seen inFigure 5. Most of the sources show a low peakto continuum value (i.e. < .
0) and a low 10/20 µ m ratio (i.e. <
50) indicating that the grainsare relatively large and have undergone signifi-cant processing. This supports the idea that thedust is constrained to a Keplerian disk. Thereare two outliers in Figure 5, TV Per and UYCMa. TV Per has a high peak to continuumvalue and a small 10/20 µ m ratio indicatingthat the dust grains are small and old. Thesmall grain size is consistent with our model,which predicts a small spherical grain volumefraction of ∼
74% for TV Per. This suggeststhat the circumstellar environment around TVPer is such that the grains are unable to growto large sizes. UY CMa has a low peak to con-tinuum value and a large 10/20 µ m ratio indi-cating that the dust grains are both large andyoung. This is also consistent with our modelwhich predicts a small grain volume fraction of ∼
50% around UY CMa. Robinson & Hyland(1977) and Mitchell & Robinson (1981) foundthat for low optical depths, some of the circum-stellar silicate dust may be in absorption ratherthan emission if it is at a low temperature. Theviewing angle of the disks will also affect thepeak-to-continuum ratio of the 10 µ m feature (Crapsi et al. 2008) As mentioned in Section4.2, factors like the grain size and shape will af-fect the observed emission features. The opticaldepth, viewing angle, temperature, and parti-cle size of the grains may mask the 10/20 µ mratio and could contribute to the high volumefraction of small grains found by our model.6.1. Crystallinity
Although Molster et al. (2002a,b) showed thatthe silicate crystallinity fraction in disk sourceswas much higher than that observed in out-flow sources, we do not see a high silicate crys-tallinity fraction in any of the objects in oursample listed as “disk” SEDs in Table 1 and forall of the FORCAST spectra there are no obvi-ous crystalline emission features present. Oneexplanation is that crystalline silicate materialis not abundant in any of the stars observed.However, crystalline olivines have been detectedaround AC Her and are thought to be present atthe ∼ −
50% level around AR Pup and U Mon(Blommaert et al. 2014; de Ruyter et al. 2005).The lack of strong crystalline silicate emissionfeatures does not necessarily indicate a lack ofpresence. If a temperature difference exists be-tween the amorphous and crystalline silicates,it is possible to include up to 40% of crystallinesilicates in the circumstellar dust without seeingcrystalline features in the spectra (Kemper et al.2001). Another possible explanation for thisobservation is that crystalline silicates are gen-erally colder than amorphous silicates, whichcould mean that the grains are not co-spatialor that they have different optical properties.In fact, when modeling the circumstellar mate-rial around AC Her, Hillen et al. (2015) foundthe spatial distribution of the forsterite to bedifferent from the amorphous dust. The differ-ence in optical properties could be due in partto the different iron content of each material,which increases the opacity in the near-IR sig-nificantly (Molster et al. 2002c; Dorschner et al.1995). Additionally, the spectral features will ust in the Winds of Proto-planetary Nebulae (a) TW Cam (b) UY CMa(c) o Cen (d) RU Cen Arneson et al. (e) SX Cen (f) SU Gem(g) AC Her (h) V441 Her ust in the Winds of Proto-planetary Nebulae (i) U Mon (j) CT Ori(k) TV Per (l) AR Pup Arneson et al. (m) R Sge (n) AI Sco(o) R Sct (p) RV Tau ust in the Winds of Proto-planetary Nebulae (q) V Vul Figure 4.
Normalized probability distribution functions of the best fit coefficients, c i , of each mineral speciesafter 5000 realizations of a Monte Carlo simulation with Gaussian noise distributions and the covariancebetween the coefficients. The dashed lines show the mean ( c i ) and the 1 σ ( σ c i ) confidence levels. Thecontours show the 1 σ confidence levels. be less prominent if the crystalline grains arelarger than the amorphous grains. Therefore,if the crystalline silicates are only moderatelyabundant (i.e. (cid:46) ∼
600 K) aroundthese two stars. If they are indeed hot, thanthe crystalline olivine abundances must be rel-atively low or the grains must be large aroundthese two stars for them to go undetected byFORCAST. 6.2.
Dual Chemistry
Our model predicts that most of the dustis carbon rich with some oxygen rich silicates. This dual formation of carbon and oxygen richminerals has been observed in several classicalnovae, namely V1280 Sco (Sakon et al. 2016),V705 Cas (Evans et al. 2005), V842 Cen (Smithet al. 1994), and QV Vul (Gehrz et al. 1992) aswell as IRAS 09425-6040, a carbon AGB starwhich shows circumstellar silicate dust features(Suh 2016). The formation of both carbon richand oxygen rich dust could be due to a chemi-cal gradient in the wind as the stars evolve fromoxygen rich to carbon rich after undergoing Cdredge-up processes due to a recent AGB ther-mal pulse. Suh (2016) successfully modeled thedust envelope around IRAS 09425-6040 with anouter oxygen rich shell and an inner carbon richshell, validating this hypothesis. Similarly, thecarbon-rich planetary nebula BD +30 ◦ Arneson et al.
Peak to Continuum C o n t i nuu m Sub t r a c t e d . / . UY CMa TV PerTW Camo Cen RU CenSX CenSU Gem AC HerV441 Her U MonCT OriAR Pup R SgeAI ScoR Sct RV TauV Vul
Figure 5.
Ratio of the continuum subtracted flux at 10 and 20 µ m versus the peak to continuum ratio of the 10 µ msilicate feature. AGB binary HR 4049 is a peculiar example of adepleted oxygen rich star with a featureless mid-IR spectrum possibly resulting from amorphouscarbon masking the spectral features from sili-cates (Acke et al. 2013). While it is possible wemay be observing the stars in transition fromoxygen rich to carbon rich, it would require thatall of these stars result from a narrow range ofmasses that terminate AGB evolution just asthe carbon exceeds the oxygen abundance. Amore plausible explanation for the dual chem-istry mineralogy is that the dust formed in acommon envelope environment of a binary sys-tem where the carbon and oxygen abundancescan rapidly change. This mechanism has beeninvoked as the possible origin of post-AGB disks(Kashi & Soker 2011; L¨u et al. 2013; Hardy et al.2016). 6.3.
Viewing Effects
Most of the FORCAST continua are well de-scribed by two Planck functions, suggesting thatwe are viewing the systems from a nearly face-on orientation and see both the inner ( ∼ ∼
250 K) regions of the disks.Our results are corroborated by Hillen et al.(2015) who used a radiative transfer code tomodel the dust around AC Her as a highlyevolved (i.e. mm sized grains), circumstellardisk with an inclination of 50 ± ◦ and foundgood agreement with observations. Bujarrabalet al. (2007) detected an extended bipolar out-flow and an unresolved, compact (presumablydisk) component around V441 Her with an in-clination of ∼ ◦ . The typical uniform diskdiameter of the N band emission region of theRV Tauri stars interferometrically observed byHillen et al. (2017) is ∼
40 mas. making it diffi- ust in the Winds of Proto-planetary Nebulae
Limitations of the Fit
It is worth mentioning the various difficul-ties encountered in the modeling of the dustspecies present around our program stars andthe limitations of our simplified model. As men-tioned in Section 3, less than half of the pro-gram stars were observed with the G6 grism(28.7–37.1 µ m). The addition of this data wouldhave aided the modeling and identification ofthe minerals as crystalline silicates have promi-nent emission features in this region. Simi-larly, the lack of coverage from 14–17 µ m be-tween grisms G3 and G5 made it more difficultto constrain the abundance of amorphous sili-cates which have emission features around 17 µ m. Amorphous carbon and metallic iron, onthe other hand, lack prominent IR features andour model could be fitting these species to theoverall dust continuum or temperature gradientthereby increasing the relative abundances.Because our model included 13 dust speciesand some of the program stars had relativelylow signal-to-noise FORCAST spectra, it wasdifficult to confirm the uniqueness of our mod-els. For spectra with high signal-to-noise ra-tios, excluding dust species from the model hada noticeably negative impact on the goodnessof fit (see Figure 6). Whereas, for low signal-to-noise spectra the exclusion of dust specieschanged the overall shape of the fit but the χ values changed very little (see Figure 7).For both high and low signal-to-noise spec-tra, adding additional dust species improves thegoodness of fit very little, if at all. We checkedthis by including Mg-rich crystalline enstatite(Jaeger et al. 1998a), iron oxide (FeO; Henninget al. 1995), amorphous alumina (porous; Bege-mann et al. 1997), and amorphous silica (Hen-ning & Mutschke 1997) to our model one at atime. The χ values either remained the sameor marginally improved for all of the program stars. The volume fraction of the added dustspecies depended on the signal-to-noise ratio ofthe spectra, with lower signal-to-noise spectragenerally including 1-4% by volume and highsignal-to-noise spectra including ≤
1% by vol-ume. Figure 8 illustrates the effect of adding ad-ditional dust species to AC Her (high signal-to-noise spectra) and Figure 9 illustrates the effectof adding additional dust species to UY CMa(low signal-to-noise spectra).6.5.
Crystallinity of the ISM
An upper limit on the degree of crystallinity ofsilicates in the diffuse ISM has been estimatedby Kemper et al. (2004) to be 0 . ± .
2% bymass. This estimate is similar to the averagecrystalline silicate fraction we find for our pro-gram stars of 0 . ± .
05% by volume. We con-cur with Kemper et al. (2004) that this suggestscrystalline material is either diluted in the ISMby other amorphous grain producing processessuch as supernovae, or there is an amorphiza-tion process that occurs in the ISM on a shortertimescale than the destruction timescale, possi-bly heavy ion bombardment. Our mineralogymodel of each star predicts that the majority ofthe dust is in the form of graphite and amor-phous carbon. On average our model predicts80% ±
1% graphite and amorphous carbon, and57% ±
1% graphite around each star by volume.This large volume fraction of graphite and car-bon around post-AGB stars may help explainthe 2175 ˚A bump observed in the interstellar ex-tinction curve which is possibly due to these twospecies (Rouleau et al. 1997; Duley & Seahra1998; Bradley et al. 2005). CONCLUSIONWe have presented a first look at data ob-tained with SOFIA FORCAST of 15 RV Tauriand 3 SRd variable stars. These data havedemonstrated the diversity of dust featurespresent in these systems, possibly tracing theevolutionary track from post-AGB star to PN.6
Arneson et al. λ F λ ( W m )
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AC Her
Carbon − largeSiC − smallGraphite − smallGraphite − largeForsterite − smallTotalFORCAST λ ( µm ) R e s i du a l (a) Pyroxene-small removed; χ = 9 . λ F λ ( W m )
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AC Her
Pyroxene − smallSiC − smallGraphite − smallGraphite − largeForsterite − smallTotalFORCAST λ ( µm ) R e s i du a l (b) Carbon-large removed; χ = 5 . λ F λ ( W m )
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AC Her
Pyroxene − smallCarbon − largeGraphite − smallGraphite − largeForsterite − smallTotalFORCAST λ ( µm ) R e s i du a l (c) SiC-small removed; χ = 2 . λ F λ ( W m )
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AC Her
Pyroxene − smallCarbon − largeSiC − smallGraphite − largeForsterite − smallTotalFORCAST λ ( µm ) R e s i du a l (d) Graphite-small removed; χ = 10 . ust in the Winds of Proto-planetary Nebulae λ F λ ( W m )
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AC Her
Pyroxene − smallCarbon − largeSiC − smallGraphite − smallForsterite − smallTotalFORCAST λ ( µm ) R e s i du a l (e) Graphite-large removed; χ = 5 . λ F λ ( W m )
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AC Her
Pyroxene − smallCarbon − largeSiC − smallGraphite − smallGraphite − largeTotalFORCAST λ ( µm ) R e s i du a l (f) Forsterite removed; χ = 1 . Figure 6.
The effect on the fit when removing dust species from the best model of AC Her.
These observations of IR excess support the hy-pothesis that the systems in question are at anadvanced stage in their transition to PNe. Ourmain conclusions can be summarized as follows:– Almost all of the stars observed display a10 µ m and/or 20 µ m emission feature. Formost of the stars observed, the FORCASTcontinua are well described by two Planckfunctions one at ∼ ∼
250 K with a majority of the dust ( ∼ Cenand V Vul, indicating that these systemsmay be in the final stages of disk dissipa-tion.– Our mineralogy model indicates the pres-ence of both carbon rich and oxygen richdust species with a majority of the dust,80% ±
1% by volume on average, in theform of amorphous carbon and graphite.All of the stars display this dual chemistrycircumstellar dust. This requires that ei- ther these stars result from a narrow rangeof masses that terminate AGB evolutionjust as the carbon exceeds the oxygenabundance or the the formation processis not single star evolution. We speculatethe formation process is common envelopeevolution.– The spectra do not exhibit any obvi-ous crystalline emission features and ourmodel only predicts UY CMa, RU Cen,and AC Her to have crystalline forsteriteat volume fractions of 4% ± . ± . ± . < .
0) and a low10/20 µ m ratio (i.e. <
50) indicating thatthe grains are relatively large and have un-dergone significant processing, support-ing the hypothesis that the dust is con-strained to a Keplerian disk and that weare viewing the heavily processed, central8
Arneson et al. λ F λ ( W m )
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UY CMa
Carbon − largeGraphite − smallGraphite − largeForsterite − smallTotalFORCAST λ ( µm ) R e s i du a l (a) Pyroxene-small removed; χ = 0 . λ F λ ( W m )
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UY CMa
Pyroxene − smallGraphite − smallGraphite − largeForsterite − smallTotalFORCAST λ ( µm ) R e s i du a l (b) Carbon-large removed; χ = 0 . λ F λ ( W m )
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UY CMa
Pyroxene − smallCarbon − largeGraphite − largeForsterite − smallTotalFORCAST λ ( µm ) R e s i du a l (c) Graphite-small removed; χ = 0 . λ F λ ( W m )
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UY CMa
Pyroxene − smallCarbon − largeGraphite − smallForsterite − smallTotalFORCAST λ ( µm ) R e s i du a l (d) Graphite-large removed; χ = 0 . ust in the Winds of Proto-planetary Nebulae λ F λ ( W m )
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UY CMa
Pyroxene − smallCarbon − largeGraphite − smallGraphite − largeTotalFORCAST λ ( µm ) R e s i du a l (e) Forsterite removed; χ = 0 . Figure 7.
The effect on the fit when removing dust species from the best model of UY CMa. regions of the disk from a nearly face-onorientation.– The average composition of the SRd vari-ables contains 8% more small carbon dustand less graphite (14% less of the smallspecies and 5% less of the large) thanthe average composition of the RV Tauristars. Of the three SRd variables mod-eled in this work, none of them containedthe large carbon species–on average theRV Tauri stars contained 13% by vol-ume. Overall the average volume fractionof large grains for the SRd variables was16% compared to 30% for the RV Tauristars. The paucity of large grains aroundany of the SRd variables supports the hy- pothesis that these stars are single starsystems.– Between the featureless IR dust species,amorphous carbon is included in more ofour models (16 out 17) than metallic iron(4 out of 17).The observations were made with the NASA/DLRStratospheric Observatory for Infrared Astron-omy (SOFIA) which is jointly operated by theUniversities Space Research Association, Inc.(USRA), under NASA contract NAS2-97001,and the Deutsches SOFIA Institut (DSI) underDLR contract 50 OK 0901 to the University ofStuttgart.
Facilities:
SOFIA (FORCAST)
Software: pymiecoated (Leinonen 2012)REFERENCES
Acke, B., Degroote, P., Lombaert, R., et al. 2013,A&A, 551, A76 Alcolea, J., & Bujarrabal, V. 1991, A&A, 245, 499 Arneson et al. λ F λ ( W m )
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AC Her
Pyroxene − smallCarbon − largeSiC − smallGraphite − smallGraphite − largeForsterite − smallEnstatite − smallTotalFORCAST λ ( µm ) R e s i du a l (a) Enstatite-small added; χ = 0 . λ F λ ( W m )
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AC Her
Pyroxene − smallCarbon − largeSiC − smallGraphite − smallGraphite − largeForsterite − smallFeO − smallTotalFORCAST λ ( µm ) R e s i du a l (b) FeO-small added; χ = 0 . λ F λ ( W m )
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AC Her
Pyroxene − smallCarbon − largeSiC − smallGraphite − smallGraphite − largeForsterite − smallAlumina − smallTotalFORCAST λ ( µm ) R e s i du a l (c) Alumina-small added; χ = 0 . λ F λ ( W m )
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AC Her
Pyroxene − smallCarbon − largeSiC − smallGraphite − smallGraphite − largeForsterite − smallSilica − smallTotalFORCAST λ ( µm ) R e s i du a l (d) Silica-small added; χ = 0 . Figure 8.
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