Draft version June 29, 2021 Typeset using LATEX twocolumn style in AASTeX62

SOFIA/FORCAST Observations of the Candidates MN 90 and HD 168625

Ryan A. Arneson,1 Dinesh Shenoy,1 Nathan Smith,2 and Robert D. Gehrz1

1Minnesota Institute for Astrophysics, School of Physics and Astronomy, University of Minnesota, 116 Church Street S.E., Minneapolis, MN 55455, USA 2Steward Observatory, University of Arizona, 933 North Cherry Avenue, Rm 336, Tucson, AZ 85721, USA

(Received December 13, 2017; Accepted July 19, 2018) Submitted to ApJ

ABSTRACT We present SOFIA/FORCAST imaging of the circumstellar dust shells surrounding the luminous blue variable (LBV) candidates MN 90 and HD 168625 to quantify the mineral abundances of the dust and to constrain the evolutionary state of these objects. Our image at 37.1 µm of MN 90 shows a limb-brightened, spherical dust shell. A least-squares fit to the spectral energy distribution of MN 90 yields a dust temperature of 59 ± 10 K, with the peak of the emission at 42.7 µm. Using 2-Dust 7 radiative transfer code, we estimate for MN 90 that mass-loss occurred at a rate of (7.3 ± 0.4) × 10− 1 1 2 M yr− × (vexp/50 km s− ) to create a dust shell with a dust mass of (3.2 ± 0.1) × 10− M . Our

images between 7.7 – 37.1 µm of HD 168625 complement previously obtained mid-IR imaging of its bipolar nebulae. The SOFIA/FORCAST imaging of HD 168625 shows evidence for the limb-brightened peaks of an equatorial torus. We estimate a dust temperature of 170 ± 40 K for the equatorial dust surrounding HD 168625, with the peak of the emission at 18.3 µm. Our 2-Dust model for HD 168625 7 1 estimates that mass-loss occurred at a rate of (3.2 ± 0.2) × 10− M yr− to create a dust torus/shell 3 with a dust mass of (2.5 ± 0.1) × 10− M .

Keywords: stars: massive — stars: mass-loss — stars: individual: HD168625 — stars: individual: MN90 — stars: circumstellar matter

1. INTRODUCTION Dor variability, with amplitudes of 1 – 2 mag occuring It is widely recognized that luminous blue variables on timescales of to decades with mass-loss rates of 5 4 1 ∼ 10− −10− M yr− . In addition, some LBVs exhibit (LBVs) represent a post-main sequence phase in which giant eruptions, η Car being the most famous example massive stars (initial mass Mi ≥ 20 M ; Langer et al.

1994) lose a considerable amount of mass via giant erup- (Humphreys et al. 1999). These giant eruptions are re- tions and minor outbursts. From the expansion ve- sponsible for producing circumstellar nebulae with sizes locities of known LBV nebulae, a dynamical age of a up to 1 – 2 pc and expansion velocities anywhere from 10 1 1 few 104 years is usually inferred, which points to a km s− to several hundred km s− for most LBVs (Weis very short-lived evolutionary phase – only about 40 are 2011) that are then shaped by wind-wind interactions arXiv:1807.08845v1 [astro-ph.SR] 23 Jul 2018 known (Clark et al. 2005; Weis 2011; Naz´eet al. 2012). (van Marle et al. 2007). In rare cases, very fast speeds 1 Although the category is still not unambiguously de- have been seen, reaching as high as 6000 km s− in the fined, these objects generally exhibit a high case of η Car (Smith 2008). These nebulae can have var- (≥ 105.5 L ), low amplitude photometric variability (∼ ious morphologies; Weis(2011) estimates 50% are bipo-

0.1 mag) on timescales ranging from weeks to months, lar, 40% spherical, and 10% are irregular. The bipolar and a larger, irregular photometric variability, called S nebulae may be formed by density gradients in the wind (Frank et al. 1995), different mass-loss episodes in which the wind changes from equatorial to polar during the Corresponding author: Ryan Arneson bistability jump (Smith et al. 2004), or the rotation of [email protected] the star (Dwarkadas & Owocki 2002; Smith & Townsend 2007). Thus, the morphology of the discloses the 2 Arneson et al. mass-loss history and evolutionary stage of the central vey of a sample of known and candidate LBVs. Mizuno star and the circumstellar environment. et al.(2010) list MN 90 as MGE029.5086-00.2090, but In this work we present 5 – 40 µm mid-IR observations other than reporting a MIPSGAL flux at 24 µm, there with the Faint Object infraRed CAmera for the SOFIA is nothing more specific about the central star. We are Telescope (FORCAST; Herter et al. 2012) instrument unaware of any other imaging or spectroscopy of MN on board the NASA Stratospheric Observatory for In- 90 that may have been obtained. Hereafter we will use frared Astronomy (SOFIA; Becklin et al. 2007; Gehrz “MN 90” to refer to both the nebula and the central et al. 2009; Young et al. 2012) of the two compact neb- star. ulae, MN 90 and HD 168625. By imaging these nebulae HD 168625 (IRAS 18184–1623) was first identified as a at a range of wavelengths, we can study their structure, candidate LBV by Chentsov & Luud(1989), who clas- estimate the dust composition, and quantify the dust sified it as spectral type B5.6 ± 0.3 with T ' 13000 ∗ temperature and total mass. These parameters will help K. Its spectral type seems to vary from B2 (Popper & to determine the physical properties of the circumstellar Seyfert 1940) to B8 (Morgan et al. 1955), although no environments and to constrain the importance of erup- dramatic light-variations have been been reported in the tive mass-loss in post-main sequence stellar evolution. last 40 years (van Genderen et al. 1992; Sterken et al. In Section2 we summarize the observations and data 1999). The lack of evidence for large variations kept reduction strategies. Section3 describes the axisym- HD 168625 from being classified as an LBV. However, it metric radiative transfer code 2-Dust and the derived was found to be LBV-like by Hutsemekers et al.(1994) dust geometry parameters and inferred mass-loss histo- who, using near-infrared (near-IR) and visible imaging ries. We discuss the results of our analysis in Section4 and spectroscopy, found a high mass-loss rate and a shell and present the conclusions in Section5. with two regions: an inner 1000 × 1300 elliptical ring and MN 90 (central star 2MASS J18455593-0308297) was a perpendicular outer horn-shaped region suggesting a discovered and catalogued by Gvaramadze et al.(2010) bipolar outflow. Nota et al.(1996) used deeper H α imag- using the Multiband Imaging Photometer (MIPS; Rieke ing of the nebula to identify faint filaments in the bipo- et al. 2004) aboard the (Werner lar structure that extended to 1600 ×2100, indicating that et al. 2004; Gehrz et al. 2007) in the MIPS Galactic a LBV-like major outburst occurred ∼ 103 years ago. Plane Survey (MIPSGAL; Carey et al. 2009), which Given its curious characteristics, HD 168625 has been 2 mapped 278 deg of the inner Galactic plane: −65◦ < the focus of several additional studies. Meixner et al. l < −10◦ and 10◦ < l < 65◦ for |b| < 1◦. It is one (1999) included it in their large, mid-infrared (mid-IR) of a large number of similar shells found in MIPSGAL proto-planetary nebula candidates survey and imaged it images that resemble the circumstellar nebulae of LBVs at 8.8, 12.5, and 20.6 µm revealing a toroidal dust shell. and late WN-type Wolf Rayet stars (WNL). The lack From mid-IR images at 4.7, 10.1, 11.6, and 19.9 µm, of optical counterparts for most of them indicates they Robberto & Herbst(1998) used an analytical spherical are highly obscured (Wachter et al. 2010). Follow-up model to derive a dust temperature of 135 K and a dust 3 spectroscopy of some of the other MIPS Nebulae has mass of 2.8×10− M . Similarly, Pasquali et al.(2002) revealed them to be LBVs, candidate LBVs or early- used mid-IR imaging at 4 and 11 µm to derive a dust type supergiants (see the summary list in Kniazev et al. temperature of 113 K, and a nebular expansion velocity 1 2015). MN 90 appears nearly circular as projected on of 19 km s− from ground-based echelle spectra. O’Hara the sky and appears to be a limb-brightened shell with et al.(2003) modeled the morphology of the toroidal a star at the center. The star is undetected in Swift UV dust region using 2-Dust and found a dust mass of 3 images, Palomar Observatory Sky Survey (POSS) vi- 2.5±0.1×10− M . Mahy et al.(2016) used far-infrared sual images, and the ground-based visual images used to (far-IR) imaging and spectroscopy with optical spectra make the HST Guide Star Catalog. The distance to MN to constrain the CNO abundances of HD 168625 and the 90 is unknown and there is little information about its surrounding nebula to determine that the central star physical parameters. Wachter et al.(2010) included MN had an initial mass between 28 – 33 M and lost its

90 along with a large sample of numerous similar shells material after the blue supergiant phase. Smith(2007) for follow-up spectroscopy but ultimately did not report presented Spitzer images showing that the bipolar lobes any spectra for this particular star. Based on the anal- and torus of HD 168625 were actually a triple-ring sys- ysis of other stars in their sample, they grouped MN 90 tem that closely resembled the ringed nebula around SN along with other shells for which they predict the stars 1987A. He pointed out that a single rotating star could are early-type. Naz´eet al.(2012) did not dectect any potentially eject an equatorial torus even if it is not ro- X-ray emission from MN 90 in their XMM-Newton sur- tating at the critical speed. Presumably this can occur MN 90 and HD 168625 3 because the stars approach or violate the classical Ed- Table 1. Summary of SOFIA/FORCAST Obser- dington limit during their giant eruptions when the mass vations of HD 168625 is ejected (see, e.g., Smith & Townsend 2007) allowing Filter λ0 ∆λ Exp. Time PSF FWHM the star’s rotation to be more influential at the result- ing lower effective gravity. Taylor et al.(2014) found (µm) (µm) (s) (arcsec) no evidence for a binary companion to HD 168625 from F077 7.7 0.47 30 2.500 monitoring and a modest rotation speed F111 11.1 0.95 30 2.500 1 of 53 km s− . The presence of a possible companion F197 19.7 5.5 16 2.700 star was detected by Aldoretta et al.(2015) using inter- F253 25.3 1.86 25 3.000 ferometric observations. A wide-orbit companion with F315 31.5 5.7 25 3.100 F336 33.6 1.9 31 3.100 a projected separation of 1.1500 was later confirmed by Martayan et al.(2016) using adaptive optics images, but F348 34.8 3.8 30 3.400 F371 37.1 3.3 26 3.5 there is no evidence that it is gravitationally bound to 00 HD 168625 and it is too distant to have any impact on the shaping of the nebula. The role companions might play in the formation and evolution of LBV and LBV- like objects is still not well understood. We observed HD 168625 on UT 2014 June 13 dur- ing Guest Investigator (GI) Cycle 2 using eight different filters which are summarized in Table1 and2. Observa- tions of Beta Andromedae provided the flux calibration 2. OBSERVATIONS & DATA REDUCTION and PSF. The targets were observed with SOFIA/FORCAST during Guest Investigator (GI) Cycles 2 and 3. Descrip- tions of the SOFIA Observatory and its science instru- 2.1. Spectral Energy Distributions ment (SI) suite have been given by Becklin et al.(2007), We present the IR spectral energy distributions Gehrz et al.(2009), and Young et al.(2012). (SEDs) of MN 90 and HD 168625 in Figure1 and FORCAST is a dual-channel mid-IR imager cover- 2, respectively. To supplement our newly acquired ing the 5 – 40 µm range. Each channel uses a 256 SOFIA/FORCAST photometry, we also gathered × 256 pixel array and provides a distortion-corrected archival data from the NASA/IPAC Infrared Science 1 3.20 × 3.20 field of view with a scale of 0.76800 pix− . Archive (IRSA; Berriman 2008) database. These in- The Short Wave Camera (SWC) uses a Si:As blocked- clude photometry from the Two Micron All-Sky Survey impurity band (BIB) array optimized for λ < 25 µm, (2MASS; Skrutskie et al. 2006) at 1.25, 1.65, and 2.17 while the Long Wave Camera’s (LWC) Si:Sb BIB array µm, the AKARI satellite (Murakami et al. 2007) at is optmized for λ > 25 µm. Observations were taken 9 and 18 µm, the Wide-field Infrared Survey Explorer in standard two-position chop-and-nod mode with the (WISE; Wright et al. 2010) at 3.4, 4.6, 12 and 22 µm, the direction of the nod matching the direction of the chop Midcourse Space Experiment (MSX ; Egan et al. 2003) (NMC). The data were reduced by the SOFIA Science at 8.3, 12.1, 14.7, and 21.3 µm the Spitzer MIPS at Center using the FORCAST Redux v1.0.1β and v1.0.6 24 µm, and the Spitzer Infrared Array Camera (IRAC) pipelines (Clarke et al. 2015) for HD 168625 and MN Galactic Legacy Infrared Midplane Survey Extraordi- 90, respectively. After correction for bad pixels and naire I (GLIMPSE I; Benjamin et al. 2003; Churchwell droop effects, the pipeline removed sky and telescope et al. 2009) program at 3.6, 4.5, 5.8, and 8 µm. Pho- background emission by first subtracting chopped image tometry from our SOFIA/FORCAST images and the pairs and then subtracting nodded image pairs. The re- Herschel (Pilbratt 2003) Photoconductor Array Cam- sulting positive images were aligned and merged. The era and Spectrometer (PACS; Poglitsch et al. 2010) at merged images were then coadded using a robust mean. 70, 100, and 160 µm were obtained by using aperture We observed MN 90 on UT 2015 July 3 during Guest photometry after sky background subtraction. For MN Investigator (GI) Cycle 3 using the F371 filter (λ0 = 37.1 90 we used an aperture of 3000 and a sky annulus with µm, ∆λ = 3.3 µm). The total coadded exposure time for an inner radius of 31.500 and outer radius of 3500. For the observation of MN 90 was 1553 sec (25.9 min). Table HD 168625 we used an aperture of 2000 and a sky an- 2 summarizes the observed flux from MN 90. Observa- nulus with an inner radius of 21.500 and outer radius tions of the asteroid Vesta provided the flux calibration of 2500. Note that in the longer wavelength Herschel and PSF, with a near-diffraction-limited FWHM at 37.1 PACS/SPIRE images of MN 90 from 160 − 500 µm µm of 3.600. the dust shell is no longer distinguishable above bright 4 Arneson et al.

Table 2. SOFIA/FORCAST Fluxes sweeps up gas and dust lost during the previous RSG from HD 168625 and MN 90 phase or by a previously ejected LBV shell that is swept up by the post eruption wind (Smith 2014). Filter Flux Error

12 2 12 2 2.3. Imaging of HD 168625 10− (W m− ) 10− (W m− ) MN 90 The FORCAST images of HD 168625 are shown in F371 2.74 0.01 Figure4. The nebula is clearly resolved, with a par- tially complete ring structure that has two peaks almost HD 168625 symmetric around the star. We concur with Meixner F077 14.75 0.04 et al.(1999); O’Hara et al.(2003) in their interpreta- F111 15.32 0.05 tion of these two peaks as limb-brightened peaks of a F197 46.76 0.18 torus of dust with a radius of ∼ 1000. The appearance is F253 28.00 0.06 consistent with previously obtained images at 8.8, 12.5, F315 22.51 0.05 and 20.6 µm from Meixner et al.(1999) and PACS 70 F336 26.03 0.05 µm images (Groenewegen et al. 2011). We stress that F348 19.43 0.03 our SOFIA/FORCAST images do not detect the outer F371 13.65 0.04 polar rings seen in Spitzer IRAC images (Smith 2007), suggesting that the rings must be cold and below the sensitivity limits of SOFIA/FORCAST. The emission detected with the ring morphology in the IRAC band 4 diffuse background emission. Only IRAC 5.8 µm pho- image was probably PAH emission or atomic line emis- tometry of the central star was available for HD 168625, sion, not thermal emission from warm dust. Figure5 the central star saturated the IRAC detector in the shows the temperature map that was derived from stack- 3.6 and 4.5 µm images, while warm circumstellar dust ing the λFλ SOFIA/FORCAST 7.7 − 37.1 µm images saturated the detector at 8.0 µm. For HD 168625 we fur- and performing a least-squares fit of the dust temper- ther include the Infrared Astronomical Satellite (IRAS; ature, Td, using the best fit modified blackbody of the 0.33 Neugebauer et al. 1984) Low Resolution Spectrometer SED (i.e. Bλ(Td) · λ ) at each pixel location. The (LRS) spectra from Volk & Cohen(1989). images were centered relative to one another by compar- We model the dust shell flux as equilibrium thermal ing the locations of the limb brightness peaks, and the emission from dust grains all at a single radius rd from 7.7−33.6 µm images were convolved with a 2D Gaussian the star. We assume the grains’ emissivity behaves as a kernel with a FWHM of 3.500 to match the resolution of β power law with Qλ ∝ λ− . For MN 90, a least-squares the 34.8 and 37.1 µm images. Our temperature map fit of a Bλ(Td) curve modified with this emissivity yields shows a large gradient in dust temperatures with inner Td = 59 ± 10 K, β = 0.76 ± 1.07, with the peak of the torus temperatures of ∼ 180 K and outer temperatures emission at λ = 42.7 µm. of ∼ 80 K, which is in agreement with the estimate of For HD 168625, a least-squares fit of a Bλ(Td) curve 170 ± 40 K obtained from our least-squares fit to the IR β modified with this emissivity Qλ ∝ λ− yields Td = excess but is slightly higher than the equilibrium tem- 170 ± 40 K, β = −0.33 ± 1.04 with the peak of the perature estimates made by Pasquali et al.(2002; 113 emission at λ = 18.3 µm. K), Robberto & Herbst(1998; 135 K) and O’Hara et al. (2003; 130 K). The inaccuracies of this temperature map 2.2. Imaging of MN 90 are due in large part to the fact that the method used Our FORCAST 37.1 µm image of MN 90 is shown in to create it assumes that emission is purely thermal and Figure3. The nebula is clearly resolved, with a radius that the dust shell is in thermal equilibrium. As noted of ∼ 1500. It appears nearly circular as projected on the previously, much of the 8.8 − 12.5 µm flux arises from sky, with brightened limbs enhanced towards the south- transient, non-equilibrium emission from PAH grains. east. The appearance is consistent with the previously Therefore, using images at these wavelengths to derive obtained MIPSGAL image at 24 µm(Carey et al. 2009). quantitative conclusions from the temperature map has Based on its appearance we treat the nebula as a thin, some limitations, however, we can interpret the maps hollow shell whose far-IR spectrum is due to equilibrium qualitatively as discussed in Section4. thermal emission from dust grains all at the same dis- tance from the star. This type of shell can result, for 2.4. IR Reddening example, when a post-red supergiant (post-RSG) star The mid-IR photometry (5 – 40 µm) must be de- has developed a fast wind during an LBV phase which reddened for comparison with the 2-Dust model SED MN 90 and HD 168625 5 MN 90

0.76 Bλ(59 K)λ− IRAC Herschel 2Dust Model AKARI SOFIA 11 2MASS WISE MIPS 10− MSX

12 10− ) 2 W m ( λ

F 13 10− λ

14 10−

15 10− 100 101 102 λ (µm)

Figure 1. Observed and model SEDs of MN 90. The reddened model SED is shown as a solid blue line. The dashed lines are the best β fitting Qλ · Bλ(Td) functions with an assumed power law emissivity Qλ ∝ λ− , which yields Td = 59 ± 10 K, β = 0.76 ± 1.07 with the peak of the emission at λ = 42.7 µm. Photometry data points are from this work (SOFIA) and archival databases. outputs, or conversely, the 2-Dust SEDs must be red- mag, using the Cardelli et al.(1989) reddening law and dened, as we have done. We used the Fritz et al.(2011) assuming RV = 3.1. extinction law as it utilizes the most recent near-IR (1 As mentioned in Section1, the distance to MN 90 is – 2.4 µm) observations of the galactic center. We ex- unknown. In the absence of further information about tend the law longward of 24 µm by adopting the Draine the star beyond the catalogued 1.2 − 8 µm photometry, (2003) interstellar extinction curve defined in Figure 10 we consider the implications of assuming that MN 90 5 of that paper, as was done by Lau et al.(2013). We is an LBV. We assume a luminosity of L? = 3 × 10 scaled the AKs of the Fritz et al.(2011) extinction law L , at the lower end of for LBVs in their based on the distance to the objects. We accomplished quiescent state between outbursts (see e.g. Figure 1 of this by utilizing the 3-D Milky Way dust map published Smith et al. 2004). This luminosity corresponds to T? ≈ β by Green et al.(2015) to estimate the extinction. We 14000 K. For dust grains with emissivity Qλ ∝ λ− , the then converted from extinction to reddening by assum- radius rd of the shell may be computed with: ing AKs = 0.320×E(B − V), as calculated by Yuan et al. (2013) for a 7000 K source spectrum at E(B − V) = 0.4  β 2 L? T? rd = 4 (1) 16πσTd Td 6 Arneson et al. HD 168625 9 10− 0.33 Bλ(170 K)λ IRAC Herschel IRAS LRS MSX IRAS 2Dust Model AKARI SOFIA 2MASS 10 10− ) 2 W

m 11 10− ( λ F λ

12 10−

13 10−

100 101 102 λ (µm)

Figure 2. Observed and model SEDs of HD 168625. The reddened model SED is shown as a solid blue line. The solid black line is the β IRAS LRS (Volk & Cohen 1989). The dashed line is the best fitting Qλ ·Bλ(Td) function with an assumed power law emissivity Qλ ∝ λ− , which yields Td = 170 ± 40 K, β = −0.33 ± 1.04 with the peak of the emission at λ = 18.3 µm. Photometry data points are from this work (SOFIA), and archival databases. where Td is the grains’ equilibrium temperature. Sub- anchor region around Brackett-γ (2.166 µm) into the stituting the assumed values for the star and the fitted visual. Td = 59 K obtained using β = 0.76 yields rd = 0.47 3. RADIATIVE TRANSFER MODELING pc. For the shell’s observed angular radius of 2000 this places MN 90 at a distance of 4.8 kpc. At this distance 3.1. 2-Dust Introduction we estimate a reddening of AKs = 0.52 for MN 90. We utilize the the axisymmetric radiative transfer The DR2 (Gaia Collaboration et al. 2018) dis- code 2-Dust (Ueta & Meixner 2003) to estimate the tance to HD 168625 is 1.61 ± 0.17 kpc or 1.55 kpc using dust mass and dust shell morphology of MN 90 and the Bayesian-inferred distance from (Bailer-Jones et al. HD 168625. The code solves the equation of radia- 2018). We adopt a distance of 1.55 kpc for HD 168625 tive transfer following the principle of long characteris- and estimate a reddening of AKs = 0.32 for HD 168625. tic (i.e. traces the radiation hitting the dust grain from Note that any visual wavelength values are subject to anywhere in the shell including the star and other dust considerable uncertainty, as Fritz et al.(2011) point out radiation) in a 2-D polar grid, while considering a 3-D that there are several possible extrapolations from their radiation field at each grid point. The dust opacities are MN 90 and HD 168625 7 calculated using Mie theory from a user-given size dis- Previous mid-IR spectra of HD 168625 by Skinner tribution and optical constants of the dust grains. It can (1997) indicate the presence of silicates in the dust be used to model a variety of axisymmetric astronom- shell of HD 168625 and previous studies of the dust ical dust systems. The dust distribution is expressed shells surrounding the LBVs Wra 751 and AG Car indi- analytically as cate that amorphous silicates are the dominant species (Voors et al. 2000). Observations of HD 168625 by F D D  B 1+C sin θ exp[ (r/rsw) ]/ exp[ (rmin/rsw) ]  r − { − − } ρ(r, θ) = ρmin Volk & Cohen(1989); Skinner(1997); Umana et al. rmin (2) (2010) have identified polycyclic aromatic hydrocarbons (PAHs), which is evident in the IRAS LRS spectra and  F × 1 + A(1 − cos θ) suggests the possible presence of carbonaceous grains E E  × {exp[−(r/rsw) ]/ exp[−(rmin/rsw) ]} as well. There is also evidence of crystalline forsterite grains being present (Blommaert et al. 2014), which where r is the radius within the limits of rmin and rmax, may contribute to the emission seen at ∼ 11 and 19 rsw is the boundary between the spherical AGB wind µm in the IRAS LRS. Therefore, we considered a com- and the axisymmetric superwind, θ is the latitude, and plex dust distribution model composed of amorphous ρmin is the dust mass density on the equatorial axis olivine (MgFeSiO4; Dorschner et al. 1995), crystalline at the inner edge of the envelope. The letters A − F forsterite (Mg2SiO4; Servoin & Piriou 1973; Scott & are input parameters that define the geometry of the Duley 1996), and amorphous carbon (Rouleau & Mar- dust density profile. A, changes the overall axisymmet- tin 1991). However, because 2-Dust cannot account for ric structure to the shell, which can be made disk-like transiently heated very small dust grains or PAH emis- or toroidal by the parameter F. The parameter B deter- sion, we ignore fitting the 2-Dust model to the 8 – 15 mines the radial fall-off of the profile and can be a func- µm region. tion of the latitudinal angle, θ, through the parameter Most estimates of the effective temperature of the cen- C. These parameters determine the toroidal structure of tral star of HD 168625 are between 12000 – 15000 K the innermost region of the shell, which is considered to (Nota et al. 1996; O’Hara et al. 2003; Mahy et al. 2016), be caused by an axisymmetric superwind at the end of and may vary by a few thousand degrees. The tempera- the AGB phase. The mid-region of the shell assumes ture and radius of the star have been adjusted to roughly a somewhat spheroidal dust distribution reflecting the match the observed SED. However, since the data points transition of mass loss geometry from spherical to axial are taken by many observers over several decades, we symmetry during the course of the AGB mass loss his- cannot model HD 168625 at any single . The in- tory. The parameters D and E control the abruptness ner radius of the dust shell is well constrained by these of the transition in the shell: small values correspond mid-IR images to be 8.500 or 0.06 pc at a distance of 1.55 to a slow transition and large values correspond to an kpc. abrupt transition. We used a Mathis et al.(1977) power As previously mentioned, little is know about MN90 law grain size distribution: and the central star. We use the SOFIA/FORCAST

3.5 37.1 µm image to constrain the angular size of rmin to n(a) = a− , amin < a < amax (3) a value of 2000 or 0.47 pc at a distance of 4.8 kpc. We started with a stellar temperature of 14000 K and ra- where a is the minimum grain size and a is the min max dius 100 R . Like HD 168625, the temperature and maximum grain sized, as specified by inputs. Because radius of the star have been adjusted to roughly match 2-Dust is axisymmetric, it is not possible to create dust the observed SED and we used a single species dust dis- shells with different size parameters in different lobes of tribution of amorphous silicates. the nebula. A more extensive discussion of the geometric parameters given in Equation (2) is given in Ueta & Meixner(2003). For more examples of the use of 2- 3.3. Model Results Dust, see Ueta et al.(2001a,b); Meixner et al.(2002); We ran approximately 150 models to obtain the best O’Hara et al.(2003); Meixner et al.(2004). fit to the SED and SOFIA/FORCAST images of both stars. When available, stellar and dust parameters were 3.2. Input Parameters taken from previous observations in the literature. We O’Hara et al.(2003) previously analyzed the morphol- started our modeling by fitting the stellar parameters ogy and parameters for the circumstellar dust around T and R . The mid-IR images constrain the inclination ∗ ∗ HD 168625 using 2-Dust, therefore, we adopt their pa- angle (θinc), inner radius of the dust shell (rmin), and rameters as initial values for our model. dust density function parameters A–F. We then fit the 8 Arneson et al.

Table 3. 2-Dust Density Function Table 4. MN 90 2-Dust Input and Derived Pa- Parameters for the Best-Fit Models rameters

Parameter MN 90 HD 168625 Input 4 L∗ d 9.3 10 L A 1 30 ∝ × T B 2 2 ∗ 16000 K R∗ d 85 R C 2.5 4 ∝ d 4.8 kpc D 0 6 ISM AKs 0.52 E 0 3 rmin d 0.35 pc ∝ F 0 4 rmax d 1.05 pc ∝ rsw d 0.52 pc θinc 0◦ 55◦ ∝ 4 3 amin 0.1 µm τ37.1µm at eq. 6 × 10− 3.3 × 10− amax 0.5 µm a −1 vexp 50 km s Amorphous Silicatesb 100%

Derived mass fraction of the mineral species, minimum (amin) 2 −2 Mdust d (3.2 0.1) 10 M and maximum grain sizes (amax), and optical depth at ∝ 2 ± ×−6 −1 M˙ dust d (7.3 0.4) 10 M yr 37.1 µm (τ37.1 ). We constrained the maximum and c ∝ ± × 4 µm τAGB d 1.0 10 yr minimum grain sizes to be between 0.001−2.0 µm. The ∝ × a best fit model was determined by eye. The best fit SEDs Typical for LBVs (Nota et al. 1995) b of MN 90 and HD 168625 are shown in Figures1 and MgFeSiO4 (Dorschner et al. 1995) 2. Input dust distribution parameters for both stars are c Timescale for mass-loss on the AGB given in Table3. Input and derived stellar and dust parameters for the best fit models of MN 90 are given in Table4. Input and derived stellar parameters for the best fit model of HD 168625 are given in Table5 with the dust parameters summarized in Table6. compare well with the FORCAST observations except The model SED for MN 90 fits the data well in the for the gap in the northern rim of the 7.7 µm emis- wavelength region λ ≥ 8 µm. The discrepancy at shorter sion and the gap in the southern rim of the 19.7, 25.3, wavelengths may be due to the choice of the anchor re- 31.5, and 33.6 µm, emission which may indicate a dif- gion for the reddening as discussed in Section 2.4, an un- ferent composition, azimuthal asymmetry in the dust derestimate in the distance, or incorrect stellar parame- density distribution around the ring, or the lack of large ters. Our model image of MN 90 compares well with the dust grains in this region. This supports the sugges- FORCAST 37.1 µm image except for the northwest edge tion that the dust shell may include small, transiently of the shell which appears brighter in the FORCAST heated dust grains that are not at thermal equilibrium image suggesting that the shell is slightly asymmetric. and are not accounted for in these models. The grain Overall, however, the morphology of the dust shell is size distribution suggests that grain sizes between 0.001 well modeled as a symmetric sphere. We found the best – 1.0 µm exist in the circumstellar environment. Our 3 fit density distribution had an amorphous silicate shell model gives a dust mass of (2.5 ± 0.1) × 10− M and 7 1 a dust mass-loss rate of (3.2 ± 0.2) × 10− M yr− . extending from rmin = 0.31 pc to 3rmin = 0.92 pc. The grain size distribution suggests that the silicates have If we rescale the dust mass estimates of O’Hara et al. grain sizes between 0.1 – 0.5 µm. Our model gives a (2003) using the Gaia DR2 distance, their dust mass 4 2 changes to (7.7 ± 0.3) × 10− M at a mass-loss rate dust mass of (3.2±0.1)×10− M and a dust mass-loss 6 1 1 of (6.7 ± 0.3) × 10 8 M yr 1 . The discrepancy can rate of (7.3 ± 0.4) × 10− M yr− × (vexp/50 km s− ). − −

The model SED for HD 168625 fits the data well in be attributed to both our larger grain size distribution most of the wavelength range except for 8 – 15 µm and complex dust distribution model which results in a as discussed in Section 3.2. We confirm the dust dis- larger optical depth in our model and therefore a larger tribution model of O’Hara et al.(2003) with only mi- dust mass estimate. Our estimate is likely an underesti- nor differences in the stellar temperature, radius and mate of the mass because we are only looking at material grain size distribution. The morphology of the dust is in the equator, ∼ 10 – 20% of the solid angle of the entire found to be a torus with an equator-to-pole density ra- nebula. The bipolar nebula seen in Spitzer images sug- tio of 30, an elliptical midshell, and an inclination angle gests that the circumstellar material at higher latitudes was ejected with higher speed, and therefore it is farther of 55◦ with the plane of the sky. Our model images MN 90 and HD 168625 9

37.1 (µm) 37.1 (µm) 30” 30” 30”

20” 20” 20”

10” 10” 10”

0 0 0

-10” -10” -10” ∆ DEC (arcsec)

-20” -20” -20”

-30” -30” -30”

-30” -20” -10” 0 10” 20” 30” -30” -20” -10” 0 10” 20” 30” ∆ RA (arcsec) ∆ RA (arcsec)

0.000 0.006 0.012 0.018 0.024 2 (Jy arcsec− )

Figure 3. Observed SOFIA/FORCAST image (left) and 2-Dust model image (right) of MN 90 at 37.1 µm with north up and east to the left. The models have been scaled to the same total flux as the observed image and convolved with a Gaussian with FWHM equal to the PSF of the SOFIA/FORCAST image (inset white circle). In the observed SOFIA/FORCAST image, the contours are spaced at 1σ, 2σ, and 3σ intervals above the background noise. In the 2-Dust model, the contours are space at 20% intervals of the peak intensity. The central star is not included in the model. from the star and cooler, which went undetected in the Table 5. HD 168625 2-Dust Input and SOFIA imaging that only detects the inner torus. The Derived Parameters model suggests that HD 168625 lost mass in a torus- Input shaped outburst, which has been suggested for all LBVs 4 L∗ d 4.5 10 L ∝ × by Hutsemekers et al.(1994). The current fast wind of T∗ 14500 K the LBV has probably interacted with this torus, creat- R∗ d 65 R ∝ ing an elliptical bubble perpendicular to the plane of the d 1.55 kpc A . torus, which is consistent with a unified model of LBV ISM Ks 0 32 rmin d 0.06 pc ∝ nebulae proposed by Nota et al.(1995). rmax d 0.23 pc ∝ rsw d 0.20 pc ∝a −1 4. DISCUSSION vexp 19 km s

As previously mentioned in Section 3.2, crystalline Derived forsterite has been detected around HD 168625. This 2 −3 Mdust d (2.5 0.1) 10 M ∝ 2 ± ×−7 −1 indicates that at least some of the circumstellar mate- M˙ dust d (3.2 0.2) 10 M yr b ∝ ± × 3 rial, probably that confined to the torus, has undergone τAGB d 1.5 10 yr ∝ × annealing and suggests that the circumstellar environ- a Pasquali et al.(2002) ment is similar to that of lower mass progenitors i.e. b proto-planetary nebulae (PPNe). We should point out Timescale for mass-loss on the AGB that our model for HD 168625 included moderately sized grains (0.1 – 1.0 µm), slightly larger than the model found by O’Hara et al.(2003; 0.001 – 1.0 µm) which 10 Arneson et al.

7.70 (µm) 7.70 (µm) 30” 30” 30”

20” 20” 20”

10” 10” 10”

0 0 0

-10” -10” -10” ∆ DEC (arcsec)

-20” -20” -20”

-30” -30” -30”

-30” -20” -10” 0 10” 20” 30” -30” -20” -10” 0 10” 20” 30” ∆ RA (arcsec) ∆ RA (arcsec)

0.00 0.05 0.10 0.15 0.20 2 (Jy arcsec− )

(a) FORCAST F077

11.1 (µm) 11.1 (µm) 30” 30” 30”

20” 20” 20”

10” 10” 10”

0 0 0

-10” -10” -10” ∆ DEC (arcsec)

-20” -20” -20”

-30” -30” -30”

-30” -20” -10” 0 10” 20” 30” -30” -20” -10” 0 10” 20” 30” ∆ RA (arcsec) ∆ RA (arcsec)

0.00 0.15 0.30 0.45 0.60 2 (Jy arcsec− )

(b) FORCAST F111 MN 90 and HD 168625 11

19.7 (µm) 19.7 (µm) 30” 30” 30”

20” 20” 20”

10” 10” 10”

0 0 0

-10” -10” -10” ∆ DEC (arcsec)

-20” -20” -20”

-30” -30” -30”

-30” -20” -10” 0 10” 20” 30” -30” -20” -10” 0 10” 20” 30” ∆ RA (arcsec) ∆ RA (arcsec)

0.0 0.5 1.0 1.5 2.0 2 (Jy arcsec− )

(c) FORCAST F197

25.3 (µm) 25.3 (µm) 30” 30” 30”

20” 20” 20”

10” 10” 10”

0 0 0

-10” -10” -10” ∆ DEC (arcsec)

-20” -20” -20”

-30” -30” -30”

-30” -20” -10” 0 10” 20” 30” -30” -20” -10” 0 10” 20” 30” ∆ RA (arcsec) ∆ RA (arcsec)

0.0 0.4 0.8 1.2 1.6 2 (Jy arcsec− )

(d) FORCAST F253 12 Arneson et al.

31.5 (µm) 31.5 (µm) 30” 30” 30”

20” 20” 20”

10” 10” 10”

0 0 0

-10” -10” -10” ∆ DEC (arcsec)

-20” -20” -20”

-30” -30” -30”

-30” -20” -10” 0 10” 20” 30” -30” -20” -10” 0 10” 20” 30” ∆ RA (arcsec) ∆ RA (arcsec)

0.0 0.3 0.6 0.9 1.2 2 (Jy arcsec− )

(e) FORCAST F315

33.6 (µm) 33.6 (µm) 30” 30” 30”

20” 20” 20”

10” 10” 10”

0 0 0

-10” -10” -10” ∆ DEC (arcsec)

-20” -20” -20”

-30” -30” -30”

-30” -20” -10” 0 10” 20” 30” -30” -20” -10” 0 10” 20” 30” ∆ RA (arcsec) ∆ RA (arcsec)

0.0 0.4 0.8 1.2 1.6 2 (Jy arcsec− )

(f) FORCAST F336 MN 90 and HD 168625 13

34.8 (µm) 34.8 (µm) 30” 30” 30”

20” 20” 20”

10” 10” 10”

0 0 0

-10” -10” -10” ∆ DEC (arcsec)

-20” -20” -20”

-30” -30” -30”

-30” -20” -10” 0 10” 20” 30” -30” -20” -10” 0 10” 20” 30” ∆ RA (arcsec) ∆ RA (arcsec)

0.0 0.4 0.8 1.2 1.6 2 (Jy arcsec− )

(g) FORCAST F348

37.1 (µm) 37.1 (µm) 30” 30” 30”

20” 20” 20”

10” 10” 10”

0 0 0

-10” -10” -10” ∆ DEC (arcsec)

-20” -20” -20”

-30” -30” -30”

-30” -20” -10” 0 10” 20” 30” -30” -20” -10” 0 10” 20” 30” ∆ RA (arcsec) ∆ RA (arcsec)

0.0 0.4 0.8 1.2 2 (Jy arcsec− )

(h) FORCAST F371

Figure 4. Observed SOFIA/FORCAST image (left) and 2-Dust model image (right) of HD 168625 at (a) 7.7 µm, (b) 11.1 µm, (c) 19.7 µm, (d) 25.3 µm, (e) 31.5 µm, (f) 33.6 µm, (g) 34.8 µm, and (h) 37.1 µm with north up and east to the left. The models have been scaled to the same total flux as the observed image and convolved with a Gaussian with FWHM equal to the matching PSF of the SOFIA/FORCAST image (inset white circle). In the observed SOFIA/FORCAST images, the contours are spaced at 1σ, 2σ, and 3σ intervals above the background noise. In the 2-Dust model, the contours are space at 20% intervals of the peak intensity. The central star is not included in the model. 14 Arneson et al.

Temperature 1 Temperature 30" 30" 30"

20" 20" 20" ) c

e 10" 10" 10" s c r a

( 0 0 0

C E D

-10" -10" -10"

-20" -20" -20"

-30" -30" -30"

-30" -20" -10" 0 10" 20" 30" -30" -20" -10" 0 10" 20" 30" RA (arcsec) RA (arcsec)

0 60 120 180 240 0 20 40 60 80 100 Temperature (K) 1 Temperature (K)

Figure 5. Temperature map (left) and 1σ temperature error map (right) of HD 168625, derived from stacking the λFλ SOFIA/FORCAST 7.7–37.1 µm images and performing a least-squares fit of the dust temperature, Td, using the best fit modified blackbody of the SED (i.e. 0.33 Bλ(Td) · λ ) at each pixel location. Pixels with errors larger than 100 K have been excluded.

Table 6. Dust Properties for Best-Fit 2-Dust Model of HD 168625

Dust Species Composition Structure Mass Fraction Density Grain Size Reference

Olivine MgFeSiO4 Amorphous 60% 3.71 0.001 1.0 Dorschner et al.(1995) − Forsterite Mg2SiO4 Crystalline 25% 3.22 0.500 2.0 Servoin & Piriou(1973); Scott & Duley(1996) − Carbon BE sample Amorphous 15% 1.44 0.001 1.0 Rouleau & Martin(1991) −

points to the nebula being relatively young and unpro- 4.1. MN 90 cessed. Aside from the similarities in morphology be- The morphology of MN 90 lacks a large equator-to- tween PPN and LBVs, a key difference may be the dura- pole mass distribution like most LBVs and is nearly tion of mass-loss. Far-IR images at 55 µm of HD 168625 spherical, similar to V4998 Sgr (Lau et al. 2014). The by O’Hara et al.(2003) suggests that the outer dust total mass (gas plus dust) lost by the star and the rate shell is no more than 5 times the inner radius. Whereas, of mass-loss are estimated to be about 3.2 ± 0.1 M and for the Egg Nebula, a well-studied PPN, 180 µm im- 4 1 1 (7.3±0.4)×10− M yr− × (vexp/50 km s− ) using the ages suggest that the outer shell is a few hundred times canonical gas-to-dust ratio of 100. Dividing our value for larger than the inner radius (Speck et al. 2000). The rmin = 0.35 pc by an assumed expansion velocity of 50 mass-loss shells of LBVs appear to be more compressed 1 km s− gives an estimated expansion time of ' 6800 yr than for PPNs, suggesting that the mass-loss occurred and using our value of rmax = 1.05 pc, we estimate the 4 in a more short-lived (< 10 yr) phase compared with mass-loss lasted for ' 1.4 × 104 yr. As noted in Table 5 PPNs (∼ 10 yr). 4, the 2-Dust estimates for the dust mass and mass-loss rate are proportional to the distance squared and given the distance to MN 90 is unknown the values we have MN 90 and HD 168625 15 estimated in this work should be interpreted as hypo- – 33 M for HD 168625 and propose that the star lost thetical. its mass during or just after the blue supergiant (BSG) Based on the mid-IR morphology and results from ra- phase and has not yet reached the red supergiant (RSG) diative transfer calculations, MN 90 is a very luminous phase. Furthermore, they found that single star evolu- star surrounded by an optically thin dust shell located at tionary tracks were able to explain the N content be- about 4.8 kpc. However, given the uncertainties in the tween the nebula and the central star. This depends distance and stellar parameters, no strong conclusions on the assumption that the star is a single star, whereas about the nature of MN 90 can be made. It is necessary Smith & Tombleson(2015) argue that LBVs may be the to observationally uncover the physical characteristics product of binary evolution. As mentioned in Section1, of the central star to determine the exact evolutionary a wide-orbit binary companion to HD 168625 has been status of MN 90. Because MN 90 is not visible in the observed, but the influence this companion has on the optical due to heavy extinction, near-IR spectroscopic evolution and morphology of HD 168625 is negligible. information is needed to constrain the physical parame- Given that no X-ray emission has been observed (Naz´e ters for the central star. Furthermore, continuous near- et al. 2012) rules out mass transfer via Roche lobe over- IR photometry to better characterize the variability of flow. The rotation rate of HD 168625 is estimated to be 1 the source would be useful. Although we interpret MN 53 km s− (Taylor et al. 2014) which is not high enough 90 as a candidate LBV star, it may be a B[e] super- to explain the bipolar structure based on rotation or giant or post-RSG star. These classes have similar stel- binary merger unless the star passed through a super- lar and dust parameters as candidate LBVs and can be Eddington phase. Furthermore, no companion has been confused with each other. Regardless of the exact evo- detected in radial velocity monitoring. lutionary status, MN 90 seems highly likely to be an The photodissociation region (PDR) detected around evolved, massive post-main sequence star. HD 168625 (Umana et al. 2010; Mahy et al. 2016) in- dicates that neutral gas makes up the majority of the 4.2. HD 168625 shell’s total mass. Mahy et al.(2016) measured a total ionized and neutral hydrogen gas mass of 1.17 M . Us- The temperature map shows a higher temperature on 3 ing our dust mass estimate of (2.5±0.1)×10− M , this the southern edge of the dust shell, ∼ 180 K, and a much corresponds to a gas-to-dust mass ratio of 470. Using lower temperature on the northern edge, ∼ 80 K. This this gas-to-dust mass ratio, we estimate a total (gas plus same temperature variation was found by O’Hara et al. 4 1 dust) shell mass-loss rate of (1.5±0.1)×10− M yr− . (2003). O’Hara et al.(2003) suggest that one possible Dividing our value for rmin = 0.06 pc by the nebular ex- explanation for the different temperatures may be that pansion velocity measured by Pasquali et al.(2002) gives the grain size distribution varies with respect to position an expansion time of ' 2800 yr, and using our value of in the nebula. Because smaller grains tend to be warmer 4 rmax = 0.23 pc, the mass-loss lasted for ' 1 × 10 yr. than larger grains, this would suggest that the small- est grains are in the southern shell with progressively larger grains towards the northern shell. Pasquali et al. (2002) interpret these distinctive optical morphologies 5. CONCLUSION as a variation in the gas-to-dust mass ratio with respect Our SOFIA/FORCAST image at 37.1 µm of MN 90 to position in the nebula (i.e. it is higher in the south). shows a limb-brightened, spherical dust shell surround- The smaller dust power-law emissivity (β = −0.33) ing the central star. A least-squares fit of a Bλ(Td) curve 0.76 we have measured for HD 168625 compared to the value with emissivity Qλ ∝ λ− to the SED of MN 90 yields used by Robberto & Herbst(1998; β = 1.2) is likely due a dust temperature of 59 ± 10 K, with the peak of the to the fact that the circumstellar dust exhibits a large emission at 42.7 µm. Our 2-Dust model supports the temperature variation (see Figure5) and emission from idea that the dust resides in a thin, spherical dust shell 2 small, transiently heated PAH grains (see IRAS LRS and estimates that MN 90 lost (3.2 ± 0.1) × 10− M of in Figure2). It is not realistic to model the dust as dust in a massive with a mass-loss rate of 6 1 1 a single temperature–a temperature gradient would be (7.3 ± 0.4) × 10− M yr− × (vexp/50 km s− ). Using more physically motivated–but in order to compare with the canonical gas-to-dust mass ratio of 100, we estimate previously measured dust temperatures of HD 168625 a total mass-loss of 3.2 ± 0.1 M for MN 90. These and to minimize the number of free parameters we only 2-Dust estimates assume that MN 90 has a luminosity use a single modified blackbody. of 9.3×104 L , at the lower end of luminosities of LBVs

Mahy et al.(2016) used Herschel/PACS spectroscopy in their quiescent state, and a distance of 4.8 kpc. Our and CNO abundances to estimate an initial mass of 28 2-Dust model of MN 90 has good agreement with ob- 16 Arneson et al. servations if we assume very small, transiently heated end of luminosities of LBVs in their quiescent state, and silicate grains. a distance of 1.55 kpc. Our SOFIA/FORCAST images between 7.7 – 37.1 µm of HD 168625 complement previously obtained mid-IR imaging. The dust temperature map that we derive from The observations were made with the NASA/DLR our observations shows a temperature variation between Stratospheric Observatory for Infrared Astronomy the northern and southern shells, suggesting different (SOFIA). SOFIA is jointly operated by the Universities grain size distributions between the two shells. A least- Space Research Association, Inc. (USRA), under NASA 0.33 squares fit of a Bλ(Td) curve with emissivity Qλ ∝ λ contract NNA17BF53C, and the Deutsches SOFIA In- to the SED of HD 168625 yields an estimated dust tem- stitut (DSI) under DLR contract 50 OK 0901 to the perature of 170 ± 40 K, with the peak of the emission at University of Stuttgart. This research was supported by 18.3 µm. Our detailed radiative transfer model using 2- NASA under USRA funding for programs 02 0101 and Dust supports the claim that the dust resides in a thin, 03 0131. RDG was supported, in part, by the United axisymmetric equatorial torus and estimates that HD States Air Force. 3 168625 lost (2.5 ± 0.1) × 10− M of dust in a massive 7 stellar wind with a mass-loss rate of (3.2 ± 0.2) × 10− 1 Facilities: SOFIA (FORCAST) M yr− . These 2-Dust estimates assume that HD

168625 has a luminosity of 4.5 × 104 L , at the lower Software: 2-Dust

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